Fos Expression in the Female Rat Brain during the Proestrous Prolactin Surge and following Mating

Abstract

A prolactin (PRL) surge occurs in the female rat during proestrus in response to elevated estradiol levels. The elevated release of ovarian steroids on the day of proestrus is also associated with sexual receptivity. Mating triggers twice-daily PRL surges that supplant the proestrous PRL surge and are responsible for maintaining luteal function during the first half of pregnancy. In order to understand the neuronal mechanisms controlling the proestrous- and mating-induced PRL surges, we examined patterns of Fos expression by immunocytochemistry in specific brain regions as a measure of neuronal 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 intromissions or mounts from males or taken directly from their homecage. On the day of proestrus, the number of neurons expressing Fos in the medial preoptic area (mPOA), medial amygdaloid nucleus (mAMYG), and ventromedial nucleus of the hypothalamus (VMH) was few and was associated with low plasma PRL levels 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 PRL surge occurred. Mating during the evening of proestrus resulted in a dramatic increase in the number of Fos-positive cells in the mPOA, mAMYG, and VMH as well as in the arcuate nucleus (ARC). Nonmated animals in diestrus showed low and unchanged PRL levels and Fos expression in all the brain areas throughout the day. These results suggest that the mPOA, mAMYG, VMH and ARC may be important brain sites for the integration of stimuli associated with proestrous- and mating-induced PRL surges.

c-<i>fos</i> Expression in the Forebrain after Mating in the Female Rat Is Altered by Adrenalectomy

“…The present results confirm previous data showing that mating with VCS induces c- fos expression in the hypothalamus and other limbic structures [3, 6, 9, 21]. As we previously showed, VCS induced a significantly greater number of FOS-IR cells in adrenal-intact animals than both the HC and MO control groups in the MePD [3, 6].…”

“…In the BNST, mPOA and VMH, 15I were needed to see significant FOS increases above HC levels, but these increases were not statistically different from the MO group. The lack of a significant increase over MO control levels after 15I differs from several previous reports [3, 6, 7, 21]. Our present findings could be the result of increased variability among the 5I and 15I animals due to differences in the numbers of mounts-without-intromission received (table 1), or other factors, such as hormonal status, time of testing, or differential handling procedures between experimenters.…”

“…Previous studies had shown that mating stimulation increased the number of FOS-IR cells present in the MePD, BNST, mPOA, VMH, PVN and ARC [3, 6, 9, 21, 22]. The MePD, BNST, mPOA, PVN and ARC have also been shown to be activated by stress [18, 19, 20, 40, 41, 42].…”

“…The BNST, MePD, PVN, ARC and mPOA are known for their involvement in hypothalamic-pituitary-adrenal axis (HPA) responses to stress [18, 19, 20]and have been shown to be activated by mating [6, 21, 22]. In addition, stress has been shown to alter PRL release in ovariectomized estrogen-treated rats by increasing its plasma level when applied in the morning, when PRL levels are low, and by suppressing it when applied in the afternoon, when the plasma PRL levels are high [23, 24].…”

Functional Internal Complexity of Amygdala: Focus on Gene Activity Mapping After Behavioral Training and Drugs of Abuse

“…It has been shown that mounts without intromissions are sufficient to induce the elevated c-Fos expression in the medial amygdala (349). However, in some studies the graded fashion of c-Fos expression in the medial amygdala with respect to the type of contact and the numbers of received intromissions was shown (102,150,375,413,520). On the other hand, Baum and Everitt (26), who compared c-Fos activation in noncopulating male rats versus males who had free access to females though did not mount versus males copulating to ejaculation, observed that the level of c-Fos expression in the medial amygdala did not depend on the degree of copulatory experience.…”