Odorant molecular length: One aspect of the olfactory code

Organic acid odorants of differing carbon number produce systematically dif ferent spatial patterns of [C-14]2-deoxyglucose uptake in the glomerular la yer of the olfactory bulb. Because increasing carbon number correlates with progressive increases in several molecular features, including hydrophobic ity, length, and volume, we determined which of these properties was most a ssociated with systematic changes in the location of an anterior, dorsomedi al module responding to fatty acids. We exposed groups of rats to two serie s of organic acids that each had the same number of carbons, but differed i n their hydrocarbon structures. These straight-chained, branched, cyclic, a nd double-bonded molecules differed independently in hydrophobicity, length , and volume. The only molecular property that was strongly correlated with the location of the module was molecular length, suggesting that this mole cular feature is the principal determinant of the chemotopic organization o f glomeruli within the module. We also found that distinct hydrocarbon stru ctures produced large differences in spatial patterns of 2-deoxyglucose upt ake in posterior parts of the bulb. Even subtly distinct structural isomers evoked posterior responses that differed greatly. The odorant 2-methylbuty ric acid evoked much greater uptake in the posterior bulb than did its stru ctural isomer 3-methylbutyric acid (isovaleric acid). These data suggest th at posterior portions of the bulb may encode specific steric features of od orant molecules and that some odorant features may have an inherent or acqu ired greater representation than do others. (C) 2000 Wiley-Liss, Inc.