Figure 4. Release probability is set by dendritic activity.

(A) Two experimental schemes: activity (left) and activity + block (right). (Left) Activity in the culture was increased by delivering APs with field stimulation for 2 h, and paired whole-cell recordings were used to access the impact of this manipulation on release probability. (Right) For activity + block condition, a similar experimental scheme was used, but excitatory synaptic activity was blocked during stimulation. (B) Example paired-pulse EPSC traces (left, averages of 20) and summary of paired-pulse EPSC amplitude ratio (right), showing an increase of PPR with activity, which is abolished by synaptic blockers. Scale bars, 15 ms, 200 pA top, 100 pA bottom. (C, D) Recordings in 1 mM Ca²⁺ / 3 mM Mg²⁺. (C, left) Frequency histogram of mEPSCs integral (white) and baseline noise (gray). Arrowhead indicates the mEPSC integral mean. (C, right) Smoothed histogram of evoked responses integral on the same postsynaptic cell, showing well defined peaks at equally spaced distances. Note that first two peaks correspond to the baseline noise and mEPSC mean in the left panel. Inset shows example traces where different number of quanta have been released. Scale bar, 2 ms, 50 pA. (D) Increasing activity leads to a significant decrease in release probability, which is abolished by blocking excitatory synaptic transmission. Smoothed integral histograms of evoked responses for example connections are shown, after increased activity alone (left), and with synaptic blockers (centre). (D, right) Summary of p_r changes for all connections.