Effect of extracellular pH on the potency of N-methyl-D-aspartic acid receptor competitive antagonists

Structure-activity analysis reveals that acidic alpha-amino acids containing an omega-PO3H2 group are more potent antagonists at N-methyl-D-aspartate (NMDA) receptors than are analogs with omega-COOH or omega-tetrazole groups. At physiological values of extra-cellular pH the omega-PO3H2 group is only partially deprotonated and the corresponding antagonists exist as ions with one or two negative charges. In contrast, competitive antagonists with omega-COOH and omega-tetrazole groups are fully ionized at physiological pH but carry only a single negative charge. Dose-inhibition analysis was performed with (2R)-AP7 and its piperidine derivative LY 257883 to determine whether ionization of the omega-PO3H2 group influences NMDA receptor antagonist potency; these experiments revealed a > 3-fold increase in potency on raising of the extracellular pH from 7.3 to pH 8.2, consistent with the increase in the relative concentration of the ionic form of the antagonist in which the omega-PO3H2 group contains two negative charges. Experiments with the omega-COOH-containing analog of LY 257883 and with SDZ EAB 515, an omega-PO3H2-containing antagonist of novel structure, revealed only 1.5- and 1.3-fold increases in potency, respectively, over the same pH range. Analysis of the kinetics of block of NMDA-activated currents resulting from rapid application of LY 257883 suggests that the increase in potency on raising of the extracellular pH results largely from an increase in the antagonist association rate constant but also from a small decrease in the dissociation rate constant. Together, these results suggest that the fully ionized forms of the R-enantiomers of AP7 and LY 257883 act as the active antagonist species at NMDA receptors.