Low nanomolar S1P increases the size of RRP without influencing mEPSC frequency. A–D, Quantification of mEPSC frequency, amplitude, and quantal charge of 14 DIV hippocampal neurons in control conditions or incubated for 40 min with S1P in the presence or absence of CAY10444. E, Representative sucrose-induced responses under the same conditions as in A. F, G, Quantitative analysis of charge transfer and number of released vesicles measured in hippocampal neurons after hypertonic solution stimulation. The number of vesicles corresponds to the ratio between the total charge transferred during the sucrose delivery and the quantal charge of mEPSCs recorded during the “baseline” period. *p < 0.05. **p < 0.01.