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The Journal of Neurophysiology Vol. 84 No. 1 July 2000, pp. 415-427
Copyright ©2000 by the American Physiological Society
1Department of Biological Sciences, University of Illinois at Chicago, Chicago 60607; and 2Department of Physiology, Northwestern University Medical School, Chicago, Illinois 60611
Schwartz, Neil E. and
Simon Alford.
Physiological Activation of Presynaptic Metabotropic Glutamate
Receptors Increases Intracellular Calcium and Glutamate Release. J. Neurophysiol. 84: 415-427, 2000. Activation of metabotropic glutamate receptors (mGluRs) has
diverse effects on the functioning of vertebrate synapses. The cellular
mechanisms that underlie these changes, however, are largely unknown.
The role of presynaptic mGluRs in modulating Ca2+ dynamics
and regulating neurotransmitter release was investigated at the
vestibulospinal-reticulospinal (VS-RS) synapse in the lamprey brain
stem. Application of the specific Group I mGluRs antagonist 7-(hydroxyimino) cyclopropa[b]chromen-1a-carboxylate ethyl ester (CPCCOEt) reduced the amplitude of consecutive high-frequency evoked
excitatory postsynaptic currents (EPSCs). A series of experiments using
techniques of electrophysiology and calcium imaging were carried out to
determine the cellular mechanisms by which this phenomenon occurs.
Concentration-dependent increases in the pre- and postsynaptic
[Ca2+]i were seen with the application of
mGluR agonists. Similarly, high-frequency stimulation of axons caused a
Group I mGluR-dependent enhancement in presynaptic Ca2+
transients. Application of mGluR agonist caused a depolarization of the
presynaptic elements, while thapsigargin decreased the high-frequency
stimulus- and agonist-induced rises in
[Ca2+]i. These data suggest that both
membrane depolarization and the release of Ca2+ from
intracellular stores potentially play a role in mGluR-induced Ca2+ signaling. To determine the effect of this modulation
of Ca2+ dynamics on spontaneous glutamate release,
miniature EPSCs were recorded from postsynaptic reticulospinal neurons.
A potent Group I mGluR agonist, (S)-homoquisqualic acid,
caused a large increase in the frequency of events. These results
demonstrate the presence of presynaptic Group I mGluRs at the VS-RS
synapse. Activation of these receptors leads to a rise in
[Ca2+]i and enhances the spontaneous and
evoked release of glutamate. Taken together, these studies highlight
the importance of synaptic activation of these facilitatory
autoreceptors in both short-term plasticity and synaptic transmission.
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