Summary. Administration of progesterone (5 or 10 mg) to pregnant rats increased serum prolactin significantly on the afternoon of Days 4, 6, 7 and 8 of pregnancy, but had no effect on later days. On Day 10 progesterone administration increased serum prolactin only in rats treated with oestrogen the day before. A similar treatment with oestrogen and progesterone was unable to stimulate secretion on the afternoon of Day 13 of pregnancy. In rats from which the corpora lutea had been unilaterally removed and hence endogenous progesterone levels were 50% of the normal values, or in those that carried 4 conceptuses, progesterone treatment after oestradiol priming was partly effective in inducing prolactin release on Day 13. However, in rats ovariectomized, with bilateral excision of the corpora lutea, or with 2 conceptuses left, treatment with ovarian steroids markedly increased serum prolactin values. By Day 13 all the rats from the ovariectomized group or with bilateral excision of the corpora lutea had aborted. On Day 13, therefore, the high serum concentrations of feto-placental factors and of progesterone are responsible for the blockade of the spontaneous and ovarian steroid-induced prolactin release. On the other hand, on Day 16 of pregnancy the decrease of circulating progesterone by excision of the corpora lutea or by ovariectomy followed by oestradiol treatment significantly increased serum prolactin on Day 17. Removal of all the conceptuses did not modify the effects of these treatments.
The present results demonstrate different roles of progesterone upon the control of prolactin secretion. After a stimulatory action during the first days of pregnancy, there is a change to an inhibitory control at the end of pregnancy. At mid-pregnancy the association of maximal circulating concentrations of fetal-placental factors and progesterone prevents the spontaneous and ovarian steroid-induced release of prolactin.