Search

Nociceptin or orphanin FQ is a novel neuropeptide that activates an opioid-like G protein-coupled receptor ORL1. This heptadecapeptide FGGFTGARKSARKLANQ resembles kappa-opioid peptide dynorphin A but exhibits an opposite effect to make animals hyperreactive to nociceptive stimulations (Meunier, J.-C., Mollereau, C., Toll, L., Suaudeau, C., Moisand, C., Alvinerie, P., Butour, J.-L., Guillemot, J.-C., Ferrara, P., Monsarrat, B., Mazarguil, H., Vassart, G., Parmentier, M., and Costentin, J. (1995) Nature 377, 532-535; Reinscheid, R. K., Nothacker, H.-P., Bourson, A., Ardati, A., Henningsen, R. A., Bunzow, J. R., Grandy, D. K., Langen, H., Monsma, F. J., Jr., and Civelli, O. (1995) Science 270, 792-794). In the present study, it was found that guinea pig brain contains receptors to which nociceptin binds much more strongly than to ORL1 receptors expressed in human 293 cells. Although the Tyr1 --Phe substitution for dynorphin A eliminates almost completely an ability to bind to opioid receptors, the Phe1 --Tyr substitution in nociceptin was found to retain almost fully both receptor binding affinity and in vivo hyperalgesic activity in tail-flick assay. Nociceptin was extremely weak to bind to opioid receptors, while Tyr1-nociceptin exhibited 10-40 times increased affinity, especially for mu receptors, due to its N-terminal sequential identity to opioid peptides. Shortened analogs of dynorphin A are known to retain receptor binding ability and analgesic activity, whereas the removal of C-terminal hexa- or decapeptide from nociceptin totally abolished the affinity for the ORL1 receptor. These results indicated that the mode of interaction between nociceptin and ORL1 receptor is quite different from that between dynorphin and opioid receptor and that the C-terminal portion of nociceptin is crucial for receptor recognition.

Intracerebroventricular (i.c.v.) injections of senktide (0.01–10 nmol), a tachykinin NK-3 agonist, had an antidiuretic action in water-loaded rats (4.5% body wt.). Pretreatment with OPC-31260 (1 mg/kg, i.v.), a non-peptide vasopressin V 2 antagonist, inhibited the antidiuretic action induced by exogenous arginine vasopressin (AVP, 0.1 μg/kg, i.v.) and senktide (0.1 nmol, i.c.v.). In addition, senktide (11.8 nmol, i.c.v.) caused a marked increased of the plasma AVP level in conscious rats. These results suggest that the central NKB analogue senktide has an antidiuretic effect by stimulating AVP secretion from the pituitary gland through the NK-3 receptor in the hypothalamus.