DIFFERENTIAL MODULATION BY DIVALENT-CATIONS OF [H-3] MK-801 BINDING IN BRAIN SYNAPTIC-MEMBRANES

Endogenous divalent cations, such as Mg2+, Ca2+, and Zn2+, differentially affected the binding of (+)-[H-3]5-methyl-10, 11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine maleate ([H-3]MK-801) to an ion channel associated with an N-methyl-D-aspartate-sensitive subclass of excitatory amino acid receptors in different preparations of brain synaptic membranes. Both Mg2+ and Ca2+ were weak inhibitors of the binding in membranes which had not been extensively washed (nonwashed membranes), over a concentration range effective in markedly potentiating the binding in the absence of any added stimulants in membranes which had been extensively washed, but not treated with a detergent (untreated membranes). In membranes extensively washed and treated with Triton X-100 (Triton-treated membranes), both cations significantly potentiated the binding in the presence of added glutamate alone. In contrast, Zn2+ was invariably active as a potent inhibitor of the binding irrespective of the membrane preparations used. In untreated membranes, Ca2+ markedly accelerated the initial association rate of [H-3]MK-801 binding without affecting the binding at equilibrium in a manner similar to that found with glycine, as well as with glutamate; Mg2+ however, facilitated the initial association rate with a concomitant reduction of the binding at equilibrium. Zn2+ was effective in accelerating the initial rapid phase of association, with the initial slow phase being delayed, and in markedly reducing the binding at equilibrium. Both Mg2+ and Ca2+ also facilitated dissociation of the bound [H-3]MK-801 and Zn2+ slowed the dissociation in untreated membranes. In the presence of Mg2+ or Ca2+, glycine was ineffective in potentiating [H-3]MK-801 binding irrespective of the membrane preparations used, whereas glutamate was an effective stimulator in Triton-treated membranes, but not in untreated membranes. However, spermidine was still active at potentiating the binding in both membrane preparations in the presence of Mg2+ or Ca2+. These results suggest that the three different divalent cations differentially modulate the opening processes of an ion channel associated with the N-methyl-D-aspartate-sensitive receptor through the respective mechanisms in the brain.