Menthol pain relief through cumulative inactivation of voltage-gated sodium channels

Menthol is a natural compound of plant origin known to produce cool sensation via the activation of the TRPM8 channel. It is also frequently part of topical analgesic drugs available in a pharmacy, although its mechanism of action is still unknown. Compelling evidence indicates that voltage-gated Na+ channels are critical for experiencing pain sensation. We tested the hypothesis that menthol may block voltage-gated Na+ channels in dorsal root ganglion (DRG) neurons. By use of a patch clamp, we evaluated the effects of menthol application on tetrodotoxin (TTX)-resistant Nav1.8 and Nav1.9 channel subtypes in DRG neurons, and on TTX-sensitive Na+ channels in immortalized DRG neuron-derived F11 cells. The results indicate that menthol inhibits Na+ channels in a concentration-, voltage-, and frequency-dependent manner. Menthol promoted fast and slow inactivation states, causing use-dependent depression of Na+ channel activity. In current clamp recordings, menthol inhibited firing at high-frequency stimulation with minimal effects on normal neuronal activity. We found that low concentrations of menthol cause analgesia in mice, relieving pain produced by a Na+ channel-targeting toxin. We conclude that menthol is a state-selective blocker of Nav1.8, Nav1.9, and TTX-sensitive Na+ channels, indicating a role for Na+ channel blockade in the efficacy of menthol as topical analgesic compound.