Synaptic plasticity underlying odor acuity learning and second-order conditioning in Drosophila
Aversive associative learning of harmful or noxious stimuli enables animals and humans to predict and avoid dangerous situations, and is, therefore, of fundamental relevance for survival. In order to accurately predict situations to be avoided, but at the same time not to transfer the response to similar, but harmless cues, the content of associative memories should be neither too specific nor too general. Therefore, classical associative learning has to ensure an appropriate balance between generalization and discrimination. This balance between generalization and discrimination itself is subject to learning: Through discriminative training animals and humans can learn to determine the exact composition of a relevant stimulus and, thereby, learn to differentiate it from similar, but not equally relevant cues. This phenomenon we refer to as acuity learning. It is characterized by a differential, bi-directional assignment of relevance to distinct parts of overlapping neuronal representations induced by similar stimuli. A related type of learning refers to second-order conditioning. Here, chains of associations between conditioned stimuli and novel, neutral stimuli are formed due to their temporal contiguity: A previously trained conditioned stimulus serves as an unconditioned stimulus in a subsequent training of second order. The neuronal circuitry and rules of synaptic plasticity underlying these very fundamental types of learning are not known. The Drosophila mushroom body provides a unique model system to analyze how these types of learning are put into effect. First, differential encoding of similar or dissimilar odor stimuli by Kenyon cell ensembles can be visualized. Second, synaptic activity and its change induced by associative learning in mushroom body-associated neurons can be monitored. Here it is proposed to use synaptically localized fluorescence sensors expressed in intrinsic and extrinsic mushroom body neurons (Kenyon cells, dopaminergic neurons and mushroom body output neurons), and to systematically analyze changes in synaptic calcium influx and transmitter release as a result of acuity learning and second-order conditioning. Furthermore, cell-specific, temperature-dependent block of transmitter release will be used to determine those synaptic connections that are required for the respective learning paradigms. The experiments will determine how acuity learning and associative learning of second-order are neuronally put into effect, and they are tightly embedded into the context of the research unit. In collaboration with project 4 (Prof. Nawrot) the results of these experiments will be implemented into an encompassing in-silico-model of the mushroom body functioning.