Mutually Independent Effects of Adrenocorticotropin on Luteinizing Hormone and Testosterone Secretion*

In this study, we examined the effect of ACTH on the sensitivity of the testes to gonadotropin and determined the role of the testosterone (T) negative feedback system in mediating the inhibitory effect of ACTH on LH secretion in adult male rats. ACTH infusion for 3 days reduced basal levels of serum T and the T response to GnRH, but did not alter basal levels of serum LH (immunoreactive) or the LH response to GnRH. These effects required the presence of the adrenal glands. Infusion of corticosterone (B) at a dose that increased serum B concentrations 9-fold had an effect similar to that of ACTH on basal serum T levels and the serum T response to GnRH. Basal levels of serum LH and the serum LH response to GnRH were not affected by B administration. These data suggest that ACTH administration reduces the sensitivity of the testes to LH, resulting in a lower basal level of T and a reduced T response to GnRH. This effect was independent of basal serum LH levels or the LH response to GnRH. It appears that B mediates the effect of ACTH on testicular sensitivity to gonadotropin. In another experiment, ACTH administration for 4 days did not alter serum LH values, but reduced serum T levels in sham-castrated male rats. In contrast, ACTH treatment blunted the increase in serum LH after castration by day 2 of treatment, despite the absence of detectable levels of serum T within 6 h after castration. These data suggest that T is not essential for the inhibitory effect of ACTH on LH secretion to occur. They do not support the hypothesis that ACTH enhances the sensitivity of the hypothalamus and/or pituitary to the negative feedback effects of T.