Biology

Sexual dimorphism

Fibromyalgia

Biochemistry

Chronic pain

Alternative medicine

Psychology

Osteoarthritis

Physical therapy

Internal medicine

MEDLINE

Medicine

Physical medicine and rehabilitation

Sex characteristics

Scopus

Pathology

Osteoarthritis and Cartilage

Females and males show clear differences in their susceptibility to many diseases, notably autoimmune diseases and malignancies, but also certain infectious diseases1Klein S.L. Flanagan K.L. Sex differences in immune responses.Nat Rev Immunol. 2016; 16: 626-638Crossref PubMed Scopus (2366) Google Scholar. In addition to sex differences in disease susceptibility, there is abundant epidemiological and experimental evidence that sex is a major determinant of pain. Women are markedly overrepresented in chronic pain states, including several painful musculoskeletal conditions such as low back pain, fibromyalgia, and myofascial pain2Fillingim R.B. King C.D. Ribeiro-Dasilva M.C. Rahim-Williams B. Riley 3rd, J.L. Sex, gender, and pain: a review of recent clinical and experimental findings.J Pain. 2009; 10: 447-485Abstract Full Text Full Text PDF PubMed Scopus (1707) Google Scholar, 3Rosen S. Ham B. Mogil J.S. Sex differences in neuroimmunity and pain.J Neurosci Res. 2017; 95: 500-508Crossref PubMed Scopus (167) Google Scholar, 4Queme L.F. Jankowski M.P. Sex differences and mechanisms of muscle pain.Curr Opin Physiol. 2019; 11: 1-6Crossref PubMed Scopus (13) Google Scholar. While there are many factors that may contribute to the observed sex differences in pain susceptibility, including sociocultural, psychological, and biological factors such as genetic or hormonal modulators (for a review, see5Mogil J.S. Sex differences in pain and pain inhibition: multiple explanations of a controversial phenomenon.Nat Rev Neurosci. 2012; 13: 859-866Crossref PubMed Scopus (560) Google Scholar), recent years have seen the emergence of an interesting paradigm that involves sexual dimorphism in neuroimmune interactions that contribute to pain in experimental animals3Rosen S. Ham B. Mogil J.S. Sex differences in neuroimmunity and pain.J Neurosci Res. 2017; 95: 500-508Crossref PubMed Scopus (167) Google Scholar. In the past decade, neuroimmune crosstalk has emerged as a key process in establishing and maintaining chronic pain, and this at different levels of the pain neuraxis6Hore Z. Denk F. Neuroimmune interactions in chronic pain - an interdisciplinary perspective.Brain Behav Immun. 2019; 79: 56-62Crossref PubMed Scopus (21) Google Scholar. In the periphery, neurons and immune cells (macrophages, T cells, mast cells) can talk to each other in the innervated tissues - for example, the synovium in an OA joint may be infiltrated by immune cells that release inflammatory mediators that can sensitize nociceptors. Neuroimmune crosstalk also occurs in dorsal root ganglia (DRG) where the cell bodies of the nociceptors reside. It has been shown that in chronic pain states, including in experimental OA, the DRGs get infiltrated with macrophages, resulting in pain-promoting feedback loops between macrophages and sensory neurons (for a recent review on the role of neuroimmune interactions in OA pain, please see7Miller R.E. AM M. M R.J. The innate immune response as a mediator of osteoarthritis pain.Osteoarthr Cartil. 2019 Dec 17; (S1063-4584(19)31291-9. [Epub ahead of print] Review)https://doi.org/10.1016/j.joca.2019.11.006Abstract Full Text Full Text PDF Scopus (41) Google Scholar). Neuroimmune interactions also occur in the central nervous system (CNS). Specifically, interactions between neurons and microglia (the resident macrophages of the CNS) and other immune cells occur in the spinal cord. Microglial activation in the dorsal horn has been documented in many pain models, including models of rheumatoid arthritis (RA) and OA, where they are thought to contribute to increased neuronal excitability and synaptic efficacy, and hence to central sensitization6Hore Z. Denk F. Neuroimmune interactions in chronic pain - an interdisciplinary perspective.Brain Behav Immun. 2019; 79: 56-62Crossref PubMed Scopus (21) Google Scholar. Finally, neuroimmune interactions may also occur in the brain, where the role of microglia has been extensively studied in neurodegenerative disorders, but also in chronic pain syndromes such as fibromyalgia6Hore Z. Denk F. Neuroimmune interactions in chronic pain - an interdisciplinary perspective.Brain Behav Immun. 2019; 79: 56-62Crossref PubMed Scopus (21) Google Scholar. These observations have sparked great interest in targeting microglia with modulators such as minocycline to treat pain8Haight E.S. Forman T.E. Cordonnier S.A. James M.L. Tawfik V.L. Microglial modulation as a target for chronic pain: from the bench to the bedside and back.Anesth Analg. 2019; 128: 737-746Crossref PubMed Scopus (33) Google Scholar. However, as is the case for the majority of studies in laboratory animals, studies pointing toward a role for microglial activation in chronic pain have largely been conducted in male animals9Mogil J.S. Chanda M.L. The case for the inclusion of female subjects in basic science studies of pain.Pain. 2005; 117: 1-5Abstract Full Text Full Text PDF PubMed Scopus (250) Google Scholar. Remarkably, in recent years it has emerged that the contribution of microglia to establishing pain in mice is sex dependent. Experimental nerve injury results in mechanical allodynia and microglial activation in both female and male mice, but inhibitors of microglial activation block allodynia only in males and not in females. By contrast, female mice use adaptive immune cells in the spinal cord to establish pain10Sorge R.E. Mapplebeck J.C. Rosen S. Beggs S. Taves S. Alexander J.K. et al.Different immune cells mediate mechanical pain hypersensitivity in male and female mice.Nat Neurosci. 2015; 18: 1081-1083Crossref PubMed Scopus (744) Google Scholar, but this is highly dependent on the hormonal context in that female mice still use microglia to establish pain when testosterone levels are elevated10Sorge R.E. Mapplebeck J.C. Rosen S. Beggs S. Taves S. Alexander J.K. et al.Different immune cells mediate mechanical pain hypersensitivity in male and female mice.Nat Neurosci. 2015; 18: 1081-1083Crossref PubMed Scopus (744) Google Scholar. In addition to these microglia findings, there have been reports on sexual dimorphism in the role of spinal Toll-like receptor 4 (TLR4) in pain, where it has been reported that intrathecal administration of the TLR4 ligand, lipopolysaccharide (LPS), causes much more robust mechanical allodynia in males than in female mice11Sorge R.E. LaCroix-Fralish M.L. Tuttle A.H. Sotocinal S.G. Austin J.S. Ritchie J. et al.Spinal cord Toll-like receptor 4 mediates inflammatory and neuropathic hypersensitivity in male but not female mice.J Neurosci. 2011; 31: 15450-15454Crossref PubMed Scopus (305) Google Scholar. Furthermore, Tlr4 deficiency or treatment with the TLR4 inhibitor, TAK-242, attenuated allodynia induced by intrathecal administration of LPS in male mice, but not in female mice12Woller S.A. Ravula S.B. Tucci F.C. Beaton G. Corr M. Isseroff R.R. et al.Systemic TAK-242 prevents intrathecal LPS evoked hyperalgesia in male, but not female mice and prevents delayed allodynia following intraplantar formalin in both male and female mice: the role of TLR4 in the evolution of a persistent pain state.Brain Behav Immun. 2016; 56: 271-280Crossref PubMed Scopus (44) Google Scholar. If translatable to humans, these divergent pathways mediating chronic pain in the two sexes may have clear implications for drug development and drug testing. Sex-dependent findings in mouse models of inflammatory and neuropathic pain reflect the growing body of literature indicating that sex as a variable has a tremendous influence on innate and adaptive immune responses, where females generally mount a more robust immune response than males. Several biological factors can contribute to this sexual dimorphism, including genetic, hormonal, and environmental influences on the immune system (for a comprehensive and fascinating review, please see1Klein S.L. Flanagan K.L. Sex differences in immune responses.Nat Rev Immunol. 2016; 16: 626-638Crossref PubMed Scopus (2366) Google Scholar). Clearly, in view of the increasing evidence for a role for innate immune pathways and TLR signaling in chronic pain in general, as well as in the pathogenesis of osteoarthritis (OA)13Griffin T.M. Scanzello C.R. Innate inflammation and synovial macrophages in osteoarthritis pathophysiology.Clin Exp Rheumatol. 2019; 37 (Suppl 120): 57-63PubMed Google Scholar and OA pain7Miller R.E. AM M. M R.J. The innate immune response as a mediator of osteoarthritis pain.Osteoarthr Cartil. 2019 Dec 17; (S1063-4584(19)31291-9. [Epub ahead of print] Review)https://doi.org/10.1016/j.joca.2019.11.006Abstract Full Text Full Text PDF Scopus (41) Google Scholar, it is highly likely that pathways mediating OA pain may differ between the sexes. In humans, female sex is a major risk factor for OA, particularly after menopause14Neogi T. Zhang Y. Epidemiology of osteoarthritis.Rheum Dis Clin N Am. 2013; 39: 1-19Abstract Full Text Full Text PDF PubMed Scopus (420) Google Scholar, for reasons that are incompletely understood. Women have a greater risk for developing knee OA than men, and women with knee OA experience more pain than men regardless of radiographic severity15Glass N. Segal N.A. Sluka K.A. Torner J.C. Nevitt M.C. Felson D.T. et al.Examining sex differences in knee pain: the multicenter osteoarthritis study.Osteoarthr Cartil. 2014; 22: 1100-1106Abstract Full Text Full Text PDF PubMed Scopus (66) Google Scholar. These sex differences in OA pain reflect the greater incidence of musculoskeletal pain in females compared to males in general. In spite of this female preponderance, experimental OA is predominantly modelled in male animals16van der Kraan P.M. Factors that influence outcome in experimental osteoarthritis.Osteoarthr Cartil. 2017; 25: 369-375Abstract Full Text Full Text PDF PubMed Scopus (27) Google Scholar. In fact, experimental knee OA induced by destabilization of the medial meniscus (DMM) is currently the most widely used mouse model of OA, but female mice are less susceptible to OA after this surgery unless they have been ovarectomized17Ma H.L. Blanchet T.J. Peluso D. Hopkins B. Morris E.A. Glasson S.S. Osteoarthritis severity is sex dependent in a surgical mouse model.Osteoarthr Cartil. 2007; 15: 695-700Abstract Full Text Full Text PDF PubMed Scopus (228) Google Scholar. Pain behaviors and pain pathways in this model have been studied for several years by different laboratories, including our own, but only in male mice18Miller R.E. Malfait A.M. Osteoarthritis pain: what are we learning from animal models?.Best Pract Res Clin Rheumatol. 2018; (In press)PubMed Google Scholar. Clearly, going forward it will be important to compare the development of pain-related behaviors in experimental OA in males and females side by side. This is precisely what the study by Temp et al. in the current issue of Osteoarthritis and Cartilage aimed to do19Temp J. Labuz L. Negrete R. Sunkara V. Machelska H. Pain and knee damage in male and female mice in the medial meniscal transection induced osteoarthritis.Osteoarthr Cartil. 2020; 28: 475-485Abstract Full Text Full Text PDF PubMed Scopus (18) Google Scholar. The authors used the medial meniscotibial transection (MMT) in C57BL/6 mice. As in other OA models, MMT surgery caused more severe cartilage damage in male mice than in female mice, while osteophyte development was similar between both sexes. In association with joint damage, male mice developed signs indicative of pain, and this in a biphasic pattern: mechanical allodynia in the ipsilateral hind paw and decreased weight-bearing on the operated limb presented in an acute phase (days 1–5), resolved during the intermediate phase (weeks 1–2) and reappeared in the chronic phase (weeks 4–12). In contrast, in female mice, both these pain-related behaviors were present throughout the entire time course of the model (day 1-week 12). Direct comparison between males and females revealed that females had greater mechanical allodynia in the intermediate period compared to males, while males had greater weight-bearing deficits at select time points in both the acute (days 1–2) and chronic (weeks 5–6) phases. However, the male mice overall had greater body weights than the female mice, which may contribute to this difference observed in weight-bearing. While the study did not investigate mechanisms underlying the observed sex differences in the development of pain behaviors, it clearly demonstrates that differences exist. Very few side by side comparisons between male and female mice in OA pain models have been reported in the literature, while a few studies report combined data for animals of both sexes. An interesting recent study investigated sex and age interactions in the rat mono-iodoacetate (MIA) model, and found that MIA induced more pronounced weightbearing deficits and knee joint hypersensitivity in older rats, and this was most pronounced in aged females, suggesting that age significantly impacts OA pain, especially in females20Ro J.Y. Zhang Y. Tricou C. Yang D. da Silva J.T. Zhang R. Age and sex differences in acute and osteoarthritis-like pain responses in rats.J Gerontol A Biol Sci Med Sci. 2019; Crossref PubMed Scopus (4) Google Scholar. The differences in susceptibility to OA joint damage between the two sexes has been appreciated for a long time, and scientists are increasingly recognizing the importance of sex -like age - as a biological variable in the pathogenesis of joint damage21Boyan B.D. Tosi L.L. Coutts R.D. Enoka R.M. Hart D.A. Nicolella D.P. et al.Addressing the gaps: sex differences in osteoarthritis of the knee.Biol Sex Differ. 2013; 4: 4Crossref PubMed Scopus (37) Google Scholar. The study by Temp et al. is one of the first to study pain behaviors in an OA model in mice of both sexes side by side. While the use of mouse models to study sexual dimorphisms in chronic pain mechanisms is severely limited because they cannot capture psychological and social factors that may contribute to the observed differences22Sluka K.A. Berkley K.J. O'Connor M.I. Nicolella D.P. Enoka R.M. Boyan B.D. et al.Neural and psychosocial contributions to sex differences in knee osteoarthritic pain.Biol Sex Differ. 2012; 3: 26Crossref PubMed Scopus (29) Google Scholar, current evidence suggests that there are basic biological pathways that diverge between the sexes. Going forward, we have to hope that the OA research community will increasingly consider these potential sex differences in susceptibility to pain and the underlying mechanisms, in order to increase translational and predictive value of animal models especially with respect to identifying new targets for OA pain. Both authors participated in the conception, drafting, revising, and approving the final version of this editorial. A.M. Malfait serves as an Associate Editor for Osteoarthritis and Cartilage. Anne-Marie Malfait (R01AR064251, R01AR060364, R61AR073576) was supported by the US National Institutes of Health/National Institute of Arthritis and Musculoskeletal and Skin Diseases (NIH/NIAMS). Rachel Miller was supported by NIAMS (K01AR070328). The funding sources had no role in the study.

Why we should study osteoarthritis pain in experimental models in both sexes