Pineal opsin: a nonvisual opsin expressed in chick pineal

Evolution of Pineal Nonvisual Opsins in Lizards and the Tuatara and Identification of Lepidopsin: A New Opsin Gene

Many lizards (Squamata), as well as the tuatara (Rhynchocephalia), are distinguished among vertebrate groups for the presence of the parietal eye, or "third eye", a structure derived from the pineal complex containing a simplified retina with photoreceptor cells. The parietal eye expresses nonvisual opsins that differ from the visual opsin repertoire of the lateral eyes. These are pinopsin (OPNP), parapinopsin (OPNPP), and parietopsin (OPNPT), all being evolutionary close to visual opsins. Here, we searched over 60 lepidosaurian genomes for pineal nonvisual opsins to check for the evolutionary trajectory of these genes in reptiles. Unexpectedly, we identified a novel opsin gene, which we termed "lepidopsin" (OPNLEP), that is present solely in the genomes of the tuatara and most lizard groups but absent from other vertebrates. Remnants of the gene are found in the coelacanth and some ray-finned fishes, implying that OPNLEP is an ancient opsin that has been repeatedly lost during vertebrate evolution. We found that the tuatara and most lizards of the Iguania, Anguimorpha, Scincoidea, and Lacertidae clades, which possess a parietal eye, harbor all pineal opsin genes. Lizards missing the parietal eye, like geckos, teiids, and a fossorial amphisbaenian, lack most or all pineal nonvisual opsins. In summary, our survey of pineal nonvisual opsins reveals (i) the persistence of a previously unknown ancient opsin gene-OPNLEP-in lepidosaurians; (ii) losses of nonvisual opsins in specific lizard clades; and (iii) a correlation between the presence of a parietal eye and the genomic repertoire of pineal nonvisual opsins.

Hypotheses in light detection by vertebrate ancient opsin in the bird brain

Extra-retinal photoreception is common across fish and avian species. In birds, the hypothalamus contains non-visual photoreceptors that detect light and regulate multiple endocrine systems. To date, light-dependent control of seasonal reproduction is one of the most well-studied systems that require deep brain photoreception. However, the precise photoreceptor(s) that detect light and the neuroendocrine connection between opsin-expressing cells and the gonadotropin-releasing hormone-1 (GnRH1) system remain poorly defined. In the past couple of decades, two opsin molecules have been proposed to link light detection with seasonal reproduction in birds: neuropsin (Opn5) and vertebrate ancient opsin (VA opsin). Only VA opsin is expressed in GnRH1 cells and has an absorption spectrum that matches the action spectrum of the avian photoperiodic reproductive response. This perspective describes how the annual change in daylength, referred to as photoperiod, regulates the neuroendocrine control of seasonal reproduction. The opsin genes are then outlined, and the cellular phototransduction cascade is described, highlighting the common feature of hyperpolarization in response to light stimulation. We then discuss the latest evidence using short-hairpin RNA to temporarily knock down VA opsin and Opn5 on transcripts involved in the neuroendocrine regulation of reproduction. Based on emerging data, we outline three theoretical scenarios in which VA opsin might regulate GnRH1 synthesis and release in birds. The models proposed provide a series of testable hypotheses that can be used to improve our understanding of avian light detection by VA opsin or other opsin-expressing cells in the brain.

An Extensive Survey of Vertebrate-specific, Nonvisual Opsins Identifies a Novel Subfamily, Q113-Bistable Opsin

A group of nonvisual opsins specific to vertebrates is essential to understand evolution of lateral eyes, one of the most prominent innovations in this lineage. Nevertheless, our knowledge of their evolutionary history remains limited. To develop an integrated view of their evolution, we surveyed these non-visual opsins (VA opsin, pinopsin, parapinopsin, parietopsin, and parapinopsin-like) in 451 vertebrate genomes. Through extensive manual curation, we completed a high-quality catalog. We could not find them in 202 mammals, supporting previous reports of their loss. VA opsins are highly conserved among nonmammals. In contrast, other opsin subfamilies experienced more dynamic molecular evolution with many secondary losses. In addition, we found a previously unreported opsin subfamily that we named Q113-Bistable (QB) opsin. We found its orthologs only in several lizards and the tuatara. Nevertheless, QB opsin pseudogenes were discovered in diverse taxa, including ray-finned fishes, indicating its ancient origin. QB opsin, parapinopsin, and parietopsin are extremely prone to be lost in the course of evolution, and loss events involving these opsins seem to occur concomitantly. Furthermore, we demonstrated the spectral properties of QB opsin as a UV-sensitive, bistable photo-pigment. This study provides the first integrated view of the entire evolutionary history of this group of opsins.

Pulsed alternating wavelength system lighting does not negatively impact production or welfare but reduces dopamine activity and may improve bone growth in grow-out Pekin ducks: Effects of PAWS lighting on meat ducks

The production and welfare of Pekin ducks can be affected by the lighting type they are housed under. There is no standard lighting system in industry and little data evaluating effects of different light systems on duck production and welfare. Pulsed Alternating Wavelength System (PAWS) is a novel LED technology that delivers multiple wavelengths of light in pulsating patterns. This study aimed to determine the effects of PAWS on brain serotonin turnover and skeletal quality in ducks. Ducks housed under PAWS were hypothesized to have lower brain serotonin turnover and equal bone quality compared to those housed under control lights (fluorescent with digital ballasts, 4500K, ∼40 lux). Ducks were placed in floor pens under PAWS or control lighting (1200 ducks/pen, n = 4 pens/treatment) at day of hatch until processing at 30 days of age (DOA). Body weights and feed intake were monitored weekly. Brains, femurs, tibiae, and humeri were collected on days 7, 14, 21 and 29 (n = 6 ducks/age/lighting type). Brain serotonin and metabolites were measured. Bone length, width, breaking strength, and ash were determined. Serotonin data were analyzed using 2-way ANOVA for age and lighting treatment with a post-hoc Fisher's LSD test. Bone data were analyzed with independent t-tests between treatments within each age. Ducks housed under PAWS were heavier by 29 DOA than controls (P < 0.001) with no differences in feed conversion. Brain analyses revealed no differences in serotonin turnover between lighting types. Early interstitial growth of PAWS femur and tibia was increased (P < 0.05), and PAWS femurs had increased bone mineral content at 29 DOA (P = 0.001). At 29 DOA, the PAWS humeri were wider than controls (P = 0.025) and had increased geometrical bone mechanical properties (P < 0.003), but no differences in breaking stress were evident. Results suggest that PAWS may have benefits for production traits and skeletal quality, however, a complete understanding of the welfare effects need further study.

Introduction and reflections on the comparative physiology of sleep and circadian rhythms

Circadian rhythms and the sleep/wake cycle allows us, and most life on Earth, to function optimally in a dynamic world, adjusting all aspects of biology to the varied and complex demands imposed by the 24-hour rotation of the Earth upon its axis. A key element in understanding these rhythms, and the success of the field in general, has been because researchers have adopted a comparative approach. Across all taxa, fundamental questions relating to the generation and regulation of sleep and circadian rhythms have been address using biochemical, molecular, cellular, system and computer modelling techniques. Furthermore, findings have been placed into an ecological and evolutionary context. By addressing both the "How" - mechanistic, and "Why" - evolutionary questions in parallel, the field has achieved remarkable successes, including how circadian rhythms are generated and regulated by light. Yet many key questions remain. In this special issue on the Comparative Physiology of Sleep and Circadian Rhythms, celebrating the 100th anniversary of the Journal of Comparative Physiology, important new discoveries are detailed. These findings illustrate the power of comparative physiology to address novel questions and demonstrate that sleep and circadian physiology are embedded within the biological framework of an organism.

Ocular and extraocular roles of neuropsin in vertebrates

The ability to detect and adapt to different levels of ambient light is critical for animal survival. Light detection is the basis of vision, but light also regulates eye development and drives several non-image-forming functions, including synchronizing circadian rhythms to the daily light/dark cycle, restricting pupils in response to changes in light intensity, and modulating mood in response to light. Until the early 2000s, these functions were thought to be solely mediated by ocular photoreceptors. However, neuropsin (OPN5), a UV-sensitive opsin, has been receiving growing attention, as new methods have revealed previously unappreciated functions of OPN5. In fact, OPN5-mediated extraocular and deep-brain photoreception have recently been described for the first time in mammals. This review aims to synthesize current knowledge of the properties and functions of OPN5 across vertebrates.

Centennial Review: History and husbandry recommendations for raising Pekin ducks in research or commercial production

By some accounts, ducks were domesticated between 400 and 10,000 yr ago and have been a growing portion of the poultry industry for decades. Ducks specifically, and waterfowl in general, have unique health, housing, nutrition and welfare concerns compared to their galliform counterparts. Although there have been many research publications in regards to health, nutrition, behavior, and welfare of ducks there have been very few reviews to provide an overview of these numerous studies, and only one text has attempted to review all aspects of the duck industry, from breeders to meat ducks. This review covers incubation, hatching, housing, welfare, nutrition, and euthanasia and highlights the needs for additional research at all levels of duck production. The purpose of this review is to provide guidelines to raise and house ducks for research as specifically related to industry practices.

Molecular economy of nature with two thyrotropins from different parts of the pituitary: pars tuberalis thyroid-stimulating hormone and pars distalis thyroid-stimulating hormone

Thyrotropin (TSH) is classically known to be regulated by negative feedback from thyroid hormones and stimulated by thyrotropin-releasing hormone (TRH) from the hypothalamus. At the end of the 1990s, studies showed that thyrotroph cells from the pars tuberalis (PT) did not have TRH receptors and their TSH regulation was independent from TRH stimulation. Instead, PT-thyrotroph cells were shown to have melatonin-1 (MT-1) receptors and melatonin secretion from the pineal gland stimulates TSH-β subunit formation in PT. Electron microscopy examinations also revealed some important differences between PT and pars distalis (PD) thyrotrophs. PT-TSH also have low bioactivity in the peripheral circulation. Studies showed that they have different glycosylations and PT-TSH forms macro-TSH complexes in the periphery and has a longer half-life. Photoperiodism affects LH levels in animals via decreased melatonin causing increased TSH-β subunit expression and induction of deiodinase-2 (DIO-2) in the brain. Mammals need a light stimulus carried into the suprachiasmatic nucleus (which is a circadian clock) and then transferred to the pineal gland to synthesize melatonin, but birds have deep brain receptors and they are stimulated directly by light stimuli to have increased PT-TSH, without the need for melatonin. Photoperiodic regulations via TSH and DIO 2/3 also have a role in appetite, seasonal immune regulation, food intake and nest-making behaviour in animals. Since humans have no clear seasonal breeding period, such studies as recent ''domestication locus'' studies in poultry are interesting. PT-TSH that works like a neurotransmitter in the brain may become an important target for future studies about humans.

The non-visual opsins expressed in deep brain neurons projecting to the retina in lampreys

In lower vertebrates, brain photoreceptor cells express vertebrate-specific non-visual opsins. We previously revealed that a pineal-related organ-specific opsin, parapinopsin, is UV-sensitive and allows pineal wavelength discrimination in lampreys and teleost. The Australian pouched lamprey was recently reported as having two parapinopsin-related genes. We demonstrate that a parapinopsin-like opsin from the Japanese river lamprey exhibits different molecular properties and distribution than parapinopsin. This opsin activates Gi-type G protein in a mammalian cell culture assay in a light-dependent manner. Heterologous action spectroscopy revealed that the opsin forms a violet to blue-sensitive pigment. Interestingly, the opsin is co-localised with green-sensitive P-opsin in the cells of the M5 nucleus of Schober (M5NS) in the mesencephalon of the river and brook lamprey. Some opsins-containing cells of the river lamprey have cilia and others an axon projecting to the retina. The opsins of the brook lamprey are co-localised in the cilia of centrifugal neurons projecting to the retina, suggesting that cells expressing the parapinopsin-like opsin and P-opsin are sensitive to violet to green light. Moreover, we found neural connections between M5NS cells expressing the opsins and the retina. These findings suggest that the retinal activity might be modulated by brain photoreception.

Brain-specific homeobox Bsx specifies identity of pineal gland between serially homologous photoreceptive organs in zebrafish

The pineal gland functioning as a photoreceptive organ in non-mammalian species is a serial homolog of the retina. Here we found that Brain-specific homeobox (Bsx) is a key regulator conferring individuality on the pineal gland between the two serially homologous photoreceptive organs in zebrafish. Bsx knock-down impaired the pineal development with reduced expression of exorh, the pineal-specific gene responsible for the photoreception, whereas it induced ectopic expression of rho, a retina-specific gene, in the pineal gland. Bsx remarkably transactivated the exorh promoter in combination with Otx5, but not with Crx, through its binding to distinct subtypes of PIRE, a DNA cis-element driving Crx/Otx-dependent pineal-specific gene expression. These results demonstrate that the identity of pineal photoreceptive neurons is determined by the combinatorial code of Bsx and Otx5, the former confers the pineal specificity at the tissue level and the latter determines the photoreceptor specificity at the cellular level.

The evolution of asymmetric photosensitive structures in metazoans and the Nodal connection

Asymmetries are observed in a great number of taxa in metazoans. More particularly, functional lateralization and neuroanatomical asymmetries within the central nervous system have been a matter of intense research for at least two hundred years. While asymmetries of some paired structures/organs (e.g. eyes, ears, kidneys, legs, arms) constitute random deviations from a pure bilateral symmetry, brain asymmetries such as those observed in the cortex and epithalamus are directional. This means that molecular and anatomical features located on one side of a given structure are observed in most individuals. For instance, in humans, the neuronal tract connecting the language areas is enlarged in the left hemisphere. When asymmetries are fixed, their molecular mechanisms can be studied using mutants displaying different phenotypes: left or right isomerism of the structure, reversed asymmetry or random asymmetry. Our understanding of asymmetry in the nervous system has been widely enriched thanks to the characterization of mutants affecting epithalamus asymmetry. Furthermore, two decades ago, pioneering studies revealed that a specific morphogen, Nodal, active only on one side of the embryo during development is an important molecule in asymmetry patterning. In this review, I have gathered important data bringing insight into the origin and evolution of epithalamus asymmetry and the role of Nodal in metazoans. After a short introduction on brain asymmetries (chapter I), I secondly focus on the molecular and anatomical characteristics of the epithalamus in vertebrates and explore some functional aspects such as its photosensitive ability related to the pineal complex (chapter II). Third, I discuss homology relationship of the parapineal organ among vertebrates (chapter III). Fourth, I discuss the possible origin of the epithalamus, presenting cells displaying photosensitive properties and/or asymmetry in the anterior part of the body in non-vertebrates (chapter IV). Finally, I report Nodal signaling expression data and functional experiments performed in different metazoan groups (chapter V).

Molecular and Neuroendocrine Mechanisms of Avian Seasonal Reproduction

Animals living outside tropical zones experience seasonal changes in the environment and accordingly, adapt their physiology and behavior in reproduction, molting, and migration. Subtropical birds are excellent models for the study of seasonal reproduction because of their rapid and dramatic response to changes in photoperiod. For example, testicular weight typically changes by more than a 100-fold. In birds, the eyes are not necessary for seasonal reproduction, and light is instead perceived by deep brain photoreceptors. Functional genomic analysis has revealed that long day (LD)-induced thyrotropin from the pars tuberalis of the pituitary gland causes local thyroid hormone (TH) activation within the mediobasal hypothalamus. This local bioactive TH, triiodothyronine (T₃), appears to regulate seasonal gonadotropin-releasing hormone (GnRH) secretion through morphological changes in neuro-glial interactions. GnRH, in turn, stimulates gonadotropin secretion and hence, gonadal development under LD conditions. In marked contrast, low temperatures accelerate short day (SD)-induced testicular regression in winter. Interestingly, low temperatures increase circulating levels of T₃ to support adaptive thermogenesis, but this induction of T₃ also triggers the apoptosis of germ cells by activating genes involved in metamorphosis. This apparent contradiction in the role of TH has recently been clarified. Central activation of TH during spring results in testicular growth, while peripheral activation of TH during winter regulates adaptive thermogenesis and testicular regression.

The 2004 Aschoff/Pittendrigh Lecture: Theory of the Origin of the Pineal Gland— A Tale of Conflict and Resolution

A theory is presented that explains the evolution of the pinealocyte from the common ancestral photoreceptor of both the pinealocyte and retinal photoreceptor. Central to the hypothesis is the previously unrecognized conflict between the two chemistries that define these cells—melatonin synthesis and retinoid recycling. At the core of the conflict is the formation of adducts composed of two molecules of retinaldehyde and one molecule of serotonin, analogous to formation in the retina of the toxic bis-retinyl ethanolamine (A2E). The hypothesis argues that early in chordate evolution, at a point before the genes required for melatonin synthesis were acquired, retinaldehyde—which is essential for photon capture—was depleted by reacting with naturally occurring arylalkylamines (tyramine, serotonin, tryptamine, phenylethylamine) and xenobiotic arylalkylamines. This generated toxic bis-retinyl arylalkylamines (A2AAs). The acquisition of arylalkylamine N-acetyltransferase (AANAT) prevented this by N-acetylating the arylalkylamines. HydroxyindoleOmethyltransferase enhanced detoxification in the primitive photoreceptor by increasing the lipid solubility of serotonin and bis-retinyl serotonin. After the serotonin. melatonin pathway was established, the next step leading toward the pinealocyte was the evolution of a daily rhythm in melatonin and the capacity to recognize it as a signal of darkness. The shift in melatonin from metabolic garbage to information developed a pressure to improve the reliability of the melatonin signal, which in turn led to higher levels of serotonin in the photodetector. This generated the conflict between serotonin and retinaldehyde, which was resolved by the cellular segregation of the two chemistries. The result, in primates, is a pineal gland that does not detect light and a retinal photodetector that does not make melatonin. High levels of AANAT in the latter tissue might serve the same function AANAT had when first acquired— prevention of A2AA formation.

Light-dependent activation of G proteins by two isoforms of chicken melanopsins

In the chicken pineal gland, light stimuli trigger signaling pathways mediated by two different subtypes, Gt and G11. These G proteins may be activated by any of the three major pineal opsins, pinopsin, OPN4-1 and OPN4-2, but biochemical evidence for the coupling has been missing except for functional coupling between pinopsin and Gt. Here we investigated the relative expression levels and the functional difference among the three pineal opsins. In the chicken pineal gland, the pinopsin mRNA level was significantly more abundant than the others, of which the OPN4-2 mRNA level was higher than that of OPN4-1. In G protein activation assays, Gt was strongly activated by pinopsin in a light-dependent manner, being consistent with previous studies, and weakly activated by OPN4-2. Unexpectedly, illuminated OPN4-2 more efficiently activated G protein(s) that was endogenously expressed in HEK293T cells in culture. On the other hand, Gq, the closest analogue of G11, was activated only by OPN4-1 although the activity was relatively weak under these conditions. These results suggest that OPN4-1 and OPN4-2 couple with Gq and Gt, respectively. Two melanopsins, OPN4-1 and OPN4-2, appear to have acquired mutually different functions through the evolution.

Adaptation to nocturnality - learning from avian genomes

The recent availability of multiple avian genomes has laid the foundation for a huge variety of comparative genomics analyses including scans for changes and signatures of selection that arose from adaptions to new ecological niches. Nocturnal adaptation in birds, unlike in mammals, is comparatively recent, a fact that makes birds good candidates for identifying early genetic changes that support adaptation to dim-light environments. In this review, we give examples of comparative genomics analyses that could shed light on mechanisms of adaptation to nocturnality. We present advantages and disadvantages of both "data-driven" and "hypothesis-driven" approaches that lead to the discovery of candidate genes and genetic changes promoting nocturnality. We anticipate that the accessibility of multiple genomes from the Genome 10K Project will allow a better understanding of evolutionary mechanisms and adaptation in general.

Clock-Controlled Regulation of the Acute Effects of Norepinephrine on Chick Pineal Melatonin Rhythms

The chicken pineal gland synthesizes and releases melatonin rhythmically in light/dark (LD) cycles, with high melatonin levels during the dark phase, and in constant darkness (DD) for several cycles before it gradually damps to arrhythmicity in DD. Daily administration of norepinephrine (NE) in vivo and in vitro prevents the damping and restores the melatonin rhythm. To investigate the role of the circadian clock on melatonin rhythm damping and of its restoration by NE, the effects of NE administration at different phases of the melatonin cycle revealed a robust rhythm in NE sensitivity in which NE efficacy in increasing melatonin amplitude peaked in late subjective night and early subjective day, suggesting a clock underlying NE sensitivity. However, NE itself had no effect on circadian phase or period of the melatonin rhythms. Transcriptional analyses indicated that even though the rhythm of melatonin output damped to arrhythmicity, messenger RNA (mRNA) encoding clock genes gper2, gper3, gBmal1, gclock, gcry1, and gcry2; enzymes associated with melatonin biosynthesis; and enzymes involved in cyclic nucleotide signaling remained robustly rhythmic. Of these, only gADCY1 (adenylate cyclase 1) and gPDE4D (cAMP-specific 3',5'-cyclic phosphodiesterase 4D) were affected by NE administration at the mRNA levels, and only ADCY1 was affected at the protein level. The data strongly suggest that damping of the melatonin rhythm in the chick pineal gland occurs at the posttranscriptional level and that a major role of the clock is to regulate pinealocytes' sensitivity to neuronal input from the brain.

Diversification of non-visual photopigment parapinopsin in spectral sensitivity for diverse pineal functions

Background

Recent genome projects of various animals have uncovered an unexpectedly large number of opsin genes, which encode protein moieties of photoreceptor molecules, in most animals. In visual systems, the biological meanings of this diversification are clear; multiple types of visual opsins with different spectral sensitivities are responsible for color vision. However, the significance of the diversification of non-visual opsins remains uncertain, in spite of the importance of understanding the molecular mechanism and evolution of varied non-visual photoreceptions.

Results

Here, we investigated the diversification of the pineal photopigment parapinopsin, which serves as the UV-sensitive photopigment for the pineal wavelength discrimination in the lamprey, linking it with other pineal photoreception. Spectroscopic analyses of the recombinant pigments of the two teleost parapinopsins PP1 and PP2 revealed that PP1 is a UV-sensitive pigment, similar to lamprey parapinopsin, but PP2 is a blue-sensitive pigment, with an absorption maximum at 460–480 nm, showing the diversification of non-visual pigment with respect to spectral sensitivity. We also found that PP1 and PP2 exhibit mutually exclusive expressions in the pineal organs of three teleost species. By using transgenic zebrafish in which these parapinopsin-expressing cells are labeled, we found that PP1-expressing cells basically possess neuronal processes, which is consistent with their involvement in wavelength discrimination. Interestingly, however, PP2-expressing cells rarely possess neuronal processes, raising the possibility that PP2 could be involved in non-neural responses rather than neural responses. Furthermore, we found that PP2-expressing cells contain serotonin and aanat2, the key enzyme involved in melatonin synthesis from serotonin, whereas PP1-expressing cells do not contain either, suggesting that blue-sensitive PP2 is instead involved in light-regulation of melatonin secretion.

Conclusions

In this paper, we have clearly shown the different molecular properties of duplicated non-visual opsins by demonstrating the diversification of parapinopsin with respect to spectral sensitivity. Moreover, we have shown a plausible link between the diversification and its physiological impact by discovering a strong candidate for the underlying pigment in light-regulated melatonin secretion in zebrafish; the diversification could generate a new contribution of parapinopsin to pineal photoreception. Current findings could also provide an opportunity to understand the “color” preference of non-visual photoreception.

Electronic supplementary material

The online version of this article (doi:10.1186/s12915-015-0174-9) contains supplementary material, which is available to authorized users.

Avian photoreceptors and their role in the regulation of daily and seasonal physiology

Birds time their activities in synchronization with daily and seasonal periodicities in the environment, which is mainly provided by changes in day length (=photoperiod). Photoperiod appears to act at different levels than simply entraining the hypothalamic clock via eyes in birds. Photoreceptor cells that transmit light information to an avian brain are localized in three independent structures, the retina of eyes, pineal gland and hypothalamus, particularly in the paraventricular organ and lateral septal area. These hypothalamic photoreceptors are commonly referred to as encephalic or deep brain photoreceptors, DBPs. Eyes and pineal are known to contribute to the circadian regulation of behavior and physiology via rhythmic melatonin secretion in several birds. DBPs have been implicated in the regulation of seasonal physiology, particularly in photoperiod induced gonadal growth and development. Here, we briefly review limited evidence that is available on the roles of these photoreceptors in the regulation of circadian and seasonal physiology, with particular emphasis placed on the DBPs.

Transcranial light affects plasma monoamine levels and expression of brain encephalopsin in the mouse

Encephalopsin (OPN3) belongs to the light-sensitive transmembrane receptor family mainly expressed in the brain and retina. It is believed that light affects mammalian circadian rhythmicity only through the retinohypothalamic tract, which transmits light information to the suprachiasmatic nucleus in the hypothalamus. However, it has been shown that light penetrates the skull. Here, we present the effect of transcranial light treatment on OPN3 expression and monoamine concentrations in mouse brain and other tissues. Mice were randomly assigned to control group, morning-light group and evening-light group, and animals were illuminated transcranially five times a week for 8 min for a total of 4 weeks. The concentrations of OPN3 and monoamines were analysed using western blotting and HPLC, respectively. We report that transcranial light treatment affects OPN3 expression in different brain areas and plasma/adrenal gland monoamine concentrations. In addition, when light was administered at a different time of the day, the response varied in different tissues. These results provide new information on the effects of light on transmitters mediating mammalian rhythmicity.

Regulation of seasonal reproduction by hypothalamic activation of thyroid hormone

Organisms living outside the tropics measure the changes in the length of the day to adapt to seasonal changes in the environment. Animals that breed during spring and summer are called long-day breeders, while those that breed during fall are called short-day breeders. Although the influence of thyroid hormone in the regulation of seasonal reproduction has been known for several decades, its precise mechanism remained unknown. Recent studies revealed that the activation of thyroid hormone within the mediobasal hypothalamus plays a key role in this phenomenon. This localized activation of the thyroid hormone is controlled by thyrotropin (thyroid-stimulating hormone) secreted from the pars tuberalis of the pituitary gland. Although seasonal reproduction is a rate-limiting factor in animal production, genes involved in photoperiodic signal transduction pathway could emerge as potential targets to facilitate domestication.

Human cellular retinaldehyde-binding protein has secondary thermal 9-cis-retinal isomerase activity

Cellular retinaldehyde-binding protein (CRALBP) chaperones 11-cis-retinal to convert opsin receptor molecules into photosensitive retinoid pigments of the eye. We report a thermal secondary isomerase activity of CRALBP when bound to 9-cis-retinal. UV/vis and (1)H NMR spectroscopy were used to characterize the product as 9,13-dicis-retinal. The X-ray structure of the CRALBP mutant R234W:9-cis-retinal complex at 1.9 Å resolution revealed a niche in the binding pocket for 9-cis-aldehyde different from that reported for 11-cis-retinal. Combined computational, kinetic, and structural data lead us to propose an isomerization mechanism catalyzed by a network of buried waters. Our findings highlight a specific role of water molecules in both CRALBP-assisted specificity toward 9-cis-retinal and its thermal isomerase activity yielding 9,13-dicis-retinal. Kinetic data from two point mutants of CRALBP support an essential role of Glu202 as the initial proton donor in this isomerization reaction.

Avian circadian organization: a chorus of clocks

In birds, biological clock function pervades all aspects of biology, controlling daily changes in sleep: wake, visual function, song, migratory patterns and orientation, as well as seasonal patterns of reproduction, song and migration. The molecular bases for circadian clocks are highly conserved, and it is likely the avian molecular mechanisms are similar to those expressed in mammals, including humans. The central pacemakers in the avian pineal gland, retinae and SCN dynamically interact to maintain stable phase relationships and then influence downstream rhythms through entrainment of peripheral oscillators in the brain controlling behavior and peripheral tissues. Birds represent an excellent model for the role played by biological clocks in human neurobiology; unlike most rodent models, they are diurnal, they exhibit cognitively complex social interactions, and their circadian clocks are more sensitive to the hormone melatonin than are those of nocturnal rodents.

Diversity of animal opsin-based pigments and their optogenetic potential

Most animal opsin-based pigments are typical G protein-coupled receptors (GPCR) and consist of a protein moiety, opsin, and 11-cis retinal as a chromophore. More than several thousand opsins have been identified from a wide variety of animals, which have multiple opsin genes. Accumulated evidence reveals the molecular property of opsin-based pigments, particularly non-conventional visual pigments including non-visual pigments. Opsin-based pigments are generally a bistable pigment having two stable and photointerconvertible states and therefore are bleach-resistant and reusable, unlike vertebrate visual pigments which become bleached. The opsin family contains Gt-coupled, Gq-coupled, Go-coupled, Gs-coupled, Gi-coupled, and Gi/Go-coupled opsins, indicating the existence of a large diversity of light-driven GPCR-signaling cascades. It is suggested that these molecular properties might contribute to different physiologies. In addition, various opsin based-pigments, especially nonconventional visual pigments having different molecular characteristics would facilitate the design and development of promising optogenetic tools for modulating GPCR-signaling, which is involved in a wide variety of physiological responses. We here introduce molecular and functional properties of various kinds of opsins and discuss their physiological function and also their potentials for optogenetic applications. This article is part of a Special Issue entitled: Retinal proteins - you can teach an old dog new tricks.

Thyroid hormone and seasonal regulation of reproduction

Organisms living outside the tropics use changes in photoperiod to adapt to seasonal changes in the environment. Several models have contributed to an understanding of this mechanism at the molecular and endocrine levels. Subtropical birds are excellent models for the study of these mechanisms because of their rapid and dramatic response to changes in photoperiod. Studies of birds have demonstrated that light is perceived by a deep brain photoreceptor and long day-induced thyrotropin (TSH) from the pars tuberalis (PT) of the pituitary gland causes local thyroid hormone activation within the mediobasal hypothalamus (MBH). The locally generated bioactive thyroid hormone, T₃, regulates seasonal gonadotropin-releasing hormone (GnRH) secretion, and hence gonadotropin secretion. In mammals, the eyes are the only photoreceptor involved in photoperiodic time perception and nocturnal melatonin secretion provides an endocrine signal of photoperiod to the PT to regulate TSH. Here, I review the current understanding of the hypothalamic mechanisms controlling seasonal reproduction in mammals and birds.

Identification of nonvisual photomotor response cells in the vertebrate hindbrain

Nonvisual photosensation enables animals to sense light without sight. However, the cellular and molecular mechanisms of nonvisual photobehaviors are poorly understood, especially in vertebrate animals. Here, we describe the photomotor response (PMR), a robust and reproducible series of motor behaviors in zebrafish that is elicited by visual wavelengths of light but does not require the eyes, pineal gland, or other canonical deep-brain photoreceptive organs. Unlike the relatively slow effects of canonical nonvisual pathways, motor circuits are strongly and quickly (seconds) recruited during the PMR behavior. We find that the hindbrain is both necessary and sufficient to drive these behaviors. Using in vivo calcium imaging, we identify a discrete set of neurons within the hindbrain whose responses to light mirror the PMR behavior. Pharmacological inhibition of the visual cycle blocks PMR behaviors, suggesting that opsin-based photoreceptors control this behavior. These data represent the first known light-sensing circuit in the vertebrate hindbrain.

The evolutionary relationship between microbial rhodopsins and metazoan rhodopsins

Rhodopsins are photoreceptive proteins with seven-transmembrane alpha-helices and a covalently bound retinal. Based on their protein sequences, rhodopsins can be classified into microbial rhodopsins and metazoan rhodopsins. Because there is no clearly detectable sequence identity between these two groups, their evolutionary relationship was difficult to decide. Through ancestral state inference, we found that microbial rhodopsins and metazoan rhodopsins are divergently related in their seven-transmembrane domains. Our result proposes that they are homologous proteins and metazoan rhodopsins originated from microbial rhodopsins. Structure alignment shows that microbial rhodopsins and metazoan rhodopsins share a remarkable structural homology while the position of retinal-binding lysine is different between them. It suggests that the function of photoreception was once lost during the evolution of rhodopsin genes. This result explains why there is no clearly detectable sequence similarity between the two rhodopsin groups: after losing the photoreception function, rhodopsin gene was freed from the functional constraint and the process of divergence could quickly change its original sequence beyond recognition.

Opsin gene duplication and divergence in ray-finned fish

Opsin gene sequences were first reported in the 1980s. The goal of that research was to test the hypothesis that human opsins were members of a single gene family and that variation in human color vision was mediated by mutations in these genes. While the new data supported both hypotheses, the greatest contribution of this work was, arguably, that it provided the data necessary for PCR-based surveys in a diversity of other species. Such studies, and recent whole genome sequencing projects, have uncovered exceptionally large opsin gene repertoires in ray-finned fishes (taxon, Actinopterygii). Guppies and zebrafish, for example, have 10 visual opsin genes each. Here we review the duplication and divergence events that have generated these gene collections. Phylogenetic analyses revealed that large opsin gene repertories in fish have been generated by gene duplication and divergence events that span the age of the ray-finned fishes. Data from whole genome sequencing projects and from large-insert clones show that tandem duplication is the primary mode of opsin gene family expansion in fishes. In some instances gene conversion between tandem duplicates has obscured evolutionary relationships among genes and generated unique key-site haplotypes. We mapped amino acid substitutions at so-called key-sites onto phylogenies and this exposed many examples of convergence. We found that dN/dS values were higher on the branches of our trees that followed gene duplication than on branches that followed speciation events, suggesting that duplication relaxes constraints on opsin sequence evolution. Though the focus of the review is opsin sequence evolution, we also note that there are few clear connections between opsin gene repertoires and variation in spectral environment, morphological traits, or life history traits.

Unexpected diversity and photoperiod dependence of the zebrafish melanopsin system

Animals have evolved specialized photoreceptors in the retina and in extraocular tissues that allow them to measure light changes in their environment. In mammals, the retina is the only structure that detects light and relays this information to the brain. The classical photoreceptors, rods and cones, are responsible for vision through activation of rhodopsin and cone opsins. Melanopsin, another photopigment first discovered in Xenopus melanophores (Opn4x), is expressed in a small subset of retinal ganglion cells (RGCs) in the mammalian retina, where it mediates non-image forming functions such as circadian photoentrainment and sleep. While mammals have a single melanopsin gene (opn4), zebrafish show remarkable diversity with two opn4x-related and three opn4-related genes expressed in distinct patterns in multiple neuronal cell types of the developing retina, including bipolar interneurons. The intronless opn4.1 gene is transcribed in photoreceptors as well as in horizontal cells and produces functional photopigment. Four genes are also expressed in the zebrafish embryonic brain, but not in the photoreceptive pineal gland. We discovered that photoperiod length influences expression of two of the opn4-related genes in retinal layers involved in signaling light information to RGCs. Moreover, both genes are expressed in a robust diurnal rhythm but with different phases in relation to the light-dark cycle. The results suggest that melanopsin has an expanded role in modulating the retinal circuitry of fish.

Membrane receptors and transporters involved in the function and transport of vitamin A and its derivatives

The eye is the human organ most sensitive to vitamin A deficiency because of vision's absolute and heavy dependence on vitamin A for light perception. Studies of the molecular basis of vision have provided important insights into the intricate mechanistic details of the function, transport and recycling of vitamin A and its derivatives (retinoid). This review focuses on retinoid-related membrane receptors and transporters. Three kinds of mammalian membrane receptors and transporters are discussed: opsins, best known as vitamin A-based light sensors in vision; ABCA4, an ATP-dependent transporter specializes in the transport of vitamin A derivative; and STRA6, a recently identified membrane receptor that mediates cellular uptake of vitamin A. The evolutionary driving forces for their existence and the wide spectrum of human diseases associated with these proteins are discussed. Lessons learned from the study of the visual system might be useful for understanding retinoid biology and retinoid-related diseases in other organ systems as well. This article is part of a Special Issue entitled Retinoid and Lipid Metabolism.

Vertebrate ancient opsin and melanopsin: divergent irradiance detectors

Both vertebrates and invertebrates respond to light by utilising a wide-ranging array of photosensory systems, with diverse photoreceptor organs expressing a characteristic photopigment, itself consisting of an opsin apoprotein linked to a light-sensitive retinoid chromophore based on vitamin A. In the eye, the pigments expressed in both cone and rod photoreceptors have been studied in great depth and mediate contrast perception, measurement of the spectral composition of environmental light, and thus classical image forming vision. By contrast, the molecular basis for non-visual and extraocular photoreception is far less understood; however, two photopigment genes have become the focus of much study, the vertebrate ancient (va) opsin and melanopsin (opn4). In this review, we discuss the history of discovery for each gene, as well as focusing on the evolution, expression profile, functional role and broader physiological significance of each photopigment. Recently, it has been suggested independently by Arendt et al. and Lamb that an ancestral opsin bifurcated in early metazoans and evolved into two quite different photopigments, one expressed in rhabdomeric photoreceptors and the other in ciliary photoreceptors. This interpretation of the evolution of the metazoan eye has provided a powerful framework for understanding photobiological organization. Their proposal, however, does not encompass all current experimental observations that would be consistent with what we term a central "Evolution of Photosensory Opsins with Common Heredity (EPOCH)" hypothesis to explain the complexity of animal photosensory systems. Clearly, many opsin genes (e.g. va opsin) simply do not fit neatly within this scheme. Thus, the review concludes with a discussion of these anomalies and their context regarding the phylogeny of photoreceptor and photopigment development.

The evolution of irradiance detection: melanopsin and the non-visual opsins

Circadian rhythms are endogenous 24 h cycles that persist in the absence of external time cues. These rhythms provide an internal representation of day length and optimize physiology and behaviour to the varying demands of the solar cycle. These clocks require daily adjustment to local time and the primary time cue (zeitgeber) used by most vertebrates is the daily change in the amount of environmental light (irradiance) at dawn and dusk, a process termed photoentrainment. Attempts to understand the photoreceptor mechanisms mediating non-image-forming responses to light, such as photoentrainment, have resulted in the discovery of a remarkable array of different photoreceptors and photopigment families, all of which appear to use a basic opsin/vitamin A-based photopigment biochemistry. In non-mammalian vertebrates, specialized photoreceptors are located within the pineal complex, deep brain and dermal melanophores. There is also strong evidence in fish and amphibians for the direct photic regulation of circadian clocks in multiple tissues. By contrast, mammals possess only ocular photoreceptors. However, in addition to the image-forming rods and cones of the retina, there exists a third photoreceptor system based on a subset of melanopsin-expressing photosensitive retinal ganglion cells (pRGCs). In this review, we discuss the range of vertebrate photoreceptors and their opsin photopigments, describe the melanopsin/pRGC system in some detail and then finally consider the molecular evolution and sensory ecology of these non-image-forming photoreceptor systems.

Evolution of opsins and phototransduction

Opsins are the universal photoreceptor molecules of all visual systems in the animal kingdom. They can change their conformation from a resting state to a signalling state upon light absorption, which activates the G protein, thereby resulting in a signalling cascade that produces physiological responses. This process of capturing a photon and transforming it into a physiological response is known as phototransduction. Recent cloning techniques have revealed the rich and diverse nature of these molecules, found in organisms ranging from jellyfish to humans, functioning in visual and non-visual phototransduction systems and photoisomerases. Here we describe the diversity of these proteins and their role in phototransduction. Then we explore the molecular properties of opsins, by analysing site-directed mutants, strategically designed by phylogenetic comparison. This site-directed mutant approach led us to identify many key features in the evolution of the photoreceptor molecules. In particular, we will discuss the evolution of the counterion, the reduction of agonist binding to the receptor, and the molecular properties that characterize rod opsins apart from cone opsins. We will show how the advances in molecular biology and biophysics have given us insights into how evolution works at the molecular level.

Time's arrow flies like a bird: two paradoxes for avian circadian biology

Biological timekeeping in birds is a fundamental feature of avian physiology, behavior and ecology. The physiological basis for avian circadian rhythmicity has pointed to a multi-oscillator system of mutually coupled pacemakers in the pineal gland, eyes and hypothalamic suprachiasmatic nuclei (SCN). In passerines, the role of the pineal gland and its hormone melatonin is particularly important. More recent molecular biological studies have pointed to a highly conserved mechanism involving rhythmic transcription and translation of "clock genes". However, studies attempting to reconcile the physiological role of pineal melatonin with molecular studies have largely failed. Recent work in our laboratory has suggested that melatonin-sensitive physiological processes are only loosely coupled to transcriptional oscillations. Similarly, although the pineal gland has been shown to be critical for overt circadian behaviors, its role in annual cycles of reproductive function appears to be minimal. Recent work on the seasonal control of birdsong, however, suggests that, although the pineal gland does not directly affect gonadal cycles, it is important for seasonal changes in song. Experimental analyses that address these paradoxes will shed light on the roles the biological clock play in birds and in vertebrates in general.

Evolution of The Vertebrate Pineal Gland: The Aanat Hypothesis

The defining feature of the pineal gland is the capacity to function as a melatonin factory that operates on a approximately 24 h schedule, reflecting the unique synthetic capacities of the pinealocyte. Melatonin synthesis is typically elevated at night and serves to provide the organism with a signal of nighttime. Melatonin levels can be viewed as hands of the clock. Issues relating to the evolutionary events leading up to the immergence of this system have not received significant attention. When did melatonin synthesis appear in the evolutionary line leading to vertebrates? When did a distinct pineal gland first appear? What were the forces driving this evolutionary trend? As more knowledge has grown about the pinealocyte and the relationship it has to retinal photoreceptors, it has become possible to generate a plausible hypothesis to explain how the pineal gland and the melatonin rhythm evolved. At the heart of the hypothesis is the melatonin rhythm enzyme arylalkylamine N-acetyltransferase (AANAT). The advances supporting the hypothesis will be reviewed here and expanded beyond the original foundation; the hypothesis and its implications will be addressed.

Probing pineal-specific gene expression with transgenic zebrafish

The pineal gland of zebrafish (Danio rerio) contains light-sensitive photoreceptor cells and plays an important role in the neuroendocrine system. The zebrafish exorhodopsin gene encodes a pineal-specific photoreceptive protein, whose promoter region harbors a cis-acting element, pineal expression-promoting element (PIPE), directing pineal-specific gene expression. For in vivo genetic studies on PIPE-binding proteins and their regulatory mechanisms, we generated a transgenic zebrafish line, Tg(P(20)-rh/P:gfp), that expresses green fluorescent protein (GFP) under the control of the zebrafish rhodopsin promoter fused with 20 PIPE repeats. In Tg(P(20)-rh/P:gfp) fish, PIPE-dependent gene expression is visualized by GFP fluorescence in the pineal gland along with PIPE-independent GFP signals in the retinal rod photoreceptors. The transgenic fish exhibit detectable and reproducible GFP fluorescence in the larval pineal gland by 5 days postfertilization. Antisense morpholino-mediated knock-down of a pineal transcription factor gene, otx5, suppresses pineal GFP expression in the transgenic line. In a pilot screen of N-ethyl-N-nitrosourea-treated fish of the GFP transgenic line, we isolated potential dominant mutations that cause attenuation of pineal GFP fluorescence with a marginal effect on the retinal GFP signal. The results suggest that the Tg(P(20)-rh/P:gfp) line will be useful for detecting deficits in PIPE-dependent gene expression in the pineal gland.

Differential expression of duplicated VAL-opsin genes in the developing zebrafish

Non-visual opsins mediate various light-dependent physiological events. Our previous search for non-visual opsin genes in zebrafish led to the discovery of VAL-opsin (VAL-opsinA) in deep brain cells and retinal horizontal cells of the adult fish. In this study, we report the identification and characterization of its duplicated gene, VAL-opsinB, in zebrafish. A molecular phylogenetic analysis indicates that VAL-opsinB is orthologous to a previously reported salmon gene and that the duplication of the VAL-opsin gene occurred in the teleost lineage. The recombinant protein of zebrafish VAL-opsinB forms a green-sensitive photopigment when reconstituted with 11-cis-retinal. VAL-opsinB expression was detected in a limited number of cells of the brain and the eye, and the expression pattern is distinct from that of the VAL-opsinA gene. Such a differential expression pattern suggests that VAL-opsinA and VAL-opsinB are involved in different physiological events in zebrafish.

Two isoforms of chicken melanopsins show blue light sensitivity

Melanopsin is a vertebrate non-visual opsin and functions as a circadian photoreceptor in mammalian retinas. Here we found the expression of two kinds of melanopsin genes in the chicken pineal gland and identified the presence of five isoforms derived from these two genes. Reconstitution of the recombinant proteins with 11-cis-retinal revealed that at least two of these melanopsin protein isoforms can function as blue-sensitive photopigments with absorption maxima at 476-484nm. These values are consistent with maximal sensitivities of action spectra determined from the physiological and behavioral studies on mammalian melanopsins. The melanopsin isoforms found in this study may function as pineal circadian photoreceptors.

NeuroD1: developmental expression and regulated genes in the rodent pineal gland

NeuroD1/BETA2, a member of the bHLH transcription factor family, is known to influence the fate of specific neuronal, endocrine and retinal cells. We report here that NeuroD1 mRNA is highly abundant in the developing and adult rat pineal gland. Pineal expression begins in the 17-day embryo at which time it is also detectable in other brain regions. Expression in the pineal gland increases during the embryonic period and is maintained thereafter at levels equivalent to those found in the cerebellum and retina. In contrast, NeuroD1 mRNA decreases markedly in non-cerebellar brain regions during development. Pineal NeuroD1 levels are similar during the day and night, and do not appear to be influenced by sympathetic neural input. Gene expression analysis of the pineal glands from neonatal NeuroD1 knockout mice identifies 127 transcripts that are down-regulated (>twofold, p < 0.05) and 16 that are up-regulated (>twofold, p < 0.05). According to quantitative RT-PCR, the most dramatically down-regulated gene is kinesin family member 5C ( approximately 100-fold) and the most dramatically up-regulated gene is glutamic acid decarboxylase 1 ( approximately fourfold). Other impacted transcripts encode proteins involved in differentiation, development, signal transduction and trafficking. These findings represent the first step toward elucidating the role of NeuroD1 in the rodent pinealocyte.

A median third eye: pineal gland retraces evolution of vertebrate photoreceptive organs

In many vertebrates, the pineal gland serves as a photoreceptive neuroendocrine organ. Morphological and functional similarities between the pineal and retinal photoreceptor cells indicate their close evolutionary relationship, and hence the comparative studies on the pineal gland and the retina are the keys to deciphering the evolutionary traces of the vertebrate photoreceptive organs. Several studies have suggested common genetic and molecular mechanisms responsible for their similarities, but largely unknown are those underlying pineal-specific development and physiological functions. Recent studies have identified several cis-acting DNA elements that participate in transcriptional control of the pineal-specific genes. Genetic approaches in the zebrafish have also contributed to elucidating the genetic network regulating the pineal development and neurogenesis. These efforts toward elucidating the molecular instrumentation intrinsic to the pineal gland, back to back with those to the retina, should lead to a comprehensive understanding of the evolutionary history of the vertebrate photoreceptive structures. This article summarizes the current status of research on these topics.

ProtoBee: hierarchical classification and annotation of the honey bee proteome

The recently sequenced genome of the honey bee (Apis mellifera) has produced 10,157 predicted protein sequences, calling for a computational effort to extract biological insights from them. We have applied an unsupervised hierarchical protein-clustering method, which was previously used in the ProtoNet system, to nearly 200,000 proteins consisting of the predicted honey bee proteins, the SWISS-PROT protein database, and the complete set of proteins of the mouse (Mus musculus) and the fruit fly (Drosophila melanogaster). The hierarchy produced by this method has been entitled ProtoBee. In ProtoBee, the proteins are hierarchically organized into 18,936 separate tree hierarchies, each representing a protein functional family. By using the mouse and Drosophila complete proteomes as reference, we are able to highlight functional groups of putative gene-loss events, putative novel proteins of unique functionality, and bee-specific paralogs. We have studied some of the ProtoBee findings and suggest their biological relevance. Examples include novel opsin genes and intriguing nuclear matches of mitochondrial genes. The organization of bee sequences into functional clusters suggests a natural way of automatically inferring functional annotation. Following this notion, we were able to assign functional annotation to about 70% of the sequences. ProtoBee is available at http://www.protobee.cs.huji.ac.il.

Parietal-eye phototransduction components and their potential evolutionary implications

The parietal-eye photoreceptor is unique because it has two antagonistic light signaling pathways in the same cell-a hyperpolarizing pathway maximally sensitive to blue light and a depolarizing pathway maximally sensitive to green light. Here, we report the molecular components of these two pathways. We found two opsins in the same cell: the blue-sensitive pinopsin and a previously unidentified green-sensitive opsin, which we name parietopsin. Signaling components included gustducin-alpha and Galphao, but not rod or cone transducin-alpha. Single-cell recordings demonstrated that Go mediates the depolarizing response. Gustducin-alpha resembles transducin-alpha functionally and likely mediates the hyperpolarizing response. The parietopsin-Go signaling pair provides clues about how rod and cone phototransduction might have evolved.

Melanopsin and other novel mammalian opsins

Within the past decade, several non-canonical opsins have been identified in mammals. These include RGR, peropsin, melanopsin, encephalopsin, and neuropsin. Although all are expressed in the eye, it is likely that they serve to mediate non-visual effects of light on physiology. Some of these opsins, however, may play an indirect role in vision by generating appropriate retinoid chromophores for the rod and cone visual pigments or by regulating the sensitivity of the visual system. Here, we survey the current state of knowledge regarding these opsins.

The opsins

Opsins, G-protein-coupled receptors including rhodopsin, are found in animals, and more than a thousand have been identified so far. Most opsins act as pigments that activate G proteins in a light-dependent manner in both visual and non-visual systems.

The photosensitive molecule rhodopsin and its relatives consist of a protein moiety - an opsin - and a non-protein moiety - the chromophore retinal. Opsins, which are G-protein-coupled receptors (GPCRs), are found in animals, and more than a thousand have been identified so far. Detailed molecular phylogenetic analyses show that the opsin family is divided into seven subfamilies, which correspond well to functional classifications within the family: the vertebrate visual (transducin-coupled) and non-visual opsin subfamily, the encephalopsin/tmt-opsin subfamily, the G_q-coupled opsin/melanopsin subfamily, the G_o-coupled opsin subfamily, the neuropsin subfamily, the peropsin subfamily and the retinal photoisomerase subfamily. The subfamilies diversified before the deuterostomes (including vertebrates) split from the protostomes (most invertebrates), suggesting that a common animal ancestor had multiple opsin genes. Opsins have a seven-transmembrane structure similar to that of other GPCRs, but are distinguished by a lysine residue that is a retinal-binding site in the seventh helix. Accumulated evidence suggests that most opsins act as pigments that activate G proteins in a light-dependent manner in both visual and non-visual systems, whereas a few serve as retinal photoisomerases, generating the chromophore used by other opsins, and some opsins have unknown functions.