Odor maps in the mammalian olfactory bulb: domain organization and odorant structural features

Abstract

Psychophysical studies indicate that structural features of odorants differentially influence their perceived odor. In the olfactory bulb (OB), odorants are represented by ensembles of activated glomeruli. Here we used optical imaging of intrinsic signals to examine how these structural features are represented spatially in the sensory map of the rat OB. We found that the dorsal OB contained two topographically fixed domains; constituent glomeruli in each domain could be activated by odorants with particular functional groups. Within each domain, other structural features such as carbon chain length and branching were represented by local differences in patterns. These results suggest that structural features are categorized into two classes, primary features (functional groups) that characterize each domain, and secondary features that are represented by local positions within each domain. Such hierarchical representations of different structural features correlate well with psychophysical structure-odor relationships.

A metric for odorant comparison

“…Furthermore, aliphatic odorants with similar high CAN values were closer to each other, according to the multidimensional metric, than aliphatic odorants with similar low CAN values. This is consistent with previous results 11,27 and supports observations made by others 12,14 .…”

“…Olfactory receptors that respond to odorants with 5 carbons are likely to respond to odorants with 4 or 6 carbons but are less likely to respond to odorants with 7 or more carbons. Similar tuning specificities have been observed in glomeruli 6,7,14,[19][20][21][22][23] . In other words, odorants that have similar CAN and functional groups elicit similar glomerular response patterns.…”

“…Because the distance between odorants in the neuronal space is estimated using the Pearson correlation, we restricted our analysis to datasets that reported the response of large sets of neurons (that is, at least 20 neurons to at least 10 odorants). We analyzed 7 studies 5,14,16,[20][21][22][23] (summarized in Table 1). In all cases except for dataset 3 (ref.…”

“…We mined data from published papers 5,11,14,16,[20][21][22][23]27 . In almost all cases, the response values had been reported as the response strength category of each olfactory sensory neuron or glomerulus.…”

“…Nevertheless, olfaction research has gravitated toward probing a limited set of axes, primarily carbon atom number (CAN) and functional group type and position [4][5][6][7][8][9][10][11][12][13] . For example, a given olfactory sensory neuron will generally respond to molecules with a sequential CAN as long as these molecules share the same functional group 5,8,[14][15][16][17][18] . Olfactory receptors that respond to odorants with 5 carbons are likely to respond to odorants with 4 or 6 carbons but are less likely to respond to odorants with 7 or more carbons.…”