Amphetamine interacts with cue stimuli to affect preference for delayed reinforcement
Rudolf Cardinal, Barry Everitt and Trevor Robbins
University of Cambridge, Downing Street, Cambridge CB2 3EB, UK.

   To examine the neuropharmacological basis of tolerance of delayed reward, three groups of food-restricted male Lister hooded rats (n = 8 for each group) were tested on a discrete trial delayed reinforcement choice task based on Evenden & Ryan (1996). Subjects initiated trials with a central nosepoke and were then offered the choice of a lever producing one pellet immediately and a lever producing four pellets after a delay. This delay increased in steps from 0 to 60 s in the course of a session, with two forced-choice trials and ten free-choice trials at each of five delays and a constant intertrial interval. Illumination of a houselight signalled the trial’s start: in the Houselight group, this remained on during the delay and feeding period. In the NoCue group, the houselight was extinguished at the moment of choice. In the Cue group, a light was illuminated over the delayed lever for the duration of the delay.
   Subjects’ choice became sensitive to the delay after several sessions of training, with the cue condition affecting their speed of acquisition but not their final choice behaviour. Once stable delay sensitivity had emerged, the rats were challenged with 0.3, 1.0 and 1.6 mg/kg d-amphetamine. The effects of amphetamine were found to interact with the cue condition such that amphetamine decreased choice of the large reinforcer in the NoCue group and increased it in the Cue group. Interpretations concerning clock and conditioned reinforcement effects are discussed.
   The rats retained sensitivity to the delay even after extended experience with the task. The effects of amphetamine will be compared with those of a benzodiazepine and a neuroleptic.

Reference
 Evenden, J. L. & Ryan, C. N. (1996). Psychopharmacology 128(2), 161-70.