In past studies in which we mapped 2-deoxyglucose uptake evoked by systematically different odorant chemicals across the entire rat olfactory bulb, glomerular responses could be related to each odorant's particular oxygen-containing functional group. In the present study we tested whether aliphatic odorants containing two such functional groups (esters, ketones, acids, alcohols, and ethers) would stimulate the combination of glomerular regions that are associated with each of the functional groups separately, or whether they would evoke unique responses in different regions of the bulb. We found that these very highly water-soluble molecules rarely evoked activity in the regions responding to the individual functional groups; instead, they activated posterior glomeruli located about halfway between the dorsal and ventral extremes in both the lateral and the medial aspects of the bulb. Additional highly water-soluble odorants, including very small molecules with single oxygenic groups, also strongly stimulated these posterior regions, resulting in a statistically significant correlation between posterior 2-deoxyglucose uptake and molecular properties associated with water solubility. By showing that highly water-soluble odorants stimulate a part of the bulb associated with peripheral and ventral regions of the epithelium, our results challenge a prevalent notion that such odorants would activate class I odorant receptors located in zone 1 of the olfactory epithelium, which projects to the dorsal aspect of the bulb.

Many naturally occurring volatile chemicals that are detected through the sense of smell contain unsaturated (double or triple) carbon-carbon bonds. These bonds can impact odors perceived by humans, yet in a prior study of unsaturated hydrocarbons we found only very minor effects of unsaturated bonds. In the present study, we tested the possibility that unsaturated bonds affect the recognition of oxygen-containing functional groups, because humans perceive odor differences between such molecules. We therefore compared spatial activity patterns across the entire glomerular layer of the rat olfactory bulb evoked by oxygen-containing odorants differing systematically in the presence, position, number, and stereochemistry of unsaturated bonds. We quantified activity patterns by mapping [ 14 C]2-deoxyglucose uptake into anatomically standardized data matrices, which we compared statistically. We found that the presence and number of unsaturated bonds consistently affected activity patterns, with the largest effect related to the presence of a triple bond. Effects of bond saturation included a loss of activity in glomeruli strongly activated by the corresponding saturated odorants, and/or the presence of activity in areas not stimulated by the corresponding saturated compounds. The position of double bonds also affected patterns of activity, but cis versus trans configuration had no measurable impact in all five sets of stereoisomers we studied. These results simultaneously indicate the importance of interactions between carbon-carbon bond types and functional groups in the neural coding of odorant chemical information and highlight the emerging concept that the rat olfactory system is more sensitive to certain types of chemical differences than others. Keywords 2-deoxyglucose; odors; imaging techniques; mappingWe have been describing systematic differences in the spatial activity patterns in the glomerular layer of the rat olfactory bulb that are evoked by odorants that differ systematically in chemical structure (e.g., carbon number, functional group, open versus closed-chain structures, enantiomers, and stereoisomerism involving different branching and functional group position; Johnson et al., 1998Johnson et al., , 1999Johnson et al., , 2002Johnson et al., , 2004Johnson et al., , 2005aJohnson et al., ,b, 2006Linster et al., 2001; Johnson and Leon 2000a,b; Ho et al., 2006a,b;Farahbod et al., 2006). Individual glomeruli in the bulb appear to receive convergent input from nasal olfactory sensory neurons that express the same single odorant receptor gene, and the location of glomeruli associated with the same odorant receptor are remarkably consistent from animal to animal (Mombaerts et al., 1996). By mapping the responses of glomeruli in the bulb to systematically related odorants, we therefore have been NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author Manuscript able to identify a number of molecular features that are particularly important in determining the response specificity of particul...