Studies of the properties of synaptic transmission have been carried out at only a few synapses. We analyzed exocytosis from rod photoreceptors with a combination of physiological and ultrastructural techniques. As at other ribbon synapses, we found that rods exhibited rapid kinetics of release, and the number of vesicles in the releasable pool is comparable to the number of vesicles tethered at ribbon-style active zones. However, unlike other previously studied neurons, we identified a highly Ca2+-sensitive pool of releasable vesicles with a relatively shallow relationship between the rate of exocytosis and [Ca2+]i that is nearly linear over a presumed physiological range of intraterminal [Ca2+]. The low-order [Ca 2+] dependence of release promotes a linear relationship between Ca2+ entry and exocytosis that permits rods to relay information about small changes in illumination with high fidelity at the first synapse in vision.
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