@phdthesis{oai:oist.repo.nii.ac.jp:00002549,
 author = {Taira, Masakazu},
 month = {2022-02-09, 2022-02-09},
 note = {Serotonin (5-HT) is an important neuromodulator in reward-driven learning and decision making. The dorsal raphe nucleus (DRN) sends diffuse 5-HT projections throughout the brain. The involvement of DRN 5-HT neurons in reward-based behaviors has been examined using various types of behavioral tasks; however, how DRN 5-HT affects computational processes of decision making remains unclear. Reinforcement learning (RL) is a theoretical framework to describe the decision making process. Previous studies based on the RL framework have proposed hypotheses on the role of 5-HT in decision making, such as temporal discounting and model-based value computation. The overall aim of this thesis is to examine these hypotheses by analyzing behaviors under optogenetic manipulation, thereby clarifying the role of DRN 5-HT neurons in reward-based behaviors. The first hypothesis is that 5-HT modulates the relative importance of future rewards. Previous behavioral studies showed that 5-HT activation enhances patience to wait for future rewards and vice versa. However, how 5-HT regulates persistence to act for future rewards remains unknown. In the first part of my thesis research, I trained mice to perform a free-operant lever-pressing task, in which motor action rather than stationary waiting was required to obtain delayed rewards. In testing the effects of optogenetic activation and inhibition of 5-HT neurons on sustained motor actions, I found that optogenetic activation or inhibition of 5-HT neurons did not affect persistence in motor actions but an effect of the activation on slowing down response vigor, suggesting a different role of 5-HT neurons in motor actions for future rewards compared to stationary waiting. The second hypothesis examined is that 5- HT affects model-based decision making. In model-based decision making, agents use their own internal models of action-outcome relationships to plan forward and to select actions. Previous computational studies proposed facilitation of model-based decision making by 5-HT neurons, but behavioral evidence of how 5-HT regulates the process is still limited. A two-step decision making task is an established behavioral task to understand model-based decision making. In the second half of my thesis project, I trained mice to perform the two-step decision making task and found that optogenetic inhibition of 5-HT neurons affected choice behaviors and reduced time to make decisions possibly reflecting the disruption of model-based decision making. By fitting behavioral data to a model-free/model-based hybrid model, I found that photoinhibition of 5-HT neurons decreased the weight of model-based decision making. These results revealed the role of 5-HT neurons reward-based behaviors and model-based computations.},
 school = {Okinawa Institute of Science and Technology Graduate University},
 title = {報酬に基づく行動におけるセロトニンの制御に関する研究},
 year = {}
}