Synaptic Ribbon Active Zones in Cone Photoreceptors Operate Independently from One Another

Cone photoreceptors depolarize in darkness to release glutamate-laden synaptic vesicles. Essential to release is the synaptic ribbon, a structure that helps organize active zones by clustering vesicles near proteins that mediate exocytosis, including voltage-gated Ca(2+) channels. Cone terminals have many ribbon-style active zones at which second-order neurons receive input. We asked whether there are functionally significant differences in local Ca(2+) influx among ribbons in individual cones. We combined confocal Ca(2+) imaging to measure Ca(2+) influx at individual ribbons and patch clamp recordings to record whole-cell I(Ca) in salamander cones. We found that the voltage for half-maximal activation (V(50)) of whole cell I(Ca) in cones averaged −38.1 mV ± 3.05 mV (standard deviation [SD]), close to the cone membrane potential in darkness of ca. −40 mV. Ca(2+) signals at individual ribbons varied in amplitude from one another and showed greater variability in V(50) values than whole-cell I(Ca), suggesting that Ca(2+) signals can differ significantly among ribbons within cones. After accounting for potential sources of technical variability in measurements of Ca(2+) signals and for contributions from cone-to-cone differences in I(Ca), we found that the variability in V(50) values for ribbon Ca(2+) signals within individual cones showed a SD of 2.5 mV. Simulating local differences in Ca(2+) channel activity at two ribbons by shifting the V(50) value of I(Ca) by ±2.5 mV (1 SD) about the mean suggests that when the membrane depolarizes to −40 mV, two ribbons could experience differences in Ca(2+) influx of >45%. Further evidence that local Ca(2+) changes at ribbons can be regulated independently was obtained in experiments showing that activation of inhibitory feedback from horizontal cells (HCs) to cones in paired recordings changed both amplitude and V(50) of Ca(2+) signals at individual ribbons. By varying the strength of synaptic output, differences in voltage dependence and amplitude of Ca(2+) signals at individual ribbons shape the information transmitted from cones to downstream neurons in vision.