Silent synapses are excitatory synapses endowed exclusively with N-methyl-D-aspartate (NMDA) responses that have been proposed to acquire alpha-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 Mg(2+) 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 Mg(2+) (1 mM). A holding potential of +60 mV removes Mg(2+) 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 (tau(w)) of NMDA-sEPSCs and decreased with development along the reported decrease of tau(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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