Modulation of ligand binding to components of the GABA(A) receptor complex by ammonia: Implications for the pathogenesis of hyperammonemic syndromes

The effects of 5-2500 mu M concentrations of neutral ammonium salts on the binding of ligands to components of the GABA(A) receptor complex were investigated. [H-3]Flunitrazepam binding to the benzodiazepine receptor was enhanced by ammonium (10-500 mu M), but not sodium tartrate with EC(50) = 98 mu M and E(max) = 31%. Further increasing ammonium tartrate concentrations (500-2500 mu M) decreased [H-3]flunitrazepam binding to control levels. The ammonium tartrate-induced increase in [H-3]flunitrazepam binding was manifested as a 50% decrease in K-d. Furthermore, GABA increased the potency of ammonium tartrate in enhancing [H-3]flunitrazepam binding by 63%. [H-3]Ro 15-1788 and [H-3]Ro 15-4513 binding to the benzodiazepine receptor was not significantly enhanced by ammonium tartrate (E(max) approximate to 13%). Ammonium tartrate also increased, then decreased the binding of 500 nM [H-3]muscimol to the GABA(A) receptor (EC(50) = 52 mu M, E(max) = 30%) in a concentration-dependent manner, but had no effect on [H-3]SR 95-531 binding (E(max) <16%). The ammonium tartrate-induced alterations in [H-3]muscimol binding were demonstrated in saturation assays as the loss of the high affinity binding site and a 27% increase in the B-max of the low affinity binding site. These results indicate that ammonia biphasically enhances, then returns ligand binding to both the GABA and benzodiazepine receptor components of the GABA(A) receptor complex to control levels in a barbiturate-like fashion. This suggests that ammonia may enhance GABAergic neurotransmission at concentrations commonly encountered in hepatic failure, an event preceding the suppression of inhibitory neuronal function observed at higher (>1 mM) ammonia concentrations. This increase in GABAergic neurotransmission is consistent with the clinical picture of lethargy, ataxia and cognitive deficits associated with liver failure and congenital hyperammonemia.