When your Brain When you need to pay attention to something important, one way it can do that is to create a burst of noradrenaline, according to a new MIT study.
Most of the brain’s noradrenaline is produced by the nucleus coeruleus at two loci, one in each cerebral hemisphere. Neurons of the locus coeruleus are marked with a green fluorescent protein.
Image credit: Gabi Drummond
This neuromodulator, generated by a structure deep in the brain called the locus coeruleus, could have brain-wide effects. In a study in mice, the MIT team discovered that an important role for noradrenaline, also known as norepinephrine, is to help the brain learn from surprising results.
“What this work shows us is that the locus coeruleus encodes unexpected events and pays attention to those events,” said Mriganka Sur, Newton Professor of Neuroscience in the Department of Brain and Cognition at MIT. surprisingly it is very important for the brain to grasp its environment. Sciences, a member of MIT’s Picower Institute for Learning and Memory, and director of the Simons Center for the Social Brain.
In addition to its role in signaling surprise, researchers have also found that noradrenaline helps stimulate behavior that leads to reward, especially in situations where it is uncertain whether the reward will be rewarded. given or not.
Sur is the senior author of new researchappear in Nature. Vincent Breton-Provencher, a former MIT postdoc now an assistant professor at Laval University, and Gabrielle Drummond, an MIT graduate student, are the lead authors of the paper.
Behavior modification
Noradrenaline is one of several neuromodulators that affect the brain, along with dopamine, serotonin, and acetylcholine. Unlike neurotransmitters, which allow cell-to-cell communication, neuromodulators are released over large regions of the brain, allowing them to produce more general effects.
“Neuromodulators are thought to perfuse large regions of the brain and thus alter the excitatory or inhibitory mechanism by which neurons are receiving in a point-like fashion- more points,” said Sur. “This suggests that they must have very important functions throughout the brain, important for survival and regulation of brain states.”
While scientists have learned much about the role of dopamine in the pursuit of motivation and reward, less is known about other neuromodulators, including noradrenaline. It has been linked to arousal and increased alertness, but too much noradrenaline can lead to anxiety.
Previous studies on the locus coeruleus, the brain’s main source of noradrenaline, have shown that it receives input from many parts of the brain and also sends signals far and wide. In the new study, the MIT team set out to investigate its role in a specific type of geometry known as reinforcement learning, or learning by trial and error.
For this study, researchers trained rats to push the lever when they heard high-frequency tones, but not when they heard low-frequency tones. When the mice responded correctly to the high-frequency tones, they received water, but if they pushed the lever when they heard the low-frequency tones, they received an unpleasant flow of air.
The mice also learned to push the lever harder when the alarm got louder. The lower the volume, the more uncertain they are about whether or not they should push. And, when the researchers inhibited the activity of the coeruleus locus, the rats became much more hesitant to push the lever when they heard a low volume, suggesting that noradrenaline boosts the chances of a reward in these situations. situations where the reward is uncertain.
“The animal is pushing because it wants a reward, and the locus coeruleus provides important signals to say, push because the reward will come,” says Sur.
The researchers also found that the neurons that produce this noradrenaline signal seem to send most of their output to the motor cortex, which provides further evidence that this signal arouses motor activity. action object.
Unexpected signal
While the first burst of noradrenaline seemed to stimulate the mice to action, the researchers also found that the second burst often occurred after the end of the trial. When the rats received an expected reward, these bursts were very small. However, when the outcome of the trial was surprising, the uproar was much greater. For example, when a rat received a blast of air instead of the reward it was expecting, the locus coeruleus sent out large amounts of noradrenaline.
In subsequent tests, that rat was less likely to push the lever when it was uncertain that it would receive a reward. “The animal is constantly adjusting its behavior,” says Sur. “Even though it has learned the task, it is adjusting its behavior based on what it just did.”
The rats also showed bursts of noradrenaline in the tests when they received an unexpected reward. These outbreaks seem to spread noradrenaline to many parts of the brain, including the prefrontal cortex, where planning and other higher cognitive functions take place.
“The unexpected coding function of the locus coeruleus seems to be much more common in the brain, and that could make sense because everything we do is unexpectedly censored,” Sur said.
The researchers now plan to explore the possible synergy between noradrenaline and other neuromodulators, especially dopamine, that also respond to unexpected rewards. They also hope to learn more about how the prefrontal cortex stores the short-term memory of input from the locus coeruleus to help animals improve their performance in future tests.
Written by Anne Trafton
Source: Massachusetts Institute of Technology
