ANTAGONISM OF N-METHYL-D-ASPARTATE RECEPTORS BY SIGMA-SITE LIGANDS - POTENCY, SUBTYPE-SELECTIVITY AND MECHANISMS OF INHIBITION

Recent studies propose that a site ligands antagonize N-methyl-D-aspar tate (NMDA) receptors by either direct, or indirect mechanisms of inhi bition. To investigate this question further we used electrical record ings to assay actions of seventeen structurally diverse a site ligands on three diheteromeric subunit combinations of cloned rat NMDA recept ors expressed in Xenopus oocytes: NR1a coexpressed with either NR2A, 2 B or 2C. The a site ligands had a wide range of potency for antagonizi ng NMDA receptor currents. Steady-state IC50 values ranged between sim ilar to 0.1 to >100 mu M. In all cases inhibition was non-competitive with respect to glycine and glutamate. Five structurally related a lig ands [eliprodil, haloperidol, ifenprodil, 4-phenyl-1-(4-phenylbutyl)-p iperidine and trifluperidol] were strongly selective for NR1a/2B recep tors. The other drugs were weakly selective or nonselective inhibitors . There was no correlation between sigma site affinity and potency of NMDA receptor antagonism for any subunit combination. Inhibition of NR 1a/2B receptors by the selective antagonists was independent of voltag e whereas inhibition by the weakly selective antagonists was voltage d ependent. Potency of 10 sigma ligands was crosschecked on NMDA current s in cultured rat cortical neurons. Them was close correspondence betw een the two assay systems. Our results argue that antagonism of NMDA r eceptor currents by the sigma ligands tested is due to direct effects on the receptor channel complex as opposed to indirect effects mediate d by sigma receptors. Inhibition occurs via sites in the NMDA receptor channel pore, or via allosteric modulatory sites associated with the NR2B subunit.