Role of Oxytocin on Prolactin Secretion During Late Pregnancy

Backgrounds/Aims: During late pregnancy, blockade of progesterone action by mifepristone (Mp) treatment induces a dopaminergic tone fall that enables naloxone (NAL) administration to release pituitary PRL. We determined whether oxytocin, that stimulates PRL secretion acting directly on anterior pituitary lactotrophs, mediates the stimulatory action of Mp and NAL on PRL secretion during late pregnancy. Methods: On day 19 of pregnancy, circulating and pituitary oxytocin and PRL levels were measured by RIA, 10, 20 and 30 min after NAL (given at 17.30h) in rats pre-treated with Mp (at 08.00h). Pituitary oxytocin receptors (OTR) expression in Mp treated rats was evaluated by RT-PCR. Activation of oxytocin neurons in Mp-NAL treated rats was measured counting double immunoreactive neurons for Fos and oxytocin (Fos-OT-ir) in SON, medial (PaMM) and lateral (PaLM) magnocellular divisions of PVN. Results: Elevated serum oxytocin and decreased pituitary oxytocin were observed 10 min after NAL administration both in vehicle- and Mp-treated rats. This PRL increase was prevented by previous ip administration of an OTR antagonist but intracerebroventricular oxytocin administration was ineffective. Mp increased pituitary OTR expression at 18.00h. Only Mp-NAL increased Fos-OT-ir neurons in the PaMM and SON. Conclusions: These findings suggest that PRL secretion induced by Mp-NAL treatment is preceded by oxytocin release. These results together with the activation of hypothalamic oxytocin neurons and the higher expression of pituitary OTR support the hypothesis that, during late pregnancy, oxytocin may act at pituitary level to facilitate PRL secretion if the inhibitory action of progesterone is blocked.



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