Effects of naloxone benzoylhydrazone on native and recombinant nociceptin/orphanin FQ receptors

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Abstract
We have studied the effects of naloxone benzoylhydrazone (NalBzoH) at recombinant human OP4 receptors expressed in Chinese hamster ovary (CHO) cells (CHOhOP4) and native OP4 sites in isolated tissues from various species. In CHOhOP4 membranes, nociceptin (NC) and NalBzoH displaced [125I]Tyr14-NC with pKi values of 10.1 and 7.3. In the presence of 100 µM GDP, NC stimulated GTPγ35S binding (pEC50 = 8.5). NalBzoH was ineffective but antagonized the effects of NC (pA2 = 6.9). At 5 µM GDP, there was an increase in potency (pEC50 = 9.3) and efficacy (4.3-fold) of NC. NalBzOH was a partial agonist (pEC50 = 7.0, Emax = 13% relative to NC). In CHOhOP4 cells, NC and NalBzoH inhibited cAMP formation with pEC50 and Emax values of 9.8 and 100% and 6.0 and 44%, respectively. In the rat vas deferens, NalBzoH (10 µM) did not modify electrically induced twitches but competitively antagonized the inhibitory action of NC (pA2 = 6.2). In the mouse vas deferens (mVD) and guinea pig ileum (gpI), NalBzoH inhibited twitches with pEC50 and Emax values of 7.6 and 78% and 8.5 and 77%, respectively. The effect of 3 µM NalBzoH was fully inhibited by 3 µM naloxone in mVD and 30 µM in gpI. Under these conditions, NalBzoH antagonized the actions of NC in both preparations with pA2 values of 6.3 and 6.8, respectively. Collectively, these data demonstrate that NalBzoH is a nonselective OP4 ligand with system-dependent behaviour.Key words: nociceptin/orphanin FQ, naloxone benzoylhydrazone, OP4 receptors, cAMP, GTPγS, bioassay.