Progesterone treatment increases Fos-immunoreactivity within some progestin receptor-containing neurons in localized regions of female rat forebrain

In female rats, the sequential release of estradiol and progesterone from the ovaries is required for the expression of sexual behavior during the estrous cycle. Many of the neuronal effects of estradiol and progesterone involve estrogen and progestin receptors. Treatment with a behaviorally-effective dose of estradiol increases Fos expression, suggestive of neuronal response, and subsequent treatment with a behaviorally-effective dose of progesterone further increases Fos expression within a few hours in female rat brain. In order to determine if neurons that respond to progesterone with increased Fos expression also contain progestin receptors, we used a double-label immunofluorescent technique to label both progestin receptors and Fos protein following progesterone or vehicle treatment of estradiol-primed female rats. As shown previously, progesterone treatment increased Fos expression in progestin receptor-containing regions, such as the ventromedial nucleus of the hypothalamus and the medial preoptic area. In addition, progesterone treatment induced a statistically-significant increase in Fos-immunoreactivity within progestin receptor-containing cells in the medial preoptic area and the ventromedial nucleus of the hypothalamus, but not in the arcuate nucleus. Therefore, many but not all of the neurons that respond to progesterone with increased Fos expression also contain progestin receptor-immunoreactivity. The progesterone-induced Fos expression within progestin receptor-containing neurons may or may not be associated with the effects of progesterone on sexual or other reproductive behaviors, as it remains to be tested. However, the Fos expression provides a useful marker to aid in identification of neurons that respond to a behaviorally-relevant dose of progesterone.