Multiple roles of a histaminergic afferent neuron in the feeding behavior of Aplysia.

The cellular and circuit properties of individual identified neurons in invertebrates can be readily studied; hence it is possible to determine how the complex properties of nerve cells function in the generation of behavior. Recent studies of the cellular basis of feeding behavior in the marine mollusc Aplysia have focused on a neuron, C2, that has a variety of complex properties that determine the behavioral functions of the neuron. C2 conveys mechanosensory information from the mouth of the animal. It receives a complex pattern of inputs during feeding behavior, and generates diverse outputs that may shape behavior. It can act to filter out slow or sporadic sensory inputs, and its own outputs can be 'gated' by synaptic input. C2 uses histamine as its transmitter, and some of its synaptic outputs are modulatory and contribute to the expression of an arousal state induced by food. Other outputs shape feeding behavior directly by affecting motor neurons, as well as presynaptically inhibiting the outputs of feeding motor programs. Thus, the complex properties of this neuron may contribute to the flexibility and adaptability of feeding in Aplysia. Studies of C2 have expanded our concepts of the properties of sensory neurons.