SHORT COMMUNICATION 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, Hirudoexhibit 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 thi