Interactions between μ-opioid receptor (μOR) and cannabinoid CB
1 receptor (CB
1R) were examined by morphological and electrophysiological methods. In baby hamster kidney (BHK) cells coexpressing μOR fused to the yellow fluorescent protein Venus and CB
1R fused to the cyan fluorescent protein Cerulean, both colors were detected on the cell surface; and fluorescence resonance energy transfer (FRET) analysis revealed that μOR and CB
1R formed a heterodimer. Coimmunoprecipitation and Western blotting analyses also confirmed the heterodimers of μOR and CB
1R. [
D-Ala
2,
N-Me-Phe
4,Gly
5-ol]enkephalin (DAMGO) or CP55,940 elicited K
+ currents in
Xenopus oocytes expressing μOR or CB
1R together with G protein activated-inwardly rectifying K
+ channels (GIRKs), respectively. In oocytes coexpressing both receptors, either of which was fused to the chimeric Gα protein G
qi5 that activates the phospholipase C pathway, both DAMGO and CP55,940 elicited Ca
2+-activated Cl
− currents, indicating that each agonist can induce responses through G
qi5 fused to either its own receptor or the other. Experiments with endogenous G
i/o protein inactivation by pertussis toxin (PTX) supported the functional heterodimerization of μOR/CB
1R through PTX-insensitive G
qi5(m) fused to each receptor. Thus, μOR and CB
1R form a heterodimer and transmit a signal through a common G protein. Our electrophysiological method could be useful for determination of signals mediated through heterodimerized G protein–coupled receptors.
View full abstract