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Transduction mechanisms were investigated in human olfactory neurons by determining characteristics of odorant-induced changes in intracellular calcium concentration ([Ca2+]i). Olfactory neurons were freshly isolated from nasal biopsies, allowed to attach to coverslips, and loaded with the calcium-sensitive indicator fura-2. Changes in [Ca2+]i were studied in response to exposure to individual odors, or odorant mixtures composed to distinguish between transduction pathways mediated by adenosine 3'5'-monophosphate (cAMP; mix A) or inositol 1,4,5-trisphosphate (InsP3; mix B). Overall, 52% of biopsies produced one or more odorant-responsive olfactory neurons, whereas 24% of all olfactory neurons tested responded to odorant exposure with a change in [Ca2+]i. As in olfactory neurons from other species, the data suggest that odorant exposure elicited calcium influx via second-messenger pathways involving cAMP or InsP3. Unlike olfactory neurons from other species that have been tested, some human olfactory neurons responded to odorants with decreases in [Ca2+]i. Also in contrast with olfactory neurons from other species, human olfactory neurons were better able to discriminate between odorant mixtures in that no neuron responded to more than one type of odor or mixture. These results suggest the presence of a previously unreported type of olfactory transduction mechanism, and raise the possibility that coding of odor qualities in humans may be accomplished to some degree differently than in other vertebrates, with the olfactory neuron itself making a greater contribution to the discrimination process.

1. The effects of acetylcholine (ACh) on granule cells freshly dissociated from rat dentate gyrus (DG) were studied using the nystatin perforated patch technique. This method allowed us to study ACh-induced currents (IACh) under voltage clamp without "run-down" of the ACh response. In some experiments, we used the conventional whole-cell method for intracellular application of drugs not permeable to cell membrane. 2. At a holding potential of -40 mV, ACh induced an outward current. The amplitude of IACh increased in a sigmoidal fashion with increasing ACh concentration. The half-maximal response and the Hill coefficient determined from the relation between ACh concentration and response were 4.98 x 10(-7) M and 1.70, respectively. 3. The reversal potential of IACh was close to the K+ equilibrium potential. The IACh was accompanied by an enhancement of the K+ current. 4. Muscarine and McN-A-343 mimicked the ACh response, whereas oxotremorine induced no response. 5. Muscarinic antagonists reversibly suppressed the IACh (10(-5) M) in a concentration-dependent manner, where the values of half-inhibition concentration (IC50) were 1.03 x 10(-6) M for pirenzepine and 2.21 x 10(-5) M for AF-DX-116. 6. Intracellular perfusion with GDP-beta S suppressed the IACh greatly. The IACh persisted in the neurons pretreated with an external solution containing pertussis toxin (IAP) for 18 h. 7. In the neurons perfused with Ca(2+)-free external solution containing 2 mM ethylene glycol-O,O'-bis (beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid and 10 mM Mg2+, the first application of ACh induced the IACh with an amplitude similar to that in the standard solution.(ABSTRACT TRUNCATED AT 250 WORDS)