DELTA-OPIOID RECEPTOR ACTIVATES CAMP-PHOSPHODIESTERASE ACTIVITIES IN NEUROBLASTOMA X GLIOMA NG108-15 HYBRID-CELLS

In neuroblastoma x glioma NG108-15 hybrid cells, opioid agonists inhib ited both basal and prostaglandin E1-stimulated adenylate cyclase acti vities assayed in the presence of the phosphodiesterase (PDE) inhibito rs isobutylmethylxanthine and ZK62711 (rolipram). However, when intrac ellular [H-3]cAMP was measured in the absence of the PDE inhibitors th e maximal inhibitory level was increased, using the opioid agonist D-A la2, D-Leu5-enkephalin. This increase in opioid activity was due to ag onist stimulation of cAMP degradation, because when the degradation ra te of [H-3] cAMP was measured in intact hybrid cells it was observed t o increase from the control value of 0.495 +/- 0.003 min-1 to 0.760 +/ - 0.003 min-1 in the presence of 1 muM D-Ala2, D-Leu5-enkephalin; this was reversed by naloxone. Dose-dependent studies with various opioid agonists, partial agonists, and antagonists revealed that there was a direct correlation between the abilities of these opioid ligands to in hibit adenylate cyclase activity and to stimulate PDE activity, with e nkephalin and its analogs being the most potent agonists. Chronic agon ist treatment also resulted in a reduction of the opioid agonist stimu lation of cAMP degradation, with an apparent decrease in the PDE activ ity upon addition of naloxone after chronic treatment. However, treatm ent of the hybrid cells with pertussis toxin, which attenuated the ago nist inhibition of adenylate cyclase activity, did not abolish this op ioid response. When selective inhibitors for various types of PDE were used, the type I PDE inhibitor W-7 attenuated the opioid effect, wher eas the type II PDE inhibitor trequinsin (HL725), the type III PDE inh ibitor indolidan, and the type IV PDE inhibitor rolipram had no effect on opioid-stimulated cAMP degradation. The stimulation of type I PDE activity by delta-opioid receptors was independent of extracellular Ca 2+ and was not observed with membrane preparations. Therefore, in NG10 8-15 cells delta-opioid receptors regulate intracellular cAMP levels b y coupling to a pertussis toxin-insensitive guanine nucleotide-binding protein, resulting in an increase in intracellular Ca2+ and in Ca2+/c almodulin-dependent PDE activity.