Post‐translational processing in model neuroendocrine systems: Precursors and products that coordinate reproductive activity in Aplysia and Lymnaea

The regulation of egg‐laying behavior in the hermaphroditic marine mollusc Aplysia has been an important model system that has provided insight into the cellular and molecular bases of neuroendocrine function. The advantages of this system derive primarily from the unique characteristics of the bag cells, two clusters of neurosecretory cells whose peptide products initiate and coordinate egg deposition. In particular, the large number of bag cell neurons in a single animal and their relative isolation from other elements of the nervous system facilitate biochemical analyses of the peptides synthesized and released by the cells. Moreover, the episodic nature of bag cell electrical activity, combined with the relative simplicity of the Aplysia nervous system and the stereotypic nature of the behaviors exhibited during egg deposition, provide unusually favorable opportunities for investigating neuropeptide action and function. This review will focus on neuropeptide synthesis and processing in the bag cells and in other organs expressing structurally related genes.