Changes in GABA metabolism in streptozotocin-induced diabetic rat retinas

The Interplay between Neurotransmitters and Calcium Dynamics in Retinal Synapses during Development, Health, and Disease

The intricate functionality of the vertebrate retina relies on the interplay between neurotransmitter activity and calcium (Ca2+) dynamics, offering important insights into developmental processes, physiological functioning, and disease progression. Neurotransmitters orchestrate cellular processes to shape the behavior of the retina under diverse circumstances. Despite research to elucidate the roles of individual neurotransmitters in the visual system, there remains a gap in our understanding of the holistic integration of their interplay with Ca2+ dynamics in the broader context of neuronal development, health, and disease. To address this gap, the present review explores the mechanisms used by the neurotransmitters glutamate, gamma-aminobutyric acid (GABA), glycine, dopamine, and acetylcholine (ACh) and their interplay with Ca2+ dynamics. This conceptual outline is intended to inform and guide future research, underpinning novel therapeutic avenues for retinal-associated disorders.

GABA-transaminase: A Key Player and Potential Therapeutic Target for Neurological Disorders

Neurological disorders such as epilepsy, autism, Huntington's disease, multiple sclerosis, and Alzheimer's disease alter brain functions like cognition, mood, movements, and language, severely compromising the well-being of persons, suffering from their negative effects. The neurotransmitters (GABA, glutamate, norepinephrine, dopamine) are found to be involved in neuronal signaling and neurotransmission. GABA, a "commanding neurotransmitter" is directly or indirectly associated with various neurological disorders. GABA is metabolized to succinic semialdehyde by a mitochondrial gamma-aminobutyric acid-transaminase (GABA-T) enzyme. Therefore, the alterations in the GABA performance in the distinct regions of the brain via GABA-T overstimulation or inhibition would play a vital role in the pathogenesis of various neurological disorders. This review emphasizes the leading participation of GABA-T in neurological disorders like Huntington's disease, epilepsy, autism, Alzheimer's disease, and multiple sclerosis. In Huntington's disease, epilepsy, and multiple sclerosis, the surfeited performance of GABA-T results in diminished levels of GABA, whereas in autism, the subsidence of GABA-T activity causes the elevation in GABA contents, which is responsible for behavioral changes in these disorders. Therefore, GABA-T inhibitors (in Huntington's disease, epilepsy, and multiple sclerosis) or agonists (in autism) can be used therapeutically. In the context of Alzheimer's disease, some researchers favor the stimulation of GABA-T activity whereas some disagree with it. Therefore, the activity of GABA-T concerning Alzheimer's disease is still unclear. In this way, studies of GABA-T enzymatic activity in contrast to neurological disorders could be undertaken to understand and be considered a therapeutic target for several GABA-ergic CNS diseases.

Visual Dysfunction in Diabetes

Although diabetic retinopathy (DR) is clinically diagnosed as a vascular disease, many studies find retinal neuronal and visual dysfunction before the onset of vascular DR. This suggests that DR should be viewed as a neurovascular disease. Prior to the onset of DR, human patients have compromised electroretinograms that indicate a disruption of normal function, particularly in the inner retina. They also exhibit reduced contrast sensitivity. These early changes, especially those due to dysfunction in the inner retina, are also seen in rodent models of diabetes in the early stages of the disease. Rodent models of diabetes exhibit several neuronal mechanisms, such as reduced evoked GABA release, increased excitatory glutamate signaling, and reduced dopamine signaling, that suggest specific neuronal deficits. This suggests that understanding neuronal deficits may lead to early diabetes treatments to ameliorate neuronal dysfunction.

Elucidating glial responses to products of diabetes-associated systemic dyshomeostasis

Diabetic retinopathy (DR) is a leading cause of blindness in working age adults. DR has non-proliferative stages, characterized in part by retinal neuroinflammation and ischemia, and proliferative stages, characterized by retinal angiogenesis. Several systemic factors, including poor glycemic control, hypertension, and hyperlipidemia, increase the risk of DR progression to vision-threatening stages. Identification of cellular or molecular targets in early DR events could allow more prompt interventions pre-empting DR progression to vision-threatening stages. Glia mediate homeostasis and repair. They contribute to immune surveillance and defense, cytokine and growth factor production and secretion, ion and neurotransmitter balance, neuroprotection, and, potentially, regeneration. Therefore, it is likely that glia orchestrate events throughout the development and progression of retinopathy. Understanding glial responses to products of diabetes-associated systemic dyshomeostasis may reveal novel insights into the pathophysiology of DR and guide the development of novel therapies for this potentially blinding condition. In this article, first, we review normal glial functions and their putative roles in the development of DR. We then describe glial transcriptome alterations in response to systemic circulating factors that are upregulated in patients with diabetes and diabetes-related comorbidities; namely glucose in hyperglycemia, angiotensin II in hypertension, and the free fatty acid palmitic acid in hyperlipidemia. Finally, we discuss potential benefits and challenges associated with studying glia as targets of DR therapeutic interventions. In vitro stimulation of glia with glucose, angiotensin II and palmitic acid suggests that: 1) astrocytes may be more responsive than other glia to these products of systemic dyshomeostasis; 2) the effects of hyperglycemia on glia are likely to be largely osmotic; 3) fatty acid accumulation may compound DR pathophysiology by promoting predominantly proinflammatory and proangiogenic transcriptional alterations of macro and microglia; and 4) cell-targeted therapies may offer safer and more effective avenues for DR treatment as they may circumvent the complication of pleiotropism in retinal cell responses. Although several molecules previously implicated in DR pathophysiology are validated in this review, some less explored molecules emerge as potential therapeutic targets. Whereas much is known regarding glial cell activation, future studies characterizing the role of glia in DR and how their activation is regulated and sustained (independently or as part of retinal cell networks) may help elucidate mechanisms of DR pathogenesis and identify novel drug targets for this blinding disease.

Time dependent effects of prolonged hyperglycemia in zebrafish brain and retina

Prolonged hyperglycemia causes long-term vision complications and an increased risk of cognitive deficits. High blood sugar also confers an osmotic load/stress to cells. We assessed behavioral and neurochemical changes in zebrafish brain and retina following prolonged hyperglycemia for 4-weeks or 8-weeks. At each time point, behavior was assessed using 3-chamber choice task and optomotor response; tissue was then collected and levels of inflammatory markers, tight junction proteins, and neurotransmitters determined using Western Blots. After 4-weeks, brain levels of v-rel reticuloendotheliosis viral oncogene homolog A (avian) (RelA; NF-kB subunit), IkB kinase (IKK), and glial fibrillary acidic protein (GFAP) were significantly elevated; differences in zonula occludens-1 (ZO-1), claudin-5, glutamic acid decarboxylase (GAD), and tyrosine hydroxylase (TH) were not significant. In retina, significant differences were observed only for TH (decreased), Rel A (increased), and GFAP (increased) levels. Glucose-specific differences in initial choice latency and discrimination ratios were also observed. After 8-weeks, RelA, GAD, and TH were significantly elevated in both tissues; IKK and GFAP levels were also elevated, though not significantly. ZO-1 and claudin-5 levels osmotically decreased in retina but displayed an increasing trend in glucose-treated brains. Differences in discrimination ratio were driven by osmotic load. OMRs increased in glucose-treated fish at both ages. In vivo analysis of retinal vasculature suggested thicker vessels after 4-weeks, but thinner vessels at 8-weeks. In vitro, glucose treatment reduced formation of nodes and meshes in 3B-11 endothelial cells, suggesting a reduced ability to form a vascular network. Overall, hyperglycemia triggered a strong inflammatory response causing initial trending changes in tight junction and neuronal markers. Most differences after 4-weeks of exposure were observed in glucose-treated fish suggesting effects on glucose metabolism independent of osmotic load. After 8-weeks, the inflammatory response remained and glucose-specific effects on neurotransmitter markers were observed. Osmotic differences impacted cognitive behavior and retinal protein levels; protein levels in brain displayed glucose-driven changes. Thus, we not only observed differential sensitivities of retina and brain to glucose-insult, but also different cellular responses, suggesting hyperglycemia causes complex effects at the cellular level and/or that zebrafish are able to compensate for the continued high blood glucose levels.

Activation of the GABA-alpha receptor by berberine rescues retinal ganglion cells to attenuate experimental diabetic retinopathy

Purpose

The aim of this study was to investigate the role and mechanism of berberine (BBR) in the protection of injured retinal ganglion cells (RGCs) in diabetic retinopathy (DR).

Methods

Experimental diabetic retinopathy rat model was successfully induced by a single intraperitoneal injection of streptozotocin (STZ, 60 mg/kg) in male SD rats with sufficient food and water for 8 weeks. Animals were randomly divided into four groups: (1) non-diabetic, (2) diabetic, (3) diabetic + BBR + PBS, and (4) diabetic + BBR + SR95531. BBR (100 mg/kg) was given daily by gavage to rats in the group (3) and group (4) for 8 weeks, and weekly intravitreal injections were conducted to rats in the group (3) with 5 μL of 1×PBS and rats in the group (4) with 5 μL of GABA-alpha receptor antagonist SR95531 to investigate the underlying mechanisms. The survival and apoptosis of RGCs were observed by fluorescence gold labeling technology and TUNEL staining. Visual function was evaluated by visual electrophysiological examination. Western blotting and immunofluorescence staining were used to analyze the expression of GABA-alpha receptors in RGCs.

Results

In an animal model, BBR can increase the survival of RGCs, reduce RGCs apoptosis, and significantly improve the visual function. The reduction of GABA, PKC-α, and Bcl-2 protein expression caused by DR can be considerably increased by BBR. SR95531 inhibits BBR's protective effect on RGC and visual function, as well as its upregulation of PKC-α and Bcl-2.

Conclusion

BBR is a promising preventive or adjuvant treatment for DR complications, and its key protective effect may involve the regulation of RGC apoptosis through the GABA-alpha receptor/protein kinase C-alpha (GABAAR/PKC-α) pathway.

Contribution of Müller Cells in the Diabetic Retinopathy Development: Focus on Oxidative Stress and Inflammation

Diabetic retinopathy is a neurovascular complication of diabetes and the main cause of vision loss in adults. Glial cells have a key role in maintenance of central nervous system homeostasis. In the retina, the predominant element is the Müller cell, a specialized cell with radial morphology that spans all retinal layers and influences the function of the entire retinal circuitry. Müller cells provide metabolic support, regulation of extracellular composition, synaptic activity control, structural organization of the blood–retina barrier, antioxidant activity, and trophic support, among other roles. Therefore, impairments of Müller actions lead to retinal malfunctions. Accordingly, increasing evidence indicates that Müller cells are affected in diabetic retinopathy and may contribute to the severity of the disease. Here, we will survey recently described alterations in Müller cell functions and cellular events that contribute to diabetic retinopathy, especially related to oxidative stress and inflammation. This review sheds light on Müller cells as potential therapeutic targets of this disease.

The role of inflammation and neurodegeneration in diabetic macular edema

The pathogenesis of diabetic macular edema (DME) is complex. Persistently high blood glucose activates multiple cellular pathways and induces inflammation, oxidation stress, and vascular dysfunction. Retinal ganglion cells, macroglial and microglial cells, endothelial cells, pericytes, and retinal pigment epithelium cells are involved. Neurodegeneration, characterized by dysfunction or apoptotic loss of retinal neurons, occurs early and independently from the vascular alterations. Despite the increasing knowledge on the pathways involved in DME, only limited therapeutic strategies are available. Besides antiangiogenic drugs and intravitreal corticosteroids, alternative therapeutic options tackling inflammation, oxidative stress, and neurodegeneration have been considered, but none of them has been currently approved.

Functional alterations of retinal neurons and vascular involvement progress simultaneously in the Psammomys obesus model of diabetic retinopathy

To investigate the progression of diabetic retinopathy (DR) in a new diurnal animal model, we monitored clinically the DR in Psammomys obesus (P. obesus) during 7 months using electroretinography (ERG) and imaging techniques. After the onset of DR, all ERG components decreased progressively. In scotopic conditions, by 3-months of disease progression, the diabetic P. obesus displayed a significant decrease in amplitude of b-max, b-wave responses, and mixed b-waves. While mixed a-wave decreased between 4 and 7 months. Significant differences of OP2 appeared following 1 month of disease. In photopic conditions, we noticed a decrease in the a-wave at 2 months, while it took more than 5 months in b-wave amplitude. The photopic negative response (PhNR) and the i-wave amplitudes decreased following 4 and 5 months. OP1 and OP2 were the first to be altered and a significant decrease in the amplitude started after 3 months. Finally, 30 Hz-flicker and photopic S-cone were impaired after 2 and 3 months, respectively. The assessment of the eye fundus of the retina revealed an abnormal vascular architecture appeared at Months 6 and 7. In addition, we noticed exudates in the superior periphery of the retina at the same stage. The retina thickness showed a significant reduction at Month 7. Our results indicate that the clinical correlates of human DR are present in diabetic P. obesus. The depressed of ERGs, disruption of retinal architecture, and the appearance of exudates may reflect vascular and neuronal damage throughout the retina as are seen in the advanced stages of human DR.

Dysfunction of retinal neurons and glia during diabetes

Diabetic retinopathy is the leading cause of blindness in those of working age. It is well known that the retinal vasculature is altered during diabetes. More recently, it has emerged that neuronal and glial dysfunction occurs in those with diabetes. Current research is directed at understanding these neuronal and glial changes because they may be an early manifestation of disease processes that ultimately lead to vascular abnormality. This review will highlight the recent advances in our understanding of the neuronal and glial changes that occur during diabetes.

The effects of early diabetes on inner retinal neurons

Diabetic retinopathy is now well understood as a neurovascular disease. Significant deficits early in diabetes are found in the inner retina that consists of bipolar cells that receive inputs from rod and cone photoreceptors, ganglion cells that receive inputs from bipolar cells, and amacrine cells that modulate these connections. These functional deficits can be measured in vivo in diabetic humans and animal models using the electroretinogram (ERG) and behavioral visual testing. Early effects of diabetes on both the human and animal model ERGs are changes to the oscillatory potentials that suggest dysfunctional communication between amacrine cells and bipolar cells as well as ERG measures that suggest ganglion cell dysfunction. These are coupled with changes in contrast sensitivity that suggest inner retinal changes. Mechanistic in vitro neuronal studies have suggested that these inner retinal changes are due to decreased inhibition in the retina, potentially due to decreased gamma aminobutyric acid (GABA) release, increased glutamate release, and increased excitation of retinal ganglion cells. Inner retinal deficits in dopamine levels have also been observed that can be reversed to limit inner retinal damage. Inner retinal targets present a promising new avenue for therapies for early-stage diabetic eye disease.

NMR metabolomics identifies over 60 biomarkers associated with Type II Diabetes impairment in db/db mice

INTRODUCTION

The rapid expansion of Type 2 Diabetes (T2D), that currently affects 90% of people suffering from diabetes, urges us to develop a better understanding of the metabolic processes involved in the disease process in order to develop better therapies. The most commonly used model for T2D research is the db/db (BKS.Cg-Dock7 < m > +/+ Lepr < db >/J) mouse model. Yet, a systematic ¹H NMR based metabolomics characterisation of most tissues in this animal model has not been published. Here, we provide a systematic organ-specific metabolomics analysis of this widely employed model using NMR spectroscopy.

OBJECTIVES

The aim of this study was to characterise the metabolic modulations associated with T2D in db/db mice in 18 relevant biological matrices.

METHODS

High-resolution ¹H-NMR and 2D-NMR spectroscopy were applied to 18 biological matrices of 12 db/db mice (WT control n = 6, db/db = 6) aged 22 weeks, when diabetes is fully established.

RESULTS

61 metabolites associated with T2D were identified. Kidney, spleen, eye and plasma were the biological matrices carrying the largest metabolomics modulations observed in established T2D, based on the total number of metabolites that showed a statistical difference between the diabetic and control group in each tissue (16 in each case) and the strength of the O-PLS DA model for each tissue. Glucose and glutamate were the most commonly associated metabolites found significantly increased in nine biological matrices. Investigated sections where no increase of glucose was associated with T2D include all intestinal segments (i.e. duodenum, jejunum, ileum and colon). Microbial co-metabolites such as acetate and butyrate, used as carbon sources by the host, were identified in excess in the colonic tissues of diabetic individuals.

CONCLUSIONS

The metabolic biomarkers identified using ¹H NMR-based metabolomics will represent a useful resource to explore metabolic pathways involved in T2D in the db/db mouse model.

Dopamine Deficiency Mediates Early Rod-Driven Inner Retinal Dysfunction in Diabetic Mice

Purpose

Electroretinograms (ERGs) are abnormal in diabetic retinas before the appearance of vascular lesions, providing a possible biomarker for diabetic vision loss. Previously, we reported that decreased retinal dopamine (DA) levels in diabetic rodents contributed to early visual and retinal dysfunction. In the current study, we examined whether oscillatory potentials (OPs) could serve as a potential marker for detecting early inner retinal dysfunction due to retinal DA deficiency.

Methods

Retinal function was tested with dark-adapted ERGs, taken at 3, 4, and 5 weeks after diabetes induction with streptozotocin. Electrical responses were analyzed and correlations were made with previously reported retinal DA levels. The effect of restoring systemic DA levels or removing DA from the retina in diabetic mice on OPs was assessed using L-3,4-dihydroxyphenylalanine (L-DOPA) treatments and retina-specific tyrosine hydroxylase (Th) knockout mice (rTHKO), respectively.

Results

Diabetic animals had significantly delayed OPs compared to control animals in response to dim, but not bright, flash stimuli. L-DOPA treatment preserved OP implicit time in diabetic mice. Diabetic rTHKO mice had further delayed OPs compared to diabetic mice with normal retinal Th, with L-DOPA treatment also providing benefit. Decreasing retinal DA levels significantly correlated with increasing OP delays mediated by rod pathways.

Conclusions

Our data suggest that inner retinal dysfunction in early-stage diabetes is mediated by rod-pathway deficits and DA deficiencies. OP delays may be used to determine the earliest functional deficits in diabetic retinopathy and to establish an early treatment window for DA therapies that may prevent progressive vision loss.

Electroretinographic evidence suggesting that the type 2 diabetic retinopathy of the sand rat Psammomys obesus is comparable to that of humans

Purpose

Type 2 diabetic retinopathy is the main cause of acquired blindness in adults. The aim of this work was to examine the retinal function of the sand rat Psammomys obesus as an animal model of diet-induced type 2 diabetes when subjected to a hypercaloric regimen.

Materials and methods

Hyperglycemia was induced in Psammomys obesus by high caloric diet (4 kcal/g). The visual function of control (n = 7) and diabetic (n = 7) adult rodents were followed up during 28 consecutive weeks with full-field electroretinogram(ERG) recordings evoked to flashes of white light according to the standard protocol of the International Society for Clinical Electrophysiology of Vision protocol (ISCEV).

Results

Twenty-eight weeks following the induction of diabetes, results revealed significantly reduced and delayed photopic and scotopic ERG responses in diabetic rats compared to control rats. More specifically, we noted a significant decrease in the amplitude of the dark-adapted 0.01ERG (62%), a- and b-wave amplitudes of the dark-adapted 3.0 ERG (33.6%, 55.1%) and the four major oscillatory potentials components (OP1-OP4) (39.0%, 75.2%, 54.8% and 53.7% respectively). In photopic conditions, diabetic rats showed a significant decrease in a- and b-wave (30.4%, 43.4%), photopic negative response (55.3%), 30 Hz flicker (63.7%), OP1-OP4(51.6%, 61.8%, 68.3% and 47.5% respectively) and S-cone (34.7%). Significantly delayed implicit times were observed for all ERG components in the diabetic animals. Results obtained are comparable to those characterizing the retinal function of patients affected with advanced stage of diabetic retinopathy.

Conclusion

Psammomys obesus is a useful translational model to study the pathophysiology of diabetic retinopathy in order to explore new therapeutic avenues in human patients.

Neurodegeneration in diabetic retinopathy: Potential for novel therapies

The complex pathology of diabetic retinopathy (DR) affects both vascular and neural tissue. The characteristics of neurodegeneration are well-described in animal models but have more recently been confirmed in the clinical setting, mostly by using non-invasive imaging approaches such as spectral domain optical coherence tomography (SD-OCT). The most frequent observations report loss of tissue in the nerve fiber layer and inner plexiform layer, confirming earlier findings from animal models. In several cases the reduction in inner retinal layers is reported in patients with little evidence of vascular lesions or macular edema, suggesting that degenerative loss of neural tissue in the inner retina can occur after relatively short durations of diabetes. Animal studies also suggest that neurodegeneration leading to retinal thinning is not limited to cell death and tissue loss but also includes changes in neuronal morphology, reduced synaptic protein expression and alterations in neurotransmission, including changes in expression of neurotransmitter receptors as well as neurotransmitter release, reuptake and metabolism. The concept of neurodegeneration as an early component of DR introduces the possibility to explore alternative therapies to prevent the onset of vision loss, including neuroprotective therapies and drugs targeting individual neurotransmitter systems, as well as more general neuroprotective approaches to preserve the integrity of the neural retina. In this review we consider some of the evidence for progressive retinal neurodegeneration in diabetes, and explore potential neuroprotective therapies.

Retinal exposure to high glucose condition modifies the GABAergic system: Regulation by nitric oxide

Diabetic retinopathy is a severe retinal complication that diabetic patients are susceptible to present. Although this disease is currently characterized as a microvascular disease, there is growing evidence that neural changes occur and maybe precede vascular impairments. Using chicken retina, an avascular tissue with no direct contact with blood vessels and neural retina, this study aimed to evaluate the influence of acute exposure to high glucose concentration in the retinal GABAergic system, and the role of nitric oxide (NO) in this modulation. Therefore, in ex vivo experiments, retinas were incubated in control (10 mM glucose) or high glucose condition (35 mM) for 30 min. By using DAF-FM to evaluate NO production, it was possible to show that high glucose (HG) significantly increased NO levels in the outer nuclear layer, inner nuclear layer (outer and inner portion), and inner plexiform layer. It was also observed that HG increased GABA immunoreactivity (IR) in amacrine and horizontal cells. HG did not change glutamic acid decarboxylase-IR, whereas it decreased GABA Transporter (GAT) 1-IR and increased GAT-3-IR. The co-treatment with 7-NI, an inhibitor of neuronal nitric oxide synthase (nNOS), blocked all changes stimulated by HG exposure. The concomitant exposure with SNAP-5114, a GAT-2/3 inhibitor, blocked the increase in GABA-IR caused by HG incubation. Therefore, our data suggest that hyperglycemia induces GABA accumulation in the cytosol by modulating GABA transporters. This response is dependent on NO production and signaling.

Early Retinal Neuronal Dysfunction in Diabetic Mice: Reduced Light-Evoked Inhibition Increases Rod Pathway Signaling

Purpose

Recent studies suggest that the neural retinal response to light is compromised in diabetes. Electroretinogram studies suggest that the dim light retinal rod pathway is especially susceptible to diabetic damage. The purpose of this study was to determine whether diabetes alters rod pathway signaling.

Methods

Diabetes was induced in C57BL/6J mice by three intraperitoneal injections of streptozotocin (STZ; 75 mg/kg), and confirmed by blood glucose levels > 200 mg/dL. Six weeks after the first injection, whole-cell voltage clamp recordings of spontaneous and light-evoked inhibitory postsynaptic currents from rod bipolar cells were made in dark-adapted retinal slices. Light-evoked excitatory currents from rod bipolar and AII amacrine cells, and spontaneous excitatory currents from AII amacrine cells were also measured. Receptor inputs were pharmacologically isolated. Immunohistochemistry was performed on whole mounted retinas.

Results

Rod bipolar cells had reduced light-evoked inhibitory input from amacrine cells but no change in excitatory input from rod photoreceptors. Reduced light-evoked inhibition, mediated by both GABA_A and GABA_C receptors, increased rod bipolar cell output onto AII amacrine cells. Spontaneous release of GABA onto rod bipolar cells was increased, which may limit GABA availability for light-evoked release. These physiological changes occurred in the absence of retinal cell loss or changes in GABA_A receptor expression levels.

Conclusions

Our results indicate that early diabetes causes deficits in the rod pathway leading to decreased light-evoked rod bipolar cell inhibition and increased rod pathway output that provide a basis for the development of early diabetic visual deficits.

Metabolomics in diabetic complications

With a global prevalence of 9%, diabetes is the direct cause of millions of deaths each year and is quickly becoming a health crisis. Major long-term complications of diabetes arise from persistent oxidative stress and dysfunction in multiple metabolic pathways. The most serious complications involve vascular damage and include cardiovascular disease as well as microvascular disorders such as nephropathy, neuropathy, and retinopathy. Current clinical analyses like glycated hemoglobin and plasma glucose measurements hold some value as prognostic indicators of the severity of complications, but investigations into the underlying pathophysiology are still lacking. Advancements in biotechnology hold the key to uncovering new pathways and establishing therapeutic targets. Metabolomics, the study of small endogenous molecules, is a powerful toolset for studying pathophysiological processes and has been used to elucidate metabolic signatures of diabetes in various biological systems. Current challenges in the field involve correlating these biomarkers to specific complications to provide a better prediction of future risk and disease progression. This review will highlight the progress that has been made in the field of metabolomics including technological advancements, the identification of potential biomarkers, and metabolic pathways relevant to macro- and microvascular diabetic complications.

High-resolution metabolite imaging of light and dark treated retina using MALDI-FTICR mass spectrometry

MS imaging (MSI) is a valuable tool for diagnostics and systems biology studies, being a highly sensitive, label-free technique capable of providing comprehensive spatial distribution of different classes of biomolecules. The application of MSI to the study of endogenous compounds has received considerable attention because metabolites are the result of the interactions of a biosystem with its environment. MSI can therefore enhance understanding of disease mechanisms and elucidate mechanisms for biological variation. We present the in situ comparative metabolomics imaging data for analyses of light- and dark-treated retina using MALDI-FTICR. A wide variety of tissue metabolites were imaged at a high spatial resolution. These include nucleotides, central carbon metabolism pathway intermediates, 2-oxocarboxylic acid metabolism, oxidative phosphorylation, glycerophospholipid metabolism, and cysteine and methionine metabolites. The high lateral resolution enabled the differentiation of retinal layers, allowing determination of the spatial distributions of different endogenous compounds. A number of metabolites demonstrated differences between light and dark conditions. These findings add to the understanding of metabolic activity in the retina.

Rodent Hyperglycemia-Induced Inner Retinal Deficits are Mirrored in Human Diabetes

PURPOSE

To evaluate the utility of low luminance stimuli to functionally probe inner retinal rod pathways in the context of diabetes mellitus in both rat and human subjects.

METHODS

Inner retinal dysfunction was assessed using oscillatory potential (OP) delays in diabetic rats. Scotopic electroretinograms (ERGs) in response to a series of increasing flash luminances were recorded from streptozotocin (STZ)-treated and control Sprague-Dawley rats after 7, 14, 20, and 29 weeks of hyperglycemia. We then evaluated OP delays in human diabetic subjects with (DR) and without (DM) diabetic retinopathy using the International Society for Clinical Electrophysiology in Vision (ISCEV) standard scotopic protocol and two additional dim test flashes.

RESULTS

Beginning 7 weeks after STZ, OP implicit times in diabetic rats were progressively delayed in response to dim, but not bright stimuli. In many diabetic subjects the standard ISCEV dim flash failed to illicit measureable OPs. However, OPs became measurable using a brighter, nonstandard dim flash (Test Flash 1, -1.43 log cd s/m²), and exhibited prolonged implicit times in the DM group compared with control subjects (CTRL).

CONCLUSIONS

Delays in scotopic OP implicit times are an early response to hyperglycemia in diabetic rats. A similar, inner retinal, rod-driven response was detected in diabetic human subjects without diabetic retinopathy, only when a nonstandard ISCEV flash intensity was employed during ERG testing.

TRANSLATIONAL RELEVANCE

The addition of a dim stimulus to standard ISCEV flashes with assessment of OP latency during ERG testing may provide a detection method for early retinal dysfunction in diabetic patients.

Identification of a novel neurotrophic factor from primary retinal Müller cells using stable isotope labeling by amino acids in cell culture (SILAC)

Retinal Müller glial cells (RMGs) have a primary role in maintaining the homeostasis of the retina. In pathological situations, RMGs execute protective and regenerative effects, but they can also contribute to neurodegeneration. It has recently been recognized that cultured primary RMGs secrete pro-survival factors for retinal neurons for up to 2 weeks in culture, but this ability is lost when RMGs are cultivated for longer durations. In our study, we investigated RMG supernatants for novel neuroprotective factors using a quantitative proteomic approach. Stable isotope labeling by amino acids in cell culture (SILAC) was used on primary porcine RMGs. Supernatants of RMGs cultivated for 2 weeks were compared with supernatants from cells that had already lost their protective capacity. Using this approach, we detected established neurotrophic factors such as transferrin, osteopontin, and leukemia inhibitory factor and identified C-X-C motif chemokine 10 (CXCL10) as a novel candidate neuroprotective factor. All factors prolonged photoreceptor survival in vitro. Ex vivo treatment of retinal explants with leukemia inhibitory factor or CXCL10 demonstrated a neuroprotective effect on photoreceptors. Western blots on CXCL10- and leukemia inhibitory factor-stimulated explanted retina and photoreceptor lysates indicated activation of pro-survival signal transducer and activator of transcription signaling and B-cell lymphoma pathways. These findings suggest that CXCL10 contributes to the supportive potential of RMGs toward retinal neurons.

Loss of synaptic connectivity, particularly in second order neurons is a key feature of diabetic retinal neuropathy in the Ins2Akita mouse

Retinal neurodegeneration is a key component of diabetic retinopathy (DR), although the detailed neuronal damage remains ill-defined. Recent evidence suggests that in addition to amacrine and ganglion cell, diabetes may also impact on other retinal neurons. In this study, we examined retinal degenerative changes in Ins2^Akita diabetic mice. In scotopic electroretinograms (ERG), b-wave and oscillatory potentials were severely impaired in 9-month old Ins2^Akita mice. Despite no obvious pathology in fundoscopic examination, optical coherence tomography (OCT) revealed a progressive thinning of the retina from 3 months onwards. Cone but not rod photoreceptor loss was observed in 3-month-old diabetic mice. Severe impairment of synaptic connectivity at the outer plexiform layer (OPL) was detected in 9-month old Ins2^Akita mice. Specifically, photoreceptor presynaptic ribbons were reduced by 25% and postsynaptic boutons by 70%, although the density of horizontal, rod- and cone-bipolar cells remained similar to non-diabetic controls. Significant reductions in GABAergic and glycinergic amacrine cells and Brn3a⁺ retinal ganglion cells were also observed in 9-month old Ins2^Akita mice. In conclusion, the Ins2^Akita mouse develops cone photoreceptor degeneration and the impairment of synaptic connectivity at the OPL, predominately resulting from the loss of postsynaptic terminal boutons. Our findings suggest that the Ins2^Akita mouse is a good model to study diabetic retinal neuropathy.

GABA and Glutamate Uptake and Metabolism in Retinal Glial (Müller) Cells

Müller cells, the principal glial cells of the retina, support the synaptic activity by the uptake and metabolization of extracellular neurotransmitters. Müller cells express uptake and exchange systems for various neurotransmitters including glutamate and γ-aminobutyric acid (GABA). Müller cells remove the bulk of extracellular glutamate in the inner retina and contribute to the glutamate clearance around photoreceptor terminals. By the uptake of glutamate, Müller cells are involved in the shaping and termination of the synaptic activity, particularly in the inner retina. Reactive Müller cells are neuroprotective, e.g., by the clearance of excess extracellular glutamate, but may also contribute to neuronal degeneration by a malfunctioning or even reversal of glial glutamate transporters, or by a downregulation of the key enzyme, glutamine synthetase. This review summarizes the present knowledge about the role of Müller cells in the clearance and metabolization of extracellular glutamate and GABA. Some major pathways of GABA and glutamate metabolism in Müller cells are described; these pathways are involved in the glutamate-glutamine cycle of the retina, in the defense against oxidative stress via the production of glutathione, and in the production of substrates for the neuronal energy metabolism.

Effect of estrogen on electroretinographic responses in streptozotocin-induced diabetic female rats

The aim of this study is to investigate the effects of estrogen on functional changes in the retinas of streptozotocin (STZ)-induced diabetic rats by using an electroretinography. Female rats were randomly divided into four treatment groups: (1) Control (sham operation and vehicle administration); (2) STZ (sham operation and STZ administration); (3) OVX (ovariectomy and vehicle administration); and (4) OVX + STZ (ovariectomy and STZ administration). Full-field electroretinograms (ERGs) were recorded before OVX and STZ administration and 4 and 12 weeks after STZ administration. At 4 weeks after STZ administration, although there were no differences in the STZ and OVX groups compared with the Control group, the amplitude of the cone-response was significantly lower in the OVX + STZ group than in the Control group (P = 0.013). At 12 weeks after STZ administration, this response showed a similar tendency in the STZ and the OVX + STZ groups. At 12 weeks after STZ administration, the implicit times of OP3 and OP4 and of the cone-response were significantly delayed in the STZ and OVX + STZ groups (OP3: P = 0.030 and 0.050, OP4: P = 0.0060 and 0.0053, cone-response: P = 0.014 and 0.039), compared with in the Control group. Thus, the retinal functions in STZ-induced diabetic female rats were aggravated by OVX. OVX-induced estrogen deficiency resulted in earlier changes in the amplitudes of cone-response, especially in the diabetes, although this is a transient effect and it is difficult to explain. Recognizing the early neurosensory change would enable a better understanding of the effect of estrogen in the retina.

Functional implication of Dp71 in osmoregulation and vascular permeability of the retina

Functional alterations of Müller cells, the principal glia of the retina, are an early hallmark of most retina diseases and contribute to their further progression. The molecular mechanisms of these reactive Müller cell alterations, resulting in disturbed retinal homeostasis, remain largely unknown. Here we show that experimental detachment of mouse retina induces mislocation of the inwardly rectifying potassium channels (Kir4.1) and a downregulation of the water channel protein (AQP4) in Müller cells. These alterations are associated with a strong decrease of Dp71, a cytoskeleton protein responsible for the localization and the clustering of Kir4.1 and AQP4. Partial (in detached retinas) or total depletion of Dp71 in Müller cells (in Dp71-null mice) impairs the capability of volume regulation of Müller cells under osmotic stress. The abnormal swelling of Müller cells In Dp71-null mice involves the action of inflammatory mediators. Moreover, we investigated whether the alterations in Müller cells of Dp71-null mice may interfere with their regulatory effect on the blood-retina barrier. In the absence of Dp71, the retinal vascular permeability was increased as compared to the controls. Our results reveal that Dp71 is crucially implicated in the maintenance of potassium homeostasis, in transmembraneous water transport, and in the Müller cell-mediated regulation of retinal vascular permeability. Furthermore, our data provide novel insights into the mechanisms of retinal homeostasis provided by Müller cells under normal and pathological conditions.

Whole genome assessment of the retinal response to diabetes reveals a progressive neurovascular inflammatory response

BACKGROUND

Despite advances in the understanding of diabetic retinopathy, the nature and time course of molecular changes in the retina with diabetes are incompletely described. This study characterized the functional and molecular phenotype of the retina with increasing durations of diabetes.

RESULTS

Using the streptozotocin-induced rat model of diabetes, levels of retinal permeability, caspase activity, and gene expression were examined after 1 and 3 months of diabetes. Gene expression changes were identified by whole genome microarray and confirmed by qPCR in the same set of animals as used in the microarray analyses and subsequently validated in independent sets of animals. Increased levels of vascular permeability and caspase-3 activity were observed at 3 months of diabetes, but not 1 month. Significantly more and larger magnitude gene expression changes were observed after 3 months than after 1 month of diabetes. Quantitative PCR validation of selected genes related to inflammation, microvasculature and neuronal function confirmed gene expression changes in multiple independent sets of animals.

CONCLUSION

These changes in permeability, apoptosis, and gene expression provide further evidence of progressive retinal malfunction with increasing duration of diabetes. The specific gene expression changes confirmed in multiple sets of animals indicate that pro-inflammatory, anti-vascular barrier, and neurodegenerative changes occur in tandem with functional increases in apoptosis and vascular permeability. These responses are shared with the clinically documented inflammatory response in diabetic retinopathy suggesting that this model may be used to test anti-inflammatory therapeutics.

Shifting of parvalbumin expression in the rat retina in experimentally induced diabetes

The AII amacrine cell, a unique rod signal integrator passing through the cone bipolar cell to ganglion cells, uses parvalbumin as a transducer of cytosolic calcium ion signals in the mammalian retina. For clarification of whether AII amacrine cell network contributes to the early neuropathogenesis of diabetic retinopathy, this study first analyzed alteration of parvalbumin expression in experimental diabetic retinas using immunohistochemical methods. Parvalbumin immunoreactivity was found in AII amacrine cells, some amacrine cells of a wide-field type, and displaced amacrine cells of the normal rat retina. During diabetes, cell density of each parvalbumin immunoreactive amacrine cell type showed no large changes despite decrease in immunoreactivity especially in AII amacrine cells. In addition to these parvalbumin immunoreactive amacrine cell types, a type of cone bipolar cells co-expressing glutamate transporter 1b and connecting electrically with AII amacrine cells appeared clearly by 4 weeks of diabetes, and thereafter sharply increased in number to that of AII amacrine cells. Protein levels of parvalbumin throughout the diabetic retinas also showed no large changes, except a transitional slight increase at 4 weeks of diabetes. These results suggest that the parvalbumin expression propagates from AII amacrine cells to a type of cone bipolar cell through electrical synapses due to dysfunction of biased mechanism in calcium ion buffering, caused by diabetic injury, and thus AII amacrine cells are closely involved in neuropathogenesis of ongoing diabetic retinopathy.

Neuronal and glial cell changes are determined by retinal vascularization in retinopathy of prematurity

We have characterized the vascular, neuronal, and glial changes in oxygen-induced retinopathy, a model of retinopathy of prematurity (ROP). Newborn Sprague-Dawley rats were exposed to either 80% +/- 2% oxygen to postnatal day P11 and then room air until P18 (ROP) or room air for the entire duration (controls). Retinal structure was examined under the light microscope and following postembedding immunocytochemistry in central, midperipheral, and peripheral regions. Müller cells were evaluated immunocytochemically with glial fibrillary acidic protein. The extent of vascularization was established histologically. ROP caused significant thinning of the inner cellular and plexiform layers, which became more pronounced in the peripheral inner nuclear layer of ROP animals (11.3% loss vs. 25.4% loss). Amacrine cell amino acid levels were particularly vulnerable in the peripheral retina; bipolar cells showed similar but less prominent changes. Müller cells had elevated glutamine levels and were most gliotic in the periphery. The vasculature extended to peripheral retinal regions at P18 in controls but not in ROP rats. The most striking pattern of change was evident in the midperipheral "transition zone" of ROP animals. Areas close to blood vessels showed neurochemical properties that were similar to those of the central retina, indicating a local protective effect of the inner retinal blood supply. We find that ROP produces complex vascular, neural, and glial changes that relate to the proximity of inner retinal blood vessels.

Streptozotocin-induced diabetes modulates GABA receptor activity of rat retinal neurons

Neural deficits suggestive of involvement of the GABA signaling pathway can often be detected early in the course of diabetic retinopathy, a leading cause of blindness in the United States. To examine in greater detail the nature of the neuronal changes associated with hyperglycemia, we investigated GABA receptor activity on retinal bipolar cells in streptozotocin-induced diabetic rats; cells from age-matched normal rats served as controls. Patch-clamp recordings from isolated rod-bipolar cells revealed that diabetes enhanced the whole cell currents elicited by GABA. Responses of the GABA(C) receptor, the predominant GABA receptor on rat rod bipolar cells, exhibited a greater sensitivity to GABA, larger maximum current responses, slower response kinetics, and a smaller single channel conductance among diabetic cells relative to those recorded from normal controls. Compared with the properties of homomeric rho1 and heteromeric rho1rho2 receptors formed in a heterologous expression system, these results suggested that there was a greater contribution from the rho1 subunit in the GABA(C) receptor-mediated response of diabetic cells. The levels of mRNA, measured with real-time RT-PCR, were consistent with this finding. There was a significant enhancement in the ratio of rho1/rho2 subunit expression in the retina of diabetic animals, although the levels of GABA rho1 subunit expression were comparable in diabetic and normal retinas. Taken together, the results suggest that diabetes modifies the subunit composition of the GABA(C) receptor on retinal neurons, most likely through its effect on the efficacy of gene transcription.

Microglial changes occur without neural cell death in diabetic retinopathy

Very early neuroglial changes have been observed to precede major vascular changes in the retina of diabetic patients and animal models. We investigated the sequence of these neuroglial changes further, in mice with alloxan-induced diabetes. Diabetes was induced by a single injection of Alloxan into C57/Bl6 mice, which subsequently received daily insulin injections. Diabetic and control animals were weighed and their blood glucose levels were determined weekly. Electroretinographic recordings and scanner laser ophthalmoscope (SLO) examinations were carried out 15 days, one month and three months after the onset of diabetes. Diabetes induction was confirmed by the presence of glucose in the urine, a tripling of blood glucose level, weight loss and an increase in glycated haemoglobin levels. Three months after diabetes onset, the electroretinogram b/a wave amplitude ratio was decreased at the highest light intensities and oscillatory potentials were delayed. The retinal fundus and vessels remained unchanged. No cell apoptosis was detected in vertical and horizontal sections of the retina by TUNEL or immunocytochemistry for the active caspase 3. No increase in GFAP-immunostaining indicative of a glial reaction was observed in Müller glial cells. By contrast, changes in the morphology of microglial cells were observed, with marked shortening of the dendrites. Thus, the microglial reaction occurs very early in progression to diabetic retinopathy, at about the same time as early electroretinographic modifications. The absence of apoptotic cells, contrasting with previous results in mice with streptozotocin-induced diabetes, is consistent with insulin neuroprotection.

Up-regulated expression of neuronal nitric oxide synthase in experimental diabetic retina

Nitric oxide (NO) can play either a neuroprotective or a neurotoxic role in diverse neurodegenerative conditions. This study investigated the differential expression of neuronal nitric oxide synthase (nNOS) in the streptozotocin-induced diabetic rat retina to clarify the involvement of NO produced from neurons in the early pathogenesis of diabetic retinopathy. A decrease in thickness of the outer retina was evident at 12 and 24 weeks after onset of diabetes. nNOS was immunolocalized in two subtypes of amacrine cells, displaced amacrine cells and in some bipolar cells in the normal retinas. The densities of each type of nNOS-expressing neuron showed no significant differences in the diabetic retinas with the exception of the bipolar cells. The numbers of nNOS bipolar cells at 12 weeks of diabetes increased threefold, showing dendritic polarity of nNOS expression. Protein levels of nNOS increased throughout the diabetic retinas reaching a peak value at 24 weeks of diabetes. Thus, diabetes up-regulates the expression of nNOS in bipolar cells, and NO from these cells may aggravate the degeneration of the outer retina in the diabetic retinas.

Glutamic Acid Decarboxylase Autoantibody Prevalence and Association with HLA Genotype in Patients with Younger-Onset Type 1 Diabetes and Proliferative Diabetic Retinopathy

OBJECTIVE

To investigate the relationship of HLA status and autoantibodies to glutamic acid decarboxylase (GAD) in proliferative diabetic retinopathy (PDR) to assess the role of autoimmunity and genetic markers in retinopathy.

DESIGN

Retrospective, nonrandomized, comparative study.

PARTICIPANTS

Patients who had suffered from type 1 diabetes for >10 years and who had been first diagnosed as diabetic under 30 years of age were studied. They were classified into 3 groups: 20 patients with diabetes and PDR (PDR group), 22 patients who had diabetes and severe nonproliferative diabetic retinopathy (SNPDR group), and 25 patients who had diabetes with no diabetic retinopathy (non-DR group).

METHODS

Blood was collected, and the relationship between HLA status and GAD autoantibody positivity in diabetic retinopathy was investigated in a cross-sectional study.

MAIN OUTCOME MEASURES

Human leukocyte antigen status and GAD autoantibody positivity.

RESULTS

The highest positive rate of GAD autoantibody was 56.0% in the non-DR group, followed by the SNPDR group (40.1%) and the PDR group (15.0%). The frequencies of the HLA-DQ4 and -DR4/-DQ4 haplotypes were significantly higher in the PDR group (75.0% and 65%, respectively) than in the SNPDR group (40.9% and 31.8%) or the non-DR group (40.0% and 28.0%) (P = 0.035 and P = 0.026, respectively). The prevalence of GAD antibodies was lower in patients with the HLA-DR4 and HLA-DQ4 alleles and -DR4/-DQ4 haplotype frequencies in the PDR group (P = 0.018, P = 0.0088, and P = 0.0031, respectively).

CONCLUSIONS

We found that the existence of GAD antibodies is inversely related and HLA status is directly related to the stage or severity of retinopathy.

Development and progression of diabetic retinopathy in patients with Type 1 diabetes who are positive for GAD autoantibody

AIMS

To investigate the relationship between autoantibodies to glutamic acid decarboxylase (GAD) and proliferative diabetic retinopathy (PDR) to assess the role of autoimmunity in retinopathy.

METHODS

Patients with Type 1 diabetes for more than 10 years who had been diagnosed under age 30 (13-28 years) were studied. They were classified into three groups. The PDR group consisted of 22 patients, the pre-PDR group was 26 patients, while the non-DR group was 32 patients who had Type 1 diabetes without retinopathy. Blood was collected to measure autoantibodies to GAD, and the relationship between PDR and GAD positivity was investigated in a cross-sectional study.

RESULTS

The highest positivity rate of GAD autoantibodies was 50.0% in the non-DR group, followed by the pre-PDR group (30.8%) and the PDR group (18.2%).

CONCLUSIONS

Production or existence of GAD autoantibodies may contribute to the prevention of retinopathy.

Modulation of the components of the rat dark-adapted electroretinogram by the three subtypes of GABA receptors

The separate components of the dark-adapted electroretinogram (ERG) are believed to reflect the electric activity of neurones in both the inner and the outer layers of the retina, although their precise origin still remains unclear. The purpose of this study was to examine whether selective blockage or stimulation of the different subtypes of GABA receptors might help further elucidate the cellular origin of the components of the dark-adapted ERG. The rat retina is of interest since the localization and physiology of GABA receptors in that retina have been examined in great detail. GABA agonists and antagonists, known to affect the responses of neurons in the inner plexiform layer, were injected into the vitreous of one eye while ERG responses evoked by flashes of white light were recorded. GABA and the GABAa agonist isoguvacine completely removed the oscillatory potentials (OPs) and reduced the amplitude of the a- and b-waves. TPMPA, a GABAC antagonist, reduced the a- and b-waves but had no significant effect on the OPs. Baclofen, a GABAb agonist, reduced the amplitude of the a- and b-waves, without having any effects on the amplitude of the OPs. The GABAb antagonist CGP35348 increased the amplitudes of the a- and b-wave without having an effect on the amplitudes of the OPs. The GABAb receptor ligands had significant and opposite effect on the latency of the OPs. These results indicate that retinal neurons, presumably a subpopulation of amacrine cells, that have GABAb receptors are not the source of the OPs of the ERG, although they may modulate these wavelets in some manner, while contributing to the generation of the dark-adapted a- and b-waves. OPs are modified by stimulation of GABAa receptors, and the a- and b-waves by stimulation of all GABA receptor subtypes.

Apoptotic death of photoreceptors in the streptozotocin-induced diabetic rat retina

AIMS/HYPOTHESIS

Neurodegenerative changes in the diabetic retina occurring before diabetic retinopathy could be inevitable by the altered energy (glucose) metabolism, in the sense that dynamic image-processing activity of the retinal neurons is exclusively dependent on glucose. We therefore investigated the morphological changes in the neural retina, including neuronal cell death, of a streptozotocin-induced model of diabetes.

METHODS

Streptozotocin was intravenously injected. Rats were maintained hyperglycaemic without insulin treatment for 1 week and 4, 8, 12, and 24 weeks, respectively. Diabetic retinas were processed for histology, electron microscopy, and immunohistochemistry using the TUNEL method.

RESULTS

A slight reduction in the thickness of the inner retina was observed throughout the diabetic retinas and a remarkable reduction was seen in the outer nuclear layer 24 weeks after the onset of diabetes. The post-synaptic processes of horizontal cells in the deep invaginations of the photoreceptors showed degeneration changes from 1 week onwards. A few necrotic ganglion cells were observed after 4 weeks. At 12 weeks, some amacrine cells and a few horizontal cells showed necrotic features. Three to seven cellular layers in the outer nuclear layer and nerve terminals, rolled by the fine processes of the Müller cells near the somata of the degenerated ganglion cells, were apparent at 24 weeks. Apoptosis appeared in a few photoreceptor cells at 4 weeks, and the number of apoptotic photoreceptors increased thereafter.

CONCLUSION/INTERPRETATION

These findings suggest that the visual loss associated with diabetic retinopathy could be attributed to an early phase of substantial photoreceptor loss, in addition to later microangiopathy.

Effect of betaxolol on aspartate aminotransferase activity in hypoxic rat retina in vitro

We investigated the effect of betaxolol on the decrease of mitochondrial aspartate aminotransferase (mAAT) activity in rat retinas induced by hypoxia in vitro. It is reported that mAAT decreases in ischemic or hypoxic retina and that the decrease is caused by Ca(2+)-dependent proteases such as calpain. Betaxolol is a compound that has beta(1)-adrenergic receptor blocking and voltage-dependent calcium channel blocking properties. The rat eye cups were maintained with Locke's solution saturated with 95% air - 5% CO(2). The eye cups were immersed in glucose-free Locke's solution saturated with 95% N(2) / 5% CO(2) (hypoxic solution). Ninety minutes of hypoxia caused a 20% decrease in mAAT activity. The eye cups incubated with the hypoxic solution containing 1 mM EGTA, 10 micro M MK-801 or 100 micro M betaxolol were protected from the decrease in mAAT activity, so that the residual mAAT activity was 50%, 45% or 40%, respectively, compared to the eye cups incubated in hypoxic solution alone, which had a 100% decrease in mAAT activity. In addition, co-incubation with EGTA and betaxolol had a greater protective effect against the mAAT decrease than a single application. This additive effect of betaxolol was dose-dependent. These results suggested that betaxolol had a protective effect against the decrease of mAAT caused by hypoxia and indicated that betaxolol might inhibit the Ca(2+) release from intracellular Ca(2+) stores.

Neurotoxic effects of GABA-transaminase inhibitors in the treatment of epilepsy: ocular perfusion and visual performance

Vigabatrin is a GABA (gamma-aminobutyric acid) transaminase inhibitor that elicits an antiepileptic effect by enhancing inhibitory neurotransmission in the brain. Vigabatrin has been previously associated with concentric peripheral visual field loss and visual electrophysiological abnormalities. Recently, visual function deficits of the central retina have been identified in a proportion of patients receiving vigabatrin; these include disturbances in colour perception, contrast sensitivity and short-wavelength automated perimetry. Consequently, it is suggested that vigabatrin-associated retinal toxicity is diffuse inducing subtle central visual dysfunction and more severe peripheral visual defects. Reductions in cerebral blood flow and cerebral metabolic rate for glucose occur in epilepsy patients receiving antiepileptic drug therapy. Despite the known cerebral haemodynamic alterations in epilepsy and the visual consequences of vigabatrin therapy, ocular blood flow has only recently been investigated in this group. We present findings from a series of novel investigations that identify compromised retinal microvascular perfusion and pulsatile ocular blood flow (POBF) in epilepsy patients. The reduction in POBF was exacerbated in epilepsy patients treated with vigabatrin compared to conventionally treated epilepsy patients. A number of theories are presented to explain compromised ocular blood flow in vigabatrin treated epilepsy patients, and the possibility of a GABAergic mechanism of toxicity is discussed.

The effects of vigabatrin on electrophysiology and visual fields in epileptics: a controlled study with a discussion of possible mechanisms

PURPOSE

To compare the visual electrophysiology and visual fields of patients taking vigabatrin to those of a control group of epileptics on other anti-epileptic drugs (AEDs).

METHODS

Fourteen epileptics treated with vigabatrin and 10 control patients treated with other AEDs underwent ERG and EOG. Goldmann visual fields were performed and analysed using standard software to measure areas contained within I4e isopters.

RESULTS

The cone and rod b-waves of the ERG, the oscillatory potential amplitudes and Arden indices were reduced in vigabatrin-treated subjects and the oscillatory potentials delayed. The Arden indices were reduced due to an increased dark trough. The areas contained within the I4e isopter of vigabatrin treated subjects were reduced compared to the control group and these areas correlated well with oscillatory potential amplitudes and b-wave amplitudes in the vigabatrin group only.

CONCLUSIONS

The use of vigabatrin is associated with a reduction of the ERG cone b-wave amplitude and oscillatory potentials which correlates with visual field loss. The Arden ratio is reduced in subjects taking vigabatrin but may recover after cessation. However, visual loss may persist in the presence of a recovered EOG. These findings suggest further effects of the drug than those mediated by GABA receptors, and support the contention that the cause of the field loss may be at least in part due to retinal effects. Possible mechanisms are discussed.

Retinal neurochemical changes following application of glutamate as a metablolic substrate

BACKGROUND

Retinal neural and glial cells share an intricate relationship that includes uptake and recycling of the amino acid neurotransmitters, glutamate and gamma-amino butyric acid (GABA), as well as metabolic links. The aim of this work was to determine the neurochemical and morphological changes induced by the removal of glucose but with the provision of exogenous glutamate in the isolated retinal preparation incubated under aerobic conditions. The carbon skeleton of glutamate can enter the tricarboxylic acid cycle as alpha-ketogluterate, providing an alternative metabolic substrate in cases of glucose deprivation.

METHODS

Isolated rat retinas were incubated in physiological media with and without glucose, using a range of glutamate concentrations to provide an alternative source of metabolic substrate. We conducted post-embedding immunocytochemistry and quantified the change in glutamate and GABA immunoreactivity within Müller cells under these different incubation conditions.

RESULTS

The provision of glutamate with normal (6 mM) glucose levels resulted in a gradual accumulation of glutamate and GABA in Müller cells, with Müller loading when exogenous glutamate concentrations were above 0.1 mM. However, when these varying levels of glutamate were applied in the absence of glucose, glutamate accumulation in Müller cells was decreased compared to the 6 mM glucose condition and GABA accumulation in Müller cells was at a minimum at moderate (0.5 and 1 mM) glutamate levels. Under hypoglycaemic conditions, exogenous glutamate (0.5 to 1 mM) is rapidly metabolised by Müller cells to the extent that no glial loading is evident, despite the high concentrations.

CONCLUSIONS

Normal neurochemical function appears to be maintained secondary to exogenous glutamate provision of 0.5 to 1 mM when glucose is not in the incubation medium, implying that glutamate can be used as an alternative metabolic substrate. We also show that Müller cells possess more rapid glutamate metabolic capabilities compared to the metabolism of GABA.

Comparisons of the amplitude size and the reproducibility of three different electrodes to record the corneal flash electroretinogram in rodents

To compare corneal electrodes commonly used in rodent eyes for repeat and left versus right eye accuracy and variability to record the flash electroretinogram (ERG). Animals studied were eight C57BL/6 mice and eight rats of the Wistar strain. Scotopic ERGs were recorded from eyes of dark-adapted anesthetized rodents to compare a custom-made gold-wire contact lens electrode (CLE), a cotton-wick silver-silver chloride electrode (SCLE), and a coiled stainless steel wire electrode (SSE). Compared to SCLE and SSE. the potentials recorded by CLE are characterized by significantly larger ERG amplitudes and oscillatory potentials in both rats and mice (p <0.0001). In analyzing test-retest data comparing the three different electrodes the coefficient of variation was smaller (range, 10.3-15.5%) and the interclass correlation coefficient (0.77-0.93) showed a better agreement for the CLE. Recording scotopic ERGs with custom-made gold-wire contact lens electrodes records large amplitudes and shows a good reproducibility and reliability to monitor retinal function in rodent eyes.

Effect of experimental diabetes on GABA-mediated inhibition of neurally induced contractions in rat isolated trachea

1. In the present study, we investigated the effect of GABA and selective GABA agonists and antagonists on neurally induced tracheal contractions in streptozotocin (STZ) diabetic rats. 2. Contractile responses to electrical field stimulation (EFS) in rat tracheal rings were completely abolished by atropine and tetrodotoxin, but were unaffected by the ganglion blocker hexamethonium, indicating that they were mediated via neuronal release of acetylcholine (ACh). 3. Contractions induced by EFS, but not by exogenous ACh, were inhibited by GABA and the selective GABA(B) receptor agonist baclofen, but not by the selective GABA(A) receptor agonist 3-aminopropane sulphonic acid. The inhibitory effects of GABA or baclofen were not affected by the GABA(A) antagonist bicuculline, but were significantly reversed by the GABA(B) antagonist phaclofen. 4. The inhibitory effects of both GABA and baclofen were found to be significantly greater in trachea from control rats compared with tissues from diabetic rats. 5. Non-adrenergic, non-cholinergic relaxation responses elicited by EFS in precontracted tracheal rings from diabetic and control rats were similar in magnitude and were unaffected by GABA or GABA analogues. 6. These results suggest that GABA decreases the response to EFS by directly inhibiting the evoked release of ACh through GABA(B) receptors in rat trachea and that STZ-induced diabetes causes an impairment in the inhibitory effect of GABA on neurally induced contractions in this tissue.

Possible mechanism for the decrease of mitochondrial aspartate aminotransferase activity in ischemic and hypoxic rat retinas

Glutamate is believed to be an excitatory amino acid neurotransmitter in the retina. Enzymes for glutamate metabolism, such as glutamate dehydrogenase, ornithine aminotransferase, glutaminase, and aspartate aminotransferase (AAT), exist mainly in the mitochondria. The abnormal increase of intracellular calcium ions in ischemic retinal cells may cause an influx of calcium ions into the mitochondria, subsequently affecting various mitochondrial enzyme activities through the activity of mitochondrial calpain. As AAT has the highest level of activity among enzymes involved in glutamate metabolism, we investigated the change of AAT activity in ischemic and hypoxic rat retinas and the protection against such activity by calpain inhibitors. We used normal RCS (rdy+/rdy+) rats. For the in vivo studies, we clamped the optic nerve of anesthetized rats to induce ischemia. In the in vitro studies, the eye cups were incubated with Locke's solution saturated with 95% N2/5% CO2. The activity of cytosolic AAT (cAAT) was about 20% of total activity, whereas mitochondrial AAT (mAAT) was about 75% in rat retina. Ninety minutes of ischemia or hypoxia caused a 20% decrease in mAAT activity, whereas cAAT activity remained unchanged. To examine the contribution of intracellular calcium ions to the degradation of mAAT, we used Ca2+-free Locke's solution containing 1 mM EGTA, ryanodine (Ca2+ channel blocker), and thapsigargin (Ca2+-ATPase inhibitor). In the present study, thapsigargin in Ca2+-free Locke's solution, but not ryanodine in this solution, was found to prevent AAT degradation. AAT degradation was also prevented by calpain inhibitors (Ca2+-dependent protease inhibitor) such as calpeptin at 1 nM, 10 nM, 0.1 microM, 1 microM and 10 microM, and by calpain inhibitor peptide, but not by other protease inhibitors (10 microM leupeptin, pepstatin, chymostatin). Additionally, we determined the subcellular localization of calpain activity and examined the change of calpain activity in ischemic rat retinas. Our results suggest that decreased activity of mAAT in ischemic and hypoxic rat retinas might be evoked by the degradation by calpain-catalyzed proteolysis in mitochondria.

Changes of GABA metabolic enzymes in acute retinal ischemia

It is reported that GABA accumulates in Müller cells in ischemic and diabetic rat retina. To investigate the mechanism of GABA accumulation in Müller cells, we localized GABA and glutamate in ischemic rat retina and measured the activity of GAD and GABA-T, enzymes involved in GABA metabolism. Using general anesthesia, we incised the bulbar conjunctiva of the rat around the limbus and clamped the left optic nerve. A sham operation was performed on the right eyes. Ocular ischemia was sustained for 30, 60 and 90 minutes. Rat eyes were enucleated immediately after ischemia and prepared for immunohistochemistry and enzyme activity measurement. Glutamate-like immunoreactivity (Glu-IR) in the sham-operated rat retina was observed in all retinal layers, showing intense staining in the nerve fiber layer (NFL), ganglion cell layer (GCL), and inner plexiform layer (IPL). Glu-IR increased in the outer plexiform layer (OPL) and outer nuclear layer (ONL) in an ischemic time-dependent manner. GABA-like immunoreactivity (GABA-IR) in sham-operated rat retina was observed in NFL, GCL, IPL and inner nuclear layer (INL). When the ischemic time was extended, GABA-IR intensely stained Müller cells. GAD activity was not changed in ischemic rat retina as compared to normal rat retina, but GABA-T activity was significantly decreased in ischemic rat retina. These results suggested that glutamate was induced by ischemia and was converted to GABA by GAD activity. Increased GABA was not metabolized because GABA-T activity was decreased. GABA accumulation in Müller cells progressed during the change in activity of these metabolic enzymes.

Electrical responses from diabetic retina

Diabetic retinopathy has long been considered to be a retinal manifestation of systemic diabetic angiopathy. Indeed, it is therapeutically true. However, the prolongation of OP peak latency in diabetic eyes without any angiographic evidence of angiopathy leads us to presume that certain neuronal disorders occur early in diabetic eyes. Even though we cannot neglect the possibility that the prolongation of the OP peak latency may derive from undetectable retinal hypoperfusion, it is still far from conventional diabetic angiopathy. Rather, the status should be properly termed "intraretinal diabetic neuropathy" in that the neurones are the disturbed cells to cause visual dysfunction. Thereafter, the OP amplitude diminishes as retinopathy advances, probably depending on the degree of retinal circulatory disturbance. Marked diminution of the OP amplitude predicts rapid progression and poor prognosis of retinopathy. Diabetic retinal pigment epitheliopathy as manifested by one of our non-photic EOG responses is another kind of early ocular involvement of diabetes. Because its mechanisms are not yet known, so far we have not succeeded in correlating it to any kind of subjective visual index. Routine fundus inspection or fluorescent fundus angiography is incapable of detecting the compromised neural retina and/or retinal pigment epithelial integrity and thus the electrophysiology of vision has the edge in ophthalmology.