Effects of menthol and its interaction with nicotine-conditioned cue on nicotine-seeking behavior in rats.

RATIONALE: Increasing clinical evidence suggests that menthol, a significant flavoring additive in tobacco products, may contribute to smoking and nicotine dependence. Relapse to smoking behavior presents a formidable challenge for the treatment of tobacco addiction. An unresolved issue is whether the mentholation of tobacco products precipitates relapse to tobacco use in abstinent smokers.

OBJECTIVES: The present study examined the effects of menthol on the perseverance and relapse of nicotine-seeking behavior in rats.

METHODS: Male Sprague-Dawley rats were trained to press a lever for intravenous nicotine self-administration (0.03 mg/kg/infusion) under a fixed-ratio five schedule of reinforcement. Each nicotine infusion was signaled by the presentation of a sensory stimulus that was established as a discrete nicotine-conditioned cue. Five minutes prior to the sessions, the rats received an intraperitoneal injection of menthol (0.1 mg/kg) or vehicle. In the subsequent extinction test sessions, nicotine was unavailable with or without menthol and/or the nicotine-conditioned cue. The reinstatement tests were performed the following day after the extinction criterion was met. Menthol was also tested on food-seeking responses. In a subset of nicotine-trained rats, a transient receptor potential melastatin 8 (TRPM8) antagonist RQ-00203078 was given prior to menthol administration.

RESULTS: Continued administration of menthol sustained responses on the previously active and nicotine-reinforced lever in the extinction tests. The readministration of menthol after extinction reinstated active lever responses. In both the extinction and the reinstatement tests, a combination of pre-session menthol administration and cue representation during the session produced a more robust behavioral effect than either menthol or the cue alone. No such effects of menthol was observed in food trained rats. RQ-00203078 did not change menthol effect on nicotine seeking.

CONCLUSION: These data demonstrated that menthol specifically sustained and reinstated nicotine-seeking behavior, and this effect was independent of TRPM8 activity. These findings suggest that menthol in most tobacco products, even not menthol labeled, may contribute to the perseverance of and relapse to tobacco-seeking behavior.