OLFACTORY AND TRIGEMINAL RESPONSES TO NICOTINE

Olfactory and trigeminal sensitivities to vapor-phase nicotine were as sessed by using psychophysical studies with normal and anosmic human s ubjects and using electrophysiological studies with rats and pigeons. This work showed that 1) psychophysical estimates of sensitivity are a pproximately tenfold higher (i.e., lower thresholds) than those based on neural recordings, with both techniques demonstrating greater olfac tory than trigeminal sensitivity for nicotine and other compounds; 2) for both chemosensory inputs, sensitivity to nicotine is at least 30-f old greater than that to several other compounds; 3) human subjects ca n discriminate qualitatively between the S-(-) and the R-(-) stereoiso mers of nicotine, although the relative importance of olfactory and tr igeminal inputs in this discriminative ability is unclear; and 4) trig eminal nerve responses in rats show similar thresholds for S-(-)- and R-(+)-nicotine but show lower suprathreshold responses to the R-(+) st ereoisomer. The olfactory epithelium and trigeminal ganglion exhibit h igh-affinity binding of S-(-)-nicotine. In addition, reverse transcrip tase-polymerase chain reaction (RT-PCR) studies have shown that many o f the nicotinic acetylcholine receptor (nAChR) subunits found in other parts of the nervous system are present in the olfactory epithelium a nd bulb and in the trigeminal ganglion. Collectively, these findings s uggest that two or more of the types of nAChRs identified in other par ts of the nervous system may serve as receptor proteins that bind nico tine-like odorants or irritants. Investigation of the pharmacology of chemosensory responses to nicotine may help to establish causal links between specific receptor proteins and the perception of odor and irri tation. (C) 1996 Wiley-Liss, Inc.