Seasonal expression of progesterone receptor in the frog brain in relation to circulating progesterone and estradiol

As a prerequisite for a central progesterone involvement in the vitellogenesis regulation, we investigated the presence of a progesterone receptor (PR), the seasonal expression in the female frog brain of Rana esculenta, at the different circulating steroid pattern and following steroids treatment. The progesterone binding moiety showed seasonally a typical characteristics of a true receptor in both cytosol and nuclear extract samples. ³H-Progesterone binding show the presence of one type of binding site with high affinity. Competition studies showed that in the nuclear extract, progesterone was the best competitor, followed by R 5020. Testosterone, deoxycorticosterone, 17α-hydroxyprogesterone and the antagonists 2914-R2 and RU486 were not effective whereas in the cytosol, progesterone, testosterone and deoxycorticosterone all competed to the same extent. The mainly located PR brain neurons and the maximum binding sites resulted in the hypothalamus at any phase of the reproductive cycle. The highest level of frog brain PR corresponds to a low levels of circulating progesterone when vitellogenin synthesis kept at its minimum and appears lower when the follicular growth proceeds and the progesterone and estradiol levels increase. Estradiol and progesterone treatments demonstrated frog brain PR down-regulation and indirectly provide evidence of progesterone involvement by receptor in the vitellogenin modulation.