The long term goal of the proposed research is to analyze the physiological mechanisms underlying the behavioral expression of arousal states. We plan to use the food-induced arousal state in Aplysia as a model system of arousal in higher animals. Specifically we propose a series of studies designed to study the effects of a serotonergic neuron (the metacerebral cell, MCC), on the modulation of feeding behavior. The first aim is to study the relationship of the type of synaptic actions of the MCC to the functional requirements of the post-synaptic neuron. The second aim is to explore the rules relating the type of synaptic action to the absence or presence of the mediation by a second messenger such as cyclic AMP. The final aim is to determine the functional role of MCC modulation of elements of the putative pattern generator that drives feeding responses. The methodology involves behavioral observation, intracellular recording, and biochemical assay techniques. The research should provide basic data relevant to the understanding of neurological disorders of movement, and psychiatric disorders, particularly those involving an affective component.