THE KINETICS OF QUANTAL TRANSMITTER RELEASE FROM RETINAL AMACRINE CELLS

Exocytosis of transmitter at most synapses is a very fast process trig gered by the entry of Ca2+ during an action potential. A reasonable ex pectation is that the fast step of exocytosis is followed by slow step s readying another vesicle for exocytosis but the identity and kinetic s of these steps are presently unclear. By voltage clamping both pre- and postsynaptic neurons in an isolated pair of retinal amacrine cells , we have measured evoked synaptic currents and responses to single ve sicles of transmitter (minis). From these currents, we have computed t he rate of exocytosis during a sustained presynaptic depolarization, W e show here that for these cells, release is consistent with a scheme of ''fire and reload.'' Large Ca2+ influx causes the rapid release of a small number of vesicles, typically approximate to 10 per presynapti c neuron, likely corresponding to those vesicles already docked. After this spike of exocytosis whose peak is 150 quanta per release site pe r s, continued Ca2+ influx sustains release at only 22 quanta per rele ase site per s, probably rate-limited by the docking of fresh vesicles .