Neural mechanisms of operant learning in zebrafish larvae
Final Report Abstract
Animal behavior can be categorized into learned and innate behaviors. A learned behavior is a behavior an individual acquires through experience. This is contrasted by an innate behavior that all members of a species naturally exhibit, and which does not require any experience. Popular examples for innate behaviors are building nests in birds or the waggle dance in honeybees. These seemingly complex behaviors are not learned, but they can, however, change too with experience: bumble bees have an innate color preference but can overcome that preference when they learn another color. Similarly, innate behaviors can be influenced by the animal’s state; Drosophila only court vigorously when they have a food source nearby to sustain the progeny. A well-studied innate behavior is the optomotor response (OMR). It is common to all fish and insects. This behavior stabilizes their course during locomotion when they are involuntarily displaced. In zebrafish larvae, a stressful experience measured as cortical level can influence innate behavior following an inverted u-shape curve. Moreover, arousal can change the sensory threshold of zebrafish larvae: animals that were exposed to a water flow for a period of time were more likely to turn towards a slow-moving OMR stimulus than before. However, this change only persisted on the time scale of minutes. The larvae were not more responsive to other sensory stimuli than those related to flow. Here we show how experience, in our case stress paired with an OMR stimulus, can affect an innate behavior. We study stress in a free-swimming paradigm using sodium chloride as a stressor. To zebrafish, salt is an aversive stimulus that they avoid in a concentration dependent manner. We show that in larval zebrafish the sensitivity to moving stimuli is dependent on contrast and spatial frequency, a phenomenon known for many animals including humans. We can influence this sensitivity by a stressful experience caused by sodium chloride exposure. This experience can enhance the innate behavior optomotor response over several hours. More specifically, the experience modulates turning behavior in zebrafish larvae such that the animals show more correct turns and less incorrect turns. A stressful experience has a multitude of effects on our bodies. In a second set of experiments we studied the effect stress on olfaction. Here we look at another innate behavior, the aversion of the odor cadaverine. We show that a stress experience, induced by a shock, can induce a state-dependent modulation of olfaction in zebrafish larvae. Zebrafish larvae are translucent and genetically emendable which allows to image activity in the whole brain in a non-invasive matter. Using two-photon imaging, we started to investigate circuit mechanisms underlying the changes in innate behavior caused by experience.
