Cannabinoid receptors can activate and inhibit G protein-coupled inwardly rectifying potassium channels in a xenopus...

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.