Responses Recorded from the Frog Olfactory Epithelium after Stimulation with R(+)- and S(-)- nicotine

The aim of this study was to determine whether the olfactorysystem is responsible for the discriminability of the stereoisomersof nicotine. The EOG was recorded after stimulation with differentconcentrations of undistilled S(–)-, distilled S(–)-and distilled R(–)-nicotine separately in three groupsof frogs (Xenopus laevis). The responses to all types of nicotineused in the experiments increased with increasing stimulus concentration.The responses to undistilled S(–)-nicotine were significantlylower compared to responses to distilled S(–)- and R(+)-nicotine,whereas no significant differences could be found when the purifiedstereoisomers of nicotine [distilled S(–)-nicotine, distilledR(+)-nicotine] were compared. Control measurements of time courseand peak concentration employing a UV-detection method demonstratedthat the differences between distilled and undistilled S(–)-nicotinecould not be explained by different nicotine concentrations. The fact that no differences between the pure nicotine stereoisomerscould be found for all concentrations used in our experimentsand that experiments in humans revealed similar detection thresholdsfor both stereoisomers points to a similar receptor affinityof R(+)- and S (–)-nicotine within the olfactory system.At this point, it cannot be determined whether the observeddifferences in the perception of nicotine enantiomers in humansare due to differences in quality coding by stereospecific receptorson the olfactory sensory cells or by specific receptors on thetrigeminal nerve endings. Chem. Senses 20: 337–344, 1995.