Delineation of Domains Within the Cannabinoid CB1 and Dopamine D2 Receptors That Mediate the Formation of the Heterodimer Complex

Both the cannabinoid CB1 receptor (CB1) and dopamine D2 receptor (D2R) are G protein-coupled receptors that are linked to inhibitory G alpha i/o protein, whereby activation of the receptor leads to the inhibition of cAMP production. Moreover, previous findings have shown evidence of cross-talk between the dopamine and endocannabinoid systems. In this report, we confirm the interaction of CB1 and D2R with co-immunoprecipitation experiments using human embryonic kidney 293T (HEK-293T) cells co-expressing both receptors. We also generated GST and His-tagged fusion proteins of the D2R and CB1 and conducted affinity purification assays and in vitro binding experiments to show that the CB1-D2R complex can be formed by a direct protein-protein interaction. This interaction is mediated by the carboxyl terminus of the CB1 receptor and the third intracellular loop of the D2 receptor. Co-transfection of an inhibitory mini-gene resulted in decreased levels of the CB1-D2R complex. Using a cAMP biosensor, we show that activation of D2R or CB1 alone in HEK-293T cells co-expressing both receptors leads to an inhibition of forskolin-stimulated cAMP accumulation. However, co-activation of both receptors resulted in a loss of this inhibition on cAMP accumulation. Our findings characterize the physical interaction between CB1 and D2R as well as demonstrate the potential functional outcome of the receptor complex.