Research reportOdor detection and odor discrimination in subadult and adult rats for two enantiomeric odorants supported by c-fos data

The study investigated the olfactory sensitivity and odor discrimination ability of male Wistar rats for the two closely related enantiomic odorants carvone and limonene. Enantiomers have identical chemical and physical properties but many are known to have different odor qualities for humans. Our psychophysical experiments revealed that subadult animals (about 2 months old) demonstrated higher olfactory sensitivities - lower detection thresholds - for d- and l-limonene than the adult conspecifics (about 6 months old). In the discrimination tasks at higher odor concentrations all animals were able to discriminate between the d- and l-odorant but subadults demonstrated lower discrimination thresholds. Within each age group no significant difference could be established.To elucidate neural correlates of our behavioral results, we have used c-fos expression to provide a global map of neuronal activity, with single cell resolution, in the olfactory bulb of d- and l-carvone exposed animals. The two carvone odors elicit different patterns of neuronal activity in the olfactory bulb, however, with partially overlapping activated areas. Our behavioral and immunhistochemical results allow to assume that the d- and l-forms of the two enantiomeric odorants have different odor qualities. Our study further demonstrates that the odor quality of an odorant depends not only on the atoms of the molecule, its chemical structure or on the presence of a functional group. Other parameters like, e.g. interaction profiles with different peptides, optical rotation of the polarized light - as seen in the enantiomers of an odorant - might also contribute to the quality of an odor.