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Corinne Beier, Jennifer Kung, Philip Huie, Roopa Dalal, Daniel V Palanker, Alexander Sher; Bipolar Cells Restructure Dendrites After Selective Ablation of Photoreceptors. Invest. Ophthalmol. Vis. Sci. 2014;55(13):4171.
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In the rabbit retina there is evidence of constructive retinal plasticity in response to focal ablation of a small patch of the photoreceptor layer by laser photocoagulation. Over a few months, healthy photoreceptors migrate inwards filling the damaged area and restoring visual sensitivity to the lesion site. We investigated the changes in the morphology of the bipolar cells beneath the lesion during the healing process in order to understand how the migrating photoreceptors connect to the deafferented bipolar cells.
Line-shaped photocoagulation lesions of Barely Visible clinical grade were produced in rabbits with a 532-nm laser, using beam diameter 100 μm, scanned along 1.5mm of superior retina. Retinal ganglion cell responses to spatio-temporal white noise stimulus were recorded on a 512-electrode array. Functional healing was characterized as a return of visual sensitivity over the lesion site. Photoreceptor migration and changes in rod bipolar cells were visually assessed using immunohistochemistry (PKCα) with confocal microscopy.
The lesioned areas of the retina, after a two-month healing period, regain almost complete visual sensitivity. Immunostaining shows no damage to the inner nuclear layer at 2 days after the procedure with rod bipolar cells showing overall normal dendritic structure. However, the rod bipolar cells within the 1-month and 2-month old lesion sites show structural changes in their dendrites. Thinner dendrites are pruned and in many cells are replaced by a single thick process reaching towards the edges of the lesion. This structural change becomes less pronounced in 2-month old lesions. In contrast, we did not observe significant changes in the dendritic morphology of the bipolar cells bordering the lesions. The rod bipolar cells maintain normal axon morphology and correct axon termination sites in the ON lamina of the IPL throughout the healing period.
The deafferented rod bipolar cells appear to be seeking viable pre-synaptic partners after the lesioning procedure. At the same time, the dendrites of the surrounding bipolar cells are not following the migrating receptors. These results suggest that the healthy photoreceptors migrate inside the damaged area, abandon their old post-synaptic partners and establish new functional connections with the deafferented bipolar cells.
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