Structure-Activity Relationships Of Nociceptin And Its Analogs And The Interaction Of ORL1, NK1 And Opioid Receptor System In Pain Modulation

Nociceptin (NC) is the endogenous ligand of the opoioid receptor like-1 (ORL1) receptor. Both the peptide sequences and their receptors of NC and opioid share high structural homology. Ulike the opioids, NC causes hyperalgesia at supraspinal level. [Nphe¹]NC(1-13)HN2, the selective and compeptitive ORL1 antagonist, when given alone intracerebroventricularly (i.c.v.), induces potent analgesic response, and it doesn't show the severe side effects such as tolerance and dependence. So we are interested in studying the structure-activity relationship of NC by introducing unnatural amino acids and pseudopeptide bond in the N-terminal end of the NC sequence. The effects of NC and its analogues on pain modulation and Guinea pig bronchus were determined.In in virto bioassays, several NC analogues lost the affinities for ORL1 receptor. Except for [Nphe⁴]NC(1-13)NH₂, but the affinity of [Nphe⁴]NC(1-13)NH₂ for ORL1 receptor is lower than NC. In addition, [F/G]NC(1-5)NH₂ not only binds to ORL1 receptor, it also has affinity for classical opioid receptors. The results imply that the change of NC conformation would contribute to the receptor affinity and selectivity.NC, [Nphe⁴]NC(l-13)NH₂ and [F/G]NC(1-5)NH₂ caused significant hyperalgesia in mice. Among them, [F/G]NC(1-5)NH₂ had the equivalent effect to NC, it played a similar role with NC in pain modulation but not with that of opioids. Though [Nphe⁴]NC(1-13)NH₂ induced hyperalgesia, but the effect was lower than that of NC. The present study suggested that [Nphe¹]NC(l-13)NH₂ and [F/G]NC(1-13)NH₂ but not NST could antagonize the effects of NC in EFS-induced constriction of guinea pig isolated bronchus, None of them did not affect the inhibitions induced by morphine and endomorphins. [Nphe⁴]NC(1-13)NH₂ showed similar but lower effects with that of NC. [F/G]NC(1-5)NH2 did not have significant effect on guinea pig isolated bronchus. These results suggested that the change of peptide bond and the distance between Phe1 and Phe4 in N-terminal are vital to the affinity and selectivity of NC to ORL1 receptor.NC at least interacts with opioid receptor system and NK1 receptor system in the modulation of pain process. Similarly, opioid receptor system and NK1 receptor system also have a direct interaction in pain modulation. We further we examined theeffects of i.c.v. nociceptin and [Nphe1]NC(l-13)NH2 on rat/mouse hemokinin (r/m HK-1, the endogenous ligand for NK1 receptor) -induced nociceptin. Moreover, we also utilized a pharmacological paradigm in which r/m HK-1 and morphine are coadministration by the i.c.v. route or coadministration of r/m HK-1 (i.c.v.) and morphine (subcutaneously, s.c). We want to further elucidate the mechanism in pain modulation and to facilitate the use for pharmacological target.R/m HK-1 had biphasic modulatory effects on supraspinal nociceptive behaviors in mice. I.c.v. administration of r/m HK-1 could produce antinociceptive effect at nanomole concentration, and naloxone significantly antagonized this effect. When injected at picomole concentration, there was significant hyperalgesic effect. Interestingly, (ORL-1) receptor antagonist [Nphel]NC(l-13)NH2 could fully abolish this hyperalgesic effect. Moreover, Analgesia induced by r/m HK-1 was reserved/blocked dose-dependently by i.c.v co-administration of orphan opioid receptor like-l(ORL-l) agonist nociceptin (NC) and [Nphe1]NC(l-13)NH2potentiated the analgesic effect elicited by r/m HK-1. These findings imply that ORL-1 receptor could modulate both descending central inhibitory pathways and excitatory pathways which were triggered by r/m HK-1.I.c.v. administration of r/m HK-1 could significantly potentiate the antinociceptive effects of morphine which was injected at peripheral and supraspinal level. These antinociceptive effects were blocked by prior treatment with the classical opioid receptors antagonist naloxone, indicating mat the potentiated analgesic response is mediated by opioid-responsive neurons. We suggest that there may be a cascade amplification mechanism in pain modulation when the two agents were co-administrated. The synergistic analgesic relationship of morphine and r/m HK-1 eatablished here supports the hypothesis that supraspinal tachykinin and peripheral and central opioid systems have a direct functional interaction in the modulation of local nociceptive responses.