Morphine could make pain last longer

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Acute pain, the result of sudden trauma, has a purpose. Pain motivates us to pull out from damaging situations, to protect a damaged body part while it heals, and to avoid similar experiences in the future. Chronic pain, on the other hand, does not go away and is resistant to most medical treatments. It may be the result of an ongoing cause (infection, etc.) but sometimes people have chronic pain for years in absence of any obvious damage. A type of chronic pain called neuropathic pain often accompanies paralysis. Following injury, the nervous system undergoes a tremendous reorganization and results sometimes in this type of chronic pain. Such pain can lead to inactivity, depression, isolation and eventually more pain.

Pain and morphine
There are numerous treatment to diminish or eliminate such pain and morphine (opioid family) is one of the compounds that is used more and more frequently. However, a recent publication in the prestigious journal PNAS from a team based in the University of Colorado (USA) unveiled a rather surprising result. Experimental data shows that a short course of morphine, starting ten days after injury, paradoxically and remarkably doubled the duration of neuropathic pain, months after the morphine treatment ceased.

A new, unidentified mechanism
This surprising effect of opioids on chronic pain was observed for the first time and this sled the team to dissect its mechanisms. Using pharmacologic and genetic approaches they discovered that the morphine activates a cascade that involves the molecules NLRP3 (NOD-like receptor protein 3 inflammasomes) and IL-1β (interleukin-1β).

Their observations led them to conclude that the long lasting pain is coming from an inflammatory response in the spinal cord. The immune system wrongly sees morphine as a threat and responds by increasing inflammation through specialized cells called microglia. Experiments that shut down this process in microglia shortened the duration of the pain.

What will happen next?
Numerous questions still remain. Scientists are still wondering if such immune reaction happens in people. Nor whether all opioid-based painkillers would behave like morphine. Since the results were revealed in a highly standardized and controlled experimental setting, the scientists also hypothesize that only certain individuals, and not the whole population, might be subject to such complications. Understanding the details of how the process works is essential in better understanding how opioid influence pain and eventually motivate the search for better treatments.