Opioids Regulate CGMP Formation in Cloned Neuroblastoma Cells

Opioid agonists caused a rapid dose-related elevation of the cGMP content of N4TG1 murine neuroblastoma cells. An excellent correlation was found between the rank order of potency of agonists in stimulating cGMP accumulation and in displacing [(3)H]etorphine ([(3)H]ETP) bound to intact cells. The narcotic antagonists naloxone and diprenorphine failed to increase cGMP content; moreover, in the presence of 5 muM naloxone, the EC(50) of ETP increased from approximately 9 nM to > 1 muM. N4TG1 cells that had been incubated for 20 min with 0.32 muM ETP and thoroughly washed displayed a marked loss in sensitivity to subsequent ETP challenge. This desensitization was characterized by a 40-50% decrease in maximal response and an increase in the apparent K(a) of ETP from 4 to 50 nM. Desensitization was complete after a 7-min incubation with 0.32 muM ETP (t((1/2)) approximately 1 min) and was only slowly reversible (t((1/2)) > 60 min). Naloxone (5 muM) and diprenorphine (0.1 muM) failed to elicit desensitization, but they blocked ETP-induced desensitization. Dextrophan and (+)-ethylketazocine were <1% as effective as levorphanol and (-)-ethylketazocine, respectively, in both stimulating cGMP accumulation and inducing desensitization. When the binding of [(3)H]ETP (0.2-20 nM) was examined under identical experimental conditions, cells that were completely desensitized by incubation with ETP (7 min with 0.32 muM or 20 min with 15 nM) showed no loss of high-affinity recognition sites. After longer incubation with ETP (0.32 muM for 20-60 min), the maximal binding of [(3)H]ETP was reduced 17-41%. The specific short-term desensitization of cGMP accumulation is not mediated or accompanied by a decrement in the number of agonist binding sites.