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Eugenol Inhibits K⁺ Currents in Trigeminal Ganglion Neurons

¹ Department of Physiology and Program in Molecular and Cellular Neuroscience, School of Dentistry and Dental Research Institute, Seoul National University, 28-2 Yeongeon-Dong Chongno-Ku, Seoul 110-749, Korea; and
² Department of Physiology, College of Medicine, Kangwon National University, Chunchon 200-710, Korea

View larger version (14K): [in this window] [in a new window]   Figure 1. Eugenol inhibited voltage-gated K+ currents in rat trigeminal ganglion neurons. (A) Using pulses from –80 mV to 60 mV for 600 ms with a holding potential of –60 mV, we obtained the representative traces under control, 1 mM eugenol, and wash-out conditions, measured at the points indicated in (B). (B) For time-course recording of voltage-gated K+ currents, we used a depolarizing potential of 20 mV from a holding of –80 mV in 10-second intervals. When trigeminal ganglion neurons were exposed to eugenol (1 mM), the voltage-gated K+ current was readily inhibited in a reversible manner. (C) The dose-response curve of the inhibition of voltage-gated K+ currents by eugenol with the IC50 of 376 ± 90 µM. The number in parentheses represents the number of cells studied.

View larger version (14K): [in this window] [in a new window]   Figure 2. Eugenol inhibited voltage-gated K+ currents in both capsaicin-sensitive and capsaicin-insensitive rat trigeminal ganglion neurons. (A) Representative current traces of voltage-gated K+ currents under control, eugenol (1 mM), and capsaicin (1 µM), respectively, in 2 trigeminal ganglion neurons. Eugenol (1 mM) inhibited voltage-gated K+ currents in both capsaicin-sensitive (left) and capsaicin-insensitive (right) trigeminal ganglion neurons. (B) Representative time-courses of the inhibition of voltage-gated K+ currents by eugenol in capsaicin-sensitive (black circle) and capsaicin-insensitive (white circle) trigeminal ganglion neurons (left). The summary of the inhibition of voltage-gated K+ currents in trigeminal ganglion neurons (right) indicated that eugenol (1 mM)-induced inhibition in capsaicin-insensitive neurons was similar to that obtained in capsaicin-sensitive neurons (mean ± SEM, p > 0.05). The number in parentheses represents the number of cells studied.

View larger version (19K): [in this window] [in a new window]   Figure 3. Inhibition of voltage-gated K+ currents by eugenol does not require TRPV1 activation. (A) Capsazepine (CZP) did not block the inhibition of voltage-gated K+ currents by eugenol in rat trigeminal ganglion (TG) neurons. The representative traces of voltage-gated K+ currents under control, CZP (10 µM), and the combined application of eugenol and CZP (left). The inhibition of voltage-gated K+ currents by the combined application of eugenol and CZP was not significantly different from that of eugenol only (mean ± SEM, p > 0.05) (right), indicating that eugenol-induced voltage-gated K+ current inhibition was TRPV1-independent. The number in parentheses represents the number of cells studied. (B) Representative current traces under control and 1 mM eugenol in Ltk– cells (left). Eugenol inhibited hKv1.5 currents in Ltk– cells. The summary of K+ current inhibition in trigeminal ganglion neurons and Ltk– cells (right). The K+ current inhibition by eugenol in trigeminal ganglion neurons was significantly greater than that in Ltk– cells (mean ± SEM, p < 0.05). (Insert) RT-PCR analysis shows no endogenous expression of TRPV1 in Ltk– cells. TRPV1 is clearly expressed in trigeminal ganglion neurons. The expected sizes of PCR products were 233 bp (TRPV1) and 293 bp (ß-actin). No PCR products were detected with H2O (lane –).