The Light Wavelength Affects the Ontogeny of Clock Gene Expression and Activity Rhythms in Zebrafish Larvae

Ontogeny of daily rhythms in the expression of metabolic factors in Nile tilapia (Oreochromis niloticus) kept at two different temperature regimes: Thermocycle and constant temperature

The daily variations of temperature are one of the main synchronizers of the circadian rhythms. In addition, water temperature influences the embryonic and larval development of fish and directly affects their metabolic processes. The application of thermocycles to fish larvae has been reported to improve growth and the maturation of the digestive system, but their effects on metabolism are poorly understood. The aim of the present study was to evaluate the effect of two different temperature regimes, cycling versus constant, on the daily rhythms of metabolic factors of Nile tilapia (Oreochromis niloticus) larvae. For this purpose, fertilized eggs were divided into two groups: one reared in a 31 °C:25 °C day:night thermocycle (TCY) and another group maintained in a constant 28 °C temperature (CTE). The photoperiod was set to a 12:12 h light/dark cycle. Samples were collected every 4 h during a 24-h cycle on days 4, 8 and 13 post fertilization (dpf). The expression levels of alanine aminotransferase (alt), aspartate aminotransferase (ast), malic enzyme, glucose-6-phosphate dehydrogenase (g6pd), phosphofructokinase (pfk) and pyruvate kinase (pk) were analyzed by qPCR. Results showed that, in 13 dpf animals, most of the genes analyzed (alt, ast, malic, g6pd and pfk) showed daily rhythms in TCY, but not in the group kept at constant temperature, with most acrophases detected during the feeding period. An increase in nutrient metabolism around feeding time can improve food utilization and thus increase larval performance. Therefore, the use of thermocycles is recommended for tilapia larviculture.

Zygotic Exposure to Venlafaxine Disrupts the Circadian Locomotor Activity Behaviour in Zebrafish Larvae

The antidepressant venlafaxine, a selective serotonin and norepinephrine reuptake inhibitor, is commonly prescribed to treat major depressive disorder and is found at high concentrations in the aquatic environment. Concerns have been raised related to the health of aquatic organisms in response to this nontargeted pharmaceutical exposure. For instance, we previously demonstrated that exposure to venlafaxine perturbs neurodevelopment, leading to behavioural alterations in zebrafish (Danio rerio). We also observed disruption in serotonin expression in the pineal and raphe, regions critical in regulating circadian rhythms, leading us to hypothesize that zygotic exposure to venlafaxine disrupts the circadian locomotor rhythm in larval zebrafish. To test this, we microinjected zebrafish embryos with venlafaxine (1 or 10 ng) and recorded the locomotor activity in 5-day-old larvae over a 24-h period. Venlafaxine deposition reduced larval locomotor activity during the light phase, but not during the dark phase of the diurnal cycle. The melatonin levels were higher in the dark compared to during the light photoperiod and this was not affected by embryonic venlafaxine deposition. Venlafaxine exposure also did not affect the transcript abundance of clock genes, including clock1a, bmal2, cry1a and per2, which showed a clear day/night rhythmicity. A notable finding was that exposure to luzindole, a melatonin receptor antagonist, decreased the locomotor activity in the control group in light, whereas the activity was higher in larvae raised from the venlafaxine-deposited embryos. Overall, zygotic exposure to venlafaxine disrupts the locomotor activity of larval zebrafish fish during the day, demonstrating the capacity of antidepressants to disrupt the circadian rhythms in behaviour. Our results suggest that disruption in melatonin signalling may be playing a role in the venlafaxine impact on circadian behaviour, but further investigation is required to elucidate the possible mechanisms in larval zebrafish.

Sodium valproate effects on the morphological and neurobehavioral phenotype of zebrafish

The anticonvulsant sodium valproate (SV) is frequently administered as a medicament but bears several negative effects in case of exposure during development. We analyzed extensively these early development effects of using the zebrafish model. Zebrafish embryos were exposed as eggs to two sublethal concentrations of SV, 10 and 25 mg/L. A general embryo toxicity analysis revealed extended anomalies in the cardiovascular system, and in the craniofacial and the spinal skeleton, as well as high mortality, in the embryos exposed to SV. The teratogenic potential of SV was confirmed in hacthed larvae by morphometric and cartilage profile analysis. Last, neurobehavioral impairments due to SV were highlighted in subjects' activity, anxiety, response to stimulations, habituation learning, and daily synchronization of locomotor activity, overall mirroring typical phenotypes associated with autistic spectrum disorders. In conclusion, our results confirmed the presence of extended and multifaced impacts of exposure to SV during development.

Maternal developmental history alters transfer of circadian clock genes to offspring in Japanese quail (Coturnix japonica)

Maternal signals shape embryonic development, and in turn post-natal phenotypes. RNA deposition is one such method of maternal signalling and circadian rhythms are one trait thought to be maternally inherited, through this mechanism. These maternal circadian gene transcripts aid development of a functioning circadian system. There is increasing evidence that maternal signals can be modified, depending on prevailing environmental conditions to optimise offspring fitness. However, currently, it is unknown if maternal circadian gene transcripts, and consequently early embryonic gene transcription, are altered by maternal developmental conditions. Here, using avian mothers who experienced either pre-natal corticosterone exposure, and/or post-natal stress as juveniles we were able to determine the effects of the timing of stress on downstream circadian RNA deposition in offspring. We demonstrated that maternal developmental history does indeed affect transfer of offspring circadian genes, but the timing of stress was important. Avian mothers who experienced stress during the first 2 weeks of post-natal life increased maternally deposited transcript levels of two core circadian clock genes, BMAL1 and PER2. These differences in transcript levels were transient and disappeared at the point of embryonic genome transcription. Pre-natal maternal stress alone was found to elicit delayed changes in circadian gene expression. After activation of the embryonic genome, both BMAL1 and PER2 expression were significantly decreased. If both pre-natal and post-natal stress occurred, then initial maternal transcript levels of BMAL1 were significantly increased. Taken together, these results suggest that developmental stress differentially produces persistent transgenerational effects on offspring circadian genes.

Light: An Extrinsic Factor Influencing Animal-based Research

Light is an environmental factor that is extrinsic to animals themselves and that exerts a profound influence on the regulation of circadian, neurohormonal, metabolic, and neurobehavioral systems of all animals, including research animals. These widespread biologic effects of light are mediated by distinct photoreceptors-rods and cones that comprise the conventional visual system and melanopsin-containing intrinsically photosensitive retinal ganglion cells (ipRGCs) of the nonvisual system that interact with the rods and cones. The rods and cones of the visual system, along with the ipRGCs of the nonvisual system, are species distinct in terms of opsins and opsin concentrations and interact with one another to provide vision and regulate circadian rhythms of neurohormonal and neurobehavioral responses to light. Here, we review a brief history of lighting technologies, the nature of light and circadian rhythms, our present understanding of mammalian photoreception, and current industry practices and standards. We also consider the implications of light for vivarium measurement, production, and technological application and provide simple recommendations on artificial lighting for use by regulatory authorities, lighting manufacturers, designers, engineers, researchers, and research animal care staff that ensure best practices for optimizing animal health and well-being and, ultimately, improving scientific outcomes.

Light-specific wavelengths differentially affect the exploration rate, opercular beat, skin color change, opsin transcripts, and the oxi-redox system of the longsnout seahorse Hippocampus reidi

Light is a strong stimulus for the sensory and endocrine systems. The opsins constitute a large family of proteins that can respond to specific light wavelengths. Hippocampus reidi is a near-threatened seahorse that has a diverse color pattern and sexual dimorphism. Over the years, H. reidi's unique characteristics, coupled with its high demand and over-exploitation for the aquarium trade, have raised concerns about its conservation, primarily due to their significant impact on wild populations. Here, we characterized chromatophore types in juvenile and adult H. reidi in captivity, and the effects of specific light wavelengths with the same irradiance (1.20 mW/cm2) on color change, growth, and survival rate. The xanthophores and melanophores were the major components of H. reidi pigmentation with differences in density and distribution between life stages and sexes. In the eye and skin of juveniles, the yellow (585 nm) wavelength induced a substantial increase in melanin levels compared to the individuals kept under white light (WL), blue (442 nm), or red (650 nm) wavelengths. In addition, blue and yellow wavelengths led to a higher juvenile mortality rate in comparison to the other treatments. Adult seahorses showed a rhythmic color change over 24 h, the highest reflectance values were obtained in the light phase, representing a daytime skin lightening for individuals under WL, blue and yellow wavelength, with changes in the acrophase. The yellow wavelength was more effective on juvenile seahorse pigmentation, while the blue wavelength exerted a stronger effect on the regulation of adult physiological color change. Dramatic changes in the opsin mRNA levels were life stage-dependent, which may infer ontogenetic opsin functions throughout seahorses' development. Exposure to specific wavelengths differentially affected the opsins mRNA levels in the skin and eyes of juveniles. In the juveniles, skin transcripts of visual (rh1, rh2, and lws) and non-visual opsins (opn3 and opn4x) were higher in individuals under yellow light. While in the juvenile's eyes, only rh1 and rh2 had increased transcripts influenced by yellow light; the lws and opn3 mRNA levels were higher in juveniles' eyes under WL. Prolonged exposure to yellow wavelength stimulates a robust increase in the antioxidant enzymes sod1 and sod2 mRNA levels. Our findings indicate that changes in the visible light spectrum alter physiological processes at different stages of life in H. reidi and may serve as the basis for a broader discussion about the implications of artificial light for aquatic species in captivity.

Modeling Sarcoglycanopathy in Danio rerio

Sarcoglycanopathies, also known as limb girdle muscular dystrophy 3-6, are rare muscular dystrophies characterized, although heterogeneous, by high disability, with patients often wheelchair-bound by late adolescence and frequently developing respiratory and cardiac problems. These diseases are currently incurable, emphasizing the importance of effective treatment strategies and the necessity of animal models for drug screening and therapeutic verification. Using the CRISPR/Cas9 genome editing technique, we generated and characterized δ-sarcoglycan and β-sarcoglycan knockout zebrafish lines, which presented a progressive disease phenotype that worsened from a mild larval stage to distinct myopathic features in adulthood. By subjecting the knockout larvae to a viscous swimming medium, we were able to anticipate disease onset. The δ-SG knockout line was further exploited to demonstrate that a δ-SG missense mutant is a substrate for endoplasmic reticulum-associated degradation (ERAD), indicating premature degradation due to protein folding defects. In conclusion, our study underscores the utility of zebrafish in modeling sarcoglycanopathies through either gene knockout or future knock-in techniques. These novel zebrafish lines will not only enhance our understanding of the disease's pathogenic mechanisms, but will also serve as powerful tools for phenotype-based drug screening, ultimately contributing to the development of a cure for sarcoglycanopathies.

Daily rhythms in the behavioural stress response of the zebrafish Danio rerio

In nature, animals are exposed to stressors that occur with different likelihood throughout the day, such as risk of predation and human disturbance. Hence, the stress response is expected to vary plastically to adaptively match these challenges. Several studies have supported this hypothesis in a wide range of vertebrate species, including some teleost fish, mostly through evidence of circadian variation in physiology. However, in teleost fish, circadian variation in behavioural stress responses is less understood. Here, we investigated the daily rhythm of stress response at the behavioural level in the zebrafish Danio rerio. We exposed individuals and shoals to an open field test every 4h over a 24h cycle, recording three behavioural indicators of stress and anxiety levels in novel environments (thigmotaxis, activity and freezing). Thigmotaxis and activity significantly varied throughout the day with a similar pattern, in line with a stronger stress response in the night phase. The same was suggested by analysis of freezing in shoals, but not in individual fish, in which variation appeared mostly driven by a single peak in the light phase. In a control experiment, we observed a set of subjects after familiarisation with the open-field apparatus. This experiment indicated that activity and freezing might present a daily rhythmicity that is unrelated to environmental novelty, and thus to stress responses. However, the thigmotaxis was constant through the day in the control condition, suggesting that the daily variation of this indicator is mostly attributable to the stress response. Overall, this research indicates that behavioural stress response of zebrafish does follow a daily rhythm, although this may be masked using behavioural indicators other than thigmotaxis. This rhythmicity can be relevant to improve welfare in aquaculture and reliability of behavioural research in fish models.

Different housing conditions for zebrafish: what are the effects?

Zebrafish is a popular experimental model in several research areas but little is known about the effects of using different strains or housing conditions. Poor control of genetic background and housing conditions could affect experimental results and data reproducibility. Here we investigated the effects of two possible sources of variation on zebrafish behaviour: fish origin and environmental parameters (light intensity, water temperature and noise). Zebrafish behaviour was then examined using the 'novel tank test', one of the most common paradigms used to assess anxiety-like behaviours in zebrafish. Our results show that an increase in light intensity alters fish behaviour, particularly freezing duration and distance from the bottom of the tank, indicating increased anxiety. Swimming activity increased at the lowest temperature (25°C). However, different levels of background noise did not cause any significant changes in behaviour. Differences were also found between zebrafish strains and populations: while the AB strain from laboratory 1 was minimally influenced by variation in holding conditions, the AB strain from laboratory 2 was highly affected by changes in temperature, light, and background noise. Our study shows that variation in strains and holding conditions can significantly influence the results of behavioural testing and should be carefully considered in the experimental design and properly reported to improve data interpretation and reproducibility.

Circadian rhythm of preferred temperature in fish: Behavioural thermoregulation linked to daily photocycles in zebrafish and Nile tilapia

Ectothermic vertebrates, e.g. fish, maintain their body temperature within a specific physiological range mainly through behavioural thermoregulation. Here, we characterise the presence of daily rhythms of thermal preference in two phylogenetically distant and well-studied fish species: the zebrafish (Danio rerio), an experimental model, and the Nile tilapia (Oreochromis niloticus), an aquaculture species. We created a non-continuous temperature gradient using multichambered tanks according to the natural environmental range for each species. Each species was allowed to freely choose their preferred temperature during the 24h cycle over a long-term period. Both species displayed strikingly consistent temporal daily rhythms of thermal preference with higher temperatures being selected during the second half of the light phase and lower temperatures at the end of the dark phase, with mean acrophases at Zeitgeber Time (ZT) 5.37 h (zebrafish) and ZT 12.5 h (tilapia). Interestingly, when moved to the experimental tank, only tilapia displayed consistent preference for higher temperatures and took longer time to establish the thermal rhythms. Our findings highlight the importance of integrating both light-driven daily rhythm and thermal choice to refine our understanding of fish biology and improve the management and welfare of the diversity of fish species used in research and food production.

Behavioural response to toxic elements, detoxification and organ accumulation are time-of-day-dependent in zebrafish

Toxic elements, such as mercury (Hg) and arsenic (As), are major pollutants in aquatic environments, posing ecological threats to living organisms due to their toxicity and bioaccumulation. This paper investigated whether zebrafish response to Hg and As displayed day/night differences. Fish were exposed to either 35 μg/L of mercury chloride for 6 h or 65 mg/L of sodium arsenate for 4 h, at two different times of the day: mid-light (day; ML) and mid-darkness (night; MD). Fish were video-recorded to investigate their behavioural response and at the end of each trial, gills and liver samples were collected for gene expression measurement. Gills, liver and brain samples were also obtained to determine Hg and As concentration. A control group (non-exposed) was video-recorded and sampled too. The effect of Hg and As on zebrafish swimming activity and the expression of antioxidant and metallothionein genes was time-of-day-dependent, with a stronger response being observed during the day than at night. However, the neurobehavioural effect of Hg was more affected by the time of exposure than the effect of As. In addition, Hg concentration in the gills was significantly higher in zebrafish exposed at ML than at MD. Altogether, these findings suggest that zebrafish response to Hg and As is time-of-day-dependent and remark the importance of considering toxicity rhythms when using this fish species as a model in toxicological research.

Loss of circadian rhythmicity in bdnf knockout zebrafish larvae

Brain-derived neurotrophic factor (BDNF) plays a pivotal role in neuronal growth and differentiation, neuronal plasticity, learning, and memory. Using CRISPR/Cas9 technology, we generated a vital Bdnf null mutant line in zebrafish and carried out its molecular and behavioral characterization. Although no defects are evident on a morphological inspection, 66% of coding genes and 37% of microRNAs turned out to be differentially expressed in bdnf^−/− compared with wild type sibling embryos. We deeply investigated the circadian clock pathway and confirmed changes in the rhythmic expression of clock (arntl1a, clock1a and clock2) and clock-controlled (aanat2) genes. The modulatory role of Bdnf on the zebrafish circadian clock was then validated by behavioral tests highlighting the absence of circadian activity rhythms in bdnf^−/− larvae. The circadian behavior was partially rescued by pharmacological treatment. The bdnf^−/− zebrafish line presented here is the first valuable and stable vertebrate model for the study of BDNF-related neurodevelopmental diseases

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Generation of a viable bdnf KO line in zebrafish

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Bdnf deficiency affects locomotor activity and thigmotaxis in larvae

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Differential RNA-seq analysis shows changes in expression of circadian clock genes

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Bdnf mutant fails in the generation of the behavioral circadian rhythmicity

Providing colored photoperiodic light stimulation during incubation: 2. Effects on early posthatch growth, immune response, and production performance in broiler chickens

Previous findings have reported that providing light during incubation can affect hatchability and chick quality. This study was conducted to investigate the effects of providing light during incubation on posthatch broiler production parameters, thermoregulation and immune response. Lights with different wavelengths were used over the course of four separate hatches. Ross 308 broiler hatching eggs were randomly distributed into 4 lighting treatments for each hatch. The incubation lighting treatments included: dark as control, white, red, or blue lights for 12 h d^-1 (200 lux at egg level). Broilers hatched from each incubator with the same gender were placed into one of 8 sets of pens (3 pens/set) and raised under 18 h d^-1 photoperiod. Six birds per pen were immunized intraocularly with AviPro ND-IB Polybanco vaccine on d 10 and 21 posthatch. Chicks hatched under white and blue lights had heavier (P < 0.05) body weight and higher (P < 0.05) feed consumption than the control group during the first 6 h postplacement. No differences in vent temperature were found among treatments at 24 h posthatch (P > 0.05). Chicks hatched with light stimulation however had more stable (P < 0.05) cloaca temperature at 36 h posthatch. No differences in average body weight gain, feed consumption or feed conversion ratio were found among lighting treatments between d 7 and d 35. On d 14 of age, birds hatched from red light had higher (P < 0.05) total IgG concentration than those hatched under dark, blue or white light. These results indicated that in ovo light stimulation with different wavelengths did not affect growth parameters of broilers at market age. Providing photoperiodic blue and white light during incubation improved the production parameters of broilers during the first week posthatch.

Finding Nemo's clock reveals switch from nocturnal to diurnal activity

Timing mechanisms play a key role in the biology of coral reef fish. Typically, fish larvae leave their reef after hatching, stay for a period in the open ocean before returning to the reef for settlement. During this dispersal, larvae use a time-compensated sun compass for orientation. However, the timing of settlement and how coral reef fish keep track of time via endogenous timing mechanisms is poorly understood. Here, we have studied the behavioural and genetic basis of diel rhythms in the clown anemonefish Amphiprion ocellaris. We document a behavioural shift from nocturnal larvae to diurnal adults, while juveniles show an intermediate pattern of activity which potentially indicates flexibility in the timing of settlement on a host anemone. qRTPCR analysis of six core circadian clock genes (bmal1, clocka, cry1b, per1b, per2, per3) reveals rhythmic gene expression patterns that are comparable in larvae and juveniles, and so do not reflect the corresponding activity changes. By establishing an embryonic cell line, we demonstrate that clown anemonefish possess an endogenous clock with similar properties to that of the zebrafish circadian clock. Furthermore, our study provides a first basis to study the multi-layered interaction of clocks from fish, anemones and their zooxanthellae endosymbionts.

Circadian regulation of muscle growth independent of locomotor activity

A long-standing question, particularly in physiotherapy and sports medicine, is whether time of day affects muscle metabolism and hence growth, either intrinsically or in response to exercise or nutrition. Answers would help to identify the best time of day to exercise, build muscle, and prevent aging- or disease-related sarcopenia. Here, we address this question in live zebrafish myotome in vivo, without interference from other circadian oscillations such as locomotor activity and food intake. We show that active muscle anabolizes more in the day and grows faster, while catabolizing more at night and growing slower. Such day/night differences remain in inactive muscle but disappear after clock disruption. We conclude that muscles display circadian differences in growth independent of activity and feeding.

Muscle tissue shows diurnal variations in function, physiology, and metabolism. Whether such variations are dependent on the circadian clock per se or are secondary to circadian differences in physical activity and feeding pattern is unclear. By measuring muscle growth over 12-h periods in live prefeeding larval zebrafish, we show that muscle grows more during day than night. Expression of dominant negative CLOCK (ΔCLK), which inhibits molecular clock function, ablates circadian differences and reduces muscle growth. Inhibition of muscle contraction reduces growth in both day and night, but does not ablate the day/night difference. The circadian clock and physical activity are both required to promote higher muscle protein synthesis during the day compared to night, whereas markers of protein degradation, murf messenger RNAs, are higher at night. Proteasomal inhibitors increase muscle growth at night, irrespective of physical activity, but have no effect during the day. Although physical activity enhances TORC1 activity, and the TORC1 inhibitor rapamycin inhibits clock-driven daytime growth, no effect on muscle growth at night was detected. Importantly, day/night differences in 1) muscle growth, 2) protein synthesis, and 3) murf expression all persist in entrained larvae under free-running constant conditions, indicating circadian drive. Removal of circadian input by exposure to either permanent darkness or light leads to suboptimal muscle growth. We conclude that diurnal variations in muscle growth and metabolism are a circadian property that is independent of, but augmented by, physical activity, at least during development.

The Influence of Behavioral, Social, and Environmental Factors on Reproducibility and Replicability in Aquatic Animal Models

The publication of reproducible, replicable, and translatable data in studies utilizing animal models is a scientific, practical, and ethical necessity. This requires careful planning and execution of experiments and accurate reporting of results. Recognition that numerous developmental, environmental, and test-related factors can affect experimental outcomes is essential for a quality study design. Factors commonly considered when designing studies utilizing aquatic animal species include strain, sex, or age of the animal; water quality; temperature; and acoustic and light conditions. However, in the aquatic environment, it is equally important to consider normal species behavior, group dynamics, stocking density, and environmental complexity, including tank design and structural enrichment. Here, we will outline normal species and social behavior of 2 commonly used aquatic species: zebrafish (Danio rerio) and Xenopus (X. laevis and X. tropicalis). We also provide examples as to how these behaviors and the complexity of the tank environment can influence research results and provide general recommendations to assist with improvement of reproducibility and replicability, particularly as it pertains to behavior and environmental complexity, when utilizing these popular aquatic models.

Relevance of Electrical Light on Circadian, Neuroendocrine, and Neurobehavioral Regulation in Laboratory Animal Facilities

Light is a key extrinsic factor to be considered in operations and design of animal room facilities. Over the past four decades, many studies on typical laboratory animal populations have demonstrated impacts on neuroendocrine, neurobehavioral, and circadian physiology. These effects are regulated independently from the defined physiology for the visual system. The range of physiological responses that oscillate with the 24 hour rhythm of the day include sleep and wakefulness, body temperature, hormonal secretion, and a wide range of other physiological parameters. Melatonin has been the chief neuroendocrine hormone studied, but acute light-induced effects on corticosterone as well as other hormones have also been observed. Within the last two decades, a new photosensory system in the mammalian eye has been discovered. A small set of retinal ganglion cells, previously thought to function as a visual output neuron, have been shown to be directly photosensitive and act differently from the classic photoreceptors of the visual system. Understanding the effects of light on mammalian physiology and behavior must take into account how the classical visual photoreceptors and the newly discovered ipRGC photoreceptor systems interact. Scientists and facility managers need to appreciate lighting impacts on circadian, neuroendocrine, and neurobehavioral regulation in order to improve lighting of laboratory facilities to foster optimum health and well-being of animals.

Photoperiod Manipulation Affects Transcriptional Profile of Genes Related to Lipid Metabolism and Apoptosis in Zebrafish (Danio rerio) Larvae: Potential Roles of Gut Microbiota

Gut microbiota plays a fundamental role in maintaining host's health by controlling a wide range of physiological processes. Administration of probiotics and manipulation of photoperiod have been suggested as modulators of microbial composition and are currently undergoing an extensive research in aquaculture as a way to improve health and quality of harvested fish. However, our understanding regarding their effects on physiological processes is still limited. In the present study we investigated whether manipulation of photoperiod and/or probiotic administration was able to alter microbial composition in zebrafish larvae at hatching stage. Our findings show that probiotic does not elicit effects while photoperiod manipulation has a significant impact on microbiota composition. Moreover, we successfully predicted lipid biosynthesis and apoptosis to be modulated by microbial communities undergoing continuous darkness. Interestingly, expression levels of caspase 3 gene (casp3) and lipid-related genes (hnf4a, npc1l1, pparγ, srebf1, agpat4 and fitm2) were found to be significantly overexpressed in dark-exposed larvae, suggesting an increase in the occurrence of apoptotic processes and a lipid metabolism impairment, respectively (p < 0.05). Our results provide the evidence that microbial communities in zebrafish at early life stages are not modulated by a short administration of probiotics and highlight the significant effect that dark photoperiod elicits on zebrafish microbiota and potentially on health.

Ontogenetic expression rhythms of visual opsins in senegalese sole are modulated by photoperiod and light spectrum

In the fish retina, rods and cones are responsible for nocturnal vision and colour perception, respectively, and exhibit a repertoire of light-sensitive opsin photopigments that permits the adaptation to different photic environment. The metamorphosis of Senegalese sole determines a migration from pelagic to benthic environments, which is accompanied by essential changes in light intensity and spectrum. In this paper, we analysed the daily expression rhythms of rod opsin and five cone opsins during sole ontogeny in animals maintained under light-dark cycles of white (LDW), blue (LDB), red (LDR) and continuous white (LL) lights. We showed that the expression of visual opsins at early stages of development was enhanced under LDB in relation to LDW, LDR and LL. Moreover, daily rhythms of opsins were more robust under LDW and LDB conditions, in particular, before and after metamorphosis. A shift in the phase of opsin rhythms was observed between hatching and pre-metamorphosis. Metamorphosis was accompanied by a transient loss in the expression rhythms for most of the opsins, which were significantly influenced by light photoperiod and spectrum. In LDR, transcript levels and rhythms were markedly affected for the majority of the opsins analysed. Under LL, most of the opsins examined exhibited endogenous rhythms, although amplitudes and acrophases changed considerably. To the best of our knowledge, this is the first study on the daily expression rhythms of visual opsins during the ontogeny of a metamorphic flatfish and further emphasises the importance of using natural lighting conditions for proper development of Senegalese sole.

Potential of the zebrafish model for the forensic toxicology screening of NPS: A comparative study of the effects of APINAC and methiopropamine on the behavior of zebrafish larvae and mice

The increased diffusion of the so-called novel psychoactive substances (NPS) and their continuous change in structure andconceivably activity has led to the need of a rapid screening method to detect their biological effects as early as possible after their appearance in the market. This problem is very felt in forensic pathology and toxicology, so the preclinical study is fundamental in the approach to clinical and autopsy cases of difficult interpretation intoxication. Zebrafish is a high-throughput suitable model to rapidly hypothesize potential aversive or beneficial effects of novel molecules. In the present study, we measured and compared the behavioral responses to two novel neuroactive drugs, namely APINAC, a new cannabimimetic drug, and methiopropamine (MPA), a methamphetamine-like compound, on zebrafish larvae (ZL) and adult mice. By using an innovative statistical approach (general additive models), it was found that the spontaneous locomotor activity was impaired by the two drugs in both species: the disruption extent varied in a dose-dependent and time-dependent manner. Sensorimotor function was also altered: i) the visual object response was reduced in mice treated with APINAC, whereas it was not after exposure to MPA; ii) the visual placing responses were reduced after treatment with both NPS in mice. Furthermore, the visual motor response detected in ZL showed a reduction after treatment with APINAC during light-dark and dark-light transition. The same pattern was found in the MPA exposed groups only at the dark-light transition, while at the transition from light to dark, the individuals showed an increased response. In conclusion, the present study highlighted the impairment of spontaneous motor and sensorimotor behavior induced by MPA and APINAC administration in both species, thus confirming the usefulness of ZL as a model for a rapid behavioural-based drug screening.

Feeding Entrainment of the Zebrafish Circadian Clock Is Regulated by the Glucocorticoid Receptor

Glucocorticoids (GCs) are steroid hormones mainly acting as key regulators of body homeostasis and stress responses. Their activities are primarily based on the binding to the GC receptor (GR), a member of the nuclear receptor family, that regulates tissue-specific sets of genes. GCs secretion follows a circadian rhythmicity with a peak linked to the animal’s activity phase. In mammals, GCs are also implicated in feeding entrainment mechanisms as internal zeitgeber. Here, we investigated, by means of behavioural and molecular approaches, the circadian clock and its regulation by light and food in wild-type (WT) and null glucocorticoid receptor (gr^−/−) zebrafish larvae, juveniles and adults. In both WT and gr^−/− larvae and adults, behavioural activity and clock gene expression were entrained to the light–dark (LD) cycle and rhythmic in constant conditions. Differences in the pattern of clock genes’ expression indicated a modulatory role of GCs. A significant role of Gr was detected in the feeding entrainment which was absent or markedly dampened in mutants. Furthermore, the expression of two clock-regulated genes involved in glucidic and lipidic metabolism was altered, highlighting the participation of GCs in metabolic processes also in fish. Taken together, our results confirmed the role of GC-mediated Gr signaling in the feeding entrainment in a non-mammalian species, the zebrafish.

Effects of the colour of photophase light on locomotor activity in a nocturnal and a diurnal South African rodent

Many physiological and behavioural responses to varying qualities of light, particularly during the night (scotophase), have been well documented in rodents. We used varying wavelengths of day-time (photophase) lighting to assess daily responses in locomotor activity in the nocturnal Namaqua rock mouse (Micaelamys namaquensis) and diurnal four-striped field mouse (Rhabdomys pumilio). Animals were exposed to three light-dark cycle regimes: a short-wavelength- (SWLC, blue), a medium-wavelength- (MWLC, green) and a long-wavelength light-dark cycle (LWLC, red). Overall, daily locomotor activity of both species changed according to different wavelengths of light: the diurnal species displayed most activity under the SWLC and the nocturnal species exhibited the highest levels of activity under the LWLC. Both species showed an increase in diurnal activity and a decrease in nocturnal activity under the LWLC. These results indicate an attenuated responsiveness to long-wavelength light in the nocturnal species, but this does not appear to be true for the diurnal species. These results emphasize that the effect of light on the locomotor activity of animals depends on both the properties of the light and the temporal organization of activity of a species.

Effect of melatonin on monochromatic light-induced changes in clock gene circadian expression in the chick liver

Light is the most prominent zeitgeber of the circadian system, which contains central and peripheral oscillators. Our previous studies found that light wavelength could influence the rhythms of melatonin synthesis and clock gene expression in the central oscillator of chicks. However, the effect of monochromatic light on the peripheral oscillator and the role of melatonin have yet to be clarified. In this study, 216 newly hatched chicks were divided into three groups (intact, sham operation and pinealectomy) and were raised under white (WL), red (RL), green (GL) or blue (BL) light for 14 days. Their plasma and livers were sampled at 6 time points with 4-h intervals. Plasma melatonin concentration and liver clock gene expression (cClock, cBmal1, cBmal2, cCry1, cCry2, cPer2, cPer3) were measured for circadian rhythm analysis. In intact and sham operation chicks under WL, all liver clock genes showed circadian expression along with oscillations in plasma melatonin. However, positive clock genes peaked at subjective night along with melatonin, while negative clock genes peaked at subjective day or the shifting time of day-night. Chick exposure to monochromatic light led to an unaltered circadian rhythmicity in plasma melatonin and liver clock genes; however, their rhythmic parameters were notably influenced. Compared to WL, GL enhanced the mesor and amplitude of melatonin and all kinds of clock genes, whereas RL had the opposite effect. Pinealectomy significantly decreased expression of liver clock genes, which was consistent with the reduction in plasma melatonin concentration, especially for the GL group, and resulted in the expression of liver clock genes showing low-mesor and low-amplitude oscillations as well as no statistically significant differences among the monochromatic light groups. Thus, we speculated that melatonin plays a key role in the effects of light wavelength on clock gene rhythm in the chick liver.

Environmental Cycles, Melatonin, and Circadian Control of Stress Response in Fish

Fish have evolved a biological clock to cope with environmental cycles, so they display circadian rhythms in most physiological functions including stress response. Photoperiodic information is transduced by the pineal organ into a rhythmic secretion of melatonin, which is released into the blood circulation with high concentrations at night and low during the day. The melatonin rhythmic profile is under the control of circadian clocks in most fish (except salmonids), and it is considered as an important output of the circadian system, thus modulating most daily behavioral and physiological rhythms. Lighting conditions (intensity and spectrum) change in the underwater environment and affect fish embryo and larvae development: constant light/darkness or red lights can lead to increased malformations and mortality, whereas blue light usually results in best hatching rates and growth performance in marine fish. Many factors display daily rhythms along the hypothalamus-pituitary-interrenal (HPI) axis that controls stress response in fish, including corticotropin-releasing hormone (Crh) and its binding protein (Crhbp), proopiomelanocortin A and B (Pomca and Pomcb), and plasma cortisol, glucose, and lactate. Many of these circadian rhythms are under the control of endogenous molecular clocks, which consist of self-sustained transcriptional-translational feedback loops involving the cyclic expression of circadian clock genes (clock, bmal, per, and cry) which persists under constant light or darkness. Exposing fish to a stressor can result in altered rhythms of most stress indicators, such as cortisol, glucose, and lactate among others, as well as daily rhythms of most behavioral and physiological functions. In addition, crh and pomca expression profiles can be affected by other factors such as light spectrum, which strongly influence the expression profile of growth-related (igf1a, igf2a) genes. Additionally, the daily cycle of water temperature (warmer at day and cooler at night) is another factor that has to be considered. The response to any acute stressor is not only species dependent, but also depends on the time of the day when the stress occurs: nocturnal species show higher responses when stressed during day time, whereas diurnal fish respond stronger at night. Melatonin administration in fish has sedative effects with a reduction in locomotor activity and cortisol levels, as well as reduced liver glycogen and dopaminergic and serotonergic activities within the hypothalamus. In this paper, we are reviewing the role of environmental cycles and biological clocks on the entrainment of daily rhythms in the HPI axis and stress responses in fish.

Carotenoid glycosides from cyanobacteria are teratogenic in the zebrafish (Danio rerio) embryo model

Toxigenicity of cyanobacteria is widely associated with production of several well-described toxins that pose recognized threats to human and ecosystem health as part of both freshwater eutrophication, and episodic blooms in freshwater and coastal habitats. However, a preponderance of evidence indicates contribution of additional bioactive, and potentially toxic, metabolites. In the present study, the zebrafish (Danio rerio) embryo was used as a model of vertebrate development to identify, and subsequently isolate and characterize, teratogenic metabolites from two representative strains of C. raciborskii. Using this approach, three chemically related carotenoids - and specifically the xanthophyll glycosides, myxol 2'-glycoside (1), 4-ketomyxol 2'-glycoside (2) and 4-hydroxymyxol 2'-glycoside (3) - which are, otherwise, well known pigment molecules from cyanobacteria were isolated as potently teratogenic compounds. Carotenoids are recognized "pro-retinoids" with retinoic acid, as a metabolic product of the oxidative cleavage of carotenoids, established as both key mediator of embryo development and, consequently, a potent teratogen. Accordingly, a comparative toxicological study of chemically diverse carotenoids, as well as apocarotenoids and retinoids, was undertaken. Based on this, a working model of the developmental toxicity of carotenoids as pro-retinoids is proposed, and the teratogenicity of these widespread metabolites is discussed in relation to possible impacts on aquatic vertebrate populations.