Narcotic painkillers prolong pain in rats, study says

Researchers say the results of the study may hold significance for using the drugs with humans.

By Stephen Feller

BOULDER, Colo., May 30 (UPI) -- Although the standard for treating pain is opioid-based painkillers, researchers suggest the drugs could prolong pain in some cases.

Researchers at the University of Colorado Boulder found treating rats with morphine caused pain to continue for several months, suggesting using the drugs for pain in humans could be making conditions worse.


Recent studies examining the effects of opioid-based painkillers, which range from morphine and OxyContin to Percocet and Vicodin, have found physical therapy patients recover faster without the drugs and that they may not be effective at treating long-term back pain.

Researchers at the University of Colorado Boulder also found in a 2013 study with rats that morphine after abdominal surgery can prolong patients' pain, potentially doubling or tripling the length of time for recovery from surgical pain.

The new study suggests the escalation in opioid use for pain in humans could be contributing to longer-term chronic pain conditions because of its effects on the spinal cord and immune cells, researchers say.

"Initially, the morphine does work and it produces great pain relief and we know that over time the ability for morphine to produce analgesia or pain relief diminishes over time," Dr. Peter Grace, an assistant research professor at the University of Colorado Boulder, said in a press release. "And what we're showing is rather than alleviating pain it's actually inducing it."


During the study, published in the Proceedings of the National Academy of Sciences, the researchers found a peripheral nerve injury in rats caused a message to be sent from nerve cells in the spinal cord to glial cells responsible for clearing threats to the body, in addition to the sensation of pain.

When researchers treated the rats' injury with five days of opioids, the cells triggered inflammation in the spinal cord, as well as other reactions, which increase the sensation of pain, suggesting the drugs actually cause an increase in pain.

Further studies are needed on whether the drugs cause the same reaction in humans, researchers say, because the experiment with rats is the first to show a pain-causing effect. They also report a method of blocking receptors on glial cells using a drug called DREADD, however they note this has not been done in humans before.

"The implications for people taking opioids like morphine, oxycodone and methadone are great, since we show the short-term decision to take such opioids can have devastating consequences of making pain worse and longer lasting," Dr. Linda Watkins, a professor at the University of Colorado Boulder, said in a press release. "This is a very ugly side to opioids that had not been recognized before."


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