Expression and Regulation of mRNA for Inhibin/Activin α- and βA-Subunits in the Granulosa Layer of the Two Largest Preovulatory Follicles during the Hen Ovulatory Cycle

In order to understand the role of inhibin and activin in regulating follicular development in the hen, the steady-state mRNA levels of inhibin/activin α- and β_A-subunits in the granulosa layer of the largest (F₁) and second largest (F₂) follicles of the hen were investigated at 4-hr intervals throughout the ovulatory cycle. In addition, because it was hypothesized that luteinizing hormone (LH) regulated β_A-subunit expression, the effect ofin vivoadministration of ovine LH (oLH) on the expression of these subunits during the early- and mid-ovulatory cycle was examined. Northern blot analysis, using³²P-labeled cDNA probes of chicken inhibin/activin α- and β_A-subunits and glyceraldehyde-3-phosphate dehydrogenase (GAPDH, internal control), revealed that in the F₁follicle, the relative level of β_A-mRNA (n = 3) was low at 23.5 hr and increased (P < 0.05) at 19.5, 15.5, and 11.5 hr before the next predicted ovulation. It then decreased (P < 0.05) at 7.5 hr and was further reduced at 3.5 and 0.5 hr prior to ovulation. In the F₂follicle, β_A-mRNA was maintained at a basal level throughout the sampling period except for a brief increase (P < 0.05) at 0.5 hr before ovulation. In contrast to the β_A-subunit, inhibin α-mRNA was abundantly expressed with no significant variations throughout the ovulatory cycle in either the F₁or the F₂follicle. When oLH was injected at 18 hr before ovulation, 200 but not 100 or 50 μg/kg (n = 3 hens per dose) significantly (P < 0.05) reduced the β_A-mRNA level in the F₁follicle by 2 hr after injection compared to the control (saline). The experiment was repeated at 12 hr before ovulation and both 100 and 200 but not 50 μg/kg oLH significantly (P < 0.05) reduced the expression of β_A-subunit mRNA with no significant difference between 100 and 200 μg/kg oLH. In contrast to the β_A-subunit, inhibin α-subunit mRNA was abundantly expressed and not affected by oLH treatment. Our data indicate that the expression of inhibin/activin β_A- but not α-subunit mRNA is developmentally regulated in the granulosa layer of the two largest follicles during the hen ovulatory cycle. In addition, LH may participate, directly or indirectly, in negative regulation of the β_A-subunit.