Termination of Pregnancy in Rats by a Prolactin Implant in Median Eminence

A prolactin-cocoa butter mixture was implanted once in the median eminence of rats on each of days 1–8 of pregnancy. Control pregnant rats were given similar implants of cocoa butter alone. When laparotomies were performed on day 15, all of the control rats were still pregnant. None of the rats implanted with prolactin-cocoa butter on days 1–6 remained pregnant, but almost all rats remained pregnant when implanted on days 7 or 8 of pregnancy. These results indicate that a prolactin implant in the median eminence, by reducing pituitary prolactin secretion, terminates pregnancy if given during the first 6 days of gestation. When prolactin was implanted in the median eminence of rats on the fourth day of gestation and 2 mg of progesterone were injected daily until day 11, pregnancy was maintained. When progesterone was withdrawn after day 11, and the rats were killed on day 15, it was evident that resorption of the fetuses had occurred. This indicates that the decrease in pituitary prolactin secretion produced by the implant of prolactin into the median eminence, resulted in luteal regression and reduced progesterone secretion.