Rapid Changes in the Expression of Inhibin α-, βA-, and βB-Subunits in Ovarian Cell Types During the Rat Estrous Cycle

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Abstract
Distributions of inhibin α-, βA-, and βB-subunits in different ovarian compartments were studied in cycling female rats by in situ hybridization with complementary RNA probes and using immunohistochemical localization with antibodies selective for each inhibin subunit. Consistent with earlier studies showing inhibin production by granulosa cells of maturing follicles, we also detected mRNAs for inhibin α-, βA-, and βB-subunits in granulosa cells of these follicles. However, based on immunohistochemistry and in situ hybridization, we found that inhibin α- is not only expressed in granulosa cells of mature follicles but in follicles at all stages of maturation, including primary to tertiary follicles. A number of primordial follicles also contained α mRNA and immunodetectable α-subunit. Interestingly, theca interna and interstitial gland cells contained inhibin α mRNA and α-subunit. Low levels of inhibin α immunoreactivity as well as specific hybridization to the complementary inhibin α mRNA probe were observed in newly formed luteal tissue. β-Subunits, on the other hand, were detected exclusively in granulosa cells of healthy tertiary follicles. The changes in expression of inhibin α-, βA-, and βB-subunits were more pronounced during the follicular phase of the cycle: inhibin α reached its highest level in granulosa cells, theca interna, and interstitial gland cells a few hours after the LH/FSH surge, while at the same time the β-subunits decreased dramatically in granulosa cells of mature follicles. Immediately before ovulation (estrus 0200 h), the α-subunit sharply declined in preovulatory follicles and was present mainly in granulosa cells from nonovulatory follicles at various stages of maturation. At that time, the βA- and βB-subunits could not be detected in preovulatory follicles but were localized mainly in small tertiary follicles (<300 μm). Unlike for the α- and βB-subunits, βA mRNA and immunoreactivity was present in large tertiary follicles (∼600 μm) immediately before ovulation. The present findings support the hypothesis that a decrease in inhibin production could be responsible for the secondary FSH surge observed early on estrus. This could be initiated by a change in the ratios of activininhibin production by decreasing first, the levels of β-subunits, second, the levels of α-subunit, and third, by a resurgence of activin A produced mainly by granulosa cells from large tertiary follicles. However, it is expected that at all stages of the estrous cycle α/β- and β/β-dimers are produced by the ovary, and that changes in circulating FSH are due to variations in the overall production of the respective subunits.

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