Distribution and regulation by estrogen of progesterone receptor in the hypothalamus of the cat.

The diencephalon is critically involved in the estrogen-dependent receptor-mediated stimulation of respiration by progesterone in cats. To identify a neuroanatomic basis for this effect of progesterone, the diencephalon of the ovariectomized cat was examined immunohistochemically with an antiprogesterone receptor (anti-PR) monoclonal antibody. No immunostaining was found in ovariectomized animals pretreated with sesame oil alone. In contrast, numerous cells in the ventromedial aspect of the hypothalamus from cats pretreated with estradiol benzoate were PR immunoreactive. Thus, PR is induced by estrogen in hypothalamic neurons of cats. In animals pretreated with estradiol benzoate, the highest density of immunostained neurons was found throughout the infundibular nucleus, especially in the region of the mammillary recess of the third ventricle. PR-immunoreactive cells were also distributed throughout the periventricular nucleus, with the highest density located rostrally and immediately above the suprachiasmatic nucleus. Notably and in contrast to a number of other species (e.g. rat and guinea pig), only very few weakly stained PR-containing cells were found in the hypothalamic ventromedial nucleus. This latter finding could reflect the progesterone independence of sexual behaviors in cat. Overall, we have identified hypothalamic areas that may subserve estrogen-dependent receptor-mediated effects of progesterone in the cat, such as the stimulation of respiration.