In the past, many drugs for killing pain have been tested on mice and found to be effective, however, researchers failed to consider one tiny factor - gender differences.

Scientists at the McGill University, Duke University and Canada's Hospital for Sick Children discovered that perception and processing of pain differ between men and women.

"This hadn't been reported before because no one ever used females, so they weren't in a position to know one way or another," says McGill University pain researcher Jeffrey Mogil.

Mogil also co-authored findings of a study published in Nature Neuroscience.

Previously, it has been thought that the immune cell microglia transmits pain signals through the nervous system to the brain, telling the brain how to react. While this is true for males, it has been found that in females, a different immune cell functions in the same way - the T cell.

According to Mogil, the reason why female animals have been previously excluded from clinical experiments is because their menstrual cycle might cause varying results. In his team's study, Mogil says that some pain drugs that are supposed to target microglia have failed in human clinical trials, probably because the biology only corresponds to half of the population.

At the University of Birmingham in Alabama, psychologist Robert Sorge and his team also tried to see how different the effects would be when pain-killing drugs are tested on both male and female mice. They induced equal levels of pain and inflammation in the mice's paws, and after seven days, injected the mice with microglia-targeting drugs. The team found that the pain-killing drugs had taken effect on the male mice, but not on the females.

Sorge's team further confirmed that males' hypersensitivity to pain is dependent on BDNF signals from microglia. BDNF - brain-derived neurotrophic factor - is a protein in microglia that prompts spinal-cord neurons whenever injuries happen, making the body sensitive to touch. It does not seem to work the same way in females.

Currently, Mogil and his team are studying pain perception and processing specifically in female mice, trying to identify their pain-sensitization pathways. They encourage other researchers to include female mice in other clinical trials, contrary to previous practice, emphasizing that the inclusion of the female mice population costs nothing.

Photo: Mario Antonio Pena Zapateria | Flickr

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