The effect of melatonin on suckling-induced oxytocin and prolactin release in the rat

There is growing evidence that melatonin (MEL) inhibits oxytocin (OT) release when used in a low dose, while higher doses stimulate the release of the hormone in the rat. In the present study we investigated the effect of exogenous MEL, administered intracerebroventricularly (ICV), on suckling-induced OT and prolactin (PRL) release in the urethane-anesthetized rat. Lactating rats suckled by 8-12 pups were studied on days 8-12 of postpartum, and lactating pups-deprived rats on the same days of postpartum served as a control. Plasma OT and PRL levels as well as hypothalamic and neurohypophyseal OT contents were measured by RIA. Suckling stimulated the secretion of both OT and PRL. The ICV injection of 1 ng/ml MEL produced a significant inhibition of suckling-induced OT as well as PRL secretion. Melatonin in doses of 100 ng/ml or 10 micrograms/ml did not modify the OT release but significantly inhibited PRL release brought about by suckling; 10 pg/ml of MEL was not effective in this regard. Thus, exogenous MEL seems to inhibit suckling-induced OT as well as PRL secretion when applied at doses regarded to be in the range of the physiological level; when applied in higher doses, it was shown not to influence the release of OT following physiological stimulation such as suckling.