FIGURE 2.

Actin promotes RRP replenishment likely by facilitating active zone clearance. (A) Sampled Cm traces induced by a pair of depol_{20 ms} at an interval of 200 ms in a control (Ctrl, left) and an Actb^–/– (right) calyx. Measurements of the capacitance jumps induced by the first and second depol_{20 ms} (ΔCm₁ and ΔCm₂) are schematically shown. (B) Left: the ratio between the second and first ΔCm (ΔCm₂/ΔCm₁) during a pair of depol_{20 ms} plotted versus paired-pulse interval at control and Actb^–/– calyces. Right: same as in left but plotting the interval between 0 and 1 s. *p < 0.05; **p < 0.01 (t-test). The data in A,B show that β-actin knockout reduces ΔCm2/ΔCm1. (C) Top: a dual pulse of 50 ms (to 0 mV) was applied at different intervals (200 and 500 ms) to the calyx. Presynaptic calcium currents (Ipre) and excitatory postsynaptic currents (EPSCs) are shown. Bottom: similar arrangement as in the top, but with the presynaptic pipette solution containing Lat A to inhibit actin polymerization. Arrows indicate that latrunculin A inhibits EPSC induced by a second pulse. (D) The Kv3.3 channel regulates EPSCs during repetitive action potential firing: sampled EPSCs (top) and the amplitude of EPSCs (bottom, mean + SEM) induced by 10 action potentials at 100 Hz at the calyx of wild-type mice, Kv3.3^–/– mice, and mice with a mutation (Kv3.3 G592R) that causes spinocerebellar ataxia 13 and inhibits F-actin nucleation at the calyx. (E) Schematic diagram showing that the F-actin cytoskeleton may facilitate active zone clearance and thus RRP replenishment. RRP replenishment may involve active zone clearance, vesicle docking, and vesicle priming that makes the docked vesicle release-ready. Panels A,B are adapted from Wu et al. (2016) with permission. Panel C is adapted from Sakaba and Neher (2003) with permission. Panel D is adapted from Wu et al. (2021) with permission.