As clinicians know, pain is a very personal experience with a unique and complex biopsychosocial pathology, thereby placing a significant burden on both physical and mental health. However, in clinical samples, the burden does not fall equally across both biological sexes, with females suffering from chronic pain more often than males.¹ In contrast, the bulk of preclinical research has relied on the use of male animals possibly due to beliefs about the negative impact of cycling hormones. In short, male animals have often been used to model conditions with a female prevalence in the assumption that the mechanisms of pain were similar between sexes. (It should be noted that the term “sex” is used herein to denote chromosomal/genetic sex and not gender in all cases.) Gender identity likely plays a significant role in the pain experience but does not have an equivalent in animals.
Across a multitude of chronic pain conditions (including back, migraine, musculoskeletal, neuropathic, oral, osteoarthritic, and chronic widespread pain), there are consistently higher prevalence rates in females compared to males.² In addition, within similar chronic pain conditions, the intensity of the pain is greater in females than males. Further, in experimental pain tasks, females have shown lower pain thresholds and tolerances to applied stimuli than males, suggesting a significant mechanistic difference.³
From Biology to Immunity
From a biological perspective, female primary afferent (sensory) nerves are more sensitive to mechanical stimuli, showing differences in their excitability and response rates compared to male afferents.⁴It is thought that this difference may be due to estrogen, with the increased amount of estrogen present leading to increased excitability of these nerves. However, pain is more complex than just sensory fibers — the immune system plays a key role in chronic pain development and persistence. Peripheral immunity involves the recognition of a pathogen by mast cells and macrophages, leading to a cascade of events that often leads to the production of many types of T cells and B cells. Central immunity (brain and spinal cord), on the other hand, utilizes microglia and astrocytes to recognize pathogens before attracting T cells into the central nervous system. With damage or infection, activation of the immune cells triggers an inflammatory response that can sensitize nerve fibers, resulting in pain. Thus, immune cells have been recognized as having a significant role in pain for some time.⁵
Unfortunately, the bulk of preclinical work has focused on male animals⁶ under the (mistaken) belief that immune system function was not sexually-dimorphic. In the authors’ experiments with mice, we showed that activation of a specific immune cell receptor in the spinal cord (toll-like receptor 4, TLR4) could elicit hypersensitivity in male but not female mice. This effect was dependent on the presence of elevated testosterone and could be seen in female mice given testosterone supplementation.⁷ We followed this work in our investigation into the microglial cells themselves and found that, whereas activation of these cells was associated with hypersensitivity, suppressing their activation had pain-relieving properties in only male mice. Once again, this effect was dependent on testosterone. However, we also found that female mice utilized T cells to mediate both inflammatory and nerve pain, but used the “male” system in the absence of T cells or the presence of elevated testosterone.⁸ These data supported the notion that both male and female mice possess both immune cell types but preferentially use one or the other based on hormone levels. Since completing our work, other researchers have shown similar effects in elegant experiments in rats⁹ and across other chronic pain conditions.¹⁰˒¹¹
Sex differences in the immune system are also present in humans. Females have higher T cell proliferation and activation in the absence of disease, fewer natural killer (NK) cells, and more T and B cells compared to males.¹²Females also have a stronger immune response to pathogens.¹³ A recent study examining cancer patients who had their dorsal root ganglia removed found that the patients demonstrated a sex-dependent gene expression profile. In short, male patients with chronic pain showed increased macrophage activity, whereas female chronic pain patients showed an increase in neuropeptides.¹⁴ Interestingly, sex differences in immune cell populations and responsivity is conserved across a variety of species,¹⁵ suggesting a common mechanism underlying the difference.
Barking Up Old Trees
Treatments for chronic pain have advanced over the past few decades, but prominent management methods are still largely based on the opium poppy (opioids), willow bark (aspirin), and coal tar (acetaminophen). The failure to implement new approaches may be attributed to several reasons. One possibility is the reliance on male animals to model a condition with a female-centric clinical presentation that critically involves a biological system that is sexually dimorphic. Together, these collected works illustrate the sex-dependent biological mechanisms that may underlie chronic pain and support the critical need for the following: 1) an increase in investigations of sex differences in the mediation and treatment of pain at the preclinical and clinical level; and (2) recognition that treatments for one biological sex may not be suitable for both.
To illustrate this point, consider that there are known sex differences in the efficacy of opioids¹⁶ and anti-inflammatory drugs¹˒⁷ with females generally receiving less relief; yet these medications are rarely differentially prescribed based on biological sex. There are sex differences in the mediation of chronic pain at a physiological level and treatments must be developed and administered with this in mind.
As a final thought, humans are complex creatures and the impact of psychological and social influences likely plays an equally large role in the pain experience. Thus, it is important to consider that, whereas biological mechanisms may be similar between species, concepts of gender identity and gender roles are not currently able to be modeled preclinically. These factors are expected to significantly impact human clinical pain, even beyond biological mechanisms.