Spatial representation of hydrocarbon odorants in the ventrolateral zones of the rat olfactory bulb.

The glomerular sheet of the olfactory bulb (OB) forms odorant receptor maps that are parceled into zones. We previously reported the molecular receptive range (MRR) property of individual glomeruli in the dorsal zone (zone 1) of the OB and showed that polar functional groups play a major role in activating glomeruli in this zone. However, the MRR property of glomeruli in zones 2-4 is not well understood yet. Using the method of intrinsic signal imaging, we recorded odorant-induced glomerular activity from the ventrolateral surface (zones 2-4) of rat OB. While hydrocarbon odorants that lack polar functional groups activate only a few glomeruli in zone 1, we found that a series of hydrocarbon odorants consistently activated many glomeruli in the ventrolateral surface. The hydrocarbon-responsive glomeruli were grouped into two clusters; glomeruli in one cluster (cluster H) responded to benzene-family hydrocarbons but not to cyclic terpene hydrocarbons. Glomeruli in the other cluster (cluster I) responded to both classes of hydrocarbons. Detailed analyses of MRR properties of individual glomeruli using hydrocarbon odorants and polar-functional-group-containing odorants showed that the common and characteristic molecular features effective in activating glomeruli in the clusters H and I are the hydrocarbon skeleton. These results suggest that ORs represented by glomeruli in these clusters recognize primarily the hydrocarbon skeleton of odorants, and thus imply a systematic difference in the manner of recognizing odorant molecular features between ORs in zone 1 and ORs in zones 2-4.