Vol. 300, Issue 1, 57-63, January 2002

[Arg¹⁴,Lys¹⁵]Nociceptin, a Highly Potent Agonist of the Nociceptin/Orphanin FQ Receptor: in Vitro and in Vivo Studies

Daniela Rizzi , Anna Rizzi, Raffaella Bigoni, Valeria Camarda, Giuliano Marzola, Remo Guerrini, Carmela De Risi, Domenico Regoli and Girolamo Calo'

Department of Experimental and Clinical Medicine, Section of Pharmacology (D.R., A.R., R.B., V.C., G.M., D.R., G.C.); and Department of Pharmaceutical Sciences and Biotechnology Center (R.G., C.D.R.), University of Ferrara, Ferrara, Italy

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 (pA₂  6) and (±)trans-1-[1-cyclooctylmethyl-3hydroxymethyl-4-piperidyl]-3-ethyl-1,3-dihydro-2H-benzimidazol-2-one (J-113397) (pA₂  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.