Distribution, density, and clustering of functional glutamate receptors before and after synaptogenesis in hippocampal neurons

Postsynaptic differentiation during glutamatergic synapse formation is poorly understood. Using a novel biophysical approach, we have investigated the distribution and density of functional glutamate receptors and characterized their clustering during synaptogenesis in cultured hippocampal neurons. We found that functional alpha- amino-3-hydroxy-5-methyl-4-isoxazolpropionate (AMPA) and N-methyl-D-aspartate (NMDA) receptors are evenly distributed in the dendritic membrane before synaptogenesis with an estimated density of 3 receptors/mu m(2). Following synaptogenesis, functional AMPA and NMDA receptors are clustered at synapses with a density estimated to be on the order of 10(4) receptors/mu m(2), which corresponds to similar to 400 receptors/synapse. Meanwhile there is no reduction in the extrasynaptic receptor density, which indicates that the aggregation of the existing pool of receptors is not the primary mechanism of glutamate receptor clustering. Furthermore our data suggest that the ratio of AMPA to NMDA receptor density may be regulated to be close to one in all dendritic locations. We also demonstrate that synaptic AMPA and NMDA receptor clusters form with a similar time course during synaptogenesis and that functional AMPA receptors cluster independently of activity and glutamate receptor activation, including following the deletion of the NMDA receptor NR1 subunit. Thus glutamate receptor activation is not necessary for the insertion, clustering, and activation of functional AMPA receptors during synapse formation, and this process is likely controlled by an activity-independent signal.