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.

Fig.1

Anatomically standardized contour charts illustrating the distribution of 2DG uptake across the entire glomerular layer in rats exposed to odorants possessing ketone groups, ester groups, or combinations of ketone and ester groups. A: Ethyl acetoacetate and comparator odorants that contain only one functional group. B: Methyl levulinate and comparator odorants. C: Acetoxyacetone and comparator odorants. Each chart represents the average of both bulbs of several rats exposed to the same odorant (Table 1). The charts are in a ventral-centered orientation as shown at bottom left. Warmer colors indicate higher uptake and cooler colors lower uptake in color steps corresponding to the number of standard deviations above or below the mean uptake across the layer as shown at bottom right. Open arrowheads indicate a glomerular module that responds to ketone odorants, black arrowheads indicate glomerular modules responding to methyl and ethyl esters, and black arrows indicate glomerular modules responding to aliphatic esters in general. Circled areas denote posterior regions responding optimally to the odorants possessing both a ketone and an ester group.

Fig. 2

Anatomically standardized contour charts illustrating the average distribution of 2DG uptake across the entire glomerular layer in rats exposed to (A) four-carbon or (B) six-carbon ketones or diketones. Orientation and color scale are the same as in Figure 1. Open arrowheads indicate glomerular modules that respond to ketone odorants. Black arrows in A show a region of uptake evoked by 2-butanone for comparison to the more caudal area activated by the corresponding diketone, butanedione (circled). Circled areas in B indicate posterior regions stimulated by six-carbon diketones but not by corresponding odorants with only one ketone group.

Fig. 3

Anatomically standardized contour charts illustrating the average distribution of 2DG uptake across the entire glomerular layer in rats exposed to (A) eight-carbon, (B) nine-carbon, or (C) seven-carbon ethyl esters or corresponding diesters. Orientation and color scale are the same as in Figure 1. Black arrowheads indicate glomerular modules responding to ethyl esters, and arrows indicate modules responding to esters in general. Circled areas indicate posterior regions activated by the diesters.

Fig. 4

Anatomically standardized contour charts illustrating the average distribution of 2DG uptake across the entire glomerular layer in rats exposed to odorants. Orientation and color scale are the same as in Figure 1. A: Odorants possessing a primary alcohol group, an ester group, or both. B: Odorants possessing a carboxylic acid group, an ester group, or both. C: Odorants possessing a secondary alcohol group, a ketone group, or both. D: Odorants possessing a carboxylic acid group or both a carboxylic acid and a ketone functional group. E: Odorants possessing various combinations of oxygenic functional groups. Open arrows indicate a glomerular module activated by alcohols and by 2-butanone. Black arrows indicate modules activated by esters in general. Black arrowheads indicate a module that responds to carboxylic acids and ethyl esters. The open arrowhead indicates a module that responds to ketone odorants. The outlined regions show the similarity in location of posterior glomeruli activated by various odorants having two oxygenic functional groups.

Fig. 5

Scatter plots showing the relationship between water solubility and 2DG uptake in posterior regions of both the medial (A) and the lateral (B) aspects of the glomerular layer. For every odorant-evoked activity pattern in our database (311 total patterns), uptake was averaged within the two regions shown as an inset in A. The value of uptake then was plotted as a function of the logarithm of water solubility, which was originally obtained in units of mg/L. Each point represents an odorant, and larger symbols are used to show the contribution made by particular classes of odorants as indicated in the key. The lines are the result of least squares, linear regressions of the data, which showed a statistically significant correlation between water solubility and uptake in both aspects of the bulb. Odorants of various classes that were highly water-soluble evoked high levels of uptake in the posterior modules.

Fig. 6

Scatter plot showing that molecular properties correlated with uptake in posterior glomeruli tend to be the same ones on both the medial and lateral aspects of the bulb. We determined the degree to which each of 13 molecular properties was correlated with average uptake in posterior modules on each aspect of the bulb, and then we plotted the correlation coefficient for the lateral module (h) relative to the correlation coefficient obtained for the medial module (H). Each point represents a single molecular property (1, log water solubility; 2, log vapor pressure; 3, hydrophilic-lipophilic balance; 4, percent hydrophilic surface; 5, hydrogen bond acceptor strength; 6, Hansen polarity; 7, hydrogen bond donor strength; 8, dipole moment; 9, number of freely rotatable bonds; 10, molecular length; 11, surface area; 12, molecular weight; 13, log P). The line is a result of least squares linear regression, the correlation coefficient for which was 0.82. Molecular properties related to water solubility and volatility were positively correlated with uptake in posterior modules both medially and laterally, while molecular properties related to size and hydrophobicity were negatively correlated with uptake in both aspects.

Fig. 7

Anatomically standardized contour charts illustrating the average distribution of 2DG uptake across the entire glomerular layer in rats exposed to other odorants that stimulate the posterior glomeruli. Orientation and color scale are the same as in Figure 1. The outlined areas represent the boundaries of the modules that were analyzed in Figure 5. Black arrowheads show the location of a module that responds to most carboxylic acids, but that is not activated by the alicyclic odorant, 2-methylcyclopropanecarboxylic acid. The open arrowhead shows a very dorsal region that shows some evidence of being activated by the secondary amine, dipropylamine.