AMMONIA PREVENTS ACTIVATION OF NMDA RECEPTORS BY GLUTAMATE IN RAT CEREBELLAR NEURONAL CULTURES

Acute ammonia toxicity is mediated by activation of NMDA receptors and is prevented by chronic moderate hyperammonaemia. The aim of this wor k was to assess whether the protective effect of chronic hyperammonaem ia is due to impaired activation of the NMDA receptor. It is shown tha t chronic hyperammonaemia in rats decreases the binding of [H-3]MK-801 to synaptosomal membranes from the hippocampus but not the amount of NMDAR1 receptor protein as determined by immunoblotting. In primary cu ltures of cerebellar neurons, long-term treatment with 1 mM ammonia al so decreased significantly the binding of [H-3]MK-801. These results s uggest that ammonia impairs NMDA receptor activation. To confirm this possibility we tested the effect of long-term treatment of the culture d neurons with 1 mM ammonia on three well known events evoked by activ ation of the NMDA receptor: neuronal death induced by glutamate, incre ase in aspartate aminotransferase activity and increase in free intrac ellular [Ca2+]. Long-term treatment with ammonia prevented noticeably the effects of glutamate or NMDA on all these parameters, These result s indicate that long-term treatment of neurons with 1 mM ammonia leads to impaired function of the NMDA receptor, which cannot be activated by glutamate or NMDA, Activation of protein kinase C by a phorbol este r restored the ability of the NMDA receptor to be activated in neurons treated with ammonia. This suggests that ammonia impairs NMDA recepto r function by decreasing protein kinase C-dependent phosphorylation.