Interaction of [3H]MK-801 with multiple states of the N-methyl-D-aspartate receptor complex of rat brain

N-Methyl-D-aspartate (N-Me-D-Asp) and phencyclidine receptors interactively mediate central nervous system processes including psychotomimetic effects of drugs as well as neurodegenerative, cognitive, and developmental events. To elucidate the mechanism of this interaction, effects of N-Me-D-Asp agonists and antagonists and of glycine-like agents upon binding of the radiolabeled phencyclidine receptor ligand [³H]MK-801 were determined in rat brain. Scatchard analysis revealed two discrete components of [³H]MK-801 binding after 4 hr of incubation. Incubation in the presence of L-glutamate led to an increase in apparent densities but not in affinities of both components of [³H]MK-801 binding as well as conversion of sites from apparent low to high affinity. Incubation in the presence of combined D-serine and L-glutamate led to an increase in the apparent density of high-affinity [³H]MK-801 binding compared with incubation in the presence of either L-glutamate of D-serine alone. These data support a model in which phencyclidine receptor ligands bind differentially to closed as well as open conformations of the N-Me-D-Asp receptor complex and in which glycine-like agents permit or facilitate agonist-induced conversion of N-Me-D-Asp receptors from closed to open conformations.