Zinc and ifenprodil allosterically inhibit two separate polyamine-sensitive sites at N-methyl-D-aspartate receptor complex

In this study, we investigated the hypothesis that inhibition of the N-meth yl-D-aspartate (NMDA) receptor complex by zinc involves a polyamine-sensiti ve regulatory site. We found that the specific binding of the open channel ligand [H-3]MK-801 to rat hippocampal membranes 1) was inhibited by low con centrations of Zn2+ (IC50 = 5.5 mu M) by 65%. 2) This high-affinity compone nt of inhibition was reversed by the polyamine spermine to an extent that c ould be reconciled with competitive interaction between Zn2+ and spermine. 3) Partial inhibition by Zn2+ was additive with partial inhibition by ifenp rodil, an inhibitor of the NMDA receptor complex supposed to act at a polya mine-sensitive regulatory site, and 4) in membranes prepared from several o ther brain regions, inhibition of [H-3]MK-801 binding by Zn2+ and by ifenpr odil was either less than additive, or superadditive. Our observation that ifenprodil, at concentrations saturating its high-affinity component of inh ibition, prevented spermine from reversing the inhibition by Zn2+ indicates that spermine did not increase [H-3]MK-801 binding by competition with Zn2 + but rather via another polyamine regulatory site not sensitive to zinc bu t sensitive to ifenprodil. We conclude that Zn2+ reduces channel opening of the NMDA receptor complex by allosteric inhibition of a polyamine-sensitiv e regulatory site different from that inhibited by ifenprodil and that thes e two allosteric sites influence each other in a manner dependent on the br ain region investigated. The different proportions of zinc/ifenprodil inhib ition in different regions could reflect different percentages of various N MDA receptor subtypes.