Incubation of nicotine craving in adolescent and adult rats. A, Timeline of the experimental procedure. B, Nicotine self-administration training in adolescents (n = 43) and adults (n = 43) rats. Data are mean ± SEM numbers of infusions and active and inactive lever presses during the 12 training days. C, Nicotine seeking in the extinction tests at different withdrawal days. Data are mean ± SEM numbers of nonreinforced presses on the previously active lever and on the inactive lever. *Different from day 1, p < 0.05 within each age group, p < 0.05. n = 7–13 rats per group. SA, Self-administation.