Green light appears to relieve pain, and a new study in mice shows why: ScienceAlert

Research suggests there may be a “simple, safe and cost-effective” way to relieve pain: the green light. And a new animal study reveals the biological basis for how it might work.

Scientists have investigated the effects of green light on pain relief at least half a decaderevealing the occasional clue as to exactly how this happens.

This new study in mice, 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 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 deactivated certain cells to see what effect this had on the animals’ sense of pain. When they deactivated the rods in some mice, those animals showed only partial relief with the green light, while mice without the cones showed no signs of pain relief at all when bathed in the green glow.

“We found that retinal cone photoreceptors are essential for green light analgesia, while rods play a secondary role,” explain Tang and colleagues in their published work.

From there, they followed the path taken by electrical signals from the eye to the brain.

Bathed in green light, the cones and rods stimulated a group of brain cells in the ventrolateral geniculate nucleuswho has previously connected 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 dorsal raphe nucleus which modulates pain, effectively turning the dial on strong pain sensations.

Various animal studies have established other mechanisms implicated in the analgesic effects of green light, such as pain receptors in the spinal cord—not surprising given how complex the experience of pain is. It involves the sensory, physical and psychological experience of stimuli and signals bouncing between the brain, spinal cord and pain receptors.

While pain relief in rodents is a long way from humans, this study succeeds in pinpointing pain circuits in the mammalian brain that respond to visual input and expands our understanding of a safe, simple way to possibly quell them.

“Although it is unclear whether color perception is comparable between humans and rodents, exposure to green light in both humans and rodents reduces pain sensitivity, suggesting the involvement of common mechanisms between the two species,” Tang and colleagues pissnoting that other brain regions are likely involved.

Exposing humans to eight hours of light therapy a day, as the researcher did with the mice in this study, is simply not practical or feasible, so it would be interesting to see if shorter cycles of green light therapy are effective in relieving pain—and for how long.

There are some encouraging results from other studies on this issue. An animal study on rats showed that green light relieves pain it can be long lastingwhich lasts four days after the treatment.

Recently Clinical trials also reported that several hours of green light therapy each day reduced pain intensity in a small group of patients with fibromyalgia and number of days with headache in migraine patients. Chronic lower back pain is another target. A green light could be useful patients after surgeryreducing their dependence on painkillers.

Although it may not work for everyone, if these studies can be replicated in more patients, it could pave the way for green light therapy to be an alternative option for total pain relief.

The ongoing challenge is that chronic pain is a beast that is difficult to tame, and not all pain is the same. Opioids are also incredibly effective at relieving pain, but they are highly addictive, and we don’t yet know what the green light is in comparison.

Beyond chronic pain, the findings add some color to our understanding of why it feels so good to spend time in nature. The cocoon of the forest, dark green in color, could calm our nervous system in several ways.

The study was published in Science translational medicine.