Painkillers Cause Chronic Pain? Rats!!!

By Pat Anson, Editor

A provocative new study is likely to stir fresh debate about the risks associated with opioid pain medication. It’s not another study about addiction or overdose, but whether opioids actually increase chronic pain, a condition known as hyperalgesia. 

An international team of researchers found that even just a few days of morphine can make chronic pain last for several months by intensifying the release of pain signals in the spinal cord.

But there’s a catch. The research was conducted on laboratory rats.

"We are showing for the first time that even a brief exposure to opioids can have long-term negative effects on pain," said Peter Grace, PhD, an assistant research professor at the University of Colorado-Boulder's Department of Psychology and Neuroscience. "We found the treatment was contributing to the problem."

Grace and his colleagues found that damaged nerve cells in rats send a message to spinal cord immune cells known as glial cells, which normally act as "housekeepers" to clear out unwanted debris and microorganisms. The first signal of nerve pain sends glial cells into alert mode, priming them for further action.

"I look at it like turning up a dimmer switch on the spinal cord," said Grace.

Nerve pain was induced in the rats by slicing open their thighs. A fine thread was then tied around a major nerve. Over the next three months, researchers poked the rats' paws with stiff nylon hairs to see how sensitive they were to pain.

Injured rats that were not treated with morphine eventually recovered and did not show pain, but those that were treated with morphine for five days remained sensitive to pain. Researchers believe the morphine stimulated their glial cells and sent them into overdrive. They liken the effect to being slapped in the face twice.

"You might get away with the first slap, but not the second," said co-author Linda Watkins, a Distinguished Professor at CU Boulder. "This one-two hit causes the glial cells to explode into action, making pain neurons go wild."

"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," said Watkins. "This is a very ugly side to opioids that had not been recognized before."

Patient advocates had a mixed reaction to the study.

“Linda Watkins is doing some awesome work. We know that glial cells are the key to pain generation. Exactly how is still poorly understood,” said Terri Lewis, PhD, a rehabilitation specialist who teaches in the field of Allied Health.  “We know that 'something' triggers inflammation and maintains it. When that trigger is turned up high, glial cells are activated."

“Generalizing from rats to humans is not okay. But if the same results are found in pigs, there is probably something to talk about,” added Lewis.

“There is enough evidence in humans that opioids work and do not make pain worse,” said Janice Reynolds, a retired nurse and patient advocate.  “Even the work in hyperalgesia has not, contrary to claims by opiophobics, translated well from rats to humans. The write up is extremely negative and tends to lead one to believe the results may be slanted or even poorly interpreted. The fact they are singling ‘chronic pain’ out is a warning sign.”

The CU-Boulder study, which is published online in the Proceedings of the National Academy of Sciences, does have an impressive pedigree, including researchers at the University of Adelaide in Australia, the University of North Carolina, the Chinese Academy of Sciences, the National Institute on Drug Abuse, the National Institute on Alcohol Abuse and Alcoholism, and Tsinghua University in Beijing.

The study was funded in part by the American Pain Society, Australia's National Health and Medical Research Council, the National Natural Science Foundation in China, the National Institute on Drug Abuse, the National Institute of Dental and Craniofacial Research and the National Institute of Alcohol Abuse and Alcoholism.