Molecular mechanisms for olfactory signal transduction

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Spike discharges of single olfactory receptor cells (ORCs) were recorded with the whole-cell patch clamp method applied to the slice preparation. In parallel, activities of transduction channels were measured under the voltage-clamp condition. Current injection induced repetitive discharges. The firing frequency was dependent on the amount of injected current, showing saturation. When cells ware stimulated by odorants, 54 out of 306 cells exhibited inward currents in response to cineole under the voltage-clamp condition (holding potential =-60 mV). The amplitude of the inward current was dependent on the stimulus period, reflecting the time-integration for the stimulus dose. The dose-response relation could be fitted by the Hill equation. Under the current-clamp condition, odorants induced repetitive spikes. In those cells spike frequency was dose-dependent, expressing saturation similar to the result obtained by current injection. Since both transduction channel and spike generative steps expressed saturation in their dose dependences, we explored what step(s) actually determines saturation in ORCs' signaling processes. By examining dose-response relations of both the current and spikes in the same cells, saturating dose was found to be dependent largely on that of the transduction step. This suggests that the dynamic range is strongly influenced by the transduction system. A simple model derived from the non-linearity of the membrane could explain that the steepness of the depolarization-concentration curve became bigger as Hill coefficient of the transduction channel was increased. In addition, it was speculated that a critical level of dynamic range was, at least in part, modified by the membrane non-linearity.In addition we made experiments on identifying second messenger mediating olfactory transduction, mechanisms for cross-adaptation and some physiological measurements from human subjects regarding olfactory perception.