Editor’s note: The North American Pain School (NAPS) took place 19-24 June 2022, in Montebello, Québec City, Canada. NAPS – an educational initiative of the International Association for the Study of Pain (IASP) and Analgesic, Anesthetic, and Addiction Clinical Trial Translations, Innovations, Opportunities, and Networks (ACTTION), and presented by the Quebec Pain Research Network (QPRN) – brings together leading experts in pain research and management to provide trainees with scientific education, professional development, and networking experiences. This year’s theme was, “Controversies in Pain Research.” Five of the trainees were also selected to serve as PRF-NAPS Correspondents, who provided firsthand reporting from the event, including interviews with NAPS’ Visiting Faculty members and Patient Partners, summaries of scientific sessions, and coverage on social media. Here, PRF-NAPS Correspondent Jessica Archibald, a PhD candidate at the University of British Columbia, Canada, provides coverage of a talk by NAPS Visiting Faculty member Kathleen A. Sluka, University of Iowa, US.
Kathleen Sluka kicked off the morning session of Day 4 at NAPS with a thought-provoking talk that urged the audience to consider sex differences in pain research. Being well versed in cell culture, molecular biology, genetic manipulations, and behavioral pharmacology in both animal and human subjects, Sluka is ideally positioned to lead this important discussion.
The most obvious work regarding sex differences has focused on reproductive systems. It is well known that sex hormones change across the lifespan, and we currently know that males and females have partial genomic differences. One model to explain sex differences is that male and female hormones differentially influence gene expression levels. This is just one piece of the puzzle, as sex differences are multifactorial – there are biological mechanisms to consider, as well as psychological and sociocultural mechanisms. Another contributing factor is different developmental stages (i.e., sex differences in early childhood vs. adulthood). So we know sex begins in the womb, every cell has a sex, and that sex affects behavior and perception, but how do sex differences affect other areas of biology? Understanding these differences makes it possible to design healthcare for individuals.
“Prevalence of pain is greater in females compared to males across the lifespan”
Sluka explained that when causing pain experimentally, females are more sensitive than males to pain, and females experience referred pain (or pain perceived at a location other than the site of the painful stimulus) more commonly than males (Tsang et al., 2008Frey Law et al., 2008). Furthermore, self-reported data show that widespread pain is greater in women and is rated higher on the numerical pain intensity scale (Glass et al., 2014). Taken together, the evidence points at prominent sex differences in pain. Why not investigate the underlying mechanisms that are responsible for these perceptual differences?
In 1989, there was a policy made by the National Institutes of Health in the US calling for the inclusion of both sexes in clinical research. In 1993, this became law. However, in preclinical research using animal and cellular models, it was only in 2016 that a similar policy was instated. A common rationale for using only one sex in animal research is authors stating they do not have adequate power to test both males and females. Along these same lines, they often mention they do not have the resources to add females to the dataset. Perhaps the most troubling argument is using males to minimize any hormonal effects. Considering that women represent 49% of the population worldwide, Sluka believes it’s imperative we examine both sexes and uncover important insights into underlying biological mechanisms.
Hormones affect the pain response in a sex-dependent manner
As a first step, Sluka’s work developing animal models to examine the underlying mechanisms of sex differences in pain is critical. Thorough investigation of outcome measures designed to identify multiple parts of the pain experience have been implemented into her experiments. One objective outcome measure she examines includes hyperalgesia (or abnormal heightened sensitivity to pain). As an example, recall the last time you had a sunburn and touched the affected area – a stimulus that was previously innocuous has become painful; that is hyperalgesia.
Sluka’s team has shown that, compared to males, female mice develop more severe, widespread, and longer-lasting hyperalgesia in a model of activity-induced muscle pain (Gregory et al., 2013). They hypothesized that testosterone might be playing a crucial role here. To test this, they performed a twofold experiment. First, if they castrated the male mice, would their pain behavior be similar to females? Second, if they provided the female mice with testosterone, would their pain behavior fit that of a male? In short, yes – their study suggested that testosterone protects against the development of widespread, long-lasting muscle pain.
They did not stop there. Using her expertise in molecular biology and genetics, Sluka and her team utilized all available techniques to acquire a mechanistic understanding of their observations. They learned that sex hormones can modulate mRNA and protein expression of the serotonin transporter in the brain. Serotonin is a brain chemical involved in complex functions such as mood, cognition, and learning, etc. Interestingly, widespread muscle pain models have been shown to increase serotonin transporter expression. When Sluka’s team examined sex differences in the distribution of the serotonin transporter in the brainstem, they found increased expression in females, and testosterone reduced this increased expression (Lesnak et al., 2020). Thus, Sluka’s team has provided strong evidence for a surrogate marker that may underlie sex differences in pain – a finding that was uncovered because they were exploring preclinical sex differences in pain.
Sex differences in the immune system and pain
Sluka went on to explain that persistent pain may reprogram the way genes work in the immune system (Massart et al., 2016). Recent evidence has shown that chronic pain not only has epigenetic effects in the brain, but also in T cells – white blood cells that are essential for immunity. Additionally, previous research has found that different immune cells mediate mechanical pain hypersensitivity in male and female mice (Sorge et al., 2016), and Sluka presented preliminary results which provide evidence of sex differences, implicating peripheral immune cells, in expressed genes at the site of muscle insult after inducing muscle pain. This type of work can open new avenues for the diagnosis and treatment of pain in humans, as specific genes or pathways can become new sex-specific targets for medications and treatments.
Consider sex as a biological variable in all biomedical research
Altogether, Sluka’s presentation and her own work provided evidence that induced pain involves the immune system, and there is a sex-specific mechanism for this activation. Centrally, she has shown testosterone protects against the development of widespread muscle pain with sex-specific mechanisms. These findings are clinically relevant, as we know women disproportionately experience increased widespread pain and pain intensity compared to men. The field of cardiology is a great example where examining sex differences is imperative. Over the past decade, scientists, healthcare providers, and policy makers have come together to improve and recognize differing symptoms of women and men suffering from cardiovascular diseases (Moska et al., 2011). Sluka’s work is providing the basis for why we should consider these variables in pain research as well.
Jessica Archibald is a PhD candidate at the University of British Columbia, Canada. You can follow her on Twitter – @ArchibaldJes.