COPPER MODULATION OF NMDA RESPONSES IN MOUSE AND RAT CULTURED HIPPOCAMPAL-NEURONS

The effect of Cu2+ on NMDA receptors was studied in cultured mouse and rat hippocampal neurons using whole-cell patch-clamp and a fast perfu sion system. Analysis of the Cu2+ concentration-response curve for inh ibition of NMDA-induced currents suggests that free Cu2+ directly inhi bits NMDA receptors with an IC50 of 0.27 mu M. Cu2+ was ineffective in blocking NMDA receptor activity when complexed with NMDA or glycine; NMDA-Cu2+ and glycine-Cu2+ complexes acted as agonists of similar pote ncy to the free amino acids. The inhibition by Cu2+ (10-100 mu M) of r esponses to 10 mu M NMDA was essentially voltage-independent. The onse t of inhibition by 100 mu M Cu2+ of responses to 2 mu M glutamate acti ng at NMDA receptors was significantly faster than NMDA receptor deact ivation evoked by a sudden decrease in the concentration of glycine or glutamate, or of both agonists. This suggests that Cu2+ acts as a non -competitive antagonist, and does not directly interfere with the bind ing of glutamate or glycine to their recognition sites on the NMDA rec eptor complex. In the absence of NMDA the apparent association rate co nstant for binding of Cu2+ to NMDA receptors, calculated from the rate of onset of block by Cu2+ of test responses to NMDA, was 19 times slo wer than in the presence of 30 mu M NMDA, suggesting that Cu2+ interac ts preferentially with agonist-bound receptors. Our results show that Cu2+ is a potent inhibitor of NMDA receptor-mediated responses.