Effects of paced mating on c-fos gene expression in the female rat brain

When estrous female rats control or pace (P) their sexual contacts with males, several neuroendocrine and behavioral responses to mating occur that are not observed or are greatly attenuated after nonpaced mating. The present study examined whether the distribution and amount of FOS immunoreactivity (FOS-IR) induced in brain by mating would be altered in females receiving paced rather than nonpaced mating stimulation. In the first experiment, females received 5 or 15 intromissions during paced mating tests (5P and 15P), 5 or 15 intromissions during nonpaced mating tests (5NP and 15NP), 15 mounts-without-intromission (MO) or remained in their homecages (HC). Selective increases 1 h after paced mating stimulation were observed in the posterodorsal medial amygdala (MePD), where significantly more FOS-IR cells were present in the 5P and 15P groups than in the respective NP groups. The 5P, 5NP and 15NP had significantly more FOS-IR than the HC, MO, and 5NP groups, and the 5P group had levels of FOS-IR which were equivalent to that seen in the 15NP group. In the posteromedial portion of the bed nucleus of the stria terminalis (BNSTpm) and the ventrolateral portion of the ventromedial nucleus of the hypothalamus (VMHvl), paced mating induced significantly greater numbers of FOS-IR cells than did either MO or HC treatments; increases induced by nonpaced mating were not statistically greater than HC controls. No differences between groups were seen in the medial preoptic area (mPOA). In the second experiment, experimentally lengthening the interintromission interval (III) as well as increasing the intromission duration to mimic the characteristics of paced mating, resulted in significant increases in FOS-IR in the MePD but not in the other three brain regions. These results demonstrate that paced mating is more effective in inducing c-fos expression than nonpaced mating, and that the MePD is particularly sensitive to differing characteristics of the mating stimuli received.