An anatomical and biochemical study of the pituitary proopiomelanocortin systems in the polypteriform fish Calamoichthys calabaricus.

Immunohistochemical and biochemical analyses were performed on the pituitary Proopiomelanocortin (POMC) systems of the polypteriform fish Calamoichthys calabaricus. Immunohistochemical staining of the pituitary revealed a clustering of ACTH immunopositive cells within the rostral pars distalis. alpha-Melanocyte stimulating hormone (alpha-MSH)-related and beta-endorphin-related immunoreactivity were found colocalized in epithelial cells of the pars intermedia. Biochemical analyses included Sephadex G-50 column chromatography, reversed-phase HPLC, and cation exchange chromatography. These analyses revealed the presence of immunoreactive forms of ACTH which stimulated glucocorticoid release when tested on isolated Bufo marinus adrenocortical tissue. Three forms of alpha-MSH were detected, and the major form had the same HPLC chromatographic properties as synthetic monoacetylated alpha-MSH. Finally, five forms of beta-endorphin were detected, and all of these forms were N-acetylated. Based on these observations, it appears that N-acetylation is a major post-translational processing event within the melanotropic cells of C. calabaricus. Given the position of Order Cladistia in the phylogeny of actinopterygian fish, it appears that N-acetylation of POMC-related products is an ancestral trait of osteichthyean fish.