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Cerebral metabolism in anoxia and the effects of some neurotropic drugs Shankar, Raj

Abstract

The effects of tetrodotoxin (TTX) and other neurotropic drugs on anaerobic glycolysis, and on transport processes, of incubated cerebral cortex slices have been studied in an effort to understand more fully cerebral metabolic processes during anoxia, and the mode of action of certain neurotropic drugs. The general approach has been to study the action of various drugs on the rates of anaerobic glycolysis of incubated brain slices under a variety of conditions. The cation (Na⁺, K⁺) contents were also studied under these conditions and the changes in these contents were related to concomitant changes in cerebral metabolism. Experiments were also carried out on the cerebral transport of amino acids and glucose under a variety of incubation conditions. Measurement of the rates of anaerobic glycolysis in the presence of TTX showed that the drug, at low concentrations such as 2 μM, enhances the rate of anaerobic glycolysis of cerebral cortex slices two to three fold, the effects being greater in the absence of Ca⁺⁺. Such an effect of TTX is far greater than that obtained on the aerobic metabolism of the cerebral cortex slices. The anaerobic glycolysis of kidney medulla slices, 2-day old rat brain slices or of acetone powder extracts from brain are not affected by the drug, indicating that the effects of TTX on the anaerobic glycolysis are specific for mature cerebral tissue, and requires integrity of the brain cells for its action. TTX has little or no effect in increasing rate of anaerobic glycolysis when it is added 10 minutes, or later, after the onset of anoxia, or when high K⁺, protoveratrine, L-glutamate or NH₄⁺ are present in the incubation medium. Under these conditions, there is either an influx of Na⁺ into, or loss of K⁺ from, the incubated cerebral tissue. In the presence of TTX under anoxic conditions, a much slower decline in the K⁺ /Na⁺ ratio of the cerebral cortex slices is observed. These and other experiments lead to the conclusion that the effects of TTX on anaerobic glycolysis are due to its action at the brain cell membrane resulting in the prevention of the changes in brain cell permeabilities to Na⁺ and K⁺ brought about by the onset of anoxia. In the presence of TTX, the initial high rate of glycolysis tends to be maintained due to only a slow decline in the cellular K⁺/Na⁺ ratio. The effects of K⁺ and Na⁺ on the anaerobic glycolysis are thought to be mediated largely by changes in the pyruvate kinase activity, which is enhanced by K⁺ and diminished by Na⁺. TTX appears to affect the aerobic and anaerobic metabolism of brain in vitro in the same way as it affects the generation action potentials i.e. by diminishing the influx of Na⁺ and efflux of K⁺ . These results lead to the conclusion that action potentials are generated in the incubated cerebral tissue at the onset of anoxia. These are blocked by TTX which manifests its effect by a higher rate of anaerobic glycolysis. The effect of TTX on the Na⁺ and K⁺ contents may be greater in the neurons than in glial cells because the former are the site of action of TTX. Consequently, the changes in the neuronal K⁺/Na⁺ ratio brought about by TTX are probably much greater than those of the K⁺/ Na⁺ ratio found in the tissue as a whole. In addition to its effects on the Na⁺ and K⁺ fluxes, TTX also prevents the efflux of amino acids from the incubated cerebral cortex slices that occurs at the onset of anoxia. This effect of TTX is independent of the activity of the transport processes normally operating on the amino acid uptake into the brain. Local anesthetics, ouabain, amytal and reserpine also increase the rate of anaerobic glycolysis of cerebral cortex slices. Local anesthetics act in a manner similar to TTX, although much higher concentrations are required. The effects of ouabain in a Ca⁺⁺-free medium are much greater than in a Ca⁺⁺-containing medium. It is suggested that the increase in the rate of anaerobic glycolysis due to ouabain is possibly mediated by an increase in cell ATP concentration under anoxia, as a result of inhibition of Na⁺ , K⁺-ATPase. The actions of amytal and reserpine on the anaerobic glycolysis of cerebral cortex slices are possibly mediated by membrane cation changes, but further work is necessary to support this conclusion.

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