A comparison of optical and electrophysiological methods for recording retinal ganglion cells during electrical stimulation

Purpose/Aim: To compare the efficacy of optical techniques with electrophysiological recordings for mapping retinal activity in response to electrical stimulation. Materials and Methods: Whole cell patch clamp, Ca ²⁺ imaging (Fluo-4-AM), and Na ⁺ imaging (CoroNa Green-AM) techniques were used to detect responses of neurons from mouse and salamander retina to electrical stimulation. Results: Synaptic currents were observed in ≥23% of retinal ganglion cells (RGCs), indicating presynaptic Ca ²⁺ increases in the inner plexiform layer (IPL). Modest depolarization with 2030mM K ⁺ consistently evoked Ca ²⁺ responses measured with Fluo4, but Ca ²⁺ responses were almost never evoked by epiretinal stimulation. In salamander retina, responses were seen in the inner nuclear layer (INL) and IPL. In mouse retina, responses were also sometimes seen in the outer pexiform layer (OPL). OPL responses showed a longer latency than IPL responses, suggesting that outer retinal circuits do not trigger synaptic responses of RGCs. Simultaneous Ca ²⁺ imaging and electrophysiological recording of synaptic currents confirmed that Fluo4-loaded retinas remained responsive to stimulation. Epiretinal stimulation evoked action potentials in ≥67% of RGCs. CoroNa Green detected Na ⁺ changes stimulated by 20mM K ⁺, but epiretinal stimulation did not evoke detectable Na ⁺ responses. Simultaneous imaging and electrophysiological recording confirmed the health of CoroNa Green-loaded retinas. We confirmed stimulation efficacy by simultaneously recording Na ⁺ changes and electrophysiological responses. Conclusions: These data demonstrate that electrophysiological recordings show greater sensitivity than Na ⁺ or Ca ²⁺ imaging in response to electrical stimulation. The paucity of Ca ²⁺ responses is consistent with limited risk for Ca ²⁺-mediated cell damage during electrical stimulation.