Using zebrafish to unravel the genetics of complex brain disorders

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.

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.

Evaluation of spontaneous recovery of behavioral and brain injury profiles in zebrafish after hypoxia

Cerebral hypoxia-ischemia can lead to motor and sensory impairments which can be dependent on the extent of infarcted regions. Since a better understanding of the neurochemical mechanisms involved in this injury is needed, the use of zebrafish as a cerebral hypoxia model has become quite promising because it could improve the knowledge about hypoxia-ischemia. In the current study, we aimed to investigate the spontaneous recovery of brain and behavioral impairments induced by hypoxia in adult zebrafish. Brain injury levels were analyzed by spectrophotometric measurement of mitochondrial dehydrogenase activity by staining with 2,3,5-triphenyltetrazolium chloride, and behavioral profiles were assessed by the open tank test. The induction of hypoxia substantially decreased mitochondrial activity in the brain and impaired behavior. The spontaneous recovery of fish subjected to hypoxia was assessed after 1, 3, 6, 24, and 48h under normoxia. The quantification of brain injury levels showed a significant increase until 24h after hypoxia, but after 48h this effect was completely reversed. Regarding behavioral parameters, we verified that locomotor activity and vertical exploration were impaired by hypoxia and these effects were reversed after 3h under normoxia. Taken together, these results show that zebrafish exhibited transient cerebral and behavioral impairments when submitted to hypoxia, and 1h under normoxic conditions was insufficient to reverse both effects. Therefore, our data help to elucidate the time window of spontaneous recovery in zebrafish after hypoxia and also the behavioral phenotypes involved in this phenomenon.

Topographical analysis of reactive zinc in the central nervous system of adult zebrafish (Danio rerio)

Reactive zinc (Zn) is crucial for neuronal signaling and is largely distributed within presynaptic vesicles of some axon terminals of distinct vertebrates. However, the distribution of reactive Zn throughout the central nervous system (CNS) is not fully explored. We performed a topographical study of CNS structures containing reactive Zn in the adult zebrafish (Danio rerio). Slices of CNS from zebrafish were stained by Neo-Timm and/or cresyl violet. The Zn specificity of Neo-Timm was evaluated with Zn chelants, N,N,N',N'-Tetrakis(2-pyridylmethyl)ethylenediamine (TPEN), sodium diethyldithiocarbamate (DEDTC), Zn sulfide washing solution, and hydrochloric acid (HCl). Unfixed slices were also immersed in the fluorescent Zn probe (zinpyr-1). Yellow-to-brown-to-black granules were revealed by Neo-Timm in the zebrafish CNS. Telencephalon exhibited slightly stained regions, while rhombencephalic structures showed high levels of staining. Although stained granules were found on the cell bodies, rhombencephalic structures showed a neuropil staining profile. The TPEN produced a mild reduction in Neo-Timm staining, while HCl and mainly DEDTC abolished the staining, indicating a large Zn content. This result was also confirmed by the application of a Zn probe. The present topographical study revealed reactive Zn throughout the CNS in adult zebrafish that should be considered in future investigation of Zn in the brain on a larger scale.

Active avoidance learning in zebrafish (Danio rerio)--the role of sensory modality and inter-stimulus interval

The zebrafish (Danio rerio) promises to meet the growing need of a high throughput model in the fields of gerontological and neurobehavioral research by possessing highly conserved anatomy and physiology with vertebrates, while having low maintenance costs. Here we further explore the conditions of active avoidance learning in zebrafish. Two pairs of distinct aversive conditioning experiments using shuttle boxes were designed to compare the effects of sensory modality and conditioned-unconditioned stimulus interval (CS-US interval) upon memory formation and retention. We found that olfactory conditioning with phenylethyl alcohol as a CS was significantly more likely to produce a successful outcome than with a visual CS. Likewise a 10 s CS-US interval yielded significantly more successful memory formation than a 15 s interval. These conditions may further facilitate the use of zebrafish to explore the genetic and neuronal base of active avoidance learning and its neuropharmacological improvement.

The role of zebrafish (Danio rerio) in dissecting the genetics and neural circuits of executive function

Zebrafish have great potential to contribute to our understanding of behavioral genetics and thus to contribute to our understanding of the etiology of psychiatric disease. However, progress is dependent upon the rate at which behavioral assays addressing complex behavioral phenotypes are designed, reported and validated. Here we critically review existing behavioral assays with particular focus on the use of adult zebrafish to explore executive processes and phenotypes associated with human psychiatric disease. We outline the case for using zebrafish as models to study impulse control and attention, discussing the validity of applying extant rodent assays to zebrafish and evidence for the conservation of relevant neural circuits.

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.

Socially-central zebrafish influence group behavior more than those on the social periphery

Certain individuals are more effective than others at using individual experience to impact group behavior. Here, we tested whether pre-training of zebrafish that are at the focal central of social group dynamics ("Key" fish) has a stronger positive impact on group performance than does pre-training of less central ("Non-Key") fish. We used very short observation periods and social network statistics to identify Key and Non-Key individuals, trained these fish to respond to an aversive stimulus, and then measured group performance after returning these now-experienced fish to a social setting. Although Key and Non-Key fish evaded the stimulus equally quickly as individuals, groups with experienced Key fish escaped the aversive stimulus more quickly than did groups with experienced Non-Key fish. The impact depended on genetic background: PN zebrafish on the social extremes (more often males) influenced the group's baseline response to the aversive stimulus, whereas experienced Scientific Hatcheries' zebrafish (both males and females) influenced the change in response over repeated trials. These results suggest that social roles are an important feature of information transfer across a group, and set the stage for future research into the genetic and evolutionary basis of social learning.

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.

In vivo assessment of the permeability of the blood-brain barrier and blood-retinal barrier to fluorescent indoline derivatives in zebrafish

Background

Successful delivery of compounds to the brain and retina is a challenge in the development of therapeutic drugs and imaging agents. This challenge arises because internalization of compounds into the brain and retina is restricted by the blood–brain barrier (BBB) and blood-retinal barrier (BRB), respectively. Simple and reliable in vivo assays are necessary to identify compounds that can easily cross the BBB and BRB.

Methods

We developed six fluorescent indoline derivatives (IDs) and examined their ability to cross the BBB and BRB in zebrafish by in vivo fluorescence imaging. These fluorescent IDs were administered to live zebrafish by immersing the zebrafish larvae at 7-8 days post fertilization in medium containing the ID, or by intracardiac injection. We also examined the effect of multidrug resistance proteins (MRPs) on the permeability of the BBB and BRB to the ID using MK571, a selective inhibitor of MRPs.

Results

The permeability of these barriers to fluorescent IDs administered by simple immersion was comparable to when administered by intracardiac injection. Thus, this finding supports the validity of drug administration by simple immersion for the assessment of BBB and BRB permeability to fluorescent IDs. Using this zebrafish model, we demonstrated that the length of the methylene chain in these fluorescent IDs significantly affected their ability to cross the BBB and BRB via MRPs.

Conclusions

We demonstrated that in vivo assessment of the permeability of the BBB and BRB to fluorescent IDs could be simply and reliably performed using zebrafish. The structure of fluorescent IDs can be flexibly modified and, thus, the permeability of the BBB and BRB to a large number of IDs can be assessed using this zebrafish-based assay. The large amount of data acquired might be useful for in silico analysis to elucidate the precise mechanisms underlying the interactions between chemical structure and the efflux transporters at the BBB and BRB. In turn, understanding these mechanisms may lead to the efficient design of compounds targeting the brain and retina.

Discrimination reversal and attentional sets in zebrafish (Danio rerio)

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