title: Membrane-delimited proteolytic regulation of opioid receptors.
abstract: Prolonged exposure of opioid receptors to agonists leads to their regulation by the classical process of clathrin-dependent internalization, followed by their intracellular degradation (down regulation). We have previously shown that the opioid agonist etorphine induced an additional process of down regulation of mu-opioid receptors (MOR) that occurred in intact MOR-transfected HEK-293 cells, as well as in isolated membranes. In the present study we show that etorphine similarly down regulated rat kappa-opioid receptors (KORs), which do not undergo the classical process of internalization and down regulation. This process was resistant to inhibitors of clathrin-coated pit formation (hypertonic sucrose, mono-dansyl-cadaverine) and was mainly mediated by membranous serine- and amino-peptidases. We further show that various opioid ligands, besides etorphine, induced down regulation of either KOR or MOR in isolated membranes. The ability of the various opioid ligands to induce membrane-delimited KOR or MOR down regulation did not correlate to their classical pharmacological profile, suggesting functional selectivity of the effect. Levorphanol, but not its stereoisomer dextrophan, induced membrane-delimited down regulation of both KOR and MOR, indicating that stereoselective binding to the receptor was necessary to initiate the process. Our findings that this proteolytic regulation of opioid receptors occurs not only in isolated membranes but also in intact cells and that it occurs even when the receptors are resistant to the conventional process of down regulation indicate its possible physiological role in the regulation of opioid activity.
authors.affiliation: Department of Physiology and Pharmacology, Sackler Faculty of Medicine, Tel-Aviv University, Tel-Aviv 69978,Israel.
authors.forename: Elena
authors.initials: E
authors.lastname: Kaplan
authors.forename: Binyamin
authors.initials: B
authors.lastname: Binyaminy
authors.forename: Mikhal
authors.initials: M
authors.lastname: Gafni
authors.forename: Ora
authors.initials: O
authors.lastname: Keren
authors.forename: Yosef
authors.initials: Y
authors.lastname: Sarne
mesh_terms: D000701:Analgesics, Opioid / Q000494:pharmacology; D000818:Animals; D001667:Binding, Competitive / Q000502:physiology; D002462:Cell Membrane / Q000187:drug effects / Q000201:enzymology*; D004847:Epithelial Cells / Q000187:drug effects / Q000201:enzymology*; D005048:Etorphine / Q000494:pharmacology; D057809:HEK293 Cells; D006801:Humans; D009292:Narcotic Antagonists; D010447:Peptide Hydrolases / Q000502:physiology*; D059748:Proteolysis / Q000187:drug effects*; D051381:Rats; D011957:Receptors, Opioid / Q000378:metabolism*; D017473:Receptors, Opioid, kappa / Q000037:antagonists & inhibitors / Q000378:metabolism; D017450:Receptors, Opioid, mu / Q000037:antagonists & inhibitors / Q000378:metabolism; D013347:Subcellular Fractions
chemical_list: D000701:Analgesics, Opioid; D009292:Narcotic Antagonists; D011957:Receptors, Opioid; D017473:Receptors, Opioid, kappa; D017450:Receptors, Opioid, mu; D005048:Etorphine; D010447:Peptide Hydrolases