Comparison of opiate inhibition of adenylate cyclase activity in neuroblastoma N18TG2 and neuroblastoma X glioma NG 108-15 hybrid cell lines

The ability of various opiate agonists to inhibit adenylate cyclase activity in neuroblastoma N18TG2 cells was investigated and compared with opiate activity in a daughter cell line, neuroblastoma X glioma NG108-15 hybrid. A high-affinity (K(diss) = 1.44 nM) binding site for ³H-labeled D-Ala²-Met⁵-enkephalinamide was observed in the membrane preparations of N18TG2 cells. The density of this binding site was 80% of that determined with the NG108-15 hybrid membrane preparations. Both the basal and the prostaglandin E₁-stimulated increase in adenylate cyclase activity were inhibited by the opioid peptide, Met⁵-enkephalin. The opiate regulation of the enzyme activity in neuroblastoma N18TG2 was observed to be stereoselective, naloxone-reversible, and GTP-dependent. A linear correlation between the potencies of various opiate agonists' inhibition of adenylate cyclase activity in N18TG2 and NG108-15 cells could be established, with etorphine being the most potent agonist in both cell lines. There are several dissimilarities in opiate inhibition of adenylate cyclase activity in these two cell lines. The maximal level of opiate inhibition (efficacy) in neuroblastoma N18TG2 was significantly lower than that in the neuroblastoma X glioma NG108-15 hybrid cells. The difference in the efficacies of various opiates tested was observed within the NG108-15 cell line also. The monovalent cation Na⁺ attenuated the opiate potency of inhibition of adenylate cyclase activity in NG108-15 cells, but not in the N18TG2 cells. Chronic exposure of these two cell lines to etorphine caused a reduced sensitivity of the cell lines to the opiate agonist. The naloxone-induced increase in adenylate cyclase activity in the NC108-15 cells chronically treated with opiate was not observed in the N18TG2 cells. These observations suggest there are differences in the molecular events which lead to opiate inhibition of the adenylate cyclase activity in these two cell lines.