Multiple pathways of inhibition shape bipolar cell responses in the retina

Brief report: effects of methylphenidate on the light adapted electroretinogram

PURPOSE

To explore changes in the electroretinogram (ERG) following methylphenidate use in attention-deficit/hyperactivity disorder (ADHD).

METHODS

Light adapted ERGs were recorded in five individuals (3 male and 2 female, age range 13.6-21.8 years) with a diagnosis of ADHD. Six flash strengths ranging from 71 to 446 Td.s were qualitatively evaluated following a minimum of 24 h without any medication and from 2 to 6 h following the individuals' standard slow-release (XL) methylphenidate dose that ranged from 18 to 60 mg.

RESULTS

Of the six flash strengths, the 178 Td.s strength revealed changes in four of the five participants with a median 27.4% increase in b-wave amplitude. For three individuals there was an increase in the a-wave amplitude and for two of the same individuals there was also a noticeable pronouncement of the oscillatory potentials. The a-wave amplitude showed a greatest median increase at the 446 Td.s flash strength of 25.8%. One individual - on the highest dose (60 mg) exhibited no morphologically distinct changes in the ERG. No differences in the time to peaks of the a- and b-wave were observed for any individual.

CONCLUSION

The a- and b-wave amplitudes of the light adapted ERG could provide insights into the effect of methylphenidate in ADHD.

pH in the vertebrate retina and its naturally occurring and pathological changes

This review summarizes the existing information on the concentration of H+ (pH) in vertebrate retinae and its changes due to various reasons. Special features of H+ homeostasis that make it different from other ions will be discussed, particularly metabolic production of H+ and buffering. The transretinal distribution of extracellular H+ concentration ([H+]o) and its changes under illumination and other conditions will be described in detail, since [H+]o is more intensively investigated than intracellular pH. In vertebrate retinae, the highest [H+]o occurs in the inner part of the outer nuclear layer, and decreases toward the RPE, reaching the blood level on the apical side of the RPE. [H+]o falls toward the vitreous as well, but less, so that the inner retina is acidic to the vitreous. Light leads to complex changes with both electrogenic and metabolic origins, culminating in alkalinization. There is a rhythm of [H+]o with H+ being higher during circadian night. Extracellular pH can potentially be used as a signal in intercellular volume transmission, but evidence is against pH as a normal controller of fluid transport across the RPE or as a horizontal cell feedback signal. Pathological and experimentally created conditions (systemic metabolic acidosis, hypoxia and ischemia, vascular occlusion, excess glucose and diabetes, genetic disorders, and blockade of carbonic anhydrase) disturb H+ homeostasis, mostly producing retinal acidosis, with consequences for retinal blood flow, metabolism and function.

Synaptotagmin-9 in mouse retina

Synaptotagmin-9 (Syt9) is a Ca2+ sensor mediating fast synaptic release expressed in various parts of the brain. The presence and role of Syt9 in retina is unknown. We found evidence for Syt9 expression throughout the retina and created mice to conditionally eliminate Syt9 in a cre-dependent manner. We crossed Syt9fl/fl mice with Rho-iCre, HRGP-Cre, and CMV-Cre mice to generate mice in which Syt9 was eliminated from rods (rodSyt9CKO), cones (coneSyt9CKO), or whole animals (CMVSyt9). CMVSyt9 mice showed an increase in scotopic electroretinogram (ERG) b-waves evoked by bright flashes with no change in a-waves. Cone-driven photopic ERG b-waves were not significantly different in CMVSyt9 knockout mice and selective elimination of Syt9 from cones had no effect on ERGs. However, selective elimination from rods decreased scotopic and photopic b-waves as well as oscillatory potentials. These changes occurred only with bright flashes where cone responses contribute. Synaptic release was measured in individual rods by recording anion currents activated by glutamate binding to presynaptic glutamate transporters. Loss of Syt9 from rods had no effect on spontaneous or depolarization-evoked release. Our data show that Syt9 acts at multiple sites in the retina and suggest that it may play a role in regulating transmission of cone signals by rods.

ElectroRetinoGraphy toward an exploration of the therapeutic potential of antidepressants in patients with major depressive disorder: A scoping review of the literature

Major Depressive Disorder (MDD) is characterized by at least one major depressive episode. It requires medical attention typically involving the prescription of antidepressants. Remission in MDD patients is often difficult to achieve because of the limited effectiveness of these drugs. Nowadays, numerous patients undergo various antidepressant treatments, with subjective changes in their personal experiences being regularly monitored. Therefore, it is essential to find clinical and objective tools that offer a more tailored approach to antidepressant selection. The neurochemistry of the retina being similar to the brain, one promising approach would be to use ElectroRetinoGraphy (ERG) measurements on MDD patients requiring antidepressant treatment. Thus, the aim of this scoping review is to highlight effects of different classes of antidepressants on retinal function evaluated by full-field ERG (ffERG), Pattern ERG (PERG) and multifocal ERG (mfERG) waveforms in MDD patients. These ERG measurements could serve as pivotal indicators in defining patient profiles, facilitating a more objective and personalized approach to therapeutic interventions, thereby advancing precision psychiatry.

Dynamic endocannabinoid-mediated neuromodulation of retinal circadian circuitry

Circadian rhythms are biological rhythms that originate from the "master circadian clock," called the suprachiasmatic nucleus (SCN). SCN orchestrates the circadian rhythms using light as a chief zeitgeber, enabling humans to synchronize their daily physio-behavioral activities with the Earth's light-dark cycle. However, chronic/ irregular photic disturbances from the retina via the retinohypothalamic tract (RHT) can disrupt the amplitude and the expression of clock genes, such as the period circadian clock 2, causing circadian rhythm disruption (CRd) and associated neuropathologies. The present review discusses neuromodulation across the RHT originating from retinal photic inputs and modulation offered by endocannabinoids as a function of mitigation of the CRd and associated neuro-dysfunction. Literature indicates that cannabinoid agonists alleviate the SCN's ability to get entrained to light by modulating the activity of its chief neurotransmitter, i.e., γ-aminobutyric acid, thus preventing light-induced disruption of activity rhythms in laboratory animals. In the retina, endocannabinoid signaling modulates the overall gain of the retinal ganglion cells by regulating the membrane currents (Ca2+, K+, and Cl- channels) and glutamatergic neurotransmission of photoreceptors and bipolar cells. Additionally, endocannabinoids signalling also regulate the high-voltage-activated Ca2+ channels to mitigate the retinal ganglion cells and intrinsically photosensitive retinal ganglion cells-mediated glutamate release in the SCN, thus regulating the RHT-mediated light stimulation of SCN neurons to prevent excitotoxicity. As per the literature, cannabinoid receptors 1 and 2 are becoming newer targets in drug discovery paradigms, and the involvement of endocannabinoids in light-induced CRd through the RHT may possibly mitigate severe neuropathologies.

Non-canonical type 1 cannabinoid receptor signaling regulates night visual processing in the inner rat retina

Type 1 cannabinoid receptors (CB1Rs) are expressed in major retinal neurons within the rod-pathway suggesting a role in regulating night visual processing, but the underlying mechanisms remain poorly understood. Using acute rat retinal slices, we show that CB1R activation reduces glutamate release from rod bipolar cell (RBC) axon terminals onto AII and A17 amacrine cells through a pathway that requires exchange proteins directly activated by cAMP (EPAC1/2) signaling. Consequently, CB1R activation abrogates reciprocal GABAergic feedback inhibition from A17 amacrine cells. Moreover, the activation of CB1Rs in vivo enhances and prolongs the time course of the dim-light rod-driven visual responses, an effect that was eliminated when both GABAA and GABAC receptors were blocked. Altogether, our findings underscore a non-canonical mechanism by which cannabinoid signaling regulates RBC dyad synapses in the inner retina to regulate dim-light visual responses to fine-tune night vision.

GABA receptors mediate adaptation and sensitization processes in mouse retinal ganglion cells

Two coordinated dynamic properties (adaptation and sensitization) are observed in retinal ganglion cells (RGCs) under the contrast stimulation. During sustained high-contrast period, adaptation decreases RGCs' responses while sensitization increases RGCs' responses. In mouse retina, adaptation and sensitization respectively show OFF- and ON-pathway-dominance. However, the mechanisms which drive the differentiation between adaptation and sensitization remain unclear. In the present study, multi-electrode recordings were conducted on isolated mouse retina under full-field contrast stimulation. Dynamic property was quantified based on the trend of RGC's firing rate during high-contrast period, light sensitivity was estimated by linear-nonlinear analysis and coding ability was estimated through stimulus reconstruction algorism. γ-Aminobutyric acid (GABA) receptors were pharmacologically blocked to explore the relation between RGCs' dynamic property and the activity of GABA receptors. It was found that GABAA and GABAC receptors respectively mediated the adaptation and sensitization processes in RGCs' responses. RGCs' dynamic property changes occurred after the blockage of GABA receptors were related to the modulation of the cells' light sensitivity. Further, the blockage of GABAA (GABAC) receptor significantly decreased RGCs' overall coding ability and eliminated the functional benefits of adaptation (sensitization). Our work suggests that the dynamic property of individual RGC is related to the balance between its GABAA-receptor-mediated inputs and GABAC-receptor-mediated inputs. Blockage of GABA receptors breaks the balance of retinal circuitry for signal processing, and down-regulates the visual information coding ability.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11571-023-09950-2.

Contribution of chemical and electrical transmission to the low delta-like intrinsic retinal oscillation in mice: A role for daylight-activated neuromodulators

Basal electroretinogram (ERG) oscillations have shown predictive value for modifiable risk factors for type 2 diabetes. However, their origin remains unknown. Here, we seek to establish the pharmacological profile of the low delta-like (δ1) wave in the mouse because it shows light sensitivity in the form of a decreased peak frequency upon photopic exposure. Applying neuropharmacological drugs by intravitreal injection, we eliminated the δ1 wave using lidocaine or by blocking all chemical and electrical synapses. The δ1 wave was insensitive to the blockade of photoreceptor input, but was accelerated when all inhibitory or ionotropic inhibitory receptors in the retina were antagonized. The sole blockade of GABAA, GABAB, GABAC, and glycine receptors also accelerated the δ1 wave. In contrast, the gap junction blockade slowed the δ1 wave. Both GABAA receptors and gap junctions contribute to the light sensitivity of the δ1 wave. We further found that the day light-activated neuromodulators dopamine and nitric oxide donors mimicked the effect of photopic exposure on the δ1 wave. All drug effects were validated through light flash-evoked ERG responses. Our data indicate that the low δ-like intrinsic wave detected by the non-photic ERG arises from an inner retinal circuit regulated by inhibitory neurotransmission and nitric oxide/dopamine-sensitive gap junction-mediated communication.

SYNAPTOTAGMIN-9 IN MOUSE RETINA

Synaptotagmin-9 (Syt9) is a Ca2+ sensor mediating fast synaptic release expressed in various parts of the brain. The presence and role of Syt9 in retina is unknown. We found evidence for Syt9 expression throughout the retina and created mice to conditionally eliminate Syt9 in a cre-dependent manner. We crossed Syt9fl/fl mice with Rho-iCre, HRGP-Cre, and CMV-cre mice to generate mice in which Syt9 was eliminated from rods (rodSyt9CKO), cones (coneSyt9CKO), or whole animals (CMVSyt9). CMVSyt9 mice showed an increase in scotopic electroretinogram (ERG) b-waves evoked by bright flashes with no change in a-waves. Cone-driven photopic ERG b-waves were not significantly different in CMVSyt9 knockout mice and selective elimination of Syt9 from cones had no effect on ERGs. However, selective elimination from rods decreased scotopic and photopic b-waves as well as oscillatory potentials. These changes occurred only with bright flashes where cone responses contribute. Synaptic release was measured in individual rods by recording anion currents activated by glutamate binding to presynaptic glutamate transporters. Loss of Syt9 from rods had no effect on spontaneous or depolarization-evoked release. Our data show that Syt9 is acts at multiple sites in the retina and suggest that it may play a role in regulating transmission of cone signals by rods.

Nfia Is Critical for AII Amacrine Cell Production: Selective Bipolar Cell Dependencies and Diminished ERG

The nuclear factor one (NFI) transcription factor genes Nfia, Nfib, and Nfix are all enriched in late-stage retinal progenitor cells, and their loss has been shown to retain these progenitors at the expense of later-generated retinal cell types. Whether they play any role in the specification of those later-generated fates is unknown, but the expression of one of these, Nfia, in a specific amacrine cell type may intimate such a role. Here, Nfia conditional knockout (Nfia-CKO) mice (both sexes) were assessed, finding a massive and largely selective absence of AII amacrine cells. There was, however, a partial reduction in type 2 cone bipolar cells (CBCs), being richly interconnected to AII cells. Counts of dying cells showed a significant increase in Nfia-CKO retinas at postnatal day (P)7, after AII cell numbers were already reduced but in advance of the loss of type 2 CBCs detected by P10. Those results suggest a role for Nfia in the specification of the AII amacrine cell fate and a dependency of the type 2 CBCs on them. Delaying the conditional loss of Nfia to the first postnatal week did not alter AII cell number nor differentiation, further suggesting that its role in AII cells is solely associated with their production. The physiological consequences of their loss were assessed using the ERG, finding the oscillatory potentials to be profoundly diminished. A slight reduction in the b-wave was also detected, attributed to an altered distribution of the terminals of rod bipolar cells, implicating a role of the AII amacrine cells in constraining their stratification.SIGNIFICANCE STATEMENT The transcription factor NFIA is shown to play a critical role in the specification of a single type of retinal amacrine cell, the AII cell. Using an Nfia-conditional knockout mouse to eliminate this population of retinal neurons, we demonstrate two selective bipolar cell dependencies on the AII cells; the terminals of rod bipolar cells become mis-stratified in the inner plexiform layer, and one type of cone bipolar cell undergoes enhanced cell death. The physiological consequence of this loss of the AII cells was also assessed, finding the cells to be a major contributor to the oscillatory potentials in the electroretinogram.

Emerging computational motifs: Lessons from the retina

The retinal neuronal circuit is the first stage of visual processing in the central nervous system. The efforts of scientists over the last few decades indicate that the retina is not merely an array of photosensitive cells, but also a processor that performs various computations. Within a thickness of only ∼200 µm, the retina consists of diverse forms of neuronal circuits, each of which encodes different visual features. Since the discovery of direction-selective cells by Horace Barlow and Richard Hill, the mechanisms that generate direction selectivity in the retina have remained a fascinating research topic. This review provides an overview of recent advances in our understanding of direction-selectivity circuits. Beyond the conventional wisdom of direction selectivity, emerging findings indicate that the retina utilizes complicated and sophisticated mechanisms in which excitatory and inhibitory pathways are involved in the efficient encoding of motion information. As will become evident, the discovery of computational motifs in the retina facilitates an understanding of how sensory systems establish feature selectivity.

IRIS: Integrated Retinal Functionality in Image Sensors

Neuromorphic image sensors draw inspiration from the biological retina to implement visual computations in electronic hardware. Gain control in phototransduction and temporal differentiation at the first retinal synapse inspired the first generation of neuromorphic sensors, but processing in downstream retinal circuits, much of which has been discovered in the past decade, has not been implemented in image sensor technology. We present a technology-circuit co-design solution that implements two motion computations-object motion sensitivity and looming detection-at the retina's output that could have wide applications for vision-based decision-making in dynamic environments. Our simulations on Globalfoundries 22 nm technology node show that the proposed retina-inspired circuits can be fabricated on image sensing platforms in existing semiconductor foundries by taking advantage of the recent advances in semiconductor chip stacking technology. Integrated Retinal Functionality in Image Sensors (IRIS) technology could drive advances in machine vision applications that demand energy-efficient and low-bandwidth real-time decision-making.

Center-surround interactions underlie bipolar cell motion sensitivity in the mouse retina

Motion sensing is a critical aspect of vision. We studied the representation of motion in mouse retinal bipolar cells and found that some bipolar cells are radially direction selective, preferring the origin of small object motion trajectories. Using a glutamate sensor, we directly observed bipolar cells synaptic output and found that there are radial direction selective and non-selective bipolar cell types, the majority being selective, and that radial direction selectivity relies on properties of the center-surround receptive field. We used these bipolar cell receptive fields along with connectomics to design biophysical models of downstream cells. The models and additional experiments demonstrated that bipolar cells pass radial direction selective excitation to starburst amacrine cells, which contributes to their directional tuning. As bipolar cells provide excitation to most amacrine and ganglion cells, their radial direction selectivity may contribute to motion processing throughout the visual system.

Calcium Channels in Retinal Function and Disease

Voltage-gated Ca2+ (Cav) channels play pivotal roles in regulating gene transcription, neuronal excitability, and neurotransmitter release. To meet the spatial and temporal demands of visual signaling, Cav channels exhibit unusual properties in the retina compared to their counterparts in other areas of the nervous system. In this article, we review current concepts regarding the specific subtypes of Cav channels expressed in the retina, their intrinsic properties and forms of modulation, and how their dysregulation could lead to retinal disease.

The glutamate/GABA system in the retina of male rats: effects of aging, neurodegeneration, and supplementation with melatonin and antioxidant SkQ1

Glutamate and -aminobutyric acid (GABA) are the most abundant amino acids in the retina. An imbalance of the glutamate/GABA system is involved in the pathogenesis of various neurodegenerative disorders. Here we for the first time analyzed alterations of expression of glutamate- and GABA-synthesizing enzymes, transporters, and relevant receptors in the retina with age in Wistar rats and in senescence-accelerated OXYS rats who develop AMD-like retinopathy. We noted consistent age-dependent expression changes of GABAergic-system proteins (GAD67, GABA-T, and GAT1) in OXYS and Wistar rats: upregulation by age 3 months and downregulation at age 18 months. At a late stage of AMD-like retinopathy in OXYS rats (18 months), there was significant upregulation of glutaminase and downregulation of glutamine synthetase, possibly indicating an increasing level of glutamate in the retina. AMD-like-retinopathy development in the OXYS strain was accompanied by underexpression of glutamate transporter GLAST. Prolonged supplementation with both melatonin and SkQ1 (separately) suppressed the progression of the AMD-like pathology in OXYS rats without affecting the glutamate/GABA system but worsened the condition of the Wistar rat's retina during normal aging. We observed decreasing protein levels of glutamine synthetase, GLAST, and GABAAR1 and an increasing level of glutaminase in Wistar rats. In summary, both melatonin and mitochondrial antioxidant SkQ1 had different effect on the retinal glutamate / GABA in healthy Wistar and senescence-accelerated OXYS rats.

Transience of the Retinal Output Is Determined by a Great Variety of Circuit Elements

Retinal ganglion cells (RGCs) encrypt stimulus features of the visual scene in action potentials and convey them toward higher visual centers in the brain. Although there are many visual features to encode, our recent understanding is that the ~46 different functional subtypes of RGCs in the retina share this task. In this scheme, each RGC subtype establishes a separate, parallel signaling route for a specific visual feature (e.g., contrast, the direction of motion, luminosity), through which information is conveyed. The efficiency of encoding depends on several factors, including signal strength, adaptational levels, and the actual efficacy of the underlying retinal microcircuits. Upon collecting inputs across their respective receptive field, RGCs perform further analysis (e.g., summation, subtraction, weighting) before they generate the final output spike train, which itself is characterized by multiple different features, such as the number of spikes, the inter-spike intervals, response delay, and the rundown time (transience) of the response. These specific kinetic features are essential for target postsynaptic neurons in the brain in order to effectively decode and interpret signals, thereby forming visual perception. We review recent knowledge regarding circuit elements of the mammalian retina that participate in shaping RGC response transience for optimal visual signaling.

Inhibition, but not excitation, recovers from partial cone loss with greater spatiotemporal integration, synapse density, and frequency

Neural circuits function in the face of changing inputs, either caused by normal variation in stimuli or by cell death. To maintain their ability to perform essential computations with partial inputs, neural circuits make modifications. Here, we study the retinal circuit’s responses to changes in light stimuli or in photoreceptor inputs by inducing partial cone death in the mature mouse retina. Can the retina withstand or recover from input loss? We find that the excitatory pathways exhibit functional loss commensurate with cone death and with some aspects predicted by partial light stimulation. However, inhibitory pathways recover functionally from lost input by increasing spatiotemporal integration in a way that is not recapitulated by partially stimulating the control retina. Anatomically, inhibitory synapses are upregulated on secondary bipolar cells and output ganglion cells. These findings demonstrate the greater capacity for inhibition, compared with excitation, to modify spatiotemporal processing with fewer cone inputs.

Lee et al. find partial cone loss triggers inhibition, but not excitation, to increase spatiotemporal integration, recover contrast gain, and increase synaptic release onto retinal ganglion cells. Natural images filtered by cone-loss receptive fields perceptually match those of controls. Thus, inhibition compensates for fewer cones to potentially preserve perception.

Transient expression of a GABA receptor subunit during early development is critical for inhibitory synapse maturation and function

Developing neural circuits, including GABAergic circuits, switch receptor types. But the role of early GABA receptor expression for establishment of functional inhibitory circuits remains unclear. Tracking the development of GABAergic synapses across axon terminals of retinal bipolar cells (BCs), we uncovered a crucial role of early GABA_A receptor expression for the formation and function of presynaptic inhibitory synapses. Specifically, early α3-subunit-containing GABA_A (GABA_Aα3) receptors are a key developmental organizer. Before eye opening, GABA_Aα3 gives way to GABA_Aα1 at individual BC presynaptic inhibitory synapses. The developmental downregulation of GABA_Aα3 is independent of GABA_Aα1 expression. Importantly, lack of early GABA_Aα3 impairs clustering of GABA_Aα1 and formation of functional GABA_A synapses across mature BC terminals. This impacts the sensitivity of visual responses transmitted through the circuit. Lack of early GABA_Aα3 also perturbs aggregation of LRRTM4, the organizing protein at GABAergic synapses of rod BC terminals, and their arrangement of output ribbon synapses.

Early Functional Impairment in Experimental Glaucoma Is Accompanied by Disruption of the GABAergic System and Inceptive Neuroinflammation

Glaucoma is a leading cause of irreversible blindness worldwide, and increased intraocular pressure (IOP) is a major risk factor. We aimed to determine if early functional and molecular differences in the glaucomatous retina manifest before significant retinal ganglion cell (RGC) loss is apparent. Adenoviral vectors expressing a pathogenic form of myocilin (Ad5.MYOC) were used to induce IOP elevation in C57BL/6 mice. IOP and pattern electroretinograms (pERG) were recorded, and retinas were prepared for RNA sequencing, immunohistochemistry, or to determine RGC loss. Ocular injection of Ad5.MYOC leads to reliable IOP elevation, resulting in significant loss of RGC after nine weeks. A significant decrease in the pERG amplitude was evident in eyes three weeks after IOP elevation. Retinal gene expression analysis revealed increased expression for 291 genes related to complement cascade, inflammation, and antigen presentation in hypertensive eyes. Decreased expression was found for 378 genes associated with the γ-aminobutyric acid (GABA)ergic and glutamatergic systems and axon guidance. These data suggest that early functional changes in RGC might be due to reduced GABAA receptor signaling and neuroinflammation that precedes RGC loss in this glaucoma model. These initial changes may offer new targets for early detection of glaucoma and the development of new interventions.

Purinergic-Glycinergic Interaction in Neurodegenerative and Neuroinflammatory Disorders of the Retina

Neurodegenerative–neuroinflammatory disorders of the retina seriously hamper human vision. In searching for key factors that contribute to the development of these pathologies, we considered potential interactions among purinergic neuromodulation, glycinergic neurotransmission, and microglia activity in the retina. Energy deprivation at cellular levels is mainly due to impaired blood circulation leading to increased release of ATP and adenosine as well as glutamate and glycine. Interactions between these modulators and neurotransmitters are manifold. First, P2Y purinoceptor agonists facilitate reuptake of glycine by glycine transporter 1, while its inhibitors reduce reverse-mode operation; these events may lower extracellular glycine levels. The consequential changes in extracellular glycine concentration can lead to parallel changes in the activity of NR1/NR2B type NMDA receptors of which glycine is a mandatory agonist, and thereby may reduce neurodegenerative events in the retina. Second, P2Y purinoceptor agonists and glycine transporter 1 inhibitors may indirectly inhibit microglia activity by decreasing neuronal or glial glycine release in energy-compromised retina. These inhibitions may have a role in microglia activation, which is present during development and progression of neurodegenerative disorders such as glaucomatous and diabetic retinopathies and age-related macular degeneration or loss of retinal neurons caused by thromboembolic events. We have hypothesized that glycine transporter 1 inhibitors and P2Y purinoceptor agonists may have therapeutic importance in neurodegenerative–neuroinflammatory disorders of the retina by decreasing NR1/NR2B NMDA receptor activity and production and release of a series of proinflammatory cytokines from microglial cells.

GABAA presynaptic inhibition regulates the gain and kinetics of retinal output neurons

Output signals of neural circuits, including the retina, are shaped by a combination of excitatory and inhibitory signals. Inhibitory signals can act presynaptically on axon terminals to control neurotransmitter release and regulate circuit function. However, it has been difficult to study the role of presynaptic inhibition in most neural circuits due to lack of cell type-specific and receptor type-specific perturbations. In this study, we used a transgenic approach to selectively eliminate GABA_A inhibitory receptors from select types of second-order neurons - bipolar cells - in mouse retina and examined how this affects the light response properties of the well-characterized ON alpha ganglion cell retinal circuit. Selective loss of GABA_A receptor-mediated presynaptic inhibition causes an enhanced sensitivity and slower kinetics of light-evoked responses from ON alpha ganglion cells thus highlighting the role of presynaptic inhibition in gain control and temporal filtering of sensory signals in a key neural circuit in the mammalian retina.

Glycinergic Inhibition Targets Specific Off Cone Bipolar Cells in Primate Retina

Adapting between scotopic and photopic illumination involves switching the routing of retinal signals between rod and cone-dominated circuits. In the daytime, cone signals pass through parallel On and Off cone bipolar cells (CBCs), that are sensitive to increments and decrements in luminance, respectively. At night, rod signals are routed into these cone-pathways via a key glycinergic interneuron, the AII amacrine cell (AII-AC). AII-ACs also provide On-pathway-driven crossover inhibition to Off-CBCs under photopic conditions. In primates, it is not known whether all Off-bipolar cell types receive functional inputs from AII-ACs. Here, we show that select Off-CBC types receive significantly higher levels of On-pathway-driven glycinergic input than others. The rise and decay kinetics of the glycinergic events are consistent with involvement of the α1 glycine receptor (GlyR) subunit, a result supported by a higher level of GLRA1 transcript in these cells. The Off-bipolar types that receive glycinergic input have sustained physiological properties and include the flat midget bipolar (FMB) cells, which provide excitatory input to the Off-midget ganglion cells (GCs; parvocellular pathway). Our results suggest that only a subset of Off-bipolar cells have the requisite receptors to respond to AII-AC input. Taken together with results in mouse retina, our findings suggest a conserved motif whereby signal output from AII-ACs is preferentially routed into sustained Off-bipolar signaling pathways.

Limited Spatial Spread Explains the Dependence of Visual Response Adaptation on Stimulus Size in Rat Superior Colliculus Neurons

Although adaptation to light occurs in the eye and mainly preserves the full dynamic range of neuronal responses during changing background illumination, it affects the entire visual system and helps to optimize visual information processing. We have shown recently that in rat superior colliculus (SC) neurons adaptation to light acts as a local low-pass filter because, in contrast to the primate SC, in rat collicular neurons adaptation to small stimuli is largely limited to the vicinity of the adaptor stimulus. However, it was unclear whether large visual stimuli would induce the same spatially limited adaptation. We addressed this question by evaluating the effects of 1.8°, 6.2° and 20.8° wide adaptor stimuli on test stimuli of variable size. Single unit recordings in the adult rat SC were employed to estimate the response amplitude. Small, 1.8° and 6.2° adaptors habituated visual responses only to stimuli smaller than the adaptive stimuli. However, the 20.8° adaptor dramatically reduced responses even to test stimuli >3 times wider than the adaptor (up to 70° wide). The latter result may be explained by a nearly complete occlusion by a large adaptor of the neuron's receptive field (RF). All these results are consistent with the idea of a limited spatial spread of adaptation in rat SC neurons that is the consequence of high convergence of retinal inputs, in which small RFs limit the spatial spread of adaptation. It is concluded that, in this limited spatial spread of adaptation, rodent SC resembles higher visual system areas in primates and indicates potential differences in visual information processing between rodents and primates.

GABAergic and glycinergic systems regulate ON-OFF electroretinogram by cooperatively modulating cone pathways in the amphibian retina

The network mechanisms underlying how inhibitory circuits regulate ON- and OFF-responses (the b- and d-waves) in the electroretinogram (ERG) remain unclear. The purpose of this study was to investigate the contribution of inhibitory circuits to the emergence of the b- and d-waves in the full-field ERG in the newt retina. To this end, we investigated the effects of several synaptic transmission blockers on the amplitudes of the b- and d-waves in the ERG obtained from newt eyecup preparations. Our results demonstrated that (a) L-APB blocked the b-wave, indicating that the b-wave arises from the activity of ON-bipolar cells (BCs) expressing type six metabotropic glutamate receptors; (b) the combined administration of UBP310/GYKI 53655 blocked the d-wave, indicating that the d-wave arises from the activity of OFF-BCs expressing kainate-/AMPA-receptors; (c) SR 95531 augmented both the b- and the d-wave, indicating that GABAergic lateral inhibitory circuits inhibit both ON- and OFF-BC pathways; (d) the administration of strychnine in the presence of SR 95531 attenuated the d-wave, and this attenuation was prevented by blocking ON-pathways with L-APB, which indicated that the glycinergic inhibition of OFF-BC pathway is downstream of the GABAergic inhibition of the ON-system; and (e) the glycinergic inhibition from the ON- to the OFF-system widens the response range of OFF-BC pathways, specifically in the absence of GABAergic lateral inhibition. Based on these results, we proposed a circuitry mechanism for the regulation of the d-wave and offered a tentative explanation of the circuitry mechanisms underlying ERG formation.

Glycinergic and GABAergic interneurons shift the location and differentially alter the size of ganglion cell receptive field centers in the mammalian retina

By using the multi-electrode array (MEA) recording technique in conjunction with white-noise checkerboard stimuli and reverse correlation methods, we studied modulatory actions of glycinergic and GABAergic interneurons on spatiotemporal profiles of ganglion cells (GCs) in dark-adapted mouse retinas. We found that application of 2 µM strychnine decreased receptive field center radii of GCs by a mean value of 11%, and shifted the GC receptive field (RF) centers by a mean distance of 28.3 µm. On the other hand, 200 µM picrotoxin + 100 µM bicuculline + 50 µM TPMPA increased GC receptive field center radii by a mean value of 19%, and shifted the GC RF centers by a mean distance of 53.7 µm. Glycinergic neurons in the mouse retina are narrow-field amacrine cells that have been shown to mediate ON-OFF crossover inhibitory synapses within the RGs' RF center, therefore they may increase the size and shift the location of GC RF center by synergistic addition to bipolar cell inputs to GCs. GABAergic neurons are wide-field amacrine cells and horizontal cells that are known to mediate antagonistic surround responses of GCs, and thus they decrease the GCs' RF center size. Our results suggest that a major global function of glycinergic and GABAergic interneurons in the mammalian retina is to provide the flexibility for adjusting the size and location of GCs' RF centers. The apparent shifts of GC RF centers suggest that the synergistic addition by GlyACs and the surround inhibition by GABAergic interneurons are not spatially symmetrical within GC RFs.

Light-evoked glutamate transporter EAAT5 activation coordinates with conventional feedback inhibition to control rod bipolar cell output

In the retina, modulation of the amplitude of dim visual signals primarily occurs at axon terminals of rod bipolar cells (RBCs). GABA and glycine inhibitory neurotransmitter receptors and the excitatory amino acid transporter 5 (EAAT5) modulate the RBC output. EAATs clear glutamate from the synapse, but they also have a glutamate-gated chloride conductance. EAAT5 acts primarily as an inhibitory glutamate-gated chloride channel. The relative role of visually evoked EAAT5 inhibition compared with GABA and glycine inhibition has not been addressed. In this study, we determine the contribution of EAAT5-mediated inhibition onto RBCs in response to light stimuli in mouse retinal slices. We find differences and similarities in the two forms of inhibition. Our results show that GABA and glycine mediate nearly all lateral inhibition onto RBCs, as EAAT5 is solely a mediator of RBC feedback inhibition. We also find that EAAT5 and conventional GABA inhibition both contribute to feedback inhibition at all stimulus intensities. Finally, our in silico modeling compares and contrasts EAAT5-mediated to GABA- and glycine-mediated feedback inhibition. Both forms of inhibition have a substantial impact on synaptic transmission to the postsynaptic AII amacrine cell. Our results suggest that the late phase EAAT5 inhibition acts with the early phase conventional, reciprocal GABA inhibition to modulate the rod signaling pathway between rod bipolar cells and their downstream synaptic targets.NEW & NOTEWORTHY Excitatory amino acid transporter 5 (EAAT5) glutamate transporters have a chloride channel that is strongly activated by glutamate, which modulates excitatory signaling. We found that EAAT5 is a major contributor to feedback inhibition on rod bipolar cells. Inhibition to rod bipolar cells is also mediated by GABA and glycine. GABA and glycine mediate the early phase of feedback inhibition, and EAAT5 mediates a more delayed inhibition. Together, inhibitory transmitters and EAAT5 coordinate to mediate feedback inhibition, controlling neuronal output.

The temporal structure of the inner retina at a single glance

The retina decomposes visual stimuli into parallel channels that encode different features of the visual environment. Central to this computation is the synaptic processing in a dense layer of neuropil, the so-called inner plexiform layer (IPL). Here, different types of bipolar cells stratifying at distinct depths relay the excitatory feedforward drive from photoreceptors to amacrine and ganglion cells. Current experimental techniques for studying processing in the IPL do not allow imaging the entire IPL simultaneously in the intact tissue. Here, we extend a two-photon microscope with an electrically tunable lens allowing us to obtain optical vertical slices of the IPL, which provide a complete picture of the response diversity of bipolar cells at a "single glance". The nature of these axial recordings additionally allowed us to isolate and investigate batch effects, i.e. inter-experimental variations resulting in systematic differences in response speed. As a proof of principle, we developed a simple model that disentangles biological from experimental causes of variability and allowed us to recover the characteristic gradient of response speeds across the IPL with higher precision than before. Our new framework will make it possible to study the computations performed in the central synaptic layer of the retina more efficiently.

Vigabatrin-Induced Retinal Functional Alterations and Second-Order Neuron Plasticity in C57BL/6J Mice

Purpose

Vigabatrin (VGB) is an effective antiepileptic that increases concentrations of inhibitory γ-aminobutyric acid (GABA) by inhibiting GABA transaminase. Reports of VGB-associated visual field loss limit its clinical usefulness, and retinal toxicity studies in laboratory animals have yielded conflicting results.

Methods

We examined the functional and morphologic effects of VGB in C57BL/6J mice that received either VGB or saline IP from 10 to 18 weeks of age. Retinal structure and function were assessed in vivo by optical coherence tomography (OCT), ERG, and optomotor response. After euthanasia, retinas were processed for immunohistochemistry, and retinal GABA, and VGB quantified by mass spectrometry.

Results

No significant differences in visual acuity or total retinal thickness were identified between groups by optomotor response or optical coherence tomography, respectively. After 4 weeks of VGB treatment, ERG b-wave amplitude was enhanced, and amplitudes of oscillatory potentials were reduced. Dramatic rod and cone bipolar and horizontal cell remodeling, with extension of dendrites into the outer nuclear layer, was observed in retinas of VGB-treated mice. VGB treatment resulted in a mean 3.3-fold increase in retinal GABA concentration relative to controls and retinal VGB concentrations that were 20-fold greater than brain.

Conclusions

No evidence of significant retinal thinning or ERG a- or b-wave deficits were apparent, although we describe significant alterations in ERG b-wave and oscillatory potentials and in retinal cell morphology in VGB-treated C57BL/6J mice. The dramatic concentration of VGB in retina relative to the target tissue (brain), with a corresponding increase in retinal GABA, offers insight into the pathophysiology of VGB-associated visual field loss.

Functional Compartmentalization within Starburst Amacrine Cell Dendrites in the Retina

Dendrites in many neurons actively compute information. In retinal starburst amacrine cells, transformations from synaptic input to output occur within individual dendrites and mediate direction selectivity, but directional signal fidelity at individual synaptic outputs and correlated activity among neighboring outputs on starburst dendrites have not been examined systematically. Here, we record visually evoked calcium signals simultaneously at many individual synaptic outputs within single starburst amacrine cells in mouse retina. We measure visual receptive fields of individual output synapses and show that small groups of outputs are functionally compartmentalized within starburst dendrites, creating distinct computational units. Inhibition enhances compartmentalization and directional tuning of individual outputs but also decreases the signal-to-noise ratio. Simulations suggest, however, that the noise underlying output signal variability is well tolerated by postsynaptic direction-selective ganglion cells, which integrate convergent inputs to acquire reliable directional information.

Novel clinical features of glycine receptor antibody syndrome: A series of 17 cases

Objective

To describe novel clinical features of GlyRα1-IgG-positive patients.

Methods

Patients with a positive serum GlyRα1-IgG were identified during a 2-year period from July 2016 to December 2018 at 2 academic centers and followed prospectively. All patients in this series were evaluated in the Neuroimmunology and Autoimmune Neurology clinics at the University of Utah or the University of Colorado.

Results

Thirteen of 17 patients had phenotypes more typically associated with glutamic acid decarboxylase (GAD65) antibody syndromes, consisting of stiff-person syndrome (SPS) with parkinsonism or cerebellar signs. One patient with parkinsonism had a presentation similar to rapidly progressive multiple system atrophy with severe dysautonomia. Ten of 17 patients had various visual symptoms including visual snow, spider web-like images forming shapes and 3-dimensional images, palinopsia, photophobia, visual hallucinations, synesthesia, and intermittent diplopia. Three of 17 patients presented with primarily autoimmune epilepsy accompanied by psychiatric symptoms.

Conclusions

Clinicians should consider testing for GlyR antibodies in GAD65 antibody-negative or low-positive GAD65 antibody patients with SPS-like presentations, especially in the setting of atypical features such as visual disturbances, parkinsonism, or epilepsy.

Voltage- and calcium-gated ion channels of neurons in the vertebrate retina

In this review, we summarize studies investigating the types and distribution of voltage- and calcium-gated ion channels in the different classes of retinal neurons: rods, cones, horizontal cells, bipolar cells, amacrine cells, interplexiform cells, and ganglion cells. We discuss differences among cell subtypes within these major cell classes, as well as differences among species, and consider how different ion channels shape the responses of different neurons. For example, even though second-order bipolar and horizontal cells do not typically generate fast sodium-dependent action potentials, many of these cells nevertheless possess fast sodium currents that can enhance their kinetic response capabilities. Ca2+ channel activity can also shape response kinetics as well as regulating synaptic release. The L-type Ca2+ channel subtype, CaV1.4, expressed in photoreceptor cells exhibits specific properties matching the particular needs of these cells such as limited inactivation which allows sustained channel activity and maintained synaptic release in darkness. The particular properties of K+ and Cl- channels in different retinal neurons shape resting membrane potentials, response kinetics and spiking behavior. A remaining challenge is to characterize the specific distributions of ion channels in the more than 100 individual cell types that have been identified in the retina and to describe how these particular ion channels sculpt neuronal responses to assist in the processing of visual information by the retina.

Synaptic inhibition tunes contrast computation in the retina

Inhibition shapes activity and signal processing in neural networks through numerous mechanisms mediated by many different cell types. Here, we examined how one type of GABAergic interneuron in the retina, the A17 amacrine cell, influences visual information processing. Our results suggest that A17s, which make reciprocal feedback inhibitory synapses onto rod bipolar cell (RBC) synaptic terminals, extend the luminance range over which RBC synapses compute temporal contrast and enhance the reliability of contrast signals over this range. Inhibition from other amacrine cells does not influence these computational features. Although A17-mediated feedback is mediated by both GABAA and GABAC receptors, the latter plays the primary role in extending the range of contrast computation. These results identify specific functions for an inhibitory interneuron subtype, as well as specific synaptic receptors, in a behaviorally relevant neural computation.

Neural Mechanisms Mediating Motion Sensitivity in Parasol Ganglion Cells of the Primate Retina

Considerable theoretical and experimental effort has been dedicated to understanding how neural circuits detect visual motion. In primates, much is known about the cortical circuits that contribute to motion processing, but the role of the retina in this fundamental neural computation is poorly understood. Here, we used a combination of extracellular and whole-cell recording to test for motion sensitivity in the two main classes of output neurons in the primate retina-midget (parvocellular-projecting) and parasol (magnocellular-projecting) ganglion cells. We report that parasol, but not midget, ganglion cells are motion sensitive. This motion sensitivity is present in synaptic excitation and disinhibition from presynaptic bipolar cells and amacrine cells, respectively. Moreover, electrical coupling between neighboring bipolar cells and the nonlinear nature of synaptic release contribute to the observed motion sensitivity. Our findings indicate that motion computations arise far earlier in the primate visual stream than previously thought.

Direct measurement of bipolar cell responses to electrical stimulation in wholemount mouse retina

OBJECTIVE

This in vitro investigation examines the response of retinal bipolar cells to extracellular electrical stimulation.

APPROACH

In vitro investigations characterizing the response of retinal neurons to electrical stimulation have primarily focused on retinal ganglion cells because they are the output neurons of the retina and their superficial position in the retina makes them readily accessible to in vitro recording techniques. Thus, the majority of information regarding the response of inner retinal neurons has been inferred from ganglion cell activity. Here we use patch clamp electrophysiology to directly record electrically-evoked activity in bipolar cells within the inner retina of normal Tg(Gng13-EGFP)GI206Gsat and degenerate rd10 Tg(Gng13-EGFP)GI206Gsat mice using a wholemount preparation.

MAIN RESULTS

Bipolar cells respond to electrical stimulation with time-locked depolarizing voltage transients. The latency of the response declines with increases in stimulation amplitude. A desensitizing response is observed during repeated stimulation with 25 ms biphasic current pulses delivered at pulse rates greater than 6 pps. A burst of long-latency (200-1000 ms) inhibitory postsynaptic potentials are evoked by the stimulus and the burst exhibits evidence of a lower and upper stimulation threshold.

SIGNIFICANCE

These results provide insights into the various types of bipolar cell activity elicited by electrical stimulation and may be useful for future retinal prosthesis stimulation protocols. This investigation uses patch clamp electrophysiology to provide direct analysis of ON-type bipolar cell responses to electrical stimulation in a wholemount retina preparation. It explores the effects of variable stimulus amplitudes, pulse widths, and frequencies in both normal and degenerate retina. The analysis adds to a body of work largely based upon indirect measurements of bipolar cell activity, and the methodology demonstrates an alternative retina preparation technique in which to acquire single-cell activity.

Quantifying the effect of light activated outer and inner retinal inhibitory pathways on glutamate release from mixed bipolar cells

Inhibition mediated by horizontal and amacrine cells in the outer and inner retina, respectively, are fundamental components of visual processing. Here, our purpose was to determine how these different inhibitory processes affect glutamate release from ON bipolar cells when the retina is stimulated with full-field light of various intensities. Light-evoked membrane potential changes (ΔV_m ) were recorded directly from axon terminals of intact bipolar cells receiving mixed rod and cone inputs (Mbs) in slices of dark-adapted goldfish retina. Inner and outer retinal inhibition to Mbs was blocked with bath applied picrotoxin (PTX) and NBQX, respectively. Then, control and pharmacologically modified light responses were injected into axotomized Mb terminals as command potentials to induce voltage-gated Ca²⁺ influx (Q^Ca ) and consequent glutamate release. Stimulus-evoked glutamate release was quantified by the increase in membrane capacitance (ΔC_m ). Increasing depolarization of Mb terminals upon removal of inner and outer retinal inhibition enhanced the ΔV_m /Q^Ca ratio equally at a given light intensity and inhibition did not alter the overall relation between Q^Ca and ΔC_m . However, relative to control, light responses recorded in the presence of PTX and PTX + NBQX increased ΔC_m unevenly across different stimulus intensities: at dim stimulus intensities predominantly the inner retinal GABAergic inhibition controlled release from Mbs, whereas the inner and outer retinal inhibition affected release equally in response to bright stimuli. Furthermore, our results suggest that non-linear relationship between Q^Ca and glutamate release can influence the efficacy of inner and outer retinal inhibitory pathways to mediate Mb output at different light intensities.

Dopamine-Dependent Sensitization of Rod Bipolar Cells by GABA Is Conveyed through Wide-Field Amacrine Cells

The vertebrate retina has the remarkable ability to support visual function under conditions of limited illumination, including the processing of signals evoked by single photons. Dim-light vision is regulated by several adaptive mechanisms. The mechanism explored in this study is responsible for increasing the light sensitivity and operational range of rod bipolar cells, the retinal neurons operating immediately downstream of rod photoreceptors. This sensitization is achieved through the sustained dopamine-dependent GABA release from other retinal neurons. Our goals were to identify the cell type responsible for the GABA release and the site of its modulation by dopamine. Previous studies have suggested the involvement of amacrine and/or horizontal cells. We now demonstrate, using mice of both sexes, that horizontal cells do not participate in this mechanism. Instead, sustained GABA input is provided by a subpopulation of wide-field amacrine cells, which stimulate the GABA_C receptors at rod bipolar cell axons. We also found that dopamine does not act directly on either of these cells. Rather, it suppresses inhibition imposed on these wide-field cells by another subpopulation of upstream GABAergic amacrine cells, thereby sustaining the GABA_C receptor activation required for rod bipolar cell sensitization.SIGNIFICANCE STATEMENT The vertebrate retina has an exquisite ability to adjust information processing to ever-changing conditions of ambient illumination, from bright sunlight to single-photon counting under dim starlight. Operation under each of these functional regimes requires an engagement of specific adaptation mechanisms. Here, we describe a mechanism optimizing the performance of the dim-light channel of vision, which consists of sensitizing rod bipolar cells by a sustained GABAergic input originating from a population of wide-field amacrine cells. Wide-field amacrine cells span large segments of the retina, making them uniquely equipped to normalize and optimize response sensitivity across distant receptive fields and preclude any bias toward local light-intensity fluctuations.

Multiplexed computations in retinal ganglion cells of a single type

In the early visual system, cells of the same type perform the same computation in different places of the visual field. How these cells code together a complex visual scene is unclear. A common assumption is that cells of a single-type extract a single-stimulus feature to form a feature map, but this has rarely been observed directly. Using large-scale recordings in the rat retina, we show that a homogeneous population of fast OFF ganglion cells simultaneously encodes two radically different features of a visual scene. Cells close to a moving object code quasilinearly for its position, while distant cells remain largely invariant to the object's position and, instead, respond nonlinearly to changes in the object's speed. We develop a quantitative model that accounts for this effect and identify a disinhibitory circuit that mediates it. Ganglion cells of a single type thus do not code for one, but two features simultaneously. This richer, flexible neural map might also be present in other sensory systems.

General features of inhibition in the inner retina

Visual processing starts in the retina. Within only two synaptic layers, a large number of parallel information channels emerge, each encoding a highly processed feature like edges or the direction of motion. Much of this functional diversity arises in the inner plexiform layer, where inhibitory amacrine cells modulate the excitatory signal of bipolar and ganglion cells. Studies investigating individual amacrine cell circuits like the starburst or A17 circuit have demonstrated that single types can possess specific morphological and functional adaptations to convey a particular function in one or a small number of inner retinal circuits. However, the interconnected and often stereotypical network formed by different types of amacrine cells across the inner plexiform layer prompts that they should be also involved in more general computations. In line with this notion, different recent studies systematically analysing inner retinal signalling at a population level provide evidence that general functions of the ensemble of amacrine cells across types are critical for establishing universal principles of retinal computation like parallel processing or motion anticipation. Combining recent advances in the development of indicators for imaging inhibition with large-scale morphological and genetic classifications will help to further our understanding of how single amacrine cell circuits act together to help decompose the visual scene into parallel information channels. In this review, we aim to summarise the current state-of-the-art in our understanding of how general features of amacrine cell inhibition lead to general features of computation.

Cell-cell communication in diabetic retinopathy

In diabetic retinopathy, high glucose (HG)-mediated breakdown in cell-cell communication promotes disruption of retinal homeostasis. Several studies indicate that HG condition alters expression of connexin genes and subsequent gap junction intercellular communication (GJIC) in retinal vascular cells and non-vascular cells. A serious consequence of disrupted cell-cell communication is apoptosis and breakdown of the blood-retinal barrier (BRB). More recently, studies suggest adverse effects from HG on retinal Müller cells. This article focuses on HG-mediated changes in connexin expression and GJIC and their subsequent effects on the breakdown of retinal homeostasis, cell death, compromised vascular permeability, and interactions between endothelial cells, pericytes and retinal Müller cells in the pathogenesis of diabetic retinopathy. Additionally, options for rectifying disrupted homeostasis under HG condition associated with diabetic retinopathy are reviewed.

Establishing Wiring Specificity in Visual System Circuits: From the Retina to the Brain

The retina is a tremendously complex image processor, containing numerous cell types that form microcircuits encoding different aspects of the visual scene. Each microcircuit exhibits a distinct pattern of synaptic connectivity. The developmental mechanisms responsible for this patterning are just beginning to be revealed. Furthermore, signals processed by different retinal circuits are relayed to specific, often distinct, brain regions. Thus, much work has focused on understanding the mechanisms that wire retinal axonal projections to their appropriate central targets. Here, we highlight recently discovered cellular and molecular mechanisms that together shape stereotypic wiring patterns along the visual pathway, from within the retina to the brain. Although some mechanisms are common across circuits, others play unconventional and circuit-specific roles. Indeed, the highly organized connectivity of the visual system has greatly facilitated the discovery of novel mechanisms that establish precise synaptic connections within the nervous system.

Inhibition decorrelates visual feature representations in the inner retina

The retina extracts visual features for transmission to the brain. Different types of bipolar cell split the photoreceptor input into parallel channels and provide the excitatory drive for downstream visual circuits. Anatomically and genetically, mouse bipolar cell types have been described at great detail, but a similarly deep understanding of their functional diversity is lacking. By imaging light-driven glutamate release from more than 13,000 bipolar cell axon terminals in the intact retina, we here show that bipolar cell functional diversity is generated by the interplay of dendritic excitatory inputs and axonal inhibitory inputs. The resultant centre and surround components of bipolar cell receptive fields interact to decorrelate bipolar cell output in the spatial and temporal domain. Our findings highlight the importance of inhibitory circuits in generating functionally diverse excitatory pathways and suggest that decorrelation of parallel visual pathways begins already at the second synapse of the mouse visual system.

Retinal biomarkers provide "insight" into cortical pharmacology and disease

The retina is an easily accessible out-pouching of the central nervous system (CNS) and thus lends itself to being a biomarker of the brain. More specifically, the presence of neuronal, vascular and blood-neural barrier parallels in the eye and brain coupled with fast and inexpensive methods to quantify retinal changes make ocular biomarkers an attractive option. This includes its utility as a biomarker for a number of cerebrovascular diseases as well as a drug pharmacology and safety biomarker for the CNS. It is a rapidly emerging field, with some areas well established, such as stroke risk and multiple sclerosis, whereas others are still in development (Alzheimer's, Parkinson's, psychological disease and cortical diabetic dysfunction). The current applications and future potential of retinal biomarkers, including potential ways to improve their sensitivity and specificity are discussed. This review summarises the existing literature and provides a perspective on the strength of current retinal biomarkers and their future potential.

Circuit Mechanisms of a Retinal Ganglion Cell with Stimulus-Dependent Response Latency and Activation Beyond Its Dendrites

Center-surround antagonism has been used as the canonical model to describe receptive fields of retinal ganglion cells (RGCs) for decades. We describe a newly identified RGC type in the mouse, called the ON delayed (OND) RGC, with receptive field properties that deviate from center-surround organization. Responding with an unusually long latency to light stimulation, OND RGCs respond earlier as the visual stimulus increases in size. Furthermore, OND RGCs are excited by light falling far beyond their dendrites. We unravel details of the circuit mechanisms behind these phenomena, revealing new roles for inhibition in controlling both temporal and spatial receptive field properties. The non-canonical receptive field properties of the OND RGC-integration of long temporal and large spatial scales-suggest that unlike typical RGCs, it may encode a slowly varying, global property of the visual scene.

Divisive suppression explains high-precision firing and contrast adaptation in retinal ganglion cells

Visual processing depends on specific computations implemented by complex neural circuits. Here, we present a circuit-inspired model of retinal ganglion cell computation, targeted to explain their temporal dynamics and adaptation to contrast. To localize the sources of such processing, we used recordings at the levels of synaptic input and spiking output in the in vitro mouse retina. We found that an ON-Alpha ganglion cell's excitatory synaptic inputs were described by a divisive interaction between excitation and delayed suppression, which explained nonlinear processing that was already present in ganglion cell inputs. Ganglion cell output was further shaped by spike generation mechanisms. The full model accurately predicted spike responses with unprecedented millisecond precision, and accurately described contrast adaptation of the spike train. These results demonstrate how circuit and cell-intrinsic mechanisms interact for ganglion cell function and, more generally, illustrate the power of circuit-inspired modeling of sensory processing.

Stereotyped initiation of retinal waves by bipolar cells via presynaptic NMDA autoreceptors

Glutamatergic retinal waves, the spontaneous patterned neural activities propagating among developing retinal ganglion cells (RGCs), instruct the activity-dependent refinement of visuotopic maps. However, its initiation and underlying mechanism remain largely elusive. Here using larval zebrafish and multiple in vivo approaches, we discover that bipolar cells (BCs) are responsible for the generation of glutamatergic retinal waves. The wave originates from BC axon terminals (ATs) and propagates laterally to nearby BCs and vertically to downstream RGCs and the optic tectum. Its initiation is triggered by the activation of and consequent glutamate release from BC ATs, and is mediated by the N-methyl-D-aspartate subtype of glutamate receptors (NMDARs) expressed at these ATs. Intercellular asymmetry of NMDAR expression at BC ATs enables the preferential initiation of waves at the temporal retina, where BC ATs express more NMDARs. Thus, our findings indicate that glutamatergic retinal waves are initiated by BCs through a presynaptic NMDA autoreceptor-dependent process.

Retinal waves are important for visual system development. However, the mechanism involved in their generation remains largely unknown. Here using in vivo two-photon imaging the authors identify the presence of retinal waves in zebrafish larvae and find that they are initiated at bipolar cells via presynaptic NMDARs.

Electrophysiological fingerprints of OFF bipolar cells in rat retina

Retinal bipolar cells (BCs) divide photoreceptor output into different channels for the parallel extraction of temporal and chromatic stimulus properties. In rodents, five types of OFF BCs have been differentiated, based on morphological and functional criteria, but their electrophysiological characterization remains incomplete. This study analyzed OFF BCs with the patch clamp technique in acute slices of rat retina. Their specific voltage-dependent currents and glutamate responses are shown to represent individual fingerprints which define the signal processing and filtering properties of each cell type and allow their unequivocal identification. Two additions to the rat BC repertoire are presented: OFF BC-2', a variation of BC-2 with wider axonal arbours and prominent Na(+) currents, is described for the first time in rodents, and OFF BC-3b, previously identified in mouse, is electrophysiologically characterized in rat. Moreover, the glutamate responses of rat OFF BCs are shown to be differentially sensitive to AMPA- and kainate-receptor blockers and to modulation by nitric oxide (NO) through a cGMP-dependent mechanism. These results contribute to our understanding of the diversity and function of bipolar cells in mammals.

Glutamatergic Monopolar Interneurons Provide a Novel Pathway of Excitation in the Mouse Retina

Excitatory and inhibitory neurons in the CNS are distinguished by several features, including morphology, transmitter content, and synapse architecture [1]. Such distinctions are exemplified in the vertebrate retina. Retinal bipolar cells are polarized glutamatergic neurons receiving direct photoreceptor input, whereas amacrine cells are usually monopolar inhibitory interneurons with synapses almost exclusively in the inner retina [2]. Bipolar but not amacrine cell synapses have presynaptic ribbon-like structures at their transmitter release sites. We identified a monopolar interneuron in the mouse retina that resembles amacrine cells morphologically but is glutamatergic and, unexpectedly, makes ribbon synapses. These glutamatergic monopolar interneurons (GluMIs) do not receive direct photoreceptor input, and their light responses are strongly shaped by both ON and OFF pathway-derived inhibitory input. GluMIs contact and make almost as many synapses as type 2 OFF bipolar cells onto OFF-sustained A-type (AOFF-S) retinal ganglion cells (RGCs). However, GluMIs and type 2 OFF bipolar cells possess functionally distinct light-driven responses and may therefore mediate separate components of the excitatory synaptic input to AOFF-S RGCs. The identification of GluMIs thus unveils a novel cellular component of excitatory circuits in the vertebrate retina, underscoring the complexity in defining cell types even in this well-characterized region of the CNS.

Mechanism of High-Frequency Signaling at a Depressing Ribbon Synapse

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

Dopamine D1 receptor modulation of calcium channel currents in horizontal cells of mouse retina

Horizontal cells form the first laterally interacting network of inhibitory interneurons in the retina. Dopamine released onto horizontal cells under photic and circadian control modulates horizontal cell function. Using isolated, identified horizontal cells from a connexin-57-iCre × ROSA26-tdTomato transgenic mouse line, we investigated dopaminergic modulation of calcium channel currents (ICa) with whole cell patch-clamp techniques. Dopamine (10 μM) blocked 27% of steady-state ICa, an action blunted to 9% in the presence of the L-type Ca channel blocker verapamil (50 μM). The dopamine type 1 receptor (D1R) agonist SKF38393 (20 μM) inhibited ICa by 24%. The D1R antagonist SCH23390 (20 μM) reduced dopamine and SKF38393 inhibition. Dopamine slowed ICa activation, blocking ICa by 38% early in a voltage step. Enhanced early inhibition of ICa was eliminated by applying voltage prepulses to +120 mV for 100 ms, increasing ICa by 31% and 11% for early and steady-state currents, respectively. Voltage-dependent facilitation of ICa and block of dopamine inhibition after preincubation with a Gβγ-blocking peptide suggested involvement of Gβγ proteins in the D1R-mediated modulation. When the G protein activator guanosine 5'-O-(3-thiotriphosphate) (GTPγS) was added intracellularly, ICa was smaller and showed the same slowed kinetics seen during D1R activation. With GTPγS in the pipette, additional block of ICa by dopamine was only 6%. Strong depolarizing voltage prepulses restored the GTPγS-reduced early ICa amplitude by 36% and steady-state ICa amplitude by 3%. These results suggest that dopaminergic inhibition of ICa via D1Rs is primarily mediated through the action of Gβγ proteins in horizontal cells.