Abstract

For all animals it is highly advantageous to associate an environmental sensory stimulus with a reinforcing experience. During associative learning, the neural representation of the sensory stimulus (conditioned stimulus; CS) converges in time and location with that of the reinforcer (unconditioned stimulus; US). The CS is then affiliated with a predictive value, altering the animal’s response towards it in following exposures. In my PhD thesis I made use of olfactory aversive conditioning in Drosophila to ask where these two different stimuli are represented and how they are processed in the nervous system to allow association. In the first part of my thesis, I investigated the presentation of the odor stimulus (CS) and its underlying neuronal pathway. CS-US association is possible even when the US is presented after the physical sensory stimulus is gone ('trace conditioning'). I compared such association of temporally non-overlapping stimuli to learning of overlapping stimuli ('delay conditioning'). I found that flies associate an odor trace with electric shock reinforcement even when they were separated with a 15 s gap. Memories after trace and delay conditioning have striking similarities: both reached the same asymptotic learning level, although at different rates, and both memories have similar decay kinetics and highly correlated generalization profiles across odors. Altogether, these results point at a common odor percept which is probably kept in the nervous system throughout and following odor presentation. In search of the physiological correlate of the odor trace, we used in vivo calcium imaging to characterize the odor-evoked activity of the olfactory receptor neurons (ORNs) in the antennal lobe (in collaboration with Alja Luedke, Konstanz University). After the offset of odor presentation, ORNs showed odor-specific response patterns that lasted for a few seconds and were fundamentally different from the response patterns during odor stimulation. Weak correlation between the behavioral odor generalization profile in trace conditioning and the physiological odor similarity profiles in the antennal lobe suggest that the odor trace used for associative learning may be encoded downstream of the ORNs. In the second part of the thesis I investigated the presentation of different aversive stimuli (USs) and their underlying neuronal pathways. I established an odor-temperature conditioning assay, comparable to the commonly used odor-shock conditioning, and compared the neural pathways mediating both memory types. I described a specific sensory pathway for increased temperature as an aversive reinforcement: the thermal sensors AC neurons, expressing dTrpA1 receptors. Despite the separate sensory pathways for odor-temperature and odor-shock conditioning, both converge to one central pathway: the dopamine neurons, generally signaling reinforcement in the fly brain. Although a common population of dopamine neurons mediates both reinforcement types, the population mediating temperature reinforcement is smaller, and probably included within the population of dopamine neurons mediating shock reinforcement. I conclude that dopamine neurons integrate different noxious signals into a general aversive reinforcement pathway. Altogether, my results contribute to our understanding of aversive olfactory conditioning, demonstrating previously undescribed behavioral abilities of flies and their neuronal representations.