Sex differences in behavior and neuropharmacology of zebrafish

The influence of sex and age differences in an adult zebrafish (Danio rerio) T-maze model of cognition

The zebrafish (Danio rerio) model is increasingly popular in neurobehavioral research, and behavioral outcomes are commonly evaluated in studies on neurodegeneration and neurotoxicity. Sex and age have been identified as important variables in cognition studies; however, these factors are often underreported in published studies that use the zebrafish model, leading to uncertainty about their impact in zebrafish T-maze experiments. In this study, we evaluated the role of sex and age in zebrafish cognitive function using a 5-day T-maze task. Our results demonstrated that female and younger zebrafish had increased learning and memory capacity. These findings highlighted the importance of considering and reporting sex and age in experimental design in zebrafish cognitive neurobehavioral studies.

Sex differences in β-N-Methylamino-L-alanine effects on zebrafish behavioral response

The β-N-methylamino-L-alanine (BMAA) is a neurotoxin produced by cyanobacteria and diatoms and related by triggered neurodegeneration. The exposure to neurotoxins has also been reported by causing emotional and neuroendocrine effects and these effects may be sex-specific. However, the effects of BMAA on emotions and pain, as well as neuroendocrine modulations remain poorly understood. Here, we evaluate potential sex differences in zebrafish behavioral responses to BMAA acute exposure on their anxiety and pain phenotypical behavioral repertoire and their neuroendocrine (cortisol) effects. Overall, sex differences in behavioral responses of adult zebrafish to BMAA exposure were demonstrated, as female fish reacted to it more strongly than males by altering their behavioral phenotype in both the novel tank and writhing -like behavior tests. In addition, sex differences were demonstrated in relation to time response, as male increased the writhing-like behavioral responses immediately after injection of BMAA, while female only 24-h after injection, reinforcing the painful stimulus caused by BMAA. However, the exposure to BMAA elevated the whole-body cortisol levels in both male and female zebrafish. Collectively, these findings emphasize the growing importance of studying sex differences in zebrafish, including the evaluation of neurotoxins effects on emotions and pain in this aquatic experimental model.

Anxiolytic-like effects of acute serotonin-releasing agents in zebrafish models of anxiety: experimental study and systematic review

Though commonly used to model affective disorders, zebrafish display notable differences in terms of the structure and function of the brain serotonin system, including responses to pharmacological interventions, as compared to mammals. For example, elevation of brain serotonin following acute administration of serotonin reuptake inhibitors (SRIs) generally has anxiogenic effects, both in the clinical situation and in rodent models of anxiety, but previous research has indicated the opposite in zebrafish. However, several issues remain unresolved. We conducted a systematic review of SRI effects in zebrafish models of anxiety and, on the basis of these results, performed a series of experiments further investigating the influence of serotonin-releasing agents on anxiety-like behaviour in zebrafish, with sex-segregated wild-type animals being administered either escitalopram, or the serotonin releaser fenfluramine, in the light-dark test. In the systematic review, we find that the available literature indicates an anxiolytic-like effect of SRIs in the novel-tank diving test. Regarding the light-dark test, most studies reported no behavioural effects of SRIs, although the few that did generally saw anxiolytic-like responses. In the experimental studies, consistent anxiolytic-like effects were observed with neither sex nor habituation influencing treatment response. We find that the general effect of acute SRI administration in zebrafish indeed appears to be anxiolytic-like, indicating, at least partly, differences in the functioning of the serotonin system as compared to mammals and that caution is advised when using zebrafish to model affective disorders.

Sex, drugs, and zebrafish: Acute exposure to anxiety-modulating compounds in a modified novel tank dive test

This study investigated the effects of anxiogenic and anxiolytic drugs on zebrafish (Danio rerio) behaviour using a modified novel tank dive test with higher walls and a narrower depth. Zebrafish were administered chondroitin sulfate, beta-carboline, delta-9-tetrahydrocannabinol (THC), ethanol, and beta-caryophyllene, and their behaviours were evaluated for geotaxis, swimming velocity, and immobility. Both anxiogenic and anxiolytic compounds generally increased bottom-dwelling behaviour, suggesting that the tank's modified dimensions significantly influence zebrafish responses. EC50 values for ethanol showed a lower threshold for velocity reduction compared to zone preference. Chondroitin sulfate uniquely caused a sex-specific increase in male swimming velocity, whereas no other sex-differences were observed with any compound. Interestingly, the presence of drug-treated fish did not alter the behaviour of observer fish, suggesting limited social buffering effects. The findings underscore the complexity of zebrafish behavioural phenotypes and highlight the need for considering tank dimensions and multiple behavioural parameters to accurately assess the effects of anxiety-modulating drugs. This study demonstrates the utility of the modified novel tank dive test in providing nuanced insights into the behavioural effects of different pharmacological agents in zebrafish.

The impacts of microplastics on zebrafish behavior depend on initial personality state

Microplastic pollution is associated with inflammation, gut dysbiosis and behavioral changes in fish. Fish have distinct personality traits but the role of personality in behavioral toxicology is rarely considered. We classified zebrafish on four behavioral axes: boldness, anxiety, sociability and exploration tendency then exposed them to low- or high- concentrations of two types of polyethylene microplastics (low- and high-density) for 28 days. Behaviors, antioxidant enzymes (catalase and superoxide dismutase), and gut microbiome were then measured. There were direct effects of microplastics on boldness, anxiety and sociability. However, fish retained their initial behavioral tendencies. Exposure to all microplastic treatments reduced average swimming speed and decreased the time spent motionless. Microplastic exposure did not affect antioxidant enzymes but did cause significant changes in the composition of the gut microbiome. This study demonstrates that environmentally realistic concentrations of microplastics can alter fish behavior, but much of the variance in response can be explained by personality.

A systematic evidence map and bibliometric analysis of the behavioural impacts of pesticide exposure on zebrafish

Pesticides are indispensable in agriculture and have become ubiquitous in aquatic environments. Pesticides in natural environments can cause many negative impacts on aquatic species, ranging from mortality to sub-lethal physiological and behavioural changes. The complex sub-lethal impacts of pesticides are routinely tested on model species, with zebrafish (Danio rerio) being regularly used as a behavioural model. Although behavioural ecotoxicology research using zebrafish is increasing rapidly, we lack quantitative evidence to support which pesticides have been tested and how study designs are carried out. This shortcoming not only limits the deliberate planning for future primary studies to fill the knowledge gaps but also hinders evidence synthesis. To provide quantitative evidence of what pesticides are currently studied and what study designs are used, we combined a systematic evidence map approach and bibliometric analysis. This novel method has been coined research weaving and allows us to elicit gaps and clusters in our evidence base, whilst showing connections between authors and institutions. The methodology can be summarised in five primary steps: literature searching, screening, extraction, data analysis and bibliometric analysis. We identified four areas where research on the sub-lethal effects of pesticide exposure on zebrafish is lacking. First, some widely used pesticides, such as neonicotinoids, are understudied. Second, most studies do not report important elements of the study design, namely the sex and the life-stage of the zebrafish. Third, some behaviours, such as impacts of pesticide exposure on zebrafish cognition, are underexplored. And last, we revealed through the bibliometric analysis that most of the research is conducted in developed countries and there is limited cross country co-authorships. Upon identifying these gaps, we offer solutions for each limitation, emphasizing the importance of diverse global research output and cross-country co-authorships. Our systematic evidence map and bibliometric analysis provide valuable insights for helping to guide future research, which can be used to help support evidence-based policy decisions.

Sex matters in preclinical research

International Women's Day 2024 has a theme of inclusion. As publishers of preclinical research, we aim to show how inclusion of females in research advances scientific rigor and improves treatment reliability. Sexual reproduction is key to all life across the plant and animal kingdoms. Biological sex takes many forms that are morphologically differentiated during development: stamens versus pistils in plants; color and plumage in birds; fallopian tubes versus vas deferens in mammals; and differences in size, for instance, males are smaller in the fruit fly Drosophila melanogaster. Physical differences may be obvious, but many traits may be more obscure, including hormonal, physiological and metabolic factors. These traits have a big influence on disease and responses to treatment. Thus, we call for improved inclusion, analysis and reporting of sex as a biological variable in preclinical animal modeling research.

Effects of curcumin to counteract levodopa-induced toxicity in zebrafish

Parkinson's disease (PD) is a neurodegenerative disorder characterized by motor dysfunction due to the death of dopaminergic neurons in the substantia nigra pars compacta. Currently, treatment of PD has focused on increasing dopamine levels, using a dopamine precursor, levodopa (L-DOPA) or stimulation of dopaminergic receptors. Prolonged use of L-DOPA is associated with the occurrence of motor complications and dyskinesia, attributed to neurotoxic effects of this drug. The aim of this study was to investigate the effects of curcumin (CUR), a lipophilic polyphenol, to counteract L-DOPA induced toxicity. Zebrafish larvae were pre-treated with CUR (0.05 µM) or vehicle dimethyl sulfoxide (DMSO) for 24 hr and subsequently exposed to L-DOPA (1 mM) or vehicle. Immediately and 24 hr after L-DOPA exposure, spontaneous swimming and dark/light behavioral tests were performed. In addition, levels of reactive oxygen species (ROS) and lipid peroxidation products were determined at the end of treatment. CUR significantly improved the motor impairment induced by 24 hr L-DOPA treatment, and reduced levels of ROS and lipoperoxidation products in zebrafish larvae. In conclusion, our results suggest that CUR acts as a neuroprotector against toxicity initiated by L-DOPA. Evidence suggests the observed effects of CUR are associated with its antioxidant properties.

Diazepam at environmentally relevant concentrations disturbed social interactions and brain neurotransmitters in adult Japanese medaka (Oryzias latipes)

Pollution by diazepam (DZP) is increasingly recognized as a major threat to aquatic organisms, but knowledge about its potential risk to fish is still limited. In this study, we exposed female and male Japanese medaka (Oryzias latipes) to environmentally relevant DZP (0.8 and 8 µg/L) for 28 days and investigated variation in their behavior (on days 7, 14, and 28) and brain neurotransmitter levels (on day 28). The results showed that DZP could be accumulated in the brain and gonads in Japanese medaka. When two fish of the same sex were placed in an aquarium, DZP exposure exhibited typical sedative effects on females (on day 7) and males (on days 7 and 14). However, these sedative effects on both sexes were no longer present after 28 days of exposure. Exposure to DZP induced sex-specific impacts on the social interactions of medaka on days 7, 14, and 28 of exposure in a time-dependent manner. When both sexes were placed into an aquarium in a ratio of 1:1, DZP could significantly alter their locomotor activity and social interaction on days 14 and 28 of the exposure. After 28 days of exposure, DZP significantly altered the levels of several neurotransmitters in the brain of medaka, also in sex-specific manners. The alterations in dopamine and serotonin levels exhibited significant correlations with the increased social interaction between females. At the same time, that of γ-aminobutyric acid significantly correlated to the decreased social interaction between males. Our findings suggest that chronic exposure to DZP, even at environmentally relevant concentrations, can accumulate in the brains and gonads of fish, and alter their behaviors by mediating brain neurotransmitter levels, which may further disturb their reproduction and population dynamics.

Experimental models of human cortical malformations: from mammals to 'acortical' zebrafish

Human neocortex controls and integrates cognition, emotions, perception and complex behaviors. Aberrant cortical development can be triggered by multiple genetic and environmental factors, causing cortical malformations. Animal models, especially rodents, are a valuable tool to probe molecular and physiological mechanisms of cortical malformations. Complementing rodent studies, the zebrafish (Danio rerio) is an important model organism in biomedicine. Although the zebrafish (like other fishes) lacks neocortex, here we argue that this species can still be used to model various aspects and brain phenomena related to human cortical malformations. We also discuss novel perspectives in this field, covering both advantages and limitations of using mammalian and zebrafish models in cortical malformation research. Summarizing mounting evidence, we also highlight the importance of translationally-relevant insights into the pathogenesis of cortical malformations from animal models, and discuss future strategies of research in the field.

Towards experimental models of delirium utilizing zebrafish

Delirium is an acute neuropsychiatric condition characterized by impaired behavior and cognition. Although the syndrome has been known for millennia, its CNS mechanisms and risk factors remain poorly understood. Experimental animal models, especially rodent-based, are commonly used to probe various pathogenetic aspects of delirium. Complementing rodents, the zebrafish (Danio rerio) emerges as a promising novel model organism to study delirium. Zebrafish demonstrate high genetic and physiological homology to mammals, easy maintenance, robust behaviors in various sensitive behavioral tests, and the potential to screen for pharmacological agents relevant to delirium. Here, we critically discuss recent developments in the field, and emphasize the developing utility of zebrafish models for translational studies of delirium and deliriant drugs. Overall, the zebrafish represents a valuable and promising aquatic model species whose use may help understand delirium etiology, as well as develop novel therapies for this severely debilitating disorder.

Sex differences in social buffering and social contagion of alarm responses in zebrafish

The alarm substance in fish is a pheromone released by injured individuals after a predator attack. When detected by other fish, it triggers fear/defensive responses, such as freezing and erratic movement behaviours. Such responses can also help other fish in the shoal to modulate their own behaviours: decreasing a fear response if conspecifics have not detected the alarm substance (social buffering) or triggering a fear response if conspecifics detected the alarm substance (social contagion). Response variation to these social phenomena is likely to depend on sex. Because males have higher-risk life-history strategies than females, they may respond more to social buffering where they risk not responding to a real predator attack, while females should respond more to social contagion because they only risk responding to a false alarm. Using zebrafish, we explored how the response of males and females to the presence/absence of the alarm substance is modified by the alarmed/unalarmed behaviour of an adjacent shoal of conspecifics. We found that, in social buffering, males decreased freezing more than females as expected, but in social contagion males also responded more than females by freezing at a higher intensity. Males were, therefore, more sensitive to visual information, while females responded more to the alarm substance itself. Because visual information updates faster than chemical information, males took more risks but potentially more benefits as well, because a quicker adjustment of a fear response allows to save energy to other activities. These sex differences provide insight into the modifying effect of life-history strategies on the use of social information.

Among-individual variation in the swimming behaviour of the amphipod Gammarus pulex under dark and light conditions

In recent years, considerable computational advancements have been made allowing automated analysis of behavioural endpoints using video cameras. However, the results of such analyses are often confounded by a large variation among individuals, making it problematic to derive endpoints that allow distinguishing treatment effects in behavioural studies. In this study, we quantitatively analysed the effects of light conditions on the swimming behaviour of the freshwater amphipod Gammarus pulex by high-throughput tracking, and attempted to unravel among individual variation using size and sex. For this, we developed the R-package Kinematics, allowing for the rapid and reproducible analysis of the swimming behaviour (speed, acceleration, thigmotaxis, curvature and startle response) of G. pulex, as well as any other organism. Our results show a considerable amount of variation among individuals (standard deviation ranging between 5 and 115 % of the average swimming behaviour). The factors size and sex and the interaction between the two only explained a minor part of this found variation. Additionally, our study is the first to quantify the startle response in G. pulex after the light is switched on, and study the variability of this response between individuals. To analyse this startle response, we established two metrics: 1) startle response magnitude (the drop in swimming velocity directly after the light switches on), and 2) startle response duration (the time it takes to recover from the drop in swimming velocity to average swimming speed). Almost 80 % of the individuals showed a clear startle response and, therefore, these metrics demonstrate a great potential for usage in behavioural studies. The findings of this study are important for the development of appropriate experimental set-ups for behavioural experiments with G. pulex.

ENVIRONMENTAL STRESSORS DIFFERENTIALLY MODULATE ANXIETY-LIKE BEHAVIOUR IN MALE AND FEMALE ZEBRAFISH

How differently male and female responds in a stressful situation is a matter of curiosity. Apart from curiosity, this opens a new arena to the synthesis of personalized/individualized medications. Here, we used zebrafish, a suitable experimental animal model to study stress and anxiety. We evaluated the differential responses in adult male and female zebrafish on the acute exposure of three different stressors: Caffeine (100mg/L), Conspecific alarm substance (3.5ml/L), and sight of sympatric predators (Leaf fish and Snakehead) with the help of two different behavioural paradigms (Novel tank test & Predator exposure). Behavioural responses were captured over 6minutes and quantified using Smart 3.0. Male zebrafish were found to be more responsive to caffeine treatment. Conspecific alarm substance-challenged males and females showed robust alarm reactions whereas females were found to be more prone to it. Female zebrafish showed statistically significant aversion to the visual representation of sympatric predators. Taken together, each stressor induced differential responses in male and female zebrafish.

Brain metabolite profiles provide insight into mechanisms for behavior sexual dimorphisms in zebrafish (Danio rerio)

The zebrafish (Danio rerio) has historically been a useful model for research in genetics, ecology, biology, toxicology, and neurobehavior. Zebrafish have been demonstrated to have brain sexual dimorphism. However, the sexual dimorphism of zebrafish behavior demands our attention, particularly. To evaluate the behavior and brain sexual dimorphisms in zebrafish, this study assessed sex differences in adult D. rerio in four behavioral domains, including aggression, fear, anxiety, and shoaling, and further compared with metabolites in the brain tissue of females and males. Our findings showed that aggression, fear, anxiety and shoaling behaviors were significantly sexually dimorphic. Interestingly, we also show through a novel data analysis method, that the female zebrafish exhibited significantly increased shoaling behavior when shoaled with male zebrafish groups and, for the first time, we offer evidence that male shoals are beneficial in dramatically alleviating anxiety in zebrafish. In addition, there were significant changes in metabolites in zebrafish brain tissue between the sexes. Furthermore, zebrafish behavioral sexual dimorphism may be associated with brain sexual dimorphism, with significant differences in brain metabolites. Therefore, to prevent the influence or even bias of behavioral sex differences on results, it is suggested that behavioral studies or behavioral-based other relevant investigations consider sexual dimorphism of behavior and brain.

Roles of the 5-HT2C receptor on zebrafish sociality

Serotonin (5-HT) receptors have been implicated in social behavior in vertebrates. Zebrafish (Danio rerio) have been increasingly being used behavioral neuroscience to study the neurobiological correlates of behavior, including sociality. Nonetheless, the role of 5-HT_2C receptors in different social functions were not yet studied in this species. Zebrafish were treated with the agonist MK-212 (2 mg/kg) or the antagonist RS-102221 (2 mg/kg) and tested in the social interaction and social novelty tests, conditional approach test, or mirror-induced aggressive displays. MK-212 increased preference for an unknown conspecific in the social investigation test, but also increased preference for the known conspecific in the social novelty test; RS-102221, on the other hand, decreased preference in the social investigation test but increased preference for the novel conspecific in the social novelty test. MK-212 also decreased predator inspection in the conditional approach test. While RS-102221 decreased time in the display zone in the mirror-induced aggressive display test, it increased display duration. Overall, these results demonstrate the complex role of 5-HT_2C receptors in different social contexts in zebrafish, revealing a participation in social plasticity in vertebrates.

Current State of Modeling Human Psychiatric Disorders Using Zebrafish

Psychiatric disorders are highly prevalent brain pathologies that represent an urgent, unmet biomedical problem. Since reliable clinical diagnoses are essential for the treatment of psychiatric disorders, their animal models with robust, relevant behavioral and physiological endpoints become necessary. Zebrafish (Danio rerio) display well-defined, complex behaviors in major neurobehavioral domains which are evolutionarily conserved and strikingly parallel to those seen in rodents and humans. Although zebrafish are increasingly often used to model psychiatric disorders, there are also multiple challenges with such models as well. The field may therefore benefit from a balanced, disease-oriented discussion that considers the clinical prevalence, the pathological complexity, and societal importance of the disorders in question, and the extent of its detalization in zebrafish central nervous system (CNS) studies. Here, we critically discuss the use of zebrafish for modeling human psychiatric disorders in general, and highlight the topics for further in-depth consideration, in order to foster and (re)focus translational biological neuroscience research utilizing zebrafish. Recent developments in molecular biology research utilizing this model species have also been summarized here, collectively calling for a wider use of zebrafish in translational CNS disease modeling.

The effects of exploratory behavior on physical activity in a common animal model of human disease, zebrafish (Danio rerio)

Zebrafish (Danio rerio) are widely accepted as a multidisciplinary vertebrate model for neurobehavioral and clinical studies, and more recently have become established as a model for exercise physiology and behavior. Individual differences in activity level (e.g., exploration) have been characterized in zebrafish, however, how different levels of exploration correspond to differences in motivation to engage in swimming behavior has not yet been explored. We screened individual zebrafish in two tests of exploration: the open field and novel tank diving tests. The fish were then exposed to a tank in which they could choose to enter a compartment with a flow of water (as a means of testing voluntary motivation to exercise). After a 2-day habituation period, behavioral observations were conducted. We used correlative analyses to investigate the robustness of the different exploration tests. Due to the complexity of dependent behavioral variables, we used machine learning to determine the personality variables that were best at predicting swimming behavior. Our results show that contrary to our predictions, the correlation between novel tank diving test variables and open field test variables was relatively weak. Novel tank diving variables were more correlated with themselves than open field variables were to each other. Males exhibited stronger relationships between behavioral variables than did females. In terms of swimming behavior, fish that spent more time in the swimming zone spent more time actively swimming, however, swimming behavior was inconsistent across the time of the study. All relationships between swimming variables and exploration tests were relatively weak, though novel tank diving test variables had stronger correlations. Machine learning showed that three novel tank diving variables (entries top/bottom, movement rate, average top entry duration) and one open field variable (proportion of time spent frozen) were the best predictors of swimming behavior, demonstrating that the novel tank diving test is a powerful tool to investigate exploration. Increased knowledge about how individual differences in exploration may play a role in swimming behavior in zebrafish is fundamental to their utility as a model of exercise physiology and behavior.

The effects of an obesogenic diet on behavior and cognition in zebrafish (Danio rerio): Trait average, variability, repeatability, and behavioral syndromes

The obesity epidemic, largely driven by the accessibility of ultra-processed high-energy foods, is one of the most pressing public health challenges of the 21st century. Consequently, there is increasing concern about the impacts of diet-induced obesity on behavior and cognition. While research on this matter continues, to date, no study has explicitly investigated the effect of obesogenic diet on variance and covariance (correlation) in behavioral traits. Here, we examined how an obesogenic versus control diet impacts means and (co-)variances of traits associated with body condition, behavior, and cognition in a laboratory population of ~160 adult zebrafish (Danio rerio). Overall, an obesogenic diet increased variation in several zebrafish traits. Zebrafish on an obesogenic diet were significantly heavier and displayed higher body weight variability; fasting blood glucose levels were similar between control and treatment zebrafish. During behavioral assays, zebrafish on the obesogenic diet displayed more exploratory behavior and were less reactive to video stimuli with conspecifics during a personality test, but these significant differences were sex-specific. Zebrafish on an obesogenic diet also displayed repeatable responses in aversive learning tests whereas control zebrafish did not, suggesting an obesogenic diet resulted in more consistent, yet impaired, behavioral responses. Where behavioral syndromes existed (inter-class correlations between personality traits), they did not differ between obesogenic and control zebrafish groups. By integrating a multifaceted, holistic approach that incorporates components of (co-)variances, future studies will greatly benefit by quantifying neglected dimensions of obesogenic diets on behavioral changes.

What do male and female zebrafish prefer? Directional and color preference in maze tasks

Studies regarding the animals' innate preferences help elucidate and avoid probable sources of bias and serve as a reference to improve and develop new behavioral tasks. In zebrafish research, data obtained in behavioral assessments are often not replicated between research groups or even inside the same laboratory raising huge concerns about replicability and reproducibility. Among the potential causes that are not well considered, sexual differences can be a probable source of bias. Thus, this study aimed to investigate the male and female zebrafish directional and color preferences in the plus-maze and T-maze behavioral tasks. Experiment 1 evaluated directional preference and experiment 2 evaluated color preference in a plus-maze task; experiment 3 evaluated preference between black or white in a T-maze task. Individual preferences were expressed as the percentage of time spent in each zone. Our results showed that male and female zebrafish demonstrated no difference in directional preference in the plus-maze task. Surprisingly, male and female zebrafish showed color preference differences in the plus-maze task; males did not show any color preference, while female zebrafish demonstrated a red preference compared to white, blue, and yellow colors. Moreover, both male and female zebrafish demonstrated a strong black color preference compared to the white color in the T-maze task. Our findings characterized the spontaneous preference of male and female zebrafish for direction and color, identifying possible biases, and providing insights that contribute to the standardization of future protocols.

Acute behavioral and Neurochemical Effects of Novel N-Benzyl-2-Phenylethylamine Derivatives in Adult Zebrafish

Hallucinogenic drugs potently affect brain and behavior and have also recently emerged as potentially promising agents in pharmacotherapy. Complementing laboratory rodents, the zebrafish (Danio rerio) is a powerful animal model organism for screening neuroactive drugs, including hallucinogens. Here, we test a battery of ten novel N-benzyl-2-phenylethylamine (NBPEA) derivatives with the 2,4- and 3,4-dimethoxy substitutions in the phenethylamine moiety and the -OCH₃, -OCF₃, -F, -Cl, and -Br substitutions in the ortho position of the phenyl ring of the N-benzyl moiety, assessing their acute behavioral and neurochemical effects in the adult zebrafish. Overall, substitutions in the Overall, substitutions in the N-benzyl moiety modulate locomotion, and substitutions in the phenethylamine moiety alter zebrafish anxiety-like behavior, also affecting the brain serotonin and/or dopamine turnover. The 24H-NBOMe(F) and 34H-NBOMe(F) treatment also reduced zebrafish despair-like behavior. Computational analyses of zebrafish behavioral data by artificial intelligence identified several distinct clusters for these agents, including anxiogenic/hypolocomotor (24H-NBF, 24H-NBOMe, and 34H-NBF), behaviorally inert (34H-NBBr, 34H-NBCl, and 34H-NBOMe), anxiogenic/hallucinogenic-like (24H-NBBr, 24H-NBCl, and 24H-NBOMe(F)), and anxiolytic/hallucinogenic-like (34H-NBOMe(F)) drugs. Our computational analyses also revealed phenotypic similarity of the behavioral activity of some NBPEAs to that of selected conventional serotonergic and antiglutamatergic hallucinogens. In silico functional molecular activity modeling further supported the overlap of the drug targets for NBPEAs tested here and the conventional serotonergic and antiglutamatergic hallucinogens. Overall, these findings suggest potent neuroactive properties of several novel synthetic NBPEAs, detected in a sensitive in vivo vertebrate model system, the zebrafish, raising the possibility of their potential clinical use and abuse.

Sex-Specific Effects of Acute Ethanol Exposure on Locomotory Activity and Exploratory Behavior in Adult Zebrafish (Danio rerio)

The zebrafish (Danio rerio) is an established model organism in pharmacology and biomedicine, including in research on alcohol use disorders and alcohol-related disease. In the past 2 decades, zebrafish has been used to study the complex effects of ethanol on the vertebrate brain and behavior in both acute, chronic and developmental exposure paradigms. Sex differences in the neurobehavioral response to ethanol are well documented for humans and rodents, yet no consensus has been reached for zebrafish. Here, we show for the first time that male zebrafish of the AB strain display more severe behavioral impairments than females for equal exposure concentrations. Adult zebrafish were immersed in 0, 1 or 2% (v/v) ethanol for 30 min, after which behavior was individually assessed in the zebrafish Multivariate Concentric Square Field™ (zMCSF) arena. Males exposed to 2% ethanol showed clear signs of sedation, including reduced activity, increased shelter seeking and reduced exploration of shallow zones. The 1% male group displayed effects in the same direction but of smaller magnitude; this group also explored the shallow areas less, but did not show a general reduction in activity nor an increase in shelter seeking. By contrast, 1 and 2% exposed females showed no alterations in explorative behavior. Females exposed to 2% ethanol did not display a general reduction in activity, rather activity gradually increased from hypoactivity to hyperactivity over the course of the test. This mixed stimulatory/depressant effect was only quantifiable when locomotory variables were analyzed over time and was not apparent from averages of the whole 30-min test, which may explain why previous studies failed to detect sex-specific effects on locomotion. Our results emphasize the importance of explicitly including sex and time as factors in pharmacological studies of zebrafish behavior. We hypothesize that the lower sensitivity of female zebrafish to ethanol may be explained by their greater body weight and associated larger distribution volume for ethanol, which may render lower brain ethanol concentrations in females.

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.

Male zebrafish (Danio rerio) do not preferentially associate with familiar over unfamiliar conspecifics

Members of several shoaling species have been shown to prefer to associate with familiar individuals, enhancing the benefits of aggregation. The authors used a series of social preference tasks in the laboratory to evaluate whether prior familiarity with potential partners influences preference of shoaling partner in male zebrafish (Danio rerio), a social species found in shallow, slow-moving waters. The authors found that though male zebrafish exhibited a strong preference for shoaling with a male conspecific as opposed to remaining alone, they exhibited no preference for familiar over unfamiliar conspecifics. This suggests that the benefits of familiarity for shoaling behaviour may not be as important for male zebrafish as has been shown in other social fish species.

Visual Function is Gradually Restored During Retina Regeneration in Adult Zebrafish

In comparison to mammals, zebrafish are able to regenerate many organs and tissues, including the central nervous system (CNS). Within the CNS-derived neural retina, light lesions result in a loss of photoreceptors and the subsequent activation of Müller glia, the retinal stem cells. Müller glia-derived progenitors differentiate and eventually restore the anatomical tissue architecture within 4 weeks. However, little is known about how light lesions impair vision functionally, as well as how and to what extent visual function is restored during the course of regeneration, in particular in adult animals. Here, we applied quantitative behavioral assays to assess restoration of visual function during homeostasis and regeneration in adult zebrafish. We developed a novel vision-dependent social preference test, and show that vision is massively impaired early after lesion, but is restored to pre-lesion levels within 7 days after lesion. Furthermore, we employed a quantitative optokinetic response assay with different degrees of difficulty, similar to vision tests in humans. We found that vision for easy conditions with high contrast and low level of detail, as well as color vision, was restored around 7–10 days post lesion. Vision under more demanding conditions, with low contrast and high level of detail, was regained only later from 14 days post lesion onwards. Taken together, we conclude that vision based on contrast sensitivity, spatial resolution and the perception of colors is restored after light lesion in adult zebrafish in a gradual manner.

Social Stress Increases Anxiety-Like Behavior Equally in Male and Female Zebrafish

Zebrafish anxiety-like behavior was assessed in the novel tank test after the formation of dominant-subordinate hierarchies. Ten pairs of animals were subjected to dyadic interactions for 5 days, and compared with control animals. After this period, a clear dominance hierarchy was established across all dyads, irrespective of sex. Social status affected parameters of anxiety-like behavior in the novel tank test, with subordinate males and females displaying more bottom-dwelling, absolute turn angle, and freezing than dominant animals and controls. The results suggest that subordinate male and female zebrafish show higher anxiety-like behavior, which together with previous literature suggests that subordination stress is conserved across vertebrates.

Long-term exposure to cyprodinil causes abnormal zebrafish aggressive and antipredator behavior through the hypothalamic-pituitary-interrenal axis

Cyprodinil, one of the main pyrimidinamine fungicides, has been used to control fungal diseases in plants and vegetables worldwide. Previous studies have investigated the influences of cyprodinil on the developmental and reproductive toxicity of fish. However, it remains unknown whether it affects fish behaviors and the underlying mechanisms. In our current study, zebrafish, an ideal model animal for behavioral studies, were exposed to cyprodinil from fertilization to 240 days postfertilization at 0.1 μg/L (environmentally relevant concentration) and 1, 10 μg/L. Firstly, we observed that aggressive behavior of zebrafish was significantly enhanced after exposure to 0.1-10 μg/L cyprodinil and antipredator behavior was decreased after exposure. Cyprodinil exposure altered the adrenocorticotropic hormone and cortisol levels, which regulate cortisol homeostasis and were significantly reduced in all exposure groups (0.1-10 μg/L). In addition, most of the key genes in the hypothalamic-pituitary-interrenal gland axis, such as corticotropin-releasing hormone and melanocortin 2 receptor, were downregulated significantly in all exposure groups, which was consistent with the hormone levels. In addition, in the hypothalamus, the number of apoptotic cells increased in a dose-dependent manner in the cyprodinil exposure groups. Moreover, these changes were potentially responsible for the increased aggression of zebrafish during the mirror-like aggressive test and for the reduced antipredator behavior during the predator avoidance test. Overall, the data provided herein further our understanding of cyprodinil toxicity and can be used to assess the ecological effects of cyprodinil on the induction of abnormal behaviors at the environmental level.

Post-Traumatic Epilepsy in Zebrafish Is Drug-Resistant and Impairs Cognitive Function

Post-traumatic epilepsy (PTE) is acquired epilepsy after traumatic brain injury (TBI). Despite the availability of more than 20 antiseizure medications (ASMs), there is no way at present to prevent epileptogenesis in TBI survivors, and many cases of PTE become drug-resistant. Importantly, the adverse effects of ASMs can significantly affect patients' quality of life. Mammalian models are commonly used for studying refractory PTE, but are expensive and laborious. Zebrafish models have become popular for studying epilepsy, but most focus on larvae, and there have been no reports to date of pharmacological screening in an adult zebrafish model of acquired epilepsy. Valid animal models are critical for understanding PTE and for developing novel therapeutics. The aim of the present study was to characterize the cognitive impairments of a zebrafish model of TBI that leads to the development of PTE. Using combined behavioral and electrophysiological approaches, we also characterized the pharmacological effects of the most commonly used ASMs to manage PTE (valproate, carbamazepine, and phenytoin). Zebrafish with PTE exhibited impairments in learning and memory, difficulty in decision making, and reduced social preference. Valproate and carbamazepine had a limited protective effect against behavioral seizures, and all three drugs failed to significantly reduce electrographical seizures. The negative impacts of TBI and ASMs in zebrafish parallel those observed in other animals, making the zebrafish model of PTE a promising high-throughput model of refractory and drug-resistant epilepsy.

5-HT2C agonists and antagonists block different components of behavioral responses to potential, distal, and proximal threat in zebrafish

Serotonin (5-HT) receptors have been implicated in responses to aversive stimuli in mammals and fish, but its precise role is still unknown. Moreover, since at least seven families of 5-HT receptors exist in vertebrates, the role of specific receptors is still debated. Aversive stimuli can be classified as indicators of proximal, distal, or potential threat, initiating responses that are appropriate for each of these threat levels. Responses to potential threat usually involve cautious exploration and increased alertness, while responses to distal and proximal threat involve a fight-flight-freeze reaction. We exposed adult zebrafish to a conspecific alarm substance (CAS) and observed behavior during (distal threat) and after (potential threat) exposure, and treated with the 5-HT_2C receptor agonists MK-212 or WAY-161503 or with the antagonist RS-102221. The agonists blocked CAS-elicited defensive behavior (distal threat), but not post-exposure increases in defensive behavior (potential threat), suggesting inhibition of responses to distal threat. MK-212 blocked changes in freezing elicited by acute restraint stress, a model of proximal threat, while RS-102221 blocked changes in geotaxis elicited this stressor. We also found that RS-102221, a 5-HT_2C receptor antagonist, produced small effect on behavior during and after exposure to CAS. Preprint: https://www.biorxiv.org/content/10.1101/2020.10.04.324202; Data and scripts: https://github.com/lanec-unifesspa/5-HT-CAS/tree/master/data/5HT2C.

Sexually dimorphic and asymmetric effects of embryonic ethanol exposure on hypocretin/orexin neurons as related to behavioral changes in zebrafish

Neurons expressing the neuropeptide hypocretin/orexin (Hcrt) in the hypothalamus promote reward-related behaviors including alcohol consumption and are shown in rodents and zebrafish to be stimulated by embryonic exposure to ethanol (EtOH). We used here in zebrafish three-dimensional analyses of the entire population of Hcrt neurons to examine how embryonic EtOH exposure at low-moderate concentrations (0.1% or 0.5% v/v) alters these neurons in relation to behavior. We found that EtOH in the water for 2 h (22-24 h post fertilization) increases the number of Hcrt neurons on the left but not right side of the brain through a stimulation of cell proliferation, this is accompanied by a decrease in locomotor activity under novel conditions but not after habituation, and these effects are evident in both larvae and adults indicating they are long lasting. Our analyses in adults revealed sexually dimorphic effects, with females consuming more EtOH-gelatin and exhibiting more freezing behavior along with an asymmetric increase in Hcrt neurons and males exhibiting increased aggression with no change in Hcrt. These findings suggest that a long lasting, asymmetric increase in Hcrt neurons induced by EtOH results from an asymmetric increase in proliferation specific to Hcrt and contributes to behavioral changes in females.

Melatonin administration attenuates acute stress by inducing sleep state in zebrafish (Danio rerio)

Melatonin plays a fundamental homeostatic role in basic biological functions, and an anti-stress role has been also proposed for this hormone. This study aimed to evaluate hormonal, enzymatic and behavioral parameters of zebrafish that received administration of melatonin and were submitted to acute stress. A total of 120 wild-type zebrafish were divided into five groups: naïve control (N), negative control group (Stress/C), positive control treated with diazepam (Stress/Diaz), treatment with melatonin at dose 1 (Stress/Melt. 1) and treatment with melatonin at dose 2 (Stress/Melt. 2). The exposure to treatments (diazepam or melatonin) was performed prior to the acute stress protocol, based on a chase by a fishing net during 5 min followed by exposure to the air for 1 min. The body cortisol levels were assessed, as well as oxidative stress (thiobarbituric acid reactive substances, reactive species of oxygen and antioxidant activity), and fish behavior (open field test). Melatonin was able to modulate acute stress effects on zebrafish by inhibiting cortisol increasing levels, reducing locomotor parameters, inducing a sleep state, reducing lipid peroxidation and stimulating antioxidant enzymatic activity.

Modulation of behavioral and neurochemical responses of adult zebrafish by fluoxetine, eicosapentaenoic acid and lipopolysaccharide in the prolonged chronic unpredictable stress model

Long-term recurrent stress is a common cause of neuropsychiatric disorders. Animal models are widely used to study the pathogenesis of stress-related psychiatric disorders. The zebrafish (Danio rerio) is emerging as a powerful tool to study chronic stress and its mechanisms. Here, we developed a prolonged 11-week chronic unpredictable stress (PCUS) model in zebrafish to more fully mimic chronic stress in human populations. We also examined behavioral and neurochemical alterations in zebrafish, and attempted to modulate these states by 3-week treatment with an antidepressant fluoxetine, a neuroprotective omega-3 polyunsaturated fatty acid eicosapentaenoic acid (EPA), a pro-inflammatory endotoxin lipopolysaccharide (LPS), and their combinations. Overall, PCUS induced severe anxiety and elevated norepinephrine levels, whereas fluoxetine (alone or combined with other agents) corrected most of these behavioral deficits. While EPA and LPS alone had little effects on the zebrafish PCUS-induced anxiety behavior, both fluoxetine (alone or in combination) and EPA restored norepinephrine levels, whereas LPS + EPA increased dopamine levels. As these data support the validity of PCUS as an effective tool to study stress-related pathologies in zebrafish, further research is needed into the ability of various conventional and novel treatments to modulate behavioral and neurochemical biomarkers of chronic stress in this model organism.

Acute Citalopram administration modulates anxiety in response to the context associated with a robotic stimulus in zebrafish

BACKGROUND

Anxiety represents one of the most urgent health challenges in Western Countries, where it is associated with major medical and societal costs. A common therapeutic approach is the use of selective serotonin reuptake inhibitors, such as Citalopram. However, this treatment of choice is characterized by incomplete efficacy and potential side effects. Preclinical research is needed to detail the mechanisms underlying therapeutic efficacy of available treatments.

METHODS

Zebrafish, a rapidly emerging model species, constitutes an excellent candidate for high-throughput studies in behavioral pharmacology. Here, we present a robotics-based experimental paradigm to investigate the effects of acute Citalopram administration on conditioned place aversion. We trained adult subjects in a three-partitioned tank, consisting of one central and two lateral compartments: the latter were associated either with a fear eliciting robotic stimulus or with an empty environment. Following training, we implemented an automated three-dimensional tracking system to assess the spatial association and detail individual phenotype in a stimulus-free test session.

RESULTS

We observed a linear dose-response profile with respect to geotaxis, with increasing Citalopram concentrations reducing the tendency to swim near the bottom of the tank. Although control subjects failed to exhibit the predicted conditioned aversion, we found preliminary evidence that Citalopram may affect sexes differentially, with male subjects showing increased conditioned aversion at low Citalopram concentration.

CONCLUSIONS

Experimental paradigms based on robotics and three-dimensional tracking can contribute methodological advancements in zebrafish behavioral psychopharmacology.

Sex differences shape zebrafish performance in a battery of anxiety tests and in response to acute scopolamine treatment

Sex differences influence human and animal behavioral and pharmacological responses. The zebrafish (Danio rerio) is a powerful, popular model system in neuroscience and drug screening. However, the impact of zebrafish sex differences on their behavior and drug responses remains poorly understood. Here, we evaluate baseline anxiety-like behavior in adult male and female zebrafish, and its changes following an acute 30-min exposure to 800-μM scopolamine, a common psychoactive anticholinergic drug. Overall, we report high baseline anxiety-like behavior and more individual variability in locomotion in female zebrafish, as well as distinct, sex-specific (anxiolytic-like in females and anxiogenic-like in males) effects of scopolamine. Collectively, these findings reinforce the growing importance of zebrafish models for studying how both individual and sex differences shape behavioral and pharmacological responses.

Unconventional anxiety pharmacology in zebrafish: Drugs beyond traditional anxiogenic and anxiolytic spectra

Anxiety is the most prevalent brain disorder and a common cause of human disability. Animal models are critical for understanding anxiety pathogenesis and its pharmacotherapy. The zebrafish (Danio rerio) is increasingly utilized as a powerful model organism in anxiety research and anxiolytic drug screening. High similarity between human, rodent and zebrafish molecular targets implies shared signaling pathways involved in anxiety pathogenesis. However, mounting evidence shows that zebrafish behavior can be modulated by drugs beyond conventional anxiolytics or anxiogenics. Furthermore, these effects may differ from human and/or rodent responses, as such 'unconventional' drugs may affect zebrafish behavior despite having no such profiles (or exerting opposite effects) in humans or rodents. Here, we discuss the effects of several putative unconventional anxiotropic drugs (aspirin, lysergic acid diethylamide (LSD), nicotine, naloxone and naltrexone) and their potential mechanisms of action in zebrafish. Emphasizing the growing utility of zebrafish models in CNS drug discovery, such unconventional anxiety pharmacology may provide important, evolutionarily relevant insights into complex regulation of anxiety in biological systems. Albeit seemingly complicating direct translation from zebrafish into clinical phenotypes, this knowledge may instead foster the development of novel CNS drugs, eventually facilitating innovative treatment of patients based on novel 'unconventional' targets identified in fish models.

Early social deprivation does not affect cortisol response to acute and chronic stress in zebrafish

Social isolation is a well-established technique for inducing early adversity but, in rodent models, the need of parental care makes it difficult to distinguish the effects of social deprivation from the consequences of nutritional deficiencies. Zebrafish do not require parental care, allowing separation of social deprivation from nutritional deprivation, and have emerged as a promising model to study ontogeny of normal and pathological behaviors relevant for human neuropsychological disorders. Previous reports of life-long isolation in zebrafish showed some consistency with mammalian literature, depicting later social deficits and locomotor hyperactivity. However, unlike reports of higher anxiety and stress behavior in isolated rodents and primates, behavioral responses were tapered in isolated fish. To examine whether life-long developmental isolation has a dampening effect on zebrafish endocrine stress response, we applied stressors to zebrafish siblings that were either isolated or socialized, and compared their whole-body cortisol levels with non-stressed control siblings kept in low-housing densities. Utilizing previously validated paradigms (exposure to novel tank and unpredictable chronic stress), we exposed separate groups (n = 9-14, mixed-sex) of social and isolated zebrafish to acute and chronic stressors and measured their cortisol levels. A univariate ANOVA and post-hoc Tukey's HSD tests confirmed that compared to socially raised control fish, developmental isolation did not increase baseline cortisol levels in zebrafish. Additionally, compared to the non-stressed condition, application of both acute and chronic stressors significantly increased cortisol levels in isolated fish and, to a similar degree, to socially raised fish. Our findings suggest that zebrafish isolation studies may help separate effects of social deprivation from nonsocial aspects of early adversity. These studies further substantiate the use of developmental isolation in zebrafish, particularly with acute and chronic stress paradigms, for modeling neuropsychological disorders.LAY SUMMARYA difficult childhood can make humans react more frequently or severely to later stress and modeling this effect in animals can help explain how and why early stress affects subsequent mental and physical health. Early social isolation does not affect later response to stressful situations in adult zebrafish, providing us with a model of psychiatric disorders that allows separation of effects of poor physical environments (lacking food, shelter, etc.) from poor social environments (lack of appropriate socialization).

[Current status of drug safety evaluation using zebrafish]

In recent years, the success rate of drug development has declined, and along with it, R&D costs have continued to rise. The rate of discontinuation of drug development due to safety reasons remains unchanged from 20 years ago. Therefore, it is important to check the safety of candidate compounds early in drug discovery in order to improve drug discovery efficiency. Under such circumstances, each company is focusing on establishing a low-cost, high-precision, and high-throughput safety screening system. The zebrafish is expected as a new experimental animal that serves as a bridge between in vitro and in vivo, and the progress of research in the last 15 years has been remarkable. At present, zebrafish are becoming a major experimental animal in Japan. At the same time, the gap between ideal and reality began to be seen, and it was time to once again understand the characteristics of zebrafish and think about its usage. This paper summarizes the points to be noted in the screening using zebrafish and introduces the use for actual safety evaluation.

Understanding neurobehavioral effects of acute and chronic stress in zebrafish

Stress is a common cause of neuropsychiatric disorders, evoking multiple behavioral, endocrine and neuro-immune deficits. Animal models have been extensively used to understand the mechanisms of stress-related disorders and to develop novel strategies for their treatment. Complementing rodent and clinical studies, the zebrafish (Danio rerio) is one of the most important model organisms in biomedicine. Rapidly becoming a popular model species in stress neuroscience research, zebrafish are highly sensitive to both acute and chronic stress, and show robust, well-defined behavioral and physiological stress responses. Here, we critically evaluate the utility of zebrafish-based models for studying acute and chronic stress-related CNS pathogenesis, assess the advantages and limitations of these aquatic models, and emphasize their relevance for the development of novel anti-stress therapies. Overall, the zebrafish emerges as a powerful and sensitive model organism for stress research. Although these fish generally display evolutionarily conserved behavioral and physiological responses to stress, zebrafish-specific aspects of neurogenesis, neuroprotection and neuro-immune responses may be particularly interesting to explore further, as they may offer additional insights into stress pathogenesis that complement (rather than merely replicate) rodent findings. Compared to mammals, zebrafish models are also characterized by increased availability of gene-editing tools and higher throughput of drug screening, thus being able to uniquely empower translational research of genetic determinants of stress and resilience, as well as to foster innovative CNS drug discovery and the development of novel anti-stress therapies.

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

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

Zebrafish models of impulsivity and impulse control disorders

Impulse control disorders (ICDs) are characterized by generalized difficulty controlling emotions and behaviors. ICDs are a broad group of the central nervous system (CNS) disorders including conduct disorder, intermittent explosive, oppositional-defiant disorder, antisocial personality disorder, kleptomania, pyromania and other illnesses. Although they all share a common feature (aberrant impulsivity), their pathobiology is complex and poorly understood. There are also currently no ICD-specific therapies to treat these illnesses. Animal models are a valuable tool for studying ICD pathobiology and potential therapies. The zebrafish (Danio rerio) has become a useful model organism to study CNS disorders due to high genetic and physiological homology to mammals, and sensitivity to various pharmacological and genetic manipulations. Here, we summarize experimental models of impulsivity and ICD in zebrafish and highlight their growing translational significance. We also emphasize the need for further development of zebrafish ICD models to improve our understanding of their pathogenesis and to search for novel therapeutic treatments.

Sex matters in neuroscience and neuropsychopharmacology

Prevalence and symptoms of most psychiatric and neurological disorders differ in men and women and there is substantial evidence that their neurobiological basis and treatment also differ by sex. This special issue sought to bring together a series of empirical papers and targeted reviews to highlight the diverse impact of sex in neuroscience and neuropsychopharmacology. This special issue emphasizes the diverse impact of sex in neuroscience and neuropsychopharmacology, including 9 review papers and 17 research articles highlighting investigation in different species (zebrafish, mice, rats, and humans). Each contribution covers scientific topics that overlap with genetics, endocrinology, cognition, behavioral neuroscience, neurology, and pharmacology. Investigating the extent to which sex differences can impact the brain and behavior is key to moving forward in neuroscience research.

Delayed behavioral and genomic responses to acute combined stress in zebrafish, potentially relevant to PTSD and other stress-related disorders: Focus on neuroglia, neuroinflammation, apoptosis and epigenetic modulation

Stress is a common trigger of stress-related illnesses, such as anxiety, phobias, depression and post-traumatic stress disorder (PTSD). Various animal models successfully reproduce core behaviors of these clinical conditions. Here, we develop a novel zebrafish model of stress (potentially relevant to human stress-related disorders), based on delayed persistent behavioral, endocrine and genomic responses to an acute severe 'combined' stressor. Specifically, one week after adult zebrafish were exposed to a complex combined 90-min stress, we assessed their behaviors in the novel tank and the light-dark box tests, as well as whole-body cortisol and brain gene expression, focusing on genomic biomarkers of microglia, astrocytes, neuroinflammation, apoptosis and epigenetic modulation. Overall, stressed fish displayed persistent anxiety-like behavior, elevated whole-body cortisol, as well as upregulated brain mRNA expression of genes encoding the glucocorticoid receptor, neurotrophin BDNF and its receptors (TrkB and P75), CD11b (a general microglial biomarker), COX-2 (an M1-microglial biomarker), CD206 (an M2-microglial biomarker), GFAP (a general astrocytal biomarker), C3 (an A1-astrocytal biomarker), S100α₁₀ (an A2-astrocytal biomarker), as well as pro-inflammatory cytokines IL-6, IL-1β, IFN-γ and TNF-α. Stress exposure also persistently upregulated the brain expression of several key apoptotic (Bax, Caspase-3, Bcl-2) and epigenetic genes (DNMT3a, DNMT3b, HAT1, HDAC4) in these fish. Collectively, the present model not only successfully recapitulates lasting behavioral and endocrine symptoms of clinical stress-related disorders, but also implicates changes in neuroglia, neuroinflammation, apoptosis and epigenetic modulation in long-term effects of stress pathogenesis in vivo.

The zebrafish tail immobilization (ZTI) test as a new tool to assess stress-related behavior and a potential screen for drugs affecting despair-like states

BACKGROUND

Affective disorders, especially depression and anxiety, are highly prevalent, debilitating mental illnesses. Animal experimental models are a valuable tool in translational affective neuroscience research. A hallmark phenotype of clinical and experimental depression, the learned helplessness, has become a key target for 'behavioral despair'-based animal models of depression. The zebrafish (Danio rerio) has recently emerged as a promising novel organism for affective disease modeling and CNS drug screening. Despite being widely used to assess stress and anxiety-like behaviors, there are presently no clear-cut despair-like models in zebrafish.

NEW METHOD

Here, we introduce a novel behavioral paradigm, the zebrafish tail immobilization (ZTI) test, as a potential tool to assess zebrafish despair-like behavior. Conceptually similar to rodent 'despair' models, the ZTI protocol involves immobilizing the caudal half of the fish body for 5 min, leaving the cranial part to move freely, suspended vertically in a small beaker with water.

RESULTS

To validate this model, we used exposure to low-voltage electric shock, alarm pheromone, selected antidepressants (sertraline and amitriptyline) and an anxiolytic drug benzodiazepine (phenazepam), assessing the number of mobility episodes, time spent 'moving', total distance moved and other activity measures of the cranial part of the body, using video-tracking. Both electric shock and alarm pheromone decreased zebrafish activity in this test, antidepressants increased it, and phenazepam was inactive. Furthermore, a 5-min ZTI exposure increased serotonin turnover, elevating the 5-hydroxyindoleacetic acid/serotonin ratio in zebrafish brain, while electric shock prior to ZTI elevated both this and the 3,4-dihydroxyphenylacetic acid/dopamine ratios. In contrast, preexposure to antidepressants sertraline and amitriptyline lowered both ratios, compared to the ZTI test-exposed fish.

COMPARISON WITH EXISTINGMETHOD(S)

The ZTI test is the first despair-like experimental model in zebrafish.

CONCLUSIONS

Collectively, this study suggests the ZTI test as a potentially useful protocol to assess stress-/despair-related behaviors, potentially relevant to CNS drug screening and behavioral phenotyping of zebrafish.

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.