Brain aminergic systems and autism-associated risk factors in zebrafish

Brain amines are neurotransmitters that modulate important functions in the central nervous system, including behavior and brain development. Several brain disorders are characterized by impairments in aminergic systems, including autism spectrum disorder (ASD). ASD is characterized by impaired social behavior, difficulty in communication and stereotypies. The etiology of ASD is poorly understood, but both environmental and genetic risk factors are known to play a role on it. Valproic acid (VPA), a drug commonly used to treat bipolar disorder, and mutations in the monoamine oxidase a gene are both environmental and genetic risk factors. Zebrafish shares relevant neurochemical aspects with humans and display a wide range of behaviors, which contribute to its appreciation as a model organism in neuroscience. Thus, this study characterized the phenotypes of zebrafish pharmacologically and genetically challenged with ASD risk factors. Additionally, functional aspects of aminergic neurotransmission in the zebrafish brain were studied through the characterization of animals that lacked vesicular monoamine transporter 2 (vmat2, also known as slc18a2) and monoamine oxidase (mao), relevant genes for aminergic vesicular transport and metabolism, respectively. In the first publication of this thesis, larval zebrafish embryonically exposed to VPA showed a reduction in the number of histaminergic neurons and in the levels of histamine when compared to control animals. The histaminergic system of VPA-exposed larvae was also affected by a downregulation of histidine decarboxylase and histamine receptors. Some of these abnormalities persisted until adulthood along with impaired social behavior. This study brings more attention to a possible involvement of the histaminergic system in the outcomes related to ASD. In the second article, the levels of dopamine, noradrenaline, serotonin and histamine of vmat2 mutants were decreased, whereas levels of dopamine and serotonin metabolites were increased, indicating elevated amine turnover. There were also fewer aminergic immunoreactive cells. Further, in mutants notch1a and pax2a were downregulated in brain proliferative zones. This mutant line may be used in the investigation of how amines transport affects brain development and function, and for use in high-throughput and drug screening. In the third article, mao−/− larvae showed a hyperserotonergic phenotype characterized by extracellular serotonin immunoreactivity that was associated with damage in aminergic systems. They also showed weaker responses to visual and acoustic stimuli, abnormal expression of developmental markers and died within 20 days post-fertilization. mao+/- fish were viable and demonstrated impaired social interactions compared with adult mao+/+ siblings. These mutants could be used in investigations aiming to assess the roles of MAOA/B and amines during brain development and to study the behavioral outcomes associated with MAOA/B deficiency. Collectively, the results of the present thesis support zebrafish as a tool to investigate mechanisms underlying ASD. Additionally, it presented two new models to study important aspects of aminergic neurotransmission in zebrafish and its role in brain function and behavior.