Chronic pain is one of the most complex medical mysteries. The body may recover from a back injury, surgery, or a pinched nerve, but the pain persists for millions of people and does not go away completely.
The question that has long puzzled doctors is: why do some pains disappear as if they never existed, while others become a constant companion for life?
A study from the University of California, Irvine offers a surprising answer, and from an unexpected source: proteins associated with Alzheimer's disease itself.
The study, published in the journal Science Translational Medicine, suggests that chronic pain may stem from a specific biological process that begins immediately after injury.
If it is proven that this same path applies to humans, the treatment strategy may change in the future from simply relieving chronic pain to preventing it from becoming intractable from the outset.
The study was led by Professor Daniele Piomelli, and his team used mice to track the chain reactions triggered by pain in the spinal cord. After injecting a mouse's paw with a substance that caused tissue damage, they observed that cells called oligodendrocytes—responsible for maintaining the protective fatty sheath around nerves—began to lose their function, reducing the production of that sheath (myelin). This loss of function, in turn, affected neighboring nerve fibers, causing them to lose their structural integrity. The nerve cells responded by overproducing amyloid proteins, leading to the formation of beta-amyloid 42, the sticky fragments notorious for forming the brain plaques associated with Alzheimer's disease.
In mice, these fragments appeared in the spinal cord at the same time that the pain was turning from a transient symptom into a permanent condition.
"We weren't looking for a link with amyloid biology, but the data led us to it," says Biomeli. "What amazed us was that this pathway wasn't just a side effect; when we stopped it, chronic pain never formed."
To determine whether beta-amyloid was merely a witness to the event or a driver of it, the team used several independent approaches: genetically modified mice lacking the amyloid precursor protein (APP), antibodies that disable beta-amyloid, three chemically different drugs, and gene inactivation to prevent the production of the precursor protein.
In all cases, inhibiting amyloid production during the critical time window prevented acute pain from becoming chronic, while the initial response to injury remained intact.
The team also discovered the key driver behind this: an enzyme called N-acylethanolamine acid amidase, which is activated in oligodendrocytes after injury. Mice born without this enzyme in those specific cells did not experience an increase in beta-amyloid and did not develop chronic pain, even in another model of nerve injury, confirming that the mechanism is not simply a coincidence in one model.
"Today we have a mechanistic explanation for exactly when this shift occurs, and a real tool for intervention," Biomelli adds. "Our goal is not to treat the pain after it has become severe, but to catch it while it is still reversible."
If these findings are confirmed in humans, the implications will be significant. Most current treatments are administered only after pain has become chronic. This perspective suggests a different approach: monitoring and treating the biological process shortly after injury to prevent pain from becoming entrenched. It also raises a profound question: Is there a common biological basis for chronic pain and neurodegenerative diseases? Months after injury, deposits resembling Alzheimer's plaques appeared in the spinal cords of mice.
Biomeli explains that the next step is to verify that this pathway works in humans and that it can be targeted safely, warning that current Alzheimer's drugs are not a safe alternative for treating pain outside of their authorized use.
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