The temperature-sensitive transient receptor potential channel, TRPM8, was recently cloned and found to be activated by cold and menthol. Whole-cell recordings show that TRPM8 is permeable to multiple cations and exhibits a strong outward rectification. Here, we examine the mechanism underlying menthol-evoked current rectification of TRPM8 transiently expressed in tsA-201 cells at room temperature (similar to 25 degrees C). Whole-cell currents (ruptured, bath: Na+, K+, Ca2+, or Ba2+; pipette: KCl) exhibited a strong outward rectification in the presence of menthol, consistent with previous studies. The outward K+ current was reduced in the presence of external Ca2+ or Ba2+. Single-channel recordings (cell-attached) showed that menthol induced brief channel openings with two conducting states in the voltage range between -80 and +60 mV. The small current (is) conducted both monovalent and divalent ions, and the large one i(S) predominantly monovalent ions. The i-V plot for Ca2+ was weakly outward rectifying, whereas those for monovalent ions were linear. The is may result in the divalent ion-induced reduction of the whole-cell outward current. The open probability (P-o) in all ion conditions tested was low at negative voltages and increased with depolarization, accounting for the small inward currents observed at the whole-cell level. In conclusion, our results indicate that menthol induced steep outward rectification of TRPM8 results from the voltage-dependent open channel probability and the permeating ion-dependent modulation of the unitary channel conductance. (c) 2005 Elsevier Inc. All rights reserved.