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-methyl-D-aspartate (NMDA) receptor complex by zinc involves a polyamine-sensitive regulatory site. We found that the specific binding of the open channel ligand [3H]MK-801 to rat hippocampal membranes 1) was inhibited by low concentrations of Zn2+ (IC50 = 5.5 microM) by 65%. 2) This high-affinity component of inhibition was reversed by the polyamine spermine to an extent that could be reconciled with competitive interaction between Zn2+ and spermine. 3) Partial inhibition by Zn2+ was additive with partial inhibition by ifenprodil, an inhibitor of the NMDA receptor complex supposed to act at a polyamine-sensitive regulatory site, and 4) in membranes prepared from several other brain regions, inhibition of [3H]MK-801 binding by Zn2+ and by ifenprodil was either less than additive, or superadditive. Our observation that ifenprodil, at concentrations saturating its high-affinity component of inhibition, prevented spermine from reversing the inhibition by Zn2+ indicates that spermine did not increase [3H]MK-801 binding by competition with Zn2+ but rather via another polyamine regulatory site not sensitive to zinc but sensitive to ifenprodil. We conclude that Zn2+ reduces channel opening of the NMDA receptor complex by allosteric inhibition of a polyamine-sensitive regulatory site different from that inhibited by ifenprodil and that these two allosteric sites influence each other in a manner dependent on the brain region investigated. The different proportions of zinc/ifenprodil inhibition in different regions could reflect different percentages of various NMDA receptor subtypes.
Berger
M L
ML
Institute of Biochemical Pharmacology, University of Vienna, Vienna, Austria. berger@univie.ac.at
Rebernik
P
P
D000494:Allosteric Regulation; D000818:Animals; D001667:Binding, Competitive; D001921:Brain / Q000378:metabolism*; D002462:Cell Membrane / Q000378:metabolism; D018891:Dentate Gyrus / Q000378:metabolism; D016291:Dizocilpine Maleate / Q000493:pharmacokinetics*; D018691:Excitatory Amino Acid Antagonists / Q000494:pharmacology*; D006624:Hippocampus / Q000378:metabolism*; D007700:Kinetics; D008297:Male; D009474:Neurons / Q000378:metabolism*; D009928:Organ Specificity; D010880:Piperidines / Q000494:pharmacology*; D011869:Radioligand Assay; D051381:Rats; D017208:Rats, Wistar; D016194:Receptors, N-Methyl-D-Aspartate / Q000737:chemistry* / Q000187:drug effects / Q000378:metabolism*; D013096:Spermine / Q000494:pharmacology*; D014316:Tritium; D015032:Zinc / Q000494:pharmacology*
D018691:Excitatory Amino Acid Antagonists; D010880:Piperidines; D016194:Receptors, N-Methyl-D-Aspartate; D014316:Tritium; D013096:Spermine; D016291:Dizocilpine Maleate; D015032:Zinc; C010739:ifenprodil