The occurrence and distribution of monoamine-(MA) containing neurons and fibres in the brain of Carassius was investigated by formaldehyde-induced fluorescence (FIF) histochemistry (Falck-Hillarp technique). Many brightly green-fluorescent nerve cell perikarya were found in the nucleus dorsolateralis and ventromedialis, in the nucleus posterioris periventricularis, in the nucleus recessus lateralis and posterioris. They also occurred in the mesencephalic nucleus lateralis valvulae, in the metencephalic nucleus gustatorius secundus and near the ventricular borders of the facial and vagal lobes in the myelencephalon.
Many fluorescent fibres and nerve terminals were localized in the frontal and medio-lateral parts of the telencephalon,
showing fluorescent connections to the caudal parts. In the diencephalon, MA-fibres branched in a horizontal and ventral tract, leading to the medulla oblongata and the hypothalamic nuclei, respectively. There were laterally situated fibres connecting the hypothalamic nuclei with the medulla and the nucleus gustatorius secundus. Many fluorescent fibres were found in the middle layers of the tectum opticum, in the torus semicircularis, in the lobus inferior and in the medulla oblongata. Considerably fewer fibres occurred in the corpus cerebelli and in the dorsal parts of the hindbrain lobes.
These results are compared with the MA-system in the brains of other fish.