Effects of castration on 24-hour periodicity of hypothalamo-pituitary gonadotropic function

Groups of 6 castrated or sham-castrated (controls) adult male rats were decapitated 7 days after the operation at 4 h intervals within a 24 h period. Serum LH, FSH and PRL were measured by radioimmunoassay and GnRH activity in medial basal hypothalamus was estimated by bioassay using donor pituitaries from normal adult rats. Finally, the responsiveness of pooled pituitaries from each experimental group to 5 ng synthetic GnRH was tested in vitro immunoreactive LH in medium being used as the end point. It was found that the castration of male rats results in the abolition of 24 h periodicity of hypothalamic GnRH content. Furthermore, a shift in the occurrence of peak levels (acrophase) and about 100 percent increase of the pituitary sensitivity to GnRH was observed. The castration also resulted in a shift in the acrophase of serum gonadotropins levels. In conclusion, the presented data suggest that the gonadal secretion possibly maintains the circadian periodicity of hypothalamo-pituitary gonadotropic function.