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The Journal of Neurophysiology Vol. 87 No. 3 March 2002, pp. 1263-1270
Copyright ©2002 by the American Physiological Society
Departments of Physiology and Biophysics, Georgetown University School of Medicine, Washington, DC 20007
Losi, Gabriele,
Kate Prybylowski,
ZhanYan Fu,
Jian Hong Luo, and
Stefano Vicini.
Silent Synapses in Developing Cerebellar Granule Neurons. J. Neurophysiol. 87: 1263-1270, 2002. Silent synapses are excitatory synapses endowed exclusively with
N-methyl-D-aspartate (NMDA) responses that
have been proposed to acquire
-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) responses
during development and after long-term potentiation (LTP). These
synapses are functionally silent because of the Mg2+ block
of NMDA receptors at resting potentials. Here we provide evidence for
the presence of silent synapses in developing cerebellar granule cells.
Using the patch-clamp technique in the whole-cell configuration, we
recorded the spontaneous excitatory postsynaptic currents (sEPSCs) from
rat cerebellar granule cells in culture and in slices at physiological
concentration of Mg2+ (1 mM). A holding potential of +60 mV
removes Mg2+ block of NMDA channels, allowing us to record
NMDA-sEPSCs. We thus compared the frequency of AMPA-sEPSCs, recorded at
60 mV, with that of NMDA-sEPSCs, recorded at +60 mV. NMDA-sEPSCs
occurred at higher frequency than the AMPA-sEPSCs in most cells
recorded in slices from rats at postnatal day (P) <13 and in culture
at 6-8 days after plating (DIV6-8). In a few cells from young rats (P6-9) and in most neurons in culture at DIV6 we recorded exclusively NMDA-sEPSCs, supporting the hypothesis of existence of functional synapses with NMDA and without AMPA receptors. Increasing glutamate release in the slice with cyclothiazide and temperature increased AMPA
and NMDA-sEPSCs frequencies but failed to alter the relative ratio of
frequency of occurrence. Frequency ratio of NMDA versus AMPA-sEPSCs in
slices was correlated with the weighted time constant of decay
(
w) of NMDA-sEPSCs and decreased with
development along the reported decrease of w.
We suggest that the prevalence of synaptic NR2A subunits that confer
faster kinetics is paralleled by the disappearance of silent synapses
early in cerebellar development.
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