Pharmacology of [Tyr(1)]nociceptin analogs: receptor binding and bioassay studies

Two series of nociceptin (NC)-related peptides with or without replacement of the N-terminal Phe by Tyr have been investigated in an attempt to obtain compounds that interact with the NC receptor (ORL1) and classic opioid receptors. When tested for their ability to displace [3H]NCNH2 ([3H]nociceptin amide; ORL1 sites) or the selective opioid receptor ligands [3H]DAMGO (m), [3H]deltorphin II (d) and [3H]U69593 (k) from their respective binding sites in guinea-pig brain membranes, [Tyr1]NCNH2 and [Tyr1]NC(1–13)NH2 showed high affinities (Ki 2nM and 5 nM, respectively) for ORL1 and approximately tenfold lower potency for m (32nM and 44nM) and k sites (42 nM and 48 nM). They also interacted, but with low potency (Ki 410 nM and 310 nM) with d sites. Shorter fragments as [Tyr1]NC(1–9)NH2 and [Tyr1]NC(1–5)NH2 were found to be inactive on ORL1, d and k sites, and extremely weak (Ki 2224 nM and 4228 nM, respectively) on m. Results of bioassays performed on the guinea-pig ileum (ORL1 and m receptors), mouse vas deferens (ORL1 and d receptors), and rabbit vas deferens (k receptor) confirmed (at least partially) the data of the binding by showing that [Tyr1]NC analogs interact with functional ORL1 as well as with classic opioid receptors. [Tyr1]NCNH2 and [Tyr1]NC(1–13)NH2 behaved as mixed ORL1/opioid receptor agonists showing similar affinities as the control NC sequence while [Tyr1]NC(1–9)NH2 and [Tyr1]NC(1–5)NH2 were inactive on ORL1 receptors but maintained some activities on opioid receptors: their effects were prevented by naloxone. The results of this study indicate that the replacement of Phe1 by Tyr in NC leads to compounds which bind both the ORL1 and m/k receptors and may represent new promising agents for use in peripheral organs.