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T. C. Jakobs, A. Koizumi, R. H. Masland; Excitatory Synaptic Inputs to Retinal Ganglion Cells Follow a Single Generic Plan. Invest. Ophthalmol. Vis. Sci. 2008;49(13):5878.
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© ARVO (1962-2015); The Authors (2016-present)
Do retinal ganglion cells of physiologically and morphologically different types show different patterns of excitatory synaptic inputs, or do they all follow the same plan? We addressed this question by analyzing the distribution of PSD95-GFP transfected into a variety of ganglion cells of adult rabbits.
Pieces of adult rabbit retina were maintained in organotypic tissue culture for four days. Retinal ganglion cells were transfected biolistically with plasmids encoding a fusion protein of PSD95 with EGFP. Immunohistochemistry with a variety of pre- and postsynaptic markers (RIBEYE, Kif3a, glutamate receptors) was used to confirm specifically synaptic localization of PSD95-GFP puncta. Fifty-six ganglion cells were imaged by confocal microscopy, and 10 were analyzed in detail using the NeuroLucida program package.
The distribution of excitatory synapses varied relatively little across the different types of cells, either in pattern or in absolute density. Across the sample of reconstructed cells the total density of PSD95-GFP puncta was 0.19 ± 0.04 puncta/linear µm of dendrite. Synapses avoided the dendrites that traversed the levels of the inner plexiform layer, i.e. those linking arbors of bistratified cells or the soma and a monostratified arbor. Otherwise, there was little difference between the patterns of excitatory input to lower order dendritic segments and higher. This was the case despite dramatic differences in the cells’ physiological type, an order of magnitude range of total dendritic length, and widely differing patterns of dendritic branching. We also found a strong (r = -0.89) inverse relationship between the area enclosed by a ganglion cell's dendritic field and the density of synapses per unit area: small cells receive denser synaptic inputs than large ones.
(1) The linear density of synaptic inputs (PSD95 sites / linear µm) varied surprisingly little and showed little specialization within the dendritic arbor. (2) The two-dimensional density of excitatory inputs across individual dendritic arbors generally peaked in a ring-shaped region surrounding the soma. This agrees with high-resolution maps of receptive field sensitivity in the rabbit. (3) The areal density scaled inversely with the total area of the dendritic arbor, so that narrow dendritic arbors receive many more synapses per unit area than large ones. This may reflect a need for cells that report upon a smaller region of visual space to receive a denser synaptic input from within that space in order to reach their action potential threshold.
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