Odorants with multiple oxygen‐containing functional groups and other odorants with high water solubility preferentially activate posterior olfactory bulb glomeruli

Abstract

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.

Spatial representations of odorants in olfactory bulbs of rats and mice: Similarities and differences in chemotopic organization

“…Iodoform, which is small and simple in structure without being very water soluble, also activated the posterior modules in rats (Fig. 3A; Johnson et al, 2007). Although iodoform evoked a small amount of uptake in a similar posterior region in mice, it also stimulated additional, more rostral glomeruli in mice (Fig.…”

“…In rats, odorants that possess very high water solubility, by virtue of either a small size or the presence of multiple oxygenic functional groups, stimulate glomeruli at the posterior extremities of the midlateral and midmedial bulbar aspects (Johnson et al, 2007). Such responses are exemplified in the patterns evoked by acetone and 2-propanol (Fig.…”

Tuning to Odor Solubility and Sorption Pattern in Olfactory Epithelial Responses

“…This similarity between the open EOG and intact sorption profiles may represent a tuning of the sensitivity of each mucosa region to the odorants normally available at that region during breathing and active sniffing. The highest flow rate of 500 ml/min we used was in the upper range of maximal sniff velocity reported by Youngentob et al (1987). It is likely that these high flow rates, which often occur during olfactory tasks, are more olfactory-relevant flow rates to which the tuning of intrinsic sensitivity is based.…”

“…Since the relative size of the EOGs does not change linearly with the calculated concentration across flow conditions, suggesting a nonlinear relationship between concentration and response, the regression model fit the four calculated concentrations for each odorant in the four conditions by assuming that the olfactory response should be sigmoidally related to the logarithm of stimulus concentration as reported in the observations of Grosmaitre et al (2006) and of Rospars et al (2008). We used the following logistic equation, Equation 2, to fit the intact response for each odorant:…”

“…We approached the comparison of the CFD and EOG results by assuming that the neural response is a sigmoidal function of the logarithm of odorant concentration (Grosmaitre et al, 2006;Rospars et al, 2008). We chose the logistic equation rather than the Hill equation because they are mathematically equivalent and the logistic equation is easier to fit in the absence of reliable estimates of the maximum response (Barlow and Blake, 1989).…”