The role of a modulatory neuron in feeding and satiation in Aplysia: effects of lesioning of the serotonergic metacerebral cells.

Food-induced arousal in Aplysia is characterized by a progressive increase in the speed and strength of biting responses elicited by a seaweed stimulus. Data from semi-intact and dissected preparations suggest that the identified, serotonergic, metacerebral cells (MCCs) of the cerebral ganglion contribute to food-induced arousal by enhancing the strength of buccal muscle contractions, and by modulating the output of the central pattern generator for biting movements. In order to test this hypothesis in intact, free-moving animals and to determine if the MCCs play a role in satiation of feeding, the behavior of animals that had their MCCs destroyed by intracellular injection of proteases was compared with that of B Cell-Lesion and Dye injection control animals (Experiment 1) or surgical control animals (Experiment 2). Nonfeeding behaviors such as defensive withdrawal responses, locomotion, and righting reflexes were unaffected by MCC lesioning. Also unaffected by MCC lesioning were appetitive feeding behaviors and the amount of food needed to satiate animals. Significant behavioral deficits in consummatory feeding behaviors, which remained stable for periods exceeding 10 d, were observed in the MCC-lesioned animals but not in controls. Lesioned animals exhibited a slowing of rate of repetitive biting responses by 40% of controls and had reduced magnitudes of repetitive bites, particularly at the end of a testing run of 10 consecutive bites. The deficit in bite magnitude was minimally evident in food-deprived animals (Experiment 1) but became more pronounced as animals were fed to satiation (Experiment 2). MCC-lesioned animals still exhibited a residual build-up of the rate and magnitude of biting responses at the onset of feeding behavior. This suggests that, in addition to the MCCs, there are other sources of modulation that contribute to plasticity of consummatory responses during the food-induced arousal state.