Conical Tomography of Ribbon Synapses of Light and Dark-Adapted Rod and Cones Photoreceptors

Abstract

Rod and cone photoreceptors undergo tonic transmitter release following changes in illumination. In both rods and cones, the fusion competent (i.e., docked and primed) synaptic vesicles are thought to be associated preferentially to a distinctive electron-dense structure referred as the “ribbon.” The aim of this study was to evaluate the location and number of docked (i.e., “hemi-fused”) and fully fused (i.e., “omega” figures) vesicles in the axon terminal of rods and cones in light and dark-adapted mice. Using conical electron tomography, we reconstructed ribbon synapses of mice exposed to steady bright light or dark-adapted for 3 to 180 minutes. The conical tomograms were analyzed using density segmentation methods based on the watershed algorithm. We found that in the light, in both rods and cones the docked vesicular pool extended alongside the entire area of contact between the axon terminals and the horizontal cell endings. In rod axon terminals, the docked pool was comprised of 460-480 hemi-fused vesicles and remained essentially unchanged during dark-adaptation. In the dark, the terminals underwent rapid but reversible changes, including an increase in plasma membrane area and the number of “omega” figures (from ∼190 to ∼1300 in rods terminals). After ∼30 minutes in the dark, the axon terminals returned to the conditions observed in mice exposed to steady light conditions. We thus conclude that in mouse rod and cone photoreceptors both ribbon-associated and non-ribbon regions of the plasma membrane of the axon terminals contribute to the shaping of signaling to post-synaptic neurons.