Parallel pathways of the moth olfactory system and their morphological and physiological characteristics
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- Institutt for psykologi 
Studying insect olfaction is interesting not only because it contributes to our general knowledge about fundamental chemosensory principles, but also because it is fascinating by itself. The opportunity of gaining insight into organization and functioning of a tiny insect brain has provided, in addition to detailed knowledge about olfactory pathways, a huge respect for these small creatures having existed on Earth more than 400 million years. Among other insect species, heliothine moths belong to a group of attractive model objects used for exploring general and specific mechanisms underlying olfactory information coding. In this study, the second-order projection neurons carrying odor information from the primary processing center, the antennal lobe, to higher brain regions are brought into focus. Previous data have reported about three main parallel tracts (medial, medio-lateral, and lateral) formed by these antennal-lobe output neurons. Since, functional characterization of any neural system is based on connection patterns of its basic neural elements, a constant refinement of the system’s anatomical organization is needed. In this study, combination of retrograde and anterograde mass staining techniques allowed detailed mapping of the parallel antennal-lobe tracts including the discovery of a new tract as well as a more precise description of the projection patterns characterizing the two most prominent tracts, the lateral and the medial. By utilizing the intracellular recording and staining technique we managed to further explore the parallel antennal lobe tracts by identifying individual projection neurons within each of totally five tracts. Reconstruction of single neurons and subsequent registration of their models into a standard brain atlas made it possible to compare the output terminals of neurons both within and across the tracts. A main conclusion which can be drawn from this comparison is the likeliness of cooperative system including the medial and mediolateral tract neurons due to their high degree of overlap in the lateral horn. The lateral tract neurons, on the other hand, terminated in several distinct regions, including an area of the lateral horn differing considerably from that of overlapping projections from the two other tracts. The fact that the direct connection between the antennal lobe and the calyces is ensured mainly by the medial tract in heliothine moths, as discovered in this study, enabled retrograde labeling the uniglomerular medial-tract neurons via dye application into the calyces. By performing calcium imaging of odor-evoked responses in the above mentioned neuron population during antennal stimulation with plant odors and pheromones, we observed a kind of Hebbian-like plasticity in the antennal lobe established by temporally pairing the two categories of odor stimuli. The plastic changes appeared as remained suppressed responses to a single plant odor after paired pheromone-plant odor stimulation. The putative role of an arrangement capable of establishing ‘memory traces’ already at the level of the antennal lobe is being discussed.