Enzymes of Cerebral GABA Metabolism and Synaptosomal GABA Uptake in Acute Liver Failure in the Rabbit: Evidence for Decreased Cerebral GABA‐Transaminase Activity

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Abstract
Measurements of the activities of the two key enzymes in cerebral GABA metabolism—glutamate decarboxylase (GAD) and GABA‐transaminase (GABA‐T)—were performed in normal rabbits and in rabbits with hepatic encephalopathy due to galactosamine‐induced liver failure. Furthermore the uptake of GABA by synaptosomes was studied. Hepatic encephalopathy was associated with a marked decrease in the activity of GAB A‐T. This decrease in activity was already apparent in galactosamine‐treated rabbits before the onset of hepatic encephalopathy. Sera and serum ultrafiltrates of rabbits with hepatic encephalopathy but not of normal rabbits or of rabbits with uremic encephalopathy were shown to inhibit GABA‐T activity in vitro. Cerebral GAD activity and synaptosomal GABA uptake in rabbits with hepatic encephalopathy and in untreated animals were not different. These later findings indicate that hepatic encephalopathy is not associated with alterations of presynaptic GABA nerve terminals in the central nervous system. The demonstration of a decrease in cortical GABA‐T activity provides indirect evidence for decreased GABA turnover in the brains of rabbits with hepatic encephalopathy and thus is compatible with augmented GABA‐ergic inhibitory neurotransmission contributing to the neural inhibition of hepatic encephalopathy.