Serotoninergic neurons in the mormyrid brain and their projection to the preelectromotor and primary electrosensory centers: immunohistochemical study.

Serotonin-containing neurons in the brain of the weak-electric fish Gnathonemus petersii (mormyridae, teleostei) were studied with the aid of immunohistochemical labeling. Study of the central serotoninergic innervation was focused on the structures subserving the command of the electric organ and the first central relay of the electrosensory system. In the midline raphe nuclei, serotoninergic neurons formed a column that stretched from the ventral caudal medulla to the dorsal midbrain, ending caudal to the cerebellar peduncle. In the dorsal tegmentum, serotoninergic neurons were found bilaterally at the anterior margin of the decussation of the lateral lemniscus. Labeled neurons were also present bilaterally immediately anterior to the cerebellar peduncle and also in the pretectal region. In the hypothalamus, many serotoninergic neurons were in contact with the ventricular wall, and a few were present in the preoptic area. This distribution of serotoninergic cell bodies showed many similarities to that in other fish and higher vertebrates but lacked the lateral spread of the serotoninergic raphe system found in the midbrain tegmentum in mammals. Labeled fibers were found in both the preelectromotor medullary relay nucleus and the electromotor command nucleus. These serotoninergic projections were traced to the posterior raphe. Serotoninergic fibers also formed a dense network in the cortex and in the nucleus of the electrosensory lobe, both of which receive primary input from electroreceptors. These results suggest that serotonin may have a role in the modulation of the intrinsic, rhythmic electromotor command and in the gating of electrosensory input.