Induction of photosensitivity by heterologous expression of melanopsin

Functional optic tract rewiring via subtype- and target-specific axonal regeneration and presynaptic activity enhancement

Mechanisms underlying functional axonal rewiring after adult mammalian central nervous system (CNS) injuries remain unclear partially due to limited models. Here we develop a mouse intracranial pre-olivary pretectal nucleus (OPN) optic tract injury model and demonstrate that Pten/Socs3 knockout and CNTF expression in retinal ganglion cells (RGCs) promotes optic tract regeneration and OPN reinnervation. Revealed by transmission electron microscopy, trans-synaptic labeling, and electrophysiology, functional synapses are formed in OPN mainly by intrinsically photosensitive RGCs, thereby partially restoring the pupillary light reflex (PLR). Moreover, combining with Lipin1 knockdown accelerates the recovery and achieves functional reconnection after chronic injury. PLR can be further boosted by increasing RGC photosensitivity with melanopsin overexpression, and it can also be enhanced by treatment of a voltage-gated calcium channel modulator to augment presynaptic release. These findings highlight the importance of neuronal types and presynaptic activity for functional reconnection after CNS injuries.

Pivotal roles of melanopsin containing retinal ganglion cells in pupillary light reflex in photopic conditions

The pupillary light reflex (PLR) is crucial for protecting the retina from excess light. The intrinsically photosensitive retinal ganglion cells (ipRGCs) in the retina are neurons that are critical to generating the PLR, receiving rod/cone photoreceptor signals and directly sensing light through melanopsin. Previous studies have investigated the roles of photoreceptors and ipRGCs in PLR using genetically-modified mouse models. Herein, we acutely ablated photoreceptors using N-nitroso-N-methylurea (MNU) to examine the roles of ipRGCs in the PLR. We conducted PLR and multiple electrode array (MEA) recordings evoked by three levels of light stimuli before and 5 days after MNU intraperitoneal (i.p..) injection using C57BL6/J wildtype (WT) mice. We also conducted these measurements using the rod & cone dysfunctional mice (Gnat1-/- & Cnga3-/-:dKO) to compare the results to published studies in which mutant mice were used to show the role of photoreceptors and ipRGCs in PLR. PLR pupil constriction increased as the light stimulus intensified in WT mice. In MNU mice, PLR was not induced by the low light stimulus, suggesting that photoreceptors induced the PLR at this light intensity. By contrast, the high light stimulus fully induced PLR, similar to the response in WT mice. In dKO mice, no PLR was evoked by the low-light stimulus and a slow-onset PLR was evoked by the high-light stimulus, consistent with previous reports. Ex vivo MEA recording in the MNU tissue revealed a population of ipRGCs with a fast onset and peak time, suggesting that they drove the fast PLR response. These results suggest that ipRGCs primarily contribute to the PLR at a high light intensity, which does not agree with the previous results shown by mutant mouse models. Our results indicate that the melanopsin response in ipRGCs generate fast and robust PLR when induced by high light.

Effect of hydrogen-rich saline on melanopsin after acute blue light-induced retinal damage in rats

Excessive exposure to blue light can cause retinal damage. Hydrogen-rich saline (HRS), one of the hydrogen therapies, has been demonstrated to be effective in eye photodamage, but the effect on the expression of melanopsin in intrinsically photosensitive retinal ganglion cells (ipRGCs) is unknown. In this study, we used a rat model of light-induced retinal injury to observe the expression of melanopsin after HRS treatment and to determine the effect of HRS on retinal ganglion cell protection. Adult SD rats were exposed to blue light (48 h) and treated with HRS for 0, 3, 7, and 14 days. Real-time polymerase chain reaction (qRT-PCR) and Western blotting (WB) were performed to find the expression of genes and proteins, respectively. The function of retinal ipRGCs was measured by pattern-evoked electroretinography (pERG). The number and morphological changes of melanopsin-positive ganglion cells in the retina were observed by immunofluorescence (IF). Acute blue light exposure caused a decrease in ipRGC function, decreased expression of melanopsin protein and the melanopsin-positive RGCs, and diminished immunoreactivity in dendrites. However, over time, melanopsin showed a tendency to self-recovery, with an increase in melanopsin protein expression and the number of melanopsin-positive RGCs, with incomplete recovery of function within two weeks. HRS treatment accelerated the recovery process, with a significant increase in melanopsin expression and the number of melanopsin-positive RGCs, and an improvement in the pERG waveform within two weeks.

Disrupted Circadian Rhythms and Substance Use Disorders: A Narrative Review

Substance use disorder is a major global health concern, with a high prevalence among adolescents and young adults. The most common substances of abuse include alcohol, marijuana, cocaine, nicotine, and opiates. Evidence suggests that a mismatch between contemporary lifestyle and environmental demands leads to disrupted circadian rhythms that impair optimal physiological and behavioral function, which can increase the vulnerability to develop substance use disorder and related problems. The circadian system plays an important role in regulating the sleep-wake cycle and reward processing, both of which directly affect substance abuse. Distorted substance use can have a reciprocal effect on the circadian system by influencing circadian clock gene expression. Considering the detrimental health consequences and profound societal impact of substance use disorder, it is crucial to comprehend its complex association with circadian rhythms, which can pave the way for the generation of novel chronotherapeutic treatment approaches. In this narrative review, we have explored the potential contributions of disrupted circadian rhythms and sleep on use and relapse of different substances of abuse. The involvement of circadian clock genes with drug reward pathways is discussed, along with the potential research areas that can be explored to minimize disordered substance use by improving circadian hygiene.

In-silico predicted mouse melanopsins with blue spectral shifts deliver efficient subcellular signaling

Melanopsin is a photopigment belonging to the G Protein-Coupled Receptor (GPCR) family expressed in a subset of intrinsically photosensitive retinal ganglion cells (ipRGCs) and responsible for a variety of processes. The bistability and, thus, the possibility to function under low retinal availability would make melanopsin a powerful optogenetic tool. Here, we aim to utilize mouse melanopsin to trigger macrophage migration by its subcellular optical activation with localized blue light, while simultaneously imaging the migration with red light. To reduce melanopsin's red light sensitivity, we employ a combination of in silico structure prediction and automated quantum mechanics/molecular mechanics modeling to predict minimally invasive mutations to shift its absorption spectrum towards the shorter wavelength region of the visible spectrum without compromising the signaling efficiency. The results demonstrate that it is possible to achieve melanopsin mutants that resist red light-induced activation but are activated by blue light and display properties indicating preserved bistability. Using the A333T mutant, we show that the blue light-induced subcellular melanopsin activation triggers localized PIP3 generation and macrophage migration, which we imaged using red light, demonstrating the optogenetic utility of minimally engineered melanopsins.

Circadian rhythms in the Drosophila eye may regulate adaptation of vision to light intensity

The sensitivity of the eye at night would lead to complete saturation of the eye during the day. Therefore, the sensitivity of the eye must be down-regulated during the day to maintain visual acuity. In the Drosophila eye, the opening of TRP and TRPL channels leads to an influx of Ca++ that triggers down-regulation of further responses to light, including the movement of the TRPL channel and Gα proteins out of signaling complexes found in actin-mediated microvillar extensions of the photoreceptor cells (the rhabdomere). The eye also exhibits a light entrained-circadian rhythm, and we have recently observed that one component of this rhythm (BDBT) becomes undetectable by antibodies after exposure to light even though immunoblot analyses still detect it in the eye. BDBT is necessary for normal circadian rhythms, and in several circadian and visual mutants this eye-specific oscillation of detection is lost. Many phototransduction signaling proteins (e.g., Rhodopsin, TRP channels and Gα) also become undetectable shortly after light exposure, most likely due to a light-induced compaction of the rhabdomeric microvilli. The circadian protein BDBT might be involved in light-induced changes in the rhabdomere, and if so this could indicate that circadian clocks contribute to the daily adaptations of the eye to light. Likewise, circadian oscillations of clock proteins are observed in photoreceptors of the mammalian eye and produce a circadian oscillation in the ERG. Disruption of circadian rhythms in the eyes of mammals causes neurodegeneration in the eye, demonstrating the importance of the rhythms for normal eye function.

Retinoid Synthesis Regulation by Retinal Cells in Health and Disease

Vision starts in retinal photoreceptors when specialized proteins (opsins) sense photons via their covalently bonded vitamin A derivative 11cis retinaldehyde (11cis-RAL). The reaction of non-enzymatic aldehydes with amino groups lacks specificity, and the reaction products may trigger cell damage. However, the reduced synthesis of 11cis-RAL results in photoreceptor demise and suggests the need for careful control over 11cis-RAL handling by retinal cells. This perspective focuses on retinoid(s) synthesis, their control in the adult retina, and their role during retina development. It also explores the potential importance of 9cis vitamin A derivatives in regulating retinoid synthesis and their impact on photoreceptor development and survival. Additionally, recent advancements suggesting the pivotal nature of retinoid synthesis regulation for cone cell viability are discussed.

Molecular Property, Manipulation, and Potential Use of Opn5 and Its Homologs

Animal opsin is a G-protein coupled receptor (GPCR) and binds retinal as a chromophore to form a photopigment. The Opsin 5 (Opn5) group within the animal opsin family comprises a diverse array of related proteins, such as Opn5m, a protein conserved across all vertebrate lineages including mammals, and other members like Opn5L1 and Opn5L2 found in non-mammalian vertebrate genomes, and Opn6 found in non-therian vertebrate genomes, along with Opn5 homologs present in invertebrates. Although these proteins collectively constitute a single clade within the molecular phylogenetic tree of animal opsins, they exhibit markedly distinct molecular characteristics in areas such as retinal binding properties, photoreaction, and G-protein coupling specificity. Based on their molecular features, they are believed to play a significant role in physiological functions. However, our understanding of their precise physiological functions and molecular characteristics is still developing and only partially realized. Furthermore, their unique molecular characteristics of Opn5-related proteins suggest a high potential for their use as optogenetic tools through more specialized manipulations. This review intends to encapsulate our current understanding of Opn5, discuss potential manipulations of its molecular attributes, and delve into its prospective utility in the burgeoning field of animal opsin optogenetics.

The cognitive impact of light: illuminating ipRGC circuit mechanisms

Ever-present in our environments, light entrains circadian rhythms over long timescales, influencing daily activity patterns, health and performance. Increasing evidence indicates that light also acts independently of the circadian system to directly impact physiology and behaviour, including cognition. Exposure to light stimulates brain areas involved in cognition and appears to improve a broad range of cognitive functions. However, the extent of these effects and their mechanisms are unknown. Intrinsically photosensitive retinal ganglion cells (ipRGCs) have emerged as the primary conduit through which light impacts non-image-forming behaviours and are a prime candidate for mediating the direct effects of light on cognition. Here, we review the current state of understanding of these effects in humans and mice, and the tools available to uncover circuit-level and photoreceptor-specific mechanisms. We also address current barriers to progress in this area. Current and future efforts to unravel the circuits through which light influences cognitive functions may inform the tailoring of lighting landscapes to optimize health and cognitive function.

Addressing Circadian Disruptions in Visually Impaired Paralympic Athletes

PURPOSE

Transmeridian travel is common for elite athletes participating in competitions and training. However, this travel can lead to circadian misalignment wherein the internal biological clock becomes desynchronized with the light-dark cycle of the new environment, resulting in performance decrement and potential negative health consequences. Existing literature extensively discusses recommendations for managing jet lag, predominantly emphasizing light-based interventions to synchronize the internal clock with the anticipated time at the destination. Nevertheless, visually impaired (VI) athletes may lack photoreceptiveness, diminishing or nullifying the effectiveness of this therapy. Consequently, this invited commentary explores alternative strategies for addressing jet lag in VI athletes.

CONCLUSIONS

VI athletes with light perception but reduced visual acuity or visual fields may still benefit from light interventions in managing jet lag. However, VI athletes lacking a conscious perception of light should rely on gradual shifts in behavioral factors, such as meal timing and exercise, to facilitate the entrainment of circadian rhythms to the destination time. Furthermore, interventions like melatonin supplementation may prove useful during and after travel. In addition, it is recommended that athlete guides adopt phase-forward or phase-back approaches to synchronize with the athlete, aiding in jet-lag management and optimizing performance.

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.

Immediate responses to ambient light in vivo reveal distinct subpopulations of suprachiasmatic VIP neurons

The circadian rhythm pacemaker, the suprachiasmatic nucleus (SCN), mediates light entrainment via vasoactive intestinal peptide (VIP) neurons (SCNVIP). Yet, how these neurons uniquely respond and connect to intrinsically photosensitive retinal ganglion cells (ipRGCs) expressing melanopsin (Opn4) has not been determined functionally in freely behaving animals. To address this, we first used monosynaptic tracing from SCNVIP neurons in mice and identified two SCNVIP subpopulations. Second, we recorded calcium changes in response to ambient light, at both bulk and single-cell levels, and found two unique activity patterns in response to high- and low-intensity blue light. The activity patterns of both subpopulations could be manipulated by application of an Opn4 antagonist. These results suggest that the two SCNVIP subpopulations connect to two types of Opn4-expressing ipRGCs, likely M1 and M2, but only one is responsive to red light. These findings have important implications for our basic understanding of non-image-forming circadian light processing.

Optogenetic control of neural activity: The biophysics of microbial rhodopsins in neuroscience

Optogenetics, the use of microbial rhodopsins to make the electrical activity of targeted neurons controllable by light, has swept through neuroscience, enabling thousands of scientists to study how specific neuron types contribute to behaviors and pathologies, and how they might serve as novel therapeutic targets. By activating a set of neurons, one can probe what functions they can initiate or sustain, and by silencing a set of neurons, one can probe the functions they are necessary for. We here review the biophysics of these molecules, asking why they became so useful in neuroscience for the study of brain circuitry. We review the history of the field, including early thinking, early experiments, applications of optogenetics, pre-optogenetics targeted neural control tools, and the history of discovering and characterizing microbial rhodopsins. We then review the biophysical attributes of rhodopsins that make them so useful to neuroscience - their classes and structure, their photocycles, their photocurrent magnitudes and kinetics, their action spectra, and their ion selectivity. Our hope is to convey to the reader how specific biophysical properties of these molecules made them especially useful to neuroscientists for a difficult problem - the control of high-speed electrical activity, with great precision and ease, in the brain.

Quantum Mechanics/Molecular mechanics calculations predict A1, not A2, is present in melanopsin (Opn4m) of red-eared slider turtles (Trachemys scripta elegans)

Melanopsin is a photopigment that plays a role in non-visual, light-driven, cellular processes such as modulation of circadian rhythms, retinal vascular development, and the pupillary light reflex (PLR). In this study, computational methods were used to understand which chromophore is harbored by melanopsin in red-eared slider turtles (Trachemys scripta elegans). In mammals, the vitamin A derivative 11-cis-retinal (A1) is the chromophore, which provides functionality for melanopsin. However, in red-eared slider turtles, a member of the reptilian class, the identity of the chromophore remains unclear. Red-eared slider turtles, similar to other freshwater vertebrates, possess visual pigments that harbor a different vitamin A derivative, 11-cis-3,4-didehydroretinal (A2), making their pigments more sensitive to red-light than blue-light, therefore, suggesting the chromophore to be the A2 derivative instead of the A1. To help resolve the chromophore identity, in this work, computational homology models of melanopsin in red-eared slider turtles were first constructed. Next, quantum mechanics/molecular mechanics (QM/MM) calculations were carried out to compare how A1 and A2 derivatives bind to melanopsin. Time dependent density functional theory (TDDFT) calculations were then used to determine the excitation energy of the pigments. Lastly, calculated excitation energies were compared to experimental spectral sensitivity data from responses by the irises of red-eared sliders. Contrary to what was expected, our results suggest that melanopsin in red-eared slider turtles is more likely to harbor the A1 chromophore than the A2. Furthermore, a glutamine (Q622.56) and tyrosine (Y853.28) residue in the chromophore binding pocket are shown to play a role in the spectral tuning of the chromophore.

The Role of Retinal Dysfunction in Myopia Development

Myopia is a refractive disorder arising from a mismatch between refractive power and relatively long axial length of the eye. With its dramatically increasing prevalence, myopia has become a pervasive social problem. It is commonly accepted that abnormal visual input acts as an initiating factor of myopia. As the first station to perceive visual signals, the retina plays an important role in myopia etiology. The retina is a fine-layered structure with multitudinous cells, processing intricate visual signals via numerous molecular pathways. Accordingly, dopaminergic mechanisms, contributions of rod and cone photoreceptors, myopic structural changes of retinal pigment epithelium (RPE) and neuro-retinal layers have all suggested a vital role of retinal dysfunction in myopia development. Herein, we separately discuss myopia-related retinal dysfunction and current dilemmas by different levels, from molecules to cells, with the hope that the comprehensive delineation could contribute to a better understanding of myopia etiology, indicate novel therapeutic targets, and inspire future studies.

Mechanisms of Cardiovascular Changes of Phototherapy in Newborns with Hyperbilirubinemia

During phototherapy of jaundiced newborns, vasodilation occurs in the skin circulation compensated by vasoconstriction in the renal and mesenteric circulation. Furthermore, there is a slight decrease in cardiac systolic volume, and blood pressure, as well as an increase in heart rate and discrete changes in the heart rate variability (HRV). The primary change during phototherapy is the skin vasodilation mediated by multiple mechanisms: 1) Passive vasodilation induced by direct skin heating effect of the body surface and subcutaneous blood vessels, modified by myogenic autoregulation. 2) Active vasodilation mediated via the mechanism provided by axon reflexes through nerve C-fibers and humoral mechanism via nitric oxide (NO) and endothelin 1 (ET-1). During and after phototherapy is a rise in the NO:ET-1 ratio. 3) Regulation of the skin circulation through the sympathetic nerves is unique, but their role in skin vasodilation during phototherapy was not studied. 4) Special mechanism is a photorelaxation independent of the skin heating. Melanopsin (opsin 4) - is thought to play a major role in systemic vascular photorelaxation. Signalling cascade of the photorelaxation is specific, independent of endothelium and NO. The increased skin blood flow during phototherapy is enabled by the restriction of blood flow in the renal and mesenteric circulation. An increase in heart rate indicates activation of the sympathetic system as is seen in the measures of the HRV. High-pressure, as well as low-pressure baroreflexes, may play important role in these adaptation responses. The integrated complex and specific mechanism responsible for the hemodynamic changes during phototherapy confirm adequate and functioning regulation of the neonatal cardiovascular system, including baroreflexes.

Short-term acute bright light exposure induces a prolonged anxiogenic effect in mice via a retinal ipRGC-CeA circuit

Light modulates mood through various retina-brain pathways. We showed that mice treated with short-term acute bright light exposure displayed anxiety-related phenotypes in a prolonged manner even after the termination of the exposure. Such a postexposure anxiogenic effect depended upon melanopsin-based intrinsically photosensitive retinal ganglion cell (ipRGC) activities rather than rod/cone photoreceptor inputs. Chemogenetic manipulation of specific central nuclei demonstrated that the ipRGC-central amygdala (CeA) visual circuit played a key role in this effect. The corticosterone system was likely to be involved in this effect, as evidenced by enhanced expression of the glucocorticoid receptor (GR) protein in the CeA and the bed nucleus of the stria terminalis and by the absence of this effect in animals treated with the GR antagonist. Together, our findings reveal a non-image forming visual circuit specifically designed for "the delayed" extinction of anxiety against potential threats, thus conferring a survival advantage.

Burning the candle at both ends: Intraretinal signaling of intrinsically photosensitive retinal ganglion cells

Intrinsically photosensitive retinal ganglion cells (ipRGCs) are photoreceptors located in the ganglion cell layer. They project to brain regions involved in predominately non-image-forming functions including entrainment of circadian rhythms, control of the pupil light reflex, and modulation of mood and behavior. In addition to possessing intrinsic photosensitivity via the photopigment melanopsin, these cells receive inputs originating in rods and cones. While most research in the last two decades has focused on the downstream influence of ipRGC signaling, recent studies have shown that ipRGCs also act retrogradely within the retina itself as intraretinal signaling neurons. In this article, we review studies examining intraretinal and, in addition, intraocular signaling pathways of ipRGCs. Through these pathways, ipRGCs regulate inner and outer retinal circuitry through both chemical and electrical synapses, modulate the outputs of ganglion cells (both ipRGCs and non-ipRGCs), and influence arrangement of the correct retinal circuitry and vasculature during development. These data suggest that ipRGC function plays a significant role in the processing of image-forming vision at its earliest stage, positioning these photoreceptors to exert a vital role in perceptual vision. This research will have important implications for lighting design to optimize the best chromatic lighting environments for humans, both in adults and potentially even during fetal and postnatal development. Further studies into these unique ipRGC signaling pathways could also lead to a better understanding of the development of ocular dysfunctions such as myopia.

Treatment with morning blue light increases left amygdala volume and sleep duration among individuals with posttraumatic stress disorder

Background

Posttraumatic stress disorder (PTSD) is associated with numerous cognitive, affective, and psychophysiological outcomes, including problems with sleep and circadian rhythms. We tested the effectiveness of a daily morning blue-light exposure treatment (BLT) versus a matched amber light treatment (ALT) to regulate sleep in individuals diagnosed with PTSD. Moreover, PTSD is also associated with reliable findings on structural neuroimaging scans, including reduced amygdala volumes and other differences in cortical gray matter volume (GMV) that may be indicative of underlying neurobehavioral dysfunctions. We examined the effect of BLT versus ALT on GMV and its association with sleep outcomes.

Methods

Seventy-six individuals (25 male; 51 female) meeting DSM-V criteria for PTSD (Age = 31.45 years, SD = 8.83) completed sleep assessments and structural neuroimaging scans, followed by random assignment one of two light groups, including BLT (469 nm; n = 39) or placebo ALT (578 nm; n = 37) light therapy daily for 30-min over 6-weeks. Participants wore a wrist actigraph for the duration of the study. After treatment, participants returned to complete sleep assessments and a structural neuroimaging scan. Neuroimaging data were analyzed using the Computational Anatomy Toolbox (CAT12) and Voxel-Based Morphometry (VBM) modules within the Statistical Parametric Mapping (SPM12) software.

Results

The BLT condition produced significant increases in total time in bed and total sleep time from actigraphy compared to the ALT condition, while ALT improved wake after sleep onset and sleep efficiency compared to BLT. Additionally, BLT led to an increase in left amygdala volume compared to ALT but did not affect hypothesized medial prefrontal regions. Finally, within group correlations showed that improvements in sleep quality and nightmare severity were correlated with increases in left amygdala volume over the course of treatment for the BLT group but not the ALT group.

Conclusion

In individuals with PTSD, daily exposure to morning blue light treatment was associated with improvements in objective sleep duration and increased volume of the left amygdala compared to amber placebo light treatment, and changes in amygdala volume correlated with subjective improvement in sleep. These findings suggest that daily morning BLT may provide an important non-pharmacologic adjunctive approach for facilitating sleep and neurobehavioral recovery from PTSD.

A neuropsin-based optogenetic tool for precise control of Gq signaling

G_q-coupled receptors regulate numerous physiological processes by activating enzymes and inducing intracellular Ca²⁺ signals. There is a strong need for an optogenetic tool that enables powerful experimental control over G_q signaling. Here, we present chicken opsin 5 (cOpn5) as the long sought-after, single-component optogenetic tool that mediates ultra-sensitive optical control of intracellular G_q signaling with high temporal and spatial resolution. Expressing cOpn5 in HEK 293T cells and primary mouse astrocytes enables blue light-triggered, G_q-dependent Ca²⁺ release from intracellular stores and protein kinase C activation. Strong Ca²⁺ transients were evoked by brief light pulses of merely 10 ms duration and at 3 orders lower light intensity of that for common optogenetic tools. Photostimulation of cOpn5-expressing cells at the subcellular and single-cell levels generated fast intracellular Ca²⁺ transition, thus demonstrating the high spatial precision of cOpn5 optogenetics. The cOpn5-mediated optogenetics could also be applied to activate neurons and control animal behavior in a circuit-dependent manner. cOpn5 optogenetics may find broad applications in studying the mechanisms and functional relevance of G_q signaling in both non-excitable cells and excitable cells in all major organ systems.

Rhodopsins: An Excitingly Versatile Protein Species for Research, Development and Creative Engineering

The first member and eponym of the rhodopsin family was identified in the 1930s as the visual pigment of the rod photoreceptor cell in the animal retina. It was found to be a membrane protein, owing its photosensitivity to the presence of a covalently bound chromophoric group. This group, derived from vitamin A, was appropriately dubbed retinal. In the 1970s a microbial counterpart of this species was discovered in an archaeon, being a membrane protein also harbouring retinal as a chromophore, and named bacteriorhodopsin. Since their discovery a photogenic panorama unfolded, where up to date new members and subspecies with a variety of light-driven functionality have been added to this family. The animal branch, meanwhile categorized as type-2 rhodopsins, turned out to form a large subclass in the superfamily of G protein-coupled receptors and are essential to multiple elements of light-dependent animal sensory physiology. The microbial branch, the type-1 rhodopsins, largely function as light-driven ion pumps or channels, but also contain sensory-active and enzyme-sustaining subspecies. In this review we will follow the development of this exciting membrane protein panorama in a representative number of highlights and will present a prospect of their extraordinary future potential.

A systematic comparison of optogenetic approaches to visual restoration

During inherited retinal degenerations (IRDs), vision is lost due to photoreceptor cell death; however, a range of optogenetic tools have been shown to restore light responses in animal models. Restored response characteristics vary between tools and the neuronal cell population to which they are delivered: the interplay between these is complex, but targeting upstream neurons (such as retinal bipolar cells) may provide functional benefit by retaining intraretinal signal processing. In this study, our aim was to compare two optogenetic tools: mammalian melanopsin (hOPN4) and microbial red-shifted channelrhodopsin (ReaChR) expressed within two subpopulations of surviving cells in a degenerate retina. Intravitreal adeno-associated viral vectors and mouse models utilising the Cre/lox system restricted expression to populations dominated by bipolar cells or retinal ganglion cells and was compared with non-targeted delivery using the chicken beta actin (CBA) promoter. In summary, we found bipolar-targeted optogenetic tools produced faster kinetics and flatter intensity-response relationships compared with non-targeted or retinal-ganglion-cell-targeted hOPN4. Hence, optogenetic tools of both mammalian and microbial origins show advantages when targeted to bipolar cells. This demonstrates the advantage of bipolar-cell-targeted optogenetics for vision restoration in IRDs. We therefore developed a bipolar-cell-specific gene delivery system employing a compressed promoter with the potential for clinical translation.

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.

Optogenetic Stimulation of Gi Signaling Enables Instantaneous Modulation of Cardiomyocyte Pacemaking

G-protein signaling pathways are central in the regulation of cardiac function in physiological and pathophysiological conditions. Their functional analysis through optogenetic techniques with selective expression of opsin proteins and activation by specific wavelengths allows high spatial and temporal precision. Here, we present the application of long wavelength-sensitive cone opsin (LWO) in cardiomyocytes for activation of the G_i signaling pathway by red light. Murine embryonic stem (ES) cells expressing LWO were generated and differentiated into beating cardiomyocytes in embryoid bodies (EBs). Illumination with red light (625 nm) led to an instantaneous decrease up to complete inhibition (84–99% effectivity) of spontaneous beating, but had no effect on control EBs. By using increasing light intensities with 10 s pulses, we determined a half maximal effective light intensity of 2.4 μW/mm² and a maximum effect at 100 μW/mm². Pre-incubation of LWO EBs with pertussis toxin completely inhibited the light effect proving the specificity for G_i signaling. Frequency reduction was mainly due to the activation of GIRK channels because the specific channel blocker tertiapin reduced the light effect by ~80%. Compared with pharmacological stimulation of M₂ receptors with carbachol with slow kinetics (>30 s), illumination of LWO had an identical efficacy, but much faster kinetics (<1 s) in the activation and deactivation demonstrating the temporal advantage of optogenetic stimulation. Thus, LWO is an effective optogenetic tool for selective stimulation of the G_i signaling cascade in cardiomyocytes with red light, providing high temporal precision.

Exposure to Blue Light Reduces Melanopsin Expression in Intrinsically Photoreceptive Retinal Ganglion Cells and Damages the Inner Retina in Rats

Purpose

The purpose of this study was to investigative the effects of blue light on intrinsically photoreceptive retinal ganglion cells (ipRGCs).

Methods

Brown Norway rats were used. Nine rats were continuously exposed to blue light (light emitting diodes [LEDs]: 463 nm; 1000 lx) for 2 days (acute exposure [AE]); 9 rats were exposed to 12 hours of blue light and 12 hours of darkness for 10 days (long-term exposure [LTE]); 6 control rats were exposed to 12 hours of white fluorescent light (1000 lx) and 12 hours of darkness for 10 days. Whole-mount retinas were immunolabelled with melanopsin antibodies; melanopsin-positive (MP) ipRGC somas and processes were counted and measured with Neuron J. To detect apoptosis, retinal cryo-sections were stained with terminal deoxynucleotidyl transferase dUTP nick-end labeling. Ultra-thin sections were visualized with transmission electron microscopy.

Results

The number of MP ipRGC somas was significantly lower in retinas from AE and LTE rats than in those from control rats (P < 0.001 and = 0.002, respectively). The mean length of MP areas of processes was significantly lower in AE rats (P < 0.001). AE rats had severe retinal damage and massive apoptosis in the outer nuclear layer; their mitochondria were damaged in the axons and dendrites of the nerve fiber layer and the inner plexiform layer. Retinal ganglion cells (RGCs) in AE rats appeared to have reduced amounts of free ribosomes and rough endoplasmic reticulum.

Conclusions

AE to blue light reduces melanopsin expression and damages RGCs, likely including ipRGCs. Changes in the axons and dendrites of RGCs suggest possible disruption of intraretinal and extraretinal signal transmission.

The Intrinsic Blue Light Responses of Avian Müller Glial Cells Imply Calcium Release from Internal Stores

The retina of vertebrates is responsible for capturing light through visual (cones and rods) and non-visual photoreceptors (intrinsically photosensitive retinal ganglion cells and horizontal cells) triggering a number of essential activities associated to image- and non-image forming functions (photic entrainment of daily rhythms, pupillary light reflexes, pineal melatonin inhibition, among others). Although the retina contains diverse types of neuronal based-photoreceptors cells, originally classified as ciliary- or rhabdomeric-like types, in recent years, it has been shown that the major glial cell type of the retina, the Müller glial cells (MC), express blue photopigments as Opn3 (encephalopsin) and Opn5 (neuropsin) and display light responses associated to intracellular Ca2 + mobilization. These findings strongly propose MC as novel retinal photodetectors (Rios et al., 2019). Herein, we further investigated the intrinsic light responses of primary cultures of MC from embryonic chicken retinas specially focused on Ca2 + mobilization by fluorescence imaging and the identity of the internal Ca2 + stores responsible for blue light responses. Results clearly demonstrated that light responses were specific to blue light of long time exposure, and that the main Ca2 + reservoir to trigger downstream responses came from intracellular stores localized in the endoplasmic reticulum These observations bring more complexity to the intrinsic photosensitivity of retinal cells, particularly with regard to the detection of light in the blue range of visible spectra, and add novel functions to glial cells cooperating with other photoreceptors to detect and integrate ambient light in the retinal circuit and participate in cell to cell communication.

Summary statement:

Non-neuronal cells in the vertebrate retina, Muller glial cells, express non-canonical photopigments and sense blue light causing calcium release from intracellular stores strongly suggesting a novel intrinsic photosensitivity and new regulatory events mediating light-driven processes with yet unknown physiological implications.

Non-visual Opsins and Novel Photo-Detectors in the Vertebrate Inner Retina Mediate Light Responses Within the Blue Spectrum Region

In recent decades, a number of novel non-visual opsin photopigments belonging to the family of G protein- coupled receptors, likely involved in a number of non-image-forming processes, have been identified and characterized in cells of the inner retina of vertebrates. It is now known that the vertebrate retina is composed of visual photoreceptor cones and rods responsible for diurnal/color and nocturnal/black and white vision, and cells like the intrinsically photosensitive retinal ganglion cells (ipRGCs) and photosensitive horizontal cells in the inner retina, both detecting blue light and expressing the photopigment melanopsin (Opn4). Remarkably, these non-visual photopigments can continue to operate even in the absence of vision under retinal degeneration. Moreover, inner retinal neurons and Müller glial cells have been shown to express other photopigments such as the photoisomerase retinal G protein-coupled receptor (RGR), encephalopsin (Opn3), and neuropsin (Opn5), all able to detect blue/violet light and implicated in chromophore recycling, retinal clock synchronization, neuron-to-glia communication, and other activities. The discovery of these new photopigments in the inner retina of vertebrates is strong evidence of novel light-regulated activities. This review focuses on the features, localization, photocascade, and putative functions of these novel non-visual opsins in an attempt to shed light on their role in the inner retina of vertebrates and in the physiology of the whole organism.

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.

Protective role of melatonin in breast cancer: what we can learn from women with blindness

PURPOSE

This review proposes an overall vision of the protective and therapeutic role of melatonin in breast cancer: from the specific cases of blind women and their reduction of breast cancer incidence to all clinical uses of the sleep hormone in breast cancer.

METHODS

We reviewed studies focused on (1) the correlation between blindness and breast cancer, (2) the correlation between melatonin and breast cancer occurrence in the general population, (3) melatonin therapeutic use in breast cancer, and (4) we discussed the properties of melatonin that could explain an anticancer effect.

RESULTS

(1) Seven studies of breast cancer risk in blind women related significant incidence decreases, up to 57%, among totally blind women. The limited number of studies and the absence of adjustment for confounding factors in most studies limit conclusions. None of these studies established melatonin profiles to determine whether blind women with a decreased breast cancer incidence produced higher levels of melatonin. (2) In the general population, 5 meta-analyses and 12 prospective-cohort studies focused on melatonin levels at recruitment and breast cancer occurrence. All reported the absence of correlation in premenopausal women, whereas in postmenopausal women, most studies showed significantly decreased risk for women with highest melatonin levels. (3) The therapeutic interest of melatonin associated with chemotherapy, radiotherapy, and hormonotherapy is poorly documented in breast cancer to conclude on a positive effect. (4) Melatonin effects on mammary carcinogenesis were only reported in in vitro and animal studies that demonstrated antiestrogenic, antioxidant, oncostatic, and immunomodulatory properties.

CONCLUSION

The preventive role of high endogenous melatonin on breast cancer as well as its beneficial therapeutic use remains to be proven.

Bioluminescent Sensors for Ca++ Flux Imaging and the Introduction of a New Intensity-Based Ca++ Sensor

Sensitive detection of biological events is a goal for the design and characterization of sensors that can be used in vitro and in vivo. One important second messenger is Ca⁺⁺ which has been a focus of using genetically encoded Ca⁺⁺ indicators (GECIs) within living cells or intact organisms in vivo. An ideal GECI would exhibit high signal intensity, excellent signal-to-noise ratio (SNR), rapid kinetics, a large dynamic range within relevant physiological conditions, and red-shifted emission. Most available GECIs are based on fluorescence, but bioluminescent GECIs have potential advantages in terms of avoiding tissue autofluorescence, phototoxicity, photobleaching, and spectral overlap, as well as enhancing SNR. Here, we summarize current progress in the development of bioluminescent GECIs and introduce a new and previously unpublished biosensor. Because these biosensors require a substrate, we also describe the pros and cons of various substrates used with these sensors. The novel GECI that is introduced here is called CalBiT, and it is a Ca⁺⁺ indicator based on the functional complementation of NanoBiT which shows a high dynamic change in response to Ca⁺⁺ fluxes. Here, we use CalBiT for the detection of Ca⁺⁺ fluctuations in cultured cells, including its ability for real-time imaging in living cells.

Spectral tuning and deactivation kinetics of marine mammal melanopsins

In mammals, the photopigment melanopsin (Opn4) is found in a subset of retinal ganglion cells that serve light detection for circadian photoentrainment and pupil constriction (i.e., mydriasis). For a given species, the efficiency of photoentrainment and length of time that mydriasis occurs is determined by the spectral sensitivity and deactivation kinetics of melanopsin, respectively, and to date, neither of these properties have been described in marine mammals. Previous work has indicated that the absorbance maxima (λmax) of marine mammal rhodopsins (Rh1) have diversified to match the available light spectra at foraging depths. However, similar to the melanopsin λmax of terrestrial mammals (~480 nm), the melanopsins of marine mammals may be conserved, with λmax values tuned to the spectrum of solar irradiance at the water's surface. Here, we investigated the Opn4 pigments of 17 marine mammal species inhabiting diverse photic environments including the Infraorder Cetacea, as well as the Orders Sirenia and Carnivora. Both genomic and cDNA sequences were used to deduce amino acid sequences to identify substitutions most likely involved in spectral tuning and deactivation kinetics of the Opn4 pigments. Our results show that there appears to be no amino acid substitutions in marine mammal Opn4 opsins that would result in any significant change in λmax values relative to their terrestrial counterparts. We also found some marine mammal species to lack several phosphorylation sites in the carboxyl terminal domain of their Opn4 pigments that result in significantly slower deactivation kinetics, and thus longer mydriasis, compared to terrestrial controls. This finding was restricted to cetacean species previously found to lack cone photoreceptor opsins, a condition known as rod monochromacy. These results suggest that the rod monochromat whales rely on extended pupillary constriction to prevent photobleaching of the highly photosensitive all-rod retina when moving between photopic and scotopic conditions.

Drosophila sensory receptors-a set of molecular Swiss Army Knives

Genetic approaches in the fruit fly, Drosophila melanogaster, have led to a major triumph in the field of sensory biology-the discovery of multiple large families of sensory receptors and channels. Some of these families, such as transient receptor potential channels, are conserved from animals ranging from worms to humans, while others, such as "gustatory receptors," "olfactory receptors," and "ionotropic receptors," are restricted to invertebrates. Prior to the identification of sensory receptors in flies, it was widely assumed that these proteins function in just one modality such as vision, smell, taste, hearing, and somatosensation, which includes thermosensation, light, and noxious mechanical touch. By employing a vast combination of genetic, behavioral, electrophysiological, and other approaches in flies, a major concept to emerge is that many sensory receptors are multitaskers. The earliest example of this idea was the discovery that individual transient receptor potential channels function in multiple senses. It is now clear that multitasking is exhibited by other large receptor families including gustatory receptors, ionotropic receptors, epithelial Na+ channels (also referred to as Pickpockets), and even opsins, which were formerly thought to function exclusively as light sensors. Genetic characterizations of these Drosophila receptors and the neurons that express them also reveal the mechanisms through which flies can accurately differentiate between different stimuli even when they activate the same receptor, as well as mechanisms of adaptation, amplification, and sensory integration. The insights gleaned from studies in flies have been highly influential in directing investigations in many other animal models.

Effects of achromatic and chromatic lights on pupillary response, endocrinology, activity, and milk production in dairy cows

Artificial light can be used as a management tool to increase milk yield in dairy production. However, little is known about how cows respond to the spectral composition of light. The aim of this study was to investigate how dairy cows respond to artificial achromatic and chromatic lights. A tie-stall barn equipped with light-emitting diode (LED) light fixtures was used to create the controlled experimental light environments. Two experiments were conducted, both using dairy cows of Swedish Red and light mixtures with red, blue or white light. In experiment I, the response to light of increasing intensity on pupil size was evaluated in five pregnant non-lactating cows. In experiment II 16h of achromatic and chromatic daylight in combination with dim, achromatic night light, was tested on pregnant lactating cows during five weeks to observe long term effects on milk production, activity and circadian rhythms. Particular focus was given to possible carry over effects of blue light during the day on activity at night since this has been demonstrated in humans. Increasing intensity of white and blue light affected pupil size (P<0.001), but there was no effect on pupil size with increased intensity of red light. Milk yield was maintained throughout experiment II, and plasma melatonin was higher during dim night light than in daylight for all treatments (P<0.001). In conclusion, our results show that LED fixtures emitting red light driving the ipRGCs indirectly via ML-cones, blue light stimulating both S-cones and ipRGCs directly and a mixture of wavelengths (white light) exert similar effects on milk yield and activity in tied-up dairy cows. This suggests that the spectral composition of LED lighting in a barn is secondary to duration and intensity.

Opsins outside the eye and the skin: a more complex scenario than originally thought for a classical light sensor

Since the discovery of melanopsin as a retinal non-visual photopigment, opsins have been described in several organs and cells. This distribution is strikingly different from the classical localization of photopigments in light-exposed tissues such as the eyes and the skin. More than 10 years ago, a new paradigm in the field was created as opsins were shown, to detect not only light, but also thermal energy in Drosophila. In agreement with these findings, thermal detection by opsins was also reported in mammalian cells. Considering the presence of opsins in tissues not reached by light, an intriguing question has emerged: What is the role of a classical light-sensor, and more recently appreciated thermo-sensor, in these tissues? To tackle this question, we address in this review the most recent studies in the field, with emphasis in mammals. We provide the present view about the role of opsins in peripheral tissues, aiming to integrate the current knowledge of the presence and function of opsins in organs that are not directly affected by light.

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.

Optogenetic Techniques for Manipulating and Sensing G Protein-Coupled Receptor Signaling

G protein-coupled receptors (GPCRs) form the largest class of membrane receptors in the mammalian genome with nearly 800 human genes encoding for unique subtypes. Accordingly, GPCR signaling is implicated in nearly all physiological processes. However, GPCRs have been difficult to study due in part to the complexity of their function which can lead to a plethora of converging or diverging downstream effects over different time and length scales. Classic techniques such as pharmacological control, genetic knockout and biochemical assays often lack the precision required to probe the functions of specific GPCR subtypes. Here we describe the rapidly growing set of optogenetic tools, ranging from methods for optical control of the receptor itself to optical sensing and manipulation of downstream effectors. These tools permit the quantitative measurements of GPCRs and their downstream signaling with high specificity and spatiotemporal precision.

Mammalian circadian systems: Organization and modern life challenges

Humans and other mammalian species possess an endogenous circadian clock system that has evolved in adaptation to periodically reoccurring environmental changes and drives rhythmic biological functions, as well as behavioural outputs with an approximately 24-hour period. In mammals, body clocks are hierarchically organized, encompassing a so-called pacemaker clock in the hypothalamic suprachiasmatic nucleus (SCN), non-SCN brain and peripheral clocks, as well as cell-autonomous oscillators within virtually every cell type. A functional clock machinery on the molecular level, alignment among body clocks, as well as synchronization between endogenous circadian and exogenous environmental cycles has been shown to be crucial for our health and well-being. Yet, modern life constantly poses widespread challenges to our internal clocks, for example artificial lighting, shift work and trans-meridian travel, potentially leading to circadian disruption or misalignment and the emergence of associated diseases. For instance many of us experience a mismatch between sleep timing on work and free days (social jetlag) in our everyday lives without being aware of health consequences that may arise from such chronic circadian misalignment, Hence, this review provides an overview of the organization and molecular built-up of the mammalian circadian system, its interactions with the outside world, as well as pathologies arising from circadian disruption and misalignment.

The Effect of a Screen Protector on Blue Light Intensity Emitted from Different Hand-held Devices

PURPOSE:

In response to growing concern about the effect of blue light on ocular tissue, companies have created mobile device screen protectors to block blue light. This project evaluates one of these screen protectors' ability to reduce blue light intensity.

METHODS:

The intensity of light at 450 nm from an iPhone 8, iPhone X, and iPad was measured in a dark room. The averages of three measurements were taken with and without the screen protector at different distances, settings of brightness, and Apple's night shift (NS) mode. Results were analyzed using paired t-tests.

RESULTS:

At 33 cm, 100% brightness, and 0% NS, the screen protector decreased intensity by 43.9%, 32.3%, and 34.9% for the iPhone 8, iPhone X, and iPad, respectively. At 33 cm and 100% brightness, increasing NS mode from 0% to 100% decreased intensity by 81.2%, 84.2%, and 86.5%. At 33 cm without NS, decreasing the brightness from 100% to 0% decreased intensity by 99.5%, 99.8%, and 97.8%.

CONCLUSIONS:

The screen protector decreased the intensity at 450 nm for every setting other than those at 0% brightness. Decreasing brightness and applying NS mode were more effective in reducing blue light. More research is needed to determine the benefits of decreasing blue light exposure from electronic devices.

The Circadian Effect Versus Mesopic Vision Effect in Road Lighting Applications

Several models on the circadian effect have been applied to indoor circadian lighting design, but applications in road lighting have not yet been clarified. Based on existing models and circadian research, we examined equivalent melanopic lux (EML), circadian light (CLA), and circadian stimulus (CS) representing the circadian effect and the S/P ratio representing the mesopic vision effect, among a dataset of light sources at photopic adaptation illuminance values of 1, 3, 10, 30, and 100 lx. The results show that the S/P ratio correlates with EML and CS (or CLA) much stronger than it correlates with color temperature. The EMLs of light sources are below 50 EML in mesopic vision, and the CSs of most light sources are below or around the threshold value of 0.05. We conclude that the circadian effect is not a significant issue in mesopic vision under most conditions and that optimization for mesopic efficiency is still a good strategy. There are quite a few light sources that may achieve both ideal mesopic efficiency and low CS. This work clarifies the circadian effect and mesopic vision effect performance of light sources in mesopic vision and will help guide choosing suitable light sources and optimization strategies for road lighting.

Coupled network of the circadian clocks: a driving force of rhythmic physiology

The circadian system is composed of coupled endogenous oscillators that allow living beings, including humans, to anticipate and adapt to daily changes in their environment. In mammals, circadian clocks form a hierarchically organized network with a 'master clock' located in the suprachiasmatic nucleus of the hypothalamus, which ensures entrainment of subsidiary oscillators to environmental cycles. Robust rhythmicity of body clocks is indispensable for temporally coordinating organ functions, and the disruption or misalignment of circadian rhythms caused for instance by modern lifestyle is strongly associated with various widespread diseases. This review aims to provide a comprehensive overview of our current knowledge about the molecular architecture and system-level organization of mammalian circadian oscillators. Furthermore, we discuss the regulatory roles of peripheral clocks for cell and organ physiology and their implication in the temporal coordination of metabolism in human health and disease. Finally, we summarize methods for assessing circadian rhythmicity in humans.

Dopaminergic Modulation of Signal Processing in a Subset of Retinal Bipolar Cells

The retina and the olfactory bulb are the gateways to the visual and olfactory systems, respectively, similarly using neural networks to initiate sensory signal processing. Sensory receptors receive signals that are transmitted to neural networks before projecting to primary cortices. These networks filter sensory signals based on their unique features and adjust their sensitivities by gain control systems. Interestingly, dopamine modulates sensory signal transduction in both systems. In the retina, dopamine adjusts the retinal network for daylight conditions (“light adaptation”). In the olfactory system, dopamine mediates lateral inhibition between the glomeruli, resulting in odorant signal decorrelation and discrimination. While dopamine is essential for signal discrimination in the olfactory system, it is not understood whether dopamine has similar roles in visual signal processing in the retina. To elucidate dopaminergic effects on visual processing, we conducted patch-clamp recording from second-order retinal bipolar cells, which exhibit multiple types that can convey different temporal features of light. We recorded excitatory postsynaptic potentials (EPSPs) evoked by various frequencies of sinusoidal light in the absence and presence of a dopamine receptor 1 (D₁R) agonist or antagonist. Application of a D₁R agonist, SKF-38393, shifted the peak temporal responses toward higher frequencies in a subset of bipolar cells. In contrast, a D₁R antagonist, SCH-23390, reversed the effects of SKF on these types of bipolar cells. To examine the mechanism of dopaminergic modulation, we recorded voltage-gated currents, hyperpolarization-activated cyclic nucleotide-gated (HCN) channels, and low-voltage activated (LVA) Ca²⁺ channels. SKF modulated HCN and LVA currents, suggesting that these channels are the target of D₁R signaling to modulate visual signaling in these bipolar cells. Taken together, we found that dopamine modulates the temporal tuning of a subset of retinal bipolar cells. Consequently, we determined that dopamine plays a role in visual signal processing, which is similar to its role in signal decorrelation in the olfactory bulb.

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.

Light adaptation in the chick retina: Dopamine, nitric oxide, and gap-junction coupling modulate spatiotemporal contrast sensitivity

Adaptation to changes in ambient light intensity, in retinal cells and circuits, optimizes visual functions. In the retina, light-adaptation results in changes in light-sensitivity and spatiotemporal tuning of ganglion cells. Under light-adapted conditions, contrast sensitivity (CS) of ganglion cells is a bandpass function of spatial frequency; in contrast, dark-adaptation reduces CS, especially at higher spatial frequencies. In this work, we aimed to understand intrinsic neuromodulatory mechanisms that underlie retinal adaptation to changes in ambient light level. Specifically, we investigated how CS is affected by dopamine (DA), nitric oxide (NO), and modifiers of electrical coupling through gap junctions, under different conditions of adapting illumination. Using the optokinetic response as a behavioral readout of direction-selective ganglion cell activity, we characterized the spatial CS of chicks under high- and low-photopic conditions and how it was regulated by DA, NO, and gap-junction uncouplers. We observed that: (1) DA D2R-family agonists and a donor of NO increased CS tested in low-photopic illumination, as if observed in the high-photopic light; whereas (2) removing their effects using either DA antagonists or NO- synthase inhibitors mimicked low-photopic CS; (3) simulation of high-photopic CS by DA agonists was abolished by NO-synthase inhibitors; and (4) selectively blocking coupling via connexin 35/36-containing gap junctions, using a "designer" mimetic peptide, increased CS, as does strong illumination. We conclude that, in the chicken retina: (1) DA and NO induce changes in spatiotemporal processing, similar to those driven by increasing illumination, (2) DA possibly acts through stimulating NO synthesis, and (3) blockade of coupling via gap junctions containing connexin 35/36 also drives a change in retinal CS functions. As a noninvasive method, the optokinetic response can provide rapid, conditional, and reversible assessment of retinal functions when pharmacological reagents are injected into the vitreous humor. Finally, the chick's large eyes, and the many similarities between their adaptational circuit functions and those in mammals such as the mouse, make them a promising model for future retinal research.

Challenging a Myth and Misconception: Red-Light Vision in Rats

Simple Summary

Light substantially influences animal physiology and behavior. Thus, it is a prerequisite to house laboratory animals under optimal light conditions. Different species possess different sets of photoreceptors, resulting in differential perception of the visible-light spectrum. While humans are trichromats with red-, green- and blue-sensitive cones, rats and mice are dichromats possessing ultraviolet- and green-sensitive cones. This led to the common assumption that red light is invisible to rodents and therefore red lights are commonly used in husbandry and experiments to observe animals during their dark phase. The retinal sensitivity of rats to red light though has never been assessed under scotopic conditions (dark-adapted) even though this mimics the setting red observation lights are being used. We examined the sensitivity to far-red light of the dark- and light-adapted rat retina. Our study demonstrates that the rat retina responds to far-red light under both conditions with great sensitivity, indicating that rats are not red-light blind. This should be taken into consideration when using red light to keep the effects of light on the retina and physiology to a minimum and will improve animal well-being and lead to better quality data by decreasing the variable light.

Abstract

Due to the lack of L-cones in the rodent retina, it is generally assumed that red light is invisible to rodents. Thus, red lights and red filter foils are widely used in rodent husbandry and experimentation allowing researchers to observe animals in an environment that is thought to appear dark to the animals. To better understand red-light vision in rodents, we assessed retinal sensitivity of pigmented and albino rats to far-red light by electroretinogram. We examined the sensitivity to red light not only on the light- but also dark-adapted retina, as red observation lights in husbandry are used during the dark phase of the light cycle. Intriguingly, both rods and cones of pigmented as well as albino rats show a retinal response to red light, with a high sensitivity of the dark-adapted retina and large electroretinogram responses in the mesopic range. Our results challenge the misconception of rodents being red-light blind. Researchers and housing facilities should rethink the use of red observation lights at night.