Pharmacological characterization of the nociceptin receptor mediating hyperalgesia in the mouse tail withdrawal assay

Abstract

1 The newly discovered neuropeptide nociceptin (NC) has recently been reported to be the endogenous ligand of the opioid-like orphan receptor. Despite its structural similarity to opioids, when injected intracerebroventricularly (i.c.v.) in the mouse, NC exerts a direct hyperalgesic e ect and reverses opioidinduced analgesia. In the present investigation, these two e ects of NC were evaluated under the same experimental conditions; in addition, a pharmacological characterization of the receptor mediating these central e ects of NC was attempted. 2 NC caused a dose dependent (0.1 ± 10 nmol/mouse), naloxone-insensitive reduction of tail withdrawal latency with a maximal e ect of about 50% of the reaction time observed in saline injected mice. In the same range of doses, NC inhibited morphine (1 nmol/mouse) induced analgesia.3 The e ects of the natural peptide were mimicked by NCNH 2 and NC(1 ± 13)NH 2 (all tested at 1 nmol/mouse) while 1 nmol NC(1 ± 9)NH 2 was found to be inactive either in reducing tail withdrawal latency or in preventing morphine analgesia. 4 [Phe 1 c(CH 2 -NH)Gly 2 ]NC(1 ± 13)NH 2 ([F/G]NC(1 ± 13)NH 2 ), which has been shown to antagonize NC e ects in the mouse vas deferens, acted as an agonist, mimicking NC e ects in both the experimental paradigms. In addition, when NC and [F/G]NC(1 ± 13)NH 2 were given together, their e ects were additive. 5 These results demonstrate that both the direct hyperalgesic action and the anti-morphine e ect of NC can be studied under the same experimental conditions in the mouse tail withdrawal assay. Moreover, the pharmacological characterization of the NC functional site responsible for these actions compared with the peripherally active site, indicates the existence of important di erences between peripheral and central NC receptors.

In vitro and in vivo studies on UFP-112, a novel potent and long lasting agonist selective for the nociceptin/orphanin FQ receptor

“…( figure 3, right panel). Thus, UFP-112 mimicked the actions of N/OFQ, approximately 100-fold more potent than the natural peptide, and produced longer lasting effects (see for comparison the dose response curves to N/OFQ previously published in [9] and [43]). In fact, the effects induced by UFP-112 in the tail-withdrawal assay were still evident at least 120 min after the i.c.v.…”

“…This was consistently observed among a large series of assays/actions. In fact, the supraspinal pronociceptive effect of N/OFQ lasted for about 15-30 min [9], while that of UFP-112 was still evident after 2 hours from i.c.v. injection; the spinal antinociceptive action of N/OFQ disappeared after 60 min from i.t.…”

“…injection of high doses of N/OFQ (i.e. 10 nmol) [9,49,50]. However, while the N/OFQ effects appeared immediately after i.c.v.…”

“…All experiments were started at 10.00 a.m. and performed according to the procedure previously described in detail [9]. Briefly, the mice were placed in a holder and the distal half of the tail was immersed in water at 48 °C.…”

[(pF)Phe4,Arg14,Lys15]N/OFQ-NH2 (UFP-102), a Highly Potent and Selective Agonist of the Nociceptin/Orphanin FQ Receptor

“…In contrast, mice treated with 0.1 and 0.3 nmol showed a decrease in locomotor activity, ataxia, and loss of the righting reflex, in a similar manner to that which occurs after i.c.v. injection of high doses of N/OFQ (i.e., 10 nmol) (Reinscheid et al, 1995;Calo et al, 1998;Rizzi et al, 2001a). However, although the N/OFQ effects seemed to occur immediately after i.c.v.…”