ALTERATION OF [H-3] MK-801 BINDING ASSOCIATED WITH THE N-METHYL-D-ASPARTATE RECEPTOR COMPLEX BY ACUTE ETHANOL IN RAT CORTEX AND HIPPOCAMPUSIN-VITRO

We investigated the effect of ethanol on specific binding of [H-3]MK-8 01 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 performe d on synaptic membrane homogenates from adult rat cortex and hippocamp us. [H-3]MK-801 binding assays were conducted under conditions of basa l, 10 mu M glutamate, or 10 mu M glutamate + 30 mu M D-serine, with an d without 50 or 100 mM ethanol. Association experiments of [H-3]MK-801 binding (5 nM) were conducted under conditions of 0 or 10 mu M glutam ate, with varying concentrations of glycine (0.01, 0.10, and 10 mu M) 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 r eceptors, but did not change high- and low-affinity constants of the t wo binding states. An ethanol-induced increase in the closed-channel r eceptor density in basal and activated conditions was suggested by the saturation experiments. Association experiments further explained thi s finding, in that ethanol (100 mM) significantly decreased fast compo nent (open-channel) [H-3]MK-801 binding in conditions of glycine (0.01 -10 mu M) only and activated conditions of glutamate + glycine (0.01-0 .10 mu M) However, the observed fast and slow kin :tic rate constants of [H-3]MK-801 binding, as well as total specific binding (fast + slow components), were not altered. Thus, ethanol seems to act as a noncom petitive antagonist upon the gating mechanism of, and ligand access to , the NMDA-coupled ion channel. These findings support previous observ ations of ethanol selectively reducing NMDA-activated calcium influx, and reducing the frequency and duration of ion channel opening in elec trophysiological studies. Similar to previous reports on NMDA-stimulat ed calcium influx and [H-3]MK-801 binding, glycine (at the maximal con centration of 10 mu M), in the presence of 10 mu M glutamate, was foun d to reverse ethanol inhibition of fast component binding.