Contribution of endogenous inhibin to the decline of the secondary surge of follicle-stimulating hormone in the rat

The involvement of inhibin in the decline of the secondary surge of follicl e-stimulating hormone (FSH) was investigated in the rat. After ovariectomy or treatment with inhibin antiserum conducted at 2300 hours during pro-oest rus, plasma concentrations of FSH were maintained at high levels compared w ith control rats. However, plasma FSH started to decline at 0500 hours duri ng oestrus in both the groups. The same treatments conducted during metoest rus markedly increased plasma FSH after 24 h (twofold compared with the tre atments during pro-oestrus), suggesting that the treatments sufficiently de pleted circulating inhibin. To examine whether the decline of plasma FSH oc curred through a transcriptional mechanism or through a translational mecha nism, FSH-beta mRNA expression and the pituitary concentration of FSH were measured. Neither ovariectomy nor inhibin immunization conducted during the night of pro-oestrus, affected the pituitary concentration of FSH after 24 h, whereas a noticeable increase was observed after the treatments conduct ed during metoestrus. In both stages, both ovariectomy and inhibin immuniza tion significantly increased FSH-beta mRNA expression compared with control rats. In contrast with the pituitary concentration of FSH, the effect of i nhibin immunization on FSH-beta mRNA expression was not different between t he stages. The present data demonstrate the involvement of inhibin in the d ecline of the secondary surge of FSH, and suggest that a factor or factors other than inhibin may also be responsible for the fall in FSH. Changes in the pituitary concentration of FSH and FSH-beta mRNA expression suggest tha t post-transcriptional mechanisms may be involved in the suppression of FSH secretion during oestrus.