Menthol preferentially inhibits persistent Na+ current mediated by NaV1.8 in small-sized dural afferent neurons of rats

Objective Menthol is widely used as a cooling agent and an adjunctive analgesic to relieve various painful conditions, such as migraine. As menthol acts as an agonist for the thermosensitive ion channel transient receptor potential melastatin 8 (TRPM8), other ion channels, such as voltage-gated Na+ channels, are also involved in the antinociceptive effect of menthol. In this study, we explored the effect of menthol on tetrodotoxin-resistant (TTX-R) Na+ channels in nociceptive sensory neurons. Methods TTX-R Na+ current (I-Na) was recorded from acutely isolated rat dural afferent neurons identified with the fluorescent dye DiI using a whole-cell patch-clamp technique. Results Under a voltage-clamp condition, menthol potently decreased the amplitude of the persistent TTX-R Na+ current (I-Na) in a concentration-dependent manner, with a minor effect on the transient current. The inhibition of persistent TTX-R I-Na by menthol was not affected by the TRPM8 antagonist. Menthol (300 mu M) (1) shifted the steady-state fast inactivation relationship to hyperpolarizing ranges without affecting the voltage-activation relationship, (2) accelerated the onset of inactivation, and (3) retarded the recovery from the inactivation of TTX-R Na+ channels. Under the current clamp condition, menthol (300 mu M) decreased the threshold for action potential generation but reduced the number of action potentials elicited by strong depolarizing current stimuli. Conclusion The results of this study suggest that menthol exerts an analgesic effect by preferentially inhibiting persistent TTX-R I-Na and modulating the inactivation and recovery kinetics of TTX-R Na+ channels.