Characterization of a triple opioid system in the human neuroblastoma NMB cell line.

The human neuroblastoma NMB cell line was found to contain the three types of opioid receptors (60% delta 25% kappa and 15% mu). The opioid receptors were negatively coupled to adenylyl-cyclase. Maximal reduction in cAMP content was achieved by selectively activating single receptor types, indicating the co-presence of the various opioid receptors in the same cells. The opioid receptors in NMB cells were up-regulated following prolonged exposure to the opioid antagonist naloxone and down-regulated following chronic treatment with the opioid agonist etorphine. Down-regulation was time-, dose- and temperature-dependent and was inhibited by colchicine and sodium azide. The NMB culture is presented as an excellent experimental model for studying the selective activation and regulation of the different opioid receptor types when they are co-expressed in the same neuron, as well as for studying interactions between the various opioid receptors.