IMMUNOCYTOCHEMICAL LOCALIZATION OF ENKEPHALINS IN THE BRAIN OF THE AFRICAN LUNGFISH, PROTOPTERUS ANNECTENS, PROVIDES EVIDENCE FOR DIFFERENTIAL DISTRIBUTION OF MET-ENKEPHALIN AND LEU-ENKEPHALIN

The distribution of various opioid peptides derived from proenkephalin A and B was studied in the brain of the African lungfish Protopterus annectens by using a series of antibodies directed against mammalian o pioid peptides. The results show that both Metenkephalin-and Leu-enkep halin-immunoreactive peptides are present in the lungfish brain. In co ntrast, enkephalin forms similar to Met-enkephalin-Arg Phe, or Met-enk ephalin-Arg-Gly-Leu, as well as mammalian alpha-neoendorphin, dynorphi n A (1-8), dynorphin A (1-13), or dynorphin A (1-17) were not detected . In all major subdivisions of the brain, the overwhelming majority of Met-enkephalin- and Leu-enkephalin-immunoreactive cells were distinct . In particular, cell bodies reacting only with Leu-enkephalin antibod ies were detected in the medial subpallium of the telencephalon, the g riseum centrale, the reticular formation, the nucleus of the solitary tract, and the visceral sensory area of the rhombencephalon. Cell bodi es reacting only with Met-enkephalin antibodies were found in the late ral subpallium of the telencephalon, the caudal hypothalamus, and the tegmentum of the mesencephalon. The preoptic periventricular nucleus o f the hypothalamus exhibited a high density of Metenkephalin-immunorea ctive neurons and only a few Leu-enkephalin-immunoreactive neurons. Th e distribution of Met-enkephalin-and Leu-enkephalin-immunoreactive cel l bodies and fibers in the lungfish brain showed similarities to the d istribution of proenkephalin A-derived peptides described previously i n the brain of land vertebrates. The presence of Met-enkephalin-and Le u-enkephalin-like peptides in distinct regions, together with the abse nce of dynorphin-related peptides, suggests that, in the lungfish, Met -enkephalin and Leu-enkephalin may originate from distinct precursors. (C) 1998 Wiley-Liss, Inc.