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The Aplysia neuroendocrine system is a particularly advantageous model for cellular and molecular studies because of the relatively small number and large size of its component neurons. Recombinant DNA techniques have been used to isolate the genes that encode the precursors of peptides expressed in identified neurons of known function. The organization and developmental expression of these genes have been examined in detail. Several of the genes encode precursors of multiple biologically active peptides that are expressed in cells which also contain classical transmitters. These studies, as well as immunohistochemical studies and the use of intracellular recording and voltage clamp techniques are the first steps toward revealing the mechanisms by which neuropeptides govern simple behaviors.

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Respiratory pumping of the gill and siphon of Aplysia californica is a fixed-action pattern coordinated by a defined set of interneurons and motor neurons. In semi-intact preparations of the gill and siphon innervated by the abdominal ganglion, respiratory pumping is facilitated for a prolonged period following activation of the peptidergic bag cell neurons. The induced changes in contractile behavior of the gill and siphon correlate with cell-specific actions of the bag cells on motor neurons regulating these organs. Our results suggest that peptidergic neurons can alter the expression of a fixed pattern of behavior by modulating the excitability of motor neurons controlling the behavior.