AN INCREASE IN ACTIVITY OF SEROTONERGIC RETZIUS NEURONS MAY NOT BE NECESSARY FOR THE CONSUMMATORY PHASE OF FEEDING IN THE LEECH HIRUDO-MEDICINALIS

During the consummatory phase of feeding, in which blood is ingested, medicinal leeches display a characteristic set of behaviours: they ext end their jaws, are less responsive to sensory input, produce mucus, r elax the body wall and exhibit waves of peristalsis that can run the l ength of the body, Earlier reports suggested that this pattern of beha viour is orchestrated by serotonin released from Retzius cells in resp onse to the appropriate sensory stimulation of the lip. We have develo ped a semi-intact preparation in which only the nervous system in the posterior half of the leech was exposed, The front half of the leech w as free to explore, bite through and feed until satiated from a blood- filled sausage casing while continuous intracellular and extracellular recordings were made from identified cells and the nerve roots of the exposed segments. Prior to attachment of the animal to the feeding de vice, the firing frequency of the Retzius cell increased transiently d uring spontaneous movements or tactile stimuli to its front or posteri or end, In contrast, Retzius cell activity decreased after the anterio r sucker attached to the membrane of the feeding device at about the t ime when ingestion was initiated. Increased activity of Leydig cells, which are known to modulate several circuits in the leech, was also as sociated with exploration, However, unlike that of Retzius cells, the activity of Leydig cells increased significantly following the onset o f consumption. These results suggest that increased activity of Retziu s cells in midbody ganglia is not a prerequisite for the consummatory phase of feeding and raises questions regarding the role of serotonin in regulating this behaviour.