Recent developments in our understanding of the avian melanocortin system: its involvement in the regulation of pigmentation and energy homeostasis

The mammalian melanocortin system has been established as a crucial regulatory component in an extraordinarily diverse number of physiological functions. In contrast, comparatively little is known about the avian melanocortin system: interest in the physiological role of alpha-MSH in birds has been limited by the fact that birds lack the intermediate lobe of the pituitary, the main source of circulating alpha-MSH in most vertebrates. Recently, however, the main avian melanocortin system genes, including POMC, AGRP, and all the melanocortin receptors, have been cloned and their physiological roles are the beginning to be elucidated. This review outlines our improved understanding of the avian melanocortin system, particularly in relation to two of the most widely studied physiological functions of the melanocortin system in mammals, the regulation of pigmentation and energy homeostasis. The data reviewed here indicate that the melanocortin system has been strongly conserved during vertebrate evolution and that alpha-MSH is produced locally in birds to act as an autocrine/paracrine hormone.