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The effects of 0.1 to 500 mM ethanol on NMDA-activated currents were studied in primary cultures of mouse cortical and hippocampal neurons. In whole-cell recordings the IC50S for inhibition of NMDA-activated currents by ethanol were 129 mM +/- 20 mM in hippocampal neurons and 126 +/- 18 mM in cortical neurons. In single-channel recordings from excised outside-out patches of cortical neurons, ethanol inhibited total charge per minute with an IC50 of 174 +/- 23 mM, which was not significantly different from the IC50S for inhibition of whole-cell current. The reduction in mean open channel lifetime by ethanol was fit by the logistic equation with an apparent IC50 of 340 +/- 28 mM. Analysis of single-channel data indicated that ethanol inhibition of NMDA currents did not involve substantial changes in fast closed state kinetics, changes in open channel conductance, or block of the open channel. At the whole-cell IC50 of ethanol, mean open channel lifetime would decrease by 28% and frequency of opening would decline by 31% to account for the reduction in current. Single-channel data were consistent with ethanol being an allosteric modulator of gating which reduces agonist efficacy.

The interaction of ethanol with glycine at the N-methyl-D-aspartate (NMDA)-activated ion channel was investigated in voltage-clamped rat cultured hippocampal neurons. As shown previously, glycine increased, and ethanol inhibited, the NMDA-activated current in these cells. Concentration-response data for glycine (0.1-100 microM) indicate that the inhibition of NMDA-activated current by ethanol does not involve a competitive interaction with glycine. Thus, ethanol appears to inhibit NMDA-activated current at a locus different from the glycine modulatory site.

The tricyclic antidepressant desmethylimipramine (DMI) interacts with the NMDA receptor/ionophore complex; however, the site of the interaction has not been clearly established. Although evidence from receptor binding assays suggests that DMI interacts with the Zn2+ binding site, other binding studies and electrophysiological studies suggest otherwise. Using the whole-cell patch clamp technique to record from cultured hippocampal neurons, we report that recovery of NMDA-activated current from block by DMI is time-dependent and this time-dependent component was not observed following preexposure of neurons to Zn2+. These observations favor the hypothesis that DMI interacts at a binding site within the NMDA receptor/complex channel pore and not at the Zn2+ binding site.