Fos expression in the female rat brain during the proestrous prolactin surge and following mating

A prolactin (PRL) surge occurs in the female rat during proestrus in respon se to elevated estradiol levels. The elevated release of ovarian steroids o n the day of proestrus is also associated with sexual receptivity. Mating t riggers twice-daily PRL surges that supplant the proestrous PRL surge and a re responsible for maintaining luteal function du ring the fi rst half of p regnancy. In order to understand the neuronal mechanisms controlling the pr oestrous- and mating-induced PRL surges, we examined patterns of Fos expres sion by immunocytochemistry in specific brain regions as a measure of neuro nal activity. Intact female rats were sacrificed at 09.00, 15.00, and 18.00 h on the day of proestrus and the day of diestrus. Brain tissues were also collected at 21.00 h on the day of proestrus from rats receiving intromiss ions or mounts from males or taken directly from their home-cage. On the da y of proestrus, the number of neurons expressing Fos in the medial preoptic area (mPOA), medial amygdaloid nucleus (mAMYG), and ventrome- dial nucleus of the hypothalamus (VMH) was few and was associated with low plasma PRL l evels at 09.00 h; however, the number of Fos-positive cells in these brain regions significantly increased at 15.00 and 18.00 h when the proestrous PR L surge occurred. Mating during the evening of proestrus resulted in a dram atic increase in the number of Fos-positive cells in the mPOA, mAMYG, and V MH as well as in the arcuate nucleus (ARC). Non-mated animals in diestrus s howed low and unchanged PRL levels and Fos expression in all the brain area s throughout the day. These results suggest that the mPOA, mAMYG, VMH and A RC may be important brain sites for the integration of stimuli associated w ith proestrous- and mating-induced PRL surges.