Serotonin integrates the feeding behavior of the medicinal leech

1. We have investigated functional roles of serotonin (5-hydroxytryptamine, 5-HT) in the feeding behavior of the medicinal leech,Hirudo medicinalis. Bathing intact leeches in serotonin (3×10−5 mol/l, 20 min) significantly alters three components of their feeding behavior. Leeches exposed to serotonin initiate swimming toward a vibrating point with a shorter latency, bite a warm surface more frequently, and ingest more blood than control animals. 2. Thermal stimulation of the prostomium evokes pharyngeal peristalsis, jaw movements and an obvious salivation by semidissected preparations. The application of serotonin (10−8 to 10−6 mol/l) directly activates and increases the rates of pharyngeal peristalsis and salivary secretion in these preparations. Serotonin also produces jaw movements and a constriction of the crop when centrally applied. 3. Vibrational stimuli, which evoke the swimming phase of feeding behavior, synaptically excite Retzius cells within segmental ganglia. Thermal stimuli, which evoke the biting phase, synaptically excite 5-HT-containing neurons within the subesophageal ganglia. Intracellular stimulation of identified 5-HT-containing neurons produces pharyngeal peristalsis reliably and enhances salivation. 4. Pharmacological lesions of particular 5-HT neurons abolishes feeding behavior. However, a brief exposure to exogenous 5-HT restores biting behavior. Well-fed leeches do not normally bite, but a brief bath in 5-HT will evoke biting behavior by them as well. 5. Serotonin is localized to a limited population of identifiable neurons in the C.N.S., and plays a pivotal and mandatory role in organizing the feeding behavior of the medicinal leech.