Further studies on nociceptin-related peptides: Discovery of a new chemical template with antagonist activity on the nociceptin receptor

Three series of nociceptin (NC)-related peptides were synthesized and their abilities (i) to bind to the NC sites expressed in mouse forebrain membran es, (ii) to inhibit the electrically evoked contraction of the mouse vas de ferens, and (iii) to inhibit forskolin-stimulated cAMP accumulation in Chin ese hamster ovary cells expressing the human recombinant NC receptor (CHONC R) were investigated. The compounds of the first series (a series) have an ordinary Xaa(1)-Gly(2) bond, those of the second series (b series) have a X aa(1)Psi(CH2-NH)Gly(2) pseudopeptide bond, and those of the third series (c series) have a peptoid (Nxaa(1)-Gly(2)) structure. The affinity values mea sured in the binding assay and in the two functional assays with the compou nds of the three series showed high levels of correlation. Thus, (I) the co mpounds of the a series in which Phe(1) was substituted with Tyr, Cha, or L eu acted as potent NC receptor agonists; (II) the b series compounds behave d as NC receptor antagonists in the mouse vas deferens and as full agonists in CHONCR cells with different potencies depending on the first amino acid residue, [Phe(1)Psi(CH2-NH)Gly(2)]NC(1-17)NH2 and [Phe(1)Psi(CH2-NH)Gly(2) ]NC(1-13)NH2 being the most potent compounds; (III) the compounds of the th ird series were all inactive both as agonists and as antagonists with the e xception of [Nphe(1)]NC(1-17)NH2 and [Nphe(1)]NC(1-13)NH2, which behaved as NC receptor antagonists both in the isolated tissue and in CHONCR cells (p K(B) 6.1-6.4). In conclusion, this study demonstrates that chemical require ments for NC receptor agonists are different from those of antagonists. Mor eover, modifications of the steric orientation of the aromatic residue Phe( 1) in the NC sequence as obtained with the pseudopeptide bond between Phe(1 ) and Gly(2) or with the displacement of the benzyl side chain by one atom, as in Nphe(1), lead respectively to reduction or elimination of efficacy. Indeed, in contrast to [Phe(1)Psi(CH2-NH)Gly(2)]NC(1-13)NH2 which has been reported to exhibit agonist activity in several assays involving either cen tral or recombinant NC receptors, [Nphe(1)]NC(1-13)NH2 antagonizes the effe ct of NC at human recombinant NC receptors and in the mouse tail withdrawal assay.