Acute and chronic alcohol dose: population differences in behavior and neurochemistry of zebrafish

Adult zebrafish in CNS disease modeling: a tank that's half-full, not half-empty, and still filling

The zebrafish (Danio rerio) is increasingly used in a broad array of biomedical studies, from cancer research to drug screening. Zebrafish also represent an emerging model organism for studying complex brain diseases. The number of zebrafish neuroscience studies is exponentially growing, significantly outpacing those conducted with rodents or other model organisms. Yet, there is still a substantial amount of resistance in adopting zebrafish as a first-choice model system. Studies of the repertoire of zebrafish neural and behavioral functions continue to reveal new opportunities for understanding the pathobiology of various CNS deficits. Although some of these models are well established in zebrafish, including models for anxiety, depression, and addiction, others are less recognized, for example, models of autism and obsessive-compulsive states. However, mounting data indicate that a wide spectrum of CNS diseases can be modeled in adult zebrafish. Here, we summarize recent findings using zebrafish CNS assays, discuss model limitations and the existing challenges, as well as outline future directions of research in this field.

Repeated ethanol exposure alters social behavior and oxidative stress parameters of zebrafish

Repeated ethanol (EtOH) consumption induces neurological disorders in humans and is considered an important public health problem. The physiological effects of EtOH are dose- and time-dependent, causing relevant changes in the social behavior. In addition, alcohol-induced oxidative stress has been proposed as a key mechanism involved in EtOH neurotoxicity. Here we investigate for the first time whether repeated EtOH exposure (REE) alters the social behavior of zebrafish and influences brain oxidation processes. Animals were exposed to water (control group) or 1% (v/v) EtOH (EtOH group) for 8 consecutive days (20min per day). EtOH was added directly to the tank water. At day 9, the social behavior and biochemical parameters were assessed. REE increased shoal cohesion by reducing inter-fish and farthest neighbor distances. SOD and CAT activities, as well as NPSH levels decreased in brain tissue. Moreover, REE increased lipid peroxidation suggesting oxidative damage. In summary, changes in oxidation processes may play a role in the CNS effects of EtOH, influencing the social behavior of zebrafish. Furthermore, in a translational neuroscience perspective, our data reinforces the utility of zebrafish to clarify the biochemical and behavioral effects of intermittent EtOH administration.

Behavioral Changes Over Time Following Ayahuasca Exposure in Zebrafish

The combined infusion of Banisteriopsis caapi stem and Psychotria viridis leaves, known as ayahuasca, has been used for centuries by indigenous tribes. The infusion is rich in N, N-dimethyltryptamine (DMT) and monoamine oxidase inhibitors, with properties similar to those of serotonin. Despite substantial progress in the development of new drugs to treat anxiety and depression, current treatments have several limitations. Alternative drugs, such as ayahuasca, may shed light on these disorders. Here, we present time-course behavioral changes induced by ayahuasca in zebrafish, as first step toward establishing an ideal concentration for pre-clinical evaluations. We exposed adult zebrafish to five concentrations of the ayahuasca infusion: 0 (control), 0.1, 0.5, 1, and 3 ml/L (n = 14 each group), and behavior was recorded for 60 min. We evaluated swimming speed, distance traveled, freezing and bottom dwelling every min for 60 min. Swimming speed and distance traveled decreased with an increase in ayahuasca concentration while freezing increased with 1 and 3 ml/L. Bottom dwelling increased with 1 and 3 ml/L, but declined with 0.1 ml/L. Our data suggest that small amounts of ayahuasca do not affect locomotion and reduce anxiety-like behavior in zebrafish, while increased doses of the drug lead to crescent anxiogenic effects. We conclude that the temporal analysis of zebrafish behavior is a sensitive method for the study of ayahuasca-induced functional changes in the vertebrate brain.

Fishing for Fetal Alcohol Spectrum Disorders: Zebrafish as a Model for Ethanol Teratogenesis

Fetal Alcohol Spectrum Disorders (FASD) describes a wide array of ethanol-induced developmental defects, including craniofacial dysmorphology and cognitive impairments. It affects ∼1 in 100 children born in the United States each year. Due to the pleiotropic effects of ethanol, animal models have proven critical in characterizing the mechanisms of ethanol teratogenesis. In this review, we focus on the utility of zebrafish in characterizing ethanol-induced developmental defects. A growing number of laboratories have focused on using zebrafish to examine ethanol-induced defects in craniofacial, cardiac, ocular, and neural development, as well as cognitive and behavioral impairments. Growing evidence supports that genetic predisposition plays a role in these ethanol-induced defects, yet little is understood about these gene-ethanol interactions. With a high degree of genetic amenability, zebrafish is at the forefront of identifying and characterizing the gene-ethanol interactions that underlie FASD. Because of the conservation of gene function between zebrafish and humans, these studies will directly translate to studies of candidate genes in human populations and allow for better diagnosis and treatment of FASD.

Alcohol-induced behavioral changes in zebrafish: The role of dopamine D2-like receptors

RATIONALE

The dopaminergic system has been proposed to mediate alcohol-induced locomotor activity, yet the mechanisms underlying this behavioral response remain poorly understood.

OBJECTIVES

This study was conducted to investigate the role of dopamine D2-like receptors in mediating alcohol-induced behavioral responses.

METHODS

In experiment 1, we examined the effects of high concentrations (0, 2.5, 5, 10 μM) of haloperidol on motor responses. In experiment 2, we examined the effects of low concentrations (0, 0.625, 1.25, 2.5 μM) of haloperidol on anxiety-like behavioral responses using the novel tank test. In experiment 3, we examined the effect of pre-treating zebrafish with different concentrations of haloperidol (0, 0.625, 2.5 μM) and subsequently exposing them to 0 or 1 % alcohol.

RESULTS

In experiment 1, haloperidol induced an inverted U-shaped concentration-dependent increase in locomotor activity. In experiment 2, haloperidol (2.5 μM) reduced the absolute turn angle and freezing behavior in a new environment. In experiment 3, acute alcohol exposure significantly increased locomotor activity and decreased anxiety-like behavioral responses. Pre-treating zebrafish with the lower dose of haloperidol (0.625 μM) abolished the alcohol-induced locomotor activity, without altering anxiety-like behavioral responses. However, pre-treating with the higher dose of haloperidol (2.5 μM) abolished both alcohol-induced increase of locomotor activity and reduction of anxiety-like behavioral responses.

CONCLUSION

The results suggest alcohol-induced locomotor hyperactivity in zebrafish is mediated via activation of dopamine D2-like receptors, whereas anxiety-like behavioral responses may only be altered by a high haloperidol concentration, at which dose the drug may affect receptors other than D2-R.

Concentration, population, and context-dependent effects of AM251 in zebrafish

RATIONALE

The function of the cannabinoid type 1 receptor (CB1-R) is poorly understood in zebrafish, and numerous inconsistent effects have been reported on it in the literature.

OBJECTIVE

The objective of the present study is to determine whether differences in the reported effects of CB1-R antagonism on anxiety-like behavioural responses, dopaminergic and serotonergic responses are due to concentration, context-dependent and/or population (genotype-related) effects.

METHOD

Two genetically distinct populations of zebrafish (AB and short fin (SF)) were treated with different concentrations of AM251 (0, 0.1, 1mg/L), and behavioural responses were quantified under two different contexts: one, following habituation and two, subsequently in a novel environment. The levels of dopamine, serotonin and their metabolites 3,4-dihydroxyindole acetic acid (DOPAC) and 5-hydroxyindoleacetic acid (5-HIAA) were quantified from whole-brain tissue.

RESULTS

We demonstrate that a 60-min exposure to AM251 (0, 0.1, 1mg/L) does not alter behavioural performance following habituation in either populations. However, when subsequently transferred to a novel environment, zebrafish that were pre-treated with the highest dose of AM251 (1mg/L) exhibited increased anxiety-like behavioural responses including elevated absolute turn angle, freezing and bottom dwelling. We found that exposure to the highest dose of AM251 (1mg/L) for 60min increased serotonin in fish of both populations tested. In contrast, exposure to 0.1mg/L AM251 decreased, whereas to 1mg/L AM251 increased dopamine, DOPAC and 5-HIAA in fish of both populations.

CONCLUSION

Our results demonstrate a genotype-independent effect of AM251 but imply that the inconsistent findings obtained after pharmacological blockade of CB1-Rs in zebrafish may be due to a combination of concentration- and environmental context-dependent effects.

Home tank water versus novel water differentially affect alcohol-induced locomotor activity and anxiety related behaviours in zebrafish

The zebrafish may be uniquely well suited for studying alcohol's mechanisms of action in vivo, since alcohol can be administered via immersion in a non-invasive manner. Despite the robust behavioural effects of alcohol administration in mammals, studies reporting the locomotor stimulant and anxiolytic effects of alcohol in zebrafish have been inconsistent. In the current study, we examined whether differences in the type of water used for alcohol exposure and behavioural testing contribute to these inconsistencies. To answer this question, we exposed zebrafish to either home water from their housing tanks or novel water from an isolated reservoir (i.e. water lacking zebrafish chemosensory and olfactory cues) with 0% or 1% v/v alcohol for 30 min, a 2 × 2 between subject experimental designs. Behavioural responses were quantified throughout the 30-minute exposure session via a video tracking system. Although control zebrafish exposed to home water and novel water were virtually indistinguishable in their behavioural responses, alcohol's effect on locomotor activity and anxiety-like behavioural responses were dependent on the type of water used for testing. Alcohol exposure in home tank water produced a mild anxiolytic and locomotor stimulant effect, whereas alcohol exposure in novel water produced an anxiogenic effect without altering locomotor activity. These results represent a dissociation between alcohol's effects on locomotor and anxiety related responses, and also illustrate how environmental factors, in this case familiarity with the water, may interact with such effects. In light of these findings, we urge researchers to explicitly state the type of water used.

Neurochemical factors underlying individual differences in locomotor activity and anxiety-like behavioral responses in zebrafish

Variation among individuals may arise for several reasons, and may have diverse underlying mechanisms. Individual differences have been studied in a variety of species, but recently a new model organism has emerged in this field that offers both sophistication in phenotypical characterization and powerful mechanistic analysis. Recently, zebrafish, one of the favorites of geneticists, have been shown to exhibit consistent individual differences in baseline locomotor activity. In the current study, we further explore this finding and examine whether individual differences in locomotor activity correlate with anxiety-like behavioral measures and with levels of dopamine, serotonin and the metabolites of these neurotransmitters. In addition, we examine whether individual differences in locomotor activity are also associated with reactivity to the locomotor stimulant effects of and neurochemical responses to acute ethanol exposure (30min long, 1% v/v ethanol bath application). Principal component analyses revealed a strong association among anxiety-like responses, locomotor activity, serotonin and dopamine levels. Furthermore, ethanol exposure was found to abolish the locomotion-dependent anxiety-like behavioral and serotonergic responses suggesting that this drug also engages a common underlying pathway. Overall, our results provide support for an important role of the serotonergic system in mediating individual differences in anxiety-like responses and locomotor activity in zebrafish and for a minor modulatory role of the dopaminergic system.

The Difference between Anxiolytic and Anxiogenic Effects Induced by Acute and Chronic Alcohol Exposure and Changes in Associative Learning and Memory Based on Color Preference and the Cause of Parkinson-Like Behaviors in Zebrafish

We describe an interdisciplinary comparison of the effects of acute and chronic alcohol exposure in terms of their disturbance of light, dark and color preferences and the occurrence of Parkinson-like behavior in zebrafish through computer visual tracking, data mining, and behavioral and physiological analyses. We found that zebrafish in anxiolytic and anxious states, which are induced by acute and chronic repeated alcohol exposure, respectively, display distinct emotional reactions in light/dark preference tests as well as distinct learning and memory abilities in color-enhanced conditional place preference (CPP) tests. Additionally, compared with the chronic alcohol (1.0%) treatment, acute alcohol exposure had a significant, dose-dependent effect on anxiety, learning and memory (color preference) as well as locomotive activities. Acute exposure doses (0.5%, 1.0%, and 1.5%) generated an “inverted V” dose-dependent pattern in all of the behavioral parameters, with 1.0% having the greatest effect, while the chronic treatment had a moderate effect. Furthermore, by measuring locomotive activity, learning and memory performance, the number of dopaminergic neurons, tyrosine hydroxylase expression, and the change in the photoreceptors in the retina, we found that acute and chronic alcohol exposure induced varying degrees of Parkinson-like symptoms in zebrafish. Taken together, these results illuminated the behavioral and physiological mechanisms underlying the changes associated with learning and memory and the cause of potential Parkinson-like behaviors in zebrafish due to acute and chronic alcohol exposure.

Differential effects of acute administration of SCH-23390, a D₁ receptor antagonist, and of ethanol on swimming activity, anxiety-related responses, and neurochemistry of zebrafish

RATIONALE

The zebrafish has become an increasingly popular animal model for investigating ethanol's actions in the brain and its effects on behavior. Acute exposure to ethanol in zebrafish has been shown to induce a dose-dependent increase of locomotor activity, to reduce fear- and anxiety-related behavioral responses, and to increase the levels of dopamine and its metabolite 3,4-dihydroxyphenylacetic acid (DOPAC).

OBJECTIVES

The objective of the present study was to investigate the role of dopamine D1 receptors (D1-R) in ethanol-induced locomotor activity in zebrafish.

METHODS

Zebrafish were pre-treated with SCH-23390 (0 or 1 mg/L bath concentration), a D1-R antagonist, and subsequently exposed to ethanol (0, 0.25, 0.5, 1.0 % v/v). To explore potential underlying mechanisms, we quantified levels of dopamine, DOPAC, serotonin, and 5-HIAA from whole-brain tissue using high-precision liquid chromatography.

RESULTS

We found pre-treatment with the D1-R antagonist to attenuate locomotor activity independent of ethanol concentration. Furthermore, unlike ethanol, D1-R antagonism did not alter behavioral responses associated with fear and anxiety. Pre-treatment with SCH-23390 decreased levels of dopamine and DOPAC, but this effect was also independent of ethanol concentration. The D1-R antagonist also reduced serotonin and 5-hydroxyindole acetic acid (5-HIAA) levels.

CONCLUSION

These results suggest a multifaceted and at least partially independent role of dopamine D1 receptors in ethanol-induced locomotor activity and anxiety-related responses as well as in the functioning of the dopaminergic and serotoninergic neurotransmitter systems in zebrafish.

Molecular psychiatry of zebrafish

Due to their well-characterized neural development and high genetic homology to mammals, zebrafish (Danio rerio) have emerged as a powerful model organism in the field of biological psychiatry. Here, we discuss the molecular psychiatry of zebrafish, and its implications for translational neuroscience research and modeling central nervous system (CNS) disorders. In particular, we outline recent genetic and technological developments allowing for in vivo examinations, high-throughput screening and whole-brain analyses in larval and adult zebrafish. We also summarize the application of these molecular techniques to the understanding of neuropsychiatric disease, outlining the potential of zebrafish for modeling complex brain disorders, including attention-deficit/hyperactivity disorder (ADHD), aggression, post-traumatic stress and substance abuse. Critically evaluating the advantages and limitations of larval and adult fish tests, we suggest that zebrafish models become a rapidly emerging new field in modern molecular psychiatry research.

Acute and chronic ethanol exposure differentially alters alcohol dehydrogenase and aldehyde dehydrogenase activity in the zebrafish liver

Chronic ethanol exposure paradigms have been successfully used in the past to induce behavioral and central nervous system related changes in zebrafish. However, it is currently unknown whether chronic ethanol exposure alters ethanol metabolism in adult zebrafish. In the current study we examine the effect of acute ethanol exposure on adult zebrafish behavioral responses, as well as alcohol dehydrogenase (ADH) and aldehyde dehydrogenase (ALDH) activity in the liver. We then examine how two different chronic ethanol exposure paradigms (continuous and repeated ethanol exposure) alter behavioral responses and liver enzyme activity during a subsequent acute ethanol challenge. Acute ethanol exposure increased locomotor activity in a dose-dependent manner. ADH activity was shown to exhibit an inverted U-shaped curve and ALDH activity was decreased by ethanol exposure at all doses. During the acute ethanol challenge, animals that were continuously housed in ethanol exhibited a significantly reduced locomotor response and increased ADH activity, however, ALDH activity did not change. Zebrafish that were repeatedly exposed to ethanol demonstrated a small but significant attenuation of the locomotor response during the acute ethanol challenge but ADH and ALDH activity was similar to controls. Overall, we identified two different chronic ethanol exposure paradigms that differentially alter behavioral and physiological responses in zebrafish. We speculate that these two paradigms may allow dissociation of central nervous system-related and liver enzyme-dependent ethanol induced changes in zebrafish.

Acute and chronic alcohol administration: effects on performance of zebrafish in a latent learning task

Alcohol abuse is a major medical problem. Zebrafish have been proposed to model alcohol related human disorders. Alcohol impairs learning and memory. Here, we analyze the effects of alcohol on performance of zebrafish in a recently developed latent learning paradigm. We employ a 2×3×2 experimental design (chronic×acute alcohol treatment×path blocked). The latent learning task had two phases: one, 30min long exploration trials (16 days, 1 trial/day) with left or right path of a complex maze blocked, and two, a subsequent probe trial with all paths open leading to a goal box that now contained stimulus fish. During the 16 days each fish received one of two chronic treatments: freshwater or 0.50% (v/v%) alcohol. Subsequently, fish were immersed for 1h in one of the following solutions: 0.00 (freshwater), 0.50% or 1.00% alcohol, the acute challenge. Behavior of fish was recorded during the probe trial that commenced immediately after the acute treatment. Path choices, latency to leave the start box and to enter the goal box, time spent in the goal box, distance traveled, and duration of freezing were quantified. We found that acute exposure to 1.00% alcohol after chronic freshwater disrupted learning performance, so did exposure to freshwater after chronic alcohol treatment (withdrawal). We also found exposure to chronic alcohol to diminish the effect of subsequent acute alcohol suggesting development of tolerance. Our results demonstrate that analysis of learning performance of zebrafish allows detection of alcohol-induced functional changes. The simplicity and scalability of the employed task also imply the utility of the zebrafish in high throughput drug screens.

Gaining translational momentum: more zebrafish models for neuroscience research

Zebrafish (Danio rerio) are rapidly becoming a popular model organism in translational neuroscience and biological psychiatry research. Here we discuss conceptual, practical and other related aspects of using zebrafish in this field ("from tank to bedside"), and critically evaluate both advantages and limitations of zebrafish models of human brain disorders. We emphasize the need to more actively develop zebrafish models for neuroscience research focusing on complex traits.

Developing zebrafish models relevant to PTSD and other trauma- and stressor-related disorders

While post-traumatic stress disorder (PTSD) and other trauma- and stress-related disorders (TSRDs) represent a serious societal and public health concern, their pathogenesis is largely unknown. Given the clinical complexity of TSRD development and susceptibility, greater investigation into candidate biomarkers and specific genetic pathways implicated in both risk and resilience to trauma becomes critical. In line with this, numerous animal models have been extensively used to better understand the pathogenic mechanisms of PTSD and related TSRD. Here, we discuss the rapidly increasing potential of zebrafish as models of these disorders, and how their use may aid researchers in uncovering novel treatments and therapies in this field.

Recent advances with a novel model organism: alcohol tolerance and sensitization in zebrafish (Danio rerio)

Alcohol abuse and dependence are a rapidly growing problem with few treatment options available. The zebrafish has become a popular animal model for behavioral neuroscience. This species may be appropriate for investigating the effects of alcohol on the vertebrate brain. In the current review, we examine the literature by discussing how alcohol alters behavior in zebrafish and how it may affect biological correlates. We focus on two phenomena that are often examined in the context of alcohol-induced neuroplasticity. Alcohol tolerance (a progressive decrease in the effect of alcohol over time) is often observed following continuous (chronic) exposure to low concentrations of alcohol. Alcohol sensitization also called reverse tolerance (a progressive increase in the effect of alcohol over time) is often observed following repeated discrete exposures to higher concentrations of alcohol. These two phenomena may underlie the development and maintenance of alcohol addiction. The phenotypical characterization of these responses in zebrafish may be the first important steps in establishing this species as a tool for the analysis of the molecular and neurobiological mechanisms underlying human alcohol addiction.

A novel method of drug administration to multiple zebrafish (Danio rerio) and the quantification of withdrawal

Anxiety testing in zebrafish is often studied in combination with the application of pharmacological substances. In these studies, fish are routinely netted and transported between home aquaria and dosing tanks. In order to enhance the ease of compound administration, a novel method for transferring fish between tanks for drug administration was developed. Inserts that are designed for spawning were used to transfer groups of fish into the drug solution, allowing accurate dosing of all fish in the group. This increases the precision and efficiency of dosing, which becomes very important in long schedules of repeated drug administration. We implemented this procedure for use in a study examining the behavior of zebrafish in the light/dark test after administering ethanol with differing 21 day schedules. In fish exposed to daily-moderate amounts of alcohol there was a significant difference in location preference after 2 days of withdrawal when compared to the control group. However, a significant difference in location preference in a group exposed to weekly-binge administration was not observed. This protocol can be generalized for use with all types of compounds that are water-soluble and may be used in any situation when the behavior of fish during or after long schedules of drug administration is being examined. The light/dark test is also a valuable method of assessing withdrawal-induced changes in anxiety.

Subcutaneous dye injection for marking and identification of individual adult zebrafish (Danio rerio) in behavioral studies

The zebrafish is increasingly utilized in behavioral brain research, as it offers a useful compromise between system complexity and practical simplicity. However, a potential drawback of this species in behavioral research is that individuals are difficult to distinguish. Here we describe a simple marking procedure, subcutaneous injection of color dyes, that may alleviate this problem. The procedure allowed us to successfully mark zebrafish and distinguish them for a period of more than 30 days, which is sufficiently long for most behavioral paradigms developed for this species. In addition, we also provide data suggesting that the injection-based marking does not significantly alter social interaction, as defined by the frequency of agonistic behaviors within shoals.

A dose for the wiser is enough: the alcohol benefits for associative learning in zebrafish

This study aimed to test seeking behavior caused by alcohol and the drug effects on learning in the zebrafish, Danio rerio. Three treatments were conducted: acute, chronic and withdrawal, using 0.10%, 0.25%, and 1.00% alcohol and control (0.00%) (vol/vol.%). For the drug seeking behavior, we used a place preference paradigm (shuttle box tank) before and after alcohol exposure in acute (single exposure) and chronic (7 days) treatments. We observed a change in the basal preference due to the association with alcohol only for 0.25% and 1.00% doses in both acute and chronic offering, indicating an alcohol-seeking behavior after the drug exposure. For the learning task, two treatments were tested: chronic alcohol exposure (26 days including the learning period) and alcohol withdrawal (15 days of alcohol exposure before the learning period). During the learning period, fish received light stimulus followed by food in a pre-defined area of the tank for 8 consecutive days. The low dose group (0.10%) learned the task by the 3rd day both in chronic and withdrawal treatments. The higher doses (0.25% and 1.00%) caused a learning impairment in the chronic treatment group, while fish from the alcohol withdrawal treatment displayed learning on the final testing day. Therefore, we suggest that high alcohol doses impair learning and cause drug seeking behavior, even after drug exposure cessation, while low doses positively affect learning and do not cause seeking behavior. Given our results we propose that the zebrafish is a promising model for identifying active compounds, antibodies or genes which modulate the alcohol dual effects: learning improvement and reinforcing behavior.

Induction of social behavior in zebrafish: live versus computer animated fish as stimuli

The zebrafish offers an excellent compromise between system complexity and practical simplicity and has been suggested as a translational research tool for the analysis of human brain disorders associated with abnormalities of social behavior. Unlike laboratory rodents zebrafish are diurnal, thus visual cues may be easily utilized in the analysis of their behavior and brain function. Visual cues, including the sight of conspecifics, have been employed to induce social behavior in zebrafish. However, the method of presentation of these cues and the question of whether computer animated images versus live stimulus fish have differential effects have not been systematically analyzed. Here, we compare the effects of five stimulus presentation types: live conspecifics in the experimental tank or outside the tank, playback of video-recorded live conspecifics, computer animated images of conspecifics presented by two software applications, the previously employed General Fish Animator, and a new application Zebrafish Presenter. We report that all stimuli were equally effective and induced a robust social response (shoaling) manifesting as reduced distance between stimulus and experimental fish. We conclude that presentation of live stimulus fish, or 3D images, is not required and 2D computer animated images are sufficient to induce robust and consistent social behavioral responses in zebrafish.

Social behavior of zebrafish: from synthetic images to biological mechanisms of shoaling

The zebrafish strikes a good balance between system complexity and practical simplicity and as a result it is becoming increasingly frequently utilized in biomedical research as a translational tool. Numerous human brain disorders are associated with abnormal social behavior and the zebrafish has been suggested for modeling such disorders. To start this line of research, however, one may need to first thoroughly examine the laboratory organism, zebrafish, and its features, social behavior in this case. Proper methods need be developed to induce and quantify social behavior. These paradigms may be able to open a window to the brain and facilitate the understanding of the biological mechanisms of social behavior and its abnormalities. This review is based on an oral paper presented at the last Measuring Behavior Conference, and as such it is mainly focused on research conducted in my own laboratory. Tracing the temporal progression of our own work, it discusses questions including what shoaling is, how it can be induced and measured and how it can be utilized in the modeling of certain human brain disorders, for example, alcohol induced abnormalities.

An integrative analysis of ethanol tolerance and withdrawal in zebrafish (Danio rerio)

The zebrafish is emerging as a popular animal model for alcohol (ethanol or EtOH) addiction due to its simplicity and practical advantages. Two phenomena associated with ethanol addiction are the development of tolerance and withdrawal. Using a multi-level approach in the current study, we characterise ethanol tolerance and withdrawal in zebrafish. We first investigate the temporal trajectory of ethanol concentration in the zebrafish brain in response to an acute exposure and during withdrawal. We report that ethanol concentrations approach a steady state within 60 min of exposure to 0.50% and 1.00% v/v ethanol and rapidly decline and return to zero within 60 min following withdrawal from chronic ethanol exposure (0.50% v/v). We characterise the changes associated with ethanol tolerance and withdrawal in zebrafish by focusing on three domains relevant to ethanol addiction: motor patterns, physiological responses (i.e. cortisol levels), and neurochemical alterations. The use of multiple domains of investigation allowed an in-depth analysis of ethanol induced changes in zebrafish.

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.

Chronic and acute alcohol administration induced neurochemical changes in the brain: comparison of distinct zebrafish populations

The zebrafish is increasingly utilized in the analysis of the effects of ethanol (alcohol) on brain function and behavior. We have shown significant population-dependent alcohol-induced changes in zebrafish behavior and have started to analyze alterations in dopaminergic and serotoninergic responses. Here, we analyze the effects of alcohol on levels of selected neurochemicals using a 2 × 3 (chronic × acute) between-subject alcohol exposure paradigm randomized for two zebrafish populations, AB and SF. Each fish first received the particular chronic treatment (0 or 0.5 vol/vol% alcohol) and subsequently the acute exposure (0, 0.5 or 1.0% alcohol). We report changes in levels of dopamine, DOPAC, serotonin, 5HIAA, glutamate, GABA, aspartate, glycine and taurine as quantified from whole brain extracts using HPLC. We also analyze monoamine oxidase and tyrosine hydroxylase enzymatic activity. The results demonstrate that compared to SF, AB is more responsive to both acute alcohol exposure and acute alcohol withdrawal at the level of neurochemistry, a finding that correlates well with prior behavioral observations and one which suggests the involvement of genes in the observed alcohol effects. We discuss correlations between the current results and prior behavioral findings, and stress the importance of characterization of zebrafish strains for future behavior genetic and psychopharmacology studies.

Acute ethanol exposure induces behavioural differences in two zebrafish (Danio rerio) strains: a time course analysis

The zebrafish has been proposed as a model organism to study genetic effects influencing behaviour and also as a tool with which the mechanisms of the action of alcohol (ethanol or EtOH) in the vertebrate brain may be investigated. In the current study we exposed zebrafish from two genetically distinct strains (WIK and TU) to a computer animated image of a natural predator of this species, the Indian leaf fish. We measured the subjects' behavioural responses in the presence of different acute doses of alcohol (0.00, 0.25, 0.50, and 1.00% vol/vol) using an observation based event-recording method. We found fish of both strains to exhibit an atypical predator inspection response during the presentation of the animated predator image coupled with a classical fear response, increased jumping frequency. We found numerous alcohol induced behavioural changes and more importantly also revealed alcohol induced strain dependent changes as well, including different dose-response trajectories for WIK vs. TU in predator inspection response, general swimming activity, location of swimming (top vs. bottom half of the tank) and freezing. The results suggest that zebrafish of the TU strain may be more tolerant at least to lower doses of alcohol as compared to WIK. The characterization of strain differences in zebrafish will aid the identification of possible molecular mechanisms involved in alcohol's actions in the vertebrate brain.

Strain dependent neurochemical changes induced by embryonic alcohol exposure in zebrafish

Fetal Alcohol Spectrum Disorder (FASD) is a preventable disease of the child resulting from alcohol (ethanol) consumption by pregnant women. Despite being preventable, FASD represents a prevalent problem throughout the world. Embryonic alcohol induced abnormalities in behavioral responses to social stimuli have been shown in humans and zebrafish. The neurobiological mechanisms underlying the abnormalities remain obscured. Here we start a mechanistic analysis by investigating the effect of embryonic alcohol exposure on the neurochemistry of zebrafish. The differing severity of symptoms seen in FASD may be partially due to genetic factors. To explore such genetic effects, here we analyzed two distinct zebrafish strains: AB and TU. Zebrafish were exposed to one of the following concentrations of alcohol, 0.00%, 0.25%, 0.50%, 0.75%, or 1.00% (vol/vol %) at 24 hours post-fertilization (hpf) for 2h. From whole brain extracts we analyzed the amount of neurotransmitters dopamine and serotonin and their metabolites across 4 different developmental time points: 15, 40, 70 and 102 days post-fertilization (dpf) using high performance liquid chromatography (HPLC). AB zebrafish exhibited a significant dose dependent embryonic alcohol exposure effect which increased in robustness with age. However, TU showed no such concentration effect: the levels of neurochemicals remained mainly unaltered by embryonic alcohol exposure in all age groups. We also analyzed the amount of alcohol reaching the embryo in the two strains and ruled out the possibility that TU has a more protective chorion. We conclude that the uncovered strain differences are due to genetic differences that protect TU from the deleterious effects of embryonic alcohol exposure.

The behavioral effects of acute Δ⁹-tetrahydrocannabinol and heroin (diacetylmorphine) exposure in adult zebrafish

The use of psychotropic drugs in clinical and translational brain research continues to grow, and the need for novel experimental models and screens is becoming widely recognized. Mounting evidence supports the utility of zebrafish (Danio rerio) for studying various pharmacological manipulations, as an alternative model complementing the existing rodent paradigms in this field. Here, we explore the effects of acute 20-min exposure to two commonly abused psychotropic compounds, Δ(9)-tetrahydrocannabinol (THC) and heroin, on adult zebrafish behavior in the novel tank test. Overall, THC administration (30 and 50 mg/L) produces an anxiogenic-like reduction of top swimming, paralleled with a slower, continuous bottom swimming. In contrast, heroin exposure (15 and 25 mg/L) evoked a hyperlocomotor response (with rapid bouts of bottom swimming and frequent 'bouncing' motions) without altering anxiety-sensitive top/bottom endpoints. The behavioral effects of these two compounds in zebrafish seem to parallel the respective rodent and human findings. Collectively, this emphasizes the growing significance of novel emerging aquatic models in translational drug abuse research and small molecule screening.

Acute ethanol administration affects zebrafish preference for a biologically inspired robot

Preclinical animal models constitute a cornerstone against which the reward processes involved in drug addiction are often studied and dissected. While rodents have traditionally represented the species of choice, a growing body of literature indicates that zebrafish are emerging as a valuable model organism. Specifically, several studies demonstrate that the effects of ethanol at the level of emotional- and cognitive-related domains can be reliably investigated using zebrafish. The rapidly evolving nature of these efforts allows substantial room for the development of novel experimental paradigms suited to this freshwater species. The field of ethorobotics may prove particularly beneficial, due to its ability to convey fully controllable and easily reproducible experimental tools. In this study, we addressed the possibility of using a biologically inspired robot to investigate the emotionally related properties of ethanol in a preference task in zebrafish. To this aim, we evaluated wild-type zebrafish preference toward a robotic stimulus and addressed whether ethanol administration (0.25% and 1.00% ethanol/water concentration) may alter such preferences. In accordance with our previous studies, we observed that zebrafish exhibit a natural attraction toward the robot. Additionally, in agreement with our predictions, we showed that ethanol administration abolishes such preferences. This work is the first to demonstrate that robotic stimuli can be used in zebrafish to investigate the reward-related properties of alcohol.

A robotics-based behavioral paradigm to measure anxiety-related responses in zebrafish

Zebrafish are gaining momentum as a laboratory animal species for the study of anxiety-related disorders in translational research, whereby they serve a fundamental complement to laboratory rodents. Several anxiety-related behavioral paradigms, which rest upon the presentation of live predatorial stimuli, may yield inconsistent results due to fatigue, habituation, or idiosyncratic responses exhibited by the stimulus itself. To overcome these limitations, we designed and manufactured a fully controllable robot inspired by a natural aquatic predator (Indian leaf fish, Nandus nandus) of zebrafish. We report that this robot elicits aversive antipredatorial reactions in a preference test and that data obtained therein correlate with data observed in traditional anxiety- and fear-related tests (light/dark preference and shelter-seeking). Finally, ethanol administration (0.25; 0.50; 1.00%) exerts anxiolytic effects, thus supporting the view that robotic stimuli can be used in the analysis of anxiety-related behaviors in zebrafish.

Quantification of locomotor activity in larval zebrafish: considerations for the design of high-throughput behavioral studies

High-throughput behavioral studies using larval zebrafish often assess locomotor activity to determine the effects of experimental perturbations. However, the results reported by different groups are difficult to compare because there is not a standardized experimental paradigm or measure of locomotor activity. To address this, we investigated the effects that several factors, including the stage of larval development and the physical dimensions (depth and diameter) of the behavioral arena, have on the locomotor activity produced by larval zebrafish. We provide evidence for differences in locomotor activity between larvae at different stages and when recorded in wells of different depths, but not in wells of different diameters. We also show that the variability for most properties of locomotor activity is less for older than younger larvae, which is consistent with previous reports. Finally, we show that conflicting interpretations of activity level can occur when activity is assessed with a single measure of locomotor activity. Thus, we conclude that although a combination of factors should be considered when designing behavioral experiments, the use of older larvae in deep wells will reduce the variability of locomotor activity, and that multiple properties of locomotor activity should be measured to determine activity level.

Time-course of behavioural changes induced by ethanol in zebrafish (Danio rerio)

The zebrafish has been proposed for the study of the effects of ethanol on the vertebrate brain. Behavioural tests have been successfully employed in the phenotypical characterization of these effects. However, the short scale (minute to minute) time course of ethanol induced changes of zebrafish behaviour has not been analyzed. The current study alleviates this need using a 2×3 chronic×acute ethanol exposure experimental design. We first expose zebrafish to ethanol chronically using a dose escalation procedure in which fish are kept in a final concentration of 0.5% vol/vol ethanol for 10 days while control fish receive identical dosing procedures but no ethanol. Subsequently, we expose zebrafish for 1h to an acute dose of ethanol (0.00, 0.50, or 1.00% vol/vol) and monitor their behaviour throughout this period. We quantify the mean and within-individual temporal variance of distance travelled, distance from bottom and angular velocity using video-tracking, and establish temporal trajectories of ethanol induced behavioural changes in zebrafish. For example, we find fish of the highest acute dose group previously not exposed to chronic ethanol to exhibit an inverted U shaped temporal trajectory in distance travelled (biphasic alcohol effect). We find this response to be blunted after chronic ethanol exposure (development of tolerance). We also describe an acute ethanol withdrawal induced increase in angular velocity. We conclude that temporal analysis of zebrafish behaviour is a sensitive method for the study of chronic and acute ethanol exposure induced functional changes in the vertebrate brain.

Acute ethanol treatment upregulates Th1, Th2, and Hdc in larval zebrafish in stable networks

Earlier studies in zebrafish have revealed that acutely given ethanol has a stimulatory effect on locomotion in fish larvae but the mechanism of this effect has not been revealed. We studied the effects of ethanol concentrations between 0.75 and 3.00% on 7-day-old larval zebrafish (Danio rerio) of the Turku strain. At 0.75-3% concentrations ethanol increased swimming speed during the first minute. At 3% the swimming speed decreased rapidly after the first minute, whereas at 0.75 and 1.5% a prolonged increase in swimming speed was seen. At the highest ethanol concentration dopamine levels decreased significantly after a 10-min treatment. We found that ethanol upregulates key genes involved in the biosynthesis of histamine (hdc) and dopamine (th1 and th2) following a short 10-min ethanol treatment, measured by qPCR. Using in situ hybridization and immunohistochemistry, we further discovered that the morphology of the histaminergic and dopaminergic neurons and networks in the larval zebrafish brain was unaffected by both the 10-min and a longer 30-min treatment. The results suggest that acute ethanol rapidly decreases dopamine levels, and activates both forms or th to replenish the dopamine stores within 30 min. The dynamic changes in histaminergic and dopaminergic system enzymes occurred in the same cells which normally express the transcripts. As both dopamine and histamine are known to be involved in the behavioral effects of ethanol and locomotor stimulation, these results suggest that rapid adaptations of these networks are associated with altered locomotor activity.

Reversed scototaxis during withdrawal after daily-moderate, but not weekly-binge, administration of ethanol in zebrafish

Alcohol abuse can lead to severe psychological and physiological damage. Little is known, however, about the relative impact of a small, daily dose of alcohol (daily-moderate schedule) versus a large, once per week dose (weekly-binge schedule). In this study, we examined the effect of each of these schedules on behavioural measures of anxiety in zebrafish (Danio rerio). Adult wild-type zebrafish were administered either 0.2% ethanol on a daily-moderate schedule or 1.4% ethanol on a weekly-binge schedule for a period of 21 days, and then tested for scototaxis (preference for darkness) during withdrawal. Compared to a control group with no alcohol exposure, the daily-moderate group spent significantly more time on the light side of the arena (indicative of decreased anxiety) on day two of withdrawal, but not day 9 of withdrawal. The weekly-binge group was not significantly different from the control group on either day of withdrawal and showed no preference for either the light or dark zones. Our results indicate that even a small dose of alcohol on a daily basis can cause significant, though reversible, changes in behaviour.

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.

Sight of conspecific images induces changes in neurochemistry in zebrafish

Zebrafish are gaining popularity in behavioural brain research as this species combines practical simplicity with system complexity. The dopaminergic system has been thoroughly investigated using mammals. Dopamine plays important roles in motor function and reward. Zebrafish have dopamine receptors homologous to mammalian counterparts, and dopamine receptor antagonists as well as alcohol have been shown to exert significant effects on this species as measured using HPLC or behavioural methods. The sight of conspecifics was previously shown to be rewarding in zebrafish but whether this stimulus affects the dopaminergic system has not been studied. Here, we present animated images of zebrafish to the experimental zebrafish subject for varying lengths of time and quantify the amount of dopamine, DOPAC, serotonin and 5HIAA extracted from the subject's brain immediately after the stimulus presentation using HPLC with electrochemical detection. We find conspecific images to induce a robust behavioural response (attraction) in experimental zebrafish. Importantly, dopamine and DOPAC levels significantly increased in response to the presentation of conspecific images but not to scrambled images. Last, serotonin and 5HIAA levels did not significantly change in response to the conspecific images. We conclude that our findings, together with pervious studies, now conclusively demonstrate that the behavioural response induced by the appearance of conspecifics is mediated, at least partly, by the dopaminergic system in zebrafish.

Behavioral screening for neuroactive drugs in zebrafish

The larval zebrafish has emerged asa vertebrate model system amenable to small molecule screens for probing diverse biological pathways. Two large-scale small molecule screens examined the effects of thousands of drugs on larval zebrafish sleep/wake and photomotor response behaviors. Both screens identified hundreds of molecules that altered zebrafish behavior in distinct ways. The behavioral profiles induced by these small molecules enabled the clustering of compounds according to shared phenotypes. This approach identified regulators of sleep/wake behavior and revealed the biological targets for poorly characterized compounds. Behavioral screening for neuroactive small molecules in zebrafish is an attractive complement to in vitro screening efforts, because the complex interactions in the vertebrate brain can only be revealed in vivo.

Arginine exposure alters ectonucleotidase activities and morphology of zebrafish larvae (Danio rerio)

Hyperargininemia is an inborn error of metabolism (IEM) characterized by tissue accumulation of arginine (Arg). Mental retardation and other neurological features are common symptoms in hyperargininemic patients. Considering purinergic signaling has a crucial role from the early stages of development and underlying mechanisms of this disease are poorly established, we investigated the effect of Arg administration on locomotor activity, morphological alterations, and extracellular nucleotide hydrolysis in larvae and adult zebrafish. We showed that 0.1 mM Arg was unable to promote changes in locomotor activity. In addition, 7-day-post-fertilization (dpf) larvae treated with Arg demonstrated a decreased body size. Arg exposure (0.1 mM) promoted an increase in ATP, ADP, and AMP hydrolysis when compared to control group. These findings demonstrated that Arg might affect morphological parameters and ectonucleotidase activities in zebrafish larvae, suggesting that purinergic system is a target for neurotoxic effects induced by Arg.

Measures of anxiety in zebrafish (Danio rerio): dissociation of black/white preference and novel tank test

The effects of wall color stimuli on diving, and the effects of depth stimuli on scototaxis, were assessed in zebrafish. Three groups of fish were confined to a black, a white, or a transparent tank, and tested for depth preference. Two groups of fish were confined to a deep or a shallow tank, and tested for black-white preference. As predicted, fish preferred the deep half of a split-tank over the shallow half, and preferred the black half of a black/white tank over the white half. Results indicated that the tank wall color significantly affected depth preference, with the transparent tank producing the strongest depth preference and the black tank producing the weakest preference. Tank depth, however, did not significantly affect color preference. Additionally, wall color significantly affected shuttling and immobility, while depth significantly affected shuttling and thigmotaxis. These results are consistent with previous indications that the diving response and scototaxis may reflect dissociable mechanisms of behavior. We conclude that the two tests are complementary rather than interchangeable, and that further research on the motivational systems underlying behavior in each of the two tests is needed.

Dopamine receptor antagonism disrupts social preference in zebrafish: a strain comparison study

Zebrafish form shoals in nature and in the laboratory. The sight of conspecifics has been found reinforcing in zebrafish learning tasks. However, the mechanisms of shoaling, and those of its reinforcing properties, are not known. The dopaminergic system has been implicated in reward among other functions and it is also engaged by drugs of abuse as shown in a variety of vertebrates including zebrafish. The ontogenetic changes in dopamine levels and, to a lesser degree, in serotonin levels, have been found to accompany the maturation of shoaling in zebrafish. Thus, we hypothesized that the dopaminergic system may contribute to shoaling in zebrafish. To test this we employed a D1-receptor antagonist and quantified behavioral responses of our subjects using a social preference (shoaling) paradigm. We found significant reduction of social preference induced by the D1-R antagonist, SCH23390, in the AB strain of zebrafish, an alteration that was not accompanied by changes in motor function or vision. We also detected D1-R antagonist-induced changes in the level of dopamine, DOPAC, serotonin and 5HIAA, respectively, in the brain of AB zebrafish as quantified by HPLC with electrochemical detection. We found the antagonist-induced behavioral changes to be absent and the levels of these neurochemicals to be lower in another zebrafish population, SF, demonstrating naturally occurring genetic variability in these traits. We conclude that this variability may be utilized to unravel the mechanisms of social behavior in zebrafish, a line of research that may be extended to other vertebrates including our own species.

Visual social preferences of lone zebrafish in a novel environment: strain and anxiolytic effects

Zebrafish (Danio rerio) have an innate tendency to join shoals. Based on this, we refined visual choice tests to focus on social interaction and novelty preference. Our design follows mouse three-chamber sociability protocols, except testing is conducted under 940 Lux fluorescent lighting. Initially, we compared performance among zebrafish strains: inbred (AB) or wild-crossbred (WIK) from Zebrafish International Resource Center, to golden and short-fin from Petco stores. AB fish exhibited a preference for shoaling; they dwelled longest near transparent boxes containing zebrafish, while short fin favored blue boxes without fish. AB and golden exhibited a strong preference for social novelty, not evident in short-fin or WIK fish. Serotonin and cannabinoids shape mammalian social behavior, and equivalents of both receptor types are expressed in the zebrafish brain. We examined the effects of the cannabinoid receptor agonist WIN 55,212-2 (1 mg/l), or serotonin 5-HT(1A) receptor agonist buspirone (10 mg/l) on Petco short-fin social choice. Fish were bath exposed to test compounds for 10 min, under these conditions [(3) H]CP55,940 (4 nm) bound to brain with a concentration of 1.9-6.4 fmol/mg 5-30 min afterward. Social approach was measured 20 min after acclimation to the test arena. WIN 55,212-2 and buspirone increased dwelling near boxed zebrafish. In zebrafish whole-brain homogenates, buspirone displaced [(3) H] 8-hydroxy-N,N-dipropylaminotetralin (dissociation constant, K(D) = 16 ± 1.2 nm) with an inhibition constant (K(i) ) of 1.8 ± 1.0 nm lower than that of WAY 100,635 (K(i) ∼1000 nm). These fish social choice tests may enhance social behavior research, and are useful for studying the effects of genetic manipulations, pharmaceuticals or environmental toxins.