The behavioural state of satiation in the leech is regulated by body distension and mimicked by serotonin depletion.

The behavioural state of an organism consists of an array of constituent behaviours or responses which may be altered differentially as the behavioural state of an animal changes (Kupferman et al. 1991). The medicinal leech, Hirudo medicinalis , exhibits a long cycle in feeding behaviour (Dickinson and Lent, 1984). When they have not fed for some months, Hirudo exhibit an array of appetitive behaviours that are associated with the state of hunger. These leeches rest near the water surface and spontaneously initiate swimming bouts. Shadows, mechanical shocks or surface ripples alert hungry Hirudo, after which they swim towards a point source of water waves. Upon contacting a surface, leeches cease to swim and will explore the surface by crawling. If warmth is encountered at this time, hungry leeches will bite; biting is typically localized in or near surface folds. During the ensuing consummatory phase of hunger, leeches ingest blood meals which increase their volume by approximately nine times and which distend their body walls. Distension acts as a stimulus that terminates feeding and produces the behavioural state of satiation (Lent and Dickinson, 1987). Recently fed leeches bite rarely, swim infrequently and appear to select greater depths in water. Hunger alternates with satiation in this behavioural cycle whose period is approximately a full year. Neuronal serotonin controls a variety of behavioural and physiological components of leech ingestive behaviour (Lent and Dickinson, 1984). Serotonin enhances spontaneously generated bouts of swimming (Willard, 1981) as well as swimming in response to water vibration (Brodfuehrer and Friesen, 1984; Lent and Dickinson, 1984). In addition, serotonin increases the frequency of biting by hungry leeches and evokes biting by satiated ones. Ganglionic serotonin levels are decreased after feeding (Lent et al. 1991) and pharmacological lesions of serotonin-containing neurones decrease biting frequency in a manner reminiscent of satiation (Lent and Dickinson, 1984; O’Gara et al. 1991). We sought to describe quantitatively these two feeding states of Hirudo by measuring a number of clearly definable behavioural responses during hunger and satiation. We quantified several behaviours (phototaxis, thigmotaxis, geotaxis, swim initiation latency and biting frequency) that are known to be or are hypothesized to be associated with the feeding status of this animal. We assessed correlation between tests of each category of