ENDOGENOUS OPIOID REGULATION OF OXYTOCIN RELEASE DURING PARTURITION IS REDUCED IN OVARIECTOMIZED RATS

Pregnant rats were ovariectomized (or sham-ovariectomized) on days 17, 18 or 21 of pregnancy and oestradiol-17beta and progesterone were rep laced. Prepartum oxytocin concentrations were significantly lower in o variectomized steroid-treated rats than in intact controls, and on day 21 of pregnancy injection of relaxin into acutely ovariectomized rats significantly increased plasma oxytocin concentrations. During partur ition, injection of the opioid antagonist naloxone induced significant increases in plasma oxytocin concentration compared with saline-injec ted rats. The naloxone-induced increase was significantly less in ovar iectomized steroid-treated rats than in rats with intact ovaries, indi cating that endogenous opioid activity is less in ovariectomized rats than in intact rats. The progress of parturition in the ovariectomized steroid-treated rats was severely disrupted compared with sham-ovarie ctomized rats despite similar plasma oxytocin levels at the birth of p up number 2; this disruption was not overcome by injection of naloxone or by the consequent increase in oxytocin secretion, indicating defic ient preparation of the uterus and birth canal in the absence of relax in. We conclude that the decreased oxytocin concentrations prepartum, the prolongation of parturition and the decrease in opioid tone in ova riectomized steroid-treated rats may be partly due to a lack of relaxi n produced by the ovary.