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A Persistence of Sex Bias in Preclinical Pain Research

A review of studies published since 2015 reveals a better balance between male and female animals in pain research, yet the existing literature shows a deep male bias in experimental effects

by Sarah Najjar


18 August 2020


PRF News

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A review of studies published since 2015 reveals a better balance between male and female animals in pain research, yet the existing literature shows a deep male bias in experimental effects

Even though chronic pain disproportionately affects women, preclinical pain research has relied heavily on data from male rodents. With increasing recognition that males and females process pain differently, some have advocated for sex equality in preclinical studies. Toward this end, the US National Institutes of Health (NIH) now requires 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 on both sexes. However, through a careful look at the existing experiments using both sexes and featuring qualitative sex differences – defined as differences in pain processing in males versus females – 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, praised the emphasis on qualitative differences in Mogil’s review. “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 article.

 

The review was published May 21, 2020, in 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 PRF 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 more than 1,100 articles published in PAIN from 2015 to 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 a pain-related dependent variable. Notably, most studies reporting quantitative differences showed that females had significantly greater pain sensitivity and males had greater analgesic sensitivity.

 

… but qualitative differences reveal a male bias

Quantitative differences were relatively easy to discern, but Mogil found that an analysis of qualitative differences – in those existing studies featuring both sexes – revealed bias in the pain literature. Here, qualitative sex differences refer to when the manipulation tested in a given study 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 more than 72% of the studies, only males showed the effect.

 

It's likely that this bias exists because experimental hypotheses 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 genetic and neural mediation of pain, as well as in cognitive, environmental, and social factors (see PRF related news story). In addition, perhaps the most high-profile examples of sex differences have been observed in the neuroimmune mediation of pain.

 

In 2015, a landmark publication from Mogil’s lab showed that chronic pain was regulated by different immune cell types in male versus female mice (see PRF related news story). This study showed that, in males only, pain hypersensitivity depended on microglia in the spinal cord; these findings have since been replicated in mice and rats. This could explain why studies show that microglia-inhibiting drugs have better pain-relieving effects in male rodents.

 

But the study of neuroimmune processing in pain has proven to be increasingly complex. In some cases, immune cell infiltration looks similar in males and females, but the signaling mechanisms are different.

 

“This is an exciting field of research,” Murphy said. “But, in addition to sex differences, we really need to consider the choice of our species and the site specificity of some of these mechanisms.” She gave the example that immune signaling in the mouse spinal cord is different from signaling in the rat brain.

 

The sexual dimorphism observed in neuroimmune studies has received the most attention over the past few years, possibly obscuring other findings. Mogil emphasized that researchers should stay up to date on sex differences discovered in genetic and neural circuit components of pain as well. Considering the vast number of recent studies showing sex differences, 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 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 more than 25 years, but Mogil’s review showed that trial results are rarely disaggregated 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 the sex distribution used in their studies and provide stratified analyses where applicable. 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.

 

To advance understanding of female pain biology, Mogil suggested that researchers go back 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.

 

Image credit: Zanfira Misbakh/123RF Stock Photo.

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