New research may explain why people who have the chronic pain condition fibromyalgia often report that they don’t respond to the types of medication that relieve other people’s pain.

Researchers from the University of Michigan Health System found that patients with fibromyalgia had reduced binding ability of a type of receptor in the brain that is the target of opioid painkiller drugs such as morphine.

The study included positron emission tomography (PET) scans of the brains of 17 women with fibromyalgia and 17 without the condition.

Results showed that the fibromyalgia patients had reduced mu-opioid receptor (MOR) availability within regions of the brain that normally process and dampen pain signals.

“The reduced availability of the receptor was associated with greater pain among people with fibromyalgia,” said lead author Richard E. Harris, research investigator in the Division of Rheumatology at the U-M Medical School's Department of Internal Medicine and a researcher at the U-M Chronic Pain and Fatigue Research Center, in a news release.

The findings appear in The Journal of Neuroscience.

Opioid pain killers work by binding to opioid receptors in the brain and spinal cord, the study explains. In addition to morphine, they include codeine, Darvocet, Vicodin, and Oxycontin.

The researchers believe that, with the lower availability of the MORs in three regions of the brains of people with fibromyalgia, such painkillers may not be able to bind as well to the receptors as they can in the brains of people without the condition.

When the painkillers cannot bind to the receptors, they cannot alleviate the patient’s pain as effectively, Harris said.

The research team also found a possible link with depression. The PET scans showed that the fibromyalgia patients with more depressive symptoms had reductions of MOR binding potential in a region of the brain thought to modulate mood and the emotional dimension of pain.

“The finding is significant because it has been difficult to determine the causes of pain in patients with fibromyalgia, to the point that acceptance of the condition by medical practitioners has been slow,” added Harris.