MAY 27, 2018 01:15 PM PDT

Why Opioids Can't Always Reduce Chronic Pain

WRITTEN BY: Carmen Leitch

While opioids have been in use in North America since at least the time of the Civil War, we still don’t know everything about how they work. New research has indicated that chronic pain alters receptors in the brain that attach to opioids. This alteration may reduce the effectiveness of opioids. The study also provided insight into how opioids can cause depression. The work, by a team of scientists at the National Institutes of Health (NIH) and McGill University, Montreal, was reported in the journal Pain.

“We know that people with chronic pain have reduced availability of opioid receptors — the molecules opioid drugs bind to — in the brain,” said study co-author Mark Pitcher, Ph.D., a visiting fellow in the Division of Intramural Research at the National Center for Complementary and Integrative Health (NCCIH). “What we haven’t known, until now, is why. Are there preexisting brain differences that might predispose some people to develop chronic pain? Or might chronic pain cause these differences? Our findings suggest that chronic pain itself is responsible.”

An imaging technique that allows researchers to see disease or injury, positron emission tomography (PET), was performed on cross-sections of rat brains. The scientists had mimicked chronic pain by making a surgical nerve injury on one group of those rats; another group was a control that had a surgery that did not cause injury. The scientists found that within three months, opioid receptor availability had gone down in several parts of the brain in the rats with the chronic pain injury; the other group was unaffected.

Other experiments explored the link between depression and chronic pain. Typically, rats will drink sugar water instead of regular water if given the option. When such animals have a symptom of depression called anhedonia, a reduced experience of pleasure, their interest in the sugar water decreases. In this work, after the injury, the rats experiencing chronic pain also had a reduced interest in the sugar water. These animals demonstrated a link between the availability of opioid receptors and their interest in sugar water.

Credit: Pxhere

“It’s well known that there’s a link between chronic pain and depression. 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,” explained study co-author M. Catherine Bushnell, Ph.D., scientific director of NCCIH’s Division of Intramural Research. 

“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 NCCIH Acting Director Dr. David Shurtleff. “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.”

Studies in humans will, of course, be needed to confirm that people experience the same effects. Chronic pain is known to cause neurological changes, however.

Sources: NIH, American Battlefield Trust, Scientific Reports, Physiology & BehaviorPain

About the Author
  • Experienced research scientist and technical expert with authorships on 28 peer-reviewed publications, traveler to over 60 countries, published photographer and internationally-exhibited painter, volunteer trained in disaster-response, CPR and DV counseling.
You May Also Like
NOV 14, 2018
NOV 14, 2018
Visualizing Embryonic Development in Zebrafish
Zebrafish share genetic features with many other organisms, and they undergo a rapid developmental process in which they can remain transparent....
NOV 20, 2018
NOV 20, 2018
What Makes Some Fats Bad
You may recall hearing at some point that there are “good fats“ and “bad fats.“ What does that really mean? But what makes one type...
NOV 26, 2018
Cell & Molecular Biology
NOV 26, 2018
New Insight Into the Machinery That Drives Cell Death
When cells are worn out and damaged or diseased, they can initiate a self-destruct sequence called apoptosis....
NOV 28, 2018
NOV 28, 2018
A Deadly Microbe Lurks on the Skin of Many People
There are some well-known bacterial pathogens that can present a major threat to health, but a lesser-known microbe may be just as big of a danger....
DEC 04, 2018
Genetics & Genomics
DEC 04, 2018
The Fallout From the First CRISPR Babies Continues
Last week the world was stunned to hear that twin babies had been born after a gene-editing experiment was conducted on human embryos....
DEC 10, 2018
DEC 10, 2018
Brain's Dopamine: The Good, The Bad and The Ugly
Dopamine in the brain is an important neurotransmitter that is often attributed to pleasure chemical. But that's not all it does; research has identified the role of dopamine in fear, emotion...
Loading Comments...