Strain-dependent differential behavioral responses of zebrafish larvae to acute MK-801 treatment

Behavioral Assays Dissecting NMDA Receptor Function in Zebrafish

Zebrafish are a powerful system to study brain development and to dissect the activity of complex circuits. One advantage is that they display complex behaviors, including prey capture, learning, responses to photic and acoustic stimuli, and social interaction (Dreosti et al., Front Neural Circuits 9:39, 2015; Bruckner et al., PLoS Biol 20:e3001838, 2022; Zoodsma et al., Mol Autism 13:38, 2022) that can be probed to assess brain function. Many of these behaviors are easily assayed at early larval stages, offering a noninvasive and high-throughput readout of nervous system function. Additionally, larval zebrafish readily uptake small molecules dissolved in water making them ideal for behavioral-based drug screens. Together, larval zebrafish and their behavioral repertoire offer a means to rapidly dissect brain circuitry and can serve as a template for high-throughput small molecule screens.NMDA receptor subunits are highly conserved in zebrafish compared to mammals (Zoodsma et al., Mol Autism 13:38, 2022; Cox et al., Dev Dyn 234:756-766, 2005; Zoodsma et al., J Neurosci 40:3631-3645, 2020). High amino acid and domain structure homology between humans and zebrafish underlie conserved functional similarities. Here we describe a set of behavioral assays that are useful to study the NMDA receptor activity in brain function.

Embryonic mercury exposure in zebrafish: Alteration of metabolites and gene expression, related to visual and behavioral impairments

The widespread presence of mercury, a heavy metal found in the environment and used in numerous industries and domestic, raises concerns about its potential impact on human health. Nevertheless, the adverse effects of this environmental toxicant at low concentrations are often underestimated. There are emerging studies showing that accumulation of mercury in the eye may contribute to visual impairment and a comorbidity between autism spectrum disorders (ASD) trait and visual impairment. However, the underlying mechanism of visual impairment in humans and rodents is challenging. In response to this issue, zebrafish larvae with a cone-dominated retinal visual system were exposed to 100 nM mercury chloride (HgCl₂), according to our previous study, followed by light-dark stimulation, a social assay, and color preference to examine the functionality of the visual system in relation to ASD-like behavior. Exposure of embryos to HgCl₂ from gastrulation to hatching increased locomotor activity in the dark, reduced shoaling and exploratory behavior, and impaired color preference. Defects in microridges as the first barrier may serve as primary tools for HgCl₂ toxicity affecting vision. Depletion of polyunsaturated fatty acids (PUFAs), linoleic acid, arachidonic acid (ARA), alpha-linoleic acid, docosahexaenoic acid (DHA), stearic acid, L-phenylalanine, isoleucine, L-lysine, and N-acetylputrescine, along with the increase of gamma-aminobutyric acid (GABA), sphingosine-1-phosphate, and citrulline assayed by liquid chromatography-mass spectrometry (LC-MS) suggest that these metabolites serve as biomarkers of retinal impairments that affect vision and behavior. Although suppression of adsl, shank3a, tsc1b, and nrxn1a gene expression was observed, among these tsc1b showed more positive correlation with ASD. Collectively, these results contribute new insights into the possible mechanism of mercury toxicity give rise to visual, cognitive, and social deficits in zebrafish.

Overexpression of miR-124-3p affects zebrafish inner ear development and hearing function via downregulation of EYA1 gene expression

The EYA1 gene is essential for normal inner ear development and affects the development and function of the inner ear in a dose-dependent manner. However, the mechanisms regulating EYA1 gene expression are not well understood. Recently, miRNAs have become recognized as important regulators of gene expression. In this study, we identified miR-124-3p through a microRNA (miRNA) target prediction website and found that miR-124-3p and its target site in the EYA1 3' untranslated region (3'UTR) are conserved in most vertebrates. Both in vivo and in vitro, the interaction of miR-124-3p with the EYA1 3'UTR exerts a negative regulatory effect. Microinjection of agomiR-124-3p into zebrafish embryos resulted in a phenotype of reduced auricular area, suggesting inner ear dysplasia. In addition, injection of agomiR-124-3p or antagomiR-124-3p caused abnormal hearing function in zebrafish. In conclusion, our results suggest that miR-124-3p can affect inner ear development and hearing function in zebrafish by regulating EYA1.

Embryonic Arsenic Exposure Triggers Long-Term Behavioral Impairment with Metabolite Alterations in Zebrafish

Arsenic trioxide (As₂O₃) is a ubiquitous heavy metal in the environment. Exposure to this toxin at low concentrations is unremarkable in developing organisms. Nevertheless, understanding the underlying mechanism of its long-term adverse effects remains a challenge. In this study, embryos were initially exposed to As₂O₃ from gastrulation to hatching under semi-static conditions. Results showed dose-dependent increased mortality, with exposure to 30-40 µM As₂O₃ significantly reducing tail-coiling and heart rate at early larval stages. Surviving larvae after 30 µM As₂O₃ exposure showed deficits in motor behavior without impairment of anxiety-like responses at 6 dpf and a slight impairment in color preference behavior at 11 dpf, which was later evident in adulthood. As₂O₃ also altered locomotor function, with a loss of directional and color preference in adult zebrafish, which correlated with changes in transcriptional regulation of adsl, shank3a, and tsc1b genes. During these processes, As₂O₃ mainly induced metabolic changes in lipids, particularly arachidonic acid, docosahexaenoic acid, prostaglandin, and sphinganine-1-phosphate in the post-hatching period of zebrafish. Overall, this study provides new insight into the potential mechanism of arsenic toxicity leading to long-term learning impairment in zebrafish and may benefit future risk assessments of other environmental toxins of concern.

Phenotypic architecture of sociality and its associated genetic polymorphisms in zebrafish

Sociality relies on motivational and cognitive components that may have evolved independently, or may have been linked by phenotypic correlations driven by a shared selective pressure for increased social competence. Furthermore, these components may be domain-specific or of general-domain across social and non-social contexts. Here, we used zebrafish to test if the motivational and cognitive components of social behavior are phenotypically linked and if they are domain specific or of general domain. The behavioral phenotyping of zebrafish in social and equivalent non-social tests shows that the motivational (preference) and cognitive (memory) components of sociality: (1) are independent from each other, hence not supporting the occurrence of a sociality syndrome; and (2) are phenotypically linked to non-social traits, forming two general behavioral modules, suggesting that sociality traits have been co-opted from general-domain motivational and cognitive traits. Moreover, the study of the association between single nucleotide polymorphisms (SNPs) and each behavioral module further supports this view, since several SNPs from a list of candidate "social" genes, are statistically associated with the motivational, but not with the cognitive, behavioral module. Together, these results support the occurrence of general-domain motivational and cognitive behavioral modules in zebrafish, which have been co-opted for the social domain.

Behavioral effects of acute ethanol in larval zebrafish (D. rerio) depend on genotype and volume of experimental well

Ethanol consumption is a worldwide problem. Sensitivity to acute effects of ethanol influences the development of chronic ethanol abuse and ethanol dependence. Environmental and genetic factors have been found to contribute to differential effects of acute ethanol. Animal models have been employed to investigate these factors. An increasingly frequently utilized animal model in ethanol research is the zebrafish. A large proportion of ethanol studies with zebrafish have been conducted with adult zebrafish. However, high throughput drug and mutation screens are particularly well adapted to larval zebrafish. These studies are often carried out using the 96-well-plate that allows monitoring large numbers of fish efficiently. Here, we investigate the effects of acute (30 min long) ethanol exposure in 8-day post-fertilization (dpf) old zebrafish. We compare four genetically distinct populations (strains) of zebrafish, measuring numerous parameters of their swim path in two well sizes, i.e., in the 96-well-plate (small volume wells) and in the 6-well-plate (large volume wells). In general, we found that the highest dose of ethanol (1% vol/vol) reduced swim speed, increased duration of immobility, increased turn angle, and increased intra-individual variance of turn angle, while the intermediate dose (0.5%) had a less strong effect, compared to control. However, we also found that these ethanol effects were strain dependent and, in general, were better detected in the larger volume well. We conclude that larval zebrafish are appropriate for quantification of acute ethanol effects and also for the analysis of environmental and genetic factors that influence these effects. We also speculate that using larger wells will likely increase sensitivity of detection and precision in screening applications.

Glutamate Nmda Receptor Antagonists With Relevance To Schizophrenia: A Review Of Zebrafish Behavioral Studies

Schizophrenia pathophysiology is associated with hypofunction of glutamate NMDA receptors (NMDAR) in GABAergic interneurons and dopaminergic hyperactivation in subcortical brain areas. The administration of NMDAR antagonists is used as an animal model that replicates behavioral phenotypes relevant to the positive, negative, and cognitive symptoms of schizophrenia. Such models overwhelmingly rely on rodents, which may lead to species-specific biases and poor translatability. Zebrafish, however, is increasingly used as a model organism to study evolutionarily conserved aspects of behavior. We thus aimed to review and integrate the major findings reported in the zebrafish literature regarding the behavioral effects of NMDAR antagonists with relevance to schizophrenia. We identified 44 research articles that met our inclusion criteria from 590 studies retrieved from MEDLINE (PubMed) and Web of Science databases. Dizocilpine (MK-801) and ketamine were employed in 29 and 10 studies, respectively. The use of other NMDAR antagonists, such as phencyclidine (PCP), APV, memantine, and tiletamine, was described in 6 studies. Frequently reported findings are the social interaction and memory deficits induced by MK-801 and circling behavior induced by ketamine. However, mixed results were described for several locomotor and exploratory parameters in the novel tank and open tank tests. The present review integrates the most relevant results while discussing variation in experimental design and methodological procedures. We conclude that zebrafish is a suitable model organism to study drug-induced behavioral phenotypes relevant to schizophrenia. However, more studies are necessary to further characterize the major differences in behavior as compared to mammals.

A fish is not a mouse: understanding differences in background genetics is critical for reproducibility

Poorly controlled background genetics in animal models contributes to the lack of reproducibility that is increasingly recognized in biomedical research. The laboratory zebrafish, Danio rerio, has been an important model organism for decades in many research areas, yet inbred strains and traditionally managed outbred stocks are not available for this species. Sometimes incorrectly referred to as 'inbred strains' or 'strains', zebrafish wild-type lines possess background genetics that are often not well characterized, and breeding practices for these lines have not been consistent over time or among institutions. In this Perspective, we trace key milestones in the history of one of the most widely used genetic backgrounds, the AB line, to illustrate the dynamic complexity within an example background that is largely invisible when reading the scientific literature. Failure to adequately control for genetic background compromises the validity of experimental outcomes. We therefore propose that authors provide as much specific detail about the origin and genetic makeup of zebrafish lines as is reasonable and possible, and that the terms used to describe background genetics be applied in a way that is consistent with other fish and mammalian model organisms. We strongly encourage the adoption of genetic monitoring for the characterization of existing zebrafish lines, to help detect genetic contamination in breeding colonies and to verify the level of genetic heterogeneity in breeding colonies over time. Careful attention to background genetics will improve transparency and reproducibility, therefore improving the utility of the zebrafish as a model organism.

DMSO effects larval zebrafish (Danio rerio) behavior, with additive and interaction effects when combined with positive controls

Embryonic and larval zebrafish (Danio rerio) behavior is commonly used to identify neurotoxic compounds. Here, we investigated whether sub-lethal exposures to the common solvents dimethyl sulfoxide (DMSO, 0.01-1%) and methanol (MeOH, 0.01-1%), or the anti-fungal agent methylene blue (MB, 0.0001-0.0005%), can influence larval behavior in a simple light/dark paradigm conducted in 96-well plates. In addition, we tested whether the media volume within the behavioral arena or the zebrafish strain, AB wild type, AB Tübingen (AB/TU), or Tüpfel long-fin (TL), could also influence larval behavior. Following the single exposures, we co-exposed larvae to DMSO and either MB or two other compounds with known behavioral effects in larval zebrafish, flutamide and perfluorooctanesulfonic acid (PFOS). We found ≥0.55% DMSO and 0.0005% MB significantly affected larval behavior, but there was no effect of MeOH. Similarly, TL showed less movement compared to AB and AB/TU strains, whereas lower media volumes also significantly reduced larval movement. However, all strains responded similarly to DMSO and MB. In the co-exposure studies, we found either additive or interaction effects between DMSO and either MB, flutamide, or PFOS, depending on the behavioral endpoint measured. In addition, media volume had no effect on the DMSO concentration response curve, but again we observed additive effects on behavior. In conclusion, methodology can lead to alterations in baseline locomotor activity and compounds can have additive or interaction effects on behavioral endpoints. However, we found no evidence that strain effects should be a concern when deciding on solvents for a simple light/dark behavioral test in larval zebrafish.

A critical review of zebrafish schizophrenia models: Time for validation?

Schizophrenia is a mental disorder that affects 1% of the population worldwide and is manifested as a broad spectrum of symptoms, from hallucinations to memory impairment. It is believed that genetic and/or environmental factors may contribute to the occurrence of this disease. Recently, the zebrafish has emerged as a valuable and attractive model for various neurological disorders including schizophrenia. In this review, we describe current pharmacological models of schizophrenia with special emphasis on providing insights into the pros and cons of using zebrafish as a behavioural model of this disease. Moreover, we highlight the advantages and utility of using zebrafish for elucidating the genetic mechanisms underlying this psychiatric disorder. We believe that the zebrafish has high potential also in the area of precision medicine and may complement the development of therapeutics, especially for pharmacoresistant patients.

Novel TRRAP mutation causes autosomal dominant non-syndromic hearing loss

Hereditary non-syndromic hearing loss is the most common inherited sensory defect in humans. More than 40 genes have been identified as causative genes for autosomal dominant non-syndromic hearing loss (ADNSHL), but there are many other candidate genes that remain to be discovered. We aimed to identify the causative gene mutation for post-lingual progressive ADNSHL in a Chinese family. Whole-exome sequencing, bioinformatic analysis, and Sanger sequencing were used to verify the co-segregation of a novel pathogenic variant (NM_ 001244580, c.511C>T, p.Arg171Cys) in the TRansformation/tRanscription domain-Associated Protein gene associated with hearing loss in a three-generation Chinese family with ADNSHL). Additionally, three more novel variants of transformation/transcription domain associated protein (TRRAP) were detected in 66 sporadic cases of hearing loss. Morpholino oligonucleotides knockdown and clustered regularly interspaced short palindromic repeats/Cas9 knockout zebrafish were constructed to validate the genetic findings. Knockdown or knockout of TRRAP resulted in significant defects in the inner ear of zebrafish, indicating that TRRAP plays an important role in inner ear development. In conclusion, TRRAP (NM_ 001244580, c.511C>T, p.Arg171Cys) co-segregated with hearing loss in a Chinese family with ADNSHL, and TRRAP deficiency caused hearing disability in zebrafish, suggesting TRRAP is a gene associated with ADNSHL.

Evaluation of development, locomotor behavior, oxidative stress, immune responses and apoptosis in developing zebrafish (Danio rerio) exposed to TBECH (tetrabromoethylcyclohexane)

Tetrabromoethylcyclohexane (TBECH), as one emerging brominated flame retardants, is ubiquitous in the environment, including water and aquatic organisms. TBECH was found to exhibit endocrine-disrupting effects in different models, whereas a survey of comprehensive toxic effects of TBECH on zebrafish is limited. In the present study, zebrafish (Danio rerio) were waterborne exposed continuously to TBECH from embryonic stage (3 h post-fertilization (hpf)) to the time when the respective parameters were evaluated. Exposure to TBECH reduced hatchability of zebrafish embryos at 72 and 96 hpf, diminished heart rate of zebrafish larvae at 48 hpf, and increased malformation in zebrafish larvae at 96 hpf. In addition, exposure to TBECH diminished free swimming distance both in the light and under a photoperiod of 10 min light/10 min dark cycles in zebrafish larvae at 6 days post-fertilization (dpf). Moreover, exposure to TBECH elevated activities of superoxide dismutase (SOD) and catalase (CAT), malondialdehyde (MDA) content, whereas it reduced glutathione (GSH) content, in zebrafish larvae at 6 dpf. Accordingly, RT-qPCR analysis demonstrated that TBECH exposure increased the mRNA levels of sod1, sod2, cat, and gpx1 in zebrafish larvae at 6 dpf. With respect to the immune aspect, the mRNA levels of pro-inflammatory genes, including il-1b, il-6, il-8, and tnfa, in larval zebrafish at 6 dpf were increased by exposure to TBECH, while pretreatment with TBECH inhibited 24 h of exposure to LPS-stimulated elevation in the mRNA levels of the abovementioned four pro-inflammatory genes in zebrafish larvae at 6 dpf. Furthermore, TBECH treatment increased caspase-3 enzyme activities and regulated apoptosis-related genes in larval zebrafish at 6 dpf. Taken together, the data obtained in this study demonstrated that TBECH caused developmental and locomotor behavioral toxicity, immunotoxicity, oxidative stress and proapoptotic effects in early life zebrafish. The present study will help to understand the comprehensive toxicity of TBECH in zebrafish.

Sound shock response in larval zebrafish: A convenient and high-throughput assessment of auditory function

Given that hearing ability can be challenged in diverse ways, it is necessary to develop an easily conducted, high-throughput method for assessing potential auditory risks. Measuring the acoustic startle response (ASR) has become a critical behavioral method in hearing research using zebrafish (Danio rerio). In this study, changes in the activity of zebrafish larvae (10 days post fertilization (dpf)) due to exposure to a sudden easily-generated broad-band noise were automatically and objectively recorded and analyzed without building sophisticated equipments. A significant increase in activity was induced by the noise stimulation and the alterations were impaired by gentamicin. In addition, a clear dose-response trend was observed between gentamicin exposure and the impaired activity, and a similar phenomenon was observed between gentamicin exposure and damage to hair cells. Our results suggested that alterations in the activity induced by a broad-band noise can potentially be used as an efficient assay for assessing hearing ability.

Development of an opioid self-administration assay to study drug seeking in zebrafish

The zebrafish (Danio rerio) has become an excellent tool to study mental health disorders, due to its physiological and genetic similarity to humans, ease of genetic manipulation, and feasibility of small molecule screening. Zebrafish have been shown to exhibit characteristics of addiction to drugs of abuse in non-contingent assays, including conditioned place preference, but contingent assays have been limited to a single assay for alcohol consumption. Using inexpensive electronic, mechanical, and optical components, we developed an automated opioid self-administration assay for zebrafish, enabling us to measure drug seeking and gain insight into the underlying biological pathways. Zebrafish trained in the assay for five days exhibited robust self-administration, which was dependent on the function of the μ-opioid receptor. In addition, a progressive ratio protocol was used to test conditioned animals for motivation. Furthermore, conditioned fish continued to seek the drug despite an adverse consequence and showed signs of stress and anxiety upon withdrawal of the drug. Finally, we validated our assay by confirming that self-administration in zebrafish is dependent on several of the same molecular pathways as in other animal models. Given the ease and throughput of this assay, it will enable identification of important biological pathways regulating drug seeking and could lead to the development of new therapeutic molecules to treat addiction.

Action sequencing in the spontaneous swimming behavior of zebrafish larvae - implications for drug development

All motile organisms need to organize their motor output to obtain functional goals. In vertebrates, natural behaviors are generally composed of a relatively large set of motor components which in turn are combined into a rich repertoire of complex actions. It is therefore an experimental challenge to investigate the organizational principles of natural behaviors. Using the relatively simple locomotion pattern of 10 days old zebrafish larvae we have here characterized the basic organizational principles governing the swimming behavior. Our results show that transitions between different behavioral states can be described by a model combining a stochastic component with a control signal. By dividing swimming bouts into a limited number of categories, we show that similar types of swimming behavior as well as stand-stills between bouts were temporally clustered, indicating a basic level of action sequencing. Finally, we show that pharmacological manipulations known to induce alterations in the organization of motor behavior in mammals, mainly through basal ganglia interactions, have related effects in zebrafish larvae. This latter finding may be of specific relevance to the field of drug development given the growing importance of zebrafish larvae in phenotypic screening for novel drug candidates acting on central nervous system targets.

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.

Exploring Hallucinogen Pharmacology and Psychedelic Medicine with Zebrafish Models

After decades of sociopolitical obstacles, the field of psychiatry is experiencing a revived interest in the use of hallucinogenic agents to treat brain disorders. Along with the use of ketamine for depression, recent pilot studies have highlighted the efficacy of classic serotonergic hallucinogens, such as lysergic acid diethylamide and psilocybin, in treating addiction, post-traumatic stress disorder, and anxiety. However, many basic pharmacological and toxicological questions remain unanswered with regard to these compounds. In this study, we discuss psychedelic medicine as well as the behavioral and toxicological effects of hallucinogenic drugs in zebrafish. We emphasize this aquatic organism as a model ideally suited to assess both the potential toxic and therapeutic effects of major known classes of hallucinogenic compounds. In addition, novel drugs with hallucinogenic properties can be efficiently screened using zebrafish models. Well-designed preclinical studies utilizing zebrafish can contribute to the reemerging treatment paradigm of psychedelic medicine, leading to new avenues of clinical exploration for psychiatric disorders.

MK-801 increases locomotor activity in a context-dependent manner in zebrafish

Zebrafish have become a popular animal model for behavioral neuroscience with an increasing number of studies examining the effects of pharmacological compounds targeting the brain. Exposure to MK-801, a non-competitive N-methyl-d-aspartate receptor antagonist has been shown to increase locomotor activity in zebrafish. However, others have failed to replicate this finding as several contradicting studies report no changes in locomotor activity following exposure to similar doses. In the current study we reconcile these behavioral reports by demonstrating that zebrafish do not exhibit changes in locomotor activity during exposure to non-sedative doses of MK-801. Interestingly, zebrafish do exhibit significant increases in locomotion if pre-treated with MK-801 followed by subsequent testing in a novel environment, which suggests the effects of MK-801 are context-dependent. In addition, we examine the potential role of the dopaminergic system in mediating MK-801's locomotor stimulant effect by quantifying the levels of dopamine and its metabolite 3,4-dihydroxyphenylacetic acid (DOPAC) in the brains of zebrafish following a 30 min exposure to 10 μM of MK-801 (the dose found to induce the largest increase in locomotor activity). Our findings indicate that the MK-801-induced increase in locomotor activity is not accompanied by changes in whole-brain levels of dopamine or DOPAC. Overall, our results suggest that MK-801's context-dependent locomotor stimulant effect may be independent of whole-brain dopaminergic activation.