Journal of Comparative Physiology A

, Volume 191, Issue 8, pp 715–732 | Cite as

Mechanisms of postinhibitory rebound and its modulation by serotonin in excitatory swim motor neurons of the medicinal leech

  • James D. Angstadt
  • Jeffrey L. Grassmann
  • Kraig M. Theriault
  • Sarah M. Levasseur
Original Paper


Postinhibitory rebound (PIR) is defined as membrane depolarization occurring at the offset of a hyperpolarizing stimulus and is one of several intrinsic properties that may promote rhythmic electrical activity. PIR can be produced by several mechanisms including hyperpolarization-activated cation current (Ih) or deinactivation of depolarization-activated inward currents. Excitatory swim motor neurons in the leech exhibit PIR in response to injected current pulses or inhibitory synaptic input. Serotonin, a potent modulator of leech swimming behavior, increases the peak amplitude of PIR and decreases its duration, effects consistent with supporting rhythmic activity. In this study, we performed current clamp experiments on dorsal excitatory cell 3 (DE-3) and ventral excitatory cell 4 (VE-4). We found a significant difference in the shape of PIR responses expressed by these two cell types in normal saline, with DE-3 exhibiting a larger prolonged component. Exposing motor neurons to serotonin eliminated this difference. Cs+ had no effect on PIR, suggesting that Ih plays no role. PIR was suppressed completely when low Na+ solution was combined with Ca2+ -channel blockers. Our data support the hypothesis that PIR in swim motor neurons is produced by a combination of low-threshold Na+ and Ca2+ currents that begin to activate near −60 mV.


Hirudo medicinalis 5-hydroxytryptamine Motor Circuit Neuromodulation PIR 





Bis(o-aminophenoxy)ethane-N,N,N′,N′-tetra-acetic acid


Central pattern generator


Dorsal excitatory motor neuron 3




Normal leech saline solution


Postinhibitory rebound


Resting membrane potential


Rebound response area




Ventral excitatory motor neuron 4


Holding potential



The authors thank Dr. Carlos Amaya for assistance with statistical analysis of the data and Dr. W. Otto Friesen for his comments on the manuscript. This work was supported by NSF grant IBN-9904431 to JDA and by a 2003 Siena College Summer Research Fellowship to JDA.


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Copyright information

© Springer-Verlag 2005

Authors and Affiliations

  • James D. Angstadt
    • 4
  • Jeffrey L. Grassmann
    • 1
  • Kraig M. Theriault
    • 2
  • Sarah M. Levasseur
    • 3
  1. 1.University of New England College of Osteopathic MedicineBiddefordUSA
  2. 2.State University of New York at AlbanyAlbanyUSA
  3. 3.Albany Medical CollegeAlbanyUSA
  4. 4.Department of BiologySiena CollegeLoudonvilleUSA

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