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Eyal Margalit, Norbert Zsolt Babai, Jianmin Luo, Wallace B. Thoreson; Inner And Outer Retinal Mechanisms Engaged By Epiretinal Stimulation In Normal and rd Mice. Invest. Ophthalmol. Vis. Sci. 2011;52(14):4935.
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© ARVO (1962-2015); The Authors (2016-present)
Retinal prosthetic devices are being developed to bypass degenerated retinal photoreceptors by directly activating other retinal neurons with electrical stimulation. However, the retinal circuitry that is activated by epiretinal electrical stimulation is not well characterized. This study was designed to understand the mechanisms by which normal and rd mice retina respond to such stimulation.
Whole cell patch clamp recordings were obtained from ganglion cells in normal and rd mice using flat-mount and retinal slice preparations. A stimulating electrode was positioned along the ganglion cell-side of the preparation at different distances from the stimulated tissue.
Pulses of cathodic current evoked action potentials in ganglion cells and less frequently evoked sustained inward currents that appeared synaptic in origin. Sustained currents reversed around ECl and were inhibited by blockade of AMPA-type glutamate receptors with NBQX, GABAa/c receptors with picrotoxinin, or glycine receptors with strychnine. Synaptic current thresholds were lower in ON ganglion cells than OFF cells, but the modest difference did not attain statistical significance. Synaptic currents were rarely observed in rd mice lacking photoreceptors compared to normal retina. In addition, confocal calcium imaging experiments in normal mice revealed that epiretinal stimulation evoked calcium increases in the outer plexiform layer.
The results suggests that epiretinal stimulation activates synaptic currents by glutamate release from bipolar cell terminals which in turn evokes release of GABA and glycine from amacrine cells.The results also imply a contribution from photoreceptor inputs to the synaptic currents observed in ganglion cells when the normal mouse retina is electrically stimulated. The paucity of synaptic responses in rd mice suggests that it is better to target retinal ganglion cells directly rather than to attempt to engage the inner retinal circuitry.
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