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

Abstract

The effects of 5-500 μM concentrations of neutral ammonium salts on the binding of ligands to components of the GABA A receptor complex were investigated. [ 3H]Flunitrazepam binding to the benzodiazepine receptor was enhanced by ammonium (10–500 μM), but not sodium tartrate with EC 50 = 98 μM and E max = 31%. Further increasing ammonium tartrate concentrations (500–2500 μM) decreased [ 3H]flunitrazepam binding to control levels. The ammonium tartrate-induced increase in [ 3H]flunitrazepam binding was manifested as a 50% decrease in K d. Furthermore, GABA increased the potency of ammonium tartrate in enhancing [ 3H]flunitrazepam binding by 63%. [ 3H]Ro 15-1788 and [ 3H]Ro 15-4513 binding to the benzodiazepine receptor was not significantly enhanced by ammonium tartrate ( E max ≈ 13%). Ammonium tartrate also increased, then decreased the binding of 500 nM [ 3H]muscimol to the GABA A receptor (EC 50 = 52 μM, E max = 30%) in a concentration-dependent manner, but had no effect on [ 3H]SR 95-531 binding ( E max < 16%). The ammonium tartrate-induced alterations in [ 3H]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.