Figure 4.

Differences in light-evoked inhibitory synaptic input, or the somatic hyperpolarization caused by such input, are insufficient to explain differences in the output of OffS and OffT RGCs. A, Peak amplitude of postsynaptic currents measured in OffS (filled circles) and OffT (open circles) RGCs at the reversal potential for EPSCs mediated by ionotropic glutamate receptors (E_Cat ∼0 mV) as a function of flash strength. Gray dashed lines indicate 0 on the vertical axis. B, Changes in the rate of AP generation elicited by dynamic clamp injection of the measured light-evoked inhibitory postsynaptic conductance (B₁) and by three different flash strengths near detection threshold (B₂; flash onset indicated by dashed vertical line) in the same OffT RGC. Raw data in B₁ represent responses to 10 presentations of the same inhibitory synaptic conductance waveform; traces in B₂ show responses to 10 successive presentations of a stimulus producing ∼0.01 Rh*/rod—i.e., the strongest flash delivered in this experiment. Injection of the light-evoked inhibitory postsynaptic conductance hyperpolarized the soma of OffT RGCs slightly more than light stimuli but did not trigger a burst of spikes. C, Effect of applying the glycine receptor antagonist strychnine (10 μm) on responses measured in the cell-attached recording configuration. The plot below shows the average firing rate produced by a family of flash strengths in the absence and presence of strychnine; flash families produced 0.004–0.064 Rh*/rod under control conditions and 0.016–0.256 Rh*/rod in the presence of strychnine. Black and gray traces above represent five consecutive responses to the strongest flash presented under control conditions (black) and in the presence of strychnine (gray), respectively.