Alistair J. Brock

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.

The utility of zebrafish to study the mechanisms by which ethanol affects social behavior and anxiety during early brain development.

Exposure to moderate levels of ethanol during brain development has a number of effects on social behavior but the molecular mechanisms that mediate this are not well understood. Gaining a better understanding of these factors may help to develop therapeutic interventions in the future. Zebrafish offer a potentially useful model in this regard. Here, we introduce a zebrafish model of moderate prenatal ethanol exposure. Embryos were exposed to 20mM ethanol for seven days (48hpf-9dpf) and tested as adults for individual social behavior and shoaling. We also tested their basal anxiety with the novel tank diving test. We found that the ethanol-exposed fish displayed reductions in social approach and shoaling, and an increase in anxiety in the novel tank test. These behavioral differences corresponded to differences in hrt1aa, slc6a4 and oxtr expression. Namely, acute ethanol caused a spike in oxtr and ht1aa mRNA expression, which was followed by down-regulation at 7dpf, and an up-regulation in slc6a4 at 72hpf. This study confirms the utility of zebrafish as a model system for studying the molecular basis of developmental ethanol exposure. Furthermore, it proposes a putative developmental mechanism characterized by ethanol-induced OT inhibition leading to suppression of 5-HT and up-regulation of 5-HT1A, which leads, in turn, to possible homeostatic up-regulation of 5-HTT at 72hpf and subsequent imbalance of the 5-HT system.

Behavioral Phenotyping of Casper Mutant and 1-Pheny-2-Thiourea Treated Adult Zebrafish

The ability to visualise neural circuits in zebrafish in vivo is one of the most useful aspects of this model organism in neuroscience. To maintain the transparency of embryos, however, drugs, such as 1-pheyl-2-thiourea (PTU) must be added, or researchers can use mutants that do not develop pigment (e.g., the casper). The behavioural characteristics of such strains, however, have not been documented. Here, we tested adult zebrafish from the casper line, as well as wild-type (Tübingen, TU) and wild-types treated as embryos with PTU on three commonly used behavioural endpoints in neuroscience: novel tank test (similar to open-field in rodents), conditioned place preference for nicotine, and social cohesion (using a new method of cluster analysis). We found no differences between the casper and the TU, but the adult TU treated with PTU as embryos showed a marked increase in anxiety during the novel tank test. These data suggest that where possible, labs interested in analysis of developmental processes involved in adult phenotypes should avoid the use of PTU in favour of transparent mutants, such as casper.

Moderate alcohol exposure during early brain development increases stimulus-response habits in adulthood

Exposure to alcohol during early central nervous system development has been shown variously to affect aspects of physiological and behavioural development. In extreme cases, this can extend to craniofacial defects, severe developmental delay and mental retardation. At more moderate levels, subtle differences in brain morphology and behaviour have been observed. One clear effect of developmental alcohol exposure is an increase in the propensity to develop alcoholism and other addictions. The mechanisms by which this occurs, however, are not currently understood. In this study, we tested the hypothesis that adult zebrafish chronically exposed to moderate levels of ethanol during early brain ontogenesis would show an increase in conditioned place preference for alcohol and an increased propensity towards habit formation, a key component of drug addiction in humans. We found support for both of these hypotheses and found that the exposed fish had changes in mRNA expression patterns for dopamine receptor, nicotinic acetylcholine receptor and μ‐opioid receptor encoding genes. Collectively, these data show an explicit link between the increased proclivity for addiction and addiction‐related behaviour following exposure to ethanol during early brain development and alterations in the neural circuits underlying habit learning.

Developmental role of acetylcholinesterase in impulse control in zebrafish

Cellular and molecular processes that mediate individual variability in impulsivity, a key behavioral component of many neuropsychiatric disorders, are poorly understood. Zebrafish heterozygous for a nonsense mutation in ache (achesb55/+) showed lower levels of impulsivity in a 5-choice serial reaction time task (5-CSRTT) than wild type and ache+∕+. Assessment of expression of cholinergic (nAChR), serotonergic (5-HT), and dopamine (DR) receptor mRNA in both adult and larval (9 dpf) achesb55/+ revealed significant downregulation of chrna2, chrna5, and drd2 mRNA in achesb55/+ larvae, but no differences in adults. Acute exposure to cholinergic agonist/antagonists had no effect on impulsivity, supporting the hypothesis that behavioral effects observed in adults were due to lasting impact of developmental alterations in cholinergic and dopaminergic signaling. This shows the cross-species role of cholinergic signaling during brain development in impulsivity, and suggests zebrafish may be a useful model for the role of cholinergic pathways as a target for therapeutic advances in addiction medicine.

Assessing the Value of the Zebrafish Conditioned Place Preference Model for Predicting Human Abuse Potential.

Regulatory agencies recommend that centrally active drugs are tested for abuse potential before approval. Standard preclinical assessments are conducted in rats or non-human primates (NHPs). This study evaluated the ability of the zebrafish conditioned place preference (CPP) model to predict human abuse outcomes. Twenty-seven compounds from a variety of pharmacological classes were tested in zebrafish CPP, categorized as positive or negative, and analyzed using standard diagnostic tests of binary classification to determine the likelihood that zebrafish correctly predict robust positive signals in human subjective effects studies (+HSE) and/or Drug Enforcement Administration drug scheduling. Results were then compared with those generated for rat self-administration and CPP, as well as NHP self-administration, using this same set of compounds. The findings reveal that zebrafish concordance and sensitivity values were not significantly different from chance for both +HSE and scheduling. Although significant improvements in specificity and negative predictive values were observed for zebrafish relative to +HSE, specificity without sensitivity provides limited predictive value. Moreover, assessments in zebrafish provided no added value for predicting scheduling. By contrast, rat and NHP models generally possessed significantly improved concordance, sensitivity, and positive predictive values for both clinical measures. Although there may be predictive value with compounds from specific pharmacological classes (e.g., µ-opioid receptor agonists, psychostimulants) for zebrafish CPP, altogether these data highlight that using the current methodology, the zebrafish CPP model does not add value to the preclinical assessment of abuse potential.

A fully automated computer based Skinner box for testing learning and memory in zebrafish

Zebrafish are an important model species with unparalleled potential to advance understanding of the genetics and neurobiology of behaviour through genetic and pharmacological screening and mutant analysis. However, advances using this species have been limited by the lack of robust, standardised methodology and equipment suitable for assessing adult behaviour. Here we describe a simple, fully automated, computer based, operant system for measuring behaviour in juvenile and adult zebrafish and provide detailed protocols for appetitive and aversive assays to assess cognitive function in adult zebrafish. Applications include the study of cognition in zebrafish (and other similar sized fish species) and in zebrafish models of psychiatric and neurodegenerative diseases (e.g., Alzheimer’s disease, schizophrenia, Huntington’s disease, frontotemporal dementia), and characterisation of the role of select brain regions, neurotransmitter systems and genes in zebrafish. Further, the scalable nature of the system makes the protocols suitable for use in pharmacological and genetic screening programmes.

Zebrafish Behavioral Models of Ageing

With lifespans rapidly increasing worldwide there has been a marked increase in age-related diseases—particularly those affecting cognition—that place a major socioeconomic burden on society. Despite this, much of what occurs during the aging process at a molecular level is poorly understood, facilitating the need for a greater understanding of the processes involved. In recent years, zebrafish have proved a useful model for the identification of genetic and cellular mechanisms affecting a variety of disease processes. Here we review the potential of zebrafish as a model for the study of cognitive ageing.

Population based forward genetic screen of mutagenized zebrafish identifies loci associated with nicotine preference and human smoking behavior

Although there is clear evidence of genetic contributions to smoking behaviour, it has proved difficult to identify causal alleles and pathways from human studies. To facilitate smoking genetics research we examined the ability of a screen of mutagenized zebrafish to predict loci affecting smoking behaviour. We identified Slit3 as a gene affecting nicotine preference in fish. Focussed SNP analysis of the homologous human locus in cohorts from UK and Finland identified two SNP variants in the SLIT3 locus that predict level of cigarette consumption and likelihood of cessation. Characterisation of Slit3 mutant larvae and adult fish revealed altered behavioural sensitivity to amisulpride, a dopaminergic and serotonergic antagonist, and increased 5htr1aa mRNA expression in mutant larvae. No effect on neuronal pathfinding was detected. These findings reveal a role for SLIT3 signalling in development of pathways affecting responses to nicotine and confirm the translational relevance of zebrafish for exploring complex human behaviours.BACKGROUND: Although there is clear evidence of genetic contributions to susceptibility to nicotine addiction, it has proved difficult to identify causal alleles and pathways from studies in humans. Mutagenesis in model species generates strong phenotypes not present in wildtype populations and can be used to identify biological mechanisms underlying quantifiable behaviours. We tested the hypothesis that a forward genetic screen for nicotine preference in zebrafish can predict loci and biological mechanisms influencing human smoking behaviour. METHODS: A population-based forward genetic screen of ethylnitrosurea- mutagenized zebrafish was used to identify lines of fish showing altered nicotine preference. Immunohistochemical, behavioral and quantitative PCR analyses were used to characterize mutant larvae. Focussed SNP analysis of the homologous human locus in cohorts from the UK and a Finnish Twin study assessed the predictive validity of the zebrafish data for human smoking behavior. RESULTS: We show nicotine preference is heritable in fish as in humans and identify loss-of-function mutations in the zebrafish Slit3 gene as associated with increased nicotine preference. Slit3 mutant larval fish showed altered sensitivity of habituation to acoustic startle to dopaminergic antagonists and increased Drd2 and Drd3 mRNA expression. Dopaminergic neuronal pathfinding was unaffected. Analysis of the SLIT3 locus in two independent human cohorts identified 2 genetic markers that predict level of cigarette consumption and likelihood of cessation. CONCLUSION: These findings suggest a role for SLIT3 signaling in development of dopaminergic pathways affecting behaviours associated with nicotine dependence and confirm the translational relevance of the zebrafish model in exploring complex human behaviors.

Treatment for tobacco dependence: a potential application for stratified medicine?

Tobacco addiction is a leading preventable cause of death worldwide and places a heavy social and financial burden on society. Therefore, ways of helping people to overcome nicotine dependence are a key element of strategies aimed at improving public health. Current treatments are only partially effective and there is a need to develop more efficient approaches to help smokers to stop. There exists a substantial genetic variability in smoking behavior and the likelihood of cessation - tailoring treatment according to an individuals genetic profile is now technologically feasible and could lead to more successful cessation attempts. Here we review studies of the genetic effects on smoking cessation in randomized controlled trials of pharmacological therapy and discuss the potential value of a personalized approach to help people stop smoking.