SYNAPTIC COEXISTENCE OF AMPA AND NMDA RECEPTORS IN THE RAT HIPPOCAMPUS - A POSTEMBEDDING IMMUNOGOLD STUDY

Synaptic distributions oft N-methyl-D-aspartate (NMDA) and alpha-amino -3-hydroxy-5-methylisoxazolepropionic acid (AMPA) receptor subunits, N MDAR1 and GluR2, respectively, were examined by electron microscopy wi th the high spatial resolution of postembedding immunogold localizatio n. We provide direct evidence for colocalization at individual axodend ritic asymmetric synapses within the CA1 subfield of rat hippocampus. AMPA/NMDA receptor colocalization was found broth in gamma-aminobutyri c acid (GABA)ergic dendrites and non-GABAergic dendritic shafts, as we ll as dendritic spines, Some asymmetric synapses were found to contain only NMDAR1 or GluR2; however, most immunopositive synapses contained both subunits. Many NMDAR1 and/or GluR2 immunopositive profiles recei ved GABAergic innervation at an adjacent synapse, providing a, substra te for GABAergic modulation of both GluR classes, These data suggest t hat excitatory neuronal transmission in CA1 neurons may generally invo lve activation of both NMDA and AMPA receptor subunits at a single syn apse, however they also offer ultrastructural evidence for NMDAR1-only synapses that might represent silent synapses, J. Neurosci. Res. 54:4 44-449, 1998, (C) 1998 Wiley-Liss, Inc.