Modulation of Rat Inhibin mRNAs in Preovulatory and Atretic Follicles
Inhibin α and βA mRNA levels can be correlated with the maturational status of the antral follicle. Two distinct classes of follicles in which the levels of inhibin α and βA mRNAs decrease have been identified by in situ hybridization. They are preovulatory follicles found on the evening of proestrus, and atretic follicles found at all times during the rat estrous cycle. Inhibin α and βA mRNA levels decline in mature follicles following the preovulatory gonadotropin surges on the evening of proestrus. Administration of a GnRH antagonist to block the primary gonadotropin surges results in elevation of both inhibin α and βA subunit mRNAs through the following morning (estrus). Replacement of exogenous LH or FSH causes a decline in inhibin α and βA subunit mRNA levels 4 h following treatment. We conclude that the primary gonadotropin surges are important for appropriate regulation of inhibin gene expression. In addition to the late proestrus decrease in inhibin mRNA levels, we have observed that inhibin mRNA levels decrease in atretic follicles. Follicles of the type 1 a class entering the earliest stages of atresia express α mRNA at low levels and βA at levels not significantly greater than background. Follicles in later stages of atresia do not express detectable levels of either inhibin subunit mRNA.
Ovarian hormones function as feedback modulators to ensure precise communication between the ovary, pituitary, and hypothalamus. Inhibin is one such gonadal hormone. Inhibin is a dimeric glycoprotein, composed of an a subunit and one of two highly related β subunits (βA or βB), that acts to suppress the secretion of the pituitary hormone FSH (1,2). We have examined the expression of inhibin in the rat ovary, using in situ hybridization to measure the levels of α and βA subunit mRNAs under a variety of physiological conditions.
We previously demonstrated that expression of the inhibin subunit mRNAs is precisely modulated throughout the rat estrous cycle (3,4). The levels of inhibin subunit mRNAs increase progressively from estrus to proestrus, culminating in mature, healthy, graafian follicles on the afternoon of proestrus, and decline between 1800 and 1900 h of the same day. The dramatic decline of the inhibin mRNAs on the evening of proestrus is coincident with stimulation of the follicles by the primary gonadotropin surges. We, therefore, investigated the role of LH and FSH in the negative regulation of the inhibin α and β mRNAs.
A second class of follicles was identified in which the inhibin subunit mRNA levels decreased, the atretic follicles. We have characterized expression of the inhibin subunit mRNAs in these follicles, and find evidence for differential expression of inhibin α and β subunit mRNAs.
KeywordsGranulosa Cell Antral Follicle GnRH Antagonist Subunit mRNAs Atretic Follicle
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