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

摘要

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.