The Temporal Relationship between Implantation and Termination of Nocturnal Prolactin Surges in Pregnant Lactating Rats*

Abstract

The timing of cessation of nocturnal prolactin (PRL) surges was studied in rats with various lengths of delay in implantation. In order to manipulate the timing of implantation, female rats were mated during the postpartum estrus and allowed to nurse their pups for varying intervals. In Exp [experiment] 1, mothers were separated from their pups either on day 0 (the day on which sperm was found in the vaginal smear), or on day 5, 7, or 9, which resulted in implantation on day 5, 7, 9 or 11, respectively. In Exp 2, mothers remained with their pups, and those mothers in which spontaneous implantation occurred after day 11 were used. To determine the exact time at which the hypophysis ceases secreting nocturnal prolactin surges, blood samples were taken via a chronic intracardiac cannula twice daily at 0200 and 0400 h for 6 consecutive days. When implantation occurred on day 5 of lactational pregnancy, the last nocturnal PRL surges occurred on day 10, as in normal pregnancy. In the groups in which implantation occurred after day 5 the nocturnal PRL surges terminated 5 days after implantation. In Exp 3, hypophysectomy was performed on the day of the last nocturnal PRL surge or on the following day in 3 groups of females corresponding to the last 3 groups in Exp 1. The 1st day on which the pituitary could be removed without causing abortion was, in each case, the 1st day on which the nocturnal PRL surge was absent. The strong correlation between the timing of implantation and the termination of pituitary PRL surges suggests that at a constant interval after implantation, rat placental lactogen levels are sufficiently high to inhibit PRL secretion by the pituitary.