The GABA hypothesis of the pathogenesis of hepatic encephalopathy: current status.

Gamma-aminobutyric acid (GABA), the principal inhibitory neurotransmitter of the mammalian brain, can induce coma. Outside the central nervous system it is synthesized by gut bacteria and catabolized largely in the liver. GABA and its agonists, as well as benzodiazepines and barbiturates, induce neural inhibition as a consequence of their interaction with specific binding sites for each of these classes of neuroactive substances on the GABA receptor complex of postsynaptic neurons. In a rabbit model of acute liver failure: (i) the pattern of postsynaptic neuronal activity in hepatic coma, as assessed by visual evoked potentials, is identical to that associated with coma induced by drugs which activate the GABA neurotransmitter system (benzodiazepines, barbiturates, and GABA agonists); (ii) the levels of GABA-like activity in peripheral blood plasma increase appreciably before the onset of hepatic encephalopathy, due at least in part to impaired hepatic extraction of gut-derived GABA from portal venous blood; (iii) the blood-brain barrier becomes abnormally permeable to an isomer of GABA, alpha-amino-isobutyric acid, before the onset of hepatic encephalopathy; and (iv) hepatic coma is associated with an increase in the density of receptors for GABA and benzodiazepines in the brain. These findings are the bases of the following hypotheses: (i) when the liver fails, gut-derived GABA in plasma crosses an abnormally permeable blood-brain barrier and by mediating neural inhibition contributes to hepatic encephalopathy; (ii) an increased number of GABA receptors in the brain found in liver failure increases the sensitivity of the brain to GABA-ergic neural inhibition; and (iii) an increased number of drug binding sites mediates the increased sensitivity to benzodiazepines and barbiturates observed in liver failure by permitting increased drug effect.