CB 1 cannabinoid (CB 1 ) and D 2 dopamine (D 2 ) receptors are known to couple to the G protein G␣ i/o . It has been reported that concurrent activation of D 2 receptors and CB 1 receptors, in primary striatal neuronal culture, promotes functional CB 1 receptor coupling to G␣ s resulting in elevations in intracellular cyclic AMP levels. We now report that in the absence of D 2 receptors, acute activation of CB 1 receptors inhibits cyclic AMP accumulation, whereas the presence of D 2 receptors promotes CB 1 -stimulated cAMP accumulation, presumably through G␣ s . This G␣ s subunit switching was not prevented by pertussis toxin treatment and occurred in the presence and absence of D 2 receptor activation. Thus, coexpression of the D 2 receptor with the CB 1 receptor was sufficient to switch the coupling of the CB 1 receptors from G␣ i/o to G␣ s . Persistent activation of D 2 receptors resulted in heterologous sensitization of adenylate cyclase to subsequent stimulation by forskolin, whereas the persistent activation of CB 1 receptors did not. Additional studies in human embryonic kidney cells cotransfected with D 2 and CB 1 receptors revealed that persistent activation (18 h) of D 2 receptors induced a switch of CB 1 receptor coupling from G␣ s to G␣ i/o . This D 2 receptor-induced effect allowed for CB 1 receptor-mediated inhibition of cyclic AMP accumulation. The present studies suggest D 2 receptors may have a significant modulatory role in determining the G protein coupling specificity of CB 1 receptors.The CB 1 cannabinoid (CB 1 ) receptor is expressed primarily in the central nervous system, especially in the basal ganglia and cortex (Herkenham et al., 1990;Matsuda et al., 1993;Tsou et al., 1998), with some expression occurring in peripheral tissues such as the uterus, testes, and ileum (Pacheco et al., 1991;Pertwee et al., 1992;Das et al., 1995). This distribution pattern in the brain suggests that the cannabinoid system and the ascending dopamine pathways may interact with one another. Indeed, functional links between dopaminergic and cannabinoid systems have been reported (Mailleux and Vanderhaeghen, 1993;Glass and Felder, 1997;Giuffrida et al., 1999). Microdialysis experiments have demonstrated that activation of D 2 dopamine receptors (D 2 ) promotes elevations in extracellular concentrations of the endocannabinoid anandamide (Giuffrida et al., 1999). Chronic treatment with D 2 receptor antagonists causes upregulation of CB 1 mRNA in striatum (Giuffrida et al., 1999).When expressed individually, activation of either the D 2 or CB 1 receptor inhibits cAMP accumulation. Several studies have shown that the CB 1 receptor inhibits adenylate cyclase activity through coupling with a pertussis toxin-sensitive G␣ i/o -protein (for review, see Howlett, 1995). The D 2 receptor also inhibits adenylate cyclase via pertussis toxin-sensitive G␣ i/o -proteins (Sibley and Monsma, 1992). Our interest in dopamine-cannabinoid interactions stems from reports that concurrent activation of D 2 and CB 1 receptors alte...