GABAC receptor-mediated inhibition in the retina

An Expanded Narrative Review of Neurotransmitters on Alzheimer's Disease: The Role of Therapeutic Interventions on Neurotransmission

Alzheimer's disease (AD) is a progressive neurodegenerative disease. The accumulation of amyloid-β (Aβ) plaques and tau neurofibrillary tangles are the key players responsible for the pathogenesis of the disease. The accumulation of Aβ plaques and tau affect the balance in chemical neurotransmitters in the brain. Thus, the current review examined the role of neurotransmitters in the pathogenesis of Alzheimer's disease and discusses the alterations in the neurochemical activity and cross talk with their receptors and transporters. In the presence of Aβ plaques and neurofibrillary tangles, changes may occur in the expression of neuronal receptors which in turn triggers excessive release of glutamate into the synaptic cleft contributing to cell death and neuronal damage. The GABAergic system may also be affected by AD pathology in a similar way. In addition, decreased receptors in the cholinergic system and dysfunction in the dopamine neurotransmission of AD pathology may also contribute to the damage to cognitive function. Moreover, the presence of deficiencies in noradrenergic neurons within the locus coeruleus in AD suggests that noradrenergic stimulation could be useful in addressing its pathophysiology. The regulation of melatonin, known for its effectiveness in enhancing cognitive function and preventing Aβ accumulation, along with the involvement of the serotonergic system and histaminergic system in cognition and memory, becomes remarkable for promoting neurotransmission in AD. Additionally, nitric oxide and adenosine-based therapeutic approaches play a protective role in AD by preventing neuroinflammation. Overall, neurotransmitter-based therapeutic strategies emerge as pivotal for addressing neurotransmitter homeostasis and neurotransmission in the context of AD. This review discussed the potential for neurotransmitter-based drugs to be effective in slowing and correcting the neurodegenerative processes in AD by targeting the neurochemical imbalance in the brain. Therefore, neurotransmitter-based drugs could serve as a future therapeutic strategy to tackle AD.

Negative modulation of the GABAAρ1 receptor function by histamine

Besides its function as a local mediator of the immune response, histamine can play a role as a neurotransmitter and neuromodulator. Histamine actions are classically mediated through four different G protein-coupled receptor subtypes but non-classical actions were also described, including effects on many ligand-gated ion channels. Previous evidence indicated that histamine acts as a positive modulator on diverse GABAA receptor subtypes, such as GABAAα1β2γ2, GABAAα2β3γ2, GABAAα3β3γ2, GABAAα4β3γ2 and GABAAα5β3γ2. Meanwhile, its effects on GABAAρ1 receptors, known to stand for tonic currents in retinal neurons, had not been examined before. The effects of histamine on the function of human homomeric GABAAρ1 receptors were studied here, using heterologous expression in Xenopus laevis oocytes followed by the electrophysiological recording of GABA-evoked Cl- currents. Histamine inhibited GABAAρ1 receptor-mediated responses. Effects were reversible, independent of the membrane potential, and strongly dependent on both histamine and GABA concentration. A rightward parallel shift in the concentration-response curve for GABA was observed in the presence of histamine, without substantial change in the maximal response or the Hill coefficient. Results were compatible with a competitive antagonism of histamine on the GABAAρ1 receptors. This is the first report of inhibitory actions exerted by histamine on an ionotropic GABA receptor.

Comparison of N-methyl-2-pyrrolidone (NMP) and the "date rape" drug GHB: behavioral toxicology in the mouse model

N-methyl-2-pyrrolidone (NMP) and γ-hydroxybutyrate acid (GHB) are synthetic solvents detected in the recreational drug market. GHB has sedative/hypnotic properties and is used for criminal purposes to compromise reaction ability and commit drug-facilitated sexual assaults and other crimes. NMP is a strong solubilizing solvent that has been used alone or mixed with GHB in case of abuse and robberies. The aim of this experimental study is to compare the acute pharmaco-toxicological effects of NMP and GHB on neurological signs (myoclonia, convulsions), sensorimotor (visual, acoustic, and overall tactile) responses, righting reflex, thermoregulation, and motor activity (bar, drag, and accelerod test) in CD-1 male mice. Moreover, since cardiorespiratory depression is one of the main adverse effects related to GHB intake, we investigated the effect of NMP and GHB on cardiorespiratory changes (heart rate, breath rate, oxygen saturation, and pulse distension) in mice. The present study demonstrates that NMP inhibited sensorimotor and motor responses and induced cardiorespiratory depression, with a lower potency and efficacy compared to GHB. These results suggest that NMP can hardly be used alone as a substance to perpetrate sexual assault or robberies.

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.

RNA-seq and GSEA identifies suppression of ligand-gated chloride efflux channels as the major gene pathway contributing to form deprivation myopia

Currently there is no consensus regarding the aetiology of the excessive ocular volume that characterizes high myopia. Thus, we aimed to test whether the gene pathways identified by gene set enrichment analysis of RNA-seq transcriptomics refutes the predictions of the Retinal Ion Driven Efflux (RIDE) hypothesis when applied to the induction of form-deprivation myopia (FDM) and subsequent recovery (post-occluder removal). We found that the induction of profound FDM led to significant suppression in the ligand-gated chloride ion channel transport pathway via suppression of glycine, GABA_A and GABA_C ionotropic receptors. Post-occluder removal for short term recovery from FDM of 6 h and 24 h, induced significant upregulation of the gene families linked to cone receptor phototransduction, mitochondrial energy, and complement pathways. These findings support a model of form deprivation myopia as a Cl⁻ ion driven adaptive fluid response to the modulation of the visual signal cascade by form deprivation that in turn affects the resultant ionic environment of the outer and inner retinal tissues, axial and vitreal elongation as predicted by the RIDE model. Occluder removal and return to normal light conditions led to return to more normal upregulation of phototransduction, slowed growth rate, refractive recovery and apparent return towards physiological homeostasis.

Altered Visual Function in a Larval Zebrafish Knockout of Neurodevelopmental Risk Gene pdzk1

Purpose

The human PDZK1 gene is located in a genomic susceptibility region for neurodevelopmental disorders. A genome-wide association study identified links between PDZK1 polymorphisms and altered visual contrast sensitivity, an endophenotype for schizophrenia and autism spectrum disorder. The PDZK1 protein is implicated in neurological functioning, interacting with synaptic molecules including postsynaptic density 95 (PSD-95), N-methyl-d-aspartate receptors (NMDARs), corticotropin-releasing factor receptor 1 (CRFR1), and serotonin 2A receptors. The purpose of the present study was to elucidate the role of PDZK1.

Methods

We generated pdzk1-knockout (pdzk1-KO) zebrafish using CRISPR/Cas-9 genome editing. Visual function of 7-day-old fish was assessed at behavioral and functional levels using the optomotor response and scotopic electroretinogram (ERG). We also quantified retinal morphology and densities of PSD-95, NMDAR1, CRFR1, and serotonin in the synaptic inner plexiform layer at 7 days, 4 weeks, and 8 weeks of age. Standard RT-PCR and nonsense-mediated decay interference treatment were also performed to assess genetic compensation in mutants.

Results

Relative to wild-type, pdzk1-KO larvae showed spatial frequency tuning functions with increased amplitude (likely due to abnormal gain control) and reduced ERG b-waves (suggestive of inner retinal dysfunction). No synaptic phenotypes, but possible morphological retinal phenotypes, were identified. We confirmed that the absence of major histological phenotypes was not attributable to genetic compensatory mechanisms.

Conclusions

Our findings point to a role for pdzk1 in zebrafish visual function, and our model system provides a platform for investigating other genes associated with abnormal visual behavior.

Modeling a population of retinal ganglion cells with restricted Boltzmann machines

The retina is a complex circuit of the central nervous system whose aim is to encode visual stimuli prior the higher order processing performed in the visual cortex. Due to the importance of its role, modeling the retina to advance in interpreting its spiking activity output is a well studied problem. In particular, it has been shown that latent variable models can be used to model the joint distribution of Retinal Ganglion Cells (RGCs). In this work, we validate the applicability of Restricted Boltzmann Machines to model the spiking activity responses of a large a population of RGCs recorded with high-resolution electrode arrays. In particular, we show that latent variables can encode modes in the RGC activity distribution that are closely related to the visual stimuli. In contrast to previous work, we further validate our findings by comparing results associated with recordings from retinas under normal and altered encoding conditions obtained by pharmacological manipulation. In these conditions, we observe that the model reflects well-known physiological behaviors of the retina. Finally, we show that we can also discover temporal patterns, associated with distinct dynamics of the stimuli.

Of neurons and pericytes: The neuro-vascular approach to diabetic retinopathy

Diabetic retinopathy (DR) is a frequent complication of diabetes mellitus and an increasingly common cause of visual impairment. Blood vessel damage occurs as the disease progresses, leading to ischemia, neovascularization, blood-retina barrier (BRB) failure and eventual blindness. Although detection and treatment strategies have improved considerably over the past years, there is room for a better understanding of the pathophysiology of the diabetic retina. Indeed, it has been increasingly realized that DR is in fact a disease of the retina's neurovascular unit (NVU), the multi-cellular framework underlying functional hyperemia, coupling neuronal computations to blood flow. The accumulating evidence reveals that both neurochemical (synapses) and electrical (gap junctions) means of communications between retinal cells are affected at the onset of hyperglycemia, warranting a global assessment of cellular interactions and their role in DR. This is further supported by the recent data showing down-regulation of connexin 43 gap junctions along the vascular relay from capillary to feeding arteriole as one of the earliest indicators of experimental DR, with rippling consequences to the anatomical and physiological integrity of the retina. Here, recent advancements in our knowledge of mechanisms controlling the retinal neurovascular unit will be assessed, along with their implications for future treatment and diagnosis of DR.

Analysis of Modulation of the ρ1 GABAA Receptor by Combinations of Inhibitory and Potentiating Neurosteroids Reveals Shared and Distinct Binding Sites

The ρ1 GABA_A receptor is prominently expressed in the retina and is present at lower levels in several brain regions and other tissues. Although the ρ1 receptor is insensitive to many anesthetic drugs that modulate the heteromeric GABA_A receptor, it maintains a rich and multifaceted steroid pharmacology. The receptor is negatively modulated by 5β-reduced steroids, sulfated or carboxylated steroids, and β-estradiol, whereas many 5α-reduced steroids potentiate the receptor. In this study, we analyzed modulation of the human ρ1 GABA_A receptor by several neurosteroids, individually and in combination, in the framework of the coagonist concerted transition model. Experiments involving coapplication of two or more steroids revealed that the receptor contains at least three classes of distinct, nonoverlapping sites for steroids, one each for the inhibitory steroids pregnanolone (3α5βP), 3α5βP sulfate, and β-estradiol. The site for 3α5βP can accommodate the potentiating steroid 5αTHDOC. The findings are discussed with respect to receptor modulation by combinations of endogenous neurosteroids. SIGNIFICANCE STATEMENT: The study describes modulation of the ρ1 GABA_A receptor by neurosteroids. The coagonist concerted transition model was used to determine overlap of binding sites for several inhibitory and potentiating steroids.

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.

Nystagmus in patients with congenital stationary night blindness (CSNB) originates from synchronously firing retinal ganglion cells

Congenital nystagmus, involuntary oscillating small eye movements, is commonly thought to originate from aberrant interactions between brainstem nuclei and foveal cortical pathways. Here, we investigated whether nystagmus associated with congenital stationary night blindness (CSNB) results from primary deficits in the retina. We found that CSNB patients as well as an animal model (nob mice), both of which lacked functional nyctalopin protein (NYX, nyx) in ON bipolar cells (BCs) at their synapse with photoreceptors, showed oscillating eye movements at a frequency of 4-7 Hz. nob ON direction-selective ganglion cells (DSGCs), which detect global motion and project to the accessory optic system (AOS), oscillated with the same frequency as their eyes. In the dark, individual ganglion cells (GCs) oscillated asynchronously, but their oscillations became synchronized by light stimulation. Likewise, both patient and nob mice oscillating eye movements were only present in the light when contrast was present. Retinal pharmacological and genetic manipulations that blocked nob GC oscillations also eliminated their oscillating eye movements, and retinal pharmacological manipulations that reduced the oscillation frequency of nob GCs also reduced the oscillation frequency of their eye movements. We conclude that, in nob mice, synchronized oscillations of retinal GCs, most likely the ON-DCGCs, cause nystagmus with properties similar to those associated with CSNB in humans. These results show that the nob mouse is the first animal model for a form of congenital nystagmus, paving the way for development of therapeutic strategies.

Differential effects of antipsychotic drugs on contrast response functions of retinal ganglion cells in wild-type Sprague-Dawley rats and P23H retinitis pigmentosa rats

Antipsychotic drugs haloperidol and clozapine have been reported to increase the sensitivity of retinal ganglion cells (RGCs) to flashes of light in the P23H rat model of retinitis pigmentosa. In order to better understand the effects of these antipsychotic drugs on the visual responses of P23H rat RGCs, I examined the responses of RGCs to a drifting sinusoidal grating of various contrasts. In-vitro multielectrode array recordings were made from P23H rat RGCs and healthy Sprague-Dawley (SD) rat RGCs. Retinas were stimulated with a drifting sinusoidal grating with eight values of contrast (0, 4, 6, 8.5, 13, 26, 51, and 83%). Contrast response functions based on response amplitudes were fitted with a hyperbolic ratio function and contrast thresholds were determined from the fitted curves. SD rat RGCs were divided into two categories, saturating and non-saturating cells, based on whether they showed saturation of responses at high contrast levels. Most SD rat RGCs (58%) were saturating cells. Haloperidol and clozapine decreased the responses of saturating SD rat RGCs to all grating contrasts, except for the highest contrast tested. Clozapine also decreased the responses of non-saturating SD rat RGCs to all grating contrasts, except for the highest contrast tested. Haloperidol did not however significantly affect the responses of non-saturating SD rat RGCs. Haloperidol and clozapine increased the contrast thresholds of both saturating and non-saturating cells in SD rat retinas. Most (73%) P23H rat RGCs could be categorized as either saturating or non-saturating cells. The remaining ‘uncategorized’ cells were poorly responsive to the drifting grating and were analyzed separately. Haloperidol and clozapine increased the responses of non-saturating and uncategorized P23H rat RGCs to most grating contrasts, including the highest contrast tested. Haloperidol and clozapine also increased the responses of saturating P23H rat RGCs to most grating contrasts but these increases were not statistically significant. Haloperidol and clozapine decreased the contrast thresholds of saturating cells, non-saturating cells and uncategorized cells in P23H rat retinas, although the decrease in contrast thresholds of saturating cells was not found to be statistically significant. Overall, the findings show that haloperidol and clozapine have differential effects on the contrast response functions of SD and P23H rat RGCs. In contrast to the effects observed on SD rat RGCs, both haloperidol and clozapine increased the responsiveness of P23H rat RGCs to both low and high contrast visual stimuli and decreased contrast thresholds.

Circadian rhythms, refractive development, and myopia

PURPOSE

Despite extensive research, mechanisms regulating postnatal eye growth and those responsible for ametropias are poorly understood. With the marked recent increases in myopia prevalence, robust and biologically-based clinical therapies to normalize refractive development in childhood are needed. Here, we review classic and contemporary literature about how circadian biology might provide clues to develop a framework to improve the understanding of myopia etiology, and possibly lead to rational approaches to ameliorate refractive errors developing in children.

RECENT FINDINGS

Increasing evidence implicates diurnal and circadian rhythms in eye growth and refractive error development. In both humans and animals, ocular length and other anatomical and physiological features of the eye undergo diurnal oscillations. Systemically, such rhythms are primarily generated by the 'master clock' in the surpachiasmatic nucleus, which receives input from the intrinsically photosensitive retinal ganglion cells (ipRGCs) through the activation of the photopigment melanopsin. The retina also has an endogenous circadian clock. In laboratory animals developing experimental myopia, oscillations of ocular parameters are perturbed. Retinal signaling is now believed to influence refractive development; dopamine, an important neurotransmitter found in the retina, not only entrains intrinsic retinal rhythms to the light:dark cycle, but it also modulates refractive development. Circadian clocks comprise a transcription/translation feedback control mechanism utilizing so-called clock genes that have now been associated with experimental ametropias. Contemporary clinical research is also reviving ideas first proposed in the nineteenth century that light exposures might impact refraction in children. As a result, properties of ambient lighting are being investigated in refractive development. In other areas of medical science, circadian dysregulation is now thought to impact many non-ocular disorders, likely because the patterns of modern artificial lighting exert adverse physiological effects on circadian pacemakers. How, or if, such modern light exposures and circadian dysregulation contribute to refractive development is not known.

SUMMARY

The premise of this review is that circadian biology could be a productive area worthy of increased investigation, which might lead to the improved understanding of refractive development and improved therapeutic interventions.

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.

Negative modulation of the GABAA ρ1 receptor function by l-cysteine

l-Cysteine is an endogenous sulfur-containing amino acid with multiple and varied roles in the central nervous system, including neuroprotection and the maintenance of the redox balance. However, it was also suggested as an excitotoxic agent implicated in the pathogenesis of neurological disorders such as Parkinson's and Alzheimer's disease. l-Cysteine can modulate the activity of ionic channels, including voltage-gated calcium channels and glutamatergic NMDA receptors, whereas its effects on GABAergic neurotransmission had not been studied before. In the present work, we analyzed the effects of l-cysteine on responses mediated by homomeric GABA_A ρ1 receptors, which are known for mediating tonic γ-aminobutyric acid (GABA) responses in retinal neurons. GABA_A ρ1 receptors were expressed in Xenopus laevis oocytes and GABA-evoked chloride currents recorded by two-electrode voltage-clamp in the presence or absence of l-cysteine. l-Cysteine antagonized GABA_A ρ1 receptor-mediated responses; inhibition was dose-dependent, reversible, voltage independent, and susceptible to GABA concentration. Concentration-response curves for GABA were shifted to the right in the presence of l-cysteine without a substantial change in the maximal response. l-Cysteine inhibition was insensitive to chemical protection of the sulfhydryl groups of the ρ1 subunits by the irreversible alkylating agent N-ethyl maleimide. Our results suggest that redox modulation is not involved during l-cysteine actions and that l-cysteine might be acting as a competitive antagonist of the GABA_A ρ1 receptors.

Contribution of GABAa, GABAc and glycine receptors to rat dark-adapted oscillatory potentials in the time and frequency domain

Retinal oscillatory potentials (OPs) consist of a series of relatively high-frequency rhythmic wavelets, superimposed onto the ascending phase of the b-wave of the electroretinogram (ERG). However, the origin of OPs is uncertain and methods of measurement of OPs are diverse. In this study, we first isolated OPs from the rat ERG and fitted them with Gabor functions and found that the envelope of the OP contained information about maximum amplitude and time-to-peak to enable satisfactory quantification of the later OPs. And the OP/b-wave ratio should be evaluated to exclude an effect of the b-wave on the OPs. Next, we recorded OPs after intravitreal injection of 2-amino-4-phosphonobutyric acid (APB), tetrodotoxin (TTX), γ-aminobutyric acid (GABA), strychnine (STR), SR95531 (SR), isoguvacine (ISO), (1,2,5,6-tetrahydropyridin-4-yl) methylphosphinic acid (TPMPA) and GABA+TPMPA. We showed that GABA and APB only removed the later OPs, when compared to control eyes. TTX delayed the peak time, and STR, SR and ISO reduced the amplitude of OPs. TPMPA delayed the peak time but increased the ratio of OPs to b-wave. Furthermore, administration of combined GABA and TPMPA caused the later OPs to increase in amplitude with time, compared with those after delivery of GABA alone. Finally, we observed that GABAc and glycine receptors contributed to a low-frequency component of the OPs, while GABAa contributed to both components. These results suggest that the early components of the OPs are mainly generated by the photoreceptors, whilst the later components are mainly regulated by GABAa, GABAc and glycine receptors.

Pan-retinal characterisation of Light Responses from Ganglion Cells in the Developing Mouse Retina

We have investigated the ontogeny of light-driven responses in mouse retinal ganglion cells (RGCs). Using a large-scale, high-density multielectrode array, we recorded from hundreds to thousands of RGCs simultaneously at pan-retinal level, including dorsal and ventral locations. Responses to different contrasts not only revealed a complex developmental profile for ON, OFF and ON-OFF responses, but also unveiled differences between dorsal and ventral RGC responses. At eye-opening, dorsal RGCs of all types were more responsive to light, perhaps indicating an environmental priority to nest viewing for pre-weaning pups. The developmental profile of ON and OFF responses exhibited antagonistic behaviour, with the strongest ON responses shortly after eye-opening, followed by an increase in the strength of OFF responses later on. Further, we found that with maturation receptive field (RF) center sizes decrease, spike-triggered averaged responses to white noise become stronger, and centers become more circular while maintaining differences between RGC types. We conclude that the maturation of retinal functionality is not spatially homogeneous, likely reflecting ecological requirements that favour earlier maturation of the dorsal retina.

Inhibitory masking controls the threshold sensitivity of retinal ganglion cells

KEY POINTS

Retinal ganglion cells (RGCs) in dark-adapted retinas show a range of threshold sensitivities spanning ∼3 log units of illuminance. Here, we show that the different threshold sensitivities of RGCs reflect an inhibitory mechanism that masks inputs from certain rod pathways. The masking inhibition is subserved by GABA_C receptors, probably on bipolar cell axon terminals. The GABAergic masking inhibition appears independent of dopaminergic circuitry that has been shown also to affect RGC sensitivity. The results indicate a novel mechanism whereby inhibition controls the sensitivity of different cohorts of RGCs. This can limit and thereby ensure that appropriate signals are carried centrally in scotopic conditions when sensitivity rather than acuity is crucial.

ABSTRACT

The responses of rod photoreceptors, which subserve dim light vision, are carried through the retina by three independent pathways. These pathways carry signals with largely different sensitivities. Retinal ganglion cells (RGCs), the output neurons of the retina, show a wide range of sensitivities in the same dark-adapted conditions, suggesting a divergence of the rod pathways. However, this organization is not supported by the known synaptic morphology of the retina. Here, we tested an alternative idea that the rod pathways converge onto single RGCs, but inhibitory circuits selectively mask signals so that one pathway predominates. Indeed, we found that application of GABA receptor blockers increased the sensitivity of most RGCs by unmasking rod signals, which were suppressed. Our results indicate that inhibition controls the threshold responses of RGCs under dim ambient light. This mechanism can ensure that appropriate signals cross the bottleneck of the optic nerve in changing stimulus conditions.

Controlling for Phylogenetic Relatedness and Evolutionary Rates Improves the Discovery of Associations Between Species' Phenotypic and Genomic Differences

The growing number of sequenced genomes allows us now to address a key question in genetics and evolutionary biology: which genomic changes underlie particular phenotypic changes between species? Previously, we developed a computational framework called Forward Genomics that associates phenotypic to genomic differences by focusing on phenotypes that are independently lost in different lineages. However, our previous implementation had three main limitations. Here, we present two new Forward Genomics methods that overcome these limitations by (1) directly controlling for phylogenetic relatedness, (2) controlling for differences in evolutionary rates, and (3) computing a statistical significance. We demonstrate on large-scale simulated data and on real data that both new methods substantially improve the sensitivity to detect associations between phenotypic and genomic differences. We applied these new methods to detect genomic differences involved in the loss of vision in the blind mole rat and the cape golden mole, two independent subterranean mammals. Forward Genomics identified several genes that are enriched in functions related to eye development and the perception of light, as well as genes involved in the circadian rhythm. These new Forward Genomics methods represent a significant advance in our ability to discover the genomic basis underlying phenotypic differences between species. Source code: https://github.com/hillerlab/ForwardGenomics/.

Postsynaptic Plasticity Triggered by Ca²⁺-Permeable AMPA Receptor Activation in Retinal Amacrine Cells

Amacrine cells are thought to be a major locus for mechanisms of light adaptation and contrast enhancement in the retina. However, the potential for plasticity in their AMPA receptor currents remains largely unknown. Using paired patch-clamp recordings between bipolar cell terminals and amacrine cells, we have simultaneously measured presynaptic membrane capacitance changes and EPSCs. Repetitive bipolar cell depolarizations, designed to maintain the same amount of exocytosis, nevertheless significantly potentiated evoked EPSCs in a subpopulation of amacrine cells. Likewise, repetitive iontophoresis (or puffs) of glutamate (or AMPA) onto the dendrites of amacrine cells also significantly potentiated evoked currents and [Ca(2+)]i rises. However, strong postsynaptic Ca(2+) buffering with BAPTA abolished the potentiation and selective antagonists of Ca(2+)-permeable AMPA receptors also blocked the potentiation of AMPA-mediated currents. Together these results suggest that Ca(2+) influx via Ca(2+)-permeable AMPA receptors can elicit a rapid form of postsynaptic plasticity in a subgroup of amacrine cell dendrites.

In vivo electroretinographic studies of the role of GABAC receptors in retinal signal processing

All three classes of receptors for the inhibitory neurotransmitter GABA (GABAR) are expressed in the retina. This study investigated roles of GABAR, especially GABACR (GABA(A)-ρ), in retinal signaling in vivo by studying effects on the mouse electroretinogram (ERG) of genetic deletion of GABACR versus pharmacological blockade using receptor antagonists. Brief full-field flash ERGs were recorded from anesthetized GABACR(-/-) mice, and WT C57BL/6 (B6) mice, before and after intravitreal injection of GABACR antagonists, TPMPA, 3-APMPA, or the more recently developed 2-AEMP; GABAAR antagonist, SR95531; GABABR antagonist, CGP, and agonist, baclofen. Intravitreal injections of TPMPA and SR95531 were also made in Brown Norway rats. The effect of 2-AEMP on GABA-induced current was tested directly in isolated rat rod bipolar cells, and 2-AEMP was found to preferentially block GABACR in those cells. Maximum amplitudes of dark (DA) and light-adapted (LA) ERG b-waves were reduced in GABACR(-/-) mice, compared to B6 mice, by 30-60%; a-waves were unaltered and oscillatory potential amplitudes were increased. In B6 mice, after injection of TPMPA (also in rats), 3-APMPA or 2-AEMP, ERGs became similar to ERGs of GABACR(-/-) mice. Blockade of GABAARs and GABABRs, or agonism of GABABRs did not alter B6 DA b-wave amplitude. The negative scotopic threshold response (nSTR) was slightly less sensitive in GABACR(-/-) than in B6 mice, and unaltered by 2-AEMP. However, amplitudes of nSTR and photopic negative response (PhNR), both of which originate from inner retina, were enhanced by TPMPA and 3-APMPA, each of which has GABAB agonist properties, and further increased by baclofen. The finding that genetic deletion of GABACR, the GABACR antagonist 2-AEMP, and other antagonists all reduced ERG b-wave amplitude, supports a role for GABACR in determining the maximum response amplitude of bipolar cells contributing to the b-wave. GABACR antagonists differed in their effects on nSTR and PhNR; antagonists with GABAB agonist properties enhanced light-driven responses whereas 2-AEMP did not.

Complex inhibitory microcircuitry regulates retinal signaling near visual threshold

Neuronal microcircuits, small, localized signaling motifs involving two or more neurons, underlie signal processing and computation in the brain. Compartmentalized signaling within a neuron may enable it to participate in multiple, independent microcircuits. Each A17 amacrine cell in the mammalian retina contains within its dendrites hundreds of synaptic feedback microcircuits that operate independently to modulate feedforward signaling in the inner retina. Each of these microcircuits comprises a small (<1 μm) synaptic varicosity that typically receives one excitatory synapse from a presynaptic rod bipolar cell (RBC) and returns two reciprocal inhibitory synapses back onto the same RBC terminal. Feedback inhibition from the A17 sculpts the feedforward signal from the RBC to the AII, a critical component of the circuitry mediating night vision. Here, we show that the two inhibitory synapses from the A17 to the RBC express kinetically distinct populations of GABA receptors: rapidly activating GABA(A)Rs are enriched at one synapse while more slowly activating GABA(C)Rs are enriched at the other. Anatomical and electrophysiological data suggest that macromolecular complexes of voltage-gated (Cav) channels and Ca(2+)-activated K(+) channels help to regulate GABA release from A17 varicosities and limit GABA(C)R activation under certain conditions. Finally, we find that selective elimination of A17-mediated feedback inhibition reduces the signal to noise ratio of responses to dim flashes recorded in the feedforward pathway (i.e., the AII amacrine cell). We conclude that A17-mediated feedback inhibition improves the signal to noise ratio of RBC-AII transmission near visual threshold, thereby improving visual sensitivity at night.

Differential alterations in the expression of neurotransmitter receptors in inner retina following loss of photoreceptors in rd1 mouse

Loss of photoreceptors leads to significant remodeling in inner retina of rd1 mouse, a widely used model of retinal degeneration. Several morphological and physiological alterations occur in the second- and third-order retinal neurons. Synaptic activity in the excitatory bipolar cells and the predominantly inhibitory amacrine cells is enhanced. Retinal ganglion cells (RGCs) exhibit hyperactivity and aberrant spiking pattern, which adversely affects the quality of signals they can carry to the brain. To further understand the pathophysiology of retinal degeneration, and how it may lead to aberrant spiking in RGCs, we asked how loss of photoreceptors affects some of the neurotransmitter receptors in rd1 mouse. Using Western blotting, we measured the levels of several neurotransmitter receptors in adult rd1 mouse retina. We found significantly higher levels of AMPA, glycine and GABAa receptors, but lower levels of GABAc receptors in rd1 mouse than in wild-type. Since GABAa receptor is expressed in several retinal layers, we employed quantitative immunohistochemistry to measure GABAa receptor levels in specific retinal layers. We found that the levels of GABAa receptors in inner plexiform layer of wild-type and rd1 mice were similar, whereas those in outer plexiform layer and inner nuclear layer combined were higher in rd1 mouse. Specifically, we found that the number of GABAa-immunoreactive somas in the inner nuclear layer of rd1 mouse retina was significantly higher than in wild-type. These findings provide further insights into neurochemical remodeling in the inner retina of rd1 mouse, and how it might lead to oscillatory activity in RGCs.

GABAergic neurotransmission and retinal ganglion cell function

Ganglion cells are the output retinal neurons that convey visual information to the brain. There are ~20 different types of ganglion cells, each encoding a specific aspect of the visual scene as spatial and temporal contrast, orientation, direction of movement, presence of looming stimuli; etc. Ganglion cell functioning depends on the intrinsic properties of ganglion cell's membrane as well as on the excitatory and inhibitory inputs that these cells receive from other retinal neurons. GABA is one of the most abundant inhibitory neurotransmitters in the retina. How it modulates the activity of different types of ganglion cells and what is its significance in extracting the basic features from visual scene are questions with fundamental importance in visual neuroscience. The present review summarizes current data concerning the types of membrane receptors that mediate GABA action in proximal retina; the effects of GABA and its antagonists on the ganglion cell light-evoked postsynaptic potentials and spike discharges; the action of GABAergic agents on centre-surround organization of the receptive fields and feature related ganglion cell activity. Special emphasis is put on the GABA action regarding the ON-OFF and sustained-transient ganglion cell dichotomy in both nonmammalian and mammalian retina.

An intracellular redox sensor for reactive oxygen species at the M3-M4 linker of GABAA ρ1 receptors

BACKGROUND AND PURPOSE

Reactive oxygen species (ROS) are normally involved in cell oxidative stress but also play a role as cellular messengers in redox signalling; for example, modulating the activity of neurotransmitter receptors and ion channels. However, the direct actions of ROS on GABAA receptors were not previously demonstrated. In the present work, we studied the effects of ROS on GABAA ρ1 receptor function.

EXPERIMENTAL APPROACH

GABAA ρ1 receptors were expressed in oocytes and GABA-evoked responses electrophysiologically recorded in the presence or absence of ROS. Chemical protection of cysteines by selective sulfhydryl reagents and site-directed mutagenesis studies were used to identify protein residues involved in ROS actions.

KEY RESULTS

GABAA ρ1 receptor-mediated responses were significantly enhanced in a concentration-dependent and reversible manner by H₂O₂. Potentiating effects were attenuated by a free radical scavenger, lipoic acid or an inhibitor of the Fenton reaction, deferoxamine. Each ρ1 subunit contains only three cysteine residues, two extracellular at the Cys-loop (C¹⁷⁷ and C¹⁹¹) and one intracellular (C³⁶⁴) at the M3-M4 linker. Mutant GABAA ρ1 receptors in which C³⁶⁴ was exchanged by alanine were completely insensitive to modulation, implying that this site, rather than a cysteine in the Cys-loop, is essential for ROS modulation.

CONCLUSION AND IMPLICATIONS

Our results show that the function of GABAA ρ1 receptors is enhanced by ROS and that the intracellular C³⁶⁴ is the sensor for ROS actions.

Benzodiazepine modulation of homomeric GABAAρ1 receptors: differential effects of diazepam and 4'-chlorodiazepam

GABA(A) receptors (GABA(A)Rs) are ligand-gated ion channels that mediate inhibitory neurotransmission in the central nervous system (CNS). They are members of the Cys-loop receptor family and display marked structural and functional heterogeneity. Many GABA(A)Rs receptor subtypes are allosterically modulated by benzodiazepines (BDZs), which are drugs extensively used as anxiolytics, sedative-hypnotics and anticonvulsants. One high-affinity site and at least three additional low-affinity sites for BDZ recognition have been identified in several heteromeric and homomeric variants of the GABA(A)Rs (e.g.: α1β2γ2, α1β2/3, β3, etc.). However, the modulation of homomeric GABA(A)ρRs by BDZs was not previously revealed, and these receptors, for a long a time, were assumed to be fully insensitive to the actions of these drugs. In the present study, human homomeric GABA(A)ρ1 receptors were expressed in Xenopus oocytes and GABA-evoked responses electrophysiologically recorded in the presence or absence of BDZs. GABA(A)ρ1 receptor-mediated responses were modulated by diazepam and 4'-chlorodiazepam in the micromolar range, in a concentration-dependent, voltage-independent and reversible manner. Diazepam produced potentiating effects on GABA-evoked Cl(-) currents and 4'-Cl diazepam induced biphasic effects depending on the GABA concentration, whereas Ro15-4513 and alprazolam were negative modulators. BDZ actions were insensitive to flumazenil. Other BDZs showed negligible activity at equivalent experimental conditions. Our results suggest that GABA(A)ρ1 receptor function can be selectively and differentially modulated by BDZs.

Ionotropic GABA Receptors and Distal Retinal ON and OFF Responses

In the vertebrate retina, visual signals are segregated into parallel ON and OFF pathways, which provide information for light increments and decrements. The segregation is first evident at the level of the ON and OFF bipolar cells in distal retina. The activity of large populations of ON and OFF bipolar cells is reflected in the b- and d-waves of the diffuse electroretinogram (ERG). The role of gamma-aminobutyric acid (GABA), acting through ionotropic GABA receptors in shaping the ON and OFF responses in distal retina, is a matter of debate. This review summarized current knowledge about the types of the GABAergic neurons and ionotropic GABA receptors in the retina as well as the effects of GABA and specific GABAA and GABAC receptor antagonists on the activity of the ON and OFF bipolar cells in both nonmammalian and mammalian retina. Special emphasis is put on the effects on b- and d-waves of the ERG as a useful tool for assessment of the overall function of distal retinal ON and OFF channels. The role of GABAergic system in establishing the ON-OFF asymmetry concerning the time course and absolute and relative sensitivity of the ERG responses under different conditions of light adaptation in amphibian retina is also discussed.

Effects of picrotoxin on light adapted frog electroretinogram are not due entirely to its action in proximal retina

In order to evaluate the site of action of picrotoxin (antagonist of ionotropic GABA receptors) on the electroretinographic (ERG) b- and d-waves, in this study we compared its effects on the intensity-response function of the ERG waves in intact light adapted frog eyecup preparations with its effects in eyecups, where the activity of proximal neurons was blocked by 1 mMN-methyl-d-aspartate (MNDA). Picrotoxin markedly enhanced the b- and d-wave amplitude and slowed the time course of the responses at all stimulus intensities in the intact eyecups. Perfusion with NMDA alone caused significant enhancement of the b-wave amplitude and diminution of the d-wave amplitude without altering their time course in the entire intensity range. When picrotoxin was applied in combination with NMDA, an enhancement of the b-wave amplitude and slowing of its time course were observed at all stimulus intensities. The increase of the b-wave amplitude was significantly higher than that seen in NMDA group. Combined application of picrotoxin and NMDA caused an enhancement of the d-wave amplitude at the lower stimulus intensities and its diminution at the higher ones, while the d-wave time course was delayed over the entire intensity range. The results obtained indicate that a part of picrotoxin effects on the amplitude and time course of the photopic ERG b- and d-waves are due to its action in the distal frog retina.

GABAergic agents modify the response of chick scleral fibroblasts to myopic and hyperopic eye cup tissues

PURPOSE

GABA antagonists inhibit experimental myopia in chick and GABA receptors have been localized to chick sclera and the retinal pigment epithelium (RPE). The RPE and the choroid alter scleral DNA and glycosaminoglycan (GAG) content in vitro; opposite effects have been observed for tissues from myopic and hyperopic eyes. The aim was to determine the effect of GABAergic agents on the DNA and GAG content of chick scleral fibroblasts directly and in co-culture with ocular tissues from myopic and hyperopic chick eyes.

MATERIALS AND METHODS

Primary cultures of fibroblastic cells expressing vimentin and α-smooth muscle actin were established. GABAergic agents were added separately (i) to the culture medium of the scleral cells and (ii) to the culture medium of the scleral cells with the addition of posterior eye cup tissue (retina, RPE, retina + RPE, choroid + RPE) to cell culture inserts. Ocular tissues were obtained from chick eyes wearing + 15D (lens-induced hyperopia, LIH) or -15D lenses (lens-induced myopia, LIM) for three days (post-hatch day 5-8) (n = 12). GAG and DNA content of scleral fibroblasts were measured.

RESULTS

GABA agents had a small direct effect on scleral cell GAG and DNA content but a larger effect was measured when GABA agents were added to the culture medium with myopic and hyperopic RPE and choroid + RPE tissues. GABA agonists increased (p = 0.002) whereas antagonists decreased (p = 0.0004) DNA content of scleral cells; effects were opposite for scleral GAG content. GABA agents significantly altered the effect of both LIM and LIH tissues (p = 0.0005) compared to control; the effects were greater for LIM tissue versus LIH tissue co-culture (p = 0.0004).

CONCLUSION

GABAergic agents affect the DNA and GAG content of scleral fibroblasts both directly and when co-cultured with ocular tissues. GABA antagonists that prevent myopia development in chick model could act via a scleral mechanism utilizing the RPE/choroid.

Neurovascular interactions in the retina: physiological and pathological roles

Increasing evidence suggests that the complex interactions among multiple cell types including neuronal, glial, and vascular cells, are critical for maintaining adequate cerebral blood flow that is necessary for normal brain function and survival. The disturbance of these interactions contributes to the pathogenesis of central nervous system disorders such as stroke and Alzheimer's disease. The retina is part of the central nervous system, and the properties of vasculature in the retina are similar to those in the brain. The interactions among multiple cell types in the retina also play an important role in the maintenance of tissue homeostasis, and the impairment of interactions can contribute to the onset and/or progression of retinal diseases. In this review, we describe the neurovascular interactions in the retina and alternations of interactions in pathological conditions such as diabetic retinopathy and glaucoma.

Identifying cell class specific losses from serially generated electroretinogram components

Purpose. Processing of information through the cellular layers of the retina occurs in a serial manner. In the electroretinogram (ERG), this complicates interpretation of inner retinal changes as dysfunction may arise from “upstream” neurons or may indicate a direct loss to that neural generator. We propose an approach that addresses this issue by defining ERG gain relationships. Methods. Regression analyses between two serial ERG parameters in a control cohort of rats are used to define gain relationships. These gains are then applied to two models of retinal disease. Results. The PIII_amp to PII_amp gain is unity whereas the PII_amp to pSTR_amp and PII_amp to nSTR_amp gains are greater than unity, indicating “amplification” (P < 0.05). Timing relationships show amplification between PIII_it to PII_it and compression for PII_it to pSTR_it and PII_it to nSTR_it, (P < 0.05). Application of these gains to ω-3-deficiency indicates that all timing changes are downstream of photoreceptor changes, but a direct pSTR amplitude loss occurs (P < 0.05). Application to diabetes indicates widespread inner retinal dysfunction which cannot be attributed to outer retinal changes (P < 0.05). Conclusions. This simple approach aids in the interpretation of inner retinal ERG changes by taking into account gain characteristics found between successive ERG components of normal animals.

Developmental regulation and activity-dependent maintenance of GABAergic presynaptic inhibition onto rod bipolar cell axonal terminals

Presynaptic inhibition onto axons regulates neuronal output, but how such inhibitory synapses develop and are maintained in vivo remains unclear. Axon terminals of glutamatergic retinal rod bipolar cells (RBCs) receive GABAA and GABAC receptor-mediated synaptic inhibition. We found that perturbing GABAergic or glutamatergic neurotransmission does not prevent GABAergic synaptogenesis onto RBC axons. But, GABA release is necessary for maintaining axonal GABA receptors. This activity-dependent process is receptor subtype specific: GABAC receptors are maintained, whereas GABAA receptors containing α1, but not α3, subunits decrease over time in mice with deficient GABA synthesis. GABAA receptor distribution on RBC axons is unaffected in GABAC receptor knockout mice. Thus, GABAA and GABAC receptor maintenance are regulated separately. Although immature RBCs elevate their glutamate release when GABA synthesis is impaired, homeostatic mechanisms ensure that the RBC output operates within its normal range after eye opening, perhaps to regain proper visual processing within the scotopic pathway.

Nitric oxide potentiation of the homomeric ρ1 GABA(C) receptor function

BACKGROUND AND PURPOSE

NO is a highly diffusible and reactive gas produced in the nervous system, which acts as a neuronal signal mediating physiological or pathological mechanisms. NO can modulate the activity of neurotransmitter receptors and ion channels, including NMDA and GABA(A) receptors. In the present work, we examined whether GABA(C) receptor function can also be regulated by NO.

EXPERIMENTAL APPROACH

Homomeric ρ1 GABA(C) receptors were expressed in oocytes and GABA-evoked responses electrophysiologically recorded in the presence or absence of the NO donor DEA. Chemical protection of cysteines by selective sulfhydryl reagents and site-directed mutagenesis were used to determine the protein residues involved in the actions of NO.

KEY RESULTS

GABAρ1 receptor responses were significantly enhanced in a dose-dependent, fast and reversible manner by DEA and the specific NO scavenger CPTIO prevented these potentiating effects. The ρ1 subunits contain only three cysteine residues, two extracellular at the Cys-loop (C177 and C191) and one intracellular (C364). Mutations of C177 and C191 render the ρ1 GABA receptors non-functional, but C364 can be safely exchanged by alanine (C364A). NEM, N-ethyl maleimide and (2-aminoethyl) methanethiosulfonate prevented the effects of DEA on GABAρ1 receptors. Meanwhile, the potentiating effects of DEA on mutant GABAρ1(C364A) receptors were similar to those observed on wild-type receptors.

CONCLUSIONS AND IMPLICATIONS

Our results suggest that the function of GABA(C) receptors can be enhanced by NO acting at the extracellular Cys-loop.

Sustained and Transient Contributions to the Rat Dark-Adapted Electroretinogram b-Wave

The most dominant feature of the electroretinogram, the b-wave, is thought to reflect ON-bipolar cell responses. However, a number of studies suggest that the b-wave is made up of several components. We consider the composition of the rat b-wave by subtracting corneal negative components obtained using intravitreal application of pharmacological agents to remove postreceptoral responses. By analyzing the intensity-response characteristic of the PII across a range of fixed times during and after a light step, we find that the rat isolated PII has 2 components. The first has fast rise and decay characteristics with a low sensitivity to light. GABAc-mediated inhibitory pathways enhance this transient-ON component to manifest increased and deceased sensitivity to light at shorter (<160 ms) and longer times, respectively. The second component has slower temporal characteristics but is more sensitive to light. GABAc-mediated inhibition enhances this sustained-ON component but has little effect on its sensitivity to light. After stimulus offset, both transient and sustained components return to baseline, and a long latency sustained positive component becomes apparent. The light sensitivities of transient-ON and sustained-OFF components are consistent with activity arising from cone ON- and OFF-bipolar cells, whereas the sustained-ON component is likely to arise from rod bipolar cells.

Paired-pulse plasticity in the strength and latency of light-evoked lateral inhibition to retinal bipolar cell terminals

Synapses in the inner plexiform layer of the retina undergo short-term plasticity that may mediate different forms of adaptation to regularities in light stimuli. Using patch-clamp recordings from axotomized goldfish Mb bipolar cell (BC) terminals with paired-pulse light stimulation, we isolated and quantified the short-term plasticity of GABAergic lateral IPSCs (L-IPSCs). Bright light stimulation evoked ON and OFF L-IPSCs in axotomized BCs, which had distinct onset latencies (∼50-80 and ∼70-150 ms, respectively) that depended on background light adaptation. We observed plasticity in both the synaptic strength and latency of the L-IPSCs. With paired light stimulation, latencies of ON L-IPSCs increased at paired-pulse intervals (PPIs) of 50 and 300 ms, whereas OFF L-IPSC latencies decreased at the 300 ms PPI. ON L-IPSCs showed paired-pulse depression at intervals <1 s, whereas OFF L-IPSCs showed depression at intervals ≤1 s and amplitude facilitation at longer intervals (1-2 s). This biphasic form of L-IPSC plasticity may underlie adaptation and sensitization to surround temporal contrast over multiple timescales. Block of retinal signaling at GABA(A)Rs and AMPARs differentially affected ON and OFF L-IPSCs, confirming that these two types of feedback inhibition are mediated by distinct and convergent retinal pathways with different mechanisms of plasticity. We propose that these plastic changes in the strength and timing of L-IPSCs help to dynamically shape the time course of glutamate release from ON-type BC terminals. Short-term plasticity of L-IPSCs may thus influence the strength, timing, and spatial extent of amacrine and ganglion cell inhibitory surrounds.

Cysteine-terminated B-domain of Staphylococcus aureus protein A as a scaffold for targeting GABA(A) receptors

This study reports the preparation and characterization of cysteine-terminated B-domain (Bd-cys) of Staphylococcus aureus protein A, in combination with immunoglobulin G (IgG) antibodies directed against the ρ1 and α1 subunits of GABA(A) receptors, for localizing reagents of interest to the target receptor. A cysteine residue was inserted at the C terminus of the cysteine-lacking B-domain (Bd) and used for conjugating maleimide-containing compounds. As determined by enzyme-linked immunosorbent assay (ELISA), binding of a Bd-cys-S-fluorescein conjugate to polyclonal guinea pig anti-GABA(A)-ρ1 and rabbit anti-GABA(A)-α1 IgG was similar to that exhibited by full-length protein A. Surface plasmon resonance analysis of the interaction of Bd-cys-S-PEG3400-biotin conjugate (where PEG is polyethylene glycol) with anti-GABA(A)-ρ1 and anti-GABA(A)-α1 yielded K(D) values of 6.4 ± 1.9 and 0.4 ± 0.1 nM, respectively. Fluorescence anisotropy analysis of the binding of Bd-cys-S-fluorescein to the two antibodies yielded EC50 values of 65 and 18 nM, respectively. As determined with biotin-reactive fluorescent reagents, Bd-cys-S-PEG3400-biotin specifically bound to the plasma membrane of Xenopus laevis oocytes that expressed α1β2γ2 or homomeric ρ1 GABA(A) receptors and were pretreated with the corresponding anti-GABA(A) IgG. The IgG-binding specificity and high affinity of Bd-cys conjugates illustrate the potential of these conjugates, in combination with a selected IgG, to localize compounds of interest at specific cell surface proteins.

Receptor targets of amacrine cells

Amacrine cells are a morphologically and functionally diverse group of inhibitory interneurons. Morphologically, they have been divided into approximately 30 types. Although this diversity is probably important to the fine structure and function of the retinal circuit, the amacrine cells have been more generally divided into two subclasses. Glycinergic narrow-field amacrine cells have dendrites that ramify close to their somas, cross the sublaminae of the inner plexiform layer, and create cross talk between its parallel ON and OFF pathways. GABAergic wide-field amacrine cells have dendrites that stretch long distances from their soma but ramify narrowly within an inner plexiform layer sublamina. These wide-field cells are thought to mediate inhibition within a sublamina and thus within the ON or OFF pathway. The postsynaptic targets of all amacrine cell types include bipolar, ganglion, and other amacrine cells. Almost all amacrine cells use GABA or glycine as their primary neurotransmitter, and their postsynaptic receptor targets include the most common GABA(A), GABA(C), and glycine subunit receptor configurations. This review addresses the diversity of amacrine cells, the postsynaptic receptors on their target cells in the inner plexiform layer of the retina, and some of the inhibitory mechanisms that arise as a result. When possible, the effects of GABAergic and glycinergic inputs on the visually evoked responses of their postsynaptic targets are discussed.

Amyloid precursor protein is required for normal function of the rod and cone pathways in the mouse retina

Amyloid precursor protein (APP) is a transmembrane glycoprotein frequently studied for its role in Alzheimer's disease. Our recent study in APP knockout (KO) mice identified an important role for APP in modulating normal neuronal development in the retina. However the role APP plays in the adult retina and whether it is required for vision is unknown. In this study we evaluated the role of APP in retinal function and morphology comparing adult wildtype (WT) and APP-KO mice. APP was expressed on neuronal cells of the inner retina, including horizontal, cone bipolar, amacrine and ganglion cells in WT mice. The function of the retina was assessed using the electroretinogram and although the rod photoreceptor responses were similar in APP-KO and WT mice, the post-photoreceptor, inner retinal responses of both the rod and cone pathways were reduced in APP-KO mice. These changes in inner retinal function did not translate to a substantial change in visual acuity as assessed using the optokinetic response or to changes in the gross cellular structure of the retina. These findings indicate that APP is not required for basic visual function, but that it is involved in modulating inner retinal circuitry.

Amacrine-to-amacrine cell inhibition: Spatiotemporal properties of GABA and glycine pathways

We measured the spatial and temporal properties of GABAergic and glycinergic inhibition to amacrine cells in the whole-mount rabbit retina. The amacrine cells were parsed into two morphological classes: narrow-field cells with processes spreading less than 200 μm and wide-field cells with processes extending more than 300 μm. The inhibition was also parsed into two types: sustained glycine and transient GABA. Narrow-field amacrine cells receive 1) very transient GABAergic inhibition with a fast onset latency of 140 ± 16 ms decaying to 30% of the peak level within 208 ± 27 ms elicited broadly over a lateral distance of up to 1500 μm and 2) sustained glycinergic inhibition with a medium onset latency of 286 ± 23 ms that was elicited over a spatial area often broader than the processes of the narrow-field amacrine cells. Wide-field amacrine cells received sustained glycinergic inhibition but no broad transient GABAergic inhibition. Surprisingly, neither of these amacrine cell classes received sustained local GABAergic inhibition, commonly found in an earlier study of ganglion cells.

Allosteric modulation of retinal GABA receptors by ascorbic acid

Ionotropic GABA receptors (GABA(A) and GABA(C)) belong to the Cys-loop receptor family of ligand-gated ion channels. GABA(C) receptors are highly expressed in the retina, mainly localized at the axon terminals of bipolar cells. Ascorbic acid, an endogenous redox agent, modulates the function of diverse proteins, and basal levels of ascorbic acid in the retina are very high. However, the effect of ascorbic acid on retinal GABA receptors has not been studied. Here we show that the function of GABA(C) and GABA(A) receptors is regulated by ascorbic acid. Patch-clamp recordings from bipolar cell terminals in goldfish retinal slices revealed that GABA(C) receptor-mediated currents activated by tonic background levels of extracellular GABA, and GABA(C) currents elicited by local GABA puffs, are both significantly enhanced by ascorbic acid. In addition, a significant rundown of GABA puff-evoked currents was observed in the absence of ascorbic acid. GABA-evoked Cl(-) currents mediated by homomeric ρ(1) GABA(C) receptors expressed in Xenopus laevis oocytes were also potentiated by ascorbic acid in a concentration-dependent, stereo-specific, reversible, and voltage-independent manner. Studies involving the chemical modification of sulfhydryl groups showed that the two Cys-loop cysteines and histidine 141, all located in the ρ(1) subunit extracellular domain, each play a key role in the modulation of GABA(C) receptors by ascorbic acid. Additionally, we show that retinal GABA(A) IPSCs and heterologously expressed GABA(A) receptor currents are similarly augmented by ascorbic acid. Our results suggest that ascorbic acid may act as an endogenous agent capable of potentiating GABAergic neurotransmission in the CNS.

The retinal hypercircuit: a repeating synaptic interactive motif underlying visual function

Abstract

The vertebrate retina generates a stack of about a dozen different movies that represent the visual world as dynamic neural images or movies. The stack is embodied as separate strata that span the inner plexiform layer (IPL). At each stratum, ganglion cell dendrites reach up to read out inhibitory interactions between three different amacrine cell classes that shape bipolar-to-ganglion cell transmission. The nexus of these five cell classes represents a functional module, a retinal ‘hypercircuit’, that is repeated across the surface of each of the dozen strata that span the depth of the IPL. Individual differences in the characteristics of each cell class at each stratum lead to the unique processing characteristics of each neural image throughout the stack. This review shows how the interactions between the morphological and physiological characteristics of each cell class generate many of the known retinal visual functions including motion detection, directional selectivity, local edge detection, looming detection, object motion and looming detection.

Effects of GABA receptor antagonists on thresholds of P23H rat retinal ganglion cells to electrical stimulation of the retina

An electronic retinal prosthesis may provide useful vision for patients suffering from retinitis pigmentosa (RP). In animal models of RP, the amount of current needed to activate retinal ganglion cells (RGCs) is higher than in normal, healthy retinas. In this study, we sought to reduce the stimulation thresholds of RGCs in a degenerate rat model (P23H-line 1) by blocking GABA receptor mediated inhibition in the retina. We examined the effects of TPMPA, a GABA(C) receptor antagonist, and SR95531, a GABA(A) receptor antagonist, on the electrically evoked responses of RGCs to biphasic current pulses delivered to the subretinal surface through a 400 µm diameter electrode. Both TPMPA and SR95531 reduced the stimulation thresholds of ON-center RGCs on average by 15% and 20% respectively. Co-application of the two GABA receptor antagonists had the greatest effect, on average reducing stimulation thresholds by 32%. In addition, co-application of the two GABA receptor antagonists increased the magnitude of the electrically evoked responses on average three-fold. Neither TPMPA nor SR95531, applied alone or in combination, had consistent effects on the stimulation thresholds of OFF-center RGCs. We suggest that the effects of the GABA receptor antagonists on ON-center RGCs may be attributable to blockage of GABA receptors on the axon terminals of ON bipolar cells.

Neurovascular interaction and the pathophysiology of diabetic retinopathy

Diabetic retinopathy (DR) is the most severe of the several ocular complications of diabetes, and in the United States it is the leading cause of blindness among adults 20 to 74 years of age. Despite recent advances in our understanding of the pathogenesis of DR, there is a pressing need to develop novel therapeutic treatments that are both safe and efficacious. In the present paper, we identify a key mechanism involved in the development of the disease, namely, the interaction between neuronal and vascular activities. Numerous pathological conditions in the CNS have been linked to abnormalities in the relationship between these systems. We suggest that a similar situation arises in the diabetic retina, and we propose a logical strategy aimed at therapeutic intervention.

Medicinal chemistry of ρ GABAC receptors

The inhibitory neurotransmitter, GABA, is a low-molecular-weight molecule that can achieve many low-energy conformations, which are recognized by GABA receptors and transporters. In this article, we assess the structure–activity relationship profiles of GABA analogs at the ionotropic ρ GABAC receptor. Such studies have significantly contributed to the design and development of potent and selective agonists and antagonists for this subclass of GABA receptors. With these tools in hand, the role of ρ GABAC receptors is slowly being realized. Of particular interest is the development of selective phosphinic acid analogs of GABA and their potential use in sleep disorders, inhibiting the development of myopia, and in improving learning and memory.

GABA(B) receptor feedback regulation of bipolar cell transmitter release

GABAergic amacrine cell feedback to bipolar cells in retina has been described, activating both GABA(A) and GABA(C) receptors. We explored whether metabotropic GABA(B) receptors also participate in this feedback pathway. CGP55845, a potent GABA(B) receptor antagonist, was employed to determine the endogenous role of these receptors. Ganglion cell EPSCs and IPSCs were monitored to measure the output of bipolar and amacrine cells. Using the tiger salamander slice preparation, we found that GABA(B) receptor pathways regulate bipolar cell release directly and indirectly. In the direct pathway, the GABA(B) receptor antagonist reduces EPSC amplitude, indicating that GABA(B) receptors cause enhanced glutamate release from bipolar cells to one set of ganglion cells. In the indirect pathway, the GABA(B) receptor antagonist reduces EPSC amplitude in another set of ganglion cells. The indirect pathway is only evident when GABA(A) receptors are inhibited, and is blocked by a glycine receptor antagonist. Thus, this second feedback pathway involves direct glycine feedback to the bipolar cell and this glycinergic amacrine cell is suppressed by GABAergic amacrine cells, through both GABA(A) and GABA(B) but not GABA(C) receptors. Overall, GABA(B) receptors do contribute to feedback regulation of bipolar cell transmitter release. However, unlike the ionotropic GABA receptor pathways, the metabotropic GABA receptor pathways act to enhance bipolar cell transmitter release. Furthermore, there are three discrete subsets of bipolar cell output regulated by GABA(B) receptor feedback (direct, indirect and null), implying three distinct, non-overlapping bipolar cell to ganglion cell circuits.