HIGH-AFFINITY [H-3] DEXTRORPHAN BINDING IN RAT-BRAIN IS LOCALIZED TO A NONCOMPETITIVE ANTAGONIST SITE OF THE ACTIVATED N-METHYL-D-ASPARTATERECEPTOR-CATION CHANNEL

[H-3]Dextrorphan recognition sites were characterized in rat brain mem branes. The pharmacological profile and regional distribution of [H-3] dextrorphan binding sites appear to distinguish these sites from those labeled either by [H-3]dextromethorphan or by putative sigma-receptor radioligands. Data from thoroughly washed forebrain membranes suggest that [H-3]dextrorphan predominantly labels a high affinity site defin ed by the activated state of the N-methyl-D-aspartate (NMDA) receptor- channel complex. Regulation of [H-3]dextrorphan binding by specific mo dulators of NMDA receptor function suggests that [H-3]dextrorphan bind ing is predominantly localized to a domain of the receptor-channel com plex also recognized by the prototypical noncompetitive antagonist rad ioligands 0,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine (MK-801) and [H-3]1-[1-(2-thienyl)cyclohexyl]piperidine (TCP). The critical rel ationship between [H-3]dextrorphan binding and activation of the NMDA receptor-complex is suggested by the profound dependence of [H-3]dextr orphan binding on glutamate in well washed membranes. Basal specific [ H-3]dextrorphan binding is nearly totally suppressed by the specific c ompetitive NMDA antagonist D(-)-2-amino-5-phosphonopentanoic acid (D-A P5), in a glutamate- but not glycine-surmountable manner. Glutamate an d glycine each stimulate [H-3]dextrorphan binding in a concentration-d ependent manner, effecting maximal increases from control of up to 30- and 14-fold, respectively. The NMDA receptor specificity of the modul ation of [H-3]dextrorphan binding by glutamate and glycine is indicate d by the sensitivity of their effects to competitive antagonism by D-A P5 and 3-amino-1-hydroxy-2-pyrrolidone (HA-966), respectively, and by the accordant rank orders of potency of glycine analogs as modulators of [H-3]dextrorphan binding and as ligands at the strychnine-insensiti ve glycine site. The divalent cations Mg2+ and Zn2+ and the polyamines spermine and spermidine regulate [H-3]dextrorphan binding in a manner consistent with radioligand interaction at the noncompetitive NMDA an tagonist domain. Mg2+ and spermidine regulate [H-3]dextrorphan binding biphasically in well washed forebrain membranes, whereas Zn2+ monoton ically inhibits [H-3]dextrorphan binding. Mg2+ and spermidine regulate [H-3]dextrorphan binding with qualitative similarity and in a contras ting fashion to their regulation of [H-3]MK-801 and [H-3]TCP binding. First, spermidine and Mg2+ are significantly more potent modulators of [H-3]dextrorphan binding than of [H-3]MK-801 and [H-3]TCP binding in well washed membranes; second, whereas the potencies of spermidine and Mg2+ as modulators of [H-3]NK-801 and [H-3]TCP binding are significan tly increased by glutamate and glycine in well washed membranes, their potencies as regulators of [H-3]dextrorphan binding appear to be unaf fected by glutamate and glycine. Furthermore, putrescine, which does n ot influence [H-3]MK-801 or [H-3]TCP binding, inhibits basal [H-3]dext rorphan binding in a manner dissimilar from that of spermidine- and sp ermine-mediated inhibition of binding. The kinetics of [H-3]dextrorpha n binding in the presence of saturating concentrations of glutamate an d glycine are complex and inadequately described by monoexponential as sociation and dissociation processes. The differential distribution of [H-3]dextrorphan recognition sites in rat brain regions and the pharm acological profile specified by the rank order of potency of an extens ive set of compounds as competitors for high affinity [H-3]dextrorphan binding unambiguously suggest that [H-3]dextrorphan binding in rat br ain membranes corresponds to the site of the NMDA antagonist activity of dextrorphan in vivo. In contrast, the pharmacological signature and distribution of high affinity [H-3]dextrorphan binding sites in rat b rain are incongruous with those of either the sigma-receptor or [H-3]d extromethorphan binding sites. Accordingly, the interaction of dextror phan and dextromethorphan at sites labeled by [H-3]dextrorphan, but no t at sites labeled by [H-3]dextromethorphan or by sigma-ligands, adequ ately accounts for the anticonvulsant and neuroprotective efficacies o f these compounds in vivo.