| Rod and cone photoreceptors encode light with different sensitivity and kinetics. Here we use optical imaging method to study the synaptic differences that affect the transmission of visual information. We find that rods tonically release synaptic vesicles in the dark at a much slower rate than cones, as measured by the release of the fluorescent vesicle indicator FMI-43. Rods signal under dimmer light, which has a lower signal-to-noise ratio than bright light. The slower release from rods matches the precision signal output to the information content of light input, which optimizes use of the limited resource of continuously cycling vesicles. To elucidate the mechanism underlying slower release from rods, we evaluate Ca2+-dependent release by simultaneously recording synaptic vesicle cycling and intraterminal Ca2+. We find that the Ca2+ sensitivity of release is indistinguishable in rods and cones, consistent with their possessing similar release machinery. However, we find the dark intraterminal Ca2+ concentration is lower in rods than in cones, as determined by 2-photon Ca 2+ imaging. Thus, the lower level of dark Ca2+ ensures that rods encode intensity with a slower vesicle release rate that is better matched to the lower information content of dim light. |
