Plasticity of retinal ribbon synapses

The Role of Sphingosine-1-Phosphate Receptor 2 in Mouse Retina Light Responses

The bioactive sphingolipid sphingosine-1-phosphate (S1P) acts as a ligand for a family of G protein-coupled S1P receptors (S1PR1-5) to participate in a variety of signaling pathways. However, their specific roles in the neural retina remain unclear. We previously showed that S1P receptor subtype 2 (S1PR2) is expressed in murine retinas, primarily in photoreceptors and bipolar cells, and its expression is altered by retinal stress. This study aims to elucidate the role of S1PR2 in the mouse retina. We examined light responses by electroretinography (ERG), structural differences by optical coherence tomography (OCT), and protein levels by immunohistochemistry (IHC) in wild-type (WT) and S1PR2 knockout (KO) mice at various ages between 3 and 6 months. We found that a- and b-wave responses significantly increased at flash intensities between 400~2000 and 4~2000 cd.s/m2, respectively, in S1PR2 KO mice relative to those of WT controls at baseline. S1PR2 KO mice also exhibited significantly increased retinal nerve fiber layer (RNFL) and outer plexiform layer (OPL) thickness by OCT relative to the WT. Finally, in S1PR2 KO mice, we observed differential labeling of synaptic markers by immunohistochemistry (IHC) and quantitative reverse transcription polymerase chain reaction (RT-qPCR). These results suggest a specific involvement of S1PR2 in the structure and synaptic organization of the retina and a potential role in light-mediated functioning of the retina.

The role of sphingosine-1-phosphate receptor 2 in mouse retina light responses

The bioactive sphingolipid sphingosine-1-phosphate (S1P) acts as a ligand for a family of G protein-coupled S1P receptors (S1PR1-5) to participate in a variety of signaling pathways. However, their specific roles in the neural retina remain unclear. We previously showed that S1P receptor subtype 2 (S1PR2) is expressed in murine retinas, primarily in photoreceptors and bipolar cells, and its expression is altered by retinal stress. This study aims to elucidate the role of S1PR2 in the mouse retina. We examined light responses by electroretinography (ERG), structural differences by optical coherence tomography (OCT), and protein levels by immunohistochemistry (IHC) in wild-type (WT) and S1PR2 knockout (KO) mice at various ages between 3 and 6 months. We found that a- and b-wave responses significantly increased at flash intensities between 400∼2000 and 4∼2,000 cd.s/m 2 respectively, in S1PR2 KO mice relative to those of WT controls at baseline. S1PR2 KO mice also exhibited significantly increased retinal nerve fiber layer (RNFL) and outer plexiform layer (OPL) thickness by OCT relative to the WT. Finally, in S1PR2 KO mice, we observed differential labeling of synaptic markers by immunohistochemistry (IHC) and quantitative reverse transcription polymerase chain reaction (RT-qPCR). These results suggest a specific involvement of S1PR2 in the structure and synaptic organization of the retina and a potential role in light-mediated functioning of the retina.

Circadian clock organization in the retina: From clock components to rod and cone pathways and visual function

Circadian (24-h) clocks are cell-autonomous biological oscillators that orchestrate many aspects of our physiology on a daily basis. Numerous circadian rhythms in mammalian and non-mammalian retinas have been observed and the presence of an endogenous circadian clock has been demonstrated. However, how the clock and associated rhythms assemble into pathways that support and control retina function remains largely unknown. Our goal here is to review the current status of our knowledge and evaluate recent advances. We describe many previously-observed retinal rhythms, including circadian rhythms of morphology, biochemistry, physiology, and gene expression. We evaluate evidence concerning the location and molecular machinery of the retinal circadian clock, as well as consider findings that suggest the presence of multiple clocks. Our primary focus though is to describe in depth circadian rhythms in the light responses of retinal neurons with an emphasis on clock control of rod and cone pathways. We examine evidence that specific biochemical mechanisms produce these daily light response changes. We also discuss evidence for the presence of multiple circadian retinal pathways involving rhythms in neurotransmitter activity, transmitter receptors, metabolism, and pH. We focus on distinct actions of two dopamine receptor systems in the outer retina, a dopamine D4 receptor system that mediates circadian control of rod/cone gap junction coupling and a dopamine D1 receptor system that mediates non-circadian, light/dark adaptive regulation of gap junction coupling between horizontal cells. Finally, we evaluate the role of circadian rhythmicity in retinal degeneration and suggest future directions for the field of retinal circadian biology.

Autophagy-Mediated Synaptic Refinement and Auditory Neural Pruning Contribute to Ribbon Synaptic Maturity in the Developing Cochlea

In rodents, massive initial synapses are formed in the auditory peripheral nervous system at the early postnatal stage, and one of the major phenomena is that the number of afferent synapses in the cochlea is significantly reduced in the duration of development. This raises the hypothesis that the number of cochlear ribbon synapses are dramatically changed with hearing development and maturation. In this study, several tracers identifying activities of autophagy were applied to estimate the level of autophagy activity in the process of ribbon synapse development in mice; further, changes in the synaptic number and spiral ganglion nerve (SGN) fibers were quantitatively measured. We found robust expression of LC3B and lysosomal-associated membrane protein 1 as well as LysoTracker in or near inner hair cells and cochlear ribbon synapses in the early stage of postnatal development. Moreover, we found a significant loss in the intensity of SGN fibers at ribbon synaptic development and hearing onset. Thus, this study demonstrates that ribbon synaptic refinement and SGN fibers pruning are closely associated with the morphological and functional maturation of ribbon synapses and that synaptic refinement and SGN fiber pruning are regulated by the robust activities of autophagy in the earlier stages of auditory development.

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.

The SNARE regulator Complexin3 is a target of the cone circadian clock

BMAL1 is a core component of the mammalian circadian clockwork. Removal of BMAL1 from the retina significantly affects visual information processing in both rod and cone pathways. To identify potential pathways and/or molecules through which BMAL1 alters signal transmission at the cone pedicle, we performed an RNA-seq differential expression analysis between cone-specific Bmal1 knockout cones (cone-Bmal1-/- ) and wild-type (WT) cones. We found 88 genes differentially expressed. Among these, Complexin3 (Cplx3), a SNARE regulator at ribbon synapses, was downregulated fivefold in the mutant cones. The purpose of this work was to determine whether BMAL1 and/or the cone clock controls CPLX3 protein expression at cone pedicles. We found that CPLX3 expression level was decreased twofold in cone-Bmal1-/- cones. Furthermore, CPLX3 expression was downregulated at night compared to the day in WT cones but remained constitutively low in mutant cones both day and night. The transcript and protein expression levels of Cplx4-the other complexin expressed in cones-were similar in WT and mutant cones; in WT cones, CPLX4 protein level did not change with the time of day. In silico analysis revealed four potential BMAL1:CLOCK binding sites upstream from exon one of Cplx3 and none upstream of exon one of Cplx4. Our results suggest that CPLX3 expression is regulated at the transcriptional level by the cone clock. The modulation of CPLX3 may be a mechanism by which the clock controls the cone synaptic transfer function to second-order cells and thereby impacts retinal signal processing during the day/night cycle.

Molecular Assembly and Structural Plasticity of Sensory Ribbon Synapses-A Presynaptic Perspective

In the mammalian cochlea, specialized ribbon-type synapses between sensory inner hair cells (IHCs) and postsynaptic spiral ganglion neurons ensure the temporal precision and indefatigability of synaptic sound encoding. These high-through-put synapses are presynaptically characterized by an electron-dense projection-the synaptic ribbon-which provides structural scaffolding and tethers a large pool of synaptic vesicles. While advances have been made in recent years in deciphering the molecular anatomy and function of these specialized active zones, the developmental assembly of this presynaptic interaction hub remains largely elusive. In this review, we discuss the dynamic nature of IHC (pre-) synaptogenesis and highlight molecular key players as well as the transport pathways underlying this process. Since developmental assembly appears to be a highly dynamic process, we further ask if this structural plasticity might be maintained into adulthood, how this may influence the functional properties of a given IHC synapse and how such plasticity could be regulated on the molecular level. To do so, we take a closer look at other ribbon-bearing systems, such as retinal photoreceptors and pinealocytes and aim to infer conserved mechanisms that may mediate these phenomena.

Synaptic ribbons foster active zone stability and illumination-dependent active zone enrichment of RIM2 and Cav1.4 in photoreceptor synapses

Rod photoreceptor synapses use large, ribbon-type active zones for continuous synaptic transmission during light and dark. Since ribbons are physically connected to the active zones, we asked whether illumination-dependent changes of ribbons influence Cav1.4/RIM2 protein clusters at the active zone and whether these illumination-dependent effects at the active zone require the presence of the synaptic ribbon. We found that synaptic ribbon length and the length of presynaptic Cav1.4/RIM2 clusters are tightly correlated. Dark-adaptation did not change the number of ribbons and active zone puncta. However, mean ribbon length and length of presynaptic Cav1.4/RIM2 clusters increased significantly during dark-adaptation when tonic exocytosis is highest. In the present study, we identified by the analyses of synaptic ribbon-deficient RIBEYE knockout mice that synaptic ribbons are (1) needed to stabilize Cav1.4/RIM2 at rod photoreceptor active zones and (2) are required for the darkness-induced active zone enrichment of Cav1.4/RIM2. These data propose a role of the ribbon in active zone stabilization and suggest a homeostatic function of the ribbon in illumination-dependent active zone remodeling.

Circadian regulation in the retina: From molecules to network

The mammalian retina is the most unique tissue among those that display robust circadian/diurnal oscillations. The retina is not only a light sensing tissue that relays light information to the brain, it has its own circadian "system" independent from any influence from other circadian oscillators. While all retinal cells and retinal pigment epithelium (RPE) possess circadian oscillators, these oscillators integrate by means of neural synapses, electrical coupling (gap junctions), and released neurochemicals (such as dopamine, melatonin, adenosine, and ATP), so the whole retina functions as an integrated circadian system. Dysregulation of retinal clocks not only causes retinal or ocular diseases, it also impacts the circadian rhythm of the whole body, as the light information transmitted from the retina entrains the brain clock that governs the body circadian rhythms. In this review, how circadian oscillations in various retinal cells are integrated, and how retinal diseases affect daily rhythms.

Circadian clock regulation of cone to horizontal cell synaptic transfer in the goldfish retina

Although it is well established that the vertebrate retina contains endogenous circadian clocks that regulate retinal physiology and function during day and night, the processes that the clocks affect and the means by which the clocks control these processes remain unresolved. We previously demonstrated that a circadian clock in the goldfish retina regulates rod-cone electrical coupling so that coupling is weak during the day and robust at night. The increase in rod-cone coupling at night introduces rod signals into cones so that the light responses of both cones and cone horizontal cells, which are post-synaptic to cones, become dominated by rod input. By comparing the light responses of cones, cone horizontal cells and rod horizontal cells, which are post-synaptic to rods, under dark-adapted conditions during day and night, we determined whether the daily changes in the strength of rod-cone coupling could account entirely for rhythmic changes in the light response properties of cones and cone horizontal cells. We report that although some aspects of the day/night changes in cone and cone horizontal cell light responses, such as response threshold and spectral tuning, are consistent with modulation of rod-cone coupling, other properties cannot be solely explained by this phenomenon. Specifically, we found that at night compared to the day the time course of spectrally-isolated cone photoresponses was slower, cone-to-cone horizontal cell synaptic transfer was highly non-linear and of lower gain, and the delay in cone-to-cone horizontal cell synaptic transmission was longer. However, under bright light-adapted conditions in both day and night, cone-to-cone horizontal cell synaptic transfer was linear and of high gain, and no additional delay was observed at the cone-to-cone horizontal cell synapse. These findings suggest that in addition to controlling rod-cone coupling, retinal clocks shape the light responses of cone horizontal cells by modulating cone-to-cone horizontal cell synaptic transmission.

Voltage-Gated Calcium Channels: Key Players in Sensory Coding in the Retina and the Inner Ear

Calcium influx through voltage-gated Ca (Ca_V) channels is the first step in synaptic transmission. This review concerns Ca_V channels at ribbon synapses in primary sense organs and their specialization for efficient coding of stimuli in the physical environment. Specifically, we describe molecular, biochemical, and biophysical properties of the Ca_V channels in sensory receptor cells of the retina, cochlea, and vestibular apparatus, and we consider how such properties might change over the course of development and contribute to synaptic plasticity. We pay particular attention to factors affecting the spatial arrangement of Ca_V channels at presynaptic, ribbon-type active zones, because the spatial relationship between Ca_V channels and release sites has been shown to affect synapse function critically in a number of systems. Finally, we review identified synaptopathies affecting sensory systems and arising from dysfunction of L-type, Ca_V1.3, and Ca_V1.4 channels or their protein modulatory elements.

Noise-Induced Cochlear Synaptopathy and Ribbon Synapse Regeneration: Repair Process and Therapeutic Target

The synapse between the inner hair cells (IHCs) and the spiral ganglion neurons (SGNs) in mammalian cochleae is characterized as having presynaptic ribbons and therefore is called ribbon synapse. The special molecular organization is reviewed in this chapter in association with the functional feature of this synapse in signal processing. This is followed by the review on noise-induced damage to this synapse with a focus on recent reports in animal models in which the effect of brief noise exposures is observed without causing significant permanent threshold shift (PTS). In this regard, the potential mechanism of the synaptic damage by noise and the impact of this damage on hearing are summarized to clarify the concept of noise-induced hidden hearing loss, which is defined as the functional deficits in hearing without threshold elevation. A controversial issue is addressed in this review as whether the disrupted synapses can be regenerated. Moreover, the review summarizes the work of therapeutic research to protect the synapses or to promote the regeneration of the synapse after initial disruption. Lastly, several unresolved issues are raised for investigation in the future.

Noise-induced Cochlear Synaptopathy and Signal Processing Disorders

Noise-induced hidden hearing loss (NIHHL) has attracted great attention in hearing research and clinical audiology since the discovery of significant noise-induced synaptic damage in the absence of permanent threshold shifts (PTS) in animal models. Although the extant evidence for this damage is based on animal models, NIHHL likely occurs in humans as well. This review focuses on three issues concerning NIHHL that are somewhat controversial: (1) whether disrupted synapses can be re-established; (2) whether synaptic damage and repair are responsible for the initial temporal threshold shifts (TTS) and subsequent recovery; and (3) the relationship between the synaptic damage and repair processes and neural coding deficits. We conclude that, after a single, brief noise exposure, (1) the damaged and the totally destroyed synapses can be partially repaired, but the repaired synapses are functionally abnormal; (2) While deficits are observed in some aspects of neural responses related to temporal and intensity coding in the auditory nerve, we did not find strong evidence for hypothesized coding-in-noise deficits; (3) the sensitivity and the usefulness of the envelope following responses to amplitude modulation signals in detecting cochlear synaptopathy is questionable.

A different ultrastructural face of ribbon synapses in the rat retina

Ribbon synapses located exclusively within retinal, cochlear and vestibular connections belong to the most interesting cellular structures but their molecular nature and functions had remained unclear. The study has provided a descriptive morphological analysis of rat eye ribbon synapses using high-resolution transmission electron microscopy (TEM). An original collection of untypical, rarely present in the literature sagittal or tangential sections through the single RIBEYE domain of the particular ribbon have been delivered.

Vesicle sub-pool organization at inner hair cell ribbon synapses

The afferent inner hair cell synapse harbors the synaptic ribbon, which ensures a constant vesicle supply. Synaptic vesicles (SVs) are arranged in morphologically discernable pools, linked via filaments to the ribbon or the presynaptic membrane. We propose that filaments play a major role in SV resupply and exocytosis at the ribbon. Using advanced electron microscopy, we demonstrate that SVs are organized in sub-pools defined by the filament number per vesicle and its connections. Upon stimulation, SVs increasingly linked to other vesicles and to the ribbon, whereas single-tethered SVs dominated at the membrane. Mutant mice for the hair cell protein otoferlin (pachanga, Otof ^Pga/Pga ) are profoundly deaf with reduced sustained release, serving as a model to investigate the SV replenishment at IHCs. Upon stimulation, multiple-tethered and docked vesicles (rarely observed in wild-type) accumulated at Otof ^Pga/Pga active zones due to an impairment downstream of docking. Conclusively, vesicles are organized in sub-pools at ribbon-type active zones by filaments to support vesicle supply, transport, and finally release.

Circadian Regulation of Synaptic Plasticity

Circadian rhythms refer to oscillations in biological processes with a period of approximately 24 h. In addition to the sleep/wake cycle, there are circadian rhythms in metabolism, body temperature, hormone output, organ function and gene expression. There is also evidence of circadian rhythms in synaptic plasticity, in some cases driven by a master central clock and in other cases by peripheral clocks. In this article, I review the evidence for circadian influences on synaptic plasticity. I also discuss ways to disentangle the effects of brain state and rhythms on synaptic plasticity.

Circadian regulation of human cortical excitability

Prolonged wakefulness alters cortical excitability, which is essential for proper brain function and cognition. However, besides prior wakefulness, brain function and cognition are also affected by circadian rhythmicity. Whether the regulation of cognition involves a circadian impact on cortical excitability is unknown. Here, we assessed cortical excitability from scalp electroencephalography (EEG) responses to transcranial magnetic stimulation in 22 participants during 29 h of wakefulness under constant conditions. Data reveal robust circadian dynamics of cortical excitability that are strongest in those individuals with highest endocrine markers of circadian amplitude. In addition, the time course of cortical excitability correlates with changes in EEG synchronization and cognitive performance. These results demonstrate that the crucial factor for cortical excitability, and basic brain function in general, is the balance between circadian rhythmicity and sleep need, rather than sleep homoeostasis alone. These findings have implications for clinical applications such as non-invasive brain stimulation in neurorehabilitation.

Cognitive performance is impaired after prolonged wakefulness, yet the contribution of circadian rhythms for proper brain function remains unclear. Here the authors show that cortical excitability measured using TMS exhibits robust circadian dynamics which is correlated with cognitive performance.

Eyes Open on Sleep and Wake: In Vivo to In Silico Neural Networks

Functional and effective connectivity of cortical areas are essential for normal brain function under different behavioral states. Appropriate cortical activity during sleep and wakefulness is ensured by the balanced activity of excitatory and inhibitory circuits. Ultimately, fast, millisecond cortical rhythmic oscillations shape cortical function in time and space. On a much longer time scale, brain function also depends on prior sleep-wake history and circadian processes. However, much remains to be established on how the brain operates at the neuronal level in humans during sleep and wakefulness. A key limitation of human neuroscience is the difficulty in isolating neuronal excitation/inhibition drive in vivo. Therefore, computational models are noninvasive approaches of choice to indirectly access hidden neuronal states. In this review, we present a physiologically driven in silico approach, Dynamic Causal Modelling (DCM), as a means to comprehend brain function under different experimental paradigms. Importantly, DCM has allowed for the understanding of how brain dynamics underscore brain plasticity, cognition, and different states of consciousness. In a broader perspective, noninvasive computational approaches, such as DCM, may help to puzzle out the spatial and temporal dynamics of human brain function at different behavioural states.

Relating structure and function of inner hair cell ribbon synapses

In the mammalian cochlea, sound is encoded at synapses between inner hair cells (IHCs) and type I spiral ganglion neurons (SGNs). Each SGN receives input from a single IHC ribbon-type active zone (AZ) and yet SGNs indefatigably spike up to hundreds of Hz to encode acoustic stimuli with submillisecond precision. Accumulating evidence indicates a highly specialized molecular composition and structure of the presynapse, adapted to suit these high functional demands. However, we are only beginning to understand key features such as stimulus-secretion coupling, exocytosis mechanisms, exo-endocytosis coupling, modes of endocytosis and vesicle reformation, as well as replenishment of the readily releasable pool. Relating structure and function has become an important avenue in addressing these points and has been applied to normal and genetically manipulated hair cell synapses. Here, we review some of the exciting new insights gained from recent studies of the molecular anatomy and physiology of IHC ribbon synapses.

Structural plasticity of the circadian timing system. An overview from flies to mammals

The circadian timing system orchestrates daily variations in physiology and behavior through coordination of multioscillatory cell networks that are highly plastic in responding to environmental changes. Over the last decade, it has become clear that this plasticity involves structural changes and that the changes may be observed not only in central brain regions where the master clock cells reside but also in clock-controlled structures. This review considers experimental data in invertebrate and vertebrate model systems, mainly flies and mammals, illustrating various forms of structural circadian plasticity from cellular to circuit-based levels. It highlights the importance of these plastic events in the functional adaptation of the clock to the changing environment.

Molecular Remodeling of the Presynaptic Active Zone of Drosophila Photoreceptors via Activity-Dependent Feedback

Neural activity contributes to the regulation of the properties of synapses in sensory systems, allowing for adjustment to a changing environment. Little is known about how synaptic molecular components are regulated to achieve activity-dependent plasticity at central synapses. Here, we found that after prolonged exposure to natural ambient light the presynaptic active zone in Drosophila photoreceptors undergoes reversible remodeling, including loss of Bruchpilot, DLiprin-α, and DRBP, but not of DSyd-1 or Cacophony. The level of depolarization of the postsynaptic neurons is critical for the light-induced changes in active zone composition in the photoreceptors, indicating the existence of a feedback signal. In search of this signal, we have identified a crucial role of microtubule meshwork organization downstream of the divergent canonical Wnt pathway, potentially via Kinesin-3 Imac. These data reveal that active zone composition can be regulated in vivo and identify the underlying molecular machinery.

Characterization of Ribeye subunits in zebrafish hair cells reveals that exogenous Ribeye B-domain and CtBP1 localize to the basal ends of synaptic ribbons

Synaptic ribbons are presynaptic structures formed by the self-association of RIBEYE–the main structural component of ribbon synapses. RIBEYE consists of two domains: a unique N-terminal A-domain and a C-terminal B-domain that is identical to the transcription co-repressor C-terminal binding protein 2 (CtBP2). Previous studies in cell lines have shown that RIBEYE A-domain alone is sufficient to form ribbon-like aggregates and that both A- and B- domains form homo-and heterotypic interactions. As these interactions are likely the basis for synaptic-ribbon assembly and structural plasticity, we wanted to examine how zebrafish Ribeye A- and B- domains interact with synaptic ribbons in vivo. To that end, we characterized the localization of exogenously expressed Ribeye A- and B- domains and the closely related protein, CtBP1, in the hair cells of transgenic zebrafish larvae. Unexpectedly, exogenously expressed Ribeye A-domain showed variable patterns of localization in hair cells; one zebrafish paralog of A-domain failed to self-associate or localize to synaptic ribbons, while the other self-assembled but sometimes failed to localize to synaptic ribbons. By contrast, Ribeye B-domain/CtBP2 was robustly localized to synaptic ribbons. Moreover, both exogenously expressed B-domain/CtBP2 and CtBP1 were preferentially localized to the basal end of ribbons adjacent to the postsynaptic density. Overexpression of B-domain/CtBP2 also appeared to affect synaptic-ribbon composition; endogenous levels of ribbon-localized Ribeye were significantly reduced as hair cells matured in B-domain/CtBP2 transgenic larvae compared to wild-type. These results reveal how exogenously expressed Ribeye domains interact with synaptic ribbons, and suggest a potential organization of elements within the ribbon body.

Sleep, clocks, and synaptic plasticity

Sleep is widely believed to play an essential role in synaptic plasticity. However, the precise mechanisms governing this presumptive function are largely unknown. There is also evidence for independent circadian oscillations in synaptic strength and morphology. Therefore, synaptic changes observed after sleep reflect interactions between state-dependent (e.g., wake versus sleep) and state-independent (circadian) processes. In this review we consider how sleep and biological clocks influence synaptic plasticity. We discuss these findings in the context of current plasticity-based theories of sleep function and propose a new model that integrates circadian and brain-state influences on synaptic plasticity.

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.

Zebrafish Cacna1fa is required for cone photoreceptor function and synaptic ribbon formation

Mutations in the human CACNA1F gene cause incomplete congenital stationary night blindness type 2 (CSNB2), a non-progressive, clinically heterogeneous retinal disorder. However, the molecular mechanisms underlying CSNB2 have not been fully explored. Here, we describe the positional cloning of a blind zebrafish mutant, wait until dark (wud), which encodes a zebrafish homolog of human CACNA1F. We identified two zebrafish cacna1f paralogs and showed that the cacna1fa transcript (the gene mutated in wud) is expressed exclusively in the photoreceptor layer. We demonstrated that Cacna1fa localizes at the photoreceptor synapse and is absent from wud mutants. Electroretinograms revealed abnormal cone photoreceptor responses from wud mutants, indicating a defect in synaptic transmission. Although there are no obvious morphological differences, we found that wud mutants lacked synaptic ribbons and that wud is essential for the development of synaptic ribbons. We found that Ribeye, the most prominent synaptic ribbon protein, was less abundant and mislocalized in adult wud mutants. In addition to cloning wud, we identified synaptojanin 1 (synj1) as the defective gene in slacker (slak), a blind mutant with floating synaptic ribbons. We determined that Cacna1fa was expressed in slak photoreceptors and that Synj1 was initially expressed wud photoreceptors, but was absent by 5 days postfertilization. Collectively, our data demonstrate that Cacna1fa is essential for cone photoreceptor function and synaptic ribbon formation and reveal a previously unknown yet critical role of L-type voltage-dependent calcium channels in the expression and/or distribution of synaptic ribbon proteins, providing a new model to study the clinical variability in human CSNB2 patients.

Presynaptic CaV1.3 channels regulate synaptic ribbon size and are required for synaptic maintenance in sensory hair cells

L-type calcium channels (Ca(V)1) are involved in diverse processes, such as neurotransmission, hormone secretion, muscle contraction, and gene expression. In this study, we uncover a role for Ca(V)1.3a in regulating the architecture of a cellular structure, the ribbon synapse, in developing zebrafish sensory hair cells. By combining in vivo calcium imaging with confocal and super-resolution structured illumination microscopy, we found that genetic disruption or acute block of Ca(V)1.3a channels led to enlargement of synaptic ribbons in hair cells. Conversely, activating channels reduced both synaptic-ribbon size and the number of intact synapses. Along with enlarged presynaptic ribbons in ca(V)1.3a mutants, we observed a profound loss of juxtaposition between presynaptic and postsynaptic components. These synaptic defects are not attributable to loss of neurotransmission, because vglut3 mutants lacking neurotransmitter release develop relatively normal hair-cell synapses. Moreover, regulation of synaptic-ribbon size by Ca(2+) influx may be used by other cell types, because we observed similar pharmacological effects on pinealocyte synaptic ribbons. Our results indicate that Ca(2+) influx through Ca(V)1.3 fine tunes synaptic ribbon size during hair-cell maturation and that Ca(V)1.3 is required for synaptic maintenance.

Evidence for synergistic and complementary roles of Bassoon and darkness in organizing the ribbon synapse

Ribbon synapses are tonically active high-throughput synapses. The performance of the ribbon synapse is accomplished by a specialization of the cytomatrix at the active zone (CAZ) referred to as the synaptic ribbon (SR). Progress in our understanding of the structure-function relationship at the ribbon synapse has come from observations that, in photoreceptors lacking a full-size scaffolding protein Bassoon (Bsn(ΔEx4/5)), dissociation of SRs coincides with perturbed signal transfer. The aim of the present study has been to elaborate the role of Bassoon as a structural organizer of the ribbon synapse and to differentiate it with regard to the ambient lighting conditions. The ultrastructure of retinal ribbon synapses has been compared between wild-type (Wt) and Bsn(ΔEx4/5) mice adapted to light (low activity) and darkness (high activity). The results obtained suggest that Bassoon and environmental illumination synergistically and complementarily act as organizers of the ribbon synapse. Thus, light-dependent and Bassoon-independent regulation involves initial SR tethering to the membrane and a basic shape transition of ribbon material from spherical to rod-like, since darkness induces these features in Bsn(ΔEx4/5) rod spherules. However, the tight anchorage of the SR via an arciform density and the proper assembly of SRs to the full-sized horseshoe-shaped complex depend on Bassoon, as these steps fail in Bsn(ΔEx4/5) rod spherules.

Role of melatonin and its receptors in the vertebrate retina

Melatonin is a chemical signal of darkness that is produced by retinal photoreceptors and pinealocytes. In the retina, melatonin diffuses from the photoreceptors to bind to specific receptors on a variety of inner retinal neurons to modify their activity. Potential target cells for melatonin in the inner retina are amacrine cells, bipolar cells, horizontal cells, and ganglion cells. Melatonin inhibits the release of dopamine from amacrine cells and increases the light sensitivity of horizontal cells. Melatonin receptor subtypes show differential, cell-specific patterns of expression that are likely to underlie differential functional modulation of specific retinal pathways. Melatonin potentiates rod signals to ON-type bipolar cells, via activation of the melatonin MT2 (Mel1b) receptor, suggesting that melatonin modulates the function of specific retinal circuits based on the differential distribution of its receptors. The selective and differential expression of melatonin receptor subtypes in cone circuits suggest a conserved function for melatonin in enhancing transmission from rods to second-order neurons and thus promote dark adaptation.

Overexpression of guanylate cyclase activating protein 2 in rod photoreceptors in vivo leads to morphological changes at the synaptic ribbon

Guanylate cyclase activating proteins are EF-hand containing proteins that confer calcium sensitivity to retinal guanylate cyclase at the outer segment discs of photoreceptor cells. By making the rate of cGMP synthesis dependent on the free intracellular calcium levels set by illumination, GCAPs play a fundamental role in the recovery of the light response and light adaptation. The main isoforms GCAP1 and GCAP2 also localize to the synaptic terminal, where their function is not known. Based on the reported interaction of GCAP2 with Ribeye, the major component of synaptic ribbons, it was proposed that GCAP2 could mediate the synaptic ribbon dynamic changes that happen in response to light. We here present a thorough ultrastructural analysis of rod synaptic terminals in loss-of-function (GCAP1/GCAP2 double knockout) and gain-of-function (transgenic overexpression) mouse models of GCAP2. Rod synaptic ribbons in GCAPs-/- mice did not differ from wildtype ribbons when mice were raised in constant darkness, indicating that GCAPs are not required for ribbon early assembly or maturation. Transgenic overexpression of GCAP2 in rods led to a shortening of synaptic ribbons, and to a higher than normal percentage of club-shaped and spherical ribbon morphologies. Restoration of GCAP2 expression in the GCAPs-/- background (GCAP2 expression in the absence of endogenous GCAP1) had the striking result of shortening ribbon length to a much higher degree than overexpression of GCAP2 in the wildtype background, as well as reducing the thickness of the outer plexiform layer without affecting the number of rod photoreceptor cells. These results indicate that preservation of the GCAP1 to GCAP2 relative levels is relevant for maintaining the integrity of the synaptic terminal. Our demonstration of GCAP2 immunolocalization at synaptic ribbons at the ultrastructural level would support a role of GCAPs at mediating the effect of light on morphological remodeling changes of synaptic ribbons.

Effect of different gentamicin dose on the plasticity of the ribbon synapses in cochlear inner hair cells of C57BL/6J mice

Faithful information transfer at the hair cell afferent synapse requires synaptic transmission to be both reliable and temporally precise. The release of neurotransmitter must exhibit both rapid on and off kinetics to accurately follow acoustic stimuli with a periodicity of 1 ms or less. To ensure such remarkable temporal fidelity, the cochlear hair cell afferent synapse undoubtedly relies on unique cellular and molecular specializations. To study effects of different doses of gentamicin on the changes of synaptic ribbons of cochlear inner hair cells (IHCs) in mice, the availability of genetic information, transgenic and knock-out animals make the C57BL/6J mouse a primary model in biomedical research. Aminoglycoside ototoxicity, however, has rarely been studied in mature mice because they are considered highly resistant to the drugs. This study presents models for gentamicin ototoxicity in adult C57BL/6J mouse strains. Five-week-old mice were injected intraperitoneally once daily with 50-300 mg gentamicin base/kg body weight for 7 days. Higher doses of gentamicin appear to be associated with earlier hearing damage in C57BL/6J mice, although not necessarily with more severe damage. At 200 mg/kg, gentamicin appears to induce significant hearing damage while not significantly affect the animal's general condition. Therefore, 200 mg/kg may be an ideal dose for ototoxicity modeling in C57BL/6J mice using gentamicin. In the early period of different dose of gentamicin effect, when the number of hair cells had not changed, the number changes of IHC ribbon synapses had taken place. Through the number of ribbon synapses changing, IHCs increased or decreased connections with spiral ganglion nerves (SGNs). The ribbon synapses played a compensatory role for gentamicin ototoxicity, while this effect was not sufficient to maintain the normal threshold of hearing.

The dynamic architecture of photoreceptor ribbon synapses: cytoskeletal, extracellular matrix, and intramembrane proteins

Rod and cone photoreceptors possess ribbon synapses that assist in the transmission of graded light responses to second-order bipolar and horizontal cells of the vertebrate retina. Proper functioning of the synapse requires the juxtaposition of presynaptic release sites immediately adjacent to postsynaptic receptors. In this review, we focus on the synaptic, cytoskeletal, and extracellular matrix proteins that help to organize photoreceptor ribbon synapses in the outer plexiform layer. We examine the proteins that foster the clustering of release proteins, calcium channels, and synaptic vesicles in the presynaptic terminals of photoreceptors adjacent to their postsynaptic contacts. Although many proteins interact with one another in the presynaptic terminal and synaptic cleft, these protein-protein interactions do not create a static and immutable structure. Instead, photoreceptor ribbon synapses are remarkably dynamic, exhibiting structural changes on both rapid and slow time scales.

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.

The cell biology of vision

Humans possess the remarkable ability to perceive color, shape, and motion, and to differentiate between light intensities varied by over nine orders of magnitude. Phototransduction—the process in which absorbed photons are converted into electrical responses—is the first stage of visual processing, and occurs in the outer segment, the light-sensing organelle of the photoreceptor cell. Studies of genes linked to human inherited blindness have been crucial to understanding the biogenesis of the outer segment and membrane-trafficking of photoreceptors.

Nicotinamide adenine dinucleotide-dependent binding of the neuronal Ca2+ sensor protein GCAP2 to photoreceptor synaptic ribbons

Guanylate cyclase activating protein 2 (GCAP2) is a recoverin-like Ca2+-sensor protein known to modulate guanylate cyclase activity in photoreceptor outer segments. GCAP2 is also present in photoreceptor ribbon synapses where its function is unknown. Synaptic ribbons are active zone-associated presynaptic structures in the tonically active photoreceptor ribbon synapses and contain RIBEYE as a unique and major protein component. In the present study, we demonstrate by various independent approaches that GCAP2 specifically interacts with RIBEYE in photoreceptor synapses. We show that the flexible hinge 2 linker region of RIBEYE(B) domain that connects the nicotinamide adenine dinucleotide (NADH)-binding subdomain with the substrate-binding subdomain (SBD) binds to the C terminus of GCAP2. We demonstrate that the RIBEYE-GCAP2 interaction is induced by the binding of NADH to RIBEYE. RIBEYE-GCAP2 interaction is modulated by the SBD. GCAP2 is strongly expressed in synaptic terminals of light-adapted photoreceptors where GCAP2 is found close to synaptic ribbons as judged by confocal microscopy and proximity ligation assays. Virus-mediated overexpression of GCAP2 in photoreceptor synaptic terminals leads to a reduction in the number of synaptic ribbons. Therefore, GCAP2 is a prime candidate for mediating Ca2+-dependent dynamic changes of synaptic ribbons in photoreceptor synapses.

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.

Ectopic synaptic ribbons in dendrites of mouse retinal ON- and OFF-bipolar cells

The ectopic distribution of synaptic ribbons in dendrites of mouse retinal bipolar cells was examined by using genetic ablation of metabotropic glutamate receptor subtype 6 (mGluR6), electron microscopy, and immunocytochemistry. Ectopic ribbons were observed in dendrites of rod and ON-cone bipolar cells in the mGluR6-deficient mouse but not in those of wild-type mice. The number of rod spherules facing the ectopic ribbons in mGluR6-deficient rod bipolar dendrites increased gradually during early growth and reached a plateau level of about 20% at 12 weeks. These ectopic ribbons were immunopositive for RIBEYE, a ribbon-specific protein, but the associated vesicles were immunonegative for synaptophysin, a synaptic-vesicle-specific protein. The presence of ectopic ribbons was correlated with an increase in the roundness of the invaginating dendrites of the rod bipolar cells. We further confirmed ectopic ribbons in dendrites of OFF-cone bipolar cells in wild-type retinas. Of the four types of OFF-cone bipolar cells (T1-T4), only the T2-type, which had a greater number of synaptic ribbons at the axon terminal and a thicker axon cylinder than the other types, had ectopic ribbons. Light-adapted experiments revealed that, in wild-type mice under enhanced-light adaptation (considered similar to the mGluR6-deficient state), the roundness in the invaginating dendrites and axon terminals of rod bipolar cells increased, but no ectopic ribbons were detected. Based on these findings and known mechanisms for neurotransmitter release and protein trafficking, the possible mechanisms underlying the ectopic ribbons are discussed on the basis of intracellular transport for the replenishment of synaptic proteins.

What drives cell morphogenesis: a look inside the vertebrate photoreceptor

Vision mediating photoreceptor cells are specialized light-sensitive neurons in the outer layer of the vertebrate retina. The human retina contains approximately 130 million of such photoreceptors, which enable images of the external environment to be captured at high resolution and high sensitivity. Rod and cone photoreceptor subtypes are further specialized for sensing light in low and high illumination, respectively. To enable visual function, these photoreceptors have developed elaborate morphological domains for the detection of light (outer segments), for changing cell shape (inner segments), and for communication with neighboring retinal neurons (synaptic terminals). Furthermore, rod and cone subtypes feature unique morphological variations of these specialized characteristics. Here, we review the major aspects of vertebrate photoreceptor morphology and key genetic mechanisms that drive their formation. These mechanisms are necessary for cell differentiation as well as function. Their defects lead to cell death.

Observations on the synaptic ribbon morphology in retinas of two human subjects at autopsy

Retinal photoreceptor and bipolar cell axon terminals possess synaptic ribbons (SR) that aid in the trafficking of synaptic vesicles at active zones. In rodents, besides SR, a number of other synaptic elements [e.g., synaptic spheres (SS)] are known to appear when exposed to altered ambient illumination. Here, we report changes of ribbon shape in photoreceptor and bipolar cell axon terminals in retinas of two persons at autopsy who suffered from brain hemorrhage. In both subjects, retinal hemorrhage was present in the outer and inner nuclear layers. SR were bent or swollen and transformed into SS. A count revealed that about 54-60% of the photoreceptor axon terminals over the nasal to temporal retina possessed SS. They were associated with synaptic triads or remained floating in cytoplasm. The bipolar cell axon terminals possessed either SR or sphere-like bodies. As these features were not seen in control retinas of donors who died of other causes, we assume that in hemorrhagic subjects, SR underwent transformation into SS, in which perhaps ischemia (caused due to vascular obstructions by hemorrhage) played a leading role.

Circadian regulation of ion channels and their functions

Ion channels are the gatekeepers to neuronal excitability. Retinal neurons of vertebrates and invertebrates, neurons of the suprachiasmatic nucleus (SCN) of vertebrates, and pinealocytes of non-mammalian vertebrates display daily rhythms in their activities. The interlocking transcription-translation feedback loops with specific post-translational modulations within individual cells form the molecular clock, the basic mechanism that maintains the autonomic approximately 24-h rhythm. The molecular clock regulates downstream output signaling pathways that further modulate activities of various ion channels. Ultimately, it is the circadian regulation of ion channel properties that govern excitability and behavior output of these neurons. In this review, we focus on the recent development of research in circadian neurobiology mainly from 1980 forward. We will emphasize the circadian regulation of various ion channels, including cGMP-gated cation channels, various voltage-gated calcium and potassium channels, Na(+)/K(+)-ATPase, and a long-opening cation channel. The cellular mechanisms underlying the circadian regulation of these ion channels and their functions in various tissues and organisms will also be discussed. Despite the magnitude of chronobiological studies in recent years, the circadian regulation of ion channels still remains largely unexplored. Through more investigation and understanding of the circadian regulation of ion channels, the future development of therapeutic strategies for the treatment of sleep disorders, cardiovascular diseases, and other illnesses linked to circadian misalignment will benefit.

Evidence that exocytosis is driven by calcium entry through multiple calcium channels in goldfish retinal bipolar cells

Ribbon-containing neurons represent a subset of neural cells that undergo graded membrane depolarizations rather than Na(+)-channel evoked action potentials. Bipolar cells of the retina are one type of ribbon-containing neuron and extensive research has demonstrated kinetically distinct pools of vesicles that are released and replenished in a calcium-dependent manner. In this study, we look at the properties of the fastest pool of releasable vesicles in these cells, often referred to as the immediately releasable pool (IRP), to investigate the relationships between vesicle release and calcium channels in these terminals. Using whole cell capacitance measurements, we monitored exocytosis in response to different magnitude and duration depolarizations, with emphasis on physiologically relevant step depolarizations. We find that release rate of the IRP increases superlinearly with membrane potential and that the IRP is sensitive to elevated EGTA concentrations in a membrane-potential-dependent manner across the physiological range of membrane potentials. Our results are best explained by a model in which multiple Ca(2+) channels act in concert to drive exocytosis of a single synaptic vesicle. Pooling calcium entering through many calcium channels may be important for reducing stochastic noise in neurotransmitter release associated with the opening of individual calcium channels.

Multiple RIBEYE-RIBEYE interactions create a dynamic scaffold for the formation of synaptic ribbons

Synaptic ribbons are large, dynamic structures in the active zone complex of ribbon synapses and important for the physiological properties of these tonically active synapses. RIBEYE is a unique and major protein component of synaptic ribbons. The aim of the present study was to understand how the synaptic ribbon is built and how the construction of the ribbon could contribute to its ultrastructural plasticity. In the present study, we demonstrate that RIBEYE self-associates using different independent approaches (yeast two-hybrid analyses, protein pull downs, synaptic ribbon-RIBEYE interaction assays, coaggregation experiments, transmission electron microscopy and immunogold electron microscopy). The A-domain [RIBEYE(A)] and B-domain [RIBEYE(B)] of RIBEYE contain five distinct sites for RIBEYE-RIBEYE interactions. Three interaction sites are present in the A-domain of RIBEYE and mediate RIBEYE(A)-RIBEYE(A) homodimerization and heterodimerization with the B-domain. The docking site for RIBEYE(A) on RIBEYE(B) is topographically and functionally different from the RIBEYE(B) homodimerization interface and is negatively regulated by nicotinamide adenine dinucleotide. The identified multiple RIBEYE-RIBEYE interactions have the potential to build the synaptic ribbon: heterologously expressed RIBEYE forms large electron-dense aggregates that are in part physically associated with surrounding vesicles and membrane compartments. These structures resemble spherical synaptic ribbons. These ribbon-like structures coassemble with the active zone protein bassoon, an interaction partner of RIBEYE at the active zone of ribbon synapses, emphasizing the physiological relevance of these RIBEYE-containing aggregates. Based on the identified multiple RIBEYE-RIBEYE interactions, we provide a molecular mechanism for the dynamic assembly of synaptic ribbons from individual RIBEYE subunits.

Active zone proteins are dynamically associated with synaptic ribbons in rat pinealocytes

Synaptic ribbons (SRs) are prominent organelles that are abundant in the ribbon synapses of sensory neurons where they represent a specialization of the cytomatrix at the active zone (CAZ). SRs occur not only in neurons, but also in neuroendocrine pinealocytes where their function is still obscure. In this study, we report that pinealocyte SRs are associated with CAZ proteins such as Bassoon, Piccolo, CtBP1, Munc13–1, and the motorprotein KIF3A and, therefore, consist of a protein complex that resembles the ribbon complex of retinal and other sensory ribbon synapses. The pinealocyte ribbon complex is biochemically dynamic. Its protein composition changes in favor of Bassoon, Piccolo, and Munc13–1 at night and in favor of KIF3A during the day, whereas CtBP1 is equally present during the night and day. The diurnal dynamics of the ribbon complex persist under constant darkness and decrease after stimulus deprivation of the pineal gland by constant light. Our findings indicate that neuroendocrine pinealocytes possess a protein complex that resembles the CAZ of ribbon synapses in sensory organs and whose dynamics are under circadian regulation.

The expression of L-type voltage-gated calcium channels in retinal photoreceptors is under circadian control

Photoreceptors are non-spiking neurons, and their synapses mediate the continuous release of neurotransmitters under the control of L-type voltage-gated calcium channels (VGCCs). Photoreceptors express endogenous circadian oscillators that play important roles in regulating photoreceptor physiology and function. Here, we report that the L-type VGCCs in chick cone photoreceptors are under circadian control. The L-type VGCC currents are greater when measured during the subjective night than during the subjective day. Using antibodies against the VGCCalpha1C and VGCCalpha1D subunits, we found that the immunofluorescence intensities of both VGCCalpha1C and VGCCalpha1D in photoreceptors are higher during the subjective night. However, the mRNA levels of VGCCalpha1D, but not VGCCalpha1C, are rhythmic. Nocturnal increases in L-type VGCCs are blocked by manumycin A, PD98059, and KN93, which suggest that the circadian output pathway includes Ras, Erk, and calcium-calmodulin dependent kinase II. In summary, four independent lines of evidence show that the L-VGCCs in cone photoreceptors are under circadian control.

Switching of Ca2+-dependent inactivation of Ca(v)1.3 channels by calcium binding proteins of auditory hair cells

Ca(V)1.3 channels comprise a vital subdivision of L-type Ca2+ channels: Ca(V)1.3 channels mediate neurotransmitter release from auditory inner hair cells (IHCs), pancreatic insulin secretion, and cardiac pacemaking. Fitting with these diverse roles, Ca(V)1.3 channels exhibit striking variability in their inactivation by intracellular Ca2+. IHCs show generally weak-to-absent Ca2+-dependent inactivation (CDI), potentially permitting audition of sustained sounds. In contrast, the strong CDI seen elsewhere likely provides critical negative feedback. Here, we explore this mysterious CDI malleability, particularly its comparative weakness in hair cells. At baseline, heterologously expressed Ca(V)1.3 channels exhibit intense CDI, wherein each lobe of calmodulin (CaM) contributes a distinct inactivation component. Because CaM-like molecules (bearing four recognizable but not necessarily functional Ca2+-binding EF hands) can perturb the Ca2+ response of molecules regulated by CaM, we asked whether such CaM-like entities could influence CDI. We find that CaM-like calcium-binding protein (CaBP) molecules are clearly expressed within the organ of Corti. In particular, the rare subtype CaBP4 is specific to IHCs, and CaBP4 proves capable of eliminating even the potent baseline CDI of Ca(V)1.3. CaBP4 thereby represents a plausible candidate for moderating CDI within IHCs.

Analysis of synaptic bodies in the Sprague-Dawley rat pineal gland under extreme photoperiods

Synaptic bodies (SBs) are small, prominent organelles in pinealocytes, most probably involved in signal transduction processes. To check the influence of the photoperiod on their shape plasticity and number we chose two extreme lighting conditions, i.e. 20h of illumination followed by 4h of darkness (LD 20:4) versus (LD 4:20). Pineal glands were assessed at 0, 4 and 13h after dark onset. Under both conditions reconstructed SBs were plates or ribbons but never spheres and there were no obvious differences in morphology. Photoperiodic changes in SB profile size and number were investigated: application of the established method for SB quantification based on single section profile counts (SSPC) of areas showed a significant increase of SB profiles under LD 20:4. However, it has to be noted that SSPC depend on both, number and size of the structures. In contrast to this, modification of the disector counting method, also applied for unbiased quantification of whole SBs, revealed that rat pinealocytes show insignificantly more SBs under LD 20:4 than under 4:20 conditions. The lengths of the SB profiles, which were first measured under different conditions in this study, depend on SB size. They increased significantly under LD 20:4. In conclusion, we detected only an increase in SB size but not in their number. We further prove that, at least for SBs, it is of no value to calculate disector levels from SSPCs.

Diurnal changes in exocytosis and the number of synaptic ribbons at active zones of an ON-type bipolar cell terminal

The number and morphology of synaptic ribbons at photoreceptor and bipolar cell terminals has been reported to change on a circadian cycle. Here we sought to determine whether this phenomenon exists at goldfish Mb-type bipolar cell terminals with the aim of exploring the role of ribbons in transmitter release. We examined the physiology and ultrastructure of this terminal around two time points: midday and midnight. Nystatin perforated-patch recordings of membrane capacitance (C(m)) revealed that synaptic vesicle exocytosis evoked by short depolarizations was reduced at night, even though Ca(2+) currents were larger. The efficiency of exocytosis (measured as the DeltaC(m) jump per total Ca(2+) charge influx) was thus significantly lower at night. The paired-pulse ratio remained unchanged, however, suggesting that release probability was not altered. Hence the decreased exocytosis likely reflects a smaller readily releasable vesicle pool at night. Electron microscopy of single sections from intact retinas averaged 65% fewer ribbons at night. Interestingly, the number of active zones did not change from day to night, only the probability of finding a ribbon at an active zone. Additionally, synaptic vesicle halos surrounding the ribbons were more completely filled at night when these on-type bipolar cells are more hyperpolarized. There was no change, however, in the physical dimensions of synaptic ribbons from day to night. These results suggest that the size of the readily releasable vesicle pool and the efficiency of exocytosis are reduced at night when fewer ribbons are present at bipolar cell terminal active zones.