The amphibian (Rana esculenta) brain progesterone receptor: relationship to plasma steroids and vitellogenic cycle during the gonadal recovery phase

We report the presence of a progesterone receptor (PR) in the brain of the female amphibian Rana esculenta and changes in the levels of this PR during the gonadal recovery phase in relation to different circulating steroid levels and steroid treatment. The highest level of the PR in the nuclear brain extract corresponded to a low level of plasma progesterone and occurred when vitellogenin synthesis was at a minimum. The lowest level was found during follicular growth, concomitant with increased plasma 17-estradiol and progesterone levels. The PR levels were significantly higher in ovariectomized females than in intact and sham-operated females. Treatment with 17-estradiol and progesterone down-regulated the frog brain PR, indicating that a possible progesterone–receptor interaction is involved in the modulation of vitellogenin.