Neuropeptide tyrosine in the brain of the African lungfish, Protopterus annectens: immunohistochemical localization and biochemical characterization.

Lungfishes, which share similarities with both fishes and amphibians, represent an interesting group in which to investigate the evolutionary transition from fishes to tetrapods. In the present study, we have investigated the localization and biochemical characteristics of neuropeptide Y (NPY)-immunoreactive material in the central nervous system of the African lungfish, Protopterus annectens. NPY-immunoreactive cell bodies were found in various regions of the brain, most notably in the telencephalon (septal area, ventral striatum, and nucleus accumbens), in the diencephalon (preoptic nucleus, periventricular region of the hypothalamus, and ventral thalamus), and in the tegmentum of the mesencephalon. A strong immunoreaction was also detected in cell bodies of the nervus terminalis. Immunoreactive nerve fibers were particularly abundant in the ventral striatum, the nucleus accumbens, the diagonal band of Broca, the hypothalamus, and the mesencephalic tegmentum. Positive fibers were also seen in the median eminence and in the neural lobe of the pituitary. The NPY-immunoreactive material localized in the brain and pituitary was characterized by combining high-performance liquid chromatography (HPLC) analysis and radioimmunological quantitation. The displacement curves obtained with synthetic porcine and frog NPY and serial dilutions of brain and pituitary extracts were parallel. Reversed-phase HPLC analysis of telencephalon, diencephalon, and pituitary extracts resolved a major NPY-immunoreactive peak that coeluted with frog NPY. The similarity between the distribution of NPY-containing neurons and the biochemical characteristics of the immunoreactive peptide in the brain of lungfish and frog strongly favors a close phylogenetic relationship between dipnoans and amphibians.