The Journal of Neuroscience, May 1, 1999, 19(9):3457-3471
Activity Patterns and Synaptic Organization of Ventrally Located
Interneurons in the Embryonic Chick Spinal Cord
Amy
Ritter,
Peter
Wenner,
Stephen
Ho,
Patrick J.
Whelan, and
Michael J.
O'Donovan
Section on Developmental Neurobiology, Laboratory of Neural
Control, National Institute of Neurological Disorders and Stroke,
National Institutes of Health, Bethesda, Maryland 20892
To investigate the origin of spontaneous activity in developing
spinal networks, we examined the activity patterns and synaptic organization of ventrally located lumbosacral interneurons, including those whose axons project into the ventrolateral funiculus (VLF), in
embryonic day 9 (E9)-E12 chick embryos. During spontaneous episodes,
rhythmic synaptic potentials were recorded from the VLF and from spinal
interneurons that were synchronized, cycle by cycle, with rhythmic
ventral root potentials. At the beginning of an episode, ventral root
potentials started before the VLF discharge and the firing of
individual interneurons. However, pharmacological blockade of recurrent
motoneuron collaterals did not prevent or substantially delay
interneuron recruitment during spontaneous episodes. The synaptic
connections of interneurons were examined by stimulating the VLF and
recording the potentials evoked in the ventral roots, in the VLF, or in
individual interneurons. Low-intensity stimulation of the VLF evoked a
short-latency depolarizing potential in the ventral roots, or in
interneurons, that was probably mediated mono- or disynaptically. At
higher intensities, long-latency responses were recruited in a highly
nonlinear manner, eventually culminating in the activation of an
episode. VLF-evoked potentials were reversibly blocked by extracellular
Co2+, indicating that they were mediated by chemical
synaptic transmission. Collectively, these findings indicate that
ventral interneurons are rhythmically active, project to motoneurons,
and are likely to be interconnected by recurrent excitatory synaptic
connections. This pattern of organization may explain the synchronous
activation of spinal neurons and the regenerative activation of spinal
networks when provided with a suprathreshold stimulus.
Key words:
spinal cord; rhythmic activity; interneurons; development; synchrony; chick
Copyright © 1999 Society for Neuroscience 0270-6474/99/1993457-15$05.00/0
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