[Studies on the feedback regulation of gonadotropin concentrations in male rats (I). The effects of hypothalamic implantation of testosterone and its metabolites on gonadotropin release in castrated male rats (author's transl)].

Testosterone (T), 5 alpha-dihydrotestosterone (5 alpha-DHT), 5 alpha-androstane-3 alpha, 17 beta-diol (3 alpha-diol) and estradiol-17 beta (E) or cholesterol (control) were implanted into the hypothalamic median eminence--arcuate nucleus (ME-ARC) region of long-term castrated male rats. Serum LH and FSH levels were measured before and after the implantation. T or E implantation resulted in a 65.5% (P less than 0.01) or 80.2% (P less than 0.01) decrease in serum LH levels respectively, within 7 days after the intrahypothalamic application. 5 alpha-DHT or 3 alpha-diol implantation in the ME-ARC region also resulted in a significant lowering of serum LH levels, although their inhibitory action was less effective than that of T or E. There were no significant differences in serum LH levels 20 min after the intraperitoneal injection of 2 micrograms/kg of LH-RH between the control group and the T, 5 alpha-DHT, 3 alpha-diol or E group. E implantation also resulted in a 34.7% (P less than 0.01) decrease in serum FSH levels at the 7th day, but other steroids failed to reduce serum FSH levels after the intrahypothalamic application. These studies suggest that aromatization of T to E is not indispensable to the negative feedback effect of androgen on the hypothalamic LH-RH release. It might also be supposed that E is one of the factors regulating the serum FSH level.