We tested the hypothesis that ERalpha and ERbeta mRNAs in the rat ovary are regulated during the post-natal period and in immature rats in response to estrogen treatment. Total ovarian ERbeta mRNA was more abundant than ERalpha mRNA and expression of ERbeta increased between post-natal days 4 and 12, coinciding with advancing folliculogenesis and an increase in granulosa cell numbers. In contrast, ERalpha mRNA levels remained relatively constant during this period. In situ hybridisation studies localised both ERalpha and ERbeta to granulosa cells of growing follicles, in 25 day old ovaries, although not all granulosa cells in a follicle or all follicles expressed the ERs. Diethylstilboestrol (DES) administered in vivo to 21 day old rats, for up to 4 days, did not significantly alter the expression of either ER as determined by RT-PCR, despite a 5.5-fold increase in granulosa cell number in these ovaries. In situ hybridisation studies established that DES-treatment down-regulated granulosa cell ER mRNAs. RT-PCR analyses on isolated granulosa cells confirmed that ERalpha was significantly down-regulated by DES. The predominance of ERbeta over ERalpha in the ovary and the regulation of ERbeta mRNA expression during ovarian development, is consistent with an important biological role for ERbeta in granulosa cell proliferation and differentiation.
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