Figure 1. Excitatory synaptic transmission from GACs to specific postsynaptic targets.

(A) Upper: collapsed two-photon images of ON (red) and OFF dendrites (blue) of an ON-OFF DSGC after whole-cell recording in a vGluT3-Cre/ChR2-YFP (green) retina. Lower: z-projection image of the same cell. Arrow: an example of ChR2-YFP-expressing GAC (green). (B) Loose-patch responses of the cell in (A) to a light bar (500 µm × 100 µm) moving at 500 µm/s in 8 directions. (C) Voltage-clamp responses (at −70 mV) of the cell in (A) to blue light flashes, showing a fast GAC-activated postsynaptic current (left, arrow), followed by a delayed BC-mediated visual response (left, asterisk). Inset: expanded view of the fast, L-AP4-resistant current. (D) Overlay of the ChR2-current in a GAC (green, at −70 mV) and the fast postsynaptic current in an ON-OFF DSGC (black, −70 mV) in response to a blue light flash. Inset: expanded view, showing a ~2-ms onset delay between the ChR2 current and the postsynaptic response. (E) Dual recording from a pair of GAC and ON-OFF DSGC in the presence of L-AP4, ACET and HEX, showing voltage-gated currents in GAC in response to depolarizing steps from −70 mV to −45, −35, −25, and −15 mV (upper traces, with the Na⁺ channel blocker QX 314 in pipette solution) and inward postsynaptic currents in the ON-OFF DSGC at −70 mV (lower traces). (F, G) Simultaneous recording (at −70 mV) from a pair of ON SAC (F, white arrow) and ON-OFF DSGC (F, yellow arrow), showing delayed, L-AP4-ACET-HEX-sensitive currents in both ON SAC and ON-OFF DSGCa (G, asterisks), and a fast, L-AP4-ACET-HEX-resistant current only in the ON-OFF DSGC, but not the ON SAC (G, arrow). The reduction in the fast response amplitude after drug application, seen in this particular case, was likely due to response rundown after a long recording period, but not to pharmacological blockade (see text related to Figure 2). (H, I) An ON DSGC in response to blue light flashes under voltage clamp at −70 mV, showing a fast synaptic response that is resistant to L-AP4 + ACET + HEX. (J) A putative W3 cell (upper, shown only in red channel), with narrow, diffuse dendritic ramification in the middle of IPL (lower, both red and green channels shown). (K) Responses (at −70 mV) of two putative W3 cells from two different retinae to blue light flashes, showing a small peak response and a sustained component. (L) Summary of the peak current response amplitudes (at −70 mV) of different cell types to blue light flashes in the presence of L-AP4 + ACET + HEX. Numbers in parentheses, cells tested. Error bars, SEM. Drug concentrations (in µM): ACET, 10; HEX, 300; and L-AP4, 20. See also Figure S1.