Bold zebrafish (Danio rerio) express higher levels of delta opioid and dopamine D2 receptors in the brain compared to shy fish

Individual differences in the boldness of female zebrafish are associated with alterations in serotonin function

One of the most prevalent axes of behavioral variation in both humans and animals is risk taking, where individuals that are more willing to take risk are characterized as bold while those that are more reserved are regarded as shy. Brain monoamines (i.e. serotonin, dopamine and noradrenaline) have been found to play a role in a variety of behaviors related to risk taking. Using zebrafish, we investigated whether there was a relationship between monoamine function and boldness behavior during exploration of a novel tank. We found a correlation between serotonin metabolism (5-HIAA:5-HT ratio) and boldness during the initial exposure to the tank in female animals. The DOPAC:DA ratio correlated with boldness behavior on the third day in male fish. There was no relationship between boldness and noradrenaline. To probe differences in serotonergic function in bold and shy fish, we administered a selective serotonin reuptake inhibitor, escitalopram, and assessed exploratory behavior. We found that escitalopram had opposing effects on thigmotaxis in bold and shy female animals: the drug caused bold fish to spend more time near the center of the tank and shy fish spent more time near the periphery. Taken together, our findings indicate that variation in serotonergic function has sex-specific contributions to individual differences in risk-taking behavior.

Locomotor Behavior and Memory Dysfunction Induced by 3-Nitropropionic Acid in Adult Zebrafish: Modulation of Dopaminergic Signaling

Huntington's disease (HD) is a progressive neurodegenerative disease characterized by neuropsychiatric disturbance, cognitive impairment, and locomotor dysfunction. In the early stage (chorea) of HD, expression of dopamine D2 receptors (D2R) is reduced, whereas dopamine (DA) levels are increased. Contrary, in the late stage (bradykinesia), DA levels and the expression of D2R and dopamine D1 receptors (D1R) are reduced. 3-Nitropropionic acid (3-NPA) is a toxin that may replicate HD behavioral phenotypes and biochemical aspects. This study assessed the neurotransmitter levels, dopamine receptor gene expression, and the effect of acute exposure to quinpirole (D2R agonist) and eticlopride (D2R antagonist) in an HD model induced by 3-NPA in adult zebrafish. Quinpirole and eticlopride were acutely applied by i.p. injection in adult zebrafish after chronic treatment of 3-NPA (60 mg/kg). 3-NPA treatment caused a reduction in DA, glutamate, and serotonin levels. Quinpirole reversed the bradykinesia and memory loss induced by 3-NPA. Together, these data showed that 3-NPA acts on the dopaminergic system and causes biochemical alterations similar to late-stage HD. These data reinforce the hypothesis that DA levels are linked with locomotor and memory deficits. Thus, these findings may suggest that the use of DA agonists could be a pharmacological strategy to improve the bradykinesia and memory deficits in the late-stage HD.

Trait sensitivity to stress and cognitive bias processes in fish: A brief overview

Like other animals, fish have unique personalities that can affect their cognition and responses to environmental stressors. These individual personality differences are often referred to as "behavioural syndromes" or "stress coping styles" and can include personality traits such as boldness, shyness, aggression, exploration, locomotor activity, and sociability. For example, bolder or proactive fish may be more likely to take risks and present lower hypothalamo-pituitary-adrenal/interrenal axis reactivity as compared to shy or reactive individuals. Likewise, learning and memory differ between fish personalities. Reactive or shy individuals tend to have faster learning and better association recall with aversive stimuli, while proactive or bold individuals tend to learn more quickly when presented with appetitive incentives. However, the influence of personality on cognitive processes other than cognitive achievement in fish has been scarcely explored. Cognitive bias tests have been employed to investigate the interplay between emotion and cognition in both humans and animals. Fish present cognitive bias processes (CBP) in which fish's interpretation of stimuli could be influenced by its current emotional state and open to environmental modulation. However, no study in fish has explored whether CBP, like in other species, can be interpreted as long-lasting traits and whether other individual characteristics may explain its variation. We hold the perspective that CBP could serve as a vulnerability factor for the onset, persistence, and recurrence of stress-related disorders. Therefore, studying fish's CBP as a state or trait and its interactions with individual variations may be valuable in future efforts to enhance our understanding of anxiety and stress neurobiology in animal models and humans.

Revisiting the role of behavior-mediated structuring in the survival of populations in hostile environments

Increasing the population density of target species is a major goal of ecosystem and agricultural management. This task is especially challenging in hazardous environments with a high abundance of natural enemies such as parasites and predators. Safe locations with lower mortality have been long considered a beneficial factor in enhancing population survival, being a promising tool in commercial fish farming and restoration of threatened species. Here we challenge this opinion and revisit the role of behavior structuring in a hostile environment in shaping the population density. We build a mathematical model, where individuals are structured according to their defensive tactics against natural enemies. The model predicts that although each safe zone enhances the survival of an individual, for an insufficient number of such zones, the entire population experiences a greater overall mortality. This is a result of the interplay of emergent dynamical behavioral structuring and strong intraspecific competition for safe zones. Non-plastic structuring in individuals' boldness reduces the mentioned negative effects. We demonstrate emergence of non-plastic behavioral structuring: the evolutionary branching of a monomorphic population into a dimorphic one with bold/shy strains. We apply our modelling approach to explore fish farming of salmonids in an environment infected by trematode parasites.

The Application of Synthetic Flavors in Zebrafish (Danio rerio) Rearing with Emphasis on Attractive Ones: Effects on Fish Development, Welfare, and Appetite

The aim of the present study was to test synthetic flavors as potential feed attractants in zebrafish (Danio rerio) during early development. Six experimental groups were set up in triplicate: (i) a CTRL group fed a zebrafish commercial diet; (ii) a PG group fed a control diet added with Propylene Glycol (PG); (iii) A1+ and A2+ groups fed a control diet added with 1% of the two attractive flavors (A1+ cheese odor made by mixing Propylene Glycol (PG) with the aromatic chemicals trimethyamine, 2-acetylpyrazine, 2-acetylpyridine, and dimethyl sulfide; and A2+ caramel odor, made of PG mixed with the aromatic chemicals vanillin, maltol, cyclotene, acetoin, butyric acid, and capric acid with traces of both gamma-octalactone and gamma-esalactone) or the repulsive flavor (A- coconut odor, made by mixing PG with the aromatic chemicals gamma-eptalactone, gamma-nonalactone, delta-esalactone, and vanillin with trace of both delta-octalactone and maltol), respectively; (iv) an ROT group fed the two attractive diets, each administered singularly in a weekly rotation scheme. All the tested synthetic flavors did not affect the overall health of larval and juvenile fish and promoted growth. Due to the longer exposure time, results obtained from the juvenile stage provided a clearer picture of the fish responses: zebrafish fed both attractive diets showed higher appetite stimulus, feed ingestion, and growth, while the brain dopaminergic activity suggested the A2+ diet as the most valuable solution for its long-lasting effect over the whole experiment (60-day feeding trial, from larvae to adults). The present study provided important results about the possible use of attractive synthetic flavors for aquafeed production, opening new sustainable and more economically valuable opportunities for the aquaculture sector.

Effects of ocean acidification on dopamine-mediated behavioral responses of a coral reef damselfish

We investigated whether CO₂-induced ocean acidification (OA) affects dopamine receptor-dependent behavior in bicolor damselfish (Stegastes partitus). Damselfish were kept in aquaria receiving flow through control (pH ~ 8.03; pCO₂ ~ 384 μatm) or OA (pH ~ 7.64; CO₂ ~ 1100 μatm) seawater at a rate of 1 L min^-1. Despite this relatively fast flow rate, fish respiration further acidified the seawater in both control (pH ~7.88; pCO₂ ~ 595 μatm) and OA (pH ~7.55; pCO₂ ~ 1450 μatm) fish-holding aquaria. After five days of exposure, damselfish locomotion, boldness, anxiety, and aggression were assessed using a battery of behavioral tests using automated video analysis. Two days later, these tests were repeated following application of the dopamine D₁ receptor agonist SKF 38393. OA-exposure induced ceiling anxiety levels that were significantly higher than in control damselfish, and SKF 38393 increased anxiety in control damselfish to a level not significantly different than that of OA-exposed damselfish. Additionally, SKF 38393 decreased locomotion and increased boldness in control damselfish but had no effect in OA-exposed damselfish, suggesting an alteration in activity of dopaminergic pathways that regulate behavior under OA conditions. These results indicate that changes in dopamine D₁ receptor function affects fish behavior during exposure to OA. However, subsequent measurements of seawater sampled using syringes during the daytime (~3-4 pm local time) from crevasses in coral reef colonies, which are used as shelter by damselfish, revealed an average pH of 7.73 ± 0.03 and pCO₂ of 925.8 ± 62.2 μatm; levels which are comparable to Representative Concentration Pathway (RCP) 8.5 predicted end-of-century mean OA levels in the open ocean. Further studies considering the immediate environmental conditions experienced by fish as well as individual variability and effect size are required to understand potential implications of the observed OA-induced behavioral effects on damselfish fitness in the wild.

Identifying knowledge gaps in understanding the effects of selective serotonin reuptake inhibitors (SSRIs) on fish behaviour

Selective serotonin reuptake inhibitors (SSRIs) are a class of antidepressants increasingly prescribed to treat patients with clinical depression. As a result of the significant negative impact of the COVID-19 pandemic on the population's mental health, its consumption is expected to increase even more. The high consumption of these substances leads to their environmental dissemination, with evidence of their ability to compromise molecular, biochemical, physiological, and behavioural endpoints in non-target organisms. This study aimed to provide a critical review of the current knowledge regarding the effects of SSRI antidepressants on fish ecologically relevant behaviours and personality-dependent traits. A literature review shows limited data concerning the impact of fish personality on their responses to contaminants and how such responses could be influenced by SSRIs. This lack of information may be attributable to a lack of widely adopted standardized protocols for evaluating behavioural responses in fish. The existing studies examining the effects of SSRIs across various biological levels overlook the intra-specific variations in behaviour and physiology associated with different personality patterns or coping styles. Consequently, some effects may remain undetected, such as variations in coping styles and the capacity to handle environmental stressors. This oversight could potentially result in long-term effects with ecological implications. Data support the need for more studies to understand the impact of SSRIs on personality-dependent traits and how they may impair fitness-related behaviours. Given the considerable cross-species similarity in the personality dimensions, the collected data may allow new insights into the correlation between personality and animal fitness.

Exposure of zebrafish to an environmental mixture of persistent organic pollutants triggers an increase in anxiety-like syndrome but does not affect boldness in unexposed offspring

Polychlorinated biphenyls (PCBs) and polybrominated diphenyl ethers (PBDEs) are persistent organic pollutants (POPs) that are present as complex mixtures in all environmental compartments, including aquatic ecosystems. However, little is known about the effects of such complex mixtures on teleost behaviour. In this study, zebrafish (Danio rerio) were chronically exposed to an environmentally relevant mixture (MIX) containing 22 PCB and 7 PBDE congeners through diet from 5 days post fertilization onwards. MIX-exposed F0 fish produced offspring (F1 and F2 generations) that were fed using plain food and grown until adulthood. In each generation, five behavioural traits (i.e. boldness, activity, sociality, exploration and anxiety) were evaluated by the mean of different experimental set-ups. Two distinct behavioural syndromes were identified: boldness, positively correlated to activity and exploration; and anxiety, associated with low sociality. F0 fish did not display any behavioural disruption resulting from POP exposure whereas F1 MIX fish were bolder than fish from other generations but did not differ significantly from F1 controls. F2 MIX fish displayed a higher anxiety syndrome than F2 controls. This is of particular importance since such behavioural changes in offspring generations may have persistent ecological consequences, may affect fitness and hence cause detrimental effects on wild fish populations exposed to POP mixtures.

Effects of different acute stressors on the regulation of appetite genes in the carp (Cyprinus carpio L.) brain

Our understanding of the timing of stress responses and specific roles of different regulatory pathways that drive stress responses is incomplete. In particular, the regulation of appetite genes as a consequence of exposure to different stressors has not been studied in sufficient detail in fish. Therefore, a stress trial was conducted with koi carp, aiming at identifying typical effects of stress on regulation of appetite genes. The stressors tank manipulation, air exposure and feed rewarding were chosen. The responses to these stressors were evaluated 10, 30 and 60 min after the stressors were applied. Orexigenic and anorexigenic genes were investigated in four different brain regions (telencephalon, hypothalamus, optic tectum and rhombencephalon). The results show that, apart from the typical appetite regulation in the hypothalamus, the different brain regions also display pronounced responses of appetite genes to the different stressors. In addition, several genes in the serotonergic, dopaminergic and gaba-related pathways were investigated. These genes revealed that rearing in pairs of two and opening of the tank lid affected anorexigenic genes, such as cart and cck, which were not changed by air exposure or feed rewarding. Moreover, distress and eustress led to limited, but distinguishable gene expression pattern changes in the investigated brain regions.

Comparative behavioral ecotoxicology of Inland Silverside larvae exposed to pyrethroids across a salinity gradient

Pyrethroids, a class of commonly used insecticides, are frequently detected in aquatic environments, including estuaries. The influence that salinity has on organism physiology and the partitioning of hydrophobic chemicals, such as pyrethroids, has driven interest in how toxicity changes in saltwater compared to freshwater. Early life exposures in fish to pyrethroids cause toxicity at environmentally relevant concentrations, which can alter behavior. Behavior is a highly sensitive endpoint that influences overall organism fitness and can be used to detect toxicity of environmentally relevant concentrations of aquatic pollutants. Inland Silversides (Menidia beryllina), a commonly used euryhaline model fish species, were exposed from 5 days post fertilization (~1-day pre-hatch) for 96 h to six pyrethroids: bifenthrin, cyfluthrin, cyhalothrin, cypermethrin, esfenvalerate and permethrin. Exposures were conducted at three salinities relevant to brackish, estuarine habitat (0.5, 2, and 6 PSU) and across 3 concentrations, either 0.1, 1, 10, and/or 100 ng/L, plus a control. After exposure, Inland Silversides underwent a behavioral assay in which larval fish were subjected to a dark and light cycle stimuli to determine behavioral toxicity. Assessment of total distanced moved and thigmotaxis (wall hugging), used to measure hyper/hypoactivity and anxiety like behavior, respectively, demonstrate that even at the lowest concentration of 0.1 ng/L pyrethroids can induce behavioral changes at all salinities. We found that toxicity decreased as salinity increased for all pyrethroids except permethrin. Additionally, we found evidence to suggest that the relationship between log K_OW and thigmotaxis is altered between the lower and highest salinities.

Evolutionary implications of size-selective mortality on the ontogenetic development of shoal cohesion: a neurochemical approach using a zebrafish, Danio rerio, harvest selection experiment

Abstract

Size-selective mortality may evolutionarily alter life-history as well as individual behavioral and physiological traits. Moreover, size-selective mortality can affect group behavioral traits, such as shoaling and collective properties (e.g., shoal cohesion), which are relevant for finding food and reducing risk of predation. Here, we present experimental evidence using selection lines of zebrafish (Danio rerio) that were exposed to positive (large-harvested), negative (small-harvested), and random (control) size-selective mortality for five generations, followed by eight generations during which harvesting was halted to remove maternal effects and to study evolutionarily fixed outcomes. We investigated changes in shoal cohesion and turnover in monoamines in zebrafish through ontogeny. To that end, we repeatedly measured inter-individual distance in groups of eight fish and the turnovers of dopamine and serotonin in brains of fish from juvenile to the adult stage at 40-day intervals. We, firstly, found that shoal cohesion was overall consistent through ontogeny at group levels suggesting the presence of collective personality. Secondly, we found a decrease in shoal cohesion through ontogeny in the small-harvested and control lines, while the large-harvested line did not show any ontogenetic change. Thirdly, the selection lines did not differ among each other in shoal cohesion at any ontogenetic stage. Fourthly, dopamine turnover increased through ontogeny in a similar way for all lines while the serotonin turnover decreased in the large-harvested and control lines, but not in the small-harvested line. The large-harvested line also had higher serotonin turnover than controls at specific time periods. In conclusion, intensive size-selective mortality left an evolutionary legacy of asymmetric selection responses in the ontogeny of shoal cohesion and the underlying physiological mechanisms in experimentally harvested zebrafish in the laboratory.

Significant statement

The evolution of animal behavior can be affected by human activities both at behavioral and physiological levels, but causal evidence is scarce and mostly focusing on single life-stages. We studied whether and to what extent size-selective harvesting, a common selection pattern in fisheries, can be an evolutionary driver of the development of shoal cohesion during ontogeny. We used a multi-generation experiment with zebrafish to study cause-and-effects of opposing size-selection patterns. We quantified shoal cohesion, and serotonin and dopamine turnover in the brain. We found that shoal cohesion emerged as a collective personality trait and that behavioral and physiological responses were asymmetrical with respect to the opposing selection patterns.

Beyond bold versus shy: Zebrafish exploratory behavior falls into several behavioral clusters and is influenced by strain and sex

Individual differences in exploratory behavior have been found across a range of taxa and are thought to contribute to evolutionary fitness. Animals that explore more of a novel environment and visit areas of high predation risk are considered bold, whereas animals with the opposite behavioral pattern are shy. Here, we determined whether this bimodal characterization of bold versus shy adequately captures the breadth of behavioral variation in zebrafish or if there are more than these two subtypes. To identify behavioral categories, we applied unsupervised machine to three-dimensional swim traces from over 400 adult zebrafish across four strains (AB, TL, TU, and WIK) and both sexes. We found that behavior stratified into four distinct clusters: previously described bold and shy behavior and two new behavioral types we call wall-huggers and active explorers. Clusters were stable across time and influenced by strain and sex where we found that TLs were shy, female TU fish were bold, male TU fish were active explorers, and male ABs were wall-huggers. Our work suggests that zebrafish exploratory behavior has greater complexity than previously recognized and lays the groundwork for the use of zebrafish in understanding the biological basis of individual differences in behavior.

Summary: Prior work described individual differences in zebrafish exploratory behavior as bold or shy. Here, we find four categories better describe their behavior: bold, shy, active explorers, and wall-huggers.

Constitutive gene expression differs in three brain regions important for cognition in neophobic and non-neophobic house sparrows (Passer domesticus)

Neophobia (aversion to new objects, food, and environments) is a personality trait that affects the ability of wildlife to adapt to new challenges and opportunities. Despite the ubiquity and importance of this trait, the molecular mechanisms underlying repeatable individual differences in neophobia in wild animals are poorly understood. We evaluated wild-caught house sparrows (Passer domesticus) for neophobia in the lab using novel object tests. We then selected a subset of neophobic and non-neophobic individuals (n = 3 of each, all females) and extracted RNA from four brain regions involved in learning, memory, threat perception, and executive function: striatum, caudal dorsomedial hippocampus, medial ventral arcopallium, and caudolateral nidopallium (NCL). Our analysis of differentially expressed genes (DEGs) used 11,889 gene regions annotated in the house sparrow reference genome for which we had an average of 25.7 million mapped reads/sample. PERMANOVA identified significant effects of brain region, phenotype (neophobic vs. non-neophobic), and a brain region by phenotype interaction. Comparing neophobic and non-neophobic birds revealed constitutive differences in DEGs in three of the four brain regions examined: hippocampus (12% of the transcriptome significantly differentially expressed), striatum (4%) and NCL (3%). DEGs included important known neuroendocrine mediators of learning, memory, executive function, and anxiety behavior, including serotonin receptor 5A, dopamine receptors 1, 2 and 5 (downregulated in neophobic birds), and estrogen receptor beta (upregulated in neophobic birds). These results suggest that some of the behavioral differences between phenotypes may be due to underlying gene expression differences in the brain. The large number of DEGs in neophobic and non-neophobic birds also implies that there are major differences in neural function between the two phenotypes that could affect a wide variety of behavioral traits beyond neophobia.

Clozapine modulation of zebrafish swimming behavior and gene expression as a case study to investigate effects of atypical drugs on aquatic organisms

Mental illnesses affect more than 150 million people in Europe and lead to an increasing consumption of neuroactive drugs during the last twenty years. The antipsychotic compound, clozapine, is one of the most used psychotropic drugs worldwide, with potentially negative consequences for the aquatic environment. Hence, the objectives of the study presented here were the quantification of clozapine induced changes in swimming behavior of exposed Danio rerio embryos and the elucidation of the molecular effects on the serotonergic and dopaminergic systems. Yolk-sac larvae were exposed to different concentrations (0.2 mg/L, 0.4 mg/L, 0.8 mg/L, 1.6 mg/L, 3.2 mg/L and 6.4 mg/L) of clozapine for 116 h post-fertilization, and changes in the swimming behavior of the larvae were assessed. Further, quantitative real-time PCR was performed to analyze the expression of selected genes. The qualitative evaluation of changes in the swimming behavior of D. rerio larvae revealed a significant decrease of the average swimming distance and velocity in the light-dark transition test, with more than a 36% reduction at the highest exposure concentration of 6.4 mg/L. Furthermore, the total larval body length was reduced at the highest concentration. An in-depth analysis based on expression of selected target genes of the serotonin (slc6a4a) and dopamine (drd2a) system showed an upregulation at a concentration of 1.6 mg/L and above. In addition, a lower increase in expression was detected for biomarkers of general stress (adra1a and cyp1a2). Our data show that exposure to clozapine during development inhibits swimming activity of zebrafish larvae, which could, in part, be due to disruption of the serotonin- and dopamine system.

Aggressive but not reproductive boldness in male green anole lizards correlates with baseline vasopressin activity

Across species, individuals within a population differ in their level of boldness in social encounters with conspecifics. This boldness phenotype is often stable across both time and social context (e.g., reproductive versus agonistic encounters). Various neural and hormonal mechanisms have been suggested as underlying these stable phenotypic differences, which are often also described as syndromes, personalities, and coping styles. Most studies examining the neuroendocrine mechanisms associated with boldness examine subjects after they have engaged in a social interaction, whereas baseline neural activity that may predispose behavioral variation is understudied. The present study tests the hypotheses that physical characteristics, steroid hormone levels, and baseline variation in Ile³-vasopressin (VP, a.k.a., Arg⁸-vasotocin) signaling predispose boldness during social encounters. Boldness in agonistic and reproductive contexts was extensively quantified in male green anole lizards (Anolis carolinensis), an established research organism for social behavior research that provides a crucial comparison group to investigations of birds and mammals. We found high stability of boldness across time, and between agonistic and reproductive contexts. Next, immunofluorescence was used to colocalize VP neurons with phosphorylated ribosomal protein S6 (pS6), a proxy marker of neural activity. Vasopressin-pS6 colocalization within the paraventricular and supraoptic nuclei of the hypothalamus was inversely correlated with boldness of aggressive behaviors, but not of reproductive behaviors. Our findings suggest that baseline vasopressin release, rather than solely context-dependent release, plays a role in predisposing individuals toward stable levels of displayed aggression toward conspecifics by inhibiting behavioral output in these contexts.

The development of behavioral sensitization induced by a single morphine exposure in adult zebrafish (Danio rerio)

BACKGROUND

Accumulating evidence suggest that behavioral sensitization is involved in the process of drug addiction. Zebrafish are sensitive to a variety of addictive drugs and are thus suitable for the study of behavioral sensitization. However, in contrast to mature rodent models of behavioral sensitization, how this phenomenon manifests in aquatic organisms, especially zebrafish, is largely unknown. In this study, we developed a morphine-induced behavioral sensitization adult zebrafish model and performed a preliminary investigation of the underlying mechanisms.

METHODS

Behavioral sensitization was established in zebrafish by observing their behavior after treatment and challenge with morphine. The effect of morphine was evaluated by a behavioral locomotor test. Different doses of morphine and withdrawal times were used to evaluate the establishment of the behavioral sensitization model.

RESULTS

Hyperlocomotion was induced after administration of morphine in adult zebrafish. After withdrawing the drug for a period, challenge with low-dose morphine evoked behavioral sensitization in zebrafish acutely pre-treated with morphine. Low-dose morphine failed to induce behavioral sensitization in zebrafish if the withdrawal time was less than 5 days or more than 7 days. Morphine induced behavioral sensitization in zebrafish may involve dopaminergic, glutamatergic and opioid systems.

CONCLUSION

A single low-dose of morphine could induce behavioral sensitization in zebrafish acutely pre-treated with morphine, and this phenomenon was highly correlated with drug dose and withdrawal time. These findings suggest that zebrafish is a suitable model for the study of behavioral sensitization.

The Opioid System in Rainbow Trout Telencephalon Is Probably Involved in the Hedonic Regulation of Food Intake

We hypothesize that opioids are involved in the regulation of food intake in fish through homeostatic and hedonic mechanisms. Therefore, we evaluated in rainbow trout (Oncorhynchus mykiss) hypothalamus and telencephalon changes in precursors, endogenous ligands and receptors of the opioid system under different situations aimed to induce changes in the homeostatic (through fasted/fed/refed fish) and hedonic (through feeding fish a control or a palatable high-fat diet) regulation of food intake. No major changes occurred in parameters assessed related with the nutritional condition of fish (fasted/fed/refed), allowing us to suggest that the opioid system seems not to have an important role in the homeostatic regulation of food intake in rainbow trout. The responses observed in telencephalon of rainbow trout fed the palatable high-fat diet included a decrease in mRNA abundance of the opioid precursor penka, in a way similar to that known in mammals, and increased mRNA abundance of the opioid receptors oprd1 and oprk1 supporting a role for telencephalic opioid system in the hedonic regulation of food intake in fish.

The potential for adverse effects in fish exposed to antidepressants in the aquatic environment

Antidepressants are one of the most commonly prescribed pharmaceutical classes for the treatment of psychiatric conditions. They act via modulation of brain monoaminergic signaling systems (predominantly serotonergic, adrenergic, dopaminergic) that show a high degree of structural conservation across diverse animal phyla. A reasonable assumption, therefore, is that exposed fish and other aquatic wildlife may be affected by antidepressants released into the natural environment. Indeed, there are substantial data reported for exposure effects in fish, albeit most are reported for exposure concentrations exceeding those occurring in natural environments. From a critical analysis of the available evidence for effects in fish, risk quotients (RQs) were derived from laboratory-based studies for a selection of antidepressants most commonly detected in the aquatic environment. We conclude that the likelihood for effects in fish on standard measured end points used in risk assessment (i.e., excluding effects on behavior) is low for levels of exposure occurring in the natural environment. Nevertheless, some effects on behavior have been reported for environmentally relevant exposures, and antidepressants can bioaccumulate in fish tissues. Limitations in the datasets used to calculate RQs revealed important gaps in which future research should be directed to more accurately assess the risks posed by antidepressants to fish. Developing greater certainty surrounding risk of antidepressants to fish requires more attention directed toward effects on behaviors relating to individual fitness, the employment of environmentally realistic exposure levels, on chronic exposure scenarios, and on mixtures analyses, especially given the wide range of similarly acting compounds released into the environment.

Developmental Exposure to Low Concentrations of Methylmercury Causes Increase in Anxiety-Related Behaviour and Locomotor Impairments in Zebrafish

Methylmercury (MeHg) is a ubiquitous pollutant shown to cause developmental neurotoxicity, even at low levels. However, there is still a large gap in our understanding of the mechanisms linking early-life exposure to life-long behavioural impairments. Our aim was to characterise the short- and long-term effects of developmental exposure to low doses of MeHg on anxiety-related behaviours in zebrafish, and to test the involvement of neurological pathways related to stress-response. Zebrafish embryos were exposed to sub-acute doses of MeHg (0, 5, 10, 15, 30 nM) throughout embryo-development, and tested for anxiety-related behaviours and locomotor activity at larval (light/dark locomotor activity) and adult (novel tank and tap assays) life-stages. Exposure to all doses of MeHg caused increased anxiety-related responses; heightened response to the transition from light to dark in larvae, and a stronger dive response in adults. In addition, impairment in locomotor activity was observed in the higher doses in both larvae and adults. Finally, the expressions of several neural stress-response genes from the HPI-axis and dopaminergic system were found to be disrupted in both life-stages. Our results provide important insights into dose-dependent differences in exposure outcomes, the development of delayed effects over the life-time of exposed individuals and the potential mechanisms underlying these effects.

Different influences of anxiety models, environmental enrichment, standard conditions and intraspecies variation (sex, personality and strain) on stress and quality of life in adult and juvenile zebrafish: A systematic review

Antagonist and long-lasting environmental manipulations (EM) have successfully induced or reduced the stress responses and quality of life of zebrafish. For instance, environmental enrichment (EE) generally reduces anxiety-related behaviours and improves immunity, while unpredictable chronic stress (UCS) and aquarium-related stressors generate the opposite effects. However, there is an absence of consistency in outcomes for some EM, such as acute exposure to stressors, social enrichment and some items of structural enrichment. Therefore, considering intraspecies variation (sex, personality, and strain), increasing intervention complexity while improving standardisation of protocols and contemplating the possibility that EE may act as a mild stressor on a spectrum between too much (UCS) and too little (standard conditions) stress intensity or stimulation, would reduce the inconsistencies of these outcomes. It would also help explore the mechanism behind stress resilience and to standardise EM protocols. Thus, this review critically analyses and compares knowledge existing over the last decade concerning environmental manipulations for zebrafish and the influences that sex, strain, and personality may have on behavioural, physiological, and fitness-related responses.

The impact of chronic unpredictable early-life stress (CUELS) on boldness and stress-reactivity: Differential effects of stress duration and context of testing

Early-life stress (ELS) has been shown to result in a diverse array of long-lasting impacts; for example, increasing vulnerability to disease or building 'resilience' in adulthood. Previously, zebrafish (Danio rerio) have been used to understand the mechanisms by which ELS induces different behavioral phenotypes in adults, with alterations in both learning and anxiety observed in exposed individuals. Here, we subjected zebrafish larvae to chronic unpredictable early-life stress (CUELS) for 7 or 14 days, to investigate the impact on boldness towards a new environment and novel object, and stress-reactivity. We observed that 7 days of CUELS resulted in increased time spent in the top of a novel tank (indicating boldness) but did not alter approach to a novel object. Although CUELS did not affect stress-reactivity in terms of cortisol levels, decreased anxiety-like response to conspecific alarm substance (CAS) was observed in both ELS groups (7 and 14 days of CUELS). Therefore, for the first time, we observe a potential negative effect of CUELS by dampening the behavioral stress response following exposure to CAS. Overall, these data support the use of zebrafish as a translational model to study the broad range of ELS-induced permanent changes in behavior. It could also be used to investigate the mechanisms underlying both the positive and the negative effects of early-life adversity.

Social boldness correlates with brain gene expression in male green anoles

Within populations, some individuals tend to exhibit a bold or shy social behavior phenotype relative to the mean. The neural underpinnings of these differing phenotypes - also described as syndromes, personalities, and coping styles - is an area of ongoing investigation. Although a social decision-making network has been described across vertebrate taxa, most studies examining activity within this network do so in relation to exhibited differences in behavioral expression. Our study instead focuses on constitutive gene expression in bold and shy individuals by isolating baseline gene expression profiles that influence social boldness predisposition, rather than those reflecting the results of social interaction and behavioral execution. We performed this study on male green anole lizards (Anolis carolinensis), an established model organism for behavioral research, which provides a crucial comparison group to investigations of birds and mammals. After identifying subjects as bold or shy through repeated reproductive and agonistic behavior testing, we used RNA sequencing to compare gene expression profiles between these groups within various forebrain, midbrain, and hindbrain regions. The ventromedial hypothalamus had the largest group differences in gene expression, with bold males having increased expression of neuroendocrine and neurotransmitter receptor and calcium channel genes compared to shy males. Conversely, shy males express more integrin alpha-10 in the majority of examined regions. There were no significant group differences in physiology or hormone levels. Our results highlight the ventromedial hypothalamus as an important center of behavioral differences across individuals and provide novel candidates for investigations into the regulation of individual variation in social behavior phenotype.

Gaps to Address in Ecological Studies of Temperament and Physiology

Ecology is a diverse field with many researchers interested in drivers and consequences of variability within populations. Two aspects of variability that have been addressed are behavioral and physiological. While these have been shown to separately influence ecological outcomes such as survival, reproductive success and fitness, combined they could better predict within-population variability in survival and fitness. Recently there has been a focus on potential fitness outcomes of consistent behavioral traits that are referred to as personality or temperament (e.g. boldness, sociability, exploration, etc.). Given this recent focus, it is an optimal time to identify areas to supplement in this field, particularly in determining the relationship between temperament and physiological traits. To maximize progress, in this perspective paper we propose that the following two areas be addressed: (1) increased diversity of species, and (2) increased number of physiological processes studied, with an eye toward using more representative and relatively consistent measures across studies. We first highlight information that has been gleaned from species that are frequently studied to determine how animal personality relates to physiology and/or survival/fitness. We then shine a spotlight on important taxa that have been understudied and that can contribute meaningful, complementary information to this area of research. And last, we propose a brief array of physiological processes to relate to temperament, and that can significantly impact fitness, and that may be accessible in field studies.

Using social rank as the lens to focus on the neural circuitry driving stress coping styles

Social hierarchy position in humans is negatively correlated with stress-related psychiatric disease risk. Animal models have largely corroborated human studies, showing that social rank can impact stress susceptibility and is considered to be a major risk factor in the development of psychiatric illness. Differences in stress coping style is one of several factors that mediate this relationship between social rank and stress susceptibility. Coping styles encompass correlated groupings of behaviors associated with differential physiological stress responses. Here, we discuss recent insights from animal models that highlight several neural circuits that can contribute to social rank-associated differences in coping style.

Acute environmental temperature variation affects brain protein expression, anxiety and explorative behaviour in adult zebrafish

This study investigated the effect of 4-d acute thermal treatments at 18 °C, 26 °C (control) and 34 °C on the nervous system of adult zebrafish (Danio rerio) using a multidisciplinary approach based on behavioural tests and brain proteomic analysis. The behavioural variations induced by thermal treatment were investigated using five different tests, the novel tank diving, light and dark preference, social preference, mirror biting, and Y-Maze tests, which are standard paradigms specifically tailored for zebrafish to assess their anxiety-like behaviour, boldness, social preference, aggressiveness, and explorative behaviour, respectively. Proteomic data revealed that several proteins involved in energy metabolism, messenger RNA translation, protein synthesis, folding and degradation, cytoskeleton organisation and synaptic vesiculation are regulated differently at extreme temperatures. The results showed that anxiety-like behaviours increase in zebrafish at 18 °C compared to those at 26 °C or 34 °C, whereas anxiety-related protein signalling pathways are downregulated. Moreover, treatments at both 18 °C and 34 °C affect the exploratory behaviour that appears not to be modulated by past experiences, suggesting the impairment of fish cognitive abilities. This study is the continuation of our previous work on the effect of 21-d chronic treatment at the same constant temperature level and will enable the comparison of acute and chronic treatment effects on the nervous system function in adult zebrafish.

Time-Dependent Effects of Acute Handling on the Brain Monoamine System of the Salmonid Coregonus maraena

The immediate stress response involves the activation of the monoaminergic neurotransmitter systems including serotonin, dopamine and noradrenaline in particular areas of the fish brain. We chose maraena whitefish as a stress-sensitive salmonid species to investigate the influence of acute and chronic handling on the neurochemistry of monoamines in the brain. Plasma cortisol was quantified to assess the activation of the stress axis. In addition, we analyzed the expression of 37 genes related to the monoamine system to identify genes that could be used as markers of neurophysiological stress effects. Brain neurochemistry responded to a single handling (1 min netting and chasing) with increased serotonergic activity 3 h post-challenge. This was accompanied by a modulated expression of monoaminergic receptor genes in the hindbrain and a significant increase of plasma cortisol. The initial response was compensated by an increased monoamine synthesis at 24 h post-challenge, combined with the modulated expression of serotonin-receptor genes and plasma cortisol concentrations returning to control levels. After 10 days of repeated handling (1 min per day), we detected a slightly increased noradrenaline synthesis and a down-regulated expression of dopamine-receptor genes without effect on plasma cortisol levels. In conclusion, the changes in serotonergic neurochemistry and selected gene-expression profiles, together with the initial plasma cortisol variation, indicate an acute response and a subsequent recovery phase with signs of habituation after 10 days of daily exposure to handling. Based on the basal expression patterns of particular genes and their significant regulation upon handling conditions, we suggest a group of genes as potential biomarkers that indicate handling stress on the brain monoamine systems.

Behavioral Screening of Alcohol Effects and Individual Differences in Zebrafish (Danio rerio)

AIM

To better understand the individual differences that make up a population, this study aimed to evaluate the effects of different alcoholic concentrations on the behavioral profiles of zebrafish (Danio rerio).

METHODS

For this purpose, adult animals were separated into two behavioral profiles: bold and shy, according to the emergence order. Bold and shy fish were individually tested for exploration after exposure to the drug. Acute exposure treatments were alcohol 0.00, 0.10, 0.25 and 0.50%. The behavioral parameters evaluated were speed while moving, maximum speed, total distance traveled and distance from the bottom of the tank.

RESULTS

For the groups that did not receive alcohol, bold animals showed higher speed while moving. Shy 0.00% and shy 0.10% had the highest maximum speed compared with other concentrations and profiles. For the distance from the bottom tank, our results showed that the increase induced by the low acute dose (0.10%) was observed for both profiles.

CONCLUSIONS

Our results corroborate with previous findings that alcohol affects the behavioral profiles of zebrafish differently, with bold animals apparently more resistant to these changes.

Tributyltin enhanced anxiety of adult male zebrafish through elevating cortisol level and disruption in serotonin, dopamine and gamma-aminobutyric acid neurotransmitter pathways

Tributyltin (TBT), a widely and persistently distributed organontin, has been well documented to disrupt reproduction and behaviors in animals due to its anti-aromatase activity. TBT has been also reported to enhance anxiety in several fish species, whereas the mechanism underlying remains largely unknown. To investigate the disruption of TBT on fish anxiety and the mechanisms possibly involved, adult male zebrafish (Danio rerio) were treated with TBT (100 and 500 ng/L) for 28 days and anxiety behavior was further investigated using a novel tank dive test. Result showed that TBT treatment significantly enhanced the total time of the fish spent in the lower half, delayed the onset time to the higher half of the tank and increased the total duration of freezing of the fish, indicating an enhanced anxiety in TBT-treated fish. Accordingly, TBT sharply elevated the cortisol levels in plasma in a concentration-dependent manner, suggesting that the elevated cortisol level might be involved in the enhanced anxiety. Although the expression of crha was significantly increased and crhbp was significantly decreased in the brain of TBT-treated fish which is consistent to the elevated cortisol level, the expressions of actha and acthb were sharply down-regulated. In contrast, the expressions of genes responsible for the synthesis and action of serotonin (5-HT) (pet1, thp2 and htr1aa), dopamine (DA) (th1, slc6a3, drd2a and drd2b) and gamma-aminobutyric acid (GABA) (gad2 and gabrg2) were all significantly inhibited. The down-regulation of these pivotal genes acting in 5-HT, DA and GABA neurotransmitter systems in response to TBT corresponded well with the TBT-enhanced anxiety in fish. It was thus strongly suggested that these neurotransmitters might be also involved in TBT-enhanced anxiety in adult male zebrafish. The present study extended our understanding of the neurotoxicity of TBT on the anxiety control and behavioral modulation in fish.

Reliability of behavioral test with fish: How neurotransmitters may exert neuromodulatory effects and alter the biological responses to neuroactive agents

Toxic agents such as pharmaceuticals and pesticides are continuously dispersed especially in the aquatic environment, as a result of human use. Their presence in the environment presents serious concerns, since these compounds interfere with the normal function of the central nervous system (CNS), causing behavior alterations, whose consequences are difficult to predict. However, behavioral responses, even those that occur after exposure to neurotoxic agents, might be modulated by the release of neurotransmitters in the brain of exposed organisms, making even more difficult to ascertain the real consequences of pollution by neurotoxic or neuroactive agents. This study aimed to understand the potential of dopamine as neuromodulator in cases of acute exposure to a pesticide (the carbamate carbofuran) and to a therapeutic agent (the benzodiazepinic drug diazepam) in the freshwater fish Gambusia holbrooki. After acute exposure to both carbofuran and to diazepam it was possible to observe deleterious alterations in the motor function, reflected by significant reductions of both average speed and distance in exposed animals. These changes were later diminished and reverted by dopamine exposure. Despite the indications obtained from our experiments, more research is needed to clarify the consequences of these behavior alterations in a more integrative perspective, namely by adding behavioral endpoints of increased ecological relevance to the adopted experimental design.

Progressive impairment of learning and memory in adult zebrafish treated by Al2O3 nanoparticles when in embryos

Al₂O₃ Nanoparticles (Al₂O₃-NPs) have been widely used because of their unique physical and chemical properties, and Al₂O₃-NPs can be released into the environment directly or indirectly. Our previous research found that 13 nm Al₂O₃-NPs can induce neural cell death and autophagy in primarily cultured neural cells in vitro. The aim of this study was to determine where Al₂O₃-NPs at 13 nm particle size can cause neural cells in vivo and assess related behavioural changes and involved potential mechanisms. Zebrafish from embryo to adult were selected as animal models. Learning and memory as functional indicators of neural cells in zebrafish were measured during the development from embryo to adult. Our results indicate that Al₂O₃-NPs treatment in zebrafish embryos stages can cause the accumulation of aluminium content in zebrafish brain tissue, leading to progressive impaired neurodevelopmental behaviours and latent learning and memory performance. Additionally, oxidative stress and disruption of dopaminergic transmission in zebrafish brain tissues are correlated with the dose-dependent and age-dependent accumulation of aluminium content. Moreover, the number of neural cells in the telencephalon tissue treated with Al₂O₃-NPs significantly declined, and the ultramicroscopic morphology indicated profound autophagy alternations. The results suggest that Al₂O₃-NPs has dose-dependent and time-dependent progressive damage on learning and memory performance in adult zebrafish when treated in embryos. This is the first study of the effects of Al₂O₃-NPs on learning and memory during the development of zebrafish from embryo to adult.

Differential effects of ethanol on behavior and GABAA receptor expression in adult zebrafish (Danio rerio) with alternative stress coping styles

Variation in stress responses between individuals are linked to factors ranging from stress coping styles to sensitivity of neurotransmitter systems. Many anxiolytic compounds (e.g. ethanol) can increase stressor engagement through modulation of neurotransmitter systems and are used to investigate stress response mechanisms. There are two alternative suites of correlated behavioral and physiological responses to stressors (stress coping styles) that differ in exploration tendencies: proactive and reactive stress coping styles. By chronically treating individuals differing in stress coping style with ethanol, a GABA-acting drug, we assessed the role of the GABAergic system on the behavioral stress response. Specifically, we investigated resulting changes in stress-related behavior (i.e. exploratory behavior) and whole-brain GABAA receptor subunits (gabra1, gabra2, gabrd, & gabrg2) in response to a novelty stressor. We found that ethanol-treated proactive individuals showed lower stress-related behaviors than their reactive counterparts. Proactive individuals showed significantly higher expression of gabra1, gabra2, and gabrg2 compared to reactive individuals and ethanol treatment resulted in upregulation of gabra1 and gabrg2 in both stress coping styles. These results suggest that impacts of ethanol on stress-related behaviors vary by stress coping style and that expression of select GABAA receptor subunits may be one of the underlying mechanisms.

Zebrafish Danio rerio shows behavioural cross-context consistency at larval and juvenile stages but no consistency between stages

Coping style is defined as a set of individual physiological and behavioural characteristics that are consistent across time and context. In the zebrafish Danio rerio, as well as in many other animals, several covariations have been established among behavioural, physiological and molecular responses. Nonetheless, not many studies have addressed the consistency in behavioural responses over time starting at the larval stage. Therefore, this study aimed to improve the understanding of behavioural consistency across contexts and over time in zebrafish from the larval to juvenile stages. Two distinct experiments were conducted: a larval stage experiment (from 8 to 21 days post fertilization, dpf) and a juvenile stage experiment (from 21 to 60 dpf). On one hand, the larval experiment allows to focus on the transition between 8 and 21 dpf, marked by significant morphological changes related to the end of larval stage and initiation of metamorphosis. On the other hand, the juvenile experiment allows to properly cover the period extending from the end of larval stage to the juvenile stage (60 dpf), including metamorphosis which is itself completed around 45 dpf. Within each experiment, boldness was determined using a group risk-taking test to identify bold and shy individuals. A novel environment test was then performed at the same age to evaluate consistency across contexts. Groups of fish (either bold or shy) were bathed in an alizarin red S solution for later identification of their initially determined coping style to evaluate behavioural consistency over time. Fish were then reared under common garden conditions and challenged again with the same behavioural tests at a later age (21 and 60 dpf in the larval and juvenile experiments, respectively). Behavioural consistency was observed across contexts, with bold fish being more active and expressing higher thigmotaxis regardless of age. There was, however, little behavioural consistency over age, suggesting behavioural plasticity during development. Moreover, the use of alizarin red S to conduct this experiment provides new perspectives for the further study of the longitudinal evolution of various traits, including behaviour, over life stages in fish.

The presence of a conspecific induces risk-taking behaviour and enlargement of somata size of dopaminergic neurons in the brain of male medaka fish

Boldness and risk-taking behaviours in animals are important traits to obtain advantages such as habitation, food resources, reproductive success and social dominance. Risk-taking behaviour is influenced by physiological and environmental conditions; however, whether individual fish become bolder by the presence of conspecifics remains unknown. In this study, a light-dark preference test was conducted using medaka fish (Oryzias latipes) with or without a neighbouring conspecific. It was found that individual medaka male fish preferred a light environment and avoided a dark environment, whereas the display of a neighbouring conspecific enhanced the time the male spent in the dark environment (i.e., this condition encouraged risk-taking). The blood glucose level increased in fish confined to the dark condition but did not increase in light-preferring fish and risk-taking fish. Large somata expressing tyrosine hydroxylase, which is the rate-limiting enzyme in dopamine synthesis, were detected in the telencephalic and diencephalic brain regions in risk-taking medaka, whereas large somata were detected in the diencephalic region in medaka confined to the dark condition. These findings indicated that medaka is a good fish model to explore the central roles of dopaminergic neurons in the telencephalon and the diencephalon, which regulate risk-taking behaviour.

Divergent neurogenomic responses shape social learning of both personality and mate preference

Behavior plays a fundamental role in shaping the origin and fate of species. Mating decisions can act to promote or restrict gene flow, as can personality traits that influence dispersal and niche use. Mate choice and personality are often both learned and therefore influenced by an individual's social environment throughout development. Likewise, the molecular pathways that shape these behaviors may also be co-expressed. In this study on swordtail fish (Xiphophorus birchmanni), we show that female mating preferences for species-typical pheromone cues are entirely dependent on social experience with adult males. Experience with adults also shapes development along the shy-bold personality axis, with shy behaviors arising from exposure to risk-averse heterospecifics as a potential stress-coping strategy. In maturing females, conspecific exposure results in a strong upregulation of olfaction and vision genes compared with heterospecific exposure, as well as immune response genes previously linked to anxiety, learning and memory. Conversely, heterospecific exposure involves an increased expression of genes important for neurogenesis, synaptic plasticity and social decision-making. We identify subsets of genes within the social decision-making network and with known stress-coping roles that may be directly coupled to the olfactory processes females rely on for social communication. Based on these results, we conclude that the social environment affects the neurogenomic trajectory through which socially sensitive behaviors are learned, resulting in adult phenotypes adapted for specific social groupings.

Physiological responses during acute stress recovery depend on stress coping style in European sea bass, Dicentrarchus labrax

Individual stress coping style (reactive, intermediate and proactive) was determined in 3 groups of 120 pit tagged European seabass using the hypoxia avoidance test. The same three groups (no change in social composition) were then reared according to the standards recommended for this species. Then, 127 days later, individuals initially characterized as reactive, intermediate or proactive were submitted to an acute confinement stress for 30 min. Blood samples were taken to measure plasma cortisol levels 30 min (Stress30) or 150 min (Stress150) after the end of the confinement stress. Individuals were then sacrificed to sample the telencephalon in order to measure the main monoamines and their catabolites (at Stress30 only). Individuals from Stress150 were sampled for whole brain for a transcriptomic analysis. The main results showed that reactive individuals had a lower body mass than intermediate individuals which did not differ from proactive individuals. The physiological cortisol response did not differ between coping style at Stress30 but at Stress150 when intermediate and proactive individuals had recovered pre stress levels, reactive individuals showed a significant higher level illustrating a modulation of stress recovery by coping style. Serotonin turnover ratio was higher in proactive and reactive individuals compared to intermediate individuals and a significant positive correlation was observed with cortisol levels whatever the coping style. Further, the confinement stress led to a general increase in the serotonin turnover comparable between coping styles. Stress150 had a significant effect on target mRNA copy number (Gapdh mRNA copy number decreased while ifrd1 mRNA copy number increased) and such changes tended to depend upon coping style.

Early-life exposure to the organophosphorus flame-retardant tris (1,3-dichloro-2-propyl) phosphate induces delayed neurotoxicity associated with DNA methylation in adult zebrafish

Early-life exposure to toxicants could affect health outcomes in adulthood. We determined the effects of early-life exposure to the organophosphorus flame-retardant tris (1,3-dichloro-2-propyl) phosphate (TDCIPP) in adult zebrafish. Embryos were exposed to TDCIPP from early embryogenesis (2 h post-fertilization) to 10 days post-fertilization (dpf), and larvae were transferred to clean water until adulthood (150 dpf). TDCIPP showed accumulation in larvae, but returned to control levels after 7 days of depuration. In adult zebrafish exposed to TDCIPP in early life, vulnerability to anxiety-like behavior was observed in females but not males, suggesting gender-dependent neurotoxicity. Decreased dopamine (DA) concentration and down-regulation of dopaminergic signaling related genes were observed in the brains of adult females. Upregulation of DNA methylation transferases (dnmt1, dnmt3a, and dnmt3b) genes were observed in larvae and brains of adult females. Further, the promoter regions of the selected key genes (bdnf, drd4b, zc4h2 and th) showed increased DNA methylation status, accompanied by down-regulation of gene transcription in larvae and brains of adult females. Our results indicate that early-life exposure to TDCIPP could cause delayed neurotoxicity in adult zebrafish.

Stress-induced changes in group behaviour

Testing animals in groups can provide valuable data for investigating behavioural stress responses. However, conventional measures typically focus on the behaviour of individual animals or on dyadic interactions. Here, we aimed to determine metrics describing the behaviour of grouping animals that can reveal differences in stress responses. Using zebrafish (Danio rerio) as a model, we observed replicated shoals both immediately and 24 hours after exposure to a novel environment, as an assessment of temporal change in response to an acute stressor. We quantified various standard behavioural measures in combination with metrics describing group structure, including different proximity, social, and spatial metrics. Firstly, we showed a high collinearity between most of the analysed metrics, suggesting that they describe similar aspects of the group dynamics. After metric selection, we found that under acute stress shoals had significantly higher shoal densities, a lower variation in nearest neighbour distances and were in closer proximity to the walls compared to the same groups tested 24 hours later, indicating a reduction in acute stress over time. Thus, the use of group metrics could allow for the refinement of behavioural protocols carried out in a range of research areas, by providing sensitive and rich data in a more relevant social context.

Boldness in Male and Female Zebrafish (Danio rerio) Is Dependent on Strain and Test

Differences in selection pressure in nature and labs have profound effects on zebrafish strains. The widely used AB strain of zebrafish has been domesticated over several decades. Recently, there has been an upsurge in the availability of genetically modified lines, e.g., the spiegeldanio (spd), which has a mutation in the fibroblast growth factor receptor 1a (fgfr1a) gene. This mutant strain (fgfr1a) has previously been reported to be bolder than fish of the Tübingen strain, from which it was generated. Our knowledge on behavioral differences between different zebrafish strains, relative to wild-caught zebrafish, is limited. In the present study we compare behaviors related to interpretation of boldness in male and female offspring (F1) of wild-caught fish, AB and fgfr1a ^-/- zebrafish. A second aim of the study was to compare the behavior of fish from these strains when tested in different behavioral assays, i.e., shelter seeking, novel tank diving and scototaxis tests. The results demonstrate that behavioral variation exists both within and between the strains, but interpretation of boldness reveals a complex pattern in which behavior differs between strains but is also related to sex and test. Therefore, a careful assessment of various strains of fish using both males and females is warranted in order to strengthen interpretation of results. This is especially important in studies where zebrafish are used as model organisms for human conditions as well as studies evaluating the effects of pharmacological or toxicological substances on behavior.

Ecotoxicity Assessment of Fe3O4 Magnetic Nanoparticle Exposure in Adult Zebrafish at an Environmental Pertinent Concentration by Behavioral and Biochemical Testing

Magnetic Nanoparticles (MNPs) are widely being investigated as novel promising multifunctional agents, specifically in the fields of development for theranostics, electronics, waste water treatment, cosmetics, and energy storage devices. Unique, superior, and indispensable properties of magnetization, heat transfer, and melting temperature make MNPs emerge in the field of therapeutics in future healthcare industries. However, MNPs ecotoxicity as well as behavioral toxicity is still unexplored. Ecotoxicity analysis may assist investigate MNPs uptake mechanism and its influence on bioavailability under a given set of environmental factors, which can be followed to investigate the biomagnification of MNPs in the environment and health risk possessed by them in an ecological food chain. In this study, we attempted to determine the behavioral changes in zebrafishes at low (1 ppm) or high (10 ppm) concentration levels of Fe₃O₄ MNPs. The synthesized Fe₃O₄ MNPs sized at 15 nm were characterized by the transmission electron microscope (TEM), the superconducting quantum interference device (SQUID) magnetometer, and the multiple behavior tests for novel tank, mirror biting, conspecific social interaction, shoaling, circadian rhythm, and short-term memory of zebrafish under MNPs chronic exposure were demonstrated. Low concentration MNP exposure did not trigger alteration for majority behavioral and biochemical tests in adult zebrafish. However, tight shoal groups were observed at a high concentration of MNPs exposure along with a modest reduction in fish exploratory behavior and a significant reduction in conspecific social interaction behavior. By using enzyme-linked immunosorbent assays (ELISA), we found a high dose of MNPs exposure significantly elevated cortisol, acetylcholine, and catalase levels while reducing serotonin, acetylcholine esterase, and dopamine levels in the brain. Our data demonstrates chronic MNPs exposure at an environmentally-relevant dose is relatively safe by supporting evidence from an array of behavioral and biochemical tests. This combinational approach using behavioral and biochemical tests would be helpful for understanding the MNPs association with anticipated colloids and particles effecting bioavailability and uptake into cells and organisms.

Coping styles in European sea bass: The link between boldness, stress response and neurogenesis

Coping styles consist of a coherent set of individual physiological and behavioral differences in stress responses that are consistent across time and context. Such consistent inter-individual differences in behavior have already been shown in European sea bass (Dicentrarchus labrax), but the associated mechanisms are still poorly understood. Here, we combine physiological measurements with individual behavioral responses in order to characterize coping styles in fish. Fish were tagged and placed in a tank for group risk-taking tests (GRT) at 8 months of age to evaluate boldness using the proxy latency of leaving a sheltered area towards an open area. A subsample of these fish were individually challenged 16 months later using an open field test (OFT), in which the boldness was assessed after being placed in a shelter within an open arena. Latency to exit the shelter, time spent in the shelter, and distance travelled were recorded for this purpose. The blood and brain were then collected to evaluate plasma cortisol concentration and neurotransmitter levels (dopamine, norepinephrine, serotonin, and related metabolites), as well as brain transcription of key genes involved in stress axis regulation (gr1, gr2, mr, crf), neurogenesis (neurod1, neurod2, pcna), and neuronal development (egr1). Fish acting bolder in the GRT were not necessarily those acting bolder in the OFT, highlighting the relatively low consistency across different types of tests performed with a 16-months interval. There was, however, a significant correlation between stress markers and boldness. Indeed, mRNA levels of mr, crf, gr2, egr1, and neurod2, as well as norepinephrine levels were higher in shy than bold fish, whereas brain serotonergic activity was lower in shy fish. Overall, our study highlights the fact that boldness was not consistent over time when testing context differed (group vs. alone). This is in agreement with previous literature suggesting that social context play a key role in boldness measurement and that the particular life history of each individual may account in shaping the personality fate of a fish.

Opioid Neurobiology, Neurogenetics and Neuropharmacology in Zebrafish

Despite the high prevalence of medicinal use and abuse of opioids, their neurobiology and mechanisms of action are not fully understood. Experimental (animal) models are critical for improving our understanding of opioid effects in vivo. As zebrafish (Danio rerio) are increasingly utilized as a powerful model organism in neuroscience research, mounting evidence suggests these fish as a useful tool to study opioid neurobiology. Here, we discuss the zebrafish opioid system with specific focus on opioid gene expression, existing genetic models, as well as its pharmacological and developmental regulation. As many human brain diseases involve pain and aberrant reward, we also summarize zebrafish models relevant to opioid regulation of pain and addiction, including evidence of functional interplay between the opioid system and central dopaminergic and other neurotransmitter mechanisms. Additionally, we critically evaluate the limitations of zebrafish models for translational opioid research and emphasize their developing utility for improving our understanding of evolutionarily conserved mechanisms of pain-related, addictive, affective and other behaviors, as well as for fostering opioid-related drug discovery.