3-Methylcrotonyl-CoA Carboxylase Converts HMB-CoA into HMG-CoA: Relevance to the Supposed Role of HMB as a Ketogenic Substrate in the Brain and Liver

This article [Woessner et al., 1958: (http://www.jbc.org/cgi/reprint/233/2/520.pdf)(http://www.ncbi.nlm.nih.gov/pubmed/13563531)] shows that biotin is required as a cofactor for the enzymatic conversion of HMB-CoA (which is also called beta-hydroxyisovaleryl-CoA or 3-hydroxyisovaleryl-CoA) into 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA), which can then be converted into ketones or cholesterol or other fatty acids. This article [Knappe et al., 1961: (http://www.ncbi.nlm.nih.gov/pubmed/14457200)] provides evidence that the biotin-dependent enzyme is, in fact, 3-methylcrotonyl-CoA carboxylase (MCCC). That's one of the four biotin-dependent carboxylase enzymes. That's pretty important, and none of the articles on HMB has ever mentioned that the utilization of HMB for cholesterol biosynthesis requires the activity of a biotin-dependent enzyme. Some articles have mentioned that HMB accumulates during biotin deficiency or in people with genetic hypofunctionality of either biotinidase, which recycles biotin and is involved in the biotin-status-dependent upregulation of the expression of biotin-dependent enzymes (biotinidase has histone biotinyltransferase activity and can thereby catalyze the biotinylation of histones, and this allows for the regulation of gene expression in interesting ways, in response to changes in biotin status/availability), deficiency (genetic deficiency or hypofunctionality) or isolated MCCC deficiency, but none of the articles has said why HMB accumulates in those situations. One reason is that MCCC is the enzyme or at least one of the enzymes that converts HMB-CoA to HMG-CoA. The articles that show that (the ones I've cited) have only been cited 2 or 3 times in 50 years, and that tells me that very few people are even aware of the reason that HMB (3-hydroxyisovalerate) accumulates in these conditions of diminished MCCC activity.

HMB also accumulates in several other genetic disorders [beta-ketothiolase deficiency, propionyl-CoA carboxylase deficiency (another disorder producing hypofunctionality of another biotin-dependent enzyme), HMG-CoA lyase deficiency, etc.], and part of the reason for the confusion is probably that the carboxylation of 3-methylcrotonyl-CoA (MC-CoA) itself, by MCCC, also would be expected to decrease the formation of HMB-CoA, in the absence of exogenous HMB, by converting MCC into 3-methylglutaconyl-CoA [Wendel and de Baulny, 2006: (http://www.springerlink.com/content/gnk7414290467321/)]. So MC-CoA (a.k.a. beta-methylcrotonyl-CoA) can either be converted into HMB-CoA by the enzyme crotonase [shown in this cited reference as beta-hydroxyisovaleryl-CoA: Bachhawat et al., 1956: (http://www.jbc.org/cgi/reprint/219/2/539.pdf)(http://www.ncbi.nlm.nih.gov/pubmed/13319276)] or into 3-methylglutaconyl-CoA, which can then also be converted into HMG-CoA by 3-methylglutaconyl-CoA hydratase (Wendel and de Baulny, 2006). This article on HMB as a nutritional supplement [Nissen and Abumrad, 1997: (http://linkinghub.elsevier.com/retrieve/pii/S095528639700048X)] identifies some of the enzymes but doesn't say, for example, that MCCC converts HMB-CoA to HMG-CoA. I think it's one of those situations in which no one does searches on HMB under its alternate names, such as 3-hydroxyisovalerate. Some of these articles, though, are a bit obscure.

These articles are important for the supposed metabolism of HMB into ketones, in astrocytes or meningeal fibroblasts, or into cholesterol in those and other cell types in the brain, etc. MCCC is expressed in many parts of the brain, and this would suggest, in my opinion, that HMB could easily be used for cholesterol formation in the brain or liver or any tissue that contains the enzymes of cholesterol biosynthesis (i.e. the skin) or for ketogenesis in the brain or liver, etc. HMB would have to be converted into a CoA thioester, obviously, but that clearly occurs readily.

Neuroprotective and Supposed Antidepressant-Like Effects of Sodium Butyrate: Relevance to HMB Research and Energy Metabolism

A lot of these articles showing that butyrate (usually administered or used in vitro as sodium butyrate, or SB), a short-chain fatty acid similar in structure to HMB (3-hydroxy-3-methylbutyrate or 3-hydroxyisovalerate, discussed in the two previous postings), reduces the degradation of numerous proteins by proteasomes are relevant to research on HMB. There are many similarities among the effects of butyrate and HMB. HMB is thought to exert its anticatabolic effects by inhibiting proteasomal activity (http://scholar.google.com/scholar?num=100&hl=en&lr=&q=proteasome+methylbutyrate+OR+%223-hydroxyisovalerate%22) and also by acting as a precursor of HMG-CoA and of cholesterol. The extent to which an HMB-induced increase in cholesterol formation contributes to the HMB-induced inhibition of proteasomal activity is unknown. SB is a nonselective inhibitor of histone deacetylase enzymes in vitro, and its histone deacetylase inhibitory effect, at least in vitro, is thought to contribute to its inhibition of TNF-alpha-induced NFkappaB (NFkB) transcription factor [a.k.a. the "Rel" family of subunits that form the dimers that comprise NFkB transcription factors: (http://scholar.google.com/scholar?num=100&hl=en&lr=&q=RelA+RelB)] activation in the cytosol (Yin et al., 2001: (http://www.jbc.org/cgi/reprint/276/48/44641)(http://www.ncbi.nlm.nih.gov/pubmed/11572859?dopt=Abstract); (http://scholar.google.com/scholar?num=100&hl=en&lr=&q=%22sodium+butyrate%22+proteasome)]. Butyrate doesn't prevent the ubiquitination of IkB proteins (inhibitors of NFkB activation) but causes them to acccumulate as ubiquitin-conjugated proteins, without being degraded in proteasomes, evidently (Yin et al., 2001). HMB is also thought to exert anti-inflammatory effects by suppressing NFkB activation, as a result of the HMB-induced suppression of "proteasomal activity" [Baxter et al., 2005: (http://www.ncbi.nlm.nih.gov/pubmed/16006030)].

I don't doubt that some of those mechanisms are important, and a decrease in the activation of NFkB transcription factors can be antiproliferative and can downregulate the expression of numerous pro-inflammatory cytokines (cytokines that suppress mitochondrial functioning), etc., but SB is produced by microorganisms in the GI tract and is known to be the major energy substrate for colonocytes in the submucosal layers (I forget the terminology) of the colon (http://scholar.google.com/scholar?num=100&hl=en&lr=&q=%22sodium+butyrate%22+energy). The in vitro research probably uses bizarre conditions and shows that SB can induce apoptosis of colon cancer cells. It looks like SB is pro-apoptotic at high but not low concentrations (0.5 mM to 2 mM) [Singh et al., 1997: (http://carcin.oxfordjournals.org/cgi/reprint/18/6/1265.pdf)(http://www.ncbi.nlm.nih.gov/pubmed/9214612)]. But SB supposedly doesn't produce very strong histone deacetylase inhibition in the brain in vivo in animals, but it does produce neuroprotective effects in all sorts of different models (http://scholar.google.com/scholar?num=100&hl=en&lr=&q=%22sodium+butyrate%22+neurodegenerative+OR+neurological+OR+neuroprotective+OR+Parkinson%27s+OR+Huntington%27s+OR+ischemia+OR+ischaemia+OR+hypoxia+OR+anoxia). Sodium butyrate has also produced some "antidepressant-like" effects in animal models of depression (http://scholar.google.com/scholar?q=%22sodium+butyrate%22+antidepressant&hl=en&lr=). Sodium butyrate is also sold as a supplement (http://www.google.com/products?q=sodium+butyrate&hl=en&aq=f).

Anyway, I just put this information up here, but I have no idea what the dosage range would be. One would obviously want to discuss this type of thing with one's doctor, and the most obvious, potential problem would be the disturbances in phosphate or calcium homeostasis in response to something like this. The infusion of 3-hydroxybutyrate, a "ketone" that doesn't have a carbonyl group but is defined as being a ketone, and acetate, for example, can increase plasma bicarbonate, and this effect appears to be the result of the metabolism of the organic acids/fatty acids and not from effects on phosphate homeostasis, in some articles. But these organic anions can just have strange effects, and it's something to be aware. Many medications can affect acid-base homeostasis and could interact with sodium butyrate or HMB. Some anticonvulsants act as carbonic anhydrase inhibitors, for example, and could interact with these types of short-chain fatty acids (such as sodium butyrate) or branched-chain organic acids/fatty acids (such as HMB).

In my opinion, sodium butyrate probably acts mostly as an energy substrate, but that doesn't exclude other mechanisms. I also think the research on sodium butyrate is likely to be relevant to future research on the mechanisms of action of HMB. Both compounds inhibit proteasomal activity and may have overlapping or similar effects, but I don't think it's going to be as simple as testing HMB as a "histone deacetylase inhibitor." Histone acetylation is extraordinarily complex and dynamic, and to think that one can treat a multitude of conditions with histone deacetylase inhibitors is not realistic, in my opinion. Vitamin D receptor activation can increase histone acetylation (http://scholar.google.com/scholar?num=100&hl=en&lr=&q=%22vitamin+D%22+histone+acetyltransferase+OR+acetylation), much as sodium butyrate supposedly does (histone deacetylase inhibition leads to increases in the acetylation of histone proteins). But it's clear, in my opinion, that a lot of the effects of sodium butyrate cannot be explained in terms of histone acetylation.

Notes on HMB; Leucine as a Supposedly-Ketogenic Amino Acid

This article [Kuhara et al., 1982: (http://www.ncbi.nlm.nih.gov/pubmed/7116632)] describes leucine as being a "potent ketogenic amino acid," and I guess there can be a significant contribution of leucine-derived branched chain organic acids, such as HMB/3-hydroxyisovalerate, to ketogenesis. About 5 percent of leucine normally is converted into HMB, apparently. The only other thing I thought of is that the slight elevation in plasma branched-chain amino acids (Holocek et al., 2009) [Holecek et al., 2009: (http://www.ncbi.nlm.nih.gov/pubmed/19056452)] might conceivably produce tryptophan (TRP) and tyrosine (TYR) and phenylalanine (PHE) depletion from the brain, in my opinion, but it looks like the effect is probably not even as pronounced as the increase in the plasma BCAA (leucine+isoleucine+valine)/(TYR+TRP+PHE) ratio from a high-protein meal. In dietary protein, 20-30 percent of the amino acids are BCAA's, if memory serves, and this causes the plasma BCAA/(TYR+TRP+PHE) ratio to increase progressively as the dietary protein intake increases [Fernstrom et al., 1979: (http://www.ajcn.org/cgi/reprint/32/9/1912.pdf)(http://www.ncbi.nlm.nih.gov/pubmed/573061)]. There's the potential that that effect, to the extent that it could occur in response to the mild, apparent "leucine-sparing" effect of HMB (Holocek et al., 2009), could, in my opinion, produce transient worsening of mood or other psychiatric complications in people. Although some of those trials with HMB suggest that the opposite effect would occur, I can't really put a lot of faith in some of these articles on the effects of BCAAs on the brain. They're used as neuroprotectives and as a treatment for mania (leucine and other BCAAs), and I doubt HMB would have the same effects. It's not an amino acid and wouldn't be expected to compete with tyrosine, tryptophan, and phenylalanine for entry into the brain. But I can't be sure about that, and it's obviously something a person would want to discuss with one's doctor. It's possible that the supposed neuroprotective effects of leucine and BCAAs, as in spinocerebellar degeneration, etc., are not mediated by glutamine or 2-oxoglutarate sparing or whatever mechanism has been suggested but are the result of a ketogenic effect of leucine in astrocytes (http://scholar.google.com/scholar?num=100&hl=en&lr=&q=BCAA+spinocerebellar). The authors of those BCAA-as-neuroprotective articles view the BCAAs as being energy substrates or as being capable of decreasing glutamate, though, I think. I don't know what the proposed mechanisms are, besides those mechanisms. HMB can also act as a precursor of fatty acids and could conceivably cause problems in people with liver disease, but those old articles show that it's, evidently, preferentially incorporated into cholesterol. I discussed the BCAA issues in a past posting (http://hardcorephysiologyfun.blogspot.com/2009/02/potential-psychiatric-pitfalls-in.html).

HMB (3-hydroxyisovalerate)

I was reading about beta-hydroxy-beta-methylbutyrate (HMB) (a.k.a. 3-hydroxyisovaleric acid or 3-hydroxy-3-methylbutyrate or beta-hydroxyisovaleric acid, etc.) again and was thinking it might be useful as an adjuctive for neuroprotective/neurotrophic or other applications. There are a few articles showing mood improvements in people taking HMB (http://scholar.google.com/scholar?q=methylbutyrate+mood+OR+%22well+being%22&hl=en&lr=), evidently for "muscle building" purposes or prevention of trauma-induced muscle wasting, and it would stand to reason that it could have some effect in that regard or produce "indirect ketogenesis" by sparing acetyl-CoA that would otherwise be used for cholesterol biosynthesis, etc. The reports of mood improvement sound bogus to me, but it's not always wise to dismiss these things. I just put these things up on the "blog" and try to give my impressions. But the absence of research, in the form of a big 10,000-person study that provides no information but that hypes up some compound, is not, in any way, evidence of the absence of validity of a compound. I don't know if HMB would have any usefulness at all for brain-related applications, but it's supposedly converted, primarily, into hydroxymethylglutaryl-CoA (HMG-CoA) in skeletal muscle myocytes and other cell types and promotes satellite cell proliferation and increases IGF-1 mRNA in satellite cells, etc. It's supposedly useful for treating catabolic conditions by virtue of its capacity to serve as a cholesterol precursor (HMG-CoA is a precursor or "building block" of cholesterol). My guess is that it may very well not be useful for brain-related applications and may, in my opinion, cause the accumulation of HMB-CoA, which could inhibit the enzymes of the glycine cleavage system or other enzymes, but I may be wrong about that.

I don't have to mention that brain cholesterol depletion (depletion of membrane cholesterol in axon terminals or postsynaptic membranes of neurons in the brain, etc.) is thought to potentially be one factor contributing to the association of low cholesterol with violent death or suicide or death by accidents [(http://scholar.google.com/scholar?num=100&hl=en&lr=&q=cholesterol+suicide); (http://scholar.google.com/scholar?num=100&hl=en&lr=&q=cholesterol+murder); (http://scholar.google.com/scholar?num=100&hl=en&lr=&q=cholesterol+%22violent+death%22); (http://scholar.google.com/scholar?num=100&hl=en&lr=&q=cholesterol+accident)]. I know this research is disturbing, but I didn't come up with the research. Don't shoot the messenger. There's considerable research suggesting that very low cholesterol levels cause people to become clumsy or impulsive or reckless and die by accidents or to kill themselves violently or to essentially be at a greater "risk" of being murdered, for unknown reasons. Presumably it's because they become aggressive or impulsive and get into some sort of confrontation that ultimately results in their being murdered, in my opinion. Violent death also includes death by suicide, and there's research showing unusually violent suicides among people with very low cholesterol. I know it's quite an unpleasant topic, but I'm just including this information to explain my reasons for not immediately dismissing something like HMB. I don't think it's possible to just dismiss such large numbers of articles, in any event. It's also relevant that cholesterol is degraded into propionate, and propionate can be anaplerotic and enter the tricarboxylic acid cycle as succinate, via succinyl-CoA. So HMB could conceivably be indirectly anaplerotic, but it could also not be. HMB could indirectly enhance ketogenesis by some mechanism, and increases in ketone availability have been suggested as approaches to treating mood disorders (http://scholar.google.com/scholar?num=100&hl=en&lr=&q=mood+ketogenic+OR+ketone).

HMB wouldn't have to elevate plasma cholesterol to conceivably contribute to cholesterol biosynthesis in the brain. Most (80+ percent) of the cholesterol in the brain is made in situ, in astrocytes and oligodendrocytes, and low plasma cholesterol could just be a sign of generalized mitochondrial dysfunction, causing impairments in beta-oxidation (fatty acid oxidation) and therefore in acetyl-CoA availability for cholesterol biosynthesis. But the extents to which plasma cholesterol normally correlates with synaptosomal cholesterol or with other variables related to brain lipid metabolism are not even known. My sense is that low plasma cholesterol can be part and parcel of HPA axis activation and could result from glucocorticoid resistance, but it may also just be a sign of mitochondrial dysfunction (derangements in energy metabolism). I know those aren't very mechanistically-rich statements.

Someone should do research to see if it improves myelination or recovery from brain injuries in animals or something like that. Acetyl-L-carnitine, for example, has been shown to increase myelination or prevent the loss of myelin during aging in animals (http://scholar.google.com/scholar?num=100&hl=en&lr=&q=%22acetyl-L-carnitine%22+myelin+OR+%22white+matter%22), but who knows if that type of effect, which is basically thought to result from its role in increasing "acetyl" group availability and maintaining beta-oxidation in astrocytes, in the brain, or in Schwann cells in the peripheral nervous system, would occur in humans. Someone should start by finding out what the actual mechanism of action is, though, because the research on HMB is astonishingly devoid of information on the actual mechanisms by which HMB even might regulate leucine metabolism. There's a recent article providing evidence that it doesn't inhibit the activity of the branched-chain alpha-keto acid dehydrogenase multienzyme complex in vivo, using labeled leucine, I think [Holecek et al., 2009: (http://www.ncbi.nlm.nih.gov/pubmed/19056452)]. But there must be some actual mechanism. Saying that it activates p70 S6 kinase is not saying anything about the mechanism, really (http://scholar.google.com/scholar?num=100&hl=en&lr=&q=p70+methylbutyrate). How does it activate the p38 MAPK and p70 S6 kinase cascades?

HMB appears to lower plasma cholesterol very slightly in humans, but my main concern with it would be the potential to either disrupt phosphate homeostasis [Sousa et al., 1996: (http://www.ncbi.nlm.nih.gov/pubmed/8852485)] or, as HMB-CoA/3-hydroxyisovaleryl-CoA (not HMG-CoA), inhibit the enzymes of the glycine cleavage system, etc. Alpha-keto acids produced by the metabolism of leucine, isoleucine, and valine are thought to be capable of inhibiting the glycine cleavage system (http://scholar.google.com/scholar?num=100&hl=en&lr=&q=%22glycine+cleavage%22+hydroxyisovaleric+OR+hydroxyisovalerate). 2-ketoisocaproate (a.k.a alpha-ketoisocaproate or 4-methyl-2-oxovaleric acid) is an example of a ketoacid metabolite (formed from leucine). Of course, the accumulation of those compounds occurs, in part, because many different inhibitory effects of toxic intermediate metabolites occur in people with genetic defects (which cause the accumulation of 3-hydroxyisovalerate). So there is likely to be not only a defect leading to overproduction of 3-hydroxyisovalerate but also a defect in the utilization of 3-hydroxyisovalerate (HMB) in people with genetic diseases. But it's still something to be aware of, in my opinion.

A lot of ketoacids, which are similar to but not the same as HMB/3-hydroxyisovalerate, can decrease serum phosphate (http://scholar.google.com/scholar?num=100&hl=en&lr=&q=ketoacid+hyperphosphatemia), and it's fairly clear, in my opinion, that they don't exert these effects just by existing as calcium salts (such as calcium beta-hydroxy-beta-methylbutyrate) in the GI tract and "binding up" phosphate (preventing phosphate absorption) or by stimulating phosphate uptake into cells (the "refeeding" syndrome or phenomenon). They use some ketoacids to treat hyperphosphatemia, and the ketoacids decrease serum phosphate and can, as a result, decrease parathyroid hormone levels, etc. There seems to be an effect of some ketoacids on acid-base regulation in the kidneys, but that's just my opinion. HMB apparently has a net charge of -1 at physiological pH values, and that would tend to suggest that it's not going to cause the kinds of issues that something like, for example, 2-oxoglutarate, which supposedly has a -2 charge in neutral solution, could cause, in my opinion. There's a toxicological study in rats [Baxter et al., 2005: (http://www.ncbi.nlm.nih.gov/pubmed/16006030)] that shows elevations in serum inorganic phosphorus, which would suggest that HMB does not cause hypophosphatemia and may actually increase serum phosphate. But I'm just saying that it's the type of thing to be aware of and discuss with one's doctor.

I hope I don't need to say this, but attempting to increase the serum cholesterol level would not be a wise or effective approach to treating brain disorders or depression. Some research shows that it's almost impossible to increase one's serum cholesterol past a certain point, with diet alone. Some people do experience increases in serum total cholesterol in response to increases in dietary cholesterol, partly because of deficient feedback inhibition of HMG-CoA reductase activity by exogenous cholesterol. Some "graphs" of LDL-cholesterol changes in response to dietary cholesterol intake show some effects [Weggemans et al., 2001: (http://www.ajcn.org/cgi/content/full/73/5/885)(http://www.ncbi.nlm.nih.gov/pubmed/11333841)], but the authors of other articles make the argument that there's very little effect of dietary cholesterol on serum cholesterol [McNamara, 2000: (http://www.jacn.org/cgi/content/full/19/suppl_5/540S)(http://www.ncbi.nlm.nih.gov/pubmed/11023005?dopt=Abstract)]. In any case, the notion that an increase in cholesterol per se would automatically exert effects on the brain doesn't make sense to me, because very little cholesterol is thought to be transported from the blood to the brain. The more likely outcome is that one would worsen atherosclerosis. The researchers studying HMB make the argument that it could be protective against cardiovascular disease. I don't know if it's true, but I just throw these suggestions out there and offer my thoughts. Extremely low cholesterol levels, in the context of some types of neurodegenerative diseases or depression, are, in my opinion, one manifestation of some other metabolic disturbance.

Note on Niacinamide and PARP Inhibition

In reference to that posting about niacinamide, I was going to mention that I don't think niacinamide is at all effective as a PARP inhibitor in vivo. I discussed some of the issues surrounding that area of controversy in a past posting (http://hardcorephysiologyfun.blogspot.com/2008/12/nonoxidative-pentose-cycle-prpp-and.html). That's just my opinion, and those issues are resurfacing again in the context of the regulation of transcription by ADP-ribosylation. I've seen articles suggesting that niacinamide or some prodrug type of derivative of niacinamide could be used to regulate ADP-ribosylation, but the capacities of niacinamide to increase poly(ADP-ribose) levels (in that old posting, I cite some research showing that) and to induce feed-forward increases in PARP activity strongly offset, in my opinion, any PARP inhibition or beneficial transcriptional changes, such as by an enhancement in ADP-ribosylation of proteins, that niacinamide may produce in vivo. I know there's excitement about influencing some of these mechanisms, and I know that the issue of niacinamide as a PARP inhibitor looked interesting and promising in the past. But, as some of that research I linked to in the old posting shows, niacinamide tends, in my opinion, to have the opposite effect in vivo. It can increase PARP activity by providing more NAD+, and it can also increase the formation of nitric oxide and reactive oxygen species by enzymes that display NADPH oxidase activity (many enzymes), etc. I'm not going to get into a discussion of the problems I see with some of these popular areas of NAD+ related research.

Uridine-Induced Maintenance of Glycogen and Total Adenosine Nucleotide Concentrations During Hypoxia: Apparent Increases In Glucose Uptake, etc.

This article is great [Rosenfeldt et al., 1998: (http://www.ncbi.nlm.nih.gov/pubmed/9794090)], and it's about uridine and not orotic acid. Orotic acid ("orotate") is a precursor of uridine but is generally toxic to the liver, in my opinion (http://scholar.google.com/scholar?q=%22fatty+liver%22+orotate+OR+orotic&hl=en&lr=), and those effects are, paradoxically, the opposite of those of uridine. Uridine has been used to treat fatty liver disease and decreases orotate formation by causing the uridine-nucleotide-mediated inhibition carbamoyl phosphate synthetase II, etc. Rosenfeldt et al. (1998) found that exogenous uridine maintained the glycogen content in the heart, increased the lactate output from the heart, and prevented much of the loss of adenine nucleotides from the heart during hypoxia. There's a typo that shows up in a couple of places, but the authors knew what they were talking about. The article is fantastic. But the concentration of uridine is listed as having been 17 mM, and the authors mean 17 uM (micromolar). The authors refer to the 17 uM concentration in the discussion section, but the mM concentration showed up in the results section. That's the Greek letter "mu," which can be an "m" in fonts other than symbol font, etc.

But the authors' analysis of the metabolic effects of uridine is really terrific. They measured the lactate output and the amount of glycogen formed and estimated that uridine had increased the rate of glucose uptake by about 50 percent but had not increased the minimal level of oxygen uptake that had occurred during the experiment. They discuss similar research and discuss the fact that the uridine-induced stimulation of glycolysis and glucose uptake (the glycogen concentration was almost double the concentration in the hearts subjected to hypoxia in the absence of uridine) is likely to have produced the uridine-induced increase in purine salvage during hypoxia. The total amount of purine nucleotides that was lost during hypoxia was almost cut in half by uridine. 17 uM is not much higher than the normal physiological plasma concentrations of uridine in humans. The major circulating pyrimidine in humans is uridine, but the major circulating pyrimidine in rats is cytidine. That's the reason the baseline plasma uridine concentration is so low in the rats. There's a lot of other research showing that nucleotides can increase glucose uptake and increase lactate output, but the effects are more complex than that.

These effects of uridine on glycogen formation and glucose uptake and even lactate output help to explain its supposed antidepressant and neuroprotective effects in humans and animals. Researchers have used uridine or its prodrugs to treat depression in a number of trials, and it's been used to treat mitochondrial disorders (encephalopathy and cardiomyopathy due to mitochondrial dysfunction, etc.). But the effects of uridine and other nucleotides are generally quite different from something like AICAR, even though other nucleotides, such as adenosine, have sometimes been shown to enhance AMPK activation (phosphorylation) and activity (by their effects of maintaining the total adenine nucleotide pool or increasing the intracellular AMP concentration more than the intracellular concentrations of other adenine nucleotides), much as AICAR activates AMPK [AMPK activation occurs when specific residues on it are phosphorylated, and the activity of AMPK is its phosphorylation of its target proteins, such as phosphofructokinase, etc.] ([Jaswal et al., 2007: (http://0-ajpheart.physiology.org.library.pcc.edu/cgi/reprint/292/4/H1978)(http://www.ncbi.nlm.nih.gov/pubmed/17172269)]. The concentration of adenosine used in that experiment was 500 uM, however, and that's a supraphysiological concentration. It's by no means a toxic concentration, because adenosine exerts various trophic effects on endothelial cells up to 1000 uM. But the extracellular adenosine concentrations don't usually exceed about 100 uM. The effects of nucleotides on AMPK activation and and activity are likely to depend on the concentrations used, and it's also important to consider the effects of nucleotides on the phosphocreatine to creatine (PCr/Cr) ratio. Many other articles show that uridine, alone or in combination with exogenous purine nucleotides, increases the PCr/Cr ratio. That effect would tend to produce allosteric inhibition of AMPK activity, etc. The main issue I have with AMPK activators is not that AMPK activation per se is "bad." In fact, the inhibition of AMPK activity or activation by specific, drug inhibitors produces toxic effects during ischemia. It's fairly clear that AMPK activation plays a role in maintaining glycolytic activity during hypoxia or ischemia. But my problem is with this assumption that "more" AMPK activation and activity is always going to be "better," and it's evident, in my opinion, that this is not always (or even usually) going to be the case, especially in the long term. This is a great article that discusses some of these issues with research on AMPK in the context of ischemia and elevated contractile activity in the heart [Dyck and Lopaschuk, 2006: (http://jp.physoc.org/content/574/1/95.full.pdf+html)(http://www.ncbi.nlm.nih.gov/pubmed/16690706?dopt=Abstract)].

Competitive Inhibitory Effects of Vitamin B6 and Vitamin B3 on NAD+ and PLP Formation; Narrow and Variable Therapeutic Margin for Vitamin B3 (and B6)

I was remembering that vitamin B6 and vitamin B3 are very structurally similar, and researchers have found that each one is capable of inhibiting the biosynthesis of the other's coenzymes. The relevance of this is that, in my opinion, a decrease in the dosage of B6 has the potential to augment the effects of B3 and vice-versa, and this importance of the ratio of B3 to B6 is potentially significant. Niacinamide and niacin are the two most-commonly supplied forms of vitamin B3, but I'm mainly discussing the effects of niacinamide, here. Niacin has other effects on lipid metabolism that niacinamide doesn't have. I'm going to refer to niacinamide as "B3" because I'm tired of typing out the long names.

I don't feel like discussing all the potential problems with high doses of B3, but B3 can, in my opinion, produce effects that are consistent with either poly(ADP)-ribose (PAR) accumulation, resulting from the utilization of B3-derived NAD+ as a substrate for poly(ADP)-ribose polymerase and other enzymes participating in ADP-ribosylation [Hassa et al., 2006: (http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=1594587)(http://www.ncbi.nlm.nih.gov/pubmed/16959969)], and the associated PRPP and ATP depletion or with increases in iNOS activity, etc. It can cause thrombocytopenia [Rottembourg et al., 2005: (http://www.ncbi.nlm.nih.gov/pubmed/16316377)], which could be a result of the hypophosphatemia that it's also been shown to cause [Muller et al., 2007: (http://cjasn.asnjournals.org/cgi/content/full/2/6/1249)(http://www.ncbi.nlm.nih.gov/pubmed/17913971); Takahashi et al., 2004: (http://www.ncbi.nlm.nih.gov/pubmed/14871431)], and also liver dysfunction [many, many references, including probably those referring to "pruritus" from niacinamide, refer to liver dysfunction from high doses of niacinamide and niacin (cholestasis commonly causes pruritus)], either by interfering with PLP formation or depleting SAM-e in the formation of N-methylniacinamide or by increasing iNOS activity or NADPH oxidase activity, etc., etc. The thrombocytopenia appears to require rather high doses, such as 1000 mg/d (Rottembourg et al., 2005), but I wouldn't assume that any dose, above some minimal dose, is absolutely not going to cause problems. (Nicotinamide is the same thing as niacinamide.) On the other hand, B3 deficiency can cause fatty liver disease. But I generally think the effects of B3 on iNOS or PARP or NADPH oxidases can get out of control very quickly, and it's sort of like vitamin B2 and ubiquinone in that regard, in my opinion. There's much more of a rationale for using somewhat higher dosages of, for example, vitamin B5, vitamin B1, and biotin, in my opinion. But even those cofactors can lower free fatty acids excessively, as in the case of vitamin B5, or produce effects, just by their normal mechanisms, that are not always going to be desirable, in my view. But they don't really have the potential to participate in these wild, redox cycling reactions that vitamins B2 and B3 (and coenzyme Q10) can, in my opinion, participate in and facilitate.

So there's a narrow dosage range (I would define a crude, therapeutic dosage range for niacinamide as 25-75 mg/d or something, but it's possible that most of the benefits would begin to plateau at doses lower than that or at the lower end of that range), and there can be a danger in, for example, reducing the dose of B6 and finding that some aberrant or undesirable effects occur. These are all just my opinions, of course. One could erroneously conclude that the effects of a decrease in the B6 dosage are "bad" because of the B6 reduction. In reality, the "bad" effects might merely be the result of a disinhibition of the biosynthesis of NAD+ from B3, resulting from the absence of such a pronounced inhibitory effect of pyridoxine or pyridoxal or PLP on nicotinamide phosphoribosyltransferase activity, etc.

People are constantly drawing inappropriate conclusions about B3 metabolism in the literature. For example, the absence of a decrease in NAD+ levels does not necessarily mean that PAR levels have not been increased in response to exogenous B3. The B3 moiety of NAD+ can be recycled (niacinamide is the main product of the PARP reactions), but this recycling could, in my opinion, amount to a kind of ATP and PRPP depleting futile cycle. PARP contains ADP-ribose but does not sequester the actual nicotinamide (B3) moiety of NAD+, but a small increase in the pool of available, recyclable nicotinamide could conceivably waste a lot of adenine nucleotides and PRPP and ATP in the *acceleration* of ADP-ribosylation reactions. NAD+ is also a cofactor of iNOS and other NADPH oxidase enzymes, and extra B3 could just augment the formation of excessive iNOS-derived nitric oxide and produce other reactive oxygen species, in my opinion. NO (nitric oxide) also activates PARP activity, etc. People seem to think that the iNOS protein concentration and activity, in a given tissue, cannot be elevated unless a person is septic or falling on the floor from some overwhelming inflammatory disease, but this is not the case, in my view. Of course, if one thinks that NAD+ levels are going to be maximized in response to an intake of 0.5 mg per day of B3, because the National Research Council says so (I forget what it's called), then one also isn't going to be able to understand the dose-response effects of B3.

These interactions between B3 and B6 have been researched in the context of "pellagra," which is defined as a B3 deficiency disease but that can actually result from either B6 or B3 deficiencies or both. The most well-known effect is the competitive inhibition of pyridoxal kinase by niacin or niacinamide or both, and this can cause pellagra-like photosensitivity (some of the kynurenine intermediates are, apart from the porphyrins, the only known endogenously-produced photosensitizing compounds) and other effects by interfering with the B6-dependent metabolism of tryptophan. These interactions can be complex, because B6 depletion disrupts the metabolism of tryptophan to niacin (niacin can be made from tryptophan in humans, in the so-called "kynurenine pathway"). This can cause intermediates in the kynurenine pathway to accumulate, and many of these can inhibit pyridoxal kinase, the enzyme that forms PLP. That further deranges the kynurenine pathway, etc. Pyridoxal kinase is inhibited by 3-hydroxykynurenine, 3-hydroxyanthranilate, xanthurenate (i.e. xanthurenic acid), and picolinate (i.e. picolinic acid) [Takeuchi and Shibata, 1984: (http://www.pubmedcentral.nih.gov/picrender.fcgi?artid=1153685&blobtype=pdf)(http://www.ncbi.nlm.nih.gov/pubmed/6466295)]. That article is actually good and may help explain some of the reports of adverse effects from free-form L-tryptophan, in my opinion. I've discussed that in past postings, but Takeuchi and Shibata (1984) discuss the very high Km values for the bindings of substrates to some of those kynurenine-pathway enzymes, etc. Some of the effects of "B3 pellagra" are, obviously, just caused, proximally, by NAD+ depletion. Essentially all niacinamide is thought to be initially converted into NAD+ in vivo, but niacin is metabolized differently. I forget the precise differences, but niacin causes hypolipidemic and vasodilating ("flushing") effects by increasing prostaglandin production (by some mechanisms that I forget). But the point is that in "B3 pellagra," there isn't enough quinolinic acid available for niacin and NAD+ synthesis (see Hassa et al., 2006). As a result, more tryptophan is diverted down the kynurenine pathway that converts tryptophan to quinolinic acid, and this increases the turnover of the PLP and causes those intermediates to build up and further deplete PLP (by decreasing its formation), etc. Here are a couple other articles(not great examples) that discuss some of these interactions [Darvay et al., 1999: (http://www.ncbi.nlm.nih.gov/pubmed/10354170); Siniscalchi et al., 2005: (http://www.ncbi.nlm.nih.gov/pubmed/16039138)]. Some drugs, such as theophylline in high doses, inhibit pyridoxal kinase also, etc.

Potential Role of Astrocyte Glycogen Depletion and "Supercompensation" to the Putative Circadian Neurobiological Effects of UVB Exposure

In reference to my previous posting, it's interesting that, apart from the neuronal pathways by which neurons in the caudal trigeminal nucleus could directly or indirectly influence hypothalamic neurons (there's quite a bit of research on the interactions of hypothalamic neurons with the trigeminal system in the context of migraines), activity in sensory neurons causes localized glycogen depletion in parts of the brain [Dienel et al., 2002: (http://www.nature.com/jcbfm/journal/v22/n12/full/9591343a.html)(http://www.ncbi.nlm.nih.gov/pubmed/12468893); (http://scholar.google.com/scholar?q=sensory+glycogen+stimulation&hl=en&lr=)]. That article showed an adaptive increase in glycogen content during "recovery" from the sensory stimulation, much as noradrenaline-induced astrocyte glycogen depletion is followed by a "rebound" increase in glycogen content (like a miniature version of glycogen "supercompensation" that can occur after glycogen-depleting exercise, when a person eats a high-carbohydrate diet, etc.). That could be an additional mechanism by which UVB exposure of the face, in particular, could regulate circadian neurobiology. Astrocyte glycogen levels (which constitute most of the brain glycogen) generally decrease progressively, during prolonged wakefulness, and UVB could intensify that pattern of glycogen depletion and overcompensation, etc., intensifying the entrainment of circadian neurobiological rhythms (http://scholar.google.com/scholar?num=100&hl=en&lr=&q=astrocyte+glycogen+sleep). The glycogen utilization, in the context of the asynchronous firing of trigeminal ganglion neurons, would produce depletion of astrocyte glycogen in various parts of the brain but could conceivably also participate in the subjective effects of UVB exposure. Maybe research on that type of mechanism would put all this business about "opioidergic" effects to rest.

Brief Summary of Mechanisms in UVB-Induced Neuroimmunomodulation via Trigeminal Ganglion and Dorsal Root Ganglion Neurons

My old paper is such a God-awful, rambling thing (http://hardcorephysiologyfun.blogspot.com/2009/05/another-old-paper-of-mine.html), that I thought I should attempt to summarize some of the mechanisms I reviewed in it. The overall concept is that UVB causes these "early mediators," such as prostaglandins and bradykinin, to be rapidly induced, within minutes of exposure. Some of these early responses are just mediated by superoxide production or by the activation of phospholipase enzymes, etc. These early-phase mediators, which cis-urocanic acid is also an example of, act on sensory nerve fibers innervating the epidermis and either directly induce axon reflex-mediated vasodilation or sensitize C-fibers and Adelta-fibers [the dorsal root ganglion (DRG) neurons] to the more potent actions of UVB-induced nerve growth factor and interleukin-6 and TNF-alpha, all of which are expressed in UVB-irradiated keratinocytes and cause the stronger, sustained effect of inducing "spontaneous" (but obviously not spontaneous) action potentials by acting directly on the peripheral terminals of DRG neurons innervating the epidermis. An axon reflex occurs when an action potential only travels to a branch point in a C-fiber, with the second "branch terminal" or arborization innervating the dermis and terminating at a blood vessel. The action potential "becomes" efferent at the branch point and then travels to the "second terminal" of the peripheral branch of the C-fiber, in the skin. Some of these apparent axon reflexes may, in fact, be dorsal root reflexes, however. There can also be weaker currents in C-fiber terminals that are not actual action potentials but that could cause neuropeptide release. But then, once DRG neurons begin to fire en masse, asynchronously, the action potentials traveling to the dorsal horn or, in response to UVB-irradiated facial skin, the caudal trigeminal nucleus and release CGRP and substance P and glutamate from their central terminals into the dorsal horn or caudal trigeminal nucleus. The CGRP and substance P then act on interneurons or spinothalamic tract neurons or spinoreticular tract or "trigeminothalamic tract" neurons, etc. As a result, the firing of these neurons can induce neurotransmitter release at segments rostral to the spinal segments at which the C-fibers releasing them entered the dorsal horn, and this type of polysynaptic transmission has been shown to occur in the spinal cord in response to UVB exposure of the skin.

It has also been shown in response to UVB exposure of the eyes of rodents, but the anatomical pathways are obscure. The action potentials probably just go to the trigeminal nucleus, from the trigeminal ganglion neurons innervating the corneas, and then cause neurons projecting from the caudal trigeminal nucleus to the hypothalamus to fire and cause hypothalamic neurons to induce the release of alpha-MSH from the pituitary, etc. (the effect could be blocked by hypophysectomy or ciliary ganglionectomy). I'm saying that because the ciliary ganglionectomy procedure may have actually cut some trigeminal ganglion nerve fibers passing through the accessory ciliary ganglion, I think it's called. I looked into the anatomical possibilities in detail but was nonetheless not able to definitively narrow down the possibilities. The anatomy is surprisingly complex, and I think the peripheral branches (sometimes referred to as "postganglionic" fibers, even though they're sensory neurons) of some trigeminal ganglion neurons travel through the ciliary ganglia and on to the caudal trigeminal nucleus. I forget the details, but it's another example of polysynaptic neurotransmission induced in response to UVB exposure to the skin. I explained the way dorsal root reflexes work in a previous posting, and it's noteworthy that GABA produces depolarization, not hyperpolarization, of the central terminals of C-fibers and Adelta fibers, upon its release from interneurons in response to other C-fiber inputs [a dorsal root reflex requires two separate C-fibers (C-type DRG neuron) or one C-fiber and an Adelta fiber, etc., and the feature that distinguishes it from an axon reflex is that the efferent action potential begins in the dorsal horn, at the central terminal of the "second" C-fiber in response to GABA receptor activation on the C-fiber].

There are obviously many other parts of the brain that would be likely to be affected, but there hasn't been much research, since the 1990s, on the changes that occur in the spinal cord. To my knowledge, no one has examined the specific phenotypic changes that would be expected to occur in the trigeminal nucleus or other parts of the midbrain or brainstem, for example, in rats exposed to UVB. Presumably, someone is interested in doing basic research in photobiology/photoneuroimmunology and learning what the effects on the circadian rhythm are. It's very likely that there are effects on circadian neurobiology, etc., and that they occur via the sensory nerve fibers.

The general idea behind systemic immunosuppression induced by UVB is that Langerhans cells, which are immature dendritic cells in the skin, are induced to migrate to regional lymph nodes in large numbers, within 6 hours or so of UVB exposure. They can induce tolerance to skin-associated or skin-derived antigens because they never mature, essentially. I'm forgetting why their migration precludes their maturation, but immature dendritic cells are tolerogenic. The infiltration of monocytes and neutrophils into the skin, at various time points after exposure, is also required for UVB-induced systemic suppression of delayed-type hypersensitivity to protein antigens injected intradermally into the UVB-exposed site, right after exposure. Different types of cells are constantly infiltrating into and migrating from the skin in response to UVB exposure, and these cells are acted upon by cytokines released from mast cells in the skin and from keratinocytes, etc. The infiltrating monocytes and neutrophils also release cytokines that act on keratinocytes and sensory nerve fibers, etc.

These are not just effects that result from increases in skin temperature or from some other nonspecific effect, because the researchers controlled for that by using cold UV sources and controlling for skin temperature and also because the phenotypic changes in the spinal cord can last up to 7-9 days. It'll probably be another 20 years before anyone does the research, though, the way things are now. It's unfortunate, because it's an interesting set of mechanisms. Obviously, these are potentially very damaging effects, and I would strongly urge anyone to talk with his or her doctor, very seriously, before receiving any sun exposure.

I'm explaining this because it's interesting and because researchers have been bullied, over the years, into not discussing it or even looking at the research, seemingly. The research has been sitting around for almost 20 years, while all the (anti-)intellectual bullying has gone on and led nowhere.

Low Solubility and Kinetics of Spontaneous Degradation of L-Glutamine in Aqueous Solution

The authors of this article [Arii et al., 1999a: (http://www.ncbi.nlm.nih.gov/pubmed/10493999)] cite research showing that L-glutamine degrades spontaneously to 5-pyrrolidone-2-carboxylic acid, at a rate of between 0.7 and 5 percent per day, in aqueous solution at pH values near 7 (neutral, as in the pH of tap water, etc.). The authors' own experiments show that, at 70 degrees C (158 degrees F), about 50 percent of the glutamine is gone in about 12 hours, at pH 7.39. That's a high temperature and would presumably accelerate the reaction rate, but it's still much slower than the degradation of the pyruvate anion in aqueous solution. Arii et al. (1999a) mention old research showing that glutamine can degrade to glutamic acid and ammonia in aqueous solution, and I remember reading some statements, by relatively unreliable sources, implying that this degradation was really rapid. It's not rapid, and the information in this article is consistent with information in other, reliable articles I've seen. The main reason free glutamine isn't commonly used in solutions for parenteral administration is that its solubility is quite low, but it's not as low as one might think. It just can't be used in "highly concentrated" solutions. I have an article that discusses this. Researchers primarily administer glutamine in the form of L-alanyl-L-glutamine, a dipeptide. In terms of its long-term "shelf life," that dipeptide is obviously much more stable than glutamine [Arii et al., 1999b: (http://www.ncbi.nlm.nih.gov/pubmed/9845788)]. I don't think that would substantially confound attempts to make subjective interpretations of data collected in people receiving intravenous glutamine, in the form of that dipeptide. The export of alanine from the skeletal muscles tends to coincide with the output of glutamine from the skeletal muscles (in vivo) [Cersosimo et al., 1986: (http://www.ncbi.nlm.nih.gov/pubmed/3513612)], and the metabolism of orally-administered glutamine by intestinal epithelial cells sometimes produces an elevation of plasma alanine, etc.

Problems With Glutamine Research

A lot of these cell culture experiments showing the effects of exogenous glutamine, in the presence or absence of other substrates, are using these "luxuriant," as some authors describe "abundance" as being, concentrations of extracellular glutamine, such as 5 mM (http://scholar.google.com/scholar?q=glutamine+%225+mM%22&hl=en&lr=) or, more commonly, 2 mM (http://scholar.google.com/scholar?num=100&hl=en&lr=&q=glutamine+%222+mM%22). Those are very high concentrations, and the extracellular fluid glutamine concentration in the brain is 400-1300 uM or so, which is 0.4-1.3 mM. I think there can be a tendency to do cell culture research and assume that cells in vivo are getting these abundant supplies of substrates, but it's not necessarily the case. I've seen that in research on magnesium, in which researchers explicitly assume that all of the ATP in vivo is going to be MgATP(2-). It's not the case, in my opinion. Also, most of the research on glutamine in humans has been done in people who have not been receiving any source of exogenous glutamine, and this is a major issue. Part of this has to do with the assumption that glutamine does not enter the brain, but there's considerable evidence that it does. The rate of efflux from the brain is almost always higher than the rate of uptake into the brain, but that says nothing about the extent to which glutamine can enter the brain. Neither does the absence of a discernable "spike" or increase in the ECF glutamine concentration, in response to the infusion of intravenous glutamine, provide any information about the extent to which glutamine has entered the brain. The glutamine-glutamate cycle is very dynamic and flexible and tends to adapt to sources of exogenous glutamine. This means that, for example, the glutamine is converted into glutamate and then either directly into 2-oxoglutarate, by glutamate dehydrogenase, or transaminated, with oxaloacetate, into 2-oxoglutarate (2-OG) and aspartate. The 2-OG can then be oxidized in the tricarboxylic acid (TCA) cycle, and its carbons can appear in all TCA cycle intermediates and in acetyl-CoA also, etc. Yudkoff et al. (1988) [Yudkoff et al., 1988: (http://www.ncbi.nlm.nih.gov/pubmed/2900878)] found that a physiological concentraion of extracellular glutamine (500 uM) caused cultured astrocytes to demonstrate no net utilization or synthesis of glutamine, and a supraphysiological concentration of 5 mM (5000 uM) was required to show a net utilization of glutamine by astrocytes. They had to remove all glutamine from the culture medium to cause the astrocytes to show a net synthesis of glutamine. The rate of synthesis is not the same thing as the rate of export, but I'm not going to get into all of that. This seems strange, but they're talking about utilization of labeled glutamine. Similarly, exogenous glutamine can spare the utilization of the existing glutamate pool for glutamine synthesis and not even elevate the total intracellular glutamate concentration [Qu et al., 2001: (http://www.ncbi.nlm.nih.gov/pubmed/11746415)]. These and other articles tend to suggest that, following ischemia, for example, the drastic increases in the oxidation of 2-OG are likely to cause exogenous glutamine to appear to exert no effect on the brain. I've discussed other mechanisms that suggest this, in recent postings. In other articles, researchers have noted that very few sites in the body demonstrate a *net* formation of glutamine, meaning that the rate of export from the tissue or cell group at large is higher than the rate of utilization of glutamine. A lot of research portrays the glutamine-glutamate-GABA cycle as if it's constantly generating all this glutamine and that there's endless glutamine being supplied and that cells can't oxidize more than 5 percent and can't even use all of it, because there's so much of it, or whatever. But large amounts of ATP are constantly being used up to maintain the cycle, and the amount of superfluous glutamine is likely to be quite small in many tissues, especially in trauma patients, etc.

Note on Terminology of Trigeminal Reflexes

I guess the term "trigeminal root reflex" isn't commonly used, but I think I'll call it that anyway. I forget what the different terms are, for the reflexes I'm thinking of. Assuming one buys into all of the "strict rules" about different processes and reflexes, there are 50 names for every other aspect of the trigeminal system.

Regulation of Cerebral Blood Flow By Trigeminal Root Reflexes or Other Causes of Efferent Action Potentials in Trigeminal Ganglion Neurons

This article [Arbab et al., 1992: (http://www.ncbi.nlm.nih.gov/pubmed/1481736)] describes the anatomical pathways that could allow ultraviolet B (some of the articles describing the use of "UVA" to induce sunburn pain are likely to be really showing effects mainly mediated by UVB; if even 1 percent of the irradiance of a source of UV radiation is in the UVB wavelengths, this small amount of UVB can be responsible for something like 95 percent of the biological effects) to influence cerebral blood flow or the degranulation of perivascular mast cells, etc. (this is in reference to a paper of mine that I posted a couple of days ago). The authors cite research showing that severing the peripheral branches of some trigeminal ganglion nerve fibers of the maxillary and ophthalmic divisions of the trigeminal nerve can reduce the diameter of the ipsilateral cerebral arteries by 25 percent, meaning that CGRP and substance P and other mediators are released, in response to efferent action potentials that are probably originating in the caudal trigeminal nucleus, from the peripheral terminals of trigeminal ganglion neurons and contribute, under baseline conditions, to the dilation of the middle cerebral artery, etc. There's also quite a bit of research on the role of neuropeptides released from C-type trigeminal ganglion neurons (the nomenclature is slightly different for different classes of trigeminal ganglion neurons, but they can still be called C-fibers, etc.) and the trigeminal nucleus in vasospasm, following subarachnoid hemorrhage (http://scholar.google.com/scholar?q=trigeminal+vasospasm+subarachnoid&hl=en&lr=). And electrical stimulation of the trigeminal ganglion can influence cerebral blood flow (http://scholar.google.com/scholar?num=100&hl=en&lr=&q=%22cerebral+blood+flow%22+trigeminal+ganglia+OR+ganglion). There are different types of trigeminal root reflexes, and they're not the same as dorsal root reflexes. But the underlying concept is similar. The concept is that two trigeminal ganglion neurons are required. An afferent action potential in a trigeminal ganglion neuron, originating from the skin of the face or the corneal epithelial cells on the surface of the eye, which is innervated by trigeminal ganglion neurons of the ophthalmic branch of the trigeminal nerve, travels to the caudal trigeminal nucleus and activates an interneuron, for example. GABA or glutamate released from the interneuron depolarizes the central terminal of a second trigeminal ganglion neuron (part of the reason this could occur relates to the anatomical relationships between some interneurons and the central branches of trigeminal ganglion neurons whose peripheral branches terminate at a wide variety of locations) and produces an efferent action potential. This can induce CGRP or substance P release at sites adjacent to the middle cerebral artery, etc., thereby causing headaches or just vasodilation or thromboses or remodeling of the smooth muscle cells, because of the release of mast cell proteases by degranulating mast cells, or any number of other effects. Dorsal root reflexes have been suggested to be a mechanism whereby sensory inputs can be "refined," so that more efferent action potentials are being induced or suppressed in some C-fibers in response to specific nociceptive or inflammatory activity in some location that is innervated by other C-fibers or Adelta fibers, etc. A similar type of "refining" effect could occur in the trigeminal nucleus, but the result could also be just inflammatory hyperalgesia and ongoing damage and inflammation, etc.

These mechanisms could, therefore, easily produce or account for pathological changes in the cerebral blood vessels, and much of this research has been done in the context of migraines and other pathological disease states that involve the trigeminal system. But basic research has become devalued to an absurd extent in some of these areas, and it seems as if these types of mechanisms are of no interest whatsoever to anyone. Of course, if one buys into some of the dogma, there is no regulation of cerebral blood flow by neuronal activity. That's obviously not true, in my opinion, although the extent to which neuronal regulation ("nervous regulation") influences cerebral blood flow is likely to be less in a person who is in perfect health than in a person who is not. And UV exposure would obviously be more likely to produce pathological effects in a person in a disease state, in my opinion, than in a person who is not in a disease state. But my main point is just that all of the crazed dogma, in some of these areas, can ultimately confound all attempts to view biological processes in an objective and "scientifically-curious" manner. There's a disturbing kind of arrogance in trying to put a value-laden spin on some of these fundamental processes in biology.

Sympathetic Activation During Resistance vs. Endurance Exercise: Relationship to Exercise-Induced Increases in Noradrenergic Transmission in the Brain

This article [Iellamo et al., 2002: (http://circ.ahajournals.org/cgi/reprint/105/23/2719)(http://www.ncbi.nlm.nih.gov/pubmed/12057984?dopt=Abstract)] is interesting, and it gives the reader at least some sense of the capacity of high-intensity exercise to produce quite different cardiovascular effects than low-intensity, aerobic exercise produces. There's a popular notion that aerobic exercise is the "heart-healthy" form of exercise and that weight training/resistance exercise is just about "muscle-building" or who knows what. The article by Iellamo et al. (2002) shows that high-intensity exercise tends to increase beta-adrenoreceptor responsiveness more than lower-intensity exercise, but the authors seem to evidently still be under the impression that this is "bad." There are just lots of problems with research in exercise. I know it's apparently impossible to research the effects of weight training in rodents, because they can only run on wheels, etc. But researchers keep doing studies on people who are in really pretty decent shape to begin with and then finding no changes or minimal changes in variables related to adrenergic functioning. In general, in my opinion, based on the information from many articles, resistance exercise produces more of an enhancement of beta-adrenoreceptor sensitivity but does not cause some kind of "pressor" effect, and endurance exercise is well-known to produce more of a pronounced increase in vagal (vagus nerve), or parasympathetic, tone, and this is not always purely beneficial to people. Sigal et al. (2004) [Sigal et al., 2004: (http://care.diabetesjournals.org/cgi/content/full/27/10/2518)(http://www.ncbi.nlm.nih.gov/pubmed/15451933)] discuss the fact that, during aerobic (endurance, lower-intensity) exercise, the regulation of glucose availability is primarily driven by these sort of subtle changes in neuroendocrine activity, and endurance exercise tends to decrease insulin levels and cause either unchanged or, actually, decreased plasma glucose levels, during exercise. In contrast, the increases in free fatty acids and plasma glucose that occur during resistance exercise are driven primarily by strong, sympathetic activation and can elevate adrenaline (epinephrine) levels by 15-fold, significantly elevate plasma growth hormone (GH) levels, and increase cortisol meaningfully, etc. So resistance exercise has a very different effect, and, in my opinion, "high-intensity" endurance exercise is not going to mimic those effects very effectively, if at all, in the long term. Reading various articles, one would think that no one has any problem with diminished sympathetic (I'm referring to adrenergic) tone, in terms of the vasoconstriction that is required for venous return to the heart, but this article discusses the high degree of prevalence of postural tachycardia (syncope can result from severe postural tachycardia/postural hypotension, and they're really talking about postural tachycardia and orthostatic intolerance in this article) [Van Lieshout et al., 2003: (http://jap.physiology.org/cgi/reprint/94/3/833)(http://www.ncbi.nlm.nih.gov/pubmed/12571122?dopt=Abstract)]. The authors discuss the fact that improving the strength of leg muscles can improve these symptoms significantly (postural tachycardia manifests itself as dizziness or as an inappropriate and prolonged increase in heart rate upon standing, etc.) and that the venous return to the heart plays a role in maintaining cerebral blood flow, etc.

It's well-known that pilots who fly some types of aircraft experience high G-forces can experience blackouts or "grayouts" tunnel vision, because of transiently diminished cerebral blood flow, and it's well known that resistance training, much more than endurance training, can reduce these symptoms by improving venous return, and Van Lieshout et al. (2003) mention some of that. The baroreflex that normally prevents orthostatic tachycardia or hypotension has multiple components and is really complex, but it's discussed, in much of the literature, as if everyone will benefit from an increase in vagal tone and that the vagal component of the baroreflex is the only relevant one. It's not, and the adrenergic activation that occurs in the brain is likely to be substantially more pronounced, in my opinion, during resistance exercise than during aerobic exercise.

The firing of muscle spindle afferent neurons increases during exercise and contributes to the activation of sympathetic neurons in the medulla, in the brainstem, and higher plasma adrenaline levels (which are obviously much higher during resistance exercise, in general, than during endurance exercise) correlate positively with larger amounts of noradrenaline release in the prefrontal cortex, in exercising rats [Pagliari et al., 1995: (http://www.ncbi.nlm.nih.gov/pubmed/7665408)]. The increases in plasma epinephrine result primarily from increases in the sympathetic outflow from the brain. The direct sympathetic innervation of the adrenal medulla allows epinephrine to be rapidly released during high-intensity exercise. ACTH is also released from the anterior pituitary gland during exercise and stimulates the release of cortisol from the adrenal cortex, during high-intensity exercise. Resistance exercise can induce prolonged elevations in plasma cortisol, at essentially all times during the increases and decreases in plasma cortisol that normally occur throughout the day. This can gradually contribute, over days, to an upregulation of beta-adrenoreceptor density and responsiveness and lead to changes in the magnitude and effects of the acute, exercise-induced increases in plasma epinephrine. Increases in plasma pCO2, during exercise, may also contribute to the activation of noradrenergic neurons in the A1/A2 adrenergic cell groups and the locus ceruleus, in the brain, during exercise [Bailey et al., 2003: (http://www.ncbi.nlm.nih.gov/pubmed/14513913)].

The release of noradrenaline in the prefrontal cortex is almost certainly a result, mainly, of increases in the firing rates of noradrenergic neurons whose cell bodies are in the locus ceruleus, although the other noradrenergic cell groups probably contribute more indirectly to that effect. The central noradrenergic activity, during exercise, is also required for brain-derived neurotrophic factor production in response to exercise, in the brain [Ivy et al., 2003 : (http://www.ncbi.nlm.nih.gov/pubmed/12759116), cited and discussed here: (http://hardcorephysiologyfun.blogspot.com/2008/12/noradrenergic-regulation-of-bdnf.html)], etc. There's a popular belief that increases in plasma beta-endorphin levels, which can occur during exercise, produce an opioidergic or morphine-like effect on the brain, etc., but an increase in plasma beta-endorphin levels is really an indication of a generalized stress response [Farrell et al., 1982: (http://www.ncbi.nlm.nih.gov/pubmed/7096149)]. To produce some kind of opioidergic reward, opioid peptides released into the blood would have to cross the blood-brain barrier, back into the brain, and somehow act selectively on opioidergic pathways that are involved in the mesolimbic reward system, and this isn't really likely to occur. There's a great deal of evidence that the increase in noradrenergic transmission, in the brain, during exercise is more likely to be a major factor regulating the subjective effects or mood elevation in response to exercise. Exercise can also increase dopamine release in the striatum (http://scholar.google.com/scholar?num=100&hl=en&lr=&q=exercise+dopamine+release), and the catecholaminergic effects of exercise are likely to be crucially important for many of its effects.

Another Old Paper of Mine

Writing this paper was an intellectual exercise, and I want to say that I strongly advise people to talk to their doctors before exposing themselves to sunlight or other UVB or UVA sources, even casually. And, in my opinion, it is not wise to use UVB or UVA for therapeutic purposes of any kind. I obviously think that sun exposure would increase the risk of skin cancers. Another implication of the mechanisms I discuss in the paper is that UV exposure could produce actual brain damage, and there are reports of people with multiple sclerosis dying after sitting in the sun. I think it would be a terrible idea for people to use UV therapeutically. In fact, UV exposure could cause brainstem hemorrhages by inducing low-frequency, spontaneous firing of trigeminal ganglion neurons, innervating the skin of the face. The low-frequency firing rates that primary afferent neurons demonstrate within ~30 minutes of exposure to UV causes CGRP and substance P to be released in the spinal cord and trigeminal nucleus, in the brain, and this could cause perivascular mast cell degranulation and thereby increase vascular permeability and cause a hemorrhage, etc. A single UV exposure can cause ~1 Hz, asynchronous firing of sensory neurons and trigeminal ganglion and dorsal horn neurons that lasts 5-7 days. But CGRP has immunomodulatory effects and could act on astrocytes and microglia, upon its release in various parts of the brain, etc. Another implication is that UV could modify cerebral blood flow (either decreasing it or increasing it) by inducing one or another type of trigeminal root reflex, meaning efferent action potentials in fibers innervating the cerebral blood vessels. UV could also modify thalamic mast cell degranulation, potentially with damaging consequences.

But the mechanisms also imply that no blood-borne photoreceptors or bilirubin (as a photoreceptor, etc.) or mysterious capacity of UVA to reach the retina would be required for UVB or UVA to modify brain function in significant ways (modify the circadian rhythm, thermoregulation, etc.). Changes in the trigeminal system can affect neurotransmission in many parts of the brain, by polysynaptic pathways, and maybe this paper would stimulate some interest in basic research on photobiology or on actual, legitimate treatments for multiple sclerosis, etc. The immunosuppressive effects of UVB are really complex and can go wrong at multiple points along the cascade of events that result from UVB exposure, and the result could be a new type of lupus-like autoimmune disease in some people. It's not a valid immunomodulatory approach. But an implication of the mechanistic analysis might be that drugs modifying CGRP receptor activation could be useful for autoimmune diseases, etc. CGRP is also a very potent vasodilator and produces NOS-independent vasodilation, for example.

In any case, I don't like anti-intellectualism in the context of basic research, and I stand by the validity of my mechanistic discussion in this paper, from an intellectual standpoint. Maybe if more people were not so terrified of discussing some of these topics in a rigorous and objective manner, it would be possible to persuade more people of various public-health-type messages. Instead, the discussion degenerates into proclamations, etc. Again, this paper is not meant to suggest any rationale for doing anything. Maybe people with multiple sclerosis or the like will be more careful about sun exposure and realize that being aware of all the mechanisms can be useful, as far as protecting oneself from the brain damage that could conceivably result from sun exposure, particularly in someone in any kind of disease state.



A Review of the Effects of Cutaneous Exposure to Ultraviolet Radiation On Primary Afferent and Dorsal Horn Neurons: Mechanisms and Effects On Immune Function and Pain
　
Abstract

Background—Following the exposure of the skin to either ultraviolet B (UVB) or ultraviolet A (UVA) radiation, the immunological responses to cutaneously-administered protein antigens or small-molecule haptens can be suppressed systemically. UVB is known to cause sensory C-fibers to release the neuropeptides -calcitonin gene-related peptide (CGRP) and substance P (SP) into the skin, and the release of these neuropeptides contributes to UVB-induced systemic immunosuppression and UVB-induced increases in neurogenic blood flow (i.e. erythema). More specifically, CGRP released from the peripheral terminals of C-fibers, in response to UVB, acts on antigen-presenting cells (APC) migrating from or infiltrating into the skin and contributes to the UVB-induced production of interleukin-10 (IL-10) by these APC. Cutaneous UVB has also been shown to increase the CGRP and SP content in the dorsal horn (DH) of the spinal cord, and this apparent release of CGRP and SP has been suggested to mediate UVB-induced sunburn pain and hyperalgesia. The immunological consequences of CGRP released in the spinal cord has never been investigated but may be relevant for understanding the etiology of multiple sclerosis, which UVB exposure may protect against but also, conceivably, worsen the course of.
Key Conclusions—Researchers have found that ultraviolet radiation (UVR) can influence the electrophysiological activities of and phenotypic expression of proteins by neurons in deeper spinal cord or brain sites (i.e. those that do not receive direct synaptic inputs from C-type, primary afferent neurons). CGRP released in the spinal cord may induce immunosuppressive cytokine production by, or reduce the antigen-presenting/costimulatory capacity of, astrocytes, microglia, or dendritic cells in the CNS. UVR induces low-frequency spontaneous, asynchronous activity in C-fibers innervating the exposed skin and also causes DH neurons to exhibit increases in spontaneous activity. This spontaneous activity releases glutamate, CGRP, and SP in the DH and contributes to UVB-induced primary and secondary hyperalgesia. These neurogenic effects may also contribute to the UVB-induced suppression of pruritus. UVB produces biphasic increases in blood flow in the UV-irradiated skin, and researchers have previously proposed that only the second peak of blood flow is neurogenic (C-fiber-mediated). Numerous pieces of evidence argue against this conclusion and suggest that UVR produces two largely neurogenic phases of increased blood flow. The first peak of blood flow is likely to be both neurogenic and non-neurogenic but is unlikely to be purely non-neurogenic. UVB has been shown to reactivate the herpes simplex virus in the trigeminal ganglia and dorsal root ganglia (DRG), and this reactivation follows a bimodal timecourse. It is likely that UVR first depletes CGRP and SP from C-fibers in the skin and subsequently induces adaptive changes in the cell bodies of DRG neurons and DH neurons, which replenish CGRP and SP stores and produce the second neurogenic phase of erythema via axon reflexes and dorsal root reflexes. UVR has also been found to produce rewarding and pain-reducing effects on the CNS. These effects have generally been attributed to UVR-induced increases in plasma opioid peptides, but it is more likely that these effects occur via purely neurogenic pathways involving the DRG, DH, and supraspinal sites (including the striatum, amygdala, etc.). UVB-induced increases in hormonal vitamin D3 production may modify the C-fiber-mediated effects of UVB and UVA. Hormonal vitamin D3 may, in part, protect against MS by increasing NGF, GDNF, and the low-affinity neurotrophin receptor (p75NTR) in the CNS. A hormonal vitamin D3 analog has been shown to increase the CGRP content in the DRG in an NGF-dependent manner, and the actions of hormonal vitamin D3 may reduce UVB-induced hyperalgesia.

Introduction

Some of the immunosuppressive effects of ultraviolet B radiation (UVB) are thought to be protective against multiple sclerosis (MS) and other autoimmune diseases (McMichael and Hall, 1997). MS is an inflammatory, demyelinating disease of the CNS that is thought to result from an autoimmune response against components of myelin (Storch et al., 1998; Lucchinetti et al., 1996; Lassman et al., 2004). The disease process in MS is also characterized by the degeneration of axons (Trapp et al., 1998) and the loss of neurons (Bozzali et al., 2002; Owens, 2003), and the extent of disability, in people with MS, is closely and positively associated with the loss of axons (Lassman et al., 2004). The demyelination in MS is believed to be largely mediated by CNS-infiltrating T-helper (CD4+) lymphocytes with a pro-inflammatory, Th1, phenotype (Lassman et al., 2004), and UVB reliably suppresses Th1-cytokine production by CD4+ T-cells (Shreedhar et al., 1998) and antigen-presenting cells (APC) (Garssen et al., 1999; Toichi et al., 2002) in regional lymph nodes. The apparent UVB-induced protection against MS has mainly been explained in terms of the immunomodulatory actions of 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] (Hayes, 2000), the hormonally active form of vitamin D3, and, to a lesser extent, -melanocyte stimulating hormone (-MSH) (Friedman, 2004). Increases in the concentrations of both -MSH and 1,25(OH)2D3 have been found to be induced, in the skin, in response to UVB exposure (Lehman et al., 2003; Funasaka et al., 2001).

The implicit assumption has been that any UVB-induced immunosuppressive effects within the CNS must be preceded by, and must also be a consequence of, immunosuppressive effects in regional lymph nodes (RLNs) that drain the UVB-irradiated skin. The UVB-induced elevation in plasma 25-hydroxyvitamin D3 [25(OH)D3] could, as noted by others, clearly produce immunological changes that would originate in the CNS (McCarty, 2006). Microglia, astrocytes, and other cell types can convert 25(OH)D3 into 1,25(OH)2D3 and respond to the newly-produced 1,25(OH)2D3 (Garcion et al., 2002), which can act in a paracrine or autocrine fashion (Garcion et al., 2002). Many, if not most, of the immunosuppressive effects of UVB are known, however, to result from the actions of mediators other than vitamin D3, mediators that include cis-urocanic acid (cis-UCA) (Holán et al., 1998; Sleijffers et al. , 2003) and numerous cytokines (Boonstra et al., 2000), or from changes that overlap in some ways with 1,25(OH)2D3-associated signalling (Lehmann et al., 2004). Although some researchers have discussed the relevance to multiple sclerosis of UVB-induced immunosuppression that is independent of 1,25(OH)2D3 (Chaudhuri, 2005; McMichael and Hall, 1997; Van der Mei et al., 2001), these discussions have frequently been narrowly focused on the effects of -MSH. The involvement of -MSH in UVB-induced immunosuppression is thought to be of somewhat secondary importance and has not been as clearly established as the involvement of other mediators (Shimizu and Streilein, 1994). Many of these immunosuppressive effects, effects that include the UVB-induced expansion of regulatory T-cell populations (Schwarz et al., 2004), have not been discussed, in detailed terms, in the context of multiple sclerosis. UVR has been shown to either prevent damage from EAE (Hauser et al., 1984) or worsen the outcome (Tsunoda et al., 2005), but the complex interactions of UVR with CNS immune privilege and EAE have not been adequately explored (Hauser et al., 1984; Tsunoda et al., 2005).

The induction of DNA damage by UVB is, for example, one effect that is immunosuppressive (Garssen et al., 2000) and that could not be induced by vitamin D3 alone. The development of UVB-induced systemic immunosuppression requires, as one component of a cascade of changes, the formation of pyrimidine dimers in the skin and the migration of DNA-damaged cells to RLNs (Garssen et al., 2000). The UVB-induced formation of cyclopyrimidine dimers in KCs is, furthermore, involved in the UVB-induced increases in IL-10 output by KCs (Nishigori et al., 1996). The enhancement of DNA repair, in UV-irradiated skin, is also known to decrease the systemic immunosuppressive effects of UVB (Garssen et al., 2000). It is notable that 1,25(OH)2D3 and its analogs increase, in various cell types, the protein content of p21CIP1/KIP1/WAF1 (Gumireddy et al., 2003), a protein that induces cell cycle arrest and allows for DNA repair to occur (Weinberg et al., 2002). In the context of DNA damage, 1,25(OH)2D3 could therefore be expected to actually lessen the immunosuppressive effects of UVB.
Apart from the effects of 1,25(OH)2D3 in the CNS (Garcion et al., 2002), UVB-induced immunosuppression has also been investigated only in lymphoid organs that drain tissues outside the brain. The suppressor cells that appear in the spleen, following UVB-irradiation of the skin (Schwarz et al., 2004), may be partially the result of APC migrating from the CNS, and some of these suppressor cells may have been specific to CNS-associated antigens. This possibility has never been evaluated. The UVB-induced expansion of various populations of regulatory T-cells (Schwarz et al., 2004), which have the potential to induce antigen-nonspecific, IL-10-dependent, bystander suppression of other T-cells (Schwarz et al., 2004), and altered APC (Dumas et al., 2000) in RLNs could indeed, as proposed by others (Sharpe, 1986), gradually reinforce tolerance to myelin-associated antigens. These "skin-to-body-to-brain" changes could occur by the mechanisms previously described (Dumas et al., 2000; Sharpe, 1986) and are likely to contribute to UVB-induced neuroimmunomodulation. An example of this type of effect was the amelioration of lupus-associated neuropsychiatric symptoms, and the normalization of the uptake of 18F-2-fluoro-2-deoxyglucose in specific brain regions, by UVA-1 phototherapy in a patient with lupus (Menon et al., 2003). These changes were accompanied by improvements in the clinical lupus scores (Menon et al., 2003). Other studies have found UVA-1-induced reductions in systemic autoantibody titres in patients with lupus (Polderman et al., 2004), reductions that have sometimes been accompanied by improvements in cognitive functioning (McGrath Jr., 2005). Thus, it is reasonable to think that the reductions in autoantibody production occurred primarily in the blood, as a result of UVA-1-induced immunosuppressive changes in the skin, and produced, for example, secondary decreases in endothelial cell activation in cerebral blood vessels. It is also understandable that researchers have traditionally examined UVB-induced changes in T-cells and APC, in the spleen or the lymph nodes draining the skin, with reference to the skin. Given that the skin is used as the initiation and elicitation site for the examination of these protein antigen-specific or hapten-specific immune responses, the focus on skin-derived APC and skin-associated immunity is entirely appropriate. However, researchers have found that UVB can increase the expression of immediate early genes (Gillardon, Wiesner, and Zimmermann, 1992) and the concentrations of neuropeptides in the dorsal horn (DH) of the spinal cord (Gillardon, Schrock, and Morano, 1992). UVR can also influence the mRNA and protein contents of neuropeptides and immediate-early gene products in primary afferent neurons, both in the cell bodies in the dorsal root ganglia (DRG) (Gillardon et al., 1991) and the peripheral branches that innervate the UV-irradiated skin (Benrath et al., 1995; Eschenfelder et al., 1995). The changes in the DH, which occur through UVR-induced actions on DRG sensory C-fibers, could cause immunosuppressive changes to proceed in a "skin-to-brain-to-body" direction. Specifically, the UVB-induced increase in the content of the neuropeptide CGRP in the DH could contribute to the apparent protection against MS.

UVB-induced systemic immunosuppression is known to depend on the neurogenic release of the neuropeptide -calcitonin gene-related peptide (CGRP, or CGRP) into the skin (Kitazawa et al., 2000; Garssen et al., 1998; Hart et al., 2002; Khalil et al., 2001; Niizeki et al., 1997; Legat et al., 2004), but the UVB-induced changes in the CGRP content in the DH and DRG have primarily been viewed in the context of UVB-induced hyperalgesia (Gillardon et al., 1991; Gillardon, Wiesner, and Zimmermann, 1992; Gillardon, Schrock, and Morano, 1992). CGRP that is released from the epidermal or dermal terminals of C-type afferent nociceptive fibers is known to exert, by direct and indirect mechanisms, immunosuppressive effects on APC in the skin (Kitazawa et al., 2000; Niizeki et al., 1997). However, the immunosuppressive implications of the UVB-induced increases in CGRP levels in the spinal cord remain unexplored.
In the context of MS, it is noteworthy that the increased CGRP concentrations, following UVB, in the vicinity of both the peripheral terminals (in the skin) and central terminals (in the DH) of DRG neurons may be partially dependent on NGF. UVB has been shown to increase the expression and release of NGF by keratinocytes (Gillardon et al., 1995) and modify the abundance of NGF receptors on keratinocytes (Bull et al., 1998) and epidermal nerve terminals (Bull et al., 1998). Researchers have also hypothesized that the increased retrograde axonal transport of NGF, from the skin to the cell bodies of DRG neurons, contributes to the UVB-induced increases in CGRP levels in the DH (Gillardon et al., 1995). Although this may be the case, other mechanisms could also be important. The release of NGF from KC, perhaps in concert with the UVB-induced changes in the p75NTR content on the peripheral branches of DRG neurons (Bull et al., 1998), could, by increasing the firing rate of DRG neurons, augment CGRP release, from the central terminals of DRG neurons, or produce phenotypic changes in the cell bodies of DRG neurons. UVR is known to induce C-fibers (Andreev et al., 1994; Eschenfelder et al., 1995; Szolcsányi, 1987) and DH neurons (Urban et al., 1993; Chapman and Dickenson, 1994) to fire, spontaneously, at low-frequencies, and the orthodromic action potentials in C-type, DRG axons are therefore the most obvious "cause" of the UVB-induced release of CGRP from the central terminals of those DRG neurons. The broader question, which will be explored in this paper, is which factors, induced by UVB in the skin, are responsible for the spontaneous action potentials and which factors may help replenish the stores of CGRP, at the central and peripheral terminals of DRG neurons, that are depleted by the UVB-induced, spontaneous activity. KC-derived NGF could exert these effects without undergoing axonal transport to the DRG. NGF, induced in the skin by inflammatory stimuli other than UVR, is known to contribute to spontaneous activity in C-fibers (Djouhri et al., 2001). In addition, exogenous NGF can exert rapid effects on C-fibers, such as the sensitization of C-fibers to capsaicin, that could contribute to hyperalgesia and spontaneous activity (Mendell et al., 2002). Some indirect evidence supports the notion that NGF participates in the UVB-induced increases in C-fiber activity (Khalil et al., 2001), but a UVB-induced increase in the axonal transport of NGF is more hypothetical.

The damage induced by experimental autoimmune encephalomyelitis (EAE) has been found to be less severe after the intracerebroventricular (i.c.v.) injection of exogenous NGF (Triaca et al., 2005) and more severe in rats either autoimmunized against NGF or injected with an anti-NGF antibody. In EAE, an NGF antibody augmented the infiltration of the CNS by pro-inflammatory cells of the immune system (Micera et al., 2000) and appeared to act on progenitor cells participating in the repair of damaged areas (Triaca et al., 2005). The expression and release of CGRP by DRG neurons and B-cells is increased by NGF (Bracci-Laudiero et al., 2002), and many of the anti-inflammatory effects of NGF may be secondary to this CGRP (Bracci-Laudiero et al., 2002). From the standpoint of UVR, the ongoing release of CGRP in the dorsal horn may be accompanied by the release of BDNF. This is because BDNF is known to be co-released with CGRP and SP from the central terminals, in the DH, of C-type, DRG neurons (Malcangio et al., 2003). CGRP has been shown to decrease the severity of experimental autoimmune diabetes (EAD) (Sun et al., 2003) and experimental autoimmune retinitis (Kezuka et al., 2004) and to exert numerous immunosuppressive effects on dendritic cells (Carucci et al., 2000), monocytes (Fox et al., 1997), and T-cells (Boudard et al., 1991). Vitamin D3 could also contribute to the UVB-induced increases in NGF and CGRP. 1,25(OH)2D3, the hormonal form of vitamin D3, and its analogs have been shown to increase the expression of NGF and the abundance of the NGF protein in various cell types, including DRG neurons (Riaz et al., 1999). In addition, the VDR is expressed in lamina I-II of the spinal cord (Stumpf et al., 1988). Thus, 1,25(OH)2D3, produced locally in the skin, or in an autocrine or paracrine fashion in the peripheral or central nervous system, may contribute to the UVR-induced effects on DRG and DH neurons.
This paper will discuss the mechanisms and immunological implications of the UVB-induced increases in the CGRP content of the spinal cord. Specifically, the increased storage and release of CGRP could produce immunosuppressive effects, in the CNS, that would be protective against multiple sclerosis. The UVB-induced increases in CGRP could decrease the release of pro-inflammatory cytokines by Th1 cells, infiltrating the spinal cord, or suppress the antigen-presenting capacity of microglia and infiltrating monocytes. In the DH, UVB-induced release of neuropeptides may also induce the migration, from the CNS, of dendritic cells that are deficient, either because of immaturity and premature migration or because of phenotypic changes induced by CGRP, in their costimulatory capacity. It is also noteworthy that UVR has been shown to exert antihyperalgesic effects in the context of chronic pain (Kaur et al., 2005b) and to produce reinforcing or reward-associated effects on humans (Gambichler et al., 2002a; Feldman et al., 2004; Kaur, Liguori, and Fleischer et al., 2006; Kaur, Liguori, and Lang et al., 2006; Zeller et al., 2006). The supposed rewarding effect of UVR has, however, never been discussed in the context of the known effects of UVR on DRG and DH neurons. There is evidence that UVB can induce the release of -MSH, from the pituitary, via a kind of neurogenic cascade involving the activation of TG neurons, fibers that innervate the cornea and comprise part of the ophthalmic branch of the trigeminal nerve, ciliary ganglion (CG) neurons, and hypothalamic neurons (Hiramoto et al., 2003). In view of these findings, it is possible that the spino-trigemino-parabrachio-amygdaloid and spinohypothalamic pathways may be involved in both the hyperalgesic and antihyperalgesic effects of UVR. The relevance of these possibilities to immune deviation (i.e. immune privilege), across multiple brain regions, will also be briefly discussed. Finally,1,25(OH)2D3, produced locally in the skin or in the DRG and spinal cord from 25(OH)D3 in the systemic circulation, may modify the effects of UVR on CGRP and on the spinal cord. The oral administration of CB1093, a vitamin D analogue, has been found to increase, in an NGF-dependent manner, the CGRP content in sciatic nerve segments of rats (Riaz et al., 1999).
Effects of UVB On Cutaneous Sensory Fibers and the Firing Rates of DRG Neurons
Exposure to UVB increases the content of CGRP alone (Seike et al., 2002) or both CGRP and SP (Legat et al., 2002; Legat et al., 2004) in cutaneous sensory nerve fibers and causes the release of these neuropeptides into the skin (Niizeki et al., 1997). The dermis and basal layer of the epidermis are innervated by the peripheral axons of primary afferent neurons (PANs), whose cell bodies are in the dorsal root ganglia (DRG) (Burbach et al., 2001; Schulze et al., 1997; Reilly et al., 1997). Approximately one million nerve fibers innervate the skin (Krogstad, 1999). UVB has been found to increase the percentages of epidermal nerve fibers that are immunoreactive for CGRP (Legat et al., 2004) and to induce the release of CGRP from nerve terminals into the epidermis (Legat et al., 2004) and the dermis (Niizeki et al., 1997). Researchers have found increases in the release of CGRP from epidermal and dermal afferent fibers, as indicated by an elevated concentration in the skin, that appear as soon as two hours after a single exposure to UVB (Gillardon et al., 1995) and persist for up to seven days (Legat et al., 2004) after the end of a course of multiple exposures. Single exposures to UVB have been shown to increase the content of CGRP in cutaneous fibers at 24 hours (Seike et al., 2002) post-irradiation. Similarly, twelve exposures to UVB over the course of a month roughly tripled the percentage of epidermal fibers containing CGRP (Legat et al., 2004). This peak in CGRP in response to chronic UVB occurred 24 hours after the final UVB treatment (Legat et al., 2004). Although Gillardon et al. (1995) found that UVB transiently reduced the concentration of CGRP in the skin of rats, this decrease was attributed to the UVB-induced release of CGRP from cutaneous nerve fibers (Gillardon et al., 1995). The timecourse of this decrease in CGRP, which emerged as soon as two hours post-irradiation (Gillardon et al., 1995) and began to rebound after 24 hours, has been interpreted as a transient depletion of stored CGRP from epidermal sensory nerve terminals (Gillardon et al., 1995). In the ears of rats exposed to UVB for four weeks, with three low-dose exposures per week, Legat et al. (2002) found data consistent with an adaptive increase in the storage of CGRP per sensory fiber in the epidermis and dermis. Seike et al. (2002) also found an increase in CGRP content within nerve fibers innervating the upper dermis and epidermis at 24 hours after single UVB exposures. These increases became proportionally more pronounced, increasing in a dose-dependent fashion, as the dose of UVB was increased from 0.3 to 0.5 J/cm2, although the increase in the CGRP content appeared to plateau as the dose was increased from 0.5 to 0.7 J/cm2 (Seike et al., 2002). Legat et al. (2002) suggested that the long-term increases in CGRP storage and release, which have been shown to appear after 24 hours and may persist for seven days after the final UVB exposure, are due to the anterograde transport of newly-expressed CGRP from the DRG.

The baseline firing rates and firing thresholds of both thinly-myelinated A-type fibers and unmyelinated C-fibers are sensitive to UV exposure (Andreev et al., 1994) and are likely to be most directly involved in the UVR-induced changes in CGRP content and release. Eschenfelder et al. (1995) found that exposure to a combination of UVB and UVA caused over 35 percent of high-threshold mechanoreceptive C-fibers in the saphenous nerve to exhibit a low-frequency (0.8-1.25 Hz), spontaneous firing pattern. This type of firing pattern, which can occur in the context of UVR exposure and other models of peripheral inflammation, is spontaneous in the sense that endogenous, physiological factors or conditions have become capable of inducing a receptor potential and, thereby, eliciting an orthodromic action potential. The firing of C-fibers at frequencies between 0.1 and 1 Hz, for example, does not produce a sensation of pain (Lynn and Shakhanbeh, 1988; Gybels et al., 1979), may produce a sensation of itch (Torebjörk, 1974), and does produce vasodilation (Lynn and Shakhanbeh, 1988). This spontaneous activity had begun at 24 hours postirradiation, had peaked after 72 hours, and was still slightly increased, above baseline, after 96 hours (Eschenfelder et al., 1995). In rabbits whose shaved ears were exposed to UVR, Szolcsányi (1987) found that polymodal nociceptive C-fibers innervating the irradiated skin of the ear developed a low-frequency pattern of activity. This background activity, at 6.64 impulses per minute or an arithmetic mean of roughly 0.1 Hz, was measurable within five hours post-irradiation in the vast majority of the C-fibers analyzed, although one of the fibers had begun to exhibit activity as soon as 30 minutes after UVR (Szolcsányi, 1987). When bradykinin was administered into the greater auricular artery of UVR-pretreated rabbits, Szolcsányi (1987) found an increase, compared with nonirradiated controls, in the total number of impulses and the "duration" of bradykinin-induced spontaneous activity in polymodal nociceptive C-fibers. These results indicate that UVR can rapidly produce both spontaneous depolarization of C-fibers and sensitization to a given concentration of a substance, such as bradykinin, that is known to induce depolarization. These concepts will subsequently be discussed in more detail. All of the fibers analyzed by Szolcsányi (1987) were fibers of the greater auricular nerve, whose cell bodies are in the cervical DRG in humans. Andreev et al. (1994) found, similarly, that UVR exposure to the rat hindpaw caused C- and A-type fibers of the saphenous nerve to exhibit low-frequency (6-108 discharges per minute, or an arithmetic mean of 0.1-1.8 Hz), spontaneous activity. This ongoing activity, measured at five days post-irradiation, was reduced by the application of morphine and other opioid receptor ligands (Andreev et al., 1994). As discussed below, other anti-hyperalgesic drugs have been shown to influence the UVR-induced sensitization of nociceptive and mechanoreceptive fibers.

Effects of UVB on Neurons in the DRG and DH

In addition to producing changes in sensory fibers in the skin, UVB has been shown to influence the expression of CGRP in the cell bodies of neurons in the DRG (Gillardon et al., 1991) and the content of CGRP in the DH of the spinal cord (Gillardon et al., 1992). Hindpaw exposure to UVB has been shown to decrease the expression of CGRP mRNA in the L3 and L4 DRG (Gillardon et al., 1991) and increase the protein content of CGRP in the medial portion of the superficial dorsal horn (i.e. laminae II-IV), in the L4-L5 lumbar segments (Gillardon et al., 1992). These changes, which were maximal at roughly the same times at which the UVB-induced erythemal skin responses were maximal, were measured in the DRG at 48 hours post-UVB (Gillardon et al., 1991) and, in the DH, from 24 to 96 hours post-UVB (Gillardon et al., 1992). The concentration of CGRP in the DH had increased to 150-160 percent of controls at the first measurement, taken at 24 hours post-UVB, and was still somewhat elevated at both 48 and 96 hours post-UVB (Gillardon et al., 1992). Compared with non-irradiated rats, the UVB-irradiated rats showed both an increase in CGRP and a relative decrement in a higher-molecular-weight CGRP "precursor" (roughly 14.4 kDa) (Gillardon et al., 1992). The increase in CGRP was therefore suggested to have been, in part, a result of the UVB-induced proteolysis of CGRP precursor peptides (Gillardon et al., 1992). The findings may also have resulted from a relative increase in the translation of CGRP, given that CGRP peptides are encoded by mRNA splice variants of mRNA transcripts of the calcitonin (CT) gene.

The effects of low-dose, daily UVB exposure on CGRP release, together with the suppressive effects of UVB on sensory fiber CGRP content in people with psoriasis, are consistent with a longer-term, adaptive decrease in neurogenic inflammation. Legat et al. (2002) found that four weeks of low-dose UVB exposures, given at three times per week, increased the content of CGRP per nerve fiber. However, as noted by the authors, this increase was not accompanied by ongoing, persistent inflammation and edema in the exposed skin (Legat et al. 2002). The authors noted that UVB may, after repeated exposures, decrease the release of CGRP from nerve fibers innervating the exposed skin (Legat et al., 2002). In patients with different subtypes of psoriasis or eczema, UVB decreased the numbers of nerve fibers containing CGRP and also decreased the overall density of nerve fibers (i.e. including those that did not contain CGRP) (Wallengren and Sundler, 2004). Given that the remaining nerve fibers were thicker, Wallengren and Sundler (2004) suggested that UVB had remodeled, rather than produced degeneration in, the sensory innervation of the epidermis and dermis. Wallengren and Sundler (2004) noted that histamine, and the mast cells releasing it, can activate sensory fibers but can also lead to desensitization and neuropeptide depletion in sensory fibers. It was also found that UVB reduced itch and inflammation in the skin of patients (Wallengren and Sundler, 2004), a finding that is consistent with the known antipruritic effects of UVB (Gilchrest et al., 1979; Lim et al., 1997; Holme and Mills, 2001; Kaptanoglu and Oskay, 2003). The suppression of itch was attributed to the UVB-induced changes in cutaneous sensory fibers (Wallengren and Sundler, 2004). Although UVB has been shown to inhibit both the weal and flare responses that are produced by mast cell-derived histamine (Fjellner and Hägermark, 1982), Wallengren and Sundler (2004) noted that UVB often suppresses pruritus in people who have not responded to antihistamines. Consistent with this assessment, Holme and Mills (2001) found that UVB reduced pruritus in a woman whose pruritis had not responded to antihistamines. Interestingly, the woman had also responded to, but had not been able to tolerate, transcutaneous electrical nerve stimulation (Holme and Mills, 2001). Kaptanoglu and Oskay (2003) also found UVB to be effective as an antipruritic in a person who was no longer responding to antihistamines. Together with other findings, which will be discussed in a subsequent section, it should become clear, as noted by Wallengren and Sundler (2004), that the UVB-induced suppression of the weal and flare, components of the so-called axon reflex, cannot be easily attributed to a process such as histamine tachyphylaxis (Wallengren and Sundler, 2004). The relative "histamine-independence" of the UVB-induced antipruritic effects may, for example, result from the central suppression of itch (i.e. in the spinal cord). When UVB is administered to only part of the body surface, the suppression of pruritus is known to be "systemic" and generalized accross the entire body surface (i.e. extending to unexposed sites) (Gilchrest et al., 1979).

UVR has also been shown to influence the concentration of substance P (SP) and the activation of one of its receptors, the neurokinin-1 receptor (NK1R), in the dorsal horn (Polgár et al., 1998; and Thompson et al., 1994). When the spinal cords of rats were removed one day after the unilateral exposure of the rats' (right) hindpaws to UVA, Polgár et al. (1998) found that the substance P content was reduced bilaterally in the L4-L5 lumbar segments and was increased, mainly in the contralateral spinal cord, within the T6-T8 thoracic segments. The distribution of SP in the irradiated rats' spinal cords was compared with the distribution found in non-irradiated controls, rather than the distribution that would have been found prior to irradiation in the experimental group (Polgár et al., 1998). The roughly 50 percent decreases in immunoreactive SP in the lumbar DH were similar on both the ipsilateral side, which contained the central branches of DRG neurons innervating and providing afferent inputs from the irradiated skin, and contralateral side and were found in laminae I and II, in deeper laminae of the DH, and in the lateral spinal nucleus (LSN) (Polgár et al., 1998). In the thoracic segments, the increased SP was more pronounced on the contralateral side than on the ipsilateral side and was most striking, both contralaterally and ipsilaterally, in laminae II and III (Polgár et al., 1998). In view of these results, Polgár et al. (1998) suggested that UVA had increased the production of SP in both the lumbar and thoracic segments of the DH but had, additionally, increased the release of SP in only certain areas. The bilateral decrease of SP in the L4-L5 lumbar DH was viewed as evidence of a UVR-induced increase in SP production and release (Polgár et al., 1998), presumably by L4-L5 DRG neurons that received afferent inputs from the ipsilateral hindpaw and entered the ipsilateral dorsal horn at the L4-L5 segments.
It should be noted that the decrease in SP content in the LSN, in the lumbar spinal cord (Polgár et al., 1998), suggests that UVR can modify the activities of neurons in supraspinal sites, such as the periaqueductal gray matter (PAG). Assuming the decrease in SP content in the LSN was due to an increase in SP release from neurons in the LSN, as proposed by Polgár et al. (1998), the release of SP could have occurred from either descending or ascending pathways (Jiang et al., 1999). Electrophysiological studies suggest that LSN neurons, specifically those that project to supraspinal sites and deliver afferent APs to those supraspinal sites, do not receive direct synaptic inputs from DRG fibers entering the spinal cord (Jiang et al., 1999). The ascending LSN neurons are nonetheless activated, in a polysynaptic manner via intervening, DH interneurons, by stimulation of dorsal root fibers (Jiang et al., 1999). LSN neurons project directly to, and form synapses with, neurons in the PAG (Harmann et al., 1988), thalamus (Battaglia and Rustioni, 1992), hypothalamus (Burstein et al., 1987), and amygdala (Burstein and Potrebic, 1993). The afferent activities of these LSN neurons are, in turn, modified, in the lumbar spinal cord, by mediators released from descending axons of neurons in the raphe nuclei and the PAG (Carlton et al., 1985; Masson et al., 1991). An example of a supraspinal pathway that may be activated by UVR is shown in Fig. 4.

The UVR-induced upregulation of the neurokinin-1 receptor (NK1-R) responsiveness of DH neurons (Thompson et al., 1994) is also consistent with an acute increase in SP-mediated effects. The UVR-induced augmentation of NMDA-R responsiveness in the DH (Thompson et al., 1994; ) also suggests that SP release is acutely increased in response to UVR, given that SP is co-released into the DH, from PAFs, with glutamate (Millan, 1999, Section 10.3). The UVR-induced release of SP in the DH could, in fact, be expected to simultaneously produce an acute decrease in SP content, as found by Polgár et al. (1998), and an increase in the responsiveness of DH neurons to NK1-R and NMDA-R activation. The NK1-R is rapidly internalized in response to SP binding, and the NK1-R is also known to also be redistributed to the plasma membrane after the degradation of SP (Millan, 1999, Section 10.3.2.2). The activation of NK1-Rs and NK2-Rs is nonetheless thought to be more important for the initiation of central sensitization than for its prolongation (Millan, 1999, Section 10.3.2.2). The activation of NK1-Rs by SP produces slow depolarization of DH neurons but can increase the fast and more sustained depolarization induced by NMDA-R activation (Millan, 1999, Section 10.3.2.2; Boxall et al., 1998b). Thus, the release of SP by UVR could account for the findings that UVR augments NMDA-R-mediated activation of DH neurons (Thompson et al., 1994; Thompson et al., 1995; Boxall et al., 1998b). For example, the binding of SP to the NK1-R can, by activating the PLC-IP3-DAG cascade, indirectly enhance the increase in intracellular calcium ([Ca2+]i) that is produced by NMDA-R activation (Millan, 1999, Section 10.3.2.2). Additionally, Boxall et al. (1998b) found evidence that NMDA-R activation, induced in the L5 DH by hindpaw UVR, can exert a "primary" role in sensitizing DH neurons to signals that produce slow depolarization. Hindpaw UVR augmented the depolarization of L5 spinal neurons that had been induced by the administration, intrathecally, of an mGluR1/mGluR5 agonist (Boxall et al., 1998b). This increased responsiveness was largely abrogated by the concurrent administration of an NMDA-R antagonist (Boxall et al., 1998b). The activation of group I mGluRs, such as by the mixed mGluR1/mGluR5 agonist that was used in UVR-treated rodents, generally produces the same slow depolarization of DH neurons and gradual elevation of [Ca2+]i that NK1R activation produces, thereby augmenting the sensitization of DH neurons and contributing to hyperalgesia (Boxall et al., 1998b; Millan, 1999, Section 10.3.2.3). Given that hindpaw UVR enhanced the responsiveness of DH neurons to mGluR1/mGluR5 activation at higher doses of the agonist but did not change the EC50 responses to the agonist, Boxall et al., (1998b) suggested that UVR had probably not upregulated the total numbers of binding sites on the DH neurons. The results of the study implied that hindpaw UVR can, as suggested by the authors (Boxall et al., 1998b), increase NMDA-R-mediated transmission in the spinal cord and thereby increase the mGluR1/mGluR5 responsiveness of DH neurons. More specifically, the authors noted that mGluR1/mGluR5 activation could serve to maintain the activation of NMDA-Rs by phosphorylating the receptors and augmenting protein kinase C (PKC) activation, in much the same way as NK1 receptor activation can augment NMDA-R responses in a PKC-dependent fashion (Boxall et al., 1998b). In contrast, agonists at mGluR3, the mRNA of which was increased by UVA in the DH (Boxall et al., 1998), have been shown to produce antinociceptive effects (Millan, 1999, Section 10.3.2.3). UVR might therefore induce both nociceptive and antinociceptive responses in the spinal cord, and the nociception that occurs through slow depolarization, as in response to CGRP or SP or mGluR receptor activation, is likely to interact with fast (i.e. ionotropic), NMDA/kainate/AMPA-R-mediated, glutamatergic transmission.
It is more difficult, however, to definitively account for the UVA-induced changes in SP content on the contralateral side of the DH. Polgár et al. (1998) implicitly suggested that the UVB-induced activation of primary afferent fibers entering and terminating in the ipsilateral lumbar spinal cord could have induced the depletion of SP from primary afferent fibers terminating in, or at least passing through, the contralateral L4-L5 DH. This is plausible and has been referred to as "volume transmission," whereby mediators are released locally but exert actions distant from the site at which the mediators have been released from (Millan, 1999, Sections 4.7 and 10.3.2.3). The contralateral changes could also have been mediated by the excitation or disinhibition of commissural interneurons, which are abundant in the spinal cord (Sugimoto et al., 1990). Neurons receiving inputs from ipsilateral DRG fibers in the superficial laminae of the DH have been shown to cross the dorsal commissure and influence the contralateral DH in a nearly symmetrical manner (Koltzenberg et al., 1999). Thus, in response to UVR, SP released ipsilaterally in the L4-L5 DH would not have had to diffuse across the midline and induce SP release from primary afferent terminals in the contralateral L4-L5 DH.

Other researchers have found that unilateral exposures to UVR can induce bilateral changes in spinal neurons or peripheral, nociceptive fibers. Thompson et al. (1994) found bilateral thermal and mechanical hyperalgesia on the hindpaws of rats that had been given unilateral, hindpaw UVA. The thermal and mechanical sensitivities were less pronounced on the contralateral hindpaws than the ipsilateral paws, but the timecourses for the changes were similar on both hindpaws (Thompson et al., 1994). Boxall et al. (1998) also found that unilateral UVA exposure to the rat hindpaw produced mechanical hyperalgesia and allodynia on both hindpaws. The peaks of hyperalgesia and allodynia in both hindpaws, measured at 24 hours postirradiation, occurred at roughly the same time that bilateral increases in the mGluR3 mRNA content were found in the lumbar dorsal horn (Boxall et al., 1998). At 24 hours post-UVA, the increase in mGluR3 mRNA was highest in laminae II-IV and lamina I of the L5 lumbar segment but was also found in laminae IV-VII (Boxall et al., 1998). The increases were restricted to laminae I-IV by 48 hours postirradiation, when the sensitivities to mechanical stimuli had begun to normalize (Boxall et al., 1998). Similarly, Gillardon et al. (1992) found that UVB exposed unilaterally to rats' hindpaws increased the concentration of junD mRNA in both the ipsilateral and contralateral sides of the lumbar spinal cord. The junD mRNA content at six hours post-UVB was increased to roughly eight times the level found in non-irradiated rats, an increase that was more or less coincident with the neurogenic vasodilation, or flare, and plasma extravasation that UVB had induced in the irradiated skin (Gillardon et al., 1992).

UVR-induced changes in B1 bradykinin receptor (B1-R) responsiveness have also been found in association with hyperalgesia on the contralateral (non-irradiated) hindpaws of rats (Perkins & Kelly, 1993b). Perkins & Kelly (1993b) found that, compared to controls, unilateral UVA increased the thermal hyperalgesia induced in both the ipsilateral and contralateral hindpaws by an intravenously-injected B1 bradykinin receptor (B1-R) agonist. In the absence of treatment with the B1-R agonist, thermal hyperalgesia was only significant on the UV-irradiated (ipsilateral) hindpaw and was still present, following its peak at 48 hours post-UVA, at 96 hours post-UVA (Perkins & Kelly, 1993b). Gougat et al. (2004) found that the thermal hyperalgesia induced by unilateral hindpaw UVA, which could be reduced by up to 85 percent by a small-molecule B1-R antagonist, was only significant on the irradiated side at 48 hours post-UVA. Although the same half-duration dose of UVA (6,210 mJ/cm2) used by both groups of investigators might account for the absence (Gougat et al., 2004) or "subclinical" character (Perkins & Kelly, 1993b) of the observed contralateral hyperalgesia, Perkins & Kelly (1993b) found that the hyperalgesia measured on the contralateral hindpaw was highest at 24 hours post-UVA and had decreased by 48 hours post-UVA. Given this earlier disappearance of BK-R hyperresponsiveness on the contralateral side, contralateral hyperalgesia may have developed in the animals studied by Gougat et al. (2004) and subsided by the 48 hour time point at which the B1-R antagonist was administered.

UVR has also been shown to induce spontaneous activity in, and increases in the excitability of, DH neurons (Urban et al., 1993; Chapman and Dickenson, 1994). In general, the spontaneous activity in DH neurons is induced by C-fiber activity but soon becomes independent of changes in C-fiber activity. In other words, the timecourse of the increases in spontaneous C-fiber activity, induced by UVR exposure, should not be assumed to parallel the increases in the firing rates of DH neurons. Szolcsányi (1987) measured spontaneous APs in PMN C-fibers that began at 30 minutes post-UVR and were well-developed across the interval of 2.5-5 hours post-UVR, but the timecourse of C-fiber activity has not been analyzed across the entire, up-to-7-day timecourse of UVR-induced hyperalgesia and blood flow increases. Eschenfelder et al. (1995) began analyzing the spontaneous APs in C-fibers, which were found to occur at 0.8-1.25 Hz, at 24 hours post-UVB and took daily measurements on each of the four subsequent days. The percentage of C-fibers showing spontaneous activity was highest at 72 hours post-UVB and declined almost to baseline by 5 days post-UVB, but the changes in the activities of C-fibers were not monitored over the first 24 hours following UVB (Eschenfelder et al., 1995). Urban et al. (1993) found that the spontaneous activity of WDR neurons, in the DH, was largely independent of C-fiber inputs at 5-7 days post-UVR, as indicated by the nonsignificant effect of dorsal rhizotomy on WDR neuron activity, but was even somewhat independent, albeit nonsignificantly, over the 1-3 day, post-UVB interval. Thus, the activities of DH neurons do not simply parallel, temporally, the activities of C-fibers. There is also some evidence that the UVR-induced changes in the spontaneous firing rates of DH neurons does not correlate, and may even vary inversely, with the excitability of DH neurons. For example, Chapman and Dickenson (1994) found that the UV-induced augmentation of one form of C-fiber wind-up, which involves measuring the excitability of DH neurons and is thought to reflect central sensitization, increased between 3 and 5 days post-UVR but was accompanied by a nonsignificant decrease in the mean frequency, from 2.5 to 1.86 Hz, of the spontaneous APs in DH neurons in the L1-L3 segments. Similarly, Chapman and Dickenson (1994) measured UVR-induced electrophysiological changes, in the DH, that were consistent with allodynia and that were disconnected from artificially-induced changes in C-fiber activity. Consistent with allodynia, Chapman and Dickenson found decreased thresholds in A fibers innervating the UV-irradiated hindpaw and increased numbers of action potentials induced in WDR neurons in response to a fixed-duration, three-times-threshold stimulation of A fibers. However, the firing of WDR neurons was not significantly augmented, compared to non-irradiated controls, in response to peripheral stimulation of C-fibers (Chapman and Dickenson, 1994). These results indicate that, in the context of central sensitization, the firing rates of DH neurons may increase or decrease in ways that do not reliably correlate with changes in spontaneous C-fiber activity.

From a practical standpoint, it should also be evident that the time post-UVR cannot be used to predict the degree to which UVR-induced hyperalgesia is peripherally-mediated or centrally-mediated. Thompson et al. (1994) found that hindpaw UVR produced an augmentation of A-wind-up, an electrophysiological change that is consistent with allodynia, in the hemisected spinal cords of rats at 24 hours post-UVR. Nociceptive responses that are consistent with central sensitization can therefore be established, in response to UVR exposure, rather quickly. Other results, apart from the effects of UVR, shed light on the capacity for C-fiber activity to rapidly induce central sensitization. Klede et al. (2003) found that 1 Hz stimulation of high-threshold, mechanically-insensitive C-fibers produced punctante SMHA and allodynia that were both centrally-mediated, but only the allodynia was clearly dependent on ongoing C-fiber stimulation. Although the results of Chapman and Dickenson (1994) imply that UVR-induced allodynia can become partially independent of C-fiber activity, the emergence of both allodynia and punctate SMHA after only 30 minutes of C-fiber stimulation (Klede et al., 2003) highlights the rapidity with which central sensitization can emerge and become, particularly in the case of punctate SMHA, partially independent of C-fiber activity.

Bradykinin and Early, UVB-Induced Action Potentials in C-fibers

Other evidence suggests that BK exerts direct effects on nociceptive PAFs, and these effects may contribute to the early induction by UVR of spontaneous activity in C-fibers. In the UVB-irradiated skin of humans, Eisenbarth et al. (2004) found that the neurogenic, axon reflex-associated vasodilation was enhanced, compared to controls, in response to the localized perfusion of B1-R and B2-R agonists into the irradiated skin. While the B1-R agonist-induced, subjective pain ratings were also enhanced at the perfusion site, the vasodilation induced by the BK-R agonists at the infusion site was not augmented by UVB (Eisenbarth et al., 2004). By inserting microdialysis catheters intracutaneously (i.e. intradermally) into the irradiated skin, Eisenbarth et al. (2004) were able to assess BK-R-induced vasodilation at both the site of BK-R agonist perfusion and the skin surrounding the perfusion site. Both sites of analysis were within the boundary of the irradiated skin, and the experiments were performed at 24 hours post-UVB (Eisenbarth et al., 2004). UVB evidently induced an increase in the B1-R or B2-R responsiveness of C-fibers or augmented the release of other C-fiber-activating mediators from KC within the perfusion site. The absence of local vasodilation largely excludes a UVB-induced increase in the responsiveness of ETC or SMC to the direct actions of BK. If UVB had induced BK-R hyperresponsiveness in both C-fibers and ETC or SMC, the BK-R agonists would be expected to have produced non-neurogenic vasodilation at the perfusion site and neurogenically-mediated vasodilation in the skin at which the flare was induced. It is noteworthy that vasodilation both within and surrounding the irradiated skin could be explained by axon reflexes induced by UVB, and Eisenbarth et al. (2004) was, therefore, evaluating the capacity of BK-R agonists to augment UVB-induced axon reflexes within the irradiated site (see Fig. 2). The effects of BK-R agonists that were unique to the UVB-irradiated subjects, and that were not found in non-irradiated controls, can be seen as "UVB-specific."

These and other results suggest that bradykinin may contribute to the spontaneous C-fiber activity induced by UVB. In rabbits whose ears had been exposed to UVR, Szolcsányi (1987) found that the administration of BK into the greater auricular artery produced a greater number and duration of spontaneous action potentials, compared to nonirradiated controls, in polymodal nociceptive C-fibers of the greater auricular nerve. In non-irradiated rabbits, bradykinin injected intra-arterially also induced "spontaneous" activity polymodal nociceptive C-fibers (Szolcsányi, 1987). In the context of the spontaneous APs that were measured in C- and A-fibers of the saphenous nerve at 5 days post-UVR, Andreev et al. (1994) noted that BK and other early mediators were unlikely to contribute directly to the activation of PAFs at such a late time point. As discussed below, however, the early activation of C-fibers by BK and other mediators may be necessary for C-fiber activity to be sustained by UVR-induced, late-phase mediators. For example, authors have proposed that NGF may sustain the release of CGRP and SP from C-fibers (Khalil et al., 2002) or be responsible for the ongoing APs in C-fibers at 5 days post-UVR (Andreev et al., 1994).

Biphasic Increases in Blood Flow: Are The Effects of Prostaglandins Non-neurogenic or Just Non-Activating?

The difficulty arises in attempting to reconcile the rapidly-induced release of CGRP and SP in the irradiated site (Benrath et al., 1995) with the apparent capsaicin-insensitivity of the first 24 hours of UVB-induced erythema. Some investigators have found that UVB induces two phases of increased blood flow within the irradiated site (Benrath et al., 1995; Benrath et al., 2001), and the early and late peaks of erythema were attributed, respectively, to non-neurogenic and neurogenic mediators (Benrath et al., 2001). The early peak increase in blood flow occurred at 1 hour post-UVB in rats (Benrath et al., 1995) and 12 hours post-UVB in humans (Benrath et al., 2001), and the second peak occurred in rats at 24 hours (Benrath et al., 1995) and in humans at 36 hours (Benrath et al., 2001) post-UVB. When human skin was treated with topical capsaicin for four days and was exposed to UVB on the day after the final capsaicin treatment, there was no reduction in blood flow in the irradiated skin until 24 hours post-UVB (Benrath et al., 2001). In part because the first peak of erythema was insensitive to capsaicin pretreatment, which depletes SP and CGRP from C-fiber terminals, it was suggested that primarily the second phase of erythema was neurogenic (Benrath et al., 2001). This neurogenic SP and CGRP release was proposed to be mediated by axon reflexes (Benrath et al., 2001). Given that the UVB-induced increases in HA and prostaglandins have been found to decrease to pre-UVB concentrations within 18-24 hours post-UVB and that the administration of COX inhibitors have been shown to only inhibit erythema within the first 24-36 hours post-UVB, the early phase of erythema was attributed to the non-neurogenic, direct vasodilatory actions of PGs and HA (Benrath et al., 2001).

Although the early phase of UVB-induced erythema is likely to be partially non-neurogenic in origin, there is considerable evidence that neurogenic effects of UVB begin almost immediately after exposure. For example, the increase in blood flow to human skin that was up to 10 mm outside the irradiated border, a neurogenically-mediated effect, began, in the absence of capsaicin pretreatment, at 9 hours post-UVB (Benrath et al., 2001). This is a more telling result than the limited attenuation of the early erythema by capsaicin, in part because the effects of topical capsaicin are less predictable than intradermal capsaicin and are less reliable in humans than in rodents (Szallasi and Blumberg, 1999). For example, Munn et al. (1997) found that topical capsaicin did not alter SP immunoreactivity in the skin of humans. In contrast, intradermal capsaicin was found to produce a pronounced decrease in SP immunoreactivity and degeneration of SP-containing nerve fibers (Szallasi and Blumberg, 1999). Szallasi and Blumberg (1999) also noted that human skin is between 4 and 8 times less permeable to topical capsaicin than rat skin. Benrath et al. (2001) also noted that the topical capsaicin preparation that was used had previously been shown to be too low in potency to completely deplete the stores of neuropeptides from sensory fibers. It is conceivable that capsaicin partially depleted the SP and CGRP stores from C-fibers and that the UVB-induced increases in BK, PGE2, TNF-, IL-1, and other early mediators depleted the remaining stores, thereby explaining the monophasic increase in blood flow, as found by Benrath et al. (2001), in capsaicin-pretreated, UVB-exposed human skin.

The finding that capsaicin pretreatment did not attenuate the UVB-induced thermal hyperalgesia within the first 24 hours post-UVB (Benrath et al., 2001) could also be interpreted as evidence of an early neurogenic effect of UVB. At first glance, this might appear to be consistent with the view that predominantly non-neurogenic mechanisms occur during the first 24 hours. Although a nonsignificant attenuation of the UVB-induced thermal hyperalgesia (THA) was apparent only after a 24 hour delay in capsaicin pretreated skin, the UVB-induced THA increased monophasically over the first 24 hours in capsaicin-pretreated skin and did not begin after a 24 hour latent period (Benrath et al., 2001). An antihyperalgesic effect of capsaicin pretreatment began to emerge after the 24-hour point (Benrath et al., 2001), and this suggests that a minimal desensitization of C-fiber responses occurred in response to capsaicin. In other words, the capsaicin pretreatment may have blunted the CGRP release produced by early, neurogenically-acting mediators. As the CGRP and SP stores were being replenished over roughly the first 24 hours post-UVB, after having been partially depleted by topical capsaicin pretreatment, the newly-replenished CGRP and SP could have produced the monophasically-emerging hyperalgesic and vasodilatory effects found by Benrath et al. (2001). This could have caused the first 24 hours of UVB-induced changes to appear "non-neurogenic," when in fact the C-fibers would have been "running on empty" and have been acted upon by BK and other early mediators.

Although the UVB-induced increases in the concentrations of PGs and HA do return to baseline levels within 24-36 hours, their early effects on C-fibers may contribute to hyperalgesia and neurogenic vasodilation at later time points. Benrath et al. (2001) suggested that UVB-induced HA, PGs and other mediators may have contributed to THA and MHA, by sensitizing C-fibers, during the first 36 hours post-irradiation but that substance P and other C-fiber-derived neuromediators may have sustained the THA after that point. Similarly, Eschenfelder et al. (1995) suggested that HA and PGs induced transiently by UVB were likely to primarily influence the erythemal and edematous responses during the time period, namely the first 8-24 hours, their concentrations were elevated. UVB-induced PGE2 could conceivably have reduced the thresholds, in C-fibers, required for C-fiber-activating stimuli (i.e. heat) or substances to induce action potentials, and this PG-mediated effect could conceivably produce THA without inducing AR-mediated vasodilation. Prostaglandins are known to sensitize C-fibers to subthreshold depolarization without, themselves, inducing APs (Millan, 1999). It is known that COX inhibitors administered more than 24-36 hours post-UVB are ineffective in reducing the erythemal response, but COX inhibitors administered immediately after UVB produced relatively stable anti-erythemal effects over 48 hours (Eschenfelder et al., 1995). It was noted that UVB-induced PGE2 and PGF2 levels may remain elevated up to 48 hours post-irradiation (Eschenfelder et al., 1995), and this could support the suggestion that a mixture of neurogenic and non-neurogenic factors contribute to UVB-induced blood flow, in animals, at the 24-hour time point (Eschenfelder et al., 1995). But given that the anti-erythemal effect of early, COX-inhibitor treatment persisted past the 36 hour point, which corresponded to the more robustly neurogenic, late blood flow peak in humans (Benrath et al., 2001), it is possible that the early effects of PGs on C-fibers facilitate the later, sustained phase of neurogenic inflammation and vasodilation. PGs and HA were not suggested to have been the exclusive or obligatory mediators of the early vasodilatory and hyperalgesic effects (Benrath et al., 2001), and PGs and HA are not the only mediators that are rapidly induced by UVB and that could act on C-fibers. BK, for example, is induced by UVB in a matter of minutes (Kang-Rotondo et al., 1996) and is known to be able to induce action potentials, and not simply induce sensitization, in C-fibers (Banik et al., 2001). Andreev et al. (1994) suggested that the pro-inflammatory cytokines and NGF could have produced the spontaneous activities of C- and A-fibers that they had measured at five days post-UVB. Additionally, the neurogenic vasodilation that was evident in humans at 9 hours post-UVB (Benrath et al., 2001) is a reliable sign that C-fibers were being depolarized, not merely sensitized, and that APs were being induced in them.

Following UVB+UVA exposure to the rat hindpaw, for example, the CGRP content of the skin decreased as soon as 2 hours post-UVR (Gillardon et al., 1995). The decrease in CGRP was nearly maximal by 6 hours post-UVB, was maximal by 12 hours post-UVB, and was starting to increase at 24 hours post-UVB (Gillardon et al., 1995). The authors suggested that UVB had depleted CGRP from the skin by inducing the release of CGRP from cutaneous nerve fibers (Gillardon et al., 1995). Although the early peak in blood flow occurred in rats at 1 hour post-UVB, the early increase had not declined, to a between-peak minimum, until 12 hours post-UVB (Benrath et al., 1995). In the same experiments, the s.c. (systemic) administration of the CGRP receptor antagonist CGRP(8-37), the NOS inhibitor L-NAME, or a combination of the two agents reduced UVB-induced blood flow as soon as 1 hour post-irradiation. The injection of the NK1-R antagonist, CP-96,345, at 1 hour post-UVB also reduced blood flow. Similarly, Eschenfelder et al. (1995) found that each of two SP receptor antagonists, administered separately and intradermally to UVA+UVB-exposed ears, decreased the resulting edema as soon as 6 hours post-irradiation and the erythemal response by 12 hours post-UVB.

When the bimodal qualities of UVR-induced herpesvirus reactivation are viewed in the context of the above controversy, UVR can be seen as inducing two phases of neurogenic changes. In experimental reactivation of HSV by UVB, for example, there is frequently a biphasic pattern to the reactivation and the appearance of lesions (Bernstein et al., 1997; Spruance and Kriesel, 2002; Burkhart and Burkhart, 2005). The first peak occurs at roughly 24 hours post-UVB and has been attributed to HSV reactivation in the skin (Burkhart and Burkhart, 2005). The second peak requires about four days, post-UVB, to occur in humans, and this has been attributed to the time required for anterograde axonal transport, as from the DRG or TG to the skin, of HSV proteins (Burkhart and Burkhart, 2005). The second neurogenic phase induced by UVB may, also in the context of CGRP release, be an "axonal transport phase," in which stores of CGRP and other proteins are transported from the cell bodies of DRG neurons to the peripheral terminals, in the skin via anterograde axonal transport, and the central terminals of DRG neurons. This second phase would replenish the depleted stores of CGRP and allow for other phenotypic changes to occur on DRG neurons.
　
Central Suppression of Itch by Pain, and Suppression of Both Itch and Pain by UVR

Given that UVB has been shown to suppress pathological itch (pruritus) and induce hyperalgesia, at least acutely, it is possible to view some of the effects of UVR in terms of the inverse relationship between itch and pain (Ikoma et al., 2003). Eisenbarth et al. (2004) found that activation of BK receptors, with the use of BK receptor agonists, did not produce itch sensation in UVB irradiated skin but did produce itch in nonirradiated controls. UVB irradiation did produce sensitization to the neurogenic effects, both in terms of the axon reflex flare and the hyperalgesia, of the BK agonists, and the authors suggested that UVB-induced hyperalgesia may have produced centrally-mediated suppression of itch (Eisenbarth et al., 2004). The generalized, inverse relationship between itch and pain is known to be tied to opiodergic effects (Ikoma et al., 2003), but it is necessary to specify the site at which the change in opiodergic signalling is occcurring. This is currently a difficult task in the context of the effects of UVR, given that the effects of UVR interact with peripheral opiodergic signalling but are also likely to affect spinally and supraspinally mediated opiodergic signalling.

Exposure to UVR has been shown to increase the expression of the proopiomelanocortin (POMC) gene and the production of -endorphin and -lipotropin by KC (Wintzen et al., 1996), and UVR also modifies the responsiveness of PAFs to opioid receptor ligands (Andreev et al., 1994). The interpretation of these findings has, however, been a source of significant confusion. In early research, exposures of large areas of skin to UVR were found to increase the plasma concentrations of -endorphin and other POMC-derived peptides (Levins et al., 1983). Given the large areas of UVR-exposed skin, the UVR-induced release of the peptides from KCs was thought to be extensive enough to increase the systemic concentrations of the peptides. Researchers have not consistently found changes in the concentrations of plasma opioid peptides in UVR-exposed humans (Wintzen et al., 2001; Gambichler et al., 2002b), and a number of relevant issues have not been addressed adequately. Some researchers have found reinforcing or reward-like effects of UVR and have explained the results in terms of an opioidergic effect (Kaur, Liguori, and Fleischer et al., 2006; Kaur, Liguori, and Lang et al., 2006). To the extent that an increase in the plasma concentration of -endorphin or another opioidergic peptide could contribute to these subjective effects, the opioid peptides would have to cross the blood-brain barrier and ultimately exert a generalized augmentation of -opioidergic activity in one or another supraspinal sites. An increase in plasma -endorphin would, most simply, not be a reliable indication of supraspinally-mediated antinociception. Apart from this issue, the increases in plasma opioids and POMC-derived mediators are more likely to be mediated by UVR-induced effects on the spinohypothalamic tract or on other neuronal populations that influence pituitary function. For example, the UVB-induced increase in plasma -MSH, following UVB exposure to the eyes alone, was found to be blocked by hypophysectomy or ciliary gangliectomy (Hiramoto et al., 2003). In addition, both the antipruritic effects of UVB and the effects of UVR on pain thresholds or C-fiber activity are broadly consistent with, at least in the short term, the antagonism of -opioidergic activity.

It is noteworthy that the notion of UVB as rewarding stimulus (Kaur, Liguori, and Fleischer et al., 2006; Kaur, Liguori, and Lang et al., 2006) is generally consistent with the augmentation of anti-nociceptive pathways and is clearly inconsistent with an escalation of centrally-mediated hyperalgesia. Researchers have found evidence that UVR can suppress pain for several hours after exposure (Kaur et al., 2005b) and that UVR can be used to prevent post-herpetic neuralgia (Jalali et al., 2006). While the investigation of UVR-induced antinociceptive and reward-associated effects is a valid avenue of research, one problem seems to be the assumption that the reward-associated effects must be primarily or exclusively opioidergic. This is not the case. Becerra et al. (2001) noted that ascending nociceptive pathways can themselves activate neurons in the ventral striatum and nucleus accumbens, meaning that nociceptive stimuli activate dopaminergically-mediated reward centers in the brain (Gear et al., 1999; Becerra et al., 2001). These effects could occur via the activation of spinothalamic tract neurons or by the direct activation of striatal neurons, given that neurons in the lateral dorsal horn of primates and rats are known to form direct synaptic connections with striatal neurons (Newman et al., 1996)
Gillardon et al. (1992) suggested, explicitly, that the apparent UVB-induced release of CGRP into the DH could both contribute to UVB-induced hyperalgesia and, implicitly, activate descending, -opioidergic, supraspinally-mediated, pathways. Although researchers have not investigated the involvement of specific supraspinal sites in the hyperalgesic or anti-hyperalgesic effects of UVB, it is likely that chronic treatment with UVB, particularly at high-doses, would activate and produce changes in neurons that exert descending influences on nocisponsive, DH neurons.

Although any UVR-induced changes in opioidergic activity in spinal or supraspinal neurons are poorly understood and are likely to be complex, UVR has produced changes in the responses of C-fibers to opioid-receptor (OR) ligands. Andreev et al. (1994) found that the application of either of two -OR agonists, morphine and DAGOL, or the -OR agonist, U-69593, to the peripheral terminals of C-fibers and A-fibers, in UV-irradiated skin, reduced the frequencies of spontaneous APs in the fibers. These reductions, measured at five days post-UVR, were naloxone-reversible (Andreev et al., 1994). These findings are relevant to a discussion of the supposed naltrexone-sensitivity of the addictive or reinforcing effects of UVR. Given that the peripheral, hyperalgesic effects of UVR are naloxone-sensitive, one would expect naloxone to disinhibit and essentially "unblind" the peripheral component of UVR-induced hyperalgesia. The naloxone would clearly be expected to amplify, both at the peripheral and spinal level, the hyperalgesic effects of ongoing UVR. Given that the behavior could be modified by merely the peripheral actions of systemically-administered naloxone, it is inappropriate to conclude that UVR produces some sort of mechanistically-nonspecific, opioidergically- and supraspinally-mediated reinforcing effect.

In the context of multiple sclerosis, it is noteworthy that IL-10, in addition to other mediators, are likely to contribute to both immunosuppressive and antihyperalgesic effects of UVR. The UVB-induced production of IL-10, by APC and other cells, is known to depend on UVB-induced CGRP release (Kitazawa et al., 2000). Given that exogenous IL-10 was found to counteract UVB-induced hyperalgesia (Saadé et al., 2000), it follows that UVB-induced endogenous IL-10 may also contribute to the supposed antihyperalgesic effects of chronic UVB. There is already some evidence that UVB can produce cytokine "cascades," in the cell bodies and central branches of trigeminal ganglion (TG) neurons, that parallel the pattern of UVB-induced cytokine production in the skin. Shimeld et al. (1999) found that UVB induced TNF- and IL-6 production, by satellite cells, in the TG of mock-inoculated mice (i.e. those that had not been infected with HSV). This transient inflammatory response is unlikely to persist, given that IL-6 knockout mice are known to have reduced IL-10 production in response to UVB (Nishimura et al., 1999). TNF-, induced in response to UVB-induced CGRP release, is also known to be required, via the TNF--induced migration of LC, for UVB-induced local immunosuppression (Niizeki et al., 1997). In addition, the UVB-induced synthesis of 1,25(OH)2D3 by KCs is known to be dependent on the UVB-induced increases in the TNF-a content in the skin (Lehman et al., 2004). A similar progression of TNF-- and IL-6-induced anti-inflammatory effects may occur in the spinal cord or other sites in the CNS.

Interactions With Vitamin D-Mediated Effects And Prospects For Further Research

Vitamin D may interact in a number of ways with the neurogenic effects of UVR. The oral administration of the vitamin D analogue CB1093, in a rat model of diabetic neuropathy, was found to increase, compared to untreated diabetic rats, the CGRP, substance P, and NGF protein concentrations in segments of the sciatic nerve (Riaz et al., 1999). In non-diabetic rats, compared to non-diabetic rats not treated with CB1093, the oral CB1093 also increased the content of CGRP and NGF in the sciatic nerve fibers, the NGF content in the soleus muscle, and the NGF mRNA in the skin from the hindlimb foot. The increases in CGRP were thought to be NGF-dependent and secondary to the CB1093-induced increase in the NGF protein content in the sciatic nerve fibers (Riaz et al., 1999).

VDR ligands are also known to induce GDNF and the low-affinity neurotrophin receptor (p75NTR) in various cell types found in the CNS, and these changes could modify the effects of UVR on the spinal cord. VDR ligands have been shown to induce the expression of the low-affinity neurotrophin receptor (p75NTR) in glioma cells (Naveilhan et al., 1996a), the expression and protein content of 75NTR in the developing brain (Eyles et al., 2003), and the p75NTR mRNA content of cultured oligodendrocytes and astrocytes (Baas et al., 2000). The p75NTR receptor binds all members of the neurotrophin family and, in concert with TrkA, is thought to be involved in the retrograde axonal transport, at least by L4 and L5 DRG neurons, of NGF (Delcroix et al., 1997). In the developing brains of rats whose mothers were depleted of dietary vitamin D3, the levels of p75NTR mRNA were reduced by 30 percent and the p75NTR protein content, in four separate brain regions, was almost completely depleted (Eyles et al., 2003). Maternal vitamin D3 depletion also reduced the concentration of the free NGF protein by 17 percent and the concentration of free GDNF by 25 percent (Eyles et al., 2003). Although VDR ligands do not appear to regulate the expression or protein content of BDNF, it is noteworthy that p75NTR is thought to be important for the trophic actions of BDNF. The induction of NGF and perhaps other neurotrophins by 1,25(OH)2D3 in the skin may also contribute to the effects of UVB on DRG neurons. Tacalcitol, a 1,25(OH)2D3 analog, has been shown to increase NGF expression the release of NGF by cultured human keratinocytes (Fukuoka et al., 2001). Fukuoka et al. (2001) suggested that VDR ligands may, by increasing neurotrophin expression in the skin, have potential in the treatment of peripheral neuropathy. It is interesting that GDNF has the potential to treat neuropathic pain (Sah et al., 2005), and GDNF has been found to upregulate CGRP expression by sensory neurons without inducing hyperalgesia (Ramer et al., 2003). 1,25(OH)2D3 has been shown to increase GDNF production by numerous cell types (Naveilhan et al., 1996b).

UVR has also been shown to modify the p75NTR content of sensory fibers (Bayerl et al., 1997; Moll et al., 1994), and these effects may or may not be partially dependent on UVB-induced 1,25(OH)2D3. For example, the p75NTR content in cutaneous nerve fibers was found to be reduced at 24 hours post-UVR in humans with UV-induced dermatitis (Bayerl et al., 1997) and also to be reduced in the dermal nerve fibers of normal humans at 48 hours post-UVB (Moll et al., 1994). The induction of NT-3 and NT-4/5 production in KCs exposed to UVB (Marconi et al., 2003), and the induction of NT-3 production in UVA-irradiated KCs (Marconi et al., 2003), are other effects of UVR that are strikingly similar to the effects of 1,25(OH)2D3 on neurotrophin production (Neveu et al., 1994). 1,25(OH)2D3 was found to upregulate NT-3 and NT-4 production by astrocytes (Neveu et al., 1994). Again, the effects of UVB and UVA on NT-3 are probably independent of 1,25(OH)2D3, but this does not preclude an effect of 1,25(OH)2D3 on KC neurotrophin production or on C-fibers in the skin. Given these remarkable similarities between the effects of VDR ligands and the effects of UVR on neurotrophin production, it is not unreasonable to suspect some local effects of UVB-induced 1,25(OH)2D3 on sensory fibers.
Interestingly, Plotnikoff and Quigley (2003) recently found that 93 percent of people who sought medical treatment for nonspecific, musculoskeletal pain were clinically deficient in vitamin D3. This was consistent with previous reports of muscle pain, occurring in conjunction with muscle weakness, in people with vitamin D3 deficiency (Plotnikoff and Quigley, 2003). Although the musculoskeletal pain in vitamin D3 deficiency was suggested to be secondary to the abnormalities in bone structure that are associated with vitamin D3 deficiency, the pain could also be the result of central sensitization and be explained in terms of the UVB-induced changes in DRG and DH neurons.

Conclusions

In summary, the UVR-induced release of CGRP in the spinal cord is likely to contribute to secondary hyperalgesia, to the antihyperalgesic effects of chronic UVR, to the suppression of pruritus by UVB, and to the reward-associated effects of UVR. CGRP released in the DH and in other spinal and supraspinal sites may also induce immunosuppressive and antihyperalgesic changes in astrocytes, microglia, or dendritic cells in the CNS. In the skin, prostaglandins are likely to sensitize C-fibers to the action-potential-inducing effects of histamine and bradykinin. The low-frequency spontaneous activity induced in C-fibers appears to induce central sensitization in DH neurons, and the firing rates of DH neurons appear to become independent of the firing rates of the C-type neurons that provide direct or indirect (i.e. converging, polysynaptic) inputs to the DH neurons. UVR-induced spontaneous activity in C-fibers has been shown to begin by 30 minutes post-irradiation, and this indicates that the timecourse alone cannot be used to distinguish between primary (peripheral) and secondary (peripheral and central) hyperalgesia. A closer examination of the timecourse, the times post-irradiation at which UVR-induced changes in the peripheral and central nervous systems occur, will nonetheless be important for future research examining changes in astrocytes or microglia. The abundance of evidence suggests that the UVR-induced changes in the CNS should be examined at time points as early as 0.5-2 hours post-irradiation and followed, at various time points, for at least five or more days post-irradiation. The immunological changes in the lymph nodes draining the CNS, such as in the posterior cervical triangle, may require considerably more time to appear.

With regard to multiple sclerosis and the generalized immunological effects of centrally-released CGRP, a number of additional conclusions follow from the experimental results discussed in this paper. First, the exposure of UVB or UVA to the eyes is likely to be especially perilous for individuals with multiple sclerosis. Although it is well known that essentially no UVB wavelengths penetrate deeper than the cornea and that little UVA penetrates deeper than the iris and lens (Sliney, 1997), the cornea is densely innervated by, for example, sensory fibers whose cell bodies are in the TG. The classical map of somatosensory "two-point discrimination" can be used as a crude indicator of the potential for direct, neurological damage induced by UVB or UVA exposure, and casual exposure of the face, and particularly the eyes, should probably be aggressively avoided by individuals with MS (i.e. given the potential, via polysynaptic changes induced by UVR through the TG, caudal trigeminal nucleus, and ciliary ganglia-to-Edinger-Westphal nucleus, for brainstem damage or neurovascular events and increases in BBB permeability). The effects of UVR on the CNS are likely to be especially potent following exposure to the eyes and, additionally, especially difficult to research, given that most sun-derived UVR reaches the surface of the eyes as diffuse UVB and UVA (i.e. after Rayleigh and Mie scattering by gases in the atmosphere of the Earth) (Sliney, 1997). Diffuse UVR would be exceptionally difficult to produce by artificial light sources, and the application of direct UVB or UVA from an artificial source would be potentially so damaging, not only to the eyes but also to the CNS, as to be unethical.

Notwithstanding these conclusions, it is possible that CGRP release, either related to or dependent on NGF trafficking from the skin, plays some role in the protection against MS among individuals younger than a certain age. Although UVR is unlikely to be useful in a therapeutic context, the involvement of NGF and CGRP in UVR-induced neuroimmunology could have implications for the use of vitamin D receptor analogs in MS. The NGF-inducing effects of VDR ligands have not previously been viewed as being relevant to the protection against MS, but the NGF-dependent augmentation, by VDR ligands, of CGRP production in the CNS is likely to have therapeutic implications for MS. Given that cutaneous UVR exposure is likely to exert complex, polysynaptic changes in the activities of various neuronal populations throughout the CNS, future research in rodents should focus on the synaptic actions, rather than increases in the plasma concentrations, of -MSH and other POMC-derived peptides.

References

Aldskogius H, Arvidsson J, and Grant G. The reaction of primary sensory neurons to peripheral nerve injury with particular emphasis on transganglionic changes. Brain Res 1985; 357(1): 27-46.
Andreev, N. et al. Opioids suppress spontaneous activity of polymodal nociceptors in rat paw skin induced by ultraviolet irradiation. Neuroscience 1994; 58(4): 793-8.
Anthoney DA, Bone I, and Evans TR. Inflammatory demyelinating polyneuropathy: a complication of immunotherapy in malignant melanoma. Ann Oncol 2000; 11(9): 1197-200.
Arvidson B. Retrograde transport of horseradish peroxidase in sensory and adrenergic neurons following injection into the anterior eye chamber. J Neurocytol 1979; 8(6): 751-64.
Avis SP and Pryse-Phillips WE. Sudden death in multiple sclerosis associated with sun exposure: a report of two cases. Can J Neurol Sci 1995; 22(4): 305-7.
Baas, D. et al. Rat oligodendrocytes express the vitamin D(3) receptor and respond to 1,25-dihydroxyvitamin D(3). Glia 2000; 31: 59-68.
Banik, R.K. et al. B2 receptor-mediated enhanced bradykinin sensitivity of rat cutaneous C-fiber nociceptors during persistent inflammation. J Neurophysiol 2001; 86: 2727-2735.
Battaglia, G., and Rustioni, A. Substance P innervation of the rat and cat thalamus. II. Cells of origin in the spinal cord. J Comp Neurol 1992; 315: 473-486.
Bayerl, C. et al. Immunohistochemical characterization of HSP, alpha-MSH, Merkel cells and neuronal markers in acute UV dermatitis and acute contact dermatitis in vivo. Inflamm Res 1997; 46(10): 409-11.
Becerra, L. et al. Reward circuitry activation by noxious thermal stimuli. Neuron 2001; 32(5): 927-46.
Benrath J, Gillardon F, and Zimmermann M. Differential time courses of skin blood flow and hyperalgesia in the human sunburn reaction following ultraviolet irradiation of the skin. Eur J Pain 2001; 5(2): 155-67.
Benrath J, Zimmermann M, and Gillardon F. Substance P and nitric oxide mediate wound healing of ultraviolet photodamaged rat skin: evidence for an effect of nitric oxide on keratinocyte proliferation. Neurosci Lett 1995; 200(1): 17-20.
Bernstein, D.I. et al. Effect of foscarnet cream on experimental UV radiation-induced herpes labialis. Antimicrobial Agents and Chemotherapy 1997; 41(9): 1961-1964.
Boonstra, A. et al. UVB irradiation modulates systemic immune responses by affecting cytokine production of antigen-presenting cells. International Immunology 2000; 12(11): 1531-1538.
Bothwell, M. Neurotrophin function in skin. J Investig Dermatol Symp Proc 1997; 2(1): 27-30.
Boudard, F. et al. Inhibition of mouse T-cell proliferation by CGRP and VIP: effects of these neuropeptides on IL-2 production and cAMP synthesis. J Neurosci Res 1991; 29: 29.
Boxall, S.J. et al. Enhanced expression of metabotropic glutamate receptor 3 messenger RNA in the rat spinal cord during ultraviolet irradiation induced peripheral inflammation. Neuroscience 1998; 82(2): 591-602.
Boxall, S.J. et al. MGluR activation reveals a tonic NMDA component in inflammatory hyperalgesia. Neuroreport 1998b; 9: 1201-1203.
Bozzali, M. et al. Quantification of brain grey matter damage in different MS phenotypes by use of diffusion tensor MR imaging. AJNR Am J Neuroradiol 2002; 23: 985-988.
Bracci-Laudiero, L. et al. NGF modulates CGRP synthesis in human B-lymphocytes: a possible anti-inflammatory action of NGF? Journal of Neuroimmunology 2002; 123: 58-65.
Bull, H.A. et al. Expression of nerve growth factor receptors in cutaneous inflammation. British Journal of Dermatology 1998; 139: 776-783.
Burbach, G.J. et al. The neurosensory tachykinins substance P and neurokinin A directly induce keratinocyte nerve growth factor. J Invest Dermatol 2001; 117: 1075-1082.
Burkhart, C.N., and Burkhart, C.G. Bimodal temporal distribution of herpetic reactivation. Mayo Clin Proc 2005; 80(2): 287-288.
Burstein, R. et al. Direct somatosensory projections from the spinal cord to the hypothalamus and telencephalon. J Neurosci 1987; 7: 4159-4164.
Burstein, R., and Potrebic, S. Retrograde labeling of neurons in the spinal cord that project directly to the amygdala or the orbital cortex in the rat. J Comp Neurol 1993; 335: 469-485.
Carlton, S.M. et al. Funicular trajectories of brainstem neurons projecting to the lumbar spinal cord in the monkey (Macaca fascicularis): a retrograde labeling study. J Comp Neurol 1985; 241: 382-404.
Carucci, J.A. et al. Calcitonin gene-related peptide decreases expression of HLA-DR and CD86 by human dendritic cells and dampens dendritic cell-driven T cell-proliferative responses via the type I calcitonin gene-related peptide receptor. J Immunol 2000; 164: 3494-3499.
Chan KY, Beck TW, Mullen P, and Dong WK. Increased axonal transport of trigeminal ganglion proteins in an electrode model. Invest Ophthalmol Vis Sci 1989; 30(9): 2050-5.
Chapman, V., and Dickenson, A.H. Enhanced responses of rat dorsal horn neurons after UV irradiation of the hindpaw; roles of the NMDA receptor. Neurosci Lett 1994; 176(1): 41-4.
Chaudhuri, A. Why we should offer routine vitamin D supplementation in pregnancy and childhood to prevent multiple sclerosis. Medical Hypotheses 2005; 64(3): 608-618.
Constantinescu CS. Melanin, melatonin, melanocyte-stimulating hormone, and the susceptibility to autoimmune demyelination: a rationale for light therapy in multiple sclerosis. Med Hypotheses 1995; 45(5): 455-8.
Davis, A.J. et al. The induction of des-Arg9-bradykinin-mediated hyperalgesia in the rat by inflammatory stimuli. Brazilian J Med Biol Res 1994; 27: 1793-1802.
Delcroix, J-D. et al. Diabetes and axotomy-induced deficits in retrograde axonal transport of nerve growth factor correlate with decreased levels of p75LNTR protein in lumbar dorsal root ganglia. Molecular Brain Research 1997; 51: 82-90.
Djouhri, L. et al. Time course and nerve growth factor dependence of inflammation-induced alterations in electrophysiological membrane properties in nociceptive primary afferent neurons. The Journal of Neuroscience 2001; 21(22): 8722-8733.
Drummond PD. Attenuation of axon reflexes to compound 48/80 after repeated iontophoresis of compound 48/80 in skin of the human forearm. Skin Pharmacol Appl Skin Physiol 2003; 16(4): 263-70.
Dumas M and Jauberteau-Marchan MO. The protective role of Langerhans' cells and sunlight in multiple sclerosis. Med Hypotheses 2000; 55(6): 517-20.
Eisenbarth, H. et al. Sensitization to bradykinin B1 and B2 receptor activation in UV-B irradiated human skin. Pain 2004; 110: 197-204.
Eschenfelder CC, Benrath J, Zimmermann M, and Gillardon F. Involvement of substance P in ultraviolet irradiation-induced inflammation in rat skin. Eur J Neurosci 1995; 7(7): 1520-6.
Eyles, D. et al. Vitamin D3 and brain development. Neuroscience 2003; 118(3): 641-53.
Feldman, S.R. et al. Ultraviolet exposure is a reinforcing stimulus in frequent indoor tanners. J Am Acad Dermatol 2004; 51: 45-51.
Ferguson IA, Schweitzer JB, and Johnson EM Jr. Basic fibroblast growth factor: receptor-mediated internalization, metabolism, and anterograde axonal transport in retinal ganglion cells. J Neurosci 1990; 10(7): 2176-89.
Fjellner, B., and Hägermark, Ö. Influence of ultraviolet light on itch and flare reactions in human skin induced by histamine and the histamine liberator compound 48/80. Acta Derm Venereol 1982; 62: 137-140.
Fox, F.E. et al. Calcitonin gene-related peptide inhibits proliferation and antigen presentation by human peripheral blood mononuclear cells: effects on B7, interleukin-10, and interleukin-12. J Invest Dermatol 1997; 108: 43-48.
Friedman, A.P. et al. Do hyporesponsive genetic variants of the melanocortin 1 receptor contribute to the etiology of multiple sclerosis? Medical Hypotheses 2004 62: 49-52.
Fukuoka, M., et al. Tacalcitol, an active vitamin D3, induces nerve growth factor production in human epidermal keratinocytes. Skin Pharmacol Appl Skin Physiol 2001; 14(4):226-33.
Funasaka Y. et al. The effect of thioredoxin on the expression of proopiomelanocortin-derived peptides, the melanocortin-1 receptor and cell survival of normal human keratinocytes. J Invest Derm Symp Proc 2001; 6: 32-37.
Gallar J, Garcia de la Rubia P, Gonzalez GG, and Belmonte C. Irritation of the anterior segment of the eye by ultraviolet radiation: influence of nerve blockade and calcium antagonists. Curr Eye Res 1995; 14(9): 827-35.
Gambichler, T. et al. Impact of UVA exposure on psychological parameters and circulating serotonin and melatonin. BMC Dermatol 2002a; 2:6.
Gambichler, T. et al. Plasma levels of opioid peptides after sunbed exposures. Br J Dermatol 2002b; 147: 1207-11.
Garcion, E. et al. New clues about vitamin D functions in the nervous system. Trends Endocrinol Metab 2002; 13: 100-105.
Garssen J, Buckley TL, and Van Loveren H. A role for neuropeptides in UVB-induced systemic immunosuppression. Photochem Photobiol 1998; 68(2): 205-10.
Garssen, J. et al. UVB exposure-induced systemic modulation of Th1 and Th2 mediated immune responses. Immunology 1999; 97: 506.
Garssen, J. et al. Transcription-coupled and global genome repair differentially influence UV-B-induced acute skin effects and systemic immunosuppression. J Immunol 2000; 164: 6199-6205.
Gear, R.W. et al. Pain-induced analgesia mediated by mesolimbic reward circuits. J Neurosci 1999; 19(16): 7175-81.
Ghadge GD, Roos RP, Kang UJ, Wollmann R, Fishman PS, Kalynych AM, Barr E, and Leiden JM. CNS gene delivery by retrograde transport of recombinant replication-defective adenoviruses. Gene Ther 1995; 2(2): 132-7.
Gilchrest, B.A. et al. Ultraviolet phototherapy of uremic pruritus. Ann Intern Med 1979; 91: 17-21.
Gillardon F, Eschenfelder C, Rush RA, and Zimmerman M. Increase in neuronal Jun immunoreactivity and epidermal NGF levels following UV exposure of rat skin. Neuroreport 1995; 6(9): 1322-4.
Gillardon F, Morano I, and Zimmermann M. Ultraviolet irradiation of the skin attenuates calcitonin gene-related peptide mRNA expression in rat dorsal root ganglion cells. Neurosci Lett 1991; 124(2): 144-7.
Gillardon F, Schrock H, Morano I, and Zimmermann M. Long-term increase in CGRP levels in rat spinal dorsal horn following skin ultraviolet irradiation. A mechanism of sunburn pain? Ann N Y Acad Sci 1992; 657: 493-6.
Gillardon F, Wiesner RJ, and Zimmermann M. Expression of the junD proto-oncogene in the rat spinal cord and skin following noxious cutaneous ultraviolet irradiation. Neurosci Lett 1992; 136(1): 87-90.
Gougat, J. et al. SSR240612 [(2R)-2-[((3R)-3-(1,3-benzodioxol-5-yl)-3-[[(6-methoxy-2-naphthyl)sulfonyl]amino]propanoyl)amino]-3-(4-[[2R,6S)-2,6-dimethylpiperidinyl]methyl]phenyl)-N-isopropyl-N-methylpropanamide hydrochloride], a new nonpeptide antagonist of the bradykinin B1 receptor: biochemical and pharmacological characterization. J Pharmacol Exp Ther 2004; 309(2): 661-9.
Gumireddy, K. et al. Anti-proliferative effects of 20-epi-vitamin-D3 analogue, KH1060 in human neuroblastoma: induction of RAR- and p21Cip1. Cancer Letters 2003; 190: 51-60.
Gybels, J. et al. A comparison between the discharge of human nociceptive nerve fibres and the subject's rating of his sensations. J Physiol (London) 1979; 292: 193-206.
Harbison JW, Calabrese VP, and Edlich RF. A fatal case of sun exposure in a multiple sclerosis patient. J Emerg Med 1989; 7(5): 465-7.
Harmann, P.A. et al. Collaterals of spinothalamic tract cells to the periaqueductal gray: a fluorescent double-labeling study in the rat. Brain Res 1988; 441: 87-97.
Hart PH, Townley SL, Grimbaldeston MA, Khalil Z, and Finlay-Jones JJ. Mast cells, neuropeptides, histamine, and prostaglandins in UV-induced systemic immunosuppression. Methods 2002; 28(1): 79-89.
Hauser, S.L. et al. Prevention of experimental allergic encephalomyelitis (EAE) in the SJL/J mouse by whole body ultraviolet radiation. J Immunol 1984; 132(3): 1276-1281.
Hayes CE, Cantorna MT, and DeLuca HF. Vitamin D and multiple sclerosis. Proc Soc Exp Biol Med 1997; 216(1): 21-7.
Hayes, C.E. Vitamin D: a natural inhibitor of multiple sclerosis. Proc Nutr Soc 2000; 59(4): 531-5.
Hiramoto, K. et al. Ultraviolet B irradiation of the eye activates a nitric oxide-dependent hypothalamopituitary proopiomelanocortin pathway and modulates functions of -melanocyte-stimulating hormone-responsive cells. J Invest Dermatol 2003; 120: 123-127.
Holán, V. et al. Urocanic acid enhances IL-10 production in activated CD4+ T cells. J Immunol 1998; 161: 3237-3241.
Holme, S.A., and Mills, C.M. Crotamiton and narrow-band UVB phototherapy: novel approaches to alleviate pruritus of breast carcinoma skin infiltration. Journal of Pain and Symptom Management 2001; 22(4): 803-805.
Ikoma, A. et al. Neurophysiology of pruritus: interaction of itch and pain. Arch Dermatol 2003; 139: 1475-1478.
Jalali, M.H. et al. Broad-band ultraviolet B phototherapy in zoster patients may reduce the incidence and severity of postherpetic neuralgia. Photodermatol Photoimmunol Photomed 2006; 22(5): 232-7.
Jiang, M.C. et al. Cellular properties of lateral spinal nucleus neurons in the rat L6–S1 spinal cord. J Neurophysiol 1999; 81: 3078-3086.
Kaptanoglu, A.F., and Oskay, T. Ultraviolet B treatment for pruritus in Hodgkin’s lymphoma. Journal of the European Academy of Dermatology and Venereology 2003; 17: 489-490.
Kang-Rotondo, C.H. et al. Upregulation of nitric oxide synthase in cultured human keratinocytes after ultraviolet B and bradykinin. Photodermatol Photoimmunol Photomed 1996; 12(2): 57-65.
Kaur, M. et al. Side effects of naltrexone observed in frequent tanners: Could frequent tanners have ultraviolet-induced high opioid levels? J Am Acad Dermatol 2005a; (May): 916.
Kaur, M. et al. Indoor tanning relieves pain. Photodermatol Photoimmunol Photomed 2005b; 21(5): 278.
Kaur, M.; Liguori, A.; Fleischer, A.B. et al. Plasma -endorphin levels in frequent and infrequent tanners before and after ultraviolet and non-ultraviolet stimuli. J Am Acad Dermatol 2006; 54(5): 919-920.
Kaur, M.; Liguori, A.; Lang, W. et al. Induction of withdrawal-like symptoms in a small randomized, controlled trial of opioid blockade in frequent tanners J Am Acad Dermatol 2006; 54: 709-11.
Kessler JA, Bell WO, and Black IB. Interactions between the sympathetic and sensory innervation of the iris. J Neurosci 1983; 3(6): 1301-7.
Kezuka, T. et al. Peritoneal exudate cells treated with calcitonin gene-related peptide suppress murine experimental autoimmune uveoretinitis via IL-10. J Immunol 2004; 173: 1454-1462.
Khalil Z, Townley SL, Grimbaldeston MA, Finlay-Jones JJ, and Hart PH. Cis-Urocanic acid stimulates neuropeptide release from peripheral sensory nerves. J Invest Dermatol 2001; 117(4): 886-91.
Kitazawa, T. et al. Hapten-specific tolerance promoted by calcitonin gene-related peptide. J Invest Dermatol 2000; 115: 942-948.
Kloos L, Sillevis Smitt P, Ang CW, Kruit W, and Stoter G. Paraneoplastic ophthalmoplegia and subacute motor axonal neuropathy associated with anti-GQ1b antibodies in a patient with malignant melanoma. J Neurol Neurosurg Psychiatry 2003; 74(4): 507-9.
Klyce SD, Beuerman RW, and Crosson CE. Alteration of corneal epithelial ion transport by sympathectomy. Invest Ophthalmol Vis Sci 1985; 26(4): 434-42.
Koltzenburg, M. et al. Does the right side know what the left is doing? Trends Neurosci 1999; 22(3): 122-7.
Krogstad, A.L. Neurogenic control of blood flow and histamine release in psoriatic skin. Acta Derm Venereol (Stockholm) 1999; 79(suppl. 203): 3-43.
Laborde JM, Dando WA, and Teetzen ML. Climate, diffused solar radiation and multiple sclerosis. Soc Sci Med 1988; 27(3): 231-8.
Lassmann, H., and Ransohoff, R.M. The CD4-Th1 model for multiple sclerosis: a critical re-appraisal. Trends Immunol 2004; 25(3): 132-7.
Legat FJ, Griesbacher T, Schicho R, and Althuber P, Schuligoi R, Kerl H, and Wolf P. Repeated subinflammatory ultraviolet B irradiation increases substance P and calcitonin gene-related peptide content and augments mustard oil-induced neurogenic inflammation in the skin of rats. Neurosci Lett 2002; 329(3): 309-13.
Legat, F.J. et al. The role of calcitonin gene-related peptide in cutaneous immunosuppression induced by repeated subinflammatory ultraviolet irradiation exposure. Experimental Dermatology 2004; 13: 242-250.
Lehmann B. et al. Demonstration of UVB-induced synthesis of 1,25-dihydroxyvitamin D3 in human skin by microdialysis. Arch Dermatol Res 2003; 295: 24-28.
Lehman B. et al. Role for tumor necrosis factor- in UVB-induced conversion of 7-dehydrocholesterol to 1,25-dihydroxyvitamin D3 in cultured keratinocytes. Journal of Steroid Biochemistry & Molecular Biology 2004; 89-90: 561-565.
Levins, P.C. et al. Plasma beta-endorphin and beta-lipotropin response to ultraviolet radiation. Lancet 1983; 2: 166.
Lim, H.W. et al. UVB phototherapy is an effective treatment for pruritus in patients infected with HIV. J Am Acad Dermatol 1997; 37: 414-417.
Liu XG, Morton CR, Azkue JJ, Zimmermann M, and Sandkuhler J. Long-term depression of C-fibre-evoked spinal field potentials by stimulation of primary afferent Adelta-fibres in the adult rat. Eur J Neurosci 1998; 10(10): 3069-75.
Livermore A, Hummel T, Pauli E, and Kobal G. Perception of olfactory and intranasal trigeminal stimuli following cutaneous electrical stimulation. Experientia 1993; 49(10): 840-2.
Lucas RM and Ponsonby AL. Ultraviolet radiation and health: friend and foe. Med J Aust 2002; 177(11-12): 594-8.
Lucchinetti, C. et al. Distinct patterns of multiple sclerosis pathology indicates heterogeneity in pathogenesis. Brain Pathol 1996; 6: 259-274.
Lynn, B. and Shakhanbeh, J. Neurogenic in£ammation in the skin of the rabbit. Agents Actions 1988; 25: 228-230.
Malcangio, M. et al. A common thread for pain and memory synapses? Brain-derived neurotrophic factor and trkB receptors. Trends in Pharmacological Sciences 2003; 24(3): 116-121.
Marconi, A. et al. Expression and function of neurotrophins and their receptors in cultured human keratinocytes. J Invest Dermatol 2003; 121: 1515-1521.
Masson, R.L., Jr. et al. Descending projections to the rat sacrocaudal spinal cord. J Comp Neurol 1991; 307: 120-130.
McCarty, M.F. Down-regulation of microglial activation may represent a practical strategy for combating neurodegenerative disorders. Med Hypoth 2006; 67(2): 251-269.
McGrath Jr., H. Elimination of anticardiolipin antibodies and cessation of cognitive decline in a UV-A1-irradiated systemic lupus erythematosus patient. Lupus 2005; 14: 859-861.
McMichael AJ and Hall AJ. Does immunosuppressive ultraviolet radiation explain the latitude gradient for multiple sclerosis? Epidemiology 1997; 8(6): 642-5.
McMichael AJ and Hall AJ. Multiple sclerosis and ultraviolet radiation: time to shed more light. Neuroepidemiology 2001; 20(3): 165-7.
Mendell, L.M. et al. Diversity of neurotrophin action in the postnatal spinal cord. Brain Research Reviews 2002; 40: 230-239.
Menetrey D and Basbaum AI. Spinal and trigeminal projections to the nucleus of the solitary tract: a possible substrate for somatovisceral and viscerovisceral reflex activation. J Comp Neurol 1987; 255(3): 439-50.
Menon, Y. et al. Reversal of brain dysfunction with UV-A1 irradiation in a patient with systemic lupus. Lupus 2003; 12: 479-482.
Micera, A. et al. Nerve growth factor antibody exacerbates neuropathological signs of experimental allergic encephalomyelitis in adult Lewis rats. J Neuroimmunol 2000; 104: 116-123.
Miki K, Fukuoka T, Tokunaga A, and Noguchi K. Calcitonin gene-related peptide increase in the rat spinal dorsal horn and dorsal column nucleus following peripheral nerve injury: up-regulation in a subpopulation of primary afferent sensory neurons. Neuroscience 1998; 82(4): 1243-52.
Millan, M.J. The induction of pain: an integrative review. Progress in Neurobiology 1999; 57: 1-164.
Moll, I. et al. Effects of ultraviolet B radiation on cytoskeletal and adhesion molecules in human epidermis. Photodermatol Photoimmunol Photomed 1994; 10(1): 26-32.
Munn, S.E. et al. The effect of topical capsaicin on substance P immunoreactivity: A clinical trial and immunohistochemical analysis. Acta Derm Venereol (Stockh) 1997; 77: 158-159.
Naveilhan, P. et al. 1,25-Dihydroxyvitamin D3 regulates the expression of the low-affinity neurotrophin receptor. Brain Res Mol Brain Res 1996a; 41: 259-268.
Naveilhan, P. et al. 1,25-Dihydroxyvitamin D3, an inducer of glial cell line-derived neurotrophic factor. Neuroreport 1996b; 7: 2171-2175.
Neveu, I. et al. 1,25-dihydroxyvitamin D3 regulates NT-3, NT-4 but not BDNF mRNA in astrocytes. Neuroreport 1994; 6: 124-126.
Newman, H.M. et al. Direct spinal projections to limbic and striatal areas: anterograde transport studies from the upper cervical spinal cord and the cervical enlargement in squirrel monkey and rat. J Comp Neurol 1996; 365(4): 640-58.
Niizeki, H. et al. Calcitonin gene-related peptide is necessary for ultraviolet B-impaired induction of contact hypersensitivity. J Immunol 1997; 159: 5183-5186.
Niizeki H, Kurimoto I, and Streilein JW. A substance P agonist acts as an adjuvant to promote hapten-specific skin immunity. J Invest Dermatol 1999; 112(4): 437-42.
Nishigori, C. et al. Evidence that DNA damage triggers interleukin-10 cytokine production in UV-irradiated murine keratinocytes. Proc Natl Acad Sci USA 1996; 93: 10354.
Nishimura, N. et al. Defective immune response and severe-skin damage following UVB irradiation in interleukin-6-deficient mice. Immunology 1999; 97: 77.
Oktar BK and Alican I. Modulation of the peripheral and central inflammatory responses by alpha-melanocyte stimulating hormone. Curr Protein Pept Sci 2002; 3(6): 623-8.
Owens, T. The enigma of multiple sclerosis: inflammation and neurodegeneration causes heterogenous dysfunction and damage. Curr Opin Neurol 2003; 16: 259-265.
Perkins, M.N. et al. Antinociceptive activity of the bradykinin B1 and B2 receptor antagonists, des-Arg9, [Leu8]-BK and HOE 140, in two models of persistent hyperalgesia in the rat. Pain 1993a; 53: 191-197.
Perkins, M.N., and Kelly, D. Induction of bradykinin B1 receptors in vivo in a model of ultra-violet irradiation-induced thermal hyperalgesia in the rat. Br J Pharmacol 1993b; 110: 1441-1444.
Pinessi L, Rainero I, de Gennaro T, Violante A, Cassano D, Matera L, and Cesano A. Cerebrospinal fluid and plasma concentrations of POMC-related peptides in multiple sclerosis. Ann N Y Acad Sci 1992; 650: 351-4.
Plotnikoff, G.A., and Quigley, J.M. Prevalence of severe hypovitaminosis D in patients with persistent, nonspecific musculoskeletal pain. Mayo Clin Proc 2003; 78(12): 1463-70.
Polderman, M.C.A. et al. Efficacy of UVA-1 cold light as an adjuvant therapy for systemic lupus erythematosus. Rheumatology 2004; 43: 1402-1404.
Polgár, E. et al. Alterations of substance P immunoreactivity in lumbar and thoracic segments of rat spinal cord in ultraviolet irradiation induced hyperalgesia of the hindpaw. Brain Res 1998; 786(1-2): 248-51.
Ponsonby AL, McMichael A, and van der Mei I. Ultraviolet radiation and autoimmune disease: insights from epidemiological research. Toxicology 2002; 181-182: 71-8.
Ramer, M.S. et al. Glial cell line-derived neurotrophic factor increases calcitonin gene-related peptide immunoreactivity in sensory and motoneurons in vivo. Eur J Neurosci 2003; 18(10): 2713-21.
Reilly, D.M. et al. The epidermal nerve fibre network. characterization of nerve fibres in human skin by confocal microscopy and assessment of racial variations. Br J Dermatol 1997; 137: 163-170.
Reynier-Rebuffel AM, Mathiau P, Callebert J, Dimitriadou V, Farjaudon N, Kacem K, Launay JM, Seylaz J, and Abineau P. Substance P, calcitonin gene-related peptide, and capsaicin release serotonin from cerebrovascular mast cells. Am J Physiol 1994; 267(5 Pt 2): R1421-9.
Riaz, S. et al. A vitamin D3 derivative (CB1093) induces nerve growth factor and prevents neurotrophic deficits in streptozotocin-diabetic rats. Diabetologia 1999; 42: 1308-1313.
Roelcke U, Hornstein C, Hund E, Schmitt HP, Siess R, Kaltenmaier M, Fassler J, and Meinck HM. "Sunbath polyneuritis": subacute axonal neuropathy in perazine-treated patients after intense sun exposure. Muscle Nerve 1996; 19(4): 438-41.
Saadé, N.E. et al. Modulation of ultraviolet-induced hyperalgesia and cytokine upregulation by interleukins 10 and 13. British Journal of Pharmacology 2000; 131: 1317-1324.
Sah, D.W. et al. New approaches for the treatment of pain: the GDNF family of neurotrophic growth factors. Curr Top Med Chem 2005; 5(6): 577-83.
Sandyk R and Awerbuch GI. Nocturnal plasma melatonin and alpha-melanocyte stimulating hormone levels during exacerbation of multiple sclerosis. Int J Neurosci 1992; 67(1-4): 173-86.
Scholzen TE, Brzoska T, Kalden DH, O'Reilly F, Armstrong CA, Luger TA, and Ansel JC. Effect of ultraviolet light on the release of neuropeptides and neuroendocrine hormones in the skin: mediators of photodermatitis and cutaneous inflammation. J Investig Dermatol Symp Proc 1999; 4(1): 55-60.
Schulze, E. et al. Immunohistochemical detection of human skin nerve fibers. Acta Histochem 1997; 99: 301-309.
Schwarz, A. et al. Ultraviolet radiation-induced regulatory T-cells not only inhibit the induction but can suppress the effector phase of contact hypersensitivity. J Immunol 2004; 172: 1036-1043.
Seiffert K and Granstein RD. Neuropeptides and neuroendocrine hormones in ultraviolet radiation-induced immunosuppression. Methods 2002; 28(1): 97-103.
Seike, M. et al. Increased synthesis of calcitonin gene-related peptide stimulates keratinocyte proliferation in murine UVB-irradiated skin. Journal of Dermatological Science 2002; 28: 135-143.
Sharpe RJ. The low incidence of multiple sclerosis in areas near the equator may be due to ultraviolet light induced suppressor cells to melanocyte antigens. Med Hypotheses 1986; 19(4): 319-23.
Shimeld, C. et al. Reactivation of herpes simplex virus type 1 in the mouse trigeminal ganglion: an in vivo study of virus antigen and cytokines. Journal of Virology 1999; 73(3): 1767-1773.
Shimizu, T., and Streilein, J.W. Influence of alpha-melanocyte stimulating hormone on induction of contact hypersensitivity and tolerance. J Dermatol Sci 1994; 8(3):187-93.
Shreedhar, V.K. et al. Origin and characteristics of ultraviolet-B radiation-induced suppressor T-lymphocytes. J Immunol 1998; 161: 1327-1335.
Sleijffers, A. et al. Epidermal cis-urocanic acid levels correlate with lower specific cellular immune responses after Hepatitis B vaccination of ultraviolet B-exposed humans. Photochemistry and Photobiology 2003; 77(3): 271-275.
Sliney, D.H. Ultraviolet radiation effects upon the eye: problems of dosimetry. Radiation Protection Dosimetry 1997; 72(3): 197-206.
Slominski A, Pisarchik A, Zbytek B, Tobin DJ, Kauser S, and Wortsman J. Functional activity of serotoninergic and melatoninergic systems expressed in the skin. J Cell Physiol 2003; 196(1): 144-53.
Spruance, S.L., and Kriesel, J.D. Treatment of herpes simplex labialis. Herpes 2002; 9(3): 64-69.
Storch, M.K. et al. Multiple sclerosis: in situ evidence for antibody and complement mediated demyelination. Ann Neurol 1998; 43: 465-471.
Streilein JW, Okamoto S, Sano Y, and Taylor AW. Neural control of ocular immune privilege. Ann N Y Acad Sci 2000; 917: 297-306.
Stumpf, W.E. et al. 1,25(OH)2 vitamin D3 sites of action in spinal cord and sensory ganglion. Anat Embryol (Berl) 1988; 177(4): 307-10.
Sun, W. et al. Intramuscular transfer of naked calcitonin gene-related peptide gene prevents autoimmune diabetes induced by multiple low-dose streptozotocin in C57BL mice. Eur J Immunol 2003; 33: 233-242.
Suzuki N, Hardebo JE, and Owman C. Trigeminal fibre collaterals storing substance P and calcitonin gene-related peptide associate with ganglion cells containing choline acetyltransferase and vasoactive intestinal polypeptide in the sphenopalatine ganglion of the rat. An axon reflex modulating parasympathetic ganglionic activity? Neuroscience 1989; 30(3): 595-604.
Szallasi, A., and Blumberg, P.M. Vanilloid (capsaicin) receptors and mechanisms. Pharmacological Reviews 1999; 51(2): 159-211.
Thompson, S.W. et al. Injury-induced plasticity of spinal reflex activity: NK1 neurokinin receptor activation and enhanced A- and C-fiber mediated responses in the rat spinal cord in vitro. J Neurosci 1994; 14(6): 3672-87.
Thompson, S.W. et al. No evidence for contribution of nitric oxide to spinal reflex activity in the rat spinal cord in vitro. Neurosci Lett 1995; 188(2): 121-4.
Toichi E. et al. Cell surface and cytokine phenotypes of skin immunocompetent cells involved in ultraviolet-induced immunosuppression. Methods 2002; 28: 104-110.
Torebjörk, H.E. Afferent C units responding to mechanical, thermal and chemical stimuli in human nonglabrous skin. Acta Physiol Scand 1974; 92: 374-390.
Townley SL, Grimbaldeston MA, Ferguson I, Rush RA, Zhang SH, Zhou XF, Conner JM, Finlay-Jones JJ, and Hart PH. Nerve growth factor, neuropeptides, and mast cells in ultraviolet-B-induced systemic suppression of contact hypersensitivity responses in mice. J Invest Dermatol 2002; 118(3): 396-401.
Trapp, B.D. et al. Axonal transection in the lesions of multiple sclerosis. N Engl J Med 1998; 338: 278-285.
Triaca, V. et al. Presence of nerve growth factor and TrkA expression in the SVZ of EAE rats: evidence for a possible functional significance. Experimental Neurology 2005; 191: 53-64.
Tsunoda, I. et al. Converting relapsing remitting to secondary progressive experimental allergic encephalomyelitis (EAE) by ultraviolet B irradiation. J Neuroimmunol 2005; 160(1-2): 122-34.
Urban, L. et al. Activity of deep dorsal horn neurons in the anaesthetized rat during hyperalgesia of the hindpaw induced by ultraviolet irradiation. Neuroscience 1993; 57(1): 167-72.
Van der Mei, I.A.F. et al. Regional variation in multiple sclerosis prevalence in Australia and its association with ambient ultraviolet radiation. Neuroepidemiology 2001; 20: 168-74.
Valyi-Nagy T, Deshmane S, Dillner A, and Fraser NW. Induction of cellular transcription factors in trigeminal ganglia of mice by corneal scarification, herpes simplex virus type 1 infection, and explantation of trigeminal ganglia. J Virol 1991; 65(8): 4142-52.
van de Meent H, Hamers FP, Lankhorst AJ, Joosten EA, and Gispen WH. Beneficial effects of the melanocortin alpha-melanocyte-stimulating hormone on clinical and neurophysiological recovery after experimental spinal cord injury. Neurosurgery 1997; 40(1): 122-30.
van der Mei IA, Ponsonby AL, Blizzard L, and Dwyer T. Regional variation in multiple sclerosis prevalence in Australia and its association with ambient ultraviolet radiation. Neuroepidemiology 2001; 20(3): 168-74.
van der Mei IA, Ponsonby AL, Dwyer T, Blizzard L, Simmons R, Taylor BV, Butzkueven H, and Kilpatrick T. Past exposure to sun, skin phenotype, and risk of multiple sclerosis: case-control study. BMJ 2003; 327(7410): 316.
Wallengren, J., and Sundler, F. Phototherapy reduces the number of epidermal and CGRP-positive dermal nerve fibres. Acta Derm Venereol 2004; 84: 111-115.
Weinberg, W.C. et al. P21WAF1 control of epithelial cell cycle and cell fate. Crit Rev Oral Biol Med 2002; 13(6): 453-464.
Weiss MD, Luciano CA, Semino-Mora C, Dalakas MC, and Quarles RH. Molecular mimicry in chronic inflammatory demyelinating polyneuropathy and melanoma. Neurology 1998; 51(6): 1738-41.
Wintzen, M. et al. Proopiomelanocortin gene product regulation in keratinocytes. J Invest Dermatol 1996; 106: 673-8.
Wintzen, M. et al. Total body exposure to ultraviolet radiation does not influence plasma levels of immunoreactive beta-endorphin in man. Photodermatol Photoimmunol Photomed 2001; 17: 256-60.
Yarnitsky D, Goor-Aryeh I, Bajwa ZH, Ransil BI, Cutrer FM, Sottile A, and Burstein R. 2003 Wolff Award: Possible parasympathetic contributions to peripheral and central sensitization during migraine. Headache 2003; 43(7): 704-14.
Yavari P, McCulloch PF, and Panneton WM. Trigeminally-mediated alteration of cardiorespiratory rhythms during nasal application of carbon dioxide in the rat. J Auton Nerv Syst 1996; 61(2): 195-200.
Yin P, Luby TM, Chen H, Etemad-Moghadam B, Lee D, Aziz N, Ramstedt U, and Hedley ML. Generation of expression constructs that secrete bioactive alphaMSH and their use in the treatment of experimental autoimmune encephalomyelitis. Gene Ther 2003; 10(4): 348-55.
Zeller, S. et al. Do adolescent indoor tanners exhibit dependency? J Am Acad Dermatol 2006; 54: 589-96.