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.
The authors suspected that the pain sensitivity was transferred through smell. To test this they exposed mice that had neither been on alcohol, nor co-housed with recovering alcoholic mice, to bedding from the different mouse groups. Bedding from either the detoxing mice, or those housed with them, induced pain sensitivity, while bedding from control mice did not. In other words, the pain sensitivity could be passed on twice, and at least one of those transmissions must have been through smell, rather than witnessing what the detoxing mice were going through.
Mice with transferred pain sensitivity did not, however, show increased anxiety, nor were they more startled by noise.
From an evolutionary perspective, the reason we feel pain is to stop us doing things that might be damaging, such as walking on an injured leg rather than giving it time to heal, or eating poisonous foods. Social animals benefit from learning from the experiences of others, so it makes sense for them to feel each others pain, at least to some extent.
Exactly how applicable the findings are to humans remains to be seen. As a species that relies much less on its nose than rodents, humans may have different mechanisms to transfer our experiences. Nevertheless, few of us are immune to the psychological effects of being around those who are suffering, and the authors speculate about possible physical implications as well.
The work also emphasizes the need for animal researchers to keep control animals separate from those they are experimenting on, lest they are accidentally affected.