Different modes of expression of AMPA and NMDA receptors in hippocampal synapses

Postembedding immunogold labeling was used to determine the relationship be tween AMPA and NMDA receptor density and size of Schaffer collateral-commis sural (SCC) synapses of the adult rat. All SCC synapses expressed NMDA rece ptors. AMPA and NMDA receptors were colocalized in at least 75% of SCC syna pses; the ratio of AMPA to NMDA receptors was a linear function of postsyna ptic density (PSD) diameter, with AMPA receptor number dropping to zero at a PSD diameter of similar to 180 nm. These findings indicate that 'silent' SCC synapses are smaller than the majority of SCC synapses at which AMPA an d NMDA receptors are colocalized. Thus synapse size may determine important properties of SCC synapses.