Serotonin is a chemical messenger in the body linked to a wide variety of behaviors, from mood and sleep to cravings and spontaneity.
In a new mouse study, Japanese researchers at the Neural Computation Unit at the Okinawa Institute of Science and Technology Graduate University (OIST) investigated how serotonin might influence one’s ability to stay patient while waiting for a reward.
The findings are published in the journal Nature Communications.
The power of serotonin over human behavior has made it a key focus in the treatment of mental health conditions such as depression, particularly with the use of selective serotonin receptor inhibitors (SSRIs). These drugs slow down the reabsorption of serotonin and keep it active in the brain.
“Serotonin has had a lot of study in pharmacology, and serotonergic drugs are commonly prescribed, but the role that serotonin has over behavior isn’t clear,” said study author Dr. Katsuhiko Miyazaki. He conducted the study with Dr. Kayoko Miyazaki.
For the study, the mice were trained to perform a task to receive a food reward: place their nose into a small hole and wait — dubbed a “nose poke.” After a predetermined period of time, the reward was delivered.
In a previous study, the research team used a method which allows scientists to use light to stimulate specific neurons with precise timing — this technique is known as optogenetics. The neurons are genetically modified to a produce a light-sensitive protein that are then stimulated by shining light along a fiber optic implanted in the brain.
In the study, serotonin-producing neurons were optogenetically stimulated in a brain region called the dorsal raphe nucleus (DRN), which outputs widely to the forebrain. The findings show that increasing the activity of serotonin neurons in the DRN significantly extended the amount of time the mice were willing to wait for a food reward.
But would mice respond similarly in circumstances when getting a reward was uncertain? In other words, would mice wait for food regardless of the probability and timing of it turning up, or would they give up?
The researchers found that there are limits to serotonin’s ability to enhance one’s patience — in fact, the certainty of the reward significantly factors into the time a mouse is willing to wait.
For example, the mice were given a nose-poke trial with a 75 percent chance of a reward and a 3-second waiting period before the reward was delivered. When these mice were given no reward, their waiting time was prolonged. However, in tests where the chance of reward delivery following a nose-poke was 50 percent or 25 percent, increasing serotonin had no effect on their’ waiting time.
“The patience effect only works when the mouse thinks there is a high probability of reward,” said Katsuhiko Miyazaki.
The findings show that the relationship between the activation of serotonin and subsequent behavior is highly dependent on the rodents’ belief about the circumstances. These results may have implications for our understanding of how humans taking serotonin boosting drugs can also be affected.
“This could help explain why combined treatment of depression with SSRIs and cognitive-behavioral therapy is more effective than just SSRIs alone,” said Katsuhiko Miyazaki. “The psychological boost of the therapy is enhanced by raised serotonin levels.”