Influence of cone pigment coexpression on spectral sensitivity and color vision in the mouse

Maps of cone opsin input to mouse V1 and higher visual areas

Studies in the mouse retina have characterized the spatial distribution of an anisotropic ganglion cell and photoreceptor mosaic, which provides a solid foundation to study how the cortex pools from afferent parallel color channels. In particular, the mouse's retinal mosaic exhibits a gradient of wavelength sensitivity along its dorsoventral axis. Cones at the ventral extreme mainly express S opsin, which is sensitive to ultraviolet (UV) wavelengths. Then, moving toward the retina's dorsal extreme, there is a transition to M-opsin dominance. Here, we tested the hypothesis that the retina's opsin gradient is recapitulated in cortical visual areas as a functional map of wavelength sensitivity. We first identified visual areas in each mouse by mapping retinotopy with intrinsic signal imaging (ISI). Next, we measured ISI responses to stimuli along different directions of the S- and M-color plane to quantify the magnitude of S and M input to each location of the retinotopic maps in five visual cortical areas (V1, AL, LM, PM, and RL). The results illustrate a significant change in the S:M-opsin input ratio along the axis of vertical retinotopy that is consistent with the gradient along the dorsoventral axis of the retina. In particular, V1 populations encoding the upper visual field responded to S-opsin contrast with 6.1-fold greater amplitude than to M-opsin contrast. V1 neurons encoding lower fields responded with 4.6-fold greater amplitude to M- than S-opsin contrast. The maps in V1 and higher visual areas (HVAs) underscore the significance of a wavelength sensitivity gradient for guiding the mouse's behavior.NEW & NOTEWORTHY Two elements of this study are particularly novel. For one, it is the first to quantify cone inputs to mouse visual cortex; we have measured cone input in five visual areas. Next, it is the first study to identify a feature map in the mouse visual cortex that is based on well-characterized anisotropy of cones in the retina; we have identified maps of opsin selectivity in five visual areas.

Euarchontan Opsin Variation Brings New Focus to Primate Origins

Debate on the adaptive origins of primates has long focused on the functional ecology of the primate visual system. For example, it is hypothesized that variable expression of short- (SWS1) and middle-to-long-wavelength sensitive (M/LWS) opsins, which confer color vision, can be used to infer ancestral activity patterns and therefore selective ecological pressures. A problem with this approach is that opsin gene variation is incompletely known in the grandorder Euarchonta, that is, the orders Scandentia (treeshrews), Dermoptera (colugos), and Primates. The ancestral state of primate color vision is therefore uncertain. Here, we report on the genes (OPN1SW and OPN1LW) that encode SWS1 and M/LWS opsins in seven species of treeshrew, including the sole nocturnal scandentian Ptilocercus lowii. In addition, we examined the opsin genes of the Central American woolly opossum (Caluromys derbianus), an enduring ecological analogue in the debate on primate origins. Our results indicate: 1) retention of ultraviolet (UV) visual sensitivity in C. derbianus and a shift from UV to blue spectral sensitivities at the base of Euarchonta; 2) ancient pseudogenization of OPN1SW in the ancestors of P. lowii, but a signature of purifying selection in those of C. derbianus; and, 3) the absence of OPN1LW polymorphism among diurnal treeshrews. These findings suggest functional variation in the color vision of nocturnal mammals and a distinctive visual ecology of early primates, perhaps one that demanded greater spatial resolution under light levels that could support cone-mediated color discrimination.

Experimental illumination of natural habitat--an experimental set-up to assess the direct and indirect ecological consequences of artificial light of different spectral composition

Artificial night-time illumination of natural habitats has increased dramatically over the past few decades. Generally, studies that assess the impact of artificial light on various species in the wild make use of existing illumination and are therefore correlative. Moreover, studies mostly focus on short-term consequences at the individual level, rather than long-term consequences at the population and community level-thereby ignoring possible unknown cascading effects in ecosystems. The recent change to LED lighting has opened up the exciting possibility to use light with a custom spectral composition, thereby potentially reducing the negative impact of artificial light. We describe here a large-scale, ecosystem-wide study where we experimentally illuminate forest-edge habitat with different spectral composition, replicated eight times. Monitoring of species is being performed according to rigid protocols, in part using a citizen-science-based approach, and automated where possible. Simultaneously, we specifically look at alterations in behaviour, such as changes in activity, and daily and seasonal timing. In our set-up, we have so far observed that experimental lights facilitate foraging activity of pipistrelle bats, suppress activity of wood mice and have effects on birds at the community level, which vary with spectral composition. Thus far, we have not observed effects on moth populations, but these and many other effects may surface only after a longer period of time.

Spectral Sensitivity Measured with Electroretinogram Using a Constant Response Method

A new method is presented to determine the retinal spectral sensitivity function S(λ) using the electroretinogram (ERG). S(λ)s were assessed in three different species of myomorph rodents, Gerbils (Meriones unguiculatus), Wistar rats (Ratus norvegicus), and mice (Mus musculus). The method, called AC Constant Method, is based on a computerized automatic feedback system that adjusts light intensity to maintain a constant-response amplitude to a flickering stimulus throughout the spectrum, as it is scanned from 300 to 700 nm, and back. The results are presented as the reciprocal of the intensity at each wavelength required to maintain a constant peak to peak response amplitude. The resulting S(λ) had two peaks in all three rodent species, corresponding to ultraviolet and M cones, respectively: 359 nm and 511 nm for mice, 362 nm and 493 nm for gerbils, and 362 nm and 502 nm for rats. Results for mouse and gerbil were similar to literature reports of S(λ) functions obtained with other methods, confirming that the ERG associated to the AC Constant-Response Method was effective to obtain reliable S(λ) functions. In addition, due to its fast data collection time, the AC Constant Response Method has the advantage of keeping the eye in a constant light adapted state.

Optogenetics in Mice Performing a Visual Discrimination Task: Measurement and Suppression of Retinal Activation and the Resulting Behavioral Artifact

Optogenetic techniques are used widely to perturb and interrogate neural circuits in behaving animals, but illumination can have additional effects, such as the activation of endogenous opsins in the retina. We found that illumination, delivered deep into the brain via an optical fiber, evoked a behavioral artifact in mice performing a visually guided discrimination task. Compared with blue (473 nm) and yellow (589 nm) illumination, red (640 nm) illumination evoked a greater behavioral artifact and more activity in the retina, the latter measured with electrical recordings. In the mouse, the sensitivity of retinal opsins declines steeply with wavelength across the visible spectrum, but propagation of light through brain tissue increases with wavelength. Our results suggest that poor retinal sensitivity to red light was overcome by relatively robust propagation of red light through brain tissue and stronger illumination of the retina by red than by blue or yellow light. Light adaptation of the retina, via an external source of illumination, suppressed retinal activation and the behavioral artifact without otherwise impacting behavioral performance. In summary, long wavelength optogenetic stimuli are particularly prone to evoke behavioral artifacts via activation of retinal opsins in the mouse, but light adaptation of the retina can provide a simple and effective mitigation of the artifact.

Neuronal Representation of Ultraviolet Visual Stimuli in Mouse Primary Visual Cortex

The mouse has become an important model for understanding the neural basis of visual perception. Although it has long been known that mouse lens transmits ultraviolet (UV) light and mouse opsins have absorption in the UV band, little is known about how UV visual information is processed in the mouse brain. Using a custom UV stimulation system and in vivo calcium imaging, we characterized the feature selectivity of layer 2/3 neurons in mouse primary visual cortex (V1). In adult mice, a comparable percentage of the neuronal population responds to UV and visible stimuli, with similar pattern selectivity and receptive field properties. In young mice, the orientation selectivity for UV stimuli increased steadily during development, but not direction selectivity. Our results suggest that, by expanding the spectral window through which the mouse can acquire visual information, UV sensitivity provides an important component for mouse vision.

Advances in understanding the molecular basis of the first steps in color vision

Serving as one of our primary environmental inputs, vision is the most sophisticated sensory system in humans. Here, we present recent findings derived from energetics, genetics and physiology that provide a more advanced understanding of color perception in mammals. Energetics of cis-trans isomerization of 11-cis-retinal accounts for color perception in the narrow region of the electromagnetic spectrum and how human eyes can absorb light in the near infrared (IR) range. Structural homology models of visual pigments reveal complex interactions of the protein moieties with the light sensitive chromophore 11-cis-retinal and that certain color blinding mutations impair secondary structural elements of these G protein-coupled receptors (GPCRs). Finally, we identify unsolved critical aspects of color tuning that require future investigation.

Inhibitory Neuron Transplantation into Adult Visual Cortex Creates a New Critical Period that Rescues Impaired Vision

The maturation of inhibitory circuits in juvenile visual cortex triggers a critical period in the development of the visual system. Although several manipulations of inhibition can alter the timing of the critical period, none have demonstrated the creation of a new critical period in adulthood. We developed a transplantation method to reactivate critical period plasticity in the adult visual cortex. Transplanted embryonic inhibitory neurons from the medial ganglionic eminence reinstate ocular dominance plasticity in adult recipients. Transplanted inhibitory cells develop cell-type-appropriate molecular characteristics and visually evoked responses. In adult mice impaired by deprivation during the juvenile critical period, transplantation also recovers both visual cortical responses and performance on a behavioral test of visual acuity. Plasticity and recovery are induced when the critical period would have occurred in the donor animal. These results reveal that the focal reactivation of visual cortical plasticity using inhibitory cell transplantation creates a new critical period that restores visual perception after childhood deprivation.

Quantifying light-dependent circadian disruption in humans and animal models

Although circadian disruption is an accepted term, little has been done to develop methods to quantify the degree of disruption or entrainment individual organisms actually exhibit in the field. A variety of behavioral, physiological and hormonal responses vary in amplitude over a 24-h period and the degree to which these circadian rhythms are synchronized to the daily light-dark cycle can be quantified with a technique known as phasor analysis. Several studies have been carried out using phasor analysis in an attempt to measure circadian disruption exhibited by animals and by humans. To perform these studies, species-specific light measurement and light delivery technologies had to be developed based upon a fundamental understanding of circadian phototransduction mechanisms in the different species. When both nocturnal rodents and diurnal humans, experienced different species-specific light-dark shift schedules, they showed, based upon phasor analysis of the light-dark and activity-rest patterns, similar levels of light-dependent circadian disruption. Indeed, both rodents and humans show monotonically increasing and quantitatively similar levels of light-dependent circadian disruption with increasing shift-nights per week. Thus, phasor analysis provides a method for quantifying circadian disruption in the field and in the laboratory as well as a bridge between ecological measurements of circadian entrainment in humans and parametric studies of circadian disruption in animal models, including nocturnal rodents.

Number and distribution of mouse retinal cone photoreceptors: differences between an albino (Swiss) and a pigmented (C57/BL6) strain

We purpose here to analyze and compare the population and topography of cone photoreceptors in two mouse strains using automated routines, and to design a method of retinal sampling for their accurate manual quantification. In whole-mounted retinas from pigmented C57/BL6 and albino Swiss mice, the longwave-sensitive (L) and the shortwave-sensitive (S) opsins were immunodetected to analyze the population of each cone type. In another group of retinas both opsins were detected with the same fluorophore to quantify all cones. In a third set of retinas, L-opsin and Brn3a were immunodetected to determine whether L-opsin⁺cones and retinal ganglion cells (RGCs) have a parallel distribution. Cones and RGCs were automatically quantified and their topography illustrated with isodensity maps. Our results show that pigmented mice have a significantly higher number of total cones (all-cones) and of L-opsin⁺cones than albinos which, in turn, have a higher population of S-opsin⁺cones. In pigmented animals 40% of cones are dual (cones that express both opsins), 34% genuine-L (cones that only express the L-opsin), and 26% genuine-S (cones that only express the S-opsin). In albinos, 23% of cones are genuine-S and the proportion of dual cones increases to 76% at the expense of genuine-L cones. In both strains, L-opsin⁺cones are denser in the central than peripheral retina, and all-cones density increases dorso-ventrally. In pigmented animals S-opsin⁺cones are scarce in the dorsal retina and very numerous in the ventral retina, being densest in its nasal aspect. In albinos, S-opsin⁺cones are abundant in the dorsal retina, although their highest densities are also ventral. Based on the densities of each cone population, we propose a sampling method to manually quantify and infer their total population. In conclusion, these data provide the basis to study cone degeneration and its prevention in pathologic conditions.

Functional architecture of the retina: development and disease

Structure and function are highly correlated in the vertebrate retina, a sensory tissue that is organized into cell layers with microcircuits working in parallel and together to encode visual information. All vertebrate retinas share a fundamental plan, comprising five major neuronal cell classes with cell body distributions and connectivity arranged in stereotypic patterns. Conserved features in retinal design have enabled detailed analysis and comparisons of structure, connectivity and function across species. Each species, however, can adopt structural and/or functional retinal specializations, implementing variations to the basic design in order to satisfy unique requirements in visual function. Recent advances in molecular tools, imaging and electrophysiological approaches have greatly facilitated identification of the cellular and molecular mechanisms that establish the fundamental organization of the retina and the specializations of its microcircuits during development. Here, we review advances in our understanding of how these mechanisms act to shape structure and function at the single cell level, to coordinate the assembly of cell populations, and to define their specific circuitry. We also highlight how structure is rearranged and function is disrupted in disease, and discuss current approaches to re-establish the intricate functional architecture of the retina.

Light transmission of the ocular media in birds and mammals

Differences in the ultraviolet (UV) cutoff of ocular media between birds and mammals have been revealed by spectrophotometric measurements of the transmission of light wavelengths by the cornea, lens and vitreous body in chickens, crows, quails, rats, rabbits and pigs. The light transmission values of the cornea were shown to be above 50% for wavelengths of 330–800 nm in birds, 300–800 nm in rat and 310–800 nm in mammals except for rat. For the lens, the light transmission values were shown to be above 50% for wavelengths of 320–800 nm in birds and rat and 390–800 nm in mammals except for rat. Thus, among the ocular media, the cornea in birds and the lens in mammals except for rat may play a role as a major UV cutoff filter.

S cones: Evolution, retinal distribution, development, and spectral sensitivity

S cones expressing the short wavelength-sensitive type 1 (SWS1) class of visual pigment generally form only a minority type of cone photoreceptor within the vertebrate duplex retina. Hence, their primary role is in color vision, not in high acuity vision. In mammals, S cones may be present as a constant fraction of the cones across the retina, may be restricted to certain regions of the retina or may form a gradient across the retina, and in some species, there is coexpression of SWS1 and the long wavelength-sensitive (LWS) class of pigment in many cones. During retinal development, SWS1 opsin expression generally precedes that of LWS opsin, and evidence from genetic studies indicates that the S cone pathway may be the default pathway for cone development. With the notable exception of the cartilaginous fishes, where S cones appear to be absent, they are present in representative species from all other vertebrate classes. S cone loss is not, however, uncommon; they are absent from most aquatic mammals and from some but not all nocturnal terrestrial species. The peak spectral sensitivity of S cones depends on the spectral characteristics of the pigment present. Evidence from the study of agnathans and teleost fishes indicates that the ancestral vertebrate SWS1 pigment was ultraviolet (UV) sensitive with a peak around 360 nm, but this has shifted into the violet region of the spectrum (>380 nm) on many separate occasions during vertebrate evolution. In all cases, the shift was generated by just one or a few replacements in tuning-relevant residues. Only in the avian lineage has tuning moved in the opposite direction, with the reinvention of UV-sensitive pigments.

Adaptive divergence in seed color camouflage in contrasting soil environments

Although adaptive plant population divergence across contrasting soil conditions is often driven by abiotic soil factors, natural enemies may also contribute. Cryptic matching to the native soil color is a form of defensive camouflage that seeds can use to avoid detection by seed predators. The legume Acmispon wrangelianus occurs across a variety of gray-green serpentine soils and brown nonserpentine soils. Quantitative digital image analysis of seed and soil colors was used to test whether genetically based seed color is a closer match to the color of the native soil than to the color of other nearby soils. Lineages bear seeds that more closely match the color of their native serpentine or nonserpentine soil type than the opposing soil type. Further, even within a soil type, lineages bear seeds with a closer color match to the soil at their native site than to other sites. The striking concordance between seed and native soil color suggests that natural selection for locally camouflaged seed color morphs, probably driven by seed predators, may maintain adaptive divergence in pigmentation, despite the opportunity for migration between soil environments.

Imaging light responses of retinal ganglion cells in the living mouse eye

This study reports development of a novel method for high-resolution in vivo imaging of the function of individual mouse retinal ganglion cells (RGCs) that overcomes many limitations of available methods for recording RGC physiology. The technique combines insertion of a genetically encoded calcium indicator into RGCs with imaging of calcium responses over many days with FACILE (functional adaptive optics cellular imaging in the living eye). FACILE extends the most common method for RGC physiology, in vitro physiology, by allowing repeated imaging of the function of each cell over many sessions and by avoiding damage to the retina during removal from the eye. This makes it possible to track changes in the response of individual cells during morphological development or degeneration. FACILE also overcomes limitations of existing in vivo imaging methods, providing fine spatial and temporal detail, structure-function comparison, and simultaneous analysis of multiple cells.

Melanopsin-based brightness discrimination in mice and humans

Photoreception in the mammalian retina is not restricted to rods and cones but extends to a small number of intrinsically photoreceptive retinal ganglion cells (ipRGCs), expressing the photopigment melanopsin [1–4]. ipRGCs are known to support various accessory visual functions including circadian photoentrainment and pupillary reflexes. However, despite anatomical and physiological evidence that they contribute to the thalamocortical visual projection [5–7], no aspect of visual discrimination has been shown to rely upon ipRGCs. Based on their currently known roles, we hypothesized that ipRGCs may contribute to distinguishing brightness. This percept is related to an object's luminance—a photometric measure of light intensity relevant for cone photoreceptors. However, the perceived brightness of different sources is not always predicted by their respective luminance [8–12]. Here, we used parallel behavioral and electrophysiological experiments to first show that melanopsin contributes to brightness discrimination in both retinally degenerate and fully sighted mice. We continued to use comparable paradigms in psychophysical experiments to provide evidence for a similar role in healthy human subjects. These data represent the first direct evidence that an aspect of visual discrimination in normally sighted subjects can be supported by inner retinal photoreceptors.

The retinal mosaics of opsin expression in invertebrates and vertebrates

Color vision is found in many invertebrate and vertebrate species. It is the ability to discriminate objects based on the wavelength of emitted light independent of intensity. As it requires the comparison of at least two photoreceptor types with different spectral sensitivities, this process is often mediated by a mosaic made of several photoreceptor types. In this review, we summarize the current knowledge about the formation of retinal mosaics and the regulation of photopigment (opsin) expression in the fly, mouse, and human retina. Despite distinct evolutionary origins, as well as major differences in morphology and phototransduction machineries, there are significant similarities in the stepwise cell-fate decisions that lead from progenitor cells to terminally differentiated photoreceptors that express a particular opsin. Common themes include (i) the use of binary transcriptional switches that distinguish classes of photoreceptors, (ii) the use of gradients of signaling molecules for regional specializations, (iii) stochastic choices that pattern the retina, and (iv) the use of permissive factors with multiple roles in different photoreceptor types.

Visual landmarks facilitate rodent spatial navigation in virtual reality environments

Because many different sensory modalities contribute to spatial learning in rodents, it has been difficult to determine whether spatial navigation can be guided solely by visual cues. Rodents moving within physical environments with visual cues engage a variety of nonvisual sensory systems that cannot be easily inhibited without lesioning brain areas. Virtual reality offers a unique approach to ask whether visual landmark cues alone are sufficient to improve performance in a spatial task. We found that mice could learn to navigate between two water reward locations along a virtual bidirectional linear track using a spherical treadmill. Mice exposed to a virtual environment with vivid visual cues rendered on a single monitor increased their performance over a 3-d training regimen. Training significantly increased the percentage of time avatars controlled by the mice spent near reward locations in probe trials without water rewards. Neither improvement during training or spatial learning for reward locations occurred with mice operating a virtual environment without vivid landmarks or with mice deprived of all visual feedback. Mice operating the vivid environment developed stereotyped avatar turning behaviors when alternating between reward zones that were positively correlated with their performance on the probe trial. These results suggest that mice are able to learn to navigate to specific locations using only visual cues presented within a virtual environment rendered on a single computer monitor.

Spectral and temporal sensitivity of cone-mediated responses in mouse retinal ganglion cells

The retina uses two photoreceptor types to encode the wide range of light intensities in the natural environment. Rods mediate vision in dim light, whereas cones mediate vision in bright light. Mouse photoreceptors include only 3% cones, and the majority of these coexpress two opsins (short- and middle-wavelength sensitive, S and M), with peak sensitivity to either ultraviolet (360 nm) or green light (508 nm). The M/S-opsin ratio varies across the retina but has not been characterized functionally, preventing quantitative study of cone-mediated vision. Furthermore, physiological and behavioral measurements suggested that mouse retina supports relatively slow temporal processing (peak sensitivity, ∼ 2-5 Hz) compared to primates; however, past studies used visible wavelengths that are inefficient at stimulating mouse S-opsin. Here, we measured the M/S-opsin expression ratio across the mouse retina, as reflected by ganglion cell responses in vitro, and probed cone-mediated ganglion cell temporal properties using ultraviolet light stimulation and linear systems analysis. From recordings in mice lacking rod function (Gnat1(-/-), Rho(-/-)), we estimate ∼ 70% M-opsin expression in far dorsal retina, dropping to <5% M-opsin expression throughout ventral retina. In mice lacking cone function (Gnat2(cpfl3)), light-adapted rod-mediated responses peaked at ∼ 5-7 Hz. In wild-type mice, cone-mediated responses peaked at ∼ 10 Hz, with substantial responsiveness up to ∼ 30 Hz. Therefore, despite the small percentage of cones, cone-mediated responses in mouse ganglion cells are fast and robust, similar to those in primates. These measurements enable quantitative analysis of cone-mediated responses at all levels of the visual system.

Chromatic bipolar cell pathways in the mouse retina

Like most mammals, mice feature dichromatic color vision based on short (S) and middle (M) wavelength-sensitive cone types. It is thought that mammals share a retinal circuit that in dichromats compares S- and M-cone output to generate blue/green opponent signals, with bipolar cells (BCs) providing separate chromatic channels. Although S-cone-selective ON-BCs (type 9 in mouse) have been anatomically identified, little is known about their counterparts, the M-cone-selective OFF-BCs. Here, we characterized cone connectivity and light responses of selected mouse BC types using immunohistochemistry and electrophysiology. Our anatomical data indicate that four (types 2, 3a/b, and 4) of the five mouse OFF-BCs indiscriminately contact both cone types, whereas type 1 BCs avoid S-cones. Light responses showed that the chromatic tuning of the BCs strongly depended on their position along the dorsoventral axis because of the coexpression gradient of M- and S-opsin found in mice. In dorsal retina, where coexpression is low, most type 2 cells were green biased, with a fraction of cells (≈ 14%) displaying strongly blue-biased responses, likely reflecting S-cone input. Type 1 cells were also green biased but did not comprise blue-biased "outliers," consistent with type 1 BCs avoiding S-cones. We therefore suggest that type 1 represents the green OFF pathway in mouse. In addition, we confirmed that type 9 BCs display blue-ON responses. In ventral retina, all BC types studied here displayed similar blue-biased responses, suggesting that color vision is hampered in ventral retina. In conclusion, our data support an antagonistically organized blue/green circuit as the common basis for mammalian dichromatic color vision.

Why different regions of the retina have different spectral sensitivities: A review of mechanisms and functional significance of intraretinal variability in spectral sensitivity in vertebrates

Vision is used in nearly all aspects of animal behavior, from prey and predator detection to mate selection and parental care. However, the light environment typically is not uniform in every direction, and visual tasks may be specific to particular parts of an animal’s field of view. These spatial differences may explain the presence of several adaptations in the eyes of vertebrates that alter spectral sensitivity of the eye in different directions. Mechanisms that alter spectral sensitivity across the retina include (but are not limited to) variations in: corneal filters, oil droplets, macula lutea, tapeta, chromophore ratios, photoreceptor classes, and opsin expression. The resultant variations in spectral sensitivity across the retina are referred to as intraretinal variability in spectral sensitivity (IVSS). At first considered an obscure and rare phenomenon, it is becoming clear that IVSS is widespread among all vertebrates, and examples have been found from every major group. This review will describe the mechanisms mediating differences in spectral sensitivity, which are in general well understood, as well as explore the functional significance of intraretinal variability, which for the most part is unclear at best.

Diversity of color vision: not all Australian marsupials are trichromatic

Color vision in marsupials has recently emerged as a particularly interesting case among mammals. It appears that there are both dichromats and trichromats among closely related species. In contrast to primates, marsupials seem to have evolved a different type of trichromacy that is not linked to the X-chromosome. Based on microspectrophotometry and retinal whole-mount immunohistochemistry, four trichromatic marsupial species have been described: quokka, quenda, honey possum, and fat-tailed dunnart. It has, however, been impossible to identify the photopigment of the third cone type, and genetically, all evidence so far suggests that all marsupials are dichromatic. The tammar wallaby is the only Australian marsupial to date for which there is no evidence of a third cone type. To clarify whether the wallaby is indeed a dichromat or trichromatic like other Australian marsupials, we analyzed the number of cone types in the “dichromatic” wallaby and the “trichromatic” dunnart. Employing identical immunohistochemical protocols, we confirmed that the wallaby has only two cone types, whereas 20–25% of cones remained unlabeled by S- and LM-opsin antibodies in the dunnart retina. In addition, we found no evidence to support the hypothesis that the rod photopigment (rod opsin) is expressed in cones which would have explained the absence of a third cone opsin gene. Our study is the first comprehensive and quantitative account of color vision in Australian marsupials where we now know that an unexpected diversity of different color vision systems appears to have evolved.

Dark light, rod saturation, and the absolute and incremental sensitivity of mouse cone vision

Visual thresholds of mice for the detection of small, brief targets were measured with a novel behavioral methodology in the dark and in the presence of adapting lights spanning ∼8 log(10) units of intensity. To help dissect the contributions of rod and cone pathways, both wild-type mice and mice lacking rod (Gnat1(-/-)) or cone (Gnat2(cpfl3)) function were studied. Overall, the visual sensitivity of mice was found to be remarkably similar to that of the human peripheral retina. Rod absolute threshold corresponded to 12-15 isomerized pigment molecules (R*) in image fields of 800 to 3000 rods. Rod "dark light" (intrinsic retinal noise in darkness) corresponded to that estimated previously from single-cell recordings, 0.012 R* s(-1) rod(-1), indicating that spontaneous thermal isomerizations are responsible. Psychophysical rod saturation was measured for the first time in a nonhuman species and found to be very similar to that of the human rod monochromat. Cone threshold corresponded to ∼5 R* cone(-1) in an image field of 280 cones. Cone dark light was equivalent to ∼5000 R* s(-1) cone(-1), consistent with primate single-cell data but 100-fold higher than predicted by recent measurements of the rate of thermal isomerization of mouse cone opsins, indicating that nonopsin sources of noise determine cone threshold. The new, fully automated behavioral method is based on the ability of mice to learn to interrupt spontaneous wheel running on the presentation of a visual cue and provides an efficient and highly reliable means of examining visual function in naturally behaving normal and mutant mice.

Light perception in two strictly subterranean rodents: life in the dark or blue?

BACKGROUND

The African mole-rats (Bathyergidae, Rodentia) are strictly subterranean, congenitally microphthalmic rodents that are hardly ever exposed to environmental light. Because of the lack of an overt behavioural reaction to light, they have long been considered to be blind. However, recent anatomical studies have suggested retention of basic visual capabilities. In this study, we employed behavioural tests to find out if two mole-rat species are able to discriminate between light and dark, if they are able to discriminate colours and, finally, if the presence of light in burrows provokes plugging behaviour, which is assumed to have a primarily anti-predatory function.

METHODOLOGY/PRINCIPAL FINDING

We used a binary choice test to show that the silvery mole-rat Heliophobius argenteocinereus and the giant mole-rat Fukomys mechowii exhibit a clear photoavoidance response to full-spectrum ("white"), blue and green-yellow light, but no significant reaction to ultraviolet or red light during nest building. The mole-rats thus retain dark/light discrimination capabilities and a capacity to perceive short to medium-wavelength light in the photopic range of intensities. These findings further suggest that the mole-rat S opsin has its absorption maximum in the violet/blue part of the spectrum. The assay did not yield conclusive evidence regarding colour discrimination. To test the putative role of vision in bathyergid anti-predatory behaviour, we examined the reaction of mole-rats to the incidence of light in an artificial burrow system. The presence of light in the burrow effectively induced plugging of the illuminated tunnel.

CONCLUSION/SIGNIFICANCE

Our findings suggest that the photopic vision is conserved and that low acuity residual vision plays an important role in predator avoidance and tunnel maintenance in the African mole-rats.

Restoration of cone vision in the CNGA3-/- mouse model of congenital complete lack of cone photoreceptor function

Congenital absence of cone photoreceptor function is associated with strongly impaired daylight vision and loss of color discrimination in human achromatopsia. Here, we introduce viral gene replacement therapy as a potential treatment for this disease in the CNGA3(-/-) mouse model. We show that such therapy can restore cone-specific visual processing in the central nervous system even if cone photoreceptors had been nonfunctional from birth. The restoration of cone vision was assessed at different stages along the visual pathway. Treated CNGA3(-/-) mice were able to generate cone photoreceptor responses and to transfer these signals to bipolar cells. In support, we found morphologically that treated cones expressed regular cyclic nucleotide-gated (CNG) channel complexes and opsins in outer segments, which previously they did not. Moreover, expression of CNGA3 normalized cyclic guanosine monophosphate (cGMP) levels in cones, delayed cone cell death and reduced the inflammatory response of Müller glia cells that is typical of retinal degenerations. Furthermore, ganglion cells from treated, but not from untreated, CNGA3(-/-) mice displayed cone-driven, light-evoked, spiking activity, indicating that signals generated in the outer retina are transmitted to the brain. Finally, we demonstrate that this newly acquired sensory information was translated into cone-mediated, vision-guided behavior.

Circadian light

The present paper reflects a work in progress toward a definition of circadian light, one that should be informed by the thoughtful, century-old evolution of our present definition of light as a stimulus for the human visual system. This work in progress is based upon the functional relationship between optical radiation and its effects on nocturnal melatonin suppression, in large part because the basic data are available in the literature. Discussed here are the fundamental differences between responses by the visual and circadian systems to optical radiation. Brief reviews of photometry, colorimetry, and brightness perception are presented as a foundation for the discussion of circadian light. Finally, circadian light (CLA) and circadian stimulus (CS) calculation procedures based on a published mathematical model of human circadian phototransduction are presented with an example.

Bat eyes have ultraviolet-sensitive cone photoreceptors

Mammalian retinae have rod photoreceptors for night vision and cone photoreceptors for daylight and colour vision. For colour discrimination, most mammals possess two cone populations with two visual pigments (opsins) that have absorption maxima at short wavelengths (blue or ultraviolet light) and long wavelengths (green or red light). Microchiropteran bats, which use echolocation to navigate and forage in complete darkness, have long been considered to have pure rod retinae. Here we use opsin immunohistochemistry to show that two phyllostomid microbats, Glossophaga soricina and Carollia perspicillata, possess a significant population of cones and express two cone opsins, a shortwave-sensitive (S) opsin and a longwave-sensitive (L) opsin. A substantial population of cones expresses S opsin exclusively, whereas the other cones mostly coexpress L and S opsin. S opsin gene analysis suggests ultraviolet (UV, wavelengths <400 nm) sensitivity, and corneal electroretinogram recordings reveal an elevated sensitivity to UV light which is mediated by an S cone visual pigment. Therefore bats have retained the ancestral UV tuning of the S cone pigment. We conclude that bats have the prerequisite for daylight vision, dichromatic colour vision, and UV vision. For bats, the UV-sensitive cones may be advantageous for visual orientation at twilight, predator avoidance, and detection of UV-reflecting flowers for those that feed on nectar.

Light alters nociceptive effects of magnetic field shielding in mice: intensity and wavelength considerations

Previous experiments with mice have shown that repeated 1 hour daily exposure to an ambient magnetic field-shielded environment induces analgesia (antinociception). The exposures were carried out in the dark (less than 2.0x1016 photonss-1m-2) during the mid-light phase of the diurnal cycle. However, if the mice were exposed in the presence of visible light (2.0x1018 photonss-1m-2, 400-750 nm), then the analgesic effects of shielding were eliminated. Here, we show that this effect of light is intensity and wavelength dependent. Introduction of red light (peak at 635 nm) had little or no effect, presumably because mice do not have photoreceptors sensitive to red light above 600 nm in their eyes. By contrast, introduction of ultraviolet light (peak at 405 nm) abolished the effect, presumably because mice do have ultraviolet A receptors. Blue light exposures (peak at 465 nm) of different intensities demonstrate that the effect has an intensity threshold of approximately 12% of the blue light in the housing facility, corresponding to 5x1016 photonss-1m-2 (integral). This intensity is similar to that associated with photoreceptor-based magnetoreception in birds and in mice stimulates photopic/cone vision. Could the detection mechanism that senses ambient magnetic fields in mice be similar to that in bird navigation?

The topography of cone photoreceptors in the retina of a diurnal rodent, the agouti (Dasyprocta aguti)

The presence, density distribution, and mosaic regularity of cone types were studied in the retina of the diurnal agouti, Dasyprocta aguti. Longwave-sensitive (L-) and shortwave-sensitive (S-) cones were detected by antibodies against the respective cone opsins. L- and S-cones were found to represent around 90 and 10% of the cone population, respectively. There was no evidence for L- and S-opsin coexpression in agouti cones. L-cone densities were highest, up to 14,000/mm2, along a horizontal visual streak located about 2-3 mm dorsal to the optic nerve, and the L-cone distribution showed a dorsoventral asymmetry with higher densities in ventral (about 10,000/mm2) than in dorsal (about 4000/mm2) retinal regions. This L-cone topography parallels the agouti's ganglion cell topography. S-cones had a peak density of 1500-2000/mm2 in the central retinal region but did not form a visual streak. Their distribution also showed a dorsoventral asymmetry with densities around 600/mm2 in dorsal and around 1000/mm2 in ventral retinal regions. The patterning of cone arrays was assessed by the density recovery profile analysis. At all eccentricities evaluated, the S-cone mosaic less efficiently packed than the L-cone mosaic. Rod densities ranged from 47,000/mm2 in peripheral to 64,000/mm2 in central retina, and rod:cone ratios were 4:1-9:1. The comparatively low rod density and high cone proportion appear well adapted to the diurnal lifestyle of the agouti.

Postreceptoral contributions to the light-adapted ERG of mice lacking b-waves

The purpose of this study was to determine the contributions of postreceptoral neurons to the light-adapted ERG of the Nob mouse, a model for complete-type congenital stationary night blindness (CSNB1) that lacks a b-wave from depolarizing bipolar cells. Ganzfeld ERGs were recorded from anesthetized adult control mice, control mice injected intravitreally with L-2-amino-4-phosphonobutyric acid (Control APB mice) to remove On pathway activity, and Nob mice. ERGs also were recorded after PDA (cis-2,3-piperidine-dicarboxylic acid, 3-5mM) was injected to block transmission to hyperpolarizing (Off) bipolar and horizontal cells, and all third-order neurons. Stimuli were brief (<4ms, 0.4-2.5log sc td s) and long (200ms, 2.5-4.6log sc td) LED flashes (lambda(max)=513nm, on a rod suppressing background (2.6log sc td). Sinusoidal modulation of the LEDs (mean, 2.6log sc td; contrast, 100%; 3-36Hz) was used to study flicker ERGs. Brief-flash ERGs of Nob mice presented as long-lasting negative waves with a positive-going intrusion that started about 50ms after the flash and peaked around 120ms. Control APB mice had similar responses, and in both cases, PDA removed the positive-going intrusion. For long flashes, PDA removed a small, slow "d-wave" after light offset. With sinusoidal stimulation, the fundamental (F1) amplitude of control mice ERG peaked at 8Hz ( approximately 70microV). For Nob mice the peak was approximately 20microV at 6Hz before PDA and approximately 10muV at 3Hz or lower after PDA. F1 responses were present up to 21Hz in control and Nob eyes and 15Hz in Nob eyes after PDA. Between 3 and 6Hz, F1 phase was 170-210 degrees more delayed in Nob than control mice; phase was hardly altered by PDA. With vector analysis, a substantial postreceptoral input to the Nob flicker ERG was revealed. In control mice, the second harmonic (F2) response showed peaks of approximately 10mocrpV at 3Hz and 13Hz. Nob mice showed almost no F2. In summary, in this study it was found that in Nob mice, postreceptoral neurons from the Off pathway make a positive-going contribution to the light-adapted flash ERG, and contribute substantially to sinusoidal flicker ERG.

Molecular mechanisms of vertebrate retina development: Implications for ganglion cell and photoreceptor patterning

Although the neural retina appears as a relatively uniform tissue when viewed from its surface, it is in fact highly patterned along its anterior-posterior and dorso-ventral axes. The question of how and when such patterns arise has been the subject of intensive investigations over several decades. Most studies aimed at understanding retinal pattern formation have used the retinotectal map, the ordered projections of retinal ganglion cells to the brain, as a functional readout of the pattern. However, other cell types are also topographically organized in the retina. The most commonly recognized example of such a topographic cellular organization is the differential distribution of photoreceptor types across the retina. Photoreceptor patterns are highly species-specific and may represent an important adaptation to the visual niche a given species occupies. Nevertheless, few studies have addressed this functional readout of pattern to date and our understanding of its development has remained superficial. Here, we review recent advances in understanding the molecular cascades that control regionalization of the eye anlage, relate these findings to the development of photoreceptor patterns and discuss common and unique strategies involved in both aspects of retinal pattern formation.

Mouse cone photoresponses obtained with electroretinogram from the isolated retina

We characterize the dark-adapted photoresponses from mouse cones intact in the isolated retina, their virtually natural environment, by isolating pharmacologically the photoreceptor light responses from the electroretinogram (ERG). Due to the different photoresponse kinetics and sensitivity of rods and cones, the cone responses were readily attained by using a rod-saturating preflash. The stimulus wavelength (544 nm) was chosen to selectively stimulate the green sensitive ("M"-)pigment. Obtained responses were monophasic, showing fast kinetics (mean t(p)=51 ms) and low sensitivity (fractional single-photon response ca. 0.23%). The amplification coefficient of cones (4.6 s(-2)) was very close to that of rods (5.6 s(-2)), while the dominant time constant of recovery was clearly smaller for cones (33 ms) than for rods (160 ms).

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

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

Cone-based vision in the aging mouse

People often experience age-related declines in cone-based visual capacities despite an absence of apparent visual pathology. Although mice are used as models of human visual pathologies associated with aging, little is known about how age impacts vision in animals with disease-free retinas since most studies have heretofore examined relatively young mice. We examined the effects of age on cone-based vision by assessing opsin gene transcription, cone densities, the flicker electroretinogram (ERG), and behavioral increment thresholds in mice. ERG measurements of cone function showed age-related declines in maximum voltage (Vmax), while opsin gene transcription, cone density, and increment thresholds were unchanged even in extremely old mice. The age-related decline in Vmax seen in mice is qualitatively similar to that documented for human subjects. It is notable that Vmax, a commonly used index of ERG activity, does not predict behavioral performance in the mouse.

Contributions of the mouse UV photopigment to the ERG and to vision

The mouse retina contains two classes of cone photopigment with respective peak sensitivities in the middle (M) wavelengths and in the ultraviolet (UV) portion of the spectrum. To examine the functional roles subserved by the UV pigment, the absorption of light by the mouse lens was measured and voltage versus intensity (V-log I) functions were derived from recordings of the flicker ERG made under test conditions designed to maximize the relative sensitivities of the two pigment types. These V-log I data accurately predict ERG-based spectral sensitivity functions, but they fail to provide a similarly accurate account of behaviorally based measurements of spectral sensitivity in that the ERG spectral sensitivity function has much higher sensitivity in the UV wavelengths than does the behavioral spectral sensitivity function. The disparity between these two is argued to be a consequence of the widespread receptor co-expression of the two types of cone pigment in the mouse and of the pattern of retinal wiring that is thought to be characteristic of all mammalian retinas.

Scotopic and photopic visual thresholds and spatial and temporal discrimination evaluated by behavior of mice in a water maze

Methods that allow specific manipulations of the mouse genome have made it possible to alter specific aspects of photoreceptor function within the mouse retina. Mice with photoreceptors that have altered photosensitivities and altered photoresponse kinetics are now available. Methods are needed that can show how those perturbations in photoreceptor response characteristics translate into perturbations in visual sensitivity and perception. We have adapted a previously described method to evaluate visual threshold, spatial discrimination and temporal discrimination in mice swimming in a water maze. In this report we describe the sensitivities of rod-mediated and cone-mediated vision using GNAT1-/-and GNAT2-/- mice. Cone-mediated vision is approximately 10,000 times less sensitive than rod-mediated vision in mice. We also demonstrate that mice can distinguish striped from solid objects in the water maze and that they can distinguish flickering from continuous illumination.

Emergence of novel color vision in mice engineered to express a human cone photopigment

Changes in the genes encoding sensory receptor proteins are an essential step in the evolution of new sensory capacities. In primates, trichromatic color vision evolved after changes in X chromosome-linked photopigment genes. To model this process, we studied knock-in mice that expressed a human long-wavelength-sensitive (L) cone photopigment in the form of an X-linked polymorphism. Behavioral tests demonstrated that heterozygous females, whose retinas contained both native mouse pigments and human L pigment, showed enhanced long-wavelength sensitivity and acquired a new capacity for chromatic discrimination. An inherent plasticity in the mammalian visual system thus permits the emergence of a new dimension of sensory experience based solely on gene-driven changes in receptor organization.

Ultraviolet light-induced and green light-induced transient pupillary light reflex in mice

PURPOSE

To study UV light-induced and green light-induced pupillary light reflex (PLR) in mice and to measure the illumination thresholds of rod-mediated and cone-mediated PLR responses.

METHODS

We measured dark-adapted transient PLR- in C57BL/6 mice elicited with ultraviolet and green light over an intensity range of 9 log units. To assist in isolating the responses mediated by rods and cones, we studied the PLRs in mouse models presenting pure cone and pure rod functions. We also characterized ERG signals in these mice under the same experimental conditions.

RESULTS

The UV light-induced transient PLR has identical intensity-response curves as green light-induced PLR in all the three mice strains. The threshold (5% PLR) of rod-driven PLR is 107 approximately 108 photons cm2 s-1, which is 1 approximately 2 log units lower than the dark-adapted ERG b-wave. The threshold of cone-driven PLR is approximately 1012.5 photons cm-2 s-1 and is similar to that of the cone ERG.

CONCLUSIONS

We demonstrated that mice have PLR responses under UV stimulation. The cone-elicited PLRs have a threshold that is approximately 5 log units higher than that of rod-elicited PLR at both UV and green wavelengths. We observed a divergence between the spectra responses in PLR and ERG. However, the mechanism and implications of this phenomenon are yet to be identified.

Chlorophyll derivatives as visual pigments for super vision in the red

The primary event in vision is light-initiated activation of visual pigments. All visual pigments consist of the protein opsin bound to 11-cis-retinal and are responsible for initiating the transformation of light into an electrical signal. In a mouse model, we show that derivatives of chlorophyll can act as visual pigments initiating the transformation of light into an electrical signal and thus change the primary event in vision to initial activation of a chlorophyll derivative. Electroretinographic b-wave amplitudes recorded in response to red and blue light were two-fold greater in mice administered chlorin e(6), which accumulated in photoreceptor outer segments.

Rod and cone function in coneless mice

Transgenic coneless mice were initially developed to study retinal function in the absence of cones. In coneless mice created by expressing an attenuated diphtheria toxin under the control of flanking sequences from the human L-cone opsin gene, a small number of cones (3-5% of the normal complement) survive in a retina that otherwise appears structurally quite normal. These cones predominantly ( approximately 87% of the total) contain UV-sensitive photopigment. ERG recordings, photoreceptor labeling, and behavioral measurements were conducted on coneless and wild-type mice to better understand how the nature of this alteration in receptor complement impacts vision. Signals from the small residual population of UV cones are readily detected in the flicker ERG where they yield signal amplitudes at saturation that are roughly proportional to the number of surviving cones. Behavioral measurements show that rod-based vision in coneless mice does not differ significantly from that of wild-type mice, nor does their rod system show any evidence of age-related deterioration. Coneless mice are able to make accurate rod-based visual discriminations at light levels well in excess of those required to reach cone threshold in wild-type mice.