Figure 6.

Selectivity of electrical coupling between ipRGC types and CRH⁺ ACs. A, Circuit diagram illustrating electrophysiological assay for ipRGC-CRH⁺ AC coupling. Optogenetic stimulation of a CRH⁺ AC (green, right) expressing ChR2 generates a photocurrent that propagates to a recorded ipRGC (blue, left) via a gap junction-mediated electrical synapse (boxed resistor symbol). B, Experimental isolation of ChR2-dependent coupling currents from melanopsin-dependent photocurrents in ipRGCs. Blue trace represents current (V_hold = −70 mV) recorded from an M4 ipRGC during repeated photostimulation of ChR2⁺ CRH⁺ ACs (Φ_max = 4.8 × 10¹⁷ Q cm⁻² s⁻¹). Initially, a melanopsin-mediated inward current (I_mel, black arrow) dominates the measured response. During later stimuli, fast inward currents are evident. Black represents the analyzed period of the response. C, Patterns of coupling currents measured in M1-M5 ipRGC types during optogenetic stimulation of CRH⁺ ACs. C₁, Green trace represents light-evoked ChR2 currents recorded in a CRH⁺ AC. Black traces represent examples of fast inward currents recorded in M2, M4, and M5 ipRGCs during optogenetic stimulation of CRH⁺ ACs. C₂, Gray traces represent examples of weak or null responses in M1-M4 ipRGCs during optogenetic stimulation of CRH⁺ ACs. D, Latencies of coupling currents in ipRGCs. Traces represent the response near stimulus onset for the cells shown in C₁. Coarse dashed lines indicate prestimulus baseline current. Fine dashed lines indicate response threshold. E, ChR2 photocurrent amplitude in CRH⁺ ACs (n = 7 cells) and coupling current amplitude in M1-M5 ipRGCs (n = 7, n = 8, n = 4, n = 5, and n = 10 cells, respectively). F, Coupling current latency for all cells in E exhibiting suprathreshold responses (CRH: n = 7; M2: n = 3; M3: n = 1; M4: n = 3; M5: n = 10). *p < 0.05. ***p < 0.001.