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Modulation of swimming behavior in the medicinal leech

IV. Serotonin-induced alteration of synaptic interactions between neurons of the swim circuit

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Abstract

Serotonin enhances the expression of swimming in the medicinal leech Hirudo medicinalis. These two reports examine the physiological causes underlying this modulation. The initial paper (Mangan et al. 1994) demonstrated that serotonin enhanced the participation of inhibitory swim motor neurons (MNs) in the generation of the swimming rhythm in the isolated nerve cord. In experiments reported here, we examined whether synaptic interactions between neurons of the swim circuit are altered by serotonin. Following exposure to 50 μM serotonin, pairwise intracellular recording revealed the presence of a time-dependent synaptic decrement. Synaptic decrement was characterized by: 1) a substantial decline in synaptic inhibition (half-decay time about 0.4 s) during constant presynaptic excitation; 2) a reduced half-time of recovery from synaptic inhibition; and 3) a strong dependence on the presynaptic neuron's membrane potential. We found little alteration in the physiology of synaptic transmission involving MNs following amine depletion in leech nerve cords. We propose that alterations in synaptic interactions resulting from exposure to elevated serotonin levels, coupled with the changes in MN cellular properties described earlier, are crucial to the increased efficacy of MNs in participating in generating and expressing the leech swimming rhythm.

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Abbreviations

AHP :

afterhyperpolarizing potential

DCC :

discontinuous current clamp

DE :

dorsal excitor motor neuron

DI :

dorsal inhibitor motor neuron

IPSP :

inhibitory postsynaptic potential

MN :

motor neuron

OIN :

oscillator interneuron

PIR :

postinhibitory rebound

VE :

ventral excitor motor neuron

VI :

ventral inhibitor motor neuron

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Author notes

  1. P. S. Mangan

    Present address: Department of Neurology, University of Virginia, Health Sciences Center, 22908, Charlottesville, VA, USA

Authors and Affiliations

  1. Department of Biology and NSF Center for Biological Timing, University of Virginia, 22901, Charlottesville, VA, USA

    P. S. Mangan, A. K. Cometa & W. O. Friesen

Authors
  1. P. S. Mangan
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  2. A. K. Cometa
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  3. W. O. Friesen
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Mangan, P.S., Cometa, A.K. & Friesen, W.O. Modulation of swimming behavior in the medicinal leech. J Comp Physiol A 175, 723–736 (1994). https://doi.org/10.1007/BF00191844

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