The effect of 5-HT receptor antagonist on reward-based decision-making.

When choosing the best action from several alternatives, we compare each value that depends on the balance between benefit and cost. Previous studies have shown that animals and humans with low brain serotonin (5-HT) level tend to choose smaller immediate reward. We used a decision-making schedule task to investigate whether 5-HT receptor is responsible for the decisions related to reward. In this task, the monkeys chose either of two different alternatives that were comprised of 1-4 drops of liquid reward (benefit) and 1-4 repeats of a color discrimination trial (workload cost), then executed the chosen schedule. By the administration of 5-HT antagonist, WAY100635, the choice tendency did not change, however, the sensitivity to the amount of reward in the schedule part was diminished. The 5-HT could have a role in maintaining reward value to keep track with the promised reward rather than modulating workload discounting of reward value.