Odorant molecular length: One aspect of the olfactory code

Abstract

Organic acid odorants of differing carbon number produce systematically different spatial patterns of [(14)C]2-deoxyglucose uptake in the glomerular layer of the olfactory bulb. Because increasing carbon number correlates with progressive increases in several molecular features, including hydrophobicity, length, and volume, we determined which of these properties was most associated with systematic changes in the location of an anterior, dorsomedial module responding to fatty acids. We exposed groups of rats to two series of organic acids that each had the same number of carbons, but differed in their hydrocarbon structures. These straight-chained, branched, cyclic, and 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 molecular feature is the principal determinant of the chemotopic organization of glomeruli within the module. We also found that distinct hydrocarbon structures produced large differences in spatial patterns of 2-deoxyglucose uptake in posterior parts of the bulb. Even subtly distinct structural isomers evoked posterior responses that differed greatly. The odorant 2-methylbutyric acid evoked much greater uptake in the posterior bulb than did its structural isomer 3-methylbutyric acid (isovaleric acid). These data suggest that posterior portions of the bulb may encode specific steric features of odorant molecules and that some odorant features may have an inherent or acquired greater representation than do others.

Long hydrocarbon chains serve as unique molecular features recognized by ventral glomeruli of the rat olfactory bulb

“…1) that had not been stimulated by any of the hundreds of odorants examined in our previous studies (Johnson et al, 1998,2005aJohnson and Leon, 2000a,b). Consistent with prior results, increasingly ventral glomeruli tended to respond to longer odorous molecules (Johnson et al, 1999(Johnson et al, ,2004Johnson and Leon, 2000b). Alkanes of greater length than were used in this homologous series probably would not be sufficiently volatile for receptor activation, which may explain why pentadecane stimulated the most ventral region of the bulb.…”

“…According to the reliable chemotopic organization previously observed (Johnson and Leon, 2000b;Johnson et al, 1999Johnson et al, , 2002Johnson et al, , 2004Johnson et al, , 2005a, and found here for smaller alkanes, the more dorsal glomerular activation by the longest alkanes in the series was surprising. Given the low vapor concentrations of these large odorants, we considered the possibility that the dorsal responses were due to an odorous contaminant in pentadecane and hexadecane.…”

“…From studies of numerous homologous series of odorants up to eleven carbons in the chain, it was clear that larger odorants systematically evoked responses that were progressively ventral (Johnson et al, 1999(Johnson et al, , 2004Johnson and Leon, 2000b). We therefore hypothesize that the previously unresponsive ventral area might respond to even larger odorants than we had yet studied (Johnson et al, 2002).…”

“…Even though responses to this homologous series had not been examined systematically before the current study, based on our previous observations (Johnson et al, 1999(Johnson et al, , 2002(Johnson et al, , 2004Johnson and Leon, 2000b), we predicted that these odorants would produce chemotopically organized responses, with increasingly ventral glomeruli being activated by increasing carbon chain length. The molecular length of the longer alkanes was greater than the odorants that we previously had studied, and we therefore expected that these alkanes would reveal a novel activation in the anterior to mid-ventral glomerular layer where other odorants had not been observed to evoke a response.…”

The fundamental role of memory in olfactory perception

“…Some aspects of synthetic coding could begin at the receptor itself, through ligand-ligand interactions with the receptor protein [29]. It is still unclear what constitutes an odorant feature [30,31], although one can assume that features correspond with ideal physiochemical ligands for receptor proteins. Thus, features could vary in their complexity from specific carbon chain lengths and functional groups to, perhaps, specific combinations of these more simple components.…”