PROGESTERONE ENHANCES AN ESTRADIOL-INDUCED INCREASE IN FOS IMMUNOREACTIVITY IN LOCALIZED REGIONS OF FEMALE RAT FOREBRAIN

In female rats, the onset of reproductive behavior depends on the sequ ential presence of estradiol followed by progesterone. Although treatm ent with high doses of estradiol has been shown to increase immunostai ning for the Fos protein, an immediate early gene product that is expr essed upon cellular activation, another report conflicts with this fin ding. However, the previous reports agree that subsequent treatment wi th progesterone has no apparent effect on Fos expression. In order to resolve this discrepancy and investigate possible effects of progester one, we used Fos immunocytochemistry combined with computer-aided imag e analysis. In experiment one, we found that treatment with 5 mu g of estradiol increased Fos immunoreactivity (Fos-IR) within a section of the medial preoptic area and the dorsal medial hypothalamus. Subsequen t treatment with 500 mu g of progesterone 1 hr before perfusion increa sed the intensity of the immunostaining within the medial preoptic are a and the dorsal medial hypothalamus, although it had no significant e ffect on Fos-IR cell number. In experiment 2, a lower concentration of Fos antiserum was used in order to diminish the immunostaining sensit ivity to a level in which no increase of Fos-IR cell number was observ ed after treatment with estradiol. Under these immunocytochemical cond itions, subsequent treatment with progesterone increased the number of Fos-IR cells in the medial preoptic area, the dorsal medial hypothala mus and the steroid receptor-rich area lateral to the ventromedial hyp othalamus. Thus, treatment with behaviorally effective doses of both e stradiol and progesterone induces Fos expression in localized regions of female rat brain.