Role of the placenta in the control of the ante-partum surge of prolactin in the rat

Abstract

ABSTRACT A nocturnal surge of prolactin secretion occurs in the dark period preceding parturition in the rat. The aim of this study was to examine the role of the placenta in the control of this prolactin surge. Plasma prolactin and progesterone were measured by radioimmunoassay in serial blood samples collected after surgical removal of conceptuses during late pregnancy, and after intracerebroventricular (i.c.v.) injection of placental lactogen (PL) before the prolactin surge. In intact control animals, prolactin secretion remained low until a nocturnal surge of secretion occurred in the dark period preceding parturition, peaking at 269±51 (s.e.m.) μg/l at 03.00 h on day 21. Progesterone levels fell from > 200 nmol/l on day 19 to <40 nmol/l by 12.00 h on day 20 of pregnancy. PL levels during late pregnancy were modified by partial or complete removal of conceptuses at 10.00 h on day 19 of pregnancy. Removal of all but one or two conceptuses did not change the normal pattern of prolactin or progesterone secretion. Removal of all conceptuses, however, induced a large nocturnal surge of prolactin secretion, peaking at 211·7±78 μg/l at 03.00 h on day 20, 24 h earlier than the surge in intact animals. Progesterone levels after removal of all conceptuses fell to <40 nmol/l by 23.00 h on day 19, approximately 12 h before the decline in intact animals. Maintenance of increased progesterone levels after conceptus removal using silicone tubing implants significantly (P <0·05) reduced the peak of the premature prolactin surge to 79·7 ±18 μg/l at 05.00 h on day 20. To determine whether PL could act at the hypothalamic level to inhibit the prolactin surge, human PL was injected into the lateral ventricle. PL injected i.c.v. at 17.00 h on day 7 of pregnancy completely abolished both the expected diurnal and nocturnal prolactin surges in the subsequent 24 h. By contrast, the same treatment on day 20 of pregnancy had no effect on the ante-partum prolactin surge. These results demonstrate two changes in the mechanisms controlling prolactin secretion on the last day of pregnancy compared with prolactin secretion during early pregnancy. First, feedback inhibition of prolactin secretion by PL which occurs at mid-pregnancy was not functional on the last day of pregnancy. Secondly, progesterone which promotes the nocturnal prolactin surges of early pregnancy, inhibited prolactin secretion during late pregnancy. Journal of Endocrinology (1991) 130, 401–407