These are two examples of a whole series of articles from the 1970s and 1980s (I found about thirty similar ones, and there are probably more) showing that depletion of 5-methyltetrahydrofolate (MTHF) from the cerebrospinal fluid can produce severe neuropathy and neurological symptoms, including reversible dementias and demyelination [Manzoor and Runcie, 1976: (http://www.pubmedcentral.nih.gov/articlerender.fcgi?blobtype=html&artid=1639752) (http://www.ncbi.nlm.nih.gov/pubmed/1268613); Botez et al., 1976: (http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=1878635) (http://www.ncbi.nlm.nih.gov/pubmed/953882)]. The full text link on one of those articles is only accessible by right-clicking and "saving as," if clicking on it doesn't work. But the articles are basically the same as the articles on cerebral folate deficiency, except these are in adults. The researchers tend to use folic acid instead of reduced folates, but they use high doses (15-30 or more mg/d). Reduced folates, such as methylfolate, would almost certainly work better, given the limited capacity for the conversion of folic acid into MTHF by the intestinal epithelial cells (extremely limited) and even by the liver.
I just can hardly believe that these articles exist, because I've never seen them cited anywhere. I think they just were lost and forgotten in the literature. I'll try to post the many, many similar articles and case reports I found from that time frame. One of the articles was cited in "Homocysteine in Health and Disease" (http://hardcorephysiologyfun.blogspot.com/2009/01/note-on-cerebral-folate-deficiency.html), and then the similar articles feature on pubmed allows one to find the rest of the related articles. I think it might be that these articles were published before much was really known about homocysteine or one-carbon metabolism. It might also be that the words the authors chose as keywords are obscure or not in line with the types of conditions people, in more recent years, had come to associate with folate depletion. These articles might not have come up in searches on a lot of topics. For example, these authors, from the two articles I linked to, use the word "neuropathy" as a descriptive term to encompass the neurological damage due to the depletion of MTHF from the CSF. Most of the research on the use of folates in neuroprotection has been done in the context of protection against ischemia or against the neurotoxic effects of homocysteine. I see that some articles published recently have cited some of the older articles [this is a really insightful one, by Peter Cole and Barton Kamen: (http://www.ncbi.nlm.nih.gov/pubmed/17061283)], but the full message that the older articles provide has, for the most part, been lost in the literature.
The articles are really important and help to "explain" or put into context a lot of research that has been published more recently. If one accepts the usual rule of thumb (as I accepted it), that folate depletion or deficiency doesn't cause the kinds of neurological damage that cobalamin (vitamin B12) depletion does, then the articles, for example, showing that serum folate correlates inversely with the extent of cortical atrophy in people as they age [Snowdon et al., 2000: (http://hardcorephysiologyfun.blogspot.com/2008/12/heres-my-old-paper-on-folate-and-more.html)] just don't make sense. The authors of these articles from the 1970s also were finding that restless legs syndrome (RLS) sometimes accompanied the frequently-severe, folate-responsive neurological damage, and I'd been wondering where the authors of the one or two recent trials of folic acid in RLS had gotten this idea to use folic acid for that. The only place those articles have apparently been cited is in one or two review articles on RLS, and that's remarkable. Although true, primary RLS is evidently due to damage or abnormalities in many parts of the brain and spinal cord, RLS is most generally produced by a decrease in D2 dopamine receptor-mediated transmission. That's an imprecise characterization, but the symptoms described in these articles strongly suggest that dopaminergic neurotransmission is sensitive to folate depletion and repletion. This is consistent with what's been learned in the 30+ years since these articles were published.
I also came across the full dosage range being used in the context of cerebral folate deficiency, and it's 0.5-3 mg/kg bw [Vincent Ramaekers and Nenad Blau, 2004: (http://www.ncbi.nlm.nih.gov/pubmed/15581159)]. Those are really large doses, and it's not clear that doses that high would need to be used in adults. But the dosages are broadly-consistent with the message that comes from these articles from the 1970s and 1980s. Researchers have used 50 mg/d of racemic methylfolate [Guaraldi et al., 1993: (http://www.ncbi.nlm.nih.gov/pubmed/8348200)], 90 mg/d of racemic methylfolate [Di Palma et al., 1994: (http://cat.inist.fr/?aModele=afficheN&cpsidt=4093099)], or up to 30 mg/d of racemic folinic acid [Alpert et al., 2002: (http://www.ncbi.nlm.nih.gov/pubmed/12046638)] to treat psychiatric symptoms in adults. Given that racemic, reduced folates were used in those studies, it is reasonable to suspect that half of those doses of L-leucovorin or (6S)-5-methylfolate, 15-45 mg/d, might be as effective (given that only half of the dose is really biologically-active) or more effective than (given the potential for the nonphysiological diastereomers to, theoretically, compete for entry into the CSF or for intracellular binding sites on folate-cofactor-dependent enzymes in cells in the central nervous system, etc.) those doses of racemic, reduced folates.
The message to come out of many of these articles, though, is that somewhat large dosages tend to be required to increase the CSF MTHF concentration significantly. The risks seem to be not very substantial, for the many reasons I've discussed in past postings (http://hardcorephysiologyfun.blogspot.com/2009/01/methylfolate-bioavailability-and.html). The upper range of the normal CSF MTHF is less than 200 nM, and the concentrations of intracellular total folates that would be expected to result from that extracellular concentration [the CSF concentration can be assumed to be comparable or identical to the intraparenchymal, interstitial fluid (ISF) MTHF concentration (http://hardcorephysiologyfun.blogspot.com/2009/01/estimating-interstitial-fluid.html)] would be expected to be relatively low and would not be expected to produce saturation of the binding sites on folate-cofactor-dependent enzymes (on their active sites or allosteric, regulatory binding sites). The concentrations of intracellular total folates in response to dosages of folic acid, in rats, of 100 ug/kg bw, 800 ug/kg bw, and 20,000 ug/kg bw can be calculated, using estimates of intracellular water content per g wet weight of brain tissue, to be 1350 nM, 1057 nM, and 1610 nM, respectively. Those are extremely low concentrations of intracellular total folates, and the results of computational studies suggest that the folate cycle does not function normally at intracellular concentrations below 5,000 nM (5 uM) [Nijhout et al., 2004: (http://hardcorephysiologyfun.blogspot.com/2008/12/heres-my-old-paper-on-folate-and-more.html)].
The brain, therefore, appears to be very resistant to strategies aimed at increasing the CSF MTHF levels through the use of folic acid alone, and reduced folates are still unlikely to produce anything approaching supraphysiological, intracellular (or extracellular) concentrations of folates in (or in the interstitial fluid surrounding) cells in the brain. Even if a dosage elevated the CSF MTHF concentration to concentrations in excess of 200 nM, for example, it is not clear that this would produce anything resembling "saturation" of intracellular folates. It's conceivable that there are mechanisms that produce, in response to the relatively low CSF MTHF levels that are normally found, unusually high accumulations of intracellular folates in neurons. But then why are the intracellular concentrations of total folates in cells in the brain, as estimated from those animal studies I discussed in my past posting, consistent with the kinds of concentrations of intracellular total folates that one would expect to result from CSF MTHF levels that are less than 200 nM (the upper limit of the normal range, which goes up to about 190 nM, I think)? The cell culture study by Watkins and Cooper (1983) (http://hardcorephysiologyfun.blogspot.com/2009/01/effects-of-reduced-folates-vs-folic.html) show that racemic folinic acid (half of which is biologically inactive, as far as I know, and which may accumulate intracellularly as biologically-inactive forms of 5-formyl-THF and 10-formyl-THF, etc.), at extracellular concentrations of 20 or 200 nM, produces concentrations of intracellular total folates that total 3,150 nM or 6,020 nM, respectively, in proliferating cells. Those ratios between the concentrations of extracellular and intracellular folates are not far off the ratios or, speaking imprecisely, "gradients" one sees between the extracellular (ISF) and intracellular fluid "compartments" in the brain.
Assuming that the CSF MTHF levels in those studies of rats, using between 100 and 20,000 ug folic acid/kg bw, were within the normal range (~60-190 nM or thereabouts) and produced such low intracellular concentrations of total folates (1350-1610 nM), I don't see why there wouldn't be considerable room for flexibility in dosages of reduced folates, as Ramaekers and Blau (2004) have found in the treatment of people with cerebral folate deficiency. And assuming neurons and astrocytes and other cell types in the brain do not have unusually low intracellular concentrations of binding sites for folate-derived cofactors [in the liver, the intracellular concentration of total folates is between 25 and 35 uM (25,000 and 35,000 nM), and the intracellular concentration of "binding sites" is between 5 and 10 times that range (http://hardcorephysiologyfun.blogspot.com/2008/12/heres-my-old-paper-on-folate-and-more.html)] or unusually large capacities to retain intracellular folate-derived cofactors, elevating the CSF/ISF MTHF concentration somewhat past the normal range would not be expected to produce toxicity. That said, the unusually low CSF homocysteine levels that exist under normal circumstances may suggest some unusual, poorly-understood aspects of the folate cycles within cells in the brain. The convulsant effects of extracellular folates on kainate and NMDA receptors could conceivably become meaningful, also, and some of the articles on cerebral folate deficiency have discussed "transient tics" in people taking high doses of reduced folates. But (and I'm not up for linking to them now) researchers have used extremely high (somewhere in the millimolar range, I think) extracellular folic acid concentrations on cultured neurons and have not found neurotoxic effects, but the convulsant effects might only exert the full degree of damage in a living animal. Obviously, those high dosage ranges are not something that one would want to use without a doctor's supervision and without monitoring one's serum cobalamin concentration, etc.
In any case, it's really fairly stunning to see those articles from the 1970s and think that they've been sitting around for 30 years. In this one [Enk et al., 1980: (http://www.ncbi.nlm.nih.gov/pubmed/6255553)], the authors describe a person who had been treated with only vitamin B12 for 20 months and hadn't gotten much benefit from the treatment. There was only improvement after the authors added one form or another of folate. There are lots of similar case reports of people who have had neuropathies or neurological problems due to cobalamin depletion and who haven't improved much from just intramuscular or intravenous cobalamin. Additionally, many of the people with the CSF MTHF depletion in these older articles also did not have megaloblastic anemia but had neurological symptoms, and that's the same type of paradox that's come out of the last 20 years of research on cobalamin-depletion-induced neurological damage.
Some of the damage that can result from CSF MTHF or cobalamin depletion may become irreversible, and I'd always thought that the cobalamin depletion had been very prolonged, in those types of cases, and had led, for example, to secondary mitochondrial DNA depletion, as cobalamin deficiency is known to be capable of producing, that had no longer been fully reversible through the use of cobalamin alone. It's conceivable that exogenous purines and pyrimidines might be applicable, in conjunction with reduced folates, to some people with more severe damage. There's reason to think that some of the neuroprotective and trophic effects of reduced folates are mediated by their effects on purines and pyrimidine metabolism, and exogenous purines and pyrimidines have been shown to exert manifold neurotrophic effects in the absence of any exogenous source of reduced folates.
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