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Regulation by estradiol of the progesterone receptor in the hypothalamus and pituitary: an immunohistochemical study in the chicken.

An antibody (IgG-RB) against the chick oviduct progesterone receptor (PR) was used to study the regulation of PR by estrogen in chicken pituitary and hypothalamus. PR was revealed exclusively in cell nuclei, whatever the hormonal state, including the absence of progesterone. In immature untreated chickens, PR was not detected in the pars distalis, while the pars tuberalis, preoptic area, and infundibular region of hypothalamus showed IgG-RB-immunoreactive cells or neurons. Estrogenic stimulation induced the appearance of PR in cells of the pars distalis and increased the immunoreactivity in the hypothalamus of young chickens of both sexes. After hormonal withdrawal, PR immunostaining returned to the level in untreated immature animals. In young hens, before they laid the first egg, PR appears progressively in pars distalis cells during the pubertal period. Antibodies to pituitary hormones (LH, FSH, GH, PRL, and ACTH) were used to characterize the secretory properties of PR-containing cells. LH- and FSH-immunoreactive cells were localized throughout the pars distalis in untreated animals. After estradiol treatment of young sexually immature chickens, immunostaining of LH and FSH was strongly reduced, up to extinction, in many gonadotropic cells, and only few PR-containing cells demonstrated some immunoreactivity in the cytoplasm. In contrast, in juvenile hens, a majority of PR-containing cells were identified as LH immunoreactive, that is gonadotrophs. There are probably two reasons for the paucity of doubly reactive cells in estradiol-treated chickens. One is technical, the optimal fixation time for both LH (greater than 20 h) and PR (less than 7 h) makes it practically impossible to reveal the hormone and the receptor at the same time. The other is related to the physiology of the system, which involves the simultaneous decrease in LH immunoreactivity and the PR induction in gonadotrophs by estradiol. The presence of PR in gonadotrophs suggests a direct feedback mechanism of sex steroids on pituitary cells in addition to the indirect effect through GnRH modulation. The hormonal content of PRL-, GH-, and ACTH-producing cells was not modified by estradiol treatment, as judged by the appropriate immunoreactivity, and their nuclei did not display PR. However, PRL cells and PR-containing cells were frequently present near each other within the same cell cords.