Eugenol Inhibits ATP-induced P2X Currents in Trigeminal Ganglion Neurons

ABSTRACT

Eugenol is widely used in dentistry to relieve pain. We have recently demonstrated voltage-gated Na+ and Ca2+ channels as molecular targets for its analgesic effects, and hypothesized that eugenol acts on P2X3, another pain receptor expressed in trigeminal ganglion (TG), and tested the effects of eugenol by whole-cell patch clamp and Ca2+ imaging techniques. In the present study, we investigated whether eugenol would modulate 5'-triphosphate (ATP)-induced currents in rat TG neurons and P2X3-expressing human embryonic kidney (HEK) 293 cells. ATP-induced currents in TG neurons exhibited electrophysiological properties similar to those in HEK293 cells, and both ATP- and alpha,beta-meATP-induced currents in TG neurons were effectively blocked by TNP-ATP, suggesting that P2X3 mediates the majority of ATP-induced currents in TG neurons. Eugenol inhibited ATP-induced currents in both capsaicin-sensitive and capsaicin-insensitive TG neurons with similar extent, and most ATP-responsive neurons were IB4-positive. Eugenol inhibited not only Ca2+ transients evoked by alpha,beta-meATP, the selective P2X3 agonist, in capsaicin-insensitive TG neurons, but also ATP-induced currents in P2X3-expressing HEK293 cells without co-expression of transient receptor potential vanilloid 1 (TRPV1). We suggest, therefore, that eugenol inhibits P2X3 currents in a TRPV1-independent manner, which contributes to its analgesic effect.