Abstract
Leeches have distinct advantages for investigating the behavioral functions of monoamines. Serotonin (5-hydroxytryptamine, 5-HT) is synthesized by a population of identifiable neurons and stored at unusually high intracellular concentrations. Identified serotonergic neurons evoke biting, salivation and pharyngeal pumping, and neuronal 5-HT is required for these physiological components of leech feeding behavior. Those stimuli which initiate and terminate ingestion by intact leeches, excite and inhibit the 5-HT neurons respectively. Hence, leech 5-HT neurons are active in contexts in which the animal feeds. In leeches representing different orders of Hirudinea, homologous 5-HT neurons appear to have similar roles in feeding. Serotonin not only affects salivation, biting and pharyngeal peristalsis in these annelids, but also in nematodes, arthropods and molluscs. These comparative data suggest that serotonin may function generally in the modulation of feeding.
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