Journal of Comparative Physiology A

, Volume 191, Issue 8, pp 715–732

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

Abstract

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.

Keywords

Hirudo medicinalis 5-hydroxytryptamine Motor Circuit Neuromodulation PIR 

Abbreviations

5-HT

5-hydroxytryptamine

BAPTA

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

CPG

Central pattern generator

DE-3

Dorsal excitatory motor neuron 3

NMDG

N-methyl-D-glucamine

NS

Normal leech saline solution

PIR

Postinhibitory rebound

RMP

Resting membrane potential

RRA

Rebound response area

TEA

Tetraethylammonium

VE-4

Ventral excitatory motor neuron 4

Vh

Holding potential

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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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