EFFECTS OF CHRONIC EXPOSURE TO SELENIUM ON SOCIAL BEHAVIOUR AND SOCIAL LEARNING IN ZEBRAFISH (Danio rerio)
Date
2021-05-31
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Journal ISSN
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ORCID
Type
Thesis
Degree Level
Doctoral
Abstract
Elevated levels of contaminants from human activities have become a major threat to animals, particularly within aquatic ecosystems. Selenium (Se) is a naturally occurring element with a narrow range of safe intake. Excessive selenium has toxicological effects, while too little causes nutritional deficits. The adverse neurobehavioural effects of Se have been investigated in both humans and fishes, but little is known about its effects on social behaviour and social learning or serotonin signalling in the brain. Considerable evidence shows that excessive exposure to Se causes toxic bioaccumulation in female fish, with Se transferring to the developing embryos leading to a diverse range of adverse health in offspring. Therefore, in this thesis, I investigated dietary exposure and transgenerational effects of chronic exposure to Se on different aspects of social behaviour including antipredator response, group preference, and social learning in zebrafish (Danio rerio), with a particular focus on alterations in the serotonergic pathway. In the first experiment of chapter one, I documented that exposure of adult zebrafish to the highest concentration (31.5 µg Se/g dry weight) of dietary selenomethionine (Se-Met), caused elevated levels of baseline fear behaviour, with fish swimming lower in the water column and in tighter shoals compared to fish in the other treatments. With this high level of baseline stress, these fish did not significantly intensify antipredator (fear) behaviours in response to exposure to chemical alarm cues. In group preference tests, when individual fish were given an opportunity to shoal with groups of different sizes, fish exposed to the highest Se-Met concentration spent significantly less time in groups. In the social learning test, I found that zebrafish exposed to the highest concentration of Se-Met (34.1 µg Se/g dry weight) displayed significantly lower escape responses compared to fish in control and lower exposure groups (3.6 and 12.8 µg Se/g dry weight). These impaired behaviours were associated with higher oxidative stress and dysregulation in mRNA expression of 5-HT receptors (htr1aa, htr1b, htr1d, htr2aa, htr2b, and htrcl1), 5-HT synthesis (tph2), its reuptake (slc6a4a) as well as monoamine oxidase (mao), an important enzyme in the degradation of 5-HT. In my final experiment, female zebrafish were treated with different concentrations of Se-Met and bred with untreated male zebrafish to produce the F1-generation required to investigate the transgenerational effects of dietary Se on social behaviour and social learning in zebrafish. Offspring were raised to adulthood (6-month-old) without Se exposure. Then, in a series of behavioural tests, I found offspring that were maternally exposed to high levels of Se showed behavioural signs of stress (although no physical impairment), had weaker group preferences, and also demonstrated impaired social learning. These neurobehavioural deficits appear to be linked to perturbations in the serotonergic system in the brain, as maternal exposure to high Se concentrations led to dysregulation of this neurotransmitter (e.g., altered transcription of 5-HT receptors). Overall, my study highlights that Se contamination impairs multiple social behaviours and social learning, and it has important trans-generational consequences even in the absence of direct Se exposure.
Description
Keywords
Selenium, Social behavior, Serotonin, antipredator response, Zebrafish, Oxidative stress
Citation
Degree
Doctor of Philosophy (Ph.D.)
Department
Biology
Program
Biology