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Journal of Neuroscience, Vol 8, 2530-2543, Copyright © 1988 by Society for Neuroscience
The development of sensorimotor synaptic connections in the lumbosacral cord of the chick embryo
MT Lee, MJ Koebbe and MJ O'Donovan
Department of Physiology and Biophysics, University of Iowa, Iowa City 52242.
We have examined the development of synaptic connections between afferents
and motoneurons in the lumbosacral spinal cord of the chick embryo between
stages 28 and 39. The central projection of afferents was visualized
following injection of dorsal root ganglia with HRP. Afferent fibers first
entered the dorsal gray matter between stages 29 and 31. They grew in a
ventrolateral direction, reaching motoneuron dendrites by stage 32.
Quantitative analysis of axon numbers suggested that individual axons did
not begin to branch extensively until they approached the lateral motor
column at stage 36. Connectivity between afferents and motoneurons was
assessed by stimulating dorsal roots or nerves supplying the femorotibialis
muscle and recording the resulting motoneuron synaptic potentials
intracellularly or from the cut ventral roots. At stages 37-39,
low-intensity stimulation produced a short- latency positive potential that
was followed at higher stimulus currents by slower positive potentials. All
of these potentials were abolished in solutions that block chemical
synaptic transmission (zero Ca2+/2 mM Mn2+). The early potential, which
includes the monosynaptic EPSP produced by muscle afferents, persisted in
the presence of the N- methyl-D-aspartate antagonist,
2-amino-5-phosphonovaleric acid (APV), but was largely eliminated by the
more general excitatory amino acid antagonist, kynurenic acid. Therefore,
in the chick, as in other species, a glutamate-like transmitter appears to
be released at the synapses between muscle afferents and motoneurons. The
APV-resistant potential was reduced in amplitude during bath application of
the glycine and GABA antagonists, strychnine and picrotoxin, suggesting
that it was composed of depolarizing inhibitory as well as excitatory
components at these stages. The monosynaptic EPSP could be recorded in
ventral roots as early as stages 32-33, when muscle afferents first grew
into the vicinity of motoneuron dendrites. The EPSP in these young embryos
was unaffected by picrotoxin and strychnine, but responded to APV and
kynurenate in a manner similar to that at later stages. Between stages 28
and 32, only long-latency, slowly rising potentials could be evoked in the
ventral roots by afferent activation. These potentials were abolished by
superfusion with zero Ca2+/2 mM Mn2+, APV, or kynurenic acid, and could be
revealed before stage 31 only by removing Mg2+ from the bath.(ABSTRACT
TRUNCATED AT 400 WORDS)
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