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, expression of the transcription factor FOS, to locate OT neurons activated during sexual behavior. Sexual behavior significantly increased the percentage of OT neurons in the hypothalamic paraventricular nucleus (PVN) expressing FOS in rats treated with estradiol and progesterone, compared to hormone-treated, nonmated controls. The OT neurons colabelled with 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 evidence, the results are consistent with the hypothesis that endogenous OT is involved in female sexual behavior, and suggest that the source of oxytocinergic innervation to the oxytocin receptors in the VMN may be neurons in the PVN. Interestingly, FOS expression in the VMN occurred in cells in the vicinity of OT fibers.