Scientists have for the first time created a sliding scale of pain for mice based on facial expressions, according to study published Sunday.
The so-called "mouse grimace scale" will speed up the development of new analgesics for humans, and could help reduce unnecessary suffering of mice and other animals in biomedical research, the researchers said.
"There are also serious implications for the improvement of veterinary care," said Jeffrey Mogil, a professor at McGill University in Montreal and the main architect of the study.
Research on pain and how to relieve it depends heavily on the use of rodents as stand-ins for humans, so accurate measurement of pain intensity in lab mice is crucial.
Up to now, however, it was not known whether degrees of discomfort and suffering in mice correspond to spontaneous facial responses, as is the case for people.
Doctors and nurses routinely use such scales to assess pain in individuals unable to communicate verbally, such as infants and the cognitively impaired.
Line drawings of faces showing different levels of discomfort are also used to help manage chronic pain in children asked to match what they feel with the appropriate images.
To find out whether rodents grimace when it hurts, Mogil and colleagues monitored and recorded facial movements before and during the injection of a substance known to cause painful inflammation.
The mice showed discomfort through facial expressions in a way similar to humans.
When pain was more intense, for example, 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.
Using an intensity scale based on changes in these five facial features, persons trained to "read" pain in expressions correctly assessed discomfort levels in the mice, based on photographs, with 80 percent accuracy.
Looking at high-resolution video images, accuracy rates went up to 97 percent.
In another set of experiments, the researchers created a so-called "knock-in" mouse with a genetic mutation known to cause migraine headaches in humans.
As expected, the mice displayed the same telltale grimaces as seen in the animals who had been injected with an inflammatory substance.
When pain-relieving drugs were administered, facial expression returned to normal.
The study, published in the journal Nature Methods, also bolsters Charles Darwin's belief that non-human animals express emotion -- including pain -- through facial expression, and that such displays emerged from the process of natural selection.
In evolutionary terms, the ability to communicate pain experience to others may benefit both the sender and receiver, such that help might be offered or a warning signal heeded.
The fact that three of the facial pain cues in mice are found in humans -- narrowing eyes, along with bulging nose and cheeks -- also support Darwin's century-old prediction that facial expressions have deep evolutionary roots.
Following up on these findings, the researchers are currently investigating whether the scale works equally well in other species, and if mice can respond to facial pain cues in each other.