Cannabinoid receptors can activate and inhibit G protein-coupled inwardly rectifying potassium channels in a Xenopus oocyte expression system

In this study, we focused on the pharmacological characterization of cannab inoid receptor coupling to G protein-gated inwardly rectifying potassium (G IRK) channels. Cannabinoids were tested on Xenopus laevis oocytes coexpress ing the CB1 receptor and GIRK1 and GIRK4 channels (CB1/GIRK1/4) or the CB2 receptor and GIRK1/4 channels (CB2/GIRK1/4). WIN 55,212-2 enhanced currents carried by GIRK channels in the CB1/GIRK1/4 and CB2/GIRK1/4 system; howeve r, the CB2 receptor did not couple efficiently to GIRK1/4 channels. In the CB1/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 poten cy was CP 55,940. WIN 55,212-2 = anandamide. The CB1-selective antagonist S R141716A alone acted as a inverse agonist by inhibiting GIRK currents in oo cytes expressing CB1/GIRK1/4, suggesting the CB1 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 CB1/GIRK1/4 or D163N/ GIRK1/4 were compared. The potency of WIN 55,212-2 at the mutant receptor w as 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 CB1 and CB2 receptors couple differently to GIRK1/4 channe ls. In the CB1/GIRK1/4 system, cannabinoids evaluated demonstrated the abil ity to enhance or inhibit GIRK currents. Furthermore, a conserved aspartate residue in the CB1 receptor is required for normal communication with GIRK channels in oocytes demonstrating the interaction between receptor and cha nnels is G protein dependent.