Localization and Specificity of Corticosteroid “Feedback Receptors” at the Hypothalamo-Hypophyseal Level; Comparative Effects of Various Steroids Implanted in the Median Eminence or the Anterior Pituitary of the Rat

In order to elucidate the localization and specificity of corticosteroid feedback receptors in the hypothalamo-hypophyseal system, various steroids were implanted in the median eminence and the anterior pituitary of female rats and their effects on stress-induced ACTH release were investigated. Suppression of ACTH release was observed after the implantation of dexamethasone, cortisol, corticosterone, 11-dehydro-corticosterone and 11-deoxycorticosterone (DOC) in the anterior median eminence. 11-Deoxycortisol (Reichstein’s substance S), tetrahydrocortisol, pregnenolone, progesterone, and testosterone were ineffective when implanted in this region. Infundibular dexamethasone, cortisol and 11-deoxycorticosterone implants suppressed ACTH release, while other steroids were not effective. Dexamethasone and 11-deoxy-corticosterone also suppressed ACTH release when implanted bilaterally in the anterior pituitary. It is concluded that corticosteroid ‘feedback receptors’ may exist, both in the hypothalamus and anterior pituitary. Failure to suppress ACTH release by some of corticosteroids when implanted in the infundibular region or anterior pituitary seems to depend on their solubility in the brain and pituitary tissues and on the amount of steroids implanted rather than on the structure of the molecule.