Serotonin-immunoreactive neurons in the brain of Eigenmannia lineata (Gymnotiformes, Teleostei)

The distribution of serotonin-immunoreactive (5-HTi) neurons was determined by the peroxidase-antiperoxidase technique. 5-HT-immunoreactive nerve cells (or perikarya) occurred in the diencephalic nucleus ventromedialis (nvm), in the so-called pre-pacemaker nucleus (ppm), in a laterally located (unnamed) nucleus (G1), in the nucleus posterior periventricularis (nppv), the nucleus recessus lateralis (nrl) and the nucleus recessus posterior (nrp). 5-HT-immunoreactivity could be localized in the pars intermedia and the proximal pars distalis of the pituitary gland. Large 5-HTi perikarya were found in the lamina VI of the mesencephalic torus semicircularis (ts). Intensely immunostained 5-HTi perikarya occurred in the raphe region. 5-HT-immunoreactivity was also located in the medullary pacemaker nucleus of the specialized electric organ (pm). Only a few 5-HTi fibres were observed in the telencephalon. Ventral to the 3rd ventricle of the diencephalon between the caudal hypothalamic nuclei most of these were fibre bundles dorsal to the nuc. recessus lateralis. Some 5-HTi fibres traversed the pituitary stalk to the proximal pars distalis of the pituitary gland. Scattered 5-HTi fibres were seen in the torus semicircularis, lobus inferior (li) and cerebellum, while more fibres appeared in the medulla oblongata (mo) and below the pacemaker nucleus. The results are compared to those in other fish.