“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.