Journal of Comparative Physiology A

, Volume 172, Issue 2, pp 223–234

Modulation of swimming behavior in the medicinal leech

I. Effects of serotonin on the electrical properties of swim-gating cell 204
  • J. D. Angstadt
  • W. O. Friesen

DOI: 10.1007/BF00189398

Cite this article as:
Angstadt, J.D. & Friesen, W.O. J Comp Physiol A (1993) 172: 223. doi:10.1007/BF00189398


The effects of serotonin on the electrical properties of swim-gating neurons (cell 204) were examined in leech (Hirudo medicinalis) nerve cords. Exposure to serotonin decreased the threshold current required to elicit swim episodes by prolonged depolarization of an individual cell 204 in isolated nerve cords. This effect was correlated with a more rapid depolarization and an increased impulse frequency of cell 204 in the first second of stimulation. In normal leech saline, brief depolarizing current pulses (1 s) injected into cell 204 failed to elicit swim episodes. Following exposure to serotonin, however, identical pulses consistently evoked swim episodes. Thus, serotonin appears to transform cell 204 from a gating to a trigger cell.

Serotonin had little effect on the steady-state currentvoltage relation of cell 204. However, serotonin altered the membrane potential trajectories in response to injected current pulses and increased the amplitude of rebound responses occurring at the offset of current pulses. These changes suggest that serotonin modulates one or more voltage dependent conductances in cell 204, resulting in a more rapid depolarization and greater firing rate in response to injected currents. Thus, modulation of intrinsic ionic conductances in cell 204 may account in part for the increased probability of swimming behavior induced by serotonin in intact leeches.

Key words

Hirudo medicinalis5-HydroxytryptamineNeuromodulationPostinhibitory reboundInward rectification



afterhyperpolarizing potential


discontinuous current clamp


dorsal posterior nerve


segmental ganglion 2


postinhibitory rebound


resting membrane potential

Copyright information

© Springer-Verlag 1993

Authors and Affiliations

  • J. D. Angstadt
    • 1
  • W. O. Friesen
    • 1
  1. 1.Department of Biology and NSF Center for Biological TimingUniversity of VirginiaCharlottesvilleUSA
  2. 2.Department of BiologySiena CollegeLoudonvilleUSA