Independence of, and interactions between, cannabinoid and opioid signal transduction pathways in N18TG2 cells

N18TG2 neuroblastoma cells co-express delta-opioid and CB1-cannabinoid receptors. Both receptors are negatively coupled to adenylyl cyclase through pertussis toxin-sensitive GTP-binding proteins. In the present study, we confirmed the independent activity of opioid and cannabinoid agonists, and investigated chronic interactions between the two signal transduction pathways in these cells. Opioid and cannabinoid agonists stimulated [35S]guanosine-5'-O-(3-thiotriphosphate) binding to N18TG2 membranes. When the opioid agonist etorphine and the cannabinoid agonist desacetyllevonantradol (DALN) were applied together, the stimulation was similar to the arithmetic sum of the two separate effects. This additivity existed even after partial ablation of the G-proteins reservoir with a low concentration of pertussis toxin, indicating that opioid and cannabinoid receptors activate different pools of G-proteins in N18TG2 cells. Chronic treatment of the cells with either opioid or cannabinoid agonists induced desensitization to the respective drug. In addition, asymmetric cross-desensitization was found: while long-term exposure to DALN induced homologous desensitization, and did not reduce the effect of etorphine, long-term exposure to etorphine attenuated the cannabinoid activation of G-proteins. Chronic exposure to either DALN or etorphine not only induced desensitization, but also elevated the basal activity of G-proteins in the exposed cells. The combination of the two drugs did not yield an additive activation, suggesting that chronic exposure of N18TG2 cultures to cannabinoid and opioid agonists modified a common responding element within the cells. This work presents the N18TG2 neuroblastoma as a suitable experimental model to study the molecular mechanism(s) underlying chronic interactions between opioid and cannabinoid drugs.