Effect of serotonin and progesterone on induced ovulation in immature rats

Serotonin (50 and 100 mg/kg), given subcutaneously, inhibited induced ovulation in immature rats treated with pregnant mare serum gonadotrophin (PMSG). A single injection was effective if given 52-55 h after the PMSG, i.e. in the 3 h period after the critical period before ovulation. The injection of serotonin inhibited the release of luteinizing hormone (LH) from the pituitary since the pituitary levels were higher than in the control animals and there was complete inhibition of the plasma LH surge. The anti-ovulatory effect was reversed by administration of progesterone and endogenous plasma progesterone levels were reduced in the late evening after serontonin treatment. The site of action of serotonin appeared to be peripheral since it inhibited induced ovulation in hypophysectomized rats but was without effect in intact rats treated intraventricularly. It is suggested that progesterone is essential for the occurrence of induced ovulation and serotonin inhibits either its secretion at the ovarian level or its passage away from the ovary.