Mammalian Retinal Ganglion Cells Research Articles

Problems and prospects for restoration of the optic nerve

Restoring visual function after damage or complete destruction of the optic nerve in adult patients has many natural barriers to neuroregeneration. Research to restore vision has focused on maintaining retinal ganglion cells (RGCs), stimulating axonal growth toward the brain, and restoring their proper synaptic connections. Unfortunately, mammalian RGC axons under normal conditions do not regenerate after injury and ultimately die. In this review, we summarize the currently known mechanisms of RGC survival and axonal regeneration in mammals, including specific intrinsic signaling pathways, key transcription factors, reprogramming genes, inflammation-related regeneration factors, and stem cell therapy. We also review the current understanding of the phenomena impeding optic nerve regeneration and possible ways to overcome these obstacles. The most important research results obtained in recent decades may be informative for the development of methods for treating the damaged visual system.

Синаптичні процеси гангліозних клітин сітківки ока ссавців

To date, mammalian retinal ganglion cells have been studied quite well, and their diversity, development, and connections have been clarified. Fifty subtypes of these cells have already been identified, each of which optimally encodes unique visual capabilities and projects visual information to various targets in the brain, where it is decoded. Some types of ganglion cells express the photopigment melanopsin (Opn4), which makes them directly sensitive to light. In this review, we discuss the diversity of ganglion cells, their characteristics and differences, and describe the structure of retinal neurons and the synaptic processes that occur in them. Since the quality of human life is seriously affected by retinal diseases that can lead to vision loss, scientists are paying close attention to the causes of visual clarity and quality and the role of retinal ganglion cells in this process, conducting multidirectional research using new methods and different objects for study.

Dual leucine zipper kinase is necessary for retinal ganglion cell axonal regeneration in Xenopus laevis.

Retinal ganglion cell (RGC) axons of the African clawed frog, Xenopus laevis, unlike those of mammals, are capable of regeneration and functional reinnervation of central brain targets following injury. Here, we describe a tadpole optic nerve crush (ONC) procedure and assessments of brain reinnervation based on live imaging of RGC-specific transgenes which, when paired with CRISPR/Cas9 injections at the one-cell stage, can be used to assess the function of regeneration-associated genes in vivo in F0 animals. Using this assay, we find that map3k12, also known as dual leucine zipper kinase (Dlk), is necessary for RGC axonal regeneration and acts in a dose-dependent manner. Loss of Dlk does not affect RGC innervation of the brain during development or visually driven behavior but does block both axonal regeneration and functional vision restoration after ONC. Dlk loss does not alter the acute changes in mitochondrial movement that occur within RGC axons hours after ONC but does completely block the phosphorylation and nuclear translocation of the transcription factor Jun within RGCs days after ONC; yet, Jun is dispensable for reinnervation. These results demonstrate that in a species fully capable of regenerating its RGC axons, Dlk is essential for the axonal injury signal to reach the nucleus but may affect regeneration through a different pathway than by which it signals in mammalian RGCs.

Modeling extracellular stimulation of retinal ganglion cells: theoretical and practical aspects

Objective. Retinal prostheses use electric current to activate inner retinal neurons, providing artificial vision for blind people. Epiretinal stimulation primarily targets retinal ganglion cells (RGCs), which can be modeled with cable equations. Computational models provide a tool to investigate the mechanisms of retinal activation, and improve stimulation paradigms. However, documentation of RGC model structure and parameters is limited, and model implementation can influence model predictions. Approach. We created a functional guide for building a mammalian RGC multi-compartment cable model and applying extracellular stimuli. Next, we investigated how the neuron’s three-dimensional shape will influence model predictions. Finally, we tested several strategies to maximize computational efficiency. Main results. We conducted sensitivity analyses to examine how dendrite representation, axon trajectory, and axon diameter influence membrane dynamics and corresponding activation thresholds. We optimized the spatial and temporal discretization of our multi-compartment cable model. We also implemented several simplified threshold prediction theories based on activating function, but these did not match the prediction accuracy achieved by the cable equations. Significance. Through this work, we provide practical guidance for modeling the extracellular stimulation of RGCs to produce reliable and meaningful predictions. Robust computational models lay the groundwork for improving the performance of retinal prostheses.

Purified regenerating retinal neurons reveal regulatory role of DNA methylation-mediated Na+/K+-ATPase in murine axon regeneration

While embryonic mammalian central nervous system (CNS) axons readily grow and differentiate, only a minority of fully differentiated mature CNS neurons are able to regenerate injured axons, leading to stunted functional recovery after injury and disease. To delineate DNA methylation changes specifically associated with axon regeneration, we used a Fluorescent-Activated Cell Sorting (FACS)-based methodology in a rat optic nerve transection model to segregate the injured retinal ganglion cells (RGCs) into regenerating and non-regenerating cell populations. Whole-genome DNA methylation profiling of these purified neurons revealed genes and pathways linked to mammalian RGC regeneration. Moreover, whole-methylome sequencing of purified uninjured adult and embryonic RGCs identified embryonic molecular profiles reactivated after injury in mature neurons, and others that correlate specifically with embryonic or adult axon growth, but not both. The results highlight the contribution to both embryonic growth and adult axon regeneration of subunits encoding the Na+/K+-ATPase. In turn, both biochemical and genetic inhibition of the Na+/K+-ATPase pump significantly reduced RGC axon regeneration. These data provide critical molecular insights into mammalian CNS axon regeneration, pinpoint the Na+/K+-ATPase as a key regulator of regeneration of injured mature CNS axons, and suggest that successful regeneration requires, in part, reactivation of embryonic signals.

Linking transcriptomes with morphological and functional phenotypes in mammalian retinal ganglion cells.

Retinal ganglion cells (RGCs) are the brain's gateway to the visual world. They can be classified into different types on the basis of their electrophysiological, transcriptomic, or morphological characteristics. Here, we characterize the transcriptomic, morphological, and functional features of 472 high-quality RGCs using Patch sequencing (Patch-seq), providing functional and morphological annotation of many transcriptomic-defined cell types of a previously established RGC atlas. We show a convergence of different modalities in defining the RGC identity and reveal the degree of correspondence for well-characterized cell types across multimodal data. Moreover, we complement some RGC types with detailed morphological and functional properties. We also identify differentially expressed genes among ON, OFF, and ON-OFF RGCs such as Vat1l, Slitrk6, and Lmo7, providing candidate marker genes for functional studies. Our research suggests that the molecularly distinct clusters may also differ in their roles of encoding visual information.

Monocyte-derived SDF1 supports optic nerve regeneration and alters retinal ganglion cells’ response to Pten deletion

Although mammalian retinal ganglion cells (RGCs) normally cannot regenerate axons nor survive after optic nerve injury, this failure is partially reversed by inducing sterile inflammation in the eye. Infiltrative myeloid cells express the axogenic protein oncomodulin (Ocm) but additional, as-yet-unidentified, factors are also required. We show here that infiltrative macrophages express stromal cell–derived factor 1 (SDF1, CXCL12), which plays a central role in this regard. Among many growth factors tested in culture, only SDF1 enhances Ocm activity, an effect mediated through intracellular cyclic AMP (cAMP) elevation and phosphatidylinositol-4,5-bisphosphate 3-kinase (PI3K) activation. SDF1 deficiency in myeloid cells (CXCL12flx/flxLysM-Cre−/+ mice) or deletion of the SDF1 receptor CXCR4 in RGCs (intraocular AAV2-Cre in CXCR4flx/flx mice) or SDF1 antagonist AMD3100 greatly suppresses inflammation-induced regeneration and decreases RGC survival to baseline levels. Conversely, SDF1 induces optic nerve regeneration and RGC survival, and, when combined with Ocm/cAMP, SDF1 increases axon regeneration to levels similar to those induced by intraocular inflammation. In contrast to deletion of phosphatase and tensin homolog (Pten), which promotes regeneration selectively from αRGCs, SDF1 promotes regeneration from non-αRGCs and enables the latter cells to respond robustly to Pten deletion; however, SDF1 surprisingly diminishes the response of αRGCs to Pten deletion. When combined with inflammation and Pten deletion, SDF1 enables many RGCs to regenerate axons the entire length of the optic nerve. Thus, SDF1 complements the effects of Ocm in mediating inflammation-induced regeneration and enables different RGC subtypes to respond to Pten deletion.

In vivo MRI evaluation of anterograde manganese transport along the visual pathway following whole eye transplantation

Background: Since adult mammalian retinal ganglion cells cannot regenerate after injury, we have recently established a whole-eye transplantation (WET) rat model that provides an intact optical system to investigate potential surgical restoration of irreversible vision loss. However, it remains to be elucidated whether physiological axoplasmic transport exists in the transplanted visual pathway.New Method: We developed an in vivo imaging model system to assess WET integration using manganese-enhanced magnetic resonance imaging (MEMRI) in rats.Since Mn2+ is a calcium analogue and an active T1-positive contrast agent, the levels of anterograde manganese transport can be evaluated in the visual pathways upon intravitreal Mn2+ administration into both native and transplanted eyes.Results: No significant intraocular pressure difference was found between native and transplanted eyes, whereas comparable manganese enhancement was observed between native and transplanted intraorbital optic nerves, suggesting the presence of anterograde manganese transport after WET. No enhancement was detected across the coaptation site in the higher visual areas of the recipient brain.Comparison with Existing Methods: Existing imaging methods to assess WET focus on either the eye or local optic nerve segments without direct visualization and longitudinal quantification of physiological transport along the transplanted visual pathway, hence the development of in vivo MEMRI.Conclusion: Our established imaging platform indicated that essential physiological transport exists in the transplanted optic nerve after WET. As neuroregenerative approaches are being developed to connect the transplanted eye to the recipient’s brain, in vivo MEMRI is well-suited to guide strategies for successful WET integration for vision restoration.

Retinal Ganglion Cell Transplantation: Approaches for Overcoming Challenges to Functional Integration.

As part of the central nervous system, mammalian retinal ganglion cells (RGCs) lack significant regenerative capacity. Glaucoma causes progressive and irreversible vision loss by damaging RGCs and their axons, which compose the optic nerve. To functionally restore vision, lost RGCs must be replaced. Despite tremendous advancements in experimental models of optic neuropathy that have elucidated pathways to induce endogenous RGC neuroprotection and axon regeneration, obstacles to achieving functional visual recovery through exogenous RGC transplantation remain. Key challenges include poor graft survival, low donor neuron localization to the host retina, and inadequate dendritogenesis and synaptogenesis with afferent amacrine and bipolar cells. In this review, we summarize the current state of experimental RGC transplantation, and we propose a set of standard approaches to quantifying and reporting experimental outcomes in order to guide a collective effort to advance the field toward functional RGC replacement and optic nerve regeneration.

Morphological identification and systematic classification of mammalian retinal ganglion cells. I. Rabbit retinal ganglion cells.

Retinal ganglion cells (RGCs) convey visual signals to 50 regions of the brain. For reasons of interest and convenience, they constitute an excellent system for the study of brain structure and function. There is general agreement that, absent a complete "parts list," understanding how the nervous system processes information will remain an elusive goal. Recent studies indicate that there are 30-50 types of ganglion cell in mouse retina, whereas only a few years ago it was still written that mice and the more visually oriented lagomorphs had less than 20 types of RGC. More than 30 years ago, I estimated that rabbits have about 40 types of RGC. The present study indicates that this number is much too low. I have employed the old but powerful method of Golgi-impregnation to rabbit retina, studying the range of component neurons in this already well-studied retinal system. Close quantitative and qualitative analyses of 1,142 RGCs in 26 retinas take into account cell body and dendritic field size, level(s) of dendritic stratification in the retina's inner plexiform layer, and details of dendritic branching. Ninety-one morphologies are recognized. Of these, at least 32 can be correlated with physiologically studied RGCs, dye-injected for morphological analysis. It is unlikely that rabbits have 91 types of RGC, but is argued here that this number lies between 60 and 70. The present study provides a "yardstick" for measuring the output of future molecular studies that may be more definitive in fixing the number of RGC types in rabbit retina.

Comparative lipidomic analysis of mammalian retinal ganglion cells and M\xfcller glia in situ and in vitro using High-Resolution Imaging Mass Spectrometry

In order to better understand retinal physiology, alterations to which underlie some ocular diseases, we set out to establish the lipid signature of two fundamental cell types in the retina, Müller Glia and Retinal Ganglion Cells (RGCs). Moreover, we compared the lipid signature of these cells in sections (in situ), as well as after culturing the cells and isolating their cell membranes (in vitro). The lipidome of Müller glia and RGCs was analyzed in porcine retinal sections using Matrix Assisted Laser Desorption Ionization Imaging Mass Spectrometry (MALDI-IMS). Isolated membranes, as well as whole cells from primary cell cultures of RGCs and Müller glia, were printed onto glass slides using a non-contact microarrayer (Nano Plotter), and a LTQ-Orbitrap XL analyzer was used to scan the samples in negative ion mode, thereafter identifying the RGCs and Müller cells immunohistochemically. The spectra acquired were aligned and normalized against the total ion current, and a statistical analysis was carried out to select the lipids specific to each cell type in the retinal sections and microarrays. The peaks of interest were identified by MS/MS analysis. A cluster analysis of the MS spectra obtained from the retinal sections identified regions containing RGCs and Müller glia, as confirmed by immunohistochemistry in the same sections. The relative density of certain lipids differed significantly (p-value ≤ 0.05) between the areas containing Müller glia and RGCs. Likewise, different densities of lipids were evident between the RGC and Müller glia cultures in vitro. Finally, a comparative analysis of the lipid profiles in the retinal sections and microarrays identified six peaks that corresponded to a collection of 10 lipids characteristic of retinal cells. These lipids were identified by MS/MS. The analyses performed on the RGC layer of the retina, on RGCs in culture and using cell membrane microarrays of RGCs indicate that the lipid composition of the retina detected in sections is preserved in primary cell cultures. Specific lipid species were found in RGCs and Müller glia, allowing both cell types to be identified by a lipid fingerprint. Further studies into these specific lipids and of their behavior in pathological conditions may well help identify novel therapeutic targets for ocular diseases.

Can We See with Melanopsin?

A small fraction of mammalian retinal ganglion cells are directly photoreceptive thanks to their expression of the photopigment melanopsin. These intrinsically photosensitive retinal ganglion cells (ipRGCs) have well-established roles in a variety of reflex responses to changes in ambient light intensity, including circadian photoentrainment. In this article, we review the growing evidence, obtained primarily from laboratory mice and humans, that the ability to sense light via melanopsin is also an important component of perceptual and form vision. Melanopsin photoreception has low temporal resolution, making it fundamentally biased toward detecting changes in ambient light and coarse patterns rather than fine details. Nevertheless, melanopsin can indirectly impact high-acuity vision by driving aspects of light adaptation ranging from pupil constriction to changes in visual circuit performance. Melanopsin also contributes directly to perceptions of brightness, and recent data suggest that this influences the appearance not only of overall scene brightness, but also of low-frequency patterns.

Strategies to Promote Long-Distance Optic Nerve Regeneration.

Mammalian retinal ganglion cells (RGCs) in the central nervous system (CNS) often die after optic nerve injury and surviving RGCs fail to regenerate their axons, eventually resulting in irreversible vision loss. Manipulation of a diverse group of genes can significantly boost optic nerve regeneration of mature RGCs by reactivating developmental-like growth programs or suppressing growth inhibitory pathways. By injury of the vision pathway near their brain targets, a few studies have shown that regenerated RGC axons could form functional synapses with targeted neurons but exhibited poor neural conduction or partial functional recovery. Therefore, the functional restoration of eye-to-brain pathways remains a greatly challenging issue. Here, we review recent advances in long-distance optic nerve regeneration and the subsequent reconnecting to central targets. By summarizing our current strategies for promoting functional recovery, we hope to provide potential insights into future exploration in vision reformation after neural injuries.

Mouse γ-Synuclein Promoter-Mediated Gene Expression and Editing in Mammalian Retinal Ganglion Cells.

Optic neuropathies are a group of optic nerve (ON) diseases caused by various insults including glaucoma, inflammation, ischemia, trauma, and genetic deficits, which are characterized by retinal ganglion cell (RGC) death and ON degeneration. An increasing number of genes involved in RGC intrinsic signaling have been found to be promising neural repair targets that can potentially be modulated directly by gene therapy, if we can achieve RGC specific gene targeting. To address this challenge, we first used adeno-associated virus (AAV)-mediated gene transfer to perform a low-throughput in vivo screening in both male and female mouse eyes and identified the mouse γ-synuclein (mSncg) promoter, which specifically and potently sustained transgene expression in mouse RGCs and also works in human RGCs. We further demonstrated that gene therapy that combines AAV-mSncg promoter with clustered regularly interspaced short palindromic repeats (CRISPR)/Cas9 gene editing can knock down pro-degenerative genes in RGCs and provide effective neuroprotection in optic neuropathies.SIGNIFICANCE STATEMENT Here, we present an RGC-specific promoter, mouse γ-synuclein (mSncg) promoter, and perform extensive characterization and proof-of-concept studies of mSncg promoter-mediated gene expression and clustered regularly interspaced short palindromic repeats (CRISPR)/Cas9 gene editing in RGCs in vivo To our knowledge, this is the first report demonstrating in vivo neuroprotection of injured RGCs and optic nerve (ON) by AAV-mediated CRISPR/Cas9 inhibition of genes that are critical for neurodegeneration. It represents a powerful tool to achieve RGC-specific gene modulation, and also opens up a promising gene therapy strategy for optic neuropathies, the most common form of eye diseases that cause irreversible blindness.

Activation of Piezo1 Mechanosensitive Channels in Retinal Ganglion Cells Leads to Cell Death

PurposePiezo1 mechanosensitive ion channels play an important role in mechanotransduction in all vertebrates and have been identified in a variety of tissues and cell types including neurons. Previously, we have identified Piezo1 mechanosensitive ion channels in zebrafish and mammalian retinal ganglion cells (RGCs). The principle aim of this study is to test whether activation of Piezo1 channels can produce neurodegeneration of zebrafish and mammalian RGCs.MethodsFor zebrafish Ca2+ imaging was performed using a genetically‐encoded Ca2+ indicator, GCaMP6f, and was targeted predominantly to RGCs using the Tg(elavl3: GCaMP6f) zebrafish line, and confirmed by backfilling the optic nerve with Texas Red hydrazide. In order to activate Piezo1 channels in RGCs, cytosolic [Ca2+]i changes were measured in response to application of the Piezo1 selective agonist, Yoda1. Immunoblotting and immunohistochemical localization for Piezo1 were used to determine the expression profile in rodent and zebrafish retina using retinal wholemounts and vertical sections. To examine apoptosis, a short‐term organotypic eyecup culture preparation was used. Eyecups were incubated in oxygenated Ringer’s solution (for 1–8 hrs) with various concentrations of Yoda1 (0.1–100μM). Following treatment with Yoda1, eyecups were fixed, sectioned, and prepared for immunohistochemistry. TUNEL assays was used to label apoptotic cells in fixed vertical retinal sections, and TUNEL‐positive cells were quantified using ImageJ software. All imaging was acquired on a Nikon C2+ confocal microscope.ResultsCalcium imaging experiments revealed that Yoda1 evokes a dose‐dependent activation of Piezo1 ion channels in GCAMP6f expressing RGCs, which was reduced in the presence of GsMTx‐4, a mechanosensitive channel antagonist. Immunohistochemical findings show strong Piezo1 immunoreactivity in RGCs and their axons in rodent and zebrafish retina. Moreover, Yoda1 evoked a concentration‐dependent increase in TUNEL‐positive labeling of RGCs in both zebrafish and rodent retina, implicating a common Ca2+‐mediated pathway for Piezo1 channel induced RGC death.ConclusionsThis study provides the first evidence for Piezo1 channel‐mediated RGC death and suggests that there is a potential link between Piezo1 channel activation on RGCs, increased [Ca2+]i influx and apoptosis. Therefore, changes in cell stiffness and pressure acting through Piezo1 mechanosensitive channels could contribute to the neurodegenerative effects observed in optic neuropathies, such as ocular injury and glaucoma.Support or Funding InformationHMC CURE Zebrafish Pilot Grant, Penn State University‐Hershey College of Medicine; Pennsylvania Department of Health, Tobacco CURE Funds

Synaptic inputs from identified bipolar and amacrine cells to a sparsely branched ganglion cell in rabbit retina.

There are more than 30 distinct types of mammalian retinal ganglion cells, each sensitive to different features of the visual environment. In rabbit retina, they can be grouped into four classes according to their morphology and stratification of their dendrites in the inner plexiform layer (IPL). The goal of this study was to describe the synaptic inputs to one type of Class IV ganglion cell, the third member of the sparsely branched Class IV cells (SB3). One cell of this type was partially reconstructed in a retinal connectome developed using automated transmission electron microscopy (ATEM). It had slender, relatively straight dendrites that ramify in the sublamina a of the IPL. The dendrites of the SB3 cell were always postsynaptic in the IPL, supporting its identity as a ganglion cell. It received 29% of its input from bipolar cells, a value in the middle of the range for rabbit retinal ganglion cells studied previously. The SB3 cell typically received only one synapse per bipolar cell from multiple types of presumed OFF bipolar cells; reciprocal synapses from amacrine cells at the dyad synapses were infrequent. In a few instances, the bipolar cells presynaptic to the SB3 ganglion cell also provided input to an amacrine cell presynaptic to the ganglion cell. There was apparently no crossover inhibition from narrow-field ON amacrine cells. Most of the amacrine cell inputs were from axons and dendrites of GABAergic amacrine cells, likely providing inhibitory input from outside the classical receptive field.

Satb1 Regulates Contactin 5 to Pattern Dendrites of a Mammalian Retinal Ganglion Cell

The size and shape of dendritic arbors are prime determinants of neuronal connectivity and function. We asked how ON-OFF direction-selective ganglion cells (ooDSGCs) in mouse retina acquire their bistratified dendrites, in which responses to light onset andlight offset are segregated to distinct strata. Wefound that the transcriptional regulator Satb1 is selectively expressed by ooDSGCs. In Satb1 mutant mice, ooDSGC dendrites lack ON arbors, and the cells selectively lose ON responses. Satb1 regulates expression of a homophilic adhesion molecule, Contactin 5 (Cntn5). Both Cntn5 and its co-receptor Caspr4 are expressed not only by ooDSGCs, but also by interneurons that form a scaffold on which ooDSGC ON dendrites fasciculate. Removing Cntn5 from either ooDSGCs or interneurons partially phenocopies Satb1 mutants, demonstrating that Satb1-dependent Cntn5 expression in ooDSGCs leads to branch-specific homophilic interactions with interneurons. Thus, Satb1 directs formation of a morphologically and functionally specialized compartment within a complex dendritic arbor.

Calcium-activated BKCa channels govern dynamic membrane depolarizations of horizontal cells in rodent retina.

Large conductance, Ca2+ -activated K+ (BKCa ) channels play important roles in mammalian retinal neurons, including photoreceptors, bipolar cells, amacrine cells and ganglion cells, but they have not been identified in horizontal cells. BKCa channel blockers paxilline and iberiotoxin, as well as Ca2+ free solutions and divalent cation Cav channel blockers, eliminate the outwardly rectifying current, while NS1619 enhances it. In symmetrical 150mm K+ , single channels had a conductance close to 250pS, within the range of all known BKCa channels. In current clamped horizontal cells, BKCa channels subdue depolarizing membrane potential excursions, reduce the average resting potential and decrease oscillations. The results show that BKCa channel activation puts a ceiling on horizontal cell depolarization and regulates the temporal responsivity of the cells. Large conductance, calcium-activated potassium (BKCa ) channels have numerous roles in neurons including the regulation of membrane excitability, intracellular [Ca2+ ] regulation, and neurotransmitter release. In the retina, they have been identified in photoreceptors, bipolar cells, amacrine cells and ganglion cells, but have not been conclusively identified in mammalian horizontal cells. We found that outward current recorded between -30 and +60mV is carried primarily in BKCa channels in isolated horizontal cells of rats and mice. Whole-cell outward currents were maximal at +50mV and declined at membrane potentials positive to this value. This current was eliminated by the selective BKCa channel blocker paxilline (100nm), iberiotoxin (10μm), Ca2+ free solutions and divalent cation Cav channel blockers. It was activated by the BKCa channel activator NS1619 (30μm). Single channel recordings revealed the conductance of the channels to be 244±11pS (n=17; symmetrical 150mm K+ ) with open probability being both voltage- and Ca2+ -dependent. The channels showed fast activation kinetics in response to Ca2+ influx and inactivation gating that could be modified by intracellular protease treatment, which suggests β subunit involvement. Under current clamp, block of BKCa current increased depolarizing membrane potential excursions, raising the average resting potential and producing oscillations. BKCa current activation with NS1619 inhibited oscillations and hyperpolarized the resting potential. These effects underscore the functional role of BKCa current in limiting depolarization of the horizontal cell membrane potential and suggest actions of these channels in regulating the temporal responsivity of the cells.

Mechanisms creating transient and sustained photoresponses in mammalian retinal ganglion cells.

Retinal neurons use sustained and transient light responses to encode visual stimuli of different frequency ranges, but the underlying mechanisms remain poorly understood. In particular, although earlier studies in retinal ganglion cells (RGCs) proposed seven potential mechanisms, all seven have since been disputed, and it remains unknown whether different RGC types use different mechanisms or how many mechanisms are used by each type. Here, we conduct a comprehensive survey in mice and rats of 12 candidate mechanisms that could conceivably produce tonic rod/cone-driven ON responses in intrinsically photosensitive RGCs (ipRGCs) and transient ON responses in three types of direction-selective RGCs (TRHR+, Hoxd10+ ON, and Hoxd10+ ON-OFF cells). We find that the tonic kinetics of ipRGCs arises from their substantially above-threshold resting potentials, input from sustained ON bipolar cells, absence of amacrine cell inhibition of presynaptic ON bipolar cells, and mGluR7-mediated maintenance of light-evoked glutamatergic input. All three types of direction-selective RGCs receive input from transient ON bipolar cells, and each type uses additional strategies to promote photoresponse transience: presynaptic inhibition and dopaminergic modulation for TRHR+ cells, center/surround antagonism and relatively negative resting potentials for Hoxd10+ ON cells, and presynaptic inhibition for Hoxd10+ ON-OFF cells. We find that the sustained nature of ipRGCs' rod/cone-driven responses depends neither on melanopsin nor on N-methyl-d-aspartate (NMDA) receptors, whereas the transience of the direction-selective cells' responses is influenced neither by α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainate receptor desensitization nor by glutamate uptake. For all cells, we further rule out spike frequency adaptation and intracellular Ca2+ as determinants of photoresponse kinetics. In conclusion, different RGC types use diverse mechanisms to produce sustained or transient light responses. Parenthetically, we find evidence in both mice and rats that the kinetics of light-induced mGluR6 deactivation determines whether an ON bipolar cell responds tonically or transiently to light.

A Mammalian Retinal Ganglion Cell Implements a Neuronal Computation That Maximizes the SNR of Its Postsynaptic Currents.

Neurons perform computations by integrating excitatory and inhibitory synaptic inputs. Yet, it is rarely understood what computation is being performed, or how much excitation or inhibition this computation requires. Here we present evidence for a neuronal computation that maximizes the signal-to-noise power ratio (SNR). We recorded from OFF delta retinal ganglion cells in the guinea pig retina and monitored synaptic currents that were evoked by visual stimulation (flashing dark spots). These synaptic currents were mediated by a decrease in an outward current from inhibitory synapses (disinhibition) combined with an increase in an inward current from excitatory synapses. We found that the SNR of combined excitatory and disinhibitory currents was voltage sensitive, peaking at membrane potentials near resting potential. At the membrane potential for maximal SNR, the amplitude of each current, either excitatory or disinhibitory, was proportional to its SNR. Such proportionate scaling is the theoretically best strategy for combining excitatory and disinhibitory currents to maximize the SNR of their combined current. Moreover, as spot size or contrast changed, the amplitudes of excitatory and disinhibitory currents also changed but remained in proportion to their SNRs, indicating a dynamic rebalancing of excitatory and inhibitory currents to maximize SNR.SIGNIFICANCE STATEMENT We present evidence that the balance of excitatory and disinhibitory inputs to a type of retinal ganglion cell maximizes the signal-to-noise ratio power ratio (SNR) of its postsynaptic currents. This is significant because chemical synapses on a retinal ganglion cell require the probabilistic release of transmitter. Consequently, when the same visual stimulus is presented repeatedly, postsynaptic currents vary in amplitude. Thus, maximizing SNR may be a strategy for producing the most reliable signal possible given the inherent unreliability of synaptic transmission.