Organization of the catecholaminergic systems in the brain of lungfishes, the closest living relatives of terrestrial vertebrates

Lungfishes are a group of sarcopterygian fishes currently considered the closest living relatives of tetrapods, and represent an interesting group for the study of evolutionary traits in the transition from fishes to tetrapods. Catecholaminergic systems in the brain are among the most carefully analyzed neurotransmitter systems in the brain of most vertebrate groups. Their organization shows major shared characteristics, although traits particular to each vertebrate class have also been found, primarily between anamniotes and amniotes. Given the relevance of lungfishes in evolutionary terms, the present study provides the first comprehensive and detailed map of the catecholaminergic structures in the brain of two representative species of lungfishes, an African lungfish (Protopterus dolloi) and the Australian lungfish (Neoceratodus forsteri), as revealed by immunohistochemistry. Distinct groups of catecholaminergic cells were observed in the olfactory bulb, pallium, and preoptic area of the telencephalon, and the subpallium is devoid of these cells. Hypothalamic and diencephalic groups were detected and, in particular, the dopaminergic nucleus of the periventricular organ was evidenced with dopamine antibodies but not with anti‐tyrosine hydroxylase. A well developed mesostriatal system was revealed formed by conspicuous groups of dopamine cells in the midbrain tegmentum and profuse innervation of the subpallium. Comparison of these results with those from other classes of vertebrates shows numerous common traits shared by most groups and also highlights particular features in lungfishes different from actinopterygian fishes that resemble those of amphibians and amniotes. The organization of the catecholaminergic systems has been analyzed in two species of lungfishes. The results show common traits with other fish groups but primarily with tetrapods, and allow to determine the extent to which this organization is comparable to that of tetrapods, which evolved from an extinct close relative of lungfishes. Photomicrograph of a transverse section through the mesencephalic tegmentum of the lungfish Protopterus dolloi illustrating the position of the putative ventral tegmental area/substantia nigra (TH positive) in relation to the oculomotor nucleus (ChAT positive).