The voltage threshold of glutamate release was low and variable (-47.11 ± 4.09 mV, mean ± SD). The area under the curve of the iGluSnFR signal (top; AUC(ΔF/F0)10ms) from (C) and corresponding whole-cell QCa (bottom) (mean ± SD). Detectable signals were defined here if the peak iGluSnFR signal was 2 times higher than baseline SD. All synapses had detectable signals in response to depolarizations ≥ -42 mV. (See also Figs. EV1-2) * p ≤ 0.05, ** p ≤ 0.01, *** p ≤ 0.001 