Kinetics of exocytosis is faster in cones than in rods

Synaptotagmins 1 and 7 in vesicle release from rods of mouse retina

Synaptotagmins are the primary Ca2+ sensors for synaptic exocytosis. Previous work suggested synaptotagmin-1 (Syt1) mediates evoked vesicle release from cone photoreceptor cells in the vertebrate retina whereas release from rods may involve another sensor in addition to Syt1. We found immunohistochemical evidence for syntaptotagmin-7 (Syt7) in mouse rod terminals and so performed electroretinograms (ERG) and single-cell recordings using mice in which Syt1 and/or Syt7 were conditionally removed from rods and/or cones. Synaptic release was measured in mouse rods by recording presynaptic anion currents activated during glutamate re-uptake and from exocytotic membrane capacitance changes. Deleting Syt1 from rods reduced glutamate release evoked by short depolarizing steps but not long steps whereas deleting Syt7 from rods reduced release evoked by long but not short steps. Deleting both sensors completely abolished depolarization-evoked release from rods. Effects of various intracellular Ca2+ buffers showed that Syt1-mediated release from rods involves vesicles close to ribbon-associated Ca2+ channels whereas Syt7-mediated release evoked by longer steps involves more distant release sites. Spontaneous release from rods was unaffected by eliminating Syt7. While whole animal knockout of Syt7 slightly reduced ERG b-waves and oscillatory potentials, selective elimination of Syt7 from rods had no effect on ERGs. Furthermore, eliminating Syt1 from rods and cones abolished ERG b-waves and additional elimination of Syt7 had no further effect. These results show that while Syt7 contributes to slow non-ribbon release from rods, Syt1 is the principal sensor shaping rod and cone inputs to bipolar cells in response to light flashes.

Cnr2 Is Important for Ribbon Synapse Maturation and Function in Hair Cells and Photoreceptors

The role of the cannabinoid receptor 2 (CNR2) is still poorly described in sensory epithelia. We found strong cnr2 expression in hair cells (HCs) of the inner ear and the lateral line (LL), a superficial sensory structure in fish. Next, we demonstrated that sensory synapses in HCs were severely perturbed in larvae lacking cnr2. Appearance and distribution of presynaptic ribbons and calcium channels (Ca_v1.3) were profoundly altered in mutant animals. Clustering of membrane-associated guanylate kinase (MAGUK) in post-synaptic densities (PSDs) was also heavily affected, suggesting a role for cnr2 for maintaining the sensory synapse. Furthermore, vesicular trafficking in HCs was strongly perturbed suggesting a retrograde action of the endocannabinoid system (ECs) via cnr2 that was modulating HC mechanotransduction. We found similar perturbations in retinal ribbon synapses. Finally, we showed that larval swimming behaviors after sound and light stimulations were significantly different in mutant animals. Thus, we propose that cnr2 is critical for the processing of sensory information in the developing larva.

Transmission at rod and cone ribbon synapses in the retina

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.

Ca2+ sensor synaptotagmin-1 mediates exocytosis in mammalian photoreceptors

To encode light-dependent changes in membrane potential, rod and cone photoreceptors utilize synaptic ribbons to sustain continuous exocytosis while making rapid, fine adjustments to release rate. Release kinetics are shaped by vesicle delivery down ribbons and by properties of exocytotic Ca2+ sensors. We tested the role for synaptotagmin-1 (Syt1) in photoreceptor exocytosis by using novel mouse lines in which Syt1 was conditionally removed from rods or cones. Photoreceptors lacking Syt1 exhibited marked reductions in exocytosis as measured by electroretinography and single-cell recordings. Syt1 mediated all evoked release in cones, whereas rods appeared capable of some slow Syt1-independent release. Spontaneous release frequency was unchanged in cones but increased in rods lacking Syt1. Loss of Syt1 did not alter synaptic anatomy or reduce Ca2+ currents. These results suggest that Syt1 mediates both phasic and tonic release at photoreceptor synapses, revealing unexpected flexibility in the ability of Syt1 to regulate Ca2+-dependent synaptic transmission.

REEP6 mediates trafficking of a subset of Clathrin-coated vesicles and is critical for rod photoreceptor function and survival

In retinal photoreceptors, vectorial transport of cargo is critical for transduction of visual signals, and defects in intracellular trafficking can lead to photoreceptor degeneration and vision impairment. Molecular signatures associated with routing of transport vesicles in photoreceptors are poorly understood. We previously reported the identification of a novel rod photoreceptor specific isoform of Receptor Expression Enhancing Protein (REEP) 6, which belongs to a family of proteins involved in intracellular transport of receptors to the plasma membrane. Here we show that loss of REEP6 in mice (Reep6^−/−) results in progressive retinal degeneration. Rod photoreceptor dysfunction is observed in Reep6^−/− mice as early as one month of age and associated with aberrant accumulation of vacuole-like structures at the apical inner segment and reduction in selected rod phototransduction proteins. We demonstrate that REEP6 is detected in a subset of Clathrin-coated vesicles and interacts with the t-SNARE, Syntaxin3. In concordance with the rod degeneration phenotype in Reep6^−/− mice, whole exome sequencing identified homozygous REEP6-E75K mutation in two retinitis pigmentosa families of different ethnicities. Our studies suggest a critical function of REEP6 in trafficking of cargo via a subset of Clathrin-coated vesicles to selected membrane sites in retinal rod photoreceptors.

Kiss-and-Run Is a Significant Contributor to Synaptic Exocytosis and Endocytosis in Photoreceptors

Accompanying sustained release in darkness, rod and cone photoreceptors exhibit rapid endocytosis of synaptic vesicles. Membrane capacitance measurements indicated that rapid endocytosis retrieves at least 70% of the exocytotic membrane increase. One mechanism for rapid endocytosis is kiss-and-run fusion where vesicles briefly contact the plasma membrane through a small fusion pore. Release can also occur by full-collapse in which vesicles merge completely with the plasma membrane. We assessed relative contributions of full-collapse and kiss-and-run in salamander photoreceptors using optical techniques to measure endocytosis and exocytosis of large vs. small dye molecules. Incubation with small dyes (SR101, 1 nm; 3-kDa dextran-conjugated Texas Red, 2.3 nm) loaded rod and cone synaptic terminals much more readily than larger dyes (10-kDa Texas Red, 4.6 nm; 10-kDa pHrodo, 4.6 nm; 70-kDa Texas Red, 12 nm) consistent with significant uptake through 2.3–4.6 nm fusion pores. By using total internal reflection fluorescence microscopy (TIRFM) to image individual vesicles, when rods were incubated simultaneously with Texas Red and AlexaFluor-488 dyes conjugated to either 3-kDa or 10-kDa dextran, more vesicles loaded small molecules than large molecules. Using TIRFM to detect release by the disappearance of dye-loaded vesicles, we found that SR101 and 3-kDa Texas Red were released from individual vesicles more readily than 10-kDa and 70-kDa Texas Red. Although 10-kDa pHrodo was endocytosed poorly like other large dyes, the fraction of release events was similar to SR101 and 3-kDa Texas Red. We hypothesize that while 10-kDa pHrodo may not exit through a fusion pore, release of intravesicular protons can promote detection of fusion events by rapidly quenching fluorescence of this pH-sensitive dye. Assuming that large molecules can only be released by full-collapse whereas small molecules can be released by both modes, our results indicate that 50%–70% of release from rods involves kiss-and-run with 2.3–4.6 nm fusion pores. Rapid retrieval of vesicles by kiss-and-run may limit membrane disruption of release site function during ongoing release at photoreceptor ribbon synapses.

Preparation of Horizontal Slices of Adult Mouse Retina for Electrophysiological Studies

Vertical slice preparations are well established to study circuitry and signal transmission in the adult mammalian retina. The plane of sectioning in these preparations is perpendicular to the retinal surface, making it ideal for the study of radially oriented neurons like photoreceptors and bipolar cells. However, the large dendritic arbors of horizontal cells, wide-field amacrine cells, and ganglion cells are mostly truncated, leaving markedly reduced synaptic activity in these cells. Whereas ganglion cells and displaced amacrine cells can be studied in a whole-mounted preparation of the retina, horizontal cells and amacrine cells located in the inner nuclear layer are only poorly accessible for electrodes in whole retina tissue. To achieve maximum accessibility and synaptic integrity, we developed a horizontal slice preparation of the mouse retina, and studied signal transmission at the synapse between photoreceptors and horizontal cells. Horizontal sectioning allows (1) easy and unambiguous visual identification of horizontal cell bodies for electrode targeting, and (2) preservation of the extended horizontal cell dendritic fields, as a prerequisite for intact and functional cone synaptic input to horizontal cell dendrites. Horizontal cells from horizontal slices exhibited tonic synaptic activity in the dark, and they responded to brief flashes of light with a reduction of inward current and diminished synaptic activity. Immunocytochemical evidence indicates that almost all cones within the dendritic field of a horizontal cell establish synapses with its peripheral dendrites. The horizontal slice preparation is therefore well suited to study the physiological properties of horizontally extended retinal neurons as well as sensory signal transmission and integration across selected synapses.

Kinetics of Inhibitory Feedback from Horizontal Cells to Photoreceptors: Implications for an Ephaptic Mechanism

Inhibitory feedback from horizontal cells (HCs) to cones generates center-surround receptive fields and color opponency in the retina. Mechanisms of HC feedback remain unsettled, but one hypothesis proposes that an ephaptic mechanism may alter the extracellular electrical field surrounding photoreceptor synaptic terminals, thereby altering Ca(2+) channel activity and photoreceptor output. An ephaptic voltage change produced by current flowing through open channels in the HC membrane should occur with no delay. To test for this mechanism, we measured kinetics of inhibitory feedback currents in Ambystoma tigrinum cones and rods evoked by hyperpolarizing steps applied to synaptically coupled HCs. Hyperpolarizing HCs stimulated inward feedback currents in cones that averaged 8-9 pA and exhibited a biexponential time course with time constants averaging 14-17 ms and 120-220 ms. Measurement of feedback-current kinetics was limited by three factors: (1) HC voltage-clamp speed, (2) cone voltage-clamp speed, and (3) kinetics of Ca(2+) channel activation or deactivation in the photoreceptor terminal. These factors totaled ∼4-5 ms in cones meaning that the true fast time constants for HC-to-cone feedback currents were 9-13 ms, slower than expected for ephaptic voltage changes. We also compared speed of feedback to feedforward glutamate release measured at the same cone/HC synapses and found a latency for feedback of 11-14 ms. Inhibitory feedback from HCs to rods was also significantly slower than either measurement kinetics or feedforward release. The finding that inhibitory feedback from HCs to photoreceptors involves a significant delay indicates that it is not due to previously proposed ephaptic mechanisms.

SIGNIFICANCE STATEMENT

Lateral inhibitory feedback from horizontal cells (HCs) to photoreceptors creates center-surround receptive fields and color-opponent interactions. Although underlying mechanisms remain unsettled, a longstanding hypothesis proposes that feedback is due to ephaptic voltage changes that regulate photoreceptor synaptic output by altering Ca(2+) channel activity. Ephaptic processes should occur with no delay. We measured kinetics of inhibitory feedback currents evoked in photoreceptors with voltage steps applied to synaptically coupled HCs and found that feedback is too slow to be explained by ephaptic voltage changes generated by current flowing through continuously open channels in HC membranes. By eliminating the proposed ephaptic mechanism for HC feedback regulation of photoreceptor Ca(2+) channels, our data support earlier proposals that synaptic cleft pH changes are more likely responsible.

Mechanism of High-Frequency Signaling at a Depressing Ribbon Synapse

Ribbon synapses mediate continuous release in neurons that have graded voltage responses. While mammalian retinas can signal visual flicker at 80-100 Hz, the time constant, τ, for the refilling of a depleted vesicle release pool at cone photoreceptor ribbons is 0.7-1.1 s. Due to this prolonged depression, the mechanism for encoding high temporal frequencies is unclear. To determine the mechanism of high-frequency signaling, we focused on an "Off" cone bipolar cell type in the ground squirrel, the cb2, whose transient postsynaptic responses recovered following presynaptic depletion with a τ of ∼0.1 s, or 7- to 10-fold faster than the τ for presynaptic pool refilling. The difference in recovery time course is caused by AMPA receptor saturation, where partial refilling of the presynaptic pool is sufficient for a full postsynaptic response. By limiting the dynamic range of the synapse, receptor saturation counteracts ribbon depression to produce rapid recovery and facilitate high-frequency signaling.

Ribbon Synapses and Visual Processing in the Retina

The first synapses transmitting visual information contain an unusual organelle, the ribbon, which is involved in the transport and priming of vesicles to be released at the active zone. The ribbon is one of many design features that allow efficient refilling of the active zone, which in turn enables graded changes in membrane potential to be transmitted using a continuous mode of neurotransmitter release. The ribbon also plays a key role in supplying vesicles for rapid and transient bursts of release that signal fast changes, such as the onset of light. We increasingly understand how the physiological properties of ribbon synapses determine basic transformations of the visual signal and, in particular, how the process of refilling the active zone regulates the gain and adaptive properties of the retinal circuit. The molecular basis of ribbon function is, however, far from clear.

Modeling and measurement of vesicle pools at the cone ribbon synapse: Changes in release probability are solely responsible for voltage-dependent changes in release

Postsynaptic responses are a product of quantal amplitude (Q), size of the releasable vesicle pool (N), and release probability (P). Voltage-dependent changes in presynaptic Ca(2+) entry alter postsynaptic responses primarily by changing P but have also been shown to influence N. With simultaneous whole cell recordings from cone photoreceptors and horizontal cells in tiger salamander retinal slices, we measured N and P at cone ribbon synapses by using a train of depolarizing pulses to stimulate release and deplete the pool. We developed an analytical model that calculates the total pool size contributing to release under different stimulus conditions by taking into account the prior history of release and empirically determined properties of replenishment. The model provided a formula that calculates vesicle pool size from measurements of the initial postsynaptic response and limiting rate of release evoked by a train of pulses, the fraction of release sites available for replenishment, and the time constant for replenishment. Results of the model showed that weak and strong depolarizing stimuli evoked release with differing probabilities but the same size vesicle pool. Enhancing intraterminal Ca(2+) spread by lowering Ca(2+) buffering or applying BayK8644 did not increase PSCs evoked with strong test steps, showing there is a fixed upper limit to pool size. Together, these results suggest that light-evoked changes in cone membrane potential alter synaptic release solely by changing release probability.

Ca2+ Diffusion through Endoplasmic Reticulum Supports Elevated Intraterminal Ca2+ Levels Needed to Sustain Synaptic Release from Rods in Darkness

In addition to vesicle release at synaptic ribbons, rod photoreceptors are capable of substantial slow release at non-ribbon release sites triggered by Ca(2+)-induced Ca(2+) release (CICR) from intracellular stores. To maintain CICR as rods remain depolarized in darkness, we hypothesized that Ca(2+) released into the cytoplasm from terminal endoplasmic reticulum (ER) can be replenished continuously by ions diffusing within the ER from the soma. We measured [Ca(2+)] changes in cytoplasm and ER of rods from Ambystoma tigrinum retina using various dyes. ER [Ca(2+)] changes were measured by loading ER with fluo-5N and then washing dye from the cytoplasm with a dye-free patch pipette solution. Small dye molecules diffused within ER between soma and terminal showing a single continuous ER compartment. Depolarization of rods to -40 mV depleted Ca(2+) from terminal ER, followed by a decline in somatic ER [Ca(2+)]. Local activation of ryanodine receptors in terminals with a spatially confined puff of ryanodine caused a decline in terminal ER [Ca(2+)], followed by a secondary decrease in somatic ER. Localized photolytic uncaging of Ca(2+) from o-nitrophenyl-EGTA in somatic ER caused an abrupt Ca(2+) increase in somatic ER, followed by a slower Ca(2+) increase in terminal ER. These data suggest that, during maintained depolarization, a soma-to-terminal [Ca(2+)] gradient develops within the ER that promotes diffusion of Ca(2+) ions to resupply intraterminal ER Ca(2+) stores and thus sustain CICR-mediated synaptic release. The ability of Ca(2+) to move freely through the ER may also promote bidirectional communication of Ca(2+) changes between soma and terminal.

SIGNIFICANCE STATEMENT

Vertebrate rod and cone photoreceptors both release vesicles at synaptic ribbons, but rods also exhibit substantial slow release at non-ribbon sites triggered by Ca(2+)-induced Ca(2+) release (CICR). Blocking CICR inhibits >50% of release from rods in darkness. How do rods maintain sufficiently high [Ca(2+)] in terminal endoplasmic reticulum (ER) to support sustained CICR-driven synaptic transmission? We show that maintained depolarization creates a [Ca(2+)] gradient within the rod ER lumen that promotes soma-to-terminal diffusion of Ca(2+) to replenish intraterminal ER stores. This mechanism allows CICR-triggered synaptic release to be sustained indefinitely while rods remain depolarized in darkness. Free diffusion of Ca(2+) within the ER may also communicate synaptic Ca(2+) changes back to the soma to influence other critical cell processes.

Weak endogenous Ca2+ buffering supports sustained synaptic transmission by distinct mechanisms in rod and cone photoreceptors in salamander retina

Differences in synaptic transmission between rod and cone photoreceptors contribute to different response kinetics in rod- versus cone-dominated visual pathways. We examined Ca²⁺ dynamics in synaptic terminals of tiger salamander photoreceptors under conditions that mimicked endogenous buffering to determine the influence on kinetically and mechanistically distinct components of synaptic transmission. Measurements of I_Cl(Ca) confirmed that endogenous Ca²⁺ buffering is equivalent to ˜0.05 mmol/L EGTA in rod and cone terminals. Confocal imaging showed that with such buffering, depolarization stimulated large, spatially unconstrained [Ca²⁺] increases that spread throughout photoreceptor terminals. We calculated immediately releasable pool (IRP) size and release efficiency in rods by deconvolving excitatory postsynaptic currents and presynaptic Ca²⁺ currents. Peak efficiency of ˜0.2 vesicles/channel was similar to that of cones (˜0.3 vesicles/channel). Efficiency in both cell types was not significantly affected by using weak endogenous Ca²⁺ buffering. However, weak Ca²⁺ buffering speeded Ca²⁺/calmodulin (CaM)-dependent replenishment of vesicles to ribbons in both rods and cones, thereby enhancing sustained release. In rods, weak Ca²⁺ buffering also amplified sustained release by enhancing CICR and CICR-stimulated release of vesicles at nonribbon sites. By contrast, elevating [Ca²⁺] at nonribbon sites in cones with weak Ca²⁺ buffering and by inhibiting Ca²⁺ extrusion did not trigger additional release, consistent with the notion that exocytosis from cones occurs exclusively at ribbons. The presence of weak endogenous Ca²⁺ buffering in rods and cones facilitates slow, sustained exocytosis by enhancing Ca²⁺/CaM-dependent replenishment of ribbons in both rods and cones and by stimulating nonribbon release triggered by CICR in rods.

Functional properties of spontaneous excitatory currents and encoding of light/dark transitions in horizontal cells of the mouse retina

As all visual information is represented in the spatio-temporal dynamics of transmitter release from photoreceptors and the combined postsynaptic responses of second-order neurons, appropriate synaptic transfer functions are fundamental for a meaningful perception of the visual world. The functional contribution of horizontal cells to gain control and organization of bipolar and ganglion cell receptive fields can only be evaluated with an in-depth understanding of signal processing in horizontal cells. Therefore, a horizontal slice preparation of the mouse retina was established to record from horizontal cell bodies with their dendritic fields intact and receiving functional synaptic input from cone photoreceptors. Horizontal cell bodies showed spontaneous excitatory currents (spEPSCs) of monophasic and more complex multi-peak waveforms. spEPSCs were induced by quantal release of glutamate from presynaptic cones with a unitary amplitude of 3 pA. Non-stationary noise analysis revealed that spEPSCs with a monoexponential decay were mediated by 7-8 glutamate receptors with a single-channel amplitude of 1.55 pA. Responses to photopic full-field illumination were characterized by reduction of a tonic inward current or hyperpolarization, inhibition of spEPSCs, followed by a fast and transient inward current at light offset. The response to periodic dark/light transitions of different frequencies was dependent on the adaptational status of the cell with a limiting frequency of 10 Hz. Both on and off components of the light response were mediated by AMPA and kainate receptors. Detailed analysis of horizontal cell synaptic physiology is a prerequisite for understanding signal coding and processing at the photoreceptor ribbon synapse.

Calmodulin enhances ribbon replenishment and shapes filtering of synaptic transmission by cone photoreceptors

At the first synapse in the vertebrate visual pathway, light-evoked changes in photoreceptor membrane potential alter the rate of glutamate release onto second-order retinal neurons. This process depends on the synaptic ribbon, a specialized structure found at various sensory synapses, to provide a supply of primed vesicles for release. Calcium (Ca(2+)) accelerates the replenishment of vesicles at cone ribbon synapses, but the mechanisms underlying this acceleration and its functional implications for vision are unknown. We studied vesicle replenishment using paired whole-cell recordings of cones and postsynaptic neurons in tiger salamander retinas and found that it involves two kinetic mechanisms, the faster of which was diminished by calmodulin (CaM) inhibitors. We developed an analytical model that can be applied to both conventional and ribbon synapses and showed that vesicle resupply is limited by a simple time constant, τ = 1/(Dρδs), where D is the vesicle diffusion coefficient, δ is the vesicle diameter, ρ is the vesicle density, and s is the probability of vesicle attachment. The combination of electrophysiological measurements, modeling, and total internal reflection fluorescence microscopy of single synaptic vesicles suggested that CaM speeds replenishment by enhancing vesicle attachment to the ribbon. Using electroretinogram and whole-cell recordings of light responses, we found that enhanced replenishment improves the ability of cone synapses to signal darkness after brief flashes of light and enhances the amplitude of responses to higher-frequency stimuli. By accelerating the resupply of vesicles to the ribbon, CaM extends the temporal range of synaptic transmission, allowing cones to transmit higher-frequency visual information to downstream neurons. Thus, the ability of the visual system to encode time-varying stimuli is shaped by the dynamics of vesicle replenishment at photoreceptor synaptic ribbons.

Endogenous calcium buffering at photoreceptor synaptic terminals in salamander retina

Calcium operates by several mechanisms to regulate glutamate release at rod and cone synaptic terminals. In addition to serving as the exocytotic trigger, Ca2+ accelerates replenishment of vesicles in cones and triggers Ca2+-induced Ca2+ release (CICR) in rods. Ca2+ thereby amplifies sustained exocytosis, enabling photoreceptor synapses to encode constant and changing light. A complete picture of the role of Ca2+ in regulating synaptic transmission requires an understanding of the endogenous Ca2+ handling mechanisms at the synapse. We therefore used the "added buffer" approach to measure the endogenous Ca2+ binding ratio (κendo ) and extrusion rate constant (γ) in synaptic terminals of photoreceptors in retinal slices from tiger salamander. We found that κendo was similar in both cell types-∼25 and 50 in rods and cones, respectively. Using measurements of the decay time constants of Ca2+ transients, we found that γ was also similar, with values of ∼100 s(-1) and 160 s(-1) in rods and cones, respectively. The measurements of κendo differ considerably from measurements in retinal bipolar cells, another ribbon-bearing class of retinal neurons, but are comparable to similar measurements at other conventional synapses. The values of γ are slower than at other synapses, suggesting that Ca2+ ions linger longer in photoreceptor terminals, supporting sustained exocytosis, CICR, and Ca2+ -dependent ribbon replenishment. The mechanisms of endogenous Ca2+ handling in photoreceptors are thus well-suited for supporting tonic neurotransmission. Similarities between rod and cone Ca2+ handling suggest that neither buffering nor extrusion underlie differences in synaptic transmission kinetics.

Rapid kinetics of endocytosis at rod photoreceptor synapses depends upon endocytic load and calcium

Release from rods is triggered by the opening of L-type Ca2+ channels that lie beneath synaptic ribbons. After exocytosis, vesicles are retrieved by compensatory endocytosis. Previous work showed that endocytosis is dynamin-dependent in rods but dynamin-independent in cones. We hypothesized that fast endocytosis in rods may also differ from cones in its dependence upon the amount of Ca2+ influx and/or endocytic load. We measured exocytosis and endocytosis from membrane capacitance (C m) changes evoked by depolarizing steps in voltage clamped rods from tiger salamander retinal slices. Similar to cones, the time constant for endocytosis in rods was quite fast, averaging <200 ms. We manipulated Ca2+ influx and the amount of vesicle release by altering the duration and voltage of depolarizing steps. Unlike cones, endocytosis kinetics in rods slowed after increasing Ca2+ channel activation with longer step durations or more strongly depolarized voltage steps. Endocytosis kinetics also slowed as Ca2+ buffering was decreased by replacing BAPTA (10 or 1 mM) with the slower Ca2+ buffer EGTA (5 or 0.5 mM) in the pipette solution. These data provide further evidence that endocytosis mechanisms differ in rods and cones and suggest that endocytosis in rods is regulated by both endocytic load and local Ca2+ levels.

Intracellular calcium stores drive slow non-ribbon vesicle release from rod photoreceptors

Rods are capable of greater slow release than cones contributing to overall slower release kinetics. Slow release in rods involves Ca²⁺-induced Ca²⁺ release (CICR). By impairing release from ribbons, we found that unlike cones where release occurs entirely at ribbon-style active zones, slow release from rods occurs mostly at ectopic, non-ribbon sites. To investigate the role of CICR in ribbon and non-ribbon release from rods, we used total internal reflection fluorescence microscopy as a tool for visualizing terminals of isolated rods loaded with fluorescent Ca²⁺ indicator dyes and synaptic vesicles loaded with dextran-conjugated pH-sensitive rhodamine. We found that rather than simply facilitating release, activation of CICR by ryanodine triggered release directly in rods, independent of plasma membrane Ca²⁺ channel activation. Ryanodine-evoked release occurred mostly at non-ribbon sites and release evoked by sustained depolarization at non-ribbon sites was mostly due to CICR. Unlike release at ribbon-style active zones, non-ribbon release did not occur at fixed locations. Fluorescence recovery after photobleaching of endoplasmic reticulum (ER)-tracker dye in rod terminals showed that ER extends continuously from synapse to soma. Release of Ca²⁺ from terminal ER by lengthy depolarization did not significantly deplete Ca²⁺ from ER in the perikaryon. Collectively, these results indicate that CICR-triggered release at non-ribbon sites is a major mechanism for maintaining vesicle release from rods and that CICR in terminals may be sustained by diffusion of Ca²⁺ through ER from other parts of the cell.

Properties of ribbon and non-ribbon release from rod photoreceptors revealed by visualizing individual synaptic vesicles

Vesicle release from rod photoreceptors is regulated by Ca(2+) entry through L-type channels located near synaptic ribbons. We characterized sites and kinetics of vesicle release in salamander rods by using total internal reflection fluorescence microscopy to visualize fusion of individual synaptic vesicles. A small number of vesicles were loaded by brief incubation with FM1-43 or a dextran-conjugated, pH-sensitive form of rhodamine, pHrodo. Labeled organelles matched the diffraction-limited size of fluorescent microspheres and disappeared rapidly during stimulation. Consistent with fusion, depolarization-evoked vesicle disappearance paralleled electrophysiological release kinetics and was blocked by inhibiting Ca(2+) influx. Rods maintained tonic release at resting membrane potentials near those in darkness, causing depletion of membrane-associated vesicles unless Ca(2+) entry was inhibited. This depletion of release sites implies that sustained release may be rate limited by vesicle delivery. During depolarizing stimulation, newly appearing vesicles approached the membrane at ∼800 nm/s, where they paused for ∼60 ms before fusion. With fusion, vesicles advanced ∼18 nm closer to the membrane. Release events were concentrated near ribbons, but lengthy depolarization also triggered release from more distant non-ribbon sites. Consistent with greater contributions from non-ribbon sites during lengthier depolarization, damaging the ribbon by fluorophore-assisted laser inactivation (FALI) of Ribeye caused only weak inhibition of exocytotic capacitance increases evoked by 200-ms depolarizing test steps, whereas FALI more strongly inhibited capacitance increases evoked by 25 ms steps. Amplifying release by use of non-ribbon sites when rods are depolarized in darkness may improve detection of decrements in release when they hyperpolarize to light.

Rapid synaptic vesicle endocytosis in cone photoreceptors of salamander retina

Following synaptic vesicle exocytosis, neurons retrieve the fused membrane by a process of endocytosis to provide a supply of vesicles for subsequent release and maintain the presynaptic active zone. Rod and cone photoreceptors use a specialized structure called the synaptic ribbon that enables them to sustain high rates of neurotransmitter release. They must also employ mechanisms of synaptic vesicle endocytosis capable of keeping up with release. While much is known about endocytosis at another retinal ribbon synapse, that of the goldfish Mb1 bipolar cell, less is known about endocytosis in photoreceptors. We used capacitance recording techniques to measure vesicle membrane fusion and retrieval in photoreceptors from salamander retinal slices. We found that application of brief depolarizing steps (<100 ms) to cones evoked exocytosis followed by rapid endocytosis with a time constant ∼250 ms. In some cases, the capacitance trace overshot the baseline, indicating excess endocytosis. Calcium had no effect on the time constant, but enhanced excess endocytosis resulting in a faster rate of membrane retrieval. Surprisingly, endocytosis was unaffected by blockers of dynamin, suggesting that cone endocytosis is dynamin independent. This contrasts with synaptic vesicle endocytosis in rods, which was inhibited by the dynamin inhibitor dynasore and GTPγS introduced through the patch pipette, suggesting that the two photoreceptor types employ distinct pathways for vesicle retrieval. The fast kinetics of synaptic vesicle endocytosis in photoreceptors likely enables these cells to maintain a high rate of transmitter release, allowing them to faithfully signal changes in illumination to second-order neurons.

Deletion of the presynaptic scaffold CAST reduces active zone size in rod photoreceptors and impairs visual processing

How size and shape of presynaptic active zones are regulated at the molecular level has remained elusive. Here we provide insight from studying rod photoreceptor ribbon-type active zones after disruption of CAST/ERC2, one of the cytomatrix of the active zone (CAZ) proteins. Rod photoreceptors were present in normal numbers, and the a-wave of the electroretinogram (ERG)--reflecting their physiological population response--was unchanged in CAST knock-out (CAST(-/-)) mice. Using immunofluorescence and electron microscopy, we found that the size of the rod presynaptic active zones, their Ca(2+) channel complement, and the extension of the outer plexiform layer were diminished. Moreover, we observed sprouting of horizontal and bipolar cells toward the outer nuclear layer indicating impaired rod transmitter release. However, rod synapses of CAST(-/-) mice, unlike in mouse mutants for the CAZ protein Bassoon, displayed anchored ribbons, normal vesicle densities, clustered Ca(2+) channels, and essentially normal molecular organization. The reduction of the rod active zone size went along with diminished amplitudes of the b-wave in scotopic ERGs. Assuming, based on the otherwise intact synaptic structure, an unaltered function of the remaining release apparatus, we take our finding to suggest a scaling of release rate with the size of the active zone. Multielectrode-array recordings of retinal ganglion cells showed decreased contrast sensitivity. This was also observed by optometry, which, moreover, revealed reduced visual acuity. We conclude that CAST supports large active zone size and high rates of transmission at rod ribbon synapses, which are required for normal vision.

Regulation of synaptic transmission at the photoreceptor terminal: a novel role for the cation-chloride co-transporter NKCC1

The Na(+)-K(+)-2Cl(-) co-transporter type 1 (NKCC1) is localized primarily throughout the outer plexiform layer (OPL) of the distal retina, a synaptic lamina that is comprised of the axon terminals of photoreceptors and the dendrites of horizontal and bipolar cells. Although known to play a key role in development, signal transmission and the gating of sensory signals in other regions of the retina and in the CNS, the contribution of NKCC1 to synaptic transmission within the OPL is largely unknown. In the present study, we investigated the function of NKCC1 at the photoreceptor-horizontal cell synapse by recording the electrical responses of photoreceptors and horizontal cells before and after blocking the activity of the transporter with bumetanide (BMN). Because NKCC1 co-transports 1 Na(+), 1 K(+) and 2 Cl(-), it is electroneutral and its activation had little effect on membrane conductance. However, recordings from postsynaptic horizontal cells revealed that inhibiting NKCC1 with BMN greatly increased glutamate release from both rod and cone terminals. In addition, we found that NKCC1 directly regulates Ca(2+)-dependent exocytosis at the photoreceptor synapse, raising the possibility that NKCC1 serves to suppress bulk release of glutamate vesicles from photoreceptor terminals in the dark and at light offset. Interestingly, NKCC1 gene and protein expressions were upregulated by light, which we attribute to the light-induced release of dopamine acting on D1-like receptors. In sum, our study reveals a new role for NKCC1 in the regulation of synaptic transmission in photoreceptors.

Structure and function of a complex sensory synapse

Vision is the most important of the senses for humans, and the retina is the first stage in the processing of light signals in the visual system. In the retina, highly specialized light-sensing neurons, the rod and cone photoreceptors, convert light into neural signals. These signals are extensively processed and filtered in the subsequent retinal network before transmitted to the higher visual centres in the brain, where the perception of viewed objects and scenes is finally constructed. A key feature of signal processing in the mammalian retina is parallel processing. Visual information is segregated in parallel pathways already at the rod and cone photoreceptor terminals, which provide multiple output synapses for the faithful encoding and transfer of the visual signals to the post-receptoral retinal network. This review aims at highlighting the current knowledge about the structural and functional pre- and post-synaptic specializations of rod and cone photoreceptor ribbon synapses, which belong to the most complex chemical synapses in the central nervous system.

Regulation of presynaptic strength by controlling Ca2+ channel mobility: effects of cholesterol depletion on release at the cone ribbon synapse

Synaptic communication requires proper coupling between voltage-gated Ca(2+) (Ca(V)) channels and synaptic vesicles. In photoreceptors, L-type Ca(V) channels are clustered close to synaptic ribbon release sites. Although clustered, Ca(V) channels move continuously within a confined domain slightly larger than the base of the ribbon. We hypothesized that expanding Ca(V) channel confinement domains should increase the number of channel openings needed to trigger vesicle release. Using single-particle tracking techniques, we measured the expansion of Ca(V) channel confinement domains caused by depletion of membrane cholesterol with cholesterol oxidase or methyl-β-cyclodextrin. With paired whole cell recordings from cones and horizontal cells, we then determined the number of Ca(V) channel openings contributing to cone Ca(V) currents (I(Ca)) and the number of vesicle fusion events contributing to horizontal cell excitatory postsynaptic currents (EPSCs) following cholesterol depletion. Expansion of Ca(V) channel confinement domains reduced the peak efficiency of release, decreasing the number of vesicle fusion events accompanying opening of each Ca(V) channel. Cholesterol depletion also inhibited exocytotic capacitance increases evoked by brief depolarizing steps. Changes in efficiency were not due to changes in I(Ca) amplitude or glutamate receptor properties. Replenishing cholesterol restored Ca(V) channel domain size and release efficiency to control levels. These results indicate that cholesterol is important for organizing the cone active zone. Furthermore, the finding that cholesterol depletion impairs coupling between channel opening and vesicle release by allowing Ca(V) channels to move further from release sites shows that changes in presynaptic Ca(V) channel mobility can be a mechanism for adjusting synaptic strength.

Release from the cone ribbon synapse under bright light conditions can be controlled by the opening of only a few Ca(2+) channels

Light hyperpolarizes cone photoreceptors, causing synaptic voltage-gated Ca(2+) channels to open infrequently. To understand neurotransmission under these conditions, we determined the number of L-type Ca(2+) channel openings necessary for vesicle fusion at the cone ribbon synapse. Ca(2+) currents (I(Ca)) were activated in voltage-clamped cones, and excitatory postsynaptic currents (EPSCs) were recorded from horizontal cells in the salamander retina slice preparation. Ca(2+) channel number and single-channel current amplitude were calculated by mean-variance analysis of I(Ca). Two different comparisons-one comparing average numbers of release events to average I(Ca) amplitude and the other involving deconvolution of both EPSCs and simultaneously recorded cone I(Ca)-suggested that fewer than three Ca(2+) channel openings accompanied fusion of each vesicle at the peak of release during the first few milliseconds of stimulation. Opening fewer Ca(2+) channels did not enhance fusion efficiency, suggesting that few unnecessary channel openings occurred during strong depolarization. We simulated release at the cone synapse, using empirically determined synaptic dimensions, vesicle pool size, Ca(2+) dependence of release, Ca(2+) channel number, and Ca(2+) channel properties. The model replicated observations when a barrier was added to slow Ca(2+) diffusion. Consistent with the presence of a diffusion barrier, dialyzing cones with diffusible Ca(2+) buffers did not affect release efficiency. The tight clustering of Ca(2+) channels, along with a high-Ca(2+) affinity release mechanism and diffusion barrier, promotes a linear coupling between Ca(2+) influx and vesicle fusion. This may improve detection of small light decrements when cones are hyperpolarized by bright light.

Acute destruction of the synaptic ribbon reveals a role for the ribbon in vesicle priming

In vision, balance and hearing, sensory receptor cells translate sensory stimuli into electrical signals whose amplitude is graded with stimulus intensity. The output synapses of these sensory neurons must provide fast signaling to follow rapidly changing stimuli while also transmitting graded information covering a wide range of stimulus intensity and must be able to sustain this signaling for long time periods. To meet these demands, specialized machinery for transmitter release, the synaptic ribbon, has evolved at the synaptic outputs of these neurons. We found that acute disruption of synaptic ribbons by photodamage to the ribbon markedly reduced both sustained and transient components of neurotransmitter release in mouse bipolar cells and salamander cones without affecting the ultrastructure of the ribbon or its ability to localize synaptic vesicles to the active zone. Our results indicate that ribbons mediate both slow and fast signaling at sensory synapses and support an additional role for the synaptic ribbon in priming vesicles for exocytosis at active zones.

Lateral mobility of presynaptic L-type calcium channels at photoreceptor ribbon synapses

At most synapses, presynaptic Ca(2+) channels are positioned near vesicle release sites, and increasing this distance reduces synaptic strength. We examined the lateral membrane mobility of presynaptic L-type Ca(2+) channels at photoreceptor ribbon synapses of the tiger salamander (Ambystoma tigrinum) retina. Movements of individual Ca(2+) channels were tracked by coupling quantum dots to an antibody against the extracellular α(2)δ(4) Ca(2+) channel subunit. α(2)δ(4) antibodies labeled photoreceptor terminals and colocalized with antibodies to synaptic vesicle glycoprotein 2 and voltage-gated Ca(2+) channel 1.4 (Ca(V)1.4) α(1) subunits. The results show that Ca(2+) channels are dynamic and move within a confined region beneath the synaptic ribbon. The size of this confinement area is regulated by actin and membrane cholesterol. Fusion of nearby synaptic vesicles caused jumps in Ca(2+) channel position, propelling them toward the outer edge of the confinement domain. Channels rebounded rapidly toward the center. Thus, although Ca(V) channels are mobile, molecular scaffolds confine them beneath the ribbon to maintain neurotransmission even at high release rates.

Quantal amplitude at the cone ribbon synapse can be adjusted by changes in cytosolic glutamate

PURPOSE

Vision is encoded at photoreceptor synapses by the number of released vesicles and size of the post-synaptic response. We hypothesized that elevating cytosolic glutamate could enhance quantal size by increasing glutamate in vesicles.

METHODS

We introduced glutamate (10-40 mM) into cone terminals through a patch pipette and recorded excitatory post-synaptic currents (EPSCs) from horizontal or OFF bipolar cells in the Ambystoma tigrinum retinal slice preparation.

RESULTS

Elevating cytosolic glutamate in cone terminals enhanced EPSCs as well as quantal miniature EPSCs (mEPSCs). Enhancement was prevented by inhibiting vesicular glutamate transport with 1S,3R-1-aminocyclopentane-1,3-dicarboxylate in the patch pipette. A low affinity glutamate receptor antagonist, γD-glutamylglycine (1 mM), less effectively inhibited EPSCs evoked from cones loaded with glutamate than control cones indicating that release from cones with supplemental glutamate produced higher glutamate levels in the synaptic cleft. Raising presynaptic glutamate did not alter exocytotic capacitance responses and exocytosis was observed after inhibiting glutamate loading with the vesicular ATPase inhibitor, concanamycin A, suggesting that release capability is not restricted by low vesicular glutamate levels. Variance-mean analysis of currents evoked by flash photolysis of caged glutamate indicated that horizontal cell AMPA receptors have a single channel conductance of 10.1 pS suggesting that ~8.7 GluRs contribute to each mEPSC.

CONCLUSIONS

Quantal amplitude at the cone ribbon synapse is capable of adjustment by changes in cytosolic glutamate levels. The small number of channels contributing to each mEPSC suggests that stochastic variability in channel opening could be an important source of quantal variability.

Conical tomography of a ribbon synapse: structural evidence for vesicle fusion

To characterize the sites of synaptic vesicle fusion in photoreceptors, we evaluated the three-dimensional structure of rod spherules from mice exposed to steady bright light or dark-adapted for periods ranging from 3 to 180 minutes using conical electron tomography. Conical tilt series from mice retinas were reconstructed using the weighted back projection algorithm, refined by projection matching and analyzed using semiautomatic density segmentation. In the light, rod spherules contained ∼470 vesicles that were hemi-fused and ∼187 vesicles that were fully fused (omega figures) with the plasma membrane. Active zones, defined by the presence of fully fused vesicles, extended along the entire area of contact between the rod spherule and the horizontal cell ending, and included the base of the ribbon, the slope of the synaptic ridge and ribbon-free regions apposed to horizontal cell axonal endings. There were transient changes of the rod spherules during dark adaptation. At early periods in the dark (3–15 minutes), there was a) an increase in the number of fully fused synaptic vesicles, b) a decrease in rod spherule volume, and c) an increase in the surface area of the contact between the rod spherule and horizontal cell endings. These changes partially compensate for the increase in the rod spherule plasma membrane following vesicle fusion. After 30 minutes of dark-adaptation, the rod spherules returned to dimensions similar to those measured in the light. These findings show that vesicle fusion occurs at both ribbon-associated and ribbon-free regions, and that transient changes in rod spherules and horizontal cell endings occur shortly after dark onset.

Fast glutamate uptake via EAAT2 shapes the cone-mediated light offset response in bipolar cells

Excitatory amino acid transporters (EAATs) are responsible for extracellular glutamate uptake within the retina, and are expressed by retinal neurons and Müller cells. Their role within glutamatergic synapses is not completely understood. In the salamander retina, five distinct EAAT-encoding genes have been cloned, making the amphibian retina an excellent system to study EAAT function. This study focused on sEAAT2, which is expressed in photoreceptor terminals and Off-bipolar cells in two isoforms, sEAAT2A and sEAAT2B. Using whole-cell patch-clamp recording, florescence imaging and antibody labelling methods, we systematically studied the functions of these two isoforms at the synapse between photoreceptors and bipolar cells, both in dark and with photic stimulation. Both sEAAT2A and sEAAT2B were sensitive to dihydrokainic acid (DHKA), a known EAAT2-specific inhibitor. Each isoform of sEAAT2 was found to play a role in tonic glutamate uptake at the cone synapse in darkness. Furthermore, presynaptic sEAAT2A strongly suppressed the rapid, transient glutamate signal from cones following light-offset. This was achieved by quickly binding exocytosed glutamate, which subsequently limited glutamate spillover to adjacent receptors at postsynaptic sites. Since the intensity and duration of photic stimulation determine the magnitude of these cone transient signals, we postulate that presynaptic cone EAATs contribute to the encoding of contrast sensitivity in cone vision.

Changes to tear cytokines of type 2 diabetic patients with or without retinopathy

PURPOSE

To investigate changes in cytokine levels in tears of type 2 diabetics with or without retinopathy.

METHODS

Tears were collected from 15 type 2 diabetics without retinopathy (DNR), 15 patients with retinopathy (DR), and 15 age and gender matched non-diabetic controls. Tear concentrations of 27 cytokines were measured by multiplex bead immunoassay. Cytokine differences between groups, ratios of type-1 T helper (Th1)/type-2 T helper (Th2) cytokines and anti-angiogenic/pro-angiogenic cytokines were analyzed statistically.

RESULTS

The most abundant cytokine detected in tears was interferon-induced protein-10 (IP-10). In comparison with controls, IP-10 and monocyte chemoattracant protein-1 (MCP-1) levels were significantly elevated in DR (p=0.016 and 0.036, respectively) and DNR groups (p=0.021 and 0.026, respectively). Interleukin-1 (IL-1) receptor antagonist (IL-1ra) levels were significantly increased in DNR (p=0.016). Th1/Th2 cytokines interferon-gamma (IFN-γ)/IL-5 and IL-2/IL-5 ratios were significantly increased in DR compared to controls (p=0.037 and 0.031, respectively). Anti-angiogenic/angiogenic cytokines IFN-γ/MCP-1 and IL-4/MCP-1 ratios in DR and DNR were significantly decreased compared to controls (p<0.05). IL-4/IL-8 and IL-12p70/IL-8 ratios were also significantly decreased in DR compared to controls (p=0.02 and 0.045, respectively). No significant correlation was demonstrated between tear cytokine concentrations and glycosylated hemoglobin (HbA1c) or fasting plasma glucose (FPG).

CONCLUSIONS

Diabetic tears exhibited elevated levels of IP-10 and MCP-1. The Th1/Th2 cytokine balance may shift to a predominantly Th1 state in DR patients. Pro-angiogenic cytokines are more highly represented than anti-angiogenic cytokines in the tears of diabetic patients.

Calcium regulates vesicle replenishment at the cone ribbon synapse

Cones release glutamate-filled vesicles continuously in darkness, and changing illumination modulates this release. Because sustained release in darkness is governed by vesicle replenishment rates, we analyzed how cone membrane potential regulates replenishment. Synaptic release from cones was measured by recording postsynaptic currents in Ambystoma tigrinum horizontal or OFF bipolar cells evoked by depolarization of simultaneously voltage-clamped cones. We measured replenishment after attaining a steady state between vesicle release and replenishment using trains of test pulses. Increasing Ca(2+) currents (I(Ca)) by changing the test step from -30 to -10 mV increased replenishment. Lengthening -30 mV test pulses to match the Ca(2+) influx during 25 ms test pulses to -10 mV produced similar replenishment rates. Reducing Ca(2+) driving force by using test steps to +30 mV slowed replenishment. Using UV flashes to reverse inhibition of I(Ca) by nifedipine accelerated replenishment. Increasing [Ca(2+)](i) by flash photolysis of caged Ca(2+) also accelerated replenishment. Replenishment, but not the initial burst of release, was enhanced by using an intracellular Ca(2+) buffer of 0.5 mm EGTA rather than 5 mm EGTA, and diminished by 1 mm BAPTA. This suggests that although release and replenishment exhibited similar Ca(2+) dependencies, release sites are <200 nm from Ca(2+) channels but replenishment sites are >200 nm away. Membrane potential thus regulates replenishment by controlling Ca(2+) influx, principally by effects on replenishment mechanisms but also by altering releasable pool size. This in turn provides a mechanism for converting changes in light intensity into changes in sustained release at the cone ribbon synapse.

Quantitative analysis of synaptic release at the photoreceptor synapse

Exocytosis from the rod photoreceptor is stimulated by submicromolar Ca(2+) and exhibits an unusually shallow dependence on presynaptic Ca(2+). To provide a quantitative description of the photoreceptor Ca(2+) sensor for exocytosis, we tested a family of conventional and allosteric computational models describing the final Ca(2+)-binding steps leading to exocytosis. Simulations were fit to two measures of release, evoked by flash-photolysis of caged Ca(2+): exocytotic capacitance changes from individual rods and postsynaptic currents of second-order neurons. The best simulations supported the occupancy of only two Ca(2+) binding sites on the rod Ca(2+) sensor rather than the typical four or five. For most models, the on-rates for Ca(2+) binding and maximal fusion rate were comparable to those of other neurons. However, the off-rates for Ca(2+) unbinding were unexpectedly slow. In addition to contributing to the high-affinity of the photoreceptor Ca(2+) sensor, slow Ca(2+) unbinding may support the fusion of vesicles located at a distance from Ca(2+) channels. In addition, partial sensor occupancy due to slow unbinding may contribute to the linearization of the first synapse in vision.

Comparative analysis of Drosophila and mammalian complexins as fusion clamps and facilitators of neurotransmitter release

The SNARE-binding protein complexin (Cpx) has been demonstrated to regulate synaptic vesicle fusion. Previous studies are consistent with Cpx functioning either as a synaptic vesicle fusion clamp to prevent premature exocytosis, or as a facilitator to directly stimulate release. Here we examined conserved roles of invertebrate and mammalian Cpx isoforms in the regulation of neurotransmitter release using the Drosophila neuromuscular junction as a model synapse. We find that SNARE binding by Cpx is required for its role as a fusion clamp. All four mammalian Cpx proteins (mCpx), which have been demonstrated to facilitate release, also function as fusion clamps when expressed in Drosophila cpx null mutants, though their clamping abilities vary between isoforms. Moreover, expression of mCpx I, II or III isoforms dramatically enhance evoked release compared to mCpx IV or Drosophila Cpx. Differences in the clamping and facilitating properties of complexin isoforms can be partially attributed to differences in the C-terminal membrane tethering domain. Our findings indicate that the function of complexins as fusion clamps and facilitators of fusion are conserved across evolution, and that these roles are genetically separable within an isoform and across different isoforms.

Hearing requires otoferlin-dependent efficient replenishment of synaptic vesicles in hair cells

Inner hair cell ribbon synapses indefatigably transmit acoustic information. The proteins mediating their fast vesicle replenishment (hundreds of vesicles per s) are unknown. We found that an aspartate to glycine substitution in the C(2)F domain of the synaptic vesicle protein otoferlin impaired hearing by reducing vesicle replenishment in the pachanga mouse model of human deafness DFNB9. In vitro estimates of vesicle docking, the readily releasable vesicle pool (RRP), Ca(2+) signaling and vesicle fusion were normal. Moreover, we observed postsynaptic excitatory currents of variable size and spike generation. However, mutant active zones replenished vesicles at lower rates than wild-type ones and sound-evoked spiking in auditory neurons was sparse and only partially improved during longer interstimulus intervals. We conclude that replenishment does not match the release of vesicles at mutant active zones in vivo and a sufficient standing RRP therefore cannot be maintained. We propose that otoferlin is involved in replenishing synaptic vesicles.

Characterisation of bipolar cell synaptic transmission in goldfish retina using paired recordings

Direct recordings from the large axon terminals of goldfish retinal bipolar cells (BCs) have revealed detailed information about the properties and regulation of exocytosis at this ribbon-type synapse. However, the relationship between BC exocytosis and evoked postsynaptic responses in amacrine and ganglion cells is not known. To address this, I have made paired recordings from BC terminals (BCTs) and neurons in the ganglion cell layer (GCL) in goldfish retinal slices. BCT depolarisation evoked short-latency, AMPA/kainate receptor-mediated EPSCs in connected GCL neurons. NMDA receptors contributed to the response at +40 mV but not at 60 mV. Evoked EPSCs contained multiple temporal components that differed in their relative amplitudes between pairs. Changing the duration or amplitude of the presynaptic stimulus affected the size and kinetics of the EPSC, with weaker stimuli slowing the EPSC activation rate. Paired-pulse stimulation caused greater depression of fast than slow EPSC components. A linear relationship was found between the amount of BCT exocytosis, measured via changes in membrane capacitance, and the charge of evoked EPSCs, whether they were mediated by AMPA/kainate receptors alone or in combination with NMDA receptors. In addition, analysis of miniature EPSCs in GCL neurons provided estimates of the quantal content of evoked EPSCs. The results demonstrate the feasibility of using this paired recording system to study synaptic transfer at ribbon synapses, and indicate that both the rapid and sustained phases of BC exocytosis are encoded in the postsynaptic response.

The making of synaptic ribbons: how they are built and what they do

Ribbon synapses in the retina and inner ear maintain tonic neurotransmitter release at high rates to transduce a broad bandwidth of stimulus intensities. In ribbon synapses, synaptic vesicles can be released by a slow, sustained mode and by fast, synchronous mechanisms. The high release rates require structural and functional specializations. The synaptic ribbon is the key structural specialization of ribbon synapses. Synaptic ribbons are large, electron-dense structures that immobilize numerous synaptic vesicles next to presynaptic release sites. A main component of synaptic ribbons is the protein RIBEYE that has the capability to build the scaffold of the synaptic ribbon via multiple RIBEYE-RIBEYE interactions. A modular assembly model of synaptic ribbons has been proposed in which synaptic ribbons are formed from individual RIBEYE subunits. The scaffold of the synaptic ribbon provides a docking site for RIBEYE-associated proteins that could execute specific synaptic ribbon functions. Multiple functions have been assigned to synaptic ribbons including roles in exocytosis, endocytosis, and synaptic membrane trafficking. Recent studies demonstrated the importance of synaptic ribbons for fast, synchronous release and emphasized the need of a tight and efficient coupling between presynaptic Ca(2+) signaling and exocytosis. The present review summarizes recent advances on structure and function of synaptic ribbons.

Vesicle pool size at the salamander cone ribbon synapse

Cone light responses are transmitted to postsynaptic neurons by changes in the rate of synaptic vesicle release. Vesicle pool size at the cone synapse constrains the amount of release and can thus shape contrast detection. We measured the number of vesicles in the rapidly releasable and reserve pools at cone ribbon synapses by performing simultaneous whole cell recording from cones and horizontal or off bipolar cells in the salamander retinal slice preparation. We found that properties of spontaneously occurring miniature excitatory postsynaptic currents (mEPSCs) are representative of mEPSCs evoked by depolarizing presynaptic stimulation. Strong, brief depolarization of the cone stimulated release of the entire rapidly releasable pool (RRP) of vesicles. Comparing charge transfer of the EPSC with mEPSC charge transfer, we determined that the fast component of the EPSC reflects release of approximately 40 vesicles. Comparing EPSCs with simultaneous presynaptic capacitance measurements, we found that horizontal cell EPSCs constitute 14% of the total number of vesicles released from a cone terminal. Using a fluorescent ribeye-binding peptide, we counted approximately 13 ribbons per cone. Together, these results suggest each cone contacts a single horizontal cell at approximately 2 ribbons. The size of discrete components in the EPSC amplitude histogram also suggested approximately 2 ribbon contacts per cell pair. We therefore conclude there are approximately 20 vesicles per ribbon in the RRP, similar to the number of vesicles contacting the plasma membrane at the ribbon base. EPSCs evoked by lengthy depolarization suggest a reserve pool of approximately 90 vesicles per ribbon, similar to the number of additional docking sites further up the ribbon.

Otoferlin is critical for a highly sensitive and linear calcium-dependent exocytosis at vestibular hair cell ribbon synapses

Otoferlin, a C2-domain-containing Ca(2+) binding protein, is required for synaptic exocytosis in auditory hair cells. However, its exact role remains essentially unknown. Intriguingly enough, no balance defect has been observed in otoferlin-deficient (Otof(-/-)) mice. Here, we show that the vestibular nerve compound action potentials evoked during transient linear acceleration ramps in Otof(-/-) mice display higher threshold, lower amplitude, and increased latency compared with wild-type mice. Using patch-clamp capacitance measurement in intact utricles, we show that type I and type II hair cells display a remarkable linear transfer function between Ca(2+) entry, flowing through voltage-activated Ca(2+) channels, and exocytosis. This linear Ca(2+) dependence was observed when changing the Ca(2+) channel open probability or the Ca(2+) flux per channel during various test potentials. In Otof(-/-) hair cells, exocytosis displays slower kinetics, reduced Ca(2+) sensitivity, and nonlinear Ca(2+) dependence, despite morphologically normal synapses and normal Ca(2+) currents. We conclude that otoferlin is essential for a high-affinity Ca(2+) sensor function that allows efficient and linear encoding of low-intensity stimuli at the vestibular hair cell synapse.

Adenosine suppresses exocytosis from cone terminals of the salamander retina

In the retina, adenosine is released in the dark and has been shown to inhibit Ca2+ influx through voltage-gated Ca2+ channels in cones. Therefore, we tested whether adenosine can inhibit exocytosis from isolated cone photoreceptors. Simultaneous measurements of membrane exocytosis and Ca2+ were made from cones using the activity-dependent dye, Synaptored-C2, and the Ca2+ indicator dye, Fluo-4. Adenosine suppressed exocytosis in cones, indicating that transmitter release is also reduced from cone terminals, and further supports an inhibitory mechanism for modulating transmitter release onto second-order neurons. Furthermore, this raises the possibility that adenosine might be neuroprotective for photoreceptors and second-order neurons by suppressing Ca2+ levels in cones and reducing exocytosis of L-glutamate, respectively.

Calcium homeostasis and cone signaling are regulated by interactions between calcium stores and plasma membrane ion channels

Calcium is a messenger ion that controls all aspects of cone photoreceptor function, including synaptic release. The dynamic range of the cone output extends beyond the activation threshold for voltage-operated calcium entry, suggesting another calcium influx mechanism operates in cones hyperpolarized by light. We have used optical imaging and whole-cell voltage clamp to measure the contribution of store-operated Ca²⁺ entry (SOCE) to Ca²⁺ homeostasis and its role in regulation of neurotransmission at cone synapses. Mn²⁺ quenching of Fura-2 revealed sustained divalent cation entry in hyperpolarized cones. Ca²⁺ influx into cone inner segments was potentiated by hyperpolarization, facilitated by depletion of intracellular Ca²⁺ stores, unaffected by pharmacological manipulation of voltage-operated or cyclic nucleotide-gated Ca²⁺ channels and suppressed by lanthanides, 2-APB, MRS 1845 and SKF 96365. However, cation influx through store-operated channels crossed the threshold for activation of voltage-operated Ca²⁺ entry in a subset of cones, indicating that the operating range of inner segment signals is set by interactions between store- and voltage-operated Ca²⁺ channels. Exposure to MRS 1845 resulted in ∼40% reduction of light-evoked postsynaptic currents in photopic horizontal cells without affecting the light responses or voltage-operated Ca²⁺ currents in simultaneously recorded cones. The spatial pattern of store-operated calcium entry in cones matched immunolocalization of the store-operated sensor STIM1. These findings show that store-operated channels regulate spatial and temporal properties of Ca²⁺ homeostasis in vertebrate cones and demonstrate their role in generation of sustained excitatory signals across the first retinal synapse.

Mechanisms that limit the light stimulus frequency following through the DL-2-amino-4-phosphonobutyric acid sensitive and insensitive rod Off-pathways

In the retina, rod signal pathways process scotopic visual information. Light decrements are mediated by two distinct groups of rod pathways in the dark-adapted retina that can be differentiated on the basis of their sensitivity to the glutamate agonist dl-2-amino-4-phosphonobutyric acid (APB). We have found that the APB sensitive and insensitive rod Off-pathways signal different light decrement information: the APB sensitive rod Off-pathway conveys slow and low frequency light signals, whereas the APB insensitive rod Off-pathways mediate fast and high frequency light signals [Wang GY (2006) Unique functional properties of the APB sensitive and insensitive rod pathways signaling light decrements in mouse retinal ganglion cells. Vis Neurosci 23:127-135]. However, the mechanisms which limit the frequency following through the APB sensitive and insensitive rod Off-pathways remain unknown. In the current study, whole-cell patch-clamp recordings were made from ganglion cells in dark and light adapted mouse retina to identify the mechanisms that limit the frequency following through the APB sensitive and insensitive rod Off-pathways. The results showed that the sites from AII amacrine cells to Off cone bipolar cells are the major mechanisms that limit the frequency following through the APB sensitive rod Off-pathway. In the APB insensitive rod Off-pathways, rods themselves limited the frequency following through these pathways. Moreover, ganglion cells were able to follow higher frequencies under photopic conditions than under scotopic conditions. The Off responses followed lower frequencies than On responses under photopic conditions. This finding was observed in cells that yielded On or Off responses only as well as in On-Off cells.

Light regulation of Ca2+ in the cone photoreceptor synaptic terminal

Retinal cones are depolarized in darkness, keeping voltage-gated Ca2+ channels open and sustaining exocytosis of synaptic vesicles. Light hyperpolarizes the membrane potential, closing Ca2+ channels and suppressing exocytosis. Here, we quantify the Ca2+ concentration in cone terminals, with Ca2+ indicator dyes. Two-photon ratiometric imaging of fura-2 shows that global Ca2+ averages approximately 360 nM in darkness and falls to approximately 190 nM in bright light. Depolarizing cones from their light to their dark membrane potential reveals hot spots of Ca2+ that co-label with a fluorescent probe for the synaptic ribbon protein ribeye, consistent with tight localization of Ca2+ channels near ribbons. Measurements with a low-affinity Ca2+ indicator show that the local Ca2+ concentration near the ribbon exceeds 4 M in darkness. The high level of Ca2+ near the ribbon combined with previous estimates of the Ca2+ sensitivity of release leads to a predicted dark release rate that is much faster than observed, suggesting that the cone synapse operates in a maintained state of synaptic depression in darkness.

Role of the synaptic ribbon in transmitting the cone light response

Cone photoreceptors distinguish small changes in light intensity while operating over a wide dynamic range. The cone synapse encodes intensity by modulating tonic neurotransmitter release, but precise encoding is limited by the quantal nature of synaptic vesicle exocytosis. Cones possess synaptic ribbons, structures that are thought to accelerate the delivery of vesicles for tonic release. Here we show that the synaptic ribbon actually constrains vesicle delivery, resulting in a maintained state of synaptic depression in darkness. Electron microscopy of cones from the lizard Anolis segrei revealed that depression is caused by the depletion of vesicles on the ribbon, indicating that resupply, not fusion, is the rate-limiting step that controls release. Responses from postsynaptic retinal neurons from the salamander Ambystoma tigrinum showed that the ribbon behaves like a capacitor, charging with vesicles in light and discharging in a phasic burst at light offset. Phasic release extends the operating range of the cone synapse to more accurately encode changes in light intensity, accentuating features that are salient to photopic vision.

Synaptic vesicle dynamics in mouse rod bipolar cells

To better understand synaptic signaling at the mammalian rod bipolar cell terminal and pave the way for applying genetic approaches to the study of visual information processing in the mammalian retina, synaptic vesicle dynamics and intraterminal calcium were monitored in terminals of acutely isolated mouse rod bipolar cells and the number of ribbon-style active zones quantified. We identified a releasable pool, corresponding to a maximum of 7 s. The presence of a smaller, rapidly releasing pool and a small, fast component of refilling was also suggested. Following calcium channel closure, membrane surface area was restored to baseline with a time constant that ranged from 2 to 21 s depending on the magnitude of the preceding Ca2+ transient. In addition, a brief, calcium-dependent delay often preceded the start of onset of membrane recovery. Thus, several aspects of synaptic vesicle dynamics appear to be conserved between rod-dominant bipolar cells of fish and mammalian rod bipolar cells. A major difference is that the number of vesicles available for release is significantly smaller in the mouse rod bipolar cell, both as a function of the total number per neuron and on a per active zone basis.

Quantal mEPSCs and residual glutamate: how horizontal cell responses are shaped at the photoreceptor ribbon synapse

At the photoreceptor ribbon synapse, glutamate released from vesicles at different positions along the ribbon reaches the same postsynaptic receptors. Thus, vesicles may not exert entirely independent effects. We examined whether responses of salamander retinal horizontal cells evoked by light or direct depolarization during paired recordings could be predicted by summation of individual miniature excitatory postsynaptic currents (mEPSCs). For EPSCs evoked by depolarization of rods or cones, linear convolution of mEPSCs with photoreceptor release functions predicted EPSC waveforms and changes caused by inhibiting glutamate receptor desensitization. A low-affinity glutamate antagonist, kynurenic acid (KynA), preferentially reduced later components of rod-driven EPSCs, suggesting lower levels of glutamate are present during the later sustained component of the EPSC. A glutamate-scavenging enzyme, glutamic-pyruvic transaminase, did not inhibit mEPSCs or the initial component of rod-driven EPSCs, but reduced later components of the EPSC. Inhibiting glutamate uptake with a low concentration of DL-threo-beta-benzoyloxyaspartate (TBOA) also did not alter mEPSCs or the initial component of rod-driven EPSCs, but enhanced later components of the EPSC. Low concentrations of TBOA and KynA did not affect the kinetics of fast cone-driven EPSCs. Under both rod- and cone-dominated conditions, light-evoked currents (LECs) were enhanced considerably by TBOA. LECs were more strongly inhibited than EPSCs by KynA, suggesting the presence of lower glutamate levels. Collectively, these results indicate that the initial EPSC component can be largely predicted from a linear sum of individual mEPSCs, but with sustained release, residual amounts of glutamate from multiple vesicles pool together, influencing LECs and later components of EPSCs.

Feedback from horizontal cells to rod photoreceptors in vertebrate retina

Retinal horizontal cells (HCs) provide negative feedback to cones, but, largely because annular illumination fails to evoke a depolarizing response in rods, it is widely believed that there is no feedback from HCs to rods. However, feedback from HCs to cones involves small changes in the calcium current (I(Ca)) that do not always generate detectable depolarizing responses. We therefore recorded I(Ca) directly from rods to test whether they were modulated by feedback from HCs. To circumvent problems presented by overlapping receptive fields of HCs and rods, we manipulated the membrane potential of voltage-clamped HCs while simultaneously recording from rods in a salamander retinal slice preparation. Like HC feedback in cones, hyperpolarizing HCs from -14 to -54, -84, and -104 mV increased the amplitude of I(Ca) recorded from synaptically connected rods and caused hyperpolarizing shifts in I(Ca) voltage dependence. These effects were blocked by supplementing the bicarbonate-buffered saline solution with HEPES. In rods lacking light-responsive outer segments, hyperpolarizing neighboring HCs with light caused a negative activation shift and increased the amplitude of I(Ca). These changes in I(Ca) were blocked by HEPES and by inhibiting HC light responses with a glutamate antagonist, indicating that they were caused by HC feedback. These results show that rods, like cones, receive negative feedback from HCs that regulates the amplitude and voltage dependence of I(Ca). HC-to-rod feedback counters light-evoked decreases in synaptic output and thus shapes the transmission of rod responses to downstream visual neurons.

How do tonic glutamatergic synapses evade receptor desensitization?

Photoreceptor output synapses are the best known tonic chemical synapses in the nervous system, in which glutamate is continuously released in darkness, activating AMPA/kainate receptors in postsynaptic neurons. It has been shown that glutamate receptors in certain types of second-order retinal cells are largely desensitized in darkness, leading to small postsynaptic currents and reduced response dynamic ranges. Here we show that the tonic glutamatergic synapses between photoreceptors and rod-dominated hyperpolarizing bipolar cells (HBC(R)s) in the salamander retina evade postsynaptic receptor desensitization by using (1) multiple invaginating ribbon junctions as releasing sites for low-frequency, synchronized multiquantal release at each site; and (2) the GluR4 AMPA receptors as the postsynaptic receptors. The multiquantal events exhibit faster decay time than the GluR4 receptor desensitization time constant and therefore self-desensitization is minimized, and the average inter-event duration in darkness is much longer than the GluR4 desensitization recovery time and thus mutual desensitization is avoided. Consequently, the HBC(R)s are not desensitized in darkness, allowing light signals to be encoded by the full operating range of the glutamate-gated postsynaptic currents. Our study illustrates for the first time how a tonic glutamatergic synapse avoids postsynaptic receptor desensitization, a strategy that may be shared by many other synapses in the nervous system that need extended operation capacity.

Kinetics of synaptic transmission at ribbon synapses of rods and cones

The ribbon synapse is a specialized structure that allows photoreceptors to sustain the continuous release of vesicles for hours upon hours and years upon years but also respond rapidly to momentary changes in illumination. Light responses of cones are faster than those of rods and, mirroring this difference, synaptic transmission from cones is also faster than transmission from rods. This review evaluates the various factors that regulate synaptic kinetics and contribute to kinetic differences between rod and cone synapses. Presynaptically, the release of glutamate-laden synaptic vesicles is regulated by properties of the synaptic proteins involved in exocytosis, influx of calcium through calcium channels, calcium release from intracellular stores, diffusion of calcium to the release site, calcium buffering, and extrusion of calcium from the cytoplasm. The rate of vesicle replenishment also limits the ability of the synapse to follow changes in release. Post-synaptic factors include properties of glutamate receptors, dynamics of glutamate diffusion through the cleft, and glutamate uptake by glutamate transporters. Thus, multiple synaptic mechanisms help to shape the responses of second-order horizontal and bipolar cells.