Examination of Age Differences in Incentive Motivation and Impulsivity as Possible Contributing Factors to a Susceptibility to the Effects of Drugs of Abuse during Adolescence

Rationale. Adolescence may be a period of susceptibility to the effects of drugs of abuse. This vulnerability may result from a convergence of psychological processes that contribute to drug addiction including impulsive action and incentive motivation during adolescence. Objectives. I examined age differences in incentive motivation, as measured by responding for a conditioned reinforcer (CR) previously paired with natural or drug rewards, and age and sex differences in impulsive action, as measured by responding on a differential reinforcement of low rates of responding (DRL) schedule or premature responding on the 2-Choice Serial Reaction Time Test (2-CSRTT), in Sprague-Dawley rats. The effects of drugs that affect these behaviours in adulthood and that act on neurochemical systems still developing during adolescence were also examined. Methods. In a first set of experiments (Chapter 3), I compared male adolescents and adults on responding for a CR previously paired with sucrose and the effect of amphetamine on this behaviour. In a second set of experiments (Chapter 4), I examined age differences in responding for a CR previously paired with passive or self-administered intravenous (IV) nicotine infusions. Subsequently, I investigated age and sex differences in responding on a DRL schedule in response to amphetamine (Chapter 5) and 2-CSRTT performance in response to amphetamine, nicotine and Ro 63-1908 (a glutamate N-Methyl-D-aspartic acid [NMDA] receptor subunit antagonist; Chapter 6). Results. Compared to adults, adolescents responded more for a CR previously paired with sucrose or passive, but not self-administered, IV nicotine infusions. Amphetamine only enhanced responding for a CR previously paired with sucrose. Adolescents responded more than adults on a DRL schedule, while adolescents made the most premature responses in the 2-CSRTT. No consistent sex differences were observed during the acquisition of either behaviour. Amphetamine increased premature responding most in adolescent males and adult females in the 2-CSRTT but not in responding on the DRL schedule. No consistent age or sex differences were observed for Ro 63-1908 or nicotine. Conclusions. Adolescents show enhanced impulsivity and incentive motivation than adults depending on the behavioural measure. Dopamine may contribute to age and sex differences in these behaviours.Ph