Distribution of monoamine-containing neurons in the brain of the weakly electric teleost, Eigenmannia lineata (Gymnotiformes: Rhamphichthyidae).

The distribution of neurons and fibres containing monoamines (MA, that is noradrenaline, dopamine, and serotonin) was studied in the brain of the weakly electric fish, Eigenmannia lineata, by formaldehyde-induced fluorescence histochemistry. Control experiments using a catecholamine-precursor (Dopa), a MA-depleting (reserpine), and a MA-accumulating (nialamide) drug, together with microspectrofluorometric measurements, allowed to distinguish specific (MA) from unspecific (for example lipofuscine) fluorescence. High Performance Liquid Chromatography (HPLC) assays of MA of untreated brains yielded 313 ± S.E. 24 ng/g brain tissue noradrenaline, 96 ± S.E. 19 ng/g dopamine, and 71 ± S.E. 12 ng/g serotonin (N = 12 in each case). The highest density of MA-perikarya was found, as in other species, in the diencephalon: in the region of the nucleus praeopticus periventricularis (npp), in the nucleus ventromedialis (nvm), in the nucleus posterior periventricularis (nppv), and in the hypothalamic nucleus recessus lateralis (nrl) and nucleus recessus posterior (nrp). The mesencephalic torus semicircularis, the "super-laminated" key structure of sensory-motor integration in gymnotids, contained monoamines in the large T-cells of layer VI. Both types of cells of the medullary pacemaker nucleus (pm) of the electric organ, the smaller pacemaker and the larger relay cells, showed specific, yellow fluorescence indicating the presence of monoamines. Fluorescent MA-fibres in the frontal, medial and lateral parts of the telencephalon connected with hypothalamic and midbrain areas. Several electroreceptive layers of the torus semicircularis (layers V, VIII, IX, XI, XIII) showed a clearly recognisable and in some parts dense network of MA-fibres. Only few fluorescent fibres occurred in the massive corpus cerebelli (which is mixed electro-/mechanoreceptive), in the second order electroreceptive lobus lineae lateralis posterior and in the medulla oblongata (ventrolateral to the raphe region, especially in the frontal medulla at the level of the torus).