Fishing for genes influencing vertebrate behavior: zebrafish making headway

Behavioral profiling of zebrafish embryos exposed to a panel of 60 water-soluble compounds

The zebrafish is a powerful whole animal model which is complementary to in vitro and mammalian models. It has been shown to be applicable to the high-throughput behavioral screening of compound libraries. We have analysed 60 water-soluble toxic compounds covering a range of common drugs, toxins and chemicals, and representing various pharmacological mechanisms. Wild-type zebrafish larvae were cultured individually in defined buffer in 96 well plates. They were exposed for a 96h period starting at 24h post fertilization (hpf). A logarithmic concentration series was used for range-finding, followed by a narrower geometric series for LC(50) determination. LC(50) values were determined at 24h intervals and behavioral testing was carried out on day 5. We used the visual motor response test, in which movement of individual larvae was analysed using automated video-tracking. For all compounds, LC(50) values were found to decrease as the embryo developed. The majority of compounds (57/60) produced an effect in both the basal (lights on) and challenge phases (lights off) of the behavioral assay. These effects were either (i) suppression of locomotor activity (monotonic concentration-response); (ii) stimulation then suppression (biphasic response); (iii) stimulation (monotonic response). We conclude that behavioral assays with zebrafish embryos could be useful for pharmaceutical efficacy and toxicity screening. The precise phenotypic outcome obtained with behavioral assay varies with compound class.

Zebrafish embryos and larvae: a new generation of disease models and drug screens

Technological innovation has helped the zebrafish embryo gain ground as a disease model and an assay system for drug screening. Here, we review the use of zebrafish embryos and early larvae in applied biomedical research, using selected cases. We look at the use of zebrafish embryos as disease models, taking fetal alcohol syndrome and tuberculosis as examples. We discuss advances in imaging, in culture techniques (including microfluidics), and in drug delivery (including new techniques for the robotic injection of compounds into the egg). The use of zebrafish embryos in early stages of drug safety-screening is discussed. So too are the new behavioral assays that are being adapted from rodent research for use in zebrafish embryos, and which may become relevant in validating the effects of neuroactive compounds such as anxiolytics and antidepressants. Readouts, such as morphological screening and cardiac function, are examined. There are several drawbacks in the zebrafish model. One is its very rapid development, which means that screening with zebrafish is analogous to "screening on a run-away train." Therefore, we argue that zebrafish embryos need to be precisely staged when used in acute assays, so as to ensure a consistent window of developmental exposure. We believe that zebrafish embryo screens can be used in the pre-regulatory phases of drug development, although more validation studies are needed to overcome industry scepticism. Finally, the zebrafish poses no challenge to the position of rodent models: it is complementary to them, especially in early stages of drug research.

In search of optimal fear inducing stimuli: Differential behavioral responses to computer animated images in zebrafish

Zebrafish has been gaining popularity in behavioral genetics and behavioral neuroscience as this species offers an excellent compromise between system complexity and practical simplicity for mechanistic analyses of brain and behavior function. Recently, a number of studies started to investigate methods with which fear responses may be induced reliably in zebrafish. The ultimate goal of these studies has been to develop zebrafish models of pathological processes and to investigate the mechanisms of fear and to eventually translate the findings to the human clinic. Previously, animated image of a sympatric predator of zebrafish was shown to induce fear responses. Here we expand on this recently gained knowledge and investigate whether other moving images may induce more robust fear responses. The images investigated include the original sympatric predator, the Indian leaf fish, another sympatric predator, the needle fish, a bird silhouette moved on the side or above the tank, an expanding dot mimicking rapid approach of an object shown on the side and from above the tank, as well as non-fear inducing images including a single and a group of zebrafish. Our results indicate that although the sympatric predators do induce some fear responses, the other images, particularly the expanding dot but also the bird silhouette shown from above are more effective. The results also reveal a stimulus dependent motor pattern response repertoire of zebrafish demonstrating that perhaps univariate quantification methods may not be appropriate for uncovering the complexity of fear or anxiety related phenotypical changes in this species.

Stimuli affecting zebrafish (Danio rerio) behavior in the light/dark preference test

Ethanol has been suggested to have an anxiolytic effect on zebrafish, primarily based on its disruption of the novel tank diving response and of some social behaviors. The light/dark preference test offers a complementary measure of anxiety-like behavior in fish, and the purpose of the current study was to determine the effects of acute ethanol exposure on behavior in the light/dark task. In Experiment 1, the stimuli used to induce light/dark preference in zebrafish were varied in order to determine how best to measure the behavior. Subjects exhibited phototaxis (preference for light) when illumination was manipulated, but scototaxis (preference for dark) when wall and substrate color were manipulated. There was a clear interaction between locomotor activity and color preference, with animals preferentially freezing in darker locations. Because of ambiguity in interpreting behavior in the open/covered version of the test, the black/white version was used in Experiment 2. In Experiment 2, zebrafish were exposed to ethanol (0.25%, 0.5%, or 1.0%) or water for 30 minutes, and then placed in a black/white preference tank containing either ethanol (same doses) or water for a 30-minute test. Ethanol exposure increased locomotor activity and reduced freezing. Additionally, there was a significant interaction between ethanol treatment and locomotor activity on side preference. Low doses of ethanol increased white avoidance in normally swimming fish, while high doses did not.

The use of the zebrafish model in stress research

The study of the causes and mechanisms underlying psychiatric disorders requires the use of non-human models for the test of scientific hypotheses as well as for use in pre-clinical drug screening and discovery. This review argues in favor of the use of zebrafish as a novel animal model to study the impact of early (stressful) experiences on the development of differential stress phenotypes in later life. This phenomenon is evolutionary conserved among several vertebrate species and has relevance to the etiology of psychiatric disorders. Why do we need novel animal models? Although significant progress has been achieved with the use of traditional mammalian models, there are major pitfalls associated with their use that impedes progress on two major fronts: 1) uncovering of the molecular mechanisms underlying aspects of compromised (stress-exposed) brain development relevant to the etiology of psychiatric disorders, and 2) ability to develop high-throughput technology for drug discovery in the field of psychiatry. The zebrafish model helps resolve these issues. Here we present a conceptual framework for the use of zebrafish in stress research and psychiatry by addressing three specific domains of application: 1) stress research, 2) human disease mechanisms, and 3) drug discovery. We also present novel methodologies associated with the development of the zebrafish stress model and discuss how such methodologies can contribute to remove the main bottleneck in the field of drug discovery.

Shoaling develops with age in Zebrafish (Danio rerio)

The biological mechanisms of human social behavior are complex. Animal models may facilitate the understanding of these mechanisms and may help one to develop treatment strategies for abnormal human social behavior, a core symptom in numerous clinical conditions. The zebrafish is perhaps the most social vertebrate among commonly used laboratory species. Given its practical features and the numerous genetic tools developed for it, it should be a promising tool. Zebrafish shoal, i.e. from a tight multimember groups, but the ontogenesis of this behavior has not been described. Analyzing the development of shoaling is a step towards discovering the mechanisms of this behavior. Here we study age-dependent changes of shoaling in zebrafish from day 7 post fertilization to over 5months of age by measuring the distance between all pairs of fish in freely swimming groups of ten subjects. Our longitudinal (repeated measure within subject) and cross sectional (non-repeated measure between subject) analyses both demonstrated a significant increase of shoaling with age (decreased distance between shoal members). Given the sophisticated genetic and developmental biology methods already available for zebrafish, we argue that our behavioral results open a new avenue towards the understanding of the development of vertebrate social behavior and of its mechanisms and abnormalities.

Let there be light: zebrafish neurobiology and the optogenetic revolution

Optogenetics has revolutionized the toolbox arsenal that neuroscientists now possess to investigate neuronal circuit function in intact and living animals. With a combination of light emitting 'sensors' and light activated 'actuators', we can monitor and control neuronal activity with minimal perturbation and unprecedented spatiotemporal resolution. Zebrafish neuronal circuits represent an ideal system to apply an optogenetic based analysis owing to its transparency, relatively small size and amenability to genetic manipulation. In this review, we describe some of the most recent advances in the development and applications of optogenetic sensors (i.e., genetically encoded calcium indicators and voltage sensors) and actuators (i.e., light activated ion channels and ion pumps). We focus mostly on the tools that have already been successfully applied in zebrafish and on those that show the greatest potential for the future. We also describe crucial technical aspects to implement optogenetics in zebrafish including strategies to drive a high level of transgene expression in defined neuronal populations, and recent optical advances that allow the precise spatiotemporal control of sample illumination.

Fighting zebrafish: characterization of aggressive behavior and winner-loser effects

Aggression is a key component of the behavioral repertoire of animals that impacts on their Darwinian fitness. The available genetic tools in zebrafish make this species a promising vertebrate neurogenetic model for the study of neural circuits underlying aggressive behavior. For this purpose, a detailed characterization of the aggressive behavior and its behavioral consequences is first needed. In this article we establish a simple protocol that reliably elicits the expression of fighting behavior in zebrafish dyads and characterized it. The agonistic behavior expressed during dyadic fighting behavior has a temporal structure, indicating the existence of an underlying architecture prone to genetic manipulation. Social interactions have consequences for subsequent behavior with a potential fitness impact, which stresses the validity of this species for the study of aggression. These effects of experience seem to be mediated by different mechanisms in winners and losers. Winners increase the probability of winning subsequent fights without changing their fighting behavior, suggesting the existence of social status cues. On the other hand, losers decrease the probability of winning subsequent fights by decreasing their motivation to escalate fights. Together, these results are a first step to the development of a quantitative framework for the study of aggressive behavior in zebrafish.

Associative learning performance is impaired in zebrafish (Danio rerio) by the NMDA-R antagonist MK-801

The zebrafish is gaining popularity in behavioral neuroscience perhaps because of a promise of efficient large scale mutagenesis and drug screens that could identify a substantial number of yet undiscovered molecular players involved in complex traits. Learning and memory are complex functions of the brain and the analysis of their mechanisms may benefit from such large scale zebrafish screens. One bottleneck in this research is the paucity of appropriate behavioral screening paradigms, which may be due to the relatively uncharacterized nature of the behavior of this species. Here we show that zebrafish exhibit good learning performance in a task adapted from the mammalian literature, a plus maze in which zebrafish are required to associate a neutral visual stimulus with the presence of conspecifics, the rewarding unconditioned stimulus. Furthermore, we show that MK-801, a non-competitive NMDA-R antagonist, impairs memory performance in this maze when administered right after training or just before recall but not when given before training at a dose that does not impair motor function, perception or motivation. These results suggest that the plus maze associative learning paradigm has face and construct validity and that zebrafish may become an appropriate and translationally relevant study species for the analysis of the mechanisms of vertebrate, including mammalian, learning and memory.

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.

Associative learning in zebrafish (Danio rerio)

The zebrafish has been one of the primary study species utilized in developmental biology. However, it is also gaining increasing amount of interest in other disciplines of biology including behavioral neuroscience; the numerous genetic tools developed and the large amount of genetic information accumulated for this species by now make it an excellent tool for the analysis of the mechanisms of complex central nervous system characteristics. Although several studies have investigated the biological and genetic underpinnings of associative learning (and memory), given the complexity of these phenomena, much remains to be discovered. In the past, the zebrafish has been employed particularly successfully in screening applications where a large number of mutations or drug effects had to be analyzed. Briefly, the practical simplicity and system complexity of the zebrafish may make this species an excellent tool also for the analysis of the mechanisms of associative learning. Screening, however, requires appropriate phenotypical (in this case behavioral) paradigms. A step in this direction is the characterization of learning abilities of zebrafish. The number of studies focused on cognitive and/or mnemonic characteristics of zebrafish is orders of magnitude smaller than those with rats or mice, but recently zebrafish has also started to be utilized in this research. The current chapter reviews these most recent developments. It also discusses certain unique features of zebrafish that must be taken into account when designing an associative learning task and how these tasks may be made high throughput.

Using zebrafish to unravel the genetics of complex brain disorders

The zebrafish has been prominently utilized in developmental biology for the past three decades and numerous genetic tools have been developed for it. Due to the accumulated genetic knowledge the zebrafish has now been considered an excellent research tool in other disciplines of biology too, including behavioral neuroscience and behavior genetics. Given the complexity of the vertebrate brain in general and the large number of human brain disorders whose mechanisms remain mainly unmapped in particular, there is a substantial need for appropriate laboratory research organisms that may be utilized to model such diseases and facilitate the analysis of their mechanisms. The zebrafish may have a bright future in this research field. It offers a compromise between system complexity (it is a vertebrate similar in many ways to our own species) and practical simplicity (it is small, easy to keep, and it is prolific). These features have made zebrafish an excellent choice, for example, for large scale mutation and drug screening. Such approaches may have a chance to tackle the potentially large number of molecular targets and mechanisms involved in complex brain disorders. However, although promising, the zebrafish is admittedly a novel research tool and only few empirical examples exist to support this claim. In this chapter, first I briefly review some of the rapidly evolving genetic methods available for zebrafish. Second, I discuss some promising examples for how zebrafish have been used to model and analyze molecular mechanisms of complex brain disorders. Last, I present some recently developed zebrafish behavioral paradigms that may have relevance for a spectrum of complex human brain disorders including those associated with abnormalities of learning and memory, fear and anxiety, and social behavior. Although at this point co-application of the genetics and behavioral approaches is rare with zebrafish, I argue that the rapid accumulation of knowledge in both of these disciplines will make zebrafish a prominent research tool for the genetic analysis of complex brain disorders.

A second-generation device for automated training and quantitative behavior analyses of molecularly-tractable model organisms

A deep understanding of cognitive processes requires functional, quantitative analyses of the steps leading from genetics and the development of nervous system structure to behavior. Molecularly-tractable model systems such as Xenopus laevis and planaria offer an unprecedented opportunity to dissect the mechanisms determining the complex structure of the brain and CNS. A standardized platform that facilitated quantitative analysis of behavior would make a significant impact on evolutionary ethology, neuropharmacology, and cognitive science. While some animal tracking systems exist, the available systems do not allow automated training (feedback to individual subjects in real time, which is necessary for operant conditioning assays). The lack of standardization in the field, and the numerous technical challenges that face the development of a versatile system with the necessary capabilities, comprise a significant barrier keeping molecular developmental biology labs from integrating behavior analysis endpoints into their pharmacological and genetic perturbations. Here we report the development of a second-generation system that is a highly flexible, powerful machine vision and environmental control platform. In order to enable multidisciplinary studies aimed at understanding the roles of genes in brain function and behavior, and aid other laboratories that do not have the facilities to undergo complex engineering development, we describe the device and the problems that it overcomes. We also present sample data using frog tadpoles and flatworms to illustrate its use. Having solved significant engineering challenges in its construction, the resulting design is a relatively inexpensive instrument of wide relevance for several fields, and will accelerate interdisciplinary discovery in pharmacology, neurobiology, regenerative medicine, and cognitive science.

An automated predator avoidance task in zebrafish

Zebrafish are becoming increasingly popular in behavioral neuroscience as investigators have started to realize the benefits of sophisticated genetic tools specifically developed for this species along with the pharmacological tools already available for other laboratory model organisms. The zebrafish has been proposed as an in vivo tool for the analysis of vertebrate fear responses as well as human psychopathological conditions such as anxiety. We have been developing behavioral tasks for zebrafish that could be utilized for screening mutation or drug induced changes in fear responses. In this paper we present a modified version of a previously developed predator avoidance paradigm that now allows the induction and quantification of avoidance reactions that we previously could not elicit. Most importantly, in the current paradigm zebrafish are now shown to respond to the appearance of a moving image of a sympatric predator, the Indian leaf fish, by increasing their distance from the image, a robust reaction that is easy to quantify in an automated manner. Unexpectedly, however, another fear response, the "diving" response, was seen robustly only at the beginning of the test but not in response to the predator stimulus. We discuss the implications of these results and conclude that although zebrafish fear responses are complex and context dependent, the current paradigm is a significant step towards high throughput screening for alterations in fear responses of zebrafish.

Toxicity, uptake kinetics and behavior assessment in zebrafish embryos following exposure to perfluorooctanesulphonicacid (PFOS)

Perfluorooctanesulphonicacid (PFOS), a persistent organic contaminant, has been widely detected in the environment, wildlife and humans, but few studies have assessed its effect on aquatic organisms. The present study evaluated the effect of PFOS on zebrafish embryos. Zebrafish embryos exhibited developmental toxicity of bent spine, uninflated swim bladder, decreased heart rate and affected spontaneous movement after exposure to various PFOS concentrations (0-8mg/L) from 6 to 120h post-fertilization (hpf). The LC(50) at 120hpf was 2.20mg/L and the EC(50) at 120hpf was 1.12mg/L. Continuous exposure to PFOS from 1 to 121hpf resulted in a steady accumulation with no evidence of elimination. PFOS induced cell death at 24hpf was consistently found in the brain, eye, and tail region of embryos. PFOS exposure induced lesions in the muscle fibers with histological examination. Behavior assessment of PFOS in zebrafish embryos elevated the basal rate of swimming after 4 days of exposure, and larvae exposed to PFOS (0.25-4mg/L) for only 1h at 6dpf swam faster with increasing PFOS concentration. Embryos/larvae exposed to 8mg/L PFOS for 24h periods from 1 to 121hpf showed the highest incidence of malformations in the 97-121hpf window. This is the first study to define uptake kinetics and to focus on behavioral consequences following PFOS exposure in zebrafish. Our results further the understanding of the toxicity of PFOS to aquatic organisms and suggest the need for additional research to identify the mode of PFOS toxicity.

High-throughput behavioral screens: the first step towards finding genes involved in vertebrate brain function using zebrafish

The zebrafish has been in the forefront of developmental biology for three decades and has become a favorite of geneticists. Due to the accumulated genetic knowledge and tools developed for the zebrafish it is gaining popularity in other disciplines, including neuroscience. The zebrafish offers a compromise between system complexity (it is a vertebrate similar in many ways to our own species) and practical simplicity (it is small, easy to keep, and prolific). Such features make zebrafish an excellent choice for high throughput mutation and drug screening. For the identification of mutation or drug induced alteration of brain function arguably the best methods are behavioral test paradigms. This review does not present experimental examples for the identification of particular genes or drugs. Instead it describes how behavioral screening methods may enable one to find functional alterations in the vertebrate brain. Furthermore, the review is not comprehensive. The behavioral test examples presented are biased according to the personal interests of the author. They will cover research areas including learning and memory, fear and anxiety, and social behavior. Nevertheless, the general principles will apply to other functional domains and should represent a snapshot of the rapidly evolving behavioral screening field with zebrafish.

Zebrafish antipredatory responses: a future for translational research?

Human neuropsychiatric conditions associated with abnormally exaggerated or misdirected fear (anxiety disorders and phobias) still represent a large unmet medical need because the biological mechanisms underlying these diseases are not well understood. Animal models have been proposed to facilitate this research. Here I review the literature with a focus on zebrafish, an upcoming laboratory organism in behavioral brain research. I argue that abnormal human fear responses are likely the result of the malfunction of neurobiological mechanisms (brain areas, circuits and/or molecular mechanisms) that originally evolved to support avoidance of predators or other harm in nature. I also argue that the understanding of the normal as well as pathological functioning of such mechanisms may be best achieved if one utilizes naturalistic experimental approaches. In case of laboratory model organisms, this may entail presenting stimuli associated with predators and measuring species-specific antipredatory responses. Although zebrafish is a relatively new subject of such inquiry, I review the recently rapidly increasing number of zebrafish studies in this area, and conclude that zebrafish is a promising research tool for the analysis of the neurobiology and genetics of vertebrate fear responses.

Associative learning in zebrafish (Danio rerio) in the plus maze

Zebrafish has been gaining increasing amount of interest in behavioral neuroscience as this species may represent a good compromise between system complexity and practical simplicity. Particularly successful have been those studies that utilized zebrafish as a screening tool. Given the complexity of the mechanisms of learning, for example, forward genetic screens with zebrafish could potentially reveal previously unknown genes and molecular pathways that subserve this function. These screens, however, require appropriate phenotypical (e.g. behavioral) paradigms. A step in this direction is the characterization of learning abilities of zebrafish. Here we employ two classical learning tasks in a plus maze. In the first, zebrafish are required to associate a visible cue with food reward irrespective of the location of this pairing. In the second, zebrafish are required to associate the spatial location of food reward irrespective of intra-maze cues. Our results demonstrate that zebrafish perform well in both tasks and show significant acquisition of the association between cue and reward as well as between location and reward. We conclude that zebrafish, similar to classical laboratory rodents, may have utility in the biological analysis of simple as well as complex forms of associative learning.

Latent learning in zebrafish (Danio rerio)

The zebrafish may represent an excellent compromise between system complexity and practical simplicity for behavioral brain research. It may be particularly appropriate for large scale screening studies whose aim is to identify mutants with altered phenotypes or novel compounds with particular efficacy. For example, the zebrafish may have utility in the analysis of the biological mechanisms of learning and memory. Although learning and memory have been extensively studied and hundreds of underlying molecular mechanisms have been identified, this number may represent only the fraction of genes involved in these complex brain functions. Thus large scale mutagenesis screens may have utility. In order for such screens to succeed, appropriate screening paradigms must be developed. The first step in this research is the characterization of learning and memory capabilities of zebrafish and the development of automatable tasks. Here we show that zebrafish is capable of latent learning, i.e. can acquire memory of their environment after being allowed to explore it. For example, we found experimental zebrafish that experienced an open left tunnel or an open right tunnel of a maze during the unrewarded exploration phase of the test to show the appropriate side bias during a probe trial when they had to swim to a group of conspecifics (the reward). Given that exploration of the maze does not require the presence of the experimenter and the probe trial, during which the subjects are video-recorded and their memory is tested, is short, we argue that the paradigm has utility in high-throughput screening.

Amphetamine recapitulates developmental programs in the zebrafish

Addictive drugs hijack the human brain's 'reward' systems. A zebrafish model of addiction has recently been used to query changes in gene expression during this process.

The synthetic substance hypoxanthine 3-N-oxide elicits alarm reactions in zebrafish (Danio rerio)

Zebrafish, one of the preferred study species of geneticists, is gaining increasing popularity in behavioral neuroscience. This small and prolific species may be an excellent tool with which the biological mechanisms of vertebrate brain function and behavior are investigated. Zebrafish has been proposed as a model organism in the analysis of fear responses and human anxiety disorders. Species-specific cues signaling the presence of predators have been successfully utilized in such research. Zebrafish has been shown to respond to its natural alarm substance with species-typical fear reactions. However, the extraction of this alarm substance and ascertaining its consistent dosing has been problematic. A synthetic substance with a known chemical identity and molecular weight would allow precise dosing and experimental control. Previously, the chemical component, hypoxanthine 3-N-oxide, common to several fish alarm substances has been identified and has been shown to elicit alarm reactions in fish species belonging to the Osteriophysan superorder. In the current study we investigate the effect of hypoxanthine 3-N-oxide by exposing zebrafish to three different concentrations of this synthetic substance. Our results show that the substance efficaciously induces species-typical fear reactions increasing the number of erratic movement episodes and jumps in zebrafish. We discuss the translational relevance of our findings and conclude that hypoxanthine 3-N-oxide will have utility to elicit fear responses in the laboratory in a precisely controlled manner in zebrafish.

Acute neuroactive drug exposures alter locomotor activity in larval zebrafish

As part of the development of a rapid in vivo screen for prioritization of toxic chemicals, we have begun to characterize the locomotor activity of zebrafish (Danio rerio) larvae by assessing the acute effects of prototypic drugs that act on the central nervous system. Initially, we chose ethanol, d-amphetamine, and cocaine, which are known, in mammals, to increase locomotion at low doses and decrease locomotion at higher doses. Wild-type larvae were individually maintained in 96-well microtiter plates at 26 degrees C, under a 14:10 h light:dark cycle, with lights on at 0830 h. At 6 days post-fertilization, ethanol (1-4% v/v), d-amphetamine sulfate (0.1-20.0 microM) or cocaine hydrochloride (0.2-50.0 microM) were administered to the larvae by immersion. Beginning 20 min into the exposure, locomotion was assessed for each animal for 70 min using 10-minute, alternating light (visible light) and dark (infrared light) periods. Low concentrations of ethanol and d-amphetamine increased activity, while higher concentrations of all three drugs decreased activity. Because ethanol effects occurred predominately during the light periods, whereas the d-amphetamine and cocaine effects occurred during the dark periods, alternating lighting conditions proved to be advantageous. These results indicate that zebrafish larvae are sensitive to neuroactive drugs, and their locomotor response is similar to that of mammals.

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

The zebrafish has been in the forefront of developmental genetics for decades and has also been gaining attention in neurobehavioral genetics. It has been proposed to model alcohol-induced changes in human brain function and behavior. Here, adult zebrafish populations, AB and SF (short-fin wild type), were exposed to chronic treatment (several days in 0.00% or 0.50% alcohol v/v) and a subsequent acute treatment (1 h in 0.00%, 0.25%, 0.50% or 1.00% alcohol). Behavioral responses of zebrafish to computer-animated images, including a zebrafish shoal and a predator, were quantified using videotracking. Neurochemical changes in the dopaminergic and serotoninergic systems in the brain of the fish were measured using high-precision liquid chromatography with electrochemical detection. The results showed genetic differences in numerous aspects of alcohol-induced changes, including, for the first time, the behavioral effects of withdrawal from alcohol and neurochemical responses to alcohol. For example, withdrawal from alcohol abolished shoaling and increased dopamine and 3,4-dihydroxyphenylacetic acid in AB but not in SF fish. The findings show that, first, acute and chronic alcohol induced changes are quantifiable with automated behavioral paradigms; second, robust neurochemical changes are also detectable; and third, genetic factors influence both alcohol-induced behavioral and neurotransmitter level changes. Although the causal relationship underlying the alcohol-induced changes in behavior and neurochemistry is speculative at this point, the results suggest that zebrafish will be a useful tool for the analysis of the biological mechanisms of alcohol-induced functional changes in the adult brain.

Long-term behavioral changes in response to early developmental exposure to ethanol in zebrafish

BACKGROUND

Zebrafish is becoming an important research tool for the analysis of brain function and behavior. It has been proposed to model human alcoholism as well as fetal alcohol syndrome. Previous studies investigating the consequences of exposure to ethanol during early development of zebrafish employed robust dosing regimens (high ethanol concentration and long exposure) that may model a rare situation in the human clinic. These studies found major structural abnormalities developing in the exposed fish.

METHODS

Here we hope to avoid such gross changes and administer only low doses of ethanol (0.00, 0.25, 0.50, 0.75, 1.00 vol/vol %) at 24-hour postfertilization and for only a short period of time (for 2 hours). We analyze the behavior of exposed fish at adult stage using computerized stimulus presentation and automated videotracking response quantification.

RESULTS

Despite the short ethanol exposure period and the modest concentrations, significant behavioral alterations were found: fish exposed to higher doses of ethanol swam at an increased distance from a computer-animated zebrafish shoal while their activity levels did not change.

CONCLUSIONS

Although the interpretation of and the mechanisms underlying this finding will require further investigation, the results suggest that zebrafish will be an appropriate model organism for the analysis of the effects of moderate to mild prenatal ethanol exposure.

Key issues concerning environmental enrichment for laboratory-held fish species

An improved knowledge and understanding of the fundamental biological requirements is needed for many of the species of fish held in captivity and, without this knowledge it is difficult to determine the optimal conditions for laboratory culture. The aim of this paper is to review the key issues concerning environmental enrichment for laboratory-held fish species and identify where improvements are required. It provides background information on environmental enrichment, describes enrichment techniques currently used in aquatic ecotoxicology studies, identifies potential restrictions in their use and discusses why more detailed and species-specific guidance is needed.

Locomotion in larval zebrafish: Influence of time of day, lighting and ethanol

The increasing use of zebrafish (Danio rerio) in developmental research highlights the need for a detailed understanding of their behavior. We studied the locomotion of individual zebrafish larva (6 days post-fertilization) in 96-well microtiter plates. Movement was recorded using a video-tracking system. Time of day results indicated locomotion, tested in darkness (infrared), decreased gradually from early morning to a stable level between 13:00 and 15:30 h. All further studies were conducted in early-to-late afternoon and lasted approximately 1 h. Each study also began with a period of darkness to minimize any unintended stimulation caused by transferring the plates to the recording platform. Locomotion in darkness increased initially to a maximum at 4 min, then decreased steadily to a low level by 20 min. Locomotion during light was initially low and then gradually increased to a stable level after 20 min. When 10-min periods of light and dark were alternated, activity was low in light and high in dark; curiously, activity during alternating dark periods was markedly higher than originally obtained during either extended dark or light. Further experiments explored the variables influencing this alternating pattern of activity. Varying the duration of the initial dark period (10-20 min) did not affect subsequent activity in either light or dark. The activity increase on return to dark was, however, greater following 15 min than 5 min of light. Acute ethanol increased activity at 1 and 2% and severely decreased activity at 4%. One-percent ethanol retarded the transition in activity from dark to light, and the habituation of activity in dark, while 2% ethanol increased activity regardless of lighting condition. Collectively, these results show that locomotion in larval zebrafish can be reliably measured in a 96-well microtiter plate format, and is sensitive to time of day, lighting conditions, and ethanol.

Strain-specific alteration of zebrafish feeding behavior in response to aversive stimuli

Behavioral management of risk, in which organisms must balance the requirements of obtaining food resources with the risk of predation, has been of considerable interest to ethologists for many years. Although numerous experiments have shown that animals alter their foraging behavior depending on the levels of perceived risk and demand for nutrients, few have considered the role of genetic variation in the trade-off between these variables. We performed a study of four zebrafish (Danio rerio (Hamilton, 1822)) strains to test for genetic variation in foraging behavior and whether this variation affected their response to both aversive stimuli and nutrient restriction. Zebrafish strains differed significantly in their latency to begin foraging from the surface of the water under standard laboratory conditions. Fish fed sooner when nutrients were restricted, although this was only significant in the absence of aversive stimuli. Aversive stimuli caused fish to delay feeding in a strain-specific manner. Strains varied in food intake and specific growth rate, and feeding latency was significantly correlated with food intake. Our results indicate significant genetic variation in foraging behavior and the perception of risk in zebrafish, with a pattern of strain variation consistent with behavioral adaptation to captivity.

Shuttle box learning in zebrafish (Danio rerio)

Zebrafish is used in forward genetic and drug screening and is gaining popularity in behavioral brain research but high throughput learning paradigms are lacking. The sight of conspecifics has been shown to be rewarding in zebrafish. Here, in a novel paradigm, subjects learn to respond to alternating presentation of computer-animated zebrafish images. The simplicity and computerization of the paradigm will make it useful for high throughput screening.

Differences in acute alcohol-induced behavioral responses among zebrafish populations

BACKGROUND

With the arsenal of genetic tools available for zebrafish, this species has been successfully used to investigate the genetic aspects of human diseases from developmental disorders to cancer. Interest in the behavior and brain function of zebrafish is also increasing as CNS disorders may be modeled and studied with this species. Alcoholism and alcohol abuse are among the most devastating and costliest diseases. However, the mechanisms of these diseases are not fully understood. Zebrafish has been proposed as a model organism to study such mechanisms. Characterization of alcohol's effects on zebrafish is a necessary step in this research.

METHODS

Here, we compare the effects of acute alcohol (EtOH) administration on the behavior of zebrafish from 4 distinct laboratory-bred populations using automated as well as observation based behavioral quantification methods.

RESULTS

Alcohol treatment resulted in significant dose-dependent behavioral changes but the dose-response trajectories differed among zebrafish populations.

CONCLUSIONS

The results demonstrate for the first time a genetic component in alcohol responses in adult zebrafish and also show the feasibility of high throughput behavioral screening. We discuss the exploration and exploitation of the genetic differences found.

The social zebrafish: behavioral responses to conspecific, heterospecific, and computer animated fish

Zebrafish has been in the forefront of developmental biology and genetics, but only recently has interest in their behavior increased. Zebrafish are small and prolific, which lends this species to high throughput screening applications. A typical feature of zebrafish is its propensity to aggregate in groups, a behavior known as shoaling. Thus, zebrafish has been proposed as a possible model organism appropriate for the analysis of the genetics of vertebrate social behavior. However, shoaling behavior is not well characterized in zebrafish. Here, using a recently developed software application, we first investigate how zebrafish respond to conspecific and heterospecific fish species that differ in coloration and/or shoaling tendencies. We found that zebrafish shoaled with their own species but not with two heterospecific species, one of which was a shoaling the other a non-shoaling species. In addition, we have started the analysis of visual stimuli that zebrafish may utilize to determine whether to shoal with a fish or not. We systematically modified the color, the location, the pattern, and the body shape of computer animated zebrafish images and presented them to experimental zebrafish. The subjects responded differentially to some of these stimuli showing preference for yellow and avoidance of elongated zebrafish images. Our results suggest that computerized stimulus presentation and automated behavioral quantification of zebrafish responses are feasible, which in turn implies that high throughput forward genetic mutation or drug screening will be possible in the analysis of social behavior with this model organism.

Quantification of shoaling behaviour in zebrafish (Danio rerio)

Zebrafish has been a favourite of developmental biologists and numerous genetic tools have been developed for this species. In recent years, zebrafish has become an increasingly popular subject of neuroscientists and behavioural scientists. One of the typical characteristics of zebrafish is shoaling, individuals forming a tight group in which fish swim together. The biological mechanisms of social behaviours are complex and not well understood in vertebrates, and zebrafish, due to its highly social nature and the genetic tools developed for it, may represent an excellent animal model with which these mechanisms may be studied. Improvement of behavioural quantification methods would facilitate research in this area. We describe a custom software application that allows the precise quantification of several parameters of group cohesion in zebrafish. We also present three experimental examples to illuminate the use of our methodology, and show how group cohesion changes in response to manipulations of the environment.

Alarm substance induced behavioral responses in zebrafish (Danio rerio)

Zebrafish (zebra danio) are becoming increasingly popular in behavioral neuroscience and behavior genetics. This small vertebrate may be utilized in modeling human brain disorders. One of the major neuropsychiatric conditions still not well understood is abnormally increased fear and anxiety. Zebrafish may be an appropriate organism with which these human diseases can be modeled and their biological mechanisms investigated. Predator induced anxiety paradigms have been suggested as useful methods in translational research. Shoaling fish, such as zebrafish, are known to respond to alarm substances with antipredatory or alarm reactions. However, these responses are not well characterized in zebrafish. In the current paper, we investigate the behavioral responses of zebrafish elicited by its alarm substance. Using observation-based as well as video-tracking aided behavior quantification methods we demonstrate significant alarm substance-induced behavioral changes that are independent of the presence of a predatory fish stimulus. The results suggest that, once refined, the use of alarm substance with zebrafish will allow the development of high throughput behavioral paradigms for drug and mutation screening aimed at the analysis of the biological mechanisms of fear in vertebrates.

Induction of hyperglycaemia in zebrafish (Danio rerio) leads to morphological changes in the retina

Diabetes affects over 16 million Americans yearly, resulting in hyperglycaemia and microvascular complications, including retinopathy, neuropathy and nephropathy. Animal models have been developed to examine the immunological aspects of type 1 diabetes and the pathogenic mechanisms associated with diabetic retinopathy, but the methods of diabetes induction raise concerns regarding these models. Zebrafish (Danio rerio) have been used extensively to study developmental processes and mutant zebrafish strains have been used to examine vision disease present in humans. In this paper, we have induced hyperglycaemia in zebrafish by alternately immersing the fish in glucose solution or water. Eyes from untreated fish or fish exposed to alternating glucose/water solutions for 28 days were dissected, sectioned and stained to visualise cell bodies in the retina. In untreated fish retinas, the inner plexiform layer (IPL) and inner nuclear layer (INL) were approximately the same thickness, whereas in fish repeatedly exposed to glucose solutions the IPL was approximately 55% the thickness of the INL. Both the IPL and INL were significantly reduced in retinas of treated fish, compared to untreated fish, similar to that seen in other animal models of diabetes and in diabetic patients. These results suggest that zebrafish may be used as an animal model in which to study diabetic retinopathy.

Zebrafish (Danio rerio) responds differentially to stimulus fish: the effects of sympatric and allopatric predators and harmless fish

The zebrafish has been an excellent model organism of developmental biology and genetics. Studying its behavior will add to the already strong knowledge of its biology and will strengthen the use of this species in behavior genetics and neuroscience. Anxiety is one of the most problematic human psychiatric conditions. Arguably, it arises as a result of abnormally exaggerated natural fear responses. The zebrafish may be an appropriate model to investigate the biology of fear and anxiety. Fear responses are expressed by animals when exposed to predators, and these responses can be learned or innate. Here we investigated whether zebrafish respond differentially to a natural predator or other fish species upon their first exposure to these fish. Naïve zebrafish were shown four species of fish chosen based on predatory status (predatory or harmless) and geographical origin (allopatric or sympatric). Our results suggest that naïve zebrafish respond differentially to the stimulus fish. Particularly interesting is the antipredatory response elicited by the zebrafish's sympatric predator, the Indian Leaf Fish, and the fact that this latter species exhibited almost no predatory attacks. The findings obtained open a new avenue of research into what zebrafish perceive as "dangerous" or fear inducing. They will also allow us to develop fear and anxiety related behavioral test methods with which the contribution of genes to, or the effects of novel anxiolytic substances on these behaviors may be analyzed.

Effects of acute and chronic ethanol exposure on the behavior of adult zebrafish (Danio rerio)

The zebrafish has been a popular subject of embryology and genetic research for the past three decades. Recently, however, the interest in its neurobiology and behavior has also increased. Nevertheless, compared to other model organisms, e.g., rodents, zebrafish behavior is understudied and very few behavioral paradigms exist for mutation or drug screening purposes. Alcoholism is one of the biggest and costliest diseases whose mechanisms are not well understood. Model organisms such as the zebrafish may be utilized in this line of research. Previously, we investigated the effects of acute ethanol exposure on adult zebrafish using four behavioral paradigms and employing manual quantification methods. Here, we study the effects of chronic ethanol exposure and analyze how it modifies the effects of acute ethanol treatment. We employ a videotracking-based automated quantification method in a predator model paradigm and show that this method is capable of detecting an avoidance reaction that is ameliorated by higher doses of ethanol, a potential anxiolytic effect. Importantly, we also demonstrate that chronic, two week long, exposure to ethanol results in significant adaptation to this substance in adult zebrafish. Overall, our results suggest that zebrafish will be an appropriate subject for high throughput screening applications aimed at the analysis of the mechanisms and pharmacology of acute and chronic ethanol induced changes in the vertebrate brain.