Research suggests there may be a “simple, safe and economical” way to relieve pain: green light. A new animal study reveals the biological basis of how it might work.
Scientists have been exploring the pain-relieving effects of green light for at least half a decade, uncovering the occasional clue as to how it happens.
This new mouse study, led by neuroscientist Yu-Long Tang of Fudan University in Shanghai, reveals eye cells and brain pathways that relieve pain felt after exposure to low-intensity green light.
In a series of studies, the researchers found that cones and rods — light-sensing eye cells — contributed to the pain-relieving effects of green light in healthy mice and mice with inflamed joints.
As scientists often do, the researchers turned off certain cells to see what effect that had on the animals’ perception of pain. When they disabled the cones in some mice, those animals showed only partial relief from the green light, while mice without cones showed no sign of pain relief when bathed in the lush glow.
“We found that retinal cone receptors are essential for green light sensing, but rods play a secondary role,” Tang and colleagues explain in their published paper.
From there, they traced the path that electrical signals took 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 is normally associated with the pain-relieving effects of bright light.
In this part of the brain, these neurons express a hormone involved in pain signaling. These cells then send the message to another part of the brain called the dorsolateral nucleus that modulates pain, effectively turning down the dial on intense pain sensations.
Various animal studies have identified other mechanisms tied to the pain-relieving effects of green light, such as pain receptors in the spinal cord—not surprising given how complex the experience of pain is. It includes the sensory, physical and psychological experience of stimuli and signals between the brain, spinal cord and pain receptors.
While pain relief in mice is a far cry from humans, this study succeeds in identifying pain circuits in the mammalian brain that respond to visual input and expands our understanding of a safe, easy way to suppress them.
“Although it is unclear whether color perception is comparable between humans and mice, exposure to green light in both humans and mice reduces pain sensitivity, suggesting the involvement of shared mechanisms between the two species,” Tang and colleagues write, noting that other brain regions. can be related.
It’s not practical or feasible to expose humans to eight hours of light therapy a day, as the researchers did to rats in this study, so it will be interesting to see if brief green light therapy is effective in relieving pain — and for how long.
At the same time, other studies have had encouraging results. An animal study in mice found that pain relief from green light can be long-lasting, lasting 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 migraine patients. Chronic low back pain is another target. Green light can be beneficial for patients after surgery, reducing their dependence on painkillers.
While it may not work for everyone, if these studies can be repeated in more patients, it could pave the way for green light therapy to become 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 incredibly effective at relieving pain but are highly addictive, and we still don’t know how the green light compares.
Chronic pain aside, the findings add some color to our understanding of why spending time in nature feels so good. The cocoon of a deep green forest can calm our nervous system in more ways than one.
The study was published in Science Translational Medicine.