Ontogenetic development of serotoninergic neurons in the brain of a teleost, the three-spined stickleback. An immunohistochemical analysis.

The ontogenetic development of serotoninergic neurons in the brain of the stickleback was investigated with the indirect immunocytochemical peroxidase-antiperoxidase technique, using a specific antibody to serotonin (5-hydroxytryptamine, 5-HT). Formation of neuronal populations takes place during embryonic development. By 80 h after fertilization, the first 5-HT perikarya have appeared in the ventricular zone of the hypothalamus (nucleus recessus lateralis) and the raphe region. At 108 h the first 5-HT perikarya can be observed in area praetectalis. At 118 h a transient group of 5-HT neurons appears rostral to the nucleus recessus lateralis, and at this same age the first 5-HT perikarya may be visualized in nucleus recessus posterioris. A group of 5-HT neurons appears in the dorsolateral tegmentum at 166 h (one day after hatching, which occurs at 120-144 h after fertilization). Differentiation of the neuronal populations, in terms of migration and formation of subdivisions, starts between 80 h and 94 h, and seems to be completed between 1 and 5 days after hatching. Raphe nuclei form an anterior group comprising nuclei raphe dorsalis, raphe medialis and a ventrolateral group, and a posterior group comprising a nucleus raphe pallidus/obscurus complex, a lateral nucleus reticularis paragigantocellularis and a ventromedial nucleus raphe magnus. The posterior and ventral raphe nuclei, which are well developed at the time of hatching, have not been visualized in the adult stickleback. While formation of 5-HT neuronal systems, as well as their primary efferent pathways, takes place during early ontogenetic development, the establishment of terminal areas and their subsequent differentiation apparently takes place during later ontogenetic stages. Most presumptive target areas are penetrated by 5-HT axons at hatching, although terminal formation does not seem to start until later. A considerable number of 5-HT neuronal groups present in the embryonic and newly hatched stickleback have not been visualized in the adult stickleback. This may be due to selective cell death, changes in transmitter phenotype or maturation of axonal transport processes during development.