Effects of Castration of Immature Rats on Serum FSH and LH, and of Various Steroid Treatments after Castration1

The intricate relationship between the gonads and pituitary gonadotropin secretion has been studied in the immature, 26-day-old rat. In male rats orchidectomized at this age, serum FSH and LH rose to significantly higher levels at 8 h postcastration. A much later response was seen in ovariectomized females: at 24 h and 48 h for FSH and LH, respectively. When groups of rats castrated at 26 days of age were treated with pharmacologic dosages of various steroids for 6 and 15 days postoperative, it was found that testosterone, 5α-dihydrotestosterone, and estradiol prevented the rise of both FSH and LH, in both sexes. A steroid-derived drug, 17α-ethinyl-testosterone-2,3-isoxazol, was also effective, while progesterone alone was unable to suppress gonadotropins in either sex. Results reaffirm that the gonadal—hypophyseal relationship is sensitive before puberty. The marked sex difference in the response to castration is undoubtedly due to different gonadal hormones (androgen or estrogen) present at the time of castration, and their contributions to this feedback process. However, it appears that hormones of either type can suppress both gonadotropins in both sexes. Results with 17α-ethinyl-testosterone-2,3-isoxazol were particularly encouraging with respect to its clinical usefulness as a gonadotropin inhibitor with little or no biologic activity as a sex steroid.