Estimation of Helix C1 and B2 excitability. (A) Representative electrophysiological recordings from C1 and B2 neurons before and after 5-HT application. Both spontaneous activity (B, C) and the response to a depolarizing current step (0.5 nA for 10 seconds; D, E) were measured. (B) B2 neurons showed a significant increase of spontaneous firing activity only after 5-HT treatment. At 24 hours after plating no activity was observed. Conversely, C1 neurons (C) were silent in all the experimental groups. During the electrical stimulation, B2 exhibited a similar firing rate (D), while a higher firing frequency was evoked in C1 neurons only after 5-HT application. (E) Comparable values were recorded 24 hours after plating.