DIRECT COMPARISON OF SEROTONIN EFFECTS ON SIPHON VERSUS TAIL SENSORY NEURONS IN APLYSIA

Modulation of the strength of siphon and tail withdrawal reflexes in A plysia involves, in part, changes in the sensory neurons that initiate these reflexes. Different observations and experiments on modulation in siphon and tail sensory neurons together contribute to the working model of mechanisms of learning and memory in Aplysia, yet no direct c omparison of modulation in these two classes of sensory neurons has ye t been made. The purpose of the present study was to directly compare modulation between siphon and tail sensory neurons in the same experim ental conditions. In particular, we focused on the effects of serotoni n on two firing properties of sensory neurons: spike duration and exci tability. We applied serotonin (5-HT) onto both siphon and tail sensor y neurons under the same conditions and found that both spike duration and excitability were significantly enhanced. This enhancement was st atistically indistinguishable between siphon and tail sensory neurons tested simultaneously in the same preparation. Thus, these two differe nt classes of sensory neurons respond to 5-HT in very similar, if not identical, ways. We conclude that if there are differences in 5-HT ind uced modulation between siphon and tail sensory neurons at the biophys ical level, such differences are not strongly manifested at the level of changes in firing properties.