BORIS HEIFETS, MONIQUE SMITH
Mice can share each other’s pain. And they appear to do it through smells.
After entering the bedding where mice in pain had slept, new mice became more sensitive to pain themselves. The experiment shows that pain can move from one animal to another. No injury or illness is required.
Scientists described their findings online October 19 in Science Advances.
The results “add to a growing body of research showing that animals communicate distress and are affected by the distress of others,” says Inbal Ben-Ami Bartal. He works at the University of California, Berkeley as a neuroscientist. Neuroscientists study how the nervous system works and how it develops. The nervous system includes the brain, spinal cord and nerve cells.
Andrey Ryabinin is a neuroscientist at Oregon Health & Sciences University in Portland. He and his colleagues hadn’t set out to study the transfer of pain from mouse to mouse. But they noticed something curious during another experiment. Mice that had become addicted to alcohol were suddenly prevented from getting any more alcohol. Soon, they developed a condition known as withdrawal. It’s a set of symptoms that arise when drugs or alcohol are no longer available to an addict. Symptoms in humans include sweating and nausea or anxiety and shakiness.
In the experiment, mice going through withdrawal had a higher sensitivity to pokes on the foot. So, it turns out, did perfectly healthy mice. But only if those healthy mice had spent time in the same room as had mice going through withdrawal.
This surprised the scientists. “We realized that there was some transfer of information about pain,” said Ryabinin. It was moving from an injured mouse to its neighbor.
He and colleagues tested this pain effect some more. They poked mice in the paw with a thin fiber. The more pain a mouse was in, the bigger its reaction. And mice grew more sensitive to the paw poke when they suffered alcohol withdrawal. The same was true when they were suffering withdrawal from morphine, a heavy-duty pain reliever. It even occurred after an injection that caused tissue inflammation. Inflammation is a physical condition in which part of the body becomes reddened, swollen, hot and often painful.
Healthy mice housed in the same room as the mice in pain themselves became more sensitive to that poke of their foot. These healthy mice also showed other signs of heightened pain sensitivity.
The results are compelling evidence for the idea that pain can be transmitted socially, says Christian Keysers. He is a neuroscientist at the Netherlands Institute for Neuroscience in Amsterdam.
A sniff of pain?
Pain’s contagion seemed to spread through the nose. That’s what further experiments revealed.
After spending time in bedding used by mice in pain, a healthy mouse’s pain sensitivity spiked. Some odor signals may have been transferred from the pained mouse onto the bedding before a mouse not experiencing pain showed up and began sniffing around. Ryabinin and colleagues are looking for what the compounds might be that could relay this pain signal.
What does this research mean for humans? No one can be sure. The sense of smell in people is far less sensitive than that of many other animals. So it’s unclear whether odors might also transmit human vulnerability to pain, Ryabinin says.
The data do suggest that scent signals can carry a pain message, Keysers says. Other senses, such as hearing or vision, may be important too, he notes. Mice might see a mouse in distress or hear its pained squeaks. Still, adds Keysers, the new paper fits with other work that shows “rodents exchange information about their states in many exciting and complex ways.”
The goal of this research is to probe the means by which animals may become more sensitive to pain. Such data might help explain more generally why pain comes and goes. The new findings suggest that sometimes pain isn’t caused by an injury, Ryabinin says. Instead, social factors or cues — such as seeing another in pain — might influence pain perception.
This idea might help researchers better understand the long-term pain that can plague some people. This chronic pain may begin mysteriously or just persist long after some injury heals.
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addict Someone unable to control the use of a habit-forming drug or to forego an unhealthy habit (such as video game playing or phone texting). It results from an illness triggered by brain changes that occur after using some drugs or engaging in some extremely pleasurable activities. People with an addiction will feel a compelling need to use a drug (which can be alcohol, the nicotine in tobacco, a prescription drug or an illegal chemical such as cocaine or heroin), even when the user knows that doing so risks severe health or legal consequences.
cell The smallest structural and functional unit of an organism. Typically too small to see with the naked eye, it consists of watery fluid surrounded by a membrane or wall. Animals are made of anywhere from thousands to trillions of cells, depending on their size. Some organisms, such as yeasts, molds, bacteria and some algae, are composed of only one cell.
chronic A condition, such as an illness (or its symptoms, including pain), that lasts for a long time.
colleague Someone who works with another; a co-worker or team member.
compound (often used as a synonym for chemical) A compound is a substance formed from two or more chemical elements united in fixed proportions. For example, water is a compound made of two hydrogen atoms bonded to one oxygen atom. Its chemical symbol is H2O.
contagious An adjective for some disease that can be spread by direct contact with an infected individual or the germs that they shed into the air, their clothes or their environment. Such diseases are referred to as contagious.
fiber Something whose shape resembles a thread or filament of some kind. (in nutrition) Components of many fibrous plant-based foods. These so-called non-digestible fiber tends to come from cellulose, lignin, and pectin — all plant constituents that resist breakdown by the body’s digestive enzymes.
inflammation The body’s response to cellular injury and obesity; it often involves swelling, redness, heat and pain. It is also an underlying feature responsible for the development and aggravation of many diseases, especially heart disease and diabetes.
nerve A long, delicate fiber that communicates signals across the body of an animal. An animal’s backbone contains many nerves, some of which control the movement of its legs or fins, and some of which convey sensations such as hot, cold, pain.
nervous system The network of nerve cells and fibers that transmits signals between parts of the body.
neuroscience The field of science that deals with the structure or function of the brain and other parts of the nervous system. Researchers in this field are known as neuroscientists.
perception The state of being aware of something — or the process of becoming aware of something — through use of the senses.
physical (adj.) A term for things that exist in the real world, as opposed to in memories or the imagination. It can also refer to properties of materials that are due to their size and non-chemical interactions (such as when one block slams with force into another).
rodent A mammal of the order Rodentia, a group that includes mice, rats, squirrels, guinea pigs, hamsters and porcupines.
social (adj.) Relating to gatherings of people; a term for animals (or people) that prefer to exist in groups. (noun) A gathering of people, for instance those who belong to a club or other organization, for the purpose of enjoying each other’s company.
spinal cord A cylindrical bundle of nerve fibers and associated tissue. It is enclosed in the spine and connects nearly all parts of the body to the brain, with which it forms the central nervous system.
symptom A physical or mental indicator generally regarded to be characteristic of a disease. Sometimes a single symptom — especially a general one, such as fever or pain — can be a sign of any of many different types of injury or disease.
tissue Any of the distinct types of material, comprised of cells, which make up animals, plants or fungi. Cells within a tissue work as a unit to perform a particular function in living organisms. Different organs of the human body, for instance, often are made from many different types of tissues. And brain tissue will be very different from bone or heart tissue.
transmit To send or pass along.
withdrawal (in medicine) An almost disease-like syndrome that can develop after animals (including people) attempt to stop using a drug (including alcohol) to which they have become addicted. Shaking, sweating, trouble sleeping, anxiety, diarrhea, vomiting, abdominal cramping, muscle aches and flu-like symptoms can occur and last for days.