(B-G) Munc18-1 null hippocampal neurons were rescued with wildtype Munc18-1a or a Munc18-1 mutant, Y473A, in which Y473 was replaced with alanine.(E) Release by hyperosmotic sucrose application (500mM, 3.5sec.) was used to assess the RRP. Left: RRP charge (M18WT: 1.68 ± 0.64 nC, n=13; M18Y473A: 0.27 ± 0.08 nC, n=13, t-test with Welch correction, p = 0.0487). Typical responses are depicted in the middle. Right: vesicular release probability per neuron (EPSC charge / RRP charge). Mean Pves (M18WT: 6.75 ± 0.77%, n=13; M18Y473A: 1.98 ± 0.72%, n=13, t-test, p = 0.0001). 