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Changes in the immunoreactivity of follistatin within preovulatory follicles after the primary gonadotrophin surge in rats

Cyclic changes in follistatin localization and the role of the primary gonadotrophin surge in regulating follistatin expression were studied in rat ovaries by immunohistochemistry. Two different polyclonal antisera were raised against synthetic peptides corresponding to amino acids 123–134 and 300–315 of human follistatin. Immunoreactive follistatin was detected in granulosa cells of secondary and mature follicles. Although immunoreactions with anti-follistatin (300–315) occurred in preovulatory follicles until immediately before ovulation (23:00 h on the day of pro-oestrus), follistatin was not immunodetected in newly formed corpora lutea (11:00 h on the day of oestrus). Granule-like immunoreactions with anti-follistatin (123–134) serum in preovulatory follicles markedly decreased in intensity on the evening of pro-oestrus, indicating the loss of follistatin production. Blocking the primary gonadotrophin surge by a pentobarbitone injection (40 mg kg^–1) at 13:30 h on the day of pro-oestrus sustained the immunoreactivity with anti-follistatin (123–134) in preovulatory follicles up to 23:00 h on the day of pro-oestrus. Injection of pentobarbitone-treated animals with exogenous LH (100 µg kg^–1) or FSH (50 µg kg^–1) at 15:30 h on the day of pro-oestrus eliminated the immunoreactions. These results indicate that the expression of follistatin in preovulatory follicles is suppressed by the primary gonadotrophin surge during pro-oestrus. Hence, it is conceivable that the cessation of follistatin production in preovulatory follicles may occur before ovulation, and that it may be caused by the primary gonadotrophin surge.

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