Streptozotocin-Induced Diabetic Rats Showed a Differential Glycine Receptor Expression in the Spinal Cord: A GlyR Role in Diabetic Neuropathy

In the spinal cord, attenuation of the inhibitory action of glycine is related to an increase in both inflammatory and diabetic neuropathic pain; however, the glycine receptor involvement in diabetic neuropathy has not been reported. We determined the expression of the glycine receptor subunits (α1-α3 and β) in streptozotocin-induced diabetic Long-Evans rats by qPCR and Western blot. The total mRNA and protein expression (whole spinal cord homogenate) of the α1, α3, and β subunits did not change during diabetes; however, the α2 subunit mRNA, but not the protein, was overexpressed 45 days after diabetes induction. By contrast, the synaptic expression of the α1 and α2 subunits decreased in all the studied stages of diabetes, but that of the α3 subunit increased on day 45 after diabetes induction. Intradermal capsaicin produced higher paw-licking behavior in the streptozotocin-induced diabetic rats than in the control animals. In addition, the nocifensive response was higher at 45 days than at 20 days. During diabetes, the expression of the glycine receptor was altered in the spinal cord, which strongly suggests its involvement in diabetic neuropathy.

Glycine neurotransmission: Its role in development

The accurate function of the central nervous system (CNS) depends of the consonance of multiple genetic programs and external signals during the ontogenesis. A variety of molecules including neurotransmitters, have been implied in the regulation of proliferation, survival, and cell-fate of neurons and glial cells. Among these, neurotransmitters may play a central role since functional ligand-gated ionic channel receptors have been described before the establishment of synapses. This review argues on the function of glycine during development, and show evidence indicating it regulates morphogenetic events by means of their transporters and receptors, emphasizing the role of glycinergic activity in the balance of excitatory and inhibitory signals during development. Understanding the mechanisms involved in these processes would help us to know the etiology of cognitive dysfunctions and lead to improve brain repair strategies.

High glucose concentrations induce oxidative stress by inhibiting Nrf2 expression in rat Müller retinal cells in vitro

Diabetic retinopathy (DR) is a complication of diabetes. Several studies have implicated oxidative stress as a fundamental factor in the progression of the disease. The nuclear factor erythroid-2-related factor 2 (Nrf2) is one of the main regulators of redox homeostasis. Glia Müller cells (MC) maintain the structural and functional stability of the retina. The objective of this study was to evaluate the effect of high glucose concentrations on reactive oxygen species (ROS) production and Nrf2 expression levels in rat MC. MC were incubated with normal (NG; 5 mM) or high glucose (HG; 25 mM) for different times. Incubation with HG increased ROS levels from 12 to 48 h but did not affect cell viability. However, exposure to 3 h of HG caused a transient decrease Nrf2 levels. At that time, we also observed a decrease in the mRNA expression of Nrf2 target genes, glutathione levels, and catalase activity, all of which increased significantly beyond initial levels after 48 h of incubation. HG exposure leads to an increase in the p65 subunit of nuclear factor-κB (NF-kB) levels, and its target genes. These results suggest that high glucose concentrations lead to alteration of the redox regulatory capacity of Nrf2 mediated by NF-kB regulation.

Retinal Nrf2 expression in normal and early streptozotocin-diabetic rats

Diabetic retinopathy is the most common cause of vision loss among diabetic patients. Although hyperglycemia produces retinal oxidative stress in long-standing diabetes, the pathogenesis mechanism is unknown. The Nuclear factor erythroid 2-related factor 2 (Nrf2) plays a central role in cell responses against oxidative damage. We used adult Long Evans rats where diabetes was induced by streptozotocin. Normal and treated rats were sacrificed at 7, 20, and 45 days after streptozotocin injection. We analyzed Nrf2 and Keap1 expression in retinal homogenates, cytoplasmic, and nuclear retinal fractions. Normal retina showed Nrf2 expression in all retina nuclear layers. We found a transitory decrease of Nrf2 mRNA and protein expression at 7 and 20 days after the streptozotocin injection that recovered later on: moreover, the protein level increased after 45 days. Keap1 immunoprecipitation revealed similar levels as Nrf2 in normal and diabetic rat retinas, indicating that the diabetic condition did not lead to dissociation of the Keap1-Nrf2 complex. Indeed, glutathione levels and superoxide dismutase activity were not altered in the treated rat retinas. These results do not support oxidative stress in the retina shortly after diabetes induction.

Mitochondrial activity in different regions of the brain at the onset of streptozotocin-induced diabetes in rats

Diabetes affects a variety of tissues including the central nervous system; moreover, some evidence indicates that memory and learning processes are disrupted. Also, oxidative stress triggers alterations in different tissues including the brain. Recent studies indicate mitochondria dysfunction is a pivotal factor for neuron damage. Therefore, we studied mitochondrial activity in three brain regions at early type I-diabetes induction. Isolated mitochondria from normal hippocampus, cortex and cerebellum revealed different rates of oxygen consumption, but similar respiratory controls. Oxygen consumption in basal state 4 significantly increased in the mitochondria from all three brain regions from diabetic rats. No relevant differences were observed in the activity of respiratory complexes, but hippocampal mitochondrial membrane potential was reduced. However, ATP content, mitochondrial cytochrome c, and protein levels of β-tubulin III, synaptophysin, and glutamine synthase were similar in brain regions from normal and diabetic rats. In addition, no differences in total glutathione levels were observed between normal and diabetic rat brain regions. Our results indicated that different regions of the brain have specific metabolic responses. The changes in mitochondrial activity we observed at early diabetes induction did not appear to cause metabolic alterations, but they might appear at later stages. Longer-term streptozotocin treatment studies must be done to elucidate the impact of hyperglycemia in brain metabolism and the function of specific brain regions.

Nitrosative Stress in the Rat Retina at the Onset of Streptozotocin-Induced Diabetes

BACKGROUND/AIMS

Nitric oxide is a multifunctional molecule that can modify proteins via nitrosylation; it can also initiate signaling cascades through the activation of soluble guanylate cyclase. Diabetic retinopathy is the leading cause of blindness, but its pathogenesis is unknown. Multiple mechanisms including oxidative-nitrosative stress have been implicated. Our main goal was to find significant changes in nitric oxide (NO) levels and determine their association with nitrosative stress in the rat retina at the onset of diabetes.

METHODS

Diabetes was induced by a single intraperitoneal administration of streptozotocin. The possible nitric oxide effects on the rat retina were evaluated by the presence of nicotinamide adenine dinucleotide phosphate diaphorase (NADPH-d), a specific marker for NO-producing neurons, detected by histochemistry performed on whole retinas and retina sections. Immunohistochemistry was also performed on retina sections for iNOS, 3-nitrotyrosine (3-NT) and glial fibrillary acidic protein (GFAP). Retinal nitric oxide levels were assessed by measuring total nitrate/nitrite concentrations. Retinal cGMP levels were determined by radioimmunoassay. Western blots for nitrotyrosine (3-NT) and oxidized proteins were performed.

RESULTS

In the present study, we found increased activity of NADPH-diaphorase and iNOS immunoreactivity in the rat retina at the onset of diabetes; this increase correlated with a remarkable increase in NO levels as early as 7 days after the onset of diabetes. However, cGMP levels were not modified by diabetes, suggesting that NO did not activate its signaling cascade. Even so, Western blots revealed a progressive increase in nitrated proteins at 7 days after diabetes induction. Likewise, positive nitrotyrosine immunolabeling was observed in the photoreceptor layer, ganglion cell layer, inner nuclear layer and some Müller cell processes in the retinas of diabetic rats. In addition, levels of oxidized proteins were increased in the retina early after diabetes induction; these levels were reduced by the administration of L-NAME. In addition, stress in Müller cells was determined by immunoreactivity to the glial fibrillary acidic protein.

CONCLUSIONS

Our findings indicated the occurrence of nitrosative stress at the onset of diabetes in the rat retina and emphasized the role of NO in retinal function and the pathogenesis of retinopathy.

Potential Role of Endoplasmic Reticulum Stress in Pathogenesis of Diabetic Retinopathy

Diabetes mellitus is a metabolic disease that leads to several complications which include retinopathy. Multiple biochemical abnormalities have been proposed to explain the development of retinopathy, including oxidative stress. Although the existence of oxidative stress has been established in the retina from long standing diabetic animals, pathogenesis and progression of retinopathy remain unclear. In order to gain insight into the pathogenesis of diabetic retinopathy, we analyzed the levels of different oxidative stress biomarkers in the retina at early stages during the progress of streptozotocin-induced diabetes. No significant changes in glutathione content, expression of NADPH-oxidase, levels of lipid peroxidation, nor production of free radicals were observed in the retina up to 45 days of diabetes induction. Likewise, a transient decrease in aconitase activity, parallel to an increase in the superoxide dismutase activity was observed at 20 days of hyperglycemia, suggesting a high capacity of retina to maintain its redox homeostasis, at least at early stages of diabetes. Nonetheless, we found an early and time-dependent increase in the levels of oxidized proteins, which was not affected by the administration of the antioxidant quercetin. Also, positive immunoreactivity to the reticulum stress protein CHOP was found in glial Müller cells of diabetic rat retinas. These findings suggest the occurrence of endoplasmic reticulum stress as a primary event in retina pathogenesis in diabetes.

In the Early Stages of Diabetes, Rat Retinal Mitochondria Undergo Mild Uncoupling due to UCP2 Activity

In order to maintain high transmembrane ionic gradients, retinal tissues require a large amount of energy probably provided by a high rate of both, glycolysis and oxidative phosphorylation. However, little information exists on retinal mitochondrial efficiency. We analyzed the retinal mitochondrial activity in ex vivo retinas and in isolated mitochondria from normal rat retina and from short-term streptozotocin-diabetic rats. In normal ex vivo retinas, increasing glucose concentrations from 5.6mM to 30mM caused a four-fold increase in glucose accumulation and CO₂ production. Retina from diabetic rats accumulated similar amounts of glucose. However, CO₂ production was not as high. Isolated mitochondria from normal rat retina exhibited a resting rate of oxygen consumption of 14.6 ± 1.1 natgO (min.mg prot)^-1 and a respiratory control of 4.0. Mitochondria from 7, 20 and 45 days diabetic rats increased the resting rate of oxygen consumption and the activity of the electron transport complexes; under these conditions the mitochondrial transmembrane potential decreased. In spite of this, the ATP synthesis was not modified. GDP, an UCP2 inhibitor, increased mitochondrial membrane potential and superoxide production in controls and at 45 days of diabetes. The role of UCP2 is discussed. The results suggest that at the early stage of diabetes we studied, retinal mitochondria undergo adaptations leading to maintain energetic requirements and prevent oxidative stress.

Immunohistochemical localization of glycogen synthase and GSK3β: control of glycogen content in retina

Glycogen has an important role in energy handling in several brain regions. In the brain, glycogen is localized in astrocytes and its role in several normal and pathological processes has been described, whereas in the retina, glycogen metabolism has been scarcely investigated. The enzyme glycogen phosphorylase has been located in retinal Müller cells; however the cellular location of glycogen synthase (GS) and its regulatory partner, glycogen synthase kinase 3β (GSK3β), has not been investigated. Our aim was to localize these enzymes in the rat retina by immunofluorescence techniques. We found both GS and GSK3β in Müller cells in the synaptic layers, and within the inner segments of photoreceptor cells. The presence of these enzymes in Müller cells suggests that glycogen could be regulated within the retina as in other tissues. Indeed, we showed that glycogen content in the whole retina in vitro was increased by high glucose concentrations, glutamate, and insulin. In contrast, retina glycogen levels were not modified by norepinephrine nor by depolarization with high KCl concentrations. Insulin also induced an increase in glycogen content in cultured Müller cells. The effect of insulin in both, whole retina and cultured Müller cells was blocked by inhibitors of phosphatidyl-inositol 3-kinase, strongly suggesting that glycogen content in retina is modulated by the insulin signaling pathway. The expression of GS and GSK3β in the synaptic layers and photoreceptor cells suggests an important role of GSK3β regulating glycogen synthase in neurons, which opens multiple feasible roles of insulin within the retina.

[The post-synaptic glycine receptor mediates in a transitory and sustained way the glycinergic inhibition in the vertebrate retina]

INTRODUCTION

Glycine and the gamma-aminobutyric acid are the principal inhibitory neurotransmitters in the vertebrate retina. The inhibitory action of glycine is mediated by the post-synaptic glycine receptor, a chloride-selective channel, constituted by three beta and two alpha subunits (alpha(1)-alpha(4)), which is antagonized by the alkaloid strychnine. In the retina, it is known that all alpha isoforms are expressed at the level of the inner synaptic layer with a very low colocalization. The glycine receptor formed by either alpha1 or alpha(3) shows rapid kinetics, whereas alpha(2) or alpha(4) receptors respond tonically. The use of transgenic mice has allowed the study of the different glycine receptor alpha subunits in the glycinegic neurotransmission of the mammalian retina.

AIM

To describe the participation of the glycine receptor in the inhibitory neurotransmission particularly in the retina.

DEVELOPMENT

In this review we describe the experiments that have allowed the localization and the involvement of the alpha subunit isoforms in specific transmission circuits of the vertebrate retina.

CONCLUSIONS

The localization of the glycine receptor conformed by different isoforms of the alpha subunit in specific neuronal types, indicate the presence of glycinergic circuits that encode information differently in the retina.

Glycine receptor internalization by protein kinases activation

Although glycine-induced currents in the central nervous system have been proven to be modulated by protein kinases A (PKA) and C (PKC), the mechanism is not well understood. In order to better comprehend the mechanism involved in this phenomenon, we tested the PKA and PKC activation effect on the specific [(3) H]glycine and [(3) H]strychnine binding to postsynaptic glycine receptor (GlyR) in intact rat retina. The specific binding constituted about 20% of the total radioligand binding. Kinetic analysis of the specific binding exhibited a sigmoidal behavior with three glycine and two strychnine binding sites and affinities of 212 nM for [(3) H]glycine and 50 nM for [(3) H]strychnine. Specific radioligand binding was decreased (60-85%) by PKA and PKC activation, an effect that was blocked by specific kinases inhibitors, as well as by cytochalasin D. GlyR expressed in the plasma membrane decreased about 50% in response to kinases activation, which was consistent with an increase of the receptor in the microsomal fraction when PKA was activated. Moreover, immunoprecipitation studies indicated that these kinases lead to a time-dependent receptor phosphorylation. Our results suggest that in retina, GlyR is cross-regulated by G protein-coupled receptors, activating PKA and PKC.

Drosophila insulin pathway mutants affect visual physiology and brain function besides growth, lipid, and carbohydrate metabolism

OBJECTIVE

Type 2 diabetes is the most common form of diabetes worldwide. Some of its complications, such as retinopathy and neuropathy, are long-term and protracted, with an unclear etiology. Given this problem, genetic model systems, such as in flies where type 2 diabetes can be modeled and studied, offer distinct advantages.

RESEARCH DESIGN AND METHODS

We used individual flies in experiments: control and mutant individuals with partial loss-of-function insulin pathway genes. We measured wing size and tested body weight for growth phenotypes, the latter by means of a microbalance. We studied total lipid and carbohydrate content, lipids by a reaction in single fly homogenates with vanillin-phosphoric acid, and carbohydrates with an anthrone-sulfuric acid reaction. Cholinesterase activity was measured using the Ellman method in head homogenates from pooled fly heads, and electroretinograms with glass capillary microelectrodes to assess performance of central brain activity and retinal function.

RESULTS

Flies with partial loss-of-function of insulin pathway genes have significantly reduced body weight, higher total lipid content, and sometimes elevated carbohydrate levels. Brain function is impaired, as is retinal function, but no clear correlation can be drawn from nervous system function and metabolic state.

CONCLUSIONS

These studies show that flies can be models of type 2 diabetes. They weigh less but have significant lipid gains (obese); some also have carbohydrate gains and compromised brain and retinal functions. This is significant because flies have an open circulatory system without microvasculature and can be studied without the complications of vascular defects.

[Cys-loop ligand-gated ion channels modulation by protein kinases A and C]

INTRODUCTION

In the nervous system, rapid chemical neurotransmission is mediated by ionotropic receptors that are activated by ligand binding. Ligand binding to its receptor promotes the selective flow of ions into the cell which changes the electrical potential of the cell membrane. Cys-loop type receptors belong to the ligand-gated ion channel superfamily including the nicotinic acetylcholine receptor, the gamma-aminobutyric acid, glycine, serotonin and zinc. Several studies showed that the activity of these receptors was modified in response to protein kinases A and C activation; the different results, apparently contradictory, could be explained by the involvement of several factors such as the type of subunits that make up these receptors, components of the cytoskeleton and sub-types of kinases and phosphatases present in nerve tissue studied.

AIM

To review the effect of protein kinases A and C on the activity of cys-loop receptors.

DEVELOPMENT

In this review we describe experiments conducted in different regions where it was determined the effect of these kinases on the function of neurotransmitter receptors mostly distributed in the nervous system.

CONCLUSIONS

The cys-loop receptors regulation by protein kinases occurs through the activation of other receptors (cross-talk) that are expressed at different stages of development and nervous system areas.

Ascorbate uptake in normal and diabetic rat retina and retinal pigment epithelium

Oxidative stress is an important causative factor in the pathogenesis of diabetic retinopathy. Therefore, it becomes important to understand the mechanisms that help maintain appropriate levels of a small molecule antioxidant such as ascorbate in the retina. The outer blood-barrier which results from the tight junctions between the retinal pigment epithelial cells (RPE) restricts the flow of nutrients reaching the retina. In this study, we characterized the transport properties of carboxyl-(14)C ascorbate (AA) in normal rat retina and RPE, and compared them with those in streptozotocin-diabetic rats. Retina and RPE accumulated AA by a temperature-sensitive and energy-dependent kinetic mechanism with an apparent K(M) of 380 and 420 microM, respectively. Accumulation of AA was significantly reduced in a sodium-free medium. Although high glucose concentrations reduced AA uptake by 40%, this was not affected by cytochalasin B. The RPE and retina of diabetic rats presented lower levels of AA accumulation. These findings suggest the presence of the specific vitamin C transporter SVCT in retina and RPE, which may be involved in the manifestation of diabetic retinopathy.

Pharmacological properties of glycine uptake in the developing rat retina

A pharmacological characterization of glycine transport was performed in the rat retina at different postnatal ages. The uptake of 3H-glycine increased during the first 2 weeks of postnatal age, reaching maximum values at 12 days; then it decreased sharply to the adult values. We found a Na+ -dependent and high-affinity transport system with a Km of 100 microM. The Na+ Hill coefficient for glycine uptake was 1.76 +/- 0.07. Although glycine uptake was insensitive to staurosporine and phorbol ester, it was reduced 40-50% by sarcosine and ALX5407. Besides, amoxapine inhibited glycine uptake by 40 and 70% in adult and immature retina, respectively. These results suggest that the Glyt1 transporter was concentrated in the nerve terminals. In addition to the presence of Glyt1 in the retina, our results provided evidence of the occurrence of Glyt2 and/or another isoform of glycine transporter, which might have had a role in the retina development.

Glucose Metabolism in Rat Retinal Pigment Epithelium

The retinal pigment epithelium (RPE) is the major transport pathway for exchange of metabolites and ions between choroidal blood supply and the neural retina. To gain insight into the mechanisms controlling glucose metabolism in RPE and its possible relationship to retinopathy, we studied the influence of different glucose concentrations on glycogen and lactate levels and CO(2) production in RPE from normal and streptozotocin-treated diabetic rats. Incubation of normal RPE in the absence of glucose caused a decrease in lactate production and glycogen content. In normal RPE, increasing glucose concentrations from 5.6 mM to 30 mM caused a four-fold increase in glucose accumulation and CO(2) yield, as well as reduction in lactate and glycogen production. In RPE from diabetic rats glucose accumulation did not increase in the presence of high glucose substrate, but it showed a four- and a seven-fold increase in CO(2) production through the mitochondrial and pentose phosphate pathways, respectively. We found high glycogen levels in RPE which can be used as an energy reserve for RPE itself and/or neural retina. Findings further show that the RPE possesses a high oxidative capacity. The large increase in glucose shunting to the pentose phosphate pathway in diabetic retina exposed to high glucose suggests a need for reducing capacity, consistent with increased oxidative stress.

Characterization of strychnine-sensitive glycine receptor in the intact frog retina: modulation by protein kinases

We studied 3H-glycine and 3H-strychnine specific binding to glycine receptor (GlyR) in intact isolated frog retinas. To avoid glycine binding to glycine uptake sites, experiments were performed at low ligand concentrations in a sodium-free medium. The binding of both radiolabeled ligands was saturated. Scatchard analysis of bound glycine and strychnine revealed a KD of 2.5 and 2.0 microM, respectively. Specific binding of glycine was displaced by beta-alanine, sarcosine, and strychnine. Strychnine binding was displaced 50% by glycine, and sarcosine. Properties of the strychnine-binding site in the GlyR were modified by sarcosine. Binding of both radioligands was considerably reduced by compounds that inhibit or activate adenylate cyclase and increased cAMP levels. A phorbol ester activator of PKC remarkably decreased glycine and strychnine binding. These results suggest modulation of GlyR in response to endogenous activation of protein kinases A and C, as well as protein phosphorylation modulating GlyR function in retina.

Pharmacological properties of glycine transport in the frog retina

The high-affinity glycine transport in neurons and glial cells is the primary means for inactivating synaptic glycine. Two different glycine transporter genes, Glyt-1 and Glyt-2, have been cloned. Glyt-1 has been reported to occur in the retina, but there is no evidence for expression of the Glyt-2 transporter. We have pharmacologically characterized glycine transport in the frog retina. 3H-Glycine uptake in the retina was insensitive to modulation by phorbol esters or changes in cAMP levels, and was partially inhibited by sarcosine. Differential sensitivity of glycine transport to sarcosine was exhibited by synaptosomal fractions from the inner and outer plexiform layers of the frog retina. The Na+ Hill coefficient of glycine uptake was 2.0, as has been reported for Glyt-2. In addition, amoxapine, a specific inhibitor of the Glyt-2a isoform, reduced by 60% glycine uptake by P2 synaptosomal fraction. Our results indicate the presence of different glycine transporter isoforms in the frog retina, acting mainly through the classical inhibitory glycine system.

Glycogen metabolism in the rat retina

It has been reported that glycogen levels in retina vary with retinal vascularization. However, the electrical activity of isolated retina depends on glucose supply, suggesting that it does not contain energetic reserves. We determined glycogen levels and pyruvate and lactate production under various conditions in isolated retina. Ex vivo retinas from light- and dark-adapted rats showed values of 44 +/- 0.3 and 19.5 +/- 0.4 nmol glucosyl residues/mg protein, respectively. The glycogen content of retinas from light-adapted animals was reduced by 50% when they were transferred to darkness. Glycogen levels were low in retinas incubated in glucose-free media and increased in the presence of glucose. The highest glycogen values were found in media containing 20 mm of glucose. A rapid increase in lactate production was observed in the presence of glucose. Surprisingly, glycogen levels were the lowest and lactate production was also very low in the presence of 30 mm glucose. Our results suggest that glycogen can be used as an immediate accessible energy reserve in retina. We speculate on the possibility that gluconeogenesis may play a protective role by removal of lactic acid.

Acetyl- and butyrylcholinesterase molecular forms in normal and streptozotocin-diabetic rat retinal pigment epithelium

We studied the composition of molecular forms of acetylcholinesterase (AChE) and butyrylcholinesterase (BChE) in normal and streptozotocin-induced diabetic rat retinal pigment epithelium (RPE). Tissues were sequentially extracted with saline (S(1)) and saline-detergent buffers (S(2)). About a 50% decrease in AChE molecular forms was observed in the diabetic RPE compared to the controls. Approximately 70% of the BChE activity in normal RPE was brought into solution and evenly distributed in S(1) and S(2). Analysis of the fractions from RPE revealed the presence of G(A)(1), G(A)(4) and a small proportion of G(H)(4) BChE forms in S(1); whereas G(A)(4) and G(A)(1) molecules predominate in S(2). A 40% decrease in the activity of G(A)(4) in S(2) was observed in the diabetic RPE. Our results show that diabetes caused a remarkable decrease in the activity of cholinesterases molecular forms in the RPE. This might be related to the alterations observed in diabetic retinopathy.

Acetyl- and butyrylcholinesterase in normal and diabetic rat retina

We studied the composition of molecular forms of acetylcholinesterase (AChE) and butyrylcholinesterase (BChE) in normal and streptozotocin-induced diabetic rat retinas. Tissues were sequentially extracted with saline (S1) and saline-detergent buffers (S2). 50% decrease in the amphiphilic G4 and G1 AChE molecular forms was observed in the diabetic retina compared to the controls. Less than 5% of the cholinesterase activity was due to BChE. 60% of the BChE activity in normal retina was brought into solution and evenly distributed between S1 and S2. In spite of the low BChE activity in the retina it was possible to detect globular forms (G(A)1, G(A)2, G(A)4, G(H)4) and a small proportion of an asymmetric form (A12) in the S1 extract. The G(A)4 and G(A)1 forms were found in the S2 extract. In the diabetic retina the activity of G(A)4 and G(A)1 BChE molecular forms was reduced 60% and 40% respectively. Our results indicate that diabetes caused a remarkable decrease in the activity of cholinesterase molecular forms in the retina. These decrease might participate in the alterations observed in the diabetic retina.

Calcium uptake, release and ryanodine binding in melanosomes from retinal pigment epithelium

High levels of calcium have been reported in pigmented tissues of the vertebrate eye, such as retinal pigment epithelium (RPE). Melanin granules also have high calcium concentrations, suggesting that melanin granules may be a calcium reservoir. Here we characterized the uptake and release of calcium in a pure melanosomal fraction obtained from frog RPE. Melanosomes take up 45Ca by a saturable system with an apparent KM of 0.5 mM. About 40% of 45Ca accumulation was insensitive to low temperature. 45Ca uptake was not affected by verapamil, nifedipine, dantrolene, vanadate, thapsigargin or cyclopiazonic acid, but it was reduced by 50% by ruthenium red, and increased by the ionophore A23187 and nigericin. Release of 45Ca-loaded was stimulated by caffeine and inositol 1,4,5 trisphosphate (IP3). Caffeine stimulated release of calcium was blocked by either ryanodine or ruthenium red, but calcium released by IP3 was not affected by heparin. No binding of 3H-IP3 was observed. The 3H-ryanodine binding sites exhibited a KB of 1.3 nM and a Bmax of 12.1 fmol/mg protein. Thus, our results suggest that melanosomes may function as intracellular organelles that regulate calcium concentration in RPE.

Effect of streptozotocin-induced diabetes on activities of cholinesterases in the rat retina

The effect of streptozotocin-induced diabetes on cholinesterases activities was studied in the retina and, for comparison, in other nervous and nonnervous tissues. Streptozotocin diabetes did not affect acetylcholinesterase activity in the retina but increased its activity in the cerebral cortex (100%) and in serum (55%), and decreased it by 30-40% in erythrocytes. The butyrylcholinesterase activity was decreased by 30-50% in retina and hippocampus and to a lesser extent in retinal pigment epithelium from rats treated with streptozotocin for one week. Changes observed in cholinesterase activities were not correlated with the fasting blood glucose concentration. The results suggest that diabetes might influence a specific subset of cells and isoforms of cholinesterases. This, in turn, could lead to alterations associated with diabetes complications.

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.

Changes in the redox state in the retina and brain during the onset of diabetes in rats

Diabetic retinopathy is thought to result from chronic changes in the metabolic pathways of the retina. Hyperglycemia leads to increased intracellular glucose concentrations, alterations in glucose degradation and an increase in lactate/pyruvate ratio. We measured lactate content in retina and other ocular and non-ocular tissues from normal and diabetic rats in the early stages of streptozotocin-induced diabetes. The intracellular redox state was calculated from the cytoplasmic [lactate]/[pyruvate] ratio. Elevated lactate concentration were found in retina and cerebral cortex from diabetic rats. These concentrations led to a significant and progressive decrease in the NAD+/NADH ratio, suggesting that altered glucose metabolism is an initial step of retinopathy. It is thus possible that tissues such as cerebral cortex have mechanisms that prevent the damaging effect of lactate produced by hyperglycemia and/or alterations of the intracellular redox state.

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.

Characterization of [2-3H]deoxy-D-glucose uptake in retina and retinal pigment epithelium of normal and diabetic rats

The outer blood-retinal barrier which results from the tight junctions between retinal pigment epithelial cells (RPE) restricts the flow of nutrients reaching the retina. We characterize the transport of [2-3H]deoxy-D-glucose (2-DG) across isolated mammalian neural retina and RPE in terms of their kinetics constants. In addition, the effect of insulin on glucose transport was studied by using streptozotocin-induced diabetic rats. RPE accumulates 2-DG by a temperature-sensitive and energy-dependent complex kinetics mechanism. The retina takes up 2-DG by an energy and Na(+)-dependent saturable system with an apparent Km of 2 mM. Insulin induced an increase of 2-DG uptake by normal retina. The retina of diabetic rats shows lower levels of 2-DG accumulation. These levels can be returned to the normal ones by exposure to insulin. Although insulin does not affect, significantly, 2-DG accumulation by RPE, 2-DG uptake of RPE from diabetic rats shows a normal saturable kinetics with an apparent Km of 20 mM. Those findings suggest the presence of different types of glucose transporters in retina and RPE. Insulin-sensitive glucose transport in retina might be involved in the manifestation of diabetic retinopathy.

Different specific binding sites of [3H]glycine and [3H]strychnine in synaptosomal membranes isolated from frog retina

Synaptosomal fractions were isolated from frog retina: a fraction enriched in photoreceptor terminals (P1) and a second one (P2) containing interneurons terminals. We compared the binding of [3H]glycine and [3H]strychnine to membranes of these synaptosomes. The binding of both radioactive ligands was saturable and Na(+)-independent. [3H]Glycine bound to a single site in P1 and P2 synaptosomal fractions, with KD = 12 and 82 nM and BMax = 3.1 and 3.06 pmol/mg protein respectively. [3H]Strychnine bound to two sites in each one of the synaptosomal fractions. For P1 KD values were 3.9 and 18.7 nM, and BMax values were 1.1 and 7.1 pmol/mg protein, respectively. Membranes from the P2 synaptosomal fraction showed KD's of 0.6 and 48 nM and BMax's of 0.4 and 4.5 pmol/mg. Specific [3H]glycine binding was displaced by beta-alanine, 1-serine, d-serine and HA966, but not by strychnine, 7-chlorokynurenic or 5,7-dichloro-kynurenic acids. Specific [3H]strychnine binding was partially displaced by glycine and related amino acids and totally displaced only by 2-NH2-strychnine. Our results indicate the presence of high affinity binding sites for glycine and strychnine in frog retinal synaptosomal membranes. The pharmacological binding pattern indicates the presence of the strychnine sensitive glycine receptor as well as other sites. These might not include the NMDA receptor-associated glycine site.

Acetyl- and butyrylcholinesterase activities in the rat retina and retinal pigment epithelium

The acetylcholinesterase (AChE) and butyrylcholinesterase (BChE) activities in the neural retina and retinal pigment epithelium (RPE) of adult rats were determined. The tissues were extracted with a saline buffer to release the soluble enzymes (S1) and the pellet re-extracted with Triton X-100 to detach the membrane-bound enzymes (S2). Less than 5% of the cholinesterase activity measured in retina and almost 30% of that assayed in RPE was due to BChE. About 20% and 10% of the AChE in retina and RPE was brought into solution with a saline buffer and the rest with a detergent-containing buffer. Main AChE molecular forms of 10.5S (hydrophilic G4H), 9.5S (amphiphilic G4A) and 3.0S (amphiphilic G1A) were identified in retina by subjecting the supernatant S1 to sedimentation analysis in sucrose gradients made with Brij 96. Amphiphilic G4 and G1 AChE were found in S2. Analysis of the soluble fractions obtained from RPE in the gradients made with Brij 96 revealed 16.0S (asymmetric A12), 10.5-10.0S (globular G4H + G4A), 4.5S (G2A), and 3.0S (G1A) AChE forms in S1, whereas G4A, G2A, and G1A enzyme molecules predominated in S2. Our results show that amphiphilic tetramers and monomers of AChE are abundant in neural retina, and enzyme tetramers, dimers, and monomers in RPE. The AChE in the neural retina might be involved in cholinergic actions. The enzyme function in the retinal pigment epithelium remains to be established.

Muscarinic receptors binding in retinal pigment epithelium during rat development

[3H]Quinuclidinyl benzylate (3H-QNB) specific binding of the developing rat retinal pigment epithelium (RPE) and neural retina has been examined. The binding of 3H-QNB to RPE was saturable and displaced by the antagonist pirenzepine. Scatchard analysis of 3H-QNB binding showed two high affinity sites to RPE, with KB = 2.6nM and 45 nM. Specific 3H-QNB binding membranes from neural retina exhibited a characteristic developmental profile. RPE showed a high density of 3H-QNB binding sites through all developmental periods studied. The major onset of binding sites is at the time of RPE differentiation. Our data open the possibility of muscarinic receptors being involved in differentiation and/or proliferation of RPE.

High affinity uptake of glutamate and aspartate in the developing rat retina

Uptake for glutamate and aspartate in both retina and synaptosomes was found to be saturable, temperature sensitive, sodium dependent and reduced by metabolic inhibitors. The P1 and P2 synaptosomal fractions showed high affinity systems for glutamate (3 and 9 microM) and aspartate (6 and 3 microM) respectively. Early after birth, glutamate accumulation was much higher than that of aspartate. It showed a rapid increase reaching the adult values about day 15. Aspartate uptake progressively increases with age up to about day 30. Our findings suggest that glutamate and aspartate may be transmitters at specific cell populations in the rat retina.

Specific interaction of glutamate with membranes from cultured retinal pigment epithelium

Excitatory amino acids (EAA) have been shown to induce phagocytosis in retinal pigment epithelial (RPE) cells. In order to explore if this action is receptor-mediated, we have identified and characterized receptors for L-glutamate through the binding of [3H]L-glutamate to membranes from chick RPE cells in primary culture. Specific binding was found saturable, with KB = 333nM and Bmax = 3.2 pmol/mg protein in frozen/thawed membranes. Na(+)-independent binding was present in cultures of 16 and 25 days in vitro, and was not affected by temperature. Pharmacological profile of analogues of EAA at different receptor types suggests the presence of a metabotropic type receptor (L-glutamate > S-2-amino-3-phosphonopropionate > 2-amino-4-phosphonobutyrate = trans-(1S,3R)-1-aminocyclopentane-1,3-dicarboxylate > quisqualate). Excitatory amino acid analogues acting at the NMDA-receptor also displaced bound L-glutamate, and a noticeable stimulation of specific binding of this ligand by glycine was shown; this effect was mimicked by D-serine and 1-hydroxy-3-aminopyrrolidone-2 (HA-966) but not by 7-chlorokynurenate, and was not inhibited by strychnine. Since taurine and GABA also increased specific binding, it is likely that modulation of EAA receptors in RPE differs from that in neurons.

Glutamate and taurine uptake by retinal pigment epithelium during rat development

1. The rat retinal pigment epithelium accumulated glutamate and taurine by saturable, temperature and Na(+)-dependent mechanisms. 2. Glutamate and taurine showed high and low affinity transport systems, with a Km of 30 microM and 80 microM, respectively. 3. The transport rates of both amino acids decreased during maturation of retinal pigment epithelial cells while their kinetic characteristics were not modified. 4. The results suggest an involvement of the retinal pigment epithelium in the regulation of glutamate and taurine levels in the neural retina and support its role as part of the blood retinal barrier.

Characterization of cholinesterase activities in primary cultures of retinal pigment epithelium

This report presents a comparative description of the acetylcholinesterase and butyrylcholinesterase activities and their molecular forms in primary cultures of retinal pigment epithelium (RPE). Acetylcholinesterase activity increases during differentiation of the cells. Sucrose sedimentation analysis of acetylcholinesterase and butyrylcholinesterase molecular forms revealed the presence of A12, G4, G2, and G1 and A8, G4, G2 and G1, respectively. RPE cells in culture release both cholinesterases into the growth medium, sedimenting as the G4 molecular form. Changes in the molecular forms of both enzymes were observed during differentiation. The results suggest a possible relationship between butyrylcholinesterase activity and cell proliferation and acetylcholinesterase activity and cell differentiation.

Zymosan induced 45Ca uptake by retinal pigment epithelial cells

45Ca uptake was studied in isolated frog retinal pigment epithelial cells in response to the phagocytic stimuli, zymosan. 45Ca uptake was strongly stimulated immediately in the presence of zymosan particles. Calcium uptake was proportional to the zymosan concentrations. After 60 min in the presence of zymosan acid phosphatase and beta-glucuronidase activities showed a 25% and 50% increase, respectively. Rod outer segments induced a similar increase of these enzyme activities. The zymosan-induced lysosomal enzyme activities was inhibited by cytochalasin B and ruthenium red. The ionophore A23187 produced a remarkable increase in 45Ca uptake but did not affect the lysosomal enzyme activities. These results suggest that in vitro RPE cells are able to respond to zymosan as phagocytosable stimuli and that calcium mediate that response.

Characterization of acetylcholinesterase and butyrylcholinesterase activities in retinal chick pigment epithelium during development

The presence of acetylcholinesterase and butyrylcholinesterase was studied in chick retinal pigment epithelium. Acetylcholinesterase activity was 13 times higher than that of butyrylcholinesterase. The former showed a Km of 290 microM and a vmax of 45.4 nmol mg-1 protein min-1, while the latter showed two apparent Km values (132 microM and 666 microM). Studies on subcellular distribution revealed that both enzymes are associated with membranes. During the embryonic development butyrylcholinesterase activity decreased, while acetylcholinesterase activity increased. The possibility that these changes are related to the proliferation and differentiation processes is discussed.

Neurochemical effects of nicotine on glutamate and GABA mechanisms in the rat brain

The effects of nicotine on gamma-aminobutyric acid (GABA) and glutamate mechanisms were studied in several rat brain regions both in vivo and in vitro. In vivo acute intermittent injections of nicotine decrease GABA utilization in the hypothalamus and glutamate levels within the nucleus caudatus and the subcortical limbic forebrain (mainly tuberculum olfactorium and nucleus accumbens). Glutamic acid decarboxylase activity was slightly increased in several regions, when the rats were treated with a single convulsant dose of nicotine and killed at the moment of the convulsions but it was not affected by a single injection nor by intermittent acute administration of non-convulsant doses of nicotine. In vitro nicotine elicited release of L-[3H]glutamate from synaptosomal preparations obtained from the frontoparietal cortex, nucleus caudatus and hypothalamus. The effect was dose-dependent and it was not blocked by mecamylamine. It was also Ca2+ independent. The possibilities are discussed that the decreased GABA utilization in the hypothalamus may be related to certain neuroendocrine actions of nicotine and that the nicotine-induced glutamate release might be involved in some of the physiological and toxicological effects of nicotine.

Taurine receptors in membranes from retinal pigment epithelium cells in culture

[3H]Taurine-specific binding to membranes from retinal pigment epithelium was demonstrated. A single saturable system was found, with KB = 237 nM and Bmax = 2.8 pmol/mg protein. Binding to freshly prepared membranes showed partial Na(+)-dependence while in frozen/thawed membranes, binding remained unchanged in the absence or presence of this ion. A 30-40% increase in binding was observed at physiological temperature (37 degrees C) compared to 4 degrees C in fresh but not in frozen membranes. Accumulation of taurine was followed during differentiation in vitro; results showed that changes in uptake and receptor binding to frozen membranes are not parallel, discarding the possibility of an interaction with uptake sites. Pharmacology of these binding sites suggests that they could be common to other amino acids, since displacement experiments showed that glycine, beta-alanine and strychnine were as potent as taurine itself in displacing [3H]taurine. Our data open the possibility of taurine being involved in the communication between the retina and the retinal pigment epithelium through an interaction with specific receptors.

Characterization of calcium uptake in chick retinal pigment epithelium

45Ca uptake was studied in isolated chick retinal pigment epithelial cells. 45Ca was accumulated by a saturable, temperature-dependent system with a KM of 400 microM and a Vmax of 0.13 mumoles2mg protein/min, which depends on the external sodium concentrations. The transport system was present early during embryonic development. RPE cells of three breeds of chicks with different degrees of pigmentation accumulated calcium proportionally to the melanin content of the cells, suggesting that pigment granules participate in the storage and regulation of intracellular calcium.

45Ca uptake by retinal pigment epithelial cells

Uptake of 45Ca was studied in isolated frog retinal pigment epithelial cells. 45Ca accumulation was found to be a saturable, temperature-dependent event. Kinetic analysis of this accumulation revealed two transport systems with apparent km of 2.0 and 0.3 mM. We found the presence of a Na-Ca exchanger mechanism that releases Ca2 under depolarized conditions. Light induced an increase of 45Ca uptake due to activation of the Na-K-ATPase and consequent decrease of extracellular potassium concentration.

A glutamate dehydrogenase-based method for the assay of L-glutamic acid: formation of pyridine nucleotide fluorescent derivatives

A method for the quantitation of L-glutamic acid in the picomole range was developed by finding conditions which allowed the production of NADH by the action of the L-glutamate dehydrogenase (EC 1.4.1.3) and its subsequent transformation to a highly fluorescent derivative. The method measures linearly glutamate from 250 pmol to 5 nmol. For its simplicity and low cost it is ideally suited to the assay of a large number of samples within a single working day. Its application to the determination of regional glutamate levels in the rat brain, as well as to the measurement of ornithine aminotransferase (EC 2.6.1.13) activity from several tissues is described. The results are similar to those obtained by different methodologies in several laboratories, but the present method offers additional advantages.

Uptake and K+-stimulated release of [14C]glycine from frog retinal synaptosomal fractions

The uptake of [14C]glycine and the effect of depolarizing potassium concentrations on its release was investigated in the whole frog retina and its synaptosomal fractions. The uptake of [14C]glycine in retina and synaptosomal fractions was found to be saturable as well as energy and Na+-dependent. The Km value for glycine uptake was found to be 46 microM for P2 fraction and 100 microM for P1 fraction, with a Vmax of 3.5 and 3.8 nmol/mg protein/min respectively. The release of [14C]glycine from P1 and P2 synaptosomal fractions was markedly increased by raising potassium concentration in the medium, in a partially Ca2+-dependent manner. Evoked glycine release was 50% reduced when calcium was omitted from the medium. The K+-stimulated release of glycine from P2 fraction was significantly reduced in the presence of TTX. The cellular origin of the P1 and P2 synaptosomal fractions releasing glycine is discussed.

Isolation and biochemical characterization of frog retinal pigment epithelium cells

The structure and function of the photoreceptor cell depends on the renewal of its outer segment. Phagocytosis of the rod outer segments by RPE is an essential part of the renewal process. Several methods have been reported in order to isolate RPE cells; however, the isolated cells are heavily contaminated by other cell types, mainly erythrocytes and rod outer segments. The primary aim of this study was the isolation of pure and viable frog RPE cells. Cells were dissociated in a calcium-free Krebs bicarbonate medium and purified by centrifugation in a ficoll density gradient. Viability of the purified cells assessed by trypan blue dye exclusion was 95%. The metabolic activity of the cells was tested by several parameters: RPE cells consume oxygen at a rate of 11.5 ngatoms/min/mg protein, transport and metabolize 14C-glucose by a sodium dependent mechanism, and are able as well to accumulate 14C-leucine and incorporate it into proteins. Results obtained in this study indicate that our isolation procedure yields a more intact preparation of RPE than those described previously; hence, it may be helpful in elucidating the biochemical and metabolic parameters involved in pigment epithelium physiology.

Effect of light on Na-Ca exchange in rod outer segments in frog

The effect of illumination on the calcium (Ca) translocation mechanisms in isolated frog rod outer segments (ROS) was studied. The ATP-dependent Ca uptake and the Ca-Ca exchange mechanisms were unaffected by light. In contrast, we report a light-evoked Ca efflux which is mediated by the Na-Ca exchange system. The ratio of released Ca to rhodopsin bleaching was measured and the stoichiometry obtained was 5 Ca molecules released per mole of rhodopsin bleached. Concomitant to the Ca release, light induced Ca uptake, which increase the total Ca content of ROS. Physiological relevance of results to the phototransduction process is discussed.