Figure 2. Presynaptic Rac1 regulates synaptic vesicles release probability and synaptic strength.

Synaptic transmission at the calyx of Held – MNTB synapse was studied using different stimulation frequencies at P28 after deletion of Rac1 at P14. (A1, B1) Representative evoked EPSCs for Rac1^+/+ (black) and Rac1^−/− (orange) at 50 Hz and 500 Hz stimulation frequency. Stimulus artifacts were blanked for clarity. (C) Magnification of the first EPSC (EPSC₁). Ablation of presynaptic Rac1 resulted in increased EPSC₁ amplitude with no change in EPSC dynamics. (A2–A4) At 50 Hz stimulation frequency, Rac1^−/− showed stronger short-term depression despite larger steady-state EPSC amplitudes. (B2–B4) At 500 Hz stimulation frequency, loss of Rac1 resulted in a lack of short-term facilitation and increased synaptic depression with no change in steady-state EPSC amplitude. (D1) Population data showing an increase in EPSC₁ amplitude in Rac1^−/−. Steady-state EPSC amplitudes were increased in Rac1^−/− at 50 Hz but not at 500 Hz stimulation frequency. (D2) Population data of the readily releasable pool (RRP) using three different estimation methods, suggesting little to no change in RRP size (D3) Population data indicating that EPSC₁ release probability in Rac1^−/− was elevated independent of estimation method. All data shown in the figure and the detailed results of statistical tests are part of the supplementary file.