Central Mechanisms of Menthol-Induced Analgesia

Abstract

Menthol is one of the most commonly used chemicals in our daily life, not only because of its fresh flavor and cooling feeling but also because of its medical benefit. Previous studies have suggested that menthol produces analgesic action in acute and neuropathic pain through peripheral mechanisms. However, the central actions and mechanisms of menthol remain unclear. Here, we report that menthol has direct effects on the spinal cord. Menthol decreased both ipsilateral and contralateral pain hypersensitivity induced by complete Freund's adjuvant in a dose-dependent manner. Menthol also reduced both first and second phases of formalin-induced spontaneous nocifensive behavior. We then identified the potential central mechanisms underlying the analgesic effect of menthol. In cultured dorsal horn neurons, menthol induced inward and outward currents in a dose-dependent manner. The menthol-activated current was mediated by Cl Ϫ and blocked by bicuculline, suggesting that menthol activates ␥-aminobutyric acid type A receptors. In addition, menthol blocked voltage-gated sodium channels and voltage-gated calcium channels in a voltage-, state-, and usedependent manner. Furthermore, menthol reduced repetitive firing and action potential amplitude, decreased neuronal excitability, and blocked spontaneous synaptic transmission of cultured superficial dorsal horn neurons. Liquid chromatography/tandem mass spectrometry analysis of brain menthol levels indicated that menthol was rapidly concentrated in the brain when administered systemically. Our results indicate that menthol produces its central analgesic action on inflammatory pain probably via the blockage of voltage-gated Na ϩ and Ca 2ϩ channels. These data provide molecular and cellular mechanisms by which menthol decreases neuronal excitability, therefore contributing to menthol-induced central analgesia.

Menthol Inhibits 5-HT3Receptor–Mediated Currents

“…Both ligand-gated and voltagegated channels are inhibited by menthol (IC 50 5 297 mM for Na 1 channels and IC 50 5 125 mM for Ca 21 channels in dorsal horn neurons) (Pan et al, 2012). Moreover, the effective concentrations observed in the current study were found to be comparable with menthol concentrations sufficient to activate TRPM8 channels, with EC 50 values ranging between 67 and 196 mM (Sherkheli et al, 2010) in X. laevis oocytes.…”

“…Recent electrophysiological studies reported that menthol inhibits the functions of Na 1 (Gaudioso et al, 2012;Pan et al, 2012) and Ca 21 channels (Pan et al, 2012) in a voltage-dependent manner. The effect of changing the membrane potential relationship between 5-HT-activated current and membrane potential before and after 10-minute menthol (300 mM) application is Menthol on 5-HT 3 Receptor shown in Fig.…”

“…Menthol undergoes extensive enterohepatic recirculation, and it is rapidly but incompletely (∼50%) metabolized to menthol glucuronide, which is excreted both in the bile and in the urine (Gelal et al, 1999). In a recent study, a high concentration of menthol (54 mg/g) was detected in brain tissue 5 minutes after 100-mg/kg intraperitoneal injection (Pan et al, 2012), indicating that menthol is rapidly absorbed into the brain. Therefore, functional modulation of 5-HT 3 receptors demonstrated in this study can mediate some of the pharmacological actions of menthol.…”

“…In earlier studies, menthol in the concentration ranges used in this study has been shown to act directly on GABA A (EC 50 5 1.1 mM) (Pan et al, 2012), glycine (100-300 mM) (Hall et al, 2004), and ryanodine receptors (1 mM) (Palade, 1987;Neumann and Copello, 2011). Both ligand-gated and voltagegated channels are inhibited by menthol (IC 50 5 297 mM for Na 1 channels and IC 50 5 125 mM for Ca 21 channels in dorsal horn neurons) (Pan et al, 2012).…”

Menthol Enhances the Desensitization of Human α3β4 Nicotinic Acetylcholine Receptors

“…Although precise concentrations of menthol in the brains of smokers are unknown, the results of an animal study show that menthol can readily penetrate the central nervous system. For example, Pan et al (2012) showed that menthol can reach high levels in the brains of mice within 5 minutes of a bolus intraperitoneal injection and was still measurable 60 minutes after injection.…”