The spatial distribution of glutamatergic inputs to dendrites of retinal ganglion cells

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Abstract
The spatial pattern of excitatory glutamatergic input was visualized in a large series of ganglion cells of the rabbit retina, by using particle‐mediated gene transfer of an expression plasmid for postsynaptic density 95‐green fluorescent protein (PSD95‐GFP). PSD95‐GFP was confirmed as a marker of excitatory input by co‐localization with synaptic ribbons (RIBEYE and kinesin II) and glutamate receptor subunits. Despite wide variation in the size, morphology, and functional complexity of the cells, the distribution of excitatory synaptic inputs followed a single set of rules: 1) the linear density of synaptic inputs (PSD95 sites/linear μm) varied surprisingly little and showed little specialization within the arbor; 2) the total density of excitatory inputs across individual arbors peaked in a ring‐shaped region surrounding the soma, which is in accord with high‐resolution maps of receptive field sensitivity in the rabbit; and 3) the areal density scaled inversely with the total area of the dendritic arbor, so that narrow dendritic arbors receive more synapses per unit area than large ones. To achieve sensitivity comparable to that of large cells, those that report upon a small region of visual space may need to receive a denser synaptic input from within that space. J. Comp. Neurol. 510:221–236, 2008.