Characterization of indole-2-carboxylate derivatives as antagonists of N-methyl-D-aspartate receptor activity at the associated glycine recognition site.

We have synthesized a series of indole-2-carboxylate derivatives and, with the use of radioligand binding, electrophysiological techniques and an in vivo transient bilateral carotid occlusion model of ischemic damage known to be sensitive to NMDA antagonists, have evaluated the indole-2-carboxylate derivatives ability to inhibit N-methyl-D-aspartate (NMDA) receptor activity through the associated glycine modulatory site. By using [3H]glycine to label this modulatory site, we found that the compounds with the highest affinity (Ki less than 1 microM) contained a chloro group at position C-6 and a polar, hydrogen-bond-accepting group at position C-3 of the indole ring. When these compounds were tested for their ability to modulate [3H]MK-801 [(+)-[3H]-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclophepten-5,10- imine maleate) binding, a functional assessment of NMDA receptor activation, binding was inhibited, indicative of NMDA receptor antagonist character. Schild regression analysis indicated that this antagonism was competitive with glycine. Next, several of these indole-2-carboxylate derivatives were analyzed electrophysiologically in rat cortex mRNA-injected Xenopus oocytes shown to express a functional NMDA receptor channel complex. These compounds inhibited NMDA receptor activity in a manner noncompetitive with NMDA. They also produced a parallel right-ward shift in the glycine dose response for potentiation of the NMDA responses in the oocytes and thus provided further evidence for a competitive interaction at the glycine site. Finally, in vivo transient bilateral carotid artery occlusion experiments revealed that these compounds were capable of reducing the damage typically associated with an ischemic insult in Mongolian gerbil hippocampal neurons.