By Pat Anson, Editor

A new study may help explain why women are more likely to have chronic pain and are more sensitive to painful sensations than men.

It’s because their brains work differently.

In experiments on laboratory animals, researchers at Georgia State University found that immune cells in female rats are more active in regions of the brain involved in pain processing. Their study, published in the Journal of Neuroscience, found that when microglia cells in the brain were blocked, the female rats responded better to opioid pain medication and matched the levels of pain relief normally seen in males.

Women suffer from a higher incidence of chronic pain conditions such as fibromyalgia and osteoarthritis. And studies have found that they often have to take more morphine than men to get the same level of analgesia.

“Indeed, both clinical and preclinical studies report that females require almost twice as much morphine as males to produce comparable pain relief,” says Hillary Doyle, a graduate student in the Neuroscience Institute of Georgia State. “Our research team examined a potential explanation for this phenomenon, the sex differences in brain microglia.”

In healthy people, microglia cells survey the brain, looking for signs of infection or pathogens like bacteria. Morphine is perceived as a pathogen and activates the cells, causing the release of inflammatory chemicals such as cytokines. Researchers say this causes "a neuroinflammatory response that directly opposes the analgesic effects of morphine."

To test their theory, researchers gave male and female rats naloxone, a drug that reverses the effects of an opioid overdose, and found that it inhibits the microglia activation triggered by morphine.

“The results of the study have important implications for the treatment of pain, and suggests that microglia may be an important drug target to improve opioid pain relief in women,” said Dr. Anne Murphy, PhD, co-author of the study and associate professor in the Neuroscience Institute at Georgia State.

Murphy says her team’s finding may also help explain why women are significantly more likely to experience chronic pain conditions than men.

A recent study at UCLA and UC Irvine found that microglial cells in both female and male rats can be activated by chronic pain.  The researchers found that brain inflammation in rodents caused by chronic nerve pain led to accelerated growth of microglia. The cells triggered chemical signals in the brain that restricted the release of dopamine, a neurotransmitter that helps control the brain's reward and pleasure centers.