D2-MSNs in the nucleus accumbens core are the primary factor limiting forced treadmill running

For many, the idea of exercise is a battle between the body's desire to move and the mind's resistance to the effort. While some people run for the sheer joy of it, others find themselves on a treadmill only because they feel they must, often leading to a sense of dread that makes them quit before they are truly exhausted. Scientists have long known that the brain contains a reward system, a network of cells that decides whether an action feels good or bad, but the precise mechanism that tells a person to stop when the activity is forced and unpleasant has remained a mystery. This question sits at the intersection of physical health and neuroscience, exploring why the brain sometimes acts as a gatekeeper, shutting down movement even when the muscles are still capable of going further.

A team of researchers at East China Normal University and the Shanghai Mental Health Center has now uncovered a specific neural circuit that appears to act as this gatekeeper. By studying mice on a treadmill, they discovered that a particular group of cells in a deep brain region called the nucleus accumbens is responsible for signaling the body to stop. This region is known to process motivation and emotion, but the researchers found that it is not the whole area that matters, but rather a specific sub-region and a specific type of cell within it. When the mice were forced to run, these cells became hyperactive, sending a strong signal that effectively told the animal to give up.

To understand how this works, the scientists first needed to see what was happening inside the brains of mice that were forced to run on a treadmill for two weeks. They compared these mice to others that simply sat on a stationary treadmill without moving. Using a technique that lights up active brain cells, they observed that the forced runners showed a significant increase in activity within a specific type of neuron called a D2-medium spiny neuron. These cells are found in the nucleus accumbens, a hub for motivation, but the researchers noticed something crucial: the activity was not uniform across the entire hub. It was concentrated in a small section known as the core, while a neighboring section called the shell showed much less change. This suggested that the core was the specific site where the brain was processing the unpleasantness of the forced exercise.

The team then looked deeper to see how these active cells were behaving. They removed tiny slices of brain tissue from the mice and measured the electrical signals of the D2-medium spiny neurons in the core. They found that after two weeks of forced running, these neurons had become much more excitable. They fired electrical spikes more frequently and received stronger excitatory signals from other parts of the brain. In contrast, the same type of neurons in the shell region did not show these changes. This indicated that the forced exercise had physically altered the wiring and sensitivity of the cells in the core, making them more sensitive to the stress of the activity.

To prove that these specific cells were actually causing the mice to stop running, the researchers performed a delicate experiment. They used a method called chemogenetics to temporarily turn off the activity of just these D2-medium spiny neurons in the core. They injected a virus into the brains of the mice that allowed them to switch these cells off with a simple drug injection. When they gave the drug to the mice before a maximum endurance test, the results were striking. The mice that had their "stop signal" turned off were able to run for significantly longer distances and for a longer time before exhaustion compared to the control mice. The drug did not make the mice faster or stronger; it simply removed the internal brake that was telling them to quit.

The researchers also checked to ensure that the drug itself or the injection process was not the cause of the improvement. They ran the same tests on mice that received a harmless injection, and those mice showed no change in their running ability. This confirmed that the extended endurance was directly due to silencing the specific neurons in the core. The study suggests that under normal circumstances, when an animal is forced to do something unpleasant, these neurons in the core of the nucleus accumbens become overactive. They act as a central brake, interpreting the stress and lack of pleasure as a reason to stop, which protects the body from overexertion but also leads to the abandonment of the exercise.

This discovery highlights a fundamental difference between how the brain handles voluntary exercise versus forced exercise. When a person or animal chooses to move, the brain's reward system encourages the behavior. But when the movement is forced, a different pathway takes over, specifically involving these D2-medium spiny neurons in the core of the nucleus accumbens. They translate the feeling of aversion into a command to stop. The study does not claim to have solved the problem of exercise adherence for humans, nor does it suggest a simple pill to make people love running. However, it provides a clear biological explanation for why forced exercise feels so difficult to sustain. It identifies a specific neural mechanism that limits persistence, offering a new target for future research into how to help people who struggle to maintain an exercise routine, particularly those who are forced into it for health reasons but lack the internal motivation to continue.