Transmission at rod and cone ribbon synapses in the retina

Abstract

Light-evoked voltage responses of rod and cone photoreceptor cells in the vertebrate retina must be converted to a train of synaptic vesicle release events for transmission to downstream neurons. This review discusses the processes, proteins, and structures that shape this critical early step in vision, focusing on studies from salamander retina with comparisons to other experimental animals. Many mechanisms are conserved across species. In cones, glutamate release is confined to ribbon release sites although rods are also capable of release at non-ribbon sites. The role of non-ribbon release in rods remains unclear. Release from synaptic ribbons in rods and cones involves at least three vesicle pools: a readily releasable pool (RRP) matching the number of membrane-associated vesicles along the ribbon base, a ribbon reserve pool matching the number of additional vesicles on the ribbon, and an enormous cytoplasmic reserve. Vesicle release increases in parallel with Ca²⁺ channel activity. While the opening of only a few Ca²⁺ channels beneath each ribbon can trigger fusion of a single vesicle, sustained release rates in darkness are governed by the rate at which the RRP can be replenished. The number of vacant release sites, their functional status, and the rate of vesicle delivery in turn govern replenishment. Along with an overview of the mechanisms of exocytosis and endocytosis, we consider specific properties of ribbon-associated proteins and pose a number of remaining questions about this first synapse in the visual system.

Keywords: Exocytosis; Photoreceptor cell; Ribbon synapse; Vertebrate retina; Vision.

Fig. 1

Rods, cones, and synaptic vesicle pools at photoreceptor ribbon synapses. a Rod and cone anatomy. b Electron micrograph of a salamander cone synapse showing two electron-dense ribbons situated above their arciform densities. The readily releasable pool (RRP) of vesicles at the base of each ribbon is colored in yellow. The ribbon-attached reserve pool is shown in blue. Many vesicles surround both sides of the ribbon, forming a cytoplasmic pool. c Confocal image of a salamander cone (yellow) and horizontal cell (magenta) filled with Lucifer yellow and sulfarhodamine B, respectively, during paired whole cell recording. d Post-synaptic current (PSC) recorded in a horizontal cell evoked by a long depolarizing step applied to a presynaptic cone. Inset shows a magnified view of the initial transient component of the PSC (adapted from [19]). e Charge transfer from the PSC in D was fit with a function consisting of two exponentials and a straight line. The initial fast component corresponds to the RRP, the second slower component likely reflects a ribbon reserve pool, and the linear function likely represents replenishment from the cytoplasmic reserve

Fig. 2

Changes in Ca²⁺ channel activity during light-evoked voltage responses of photoreceptors. a Ca²⁺ channel openings recorded in the cell-attached patch mode from a photoreceptor terminal in salamander retina with 82 mM Ba²⁺ in the pipette. b Overlaid series of voltage responses evoked in a salamander cone by light flashes of increasing intensity. c Whole-cell Ca²⁺ current (I_Ca) plotted against voltage evoked by a ramp voltage protocol in a cone. Voltage was corrected for access resistance and liquid junction potential [79]. The vertical dashed line indicates the typical photoreceptor resting potential in darkness of − 40 mV. The arrow shows the reduction in I_Ca that would accompany a bright light flash

Fig. 3

Diagram of the vesicle cycle at a photoreceptor ribbon synapse. Vesicles in the readily releasable pool (RRP)at the base of the ribbon dock at the membrane and can fuse within ~ 50 ms. Vesicular proteins and lipids released into the membrane must then be removed to prepare the release site for further release. This involves rapid endocytosis with a time constant (τ) of ~ 400 ms. After reforming and refilling vesicles with glutamate, they are returned to the large cytoplasmic pool of vesicles. Vacant sites on the ribbon are replenished from the cytoplasmic pool via both a rapid Ca²⁺-/CaM-dependent mechanism (τ ~ 700 ms) and slower Ca²⁺-independent mechanism (τ ~ 13 s). As vesicles move down the ribbon, they are primed for release. Illustration redrawn and adapted from [230]

Fig. 4

Imaging synaptic vesicle approach to the membrane in a salamander rod terminal. Total internal reflectance fluorescence microscopy (TIRFM) was used to image a vesicle approaching the membrane and then fusing at a rod synapse. The vesicle was loaded with 3kD dextran-conjugated AlexaFluor488. The diagram at the top illustrates a vesicle becoming progressively brighter as it advances towards the membrane through the evanescent field generated by TIRF illumination (length constant = 60 nm). As shown by the TIRFM images at the bottom, after approaching the membrane, the vesicle fused and rapidly released its contents, disappearing within a single 40 ms frame [244]

Fig. 5

Spontaneous and evoked glutamate release from a mouse rod. Glutamate release was assessed in rods by recording anion currents activated during glutamate re-uptake through presynaptic glutamate transporters. When the rod was held at − 70 mV, release consisted of occasional spontaneous, uniquantal events (bottom trace). When this rod was held at − 40 mV, release transitioned to a semi-regular series of multiquantal bursts consisting of ~ 17 vesicles apiece (top trace)