Modeling consequences of prolonged strong unpredictable stress in zebrafish: Complex effects on behavior and physiology

Towards zebrafish models to unravel translational insights of obsessive-compulsive disorder: A neurobehavioral perspective

Obsessive-compulsive disorder (OCD) is a chronic and debilitating illness that has been considered a polygenic and multifactorial disorder, challenging effective therapeutic interventions. Although invaluable advances have been obtained from human and rodent studies, several molecular and mechanistic aspects of OCD etiology are still obscure. Thus, the use of non-traditional animal models may foster innovative approaches in this field, aiming to elucidate the underlying mechanisms of disease from an evolutionary perspective. The zebrafish (Danio rerio) has been increasingly considered a powerful organism in translational neuroscience research, especially due to the intrinsic features of the species. Here, we outline target mechanisms of OCD for translational research, and discuss how zebrafish-based models can contribute to explore neurobehavioral aspects resembling those found in OCD. We also identify possible advantages and limitations of potential zebrafish-based models, as well as highlight future directions in both etiological and therapeutic research. Lastly, we reinforce the use of zebrafish as a promising tool to unravel the biological basis of OCD, as well as novel pharmacological therapies in the field.

Parental preconception stress in zebrafish induces long-lasting anxiety in offspring

The growth and function of the vertebrate brain are impacted by environmental stimuli and early life stress. Adults who experience chronic stress during early life are more likely to suffer various neurodevelopmental and health issues. However, our understanding of how these specific environmental signals at different developmental stages affect brain development is poorly understood. In this study, we investigated if stress in parents prior to conception modulates neurodevelopment in offspring. We used a chronic unpredictable stress model adapted to zebrafish, which is an increasingly popular vertebrate model in neuroscience research to investigate the effects of both maternal and paternal preconception stress on offspring behavior. We evaluated the responsiveness of three anxiety-related behavioral paradigms in zebrafish: the novel tank test, thigmotaxis, and shoaling behavior. We found larvae from stressed females exhibited anxiety-like behavior in a thigmotaxis assay. As these larvae matured into adults, they continued to exhibit anxiety-like behavior in a novel tank and shoaling behavioral assay. These studies indicate preconception stress exposure in parents can induce life-long alterations in offspring neurodevelopment. Further, these results expand the hypothesis that chronically elevated glucocorticoid signaling not only in stressed mothers, but also stressed dads can affect neurodevelopment in offspring. We propose that zebrafish may be a useful model to study the transgenerational effects of chronic stress mediated via the maternal and paternal line.

Structural Characterization and Antidepressant-like Effects of Polygonum sibiricum Polysaccharides on Regulating Microglial Polarization in Chronic Unpredictable Mild Stress-Induced Zebrafish

Polysaccharides are one of the main active ingredients of Polygonum sibiricum (PS), which is a food and medicine homolog used throughout Chinese history. The antidepressant-like effects of PSP and its underlying mechanisms remain elusive, especially the regulation of microglial polarization. The current study determined the chemical composition and structural characteristics of PSP. Then, the chronic unpredictable mild stress (CUMS) procedure was carried out on the zebrafish for 5 weeks, and PSP was immersed for 9 days (1 h/d). The body weight of zebrafish was monitored, and behavioral tests, including the novel tank test and light and dark tank test, were performed to evaluate the antidepressant-like effects of PSP. Then, the function of the hypothalamic-pituitary-interrenal (HPI) axis, the levels of peripheral inflammation, neuronal and blood-brain barrier damage in the mesencephalon and telencephalon, and the mRNA expression of M1/M2 phenotype genes in the brain were examined. PSP samples had the typical structural characteristics of polysaccharides, consisting of glucose, mannose, and galactose, with an average Mw of 20.48 kDa, which presented porous and agglomerated morphologies. Compared with untreated zebrafish, the depression-like behaviors of CUMS-induced zebrafish were significantly attenuated. PSP significantly decreased the levels of cortisol and pro-inflammatory cytokines and increased the levels of the anti-inflammatory cytokines in the body of CUMS-induced depressive zebrafish. Furthermore, PSP remarkably reversed the neuronal and blood-brain barrier damage in the mesencephalon and telencephalon and the mRNA expression of M1/M2 phenotype genes in the brain. These findings indicated that the antidepressant-like effects of PSP were related to altering the HPI axis hyperactivation, suppressing peripheral inflammation, inhibiting neuroinflammation induced by microglia hyperactivation, and modulating microglial M1/M2 polarization. The current study provides the foundations for future examinations of PSP in the functional foods of emotional regulation.

Developing Peripheral Biochemical Biomarkers of Brain Disorders: Insights from Zebrafish Models

High prevalence of human brain disorders necessitates development of the reliable peripheral biomarkers as diagnostic and disease-monitoring tools. In addition to clinical studies, animal models markedly advance studying of non-brain abnormalities associated with brain pathogenesis. The zebrafish (Danio rerio) is becoming increasingly popular as an animal model organism in translational neuroscience. These fish share some practical advantages over mammalian models together with high genetic homology and evolutionarily conserved biochemical and neurobehavioral phenotypes, thus enabling large-scale modeling of human brain diseases. Here, we review mounting evidence on peripheral biomarkers of brain disorders in zebrafish models, focusing on altered biochemistry (lipids, carbohydrates, proteins, and other non-signal molecules, as well as metabolic reactions and activity of enzymes). Collectively, these data strongly support the utility of zebrafish (from a systems biology standpoint) to study peripheral manifestations of brain disorders, as well as highlight potential applications of biochemical biomarkers in zebrafish models to biomarker-based drug discovery and development.

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.

Fluoxetine rescues the depressive-like behaviour induced by reserpine and the altered emotional behaviour induced by nicotine withdrawal in zebrafish: Involvement of tyrosine hydroxylase

BACKGROUND

Nicotine cessation leads to anxiety and depression.

AIMS

The suitability of the zebrafish model of anhedonia using reserpine and fluoxetine was evaluated. Fluoxetine was also used to reduce nicotine withdrawal-induced anhedonic state.

METHODS

Zebrafish were exposed to reserpine (40 mg/l) and then to fluoxetine (0.1 mg/l) for 1 week. Anhedonia was evaluated in the Novel Tank Diving and Compartment Preference tests. Another group was exposed to nicotine (1 mg/l/2 weeks) and then exposed to fluoxetine. Anxiety and anhedonia were evaluated 2-60 days after. Tyrosine hydroxylase (TH) immunoreactivity and microglial morphology (labelled by 4C4 monoclonal antibody) in the parvocellular pretectal nucleus (PPN), dorsal part, and of calcitonin gene-related peptide (CGRP) in the hypothalamus were also analysed.

RESULTS

Less time in the top and increased latency to the top in reserpine compared to a drug-free group was found. Fluoxetine rescued reserpine-induced the reduced time in the top. Seven and 30 days after nicotine withdrawal more time in the bottom and similar time in the Compartment Preference test, rescued by fluoxetine, were shown. In the PPN, 30-day withdrawal induced an increase in TH immunoreactivity, but fluoxetine induced a further significant increase. No changes in PPN microglia morphology and hypothalamic CGRP were detected.

CONCLUSIONS

Our findings validate the suitability of the zebrafish model of anhedonia using the reserpine-induced depression-like behaviour and the predictivity using fluoxetine. Fluoxetine rescued nicotine withdrawal-induced anhedonic state, opening the possibility to screen new drugs to alleviate anxiety and depression in smokers during abstinence.

Systematic review and meta-analysis of 10 years of unpredictable chronic stress in zebrafish

The zebrafish (Danio rerio) is a model animal that is being increasingly used in neuroscience research. A decade ago, the first study on unpredictable chronic stress (UCS) in zebrafish was published, inspired by protocols established for rodents in the early 1980s. Since then, several studies have been published by different groups, in some cases with conflicting results. Here we conducted a systematic review to identify studies evaluating the effects of UCS in zebrafish and meta-analytically synthetized the data of neurobehavioral outcomes and relevant biomarkers. Literature searches were performed in three databases (PubMed, Scopus and Web of Science) with a two-step screening process based on inclusion/exclusion criteria. The included studies underwent extraction of qualitative and quantitative data, as well as risk-of-bias assessment. Outcomes of included studies (n = 38) were grouped into anxiety/fear-related behavior, locomotor function, social behavior or cortisol level domains. UCS increased anxiety/fear-related behavior and cortisol levels while decreasing locomotor function, but a significant summary effect was not observed for social behavior. Despite including a substantial number of studies, the high heterogeneity and the methodological and reporting problems evidenced in the risk-of-bias analysis made it difficult to assess the internal validity of most studies and the overall validity of the model. Our review thus evidences the need to conduct well-designed experiments to better evaluate the effects of UCS on diverse behavioral patterns displayed by zebrafish.

The zebrafish for preclinical psilocybin research

In this review, we discuss the possible utility of zebrafish in research on psilocybin, a psychedelic drug whose recreational use as well as possible clinical application are gaining increasing interest. First, we review behavioral tests with zebrafish, focussing on anxiety and social behavior, which have particular relevance in the context of psilocybin research. Next, we briefly consider methods of genetic manipulations with which psilocybin's phenotypical effects and underlying mechanisms may be investigated in zebrafish. We briefly review the known mechanisms of psilocybin, and also discuss what we know about its safety and toxicity profile. Last, we discuss examples of how psilocybin may be employed for testing treatment efficacy in preclinical research for affective disorders in zebrafish. We conclude that zebrafish has a promising future in preclinical research on psychedelic drugs.

Sulfonamides (SAs) exposure causes neurobehavioral toxicity at environmentally relevant concentrations (ERCs) in early development of zebrafish

Antibiotics, due to their stability and persistence in the environment, can have chronic impacts on various ecosystems and organisms. However, the molecular mechanisms underlying antibiotic toxicity at environmental concentrations, particularly the neurotoxic effects of sulfonamides (SAs), remain poorly understood. In this study, we assessed the neurotoxicity of six SAs including the sulfadiazine (SD), sulfathiazole (ST), sulfamethoxazole (SMX), sulfisoxazole (SIZ), sulfapyridine (SPD), and sulfadimethoxine (SDM) by exposing zebrafish to environmentally relevant concentrations (ERCs). The SAs exhibited concentration-dependent effects on zebrafish behavior, including spontaneous movement, heartbeat, survival rate, and body metrics, ultimately leading to depressive-like symptoms and sublethal toxicity during early life stages. Notably, even the lowest SA concentration (0.05 μg/L) induced neurotoxicity and behavioral impairment in zebrafish. We observed a dose-dependent increase in melancholy behavior as indicated by increased resting time and decreased motor activity in zebrafish larvae. Following exposure to SAs from 4 to 120 h post-fertilization (hpf), key genes involved in folate synthesis [sepiapterin reductase a (spra), phenylalanine hydroxylase (pah), tyrosine hydroxylase (th), and tryptophan hydroxylase 1 (tryptophan 5-monooxygenase) a tryptophan hydroxylase (tph1a)] and carbonic anhydrase (CA) metabolism [carbonic anhydrase II (ca2), carbonic anhydrase IV a (ca4a), carbonic anhydrase VII (ca7), and carbonic anhydrase XIV (ca14)] were significantly downregulated or inhibited at different concentrations. Our findings demonstrate that acute exposure to six SAs at environmentally relevant concentrations induces developmental and neurotoxic effects in zebrafish, impacting folate synthesis pathways and CA metabolism. These results provide valuable insights into the potential role of antibiotics in depressive disorders and neuroregulatory pathways.

Altered GABAergic, glutamatergic and endocannabinoid signaling is accompanied by neuroinflammatory response in a zebrafish model of social withdrawal behavior

Introduction

Deficits in social communication are in the core of clinical symptoms characterizing many neuropsychiatric disorders such as schizophrenia and autism spectrum disorder. The occurrence of anxiety-related behavior, a common co-morbid condition in individuals with impairments in social domain, suggests the presence of overlapping neurobiological mechanisms between these two pathologies. Dysregulated excitation/inhibition balance and excessive neuroinflammation, in specific neural circuits, are proposed as common etiological mechanisms implicated in both pathologies.

Methods and Results

In the present study we evaluated changes in glutamatergic/GABAergic neurotransmission as well as the presence of neuroinflammation within the regions of the Social Decision-Making Network (SDMN) using a zebrafish model of NMDA receptor hypofunction, following sub-chronic MK-801 administration. MK-801-treated zebrafish are characterized by impaired social communication together with increased anxiety levels. At the molecular level, the behavioral phenotype was accompanied by increased mGluR5 and GAD67 but decreased PSD-95 protein expression levels in telencephalon and midbrain. In parallel, MK-801-treated zebrafish exhibited altered endocannabinoid signaling as indicated by the upregulation of cannabinoid receptor 1 (CB1R) in the telencephalon. Interestingly, glutamatergic dysfunction was positively correlated with social withdrawal behavior whereas defective GABAergic and endocannabinoid activity were positively associated with anxiety-like behavior. Moreover, neuronal and astrocytic IL-1β expression was increased in regions of the SDMN, supporting the role of neuroinflammatory responses in the manifestation of MK-801 behavioral phenotype. Colocalization of interleukin-1β (IL-1β) with β₂-adrenergic receptors (β₂-ARs) underlies the possible influence of noradrenergic neurotransmission to increased IL-1β expression in comorbidity between social deficits and elevated anxiety comorbidity.

Discussion

Overall, our results indicate the contribution of altered excitatory and inhibitory synaptic transmission as well as excessive neuroinflammatory responses in the manifestation of social deficits and anxiety-like behavior of MK-801-treated fish, identifying possible novel targets for amelioration of these symptoms.

Structural environmental enrichment and the way it is offered influence cognitive judgement bias and anxiety-like behaviours in zebrafish

Environmental enrichment in zebrafish generally reduces anxiety-related behaviours, improves learning in maze trials and increases health and biological fitness. However, certain types of enrichment or certain conditions induce the opposite effects. Therefore, it is essential to study the characteristics of environmental enrichment that modulate these effects. This study aims to investigate if structural environmental enrichment and the way it is offered influence cognitive judgement bias and anxiety-like behaviours in adult zebrafish. The fish were assigned to six housing manipulations: constant barren, constant enrichment, gradual gain of enrichment, gradual loss of enrichment, sudden gain of enrichment and sudden loss of enrichment. We then transposed the cognitive judgment bias paradigm, formerly used in studies on other animals to measure the link between emotion and cognition, to objectively assess the impact of these manipulations on the zebrafish's interpretation of ambiguous stimuli, considering previous experiences and related emotional states. We used two battery tests (light/dark and activity tests), which measured anxiety-related behaviours to check if these tests covariate with cognitive bias results. The fish with a sudden gain in enrichment showed a pessimistic bias (interpreted ambiguous stimuli as negative). In addition, the fish that experienced a sudden gain and a gradual loss in enrichment showed more anxiety-like behaviours than the fish that experienced constant conditions or a gradual gain in enrichment. The data provide some proof that structural environmental enrichment and the way it is presented can alter zebrafish's cognitive bias and anxiety-like behaviours.

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.

Centella asiatica (L.) Urb. Extract ameliorates branched-chain amino acid (BCAA) metabolism in acute reserpine-induced stress zebrafish model via 1H Nuclear Magnetic Resonance (NMR)-based metabolomics approach

Depression is a common mental disorder that can adversely affect psychosocial function and quality of life. However, the exact aetiology and pathogenesis of depression are still unclear. Stress plays a major role in the pathogenesis of depression. The use of currently prescribed antidepressants has many side effects. Centella asiatica (C. asiatica) has shown promising antidepressant activity in rodent models. Here, we developed a reserpine-induced zebrafish stress-like model and performed behavioural analysis, cortisol measurement and ¹H-Nuclear Magnetic Resonance (¹H NMR) spectroscopy-based metabolomics analysis to test the anti-stress activity of ethanolic extract of C. asiatica (RECA). A significant increase in total distance travelled (F_(8,8) = 8.905, p = 0.0054) and a reduction in freezing duration (F_{(9, 9)} = 10.38, p = 0.0018) were found in the open field test (OFT). Asiaticoside, one of tested C.asiatica's triterpenoid gives a significant increase in contact duration (F_(5,5) = 142.3, (p = 0.0330) at 2.5 mg/kg). Eight biomarkers were found, i.e. ß-hydroxyisovaleric acid, leucine, threonine, scylloinositol, lactate, betaine, valine, choline and l-fucose, to be responsible for the class separation between stress and RECA-treated groups. Metabolic pathway alteration in zebrafish brain upon treatment with RECA was identified as valine, leucine and isoleucine biosynthesis, while alanine, aspartate, glutamate and glycerophospholipid metabolism was involved after fluoxetine treatment.

Suppression of Chronic Unpredictable Stress-Persuaded Increased Monoamine Oxidase Activity by Taurine Promotes Significant Neuroprotection in Zebrafish Brain

Neuropsychiatric upshots following chronic exposure to unpredictable adverse stressors have been well documented in the literature. Considering the significant impact of chronic unpredictable stress (CUS), the literature is elusive regarding the neuroprotective efficacy of taurine against CUS-induced oxidative stress and chromatin condensation in the zebrafish brain. In this study, to ameliorate CUS-persuaded neurological outcomes, waterborne treatment of taurine as a prophylactic intervention was undertaken. Further, our approach also focused on the gross neurobehavioral response of zebrafish, oxidative stress indices and neuromorphology of the zebrafish brain following CUS exposure with taurine treatment. Because taurine provides significant neuroprotection against oxidative insult, the cytosolic level of monoamine oxidase (MAO) in the zebrafish brain following CUS exposure is worth investigating. Further, as heightened MAO activity is associated with augmented oxidative and chromatin condensation, the focus of this study was on whether taurine provides neuroprotection by downregulating MAO levels in the brain. Our findings show that CUS-persuaded altered neurobehavioral response was significantly rescued by taurine. Moreover, our findings firmly support the hypothesis that taurine acts as a radical neuroprotector by restoring glutathione biosynthesis in the zebrafish brain subsequent to CUS exposure. Additionally, the rising level of brain MAO following chronic exposure to CUS is ameliorated by taurine treatment. These findings strongly advocate the role of taurine as a natural MAO inhibitor through the neuroprotection it provides against CUS-instigated oxidative stress in zebrafish. However, the fundamental neuroprotective mechanism of such natural compounds needs to be elucidated to determine their neuroprotective efficacy against stress regimens.

Animal Inflammation-Based Models of Neuropsychiatric Disorders

Mounting evidence links psychiatric disorders to central and systemic inflammation. Experimental (animal) models of psychiatric disorders are important tools for translational biopsychiatry research and CNS drug discovery. Current experimental models, most typically involving rodents, continue to reveal shared fundamental pathological pathways and biomarkers underlying the pathogenetic link between brain illnesses and neuroinflammation. Recent data also show that various proinflammatory factors can alter brain neurochemistry, modulating the levels of neurohormones and neurotrophins in neurons and microglia. The role of "active" glia in releasing a wide range of proinflammatory cytokines also implicates glial cells in various psychiatric disorders. Here, we discuss recent animal inflammation-related models of psychiatric disorders, focusing on their translational perspectives and the use of some novel promising model organisms (zebrafish), to better understand the evolutionally conservative role of inflammation in neuropsychiatric conditions.

Evolutionarily conserved gene expression patterns for affective disorders revealed using cross-species brain transcriptomic analyses in humans, rats and zebrafish

Widespread, debilitating and often treatment-resistant, depression and other stress-related neuropsychiatric disorders represent an urgent unmet biomedical and societal problem. Although animal models of these disorders are commonly used to study stress pathogenesis, they are often difficult to translate across species into valuable and meaningful clinically relevant data. To address this problem, here we utilized several cross-species/cross-taxon approaches to identify potential evolutionarily conserved differentially expressed genes and their sets. We also assessed enrichment of these genes for transcription factors DNA-binding sites down- and up- stream from their genetic sequences. For this, we compared our own RNA-seq brain transcriptomic data obtained from chronically stressed rats and zebrafish with publicly available human transcriptomic data for patients with major depression and their respective healthy control groups. Utilizing these data from the three species, we next analyzed their differential gene expression, gene set enrichment and protein-protein interaction networks, combined with validated tools for data pooling. This approach allowed us to identify several key brain proteins (GRIA1, DLG1, CDH1, THRB, PLCG2, NGEF, IKZF1 and FEZF2) as promising, evolutionarily conserved and shared affective 'hub' protein targets, as well as to propose a novel gene set that may be used to further study affective pathogenesis. Overall, these approaches may advance cross-species brain transcriptomic analyses, and call for further cross-species studies into putative shared molecular mechanisms of affective pathogenesis.

The Effect of Unpredictable Chronic Stress on Rare Minnow (Gobiocypris rarus): Growth, Behaviour and Physiology

Fishes often adjust their behaviour patterns and physiological responses to cope with changing environments, and different life experiences affect them differently. Fishes might adapt to short-term stress, whereas long-term unpredictable stress may lead to various adverse effects. Although some studies have constructed unpredictable stress models of fish, the effect of unpredictable chronic stress (UCS) in the laboratory is poorly understood in fishes. In the current study, we exposed adult rare minnow to an unpredictable chronic stress protocol over 7 and 14 days and measured their response in terms of growth performance, cortisol, neurotransmitter levels (DA, 5-HT, and related metabolites), and behaviour patterns to comprehensively assess the effects of UCS on laboratory rare minnow. We discovered that specific growth rates were significantly decreased, and cortisol levels were lowered in both 7-days and 14-days stress groups. In the behaviour test, the activity level of the 14-days stress group increased, but there was no significant difference in the number of crossings to the center areas, time spent in the center areas, or the speed. In addition, the levels of DA and 5-HT did not change in the stress groups, but the DOPAC and 5-HIAA levels in the 14 days stress group were significantly higher than those in the control group. These results suggested that UCS influences rare minnow growth performance, behaviour patterns, and cortisol levels, and similar stress should be minimised in the laboratory.

Physical exercise prevents behavioral alterations in a reserpine-treated zebrafish: A putative depression model

Major depressive disorder (MDD) has increasingly reached the world population with an expressive increase in recent years due to the COVID-19 pandemic. Here we used adult zebrafish (Danio rerio) as a model to verify the effects of reserpine on behavior and neurotransmitter levels. We observed an increase in the immobile time and time spent in the bottom zone of the tank in reserpine-exposed animals. The results demonstrated a decrease in distance traveled and velocity. Reserpine exposure did not induce changes in memory and social interaction compared to the control group. We also evaluated the influence of exposure to fluoxetine, a well-known antidepressant, on the behavior of reserpine-exposed animals. We observed a reversal of behavioral alterations caused by reserpine. To verify whether behavioral alterations in the putative depression model induced by reserpine could be prevented, the animals were subjected to physical exercise for 6 weeks. The results showed a protective effect of the physical exercise against the behavioral changes caused by reserpine in zebrafish. In addition, we observed a reduction in dopamine and serotonin levels and an increase in the 3,4-dihydroxyphenylacetic acid (DOPAC) levels in the brain. Physical exercise was able to prevent the changes in dopamine and serotonin levels, reinforcing that the preventive effect promoted by physical exercise is related to the modulation of neurotransmitter levels. Our findings showed that reserpine was effective in the induction of a putative depression model in zebrafish and that physical exercise may be an alternative to prevent the effects induced by reserpine.

Changes in Dendritic Spine Morphology and Density of Granule Cells in the Olfactory Bulb of Anguilla anguilla (L., 1758): A Possible Way to Understand Orientation and Migratory Behavior

Simple Summary

The olfactory bulb can process odour cues through granular cells (GCs) and dendritic spines, changing their synaptic plasticity properties and their morphology. The GCs’ dendritic spines density and morphology were analysed in Anguilla anguilla, considering the olfaction as a driver involved in fish orientation and migration. For the head and neck morphology, spines were classified as mushroom, long thin, stubby, and filopodia. Spines’ density decreased from juvenile migrants to no-migrant stages and increased in the adult migrants. Spines’ density was comparable between glass and silver eels as an adaptation to migration, while at non-migrating phases, spines’ density decreased. For its phylogenetic Elopomorph attribution and its complex life cycle, A. anguilla could be recommended as a model species to study the development of dendritic spines in GCs of the olfactory bulb. Considering the role of olfaction in the orientation and migration of A. anguilla, the modification of environmental stimuli (ocean alterations and climate change) could represent contributing factors that threaten this critically endangered species.

Abstract

Olfaction could represent a pivotal process involved in fish orientation and migration. The olfactory bulb can manage olfactive signals at the granular cell (GC) and dendritic spine levels for their synaptic plasticity properties and changing their morphology and structural stability after environmental odour cues. The GCs’ dendritic spine density and morphology were analysed across the life stages of the catadromous Anguilla anguilla. According to the head and neck morphology, spines were classified as mushroom (M), long thin (LT), stubby (S), and filopodia (F). Total spines’ density decreased from juvenile migrants to no-migrant stages, to increase again in the adult migrant stage. Mean spines’ density was comparable between glass and silver eels as an adaptation to migration. At non-migrating phases, spines’ density decreased for M and LT, while M, LT, and S density increased in silver eels. A great dendritic spine development was found in the two migratory phases, regressing in trophic phases, but that could be recreated in adults, tracing the migratory memory of the routes travelled in juvenile phases. For its phylogenetic Elopomorph attribution and its complex life cycle, A. anguilla could be recommended as a model species to study the development of dendritic spines in GCs of the olfactory bulb as an index of synaptic plasticity involved in the modulation of olfactory stimuli. If olfaction is involved in the orientation and migration of A. anguilla and if eels possess a memory, these processes could be influenced by the modification of environmental stimuli (ocean alterations and rapid climate change) contributing to threatening this critically endangered species.

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.

Behavioral and cortisol analysis of the anti-stress effect of Polygonum minus (Huds) extracts in chronic unpredictable stress (CUS) zebrafish model

The World Health Organization (WHO) recorded approximately 350 million people worldwide have suffered from mental health disorders, such as depression, anxiety, schizophrenia, and addictive behaviors. The search for new drugs from nature has drawn on many biological resources and human practices. In this study, leaves of Polygonum minus standardized extract (Biokesum®), 1 and 100 mg/L were used to evaluate the anti-stress effect in the chronic unpredictable stress (CUS) zebrafish model. Five groups of zebrafish were manipulated in this study, comprising control, chronic unpredictable stress (CUS), CUS + Biokesum® 1 mg/L (4 days, 20 min/day, immersion) CUS + Biokesum® 100 mg/L (4 days, 20 min/day, immersion) and CUS + fluoxetine 0.6 mg/L (4 days, 20 min/day, immersion). Four different behavioral tests were used, i.e. open-field test, social interaction test, light and dark test, and exploratory test. After four consecutive days of treatment, the zebrafish were sacrificed for whole-body cortisol analysis. The exploratory test showed a significant change upon P. minus treatment (one-way ANOVA; p = 0.0011). Cortisol analysis showed a decrease of cortisol level after treatment with the extract and fluoxetine, without significant difference. These results showed that zebrafish is a reliable model to study the anti-stress effect of compounds or herbal extract.

Effect of aerobic exercise as a treatment on type 2 diabetes mellitus with depression-like behavior zebrafish

BACKGROUND

Depression is the most known complication of type 2 diabetes mellitus (T2DM). Aerobic exercise improves glycemic control in T2DM, although the underlying mechanisms of comorbid depression-like behaviors in T2DM have not yet been fully elucidated.

METHODS

120 zebrafish were randomly assigned to four groups: Control, T2DM, T2DM + metformin, and T2DM + aerobic exercise. Then, all animals except the control group were fed with high glucose fairy shrimp (~40 g/kg/day) and exposed reserpine (40 μg/ml for 20 min) for 10 days. Here, behavioral tests were used for model verification. Following the verification, all groups were treated as before. Additionally, the T2DM + metformin group received metformin (~10.6 mg/kg/day) at the same time, while the T2DM + aerobic exercise group received aerobic exercise 30 min/day. Finally, blood glucose and behavioral tests, as well as protein and molecular levels were determined at Day 11 and 12.

RESULTS

Aerobic exercise alleviated depressive-like behavior and enhanced the levels of antidepressant biomarkers (NE, 5-HIAA) in zebrafish after 10 consecutive days of exercise. Additionally, 10 consecutive days of aerobic exercise decreased the levels of inflammatory biomarkers (IFN-γ, IL-1, IL-4) and depressive biomarkers (cortisol). Meanwhile, it also aided in the reduction of CD11b, IL-6, IL-6R, and caspase-3 expression to combat the neuroinflammation induced by T2DM, mediated the BDNF-TrkB pathway, and increased Bcl-2/Bax levels.

CONCLUSION

Given the remarkable similarity in neurochemistry between humans and zebrafish, this study supports the effectiveness of aerobic exercise as clinical guidance in preventing and treating T2DM complicated with depression.

The Effect of Environmental Enrichment on Laboratory Rare Minnows (Gobiocypris rarus): Growth, Physiology, and Behavior

Simple Summary

Environmental enrichment is an important part of animal welfare. In this study, rare minnow in different rearing conditions underwent comprehensive evaluation regarding growth, anxiety-like behavior, and physiology parameters. Results showed that there were no differences in SGR, anxiety-like behavior, DA, DOPAC, and 5-HIAA levels between control and enriched groups. However, the enriched group had higher cortisol and 5-HT levels. Therefore, researchers should focus on the effect of environmental enrichment regarding the welfare of rare minnow and how it effects the validity of data from laboratory studies.

Abstract

Environmental enrichment is a method to increase environmental heterogeneity, which may reduce stress and improve animal welfare. Previous studies have shown that environmental enrichment can increase the growth rate, decrease aggressive and anxiety-like behaviors, improve learning ability and agility, and reduce cortisol levels in animals. These effects usually differ between species. Unfortunately, habitat enrichment on laboratory fish is poorly studied and seldom adopted in care guidance. Rare minnows (Gobiocypris rarus) have been cultured as a native laboratory fish in China in barren banks without environmental enrichment since 1990; they have been widely used in studies on ecotoxicology, environmental science, and other topics. The purpose of this study was to investigate the effect of environment enrichment on the growth, physiological status, and anxiety-like behavior of laboratory rare minnows. We observed and analyzed SGR, cortisol levels, DA, DOPAC, 5-HT and 5-HIAA, and anxiety-like behavior indexes after one month of treatment in barren (control) and enrichment tanks. We found that there were no significant differences in SGR, anxiety-like behavior, DA, DOPAC, or 5-HIAA levels between the two treatments. However, higher cortisol and 5-HT levels were observed in the enrichment tanks. This study suggests that rare minnows might be influenced by their living environment, and future related studies should consider their environmental enrichment.

Towards Modeling Anhedonia and Its Treatment in Zebrafish

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

Application and advantages of zebrafish model in the study of neurovascular unit

The concept of "Neurovascular Unit" (NVU) was put forward, so that the research goal of Central Nervous System (CNS) diseases gradually transitioned from a single neuron to the structural and functional integrity of the NVU. Zebrafish has the advantages of high homology with human genes, strong reproductive capacity and visualization of neural circuits, so it has become an emerging model organism for NVU research and has been applied to a variety of CNS diseases. Based on CNKI (https://www.cnki.net/) and PubMed (https://pubmed.ncbi.nlm.nih.gov/about/) databases, the author of this article sorted out the relevant literature, analyzed the construction of a zebrafish model of various CNS diseases，and the use of diagrams showed the application of zebrafish in the NVU, revealed its relationship, which would provide new methods and references for the treatment and research of CNS diseases.

Understanding how stress responses and stress-related behaviors have evolved in zebrafish and mammals

Stress response is essential for the organism to quickly restore physiological homeostasis disturbed by various environmental insults. In addition to well-established physiological cascades, stress also evokes various brain and behavioral responses. Aquatic animal models, including the zebrafish (Danio rerio), have been extensively used to probe pathobiological mechanisms of stress and stress-related brain disorders. Here, we critically discuss the use of zebrafish models for studying mechanisms of stress and modeling its disorders experimentally, with a particular cross-taxon focus on the potential evolution of stress responses from zebrafish to rodents and humans, as well as its translational implications.

Understanding the effects of Moringa oleifera in chronic unpredictable stressed zebrafish using metabolomics analysis

ETHNOPHARMACOLOGICAL RELEVANCE

Moringa leaves have been used for thousands of years to maintain skin health and mental fitness. People also use it to relieves pain and stress.

AIM OF THE STUDY

To determine the effects of Moringa oleifera leaves ethanol-aqueous (ratio 7:3) extract (MOLE) on the chronically stressed zebrafish.

METHOD

The changes in the stress-related behaviour and the metabolic pathways in response to MOLE treatment in zebrafish were studied. A chronic unpredictable stress model was adopted in which zebrafish were induced with different stressors for 14 days. Stress-related behaviour was assessed using a depth-preference test and a light and dark test. Three doses of MOLE (500, 1000, and 2000 mg/L) were administered to the zebrafish. Upon sacrifice, the brains were harvested and processed for LC-MS QTOF based, global metabolomics analysis.

RESULTS

We observed significant changes in the behavioural parameters, where the swimming time at the light phase and upper phase of the tank were increased in the chronically stressed zebrafish treated with MOLE compared to those zebrafish which were not treated. Further, distinctive metabolite profiles were observed in zebrafish with different treatments. Several pathways that shed light on effects of MOLE were identified. MOLE is believed to relieve stress by regulating pathways that are involved in the metabolism of purine, glutathione, arginine and proline, D-glutamine, and D-glutamate.

CONCLUSION

MOLE is potentially an effective stress reliever. However, its effects in human needs to be confirmed with a systematic randomised control trial.

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.

Zebrafish as an Animal Model for Testing Agents with Antidepressant Potential

Depression is a serious mental disease that, according to statistics, affects 320 million people worldwide. Additionally, a current situation related to the COVID-19 pandemic has led to a significant deterioration of mental health in people around the world. So far, rodents have been treated as basic animal models used in studies on this disease, but in recent years, Danio rerio has emerged as a new organism that might serve well in preclinical experiments. Zebrafish have a lot of advantages, such as a quick reproductive cycle, transparent body during the early developmental stages, high genetic and physiological homology to humans, and low costs of maintenance. Here, we discuss the potential of the zebrafish model to be used in behavioral studies focused on testing agents with antidepressant potential.

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.

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.

Zebrafish Tools for Deciphering Habenular Network-Linked Mental Disorders

Simple Summary

Everything that we think, feel or do depends on the function of neural networks in the brain. These are highly complex structures made of cells (neurons) and their interconnections (axons), which develop dependent on precisely coordinated interactions of genes. Any gene mutation can result in unwanted alterations in neural network formation and concomitant brain disorders. The habenula neural network is one of these important circuits, which has been linked to autism, schizophrenia, depression and bipolar disorder. Studies using the zebrafish have uncovered genes involved in the development of this network. Intriguingly, some of these genes have also been identified as risk genes of human brain disorders highlighting the power of this animal model to link risk genes and the affected network to human disease. But can we use the advantages of this model to identify new targets and compounds with ameliorating effects on brain dysfunction? In this review, we summarise the current knowledge on techniques to manipulate the habenula neural network to study the consequences on behavior. Moreover, we give an overview of existing behavioral test to mimic aspects of mental disorders and critically discuss the applicability of the zebrafish model in this field of research.

Abstract

The prevalence of patients suffering from mental disorders is substantially increasing in recent years and represents a major burden to society. The underlying causes and neuronal circuits affected are complex and difficult to unravel. Frequent disorders such as depression, schizophrenia, autism, and bipolar disorder share links to the habenular neural circuit. This conserved neurotransmitter system relays cognitive information between different brain areas steering behaviors ranging from fear and anxiety to reward, sleep, and social behaviors. Advances in the field using the zebrafish model organism have uncovered major genetic mechanisms underlying the formation of the habenular neural circuit. Some of the identified genes involved in regulating Wnt/beta-catenin signaling have previously been suggested as risk genes of human mental disorders. Hence, these studies on habenular genetics contribute to a better understanding of brain diseases. We are here summarizing how the gained knowledge on the mechanisms underlying habenular neural circuit development can be used to introduce defined manipulations into the system to study the functional behavioral consequences. We further give an overview of existing behavior assays to address phenotypes related to mental disorders and critically discuss the power but also the limits of the zebrafish model for identifying suitable targets to develop therapies.

Effects of acute and chronic arecoline in adult zebrafish: Anxiolytic-like activity, elevated brain monoamines and the potential role of microglia

Arecoline is a naturally occurring psychoactive alkaloid with partial agonism at nicotinic and muscarinic acetylcholine receptors. Arecoline consumption is widespread, making it the fourth (after alcohol, nicotine and caffeine) most used substance by humans. However, the mechanisms of acute and chronic action of arecoline in-vivo remain poorly understood. Animal models are a valuable tool for CNS disease modeling and drug screening. Complementing rodent studies, the zebrafish (Danio rerio) emerges as a promising novel model organism for neuroscience research. Here, we assessed the effects of acute and chronic arecoline on adult zebrafish behavior and physiology. Overall, acute and chronic arecoline treatments produced overt anxiolytic-like behavior (without affecting general locomotor activity and whole-body cortisol levels), with similar effects also caused by areca nut water extracts. Acute arecoline at 10 mg/L disrupted shoaling, increased social preference, elevated brain norepinephrine and serotonin levels and reduced serotonin turnover. Acute arecoline also upregulated early protooncogenes c-fos and c-jun in the brain, whereas chronic treatment with 1 mg/L elevated brain expression of microglia-specific biomarker genes egr2 and ym1 (thus, implicating microglial mechanisms in potential effects of long-term arecoline use). Finally, acute 2-h discontinuation of chronic arecoline treatment evoked withdrawal-like anxiogenic behavior in zebrafish. In general, these findings support high sensitivity of zebrafish screens to arecoline and related compounds, and reinforce the growing utility of zebrafish for probing molecular mechanisms of CNS drugs. Our study also suggests that novel anxiolytic drugs can eventually be developed based on arecoline-like molecules, whose integrative mechanisms of CNS action may involve monoaminergic and neuro-immune modulation.

Sleep Disturbances and Depression Are Co-morbid Conditions: Insights From Animal Models, Especially Non-human Primate Model

The incidence rates of depression are increasing year by year. As one of the main clinical manifestations of depression, sleep disorder is often the first complication. This complication may increase the severity of depression and lead to poor prognosis in patients. In the past decades, there have been many methods used to evaluate sleep disorders, such as polysomnography and electroencephalogram, actigraphy, and videography. A large number of rodents and non-human primate models have reproduced the symptoms of depression, which also show sleep disorders. The purpose of this review is to examine and discuss the relationship between sleep disorders and depression. To this end, we evaluated the prevalence, clinical features, phenotypic analysis, and pathophysiological brain mechanisms of depression-related sleep disturbances. We also emphasized the current situation, significance, and insights from animal models of depression, which would provide a better understanding for the pathophysiological mechanisms between sleep disturbance and depression.

Acute and chronic stress differently alter the expression of cytokine and neuronal markers genes in zebrafish brain

We report the effects of acute and chronic stress on the expression of selective immune-related genes and markers of neuronal function in the brain of the zebrafish (Danio rerio). Fish were distributed into three groups: the non-stressed control group; the acute stress (AS) group, submitted to a single stressing episode; and the unpredictable chronic stress (UCS) group, submitted to two daily stressing episodes of alternating times and types of stress. The stressing protocols were applied for a period of 14 days. The UCS protocol triggered the expression of the pro-inflammatory cytokine genes IL-1β and TNF-α, the anti-inflammatory cytokine IL-10 (negative feedback from the immune system), reduction in cFOS gene expression, and caused neuro-inflammation. The AS protocol had no effect on gene expression. Altered expression of cytokine genes, as observed in our study, correlates with several pathologies associated with neuro-inflammation, and the reduction of cFOS gene expression may indicate the occurrence of reduced neuronal plasticity. Our study further extends our knowledge about the interaction of the immune system and the different forms of stress.

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.

An acetylcholinesterase inhibitor, donepezil, increases anxiety and cortisol levels in adult zebrafish

BACKGROUND

A potent acetylcholinesterase inhibitor, donepezil is a cognitive enhancer clinically used to treat neurodegenerative diseases. However, its complete pharmacological profile beyond cognition remains unclear. The zebrafish (Danio rerio) is rapidly becoming a powerful novel model organism in neuroscience and central nervous system drug screening.

AIM

Here, we characterize the effects of 24-h donepezil administration on anxiety-like behavioral and endocrine responses in adult zebrafish.

METHODS

We evaluated zebrafish anxiety-like behaviors in the novel tank, the light-dark and the shoaling tests, paralleled by assessing brain acetylcholinesterase activity and whole-body cortisol levels.

RESULTS

Overall, donepezil dose-dependently decreased zebrafish locomotor activity in the novel tank test and reduced time in light in the light-dark test, likely representing hypolocomotion and anxiety-like behaviors. Donepezil predictably decreased brain acetylcholinesterase activity, also increasing whole-body cortisol levels, thus further linking acetylcholinesterase inhibition to anxiety-like behavioral and endocrine responses.

CONCLUSION

Collectively, these findings suggest negative modulation of zebrafish affective behavior by donepezil, support the key role of cholinergic mechanisms in behavioral regulation in zebrafish, and reinforce the growing utility of zebrafish models for studying complex behavioral processess and their neuroendocrine and neurochemical regulation.

Understanding complex dynamics of behavioral, neurochemical and transcriptomic changes induced by prolonged chronic unpredictable stress in zebrafish

Stress-related neuropsychiatric disorders are widespread, debilitating and often treatment-resistant illnesses that represent an urgent unmet biomedical problem. Animal models of these disorders are widely used to study stress pathogenesis. A more recent and historically less utilized model organism, the zebrafish (Danio rerio), is a valuable tool in stress neuroscience research. Utilizing the 5-week chronic unpredictable stress (CUS) model, here we examined brain transcriptomic profiles and complex dynamic behavioral stress responses, as well as neurochemical alterations in adult zebrafish and their correction by chronic antidepressant, fluoxetine, treatment. Overall, CUS induced complex neurochemical and behavioral alterations in zebrafish, including stable anxiety-like behaviors and serotonin metabolism deficits. Chronic fluoxetine (0.1 mg/L for 11 days) rescued most of the observed behavioral and neurochemical responses. Finally, whole-genome brain transcriptomic analyses revealed altered expression of various CNS genes (partially rescued by chronic fluoxetine), including inflammation-, ubiquitin- and arrestin-related genes. Collectively, this supports zebrafish as a valuable translational tool to study stress-related pathogenesis, whose anxiety and serotonergic deficits parallel rodent and clinical studies, and genomic analyses implicate neuroinflammation, structural neuronal remodeling and arrestin/ubiquitin pathways in both stress pathogenesis and its potential therapy.

Decoding the role of zebrafish neuroglia in CNS disease modeling

Neuroglia, including microglia and astrocytes, is a critical component of the central nervous system (CNS) that interacts with neurons to modulate brain activity, development, metabolism and signaling pathways. Thus, a better understanding of the role of neuroglia in the brain is critical. Complementing clinical and rodent data, the zebrafish (Danio rerio) is rapidly becoming an important model organism to probe the role of neuroglia in brain disorders. With high genetic and physiological similarity to humans and rodents, zebrafish possess some common (shared), as well as some specific molecular biomarkers and features of neuroglia development and functioning. Studying these common and zebrafish-specific aspects of neuroglia may generate important insights into key brain mechanisms, including neurodevelopmental, neurodegenerative, neuroregenerative and neurological processes. Here, we discuss the biology of neuroglia in humans, rodents and fish, its role in various CNS functions, and further directions of translational research into the role of neuroglia in CNS disorders using zebrafish models.

17-α-Ethinylestradiol modulates endocrine and behavioral responses to stress in zebrafish

The synthetic estrogen, 17-α-ethinylestradiol (EE2), present in contraceptive pills, is an endocrine-disrupting chemical (EDC) that can be found in the aquatic environment. We examined the impacts of EE2 on zebrafish behavioral and physiological responses through the novel tank test (NTT), which measures anxiety-like behavior; the mirror-induced aggression (MIA) test, which measures aggressiveness; and the social preference test (SPT), which measures social cohesion. The steroid hormone levels were also measured. Here, we show that exposure to EE2 impairs stress responses by regulating the levels of specific hormones and eliciting an anxiolytic response, increasing aggression, and reducing social preference in zebrafish. In nature, these changes in behavior compromise reproduction and anti-predator behaviors, which, in turn, affects species survival. The maintenance of an intact behavioral repertoire in zebrafish is essential for their survival. Thus, our results point to the danger of environmental contamination with EE2 as it may alter the dynamics of the prey-predator relationship.

Moderate early life stress improves adult zebrafish (Danio rerio) working memory but does not affect social and anxiety-like responses

Early life stress (ELS) is defined as a short or chronic period of trauma, environmental or social deprivation, which can affect different neurochemical and behavioral patterns during adulthood. Zebrafish (Danio rerio) have been widely used as a model system to understand human neurodevelopmental disorders and display translationally relevant behavioral and stress-regulating systems. In this study, we aimed to investigate the effects of moderate ELS by exposing young animals (6-weeks postfertilization), for 3 consecutive days, to three stressors, and analyzing the impact of this on adult zebrafish behavior (16-week postfertilization). The ELS impact in adults was assessed through analysis of performance on tests of unconditioned memory (free movement pattern Y-maze test), exploratory and anxiety-related task (novel tank diving test), and social cohesion (shoaling test). Here, we show for the first time that moderate ELS increases the number of alternations in turn-direction compared to repetitions in the unconditioned Y-maze task, suggesting increased working memory, but has no effect on shoal cohesion, locomotor profile, or anxiety-like behavior. Overall, our data suggest that moderate ELS may be linked to adaptive flexibility which contributes to build "resilience" in adult zebrafish by improving working memory performance.

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.

Non-pharmacological and pharmacological approaches for psychiatric disorders: Re-appraisal and insights from zebrafish models

Acute and chronic stressors are common triggers of human mental illnesses. Experimental animal models and their cross-species translation to humans are critical for understanding of the pathogenesis of stress-related psychiatric disorders. Mounting evidence suggests that both pharmacological and non-pharmacological approaches can be efficient in treating these disorders. Here, we analyze human, rodent and zebrafish (Danio rerio) data to compare the impact of non-pharmacological and pharmacological therapies of stress-related psychopathologies. Emphasizing the likely synergism and interplay between pharmacological and environmental factors in mitigating daily stress both clinically and in experimental models, we argue that environmental enrichment emerges as a promising complementary therapy for stress-induced disorders across taxa. We also call for a broader use of novel model organisms, such as zebrafish, to study such treatments and their potential interplay.

Garcinol pacifies acrylamide induced cognitive impairments, neuroinflammation and neuronal apoptosis by modulating GSK signaling and activation of pCREB by regulating cathepsin B in the brain of zebrafish larvae

The presence of acrylamide (ACR) in food results in evident cognitive decline, accumulation of misfolded proteins, neurotoxicity, neuroinflammation, and neuronal apoptosis leading to progressive neurodegeneration. Here, we used 4 dpf zebrafish larvae exposed to ACR (1mM/3days) as our model, and neuronal proteins were analyzed. Next, we tested the effect of garcinol (GAR), a natural histone-acetylation inhibitor, whose neuroprotection mechanism of action remains to be fully elucidated. Our result revealed that ACR exposure significantly impaired cognitive behavior, downregulated oxidative repair machinery, and enhanced microglia-induced neuronal apoptosis. Moreover, ACR mediated cathepsin-B (CAT-B) translocation acted as the intracellular secretase for the processing of amyloid precursor protein (APP) and served as an additional risk factor for tau hyper-phosphorylation. Here, GAR suppresses ACR mediated CATB translocation as similar with standard inhibitor CA-074. And, this pharmacological repression helped in inhibiting amyloidogenic APP processing and downstream tau hyper-phosphorylation. GAR neuroprotection was accompanied by CREB, ATF1, and BDNF activation promoting neuronal survival. At the same time, GAR subdued cdk5 and GSK3β, the link between APP processing and tau hyper-phosphorylation. Taken together, our findings indicate that GAR rescued from ACR mediated behavioral defects, oxidative injury, neuroinflammation, undesirable APP processing, tau hyper-phosphorylation which in turn found to be CATB dependent.

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.

Effects of chronic prazosin, an alpha-1 adrenergic antagonist, on anxiety-like behavior and cortisol levels in a chronic unpredictable stress model in zebrafish (Danio rerio)

Post-traumatic stress disorder (PTSD) is often associated with significant neuroendocrine dysfunction and a variety of other symptoms. Today, there are limited efficacious treatment options for PTSD, none of which directly target the dysfunction observed with the hypothalamic-pituitary-adrenal (HPA) axis. The development of new pharmacological treatments is expensive and time consuming; thus, there is utility in repurposing compounds already approved for use in other conditions. One medication in particular that has shown promise for the alleviation of PTSD symptoms is prazosin, an alpha-1 adrenergic receptor antagonist used to treat hypertension. While there have been many studies indicating the efficacy of prazosin in the treatment of PTSD symptoms, no studies fully elucidate mechanisms elicited by this treatment, nor is it clear if prazosin normalizes neuroendocrine dysfunction associated with trauma exposure. The use of zebrafish (Danio rerio) has been growing in popularity, in part, due to the homology of the stress response system with mammals. In this study, the zebrafish model was utilized to determine behavioral and biological changes induced by chronic unpredictable stress (CUS) and how these effects could be modulated by chronic prazosin treatment. The results indicated that 7d of CUS increased anxiety-like behavior in the novel tank test and decreased basal levels of cortisol. Chronic (7d) prazosin treatment decreased anxiety-like behaviors overall but did not appear to affect CUS-induced changes in behavior and basal cortisol levels. This suggests that the clinical effectiveness of prazosin may not normalize dysregulated stress responses prevalent in many patients with PTSD, but that prazosin-induced relief from anxiety in stress-related conditions may involve an alternative mechanism other than by normalizing neuroendocrine dysfunction.

High-glucose/high-cholesterol diet in zebrafish evokes diabetic and affective pathogenesis: The role of peripheral and central inflammation, microglia and apoptosis

Neuroinflammation and metabolic deficits contribute to the etiology of human affective disorders, such as anxiety and depression. The zebrafish (Danio rerio) has recently emerged as a powerful new model organism in CNS disease modeling. Here, we exposed zebrafish to 2% glucose and 10% cholesterol for 19 days to experimentally induce type 2 diabetes (DM) and to assess stress responses, microglia, inflammation and apoptosis. We analyzed zebrafish anxiety-like behavior in the novel tank and light-dark box (Days 15-16) tests, as well as examined their biochemical and genomic biomarkers (Day 19). Confirming DM-like state in zebrafish, we found higher whole-body glucose, triglyceride, total cholesterol, low-density lipoprotein levels and glucagon mRNA expression, and lower high-density lipoprotein levels. DM zebrafish also showed anxiety-like behavior, elevated whole-body cortisol and cytokines IFN-γ and IL-4, as well as higher brain mRNA expression of the glucocorticoid receptor, CD11b (a microglial biomarker), pro-inflammatory cytokines IL-6 and TNF-α (but not IL-1β or anti-inflammatory cytokines IL-4 and IL-10), GFAP (an astrocytal biomarker), neurotrophin BDNF, its receptors p75 and TrkB, as well as apoptotic Bax and Caspase-3 (but not BCl-2) genes. Collectively, this supports the overlapping nature of DM-related affective pathogenesis and emphasizes the role of peripheral and central inflammation and apoptosis in DM-related affective and neuroendocrine deficits in zebrafish.