In young adult female rats, the patterns of inhibin subunit mRNA expression during the estrous cycle are regulated by cyclic changes in gonadotropin secretion and follicular development. Since there are distinct alterations in profiles of both hormone secretion and follicu-logenesis during the reproductive lifespan of the female rat, we have characterized the gene expression and distribution of inhibin subunit mRNAs in immature (22 days), young adult cyclic (3-4 months), middle-aged cyclic (9-10 months), and old (12-13 months) persistent estrous (PE) rat ovaries. Northern blot analyses revealed that in contrast to young adult cyclic rats, inhibin a- and /3A-subunit mRNA levels in the ovaries of middle-aged cyclic rats remained substantially elevated after the proestrous gonadotropin surges and ovulation. Likewise, acyclic immature and old PE rats showed high levels of inhibin α- and βA-subunit transcripts in their ovaries. In situ hybridization analyses demonstrated that inhibin α- and βA mRNAs were abundantly expressed in maturing follicles of both young and middle-aged cyclic females, while high levels of α-subunit transcripts were only detected in the ovarian stroma of middle-aged animals. The ovaries of old PE rats had numerous large cystic follicles (with variable layers of granulosa cells) and few degenerating cyst-like structures (completely devoid of granulosa cells). Inhibin subunit transcripts were expressed abundantly in both the granulosa (α- and βA -subunits) and theca interna (α-subunit only) layers of large follicles, but were absent from degenerating cysts devoid of granulosa cells. The ovarian stroma of PE rats also expressed very high levels of inhibin-α, but not βA mRNA. The ovaries of immature rats contained large numbers of uniformly developing secondary follicles. High levels of inhibin-α mRNA were expressed homogeneously in the granulosa layer of all growing follicles, whereas inhibin βA mRNAs were only detected in selectively larger follicles with multiple layers of granulosa cells. Hormone RIAs of serum samples from these same groups of animals showed that basal levels of serum FSH were substantially higher in immature, middle-aged cyclic, and old PE rats than in young adult rats. These results demonstrate that enhanced ovarian inhibin subunit gene expression in the female rat is associated with increased serum FSH levels regardless of chronological age. On the other hand, aging appears to selectively enhance inhibin α, but not βA, gene expression in the ovarian stroma, such that it may gradually become a major secondary site of a-subunit mRNA production in addition to the follicular compartments.
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