Olfactory coding strategies visualized by in vivo calcium-sensitive dye imaging

In the olfactory system, odors are represented by spatially-organized patterns of neural activity in the olfactory bulb, the first synaptic level of the olfactory pathway. We have visualized these patterns by loading mouse olfactory receptor neurons with calcium-sensitive dye and imaging odorant-evoked fluorescence signals from their axon terminals. This approach allows us to characterize how odors are represented at the level of input to the olfactory bulb. Receptor neurons converge onto and terminate within spherical neuropile called glomeruli. Using two-photon imaging, we found that all neurons innervating a glomerulus show similar odorant response properties, confirming the idea that glomeruli act as functional units in representing odors. With wide-field imaging, we found that odorant representations tend to involve input to many, widely distributed glomeruli, and that these representations change dramatically with odorant concentration. We also found that input to glomeruli is temporally complex, with receptor neuron input showing both glomerulus- and odorant-specific kinetics. Thus, representations of odorant information in the olfactory bulb are spatially distributed, tend to involve large numbers of glomeruli, and show complex but stereotyped temporal dynamics. We hypothesize that all of these parameters are important in coding information about both odorant identity and concentration.