Serotonin-producing neurons are mainly located in the brainstem. From there, they send axons all over the brain and spinal cord, reaching the hypothalamus, hippocampus, cerebral cortex, cerebellum, and pretty much every other region of the central nervous system. Image Credit: ResearchGate
Serotonin is widely thought to influence mood, but a new study suggests that it also contributes to how we make decisions and process rewards through brain imaging.
Researchers used simultaneous brain imaging (fPET and fMRI) to study individuals who performed monetary incentive tasks in which they could gain or lose money. This allowed them to measure changes in serotonin synthesis rates during different phases of the task.
Their results show that the effects of serotonin changes based on which part of the brain is involved and the phase of the task. In the ventral striatum, a brain region involved in reward processing and outcome evaluation, serotonin synthesis spiked by 21%, and these changes seem to be most closely associated with the moment when an individual receives an outcome (e.g., won/lost money). However, in the anterior insula, a region involved in processing salience and anticipation, serotonin synthesis surged by 41%, and its effect was more closely related to anticipation (waiting for results) than to receiving the outcome. Effectively, these surges had an inhibitory effect, meaning the serotonin was quieting down background brain noise so the mind could focus on the task.
These results suggest that serotonin is involved in both anticipation of rewards and processing outcomes, though these occur in different brain regions.
In addition, rather than regulating happiness and/or sadness, the role of serotonin appears more complex and regulatory, being linked to how the brain responds to important events, as well as being associated with motivation, attention, learning from experience and responding to salient events.
Thus, this research has expanded our understanding of serotonin beyond its traditional association with mood, showing it operates in much more flexible and dynamic ways, particularly in how we anticipate rewards, process outcomes, stay motivated, and adapt to important events.
Additional research will need to be performed to understand how these changes may differ in individuals suffering from various disorders (e.g., major depression, substance abuse/misuse).
Overall, this study demonstrates that serotonin is involved in how the brain manages rewards and behaviour in much more flexible ways than we previously understood.
Reference
Hahn, A., Reed, M. B., Murgaš, M., et al. (2025). Dynamics of human serotonin synthesis differentially link to reward anticipation and feedback. Molecular Psychiatry. https://doi.org/10.1038/s41380-024-02696-1
