Pain is not pleasant. We try to avoid it at all costs. Even witnessing another in pain can cause one to feel pain, particularly if it is someone we know. This ability to empathize has been thought to be uniquely human. However, Dale Langford and colleagues provide evidence in a recent Science publication that this ability may also exist in mice. Other studies had shown that pain-related distress of a conspecific can serve as an aversive stimulus in non-primates, but Langford and colleagues have demonstrated that the degree of pain can be modulated by the presence or absence of a conspecific in similar distress.
The authors first injected mice with a substance that induced pain behaviour, in this case writhing. One group of mice was left in isolation while individuals from a second group were placed in the company of another mouse that had either been injected with the same substance or was uninjected. The team noticed that the injected mice that were left alone and the mice that were placed with an uninjected mouse both exhibited the same levels of pain behaviour. However, mice placed in the company of another injected mouse displayed significantly more pain behaviour than the other groups; i.e. seeing another mouse in pain increased their own pain behaviour. There was just one catch though: pain behaviour only increased if the accompanying injected mouse was a cagemate and not if it was a stranger. Thus, injection and the mere observation of a cagemate in similar distress produced significant hyperalgesia, suggesting that the pain system can be sensitized by the observation of pain in someone familiar.
In a separate test, another pain behaviour, paw licking, was induced by injecting either a low or high dose of formalin. In those that received a low dose and observed a cagemate that received a high dose, there was an increase in pain behaviour compared to mice that received a low dose and were left in isolation; an effect similar to that seen in the previous experiment. Again,the changes in pain behaviour were only seen if the other mouse was familiar. Surprisingly, there was a reduction in pain behaviour in those mice that received a high dose but observed a cagemate that received a low dose. Thus,observation not only enhances pain but can also reduce it.
What would be the advantage of not just identifying and feeling the pain of another, but to have your own pain compounded? Perhaps it evolved from a need to recognize pain in others in order to avoid noxious stimuli, an ability valuable to all organisms. There have been reports of `mirror neurons' in the brain, which fire not only when one is performing an action but also when the same action is performed by another, and may provide a possible mechanism for the `mirror pain' seen here. Interestingly, autistic children show no mirror neuron activity while imitating and observing emotional expressions and it has been suggested that a dysfunctional mirror neuron system may underlie the social deficits observed in autism.
