Effects of SCH 23390 and sulpiride on the reinforced responding of the young rat.

Reinforced responding of 17-day-old rat pups was assessed after blockade of dopamine D1 and D2 receptor systems. Rat pups were trained to traverse a straight alley for nipple attachment reward and then injected with various doses of either sulpiride (D2 antagonist), SCH 23390 (D1 antagonist), or a combination of sulpiride and SCH 23390. The approach performance of drug-treated pups was then compared with vehicle-treated pups on both reinforcement and extinction trials. SCH 23390, but not sulpiride, depressed the appetitive approach responding of the pups. During extinction testing, SCH 23390-treated pups had longer response latencies than pups given vehicle. "Best score" data suggested that SCH 23390 primarily affected reward processes and not motor capability. Most important, the effects of SCH 23390 on reinforced responding were potentiated by sulpiride, suggesting that both the D1 and D2 receptor systems are involved in reward processes.