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 α and β(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 α and β(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 α and β(A) subunit mRNAs compared to those of PE controls and young cyclic females. In situ hybridization revealed that the decreased inhibin α subunit mRNA after P-implants was due to decreased gene expression in the granulosa cells of large preovulatory follicles and to a complete absence of gene expression in large, cystic follicles devoid of granulosa cells and oocytes. In addition, inhibin α subunit mRNA was expressed in the newly developed follicles after implant removal. The β-(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 α and β(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 α and β(A) mRNAs in large and anovulatory follicles and that reinstatement of ovarian cycles in aged rats restores inhibin gene expression to normal levels.