Eugenol Inhibits ATP-induced P2X Currents in Trigeminal Ganglion Neurons

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 α ,β-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 α ,β-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.