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, capacitan ce increases to a plateau of similar to 30 fF with a time constant of similar to 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 wit h a slower time constant of similar to 300 ms. EGTA (5 mM) depresses t he slower capacitance rise but leaves the ultrafast phase intact. BAPT A (5 mM) depresses both components of exocytosis. With paired-pulse st imulation, the ultrafast pool recovers from depletion with a time cons tant of similar to 4 s. The ultrafast component may represent fusion o f docked vesicles at the base of the synaptic ribbon, while the slower component represents more distal vesicles on the ribbon.