INDUCTION OF FOS IMMUNOREACTIVITY IN OXYTOCIN NEURONS AFTER SEXUAL-ACTIVITY IN FEMALE RATS

Several lines of evidence suggest that centrally released oxytocin (OT ) may act in the hypothalamic ventromedial nucleus (VMN) to facilitate sexual behavior in female rats primed with estradiol and progesterone . The present experiment used a marker of neuronal activation, express ion of the transcription factor FOS, to locate OT neurons activated du ring sexual behavior. Sexual behavior significantly increased the perc entage of OT neurons in the hypothalamic paraventricular nucleus (PVN) expressing FOS in rats treated with estradiol and progesterone, compa red to hormone-treated, nonmated controls. The OT neurons colabelled w ith FOS immunoreactivity were found in regions of the PVN, but not in the supraoptic nucleus. As reported by others, FOS expression also was induced in the preoptic area and the VMN. Taken together with other e vidence, the results are consistent with the hypothesis that endogenou s OT is involved in female sexual behavior, and suggest that the sourc e of oxytocinergic innervation to the oxytocin receptors in the VMN ma y be neurons in the PVN. Interestingly, FOS expression in the VMN occu rred in cells in the vicinity of OT fibers.