Photoreceptor light adaptation is mediated by cytoplasmic calcium concentration

Distribution of intracellular Ca2+-ATPases in the mouse retina and their involvement in light-induced cone degeneration

Calcium signalling is involved in many processes in mammalian retina, from development to mature functions and neurodegeneration. Although proteins involved in Ca2+ entry in retinal cells have been well studied, less is known about Ca2+-clearance. Among the Ca2+ pumps, plasma membrane Ca2+-ATPases (PMCAs) have been identified as key proteins extruding Ca2+ across the plasma membrane with specific distribution in developing and adult retina. However, the two main isoforms of intracellular Ca2+-ATPases in the central nervous system, the sarco(endo)plasmic reticulum (ER) Ca2+-ATPase 2b (SERCA2b) and the secretory pathway Ca2+-ATPase 1 (SPCA1), which remove cytosolic Ca2+ into intracellular stores, have been less or not at all analysed, respectively. In this study, we described for the first time the SPCA1 localisation in adult mouse retina and we report differential distributions of SERCA2b and SPCA1 transporters within various classes of retinal neurons and distinct subcellular localisations. In addition, we studied the expression and localisation of both Ca2+ pumps in 661W cells, a cone photoreceptor-derived cell line. Since continuous exposure to high light intensity induces photodegeneration, we analysed the effect of LED light exposure on these cells and SERCA2b and SPCA1 distribution. We found that continuous mild LED-light exposure compromised cell survival and produced stress in the ER and Golgi, the Ca2+ stores where the two pumps are localised. These effects were reversed after halting light exposure and washing. This study demonstrates that Ca2+ signalling may be involved in light-induced photoreceptor cell damage and points to previously unrecognised functions of intracellular Ca2+-ATPases in retina physiology.

Analysis of dim-light responses in rod and cone photoreceptors with altered calcium kinetics

Rod and cone photoreceptors in the retina of vertebrates are the primary sensory neurons underlying vision. They convert light into an electrical current using a signal transduction pathway that depends on Ca[Formula: see text] feedback. It is known that manipulating the Ca[Formula: see text] kinetics affects the response shape and the photoreceptor sensitivity, but a precise quantification of these effects remains unclear. We have approached this task in mouse retina by combining numerical simulations with mathematical analysis. We consider a parsimonious phototransduction model that incorporates negative Ca[Formula: see text] feedback onto the synthesis of cyclic GMP, and fast buffering reactions to alter the Ca[Formula: see text] kinetics. We derive analytic results for the photoreceptor functioning in sufficiently dim light conditions depending on the photoreceptor type. We exploit these results to obtain conceptual and quantitative insight into how response waveform and amplitude depend on the underlying biophysical processes and the Ca[Formula: see text] feedback. With a low amount of buffering, the Ca[Formula: see text] concentration changes in proportion to the current, and responses to flashes of light are monophasic. With more buffering, the change in the Ca[Formula: see text] concentration becomes delayed with respect to the current, which gives rise to a damped oscillation and a biphasic waveform. This shows that biphasic responses are not necessarily a manifestation of slow buffering reactions. We obtain analytic approximations for the peak flash amplitude as a function of the light intensity, which shows how the photoreceptor sensitivity depends on the biophysical parameters. Finally, we study how changing the extracellular Ca[Formula: see text] concentration affects the response.

Retinal Encoding of Natural Scenes

An ultimate goal in retina science is to understand how the neural circuit of the retina processes natural visual scenes. Yet most studies in laboratories have long been performed with simple, artificial visual stimuli such as full-field illumination, spots of light, or gratings. The underlying assumption is that the features of the retina thus identified carry over to the more complex scenario of natural scenes. As the application of corresponding natural settings is becoming more commonplace in experimental investigations, this assumption is being put to the test and opportunities arise to discover processing features that are triggered by specific aspects of natural scenes. Here, we review how natural stimuli have been used to probe, refine, and complement knowledge accumulated under simplified stimuli, and we discuss challenges and opportunities along the way toward a comprehensive understanding of the encoding of natural scenes. Expected final online publication date for the Annual Review of Vision Science, Volume 8 is September 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Molecular basis of rod and cone differences

In the vertebrate retina, rods and cones both detect light, but they are different in functional aspects such as light sensitivity and time resolution, for example, and in some of cell biological aspects. For functional aspects, both photoreceptors are known to share a common mechanism, phototransduction cascade, consisting of a series of enzyme reactions to convert a photon-capture signal to an electrical signal. To understand the mechanisms of the functional differences between rods and cones at the molecular level, we compared biochemically each of the reactions in the phototransduction cascade between rods and cones using the cells isolated and purified from carp retina. Although proteins in the cascade are functionally similar between rods and cones, their activities together with their expression levels are mostly different between these photoreceptors. In general, reactions to generate a response are slightly less effective, as a total, in cones than in rods, but each of the reactions for termination and recovery of a response are much more effective in cones. These findings explain lower light sensitivity and briefer light responses in cones than in rods. In addition, our considerations suggest that a Ca²⁺-binding protein, S-modulin or recoverin, has a currently unnoticed role in shaping light responses. With comparison of the expression levels of proteins and/or mRNAs using purified cells, several proteins were found to be specifically or predominantly expressed in cones. These proteins would be of interest for future studies on the difference between rods and cones.

Biochemistry and physiology of zebrafish photoreceptors

All vertebrates share a canonical retina with light-sensitive photoreceptors in the outer retina. These photoreceptors are of two kinds: rods and cones, adapted to low and bright light conditions, respectively. They both show a peculiar morphology, with long outer segments, comprised of ordered stacks of disc-shaped membranes. These discs host numerous proteins, many of which contribute to the visual transduction cascade. This pathway converts the light stimulus into a biological signal, ultimately modulating synaptic transmission. Recently, the zebrafish (Danio rerio) has gained popularity for studying the function of vertebrate photoreceptors. In this review, we introduce this model system and its contribution to our understanding of photoreception with a focus on the cone visual transduction cascade.

Clinical vision and molecular loss: Integrating visual psychophysics with molecular genetics reveals key details of normal and abnormal visual processing

Over the past two decades we have developed techniques and models to investigate the ways in which known molecular defects affect visual performance. Because molecular defects in retinal signalling invariably alter the speed of visual processing, our strategy has been to measure the resulting changes in flicker sensitivity. Flicker measurements provide not only straightforward clinical assessments of visual performance but also reveal fundamental details about the functioning of both abnormal and normal visual systems. Here, we bring together our past measurements of patients with pathogenic variants in the GNAT2, RGS9, GUCA1A, RPE65, OPA1, KCNV2 and NR2E3 genes and analyse the results using a standard model of visual processing. The model treats flicker sensitivity as the result of the actions of a sequence of simple processing steps, one or more of which is altered by the genetic defect. Our analyses show that most defects slow down the visual response directly, but some speed it up. Crucially, however, other steps in the processing sequence can make compensatory adjustments to offset the abnormality. For example, if the abnormal step slows down the visual response, another step is likely to speed up or attenuate the response to rebalance system performance. Such compensatory adjustments are probably made by steps in the sequence that usually adapt to changing light levels. Our techniques and modelling also allow us to tease apart stationary and progressive effects, and the localised molecular losses help us to unravel and characterise individual steps in the normal and abnormal processing sequences.

Evolution of the genes mediating phototransduction in rod and cone photoreceptors

This paper reviews current knowledge of the evolution of the multiple genes encoding proteins that mediate the process of phototransduction in rod and cone photoreceptors of vertebrates. The approach primarily involves molecular phylogenetic analysis of phototransduction protein sequences, combined with analysis of the syntenic arrangement of the genes. At least 35 of these phototransduction genes appear to reside on no more than five paralogons - paralogous regions that each arose from a common ancestral region. Furthermore, it appears that such paralogs arose through quadruplication during the two rounds of genome duplication (2R WGD) that occurred in a chordate ancestor prior to the vertebrate radiation, probably around 600 millions years ago. For several components of the phototransduction cascade, it is shown that distinct isoforms already existed prior to WGD, with the likely implication that separate classes of scotopic and photopic photoreceptor cells had already evolved by that stage. The subsequent quadruplication of the entire genome then permitted the refinement of multiple distinct protein isoforms in rods and cones. A unified picture of the likely pattern and approximate timing of all the important gene duplications is synthesised, and the implications for our understanding of the evolution of rod and cone phototransduction are presented.

Pathway-specific light adaptation in human electroretinograms

The cellular origins of slow ERG changes during light adaptation following a dark-adapted state are still unclear. To study light adaptation, six healthy, normal trichromats were dark-adapted for 30 min prior to full-field ERG recordings to sinusoidal stimuli that isolate responses of the L- or M-cones or that stimulate luminance and chromatic mechanisms at 12 or 36 Hz. Recordings were performed for 16 min with 2-min intervals after onset of a constant background. Generally, the responses were sine-wave-like, and the first harmonic (fundamental) component dominated the Fourier spectrum except for the 12-Hz luminance stimulus in which two components, a sine-wave-like component and a transient component, determined the response profiles, leading to large second harmonic components. The amplitude of the first harmonic component (F) increased as a function of the light-adaptation time except for the 12-Hz luminance stimulus at which the F component decreased as a function of the light-adaptation period. The phase of the first harmonic component changed only slightly (less than 30°) during the light-adaptation period for all stimuli conditions. The L/M ratio in luminance reflecting ERGs decreased with increasing adaptation time. Our present data suggest that the light-adaptation process mainly reflects changes in the luminance pathway. The responses to 12-Hz luminance stimuli are determined by two different luminance driven pathways with different adaptation characteristics.

Rod Photoresponse Kinetics Limit Temporal Contrast Sensitivity in Mesopic Vision

The mammalian visual system operates over an extended range of ambient light levels by switching between rod and cone photoreceptors. Rod-driven vision is sluggish, highly sensitive, and operates in dim or scotopic lights, whereas cone-driven vision is brisk, less sensitive, and operates in bright or photopic lights. At intermediate or mesopic lights, vision transitions seamlessly from rod-driven to cone-driven, despite the profound differences in rod and cone response dynamics. The neural mechanisms underlying such a smooth handoff are not understood. Using an operant behavior assay, electrophysiological recordings, and mathematical modeling we examined the neural underpinnings of the mesopic visual transition in mice of either sex. We found that rods, but not cones, drive visual sensitivity to temporal light variations over much of the mesopic range. Surprisingly, speeding up rod photoresponse recovery kinetics in transgenic mice improved visual sensitivity to slow temporal variations, in the range where perceptual sensitivity is governed by Weber's law of sensation. In contrast, physiological processes acting downstream from phototransduction limit sensitivity to high frequencies and temporal resolution. We traced the paradoxical control of visual temporal sensitivity to rod photoresponses themselves. A scenario emerges where perceptual sensitivity is limited by: (1) the kinetics of neural processes acting downstream from phototransduction in scotopic lights, (2) rod response kinetics in mesopic lights, and (3) cone response kinetics as light levels rise into the photopic range.SIGNIFICANCE STATEMENT Our ability to detect flickering lights is constrained by the dynamics of the slowest step in the visual pathway. Cone photoresponse kinetics limit visual temporal sensitivity in bright (photopic) lights, whereas mechanisms in the inner retina limit sensitivity in dim (scotopic) lights. The neural mechanisms underlying the transition between scotopic and photopic vision in mesopic lights, when both rods are cones are active, are unknown. This study provides a missing link in this mechanism by establishing that rod photoresponse kinetics limit temporal sensitivity during the mesopic transition. Surprisingly, this range is where Weber's Law of Sensation governs temporal contrast sensitivity in mouse. Our results will help guide future studies of complex and dynamic interactions between rod-cone signals in the mesopic retina.

Investigating the Ca2+-dependent and Ca2+-independent mechanisms for mammalian cone light adaptation

Vision is mediated by two types of photoreceptors: rods, enabling vision in dim light; and cones, which function in bright light. Despite many similarities in the components of their respective phototransduction cascades, rods and cones have distinct sensitivity, response kinetics, and adaptation capacity. Cones are less sensitive and have faster responses than rods. In addition, cones can function over a wide range of light conditions whereas rods saturate in moderately bright light. Calcium plays an important role in regulating phototransduction and light adaptation of rods and cones. Notably, the two dominant Ca²⁺-feedbacks in rods and cones are driven by the identical calcium-binding proteins: guanylyl cyclase activating proteins 1 and 2 (GCAPs), which upregulate the production of cGMP; and recoverin, which regulates the inactivation of visual pigment. Thus, the mechanisms producing the difference in adaptation capacity between rods and cones have remained poorly understood. Using GCAPs/recoverin-deficient mice, we show that mammalian cones possess another Ca²⁺-dependent mechanism promoting light adaptation. Surprisingly, we also find that, unlike in mouse rods, a unique Ca²⁺-independent mechanism contributes to cone light adaptation. Our findings point to two novel adaptation mechanisms in mouse cones that likely contribute to the great adaptation capacity of cones over rods.

Evolution of the calcium feedback steps of vertebrate phototransduction

We examined the genes encoding the proteins that mediate the Ca-feedback regulatory system in vertebrate rod and cone phototransduction. These proteins comprise four families: recoverin/visinin, the guanylyl cyclase activating proteins (GCAPs), the guanylyl cyclases (GCs) and the sodium/calcium-potassium exchangers (NCKXs). We identified a paralogon containing at least 36 phototransduction genes from at least fourteen families, including all four of the families involved in the Ca-feedback loop (recoverin/visinin, GCAPs, GCs and NCKXs). By combining analyses of gene synteny with analyses of the molecular phylogeny for each of these four families of genes for Ca-feedback regulation, we have established the likely pattern of gene duplications and losses underlying the expansion of isoforms, both before and during the two rounds of whole-genome duplication (2R WGD) that occurred in early vertebrate evolution. Furthermore, by combining our results with earlier evidence on the timing of duplication of the visual G-protein receptor kinase genes, we propose that specialization of proto-vertebrate photoreceptor cells for operation at high and low light intensities preceded the emergence of rhodopsin, which occurred during 2R WGD.

Regulation of calcium homeostasis in the outer segments of rod and cone photoreceptors

Calcium plays important roles in the function and survival of rod and cone photoreceptor cells. Rapid regulation of calcium in the outer segments of photoreceptors is required for the modulation of phototransduction that drives the termination of the flash response as well as light adaptation in rods and cones. On a slower time scale, maintaining proper calcium homeostasis is critical for the health and survival of photoreceptors. Decades of work have established that the level of calcium in the outer segments of rods and cones is regulated by a dynamic equilibrium between influx via the transduction cGMP-gated channels and extrusion via rod- and cone-specific Na⁺/Ca²⁺, K⁺ exchangers (NCKXs). It had been widely accepted that the only mechanism for extrusion of calcium from rod outer segments is via the rod-specific NCKX1, while extrusion from cone outer segments is driven exclusively by the cone-specific NCKX2. However, recent evidence from mice lacking NCKX1 and NCKX2 have challenged that notion and have revealed a more complex picture, including a NCKX-independent mechanism in rods and two separate NCKX-dependent mechanisms in cones. This review will focus on recent findings on the molecular mechanisms of extrusion of calcium from the outer segments of rod and cone photoreceptors, and the functional and structural changes in photoreceptors when normal extrusion is disrupted.

Vision Made Easy: Cubozoans Can Advance Our Understanding of Systems-Level Visual Information Processing

Animals relying on vision as their main sensory modality reserve a large part of their central nervous system to appropriately navigate their environment. In general, neural involvement correlates to the complexity of the visual system and behavioural repertoire. In humans, one third of the available neural capacity supports our single-chambered general-purpose eyes, whereas animals with less elaborate visual systems need less computational power, and generally have smaller brains, and thereby lack in visual behaviour. As a consequence, both traditional model animals (mice, zebrafish, and flies) and more experimentally tractable animals (Hydra, Planaria, and C. elegans) cannot contribute to our understanding of systems-level visual information processing-a Goldilocks case of too big and too small.However, one animal, the box jellyfish Tripedalia cystophora, possesses a rather complex visual system, displays multiple visual behaviours, yet processes visual information by means of a relatively simple central nervous system. This-just right-model system could not only provide information on how visual stimuli are processed through distinct combinations of neural circuitry but also provide a processing algorithm for extracting specific information from a complex visual scene.

The Na+/Ca2+, K+ exchanger NCKX4 is required for efficient cone-mediated vision

Calcium (Ca2+) plays an important role in the function and health of neurons. In vertebrate cone photoreceptors, Ca2+ controls photoresponse sensitivity, kinetics, and light adaptation. Despite the critical role of Ca2+ in supporting the function and survival of cones, the mechanism for its extrusion from cone outer segments is not well understood. Here, we show that the Na+/Ca2+, K+ exchanger NCKX4 is expressed in zebrafish, mouse, and primate cones. Functional analysis of NCKX4-deficient mouse cones revealed that this exchanger is essential for the wide operating range and high temporal resolution of cone-mediated vision. We show that NCKX4 shapes the cone photoresponse together with the cone-specific NCKX2: NCKX4 acts early to limit response amplitude, while NCKX2 acts late to further accelerate response recovery. The regulation of Ca2+ by NCKX4 in cones is a novel mechanism that supports their ability to function as daytime photoreceptors and promotes their survival.

Light adaptation and the evolution of vertebrate photoreceptors

KEY POINTS

Lamprey are cyclostomes, a group of vertebrates that diverged from lines leading to jawed vertebrates (including mammals) in the late Cambrian, 500 million years ago. It may therefore be possible to infer properties of photoreceptors in early vertebrate progenitors by comparing lamprey to other vertebrates. We show that lamprey rods and cones respond to light much like rods and cones in amphibians and mammals. They operate over a similar range of light intensities and adapt to backgrounds and bleaches nearly identically. These correspondences are pervasive and detailed; they argue for the presence of rods and cones very early in the evolution of vertebrates with properties much like those of rods and cones in existing vertebrate species.

ABSTRACT

The earliest vertebrates were agnathans - fish-like organisms without jaws, which first appeared near the end of the Cambrian radiation. One group of agnathans became cyclostomes, which include lamprey and hagfish. Other agnathans gave rise to jawed vertebrates or gnathostomes, the group including all other existing vertebrate species. Because cyclostomes diverged from other vertebrates 500 million years ago, it may be possible to infer some of the properties of the retina of early vertebrate progenitors by comparing lamprey to other vertebrates. We have previously shown that rods and cones in lamprey respond to light much like photoreceptors in other vertebrates and have a similar sensitivity. We now show that these affinities are even closer. Both rods and cones adapt to background light and to bleaches in a manner almost identical to other vertebrate photoreceptors. The operating range in darkness is nearly the same in lamprey and in amphibian or mammalian rods and cones; moreover background light shifts response-intensity curves downward and to the right over a similar range of ambient intensities. Rods show increment saturation at about the same intensity as mammalian rods, and cones never saturate. Bleaches decrease sensitivity in part by loss of quantum catch and in part by opsin activation of transduction. These correspondences are so numerous and pervasive that they are unlikely to result from convergent evolution but argue instead that early vertebrate progenitors of both cyclostomes and mammals had photoreceptors much like our own.

Distinct subcomponents of mouse retinal ganglion cell receptive fields are differentially altered by light adaptation

The remarkable dynamic range of vision is facilitated by adaptation of retinal sensitivity to ambient lighting conditions. An important mechanism of sensitivity adaptation is control of the spatial and temporal window over which light is integrated. The retina accomplishes this by switching between parallel synaptic pathways with differing kinetics and degrees of synaptic convergence. However, the relative shifts in spatial and temporal integration are not well understood - particularly in the context of the antagonistic spatial surround. Here, we resolve these issues by characterizing the adaptation-induced changes to spatiotemporal integration in the linear receptive field center and surround of mouse retinal ganglion cells. While most ganglion cells lose their antagonistic spatial surround under scotopic conditions, a strong surround is maintained in a subset. We then applied a novel technique that allowed us to analyze the receptive field as a triphasic temporal filter in the center and a biphasic filter in the surround. The temporal tuning of the surround was relatively maintained across adaptation conditions compared to the center, which greatly increased its temporal integration. Though all phases of the center's triphasic temporal response slowed, some shifted significantly less. Additionally, adaptation differentially shifted ON and OFF pathway temporal tuning, reducing their asymmetry under scotopic conditions. Finally, spatial integration was significantly increased by dark adaptation in some cells while it decreased it in others. These findings provide novel insight into how adaptation adjusts visual information processing by altering fundamental properties of ganglion cell receptive fields, such as center-surround antagonism and space-time integration.

The Na(+)/Ca(2+), K(+) exchanger 2 modulates mammalian cone phototransduction

Calcium ions (Ca²⁺) modulate the phototransduction cascade of vertebrate cone photoreceptors to tune gain, inactivation, and light adaptation. In darkness, the continuous current entering the cone outer segment through cGMP-gated (CNG) channels is carried in part by Ca²⁺, which is then extruded back to the extracellular space. The mechanism of Ca²⁺ extrusion from mammalian cones is not understood. The dominant view has been that the cone-specific isoform of the Na⁺/Ca²⁺, K⁺ exchanger, NCKX2, is responsible for removing Ca²⁺ from their outer segments. However, indirect evaluation of cone function in NCKX2-deficient (Nckx2^−/−) mice by electroretinogram recordings revealed normal photopic b-wave responses. This unexpected result suggested that NCKX2 may not be involved in the Ca²⁺ homeostasis of mammalian cones. To address this controversy, we examined the expression of NCKX2 in mouse cones and performed transretinal recordings from Nckx2^−/− mice to determine the effect of NCKX2 deletion on cone function directly. We found that Nckx2^−/− cones exhibit compromised phototransduction inactivation, slower response recovery and delayed background adaptation. We conclude that NCKX2 is required for the maintenance of efficient Ca²⁺ extrusion from mouse cones. However, surprisingly, Nckx2^−/− cones adapted normally in steady background light, indicating the existence of additional Ca²⁺-extruding mechanisms in mammalian cones.

Why are rods more sensitive than cones?

One hundred and fifty years ago Max Schultze first proposed the duplex theory of vision, that vertebrate eyes have two types of photoreceptor cells with differing sensitivity: rods for dim light and cones for bright light and colour detection. We now know that this division is fundamental not only to the photoreceptors themselves but to the whole of retinal and visual processing. But why are rods more sensitive, and how did the duplex retina first evolve? Cells resembling cones are very old, first appearing among cnidarians; the emergence of rods was a key step in the evolution of the vertebrate eye. Many transduction proteins have different isoforms in rods and cones, and others are expressed at different levels. Moreover rods and cones have a different anatomy, with only rods containing membranous discs enclosed by the plasma membrane. These differences must be responsible for the difference in absolute sensitivity, but which are essential? Recent research particularly expressing cone proteins in rods or changing the level of expression seem to show that many of the molecular differences in the activation and decay of the response may have each made a small contribution as evolution proceeded stepwise with incremental increases in sensitivity. Rod outer-segment discs were not essential and developed after single-photon detection. These experiments collectively provide a new understanding of the two kinds of photoreceptors and help to explain how gene duplication and the formation of rod-specific proteins produced the duplex retina, which has remained remarkably constant in physiology from amphibians to man.

Distinct signaling of Drosophila chemoreceptors in olfactory sensory neurons

In Drosophila, olfactory sensory neurons (OSNs) rely primarily on two types of chemoreceptors, odorant receptors (Ors) and ionotropic receptors (Irs), to convert odor stimuli into neural activity. The cellular signaling of these receptors in their native OSNs remains unclear because of the difficulty of obtaining intracellular recordings from Drosophila OSNs. Here, we developed an antennal preparation that enabled the first recordings (to our knowledge) from targeted Drosophila OSNs through a patch-clamp technique. We found that brief odor pulses triggered graded inward receptor currents with distinct response kinetics and current-voltage relationships between Or- and Ir-driven responses. When stimulated with long-step odors, the receptor current of Ir-expressing OSNs did not adapt. In contrast, Or-expressing OSNs showed a strong Ca(2+)-dependent adaptation. The adaptation-induced changes in odor sensitivity obeyed the Weber-Fechner relation; however, surprisingly, the incremental sensitivity was reduced at low odor backgrounds but increased at high odor backgrounds. Our model for odor adaptation revealed two opposing effects of adaptation, desensitization and prevention of saturation, in dynamically adjusting odor sensitivity and extending the sensory operating range.

A new mouse model for stationary night blindness with mutant Slc24a1 explains the pathophysiology of the associated human disease

Mutations that affect calcium homeostasis (Ca(2+)) in rod photoreceptors are linked to retinal degeneration and visual disorders such as retinitis pigmentosa and congenital stationary night blindness (CSNB). It is thought that the concentration of Ca(2+) in rod outer segments is controlled by a dynamic balance between influx via cGMP-gated (CNG) channels and extrusion via Na(+)/Ca(2+), K(+) exchangers (NCKX1). The extrusion-driven lowering of rod [Ca(2+)]i following light exposure controls their light adaptation and response termination. Mutant NCKX1 has been linked to autosomal-recessive stationary night blindness. However, whether NCKX1 contributes to light adaptation has not been directly tested and the mechanisms by which human NCKX1 mutations cause night blindness are not understood. Here, we report that the deletion of NCKX1 in mice results in malformed outer segment disks, suppressed expression and function of rod CNG channels and a subsequent 100-fold reduction in rod responses, while preserving normal cone responses. The compensating loss of CNG channel function in the absence of NCKX1-mediated Ca(2+) extrusion may prevent toxic Ca(2+) buildup and provides an explanation for the stationary nature of the associated disorder in humans. Surprisingly, the lack of NCKX1 did not compromise rod background light adaptation, suggesting additional Ca(2+)-extruding mechanisms exist in these cells.

Regulation of mammalian cone phototransduction by recoverin and rhodopsin kinase

Cone photoreceptors function under daylight conditions and are essential for color perception and vision with high temporal and spatial resolution. A remarkable feature of cones is that, unlike rods, they remain responsive in bright light. In rods, light triggers a decline in intracellular calcium, which exerts a well studied negative feedback on phototransduction that includes calcium-dependent inhibition of rhodopsin kinase (GRK1) by recoverin. Rods and cones share the same isoforms of recoverin and GRK1, and photoactivation also triggers a calcium decline in cones. However, the molecular mechanisms by which calcium exerts negative feedback on cone phototransduction through recoverin and GRK1 are not well understood. Here, we examined this question using mice expressing various levels of GRK1 or lacking recoverin. We show that although GRK1 is required for the timely inactivation of mouse cone photoresponse, gradually increasing its expression progressively delays the cone response recovery. This surprising result is in contrast with the known effect of increasing GRK1 expression in rods. Notably, the kinetics of cone responses converge and become independent of GRK1 levels for flashes activating more than ∼1% of cone pigment. Thus, mouse cone response recovery in bright light is independent of pigment phosphorylation and likely reflects the spontaneous decay of photoactivated visual pigment. We also find that recoverin potentiates the sensitivity of cones in dim light conditions but does not contribute to their capacity to function in bright light.

Fixational eye movements in the earliest stage of metazoan evolution

All known photoreceptor cells adapt to constant light stimuli, fading the retinal image when exposed to an immobile visual scene. Counter strategies are therefore necessary to prevent blindness, and in mammals this is accomplished by fixational eye movements. Cubomedusae occupy a key position for understanding the evolution of complex visual systems and their eyes are assumedly subject to the same adaptive problems as the vertebrate eye, but lack motor control of their visual system. The morphology of the visual system of cubomedusae ensures a constant orientation of the eyes and a clear division of the visual field, but thereby also a constant retinal image when exposed to stationary visual scenes. Here we show that bell contractions used for swimming in the medusae refresh the retinal image in the upper lens eye of Tripedalia cystophora. This strongly suggests that strategies comparable to fixational eye movements have evolved at the earliest metazoan stage to compensate for the intrinsic property of the photoreceptors. Since the timing and amplitude of the rhopalial movements concur with the spatial and temporal resolution of the eye it circumvents the need for post processing in the central nervous system to remove image blur.

Origin and control of the dominant time constant of salamander cone photoreceptors

Recovery of the light response in vertebrate photoreceptors requires the shutoff of both active intermediates in the phototransduction cascade: the visual pigment and the transducin–phosphodiesterase complex. Whichever intermediate quenches more slowly will dominate photoresponse recovery. In suction pipette recordings from isolated salamander ultraviolet- and blue-sensitive cones, response recovery was delayed, and the dominant time constant slowed when internal [Ca²⁺] was prevented from changing after a bright flash by exposure to 0Ca²⁺/0Na⁺ solution. Taken together with a similar prior observation in salamander red-sensitive cones, these observations indicate that the dominance of response recovery by a Ca²⁺-sensitive process is a general feature of amphibian cone phototransduction. Moreover, changes in the external pH also influenced the dominant time constant of red-sensitive cones even when changes in internal [Ca²⁺] were prevented. Because the cone photopigment is, uniquely, exposed to the external solution, this may represent a direct effect of protons on the equilibrium between its inactive Meta I and active Meta II forms, consistent with the notion that the process dominating recovery of the bright flash response represents quenching of the active Meta II form of the cone photopigment.

Adaptation of mammalian photoreceptors to background light: putative role for direct modulation of phosphodiesterase

All sensory receptors adapt. As the mean level of light or sound or odor is altered, the sensitivity of the receptor is adjusted to permit the cell to function over as wide a range of ambient stimulation as possible. In a rod photoreceptor, adaptation to maintained background light produces a decrease (or "sag") in the response to the prolonged illumination, as well as an acceleration in response decay time and a Weber-Fechner-like decrease in sensitivity. Earlier work on salamander indicated that adaptation is controlled by the intracellular concentration of Ca(2+). Three Ca(2+)-dependent mechanisms were subsequently identified, namely, regulation of guanylyl cyclase, modulation of activated rhodopsin lifetime, and alteration of channel opening probability, with the contribution of the cyclase thought to be the most important. Later experiments on mouse that exploit the powerful techniques of molecular genetics have shown that cyclase does indeed play a significant role in mammalian rods, but that much of adaptation remains even when regulation of cyclase and both of the other proposed pathways have been genetically deleted. The identity of the missing mechanism or mechanisms is unclear, but recent speculation has focused on direct modulation of spontaneous and light-activated phosphodiesterase.

Role of guanylyl cyclase modulation in mouse cone phototransduction

A negative phototransduction feedback in rods and cones is critical for the timely termination of their light responses and for extending their function to a wide range of light intensities. The calcium feedback mechanisms that modulate phototransduction in rods have been studied extensively. However, the corresponding modulation mechanisms that enable cones to terminate rapidly their light responses and to adapt in bright light, properties critical for our daytime vision, are still not understood. In cones, calcium feedback to guanylyl cyclase is potentially a key step in phototransduction modulation. The guanylyl cyclase activity is modulated by the calcium-binding guanylyl cyclase activating proteins (GCAP1 and GCAP2). Here, we used single-cell and transretinal recordings from mouse to determine how GCAPs modulate dark-adapted responses as well as light adaptation in mammalian cones. Deletion of GCAPs increased threefold the amplitude and dramatically prolonged the light responses in dark-adapted mouse cones. It also reduced the operating range of mouse cones in background illumination and severely impaired their light adaptation. Thus, GCAPs exert powerful modulation on the mammalian cone phototransduction cascade and play an important role in setting the functional properties of cones in darkness and during light adaptation. Surprisingly, despite their better adaptation capacity and wider calcium dynamic range, mammalian cones were modulated by GCAPs to a lesser extent than mammalian rods. We conclude that a disparity in the strength of GCAP modulation cannot explain the differences in the dark-adapted properties or in the operating ranges of mammalian rods and cones.

Channel modulation and the mechanism of light adaptation in mouse rods

Vertebrate photoreceptors are thought to adapt to light by a change in Ca(2+), which is postulated to mediate modulation of (1) excited rhodopsin (Rh*) by Ca(2+)-dependent binding of recoverin, (2) guanylyl cyclase activity via Ca(2+)-dependent GCAP proteins, and (3) cyclic nucleotide-gated channels by binding of Ca(2+)-calmodulin. Previous experiments genetically deleted recoverin and the GCAPs and showed that significant regulation of sensitivity survives removal of (1) and (2). We genetically deleted the channel Ca(2+)-calmodulin binding site in the mouse Mus musculus and found that removal of (3) alters response waveform, but removal of (3) or of (2) and (3) together still leaves much of adaptation intact. These experiments demonstrate that an important additional mechanism is required, which other experiments indicate may be regulation of phosphodiesterase 6 (PDE6). We therefore constructed a kinetic model in which light produces a Ca(2+)-mediated decrease in PDE6 decay rate, with the novel feature that both spontaneously activated and light-activated PDE6 are modulated. This model, together with Ca(2+)-dependent acceleration of guanylyl cyclase, can successfully account for changes in sensitivity and response waveform in background light.

Photopigment quenching is Ca2+ dependent and controls response duration in salamander L-cone photoreceptors

The time scale of the photoresponse in photoreceptor cells is set by the slowest of the steps that quench the light-induced activity of the phototransduction cascade. In vertebrate photoreceptor cells, this rate-limiting reaction is thought to be either shutoff of catalytic activity in the photopigment or shutoff of the pigment's effector, the transducin-GTP–phosphodiesterase complex. In suction pipette recordings from isolated salamander L-cones, we found that preventing changes in internal [Ca²⁺] delayed the recovery of the light response and prolonged the dominant time constant for recovery. Evidence that the Ca²⁺-sensitive step involved the pigment itself was provided by the observation that removal of Cl⁻ from the pigment's anion-binding site accelerated the dominant time constant for response recovery. Collectively, these observations indicate that in L-cones, unlike amphibian rods where the dominant time constant is insensitive to [Ca²⁺], pigment quenching rate limits recovery and provides an additional mechanism for modulating the cone response during light adaptation.

Kinetics of turn-offs of frog rod phototransduction cascade

The time course of the light-induced activity of phototrandsuction effector enzyme cGMP-phosphodiesterase (PDE) is shaped by kinetics of rhodopsin and transducin shut-offs. The two processes are among the key factors that set the speed and sensitivity of the photoresponse and whose regulation contributes to light adaptation. The aim of this study was to determine time courses of flash-induced PDE activity in frog rods that were dark adapted or subjected to nonsaturating steady background illumination. PDE activity was computed from the responses recorded from solitary rods with the suction pipette technique in Ca²⁺-clamping solution.

A flash applied in the dark-adapted state elicits a wave of PDE activity whose rising and decaying phases have characteristic times near 0.5 and 2 seconds, respectively. Nonsaturating steady background shortens both phases roughly to the same extent. The acceleration may exceed fivefold at the backgrounds that suppress ≈70% of the dark current.

The time constant of the process that controls the recovery from super-saturating flashes (so-called dominant time constant) is adaptation independent and, hence, cannot be attributed to either of the processes that shape the main part of the PDE wave. We hypothesize that the dominant time constant in frog rods characterizes arrestin binding to rhodopsin partially inactivated by phosphorylation. A mathematical model of the cascade that considers two-stage rhodopsin quenching and transducin inactivation can mimic experimental PDE activity quite well. The effect of light adaptation on the PDE kinetics can be reproduced in the model by concomitant acceleration on both rhodopsin phosphorylation and transducin turn-off, but not by accelerated arrestin binding. This suggests that not only rhodopsin but also transducin shut-off is under adaptation control.

How vision begins: an odyssey

Retinal rods and cones, which are the front-end light detectors in the eye, achieve wonders together by being able to signal single-photon absorption and yet also able to adjust their function to brightness changes spanning 10(9)-fold. How these cells detect light is now quite well understood. Not surprising for almost any biological process, the intial step of seeing reveals a rich complexity as the probing goes deeper. The odyssey continues, but the knowledge gained so far is already nothing short of remarkable in qualitative and quantitative detail. It has also indirectly opened up the mystery of odorant sensing. Basic science aside, clinical ophthalmology has benefited tremendously from this endeavor as well. This article begins by recapitulating the key developments in this understanding from the mid-1960s to the late 1980s, during which period the advances were particularly rapid and fit for an intricate detective story. It then highlights some details discovered more recently, followed by a comparison between rods and cones.

Photoreceptor adaptation in intrinsically photosensitive retinal ganglion cells

A rare type of mammalian retinal ganglion cell (RGC) expresses the photopigment melanopsin and is a photoreceptor. These intrinsically photosensitive RGCs (ipRGCs) drive circadian-clock resetting, pupillary constriction, and other non-image-forming photic responses. Both the light responses of ipRGCs and the behaviors they drive are remarkably sustained, raising the possibility that, unlike rods and cones, ipRGCs do not adjust their sensitivity according to lighting conditions ("adaptation"). We found, to the contrary, that ipRGC sensitivity is plastic, strongly influenced by lighting history. When exposed to a constant, bright background, the background-evoked response decayed, and responses to superimposed flashes grew in amplitude, indicating light adaptation. After extinction of a light-adapting background, sensitivity recovered progressively in darkness, indicating dark adaptation. Because these adjustments in sensitivity persisted when synapses were blocked, they constitute "photoreceptor adaptation" rather than "network adaptation." Implications for the mechanisms generating various non-image-forming visual responses are discussed.

Toward a unified model of vertebrate rod phototransduction

Recently, we introduced a phototransduction model that was able to account for the reproducibility of vertebrate rod single-photon responses (SPRs) (Hamer et al., 2003). The model was able to reproduce SPR statistics by means of stochastic activation and inactivation of rhodopsin (R*), transducin (G alpha ), and phosphodiesterase (PDE). The features needed to capture the SPR statistics were (1) multiple steps of R* inactivation by means of multiple phosphorylations (followed by arrestin capping) and (2) phosphorylation dependence of the affinity between R* and the three molecules competing to bind with R* (G alpha, arrestin, and rhodopsin kinase). The model was also able to account for several other rod response features in the dim-flash regime, including SPRs obtained from rods in which various elements of the cascade have been genetically disabled or disrupted. However, the model was not tested under high light-level conditions. We sought to evaluate the extent to which the multiple phosphorylation model could simultaneously account for single-photon response behavior, as well as responses to high light levels causing complete response saturation and/or significant light adaptation (LA). To date no single model, with one set of parameters, has been able to do this. Dim-flash responses and statistics were simulated using a hybrid stochastic/deterministic model and Monte-Carlo methods as in Hamer et al. (2003). A dark-adapted flash series, and stimulus paradigms from the literature eliciting various degrees of light adaptation (LA), were simulated using a full differential equation version of the model that included the addition of Ca2+-feedback onto rhodopsin kinase via recoverin. With this model, using a single set of parameters, we attempted to account for (1) SPR waveforms and statistics (as in Hamer et al., 2003); (2) a full dark-adapted flash-response series, from dim flash to saturating, bright flash levels, from a toad rod; (3) steady-state LA responses, including LA circulating current (as in Koutalos et al., 1995) and LA flash sensitivity measured in rods from four species; (4) step responses from newt rods ( Forti et al., 1989) over a large dynamic range; (5) dynamic LA responses, such as the step-flash paradigm of Fain et al. (1989), and the two-flash paradigm of Murnick and Lamb (1996); and (6) the salient response features from four knockout rod preparations. The model was able to meet this stringent test, accounting for almost all the salient qualitative, and many quantitative features, of the responses across this broad array of stimulus conditions, including SPR reproducibility. The model promises to be useful in testing hypotheses regarding both normal and abnormal photoreceptor function, and is a good starting point for development of a full-range model of cone phototransduction. Informative limitations of the model are also discussed.

Opsin activation of transduction in the rods of dark-reared Rpe65 knockout mice

Rpe65 knockout mice (Rpe65-/-) are unable to synthesize the visual pigment chromophore 11-cis retinal; however, if these animals are reared in complete darkness, the rod photoreceptors accumulate a small amount of 9-cis retinal and its corresponding visual pigment isorhodopsin. Suction-electrode recording of single rods from dark-reared Rpe65-/- mice showed that the rods were about 400 times less sensitive than wild-type control rods and that the maximum responses were much smaller in amplitude. Spectral sensitivity measurements indicated that Rpe65-/- rod responses were generated by isorhodopsin rather than rhodopsin. Sensitivity and pigment concentration were compared in the same mice by measuring light responses from rods of one eye and pigment concentration from the retina of the other eye. Retinas had 11-35% of the normal pigment level, but the rods were of the order of 20-30 times less sensitive than could be accounted for by the loss in quantum catch. This extra desensitization must be caused by opsin-dependent activation of the visual cascade, which leads to a state equivalent to light adaptation in the dark-adapted rod. By comparing the sensitivity of dark-reared Rpe65-/- rods to that produced in normal rods by background light, we estimate that Rpe65-/- opsin is of the order of 2.5x10(-5) as efficient in activating transduction as photoactivated rhodopsin (Rh*) in WT mice. Dark-reared Rpe65-/- rods are less desensitized than rods from cyclic light-reared Rpe65-/- mice, have about 50% more photocurrent and degenerate at a slower rate. Retinas sectioned after 9 months in darkness show a larger number of photoreceptor nuclei in dark-reared animals than in cyclic light-reared animals, though both have fewer nuclei than in cyclic light-reared wild-type retinas. Both also have shorter outer segments and a lower free-Ca2+ concentration. These experiments provide the first quantitative measurement of opsin activation in physiologically responding mammalian rods.

The relationship between ambient lighting conditions, absolute dark-adapted thresholds, and rhodopsin in black and hypopigmented mice

Significant variation in absolute dark-adapted thresholds is observed both within and between strains of mice with differing ocular pigmentation levels. Differences in threshold within a single strain are related to the Williams' photostasis effect, that is, photoreceptor rhodopsin levels are dependent upon ambient lighting conditions. To examine threshold differences among strains, we equalized rhodopsin levels by maintaining albino mice (c2J/c2J) at 2 × 10−4 cd/m2 (dim light) and black mice at 2 × 102 cd/m2 (bright light). This resulted in ocular rhodopsin levels for albino mice (albino—dim) of 494 ± 11 pmoles/eye and rhodopsin levels for black mice (black—bright) of 506 ± 25 pmoles/eye. For comparison, rhodopsin levels in black mice maintained in dim light are 586 ± 46 pmoles/eye and 217 ± 46 pmoles/eye in albino mice maintained in bright light. We found similar dark-adapted thresholds (6.38 log cd/m2vs. 6.47 log cd/m2)) in albino and black mice with equivalent rhodopsin determined with a water maze test. This suggests that dark-adapted thresholds are directly related to rhodopsin levels regardless of the level of ocular melanin. The number of photoreceptors, photoreceptor layer thickness, and outer segment length did not differ significantly between albino (dark) and black mice (bright). These results demonstrate that the visual sensitivity defect found in hypopigmented animals is secondary to abnormal rhodopsin regulation and that hypopigmented animals have either an improper input to the photostasis mechanism or that the photostasis mechanism is defective.

The potential of Na+/Ca2+ exchange blockers in the treatment of cardiac disease

The Na(+)/Ca(2+) exchanger (NCX), a surface membrane antiporter, is the primary pathway for Ca(2+) efflux from the cardiac cell and a determinant of both the electrical and contractile state of the heart. Enhanced expression of NCX has recently been recognised as one of the molecular mechanisms that contributes to reduced Ca(2+) release, impaired contractility and an increased risk of arrhythmias during the development of cardiac hypertrophy and failure. The NCX has also been implicated in the mechanism of arrhythmias and cellular injury associated with ischaemia and reperfusion. Hence, NCX blockade represents a potential therapeutic strategy for treating cardiac disease, however, its reversibility and electrogenic properties must be taken into consideration when predicting the outcome. NCX inhibition has been demonstrated to be protective against ischaemic injury and to have a positive inotropic and antiarrhythmic effect in failing heart cells. However, progress has been impaired by the absence of clinically useful agents. Two drugs, KB-R7943 and SEA-0400, have been developed as NCX blockers but both lack specificity. Selective peptide inhibitors have been well characterised but are active only when delivered to the intracellular space. Gene therapy strategies may circumvent the latter problem in the future. This review discusses the effects of NCX blockade, supporting its potential as a new cardiovascular therapeutic strategy.

Visual pigment phosphorylation but not transducin translocation can contribute to light adaptation in zebrafish cones

The ability of cone photoreceptors to adapt to light is extraordinary. In this study we evaluated two biochemical processes, visual pigment phosphorylation and transducin translocation, for their ability to contribute to light adaptation in zebrafish cones. Since cytoplasmic Ca2+ regulates light adaptation, the sensitivities of these processes to both light and Ca2+ were examined. Cytoplasmic Ca2+ regulates the sites of light-stimulated phosphorylation. Unexpectedly, we found that Ca2+ also regulates the extent of phosphorylation of unbleached cone pigments. Immunocytochemical analyses revealed that neither light nor cytoplasmic Ca2+ influences the localization of transducin in zebrafish cones.

Step response of mouse rod photoreceptors modeled in terms of elemental photic signals

The process of light adaptation in rod photoreceptors enables these sensory cells of the retina to remain responsive to photic stimuli over a broad range of light intensity. Recent studies have employed the technique of paired-flash electroretinography to determine properties of phototransduction, and of light and dark adaptation, in rod photoreceptors in the living eye. Building on these studies, we have developed a theoretical model aimed at explaining the rod electrical response to a step of light based on known physiology. The central feature of the model is its description of the macroscopic (i.e., measured) response in terms of a time-evolving, weighted sum of elemental responses determined under dark-adapted and near fully light-adapted conditions. The model yields a time-dependent function that describes the course of desensitization and putatively represents the cumulative dynamics of underlying biochemical processes involved in light adaptation of the rod.