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Screening for the neuronal substrate of the inhibitory interaction between two genetically different learning systems in Drosophila

Learning is an essential component for an animal’s survival as it enables for better future decisions: such as finding more nutritious food or avoiding harmful stimuli. While the molecular mechanisms underlying learning and memory have been studied extensively, only little is known about the neuronal architecture and the mechanisms for how the different learning systems interact in composite learning tasks. There is experimental evidence that at least two types of learning are mediated by distinct molecular mechanisms in humans, mice, songbirds, snails and in fruit flies, where one is involved in the effects of the animal’s own behavior on the world (self-learning) and a separate one for the relationship of events in the world (world-learning). A fly primarily uses the world-learning system as it explores its environment and learns about the stimuli associated with it. For this initial phase, the world-learning system inhibits self-learning ensuring a flexible memory, independent of the behavioral context within which it was acquired. This inhibition is mediated by the mushroom bodies (MB) in fruit flies: blocking the synaptic output of the mushroom bodies impairs their ability to inhibit self-learning, leading to premature habit formation. For the purpose of this study, we used a two-phase experimental design. The first phase consists of a composite learning task containing both self- and world-learning components, followed by a test phase that isolates the self-learning component. The MB is composed of 21 different types of output neurons (MBONs) which receive the neuronal activity from the MB for further processing. To map the MBON pathway through which the Mbs inhibit self-learning, we performed a comprehensive screen blocking each of the 21 different types of MBONs and testing them for a loss of the ability to inhibit self-learning. The results from this screen allow us to propose downstream MBON targets which must be associated with the neurons mediating operant self-learning.

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