Biophysical properties of menthol-activated cold receptor TRPM8 channels

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 (∼25 °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 (iL) 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 (Po) 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.