Take 2 studs and call me in the morning (how male docs inhibit pain in mice)

Researchers who were working on the mechanisms of pain began to notice something strange in their test animals. Sometimes, when they injected the animals to produce a painful response, the animals barely responded. They didn’t grimace or lick the site of the injection.

At other times, the animals did grimace and frequently lick the site.

Some of the investigators began to wonder why. They also noticed that, when the humans left the room, the animals then began to display more behaviors to indicate that they were in pain.

Over the course of time the scientists ferreted out the reason. They noted that when only male researchers were present, the mice tended to show lower levels of pain than when only females were present. This behavior was essentially independent of the sex of the mice used.

What was this “of mice and men” response? The researchers were puzzled as to why the mice reacted to male researchers but not to female researchers. They tried an experiment where they left used t-shirts from the researchers with the injected mice. They found that worn male investigators’ t-shirts (minus the male investigator), when left with the mice, caused decreased pain behaviors. T-shirts worn by female investigators, when left with the injected mice, didn’t decrease their pain behaviors.

Something about the scent of a man seemed to cause the mice to ignore their distress.

Tell me if it hurts. (Hot doc via Shutterstock)

Tell me if it hurts. (Hot doc via Shutterstock)

Further study indicates that the testosterone in male sweat inhibited the pain behaviors in mice (females also secrete testosterone in sweat, but in lower quantities.) Similar results were found when they tried placing the mice in bedding that had been used by other male animals that were not mice (e.g., bedding from dogs.) The mice exhibited less pain when in the bedding from male animals, but not the bedding from female animals.

The investigators think that perhaps the smell of testosterone stresses the mice. The stress response may lead to an endorphin-like release in the mouse brain, therefore damping-down the pain sensation. When the testosterone smell (or male investigator) leaves, no more stressor and the pain returns.

They have hypothesized that, perhaps, not showing pain has a survival benefit for mice in the wild. A mouse who displays pain could be viewed by a predator as having been injured. An injured mouse could be viewed as weakened and easier to attack.

This research will need to be repeated in mice and other species to see if it is reproducible. If it does turn out to be correct, it may prove useful in helping to control pain in humans. In the future, when you have a painful injury, the doctor may just say, “Take two studs and call me in the morning.”

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Mark Thoma, MD, is a physician who did his residency in internal medicine. Mark has a long history of social activism, and was an early technogeek, and science junkie, after evolving through his nerd phase. Favorite quote: “The most exciting phrase to hear in science... is not 'Eureka!' (I found it!) but 'That's funny.'” - Isaac Asimov

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