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.

Original languageEnglish
Pages (from-to)1241-1249
Number of pages9
Issue number6
StatePublished - Dec 1996


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