Prolactin secretory surge during estrus coincides with increased dopamine activity in the hypothalamus and preoptic area and is not altered by ovariectomy on proestrus

The hormonal factors associated with converting a corpus luteum of estrous cycle into a corpus luteum of pseudopregnancy were studied by measuring LH and FSH prolactin, estradiol and progesterone levels in decapitated rats during the 4-day estrous cycle and a comparable time of pseudopregnancy (lights on 0600-0800 hr.). During the estrous cycle, prolactin, LH and FSH remained low and unchanging except on the afternoon of proestrus, when typical proestrous surges were observed. In contrast, estradiol levels began to increase on D-1, from baseline values of 7 pg/ml to approximately 15-20 pg/ml. These levels were maintained until the afternoon of D-2 when estradiol further increased to reach peak levels of 40-50 pg/ml by 0900 hr on proestrus. Estradiol then declined in relation to the increase in LH secreation and had returned to baseline by estrus. Progesterone secretion by the corpora lutea of the cycle also increased on the afternoon of D-1 and reached a maximum value of 25-30 ng/ml early on the morning of D-2. At this time, a precipitious fall in progesterone occurred, returning to baseline values of 5-1- ng/ml by 0700 on D-2 signifying the regression of the corpora lutea of the cycle. Progesterone remained low thereafter until the afternoon of proestrus when levels increased in response to the proestrus when levels increased in response to the proestrous surge of LH. Following cervical stimulation at 1900 hr on proestrus, no differences were noted, with respect to the estrous cycle, in LH, FSH or estradiol secreation through the afternoon of D-2. Surprisingly, progesterone levels did not differ in the cycle and pseudopregnancy until the early morning of D-29 instead of progesterone levels falling to baseline as they had during the cycle, the corpora lutea of pseudopregnancy were rescused, progesterone increasing dramatically to reach levels of 45-50 ng/ml by 1700 hr on that same day. The only difference in hormone secretion that was noted which could account for this marked divergence in progesterone secretion was the pattern of prolactin secretion following cervical stimulation. In contrast to the low levels seen during the estrous cycle, biphasio surges of prolactin secretion occured each day, one being nocturnal (0100-0900 hr) and the other diurnal (1500-2100 hr). The rescue of the corpus luteum occured in association with the nocturnal surge on D-2. These results suggest that nocturnal surge on D-2, PROLACTIN IS THE MAJOR Luteotropic stimulus which transforms and estrous cycle into pseudopregnancy by prolonging progesterone secretion from the corpus luteum. Moreover, if LH is important for progesterone secretion, no changes were observed in the pattern of LH secretion which can account for the rescue of the corpus luteum.

Although there is extensive evidence for effects of prolactin (PRL) on the brain, knowledge about the PRL receptor (PRL-R) in the brain is limited. By using monoclonal antibodies raised against purified rat liver PRL-R, the distribution of PRL-R was investigated by immunohistochemistry in brains of the estrogen-treated ovariectomized (OVX+E) rat and the adult male rat. Immunohistochemistry was performed by using the avidin biotinylated horse radish peroxidase macromolecular complex method. In both male and OVX+E rats, strong immunostaining was detected in the choroid plexus of all cerebral ventricles. This immunostaining was localized predominately on epithelial cell membranes. In the OVX+E female rat, scattered immunoreactive perikarya were observed in the arcuate nucleus, periventricular hypothalamic nucleus, preoptic area, suprachiasmatic nucleus, and supraoptic nucleus of the hypothalamus. Immunostaining in hypothalamic nuclei was localized on neuronal cell bodies as well as on neuronal processes. In addition, there was extensive PRL-R immunoreactivity throughout the globus pallidus and ventral pallidum. Immunostaining in these striatal regions was not associated with neuronal cell bodies but appeared to be localized on processes or glial cells. In the male rat, less immunostaining was observed in the hypothalamus, and there was no immunostaining in the corpus striatum. No significant staining was observed in the cerebral cortex, thalamus, or hindbrain of either male or OVX+E rats. The implication of PRL-R existence in these brain regions remains to be investigated.