Effects of sub-chronic methylphenidate on risk-taking and sociability in zebrafish (Danio rerio)

Methylphenidate exposure in juvenile period elicits locomotion changes and anxiolytic-like behavior in adulthood: Evidence using zebrafish as a translational model

Methylphenidate (MPH) is a central nervous system stimulant that is mainly used for Attention-Deficit/Hyperactivity Disorder (ADHD). It is well known that there is a high rate of ADHD misdiagnosis, leading to a great number of neurotypical children chronically exposed to MPH in early periods of life. This increase raises concern about possible long-lasting effects of this exposure. We aimed to evaluate whether exposure to MPH during childhood might impact adult behavioral pattern. For this purpose, we used zebrafish as a translational model considering its robustness as experimental model and fast life cycle. Fish were exposed during juvenile period (from 30 to 60 post-natal day) at MPH therapeutic concentration (2 mg L-1), and behavioral tests were performed at fish adulthood (120 post-natal day). MPH provoked slight anxiolytic-like effects and hyperlocomotion, and no differences on sociability and cortisol levels were observed. Moreover, sex did not affect any of the parameters evaluated. These results demonstrate that early chronic exposure to MPH leads to neurobehavioral adaptations that persist into adulthood in zebrafish regardless of sex, suggesting that the misuse of MPH during childhood and adolescence can alter neurobehavioral plasticity and these alterations might persist until adulthood.

A new mechanistic insight into the association between environmental perfluorooctane sulfonic acid (PFOS) exposure and attention deficit and hyperactivity disorder (ADHD)-like behavior

Perfluorooctane sulfonic acid (PFOS) is one of the main residual environmental pollutants that threaten human health. PFOS exposure is positively correlated with the prevalence of attention deficit hyperactivity disorder (ADHD); however, the underlying mechanism is unknown. Given that dopamine (DA) is a crucial target for PFOS and that its dysfunction is a key role in ADHD development, it is speculated that PFOS exposure contributes to the occurrence of ADHD to some extent by disrupting DA homeostasis. To establish the relationship between PFOS exposure, DA dysfunction, and ADHD-like behavior, adult zebrafish were exposed to PFOS for 21 days using PFOS concentrations in the serum of patients with ADHD as the reference exposure dose. Results showed that PFOS caused ADHD-like behaviors, with the presence of the slightly elevated percentage of time spent in movement and prolonged time spent in reaching the target zone in the T-maze. Hyperactivity and cognitive ability impairment were more severe with increasing PFOS concentrations. Further investigation showed that PFOS exposure resulted in a decrease in the DA content, accompanied by a decrease in the number of dopaminergic neurons and a disturbance in the transcription profiles of genes associated with the dopaminergic system. Treatment with Ritalin effectively alleviated PFOS-induced ADHD-like behavior and restored DA levels, number of dopaminergic neurons, and expression of DA metabolism-related genes, suggesting that PFOS exposure induced ADHD-like behavior by triggering DA secretion disorder. This study enriches our understanding of the pathogenic mechanisms underlying ADHD development and emphasizes the importance of focusing on the health risks pertaining to environmental exposure.

The translational genetics of ADHD and related phenotypes in model organisms

Attention-deficit/hyperactivity disorder (ADHD) is a highly prevalent neurodevelopmental disorder resulting from the interaction between genetic and environmental risk factors. It is well known that ADHD co-occurs frequently with other psychiatric disorders due, in part, to shared genetics factors. Although many studies have contributed to delineate the genetic landscape of psychiatric disorders, their specific molecular underpinnings are still not fully understood. The use of animal models can help us to understand the role of specific genes and environmental stimuli-induced epigenetic modifications in the pathogenesis of ADHD and its comorbidities. The aim of this review is to provide an overview on the functional work performed in rodents, zebrafish and fruit fly and highlight the generated insights into the biology of ADHD, with a special focus on genetics and epigenetics. We also describe the behavioral tests that are available to study ADHD-relevant phenotypes and comorbid traits in these models. Furthermore, we have searched for new models to study ADHD and its comorbidities, which can be useful to test potential pharmacological treatments.

Modelling ADHD-Like Phenotypes in Zebrafish

The use of multiple species to model complex human psychiatric disorders, such as ADHD, can give important insights into conserved evolutionary patterns underlying multidomain behaviors (e.g., locomotion, attention, and impulsivity). Here we discuss the advantages and challenges in modelling ADHD-like phenotypes in zebrafish (Danio rerio), a vertebrate species that has been widely used in neuroscience and behavior research. Moreover, multiple behavioral tasks can be used to model the core symptoms of ADHD and its comorbidities. We present a critical review of current ADHD studies in zebrafish, and how this species might be used to accelerate the discovery of new drug treatments for this disorder.

Sociability as a personality trait in animals: methods, causes and consequences

Within animal populations there is variation among individuals in their tendency to be social, where more sociable individuals associate more with other individuals. Consistent inter-individual variation in 'sociability' is considered one of the major axes of personality variation in animals along with aggressiveness, activity, exploration and boldness. Not only is variation in sociability important in terms of animal personalities, but it holds particular significance for, and can be informed by, two other topics of major interest: social networks and collective behaviour. Further, knowledge of what generates inter-individual variation in social behaviour also holds applied implications, such as understanding disorders of social behaviour in humans. In turn, research using non-human animals in the genetics, neuroscience and physiology of these disorders can inform our understanding of sociability. For the first time, this review brings together insights across these areas of research, across animal taxa from primates to invertebrates, and across studies from both the laboratory and field. We show there are mixed results in whether and how sociability correlates with other major behavioural traits. Whether and in what direction these correlations are observed may differ with individual traits such as sex and body condition, as well as ecological conditions. A large body of evidence provides the proximate mechanisms for why individuals vary in their social tendency. Evidence exists for the importance of genes and their expression, chemical messengers, social interactions and the environment in determining an individual's social tendency, although the specifics vary with species and other variables such as age, and interactions amongst these proximate factors. Less well understood is how evolution can maintain consistent variation in social tendencies within populations. Shifts in the benefits and costs of social tendencies over time, as well as the social niche hypothesis, are currently the best supported theories for how variation in sociability can evolve and be maintained in populations. Increased exposure to infectious diseases is the best documented cost of a greater social tendency, and benefits include greater access to socially transmitted information. We also highlight that direct evidence for more sociable individuals being safer from predators is lacking. Variation in sociability is likely to have broad ecological consequences, but beyond its importance in the spread of infectious diseases, direct evidence is limited to a few examples related to dispersal and invasive species biology. Overall, our knowledge of inter-individual variation in sociability is highly skewed towards the proximate mechanisms. Our review also demonstrates, however, that considering research from social networks and collective behaviour greatly enriches our understanding of sociability, highlighting the need for greater integration of these approaches into future animal personality research to address the imbalance in our understanding of sociability as a personality trait.

CHRONIC EXPOSURE TO METHYLPHENIDATE-CONTAMINATED WATER ELICITS SOCIAL IMPAIRMENT TO ZEBRAFISH

Residual contamination of water with MPH represents a severe environmental issue because it can affect non-target animals. Here we describe the behavioral effects in zebrafish after chronic contamination of water containing residues of MPH (0.1875, 1.875 and 3 ug/L). These doses are environmentally relevant since they reflect those found in wastewaters. We evaluated the behavioral effect through the novel tank test (NTT) and social preference test (SPT), and after euthanasia we analyzed oxidative stress parameters. Zebrafish exposed to MPH presented a social impairment, avoiding the conspecifics segment in the social preference test. In addition, MPH in the lowest concentration provoked an anxiolytic effect in the novel tank test. Oxidative stress is not related to these changes. Since the maintenance of an intact behavioral repertoire is crucial for species survival and fitness, our results demonstrate that residual contamination of water by MPH can be a threat to zebrafish, impacting directly to its well-being and survival in the aquatic environment.