By Pat Anson, PNN Editor

Why are women more likely than men to suffer from fibromyalgia, osteoarthritis, irritable bowel syndrome, and other chronic pain conditions?  

Various theories have been proposed over the years, such as gender bias in healthcare, the lingering effects of childhood trauma, and women “catastrophizing” about their pain more than men.

Now there’s a new theory, which could radically change how men and women are treated for pain.

In a groundbreaking study published in the journal BRAIN, researchers at University of Arizona Health Sciences identified two substances – prolactin and orexin B – that appear to make mice, monkeys and humans more sensitized to pain. Prolactin is a hormone that promotes breast development and lactation in females; while orexin B is a neurotransmitter that helps keep us awake and stimulates appetite.

Both males and females have prolactin and orexin, but females have much higher levels of prolaction and males have more orexin.  In addition to promoting lactation and wakefulness, both substances also appear to play a role in regulating nociceptors, specialized nerve cells near the spinal cord that produce pain when they are activated by a disease or injury.

“Until now, the assumption has been that the driving mechanisms that produce pain are the same in men and women,” says Frank Porreca, PhD, research director of the Comprehensive Center for Pain & Addiction at UA Health Sciences. “What we found is that the basic, underlying mechanisms that result in the perception of pain are different in male and female mice, in male and female nonhuman primates, and in male and female humans.”

Porreca and his colleagues made their discovery while researching the relationship between chronic pain and sleep.  Using tissue samples from male and female mice, rhesus monkeys and humans, they found that prolactin only sensitizes nociceptors in females, regardless of species, while orexin B only sensitizes the nociceptors of males.

The research team then tried blocking prolactin and orexin B signaling, and found that blocking prolactin reduced nociceptor activation only in female cells, while blocking orexin B only affected the nerve cells of males. In effect, they found that there are distinctive “male” and “female” nociceptors.  

“The nociceptor is actually different in men and women, different in male and female rodents, and different in male and female non human primates. That’s a remarkable concept, because what it's really telling us is that the things that promote nociceptive sensitization in a man or a woman could be totally different,” Porreca told PNN. “These are two mechanisms that we identified, but there are likely to be many, many more that have yet to be identified.”

Once such mechanism could be calcitonin gene-related peptides (CGRPs), a protein that binds to nerve receptors in the brain and trigger migraine pain. In a recent study, Porreca suggested that sexual differences may be the reason why migraine drugs that block CGRPs are effective in treating migraine pain in women, but are far less effective in men.  

Until these differences are more fully understood, Porreca says clinical trials should be designed to have an equal number of men and women. That way differences between the sexes could be more easily recognized and applied in clinical practice.

For example, therapies that block prolactin may be an effective way to treat fibromyalgia in women, while drugs that block orexin B might be a better way to treat certain pain conditions in men.

“We have an opportunity to develop therapies that could be more effective in treating pain in a man or in a woman than the generalized kinds of therapies that we use now,” said Porreca. ‘I think it's critically important that these pain syndromes really be taken very seriously. And that we find better ways of treating female pain and also male pain.”