REINFORCED RESPONDING OF THE 11-DAY-OLD RAT PUP - SYNERGISTIC INTERACTION OF D1 AND D2 DOPAMINE-RECEPTORS

Reinforced responding of 11-day-old rat pups was assessed after blockade of D1 and D2 dopamine receptors. Initially, rat pups were trained to traverse a straight alley for nipple attachment reward. Rat pups were than injected IP with either the D1 antagonist SCH 23390 (0.01, 0.015, 0.03, or 0.1 mg/kg), the D2 antagonist sulpiride (15 or 50 mg/kg), or a combination of SCH 23390 (0.015 mg/kg) and sulpiride (15 mg/kg). The approach performance of drug-treated pups was then compared to vehicle-treated pups on both reinforcement and extinction trials. Sulpiride (15 mg/kg) did not affect either the extinction or reinforced responding of 11-day-old rat pups. In contrast, SCH 23390-treated pups showed significantly longer response latencies than the vehicle controls in both extinction and reinforcement conditions. Combined treatment with SCH 23390 and sulpiride produced the longest response latencies. Analyses of "best score" and frequency data indicated that the drug-induced decline in responding was due to effects on both reward processes and motor capability. The combined results indicate that D1 and D2 receptors interact complexly to affect reinforced responding.