[3H]MK801 binding to the NMDA receptor/ionophore complex is regulated by divalent cations: evidence for multiple regulatory sites.

We have examined the ability of divalent and trivalent cations to regulate [3H]MK801 binding to the N-methyl-D-aspartate operated ionophore of rat brain membranes. In EDTA-washed membranes that are nominally free of glutamate and glycine the cations Ba2+, Ca2+, Co2+, La3+, Mn2+ and Sr2+ increased [3H]MK801 binding in the range 0.01-1.0 mM, depending on the cation studied. At higher concentrations (0.1-30 mM) these cations all inhibited binding. In contrast, Cd2+, Hg2+, Mg2+, Ni2+ and Zn2+ inhibited binding at all concentrations tested. The addition of maximally effective concentrations of glutamate (100 microM) and glycine (30 microM) increased binding by some 200% above control. In the presence of glutamate and glycine all cations except Sr2+ only inhibited binding, while the stimulation produced by Sr2+ was markedly diminished. The potency of most of the divalent cations tested was increased in the presence of glutamate and glycine. In contrast, Cd2+ and Zn2+ became less potent, while the potency of Hg2+ did not change. Thus, it appears that cations regulate the function of the N-methyl-D-aspartate receptor/ionophore complex by interacting with at least two separate sites.