Animal model for subacute combined degeneration

Multiple Sclerosis and Other Acquired Demyelinating Diseases of the Central Nervous System

Acquired demyelinating diseases of the central nervous system (CNS) comprise inflammatory conditions, including multiple sclerosis (MS) and related diseases, as well as noninflammatory conditions caused by toxic, metabolic, infectious, traumatic, and neurodegenerative insults. Here, we review the spectrum of diseases producing acquired CNS demyelination before focusing on the prototypical example of MS, exploring the pathologic mechanisms leading to myelin injury in relapsing and progressive MS and summarizing the mechanisms and modulators of remyelination. We highlight the complex interplay between the immune system, oligodendrocytes and oligodendrocyte progenitor cells (OPCs), and other CNS glia cells such as microglia and astrocytes in the pathogenesis and clinical course of MS. Finally, we review emerging therapeutic strategies that exploit our growing understanding of disease mechanisms to limit progression and promote remyelination.

Neurodegeneration in a domestic rabbit with severe malnourishment and low hepatic copper

Copper is a trace element that plays an essential role in neurodevelopment and neurologic function. Acquired copper deficiency has a range of neurologic manifestations, with myelopathy being the most common association. We describe here the clinical, radiologic, histopathologic, and toxicologic findings of a rabbit with malnutrition, neurodegeneration, and suspected copper deficiency. A stray, adult female dwarf rabbit cross (Oryctolagus cuniculus) in poor body condition developed ataxia and pelvic limb weakness progressing to lateral recumbency and urine retention. The clinical findings suggested multifocal brainstem disease with right-sided central vestibular involvement; however, microscopic examination identified thoracic and lumbosacral spinal cord myelopathy. Differentials for the spinal cord changes included neurodegenerative disease, nutritional deficiency, neurotoxin, trauma to the lumbosacral region, and ischemia. Hepatic copper levels were suboptimal at 18 ppm dry weight (RI: 24-150 ppm dry weight). While speculative, copper-deficiency myelopathy is a treatable cause of non-compressive myelopathy that may occur in this species.

Neurological Manifestations Induced by Nitrous Oxide Abuse: A Case Series and Review of Literature

INTRODUCTION

Nitrous oxide (NO) abuse is increasing among young people. This can result in severe neurological disorders such as myelopathy and/or peripheral neuropathy. We report the clinical presentations, biological, radiologic and electrophysiological findings of 5 patients hospitalized with neurological symptoms consecutive to NO abuse. In addition, a literature review was conducted to describe the neurological characteristics and to identify factors associated with a poor recovery.

CASE REPORT

Among the 5 patients included, 2 had a myeloneuropathy, 2 had a sensorimotor neuropathy, and 1 had a normal spinal cord magnetic resonance imaging and electromyography despite neurological manifestations consistent with myeloneuropathy. After vitamin B 12 supplementation, recovery was reported in 4 patients, and 1 was lost to follow-up.From the literature review, 154 patients were included [94 males; median age 22 (19 to 26) y; NO exposure 9 (3 to 18) mo]. A myelopathy was identified in 116 patients (75%) and a peripheral neuropathy was documented in 89 patients (58%). Compared with patients who recovered, those with sequelae were more likely to have a motor deficit at presentation ( P <0.001), to use NO regularly ( P <0.001), to have a lower vitamin B 12 level ( P =0.04), and a higher concentration of homocysteine ( P =0.04). A less extensive myelopathy was more frequently found in the group with favorable outcomes ( P =0.002).

CONCLUSION

Neurological disorders caused by NO may be challenging with severe clinical patterns. We identified several factors associated with a poor recovery, to make clinicians aware of NO-induced neurotoxicity.

Vitamin B12 Deficiency and the Nervous System: Beyond Metabolic Decompensation-Comparing Biological Models and Gaining New Insights into Molecular and Cellular Mechanisms

Vitamin B12 (VitB12) is a micronutrient and acts as a cofactor for fundamental biochemical reactions: the synthesis of succinyl-CoA from methylmalonyl-CoA and biotin, and the synthesis of methionine from folic acid and homocysteine. VitB12 deficiency can determine a wide range of diseases, including nervous system impairments. Although clinical evidence shows a direct role of VitB12 in neuronal homeostasis, the molecular mechanisms are yet to be characterized in depth. Earlier investigations focused on exploring the biochemical shifts resulting from a deficiency in the function of VitB12 as a coenzyme, while more recent studies propose a broader mechanism, encompassing changes at the molecular/cellular levels. Here, we explore existing study models employed to investigate the role of VitB12 in the nervous system, including the challenges inherent in replicating deficiency/supplementation in experimental settings. Moreover, we discuss the potential biochemical alterations and ensuing mechanisms that might be modified at the molecular/cellular level (such as epigenetic modifications or changes in lysosomal activity). We also address the role of VitB12 deficiency in initiating processes that contribute to nervous system deterioration, including ROS accumulation, inflammation, and demyelination. Consequently, a complex biological landscape emerges, requiring further investigative efforts to grasp the intricacies involved and identify potential therapeutic targets.

Neuropathy with Cerebral Features Induced by Nitrous Oxide Abuse-A Case Report

Nitrous oxide abuse may cause functional cobalamin deficiency and subsequent damage to the peripheral nerves, the spinal cord, and the brain, a symptom complex best described by the term cobalamin neuropathy. Here, we report a case of cobalamin neuropathy with uncommon cerebral symptomatology following nitrous oxide intoxication and contextualize the symptomatology. A 22-year-old male with a history of mixed drug dependency presented at the emergency room after inhaling six 615 g cylinders, equal to ~1800 L, of nitrous oxide daily for two weeks. His main complaints were rapidly progressing paresthesias and gait difficulties, but he was also found to suffer from memory impairment and signs of extrapyramidal pathology in the form of dystonic posturing and athetosis. Neuroimaging demonstrated spinal cord hyperintensities consistent with subacute combined degeneration. The patient had low serum cobalamin and high plasma homocysteine, suggesting cobalamin neuropathy. After commencing treatment with parenteral hydroxocobalamin, plasma homocysteine normalized. The extrapyramidal symptoms disappeared during the first days of treatment, whereas the cognitive and peripheral symptoms only partially resolved over the following 20 days. This case highlights how neurological symptoms such as hyperkinetic movements and memory impairment may be associated with chronic nitrous oxide abuse. It is unclear to what extent these and other symptoms of cobalamin neuropathy are reversible, which underscores the public health concern.

Scientific opinion on the tolerable upper intake level for folate

Following a request from the European Commission (EC), the EFSA Panel on Nutrition, Novel Foods and Food Allergens (NDA) was asked to deliver a scientific opinion on the revision of the tolerable upper intake level (UL) for folic acid/folate. Systematic reviews of the literature were conducted to assess evidence on priority adverse health effects of excess intake of folate (including folic acid and the other authorised forms, (6S)-5-methyltetrahydrofolic acid glucosamine and l-5-methyltetrahydrofolic acid calcium salts), namely risk of cobalamin-dependent neuropathy, cognitive decline among people with low cobalamin status, and colorectal cancer and prostate cancer. The evidence is insufficient to conclude on a positive and causal relationship between the dietary intake of folate and impaired cognitive function, risk of colorectal and prostate cancer. The risk of progression of neurological symptoms in cobalamin-deficient patients is considered as the critical effect to establish an UL for folic acid. No new evidence has been published that could improve the characterisation of the dose-response between folic acid intake and resolution of megaloblastic anaemia in cobalamin-deficient individuals. The ULs for folic acid previously established by the Scientific Committee on Food are retained for all population groups, i.e. 1000 μg/day for adults, including pregnant and lactating women, 200 μg/day for children aged 1-3 years, 300 μg/day for 4-6 years, 400 μg/day for 7-10 years, 600 μg/day for 11-14 years and 800 μg/day for 15-17 years. A UL of 200 μg/day is established for infants aged 4-11 months. The ULs apply to the combined intake of folic acid, (6S)-5-methyltetrahydrofolic acid glucosamine and l-5-methyltetrahydrofolic acid calcium salts, under their authorised conditions of use. It is unlikely that the ULs for supplemental folate are exceeded in European populations, except for regular users of food supplements containing high doses of folic acid/5-methyl-tetrahydrofolic acid salts.

Nitrous oxide abuse leading to extreme homocysteine levels and thrombosis in young adults: a case series

BACKGROUND

Neurologic complications from recreational use of nitrous oxide (N₂O), which are attributed to vitamin B12 deficiency, have been well documented. With increasing dosages and frequency of N₂O use, an additional association with thromboembolisms is becoming apparent.

OBJECTIVES

To assess thrombotic complications of recreational N₂O use.

METHODS

All medical charts at the largest hospital in Amsterdam were searched for N₂O use and subsequent neurologic and/or thrombotic events. For patients with thrombotic events, we extracted data on the risk factors for arterial and venous thrombosis as well as serum vitamin B12 and homocysteine concentrations.

RESULTS

Between January 2015 and May 2021, 326 patients who reported recreational use of N₂O were identified; of these, 17 (5%) patients presented with severe thrombotic events associated with N₂O (71% men; median age, 26 years [range, 18-53 years]), 5 patients presented with arterial thrombosis (3 with acute coronary syndrome, 1 with femoral artery thrombosis, and 1 with middle cerebral artery thrombus), and 12 patients presented with venous thromboembolisms (10 with pulmonary embolisms, 1 with portal vein thrombosis and 1 with cerebral vein thrombosis). Additionally, homocysteine were concentrations severely increased (median, 125 μmol/L [range, 22-253 μmol/L]; reference, <15 μmol/L). Patients reported use of 400 to 6000 g (ie, 50-750 balloons) of N₂O in 1 day. Fifty percent of these patients had experienced neurologic symptoms before the thrombotic event.

CONCLUSION

We describe an alarming incidence of serious thrombotic events among young adults after excessive recreational use of N₂O, accompanied by extremely high homocysteine concentrations. The upward trend in the recreational use of N₂O warrants more awareness of its dangers among both users and medical professionals. Furthermore, these findings could reopen the discussion on possible associations between hyperhomocysteinemia and thrombosis mediated through N₂O.

Global Burden Related to Nitrous Oxide Exposure in Medical and Recreational Settings: A Systematic Review and Individual Patient Data Meta-Analysis

The risk of adverse effects of nitrous oxide (N₂O) exposure is insufficiently recognized despite its widespread use. These effects are mainly reported through case reports. We conducted an individual patient data meta-analysis to assess the prevalence of clinical, laboratory, and magnetic resonance findings in association with N₂O exposure in medical and recreational settings. We calculated the pooled estimates for the studied outcomes and assessed the potential bias related to population stratification using principal component analysis. Eighty-five publications met the inclusion criteria and reported on 100 patients with a median age of 27 years and 57% of recreational users. The most frequent outcomes were subacute combined degeneration (28%), myelopathy (26%), and generalized demyelinating polyneuropathy (23%). A T2 signal hyperintensity in the spinal cord was reported in 68% (57.2–78.8%) of patients. The most frequent clinical manifestations included paresthesia (80%; 72.0–88.0%), unsteady gait (58%; 48.2–67.8%), and weakness (43%; 33.1–52.9%). At least one hematological abnormality was retrieved in 71.7% (59.9–83.4%) of patients. Most patients had vitamin B12 deficiency: vitamin B12 <150 pmol/L (70.7%; 60.7–80.8%), homocysteine >15 µmol/L (90.3%; 79.3–100%), and methylmalonic acid >0.4 µmol/L (93.8%; 80.4–100%). Consistently, 85% of patients exhibited a possibly or probably deficient vitamin B12 status according to the cB12 scoring system. N₂O can produce severe outcomes, with neurological or hematological disorders in almost all published cases. More than half of them are reported in the setting of recreational use. The N₂O-related burden is dominated by vitamin B12 deficiency. This highlights the need to evaluate whether correcting B12 deficiency would prevent N₂O-related toxicity, particularly in countries with a high prevalence of B12 deficiency.

The Neurology of Cobalamin

The following review indicates that the impact of cobalamin on neurologic disease extends far beyond the traditional myelopathy of classical pernicious anemia. The delineation of a broad spectrum of inherited disorders of cobalamin processing has served to illustrate and precisely define each step in the normal absorption, transport and intracellular metabolism of this essential vitamin. Recent clinical work has extended the boundaries of acquired cobalamin deficiency to encompass a variety of neuropsychiatric disturbances without identifiable concomitant hematologic derangements and emphasized the utility and sensitivity of new laboratory tests. These findings will demand increased vigilance from clinicians so that atypical and subtle cobalamin deficiency states will be readily diagnosed. The wide range of neurologic dysfunction observed in both inherited and acquired disorders of cobalamin metabolism challenges basic scientists to delineate cobalamin’s presumed important role in the normal development and homeostasis of the nervous system.

Professor John Scott, folate and neural tube defects

John Scott (1940-2013) was born in Dublin where he was to spend the rest of his career, both as an undergraduate and subsequently Professor of Biochemistry and Nutrition at Trinity College. His research with the talented group of scientists and clinicians that he led has had a substantial impact on our understanding of folate metabolism, mechanisms of its catabolism and deficiency. His research established the leading theory of folate involvement with vitamin B12 in the pathogenesis of vitamin B12 neuropathy. He helped to establish the normal daily intake of folate and the increased requirements needed either in food or as a supplement before and during pregnancy to prevent neural tube defects. He also suggested a dietary supplement of vitamin B12 before and during pregnancy to reduce the risk of neural tube defects. It would be an appropriate epitaph if fortification of food with folic acid became mandatory in the UK and Ireland, as it is in over 70 other countries.

Degenerative myelopathy in 18 Pembroke Welsh Corgi dogs

Postmortem examination was performed on 18 Pembroke Welsh Corgi dogs (mean age 12.7 years) with clinical signs and antemortem diagnostic tests compatible with a diagnosis of degenerative myelopathy. Tissue sections from specific spinal cord and brain regions were systematically evaluated in all dogs. Axonal degeneration and loss were graded according to severity and subsequently compared across different spinal cord segments and funiculi. White matter lesions were identified in defined regions of the dorsal, lateral, and ventral funiculi. The dorsolateral portion of the lateral funiculus was the most severely affected region in all cord segments. Spinal cord segment T12 exhibited the most severe axonal loss. Spinal nerve roots, peripheral nerves, and brain sections were within normal limits, with the exception of areas of mild astrogliosis in gray matter of the caudal medulla. Dogs with more severe lesions showed significant progression of axonal degeneration and loss at T12 and at cord segments cranial and caudal to T12. Severity of axonal loss in individual dogs positively correlated with the duration of clinical signs. The distribution of axonal degeneration resembled that reported in German Shepherd Dog degenerative myelopathy but differed with respect to the transverse and longitudinal extent of the lesions within more clearly defined funicular areas. Although these lesion differences might reflect disease longevity, they could also indicate a form of degenerative myelopathy unique to the Pembroke Welsh Corgi dog.

Cobalamin (vitamin B12) in subacute combined degeneration and beyond: traditional interpretations and novel theories

Subacute combined degeneration (SCD) is a neuropathy due to cobalamin (Cbl) (vitamin B(12)) deficiency acquired in adult age. Hitherto, the theories advanced to explain the pathogenesis of SCD have postulated a causal relationship between SCD lesions and the impairment of either or both of two Cbl-dependent reactions. We have identified a new experimental model, the totally gastrectomized rat, to reproduce the key morphological features of the disease [spongy vacuolation, intramyelinic and interstitial edema of the white matter of the central nervous system (CNS), and astrogliosis], and found new mechanisms responsible for the pathogenesis of SCD: the neuropathological lesions in TGX rats are not only due to mere vitamin withdrawal but also to the overproduction of the myelinolytic tumor necrosis factor (TNF)-alpha and the reduced synthesis of the two neurotrophic agents, epidermal growth factor (EGF) and interleukin-6. This deregulation of the balance between TNF-alpha and EGF synthesis induced by Cbl deficiency has been verified in the sera of patients with pernicious anemia (but not in those with iron-deficient anemia), and in the cerebrospinal fluid (CSF) of SCD patients. These new functions are not linked to the coenzyme functions of the vitamin, but it is still unknown whether they involve genetic or epigenetic mechanisms. Low Cbl levels have also been repeatedly observed in the sera and/or CSF of patients with Alzheimer's disease or multiple sclerosis, but whether Cbl deficit plays a role in the pathogenesis of these diseases is still unclear.

Hypothesis on the pathogenesis of vacuolar myelopathy, dementia, and peripheral neuropathy in AIDS

Certain aspects of the clinical syndrome of dementia, cerebral atrophy, predominantly sensory neuropathy, and vacuolar myelopathy in AIDS resemble those seen in vitamin B12 deficiency. Pathologically, there are similarities not only in the changes in the spinal cord, but also in the brain and peripheral nerves. The pathogenesis of vacuolar myelopathy may be secondary to a combination of immune mediated myelin and oligodendrocyte injury, and simultaneous impairment of repair mechanisms due to a deficiency of S-adenosylmethionine (SAM). Products derived from macrophages may interfere directly with the methyl transfer cycle through the generation of reactive oxygen intermediates and reactions involving nitric oxide and peroxynitrite which may limit the supply of methionine for conversion to SAM, both by direct interaction as well as through inhibition of methionine synthase. Macrophage activation with secretion of cytokines and other biologically reactive substances within the nervous system is sustained in the late stages of HIV infection by the general effects of immune depletion, including loss of T cells (with concomitant reduction of macrophage regulatory molecules) and recurrent opportunistic infections, and may be further augmented by the local presence of the virus itself (or its surface glycoprotein gp120). This would account for the common, but not exclusive, occurrence of vacuolar myelopathy in AIDS. The ability of the virus and its products to stimulate macrophage and microglial activation may also explain the association between severity of vacuolar myelopathy and the presence of HIV encephalitis. A similar mechanism may underlie the pathogenesis of dementia, cerebral atrophy, and peripheral neuropathy. Local factors or differential susceptibility between the central and peripheral nervous system may determine whether myelinotoxic or neurotoxic processes predominate; the prominence of myelin involvement in the spinal cord, and axonal involvement peripherally may reflect both ends of this range, with the brain manifesting a more equal balance of both processes.

The biochemical basis of the neuropathy in cobalamin deficiency

The pathogenesis of the neuropathy associated with vitamin B12 deficiency (subacute combined degeneration (SCD)) is now thought to be related to interference with the methylation reactions in the CNS. The methylation reactions are processed by S-adenosylmethionine (SAM), which is controlled by its product, S-adenosylhomocysteine (SAH). The relationship of these two compounds is termed the methylation ratio. It has been demonstrated that if the ratio falls, the methylation reactions are inhibited leading to a state of CNS hypomethylation. The ratio can fall either due to a rise in SAH or a fall in SAM. It is suggested that for clinical signs to develop in animals who are susceptible to the lesion, both events are usually required. Inhibition of the vitamin B12-dependent enzyme, methionine synthase, leads to a rapid fall in the ratio in the CNS, since unlike other organs such as the liver, it does not have an alternative method of re-methylating homocysteine to maintain the endogenous synthesis of SAM. The supply of methyl groups necessary for the re-methylation reactions is controlled by a series of enzymes, which include methionine synthase. The inborn errors of metabolism that produce deficiency or impairment of these enzymes are described. Neurological syndromes associated with deficiency of these enzymes have close associations with SCD. The other clinical evidence and animal experiments that support this hypothesis are also described.

Reversible myeloneuropathy of nitrous oxide abuse: serial electrophysiological studies

Detailed electrophysiological studies were performed in 4 patients with myeloneuropathy induced by abuse of nitrous oxide for 1 to 4 years. All presented with paresthesias, weakness, and Lhermitte's phenomena, and exhibited signs of sensorimotor polyneuropathy, ataxia, and arreflexia. Two had subnormal serum vitamin B12 levels. Baseline electrophysiologic testing revealed reduced motor unit potentials, prolonged F wave latencies, absent H reflexes, denervation potentials, and delays in motor and sensory conduction. Three had peripheral and nuchal delay after median nerve stimulation. All were reevaluated after 3 to 12 months' abstinence and treatment with vitamin B12, and all showed substantial clinical improvement. Parallel improvement in electrophysiologic findings occurred, but residual minor conduction delays, loss of H reflexes, electromyographic evidence of denervation, or abnormalities of posterior tibial SEP were noted. These findings confirm the reversibility of myeloneuropathy of nitrous oxide abuse and describe the profile of electrophysiologic recovery in subjects who abstain from further neurotoxic exposure.

The inhibitory effects of nitrous oxide and methylmercury in vivo on methionine synthase (EC 2.1.1.13) activity in the brain, liver, ovary and spinal cord of the rat

1. Baseline levels of methionine synthase (MS) activity in spinal cord and ovary from female rats were comparable to activities seen in brain and liver. 2. Exposure (1 hr) of female rats to 50% nitrous oxide (N2O) caused significant decreases in MS in brain, liver, and spinal cord and a sharp drop in ovary. 3. Four daily injections of 6 mg/kg methylmercury (MM) caused a significant decrease in MS activity in brain and a decrease in mean MS activity in ovary and spinal cord. 4. Results from a combined treatment with MM and N2O suggests additive inhibition of MS by these agents.

The fruit bat as an experimental model of the neuropathy of cobalamin deficiency

The fruit bat provides a unique small mammal model of the neurological changes associated with cobalamin deficiency. Work with this model has shown that methionine moderates the development of the neurological impairment. This action does not appear to be via the methyl donor S-adenosylmethionine, but its role in the provision of formate is not excluded. Furthermore, methylation reactions in the nervous system are not impaired in severe cobalamin deficiency, despite low levels of methionine synthetase activity. The accumulation of physiologically inactive analogues of cobalamin also do not appear to be aetiologically important in the neuropathy. Brain folates are minimally affected by severe cobalamin deficiency, although liver folates decrease significantly. Deranged GABA function in the brain may play a role in the symptomatology of cobalamin deficiency. There is some evidence for the hypothesis that deranged fatty acid metabolism in neural tissue contributes to altered membrane structure and hence function. Changes in the properties of membrane proteins may play a contributory role. The biochemical basis of the neuropathy has still to be fully elucidated.

Beta-adrenergic stimulation of protein (arginine) methyltransferase activity in cultured cerebral cells from embryonic mice

Several adrenergic effectors and neurotransmitters were tested as potential regulators of myelin basic protein (MBP) and histone methyltransferase activities. Both enzymes were specifically activated by beta-adrenergic agonists in a stereospecific manner. Cyclic AMP (but not AMP) stimulated the enzymes to the same extent as did the beta-adrenergic agonist, (-) isoproterenol. The studies suggest that beta-adrenergic agonists stimulate adenylate cyclase thereby causing an increased production of cyclic AMP which stimulates the methyltransferases. Cycloheximide addition to the reaction mixture did not affect the stimulation due to cyclic AMP, indicating that new protein synthesis is not involved in the cyclic AMP stimulation of the methyltransferases. Thyroid hormone (T3) has been shown to stimulate MBP methyltransferase [Amur et al, 1984] and could exert its stimulatory effect through beta-adrenergic-dependent systems. But the beta-adrenergic antagonist, propranolol, did not block the stimulation by T3, suggesting that the effect of T3 is not mediated through beta-adrenergic-dependent systems. Thus, the methylation of MBP seems to be regulated both by T3 and by neurotransmitters and/or hormones mediating their effects through cyclic AMP production, whereas the methylation of histones seems to be regulated only by the latter.

Dissociation of methionine synthetase (EC 2.1.1.13) activity and impairment of DNA synthesis in fruit bats (Rousettus aegyptiacus) with nitrous oxide-induced vitamin B12 deficiency

1. The effect of methylcobalamin inactivation by the anaesthetic gas nitrous oxide on the activity of the cobalamin-dependent methionine synthetase (5-methyltetrahydrofolate homocysteine methyltransferase; EC 2.1.1.13) reaction, and on DNA synthesis, in the fruit bat Rousettus aegyptiacus, was examined. 2. Methionine synthetase activity in the liver of bats exposed to N2O-oxygen (50:50, v/v) for 90 min/d averaged 32% of that of controls after 4 d of exposure and only 5% after 12-14 weeks of exposure. 3. DNA synthesis in the bone marrow, as reflected by the deoxyuridine suppression test, was unaffected by 4 d of exposure to N2O and only minimally affected after 5-10 weeks of exposure. 4. These results suggest that DNA synthesis in the fruit bat is unusually resistant to inhibition of methionine synthetase and imply the existence of a non-methylated circulating folate pool in this species.

Regulation of myelin basic protein (arginine) methyltransferase by thyroid hormone in myelinogenic cultures of cells dissociated from embryonic mouse brain

The ontogenetic expression of myelin basic protein (arginine) methyltransferase in myelinogenic cultures of cells dissociated from embryonic mouse brain is highly dependent on the presence of thyroid hormone. Restoration of myelin basic protein methyltransferase to normal activities occurred 16 h after the addition of 100 nM L-3,5,3'-triiodothyronine to hypothyroid medium. These data demonstrate that thyroid hormone can regulate a posttranslational event. On the other hand, histone (arginine) methyltransferase has a different temporal activity pattern, which is not coordinated with myelination, and is not influenced by the lack of thyroid hormone. These data, which suggest the existence of two methyltransferases, were substantiated by demonstrating that the total amount of methylation of added myelin basic protein and histone is the same whether they are incubated together or separately. The requirement of thyroid hormone for the expression of the myelin basic protein methyltransferase and not for histone methyltransferase suggests that thyroid hormone preferentially regulates myelin-associated events in these cultures.

Tissue S-adenosylmethionine levels in fruit bats (Rousettus aegyptiacus) with nitrous oxide-induced neuropathy

The effect of cobalamin inactivation by the anaesthetic gas nitrous oxide on the concentration of S-adenosylmethionine (Ado Met) in brain and liver of fruit bats (Rousettus aegyptiacus) was examined. Test animals exposed to N2O-oxygen (50:50, v/v) developed ataxia and paralysis leading to death after an average of 9.8 weeks (n6). Animals receiving pteroylmonoglutamic acid supplements in the diet became ataxic earlier (mean 8.8 weeks) while those receiving methionine supplements survived for significantly longer periods (12.5 weeks, P less than 0.01). Plasma cobalamin levels indicated severe depletion of cobalamin stores in N2O-exposed animals. The mean concentration of Ado Met in the brain of N2O-treated bats was nearly 50% higher than that of untreated controls. Ado Met levels in treated bats receiving pteroylmonoglutamic acid or methionine supplements were respectively 18 and 25% higher than in controls. In contrast, the concentration of Ado Met in the liver of all the N2O-treated groups was slightly lower than in controls. These results suggest that the N2O-induced neuropathy in the fruit bat is not related to a depletion of Ado Met in the nervous system.

Local cerebral glucose utilization in two models of B12 deficiency

Local cerebral glucose utilization (LCGU), as measured by the 2-deoxy-D-[1-14C]glucose technique, reflects local cerebral functional activity. In an effort to elucidate mechanisms of the encephalopathy associated with deficiency of vitamin B12, LCGU was determined in two recently described models of effective B12 deficiency: exposure of rats to subanesthetic doses of nitrous oxide (N2O) and/or administration of 1-amino-cyclopentane-1-carboxylic acid (cycloleucine). Our results show that exposure of adult rats to N2O depresses LCGU selectively in cortical, auditory, and limbic structures, in association with a depression in whole-brain activities of the vitamin B12-dependent methyltetrahydrofolate-homocysteine methyl-transferase (EC 2.1.1.13, methionine synthetase). Cycloleucine has no discernible effect on LCGU in the adult rat and does not change the cerebral activity of methionine synthetase.

The role of methionine in the intracellular accumulation and function of folates

It is suggested that mammalian cells have evolved to respond to methionine deficiency since in such circumstances vital methylation reactions are put at risk, due to decreased levels of S-adenosyl-methionine. Enzymatic changes occurring as a result of decreased methionine, S-adenosylmethionine and S-adenosylhomocysteine, optimize the remethylation of homocysteine to methionine by decreasing homocysteine catabolism and channelling cellular folates into 5-methyltetrahydropteroylglutamate (5-CH3-H4 PteGlu). The latter, in addition to optimising the remethylation cycle, directs the folate cofactors away from purine and pyrimidine biosynthesis and decreases the rate of proliferation of rapidly dividing cells thus reducing competition for methionine incorporation into proteins. Decreased cellular homocysteine, as a result of decreased methionine, would also restrict cell division by decreased conversion of plasma 5-CH3-H4PteGlu into intracellular polyglutamates. Cobalamin deficiency, either nutritional or due to exposure to the Co (I) cobalamin inactivating agent nitrous oxide, prevents the demethylation of 5-CH3-H4PteGlu, which even in the presence of adequate amounts of homocysteine and methionine prevents rapidly proliferating cells from converting enough of the plasma 5-CH3-H4 PteGlu into folylpolyglutamate forms to permit normal DNA biosynthesis and cell replication. This, together with the trapping of the cellular folate cofactors in the 5-CH3-H4PteGlu form, results in megaloblastic changes occurring in tissues such as the marrow. The vital role of the methylation reactions was demonstrated by exposing monkeys to nitrous oxide which inactivated their methionine synthetase. The resultant ataxia and severe demyelination was prevented and diminished by methionine supplementation. When methionine synthetase was similarly inactivated in mice it was shown that while 5-CH3-H4PteGlu enters mammalian cells, it is not converted into a polyglutamyl form and subsequently leaves the cell unmetabolised. In similar experiments in rats methionine was found to have only a small effect in restoring folylpolyglutamate biosynthesis, contrary to previous reports using nutritionally cobalamin deficient animals. It was found that a decrease in the deoxythymidine salvage pathway by methionine, under the experimental conditions used, has led others to the mistaken conclusion that methionine has an 'anti-folate' effect in bone marrow, i.e. that it decreases folate availability for thymidylate synthetase.

Cobalamin inactivation by nitrous oxide produces severe neurological impairment in fruit bats : protection by methionine and aggravation by folates

Nitrous oxide, which inactivates cobalamin when administered to fruit bats, results in severe neurological impairment leading to ataxia, paralysis and death. This occurs after about 6 weeks in animals depleted of cobalamin by dietary restriction, and after about 10 weeks in cobalamin replete bats. Supplementation of the diet with pteroylglutamic acid caused acceleration of the neurological impairment - the first unequivocal demonstration of aggravation of the neurological lesion in cobalamin deficiency by pteroylglutamic acid. The administration of formyltetrahydropteroylglutamic acid produced similar aggravation of the neurological lesion. Supplementation of the diet with methionine protected the bats from neurological impairment, but failed to prevent death. Methionine supplementation protected against the exacerbating effect of folate, preventing the development of neurological changes. These findings lend support to the hypothesis that the neurological lesion in cobalamin deficiency may be related to a deficiency in the methyl donor S-adenosylmethionine which follows diminished synthesis of methionine.

The effects of nitrous oxide on cobalamins, folates, and on related events

The anaesthetic gas nitrous oxide (N2O), when inhaled for longer than 6 hr, produces megaloblastic anemia in man. Longer term inhalation, as in addicts, produces a syndrome similar to that due to B12 neuropathy, and long term exposure to low concentrations results in an increased abortion rate and neuropathy, particularly in dental personnel. N2O acts by oxidizing vitamin B12 from the active reduced cob[I]alamin form to the inactive cob[III]alamin form. In turn, this inactivates the enzyme methionine synthetase which requires both B12 and folate as cofactors. In the rat, hepatic methionine synthetase is completely inactivated after 3 hr exposure to a mixture of equal parts of N2O/O2. There is an impared uptake of folate analogues by the liver so that the plasma folate level rises and thereafter there is a considerable loss of folate into the urine. Hepatic folate concentration falls to 25% within 10 days of N2O exposure. There is a failure to synthesize folate polyglutamate (the active folate coenzyme) from all other than formyltetrahydrofolate. As oxidization of the methyl of methionine is an important source of formyl, the failure of methionine synthesis in turn appears to lead to the failure in supply of formate and, hence, a lack of the formylfolate substrate.

Neurological involvement in hereditary transcobalamin II deficiency

A case of hereditary transcobalamin II deficiency with neurological involvement is described. The patient presented in early infancy with megaloblastic anaemia and was treated with folinic acid from 6 weeks of age. The diagnosis of transcobalamin II deficiency was not made until he was 2 years old when he showed severely retarded intellectual development, ataxia and pyramidal deficit in the limbs. Following treatment with hydroxocobalamin, his condition has slowly improved but he has remained with a severe neurological deficit. The consequences of vitamin B12 deficiency on neurological development in infancy are discussed.

Nitrous oxide-induced myeloneuropathy: report of cases

The development of neurologic symptoms, especially a myeloneuropathy, remains a serious hazard of nitrous oxide exposure. The need to use it in a well-ventilated environment with a scavenger system and the avoidance of its self-use are clear from the cases reported. The findings of the questionnaire disclose that the majority of dentists have experienced none of these problems while using nitrous oxide in a professional manner; however, self-use of nitrous oxide, with its potential neurologic sequelae, may be a serious problem among dental personnel.

Neurotoxicity of folates: implications for vitamin B12 deficiency and Huntington's chorea

Recent work has shown that several folates interact with excitatory kainic acid receptors in the mammalian brain and appear to have agonist activity at these receptors. Since kainic acid is a potent neurotoxin it is possible that folates share this toxicity and that high levels of folates result in neuronal damage. Levels of methyltetrahydrofolate are markedly elevated in vitamin B12 deficiency, a disease associated with neuronal destruction. We propose that this destruction occurs as a result of a neurotoxic action of methyltetrahydrofolate. Injection of kainic acid into the basal ganglia of experimental animals produces a pattern of damage similar to that found in patients dying of Huntington's chorea. It is possible that the underlying defect in this disease resides in the pathways of folate metabolism such that a neurotoxic excess of folates accumulates in the central nervous system. Such a disease process might be arrested by antifolate drugs.

Evidence that the decreased liver folate status following vitamin B-12 inactivation in the mouse is due to increased loss rather than impaired uptake

Using N2O to inactivated B-12, mouse liver polyglutamate distribution has been examined at short time intervals after injection of [3H]pteroyglutamate. An increased monoglutamate pool is found initially which is lost by 24 h. This finding strongly supports the suggestion that the reduction in total liver folate in vitamin B12 deficiency is due to increased loss rather than decreased uptake.

Vitamin B12 regulates folate metabolism by the supply of formate

Nitrous oxide (N2O) inactivates the B12 coenzyme involved in methionine synthesis and interrupts formation of the folate coenzyme (folate polyglutamate). Normal synthesis of folate polyglutamate is restored in the N2O-treated rat when folate carrying a single carbon unit is supplied at the formate level of oxidation. The activity of the enzyme, formyl synthetase, which links formate to tetrahydrofolate, is increased after exposure to nitrous oxide. Formate is normally derived from the oxidation of methyl groups, methionine being an important source. It is suggested that failure of methionine synthesis leads to a paucity of formate and in turn to inadequate formylation of tetrahydrofolate. Formyltetrahydrofolate is the required substrate for the synthesis of folate polyglutamate, and impairment of this step in turn compromises general folate metabolism.

Occupational disease in dentistry and chronic exposure to trace anesthetic gases

A mail survey of 30,650 dentists and 30,547 chairside assistants grouped according to occupational exposure to inhalation anesthetic and sedatives in the dental operatory indicated increased general health problems and reproductive difficulties among respondents exposed to anesthetics. For male dentists who were heavily exposed to anesthetics, the increase in liver disease was 1.7-fold, kidney disease was 1.2-fold, and neurological disease was 1.9-fold. For wives of male dentists who were heavily exposed to anesthetics, the increase in spontaneous abortion rate was 1.5-fold. Among female chairside assistants who were heavily exposed to anesthetics, the increase in liver disease was 1.6-fold, kidney disease was 1.7-fold, and neurological disease was 2.8-fold. The increase in spontaneous abortion rate among assistants who were heavily exposed was 2.3-fold. Cancer rates in women heavily exposed to inhalation anesthetics were increased 1.5-fold but this finding was not statistically significant (P = .06). Separate analysis of the data for disease rates and birth difficulties by type of inhalation anesthetic indicates that in both dentists and chairside assistants chronic exposure to nitrous oxide alone is associated with an increase rate of adverse response.

Effects of low cobalamin diet and chronic cyanide toxicity in baboons

The effects of a low cobalamin (Cbl) diet, together with chronic cyanide or thiocyanate administration in some animals have been investigated in baboons over a period of 42 months. All animals remained healthy throughout the study and gained weight at a similar rate. None became anaemic or showed major haematological changes and there were no major neurological changes. Plasma total Cbl in the animals on the low Cbl diet fell within 9 months to values below the lower limit in man and were lowest at 24 months in baboons not receiving cyanide or thiocyanate. A striking feature in all animals, however, was an apparently seasonal increase in the plasma total Cbl each autumn with a corresponding decrease the following spring. This fluctuation was detected by radioisotopic assay but not by Euglena. Methylmalonic (MMA) excretion after oral valine ranged from 0.1--8.4 mg/24 h and was greatest in animals on the low Cbl diet and not receiving cyanide or thiocyanate. The results suggested an inverse relationship between MMA excretion and plasma total Cbl. Plasma thiocyanate was consistently higher in animals receiving cyanide or thiocyanate and at the end of the study plasma cyanide was highest in animals on the low Cbl diet receiving cyanide. The results support the suggestions that cyanide affects bodily handling of Cbl and that hydroxo-cobalamin plays a part in detoxication of cyanide.