Abstract: The 5-HT immunoreactive fibers(5-HT-IR fibers)in pleural sensory cluster of aplysia were examined at the light microscopic level using ABC immunohistochemical method.Both the cell bodies and the proximal axon segments of pleural sensory neurons were surrounded by 5-HT-IR fibers with numerous 5-HT-IR varicosities.To analyze the size distribution of the 5-HT-IR varieosities quantitatively,4 microscopic fields of approximately 32000μm were drawn with a camera lueida.The size of individural 5-HT-IR varicosities was then estimated from the camera lucida drawings by measuring their diameters along the short axis.The average short axis diameter of the varicosities was 1.1±0.6 μm(mean±S.D.;n=574),ranging from 0.2～3.8 μm.The shape of the size distribution of the 5-HT-IR varicosities was skewed towards the smaller diameter.Based on the size distribution of the varicosities along a strand of fibers,at least four different types of 5-HT-IR fibers were ob- served.Some fibers sent out branches,which usually emerged from a varicosity.Abranched varicosity might generate 2～4 daugher branches with sub-varicosity.Aaverage short axis diameter of branched varieosities was 1.7±0.5 μm(n=35),ranging from 1.0 μm to 2.4 μm.Some small varicosities seemed to contact with the somata surface of pleural sensory neurons.But this level of resolution was not sufficient to determine if these immunoreactive varicosities actually contact with pleural sensory neurons in synaptic.The cell body of the sensory neuron was covered by a thin sheath of glial cells that might hinder access of the transmitter secreted from the 5-HT-IR varicosities.

Key words: serotonin, sensory neuron, immunocytochemistry, aplysia