Why pain after injury becomes chronic

An injury can stop being acute while the pain remains. Why pain after injury becomes chronic is still only partly understood. A new mouse study points to an unexpected sequence of events in the spinal cord: injury alters cells that maintain nerve fibers, myelin becomes disrupted, and neurons begin producing Aβ42, an amyloid peptide best known from research on Alzheimer’s disease.

The study, published in Science Translational Medicine, focused not simply on acute pain but on the biological transition from an immediate response to persistent pain-related hypersensitivity. That transition matters because acute pain can serve a protective function, whereas persistent pain may continue long after that immediate function is useful.

The findings do not establish the mechanism of chronic pain in people. The key experiments were performed in mice. What makes the study notable is that the researchers did more than identify a molecular change associated with persistent pain: they disrupted the proposed pathway in several ways and prevented pain chronification in their animal models.

What changes after injury

In the main model, mice received a formalin injection into a hind paw to induce tissue injury and persistent pain-related hypersensitivity. On day four, the researchers analyzed individual nuclei from cells in the spinal cord and found particularly prominent metabolic changes in oligodendrocytes.

Oligodendrocytes are cells that produce and maintain myelin, the insulating material surrounding nerve fibers. After peripheral injury, these cells reduced expression of genes involved in producing myelin proteins while increasing pathways involved in lipid synthesis. The study suggests that these resources were being redirected toward neural remodeling rather than normal myelin maintenance.

The composition of spinal myelin changed, and axonal integrity became compromised. Then came the more unexpected part of the pathway.

Neurons accumulated amyloid precursor protein, or APP. At the same time, levels of BACE1, an enzyme that processes APP along the pathway that produces amyloid peptides, increased. During the critical period when persistent pain was developing, insoluble Aβ42 also increased in the spinal cord.

Aβ42 is widely studied because of its role in amyloid deposits associated with Alzheimer’s disease. The presence of the same peptide here does not make chronic pain a form of Alzheimer’s disease. The study instead identifies a localized amyloid-related process in the spinal cord of mice following peripheral injury.

What happened when the pathway was interrupted

The crucial question was whether Aβ42 was simply appearing alongside persistent pain or was helping drive the transition.

The researchers interfered with Aβ42 production, including through inhibition of BACE1, and also delivered an antibody targeting amyloid into the spinal fluid. These interventions prevented persistent pain-related hypersensitivity after injury while leaving the initial acute pain response intact.

That distinction is important. The interventions did not simply eliminate every response to a painful stimulus. In these models, they interfered specifically with the process by which an acute response became persistent.

The researchers also identified an upstream component of the pathway: N-acylethanolamine acid amidase, or NAAA, an enzyme involved in lipid metabolism. When NAAA was deleted specifically from oligodendrocytes, the injury-induced increase in Aβ42 was prevented and chronic pain-related behavior did not develop.

The result was reproduced in a second injury model involving ligation of the sciatic nerve. That makes it less likely that the pathway is merely an unusual feature of the formalin model.

Could chronic pain be stopped before it takes hold?

The study suggests a different way of thinking about pain chronification. After some injuries, there may be a limited period in which the nervous system is still being remodeled and persistent pain can potentially be prevented by disrupting the biological process that stabilizes it, rather than simply suppressing pain signals after chronic pain is established.

That possibility remains preclinical. It is not yet known whether people experience the same sequence of oligodendrocyte remodeling, myelin disruption, APP accumulation, BACE1 activation, and Aβ42 production after injury. Researchers would also need to establish when such a process occurs and whether it can be targeted safely.

Nor does the study provide a rationale for using currently available anti-amyloid Alzheimer’s drugs to treat chronic pain. The authors emphasize that the pathway first needs to be demonstrated and understood in humans.

The broader implication is more fundamental. In these experiments, chronic pain was not merely acute pain that lasted longer. There was a biological transition between the two, and that transition could be interrupted. If a comparable mechanism exists in humans, pain medicine may eventually be able to ask not only how established chronic pain can be treated, but whether it can sometimes be prevented from becoming chronic in the first place.