Universität zu Köln

Berger, Michael (2023). Factors Influencing Reinforcer Properties in Olfactory Associative Learning and Memory in Drosophila melanogaster Larvae and Adults. PhD thesis, Universität zu Köln.

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Abstract

Drosophila larvae are attracted to ethanol. It provides them with a variety of benefits, such as protection from predatory wasps, nutrition, and a positive teaching signal. However, high ethanol concentrations are detrimental, resulting in death, decreased pupation rate and deformations. This thesis addresses the question, whether ethanol can be used as a reinforcer in olfactory conditioning in larvae. Indeed, the results show that larvae are attracted to the ethanol smell in a concentration dependent manner and that larvae prefer low ethanol doses mixed in agarose as a crawling surface. Ethanol affects attraction to other odorants in a concentration dependent manner. Furthermore, fructose in the agarose plate blocks attraction to ethanol and some key odorants for fermentation processes. The results suggest that larvae might be indifferent to low doses of ethanol because the behaviour after conditioning was comparable to conditioning without reinforcement. Taken together, the context of the ethanol encounter significantly affects whether a larva perceives ethanol as rewarding. Indeed, ethanol might primarily function as an olfactory guide to a carbohydrate food source and otherwise be not attractive to the animal. Drosophila melanogaster TβhnM18 mutant is accepted to have a defect in the formation of different types of memory and to show changes in their metabolic traits. This thesis addresses the question, whether TβhnM18 flies are indeed defective for appetitive STM formation and how appetitive behaviour is regulated by the internal state. Indeed, TβhnM18 flies show appetitive STM under 40 h starvation or with yeast reinforcer. Under 16 h starvation TβhnM18 flies form only LTM after STM conditioning. In w1118, 40 h starvation shifts appetitive STM into LTM and into ARM in TβhnM18 flies. Glycogen storage depletion and blocking of InR signalling onto OA neurons both induce appetitive STM in TβhnM18 flies. Additionally, the function of OA as a switch in learning and memory formation was uncovered, where OA pulses at certain time points result in blocking of LTM and STM, while blocking OA pulses shuttled STM to LTM after STM conditioning. Taken together, this thesis provides insight into the regulation of learning and memory through OA signalling and the role of the internal state in appetitive behaviour regulation.