Regulation of corticotropin-releasing hormone secretion by ACTH at different times after adrenalectomy

RESUMO

This study was undertaken to determine the short-loop negative feedback at the hypothalamic site after adrenalectomy (ADX), when the hypothalamic-pituitary axis was at different set-points. Rats were submitted to the experimental procedure 3 h and 1, 3, and 14 days after bilateral ADX or sham surgery under ether anesthesia by the dorsal approach. ADX rats were supplied with 1.5 per cent NaCl. After decapitation, plasma samples were collected and corticosterone (B) and adrenocorticotrophic hormone (ACTH) concentrations were determined by radioimmunoassay (RIA). Using in vitro incubation of the median basal hypothalamus (MBH) fragment we studied the hypothalamic corticotrophin-releasing hormone (CRH) release under basal conditions and under the inhibitory effect of ACTH. Hypothalamic CRH secretion, determined by direct RIA, with out extraction and reported as pg CRH/MBH per 1 h incubation, was inhibited by ACTH administration in a dose-dependent manner, demonstrated by a significant negative correlation obtained by linear regression analysis of CRH secretion as a function of ACTH dos However, the percentage of inhibition of CRH secretion was higher at 1 and 3 days than at 14 days after ADX. In conclusion, these in vitro studies demonstrated that ACTH suppressed basal CRH secretion in a dose dependent manner, an effect compatible with a short-loop feedback mechanism. Furthermore, we demonstrated variable suppressibility of CHR secretion by ACTH at different times after ADX.