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 f rom the ovaries is required for the expression of sexual behavior duri ng the estrous cycle. Many of the neuronal effects of estradiol and pr ogesterone involve estrogen and progestin receptors. Treatment with a behaviorally-effective dose of estradiol increases Fos expression, sug gestive of neuronal response, and subsequent treatment with a behavior ally-effective dose of progesterone further increases Fos expression w ithin a few hours in female rat brain. In order to determine if neuron s that respond to progesterone with increased Fos expression also cont ain progestin receptors, we used a double-label immunofluorescent tech nique to label both progestin receptors and Fos protein following prog esterone or vehicle treatment of estradiol-primed female rats. As show n previously, progesterone treatment increased Fos expression in proge stin receptor-containing regions, such as the ventromedial nucleus of the hypothalamus and the medial preoptic area. In addition, progestero ne treatment induced a statistically-significant increase in Fos-immun oreactivity within progestin receptor-containing cells in the medial p reoptic area and the ventromedial nucleus of the hypothalamus, but not in the arcuate nucleus. Therefore, many but not all of the neurons th at respond to progesterone with increased Fos expression also contain progestin receptor-immunoreactivity. The progesterone-induced Fos expr ession within progestin receptor-containing neurons may or may not be associated with the effects of progesterone on sexual or other reprodu ctive behaviors, as it remains to be tested. However, the Fos expressi on provides a useful marker to aid in identification of neurons that r espond to a behaviorally-relevant dose of progesterone.