Rhodopsin kinetics in the human eye

Visual pigment concentration and photoreceptor outer segment length in the human retina

PURPOSE

The Beer-Lambert law suggests that visual pigment optical density (OD) should be linearly related to the length of photoreceptor outer segments (POSs). Mammalian studies indicate that visual pigment concentration increases with POS length, but the nature of this relationship may vary due to factors such as visual pigment packing density or retinal eccentricity, and may not necessarily be linearly related. The purpose of this study was to establish the relationship between OD and POS length in humans.

METHODS

Spectral domain optical coherence tomography (OCT) was used to image POS, and imaging retinal densitometry (IRD) was used to measure OD at corresponding locations in 19 healthy participants (age range 25-82 years). POS length and OD measurements were extracted from OCT and IRD images at 23 discrete locations spanning the central 9° of the retina. The averaged data from all participants were fitted with models based on the Beer-Lambert law to establish the relationship between OD and POS length.

RESULTS

Visual pigment OD increased monotonically with POS length, but the relationship was non-linear, and a straight-line fit, based on a simple interpretation of the Beer-Lambert law, provided a poor description. A model allowing for different rod and cone visual pigment concentrations provided a superior fit. Specifically, the data were well described by a model where the molar concentration of visual pigment in cones and rods were 3.8 × 10-3 mol/L and 1.8 × 10-3mol/L, respectively.

CONCLUSIONS

In accordance with the Beer-Lambert law, the results indicate that OD increases monotonically with POS length in humans, but the precise relationship is dependent on photoreceptor type. These results suggest that visual pigment concentration in rods is only about 48% of that found in cones. This may be due to the ubiquitous nature of artificial light that works to reduce the concentration of rhodopsin in rod photoreceptors.

Rhodopsin, light-sensor of vision

The light sensor of vertebrate scotopic (low-light) vision, rhodopsin, is a G-protein-coupled receptor comprising a polypeptide chain with bound chromophore, 11-cis-retinal, that exhibits remarkable physicochemical properties. This photopigment is extremely stable in the dark, yet its chromophore isomerises upon photon absorption with 70% efficiency, enabling the activation of its G-protein, transducin, with high efficiency. Rhodopsin's photochemical and biochemical activities occur over very different time-scales: the energy of retinaldehyde's excited state is stored in <1 ps in retinal-protein interactions, but it takes milliseconds for the catalytically active state to form, and many tens of minutes for the resting state to be restored. In this review, we describe the properties of rhodopsin and its role in rod phototransduction. We first introduce rhodopsin's gross structural features, its evolution, and the basic mechanisms of its activation. We then discuss light absorption and spectral sensitivity, photoreceptor electrical responses that result from the activity of individual rhodopsin molecules, and recovery of rhodopsin and the visual system from intense bleaching exposures. We then provide a detailed examination of rhodopsin's molecular structure and function, first in its dark state, and then in the active Meta states that govern its interactions with transducin, rhodopsin kinase and arrestin. While it is clear that rhodopsin's molecular properties are exquisitely honed for phototransduction, from starlight to dawn/dusk intensity levels, our understanding of how its molecular interactions determine the properties of scotopic vision remains incomplete. We describe potential future directions of research, and outline several major problems that remain to be solved.

Light adaptation characteristics of melanopsin

Following photopigment bleaching, the rhodopsin and cone-opsins show a characteristic exponential regeneration in the dark with a photocycle dependent on the retinal pigment epithelium. Melanopsin pigment regeneration in animal models requires different pathways to rods and cones. To quantify melanopsin-mediated light adaptation in humans, we first estimated its photopigment regeneration kinetics through the photo-bleach recovery of the intrinsic melanopsin pupil light response (PLR). An intense broadband light (~120,000 Td) bleached 43% of melanopsin compared to 86% of the cone-opsins. Recovery from a 43% bleach was 3.4X slower for the melanopsin than cone-opsin. Post-bleach melanopsin regeneration followed an exponential growth with a 2.5 min time-constant (τ) that required 11.2 min for complete recovery; the half-bleaching level (I_p) was ~ 4.47 log melanopic Td (16.10 log melanopsin effective photons.cm^-2.s^-1; 8.25 log photoisomerisations.photoreceptor^-1.s^-1). The effect on the cone-directed PLR of the level of the melanopsin excitation during continuous light adaptation was then determined. We observed that cone-directed pupil constriction amplitudes increased by ~ 10% when adapting lights had a higher melanopic excitation but the same mean photometric luminance. Our findings suggest that melanopsin light adaptation enhances cone signalling along the non-visual retina-brain axis. Parameters τ and I_p will allow estimation of the level of melanopsin bleaching in any light units; the data have implications for quantifying the relative contributions of putative melanopsin pathways to regulate the post-bleach photopigment regeneration and adaptation.

Rod Photoreceptors Avoid Saturation in Bright Light by the Movement of the G Protein Transducin

Rod photoreceptors can be saturated by exposure to bright background light, so that no flash superimposed on the background can elicit a detectable response. This phenomenon, called increment saturation, was first demonstrated psychophysically by Aguilar and Stiles and has since been shown in many studies to occur in single rods. Recent experiments indicate, however, that rods may be able to avoid saturation under some conditions of illumination. We now show in ex vivo electroretinogram and single-cell recordings that in continuous and prolonged exposure even to very bright light, the rods of mice from both sexes recover as much as 15% of their dark current and that responses can persist for hours. In parallel to recovery of outer segment current is an ∼10-fold increase in the sensitivity of rod photoresponses. This recovery is decreased in transgenic mice with reduced light-dependent translocation of the G protein transducin. The reduction in outer-segment transducin together with a novel mechanism of visual-pigment regeneration within the rod itself enable rods to remain responsive over the whole of the physiological range of vision. In this way, rods are able to avoid an extended period of transduction channel closure, which is known to cause photoreceptor degeneration.SIGNIFICANCE STATEMENT Rods are initially saturated in bright light so that no flash superimposed on the background can elicit a detectable response. Frederiksen and colleagues show in whole retina and single-cell recordings that, if the background light is prolonged, rods slowly recover and can continue to produce significant responses over the entire physiological range of vision. Response recovery occurs by translocation of the G protein transducin from the rod outer to the inner segment, together with a novel mechanism of visual-pigment regeneration within the rod itself. Avoidance of saturation in bright light may be one of the principal mechanisms the retina uses to keep rod outer-segment channels from ever closing for too long a time, which is known to produce photoreceptor degeneration.

The Effect of Blue-blocking Lenses on Photostress Recovery Times

SIGNIFICANCE

The selective reduction in visible wavelengths transmitted through commercially available blue-blocking lenses (BBLs) is known to influence the appearance and contrast detection of objects, particularly at low light levels. This influence may impair the human retinal receptor response time to dynamic light changes during photostress events.

PURPOSE

This study aimed to assess whether BBLs selectively affect photostress recovery times (PSRTs) for chromatic and achromatic stimuli of different Weber contrasts that were viewed on a dark black background.

METHODS

Photostress recovery times were measured in 12 younger participants (18 to 39 years old) with no history of ocular disease or abnormal vision. Photostress recovery times were evaluated for four brands of BBLs, which were compared with a control lens. In these experiments, after exposure to an intense light source for 5 seconds, the time taken to recover vision and correctly identify a computer-generated letter stimulus viewed under low and high luminance levels was determined, which means perception is likely to be governed by mesopic and photopic conditions. Across conditions, the letter stimulus was achromatic and chromatic and could differ in luminance contrast.

RESULTS

Under photopic stimulus conditions, although reducing luminance contrast increased PSRTs, BBLs had no significant effect on PSRTs relative to control lens. However, under mesopic stimulus conditions, BBLs significantly affect PSRTs for both achromatic (F2.006,8.02 = 61.95, P < .0001) and chromatic stimuli (F3,16 =139.01, P < .0001), particularly for blue targets, which had considerably longer PSRTs (38.40 seconds). The brand of BBL was also shown to selectively affect PSRTs, with those with transmittance profiles that block the most blue light having longer PSRTs.

CONCLUSIONS

The present study suggests that, although the color and contrast of the target stimuli affected recovery times, the difference in recovery times between different types of BBLs was noticed only under low-light-level stimulus conditions.

Assessing the temporal uniformity of CIELAB hue angle

In recent work [J. Opt. Soc. Am. A36, 1022 (2019)JOAOD60740-323210.1364/JOSAA.36.001022], we found that $\Delta {E^*_\textit{ab}}/{\rm s}$ΔEab∗/s in CIELAB is not suitable for describing the perceived speed of temporal color changes in full-room illumination. Two hue transitions with the same physical speed of change, in terms of $\Delta {E^*_\textit{ab}}/{\rm s}$ΔEab∗/s, were not perceived to change at the same speed. This is not really surprising, since CIELAB was not designed to characterize the perception of temporal color transitions in illumination. In this study, we further investigate the temporal uniformity of CIELAB. The stimuli were presented in a square of 4.3° visual angle surrounded by a 4000 K adapting field, similar to the viewing condition for which CIELAB was designed (i.e., where color stimuli are presented on-axis surrounded by a static adaptation field). The human observers viewed pairs of temporal color transitions which were presented sequentially, and were asked to select the one that appeared to change faster. The results confirmed that under these conditions CIELAB was also not temporally uniform. We present preliminary attempts to improve the temporal uniformity for both CIELAB and cone-excitation spaces (i.e., LMS and DKL (Derrington-Krauskopf-Lennie [J. Physiol.357, 241 (1984)JPHYA70022-375110.1113/jphysiol.1984.sp015499]).

Functional Imaging of the Outer Retinal Complex using High Fidelity Imaging Retinal Densitometry

We describe a new technique, high fidelity Imaging Retinal Densitometry (IRD), which probes the functional integrity of the outer retinal complex. We demonstrate the ability of the technique to map visual pigment optical density and synthesis rates in eyes with and without macular disease. A multispectral retinal imaging device obtained precise measurements of retinal reflectance over space and time. Data obtained from healthy controls and 5 patients with intermediate AMD, before and after photopigment bleaching, were used to quantify visual pigment metrics. Heat maps were plotted to summarise the topography of rod and cone pigment kinetics and descriptive statistics conducted to highlight differences between those with and without AMD. Rod and cone visual pigment synthesis rates in those with AMD (v = 0.043 SD 0.019 min⁻¹ and v = 0.119 SD 0.046 min⁻¹, respectively) were approximately half those observed in healthy controls (v = 0.079 SD 0.024 min⁻¹ for rods and v = 0.206 SD 0.069 min⁻¹ for cones). By mapping visual pigment kinetics across the central retina, high fidelity IRD provides a unique insight into outer retinal complex function. This new technique will improve the phenotypic characterisation, diagnosis and treatment monitoring of various ocular pathologies, including AMD.

Delayed S-cone sensitivity losses following the onset of intense yellow backgrounds linked to the lifetime of a photobleaching product?

Thirty years ago, Mollon, Stockman, & Polden (1987) reported that after the onset of intense yellow 581-nm backgrounds, S-cone threshold rose unexpectedly for several seconds before recovering to the light-adapted steady-state value-an effect they called: "transient-tritanopia of the second kind" (TT2). Given that 581-nm lights have little direct effect on S-cones, TT2 must arise indirectly from the backgrounds' effects on the L- and M-cones. We attribute the phenomenon to the action of an unknown L- and M-cone photobleaching product, X, which acts at their outputs like an "equivalent" background light that then inhibits S-cones at a cone-opponent, second-site. The time-course of TT2 is similar in form to the lifetime of X in a two-stage, first-order biochemical reaction A→X→C with successive best-fitting time-constants of 3.09 ± 0.35 and 7.73 ± 0.70 s. Alternatively, with an additional slowly recovering exponential "restoring-force" with a best-fitting time-constant 23.94 ± 1.42 s, the two-stage best-fitting time-constants become 4.15 ± 0.62 and 6.79 ± 1.00 s. Because the time-constants are roughly independent of the background illumination, and thus the rate of photoisomerization, A→X is likely to be a reaction subsidiary to the retinoid cycle, perhaps acting as a buffer when the bleaching rate is too high. X seems to be logarithmically related to S-cone threshold, which may result from the logarithmic cone-opponent, second-site response compression after multiplicative first-site adaptation. The restoring-force may be the same cone-opponent force that sets the rate of S-cone recovery following the unusual threshold increase following the offset of dimmer yellow backgrounds, an effect known as "transient-tritanopia" (TT1).

Rapid Adaptation of Night Vision

Apart from the well-known loss of color vision and of foveal acuity that characterizes human rod-mediated vision, it has also been thought that night vision is very slow (taking up to 40 min) to adapt to changes in light levels. Even cone-mediated, daylight, vision has been thought to take 2 min to recover from light adaptation. Here, we show that most, though not all adaptation is rapid, taking less than 0.6 s. Thus, monochrome (black-white-gray) images can be presented at mesopic light levels and be visible within a few 10th of a second, even if the overall light level, or level of glare (as with passing headlamps while driving), changes abruptly.

Formation and Clearance of All-Trans-Retinol in Rods Investigated in the Living Primate Eye With Two-Photon Ophthalmoscopy

Purpose

Two-photon excited fluorescence (TPEF) imaging has potential as a functional tool for tracking visual pigment regeneration in the living eye. Previous studies have shown that all-trans-retinol is likely the chief source of time-varying TPEF from photoreceptors. Endogenous TPEF from retinol could provide the specificity desired for tracking the visual cycle. However, in vivo characterization of native retinol kinetics is complicated by visual stimulation from the imaging beam. We have developed an imaging scheme for overcoming these challenges and monitored the formation and clearance of retinol.

Methods

Three macaques were imaged by using an in vivo two-photon ophthalmoscope. Endogenous TPEF was excited at 730 nm and recorded through the eye's pupil for more than 90 seconds. Two-photon excited fluorescence increased with onset of light and plateaued within 40 seconds, at which point, brief incremental stimuli were delivered at 561 nm. The responses of rods to stimulation were analyzed by using first-order kinetics.

Results

Two-photon excited fluorescence resulting from retinol production corresponded to the fraction of rhodopsin bleached. The photosensitivity of rhodopsin was estimated to be 6.88 ± 5.50 log scotopic troland. The rate of retinol clearance depended on intensity of incremental stimulation. Clearance was faster for stronger stimuli and time constants ranged from 50 to 300 seconds.

Conclusions

This study demonstrates a method for rapidly measuring the rate of clearance of retinol in vivo. Moreover, TPEF generated due to retinol can be used as a measure of rhodopsin depletion, similar to densitometry. This enhances the utility of two-photon ophthalmoscopy as a technique for evaluating the visual cycle in the living eye.

Small field tritanopia in the peripheral retina

If stimuli are made sufficiently small, color-normal individuals report a loss in hue perception, in particular a decrease in the perception of green, in both the fovea and peripheral retina. This effect is referred to as small field tritanopia. It is not clear, however, how rod input may alter the dynamics of small field tritanopia in the peripheral retina. This paper looks at peripheral hue-naming data obtained for small stimuli at mesopic and photopic retinal illuminances under conditions that minimize (bleach) and maximize (no bleach) rod contribution. The data show that attenuation in the perception of green occurs with larger stimuli in the no-bleach condition than in the bleach condition. As retinal illuminance increases, the stimulus size that elicits small field tritanopia decreases, but the stimulus size is still larger under the no-bleach condition. Small field tritanopia in both the bleach and no-bleach conditions may be related to short-wavelength-sensitive (S) cone activity and its potential role in the mediation of the perception of green. The differences in stimulus size for small field tritanopia may be explained by rod input into the magnocellular and koniocellular pathways, which compromises the strength of the chromatic signals and creates a differential loss in the perception of green as compared to the other elemental hues.

The Photosensitivity of Rhodopsin Bleaching and Light-Induced Increases of Fundus Reflectance in Mice Measured In Vivo With Scanning Laser Ophthalmoscopy

PURPOSE

To quantify bleaching-induced changes in fundus reflectance in the mouse retina.

METHODS

Light reflected from the fundus of albino (Balb/c) and pigmented (C57Bl/6J) mice was measured with a multichannel scanning laser ophthalmoscopy optical coherence tomography (SLO-OCT) optical system. Serial scanning of small retinal regions was used for bleaching rhodopsin and measuring reflectance changes.

RESULTS

Serial scanning generated a saturating reflectance increase centered at 501 nm with a photosensitivity of 1.4 × 10-8 per molecule μm2 in both strains, 2-fold higher than expected were irradiance at the rod outer segment base equal to that at the retinal surface. The action spectrum of the reflectance increase corresponds to the absorption spectrum of mouse rhodopsin in situ. Spectra obtained before and after bleaching were fitted with a model of fundus reflectance, quantifying contributions from loss of rhodopsin absorption with bleaching, absorption by oxygenated hemoglobin (HbO2) in the choroid (Balb/c), and absorption by melanin (C57Bl/6J). Both mouse strains exhibited light-induced broadband reflectance changes explained as bleaching-induced reflectivity increases at photoreceptor inner segment/outer segment (IS/OS) junctions and OS tips.

CONCLUSIONS

The elevated photosensitivity of rhodopsin bleaching in vivo is explained by waveguide condensing of light in propagation from rod inner segment (RIS) to rod outer segment (ROS). The similar photosensitivity of rhodopsin in the two strains reveals that little light backscattered from the sclera can enter the ROS. The bleaching-induced increases in reflectance at the IS/OS junctions and OS tips resemble results previously reported in human cones, but are ascribed to rods due to their 30/1 predominance over cones in mice and to the relatively minor amount of cone M-opsin in the regions scanned.

Role of extrinsic noise in the sensitivity of the rod pathway: rapid dark adaptation of nocturnal vision in humans

Rod-mediated 500 nm test spots were flashed in Maxwellian view at 5 deg eccentricity, both on steady 10.4 deg fields of intensities (I) from 0.00001 to 1.0 scotopic troland (sc td) and from 0.2 s to 1 s after extinguishing the field. On dim fields, thresholds of tiny (5') tests were proportional to √I (Rose-DeVries law), while thresholds after extinction fell within 0.6 s to the fully dark-adapted absolute threshold. Thresholds of large (1.3 deg) tests were proportional to I (Weber law) and extinction thresholds, to √I.

CONCLUSIONS

rod thresholds are elevated by photon-driven noise from dim fields that disappears at field extinction; large spot thresholds are additionally elevated by neural light adaptation proportional to √I. At night, recovery from dimly lit fields is fast, not slow.

Vitamin A and Vision

Visual systems detect light by monitoring the effect of photoisomerization of a chromophore on the release of a neurotransmitter from sensory neurons, known as rod and cone photoreceptor cells in vertebrate retina. In all known visual systems, the chromophore is 11-cis-retinal complexed with a protein, called opsin, and photoisomerization produces all-trans-retinal. In mammals, regeneration of 11-cis-retinal following photoisomerization occurs by a thermally driven isomerization reaction. Additional reactions are required during regeneration to protect cells from the toxicity of aldehyde forms of vitamin A that are essential to the visual process. Photochemical and phototransduction reactions in rods and cones are identical; however, reactions of the rod and cone visual pigment regeneration cycles differ, and perplexingly, rod and cone regeneration cycles appear to use different mechanisms to overcome the energy barrier involved in converting all-trans- to 11-cis-retinoid. Abnormal processing of all-trans-retinal in the rod regeneration cycle leads to retinal degeneration, suggesting that excessive amounts of the retinoid itself or its derivatives are toxic. This line of reasoning led to the development of various approaches to modifying the activity of the rod visual cycle as a possible therapeutic approach to delay or prevent retinal degeneration in inherited retinal diseases and perhaps in the dry form of macular degeneration (geographic atrophy). In spite of great progress in understanding the functioning of rod and cone regeneration cycles at a molecular level, resolution of a number of remaining puzzling issues will offer insight into the amelioration of several blinding retinal diseases.

Modeling Photo-Bleaching Kinetics to Create High Resolution Maps of Rod Rhodopsin in the Human Retina

We introduce and describe a novel non-invasive in-vivo method for mapping local rod rhodopsin distribution in the human retina over a 30-degree field. Our approach is based on analyzing the brightening of detected lipofuscin autofluorescence within small pixel clusters in registered imaging sequences taken with a commercial 488nm confocal scanning laser ophthalmoscope (cSLO) over a 1 minute period. We modeled the kinetics of rhodopsin bleaching by applying variational optimization techniques from applied mathematics. The physical model and the numerical analysis with its implementation are outlined in detail. This new technique enables the creation of spatial maps of the retinal rhodopsin and retinal pigment epithelium (RPE) bisretinoid distribution with an ≈ 50μm resolution.

New wrinkles in retinal densitometry

PURPOSE

Retinal densitometry provides objective information about retinal function. But, a number of factors, including retinal reflectance changes that are not directly related to photopigment depletion, complicate its interpretation. We explore these factors and suggest a method to minimize their impact.

METHODS

An adaptive optics scanning light ophthalmoscope (AOSLO) was used to measure changes in photoreceptor reflectance in monkeys before and after photopigment bleaching with 514-nm light. Reflectance measurements at 514 nm and 794 nm were recorded simultaneously. Several methods of normalization to extract the apparent optical density of the photopigment were compared.

RESULTS

We identified stimulus-related fluctuations in 794-nm reflectance that are not associated with photopigment absorptance and occur in both rods and cones. These changes had a magnitude approaching those associated directly with pigment depletion, precluding the use of infrared reflectance for normalization. We used a spatial normalization method instead, which avoided the fluctuations in the near infrared, as well as a confocal AOSLO designed to minimize light from layers other than the receptors. However, these methods produced a surprisingly low estimate of the apparent rhodopsin density (animal 1: 0.073 ± 0.006, animal 2: 0.032 ± 0.003).

CONCLUSIONS

These results confirm earlier observations that changes in photopigment absorption are not the only source of retinal reflectance change during dark adaptation. It appears that the stray light that has historically reduced the apparent density of cone photopigment in retinal densitometry arises predominantly from layers near the photoreceptors themselves. Despite these complications, this method provides a valuable, objective measure of retinal function.

Rod photopigment kinetics after photodisruption of the retinal pigment epithelium

PURPOSE

Advances in retinal imaging have led to the discovery of long-lasting retinal changes caused by light exposures below published safety limits, including disruption of the RPE. To investigate the functional consequences of RPE disruption, we combined adaptive optics ophthalmoscopy with retinal densitometry.

METHODS

A modified adaptive optics scanning light ophthalmoscope (AOSLO) measured the apparent density and regeneration rate of rhodopsin in two macaques before and after four different 568-nm retinal radiant exposures (RREs; 400-3200 J/cm(2)). Optical coherence tomography (OCT) was used to measure the optical path length through the photoreceptor outer segments before and after RPE disruption.

RESULTS

All tested RREs caused visible RPE disruption. Apparent rhodopsin density was significantly reduced following 1600 (P = 0.01) and 3200 J/cm(2) (P = 0.007) exposures. No significant change in apparent density was observed in response to 800 J/cm(2). Surprisingly, exposure to 400 J/cm(2) showed a significant increase in apparent density (P = 0.047). Rhodopsin recovery rate was not significantly affected by these RREs. Optical coherence tomography measurements showed a significant decrease in the optical path length through the photoreceptor outer segments for RREs above 800 J/cm(2) (P < 0.001).

CONCLUSIONS

At higher RREs, optical path length through the outer segments was reduced. However, the rate of photopigment regeneration was unchanged. While some ambiguity remains as to the correlation between measured reflectivity and absolute rhodopsin density; at the lowest RREs, RPE disruption appears not to be accompanied by a loss of apparent rhodopsin density, which would have been indicative of functional loss.

Not just signal shutoff: the protective role of arrestin-1 in rod cells

The retinal rod cell is an exquisitely sensitive single-photon detector that primarily functions in dim light (e.g., moonlight). However, rod cells must routinely survive light intensities more than a billion times greater (e.g., bright daylight). One serious challenge to rod cell survival in daylight is the massive amount of all-trans-retinal that is released by Meta II, the light-activated form of the photoreceptor rhodopsin. All-trans-retinal is toxic, and its condensation products have been implicated in disease. Our recent work has developed the concept that rod arrestin (arrestin-1), which terminates Meta II signaling, has an additional role in protecting rod cells from the consequences of bright light by limiting free all-trans-retinal. In this chapter we will elaborate upon the molecular mechanisms by which arrestin-1 serves as both a single-photon response quencher as well as an instrument of rod cell survival in bright light. This discussion will take place within the framework of three distinct functional modules of vision: signal transduction, the retinoid cycle, and protein translocation.

Scanning laser ophthalmoscope measurement of local fundus reflectance and autofluorescence changes arising from rhodopsin bleaching and regeneration

PURPOSE

We measured the bleaching and regeneration kinetics of rhodopsin in the living human eye with two-wavelength, wide-field scanning laser ophthalmoscopy (SLO), and investigated the effect of rhodopsin bleaching on autofluorescence intensity.

METHODS

The retina was imaged with an Optos P200C SLO by its reflectance of 532 and 633 nm light, and its autofluorescence excited by 532 nm light, before and after exposure to lights calibrated to bleach rhodopsin substantially. Bleaching was confined to circular retinal regions of 4.8° visual angle located approximately 16° superotemporal and superonasal to fixation. Images were captured as 12-bit tiff files and postprocessed to extract changes in reflectance and autofluorescence.

RESULTS

At the locus of bleaching transient increases in reflectance of the 532 nm, but not the 633 nm beam were observed readily and quantified. A transient increase in autofluorescence also occurred. The action spectrum, absolute sensitivity, and recovery of the 532 nm reflectance increase were consistent with previous measurements of human rhodopsin's spectral sensitivity, photosensitivity, and regeneration kinetics. The autofluorescence changes closely tracked the changes in rhodopsin density.

CONCLUSIONS

The bleaching and regeneration kinetics of rhodopsin can be measured locally in the human retina with a widely available SLO. The increased autofluorescence excited by 532 nm light upon bleaching appears primarily due to transient elimination of rhodopsin's screening of autofluorescent fluorochromes in the RPE. The spatially localized measurement with a widely available SLO of rhodopsin, the most abundant protein in the retina, could be a valuable adjunct to retinal health assessment.

Aging and vision

Given the increasing size of the older adult population in many countries, there is a pressing need to identify the nature of aging-related vision impairments, their underlying mechanisms, and how they impact older adults' performance of everyday visual tasks. The results of this research can then be used to develop and evaluate interventions to slow or reverse aging-related declines in vision, thereby improving quality of life. Here we summarize salient developments in research on aging and vision over the past 25 years, focusing on spatial contrast sensitivity, vision under low luminance, temporal sensitivity and motion perception, and visual processing speed.

Unique hue loci differ with methodology

BACKGROUND

Studies investigating the effect of rods on unique hue loci in the peripheral retina generally obtain measures at two time points associated with the dark adaptation function - the cone plateau and the rod plateau. In comparison, this study used a color-naming procedure to identify the loci of unique green and unique yellow as a function of time associated with the entire dark adaptation function. The unique hue loci derived by this procedure were then compared to those obtained directly with a staircase procedure.

METHOD

Hue-scaling functions were obtained for monochromatic stimuli for four observers using the '4 + 1' procedure. Data were collected every 4 min following extinction of a bleaching stimulus. These hue-scaling functions were then converted to uniform appearance diagrams (UADs) to derive unique green and unique yellow loci. Unique green and unique yellow loci were also obtained from the same observers via a staircase procedure at 4-9 min post-bleach (minimal rod input) and after 28 min dark adaptation (maximal rod input). Measurements were made in the peripheral retina at 10° temporal retinal eccentricity and at the fovea.

RESULTS

Unique green loci derived from UADs are at longer wavelengths compared to those measured directly with the staircase procedure. In addition, unique green loci derived from UADs show a progressive shift to longer wavelengths as time post-bleach increases; whereas, unique green loci obtained from the staircase procedure differ little between the rod-bleach and no-bleach conditions. Unique yellow loci are similar across both experimental procedures.

CONCLUSION

Unique green loci derived from UADs are not the same as those measured with traditional psychophysical procedures. These differences may be due to the different response criteria used by observers in the color-naming and staircase procedures. The unique green loci obtained from UADs indicate that rod signals shift unique green loci to longer wavelengths as time post-bleach increases. More direct methods need to be employed to determine if this rod effect is valid.

Novel snapshot imaging of photoreceptor bleaching in macaque and human retinas

PURPOSE

Various methods have been used to obtain a topographic map of bleached photopigments in human retinas in the past. The purpose of this study was to determine whether the bleaching topography of the photoreceptors could be obtained by snapshot imaging reflectometry.

METHODS

Four to five fundus photographs of one rhesus monkey and three healthy human subjects were taken by white flashes at intervals of 4 s, with a commercial fundus camera with minimal modifications. The flash-induced reflectance increases (bleaching) were calculated by dividing the reflectance of the first image into the subsequent images, pixel by pixel.

RESULTS

The topography of the bleached macula corresponded well with the anatomical distribution of the cones. The ratio of reflectance changes in the center to that in the surrounding tissue was high for red and low for green and blue images. These results indicate that the reflectivity changes were not artifacts but were derived from changes in the photopigment density in the cones and rods.

CONCLUSIONS

The topography of bleached photoreceptors obtained with a commercial fundus camera from one monkey and three healthy human subjects showed that this technique has potential as a new clinical method for examining photoreceptor function in both normal and diseased retinas.

Distinct contributions of rod, cone, and melanopsin photoreceptors to encoding irradiance

Photoreceptive, melanopsin-expressing retinal ganglion cells (mRGCs) encode ambient light (irradiance) for the circadian clock, the pupillomotor system, and other influential behavioral/physiological responses. mRGCs are activated both by their intrinsic phototransduction cascade and by the rods and cones. However, the individual contribution of each photoreceptor class to irradiance responses remains unclear. We address this deficit using mice expressing human red cone opsin, in which rod-, cone-, and melanopsin-dependent responses can be identified by their distinct spectral sensitivity. Our data reveal an unexpectedly important role for rods. These photoreceptors define circadian responses at very dim “scotopic” light levels but also at irradiances at which pattern vision relies heavily on cones. By contrast, cone input to irradiance responses dissipates following light adaptation to the extent that these receptors make a very limited contribution to circadian and pupillary light responses under these conditions. Our data provide new insight into retinal circuitry upstream of mRGCs and optimal stimuli for eliciting irradiance responses.

Chromatic perceptive field sizes measured at 10 degrees eccentricity along the horizontal and vertical meridians

The different hemifields in the retina are known to vary in photoreceptor density as well as in the number of photoreceptors converging onto one ganglion cell. The effect of these differences among the retinal hemifields at 10 degrees retinal eccentricity was investigated using a color-naming procedure to derive perceptive field sizes for the hue terms of blue, green, yellow, and red. Color-naming data were obtained under two conditions: (1) after a bleach condition, chosen to minimize rod contribution, and (2) after 30 min dark adaptation, chosen to maximize rod contribution. Perceptive field sizes measured in the bleach condition were consistent with degree of neural convergence of cones to ganglion cells across the retina rather than differences in cone density. Rod densities relative to cone densities correlated with the size of perceptive fields in the no-bleach condition, i.e., the greater the rod:cone ratio, the larger the perceptive field.

The macular automated photostress test

PURPOSE

To introduce a standardized macular photostress test using an automated perimeter as a method to quantify macular disease severity and as a tool to distinguish optic neuropathy from macular pathology.

DESIGN

Prospective interventional pilot study.

METHODS

Twenty-five bilaterally pseudophakic subjects aged range, 65 to 84: 15 patients with varying severity of non-neovascular age-related macular degeneration (AMD), five patients with no ocular disease, and five patients with moderate primary open-angle glaucoma (POAG). Previously reported normative values served as controls for this study. Patients underwent foveal threshold testing using the Humphrey Visual Field Perimeter Model 750 (Carl Zeiss Meditec, Inc, Dublin, California, USA). Baseline measurements were compared to threshold sensitivity after photostress at one minute and then two-minute intervals until sensitivity returned to baseline. Main outcome measures were baseline foveal threshold sensitivity, foveal threshold depression, and recovery following photostress.

RESULTS

Automated macular photostress testing in macular disease (AMD) causes a decrease (P < .001) in baseline foveal sensitivity and a delay (P < .001) in recovery time to baseline sensitivity. Optic nerve pathology (POAG) does not affect (P = .343) the foveal response curve.

CONCLUSIONS

The macular automated photostress (MAP) test is an inexpensive, noninvasive, and readily accessible adjunct for evaluating patients with macular disease. This standardized protocol is useful in objectively defining disease severity, may be used to follow response to treatment, and could aid in distinguishing optic neuropathy from macular pathology.

Retinol dehydrogenase (RDH12) protects photoreceptors from light-induced degeneration in mice

RDH12 has been suggested to be one of the retinol dehydrogenases (RDH) involved in the vitamin A recycling system (visual cycle) in the eye. Loss of function mutations in the RDH12 gene were recently reported to be associated with autosomal recessive childhood-onset severe retinal dystrophy. Here we show that RDH12 localizes to the photoreceptor inner segments and that deletion of this gene in mice slows the kinetics of all-trans-retinal reduction, delaying dark adaptation. However, accelerated 11-cis-retinal production and increased susceptibility to light-induced photoreceptor apoptosis were also observed in Rdh12(-/-) mice, suggesting that RDH12 plays a unique, nonredundant role in the photoreceptor inner segments to regulate the flow of retinoids in the eye. Thus, severe visual impairments of individuals with null mutations in RDH12 may likely be caused by light damage(1).

Role of photoreceptor-specific retinol dehydrogenase in the retinoid cycle in vivo

The retinoid cycle is a recycling system that replenishes the 11-cis-retinal chromophore of rhodopsin and cone pigments. Photoreceptor-specific retinol dehydrogenase (prRDH) catalyzes reduction of all-trans-retinal to all-trans-retinol and is thought to be a key enzyme in the retinoid cycle. We disrupted mouse prRDH (human gene symbol RDH8) gene expression by targeted recombination and generated a homozygous prRDH knock-out (prRDH-/-) mouse. Histological analysis and electron microscopy of retinas from 6- to 8-week-old prRDH-/- mice revealed no structural differences of the photoreceptors or inner retina. For brief light exposure, absence of prRDH did not affect the rate of 11-cis-retinal regeneration or the decay of Meta II, the activated form of rhodopsin. Absence of prRDH, however, caused significant accumulation of all-trans-retinal following exposure to bright lights and delayed recovery of rod function as measured by electroretinograms and single cell recordings. Retention of all-trans-retinal resulted in slight overproduction of A2E, a condensation product of all-trans-retinal and phosphatidylethanolamine. We conclude that prRDH is an enzyme that catalyzes reduction of all-trans-retinal in the rod outer segment, most noticeably at higher light intensities and prolonged illumination, but is not an essential enzyme of the retinoid cycle.

From candelas to photoisomerizations in the mouse eye by rhodopsin bleaching in situ and the light-rearing dependence of the major components of the mouse ERG

To quantify the rate at which light in a ganzfeld produces photoisomerizations in mouse rods in situ, we measured the rate of rhodopsin bleaching in eyes of recently euthanized mice with fully dilated pupils. The amount of rhodopsin declined as a first-order (exponential) function of the duration of the exposure at the luminance of 920 scot cd m(-2): the rate constants of bleaching were 8.3 x 10(-6) and 2.8 x 10(-5) s(-1) (scot cd(-1)m2)(-1) for C57B1/6 and 129P3/J mice, respectively. When the approximately 3-fold difference in effective areas of the pupils of the mice are taken into consideration, the bleaching rates for both strains become essentially the same, 2.6 x 10(-6) fraction rhodopsin (scot Td s)(-1). Assuming 7 x 10(7) rhodopsin molecules per rod, this bleaching rate yields the result that a flash of 1 scot Td s produces 181 photoisomerizations per rod, a value close to that derived from analysis of the collecting area of the rod for axially propagating light. We measured the electroretinograms of mice of the two strains reared under controlled illumination conditions (2 and 100 lux), and compared their properties, using the calibrations to determine the absolute sensitivities of the b-wave and a-waves. The intensity that produces a half-saturating rod b-wave response is 0.3-0.6 photoisomerizations rod(-1), and the amplification constant of the rod a-wave is 5-6 s(-2) photoisomerization(-1), with little dependence on the strain.

Dark adaptation and the retinoid cycle of vision

Following exposure of our eye to very intense illumination, we experience a greatly elevated visual threshold, that takes tens of minutes to return completely to normal. The slowness of this phenomenon of "dark adaptation" has been studied for many decades, yet is still not fully understood. Here we review the biochemical and physical processes involved in eliminating the products of light absorption from the photoreceptor outer segment, in recycling the released retinoid to its original isomeric form as 11-cis retinal, and in regenerating the visual pigment rhodopsin. Then we analyse the time-course of three aspects of human dark adaptation: the recovery of psychophysical threshold, the recovery of rod photoreceptor circulating current, and the regeneration of rhodopsin. We begin with normal human subjects, and then analyse the recovery in several retinal disorders, including Oguchi disease, vitamin A deficiency, fundus albipunctatus, Bothnia dystrophy and Stargardt disease. We review a large body of evidence showing that the time-course of human dark adaptation and pigment regeneration is determined by the local concentration of 11-cis retinal, and that after a large bleach the recovery is limited by the rate at which 11-cis retinal is delivered to opsin in the bleached rod outer segments. We present a mathematical model that successfully describes a wide range of results in human and other mammals. The theoretical analysis provides a simple means of estimating the relative concentration of free 11-cis retinal in the retina/RPE, in disorders exhibiting slowed dark adaptation, from analysis of psychophysical measurements of threshold recovery or from analysis of pigment regeneration kinetics.

A new look at the Bezold–Brücke hue shift in the peripheral retina

Experiments were conducted with a bipartite field to better understand the Bezold-Brücke hue shift in the peripheral retina. The first experiment measured hue shift in the fovea and at 1 degrees and 8 degrees along the horizontal meridian of the nasal retina for nominal test wavelengths of 430, 450, 490, 520 and 610 nm. Peripheral measurements were obtained under two adaptation conditions: after 30 min dark adaptation and following a rod-bleach. Results indicated that foveal hue shifts differed from those obtained after a rod-bleach. Data from the rod-bleach and no-bleach conditions in the periphery were similar, indicating that rods could not account for the differences between the foveal data and the rod-bleach peripheral data. Hue shifts obtained for the 520 nm test stimulus, and to a smaller extent other test wavelengths, at 8 degrees nasal retinal eccentricity revealed that the wavelength of the matching stimulus depended upon the lateral position of the matching and test fields, and this effect was greater in the no-bleach condition than the rod-bleach condition. Several factors were investigated in experiments 2 and 3 to explain the results with the 520 nm test field. It appears that differential rod density under the two half fields and the compression of photoreceptors by the optic disk may partially, but not fully, account for the 520 nm effect.

Recovery of the human photopic electroretinogram after bleaching exposures: estimation of pigment regeneration kinetics

We used a fibre electrode in the lower conjunctival sac of the human eye to record the a-wave of the photopic electroretinogram elicited in response to dim red flashes, delivered in the presence of a rod-saturating blue background, before and after exposure of the eye to bright white illumination that bleached a significant fraction of cone photopigment. Responses were recorded from two normal subjects whose pupils were maximally dilated. A range of intensities of bleaching light were used, from 500 to 3000 photopic cd m(-2), and exposures were made sufficiently long in duration to achieve a steady-state bleach. In addition, responses were also recorded following shorter durations of exposures to the highest intensity (3000 cd m(-2)); these durations ranged from 5 to 60 s. The amplitude of the a-wave response to dim flashes was reduced following the exposures, with brighter or longer exposures causing greater reduction. The amplitude then recovered within about 4 min to the prebleach level. The amplitudes measured at ca 15 ms after the flash were used to derive the effective intensity of the flashes, thereby quantifying the fraction of photopigment available at the time of delivery of each flash. Recovery from all exposures in both subjects followed a common time course, which could be described well by a model of pigment kinetics based on rate-limited regeneration, where the initial rate of recovery following a total bleach was ca 50% of the total pigment per minute, and the residual pigment level for half the maximal rate was ca 20% of the total pigment. The same parameters, together with a fixed photosensitivity, could account for the steady-state pigment levels seen at each bleaching intensity, and also for the fraction of pigment bleached following exposures of different duration at the highest intensity. The dim-flash ERG thus provides a novel method for assessing pigment regeneration in vivo. Our finding that pigment regeneration follows rate-limited kinetics may explain previous reports of pigment regeneration deviating from first order kinetics. We present a model of regeneration in which the rate limit arises from a limitation in the delivery of 11-cis-retinoid to the photoreceptor outer segments.

Mechanistic studies of ABCR, the ABC transporter in photoreceptor outer segments responsible for autosomal recessive Stargardt disease

ABCR is an ABC transporter that is found exclusively in vertebrate photoreceptor outer segments. Mutations in the human ABCR gene are responsible for autosomal recessive Stargardt disease, the most common cause of early onset macular degeneration. In this paper we review our recent work with purified and reconstituted ABCR derived from bovine retina and from cultured cells expressing wild type or site-directed mutants of human ABCR. These experiments implicate all-trans-retinal (or Schiff base adducts between all-trans-retinal and phosphatidylethanolamine) as the transport substrate, and they reveal asymmetric roles for the two nucleotide binding domains in the transport reaction. A model for the retinal transport reaction is presented which accounts for these experimental observations.

New developments in the visual cycle: functional role of 11-cis retinyl esters in the retinal pigment epithelium

Although both 11-cis and all-tmns retinyl esters exist in the retinal pigment epithelium, the relative importance of each in the visual cycle has been unclear. Recent data indicate that there are 2 biochemical pathways leading to the formation of 11-cis retinoids from the retinal pigment epithelium pool of retinyl esters. One well-established pathway is located in the endoplasmic reticulum where all-trans retinyl esters are hydrolyzed, isomerized, and then oxidized to form 11-cis retinal (endoplasmic reticulum pathway). A more recently identified pathway resides within the plasma membrane where 11-cis retinyl esters are hydrolyzed directly to 11-cis retinol (plasma membrane pathway). Either or both pathways may provide 11-cis retinoids for regeneration of rod and cone visual pigments. Recent reports have suggested that the regeneration of rod and cone pigments are carried out by different mechanisms, and that 11-cis retinyl esters (plasma membrane pathway) in the retinal pigment epithelium may be specific to cone pigment regeneration. In this paper we review both visual pathways and consider data in support of the hypothesis that 1 of these 2 pathways' (the plasma membrane pathway) functions to provide visual pigment chromophores selectively for cone photoreceptors.

A photic visual cycle of rhodopsin regeneration is dependent on Rgr

During visual excitation, rhodopsin undergoes photoactivation and bleaches to opsin and all-trans-retinal. To regenerate rhodopsin and maintain normal visual sensitivity, the all-trans isomer must be metabolized and reisomerized to produce the chromophore 11-cis-retinal in biochemical steps that constitute the visual cycle and involve the retinal pigment epithelium (RPE; refs. 3-8). A key step in the visual cycle is isomerization of an all-trans retinoid to 11-cis-retinol in the RPE (refs. 9-11). It could be that the retinochrome-like opsins, peropsin, or the retinal G protein-coupled receptor (RGR) opsin12-16 are isomerases in the RPE. In contrast to visual pigments, RGR is bound predominantly to endogenous all-trans-retinal, and irradiation of RGR in vitro results in stereospecific conversion of the bound all-trans isomer to 11-cis-retinal. Here we show that RGR is involved in the formation of 11-cis-retinal in mice and functions in a light-dependent pathway of the rod visual cycle. Mutations in the human gene encoding RGR are associated with retinitis pigmentosa.

Interaction of 11-cis-retinol dehydrogenase with the chromophore of retinal g protein-coupled receptor opsin

Vertebrate opsins in both photoreceptors and the retinal pigment epithelium (RPE) have fundamental roles in the visual process. The visual pigments in photoreceptors are bound to 11-cis-retinal and are responsible for the initiation of visual excitation. Retinochrome-like opsins in the RPE are bound to all-trans-retinal and play an important role in chromophore metabolism. The retinal G protein-coupled receptor (RGR) of the RPE and Müller cells is an abundant opsin that generates 11-cis-retinal by stereospecific photoisomerization of its bound all-trans-retinal chromophore. We have analyzed a 32-kDa protein (p32) that co-purifies with bovine RGR from RPE microsomes. The co-purified p32 was identified by mass spectrometric analysis as 11-cis-retinol dehydrogenase (cRDH), and enzymatic assays have confirmed the isolation of an active cRDH. The co-purified cRDH showed marked substrate preference to 11-cis-retinal and preferred NADH rather than NADPH as the cofactor in reduction reactions. cRDH did not react with endogenous all-trans-retinal bound to RGR but reacted specifically with 11-cis-retinal that was generated by photoisomerization after irradiation of RGR. The reduction of 11-cis-retinal to 11-cis-retinol by cRDH enhanced the net photoisomerization of all-trans-retinal bound to RGR. These results indicate that cRDH is involved in the processing of 11-cis-retinal after irradiation of RGR opsin and suggest that cRDH has a novel role in the visual cycle.

Effect of NADPH on formation and decay of human metarhodopsin III at physiological temperatures

Difference absorption spectra were recorded during the formation and decay of metarhodopsin III after sonicated membrane suspensions of rhodopsin were bleached at 37 degrees C. The data were analyzed using SVD, spectral decomposition and global exponential fitting. By comparison of the results in the presence or absence of 70 microM NADPH and those for bovine or human rhodopsin, a single comprehensive scheme was fit to all the data, including reduction of retinal to retinol by the intrinsic retinol dehydrogenase. On the time scale studied the mechanism involves two 382 nm absorbing species and two 468 nm, absorbing species, supporting the notion that human metarhodopsin III is not a homogeneous species. The results confirm that metarhodopsin III forms and persists sufficiently long in the human retina under physiological conditions that it could undergo secondary photoisomerization.

Human cone photoreceptor responses measured by the electroretinogram [correction of electoretinogram] a-wave during and after exposure to intense illumination

We recorded the a-wave of the electroretinogram from human subjects with normal vision, using a corneal fibre electrode and ganzfeld stimulation under photopic conditions, so as to extract the parameters of cone phototransduction. The amplitude of bright flash responses provided a measure of the massed circulating current of the cones, while the amplitude of dim flash responses provided a measure of the product of the fraction of cone photopigment present, and the amplification constant of transduction within the cones. In the presence of steady background illumination, the cone circulating current declined to half at 3000 photopic trolands, and to a quarter at 20 000 photopic trolands. At very early times after the delivery of a near-total bleach, we could not determine the level of circulating current as our bright flashes did not appear to saturate the a-wave (presumably because so little pigment was present). However, by 20-30 s after a total bleach, the cone circulating current had returned to its dark-adapted level. Following smaller bleaches (when ca 50 % of the pigment remained present) the bright flashes were able to saturate the a-wave even at very early times. Within 3 s of extinction of the illumination, the cone circulating current had returned to its dark-adapted level. This is at least a factor of 300 times faster than the period of ca 15 min required for full recovery of rods exposed to the same level of bleach, and indicates a major difference between rods and cones in the way that they cope with the photoproducts of bleaching. Despite the very rapid recovery of circulating current after bleaches, the recovery of dim-flash sensitivity was much slower, with a time constant of ca 1.5 min after a near-total bleach. This time course is very similar to previous measurements of the regeneration of cone photopigment, and it seems highly probable that the reduction in dim-flash sensitivity results from pigment depletion.

The spatial arrangement of L and M cones in the peripheral human retina

The spatial arrangement of L and M cones in the human peripheral retina was estimated from red-green color naming of small test flashes (0.86 min of arc, 555 nm, constant intensity) presented at different locations (grid with 1.5 min of arc steps) centered at 17 degrees temporal eccentricity. Simulated red-green color naming ratings were generated by a model based on an ideal observer for all possible patterns of placement and relative numerosities of L and M cones, constrained by the anatomical data on the statistics of cone spacing at this retinal location. The best matching simulated performance as compared to the human observer's data determined the cone array most likely to produce that observer's color naming results. The mosaics for two color normal observers showed L and M cones randomly arrayed over this retinal region. Consequences of random cone placements for spectral sampling and color opponency are discussed.

Identification and characterization of all-trans-retinol dehydrogenase from photoreceptor outer segments, the visual cycle enzyme that reduces all-trans-retinal to all-trans-retinol

Retinol dehydrogenase (RDH), the enzyme that catalyzes the reduction of all-trans-retinal to all-trans-retinol within the photoreceptor outer segment, was the first visual cycle enzymatic activity to be identified. Previous work has shown that this enzyme utilizes NADPH, shows a marked preference for all-trans-retinal over 11-cis-retinal, and is tightly associated with the outer segment membrane. This paper reports the identification of a novel member of the short chain dehydrogenase/reductase family, photoreceptor RDH (prRDH), using subtraction and normalization of retina cDNA, high throughput sequencing, and data base homology searches to detect retina-specific genes. Bovine and human prRDH are highly homologous and are most closely related to 17-beta-hydroxysteroid dehydrogenase 1. The enzymatic properties of recombinant bovine prRDH closely match those previously reported for RDH activity in crude bovine rod outer segment preparations. In situ hybridization and RNA blotting show that the PRRDH gene is expressed specifically in photoreceptor cells, and protein blotting and immunocytochemistry show that prRDH localizes exclusively to both rod and cone outer segments and that prRDH is tightly associated with outer segment membranes. Taken together, these data indicate that prRDH is the enzyme responsible for the reduction of all-trans-retinal to all-trans-retinol within the photoreceptor outer segment.

L and M cone relative numerosity and red-green opponency from fovea to midperiphery in the human retina

The relative numerosity of the long-wavelength-sensitive (L) and middle-wavelength-sensitive (M) cones and the red-green color appearance, as assessed by means of unique yellow, are stable from fovea to midperiphery (+/- 28 deg nasotemporal). As foveal tests decrease in size, unique yellow progressively shifts toward longer wavelengths, favoring a model of red-green opponency carried by cells whose centers receive input from either L or M cones and whose surrounds receive mixed contributions from both. Individual differences in unique yellow over a 20-nm range and the relative numerosity of L and M cones can be linked by means of this model, suggesting that the relative number of L and M cones is a factor that regulates individual variations in red-green color appearance.

Effect of the short-wavelength-sensitive-cone mosaic and rods on the locus of unique green

A primary goal of this study was to establish whether the magnitude of the short-wavelength-sensitive- (S-) cone signal into the yellow/blue (Y/B) mechanism was influenced by the absolute or the relative numbers of S cones. This was assessed by measuring the locus of unique green for various test sizes at four eccentric locations chosen to exploit differences in the underlying mosaic of S cones. In general, the locus of unique green was unaffected by test size, retinal quadrant, or rod input but was influenced by retinal eccentricity. The locus of unique green shifted to shorter wavelengths as retinal eccentricity increased from 1 degrees to 8 degrees. The data do not support a model whereby the S-cone signal is determined by the absolute number of S cones, but a model based on the relative number of S cones cannot be eliminated.

Aging and dark adaptation

Older adults have serious difficulty seeing under low illumination and at night, even in the absence of ocular disease. Optical changes in the aged eye, such as pupillary miosis and increased lens density, cannot account for the severity of this problem, and little is known about its neural basis. Dark adaptation functions were measured on 94 adults ranging in age from the 20s to the 80s to assess the rate of rod-mediated sensitivity recovery after exposure to a 98% bleach. Fundus photography and a grading scale were used to characterize macular health in subjects over age 49 in order to control for macular disease. Thresholds for each subject were corrected for lens density based on individual estimates, and pupil diameter was controlled. Results indicated that during human aging there is a dramatic slowing in rod-mediated dark adaptation that can be attributed to delayed rhodopsin regeneration. During the second component of the rod-mediated phase of dark adaptation, the rate of sensitivity recovery decreased 0.02 log unit/min per decade, and the time constant of rhodopsin regeneration increased 8.4 s/decade. The amount of time to reach within 0.3 log units of baseline scotopic sensitivity increased 2.76 min/decade. These aging-related changes in rod-mediated dark adaptation may contribute to night vision problems commonly experienced by the elderly.

Light adaptation and dark adaptation of human rod photoreceptors measured from the a-wave of the electroretinogram

1. We recorded the a-wave of the human electroretinogram from subjects with normal vision, using a corneal electrode and ganzfeld (full-field) light stimulation. From analysis of the rising phase of rod-isolated flash responses we determined the maximum size (amax) of the a-wave, a measure of the massed circulating current of the rods, and the amplification constant (A) of transduction within the rod photoreceptors. 2. During light adaptation by steady backgrounds the maximal response was reduced, as reported previously. amax declined approximately as I0/(I0 + IB), where IB is retinal illuminance and I0 is a constant. In different subjects I0 ranged from 40 to 100 trolands, with a mean of 70 trolands, corresponding to about 600 photoisomerizations s-1 per rod. (1 troland is the retinal illuminance that results when a surface luminance of 1 cd m-2 is viewed through a pupil area of 1 mm2.) The amplification constant A decreased only slightly in the presence of steady backgrounds. 3. Following a full bleach amax recovered along an S-shaped curve over a period of 30 min. There was no detectable response for the first 5 min, and half-maximal recovery took 13-17 min. 4. The apparent amplification constant decreased at early times after large bleaches. However, upon correction for reduced light absorption due to loss of pigment, with regeneration of rhodopsin occurring with a time constant of 9-15 min in different subjects, it appeared that the true value of A was probably unchanged by bleaching. 5. The recovery of amax following a bleach could be converted into recovery of equivalent background intensity, using a 'Crawford transformation' derived from the light adaptation results. Following bleaches ranging from 10 to > 99 %, the equivalent background intensity decayed approximately exponentially, with a time constant of about 3 min. 6. The time taken for amax to recover to a fixed proportion of its original level increased approximately linearly (rather than logarithmically) with fractional bleach, with a slope of about 12 min per 100 % bleach. Similar behaviour has previously been seen in psychophysical dark adaptation experiments, for the dependence of the 'second component' of recovery on the level of bleaching.

Analysis of red/green color discrimination in subjects with a single X-linked photopigment gene

Many subjects despite having only a single X-linked pigment gene (single-L/M-gene subjects) are able to make chromatic discriminations by Rayleigh matching, especially when large fields are used. We used a combination of psychophysics (Rayleigh match), electroretinograms (ERG), and molecular genetic techniques to rule out several possible explanations of this phenomenon. Use of rods for chromatic discrimination was unlikely since strong adapting fields were employed and the large-field match results were not consistent with rod participation. A putative mid- to long-wavelength photopigment that escapes detection by current molecular genetic analysis was ruled out by finding only a single L/M photopigment in flicker ERGs from 16 single-L/M-gene subjects. Large-field match results were not consistent with participation of S cones. Amino acid sequence polymorphisms in the S-pigment gene that might have shifted the S cone spectrum towards longer wavelengths were not found on sequencing. The mechanism of chromatic discrimination in the presence of a single photopigment therefore remains unknown. Further possible explanations such as variations in cone pigment density and retinal inhomogeneities are discussed.

Effect of the S-cone mosaic and rods on red/green equilibria

The loci of unique blue and unique yellow were measured with and without a rod bleach for various test sizes in the fovea and at 1 and 8 deg nasal and superior retinal eccentricities. Test sizes and retinal positions were selected to systematically manipulate the absolute and relative numbers of S cones underlying the test stimuli. The results revealed the following: (1) The locus of unique blue shifted to longer wavelengths as the absolute number of S cones underlying the test stimulus increased, suggesting that the S-cone neural weighting factor of the red/green (R/G) opponent model is linked to the absolute number of S cones. (2) In general, the locus of unique yellow remained invariant, although changes were observed in the superior retina. This finding indicates that either the L-to-M-cone ratio may not be invariant across all retinal quadrants or that this ratio may not determine the locus of unique yellow. (3) Rod signals affected the locus of the unique hues, especially at small test sizes, demonstrating an influence of rods on the R/G opponent mechanism.

Molecular characterization of a novel short-chain dehydrogenase/reductase that reduces all-trans-retinal

The reduction of all-trans-retinal in photoreceptor outer segments is the first step in the regeneration of bleached visual pigments. We report here the cloning of a dehydrogenase, retSDR1, that belongs to the short-chain dehydrogenase/reductase superfamily and localizes predominantly in cone photoreceptors. retSDR1 expressed in insect cells displayed substrate specificities of the photoreceptor all-trans-retinol dehydrogenase. Homology modeling of retSDR1 using the carbonyl reductase structure as a scaffold predicted a classical Rossmann fold for the nucleotide binding, and an N-terminal extension that could facilitate binding of the enzyme to the cell membranes. The presence of retSDR1 in a subset of inner retinal neurons and in other tissues suggests that the enzyme may also be involved in retinol metabolism outside of photoreceptors.

Reduction of all-trans-retinal limits regeneration of visual pigment in mice

Absorption of photons by pigments in photoreceptor cells results in photoisomerization of the chromophore, 11-cis-retinal, to all-trans-retinal and activation of opsin. Photolysed chromophore is converted back to the 11-cis-configuration via several enzymatic steps in photoreceptor and retinal pigment epithelial cells. We investigated the levels of retinoids in mouse retina during constant illumination and regeneration in the dark as a means of obtaining more information about the rate-limiting step of the visual cycle and about cycle intermediates that could be responsible for desensitization of the visual system. All-trans-retinal accumulated in the retinas during constant illumination and following flash illumination. Decay of all-trans-retinal in the dark following constant illumination occurred without substantial accumulation of all-trans-retinal, generated by constant approximately equal to visual pigment regeneration (t1/2 approximately 5 and t1/2 approximately 7 min, respectively). All-trans-retinal, generated by constant illumination, decayed approximately 3 times more rapidly than that generated by a flash and, as shown previously, the rate of rhodopsin regeneration following a flash was approximately 4 times slower than after constant illumination. The retinyl ester pool (> 95% all-trans-retinyl ester) did not show a statistically significant change in size or composition during illumination. In addition, constant illumination increased the amount of photoreceptor membrane-associated arrestin. The results suggest that the rate-limiting step of the visual cycle is the reduction of all-trans-retinal to all-trans-retinol by all-trans-retinol dehydrogenase. The accumulation of all-trans-retinal during illumination may be responsible, in part, for the reduction in sensitivity of the visual system that accompanies photobleaching and may contribute to the development of retinal pathology associated with light damage and aging.