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. 2020 Nov 26;14:604163. doi: 10.3389/fncel.2020.604163

Figure 1.

Figure 1

Optogenetic stimulation of starburst amacrine cells evokes postsynaptic currents in ON-OFF direction-selective ganglion cells. (A) An ON-OFF DSGC receives excitatory cholinergic and inhibitory GABAergic input from ON and OFF SACs. Neighboring SACs also provide GABAergic inhibition to each other. SACs express channelrhodopsin-2 (ChR2, blue), which is activated after blocking conventional glutamatergic transmission from photoreceptors and cone bipolar cells (CBCs; see Materials and Methods). (B) ChR2-evoked postsynaptic currents (PSCs) depend on voltage-gated calcium channels. Left, ChR2-evoked GABAergic IPSCs (IGABA) and cholinergic EPSCs (IACh) recorded in separate ON-OFF DSGCs before (ctrl) and during bath application of cadmium (Cd2+, 200 μM). Stimulus intensity, 4.8 × 1017 quanta (Q) cm−2 s−1. Right, peak amplitude of ChR2-evoked PSCs before and during Cd2+ application (n = 5 cells). IGABA–ctrl vs. Cd2+: *p = 0.014, t = 4.2; IACh–ctrl vs. Cd2+: **p = 0.003, t = 6.6 (paired t-tests). Error bars here and below indicate ± SEM across cells. (C) ChR2-mediated modulation of SAC Vm. Left, ChR2-modulated Vm of ON SAC before and during bath application of Cd2+. Right, mean ChR2-evoked depolarization before and during Cd2+ application (n = 6 cells). (D) Dynamic range of ChR2-mediated responses. Within a range of stimulus intensity [20-100% Φmax (4.8 × 1017 Q cm−2 s−1)], ON SAC Vm remains saturated while IPSCs increase. (E) Normalized input-output relations for ChR2-modulated ON SAC Vm (n = 4 cells) and ON-OFF DSGC IPSCs (n = 4 cells); light traces indicate individual cells. r(Φ) is the response to a stimulus of intensity Φ.