Figure 6. Local stimulation decreases release probability selectively.

(A, B) Theta-glass pipette stimulation produces a localized synaptic response. (A) Synapses were labeled with FM-dye (red), and a whole-cell recording established. A EPSC (inset trace) was evoked by positioning the stimulating pipette in front of a group of synapses on the dendrite of the recorded cell (colored in blue for clarity), confirming successful synaptic stimulation. (B) The same synapses were stimulated by 1200 APs at 20 Hz, and FM-dye fluorescence monitored. Inset graph shows that only the group of synapses directly in front of the pipette lost FM-dye fluorescence, indicating high spatial selectivity of the stimulus. (C) Local stimulation was used to increase synaptic activity in a restricted part of a dendritic branch, and p_r estimated subsequently by loading synapses with FM-dye. (D) Example DIC image of a dendritic branch with a group of synapses, which were stimulated for 2 h, with superimposed pseudocolored FM4-64 puncta. Fluorescence intensity represents p_r. Scale bar, 5 μm. (E) Data summary showing that p_r decreases only in stimulated synapses, and that this effect is abolished by synaptic blockers. (F) Another example image of the same experiment shown in D, where synapses of interest have been categorized according to the axon they belong to (color dots, see Fig. S6 for details on axon tracing procedure). Scale bar, 10 μm. (G) Data summary showing that local stimulation selectively decreases p_r even if synapses on the dendritic branch belong to the same presynaptic input. (H) Summary data demonstrating that p_r of synapses from different inputs becomes similar after stimulation. Error bars are ± s.e.m.