New Brain Circuit Discovery Explains Why Chronic Pain Won’t Shut Off, Points To Possible Fix

Stub a toe and it hurts for a minute. For roughly 50 million Americans, that persistent, aching sensation never fully goes away, not after days, not after months, sometimes not after years. Scientists have struggled for decades to explain why the body’s alarm system sometimes gets permanently stuck in the “on” position. A team at Stanford University may have found the answer.

Working in mice, the researchers traced a previously unknown circuit loop in the nervous system that appears to keep chronic pain alive long after an injury has healed. It runs from the spinal cord up through the brain and back down again, functioning like a faulty feedback system: once hijacked by injury, it locks the body into a state of lasting sensitivity, particularly to touch and pressure. Shutting down any single stop along this circuit eliminated chronic pain in mice, while normal, protective pain stayed intact.

Published in the journal Nature, the work opens a new window into one of medicine’s most stubborn problems and points toward potential treatments that would work very differently from opioids.

Scientists Had a Suspect but No Proof

Pain researchers have long known that a small cluster of nerve cells in the brainstem, in a region called the rostral ventromedial medulla, plays some role in chronic pain. Known as “on-cells,” these neurons fire when something hurts and go quiet when morphine is administered. But no one had been able to selectively switch them on and off in living animals to test what they actually do, and how injury signals from the body were reaching them had not been clearly mapped.

Senior author Xiaoke Chen and co-first authors Qian Wang, Joo Han Lee, and Gregory Nachtrab built new genetic tools to answer both questions. Using a custom-engineered virus, one designed to travel backward along neurons and let researchers trace their connections, they gained precise access to these brainstem neurons in mice, able to record them, silence them, activate them, or destroy them without disturbing neighboring cells.

What Happened When the Chronic Pain Circuit Was Silenced

After surgically inducing nerve damage, a procedure that mimics the kind of injury behind many human chronic pain conditions, the brainstem neurons became dramatically more reactive. Responses to pressure and touch grew larger over the first week and stayed elevated for at least 28 days.

Destroying these neurons before injury prevented chronic mechanical pain. Silencing them after it was already established, even 28 days out, a point when this type of pain typically stops responding to morphine, returned thresholds close to normal levels. Spontaneous pain behaviors including paw licking and flinching dropped off. Facial grimacing scores, measured by an automated scale, normalized.

Activating or silencing these neurons in healthy, uninjured mice had almost no effect. They appeared to matter only when the pain system was already stuck in overdrive.

An Unexpected Brain Region Drives the Loop

Conventional wisdom pointed to a structure called the periaqueductal gray, long considered the command center of pain control, as the likely source of input driving these brainstem neurons. When the researchers silenced that connection, it had little to no effect on chronic pain.

Instead, the critical signal came from somewhere unexpected: the lateral superior colliculus, a region better known for helping orient the eyes and head toward objects of interest. Silencing its connection to the brainstem neurons eliminated chronic mechanical pain in injured mice. Activating it once daily for seven consecutive days, but not in a single session, created lasting mechanical pain sensitivity in healthy, uninjured mice where none had existed before.

Tracing further upstream, the researchers found that the lateral superior colliculus was receiving pain-related signals from the brain’s primary touch-processing center, which in turn was getting injury information from the spinal cord through two parallel routes via the thalamus, a deep brain relay station.

Why Chronic Pain Is So Hard to Override

Put together, the circuit forms a continuous loop: spinal cord to thalamus to touch cortex to lateral superior colliculus to brainstem and back to the spinal cord. Pain signals travel up, get processed, and a descending command returns, sending signals back that amplify pain sensitivity. A self-reinforcing cycle that can outlast any injury.

One of the more sobering findings involves those two parallel thalamic routes. Blocking either one alone had no effect on chronic pain, because the other compensated. Only silencing both simultaneously broke the cycle. That built-in redundancy may help explain why chronic pain resists treatment so stubbornly.

Critically, disrupting the circuit left normal pain sensation untouched. Mice could still feel dangerous stimuli and still distinguish between a smooth surface and sandpaper, basic sensory abilities that rely on separate pathways. Separating chronic pain from protective pain has been a core goal of pain medicine for decades.

Mouse studies do not automatically translate to human treatments, and the researchers frame this as foundational circuit mapping rather than a near-term therapy. But the potential is hard to dismiss. In these experiments, targeting any single node in the loop was enough to break the cycle. Researchers also identified a nearby brain structure containing mostly inhibitory neurons that connects to two separate nodes of the descending pathway at once. Activating those inputs could, in theory, suppress the entire loop at two points simultaneously, making it what the authors describe as “a powerful therapeutic target for alleviating chronic mechanical pain.”

Chronic pain is the leading cause of disability worldwide. For the millions living with it, a therapy that targets the circuit keeping them trapped rather than numbing the entire nervous system could mark a major shift in how medicine approaches suffering.

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Disclaimer: This article describes research conducted entirely in mice. Findings have not been tested in humans and do not constitute medical advice. Consult a qualified healthcare provider for questions about chronic pain or treatment options.

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