Alteration of [3H]MK‐801 Binding Associated with the JV‐Methyl‐d‐Aspartate Receptor Complex by Acute Ethanol in Rat Cortex and Hippocampus In Vitro

We investigated the effect of ethanol on specific binding of [ 3 H]MK-801 to the intrachannel phencyclidine (PCP) receptor site, as an index of change in the functional response of the N-methyl-D-Aspartate (NMDA)-associated ion channel. Saturation binding experiments were performed on synaptic membrane homogenates from adult rat cortex and hippocampus. [ 3 H]MK-801 binding assays were conducted under conditions of basal, 10 μm glutamate, or 10 μm glutamate +30 μ m D-serine, with and without 50 or 100 mm ethanol. Association experiments of [ 3 H]MK-801 binding (5 nm) were conducted under conditions of 0 or 10 μm glutamate, with varying concentrations of glycine (0.01, 0.10, and 10 μm) with and without 100 mm ethanol. Ethanol (50 and 100 mm) significantly decreased the percentage of high-affinity (open-channel state) MK-801 receptors with a concomitant increase in percentage of low-affinity receptors, but did not change high- and low-affinity constants of the two binding states. An ethanol-induced increase in the closed-channel receptor density in basal and activated conditions was suggested by the saturation experiments. Association experiments further explained this finding, in that ethanol (100 mm) significantly decreased fast component (openchannel) [ 3 H]MK-801 binding in conditions of glycine (0.01-10 μm) only and activated conditions of glutamate+glycine (0.01-0.10 μm). However, the observed fast and slow kinetic rate constants of [ 3 H]MK-801 binding, as well as total specific binding (fast+slow components), were not altered. Thus, ethanol seems to act as a noncompetitive antagonist upon the gating mechanism of, and liBand access to, the NMDA-coupled ion channel. These findings support previous observations of ethanol selectively reducing NMDA-activated calcium influx, and reducing the frequency and duration of ion channel opening in electrophysiological studies. Similar to previous reports on NMDA-stimulated calcium influx and [ 3 H]MK-801 binding, glycine (at the maximal concentration of 10 μm), in the presence of 10 μm glutamate, was found to reverse ethanol inhibition of fast component binding