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Journal of Neuroscience, Vol 15, 5900-5911, Copyright © 1995 by Society for Neuroscience
Blocking Ca(2+)-dependent synaptic release delays motoneuron differentiation in the rat spinal cord
H Xie and L Ziskind-Conhaim
Department of Physiology, University of Wisconsin Medical School, Madison 53706, USA.
Development of motoneuron electrical properties and excitability was
studied in spinal cord explants of rat embryos cultured for 1-3 weeks. The
morphological organization of the spinal cord and synaptic inputs onto
motoneurons were maintained in organ culture. The rate of differentiation
of motoneuron resting potential and increase in membrane excitability was
similar in vitro and in vivo, suggesting that these properties were
regulated by cellular signals or extracellular differentiation-promoting
factors that were preserved in culture. However, maturation of input
resistance, action potential threshold and action potential maximum rate of
rise was slower than in vivo. Culturing spinal cord explants with their
dorsal root ganglia attached did not facilitate motoneuron differentiation.
The role of newly formed synaptic pathways in regulating the changes in
motoneuron electrical properties was studied in the presence of blockers of
synaptic transmission. Motoneuron differentiation was delayed in spinal
cords cultured in the presence of TTX, indicating that electrical activity
influenced the time course of their development. However, blocking synaptic
transmission with antagonists of glutamate, glycine, and GABAA receptors
did not affect the rate of motoneuron differentiation, suggesting that
maturation of motoneuron phenotype was independent of activation of these
transmitter-gated channels. Incubating spinal cords in medium containing
high-K+, which increased the frequency of spontaneous potentials, reversed
the inhibitory effect of TTX. Similar to TTX action, motoneuron development
was retarded when synaptic release was chronically blocked with either
tetanus toxin or omega- conotoxin, a Ca2+ channel blocker. These findings
suggested that electrical activity in spinal cord explants modulated
motoneuron differentiation via Ca(2+)-dependent synaptic release of
neurotransmitters or neurotrophic factors.
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