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Pyrithiamine was administered to newborn rats throughout the vulnerable period for myelinogenesis. A major metabolic defect was produced in the cerebral activities of the thiamine-dependent enzymes, transketolase and pyruvate decarboxylase. In spite of a defect in carbohydrate metabolism which is lethal in adult rats, overall development, and myelination as indicated by biochemical and morphological criteria, proceeded at an essentially normal rate. These findings indicate alternative metabolic pathways may be operational in newborn rat brain enabling it to circumvent major blockage in thiamine-dependent reactions.

Previous studies have shown that transketolase activity is decreased in the brains of thiamin deficient rats. This study assesses the effect of decreased transketolase levels on the activity of the pentose phosphate cycle in murine thiamin deficient cortex and brainstem. Thiamin deficiency was produced in newborn and adult rats by either pyrithiamin administration or by feeding a low thiamin diet. Newborn rats were killed at 22 days of age, and adults were killed at the onset of moderate to severe nurological signs. Cortices and brainstems from thiamin deficient and control rats were analyzed for activity of the two regulatory enzymes of the pentose phosphate cycle, glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase. Flux through the pathway was measured by the differentially labeled glucose technique in the brainstems of deficient and control adult rats. In both the brainstem and cortex of thiamin dificient rats, areas in which transketolase activity was decreased up to 65%, the activities of the two regulatory enzymes, glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase, were unaltered. Further, flux through the pentose phosphate cycle was not decreased as compared to pair-fed control rats. These data do not support the hypothesis that in thiamin dificient rats, a decrease in cerebral transketolase activity leads to a diminished pentose phosphate cycle activity.

Interference with cerebral energy metabolism due to excess ammonia has been postulated as a cause of hepatic encephalopathy. Furthermore, consideration of the neurologic basis of such features of hepatic encephalopathy as asterixis, decerebrate rigidity, hyperpnea, and coma suggests a malfunction of structures in the base of the brain and their cortical connections. The three major sources of intracerebral energy, adenosine triphosphate (ATP), phosphocreatine, and glucose, as well as glycogen, were assayed in brain cortex and base of rats given ammonium acetate with resultant drowsiness at 5 minutes and subsequent coma lasting at least 30 minutes. Cortical ATP and phosphocreatine remained unaltered during induction of coma. By contrast, basilar ATP, initially 1.28 +/- 0.15 mumoles per g, was unchanged at 2.5 minutes but fell by 28.1, 27.3, and 26.6% (p < 0.001) at 5, 15, and 30 minutes after NH(4)Ac. At comparable times, basilar phosphocreatine fell more strikingly by 62.2, 96, 77.1, and 71.6% (p < 0.001) from a control level of 1.02 +/- 0.38 mumoles per g. These basilar changes could not be induced by anesthesia, psychomotor stimulation, or moderate hypoxia and were not due to increased accumulation of ammonia in the base. Glucose and glycogen concentrations in both cortex and base fell significantly but comparably during development of stupor, and prevention of the cerebral glucose decline by pretreatment with glucose did not obviate ammonia-induced coma or the basilar ATP fall. These findings represent the first direct evidence that toxic doses of ammonia in vivo acutely affect cerebral energy metabolism and that this effect is preferentially localized to the base of the brain.

Thiamine-deficient encephalopathy is characterized by morphologic lesions in the brainstem and less extensively in the cerebellum, but the early neurologic signs reverse rapidly and fully with thiamine, indicating a metabolic disorder. The suggested causal mechanisms of the encephalopathy involve two thiamine-dependent enzymes: (a) impairment of pyruvate decarboxylase activity with decreased cerebral energy (ATP) synthesis, and (b) reduction of transketolase activity with possible impairment of the hexose monophosphate shunt and subsequent decrease in NADPH formation. The latter may be important in maintaining glutathione in a reduced form (GSH), which apparently functions by keeping enzymes in a reduced (active) conformation. To examine some of these postulated mechanisms, in this study we measured pyruvate decarboxylase and transketolase activity, lactate, ATP and GSH levels in the cerebral cortex, cerebellum, and brainstem, and thiamine concentration in whole brain of rats with diet-induced low thiamine encephalopathy. Pair-fed and normally fed asymptomatic control animals were similarly investigated. To assess the functional importance of some of our results, we repeated the studies in rats, immediately (16-36 hr) after reversal of the neurological signs with thiamine administration. THE DATA OBTAINED LED TO THE FOLLOWING CONCLUSIONS: (a) Brain contains a substantial reserve of thiamine in that thiamine level has to fall to below 20% of normal before the onset of overt encephalopathy and an increase in brain thiamine to only 26% of normal results in rapid reversal of neurologic signs. (b) Both cerebral transketolase and pyruvate decarboxylase activities are impaired in low thiamine encephalopathy and the abnormality in the pyruvate decarboxylase is reflected in a rise in brain lactate. These biochemical abnormalities occur primarily in the brainstem and cerebellum, the sites of the morphologic changes. (c) Although the fall in cerebral transketolase is about twofold greater than that of pyruvate decarboxylase activity during encephalopathy, both enzymes rise on reversal of neurologic signs and the degree of the transketolase rise is slight. Accordingly, this study cannot ascertain the relative functional importance of these two pathways in the induction of the encephalopathy. The data suggest, however, that the depression of transketolase is not functionally important per se, but may only be an index of some other critical aspect of the hexose monophosphate shunt. (d) The normal cerebral ATP concentration and small GSH fall during encephalopathy, with little GSH rise on reversal of neurologic signs, suggest that a depletion of neither substance is instrumental in inducing thiamine-deficient encephalopathy.