Excitation and desensitization of mouse rod photoreceptors in vivo following bright adapting light

Sex-Specific Retinal Anomalies Induced by Chronic Social Defeat Stress in Mice

Major depressive disorder (MDD) is one of the most common consequences of chronic stress. Still, there is currently no reliable biomarker to detect individuals at risk to develop the disease. Recently, the retina emerged as an effective way to investigate psychiatric disorders using the electroretinogram (ERG). In this study, cone and rod ERGs were performed in male and female C57BL/6 mice before and after chronic social defeat stress (CSDS). Mice were then divided as susceptible or resilient to stress. Our results suggest that CSDS reduces the amplitude of both oscillatory potentials and a-waves in the rods of resilient but not susceptible males. Similar effects were revealed following the analysis of the cone b-waves, which were faster after CSDS in resilient mice specifically. In females, rod ERGs revealed age-related changes with no change in cone ERGs. Finally, our analysis suggests that baseline ERG can predict with an efficacy up to 71% the expression of susceptibility and resilience before stress exposition in males and females. Overall, our findings suggest that retinal activity is a valid biomarker of stress response that could potentially serve as a tool to predict whether males and females will become susceptible or resilient when facing CSDS.

Mouse Cones Adapt Fast, Rods Slowly In Vivo

Purpose

To study rod- and cone-driven adaptation dynamics separately, we used the silent substitution technique to selectively stimulate rods or cones in the Opn1lwLIAIS (LIAIS) mouse, in which the native M-cone pigment is replaced by a human L-cone pigment (L*).

Methods

ERG recordings were performed on anesthetized LIAIS mice. ERG stimuli were sinusoidally modulated. After 10 minutes of adaptation to 0.4 candela per square meter (cd/m2) ERGs were measured, followed by 11-minute adaptation to 8.8 cd/m2 background and recordings directly after the luminance increase and every second minute. Finally, during adaptation to 0.4 cd/m2 for 32 minutes, ERG responses were recorded directly after the change in background and every second minute. This protocol was repeated with rod-isolating stimuli (8 Hz; 75% rod contrast), L*-cone-isolating stimuli (12 Hz; 55% cone contrast) and white light (8 Hz and 12 Hz; 100% Michelson contrast).

Results

At 8.8 cd/m2, responses directly displayed photopic response properties without further changes in either cone or white light responses. Rod-driven responses were very small. After the return to 0.4 cd/m2, both rod-driven and white light responses increased over a time course of about 30 minutes. Cone-driven responses were very small. Response phases changed directly after a change in background without further alterations.

Conclusions

Rod- and cone-driven signal pathways display strongly different adaptation characteristics: adaptation of cone-driven responses to photopic conditions is very fast, whereas rod-driven responses change with a time course up to 30 minutes during scotopic conditions. Luminance responses are cone-driven at 8.8 cd/m2 and rod-driven at 0.4 cd/m2.

Restoration of high-sensitivity and adapting vision with a cone opsin

Inherited and age-related retinal degenerative diseases cause progressive loss of rod and cone photoreceptors, leading to blindness, but spare downstream retinal neurons, which can be targeted for optogenetic therapy. However, optogenetic approaches have been limited by either low light sensitivity or slow kinetics, and lack adaptation to changes in ambient light, and not been shown to restore object vision. We find that the vertebrate medium wavelength cone opsin (MW-opsin) overcomes these limitations and supports vision in dim light. MW-opsin enables an otherwise blind retinitis pigmenotosa mouse to discriminate temporal and spatial light patterns displayed on a standard LCD computer tablet, displays adaption to changes in ambient light, and restores open-field novel object exploration under incidental room light. By contrast, rhodopsin, which is similar in sensitivity but slower in light response and has greater rundown, fails these tests. Thus, MW-opsin provides the speed, sensitivity and adaptation needed to restore patterned vision.

In Vivo Efficacy of an Injectable Microsphere-Hydrogel Ocular Drug Delivery System

PURPOSE

Demonstrate in vivo that controlled and extended release of a low dose of anti-vascular endothelial growth factor (anti-VEGF) from a microsphere-hydrogel drug delivery system (DDS) has a therapeutic effect in a laser-induced rat model of choroidal neovascularization (CNV).

METHODS

Anti-VEGF (ranibizumab or aflibercept) was loaded into poly(lactic-co-glycolic acid) microspheres that were then suspended within an injectable poly(N-isopropylacrylamide)-based thermo-responsive hydrogel DDS.The DDS was shown previously to release bioactive anti-VEGF for ~200 days. CNV was induced using an Ar-green laser. The four experimental groups were as follows: (i) non-treated, (ii) drug-free DDS, (iii) anti-VEGF-loaded DDS, and (iv) bolus injection of anti-VEGF. CNV lesion areas were measured based on fluorescein angiograms and quantified using a multi-Otsu thresholding technique. Intraocular pressure (IOP) and dark-adapted electroretinogram (ERG) were also obtained pre- and post-treatment (1, 2, 4, 8, and 12 weeks).

RESULTS

The anti-VEGF-loaded DDS group had significantly smaller (60%) CNV lesion areas than non-treated animals throughout the study. A small transient increase in IOP was seen immediately after injection; however, all IOP measurements at all time points were within the normal range. There were no significant changes in ERG maximal response compared to pre-treatment measurements for the drug-loaded DDS, which suggests no adverse effects on retinal cellular function.

CONCLUSIONS

The current study demonstrates that the DDS can effectively decrease laser-induced CNV lesions in a murine model. Controlled and extended release from our DDS achieved greater treatment efficacy using an order of magnitude less drug than what is required with bolus administration. This suggests that our DDS may provide a significant advantage in the treatment of posterior segment eye diseases.

Deficient glucose and glutamine metabolism in Aralar/AGC1/Slc25a12 knockout mice contributes to altered visual function

PURPOSE

To characterize the vision phenotype of mice lacking Aralar/AGC1/Slc25a12, the mitochondrial aspartate-glutamate carrier mutated in global cerebral hypomyelination (OMIM 612949).

METHODS

We tested overnight dark-adapted control and aralar-deficient mice for the standard full electroretinogram (ERG) response. The metabolic stress of dark-adaptation was reduced by 5 min illumination after which the ERG response was monitored in darkness. We used the electrical response to two identical saturating light flashes (paired-flash stimulation) to isolate the inner retina and photoreceptor responses. Retinal morphology was examined with hematoxylin and eosin staining, immunohistochemistry of antibodies against retinal cells, and 4',6-diamidino-2-phenylindole (DAPI) labeling.

RESULTS

Aralar plays a pivotal role in retina metabolism as aralar provides de novo synthesis pathway for glutamine, protects glutamate from oxidation, and is required for efficient glucose oxidative metabolism. Aralar-deficient mice are not blind as their retinas have light-evoked activity. However, we report an approximate 50% decrease in the ERG amplitude response in the light-evoked activity of dark-adapted retinas from aralar-deficient mice, in spite of normal retina histology. The defective response is partly reversed by exposure to a brief illumination period, which lowers the metabolic stress of dark-adaptation. The metabolic stress and ERG alteration takes place primarily in photoreceptors, but the response to two flashes applied in fast succession also revealed an alteration in synaptic transmission consistent with an imbalance of glutamate and an energy deficit in the inner retina neurons.

CONCLUSIONS

We propose that compromised glucose oxidation and altered glutamine and glutamate metabolism in the absence of aralar are responsible for the phenotype reported.

Adaptive potentiation in rod photoreceptors after light exposure

Photoreceptors adapt to changes in illumination by altering transduction kinetics and sensitivity, thereby extending their working range. We describe a previously unknown form of rod photoreceptor adaptation in wild-type (WT) mice that manifests as a potentiation of the light response after periods of conditioning light exposure. We characterize the stimulus conditions that evoke this graded hypersensitivity and examine the molecular mechanisms of adaptation underlying the phenomenon. After exposure to periods of saturating illumination, rods show a 10-35% increase in circulating dark current, an adaptive potentiation (AP) to light exposure. This potentiation grows as exposure to light is extended up to 3 min and decreases with longer exposures. Cells return to their initial dark-adapted sensitivity with a time constant of recovery of ∼7 s. Halving the extracellular Mg concentration prolongs the adaptation, increasing the time constant of recovery to 13.3 s, but does not affect the magnitude of potentiation. In rods lacking guanylate cyclase activating proteins 1 and 2 (GCAP(-/-)), AP is more than doubled compared with WT rods, and halving the extracellular Mg concentration does not affect the recovery time constant. Rods from a mouse expressing cyclic nucleotide-gated channels incapable of binding calmodulin also showed a marked increase in the amplitude of AP. Application of an insulin-like growth factor-1 receptor (IGF-1R) kinase inhibitor (Tyrphostin AG1024) blocked AP, whereas application of an insulin receptor kinase inhibitor (HNMPA(AM)3) failed to do so. A broad-acting tyrosine phosphatase inhibitor (orthovanadate) also blocked AP. Our findings identify a unique form of adaptation in photoreceptors, so that they show transient hypersensitivity to light, and are consistent with a model in which light history, acting via the IGF-1R, can increase the sensitivity of rod photoreceptors, whereas the photocurrent overshoot is regulated by Ca-calmodulin and Ca(2+)/Mg(2+)-sensitive GCAPs.

Mouse b-wave mutants

The b-wave is a major component of the electroretinogram that reflects the activity of depolarizing bipolar cells (DBCs). The b-wave is used diagnostically to identify patients with defects in DBC signaling or in transmission from photoreceptors to DBCs. In mouse models, an abnormal b-wave has been used to demonstrate a critical role of a particular protein in the release of glutamate from photoreceptor terminals, in establishing the structure of the photoreceptor-to-DBC synapse, in DBC signal transduction, and also in DBC development, survival, or metabolic support. The purpose of this review is to summarize these models and how they have advanced our understanding of outer retinal function.

Recovery of rod photoresponses in ABCR-deficient mice

PURPOSE

The ABCR protein of the rod outer segment is thought to facilitate movement of the all-trans retinal photoproduct of rhodopsin bleaching out of the disc lumen. This study was undertaken to investigate the extent to which ABCR deficiency affects the post-bleach recovery of the rod photoresponse in ABCR-deficient (abcr-/-) mice.

METHODS

Electroretinographic (ERG) a-wave responses were recorded from abcr-/- mice and two control strains. A bright probe flash was used to examine the course of rod recovery after fractional rhodopsin bleaches of approximately 10(-6), approximately 3 x 10(-5), approximately 0.03, and approximately 0.30 to approximately 0.40.

RESULTS

Dark-adapted abcr-/- mice and control animals exhibited similar normalized near-peak amplitudes of the paired-flash-ERG-derived, weak-flash response. Response recovery after approximately 10(-6) bleaching exhibited an average exponential time constant of 319, 171, and 213 ms, respectively, in the abcr-/- and the two control strains. Recovery time constants determined for approximately 3 x 10(-5) bleaching did not differ significantly among strains. However, those determined for the approximately 0.03 bleach indicated significantly faster recovery in abcr-/- mice (2.34 +/- 0.74 minutes) than in the controls (5.36 +/- 2.20 and 5.92 +/- 2.44 minutes). After approximately 0.30 to approximately 0.40 bleaching, the initial recovery in the abcr-/- mice was, on average, faster than in control mice.

CONCLUSIONS

By comparison with control animals, abcr-/- mice exhibit faster rod recovery after a bleach of approximately 0.03. The data suggest that ABCR in normal rods may directly or indirectly prolong all-trans retinal clearance from the disc lumen over a significant bleaching range, and that the essential function of ABCR may be to promote the clearance of residual amounts of all-trans retinal that remain in the discs long after bleaching.

Test of the paired-flash electroretinographic method in mice lacking b-waves

Previous studies of rod photoreceptors in vivo have employed a paired-flash electroretinographic (ERG) technique to determine rod response properties. To test whether absence versus presence of the ERG b-wave affects the photoreceptor response derived by the paired-flash method, we examined paired-flash-derived responses obtained from nob mice, a mutant strain with a defect in signal transduction between photoreceptors and ON bipolar cells that causes a lack of the b-wave. Normal littermates of the nob mice served as controls. The normalized amplitude-intensity relation of the derived response determined in nob mice at the near-peak time of 86 ms was similar to that determined for the controls. The full time course of the derived rod response was obtained for test flash strengths ranging from 0.11 to 17.38 scotopic cd s m(-2) (sc cd s m(-2)). Time-course data obtained from nob and control mice exhibited significant but generally modest differences. With saturating test flash strengths, half-recovery times for the derived response of nob versus control mice differed by approximately 60 ms or less about the combined (nob and control) average respective values. Time course data also were obtained before versus after intravitreal injection of L-2-amino-4-phosphonobutyrate (APB) (which blocks transmission from photoreceptors to depolarizing bipolar cells) and of cis 2,3-piperidine dicarboxylic acid (PDA) (which blocks transmission to OFF bipolar cells, and to horizontal, amacrine and ganglion cells). Neither APB nor PDA substantially affected derived responses obtained from nob or control mice. The results provide quantitative information on the effect of b-wave removal on the paired-flash-derived response in mouse. They argue against a substantial skewing effect of the b-wave on the paired-flash-derived response obtained in normal mice and are consistent with the notion that, to good approximation, this derived response represents the isolated flash response of the photoreceptors in both nob and normal mice.

An adaptive ERG technique to measure normal and altered dark adaptation in the mouse

The time-course of dark adaptation provides valuable insights into the function and interactions between the rod and cone pathways in the retina. Here we describe a technique that uses the flash electroretinogram (ERG) response to probe the functional integrity of the cone and rod pathways during the dynamic process of dark adaptation in the mouse. Retinal sensitivity was estimated from the stimulus intensity required to maintain a 30 microV criterion b-wave response during a 40 min period of dark adaptation. When tracked in this manner, dark adaptation functions in WT mice depended upon the bleaching effects of initial background adaptation conditions. Altered dark adaptation functions, commensurate with the functional deficit were recorded in pigmented mice that lacked cone function (Gnat2 ( cplf3 )) and in WT mice injected with a toxin, sodium iodate (NaIO(3)), which targets the retinal pigment epithelium and also has downstream effects on photoreceptors. These data demonstrate that this adaptive tracking procedure measures retinal sensitivity and the contributions of the rod and/or cone pathways during dark adaptation in both WT control and mutant mice.

Results from screening over 9000 mutation-bearing mice for defects in the electroretinogram and appearance of the fundus

Random mutagenesis combined with phenotypic screening using carefully crafted functional tests has successfully led to the discovery of genes that are essential for a number of functions. This approach does not require prior knowledge of the identity of the genes that are involved and is a way to ascribe function to the nearly 6000 genes for which knowledge of the DNA sequence has been inadequate to determine the function of the gene product. In an effort to identify genes involved in the visual system via this approach, we have tested over 9000 first and third generation offspring of mice treated with the mutagen N-ethyl-N-nitrosourea (ENU) for visual defects, as evidenced by abnormalities in the electroretinogram and appearance of the fundus. We identified 61 putative mutations with this procedure and outline the steps needed to identify the affected genes.

Contribution of cone photoreceptors and post-receptoral mechanisms to the human photopic electroretinogram

We recorded the electroretinogram (ERG) from human subjects with normal vision, using ganzfeld stimulation in the presence of rod-suppressing blue background light. In families of responses to flashes of increasing intensity, we investigated features of both receptoral and post-receptoral origin. Firstly, we found that the oscillatory potentials (OPs, that have long been known to be post-receptoral) exhibited a time course that was invariant over a range of bright flash intensities. Secondly, we found that the photopic b-wave (which probably originates in cone ON bipolar cells) was most pronounced after test flashes of around 20 Td s, and could be suppressed either by increasing the test flash intensity or by applying a second flash after the test flash. We obtained estimates of the time course of the cone photoreceptor response using the paired-flash technique, in which an intense 'probe' flash was delivered at different times after a test flash. The response to the probe flash was recorded and, its amplitude was measured at early times after the probe flash. Estimates obtained in this way were of normalized amplitude, but could be scaled to an absolute amplitude by making an assumption about the level of probe-flash response that corresponded to complete suppression of photoreceptor current. For moderately bright test flashes the estimated cone photoreceptor response at early times coincided closely with the a-wave of the test flash ERG. However, the maximal size of this estimated response accounted for only about 70% of the peak a-wave amplitude in the case of bright flashes, and for an even smaller proportion after flashes of lower intensity, and we take this to indicate the existence of a third substantial post-receptoral contribution to the a-wave. For dim flashes, the time-to-peak of the cone response was around 15-20 ms, and for saturating flashes the dominant time constant of recovery was about 18 ms. The intensity dependence of the estimated cone response amplitude at fixed times followed an exponential saturation relation. We provide a comparison between our estimates of photoreceptor responses from human cones, and recent estimates from monkey cones obtained using related ERG approaches, and earlier single-cell measurements from isolated primate cones.

Bleaching desensitization: background and current challenges

"Bleaching desensitization" in rod photoreceptors refers to the prolonged depression of phototransduction sensitivity exhibited by rods after their exposure to bright light, i.e., after photolysis (bleaching) of a substantial fraction of rhodopsin in the outer segments. Rod recovery from bleaching desensitization depends critically on operation of the retinoid visual cycle: in particular, on the removal of all-trans retinal bleaching product from opsin and on the delivery of 11-cis retinal to opsin's chromophore binding site. The present paper summarizes representative findings that address the mechanism of bleaching desensitization.

Availability of 11-cis retinal and opsins without chromophore as revealed by small bleaches of rhodopsin in excised albino mouse eyes

Small bleaches were used to study the rhodopsin regeneration process. At bleaches from 5.2% to 24.7%, the rhodopsin regenerations were consistent with a one-for-one recovery of bleached molecules. At response saturation rod photoreceptors exhibit a bleach level of only 5%. Major increases in rhodopsin regeneration were observed at bleach levels between 1.3% and 5.2%. The rhodopsin regenerations exhibited a linear relationship that was 4-times the bleach (dark adaptations of 0.75 and 1.5 h). The data show that the bleach initiates the availability, and possibly production, of 11-cis retinal in amounts that are 4-times the number of bleached molecules within the functional range of the rod photoreceptors. Rhodopsin regeneration also requires the presence of opsins without chromophore. Regenerations beyond the bleach indicate the presence of such opsins prior to the bleach. The opsin amounts were 8.1%, 8.6%, 3.1% and 0% of the total visual pigment at dark adaptation times of 0.75, 1.5, 24 and 48 h, respectively. Those opsins, as well as the ones produced by the bleach, may be regenerated to rhodopsin following a small bleach or with additional time in the dark.

Electroretinographic evidence for altered phototransduction gain and slowed recovery from photobleaches in albino mice with a MET450 variant in RPE65

Our purpose was to investigate the physiological phenotype of albino mice with a variation in the Rpe65 gene encoding either methionine or leucine at amino acid #450. Full-field electroretinograms (ERGs) were recorded from C57BL/6J-c(2J) albino mice with MET450 and BALB/cByJ albino mice with LEU450. Recordings from pigmented mice (C57BL/6J) served as controls. Rod ERG a-waves were fitted with a model to estimate parameters of activation. Recovery of function following a photobleach was studied by monitoring the return to pre-bleach a- or b-wave amplitudes of the dark-adapted electroretinogram. The parameter, S, derived from the fit of the rod model, was significantly higher for albino mice compared to pigmented controls. Between the albino mice, S was highest for BALB/cByJ compared to C57BL/6J-c(2J). The parameters t(d) and Rm(P3) were not different across the three strains. The difference in S between the BALB/cByJ and C57BL/6J-c(2J) albino strains is interpreted to reflect differences in intrinsic phototransduction gain. Recovery from a photobleach was also slower for the C57BL/6J-c(2J) albino mice compared with BALB/cByJ albino mice, consistent with prior studies showing slowed rhodopsin regeneration in mice with the RPE65-METH450 variant. ERG recordings show that C57BL/6J-c(2J) albino mice with the MET450 variant of the RPE65 protein have a lower gain of activation and slower recovery from photobleach than do the BALB/cByJ albino mice with LEU450. Both the slower recovery from photobleach and lower gain of activation characteristic of the C57BL/6J-c(2J) strain may contribute to the mechanism by which it is protected from light-induced photoreceptor death relative to BALB/c.

Arrestin migrates in photoreceptors in response to light: a study of arrestin localization using an arrestin-GFP fusion protein in transgenic frogs

Subcellular translocation of phototransduction proteins in response to light has previously been detected by immunocytochemistry. This movement is consistent with the hypothesis that migration is part of a basic cellular mechanism regulating photoreceptor sensitivity. In order to monitor the putative migration of arrestin in response to light, we expressed a functional fusion between the signal transduction protein arrestin and green fluorescent protein (GFP) in rod photoreceptors of transgenic Xenopus laevis. In addition to confirming reports that arrestin is translocated, this alternative approach generated unique observations, raising new questions regarding the nature and time scale of migration. Confocal fluorescence microscopy was performed on fixed frozen retinal sections from tadpoles exposed to three different lighting conditions. A consistent pattern of localization emerged in each case. During early light exposure, arrestin-GFP levels diminished in the inner segments (ISs) and simultaneously increased in the outer segments (OSs), initially at the base and eventually at the distal tips as time progressed. Arrestin-GFP reached the distal tips of the photoreceptors by 45-75 min at which time the ratio of arrestin-GFP fluorescence in the OSs compared to the ISs was maximal. When dark-adaptation was initiated after 45 min of light exposure, arrestin-GFP rapidly re-localized to the ISs and axoneme within 30 min. Curiously, prolonged periods of light exposure also resulted in re-localization of arrestin-GFP. Between 150 and 240 min of light adaptation the arrestin-GFP in the ROS gradually declined until the pattern of arrestin-GFP localization was indistinguishable from that of dark-adapted photoreceptors. This distribution pattern was observed over a wide range of lighting intensity (25-2700 lux). Immunocytochemical analysis of arrestin in wild-type Xenopus retinas gave similar results.