Researchers have begun to unravel how serotonin acts, based on data collected in a first-of-its-kind experiment that utilized electrochemical probes implanted into the brain of awake human beings. The neurotransmitter serotonin is associated with mood and helps shape the decisions they make. The study is published in the journal Neuropsychopharmacology.
The readings were collected during brain surgery as patients played an investment game before receiving deep brain stimulation as a treatment to attempt to alleviate symptoms of Parkinson's disease. The research provides the first-ever recordings of simultaneous sub-second fluctuations in dopamine and serotonin during active decision-making in a conscious human subject.
The analysis provides new understanding of serotonin's role in regulating human choice and how it operates alongside dopamine, a neurotransmitter long associated with reward and its reinforcement. They take measurements of patients undergoing deep brain stimulation who volunteered to take part in the study. As the neurotransmitter is implicated in prevalent neuropsychiatric disorders, including depression, the researchers aim to uncover how the chemical aids humans in developing adaptive actions.
They found that serotonin is highly active in the part of the brain that helps us to navigate bad outcomes in a way that ensures we don't overreact to them, said Rosalyn Moran. "Serotonin acts in a way that reminds us to pay attention and learn from bad things, and to promote behaviours that are less risk-seeking but also less risk averse," Moran said. "When there's an imbalance of serotonin, you might hide in a corner or run towards the fire, when you should be doing something in between."
The researchers refer to this middle-of-the-road behaviour promotion as a "keep calm and carry on" motif. Here, serotonin appears to temper excitement over positive outcomes while softening the potential disappointment of negative outcomes. This process can go awry when the neurotransmitter levels aren't in balance.
According to the National Institute of Mental Health, more than 10 million adults in the United States suffered at least one major depressive episode. About half of those people take antidepressants, which primarily consist of selective serotonin reuptake inhibitors. The drugs are designed to keep serotonin at elevated levels in a person's brain by limiting its reabsorption.
The team will soon begin data collection at Carilion Clinic, through collaboration with Mark Witcher, a functional neurosurgeon. Witcher, who is also an author on the paper, became a collaborator on the project when he was the chief surgery resident at Wake Forest School of Medicine before becoming an attending physician at Carilion Clinic and assistant professor of surgery in the division of neurosurgery at the Virginia Tech Carilion School of Medicine.