ESTRADIOL AND PROGESTERONE INFLUENCE THE RESPONSE OF VENTROMEDIAL HYPOTHALAMIC NEURONS TO TACTILE STIMULI ASSOCIATED WITH FEMALE REPRODUCTION

Stimulation of the vagina and cervix, provided by the male during copu lation or manually with a probe, causes many behavioral and endocrine changes associated with female reproduction in rats. Previously, we fo und that vaginal-cervical stimulation (VCS), by mating or manual probi ng, increases the expression of Fos-immunoreactivity (Fos-IR) in discr ete populations of neurons in the preoptic area, mediobasal hypothalam us and midbrain, suggesting that these neurons respond to VCS. The pur pose of the present study was to determine if hormonal priming would i ncrease the number of Fos-IR cells following VCS. Contrary to our hypo thesis, in Experiment 1 priming animals with a behaviorally effective dose of 17 beta P-estradiol benzoate followed 48 h later by progestero ne caused a trend towards a decrease in the number of VCS-induced Fos- IR cells in the ventromedial hypothalamus. In Experiment 2, which was done to confirm this decrease in VCS-induced Fos-IR neurons by hormone s, this effect was found to be statistically significant. Furthermore, this hormone-induced decrease in VCS-responsive cells was localized t o the ventromedial nucleus of the hypothalamus, an area rich in estrog en and progestin receptors. No effects of hormone treatment on VCS-ind uced Fos-IR were observed in any other brain regions analyzed. These f indings suggest that steroid hormones may elicit some of their effects on female reproductive behavior and physiology by altering the respon siveness of ventromedial nucleus neurons to vaginal-cervical stimulati on.