Progesterone receptor immunoreactivity in the brains of ovariectomized aged rats.

We examined the induction of progesterone receptor-immunoreactive (PR-ir) cells by estrogen in the rat preoptic area and ventromedial hypothalamic nucleus. Ovariectomized young (3-month-old) and old (24-month-old) female rats were treated with estrogen or cholesterol for 4 days. Estrogen significantly increased PR-ir cells in the preoptic area and ventromedial hypothalamic nucleus in young rats. Cholesterol-treated old rats had very few PR-ir cells; estrogen treatment significantly increased the number of PR-ir cells in both the preoptic area and the ventromedial hypothalamic nucleus in old rats, although less than in young rats. Therefore, the ability of estrogen to induce PR immunoreactivity in the hypothalamus in ovariectomized rats is attenuated in old rats compared with young rats.