Effect of zinc on NMDA receptor-mediated channel currents in cortical neurons

Abstract

Recent data have indicated that the divalent cation Zn2+ can selectively block central neuronal excitation mediated by N-methyl-D-aspartate (NMDA) receptors. The present experiments were conducted to determine the action of Zn2+ at the single-channel level. Outside-out membrane patches were prepared from cultured murine cortical neurons. Glutamate, 3μM, in the presence of 5 μM glycine activated channels with a main conductance state of about 50 pS which were blocked in a voltage-dependent manner by Mg2+ Zn2+ appeared to have 2 effects on these NMDA receptor-activated channels. First, at concentrations as low as 1-10 μM, Zn2+ produced a concentration-dependent reduction in channel open probability, insensitive to membrane voltage between -60 and +40 mV; about 50% reduction in open probability was produced by 3 μM Zn2+. This reduction was mostly due to a decrease in opening frequency and only weakly mimicked by Mg2+. Second, at higher concentrations (10-100 μM) and negative membrane voltages, Zn2+ additionally produced an apparent reduction in single-channel amplitude, associated with an increase in channel noise, suggestive of a fast channel block. The amplitude reduction was voltage-dependent, with a δof 0.51; amplitude distribution analysis suggested that this voltage dependence was primarily contributed by the "on" blocking rate constant, with little contribution from the "off" rate constant. The channel block produced by Zn2+ was faster than that of Mg2+, which at 100 μM and negative membrane voltages induces flickering of the NMDA receptor-activated channel without changing apparent channel amplitude. Thus, Zn2+ may reduce NMDA receptor-activated channel currents on cortical neurons by acting at 2 different sites, one outside the membrane field and affecting opening frequency, and the other inside the channel and interfering directly with the passage of ions. In this manner, Zn2+ coreleased with glutamate from presynaptic terminals may dynamically modulate NMDA receptor-activated currents.