Indispensable Role of Peripheral Progesterone Level for the Occurrence of Prolactin Surges in Pseudopregnant Rats

Abstract

Ovariectomy was performed in pseudopregnant (psp) rats on Day 1, 4 or 6 of psp (ovx-1, ovx-4 or ovx-6). Daily prolactin surges measured at 0500 h in ovx-1 rats occurred for 6 days similar to surges in chronically ovariectomized rats. However, prolactin surges at 0500 h in ovx-4 and ovx-6 rats stopped occurring immediately after ovariectomy. Progesterone supplementation by Silastic implants before the end of normal psp extended the occurrence of prolactin surges if the peripheral progesterone level was elevated to 80 ng/ml serum, but not when elevated to 20 ng/ml serum. Pseudopregnancy was extended up to 20 days by induction of ovulation on Day 6 of psp by hCG treatment. In these rats, prolactin surges lasted as long as peripheral progesterone stayed at an elevated level, namely until Day 18. These results suggest that neuroendocrine programming of cervical stimulation for the induction of prolactin surges is not retained for more than 4 days under a usual psp condition where corpora lutea start to secrete elevated amounts of progesterone from the afternoon of Day 3. After this period, the peripheral progesterone level must be solely responsible for the further maintenance of prolactin surges.