The vesicular release probability sets the strength of individual Schaffer collateral synapses
Abstract Information processing in the brain is controlled by quantal release of neurotransmitters, a tightly regulated process. Even in a single axon, presynaptic boutons differ in the number of docked vesicles, but it is not known if the vesicular release probability (pves) is homogenous or variable between individual boutons. We optically measured evoked transmitter release at individual Schaffer collateral synapses using the genetically encoded glutamate sensor iGluSnFR, localizing the fusion site on the bouton with high spatiotemporal precision. Fitting a binomial model to measured response amplitude distributions allowed us to extract the quantal parameters N, pves, and q. Schaffer collateral boutons typically released only a single vesicle under low pves conditions and switched to multivesicular release in high calcium saline. We found that pves was highly variable between individual boutons and had a dominant impact on presynaptic output.