Journal of Neuroscience, Vol 14, 4684-4693, Copyright © 1994 by Society for Neuroscience
Heterosynaptic suppression of developing neuromuscular synapses in culture
YJ Lo and MM Poo
Department of Biological Sciences, Columbia University, New York, New York 10027.
Activity-dependent competition between two spinal neurons coinnervating an
embryonic myocyte was studied in Xenopus cell culture. We have
characterized in detail the phenomenon of heterosynaptic suppression by
which tetanic stimulation of one neuron results in functional suppression
of the synapse made by the untetanized neuron (Lo and Poo, 1991).
Fluorescence labeling of the neurons using two different fluorophores
revealed that the coinnervating nerve terminals on the spherical myocyte
were in close proximity. Heterosynaptic suppression could be induced when
the postsynaptic cell was held under either current-clamp or voltage-clamp
conditions during the tetanic stimulation. This finding, together with the
observation that repetitive postsynaptic depolarization of the myocyte by
direct current injection was much less effective in inducing synaptic
depression, suggests that postsynaptic ACh receptor activation plays a
dominant role in the induction of heterosynaptic suppression. The
heterosynaptic suppression appears to be mediated by a rise of Ca2+ levels
in the postsynaptic cell, since it was not observed when the cytosolic Ca2+
concentration of the myocyte was buffered at a low level with intracellular
loading of a Ca2+ chelator, 1,2-bis(2-aminophenoxy)ethane-
N,N,N',N'-tetra-acetic acid (BAPTA). The dependence of heterosynaptic
suppression on the pattern of tetanic stimulation was also studied. At a
stimulation frequency of 2 Hz, detectable heterosynaptic suppression could
be induced after 20 repetitive stimuli were applied to one of the
presynaptic neurons and the suppression was more effective with increasing
number of stimuli. Over the range of 0.5-5 Hz, the extent of suppression
was independent of the frequency of tetanic stimulation and, in some cells,
detectable suppression could be induced at a frequency as low as 0.05 Hz.
Except for a few cases, heterosynaptic suppression was found to last for as
long as the recording was made after tetanus (up to 1 hr). The fact that
the mean amplitude of spontaneous synaptic currents remained the same
before and after the suppression while the evoked synaptic currents
exhibited higher fluctuation after suppression suggests that the observed
synaptic suppression involves a reduction of evoked ACh release from the
nerve terminal, although postsynaptic changes have not been excluded.
Finally we found that spontaneous synaptic activity may also contribute in
part to the synaptic competition between coinnervating nerve terminals.
Taken together, these findings provide a quantitative basis for further
understanding of activity-dependent competition between developing
neuromuscular synapses.
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