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Abstract
Hungry medicinal leeches, Hirudo medicinalis, bite warm surfaces and ingest blood
meals averaging 890% of their weight. Satiation lasts 12-18 months during which leeches
avoid warm surfaces and will not bite. The segmental nervous system of the leech is
distinguished by a population of neurons which contain serotonin (5-Hydroxytryptamine,
5-HT) at high concentrations. Some of these identified 5-HT neurons directly activate
the effectors responsible for three physiological components of feeding: salivary
secretion, bite-like movements and pharyngeal peristalsis. A localized warming of
the lip is sufficient to initiate ingestion and synaptically excites anterior 5-HT
cells into high frequency impulses or bursts. Distension of the body wall terminates
ingestion and also hyperpolarizes these 5-HT neurons. Serotonin treatment produces
hyperphagic behavior by the leech, while a specific pharmacological lesion of its
5-HT cell produces the anorexic behavior of satiation. This anorexia is transiently
reversed by 5-HT treatment. Serotonin plays an obligatory role in the initiation and
expression of leech feeding behavior by its differential modulation of central neuronal
networks and peripheral glands and muscles.