Direct Comparison of Serotonin Effects on Siphon Versus Tail Sensory Neurons in Aplysia

Modulation of the strength of s iphon and tail withdrawal reflexes in Aplysia involves, in part, changes in the sensory neurons that initiate these reflexes. Different observations and experiments on modulat ion in s iphon and tail sensory neurons together contribute to the working model of mechanisms of learning and memory in Aplysia, yet no direct comparison of modulat ion in these two classes of sensory neurons has yet been made. The purpose of the present study was to directly compare modulat ion between siphon and tail sensory neurons in the same experimental conditions. In particular, we focused on the effects of serotonin on two firing properties of sensory neurons: spike duration and excitability. We appUed serotonin (5-HT) onto both siphon and tail sensory neurons under the same conditions and found that both spike duration and excitability were significantly enhanced. This enhancement was statistically indistinguishable between siphon and tail sensory neurons tested simultaneously in the same preparation. Thus, these two different 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 induced modulat io