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Neurobiology of Chronic Muscle Pain

In a study funded by the National Institute of Neurological Disorders and Stroke and co-funded by the National Institute of Arthritis and Musculoskeletal and Skin Diseases and led by Kathleen Sluka, PhD, investigators looked to develop a new model of chronic, widespread pain and then use it to identify responsible mechanisms in the peripheral or central nervous systems.

Though the onset of fibromyalgia is often linked to injury or trauma, researchers believe the pain is maintained by changes in the central nervous system as opposed to any ongoing tissue damage. To better understand how injury might trigger fibromyalgia pain and which central nervous system mechanisms might perpetuate it, Dr. Sluka and her colleagues attempted to create a fibromyalgia-like pain condition in laboratory mice and rats. To do so, the researchers injected the rodents' calf muscles with an acidic saline solution. They then measured the rodents' pain sensitivity using a test in which pressure is applied to the rodents' paws, much as doctors apply pressure to potential tender points on people's bodies to diagnose fibromyalgia.

The team found that two injections, given 2 days to 5 days apart, produced a long-lasting, hypersensitivity that affected pain on both sides of the mice and rats' bodies, and that certain drugs (such as morphine or compounds injected directly into the spinal cord to block a protein called N-methyl-D-aspartate, or NMDA) reversed the problem.

Now that they have created and confirmed a mouse and rat model of chronic pain, the researchers are using the model to test potential pain mechanisms, including the role of NMDA receptors and other proteins believed to play a role in pain-processing in the spinal cord. The researchers also are investigating the possibility that changes occur in the genes of people with chronic pain conditions that lead to the production of more pain-producing proteins. The researchers' eventual goal is to find ways to block the production or actions of these proteins.