title: Guanine nucleotide-binding protein-coupled and -uncoupled states of opioid receptors and their relevance to the determination of subtypes.
abstract: Opioid receptors are currently classified as mu, delta, and kappa types, but various subtypes have also been proposed. We have investigated whether subtypes exist by using [3H]bremazocine. [3H]Bremazocine binds to twice as many naloxone-sensitive sites as other nonselective opioid agonists, as shown in four membrane types that have very different ratios of mu, delta, and kappa receptor types. [3H]Bremazocine binding is completely inhibited by an excess (in unlabeled form) of other opioid ligands, with Hill coefficients of 0.8-0.95. These paradoxes can be explained if there are high- and low-affinity states of the mu, delta, and kappa receptors and bremazocine binds with similar affinities to both states. We propose that these states are the guanine nucleotide-binding protein (G-protein)-coupled form and the uncoupled form of each receptor. As evidence for this proposal, the [3H]bremazocine binding suffered little or no loss with G-protein-uncoupling treatments, whereas binding of other opioid agonists was fully sensitive. We conclude that [3H]bremazocine offers a tool for the measurement of the total pools of coupled and uncoupled opioid receptors and that much of the previous characterization of opioid receptor subtypes reflects, instead, a significant pool of G-protein-uncoupled opioid receptors.
authors.affiliation: Medical Research Council Molecular Neurobiology Unit, Medical Research Council Centre, Cambridge, United Kingdom.
authors.forename: A
authors.initials: A
authors.lastname: Richardson
authors.forename: C
authors.initials: C
authors.lastname: Demoliou-Mason
authors.forename: E A
authors.initials: EA
authors.lastname: Barnard
mesh_terms: D000700:Analgesics / Q000378:metabolism*; D000818:Animals; D044482:Benzeneacetamides; D001575:Benzomorphans / Q000378:metabolism*; D001667:Binding, Competitive; D002462:Cell Membrane / Q000378:metabolism; D002531:Cerebellum / Q000378:metabolism; D020875:Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; D004743:Enkephalin, Leucine / Q000031:analogs & derivatives / Q000378:metabolism; D004745:Enkephalins / Q000378:metabolism; D005048:Etorphine / Q000378:metabolism; D005260:Female; D019204:GTP-Binding Proteins / Q000378:metabolism*; D006168:Guinea Pigs; D006801:Humans; D007700:Kinetics; D008024:Ligands; D010920:Placenta / Q000378:metabolism; D011247:Pregnancy; D016548:Prosencephalon / Q000378:metabolism; D011485:Protein Binding; D011759:Pyrrolidines / Q000378:metabolism; D051381:Rats; D011957:Receptors, Opioid / Q000145:classification / Q000378:metabolism*
chemical_list: D000700:Analgesics; D044482:Benzeneacetamides; D001575:Benzomorphans; D004745:Enkephalins; D008024:Ligands; D011759:Pyrrolidines; D011957:Receptors, Opioid; D020875:Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; D005048:Etorphine; D004743:Enkephalin, Leucine; C034318:enkephalin, Ser(2), Leu(5), Thr(6)-; D019204:GTP-Binding Proteins; C027248:bremazocine; C045444:U 69593