ABSTRACT
The nervous system consists of a vast network of cells, many of which are anatomically and functionally unique. The simple nervous system of the marine mollusc Aplysia is particularly suitable for analyzing neuron-specific gene expression because it contains only about 20,000 central nerve cells which are collected into four pairs of symmetric ganglia and a single asymmetric abdominal ganglion. During the egg-laying season, more than half of the protein synthetic machinery of the bag cells is devoted to the production of polypeptide precursors of egg-laying hormone (ELH) neuroactive peptide ELH. The immunofluorescent studies of Chiu and Strumwasser based on serial sections have shown that occasional unidentified neurons located outside the bag cell cluster express ELH-related peptides. The conventional transmitter substances, neuroactive peptides can act locally as neurotransmitters on neighboring neurons. The fact that active peptide sequences are interspersed in the midst of nonfunctional protein sequences enhances the potential for evolutionary change in peptides and therefore in behavior.
