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Bywalez, Wolfgang (2016): Physiology of rodent olfactory bulb interneurons. Dissertation, LMU München: Graduate School of Systemic Neurosciences (GSN)



The sense of olfaction is a central gateway of perceiving and evaluating an animal’s environment filled with volatile chemicals. It affects individual and social behavior in an evaluative way, i.e. by helping to find food sources, warning from dangers like toxins or predators or influencing mating choice. Already the first central station for vertebrate olfactory processing, the olfactory bulb (OB), is astonishingly complex. Its structure features several horizontal layers of signal transformation that includes a large variety of local interneurons (INs). Most of these cells are subject to adult neurogenesis, which rejuvenates and remodels the circuitry throughout life. One of those interneuron subtypes, the granule cell (GC), poses the most numerous cell type of the olfactory bulb. As the major synaptic connection of the bulb, linking different glomerular units, it participates in numerous reported tasks like odor discrimination or memory formation. Many of those capacities are attributable to the function of peculiar spines with long necks and enormous bulbar heads called gemmules. They accommodate pre- and postsynaptic specializations of the reciprocal synapse with mitral cells (MCs) that are topographically and functionally linked and feature many modes of signal integration and transmission. As of yet, the mechanistic underpinnings of activation and neurotransmitter release are not yet resolved in great detail. This gave rise to the first project of this thesis, which focusses on the detailed granule cell gemmule physiology during local glutamatergic activation. With the help of two-photon glutamate uncaging and concomitant calcium imaging, the spine could be selectively stimulated and its physiological dynamics tested. By the use of different pharmacological agents, we could verify the importance of voltage gated sodium channels (Nav) for local signal amplification and the involvement of NMDA and high voltage activated calcium channels (HVACCs) in the calcium elevation during local stimulation, which is important for γ-aminobutyric acid (GABA) release from the spine. The superthreshold depolarizing signal and strong calcium elevation during local input are exclusively restricted to the spine, which affirms the chemical and electrical isolation of gemmules from the rest of the cell. In this study we thereby confirmed the theoretical prediction of active computation within single spines in our system, emphasizing the functional importance of morphological compartmentalization for the cell’s physiology. The second largest population of interneurons in the olfactory bulb is located in the glomerular layer (GL) of the olfactory bulb and subsumes a plethora of different cell types, categorized in terms of molecular characteristics (mostly neurotransmitter), morphology and function. Among those, dopaminergic (DAergic) juxtaglomerular cells (JGCs) form a subpopulation, which the second part of this thesis is focused on. Innervated by the first or second synapse in the olfactory pathway, these cells exert strong influence in very early stages of olfactory signaling. The gating and transformation of inputs locally and very importantly also laterally over large distances originate from several factors. This cell grouping usually expresses two neurotransmitters at the same time, GABA and dopamine (DA), and encompass many different morphologies and synaptic arrangements with other cell types. Utilizing dopamine transporter (DAT) based staining methods in three animal populations differing in age and species, this study revealed a larger diversity of dopaminergic cell types in the glomerular layer. New ‘uniglomerular’ and a ‘clasping’ cell types were discriminated, showing distinct dendritic formations and glomerulus innervations, which was assessed with a new morphometric tool kit. The clasping cell type features dendritic specializations, densely clasping around single cell bodies. These morphological traits occur in higher abundance and complexity specifically among adult animals and could be structures of neurotransmitter output since they show strong calcium influx upon soma depolarization. Comparisons of the three animal populations showed age- and/or species-dependent changes in the subtype composition of dopaminergic JGCs. Concordant with recent research, the inclusion of age-dependent comparisons in bulbar studies turned out to be of great significance.