In a research study published in Current Biology, it is described that memory of earlier pain – which is believed to drive a chronic pain–may vary based on sex, in both mice and humans.
A team researchers from McGill and University of Toronto Mississauga has discovered that men as well as male mice recall earlier painful experiences clearly, resulting in hypertensive and stressful reactions to later pain when returned to body region where it had previously experienced. On the other hand, women and female mice did not appear to be stressed by their earlier painful experiences.
According to the research team, robust translation nature of their findings would be promising to help scientists move ahead in their search of potential chronic pain treatment in the future.
Robust Results in Mice and Men
In their experiments, the researchers looked at pain hypersensitivity in mice and discovered the differences in levels of stress between male and female mice. They extended their investigations to humans to check whether the results are similar. Surprisingly, there seemed to be similar differences between men and women as it was observed in mice, senior author Jeffrey Mogil said.
What’s even more surprising was that male species reacted more, according to first author Loren Martin, because it is commonly known that females are more sensitive to pain compared to men and are usually more stressed out.
Developing Memories of Pain in Humans and Mice
In experiments with mice and humans, the team included a total of 79 participants, of which, 41 men and 38 women between the ages of 18-40 (for humans). The subjects were taken to a particular room (or testing container) where they experienced low levels of pain due to heat delivered to their forearm (humans) and hind paw (mice).
The pain level was rated on a 100-point scale for humans, and for mice, it was rated on how quickly they moved away from the source of heat. Immediately after the initial low level of pain experience, the study participants experienced highly intense pain developed to serve as a Pavlovian conditioning stimuli. The human participants were assigned to wear tightly inflated blood pressure cuff followed by arm exercise for around 20 minutes. According to the researchers, it is extremely painful and just 7 in 79 participants rated it below 50 on the scale. In case of mice, each individual were injected with diluted vinegar which causes stomach ache for up to 30 minutes.
Blocking Memory Makes the Pain Go Away
In order to determine the role of memory in painful experiences, on the following day, the subjects were sent to the specific room or testing container (depending on the species), and heat was again delivered to their forearms and hind paws.
It was found that men rated the pain of heat higher than the previous day and also higher than their female counterparts did. Similarly, male mice, but not female, exhibited a higher pain response to heat, when returned to the same environment.
To confirm that the pain was more due to memories of earlier pain, the team inhibited the memory by injecting the male mice’s brains with a drug called ZIP that blocks memory. As a result, in a pain memory experiment, the mice did not show any remembered pain.
According to the researchers, the study supports the idea that memory of earlier pain can impact later pain which may be useful for chronic pain treatment in the future.