Supplementary Figure 7: A chloride-conducting channelrhodopsin inhibits somatic spiking but triggers neurotransmitter release in presynaptic terminals and fails to attenuate evoked release.

(a) Light-evoked outward photocurrents recorded in a cultured neuron transduced with GtACR1 and clamped to 0 mV. Gray value indicates irradiance. Inset: average peak and steady-state photocurrents recorded at a light power density of 2 mW mm^–2 (n = 7). (b) Attenuation of spiking in cultured neurons during illumination with increasing light power densities. Neurons were recorded in whole-cell current clamp mode and current injections were performed through the patch pipette to evoke action potentials. The minimal current required to elicit spikes (rheobase) was measured under light pulses of varying light power density. (c) Light-evoked release in GtACR1-expressing thalamocortical terminals. Experiments were conducted in the same configuration as shown in Fig. 2. Depicted are light-evoked EPSC (left) and microstimulation-evoked EPSCs (right). (d) Comparison of light- and microstimulation-evoked EPSCs recorded in the same cells under the same configuration described in c (n = 8). (e) To evaluate the effect of GtACR1 activation in presynaptic terminals, we compared the responses to microstimulation of the thalamocortical fibers following a 10 ms light pulse (top) and under continuous light stimulation (bottom). This was necessary because of the light onset-associated EPSCs, which caused adaptation in the subsequent microstimulation-evoked responses. (f) Summary of GtACR1 experiments, performed as depicted in e (n = 4), showing no effect of light administration on microstimulation-evoked release from thalamocortical terminals. Error bars indicate s.e.m. (see Supplementary Table 1 for statistics).