Differential binding properties of [3H]dextrorphan and [3H]MK-801 in heterologously expressed NMDA receptors.
The N-methyl-D-aspartate receptor (NMDAR) antagonists: MK-801, phencyclidine and ketamine are open-channel blockers with limited clinical value due to psychotomimetic effects. Similarly, the psychotomimetic effects of the dextrorotatory opioids, dextromethorphan and its metabolite dextrorphan, derive from their NMDAR antagonist actions. Differences in the use dependency of blockade, however, suggest that the binding sites for MK-801 and dextrorphan are distinct. In the absence of exogenous glutamate and glycine, the rate of association of [3H]MK-801 with wild-type NR1-1a/NR2A receptors was considerably slower than that for [3H]dextrorphan. Glutamate individually, and in the presence of the co-agonist glycine, had substantial effects on the specific binding of [3H]MK-801, while the binding of [3H]dextrorphan was not affected. Mutation of residues N616 and A627 in the NR1 subunit had a profound effect on [3H]MK-801 binding affinity, while that of [3H]dextrorphan was unaltered. In contrast, NR1 residues, W611 and N812, were critical for specific binding of [3H]dextrorphan to NR1-1a/NR2A complexes with no corresponding influence on that of [3H]MK-801. Thus, [3H]dextrorphan and [3H]MK-801 have distinct molecular determinants for high-affinity binding. The ability of [3H]dextrorphan to bind to a closed channel, moreover, indicates that its recognition site is shallower in the ion channel domain than that of MK-801 and may be associated with the extracellular vestibule of the NMDAR.
Department of Physiology and Pharmacology, The University of Georgia, College of Veterinary Medicine, Athens, GA 30602, USA. klepage@uga.edu
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D000818:Animals; D001665:Binding Sites / Q000187:drug effects; D019556:COS Cells; D002462:Cell Membrane / Q000187:drug effects; D002522:Chlorocebus aethiops; D003164:Competitive Bidding / Q000379:methods; D003917:Dextrorphan / Q000494:pharmacology*; D016291:Dizocilpine Maleate / Q000494:pharmacology*; D004305:Dose-Response Relationship, Drug; D004347:Drug Interactions; D018691:Excitatory Amino Acid Antagonists / Q000494:pharmacology*; D018698:Glutamic Acid / Q000494:pharmacology; D005998:Glycine / Q000494:pharmacology; D008954:Models, Biological; D016296:Mutagenesis / Q000502:physiology; D011869:Radioligand Assay / Q000379:methods; D016194:Receptors, N-Methyl-D-Aspartate / Q000187:drug effects / Q000235:genetics / Q000378:metabolism*; D016415:Sequence Alignment; D017421:Sequence Analysis / Q000379:methods; D014162:Transfection / Q000379:methods; D014316:Tritium / Q000378:metabolism / Q000493:pharmacokinetics
D018691:Excitatory Amino Acid Antagonists; D016194:Receptors, N-Methyl-D-Aspartate; D003917:Dextrorphan; D014316:Tritium; D018698:Glutamic Acid; D016291:Dizocilpine Maleate; D005998:Glycine