Mouse Grimace Scale Can Identify Pain
Scientists have created a sliding scale to measure the pain of mice based on their facial expressions, according to a study published on Sunday.
The “mouse grimace scale” will speed up the development of new pain remedies for humans, and could help reduce the unnecessary suffering of mice and other animals used in biomedical research, scientists said.
Research on pain and how to relieve it relies heavily on the use of rodents as stand-ins for humans, so it is crucial to accurately measure pain intensity in lab mice. Up to now it was never known whether degrees of discomfort and suffering in mice corresponded to spontaneous facial responses, as is the case with humans.
Medical workers often use such scales to assess pain in patients that are unable to communicate verbally, such as infants and the cognitively impaired. Line drawings of faces showing different expressions of discomfort are also used to help manage pain in children asked to match what they feel with the proper image.
Jeffrey Mogil, a professor at McGill University in Montreal and the main architect of the study, monitored and recorded facial movements of mice before and during injections of a particular pain inducing substance to see whether they grimace when it hurts or not.
The researchers found that the mice showed discomfort through facial expressions in a way similar to humans. When pain was more intense, the eyes narrowed, the bridge of the nose and cheeks bulged, the ears moved down and back, and the whiskers either bunched up or flattened out against the face.
The researchers used an intensity scale that measured changes in five facial features. Those trained to distinguish pain by expressions correctly assessed discomfort levels in the mice, based on photographs, with 80 percent accuracy. Accuracy rates went up to 97 percent when high-resolution video was used.
In another set of experiments, researchers created a so-called “knock-in” mouse with a genetic mutation known to cause migraine headaches in humans. The mice displayed the same telltale grimaces as seen in the animals that had been injected with an inflammatory substance.
When the researchers administered pain-relieving drugs, facial expressions returned to normal.
The study reinforces Charles Darwin’s belief that non-human animals express emotions — including pain — through facial expression, and that such displays evolved through the process of natural selection.
In terms of evolution, the ability to communicate pain experience to others may benefit both the sender and the receiver, in such ways that help might be offered or a warning signal paid attention to.
The fact that mice and humans share three facial expressions — narrowing eyes and bulging nose and cheeks — when it comes to pain, supports Darwin’s prediction that facial expressions have deep evolutionary ties.
The researchers are now investigating if the scale works equally well with other species, and if mice can respond to facial expressions in each other.
The study was published in the journal Nature Methods on Sunday May 9.
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