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 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 EC₅₀ = 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 (EC₅₀ = 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.