Light-evoked calcium responses of isolated melanopsin-expressing retinal ganglion cells

Melanopsin activates divergent phototransduction pathways in intrinsically photosensitive retinal ganglion cell subtypes

Melanopsin signaling within intrinsically photosensitive retinal ganglion cell (ipRGC) subtypes impacts a broad range of behaviors from circadian photoentrainment to conscious visual perception. Yet, how melanopsin phototransduction within M1-M6 ipRGC subtypes impacts cellular signaling to drive diverse behaviors is still largely unresolved. The identity of the phototransduction channels in each subtype is key to understanding this central question but has remained controversial. In this study, we resolve two opposing models of M4 phototransduction, demonstrating that hyperpolarization-activated cyclic nucleotide-gated (HCN) channels are dispensable for this process and providing support for a pathway involving melanopsin-dependent potassium channel closure and canonical transient receptor potential (TRPC) channel opening. Surprisingly, we find that HCN channels are likewise dispensable for M2 phototransduction, contradicting the current model. We instead show that M2 phototransduction requires TRPC channels in conjunction with T-type voltage-gated calcium channels, identifying a novel melanopsin phototransduction target. Collectively, this work resolves key discrepancies in our understanding of ipRGC phototransduction pathways in multiple subtypes and adds to mounting evidence that ipRGC subtypes employ diverse phototransduction cascades to fine-tune cellular responses for downstream behaviors.

S-Cone Photoreceptors Regulate Daily Rhythms and Light-Induced Arousal/Wakefulness in Diurnal Grass Rats (Arvicanthis niloticus)

Beyond visual perception, light has non-image-forming effects mediated by melanopsin-expressing, intrinsically photosensitive retinal ganglion cells (ipRGCs). The present study first used multielectrode array recordings to show that in a diurnal rodent, Nile grass rats (Arvicanthis niloticus), ipRGCs generate rod/cone-driven and melanopsin-based photoresponses that stably encode irradiance. Subsequently, two ipRGC-mediated non-image-forming effects, namely entrainment of daily rhythms and light-induced arousal, were examined. Animals were first housed under a 12:12 h light/dark cycle (lights-on at 0600 h) with the light phase generated by a low-irradiance fluorescent light (F12), a daylight spectrum (D65) stimulating all photoreceptors, or a narrowband 480 nm spectrum (480) that maximized melanopsin stimulation and minimized S-cone stimulation (λmax 360 nm) compared to D65. Daily rhythms of locomotor activities showed onset and offset closer to lights-on and lights-off, respectively, in D65 and 480 than in F12, and higher day/night activity ratio under D65 versus 480 and F12, suggesting the importance of S-cone stimulation. To assess light-induced arousal, 3-h light exposures using 4 spectra that stimulated melanopsin equally but S-cones differentially were superimposed on F12 background lighting: D65, 480, 480 + 365 (narrowband 365 nm), and D65 - 365. Compared to the F12-only condition, all four pulses increased in-cage activity and promoted wakefulness, with 480 + 365 having the greatest and longest-lasting wakefulness-promoting effects, again indicating the importance of stimulating S-cones as well as melanopsin. These findings provide insights into the temporal dynamics of photoreceptor contributions to non-image-forming photoresponses in a diurnal rodent that may help guide future studies of lighting environments and phototherapy protocols that promote human health and productivity.

Chronic restraint stress-induced depression-like behavior is mediated by upregulation of melanopsin expression in C57BL/6 mice retina

BACKGROUND

Depression is associated with circadian disturbances in which melanopsin was a key mechanism. Further studies have demonstrated that melanopsin gene variations are associated with some depressive disorders and aberrant light can impair mood through melanopsin-expressing retinal ganglion cells (mRGCs). The goal of this study was to explore the direct relationship between depression and melanopsin.

METHODS

Adult C57BL/6 male mice were physically restrained for 16 h in a 50-ml polypropylene centrifuge tube and all behavioral tests were performed after CRS treatment. Western blot analysis and immunofluorescence were used to detect melanopsin expression in the retina of C57BL/6 mice. And we observed the change of the electrophysiological function and release of glutamate of mRGCs.

RESULTS

The melanopsin expression upregulate in mRGCs of chronic restraint stress (CRS)-treating mice which exhibit depression-like behavior. The frequency of blue light-induced action potentials and light-induced glutamate release mediated by melanopsin also increase significantly. This change of melanopsin is mediated by the CRS-induced glucocorticoid.

CONCLUSIONS

CRS may induce the depression-like behavior in mice via glucocorticoid-melanopsin pathway. Our findings provide a novel mechanistic link between CRS-induced depression and melanopsin in mice.

Retinal TRP channels: Cell-type-specific regulators of retinal homeostasis and multimodal integration

Transient receptor potential (TRP) channels are a widely expressed family of 28 evolutionarily conserved cationic ion channels that operate as primary detectors of chemical and physical stimuli and secondary effectors of metabotropic and ionotropic receptors. In vertebrates, the channels are grouped into six related families: TRPC, TRPV, TRPM, TRPA, TRPML, and TRPP. As sensory transducers, TRP channels are ubiquitously expressed across the body and the CNS, mediating critical functions in mechanosensation, nociception, chemosensing, thermosensing, and phototransduction. This article surveys current knowledge about the expression and function of the TRP family in vertebrate retinas, which, while dedicated to transduction and transmission of visual information, are highly susceptible to non-visual stimuli. Every retinal cell expresses multiple TRP subunits, with recent evidence establishing their critical roles in paradigmatic aspects of vertebrate vision that include TRPM1-dependent transduction of ON bipolar signaling, TRPC6/7-mediated ganglion cell phototransduction, TRP/TRPL phototransduction in Drosophila and TRPV4-dependent osmoregulation, mechanotransduction, and regulation of inner and outer blood-retina barriers. TRP channels tune light-dependent and independent functions of retinal circuits by modulating the intracellular concentration of the 2nd messenger calcium, with emerging evidence implicating specific subunits in the pathogenesis of debilitating diseases such as glaucoma, ocular trauma, diabetic retinopathy, and ischemia. Elucidation of TRP channel involvement in retinal biology will yield rewards in terms of fundamental understanding of vertebrate vision and therapeutic targeting to treat diseases caused by channel dysfunction or over-activation.

Divergent G-protein selectivity across melanopsins from mice and humans

Melanopsin is an opsin photopigment and light-activated G-protein-coupled receptor; it is expressed in photoreceptive retinal ganglion cells (mRGCs) and can be employed as an optogenetic tool. Mammalian melanopsins can signal via G_q/11 and G_i/o/t heterotrimeric G proteins, but aspects of the mRGC light response appear incompatible with either mode of signalling. We use live-cell reporter assays in HEK293T cells to show that melanopsins from mice and humans can also signal via G_s. We subsequently show that this mode of signalling is substantially divergent between species. The two established structural isoforms of mouse melanopsin (which differ in the length of their C-terminal tail) both signalled strongly through all three G-protein classes (G_q/11, G_i/o and G_s), whereas human melanopsin showed weaker signalling through G_s. Our data identify G_s as a new mode of signalling for mammalian melanopsins and reveal diversity in G-protein selectivity across mammalian melanopsins.

Summary: The photopigment melanopsin (OPN4), which provides inner retinal photoreception in mammals, shows light-dependent activation of G_s G protein that is more pronounced for mouse than human photopigment.

Melanopsin phototransduction: beyond canonical cascades

Melanopsin is a visual pigment that is expressed in a small subset of intrinsically photosensitive retinal ganglion cells (ipRGCs). It is involved in regulating non-image forming visual behaviors, such as circadian photoentrainment and the pupillary light reflex, while also playing a role in many aspects of image-forming vision, such as contrast sensitivity. Melanopsin was initially discovered in the melanophores of the skin of the frog Xenopus, and subsequently found in a subset of ganglion cells in rat, mouse and primate retinas. ipRGCs were initially thought to be a single retinal ganglion cell population, and melanopsin was thought to activate a single, invertebrate-like Gq/transient receptor potential canonical (TRPC)-based phototransduction cascade within these cells. However, in the 20 years since the discovery of melanopsin, our knowledge of this visual pigment and ipRGCs has expanded dramatically. Six ipRGC subtypes have now been identified in the mouse, each with unique morphological, physiological and functional properties. Multiple subtypes have also been identified in other species, suggesting that this cell type diversity is a general feature of the ipRGC system. This diversity has led to a renewed interest in melanopsin phototransduction that may not follow the canonical Gq/TRPC cascade in the mouse or in the plethora of other organisms that express the melanopsin photopigment. In this Review, we discuss recent findings and discoveries that have challenged the prevailing view of melanopsin phototransduction as a single pathway that influences solely non-image forming functions.

Melanopsin Signaling Pathway in HEK293 Cell Line with Stable Expression of Human Melanopsin: Possible Participation of Phospholipase C beta 4 and Diacylglycerol

Melanopsin, a member of the G protein-coupled receptors family, is involved in non-image-forming functions including circadian rhythm, sleep regulation and pupil response. In spite of significant research efforts, the signaling cascade involving melanopsin photoactivation remains poorly characterized. Here, we analyzed the effects of photoactivation of melanopsin on phospholipase C (PLC) and diacylglycerol. As an in vitro model, HEK293 cells with stable expression of human melanopsin were used. Although both the PLC_β1 and PLC_β4 subtypes were activated by the cell exposure to blue light, only PLC_β4 appeared to play a significant role in the studied melanopsin signaling pathway. We have demonstrated, for the first time, that cells expressing human melanopsin and enriched with 11-cis-retinal exhibited significantly increased diacylglycerol level. To determine the role of phospholipase C and involvement of diacylglycerols, two approaches were employed: inhibition of the G protein and phospholipase C (using the BIM-46187 and U73122 inhibitors, respectively), and gene silencing using siRNA of PLC_β1 and PLC_β4 . While silencing the PLC_β4 gene and using U73122 inhibited the diacylglycerol and calcium ion responses, the FOS gene expression level was only partially reduced. These results may facilitate a better understanding of the role of phospholipase C and diacylglycerols in the melanopsin signaling pathway.

Diversity of intrinsically photosensitive retinal ganglion cells: circuits and functions

The melanopsin-expressing, intrinsically photosensitive retinal ganglion cells (ipRGCs) are a relatively recently discovered class of atypical ganglion cell photoreceptor. These ipRGCs are a morphologically and physiologically heterogeneous population that project widely throughout the brain and mediate a wide array of visual functions ranging from photoentrainment of our circadian rhythms, to driving the pupillary light reflex to improve visual function, to modulating our mood, alertness, learning, sleep/wakefulness, regulation of body temperature, and even our visual perception. The presence of melanopsin as a unique molecular signature of ipRGCs has allowed for the development of a vast array of molecular and genetic tools to study ipRGC circuits. Given the emerging complexity of this system, this review will provide an overview of the genetic tools and methods used to study ipRGCs, how these tools have been used to dissect their role in a variety of visual circuits and behaviors in mice, and identify important directions for future study.

Retina and melanopsin neurons

Melanopsin retinal ganglion cells (mRGCs) are the third class of retinal photoreceptors with unique anatomical, electrophysiological, and biological features. There are different mRGC subtypes with differential projections to the brain. These cells contribute to many nonimage-forming functions of the eye, the most relevant being the photoentrainment of circadian rhythms through the projections to the suprachiasmatic nucleus of the hypothalamus. Other relevant biological functions include the regulation of the pupillary light reflex, mood, alertness, and sleep, as well as a possible role in formed vision. The relevance of the mRGC-related pathways in the brain is highlighted by the role that the dysfunction and/or loss of these cells may play in affecting circadian rhythms and sleep in many neurodegenerative disorders including Alzheimer's, Parkinson's and Huntington's disease and in aging. Moreover, the occurrence of circadian dysfunction is a known risk factor for dementia. In this chapter, the anatomy, physiology, and functions of these cells as well as their resistance to neurodegeneration in mitochondrial optic neuropathies or their predilection to be lost in other neurodegenerative disorders will be discussed.

The Effect of Refractive Error on Melanopsin-Driven Pupillary Responses

Purpose

Human and animal studies suggest that light-mediated dopamine release may underlie the protective effect of time outdoors on myopia development. Melanopsin-containing retinal ganglion cells may be involved in this process by integrating ambient light exposure and regulating retinal dopamine levels. The study evaluates this potential involvement by examining whether melanopsin-driven pupillary responses are associated with adult refractive error.

Methods

Subjects were 45 young adults (73% female, 24.1 ± 1.8 years) with refractive errors ranging from –6.33 D to +1.70 D. The RAPDx (Konan Medical) pupillometer measured normalized pupillary responses to three forms of square-wave light pulses alternating with darkness at 0.1 Hz: alternating long wavelength (red, peak at 608 nm) and short wavelength (blue, peak at 448 nm), followed by red only and then blue only.

Results

Non-myopic subjects displayed greater pupillary constriction in the blue-only condition and slower redilation following blue light offset than subjects with myopia (P = 0.011). Pupillary responses were not significantly different between myopic and non-myopic subjects in the red-only condition (P = 0.15). More hyperopic/less myopic refractive error as a continuous variable was linearly related to larger increases in pupillary constriction in response to blue-only stimuli (r = 0.48, P = 0.001).

Conclusions

Repeated light exposures to blue test stimuli resulted in an adaptation in the pupillary response (more constriction and slower redilation), presumably due to increased melanopsin-mediated input in more hyperopic/less myopic adults. This adaptive property supports a possible role for these ganglion cells in the protective effects of time outdoors on myopia development.

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.

Circadian regulation in the retina: From molecules to network

The mammalian retina is the most unique tissue among those that display robust circadian/diurnal oscillations. The retina is not only a light sensing tissue that relays light information to the brain, it has its own circadian "system" independent from any influence from other circadian oscillators. While all retinal cells and retinal pigment epithelium (RPE) possess circadian oscillators, these oscillators integrate by means of neural synapses, electrical coupling (gap junctions), and released neurochemicals (such as dopamine, melatonin, adenosine, and ATP), so the whole retina functions as an integrated circadian system. Dysregulation of retinal clocks not only causes retinal or ocular diseases, it also impacts the circadian rhythm of the whole body, as the light information transmitted from the retina entrains the brain clock that governs the body circadian rhythms. In this review, how circadian oscillations in various retinal cells are integrated, and how retinal diseases affect daily rhythms.

Transcriptomic Signatures of Postnatal and Adult Intrinsically Photosensitive Ganglion Cells

Intrinsically photosensitive retinal ganglion cells (ipRGCs) are rare mammalian photoreceptors essential for non-image-forming vision functions, such as circadian photoentrainment and the pupillary light reflex. They comprise multiple subtypes distinguishable by morphology, physiology, projections, and levels of expression of melanopsin (Opn4), their photopigment. The molecular programs that distinguish ipRGCs from other ganglion cells and ipRGC subtypes from one another remain elusive. Here, we present comprehensive gene expression profiles of early postnatal and adult mouse ipRGCs purified from two lines of reporter mice that mark different sets of ipRGC subtypes. We find dozens of novel genes highly enriched in ipRGCs. We reveal that Rasgrp1 and Tbx20 are selectively expressed in subsets of ipRGCs, though these molecularly defined groups imperfectly match established ipRGC subtypes. We demonstrate that the ipRGCs regulating circadian photoentrainment are diverse at the molecular level. Our findings reveal unexpected complexity in gene expression patterns across mammalian ipRGC subtypes.

μ-Opioid Receptor Activation Directly Modulates Intrinsically Photosensitive Retinal Ganglion Cells

Intrinsically photosensitive retinal ganglion cells (ipRGCs) encode light intensity and trigger reflexive responses to changes in environmental illumination. In addition to functioning as photoreceptors, ipRGCs are post-synaptic neurons in the inner retina, and there is increasing evidence that their output can be influenced by retinal neuromodulators. Here we show that opioids can modulate light-evoked ipRGC signaling, and we demonstrate that the M1, M2 and M3 types of ipRGCs are immunoreactive for μ-opioid receptors (MORs) in both mouse and rat. In the rat retina, application of the MOR-selective agonist DAMGO attenuated light-evoked firing ipRGCs in a dose-dependent manner (IC50 < 40 nM), and this effect was reversed or prevented by co-application of the MOR-selective antagonists CTOP or CTAP. Recordings from solitary ipRGCs, enzymatically dissociated from retinas obtained from melanopsin-driven fluorescent reporter mice, confirmed that DAMGO exerts its effect directly through MORs expressed by ipRGCs. Reduced ipRGC excitability occurred via modulation of voltage-gated potassium and calcium currents. These findings suggest a potential new role for endogenous opioids in the mammalian retina and identify a novel site of action-MORs on ipRGCs-through which opioids might exert effects on reflexive responses to environmental light.

Observer-perceived light aversion behaviour in photophobic subjects with traumatic brain injury

BACKGROUND

Photophobia is a common sequela of traumatic brain injury (TBI). Diagnostic tools for this debilitating condition are lacking. This investigation sought to determine whether masked observers can distinguish subjects with TBI-associated photophobia from matched controls based on video recordings of their ocular responses to light stimulation.

METHODS

Cohorts of students (n = 20), photophobic TBI subjects (n = 28) and their matched control subjects (n = 12) were recruited. A custom pupillometer delivered bright (10¹³ -10¹⁴ photons/s/cm² ), flashing (0.10 Hz) red (625 nm) and blue (470 nm) light stimuli to subjects, and consensual pupil light responses were recorded. Using a five-point scale, masked observers later graded light aversion behaviour in the pupil video recordings obtained from the student cohort based on observed blinking, tearing and squinting. A grading scale was developed and used by masked observers to grade light aversion behaviour in videos obtained from subjects with post-TBI photophobia and the matched controls. These subjects also scored their perceived discomfort during each light pulse using a five-point scale.

RESULTS

The subjects in the TBI cohort scored both the blue and red flashing stimuli as evoking more discomfort, relative to control subjects, consistent with their reported photophobia. There was strong agreement among the masked observers for their grades of light aversion behaviour in the videos of ocular light stimulation (interclass correlation co-efficient = 0.78; 29 per cent perfect concordance). However, the median grades for the videos obtained from the TBI subject cohort were not significantly different from those for the control group.

CONCLUSIONS

Clinicians cannot diagnose TBI-related photophobia based solely on video recordings of ocular responses to light. The need remains for an objective test to diagnose and manage this prevalent post-TBI symptom.

Melanopsin Phototransduction Is Repurposed by ipRGC Subtypes to Shape the Function of Distinct Visual Circuits

Melanopsin is expressed in distinct types of intrinsically photosensitive retinal ganglion cells (ipRGCs), which drive behaviors from circadian photoentrainment to contrast detection. A major unanswered question is how the same photopigment, melanopsin, influences such vastly different functions. Here we show that melanopsin's role in contrast detection begins in the retina, via direct effects on M4 ipRGC (ON alpha RGC) signaling. This influence persists across an unexpectedly wide range of environmental light levels ranging from starlight to sunlight, which considerably expands the functional reach of melanopsin on visual processing. Moreover, melanopsin increases the excitability of M4 ipRGCs via closure of potassium leak channels, a previously unidentified target of the melanopsin phototransduction cascade. Strikingly, this mechanism is selective for image-forming circuits, as M1 ipRGCs (involved in non-image forming behaviors), exhibit a melanopsin-mediated decrease in excitability. Thus, melanopsin signaling is repurposed by ipRGC subtypes to shape distinct visual behaviors.

Circadian Behavioral Responses to Light and Optic Chiasm-Evoked Glutamatergic EPSCs in the Suprachiasmatic Nucleus of ipRGC Conditional vGlut2 Knock-Out Mice

Intrinsically photosensitive retinal ganglion cells (ipRGCs) innervate the hypothalamic suprachiasmatic nucleus (SCN), a circadian oscillator that functions as a biological clock. ipRGCs use vesicular glutamate transporter 2 (vGlut2) to package glutamate into synaptic vesicles and light-evoked resetting of the SCN circadian clock is widely attributed to ipRGC glutamatergic neurotransmission. Pituitary adenylate cyclase-activating polypeptide (PACAP) is also packaged into vesicles in ipRGCs and PACAP may be coreleased with glutamate in the SCN. vGlut2 has been conditionally deleted in ipRGCs in mice [conditional knock-outs (cKOs)] and their aberrant photoentrainment and residual attenuated light responses have been ascribed to ipRGC PACAP release. However, there is no direct evidence that all ipRGC glutamatergic neurotransmission is eliminated in vGlut2 cKOs. Here, we examined two lines of ipRGC vGlut2 cKO mice for SCN-mediated behavioral responses under several lighting conditions and for ipRGC glutamatergic neurotransmission in the SCN. Circadian behavioral responses varied from a very limited response to light to near normal photoentrainment. After collecting behavioral data, hypothalamic slices were prepared and evoked EPSCs (eEPSCs) were recorded from SCN neurons by stimulating the optic chiasm. In cKOs, glutamatergic eEPSCs were recorded and all eEPSC parameters examined (stimulus threshold, amplitude, rise time or time-to-peak and stimulus strength to evoke a maximal response) were similar to controls. We conclude that a variable number but functionally significant percentage of ipRGCs in two vGlut2 cKO mouse lines continue to release glutamate. Thus, the residual SCN-mediated light responses in these cKO mouse lines cannot be attributed solely to ipRGC PACAP release.

Eyeing up the Future of the Pupillary Light Reflex in Neurodiagnostics

The pupillary light reflex (PLR) describes the constriction and subsequent dilation of the pupil in response to light as a result of the antagonistic actions of the iris sphincter and dilator muscles. Since these muscles are innervated by the parasympathetic and sympathetic nervous systems, respectively, different parameters of the PLR can be used as indicators for either sympathetic or parasympathetic modulation. Thus, the PLR provides an important metric of autonomic nervous system function that has been exploited for a wide range of clinical applications. Measurement of the PLR using dynamic pupillometry is now an established quantitative, non-invasive tool in assessment of traumatic head injuries. This review examines the more recent application of dynamic pupillometry as a diagnostic tool for a wide range of clinical conditions, varying from neurodegenerative disease to exposure to toxic chemicals, as well as its potential in the non-invasive diagnosis of infectious disease.

Phototransduction in Retinal Ganglion Cells

The mammalian retina contains a small number of retinal ganglion cells that express melanopsin, a retinal based visual pigment, and generate a depolarizing response to light in the absence of rod and cone driven synaptic input; hence they are referred to as intrinsically photosensitive retinal ganglion cells (ipRGCs). They have been shown to be comprised of a number of sub-types and to provide luminance information that participates primarily in a variety of non-imaging forming visual functions. Here I review what is currently known about the cascade of events that couple the photoisomerization of melanopsin to the opening of a non-selective cation channel. While these events conform in a general sense to the prevailing model for invertebrate phototransduction, in which visual pigment signals through a G protein of the Gq class and a phospholipase C cascade to open a TRPC type ion channel, none of the molecular elements in the melanopsin transduction process have been unequivocally identified. This has given rise to the possibility that the underlying mechanism responsible for intrinsic photosensitivity is not same in all ipRGC sub-types and to the recognition that signal transduction in ipRGCs is more complex than originally thought.

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.

A Population Representation of Absolute Light Intensity in the Mammalian Retina

Environmental illumination spans many log units of intensity and is tracked for essential functions that include regulation of the circadian clock, arousal state, and hormone levels. Little is known about the neural representation of light intensity and how it covers the necessary range. This question became accessible with the discovery of mammalian photoreceptors that are required for intensity-driven functions, the M1 ipRGCs. The spike outputs of M1s are thought to uniformly track intensity over a wide range. We provide a different understanding: individual cells operate over a narrow range, but the population covers irradiances from moonlight to full daylight. The range of most M1s is limited by depolarization block, which is generally considered pathological but is produced intrinsically by these cells. The dynamics of block allow the population to code stimulus intensity with flexibility and efficiency. Moreover, although spikes are distorted by block, they are regularized during axonal propagation.

Attenuation of short wavelengths alters sleep and the ipRGC pupil response

PURPOSE

Exposure to increasing amounts of artificial light during the night may contribute to the high prevalence of reported sleep dysfunction. Release of the sleep hormone melatonin is mediated by the intrinsically photosensitive retinal ganglion cells (ipRGCs). This study sought to investigate whether melatonin level and sleep quality can be modulated by decreasing night-time input to the ipRGCs.

METHODS

Subjects (ages 17-42, n = 21) wore short wavelength-blocking glasses prior to bedtime for 2 weeks. The ipRGC-mediated post illumination pupil response was measured before and after the experimental period. Stimulation was presented with a ganzfeld stimulator, including one-second and five-seconds of long and short wavelength light, and the pupil was imaged with an infrared camera. Pupil diameter was measured before, during and for 60 s following stimulation, and the six-second and 30 s post illumination pupil response and area under the curve following light offset were determined. Subjects wore an actigraph device for objective measurements of activity, light exposure, and sleep. Saliva samples were collected to assess melatonin content. The Pittsburgh Sleep Quality Index (PSQI) was administered to assess subjective sleep quality.

RESULTS

Subjects wore the blue-blocking glasses 3:57 ± 1:03 h each night. After the experimental period, the pupil showed a slower redilation phase, resulting in a significantly increased 30 s post illumination pupil response to one-second short wavelength light, and decreased area under the curve for one and five-second short wavelength light, when measured at the same time of day as baseline. Night time melatonin increased from 16.1 ± 7.5 pg mL^-1 to 25.5 ± 10.7 pg mL^-1 (P < 0.01). Objectively measured sleep duration increased 24 min, from 408.7 ± 44.9 to 431.5 ± 42.9 min (P < 0.001). Mean PSQI score improved from 5.6 ± 2.9 to 3.0 ± 2.2.

CONCLUSIONS

The use of short wavelength-blocking glasses at night increased subjectively measured sleep quality and objectively measured melatonin levels and sleep duration, presumably as a result of decreased night-time stimulation of ipRGCs. Alterations in the ipRGC-driven pupil response suggest a shift in circadian phase. Results suggest that minimising short wavelength light following sunset may help in regulating sleep patterns.

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.

M1 ipRGCs Influence Visual Function through Retrograde Signaling in the Retina

Melanopsin-expressing intrinsically photosensitive retinal ganglion cells (ipRGCs, with five subtypes named M1-M5) are a unique subclass of RGCs with axons that project directly to many brain nuclei involved in non-image-forming functions such as circadian photoentrainment and the pupillary light reflex. Recent evidence suggests that melanopsin-based signals also influence image-forming visual function, including light adaptation, but the mechanisms involved are unclear. Intriguingly, a small population of M1 ipRGCs have intraretinal axon collaterals that project toward the outer retina. Using genetic mouse models, we provide three lines of evidence showing that these axon collaterals make connections with upstream dopaminergic amacrine cells (DACs): (1) ipRGC signaling to DACs is blocked by tetrodotoxin both in vitro and in vivo, indicating that ipRGC-to-DAC transmission requires voltage-gated Na(+) channels; (2) this transmission is partly dependent on N-type Ca(2+) channels, which are possibly expressed in the axon collateral terminals of ipRGCs; and (3) fluorescence microscopy reveals that ipRGC axon collaterals make putative presynaptic contact with DACs. We further demonstrate that elimination of M1 ipRGCs attenuates light adaptation, as evidenced by an impaired electroretinogram b-wave from cones, whereas a dopamine receptor agonist can potentiate the cone-driven b-wave of retinas lacking M1 ipRGCs. Together, the results strongly suggest that ipRGCs transmit luminance signals retrogradely to the outer retina through the dopaminergic system and in turn influence retinal light adaptation.

SIGNIFICANCE STATEMENT

Melanopsin-expressing intrinsically photosensitive retinal ganglion cells (ipRGCs) comprise a third class of retinal photoreceptors that are known to mediate physiological responses such as circadian photoentrainment. However, investigation into whether and how ipRGCs contribute to vision has just begun. Here, we provide convergent anatomical and physiological evidence that axon collaterals of ipRGCs constitute a centrifugal pathway to DACs, conveying melanopsin-based signals from the innermost retina to the outer retina. We further demonstrate that retrograde signals likely influence visual processing because elimination of axon collateral-bearing ipRGCs impairs light adaptation by limiting dopamine-dependent facilitation of the cone pathway. Our findings strongly support the hypothesis that retrograde melanopsin-based signaling influences visual function locally within the retina, a notion that refutes the dogma that RGCs only provide physiological signals to the brain.

Melanopsin-expressing retinal ganglion cells are resistant to cell injury, but not always

Melanopsin retinal ganglion cells (mRGCs) are intrinsically photosensitive RGCs deputed to non-image forming functions of the eye such as synchronization of circadian rhythms to light-dark cycle. These cells are characterized by unique electrophysiological, anatomical and biochemical properties and are usually more resistant than conventional RGCs to different insults, such as axotomy and different paradigms of stress. We also demonstrated that these cells are relatively spared compared to conventional RGCs in mitochondrial optic neuropathies (Leber's hereditary optic neuropathy and Dominant Optic Atrophy). However, these cells are affected in other neurodegenerative conditions, such as glaucoma and Alzheimer's disease. We here review the current evidences that may underlie this dichotomy. We also present our unpublished data on cell experiments demonstrating that melanopsin itself does not explain the robustness of these cells and some preliminary data on immunohistochemical assessment of mitochondria in mRGCs.

Evaluating retinal ganglion cell loss and dysfunction

Retinal ganglion cells (RGC) bear the sole responsibility of propagating visual stimuli to the brain. Their axons, which make up the optic nerve, project from the retina to the brain through the lamina cribrosa and in rodents, decussate almost entirely at the optic chiasm before synapsing at the superior colliculus. For many traumatic and degenerative ocular conditions, the dysfunction and/or loss of RGC is the primary determinant of visual loss and are the measurable endpoints in current research into experimental therapies. To actually measure these endpoints in rodent models, techniques must ascertain both the quantity of surviving RGC and their functional capacity. Quantification techniques include phenotypic markers of RGC, retrogradely transported fluorophores and morphological measurements of retinal thickness whereas functional assessments include electroretinography (flash and pattern) and visual evoked potential. The importance of the accuracy and reliability of these techniques cannot be understated, nor can the relationship between RGC death and dysfunction. The existence of up to 30 types of RGC complicates the measuring process, particularly as these may respond differently to disease and treatment. Since the above techniques may selectively identify and ignore particular subpopulations, their appropriateness as measures of RGC survival and function may be further limited. This review discusses the above techniques in the context of their subtype specificity.

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.

In vitro transdifferentiation of human peripheral blood mononuclear cells to photoreceptor-like cells

Direct reprogramming is a promising, simple and low-cost approach to generate target cells from somatic cells without using induced pluripotent stem cells. Recently, peripheral blood mononuclear cells (PBMCs) have attracted considerable attention as a somatic cell source for reprogramming. As a cell source, PBMCs have an advantage over dermal fibroblasts with respect to the ease of collecting tissues. Based on our studies involving generation of photosensitive photoreceptor cells from human iris cells and human dermal fibroblasts by transduction of photoreceptor-related transcription factors via retrovirus vectors, we transduced these transcription factors into PBMCs via Sendai virus vectors. We found that retinal disease-related genes were efficiently detected in CRX-transduced cells, most of which are crucial to photoreceptor functions. In functional studies, a light-induced inward current was detected in some CRX-transduced cells. Moreover, by modification of the culture conditions including additional transduction of RAX1 and NEUROD1, we found a greater variety of retinal disease-related genes than that observed in CRX-transduced PBMCs. These data suggest that CRX acts as a master control gene for reprogramming PBMCs into photoreceptor-like cells and that our induced photoreceptor-like cells might contribute to individualized drug screening and disease modeling of inherited retinal degeneration.

Muscarinic acetylcholine receptor-mediated stimulation of retinal ganglion cell photoreceptors

Melanopsin-dependent phototransduction in intrinsically photosensitive retinal ganglion cells (ipRGCs) involves a Gq-coupled phospholipase C (PLC) signaling cascade. Acetylcholine, released in the mammalian retina by starburst amacrine cells, can also activate Gq-PLC pathways through certain muscarinic acetylcholine receptors (mAChRs). Using multielectrode array recordings of rat retinas, we demonstrate that robust spiking responses can be evoked in neonatal and adult ipRGCs after bath application of the muscarinic agonist carbachol. The stimulatory action of carbachol on ipRGCs was a direct effect, as confirmed through calcium imaging experiments on isolated ipRGCs in purified cultures. Using flickering (6 Hz) yellow light stimuli at irradiances below the threshold for melanopsin activation, spiking responses could be elicited in ipRGCs that were suppressed by mAChR antagonism. Therefore, this work identified a novel melanopsin-independent pathway for stimulating sustained spiking in ganglion cell photoreceptors. This mAChR-mediated pathway could enhance ipRGC spiking responses in conditions known to evoke retinal acetylcholine release, such as those involving flickering or moving visual stimuli. Furthermore, this work identifies a pharmacological approach for light-independent ipRGC stimulation that could be targeted by mAChR agonists.

The contribution of inner and outer retinal photoreceptors to infra-slow oscillations in the rat olivary pretectal nucleus

A subpopulation of olivary pretectal nucleus (OPN) neurons discharges action potentials in an oscillatory manner, with a period of approximately two minutes. This 'infra-slow' oscillatory activity depends on synaptic excitation originating in the retina. Signals from rod-cone photoreceptors reach the OPN via the axons of either classic retinal ganglion cells or intrinsically photosensitive retinal ganglion cells (ipRGCs), which use melanopsin for photon capturing. Although both cell types convey light information, their physiological functions differ considerably. The aim of the present study was to disentangle how rod-cone and melanopsin photoresponses contribute to generation of oscillatory activity. Pharmacological manipulations of specific phototransduction cascades were used whilst recording extracellular single-unit activity in the OPN of anaesthetized rats. The results show that under photopic conditions (bright light), ipRGCs play a major role in driving infra-slow oscillations, as blocking melanopsin phototransmission abolishes or transiently disturbs oscillatory firing of the OPN neurons. On the other hand, blocking rod-cone phototransmission does not change firing patterns in photopic conditions. However, under mesopic conditions (moderate light), when melanopsin phototransmission is absent, blocking rod-cone signalling causes disturbances or even the disappearance of oscillations implying that classic photoreceptors are of greater importance under moderate light. Evidence is provided that all photoreceptors are required for the generation of oscillations in the OPN, although their roles in driving the rhythm are determined by the lighting conditions, consistent with their relative sensitivities. The results further suggest that maintained retinal activity is crucial to observe infra-slow oscillatory activity in the OPN.

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.

Orexin-A potentiates L-type calcium/barium currents in rat retinal ganglion cells

Two neuropeptides, orexin-A and orexin-B (also called hypocretin-1 and -2), have been implicated in sleep/wake regulation, feeding behaviors via the activation of two subtypes of G-protein-coupled receptors: orexin 1 and orexin 2 receptors (OX1R and OX2R). While the expression of orexins and orexin receptors is immunohistochemically revealed in retinal neurons, the function of these peptides in the retina is largely unknown. Using whole-cell patch-clamp recordings in rat retinal slices, we demonstrated that orexin-A increased L-type-like barium currents (IBa,L) in ganglion cells (GCs), and the effect was blocked by the selective OX1R antagonist SB334867, but not by the OX2R antagonist TCS OX2 29. The orexin-A effect was abolished by intracellular dialysis of GDP-β-S/GPAnt-2A, a Gq protein inhibitor, suggesting the mediation of Gq. Additionally, during internal dialysis of the phosphatidylinositol (PI)-phospholipase C (PLC) inhibitor U73122, orexin-A did not change the IBa,L of GCs, whereas the orexin-A effect persisted in the presence of the phosphatidylcholine (PC)-PLC inhibitor D609. The orexin-A-induced potentiation was not seen with internal infusion of Ca(2+)-free solution or when inositol 1,4,5-trisphosphate (IP3)-sensitive Ca(2+) release from intracellular stores was blocked by heparin/xestospongins-C. Moreover, the orexin-A effect was mimicked by the protein kinase C (PKC) activator phorbol 12-myristate 13-acetate, but was eliminated when PKC was inhibited by bisindolylmaleimide IV (Bis-IV)/Gö6976. Neither adenosine 3',5'-cyclic monophosphate (cAMP)-protein kinase A (PKA) nor guanosine 3',5'-cyclic monophosphate (cGMP)-protein kinase G (PKG) signaling pathway was likely involved, as orexin-A persisted to potentiate the IBa,L of GCs no matter these two pathways were activated or inhibited. These results suggest that, by activating OX1R, orexin-A potentiates the IBa,L of rat GCs through a distinct Gq/PI-PLC/IP3/Ca(2+)/PKC signaling pathway.

TRPM3 expression in mouse retina

Transient receptor potential (TRP) channels constitute a large family of cation permeable ion channels that serve crucial functions in sensory systems by transducing environmental changes into cellular voltage and calcium signals. Within the retina, two closely related members of the melastatin TRP family, TRPM1 and TRPM3, are highly expressed. TRPM1 has been shown to be required for the depolarizing response to light of ON-bipolar cells, but the role of TRPM3 in the retina is unknown. Immunohistochemical staining of mouse retina with an antibody directed against the C-terminus of TRPM3 labeled the inner plexiform layer (IPL) and a subset of cells in the ganglion cell layer. Within the IPL, TRPM3 immunofluorescence was markedly stronger in the OFF sublamina than in the ON sublamina. Electroretinogram recordings showed that the scotopic and photopic a- and b-waves of TRPM3(-/-) mice are normal indicating that TRPM3 does not play a major role in visual processing in the outer retina. TRPM3 activity was measured by calcium imaging and patch-clamp recording of immunopurified retinal ganglion cells. Application of the TRPM3 agonist, pregnenolone sulfate (PS), stimulated increases in intracellular calcium in ~40% of cells from wild type and TRPM1(‑/‑) mice, and the PS-stimulated increases in calcium were blocked by co-application of mefenamic acid, a TRPM3 antagonist. No PS-stimulated changes in fluorescence were observed in ganglion cells from TRPM3(-/-) mice. Similarly, PS-stimulated currents that could be blocked by mefenamic acid were recorded from wild type retinal ganglion cells but were absent in ganglion cells from TRPM3-/- mice.

Photic regulation of clock systems

Circadian rhythms in physiology and behavior provide a selective advantage by enabling organisms to anticipate rhythmic changes in their environment. These rhythms are based upon a molecular clock generated via an intracellular transcriptional-translational feedback loop involving a number of key clock genes. However, to be of practical use, circadian rhythms need to be entrained to the external environment. In mammals, the primary signal for entrainment is light detected by the photoreceptors of the eye. Research on the mechanisms of photic entrainment has identified a novel photoreceptor system in the retina, consisting of photosensitive retinal ganglion cells expressing the photopigment melanopsin. Light input from these retinal photoreceptors reaches the master circadian pacemaker in the suprachiasmatic nuclei (SCN) via the retinohypothalamic tract, where it then interacts with the molecular clock to bring about entrainment. This chapter focuses on the retinal photoreceptors mediating entrainment, and how light information from the retina is transmitted to the SCN, before detailing recent advances in our understanding of how the molecular clock within the SCN is regulated by light input. Finally, the primary assays that have been used to measure photic entrainment are described.

The injury resistant ability of melanopsin-expressing intrinsically photosensitive retinal ganglion cells

Neurons in the mammalian retina expressing the photopigment melanopsin have been identified as a class of intrinsically photosensitive retinal ganglion cells (ipRGCs). This discovery more than a decade ago has opened up an exciting new field of retinal research, and following the initial identification of photosensitive ganglion cells, several subtypes have been described. A number of studies have shown that ipRGCs subserve photoentrainment of circadian rhythms. They also influence other non-image forming functions of the visual system, such as the pupillary light reflex, sleep, cognition, mood, light aversion and development of the retina. These novel photosensitive neurons also influence form vision by contributing to contrast detection. Furthermore, studies have shown that ipRGCs are more injury-resistant following optic nerve injury, in animal models of glaucoma, and in patients with mitochondrial optic neuropathies, i.e., Leber’s hereditary optic neuropathy and dominant optic atrophy. There is also an indication that these cells may be resistant to glutamate-induced excitotoxicity. Herein we provide an overview of ipRGCs and discuss the injury-resistant character of these neurons under certain pathological and experimental conditions.

Phosphorylation of rat melanopsin at Ser-381 and Ser-398 by light/dark and its importance for intrinsically photosensitive ganglion cells (ipRGCs) cellular Ca2+ signaling

The G protein-coupled light-sensitive receptor melanopsin is involved in non-image-forming light responses including circadian timing. The predicted secondary structure of melanopsin indicates a long cytoplasmic tail with many potential phosphorylation sites. Using bioinformatics, we identified a number of amino acids with a high probability of being phosphorylated. We generated antibodies against melanopsin phosphorylated at Ser-381 and Ser-398, respectively. The antibody specificity was verified by immunoblotting and immunohistochemical staining of HEK-293 cells expressing rat melanopsin mutated in Ser-381 or Ser-398. Using the antibody recognizing phospho-Ser-381 melanopsin, we demonstrated by immunoblotting and immunohistochemical staining in HEK-293 cells expressing rat melanopsin that the receptor is phosphorylated in this position during the dark and dephosphorylated when light is turned on. On the contrary, we found that melanopsin at Ser-398 was unphosphorylated in the dark and became phosphorylated after light stimulation. The light-induced changes in phosphorylation at both Ser-381 and Ser-398 were rapid and lasted throughout the 4-h experimental period. Furthermore, phosphorylation at Ser-381 and Ser-398 was independent of each other. The changes in phosphorylation were confirmed in vivo by immunohistochemical staining of rat retinas during light and dark. We further demonstrated that mutation of Ser-381 and Ser-398 in melanopsin-expressing HEK-293 cells affected the light-induced Ca²⁺ response, which was significantly reduced as compared with wild type. Examining the light-evoked Ca²⁺ response in a melanopsin Ser-381 plus Ser-398 double mutant provided evidence that the phosphorylation events were independent.

Altered entrainment to the day/night cycle attenuates the daily rise in circulating corticosterone in the mouse

The suprachiasmatic nucleus (SCN) is a circadian oscillator entrained to the day/night cycle via input from the retina. Serotonin (5-HT) afferents to the SCN modulate retinal signals via activation of 5-HT1B receptors, decreasing responsiveness to light. Consequently, 5-HT1B receptor knockout (KO) mice entrain to the day/night cycle with delayed activity onsets. Since circulating corticosterone levels exhibit a robust daily rhythm peaking around activity onset, we asked whether delayed entrainment of activity onsets affects rhythmic corticosterone secretion. Wheel-running activity and plasma corticosterone were monitored in mice housed under several different lighting regimens. Both duration of the light:dark cycle (T cycle) and the duration of light within that cycle was altered. 5-HT1B KO mice that entrained to a 9.5L:13.5D (short day in a T = 23 h) cycle with activity onsets delayed more than 4 h after light offset exhibited a corticosterone rhythm in phase with activity rhythms but reduced 50% in amplitude compared to animals that initiated daily activity <4 h after light offset. Wild type mice in 8L:14D (short day in a T = 22 h) conditions with highly delayed activity onsets also exhibited a 50% reduction in peak plasma corticosterone levels. Exogenous adrenocorticotropin (ACTH) stimulation in animals exhibiting highly delayed entrainment suggested that the endogenous rhythm of adrenal responsiveness to ACTH remained aligned with SCN-driven behavioral activity. Circadian clock gene expression in the adrenal cortex of these same animals suggested that the adrenal circadian clock was also aligned with SCN-driven behavior. Under T cycles <24 h, altered circadian entrainment to short day (winter-like) conditions, manifest as long delays in activity onset after light offset, severely reduces the amplitude of the diurnal rhythm of plasma corticosterone. Such a pronounced reduction in the glucocorticoid rhythm may alter rhythmic gene expression in the central nervous system and in peripheral organs contributing to an array of potential pathophysiologies.

Adenosine modulates light responses of rat retinal ganglion cell photoreceptors througha cAMP-mediated pathway

Adenosine is an established neuromodulator in the mammalian retina, with A1 adenosine receptors being especially prevalent in the innermost ganglion cell layer. Activation of A1 receptors causes inhibition of adenylate cyclase, decreases in intracellular cyclic AMP (cAMP) levels and inhibition of protein kinase A (PKA). In this work, our aim was to characterize the effects of adenosine on the light responses of intrinsically photosensitive retinal ganglion cells (ipRGCs) and to determine whether these photoreceptors are subject to neuromodulation through intracellular cAMP-related signalling pathways. Using multielectrode array recordings from postnatal and adult rat retinas, we demonstrated that adenosine significantly shortened the duration of ipRGC photoresponses and reduced the number of light-evoked spikes fired by these neurons. The effects were A1 adenosine receptor-mediated, and the expression of this receptor on melanopsin-containing ipRGCs was confirmed by calcium imaging experiments on isolated cells in purified cultures. While inhibition of the cAMP/PKA pathway by adenosine shortened ipRGC light responses, stimulation of this pathway with compounds such as forskolin had the opposite effect and lengthened the duration of ipRGC spiking. Our findings reveal that the modification of ipRGC photoresponses through a cAMP/PKA pathway is a general feature of rat ganglion cell photoreceptors, and this pathway can be inhibited through activation of A1 receptors by adenosine. As adenosine levels in the retina rise at night, adenosinergic modulation of ipRGCs may serve as an internal regulatory mechanism to limit transmission of nocturnal photic signals by ipRGCs to the brain. Targeting retinal A1 adenosine receptors for ipRGC inhibition represents a potential therapeutic target for sleep disorders and migraine-associated photophobia.

Using siRNA to define functional interactions between melanopsin and multiple G Protein partners

Melanopsin expressing photosensitive retinal ganglion cells (pRGCs) represent a third class of ocular photoreceptors and mediate a range of non-image forming responses to light. Melanopsin is a G protein coupled receptor (GPCR) and existing data suggest that it employs a membrane bound signalling cascade involving Gnaq/11 type G proteins. However, to date the precise identity of the Gα subunits involved in melanopsin phototransduction remains poorly defined. Here we show that Gnaq, Gna11 and Gna14 are highly co-expressed in pRGCs of the mouse retina. Furthermore, using RNAi based gene silencing we show that melanopsin can signal via Gnaq, Gna11 or Gna14 in vitro, and demonstrate that multiple members of the Gnaq/11 subfamily, including Gna14 and at least Gnaq or Gna11, can participate in melanopsin phototransduction in vivo and contribute to the pupillary light responses of mice lacking rod and cone photoreceptors. This diversity of G protein interactions suggests additional complexity in the melanopsin phototransduction cascade and may provide a basis for generating the diversity of light responses observed from pRGC subtypes.

Light-induced rapid Ca²⁺ response and MAPK phosphorylation in the cells heterologously expressing human OPN5

Molecular imaging is a powerful tool for investigating intracellular signalling, but it is difficult to acquire conventional fluorescence imaging from photoreceptive cells. Here we demonstrated that human opsin5 (OPN5) photoreceptor mediates light-induced Ca(2+) response in human embryonic kidney (HEK293) and mouse neuroblastoma (Neuro2a) cell lines using a luminescence imaging system with a fluorescent indicator and a newly synthesized bioluminescent indicator. Weak light fluorescence and bioluminescence imaging revealed rapid and transient light-stimulated Ca(2+) release from thapsigargin-sensitive Ca(2+) stores, whereas long-lasting Ca(2+) elevation was observed using a conventional fluorescence imaging system. Bioluminescence imaging also demonstrated that OPN5 activation in HEK293 cells induced a decrease in pertussis toxin-sensitive cAMP, confirming previous reports. In addition, ultraviolet radiation induced the phosphorylation of mitogen-activated protein kinases when OPN5 was stimulated in Neuro2a cells. These findings suggest that the combination of these imaging approaches may provide a new means to investigate the physiological characteristics of photoreceptors.

Transient receptor potential canonical 7: a diacylglycerol-activated non-selective cation channel

Transient receptor potential canonical 7 (TRPC7) channel is the seventh member of the mammalian TRPC channel family. TRPC7 mRNA, protein, and channel activity have been detected in many tissues and organs from the mouse, rat, and human. TRPC7 has high sequence homology with TRPC3 and TRPC6, and all three channels are activated by membrane receptors that couple to isoforms of phospholipase C (PLC) and mediate non-selective cation currents. TRPC7, along with TRPC3 and TRPC6, can be activated by direct exogenous application of diacylglycerol (DAG) analogues and by pharmacological maneuvers that increase endogenous DAG in cells. TRPC7 shows distinct properties of activation, such as constitutive activity and susceptibility to negative regulation by extracellular Ca(2+) and by protein kinase C. TRPC7 can form heteromultimers with TRPC3 and TRPC6. Although TRPC7 remains one of the least studied TRPC channel, its role in various cell types and physiological and pathophysiological conditions is beginning to emerge.

Simultaneous electrophysiological recording and calcium imaging of suprachiasmatic nucleus neurons

Simultaneous electrophysiological and fluorescent imaging recording methods were used to study the role of changes of membrane potential or current in regulating the intracellular calcium concentration. Changing environmental conditions, such as the light-dark cycle, can modify neuronal and neural network activity and the expression of a family of circadian clock genes within the suprachiasmatic nucleus (SCN), the location of the master circadian clock in the mammalian brain. Excitatory synaptic transmission leads to an increase in the postsynaptic Ca(2+) concentration that is believed to activate the signaling pathways that shifts the rhythmic expression of circadian clock genes. Hypothalamic slices containing the SCN were patch clamped using microelectrodes filled with an internal solution containing the calcium indicator bis-fura-2. After a seal was formed between the microelectrode and the SCN neuronal membrane, the membrane was ruptured using gentle suction and the calcium probe diffused into the neuron filling both the soma and dendrites. Quantitative ratiometric measurements of the intracellular calcium concentration were recorded simultaneously with membrane potential or current. Using these methods it is possible to study the role of changes of the intracellular calcium concentration produced by synaptic activity and action potential firing of individual neurons. In this presentation we demonstrate the methods to simultaneously record electrophysiological activity along with intracellular calcium from individual SCN neurons maintained in brain slices.

Small-molecule antagonists of melanopsin-mediated phototransduction

Melanopsin, expressed in a subset of retinal ganglion cells, mediates behavioral adaptation to ambient light and other non-image forming photic responses. This has raised the possibility that pharmacological manipulation of melanopsin can modulate several CNS responses including photophobia, sleep, circadian rhythms and neuroendocrine function. Here we describe the identification of a potent synthetic melanopsin antagonist with in vivo activity. Novel sulfonamide compounds inhibiting melanopsin (opsinamides) compete with retinal binding to melanopsin and inhibit its function without affecting rod/cone mediated responses. In vivo administration of opsinamides to mice specifically and reversibly modified melanopsin-dependent light responses including the pupillary light reflex and light aversion. The discovery of opsinamides raises the prospect of therapeutic control of the melanopsin phototransduction system to regulate light-dependent behavior and remediate pathological conditions.

Adaptation to steady light by intrinsically photosensitive retinal ganglion cells

Intrinsically photosensitive retinal ganglion cells (ipRGCs) are recently discovered photoreceptors in the mammalian eye. These photoreceptors mediate primarily nonimage visual functions, such as pupillary light reflex and circadian photoentrainment, which are generally expected to respond to the absolute light intensity. The classical rod and cone photoreceptors, on the other hand, mediate image vision by signaling contrast, accomplished by adaptation to light. Experiments by others have indicated that the ipRGCs do, in fact, light-adapt. We found the same but, in addition, have now quantified this light adaptation for the M1 ipRGC subtype. Interestingly, in incremental-flash-on-background experiments, the ipRGC's receptor current showed a flash sensitivity that adapted in background light according to the Weber-Fechner relation, well known to describe the adaptation behavior of rods and cones. Part of this light adaptation by ipRGCs appeared to be triggered by a Ca(2+) influx, in that the flash response elicited in the absence of extracellular Ca(2+) showed a normal rising phase but a slower decay phase, resulting in longer time to peak and higher sensitivity. There is, additionally, a prominent Ca(2+)-independent component of light adaptation not typically seen in rods and cones or in invertebrate rhabdomeric photoreceptors.