Rats, Depression and Chronic Pain

By Pat Anson, Editor

An unusual study involving rats, depression and chronic pain is making headlines – the latest in a long line of flawed research studies being used to debunk the effectiveness of opioid pain medication.

“NIH study suggests opioid therapy not effective against chronic pain,” is the headline in UPI.

“Pain-induced changes in the brain explain the limited effectiveness of opioid therapy,” is how the Tech Explorist put it.

At issue is a small study by the National Institutes of Health (NIH) and McGill University in Montreal on pain-induced changes in 17 laboratory rats. That's right, 17 rats. The study findings, published in the journal Pain, concluded that chronic pain reduced the number of opioid receptors – the molecules that opioids bind to -- in the rats’ brains. In theory at least, that would make the rats less responsive to opioid pain medication.

Note that the research did not include any people, the rats were not given any opioids, and the effectiveness of opioids wasn't even measured in the rats. But that didn’t stop the NIH from drawing some sweeping conclusions.

“These results provide insights into why we see limited effectiveness of opioid therapy in chronic pain and the mechanism of the depression that may accompany it,” said David Shurtleff, PhD, acting director at the National Center for Complementary and Integrative Health (NCCIH).

“These basic research findings support NIH’s efforts to better understand chronic pain and comorbid symptoms and to develop better ways to help chronic pain patients effectively manage their pain.”

McGill University was more cautious, saying further studies were needed in humans to confirm the study findings.

“Although the study… was conducted in rats, and the results of animal studies may not be directly applicable to people, the findings provide new insights into how the brain may respond to pain and opioids,” a McGill press release states. “These findings, if confirmed in people, will enhance the understanding of the impact of chronic pain on the brain, its relation to depression, and the effects of opioids.”

Researchers have many theories about the origins and treatment of chronic pain, but conducting tests on humans to prove them is problematic. Laboratory animals are often used as an imperfect substitute.

In the NIH/McGill study, 17 rats had brain surgeries to produce a nerve injury that causes chronic pain, while another group of rats had sham surgeries (a similar procedure that did not cause chronic pain). Three months later, PET scan imaging showed opioid receptors had decreased in multiple regions of the brain in the nerve-injured rats, but no changes occurred in the sham-surgery rats.

These results suggest that pain itself, not treatment or pre-existing trauma, altered the brain’s opioid system. Other tests showed a weaker link between chronic pain and depression in the nerve-injured rats.

How did researchers determine the rats were depressed? 

When given a choice, healthy rats will normally drink water sweetened with sugar rather than plain water. But animals with a decreased ability to experience pleasure, a recognized symptom of depression, may not. The rats in the study with chronic pain showed a decreased preference for sugar water over plain water, while rats in the sham group still showed a preference for sweetened water. This, the researchers believe, was enough evidence to conclude the nerve-injured rates were depressed.

“It’s well known that there’s a link between chronic pain and depression,” explained co-author M. Catherine Bushnell, PhD, scientific director of NCCIH’s Division of Intramural Research.  “The results of this study indicate that pain-induced changes in the brain’s opioid system may play a role in this association. Animals with the greatest decrease in opioid receptor availability showed the greatest increase in depression-like symptoms after experiencing chronic pain.”

While intriguing, the results of this rat study are far from definitive and do not prove that opioids are an ineffective treatment for chronic pain in people. What they do show is that we need more and better research about opioids and chronic pain, not more misleading headlines and statements from the NIH.

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.