The organization of monoamine‐containing neurons in the brain of the sunfish (Lepomis gibbosus) as revealed by fluorescence microscopy

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Abstract
The morphological organization of the monoamine‐containing neurons in the brain of the sunfish (Lepomis gibbosus) was studied by means of the Falck‐Hillarp histofluorescence method. No attempt was made to distinguish between norepinephrine and dopamine, both primary catecholamines (CA) yielding a similar yellow‐green fluorescence after paraformaldehyde treatment. In the brain stem of this teleost fish, three groups of CA‐containing neuronal somata have been found. First, there is a small collection of CA perikarya located just caudal to the obex of the fourth ventricle. The neurons of this medullo‐spinal group give rise to numerous CA fibers many of which ascend within the central portion of the medulla. Intermingled with these CA fibers are some CA cells that constitute the central medullary group. The CA perikarya of this group are scattered between the levels of cranial nerves X and VIII. The tegmentum of the isthmus also contains a small group of very closely packed CA neurons. The large‐sized CA cells of the isthmal group are located dorsolateral to the medial longitudinal fasciculus, partly within the periventricular gray. High densities of CA varicosities were also disclosed in various brain stem structures such as the optic tectum, the torus semicircularis and the cerebellar valvula. In addition, numerous serotonin (5‐HT)‐type neuronal somata were found in the raphe region of the brain stem, particularly at caudal mesencephalic, isthmal and rostral medullar levels. A large number of CA cell bodies were visualized in the sunfish hypothalamus. Most of them form two populations of small, round cells that are located along and partly within the ependymal walls of the posterior and lateral recesses of the third ventricle. These bipolar cells possess one short club‐like process protruding into the ventricle and their thin ependymofugal processes contribute to the CA innervation of numerous hypothalamic regions. Large CA neurons apparently without direct CSF contact also occur in the area of nucleus posterior tuberis, at the level of the mesodiencephalic junction. Although the hypothalamic inferior lobes are devoid of CA cell bodies they are heavily innervated by CA axons. The sunfish telencephalon also receives a strikingly massive and complex monoaminergic innervation. Numerous CA fibers which are first observed at the level of the preoptic area, ascend through the central zone of the telencephalon and arborize profusely particularly within the medial zone of area dorsalis telencephali. Other CA fibers, as well as abundant fine 5‐HT varicosities were found in the lateral zone of area dorsalis. Although the exact origin of the telencephalic CA afferents in Lepomis is not known, part of it may arise from the isthmal CA cell group which appears similar to the locus coeruleus of reptiles, birds and mammals.