The colored section of the rat's brain PET scan shows less opioid receptor availability. Researchers say the new study suggests opioid painkillers are not useful against chronic pain. Photo by Scott J. Thompson/NIH

May 25 (UPI) -- Opioid therapy may not always be effective against chronic pain and could lead to depression, according to a National Institutes of Health Study.

The study was conducted by the NIH in collaboration with Montreal's McGill University and published Thursday in the medical journal Pain.


Using laboratory rats, the research suggests that pain-induced changes in the brain's opioid receptor system can limit the effectiveness of therapy with the drugs. The study also demonstrates the impact of chronic pain on the brain and its relationship to depression.

"We know that people with chronic pain have reduced availability of opioid receptors, the molecules opioid drugs bind to, in the brain," Dr. Mark Pitcher, a researcher at NIH and an author of the study, said in a press release. "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."

Tests indicated that rats that underwent brain surgery to simulate a nerve injury were less likely to drink sugared water, which was preferred by healthy rats, over regular water. It was a sign of a decreased ability to experience pleasure, a recognized symptom of depression named anhedonia.

Changes associated with chronic pain are described as neuroplasticity. The study suggests that changes to opioid receptors in the brain can impact depression.

"It's well known that there's a link between chronic pain and depression," said study co-author Dr. M. Catherine Bushnell. "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."

A study of patients at Minnesota Veterans Affairs clinics, released in March, indicated that opioid medications were not better than non-opioid treatments to improve chronic back, hip or knee pain.