Characterization of sickness behavior in zebrafish

'Environmental standard limit concentration' arsenic exposure is associated with anxiety, depression, and autism-like changes in early-life stage zebrafish

Arsenic is a worldwide environmental pollutant that can impair human health. Previous studies have identified mental disorders induced by arsenic, but the environmental exposure concentrations in the early life stages associated with these disorders are poorly understood. In the present study, early-life stage zebrafish were used to explore the effects on mental disorders under 'environmental standard limit concentrations' arsenic exposures of 5, 10, 50, 150, and 500 μg/L. The results showed that arsenic exposure at these concentrations changed the locomotor behavior in larval zebrafish and was further associated with anxiety, depression, and autism-like behavior in both larval and juvenile zebrafish. Changes were noted at benchmark dose limit (BMDL) concentrations as low as 0.81 μg/L. Transcriptomics showed that immediate early genes (IEGs) fosab, egr1, egr2a, ier2b, egr3, and jund were decreased after arsenic exposure in larval and juvenile zebrafish. Nervous system impairment and anxiety, depression, and autism-like behaviors in early-life stage zebrafish at 'environmental standard limit concentrations' may be attributed to the downregulation of IEGs. These findings in zebrafish provided new experimental support for an arsenic toxicity threshold for mental disorders, and they suggest that low levels of environmental chemicals may be causative developmental factors for mental disorders.

Mixture of pesticides based on dimethylamine and imidacloprid affects locomotion of adult zebrafish

Numerous chemical compounds are found in aquatic environments; among them are pesticides. Pesticides are widely used worldwide, and this use has progressively increased in recent decades, resulting in the accumulation of potentially toxic compounds in surface waters. Dimethylamine-based herbicides (DBH) and imidacloprid-based insecticides (IBI) have low soil absorption and high water solubility, facilitating the arrival of these compounds in aquatic environments. In this study, our objective was to analyze whether two pesticides, DBH and IBI at environmentally relevant concentrations of 320 μg/L for each compound, and their mixtures impact the behavioral and endocrine parameters of adult zebrafish, verifying the effect of pesticides on exploratory behavior and social and analyzing hormonal parameters related to stress. Acute exposure to the mixture of pesticides reduced fish locomotion. Pesticides alone and in combination did not affect cortisol levels in exposed animals. Pesticides, when tested together, can cause different effects on non-target organisms, and the evaluation of mixtures of these compounds is extremely important.

Methylphenidate exposure in juvenile period elicits locomotion changes and anxiolytic-like behavior in adulthood: Evidence using zebrafish as a translational model

Methylphenidate (MPH) is a central nervous system stimulant that is mainly used for Attention-Deficit/Hyperactivity Disorder (ADHD). It is well known that there is a high rate of ADHD misdiagnosis, leading to a great number of neurotypical children chronically exposed to MPH in early periods of life. This increase raises concern about possible long-lasting effects of this exposure. We aimed to evaluate whether exposure to MPH during childhood might impact adult behavioral pattern. For this purpose, we used zebrafish as a translational model considering its robustness as experimental model and fast life cycle. Fish were exposed during juvenile period (from 30 to 60 post-natal day) at MPH therapeutic concentration (2 mg L-1), and behavioral tests were performed at fish adulthood (120 post-natal day). MPH provoked slight anxiolytic-like effects and hyperlocomotion, and no differences on sociability and cortisol levels were observed. Moreover, sex did not affect any of the parameters evaluated. These results demonstrate that early chronic exposure to MPH leads to neurobehavioral adaptations that persist into adulthood in zebrafish regardless of sex, suggesting that the misuse of MPH during childhood and adolescence can alter neurobehavioral plasticity and these alterations might persist until adulthood.

Methamphetamine Shows Different Joint Toxicity for Different Types of Microplastics on Zebrafish Larvae by Mediating Oxidative Stress

The coexistence of polystyrene (PS) and polypropylene (PVC) microplastics (MPs) and methamphetamine (METH) in aquatic systems is evident. However, the joint toxicity is unclear. Here, zebrafish larvae were exposed to single PS and PVC MPs (20 mg L-1) and combined with METH (250 and 500 μg L-1) for 10 days. The results indicated that acute exposure to PS and PVC MPs induced lethal effects on zebrafish larvae (10-20%). Treatment with MPs markedly suppressed the locomotion of zebrafish, showing as the lengthy immobility (51-74%) and lower velocity (0.09-0.55 cm s-1) compared with the control (1.07 cm s-1). Meanwhile, histopathological analysis revealed pronounced depositions of MPs particles in fish's intestinal tract, triggering inflammatory responses (histological scores: 1.6-2.0). In the coexposure groups, obviously inflammatory responses were found. Furthermore, the up-regulations of the genes involved in the oxidative kinase gene and inflammation related genes implied that oxidative stress triggered by MPs on zebrafish larvae might be responsible for the mortality and locomotion retardant. The antagonistic and stimulatory effects of METH on the expression changes of genes found in PVC and PS groups implied the contrary combined toxicity of PS/PVC MPs and METH. This study for the first time estimated the different toxicity of PS and PVC MPs on fish and the joint effects with METH at high environmental levels. The results suggested PS showed stronger toxicity than PVC for fish larvae. The addition of METH stimulated the effects of PS but antagonized the effects of PVC, promoting control strategy development on MPs and METH in aquatic environments.

The neurobehavioral impacts of typical antibiotics toward zebrafish larvae

Due to the widely usage in livestock, aquaculture and clinics, antibiotic residues are existed in aqueous environments and their potential toxicity to aquatic organisms is concerning. Here, we used zebrafish as the model to investigate the neurotoxicity and involved mechanism of seven antibiotics that were frequently detected in surface waters. The results revealed that the short-term exposure to clarithromycin (CLA), chlortetracycline (CTC) and roxithromycin (ROX) induced behavioral effects, with effective concentration of 1 μg/L (CTC and ROX) and 100 μg/L (CLA, CTC and ROX) respectively. A significant decrease in the travel distance and velocity as well as an increase in turn angle was measured. TUNEL assay identified increased cell apoptosis in brain sections of larvae exposed to three neurotoxic antibiotics, which raised the possibility that the behavioral symptoms were associated with neural damage. Transcriptome sequencing showed that the three antibiotics could affect the nervous system of zebrafish including the alteration of synaptogenesis and neurotransmission. Additionally, ROX and CTC affected pathways involved in mitochondrial stress response and endocrine system in zebrafish larvae. Besides, BDNF, ASCL1, and CREBBP are potential upstream regulatory factors that mediated these impacts. These findings indicated that exposure of CTC, ROX and CLA may cause abnormal behavior toward zebrafish larvae under environmental relevant concentration and revealed the potential role of neural cell apoptosis and synaptogenesis signaling in mediating this effect.

Immune challenge affects reproductive behaviour in the guppy (Poecilia reticulata)

Immunocompetence and reproduction are among the most important determinants of fitness. However, energetic and metabolic constraints create conflict between these two life-history traits. While many studies have explored the relationship between immune activity and reproductive fitness in birds and mammals inoculated with bacterial endotoxin, very few have focused on fish. Fish have been neglected in this area due, in part, to the claim that they are largely resistant to the immune effects of endotoxins. However, the present study suggests that they are susceptible to significant effects with respect to reproductive behaviour. Here, we examined the reproductive behaviour of male guppies following exposure to bacterial lipopolysaccharides (LPS) in comparison to that of male guppies in a control treatment. Additionally, we investigated the responses of females to these males. We show that although immune challenge does not suppress general activity in male guppies, it significantly reduces mating effort. While females showed no difference in general activity as a function of male treatments, they did exhibit reduced group cohesion in the presence of LPS-exposed males. We discuss this in the context of sickness behaviours, social avoidance of immune-challenged individuals and the effects of mounting an immune response on reproductive behaviour.

Behavioral, biochemical, and endocrine responses of zebrafish to 30-min exposure with environmentally relevant concentrations of imidacloprid-based insecticide

The imidacloprid-based insecticides (IBIs) are among the most used insecticides worldwide, and chronic and acute toxic effects (days exposure protocols) have been reported in several species in studies of IBIs at lethal concentrations. However, there is little information on shorter time exposures and environmentally relevant concentrations. In this study, we investigated the effect of a 30-min exposure to environmentally relevant concentrations of IBI on the behavior, redox status, and cortisol levels of zebrafish. We showed that the IBI decreased fish locomotion and social and aggressive behaviors and induced an anxiolytic-like behavior. Furthermore, IBI increased cortisol levels and protein carbonylation and decreased nitric oxide levels. These changes were mostly observed at 0.013 and 0.0013 µg·L^-1 of IBI. In an environmental context, these behavioral and physiological disbalances, which were immediately triggered by IBI, can impair the ability of fish to evade predators and, consequently, affect their survival.

Equine Social Behaviour: Love, War and Tolerance

Sociality is an ethological need of horses that remained unchanged by domestication. Accordingly, it is essential to include horses' social behavioural requirements and the opportunity to establish stable affiliative bonds in equine management systems and welfare assessment. Thus, this systematic review aims to provide an up-to-date analysis of equine intraspecific social ethograms. A literature review yielded 27 papers that met the inclusion criteria by studying adult (≥2 years) equine social behaviour with conspecifics using a well-defined ethogram. Social interactions were observed in 851 horses: 320 (semi-)feral free-ranging, 62 enclosed (semi-)feral and 469 domesticated, living in groups averaging 9.1 (mean +/- 6.8 s.d., range: 2-33) horses. The ethograms detailed in these 27 studies included a total of 40 (mean: 12.8/paper, range: 2-23) social behaviours, of which 60% (24/40) were agonistic, 30% (12/40) affiliative, 7.5% (3/40) investigative and 2.5% (1/40) neutral. The 27 publications included 67.7% agonistic and only 26% affiliative, 5.1% investigative and 1.2% neutral social behaviours in their methodology, thus focusing predominantly on socio-negative interactions. The strong emphasis on agonistic behaviours in equine ethology starkly contrasts with the rare occurrence of agonistic behaviours in stable horse groups and the well-established importance of affiliative interactions for equine welfare. The nuanced and complex equine social behaviour requires refinement of the ethogram with a greater focus on affiliative, ambivalent and indifferent interactions and the role of social tolerance in equine social networks to advance equine welfare assessment.

Luteolin, a promising quorum quencher mitigates virulence factors production in Pseudomonas aeruginosa - In vitro and in vivo approach

Pseudomonas aeruginosa (PA) is an opportunistic pathogen that causes healthcare-associated infection and high mortality in immunocompromised patients. It produces several virulence factors through quorum sensing (QS) mechanisms that is essential for subverting host immune system. Even front-line antibiotics are unable to control PA pathogenicity due to the emergence of antibiotic resistance. Luteolin is a naturally derived compound that has proven to be the effective drug to annihilate pathogens through quorum quenching mechanism. In this study, the protective effect of luteolin against the PA-mediated inflammation was demonstrated using zebrafish model. Luteolin protects zebrafish from PA infection and increases their survival rate. It was found that PA-mediated ROS, lipid peroxidation, and apoptosis were also significantly reduced in luteolin-treated zebrafish larvae. Open field test (OFT) reveals that luteolin rescued PA-infected zebrafish from retarded swimming behavior. Furthermore, luteolin increases SOD and CAT levels and decreases LDH and NO levels in PA-infected zebrafish compare to control group. Histological and gene expression analysis reveals that luteolin protects PA-infected zebrafish by decreasing gut inflammation and altering the expression of inflammatory (TNF-α, IL-1β, IL-6) and antioxidant markers (iNOS, SOD, CAT). Thus, luteolin was found to have dual effect in protecting PA-infected zebrafish by decreasing virulence factors production in PA and stimulating host immune system. This is the first study demonstrating the protective effect of luteolin using animal model. Hence, luteolin could be used as a future therapeutic drug to control multi-drug resistant PA.

One messenger shared by two systems: How cytokines directly modulate neurons

Cytokines are small, secreted proteins that are known for their roles in the immune system. An accumulating body of evidence indicates that cytokines also work as neuromodulators in the central nervous system (CNS). Cytokines can access the CNS through multiple routes to directly impact neurons. The neuromodulatory effects of cytokines maintain the overall homeostasis of neural networks. In addition, cytokines regulate a diverse repertoire of behaviors both at a steady state and in inflammatory conditions by acting on discrete brain regions and neural networks. In this review, we discuss recent findings that provide insight into how combinatorial codes of cytokines might mediate neuro-immune communications to orchestrate functional responses of the brain to changes in immunological milieus.

Household based-pyrethroids on adult zebrafish (Danio rerio) exert behavioral and cholinergic changes in different brain regions

Pyrethroid-based insecticides are largely used for mosquito control. These compounds have household and agricultural applications with different formulations. Two important compounds used as household insecticides are prallethrin and transfluthrin, both from the pyrethroid chemical group. With the mode of action centered on sodium channels, pyrethroids keep the ionic sodium channels open for a long time causing the death of the insect by nervous hyperexcitability. Given the increased use of household insecticides by humans and the incidence of disease outbreaks with unknown etiology such as autism spectrum disease, schizophrenia, and Parkinson's disease we investigate some physiological inputs of these compounds on zebrafish. In this study, we evaluated the social interaction, shoaling formation, and anxiety-like behavior of zebrafish exposed chronically to transfluthrin- and prallthrin-based insecticides (T-BI and P-BI). In addition, we quantified the activity of the enzyme acetylcholinesterase (AChE) in different brain regions. We observed that both compounds caused anxiolytic behavior and reduced shoaling formation and social interaction. Their behavioral biomarkers indicated a harmful ecological effect on the specie as well as a possible impact of these compounds on autism spectrum disorder (ASD) and schizophrenia (SZP). In addition, the AChE activity would change its activity in different brain regions modulating the anxiety-like behavior and social behavior in zebrafish. We conclude that P-BI and T-BI make us alert about the relationship of these compounds with nervous diseases related to cholinergic signaling.

Tyramine-induced gastrointestinal dysregulation is attenuated via estradiol associated mechanisms in a zebrafish larval model

Development of targeted therapeutics to alleviate gastrointestinal (GI) inflammation and its debilitating consequences are required. In this context, the trace aminergic system may link together sex, diet and inflammation. Utilising a zebrafish larval model of GI inflammation, the current study aimed to investigate mechanisms by which excess amounts of trace amines (TAs) may influence GI health. In addition, we probed the potential role of 17β-estradiol (E2) and its receptors, given the known female-predominance of many GI disorders. To assess GI functionality and integrity, live imaging techniques (neutral red staining) and post-mortem immunofluorescent staining of tight junction proteins (occludin and ZO-1) were analyzed respectively. In addition, behavioural assays, as an indication of overall wellbeing, as well as whole body H₂O₂ and prostaglandin E2 assays were performed to inform on oxidative and inflammatory status. Excess β-phenethylamine (PEA), tryptamine (TRP) and ρ-tyramine (TYR) resulted in adverse GI and systemic effects. In this regard, clear beneficial effects of E2 to modulate the effects of PEA, TRP and TYR was evident. Moreover, agmatine displayed potential protective effects on GI epithelium and whole body oxidative status, however, potential to induce systemic inflammation suggests the importance of dosage and administration optimisation. Taken together, TYR seems like the most prominent TA to have damaging GI effects, feasibly exacerbating GI inflammation. In this context, the relative lack of E2 may provide mechanistic insights into the reported female-predominance of GI disorders. Moreover, an effective therapeutic in this context may be required to maintain GI TA load despite fluctuating E2 levels.

Fluoxetine and Curcumin Prevent the Alterations in Locomotor and Exploratory Activities and Social Interaction Elicited by Immunoinflammatory Activation in Zebrafish: Involvement of BDNF and Proinflammatory Cytokines

The increase in proinflammatory cytokine expression causes behavioral changes consistent with sickness behavior, and this led to the suggestion that depression might be a psychoneuroimmunological phenomenon. Here, we evaluated the effects of the pretreatment with fluoxetine (10 mg/kg, i.p.) and curcumin (0.5 mg/kg, i.p.) on the immune response elicited by the inoculation of an Aeromonas hydrophila bacterin in zebrafish. Non-pretreated but A. hydrophila-inoculated and sham-inoculated groups of fish served as controls. The social preference, locomotor, exploratory activities, and cerebral expression of il1b, il6, tnfa, and bdnf mRNA were compared among the groups. Behavioral changes characteristic of sickness behavior and a significant increase in the expression of il1b and il6 cytokines were found in fish from the immunostimulated group. The behavioral alterations caused by the inflammatory process were different between males and females, which was coincident with the increased expression of cerebral BDNF. Fluoxetine and curcumin prevented the sickness behavior induced by A. hydrophila and the increased expression of proinflammatory cytokines. Our results point to the potential of zebrafish as a translational model in studies related to neuroinflammation and demonstrate for the first time the effects of fluoxetine and curcumin on zebrafish sickness behavior.

Investigating the effects of chronic low-dose radiation exposure in the liver of a hypothermic zebrafish model

Mankind's quest for a manned mission to Mars is placing increased emphasis on the development of innovative radio-protective countermeasures for long-term space travel. Hibernation confers radio-protective effects in hibernating animals, and this has led to the investigation of synthetic torpor to mitigate the deleterious effects of chronic low-dose-rate radiation exposure. Here we describe an induced torpor model we developed using the zebrafish. We explored the effects of radiation exposure on this model with a focus on the liver. Transcriptomic and behavioural analyses were performed. Radiation exposure resulted in transcriptomic perturbations in lipid metabolism and absorption, wound healing, immune response, and fibrogenic pathways. Induced torpor reduced metabolism and increased pro-survival, anti-apoptotic, and DNA repair pathways. Coupled with radiation exposure, induced torpor led to a stress response but also revealed maintenance of DNA repair mechanisms, pro-survival and anti-apoptotic signals. To further characterise our model of induced torpor, the zebrafish model was compared with hepatic transcriptomic data from hibernating grizzly bears (Ursus arctos horribilis) and active controls revealing conserved responses in gene expression associated with anti-apoptotic processes, DNA damage repair, cell survival, proliferation, and antioxidant response. Similarly, the radiation group was compared with space-flown mice revealing shared changes in lipid metabolism.

Social behaviors as welfare indicators in teleost fish

Animal welfare is a key issue not only for aquaculture industry and food production, but also for daily husbandry practices in research topics related to physiology in wild and farmed animals. In this context, teleost fish constitute interesting models to assess alternative welfare indicators because of their wide diversity in reproductive and social structures. Any framework for assessing teleost fish welfare needs to account for the physiological mechanisms involved in each species as a first step. A comprehensive approach should also take into account how these physiological and behavioral parameters can be altered by environmental enrichment considering the specific requirements in each case and identifying intrinsic biological characteristics of individual species. This review will show how cortisol and sex steroids regulate social behavior in teleost fish, and how different aspects of social behavior can be employed as welfare indicators according to specific characteristics in each case. This article will consider evidence in teleost fish, including cichlids, characids and cyprinids with different reproductive strategies and social structures (e.g., territorial social hierarchies or shoaling behavior). Neotropical species will be particularly emphasized. The main laboratory-based animal welfare indicators are cortisol, a classical stress hormone, together with sex steroids. Considering that the endocrine landscape is intrinsically related to social behavior, reproductive and agonistic behavioral traits such as aggression, anxiety and courtship are key elements to assess welfare under housing and culture conditions. This review highlights the importance of assessing physiological mechanisms and identifying behavioral characteristics in teleost fish, especially in Neotropical species, as a baseline to understand which environmental enrichment can improve animal welfare in each individual species.

Evolutionary Aspects of Infections: Inflammation and Sickness Behaviors

Sickness behavior was conceptualized initially as the behavioral counterpart of the fever response to infectious pathogens. It helps to raise body temperature to its higher setpoint and to maintain it at this new level and it has the additional benefit of enabling a weakened organism to protect itself from other dangers. The discovery of the behavioral effects of proinflammatory cytokines produced by activated immune cells provided a cellular and molecular basis to this phenomenon. The administration of cytokines or cytokine inducers like lipopolysaccharide to healthy rodents allowed to reveal the similarities and differences between inflammation-induced sickness behavior and the fever response. It also led to the understanding of how the inflammatory response that is triggered at the periphery can propagate into the brain and induce the behavioral manifestations of sickness. At the behavioral level, the demonstration that sickness behavior is the expression of a motivational state that reorganizes perception and action in face of a microbial pathogen just like fear in face of a predator appeared at first glance to strengthen the adaptive value of this behavior. However, all aspects of sickness behavior are not always favorable for the organism. This is the case for anorexia that is beneficial in the context of bacterial infection but detrimental in the context of viral infection. In addition, studies of sickness behavior in natural conditions revealed that like any other defensive behavior, sickness behavior requires trade-offs between its survival benefits for the sick individual and the costs incurred especially in the context of gregarious groups. Thanks to these studies, evidence is emerging that sickness behavior is much more variable in its expression than initially thought, and that part of this variability depends not only on the pathogen and the social context in which the infection develops but also on individual factors including species, sex, age, nutrition, and physiological status.

Persistent and transgenerational effects of pesticide residues in zebrafish

Highly toxic chemical compounds are present in rivers and lakes, endangering the survival of non-target species. To evaluate the effects of environmental contamination on non-target species, we used the zebrafish as an animal model. Environmental concentrations of the widely used pesticides, glyphosate (GBH) at 4.8 μg·L^-1 and 2,4-dichlorophenoxyacetic acid (DBH) at 3.4 μg·L^-1, were used. The animals were exposed during the entire period of organogenesis and evaluated in our previous study regarding initial developmental parameters. In the present study, we evaluate these fish when achieve the adult phase, using the novel tank test (NTT) and the aversivity test. In the second step, the animals were allowed to reproduce, and the initial parameters of development, behavioral parameters in the open field test (OFT) and in the aversivity test (AST), and biochemical biomarkers as acetylcholinesterase (AChE), catalase (CAT), and superoxide dismutase (SOD) in the F1 generation were studied. Fish exposed to GBH showed hypermobility, and their anti-predatory reaction was impaired during adulthood, indicating a persistent effect. We also showed that fish had impaired behavioral and survival changes in the F1 generation as well as effects on AChE activity and antioxidant enzymes, characterizing a transgenerational effect. The fish did not show persistent effects in adulthood due to DBH exposure; however, they were unable to reproduce. Our findings demonstrate the serious impact of pesticides on fish, where the effects of contamination can affect future generations and compromise the species' survival.

Exposure to levonorgestrel-based birth control pill in early life and its persistent effects in zebrafish

The consumption of progestins has increased considerably in recent decades, as has their disposal into the environment. These substances can negatively affect the reproduction, physiology, and behavior of non-target organisms, such as fish. We aimed to evaluate the effects of exposure to environmentally relevant concentrations of levonorgestrel-control birth based (1.3, 13.3, 133, and 1330 ng/L) on the development and behavior of zebrafish (Danio rerio) in terms of mortality, hatching, spontaneous movement, and larval and adult behavioral tests. Exposure caused anxiogenic-like behavior in larvae, which persisted in adults, as demonstrated by the light-dark test. In contrast, it caused anxiolytic-like behavior in the novel tank test. There was a high mortality rate at all tested concentrations and increases in the hormone cortisol at 13.3 ng/L that affected the sex ratio. These changes may lead to an ecological imbalance, emphasizing the risk of early exposure to progestins in the environment.

Age-associated suppression of exploratory activity during sickness is linked to meningeal lymphatic dysfunction and microglia activation

Peripheral inflammation triggers a transient, well-defined set of behavioral changes known as sickness behavior^1-3, but the mechanisms by which inflammatory signals originating in the periphery alter activity in the brain remain obscure. Emerging evidence has established meningeal lymphatic vasculature as an important interface between the central nervous system (CNS) and the immune system, responsible for facilitating brain solute clearance and perfusion by cerebrospinal fluid (CSF)^4,5. Here, we demonstrate that meningeal lymphatics both assist microglial activation and support the behavioral response to peripheral inflammation. Ablation of meningeal lymphatics results in a heightened behavioral response to IL-1β-induced inflammation and a dampened transcriptional and morphological microglial phenotype. Moreover, our findings support a role for microglia in tempering the severity of sickness behavior with specific relevance to aging-related meningeal lymphatic dysfunction. Transcriptional profiling of brain myeloid cells shed light on the impact of meningeal lymphatic dysfunction on microglial activation. Furthermore, we demonstrate that experimental enhancement of meningeal lymphatic function in aged mice is sufficient to reduce the severity of exploratory abnormalities but not pleasurable consummatory behavior. Finally, we identify dysregulated genes and biological pathways, common to both experimental meningeal lymphatic ablation and aging, in microglia responding to peripheral inflammation that may result from age-related meningeal lymphatic dysfunction.

Fish Behavior as a Neural Proxy to Reveal Physiological States

Behaviors are the integrative outcomes of the nervous system, which senses and responds to the internal physiological status and external stimuli. Teleosts are aquatic organisms which are more easily affected by the surrounding environment compared to terrestrial animals. To date, behavioral tests have been widely used to assess potential environmental risks using fish as model animals. In this review, we summarized recent studies regarding the effects of internal and external stimuli on fish behaviors. We concluded that behaviors reflect environmental and physiological changes, which have possible implications for environmental and physiological assessments.

Zebrafish Larvae Behavior Models as a Tool for Drug Screenings and Pre-Clinical Trials: A Review

To discover new molecules or review the biological activity and toxicity of therapeutic substances, drug development, and research relies on robust biological systems to obtain reliable results. Phenotype-based screenings can transpose the organism’s compensatory pathways by adopting multi-target strategies for treating complex diseases, and zebrafish emerged as an important model for biomedical research and drug screenings. Zebrafish’s clear correlation between neuro-anatomical and physiological features and behavior is very similar to that verified in mammals, enabling the construction of reliable and relevant experimental models for neurological disorders research. Zebrafish presents highly conserved physiological pathways that are found in higher vertebrates, including mammals, along with a robust behavioral repertoire. Moreover, it is very sensitive to pharmacological/environmental manipulations, and these behavioral phenotypes are detected in both larvae and adults. These advantages align with the 3Rs concept and qualify the zebrafish as a powerful tool for drug screenings and pre-clinical trials. This review highlights important behavioral domains studied in zebrafish larvae and their neurotransmitter systems and summarizes currently used techniques to evaluate and quantify zebrafish larvae behavior in laboratory studies.

Social networks respond to a disease challenge in calves

Changes in network position and behavioral interactions have been linked with infectious disease in social animals. Here, we investigate the effects of an experimental disease challenge on social network centrality of group-housed Holstein bull dairy calves. Within group-housed pens (6/group) calves were randomly assigned to either a previously developed challenge model, involving inoculation with Mannheimia haemolytia (n = 12 calves; 3 calves/group) or a control involving only saline (n = 12 calves; 3 calves/group). Continuous behavioral data were recorded from video on pre-treatment baseline day and for 24 h following inoculation to describe social lying frequency and duration and all active social contact between calves. Mixed-model analysis revealed that changes in network position were related to the challenge. Compared to controls, challenged calves had reduced centrality and connectedness, baseline to challenge day. On challenge day, challenged calves were less central in the directed social contact networks (lower degree, strength and eigenvector centrality), and initiated contact (higher out-degree) with more penmates, compared to healthy calves. This finding suggests that giving rather than receiving affiliative social contact may be more beneficial for challenged calves. This is the first study demonstrating that changes in social network position coincide with an experimental challenge of a respiratory pathogen in calves.

Improving zebrafish laboratory welfare and scientific research through understanding their natural history

Globally, millions of zebrafish (Danio rerio) are used for scientific laboratory experiments for which researchers have a duty of care, with legal obligations to consider their welfare. Considering the growing use of the zebrafish as a vertebrate model for addressing a diverse range of scientific questions, optimising their laboratory conditions is of major importance for both welfare and improving scientific research. However, most guidelines for the care and breeding of zebrafish for research are concerned primarily with maximising production and minimising costs and pay little attention to the effects on welfare of the environments in which the fish are maintained, or how those conditions affect their scientific research. Here we review the physical and social conditions in which laboratory zebrafish are kept, identifying and drawing attention to factors likely to affect their welfare and experimental science. We also identify a fundamental lack knowledge of how zebrafish interact with many biotic and abiotic features in their natural environment to support ways to optimise zebrafish health and well-being in the laboratory, and in turn the quality of scientific data produced. We advocate that the conditions under which zebrafish are maintained need to become a more integral part of research and that we understand more fully how they influence experimental outcome and in turn interpretations of the data generated.

Peripheral NOD-like receptor deficient inflammatory macrophages trigger neutrophil infiltration into the brain disrupting daytime locomotion

Inflammation is known to disrupt normal behavior, yet the underlying neuroimmune interactions remain elusive. Here, we investigated whether inappropriate macrophage-evoked inflammation alters CNS control of daily-life animal locomotion using a set of zebrafish mutants selected for specific macrophage dysfunction and microglia deficiency. Large-scale genetic and computational analyses revealed that NOD-like receptor nlrc3l mutants are capable of normal motility and visuomotor response, but preferentially swim less in the daytime, suggesting possible low motivation rather than physical impairment. Examining their brain activities and structures implicates impaired dopaminergic descending circuits, where neutrophils abnormally infiltrate. Furthermore, neutrophil depletion recovered daytime locomotion. Restoring wild-type macrophages reversed behavioral and neutrophil aberrations, while three other microglia-lacking mutants failed to phenocopy nlrc3l mutants. Overall, we reveal how peripheral inflammatory macrophages with elevated pro-inflammatory cues (including il1β, tnfα, cxcl8a) in the absence of microglia co-opt neutrophils to infiltrate the brain, thereby potentially enabling local circuitry modulation affecting daytime locomotion.

Embryonic exposure to genistein induces anxiolytic and antisocial behavior in zebrafish: persistent effects until the adult stage

Genistein is a phytoestrogen, which is structurally similar to 17β-estradiol. It is present in plants, food, and as a contaminant in effluents. In this article, we demonstrate the effects of embryonic exposure to three different concentrations of genistein (10 μg/L, 40 μg/L, and 80 μg/L) which is similar to those found in effluents. Zebrafish eggs were exposed during the first 72 h post-fertilization (hpf). Heart rate was evaluated at 48 hpf and mortality rate was assessed during the first 72 hpf. The light/dark (LDT) and open field (OFT) behavioral tests were applied to the larvae (6 dpf), and the novel tank (NTT), social preference (SPT), light-dark (LDT), and sexing tests were performed on adult fish (90 dpf). Embryonic exposure to genistein caused anxiolytic-like behavior in both larvae and adult animals. In adult stage, we observed an increase in locomotor activity and antisocial behavior in the concentration of 40 μg/L. There was an increase in the mortality rate in all concentrations when compared to the control and an increase in heart rate at the concentration of 80 μg/L. Exposure to 10 μg/L generated a higher frequency of females when compared to the control group. Our results show that exposure to genistein during the embryonic phase brings damage in the short and long term as it increases the mortality rate and leads to behavioral disorders both in the larval stage, with perpetuation until adult stage. The anxiolytic-like effect and less social interaction are effects that harm fish survival.

Towards Modeling Anhedonia and Its Treatment in Zebrafish

Mood disorders, especially depression, are a major cause of human disability. The loss of pleasure (anhedonia) is a common, severely debilitating symptom of clinical depression. Experimental animal models are widely used to better understand depression pathogenesis and to develop novel antidepressant therapies. In rodents, various experimental models of anhedonia have already been developed and extensively validated. Complementing rodent studies, the zebrafish (Danio rerio) is emerging as a powerful model organism to assess pathobiological mechanisms of affective disorders, including depression. Here, we critically discuss the potential of zebrafish for modeling anhedonia and studying its molecular mechanisms and translational implications.

Sickness and the social brain: How the immune system regulates behavior across species

Many instances of sickness critically involve the immune system. The immune system talks to the brain in a bi-directional loop. This discourse affords the immune system immense control, such that it can influence behavior and optimize recovery from illness. These behavioral responses to infection are called sickness behaviors and can manifest in many ways, including changes in mood, motivation, or energy. Fascinatingly, most of these changes are conserved across species, and most organisms demonstrate some form of sickness behaviors. One of the most interesting sickness behaviors, and not immediately obvious, is altered sociability. Here, we discuss how the immune system impacts social behavior, by examining the brain regions and immune mediators involved in this process. We first outline how social behavior changes in response to infection in various species. Next, we explore which brain regions control social behavior and their evolutionary origins. Finally, we describe which immune mediators establish the link between illness and social behavior, in the context of both normal development and infection. Overall, we hope to make clear the striking similarities between the mechanisms that facilitate changes in sociability in derived and ancestral vertebrate, as well as invertebrate, species.

Tilapia Lake Virus-Induced Neuroinflammation in Zebrafish: Microglia Activation and Sickness Behavior

In mammals, the relationship between the immune system and behavior is widely studied. In fish, however, the knowledge concerning the brain immune response and behavioral changes during brain viral infection is very limited. To further investigate this subject, we used the model of tilapia lake virus (TiLV) infection of zebrafish (Danio rerio), which was previously developed in our laboratory. We demonstrated that TiLV persists in the brain of adult zebrafish for at least 90 days, even when the virus is not detectable in other peripheral organs. The virions were found in the whole brain. During TiLV infection, zebrafish displayed a clear sickness behavior: decreased locomotor activity, reduced food intake, and primarily localizes near the bottom zone of aquaria. Moreover, during swimming, individual fish exhibited also unusual spiral movement patterns. Gene expression study revealed that TiLV induces in the brain of adult fish strong antiviral and inflammatory response and upregulates expression of genes encoding microglia/macrophage markers. Finally, using zebrafish larvae, we showed that TiLV infection induces histopathological abnormalities in the brain and causes activation of the microglia which is manifested by changes in cell shape from a resting ramified state in mock-infected to a highly ameboid active state in TiLV-infected larvae. This is the first study presenting a comprehensive analysis of the brain immune response associated with microglia activation and subsequent sickness behavior during systemic viral infection in zebrafish.

Neuromodulation by the immune system: a focus on cytokines

Interactions between the immune system and the nervous system have been described mostly in the context of diseases. More recent studies have begun to reveal how certain immune cell-derived soluble effectors, the cytokines, can influence host behaviour even in the absence of infection. In this Review, we contemplate how the immune system shapes nervous system function and how it controls the manifestation of host behaviour. Interactions between these two highly complex systems are discussed here also in the context of evolution, as both may have evolved to maximize an organism's ability to respond to environmental threats in order to survive. We describe how the immune system relays information to the nervous system and how cytokine signalling occurs in neurons. We also speculate on how the brain may be hardwired to receive and process information from the immune system. Finally, we propose a unified theory depicting a co-evolution of the immune system and host behaviour in response to the evolutionary pressure of pathogens.

Sickness behaviors across vertebrate taxa: proximate and ultimate mechanisms

There is nothing like a pandemic to get the world thinking about how infectious diseases affect individual behavior. In this respect, sick animals can behave in ways that are dramatically different from healthy animals: altered social interactions and changes to patterns of eating and drinking are all hallmarks of sickness. As a result, behavioral changes associated with inflammatory responses (i.e. sickness behaviors) have important implications for disease spread by affecting contacts with others and with common resources, including water and/or sleeping sites. In this Review, we summarize the behavioral modifications, including changes to thermoregulatory behaviors, known to occur in vertebrates during infection, with an emphasis on non-mammalian taxa, which have historically received less attention. We then outline and discuss our current understanding of the changes in physiology associated with the production of these behaviors and highlight areas where more research is needed, including an exploration of individual and sex differences in the acute phase response and a greater understanding of the ecophysiological implications of sickness behaviors for disease at the population level.

Stimulants cocktail: Methylphenidate plus caffeine impairs memory and cognition and alters mitochondrial and oxidative status

Methylphenidate (MPH) is a psychostimulant widely misused to increase wakefulness by drivers and students. Also, MPH can be found in dietary supplements in a clandestine manner aiming to burst performance of physical exercise practitioners. The abusive use of high doses of caffeine (CAF) in these contexts is equally already known. Here, we demonstrate the behavioral, oxidative and mitochondrial effects after acute exposure to high doses of MPH (80 mg/L) and CAF (150 mg/L), alone or associated (80 mg/L + 150 mg/L, respectively). We used zebrafish as animal model due to its high translational relevance. We evaluated the behavioral effects using the Novel Tank Test (NTT), Social Preference Test (SPT) and Y-maze Task and analyzed biomarkers of oxidative stress and activity of mitochondrial respiratory chain complexes. MPH alone induced antisocial behavior. MPH inhibited lipid peroxidation. The association of MPH + CAF presented memory impairment and anxiogenic behavior. In oxidative status, it inhibited lipid peroxidation, increased protein carbonylation and mitochondrial complex II, III and IV activity. Our results demonstrate that MPH and CAF alone negatively impact the typical behavioral of zebrafish. When associated, changes in cognition, memory, oxidative and mitochondrial status are more relevant.

Effects of Metformin on Life Span, Cognitive Ability, and Inflammatory Response in a Short-Lived Fish

Metformin, an oral antidiabetic drug, prolongs the life span in nematode, silkworm, and other transgenic rodents, but its effects on longevity and aging-related cognitive ability using natural aging vertebrate models remain poorly understood. The genus of annual fish Nothobranchius show accelerated growth and expression of aging biomarkers. Here, using the short-lived fish Nothobranchius guentheri, we investigated effects of metformin on life span and aging-related cognitive ability and inflammation. Total of 145 fish, 72 fish were fed with metformin in the concentration of 2 mg/g food and 73 fish without metformin from 16 weeks of age until the end of their lives. The chronic feeding with metformin prolonged the life span of the fish and delayed aging with retarded accumulation of lipofuscin in liver, senescence-associated beta-galactosidase (SA-β-gal) activity in skin and serum levels of cholesterol and triglyceride significantly in the 10-month-old fish. Furthermore, metformin improved motor, learning, and memory skills by behavior tests accompanying with reduction of SA-β-gal activity and neurofibrillary degeneration and inhibition of inflammatory response including downregulated NF-κB and proinflammatory cytokines IL-8, TNF-α, and IL-1β expression and enhanced anti-inflammatory cytokine IL-10 level in brain. These findings demonstrate that metformin prolongs the life span and exerts neuroprotective and anti-inflammation function to improve cognitive ability in annual fish. It might be an effective strategy by using metformin to raise the possibility of promoting healthy aging of old population in aging process.

Molecular links between endocrine, nervous and immune system during chronic stress

INTRODUCTION

The stress response is different in various individuals, however, the mechanisms that could explain these distinct effects are not well known and the molecular correlates have been considered one at the time. Particular harmful conditions occur if the subject, instead to cope the stressful events, succumb to them, in this case, a cascade reaction happens that through different signaling causes a specific reaction named "sickness behaviour." The aim of this article is to review the complex relations among important molecules belonging to Central nervous system (CNS), immune system (IS), and endocrine system (ES) during the chronic stress response.

METHODS

After having verified the state of art concerning the function of cortisol, norepinephrine (NE), interleukin (IL)-1β and melatonin, we describe as they work together.

RESULTS

We propose a speculative hypothesis concerning the complex interplay of these signaling molecules during chronic stress, highlighting the role of IL-1β as main biomarker of this effects, indeed, during chronic stress its increment transforms this inflammatory signal into a nervous signal (NE), in turn, this uses the ES (melatonin and cortisol) to counterbalance again IL-1β. During cortisol resistance, a vicious loop occurs that increments all mediators, unbalancing IS, ES, and CNS networks. This IL-1β increase would occur above all when the individual succumbs to stressful events, showing the Sickness Behaviour Symptoms. IL-1β might, through melatonin and vice versa, determine sleep disorders too.

CONCLUSION

The molecular links here outlined could explain how stress plays a role in etiopathogenesis of several diseases through this complex interplay.

Social Preference Tests in Zebrafish: A Systematic Review

The use of animal models in biology research continues to be necessary for the development of new technologies and medicines, and therefore crucial for enhancing human and animal health. In this context, the need to ensure the compliance of research with the principles Replacement, Reduction and Refinement (the 3 Rs), which underpin the ethical and human approach to husbandry and experimental design, has become a central issue. The zebrafish (Danio rerio) is becoming a widely used model in the field of behavioral neuroscience. In particular, studying zebrafish social preference, by observing how an individual fish interacts with conspecifics, may offer insights into several neuropsychiatric and neurodevelopmental disorders. The main aim of this review is to summarize principal factors affecting zebrafish behavior during social preference tests. We identified three categories of social research using zebrafish: studies carried out in untreated wild-type zebrafish, in pharmacologically treated wild-type zebrafish, and in genetically engineered fish. We suggest guidelines for standardizing social preference testing in the zebrafish model. The main advances gleaned from zebrafish social behavior testing are discussed, together with the relevance of this method to scientific research, including the study of behavioral disorders in humans. The authors stress the importance of adopting an ethical approach that considers the welfare of animals involved in experimental procedures. Ensuring a high standard of animal welfare is not only good for the animals, but also enhances the quality of our science.

Using zebrafish (Danio rerio) models to understand the critical role of social interactions in mental health and wellbeing

Social behavior represents a beneficial interaction between conspecifics that is critical for maintaining health and wellbeing. Dysfunctional or poor social interaction are associated with increased risk of physical (e.g., vascular) and psychiatric disorders (e.g., anxiety, depression, and substance abuse). Although the impact of negative and positive social interactions is well-studied, their underlying mechanisms remain poorly understood. Zebrafish have well-characterized social behavior phenotypes, high genetic homology with humans, relative experimental simplicity and the potential for high-throughput screens. Here, we discuss the use of zebrafish as a candidate model organism for studying the fundamental mechanisms underlying social interactions, as well as potential impacts of social isolation on human health and wellbeing. Overall, the growing utility of zebrafish models may improve our understanding of how the presence and absence of social interactions can differentially modulate various molecular and physiological biomarkers, as well as a wide range of other behaviors.

Dose- and time-dependent effects of an immune challenge on fish across biological levels

Due to global changes, fish are increasingly exposed to immune challenges associated with disease outbreaks in aquatic ecosystems. Adjustments in physiology and behavior are generally critical to maintaining homeostasis after an immune challenge, but there is limited knowledge on the specific thresholds and dynamics of responses across levels of biological organization in fish. In this study, we tested how different concentrations of an antigens mixture (phytohemagglutinin and lipopolysaccharide) affected innate immunity with potential consequences on oxidative stress, energy reserves, body condition, and behavior across time, using the common gudgeon (Gobio sp.) as model species. The immune challenge induced a transitory increase in lytic enzyme activity (i.e., lysozyme) and local immune response (i.e., skin swelling) 2 days after the antigen injection. The available energy stored in muscle was also reduced 4 days after injection, without inducing oxidative stress at the cellular level. Overall, the immune challenge induced limited costs at the molecular and cellular levels but had strong effects at the whole organism level, especially on behavior. Indeed, fish swimming activity and sociability were affected in a dose- and time-dependent manner. These results suggest that immune challenges have dose-dependent effects across levels of biological organization and that behavior is a key response trait to cope with pathogen-induced immune costs in the wild, although fitness consequences remain to be tested.

CHRONIC EXPOSURE TO METHYLPHENIDATE-CONTAMINATED WATER ELICITS SOCIAL IMPAIRMENT TO ZEBRAFISH

Residual contamination of water with MPH represents a severe environmental issue because it can affect non-target animals. Here we describe the behavioral effects in zebrafish after chronic contamination of water containing residues of MPH (0.1875, 1.875 and 3 ug/L). These doses are environmentally relevant since they reflect those found in wastewaters. We evaluated the behavioral effect through the novel tank test (NTT) and social preference test (SPT), and after euthanasia we analyzed oxidative stress parameters. Zebrafish exposed to MPH presented a social impairment, avoiding the conspecifics segment in the social preference test. In addition, MPH in the lowest concentration provoked an anxiolytic effect in the novel tank test. Oxidative stress is not related to these changes. Since the maintenance of an intact behavioral repertoire is crucial for species survival and fitness, our results demonstrate that residual contamination of water by MPH can be a threat to zebrafish, impacting directly to its well-being and survival in the aquatic environment.

Effects of Pseudoloma neurophilia infection on the brain transcriptome in zebrafish (Danio rerio)

Laboratory zebrafish are commonly infected with the intracellular, brain-infecting microsporidian parasite Pseudoloma neurophilia. Chronic P. neurophilia infections induce inflammation in meninges, brain and spinal cord, and have been suggested to affect neural functions since parasite clusters reside inside neurons. However, underlying neural and immunological mechanisms associated with infection have not been explored. Utilizing RNA-sequencing analysis, we found that P. neurophilia infection upregulated 175 and downregulated 45 genes in the zebrafish brain, compared to uninfected controls. Four biological pathways were enriched by the parasite, all of which were associated with immune function. In addition, 14 gene ontology (GO) terms were enriched, eight of which were associated with immune responses and five with circadian rhythm. Surprisingly, no differentially expressed genes or enriched pathways were specific for nervous system function. Upregulated immune-related genes indicate that the host generally show a pro-inflammatory immune response to infection. On the other hand, we found a general downregulation of immune response genes associated with anti-pathogen functions, suggesting an immune evasion strategy by the parasite. The results reported here provide important information on host-parasite interaction and highlight possible pathways for complex effects of parasite infections on zebrafish phenotypes.

Comparison of bacterial lipopolysaccharide-induced sickness behavior in rodents and humans: Relevance for symptoms of anxiety and depression

Increasing evidence from animal and human studies suggests that inflammation may be involved in mood disorders. Sickness behavior and emotional changes induced by experimental inflammatory stimuli have been extensively studied in humans and rodents to better understand the mechanisms underlying inflammation-driven mood alterations. However, research in animals and humans have remained compartmentalized and a comprehensive comparison of inflammation-induced sickness and depressive-like behavior between rodents and humans is lacking. Thus, here, we highlight similarities and differences in the effects of bacterial lipopolysaccharide administration on the physiological (fever and cytokines), behavioral and emotional components of the sickness response in rodents and humans, and discuss the translational challenges involved. We also emphasize the differences between observable sickness behavior and subjective sickness reports, and advocate for the need to obtain both subjective reports and objective measurements of sickness behavior in humans. We aim to provide complementary insights for translational clinical and experimental research on inflammation-induced behavioral and emotional changes, and their relevance for mood disorders such as depression.

The zebrafish subcortical social brain as a model for studying social behavior disorders

Social behaviors are essential for the survival and reproduction of social species. Many, if not most, neuropsychiatric disorders in humans are either associated with underlying social deficits or are accompanied by social dysfunctions. Traditionally, rodent models have been used to model these behavioral impairments. However, rodent assays are often difficult to scale up and adapt to high-throughput formats, which severely limits their use for systems-level science. In recent years, an increasing number of studies have used zebrafish (Danio rerio) as a model system to study social behavior. These studies have demonstrated clear potential in overcoming some of the limitations of rodent models. In this Review, we explore the evolutionary conservation of a subcortical social brain between teleosts and mammals as the biological basis for using zebrafish to model human social behavior disorders, while summarizing relevant experimental tools and assays. We then discuss the recent advances gleaned from zebrafish social behavior assays, the applications of these assays to studying related disorders, and the opportunities and challenges that lie ahead.