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The secretory patterns of prolactin and progesterone and the participation of ovarian steroids for their secretion in the early phases of delayed-pseudopregnancy (D-PSP) were studied in rats. D-DSP or immediate pseudopregnancy (I-PSP) was induced by vaginal stimulation (VS) given at 14:00 on diestrus 2. Blood for prolactin and progesterone analyses was obtained by decapitation performed every 2 hours between 13:00 on diestrus 2 and 13:00 on proestrus of the estrous cycle and during the corresponding stages of PSP. After the VS given on diestrus 2, concentrations of plasma prolactin and progesterone in D-PSP rats were the same as those of normal cyclic rats. In the rats given VS on estrus (Day 0) to produce I-PSP, a nocturnal surge of prolactin was obvious in the morning of Day 3. Plasma progesterone levels were also high in these rats. However, in the ovariectomized-adrenalectomized (OVX-ADX) rats, on which the operation was performed at various stages of the estrous cycle, a nocturnal surge of prolactin was clearly observed after the VS given at 14:00 on diestrus 2. The appearance of a nocturnal prolactin surge by the VS in the OVX-ADX rats was inhibited by the administration of estradiol-17 beta (1 microgram/rat). The nocturnal surge of prolactin in the early phase of I-PSP was also diminished by the administration of the steroid. Concomitant administration of progesterone with estrogen, however, overcame the inhibition of the nocturnal surge of prolactin by estrogen. These results indicate that the stabbing secretion of prolactin observed in the early phases of pregnancy or pseudopregnancy is modulated by the ovarian steroids and also suggests that the increasing secretion of estrogen from diestrus 2 to proestrus in the normal estrous cycle suppresses the prolactin surge expected by VS and resulting in D-PSP.

The Control of Prolactin Surge by Ovarian Steroids in the Early Phase of Pseudopregnancy in Rats