2-3-dioxo-6-nitro-7-sulfamoylbenzo(f)quinoxaline and Retinal-Degeneration

Retinal prosthetic devices are being developed to bypass degenerated retinal photoreceptors by directly activating retinal neurons with electrical stimulation. However, the retinal circuitry that is activated by epiretinal stimulation is not well characterized. 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 E(Cl) and were inhibited by blockade of α-amino-3-hydroxyl-5-methyl-4-isoxazole-proprionate (AMPA)-type glutamate receptors with 2,3-dihydroxy-6-nitro-sulfamoyl-benzo(f)-quinoxaline-2,3-dione (NBQX), γ aminobutyric acid a/c (GABA(a/c)) receptors with picrotoxinin, or glycine receptors with strychnine. This suggests that epiretinal stimulation activates glutamate release from bipolar cell terminals, which in turn evokes release of GABA and glycine from amacrine cells. 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 retina slices revealed that epiretinal stimulation evoked calcium increases in the outer plexiform layer. These results imply a contribution from photoreceptor inputs to the synaptic currents observed in ganglion cells. The paucity of synaptic responses in rd mice retina slices suggests that it is better to target retinal ganglion cells directly rather than to attempt to engage the inner retinal circuitry.

Topics: Animals; Biophysics; Calcium; Disease Models, Animal; Electric Stimulation; Evoked Potentials; Excitatory Amino Acid Antagonists; Excitatory Postsynaptic Potentials; GABA-A Receptor Antagonists; Glycine Agents; In Vitro Techniques; Mice; Mice, Inbred C57BL; Mice, Mutant Strains; Microscopy, Confocal; Patch-Clamp Techniques; Picrotoxin; Quinoxalines; Retina; Retinal Degeneration; Retinal Ganglion Cells; Sesterterpenes; Strychnine; Visual Pathways

Genetically encoded optical neuromodulators create an opportunity for circuit-specific intervention in neurological diseases. One of the diseases most amenable to this approach is retinal degeneration, where the loss of photoreceptors leads to complete blindness. To restore photosensitivity, we genetically targeted a light-activated cation channel, channelrhodopsin-2, to second-order neurons, ON bipolar cells, of degenerated retinas in vivo in the Pde6b(rd1) (also known as rd1) mouse model. In the absence of 'classical' photoreceptors, we found that ON bipolar cells that were engineered to be photosensitive induced light-evoked spiking activity in ganglion cells. The rescue of light sensitivity was selective to the ON circuits that would naturally respond to increases in brightness. Despite degeneration of the outer retina, our intervention restored transient responses and center-surround organization of ganglion cells. The resulting signals were relayed to the visual cortex and were sufficient for the animals to successfully perform optomotor behavioral tasks.

Topics: Animals; Behavior, Animal; Disease Models, Animal; Electroporation; Evoked Potentials, Visual; Excitatory Amino Acid Antagonists; Gene Expression Regulation; Light; Luminescent Proteins; Mice; Mice, Inbred C57BL; Mice, Transgenic; Motor Activity; Patch-Clamp Techniques; Photic Stimulation; Piperazines; Quinoxalines; Retinal Bipolar Cells; Retinal Degeneration; Retinal Ganglion Cells; Rhodopsin; Time Factors; Vision, Ocular; Visual Pathways