INGESTIVE SENSORY INPUTS EXCITE SEROTONIN EFFECTOR NEURONS AND PROMOTE SEROTONIN DEPLETION FROM THE LEECH CENTRAL-NERVOUS-SYSTEM AND PERIPHERY

Thermal and chemical stimuli known to ingestive behaviours in the medi cinal leech medicinalis were tested for their physiological effects on Retzius neurones and for their biochemical effects on serotonin level s in the central nervous system, pharynx and body wall. Retzius neuron es throughout the leech nerve cord receive excitatory synaptic input d uring thermal or chemical stimulation of the prostomial lip. These neu rones respond to the rate of change of temperature as well as to absol ute temperature at the lip. Exposure of the lip to sodium chloride exc ites Retzius neurones, whereas exposure to arginine has little effect. Thermal stimulation of the lip elicits a more rapid but less prolonge d excitation of Retzius neurones than does chemical stimulation. Stimu lation of the prostomial lip is associated with afferent activities in the cephalic nerves D-1, D-2 and V-1-2. Thermal stimulation of the pr ostomial lip results in depletion of serotonin from midbody ganglia, w hereas chemical stimulation has no effect. Conversely, chemical stimul ation of the lip results in depletion of serotonin from the body wall, whereas thermal stimulation does not. Pharyngeal serotonin content is decreased with either modality. These data distinguish two important feeding-related sensory input pathways to central serotonergic effecto r neurones in Hirudo medicinalis.