Origin and sensitivity of the light peak in the intact cat eye

A Splicing Mutation in Slc4a5 Results in Retinal Detachment and Retinal Pigment Epithelium Dysfunction

Fluid and solute transporters of the retinal pigment epithelium (RPE) are core components of the outer blood-retinal barrier. Characterizing these transporters and their role in retinal homeostasis may provide insights into ocular function and disease. Here, we describe RPE defects in tvrm77 mice, which exhibit hypopigmented patches in the central retina. Mapping and nucleotide sequencing of tvrm77 mice revealed a disrupted 5' splice donor sequence in Slc4a5, a sodium bicarbonate cotransporter gene. Slc4a5 expression was reduced 19.7-fold in tvrm77 RPE relative to controls, and alternative splice variants were detected. SLC4A5 was localized to the Golgi apparatus of cultured human RPE cells and in apical and basal membranes. Fundus imaging, optical coherence tomography, microscopy, and electroretinography (ERG) of tvrm77 mice revealed retinal detachment, hypopigmented patches corresponding to neovascular lesions, and retinal folds. Detachment worsened and outer nuclear layer thickness decreased with age. ERG a- and b-wave response amplitudes were initially normal but declined in older mice. The direct current ERG fast oscillation and light peak were reduced in amplitude at all ages, whereas other RPE-associated responses were unaffected. These results link a new Slc4a5 mutation to subretinal fluid accumulation and altered light-evoked RPE electrophysiological responses, suggesting that SLC4A5 functions at the outer blood-retinal barrier.

Is white the right light for the clinical electrooculogram?

PURPOSE

To investigate if a lower luminance monochromatic LED stimulus could be used as an alternative to a high luminance white light for the clinical electrooculogram.

METHODS

Clinical electrooculograms were recorded in color normal participants (N = 23) aged 22.6 ± 1.2 years, 7 male and 16 female using the standard 100 cd.m^-2 white illuminant and four monochromatic LEDs with peak wavelengths of 448, 534, 596 and 634 nm at 30 cd.m^-2. Pupils were dilated and there was a 30 cd.m^-2pre-adaptation to white light for 2 min followed by 15 min dark adaptation and 20 min recording in the light stimulus using a Ganzfeld stimulator.

RESULTS

The normalized LP:DTratio for the short wavelength LED (448 nm) was equivalent in amplitude and timing to the ISCEV standard EOG (p = .99). The LP:DTratio for the white (100 cd.m^-2) and 448 nm (30 cd.m^-2) were (median ± SEM): 2.49 ± .11 and 2.47 ± .11. The time to light-rise peak was also equivalent being 9.0 ± .2 and 8.0 ± .4 min (p = .54).

CONCLUSIONS

Consideration may be given to using a short wavelength monochromatic stimulus that is more comfortable for the subject than the current 100 cd.m^-2 illuminant.

High-yield, automated intracellular electrophysiology in retinal pigment epithelia

BACKGROUND

Recent advancements with induced pluripotent stem cell-derived (iPSC) retinal pigment epithelium (RPE) have made disease modeling and cell therapy for macular degeneration feasible. However, current techniques for intracellular electrophysiology - used to validate epithelial function - are painstaking and require manual skill; limiting experimental throughput.

NEW METHOD

A five-stage algorithm, leveraging advances in automated patch clamping, systematically derived and optimized, improves yield and reduces skill when compared to conventional, manual techniques.

RESULTS

The automated algorithm improves yield per attempt from 17% (manually, n = 23) to 22% (automated, n = 120) (chi-squared, p = 0.004). Specifically for RPE, depressing the local cell membrane by 6 μm and electroporating (buzzing) just prior to this depth (5 μm) maximized yield.

COMPARISON WITH EXISTING METHOD

Conventionally, intracellular epithelial electrophysiology is performed by manually lowering a pipette with a micromanipulator, blindly, towards a monolayer of cells and spontaneously stopping when the magnitude of the instantaneous measured membrane potential decreased below a predetermined threshold. The new method automatically measures the pipette tip resistance during the descent, detects the cell surface, indents the cell membrane, and briefly buzzes to electroporate the membrane while descending, overall achieving a higher yield than conventional methods.

CONCLUSIONS

This paper presents an algorithm for high-yield, automated intracellular electrophysiology in epithelia; optimized for human RPE. Automation reduces required user skill and training while, simultaneously, improving yield. This algorithm could enable large-scale exploration of drug toxicity and physiological function verification for numerous kinds of epithelia.

Use of Direct Current Electroretinography for Analysis of Retinal Pigment Epithelium Function in Mouse Models

A monolayer of pigmented epithelial cells, the retinal pigment epithelium (RPE), supports photoreceptor function in many ways. Consistent with these roles, RPE dysfunction underlies a number of hereditary retinal disorders. To monitor RPE function in vivo models for these conditions, we adapted an electroretinographic (ERG) technique based on direct current amplification (DC-ERG). This chapter describes the main features of this approach and its application to mouse models involving the RPE.

Noninvasive Electroretinographic Procedures for the Study of the Mouse Retina

Overall retinal function can be monitored by recording the light-evoked response of the eye at the corneal surface. The major components of the electroretinogram (ERG) provide important information regarding the functional status of many retinal cell types including rod photoreceptors, cone photoreceptors, bipolar cells, and the retinal pigment epithelium (RPE). The ERG can be readily recorded from mice, and this unit describes procedures for mouse anesthesia and the use of stimulation and recording procedures for measuring ERGs that reflect the response properties of different retinal cell types. Through these, the mouse ERG provides a noninvasive approach to measure multiple aspects of outer retinal function, including the status of the initial rod and cone pathways, rod photoreceptor deactivation, rod dark adaptation, the photoreceptor-to-bipolar cell synapse, and the RPE. © 2018 by John Wiley & Sons, Inc.

Early Functional and Morphologic Abnormalities in the Diabetic Nyxnob Mouse Retina

Purpose

The electroretinogram c-wave is generated by the summation of the positive polarity hyperpolarization of the apical RPE membrane and a negative polarity slow PIII response of Müller glia cells. Therefore, the c-wave reduction noted in prior studies of mouse models of diabetes could reflect a reduction in the RPE component or an increase in slow PIII. The present study used a genetic approach to distinguish between these two alternatives.

Methods

Nyx^nob mice lack the ERG b-wave, revealing the early phase of slow PIII. To visualize changes in slow PIII due to diabetes, Nyx^nob mice were given streptozotocin (STZ) injections to induce diabetes or received vehicle as a control. After 1, 2, and 4 weeks of sustained hyperglycemia (>250 mg/dL), standard strobe flash ERG and dc-ERG testing were conducted. Histological analysis of the retina was performed.

Results

A reduced c-wave was noted at the 1 week time point, and persisted at later time points. In comparison, slow PIII amplitudes were unaffected after 1 week of hyperglycemia, but were significantly reduced in STZ mice at the 2-week time point. The decrease in amplitude occurred before any identifiable decrease to the a-wave. At the later time point, the a-wave became involved, although the slow PIII reductions were more pronounced. Morphological abnormalities in the RPE, including increased thickness and altered melanosome distribution, were identified in diabetic animals.

Conclusions

Because the c-wave and slow PIII were both reduced, these results demonstrated that diabetes-induced reductions to the c-wave cannot be attributed to an early increase in the Müller glia-derived potassium conductance. Furthermore, because the a-wave, slow PIII and c-wave reductions were not equivalent, and varied in their onset, the reductions cannot reflect the same mechanism, such as a change in membrane resistance. The presence of small changes to RPE architecture indicate that the c-wave reductions present in diabetic mice likely represents a primary change in the RPE induced by hyperglycemia.

The progress in understanding and treatment of diabetic retinopathy

Diabetic retinopathy is the most frequently occurring complication of diabetes mellitus and remains a leading cause of vision loss globally. Its aetiology and pathology have been extensively studied for half a century, yet there are disappointingly few therapeutic options. Although some new treatments have been introduced for diabetic macular oedema (DMO) (e.g. intravitreal vascular endothelial growth factor inhibitors ('anti-VEGFs') and new steroids), up to 50% of patients fail to respond. Furthermore, for people with proliferative diabetic retinopathy (PDR), laser photocoagulation remains a mainstay therapy, even though it is an inherently destructive procedure. This review summarises the clinical features of diabetic retinopathy and its risk factors. It describes details of retinal pathology and how advances in our understanding of pathogenesis have led to identification of new therapeutic targets. We emphasise that although there have been significant advances, there is still a pressing need for a better understanding basic mechanisms enable development of reliable and robust means to identify patients at highest risk, and to intervene effectively before vision loss occurs.

Early retinal pigment epithelium dysfunction is concomitant with hyperglycemia in mouse models of type 1 and type 2 diabetes

In the diabetic retina, cellular changes in the retinal pigment epithelium (RPE) and neurons occur before vision loss or diabetic retinopathy can be identified clinically. The precise etiologies of retinal pathology are poorly defined, and it remains unclear if the onset and progression of cellular dysfunction differ between type 1 and type 2 diabetes. Three mouse models were used to compare the time course of RPE involvement in type 1 and type 2 diabetes. C57BL/6J mice injected with streptozotocin (STZ mice) modeled type 1 diabetes, whereas Lepr(db/db) mice on both BKS and B6.BKS background strains modeled type 2 diabetes. Electroretinogram (ERG)-based techniques were used to measure light-evoked responses of the RPE (direct current-coupled ERG, dc-ERG) and the neural retina (a-wave, b-wave). Following onset of hyperglycemia, a-wave and b-wave amplitudes of STZ mice declined progressively and by equivalent degrees. Components of the dc-ERG were also altered, with the largest reduction seen in the c-wave. Lepr(db/db) mice on the BKS strain (BKS.Lepr) displayed sustained hyperglycemia and a small increase in insulin, whereas Lepr(db/db) mice on the B6.BKS background (B6.BKS.Lepr) were transiently hyperglycemic and displayed severe hyperinsulinemia. BKS.Lepr mice exhibited sustained reductions in the dc-ERG c-wave, fast oscillation, and off response that were not attributable to reduced photoreceptor activity; B6.BKS.Lepr mice displayed transient reductions in the c-wave and fast oscillation that correlated with hyperglycemia and magnitude of photoreceptor activity. In summary, all mouse models displayed altered RPE function concomitant with the onset of hyperglycemia. These results suggest that RPE function is directly reduced by elevated blood glucose levels. That RPE dysfunction was reversible and mitigated in hyperinsulinemic B6.BKS.Lepr mice provides insight into the underlying mechanism.

Disease-causing mutations in BEST1 gene are associated with altered sorting of bestrophin-1 protein

Mutations in BEST1 gene, encoding the bestrophin-1 (Best1) protein are associated with macular dystrophies. Best1 is predominantly expressed in the retinal pigment epithelium (RPE), and is inserted in its basolateral membrane. We investigated the cellular localization in polarized MDCKII cells of disease-associated Best1 mutant proteins to study specific sorting motifs of Best1. Real-time PCR and western blots for endogenous expression of BEST1 in MDCK cells were performed. Best1 mutant constructs were generated using site-directed mutagenesis and transfected in MDCK cells. For protein sorting, confocal microscopy studies, biotinylation assays and statistical methods for quantification of mislocalization were used. Analysis of endogenous expression of BEST1 in MDCK cells revealed the presence of BEST1 transcript but no protein. Confocal microscopy and quantitative analyses indicate that transfected normal human Best1 displays a basolateral localization in MDCK cells, while cell sorting of several Best1 mutants (Y85H, Q96R, L100R, Y227N, Y227E) was altered. In contrast to constitutively active Y227E, constitutively inactive Y227F Best1 mutant localized basolaterally similar to the normal Best1 protein. Our data suggest that at least three basolateral sorting motifs might be implicated in proper Best1 basolateral localization. In addition, non-phosphorylated tyrosine 227 could play a role for basolateral delivery.

Exclusion of aldose reductase as a mediator of ERG deficits in a mouse model of diabetic eye disease

Streptozotocin (STZ)-induced diabetes is associated with reductions in the electrical response of the outer retina and retinal pigment epithelium (RPE) to light. Aldose reductase (AR) is the first enzyme required in the polyol-mediated metabolism of glucose, and AR inhibitors have been shown to improve diabetes-induced electroretinogram (ERG) defects. Here, we used control and AR -/- mice to determine if genetic inactivation of this enzyme likewise inhibits retinal electrophysiological defects observed in a mouse model of type 1 diabetes. STZ was used to induce hyperglycemia and type 1 diabetes. Diabetic and age-matched nondiabetic controls of each genotype were maintained for 22 weeks, after which ERGs were used to measure the light-evoked components of the RPE (dc-ERG) and the neural retina (a-wave, b-wave). In comparison to their nondiabetic controls, wildtype (WT) and AR -/- diabetic mice displayed significant decreases in the c-wave, fast oscillation, and off response components of the dc-ERG but not in the light peak response. Nondiabetic AR -/- mice displayed larger ERG component amplitudes than did nondiabetic WT mice; however, the amplitude of dc-ERG components in diabetic AR -/- animals were similar to WT diabetics. ERG a-wave amplitudes were not reduced in either diabetic group, but b-wave amplitudes were lower in WT and AR -/-diabetic mice. These findings demonstrate that the light-induced responses of the RPE and outer retina are disrupted in diabetic mice, but these defects are not due to photoreceptor dysfunction, nor are they ameliorated by deletion of AR. This latter finding suggests that benefits observed in other studies utilizing pharmacological inhibitors of AR might have been secondary to off-target effects of the drugs.

Light-evoked responses of the retinal pigment epithelium: changes accompanying photoreceptor loss in the mouse

Mutations in genes expressed in the retinal pigment epithelium (RPE) underlie a number of human inherited retinal disorders that manifest with photoreceptor degeneration. Because light-evoked responses of the RPE are generated secondary to rod photoreceptor activity, RPE response reductions observed in human patients or animal models may simply reflect decreased photoreceptor input. The purpose of this study was to define how the electrophysiological characteristics of the RPE change when the complement of rod photoreceptors is decreased. To measure RPE function, we used an electroretinogram (dc-ERG)-based technique. We studied a slowly progressive mouse model of photoreceptor degeneration (Prph(Rd2/+)), which was crossed onto a Nyx(nob) background to eliminate the b-wave and most other postreceptoral ERG components. On this background, Prph(Rd2/+) mice display characteristic reductions in a-wave amplitude, which parallel those in slow PIII amplitude and the loss of rod photoreceptors. At 2 and 4 mo of age, the amplitude of each dc-ERG component (c-wave, fast oscillation, light peak, and off response) was larger in Prph(Rd2/+) mice than predicted by rod photoreceptor activity (Rm(P3)) or anatomical analysis. At 4 mo of age, the RPE in Prph(Rd2/+) mice showed several structural abnormalities including vacuoles and swollen, hypertrophic cells. These data demonstrate that insights into RPE function can be gained despite a loss of photoreceptors and structural changes in RPE cells and, moreover, that RPE function can be evaluated in a broader range of mouse models of human retinal disease.

Bestrophins and retinopathies

Best vitelliform macular dystrophy (BVMD, also called Best's disease) is a dominantly inherited, juvenile-onset form of macular degeneration, which is characterized by abnormal accumulation of yellow pigment in the outer retina and a depressed electro-oculogram light peak (LP). Over 100 disease-causing mutations in human bestrophin-1 (hBest1) are closely linked to BVMD and several other retinopathies. However, the physiological role of hBest1 and the mechanisms of retinal pathology remain obscure partly because hBest1 has been described as a protein with multiple functions including a Ca2+-activated Cl- channel, a Ca2+ channel regulator, a volume-regulated Cl- channel, and a HCO3- channel. This review focuses on how dysfunction of hBest1 is related to the accumulation of yellow pigment and a decreased LP. The dysfunction of hBest1 as a HCO3- channel or a volume-regulated Cl- channel may be associated with defective regulation of the subretinal fluid or phagocytosis of photoreceptor outer segments by retinal pigment epithelium cells, which may lead to fluid and pigment accumulation.

Dysregulation of human bestrophin-1 by ceramide-induced dephosphorylation

Best vitelliform macular dystrophy is an inherited autosomal dominant, juvenile onset form of macular degeneration caused by mutations in a chloride ion channel, human bestrophin-1 (hBest1). Mutations in Best1 have also been linked to several other forms of retinopathy. In addition to mutations, hBest1 dysfunction might come about by disruption of other processes that regulate Best1 function. Here we show that hBest1 chloride channel activity is regulated by ceramide and phosphorylation. We have identified a protein kinase C (PKC) phosphorylation site (serine 358) in hBest1 that is important for sustained channel function. Channel activity is maintained by PKC activators, protein phosphatase inhibitors, or pseudo-phosphorylation by substitution of glutamic acid for serine 358. When ceramide levels are elevated by exogenous addition of ceramide to the bath, by addition of bacterial sphingomyelinase, or by hypertonic stress, S358 is rapidly dephosphorylated. The dephosphorylation is mediated by protein phosphatase 2A. Hypertonic stress-induced dephosphorylation is blocked by a dihydroceramide, an inactive form of ceramide, and manumycin, an inhibitor of neutral sphingomyelinase. Our results support a model in which ceramide accumulation during early stages of retinopathy inhibits hBest1 function, leading to abnormal fluid transport across the retina, and enhanced inflammation.

Electrophysiology in pigment epithelial changes

The c-wave of the ERG and the slow SP variations reflect mainly the activity of the pigment epithelium. However, both potentials are dependent upon the photoreceptors and/or the inner retina as well. In pigment epithelial abnormalities the c-wave is reduced or abolished, and the slow SP variations, d.c. recorded directly or investigated with the EOG, reduced or abolished as well.

Molecular physiology of bestrophins: multifunctional membrane proteins linked to best disease and other retinopathies

This article reviews the current state of knowledge about the bestrophins, a newly identified family of proteins that can function both as Cl(-) channels and as regulators of voltage-gated Ca(2+) channels. The founding member, human bestrophin-1 (hBest1), was identified as the gene responsible for a dominantly inherited, juvenile-onset form of macular degeneration called Best vitelliform macular dystrophy. Mutations in hBest1 have also been associated with a small fraction of adult-onset macular dystrophies. It is proposed that dysfunction of bestrophin results in abnormal fluid and ion transport by the retinal pigment epithelium, resulting in a weakened interface between the retinal pigment epithelium and photoreceptors. There is compelling evidence that bestrophins are Cl(-) channels, but bestrophins remain enigmatic because it is not clear that the Cl(-) channel function can explain Best disease. In addition to functioning as a Cl(-) channel, hBest1 also is able to regulate voltage-gated Ca(2+) channels. Some bestrophins are activated by increases in intracellular Ca(2+) concentration, but whether bestrophins are the molecular counterpart of Ca(2+)-activated Cl(-) channels remains in doubt. Bestrophins are also regulated by cell volume and may be a member of the volume-regulated anion channel family.

Experimental hypoxia in human eyes: Implications for ischaemic disease

OBJECTIVE

This study investigated neuroretinal activity under normoxic and hypoxic conditions with the multifocal electroretinogram (mfERG).

METHODS

We used two mfERG paradigms, the fast flicker and slow flash stimulation modes, to measure neuroretinal activity in five healthy participants who breathed room air and a reduced oxygen mixture (14% oxygen, balance nitrogen). We analysed concentric ring N1P1 and P1N2 response density amplitudes, the P1 implicit times as well as the local scalar product (SP) response densities.

RESULTS

During hypoxia there was a significant reduction of the scalar product response density for the fast flicker (p<0.001) and for the slow flash mfERG (p<0.001). The N1P1 and P1N2 response densities were lower especially for the central three rings; although these reductions were not significant between the two oxygen conditions, they indicated an overall distortion of the mfERG waveform.

CONCLUSIONS

It is demonstrated that a post-receptoral, primarily ON and OFF bipolar cell deficit is evident in the central retina of healthy young people during short term hypoxia.

SIGNIFICANCE

Our findings suggest that persons with pre-existing ischaemic eye disease may be at risk when exposed to hypoxic conditions.

Single Cl- channels activated by Ca2+ in Drosophila S2 cells are mediated by bestrophins

Mutations in human bestrophin-1 (VMD2) are genetically linked to several forms of retinal degeneration but the underlying mechanisms are unknown. Bestrophin-1 (hBest1) has been proposed to be a Cl⁻ channel involved in ion and fluid transport by the retinal pigment epithelium (RPE). To date, however, bestrophin currents have only been described in overexpression systems and not in any native cells. To test whether bestrophins function as Ca²⁺-activated Cl⁻ (CaC) channels physiologically, we used interfering RNA (RNAi) in the Drosophila S2 cell line. S2 cells express four bestrophins (dbest1–4) and have an endogenous CaC current. The CaC current is abolished by several RNAi constructs to dbest1 and dbest2, but not dbest3 or dbest4. The endogenous CaC current was mimicked by expression of dbest1 in HEK cells, and the rectification and relative permeability of the current were altered by replacing F81 with cysteine. Single channel analysis of the S2 bestrophin currents revealed an ∼2-pS single channel with fast gating kinetics and linear current–voltage relationship. A similar channel was observed in CHO cells transfected with dbest1, but no such channel was seen in S2 cells treated with RNAi to dbest1. This provides definitive evidence that bestrophins are components of native CaC channels at the plasma membrane.

The electro-oculogram

The retinal pigment epithelium (RPE) lying distal to the retina regulates the extracellular environment and provides metabolic support to the outer retina. RPE abnormalities are closely associated with retinal death and it has been claimed several of the most important diseases causing blindness are degenerations of the RPE. Therefore, the study of the RPE is important in Ophthalmology. Although visualisation of the RPE is part of clinical investigations, there are a limited number of methods which have been used to investigate RPE function. One of the most important is a study of the current generated by the RPE. In this it is similar to other secretory epithelia. The RPE current is large and varies as retinal activity alters. It is also affected by drugs and disease. The RPE currents can be studied in cell culture, in animal experimentation but also in clinical situations. The object of this review is to summarise this work, to relate it to the molecular membrane mechanisms of the RPE and to possible mechanisms of disease states.

Functional abnormalities in the retinal pigment epithelium of CFTR mutant mice

In response to light, the mouse retinal pigment epithelium (RPE) generates a series of slow changes in potential that are referred to as the c-wave, fast oscillation (FO) and light peak (LP) of the electroretinogram (ERG). While the FO is known to reflect a Cl(-) conductance generated at the basal membrane of the RPE, the specific channel (s) underlying this potential has not been identified. In the present study we examined two strains of mice with cftr mutations to define the contribution that cystic fibrosis transmembrane regulator (CFTR)-mediated Cl(-) conductance makes to the mouse ERG. Responses obtained from cftr(Delta508/Delta508) mice exhibited an overall reduction in all components generated by the RPE in response to light without alteration of the luminance response function. Responses obtained from cftr(-/-) mice were also reduced in amplitude. These results illustrate the usefulness of ERG analysis of mice deficient in ion channels that are expressed in the RPE, and indicate that CFTR contributes to the generation of RPE-driven ERG components, but that it is not the sole generator of any one of these components.

Looking chloride channels straight in the eye: bestrophins, lipofuscinosis, and retinal degeneration

Recent evidence suggests that Cl(-) ion channels are important for retinal integrity. Bestrophin Cl(-) channel mutations in humans are genetically linked to a juvenile form of macular degeneration, and disruption of some ClC Cl(-) channels in mice leads to retinal degeneration. In both cases, accumulation of lipofuscin pigment is a key feature of the cellular degeneration. Because Cl(-) channels regulate the ionic environment inside organelles in the endosomal-lysosomal pathway, retinal degeneration may result from defects in lysosomal trafficking or function.

Two components of the human alcohol electro-oculogram

Light onset or drinking alcohol causes the standing potential of the eye to rise and then fall to a trough (the EOG). After allowing for the time for the alcohol to be absorbed into the blood stream, the changes of current with time are identical for the two agents but each acts through a separate pathway, on the same effector mechanism. We have shown that +ve and -ve processes of the alcohol-EOG may be differentially affected in disease. We have now determined the separate dose-response relationship of the two voltage changes. Alcohol diluted with water was given by mouth to fasting dark-adapted subjects. Recordings continued until both the positive peak and the later negative trough were well-characterised. Doses of alcohol ranged from 3.54 to 450 mg Kg(-1) of body weight. Experiments were carried out on three normal subjects, 4-8th decade. The results are consistent with the hypothesis that each voltage change is determined by the relation: [EtOH] x [R] <--> [EtOH.R], where <--> represents a reversible reaction. For the +ve peak, semi-saturation occurs at approximately 35 mg Kg(-l). For the -ve trough it is smaller, 11 mg Kg(-l). Therefore the result is consistent with there being 2 distinct processes, and the human EOG cannot be a single 'damped oscillation'. During the short period when change of blood alcohol concentration is effective in causing the EOG sequence (using doses which provoke large voltage changes), the computed blood concentration varies from 0.01 to 0.1 mM, i.e. is > 2 orders of magnitude less than the levels required for intoxication.

A model of best vitelliform macular dystrophy in rats

PURPOSE

The VMD2 gene, mutated in Best macular dystrophy (BMD) encodes bestrophin, a 68-kDa basolateral plasma membrane protein expressed in retinal pigment epithelial (RPE) cells. BMD is characterized by a depressed light peak (LP) in the electro-oculogram. Bestrophin is thought to be the Cl channel that generates the LP. The goal was to generate an animal model of BMD and to determine the effects of bestrophin overexpression on the RPE-generated components of the ERG.

METHODS

Bestrophin or bestrophin mutants (W93C or R218C) were overexpressed in the RPE of rats by injection of replication-defective adenovirus. Immunofluorescence microscopy and ERG recordings were used to study subsequent effects.

RESULTS

Bestrophin was confined to the basolateral plasma membrane of the RPE. Neither wild-type (wt) nor mutant bestrophin affected the a- or b-waves of the ERG. Wt bestrophin, however, increased the c-wave and fast oscillation (FO), but not the LP. In contrast, both mutants had little or no effect on the c-wave and FO, but did reduce LP amplitude. LP amplitudes across a range of stimuli were not altered by wt bestrophin, though the luminance response function was desensitized. LP response functions were unaffected by bestrophin R218C but were significantly altered by bestrophin W93C.

CONCLUSIONS

A model of BMD was developed in the present study. Because overexpression of wt bestrophin shifted luminance response but did not alter the range of LP response amplitudes, the authors conclude that the rate-limiting step for generating LP amplitude occurs before activation of bestrophin or that bestrophin does not directly generate the LP conductance.

Light-evoked responses of the mouse retinal pigment epithelium

In response to light, the retinal pigment epithelium (RPE) generates a series of slow potentials that can be recorded as the c-wave, fast oscillation (FO), and light peak (LP) of the electroretinogram (ERG). As these potentials can be related to specific cellular events, they provide information about RPE function and how that may be altered by disease or experimental manipulation. In the present study we describe a noninvasive means for recording the light-evoked responses of the mouse RPE and use this to define the stimulus-response properties of the major components in three inbred strains of mice (BALBc/ByJ, C57BL/6J, and 129/SvJ) and two mouse mutants that reduce activity in the rod pathway. All of the major ERG components generated by the RPE are readily measured in the mouse. In albino strains (BALBc/ByJ and 129/SvJ) the intensity-response functions for the c-wave, FO, and LP are shifted toward lower intensities in comparison to those for C57BL/6J mice. Each of these components was markedly reduced in mice lacking transducin in which rod phototransduction is interrupted, indicating that they reflect primarily rod photoreceptor activity. All components were observed in no b-wave (nob) mutant mice, indicating that inner retinal activity does not make a major contribution to these potentials. Further studies of mutant mice will allow us to define the functional consequences of gene manipulation on RPE function and to evaluate specific hypotheses regarding the generation of ERG components.

Concentration-dependent effects of dopamine on the direct current electroretinogram of pigmented rabbits during prolonged intermittent recording

The direct-current (DC) electroretinogram (ERG) was studied in 24 pigmented rabbits. Four experiments were performed, each including six animals. One eye was injected intravitreally with 0.1 ml dopamine (DA) with an estimated concentration in the vitreous body of 0.0025 mM, 0.025 mM, 0.25 mM and 2.5 mM, respectively. The contralateral eye was injected with the same amount of saline. Following the injection the animals were dark adapted for 30 min and then exposed to repeated light stimuli of low intensity for almost 3 hours (series I: 1 stimulus per 3 min, 10 s duration, light intensity 6.8 x 10(2) lux). After another 30 min period of dark adaptation repeated light stimuli of high intensity were presented to the eyes (series II: 1 stimulus per 70 s, 10 s duration, light intensity 6.8 x 10(4) lux) for 33 min. In the control eyes, a slow increase with time of the a-, b- and c-wave amplitudes was observed during series I. During series II, the a- b- and c-wave amplitudes were markedly reduced between the first and the second light stimulus, but subsequently grew to a peak. The behavior of the ERG in the eyes injected with dopamine was not different from that observed in the control eyes at the lowest concentration of the drug. At higher concentrations the b- and c-wave amplitudes were reduced compared with the control eyes, and did not show the slow increase with time observed in these eyes during series I. Peak responses observed during series II in the control eyes was increasingly suppressed in the eyes treated with dopamine. Results of ERG recordings suggest that dopamine influences retinal adaptation in rabbits in a dose dependent manner.

Retinal pigment epithelial function: a role for CFTR?

In the vertebrate eye, the photoreceptor outer segments and the apical membrane of the retinal pigment epithelium (RPE) are separated by a small extracellular (subretinal) space whose volume and chemical composition varies in the light and dark. Light onset triggers relatively fast (ms) retinal responses and much slower voltage and resistance changes (s to min) at the apical and basolateral membranes of the RPE. Two of these slow RPE responses, the fast oscillation (FO) and the light peak, are measured clinically as part of the electrooculogram (EOG). Both EOG responses are mediated in part by apical and basolateral membranes proteins that form a pathway for the movement of salt and osmotically obliged fluid across the RPE, from retina to choroid. This transport pathway serves to control the volume and chemical composition of the subretinal and choroidal extracellular spaces. In human fetal RPE, we have identified one of these proteins, the cystic fibrosis transmembrane conductance regulator (CFTR) by RT-PCR, immunolocalization, and electrophysiological techniques. Evidence is presented to suggest that the FO component of the EOG is mediated directly or indirectly by CFTR.

Effect of alcohol and light on the retinal pigment epithelium of normal subjects and patients with retinal dystrophies

BACKGROUND

Light absorbed by photoreceptors causes oscillations in the voltage across the retinal pigment epithelium (RPE). This is the basis of the clinical test, electro-oculography (EOG). We have previously shown that alcohol causes a sequence of voltage changes which are so precisely the same as those caused by light that they must be produced by the same RPE machinery. There is good evidence that alcohol produces its effect by a direct action on the RPE. Consequently, in diseases associated with loss of photoreceptors, alcohol should continue to produce the voltage changes of the EOG unless secondary changes have occurred in the RPE.

METHODS

The alcohol response in patients with retinitis pigmentosa (RP) was investigated using EOG.

RESULTS

In no patient with RP was there any alcohol rise.

CONCLUSION

In patients with RP secondary abnormalities of function of the RPE must occur.

Extracellular ATP activates calcium signaling, ion, and fluid transport in retinal pigment epithelium

The presence of receptors for ATP has not been established in any native preparation of retinal neurons or glia. In the present study, we used conventional electrophysiological and [Ca2+]in fluorescence imaging techniques to investigate the effects of ATP added to Ringer's solution perfusing the retinal-facing (apical) membrane of freshly isolated monolayers of bovine retinal pigment epithelium (RPE). ATP (or UTP) produced large, biphasic voltage and resistance changes with a Kd of approximately 5 microM for ATP and approximately 1 microM for UTP. Electrical and pharmacological evidence indicates that the first and second phases of the response are attributable to an increase in basolateral membrane Cl conductance and a decrease in apical membrane K conductance, respectively. The ATP-induced responses were not affected by adenosine, but were reduced by the P2-purinoceptor blocker suramin. ATP also produced a large, transient increase in [Ca2+]in that was blocked by cyclopiazonic acid, an inhibitor of endoplasmic reticulum Ca2+-ATPases. The calcium buffer BAPTA attenuated the voltage effects of ATP. We also found that apical DIDS significantly inhibited the ATP-evoked [Ca2+]in and electrical responses, suggesting that DIDS blocked the purinoceptor. Measurements of fluid movement across the RPE using the capacitance probe technique demonstrated a significant increase in fluid absorption by apical UTP. These data indicate the presence of metabotropic P2Y/P2U-purinoceptors at the RPE apical membrane and implicate extracellular ATP in vivo as a retinal signaling molecule that could help regulate the hydration and chemical composition of the subretinal space.

Effects of high doses of toluene on color vision

High exposure to toluene may cause optic neuropathy and retinopathy, both associated with dyschromatopsia. Another solvent, ethanol, is known to induce acute blue-yellow dyschromatopsia. This study investigated the acute effects of high doses of toluene on color vision. Eight male printshop workers were examined before and after cleaning printing containers with pure toluene. After cleaning, concentrations of toluene in blood were between 3.61 and 7.37 mg/l. Color vision was tested with the Farnsworth panel D-15 test, the Lanthony desaturated panel D-15 test, and the Standard Pseudoisochromatic Plates part 2. For control of possible acute effects, eight workers of a metal-working factory without any neurotoxic exposure were tested according to the same procedure. Acute exposure to toluene did not cause impairment of color vision. However, statistical power is limited due to the small number of exposed subjects. Color vision of the printshop workers tested before cleaning was slightly impaired (statistically not significant) when compared with unexposed subjects.

Extracellular ATP activates calcium signaling, ion, and fluid transport in retinal pigment epithelium

The presence of receptors for ATP has not been established in any native preparation of retinal neurons or glia. In the present study, we used conventional electrophysiological and [Ca2+]in fluorescence imaging techniques to investigate the effects of ATP added to Ringer's solution perfusing the retinal-facing (apical) membrane of freshly isolated monolayers of bovine retinal pigment epithelium (RPE). ATP (or UTP) produced large, biphasic voltage and resistance changes with a Kd of approximately 5 microM for ATP and approximately 1 microM for UTP. Electrical and pharmacological evidence indicates that the first and second phases of the response are attributable to an increase in basolateral membrane Cl conductance and a decrease in apical membrane K conductance, respectively. The ATP-induced responses were not affected by adenosine, but were reduced by the P2-purinoceptor blocker suramin. ATP also produced a large, transient increase in [Ca2+]in that was blocked by cyclopiazonic acid, an inhibitor of endoplasmic reticulum Ca2+-ATPases. The calcium buffer BAPTA attenuated the voltage effects of ATP. We also found that apical DIDS significantly inhibited the ATP-evoked [Ca2+]in and electrical responses, suggesting that DIDS blocked the purinoceptor. Measurements of fluid movement across the RPE using the capacitance probe technique demonstrated a significant increase in fluid absorption by apical UTP. These data indicate the presence of metabotropic P2Y/P2U-purinoceptors at the RPE apical membrane and implicate extracellular ATP in vivo as a retinal signaling molecule that could help regulate the hydration and chemical composition of the subretinal space.

Changes in the Direct-Current Electroretinogram of Albino Rabbits during Prolonged Intermittent Recording

The study investigated the pattern of the direct-current electroretinogram (ERG) of albino rabbits during prolonged intermittent recording, and whether different initial dark adaptation periods or starting the experiments at different times influenced the results. We examined 27 rabbits under general anesthesia, in three experiments, each comprising nine animals. Five series (experiments 1 and 3) or four series (experiment 2) of ten repeated light stimuli were presented to the eyes with 30 minutes of dark adaptation before experiments 1 and 3, and 90 minutes before experiment 2. The dark adaptation of experiments 1 and 2 began at 10.30 a.m. and that of experiment 3 at 3.30 p.m. The interval between consecutive series of light stimuli was 33 minutes. Stimulus intensity was 680 lux, stimulus duration 10 seconds, and the interval between stimuli 3 minutes. The mean b-wave amplitude of the ten recordings in each series of stimuli increased up to the series beginning 3.5 hours (experiments 1 and 2) or 2.5 hours (experiment 3) after the start of dark adaptation. The mean c-wave amplitude increased throughout experiments 1 and 3, and up to the series beginning 3.5 hours after the start of dark adaptation in experiment 2. The mean a-wave amplitude was more stable. It seemed irrelevant for the long-term development of the mean ERG amplitudes whether the eye was dark adapted (experiment 2) or exposed to repeated light stimuli (experiments 1 and 3) during the first part of the experiment, and whether the experiments started in the morning or in the afternoon.

Effects of season on electro-oculographic ratio in winter seasonal affective disorder

Low electro-oculographic (EOG) ratios have been reported in patients with seasonal affective disorder (SAD) during the winter. This study evaluated the effects of the changing seasons on EOG ratios in SAD patients. Sixteen outpatients with SAD and 16 age-, sex-, and medication-matched normal volunteers had EOG testing during the winter and again during the summer. There was a significant season x group interaction in EOG ratios, with normal subjects showing higher ratios in winter than in summer--a seasonal variation not observed in SAD patients. SAD patients may have a subsensitivity to environmental light that leads them to experience symptoms during the winter.

Effects of adenosine on chick retinal pigment epithelium: membrane potentials and light-evoked responses

We examined the effects of adenosine, a putative mediator of neuroprotection during cerebral ischemia, on the electrophysiological characteristics of retina-retinal pigment epithelium-choroid preparations obtained from 1-7 day-old chick and maintained in vitro. Our experiments produced the following results. First, superfusion of the retinal surface with adenosine (0.1 mM) increased the trans-tissue potential. The trans-epithelial (but not the trans-retinal) potential was also increased to the same magnitude with a time-course similar to that of the trans-tissue potential. Second, adenosine produced a depolarization of the epithelial basal plasma membrane with a concomitant decrease in its basal membrane resistance. Third, the trans-epithelial (but not the trans-retinal) c-wave in response to a light stimulus was augmented by adenosine. Adenosine reduced the hyperpolarization of the epithelial basal membrane, but had no effect on the extracellular concentration of K+ in the subretinal region. Fourth, the light-peak that was elicited with a 300 s light stimulus was also depressed by adenosine. Fifth, when 4,4'-diisothiocy anostilbene-2,2'-disulfonate (DIDS), a relatively selective inhibitor of Cl- channels, was perfused at 50 microM on the choroidal surface, adenosine-induced increases in the trans-tissue potential and the c-wave were both abolished. These results suggest that adenosine increased the Cl- conductance of the basal plasma membrane of the retinal pigment epithelium and thereby augmented the standing potential as well as the light-elicited membrane potentials of the retinal pigment epithelium, which seems to be involved in the pathophysiology of retinal ischemia.

The delayed basolateral membrane hyperpolarization of the bovine retinal pigment epithelium: mechanism of generation

1. Conventional and ion-selective double-barrelled microelectrodes were used in an in vitro preparation of bovine retinal pigment epithelium (RPE)-choroid to measure the changes in membrane voltage, resistance and intracellular Cl- activity (aCli) produced by small, physiological changes in extracellular potassium concentration ([K+]o). These apical [K+]o changes approximate those produced in the extracellular (subretinal) space between the photoreceptors and the RPE following transitions between light and dark. 2. Changing apical [K+]o from 5 to 2 mM in vitro elicited membrane voltage responses with three distinct phases. The first phase was generated by an apical membrane hyperpolarization, followed by a (delayed) basolateral membrane hyperpolarization (DBMH); the third phase was an apical membrane depolarization. The present experiments focus on the membrane and cellular mechanisms that generate phase 2 of the response, the DBMH. 3. The DBMH was abolished in the presence of apical bumetanide (100 microM); this response was completely restored after bumetanide removal. 4. Reducing apical [K+]o, adding apical bumetanide (500 mM), or removing apical Cl- decreased aCli by 25 +/- 6 (n = 8), 28 +/- 1 (n = 2) and 26 +/- 5 mM (n = 3), respectively; adding 100 microM apical bumetanide decreased aCli by 12 +/- 2 mM (n = 3). Adding apical bumetanide or removing apical bath Cl- hyperpolarized the basolateral membrane and decreased the apparent basolateral membrane conductance (GB). 5. DIDS (4,4'-diisothiocyanostilbene-2,2'-disulphonic acid) blocked the RPE basolateral membrane Cl- conductance and inhibited the DBMH and the basolateral membrane hyperpolarization produced by apical bumetanide addition or by removal of apical Cl-o. The present results show that the DBMH is caused by delta[K]o-induced inhibition of the apical membrane Na(+)-K(+)-2Cl- cotransporter; the subsequent decrease in aCli generated a hyperpolarization at the basolateral membrane Cl- channel.

Light-dependent hydration of the space surrounding photoreceptors in the cat retina

We have studied the effect of retinal illumination on the concentration of the extracellular space marker tetramethylammonium (TMA+) in the dark-adapted cat retina using double-barreled ion-selective microelectrodes. The retina was loaded with TMA+ by a single intravitreal injection. Retinal illumination produced a slow decrease in [TMA+]o, which was maximal in amplitude in the most distal portion of the space surrounding photoreceptors, the subretinal space. The light-evoked decrease in [TMA+]o was considerably slower and of a different overall time course than the light-evoked decrease in [K+]o, also recorded in the subretinal space. [TMA+]o decreased to a peak at 38 s after the onset of illumination, then slowly recovered towards the baseline, and transiently increased following the offset of illumination. It resembled the light-evoked [TMA+]o decreases previously recorded in the in vitro preparations of frog (Huang & Karwoski, 1990, 1992) and chick (Li et al., 1992, 1994) but was considerably larger in amplitude, 22% compared with 7%. As in frog, where it was first recorded, the light-evoked [TMA+]o decrease is considered to originate from a light-evoked increase in the volume of the subretinal space (or subretinal hydration). A mathematical model accounting for [TMA+]o diffusion predicted that the volume increase underlying the response was 63% on average and could be as large as 95% and last for minutes. The estimated volume increase was then used to examine its effect on K+ concentration in the subretinal space. We conclude that a light-dependent hydration of the subretinal space represents a significant physiological event in the intact cat eye, which should affect the organization of the interphotoreceptor matrix, and the concentrations of all ions and metabolites located in the subretinal space.

Effect of kainic acid and NMDA on the pattern electroretinogram, the scotopic threshold response, the oscillatory potentials and the electroretinogram in the urethane anaesthetized cat

Kainic acid (KA, 12.5-100 nmol) or N-methyl-D-aspartate (NMDA 25-250 nmol) was injected into the vitreous of one eye of urethane anaesthetized cats. Pattern electroretinograms (PERGs) were recorded to transient contrast reversing bars. Scotopic luminance electroretinograms (ERGs) were recorded to blue flashes. All doses of KA reduced the oscillatory potentials (OPs), PERG and focal ERG (FERG). At 50 nmol KA, the b-wave and scoptic threshold response (STR) were normal. At 100 nmol KA, the STR was absent and the b-wave reduced by over 50%. OPs and STRs were reduced in all NMDA injected eyes. NMDA at 25 nmol enhanced the FERG, PERG, and b-wave and high doses (above 150 nmol) reduced them. Light microscopic examination of retinas showed 25 nmol KA only damaged dendrites of ganglion cells. NMDA damage was slight with < 200 nmol. These data show that the cat PERG has a proximal component which is very sensitive to low doses of KA; the PERG and FERG are very similar; the STR and PERG are generated by different structures and that the OPs and the FERG and PERG are all generated close to the ganglion cell layer, proximal to the STR.

Effects of phototherapy on electrooculographic ratio in winter seasonal affective disorder

Low electrooculographic (EOG) ratios have been reported in patients with seasonal affective disorder (SAD). This study was undertaken to replicate these results and to consider the effects of light therapy on the EOG in SAD patients. Sixteen outpatients with SAD and 16 age-, sex-, and medication-matched control subjects had EOG testing before and after 1 week of light therapy during the winter. The EOG ratios in the SAD patients were only marginally lower than those in the normal control subjects. These differences persisted after light therapy. Although the slightly decreased EOG ratios in SAD patients might have resulted from an artifact of test variability, drowsiness, or other confounding factors, the difference between patients and control subjects raises the possibility of retinal abnormality in SAD.

The c-wave of the direct-current electroretinogram and the standing potential of the albino rabbit eye in response to repeated series of light stimuli with different interstimulus intervals

The direct-current electroretinogram and the standing potential of the eye of seven albino rabbits were recorded in response to repeated light stimuli, which were presented in four consecutive series. The intervals between the beginning of succeeding stimuli were 8 minutes, 4 minutes, 2 minutes and 70 seconds (series 1, 2, 3 and 4, respectively). Stimulus duration (10 seconds) and light intensity (6.8 x 10(4) lux) were constant during the experiments. The series lasted for 36-40 minutes, and each was preceded by 30 minutes of dark adaptation. During series 1, the end amplitudes of the a-, b- and c-waves were not significantly changed compared with the initial levels. During series 2, 3 and 4, the a-, b- and c-wave amplitudes were markedly reduced immediately after the first electroretinogram recording. The a- and b-waves then recovered to a limited extent, but the c-wave was more fully restored. A slight peak in the c-wave amplitude could be discerned 16-20 minutes after the start of recording. A decrease in the standing potential was seen 50-54 seconds after the start of light stimulation during all four series, and a peak occurred 12-16 minutes after the start of recording. The similarity in behavior between the c-wave and the standing potential suggests the operation of a pigment epithelial mechanism behind the more complete recovery of the c-wave amplitude. When electroretinogram amplitudes and standing potential levels are discussed, and when one experiment is compared with another one, it is important that adaptational and stimulus conditions, as well as time course, are well controlled and clearly specified.

Dopamine and melatonin interactions in the intact chicken eye. Electrooculographic and biochemical study

Electrophysiological and biochemical techniques were used to investigate the interactions between dopamine (DA) and melatonin (MEL) in the intact chicken eye. Endogenous DA depletion induced by intraocular administration of alpha-methyl-para-tyrosine (alpha-MPT), a selective tyrosine hydroxylase inhibitor, decreases the transepithelial potential (TEP) of the retinal pigment epithelium and reduces the light peak (LP) recorded by an indirect electro-oculographic (EOG) method. An intraocular injection of MEL also reduces the TEP but does not reduce the LP. Retinal MEL is increased after endogenous DA depletion and a tight inverse correlation between DA and MEL contents was found. The present data, together with other findings support the hypothesis (1) that in the intact chicken eye, DA and MEL play respectively a role of light and dark signals on the TEP, and (2) that a balance between these two neurohormones may be responsible for the regulation of RPE events which are dependent on light-dark conditions.

Effects of intraocular pressure on pH outside rod photoreceptors in the cat retina

Double-barreled H(+)-selective microelectrodes were used to study the effect of raising intraocular pressure (ocular hypertension) on intraretinal pH in the cat. A surprising alkalinizing response was observed in the subretinal space on the first experimental day in seven of 11 experiments. Of the three retinal regions studied, the alkalinizing response was present in the area centralis and in the adjoining near-discal region, but was absent in the superior temporal periphery. The alkalinizing response was relatively fragile, diminishing and then extinguishing with repetition of episodes of ocular hypertension. It was also never observed on the second experimental day. The alkalinizing response had a relatively low threshold, appearing with 10 mmHg elevations of intraocular pressure, i.e. when the perfusion pressure was still relatively high, but was also present across a wide range of perfusion pressures. It was maximal in amplitude in the most distal portion of the subretinal space and consisted of two components, an 'on' alkalinization during the episode of ocular hypertension, and an 'off' alkalinization following it. When the alkalinizing response was present in the subretinal space, it also could be recorded with placement of the microelectrode in the choroid. Elevations of systemic arterial blood pressure with epinephrine (intravenous), by raising perfusion pressure, also alkalinized the subretinal space, indicating that an increase in choroidal blood flow can produce this type of pH response. While pH recordings do not directly monitor blood flow, we tentatively interpreted the alkalinizing response with ocular hypertension as originating from a regional increase in choroidal blood flow, but we cannot rule out other explanations. Acidification of the subretinal space in response to ocular hypertension replaced the alkalinizing response when the latter extinguished, or acidification was the sole response in the absence of an alkalinization. The acidification also had a low threshold, was maximal in the most distal portion of the subretinal space, and increased in size with decreases in perfusion pressure. The acidification was reduced in amplitude by breathing 100% oxygen (hyperoxia). This was interpreted as suppression of the component of acidification that originated from an increase in glycolysis by the rod photoreceptors during ocular hypertension.

Effects of light and darkness on pH outside rod photoreceptors in the cat retina

We recorded pH in the extracellular space surrounding rod photoreceptors in the dark-adapted eye of the cat and during illumination with double-barreled H(+)-selective microelectrodes. A pH of 7.17 was recorded in the vitreous at the retinal surface of the dark-adapted eye and this became more alkaline during light adaptation. In dark adaptation, a pH close to 7.00 was recorded in a region of maximal acidity in the extracellular space surrounding rods in the outer nuclear layer (ONL). pH steeply alkalinized as the microelectrode was moved more distally towards the retinal pigment epithelium (RPE), and almost reached the pH of the arterial blood at the apical surface of the RPE. Illumination produced an intraretinal alkalinization that was largest (up to 0.2 pH units) in the ONL, maximal in amplitude at rod-saturating intensities, and that was sustained during steady background illumination. The light-evoked alkalinization was relatively slow in onset, having a time constant (1/e) of 64 sec, and took 8-12.5 min to return to the dark-adapted level after the offset of maintained illumination. These results show that acid production by cat rods is highest in the dark, reflecting a high rate of energy metabolism, and suggest that glycolysis is required to support the dark current. Illumination, by suppressing both glycolysis and respiration, alkalinizes the extracellular space surrounding rods. The substantial change in pH outside rods from dark to light could alter pH dependent properties of the interphotoreceptor matrix.

Effects of systemic hypoxia on pH outside rod photoreceptors in the cat retina

We studied the effect of systemic hypoxia on intraretinal pH in the intact cat eye using double-barreled H(+)-sensitive microelectrodes. Hypoxia in the dark further acidified the extracellular space surrounding rods in the distal retina and this effect was maximal in the outer nuclear layer (ONL). An acidification occurred in response to essentially any decrease in PaO2 below the normoxic level. Light-evoked alkalinizations in the ONL were larger in amplitude during hypoxia than in normoxia and this difference increased with the severity of hypoxia. Background illumination suppressed the hypoxic acidification of the ONL, completely inhibiting it at rod saturating intensities, at levels of hypoxia down to PaO2s of 40 mmHg. Systemic hyperoxia produced a small alkalinization in the ONL, and a reduction in the amplitude of the light-evoked alkalinizations. This suggests that systemic hyperoxia can partially suppress the ongoing glycolysis of dark-adapted rods. Changes in blood flow during hypoxia also altered intraretinal pH. Hypoxia led to an alkalinization in the choroid in both dark and light adaptation that spread into the distal retina. This alkalinization is most likely caused by the increase in CO2 removal that occurs as systemic blood pressure, and as a consequence, choriocapillaris blood flow increase during hypoxia. The alkalinization attenuated the acidification that was observed outside rods during hypoxia. There was also an alkalinization of the proximal portion of the retina, which spread into the vitreous. This alkalinization was attributed to the autoregulatory increase in blood flow that occurs in the retinal vessels during hypoxia. These findings provide further evidence for the hypothesis that the energy metabolism of dark-adapted rods is exquisitely sensitive to systemic hypoxia so that any small decrease in PaO2 increases rod glycolysis. Rod-saturating illumination can completely suppress this increase in glycolysis for all but severe hypoxia. An increase in blood flow in the choriocapillaris during hypoxia appears to mitigate the effects of hypoxia on the distal retina.

Effects of intravenous acetazolamide on retinal pH in the cat

Double-barreled H(+)-selective microelectrodes were used to study the effect on intravenous acetazolamide on intraretinal pH in the cat. Acetazolamide (11.4-27.8 mg kg-1 intravenously) caused a rapid acidification of the subretinal space. This change in pH originated in the most distal portion of the subretinal space and could not be attributed to a change in pH or PCO2 of the arterial blood. Slow light-evoked alkalinizations in distal retina, attributable to a decrease in rod photoreceptor energy metabolism, were relatively unaltered by acetazolamide. This result indicated that acetazolamide had not crossed the blood-retinal barrier in sufficient amounts to change this response. In time, following intravenous perfusion of acetazolamide, continuous depth profiles of intraretinal pH showed an acidification of the entire retina and the vitreous also became more acidic. These results indicate that the rapid or primary effect of acetazolamide is an acidification of the distal portion of the subretinal space, which is thought to originate in a change in the transport of H+ or HCO3- by the retinal pigment epithelium. This is followed by an acidification of the entire retina and vitreous, presumably due to diffusion of acid from the distal retina, although there could be additional causes.

Potassium-evoked responses from the retinal pigment epithelium of the toad Bufo marinus

Changes in the apical and basal membrane potentials and the resultant changes in the transepithelial potential were recorded from the isolated retinal pigment epithelium of the toad Bufo marinus while the potassium concentration superfusing the apical membrane was changed. Lowering apical potassium caused an initial apically-generated hyperpolarization that increased the transepithelial potential which was usually followed by a delayed basally-generated hyperpolarization that decreased the transepithelial potential. Light evoked a similar pattern of apical and basal responses in a preparation of neural retina-retinal pigment epithelium-choroid. The delayed basal hyperpolarization was accompanied by an apparent increase in basal membrane resistance, and was inhibited by adding the anion transport blocker 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid or the metabolic inhibitor dinitrophenol to the solution superfusing the choroidal side of the retinal pigment epithelium (RPE). The results suggest that a change in the chloride equilibrium potential or chloride conductance of the basal membrane mediates the delayed basal response.

Light and dark induced variations of the c-wave voltage of the chicken eye after treatment with sodium aspartate

Light and dark-induced variations of the ERG c-wave voltage were recorded in control chickens and after intravitreal injection of Na aspartate, a treatment whose main effect is to functionally disconnect the pigment epithelium-photoreceptor complex from second order neurons. After aspartate, the fast light rise which characterizes this preparation is no longer observed; it is substituted for by a potential variation of much slower time course and of lower magnitude. The data totally confirm previous findings obtained through an indirect EOG technique and suggest the participation of inner retinal layers in the generation of the light peak in the chicken eye.

Is dopamine involved in the generation of the light peak in the intact chicken eye?

The implication for dopamine (DA) in the modulation of the standing potential (SP) and the light peak (LP) was tested in intact chickens using an indirect EOG method. After an intravenous or intravitreal injection of DA, a transient, dose-dependent increase in the SP was observed. The LP, recorded after an intravenous injection, was preserved. But after an intravitreal injection, the LP was strongly reduced or even abolished depending on the dose of DA, whereas the photoreceptor response was unchanged. The data supports the hypothesis that the light peak, which is generated by a neural retina-pigment epithelium interaction, could be triggered by dopamine released at light onset from the inner retinal layers.

Effects of intravitreal and intravenous administrations of dopamine on the standing potential and the light peak in the intact chicken eye

We studied the modifications of the standing potential (SP) of the eye and of the light peak (LP) after exposure to dopamine, a neurotransmitter released at light by the inner retina and known to affect electrical properties of the retinal pigment epithelium. Intravenous or intravitreal injections of dopamine (DA) were performed on intact chickens. "Choroidal" application (through an intravenous injection) induced a transient increase of the SP and the LP was preserved. On the other hand, "apical" applications of DA (through an intraocular injection) also increased the SP but considerably depressed the LP. These results are in agreement with the hypothesis that the light-induced release of dopamine from the neuroretina may be responsible for the LP generation in the intact chicken eye.

Active ion transport pathways in the bovine retinal pigment epithelium

1. Radioactive tracer flux measurements demonstrate that active ion transport across the isolated bovine retinal pigment epithelium (RPE)-choroid preparation can be maintained for hours after the eye is enucleated and the tissue removed from the eye. 2. It has been shown that 86Rb tracer fluxes can be used to monitor potassium (K+) transport across bull-frog RPE. In bovine RPE, net 86Rb (K+) absorption is zero. Apical barium (Ba2+) elevated active K+ absorption from zero to approximately 0.3 mu equiv cm-2 h-1. This Ba2(+)-induced increase in active K+ absorption was inhibited either by ouabain or bumetanide in the apical bath. 3. In control Ringer solution, buffered with bicarbonate and CO2, the RPE-choroid actively absorbs chloride (Cl-) at a rate of approximately 0.5 mu equiv cm-2 h-1. In contrast, sodium (Na+) is secreted at a rate of approximately 0.5 mu equiv cm-2 h-1. Chloride absorption was inhibited by apical bumetanide, and Na+ secretion was inhibited by apical ouabain. These drugs were only effective when placed in the solution bathing the apical or retinal side of the tissue. 4. Net Cl- absorption requires an exit mechanism at the basolateral membrane. DIDS (4,4'-diisothiocyanostilbene-2,2'-disulphonic acid) in the basal bath completely inhibited net Cl- absorption in bicarbonate-free Ringer solution. 5. These experiments show that the chloride transport pathway contains at least two components: (1) a bumetanide-sensitive uptake mechanism at the apical membrane; and (2) an efflux mechanism at the basolateral membrane that is blocked by DIDS. 6. Three apical membrane mechanisms were identified that could help modulate [K+]o in the subretinal or extracellular space that separates the distal retina and the RPE apical membrane. They are: (1) an ouabain-sensitive Na(+)-K+ pump; (2) a bumetanide-sensitive mechanism, the putative Na(+)-K(+)-Cl- co-transporter; (3) a barium-sensitive K+ channel that recycles, to the apical bath, most or all of the potassium that is actively taken up by the Na(+)-K+ pump and the co-transporter. 7. These data suggest that light-induced alterations in subretinal potassium that occur in vivo can activate the chloride transport pathway and help modulate RPE intracellular Cl- during transitions between the light and dark.

Electrophysiological consequences of retinal hypoxia

Experiments on cats show that electrical activity of the inner (proximal) retina is unaffected during systemic hypoxia as long as arterial oxygen tension (PaO2) is above 40 mm Hg. This is due to effective regulation of inner retinal tissue PO2 by the retinal circulation. In contrast, some electrical signals generated in the outer (distal) retina begin to change when PaO2 falls below 70-80 mmHg. The outer retinal responses are generated by the retinal pigment epithelium, but their susceptibility to hypoxia results primarily from their dependence on photoreceptors. Photoreceptor metabolism is sensitive to hypoxia because of the high oxygen consumption of photoreceptors and their reliance on the choroidal circulation, which cannot regulate PO2 in the outer retina. Retinal electrophysiology and oxygen distribution are altered by acutely elevated intraocular pressure just as by hypoxia. These results raise the question as to how inner retinal function can be preserved when outer retinal function is altered. The explanations proposed relate to (1) differences in conditions of light adaptation in different studies, (2) the possible inappropriateness of the previous measurements in the inner retina for revealing photoreceptor dysfunction, and (3) a possible preservation of photoreceptor electrical responses when their metabolism is altered. Comparison of cat and human studies suggests that the human retina is affected in much the same way during hypoxia as the cat retina, but further experiments are required for an understanding of the role of hypoxia in human disease.