Sexual stimulation induces Fos immunoreactivity within GnRH neurons of the female rat preoptic area: interaction with steroid hormones.

We have shown previously that sexual stimulation (copulation with intromission or vaginocervical stimulation) induces c-fos mRNA and Fos-like immunoreactivity (IR) within estrogen-concentrating and nonconcentrating regions of the female rat forebrain, including regions that contain gonadotropin-releasing hormone (GnRH) neurons in septum and anterior preoptic area. The overall induction of Fos-like IR within these regions was specific to afferent sensory stimulation and did not require treatment with estrogen and progesterone. Because vaginocervical stimulation facilitates lordosis and increases the release of luteinizing hormone, the present study examined whether hormone treatment that promotes sexual receptivity, with or without sexual stimulation, increases Fos-like IR specifically within GnRH-containing neurons. Sexually experienced ovariectomized rats were administered estradiol benzoate (10 micrograms) 48 h and progesterone (500 micrograms) 4 h before either 1 h of paced copulation with a sexually vigorous male, 50 vaginocervical stimulations with a glass rod distributed over 1 h, or no stimulation. Control rats received injections of the oil vehicle. Fos-like IR was found within a significant number of GnRH-positive neurons in the anterior preoptic area caudal to the organum vasculosum following copulation with intromission or vaginocervical stimulation as compared with no stimulation. Although few GnRH cells coexpressed Fos following hormone treatment alone, this treatment enhanced the number of GnRH neurons that coexpressed Fos following vaginocervical stimulation as compared with the effect of vaginocervical stimulation in oil-treated rats. Together, these data indicate that estrogen and progesterone can augment the responsiveness of certain GnRH neurons to vaginocervical stimulation, consistent with the effects of sexual activity on GnRH release.