We employed several ACTH immunoassays using antisera with affinities for differing regions of the molecule as well as an ACTH bioassay to further study the distribution and character of ACTH in rat, cat, and monkey brain. Acid extracts of brain tissues were examined for N-terminal and midportion ACTH concentrations and MSH content. In each species, ACTH concentrations were greatest in the hypothalamus, with lesser concentrations in more peripheral regions. Hypophysectomy, dexamethasone suppression, or adrenalectomy had only minor effects on ACTH content and distribution. In all species, the concentrations of N-terminal ACTH immunoreactivity were several times greater than those of midportion immunoreactivity. aMSH was present in low concentrations and could account for only a small fraction of N-terminal immunoreactivity. ACTH bioactivity was present only in central areas and correlated well with midportion immunoreactivity. A sample of larger molecular weight ACTH (31, 000 daltons) reacted approximately 10–15 times more effectively with the N-terminal ACTH antibody than with the midportion antibody. Moreover, gel filtration of brain extracts revealed the presence of large molecular weight substances with ACTH immunoreactivity. These results indicate that extrapituitary areas of brain contain ACTH and that a portion of the ACTH immunoreactivity found in those regions is attributable to relatively bioinactive high molecular weight ACTH precursors.
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