Schild plot analysis of glycine and kynurenic acid at the N-methyl- D-aspartate excitatory amino acid receptor

Glycine enhanced theN-methyl- D-aspartate (NMDA)-stimulated sodium flux in rat hippocampal slides in a concentration-dependent manner. The potentiation by glycine was apparently not competitive to NMDA with a maximal effect of about 50% enhancement at approximately 150 μM glycine. Glycine also reversed the kynurenic acid inhibition of NMDA-stimulated sodium flux, increasing flux two-fold. In the absence of glycine, the kynurenic acid inhibition of NMDA-stimulated sodium flux appeared not competitive to NMDA, with a Schild plot slope of0.6 ± 0.1, significantly less than 1(P s$ˇ0.05). Addition of 100 μM glycine gave a Schild plot slope of1.0 ± 0.3, classically defining competitive inhibition. However, the pK b of 4.5 obtained from the x intercept gives an inaccurate estimate of the affinity of kynurenic acid for the NMDA recognition site since glycine is present. Addition of 400 μM glycine resulted in a Schild plot slope of1.9 ± 0.2. By varying the concentrations of glycine, we have apparently reproduced a portion of the theoretical set of curves for various ratios of affinities of the antagonist for the agonist recognition site and a second site. These results support a model where glycine modulates NMDA receptor function and where kynurenic acid acts both as a competitive antagonist to NMDA and at a second site from which it is displaced by glycine.