Dissecting a role for melanopsin in behavioural light aversion reveals a response independent of conventional photoreception

Preservation of Intrinsically Photosensitive Retinal Ganglion Cells (ipRGCs) in Late Adult Mice: Implications as a Potential Biomarker for Early Onset Ocular Degenerative Diseases

Purpose

Intrinsically photosensitive retinal ganglion cells (ipRGCs) play a crucial role in non-image-forming visual functions. Given their significant loss observed in various ocular degenerative diseases at early stages, this study aimed to assess changes in both the morphology and associated behavioral functions of ipRGCs in mice between 6 (mature) and 12 (late adult) months old. The findings contribute to understanding the preservation of ipRGCs in late adults and their potential as a biomarker for early ocular degenerative diseases.

Methods

Female and male C57BL/6J mice were used to assess the behavioral consequences of aging to mature and old adults, including pupillary light reflex, light aversion, visual acuity, and contrast sensitivity. Immunohistochemistry on retinal wholemounts from these mice was then conducted to evaluate ipRGC dendritic morphology in the ganglion cell layer (GCL) and inner nuclear layer (INL).

Results

Morphological analysis showed that ipRGC dendritic field complexity was remarkably stable through 12 months old of age. Similarly, the pupillary light reflex, visual acuity, and contrast sensitivity were stable in mature and old adults. Although alterations were observed in ipRGC-independent light aversion distinct from the pupillary light reflex, aged wild-type mice continuously showed enhanced light aversion with dilation. No effect of sex was observed in any tests.

Conclusions

The preservation of both ipRGC morphology and function highlights the potential of ipRGC-mediated function as a valuable biomarker for ocular diseases characterized by early ipRGC loss. The consistent stability of ipRGCs in mature and old adult mice suggests that detected changes in ipRGC-mediated functions could serve as early indicators or diagnostic tools for early-onset conditions such as Alzheimer's disease, Parkinson's disease, and diabetes, where ipRGC loss has been documented.

Association of eye strain with dry eye and retinal thickness

PURPOSE

The purpose of this cohort study was to investigate the association between the prevalence of abnormal ocular examination results and the common visual symptoms of eye strain, blurred vision and photophobia.

METHODS

Consecutive first-visit outpatients with best-corrected visual acuity better than 20/30 in both eyes were enrolled and those with a history of intra-ocular lens implantation and glaucoma were excluded. Dry eye-related examinations and retinal thickness measurement were conducted. The odds ratio (OR) was calculated with logistic regression analyses of ocular data in relation to the presence of visual symptoms.

RESULTS

A total of 6078 patients (3920 women, mean age 49.0 ± 20.4 years) were analyzed. The prevalence of each symptom was 31.8% for eye strain, 22.5% for blurred vision and 16.0% for photophobia. A significant risk factor for eye strain was short tear break-up time (TBUT) (OR 1.88), superficial punctate keratitis (SPK) (OR 1.44), and thickness of ganglion cell complex (GCC) (OR 1.30). Risk factors for blurred vision were short TBUT (OR 1.85), SPK (OR 1.24) and GCC (OR 0.59). Risk factors for photophobia were short TBUT (OR 1.77) and SPK (OR 1.32). Schirmer test value, peripapillary nerve fiber layer thickness and full macular thickness were not associated with the tested symptoms.

CONCLUSION

The current study successfully identified female gender, short TBUT, and SPK as significant risk factors for eye strain, blurred vision, and photophobia with considerable ORs.

Regulation of mouse exploratory behaviour by irradiance and cone-opponent signals

BACKGROUND

Animal survival depends on the ability to adjust behaviour according to environmental conditions. The circadian system plays a key role in this capability, with diel changes in the quantity (irradiance) and spectral content ('colour') of ambient illumination providing signals of time-of-day that regulate the timing of rest and activity. Light also exerts much more immediate effects on behaviour, however, that are equally important in shaping daily activity patterns. Hence, nocturnal mammals will actively avoid light and dramatically reduce their activity when light cannot be avoided. The sensory mechanisms underlying these acute effects of light are incompletely understood, particularly the importance of colour.

RESULTS

To define sensory mechanisms controlling mouse behaviour, we used photoreceptor-isolating stimuli and mice with altered cone spectral sensitivity (Opn1mwR), lacking melanopsin (Opn1mwR; Opn4-/-) or cone phototransduction (Cnga3-/-) in assays of light-avoidance and activity suppression. In addition to roles for melanopsin-dependent irradiance signals, we find a major influence of spectral content in both cases. Hence, remarkably, selective increases in S-cone irradiance (producing a blue-shift in spectrum replicating twilight) drive light-seeking behaviour and promote activity. These effects are opposed by signals from longer-wavelength sensitive cones, indicating a true spectrally-opponent mechanism. Using c-Fos-mapping and multielectrode electrophysiology, we further show these effects are associated with a selective cone-opponent modulation of neural activity in the key brain site implicated in acute effects of light on behaviour, the subparaventricular zone.

CONCLUSIONS

Collectively, these data reveal a mechanism whereby blue-shifts in the spectrum of environmental illumination, such as during twilight, promote mouse exploratory behaviour.

Phenotype Characterization of a Mice Genetic Model of Absolute Blindness

Recent technological development requires new approaches to address the problem of blindness. Such approaches need to be able to ensure that no cells with photosensitive capability remain in the retina. The presented model, Opn4^−/− × Pde6b^rd10/rd10 (O×Rd) double mutant murine, is a combination of a mutation in the Pde6b gene (photoreceptor degeneration) together with a deletion of the Opn4 gene (responsible for the expression of melanopsin in the intrinsically photosensitive retinal ganglion cells). This model has been characterized and compared with those of WT mice and murine animal models displaying both mutations separately. A total loss of pupillary reflex was observed. Likewise, behavioral tests demonstrated loss of rejection to illuminated spaces and a complete decrease in visual acuity (optomotor test). Functional recordings showed an absolute disappearance of various wave components of the full-field and pattern electroretinogram (fERG, pERG). Likewise, visual evoked potential (VEP) could not be recorded. Immunohistochemical staining showed marked degeneration of the outer retinal layers and the absence of melanopsin staining. The combination of both mutations has generated an animal model that does not show any photosensitive element in its retina. This model is a potential tool for the study of new ophthalmological approaches such as optosensitive agents.

Optogenetics for visual restoration: From proof of principle to translational challenges

Degenerative retinal disorders are a diverse family of diseases commonly leading to irreversible photoreceptor death, while leaving the inner retina relatively intact. Over recent years, innovative gene replacement therapies aiming to halt the progression of certain inherited retinal disorders have made their way into clinics. By rendering surviving retinal neurons light sensitive optogenetic gene therapy now offers a feasible treatment option that can restore lost vision, even in late disease stages and widely independent of the underlying cause of degeneration. Since proof-of-concept almost fifteen years ago, this field has rapidly evolved and a detailed first report on a treated patient has recently been published. In this article, we provide a review of optogenetic approaches for vision restoration. We discuss the currently available optogenetic tools and their relative advantages and disadvantages. Possible cellular targets will be discussed and we will address the question how retinal remodelling may affect the choice of the target and to what extent it may limit the outcomes of optogenetic vision restoration. Finally, we will analyse the evidence for and against optogenetic tool mediated toxicity and will discuss the challenges associated with clinical translation of this promising therapeutic concept.

The Retinal Basis of Light Aversion in Neonatal Mice

Aversive responses to bright light (photoaversion) require signaling from the eye to the brain. Melanopsin-expressing intrinsically photosensitive retinal ganglion cells (ipRGCs) encode absolute light intensity and are thought to provide the light signals for photoaversion. Consistent with this, neonatal mice exhibit photoaversion before the developmental onset of image vision, and melanopsin deletion abolishes photoaversion in neonates. It is not well understood how the population of ipRGCs, which constitutes multiple physiologically distinct types (denoted M1-M6 in mouse), encodes light stimuli to produce an aversive response. Here, we provide several lines of evidence that M1 ipRGCs that lack the Brn3b transcription factor drive photoaversion in neonatal mice. First, neonatal mice lacking TRPC6 and TRPC7 ion channels failed to turn away from bright light, while two photon Ca2+ imaging of their acutely isolated retinas revealed reduced photosensitivity in M1 ipRGCs, but not other ipRGC types. Second, mice in which all ipRGC types except for Brn3b-negative M1 ipRGCs are ablated exhibited normal photoaversion. Third, pharmacological blockade or genetic knockout of gap junction channels expressed by ipRGCs, which reduces the light sensitivity of M2-M6 ipRGCs in the neonatal retina, had small effects on photoaversion only at the brightest light intensities. Finally, M1s were not strongly depolarized by spontaneous retinal waves, a robust source of activity in the developing retina that depolarizes all other ipRGC types. M1s therefore constitute a separate information channel between the neonatal retina and brain that could ensure behavioral responses to light but not spontaneous retinal waves.SIGNIFICANCE STATEMENT At an early stage of development, before the maturation of photoreceptor input to the retina, neonatal mice exhibit photoaversion. On exposure to bright light, they turn away and emit ultrasonic vocalizations, a cue to their parents to return them to the nest. Neonatal photoaversion is mediated by intrinsically photosensitive retinal ganglion cells (ipRGCs), a small percentage of the retinal ganglion cell population that express the photopigment melanopsin and depolarize directly in response to light. This study shows that photoaversion is mediated by a subset of ipRGCs, called M1-ipRGCs. Moreover, M1-ipRGCs have reduced responses to retinal waves, providing a mechanism by which the mouse distinguishes light stimulation from developmental patterns of spontaneous activity.

Photoaversion in inherited retinal diseases: clinical phenotypes, biological basis, and qualitative and quantitative assessment

Severe light sensitivity is a feature common to a range of ophthalmological and neurological diseases. In inherited retinal diseases (IRDs) particularly, this may be accompanied by significant visual disruption. These symptoms are extremely debilitating for affected individuals and have significant implications in terms of day-to-day activities. Underlying mechanisms remain to be fully elucidated. Currently, there are many assessments of photoaversion (PA), however, all have limitations, with quantitative measurement in particular needing further evaluation. To understand the complexities associated with photoaversion from different pathologies, qualitative and quantitative assessments of the light aversion response must be standardized. There is no treatment to date, and strategies to alleviate symptoms focus on light avoidance. With respect to IRDs, however, gene therapy is currently being investigated in clinical trials and promising and further treatments may be on the horizon. The better characterization of these symptoms is an important end point measure in IRD gene therapy trials.

Reflexive Eye Closure in Response to Cone and Melanopsin Stimulation: A Study of Implicit Measures of Light Sensitivity in Migraine

BACKGROUND AND OBJECTIVES

To quantify interictal photophobia in migraine with and without aura using reflexive eye closure as an implicit measure of light sensitivity and to assess the contribution of melanopsin and cone signals to these responses.

METHODS

Participants were screened to meet criteria for 1 of 3 groups: headache-free (HF) controls, migraine without aura (MO), and migraine with visual aura (MA). MO and MA participants were included if they endorsed ictal and interictal photophobia. Exclusion criteria included impaired vision, inability to collect usable pupillometry, and history of either head trauma or seizure. Participants viewed light pulses that selectively targeted melanopsin, the cones, or their combination during recording of orbicularis oculi EMG (OO-EMG) and blinking activity.

RESULTS

We studied 20 participants in each group. MA and MO groups reported increased visual discomfort to light stimuli (discomfort rating, 400% contrast, MA: 4.84 [95% confidence interval 0.33, 9.35]; MO: 5.23 [0.96, 9.50]) as compared to HF controls (2.71 [0, 6.47]). Time course analysis of OO-EMG and blinking activity demonstrated that reflexive eye closure was tightly coupled to the light pulses. The MA group had greater OO-EMG and blinking activity in response to these stimuli (EMG activity, 400% contrast: 42.9%Δ [28.4, 57.4]; blink activity, 400% contrast: 11.2% [8.8, 13.6]) as compared to the MO (EMG activity, 400% contrast: 9.9%Δ [5.8, 14.0]; blink activity, 400% contrast: 4.7% [3.5, 5.9]) and HF control (EMG activity, 400% contrast: 13.2%Δ [7.1, 19.3]; blink activity, 400% contrast: 4.5% [3.1, 5.9]) groups.

DISCUSSION

Our findings suggest that the intrinsically photosensitive retinal ganglion cells (ipRGCs), which integrate melanopsin and cone signals, provide the afferent input for light-induced reflexive eye closure in a photophobic state. Moreover, we find a dissociation between implicit and explicit measures of interictal photophobia depending on a history of visual aura in migraine. This implies distinct pathophysiology in forms of migraine, interacting with separate neural pathways by which the amplification of ipRGC signals elicits implicit and explicit signs of visual discomfort.

Neuroprotective effects of bone marrow Sca-1+ cells against age-related retinal degeneration in OPTN E50K mice

Glaucoma is characterized by retinal ganglion cell (RGC) death, the underlying mechanisms of which are still largely unknown. An E50K mutation in the Optineurin (OPTN) gene is a leading cause of normal-tension glaucoma (NTG), which directly affects RGCs in the absence of high intraocular pressure and causes severe glaucomatous symptoms in patients. Bone marrow (BM) stem cells have been demonstrated to play a key role in regenerating damaged tissue during ageing and disease through their trophic effects and homing capability. Here, we separated BM stem cells into Sca-1⁺ and Sca-1^- cells and transplanted them into lethally irradiated aged OPTN E50K mice to generate Sca-1⁺ and Sca-1^- chimaeras, respectively. After 3 months of BM repopulation, we investigated whether Sca-1⁺ cells maximized the regenerative effects in the retinas of NTG model mice with the OPTN E50K mutation. We found that the OPTN E50K mutation aggravated age-related deficiency of neurotrophic factors in both retinas and BM during NTG development, leading to retinal degeneration and BM dysfunction. Sca-1⁺ cells from young healthy mice had greater paracrine trophic effects than Sca-1^- cells and Sca-1⁺ cells from young OPTN E50K mice. In addition, Sca-1⁺ chimaeras demonstrated better visual functions than Sca-1^- chimaeras and untreated OPTN E50K mice. More Sca-1⁺ cells than Sca-1^- cells were recruited to repair damaged retinas and reverse visual impairment in NTG resulting from high expression levels of neurotrophic factors. These findings indicated that the Sca-1⁺ cells from young, healthy mice may have exhibited an enhanced ability to repair retinal degeneration in NTG because of their excellent neurotrophic capability.

Color and contrast vision in mouse models of aging and Alzheimer's disease using a novel visual-stimuli four-arm maze

We introduce a novel visual-stimuli four-arm maze (ViS4M) equipped with spectrally- and intensity-controlled LED emitters and dynamic grayscale objects that relies on innate exploratory behavior to assess color and contrast vision in mice. Its application to detect visual impairments during normal aging and over the course of Alzheimer’s disease (AD) is evaluated in wild-type (WT) and transgenic APP_SWE/PS1_∆E9 murine models of AD (AD⁺) across an array of irradiance, chromaticity, and contrast conditions. Substantial color and contrast-mode alternation deficits appear in AD⁺ mice at an age when hippocampal-based memory and learning is still intact. Profiling of timespan, entries and transition patterns between the different arms uncovers variable AD-associated impairments in contrast sensitivity and color discrimination, reminiscent of tritanomalous defects documented in AD patients. Transition deficits are found in aged WT mice in the absence of alternation decline. Overall, ViS4M is a versatile, controlled device to measure color and contrast-related vision in aged and diseased mice.

Photosensitive ganglion cells: A diminutive, yet essential population

Our visual system has evolved to provide us with an image of the scene that surrounds us, informing us of its texture, colour, movement, and depth with an enormous spatial and temporal resolution, and for this purpose, the image formation (IF) dedicates the vast majority of our retinal ganglion cell (RGC) population and much of our cerebral cortex. On the other hand, a minuscule proportion of RGCs, in addition to receiving information from classic cone and rod photoreceptors, express melanopsin and are intrinsically photosensitive (ipRGC). These ipRGC are dedicated to non-image-forming (NIF) visual functions, of which we are unaware, but which are essential for aspects related to our daily physiology, such as the timing of our circadian rhythms and our pupillary light reflex, among many others. Before the discovery of ipRGCs, it was thought that the IF and NIF functions were distinct compartments regulated by different RGCs, but this concept has evolved in recent years with the discovery of new types of ipRGCs that innervate subcortical IF regions, and therefore have IF visual functions. Six different types of ipRGCs are currently known. These are termed M1-M6, and differ in their morphological, functional, molecular properties, central projections, and visual behaviour responsibilities. A review is presented on the melanopsin visual system, the most active field of research in vision, for which knowledge has grown exponentially during the last two decades, when RGCs giving rise to this pathway were first discovered.

Circadian Photoentrainment in Mice and Humans

Light around twilight provides the primary entrainment signal for circadian rhythms. Here we review the mechanisms and responses of the mouse and human circadian systems to light. Both utilize a network of photosensitive retinal ganglion cells (pRGCs) expressing the photopigment melanopsin (OPN4). In both species action spectra and functional expression of OPN4 in vitro show that melanopsin has a λ_max close to 480 nm. Anatomical findings demonstrate that there are multiple pRGC sub-types, with some evidence in mice, but little in humans, regarding their roles in regulating physiology and behavior. Studies in mice, non-human primates and humans, show that rods and cones project to and can modulate the light responses of pRGCs. Such an integration of signals enables the rods to detect dim light, the cones to detect higher light intensities and the integration of intermittent light exposure, whilst melanopsin measures bright light over extended periods of time. Although photoreceptor mechanisms are similar, sensitivity thresholds differ markedly between mice and humans. Mice can entrain to light at approximately 1 lux for a few minutes, whilst humans require light at high irradiance (>100’s lux) and of a long duration (>30 min). The basis for this difference remains unclear. As our retinal light exposure is highly dynamic, and because photoreceptor interactions are complex and difficult to model, attempts to develop evidence-based lighting to enhance human circadian entrainment are very challenging. A way forward will be to define human circadian responses to artificial and natural light in the “real world” where light intensity, duration, spectral quality, time of day, light history and age can each be assessed.

Persistently Worsened Tear Break-up Time and Keratitis in Unilateral Pseudophakic Eyes after a Long Postoperative Period

Dry eye disease may develop and persist after cataract surgery; however, unilateral cases have not been fully documented. This cross-sectional, observational study was conducted in five eye clinics in Japan. A total of 1023 outpatients were initially enrolled, and 89 unilateral pseudophakic subjects with 1+ year of follow-up after uncomplicated cataract surgery were included. The tear break-up times (TBUTs) and keratoconjunctival staining results were compared between phakic and pseudophakic eyes. The mean age of the patients was 69.3 ± 10.4 years (32 men, 36.0%), and the mean postoperative period was 4.6 ± 4.4 (1–20) years. For the ophthalmic parameters, the TBUTs were 4.4 ± 1.9 and 3.8 ± 1.9 s (p < 0.001), the keratoconjunctival staining scores were 0.11 ± 0.38 and 0.22 ± 0.56 (p = 0.02), the spherical equivalents were −1.27 ± 2.51 and −0.99 ± 1.45 D (p = 0.21), the astigmatic errors were 0.79 ± 0.66 and 0.78 ± 0.58 D (p = 0.80), and the intraocular pressures were 13.6 ± 2.9 and 13.5 ± 2.6 mmHg (p = 0.62) for the phakic and pseudophakic eyes, respectively. The corneal status was significantly worse in the pseudophakic eyes than in the contralateral phakic eyes, even after more than one year after implant surgery. The present results suggested that long-term ocular surface problems should be examined further since they may not originate only from surgery or postoperative ocular surface diseases.

Light-induced stress as a primary evolutionary driver of eye origins

Eyes are quintessential complex traits and our understanding of their evolution guides models of trait evolution in general. A long-standing account of eye evolution argues natural selection favors morphological variations that allow increased functionality for sensing light. While certainly true in part, this focus on visual performance does not entirely explain why diffuse photosensitivity persists even after eyes evolve, or why eyes evolved many times, each time using similar building blocks. Here, we briefly review a vast literature indicating most genetic components of eyes historically responded to stress caused directly by light, including ultraviolet damage of DNA, oxidative stress, and production of aldehydes. We propose light-induced stress had a direct and prominent role in the evolution of eyes by bringing together genes to repair and prevent damage from light-stress, both before and during the evolution of eyes themselves. Stress-repair and stress-prevention genes were perhaps originally deployed as plastic responses to light and/or as beneficial mutations genetically driving expression where light was prominent. These stress-response genes sense, shield, and refract light but only as reactions to ongoing light stress. Once under regulatory-genetic control, they could be expressed before light stress appeared, evolve as a module, and be influenced by natural selection to increase functionality for sensing light, ultimately leading to complex eyes and behaviors. Recognizing the potentially prominent role of stress in eye evolution invites discussions of plasticity and assimilation and provides a hypothesis for why similar genes are repeatedly used in convergent eyes. Broadening the drivers of eye evolution encourages consideration of multi-faceted mechanisms of plasticity/assimilation and mutation/selection for complex novelties and innovations in general.

Mitochondria as Potential Targets and Initiators of the Blue Light Hazard to the Retina

Commercially available white light-emitting diodes (LEDs) have an intense emission in the range of blue light, which has raised a range of public concerns about their potential risks as retinal hazards. Distinct from other visible light components, blue light is characterized by short wavelength, high energy, and strong penetration that can reach the retina with relatively little loss in damage potential. Mitochondria are abundant in retinal tissues, giving them relatively high access to blue light, and chromophores, which are enriched in the retina, have many mitochondria able to absorb blue light and induce photochemical effects. Therefore, excessive exposure of the retina to blue light tends to cause ROS accumulation and oxidative stress, which affect the structure and function of the retinal mitochondria and trigger mitochondria-involved death signaling pathways. In this review, we highlight the essential roles of mitochondria in blue light-induced photochemical damage and programmed cell death in the retina, indicate directions for future research and preventive targets in terms of the blue light hazard to the retina, and suggest applying LED devices in a rational way to prevent the blue light hazard.

Measuring vision using innate behaviours in mice with intact and impaired retina function

Measuring vision in rodents is a critical step for understanding vision, improving models of human disease, and developing therapies. Established behavioural tests for perceptual vision, such as the visual water task, rely on learning. The learning process, while effective for sighted animals, can be laborious and stressful in animals with impaired vision, requiring long periods of training. Current tests that that do not require training are based on sub-conscious, reflex responses (e.g. optokinetic nystagmus) that don't require involvement of visual cortex and higher order thalamic nuclei. A potential alternative for measuring vision relies on using visually guided innate defensive responses, such as escape or freeze, that involve cortical and thalamic circuits. In this study we address this possibility in mice with intact and degenerate retinas. We first develop automatic methods to detect behavioural responses based on high dimensional tracking and changepoint detection of behavioural time series. Using those methods, we show that visually guided innate responses can be elicited using parametisable stimuli, and applied to describing the limits of visual acuity in healthy animals and discriminating degrees of visual dysfunction in mouse models of retinal degeneration.

Transplantation of photoreceptors into the degenerative retina: Current state and future perspectives

The mammalian retina displays no intrinsic regenerative capacities, therefore retinal degenerative diseases such as age-related macular degeneration (AMD) or retinitis pigmentosa (RP) result in a permanent loss of the light-sensing photoreceptor cells. The degeneration of photoreceptors leads to vision impairment and, in later stages, complete blindness. Several therapeutic strategies have been developed to slow down or prevent further retinal degeneration, however a definitive cure i.e. replacement of the lost photoreceptors, has not yet been established. Cell-based treatment approaches, by means of photoreceptor transplantation, have been studied in pre-clinical animal models over the last three decades. The introduction of pluripotent stem cell-derived retinal organoids represents, in principle, an unlimited source for the generation of transplantable human photoreceptors. However, safety, immunological and reproducibility-related issues regarding the use of such cells still need to be solved. Moreover, the recent finding of cytoplasmic material transfer between donor and host photoreceptors demands reinterpretation of several former transplantation studies. At the same time, material transfer between healthy donor and dysfunctional patient photoreceptors also offers a potential alternative strategy for therapeutic intervention. In this review we discuss the history and current state of photoreceptor transplantation, the techniques used to assess rescue of visual function, the prerequisites for effective transplantation as well as the main roadblocks, including safety and immune response to the graft, that need to be overcome for successful clinical translation of photoreceptor transplantation approaches.

Young bone marrow Sca-1 cells protect aged retina from ischaemia-reperfusion injury through activation of FGF2

Retinal ganglion cell apoptosis and optic nerve degeneration are prevalent in aged patients, which may be related to the decrease in bone marrow (BM) stem cell number/function because of the possible cross‐talk between the two organs. This pathological process is accelerated by retinal ischaemia‐reperfusion (I/R) injury. This study investigated whether young BM stem cells can regenerate and repair the aged retina after acute I/R injury. Young BM stem cell antigen 1 positive (Sca‐1⁺) or Sca‐1⁻ cells were transplanted into lethally irradiated aged recipient mice to generate Sca‐1⁺ and Sca‐1⁻ chimaeras, respectively. The animals were housed for 3 months to allow the young Sca‐1 cells to repopulate in the BM of aged mice. Retinal I/R was then induced by elevation of intraocular pressure. Better preservation of visual function was found in Sca‐1⁺ than Sca‐1⁻ chimaeras 7 days after injury. More Sca‐1⁺ cells homed to the retina than Sca‐1⁻ cells and more cells differentiated into glial and microglial cells in the Sca‐1⁺ chimaeras. After injury, Sca‐1⁺ cells in the retina reduced host cellular apoptosis, which was associated with higher expression of fibroblast growth factor 2 (FGF2) in the Sca‐1⁺ chimaeras. Young Sca‐1⁺ cells repopulated the stem cells in the aged retina and diminished cellular apoptosis after acute I/R injury through FGF2 and Akt signalling pathways.

Splice-Modulating Oligonucleotide QR-110 Restores CEP290 mRNA and Function in Human c.2991+1655A>G LCA10 Models

Leber congenital amaurosis type 10 (LCA10) is a severe inherited retinal dystrophy associated with mutations in CEP290. The deep intronic c.2991+1655A>G mutation in CEP290 is the most common mutation in LCA10 individuals and represents an ideal target for oligonucleotide therapeutics. Here, a panel of antisense oligonucleotides was designed to correct the splicing defect associated with the mutation and screened for efficacy and safety. This identified QR-110 as the best-performing molecule. QR-110 restored wild-type CEP290 mRNA and protein expression levels in CEP290 c.2991+1655A>G homozygous and compound heterozygous LCA10 primary fibroblasts. Furthermore, in homozygous three-dimensional iPSC-derived retinal organoids, QR-110 showed a dose-dependent restoration of mRNA and protein function, as measured by percentage and length of photoreceptor cilia, without off-target effects. Localization studies in wild-type mice and rabbits showed that QR-110 readily reached all retinal layers, with an estimated half-life of 58 days. It was well tolerated following intravitreal injection in monkeys. In conclusion, the pharmacodynamic, pharmacokinetic, and safety properties make QR-110 a promising candidate for treating LCA10, and clinical development is currently ongoing.

A Robust Optomotor Assay for Assessing the Efficacy of Optogenetic Tools for Vision Restoration

Purpose

To develop an animal behavioral assay for the quantitative assessment of the functional efficacy of optogenetic therapies.

Methods

A triple-knockout (TKO) mouse line, Gnat1^−/−Cnga3^−/−Opn4^−/−, and a double-knockout mouse line, Gnat1^−/−Cnga3^−/−, were employed. The expression of channelrhodopsin-2 (ChR2) and its three more light-sensitive mutants, ChR2-L132C, ChR2-L132C/T159C, and ChR2-132C/T159S, in inner retinal neurons was achieved using rAAV2 vectors via intravitreal delivery. Pupillary constriction was assessed by measuring the pupil diameter. The optomotor response (OMR) was examined using a homemade optomotor system equipped with light-emitting diodes as light stimulation.

Results

A robust OMR was restored in the ChR2-mutant-expressing TKO mice; however, significant pupillary constriction was observed only for the ChR2-L132C/T159S mutant. The ability to evoke an OMR was dependent on both the light intensity and grating frequency. The most light-sensitive frequency for the three ChR2 mutants was approximately 0.042 cycles per degree. Among the three ChR2 mutants, ChR2-L132C/T159S was the most light sensitive, followed by ChR2-L132C/T159C and ChR2-L132C. Melanopsin-mediated pupillary constriction resulted in a substantial reduction in the light sensitivity of the ChR2-mediated OMR.

Conclusions

The OMR assay using TKO mice enabled the quantitative assessment of the efficacy of different optogenetic tools and the properties of optogenetically restored vision. Thus, the assay can serve as a valuable tool for developing effective optogenetic therapies.

Melanopsin expression in the cornea

A unique class of intrinsically photosensitive retinal ganglion cells in mammalian retinae has been recently discovered and characterized. These neurons can generate visual signals in the absence of inputs from rods and cones, the conventional photoreceptors in the visual system. These light sensitive ganglion cells (mRGCs) express the non-rod, non-cone photopigment melanopsin and play well documented roles in modulating pupil responses to light, photoentrainment of circadian rhythms, mood, sleep and other adaptive light functions. While most research efforts in mammals have focused on mRGCs in retina, recent studies reveal that melanopsin is expressed in non-retinal tissues. For example, light-evoked melanopsin activation in extra retinal tissue regulates pupil constriction in the iris and vasodilation in the vasculature of the heart and tail. As another example of nonretinal melanopsin expression we report here the previously unrecognized localization of this photopigment in nerve fibers within the cornea. Surprisingly, we were unable to detect light responses in the melanopsin-expressing corneal fibers in spite of our histological evidence based on genetically driven markers and antibody staining. We tested further for melanopsin localization in cell bodies of the trigeminal ganglia (TG), the principal nuclei of the peripheral nervous system that project sensory fibers to the cornea, and found expression of melanopsin mRNA in a subset of TG neurons. However, neither electrophysiological recordings nor calcium imaging revealed any light responsiveness in the melanopsin positive TG neurons. Given that we found no light-evoked activation of melanopsin-expressing fibers in cornea or in cell bodies in the TG, we propose that melanopsin protein might serve other sensory functions in the cornea. One justification for this idea is that melanopsin expressed in Drosophila photoreceptors can serve as a temperature sensor.

Long-term restoration of visual function in end-stage retinal degeneration using subretinal human melanopsin gene therapy

Optogenetic strategies to restore vision in patients who are blind from end-stage retinal degenerations aim to render remaining retinal cells light sensitive once photoreceptors are lost. Here, we assessed long-term functional outcomes following subretinal delivery of the human melanopsin gene (OPN4) in the rd1 mouse model of retinal degeneration using an adeno-associated viral vector. Ectopic expression of OPN4 using a ubiquitous promoter resulted in cellular depolarization and ganglion cell action potential firing. Restoration of the pupil light reflex, behavioral light avoidance, and the ability to perform a task requiring basic image recognition were restored up to 13 mo following injection. These data suggest that melanopsin gene therapy via a subretinal route may be a viable and stable therapeutic option for the treatment of end-stage retinal degeneration in humans.

The TRPM1 Channel Is Required for Development of the Rod ON Bipolar Cell-AII Amacrine Cell Pathway in the Retinal Circuit

Neurotransmission plays an essential role in neural circuit formation in the central nervous system (CNS). Although neurotransmission has been recently clarified as a key modulator of retinal circuit development, the roles of individual synaptic transmissions are not yet fully understood. In the current study, we investigated the role of neurotransmission from photoreceptor cells to ON bipolar cells in development using mutant mouse lines of both sexes in which this transmission is abrogated. We found that deletion of the ON bipolar cation channel TRPM1 results in the abnormal contraction of rod bipolar terminals and a decreased number of their synaptic connections with amacrine cells. In contrast, these histological alterations were not caused by a disruption of total glutamate transmission due to loss of the ON bipolar glutamate receptor mGluR6 or the photoreceptor glutamate transporter VGluT1. In addition, TRPM1 deficiency led to the reduction of total dendritic length, branch numbers, and cell body size in AII amacrine cells. Activated Goα, known to close the TRPM1 channel, interacted with TRPM1 and induced the contraction of rod bipolar terminals. Furthermore, overexpression of Channelrhodopsin-2 partially rescued rod bipolar cell development in the TRPM1^-/- retina, whereas the rescue effect by a constitutively closed form of TRPM1 was lower than that by the native form. Our results suggest that TRPM1 channel opening is essential for rod bipolar pathway establishment in development.SIGNIFICANCE STATEMENT Neurotransmission has been recognized recently as a key modulator of retinal circuit development in the CNS. However, the roles of individual synaptic transmissions are not yet fully understood. In the current study, we focused on neurotransmission between rod photoreceptor cells and rod bipolar cells in the retina. We used genetically modified mouse models which abrogate each step of neurotransmission: presynaptic glutamate release, postsynaptic glutamate reception, or transduction channel function. We found that the TRPM1 transduction channel is required for the development of rod bipolar cells and their synaptic formation with subsequent neurons, independently of glutamate transmission. This study advances our understanding of neurotransmission-mediated retinal circuit refinement.

A novel intrinsic electroretinogram response in isolated mouse retina

Since the discovery of intrinsic photosensitive retinal ganglion cell (ipRGC) was reported in 2002, many features specific to this cell type have been described. However, scare information is available on the retinographic components directly reflecting ipRGC activity. In this study, we identified the electroretinogram (microERG) that reflects the photoresponses by ipRGCs in ex vivo preparations of the mouse retina, in which classical photoreceptors (cones and rods) were ablated mechanically and photochemically. MicroERG consisted of three components: a large transient ON response, a small and lazy hump 19s after the onset of the light, and a large transient OFF response. A complete microERG recording required at least 30s of light exposure. MicroERG showed the highest spectral photosensitivity at 478nm. This wavelength corresponds to the peak wavelength in the ipRGCs' photosensitive curve. The psychophysical test using a blue light-emitting diode (LED) light (470nm) revealed that the absolute threshold illuminance for microERG was greater than 12.26 log photons/s/cm² in both ON and OFF responses, whereas microERG was not adapted for dark. The amplitude of microERG increased linearly with irradiance. The sensitivity of temporal frequency was high in microERG (at least 100Hz), as suggested by the study on melatonin suppression by flickering light in human subjects (Zelter et al., 2014). Melatonin secretion was suppressed by light via ipRGCs and the suprachiasmatic nucleus. These properties of the photoresponse indicate that microERG may reflect the functions of ipRGC as a luminance detector in the mouse retina.

Unconventional Roles of Opsins

Rhodopsin is the classical light sensor. Although rhodopsin has long been known to be important for image formation in the eye, the requirements for opsins in non-image formation and in extraocular light sensation were revealed much later. Most recent is the demonstration that an opsin in the fruit fly, Drosophila melanogaster, is expressed in pacemaker neurons in the brain and functions in light entrainment of circadian rhythms. However, the biggest surprise is that opsins have light-independent roles, countering more than a century of dogma that they function exclusively as light sensors. Through studies in Drosophila, light-independent roles of opsins have emerged in temperature sensation and hearing. Although these findings have been uncovered in the fruit fly, there are hints that opsins have light-independent roles in a wide array of animals, including mammals. Thus, despite the decades of focus on opsins as light detectors, they represent an important new class of polymodal sensory receptor.

Modulation of recognition memory performance by light requires both melanopsin and classical photoreceptors

Acute light exposure exerts various effects on physiology and behaviour. Although the effects of light on brain network activity in humans are well demonstrated, the effects of light on cognitive performance are inconclusive, with the size, as well as direction, of the effect depending on the nature of the task. Similarly, in nocturnal rodents, bright light can either facilitate or disrupt performance depending on the type of task employed. Crucially, it is unclear whether the effects of light on behavioural performance are mediated via the classical image-forming rods and cones or the melanopsin-expressing photosensitive retinal ganglion cells. Here, we investigate the modulatory effects of light on memory performance in mice using the spontaneous object recognition task. Importantly, we examine which photoreceptors are required to mediate the effects of light on memory performance. By using a cross-over design, we show that object recognition memory is disrupted when the test phase is conducted under a bright light (350 lux), regardless of the light level in the sample phase (10 or 350 lux), demonstrating that exposure to a bright light at the time of test, rather than at the time of encoding, impairs performance. Strikingly, the modulatory effect of light on memory performance is completely abolished in both melanopsin-deficient and rodless-coneless mice. Our findings provide direct evidence that melanopsin-driven and rod/cone-driven photoresponses are integrated in order to mediate the effect of light on memory performance.

Peripheral Sensory Neurons Expressing Melanopsin Respond to Light

The ability of light to cause pain is paradoxical. The retina detects light but is devoid of nociceptors while the trigeminal sensory ganglia (TG) contain nociceptors but not photoreceptors. Melanopsin-expressing intrinsically photosensitive retinal ganglion cells (ipRGCs) are thought to mediate light-induced pain but recent evidence raises the possibility of an alternative light responsive pathway independent of the retina and optic nerve. Here, we show that melanopsin is expressed in both human and mouse TG neurons. In mice, they represent 3% of small TG neurons that are preferentially localized in the ophthalmic branch of the trigeminal nerve and are likely nociceptive C fibers and high-threshold mechanoreceptor Aδ fibers based on a strong size-function association. These isolated neurons respond to blue light stimuli with a delayed onset and sustained firing, similar to the melanopsin-dependent intrinsic photosensitivity observed in ipRGCs. Mice with severe bilateral optic nerve crush exhibit no light-induced responses including behavioral light aversion until treated with nitroglycerin, an inducer of migraine in people and migraine-like symptoms in mice. With nitroglycerin, these same mice with optic nerve crush exhibit significant light aversion. Furthermore, this retained light aversion remains dependent on melanopsin-expressing neurons. Our results demonstrate a novel light-responsive neural function independent of the optic nerve that may originate in the peripheral nervous system to provide the first direct mechanism for an alternative light detection pathway that influences motivated behavior.

Characterisation of light responses in the retina of mice lacking principle components of rod, cone and melanopsin phototransduction signalling pathways

Gnat(-/-), Cnga3(-/-), Opn4(-/-) triple knockout (TKO) mice lack essential components of phototransduction signalling pathways present in rods, cones and photosensitive retinal ganglion cells (pRGCs), and are therefore expected to lack all sensitivity to light. However, a number of studies have shown that light responses persist in these mice. In this study we use multielectrode array (MEA) recordings and light-induced c-fos expression to further characterise the light responses of the TKO retina. Small, but robust electroretinogram type responses are routinely detected during MEA recordings, with properties consistent with rod driven responses. Furthermore, a distinctive pattern of light-induced c-fos expression is evident in the TKO retina, with c-fos expression largely restricted to a small subset of amacrine cells that express disabled-1 (Dab1) but lack expression of glycine transporter-1 (GlyT-1). Collectively these data are consistent with the persistence of a novel light sensing pathway in the TKO retina that originates in rod photoreceptors, potentially a rare subset of rods with distinct functional properties, and which is propagated to an atypical subtype of AII amacrine cells. Furthermore, the minimal responses observed following UV light stimulation suggest only a limited role for the non-visual opsin OPN5 in driving excitatory light responses within the mouse retina.

Melanopsin Regulates Both Sleep-Promoting and Arousal-Promoting Responses to Light

Light plays a critical role in the regulation of numerous aspects of physiology and behaviour, including the entrainment of circadian rhythms and the regulation of sleep. These responses involve melanopsin (OPN4)-expressing photosensitive retinal ganglion cells (pRGCs) in addition to rods and cones. Nocturnal light exposure in rodents has been shown to result in rapid sleep induction, in which melanopsin plays a key role. However, studies have also shown that light exposure can result in elevated corticosterone, a response that is not compatible with sleep. To investigate these contradictory findings and to dissect the relative contribution of pRGCs and rods/cones, we assessed the effects of light of different wavelengths on behaviourally defined sleep. Here, we show that blue light (470 nm) causes behavioural arousal, elevating corticosterone and delaying sleep onset. By contrast, green light (530 nm) produces rapid sleep induction. Compared to wildtype mice, these responses are altered in melanopsin-deficient mice (Opn4^-/-), resulting in enhanced sleep in response to blue light but delayed sleep induction in response to green or white light. We go on to show that blue light evokes higher Fos induction in the SCN compared to the sleep-promoting ventrolateral preoptic area (VLPO), whereas green light produced greater responses in the VLPO. Collectively, our data demonstrates that nocturnal light exposure can have either an arousal- or sleep-promoting effect, and that these responses are melanopsin-mediated via different neural pathways with different spectral sensitivities. These findings raise important questions relating to how artificial light may alter behaviour in both the work and domestic setting.

Light can produce either sleep or arousal in mice. This study reveals that these opposing effects depend upon the wavelength of light and appear to involve separate pathways, both modulated by the photopigment melanopsin.

Light exerts profound effects on our physiology and behaviour, setting our biological clocks to the correct time and regulating when we are asleep and we are awake. The photoreceptors mediating these responses include the rods and cones involved in vision, as well as a subset of photosensitive retinal ganglion cells (pRGCs) expressing the blue light-sensitive photopigment melanopsin. Previous studies have shown that mice lacking melanopsin show impaired sleep in response to light. However, other studies have shown that light increases glucocorticoid release—a response typically associated with stress. To address these contradictory findings, we studied the responses of mice to light of different colours. We found that blue light was aversive, delaying sleep onset and increasing glucocorticoid levels. By contrast, green light led to rapid sleep onset. These different behavioural effects appear to be driven by different neural pathways. Surprisingly, both responses were impaired in mice lacking melanopsin. These data show that light can promote either sleep or arousal. Moreover, they provide the first evidence that melanopsin directly mediates the effects of light on glucocorticoids. This work shows the extent to which light affects our physiology and has important implications for the design and use of artificial light sources.

Signalling by melanopsin (OPN4) expressing photosensitive retinal ganglion cells

Over the past two decades there have been significant advances in our understanding of both the anatomy and function of the melanopsin system. It has become clear that rather than acting as a simple irradiance detector the melanopsin system is in fact far more complicated. The range of behavioural systems known to be influenced by melanopsin activity is increasing with time, and it is now clear that melanopsin contributes not only to multiple non-image forming systems but also has a role in visual pathways. How melanopsin is capable of driving so many different behaviours is unclear, but recent evidence suggests that the answer may lie in the diversity of melanopsin light responses and the functional specialisation of photosensitive retinal ganglion cell (pRGC) subtypes. In this review, we shall overview the current understanding of the melanopsin system, and evaluate the evidence for the hypothesis that individual pRGC subtypes not only perform specific roles, but are functionally specialised to do so. We conclude that while, currently, the available data somewhat support this hypothesis, we currently lack the necessary detail to fully understand how the functional diversity of pRGC subtypes correlates with different behavioural responses, and ultimately why such complexity is required within the melanopsin system. What we are lacking is a cohesive understanding of how light responses differ between the pRGC subtypes (based not only on anatomical classification but also based on their site of innervation); how these diverse light responses are generated, and most importantly how these responses relate to the physiological functions they underpin.

Local and systemic responses following intravitreous injection of AAV2-encoded modified Volvox channelrhodopsin-1 in a genetically blind rat model

We previously designed a modified channelrhodopsin-1 (mVChR1) protein chimera with a broader action than that of Chlamydomonas channelrhodopsin-2 and reported that its transduction into retinal ganglion cells can restore visual function in genetically blind, dystrophic Royal College of Surgeons (RCS) rats, with photostimuli ranging from 486 to 640 nm. In the current study, we sought to investigate the safety and influence of mVChR1 transgene expression. Adeno-associated virus type 2 encoding mVChR1 was administered by intravitreous injection into dystrophic RCS rats. Reverse-transcription PCR was used to monitor virus and transgene dissemination and the results demonstrated that their expression was restricted specifically within the eye tissues, and not in non-target organs. Moreover, examination of the blood, plasma and serum revealed that no excess immunoreactivity was present, as determined using standard clinical hematological parameters. Serum antibodies targeting the recombinant adeno-associated virus (rAAV) capsid increased after the injection; however, no increase in mVChR1 antibody was detected during the observation period. In addition, retinal histological examination showed no signs of inflammation in rAAV-injected rats. In conclusion, our results demonstrate that mVChR1 can be exogenously expressed without harmful immunological reactions in vivo. These findings will aid in studies of AAV gene transfer to restore vision in late-stage retinitis pigmentosa.

Isoforms of Melanopsin Mediate Different Behavioral Responses to Light

Melanopsin (OPN4) is a retinal photopigment that mediates a wide range of non-image-forming (NIF) responses to light including circadian entrainment, sleep induction, the pupillary light response (PLR), and negative masking of locomotor behavior (the acute suppression of activity in response to light). How these diverse NIF responses can all be mediated by a single photopigment has remained a mystery. We reasoned that the alternative splicing of melanopsin could provide the basis for functionally distinct photopigments arising from a single gene. The murine melanopsin gene is indeed alternatively spliced, producing two distinct isoforms, a short (OPN4S) and a long (OPN4L) isoform, which differ only in their C terminus tails. Significantly, both isoforms form fully functional photopigments. Here, we show that different isoforms of OPN4 mediate different behavioral responses to light. By using RNAi-mediated silencing of each isoform in vivo, we demonstrated that the short isoform (OPN4S) mediates light-induced pupillary constriction, the long isoform (OPN4L) regulates negative masking, and both isoforms contribute to phase-shifting circadian rhythms of locomotor behavior and light-mediated sleep induction. These findings demonstrate that splice variants of a single receptor gene can regulate strikingly different behaviors.

Retinal neurodegeneration in experimental glaucoma

In rats and mice, limbar tissues of the left eye were laser-photocoagulated (LP) and ocular hypertension (OHT) effects were investigated 1 week to 6 months later. To investigate the innermost layers, retinas were examined in wholemounts using tracing from the superior colliculi to identify retinal ganglion cells (RGCs) with intact retrograde axonal transport, melanopsin immunodetection to identify intrinsically photosensitive RGCs (m(+)RGC), Brn3a immunodetection to identify most RGCs but not m(+)RGCs, RECA1 immunodetection to examine the inner retinal vessels, and DAPI staining to detect all nuclei in the GC layer. The outer retinal layers (ORLs) were examined in cross sections analyzed morphometrically or in wholemounts to study S- and L-cones. Innervation of the superior colliculi was examined 10 days to 14 weeks after LP with orthogradely transported cholera toxin subunit B. By 2 weeks, OHT resulted in pie-shaped sectors devoid of FG(+)RGCs or Brn3a(+)RGCs but with large numbers of DAPI(+)nuclei. Brn3a(+)RGCs were significantly greater than FG(+)RGCs, indicating the survival of large numbers of RGCs with their axonal transport impaired. The inner retinal vasculature showed no abnormalities that could account for the sectorial loss of RGCs. m(+)RGCs decreased to approximately 50-51% in a diffuse loss across the retina. Cross sections showed focal areas of degeneration in the ORLs. RGC loss at 1m diminished to 20-25% and did not progress further with time, whereas the S- and L-cone populations diminished progressively up to 6m. The retinotectal projection was reduced by 10 days and did not progress further. LP-induced OHT results in retrograde degeneration of RGCs and m(+)RGCs, severe damage to the ORL, and loss of retinotectal terminals.

Light aversion and corneal mechanical sensitivity are altered by intrinscally photosensitive retinal ganglion cells in a mouse model of corneal surface damage

Animal models of corneal surface damage reliably exhibit altered tear quality and quantity, apoptosis, nerve degeneration, immune responses and many other symptoms of dry eye disease. An important clinical symptom of dry eye disease is photoallodynia (photophobia), which can be modeled in mice using behavioral light aversion as a surrogate. Intrinsically photosensitive retinal ganglion cells (ipRGCs) function as irradiance detectors. They have been shown to mediate innate light aversion and are ideal candidates to initiate or modulate light aversion in disease or dysfunctional states. This study addresses the relationship between light aversion, corneal mechanical sensitivity and corneal surface damage in a preclinical mouse model using bilateral topical application of benzalkonium chloride (BAC). Corneal application of BAC resulted in similar levels of corneal surface damage by fluorescein staining in both wild type mice and mice lacking ipRGCs. Light aversion was an early symptom of corneal surface damage, was proportional to the level of corneal damage and dependent on melanopsin-expressing cells. A decrease in both corneal mechanosensitivity and light aversion was observed in mice lacking melanopsin-expressing cells, suggesting a connection in the neural circuits mediating the two most common symptoms of corneal surface damage.

Effects of ocular hypertension in the visual system of pigmented mice

To study the effects of ocular hypertension (OHT) on the visual system of C57BL/6 pigmented mice, the limbal and episcleral veins of the left eye were laser photocoagulated (LP). LP increased the intraocular pressure during the first five days (d), reaching basal values at 7d. To investigate the effect of OHT on the retinal ganglion cell (RGC) retrograde axonal transport, hydroxistilbamidine methanesulfonate (OHSt) was applied to both superior colliculi (SCi) and the retinas were dissected 2 or 4 weeks after LP. To determine RGC survival, these same retinas were immunoreacted against Brn3a (general RGC population) and melanopsin (intrinsically photosensitive RGCs, m⁺RGCs). To study whether OHT affected non-RGC neurons in the ganglion cell layer (GCL), RGCs were immunodetected with Brn3a and all GCL nuclei counterstained with DAPI in a group of animals examined 4 weeks post-LP. Innervation of the SCi was examined at 10 days, 8 or 14 weeks after LP with the orthogradely transported cholera toxin subunit-B. OHT resulted in diffuse and sectorial loss of OHSt+RGCs (50% at 2 weeks and 62% at 4 weeks) and in a comparable loss of Brn3a+RGCs at the same time intervals. m+RGCs decreased to 59% at 2 weeks and to 46% at 4 weeks, such loss was diffuse, did not parallel the sectorial loss of the general RGC population and was more severe in the superior-temporal retina. In the GCL, cell loss is selective for RGCs and does not affect other non-RGC neurons. The retinotectal innervation appeared significantly reduced at 10 days (55.7%) and did not progress further up to 14 weeks (46.6%). Thus, LP-induced OHT results in retrograde degeneration of RGCs and m+RGCs, as well as in the loss of CTB-labelled retinotectal terminals.

A role for the outer retina in development of the intrinsic pupillary light reflex in mice

Mice do not require the brain in order to maintain constricted pupils. However, little is known about this intrinsic pupillary light reflex (iPLR) beyond a requirement for melanopsin in the iris and an intact retinal ciliary marginal zone (CMZ). Here, we study the mouse iPLR in vitro and examine a potential role for outer retina (rods and cones) in this response. In wild-type mice the iPLR was absent at postnatal day 17 (P17), developing progressively from P21-P49. However, the iPLR only achieved ∼ 30% of the wild-type constriction in adult mice with severe outer retinal degeneration (rd and rdcl). Paradoxically, the iPLR increased significantly in retinal degenerate mice >1.5 years of age. This was accompanied by an increase in baseline pupil tone in the dark to levels indistinguishable from those in adult wild types. This rejuvenated iPLR response was slowed by atropine application, suggesting the involvement of cholinergic neurotransmission. We could find no evidence of an increase in melanopsin expression by quantitative PCR in the iris and ciliary body of aged retinal degenerates and a detailed anatomical analysis revealed a significant decline in melanopsin-positive intrinsically photosensitive retinal ganglion cells (ipRGCs) in rdcl mice >1.5 years. Adult mice lacking rod function (Gnat1(-/-)) also had a weak iPLR, while mice lacking functional cones (Cpfl5) maintained a robust response. We also identify an important role for pigmentation in the development of the mouse iPLR, with only a weak and transient response present in albino animals. Our results show that the iPLR in mice develops unexpectedly late and are consistent with a role for rods and pigmentation in the development of this response in mice. The enhancement of the iPLR in aged degenerate mice was extremely surprising but may have relevance to behavioral observations in mice and patients with retinitis pigmentosa.

Distribution of melanopsin positive neurons in pigmented and albino mice: evidence for melanopsin interneurons in the mouse retina

Here we have studied the population of intrinsically photosensitive retinal ganglion cells (ipRGCs) in adult pigmented and albino mice. Our data show that although pigmented (C57Bl/6) and albino (Swiss) mice have a similar total number of ipRGCs, their distribution is slightly different: while in pigmented mice ipRGCs are more abundant in the temporal retina, in albinos the ipRGCs are more abundant in superior retina. In both strains, ipRGCs are located in the retinal periphery, in the areas of lower Brn3a⁺RGC density. Both strains also contain displaced ipRGCs (d-ipRGCs) in the inner nuclear layer (INL) that account for 14% of total ipRGCs in pigmented mice and 5% in albinos. Tracing from both superior colliculli shows that 98% (pigmented) and 97% (albino) of the total ipRGCs, become retrogradely labeled, while double immunodetection of melanopsin and Brn3a confirms that few ipRGCs express this transcription factor in mice. Rather surprisingly, application of a retrograde tracer to the optic nerve (ON) labels all ipRGCs, except for a sub-population of the d-ipRGCs (14% in pigmented and 28% in albino, respectively) and melanopsin positive cells residing in the ciliary marginal zone (CMZ) of the retina. In the CMZ, between 20% (pigmented) and 24% (albino) of the melanopsin positive cells are unlabeled by the tracer and we suggest that this may be because they fail to send an axon into the ON. As such, this study provides the first evidence for a population of melanopsin interneurons in the mammalian retina.

Photophobia and abnormally sustained pupil responses in a mouse model of bradyopsia

PURPOSE

Mutations in the RGS9 gene cause the visual disorder bradyopsia, which includes difficulty adapting to changes in light and photophobia. The purpose of this study was to determine whether lack of Rgs9 also caused photophobia-like behavior in Rgs9 knockout (Rgs9-/-) mice and to identify useful diagnostic measures of Rgs9 dysfunction.

METHODS

We measured two behavioral responses to light and the pupillary light reflex to determine the form and basis of photophobia in Rgs9-/- mice.

RESULTS

Rgs9-/- mice spent less time than wild-type mice in both dim and bright light. The mice also showed increased sensitivity to light in negative masking behavior, with a half maximal response at 0.08 lux (0.01 μW·cm(-2)) in Rgs9-/- mice compared to 5.0 lux (0.85 μW·cm(-2)) in wild-type mice. These behaviors were not due to increased anxiety or increased pupil size causing more light to enter the eye. Rather, constriction of the pupil showed that Rgs9-/- mice had an abnormally sustained response to light across multiple irradiance measurement pathways.

CONCLUSIONS

Rgs9-/- mice recapitulate a photophobia phenotype of bradyopsia, and the pupil light reflex identifies a simple means to screen for irradiance measurement abnormalities in bradyopsia and potentially other genetic disorders involving photophobia.

A role for the ciliary marginal zone in the melanopsin-dependent intrinsic pupillary light reflex

Maintenance of pupillary constriction in light-adapted rodents has traditionally been thought to involve a reflex between retina, brain and iris, with recent work identifying the melanopsin-expressing intrinsically photosensitive retinal ganglion cells (ipRGCs) as the major conduits for retinal input to the brain. There is also a less well-understood phenomenon whereby the iris of some mammals, including mice, will constrict to light when either the eye, or the iris itself is physically isolated from the brain. The intrinsic pupillary light reflex (iPLR) is the term given to pupil constriction in the absence of retinal input to the brain. Here, using an intraocular axotomy approach, we show that the iPLR in conscious mice spans a dynamic range over 3 log units of irradiance. This iPLR response is absent in melanopsin knockout (MKO) mice and can be significantly inhibited by atropine. Immunohistochemistry for cfos and melanopsin, in combination with light exposure revealed a population of small ipRGCs in the retinal ciliary marginal zone (CMZ), which remain responsive to light in axotomised mice. We report that damage to the CMZ in a novel in vitro preparation removes a significant component of the iPLR response, while a detailed immunohistochemical analysis of the CMZ in wildtype mice revealed a melanopsin-rich plexus, which was consistently most intense in nasal retina. There were clear examples of melanopsin-positive, direct retino-ciliary projections, which appear to emanate from Brn3b negative, M1 type ipRGCs. These cells are clustered along the melanopsin-rich plexus nasally and may channel ipRGC signals from retina into the iris via ciliary body. Comparison between wildtype and MKO mice reveals that the ciliary body is also weakly stained for melanopsin. Our results show that the full extent of iPLR in mice requires cholinergic neurotransmission and intact signalling at the CMZ/ciliary body. This response may be mediated to some extent by ipRGCs, which send direct projections from the retina into ciliary body. In addition to the melanopsin-mediated iris sphincter constriction suggested by others, we propose a new mechanism, which may involve constriction of the ciliary body and ipRGC-mediated relaxation of the iris dilator muscle.

Genetic advances in ophthalmology: the role of melanopsin-expressing, intrinsically photosensitive retinal ganglion cells in the circadian organization of the visual system

Daily changes in the light-dark cycle are the principal environmental signal that enables organisms to synchronize their internal biology with the 24-hour day-night cycle. In humans, the visual system is integral to photoentrainment and is primarily driven by a specialized class of intrinsically photosensitive retinal ganglion cells (ipRGCs) that express the photopigment melanopsin (OPN4) in the inner retina. These cells project through the retinohypothalamic tract (RHT) to the suprachiasmatic nuclei (SCN) of the hypothalamus, which serves as the body's master biological clock. At the same time, the retina itself possesses intrinsic circadian oscillations, exemplified by diurnal fluctuations in visual sensitivity, neurotransmitter levels, and outer segment turnover rates. Recently, it has been noted that both central and peripheral oscillators share a molecular clock consisting of an endogenous, circadian-driven, transcription-translation feedback loop that cycles with a periodicity of approximately 24 hours. This review will cover the role that melanopsin and ipRGCs play in the circadian organization of the visual system.

Blurring the boundaries of vision: novel functions of intrinsically photosensitive retinal ganglion cells

Mammalian vision consists of the classic image-forming pathway involving rod and cone photoreceptors interacting through a neural network within the retina before sending signals to the brain, and a non image-forming pathway that uses a photosensitive cell employing an alternative and evolutionary ancient phototransduction system and a direct connection to various centers in the brain. Intrinsically photosensitive retinal ganglion cells (ipRGCs) contain the photopigment melanopsin, which is independently capable of photon detection while also receiving synaptic input from rod and cone photoreceptors via bipolar cells. These cells are the retinal sentry for subconscious visual processing that controls circadian photoentrainment and the pupillary light reflex. Classified as irradiance detectors, recent investigations have led to expanding roles for this specific cell type and its own neural pathways, some of which are blurring the boundaries between image-forming and non image-forming visual processes.

Slow angled-descent forepaw grasping (SLAG): an innate behavioral task for identification of individual experimental mice possessing functional vision

Background

There is significant interest in the generation of improved assays to clearly identify experimental mice possessing functional vision, a property that could qualify mice for inclusion in behavioral and neuroscience studies. Widely employed current methods rely on mouse responses to visual cues in assays of reflexes, depth perception, or cognitive memory. However, commonly assessed mouse reflexes can sometimes be ambiguous in their expression, while depth perception assays are sometimes confounded by variation in anxiety responses and exploratory conduct. Furthermore, in situations where experimental groups vary in their cognitive memory capacity, memory assays may not be ideal for assessing differences in vision.

Results

We have optimized a non-invasive behavioral assay that relies on an untrained, innate response to identify individual experimental mice possessing functional vision: slow angled-descent forepaw grasping (SLAG). First, we verified that SLAG performance depends on vision and not olfaction. Next, all members of an age-ranged cohort of 158 C57BL/6 mice (57 wild-type, 101 knockout, age range 44–241 days) were assessed for functional vision using the SLAG test without training or conditioning. Subjecting the population to a second innate behavioral test, Dark Chamber preference, corroborated that the functional vision assessment of SLAG was valid.

Conclusions

We propose that the SLAG assay is immediately useful to quickly and clearly identify experimental mice possessing functional vision. SLAG is based on a behavioral readout with a significant innate component with no requirement for training. This will facilitate the selection of mice of known sighted status in vision-dependent experiments that focus on other types of behavior, neuroscience, and/or cognitive memory.

Number and spatial distribution of intrinsically photosensitive retinal ganglion cells in the adult albino rat

Intrinsically photosensitive retinal ganglion cells (ipRGCs) respond directly to light and are responsible of the synchronization of the circadian rhythm with the photic stimulus and for the pupillary light reflex. To quantify the total population of rat-ipRGCs and to assess their spatial distribution we have developed an automated routine and used neighbour maps. Moreover, in all analysed retinas we have studied the general population of RGCs - identified by their Brn3a expression - and the population of ipRGCs - identified by melanopsin immunodetection - thus allowing the co-analysis of their topography. Our results show that the total mean number ± standard deviation of ipRGCs in the albino rat is 2047 ± 309. Their distribution in the retina seems to be complementary to that of Brn3a(+)RGCs, being denser in the periphery, especially in the superior retina where their highest densities are found in the temporal quadrant, above the visual streak. In addition, by tracing the retinas from both superior colliculi, we have also determined that 90.62% of the ipRGC project to these central targets.

Modulation of CGRP-induced light aversion in wild-type mice by a 5-HT(1B/D) agonist

The neuropeptide calcitonin gene-related peptide (CGRP) plays a critical role in the pathophysiology of migraine. We have focused on the role of CGRP in photophobia, which is a common migraine symptom. We previously used an operant-based assay to show that CGRP-sensitized transgenic (nestin/hRAMP1), but not control, mice exhibited light aversion in response to an intracerebroventricular CGRP injection. A key question was whether the transgenic phenotype was due to overexpression of the CGRP receptor at endogenous or novel expression sites. We reasoned that if endogenous receptor sites were sufficient for light-aversive behavior, then wild-type mice should also show the phenotype when given a sufficiently strong stimulus. In this study, we report that mice with normal levels of endogenous CGRP receptors demonstrate light avoidance following CGRP administration. This phenotype required the combination of two factors: higher light intensity and habituation to the testing chamber. Control tests confirmed that light aversion was dependent on coincident exposure to CGRP and light and cannot be fully explained by increased anxiety. Furthermore, CGRP reduced locomotion only in the dark, not in the light. Coadministration of rizatriptan, a 5-HT(1B/D) agonist anti-migraine drug, attenuated the effects of exogenous CGRP on light aversion and motility. This suggests that triptans can act by mechanisms that are distinct from inhibition of CGRP release. Thus, we demonstrate that activation of endogenous CGRP receptors is sufficient to elicit light aversion in mice, which can be modulated by a drug commonly used to treat migraine.

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.

Melanopsin-positive intrinsically photosensitive retinal ganglion cells: from form to function

Melanopsin imparts an intrinsic photosensitivity to a subclass of retinal ganglion cells (ipRGCs). Generally thought of as irradiance detectors, ipRGCs target numerous brain regions involved in non-image-forming vision. ipRGCs integrate their intrinsic, melanopsin-mediated light information with rod/cone signals relayed via synaptic connections to influence light-dependent behaviors. Early observations indicated diversity among these cells and recently several specific subtypes have been identified. These subtypes differ in morphological and physiological form, controlling separate functions that range from biological rhythm via circadian photoentrainment, to protective behavioral responses including pupil constriction and light avoidance, and even image-forming vision. In this Mini-Symposium review, we will discuss some recent findings that highlight the diversity in both form and function of these recently discovered atypical photoreceptors.