Hypersensitivity to pain can be transferred amongst mice. The discovery raises questions about whether similar empathy pathways exist in humans, while also indicating a possible source of error in animal experiments.
The human experience of pain is influenced by environmental factors and our social environment: “when you win, nothing hurts,” as sporting coaches will tell you.
In Science Advances, a team at Oregon Health and Science University note that the influence of these factors in other social animals is relatively unexplored. The authors were struck, however, by the finding that uninjured mice, housed with those that have suffered a peripheral nerve injury, react more strongly to exposure to mild acids.
To test this further the authors used established techniques to induce hypersensitivity to pain in mice. These included injecting a single paw with chemicals that cause inflammation, and making their subjects go cold turkey from morphine or alcohol. The unfortunate mice shared cages with others of their species that were spared the unpleasant treatment.
After two weeks these cage-mates also showed increased sensitivity to pain, in a way separately housed control mice did not. Indeed, while the uninjured cage-mates did not become as hypersensitive as those that experienced an injury, their reactions were closer to the treated mice than to the controls, Moreover, sharing a cage with a detoxing mouse produced responses that were not significantly different from undergoing morphine detox directly. Female mice were more likely to suffer in sympathy with their cage mates than males.