Peripheral post-training administration of 4-OH amphetamine enhances retention of a reduction in reward magnitude

This experiment examined the effect of post-training systemic (sc) injections of 4-OH amphetamine on memory for a reduction in reward. Rats were trained to run in a straight alley (six trials per day, 30-s intertrial interval) and received either 1 or 10 45-mg sucrose pellets on each trial. On Day 8, rats receiving 10 pellets were shifted to the 1-pellet reward. Shifted rats displayed a characteristic increase in response latencies typically attributed to an aversive emotional response to reward reduction. Immediately after the reward shift, all rats received an injection of either 2 mg/kg of 4-OH amphetamine or the equivalent volume of physiological saline. The rats were then returned to their home cages for 6 days without further training. On Day 15 the rats were returned to training, all with 1 sucrose pellet as reward. The performance of unshifted rats was comparable to that on Day 8 and the first trial performance of shifted rats given post-training saline was not significantly different from that of unshifted rats. Further, the shifted saline group's mean latencies for the entire training block on Day 15 were comparable to their mean latencies on Day 8. These findings suggest that the shifted saline rats did not remember the prior reward reduction on Day 8. In contrast, the shifted rats given post-training 4-OH amphetamine displayed first trial latencies that were longer than those of all other groups on Day 15. Additionally, on the entire training block on Day 15 the latencies of the shifted 4-OH amphetamine group were longer than those recorded on Day 8, indicating that shifted 4-OH amphetamine rates retained memory of the prior reward reduction. These findings suggest that post-training activation of peripheral adrenergic systems by 4-OH amphetamine enhances the retention for aversive emotional consequences elicited by reward reduction.