Olfactory Perception and Physiology in Drosophila melanogaster

All animals are able to evaluate an odor by its consequences depending on prior experiences. This includes the possibility to show a resembling response to similar odors with the same consequence as well as a distinct response to similar odors with a different consequence. To accomplish these different tasks, the animal has to change its olfactory acuity, dependent on the relevance of olfactory stimuli and their consequences. Olfactory systems require mechanisms to combine and separate similar, yet slightly distinct patterns of neuronal activity. In the present study, it was investigated whether the olfactory acuity of the fruit fly Drosophila melanogaster could be altered depending on the situation. In particular, it was tried to decrease or increase the olfactory "acuity" by different conditioning paradigms. To this means, two structurally similar odorants were identified. It was confirmed on a behavioral and physiological level that this chemical similarity was also conveyed to the flies nervous system. A generalization of the two odorants after absolute training of one of them showed a perceptual similarity. Activity measurements with calcium imaging in the antennal lobe and the mushroom body confirmed the similarity on a physiological level. In the antennal lobe, the glomerular activation pattern of the similar odorants was largely overlapping, whereas a dissimilar control odor showed a distinct pattern. The similarity in the mushroom body was proven by a stronger correlation between the similar odorants when compared to the correlations with the dissimilar control. An enhancement of the olfactory acuity of the fly could be shown after differential training. After training the flies to discriminate between the two similar odorants by using one as a CS+ and the other as a CS- during differential conditioning, the similarity of the odorants was decreased. This was measured on a behavioral level by a choice test between the two similar odors. Whereas absolute training showed a weak avoidance response of the punished odorant, the avoidance was substantially increased after differential training. It could be shown that this increased avoidance can be ascribed to an effect of conditioned inhibition. A closer look in the antennal lobe network, in particular the type I inhibitory local interneurons, could show the necessity of a functional inhibition in the antennal lobe during the olfactory discrimination. Without functional synaptic transmission of type I inhibitory local interneurons, a discrimination of the similar odorants after differential training was impossible. Interestingly, generalization of the similar odorants and discrimination of two dissimilar odorants was not impaired. A neuronal correlate for the learned discrimination could be localized in the mushroom body horizontal lobes. Absolute training resulted in an even further increased similarity of the activation patterns evoked by two similar odorants in the beta prime-lobe. Differential training on the contrary led to a decreased similarity of the activation patterns in the shaft region of the gamma-lobe. It can be concluded that an enhancement of the olfactory acuity in Drosophila is possible when the animals are explicitly trained to discriminate between two similar odorants. In order to fulfill this task, local interneurons in the antennal lobe play a crucial role. Neuronal properties in the mushroom body are changing during the differential conditioning process so that the activation pattern of previously similar odorants become more dissimilar.