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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.

Single nerve cells can use more than one substance as extracellular chemical messengers. Classical transmitters have been shown to coexist in the same neuron and possibly even in the same vesicle as neuroactive peptides. Furthermore, multiple neuroactive peptides, which are thought to be coreleased, are often encoded in the same precursor assuring stoichiometric synthesis. The precise organization of multiple message systems and the physiological significance of the coexistence is poorly understood. The abdominal ganglion of the gastropod mollusc Aplysia contains a number of identified neurons that are cotransmitter candidates. One such cell, Lii, is cholinergic and probably also uses biologically active peptides. Differential screening with labeled cDNA was used to isolate cDNA clones expressed specifically in the bag cells and abdominal ganglion neurons L11 or R15. Analysis of an L11-specific clone suggests that it encodes a 14.7-kDa protein that is the precursor for the secreted peptides. The poly(A)+ RNA transcript is -1.2 kilobases and there are 1-3 copies of this gene in the Aplysia haploid genome.The behavioral potential of an organism is determined by the specific interactions between cells of the nervous system. Intercellular communication in the brain is generated by two general mechanisms. First, a presynaptic neuron can send an axon to an appropriate position and release a chemical messenger, which traverses a short distance to bind a receptor on a target cell. The specialized membrane structures at the synapse juxtapose release sites with receptors and often include efficient mechanisms for removing the transmitter from the synaptic cleft. The chemical messengers at synapses are normally classic neurotransmitters, such as acetylcholine and y-aminobutyric acid. Neurons can also influence electrical and metabolic activities of target cells some distance from the site of messenger release. This form of intercellular communication is often mediated by biologically active peptides, which are carried to target tissues through the circulation or simply through passive diffusion. The specificity of these interactions is not determined by spatial constraints but by the distribution of highly specific receptors for the messengers. In addition, the concentration of this type of extracellular messenger necessary to achieve a biological response is often several orders of magnitude lower than that of classical transmitters. Thus, in thinking of neural circuitry, one must superimpose these two forms of communication between cells to generate an accurate representation of neuronal interactions. These two general forms of intercellular communication are of course not as distinct as outlined above. Recent investigations suggest that a single neuron may use more than one transmitter (1), frequently a classic neurotransmitter and a peptide, to elicit a variety of responses (2, 3).To clarify these and other issues, we use the gastropod Fig. 1). In this report, we extend these studies to a number of other pepti...

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