ELECTROPHYSIOLOGICAL CHARACTERIZATION OF THE ANTAGONIST PROPERTIES OF2 NOVEL NMDA RECEPTOR GLYCINE SITE ANTAGONISTS, L-695,902 AND L-701,324

The pharmacological effects of two novel N-methyl-D-aspartate (NMDA) r eceptor glycine site antagonists, L-701,324 and L-695,902 were examine d on whole-cell voltage-clamped cells and compared to a prototypic ant agonist, 7-chlorokynurenic acid. Both L-701,324 and L-695,902 non-comp etitively antagonised NMDA responses elicited in rat cultured cortical neurones, this was shown to be due to a competitive interaction at th e glycine co-agonist site on the receptor complex (K-b values: 19 nM a nd 2.6 mu M, respectively). Inhibition curves for the antagonism of re sponses to combined applications of NMDA and glycine showed that both antagonists were devoid of any intrinsic activity i.e. ''full'' antago nists and were, therefore, capable of completely abolishing inward cur rents. Despite this fact, both of these novel antagonists apparently m odulated glutamate affinity for its recognition site-a property hither to associated only with glycine site partial agonists. Human recombina nt NMDA receptors comprising NR1a/NR2A and NR1a/NR2B subunits expresse d in mouse fibroblast cells were also used to determine whether L-701, 324 and L-695,902 were capable of discriminating between subtypes of N MDA receptor. Inhibition curves to each antagonist showed no differenc e in affinity for either of these subunit assemblies (mK(i) values: L- 701,324 = 0.005 mu M on both assemblies; L-695,902 = 4.37 and 3.7 mu M on NR1a/NR2A and NR1a/NR2B, respectively). Kinetic analysis of the of f-rates of antagonism with L-701,324 revealed that the high affinity o f this compound compared to 7-chlorokynurenic acid were attributable t o an exceptionally slow dissociation of the antagonist from the recept or. Copyright (C) 1996 Elsevier Science Ltd.