A breakthrough research found that male and female mice use different cells to signal pain. This could explain why both more women suffer from chronic pain than men, and pain relief medication seems to respond differently in women.

Men and women respond to pain differently.

The international team of researchers set out to test a long-standing hypothesis: that the pain signal is transmitted to the brain from the site of injury or inflammation microglia – immune cells found in the brain and spinal chord. But when drugs were administered that inhibited the microglia cells, only the male mice showed reduce pain response. The female mice were completely unaffected, suggesting pain is transmitted via a different mechanism. The researchers hypothesis that T cell, fundamentally different immune cells, relay pain the female mice. This needs to be confirmed.

“Understanding the pathways of pain and sex differences is absolutely essential as we design the next generation of more sophisticated, targeted pain medications,” says co-senior author Michael Salter, a professor at the University of Toronto.

“We believe that mice have very similar nervous systems to humans, especially for a basic evolutionary function like pain, so these findings tell us there are important questions raised for human pain drug development.”

In 2009, single-sex studies of male animals outnumbering those of females 5.5 to 1. Prompted by a growing body of evidence that suggests males and females respond differently, the  US National Institutes of Health issued a new policy which mandates preclinical research to study tissue from both sexes.

“For the past 15 years scientists have thought that microglia controlled the volume knob on pain, but this conclusion was based on research using almost exclusively male mice,” Mogil says. “This finding is a perfect example of why this policy, and very carefully designed research, is essential if the benefits of basic science are to serve everyone.”