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A. Koizumi, T. C. Jakobs, R. H. Masland; Spatial Regularity in the Arrangement of Excitatory Synapses Within the Inner Plexiform Layer. Invest. Ophthalmol. Vis. Sci. 2010;51(13):894.
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© ARVO (1962-2015); The Authors (2016-present)
In the inner plexiform layer (IPL) of the retina, excitatory outputs from functionally distinct types of bipolar cells make synapses upon the dendrites of ganglion and amacrine cells. These contacts occur between defined types of cells and within precisely defined laminae. Here we asked whether the arrangement of synapses onto ganglion cells within the same lamina manifests any discernible spatial regularity.
Pieces of adult rabbit retina were maintained in organotypic tissue culture for four days. Retinal ganglion cells were transfected with plasmids encoding a fusion protein of PSD95 with EGFP using a gene gun. Fifty-six ganglion cells were imaged by confocal microscopy, and 10 were chose for detailed analysis in using the NeuroLucida program package. Synapse distributions were analyzed for non-randomness using nearest neighbor analysis and a modified version of the density recovery profile test.
The absolute density of excitatory synapses throughout the whole sample of reconstructed cells was 0.19 ± 0.04 puncta/linear µm of dendrite and varied very little across the different types of cells, or across dendrites of different order. Even upon casual inspection, the synaptic inputs to a ganglion cell did not appear to be random. Statistical analysis showed that the synaptic puncta are spaced regularly along the dendrites with mean inter-synapse interval ranging from 2 to 4 µm. This was true for ganglion cells of many different functional types, for starburst amacrine cells, and for the varicosities of bipolar axon terminals.
(1) The linear density of synaptic inputs (PSD95 sites / linear µm) varied very little between ganglion cells of different functional types. (2) The excitatory synapses upon the dendrites of retinal ganglion (and amacrine) cells are regularly spaced along the dendrites: the presence of a PSD95 zone decreases the probability that another excitatory synapse lies nearby. The mean inter-synapse interval - the exclusion zone -- ranges from 2 to 4 µm for different cells. (3) The same spacing regularity was observed for the varicosities of bipolar axon terminals. (4) Thus it appears that the regular spacing of whole cells across the retinal surface has a correspondence on the smaller scale of the individual synaptic sites within the IPL.
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