A new study has unraveled the neural circuits that explain how green light can relieve some cases of chronic pain. The neurons ultimately activate the brain’s own opioid system.
The effect of light on pain reduction has been well-established in pre-clinical models, so the researchers’ first step was to confirm these earlier findings by showing that green light exposure in mice with a form of arthritis reduced pain. They then explored how this effect was altered in mice that lacked various structures in their eyes that detected visual input.
Next, the study authors sought to identify the neural pathways that convert the signals sent by cone cells into pain relief. Imaging experiments showed that once green light was detected by these cones, a flurry of information was transmitted to the ventral lateral geniculate nucleus (vLGN), which is located in a key brain structure called the thalamus.
The researchers therefore repeated their experiments on mice that lacked the ability to produce these proteins and found that green light provided absolutely no pain relief to these PENK-less rodents. Similarly, blocking opioid receptors in regular mice also eliminated the analgesic effects of the color green.
"Exposure to an environment rich in the color green (such as [the Japanese and Chinese practice] forest bathing) can decrease physiological and psychological pain. Psychology studies have shown that “green” conveys positive information related to happiness," the authors write.
