Rationale: Peak initiation of smoking occurs during adolescence and early onset of smoking is associated with a reduced probability of quitting and greater risk of relapse compared to later onset. Considering the epidemiological evidence, adolescents may exhibit a unique biological susceptibility to nicotine taking, in addition to the behavioural and psychosocial factors known to influence adolescent smoking. Objectives: The current series of experiments, using a rat model of adolescent nicotine taking, was designed to investigate age differences in the processes involved in the acquisition and maintenance of nicotine taking that might account for the elevated initiation rates of smoking during adolescence. Methods: We first investigated age differences in the neural response to acute nicotine administration using c-fos mRNA expression. We then examined age differences in the rewarding and aversive effects of nicotine in the conditioned place preference (CPP) and conditioned taste avoidance (CTA) paradigms, respectively. The direct reinforcing effects of nicotine were tested in adolescent and adult rats under a variety of reinforcement schedules in the operant intravenous self-administration paradigm; extinction and nicotine priming-induced reinstatement were also examined. Finally, age differences in nicotine withdrawal precipitated by mecamylamine were assessed. Results: Nicotine had greater activational effects on c-fos mRNA expression in reward-related neural substrates of adolescent compared to adult brain. Adolescent rats were also more sensitive to the rewarding effects of nicotine (CPP) yet less sensitive to its aversive effects (CTA) compared to adult rats. Nicotine was equally reinforcing in adolescents and adults self-administering under simple reinforcement schedules, but adults were more motivated to obtain nicotine under higher reinforcement schedules. Adults were more resistant to extinction, yet both age groups demonstrated similar priming-induced reinstatement of nicotine seeking. Under spontaneous acquisition conditions, adults were more sensitive to the reinforcing effects of a low nicotine infusion dose. The aversive effects of nicotine withdrawal were also more prominent in adults compared to adolescents. Conclusions: These findings have important implications since they demonstrate a unique susceptibility to the conditioned rewarding effects of nicotine that would promote acquisition of smoking behaviour during adolescence, whereas adults may be more vulnerable to processes involved in its maintenance.

Rationale: Peak initiation of smoking occurs during adolescence and early onset of smoking is associated with a reduced probability of quitting and greater risk of relapse compared to later onset. Considering the epidemiological evidence, adolescents may exhibit a unique biological susceptibility to nicotine taking, in addition to the behavioural and psychosocial factors known to influence adolescent smoking. Objectives: The current series of experiments, using a rat model of adolescent nicotine taking, was designed to investigate age differences in the processes involved in the acquisition and maintenance of nicotine taking that might account for the elevated initiation rates of smoking during adolescence. Methods: We first investigated age differences in the neural response to acute nicotine administration using c-fos mRNA expression. We then examined age differences in the rewarding and aversive effects of nicotine in the conditioned place preference (CPP) and conditioned taste avoidance (CTA) paradigms, respectively. The direct reinforcing effects of nicotine were tested in adolescent and adult rats under a variety of reinforcement schedules in the operant intravenous self-administration paradigm; extinction and nicotine priming-induced reinstatement were also examined. Finally, age differences in nicotine withdrawal precipitated by mecamylamine were assessed. Results: Nicotine had greater activational effects on c-fos mRNA expression in reward-related neural substrates of adolescent compared to adult brain. Adolescent rats were also more sensitive to the rewarding effects of nicotine (CPP) yet less sensitive to its aversive effects (CTA) compared to adult rats. Nicotine was equally reinforcing in adolescents and adults self-administering under simple reinforcement schedules, but adults were more motivated to obtain nicotine under higher reinforcement schedules. Adults were more resistant to extinction, yet both age groups demonstrated similar priming-induced reinstatement of nicotine seeking. Under spontaneous acquisition conditions, adults were more sensitive to the reinforcing effects of a low nicotine infusion dose. The aversive effects of nicotine withdrawal were also more prominent in adults compared to adolescents. Conclusions: These findings have important implications since they demonstrate a unique susceptibility to the conditioned rewarding effects of nicotine that would promote acquisition of smoking behaviour during adolescence, whereas adults may be more vulnerable to processes involved in its maintenance.