Effect of forskolin and isobutylmethylxanthine on delta-opioid receptor activity in neuroblastoma x glioma NG108-15 cells

Continuous elevation of intracellular cyclic AMP (cAMP) by culturing neuroblastoma x glioma NG108-15 hybrid cells in the presence of forskolin and isobutylmethylxanthine (IBMX) in a chemically defined medium resulted in differentiation of the hybrid cells, as indicated by extension of neurite- like structures and induction of a subclass of G-protein, Go, as monitored by Western analysis. This cellular differentiation also resulted in an initial 25 to 30% increase in [³H]diprenorphine binding 3 hr after forskolin and IBMX treatment, followed by a decrease in opioid receptor density to the maximal level of 35% of control 4 days later. However, the potencies and maximal inhibitory levels of various opioid agonists to inhibit adenylate cyclase activity was not altered during differentiation. When the differentiated hybrid cells were treated with DADLE chronically, an apparent decrease in the ability of the agonist to desensitize and to down-regulate the delta-opioid receptor was observed. It is unlikely that this observed attenuation was due to activation of cAMP-dependent protein kinase A, because (1) attenuation of DADLE desensitization was time-dependent, reaching maximal effects 48 hr after the initiation of treatment; and (2) pretreatment of NG108-15 cells with forskolin and IBMX resulted in attenuation of forskolin's ability to stimulate adenylate cyclase activity and parallel decrease in the ability of forskolin to activate the cAMP-dependent protein kinases in these cells was also observed. Thus, it is unlikely that the activation of protein kinase A by forskolin and IBMX is the cause for the attenuation of DADLE- induced delta-opioid receptor desensitization in differentiated NG108-15 cells.