Rodents Feel Less Pain When Men Are Around - And That's Worrying To Scientists

Krishnendu Halder/ReutersA pharmacologist in India checks the effect of cancer drugs on a mouse.Those who study animal behaviour have long feared that their mere presence might affect the outcomes of their experiments.

Few attempts have been made to see if it actually does so - but a study just published in Nature Methods by Jeffrey Mogil of McGill University in Montreal and his colleagues has tried to correct that deficiency.

Dr Mogil's team injected the ankles of mice with zymosan A, an inflammatory agent, and monitored what happened when, in some cases, an experimenter stayed in the room, sitting about half a metre away, and when in others, he or she did not. They evaluated evidence of murine pain, caught on videotape, using the "mouse grimace scale". This measures ear and whisker position, eye-squeezing and the bulging of noses and cheeks to gauge an animal's level of distress.

Their suspicion that an experimenter's presence would affect distress levels in some way turned out to be true - but only half the time. Both when the mice were left on their own and when they had any of four female researchers in the room with them, the injections induced visible distress. But when any of four male researchers was there, the animals showed significantly fewer signs of pain. The same was true when the team did the experiment on rats, and also when they used a different pain stimulus, formalin.

Nor was the actual presence of a human required. When the team substituted the four men with T-shirts that those men had slept in the night before, the rodents responded in the same way. Women's T-shirts, like women themselves, did not reduce apparent pain - indeed, when placed in the room next to a man's shirt, a woman's shirt was able to abolish its effect. Nor was the phenomenon caused only by the scent of human males. Bedding slept in by male dogs, cats, rats and even guinea pigs all produced a similar response.

The scent of a man

The T-shirt experiments led the team to suspect that the effect they observed is caused by something in male sweat. They therefore tested mice with gauze soaked in either androstenone or androstadienone (steroids found in male sweat and urine). The same thing happened and, moreover, the amount of grimacing the animals showed was inversely related to the concentration of the steroids.

The question remained of whether the mice were actually feeling less pain, a phenomenon called stress-induced analgesia, or just masking it better in the presence of men. Further experiments, which measured levels of corticosterone, a stress hormone, in the animals' blood, showed that they were indeed stressed by the mere smell of a man. And examination of gene activity in their pain-producing nerve cells confirmed that these cells temporarily shut up shop at the same time. Simply put, the animals were being scared painless. (A significant increase in faecal pellets suggested they were scared shitless as well.)

This is an important finding. At the least, it may account for some failures to replicate results in animal experiments, a perennial problem in the field. And, because stress affects numerous bodily responses besides pain, research in many areas may be affected. Dr Mogil suggests, therefore, that the sex of those who conduct experiments needs to be a matter of record, included routinely in the methods sections of research papers.

His result, though, has wider ramifications even than that, for there are likely to be situations when it is the mere presence of a human being, regardless of sex, which is the problem. Ethologists - those who study the behaviour of wild animals - have known this for a long time and thus try either to make themselves invisible to the objects of their attention, or else to become so familiar that they are ignored as a harmless part of the environment.

This process of habituation, unfortunately, is time-consuming and therefore difficult in a high-throughput physiology laboratory. But Dr Mogil's study is a warning that such labs need to examine their procedures carefully. Looking at ethology's methods may not be a bad start.

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