Topographic representation of odorant molecular features in the rat olfactory bulb.

Individual glomeruli in the mammalian olfactory bulb (OB) most probably represent a single odorant receptor (OR). The assembly of glomeruli thus forms the maps of ORs. How is the approximately 1,000 ORs represented spatially in the glomerular map? Using the method of optical imaging of intrinsic signals and systematic panels of stimulus odorants, we recorded odorant-induced glomerular activity from the dorsal and dorsolateral areas of the rat OB, and examined the molecular receptive range (MRR) of individual glomeruli. We then deduced the characteristic molecular features that were shared by odorants effective in activating individual glomeruli. Analysis of the spatial representation of the MRR showed that glomeruli with similar MRRs gathered in close proximity and formed molecular feature clusters and subclusters. Although the shape of the clusters varied among different OBs, the clusters were arranged at stereotypical positions in relation to the zonal organization of the OB. Examination of the spatial representation of the characteristic molecular features of odorants using structurally semirigid aromatic compounds suggest a systematic and gradual change in the characteristic molecular features according to the position of subclusters in the map. The topographic map of the characteristic molecular features may reflect a systematic spatial representation of the ORs and may participate in the neural bases for the odorant structure-odor quality relationship.