Effects of Eugenol on T-type Ca2+ Channel Isoforms

Eugenol has been used as an analgesic in dentistry. Previous studies have demonstrated that voltage-gated Na+ channels and high-voltage-activated Ca2+ channels expressed in trigeminal ganglion (TG) neurons sensing dental pain are molecular targets of eugenol for its analgesic effects. However, it has not been investigated whether eugenol can affect T-type Ca2+ channels, which are known to be detected in the afferent neurons. In this report, we investigate how eugenol can influence cloned T-type channel isoforms expressed in HEK293 cells, using whole-cell patch clamp. Application of eugenol inhibited Ca(v)3.1, Ca(v)3.2, and Ca(v)3.3 currents in a concentration-dependent manner with IC50 values of 463, 486, and 708 mu M, respectively. Eugenol was found to negatively shift the steady-state inactivation curves of the T-type channel isoforms, but it did not shift their activation curves. In addition, eugenol had little effect on the current kinetics of Ca(v)3.1 and Ca(v)3.2, but it accelerated the inactivation kinetics of Ca(v)3.3 currents. Reduction of channel availability enhanced eugenol inhibition sensitivity for Ca(v)3.1 and Ca(v)3.2, but not for Ca(v)3.3. Moreover, eugenol inhibition of T-type channel isoforms was found to be use dependent. Finally, we show that the T-type currents recorded from rat TG neurons were inhibited by eugenol with a similar potency to Ca(v)3.1 and Ca(v)3.2 isoforms. Taken together, our findings suggest that T-type Ca2+ channels are additional molecular targets for the pain-relieving effects of eugenol.