Scientists increasingly believe that one of the driving forces of chronic pain, the number one health problem in prevalence and overload, seems to be the reminder of the previous pain. Research published in Current Biology suggests that there may be sex-based variations in how pain is remembered in rats and humans.
The research team, led by colleagues at McGill and Toronto Mississauga universities in Canada, found that men (and male mice) clearly remember previous painful experiences. As a result, they were stressed and hypersensitive to subsequent pain when they returned to where they had previously experienced.
Women (and women) did not seem to feel stressed by their previous experiences of pain. Researchers believe that the robust translational nature of the results, from mice to men, will potentially help scientists move forward in the search for future treatments for chronic pain.
It was a discovery that totally surprised. "We set out to do an experiment to observe the hypersensitivity to pain in mice and found these surprising differences in stress levels between male and female mice," explains lead study author Jeffrey Mogil, professor of pain studies at the Department of Psychology of McGill and Alan Edwards Center for Pain Research.
And he continues: "So we decided to extend the experiment to humans to see if the results would be similar." We were surprised when we saw that there seemed to be the same differences between men and women we saw in mice.
"What was even more surprising was that men reacted more, because it is well known that women are more sensitive to pain than men and that they are usually more stressed," explains Loren Martin, the first author of the article and assistant professor. of Psychology at the University of Toronto Mississauga.
In experiments with humans and mice, subjects (41 men and 38 women between the ages of 18-40 in the case of humans) were taken to a specific room (or placed in a test container somewhat, depending on the species), where they experienced low levels of pain caused by the heat that was supplied to the hind paw or forearm.
Humans assessed the pain level on a 100-point scale and the rats "assessed" the pain by how quickly they moved away from the heat source. Immediately after this initial experience of low pain, subjects experienced a more intense pain designed to act as a Pavlovian conditioning stimulus.
Human subjects were asked to use a firmly inflated blood pressure cuff and to exercise their arms for 20 minutes. This is unbearable and only seven of the 80 subjects scored less than 50 on a 100-point scale. Each rat received a diluted injection of vinegar designed to cause a stomach ache for about 30 minutes.
BLOCKING MEMORIES MAKE DISAPPEARING PAIN
To observe the role of memory in the experience of pain, the next day the subjects returned to the same room or to a different room, to the same room or to a different testing room. Once again the heat was applied to his arms or hind legs.
When (and only when) they were taken to the same room as in the previous test, men rated heat pain higher than the day before and larger than women. Similarly, male rats, but not female rats, who returned to the same environment showed a greater response to heat pain, while rodents placed in a new, neutral environment did not.
"We believe that rats and men anticipated the cuff or vinegar, and for men, the stress of that anticipation caused a greater sensitivity to pain," says Mogil, "there was a reason to expect us to perceive greater sensitivity to pain." second day, but there was no reason to expect it to be specific to men, this was a complete surprise. "
To confirm that the pain increased due to memories of previous pain, the researchers interfered in memory by injecting into the brains of male mice a medication called ZIP that is known to block memory. When the scientists performed the pain recall experiment, these mice showed no signs of remembering pain.
"This is an important finding because there is growing evidence to suggest that chronic pain is a problem as long as you remember, and this study is the first time it shows a pain memory using a translation approach, both rodents as well as the human beings, "says Martin, who is also a Canadian Level II Research Professor in Translational Pain Research.
He adds, "If remembered pain is a driving force of chronic pain and we understand how pain is remembered, we can help some patients by directly treating the mechanisms behind memories." "This research supports the idea that pain memory can affect later pain," concludes Mogil.