Distribution of serotonin-immunoreactivity in juvenile Aplysia

Serotonin-immunocytochemistry has been applied to whole mounts of the central nervous system and of several peripheral tissues from stage 12 juvenile Aplysia californica. The small size of animals at this stage permits visualization of the three-dimensional distribution of structures containing serotonin-immunoreactivity in unsectioned tissues. Many neuronal cell bodies are stained in addition to the giant cerebral neuron of the cerebral ganglion and cells in the RB cluster of the abdominal ganglia which previously had been characterized biochemically and pharmacologically as being serotoninergic. Neuronal cell bodies, both in central ganglia and in the wall of the gut, are encircled by plexuses of serotoninergic varicosities. The neuropil of ganglia and the eye also contain fine, immunoreactive axons bearing varicosities. Intraganglionic connectives and nerves contain many stout fluorescent axons. Serotoninergic varicosities are also observed in the connective tissue sheath surrounding central ganglia and nerves, as well as in heart and body muscle, blood vessels and gut.