Manganese enhanced MRI reveals functional circuitry in response to odorant stimuli

To investigate the circuitry involved in detecting odorants in the rodent brain, we developed a method using manganese-enhanced MRI (MEMRI) to map the flow of neural information from the olfactory sensory neurons (OSNs) to the central layers of the olfactory bulb. Studies have shown that Mn2+ enters active neurons and is transported anterogradely to axon terminals where it can cross synapses to functionally trace neural networks. Thus, by delivering MnCl2 directly into the nasal cavity of mice and then exposing them to defined odorants, Mn2+ is preferentially taken up by activated OSNs. Using the time course of the MRI signal, we generated maps of Mn2+ accumulation in the olfactory bulb for both glomerular and mitral cell layers. Results demonstrated that overlapping yet distinct enhancement patterns were produced by exposure to either octanal, acetophenone, or carvone. Notably, areas of Mn2+ accumulation in the mitral cell layer were similar to those in the glomerular layer consistent with neural information that passes from specific OSNs to specific mitral cells. Finally, by correlating specific Mn2+ signal peaks to genetically labeled glomeruli that are known to be activated by the odorant octanal, we show that MEMRI maps can be resolved at the level of individual glomeruli.