EVIDENCE THAT VESICLES ON THE SYNAPTIC RIBBON OF RETINAL BIPOLAR NEURONS CAN BE RAPIDLY RELEASED

We relate the ultrastructure of the giant bipolar synapse in goldfish retina to the jump in capacitance that accompanies depolarization-evok ed exocytosis. Mean vesicle diameter is 29 +/- 4 nm, giving 26.4 aF/ve sicle, so the maximum evoked capacitance (150 fF within 200 ms) repres ents fusion of about 5700 vesicles. Two terminals contained, respectiv ely, 45 and 65 ribbon-type synaptic outputs, and a fully loaded ribbon tethers about 110 vesicles. Thus, the tethered pool, about 6000 vesic les, corresponds to the rapidly released pool. Further, the difference between small and large terminals in number of tethered vesicles matc hes their difference in capacitance jump. This suggests, within a ''fi re and reload'' model of exocytosis, that the ribbon translocates syna ptic vesicles very rapidly to membrane docking sites, supporting a max imum release rate of 500 vesicles/active zone/s, until the population of tethered vesicles is exhausted.