Estrogen-independent and estrogen-induced progesterone receptors, and their regulation by progestins in the hypothalamus and pituitary of the chick embryo: an immunohistochemical study

Abstract

The effects of an anti-estrogen and of progestins on the progesterone receptor (PR) in the hypothalamus and pituitary of 10-day-old chick embryos were studied by immunohistochemistry with an antibody to the receptor. In a first experiment, to determine if endogenous estrogens are responsible for the early appearance of PR in the chick embryo, a continuous treatment with the anti-estrogen, Tamoxifen, was applied from day 0 of incubation. In the hypothalamus and pars distalis of the pituitary a Tamoxifen treatment (10 μg every other day from day 0) did not modify the distribution of PR-positive cells or the intensity of PR immunoreactivity (PR-IR), compared to oil-injected embryos. In contrast, the same treatment totally blocked the increase of PR-IR in embryos administered estradiol (10 μg on day 7). Thus, the estradiol-induced PR-IR is inhibited by Tamoxifen, whereas the natural appearance of PR is not. We conclude that, in the chick embryo, the basal expression of PR is estradiol independent. In a second experiment, the regulation of PR by its own ligands (progesterone and the synthetic progestin, R5020) was studied. Progesterone (150 μg), administered to embryos 16 or 48 h before sacrifice on day 10, induced a slight increase in PR-IR in hypothalamus and anterior pituitary, whereas R5020 had no effect. In embryos treated with estradiol on day 7, R5020 decreased the PR-IR to a level comparable to that of control embryos. Thus R5020, a non metabolizable progestin, down-regulates the estradiol-induced PR-IR, but has no effect on the estradiol-independent, naturally expressed PR. These results demonstrate that, in the hypothalamus and pars distalis of pituitary, the early basal level of expression of PR is independent of both estradiol and progestins. However, the two hormones exert a regulatory effect above the constitutive basal level: up-regulation by estradiol and down-regulation by progestins. The early appearance of PR in the central nervous system and its sensitive regulation by estradiol and progestins bring experimental evidence of a possible physiological role of progesterone during embryonic development.