Characterization of human cannabinoid CB2 receptor coupled to chimeric Galpha(qi5) and Galpha(qo5) proteins.

Cannabinoid CB(2) receptors may couple to a variety of G proteins and intracellular effector systems to regulate physiological and pathophysiological processes involved in inflammatory and neuropathic pain. In this study, the coupling of cannabinoid hCB(2) receptors to Galpha(qo5) and Galpha(qi5) proteins was studied and compared by investigating the pharmacological properties of HEK-293 cells co-expressing cannabinoid hCB(2) with chimeric Galpha(qo5) (HEK-hCB(2)-G(qo5)) or Galpha(qi5) (HEK-hCB(2)-G(qi5)). Both cell lines were found to be amendable for measuring cannabinoid CB(2) receptor agonist evoked Ca(2+) mobilization in a high-throughput manner. Comparison of binding affinities of ligands in homogenates prepared from both cell lines revealed similar affinities for [(3)H]CP55,940 displacement with the following rank order: CP55,940 approximately WIN55,212-2 > SR144528 > JWH015approximatelyAM1241approximately AM630 > SR141617A approximately AM251. In comparison at cannabinoid hCB(1) receptors: the rank order was: SR141617A approximately CP55,940 > AM251 > WIN55,212-2 > AM1241approximatelySR144528 > JWH015approximatelyAM630. No significant differences in cannabinoid receptor agonist (CP55,940 approximately WIN55,212-2 > JWH015) or antagonist(SR144528 approximately AM1241 > AM630 > AM251 approximately SR141617A) profiles were observed in HEK-hCB(2)-G(qo5) and HEK-hCB(2)-G(qi5) cells as determined using intracellular Ca(2+) measurements. Experiments with HEK-hCB(2)-G(qi5) cells carried out by investigating interactions among CP55,940, carbachol, thapsigargin, and U73122 revealed that the mechanism of cannabinoid hCB(2) receptor coupling via chimeric G proteins to Ca(2+) mobilization involves phospholipase C-inositol trisphosphate (PLC-IP(3)) and that it is less efficient in comparison to the endogenous muscarinic mediated PLC-IP(3)-Ca(2+) pathway. This study demonstrates that expressed cannabinoid CB(2) receptors couple equally well to Galpha(qo5) and Galpha(qi5) proteins and that receptor agonist or antagonist pharmacology is not influenced by the nature of these coupled G proteins when heterologously expressed.