Synaptically released glutamate activates extrasynaptic NMDA receptors on cells in the ganglion cell layer of rat retina

NMDA Receptors: Distribution, Role, and Insights into Neuropsychiatric Disorders

BACKGROUND

N-methyl-D-aspartate receptors (NMDARs) are members of the ionotropic glutamate receptor family. These ligand-gated channels are entwined with numerous fundamental neurological functions within the central nervous system (CNS), and numerous neuropsychiatric disorders may arise from their malfunction.

METHODS

The purpose of the present review is to provide a detailed description of NMDARs by addressing their molecular structures, activation mechanisms, and physiological roles in the mammalian brain. In the second part, their role in various neuropsychiatric disorders including stroke, epilepsy, anti-NMDA encephalitis, Alzheimer's and Huntington's diseases, schizophrenia, depression, neuropathic pain, opioid-induced tolerance, and hyperalgesia will be covered.

RESULTS

Finally, through a careful exploration of the main non-competitive NMDARs antagonists (channel-blockers).

CONCLUSION

We discuss the strengths and limitations of the various molecular structures developed for diagnostic or therapeutic purposes.

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.

Deficits in integrative NMDA receptors caused by Grin1 disruption can be rescued in adulthood

Glutamatergic NMDA receptors (NMDAR) are critical for cognitive function, and their reduced expression leads to intellectual disability. Since subpopulations of NMDARs exist in distinct subcellular environments, their functioning may be unevenly vulnerable to genetic disruption. Here, we investigate synaptic and extrasynaptic NMDARs on the major output neurons of the prefrontal cortex in mice deficient for the obligate NMDAR subunit encoded by Grin1 and wild-type littermates. With whole-cell recording in brain slices, we find that single, low-intensity stimuli elicit surprisingly-similar glutamatergic synaptic currents in both genotypes. By contrast, clear genotype differences emerge with manipulations that recruit extrasynaptic NMDARs, including stronger, repetitive, or pharmacological stimulation. These results reveal a disproportionate functional deficit of extrasynaptic NMDARs compared to their synaptic counterparts. To probe the repercussions of this deficit, we examine an NMDAR-dependent phenomenon considered a building block of cognitive integration, basal dendrite plateau potentials. Since we find this phenomenon is readily evoked in wild-type but not in Grin1-deficient mice, we ask whether plateau potentials can be restored by an adult intervention to increase Grin1 expression. This genetic manipulation, previously shown to restore cognitive performance in adulthood, successfully rescues electrically-evoked basal dendrite plateau potentials after a lifetime of NMDAR compromise. Taken together, our work demonstrates NMDAR subpopulations are not uniformly vulnerable to the genetic disruption of their obligate subunit. Furthermore, the window for functional rescue of the more-sensitive integrative NMDARs remains open into adulthood.

Glutamate transporter contribution to retinal ganglion cell vulnerability in a rat model of multiple sclerosis

Glial glutamate transporters actively participate in neurotransmission and have a fundamental role in determining the ambient glutamate concentration in the extracellular space. Their expression is dynamically regulated in many diseases, including experimental autoimmune encephalomyelitis (EAE), a model of multiple sclerosis. In EAE, a downregulation has been reported which may render neurons more susceptible to glutamate excitotoxicity. In this study, we have investigated the expression of GLAST (EAAT1) and GLT-1 (EAAT2) in the retina of Brown Norway rats following induction of myelin oligodendrocyte glycoprotein (MOG)-EAE, which results in retinal ganglion cell (RGC) degeneration and dysfunction. In addition, we tested whether AAV-mediated overexpression of GLAST in the retina can protect RGCs from degeneration. To address the impact of glutamate transporter modulation on RGCs, we performed whole-cell recordings and measured tonic NMDA receptor-mediated currents in the absence and presence of a glutamate-uptake blocker. We report that αOFF-RGCs show larger tonic glutamate-induced currents than αON-RGCs, in line with their greater vulnerability under neuroinflammatory conditions. We further show that increased AAV-mediated expression of GLAST in the retina does indeed protect RGCs from degeneration during the inflammatory disease. Collectively, our study highlights the neuroprotective role of glutamate transporters in the EAE retina and provides a characterization of tonic glutamate-currents of αRGCs. The larger effects of increased extracellular glutamate concentration on the αOFF-subtype may underlie its enhanced vulnerability to degeneration.

Activation of retinal glial cells contributes to the degeneration of ganglion cells in experimental glaucoma

Elevation of intraocular pressure (IOP) is a major risk factor for neurodegeneration in glaucoma. Glial cells, which play an important role in normal functioning of retinal neurons, are well involved into retinal ganglion cell (RGC) degeneration in experimental glaucoma animal models generated by elevated IOP. In response to elevated IOP, mGluR I is first activated and Kir4.1 channels are subsequently inhibited, which leads to the activation of Müller cells. Müller cell activation is followed by a complex process, including proliferation, release of inflammatory and growth factors (gliosis). Gliosis is further regulated by several factors. Activated Müller cells contribute to RGC degeneration through generating glutamate receptor-mediated excitotoxicity, releasing cytotoxic factors and inducing microglia activation. Elevated IOP activates microglia, and following morphological and functional changes, these cells, as resident immune cells in the retina, show adaptive immune responses, including an enhanced release of pro-inflammatory factors (tumor neurosis factor-α, interleukins, etc.). These ATP and Toll-like receptor-mediated responses are further regulated by heat shock proteins, CD200R, chemokine receptors, and metabotropic purinergic receptors, may aggravate RGC loss. In the optic nerve head, astrogliosis is initiated and regulated by a complex reaction process, including purines, transmitters, chemokines, growth factors and cytokines, which contributes to RGC axon injury through releasing pro-inflammatory factors and changing extracellular matrix in glaucoma. The effects of activated glial cells on RGCs are further modified by the interplay among different types of glial cells. This review is concluded by presenting an in-depth discussion of possible research directions in this field in the future.

Homeostatic plasticity in the retina

Vision begins in the retina, whose intricate neural circuits extract salient features of the environment from the light entering our eyes. Neurodegenerative diseases of the retina (e.g., inherited retinal degenerations, age-related macular degeneration, and glaucoma) impair vision and cause blindness in a growing number of people worldwide. Increasing evidence indicates that homeostatic plasticity (i.e., the drive of a neural system to stabilize its function) can, in principle, preserve retinal function in the face of major perturbations, including neurodegeneration. Here, we review the circumstances and events that trigger homeostatic plasticity in the retina during development, sensory experience, and disease. We discuss the diverse mechanisms that cooperate to compensate and the set points and outcomes that homeostatic retinal plasticity stabilizes. Finally, we summarize the opportunities and challenges for unlocking the therapeutic potential of homeostatic plasticity. Homeostatic plasticity is fundamental to understanding retinal development and function and could be an important tool in the fight to preserve and restore vision.

Cellular mechanisms mediating activity-dependent extracellular space shrinkage in the retina

Volume transmission plays an essential role in CNS function, with neurotransmitters released from synapses diffusing through the extracellular space (ECS) to distant sites. Changes in the ECS volume fraction (α) will influence the diffusion and the concentration of transmitters within the ECS. We have recently shown that neuronal activity evoked by physiological photic stimuli results in rapid decreases in ECS α as large as 10% in the retina. We now characterize the cellular mechanisms responsible for this ECS shrinkage. We find that block of inwardly rectifying K⁺ channels with Ba²⁺, inhibition of the Na⁺/K⁺/2Cl⁻ cotransporter with bumetanide, or block of AQP4 water channels with TGN‐020 do not diminish the light‐evoked ECS decrease. Inhibition of the Na⁺/HCO₃⁻ cotransporter by removing HCO₃⁻ from the superfusate, in contrast, reduces the light‐evoked ECS decrease by 95.6%. Inhibition of the monocarboxylate transporter with alpha‐cyano‐4‐hydroxycinnamate (4‐CIN) also reduces the ECS shrinkage, but only by 32.5%. We tested whether the swelling of Müller cells, the principal glial cells of the retina, is responsible for the light‐evoked ECS shrinkage. Light stimulation evoked a 6.3% increase in the volume of the fine processes of Müller cells. This volume increase was reduced by 97.1% when HCO₃⁻ was removed from the superfusate. We conclude that a large fraction of the activity‐dependent decrease in ECS α is generated by the activation of the Na⁺/HCO₃⁻ cotransporter in Müller cells. The monocarboxylate transporter may also contribute to the response.

Retinal Glutamate Neurotransmission: From Physiology to Pathophysiological Mechanisms of Retinal Ganglion Cell Degeneration

Glutamate neurotransmission and metabolism are finely modulated by the retinal network, where the efficient processing of visual information is shaped by the differential distribution and composition of glutamate receptors and transporters. However, disturbances in glutamate homeostasis can result in glutamate excitotoxicity, a major initiating factor of common neurodegenerative diseases. Within the retina, glutamate excitotoxicity can impair visual transmission by initiating degeneration of neuronal populations, including retinal ganglion cells (RGCs). The vulnerability of RGCs is observed not just as a result of retinal diseases but has also been ascribed to other common neurodegenerative and peripheral diseases. In this review, we describe the vulnerability of RGCs to glutamate excitotoxicity and the contribution of different glutamate receptors and transporters to this. In particular, we focus on the N-methyl-d-aspartate (NMDA) receptor as the major effector of glutamate-induced mechanisms of neurodegeneration, including impairment of calcium homeostasis, changes in gene expression and signalling, and mitochondrial dysfunction, as well as the role of endoplasmic reticular stress. Due to recent developments in the search for modulators of NMDA receptor signalling, novel neuroprotective strategies may be on the horizon.

Cocaine experience induces functional adaptations in astrocytes: Implications for synaptic plasticity in the nucleus accumbens shell

Astrocytes have become established as an important regulator of neuronal activity in the brain. Accumulating literature demonstrates that cocaine self-administration in rodent models induces structural changes within astrocytes that may influence their interaction with the surrounding neurons. Here, we provide evidence that cocaine impacts astrocytes at the functional level and alters neuronal sensitivity to astrocyte-derived glutamate. We report that a 14-day period of short access to cocaine (2 h/day) decreases spontaneous astrocytic Ca²⁺ transients and precipitates changes in astrocyte network activity in the nucleus accumbens shell. This is accompanied by increased prevalence of slow inward currents, a physiological marker of neuronal activation by astrocytic glutamate, in a subset of medium spiny neurons. Within, but not outside, of this subset, we observe an increase in duration and frequency of N-methyl-D-aspartate (NMDA) receptor-mediated synaptic events. Additionally, we find that the link between synaptic NMDA receptor plasticity and neuron sensitivity to astrocytic glutamate is maintained independent of drug exposure and is observed in both cocaine and saline control animals. Imaging analyses of neuronal Ca²⁺ activity show no effect of cocaine self-administration on individual cells or on neuronal network activity in brain slices. Therefore, our data indicate that cocaine self-administration promotes astrocyte-specific functional changes that can be linked to increased glutamate-mediated coupling with principal neurons in the nucleus accumbens. Such coupling may be spatially restricted as it does not result in a broad impact on network structure of local neuronal circuits.

Structure, Function, and Pharmacology of Glutamate Receptor Ion Channels

Many physiologic effects of l-glutamate, the major excitatory neurotransmitter in the mammalian central nervous system, are mediated via signaling by ionotropic glutamate receptors (iGluRs). These ligand-gated ion channels are critical to brain function and are centrally implicated in numerous psychiatric and neurologic disorders. There are different classes of iGluRs with a variety of receptor subtypes in each class that play distinct roles in neuronal functions. The diversity in iGluR subtypes, with their unique functional properties and physiologic roles, has motivated a large number of studies. Our understanding of receptor subtypes has advanced considerably since the first iGluR subunit gene was cloned in 1989, and the research focus has expanded to encompass facets of biology that have been recently discovered and to exploit experimental paradigms made possible by technological advances. Here, we review insights from more than 3 decades of iGluR studies with an emphasis on the progress that has occurred in the past decade. We cover structure, function, pharmacology, roles in neurophysiology, and therapeutic implications for all classes of receptors assembled from the subunits encoded by the 18 ionotropic glutamate receptor genes. SIGNIFICANCE STATEMENT: Glutamate receptors play important roles in virtually all aspects of brain function and are either involved in mediating some clinical features of neurological disease or represent a therapeutic target for treatment. Therefore, understanding the structure, function, and pharmacology of this class of receptors will advance our understanding of many aspects of brain function at molecular, cellular, and system levels and provide new opportunities to treat patients.

Different glutamate sources and endogenous co-agonists activate extrasynaptic NMDA receptors on amacrine cells of the rod pathway microcircuit

The NMDA receptors (NMDARs) expressed by AII and A17 amacrine cells, the two main inhibitory interneurons of the rod pathway microcircuit in the mammalian retina, are exclusively extrasynaptic, activated by ambient levels of glutamate, and molecularly distinct, with AII and A17 amacrines expressing GluN2B- and GluN2A-containing receptors, respectively. This important sensory microcircuit thus provides a unique model to study the activation and function of extrasynaptic NMDARs. Here, we investigated the sources of glutamate and the endogenous co-agonists (d-serine or glycine) that activate these distinct populations of NMDARs. With acute slices from rat retina, we used whole-cell voltage-clamp recording and measurement of current noise to monitor levels of NMDAR activity. Pre-incubation of retina with bafilomycin A1 (an inhibitor of neurotransmitter uptake into synaptic vesicles) abolished NMDAR-mediated noise in AII, but not A17 amacrines, suggesting a vesicular source of glutamate activates AII NMDARs, whereas a non-vesicular source activates A17 NMDARs. Pre-incubation of retina with l-methionine sulfoximine (an inhibitor of glutamine synthetase) also abolished NMDAR-mediated noise in AII, but not A17 amacrines, suggesting a neuronal source of glutamate activates AII NMDARs, whereas a glial source activates A17 NMDARs. Enzymatic breakdown of d-serine reduced NMDAR-mediated noise in AII, but not A17 amacrines, suggesting d-serine is the endogenous co-agonist at AII, but not A17 NMDARs. Our results reveal unique characteristics of these two populations of extrasynaptic NMDARs. The differential and independent activation of these receptors is likely to provide specific contributions to the signal processing and plasticity of the cellular components of the rod pathway microcircuit.

Cholinergic and Adenosinergic Modulation of Synaptic Release

In this review we will discuss the effect of two neuromodulatory transmitters, acetylcholine (ACh) and adenosine, on the synaptic release probability and short-term synaptic plasticity. ACh and adenosine differ fundamentally in the way they are released into the extracellular space. ACh is released mostly from synaptic terminals and axonal bouton of cholinergic neurons in the basal forebrain (BF). Its mode of action on synaptic release probability is complex because it activate both ligand-gated ion channels, so-called nicotinic ACh receptors and G-protein coupled muscarinic ACh receptors. In contrast, adenosine is released from both neurons and glia via nucleoside transporters or diffusion over the cell membrane in a non-vesicular, non-synaptic fashion; its receptors are exclusively G-protein coupled receptors. We show that ACh and adenosine effects are highly specific for an identified synaptic connection and depend mostly on the presynaptic but also on the postsynaptic receptor type and discuss the functional implications of these differences.

PSD-95 Serine 73 phosphorylation is not required for induction of NMDA-LTD

PSD-95 is a major scaffolding protein of the post-synaptic density (PSD) of a glutamatergic synapse. PSD-95, via interactions with stargazin, anchors AMPA receptors at the synapse and regulates AMPAR currents. The expression of PSD-95 is regulated during synaptic plasticity. It is, however, unknown whether this regulation is required for induction of functional plasticity of glutamatergic synapses. Here, we show that NMDA-induced long-term depression of synaptic transmission (NMDA-LTD) is accompanied by downregulation of PSD-95 protein levels. Using pharmacologic and molecular manipulations, we further demonstrate that the NMDA-induced downregulation of PSD-95 depends on the activation of CaMKII and CaMKII-driven phosphorylation of PSD-95 serine 73. Surprisingly, neither CaMKII activity nor CaMKII-dependent phosphorylation of PSD-95 serine 73 are required for the expression of NMDA-LTD. These results support the hypothesis that synaptic plasticity of AMPARs may occur without dynamic regulation of PSD-95 protein levels.

ATP11B deficiency leads to impairment of hippocampal synaptic plasticity

Synaptic plasticity is known to regulate and support signal transduction between neurons, while synaptic dysfunction contributes to multiple neurological and other brain disorders; however, the specific mechanism underlying this process remains unclear. In the present study, abnormal neural and dendritic morphology was observed in the hippocampus following knockout of Atp11b both in vitro and in vivo. Moreover, ATP11B modified synaptic ultrastructure and promoted spine remodeling via the asymmetrical distribution of phosphatidylserine and enhancement of glutamate release, glutamate receptor expression, and intracellular Ca2+ concentration. Furthermore, experimental results also indicate that ATP11B regulated synaptic plasticity in hippocampal neurons through the MAPK14 signaling pathway. In conclusion, our data shed light on the possible mechanisms underlying the regulation of synaptic plasticity and lay the foundation for the exploration of proteins involved in signal transduction during this process.

Cognitive and physical disability in Egyptian patients with multiple sclerosis: genetic and optical coherence tomography study

Objectives: The aim of this study was to explore the relationship between cognitive dysfunction, neurodegeneration, and genetic factors among multiple sclerosis (MS) patients. Methods: Fifty patients of definite MS were included. Physical disability was assessed by expanded disability status scale (EDSS). Cognitive functions were assessed by using the Brief International Cognitive Assessment for Multiple Sclerosis (BICAMS). For each eye, optical coherence tomography (OCT) was used to track thickness of retinal nerve fiber layer (RNFL) and ganglion cell complex (GCC), respecting the previous history of optic neuritis (ON). All patients were genotyped for glutamate N-methyl-D-aspartate receptors (NMDARs). Results: A statistically significant negative correlation was found between scores of EDSS and each of neuropsychological tests scores and thickness of both RNFL and GCC. The predictor for progressive disability assessed by EDSS was Symbol Digit Modalities Test (SDMT) (P = 0.021), that is dependent on the educational level of the patients (P = 0.016). A statistically significant positive correlation was found between scores of all neuropsychological tests and the thickness of both RNFL and GCC. Eighty-three percent of MS patients with CC genotype reported previous attacks of ON with significant thinning in RNFL and GCC despite their higher cognitive performance in comparison to other genotypes. Discussion: Deficit in information processing speed measured by SDMT is a predictor of early progressive disability in MS patients. Thinning of RNFL and GCC is a potential biomarker for cognitive and physical disability in MS. The CC genotype of glutamate NMDAR gene has a divergent effect on visual and cognitive functions.

Hyperhomocysteinemia leads to exacerbation of ischemic brain damage: Role of GluN2A NMDA receptors

Hyperhomocysteinemia has been implicated in several neurodegenerative disorders including ischemic stroke. However, the pathological consequences of ischemic insult in individuals predisposed to hyperhomocysteinemia and the associated etiology are unknown. In this study, we evaluated the outcome of transient ischemic stroke in a rodent model of hyperhomocysteinemia, developed by subcutaneous implantation of osmotic pumps containing L-homocysteine into male Wistar rats. Our findings show a 42.3% mortality rate in hyperhomocysteinemic rats as compared to 7.7% in control rats. Magnetic resonance imaging of the brain in the surviving rats shows that mild hyperhomocysteinemia leads to exacerbation of ischemic injury within 24 h, which remains elevated over time. Behavioral studies further demonstrate significant deficit in sensorimotor functions in hyperhomocysteinemic rats compared to control rats. Using pharmacological inhibitors targeting the NMDAR subtypes, the study further demonstrates that inhibition of GluN2A-containing NMDARs significantly reduces ischemic brain damage in hyperhomocysteinemic rats but not in control rats, indicating that hyperhomocysteinemia-mediated exacerbation of ischemic brain injury involves GluN2A-NMDAR signaling. Complementary studies in GluN2A-knockout mice show that in the absence of GluN2A-NMDARs, hyperhomocysteinemia-associated exacerbation of ischemic brain injury is blocked, confirming that GluN2A-NMDAR activation is a critical determinant of the severity of ischemic damage under hyperhomocysteinemic conditions. Furthermore, at the molecular level we observe GluN2A-NMDAR dependent sustained increase in ERK MAPK phosphorylation under hyperhomocysteinemic condition that has been shown to be involved in homocysteine-induced neurotoxicity. Taken together, the findings show that hyperhomocysteinemia triggers a unique signaling pathway that in conjunction with ischemia-induced pathways enhance the pathology of stroke under hyperhomocysteinemic conditions.

Extrasynaptic NMDA Receptors on Rod Pathway Amacrine Cells: Molecular Composition, Activation, and Signaling

In the rod pathway of the mammalian retina, axon terminals of glutamatergic rod bipolar cells are presynaptic to AII and A17 amacrine cells in the inner plexiform layer. Recent evidence suggests that both amacrines express NMDA receptors, raising questions concerning molecular composition, localization, activation, and function of these receptors. Using dual patch-clamp recording from synaptically connected rod bipolar and AII or A17 amacrine cells in retinal slices from female rats, we found no evidence that NMDA receptors contribute to postsynaptic currents evoked in either amacrine. Instead, NMDA receptors on both amacrine cells were activated by ambient glutamate, and blocking glutamate uptake increased their level of activation. NMDA receptor activation also increased the frequency of GABAergic postsynaptic currents in rod bipolar cells, suggesting that NMDA receptors can drive release of GABA from A17 amacrines. A striking dichotomy was revealed by pharmacological and immunolabeling experiments, which found GluN2B-containing NMDA receptors on AII amacrines and GluN2A-containing NMDA receptors on A17 amacrines. Immunolabeling also revealed a clustered organization of NMDA receptors on both amacrines and a close spatial association between GluN2B subunits and connexin 36 on AII amacrines, suggesting that NMDA receptor modulation of gap junction coupling between these cells involves the GluN2B subunit. Using multiphoton Ca2+ imaging, we verified that activation of NMDA receptors evoked an increase of intracellular Ca2+ in dendrites of both amacrines. Our results suggest that AII and A17 amacrines express clustered, extrasynaptic NMDA receptors, with different and complementary subunits that are likely to contribute differentially to signal processing and plasticity.SIGNIFICANCE STATEMENT Glutamate is the most important excitatory neurotransmitter in the CNS, but not all glutamate receptors transmit fast excitatory signals at synapses. NMDA-type glutamate receptors act as voltage- and ligand-gated ion channels, with functional properties determined by their specific subunit composition. These receptors can be found at both synaptic and extrasynaptic sites on neurons, but the role of extrasynaptic NMDA receptors is unclear. Here, we demonstrate that retinal AII and A17 amacrine cells, postsynaptic partners at rod bipolar dyad synapses, express extrasynaptic (but not synaptic) NMDA receptors, with different and complementary GluN2 subunits. The localization of GluN2A-containing receptors to A17s and GluN2B-containing receptors to AIIs suggests a mechanism for differential modulation of excitability and signaling in this retinal microcircuit.

Atypical Expression and Activation of GluN2A- and GluN2B-Containing NMDA Receptors at Ganglion Cells during Retinal Degeneration

Cellular communication through chemical synapses is determined by the nature of the neurotransmitter and the composition of postsynaptic receptors. In the excitatory synapse between bipolar and ganglion cells of the retina, postsynaptic AMPA receptors mediate resting activity. During evoked response, however, more abundant and sustained levels of glutamate also activate GluN2B-containing NMDA receptors (NMDARs). This phasic recruitment of distinct glutamate receptors is essential for visual discrimination; however, the fidelity of this basic mechanism under elevated glutamate levels due to aberrant activity, a common pathophysiology, is not known. Here, in both male and female mice with retinal degeneration (rd10), a condition associated with elevated synaptic activity, we reveal that changes in synaptic input to ganglion cells altered both composition and activation of NMDARs. We found that, in contrast to wild type, the spontaneous activity of rd10 cells was largely NMDAR-dependent. Surprisingly, this activity was driven primarily by atypical activation of GluN2A -containing NMDARs, not GluN2B-NMDARs. Indeed, immunohistochemical analyses and Western blot showed greater levels of the GluN2A-NMDAR subunit expression in rd10 retina compared to wild type. Overall, these results demonstrate how aberrant signaling leads to pathway-specific alterations in NMDAR expression and function.

Elevated Pressure Increases Ca2+ Influx Through AMPA Receptors in Select Populations of Retinal Ganglion Cells

The predominate type of AMPA receptor expressed in the CNS is impermeable to Ca2+ (CI-AMPAR). However, some AMPA receptors are permeable to Ca2+ (CP-AMPAR) and play important roles in development, plasticity and disease. In the retina, ganglion cells (RGCs) are targets of disease including glaucoma and diabetic retinopathy, but there are many types of RGCs and not all types are targeted equally. In the present study, we sought to determine if there are differences in expression of AMPARs amongst RGC subtypes, and if these differences might contribute to differential vulnerability in a model of stress. Using cultured RGCs we first show that acute exposure to elevated pressure increased expression of Ca2+-permeable AMPA receptors (CP-AMPARs) in some, but not all classes of RGCs. When RGCs were sampled without regard to subtype, AMPA currents, measured using patch clamp recording, were blocked by the CP-AMPAR blocker PhTX-74 to a greater extent in pressure-treated RGCs vs. control. Furthermore, imaging experiments revealed an increase in Ca2+ influx during AMPA application in pressure-treated RGCs. However, examination of specific RGC subtypes using reporter lines revealed striking differences in both baseline AMPAR composition and modulation of this baseline composition by stress. Notably, ON alpha RGCs identified using the Opn4 mouse line and immunohistochemistry, had low expression of CP-AMPARs. Conversely, an ON-OFF direction selective RGC and putative OFF alpha RGC each expressed high levels of CP-AMPARs. These differences between RGC subtypes were also observed in RGCs from whole retina. Elevated pressure further lowered expression of CP-AMPARs in ON alpha RGCs, but raised expression in ON-OFF and OFF RGCs. Changes in CP-AMPAR expression following challenge with elevated pressure were correlated with RGC survival: ON alpha RGCs were unaffected by application of pressure, while the number of putative OFF alpha RGCs declined by approximately 50% following challenge with pressure. Differences in expression of CP-AMPARs between RGC subtypes may form the underpinnings for subtype-specific synaptic plasticity. Furthermore, the differential responses of these RGC subtypes to elevated pressure may contribute to the reported resistance of ON alpha, and susceptibility of OFF and ON-OFF RGCs to injury in models of glaucoma.

Quinolinic acid and glutamatergic neurodegeneration in Caenorhabditis elegans

Quinolinic acid (QUIN) is an endogenous neurotoxin that acts as an N-methyl-D-aspartate receptor (NMDAR) agonist generating a toxic cascade, which can lead to neurodegeneration. The action of QUIN in Caenorhabditis elegans and the neurotoxins that allow the study of glutamatergic system disorders have not been carefully addressed. The effects of QUIN on toxicological and behavioral parameters in VM487 and VC2623 transgenic, as well as wild-type (WT) animals were performed to evaluate whether QUIN could be used as a neurotoxin in C. elegans. QUIN reduced survival of WT worms in a dose-dependent manner. A sublethal dose of QUIN (20 mM) increased reactive oxygen species (ROS) levels in an nmr-1/NMDAR-dependent manner, activated the DAF-16/FOXO transcription factor, and increased expression of the antioxidant enzymes, superoxide dismutase-3, glutathione S-transferase-4, and heat shock protein-16.2. QUIN did not change motor behavioral parameters, but altered the sensory behavior in N2 and VM487 worms. Notably, the effect of QUIN on the sensory behavioral parameters might occur, at least in part, secondary to increased ROS. However, the touch response behavior indicates a mechanism of action that is independent of ROS generation. In addition, non-lethal doses of QUIN triggered neurodegeneration in glutamatergic neurons. Our findings indicate that C. elegans might be useful as a model for studies of QUIN as a glutamatergic neurotoxin in rodent models.

A Novel Tetramethylpyrazine Derivative Prophylactically Protects against Glutamate-Induced Excitotoxicity in Primary Neurons through the Blockage of N-Methyl-D-aspartate Receptor

The over-activation of NMDA receptor via the excessive glutamate is believed to one of the most causal factors associated with Alzheimer’s disease (AD), a progressive neurodegenerative brain disorder. Molecules that could protect against glutamate-induced neurotoxicity may hold therapeutic values for treating AD. Herein, the neuroprotective mechanisms of dimeric DT-010, a novel derivative of naturally occurring danshensu and tetramethylpyrazine, were investigated using primary rat cerebellar granule neurons (CGNs) and hippocampal neurons. It was found that DT-010 (3–30 μM) markedly prevented excitotoxicity of CGNs caused by glutamate, as evidenced by the promotion of neuronal viability as well as the reversal of abnormal morphological changes. While its parent molecules did not show any protective effects even when their concentration reached 50 μM. Additionally, DT-010 almost fully blocked intracellular accumulation of reactive oxygen species caused by glutamate and exogenous oxidative stimulus. Moreover, Western blot results demonstrated that DT-010 remarkably attenuated the inhibition of pro-survival PI3K/Akt/GSK3β pathway caused by glutamate. Ca²⁺ imaging with Fluo-4 fluorescence analysis further revealed that DT-010 greatly declined glutamate-induced increase in intracellular Ca²⁺. Most importantly, with the use of whole-cell patch clamp electrophysiology, DT-010 directly inhibited NMDA-activated whole-cell currents in primary hippocampal neurons. Molecular docking simulation analysis further revealed a possible binding mode that inhibited NMDA receptor at the ion channel, showing that DT-010 favorably binds to Asn602 of NMDA receptor via arene hydrogen bond. These results suggest that DT-010 could be served as a novel NMDA receptor antagonist and protect against glutamate-induced excitotoxicity from blocking the upstream NMDA receptors to the subsequent Ca²⁺ influx and to the downstream GSK3β cascade.

Notch Signaling Activation Enhances Human Adipose-Derived Stem Cell Retinal Differentiation

Retinal disease treatment by stem cell-based replacement relies on stem cell differentiation into retinal cells. We previously demonstrated that human periodontal ligament-derived stem cells can be directed into retinal lineage upon induction. Here, we report the transdifferentiation potential of human adipose-derived stem cells (ASCs) into retinal lineage and its enhancement by Notch signaling modulation. Human ASCs, isolated from abdominal fat, expressed mesenchymal but not hematopoietic stem cell markers, and they can differentiate into adipocytes, chondrocytes, and osteoblasts in vitro. Upon noggin/Dkk-1/IGF-1 induction, the treated ASCs showed elevated expression of retinal progenitor, retinal ganglion, and photoreceptor cell markers as well as the glutamate-evoked calcium response, which was not observed in the noninduced cells. Compared to the regular induction treatment, Notch signaling activation by JAG1 enhanced the expression of retinal progenitor and precursor markers without affecting the glutamate-evoked calcium response. In contrast, Notch signaling inhibition by DAPT showed more retinal ganglion cells, but delayed the response to glutamate stimulation. In summary, our results revealed that human ASCs possess a retinal transdifferentiation potential upon noggin/Dkk-1/IGF-1 induction, which can further be enhanced by Notch signaling activation.

NMDA Receptors Contribute to Retrograde Synaptic Transmission from Ganglion Cell Photoreceptors to Dopaminergic Amacrine Cells

Recently, a line of evidence has demonstrated that the vertebrate retina possesses a novel retrograde signaling pathway. In this pathway, phototransduction is initiated by the photopigment melanopsin, which is expressed in a small population of retinal ganglion cells. These ganglion cell photoreceptors then signal to dopaminergic amacrine cells (DACs) through glutamatergic synapses, influencing visual light adaptation. We have previously demonstrated that in Mg²⁺-containing solution, α-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate (AMPA) receptors mediate this glutamatergic transmission. Here, we demonstrate that removing extracellular Mg²⁺ enhances melanopsin-based DAC light responses at membrane potentials more negative than −40 mV. Melanopsin-based responses in Mg²⁺-free solution were profoundly suppressed by the selective N-methyl-D-aspartate (NMDA) receptor antagonist D-AP5. In addition, application of NMDA to the retina produced excitatory inward currents in DACs. These data strongly suggest that DACs express functional NMDA receptors. We further found that in the presence of Mg²⁺, D-AP5 reduced the peak amplitude of melanopsin-based DAC responses by ~70% when the cells were held at their resting membrane potential (−50 mV), indicating that NMDA receptors are likely to contribute to retrograde signal transmission to DACs under physiological conditions. Moreover, our data show that melanopsin-based NMDA-receptor-mediated responses in DACs are suppressed by antagonists specific to either the NR2A or NR2B receptor subtype. Immunohistochemical results show that NR2A and NR2B subunits are expressed on DAC somata and processes. These results suggest that DACs express functional NMDA receptors containing both NR2A and NR2B subunits. Collectively, our data reveal that, along with AMPA receptors, NR2A- and NR2B-containing NMDA receptors mediate retrograde signal transmission from ganglion cell photoreceptors to DACs.

Selective synaptic connections in the retinal pathway for night vision

The mammalian retina encodes visual information in dim light using rod photoreceptors and a specialized circuit: rods→rod bipolar cells→AII amacrine cell. The AII amacrine cell uses sign-conserving electrical synapses to modulate ON cone bipolar cell terminals and sign-inverting chemical (glycinergic) synapses to modulate OFF cone cell bipolar terminals; these ON and OFF cone bipolar terminals then drive the output neurons, retinal ganglion cells (RGCs), following light increments and decrements, respectively. The AII amacrine cell also makes direct glycinergic synapses with certain RGCs, but it is not well established how many types receive this direct AII input. Here, we investigated functional AII amacrine→RGC synaptic connections in the retina of the guinea pig (Cavia porcellus) by recording inhibitory currents from RGCs in the presence of ionotropic glutamate receptor (iGluR) antagonists. This condition isolates a specific pathway through the AII amacrine cell that does not require iGluRs: cone→ON cone bipolar cell→AII amacrine cell→RGC. These recordings show that AII amacrine cells make direct synapses with OFF Alpha, OFF Delta and a smaller OFF transient RGC type that co-stratifies with OFF Alpha cells. However, AII amacrine cells avoid making synapses with numerous RGC types that co-stratify with the connected RGCs. Selective AII connections ensure that a privileged minority of RGC types receives direct input from the night-vision pathway, independent from OFF bipolar cell activity. Furthermore, these results illustrate the specificity of retinal connections, which cannot be predicted solely by co-stratification of dendrites and axons within the inner plexiform layer.

Expression and localisation of two-pore domain (K2P) background leak potassium ion channels in the mouse retina

Two-pore domain (K2P) potassium channels perform essential roles in neuronal function. These channels produce background leak type potassium currents that act to regulate resting membrane potential and levels of cellular excitability. 15 different K2P channels have been identified in mammals and these channels perform important roles in a wide number of physiological systems. However, to date there is only limited data available concerning the expression and role of K2P channels in the retina. In this study we conduct the first comprehensive study of K2P channel expression in the retina. Our data show that K2P channels are widely expressed in the mouse retina, with variations in expression detected at different times of day and throughout postnatal development. The highest levels of K2P channel expression are observed for Müller cells (TWIK-1, TASK-3, TRAAK, and TREK-2) and retinal ganglion cells (TASK-1, TREK-1, TWIK-1, TWIK-2 and TWIK-3). These data offer new insight into the channels that regulate the resting membrane potential and electrical activity of retinal cells, and suggests that K2P channels are well placed to act as central regulators of visual signalling pathways. The prominent role of K2P channels in neuroprotection offers novel avenues of research into the treatment of common retinal diseases.

Molecular and cellular physiology of sodium-dependent glutamate transporters

Glutamate is the major excitatory transmitter in the vertebrate brain. After its release from presynaptic nerve terminals, it is rapidly taken up by high-affinity sodium-dependent plasma membrane transporters. While both neurons and glial cells express these excitatory amino acid transporters (EAATs), the majority of glutamate uptake is accomplished by astrocytes, which convert synaptically-released glutamate to glutamine or feed it into their own metabolism. Glutamate uptake by astrocytes not only shapes synaptic transmission by regulating the availability of glutamate to postsynaptic neuronal receptors, but also protects neurons from hyper-excitability and subsequent excitotoxic damage. In the present review, we provide an overview of the molecular and cellular characteristics of sodium-dependent glutamate transporters and their associated anion permeation pathways, with a focus on astrocytic glutamate transport. We summarize their functional properties and roles within tripartite synapses under physiological and pathophysiological conditions, exemplifying the intricate interactions and interrelationships between neurons and glial cells in the brain.

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.

Organization, control and function of extrasynaptic NMDA receptors

N-methyl D-aspartate receptors (NMDARs) exist in different forms owing to multiple combinations of subunits that can assemble into a functional receptor. In addition, they are located not only at synapses but also at extrasynaptic sites. There has been intense speculation over the past decade about whether specific NMDAR subtypes and/or locations are responsible for inducing synaptic plasticity and excitotoxicity. Here, we review the latest findings on the organization, subunit composition and endogenous control of NMDARs at extrasynaptic sites and consider their putative functions. Because astrocytes are capable of controlling NMDARs through the release of gliotransmitters, we also discuss the role of the glial environment in regulating the activity of these receptors.

Mechanisms regulating spill-over of synaptic glutamate to extrasynaptic NMDA receptors in mouse substantia nigra dopaminergic neurons

N‐Methyl‐d‐aspartate glutamate receptors (NMDARs) contribute to neural development, plasticity and survival, but they are also linked with neurodegeneration. NMDARs at synapses are activated by coincident glutamate release and depolarization. NMDARs distal to synapses can sometimes be recruited by ‘spill‐over’ of glutamate during high‐frequency synaptic stimulation or when glutamate uptake is compromised, and this influences the shape of NMDAR‐mediated postsynaptic responses. In substantia nigra dopamine neurons, activation of NMDARs beyond the synapse during different frequencies of presynaptic stimulation has not been explored, even though excitatory afferents from the subthalamic nucleus show a range of firing frequencies, and these frequencies change in human and experimental Parkinson's disease. This study reports that high‐frequency stimulation (80 Hz/200 ms) evoked NMDAR‐excitatory postsynaptic currents (EPSCs) that were larger and longer lasting than those evoked by single stimuli at low frequency (0.1 Hz). MK‐801, which irreversibly blocked NMDAR‐EPSCs activated during 0.1‐Hz stimulation, left a proportion of NMDAR‐EPSCs that could be activated by 80‐Hz stimulation and that may represent activity of NMDARs distal to synapses. TBOA, which blocks glutamate transporters, significantly increased NMDAR‐EPSCs in response to 80‐Hz stimulation, particularly when metabotropic glutamate receptors (mGluRs) were also blocked, indicating that recruitment of NMDARs distal to synapses is regulated by glutamate transporters and mGluRs. These regulatory mechanisms may be essential in the substantia nigra for restricting glutamate diffusion from synaptic sites and keeping NMDAR‐EPSCs in dopamine neurons relatively small and fast. Failure of glutamate transporters may contribute to the declining health of dopamine neurons during pathological conditions.

The ubiquitous nature of multivesicular release

'Simplicity is prerequisite for reliability' (E.W. Dijkstra [1]) Presynaptic action potentials trigger the fusion of vesicles to release neurotransmitter onto postsynaptic neurons. Each release site was originally thought to liberate at most one vesicle per action potential in a probabilistic fashion, rendering synaptic transmission unreliable. However, the simultaneous release of several vesicles, or multivesicular release (MVR), represents a simple mechanism to overcome the intrinsic unreliability of synaptic transmission. MVR was initially identified at specialized synapses but is now known to be common throughout the brain. MVR determines the temporal and spatial dispersion of transmitter, controls the extent of receptor activation, and contributes to adapting synaptic strength during plasticity and neuromodulation. MVR consequently represents a widespread mechanism that extends the dynamic range of synaptic processing.

Properties of a Glutamatergic Synapse Controlling Information Output from Retinal Bipolar Cells

One general categorization of retinal ganglion cells is to segregate them into tonically or phasically responding neurons, each conveying discrete aspects of the visual scene. Although best identified in the output signals of the retina, this distinction is initiated at the first synapse: between photoreceptors and the dendrites of bipolar cells. In this study we found that the output synapses of bipolar cells also contribute to separate these pathways. Both transient and sustained ganglion cells can produce maintained spike activity, but bipolar cell glutamate release exhibits a divergence that corresponds to the response characteristics of the ganglion cells. Comparing light intensity coding in the sustained and transient ON pathways revealed that they shared the intensity spectrum. The transient pathway had greater sensitivity but smaller dynamic range, and switched from intensity coding to event detection at light levels where sustained pathway sensitivity began to rise. The distinctive properties of the sustained pathway depended upon inhibition and shifted toward those of the transient pathway in the absence of inhibition. The transient system was comparatively unaffected by the loss of inhibition and this was due to the concomitant activation of perisynaptic NMDA receptors. Overall, the properties of bipolar cell dendritic and axon terminals both contribute to the formation of key aspects of the sustained/transient dichotomy normally associated with ganglion cells.

NMDA receptor activity determines neuronal fate: location or number?

It is widely believed that the proper activation of N-methyl-D-aspartate (NMDA) receptors (NMDARs) promotes neuronal survival, whereas an excessive activation of NMDARs leads to neuronal damage. NMDARs are found at both synaptic and extrasynaptic sites. One current prevailing theory proposes the dichotomy of NMDAR activity. The role of the two population receptors is mutual antagonism. The activation of synaptic NMDARs, such as synaptic activity at physiological levels, promotes neuronal survival. However, the activation of extrasynaptic NMDARs occurring during stroke, brain injury, and chronic neurological diseases contributes to neuronal death. Thus, the location of NMDARs determines the neuronal fate. However, the theory is greatly challenged. Several studies suggested that synaptic NMDARs are involved in neuronal death. Recently, our work further showed that the coactivation of synaptic and extrasynaptic NMDARs contributes to neuronal death under neuronal insults. Therefore, we propose that the magnitude and duration of NMDAR activation determines the neuronal fate. More interestingly, there appears to be some subtle differences in the affinity between synaptic and extrasynaptic NMDARs, shedding light on the development of selective drugs to block extrasynaptic NMDARs.

Neuroprotective effect of astaxanthin against rat retinal ganglion cell death under various stresses that induce apoptosis and necrosis

PURPOSE

Astaxanthin is a type of carotenoid known to have strong antioxidant effects. The purpose of this study was to investigate whether astaxanthin confers a neuroprotective effect against glutamate stress, oxidative stress, and hypoxia-induced apoptotic or necrotic cell death in primary cultures of rat retinal ganglion cells (RGCs).

METHODS

Purified rat RGCs were exposed to three kinds of stressors induced by 25 μM glutamate for 72 h, B27 medium without an antioxidant for 4 h, and a reduced oxygen level of 5% for 12 h. Each assay was repeated 12 times, with or without 1 nM, 10 nM, and 100 nM astaxanthin. The number of live RGCs was then counted using a cell viability assay. RGC viability in each condition was evaluated and compared with controls. In addition, we measured apoptosis and DNA damage.

RESULTS

We found that under glutamate stress, RGC viability was reduced to 58%. Cultures with 1 nM, 10 nM, and 100 nM astaxanthin showed an increase in RGC viability of 63%, 74%, and 84%, respectively. Under oxidative stress, RGC viability was reduced to 40%, and astaxanthin administration resulted in increased viability of 43%, 50%, and 67%, respectively. Under hypoxia, RGC viability was reduced to 66%, and astaxanthin administration resulted in a significant increase in viability to 67%, 77%, and 93%, respectively. These results indicate that 100 nM astaxanthin leads to a statistically significant increase in RGC viability under the three kinds of stressors tested, compared to controls (Dunnett's test, p<0.05). The apoptotic activity of RGCs under glutamate stress increased to 32%, but was reduced to 15% with 100 nM astaxanthin administration. Glutamate stress led to a 58% increase in DNA damage, which was reduced to 43% when cultured with 100 nM astaxanthin. Thus, 100 nM astaxanthin showed a statistically significant reduction in apoptosis and DNA damage in RGCs (Wilcoxon rank-sum test, p<0.05).

CONCLUSIONS

Our results suggest that astaxanthin has a neuroprotective effect against RGC death induced by glutamate stress, oxidative stress, and hypoxia, which induce apoptotic and necrotic cell death.

Activity-dependent modulation of inhibitory synaptic kinetics in the cochlear nucleus

Spherical bushy cells (SBCs) in the anteroventral cochlear nucleus respond to acoustic stimulation with discharges that precisely encode the phase of low-frequency sound. The accuracy of spiking is crucial for sound localization and speech perception. Compared to the auditory nerve input, temporal precision of SBC spiking is improved through the engagement of acoustically evoked inhibition. Recently, the inhibition was shown to be less precise than previously understood. It shifts from predominantly glycinergic to synergistic GABA/glycine transmission in an activity-dependent manner. Concurrently, the inhibition attains a tonic character through temporal summation. The present study provides a comprehensive understanding of the mechanisms underlying this slow inhibitory input. We performed whole-cell voltage clamp recordings on SBCs from juvenile Mongolian gerbils and recorded evoked inhibitory postsynaptic currents (IPSCs) at physiological rates. The data reveal activity-dependent IPSC kinetics, i.e., the decay is slowed with increased input rates or recruitment. Lowering the release probability yielded faster decay kinetics of the single- and short train-IPSCs at 100 Hz, suggesting that transmitter quantity plays an important role in controlling the decay. Slow transmitter clearance from the synaptic cleft caused prolonged receptor binding and, in the case of glycine, spillover to nearby synapses. The GABAergic component prolonged the decay by contributing to the asynchronous vesicle release depending on the input rate. Hence, the different factors controlling the amount of transmitters in the synapse jointly slow the inhibition during physiologically relevant activity. Taken together, the slow time course is predominantly determined by the receptor kinetics and transmitter clearance during short stimuli, whereas long duration or high frequency stimulation additionally engage asynchronous release to prolong IPSCs.

NMDA and AMPA receptors contribute similarly to temporal processing in mammalian retinal ganglion cells

Postsynaptic AMPA- and NMDA-type glutamate receptors (AMPARs, NMDARs) are commonly expressed at the same synapses. AMPARs are thought to mediate the majority of fast excitatory neurotransmission whereas NMDARs, with their relatively slower kinetics and higher Ca(2+) permeability, are thought to mediate synaptic plasticity, especially in neural circuits devoted to learning and memory. In sensory neurons, however, the roles of AMPARs and NMDARs are less well understood. Here, we tested in the in vitro guinea pig retina whether AMPARs and NMDARs differentially support temporal contrast encoding by two ganglion cell types. In both OFF Alpha and Delta ganglion cells, contrast stimulation evoked an NMDAR-mediated response with a characteristic J-shaped I-V relationship. In OFF Delta cells, AMPAR- and NMDAR-mediated responses could be modulated at low frequencies but were suppressed during 10 Hz stimulation, when responses were instead shaped by synaptic inhibition. With inhibition blocked, both AMPAR- and NMDAR-mediated responses could be modulated at 10 Hz, indicating that NMDAR kinetics do not limit temporal encoding. In OFF Alpha cells, NMDAR-mediated responses followed stimuli at frequencies up to ∼18 Hz. In both cell types, NMDAR-mediated responses to contrast modulation at 9-18 Hz showed delays of <10 ms relative to AMPAR-mediated responses. Thus, NMDARs combine with AMPARs to encode rapidly modulated glutamate release, and NMDAR kinetics do not limit temporal coding by OFF Alpha and Delta ganglion cells substantially. Furthermore, glutamatergic transmission is differentially regulated across bipolar cell pathways: in some, release is suppressed at high temporal frequencies by presynaptic inhibition.

Extrasynaptic NMDA Receptor in Excitotoxicity: Function Revisited

It is generally accepted that proper activation of N-methyl-d-aspartate receptors (NMDARs) promotes neuronal survival and supports neuroplasticity, and excessive NMDAR activation leads to pathological outcomes and neurodegeneration. As NMDARs are found at both synaptic and extrasynaptic sites, there is significant interest in determining how NMDARs at different subcellular locations differentially regulate physiological as well as pathological functions. Better understanding of this issue may support the development of therapeutic strategies to attenuate neuronal death or promote normal brain function. Although the current prevailing theory emphasizes the major role of extrasynaptic NMDARs in neurodegeneration, there is growing evidence indicating the involvement of synaptic receptors. It is also evident that physiological functions of the brain also involve extrasynaptic NMDARs. Our recent studies demonstrate that the degree of cell death following neuronal insults depends on the magnitude and duration of synaptic and extrasynaptic receptor co-activation. These new results underscore the importance of revisiting the function of extrasynaptic NMDARs in cell fate. Furthermore, the development of antagonists that preferentially inhibit synaptic or extrasynaptic receptors may better clarify the role of NMDARs in neurodegeneration.

Synthetic conantokin peptides potently inhibit N-methyl-D-aspartate receptor-mediated currents of retinal ganglion cells

Retinal ganglion cells (RGCs), which are the sole output neurons of the retina, express N-methyl-D-aspartate receptors (NMDARs), rendering these cells susceptible to glutamate excitotoxicity, with implications for loss of normal RGC excitatory responses in disorders such as glaucoma and diabetic retinopathy. Therefore, antagonists that inhibit NMDAR-mediated currents specifically by targeting the GluN2B component of the ion channel have the potential to serve as a basis for developing potential therapeutics. The roles of peptidic conantokins, which are potent brain neuronal NMDAR inhibitors, were studied. By using patch-clamp whole-cell analyses in dissociated RGCs and retinal whole-mount RGCs, we evaluated the effects of synthetic conantokin-G (conG) and conantokin-T (conT), which are small γ-carboxyglutamate-containing peptides, on NMDA-mediated excitatory responses in mouse RGCs. Both conG and conT inhibited the NMDA-mediated currents of dark-adapted dissociated and whole-mount RGCs in a dose-dependent, reversible, noncompetitive manner. Inhibition of NMDA-mediated steady-state currents by NMDAR nonsubunit-selective conT was approximately threefold greater than GluN2B-selective conG or ifenprodil, demonstrating its potential ability to inhibit both GluN2A- and GluN2B-containing ion channels in RGCs. Because the extent of inhibition of NMDA-evoked currents by conG and the pharmacologic GluN2B-selective inhibitor ifenprodil were similar (40-45%) to that of the GluN2A-selective antagonist NVP-AAM0077, we conclude that the levels of GluN2A and GluN2B subunits are similar in RGCs. These results provide a novel basis for developing effective neuroprotective agents to aid in the prevention of undesired glutamatergic excitotoxicity in neurodegenerative diseases of the retina and demonstrate functional assembly of NMDARs in RGCs.

Functional architecture of the retina: development and disease

Structure and function are highly correlated in the vertebrate retina, a sensory tissue that is organized into cell layers with microcircuits working in parallel and together to encode visual information. All vertebrate retinas share a fundamental plan, comprising five major neuronal cell classes with cell body distributions and connectivity arranged in stereotypic patterns. Conserved features in retinal design have enabled detailed analysis and comparisons of structure, connectivity and function across species. Each species, however, can adopt structural and/or functional retinal specializations, implementing variations to the basic design in order to satisfy unique requirements in visual function. Recent advances in molecular tools, imaging and electrophysiological approaches have greatly facilitated identification of the cellular and molecular mechanisms that establish the fundamental organization of the retina and the specializations of its microcircuits during development. Here, we review advances in our understanding of how these mechanisms act to shape structure and function at the single cell level, to coordinate the assembly of cell populations, and to define their specific circuitry. We also highlight how structure is rearranged and function is disrupted in disease, and discuss current approaches to re-establish the intricate functional architecture of the retina.

Disinhibitory recruitment of NMDA receptor pathways in retina

Glutamate release at bipolar to ganglion cell synapses activates NMDA and AMPA/kainic acid (KA) ionotropic glutamate receptors. Their relative strength determines the output signals of the retina. We found that this balance is tightly regulated by presynaptic inhibition that preferentially suppresses NMDA receptor (NMDAR) activation. In transient ON-OFF neurons, block of GABA and glycine feedback enhanced total NMDAR charge by 35-fold in the ON response and 9-fold in the OFF compared with a 1.7-fold enhancement of AMPA/KA receptors. Blocking only glycine receptors enhanced the NMDAR excitatory postsynaptic current 10-fold in the ON and 2-fold in the OFF pathway. Blocking GABA(A) or GABA(C) receptors (GABA(C)Rs or GABA(A)Rs) produced small changes in total NMDAR charge. When both GABA(A)Rs and GABA(C)Rs were blocked, the total NMDAR charge increased ninefold in the ON and fivefold in the OFF pathway. This exposed a strong GABA(C)R feedback to bipolar cells that was suppressed by serial amacrine cell synapses mediated by GABA(A)Rs. The results indicate that NMDAR currents are large but latent, held in check by dual GABA and glycine presynaptic inhibition. One example of this controlled NMDAR activation is the cross talk between ON and OFF pathways. Blocking the ON pathway increased NMDAR relative strength in the OFF pathway. Stimulus prolongation similarly increased the NMDAR relative strength in the OFF response. This NMDAR enhancement was produced by a diminution in GABA and glycine feedback. Thus the retinal network recruits NMDAR pathways through presynaptic disinhibition.

Correlated spontaneous activity persists in adult retina and is suppressed by inhibitory inputs

Spontaneous rhythmic activity is a hallmark feature of the developing retina, where propagating retinal waves instruct axonal targeting and synapse formation. Retinal waves cease around the time of eye-opening; however, the fate of the underlying synaptic circuitry is unknown. Whether retinal waves are unique to the developing retina or if they can be induced in adulthood is not known. Combining patch-clamp techniques with calcium imaging, we demonstrate that propagative events persist in adult mouse retina when it is deprived of inhibitory input. This activity originates in bipolar cells, resembling glutamatergic stage III retinal waves. We find that, as it develops, the network interactions progressively curtail this activity. Together, this provides evidence that the correlated propagative neuronal activity can be induced in adult retina following the blockade of inhibitory interactions.

The first stage of cardinal direction selectivity is localized to the dendrites of retinal ganglion cells

Inferring the direction of image motion is a fundamental component of visual computation and essential for visually guided behavior. In the retina, the direction of image motion is computed in four cardinal directions, but it is not known at which circuit location along the flow of visual information the cardinal direction selectivity first appears. We recorded the concerted activity of the neuronal circuit elements of single direction-selective (DS) retinal ganglion cells at subcellular resolution by combining GCaMP3-functionalized transsynaptic viral tracing and two-photon imaging. While the visually evoked activity of the dendritic segments of the DS cells were direction selective, direction-selective activity was absent in the axon terminals of bipolar cells. Furthermore, the glutamate input to DS cells, recorded using a genetically encoded glutamate sensor, also lacked direction selectivity. Therefore, the first stage in which extraction of a cardinal motion direction occurs is the dendrites of DS cells.

Light-evoked currents in retinal ganglion cells from dystrophic RCS rats

PURPOSE

To study the electrophysiological properties of the light-evoked currents in ganglion cells in situations of retinal degeneration.

METHODS

We investigated light-evoked currents in ganglion cells by performing whole-cell patch-clamp recordings from ganglion cells using a retina-stretched preparation from Royal College of Surgeons (RCS) rats, a model of retinal degeneration and congenic controls at different ages. Pharmacological inhibitors of the AMPA receptor (NBQX), GABA receptor (BMI), and sodium channels (TTX) were used to identify the components of the light-evoked currents in ON, OFF and ON-OFF retinal ganglion cells.

RESULTS

We found that the light-evoked currents in ganglion cells from control rats were inhibited by NBQX, BMI and TTX, suggesting that AMPA receptors, GABA receptors and sodium channels contribute to these currents in ganglion cells. However, only AMPA receptor-mediated currents were recorded in RCS rats. Light-evoked inward currents were absent in the majority of ganglion cells from RCS rats, particularly at the later stages of retinal degeneration. At earlier stages of retinal degeneration, we found that both the timing and amplitude of light-evoked currents are significantly different in ganglion cells from RCS and control rats.

CONCLUSIONS

Our study furthers the understanding of the electrophysiological characteristics of retinal ganglion cells during retinal degeneration, and provides insight into the optimal timing for the treatment of retinal degeneration.

Alterations of sodium and potassium channels of RGCs in RCS rat with the development of retinal degeneration

All know that retinitis pigmentosa (RP) is a group of hereditary retinal degenerative diseases characterized by progressive dysfunction of photoreceptors and associated with progressive cells loss; nevertheless, little is known about how rods and cones loss affects the surviving inner retinal neurons and networks. Retinal ganglion cells (RGCs) process and convey visual information from retina to visual centers in the brain. The healthy various ion channels determine the normal reception and projection of visual signals from RGCs. Previous work on the Royal College of Surgeons (RCS) rat, as a kind of classical RP animal model, indicated that, at late stages of retinal degeneration in RCS rat, RGCs were also morphologically and functionally affected. Here, retrograde labeling for RGCs with Fluorogold was performed to investigate the distribution, density, and morphological changes of RGCs during retinal degeneration. Then, patch clamp recording, western blot, and immunofluorescence staining were performed to study the channels of sodium and potassium properties of RGCs, so as to explore the molecular and proteinic basis for understanding the alterations of RGCs membrane properties and firing functions. We found that the resting membrane potential, input resistance, and capacitance of RGCs changed significantly at the late stage of retinal degeneration. Action potential could not be evoked in a part of RGCs. Inward sodium current and outward potassium current recording showed that sodium current was impaired severely but only slightly in potassium current. Expressions of sodium channel protein were impaired dramatically at the late stage of retinal degeneration. The results suggested that the density of RGCs decreased, process ramification impaired, and sodium ion channel proteins destructed, which led to the impairment of electrophysiological functions of RGCs and eventually resulted in the loss of visual function.

Spillover-mediated feedforward inhibition functionally segregates interneuron activity

Neurotransmitter spillover represents a form of neural transmission not restricted to morphologically defined synaptic connections. Communication between climbing fibers (CFs) and molecular layer interneurons (MLIs) in the cerebellum is mediated exclusively by glutamate spillover. Here, we show how CF stimulation functionally segregates MLIs based on their location relative to glutamate release. Excitation of MLIs that reside within the domain of spillover diffusion coordinates inhibition of MLIs outside the diffusion limit. CF excitation of MLIs is dependent on extrasynaptic NMDA receptors that enhance the spatial and temporal spread of CF signaling. Activity mediated by functionally segregated MLIs converges onto neighboring Purkinje cells (PCs) to generate a long-lasting biphasic change in inhibition. These data demonstrate how glutamate release from single CFs modulates excitability of neighboring PCs, thus expanding the influence of CFs on cerebellar cortical activity in a manner not predicted by anatomical connectivity.

Extracellular sheets and tunnels modulate glutamate diffusion in hippocampal neuropil

Although the extracellular space in the neuropil of the brain is an important channel for volume communication between cells and has other important functions, its morphology on the micron scale has not been analyzed quantitatively owing to experimental limitations. We used manual and computational techniques to reconstruct the 3D geometry of 180 μm(3) of rat CA1 hippocampal neuropil from serial electron microscopy and corrected for tissue shrinkage to reflect the in vivo state. The reconstruction revealed an interconnected network of 40-80 nm diameter tunnels, formed at the junction of three or more cellular processes, spanned by sheets between pairs of cell surfaces with 10-40 nm width. The tunnels tended to occur around synapses and axons, and the sheets were enriched around astrocytes. Monte Carlo simulations of diffusion within the reconstructed neuropil demonstrate that the rate of diffusion of neurotransmitter and other small molecules was slower in sheets than in tunnels. Thus, the non-uniformity found in the extracellular space may have specialized functions for signaling (sheets) and volume transmission (tunnels).