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

Summary

• 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).