Why do we experience chronic pain long after an acute injury? A new study in mice shows that our bodies own defenses against pain may be the cause of chronic pain and even symptoms of opioid withdrawal. When an injury occurs, the body responds by releasing natural opiod compounds that have an immediate pain relieving effect. This makes it easier to cope with serious injuries. However, our own bodies defenses against pain may come with a double edged sword. According to a study published in Science, the process of naturally relieving pain in mice appears to put the natural opoid receptor into a chronically “on” position. When the biological switch is turned off, this leads to a reoccurrence of pain, even months after the injury has occurred. This could show some insight into why humans have chronic pain, and could lead to a research pathway to treat these symptoms. Chronic pain can arise after an acute injury. Chronic back pain, for example, is often felt after lower back injuries. When and why acute pain transforms into chronic pain, however, has been largely unknown. Sensory neurobiologist Bradley Taylor and his colleagues at Univerisity of Kentucky Medical Center questioned whether the answer might lie in the body’s mechanisms for controlling pain in the first place. “We know that the body can control acute pain,” Taylor said, “but we asked whether the body could control chronic pain.” To probe the body’s system for dealing with acute injuries, seeking to see how this affects to chronic pain, Taylor and his research team induced inflammatory pain into the paws of mice. They waited for initial pain to subside then began to interfere with the opioid pathway. After six months, when the injury had healed the researchers administered naltrexone methobbromide, a drug that blocks the receptors that bind to opioids. Blocking the receptors activity caused pain to return, surprisingly to the scientists. The mice also exhibited signs of opioid withdrawal. “We think it’s possible that the body becomes dependent on the endogenous opioid system after an injury,” says Taylor, “but this is speculative….If these results are further validated, this could be the first recorded expression of what the researchers call ‘endogenous opioid withdrawal”. After this initial discovery, the research team was able to determine the specific receptor involved using in vitro tests. Muopioid reciptor (MOR) was found to be the culprit. It was found that the paw injury set the MOR into a state of constant activity called Constitutive activity (where the receptor remains active even in absence of opioids). Taylor and his team think that extended MOR activity may lead to the pain pathways becoming oversensitive, leading to chronic pain. The hope is that further research on the MOR receptor and its role in chronic pain will lead to ways to prevent the transition from acute to chronic pain in humans. Further questions that neeed to be answered are: why does MOR continue to signal? Is it independent of endogenous opioids or what opioids come into play? Hopefully as we learn more about chronic pain we will find ways to not only treat it but prevent it. blogs.nature/spoonful/2013/09/receptors-involved-in-modulating-pain-remain-active.html
Posted on: Fri, 27 Sep 2013 16:31:58 +0000
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