Opioid and cannabinoid receptors share a common pool of GTP-binding proteins in cotransfected cells, but not in cells which endogenously coexpress the receptors

Opioid (mu. delta, kappa) and cannabinoid (CB1, CB2) receptors are coupled mainly to G(i)/G(o) GTP-binding proteins. The goal of the present study was to determine whether different subtypes of opioid and cannabinoid receptor s, when coexpressed in the same cell, share a common reservoir. or utilize different pools, of G proteins. The stimulation of [S-35]GTP gamma S binding by selective opioid and cannab inoid agonists was tested in transiently transfected COS-7 cells, as well a s in neuroblastoma cell lines. In COS-7 cells, cotransfection of mu- and de lta-opioid receptors led to stimulation of [S-35]GTP gamma S binding by eit her mu-selective (DAMGO) or delta-selective (DPDPE) agonists. The combined effect of the two agonists was similar to the effect of either DAMGO or DPD PE alone, suggesting the activation of a common G-protein reservoir by the two receptor subtypes. The same phenomenon was observed when COS-7 cells were cotransfected with C B1 cannabinoid receptors and either mu- or delta-opioid receptors. On the other hand. in N18STG2 neuroblastoma cells, which endogenously coexp ress CB1 and delta-opioid receptors, as well as in SK-N-SH neuroblastoma ce lls, which coexpress mu- and delta-opioid receptors, the combined effects o f the various agonists (the selective cannabinoid DALN and the selective op ioids DPDPE and DAMGO) were additive, implying the activation of different pools of G proteins by each receptor subtype. These results suggest a fundamental difference between native and artificia lly transfected cells regarding the compartmentalization of receptors and G TP-binding proteins.