Rapid mobilization of AMPA receptors into silent synapses via the activation of NMDA receptors in low-density hippocampal cultures. A, Double staining of low-density hippocampal neurons that had been treated chronically withd,l-APV by antibodies against an NMDA receptor subunit (NR1) and AMPA receptor subunits (GluR1/2/3). Top two panels, A neuron that had been incubated in aCSF with 100 μm glycine, 200 μm d,l-APV, and no added Mg²⁺ for 15 min. Bottom two panels, A neuron that had been incubated in similar aCSF without d,l-APV. B, The number of AMPA clusters was increased significantly via the activation of NMDA receptors by glycine and 0 Mg²⁺.C, The proportion of silent synapses was decreased significantly via the activation of NMDA receptors. D–Fare similar to A–C, with the use of neurons that had been grown in culture medium in the absence of d,l-APV.Open bars, Neurons that had been incubated in aCSF containing glycine, d,l-APV, and no added Mg²⁺; filled bars, neurons that had been incubated in aCSF containing glycine, no d,l-APV, and no added Mg²⁺. ***p < 0.001.