Figure 1.

Bidirectional influence of PC discharges by short GrC burst stimuli. A, Left, Diagram of the feedforward inhibitory circuit in the cerebellar cortex. GrCs were stimulated 1, 3, and 7 times during irregular activation of many ChR2-expressing MFs by blue light illumination (optogenetic stim). PCs were first recorded in juxtacellular mode to monitor discharge (top right), then whole-cell patch-clamped to measure synaptic weights and E/I balance (bottom right). Excitation was recorded at V_m = −60 mV, inhibition at V_m = 0 mV. B, Confocal image of a transverse cerebellar section of a Thy1-ChR2-eYFP adult mouse showing strong eYFP fluorescence in MF rosettes. C, Top, PCs were recorded in juxtacellular, then whole-cell patch-clamped (bottom traces) before and during MF photostimulation (No ill and ill, respectively). Irregular firing and an increase in excitatory and inhibitory synaptic noise were observed. D, Quantitative analyses of PC discharge irregularities. The CV2, the frequency-independent LvR parameter, and the mean baseline frequency significantly increased during MF photostimulation (see Materials and Methods) ***p < 0.001, see text. Error bars indicate SEM. E, Left, Example of a biphasic modulation of PC discharge by GrC stimulation and determination of the spike gain. From the raster plots, a mean PSTH was determined and the baseline was subtracted. The spike gain was estimated by integrating this subtracted mean PSTH. Right, Two opposite examples of modulation of PC discharge by GrC stimulations.