Effect of Anions on [3H]CGP 39653 Binding to NMDA Receptors in Brain

Abstract

We previously demonstrated biphasic effects of salts on the binding of [3H]CGP 39653, a ligand for NMDA recognition sites, in well-washed membranes from rat cerebral cortex. Low concentrations of the salts increased binding, and higher concentrations produced inhibition. The increase in binding was due to conversion of NMDA recognition sites from a low- to a high-affinity state by cations (Soc. Neurosci. Abstr. 19:1784). In the present study, we evaluated the effects of sodium salts of different anions on [3H]CGP 39653 binding to NMDA recognition sites under conditions in which all binding sites were transformed from the low- to the high-affinity state. In well-washed rat cortical synaptic membranes, all anions studied inhibited [3H]CGP 39653 binding with different potencies. The range of IC50 values was from 2 mM to 300 mM. The order of potencies was as follows: citrate > EDTA > phosphate > carbonate > iodinate > nitrate > bromide > chloride > acetate > fluoride. Of the anions tested, HEPES had the lowest potency (IC50 > 400 mM). These findings have implications for the design of in vitro binding assays. They also suggest that in vivo conditions are complicated by the presence of endogenous anions which can alter interactions of ligands with NMDA recognition sites.