Menthol decreases oral nicotine aversion in C57BL/6 mice through a TRPM8-dependent mechanism.

Background Nicotine is a major oral irritant in smokeless tobacco products and has an aversive taste. Mentholated smokeless tobacco products are highly popular, suggesting that menthol increases their palatability and may facilitate initiation of product use. While menthol is known to reduce respiratory irritation by tobacco smoke irritants, it is not known whether this activity extends to oral nicotine and its aversive effects. Study design The two-bottle choice drinking assay was used to characterise aversion and preference in C57BL/6 mice to a range of menthol concentrations (10-200 mu g/mL). Then, effects of menthol on oral nicotine aversion were determined. Responses were compared with those in mice deficient in the cold/menthol receptor, TRPM8, expressed in trigeminal sensory neurons innervating the oral cavity. Results Mice showed aversion to menthol concentrations of 100 mu g/mL and above. When presented with a highly aversive concentration of nicotine (200 mu g/mL), mice preferred solutions with 50 or 100 mu g/mL menthol added over nicotine alone. In contrast to wild-type mice, Trpm8-/- showed a strong aversion to mentholated (100 mu g/mL) nicotine (200 mu g/mL) and preferred nicotine alone. Trpm8-/- mice show aversion to lower concentrations of menthol than wild-type mice. Conclusions Oral menthol can reduce the aversive effects of oral nicotine and, at higher concentrations, acts as an irritant by itself. Menthol's effects in relation to nicotine require TRPM8, the cool temperature sensing ion channel that activates analgesic and counterirritant mechanisms. These mechanisms may underlie preference for menthol-containing smokeless tobacco products and may facilitate initiation of product use.