Abstract

The nociceptin (NC)/orphanin FQ analog, [Arg¹⁴,Lys¹⁵]NC, has been recently demonstrated to behave as a potent agonist at the human recombinant NC receptors (OP₄). In this study, we evaluated the pharmacological profile of [Arg¹⁴,Lys¹⁵]NC in vitro on the native OP₄ receptors expressed in isolated tissues and in vivo in the locomotor activity and the tail-withdrawal assays in mice. On isolated tissues, [Arg¹⁴,Lys¹⁵]NC mimicked the effects of NC, showing similar maximal effects but higher potencies (17-fold in the mouse vas deferens, 10-fold in the rat vas deferens, and about 5-fold in the guinea pig ileum and mouse colon). In these preparations, the effects of [Arg¹⁴,Lys¹⁵]NC were not modified by 1 μM naloxone, although antagonized by the OP₄ receptor antagonists [Nphe¹]NC(1–13)NH₂(pA ₂ ≅ 6) and (±)trans-1-[1-cyclooctylmethyl-3hydroxymethyl-4-piperidyl]-3-ethyl-1,3-dihydro-2H-benzimidazol-2-one (J-113397) (pA ₂ ≅ 8). In the rat vas deferens, a cocktail of peptidase inhibitors increased the maximal effects of NC, its analog, and the pEC₅₀ of NC (by 4-fold); the potency of [Arg¹⁴,Lys¹⁵]NC was not significantly modified by peptidase inhibitors. In in vivo experiments, [Arg¹⁴,Lys¹⁵]NC mimicked the effects of NC, producing, after intracerebroventricular administration, pronociceptive effects in the tail-withdrawal assay and inhibiting the locomotor activity of the mice. In both assays, [Arg¹⁴,Lys¹⁵]NC was about 30-fold more potent than NC and produced longer lasting effects. Taken together, the present data demonstrate that [Arg¹⁴,Lys¹⁵]NC behaves as a highly potent agonist of the OP₄ receptor and is able to produce long-lasting effects in vivo, compared with the natural ligand NC.