Cannabinoid Receptors Can Activate and Inhibit G Protein-Coupled Inwardly Rectifying Potassium Channels in a Xenopus Oocyte Expression System

• Published in: The Journal of pharmacology and experimental therapeutics 1999-11, Vol.291 (2), p.618
• Main Authors: McAllister, S D, Griffin, G, Satin, L S, Abood, M E
• Format: Article
• Language: English
• Subjects: Analgesics - pharmacology; Animals; Arachidonic Acids - pharmacology; Asparagine; Benzoxazines; Calcium Channel Blockers - pharmacology; Cyclohexanols - pharmacology; Dose-Response Relationship, Drug; Endocannabinoids; GTP-Binding Proteins - metabolism; Morpholines - pharmacology; Mutation; Naphthalenes - pharmacology; Oocytes - physiology; Polyunsaturated Alkamides; Potassium Channels - drug effects; Potassium Channels - genetics; Receptors, Cannabinoid; Receptors, Drug - classification; Receptors, Drug - physiology; RNA, Complementary - chemical synthesis; Time Factors; Xenopus
• ISSN: 0022-3565; 1521-0103

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.