Signaling Through Homomeric and Heteromeric Cannabinoid CB1 receptors

Cannabis (Marijuana) has multiple effects on the human body, such as analgesia, euphoria and memory impairment. Delta-9 tetrahydrocannabinol (D9-THC), the active ingredient in cannabis, binds to cannabinoid receptors, seven-transmembrane G protein-coupled receptors (GPCRs) that mediate a variety of physiological functions. GPCRs were believed to function only in homomeric forms, however, recent findings show that different GPCRs can also form heteromeric complexes that may expand their signaling properties. In this study, we focused on Cannabinoid CB1 receptor (CB1R) heteromers with the mu-opioid receptor (MOR) and the Dopamine type 2 receptor (D2R), respectively. We utilized a variety of techniques, such as the calcium mobilization assay, a luciferase complementation assay and an electrophysiology assay to study the pharmacology of the CB1R-MOR and CB1R-D2R heteromers. Our data demonstrate that co-expression of CB1R enhances the Gi signaling through MOR and inhibits the beta-arrestin recruitment to MOR. We also show that co-application of CB1R ligands can further accentuate the MOR signaling modulation. Co-expression of a CB1R transmembrane domain 5 (TM5), but not a TM1, mini-gene abrogated the signaling change suggesting that it is likely due to heteromerization of MOR and CB1R. Utilizing this herteromeric signaling could provide a novel therapeutic approach that may yield potent analgesic effects with reduced side effects. We have also found that CB1R switched its signaling specificity from Gi to Gs upon its heteromerizaiton with D2R. In conclusion, our data show that CB1R expands its signaling repertory and modulates the partner receptor signaling upon heteromerization