title: Cannabinoid receptors can activate and inhibit G protein-coupled inwardly rectifying potassium channels in a xenopus oocyte expression system.
abstract: In this study, we focused on the pharmacological characterization of cannabinoid receptor coupling to G protein-gated inwardly rectifying potassium (GIRK) channels. Cannabinoids were tested on Xenopus laevis oocytes coexpressing the CB(1) receptor and GIRK1 and GIRK4 channels (CB(1)/GIRK1/4) or the CB(2) receptor and GIRK1/4 channels (CB(2)/GIRK1/4). WIN 55,212-2 enhanced currents carried by GIRK channels in the CB(1)/GIRK1/4 and CB(2)/GIRK1/4 system; however, the CB(2) receptor did not couple efficiently to GIRK1/4 channels. In the CB(1)/GIRK1/4 system, WIN 55,212-2 was the most efficacious compound tested. CP 55,940 and anandamide acted as partial agonists. The rank order of potency was CP 55,940 > WIN 55,212-2 = anandamide. The CB(1)-selective antagonist SR141716A alone acted as a inverse agonist by inhibiting GIRK currents in oocytes expressing CB(1)/GIRK1/4, suggesting the CB(1) receptor is constitutively activated. A conserved aspartate residue, which was previously shown to be critical for G protein coupling in cannabinoid receptors, was mutated (to asparagine, D163N) and analyzed. Oocytes coexpressing CB(1)/GIRK1/4 or D163N/GIRK1/4 were compared. The potency of WIN 55, 212-2 at the mutant receptor was similar to wild type, but its efficacy was substantially reduced. CP 55,940 did not elicit currents in oocytes expressing D163N/GIRK1/4. In summary, it appears the CB(1) and CB(2) receptors couple differently to GIRK1/4 channels. In the CB(1)/GIRK1/4 system, cannabinoids evaluated demonstrated the ability to enhance or inhibit GIRK currents. Furthermore, a conserved aspartate residue in the CB(1) receptor is required for normal communication with GIRK channels in oocytes demonstrating the interaction between receptor and channels is G protein dependent.
authors.affiliation: Department of Pharmacology and Toxicology, Medical College of Virginia Campus of Virginia Commonwealth University, Richmond, Virginia, USA.
authors.forename: S D
authors.initials: SD
authors.lastname: McAllister
authors.forename: G
authors.initials: G
authors.lastname: Griffin
authors.forename: L S
authors.initials: LS
authors.lastname: Satin
authors.forename: M E
authors.initials: ME
authors.lastname: Abood
mesh_terms: D000700:Analgesics / Q000494:pharmacology; D000818:Animals; D001095:Arachidonic Acids / Q000494:pharmacology; D001216:Asparagine; D048588:Benzoxazines; D002121:Calcium Channel Blockers / Q000494:pharmacology*; D003511:Cyclohexanols / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D063388:Endocannabinoids; D019204:GTP-Binding Proteins / Q000378:metabolism*; D009025:Morpholines / Q000494:pharmacology*; D009154:Mutation; D009281:Naphthalenes / Q000494:pharmacology*; D009865:Oocytes / Q000502:physiology; D053284:Polyunsaturated Alkamides; D015221:Potassium Channels / Q000187:drug effects / Q000235:genetics*; D018075:RNA, Complementary / Q000138:chemical synthesis; D043882:Receptors, Cannabinoid; D011955:Receptors, Drug / Q000145:classification / Q000502:physiology*; D013997:Time Factors; D014981:Xenopus
chemical_list: D000700:Analgesics; D001095:Arachidonic Acids; D048588:Benzoxazines; D002121:Calcium Channel Blockers; D003511:Cyclohexanols; D063388:Endocannabinoids; D009025:Morpholines; D009281:Naphthalenes; D053284:Polyunsaturated Alkamides; D015221:Potassium Channels; D018075:RNA, Complementary; D043882:Receptors, Cannabinoid; D011955:Receptors, Drug; C070417:(3R)-((2,3-dihydro-5-methyl-3-((4-morpholinyl)methyl)pyrrolo-(1,2,3-de)-1,4-benzoxazin-6-yl)(1-naphthalenyl))methanone; D001216:Asparagine; C054649:3-(2-hydroxy-4-(1,1-dimethylheptyl)phenyl)-4-(3-hydroxypropyl)cyclohexanol; D019204:GTP-Binding Proteins; C078814:anandamide