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Facial expressions quantify pain in lab mice

Erin Podolak

The mouse grimace scale provides researchers a quantifiable method for determining how mice express pain.

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Researchers from McGill University have created a new method that determines the severity of pain experienced by laboratory mice. The technique will help reduce animal discomfort in biomedical research and advance the development of new pain medications.

The method, called the mouse grimace scale, uses five facial clues that indicate the severity of a mouse’s pain. Three characteristics are adapted from a similar scale for human babies: orbital tightening, nose bulge, and cheek bulge. Two other characteristics are unique to the mouse scale: ear position and whisker change.

The mouse grimace scale. Source: Jeffrey Mogil.

“I think if humans had whiskers and could move their ears, these would be useful in human pain studies too,” lead author Jeffrey Mogil, leader of the pain genetics laboratory at McGill University, told BioTechniques.

To develop the scale, Mogil and collegues provoked laboratory mice with an injection that caused painful inflammation. The researchers captured still frames from a high-resolution video of the mice’s facial expressions before and after the injection. In a blind analysis, the researchers were able to identify the mice experiencing pain from the injection with 80% accuracy in the still images and 97% accuracy in the video.

“Darwin wrote a whole book about [the physical expression of pain], so the amazing thing isn’t that we found this to be true, the amazing thing is that no one thought to come up with this before,” said Mogil. “Vets have known it to be true, but now we can quantify it.”

Reducing animal pain in biomedical research

Although animal rights activists have accused Mogil of torturing the mice in the experiment, Mogil said the findings could actually help reduce the pain currently experienced by laboratory animals.

“This finding alone, among pain research in the last few decades, actually has a chance to relieve the pain in animals, but these animal rights activists don’t realize that,” said Mogil. “These tests are mild—nothing that Tylenol or pain relievers wouldn’t fix—but you can never know how much pain an animal is in the way you can’t feel another person’s pain experience.”

Veterinarians and animal researchers could use the scale to make procedures such as surgeries and implantations less uncomfortable for animals. The scale could provide researchers with an indication of when to administer pain relievers and how much. “Now veterinarians will be able to tell the mice are in pain, so it makes [the administration of pain relievers] more precise,” said Mogil.

The researchers now intend to re-explore a study published by Mogil and colleagues in 2006 showing that the pain behavior of mice is increased if a mouse sees another mouse that it knows in pain. This study inspired Mogil to explore this empathy effect in mice; to see if pain signals were transmitted visually between the mice. He deviated from this original line of inquiry for this research analyzing facial expressions to see if humans can identify pain cues in mice. He will now continue his initial line of inquiry into whether or not mice can read each other’s facial cues to tell if they are in pain. The researchers will repeat some experiments used in the field of pain research to determine if the results change when using the grimace scale.

The team also intends to find out if the pain scale can be used in other animals. “This could work its way into veterinary practice if it is pretty similar in cats and dogs,” said Mogil. “Right now, we are doing the same thing in rats, and we are getting responses similar to the mouse study. The scale may need tweaking, but it’s promising.”

The paper, “Coding of facial expressions of pain in the laboratory mouse,” was published May 9, in Nature Methods.