Pain researchers know that, in the immediate aftermath of a severe injury, pain sensitization pathways become active, causing the body to produce opioids—naturally occurring chemicals that inhibit pain by activating receptors. But a mouse study published today in Science reveals that a specific type of opioid receptor found on the surface of nerve cells remains hyperactive months after an injury has healed—a period much longer than previously thought. Moreover, blocking this receptor from binding opioids can produce opioid withdrawal, much like that experienced by people addicted to heroin or codeine. This finding suggests that opioids only serve to mask underlying pain, shedding light on why some chronic nature of pain disorders.
To study the long-term role of ‘endogenous’ opioids produced naturally by the body, scientists created an inflammatory response in the paws of mice by injecting a dose of toxic bacteria fragments. The researchers then allowed the injury to heal for a period of three weeks or more, all the while monitoring how sensitive the area was by using molecular biomarkers and recording how often the rodents made facial grimaces or withdrew their paws when the injured area was touched.
Six months on, once the injury had healed, the researchers administered naltrexone methobromide, a drug that blocks the receptors that can normally bind to opioids. Surprisingly, blocking the receptor’s activity caused the pain to return, even though considerable time had passed. Moreover, the mice also exhibited the telltale signs of opioid withdrawal, such as jumping, shaking and teeth chattering. “We think it’s possible that the body becomes dependent on the endogenous opioid system after an injury,” says neurobiologist and co-author Bradley Taylor, of the University of Kentucky in Lexington, “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’.