Short title: Nociceptin inhibits pregnant uterine contractions Summary sentence: Nociceptin has been identified in pregnant rat uterus with uterus- relaxing function through cAMP and potassium channel signaling.

The actions of the endogenous peptide nociceptin (PNOC previously abbreviated as N/OFQ) on the myometrium have not been investigated previously. Our aim was to study the presence and functional role of PNOC in the modulation of uterine contractility in pregnant rats at term. The presence of PNOC and its receptors (OPRL1 previously called NOP) in the uterus were detected by radioimmunoassay (RIA) and radioligand-binding experiments. The PNOC- stimulated G protein activation was assessed by a ( 32 S)GTPgammaS-binding technique. The effects of PNOC in uterine rings precontracted with KCl or oxytocin were also tested in vitro. Uterine levels of cAMP were measured by enzyme immunoassay (EIA). The K + channel blockers tetraethylammonium and paxilline were used to study the role of K + channels in mediating the uterine effects of PNOC. Both PNOC and OPRL1 were present in the uterus. PNOC revealed a maximum contraction inhibition of ~30%, which was increased to 40% by naloxone. Naloxone and pertussis toxin significantly attenuated the G protein-stimulating effect of PNOC. The uterine cAMP levels were found to be elevated by PNOC and naloxone, and after preincubation with pertussis toxin. Tetraethylammonium and paxilline reduced the contraction- inhibiting effect of PNOC and naloxone to ~10 and 15%. We presume that PNOC plays a role in regulating uterine contractility at term. Its effect is mediated partly by stimulatory heterotrimeric G (Gs) proteins coupled to OPRL1 receptors and elevated cAMP levels, and also by Ca 2+ - dependent K + channels. Our results demonstrate a novel action and signaling pathway for PNOC