Interactions between odorant functional group and hydrocarbon structure influence activity in glomerular response modules in the rat olfactory bulb.

To investigate the effect of odorant hydrocarbon structure on spatial representations in the olfactory bulb systematically, we exposed rats to odorant chemicals possessing one of four different oxygen-containing functional groups on one of five different hydrocarbon backbones. We also used several hydrocarbon odorants lacking other functional groups. Hydrocarbon structural categories included straight-chained, branched, double-bonded, alicyclic, and aromatic features. Activity throughout the entire glomerular layer was measured as uptake of [(14)C]2-deoxyglucose and was mapped into anatomically standardized data matrices for statistical comparisons across different animals. Patterns evoked by straight-chained aliphatic odorants confirmed an association of activity in particular glomerular response modules with particular functional groups. However, the amount of activity in these same modules also was affected significantly by differences in hydrocarbon structure. Thus, the molecular features recognized by receptors projecting to these response modules appear to involve both functional group and hydrocarbon structural elements. In addition, particular benzyl and cyclohexyl odorants evoked activity in dorsal modules previously associated with the ketone functional group, which represents an exception to the rule of one feature per response module that had emerged from our previous studies. These dorsal modules also responded to nitrogen-containing aromatic compounds involving pyridine and pyrazine rings. The unexpected overlap in modular responses to ketones and odorants seemingly unrelated to ketones may reflect some covert shared molecular feature, the existence of odorant sensory neurons with multiple specificities, or a mosaic of sensory neuron projections to these particular modules.