RESPONSES RECORDED FROM THE FROG OLFACTORY EPITHELIUM AFTER STIMULATION WITH R(- AND S(-)- NICOTINE())

The aim of this study was to determine whether the olfactory system is responsible for the discriminability of the stereoisomers of nicotine . The EOG was recorded after stimulation with different concentrations of undistilled S(-)-, distilled S(-)- and distilled R(-)-nicotine sep arately in three groups of frogs (Xenopus laevis). The responses to al l types of nicotine used in the experiments increased with increasing stimulus concentration. The responses to undistilled S(-)-nicotine wer e significantly lower compared to responses to distilled S(-)- and R()-nicotine, whereas no significant differences could be found when the purified stereoisomers of nicotine [distilled S(-)-nicotine, distille d R(+)-nicotine] were compared. Control measurements of time course an d peak concentration employing a UV-detection method demonstrated that the differences between distilled and undistilled S(-)-nicotine could not be explained by different nicotine concentrations. The fact that no differences between the pure nicotine stereoisomers could be found for all concentrations used in our experiments and that experiments in humans revealed similar detection thresholds for both stereoisomers p oints to a similar receptor affinity of R(+)- and S(-)-nicotine within the olfactory system. At this point, it cannot be determined whether the observed differences in the perception of nicotine enantiomers in humans are due to differences in quality coding by stereospecific rece ptors on the olfactory sensory cells or by specific receptors on the t rigeminal nerve endings.