The bodies of mammals, including humans, respond to injury by releasing endogenous opioids, mitigating acute pain. A team of researchers has found a surprisingly long-lasting opioid mechanism of natural chronic pain control, and have observed a development of both physical and physiological dependence on this opioid system, akin to that of opioid narcotics, in the bodies of test animals.
The scientists, led by Bradley Taylor from the University of Kentucky, examined opioid function at sites of pain modulation in the spinal cord. Researchers have known for a while that blocking opioid receptors can increase the intensity of acute pain, occurring immediately after injury, but have been unsure whether blocking opioids could increase chronic, long-term pain.
The researchers simulated human injury in a mouse model, then waited several weeks for signs of pain-like behaviors to subside. They then administered opioid receptor blockers.
When the opioid system (called mu opioid receptor constitutive activity (MORCA) by the authors) was blocked, the mice reverted to a set of behaviors associated with the experience of pain. Surprisingly, they also experienced symptoms similar to the known effects of opioid withdrawal in a drug addict: tremor, jumping and shakiness.
These results were observed even up to six and a half months after pain had seemingly resolved. The long-lasting nature of the phenomenon suggests that endogenous opioid analgesia silently continues long after an injury has healed.
Because the body appears to develop a reliance on MORCA, and because stress is a key factor in causing relapse in opioid addicts, it follows that stress may also be a key factor in relapse in chronic pain patients. Thus, the authors speculate that stress could interfere with endogenous MORCA analgesia, leading to the emergence of widespread, rampant chronic pain such as is observed in a range of conditions, including fibromyalgia.
More studies are needed to fully understand this mechanism.