Regulation of Nocturnal Prolactin Surges during Pregnancy in the Rat*

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Abstract
The abrupt termination of the nocturnal PRL [prolactin] surge on day 11 of pregnancy in the rat may be caused by secretions from the uterine-placental unit. Removal of the uterus and its contents on day 6, 8 or 10 of pregnancy prolonged for several days the occurrence of the nocturnal PRL surge, measured at 0300 h. Even after the surge had disappeared in intact pregnant rats, hysterectomy on day 11 or 12 resulted in a resumption of the surge for several days. To determine whether PRL or PRL-like materials would inhibit the surge, bovine PRL or rat placental extract was injected into pregnant rats hysterectomized on day 10. Neither of these 2 treatments altered serum PRL levels. Implantation of human placental lactogen into the median eminence on day 8 of pregnancy abruptly terminated the nocturnal PRL surge and caused the pups to be aborted. To determine whether progesterone is responsible for the extended nocturnal PRL surges seen in pregnant rats after hysterectomy, progesterone in silastic tubing was implanted s.c. In control pregnant rats hysterectomized and ovariectomized on day 12, no nocturnal surge was present. In similarly treated rats which received progesterone, the nocturnal surge returned and was similar to that in hysterectomized rats with intact ovaries. Progesterone apparently is the signal to the brain to maintain the nocturnal PRL surge if the inhibitory factor from the uterine-placental unit is absent.