Amygdala modulates memory for changes in reward magnitude: reversible post-training inactivation with lidocaine attenuates the response to a reduction in reward.

The present study used a reward reduction paradigm to examine the role of the amygdala in memory for reduction in reward magnitude. Male Sprague-Dawley rats were implanted with bilateral amygdala cannulae and trained to run a straight alley (6 trials/day) for either ten or one 45-mg food pellets. On Day 10 of training, half the animals in the 10 pellet reward group were shifted to a one pellet reward. Immediately following shifted trials, the animals received an intra-amygdala injection of either a 2% lidocaine solution or phosphate buffer (0.5 microliter/side). Shifted training continued for 2 more days and no further injections were given. Shifted animals that received a buffer injection displayed a sharp increase in response latencies compared to unshifted controls on the second day of shifted training. In contrast, shifted animals that received intra-amygdala injections of lidocaine exhibited significantly lower latencies compared to the shifted vehicle group on the second day of shifted training. The findings indicate that post-training inactivation of the amygdala attenuates the response to reward reduction, suggesting that the amygdala modulates the storage for a reduction in reward magnitude.