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R. J. Clarke, D. J. Ehrlich, S. Elkabes, E. Townes-Anderson; Go Is Upregulated in Bipolar Cells Following the Loss of Their Synaptic Connections With Photoreceptors. Invest. Ophthalmol. Vis. Sci. 2007;48(13):4524.
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Expression of mGluR6 glutamate receptors in ON bipolar cells is decreased and shows spatial displacement in mutant mice with photoreceptor degeneration (Strettoi et al. Vision Res, 43:867, 2003). Here, we studied the fate of the G protein Goα, which links mGluR6 to its function closing cation channels in ON bipolar cells, in mutant mice and also in bipolar cells from healthy, normal salamander retinas that have been isolated from their photoreceptor inputs by retinal dissociation.
Immunocytochemistry, using a monoclonal antibody against Goα, was performed on paraformaldehyde-fixed retinas of normal mice and mice in which photoreceptors had been lost through disease. Images were acquired from 10µm frozen sections and staining intensity measured between scan lines placed vertically through all the layers of the retina. In addition, the proportion of bipolar cells with Goα staining was studied in salamander retinal cell cultures which contain isolated bipolar cells. A sample of 15-20 bipolar cells was identified by their morphologically distinct Landolt club at time of plating and then Goα immuno-staining, if present, was determined for each cell after 1hr, 24hrs, and 7 days in vitro.
Goα staining was increased 2-4 fold in the outer part of the inner nuclear layer in all mutant mice compared to mice with an intact photoreceptor layer. Goα staining was also greater in the inner plexiform layer in these mice. In salamander retinal cell cultures, the proportion of Goα -positive bipolar cells increased from 45% after 1 hour to a maximum of 74% after 24 hours in culture. Goα staining remained in 66% of bipolar cells over the next 7 days in culture. Comparing these results with the intact salamander retina where only 41% of bipolar cells stain for Goα (Zhang and Wu. J Comp Neurol, 461:276, 2003), suggests that at least 24-33% of OFF bipolar cells, which use AMPA glutamate receptors and normally do not stain for Goα, are expressing Goα for the first time as a result of injury.
The similarity between the fate of Goα from mutant mouse retinas and salamander retinal cultures suggests that loss of photoreceptor-bipolar synaptic contacts is responsible for the upregulation of Goα in bipolar cells. The expression of Goα in OFF bipolar cells may have implications for their physiological function.
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