Synaptic plasticity and visual tuning properties in the zebrafish optic tectum

The optic tectum (OT) is the structure in charge of processing vision which is one of the primary senses used for survival in non-mammalian vertebrates. Synaptic plasticity is the ability that allows the brain to learn and forget all the time while, for instance, you are reading this abstract. This thesis aims to contribute with the understanding of the mechanisms of neural plasticity in visual short-term memory such as adaptation and habituation, and how novel visual cues are processed in the zebrafish CNS, before deciding to trigger or not a response. First, I reviewed different behavioural methods to assess memory formation in late-stage ZF larvae. These methods included the Novel object recognition (NOR) paradigm, with variants such as the visual lateralization NOR (VLNOR) tests. After excluding subjective bias using automatized analysis, I found that there is no statistically significant difference in the eye preferred to explore new objects. Thus, other parameters can be better used than those suggested in the literature, such as the distance kept from the novel object and exploration patterns. Then, I used traditional and fluorescent in situ hybridization using c-fos to visualize the brain areas that activate when a novel object is presented. I found activation in the olfactory bulb and superficial tectal regions in fish exposed to a novelty compared with controls. Additionally, these protocols were optimized to match the latest larval stage of zebrafish suitable for calcium imaging (21 days post-fertilization). Finally, I used calcium imaging in vivo to provide an initial characterization of how visual information is processed downstream of the visual system. We found that the processing mechanisms are multi-layered, and individual cells differ in degree and speed of adaptation and habituation to different stimuli. I demonstrate the diversity of tuning properties in the neuropil, broadly categorized into at least seven classes in relation to their selectivity towards different frequencies and contrasts: high-pass, band-pass, low-pass; high, medium, and low contrast sensitive, and contrast-suppressed cells.