There are significant differences between rats and mice in the gonadal regulation of several aspects of gonadotroph function. To investigate whether these extend to the pretranslational regulation of FSH synthesis by gonadal steroids, we have measured FSH-β mRNA levels following gonadectomy and sex-steroid replacement and have related these to serum and pituitary FSH as a reflection of overall hormone synthesis.
In ovariectomized rats, FSH-β mRNA levels increased by 8 h, decreased, and then rose progressively over the next 28 days. A similar pattern of response was observed in orchidectomized rats. In mice, there were progressive increases in FSH-β mRNA levels in both males and females following gonadectomy, without evidence of the early peaks observed in rats. In both species, the change in FSH-β mRNA levels after gonadectomy was greater in females than in males. These changes in FSH-β mRNA following gonadectomy were paralleled by changes in the serum FSH concentration. In ovariectomized female rats and mice, pituitary FSH stores increased by 8 h and 3 days respectively, whereas in male rats, pituitary FSH content did not rise until 10 days after orchidectomy. The most striking species difference was the marked and prolonged reduction of pituitary FSH after orchidectomy of mice.
Treatment of rats and mice from the time of ovariectomy, with a dose of oestradiol that prevents increases in serum LH, only partially attenuated the rises in FSH-β mRNA and serum FSH and did not prevent the increase in pituitary FSH content. Treatment of intact or orchidectomized rats with testosterone suppressed FSH-β mRNA levels to 50% below intact control values without affecting pituitary FSH content. In mice, testosterone treatment for 10 days reduced the post-castration increase in FSH-β mRNA by only 26%, and prevented the fall in pituitary FSH content, although the increased serum concentration of FSH was unaffected.
In conclusion: (1) there is a good correlation between FSH-β mRNA levels and overall FSH biosynthesis in male and female rats and female mice, but this relationship is less obvious in male mice where pituitary FSH stores are not increased; (2) the inability of oestradiol to prevent completely the post-ovariectomy increase in FSH-β mRNA and FSH synthesis in female rats and mice indicates either that other gonadal products are necessary or that higher doses of oestradiol are required than for complete suppression of LH synthesis; (3) whilst the post-gonadectomy increases in FSH-β mRNA are larger in the female of both species, there are no major differences between rats and mice in the regulation of FSH-β gene expression by sex steroids.