The next big wave in pain treatment might come in the form of light waves—green light in particular.
New research shows that healthy rats and those with experimental nerve injury feel less pain following exposure to green light, which affected pain processing in the brain. The pain-dampening effect required activation of the visual system, and also the release of endogenous opioids, the brain’s own morphine-like chemicals.
If the new research in rats holds up in people, patients might one day don green contact lenses or goggles to alleviate pain.
The work from researchers at University of Arizona, Tucson, US, was published in the February issue of the journal Pain.
From green light to pain relief
Researcher Mohab Ibrahim first tested the effects of various types of light on rats’ responses to pain. He placed light-emitting diodes (LED) on the outside of the rats’ clear plastic cages and exposed the animals to light for eight hours per day for five days. (Aside from the colored LEDs, the room was dark.)
Rats exposed to normal ambient light (fluorescent lights and light from a window), white LEDs, or darkness did not show any changes in the time it took them to withdrawal their paw from a hot surface—a commonly used indicator of pain tolerance. But green light had an unexpected effect.
“Much to my surprise, green light was anti-nociceptive [pain relieving] in the rats,” Ibrahim said.
The paw withdrawal time increased by about 25 percent, indicating increased tolerance to pain, and lasted for days after green light exposure ended. Blue light also had a smaller but significant effect. Ibrahim tested different intensities of green light ranging from 4 to 110 lux (a measure of how strong light is) and found that the lowest intensity light was sufficient to increase pain tolerance.
Importantly, green light also dampened pain in rats with a spinal nerve injury, an experimental model of chronic pain. Seven days following injury, rats displayed greater sensitivity to heat and mechanical pokes than uninjured rats, as expected. But that sensitivity was reversed completely after four days of green light.
The finding that such a low-intensity light—just 4 lux—was capable of producing the pain-relieving effect is notable, says Andrew Ahn, a neurologist and researcher at the pharmaceutical company Eli Lilly, Indianapolis, US.
“Rodents are exquisitely sensitive to light,” so the researchers were right to use a very low light level in the animals. When the effect is tested in people, it might require a higher intensity, he says.
Opioid release required
Endogenous opioids are the body’s natural morphine-like chemicals released in the brain to quell pain signals. After eight hours of green light exposure, rats were given a drug to counteract any endogenous opioids that might have been released in response to the light. That drug, naloxone, reversed the light’s pain-relieving effect, indicating that green light dampened pain by triggering endogenous opioids.
The authors further traced the source of opioid release to the rostral ventromedial medulla, a key area in the brain’s pain-control system.
Light in the brain
The team next wanted to confirm that vision was critical to green light’s effect on pain, so technician Kerry Gilbraith, whom Ibrahim called “amazing and extremely resourceful,” fabricated tiny plastic contact lenses for the rats.
Black, opaque lenses completely blocked light from entering rats’ eyes—and the pain-relieving effect of green LEDs. Rats wearing clear lenses and exposed to green light, in contrast, showed less pain just as control rats without lenses did.
Strikingly, pain relief was also evident in rats wearing green contact lenses, which pass light only in the green wavelength, and exposed to regular room light. This confirmed that the light’s pathway into the nervous system was through the eyes.
As for how the light coming through the eyes influences areas of the brain that regulate pain remains a complete mystery. However, Ahn says that the effect probably does not work through the classic visual system that allows us to read and see, but perhaps on another pathway.
“There is a very primitive light detection system that’s not associated with visual images,” according to Ahn. Instead, that system influences our sleep cycle and our emotions. One could imagine that such a neural pathway between the eye and brain areas that control emotions could influence pain, he says, but further studies will be needed to determine whether the current work involves those pathways.
Light has long been recognized to affect emotional state and even pain: people with migraine headaches experience light as aversive and even painful, and bright white light therapy has been used to treat depression. But no previous studies have investigated the effects of colored light on pain, particularly at low intensity.
Green light for people?
Will green light’s soothing effect translate to humans? Ibrahim and colleagues have begun a clinical trial in people with fibromyalgia. The preliminary results are promising, he said, but it will take time before they can make any conclusions. Meanwhile friends and family are asking him how they can apply the technique. Self-treating with green light is probably premature, he said, “but this is not invasive, and there is no risk.”
And, said Ahn, “given that the current drug treatments for chronic pain are inadequate and leave people with serious unmet need, exploring non-pharmacological therapies is really important.” Nevertheless, Ahn added, the work might suggest new opportunities for drugs to enhance the effect of light on pain.
Meanwhile, go sit outside among the trees—it can’t hurt.
To read about the research in more detail, see the related Pain Research Forum news story here.
Stephani Sutherland, PhD, is a neuroscientist, yogi, and freelance writer in Southern California.