Many morphological silent synapses exist in high-density cultured cortical neurons chronically treated withd,l-APV. A, Double staining of high-density cultured cortical neurons by antibodies against AMPA receptor subunits (GluR1/2/3, left) and an NMDA receptor subunit (NR1,right). B, Enlarged images (2×) fromA [left, GluR1/2/3;middle, NR1; right, overlay of GluR1+GluR2 (red) and NR1 (green)]. Scale bar: B, D,F, 10 μm. C, Double staining of AMPA receptor subunits (GluR1/2/3, left) and synaptophysin (right). D, Enlarged images (2×) fromC [left, GluR1+GluR2;middle, synaptophysin; right, overlay of GluR1+GluR2 (red) and synaptophysin (green)]. E, Double staining of NMDA receptor subunit (NR1, left) and synaptophysin (right). F, Enlarged images (2×) fromE [left, NR1; middle, synaptophysin; right, overlay of NR1 (red) and synaptophysin (green)].