Corticotropin-like immunoreactivity in the brain of intact, hypophysectomized, cortisol- and metopirone-treated eels. Comparison with changes in pituitary corticotropes and brain corticotropin-releasing factor.

An ACTH-like peptidergic system was demonstrated in the brain of three teleost species by immunocytochemistry. In order to investigate the origin of brain ACTH and factors modulating its synthesis, similar techniques were applied to the brain of eels (1) submitted to hypophysectomy in order to suppress pituitary ACTH and plasma cortisol, (2) injected with cortisol to inhibit pituitary ACTH synthesis and release, and (3) injected with metopirone to block cortisol synthesis and stimulate ACTH synthesis and release. Hypophysectomized eels showed a normal distribution of immunoreactive perikarya in the ventral hypothalamus and fibers in the brain, suggesting that brain ACTH does not arise from the pituitary. In cortisol-treated eels immunostaining was markedly reduced in brain perikarya and pituitary corticotropes, suggesting a reduced synthesis. In metopirone-injected eels, one third of the animals showed an increased immunostaining in perikarya and a dense network of immunoreactive fibers, suggesting that ACTH synthesis was increased. Brain ACTH was not affected in other animals. Pituitary corticotropes were rapidly degranulated. Responses of ACTH in the brain and pituitary occur independently when cortisol synthesis is inhibited. These responses are compared to those of the corticotropin-releasing factor system in the same eels.