Temperature-independent binding of[ 3H](±)-3-(2-car☐ypiperazin-4-yl)propyl-1-phosphonic acid in brain synaptic membranes treated by Triton X-100

Abstract

Specific binding of[ 3H](±)-3-(2-car☐ypiperazin-4-yl)propyl-1-phosphonic acid (CPP), a highly selective antagonist forN-methyl- d-aspartic acid (NMDA) receptors, was examined in brain synaptic membranes treated with Triton X-100 by using a filtration assay method. Elevation of incubation temperature from 2 to 30 °C markedly diminished the binding. The binding reached a plateau within 5 min after the initiation of incubation at 2 °C, while the time required to attain an equilibrium was 1 min at 30 °C. The binding at 2 °C was rapidly dissociated by the addition of an excess of unlabeled CPP, NMDA and l-glutamic acid ( l-Glu). The binding was also saturable with increasing concentrations of the ligand and displaced by various amino acids structurally related to l-Glu in a stereospecific manner. Competitive but not noncompetitive antagonists for the NMDA receptors invariably inhibited the binding. However, the binding was not prominently affected by agonists for the other subclasses of the brain excitatory amino acid receptors. Both reduced and oxidized forms of glutathione significantly displaced the binding. Scatchard analysis revealed that Triton treatment increased the affinity and density of binding sites which consisted of a single component. Among some endogenous tryptophan metabolites, kynurenic, anthranilic and quinolinic acids inhibited the binding. These results suggest that a filtration assay method is also useful to detect the binding of NMDA receptors in the brain.