The effects of d-amphetamine, chlordiazepoxide, alpha-flupenthixol and behavioural manipulations on choice of signalled and unsignalled delayed reinforcement in rats

Rationale. Inability to tolerate delays to reward is an important component of impulsive behaviour, and has been suggested to reflect dysfunction of d opamine systems. Objectives: The present experiments examined the effects o f signalling a delayed, large reward on rats' ability to choose it over a s mall, immediate reward, and on the response to amphetamine, a dopamine rece ptor antagonist, and a benzodiazepine. Methods. Three groups of Lister hood ed rats were tested on a two-lever discrete-trial delayed reinforcement tas k in which they chose one pellet delivered immediately or four pellets deli vered after a delay. This delay increased from 0 to 60 s during each sessio n. Trials began with illumination of a houselight: in the Houselight group, this remained on during the delay and feeding period. In the No Cue group, the houselight was extinguished at the moment of choice. In the Cue group, a stimulus light was illuminated during the delay. Once trained, the rats were challenged with d-amphetamine (0.3, 1.0, 1.6 mg/kg), chlordiazepoxide (1.0, 3.2, 5.6, 10 mg/kg), alpha -flupenthixol (0.125, 0.25, 0.5 mg/kg), an d various behavioural manipulations. Results: Subjects' choice became and r emained sensitive to the delay; the cue speeded learning. Amphetamine decre ased choice of the large reinforcer in the No Cue group and increased it in the Cue group, alpha -Flupenthixol and chlordiazepoxide generally decrease d preference for the delayed reinforcer; flupenthixol reduced the cue's eff ects, but chlordiazepoxide did not interact with the cue condition. Conclus ions: Signals present during a delay can enhance the ability of amphetamine to promote choice of delayed rewards.