Melatonin-induced changes in blood and pituitary luteinizing hormone and prolactin levels during the perinatal period in rat dams.

Following melatonin implants into pregnant rats throughout the entire period of gestation, the prenatal level of LH was significantly lowered in serum and the pituitary, while that of prolactin was enhanced in serum and diminished in the pituitary. After parturition, most of the melatonin-induced changes in LH and prolactin disappeared, being replaced by compensatory stimulation of release and synthesis of LH and inhibition of release of prolactin. Neither the abrupt physiological decline of serum progesterone, nor the enhanced level of oestrogen present during the period preceding delivery seem to be affected by melatonin. The findings were observed in both sham-operated (control) and pinealectomized animals, thus suggesting that melatonin exerts its inhibitory action on hypothalamic LH-RH and PIF not by utilizing an additional antigonadotropic pineal factor and not by the steroid sex hormones.