Identification of circadian brain photoreceptors mediating photic entrainment of behavioural rhythms in lizards

An Expanding Role for Nonvisual Opsins in Extraocular Light Sensing Physiology

We live on a planet that is bathed in daily and seasonal sunlight cycles. In this context, terrestrial life forms have evolved mechanisms that directly harness light energy (plants) or decode light information for adaptive advantage. In animals, the main light sensors are a family of G protein-coupled receptors called opsins. Opsin function is best described for the visual sense. However, most animals also use opsins for extraocular light sensing for seasonal behavior and camouflage. While it has long been believed that mammals do not have an extraocular light sensing capacity, recent evidence suggests otherwise. Notably, encephalopsin (OPN3) and neuropsin (OPN5) are both known to mediate extraocular light sensing in mice. Examples of this mediation include photoentrainment of circadian clocks in skin (by OPN5) and acute light-dependent regulation of metabolic pathways (by OPN3 and OPN5). This review summarizes current findings in the expanding field of extraocular photoreception and their relevance for human physiology.

A Comparative Perspective on Extra-retinal Photoreception

Ubiquitous in non-mammalian vertebrates, extra-retinal photoreceptors (ERPs) have been linked to an array of physiological, metabolic, behavioral, and morphological changes. However, the mechanisms and functional roles of ERPs remain one of the enduring questions of modern biology. In this review article, we use a comparative framework to identify conserved roles and distributions of ERPs, highlighting knowledge gaps. We conclude that ERP research can be divided into two largely unconnected categories: (i) identification and localization of photoreceptors and (ii) linkage of non-retinal light reception to behavioral and physiological processes, particularly endocrine systems. However, the emergence of novel gene editing and silencing techniques is enabling the unification of ERP research by allowing the bridging of this divide.

Neuronal Organization of Deep Brain Opsin Photoreceptors in Adult Teleosts

Biological impacts of light beyond vision, i.e., non-visual functions of light, signify the need to better understand light detection (or photoreception) systems in vertebrates. Photopigments, which comprise light-absorbing chromophores bound to a variety of G-protein coupled receptor opsins, are responsible for visual and non-visual photoreception. Non-visual opsin photopigments in the retina of mammals and extra-retinal tissues of non-mammals play an important role in non-image-forming functions of light, e.g., biological rhythms and seasonal reproduction. This review highlights the role of opsin photoreceptors in the deep brain, which could involve conserved neurochemical systems that control different time- and light-dependent physiologies in in non-mammalian vertebrates including teleost fish.

The hypothalamic photoreceptors regulating seasonal reproduction in birds: a prime role for VA opsin

Extraretinal photoreceptors located within the medio-basal hypothalamus regulate the photoperiodic control of seasonal reproduction in birds. An action spectrum for this response describes an opsin photopigment with a λmax of ∼ 492 nm. Beyond this however, the specific identity of the photopigment remains unresolved. Several candidates have emerged including rod-opsin; melanopsin (OPN4); neuropsin (OPN5); and vertebrate ancient (VA) opsin. These contenders are evaluated against key criteria used routinely in photobiology to link orphan photopigments to specific biological responses. To date, only VA opsin can easily satisfy all criteria and we propose that this photopigment represents the prime candidate for encoding daylength and driving seasonal breeding in birds. We also show that VA opsin is co-expressed with both gonadotropin-releasing hormone (GnRH) and arginine-vasotocin (AVT) neurons. These new data suggest that GnRH and AVT neurosecretory pathways are endogenously photosensitive and that our current understanding of how these systems are regulated will require substantial revision.

Encephalic photoreception and phototactic response in the troglobiont Somalian blind cavefish Phreatichthys andruzzii

Many physiological and behavioural responses to changes in environmental lighting conditions are mediated by extraocular photoreceptors. Here we investigate encephalic photoreception in Phreatichthys andruzzii, a typical cave-dwelling fish showing an extreme phenotype with complete anophthalmy and a reduction in size of associated brain structures. We firstly identified two P. andruzzii photopigments, orthologues of rod opsin and exo-rod opsin. In vitro, both opsins serve as light-absorbing photopigments with λ(max) around 500 nm when reconstituted with an A(1) chromophore. When corrected for the summed absorption from the skin and skull, the spectral sensitivity profiles shifted to longer wavelengths (rod opsin: 521 nm; exo-rod opsin: 520 nm). We next explored the involvement of both opsins in the negative phototaxis reported for this species. A comparison of the spectral sensitivity of the photophobic response with the putative A(2) absorbance spectra corrected for skin/skull absorbance indicates that the A(2) versions of either or both of these pigments could explain the observed behavioural spectral sensitivity.

An enzymatic mechanism for generating the precursor of endogenous 13-cis retinoic acid in the brain

13-cis Retinoic acid (13cRA), a stereoisomeric form of retinoic acid, is naturally generated in the body and is also used clinically to treat acute promyelocytic leukemia, some skin diseases and cancer. Furthermore, it has been suggested that 13cRA modulates brain neurochemical systems because increased 13cRA levels are correlated with depression and increased suicidal tendencies. However, the mechanism for the generation of endogenous 13cRA is not well understood. The present study identified and characterized a novel enzyme in zebrafish brain, 13-cis isomerohydrolase (13cIMH) (EC 5.2.1.7), which exclusively generated 13-cis retinol and can be oxidized to 13cRA. 13cIMH shares 74% amino acid sequence identity with human retinal pigment epithelium specific 65 kDa protein (RPE65), an 11-cis isomerohydrolase in the visual cycle, and retains the key residues essential for the isomerohydrolase activity of RPE65. Similar to RPE65, 13cIMH is a membrane-associated protein, requires all-trans retinyl ester as its intrinsic substrate, and its enzymatic activity is dependent on iron. The purified 13cIMH converted all-trans retinyl ester exclusively to 13-cis retinol with K(m)  = 2.6 μm and k(cat) = 4.4 × 10(-4) ·s(-1) . RT-PCR, western blot analysis and immunohistochemistry detected 13cIMH expression in the brain. These results suggest that 13cIMH may play a key role in the generation of 13cRA, as well as in the modulation of neuronal functions in the brain.

A sky polarization compass in lizards: the central role of the parietal eye

The present study first examined whether ruin lizards Podarcis sicula are able to orientate using the e-vector direction of polarized light. Ruin lizards were trained and tested indoors, inside a hexagonal Morris water maze, positioned under an artificial light source producing plane polarized light with a single e-vector, which provided an axial cue. Lizards were subjected to axial training by positioning two identical goals in contact with the centre of two opposite side walls of the Morris water maze. Goals were invisible because they were placed just beneath the water surface, and water was rendered opaque. The results showed that the directional choices of lizards meeting learning criteria were bimodally distributed along the training axis, and that after 90 deg rotation of the e-vector direction of polarized light the lizards directional choices rotated correspondingly, producing a bimodal distribution which was perpendicular to the training axis. The present results confirm in ruin lizards results previously obtained in other lizard species showing that these reptiles can use the e-vector direction of polarized light in the form of a sky polarization compass. The second step of the study aimed at answering the still open question of whether functioning of a sky polarization compass would be mediated by the lizard parietal eye. To test this, ruin lizards meeting learning criteria were tested inside the Morris water maze under polarized light after their parietal eyes were painted black. Lizards with black-painted parietal eyes were completely disoriented. Thus, the present data show for the first time that the parietal eye plays a central role in mediating the functioning of a putative sky polarization compass of lizards.

Current knowledge on the melatonin system in teleost fish

Melatonin is a much conserved feature in vertebrates that plays a central role in the entrainment of daily and annual physiological rhythms. Investigations aiming at understanding how melatonin mediates the effects of photoperiod on crucial functions and behaviors have been very active in the last decades, particularly in mammals. In fish a clear-cut picture is still missing. Here we review the available data on (i) the sites of melatonin production in fish, (ii) the mechanisms that control its daily and annual rhythms of production and (iii) the characterization of its different receptor subtypes, their location and regulation. The in vivo and in vitro data on melatonin effects on crucial neuroendocrine regulations, including reproduction, growth, feeding and behavioral responses, are also reviewed. Finally we discuss how manipulation of the photic cues impact on fish circannual clock and annual cycle of reproduction, and how this can be used for aquaculture purposes.

Orientation of lizards in a Morris water-maze: roles of the sun compass and the parietal eye

The present study examined for the first time whether a Morris water-maze can be used to explore compass and other orientation mechanisms in the ruin lizard Podarcis sicula. In the open field, during sunny days, lizards were individually trained to swim from the center of the water maze onto a hidden platform (the goal), positioned at the periphery of the maze in a single compass direction. The goal was invisible because it was placed just beneath the water surface and the water was rendered opaque. The results showed that lizards learn to swim directly towards the hidden goal under the sun in the absence of visual feature cues. We further examined whether the observed orientation response would be due to lizards learning the spatial position of the goal relative to the sun's azimuth, i.e. to the use of a time-compensated sun compass. Lizards reaching learning criteria were subjected to 6 h clock-shift (fast or slow), and tested for goal orientation in the Morris water-maze. Results demonstrated that the learned orientation response is mediated by a time-compensated sun compass. Further investigations provided direct evidence that in ruin lizards an intact parietal eye is required to perform goal orientation under the sun inside a Morris water-maze,and that other brain photoreceptors, like the pineal or deep brain photoreceptors, are not involved in orientation.

Circadian photoreception in vertebrates

To be adaptively useful, internal circadian clocks must be entrained (synchronized) to daily rhythms in the external world. The entraining process adjusts the period of the internal clock to 24 hours and its phase to a value that determines the organism's temporal niche (e.g., diurnal and nocturnal). For most vertebrates, the dominant environmental synchronizer is light. All vertebrates employ specialized photoreceptor cells to perceive synchronizing light signals, but mammals and nonmammalian vertebrates do this differently. Mammals concentrate circadian photoreceptors in the retina, employing rods, cones, and a subset of retinal ganglion cells that are directly photosensitive and contain an unusual photopigment (melanopsin). Nonmammalian vertebrates use photoreceptors located deep in the brain and in the pineal gland as well as others in the retina. Such photoreceptor extravagance is difficult to explain. It seems likely that the different photoreceptor classes in this elaborate sensory system may have specialized roles in entrainment. There is some evidence that this is in fact the case. Furthermore, this nonvisual "circadian" photoreceptive system also controls acute behavioral responses to light (masking), pupillary constriction, and photoperiodic regulation of reproductive state. We review some of the early work on birds and describe new findings that indicate specific roles for retinal rods, cones, and photosensitive retinal ganglion cells in mammals.

Retinal, pineal and diencephalic expression of frog arylalkylamine N-acetyltransferase-1

The arylalkylamine N-acetyltransferase (AANAT) is a key enzyme in the rhythmic production of melatonin. Two Aanats are expressed in Teleost fish (Aanat1 in the retina and Aanat2 in the pineal organ) but only Aanat1 is found in tetrapods. This study reports the cloning of Aanat1 from R. perezi. Transcripts were mainly expressed in the retina, diencephalon, intestine and testis. In the retina and pineal organ, Aanat1 expression was in the photoreceptor cells. Expression was also seen in ependymal cells of the 3rd ventricle and discrete cells of the suprachiasmatic area. The expression of Aanat1 in both the retina and pineal organ, and the absence of Aanat2 suggests that green frog resembles more to birds and mammals than to Teleost fish, as far as Aanat is concerned. The significance of Aanat1 in extra-pineal and extra-retinal tissues remains to be elucidated; in the diencephalon, it might be associated to the so-called deep brain photoreceptor cells.

Isolation and characterization of melanopsin and pinopsin expression within photoreceptive sites of reptiles

Non-mammalian vertebrates have multiple extraocular photoreceptors, mainly localised in the pineal complex and the brain, to mediate irradiance detection. In this study, we report the full-length cDNA cloning of ruin lizard melanopsin and pinopsin. The high level of identity with opsins in both the transmembrane regions, where the chromophore binding site is located, and the intracellular loops, where the G-proteins interact, suggests that both melanopsin and pinopsin should be able to generate a stable photopigment, capable of triggering a transduction cascade mediated by G-proteins. Phylogenetic analysis showed that both opsins are located on the expected branches of the corresponding sequences of ortholog proteins. Subsequently, using RT-PCR and RPA analysis, we verified the expression of ruin lizard melanopsin and pinopsin in directly photosensitive organs, such as the lateral eye, brain, pineal gland and parietal eye. Melanopsin expression was detected in the lateral eye and all major regions of the brain. However, different from the situation in Xenopus and chicken, melanopsin is not expressed in the ruin lizard pineal. Pinopsin mRNA expression was only detected in the pineal complex. As a result of their phylogenetic position and ecology, reptiles provide the circadian field with some of the most interesting models for understanding the evolution of the vertebrate circadian timing system and its response to light. This characterization of melanopsin and pinopsin expression in the ruin lizard will be important for future studies aimed at understanding the molecular basis of circadian light detection in reptiles.

Neural Interaction of Gonadotropin-regulating Hormone Immunoreactive Neurons and the Suprachiasmatic Nucleus with the Paraventricular Organ in the Japanese Grass Lizard (Takydromus tachydromoides)

Our previous study demonstrated that the paraventricular organ (PVO) in the hypothalamus of the Japanese grass lizard (Takydromus tachydromoides) showed immunoreactivity against the light signal-transducing G-protein, transducin. This finding suggested that the PVO was a candidate for the deep-brain photoreceptor in this species. To understand functions of the PVO, we investigated distributions of transducin, serotonin, gonadotropin-releasing hormone (GnRH), and gonadotropin-inhibitory hormone (GnIH) in the lizard's brain. We immunohistochemically confirmed co-localization of transducin and serotonin in PVO neurons that showed structural characteristics of cerebrospinal fluid (CSF)-contacting neurons. GnRH-immunoreactive (ir) cells were localized in the posterior commissure and lateral hypothalamic area. Some of the serotonin-ir fibers extending from the PVO to the lateral hypothalamic area contacted the GnRH-ir cell bodies. GnIH-ir cells were localized in the nucleus accumbens, paraventricular nucleus, and upper medulla, and GnIH-ir fibers from the paraventricular nucleus contacted the lateral processes of serotonin-ir neurons in the PVO. In addition, we found that serotonin-ir fibers from the PVO extended to the suprachiasmatic nucleus (SCN), and the retrograde transport method confirmed the PVO projections to the SCN. These findings suggest that the PVO, by means of innervation mediated by serotonin, plays an important role in the regulation of pituitary function and the biological clock in the Japanese grass lizard.

Extraocular photoreception and circadian entrainment in nonmammalian vertebrates

In mammals both the regulation of circadian rhythms and photoperiodic responses depend exclusively upon photic information provided by the lateral eyes; however, nonmammalian vertebrates can also rely on multiple extraocular photoreceptors to perform the same tasks. Extraocular photoreceptors include deep brain photoreceptors located in several distinct brain sites and the pineal complex, involving intracranial (pineal and parapineal) and extracranial (frontal organ and parietal eye) components. This review updates the research field of the most recent acquisitions concerning the roles of extraocular photoreceptors on circadian physiology and behavior, particularly photic entrainment and sun compass orientation.

Nonvisual Photoreception in the Chick Iris

The embryonic chicken iris constricts to light ex vivo, but with characteristics atypical of visual phototransduction. The chick iris was most sensitive to short-wavelength light, demonstrating an action spectrum consistent with cryptochrome rather than with opsin pigments. Pupillary responses did not attenuate after saturating light exposure, but showed paradoxical potentiation. Iris photosensitivity was not affected by retinoid depletion or inhibitors of visual phototransduction. Knockdown of cryptochrome expression, but not of melanopsin expression, decreased iris photosensitivity. These data characterize a non-opsin photoreception mechanism in a vertebrate eye and suggest a conserved photoreceptive role for cryptochromes in vertebrates.