Serotonin-immunoreactivity in peripheral tissues of the opisthobranch molluscs Pleurobranchaea californica and Tritonia diomedea.

The distribution of serotonin (5-HT)-immunoreactive elements in peripheral organs of the sea-slugs Pleurobranchaea californica and Tritonia diomedea was studied in cryostat sections. For Pleurobranchaea, 5-HT-immunoreactive (5-HT-IR) neuron cell bodies were found only in the central nervous system (CNS); 5-HT-IR cell bodies were not observed in foot, tentacles, rhinophores, oral veil, mouth, buccal mass, esophagus, gills, salivary glands, skin, reproductive system, and acidic glands, nor in peripheral tentacle and rhinophore ganglia. However, 5-HT-IR neuronal processes were widely distributed in these structures and the patterns of 5-HT-IR elements were characteristic for each particular peripheral tissue. 5-HT-IR elements were most dense in the sole of the foot and the reproductive system, followed by rhinophores, tentacles, oral veil, mouth, buccal mass, and esophagus. The sensory epithelium of rhinophores, tentacles, and mouth showed a highly structured glomerular organization of 5-HT-IR fibers, suggesting a role for 5-HT in sensory signaling. A much lower density of 5-HT-IR innervation was observed in gills, skin, salivary, and acidic glands. 5-HT-IR was observed in neuropil of tentacle and rhinophore ganglia with many transverse 5-HT-IR axons running to peripheral sensory areas. The distribution of 5-HT-IR elements in Tritonia was similar to that of Pleurobranchaea. A significant suggestion of the data is that central serotonergic neurons may modulate afferent pathways from sensory epithelia at the periphery.