Multimodal imaging of sensory inputs and neuronal coding in the rodent olfactory bulb

The glomeruli of the olfactory bulb form a sensory map of odor space. I explored this map by measuring the responses of glomeruli on the dorsal bulb to hundreds of odors, in both rat and mouse. Identified glomeruli could be matched across individuals and even across species by their odor response profile. By comparing the layout in different individuals I found that the map has high precision: the positions of glomeruli varied with a standard deviation of <1 glomerular spacing medio-lateral, and slightly more antero-posterior. Furthermore, mouse and rat share many glomeruli with apparently identical odor tuning, and they are arranged in a similar layout. I identified a coarse relationship that specifies the position of a glomerulus from its odor tuning with a precision of-5 glomerular spacings. However, no chemotopic order was apparent on a finer scale, and nearby glomeruli were almost as diverse in their odor tuning as distant ones. I conclude that glomeruli are positioned much more precisely than any recognizable chemotopic structure on the bulb, and that the local diversity of chemical sensitivities is a feature, not a flaw, of the olfactory map. In a second project, I investigated how the odor map develops functionally postnatally. The relative positions of functionally unique glomeruli were equally precise at all ages examined (1 week to adulthood). Fewer glomeruli were activated by odors in 1 to 3 week-old mice than in adults. Less than 40% of pup glomeruli were functionally similar to adult glomeruli. Finally, fewer pup glomeruli could be reliably identified across multiple individuals in contrast to the reliable identification in adults. Therefore, the functional odor map of inputs in the olfactory bulb undergoes significant postnatal refinement during the period of maternal care. In a third project I explored the functional architecture of individual glomeruli mice lacking the olfactory marker protein (OMP) via multiphoton microscopy. I found that many individual glomeruli contain functionally distinct microdomains in mice lacking OMP. These morpho-functional microdomains persist after postsynaptic activity is blocked, indicating that they arise from heterogeneity in input axons innervating single glomeruli rather than postsynaptic processing of odor input.