Free amino acids in developing rat retina

Visual fatigue a comprehensive review of mechanisms of occurrence, animal model design and nutritional intervention strategies

When the eyes work intensively, it is easy to have eye discomfort such as blurred vision, soreness, dryness, and tearing, that is, visual fatigue. Visual fatigue not only affects work and study efficiency, but long-term visual fatigue can also easily affect physical and mental health. In recent years, with the popularization of electronic products, although it has brought convenience to the office and study, it has also caused more frequent visual fatigue among people who use electronic devices. Moreover, studies have reported that the number of people with visual fatigue is showing a trend of increasing year by year. The range of people involved is also extensive, especially students, people who have been engaged in computer work and fine instruments (such as microscopes) for a long time, and older adults with aging eye function. More and more studies have proposed that supplementation with the proper nutrients can effectively relieve visual fatigue and promote eye health. This review discusses the physiological mechanisms of visual fatigue and the design ideas of animal experiments from the perspective of modern nutritional science. Functional food ingredients with the ability to alleviate visual fatigue are discussed in detail.

Effects of Hydrostatic Pressure on Electrical Retinal Activity in a Multielectrode Array-Based ex vivo Glaucoma Acute Model

Glaucoma is a heterogeneous eye disease causing atrophy of the optic nerve head (ONH). The optic nerve is formed by the axons of the retinal ganglion cells (RGCs) that transmit visual input to the brain. The progressive RGC loss during glaucoma leads to irreversible vision loss. An elevated intraocular pressure (IOP) is described as main risk factor in glaucoma. In this study, a multielectrode array (MEA)-based ex vivo glaucoma acute model was established and the effects of hydrostatic pressure (10, 30, 60, and 90 mmHg) on the functionality and survival of adult male and female wild-type mouse (C57BL/6) retinae were investigated. Spontaneous activity, response rate to electrical and light stimulation, and bursting behavior of RGCs was analyzed prior, during, and after pressure stress. No pressure related effects on spontaneous firing and on the response rate of the RGCs were observed. Even a high pressure level (90 mmHg for 2 h) did not disturb the RGC functionality. However, the cells’ bursting behavior significantly changed under 90 mmHg. The number of spikes in bursts doubled during pressure application and stayed on a high level after pressure stress. Addition of the amino sulfonic acid taurine (1 mM) showed a counteracting effect. OFF ganglion cells did not reveal an increase in bursts under pressure stress. Live/dead staining after pressure application showed no significant changes in RGC survival. The findings of our ex vivo model suggest that RGCs are tolerant toward high, short-time pressure stress.

Magnesium acetyltaurate prevents retinal damage and visual impairment in rats through suppression of NMDA-induced upregulation of NF-κB, p53 and AP-1 (c-Jun/c-Fos)

Magnesium acetyltaurate (MgAT) has been shown to have a protective effect against N-methyl-D-aspartate (NMDA)-induced retinal cell apoptosis. The current study investigated the involvement of nuclear factor kappa-B (NF-κB), p53 and AP-1 family members (c-Jun/c-Fos) in neuroprotection by MgAT against NMDA-induced retinal damage. In this study, Sprague-Dawley rats were randomized to undergo intravitreal injection of vehicle, NMDA or MgAT as pre-treatment to NMDA. Seven days after injections, retinal ganglion cells survival was detected using retrograde labelling with fluorogold and BRN3A immunostaining. Functional outcome of retinal damage was assessed using electroretinography, and the mechanisms underlying antiapoptotic effect of MgAT were investigated through assessment of retinal gene expression of NF-κB, p53 and AP-1 family members (c-Jun/c-Fos) using reverse transcription-polymerase chain reaction. Retinal phospho-NF-κB, phospho-p53 and AP-1 levels were evaluated using western blot assay. Rat visual functions were evaluated using visual object recognition tests. Both retrograde labelling and BRN3A immunostaining revealed a significant increase in the number of retinal ganglion cells in rats receiving intravitreal injection of MgAT compared with the rats receiving intravitreal injection of NMDA. Electroretinography indicated that pre-treatment with MgAT partially preserved the functional activity of NMDA-exposed retinas. MgAT abolished NMDA-induced increase of retinal phospho-NF-κB, phospho-p53 and AP-1 expression and suppressed NMDA-induced transcriptional activity of NF-κB, p53 and AP-1 family members (c-Jun/c-Fos). Visual object recognition tests showed that MgAT reduced difficulties in recognizing the visual cues (i.e. objects with different shapes) after NMDA exposure, suggesting that visual functions of rats were relatively preserved by pre-treatment with MgAT. In conclusion, pre-treatment with MgAT prevents NMDA induced retinal injury by inhibiting NMDA-induced neuronal apoptosis via downregulation of transcriptional activity of NF-κB, p53 and AP-1-mediated c-Jun/c-Fos. The experiments were approved by the Animal Ethics Committee of Universiti Teknologi MARA (UiTM), Malaysia, UiTM CARE No 118/2015 on December 4, 2015 and UiTM CARE No 220/7/2017 on December 8, 2017 and Ethics Committee of Belgorod State National Research University, Russia, No 02/20 on January 10, 2020.

A multielectrode array-based hypoxia model for the analysis of electrical activity in murine retinae

Several eye diseases, for example, retinal artery occlusion, diabetic retinopathy, and glaucoma, are associated with retinal hypoxia. The lack of oxygen in the retina, especially in retinal ganglion cells (RGCs), causes cell damage up to cell degeneration and leads to blindness. Using multielectrode array recordings, an ex vivo hypoxia acute model was established to analyze the electrical activity of murine wild-type retinae under hypoxic stress conditions. Hypoxia was induced by exchanging the perfusion with oxygen-saturated medium by nitrogen-saturated medium. Hypoxic periods of 0 min (control) up to 60 min were tested on the retinae of adult female C57BL/6J mice. The electrical RGC activity vanished during hypoxia, but conditionally returned after the reestablishment of conventional test conditions. With increasing duration of hypoxia, the returning RGC activity decreased. After a hypoxic period of 30 min and a subsequent recovery time of 30 min, 59.43 ± 11.35% of the initially active channels showed a restored RGC activity. The survival rate of retinal cells after hypoxic stress was analyzed by a live/dead staining assay using two-photon laser scanning microscopy. For detailed information about molecular changes caused by hypoxia, a microarray gene expression analysis was performed. Furthermore, the effect of 2-aminoethanesulfonic acid (taurine, 1 mM) on retinae under hypoxic stress was tested. Treatment with taurine resulted in an increase in the RGC response rate after hypoxia and also increased the survival rate of retinal cells under hypoxic stress, confirming its potential as promising candidate for neuroprotective therapies of eye diseases.

Antiapoptotic effect of taurine against NMDA-induced retinal excitotoxicity in rats

OBJECTIVE

N-methyl-D-aspartate (NMDA) excitotoxicity has been proposed to mediate apoptosis of retinal ganglion cells (RGCs) in glaucoma. Taurine (TAU) has been shown to have neuroprotective properties, thus we examined anti-apoptotic effect of TAU against retinal damage after NMDA exposure.

METHODOLOGY

Sprague-Dawley rats were divided into 5 groups of 33 each. Group 1 was administered intravitreally with PBS and group 2 was similarly injected with NMDA (160 nmol). Groups 3, 4 and 5 were injected with TAU (320 nmol) 24 hours before (pre-treatment), in combination (co-treatment) and 24 hours after (post-treatment) NMDA exposure respectively. Seven days after injection, rats were sacrificed; eyes were enucleated, fixed and processed for morphometric analysis, TUNEL and caspase-3 staining. Optic nerve morphology assessment was done using toluidine blue staining. The estimation of BDNF, pro/anti-apoptotic factors (Bax/Bcl-2) and caspase-3 activity in retina was done using ELISA technique.

RESULTS

Severe degenerative changes were observed in retinae after intravitreal NMDA exposure. The retinal morphology in the TAU pre-treated group appeared more similar to the control retinae and demonstrated a higher number of nuclei than the NMDA group both per 100 μm length (by 1.5-fold, p < 0.001) and per 100 μm² area (by 1.41-fold, p < 0.05) of the GCL. After NMDA exposure, visible axonal swelling was observed in optic nerve sections. In comparison with the changes observed in the NMDA treated group, the TAU treated group showed fewer prominent changes; axonal swelling was less frequent and less marked. Additionally, no marked glial cell changes were observed in the TAU-pretreated group. All TAU treated groups, particularly the pre-treated group, showed a significant decrease in the NMDA-induced optic nerve damage, with a 50% reduction (p < 0.001) in the mean grading compared to NMDA group. For the same, there was 25% decrease in co- and post-treatment groups, as compared with the NMDA group. Pre-treatment with TAU abolished apoptotic response to NMDA as indicated by decrease in the number of TUNEL- and caspase-3-positive cells. TAU pre-treatment also increased the Bcl-2 level (by 2.80-fold, p < 0.001) and decreased the level of Bax (by 34%, p < 0.01), and activity of caspase-3 (by 36%, p < 0.001) compared to NMDA group.

IN CONCLUSION

our study revealed that pre-treatment with TAU prevents NMDA-induced retinal cell apoptosis more effectively than co- and post-treatment with TAU.

Rational Basis for Nutraceuticals in the Treatment of Glaucoma

BACKGROUND

Glaucoma, the second leading cause of blindness worldwide, is a chronic optic neuropathy characterized by progressive retinal ganglion cell (RGC) axons degeneration and death. Primary open-angle glaucoma (OAG), the most common type, is often associated with increased intraocular pressure (IOP), however other factors have been recognized to partecipate to the patogenesis of the optic neuropathy. IOP-independent mechanisms that contribute to the glaucoma-related neurodegeneration include oxidative stress, excitotoxicity, neuroinflammation, and impaired ocular blood flow. The involvement of several and diverse factors is one of the reasons for the progression of glaucoma observed even under efficient IOP control with the currently available drugs.

METHODS

Current research and online content related to the potential of nutritional supplements for limiting retinal damage and improving RGC survival is reviewed.

RESULTS

Recent studies have suggested a link between dietary factors and glaucoma risk. Particularly, some nutrients have proven capable of lowering IOP, increase circulation to the optic nerve, modulate excitotoxicity and promote RGC survival. However, the lack of clinical trials limit their current therapeutic use. The appropriate use of nutraceuticals that may be able to modify the risk of glaucoma may provide insight into glaucoma pathogenesis and decrease the need for, and therefore the side effects from, conventional therapies.

CONCLUSION

The effects of nutrients with anti-oxidant and neuroprotective properties are of great interest and nutraceuticals may offer some therapeutic potential although a further rigorous evaluation of nutraceuticals in the treatment of glaucoma is needed to determine their safety and efficacy.

The Benefits of the Citrus Flavonoid Diosmin on Human Retinal Pigment Epithelial Cells under High-Glucose Conditions

We investigate diosmin for its effect on the ARPE-19 human retinal pigment epithelial cells exposed to high glucose, a model of diabetic retinopathy (DR). After incubation for 4 days with a normal (5 mmol/L) concentration of D-glucose, ARPE-19 cells were exposed separately to normal or high concentrations of D-glucose (30 mmol/L) with or without diosmin at different concentrations (0.1, 1, 10 μg/mL) for another 48 h. Next, we assessed cell viability, reactive oxygen species (ROS) generation and antioxidant enzyme activities. In order to examine the underlying molecular mechanisms, we meanwhile analyzed the expressions of Bax, Bcl-2, total and phosphorylated JNK and p38 mitogen-activated protein kinase (MAPK). Diosmin dose dependently enhanced cell viability following high glucose treatment in ARPE-19 cells. The activities of superoxide dismutase and glutathione peroxidase, as well as the levels of reduced glutathione were decreased, while it was observed that levels of ROS in high glucose cultured ARPE-19 cells increased. High glucose also disturbed Bax and Bcl-2 expression, interrupted Bcl-2/Bax balance, and triggered subsequent cytochrome c release into the cytosol and activation of caspase-3. These detrimental effects were ameliorated dose dependently by diosmin. Furthermore, diosmin could abrogate high glucose-induced apoptosis as well as JNK and P38 MAPK phosphorylation in ARPE-19 cells. Our results suggest that treatment ARPE-19 cells with diosmin halts hyperglycemia-mediated oxidative damage and thus this compound may be a candidate for preventing the visual impairment caused by DR.

Effects of diabetic retinopathy on the barrier functions of the retinal pigment epithelium

Diabetic retinopathy is a debilitating microvascular complication of diabetes mellitus. A rich literature describes the breakdown of retinal endothelial cells and the inner blood-retinal barrier, but the effects of diabetes on the retinal pigment epithelium (RPE) has received much less attention. RPE lies between the choroid and neurosensory retina to form the outer blood-retinal barrier. RPE's specialized and dynamic barrier functions are crucial for maintaining retinal health. RPE barrier functions include a collection of interrelated structures and activities that regulate the transepithelial movement of solutes, including: diffusion through the paracellular spaces, facilitated diffusion through the cells, active transport, receptor-mediated and bulk phase transcytosis, and metabolic processing of solutes in transit. In the later stages of diabetic retinopathy, the tight junctions that regulate the paracellular space begin to disassemble, but there are earlier effects on the other aspects of RPE barrier function, particularly active transport and metabolic processing. With advanced understanding of RPE-specific barrier functions, and more in vivo-like culture models, the time is ripe for revisiting experiments in the literature to resolve controversies and extend our understanding of how diabetes affects the outer blood-retinal barrier.

Taurine: the comeback of a neutraceutical in the prevention of retinal degenerations

Taurine is the most abundant amino acid in the retina. In the 1970s, it was thought to be involved in retinal diseases with photoreceptor degeneration, because cats on a taurine-free diet presented photoreceptor loss. However, with the exception of its introduction into baby milk and parenteral nutrition, taurine has not yet been incorporated into any commercial treatment with the aim of slowing photoreceptor degeneration. Our recent discovery that taurine depletion is involved in the retinal toxicity of the antiepileptic drug vigabatrin has returned taurine to the limelight in the field of neuroprotection. However, although the retinal toxicity of vigabatrin principally involves a deleterious effect on photoreceptors, retinal ganglion cells (RGCs) are also affected. These findings led us to investigate the possible role of taurine depletion in retinal diseases with RGC degeneration, such as glaucoma and diabetic retinopathy. The major antioxidant properties of taurine may influence disease processes. In addition, the efficacy of taurine is dependent on its uptake into retinal cells, microvascular endothelial cells and the retinal pigment epithelium. Disturbances of retinal vascular perfusion in these retinal diseases may therefore affect the retinal uptake of taurine, resulting in local depletion. The low plasma taurine concentrations observed in diabetic patients may further enhance such local decreases in taurine concentration. We here review the evidence for a role of taurine in retinal ganglion cell survival and studies suggesting that this compound may be involved in the pathophysiology of glaucoma or diabetic retinopathy. Along with other antioxidant molecules, taurine should therefore be seriously reconsidered as a potential treatment for such retinal diseases.

Neuroprotective Mechanisms of Taurine against Ischemic Stroke

Ischemic stroke exhibits a multiplicity of pathophysiological mechanisms. To address the diverse pathophysiological mechanisms observed in ischemic stroke investigators seek to find therapeutic strategies that are multifaceted in their action by either investigating multipotential compounds or by using a combination of compounds. Taurine, an endogenous amino acid, exhibits a plethora of physiological functions. It exhibits antioxidative properties, stabilizes membrane, functions as an osmoregulator, modulates ionic movements, reduces the level of pro-inflammators, regulates intracellular calcium concentration; all of which contributes to its neuroprotective effect. Data are accumulating that show the neuroprotective mechanisms of taurine against stroke pathophysiology. In this review, we describe the neuroprotective mechanisms employed by taurine against ischemic stroke and its use in clinical trial for ischemic stroke.

Taurine provides neuroprotection against retinal ganglion cell degeneration

Retinal ganglion cell (RGC) degeneration occurs in numerous retinal diseases leading to blindness, either as a primary process like in glaucoma, or secondary to photoreceptor loss. However, no commercial drug is yet directly targeting RGCs for their neuroprotection. In the 70s, taurine, a small sulfonic acid provided by nutrition, was found to be essential for the survival of photoreceptors, but this dependence was not related to any retinal disease. More recently, taurine deprivation was incriminated in the retinal toxicity of an antiepileptic drug. We demonstrate here that taurine can improve RGC survival in culture or in different animal models of RGC degeneration. Taurine effect on RGC survival was assessed in vitro on primary pure RCG cultures under serum-deprivation conditions, and on NMDA-treated retinal explants from adult rats. In vivo, taurine was administered through the drinking water in two glaucomatous animal models (DBA/2J mice and rats with vein occlusion) and in a model of Retinitis pigmentosa with secondary RGC degeneration (P23H rats). After a 6-day incubation, 1 mM taurine significantly enhanced RGCs survival (+68%), whereas control RGCs were cultured in a taurine-free medium, containing all natural amino-acids. This effect was found to rely on taurine-uptake by RGCs. Furthermore taurine (1 mM) partly prevented NMDA-induced RGC excitotoxicity. Finally, taurine supplementation increased RGC densities both in DBA/2J mice, in rats with vein occlusion and in P23H rats by contrast to controls drinking taurine-free water. This study indicates that enriched taurine nutrition can directly promote RGC survival through RGC intracellular pathways. It provides evidence that taurine can positively interfere with retinal degenerative diseases.

Actions of guanidinoethane sulfonate on taurine concentration, retinal morphology and seizure threshold in the neonatal rat

Administration of the taurine transport inhibitor, guanidinoethane sulfonate (GES) to pregnant rats depleted taurine concentrations to approximately one-half of normal values in the newborn progeny. By 5 days of age taurine concentrations had returned to normal in all organs tested with the exception of the lungs. Longer postnatal exposure to GES significantly depressed tissue taurine levels. Prenatal exposure to GES had no effect on fetal development or the capability of the newborn rat to biosynthesize or transport taurine. Pre- and postnatal exposure to GES produced a degeneration of the photoreceptor layer of the retina similar to that observed in cats fed a taurine deficient diet. The pentylene tetrazole chemoshock threshold in GES-treated pups was greater than that in control pups. These results indicate that prenatal exposure to GES deplete taurine concentrations in the newborn rat. Morphological changes are thereby produced in the retina of rat that are similar to those observed in animals having limited ability to synthesize taurine which are maintained on a taurine-free diet.

Taurine deficiency damages retinal neurones: cone photoreceptors and retinal ganglion cells

In 1970s, taurine deficiency was reported to induce photoreceptor degeneration in cats and rats. Recently, we found that taurine deficiency contributes to the retinal toxicity of vigabatrin, an antiepileptic drug. However, in this toxicity, retinal ganglion cells were degenerating in parallel to cone photoreceptors. The aim of this study was to re-assess a classic mouse model of taurine deficiency following a treatment with guanidoethane sulfonate (GES), a taurine transporter inhibitor to determine whether retinal ganglion cells are also affected. GES treatment induced a significant reduction in the taurine plasma levels and a lower weight increase. At the functional level, photopic electroretinograms were reduced indicating a dysfunction in the cone pathway. A change in the autofluorescence appearance of the eye fundus was explained on histological sections by an increased autofluorescence of the retinal pigment epithelium. Although the general morphology of the retina was not affected, cell damages were indicated by the general increase in glial fibrillary acidic protein expression. When cell quantification was achieved on retinal sections, the number of outer/inner segments of cone photoreceptors was reduced (20 %) as the number of retinal ganglion cells (19 %). An abnormal synaptic plasticity of rod bipolar cell dendrites was also observed in GES-treated mice. These results indicate that taurine deficiency can not only lead to photoreceptor degeneration but also to retinal ganglion cell loss. Cone photoreceptors and retinal ganglion cells appear as the most sensitive cells to taurine deficiency. These results may explain the recent therapeutic interest of taurine in retinal degenerative pathologies.

Age-related retinal degeneration in animal models of aging: possible involvement of taurine deficiency and oxidative stress

There is strong evidence that the retina degenerates with age. Electroretinogram deficits and photoreceptor cell death and structural abnormalities have been observed in both animal and human studies of aging. The mechanism behind this phenomenon is a very interesting area for scientific and medical study. Current data support the link between retinal degeneration and increased oxidative stress. Taurine is a free amino acid found in high millimolar concentrations in the retina, and age-related deficiency in retinal levels of taurine may contribute to the retinal degeneration associated with age. Taurine acts as an antioxidant and taurine replenishment is known to alleviate oxidative stress in the retina. Thus taurine supplementation may be useful in the treatment of age-related retinal dysfunction.

A Role for Ligand-Gated Ion Channels in Rod Photoreceptor Development

Neurotransmitter receptors are central to communication at synapses. Many components of the machinery for neurotransmission are present prior to synapse formation, suggesting a developmental role. Here, evidence is presented that signaling through glycine receptor alpha2 (GlyRalpha2) and GABA(A) receptors plays a role in photoreceptor development in the vertebrate retina. The signaling is likely mediated by taurine, which is present at high levels throughout the developing central nervous system (CNS). Taurine potentiates the production of rod photoreceptors, and this induction is inhibited by strychnine, an antagonist of glycine receptors, and bicuculline, an antagonist of GABA receptors. Gain-of-function experiments showed that signaling through GlyRalpha2 induced exit from mitosis and an increase in rod photoreceptors. Furthermore, targeted knockdown of GlyRalpha2 decreased the number of photoreceptors while increasing the number of other retinal cell types. These data support a previously undescribed role for these ligand-gated ion channels during the early stages of CNS development.

Inhibition of taurine transport by high concentration of glucose in cultured rat cardiomyocytes

Cultured rat cardiomyocytes were treated with 10, 20, and 30 mmol/L glucose and 30 mmol/L glucose plus protein kinase C (PKC) inhibitor, Chelerythrine. In the 20 and 30 mmol/L glucose-treated cells, taurine contents reduced by 15% and 27% (P<.05), respectively, taurine transporter (TAUT) mRNA levels reduced by 47% and 64% (P<.05), respectively, and cysteine sulfinate decarboxylase (CSD) mRNA reduced slightly, but not significantly. Time-dependent taurine uptakes reduced in the 10, 20, and 30 mmol/L glucose-treated cells, and time-dependent taurine release reduced in the 30 mmol/L glucose-treated cells. The Vmax of taurine transport decreased by 18%, 30%, and 35% (P<.05) in the 10, 20, and 30 mmol/L glucose-treated cells, respectively, while Km of taurine transport remained unchanged. When PKC inhibitor, Chelerythrine, combined with 30 mmol/L glucose was treated with the cells, the lowered taurine content, taurine uptake, taurine release, and Vmax of taurine transport caused by 30 mmol/L glucose were eliminated. These results demonstrate that high glucose considerably and specifically decreases intracellular taurine content, taurine transport activity, and TAUT mRNA, possibly through PKC-mediated transcriptional and posttranslational pathways.

Taurine and its trophic effects in the retina

The sulphur amino acid taurine possesses variable functions during development and regeneration of the central nervous system. The retina synthesize and uptake taurine, which is the amino acid present in the highest concentration in this tissue. Deficiency of taurine alters the structure and the function of the cerebral and cerebellar cortex, as well as the retina. Taurine increases outgrowth of postcrush goldfish retina in culture, partially by elevating calcium influx, and also by the modulation of protein phosphorylation. Its concentration increases in the retina after the lesion of the optic nerve, and the intraocular injection of it, between the crush and the explantation, stimulates the outgrowth of neurites. Taken together, although there are a great number of unresolved questions on the mechanisms of action of this amino acid as a trophic substance, the results support the role of taurine during regeneration of the optic nerve.

Insulin-stimulated taurine uptake in rat retina and retinal pigment epithelium

Taurine is found at millimolar concentration in the retina and retinal pigment epithelium. High concentrations of taurine are essential for maintenance of retinal function. Taurine uptake by retina and retinal pigment epithelium was significantly enhanced by physiological concentrations of insulin as well as by high glucose concentrations. The results indicate that both, glucose and insulin enhanced taurine uptake occur through an increase in transport capacity which offset an additional, small decrease in affinity of the taurine carrier. Similar results were observed in retina and retinal pigment epithelium from streptozotocin-induced diabetic rats, suggesting that glucose and insulin regulate the taurine carrier through the same mechanism.

Taurine uptake activity in the rat retina: protein kinase C-independent inhibition by chelerythrine

Taurine, a regulatory amino acid of various biochemical processes in the retina, requires an efficient uptake system to maintain the high physiological concentration of taurine in the retina. Taurine uptake was characterized in both whole retinal preparations and in isolated rod outer segments (ROS) in terms of uptake kinetics and possible protein kinase C (PKC)-dependent regulation. Two uptake systems, a high- and a low-affinity system, were found in whole retinal preparations while only the high-affinity system was found in the isolated ROS. All the uptake systems characterized were inhibited by guanidinoethane sulfonate (GES), a well-known competitive inhibitor of taurine uptake. Stimulation and inhibition of PKC activity with phorbol myristate acetate and with staurosporine, respectively, produced no significant effect on taurine uptake. On the other hand, chelerythrine (CHT), a documented potent PKC inhibitor, was found to cause significant inhibition of the two taurine uptake systems, presumably through a PKC-independent mechanism. The data demonstrate that CHT may be a useful tool in studying taurine uptake in the retina and specifically in the ROS.

Histidine-like immunoreactivity in the rat retina

PURPOSE

Histidine is an indispensable amino acid with an imidazole ring and it is a precursor of histamine, carnosine, and anserine. Histidine has been proposed to act as a neurotransmitter or neuromodulator in mammalian central nervous system (CNS), including retina. In this study, we report histidine-like immunoreactivity in the rat retina with the use of antibodies raised against histidine coupled to bovine serum albumin (BSA) with glutaraldehyde.

METHODS

In order to confirm the specificity of the antiserum toward histidine, immunodots were carried out. Only the histidine conjugate showed immunoreactivity. The rat retinas fixed with glutaraldehyde were used for immunocytochemistry.

RESULTS

Histidine immunoreactivity was identified in the ganglion cell layer (GCL), inner nuclear layer (INL), inner plexiform layer (IPL), and Müller cells of rat retina.

CONCLUSIONS

Histidine may be a precursor of histamine in the inner retina, and Müller cells may play some role in the metabolism of histidine.

Effect of diabetes on levels and uptake of putative amino acid neurotransmitters in rat retina and retinal pigment epithelium

Free amino acid levels and high affinity uptake of glutamate, aspartate, gamma-aminobutyrate, glycine and taurine were studied in retina and retinal pigment epithelium of streptozotocin diabetic rats. Results show that experimental diabetes produces a generalized fall in the content of free amino acids in both retina and retinal pigment epithelium. With regard to the high affinity uptake, in the two tissues of diabetic animals showed decreased aspartate uptake, enhanced taurine and gamma-aminobutyrate uptake, whereas that of glycine and glutamate was unchanged. These results might suggest that diabetes causes alterations of specific amino acid transport systems and/or alterations of some cell populations.

Postnatal developmental expression of glutamine and related amino acids in the rat retinas

PURPOSE

To evaluate postnatal developmental changes in the amounts of retinal glutamate, glutamine and GABA, and in the distribution of retinal glutamine in the rat.

METHODS

Free amino acids were extracted from rat retinas of different postnatal stages, and the concentrations of glutamate, glutamine and GABA were determined by HPLC. Also, anti-glutamine antibody was raised and an immunocytochemistry was performed with paraffin-embedded retinal sections in parallel with free amino acid analyses.

RESULTS

Glutamate occurred in high concentrations at the birth and showed a stable pool, while glutamine and GABA remained low until postnatal day 3 or 5, and gradually increased in the developing rat retinas. Glutamine immunolabeling was observed in the retinal pigment epithelium and in a subpopulation of presumed amacrine cells in the early postnatal days. It was also found in Muller cells and in some ganglion cells or displaced amacrine cells in the ganglion cells layer. Glutamine immunolabeling was transiently observed also in horizontal cells. Finally, the immunolabeling was dominant in the inner and outer plexiform layers in the adult retinas.

CONCLUSIONS

Postnatal developmental increase in the levels of glutamine and GABA might be dependent on the maturation of neurons or glial cells that possess the activity of the key enzymes of each amino acid. It was suggested that an expression of glutamine immunolabeling can be a marker of neurons that utilize glutamine as a precursor for glutamate or GABA, and of Müller cell maturations in postnatal early stage of the retina, while it changes to demonstrate the locations of glutamine cycle in the retina with adult characteristics.

Taurine in gerbil retina: changes during ischemia reperfusion/insult (I.R.I.) and aging

The retina contains a high amount of taurine which suggests a role in retinal function. The mongolian gerbil is well studied in stroke because of its incomplete circle of Willis. Two groups of gerbils were used: one served as control and the other was subjected to unilateral left carotid occlusion during 30 minutes, followed by 60 minutes of reperfusion. Gerbils were selected by ocular fundus and only sensitive gerbils were retained for the experimentation. We studied the level of taurine in gerbil retina of different ages: 3, 9, 15 and 24 months old (sham operated and ischemic groups). Level of taurine was determined by HPLC/electrochemical method. Compared to sham operated groups, level of taurine was significantly increased in ischemic groups for all ages studied. In the sham operated groups, level of taurine was low at birth, reached a plateau, and then decreased with aging. In the ischemic groups, level of taurine regularly increased from 3 to 24 months of age. With comparison evaluated for each age (modification ischemic versus sham operated groups expressed in percentage), level of taurine was quite equal at 3 and 9 months of age, but increased in 15 and 24 months old gerbils.

K(+)-, hypoosmolarity-, and NH4(+)-induced taurine release from cultured rabbit Müller cells: role of Na+ and Cl- ions and relation to cell volume changes

The release of preloaded radiolabeled taurine (TAU) from cultured rabbit Müller cells [14-21 days in vitro (DIV)] was measured before and after treatment with the following stimuli: 1) isoosmotic 65 mM KCl; 2) a medium made hypoosmotic by uncompensated lowering of Na+ by 40-100 mM; and 3) NH4Cl ranging from 0.25 to 5 mM. The same stimuli were tested for their effect on the cell volume by the 3-O-methyl-D-glucose (OMG) uptake method of Kletzien et al. (Anal Biochem 68:537, 1975). Hypoosmotic media and 65 mM KCl stimulated TAU release, and the release was well correlated with the increase of cell volume. The stimulatory effect of 65 mM KCl was abolished by isotonic removal of Cl- or Na+, and omission of either ion markedly enhanced the basal release of TAU. The results are roughly consistent with the characteristics of the swelling-induced TAU release reported for cultured astrocytes and neurons of various CNS regions, and also for freshly isolated, nondissociated retina. Taken together, the results are indicative of a significant role of TAU release from Müller cells, in the osmosensory response of the retina. Ammonium chloride stimulated TAU release in a dose-dependent manner, a significant stimulation being already observed at 0.5 mM, a concentration that is frequently measured in brain during acute hyperammonemia. The effect of NH4Cl was strictly chloride dependent at 0.5-2 mM, but partly Cl- independent at 5 mM. The Kletzien's method did not appear to be well suited for measuring cell volume in the presence of ammonium ions.(ABSTRACT TRUNCATED AT 250 WORDS)

Taurine: retinal function

The status and potential functions of taurine in the retina have been reviewed. Taurine is present in high concentrations in the retina of all species tested, while the retinal concentrations of the enzymes necessary to synthesize taurine are presumed to vary among those species. The documented low activity of cysteinesulfinic acid decarboxylase, a key enzyme in taurine biosynthesis, in the livers of the cat, monkey and human possibly reflect low activity in their retinas, indicating reliance on the diet as an important source of taurine. Both high- and low-affinity binding proteins and uptake systems have been described for taurine in retinal tissue. Evoked release of taurine by light and other depolarizing stimuli have been well documented. Retinal pathologies including diminished ERGs and morphologic changes have been reported for animals and man deficient in taurine. Possible functions for taurine in the retina include: (1) protection of the photoreceptor - based on the shielding effects of taurine on rod outer segments exposed to light and chemicals; (2), regulation of Ca2+ transport - based on the modulatory effects of taurine on Ca2+ fluxes in the presence and absence of ATP; and (3) regulation of signal transduction - based on the inhibitory effects of taurine on protein phosphorylation.

Taurine effect on neuritic growth from goldfish retinal explants

Explants from retinas from which the optic nerve was previously crushed exhibited neuritic outgrowth after 4-5 days in culture, and up to 10 days in the presence of the same nutrient medium. Intact retinas did not exhibit vigorous outgrowth. Taurine added in the medium to post-crush explants accelerated the emission of neurites in 1-2 days, and increased the nerve growth index in a concentration-dependent manner up to 4 mM. Greater concentration did not produce any effect on the nerve growth index. The effect of taurine was produced between 0 and 5 days in culture; from day 5 to day 10 explants did not exhibit further growth in the presence of the amino acid. However, post-crush control explants exhibited growth between 5 and 10 days. Taurine effect is blocked in the absence of fetal calf serum, and is reduced in the presence of dialysed serum. Providing new medium on days 1 and 2, but not later, elevated the nerve growth index with respect to explants regenerating in the same medium since day 0. In addition, taurine supplied at various days after plating produced the same pattern of growth as that observed by adding fresh medium, but the nerve growth index was statistically greater in the presence of the amino acid. The mechanisms for these observations remain to be elucidated.

Taurine and other free amino acids in the retina, vitreous, lens, iris-ciliary body, and cornea of the rat eye

Levels of free amino acids were determined quantitatively in whole ocular tissues of the rat eye with aid of a sensitive amino acid analyzer. The tissues studied were the retina, vitreous, lens, iris-ciliary body, and cornea. The retina and lens contained a more concentrated free amino acid pool than other tissues. The neuroactive amino acids taurine. GABA, glutamic acid, aspartic acid, and glycine were clearly enriched in the retina. Taurine was the most abundant amino acid in all five tissues studied, and its high concentration in non-neural tissues, especially the lens, suggests that it must have other functions as well as neurotransmitter ones in the rat eye.

Altered time course of changes in the hippocampal concentration of excitatory and inhibitory amino acids during kainate-induced epilepsy

The temporal sequence of electrophysiological and biochemical correlates of epilepsy induced by systemic injection of kainic acid (15 mg/kg i.p.) was investigated in male rats. A significant decrease in the hippocampal concentration of glutamate and aspartate was observed 20 min after the injection. These decreases preceded both electrographic and behavioral manifestations of epilepsy, thus suggesting a causal relationship between acidic amino acid changes and the genesis of kainate-induced hyperactivity. About 30-45 min after kainate injection, a decrease in glutamate, aspartate, glycine and taurine and no change in GABA concentration were observed. Bioelectrical activity, recorded in the regio inferior (CA3) of the hippocampus or in the fascia dentata revealed the presence of high frequency bursts separated by a long-lasting depression of discharge. About 55-75 min after the injection, the number of spikes in each burst increased and the duration and frequency of interictal pauses decreased. This stage was characterized by a decrease in glutamate and aspartate, restoration to normal of glutamine, glycine and taurine and a decrease in GABA.

Retina maturation following administration of thyroxine in developing rats: effects on polyamine metabolism and glutamate decarboxylase

The effects of subcutaneous daily treatment with thyroxine on cell proliferation, differentiation, polyamines, and gamma-aminobutyric acid metabolism in the rat retina were studied during the first 20 postnatal days. The retinal layers of the treated rats displayed an enhanced cell differentiation which reached its maximum 9-12 days from birth; but this effect stopped very quickly and was finished by the 20th postnatal day. Primarily there was an increase in ornithine decarboxylase activity which was accompanied by an increase in putrescine, spermidine, and spermine levels. S-Adenosylmethionine decarboxylase was induced later than ODC; corresponding with the enhanced synaptogenesis, glutamate decarboxylase increased 15-fold between the fourth and 15th days. Our data are consistent with the hypothesis that thyroxine may exert some of its effects by inducing the enzymes which regulate polyamine metabolism and synaptogenesis.

Phenolic and tyrosyl ring deiodination in thyroxine from rat retina during postnatal development

To elucidate tetraiodothyronine (T4) metabolism in developing rat retina 5-monodeiodinating and 5'-monodeiodinating activities were studied. T4 was incubated with aliquots of homogenate or crude primary subcellular fractions, and the 3,3',5'-triiodothyronine (rT3) or 3,5,3'-triiodothyronine (T3) produced were measured by radioimmunoassay. Reaction rates were dependent on incubation time, tissue amount, temperature and pH. The optimum pH values were 7.8 and 7.2 respectively for rT3-forming and T3-forming systems. Conversion of T4 to either T3 or rT3 was dependent on dithiothreitol concentration, and the T4-5'-deiodinating activity was inhibited by propylthiouracil. Deiodinase activities were mainly found in the crude microsomes. The retinal 5'-monodeiodination rate of T4 was immeasurably low by the 2nd day and the highest values were reached on 15th day of postnatal development. On the other hand deiodination of the T4 tyrosyl ring shows a progressive decline from birth, and adult values were reached on the 15th day. Data support the hypothesis that, in developing rat thyroxine, phenolic and tyrosyl-ring deiodinase activities are present in the retina and their reciprocal changes may regulate morphological and biochemical cell maturation.

Identification of strychnine binding sites in the rat retina

[3H]Strychnine specifically binds to membrane fractions isolated from rat retinae. The binding is saturable, with an apparent dissociation constant, KD, of 14.3 x 10(-9) M and 205 fmol bound/mg protein. Specific binding is time-dependent and proportional to protein concentration. Glycine and taurine are equally potent inhibitors of [3H]strychnine binding (Ki = 4 x 10(-5) M); no other amino acids endogenously present in the retina inhibited [3H]strychnine binding.

Photochemical damage in the albino rat retina: morphological changes and endogenous amino acids

Endogenous amino acids were measured in retinas of rats exposed for up to 48 h to fluorescent light. Typical light damage was seen in photoreceptor cells after 30 h exposure to a maximum luminance of 1544 scotopic lux; and, from this time, taurine levels were significantly reduced. In contrast, the concentrations of other amino acids increased. After 18 h exposure to light, GABA, glycine, glutamate, and aspartate levels were raised in the photoreceptor cells, and GABA, glutamate, and glutamine levels in the inner retina. When 'exposed' animals were returned to their normal environment for 72 h, photoreceptor degeneration progressed and taurine concentrations were further reduced: the results suggest that the loss was from damaged photoreceptor cells. At this time the concentrations of the other amino acids measured had, in general, returned to normal.

High-affinity taurine uptake in developing retina

A specific system for taurine transport is present at the early stages of development in both chick and rat retinas. The results obtained with taurine analogs indicate a high degree of specificity of taurine uptake. Two transport systems were detected for the adult rat retina: a high-affinity (Km 21 microM) and a low-affinity transport system (Km 312 microM). On the other hand, in the adult chick retina, only a low-affinity transport system (Km 580 microM) could be detected. Nevertheless, embryo chick retina accumulated [3H]taurine by two different kinetic mechanisms with Kms of 242 microM and 21 microM for the low- and high-affinity processes, respectively. Taurine uptake systems were absolutely Na+ dependent. The sodium-dependence curve for taurine uptake was sigmoid. These mechanisms appear not to be mediated by a Na+ cotransport system. In spite of the differences observed in taurine uptake in both species, in each of them it closely parallels the changes brought about by the morphological and functional maturation of the retina.

Taurine effects on 45Ca2+ transport in retinal subcellular fractions

The effect of taurine on 45Ca2+ transport by subcellular fractions from the chick retina was examined. An inhibitory action of taurine on 45Ca2+ uptake was observed in retinal fractions incubated for 1--5 min in a Krebs--bicarbonate medium, pH 7.4. In the crude nuclear fraction, 25 mM taurine produced a decrease of 50% in 45Ca2+ uptake; in the crude synaptosomal fraction, taurine reduced 45Ca2+ accumulation by 70%; the maximum inhibitory effect of taurine on 45Ca2+ uptake (80%) was observed in a fraction containing outer segments and pigment epithelium cells. Taurine effect was specific, dose-dependent and related to osmotically sensitive particles. The results suggest a role for taurine in the regulation of calcium fluxes in the retina.

Taurine in the developing rabbit visual system: changes in concentration and axonal transport including a comparison with axonally transported proteins

[35S]Taurine injected intravitreally into rabbits was transported axonally to the optic nerve terminals. Considerably more [35S]taurine was transported in young rabbits than in mature rabbits. The time course of taurine transport did not parallel that of proteins labeled with [3H]proline in the same system. The concentration of taurine in all components of the visual system, except retina, was greater in young animals than in mature animals, and was especially high in optic nerve. The possible functions of the high concentrations of taurine and the greater amount of axonally transported taurine in developing mammalian CNS are discussed.

Branched chain amino acid metabolism in the retina of diabetic rats

Diabetes is known to produce increased levels of the branched chain amino acids in plasma, heart and muscle as well as increased oxidation of [14C]-leucine by nerves and muscles from rats. Plasma and retinas from streptozotocin diabetic rats had significant elevations in branched chain amino acid levels compared to control. Retinas from diabetic rats have been found to oxidize significantly more of the branched chain amino acids, leucine, isoleucine and valine than did control retinas when incubated in media containing 16.5 mmol/l glucose. Neither the extracellular space nor the tissue pool of leucine was significantly different in the two groups. The addition of 19 amino acids, at normal plasma concentrations, to the incubation media resulted in 80 percent suppression of leucine oxidation without significant change in incorporation of [14C] into protein. These results suggest that the major role for the branched chain amino acids in the rat retina is in protein synthesis which is not affected by short-term diabetes.