Peptides controlling behavior in Aplysia.
Figure 11 summarizes our present understanding of the relationships between the bag cells, the atrial gland, their respective peptides, the central nervous system, and reproductive behavior. There are some interesting aspects of the overall organization of the system. The three hormonal peptides (ELH and the two atrial gland peptides) have specific actions on the central nervous system not unlike what we are currently learning from mammalian systems (e.g., LHRH and TRH). ELH, in addition, has several specific peripheral targets, the details of which remain to be worked out. The fact that ELH and other hormones have multiple targets within the central nervous system as well as nonnervous peripheral targets raises the question of whether one or more different receptors exist for single hormone. We suggest that peptides larger than perhaps five residues may carry several "messages" or receptor binding sites encoded within the one molecule. Large peptides such as ELH could obviously have separate domains of charge distribution within the molecule, and these would have the advantage, over the classical small molecule transmitters, of activating a variety of very different targets. The atrial gland is a peripheral source of peptides with potent nervous system actions; this is reminiscent of peptides in mammals, e.g., substance P, gastrin, and somatostatin, all of which were initially isolated from the gut and which are now being found in and also have actions on the central nervous system. Such resemblances in the principles of organization between mammals and molluscs are constant reminders that neuropeptidergic systems are old tricks in the evolutionary bag and that what we learn from molluscs and other invertebrates about mechanisms and organization will likely apply to mammals.