Enhanced incentive motivation for sucrose-paired cues in adolescent rats: possible roles for dopamine and opioid systems.

Vulnerability to the effects of drugs of abuse during adolescence may be related to altered incentive motivation, a process believed to be important in addiction. Incentive motivation can be seen when a neutral stimulus acquires motivational properties through repeated association with a primary reinforcer. We compared adolescent (postnatal day (PND) 24-50) and adult (>PND 70) rats on a measure of incentive motivation: responding for a conditioned reinforcer (CR). Rats learned to associate the delivery of 0.1 ml of 10% sucrose with a conditioned stimulus (CS; light and tone); 30 pairings per day were given over 14 days. Then, we measured responding on a lever delivering the CS (now a CR) after injections of amphetamine (0, 0.25 or 0.5 mg/kg). We also examined responding for CR when the CS and sucrose were paired or unpaired during conditioning, and responding for the primary reinforcer (10% sucrose) in control experiments. Finally, we examined the effects of D(1) and D(2) dopamine receptor antagonists (SCH 39166 and eticlopride, respectively) and an opioid receptor antagonist (naltrexone) on responding for a CR in adolescent rats. Adolescents but not adults acquired responding for a CR, but adolescents responded less than adults for the primary reinforcer. Responding for a CR depended upon the pairing of the CS and sucrose during conditioning. Both dopamine and opioid receptor antagonists reduced responding for the CR. Therefore, incentive motivation may be enhanced in adolescents compared with adults, and incentive motivation may be mediated in part by both dopamine and opioid systems.