Effects of dopaminergic drugs on delayed reward as a measure of impulsive behavior in rats

Rationale: Impulsive individuals exhibit an exaggerated preference for imme diate rewards over delayed but larger rewards, perhaps because they value t he delayed rewards less. Dopamine (DA) has been postulated to mediate the i ncentive value of rewards, thus it may also mediate the exaggerated prefere nce for immediate rewards in impulsive individuals. In this paper, we inves tigate the effects of DA agonists and antagonists on the value of delayed v ersus immediate rewards. Objectives: The study had three main objectives: ( 1) to determine the effects of D-1 and D-2 type DA antagonists on the value of delayed rewards, (2) to determine the effect of the indirect DA agonist d-amphetamine on the value of delayed rewards, (3) to determine the sensit ivity of the adjusting amount (AdjAmt) procedure to acute one-day changes i n delay to reward, amount of reward, deprivation level and starting amount. Methods: In the AdjAmt procedure, rats choose between an adjusting amount of water given immediately (adjusting alternative) and a constant 150 mu l water delayed by 4 s (standard alternative). The immediate amount of water is adjusted across trials until the rat chooses both alternatives with equa l frequency (indifference point). The final adjusted amount is an indicator of the subjective value of the standard alternative. Results: The D-1/D-2 antagonist flupenthixol (25, 50, and 100 mu g/2 kg) and the D-2 antagonist raclopride (40, 80, and 120 mu g/kg), decreased the indifference points, wh ereas the D-1 antagonist SCH 23390 (5, 10, and 20 mu g/kg) did not affect t he indifference point. All three DA antagonists slowed responding. The indi rect DA agonist amphetamine (0.5, and 1.0 mg/kg) had effects opposite to th ose of the DA antagonists, it decreased choice latency, increased the numbe r of trials completed and increased the indifference point. Decreasing the water deprivation level (6, 24, and 48 h) had no effect on the indifference points but slowed responding. Increasing the delay to reward (2, 4, and 8 s) and decreasing the amount of water available for choosing the standard a lternative (300, 150, 75 mu l) decreased the indifference point. Conclusion s: The results indicate that amphetamine increased the value of delayed rew ards (decreased impulsivity) and that D-2 but not D-1 receptor antagonists decreased the value of delayed rewards (increased impulsivity). The procedu re was sensitive to acute I-day changes in delay and magnitude of reward.