Estrogen receptors and aromatase activity in the hypothalamus of the female frog, Rana esculenta. Fluctuations throughout the reproductive cycle

It is well known that certain actions of androgen are mediated through in s itu aromatization to estrogen in neural target tissues. This study was unde rtaken to investigate androgen utilization in the hypothalamus of the femal e frog, Rana esculenta, through a quantification of estrogen receptors and aromatase activity during the reproductive cycle, H-3-estradiol-binding mol ecules were present in both the cytosol and the nuclear extract of the hypo thalamus. These molecules bound specifically H-3-estradiol with high affini ty (Kd 10(-10) M) and low capacity (cytosol: 1.2 +/- 0.4 fmol/mg protein; n uclear extract: 7.9 +/- 0.6 fmol/mg protein). Aromatase activity was detect ed in the microsomal fraction of the hypothalamus using a sensitive in vitr o radiometric assay. Both aromatase activity and nuclear estrogen receptor binding fluctuated in synchrony throughout the reproductive cycle. Western blot analysis of aromatase protein revealed one immunoreactive band with a molecular weight of approximately 56 kDa. In contrast to aromatase enzyme a ctivity, the relative levels of aromatase protein changed little during the reproductive cycle suggesting that post-translational mechanisms may be in volved in regulating estrogen synthesis in the frog brain. A possible role for estrogens in the modulation of the reproductive behavior in this specie s is suggested. (C) 2000 Elsevier Science B.V. All rights reserved.