| dc.description.abstracteng | Drosophila senses diverse kinds of olfactory stimuli in the external environment. These stimuli are often not pure chemicals, but rather mixtures of many odorants. The fly olfactory system, however, needs to extract the requisite information from this complex stimulus. This is done by converting odor representations into meaningful, discrete categories in the perceptual space. The first olfactory relay system in the Drosophila brain is the antennal lobe (AL) which consists of olfactory sensory neurons (OSNs), projection neurons (PNs) and local interneurons (LNs), and these three populations of neurons interact and process the odor stimuli. To observe any transformations of odor representations in terms of odor classification, linearly varying odor stimuli were presented. This was achieved using binary mixtures such that one odor was "morphed" into another. Three binary mixtures were made from combinations of three distinct odorants ,i.e., 4-methylcyclohexanol (MCH), 3-Octanol (3-Oct) and Pentyl acetate (PA). In the first step, I determined and confirmed that each single odor-evoked distinct odor representations across its different concentrations in the AL, especially in the OSNs. A second requirement was to determine the degree of similarity between the three odors in both OSNs and PNs. From their odor representations, it was found that PA was more similar to 3-Oct compared to MCH in OSNs. Moreover, the similarity between PA and 3-Oct became even stronger in PNs. After determining the prerequisites of using PA, 3-Oct and MCH for binary mixtures, odor-evoked Ca2+ activity patterns for the three different types of binary mixtures were determined in both OSNs and PNs. Each binary odor mixture showed a different kind of odor classification. The MCH/3-Oct binary mixtures showed no odor classification in OSNs, but in PNs two distinct categories were observed. However, the same could not be found in either of the other two odor mixtures. For MCH/PA binary mixtures, no odor classification was found in OSNs. But in PNs, all mixtures having greater than 0% PA got categorized and distinctly separated from the 100% MCH-0% PA. This was referred to as odor dominance of PA. The last odor mixture was of the similar odor pair, 3-Oct/PA, in which all the intermediate mixtures formed one cluster in both OSNs and PNs. This was due to the high degree of similarity already present between the individual odors in OSNs and PNs. The glomeruli in the AL are interconnected by a population of GABAergic LNs and they are known from previous studies to be involved in odor processing. GABA binds to two kinds of receptors: GABAA and GABAB. GABAA receptors are known to be involved in the early phase of the odor processing and they can be pharmacologically blocked by picrotoxin. Therefore, fly brains were subjected to picrotoxin solution and its effects were observed on the three types of odor classification found with the combination of the selected odors. GABAergic inhibition mediated via GABAA receptors was found to be required for the odor classification of binary mixtures of MCH/3-Oct. However, no changes were found due to picrotoxin on MCH/PA and 3-Oct/PA binary mixtures. In conclusion, different kinds of odor classification could be observed for different odors in the AL of the fly. GABAergic inhibition has been found to play a role in mediating odor classification of different stimuli. However, not every odor classification requires GABAergic inhibition, as was observed in the case of MCH/PA binary mixtures, thereby also suggesting an existence of other possible mechanisms for odor classification. | de |