A New Understanding of Chronic Pain
Researchers at the Salk Institute and Harvard Medical School have identified a mechanism that sends “erroneous pain signals” to the brain. And that means potentially better identification of pain disorders that have no clear physical cause.
People who suffer from chronic pain are often sensitive to stimuli that don’t normally cause pain, such as a light touch to the hand or a change in skin temperature. These conditions, generally referred to as forms of allodynia, include fibromyalgia as well as nerve damage caused by diabetes, cancer and autoimmune disorders.
Other cases occur after the amputation of a limb; pain seems to be focused in the missing appendage. Although those sensations often subside, they can often linger indefinitely and cause chronic pain.
“These disorders are extremely frustrating for patients, because there is still no effective treatment for such chronic pain disorders,” said Qiufu Ma, a professor of neurobiology at Harvard Medical School and co-senior author on the paper.
In the study, published in the journal Cell, the investigators used a mouse model to look at the spinal circuits that process and transmit pain signals.
“Until now, the spinal cord circuitry involved in processing pain has remained a black box,” says Martyn Goulding, Salk professor in the Molecular Neurobiology Laboratory and a co-senior author of the paper. Goulding said that “Identifying the neurons that make up these circuits” marks the beginning of understanding how chronic pain can stem from erroneous neural processing.
Scientists have long theorized that pain signals are sent from sensory neurons in the limbs and other extremities to transmission neurons in the spinal cord, which then relay the information to the brain, according to a news release from the Salk Institute. At each step, the pain information can be altered or even blocked. The spinal-cord circuitry is especially important, the researchers said, because it relays the most crucial pain signals to the brain.
In their new study, the Salk and Harvard researchers set out to precisely identify the sensory neurons that connect with the spinal cord. They identified “inhibitory neurons” that seem to control whether tough activates other neurons to send pain signals to the brain.
This finding begins to explain how a light touch can cause discomfort in someone with allodynia: if something is wrong in the pain circuitry, then the sensations of touch could instead activate other neurons that trigger a pain signal. Goulding said the discovery “really opens the door to understanding what’s happening in these pain disorders where the cause of the pain is seemingly innocuous or not known. It could be that something has gone awry in how this spinal circuitry is operating, so sensations become jumbled together and emerge as pain.”