Control of gonadotrophin secretion during the pubertal and seasonal transitions in the male sheep.
Castration of spring-born, male Suffolk lambs at 2 weeks of age resulted in a rapid increase in gonadotrophin secretion. LH pulse frequency stabilized by 6 weeks of age and did not change throughout the ensuing summer and autumn. In contrast, LH pulse frequency in castrated, adult rams decreased during the summer and increased in the autumn. Testosterone or oestradiol replacement initially suppressed the post-castration increase in gonadotrophin secretion in developing lambs. Responsiveness to steroid inhibition was lost in late spring and summer (8-15 weeks of age). In steroid-treated castrated mature rams, however, an increase in response to the inhibitory effects of testosterone and oestradiol occurred during the summer, as indicated by slower LH pulse frequencies in June, July and August, than at other times of the year. A decrease in response to inhibition by steroids then occurred in the autumn, months after the springtime reactivation of reproductive function (testicular growth, testosterone and gonadotrophin secretion) in testes-intact adult rams. The results reveal fundamental differences between the regulation of gonadotrophin secretion in ram lambs and adults. The pubertal decrease in response to inhibition by steroids, which is not coincident with a steroid-independent increase in gonadotrophin secretion, may play a key role in increasing gonadotrophin secretion to initiate gonadal activity in the spring. The post-pubertal ram also exhibits the first signs of the seasonal resumption of reproductive activity in the spring. However, it is not until the autumn that a decrease in response to inhibitory feedback by steroids occurs, in concert with a steroid-independent increase in gonadotrophin release. Therefore, in the sexually mature ram, rather than stimulating the initial springtime recovery of gonadal function, these two mechanisms may act in concert to achieve gonadotrophin concentrations and/or patterns necessary to optimize reproductive competence in the autumn.
D000818:Animals; D004958:Estradiol / Q000494:pharmacology; D005640:Follicle Stimulating Hormone / Q000378:metabolism; D007986:Luteinizing Hormone / Q000378:metabolism*; D008297:Male; D009919:Orchiectomy; D012621:Seasons; D012741:Sexual Maturation; D012756:Sheep / Q000502:physiology*; D013739:Testosterone / Q000494:pharmacology
D013739:Testosterone; D004958:Estradiol; D007986:Luteinizing Hormone; D005640:Follicle Stimulating Hormone