Vol. 282, Issue 1, 326-338, 1997

Antagonism of N-Methyl-D-Aspartate Receptors by  Site Ligands: Potency, Subtype-Selectivity and Mechanisms of Inhibition

E. R. Whittemore, V. I. Ilyin and R. M. Woodward

CoCensys Pharmaceuticals Inc., California

Recent studies propose that  site ligands antagonize N-methyl-D-aspartate (NMDA) receptors by either direct, or indirect mechanisms of inhibition. To investigate this question further we used electrical recordings to assay actions of seventeen structurally diverse  site ligands on three diheteromeric subunit combinations of cloned rat NMDA receptors expressed in Xenopus oocytes: NR1a coexpressed with either NR2A, 2B or 2C. The  site ligands had a wide range of potency for antagonizing NMDA receptor currents. Steady-state IC₅₀ values ranged between ~0.1 to >100 µM. In all cases inhibition was non-competitive with respect to glycine and glutamate. Five structurally related  ligands [eliprodil, haloperidol, ifenprodil, 4-phenyl-1-(4-phenylbutyl)-piperidine and trifluperidol] were strongly selective for NR1a/2B receptors. The other drugs were weakly selective or nonselective inhibitors. There was no correlation between  site affinity and potency of NMDA receptor antagonism for any subunit combination. Inhibition of NR1a/2B receptors by the selective antagonists was independent of voltage whereas inhibition by the weakly selective antagonists was voltage dependent. Potency of 10  ligands was cross-checked on NMDA currents in cultured rat cortical neurons. There was close correspondence between the two assay systems. Our results argue that antagonism of NMDA receptor currents by the  ligands tested is due to direct effects on the receptor channel complex as opposed to indirect effects mediated by  receptors. Inhibition occurs via sites in the NMDA receptor channel pore, or via allosteric modulatory sites associated with the NR2B subunit.