Recognition and activation of the opioid receptor-like ORL1 receptor by nociceptin, nociceptin analogs and opioids

Nociceptin, also known as orphanin FQ, was recently identified as the naturally occurring agonist of orphan opioid receptor-like ORL1 receptor (Meunier et al., 1995, Nature 377, 532; Reinscheid et al., 1995, Science 270, 792). Nociceptin is a heptadecapeptide which, although it resembles dynorphin A, the endogenous agonist of the kappa-opioid receptor, displays very low potency in competing with binding of [H-3]diprenorphine to or inhibiting adenylate cyclase via mu-, delta- and kappa-opioid receptors. Tritium-labeled nociceptin ([H-3]nociceptin) was used here to establish a pharmacological profile in vitro of the ORL1 receptor. In membranes from recombinant Chinese hamster ovary (CHO) cells expressing the ORL1 receptor, equilibrium binding of [H-3]nociceptin is highly specific, saturable (B-max in the range 1.3-1.8 pmol/mg protein) and of high affinity (K-d approximate to 0.1 nM). It is selectively decreased in the presence of Na+ ions and/or of the GTP analog 5'-guanylylimido-dophosphate, an allosteric regulation that is analogous to that oc opiate binding to opioid receptors. A few opiates, namely lofentanil, a 4-anilinopiperidine derivative and etorphine, a 6,14-endo-ethenotetrahydrothebaine derivative, were found to be quite potent not only in competing with binding of [H-3]nociceptin at the ORL1 receptor but also in inhibiting forskolin-induced accumulation of cyclic AMP in intact recombinant CHO cells. In a preliminary attempt to delineate active parts of the neuropeptide, nociceptin analogs were also tested, including N- and C-terminal truncation products. Our results suggest that the highly basic, internal core of nociceptin might be essential in conferring on the peptide both affinity for and activity at the ORL1 receptor. In this respect, the message and address division of dynorphin A, nociceptin's closest structural analog, do not seem to apply to nociceptin.