A brain region normally associated with the body’s natural ability to suppress pain can change roles after nerve injury, becoming a driver of chronic pain instead. New research in mice suggests that restoring a specific opioid signaling system in this region can reverse pain hypersensitivity, potentially pointing toward more targeted treatments for neuropathic pain.
The study, published in Current Biology, focused on the locus coeruleus (LC), a small brainstem region that produces norepinephrine and helps regulate alertness, stress responses, and pain. Although the LC can suppress acute pain, previous research has suggested that nerve injury can alter its activity so that it instead helps sustain chronic pain.
Researchers at Washington University School of Medicine investigated how that transformation occurs and whether the process could be reversed. Using a mouse model of long-term nerve injury, the researchers first used optogenetics to temporarily inhibit LC neurons. Four weeks after injury, shutting down LC activity reduced hypersensitivity, supporting the idea that the region had become a pain generator. The effect was particularly associated with LC neurons projecting to the medial prefrontal cortex, a brain region involved in cognition, emotion, and pain processing.
The investigators then examined mu opioid receptors (MORs) on LC neurons. These receptors respond to the body’s endogenous opioids and normally inhibit LC activity. When researchers selectively removed MORs from different LC circuits, they found strikingly different effects: loss of MOR signaling in LC neurons projecting to the medial prefrontal cortex increased pain sensitivity, while removing the receptors from LC neurons projecting to the spinal cord reduced it.
The findings suggest that opioid signaling within the LC does not operate as a simple on-off switch. Instead, its effect on pain depends on which neural pathway is being regulated.
Most importantly, restoring MOR signaling—or restoring the receptors themselves—in LC neurons reversed hypersensitivity following spared nerve injury in mice. The results suggest that nerve injury may weaken a natural opioid-mediated brake that ordinarily prevents the LC from sustaining pain.
“Importantly, our work also adds critical new evidence that loss of LC-MOR may underlie the transition from an acute analgesia system to a chronic pain generator,” the authors wrote.
The ability to reverse hypersensitivity by restoring the receptors also indicates that the body’s endogenous opioids remain available after injury and can activate the reinstated receptors.
The work remains preclinical, and researchers will need to determine how LC opioid signaling changes over the course of neuropathic pain and whether the pathway can be safely and selectively targeted in humans. But the findings raise the possibility of treating chronic pain by restoring localized opioid signaling rather than broadly activating opioid receptors throughout the nervous system.
“Altogether, these results suggest rescued MOR function in the LC may be a therapeutic target for the treatment of chronic pain,” the authors concluded.
