The Glycine Site of the N‐Methyl‐D‐Aspartate Receptor Channel: Differences Between the Binding of HA‐966 and of 7‐Chlorokynurenic Acid

Abstract: The mechanisms of action of three different glycinesite antagonists of the N‐methyl‐D‐aspartate (NMDA)‐receptor channel were analyzed employing [³H]glycine direct binding assays, as well as functional glycine‐ and glutamate‐induced uncompetitive blocker binding assays. The latter assays measure apparent channel opening. All three antagonists tested, viz., 7‐chlorokynurenic acid (7‐Cl‐KYNA), kynurenic acid (KYNA), and 1‐hydroxy‐3‐aminopyrrolidone‐2 (HA‐966), inhibited the binding of [³H]glycine to the NMDA receptor in a dose‐dependent manner. These antagonists also inhibited the glycine‐induced increase in accessibility of the uncompetitive blocker [³H]N‐[1‐(2‐thienyl)cyclohexyl]‐piperidine ([³H]TCP) to the channel. 7‐Cl‐KYNA and KYNA, but not HA‐966, completely blocked the glutamate‐induced binding of [³H]TCP, in a manner similar to the non‐competitive manner in which the selective NMDA antagonist D‐(‐)‐2‐amino‐5‐phosphonovaleric acid (AP‐5) inhibited glycine‐induced [³H]TCP binding. The inhibitory effects of HA‐966 and of AP‐5 on glutamate‐induced [³H]TCP binding were overcome when glutamate concentrations were increased. Of the three antagonists, 7‐Cl‐KYNA appears to be the most potent (K_i= 0.4–1.0 μM) and the most selective glycine antagonist. KYNA was found to act at both the glycine (K_i= 40–50 μM) and the glutamate sites. In contrast, HA‐966 (K_i= 6–17 μM) appears to act either on a domain distinct from the glutamate and the glycine sites, but tightly associated with the latter, or at the glycine site, but according to a mechanism distinct from that of 7‐Cl‐KYNA. The results also show that simultaneous occupancy of the glycine and the glutamate sites of the NMDA receptor is essential for their functional activation, and support two concepts suggested by previous electrophysiological experiments: that glycine is a co‐agonist of glutamate, and that glycine is at least an M‐type allosteric effector of glutamate (i.e., it increases the maximal glutamate response without affecting its apparent binding constant).