Spatial representation of odorant structure in the moth antennal lobe: a study of structure-response relationships at low doses.

How odorant structure and concentration are spatially represented within the primary olfactory integration center, the antennal lobe (AL) or olfactory bulb (OB) in invertebrates and vertebrates, respectively, is currently a topic of high interest. Here, we show the spatial representation of odorant structure in the antennal lobe of the moth Spodoptera littoralis by imaging calcium activity evoked by straight chain aliphatic alcohols and aldehydes at low doses. Activity patterns of a given odor were most similar to compounds with the same functional group, differing in chain length by only one carbon atom. A chain length dependency was present as the most activated glomerulus in the lobe shifted from a medial to a lateral position with increasing chain length of the molecule. Statistical analysis revealed that in both classes of chemicals the chain length of the molecule was represented in a similar way. No topographically fixed domains were observed for any of the classes. However, activity patterns evoked by lower chain length molecules were spatially more distinct than patterns evoked by higher chain length molecules. The number of activated glomeruli for both classes of chemicals increased with increasing chain length to reach a maximum at eight or nine C atoms followed by a decrease as the chain length further increased.