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The microglia layers response to electrical overstimulation of the retina imaged under epiretinal stimulation electrodes

Ethan Cohen1, Alula Yohannes1, Wai Wong2, K. Shea3, Christopher Jung4, and Alexander Beylin1.
1 CDRH/FDA. 2 NEI/NIH. 3 CDER/FDA, 4 Univ. of Md. Baltimore Co.

Purpose: We investigated using the morphological responses of retinal microglia as indicators of tissue damage from electrical overstimulation by imaging them through an optically transparent stimulus electrode. We examined how the different layers of retinal microglia responded to overstimulation of the retinal tissue and determined how the microglia respond spatially to the injury.

Methods: To track the microglia, we used a transgenic mouse where the microglia expressed a water soluble green fluorescent protein (GFP). A clear stimulus electrode was placed epiretinally on the inner limiting membrane and the microglia layers were imaged using time-lapse confocal microscopy. We examined how the microglia responded both temporally and spatially to local overstimulation of the retinal tissue. Using confocal microscope vertical image stacks, the microglia under the electrode were imaged at 2.5min intervals. The retina was overstimulated for a 5 minute period using 1msec 749μC/cm2/ph biphasic current pulses and changes in the microglia morphology were followed for 1 hour post stimulation.

Results: The microglia response to epiretinal overstimulation depended on their spatial location relative to the electrode lumen and could result in 3 different morphological responses. Some microglia were severely injured and became a series of immotile ball-like fluorescent processes. Other microglia survived, and reacted rapidly to the injury by extending filopodia oriented toward the damage zone. This response was seen in inner retinal microglia outside the stimulus electrode edge. A third effect, seen with the deeper outer microglia under the electrode, was a fading of their fluorescent image.

Conclusions: The microglial morphological responses to electrical overstimulation injury occur rapidly and can show both direct and indirect effects of the stimulus electrode injury.

Financial disclosure: None

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