When mice trained to navigate a virtual maze suddenly faced a changed reward pathway, their brains flooded with a chemical messenger called acetylcholine—and they quickly changed course. That small moment of neural adjustment, captured in real time by researchers at Japan's Okinawa Institute of Science and Technology (OIST), may hold secrets to helping millions of people break free from addiction, obsessive compulsive disorder, and the rigid behavior patterns of Parkinson's disease.
Behavioral flexibility—the ability to abandon an old strategy when circumstances shift—feels intuitive to most of us. When a job interview takes an unexpected turn, or a familiar route is suddenly closed, we pivot. But neuroscience has long struggled to explain how the brain performs this seemingly simple feat. The answer, it turns out, lies in acetylcholine, a neurotransmitter released by specialized brain cells called cholinergic interneurons, located in a region called the striatum.
"The brain mechanisms behind changing behaviors have remained elusive, because adapting to a given scenario is very neurologically complex. It requires interconnected activity across multiple areas of the brain," explains Professor Jeffery Wickens, head of the Neurobiology Research Unit at OIST and co-author of the study, which was published in Nature Communications. Using advanced two-photon microscopy, the team watched acetylcholine release in action as mice confronted the shock of a broken expectation. When the reward disappeared, acetylcholine surged in specific brain regions, and the mice showed what researchers call "lose-shift" behavior—they tried a different path through the maze instead of stubbornly repeating the old route.
The more acetylcholine that flooded the brain, the more likely the mice were to change their future choices. Dr. Gideon Sarpong, the study's first author, describes the elegant clarity of the finding: "The greater the increase in acetylcholine the more likely the mice were to change their future choices. Our results demonstrated the importance of acetylcholine in breaking habits and enabling new choices to be made." To confirm acetylcholine's causal role, researchers reduced the animals' ability to produce it—and the mice became stubborn, far less likely to adjust their decisions after an unexpected disappointment.
What makes the discovery particularly intriguing is that not all cholinergic neurons responded identically. While most released more acetylcholine, some small clusters actually decreased their activity. This nuance suggests the brain doesn't simply erase old successful strategies; it preserves them as backup information in case the world changes again. "This indicates that the mice may not necessarily forget the previous pathway to reward, but retain this information in case the situation changes again," says Dr. Sarpong.
The implications extend far beyond laboratory mice. Conditions marked by behavioral rigidity—addiction, obsessive compulsive disorder, Parkinson's disease—often involve disrupted acetylcholine signaling. Professor Wickens emphasizes that behavioral flexibility involves a much larger network of brain regions and chemical systems working in concert. But understanding acetylcholine's specific role opens a door. "In particular, with conditions such as addiction and obsessive-compulsive disorder we see a difficulty in breaking habits and shifting behavior," he notes. "So, understanding the mechanics of behavioral flexibility may one day help us develop better treatments."
The research reveals that the brain's capacity to adapt isn't magic—it's chemistry, and now scientists have a clearer map of how it works.