Researchers at Fudan University have identified a biological "emotion gate" in the brain that determines whether chronic pain will evolve into clinical depression, offering a potential breakthrough for millions burdened by both physical and mental illness.

The study, published on Friday in the journal Science, addresses a critical global health issue: nearly 60 percent of individuals with chronic pain also suffer from depression. While the link between the two has long been observed, the specific biological "switch" that triggers this emotional decline has remained a mystery until now.

The research team, based at Fudan University's Institute of Science and Technology for Brain-inspired Intelligence, found that this gate is located within the hippocampus. While generally known as the brain's center for memory and navigation, the hippocampus also acts as a primary hub for mood regulation.

By analyzing neuroimaging data from 30,000 people alongside animal models, the team discovered that the brain initially attempts to defend itself against pain.

"In the early stages of pain, the hippocampus temporarily enlarges and its functional state enhances," said Xiao Xiao, a co-corresponding author of the paper and executive director of the institute's behavioral and cognitive neuroscience center."However, as pain persists and depression sets in, the hippocampus gradually atrophies — meaning it wastes away — and its regulatory capacity declines."

The researchers narrowed the gate down to a specific sub-region called the dentate gyrus, or DG. This area is unique because it can produce new neurons, a process called plasticity that helps the brain adapt to stress.

The study found that in the early stages of pain, these new neurons are highly active. But as pain becomes chronic, microglia — the brain's primary immune cells — interfere. These cells, which normally act as the brain's "cleanup crew", become overactive and disrupt the balance of electrical signals in the brain. This disruption causes the "emotion gate" to fail, allowing physical pain to transform into a persistent negative emotional state.

The discovery suggests a new way to prevent depression in pain patients. Rather than just treating the pain itself, doctors could use brain imaging to monitor the health of the hippocampus and intervene before it begins to shrink.

The team found that while inhibiting new neurons could stop emotional decline, it caused other cognitive side effects. However, targeting the overactive microglia proved more effective.

"In our study, we used an established drug widely used clinically to regulate microglia, which could be transitioned to clinical use for patients more effectively and swiftly," Xiao said.

The researchers, who began this work in 2018, now plan to use artificial intelligence to screen for new small-molecule drugs that can target these specific brain cells. They believe that genetic factors and environmental influences likely determine why some patients' emotion gates are more resilient than others.