Microbiota modulation of behavior and stress responses : implications for neuro-immune research in zebrafish
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The gut microbiota (GM) consists of a large microbial community whose collective set of genes encodes a vast array of functions. These microbes play a major role in many physiological processes within the host and are essential for survival. Ongoing research suggests that the GM is not only involved in gut physiology but may also have significant effects on brain function and behavior. The gut-brain axis is a bidirectional route of communication between the gastrointestinal tract and the central nervous system via neural, hormonal, and immunological pathways. The influence of the GM on gut-brain signaling is not well characterized. This dissertation research is aimed at investigating the role of the GM on stress-related brain function and behavior, as well as developing unique approaches for neuroimmune research. My first project was to investigate the influence the GM has on behavior in genetically identical mice. First, I isolated a subset of bacteria that colonize the ileum of mice from a particular vendor that has been shown to produce mice with very diverse GM. Isogenic colonies of mice were then generated with and without this subset of microbes and subjected to stress testing and behavioral analysis. From this study we were able to determine that the GM does play a significant role in stress-related brain function and behavior. However, dissecting out individual microbes and mechanisms by which the GM uses to communicate through the gut-brain axis proves to be a challenging task. In order to address some of these questions we began to explore more basic model systems, such as zebrafish. Zebrafish are an emerging high-throughput model system that exhibit many behaviors that have been correlated with those seen in human neurological disorders. The ability to easily control and manipulate the microbial environment in developing zebrafish makes neuroimmune research in zebrafish advantageous over other model systems. This research characterizes the ability of the GM to alter stress- and anxiety-related behavior and to mitigate stress responses in zebrafish. Furthermore, this research investigates potential pathways of communication to provide insight for mechanisms by which the GM influences the gut-brain axis. Since neurobehavioral research in zebrafish is an evolving field, my initial zebrafish project was to determine an optimal euthanasia agent for use in stress studies. This was crucial for eliminating euthanasia-induced stress responses which could confound results in our experiments. My next project was to generate gnotobiotic zebrafish in order to examine the significance of the GM in anxiety-related behavior and stress responses. Lastly, through bacterial metabolite experiments and probiotic studies, we began to investigate mechanisms of communication through the GM-gut-brain axis. This research sets the foundation for using zebrafish for neuroimmune research and will help elucidate the role that the GM plays in modulating brain function and behavior. A better understanding of the influence microbes have on the gut-brain axis will facilitate the potential for probiotic therapeutics targeting neurological disorders.