Alpha-melanocyte-stimulating hormone (alpha-MSH) in the brain of the African lungfish, Protopterus annectens: immunohistochemical localization and biochemical characterization.

Abstract

The distribution of alpha-melanocyte-stimulating hormone (alpha-MSH) containing neurons and the molecular forms of alpha-MSH-related peptides exhibit substantial differences in the brains of fish and amphibians. Lungfishes, which share similarities with both fishes and tetrapods, represent a valuable group in which to investigate the neuroanatomical and neurochemical facets of evolution. In the present study, we have localized and characterized alpha-MSH-immunoreactive peptides in the central nervous system of the African lungfish Protopterus annectens. Perikarya exhibiting alpha-MSH-like immunoreactivity were observed in two distinct regions of the hypothalamus: the rostral part of the preoptic nucleus and the caudal part of the hypothalamus. In the caudal hypothalamus most alpha-MSH-immunopositive perikarya were located in both the subependymal and deepest layers of the ventral periventricular region. Scattered alpha-MSH-immunopositive cells were occasionally detected in the dorsal side of the caudal hypothalamus. The alpha-MSH-immunoreactive material localized in the brain was characterized by combining high-performance liquid chromatography (HPLC) analysis and radioimmunological detection. The displacement curves obtained with synthetic alpha-MSH and serial dilutions of brain and pituitary extracts were parallel. HPLC analysis of lungfish hypothalamic extracts showed that the major immunoreactive peak coeluted with synthetic desacetyl alpha-MSH and its sulfoxide derivative. An additional peak coeluted with synthetic sulfoxide alpha-MSH. In contrast, in the pituitary, the predominant form of alpha-MSH-like material coeluted with the N,O-diacetyl alpha-MSH standard. These results provide the first evidence for the presence of alpha-MSH-related peptides in the brain of a lungfish. The distribution of alpha-MSH neuronal systems in the lungfish is very similar to that reported in amphibians, supporting the existence of phylogenetic convergences between these two vertebrate groups.