Termination of prolactin surges with development of placental lactogen secretion in the pregnant rat.

ABSTRACT

It has been hypothesized that it is rat placental lactogen (rPL) which causes PRL surges to terminate at mid-pregnancy. Using a hormonally induced model of delayed implantation, the temporal relationship between the secretion of rPL and the nocturnal PRL surges was followed. Implantation was prevented by removing the ovaries, the source of estrogen, on day 3 of pregnancy. Blastocysts were maintained free-floating in the uterine lumen by sc injection of 4 mg progesterone in oil daily for 0, 5, 7, or 9 days. Implantation was induced and the subsequent pregnancy was maintained with 1 microgram estrone plus 4 mg progesterone daily. Nocturnal PRL surges (0500 h) were followed for 10 days after the first estrone injection. Control animals last exhibited PRL surges on day 10. Animals with 5, 7, or 9 days of implantation delay had their last PRL surge on days 15, 17, and 18, respectively. Levels of rPL in control animals, as measured by Nb2 lymphoma cell bioassay, were low on day 6, slightly higher on day 10, and significantly elevated on day 12. Delaying implantation delayed the increase in rPL secretion in the experimental groups. This proportionately prolonged the number of days the PRL surges were present. These data suggest that the termination of the nocturnal PRL surges requires the secretion of rPL by the developing conceptus.