Sex differences and mechanisms of muscle pain.

Abstract

Clinical conditions resulting in musculoskeletal pain show important sex differences in both prevalence and degree of functional disability. The underlying mechanisms for these distinctions in pain manifestation are not fully known. However, recent preclinical studies have shown at the primary afferent level that males and females present fundamental differences in their peripheral response properties and injury-related gene expression patterns that may underlie observed afferent sensitization. At the spinal cord level, studies in various models of pain suggest important roles for the immune system, glutamate signaling and hormones in modulating sex differences. While preclinical studies have been able to characterize some of the basic underlying molecular mechanisms of sex differences in muscle pain, human studies have relied mainly on functional brain imaging studies to explain differences. Further complicating our understanding of how sex influences muscle pain is the notion that the type of injury sustained, or clinical condition may differentially activate distinct mechanisms of muscle pain development in males versus females. More research is necessary to better understand how the sexes differ in their perception of muscle pain. This review highlights recent advances in both human and animal studies of sex differences in muscle pain.

Introduction

Many clinical conditions that include musculoskeletal pain as a component, such as fibromyalgia, complex regional pain syndrome (CRPS), low back pain or myofascial pain have a higher prevalence in females [1–5]. Women also often report greater functional impairment compared to men [6]. While the sex differences in the epidemiology of musculoskeletal pain have been known for quite some time, only recently have clinical and preclinical studies begun to focus on these differences in multiple muscle pain conditions or in animal models. Earlier studies have often only focused on males and few have centered on the basic mechanisms of musculoskeletal pain or its transition to chronic pain.

In this review, we will focus on recently reported findings from patients living with painful musculoskeletal conditions or in healthy human subjects undergoing experimental muscle pain. This will be paired with a review of recent literature on animal models of myalgia. We will highlight recent advances in our basic understanding of sex differences in muscle pain mechanisms and emphasize areas in need of further investigation.

Sex differences in human musculoskeletal pain

In humans, there are sex differences in pain perception at multiple levels, but whether men or women display distinctions in pain appears to be context dependent. In general, women are often reported to have reduced tolerance and increased responses to painful stimuli [7–10]. For example, in a study with patients experiencing shoulder pain, women reported greater clinical pain and had enhanced sensitivity to pressure pain [11]. Interestingly while both men and women can report the same musculoskeletal pain severity, women often report a significantly higher level of activity and pain acceptance. Men experience higher fear of movement, and mood disturbances tied to lower activity levels [12]. Recent reports also show differences in how the experience of pain is remembered. Males, but not females, exhibit increased pain hypersensitivity if placed in a context that similar to previous painful experiences, and this may be mediated by testosterone [13].

Experimental musculoskeletal pain in human subjects has also provided context dependent results. Studies analyzing sex differences in humans undergoing delayed onset muscle soreness have not found any significant differences between men and women [14, 15]. Other experimental investigations using hypertonic saline-evoked muscle pain, however, have reported increased pressure point thresholds in males but not in females [16]. In a model of endotoxemia, one way to induce widespread inflammatory pain, a study revealed decreased pressure pain thresholds (PPT) in women at baseline, but no sex differences after inflammation [17].

This variability could be due to many factors. Age may be one important element as recent work did not find differences in pain sensitivity or pain affect between older males and females, [18], in contrast with the findings reported in younger adults [6, 10, 11, 19]. Another element commonly observed with myalgia is an altered cardiovascular response to muscle contraction; also referred to as the exercise pressor reflex (EPR). Women have been reported to have a blunted metaboreflex compared to men under normal conditions [20]. However, some studies have shown increased vasomotor responses and pain in females after intramuscular injection of hypertonic saline [21].

From a mechanistic point of view variations could also be tied to sex differences in brain activation patterns induced by muscle pain as previously reported using fMRI. Significant changes in signal intensity in the mid-cingulate cortex and dorsolateral prefrontal cortex, occur in a sex-dependent dimorphic pattern, suggesting important differences in the emotional processing of pain [22]. While sex differences have been detected in healthy human subjects and in chronic muscle pain patients, more studies are necessary to elucidate the underlying mechanisms behind these phenomena in both the periphery and central nervous system. Preclinical animal studies are also crucial to enhance our mechanistic understanding of sex differences in muscle pain, in addition to clinical investigations.

Sex differences in preclinical models of muscle pain.

Only recently have preclinical studies begun to dissect the mechanisms of muscle pain and the role of sex on its expression. Like human studies, preclinical assessments also appear to provide context specific results regarding sex and myalgia. Earlier reports analyzing inflammatory models of muscle pain have not detected sex differences in evoked hypersensitivity [23]. This was not the case in other models, where female mice that were exposed to muscle fatigue show increased response to mechanical stimulation of the paw; an effect that was not present in ovariectomized mice [24]. Interestingly, when muscle inflammation was paired with fatigue, sex differences were not observed [24]. In contrast, in a more recent mouse model of localized fatigue paired with a low- pH (5.0) injections, mechanical hyperalgesia was observed in both male and female mice. However, males resolved by day 14 while females did not return to baseline levels until 42 days post insult. Furthermore, males required the insult to occur immediately after fatigue induction to develop mechanical hyperalgesia while females developed hypersensitivity even when the insult was delivered 24h after muscle fatigue [25]. Estrogen did not appear to mediate these outcomes as ovariectomized females behaved similar to their intact counterparts [26]. Macrophages, however, may be part of the sensitization process as local depletion using clondronate liposomes, prevented the development of mechanical sensitization [26]. Sex differences have also been observed in a mouse model of CRPS [27]. In this study, female mice that experience a fracture injury and subsequent casting displayed lower nociceptive thresholds compared to males. Female mice also showed exaggerated signs of motor dysfunction, and latent sensitization.

Differences in sensitization likely involve many components of the pain pathway. Group III/IV primary afferents are the main transducers of nociceptive signals from muscle tissue. Their sensory detection capabilities are varied as they can respond to mechanical stimuli in the noxious and non-noxious ranges, can have responsiveness to heat or cold, and can be activated by chemical stimuli that often include varying combinations of lactate, ATP and protons. These afferents can be single modality units as well as polymodal [28]. The chemosensory functions of these afferents in particular, have been frequently associated with sensations of fatigue or pain [28, 29]. Furthermore, primary muscle afferents also comprise the sensory arm of the exercise pressor reflex [30, 31].

Electrophysiological analysis of uninjured primary muscle afferents shows that certain response properties are distinct between males and females. For example, females appear to have greater numbers of mechanically sensitive group III/IV muscle afferents at baseline, which often display increased firing to mechanical deformation of the muscles after stimulation with metabolites mixtures containing lactate, ATP and protons. In addition, female afferents also seem to respond to heat stimuli with higher firing rates [32]. Under certain injury conditions such as ischemia with reperfusion injury (I/R), female primary muscle afferents display higher firing to cold stimuli while males do not [32, 33].

Gene expression analysis of DRGs after ischemic muscle injury, for example suggest that females display specific upregulation of the heat and proton sensing channel, transient receptor potential vanilloid type 1 (TRPV1) and the cold responsive receptor, TRP melastatin 8 (TRPM8) [32]. Males appear to distinctly upregulate acid sensing ion channel 3 (ASIC3). Both males and females upregulate the ATP receptors, P2X3 and P2X5 after muscle ischemia [31, 33, 34]. Each of these changes in gene expression have been correlated with exacerbated pain related-behaviors [33]. ASIC3 has been linked to pain sensitization in multiple models of muscle pain [23, 26, 33, 35] and it has been suggested that P2X5 can modulate the pH sensitivity of ASIC3 [36]. TRPV1 has also been associated with hypersensitivity in multiple animal models, including, inflammation [37–39], incisional pain [40, 41], delayed onset muscle soreness [42, 43], and ischemic myalgia [44, 45]. In trigeminal neurons, it has been reported to interact with P2X3 in order to modulate afferent sensitization [46]. While results from ischemic injuries suggest a role for TRPV1 in female muscle afferent sensitization, the context of the injury again may be an important factor by which hypersensitivity is induced. One report evaluating mechanical hypersensitivity after carrageenan-induced muscle inflammation in male mice suggested that TRPV1 modulates mechanical sensitivity [38]. Nevertheless, results strongly imply that even before injury, male and female primary muscle afferents are fundamentally different. Moreover, while both sexes experience sensitization and behavioral changes associated with muscle pain after injury, the underlying molecular mechanisms for altered nociception are also likely distinct.

Sex differences in mechanisms of muscle pain development are not only observed at the DRG level but are also found in the spinal cord. This is important as primary muscle afferents have been known for decades to be highly efficient at evoking central sensitization. Repetitive stimulation of the gastrocnemius nerve causes greater long-term potentiation (LTP) of C fiber-evoked field potentials in the spinal dorsal horn compared with stimulation of a cutaneous nerve [48]. Furthermore, compared to cutaneous afferents, muscle sensory neurons are more effective at increasing the excitability of spinal flexion reflexes [49, 50]. In models of CRPS, the levels of the glutamate receptor NR2b, were decreased in males but not in females and this was thought to modulate central sensitization [27].

In a model of peripheral nerve injury, other important sex differences involving the immune system have been shown to regulate the development of pain. In both male and female mice, microglia proliferate in the spinal cord after peripheral nerve injury, but only males use microglia as mediators for the development of prolonged hypersensitivity [51], possibly through a mechanism dependent on the purinergic receptor, P2X4 [52]. Females do not upregulate P2X4 receptors but use a microglia-independent pathway to mediate pain hypersensitivity, potentially involving T cells. The unique functions of microglia may be applicable to muscle pain as studies in a rat model of chronic myositis report increased microglia immunoreactivity in the spinal cord associated with mechanical hyperalgesia that was effectively reversed by gabapentin administration [53]. Glial cells have also been linked to mechanical hypersensitivity in a rat model of low back pain as glial inhibitors, minocycline and fluorocitrate were effective at preventing latent spinal sensitization [54].

Microglia may also influence the observed sex differences in morphine-induced analgesia. A study using peripheral administration of LPS to activate periaqueductal gray (PAG) microglia showed that greater activation in females compared to males was accompanied by increased transcriptional levels of interleukin 1β (IL-1β) and a significant rightward shift in the morphine dose–response curve [55]. In the same study, naloxone effectively potentiated morphine anti-nociception in females such that no sex differences in were observed. Other research on masseter muscle myositis showed that only males upregulated the μ-opioid receptor in the trigeminal ganglion and that orchidectomy prevented this upregulation [47]. Overall, these findings support a crucial role for immune cells in the sex dependent mechanisms of muscle pain development.

There is further evidence suggesting that sex hormones can directly play a role in pain sensitization. In many cases, the effects mediated by hormones can be striking, as exemplified by a recent report showing that male rodents can experience testosterone-mediated conditioned pain hypersensitivity. This effect could also be translated to humans [13]. One report has shown in a mouse model of reserpine-induced widespread muscle pain, that mechanical hyperalgesia was enhanced by ovariectomy and partially rescued in a dose dependent fashion by the administration of 17β-estradiol [56]. On the other hand, testosterone has been shown to regulate gene expression in a mouse model of CFA-induced masseter muscle inflammation. Female and orchiectomized male rats displayed TRPV1 upregulation after inflammation, and replacement of testosterone in the orchiectomized males was effective in preventing the observed upregulation of TRPV1 [37]. This data may help explain results observed in ischemic muscle pain whereby TRPV1 may play a stronger role in hypersensitivity in females than males [32]. Collectively, this suggests multiple sex differences in mechanisms of peripheral and central sensitization after muscle injury.

Final considerations

Musculoskeletal pain is a complex phenomenon and sex differences have been reported at multiple steps in the nociceptive signaling pathway. Primary sensory afferents, spinal cord modulation and brain processing all present variations between sexes (Fig. 1). These need to be taken into account in future research aimed at developing therapies targeting the complex pathologies that drive muscle pain between men and women.

Figure 1: Sex-related mechanisms of muscle pain.

In certain diseases or upon injury, muscle tissue releases a variety of metabolites, cytokines and grow factors that can be accompanied by immune cell infiltration. These signals are paired with differential gene expression patterns and receptor interactions between males and females in the DRGs. Meanwhile in the spinal cord, the increased signals from muscle afferents are possibly modulated by increased immune reactivity of microglia in males and T-cells in females. The perception of pain in the brain can be further influenced by sex-specific psychological and emotional factors and may lead to the distinct sensations of pain in men versus women.

Highlights.

• Men and women experience muscle pain differently. The underlying mechanisms for this difference are still not fully understood.

• Peripheral mechanisms of muscle pain development may be sex and injury dependent and involve multiple receptor pathways.

• In the central nervous system, sex differences have been detected in immune signaling, circuit activation and higher order brain structures.