Nonselective Suppression of Voltage-gated Currents by Odorants in the Newt Olfactory Receptor Cells

Effects of odorants on voltage-gated ionic channels were investigated in isolated newt olfactory receptor cells by using the whole cell version of the patch–clamp technique. Under voltage clamp, membrane depolarization to voltages between −90 mV and +40 mV from a holding potential (V_h) of −100 mV generated time-and voltage-dependent current responses; a rapidly (< 15 ms) decaying initial inward current and a late outwardcurrent. When odorants (1 mM amyl acetate, 1 mM acetophenone, and 1 mM limonene) were applied to the recorded cell, the voltage-gated currents were significantly reduced. The dose-suppression relations of amyl acetatefor individual current components (Na⁺ current: I_Na, T-type Ca²⁺ current: I_Ca,T, L-type Ca²⁺ current: I_Ca,L, delayedrectifier K⁺ current: I_Kv and Ca²⁺-activated K⁺ current: I_K(Ca)) could be fitted by the Hill equation. Half-blockingconcentrations for each current were 0.11 mM (I_Na), 0.15 mM (I_Ca,T), 0.14 mM (I_Ca,L), 1.7 mM (I_Kv), and 0.17 mM(I_K(Ca)), and Hill coefficient was 1.4 (I_Na), 1.0 (I_Ca,T), 1.1 (I_Ca,L), 1.0 (I_Kv), and 1.1 (I_K(Ca)), suggesting that the inward current is affected more strongly than the outward current. The activation curve of I_Na was not changed significantly by amyl acetate, while the inactivation curve was shifted to negative voltages; half-activation voltages were−53 mV at control, −66 mV at 0.01 mM, and −84 mV at 0.1 mM. These phenomena are similar to the suppressiveeffects of local anesthetics (lidocaine and benzocaine) on I_Na in various preparations, suggesting that both types ofsuppression are caused by the same mechanism. The nonselective blockage of ionic channels observed here isconsistent with the previous notion that the suppression of the transduction current by odorants is due to the direst blockage of transduction channels.