Published September 9, 2020
Even though chronic pain disproportionately affects women, pain research has relied heavily on data from male rats and mice. With increasing recognition that males and females process pain differently, some have advocated for sex equality in laboratory research that uses animals (known as preclinical studies). Towards this end, the US National Institutes of Health (NIH) began to require equal representation of male and female animals in federally funded research.
A new review article by Jeffrey Mogil, McGill University, Montreal, Canada, has now reviewed the preclinical literature on sex differences in pain processing. At first, results appeared promising, as they pointed to a notable decrease in the number of male-only studies along with an increase in studies reporting results from both sexes. However, through a careful look at the existing experiments using both sexes, Mogil found that experimental effects were seen much more often in males than in females, revealing a male bias that infects the entire pain literature.
Anne Murphy, Georgia State University, Atlanta, US, noted that this type of analysis of previous studies was necessary. “We have to move beyond the behavioral studies and really start to look at what’s driving the sex differences,” she said. “If you don’t see a sex difference in the behavioral outcome, that doesn’t mean there are no differences in the anatomical or physiological mechanisms underlying it.” Murphy was not involved with the new review.
The review was published May 21, 2020, in the scientific journal Nature Reviews Neuroscience.
Quantitative differences show progress towards sex equality…
In 2014, the NIH announced a policy requiring that studies funded by the agency have equal representation of males and females in preclinical research (see Pain Research Forum related news story) and began implementing this policy in 2016. Similar “sex as a biological variable” (SABV) policies had already been in place in Canada and some European countries. According to Mogil’s review, these policies have been effective.
Taking into consideration over 1,100 articles published in the journal PAIN from 2015-2019, Mogil found a decrease in the number of studies that included only male animals, and an increase in the number of studies that used animals of both sexes and reported on sex differences. For many years prior to 2016, about 80% of studies in PAIN used only males. In 2019, the proportion of male-only studies dropped to about 50%.
“The good news is that this statistic that was completely stubborn has finally started to budge,” said Mogil.
Mogil then used a wider range of the pain literature to analyze both quantitative and qualitative findings in studies of sex differences. Quantitative differences are those where there was a statistically significant difference between males and females in pain-related outcomes, that is, in how the sexes responded to different experimental manipulations that can have an effect on pain. Most studies reporting quantitative differences showed that females had significantly greater pain sensitivity and males responded better to pain-relieving drugs.
…but qualitative differences reveal a male bias
Quantitative differences were relatively easy to discern, but Mogil found that an analysis of qualitative differences revealed bias in the pain literature. Here, qualitative sex differences refer to when an experiment had an effect in one sex but not the other. Mogil said that if there was no bias, then there would be a 50/50 distribution of males and females showing an experimental effect. However, in over 72% of the studies, only males showed the effect.
It’s likely that this bias exists because ideas for experiments have been generated based on decades of research performed only in males. The male bias may not come as a shock to pain researchers but, having found a way to quantify it for the first time, Mogil was surprised by the extent of it.
“We have a bigger hole to dig ourselves out of than we thought,” Mogil said.
The review article described recent findings of qualitative sex differences in the genetics and neurobiology of pain, as well as in environmental and social factors that contribute to pain (see Pain Research Forum related news story).
In addition, perhaps the most high-profile examples of sex differences have been observed in how the body’s immune system is involved in pain. In 2015, an important publication from Mogil’s lab showed that chronic pain was regulated by different types of immune cells in male vs. female mice (see Pain Research Forum related news story). As a result, subsequent studies showed that drugs blocking specific immune cells had better pain-relieving effects in male rodents.
The sex differences observed in these immune system studies have received the most attention over the past few years, but Mogil emphasized that researchers should stay up to date on sex differences discovered in genetic and neurobiological components of pain as well.
Considering the vast amount of recent studies showing sex differences (often referred to as sexual dimorphism), Mogil said, “It raises the big question of whether there’s something specific about pain that makes it particularly sexually dimorphic, or whether everything is that sexually dimorphic and we in the pain field are just a little bit ahead of the curve in demonstrating it.” It remains to be seen whether other fields of biomedicine have such robust sex differences.
Will fixing sex bias lead to better pain treatments?
In addition to equal sex representation in preclinical rodent studies, there is a need for better analyses of sex differences in clinical trials. The NIH has required equal numbers of men and women in its funded trials for over 25 years, but Mogil’s review showed that trial results are rarely separated by sex. This is especially problematic since drugs currently in the pipeline for pain treatment have been developed based on male-biased studies.
“Drugs are failing that may actually work in men. When these drugs don’t get approved, everyone thinks it’s a failure and the biology was wrong, but the biology may have been perfectly right for males,” according to Mogil.
The question of how sex differences in pain could affect the results of drug studies in people is important for the pain field, considering that clinical trials of new pain therapeutics have been largely unsuccessful over the past few decades.
Mogil is hopeful that continued implementation of SABV policies will reveal more about the extent of male bias in the pain literature and lead to a better understanding of how males and females process pain differently. In order for this progress to occur, investigators must provide clear details about their studies. Both Mogil and Murphy advocate for clearer reporting practices and Murphy stressed that, at the very least, information regarding sex and species should be included in study titles.
Finally, to advance understanding of female pain biology, Mogil suggested that researchers return to the drawing board.
“We need to go back to basics and revisit some of our assumptions about how pain biology works,” he said. “For some of the foundational findings in pain research, it would be good if people went back and made sure these were true in female subjects.”
Sarah Najjar is a postdoctoral fellow at the University of Pittsburgh, US.
This story first appeared on the IASP Pain Research Forum and has been lightly edited for RELIEF.