Processing of different sensory qualities in the olfactory bulb of Xenopus laevis studied by advanced line illumination microscopy

Abstract

The olfactory system is renowned for its ability to sample and process a tremendously large odorant space. Lately, it has been shown that other sensory qualities such as temperature and pressure are detected in the olfactory system. However, little is known about how they are processed within the olfactory bulb and how this multisensory information is integrated in the context of olfaction. In this thesis, two specific olfactory glomeruli — the mechanosensitive beta- and the thermosensitive gamma -glomerulus — in the olfactory bulb of Xenopus laevis tadpoles were analyzed by means of a custom-built line illumination microscope. The line illumination technique was combined with subtraction imaging and pixel reassignment to perform fast calcium recordings in three dimensions. These recordings, covering first- and second-order neurons in the olfactory bulb, revealed a strong interconnection of both sensitivity to temperature drops as well as mechanosensitivity with olfaction. Dual-sensitive mitral/tufted cells responding to amino acids as well as to temperature drops were located in the proximity of the -glomerulus. A dose-response curve for the mechanosensitivity of the beta-glomerulus was measured. Additionally, the same glomerulus showed sensitivity to a surprisingly broad spectrum of single amino acids, thereby combining mechano- and chemosensitivity in the same structure. The axon separation of different olfactory receptor neurons within a single glomerulus was successful using double staining electroporation. Recordings obtained from such stainings suggest that the sensitivity of the beta-glomerulus to amino acids was caused by a single broadly tuned olfactory receptor. Lastly, a sufficient resolution for single axon as well as for single dendrite recordings within the beta-glomerulus was achieved by line illumination microscopy. The demonstrated increase in efficiency of the line scanning technique enables future experiments with two calcium indicators to analyze the signal transmission within a single glomerulus