Serotonin-containing neurons in Drosophila melanogaster: development and distribution.

Antibodies made against serotonin (5HT) were used to identify the serotonin neuronal system in the developing and adult nervous system of Drosophila melanogaster. The 5HT neuronal pattern is composed of a small number of neurons, 84 in larvae and 106 in adults, distributed in clusters composed of one to five neurons in the CNS; 5HT immunoreactive (5HT-IR) neurons appear to be predominantly intrasegmental interneurons; however, intersegmental 5HT-IR fibers are observed and at least some neurons send peripheral fibers. Acquisition of 5HT immunoreactivity in the CNS occurs late in embryogenesis, by 16-18 hours, and most if not all the 5HT neurons appear to persist into adulthood. During early metamorphosis, the intensity of 5HT-IR neuropil transiently decreases. Other changes in the CNS during this period are reflected in the appearance of two new 5HT clusters and 5HT-IR neuropil in the developing optic lobes. Comparison of the 5HT-IR pattern with other transmitter systems in Drosophila as well as comparison of the 5HT-IR pattern within different insect species is presented.