High affinity [3H]dextrorphan binding in rat brain is localized to a noncompetitive antagonist site of the activated N-methyl-D-aspartate receptor-cation channel.
[3H]Dextrorphan recognition sites were characterized in rat brain membranes. The pharmacological profile and regional distribution of [3H]dextrorphan binding sites appear to distinguish these sites from those labeled either by [3H]dextromethorphan or by putative sigma receptor radioligands. Data from thoroughly washed forebrain membranes suggest that [3H]dextrorphan predominantly labels a high affinity site defined by the activated state of the N-methyl-D-aspartate (NMDA) receptor-channel complex. Regulation of [3H]dextrorphan binding by specific modulators of NMDA receptor function suggests that [3H]dextrorphan binding is predominantly localized to a domain of the receptor-channel complex also recognized by the prototypical noncompetitive antagonist radioligands (+)-[3H]5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imi ne (MK-801) and [3H]1-[1-(2-thienyl)cyclohexyl]piperidine (TCP). The critical relationship between [3H]dextrorphan binding and activation of the NMDA receptor-complex is suggested by the profound dependence of [3H]dextrorphan binding on glutamate in well washed membranes. Basal specific [3H]dextrorphan binding is nearly totally suppressed by the specific competitive NMDA antagonist D(-)-2-amino-5-phosphonopentanoic acid (D-AP5), in a glutamate- but not glycine-surmountable manner. Glutamate and glycine each stimulate [3H]dextrorphan binding in a concentration-dependent manner, effecting maximal increases from control of up to 30- and 14-fold, respectively. The NMDA receptor specificity of the modulation of [3H]dextrorphan binding by glutamate and glycine is indicated by the sensitivity of their effects to competitive antagonism by D-AP5 and 3-amino-1-hydroxy-2-pyrrolidone (HA-966), respectively, and by the accordant rank orders of potency of glycine analogs as modulators of [3H]dextrorphan binding and as ligands at the strychnine-insensitive glycine site. The divalent cations Mg2+ and Zn2+ and the polyamines spermine and spermidine regulate [3H]dextrorphan binding in a manner consistent with radioligand interaction at the noncompetitive NMDA antagonist domain. Mg2+ and spermidine regulate [3H]dextrorphan binding biphasically in well washed forebrain membranes, whereas Zn2+ monotonically inhibits [3H]dextrorphan binding. Mg2+ and spermidine regulate [3H]dextrorphan binding with qualitative similarity and in a contrasting fashion to their regulation of [3H]MK-801 and [3H]TCP binding. First, spermidine and Mg2+ are significantly more potent modulators of [3H]dextrorphan binding than of [3H]MK-801 and [3H]TCP binding in well washed membranes; second, whereas the potencies of spermidine and Mg2+ as modulators of [3H]MK-801 and [3H]TCP binding are significantly increased by glutamate and glycine in well washed membranes, their potencies as regulators of [3H]dextrorphan binding appear to be unaffected by glutamate and glycine.(ABSTRACT TRUNCATED AT 400 WORDS)
College of Pharmacy, Oregon State University, Corvallis 97331.
P H
PH
Franklin
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Murray
D000818:Animals; D001667:Binding, Competitive; D001921:Brain / Q000378:metabolism*; D002412:Cations / Q000378:metabolism*; D003917:Dextrorphan / Q000378:metabolism* / Q000493:pharmacokinetics; D005971:Glutamates / Q000494:pharmacology; D018698:Glutamic Acid; D005998:Glycine / Q000494:pharmacology; D007473:Ion Channels / Q000187:drug effects / Q000378:metabolism*; D007700:Kinetics; D008274:Magnesium / Q000378:metabolism / Q000494:pharmacology; D008297:Male; D011073:Polyamines / Q000494:pharmacology; D051381:Rats; D011919:Rats, Inbred Strains; D016194:Receptors, N-Methyl-D-Aspartate / Q000037:antagonists & inhibitors / Q000378:metabolism* / Q000502:physiology; D014018:Tissue Distribution; D014316:Tritium; D015032:Zinc / Q000378:metabolism / Q000494:pharmacology
D002412:Cations; D005971:Glutamates; D007473:Ion Channels; D011073:Polyamines; D016194:Receptors, N-Methyl-D-Aspartate; D003917:Dextrorphan; D014316:Tritium; D018698:Glutamic Acid; D008274:Magnesium; D015032:Zinc; D005998:Glycine