An Ovarian Role in Prolonging and Terminating the Two Surges of Prolactin in Pseudopregnant Rats*

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Abstract
Daily nocturnal (N) and diurnal (D) surges of PRL are characteristic of a pseudopregnancy (psp) induced by cervical stimulation (CS). These semicircadian surges persist for 13–14 days, after which time they cease, and psp ends. The purpose of this study was to describe the involvement of the ovarian steroids in the maintenance and termination of the N and D surges of PRL. CS of long term ovariectomized (OVX) rats resulted in N and D surges of PRL lasting 14 days. By day 16 after CS, neither surge of PRL was present. The sc placement of Silastic implants containing large quantities of progesterone in long term OVX-CS rats sustained and enhanced the N surge of PRL until at least day 16. The D surge was also sustained until day 16, but its magnitude was normal. Estradiol alone was capable of maintaining the D but not the N surge for this same time interval. However, pharmacological quantities of estradiol restricted the magnitude of the N surge generated in the progesterone-implanted animal. Since neither surge of PRL was present by day 14 after CS in intact rats, but at least the nocturnal surge was still present at this time in long term or acutely OVX rats, this implies an active role of ovarian steroids in the termination of PRL surges at the end of psp. At the end of psp, luteal progesterone secretion declines and follicular estradiol secretion increases. Varying sizes of Silastic implants containing progesterone or estrogen were placed into psp animals after acute ovariectomy on day 9 to approximate this ovarian steroid secretory pattern of intact psp rats. Only when the decrease in progesterone was coupled with a modest increase in estrogen was there a total cessation of the two daily surges of PRL in OVX-CS rats. The data suggest that the PRL surges end in the intact psp rat due to the waning response to CS as well as the shifting steroid ratio represented by the fall in progesterone in the presence of a small amount of estrogen.