Characterization of the non-competitive antagonist binding site of the NMDA receptor in dark Agouti rats

The ability of non-competitive NMDA antagonists and other selected compounds to inhibit 3H]MK-801 binding to the NMDA receptor in brain membranes was evaluated in female, dark Agouti rats. In homologous competition binding studies the average apparent affinity (KD) of 3H]MK-801 for its binding site was 5.5 nM and the binding site density (Bmax) was 1.83 pmol/mg protein. Inhibition of 3H]MK-801 binding by non-competitive NMDA antagonists was best described with a one-site competition model and the average Hill coefficients were -1. A series of eight non-competitive NMDA antagonists inhibited 3H]MK-801 binding with the following rank order of affinity (K(i), nM): MK-801 (5.5) > dexoxadrol (21.5) > or = TCP (24.2) > phencyclidine (100.8) > (+)-SKF 10,047 (357.7) > dextrorphan (405.2) > ketamine (922.2) > dextromethorphan (2913). These inhibition binding constants determined in dark Agouti rat brain membranes were significantly correlated (P = 0.0002; r2 = 0.95) with previously reported values determined in Sprague-Dawley rats Wong et al., 1988, J. Neurochem. 50, 274-281]. Despite significant differences in metabolic capability between these strains, the central nervous system NMDA receptor ion channel shares similar characteristics.