Pain is a powerful teaching tool. Memories of past painful experiences can shape future encounters—even unconsciously. New research in mice and humans now shows that males but not females of both species developed pain hypersensitivity in an environment where they had previously been through a painful experience, likely the result of increased stress that the males felt.
The research, published January 21, 2019, in the journal Current Biology, is the latest in a growing number of studies to show that pain processing might be fundamentally different between males and females.
The work comes from Jeffrey Mogil, McGill University, Montreal, Canada, and Loren Martin, University of Toronto, Canada. Mogil and colleagues have led the charge in studying rodents of both sexes in pain studies—and they’re finding differences everywhere they look.
“Why pain is so riddled with sex differences, we just don’t know,” Mogil said.
In a commentary accompanying the new study, pain researchers Theodore Price and Stephanie Shiers, University of Texas at Dallas, US, wrote: “Leading up to this work, Mogil and colleagues have been the torch runners for the study of sex differences in pain. Their groundbreaking research has revolutionized the pain field and changed the way pain researchers assess pain and pain plasticity,” referring to how the nervous system changes during pain. The researchers “have now added an important new chapter to the phenomenon of pain memory,” they wrote.
What’s past is prologue—at least for male mice
The researchers first wanted to test whether being in the setting of a previously painful experience would increase future pain sensitivity in mice.
To do that, they placed the animals in in a Plexiglas cylinder and tested how quickly the mice withdrew their paws in response to painful heat. Immediately afterward, the mice were injected in the abdomen with acetic acid (a chemical component of vinegar), which causes painful abdominal cramps and writhing behavior for 20 to 30 minutes, and then returned to their home cages after 30 minutes.
A day later, the mice were placed either back in the same cylinder in the same room, or in a novel cubicle in a different room, and again tested for heat pain sensitivity.
Remarkably, Mogil said, “we found that only males tested back in the same room were more sensitive on day two than on day one. In every other group, we saw no change.”
Of (male) mice AND men
Martin then carried out a similar experiment in people. Forty-one male and 38 female study subjects made continuous ratings of pain caused by a thermal probe to the inner forearm. After the testing was complete, they underwent a much more painful experience: 20 minutes with a blood pressure cuff pumped up significantly higher than for a routine exam—while they exercised the arm.
“It’s excruciating,” Martin said. “I’ve done it, and I’ll never do it again.” Mogil concurred.
The next day, subjects returned for a new round of thermal testing; this was to improve the accuracy of the study, the researchers told them. Half the subjects came back to the same room and underwent re-testing by the same experimenter as the day before. But, for the other half of subjects, the investigators told them there had been a flood in the previous day’s testing room, and that the experimenter who tested them earlier had fallen ill, so these folks were retested in a different building by a different experimenter—a novel environment.
“For men, much like in mice, those tested in the same location showed a hypersensitive response when tested on the second day—they rated their pain as being a little bit more intense and unpleasant,” Martin said.
Men tested in the novel environment and women tested in either environment had similar pain ratings as the day before. And, in neither people nor mice were there sex differences in pain on the first day.
The stress effect on pain memories
The researchers went on to link the pain hypersensitivity in males to testosterone, as castrated mice failed to show such hypersensitivity. They also showed that stress played a role, as levels of corticosterone, a stress hormone, were elevated only in male mice that were re-tested in the same environment and not in any other mice.
Similarly, men re-tested in the same environment reported significantly higher stress levels than the day before. And, “how stressed the men were predicted how much more sensitive they were to the pain, suggesting that stress caused the hypersensitivity itself,” Mogil said.
Ruth Drdla-Schutting, a pain investigator at the Medical University of Vienna, Austria, who was not involved in the research, wrote in an email that she was not surprised that mice would develop increased pain sensitivity from stress having previously been through a painful experience like the one in this study. “What is extremely surprising, though, is that this phenomenon was only observed in males.”
To understand what changes in the nervous system might explain the sex difference in pain, the researchers blocked a molecule in mice thought to be important for memory formation. This prevented the sensitivity to pain seen in male mice.
Interestingly, the inhibitor worked when it was injected into the brain, as the researchers expected, but also when it was injected directly into the spinal cord. That suggests that some element of pain memory formation takes place in the spinal cord.
“I’m always astounded by just how much pain physiology is happening in the spinal cord,” Mogil said. “I’m starting to believe that—for whatever reason—it’s all about the spinal cord.”
“Pain memories are implicit,” according to Drdla-Schutting. “They may be stored in the nervous system at a level precluding consciousness,” but they may still powerfully shape responses to future, potentially painful events, she said.
Does evolution hold the answer?
Why would nature produce the newly identified sex difference in the first place?
One might expect that females, not males, would have developed pain hypersensitivity. According to previous studies, Drdla-Schutting noted, “women have a higher prevalence of developing chronic pain disorders, and stress-related pain disorders such as fibromyalgia or irritable bowel syndrome are more frequent in females.” So she wondered whether an evolutionary perspective might provide an explanation.
Meanwhile, Mogil wondered why men but not women would experience stress. “One possibility is that although women have more sensitivity to pain, women have a lot more experience with pain—even at age 20,” so men may have been more stressed by the pain’s novelty. Another idea, he said, is that the phenomenon by which women have been said to forget the pain of childbirth is a more global mechanism. “Maybe women really do not remember pain as well as men.”
Looking back on evolution, males and females have experienced distinctly different types of pain. Women may begin experiencing regular monthly visceral pain in adolescence with their menstrual cycle, whereas males may have been more likely to be injured in an attack by a predator or violent adversary.
“It may be more advantageous for a male to remember where they encountered physical pain if their survival depends on it, even if it’s unconsciously,” Martin said. For women, on the other hand, “their pain happens wherever they are, so the environmental context doesn’t really matter too much,” Mogil said.
Regardless, “this study highlights that a patient’s personal history might actually contribute in a very specific way to chronic pain development,” Martin said. “That’s often something not taken into account.”
This story first appeared on the Pain Research Forum and has been adapted for RELIEF.
Stephani Sutherland, PhD, is a neuroscientist and freelance journalist in Southern California. Follow her on Twitter @SutherlandPhD