Research suggests there could be a "simple, safe, and economical" way to relieve pain: green light. And a new animal study reveals the biological underpinnings of how it might work.
Scientists have been exploring the pain-relieving effects of green light for at least half a decade, uncovering an occasional clue on exactly how it happens.
This new mouse study, led by neuroscientist Yu-Long Tang of Fudan University in Shanghai, reveals the eye cells and brain pathways that underpin the pain relief sometimes felt after exposure to low-intensity green light.
In a series of experiments, the researchers discovered that cones and rods – the eye cells that sense light – contributed to the pain-relieving effects of green light in healthy mice and mice with inflamed joints.
As scientists often do, the researchers inactivated certain cells to see what effect that had on the animals' sense of pain. When they inactivated the rods in some mice, those animals showed only partial relief with green light, while mice devoid of cones showed no signs of pain relief at all when bathed in a verdant glow.
"We found that retinal cone photoreceptors are essential for green light analgesia, whereas rods play a secondary role," explain Tang and colleagues in their published paper.
From there, they chased the path taken by electrical signals from the eye through the brain.
Bathed in green light, the cones and rods stimulated a group of brain cells in the ventrolateral geniculate nucleus, which has previously been linked to the analgesic effects of bright light in general.
In this part of the brain, these neurons express a hormone involved in pain signaling. These cells then relay the message to another part of the brain called the dorsal raphe nucleus that modulates pain, effectively turning down the dial on severe pain sensations.
Different animal studies have identified other mechanisms entwined in the analgesic effects of green light, such as pain receptors in the spinal cord – which is not surprising given how complex the experience of pain is. It involves the sensory, bodily, and psychological experience of stimuli and signals that bounce between the brain, spinal cord, and pain receptors.
While pain relief in rodents is a far cry from humans, this study succeeds in pinpointing pain circuits in the mammalian brain that respond to visual inputs and expands our understanding of a safe, easy way to possibly quell them.
"Although it is unclear whether color perception is comparable between humans and rodents, green light exposure in both humans and rodents reduces pain sensitivity, suggesting the involvement of shared mechanisms between the two species," Tang and colleagues write, noting that other brain regions are likely involved.
Exposing people to eight hours of light therapy a day, as the researcher did to the mice in this study, is just not practical or feasible, so it would be interesting to see whether shorter bouts of green light therapy are effective at relieving pain – and for how long.
On that point, there are some encouraging results from other studies. An animal study in rats suggested that pain relief from green light might be long-lasting, persisting for four days after treatment.
Recent clinical trials have also reported that a few hours of green light therapy each day reduced pain intensity in a small group of fibromyalgia patients and the number of headache days in migraineurs. Chronic low back pain is another target. Green light could benefit patients following surgery, reducing their dependence on painkillers.
Though it might not work for everyone, if these studies can be repeated in more patients, it might pave the way for green light therapy to be an alternative option for pain relief altogether.
The enduring challenge is that chronic pain is a tricky beast to tame, and not all pain is the same. Opioids are also incredibly effective at relieving pain but highly addictive, and we don't know yet how green light compares.
Aside from chronic pain, the findings add some color to our understanding of why spending time in nature feels so good. The cocoon of a forest, deep green in color, might calm our nervous system in more ways than one.
The study was published in Science Translational Medicine.