Enhanced Optogenetic Stimulation of Retinal Ganglion Cells With Assistive Electric Stimulation for Low Optical Power Artificial Vision

High optical power of optogenetic stimulation may cause phototoxicity during chronic application. To lower the optical power, a hybrid approach which combines optogenetic and electric modalities has been proposed. However, the hybrid stimulation effect has not been well studied in the retina, which would be an ideal target of optogenetic intervention for sight restoration. Here, we investigated the assistive effect of electric pulses while optogenetic stimulation in both wild-type (wt) and retinal degeneration 10 (rd10) mouse retinas. We injected AAV2-CAG-ChR2(H134R)-EGFP into the 4-week-old mouse eyeballs. After $\gt {4}\,{}$ weeks, spiking activities of retinal ganglion cells of ex-vivo retinas were recorded using a cell-attached patch clamping in response to hybrid stimulation: 3 light intensities (i.e., Levels 1, 2, and 3) for optogenetic stimulation and 3 types of assistive electric pulses (i.e., -5 and $-\,{10}\,{\mu }$ A square pulses, and $-\,{20}\,{\mu }$ A increasing ramping current) were tested. Notably, in wt retina, the hybrid stimulation with $-\,{10}\,{\mu }$ A square pulse evoked significantly more spikes compared to the optogenetic-only case, showing average increases of ${}\,{1.89}\,{\pm }\,{2.34}\,{}$ , ${}\,{2.49}\,{\pm }\,{1.92}\,{}$ , and ${}\,{2.50}\,{\pm }\,{1.61}\,{}$ spikes for the Levels 1, 2 and 3, respectively. For the same conditions, spiking latencies were reduced by ${}\,{35.27}\,{\pm }\,{41.34}\,{}$ , ${}\,{10.62}\,{\pm }\,{1}{3.73}\,{}$ , and ${}\,{8.64}\,{\pm }\,{15.33}\,{}$ ms. These results demonstrate hybrid stimulation can enhance spiking magnitude and reduce temporal delay. Also, our results indicate assistive electric pulse is more effective for lower power optogenetic stimulation than higher one but the assistive effect was reduced in rd10 retinas. Our study suggests hybrid stimulation holds promise for enhancing chronic applicability of optogenetic approaches for vision restoration by lengthening battery life through the reduced optical power requirement.