[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 0.01-10 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 μM) and glycine (30 μM) 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.