Abstract: The morphological properties of serotoninergic immunoreactive fibers and their spatial relationships to pleural sensory neurons of Aplysia were studied with an immunocytochemical ABC method under light and transmission electron microscopy.The data show that 5-HT-IR varicosities make apparent synaptic contacts with pleural sensory neurons of aplysia.In light microscopy,it was observed numerous 5-HT-IR fibers containing varicosities around the somata and the initial axon segment of pleural sensory neurons.Labeled varicosities(n=574)might be classfied into three groups based on their size:i.e.small(0.2～1.2μm, 79%),medium(1.4～2.4μm,18%),and large(>2.6μm,3%)groups.In electron microscopy,it was observed axosomatic,axoaxonic,axoglial synaptic contacts in the pleural sensory clusters.Two types of synaptic contacts,flat and invaginated,were found.After measuring the shortest diameter of the varicosity(0.1～1.0μm,n=42),it appeared that only the small varicosities formed synaptic contacts.These results suggest that under appropriated physiological conditions,serotonin might be released from varicosities directly onto perikayra of pleural sensory neurons eliciting the cellular changes correlated with sensitization.Two types of synaptic contacts,flat and invaginatcd,could represent two different functional states,the invaginated type more effective than the other.Thus,pleural sensory neurons con- tributing to sensitization of tail withdrawal reflex in aplysia could be activated by serotonin- ergic neurons.

Key words: serotonin, sensory neuron, immunocytochemistry, aplysia