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  Objective  To establish a nicotine intravenous self-administration rat model, and to examine, with this model, the effects of two flavoring additives, menthol and cineole, on nicotine dependence.

  Methods  Thirty male Sprague-Dawley (SD) rats were included in the study. After jugular venous catheterization was performed, fixed concentration of nicotine was administered in order to train the rats and establish the rat model of intravenous self-administration groups, receiving intraperitoneal injection of menthol, cineole, and dimethyl sulfoxide (DMSO), the vehicle that was used for the control group. The rats were tested with different fixed-ratio (FR) schedules, including FR1 schedule, in which the rat received one nicotine infusion for every active nose poke, FR2 schedule, in which the rat received one nicotine infusion for every two active nose pokes, and FR5 schedule, in which the rat received one nicotine infusion for every five active nose pokes. The number of active and inactive poke responses and the number of nicotine infusion were documented accordingly.

  Results  After 10 days of training in nicotine self-administration, the 30 rats demonstrated significant increase in the number of active poke responses and the number of nicotine infusion, which were maintained at a stable and relatively high level. The number of active poke responses was significantly higher that of inactive poke responses (P< 0.001). The rat model of intravenous nicotine self-administration was successfully established. In the testing phase, under the FR2 schedule, the menthol group showed a reduced number of active poke responses (P=0.020). Under the FR5 schedule, the groups showed obvious interaction between time and the number of active poke responses (P<0.011), with the menthol group showing reduced number of active poke responses on day three (P=0.011) and the cineole group showing rising number of active poke responses on day three (P=0.003). The DMSO control group did not show any significant change.

  Conclusions  Menthol and cineole are shown to have an effect on nicotine dependence. When there is relative difficulty involved in obtaining nicotine, menthol suppresses nicotine dependence, whereas cineole enhances nicotine dependence.