Figure 4.

Transmitter-specific postsynaptic filtering in SAC→DSGC transmission. (A) Isolation and measurement of spontaneous cholinergic EPSCs in ON-OFF DSGCs. Top, spontaneous EPSCs (sEPSCs) recorded from an ON-OFF DSGC during blockade of AMPA (50 μM DNQX) and NMDA (50 μM D-AP5) receptors. Bottom left, expanded view of boxed region. Dots indicate individual spontaneous EPSCs. Bottom right, alignment and averaging of cholinergic sEPSCs. One hundred individual sEPSCs (light gray) superimposed with the mean (green) of all sEPSCs. Dotted line indicates baseline holding current. (B) Schematic diagram illustrating experimental generation and recording of evoked monophasic IPSCs (emIPSCs) in an ON-OFF DSGC. Brief (<10 ms) optogenetic stimulation of presynaptic SACs evokes a small, monophasic IPSC (red). Following m trials, n monophasic events are distinguished from multiphasic events and failures. (C) Comparison of all sIPSCs and emIPSCs recorded in DSGCs. Amplitude is plotted against decay time constant (τ_decay) for all sIPSCs (n = 884 events in 4 ON-OFF DSGCs) and emIPSCs (n = 212 events from 4 ON-OFF DSGCs and 1 ON DSGC). Marginal probability distributions of amplitude and τ_decay are shown at right and above, respectively. PSC amplitude—sEPSC vs. emIPSC: ***p < 0.001, D = 0.82 (Kolmogorov-Smirnov test). (D) Averaged time courses of sIPSCs and emIPSCs. The average of all sIPSCs (green) is fit (black, dashed; see Materials and Methods) with a single exponential decay term. The average of all emIPSCs (inverted, magenta) is fit with two exponential decay terms. Traces are normalized to their respective maxima.