Activity of an Identified Histaminergic Neuron, and Its Possible Role in Arousal of Feeding Behavior in Semi-intact Aplysia

The possible functions of histaminergic neuron C2 were studied in an isolated head preparation from which it was possible to obtain intracellular recordings while the buccal mass exhibited feeding-like responses. Application of food to the lips of the isolated head preparation elicited rhythmic buccal movements that appeared to be ingestion responses, since they moved seaweed into the buccal cavity and towards the esophagus, and their frequency and regularity was similar to the ingestion responses studied in a group of intact animals. The ingestion responses of the buccal mass consisted of 2 main phases of movement of the radula from a middle rest position: forward and return to rest, and backward and return to rest. The relative magnitudes and timing of these 2 phases were variable. Intracellular recordings from C2 in the isolated head revealed that C2 is silent when the buccal mass is quiescent, but that it can be excited into spike activity, either by mechanical stimulation of the perioral zone or by chemostimulation that results in rhythmic movement. C2 fires a burst of spikes in phase with each protraction-retraction cycle, and, if the movements continue, C2 fires even when the eliciting stimulus has been removed. Activity of the cell was usually preceded by fast depolarizing responses that appeared to be blocked axon spikes. The evidence suggests that C2 is part of a positive feedback loop that may help maintain the persistence of arousal of feeding behavior beyond the time that food stimuli are removed.