The Journal of Neuroscience, April 15, 2003, 23(8):3154
Variable Properties in a Single Class of Excitatory Spinal
Synapse
David
Parker
Department of Zoology, University of Cambridge, Cambridge CB2 3EJ,
United Kingdom
Although synaptic properties are specific to the type of synapse
examined, there is evidence to suggest that properties can vary in
individual synaptic populations. Here, a large sample of monosynaptic
connections made by excitatory interneurons (EINs) onto motor neurons
in the lamprey spinal cord locomotor network has been used to examine
the properties of a single class of spinal synapse in detail.
The properties and activity-dependent plasticity of EIN-evoked EPSPs
varied considerably. This variability occurred at convergent inputs
made by several EINs onto single motor neurons. This suggests that it
was an intrinsic network property and not simply related to differences
between animals or experiments. The activity-dependent plasticity of
EIN-evoked EPSPs could be negatively or positively related to the
initial EPSP amplitude (P1 and P2 connections, respectively). This
reflected the development of facilitation and depression from either
small or large initial EPSPs.
To identify differences in presynaptic properties that could contribute
to the synaptic variability, the quantal amplitude, release
probability, number of release sites, and size of the available vesicle
pool were examined. This analysis suggested that the variable amplitude
and plasticity of EPSPs at P1 and P2 connections reflected an
interaction between the release probability and the size of the
available transmitter store.
There is thus significant functional variability in EIN synaptic
properties. Synapses ranged from strong (evoked postsynaptic spikes) to
weak (small depressing EPSPs). The selection of interneurons with
different synaptic properties could provide an intrinsic mechanism for
modifying excitatory network interactions and the locomotor network output.
Key words:
interneuron; spinal cord; synaptic plasticity; neural network; motor neuron; lamprey
Copyright © 2003 Society for Neuroscience 0270-6474/03/2383154-10$05.00/0
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