Figure 4. Postsynaptic silence demonstrated by two-photon laser glutamate uncaging.

a | The left-hand panel shows a spine and its parent dendrite, with sites of glutamate uncaging indicated by asterisks; the show the average| AMPAR-mediated unitary excitatory postsynaptic current (uEPSC) at each site. On each spine tested, an area of maximal response (in this case the site indicated by the lower asterisk) could be located, illustrating the precision of the technique of glutamate uncaging. The right-hand panel shows a lower-magnification view of the basal dendrite of a layer II/III cortical pyramidal neuron from a postnatal day (P) 9 rat, showing examples of mature spines (indicated by the white arrowheads) and a filopodium (indicated by the red arrowhead). b | At P9, glutamate uncaging at a spine revealed no uEPSC at −70 mV but did reveal a slow, NMDAR-mediated uEPSC at +40 mV; five individual trials (grey traces) and the corresponding averages (blue traces) are shown. This is a silent synapse that clearly lacks functional AMPARs in the postsynaptic membrane. c | NMDAR-mediated uEPSC amplitudes versus AMPAR-mediated uEPSC amplitudes are plotted in control conditions (n = 72 spines, P9–13) or in the presence of the AMPAR antagonist NBQX (n = 13 spines, P9–15). The dashed line marks the expected position for silent spines lacking AMPARs. Many control spines fall along this line, exhibiting AMPAR-mediated uEPSCs similar to those recorded in the presence of NBQX, illustrating the large number of silent synapses in these young animals. During the second postnatal week, silent synapses became increasingly hard to find and had virtually disappeared by P14–17 (not shown). Parts a and b reproduced and modified (respectively), with permission, from REF. 100 © (2008) Cambridge University Press.