Extended Data Fig. 7: Climbing fibers (CFs) excite candelabrum cells via glutamate spillover.
From: Candelabrum cells are ubiquitous cerebellar cortex interneurons with specialized circuit properties
Previous studies have shown that CFs do not synapse directly onto molecular layer interneurons (MLIs), but that CF activation evokes a slow spillover current in MLIs and GCs. We performed similar experiments. We placed a stimulus electrode in the granular layer to activate axons and then assessed the properties of the inputs by stimulating with multiple trials for a range of stimulus intensities and determining the rise time and decay times of EPSCs. Experiments were performed with inhibitory transmission blocked. As in previous studies, we identified putative CF spillover currents based on slow rise and decay times, all or none activation and pronounced depression. If it satisfied these criteria we went on to determine its sensitivity to inhibiting glutamate uptake with TBOA. Many stimulus sites and many EPSCs were tested for each cell and CF inputs to MLIs were observed in 2 of 4 cells, whereas they were observed in 3 of 25 CCs. ai) Example of CF spillover excitation for an MLI. Individual traces shown in gray and average in black. ii) Responses evoked by a stimulation of increasing intensities for the same MLI, with 4 traces overlaid for each intensity. iii) Summary of EPSC amplitudes as a function of stimulus intensity for ai. iv) Slow EPSCs before and after bath application of 50 µM DL-TBOA. Shaded error bars show the average + /- SEM for 10 trials. bi-iv) Same as panel a for a candelabrum cell. c. Summary of amplitude, kinetic parameters, and paired pulse ratio for CF spillover excitation onto CCs. n = 2 cells (MLIslow), 3 cells (CCslow) and 10 cells (CCfast). d. Summary of the effect of TBOA on the amplitudes, rise-times and decay times of synaptic responses. n is the same as in panel c. In all box plots, box outlines and center line indicate median and interquartile range, and the whiskers indicate data points up to 1.5x interquartile range. Whenever n < 5, only the mean value is indicated with a horizontal line.
