Wiring and information processing in the olfactory bulb of larval Xenopus laevis
by Thomas Offner
Date of Examination:2019-12-18
Date of issue:2020-02-04
Advisor:Prof. Dr. Ivan Manzini
Referee:Prof. Dr. Ivan Manzini
Referee:Prof. Dr. Thomas Dresbach
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Abstract
English
Olfactory systems of vertebrates have adapted to the various challenges of their environments on both the level of odorant receptors and neuronal wiring. Amphibians like the African clawed frog Xenopus laevis offer a powerful model to investigate how different neuronal wiring and odor information processing strategies emerged among vertebrates during water-to-land transition. In the olfactory bulb the sensory information from the peripheral olfactory organs is conveyed to mitral/tufted cells and their associated neuronal network in anatomical and functional units called olfactory glomeruli. My work aimed to answer the question, how olfactory sensory information is organized on the glomerular level and relayed to the mitral/tufted cells in the lateral olfactory bulb of larval Xenopus laevis. I could reveal the full extent of the olfactory receptor neuron population that innervates the lateral glomerular cluster and provide evidence for an anatomical and possibly functional finer subdivision of the lateral olfactory subsystem. I characterized sulfated steroids as suitable stimuli for mitral/tufted cells of the accessory and the lateral main olfactory bulb. The glomerular odor representation of the postsynaptic network to single amino acid stimuli appears to be very variable and non-stereotypical between animals. I found several similarities of the larval Xenopus lateral glomerular cluster to the rodent accessory olfactory system. Among those were the lack of chemotopy and the independence of tuning and spatial clustering between glomeruli. I characterized the population of mitral/tufted cells associated with the lateral glomerular cluster and found major differences in morphology and glomerular connectivity between them. The equal ratio between uni- and multi-glomerular MTCs imply the co-existence of several wiring logics in the lateral olfactory bulb. Neuronal beta tubulin II expression defines a subset of mitral/tufted cells of unknown maturation status, that was in parts responsive to amino acids. Overall, my work suggests, that the lateral olfactory subsystem incorporates several different wiring logics and resembles a rodent ‘accessory olfactory system-like’ subsystem that is still undergoing developmental processes. My work and further work from our laboratory might hint to the existence of a yet undescribed ‘threaded labeled line’ wiring logic in the olfactory system of amphibians.The actual connectivity and purpose of these potential wiring and odor information processing strategies remains to be determined.
Keywords: olfaction; olfactory bulb; glomeruli; Xenopus; mitral/tufted cells; odor map