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 dopamine systems.

OBJECTIVES

The present experiments examined the effects of signalling a delayed, large reward on rats' ability to choose it over a small, immediate reward, and on the response to amphetamine, a dopamine receptor antagonist, and a benzodiazepine.

METHODS

Three groups of Lister hooded rats were tested on a two-lever discrete-trial delayed reinforcement task in which they chose one pellet delivered immediately or four pellets delivered after a delay. This delay increased from 0 to 60 s during each session. 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), and various behavioural manipulations.

RESULTS

Subjects' choice became and remained sensitive to the delay; the cue speeded learning. Amphetamine decreased choice of the large reinforcer in the No Cue group and increased it in the Cue group. alpha-Flupenthixol and chlordiazepoxide generally decreased preference for the delayed reinforcer; flupenthixol reduced the cue's effects, but chlordiazepoxide did not interact with the cue condition.

CONCLUSIONS

Signals present during a delay can enhance the ability of amphetamine to promote choice of delayed rewards.