Ultrafast exocytosis elicited by calcium current in synaptic terminals of retinal bipolar neurons

Using high resolution capacitance measurements, we have characterized an ultrafast component of transmitter release in ribbon-type synaptic terminals of retinal bipolar neurons. During depolarization, capacitance increases to a plateau of ~30 fF with a time constant of ~1.5 ms. When not limited by activation kinetics of calcium current, the small pool is depleted even faster, with a time constant of 0.5 ms. After the ultrafast pool is depleted, capacitance rises with a slower time constant of ~300 ms. EGTA (5 mM) depresses the slower capacitance rise but leaves the ultrafast phase intact. BAPTA (5 mM) depresses both components of exocytosis. With paired-pulse stimulation, the ultrafast pool recovers from depletion with a time constant of ~4 s. The ultrafast component may represent fusion of docked vesicles at the base of the synaptic ribbon, while the slower component represents more distal vesicles on the ribbon.