A Direct Effect of the Uterus on the Surges of Prolactin Induced by Cervical Stimulation in the Rat*

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Abstract
Electrical stimulation of the uterine cervix eventuates in a 13-day pseudopregnancy characterized by nocturnal and diurnal surges of PRL secretion and constantly elevated levels of serum progesterone. Upon removal of the uterus shortly after cervical stimulation (CS), both surges of PRL are continued beyond day 13, and decline of progesterone secretion by the pereisting corpora lutea is prevented. This study describes the effects of absence of the uterus and persistent secretion of progesterone on continuation of daily PRL surges. CS on proestrus (day 0) and estrus (day 1) and hysterectomy on day 5 resulted in nocturnal and diurnal PRL surges prolonged through day 16, while sham-hysterectomized (hyst) rats no longer showed surges on day 16. Cervically stimulated ovariectomized (OV) rats which were hyst on day 5 presented attenuated nocturnal surges of PRL until day 16. However, no diurnal surges were present at this time. Neither surge continued until day 16 in OV-CS-shamhyst rats. OV-CS rats with intact uteri responded to sc placement of progesterone-containing Silastic implants by presenting attenuated nocturnal surges of PRL until day 16. Diurnal surges were not present at that time. Neither surge was continued until day 16 in OV-CS rats bearing blank implants. However, in OV-CS rats, which were hyst and implanted with progesterone, magnified nocturnal surges and normal diurnal surges continued until day 16. If the nocturnal surges are to persist, either disinhibition from uterine influence or sufficient stimulation by progesterone in the presence of the uterus is required. On the other hand, both disinhibition from uterine influence and the presence of progesterone secreted by the maintained corpora lutea are required for prolongation of normal diurnal PRL surges.