EXPRESSION OF FOLLISTATIN MESSENGER-RNA BY SOMATOTROPES AND MAMMOTROPES EARLY IN THE RAT ESTROUS-CYCLE

We previously found follistatin (FS) mRNA in gonadotropes [predominantly in cells with luteinizing hormone (LH) antigens] and folliculostellate cells (with S100 antigens) in diestrous rats pituitaries. However, earlier in the cycle, when percentages of gonadotropes are lowest, percentages of cells expressing FS are 1.5-2-fold higher than in diestrus. This study was designed to detect FS mRNA and other pituitary antigens to identify the additional cells with dual in situ hybridization and immunolabeling protocols. The mRNA was detected with biotinylated complementary oligonucleotide probes and avidin-biotin-peroxidase complexes. Significant labeling for FS mRNA was found in cells with the following antigens: growth hormone (GH) (7% of pituitary cells); prolactin (PRL) (5%); S100 protein (5%); follicle-stimulating hormone (FSHbeta) (4%); LHbeta (3%); and thyroid-stimulating hormone (TSHbeta) (3%). Optimal conditions for detection included: overnight plating of >50,000 cells/well (24-well tray) in media containing 10% fetal bovine serum; hybridization at 37-degrees-C; and fixation in 2% glutaraldehyde. Whereas FS is expressed predominantly by LH gonadotropes at mid-cycle, FS mRNA can be expressed by all types of antigen-bearing cells earlier in the cycle. Its function in the pituitary may relate to its role in binding activin, which would result in inhibition of FSH release. However, since activin inhibits secretion of GH, PRL, and adrenocorticotropin (ACTH), FS may also control activin's effects on these cells. The FS-expressing cells may therefore be paracrine or autocrine regulators.