New research from the University of Colorado Boulder points to a little-known brain circuit that may determine whether transient pain subsides or becomes a long-lasting problem. The findings suggest that this circuit plays a key role in turning transient pain into chronic pain that can last for months or even years. The study, conducted in animals and published in the Journal of Neuroscience, focused on a region called the caudal granular insular cortex (CGIC). The researchers found that interrupting this circuit can both prevent the onset of chronic pain and stop it after it has already begun.
“In our study, we used several state-of-the-art methods to identify the specific brain circuit that is critical to whether pain becomes chronic and that instructs the spinal cord to do so,” said lead author Linda Watkins, distinguished professor of behavioral neuroscience in the College of Arts and Sciences. “If this crucial decision maker is turned off, chronic pain does not occur. If it is already present, it subsides.”
New Tools are Driving a “Neuroscience Gold Rush”
The paper comes at a time of rapid progress in brain research. First author Jayson Ball describes the current situation as a “neuroscience gold rush”, driven by advanced tools that allow scientists to precisely control specific groups of brain cells. This refers to modern methods such as chemogenetics, optogenetics and targeted genetic manipulation, which can be used to specifically switch individual neuronal circuits on or off. This makes it possible for the first time not only to examine broad regions of the brain, but also to identify very specifically the functional “decision-making units” within these regions that are responsible for diseases such as chronic pain.
Using these techniques, researchers can now localize the exact neural pathways involved in complex diseases and even specifically influence their activity. This high level of detail could significantly accelerate the development of new treatment methods. For example, targeted pharmacological infusions that only affect certain neuronal populations or sophisticated brain-machine interfaces that can measure and modulate activity patterns in the brain in real time are conceivable. In the long term, such approaches could represent safer alternatives to traditional painkillers such as opioids, which work effectively but are associated with side effects and risks of dependency.
“This study adds an important leaf to the tree of knowledge about chronic pain,” said Ball, who received his PhD in Watkins’ lab in May and now works for Neuralink, a California-based start-up that develops brain-machine interfaces for human health. Particularly important is the prospect that chronic pain could in future be addressed not only pharmacologically, but possibly also technically – for example through systems that specifically regulate neuronal activity and thus interrupt pathological pain loops.
When Pain Signals No Longer Stop
Chronic pain is a widespread problem. According to the Centers for Disease Control, about one in four adults suffers from it, and nearly one in ten say it interferes with their daily lives. A common feature of nerve-related pain is allodynia, a condition in which even light touch can feel painful.
Short-term and long-term pain behave very differently. Acute pain acts as a warning signal and occurs when injured tissue, such as a stubbed toe, sends signals to the brain via the spinal cord. This system serves to protect the body and normally switches off again as soon as the cause of the injury has been rectified. Chronic pain, on the other hand, persists even after the injury has healed and creates a kind of false alarm that can last for weeks, months or years. This leads to a permanent change in the nervous system: pain-conducting pathways remain overactive, while inhibitory mechanisms that normally dampen pain are weakened. As a result, the brain can interpret even harmless stimuli as painful, as if the body’s warning system is “stuck” and no longer returns to its normal state.
“Why and how pain does not subside and lead to chronic pain is a central question to which no answer has yet been found,” said Watkins. This uncertainty is also due to the fact that chronic pain is not only a physical, but also a neurobiological and partly psychological phenomenon. Research suggests that a kind of “pain memory” can develop in the spinal cord and brain, in which nerve cells remain permanently sensitized. At the same time, inflammatory processes following an injury can increase the sensitivity of the nervous system to stimuli in the long term. Emotional factors such as stress or anxiety also intensify these processes by influencing pain processing in the brain. This results in a complex interplay of overactive nerve pathways, altered brain networks and biological stress reactions, which explains why chronic pain is so difficult to break through again.
Targeting the Neural Pathway in the Brain that Perpetuates Pain
Earlier work from Watkins’ lab in 2011 pointed to the CGIC as an important factor in pain sensitivity. This small region, about the size of a sugar cube, is located deep in the insula, a part of the brain involved in processing sensory input, body awareness and emotional appraisal. Studies in humans have shown that this area tends to be overactive in people with chronic pain, suggesting that it plays a central role in maintaining persistent pain sensations. However, until recently it has been difficult to study this region in detail, as the only way to directly influence it was to surgically remove it – which of course is not a realistic treatment option.
In the new study, the team first used fluorescent proteins to visualize which nerve cells become active after a rat suffered an injury to the sciatic nerve. This allowed them to precisely track which neuronal networks remain activated in the post-injury phase. They then applied advanced “chemogenetic” methods, in which certain genes can be specifically switched on or off in selected neurons. This technique makes it possible to very specifically control the function of individual cell groups in the brain without affecting other areas.
The results revealed a surprising pattern: the CGIC appears to play only a subordinate role in the immediate processing of pain after an injury. Acute pain – i.e. the body’s rapid warning reaction – is primarily controlled by other neuronal networks. However, the CGIC is crucial for the long-term maintenance of pain. In other words: While other brain regions “report” the acute pain, the CGIC appears to be responsible for ensuring that this pain signal does not switch off again and turn into a chronic state.
This finding is particularly important because it shows that chronic pain is not simply an intensified form of acute pain, but is based on its own stable neuronal circuits. This opens up the possibility of specifically influencing precisely these long-term active networks without impairing the normal protective function of acute pain. This is precisely where new therapeutic approaches come in, which attempt to specifically modulate the CGIC or similar structures in order to interrupt chronic pain processes without switching off the entire pain system.
How the Brain Maintains Pain
The researchers found that the CGIC sends signals to the somatosensory cortex, which processes touch and pain and gives them a conscious meaning. This area is in turn connected to the spinal cord and influences the transmission of pain signals there. This creates a feedback loop that actively maintains the transmission of pain instead of stopping it again after an injury.
“We found that activating this signaling pathway stimulates the part of the spinal cord that transmits touch and pain to the brain, which means that touch is now also perceived as pain,” says Jayson Ball. This principle also explains allodynia, in which harmless stimuli have a painful effect because the nervous system has become hypersensitive.
If this signaling pathway is blocked shortly after an injury, the pain disappears normally again after healing. Even chronic pain could be stopped in the experiments by deactivating the circuit. “Our research provides clear evidence that certain signaling pathways in the brain can be targeted to modulate pain,” said Ball.
On the Way to New Treatment Methods for Chronic Pain
Researchers do not yet know what causes the CGIC to send out persistent pain signals, and further studies are needed before these findings can be transferred to humans. In particular, the question remains as to why certain neuronal cell groups remain permanently active after an original injury or illness, thereby maintaining a chronic pain signal even though the actual trigger has long since healed. This lack of “switching off” indicates that stable dysregulation mechanisms can develop in the brain that make the pain independent, so to speak.
Nevertheless, this work points to new treatment options. Ball envisions a future in which doctors use targeted injections or infusions to influence specific brain cells without the far-reaching side effects and addictive potential associated with opioids. In contrast to classic painkillers, which dampen the entire nervous system non-specifically, such approaches could modulate individual neuronal networks that are responsible for maintaining chronic pain signals in a very targeted manner. This could potentially allow pain to be regulated directly at its source without severely impairing other important brain functions.
He also points out that brain-machine interfaces, whether implanted or worn externally, could help in the treatment of severe chronic pain. In the future, these technologies could not only measure brain activity, but also specifically alter it and thus “recalibrate” disturbed pain networks. “Now that we have tools to manipulate the brain – not just at the level of a general region, but at the level of specific cell groups – the search for new treatments is progressing much faster,” he said. Overall, this development points to a fundamental change in pain therapy: away from broad-acting drugs towards highly precise, cell-based and possibly also technology-supported forms of treatment.