Enhanced ovarian inhibin subunit gene expression in aging rats is due to chronic anovulation

The persistent-estrous (PE) state in aging rats, characterized by a lack of ovulation and absence of estrous cycles, is associated with enhanced inhibin alpha and beta A subunit mRNA expression in the ovaries. It has been shown that the PE state can be interrupted by successive treatments with a progesterone implant (P-implant) and that estrous cycles can be transiently restored after implant removal. The present study examined whether restoration of estrous cycles in PE rats could reverse the altered ovarian inhibin alpha and beta A subunit gene expression. PE rats were treated with subcutaneous P-implants for 6 wk. After implant removal, the return of estrous cyclicity was confirmed by characteristic cyclic changes in vaginal cytology. Ovaries collected from the P-implant-treated animals at 1100 h on diestrus Day 2 or proestrus showed significantly decreased levels of both inhibin alpha and beta A subunit mRNAs compared to those of PE controls and young cyclic females. In situ hybridization revealed that the decreased inhibin alpha subunit mRNA after P-implants was due to decreased gene expression in the granulosa cells of large preovulatory follicles and to a compete absence of gene expression in large, cystic follicles devoid of granulosa cells and oocytes. In addition, inhibin alpha subunit mRNA was expressed in the newly developed follicles after implant removal. The beta A subunit mRNA was detected only in maturing follicles, not in newly developing follicles or in the large cystic follicles. The patterns of ovarian inhibin alpha and beta A subunit gene expression mimicked those of cyclic animals. These data indicate that loss of estrous cycles in aging rats results in an overexpression of inhibin alpha and beta A mRNAs in large and anovulatory follicles and that reinstatement of ovarian cycles in aged rats restores inhibin gene expression to normal levels.