ADDRESS AND MESSAGE SEQUENCES FOR THE NOCICEPTIN RECEPTOR - A STRUCTURE-ACTIVITY STUDY OF NOCICEPTIN-(1-13)-PEPTIDE AMIDE

Nociceptin (NC) and some of its fragments as well as nociceptin-(1-13) -peptide amide [NC(1-13)-NH2] and a series of its analogues were prepa red and tested in the mouse vas deferens in an attempt to identify the sequences involved in the activation (message) and in the binding (ad dress) of nociceptin to its receptor. The NC receptor that inhibits th e electrically evoked twitches of the mouse vas deferens was demonstra ted to be distinct from the delta opioid receptor, since naloxone and Dmt-Tic-OH (a selective delta opioid receptor antagonist) block the de lta opioid receptor but have no effect on the nociceptin receptor. Res ults from structure-activity experiments suggest that (a) the entire s equence of NC may not be required for full biological activities, sinc e NC(1-13)-NH2 is as active as NC; (b) fragments of NC have however to be amidated as in NC(1-13)-NH2 in order to be protected from degradat ion by proteases; (c) cationic residues (as Args(8,12), Lys(9,13)) app ear to play a functional role, since their replacement with Ala in the sequence of NC(1-13)-NH2 leads to inactivity; (d) the N-terminal tetr apeptide Phe-Gly-Gly-Phe is essential for activity: its full length an d flexibility appear to be required for NC receptor activation and/or occupation; (e) Phe(4) and not Phe(1) appears to be the residue involv ed in receptor activation, since the replacement of Phe(1) with Leu ha s no effect, while that of Phe(4) leads to inactivity. Results summari zed in this paper indicate that the structural requirements of NC for occupation and activation of its receptor are different from that of o pioids, particularly delta agonists.