When we become injured, our bodies release natural opioids that attach to receptors in the brain and send signals that block pain, slow breathing, and produce a general feeling of calm. Now, a new study finds the body can become dependent on these natural opioids, and this may contribute to the emergence of chronic pain.

Role of Opioids Explored in Experiments

Without natural opioids, acute pain after an injury or surgery would be much worse. Yet, unlike synthetic opioids, the body will never produce enough to cause an overdose. Can the body produce enough to cause addiction, though?

That is precisely what scientists suggest in a new study conducted by researchers in the U.S. and Canada. Moreover, they hypothesize that it might be possible to prevent the transition from acute to chronic pain by blocking the body’s overuse of opioids.

No one knows exactly how chronic pain develops, though sometimes it results from a single incident of acute pain that matures into an ongoing condition. Worse, there are no treatments to end chronic pain, and those who suffer do so with little relief. To explore the part played by natural opioids in chronic pain, Gregory Corder and his colleagues produced inflammation in the paws of mice and then allowed their pain-like behavior to naturally subside over several days to weeks. (Undoubtedly, the animal’s natural opioids helped in this process.) Next, they gave the mice opioid receptor blockers, in some cases up to six months after the original injury.

Oddly, these drugs that block opioids revived the same pain-like behavior demonstrated when the mice were first injured. Additionally, the researchers observed pain neuron activation in the spines of these animals. After this, the researchers identified a particular opioid receptor, known as the μ-opioid receptor (or MOR).

“In mice with chronic pain, MOR seemed to be stuck in a constant “on” state,” the authors wrote. So what would happen if that switch were turned off?

Stopping the Flow of Opioids

When given an opioid blocker to turn off the MOR, the mice displayed classic symptoms of withdrawal seen in addicts when they stop taking drugs: shaking and tremors. Those mice that did not receive the opioid blocker to disrupt the flow of natural analgesia did not exhibit this behavior. Researchers found that stopping the flow of MOR seemed to initiate not only pain and pain transmission, but also the production of a protein found in the spinal cord: adenylyl cyclase type 1. And it just happens to be the case that this particular protein is known to contribute to both addiction and chronic pain.

“Together, these results suggest that while MOR keeps acute pain under control after injury, it also can cause the body to become dependent on its own opioids, which contributes to the development of chronic pain,” wrote the authors.

Source: Corder G, Doolen S, Donahue RR, et al. Constitutive μ-Opioid Receptor Activity Leads to Long-Term Endogenous Analgesia and Dependence. Science. 2013.