Allosteric modulation of the glutamate site on the NMDA receptor by four novel glycine site antagonists

Using radioligand binding studies, we have investigated the binding properties of four 4-hydroxy-2-quinolones, a novel series of selective antagonists for the glycine site on the $\text{N-}\text{methyl}\text{-}\text{d}\text{-}\text{aspartate}$ (NMDA) receptor. L-701,324, L-703,717, L-698,532 and L-695,902 inhibited [³H]L-689,560 (glycine site antagonist) binding to rat cortex/hippocampus P₂ membranes with IC₅₀ values of 1.97, 4.47, 209 and 6448 nM, respectively, whilst also inhibiting non-equilibrium [³H]dizocilpine binding to the NMDA receptor ion-channel. All four compounds partially inhibited l-[³H]glutamate (∼ 50% inhibition; agonist) binding and enhanced [³H]cis-4-phosphonomethyl-2-piperidine carboxylate ([³H]CGS-19755; 41–81% enhancement; ‘C-5’ antagonist) and [³H]3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonate ([³H]CPP; 28–66% enhancement; ‘C-7’ antagonist) binding to the glutamate recognition site of the NMDA receptor with EC₅₀ values similar to those observed for [³H]L-689,560 binding. These results provide further evidence for allosteric interactions between the glutamate and glycine recognition sites of the NMDA receptor complex, and as the 4-hydroxy-2-quinolones are ‘full’ antagonists at the glycine site, indicate that these interactions are not caused by the intrinsic activity of a compound.