Comparison of the effects of [Phe1Ψ(CH2-NH)Gly2]nociceptin (1-13)NH2 in rat brain, rat vas deferens and CHO cells expressing recombinant human nociceptin receptors

1 Nociceptin(NC) is the endogenous ligand for the opioid receptor like-1 receptor (NC-receptor). [Phe'Psi(CH2-NH)Gly(2)]Nociceptin(1-13)NH2 ([F/G]NC(1-13)NH2) has been reported to antagonize NC actions in peripheral guinea-pig and mouse tissues. In this study, we investigated the effects of a range of NC C-terminal truncated fragments and [F/G]NC(1-13)NH2 on NC receptor binding, glutamate release from rat cerebrocortical slices (rCX), inhibition of cyclic AMP accumulation in CIIO cells expressing the NC receptor (CHONCR) and electrically evoked contractions of the rat vas deferens (rVD). 2 In radioligand binding assays, a range of ligands inhibited [I-125]-Tyr(14)-NC binding in membranes from rCX and CHONCR cells. As the peptide was truncated there was a general decline in pK(i). [F/G]NC(1-13)NH2 was as potent as NC(1-13)NH2. 3 The order of potency for NC fragments to inhibit cyclic AMP accumulation in whole CHONCR cells was NCNH2 greater than or equal to NC = NC(1-13)NH2 > NC(1-12)NH2 > > NC(1-11)NH2. [F/G]NC(1-13)NH2 was a full agonist with a pEC(50) value of 8.65. 4 NCNH2 and [F/G]NC(1-13)NH2 both inhibited K+ evoked glutamate release from rCX with pEC(50) and maximum inhibition of 8.16, 48.5+/-4.9% and 7.39, 58.9+/-6.8% respectively. 5 In rVD NC inhibited electrically evoked contractions with a pEC(50) of 6.63. Although [F/G]NC(1-13)NH2, displayed a small (instrinsic activity alpha=0.19) but consistent residual agonist activity, it acted as a competitive antagonist (pA(2) 6.76) in the rVD. 6 The differences between [F/G]NC(1-13)NH2 action on central and peripheral NC signalling could be explained if [F/G]NC(1-13)NH2 was a partial agonist with high strength of coupling in the CNS and low in the periphery. An alternative explanation could be the existence of central and peripheral receptor isoforms.