H. Vongersdorff et al., EVIDENCE THAT VESICLES ON THE SYNAPTIC RIBBON OF RETINAL BIPOLAR NEURONS CAN BE RAPIDLY RELEASED, Neuron, 16(6), 1996, pp. 1221-1227
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.