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.     

Daily variations of immunoreactive melatonin in the visual system of crayfish

In crustaceans, melatonin has been detected in the central nervous system and some other organs. The aim of this study was to analyze the melatonin content in the visual system of Procambarus clarkii, by means of radioimmunoassay, at different day-night phases. We have also studied the action of exogenous melatonin on the main properties of the electroretinogram (ERG) circadian rhythm. Experiments were conducted with 25 specimens maintained under controlled conditions of 16°C and 12 h of light alternating with 12 h of darkness. Eyes where cut in dim red light and shock frozen with liquid nitrogen and pulverized in a mortar until a homogenous powder was obtained. Melatonin was extracted with acetone, followed by centrifugation, diluted with an equal volume of equa bidest to ensure freezing at ?80°C for at least 90 min and lyophilization at the same temperature. Lyophilizates, after having been dissolved in RIA buffer, were used for determinations of melatonin. Long-term recordings of electrical responses to light (ERG) were obtained for 10 or more consecutive days. At the 5th day, a single dose of melatonin was injected and its effects on amplitude and period of the ERG circadian rhythm were measured. Melatonin concentrations differed considerably depending on the circadian time and attained a maximum during dark phase. Among the crustaceans, Procambarus clarkii represents the first case in which melatonin peaks during the night following the typical pattern known in the majority of organisms. After melatonin injection, period and amplitude of the ERG circadian rhythm were increased. This effect suggests the involvement of melatonin in the oscillators underlying the generation and expression of circadian rhythms in crayfish.