Vertebrate ancient (VA) opsin and extraretinal photoreception in the Atlantic salmon (Salmo salar)

Light conditions during Atlantic salmon embryogenesis affect key neuropeptides in the melanocortin system during transition from endogenous to exogenous feeding

During the first feeding period, fish will adapt to exogenous feeding as their endogenous source of nutrients is depleted. This requires the development of a functional physiological system to control active search for food, appetite, and food intake. The Atlantic salmon (Salmo salar) melanocortin system, a key player in appetite control, includes neuronal circuits expressing neuropeptide y (npya), agouti-related peptide (agrp1), cocaine- and amphetamine-regulated transcript (cart), and proopiomelanocortin (pomca). Little is known about the ontogeny and function of the melanocortin system during early developmental stages. Atlantic salmon [0-730 day degrees (dd)] were reared under three different light conditions (DD, continuous darkness; LD, 14:10 Light: Dark; LL, continuous light) before the light was switched to LD and the fish fed twice a day. We examined the effects of different light conditions (DD _LD , LD _LD , and LL _LD ) on salmon growth, yolk utilization, and periprandial responses of the neuropeptides npya1, npya2, agrp1, cart2a, cart2b, cart4, pomca1, and pomca2. Fish were collected 1 week (alevins, 830 dd, still containing yolk sac) and 3 weeks (fry, 991 dd, yolk sac fully consumed) into the first feeding period and sampled before (-1 h) and after (0.5, 1.5, 3, and 6 h) the first meal of the day. Atlantic salmon reared under DD _LD , LD _LD , and LL _LD had similar standard lengths and myotome heights at the onset of first feeding. However, salmon kept under a constant light condition during endogenous feeding (DD _LD and LL _LD ) had less yolk at first feeding. At 830 dd none of the neuropeptides analyzed displayed a periprandial response. But 2 weeks later, and with no yolk remaining, significant periprandial changes were observed for npya1, pomca1, and pomca2, but only in the LD _LD fish. This suggests that these key neuropeptides serve an important role in controlling feeding once Atlantic salmon need to rely entirely on active search and ingestion of exogenous food. Moreover, light conditions during early development did not affect the size of salmon at first feeding but did affect the mRNA levels of npya1, pomca1, and pomca2 in the brain indicating that mimicking natural light conditions (LD _LD ) better stimulates appetite control.

Deep Diversity: Extensive Variation in the Components of Complex Visual Systems across Animals

Understanding the molecular underpinnings of the evolution of complex (multi-part) systems is a fundamental topic in biology. One unanswered question is to what the extent do similar or different genes and regulatory interactions underlie similar complex systems across species? Animal eyes and phototransduction (light detection) are outstanding systems to investigate this question because some of the genetics underlying these traits are well characterized in model organisms. However, comparative studies using non-model organisms are also necessary to understand the diversity and evolution of these traits. Here, we compare the characteristics of photoreceptor cells, opsins, and phototransduction cascades in diverse taxa, with a particular focus on cnidarians. In contrast to the common theme of deep homology, whereby similar traits develop mainly using homologous genes, comparisons of visual systems, especially in non-model organisms, are beginning to highlight a "deep diversity" of underlying components, illustrating how variation can underlie similar complex systems across taxa. Although using candidate genes from model organisms across diversity was a good starting point to understand the evolution of complex systems, unbiased genome-wide comparisons and subsequent functional validation will be necessary to uncover unique genes that comprise the complex systems of non-model groups to better understand biodiversity and its evolution.

Harbours as unique environmental sites of multiple anthropogenic stressors on fish hormonal systems

Fish development and acclimation to environmental conditions are strongly mediated by the hormonal endocrine system. In environments contaminated by anthropogenic stressors, hormonal pathway alterations can be detrimental for growth, survival, fitness, and at a larger scale for population maintenance. In the context of increasingly contaminated marine environments worldwide, numerous laboratory studies have confirmed the effect of one or a combination of pollutants on fish hormonal systems. However, this has not been confirmed in situ. In this review, we explore the body of knowledge related to the influence of anthropogenic stressors disrupting fish endocrine systems, recent advances (focusing on thyroid hormones and stress hormones such as cortisol), and potential research perspectives. Through this review, we highlight how harbours can be used as "in situ laboratories" given the variety of anthropogenic stressors (such as plastic, chemical, sound, light pollution, and invasive species) that can be simultaneously investigated in harbours over long periods of time.

Characterization of tmt-opsin2 in Medaka Fish Provides Insight Into the Interplay of Light and Temperature for Behavioral Regulation

One of the big challenges in the study of animal behavior is to combine molecular-level questions of functional genetics with meaningful combinations of environmental stimuli. Light and temperature are important external cues, influencing the behaviors of organisms. Thus, understanding the combined effect of light and temperature changes on wild-type vs. genetically modified animals is a first step to understand the role of individual genes in the ability of animals to cope with changing environments. Many behavioral traits can be extrapolated from behavioral tests performed from automated motion tracking combined with machine learning. Acquired datasets, typically complex and large, can be challenging for subsequent quantitative analyses. In this study, we investigate medaka behavior of tmt-opsin2 mutants vs. corresponding wild-types under different light and temperature conditions using automated tracking combined with a convolutional neuronal network and a Hidden Markov model-based approach. The temperatures in this study can occur in summer vs. late spring/early autumn in the natural habitat of medaka fish. Under summer-like temperature, tmt-opsin2 mutants did not exhibit changes in overall locomotion, consistent with previous observations. However, detailed analyses of fish position revealed that the tmt-opsin2 mutants spent more time in central locations of the dish, possibly because of decreased anxiety. Furthermore, a clear difference in location and overall movement was obvious between the mutant and wild-types under colder conditions. These data indicate a role of tmt-opsin2 in behavioral adjustment, at least in part possibly depending on the season.

Neural activation in photosensitive brain regions of Atlantic salmon (Salmo salar) after light stimulation

Photoreceptive inputs to the teleost brain are perceived as image of the visual world and as photo-modulation of neuroendocrine and neuronal signals. The retina and pineal organ are major receptive organs with projections to various parts of the brain, but in the past decades deep brain photoreceptors have emerged as candidates for photoreceptive inputs, either independent or in combination with projections from light sensory organs. This study aimed to test the effects of narrow bandwidth light using light-emitting diodes technology on brain neural activity through putative opsin stimulation in Atlantic salmon. The expression of c-fos, a known marker of neural activity, was compared in situ between dark-adapted salmon parr and following light stimulation with different wavelengths. c-fos expression increased with duration of light stimulation and the strongest signal was obtained in fish exposed to light for 120 minutes. Distinct and specific brain regions were activated following dark to light stimulation, such as the habenula, suprachiasmatic nucleus, thalamus, and hypothalamus. The c-fos expression was overlapping with photoreceptors expressing melanopsin and/or vertebrate ancient opsin, suggesting a potential direct activation by light. Interestingly in the habenula, a distinct ring of vertebrate ancient opsin and melanopsin expressing cells is overlapping with c-fos expression after neural activation. Salmon exposed to different spectra had neural activation in similar brain regions. The most apparent difference was melanopsin expression in the lateral cells of the lateral tuberal nuclus in the hypothalamus, which appeared to be specifically activated by red light. Light-stimulated neuronal activity in the deep brain was limited to subpopulations of neurons, mainly in regions with neuronal modulation activity, retinal and pineal innervations and known presence of nonvisual photoreceptors. The overlapping expression patterns of c-fos and nonvisual opsins support direct light stimulation of deep brain photoreceptors and the importance of these systems in light induced brain activity.

Behavioural red-light sensitivity in fish according to the optomotor response

Various procedures have been adopted to investigate spectral sensitivity of animals, e.g. absorption spectra of visual pigments, electroretinography, optokinetic response, optomotor response (OMR) and phototaxis. The use of these techniques has led to various conclusions about animal vision. However, visual sensitivity should be evaluated consistently for a reliable comparison. In this study, we retrieved behavioural data of several fish species using a single OMR procedure and compared their sensitivities to near-infrared light. Besides cavefish that lack eyes, some species were not appropriate for the OMR test because they either stayed still or changed swimming direction frequently. Eight of 13 fish species tested were OMR positive. Detailed analyses using medaka, goldfish, zebrafish, guppy, stickleback and cichlid revealed that all the fish were sensitive to light at a wavelength greater than or equal to 750 nm, where the threshold wavelengths varied from 750 to 880 nm. Fish opsin repertoire affected the perception of red light. By contrast, the copy number of long-wavelength-sensitive (LWS) genes did not necessarily improve red-light sensitivity. While the duplication of LWS and other cone opsin genes that has occurred extensively during fish evolution might not aid increasing spectral sensitivity, it may provide some other advantageous ophthalmic function, such as enhanced spectral discrimination.

What Goes Wrong during Early Development of Artificially Reproduced European Eel Anguilla anguilla? Clues from the Larval Transcriptome and Gene Expression Patterns

Simple Summary

Closing the life cycle of the European eel in captivity is urgently needed to gain perspective for the commercial production of juvenile glass eels. Larvae are produced weekly at our facilities, but large variations in larval mortality are observed during the first week after hatching. Although much effort has been devoted to investigating ways to prevent early larval mortality, it remains unclear what the causes are. The aim of this study was to perform a transcriptomic study on European eel larvae in order to identify genes and physiological pathways that are differentially regulated in the comparison of larvae from batches that did not survive for longer than three days vs. larvae from batches that survived for at least a week up to 22 days after hatching (non-viable vs. viable larvae). In contrast to earlier published studies on European eel, we conclude that larvae exhibit immune competency. Non-viable larvae initiated an inflammatory and host protection immune response and tried to maintain osmoregulatory homeostasis. As a perspective, microbial control and salinity reduction might benefit eel larvae in terms of lower mortality and improved development by lowering the costs of immune functioning and osmoregulation.

Abstract

In eels, large variations in larval mortality exist, which would impede the viable production of juvenile glass eels in captivity. The transcriptome of European eel larvae was investigated to identify physiological pathways and genes that show differential regulation between non-viable vs. viable larvae. Expression of genes involved in inflammation and host protection was higher, suggesting that non-viable larvae suffered from microbial infection. Expression of genes involved in osmoregulation was also higher, implying that non-viable larvae tried to maintain homeostasis by strong osmoregulatory adaptation. Expression of genes involved in myogenesis, neural, and sensory development was reduced in the non-viable larvae. Expression of the major histocompatibility complex class-I (mhc1) gene, M-protein (myom2), the dopamine 2B receptor (d2br), the melatonin receptor (mtr1), and heat-shock protein beta-1 (hspb1) showed strong differential regulation and was therefore studied in 1, 8, and 15 days post-hatch (dph) larvae by RT-PCR to comprehend the roles of these genes during ontogeny. Expression patterning of these genes indicated the start of active swimming (8 dph) and feed searching behavior (15 dph) and confirmed immunocompetence immediately after hatching. This study revealed useful insights for improving larval survival by microbial control and salinity reduction.

Can skyglow reduce nocturnal melatonin concentrations in Eurasian perch?

Artificial light at night (ALAN) changes the natural rhythm of light and darkness and can impair the biorhythms of animals, for example the nocturnal melatonin production of vertebrates, which serves as a proxy for daily physiological rhythms. Freshwater fish are exposed to ALAN in large urban and suburban areas in the form of direct light or in the form of skyglow, a diffuse brightening of the night sky through the scattered light reflected by clouds, atmospheric molecules, and particles in the air. However, investigations on the sensitivity of melatonin production of fish towards low intensities of ALAN in the range of typical skyglow are rare. Therefore, we exposed Eurasian perch (Perca fluviatilis) to nocturnal illumination levels of 0.01 lx, 0.1 lx and 1 lx and a control group with dark nights and daylight intensities of 2900 lx in all groups. After ten days of exposure to the experimental conditions, tank water was non-invasively sampled every 3 h over a 24 h period and melatonin was measured by ELISA. Melatonin was gradually reduced in all treatments with increasing intensity of ALAN whereas rhythmicity was maintained in all treatment groups although at 1 lx not all evaluated parameters confirmed rhythmicity. These results show a high sensitivity of Eurasian perch towards ALAN indicating that low light intensities of 0.01 lx and 0.1 lx as they occur in urban and suburban areas in the form of skyglow can affect the physiology of Eurasian perch. Furthermore, we highlight how this may impact perch in their sensitivity towards lunar rhythms and the role of skyglow for biorhythms of temperate freshwater fish.

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.

Influence of light intensity and spectral composition of artificial light at night on melatonin rhythm and mRNA expression of gonadotropins in roach Rutilus rutilus

In this study we investigated the influence of artificial light at night (ALAN) of different intensities (0, 1, 10, 100 lx) and different colours (blue, green, red) on the daily melatonin rhythm and mRNA expression of gonadotropins in roach Rutilus rutilus, a ubiquitous cyprinid, which occur in standing and moderately flowing freshwater habitats of central Europe. Melatonin concentrations were significantly lowered under nocturnal white light already at 1 lx. Low intensity blue, green and red ALAN lowered the melatonin levels significantly in comparison to a dark control. We conclude that ALAN can disturb melatonin rhythms in roach at very low intensities and at different wavelengths and thus light pollution in urban waters has the potential to impact biological rhythms in fish. However, mRNA expression of gonadotropins was not affected by ALAN during the period of the experiments. Thus, suspected implications of ALAN on reproduction of roach could not be substantiated.

Deep Brain Photoreceptor (val-opsin) Gene Knockout Using CRISPR/Cas Affects Chorion Formation and Embryonic Hatching in the Zebrafish

Non-rod non-cone photopigments in the eyes and the brain can directly mediate non-visual functions of light in non-mammals. This was supported by our recent findings on vertebrate ancient long (VAL)-opsin photopigments encoded by the val-opsinA (valopa) and val-opsinB (valopb) genes in zebrafish. However, the physiological functions of valop isoforms remain unknown. Here, we generated valop-mutant zebrafish using CRISPR/Cas genome editing, and examined the phenotypes of loss-of-function mutants. F0 mosaic mutations and germline transmission were confirmed via targeted insertions and/or deletions in the valopa or valopb gene in F1 mutants. Based on in silico analysis, frameshift mutations converted VAL-opsin proteins to non-functional truncated forms with pre-mature stop codons. Most F1 eggs or embryos from F0 female valopa/b mutants showed either no or only partial chorion elevation, and the eggs or embryos died within 26 hour-post-fertilization. However, most F1 embryos from F0 male valopa mutant developed but hatched late compared to wild-type embryos, which hatched at 4 day-post-fertilization. Late-hatched F1 offspring included wild-type and mutants, indicating the parental effects of valop knockout. This study shows valop gene knockout affects chorion formation and embryonic hatching in the zebrafish.

Two UV-Sensitive Photoreceptor Proteins, Opn5m and Opn5m2 in Ray-Finned Fish with Distinct Molecular Properties and Broad Distribution in the Retina and Brain

Opn5 is a group within the opsin family of proteins that is responsible for visual and non-visual photoreception in animals. It consists of several subgroups, including Opn5m, the only subgroup containing members found in most vertebrates, including mammals. In addition, recent genomic information has revealed that some ray-finned fishes carry paralogous genes of Opn5m while other fishes have no such genes. Here, we report the molecular properties of the opsin now called Opn5m2 and its distributions in both the retina and brain. Like Opn5m, Opn5m2 exhibits UV light-sensitivity when binding to 11-cis-retinal and forms a stable active state that couples with Gi subtype of G protein. However, Opn5m2 does not bind all-trans-retinal and exhibits exclusive binding to 11-cis-retinal, whereas many bistable opsins, including fish Opn5m, can bind directly to all-trans-retinal as well as 11-cis-retinal. Because medaka fish has lost the Opn5m2 gene from its genome, we compared the tissue distribution patterns of Opn5m in medaka fish, zebrafish, and spotted gar, in addition to the distribution patterns of Opn5m2 in zebrafish and spotted gar. Opn5m expression levels showed a gradient along the dorsal–ventral axis of the retina, and preferential expression was observed in the ventral retina in the three fishes. The levels of Opn5m2 showed a similar gradient with preferential expression observed in the dorsal retina. Opn5m expression was relatively abundant in the inner region of the inner nuclear layer, while Opn5m2 was expressed in the outer edge of the inner nuclear layer. Additionally, we could detect Opn5m expression in several brain regions, including the hypothalamus, of these fish species. Opn5m2 expression could not be detected in zebrafish brain, but was clearly observed in limited brain regions of spotted gar. These results suggest that ray-finned fishes can generally utilize UV light information for non-image-forming photoreception in a wide range of cells in the retina and brain.

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.

Impact of different colours of artificial light at night on melatonin rhythm and gene expression of gonadotropins in European perch

The distribution and intensity of artificial light at night, commonly referred to as light pollution, is consequently rising and progressively also ecological implications come to light. Low intensity light is known to suppress nocturnal melatonin production in several fish species. This study aims to examine the least suppressive light colour for melatonin excreted into the holding water and the influence of different light qualities and quantities in the night on gene expression of gonadotropins in fish. European perch (Perca fluviatilis) were exposed to light of different wavelengths during the night (blue, green, and red). Melatonin concentrations were measured from water samples every 3h during a 24h period. Gene expression of gonadotropins was measured in perch exposed to different light colours and was additionally examined for perch subjected to different intensities of white light (0 lx, 1 lx, 10 lx, 100 lx) during the night. All different light colours caused a significant drop of melatonin concentration; however, blue light was least suppressive. Gene expression of gonadotropins was not influenced by nocturnal light of different light colours, but in female perch gonadotropin expression was significantly reduced by white light already at the lowest level (1 lx). We conclude that artificial light with shorter wavelengths at night is less effective in disturbing biological rhythms of perch than longer wavelengths, coinciding with the light situation in freshwater habitats inhabited by perch. Different light colours in the night showed no significant effect on gonadotropin expression, but white light in the night can disturb reproductive traits already at very low light intensities. These findings indicate that light pollution has not only the potential to disturb the melatonin cycle but also the reproductive rhythm and may therefore have implications on whole species communities.

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.

Functional characterisation of the chromatically antagonistic photosensitive mechanism of erythrophores in the tilapia Oreochromis niloticus

Non-visual photoreceptors with diverse photopigments allow organisms to adapt to changing light conditions. Whereas visual photoreceptors are involved in image formation, non-visual photoreceptors mainly undertake various non-image-forming tasks. They form specialised photosensory systems that measure the quality and quantity of light and enable appropriate behavioural and physiological responses. Chromatophores are dermal non-visual photoreceptors directly exposed to light and they not only receive ambient photic input but also respond to it. These specialised photosensitive pigment cells enable animals to adjust body coloration to fit environments, and play an important role in mate choice, camouflage and ultraviolet (UV) protection. However, the signalling pathway underlying chromatophore photoresponses and the physiological importance of chromatophore colour change remain under-investigated. Here, we characterised the intrinsic photosensitive system of red chromatophores (erythrophores) in tilapia. Like some non-visual photoreceptors, tilapia erythrophores showed wavelength-dependent photoresponses in two spectral regions: aggregations of inner pigment granules under UV and short-wavelengths and dispersions under middle- and long-wavelengths. The action spectra curve suggested that two primary photopigments exert opposite effects on these light-driven processes: SWS1 (short-wavelength sensitive 1) for aggregations and RH2b (rhodopsin-like) for dispersions. Both western blot and immunohistochemistry showed SWS1 expression in integumentary tissues and erythrophores. The membrane potential of erythrophores depolarised under UV illumination, suggesting that changes in membrane potential are required for photoresponses. These results suggest that SWS1 and RH2b play key roles in mediating intrinsic erythrophore photoresponses in different spectral ranges and this chromatically dependent antagonistic photosensitive mechanism may provide an advantage to detect subtle environmental photic change.

Characterization of visual pigments, oil droplets, lens and cornea in the whooping crane Grus americana

Vision has been investigated in many species of birds, but few studies have considered the visual systems of large birds and the particular implications of large eyes and long-life spans on visual system capabilities. To address these issues we investigated the visual system of the whooping crane Grus americana (Gruiformes, Gruidae), which is one of only two North American crane species. It is a large, long-lived bird in which UV sensitivity might be reduced by chromatic aberration and entrance of UV radiation into the eye could be detrimental to retinal tissues. To investigate the whooping crane visual system we used microspectrophotometry to determine the absorbance spectra of retinal oil droplets and to investigate whether the ocular media (i.e. the lens and cornea) absorb UV radiation. In vitro expression and reconstitution was used to determine the absorbance spectra of rod and cone visual pigments. The rod visual pigments had wavelengths of peak absorbance (λmax) at 500 nm, whereas the cone visual pigment λmax values were determined to be 404 nm (SWS1), 450 nm (SWS2), 499 nm (RH2) and 561 nm (LWS), similar to other characterized bird visual pigment absorbance values. The oil droplet cut-off wavelength (λcut) values similarly fell within ranges recorded in other avian species: 576 nm (R-type), 522 nm (Y-type), 506 nm (P-type) and 448 nm (C-type). We confirm that G. americana has a violet-sensitive visual system; however, as a consequence of the λmax of the SWS1 visual pigment (404 nm), it might also have some UV sensitivity.

The pineal complex in the cichlid Cichlasoma dimerus: effect of different photoperiods on its cell morphology

This research describes the pineal complex histology in juvenile and adult Cichlasoma dimerus, and the effect of different photoperiods on its cell morphology. In both juveniles and adults, the pineal complex of C. dimerus has three components: the pineal organ, consisting of a pineal vesicle (PV) and a pineal stalk, the parapineal organ and the dorsal sac. Although a strong morphological resemblance exists between the two stages, different synthesis patterns of cone and rod opsins were detected in the two life stages. An effect of the photoperiod length was observed on putative pinealocytes' activity from the PV, measured indirectly through nuclear area morphometry. Individuals exposed to a natural photoperiod (14L:10D) had smaller nuclear areas (mean ± s.e. = 13·82 ± 1·52 µm(2) ) than those exposed to a short photoperiod (8:16) (21·45 ± 2·67 µm(2) ; P < 0·001). Eventually, the nuclear area of pinealocytes could be used as a putative indicator of melatonin synthesis in fishes where it is difficult to obtain plasma samples, e.g. due to its small size or age. This work constitutes one of the few comparative descriptions of the pineal complex of juvenile and adult teleost and suggests potential approaches for the study of melatonin synthesis in fish larvae or small adult fishes.

Localization and characterization of val-opsin isoform-expressing cells in the brain of adult zebrafish

In addition to vision, light information is used to regulate a range of animal physiology. Such nonimage-forming functions of light are mediated by nonvisual photoreceptors expressed in distinct neurons in the retina and the brain in most vertebrates. A nonvisual photoreceptor vertebrate ancient long opsin (VAL-opsin) possesses two functional isoforms in the zebrafish, encoded by valopa and valopb, which has received little attention. To delineate the neurochemical identities of valop cells and to test for colocalization of the valop isoforms, we used in situ hybridization to characterize the expression of the valop genes along with that of neurotransmitters and a neuropeptide known to be present at the sites of valop expression. Double labeling showed that the thalamic valop population coexpresses valopa and valopb. All the thalamic valop cells overlapped with a GABAergic cell mass that continues from the anterior nucleus to the intercalated thalamic nucleus. A novel valopa cell population found in the superior raphe was serotonergic in nature. A valopb cell population in the Edinger-Westphal nucleus was identified as containing thyrotropin-releasing hormone. Valopb cells localized in the hindbrain intermediate reticular formation were noncholinergic in nature (nonmotorneurons). Thus, the presence of valop cell populations in different brain regions with coexpression of neurotransmitters and neuropeptides and the colocalization of valop isoforms in the thalamic cell population indicate regulatory and functional complexity of VAL-opsin in the brain of the zebrafish.

Sensitivity differences in fish offer near-infrared vision as an adaptable evolutionary trait

Near-infrared (NIR) light constitutes an integrated part of solar radiation. The principal ability to sense NIR under laboratory conditions has previously been demonstrated in fish. The availability of NIR in aquatic habitats, and thus its potential use as a cue for distinct behaviors such as orientation and detection of prey, however, depends on physical and environmental parameters. In clear water, blue and green light represents the dominating part of the illumination. In turbid waters, in contrast, the relative content of red and NIR radiation is enhanced, due to increased scattering and absorption of short and middle range wavelengths by suspended particles and dissolved colored materials. We have studied NIR detection thresholds using a phototactic swimming assay in five fish species, which are exposed to different NIR conditions in their natural habitats. Nile and Mozambique tilapia, which inhabit waters with increased turbidity, displayed the highest spectral sensitivity, with thresholds at wavelengths above 930 nm. Zebrafish, guppy and green swordtail, which prefer clearer waters, revealed significantly lower thresholds of spectral sensitivity with 825–845 nm for green swordtail and 845–910 nm for zebrafish and guppy. The present study revealed a clear correlation between NIR sensation thresholds and availability of NIR in the natural habitats, suggesting that NIR vision, as an integral part of the whole spectrum of visual abilities, can serve as an evolutionarily adaptable trait in fish.

Isolation and characterization of melanopsin photoreceptors of Atlantic salmon (Salmo salar)

Melanopsins constitute a recently described group of vertebrate opsin photoreceptors that are involved in nonvisual photoreception. Here we describe the identification of six melanopsin genes of Atlantic salmon (Salmo salar), a valuable teleost model for studying nonvisual photoreception and the basis of photoperiodism. The results show that genes belonging to two different groups, the mammalian-like (Opn4m) and the Xenopus-like (Opn4x) melanopsins have been duplicated in teleosts. In addition, two pairs of salmon duplicates were identified, presumably originating from the salmon lineage whole genome duplication event. The expression pattern of melanopsins was studied by in situ hybridization. The results show that Opn4m and Opn4x melanopsins are differentially expressed in the brain and retina, indicating a functional divergence. In the retina, Opn4m and Opn4x melanopsin are differentially expressed in ganglion, amacrine, and horizontal cells. In the brain, Opn4m is expressed in the dorsal thalamus and in the nucleus lateralis tuberis of the hypothalamus, which is closely connected to and involved in the regulation of pituitary function. Opn4x melanopsins are expressed in the dopaminergic, hypophysiotrophic cell population of the suporaoptic/chiasmatic nucleus and in the serotonergic cell population of the left habenula. The results suggest that melanopsin photoreceptors can be involved in signaling of photoperiodic information through multiple pathways, involving both the retina and possibly as deep-brain photoreceptors directly transmitting photoperiodic information to the hypothalamus-pituitary axis.

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.

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.

Conservation of the photoperiodic neuroendocrine axis among vertebrates: evidence from the teleost fish, Gasterosteus aculeatus

Photoperiod, or length of day, has a predictable annual cycle, making it an important cue for the timing of seasonal behavior and development in many organisms. Photoperiod is widely used among temperate and polar animals to regulate the timing of sexual maturation. The proper sensing and interpretation of photoperiod can be tightly tied to an organism's overall fitness. In photoperiodic mammals and birds the thyroid hormone pathway initiates sexual maturation, but the degree to which this pathway is conserved across other vertebrates is not well known. We use the threespine stickleback Gasterosteus aculeatus, as a representative teleost to quantify the photoperiodic response of key genes in the thyroid hormone pathway under controlled laboratory conditions. We find that the photoperiodic responses of the hormones are largely consistent amongst multiple populations, although differences suggest physiological adaptation to various climates. We conclude that the thyroid hormone pathway initiates sexual maturation in response to photoperiod in G. aculeatus, and our results show that more components of this pathway are conserved among mammals, birds, and teleost fish than was previously known. However, additional endocrinology, cell biology and molecular research will be required to define precisely which aspects of the pathway are conserved across vertebrates.

Identification of two arylalkylamine N-acetyltranferase 1 genes with different developmental expression profiles in the flatfish Solea senegalensis

The existence of two arylalkylamine N-acetyltransferase 1 (Aanat1) genes in the genome of some teleosts has been reported recently by in silico analysis. However, there are no data concerning the similarities and/or differences between them and many questions remain to be answered, such as their expression sites, development, or kinetics. Here, we report the cloning of Aanat1a and Aanat1b cDNAs from the sole retina and show for the first time that at least three Aanat genes are expressed in a vertebrate species. Because melatonin is involved in fish ontogeny, we analyzed the developmental transcript levels of Aanat1a and Aanat1b by quantitative real-time PCR, showing their inverse and stage-specific expression patterns. Aanat1a was more abundant during early than late larval stages. Before metamorphosis, nocturnal expression was higher. At metamorphosis, Aanat1a expression decreased and lost these day-night variations. In contrast, the abundance of Aanat1b transcripts, low during early developing stages, rose significantly throughout metamorphosis. This situation seemed to apply to the adult because Aanat1a expression was lower than Aanat1b expression in the retina of adults, where the former did not exhibit day-night variations, while the latter did so with much higher nocturnal transcript levels. In situ hybridization analysis detected Aanat1a and Aanat1b messengers in the outer and inner nuclear layers of retina. The differences in abundance and distinct day-night expression patterns between Aanat1a and Aanat1b during sole development suggest different functions for these two enzymes as well as the existence of interactions between the melatoninergic and thyroid hormone systems during flatfish metamorphosis.

Molecular cloning and expression of long-wavelength-sensitive cone opsin in the brain of a tropical damselfish

Ovarian development of the sapphire devil, Chrysiptera cyanea, exhibits photoperiodism and is stimulated under long-day conditions. Previous studies suggest that red light is more effective than green and blue lights for inducing ovarian development. In addition, the extra-retinal photoreception involved in the ovarian development is suggested in this species. The present study aimed to clone the red-light-sensitive cone opsin (sdLWS) of this species, to demonstrate its expression in the brain, and to confirm the effectiveness of red light on the initiation of ovarian development. A phylogenetic analysis revealed that sdLWS belongs to the long-wavelength-sensitive opsin (LWS opsin) group, showing high identity (77-92%) with the LWS opsin of other fishes. RT-PCR showed that sdLWS is expressed in the eye, brain, and ovary. In situ hybridization indicated that sdLWS is expressed in the third ventricle periventricular area in the anterior hypothalamus. Exposing fish to long-day conditions of red light resulted in the appearance of vitellogenic oocytes in the ovary and an increase in the gonadosomatic index within 2weeks, suggesting that fish immediately undergo oocyte development under red light conditions. These results indicate that sdLWS is a possible candidate of deep brain photoreceptor molecule involved in photoperiodic ovarian development in the sapphire devil.

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.

Importance of light in temporal organization of photoreceptor proteins and melatonin-producing system in the pineal of carp Catla catla

The importance of light in the temporal organization of photoreceptor proteins and melatonin-producing system has been investigated for the first time in the pineal of a tropical fish. In this study, an identical experimental paradigm was followed during the four distinct phases of an annual cycle in adult carps (Catla catla) maintained either under natural photoperiod (NP) or continuous illumination (LL) or darkness (DD) for 30 days. At the end of each experiment, the pineal from fish in each experimental group was collected either at 06:00, 12:00, 18:00, or 24:00 in a daily cycle and assessed by Western blot analysis for pineal rod-like opsin, alpha-transducin, and AANAT. The same animals were also used for measurement of serum melatonin levels, and the serum as well as intra-pineal Ca(++) levels at each timepoint. The study revealed a daily rhythmicity with a peak at 12:00 h and nadir at 24:00 h in the band intensity of pineal rod-like opsin and alpha-transducin in NP fish, while the band intensities of these photo-pigment proteins remained high under LL and low under DD, irrespective of clock hour during the 24 h cycle. The band intensity of pineal AANAT, levels of serum melatonin, and both serum Ca(++) and intra-pineal Ca(++) were maximum at 24:00 h and minimum at 12:00h in NP fish, and they were significantly lower under LL and higher under DD at each point of study. The results showed loss of daily rhythm in each studied variable in both LL and DD carps, suggesting that their circadian organization is dependent on the external light-dark conditions, rather than an endogenous circadian oscillator in the pineal.

Neuronal regulation of photo-induced pineal photoreceptor proteins in carp Catla catla

In the present in vitro study on the pineal in carp Catla catla, specific agonist and antagonists of receptors for different neuronal signals and regulators of intra-cellular Ca(++) and cAMP were used to gather basic information on the neuronal signal transduction cascade mechanisms in the photo-induced expression of rod-like opsin and alpha-transducin-like proteins in any fish pineal. Western-blot analysis followed by quantitative analysis of respective immunoblot data for both the proteins revealed that photo-induced expression of each protein was stimulated by cholinergic (both nicotinic and muscarinic) agonists and a dopaminergic antagonist, inhibited by both cholinergic antagonists and a dopaminergic agonist, but not affected by any agonists or antagonists of adrenergic (alpha(1), alpha(2) and beta(1)) receptors. Moreover, expression of each protein was stimulated by voltage gated L type calcium channel blocker, adenylate cyclase inhibitor and phosphodiesterase activator; but suppressed by the activators of both calcium channel and adenylate cyclase, and by phosphodiesterase inhibitor. Collectively, we report for the first time that both cholinergic and dopaminergic signals play an important, though antagonistic, role in the photo-induced expression of photoreceptor proteins in the fish pineal through activation of a signal transduction pathway in which both calcium and cAMP may act as the intracellular messengers.

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.

Current knowledge on the photoneuroendocrine regulation of reproduction in temperate fish species

Seasonality is an important adaptive trait in temperate fish species as it entrains or regulates most physiological events such as reproductive cycle, growth profile, locomotor activity and key life-stage transitions. Photoperiod is undoubtedly one of the most predictable environmental signals that can be used by most living organisms including fishes in temperate areas. This said, however, understanding of how such a simple signal can dictate the time of gonadal recruitment and spawning, for example, is a complex task. Over the past few decades, many scientists attempted to unravel the roots of photoperiodic signalling in teleosts by investigating the role of melatonin in reproduction, but without great success. In fact, the hormone melatonin is recognized as the biological time-keeping hormone in fishes mainly due to the fact that it reflects the seasonal variation in daylength across the whole animal kingdom rather than the existence of direct evidences of its role in the entrainment of reproduction in fishes. Recently, however, some new studies clearly suggested that melatonin interacts with the reproductive cascade at a number of key steps such as through the dopaminergic system in the brain or the synchronization of the final oocyte maturation in the gonad. Interestingly, in the past few years, additional pathways have become apparent in the search for a fish photoneuroendocrine system including the clock-gene network and kisspeptin signalling and although research on these topics are still in their infancy, it is moving at great pace. This review thus aims to bring together the current knowledge on the photic control of reproduction mainly focusing on seasonal temperate fish species and shape the current working hypotheses supported by recent findings obtained in teleosts or based on knowledge gathered in mammalian and avian species. Four of the main potential regulatory systems (light perception, melatonin, clock genes and kisspeptin) in fish reproduction are reviewed.

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.

VA opsin-based photoreceptors in the hypothalamus of birds

Studies in the 1930s demonstrated that birds possess photoreceptors that are located within the hypothalamus and regulate photoperiodic responses to day length. Most recently, photoperiod has been shown to alter the activity of the pars tuberalis to release thyrotrophin, which ultimately drives a reproductive response. Despite these significant findings, the cellular and molecular identity of the hypothalamic photoreceptors has remained a mystery. Action spectra implicated an opsin-based photopigment system, but further identification based on rod- or cone-opsin probes failed, suggesting the utilization of a novel opsin. The vertebrate ancient (VA) opsin photopigments were isolated in 1997 but were thought to have a restricted taxonomic distribution, confined to the agnatha and teleost fish. Here, we report the isolation of VA opsin from chicken and show that the two isoforms spliced from this gene (cVAL and cVA) are capable of forming functional photopigments. Further, we show that VA opsin is expressed within a population of hypothalamic neurons with extensive projections to the median eminence. These results provide the most complete cellular and molecular description of a deep brain photoreceptor in any vertebrate and strongly implicate VA opsin in mediating the avian photoperiodic response.

Zebrafish cell clocks feel the heat and see the light!

The zebrafish has rapidly become established as one of the most valuable vertebrate models for studying circadian clock function. A major initial attraction was its utility in large-scale genetic screens. It subsequently emerged that most zebrafish cells possess circadian clocks that can be entrained directly by exposure to temperature or light dark cycles, a property shared by several zebrafish cell lines. This is not the case for mammals, where the retina is the primary source of light input to the clock. Furthermore, mammalian cell culture clocks can only be entrained by acute culture treatments such as serum shocks. Thus, the zebrafish is proving invaluable to study light and temperature input to the vertebrate clock. In addition, the accessibility of its early developmental stages has placed the zebrafish at the forefront of studies aimed at understanding how the circadian clock is established during embryogenesis.

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.

Clockwork blue: on the evolution of non-image-forming retinal photoreceptors in marine and terrestrial vertebrates

This paper presents a hypothesis that could explain why blue light appears to dominate non-image-forming (NIF) ocular photoreception in marine as well as terrestrial vertebrates. Indeed, there is more and more evidence suggesting that 'novel' retinal photoreceptors, which are sensitive to blue light and were only discovered in the 1990s, could be a feature shared by all vertebrates. In our view, blue light photoreception evolved and persisted as NIF photoreception because it has been useful in the colonisation of extensive photo-dependent oceanic habitats and facilitated the move of vertebrates from an aquatic to a terrestrial environment. Because the available scattered evidence is compatible with the validity of our hypothesis, we hope that our rationale will be followed up. Indeed, it (1) involves testable predictions, (2) provides plausible explanations for previous observations, (3) unites phenomena not previously considered related to one another and (4) suggests tests that have not been carried out before. Overall, our approach not only embraces cross-disciplinary links; it, moreover, serves as a reminder of an all-embracing evolutionary history, especially with regard to a ubiquitous photoreceptive 'clockwork-blue' in marine and terrestrial vertebrates.

Melatonin effects on the hypothalamo-pituitary axis in fish

Melatonin, a hormonal output signal of vertebrate circadian clocks, contributes to synchronizing behaviors and neuroendocrine regulations with the daily and annual variations of the photoperiod. Conservation and diversity characterize the melatonin system: conservation because its pattern of production and synchronizing properties are a constant among vertebrates; and diversity because regulation of both its synthesis and modes of action have been profoundly modified during vertebrate evolution. Studies of the targets and modes of action of melatonin in fish, and their parallels in mammals, are of interest to our understanding of time-related neuroendocrine regulation and its evolution from fish to mammals, as well as for aquacultural purposes.

Spectral sensitivity of melatonin suppression in the zebrafish pineal gland

The pineal gland of the zebrafish (Danio rerio) is a clock-containing photoreceptive organ. Superfused pineal glands kept in darkness display rhythmic melatonin production that lasts for days, with high melatonin levels during the night and low levels during the day. Nocturnal light, however, evokes an acute suppression of melatonin synthesis in the photoreceptor cells. Towards characterizing zebrafish pineal photopigment that is involved in the acute melatonin suppression we have measured the spectral sensitivity of melatonin-suppression response in superfused pineal glands. The effect of 2 h light exposure of seven wavelengths (lambdaavg 408, 460, 512, 560, 608, 660 and 697+/-10-15 nm) at multiple irradiances (10(7)-10(14) photons/cm2/s) was determined, and an action spectrum was plotted. The resultant action spectrum provides evidence for the involvement of multiple photopigments in melatonin suppression. The most efficient melatonin-suppression response was achieved by exposure to light of around 512 nm; however, another peak of lower irradiance sensitivity was observed in the middle to long wavelengths. Opsins-specific RT-PCR analysis confirmed the expression of exo-rhodopsin and visual red-sensitive opsin in the pineal gland, while other zebrafish visual opsins as well as VA and VAL opsins were not detected. Dartnall monograms for exo-rhodopsin and visual red-sensitive opsin account for most but not all of the spectral sensitivity features. Therefore, additional pineal photopigments may contribute to the melatonin-suppression response in the pineal gland.

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.

The interplay between the pineal complex and the habenular nuclei in lower vertebrates in the context of the evolution of cerebral asymmetry

This paper presents an overview on the epithalamus of vertebrates, with particular reference to the pineal and to the asymmetrical organization of the habenular nuclei in lower vertebrates. The relationship between the pineal and the habenulae in the course of phylogenesis is here emphasized, taking data in the frog as example. Altogether the data support the hypothesis, put forward also in earlier studies, of a correlation of habenular asymmetry in lower vertebrates with phylogenetic modification of the pineal complex. The present re-visitation was also stimulated by recent data on the asymmetrical expression of Nodal genes, which involves the pineal and habenular structures in zebrafish. The comparative analysis of data, from cyclostomes to mammals, suggests that transformation of epithalamic structures may play an important role in brain evolution. In addition, in mammals, including rodents, a remarkable complexity has evolved in the organization of the habenulae and their functional interactions with the pineal gland. The evolution of these two epithalamic structures seems to open also new perspectives of knowledge on their implication in the regulation of biological rhythms.

Existence of an extra-retinal and extra-pineal photoreceptive organ that regulates photoperiodism in gonadal development of an Osmerid teleost, ayu (Plecoglossus altivelis)

Ayu (Plecoglossus altivelis) is an Osmerid teleost whose gonadal development exhibits clear photoperiodism: it is stimulated and prevented under short and long photoperiod, respectively. However, the photoreceptor organ involved in this process remains to be identified. In the present study, we examined whether gonads of ophthalmectomized (Ex) and pinealectomized (Px) ayu respond to short photoperiod to test whether photoreceptors other than lateral eyes and pineal complex are involved in the photoperiodic response of gonadal development. Gonadosomatic index (GSI) and plasma levels of sex steroids (testosterone and 11-ketotestosterone for males and testosterone and estradiol 17-beta for females) were significantly increased in the Ex+Px ayu kept under short photoperiod in both males and females as compared with the initial control. On the other hand, there were no significant increases in GSI and sex steroids in the Ex+Px ayu kept under long photoperiod. Histological observation of gonads in the Ex+Px ayu revealed that oocytes undergoing final maturation in females and proliferation of germ cells in males were observed under short photoperiod but not under long photoperiod. These results indicate that extra-retinal and extra-pineal photoreceptive organ regulates photoperiodic gonadal development in this species.

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.

Patterns of cell proliferation and cell death in the developing retina and optic tectum of the brown trout

We have analyzed the patterns of cell proliferation and cell death in the retina and optic tectum of the brown trout (Salmo trutta fario) throughout embryonic and postembryonic stages. Cell proliferation was detected by immunohistochemistry with an antibody against the proliferating cell nuclear antigen (PCNA), and apoptosis by means of the TUNEL method. Haematoxylin and DAPI staining were also used to demonstrate apoptotic cells. Photoreceptor cell differentiation was assessed by immunohistochemistry with antibodies against opsins. Throughout embryonic development, PCNA-immunoreactive (PCNA-ir) cells become progressively restricted to the peripheral growth zone of the retina, which appears to be the principal source of new retinal cells from late embryos to adults. However, some PCNA-ir cells are observed secondarily in the differentiated retina, first in the inner nuclear layer of 15-mm alevins and later in the outer nuclear layer of 16-mm alevins, after differentiation of the first rods in the central retina, as demonstrated with opsin immunocytochemistry. Our observations also support the view that the PCNA-ir cells observed secondarily in the INL of the central retina of alevins are photoreceptor precursors. The number and distribution of apoptotic cells in the retina and optic tectum of the trout change throughout development, allowing distinction of several waves of apoptosis. Cell death is detected in proliferating areas at early stages, then in postmitotic or differentiating areas, and later concurring temporal and spatially with the establishment of visual circuits, thus indicating a relationship between apoptosis and proliferation, differentiation and synaptogenesis.