Organization of the catecholaminergic systems in two basal actinopterygian fishes, Polypterus senegalus and Erpetoichthys calabaricus (Actinopterygii: Cladistia)

Cladistians are a group of basal nonteleost actinopterygian fishes that represent an interesting group for the study of primitive brain features, most likely present in the ancestral Osteichthyes. We have investigated the catecholaminergic (CA) systems in the brain of two representative cladistian species, the bichir Polypterus senegalus and the reedfish Erpetoichthys calabaricus, by means of antibodies against tyrosine hydroxylase (TH; the first enzyme in the synthesis of catecholamines) and dopamine (DA). Double immunohistofluorescence was performed for simultaneous detection of TH with nitric oxide synthase, choline acetyltransferase, calbindin, calretinin, and serotonin, aiming to accurately establish the localization of the CA neurons and to assess possible interactions between these neuroactive substances. All forebrain CA groups of cladistians are dopaminergic, whereas noradrenergic cells are located within the rhombencephalon. Distinct groups of DA immunoreactive (DA-ir) cells were observed in the olfactory bulb, subpallium, and preoptic area of the telencephalon. Hypothalamic groups were detected in the suprachiasmatic nucleus, retrotuberal and retromamillary areas and, in particular, the paraventricular organ showed immunoreactivity to dopamine but not to TH. Diencephalic DA-ir groups were detected in the prethalamus, posterior tubercle, and pretectum. A small DA-ir cell population was observed in the midbrain tegmentum only in Polypterus. CA cell groups were also located in the locus coeruleus, solitary tract nucleus, and area postrema within the rhombencephalon, the spinal cord, and the retina. The comparison of these results with other vertebrates, using a neuromeric analysis, shows highly conserved traits in all vertebrates studied but also evidences particular characteristics of actinopterygian fishes.