Differential effects of methamphetamine and cocaine on conditioned place preference and locomotor activity in adult and adolescent male rats

Human and animal laboratory studies show that adolescents and adults respond differently to drugs and that drug administration during adolescence leads to different behavioral effects than during adulthood. Although there are a number of studies on the effects of cocaine, little is known about the effects of methamphetamine in adolescent vs adult rats. In the present study, sensitivity to the conditioned reward of multiple doses of methamphetamine or cocaine was evaluated in male adolescent (PND 34) and adult (PND 66) rats using a conditioned place preference (CPP) paradigm. In addition, the locomotor-activating effects of methamphetamine were determined across a 5-day period of administration. After 3 days of training with cocaine, both adolescent and adult male rats developed CPP to cocaine, however, the dose-effect curve for cocaine CPP was shifted to the left in adolescent compared to adult rats. In contrast to the development of CPP to cocaine in both groups after 3 days of conditioning, methamphetamine CPP occurred only in adolescent, and not in adult rats. After 5 days of training, however, both adolescent and adult rats exhibited identical responses to multiple doses of methamphetamine and a significant CPP was observed in both groups. Daily administration of methamphetamine increased locomotor activity in both adolescent and adult rats, with a greater effect seen in the adults. In neither group, was there evidence of a significant sensitization to the locomotor-activating effects of methamphetamine. These data show that adolescents are more sensitive to psychostimulant reward and thus to the conditioned rewarding properties of cocaine or methamphetamine than adults. A better understanding of this difference may lead to age-specific preventions and treatments for psychostimulant abuse.