Synaptic transmission mediated by internal calcium stores in rod photoreceptors

The endoplasmic reticulum: Homeostasis and crosstalk in retinal health and disease

The endoplasmic reticulum (ER) is the largest intracellular organelle carrying out a broad range of important cellular functions including protein biosynthesis, folding, and trafficking, lipid and sterol biosynthesis, carbohydrate metabolism, and calcium storage and gated release. In addition, the ER makes close contact with multiple intracellular organelles such as mitochondria and the plasma membrane to actively regulate the biogenesis, remodeling, and function of these organelles. Therefore, maintaining a homeostatic and functional ER is critical for the survival and function of cells. This vital process is implemented through well-orchestrated signaling pathways of the unfolded protein response (UPR). The UPR is activated when misfolded or unfolded proteins accumulate in the ER, a condition known as ER stress, and functions to restore ER homeostasis thus promoting cell survival. However, prolonged activation or dysregulation of the UPR can lead to cell death and other detrimental events such as inflammation and oxidative stress; these processes are implicated in the pathogenesis of many human diseases including retinal disorders. In this review manuscript, we discuss the unique features of the ER and ER stress signaling in the retina and retinal neurons and describe recent advances in the research to uncover the role of ER stress signaling in neurodegenerative retinal diseases including age-related macular degeneration, inherited retinal degeneration, achromatopsia and cone diseases, and diabetic retinopathy. In some chapters, we highlight the complex interactions between the ER and other intracellular organelles focusing on mitochondria and illustrate how ER stress signaling regulates common cellular stress pathways such as autophagy. We also touch upon the integrated stress response in retinal degeneration and diabetic retinopathy. Finally, we provide an update on the current development of pharmacological agents targeting the UPR response and discuss some unresolved questions and knowledge gaps to be addressed by future research.

Robust expression of the TRPC1 channel associated with photoreceptor loss in the rat retina

Baseline intracellular calcium levels are significantly higher in neuronal and glial cells of rat retinas with retinitis pigmentosa (RP). Although this situation could initiate multiple detrimental pathways that lead to cell death, we considered the possibility of TRPC1 being involved in maintaining calcium homeostasis in the retina by acting as a component of store-operated calcium (SOC) channels with special relevance during photoreceptor degeneration. In this study, we examined by Western blot the expression of TRPC1 in healthy control rat retinas (Sprague-Dawley, SD) and retinas with RP (P23H-1 rats). We also analyzed its specific cellular distribution by immunofluorescence to recognize changes during neurodegeneration and to determine whether its presence is consistent with high basal calcium levels and cellular survival in degenerating retinas. We found that TRPC1 immunostaining was widely distributed across the retina in both rat strains, SD and P23H, and its expression levels significantly increased in the retinas with advanced degeneration compared to the age-control SD rats. In the outer retina, TRPC1 immunoreactivity was distributed in pigment epithelium cells, the photoreceptor inner segments of older animals, and the outer plexiform layer. In the inner retina, TRPC1 labeling was detected in horizontal cells, specific somata of bipolar and amacrine cells, and cellular processes in all the strata of the inner plexiform layer. Somata and processes were also highly immunoreactive in the ganglion cell layer and astrocytes in the nerve fiber layer in all animals. In the P23H rat retinas, the TRPC1 distribution pattern changed according to advancing photoreceptor degeneration and the gliosis reaction, with TRPC1 immunoreactive Müller cells mainly in advanced stages of disease. The cellular TRPC1 immunoreactivity found in this work suggests different mechanisms of activation of these channels depending on the cell type. Furthermore, the results support the idea that photoreceptor loss due to RP is associated with robust TRPC1 protein expression in the rat inner retina and raise the possibility of TRPC1 channels contributing to maintain high basal calcium levels during neurodegeneration and/or maintenance processes of the inner retina.

Retinal TRP channels: Cell-type-specific regulators of retinal homeostasis and multimodal integration

Transient receptor potential (TRP) channels are a widely expressed family of 28 evolutionarily conserved cationic ion channels that operate as primary detectors of chemical and physical stimuli and secondary effectors of metabotropic and ionotropic receptors. In vertebrates, the channels are grouped into six related families: TRPC, TRPV, TRPM, TRPA, TRPML, and TRPP. As sensory transducers, TRP channels are ubiquitously expressed across the body and the CNS, mediating critical functions in mechanosensation, nociception, chemosensing, thermosensing, and phototransduction. This article surveys current knowledge about the expression and function of the TRP family in vertebrate retinas, which, while dedicated to transduction and transmission of visual information, are highly susceptible to non-visual stimuli. Every retinal cell expresses multiple TRP subunits, with recent evidence establishing their critical roles in paradigmatic aspects of vertebrate vision that include TRPM1-dependent transduction of ON bipolar signaling, TRPC6/7-mediated ganglion cell phototransduction, TRP/TRPL phototransduction in Drosophila and TRPV4-dependent osmoregulation, mechanotransduction, and regulation of inner and outer blood-retina barriers. TRP channels tune light-dependent and independent functions of retinal circuits by modulating the intracellular concentration of the 2nd messenger calcium, with emerging evidence implicating specific subunits in the pathogenesis of debilitating diseases such as glaucoma, ocular trauma, diabetic retinopathy, and ischemia. Elucidation of TRP channel involvement in retinal biology will yield rewards in terms of fundamental understanding of vertebrate vision and therapeutic targeting to treat diseases caused by channel dysfunction or over-activation.

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.

Resting and stimulated mouse rod photoreceptors show distinct patterns of vesicle release at ribbon synapses

The vertebrate visual system can detect and transmit signals from single photons. To understand how single-photon responses are transmitted, we characterized voltage-dependent properties of glutamate release in mouse rods. We measured presynaptic glutamate transporter anion current and found that rates of synaptic vesicle release increased with voltage-dependent Ca2+ current. Ca2+ influx and release rate also rose with temperature, attaining a rate of ∼11 vesicles/s/ribbon at -40 mV (35°C). By contrast, spontaneous release events at hyperpolarized potentials (-60 to -70 mV) were univesicular and occurred at random intervals. However, when rods were voltage clamped at -40 mV for many seconds to simulate maintained darkness, release occurred in coordinated bursts of 17 ± 7 quanta (mean ± SD; n = 22). Like fast release evoked by brief depolarizing stimuli, these bursts involved vesicles in the readily releasable pool of vesicles and were triggered by the opening of nearby ribbon-associated Ca2+ channels. Spontaneous release rates were elevated and bursts were absent after genetic elimination of the Ca2+ sensor synaptotagmin 1 (Syt1). This study shows that at the resting potential in darkness, rods release glutamate-filled vesicles from a pool at the base of synaptic ribbons at low rates but in Syt1-dependent bursts. The absence of bursting in cones suggests that this behavior may have a role in transmitting scotopic responses.

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.

Angiotensin-Receptor-Associated Protein Modulates Ca2+ Signals in Photoreceptor and Mossy Fiber cells

Fast, precise and sustained neurotransmission requires graded Ca²⁺ signals at the presynaptic terminal. Neurotransmitter release depends on a complex interplay of Ca²⁺ fluxes and Ca²⁺ buffering in the presynaptic terminal that is not fully understood. Here, we show that the angiotensin-receptor-associated protein (ATRAP) localizes to synaptic terminals throughout the central nervous system. In the retinal photoreceptor synapse and the cerebellar mossy fiber-granule cell synapse, we find that ATRAP is involved in the generation of depolarization-evoked synaptic Ca²⁺ transients. Compared to wild type, Ca²⁺ imaging in acutely isolated preparations of the retina and the cerebellum from ATRAP knockout mice reveals a significant reduction of the sarcoendoplasmic reticulum (SR) Ca²⁺-ATPase (SERCA) activity. Thus, in addition to its conventional role in angiotensin signaling, ATRAP also modulates presynaptic Ca²⁺ signaling within the central nervous system.

Spontaneous action potentials in retinal horizontal cells of goldfish (Carassius auratus) are dependent upon L-type Ca2+ channels and ryanodine receptors

Horizontal cells (HCs) are interneurons of the outer retina that undergo graded changes in membrane potential during the light response and provide feedback to photoreceptors. We characterized spontaneous Ca²⁺-based action potentials (APs) in isolated goldfish (Carassius auratus) HCs with electrophysiological and intracellular imaging techniques. Transient changes in intracellular Ca²⁺ concentration ([Ca²⁺]_i) were observed with fura-2 and were abolished by removal of extracellular Ca²⁺ or by inhibition of Ca²⁺ channels by 50 µM Cd²⁺ or 100 µM nifedipine. Inhibition of Ca²⁺ release from stores with 20 µM ryanodine or 50 µM dantrolene abolished Ca²⁺ transients and increased baseline [Ca²⁺]_i. This increased baseline was prevented by blocking L-type Ca²⁺ channels with nifedipine, suggesting that Ca²⁺-induced Ca²⁺ release from stores may be needed to inactivate membrane Ca²⁺ channels. Caffeine (3 mM) increased the frequency of Ca²⁺ transients, and the store-operated channel antagonist 2-aminoethyldiphenylborinate (100 μM) counteracted this effect. APs were detected with voltage-sensitive dye imaging (FluoVolt) and current-clamp electrophysiology. In current-clamp recordings, regenerative APs were abolished by removal of extracellular Ca²⁺ or in the presence of 5 mM Co²⁺ or 100 µM nifedipine, and APs were amplified with 15 mM Ba²⁺. Collectively, our data suggest that during APs Ca²⁺ enters through L-type Ca²⁺ channels and that Ca²⁺ stores (gated by ryanodine receptors) contribute to the rise in [Ca²⁺]_i. This work may lead to further understanding of the possible role APs have in vision, such as transitioning from light to darkness or modulating feedback from HCs to photoreceptors.NEW & NOTEWORTHY Horizontal cells (HCs) are interneurons of the outer retina that provide inhibitory feedback onto photoreceptors. HCs respond to light via graded changes in membrane potential. We characterized spontaneous action potentials in HCs from goldfish and linked action potential generation to a rise in intracellular Ca²⁺ via plasma membrane channels and ryanodine receptors. Action potentials may play a role in vision, such as transitioning from light to darkness, or in modulating feedback from HCs to photoreceptors.

Simultaneous Release of Multiple Vesicles from Rods Involves Synaptic Ribbons and Syntaxin 3B

First proposed as a specialized mode of release at sensory neurons possessing ribbon synapses, multivesicular release has since been described throughout the central nervous system. Many aspects of multivesicular release remain poorly understood. We explored mechanisms underlying simultaneous multivesicular release at ribbon synapses in salamander retinal rod photoreceptors. We assessed spontaneous release presynaptically by recording glutamate transporter anion currents (I_A(glu)) in rods. Spontaneous I_A(glu) events were correlated in amplitude and kinetics with simultaneously measured miniature excitatory postsynaptic currents in horizontal cells. Both measures indicated that a significant fraction of events is multiquantal, with an analysis of I_A(glu) revealing that multivesicular release constitutes ∼30% of spontaneous release events. I_A(glu) charge transfer increased linearly with event amplitude showing that larger events involve greater glutamate release. The kinetics of large and small I_A(glu) events were identical as were rise times of large and small miniature excitatory postsynaptic currents, indicating that the release of multiple vesicles during large events is highly synchronized. Effects of exogenous Ca²⁺ buffers suggested that multiquantal, but not uniquantal, release occurs preferentially near Ca²⁺ channels clustered beneath synaptic ribbons. Photoinactivation of ribbons reduced the frequency of spontaneous multiquantal events without affecting uniquantal release frequency, showing that spontaneous multiquantal release requires functional ribbons. Although both occur at ribbon-style active zones, the absence of cross-depletion indicates that evoked and spontaneous multiquantal release from ribbons involve different vesicle pools. Introducing an inhibitory peptide into rods to interfere with the SNARE protein, syntaxin 3B, selectively reduced multiquantal event frequency. These results support the hypothesis that simultaneous multiquantal release from rods arises from homotypic fusion among neighboring vesicles on ribbons and involves syntaxin 3B.

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.

Endocytosis sustains release at photoreceptor ribbon synapses by restoring fusion competence

Endocytosis is an essential process at sites of synaptic release. Not only are synaptic vesicles recycled by endocytosis, but the removal of proteins and lipids by endocytosis is needed to restore release site function at active zones after vesicle fusion. Synaptic exocytosis from vertebrate photoreceptors involves synaptic ribbons that serve to cluster vesicles near the presynaptic membrane. In this study, we hypothesize that this clustering increases the likelihood that exocytosis at one ribbon release site may disrupt release at an adjacent site and therefore that endocytosis may be particularly important for restoring release site competence at photoreceptor ribbon synapses. To test this, we combined optical and electrophysiological techniques in salamander rods. Pharmacological inhibition of dynamin-dependent endocytosis rapidly inhibits release from synaptic ribbons and slows recovery of ribbon-mediated release from paired pulse synaptic depression. Inhibiting endocytosis impairs the ability of second-order horizontal cells to follow rod light responses at frequencies as low as 2 Hz. Inhibition of endocytosis also increases lateral membrane mobility of individual Ca2+ channels, showing that it changes release site structure. Visualization of single synaptic vesicles by total internal reflection fluorescence microscopy reveals that inhibition of endocytosis reduces the likelihood of fusion among vesicles docked near ribbons and increases the likelihood that they will retreat from the membrane without fusion. Vesicle advance toward the membrane is also reduced, but the number of membrane-associated vesicles is not. Endocytosis therefore appears to be more important for restoring later steps in vesicle fusion than for restoring docking. Unlike conventional synapses in which endocytic restoration of release sites is evident only at high frequencies, endocytosis is needed to maintain release from rod ribbon synapses even at modest frequencies.

Mechanisms, pools, and sites of spontaneous vesicle release at synapses of rod and cone photoreceptors

Photoreceptors have depolarized resting potentials that stimulate calcium-dependent release continuously from a large vesicle pool but neurons can also release vesicles without stimulation. We characterized the Ca(2+) dependence, vesicle pools, and release sites involved in spontaneous release at photoreceptor ribbon synapses. In whole-cell recordings from light-adapted horizontal cells (HCs) of tiger salamander retina, we detected miniature excitatory post-synaptic currents (mEPSCs) when no stimulation was applied to promote exocytosis. Blocking Ca(2+) influx by lowering extracellular Ca(2+) , by application of Cd(2+) and other agents reduced the frequency of mEPSCs but did not eliminate them, indicating that mEPSCs can occur independently of Ca(2+) . We also measured release presynaptically from rods and cones by examining quantal glutamate transporter anion currents. Presynaptic quantal event frequency was reduced by Cd(2+) or by increased intracellular Ca(2+) buffering in rods, but not in cones, that were voltage clamped at -70 mV. By inhibiting the vesicle cycle with bafilomycin, we found the frequency of mEPSCs declined more rapidly than the amplitude of evoked excitatory post-synaptic currents (EPSCs) suggesting a possible separation between vesicle pools in evoked and spontaneous exocytosis. We mapped sites of Ca(2+) -independent release using total internal reflectance fluorescence (TIRF) microscopy to visualize fusion of individual vesicles loaded with dextran-conjugated pHrodo. Spontaneous release in rods occurred more frequently at non-ribbon sites than evoked release events. The function of Ca(2+) -independent spontaneous release at continuously active photoreceptor synapses remains unclear, but the low frequency of spontaneous quanta limits their impact on noise.

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.

Calcium-induced calcium release supports recruitment of synaptic vesicles in auditory hair cells

Hair cells from auditory and vestibular systems transmit continuous sound and balance information to the central nervous system through the release of synaptic vesicles at ribbon synapses. The high activity experienced by hair cells requires a unique mechanism to sustain recruitment and replenishment of synaptic vesicles for continuous release. Using pre- and postsynaptic electrophysiological recordings, we explored the potential contribution of calcium-induced calcium release (CICR) in modulating the recruitment of vesicles to auditory hair cell ribbon synapses. Pharmacological manipulation of CICR with agents targeting endoplasmic reticulum calcium stores reduced both spontaneous postsynaptic multiunit activity and the frequency of excitatory postsynaptic currents (EPSCs). Pharmacological treatments had no effect on hair cell resting potential or activation curves for calcium and potassium channels. However, these drugs exerted a reduction in vesicle release measured by dual-sine capacitance methods. In addition, calcium substitution by barium reduced release efficacy by delaying release onset and diminishing vesicle recruitment. Together these results demonstrate a role for calcium stores in hair cell ribbon synaptic transmission and suggest a novel contribution of CICR in hair cell vesicle recruitment. We hypothesize that calcium entry via calcium channels is tightly regulated to control timing of vesicle fusion at the synapse, whereas CICR is used to maintain a tonic calcium signal to modulate vesicle trafficking.

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.

Psychophysical testing in rodent models of glaucomatous optic neuropathy

Processing of visual information begins in the retina, with photoreceptors converting light stimuli into neural signals. Ultimately, signals are transmitted to the brain through signaling networks formed by interneurons, namely bipolar, horizontal and amacrine cells providing input to retinal ganglion cells (RGCs), which form the optic nerve with their axons. As part of the chronic nature of glaucomatous optic neuropathy, the increasing and irreversible damage and ultimately loss of neurons, RGCs in particular, occurs following progressive damage to the optic nerve head (ONH), eventually resulting in visual impairment and visual field loss. There are two behavioral assays that are typically used to assess visual deficits in glaucoma rodent models, the visual water task and the optokinetic drum. The visual water task can assess an animal's ability to distinguish grating patterns that are associated with an escape from water. The optokinetic drum relies on the optomotor response, a reflex turning of the head and neck in the direction of the visual stimuli, which usually consists of rotating black and white gratings. This reflex is a physiological response critical for keeping the image stable on the retina. Driven initially by the neuronal input from direction-selective RGCs, this reflex is comprised of a number of critical sensory and motor elements. In the presence of repeatable and defined stimuli, this reflex is extremely well suited to analyze subtle changes in the circuitry and performance of retinal neurons. Increasing the cycles of these alternating gratings per degree, or gradually reducing the contrast of the visual stimuli, threshold levels can be determined at which the animal is no longer tracking the stimuli, and thereby visual function of the animal can be determined non-invasively. Integrating these assays into an array of outcome measures that determine multiple aspects of visual function is a central goal in vision research and can be realized, for example, by the combination of measuring optomotor reflex function with electroretinograms (ERGs) and optical coherence tomography (OCT) of the retina. These structure-function correlations in vivo are urgently needed to identify disease mechanisms as potential new targets for drug development. Such a combination of the experimental assessment of the optokinetic reflex (OKR) or optomotor response (OMR) with other measures of retinal structure and function is especially valuable for research on GON. The chronic progression of the disease is characterized by a gradual decrease in function accompanied by a concomitant increase in structural damage to the retina, therefore the assessment of subtle changes is key to determining the success of novel intervention strategies.

Elementary properties of Ca(2+) channels and their influence on multivesicular release and phase-locking at auditory hair cell ribbon synapses

Voltage-gated calcium (Ca_v1.3) channels in mammalian inner hair cells (IHCs) open in response to sound and the resulting Ca²⁺ entry triggers the release of the neurotransmitter glutamate onto afferent terminals. At low to mid sound frequencies cell depolarization follows the sound sinusoid and pulses of transmitter release from the hair cell generate excitatory postsynaptic currents (EPSCs) in the afferent fiber that translate into a phase-locked pattern of action potential activity. The present article summarizes our current understanding on the elementary properties of single IHC Ca²⁺ channels, and how these could have functional implications for certain, poorly understood, features of synaptic transmission at auditory hair cell ribbon synapses.

Big minis from hair cells: mechanism and function

What underlies the large variation in mEPSC amplitude in the auditory system? And is this variability important? In this issue of Neuron, Li et al. (2014) address the significance of large mEPSCs to auditory processing and Chapochnikov et al. (2014) describe a novel mechanism underlying them.

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.

Role of intracellular calcium stores in hair-cell ribbon synapse

Intracellular calcium stores control many neuronal functions such as excitability, gene expression, synaptic plasticity, and synaptic release. Although the existence of calcium stores along with calcium-induced calcium release (CICR) has been demonstrated in conventional and ribbon synapses, functional significance and the cellular mechanisms underlying this role remains unclear. This review summarizes recent experimental evidence identifying contribution of CICR to synaptic transmission and synaptic plasticity in the CNS, retina and inner ear. In addition, the potential role of CICR in the recruitment of vesicles to releasable pools in hair-cell ribbon synapses will be specifically discussed.

Presynaptic [Ca(2+)] and GCAPs: aspects on the structure and function of photoreceptor ribbon synapses

Changes in intracellular calcium ions [Ca²⁺] play important roles in photoreceptor signaling. Consequently, intracellular [Ca²⁺] levels need to be tightly controlled. In the light-sensitive outer segments (OS) of photoreceptors, Ca²⁺ regulates the activity of retinal guanylate cyclases thus playing a central role in phototransduction and light-adaptation by restoring light-induced decreases in cGMP. In the synaptic terminals, changes of intracellular Ca²⁺ trigger various aspects of neurotransmission. Photoreceptors employ tonically active ribbon synapses that encode light-induced, graded changes of membrane potential into modulation of continuous synaptic vesicle exocytosis. The active zones of ribbon synapses contain large electron-dense structures, synaptic ribbons, that are associated with large numbers of synaptic vesicles. Synaptic coding at ribbon synapses differs from synaptic coding at conventional (phasic) synapses. Recent studies revealed new insights how synaptic ribbons are involved in this process. This review focuses on the regulation of [Ca²⁺] in presynaptic photoreceptor terminals and on the function of a particular Ca²⁺-regulated protein, the neuronal calcium sensor protein GCAP2 (guanylate cyclase-activating protein-2) in the photoreceptor ribbon synapse. GCAP2, an EF-hand-containing protein plays multiple roles in the OS and in the photoreceptor synapse. In the OS, GCAP2 works as a Ca²⁺-sensor within a Ca²⁺-regulated feedback loop that adjusts cGMP levels. In the photoreceptor synapse, GCAP2 binds to RIBEYE, a component of synaptic ribbons, and mediates Ca²⁺-dependent plasticity at that site. Possible mechanisms are discussed.

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.

Zinc modulation of calcium activity at the photoreceptor terminal: a calcium imaging study

There is abundant experimental evidence that zinc ions (Zn(2+)) are present in the synaptic vesicles of vertebrate photoreceptors, and that they are co-released with glutamate. Here we show that increasing the concentration of extracellular zinc (2 μM-2 mM) suppresses the entry of calcium into the synaptic terminals of isolated salamander double cones. The resultant dose-dependent curve was fit by an inverse Hill equation having an IC50 of 38 μM, and Hill coefficient of 1.1. Because there is currently no reliable way to measure the concentration of extracellular zinc, it is not known whether the zinc released under normal circumstances is of physiological significance. In an attempt to circumvent this problem we used zinc chelators to reduce the available pool of endogenous zinc. This enabled us to determine how the absence of zinc affected calcium entry. We found that when intra- or extra-cellular zinc was chelated by 250 μM of membrane-permeable TPEN or 500 μM of membrane-impermeable histidine, there was a significant rise in the depolarization-induced intracellular calcium level within photoreceptor terminals. This increase in internal [Ca(2+)] will undoubtedly lead to a concomitant increase in glutamate release. In addition, we found that blocking the L-type calcium channels that are expressed on the synaptic terminals of photoreceptors with 50 μM nicardipine or 100 μM verapamil abolished the effects of zinc chelation. These findings are a good indication that, when released in vivo, the zinc concentration is sufficient to suppress voltage-gated calcium channels, and reduce the rate of glutamate release from photoreceptor terminals.

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.

Photoreceptor inner and outer segments

Photoreceptors are exquisitely adapted to transform light stimuli into electrical signals that modulate neurotransmitter release. These cells are organized into several compartments including the unique outer segment (OS). Its whole function is to absorb light and transduce this signal into a change of membrane potential. Another compartment is the inner segment where much of metabolism and regulation of membrane potential takes place and that connects the OS and synapse. The synapse is the compartment where changes in membrane potentials are relayed to other neurons in the retina via release of neurotransmitter. The composition of the plasma membrane surrounding these compartments varies to accommodate their specific functions. In this chapter, we discuss the organization of the plasma membrane emphasizing the protein composition of each region as it relates to visual signaling. We also point out examples where mutations in these proteins cause visual impairment.

EF hand-mediated Ca- and cGMP-signaling in photoreceptor synaptic terminals

Photoreceptors, the light-sensitive receptor neurons of the retina, receive and transmit a plethora of visual informations from the surrounding world. Photoreceptors capture light and convert this energy into electrical signals that are conveyed to the inner retina. For synaptic communication with the inner retina, photoreceptors make large active zones that are marked by synaptic ribbons. These unique synapses support continuous vesicle exocytosis that is modulated by light-induced, graded changes of membrane potential. Synaptic transmission can be adjusted in an activity-dependent manner, and at the synaptic ribbons, Ca²⁺- and cGMP-dependent processes appear to play a central role. EF-hand-containing proteins mediate many of these Ca²⁺- and cGMP-dependent functions. Since continuous signaling of photoreceptors appears to be prone to malfunction, disturbances of Ca²⁺- and cGMP-mediated signaling in photoreceptors can lead to visual defects, retinal degeneration (rd), and even blindness. This review summarizes aspects of signal transmission at the photoreceptor presynaptic terminals that involve EF-hand-containing Ca²⁺-binding proteins.

Calcium stores in vertebrate photoreceptors

This review lays out the emerging evidence for the fundamental role of Ca(2+) stores and store-operated channels in the Ca(2+) homeostasis of rods and cones. Calcium-induced calcium release (CICR) is a major contributor to steady-state and light-evoked photoreceptor Ca(2+) homeostasis in the darkness whereas store-operated Ca(2+) channels play a more significant role under sustained illumination conditions. The homeostatic response includes dynamic interactions between the plasma membrane, endoplasmic reticulum (ER), mitochondria and/or outer segment disk organelles which dynamically sequester, accumulate and release Ca(2+). Coordinated activation of SERCA transporters, ryanodine receptors (RyR), inositol triphosphate receptors (IP3Rs) and TRPC channels amplifies cytosolic voltage-operated signals but also provides a memory trace of previous exposures to light. Store-operated channels, activated by the STIM1 sensor, prevent pathological decrease in [Ca(2+)]i mediated by excessive activation of PMCA transporters in saturating light. CICR and SOCE may also modulate the transmission of afferent and efferent signals in the outer retina. Thus, Ca(2+) stores provide additional complexity, adaptability, tuneability and speed to photoreceptor signaling.

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.

Postsynaptic recordings at afferent dendrites contacting cochlear inner hair cells: monitoring multivesicular release at a ribbon synapse

The afferent synapse between the inner hair cell (IHC) and the auditory nerve fiber provides an electrophysiologically accessible site for recording the postsynaptic activity of a single ribbon synapse ^1-4. Ribbon synapses of sensory cells release neurotransmitter continuously, the rate of which is modulated in response to graded changes in IHC membrane potential ⁵. Ribbon synapses have been shown to operate by multivesicular release, where multiple vesicles can be released simultaneously to evoke excitatory postsynaptic currents (EPSCs) of varying amplitudes ^{1, 4, 6-11}. Neither the role of the presynaptic ribbon, nor the mechanism underlying multivesicular release is currently well understood.

The IHC is innervated by 10-20 auditory nerve fibers, and every fiber contacts the IHC with a unmyelinated single ending to form a single ribbon synapse. The small size of the afferent boutons contacting IHCs (approximately 1 μm in diameter) enables recordings with exceptional temporal resolution to be made. Furthermore, the technique can be adapted to record from both pre- and postsynaptic cells simultaneously, allowing the transfer function at the synapse to be studied directly ². This method therefore provides a means by which fundamental aspects of neurotransmission can be studied, from multivesicular release to the elusive function of the ribbon in sensory cells.

Location of release sites and calcium-activated chloride channels relative to calcium channels at the photoreceptor ribbon synapse

Vesicle release from photoreceptor ribbon synapses is regulated by L-type Ca(2+) channels, which are in turn regulated by Cl(-) moving through calcium-activated chloride [Cl(Ca)] channels. We assessed the proximity of Ca(2+) channels to release sites and Cl(Ca) channels in synaptic terminals of salamander photoreceptors by comparing fast (BAPTA) and slow (EGTA) intracellular Ca(2+) buffers. BAPTA did not fully block synaptic release, indicating some release sites are <100 nm from Ca(2+) channels. Comparing Cl(Ca) currents with predicted Ca(2+) diffusion profiles suggested that Cl(Ca) and Ca(2+) channels average a few hundred nanometers apart, but the inability of BAPTA to block Cl(Ca) currents completely suggested some channels are much closer together. Diffuse immunolabeling of terminals with an antibody to the putative Cl(Ca) channel TMEM16A supports the idea that Cl(Ca) channels are dispersed throughout the presynaptic terminal, in contrast with clustering of Ca(2+) channels near ribbons. Cl(Ca) currents evoked by intracellular calcium ion concentration ([Ca(2+)](i)) elevation through flash photolysis of DM-nitrophen exhibited EC(50) values of 556 and 377 nM with Hill slopes of 1.8 and 2.4 in rods and cones, respectively. These relationships were used to estimate average submembrane [Ca(2+)](i) in photoreceptor terminals. Consistent with control of exocytosis by [Ca(2+)] nanodomains near Ca(2+) channels, average submembrane [Ca(2+)](i) remained below the vesicle release threshold (∼ 400 nM) over much of the physiological voltage range for cones. Positioning Ca(2+) channels near release sites may improve fidelity in converting voltage changes to synaptic release. A diffuse distribution of Cl(Ca) channels may allow Ca(2+) influx at one site to influence relatively distant Ca(2+) channels.

Nanodomain control of exocytosis is responsible for the signaling capability of a retinal ribbon synapse

Primary sensory circuits encode both weak and intense stimuli reliably, requiring that their synapses signal over a wide dynamic range. In the retinal circuitry subserving night vision, processes intrinsic to the rod bipolar (RB) cell presynaptic active zone (AZ) permit the RB synapse to encode signals generated by the absorption of single photons as well as by more intense stimuli. In a study using an in vitro slice preparation of the mouse retina, we provide evidence that the location of Ca channels with low open probability within nanometers of the release sites is a critical determinant of the physiological behavior of the RB synapse. This gives rise to apparent one-to-one coupling between Ca channel opening and vesicle release, allowing presynaptic potential to be encoded linearly over a wide dynamic range. Further, it permits a transition from univesicular to multivesicular release (MVR) when two Ca channels/AZ open at potentials above the threshold for exocytosis. MVR permits small presynaptic voltage changes to elicit postsynaptic responses larger than quantal synaptic noise.

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.

Two modes of release shape the postsynaptic response at the inner hair cell ribbon synapse

Cochlear inner hair cells (IHCs) convert sounds into receptor potentials and via their ribbon synapses into firing rates in auditory nerve fibers. Multivesicular release at individual IHC ribbon synapses activates AMPA-mediated EPSCs with widely ranging amplitudes. The underlying mechanisms and specific role for multivesicular release in encoding sound are not well understood. Here we characterize the waveforms of individual EPSCs recorded from afferent boutons contacting IHCs and compare their characteristics in immature rats (postnatal days 8-11) and hearing rats (postnatal days 19-21). Two types of EPSC waveforms were found in every recording: monophasic EPSCs, with sharp rising phases and monoexponential decays, and multiphasic EPSCs, exhibiting inflections on rising and decaying phases. Multiphasic EPSCs exhibited slower rise times and smaller amplitudes than monophasic EPSCs. Both types of EPSCs had comparable charge transfers, suggesting that they were activated by the release of similar numbers of vesicles, which for multiphasic EPSCs occurred in a less coordinated manner. On average, a higher proportion of larger, monophasic EPSCs was found in hearing compared to immature rats. In addition, EPSCs became significantly faster with age. The developmental increase in size and speed could improve auditory signaling acuity. Multiphasic EPSCs persisted in hearing animals, in some fibers constituting half of the EPSCs. The proportion of monophasic versus multiphasic EPSCs varied widely across fibers, resulting in marked heterogeneity of amplitude distributions. We propose that the relative contribution of two modes of multivesicular release, generating monophasic and multiphasic EPSCs, may underlie fundamental characteristics of auditory nerve fibers.

Control of intracellular calcium signaling as a neuroprotective strategy

Both acute and chronic degenerative diseases of the nervous system reduce the viability and function of neurons through changes in intracellular calcium signaling. In particular, pathological increases in the intracellular calcium concentration promote such pathogenesis. Disease involvement of numerous regulators of intracellular calcium signaling located on the plasma membrane and intracellular organelles has been documented. Diverse groups of chemical compounds targeting ion channels, G-protein coupled receptors, pumps and enzymes have been identified as potential neuroprotectants. The present review summarizes the discovery, mechanisms and biological activity of neuroprotective molecules targeting proteins that control intracellular calcium signaling to preserve or restore structure and function of the nervous system. Disease relevance, clinical applications and new technologies for the identification of such molecules are being discussed.

A fast rod photoreceptor signaling pathway in the mammalian retina

Rod photoreceptors were recently shown to contact Off cone bipolar cells, providing a novel pathway for rod signal flow in the mammalian retina. By recording from pairs of rods and Off cone bipolar cells in the ground squirrel, we measured the synaptic responses of mammalian rods unfiltered by the slow kinetics of the rod bipolar cell response. We show that vesicle fusion and turnover in mammalian rods is fast, and that this new pathway can mediate rapid signaling.

Calcium-induced calcium release contributes to synaptic release from mouse rod photoreceptors

We tested whether calcium-induced calcium release (CICR) contributes to synaptic release from rods in mammalian retina. Electron micrographs and immunofluorescent double labeling for the sarco/endoplasmic reticulum Ca(2+)-ATPase (SERCA2) and synaptic ribbon protein, ribeye, showed a close association between ER and synaptic ribbons in mouse rod terminals. Stimulating CICR with 10 microM ryanodine evoked Ca(2+) increases in rod terminals from mouse retinal slices visualized using confocal microscopy with the Ca(2+)-sensitive dye, Fluo-4. Ryanodine also stimulated membrane depolarization of individual mouse rods. Inhibiting CICR with a high concentration of ryanodine (100 microM) reduced the electroretinogram (ERG) b-wave but not a-wave consistent with inhibition of synaptic transmission from rods. Ryanodine (100 microM) also inhibited light-evoked voltage responses of individual rod bipolar cells (RBCs) and presumptive horizontal cells recorded with perforated patch recording techniques. A presynaptic site of action for ryanodine's effects is further indicated by the finding that ryanodine (100 microM) did not alter currents evoked in voltage-clamped RBCs by puffing the mGluR6 antagonist, (RS)-alpha-cyclopropyl-4-phosphonophenylglycine (CPPG), onto bipolar cell dendrites in the presence of the mGluR6 agonist L-(+)-2-amino-4-phosphonobutyric acid (L-AP4). Ryanodine (100 microM) also inhibited glutamatergic outward currents in RBCs evoked by electrical stimulation of rods using electrodes placed in the outer segment layer. Together, these results indicate that, like amphibian retina, CICR contributes to synaptic release from mammalian (mouse) rods. By boosting synaptic release in darkness, CICR may improve the detection of small luminance changes by post-synaptic neurons.

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.

Caffeine inhibition of ionotropic glycine receptors

We found that caffeine is a structural analogue of strychnine and a competitive antagonist at ionotropic glycine receptors (GlyRs). Docking simulations indicate that caffeine and strychnine may bind to similar sites at the GlyR. The R131A GlyR mutation, which reduces strychnine antagonism without suppressing activation by glycine, also reduces caffeine antagonism. GlyR subtypes have differing caffeine sensitivity. Tested against the EC(50) of each GlyR subtype, the order of caffeine potency (IC(50)) is: alpha2beta (248 +/- 32 microm) alpha3beta (255 +/- 16 microm) > alpha4beta (517 +/- 50 microm) > alpha1beta(837 +/- 132 microm). However, because the alpha3beta GlyR is more than 3-fold less sensitive to glycine than any of the other GlyR subtypes, this receptor is most effectively blocked by caffeine. The glycine dose-response curves and the effects of caffeine indicate that amphibian retinal ganglion cells do not express a plethora of GlyR subtypes and are dominated by the alpha1beta GlyR. Comparing the effects of caffeine on glycinergic spontaneous and evoked IPSCs indicates that evoked release elevates the glycine concentration at some synapses whereas summation elicits evoked IPSCs at other synapses. Caffeine serves to identify the pharmacophore of strychnine and produces near-complete inhibition of glycine receptors at concentrations commonly employed to stimulate ryanodine receptors.

TMEM16B, a novel protein with calcium-dependent chloride channel activity, associates with a presynaptic protein complex in photoreceptor terminals

Photoreceptor ribbon synapses release glutamate in response to graded changes in membrane potential evoked by vast, logarithmically scalable light intensities. Neurotransmitter release is modulated by intracellular calcium levels. Large Ca(2+)-dependent chloride currents are important regulators of synaptic transmission from photoreceptors to second-order neurons; the molecular basis underlying these currents is unclear. We cloned human and mouse TMEM16B, a member of the TMEM16 family of transmembrane proteins, and show that it is abundantly present in the photoreceptor synaptic terminals in mouse retina. TMEM16B colocalizes with adaptor proteins PSD95, VELI3, and MPP4 at the ribbon synapses and contains a consensus PDZ class I binding motif capable of interacting with PDZ domains of PSD95. Furthermore, TMEM16B is lost from photoreceptor membranes of MPP4-deficient mice. This suggests that TMEM16B is a novel component of a presynaptic protein complex recruited to specialized plasma membrane domains of photoreceptors. TMEM16B confers Ca(2+)-dependent chloride currents when overexpressed in mammalian cells as measured by halide sensitive fluorescent protein assays and whole-cell patch-clamp recordings. The compartmentalized localization and the electrophysiological properties suggest TMEM16B to be a strong candidate for the long sought-after Ca(2+)-dependent chloride channel in the photoreceptor synapse.

The molecular architecture of ribbon presynaptic terminals

The primary receptor neurons of the auditory, vestibular, and visual systems encode a broad range of sensory information by modulating the tonic release of the neurotransmitter glutamate in response to graded changes in membrane potential. The output synapses of these neurons are marked by structures called synaptic ribbons, which tether a pool of releasable synaptic vesicles at the active zone where glutamate release occurs in response to calcium influx through L-type channels. Ribbons are composed primarily of the protein, RIBEYE, which is unique to ribbon synapses, but cytomatrix proteins that regulate the vesicle cycle in conventional terminals, such as Piccolo and Bassoon, also are found at ribbons. Conventional and ribbon terminals differ, however, in the size, molecular composition, and mobilization of their synaptic vesicle pools. Calcium-binding proteins and plasma membrane calcium pumps, together with endomembrane pumps and channels, play important roles in calcium handling at ribbon synapses. Taken together, emerging evidence suggests that several molecular and cellular specializations work in concert to support the sustained exocytosis of glutamate that is a hallmark of ribbon synapses. Consistent with its functional importance, abnormalities in a variety of functional aspects of the ribbon presynaptic terminal underlie several forms of auditory neuropathy and retinopathy.

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.

Anatomical and neurochemical characterization of dopaminergic interplexiform processes in mouse and rat retinas

Dopaminergic (DA) neurons of mouse and rat retinas are of the interplexiform subtype (DA-IPC), i.e., they send processes distally toward the outer retina, exhibiting numerous varicosities along their course. The primary question we addressed was whether distally located DA-IPC varicosities, identified by tyrosine hydroxylase (TH) immunoreactivity, had the characteristic presynaptic proteins associated with calcium-dependent vesicular release of neurotransmitter. We found that TH immunoreactive varicosities in the outer retina possessed vesicular monoamine transporter 2 and vesicular GABA transporter, but they lacked immunostaining for any of nine subtypes of voltage-dependent calcium channel. Immunoreactivity for other channels that may permit calcium influx such as certain ionotropic glutamate receptors and canonical transient receptor potential channels (TRPCs) was similarly absent, although DA-IPC varicosities did show ryanodine receptor immunoreactivity, indicating the presence of intracellular calcium stores. The synaptic vesicle proteins sv2a and sv2b and certain other proteins associated with the presynaptic membrane were absent from DA-IPC varicosities, but the vesicular SNARE protein, vamp2, was present in a fraction of those varicosities. We identified a presumed second class of IPC that is GABAergic but not dopaminergic. Outer retinal varicosities of this putative GABAergic IPC did colocalize synaptic vesicle protein 2a, suggesting they possessed a conventional vesicular release mechanism.

The dihydropyridine isradipine inhibits the murine but not the bovine A-wave response of the electroretinogram

PURPOSE

In order to record light-evoked responses from photoreceptor cells and the higher neuronal retinal network, the isolated vertebrate retina represents a sensitive tool for basic research of retinal function and for testing the toxicity of ocular therapeutics. In the past, this in vitro technique was optimized for frog and bovine retina; it should be transferred now to the isolated murine retina because the model could allow for functional testing of genes involved in retinal signalling using wild-type and gene-inactivated mice. Thus, alterations in the electroretinogram (ERG) may reveal differences in retinal information processing because of the inactivation of a specific gene.

METHODS

We used a superfused vertebrate retina assay to test bovine and murine retina.

RESULTS

In order to evaluate the sensitivity of the ERG recording technique from the isolated murine retina, we first determined the light intensity response and the stability of the a-wave amplitude during ERG recording, which did not differ between the species. However, testing the dihydropyridine sensitivity of the a-wave, we found that the murine a-wave was highly sensitive towards racemic isradipine (8-25 nM) but the bovine retina a-wave was not. A similar species-dependent difference was observed for mibefradil (10 muM).

CONCLUSION

Murine and bovine retina differ with respect to transretinal signalling. At the level of photoreceptor cells, the ERG/a-wave is modulated by isradipine-sensitive voltage-gated Ca(2+) channels, which trigger feedback signalling to photoreceptors.

Depletion of calcium stores regulates calcium influx and signal transmission in rod photoreceptors

Tonic synapses are specialized for sustained calcium entry and transmitter release, allowing them to operate in a graded fashion over a wide dynamic range. We identified a novel plasma membrane calcium entry mechanism that extends the range of rod photoreceptor signalling into light-adapted conditions. The mechanism, which shares molecular and physiological characteristics with store-operated calcium entry (SOCE), is required to maintain baseline [Ca(2+)](i) in rod inner segments and synaptic terminals. Sustained Ca(2+) entry into rod cytosol is augmented by store depletion, blocked by La(3+) and Gd(3+) and suppressed by organic antagonists MRS-1845 and SKF-96365. Store depletion and the subsequent Ca(2+) influx directly stimulated exocytosis in terminals of light-adapted rods loaded with the activity-dependent dye FM1-43. Moreover, SOCE blockers suppressed rod-mediated synaptic inputs to horizontal cells without affecting presynaptic voltage-operated Ca(2+) entry. Silencing of TRPC1 expression with small interference RNA disrupted SOCE in rods, but had no effect on cone Ca(2+) signalling. Rods were immunopositive for TRPC1 whereas cone inner segments immunostained with TRPC6 channel antibodies. Thus, SOCE modulates Ca(2+) homeostasis and light-evoked neurotransmission at the rod photoreceptor synapse mediated by TRPC1.

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.

Ca2+-induced Ca2+ release in Aplysia bag cell neurons requires interaction between mitochondrial and endoplasmic reticulum stores

Intracellular Ca2+ is influenced by both Ca2+ influx and release. We examined intracellular Ca2+ following action potential firing in the bag cell neurons of Aplysia californica. Following brief synaptic input, these neuroendocrine cells undergo an afterdischarge, resulting in elevated Ca2+ and the secretion of neuropeptides to initiate reproduction. Cultured bag cell neurons were injected with the Ca2+ indicator, fura-PE3, and subjected to simultaneous imaging and electrophysiology. Delivery of a 5-Hz, 1-min train of action potentials (mimicking the fast phase of the afterdischarge) produced a Ca2+ rise that markedly outlasted the initial influx, consistent with Ca2+-induced Ca2+ release (CICR). This response was attenuated by about half with ryanodine or depletion of the endoplasmic reticulum (ER) by cyclopiazonic acid. However, depletion of the mitochondria, with carbonyl cyanide 4-(trifluoromethoxy) phenylhydrazone, essentially eliminated CICR. Dual depletion of the ER and mitochondria did not reduce CICR further than depletion of the mitochondria alone. Moreover, tetraphenylphosphonium, a blocker of mitochondrial Ca2+ release, largely prevented CICR. The Ca2+ elevation during and subsequent to a stimulus mimicking the full afterdischarge was prominent and enhanced by protein kinase C activation. Traditionally, the ER is seen as the primary Ca2+ source for CICR. However, bag cell neuron CICR represents a departure from this view in that it relies on store interaction, where Ca2+ released from the mitochondria may in turn liberate Ca2+ from the ER. This unique form of CICR may be used by both bag cell neurons, and other neurons, to initiate secretion, activate channels, or induce gene expression.

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.

Voltage-activated ion channels and Ca(2+)-induced Ca (2+) release shape Ca (2+) signaling in Merkel cells

Ca(2+) signaling and neurotransmission modulate touch-evoked responses in Merkel cell-neurite complexes. To identify mechanisms governing these processes, we analyzed voltage-activated ion channels and Ca(2+) signaling in purified Merkel cells. Merkel cells in the intact skin were specifically labeled by antibodies against voltage-activated Ca(2+) channels (Ca(V)2.1) and voltage- and Ca(2+)-activated K(+) (BK(Ca)) channels. Voltage-clamp recordings revealed small Ca(2+) currents, which produced Ca(2+) transients that were amplified sevenfold by Ca(2+)-induced Ca(2+) release. Merkel cells' voltage-activated K(+) currents were carried predominantly by BK(Ca) channels with inactivating and non-inactivating components. Thus, Merkel cells, like hair cells, have functionally diverse BK(Ca) channels. Finally, blocking K(+) channels increased response magnitude and dramatically shortened Ca(2+) transients evoked by mechanical stimulation. Together, these results demonstrate that Ca(2+) signaling in Merkel cells is governed by the interplay of plasma membrane Ca(2+) channels, store release and K(+) channels, and they identify specific signaling mechanisms that may control touch sensitivity.