Effects of the κ‐opioid agonist U50,488 on parturition in rats

1 The effects of the K-opioid agonist U50,488 on parturition were studied in the rat. 2 Given directly after the birth of the second pup U50,488 (5 mg or 10 mg kg−1, i.p.) delayed the birth of the subsequent 4 pups by ca. 100 min, acting like morphine (10 mg kg−1, i.p.). In controls given the vehicle i.p., the birth of the 4 pups after treatment took 45.4 ± 4.6 min. The effects of U50,488 could be prevented by simultaneous naloxone injection (10 mg kg−1). Injection of either U50,488 or morphine at 1 mg kg−1, i.v. also significantly delayed parturition. The effects of U50,488 but not of morphine were fully prevented by preinjection with nor-binaltorphimine (0.5 mg kg−1, i.v.) showing selective κ-opioid receptor-mediated inhibition by U50,488 of established parturition. 3 In rats with an indwelling jugular venous cannula, i.v. injection of U50,488 (5 mg kg−1) after the birth of the second pup slowed parturition in a similar way to i.p. injection and significantly reduced blood plasma oxytocin concentration measured by radioimmunoassay compared with vehicle-injected controls. 4 Bolus i.v. injections of oxytocin (4 mu once per 5 min) significantly reduced the delay in parturition caused by i.v. U50,488, but continuous i.v. infusion of oxytocin (4 mu 5 min−1) was less effective. 5 Since i.v. oxytocin did not immediately reverse the effects of U50,488 on parturition, direct effects of U50,488 on isometric uterine contractions in vitro were sought. U50,488 inhibited spontaneous or oxytocin-stimulated contractions of uteri from rats within 24 h after parturition in a dose-related manner; the inhibitory effect was not naloxone-reversible. 6 Thus U50,488 inhibited established parturition in the rat in a κ-opioid selective manner by reducing oxytocin secretion. The inhibitory effect may well have been potentiated by a direct non-opioid depressant action on contractile activity of the uterus.