The Effect of SCH 39166, Eticlopride, and Naltrexone on Responding for a CR in Adolescent Rats. SCH 39166 (0.06 mg/kg; D1 dopamine receptor), eticlopride (0.05 and 0.1 mg/kg; D2 dopamine receptor antagonist), and naltrexone (2.0 mg/kg; opioid receptor antagonist) decreased CR lever responding compared with saline as shown in panels a, b, and c, respectively. (**p<0.01: CR saline vs CR antagonist) The values are the means±SEM. CR, lever delivering light and tone; NCR, lever with no programmed consequence. SCH 39166, n=12; eticlopride, n=12; naltrexone n=12.