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

[Systemic analgesia during labour.]

Systemic analgesia is used in obstetrics to alleviate the pain in labour and to prevent adverse effects on the fetus due to maternal pain and stress and subsequent complications such as prolonged labour. To supplement psychological support tranquillizers such as diazepam are useful in allaying anxiety and increasing patients' acceptance of labour. Possible side-effects include neonatal hypothermia and poor muscle tone of the newborn when large doses are given. When pain is more severe opioids should be administered, since less potent analgesics cannot provide adequate pain relief in obstetrics. The most important side effect of opioids is the dose-related respiratory depression, which can be harmful to the neonate. Therefore, large doses (>150 mg pethidine) should be avoided. However, even moderate doses (100 mg pethidine) may adversely affect infant neurobehaviour. Thus, in many cases adequate pain relief afforded to parturients by systemic analgesia may result in altered adaptive functions of the newborn. This makes it reasonable to consider alternative methods, including epidural anaesthesia, which is highly effective and fairly unproblematic. Drug administration in the management of labour pain can be recommended if only small doses are needed and in parturients who refuse regional anaesthesia or for whom it is contraindicated or not available.

Effect of nitrous oxide on nickel deprivation in rats

Because nickel may have a biological function in a pathway in which vitamin B12 is important, an experiment was performed to determine the effects of nitrous oxide exposure in rats deprived of nickel. Exposure to nitrous oxide (N2O) causes inactivation of cobalamin and a subsequent decrease in the vitamin B12-dependent enzymes methionine synthase and methylmalonyl CoA mutase. Rats were assigned to dietary groups of 12 in a factorially arranged experiment with dietary variables of nickel (0 or 1 microgram/g) and vitamin B12 (0 or 50 ng/g). After 6 wk, one-half of the rats from each dietary group were exposed to 50% N2O/50% O2 for 90 min/d for the last 28 d of the experiment. Vitamin B12, N2O, or their interaction had numerous effects; classical findings included N2O-induced reduction in plasma vitamin B12 and decreases in the vitamin B12-dependent enzymes. Inactivation of vitamin B12 by N2O, however, did not exacerbate signs of nickel deprivation, possibly because the rats were able to metabolically compensate to N2O exposure.

Tissue folates in fruit bats (Rousettus aegyptiacus) with nitrous oxide-induced vitamin B12 deficiency and neurological impairment

1. Long-term exposure of the fruit bat Rousettus aegyptiacus to nitrous oxide, which inactivates methylcobalamin, leads to neurological impairment and ataxia. 2. In N2O-exposed animals, liver concentrations of total folates and methyl folates decreased to less than one-fifth that of control animals. Pediococcus cerevisiae-active folates were also reduced. 3. In brain, there were no changes in total or methyl folates, but P. cerevisiae-active folates were lower in N2O-exposed animals. 4. Supplementation with methionine retarded the development of neurological impairment and the fall in liver total and methyl folates, but not that in P. cerevisiae-active folates. 5. Supplementation with serine failed to retard the development of neurological impairment or fall in hepatic folates. 6. The present results suggest that the N2O-induced neurological impairment in the bat is not related to depletion of cerebral folates, but do not exclude changes in the subcellular distribution of folates.

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.