Medical researchers have identified a class of cells involved with a strange and sometimes debilitating form of chronic pain. Better yet, they have developed a treatment using the application of a light-sensitive chemical and infrared radiation, offering hope that sufferers may get pain-free lives back.
Neuropathic pain occurs when something goes wrong with the nervous system. Even without damage to the body's tissues, nerves can fire as if stabbed, burned, or suffering from an electric shock. In more subtle cases, the sensation is one of numbness or itching. Although 7-8 percent of the European population suffers from neuropathic pain, most cases are mild or intermittent. For a small minority, however, the pain is so severe even the movement of a single hair on one's arm can be agonizing.
The pain can be either central, from the spinal cord or the brain, or peripheral, where the slightest touch to the skin triggers anything from irritation to agony. It is this peripheral pain that Dr Paul Heppenstall of the European Molecular Biology Laboratory in Rome has found a novel way to treat.
The skin has a variety of nerve cells, each of which responds to different stimuli. For the first time, the team showed that only TrkB-positive nerve cells cause peripheral neuropathic pain, and set out to fix them.
Heppenstall designed a molecule that binds to TrkB-positive neurons but has no reaction to other types of cells, including other nerve cells. Modifications to this molecule made it sensitive to light. When a patch of mouse skin susceptible to neuropathic pain was injected with the molecule and exposed to near-infrared light, the nerve cells retracted, leaving them buried more deeply in the skin and less responsive to slight sensations.
The idea of removing the ends of nerve cells sounds horrific, but they report in Nature Communications that the technique worked, at least with mice. Those who had previously shown signs of suffering pain from even gentle touching responded normally after the therapy.
"It's like eating a strong curry, which burns the nerve endings in your mouth and desensitizes them for some time," Heppenstall said in a statement.
Indeed, applying capsaicin, the active component in chili peppers, is already in use as a less targeted way to cause skin neuron contraction.
Unfortunately, the nerves regrow. For mice, the therapy would need to be repeated every three weeks, though humans might have longer cycles. Although Heppenstall has experimented on human skin tissue in vitro, he said: “A lot of work needs to be done before we can do a similar study in people with neuropathic pain.”
Sufferers are so desperate for relief, however, that the authors are hopeful they will find partners to facilitate this work.
