Fast kinetics of exocytosis revealed by simultaneous measurements of presynaptic capacitance and postsynaptic currents at a central synapse

The rate of release from nerve terminals depends on both the number of rele ase sites and the rate of release at each site. The latter remains largely unknown at central synapses. We addressed this issue by simultaneously meas uring the nerve terminal membrane capacitance and the postsynaptic current at single calyceal synapses in rat brainstem. We found that a 10 ms presyna ptic step depolarization depleted a releasable pool containing 3300-5200 ve sicles. Released vesicles were endocytosed with a time constant of a few se conds to tens of seconds. Release of only one third of this pool saturated both postsynaptic AMPA and NMDA receptors. A release site can release more than three vesicles in 10 ms (>300 vesicles per second). We conclude that b oth a large number of release sites and a fast release rate at each site en able synapses to release at a high rate.