Mating-induced increases in FOS protein in preoptic area and medial amygdala of cycling female rats.

Genitosensory stimulation received during mating initiates neural and endocrine changes necessary for pregnancy. The present study examined sites of induction of the proto-oncogene, c-Fos, after mating in the cycling female rat to determine neural sites activated by such stimulation that might be involved in pregnancy initiation. Two groups of cycling female rats were exposed to males on the evening of proestrus and remained with the male until receiving either 14 mounts-with-intromission including ejaculations (Intromissions) or a control number of mounts-without-intromission (Mounts-Only). These two mating treatments were previously shown to induce pregnancy/pseudopregnancy in 100% and 0% of the animals, respectively. Seventy-five minutes after mating, females were perfused intracardially with 2% paraformaldehyde/2.5% acrolein, and the brains were processed for FOS immunocytochemistry using standard procedures. FOS-immunoreactive cells (FOS-IR) were counted in standard template quadrilaterals in the preoptic area (POA), medial amygdala (mAMYG), and the paraventricular (PVN), ventromedial (VMN), and dorsomedial (DMN) nuclei of the hypothalamus at 50 x magnification using a camera lucida. Significantly higher numbers of FOS-IR cells were seen in Intromissions females above Mounts-Only females in the POA and mAMYG, demonstrating that c-Fos expression in these areas depended upon cervical-vaginal stimulation rather than on cutaneous somatosensory input received in Mounts-Only tests. The cells within the POA and mAMYG that are activated in response to the intromittive stimulus may be directly involved in pathways triggering one or several of the neuroendocrine responses to mating in the female rat.