Abstract

There is still an unmet need for the discovery and development of neuroactive drugs to treat psychiatric illness. The list of compounds used for treatment today is long, yet the often times reported lack of efficacy and the broad spectrum of unpleasant side effects are concerning. With my work presented in this thesis I aim to explore new routs of drug discovery using the zebrafish model system as valuable tool to bridge the gap between bench and bedside. In the first and second study I exploit the transparency of zebrafish to test the utility of synthetic photochemical compounds to modify behavior in a targeted fashion. Small molecules that can be activated and inactivated by light are attractive targets for drug development since they offer spatiotemporal control and their dose can potentially be calibrated instantly and interactively during treatment. I explored remote optical control of neuronal activity and behavior and implemented a rapid behavioral assay that is designed to provide a readout of neurotropic effects in zebrafish larvae (Barber et al., 2016; Trads et al., 2017). In the third project I studied the grs357 zebrafish mutant as a tool for drug discovery. Dysregulation of the hypothalamic-pituitary-adrenal (HPA) axis is highly correlated with depression and is thought to contribute to the etiology and progression of the disease. The grs357zebrafish mutant has a missense mutation that abolishes glucocorticoid receptor transcriptional activity and results in a chronically elevated stress axis together with behavioral endophenotypes of depression. This mutant therefore provides an entry point for research into the pathogenesis of depression and for the development of potential drug treatments. To examine how known antidepressant compounds affect brain activity of larval zebrafish, I employed brain- wide, cellular-resolution light-sheet microscopy. With a graph-theoretical approach, I extracted network parameters of neuronal activity in both wildtype and grs357 mutant fish, following treatment with fluoxetine, ketamine and cycloserine. Consistent with broad expression of the glucocorticoid receptor throughout the brain, I showed that the mutant fish exhibit an altered correlational structure of resting- state brain activity. Antidepressants differentially affected particular metrics of functional connectivity. Intriguingly, in grs357 mutant fish, an increased ‘modularity’, which represents the degree of segregation of the network into highly clustered modules, with fewer functional connections among them, was restored by fluoxetine to wildtype levels. With this project I showed that light-sheet imaging of zebrafish brain activity combined with graph-theoretical analysis of functional connectivity provides a content-rich and scalable approach for studying the neural consequences of drug x genotype interactions (Burgstaller et al., 2019). In summary, the work presented here took advantage of cutting edge methods like photopharmacology and cellular-resolution light-sheet microscopy for drug discovery in the larval zebrafish. Importantly, apart from the biological findings, these methods can be applied to study various compounds and animal models of psychiatric disorders.