Chemical and thermal stimuli have short-lived effects on the retzius cell in the medicinal leech

During the appetitive phase of feeding, hungry leeches detect a prey by the integration of signals perceived by different sensory systems. Earlier reports suggested that chemical or thermal sensory stimulation of the lip was associated with increased afferent activity in cephalic nerves connecting the lip to the central nervous system. These authors further suggested that this activity was relayed to Retzius cells in segmental ganglia, which then released serotonin to initiate and control all aspects of feeding behavior. In this study, we show that chemosensory or thermal activation of the lip lasting for at least 5 min produces a distinct signal in the cephalic nerves consisting of action potentials of low amplitude. These small amplitude signals are clearly distinguishable from the large action potentials evoked by mechanosensory stimuli applied to the same area of the lip. Both types of sensory stimuli also evoke an increase in the firing frequency of the Retzius cells in segmental ganglia. However, the response recorded in the nerves and the Retzius cells during a maintained stimulus is not constant but decreases with an exponential time course. These results agree with our earlier observations on a semi-intact feeding preparation in which we showed that the firing frequency of the Retzius cell decreased as soon as the leech began to ingest its meal. Therefore, our data provide further evidence suggesting that it is unlikely that heat or chemical cues maintain the Retzius cell in an active state throughout the consummatory phase of feeding.