(A), schematic illustrating synaptic transmission at rod-RBC synapses. In the dark, rods are depolarized, which activates Cav1.4 and Ca2+-triggered glutamate (E) release. Activation of the mGluR6/G-protein pathway by glutamate or L-AP4 inhibits TRPM1-dependent channels. Light signals or the mGluR6 antagonist CPPG activate the ON pathway by disinhibiting the TRPM1-dependent conductance. (B–D), confocal micrographs of the OPL of WT, G369i KI and Cav1.4 KO mice triple-labeled with antibodies against Ribeye, mGluR6, and TRPM1. In B,C, only signals for Ribeye and mGluR6 (B) or TRPM1 (C) are shown for clarity. (C) Higher magnification views of boxed regions are shown in the merged image (D). Arrows and arrowheads depict ribbons and spheres, respectively. (E–F), whole-cell patch clamp electrophysiology of RBCs recorded in WT, G369i KI, and Cav1.4 KO mouse retinal slices. E, left, representative image of Lucifer Yellow-filled RBC showing typical lobular-shaped terminal (arrowhead). Right, representative current responses to CPPG puffs in RBCs held at −60 mV and +60 mV. Lines within each current trace are low-pass FFT filtered data. F, peak amplitudes of CPPG-evoked EPSCs. Each shape represents individual cells recorded at −60 and +60 mV. Bars represent mean ± SEM. ****p<0.0001, **p<0.01, *p<0.05, by one-way ANOVA with Fishers least significant difference post-hoc test. Scale bars, 2 μm in B,C, 1 μm in D, 15 μm in E.