Zebrafish behavioural assays of translational relevance for the study of psychiatric disease

NemoTrainer: Automated Conditioning for Stimulus-Directed Navigation and Decision Making in Free-Swimming Zebrafish

Current methods for associative conditioning in animals involve human intervention that is labor intensive, stressful to animals, and introduces experimenter bias in the data. Here, we describe a simple apparatus and a flexible, microcontroller-based conditioning paradigm that minimizes human intervention. Our methodology exploits directed movement towards a target that depends on spatial working memory, including processing of sensory inputs, motivational drive, and attentional mechanisms. Within a stimulus-driven conditioning paradigm designed to train zebrafish, we present a localized pulse of light via LEDs and/or sounds via an underwater transducer. A webcam placed below a glass tank records fish-swimming behavior. For classical conditioning, animals simply associate a sound or light with an unconditioned stimulus, such as a small food reward presented at a fixed location, and swim towards that location to obtain a few grains of food dispensed automatically via a sensor-triggered, stepper motor. During operant conditioning, a fish must first approach a proximity sensor at a remote location and then swim to the reward location. For both types of conditioning, a timing-gated interrupt activates stepper motors via custom software embedded within a microcontroller (Arduino). "Ardulink", a Java facility, implements Arduino-computer communication protocols. In this way, a Java-based user interface running on a host computer can provide full experimental control. Alternatively, a similar level of control is achieved via an Arduino script communicating with an event-driven application controller running on the host computer. Either approach can enable precise, multi-day scheduling of training, including timing, location, and intensity of stimulus parameters; and the feeder. Learning can be tracked by monitoring turning, location, response times, and directional swimming of individual fish. This facilitates the comparison of performance within and across a cohort of animals. Our scheduling and control software and apparatus ("NemoTrainer") can be used to study multiple aspects of species-specific behaviors as well as the effects on them of various interventions.

Using zebrafish to understand reciprocal interactions between the nervous and immune systems and the microbial world

Animals rely heavily on their nervous and immune systems to perceive and survive within their environment. Despite the traditional view of the brain as an immunologically privileged organ, these two systems interact with major consequences. Furthermore, microorganisms within their environment are major sources of stimuli and can establish relationships with animal hosts that range from pathogenic to mutualistic. Research from a variety of human and experimental animal systems are revealing that reciprocal interactions between microbiota and the nervous and immune systems contribute significantly to normal development, homeostasis, and disease. The zebrafish has emerged as an outstanding model within which to interrogate these interactions due to facile genetic and microbial manipulation and optical transparency facilitating in vivo imaging. This review summarizes recent studies that have used the zebrafish for analysis of bidirectional control between the immune and nervous systems, the nervous system and the microbiota, and the microbiota and immune system in zebrafish during development that promotes homeostasis between these systems. We also describe how the zebrafish have contributed to our understanding of the interconnections between these systems during infection in fish and how perturbations may result in pathology.

Translational relevance of forward genetic screens in animal models for the study of psychiatric disease

Psychiatric disorders represent a significant burden in our societies. Despite the convincing evidence pointing at gene and gene-environment interaction contributions, the role of genetics in the etiology of psychiatric disease is still poorly understood. Forward genetic screens in animal models have helped elucidate causal links. Here we discuss the application of mutagenesis-based forward genetic approaches in common animal model species: two invertebrates, nematodes (Caenorhabditis elegans) and fruit flies (Drosophila sp.); and two vertebrates, zebrafish (Danio rerio) and mice (Mus musculus), in relation to psychiatric disease. We also discuss the use of large scale genomic studies in human populations. Despite the advances using data from human populations, animal models coupled with next-generation sequencing strategies are still needed. Although with its own limitations, zebrafish possess characteristics that make them especially well-suited to forward genetic studies exploring the etiology of psychiatric disorders.

Attractin Participates in Schizophrenia by Affecting Testosterone Levels

Attractin (ATRN) is a widely expressed glycoprotein that is involved in energy homeostasis, neurodevelopment, and immune response. It is encoded by a gene spanning 180 kb on chromosome 20p13, a region previously implicated in schizophrenia by linkage studies. To address a possible role of ATRN in disorders of the central nervous system, we created an atrn knockout zebrafish line and performed behavioral tests. Adult atrn^–/– zebrafish exhibited more pronounced attack behavior relative to wild-type control zebrafish in a tracking analysis. Biochemical analysis revealed elevated testosterone levels in atrn^–/– zebrafish. At the gene expression level, we noted an upregulation of cyp51 and hsd17b7, key proteins in testosterone synthesis in the brains of both adult and larvae of atrn^–/– zebrafish. In order to further elucidate the relationship between testosterone and behavioral syndromes, we then compared testosterone levels of 9,008 psychiatric patients and 247 healthy controls from the same catchment area. Of all subjects examined, male subjects with schizophrenia exhibited lower testosterone levels compared with controls. In contrast, female subjects with a diagnosis of schizophrenia or bipolar disorder featured higher testosterone levels than did same sex controls. Purposeful sampling of extreme groups showed reduced ATRN expression in a subset of these subjects. Finally, we identified 14 subjects with ATRN mutations. All of whom displayed abnormal testosterone levels. In summary, the interplay of ATRN and testosterone may help to explain sexual dimorphisms in selected behavioral phenotypes.

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.

Production of bioactive recombinant human fibroblast growth factor 12 using a new transient expression vector in E. coli and its neuroprotective effects

In recent years, an increasing number of studies have shown that fibroblast growth factor 12 (FGF12) plays important roles in regulating neural development and function. Importantly, changes of FGF12 expression are thought to be related to the pathophysiology of many neurological diseases. However, little research has been performed to explore the protective effect of FGF12 on nerve damage. This study aims to explore its neuroprotective effects using our recombinant humanized FGF12 (rhFGF12). The hFGF12 gene was cloned and ligated into an expression vector to construct a recombinant plasmid pET-3a-hFGF12. Single colonies were screened to obtain high expression engineering strains, and fermentation and purification protocols for rhFGF12 were designed and optimized. The biological activities and related mechanisms of rhFGF12 were investigated by MTT assay using NIH3T3 and PC12 cell lines. The in vitro neurotoxicity model of H₂O₂-induced oxidative injury in PC12 cells was established to explore the protective effects of rhFGF12. The results indicate that the beneficial effects of rhFGF12 were most likely achieved by promoting cell proliferation and reducing apoptosis. Moreover, a transgenic zebrafish (islet) with strong GFP fluorescence in the motor neurons of the hindbrain was used to establish a central injury model caused by mycophenolate mofetil (MMF). The results suggested that rhFGF12 could ameliorate central injury induced by MMF in zebrafish. In conclusion, we have established an efficient method to express and purify active rhFGF12 using an Escherichia coli expression system. Besides, rhFGF12 plays a protective effect of on nerve damage, and it provides a promising therapeutic approach for nerve injury. KEY POINTS: • Effective expression and purification of bioactive rhFGF12 protein in E. coli. • ERK/MAPK pathway is involved in rhFGF12-stimulated proliferation on PC12 cells. • The rhFGF12 has the neuroprotective effects by inhibiting apoptosis.

Parkinson's Disease-Induced Zebrafish Models: Focussing on Oxidative Stress Implications and Sleep Processes

The complex yet not fully understood pathophysiology of Parkinson's disease includes an important molecular component consisting of oxidative status changes, thus leading to oxidative stress occurrence. While no particular evidence has been reported that describes the relationship between oxidative stress and the molecular mechanisms behind Parkinson's disease development, animal model studies has shown that oxidative stress induction could modulate Parkinson's disease symptomatology. Despite the inability to perfectly replicate human disease in animals and despite that Parkinson's disease has not been reported in any animal species, animal modeling is one of the most important tools in understanding the complex mechanisms of human disorders. In this way, this study is aimed at detailing this particular relationship and describing the molecular mechanisms underlying Parkinson's disease in animal models, focusing on the potential advantages and disadvantages of zebrafish in this context. The information relevant to this topic was gathered using major scientific database research (PubMed, Google Scholar, Web of Science, and Scopus) based on related keywords and inclusion criteria. Thus, it was observed that oxidative stress possesses an important role in Parkinson's disease as shown by numerous animal model studies, many of which are based on rodent experimental models. However, an emerging impact of the zebrafish model was observed in the research of Parkinson's disease pathological mechanisms with regard to disease development factors and the cause-effect relationship between oxidative stress and comorbidities (such as depression, hyposmia, fatigue, sleep disturbances, and cognitive deficits) and also with regard to the pharmacological potential of antioxidant molecules in Parkinson's disease treatment.

The ontogeny of continuous quantity discrimination in zebrafish larvae (Danio rerio)

Several studies have investigated the ontogeny of the capacity to discriminate between discrete numerical information in human and non-human animals. Contrarily, less attention has been devoted to the development of the capacity to discriminate continuous quantities. Recently, we set up a fast procedure for screening continuous quantity abilities in adult individuals of an animal model in neurodevelopmental research, the zebrafish. Two different sized holes are presented in a wall that divides the home tank in two halves and the spontaneous preference of fish for passing through the larger hole is exploited to measure their discrimination ability. We tested zebrafish larvae in the first, second and third week of life varying the relative size of the smaller circle (0.60, 0.75, 0.86, 0.91 area ratio). We found that the number of passages increased across the age. The capacity to discriminate the larger hole decreased as the ratio between the areas increased. No difference in accuracy was found as a function of age. The accuracy of larval zebrafish almost overlaps that found in adults in a previous study, suggesting a limited role of maturation and experience on the ability to estimate areas in this species.

Neuroinflammation: An overview of neurodegenerative and metabolic diseases and of biotechnological studies

Neuroinflammation is an important factor contributing to cognitive impairment and neurodegenerative diseases, including Alzheimer's disease (AD), Parkinson's disease (PD), Amyotrophic lateral sclerosis (ALS), ischemic injury, and multiple sclerosis (MS). These diseases are characterized by inexorable progressive injury of neuron cells, and loss of motor or cognitive functions. Microglia, which are the resident macrophages in the brain, play an important role in both physiological and pathological conditions. In this review, we provide an updated discussion on the role of ROS and metabolic disease in the pathological mechanisms of activation of the microglial cells and release of cytotoxins, leading to the neurodegenerative process. In addition, we also discuss in vivo models, such as zebrafish and Caenorhabditis elegans, and provide new insights into therapeutics bioinspired by neuropeptides from venomous animals, supporting high throughput drug screening in the near future, searching for a complementary approach to elucidating crucial mechanisms associated with neurodegenerative disorders.

The Impact of Brain Lateralization and Anxiety-Like Behaviour in an Extensive Operant Conditioning Task in Zebrafish (Danio rerio)

Several studies in mammals, birds, and fish have documented better cognitive abilities associated with an asymmetrical distribution of cognitive functions in the two halves of the brain, also known as ‘functional brain lateralization’. However, the role of brain lateralization in learning abilities is still unclear. In addition, although recent studies suggest a link between some personality traits and accuracy in cognitive tasks, the relation between anxiety and learning skills in Skinner boxes needs to be clarified. In the present study, we tested the impact of brain lateralization and anxiety-like behaviour in the performance of an extensive operant conditioning task. Zebrafish tested in a Skinner box underwent 500 trials in a colour discrimination task (red vs. yellow and green vs. blue). To assess the degree of lateralization, fish were observed in a detour test in the presence of a dummy predator, and anxiety-like behaviour was studied by observing scototaxis response in an experimental tank divided into light and dark compartments. Although the low performance in the colour discrimination task did not permit the drawing of firm conclusions, no correlation was found between the accuracy in the colour discrimination task and the behaviour in the detour and scototaxis tests. This suggests that neither different degrees of asymmetries in brain lateralization nor anxiety may significantly impact the learning skills of zebrafish.

Single and Synergistic Effects of Cannabidiol and Δ-9-Tetrahydrocannabinol on Zebrafish Models of Neuro-Hyperactivity

In this study, we aimed to investigate the effect of the two main active cannabinoids extracted from cannabis: Δ-9-tetrahydrocannabinol (THC) and cannabidiol (CBD) on two distinct behavioral models of induced neuro-hyperactivity. We have taken advantage of two previously developed zebrafish models of neuro-hyperactivity: a chemically induced pentylenetetrazole model and a genetic model caused by loss-of-function mutations in the GABA receptor subunit alpha 1 (GABRA1−/−). Both CBD and THC have a significant effect on the behavioral changes induced by both models. Importantly, we have also shown that when applied together at different ratios of THC to CBD (1:1, 1:5, and 1:10), there was a synergistic effect at a ratio of 1:1. This was particularly important for the genetically induced neuro-hyperactivity as it brought the concentrations of THC and CBD required to oppose the induced behavioral changes to levels that had much less of an effect on baseline larval behavior. The results of this study help to validate the ability of THC and CBD to oppose neuro-hyperactivity linked to seizure modalities. Additionally, it appears that individually, each cannabinoid may be more effective against the chemically induced model than against the GABRA1−/− transgenic model. However, when applied together, the concentration of each compound required to oppose the GABRA1−/− light-induced activity was lowered. This is of particular interest since the use of cannabinoids as therapeutics can be dampened by their side-effect profile. Reducing the level of each cannabinoid required may help to prevent off target effects that lead to side effects. Additionally, this study provides a validation of the complimentary nature of the two zebrafish models and sets a platform for future work with cannabinoids, particularly in the context of neuro-hyperactivity disorders such as epilepsy.

Taurine modulates the stress response in zebrafish

The zebrafish (Danio rerio) is used as an emergent model organism to investigate the behavioral and physiological responses to stress. The anxiolytic-like effects of taurine in zebrafish support the existence of different mechanisms of action, which can play a role in preventing stress-related disorders (i.e., modulation of GABA_A, strychnine-sensitive glycine, and NMDA receptors, as well as antioxidant properties). Herein, we investigate whether taurine modulates some behavioral and biochemical responses in zebrafish acutely submitted to chemical and mechanical stressors. We pretreated zebrafish for 1 h in beakers at 42, 150, and 400 mg/L taurine. Fish were later acutely exposed to a chemical stressor (conspecific alarm substance) or to a mechanical stressor (net chasing), which elicits escaping responses and aversive behaviors. Locomotion, exploration, and defensive-like behaviors were measured using the novel tank and the light-dark tests. Biochemical (brain oxidative stress-related parameters) and whole-body cortisol levels were also quantified. We showed that taurine prevents anxiety/fear-like behaviors and protein carbonylation and dampens the cortisol response following acute stress in zebrafish. In summary, our results demonstrate a protective role of taurine against stress-induced behavioral and biochemical changes, thereby reinforcing the growing utility of zebrafish models to investigate the neuroprotective actions of taurine in vertebrates.

Zebrafish models in neuropsychopharmacology and CNS drug discovery

Despite the high prevalence of neuropsychiatric disorders, their aetiology and molecular mechanisms remain poorly understood. The zebrafish (Danio rerio) is increasingly utilized as a powerful animal model in neuropharmacology research and in vivo drug screening. Collectively, this makes zebrafish a useful tool for drug discovery and the identification of disordered molecular pathways. Here, we discuss zebrafish models of selected human neuropsychiatric disorders and drug-induced phenotypes. As well as covering a broad range of brain disorders (from anxiety and psychoses to neurodegeneration), we also summarize recent developments in zebrafish genetics and small molecule screening, which markedly enhance the disease modelling and the discovery of novel drug targets.

Behavioral Analysis of Dopaminergic Activation in Zebrafish and Rats Reveals Similar Phenotypes

Zebrafish is emerging as a complement to mammals in behavioral studies; however, there is a lack of comparative studies with rodents and humans to establish the zebrafish as a predictive translational model. Here we present a detailed phenotype evaluation of zebrafish larvae, measuring 300-3000 variables and analyzing them using multivariate analysis to identify the most important ones for further evaluations. The dopamine agonist apomorphine has previously been shown to have a complex U-shaped dose-response relationship in the variable distance traveled. In this study, we focused on breaking down distance traveled into more detailed behavioral phenotypes for both zebrafish and rats and identified in the multivariate analysis low and high dose phenotypes with characteristic behavioral features. Further analysis of single parameters also identified an increased activity at the lowest concentration indicative of a U-shaped dose-response. Apomorphine increased the distance of each swim movement (bout) at both high and low doses, but the underlying behavior of this increase is different; at high dose, both bout duration and frequency increased whereas bout max speed was higher at low dose. Larvae also displayed differences in place preference. The low dose phenotype spent more time in the center, indicative of an anxiolytic effect, while the high-dose phenotype had a wall preference. These dose-dependent effects corroborated findings in a parallel rat study and previous observations in humans. The translational value of pharmacological zebrafish studies was further evaluated by comparing the amino acid sequence of the dopamine receptors (D1-D4), between zebrafish, rats and humans. Humans and zebrafish share 100% of the amino acids in the binding site for D1 and D3 whereas D2 and D4 receptors share 85-95%. Molecular modeling of dopamine D2 and D4 receptors indicated that nonconserved amino acids have limited influence on important ligand-receptor interactions.

Challenges in understanding psychiatric disorders and developing therapeutics: a role for zebrafish

The treatment of psychiatric disorders presents three major challenges to the research and clinical community: defining a genotype associated with a disorder, characterizing the molecular pathology of each disorder and developing new therapies. This Review addresses how cellular and animal systems can help to meet these challenges, with an emphasis on the role of the zebrafish. Genetic changes account for a large proportion of psychiatric disorders and, as gene variants that predispose to psychiatric disease are beginning to be identified in patients, these are tractable for study in cellular and animal systems. Defining cellular and molecular criteria associated with each disorder will help to uncover causal physiological changes in patients and will lead to more objective diagnostic criteria. These criteria should also define co-morbid pathologies within the nervous system or in other organ systems. The definition of genotypes and of any associated pathophysiology is integral to the development of new therapies. Cell culture-based approaches can address these challenges by identifying cellular pathology and by high-throughput screening of gene variants and potential therapeutics. Whole-animal systems can define the broadest function of disorder-associated gene variants and the organismal impact of candidate medications. Given its evolutionary conservation with humans and its experimental tractability, the zebrafish offers several advantages to psychiatric disorder research. These include assays ranging from molecular to behavioural, and capability for chemical screening. There is optimism that the multiple approaches discussed here will link together effectively to provide new diagnostics and treatments for psychiatric patients.

Summary: In this review, we discuss strengths and limitations of prevalent laboratory models that are used for understanding psychiatric disorders and developing therapeutics, with emphasis on the zebrafish.

Using visual lateralization to model learning and memory in zebrafish larvae

Impaired learning and memory are common symptoms of neurodegenerative and neuropsychiatric diseases. Present, there are several behavioural test employed to assess cognitive functions in animal models, including the frequently used novel object recognition (NOR) test. However, although atypical functional brain lateralization has been associated with neuropsychiatric conditions, spanning from schizophrenia to autism, few animal models are available to study this phenomenon in learning and memory deficits. Here we present a visual lateralization NOR model (VLNOR) in zebrafish larvae as an assay that combines brain lateralization and NOR. In zebrafish larvae, learning and memory are generally assessed by habituation, sensitization, or conditioning paradigms, which are all representatives of nondeclarative memory. The VLNOR is the first model for zebrafish larvae that studies a memory similar to the declarative memory described for mammals. We demonstrate that VLNOR can be used to study memory formation, storage, and recall of novel objects, both short and long term, in 10-day-old zebrafish. Furthermore we show that the VLNOR model can be used to study chemical modulation of memory formation and maintenance using dizocilpine (MK-801), a frequently used non-competitive antagonist of the NMDA receptor, used to test putative antipsychotics in animal models.

Zebrafish models for translational neuroscience research: from tank to bedside

The zebrafish (Danio rerio) is emerging as a new important species for studying mechanisms of brain function and dysfunction. Focusing on selected central nervous system (CNS) disorders (brain cancer, epilepsy, and anxiety) and using them as examples, we discuss the value of zebrafish models in translational neuroscience. We further evaluate the contribution of zebrafish to neuroimaging, circuit level, and drug discovery research. Outlining the role of zebrafish in modeling a wide range of human brain disorders, we also summarize recent applications and existing challenges in this field. Finally, we emphasize the potential of zebrafish models in behavioral phenomics and high-throughput genetic/small molecule screening, which is critical for CNS drug discovery and identifying novel candidate genes.

Larval zebrafish model for FDA-approved drug repositioning for tobacco dependence treatment

Cigarette smoking remains the most preventable cause of death and excess health care costs in the United States, and is a leading cause of death among alcoholics. Long-term tobacco abstinence rates are low, and pharmacotherapeutic options are limited. Repositioning medications approved by the U.S. Food and Drug Administration (FDA) may efficiently provide clinicians with new treatment options. We developed a drug-repositioning paradigm using larval zebrafish locomotion and established predictive clinical validity using FDA-approved smoking cessation therapeutics. We evaluated 39 physician-vetted medications for nicotine-induced locomotor activation blockade. We further evaluated candidate medications for altered ethanol response, as well as in combination with varenicline for nicotine-response attenuation. Six medications specifically inhibited the nicotine response. Among this set, apomorphine and topiramate blocked both nicotine and ethanol responses. Both positively interact with varenicline in the Bliss Independence test, indicating potential synergistic interactions suggesting these are candidates for translation into Phase II clinical trials for smoking cessation.

Zebrafish as an emerging model for studying complex brain disorders

The zebrafish (Danio rerio) is rapidly becoming a popular model organism in pharmacogenetics and neuropharmacology. Both larval and adult zebrafish are currently used to increase our understanding of brain function, dysfunction, and their genetic and pharmacological modulation. Here we review the developing utility of zebrafish in the analysis of complex brain disorders (including, e.g., depression, autism, psychoses, drug abuse, and cognitive deficits), also covering zebrafish applications towards the goal of modeling major human neuropsychiatric and drug-induced syndromes. We argue that zebrafish models of complex brain disorders and drug-induced conditions are a rapidly emerging critical field in translational neuroscience and pharmacology research.

Using an automated 3D-tracking system to record individual and shoals of adult zebrafish

Like many aquatic animals, zebrafish (Danio rerio) moves in a 3D space. It is thus preferable to use a 3D recording system to study its behavior. The presented automatic video tracking system accomplishes this by using a mirror system and a calibration procedure that corrects for the considerable error introduced by the transition of light from water to air. With this system it is possible to record both single and groups of adult zebrafish. Before use, the system has to be calibrated. The system consists of three modules: Recording, Path Reconstruction, and Data Processing. The step-by-step protocols for calibration and using the three modules are presented. Depending on the experimental setup, the system can be used for testing neophobia, white aversion, social cohesion, motor impairments, novel object exploration etc. It is especially promising as a first-step tool to study the effects of drugs or mutations on basic behavioral patterns. The system provides information about vertical and horizontal distribution of the zebrafish, about the xyz-components of kinematic parameters (such as locomotion, velocity, acceleration, and turning angle) and it provides the data necessary to calculate parameters for social cohesions when testing shoals.

Behavioral and molecular analysis of nicotine-conditioned place preference in zebrafish

Studies using mice and rats have demonstrated that nicotine induces a conditioned place preference (CPP), with more effective results obtained by using biased procedures. Zebrafish have also been used as a model system to identify factors influencing nicotine-associated reward by using an unbiased design. Here, we report that zebrafish exhibited putative nicotine biased CPP to an initially aversive compartment (nicotine-paired group). A counterbalanced nicotine-exposed control group did not show a significant preference shift, providing evidence that the preference shift in the nicotine-paired group was not due to a reduction of aversion for this compartment. Zebrafish preference was corroborated by behavioral analysis of several indicators of drug preference, such as time spent in the drug-paired side, number of entries to the drug-paired side, and distance traveled. These results provided strong evidence that zebrafish may actually develop a preference for nicotine, although the drug was administrated in an aversive place for the fish, which was further supported by molecular studies. Reverse transcription-quantitative real-time PCR analysis depicted a significant increase in the expression of α7 and α6 but not α4 and β2 subunits of the nicotinic receptor in nicotine-paired zebrafish brains. In contrast, zebrafish brains from the counterbalanced nicotine group showed no significant changes. Moreover, CREB phosphorylation, an indicator of neural activity, accompanied the acquisition of nicotine-CPP. Our studies offered an incremental value to the drug addiction field, because they further describe behavioral features of CPP to nicotine in zebrafish. The results suggested that zebrafish exposed to nicotine in an unfriendly environment can develop a preference for that initially aversive place, which is likely due to the rewarding effect of nicotine. Therefore, this model can be used to screen exogenous and endogenous molecules involved in nicotine-associated reward in vertebrates.

The first mecp2-null zebrafish model shows altered motor behaviors

Rett syndrome (RTT) is an X-linked neurodevelopmental disorder and one of the most common causes of mental retardation in affected girls. Other symptoms include a rapid regression of motor and cognitive skills after an apparently early normal development. Sporadic mutations in the transcription factor MECP2 has been shown to be present in more than 90% of the patients and several models of MeCP2-deficient mice have been created to understand the role of this gene. These models have pointed toward alterations in the maintenance of the central nervous system rather than its development, in line with the late onset of the disease in humans. However, the exact functions of MeCP2 remain difficult to delineate and the animal models have yielded contradictory results. Here, we present the first mecp2-null allele mutation zebrafish model. Surprisingly and in contrast to MeCP2-null mouse models, mecp2-null zebrafish are viable and fertile. They present nonetheless clear behavioral alterations during their early development, including spontaneous and sensory-evoked motor anomalies, as well as defective thigmotaxis.

Differential reinforcement of an approach response in zebrafish (Danio rerio)

Five zebrafish were trained to approach a target using a fully automated training procedure. During a training session, if the distance between the fish and the target was closer than an arbitrarily set distance, the approach response was reinforced by food. The fish continued to respond under this reinforcement contingency and the distance criterion could be shortened up to eighty times within a 1h session. The initial distance limit was then shortened for the next test training session. Once the initial distance criterion was reduced to a final minimum distance, the distance criterion was fixed at this value for the next nine successive sessions. In a second experiment using different fish, we manipulated approach distances in three conditions. The first condition was identical to the changing criterion training as in Experiment 1. In the second condition, only response distances under a distance criterion were reinforced. And in the last condition, only response distances over the distance criterion were reinforced. Results show that zebrafish can control the distance between themselves and a target. In other words, zebrafish are sensitive to the spatial consequences of their behavior. The present results show that a differential reinforcement paradigm can be successfully applied to zebrafish which therefore enhances their value as a vertebrate model for studies of complex behavior including visuomotor learning.

Dissociating the effects of habituation, black walls, buspirone and ethanol on anxiety-like behavioral responses in shoaling zebrafish. A 3D approach to social behavior

Understanding the different patterns of anxiety-like behavioral responses is of great interest for pharmacological and genetic research. Here we report the effects of 3.5-hr habituation, buspirone and ethanol on those responses in shoaling zebrafish (Danio rerio). Since in these experiments we used a container with white walls, the effects of black-vs.-white walls were tested in a separate experiment. An important objective was to determine whether factors unrelated to anxiety played a role in modulating the responses. The anxiety-like behavioral responses studied here are social cohesion, distance from bottom and bottom-dwell time, radial distribution (to study thigmotaxis), transparent-wall preference (to study escape responses), locomotion and freezing. The experimental conditions yielded distinctly different response patterns. Thigmotaxis was the most obvious response to white walls and it was significantly reduced after 3.5-hr habituation. It was not affected by any of the drugs. The reduction of social cohesion after 3.5-hr habituation and in the 0.5% ethanol group was probably the most interesting effect seen in this study. A role of anxiety herein was suggested but could not be established with certainty. Other hypotheses were also discussed. The large increase of distance-from-bottom resulting in swimming close to the water surface, which occurred in both buspirone groups and in the 0.5%-ethanol group, is most likely not an anxiolytic response, because of the discrepancy with the in the literature well-established time-course and the absence of any effect of 3.5-hr habituation or black walls on vertical measures. Finally, locomotion and duration freezing could not be specifically taken as indicators for the state of anxiety and the results concerning transparent-wall preference were not sufficient clear. We conclude that the neuronal and ethological mechanisms underlying the effects of habituation, white-aversion, buspirone and ethanol on anxiety-like behavioral responses are complex and need further exploration.

Maturation of shoaling in two zebrafish strains: a behavioral and neurochemical analysis

Abnormal social behavior is a hallmark of several human neuropsychiatric and neurodevelopmental disorders for which appropriate treatment is lacking. The zebrafish has been proposed as a tool with which these disorders may be modeled and their mechanisms analyzed. A potential starting point of such analyses is the identification of genetic differences between distinct zebrafish strains. Here we compare AB and TU, two well established zebrafish strains, and characterize the developmental trajectories of their shoaling (social) behavior and of the levels of dopamine, serotonin as well as a metabolite of each of these neurotransmitters, DOPAC and 5HIAA from whole brain extracts. Using a novel video-tracking software application, we demonstrate significant strain dependent changes in the maturation of shoaling between day 7 and day 87 post-fertilization. Using high-precision liquid chromatography specifically adapted to zebrafish, we uncover a significant age×strain interaction in dopamine and DOPAC that apparently correlates well with the behavioral differences found between the strains. We also report on strain differences in serotonin and 5HIAA. We discuss possible mechanistic analyses that will address causality and conclude that zebrafish will be a useful tool with which the neurobiological and genetic bases of social behavior may be analyzed in vertebrates.

Hooked! Modeling human disease in zebrafish

Zebrafish have been widely used as a model system for studying developmental processes, but in the last decade, they have also emerged as a valuable system for modeling human disease. The development and function of zebrafish organs are strikingly similar to those of humans, and the ease of creating mutant or transgenic fish has facilitated the generation of disease models. Here, we highlight the use of zebrafish for defining disease pathways and for discovering new therapies.

Discrimination reversal and attentional sets in zebrafish (Danio rerio)

The potential of zebrafish as a comparative model in behavioural neuroscience is currently hampered only by the lack of reliable and validated behavioural assays available to researchers. In the present experiment, we describe the performance of zebrafish in a test of attentional set formation. The fish were initially trained on a two-choice colour discrimination. Upon reaching acquisition criterion, the reinforced alternative was switched to the previously unreinforced alternative. Again, upon reaching criterion, the cues were replaced with a novel pair of colours (intra-dimensional shift) and reversed again on reaching criteria. We found that zebrafish show a steady decrease in trials-to-criteria over the four phases of the experiment, suggesting that they are forming and maintaining an attentional set, as has previously been demonstrated with mammals. Reversal learning deficits have been implicated in a variety of human psychological disorders (e.g., disorders of impulse control) and as such, we propose that performance of zebrafish in this procedure may represent a useful comparative model to complement existing rodent models.

A larval zebrafish model of bipolar disorder as a screening platform for neuro-therapeutics

Modelling neurological diseases has proven extraordinarily difficult due to the phenotypic complexity of each disorder. The zebrafish has become a useful model system with which to study abnormal neurological and behavioural activity and holds promise as a model of human disease. While most of the disease modelling using zebrafish has made use of adults, larvae hold tremendous promise for the high-throughput screening of potential therapeutics. The further development of larval disease models will strengthen their ability to contribute to the drug screening process. Here we have used zebrafish larvae to model the symptoms of bipolar disorder by treating larvae with sub-convulsive concentrations of the GABA antagonist pentylenetetrazol (PTZ). A number of therapeutics that act on different targets, in addition to those that have been used to treat bipolar disorder, were tested against this model to assess its predictive value. Carbamazepine, valproic acid, baclofen and honokiol, were found to oppose various aspects of the PTZ-induced changes in activity. Lidocaine and haloperidol exacerbated the PTZ-induced activity changes and sulpiride had no effect. By comparing the degree of phenotypic rescue with the mechanism of action of each therapeutic we have shown that the low-concentration PTZ model can produce a number of intermediate phenotypes that model symptoms of bipolar disorder, may be useful in modelling other disease states, and will help predict the efficacy of novel therapeutics.

EuFishBioMed (COST Action BM0804): a European network to promote the use of small fishes in biomedical research

Small fresh water fishes such as the zebrafish (Danio rerio) have become important model organisms for biomedical research. They currently represent the best vertebrate embryo models in which it is possible to derive quantitative data on gene expression, signaling events, and cell behavior in real time in the living animal. Relevant phenotypes in fish mutants are similar to those of other vertebrate models and human diseases. They can be analyzed in great detail and much faster than in mammals. In recent years, approximately 2500 genetically distinct fish lines have been generated by European research groups alone. Their potential, including their possible use by industry, is far from being exploited. To promote zebrafish research in Europe, EuFishBioMed was founded and won support by the EU COST programme ( http://www.cost.esf.org/ ). The main objective of EuFishBioMed is to establish a platform of knowledge exchange for research on small fish models with a strong focus on widening its biomedical applications and an integration of European research efforts and resources. EuFishBioMed currently lists more than 300 member laboratories in Europe, offers funding for short-term laboratory visits, organizes and co-sponsors meetings and workshops, and has successfully lobbied for the establishment of a European Zebrafish Resource Centre. To maintain this network in the future, beyond the funding period of the COST Action, we are currently establishing the European Society for Fish Models in Biology and Medicine.

Development and implementation of a three-choice serial reaction time task for zebrafish (Danio rerio)

Zebrafish are an established and widely utilized developmental genetic model system, but limitations in developed behavioral assays have meant that their potential as a model in behavioral neuroscience has yet to be fully realized. Here, we describe the development of a novel operant behavioral assay to examine a variety of aspects of stimulus control in zebrafish using a 3 choice serial reaction time task (3 CSRTT). Fish were briefly exposed to three spatially distinct, but perceptually identical stimuli, presented in a random order after a fixed-time inter-trial interval (ITI). Entries to the correct response aperture either during the stimulus presentation, or within a brief limited hold period following presentation, were reinforced with illumination of the magazine light and delivery of a small food reward. Following training, premature responding was probed with a long-ITI session three times; once at baseline, once following a saline injection and once following an injection of a low dose of amphetamine (AMPH; 0.025 mg/kg). We predicted that if premature responding was related to impulsivity (as in rodents) it would be reduced following the AMPH injection. Results confirmed that zebrafish could learn to perform a complex operant task similar to tasks developed for rodents which are used to probe sustained attention and impulsivity, but the results from the AMPH trials were inconclusive. This study provides the foundations for development and further validation of this species as a model for some aspects of human attentional and impulse control disorders, such as substance abuse disorder.

Molecular-genetic mapping of zebrafish mutants with variable phenotypic penetrance

Forward genetic screens in vertebrates are powerful tools to generate models relevant to human diseases, including neuropsychiatric disorders. Variability in phenotypic penetrance and expressivity is common in these disorders and behavioral mutant models, making their molecular-genetic mapping a formidable task. Using a ‘phenotyping by segregation’ strategy, we molecularly map the hypersensitive zebrafish houdini mutant despite its variable phenotypic penetrance, providing a generally applicable strategy to map zebrafish mutants with subtle phenotypes.

Imaging escape and avoidance behavior in zebrafish larvae

This review provides an overview of the assays that are used for measuring escape and avoidance behavior in zebrafish, with a specific focus on zebrafish larvae during the first week of development. Zebrafish larvae display a startle response when exposed to tactile, acoustic, or visual stimuli and will avoid dark areas, moving objects, conspecifics, and open spaces. Emotional states such as fear and anxiety might be induced when larvae are exposed to stimuli that they would normally escape from or avoid. Although these emotional states probably differ between species and change during development, much can be learned about human fear and anxiety using zebrafish as a model system. The molecular mechanisms of fear and anxiety are highly conserved in vertebrates and are present during early zebrafish development. Larvae during the first week of development display elevated cortisol levels in response to stress and are sensitive to the same anxiolytics that are used for the management of anxiety in humans. Zebrafish larvae are well suited for high-throughput analyses of behavior, and automated systems have been developed for imaging and analyzing the behavior of zebrafish larvae in multiwell plates. These high-throughput analyses will not only provide a wealth of information on the genes and environmental factors that influence escape and avoidance behaviors and the emotional states that might accompany them but will also facilitate the discovery of novel pharmaceuticals that could be used in the management of anxiety disorders in humans.

Behavioral performance altering effects of MK-801 in zebrafish (Danio rerio)

MK-801, a non-competitive NMDA-R antagonist, has been utilized in the analysis of mammalian learning and memory. The zebrafish is a novel vertebrate study species that has been proposed for the analysis of the mechanisms of learning and memory. Although learning paradigms have been developed for this species, psychopharmacological characterization of its behavioral responses is rudimentary. Before one attempts the analysis of the effects of MK-801 on learning and memory in zebrafish, one needs to know whether this drug affects motor function, perception and/or motivation, factors that may influence performance in learning tasks. Here we conduct dose response analyses investigating the effects of 0, 2, 20 and 100 μM MK-801 administered 24h or 30 min before the behavioral test, or during the test. We analyze responses in the open tank to measure motor and posture patterns, in the light dark paradigm to evaluate visual perception, and in a group preference task to attempt to quantify motivation. Our results show a significant performance alteration only in the highest (100 μM) dose groups. These fish spent more time on the bottom of their tank, showed elevated Erratic movement, increased their clockwise and counterclockwise turning frequency, and reduced the time spent near a shoal stimulus, behavioral alterations that also depended upon the timing of drug administration. Thus, using the current delivery procedures and outbred zebrafish population, the highest dose that may not lead to significant performance deficits is 20 μM, a concentration we propose to use in a future learning study in zebrafish.