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Increasing evidence suggests that glia cells are important players in different brain functions. Aberrant glia-neuron communication can lead to a variety of brain diseases, including epilepsy. Hence, glia cells could be a potential therapeutic target increasing the responsiveness of patients that cannot be helped today. Unfortunately, there is little mechanistic insight into the communication between glia and neurons. Therefore, this thesis aimed to provide more understanding in the glia-to-neuron connectivity focusing on the telencephalon of the juvenile zebrafish brain. First, it was observed that glia cells cover the whole telencephalon using confocal imaging. In addition, two distinct morphological subtypes were found where one has the typical radial glia morphology and the other subtype has similarities with the mammalian astrocyte. Next, by optogenetically depolarizing a small group of glia cells, a significant and transient excitation of nearby neurons could be induced. Moreover, when blocking α-amino-3-hydroxy-5-methyl-4- isoxazole-propionate (AMPA) and N-methyl-D-aspartate (NMDA) receptors, the significant excitation was eliminated in neurons that were excited with a delay and was still present in neurons that were excited without a delay. This indicates that there are different mechanisms, where the slow responsive neurons are suggested to communicate with glia cells through multiple connections and the fast responsive neurons might communicate with glia cells via gap junctions. Lastly, in order to test if and how the connectivity between glia and neurons alters during epileptic seizures, a pentylenetetrazole-induced seizure model was used. However, this did not reveal significant changes of the connectivity during seizures. Taken together, this data provides more evidence for glia being important players in brain functioning. Nevertheless, more research needs to be done to fully characterize the interactions between glia and neurons in physiological and pathological conditions.

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Fragile X syndrome patients present neuronal alterations that lead to severe intellectual disability, but the underlying neuronal circuit mechanisms are poorly understood. An exciting hypothesis postulates that reduced GABAergic inhibition of excitatory neurons is a key component in the pathophysiology of fragile X syndrome. Here, I directly test this idea. First, I show that a Drosophila melanogaster model of fragile X syndrome exhibits strongly impaired olfactory behaviors. In line with this, olfactory representations are less odor-specific due to broader response tuning of excitatory projection neurons. I find that impaired inhibitory interactions underlie reduced specificity in olfactory representations. Finally, I show that defective lateral inhibition across projection neurons is caused by weaker inhibition from GABAergic interneurons. I provide direct evidence that deficient inhibition impairs sensory computations and behavior in an in vivo model of fragile X syndrome. Together with evidence of impaired inhibition in autism and Rett syndrome, these findings suggest a potentially general mechanism for intellectual disability.

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How does our brain work? Understanding how the sensory information is being processed in the brain and generates behaviors is one of the greatest challenges of science in our century.Activity of single neurons have been investigated in great detail. However, neurons do not work in isolation. They are organized in circuits that process key information to perform higher brain functions. In order to understand these brain circuits the use of functional imaging has been established as an excellent tool for monitoring network activity with high spatial and temporal resolution. Therefore, during the first part of my thesis, I developed a framework of algorithms to extract relevant information from functional imaging in order to understand the activity of thousands of neurons. The second aim of this thesis is to study neuronal coding, how neurons process sensory information and react to external stimuli. As means to achieve that aim, I selected to study the gustatory system, a well-defined and simple sensory system which coding strate gies are still a matter of debate.In order to study the neural mechanisms inherent to gustatory information processing, I focused on the brainstem, the first relay of taste processing in the central nervous system. Brainstem circuits are a very attractive system for studying the basic computations underlying simple reflex motor outputs. Moreover, in the particular organization of the zebrafish, this circuits are rather accessible.My results showed that taste categories (sweet, sour , bitter, salt and umami) are represented by dissimilar brainstem responses and generate different behaviors. I also showed that the intensity perception of different categories is encoded by different principles.On the other hand, the food we consume is a combination of different tastants from different categories. Then, how does the brainstem cope with such mixtures? I observed that taste mixtures generates non linearities in the activity of the brainstem circuits that may suggest the presence of complex coding mechanisms such as dynamic gain modulation and neural attractors. These mechanisms might be used for a better food detection or more efficient rejection of inedible substances.For the first time, to the best of my knowledge, it has been shown that these kind of higher level computations can occur in this primitive but evolutionary conserved taste processing center in the brainstem.

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Epilepsy is a group of disorders that affect over 1% of the worldwide population and that are characterized by the occurrence of seizures; transient periods of abnormally synchronized and hyperactive neuronal firing. Although being among the most common diseases of the central nervous system, epileptic disorders remain poorly understood. To create a better understanding of the mechanisms underlying seizures, zebrafish models are of great importance since they provide easy brain access and allow high-throughput studies. However, an improved characterization of epileptic zebrafish models is needed. Therefore, I aimed to define three pharmacological models to study acute generalized seizure occurrence in 5 days post fertilization (dpf) zebrafish larvae. I analysed pentylenetetrazol (PTZ), picrotoxin and pilocarpine-induced epileptic seizures based on brain activity measured by calcium levels. Calcium fluctuations were observed by combining the fluorescent genetically encoded calcium indicator GCaMP6s with epifluorescence microscopy. PTZ, picrotoxin and pilocarpine were all found to be highly efficient proconvulsive agents, but all showed a high level of interindividual variability. However, in general, PTZ evoked generalized seizures with an earlier onset and both PTZ and pilocarpine evoked clearly distinguishable transitions from baseline to preictal activity to seizures invading the entire brain. Additionally, an increase in glial activity was observed prior to the seizures for all three proconvulsive agents. Subsequently, I aimed to characterize the importance of this increased glial preictal activity in PTZ treated animals. Therefore, I both inhibited and activated the purinergic P2Y1 receptor, known to be involved in mediating astrocytic hyperactivity. Here, P2Y1 activation appeared to decrease seizure occurrence, which might imply a protective role for glial cells in epilepsy.

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Autosomal Dominant Sleep-related Hypermotor Epilepsy (ADSHE) is a rare genetic disease that results in seizures related to sleep. Nicotinic acetylcholine receptor subunit alpha 4 (CHRNA4) is one of several genes that is related to this type of epilepsy. Molecular mechanisms underlying this disease still need to be unraveled. Mutations in the gene could lead to anatomical alterations of the brain, which could have an impact on the brain activity and eventually lead to behavioral changes. Unravelling all these steps could provide more understanding of the disease. For this, mutant and transgenic zebrafish models are used to investigate the functionality of the gene and characterization of the brain anatomy. Two different approaches were used to establish ADSHE models, a CRISPR/Cas9-mediated knock-in model and a transgenic model. Gal4 knock-in at the endogenous locus of Chrna4 would be used to investigate the expression pattern, and contribution of the gene to ADSHE by gene manipulation. Despite the different injection approaches, the specific knock-in of Gal4 at the endogenous locus of Chrna4 has not yet been established. On the other hand, the transgenic model expressing the human CHRNA4 containing a specific mutation found in several patients, is established in a mosaic manner. This model has to be raised and used for further research about ADSHE. At the same time, characterization of the brain anatomy of both Chrna4a and Chrna4b loss of function zebrafish, indicated absence of major changes at glutamatergic and ɣ-amino butyric acid (GABA)ergic neuron populations and acetylcholine (ACh)-producing cells at the age of 4 days post fertilization. These findings are based on comparison of confocal images of the mutant zebrafish brains with wild type fish. Behavior analysis of both the single and double mutant fish during normal circadian rhythm revealed no epileptic phenotype. Neither were stringent changes observed between the mutant and wild type fish. Altogether these findings suggest the idea that ADSHE is caused by a gain of function rather than a loss of function. Further research on the transgenic zebrafish model could provide more insight on the effect of the mutation to the functionality of Chrna4.