Fetal prolactin levels respond to a maternal melatonin implant

Seasonal PRL changes in adult sheep are controlled by photoperiod. The ability to detect photoperiod is mediated by the pineal gland through melatonin secretion. A rhythm in plasma melatonin has been described in fetal sheep. In this work we investigated whether the fetus responds to changes in circulating levels of melatonin. Fetal plasma PRL was measured every 2 h for 24 h, 7 days after the mother had received a sc Silastic implant containing approximately 1 g melatonin. Control fetuses received an empty implant. Melatonin is slowly released from implants, and it is known to cross the sheep placenta. Fetal plasma melatonin increased 10-fold after the implant. Plasma PRL in fetuses treated with melatonin was a third of that of control fetuses. Fetal plasma PRL concentrations were 56.6 +/- 5.7 ng/ml (mean +/- SE) in control and 18.1 +/- 2.7 ng/ml 7 days after the melatonin implant. We conclude from these data that the ability to respond to melatonin is present in sheep during fetal life.