Increased Retinal Metabolism Induced by Flicker in the Isolated Mouse Retina

Both the retina and brain exhibit neurovascular coupling, increased blood flow during increased neural activity. In the retina increased blood flow can be evoked by flickering light, but the magnitude of the metabolic change that underlies this is not known. Local changes in oxygen consumption (QO2) are difficult to measure in vivo when both supply and demand are changing. Here we isolated the C57BL/6J mouse retina and supplied it with oxygen from both sides of the tissue. Microelectrode recordings of PO2 were made in darkness and during 20 s of high scotopic flickering light at 1 Hz. Flicker led to a PO2 increase in the outer retina and a decrease in the inner retina, indicating that outer retinal QO2 (QOR) decreased and inner retinal QO2 (QIR) increased. A four-layer oxygen diffusion model was fitted to PO2 values obtained in darkness and at the end of flicker to determine the values of QOR and QIR. QOR in flicker was 76 ± 14% (mean and SD, n = 10) of QOR in darkness. The increase in QIR was smaller, 6.4 ± 5.0%. These metabolic changes are likely smaller than the maximum changes, because with no regeneration of pigment in the isolated retina, we limited the illumination. Further modeling indicated that at high illumination, QIR could increase by up to 45%, which is comparable to the magnitude of flow changes. This suggests that the blood flow increase is at least roughly matched to the increased metabolic demands of activity in the retina.

Diabetes-Induced Changes of the Rat ERG in Relation to Hyperglycemia and Acidosis

PURPOSE

To understand the mechanism of changes in the c-wave of the electroretinogram (ERG) in diabetic rats, and to explore how glucose manipulations affect the c-wave.

METHODS

Vitreal ERGs were recorded in control and diabetic Long-Evans rats, 3-60 weeks after IP vehicle or streptozotocin. A few experiments were performed on Brown Norway rats. Voltage responses to current pulses were used to measure the transepithelial resistance of the retinal pigment epithelium (RPE).

RESULTS

During development of diabetes the b-wave amplitude progressively decreased to about half of the initial amplitude after a year. In contrast, the c-wave was strongly affected from the very beginning (3 weeks) of diabetes. In control rats, the c-wave was cornea-positive at lower illuminations but was cornea-negative at higher (photopic) illumination. In diabetics, the whole amplitude-intensity curve was shifted toward negativity. The magnitude of this shift was markedly affected by acute glucose manipulations in diabetics but not in controls. Increased blood glucose made the c-wave more negative, and decreased blood glucose with insulin had the opposite effect. Experimentally induced acidification of the retina had a small effect that was different from diabetes, shifting the c-wave toward positivity, slightly in controls and more noticeably in diabetics. One reason for the significant negativity of the diabetic ERG was a decrease of the cornea-positive response of the RPE due to a decrease of the transepithelial resistance.

CONCLUSIONS

The ERG c-wave is more negative in diabetics than in control animals, and is far more sensitive to changes in blood glucose. The increased negativity is largely if not entirely due to changes in the transepithelial resistance of the RPE, an electrical analog of the breakdown of the blood-retinal barrier observed in other studies. The sensitivity of the c-wave to glucose in diabetics may also be due to changes in transepithelial resistance.

Oxygen profiles and oxygen consumption in the isolated mouse retina

The retina has a large demand for oxygen, but there is only limited information on differences between oxygen utilization (QO2) in the inner and outer retina, and limited data on mouse, which has become a prevalent animal model. This study utilized the isolated mouse retina, which allowed more detailed spatial analysis of QO2 than other methods. Oxygen sensitive microelectrodes were used to obtain profiles of oxygen tension across the isolated mouse retina, and mathematical models of retinal oxygen diffusion with four and five layers were fitted to the data to obtain values for QO2 of the outer retina (QOR) and inner retina (QIR). The boundaries between layers were free parameters in these models. The five-layer model resulted in lower error between the model and data, and agreed better with known anatomy. The three layers for the outer retina occupied half of the retina, as in prior work on rat, cat, and monkey, and the inner half of the retina could be divided into two layers, in which the one closer to the vitreous (layer 5) had much lower QO2 than the more distal inner retina (layer 4). QIR in darkness was 3.9 ml O2-100 g-1-min-1, similar to the value for intact cat retina, and did not change during light. QOR in darkness was 2.4 ml O2-100 g-1-min-1, lower than previous values in cat and rat, possibly because of damage to photoreceptors during isolation. There was a tendency for QOR to be lower in light, but it was not significant in this preparation.

Brain Tissue Oxygen and BOLD fMRI Under Different Levels of Neuronal Activity

Localized increases in neuronal activity are supported by the hemodynamic response, which delivers oxygen to the brain tissue to support synaptic functions, action potentials and other neuronal processes. However, it remains unknown if changes in baseline neuronal activity, which are expected to reflect neuronal metabolic demand, alter the relationship between the local hemodynamic and oxygen behaviour. In order to better characterize this system, we examine here the relationship between brain tissue oxygen (PO2) and hemodynamic responses (BOLD functional MRI) under different levels of neuronal activity. By comparing the stimulus-evoked responses during different levels of baseline neuronal activity, the awake state vs isoflurane anesthesia, we were able to measure how a known change in neuronal demand affected tissue PO2 as well as the hemodynamic response to stimulation. We observed a high correlation between stimulus-evoked PO2 and BOLD responses in the awake state. Moreover, we found that the evoked PO2 and BOLD responses were still present despite the elevated tissue oxygen baseline and decreased baseline of neuronal activity under low concentration isoflurane, and that the magnitudes of these responses decreased by similar proportions but the relationship between these signals was distorted. Our findings point to distortion of the BOLD-PO2 relationship due to anesthesia. The feedback mechanism to adjust the level of brain tissue oxygen, as well as the correlation between BOLD and PO2 responses, are impaired even by a small dose of anesthetics.

Brain tissue oxygen dynamics while mimicking the functional deficiency of interneurons

The dynamic interaction between excitatory and inhibitory activity in the brain is known as excitatory-inhibitory balance (EIB). A significant shift in EIB toward excitation has been observed in numerous pathological states and diseases, such as autism or epilepsy, where interneurons may be dysfunctional. The consequences of this on neurovascular interactions remains to be elucidated. Specifically, it is not known if there is an elevated metabolic consumption of oxygen due to increased excitatory activity. To investigate this, we administered microinjections of picrotoxin, a gamma aminobutyric acid (GABA) antagonist, to the rabbit cortex in the awake state to mimic the functional deficiency of GABAergic interneurons. This caused an observable shift in EIB toward excitation without the induction of seizures. We used chronically implanted electrodes to measure both neuronal activity and brain tissue oxygen concentrations (PO₂) simultaneously and in the same location. Using a high-frequency recording rate for PO₂, we were able to detect two important phenomena, (1) the shift in EIB led to a change in the power spectra of PO₂ fluctuations, such that higher frequencies (8-15 cycles per minute) were suppressed and (2) there were brief periods (dips with a duration of less than 100 ms associated with neuronal bursts) when PO₂ dropped below 10 mmHg, which we defined as the threshold for hypoxia. The dips were followed by an overshoot, which indicates either a rapid vascular response or decrease in oxygen consumption. Our results point to the essential role of interneurons in brain tissue oxygen regulation in the resting state.

Extracellular K+ reflects light-evoked changes in retinal energy metabolism

Retinal neurons spend most of their energy to support the transmembrane movement of ions. Light-induced electrical activity is associated with a redistribution of ions, which affects the energy demand and results in a change in metabolism. Light-induced metabolic changes are expected to be different in distal and proximal retina due to differences in the light responses of different retinal cells. Extracellular K⁺ concentration ([K⁺]_o) is a reliable indicator of local electrophysiological activity, and the purpose of this work was to compare [K⁺]_o changes evoked by steady and flickering light in distal and proximal retina. Data were obtained from isolated mouse (C57Bl/6J) retinae. Double-barreled K⁺-selective microelectrodes were used to simultaneously record [K⁺]_o and local ERGs. In the distal retina, photoreceptor hyperpolarization led to suppression of ion transfer, a decrease in [K⁺]_o by 0.3-0.5 mM, reduced energy demand, and, as previously shown in vivo, decreased metabolism. Flickering light had the same effect on [K⁺]_o in the distal retina as steady light of equivalent illumination. The conductance and voltage changes in postreceptor neurons are cell-specific, but the overall effect of steady light in the proximal retina is excitation, which is reflected in a [K⁺]_o increase there (by a maximum of 0.2 mM). In steady light the [K⁺]_o increase lasts only 1-2 s, but a sustained [K⁺]_o increase is evoked by flickering light. A squarewave low frequency (1 Hz) flicker of photopic intensity produced the largest increases in [K⁺]_o. Judging by measurements of [K⁺]_o, steady illumination decreases energy metabolism in the distal retina, but not in the proximal retina (except for the first few seconds). Flickering light evokes the same decrease in the distal retina, but also evokes a sustained [K⁺]_o increase in the proximal retina, suggesting an increase of metabolic demand there, especially at 1 Hz, when neurons of both on- and off-pathways appear to contribute maximally. This proximal retinal metabolic response to flicker correlates to the increase in blood flow during flicker that constitutes neurovascular coupling.

K+-dependent Müller cell-generated components of the electroretinogram

The electroretinogram (ERG) has been employed for years to collect information about retinal function and pathology. The usefulness of this noninvasive test depends on our understanding of the cell sources that generate the ERG. Important contributors to the ERG are glial Müller cells (MCs), which are capable of generating substantial transretinal potentials in response to light-induced changes in extracellular K+ concentration ([K+]o). For instance, the MCs generate the slow PIII (sPIII) component of the ERG as a reaction to a photoreceptor-induced [K+]o decrease in the subretinal space. Similarly, an increase of [K+]o related to activity of postreceptor retinal neurons also produces transretinal glial currents, which can potentially influence the amplitude and shape of the b-wave, one of the most frequently analyzed ERG components. Although it is well documented that the majority of the b-wave originates from On-bipolar cells, some contribution from MCs was suggested many years ago and has never been experimentally rejected. In this work, detailed information about light-evoked [K+]o changes in the isolated mouse retina was collected and then analyzed with a relatively simple linear electrical model of MCs. The results demonstrate that the cornea-positive potential generated by MCs is too small to contribute noticeably to the b-wave. The analysis also explains why MCs produce the large cornea-negative sPIII subcomponent of the ERG, but no substantial cornea-positive potential.

Intravenous Immunomodulatory Nanoparticle Treatment for Traumatic Brain Injury

OBJECTIVE

There are currently no definitive disease-modifying therapies for traumatic brain injury (TBI). In this study, we present a strong therapeutic candidate for TBI, immunomodulatory nanoparticles (IMPs), which ablate a specific subset of hematogenous monocytes (hMos). We hypothesized that prevention of infiltration of these cells into brain acutely after TBI would attenuate secondary damage and preserve anatomic and neurologic function.

METHODS

IMPs, composed of US Food and Drug Administration-approved 500nm carboxylated-poly(lactic-co-glycolic) acid, were infused intravenously into wild-type C57BL/6 mice following 2 different models of experimental TBI, controlled cortical impact (CCI), and closed head injury (CHI).

RESULTS

IMP administration resulted in remarkable preservation of both tissue and neurological function in both CCI and CHI TBI models in mice. After acute treatment, there was a reduction in the number of immune cells infiltrating into the brain, mitigation of the inflammatory status of the infiltrating cells, improved electrophysiologic visual function, improved long-term motor behavior, reduced edema formation as assessed by magnetic resonance imaging, and reduced lesion volumes on anatomic examination.

INTERPRETATION

Our findings suggest that IMPs are a clinically translatable acute intervention for TBI with a well-defined mechanism of action and beneficial anatomic and physiologic preservation and recovery. Ann Neurol 2020;87:442-455.

Emixustat Reduces Metabolic Demand of Dark Activity in the Retina

Purpose

In the dark, photoreceptor outer segments contain high levels of cyclic guanosine 3'-5' monophosphate (cGMP), which binds to ion channels, holding them open and allowing an influx of cations. Ion pumping activity, which balances cation influx, uses considerable amounts of adenosine triphosphate (ATP) and oxygen. Light reduces cation influx and thereby lowers metabolic demand. Blood vessels are compromised in the diabetic retina and may not be able to meet the higher metabolic demand in darkness. Emixustat is a visual cycle modulator (VCM) that reduces chromophore levels and, therefore, may mimic light conditions. We evaluated the effect of emixustat on oxygen consumption and cation influx in dark conditions.

Methods

Cation influx was measured in rats using Mn2+-magnetic resonance imaging (MEMRI). Retinal oxygen profiles were recorded to evaluate oxygen consumption. In the MEMRI protocol, animals were treated with either emixustat or vehicle. In the oxygen protocol, animals were untreated or treated with emixustat.

Results

In vehicle-treated animals, cation channel activity increased in the dark. Emixustat treatment reduced cation channel activity; activity was comparable to vehicle-treated controls in light conditions. In vehicle-treated animals, minimum retinal oxygen tension decreased as the retina recovered from a photobleach, indicating that more oxygen was being consumed. Emixustat treatment prevented the decrease in oxygen pressure after photobleach.

Conclusions

Emixustat reduced the cation influx and retinal oxygen consumption associated with dark conditions. VCMs are a promising potential treatment for ischemic retinal neovascularization, such as that in diabetic retinopathy.

Improved Macular Capillary Flow on Optical Coherence Tomography Angiography After Panretinal Photocoagulation for Proliferative Diabetic Retinopathy

PURPOSE

This study evaluated the macular microvascular changes in eyes with proliferative diabetic retinopathy (PDR) following panretinal photocoagulation (PRP).

DESIGN

Using optical coherence tomographic angiography (OCTA), we prospectively studied 10 eyes of 10 subjects with high-risk PDR immediately before, at 1 month, and at 3-6 months following PRP, using a 3- × 3-mm OCTA scan at each visit.

METHODS

The following parameters were calculated for the superficial (SCP), middle (MCP), and deep capillary plexuses (DCP): parafoveal vessel density (VD), adjusted flow index (AFI), and percent area of nonperfusion (PAN). Parafoveal SCP vessel-length density (VLD) was also evaluated. We performed univariate and multivariable statistics, adjusting for age and signal strength. To model the hemodynamic effect of PRP, we also present a mathematical model based on electrical circuits.

RESULTS

We found no significant difference for the vascular density parameters following PRP, except for decreased density at the MCP at the latest timepoint in the adjusted multivariable model. PAN, a metric of nonperfusion adjusted for noise, and AFI, a surrogate metric of blood flow, showed significant increases at all capillary levels in the adjusted model. Our mathematical model explained how PRP would increase macular blood flow.

CONCLUSIONS

Using OCTA, we found an overall increase in the flow metrics of all capillary layers in the macula following PRP, unrelated to macular edema or thickening, in line with the mathematical model. Our results suggest an overall redistribution of blood flow to the posterior pole following PRP, adding a new dimension to our understanding of the complex biologic effects of PRP in PDR. NOTE: Publication of this article is sponsored by the American Ophthalmological Society.

Diabetes Alters pH Control in Rat Retina

Purpose

The purpose of this study was to determine whether the ability of the rat retina to control its pH is affected by diabetes.

Methods

Double-barreled H+-selective microelectrodes were used to measure extracellular [H+] in the dark-adapted retina of intact control and diabetic Long-Evans rats 1 to 6 months after intraperitoneal injection of vehicle or streptozotocin, respectively. Two manipulations-increasing of blood glucose and intravenous injection of the carbonic anhydrase blocker dorzolamide (DZM)-were used to examine their effects on retinal pH regulation.

Results

An increase of retinal acidity was correlated with the diabetes-related increase in blood glucose, but only between 1 and 3 months of diabetes, not earlier or later. Adding intravenous glucose had no noticeable effect on the retinal acidity of control animals. In contrast, similar injections of glucose in diabetic rats significantly increased the acidity of the retina. Again, the largest increase of retinal acidity due to artificially elevated blood glucose was observed at 1 to 3 months of diabetes. Suppression of carbonic anhydrase by DZM dramatically increased the retinal acidity in both control and diabetic retinas to a similar degree. However, in controls, the strongest effect of DZM was recorded within 10 minutes after the injection, but in diabetics, the effect tended to increase with time and after 2 hours could be two to three times larger than at the beginning.

Conclusions

During development of diabetes in rats, the control over retinal pH is partly compromised so that conditions that perturb retinal pH lead to larger and/or more sustained changes than in control animals.

The logic of ionic homeostasis: Cations are for voltage, but not for volume

Neuronal activity is associated with transmembrane ionic redistribution, which can lead to an osmotic imbalance. Accordingly, activity-dependent changes of the membrane potential are sometimes accompanied by changes in intracellular and/or extracellular volume. Experimental data that include distributions of ions and volume during neuronal activity are rare and rather inconsistent partly due to the technical difficulty of performing such measurements. However, progress in understanding the interrelations among ions, voltage and volume has been achieved recently by computational modelling, particularly “charge-difference” modelling. In this work a charge-difference computational model was used for further understanding of the specific roles for cations and anions. Our simulations show that without anion conductances the transmembrane movements of cations are always osmotically balanced, regardless of the stoichiometry of the pump or the ratio of Na⁺ and K⁺ conductances. Yet any changes in cation conductance or pump activity are associated with changes of the membrane potential, even when a hypothetically electroneutral pump is used in calculations and K⁺ and Na⁺ conductances are equal. On the other hand, when a Cl^- conductance is present, the only way to keep the Cl^-equilibrium potential in accordance with the changed membrane potential is to adjust cell volume. Importantly, this voltage-evoked Cl^--dependent volume change does not affect intracellular cation concentrations or the amount of energy that is necessary to support the system. Taking other factors into consideration (i.e. the presence of internal impermeant poly-anions, the activity of cation-Cl^- cotransporters, and the buildup of intra- and extracellular osmolytes, both charged and electroneutral) adds complexity, but does not change the main principles.

We have developed software that calculates membrane potential and cell volume that result from redistribution of principal ions (K⁺, Na⁺, and Cl^-) during normal cellular activity and experimental manipulations. Calculations in the model are done by an iterative charge-difference method that makes few assumptions about governing equations. Most of the features that were considered to be important for volume and voltage regulation were incorporated in the model, including the unique capability to perform calculations with different values of transmembrane water permeability. We have used the program to reexamine interactions between ionic fluxes, membrane potential, and cell volume and found that there was a previously unappreciated difference in the way that the distribution of cations and anions affect the cell. Na⁺ and K⁺, which are distributed unevenly across the membrane by the Na⁺/K⁺-ATPase, are primarily responsible for the membrane potential, but, contrary to popular belief, do not directly participate in volume regulation. On the other hand, the Cl^- conductance determines the extent of volume changes, because Cl^- has to follow the changes of membrane potential, which inevitably leads to changes in cell volume. The software is available to download and use for other investigations.

Retinal pH and Acid Regulation During Metabolic Acidosis

PURPOSE

Changes in retinal pH may contribute to a variety of eye diseases. To study the effect of acidosis alone, we induced systemic metabolic acidosis and hypothesized that the retina would respond with altered expression of genes involved in acid/base regulation.

METHODS

Systemic metabolic acidosis was induced in Long-Evans rats for up to 2 weeks by adding NH4Cl to the drinking water. After 2 weeks, venous pH was 7.25 ± 0.08 (SD) and [HCO3-] was 21.4 ± 4.6 mM in acidotic animals; pH was 7.41 ± 0.03 and [HCO3-] was 30.5 ± 1.0 mM in controls. Retinal mRNAs were quantified by quantitative reverse transcription polymerase chain reaction. Protein was quantified with Western blots and localized by confocal microscopy. Retinal [H+]o was measured in vivo with pH microelectrodes in animals subjected to metabolic acidosis and in controls.

RESULTS

NH4Cl in drinking water or given intravenous was effective in acidifying the retina. Cariporide, a blocker of Na+/H+ exchange, further acidified the retina. Metabolic acidosis for 2 weeks led to increases of 40-100% in mRNA for carbonic anhydrase isoforms II (CA-II) and XIV (CA-XIV) and acid-sensing ion channels 1 and 4 (ASIC1 and ASIC4) (all p < 0.005). Expression of anion exchange protein 3 (AEP-3) and Na+/H+ exchanger (NHE)-1 also increased by ≥50% (both p < 0.0001). Changes were similar after 1 week of acidosis. Protein for AEP-3 doubled. NHE-1 co-localized with vascular markers, particularly in the outer plexiform layer. CA-II was located in the neural parenchyma of the ganglion cell layer and diffusely in the rest of the inner retina.

CONCLUSIONS

The retina responds to systemic acidosis with increased expression of proton and bicarbonate exchangers, carbonic anhydrase, and ASICs. While responses to acidosis are usually associated with renal regulation, these studies suggest that the retina responds to changes in local pH presumably to control its acid/base environment in response to systemic acidosis.

Increased Retinal Oxygen Metabolism Precedes Microvascular Alterations in Type 1 Diabetic Mice

Purpose

To investigate inner retinal oxygen metabolic rate (IRMRO₂) during early stages of type 1 diabetes in a transgenic mouse model.

Methods

In current study, we involved seven diabetic mice (Akita/+, TSP1^−/−) and seven control mice (TSP1^−/−), and applied visible-light optical coherence tomography (vis-OCT) to image functional parameters including retinal blood flow rate, oxygen saturation (sO₂) and the IRMRO₂ value longitudinally from 5 weeks of age to 13 weeks of age. After imaging at 13 weeks of age, we analyzed the imaging results, and examined histology of mouse retina.

Results

Between diabetic mice and the control group, we observed significant differences in venous sO₂ from 9 weeks of age (P = 0.006), and significant increment in IRMRO₂ from 11 weeks of age (P = 0.001) in diabetic mice compared with control group. We did not find significant differences in retinal blood flow rate as well as arterial sO₂ during imaging between diabetic and control mice. Histologic examination of diabetic and control mice at 13 weeks of age also revealed no anatomical retinal alternations.

Conclusions

In diabetic retinopathy, complications in retinal oxygen metabolism may occur before changes of retinal anatomical structure.

Retinal oxygen: from animals to humans

This article discusses retinal oxygenation and retinal metabolism by focusing on measurements made with two of the principal methods used to study O₂ in the retina: measurements of PO₂ with oxygen-sensitive microelectrodes in vivo in animals with a retinal circulation similar to that of humans, and oximetry, which can be used non-invasively in both animals and humans to measure O₂ concentration in retinal vessels. Microelectrodes uniquely have high spatial resolution, allowing the mapping of PO₂ in detail, and when combined with mathematical models of diffusion and consumption, they provide information about retinal metabolism. Mathematical models, grounded in experiments, can also be used to simulate situations that are not amenable to experimental study. New methods of oximetry, particularly photoacoustic ophthalmoscopy and visible light optical coherence tomography, provide depth-resolved methods that can separate signals from blood vessels and surrounding tissues, and can be combined with blood flow measures to determine metabolic rate. We discuss the effects on retinal oxygenation of illumination, hypoxia and hyperoxia, and describe retinal oxygenation in diabetes, retinal detachment, arterial occlusion, and macular degeneration. We explain how the metabolic measurements obtained from microelectrodes and imaging are different, and how they need to be brought together in the future. Finally, we argue for revisiting the clinical use of hyperoxia in ophthalmology, particularly in retinal arterial occlusions and retinal detachment, based on animal research and diffusion theory.

Development of diabetes-induced acidosis in the rat retina

We hypothesized that the retina of diabetic animals would be unusually acidic due to increased glycolytic metabolism. Acidosis in tumors and isolated retina has been shown to lead to increased VEGF. To test the hypothesis we have measured the transretinal distribution of extracellular H(+) concentration (H(+)-profiles) in retinae of control and diabetic dark-adapted intact Long-Evans rats with ion-selective electrodes. Diabetes was induced by intraperitoneal injection of streptozotocin. Intact rat retinae are normally more acidic than blood with a peak of [H(+)]o in the outer nuclear layer (ONL) that averages 30 nM higher than H(+) in the choroid. Profiles in diabetic animals were similar in shape, but diabetic retinae began to be considerably more acidic after 5 weeks of diabetes. In retinae of 1-3 month diabetics the difference between the ONL and choroid was almost twice as great as in controls. At later times, up to 6 months, some diabetics still demonstrated abnormally high levels of [H(+)]o, but others were even less acidic than controls, so that the average level of acidosis was not different. Greater variability in H(+)-profiles (both between animals and between profiles recorded in one animal) distinguished the diabetic retinae from controls. Within animals, this variability was not random, but exhibited regions of higher and lower H(+). We conclude that retinal acidosis begins to develop at an early stage of diabetes (1-3 months) in rats. However, it does not progress, and the acidity of diabetic rat retina was diminished at later stages (3-6 months). Also the diabetes-induced acidosis has a strongly expressed local character. As result, the diabetic retinas show much wider variability in [H(+)] distribution than controls. pH influences metabolic and neural processes, and these results suggest that local acidosis could play a role in the pathogenesis of diabetic retinopathy.

Light-induced pH changes in the intact retinae of normal and early diabetic rats

Double-barreled H(+)-selective microelectrodes were used to measure local extracellular concentration of H(+) ([H(+)]o) in the retina of dark-adapted anesthetized Long-Evans rats. The microelectrode advanced in steps of 30 μm throughout the retina from the vitreal surface to retinal pigment epithelium and then to the choroid, recording changes in [H(+)]o evoked by light stimulation. Recordings were performed in diabetic rats 1-3 months after intraperitoneal injection of streptozotocin and the results were compared with data obtained in age-matched control animals. Brief light stimulation (2.5 s) evoked changes of [H(+)]o with amplitudes of a few nM. Throughout the retina, there was a transient initial acidification for ∼200 ms followed by steady alkalinization, although amplitudes and kinetics of these components were slightly variable in different retinal layers. No significant difference was found when the light-induced [H(+)]o changes recorded in various retinal layers of early diabetic rats were compared with the [H(+)]o changes from corresponding layers of control animals. Also, when H(+)-selective microelectrodes were located in the retinal pigment epithelium (RPE) layer, an increase in H(+) was recorded, whose time course and amplitude were similar in control and diabetic rats. However, a striking difference between light-induced [H(+)]o changes in controls and diabetics was observed in the choriocapillaris, in the thin layer (10-20 μm) distal to the basal membrane of the RPE. In control rats, choroidal [H(+)]o decreased in a few cases, but much more often practically did not change. In contrast, diabetic rats demonstrated either an increase (in half of the cases) or no change in choroidal [H(+)]o. The data suggest that the active participation of the choroidal blood supply in stabilization of [H(+)]o could be partially compromised already at early stages of diabetes in rats. Interestingly, it appeared that the acid removal by the choroidal circulation was compromised most after 1 month of diabetes and tended to improve later.

Spontaneous Fluctuations of PO2 in the Rabbit Somatosensory Cortex

In many tissues, PO2 fluctuates spontaneously with amplitudes of a few mmHg. Here we further characterized these oscillations. PO2 recordings were made from the whisker barrel cortex of six rabbits with acutely or chronically placed polarographic electrodes. Measurements were made while rabbits were awake and while anesthetized with isoflurane, during air breathing, and during 100% oxygen inspiration. In awake rabbits, 90% of the power was between 0 and 20 cycles per minute (cpm), not uniformly distributed over this range, but with a peak frequently near 10 cpm. This was much slower than heart or respiratory rhythms and is similar to the frequency content observed in other tissues. During hyperoxia, total power was higher than during air-breathing, and the dominant frequencies tended to shift toward lower values (0-10 cpm). These observations suggest that at least the lower frequency fluctuations represent efforts by the circulation to regulate local PO2. There were no consistent changes in total power during 0.5 or 1.5% isoflurane anesthesia, but the power shifted to lower frequencies. Thus, both hyperoxia and anesthesia cause characteristic, but distinct, changes in spontaneous fluctuations. These PO2 fluctuations may be caused by vasomotion, but other factors cannot be ruled out.

Association of Diabetic Macular Nonperfusion With Outer Retinal Disruption on Optical Coherence Tomography

IMPORTANCE

Diabetic macular nonperfusion leads to decreased perifoveal capillary blood flow, which in turn causes chronic ischemia of the retinal tissue. Using point-to-point correlation between spectral-domain optical coherence tomography (SD-OCT) and nonperfusion on fluorescein angiography, we observed that retinal capillary nonperfusion is associated with photoreceptor compromise on OCT. This study highlights a new concept of a possible contribution of the retinal deep capillary plexus to photoreceptor compromise in diabetic retinopathy in the absence of diabetic macular edema.

OBJECTIVE

To report outer retinal structural changes associated with enlargement of the foveal avascular zone and/or capillary nonperfusion in the macular area of diabetic patients.

DESIGN, SETTING, AND PARTICIPANTS

Retrospective observational cross-sectional study in 9 patients who were diagnosed as having diabetic retinopathy without diabetic macular edema and underwent fluorescein angiography and SD-OCT for diabetic retinopathy from July 8, 2014, to December 1, 2014, at a tertiary academic referral center. This analysis was conducted between December 2, 2014, and January 31, 2015.

MAIN OUTCOMES AND MEASURES

Outer retinal changes on SD-OCT in areas of macular ischemia.

RESULTS

The study included 13 eyes of 9 diabetic patients (4 men and 5 women aged 34-58 years) with a mean duration of diabetes mellitus of 14.5 years. Nine eyes showed outer retinal disruption revealed by SD-OCT that colocalized to areas of enlargement of the foveal avascular zone and macular capillary nonperfusion. Four fellow eyes with normal foveal avascular zones did not show any retinal changes on SD-OCT.

CONCLUSIONS AND RELEVANCE

Macular ischemia in diabetic patients can be associated with photoreceptor compromise. The presence of disruption of the photoreceptors on OCT in diabetic patients can be a manifestation of underlying capillary nonperfusion in eyes without diabetic macular edema. Ischemia at the deep capillary plexus may play an important role in these outer retinal changes.

Simultaneous optical coherence tomography angiography and fluorescein angiography in rodents with normal retina and laser-induced choroidal neovascularization

Fluorescein angiography (FA) is the current clinical imaging standard for vascular related retinal diseases such as macular degeneration and diabetic retinopathy. However, FA is considered invasive and can provide only two-dimensional imaging. In comparison, optical coherence tomography angiography (OCTA) is noninvasive and can generate three-dimensional imaging; investigations of OCTA already demonstrated great promise in retinal vascular imaging. Yet, to further develop and apply OCTA, strengths and weaknesses between OCTA and FA need to be thoroughly compared. To avoid complications in image registration, an ideal comparison requires co-registered and simultaneous imaging by both FA and OCTA. In this Letter, we developed a system with integrated laser-scanning ophthalmoscope FA (SLO-FA) and OCTA, and conducted simultaneous dual-modality retinal vascular imaging in rodents. In imaging healthy rodent eyes, OCTA can resolve retinal capillaries better than SLO-FA does, particularly deep capillaries. In imaging rodent eyes with laser-induced choroidal neovascularization (CNV), OCTA can identify CNV that eludes SLO-FA detection.

Visible light optical coherence tomography measures retinal oxygen metabolic response to systemic oxygenation

The lack of capability to quantify oxygen metabolism noninvasively impedes both fundamental investigation and clinical diagnosis of a wide spectrum of diseases including all the major blinding diseases such as age-related macular degeneration, diabetic retinopathy, and glaucoma. Using visible light optical coherence tomography (vis-OCT), we demonstrated accurate and robust measurement of retinal oxygen metabolic rate (rMRO₂) noninvasively in rat eyes. We continuously monitored the regulatory response of oxygen consumption to a progressive hypoxic challenge. We found that both oxygen delivery, and rMRO₂ increased from the highly regulated retinal circulation (RC) under hypoxia, by 0.28 ± 0.08 μL min^-1 (p < 0.001), and 0.20 ± 0.04 μL min^-1 (p < 0.001) per 100 mmHg systemic pO₂ reduction, respectively. The increased oxygen extraction compensated for the deficient oxygen supply from the poorly regulated choroidal circulation. Results from an oxygen diffusion model based on previous oxygen electrode measurements corroborated our in vivo observations. We believe that vis-OCT has the potential to reveal the fundamental role of oxygen metabolism in various retinal diseases.

Increased intraretinal PO2 in short-term diabetic rats

In diabetic retinopathy, neovascularization is hypothesized to develop due to hypoxia in the retina. However, evidence for retinal hypoxia is limited, and the progressive changes in oxygenation are unknown. The objective of this study was to determine if retinal hypoxia occurs early in the development of diabetes. Intraretinal oxygen (PO2) profiles were recorded with oxygen-sensitive microelectrodes in control and diabetic Long-Evans rats at 4 and 12 weeks after induction of diabetes. Diabetes did not affect oxygen consumption in the photoreceptors in either dark or light adaptation. Oxygenation of the inner retina was not affected after 4 weeks of diabetes, although vascular endothelial growth factor levels increased. At 12 weeks, average inner retinal PO2, normalized to choriocapillaris PO2, was higher in diabetic rats than in age-matched controls, which was opposite to what was expected. Thus retinal hypoxia is not a condition of early diabetes in rat retina. Increased inner retinal PO2 may occur because oxygen consumption decreases in the inner retina.

Diabetes changes expression of genes related to glutamate neurotransmission and transport in the Long-Evans rat retina

PURPOSE

This study investigated changes in the transcript levels of genes related to glutamate neurotransmission and transport as diabetes progresses in the Long-Evans rat retina. Transcript levels of vascular endothelial growth factor (VEGF), erythropoietin, and insulin-like growth factor binding protein 3 (IGFBP3) were also measured due to their protective effects on the retinal vasculature and neurons.

METHODS

Diabetes was induced in Long-Evans rats with a single intraperitoneal (IP) injection of streptozotocin (STZ; 65 mg/kg) in sodium citrate buffer. Rats with blood glucose >300 mg/dl were deemed diabetic. Age-matched controls received a single IP injection of sodium citrate buffer only. The retinas were dissected at 4 and 12 weeks after induction of diabetes, and mRNA and protein were extracted from the left and right retinas of each rat, respectively. Gene expression was analyzed using quantitative real-time reverse-transcription PCR. Enzyme-linked immunosorbent assay was used to quantify the concentration of VEGF protein in each retina. Statistical significance was determined using 2×2 analysis of variance followed by post-hoc analysis using Fisher's protected least squares difference.

RESULTS

Transcript levels of two ionotropic glutamate receptor subunits and one glutamate transporter increased after 4 weeks of diabetes. In contrast, 12 weeks of diabetes decreased the transcript levels of several genes, including two glutamate transporters, four out of five N-methyl-D-aspartate (NMDA) receptor subunits, and all five kainate receptor subunits. Diabetes had a greater effect on gene expression of NMDA and kainate receptor subunits than on the α-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) receptor subunits, for which only GRIA4 significantly decreased after 12 weeks. VEGF protein levels were significantly increased in 4-week diabetic rats compared to age-matched control rats whereas the increase was not significant after 12 weeks. Transcript levels of VEGF and VEGF receptors were unchanged with diabetes. Erythropoietin and IGFBP3 mRNA levels significantly increased at both time points, and IGFBP2 mRNA levels increased after 12 weeks.

CONCLUSIONS

Diabetes caused significant changes in the transcriptional expression of genes related to ionotropic glutamate neurotransmission, especially after 12 weeks. Most genes with decreased transcript levels after 12 weeks were expressed by retinal ganglion cells, which include glutamate transporters and ionotropic glutamate receptors. Two genes expressed by retinal ganglion cells but unrelated to glutamate neurotransmission, γ-synuclein (SNCG) and adenosine A1 receptor (ADORA1), also had decreased mRNA expression after 12 weeks. These findings may indicate ganglion cells were lost as diabetes progressed in the retina. Decreased expression of the glutamate transporter SLC1A3 would lead to decreased removal of glutamate from the extracellular space, suggesting that diabetes impairs this function of Müller cells. These findings suggest that ganglion cells were lost due to glutamate excitotoxicity. The changes at 12 weeks occurred without significant changes in retinal VEGF protein or mRNA, although higher VEGF protein levels at 4 weeks may be an early protective response. Increased transcript levels of erythropoietin and IGFBP3 may also be a protective response.

24-h core temperature in obese and lean men and women

Maintenance of core temperature is a major component of 24-h energy expenditure, and its dysregulation could contribute to the pathophysiology of obesity. The relationship among temperature, sex, and BMI, however, has not been fully elucidated in humans. This study investigated core temperature in obese and lean individuals at rest, during 20-min exercise, during sleep, and after food consumption. Twelve lean (18.5-24.9 kg/m(2)) and twelve obese (30.0-39.9 kg/m(2)) healthy participants, ages 25-40 years old, were admitted overnight in a clinical research unit. Females were measured in the follicular menstrual phase. Core temperature was measured every minute for 24 h using the CorTemp system, a pill-sized sensor that measures core temperature while in the gastrointestinal tract and delivers the measurement via a radio signal to an external recorder. Core temperature did not differ significantly between the obese and lean individuals at rest, postmeals, during exercise, or during sleep (P > 0.5), but core temperature averaged over the entire study was significantly higher (0.1-0.2 °C) in the obese (P = 0.023). Each individual's temperature varied considerably during the study, but at all times, and across the entire study, women were ~0.4 °C warmer than men (P < 0.0001). These data indicate that obesity is not associated with a lower core temperature but that women have a higher core temperature than men at rest, during sleep, during exercise, and after meals.

Oxygen consumption and distribution in the Long-Evans rat retina

The purpose of this study was to investigate the oxygen distribution and consumption in the pigmented Long-Evans rat retina in vivo during dark and light adaptation, and to compare these results to previous work on cat and albino rat. Double-barreled microelectrodes recorded both intraretinal PO(2) depth profiles and the electroretinogram (ERG), which was used to identify the boundaries of the retina. Light adaptation decreased photoreceptor oxygen consumption per unit volume (Q(av)) from 3.0 ± 0.4 ml·100 g(-1) min(-1) (mean ± SEM) in darkness to 1.8 ± 0.2 ml·100 g(-1) min(-1) and increased minimum outer retinal PO(2) at the inner segments (P(min)) from 17.4 ± 3.0 to 29.9 ± 5.3 mmHg. The effects of light on outer retinal PO(2) and Q(av) were similar to those previously observed in cat, monkey, and albino rats; however, dark-adapted P(min) was higher in rat than cat. The parameters derived from fitting the oxygen diffusion model to the rat data were compared to those from cat. Oxygen consumption of the inner segments (Q(2)) and choroidal PO(2) (P(C)) in rat and cat were similar. P(min) was higher in rat than in cat for two reasons: first, rat photoreceptors have a shorter oxygen consuming region; and second, the retinal circulation supplied a greater fraction of consumed oxygen to rat photoreceptors. The average PO(2) across the inner retina (P(IR)) was not different in dark adaptation (25.4 ± 4.8 mmHg) and light adaptation (28.8 ± 5.4 mmHg) when measured from PO(2) profiles. However, with the microelectrode stationary at 9-18% retinal depth, a small consistent decrease in PO(2) occurred during illumination. Flickering light at 6 Hz decreased inner retinal PO(2) significantly more than an equivalent steady illumination, suggesting that changes in blood flow did not completely compensate for increased metabolism. This study comprehensively characterized rat retinal oxygenation in both light and dark, and determined the similarities and differences between rat and cat retinas.

Effect of isoflurane on brain tissue oxygen tension and cerebral autoregulation in rabbits

Oxygen tension (PO(2)) was measured in rabbit whisker barrel cortex using oxygen sensitive electrodes to investigate the impact of isoflurane anesthesia on oxygen autoregulation. Responses to 90s episodes of 100% oxygen inspiration were obtained from rabbits before anesthesia, and then when the animals were anesthetized with 0.5% or 1.5% isoflurane. For each episode, ΔPO(2) (i.e., hyperoxic PO(2) minus baseline PO(2)) was computed. Compared to the conscious state, brain ΔPO(2) increased during anesthesia with 1.5% isoflurane (0.73 MAC) by an average of 116%, whereas 0.5% isoflurane produced an insignificant average increase of 31%. The results suggest that moderate levels of isoflurane impaired autoregulation of brain tissue oxygen tension.

Decreased circulation in the feline choriocapillaris underlying retinal photocoagulation lesions

PURPOSE

To investigate the effects of argon laser photocoagulation on the choroidal circulation in cats.

METHODS

Three sizes of argon laser lesions designed to damage the outer retina were created in six cats: larger than 1 mm, 500 μm, and 200 μm. At least 1 month after the lesions, damage to the choroidal vasculature was studied in two ways. First, scanning laser ophthalmoscopy was used to obtain infrared reflectance (IR) photographs and indocyanine green (ICG) angiograms. Second, fluorescent microspheres (15 μm) were injected into the left ventricle. The globes were fixed, the choroid was flat mounted, and images were taken with a fluorescence microscope. Retinal histology was assessed in comparable lesions.

RESULTS

Histology showed that the inner retina was preserved, but the choroid, tapetum, and outer retina were damaged. ICG angiograms revealed choriocapillaris loss in large lesions and in some 500-μm lesions, whereas the larger vessels were preserved; in 200 μm lesions, choriocapillaris loss was not detectable. However, in all lesions, the distribution of microspheres revealed little if any choriocapillaris flow. In larger lesions, the damaged region was surrounded by an area in which the number of microspheres was higher than in the lesion but lower than in the normal retina.

CONCLUSIONS

Under lesions that destroyed photoreceptors, the choriocapillaris was also compromised, even when no change could be detected with ICG angiography. Panretinal photocoagulation is designed to increase retinal PO2 by allowing choroidal oxygen to reach the inner retina, but its effectiveness may be limited by damage to the choriocapillaris.

Metabolic responses to light in monkey photoreceptors

PURPOSE

Transient changes in intraretinal oxygen tension (PO(2)) in response to light stimuli were studied in order to understand the dynamics of light-evoked changes in photoreceptor oxidative metabolism.

METHODS

PO(2) changes during illumination were recorded by double-barreled microelectrodes in the outer part of the perifoveal retina in five macaques (Rhesus and Cynomolgus) and were fitted to a single exponential equation to obtain the time constant (tau) and maximum PO(2) change.

RESULTS

At the onset of light, PO(2) increased at all illuminations in all animals. The magnitude of the light-evoked PO(2) change increased with increasing illumination over 3-4 log units but decreased in all animals at the maximum illumination. The median time constant of the PO(2) change (tau) was 26 sec and was not correlated with illumination. The time constant for the return to darkness was similar for illuminations below rod saturation. Since O(2) diffusion is fast over the short distance from the choroid to the inner segments, tau reflects the time course of the underlying change in oxidative metabolism.

CONCLUSIONS

Previous results suggested that two competing processes influence the change in photoreceptor oxidative metabolism with light, Na(+)/K(+) pumping and cyclic guanosine monophosphate (cGMP) turnover. Because a single exponential fitted the PO(2) data, it appears that these processes have time constants that differ by no more than a few seconds in primate. In monkeys, tau is longer than previously reported values for cat and rat. Longer time constants are related to larger photoreceptor volume, possibly because metabolic rate is controlled by intracellular Na(+), and a change in intracellular Na(+) after the onset of illumination occurs more slowly in larger photoreceptors. The "metabolic threshold" illumination that reduced oxygen consumption by about 10% is approximately the same as the illumination that closes 10% of the light-dependent cation channels that are open in the dark.

Is obesity associated with lower body temperatures? Core temperature: a forgotten variable in energy balance

The global increase in obesity, along with the associated adverse health consequences, has heightened interest in the fundamental causes of excessive weight gain. Attributing obesity to "gluttony and sloth", blaming the obese for overeating and limiting physical activity, oversimplifies a complex problem, since substantial differences in metabolic efficiency between lean and obese have been decisively demonstrated. The underlying physiological basis for these differences have remained poorly understood. The energetic requirements of homeothermy, the maintenance of a constant core temperature in the face of widely divergent external temperatures, accounts for a major portion of daily energy expenditure. Changes in body temperature are associated with significant changes in metabolic rate. These facts raise the interesting possibility that differences in core temperature may play a role in the pathophysiology of obesity. This review explores the hypothesis that lower body temperatures contribute to the enhanced metabolic efficiency of the obese state.

Gene expression patterns in hypoxic and post-hypoxic adult rat retina with special reference to the NMDA receptor and its interactome

PURPOSE

A gene expression analysis of hypoxic rat retina was undertaken to gain a deeper understanding of the possible molecular mechanisms that underlie hypoxia-induced retinal pathologies and identify possible therapeutic targets.

METHODS

Rats were made severely hypoxic (6%-7% O(2)) for 3 h. Some rats were sacrificed at this time, and others were allowed to recover for 24 h under normoxic conditions. A focused oligonucleotide microarray of 1,178 genes, qRT-PCR of selected transcripts, and western analysis of hypoxia inducible factor-1alpha (HIF-1alpha) were used to compare retinas from the hypoxic and recovery groups to control animals that were not made hypoxic. SAM analysis was used to identify statistically significant changes in microarray data, and the bioinformatics programs GoMiner, Gene Set Enrichment Analysis (GSEA), and HiMAP were used to identify significant ontological categories and analyze the N-methyl-D-aspartate (NMDA) receptor interactome.

RESULTS

HIF-1alpha protein, but not mRNA, was elevated up to 15-fold during hypoxia, beginning at 0.5 h, the shortest duration examined. Of the total of 1,178 genes examined by microarray, 119 were significantly upregulated following hypoxia. Of these, 86 were still significantly upregulated following recovery. However, 24 genes were significantly downregulated following hypoxia, with 12 still significantly downregulated after recovery. Of the 1035 genes that did not change with hypoxia, the expression of 36 genes was significantly changed after recovery. Ontological analyses showed significant upregulation of a large number of genes in the glutamate receptor family, including 3 of the 5 NMDA subunits. qRT-PCR analysis further corroborated these findings. Genes known to directly interact specifically with the NR1 subunit of the NMDA receptor were identified using HiMAP databases. GSEA analysis revealed that these genes were not affected by either hypoxia or altered after recovery.

CONCLUSIONS

The identification of gene expression alterations as a function of hypoxia and recovery from hypoxia is important to understand the molecular mechanisms underlying retinal dysfunction associated with a variety of diseases. Gene changes were identified in hypoxic retina that could be linked to specific networks. Retinas recovering from hypoxia also showed distinct patterns of gene expression that were different from both normoxic control retinas and hypoxic retinas, indicating that hypoxia initiates a complex pattern of gene expression. Diseases of which hypoxia is a component may exhibit the several changes found here. Several potential therapeutic targets have been identified by our approach, including modulation of NMDA receptor expression and signaling, which until now have only been shown to play a role in responding to ischemia.

Do the obese have lower body temperatures? A new look at a forgotten variable in energy balance

Understanding the pathogenesis of obesity is now more important than ever, given the remarkable world-wide epidemic. This paper explores the potential role of core temperature in energy balance, and develops the hypothesis that basal temperature and changes in the temperature response in various situations contribute to the enhanced metabolic efficiency of the obese state. The argument is based on the important contribution that heat production makes in establishing the basal or resting metabolic rate, as well as on an analysis of the adaptive role played by changes in temperature in response to environmental challenge. If this hypothesis is validated, new therapeutic approaches may ensue.

Effect of carbogen (95% O2/5% CO2) on retinal oxygenation in dark-adapted anesthetized cats

This work assessed the relative effects of inspiring carbogen (95% O(2)/5% CO2) and 100% O2 on intraretinal PO2 and oxygen consumption in the cat retina. Oxygen microelectrodes were used to measure the distribution of oxygen in the central retina of dark-adapted anesthetized cats during normoxia, 100% O2 breathing, and carbogen breathing. Profiles of oxygen tension (PO2) as a function of retinal depth were recorded. Changes in PO2 caused by the transient administration of carbogen and 100% oxygen were also measured at selected retinal depths. Average PO2 values at the choroid, at the boundary between the inner and outer retina, and across the inner retina were significantly higher during inspiration of carbogen than 100% O2. There were no significant differences among conditions in outer retinal oxygen consumption. During the transient gas administration, average changes in PO2 generally increased with depth. At the end of gas administration, the decay of PO2 within the retina occurred quickly, meaning that short-term gas administration may have little therapeutic value.

Oxygen distribution and consumption in the macaque retina

The oxygen distribution in the retina of six anesthetized macaques was investigated as a model for retinal oxygenation in the human retina in and adjacent to the fovea. Po2 was measured as a function of retinal depth under normal physiological conditions in light and dark adaptation with O2 microelectrodes. Oxygen consumption (Qo2) of the photoreceptors was extracted by fitting a steady-state diffusion model to Po2 measurements. In the perifovea, the Po2 was 48 ± 13 mmHg (mean and SD) at the choroid and fell to a minimum of 3.8 ± 1.9 mmHg around the photoreceptor inner segments in dark adaptation, rising again toward the inner retina. The Po2 in the inner half of the retina in darkness was 17.9 ± 7.8 mmHg. When averaged over the outer retina, photoreceptor Qo2 (called Qav) was 4.6 ± 2.3 ml O2·100 g−1·min−1 under dark-adapted conditions. Illumination sufficient to saturate the rods reduced Qav to 72 ± 11% of the dark-adapted value. Both perifoveal and foveal photoreceptors received most of their O2 from the choroidal circulation. While foveal photoreceptors have more mitochondria, the Qo2 of photoreceptors in the fovea was 68% of that in the perifovea. Oxygenation in macaque retina was similar to that previously found in cats and other mammals, reinforcing the relevance of nonprimate animal models for the study of retinal oxygenation, but there was a smaller reduction in Qo2 with light than observed in cats, which may have implications for understanding the influence of light under some clinical conditions.

Hyperoxia improves oxygen consumption in the detached feline retina

PURPOSE

To investigate the effects of hyperoxia on retinal oxygenation and oxygen consumption in the detached feline retina.

METHODS

Retinal detachment was created in nine intact anesthetized cats by injecting 0.25% sodium hyaluronate in balanced salt solution into the subretinal space. Oxygen microelectrodes were used to collect spatial profiles of retinal Po(2) in both the attached and detached retina. A diffusion model was fitted to quantify photoreceptor oxygen consumption (Q(av)).

RESULTS

In the detached retina, the Po(2) at the border between the retina and the fluid layer under the retina decreased; hyperoxia increased it to a level that was not significantly different from the control (attached retina, air breathing). Detachment did not change the Po(2) at the border between the avascular and vascularized retina; hyperoxia significantly increased the level. Oxygen consumption decreased to 47% +/- 18% of the control value in the detached retina during normoxia; hyperoxia increased Q(av) to 68% +/- 17% of control. Hyperoxia increased the average inner retinal Po(2) (P(IR)) in the detached retina to a level higher than that during normoxia. Detachment did not change P(IR) during normoxia.

CONCLUSIONS

Hyperoxia has been shown to improve photoreceptor survival in the detached retina. The present work suggests that hyperoxia is protective because it allowed increased photoreceptor oxygen consumption. Whereas normal Po(2)s were maintained at the inner and outer border of the avascular region during hyperoxia, Q(av) was not restored to normal, suggesting that other factors are involved in photoreceptor dysfunction during detachment in addition to insufficient oxygen delivery.

Determination of the core undergraduate BME curriculum--the 1st step in a Delphi study

The VaNTH Engineering Research Center for Bioengineering Education Technologies has completed the first round of a Delphi study to determine the key concepts that comprise the core curriculum of undergraduate programs in biomedical engineering. The study was conducted as a Web-based survey, consisting of eighty questions divided among nineteen topics, including eleven biomedical engineering domains, four biology domains, and mathematical and scientific prerequisites. Participants included representatives from academia, industry, and young alumni of undergraduate BME programs. Results from the survey will be available at: http://www.vanth.org/curriculum/.

The Electroretinogram Components in Abyssinian Cats with Hereditary Retinal Degeneration

PURPOSE

To examine phototransduction using the a-wave and other aspects of retinal function with the intraretinal b- and c-waves at different stages of an inherited photoreceptor degeneration in Abyssinian cats.

METHODS

Vitreal and intraretinal ERGs were recorded from eight dark-adapted, anesthetized Abyssinian cats. Brief bright flashes were used to elicit vitreal a- and b-waves. Longer, weaker flashes were used to elicit intraretinal b- and c-waves. Stages 1 through 4 of the disease were characterized ophthalmoscopically. Parameters of the Lamb and Pugh a-wave model (a(max), A, and t(eff)) for the Abyssinian cats were compared with those for normal cats. Light microscopy was used to count photoreceptor nuclei.

RESULTS

The maximum a-wave amplitude, a(max), was significantly smaller in stage 1, and continued to decrease (stage 1: 50% of normal, stage 2: 28%, stage 3: 27%; and stage 4: unrecordable). There was a small, but not significant, decrease in the amplification constant A from 0.24 +/- 0.11 s(-2) in normal cats to 0.16 +/- 0.08 s(-2) in Abyssinian cats. The intraretinal b- and c-wave amplitudes decreased most dramatically during the early stage of the disease. Affected animals had fewer photoreceptors than unaffected Abyssinians or control animals. The number of photoreceptors declined most rapidly in the inferior periphery.

CONCLUSIONS

The amplitudes of all ERG components were already reduced significantly by stage 1 and progressively declined. The lack of major changes in a-wave model parameters indicates that the degeneration is probably not due to a mutation in transduction proteins. Losses of photoreceptor function were larger than losses of photoreceptor nuclei.

Retinal oxygenation and oxygen metabolism in Abyssinian cats with a hereditary retinal degeneration

PURPOSE

To investigate the effects of a hereditary retinal degeneration on retinal oxygenation and determine whether it is responsible for the severe attenuation of retinal circulation in hereditary photoreceptor degenerations.

METHODS

Seven adult Abyssinian cats affected by hereditary retinal degeneration were studied. Oxygen microelectrodes were used to collect spatial profiles of retinal oxygenation in anesthetized animals. A one-dimensional model of oxygen diffusion was fitted to the data to quantify photoreceptor oxygen utilization (Qo(2)).

RESULTS

Photoreceptor Qo(2) progressively decreased until it reached zero in the end stage of the disease. Average inner retinal oxygen tension remained within normal limits at all disease stages, despite the observed progressive retinal vessel attenuation. Light affected photoreceptors normally, decreasing Qo(2) by approximately 50% at all stages of the disease.

CONCLUSIONS

Loss of photoreceptor metabolism allows choroidal oxygen to reach the inner retina, attenuating the retinal circulation in this animal model of retinitis pigmentosa (RP) and probably also in human RP. As the degeneration progresses, there is a strong relationship between changes in the a-wave of the ERG and changes in rod oxidative metabolism, indicating that these two functional measures change together.

Effect of hypoxemia and hyperglycemia on pH in the intact cat retina

OBJECTIVE

To examine the effects of acute hypoxemia and hyperglycemia on retinal pH to understand hyperglycemia-induced changes in the normal intact cat retina.

METHODS

Spatial profiles of extracellular hydrogen ion (H+) concentration were obtained from the cat retina, in vivo, using pH-sensitive microelectrodes during normoxia (arterial partial pressure of oxygen [PaO2] = 114.5 +/- 7.9 mm Hg), normoglycemia (plasma glucose concentration, 117 +/- 19 mg/dL), acute hypoxemia (PaO2 = 29.5 +/- 2.2 mm Hg), and acute hyperglycemia (plasma glucose concentration, 303 +/- 67 mg/dL). An H+ diffusion model was fitted to the outer retinal data to quantify photoreceptor H+ production. The inner retinal pH was also examined.

RESULTS

Hypoxemia induced a mean acute panretinal acidification of 0.16 pH units that originated from a 2.55-fold increase in net photoreceptor H+ production. Hyperglycemia induced an acute panretinal acidification of 0.12 pH units; however, photoreceptor H+ production levels remained unchanged. Retinal pH changes followed the course of arterial PaO2 and blood glucose changes.

CONCLUSIONS

The increase in photoreceptor H+ production during hypoxemia confirms the importance of glycolysis in the retina. Hyperglycemia-induced pH changes resulted from either increased inner retinal H+ production or decreased H+ clearance/neutralization. Clinical Relevance The hyperglycemia-induced acidification that originates in the inner retina suggests that retinal acidosis may contribute to the development of diabetic retinal disease.

Retinal arterial occlusion leads to acidosis in the cat

This study investigated the changes in pH during retinal artery occlusion by means of extracellular H+ concentration ([H+]o) measurements in the retina under both air and 100% O2 ventilation. Occlusion was produced in intact anesthetised cats by pressing with a probe onto a retinal artery. [H+]o profiles were recorded across the retina with pH sensitive microelectrodes. The average inner retinal [H+]o increased during occlusion, resulting in an acidification of as much as 0.10 pH units, even under 100% O2 ventilation. The inner retinal H+ profile magnitude decreased during occlusion due to impaired clearance. The average outer retinal H+ profile magnitude also increased even though outer retinal H+ production did not increase during occlusion. This might be due to H+ diffusion from the inner retina to the outer retina, which is opposite to the flux in the normal retina. After reperfusion, [H+]o returned to its preocclusion value. In conclusion, arterial occlusion leads to acidification of the retina. Enhanced oxygenation during occlusion did not decrease this acidification. This may explain why increasing PO2 in the retina by enhanced O2 breathing improves retinal function during and after occlusion, but does not totally reverse the effect of occlusion.

Hyperoxia Promotes Electroretinogram Recovery after Retinal Artery Occlusion in Cats

PURPOSE

This work assessed the hypotheses that (1) hyperoxia is preferable to air breathing during retinal arterial occlusion, (2) hyperoxia during occlusion is beneficial in promoting recovery from arterial occlusion, and (3) hyperoxia has value even if it is delayed relative to the onset of the occlusion.

METHODS

Reversible branch retinal artery occlusion was produced by pressing with a glass probe onto an artery emerging from the superior part of the optic disc in the retina of anesthetized cats. During 2-hour occlusion episodes, the cats breathed 100% O(2), 1 hour of air and 1 hour of 100% O(2), 1 hour of air and 1 hour of 70% O(2), or air. Intraretinal ERGs were recorded before, during, and after the occlusion.

RESULTS

Hyperoxia during occlusion preserved intraretinal b-wave amplitude at 86% +/- 12% of normal; longer durations of increased oxygenation maintained the b-wave at higher levels during occlusion and increased the probability of b-wave recovery after occlusion; higher O(2) content in the breathing gas increased b-wave amplitude during recovery; and hyperoxia during occlusion decreased the time it took for the b-wave to recover after the occlusion.

CONCLUSIONS

Hyperoxia is preferable to air breathing during retinal arterial occlusion not only for maintaining b-wave amplitude during occlusion, but also for providing a shorter recovery time and better percentage recovery after the end of the occlusion. Even if it is not possible to begin hyperoxia at the onset of occlusion, it may still be valuable.

Retinal oxygen: fundamental and clinical aspects

We reviewed research on retinal oxygen (O2) distribution and use, focusing on O2 microelectrode studies in animals with circulatory patterns similar to those of humans. The inner and outer halves of the retina are different domains in terms of O2. Understanding their properties can suggest mechanisms of and therapies for retinal diseases. Inner retinal PO2 averages about 20 mm Hg. Effective O2 autoregulation of the retinal circulation ensures that inner retinal PO2 is relatively uninfluenced by systemic hypoxia and hyperoxia and increased intraocular pressure in healthy animals. Failures of the retinal circulation lead to tissue hypoxia that underlies the vasoproliferation in diabetic retinopathy and retinopathy of prematurity. Choroidal blood flow is not regulated metabolically, so systemic hypoxia and elevated intraocular pressure lead to decreases in choroidal PO2 and photoreceptor O2 consumption. The same lack of regulation allows choroidal PO2 to increase dramatically during hyperoxia, offering the potential for O2 to be used therapeutically in retinal vascular occlusive diseases and retinal detachment.

Effect of acute hyperglycemia on oxygen and oxidative metabolism in the intact cat retina

PURPOSE

The Crabtree effect is the phenomenon of inhibition of respiration by glycolysis, as a result of elevated glucose levels. It is not certain whether the Crabtree effect occurs in the retina, which has a high glycolytic capacity. In the current study, in vivo photoreceptor oxygen consumption was examined during the normo- and hyperglycemic states in the dark-adapted cat retina to determine whether the Crabtree effect occurs in the outer retina.

METHODS

Spatial profiles of oxygen tension were obtained in the cat retina, in vivo, with the use of oxygen microelectrodes during control conditions and acute (5.19 +/- 0.83 hour) episodes of hyperglycemia (blood glucose, >350 mg/dL). The outer retinal portions of the profiles were fitted to a model of oxygen diffusion to quantify photoreceptor oxygen consumption.

RESULTS

Photoreceptor oxygen consumption did not significantly change during hyperglycemia compared with control conditions. Choroidal PO(2) decreased during hyperglycemia by an average of 5.8 +/- 7.4 mm Hg. This led to an increase in the fraction of O(2) used by the photoreceptors that was derived from the inner retina. Choroidal PO(2) did not recover when blood glucose levels were returned to normal. Average inner retinal PO(2) was not affected by the episodes of hyperglycemia.

CONCLUSIONS

The Crabtree effect does not occur to any significant degree in the outer retina, because hyperglycemia did not affect photoreceptor oxygen consumption. Choroidal PO(2) decreased during hyperglycemia, and the oxygen deficit was made up by the retinal circulation.

Quantification of in vivo anaerobic metabolism in the normal cat retina through intraretinal pH measurements

We examined intraretinal [H+] in the intact retina of anesthetized cats using H+-sensitive microelectrodes to obtain spatial profiles of extracellular [H+]. One H+ is produced when an anaerobically generated ATP is utilized. We theorized that H+ production directly reflects anaerobic glucose consumption. From the choroid (pH approximately 7.40), [H+]o steadily increased to a maximum concentration in the proximal portion of the outer nuclear layer (pH approximately 7.20). The shape of the profile was always concave down, indicating that a net production of H+ occurred across the avascular outer retina. A three-layer diffusion model of the outer retina was developed and fitted to the data to quantify photoreceptor H+ extrusion into the extracellular space (Q(OR-H+)). It was determined that the outer segment (OS) layer had negligible H+ extrusion. The data were then refitted to a special three-layer model in which the OS layer Q(H+) was set equal to zero, but in which the inner segments and outer nuclear layer produced H+. The resulting Q(OR-H+) was several orders of magnitude lower than previous measurements of Q(OR-lactate), which were based on choroidal mass balances of lactate. Stoichiometrically, one H+ is produced for each lactate produced, so we concluded that Q(OR-H+) is a measure of net rather than total H+ production. Because retinal acid production is so high, the retina must contain efficient H+ clearance and/or neutralization mechanisms that prevent severe acidosis. The effect of light on retinal extracellular [H+] and Q(OR-H+) was also examined. As expected, light adaptation caused a retinal alkalinization that resulted from a 52% reduction in Q(OR-H+). This is in agreement with previous studies that have shown that both oxidative (e.g. Haugh et al., 1990) and glycolytic metabolism (Wang et al., 1997a,c) in the photoreceptor are decreased by a factor of 2 during light adaptation. Although we could not obtain absolute values for outer retinal glycolysis, changes in Q(OR-H+) appear to directly reflect changes in glycolytic metabolism.

Photoreceptor inner segments in monkey and human retina: mitochondrial density, optics, and regional variation

The present work quantifies aspects of photoreceptor structure related to mitochondria, inner segment dimensions, and optical properties, as a basis for furthering our understanding of rod and cone function. Electron-microscopic analyses were performed on the retina of one stumptail macaque (Macaca arctoides) to obtain stereological measurements of ellipsoid mitochondrial density, and sizes and shapes of outer and inner segments. In addition, the distribution of mitochondria and the optical properties of human foveal cones were examined with electron microscopy and Nomarski differential interference contrast (NDIC) imaging. Mitochondria comprised 74-85% of cone ellipsoids and 54-66% of rod ellipsoids in macaque. Ellipsoid volume increased with eccentricity by 2.4-fold for rods and more than 6-fold for cones over eccentricities to 12.75 mm, while the volume of the outer segment supported by the ellipsoid was essentially constant for both rods and cones. Per unit volume of outer segment, cones contained ten times as much mitochondria as rods. In human fovea, as in the rest of the retina, most cone mitochondria were located in the distal inner segment. In the foveal center, however, there are also mitochondria in the myoid, as well as in the outer fiber, proximal to the external limiting membrane (ELM). Analyses of the optical aperture of human foveal cones, the point at which their refractive index clearly differs from the extrareceptoral space, showed that it correlated well with the location of mitochondria, except in the foveal center, where the aperture appeared proximal to the ELM. While mitochondria have an important metabolic function, we suggest that the striking differences between rods and cones in mitochondrial content are unlikely to be determined by metabolic demand alone. The numerous cone mitochondria may enhance the waveguide properties of cones, particularly in the periphery.