Stimulation of mesocerebrum in Helix aspersa inhibits the neural network underlying avoidance behavior

Abstract

1. During mating, snails exhibit a marked suppression of the defensive withdrawal responses normally triggered by tactile contacts. Because previous work has identified the mesocerebrum as a center for the control of mating behavior, the present experiments tested whether the mesocerebrum can mediate the suppression of withdrawal responses. 2. A group of large neurons was identified in the parietal ganglion as part of a command circuit for avoidance behavior, including withdrawal (Figs. 2–4). 3. Extracellular stimulation of the mesocerebrum (20 Hz for 1–5 s) reduced the synaptic drive evoked in the parietal command neurons by a tactile stimulus (Fig. 5). In semi-intact preparations, there was a concurrent decrease in the number of action potentials evoked in the parietal cells and the extent of closure of the pneumostome, the latter being a measure of the behavioral response (Figs. 5 and 6). 4. The mesocerebrum stimulus caused a hyperpolarization in parietal command neurons (Figs. 5 and 7). In addition, FMRFamide mediates an unidentified mechanism that contributes to the inhibition of evoked spiking in avoidance command cells (Figs. 9 and 10). 5. In parallel with the foregoing, stimulation of the mesocerebrum inhibited a group of serotonergic neurons in the pedal ganglion that is capable of facilitating the afferent excitation of the avoidance command cells (Fig. 8).