Musculoskeletal Pain

Kathryn Albers Mechanisms and Clinical Presentation of Pain November 4, 2019

Queme et al., (2017). Peripheral mechanisms of ischemic myalgia. Frontiers in Cellular Neuroscience.

Mense et al., (2010). Functional anatomy of muscle: Muscle, nociceptors and afferent fibers. In MusclePain: Understanding the Mechanisms. The musculoskeletal system consists of the body's bones, muscles, tendons, ligaments, joints, and cartilage.

A tendon is a fibrous connective tissue that attaches muscle to bone (serves to move the bone). A ligament is a fibrous connective tissue that attaches bone to bone (serves to hold structures together). Major health problems presenting with muscle ache/pain are addressed by NIAMS, National Institutes of Arthritis, Musculoskeletal and Skin Diseases.

Neck pain

Temperomandibular joint pain Fibromyalgia

Shoulder pain

Low back pain Skeletal muscle comprises 40% of body weight.

Muscles produce several hundred myokines; cytokines, growth factors, proteoglycan peptides released by muscle cells (myocytes) in response to muscular contractions. They have autocrine, paracrine and/or endocrine effects on muscle mass, fat metabolism, inflammation….

Lee and Jun. (2019) Role of myokines in regulating skeletal muscle mass and function. Frontiers in Physiology 10. Musculoskeletal Pain Overview Physical activity leads to contraction-induced mechanical and metabolic stimuli in muscle tissue. These stimuli activate receptors on terminals of thinly myelinated and unmyelinated DRG neurons that project to the DH of the spinal cord.

• Chronic muscle pain can be regional (back or neck) or whole body with tender points spread over the body (fibromyalgia). • In contrast to cutaneous nociceptive stimuli, sensations from deep tissue (muscle, vascular, fascia) pain are dull, aching and poorly localized. Muscle pain can also elicit a drop in blood pressure, sweating and nausea. • Pain can relate to dysfunction of peripheral perfusion (e.g., peripheral vascular disease) and are often comorbid with altered cardiovascular responses to muscle contraction/exercise (exercise pressor reflex, EPR). Musculoskeletal Pain Overview, continued

• Muscle pain (myalgia) can present as • Inflammatory, e.g., rheumatoid arthritis (autoimmune, widespread), exercise-induced • Degenerative/inflammatory – osteoarthritis (localized to joints) • Possibly inflammatory – • fibromyalgia, • low back pain • myofascial (muscle-connective tissue) pain syndrome, where pressure on sensitive points in the muscle (knots) is painful. MPS typically caused by repetitive motions or stress-related muscle tension. Cellular basis of muscle pain:

1. Myositis – general term for inflammation of the muscle. - Injury, infection or autoimmune disease can cause polymyositis. Immune activation generates interleukins, TNF, chemokines, growth factors.

2. Tissue acidosis, a decrease in local tissue pH. - Pain correlates with increased protons (H+) and lactate buildup in muscle/synovial fluid - Infusion of acidic solutions (pH 5.2) into muscle causes mechanical hyperalgesia in human and in rodent models.

3. Dysregulation of metabolite levels - ATP, lactate and protons. Infusion of metabolite soup into muscle is painful. 10 volunteers (6 men/4 women) 0.2ml protons/ATP/lactate infused over 30s under fascia of thumb muscle.

Exp Physiol 99.2(2014)368.

Findings: • Low conc of protons, lactate and ATP evoked sensations related to fatigue. • Higher conc evoked pain. • Single metabolites evoked no sensations • Interpretations: - Two types of sensory neurons encode metabolites; one detects low concentrations, signals fatigue. The other detects higher conc and signals ‘ache and hot’. - In human, metabolites normally produced by exercise act in combination to activate sensory neurons that signal sensations of fatigue and muscle pain.

Do metabolites enhance sensitivity of muscle afferents in chronic disease states, e.g., fibromyalgia? Muscle Morphology

Straited (skeletal) – voluntary, ~40% of body weight Cardiac - involuntary Smooth - involuntary

connective tissue

connective tissue connective tissue Fig. 2.1 From S. Mense et al., Functional Anatomy of Muscle: Muscle, Muscle Nociceptors and Afferent Fibers. S. Mense and R.D. Gerwin (eds.), Muscle Pain: Understanding the Mechanisms, DOI 10.1007/978-3-540-85021-2_2, # Springer-Verlag Berlin Heidelberg 2010 Muscle Innervation

e.g., knee

Group I-IV nomenclature introduced by DPC Lloyd, 1943.

Afferent fiber types • Group Ia, II – assoc with muscle spindles; respond to stretch, changes in muscle length. • Group III – thinly myelinated A delta fibers • Group IV - unmyelinated C fibers Group III/IV afferents primarily project to sc lamina I, II (less so to IV-V). Efferent motor neurons: Alpha – innervate extrafusal fibers and intiate muscle contraction. Gamma – innervate intrafusal muscle fibers in the muscle spindle. Beta motor neurons – innervate both extra- and intra –fusal fibers. Very few of these. Properties of Muscle Nociceptors • Relatively slow conducting, below 20-30 m/s (species dependent) (fastest fibers reach 100 m/s).

• Thin myelinated, group III fibers – correspond in conduction velocity to cutaneous Ad fibers. 1.2 - 14 m/s (in mouse)

• Unmyelinated group IV fibers – correspond in conduction velocity to cutaneous C fibers. <1.2 m/s (mouse) a-CGRP • Group III and group IV fibers function as chemo-, thermo- and mechano-receptors, functional overlap unclear *. arteriole

• Many Group III/IV fibers innervate adventitia of vasculature and perimysium; few innervate muscle fibers**.

*Light AR, Perl ER (2003) Unmyelinated afferent fibers are not only for pain anymore. J Comp Neurol 461:140–150. Discusses muscle LTMRs.

**Reinert A, Kaske A, Mense S (1998) Inflammation-induced increase in the density of neuropeptide-immunoreactive nerve endings in rat skeletal muscle. Exp Brain Res 121:174–180 Fig. 2.5 From Mense et al., Muscle Pain: Understanding the Mechanisms, DOI 10.1007/978- 3-540-85021-2_2, 2010 That muscle fibers themselves are not supplied by free nerve endings may relate to the clinical experience that muscle cell death is usually not painful, at least not if it occurs slowly, as during muscular dystrophy, polymyositis, or dermatomyositis (skin rash with muscle inflammation).

A different situation is tearing of a muscle fiber bundle, which can be extremely painful. Many muscle cells are destroyed simultaneously and release their contents (e.g., K+ ions, ATP) in the interstitial space, from where they can diffuse to nociceptive endings.

S. Mense Review: Functional Anatomy of Muscle: Muscle, Nociceptors and Afferent Fibers Membrane receptors associated with nociception are expressed by muscle afferents. Most studied are: Acid Sensing Ion Channels - sensitive to H+; activated by change in pH produced by exercise, ischemia, inflammation.

TRPV1 – sensitive to H+, lipids (anadamide), heat

P2X3/5 – activated by ATP

Evidence suggests interaction of receptor types on skeletal muscle afferent endings, e.g., P2X/ASICs Birdsong et al (2010) Sensing muscle ischemia: coincident detection of acid and ATP via interplay of two ion channels. Neuron 68:739. Models of Muscle Pain • Injection/continuous infusion of algogenic compounds: (carrageenan, mustard oil, immune modulators). • Acid model – Injection(s) of acidic saline causes mech allodynia via activation of acid sensing ion channels (ASICs). - Non-inflammatory - Inhibition of acid pain by opioids, NMDA blockers, pregabalin and exercise but not to NSAIDs. • Exercise induced: delayed onset of muscle soreness model. Produces micro tears that cause pain and increased lactate, glutamate, PGE2, SubP. • Exercise (wheel running) plus acid, e.g., pH 5 • Injection of metabolites produced by muscle contraction - combination of ATP, lactate, protons (H+) act synergistically to activate receptors. • Prolonged ischemia (e.g., 6h surgical occlusion, brachial artery) – models pain associated with conditions that can produce ischemia/reperfusion injuries: complex regional pain syndrome (CRPS), peripheral vascular disease (PVD), sickle cell anemia and fibromyalgia. • Clinical features of ischemic myalgia include decreased activity, ongoing pain, hypersensitivity, and weakness in the affected muscle tissue. In contrast to nociceptive stimuli from skin, muscle activity/pain can elicit a cardiovascular (exercise pressor) reflex, i.e., increased heart rate, blood pressure, sweating.

Activation of Group III/IV afferents - transmit muscle pain and affect autonomic (sympathetic) responses. • Ablation/anesthetic block of group III/IV neurons abolishes the EPR in response to muscle contraction.

• Patients with CRPS or vascular disease have exaggerated EPRs.

• Both pain and change in cardiovascular reflexes in response to ischemic damage involves upregulation of P2Y1 in primary afferents. Queme, Queme, Ross andJankowski. Peripheral mechanisms2017. of ischemic myalgia. Frontiers inCellular Neuroscience. Acid activates Acid Sensing Ion Channels (ASICs), a subfamily of the epithelial sodium channel (ENaC/degenerin) ion channel superfamily Voltage independent, proton-gated sodium channels that detect change in pH (5-8; tissue acidification) under pathological and non- pathological conditions: ischemia, fatiguing muscle exercise, cancer, neuronal activity, inflammation.

ASICs1-5 genes have splice variants and protein isoforms. Most common ASIC1a, ASIC1a/2a, ASIC3.

Studies support major role for ASIC3 in nociception: Injection of acid causes pain; pain is absent in ASIC3 KO and blocked by amiloride 10.5483/bmbrep.2013.46.6.121 (Na+ channel inhibitor). BMB reports 2013 DOI: ASICs in muscle pain signaling:

ASIC3 primary involvement  mechanical hyperalgesia induced by repeated acid injection is absent in ASIC3 KOs but not ASIC2 KOs.

In response to ischemic injury, ASIC3 expression in muscle afferents is dependent on production of the inflammatory cytokine IL1b by muscle.

ASICs can be activated by nonproton ligands, e.g., venom peptides (coral snake, black mamba, spider) and polyamine compounds (agmatine). ASICs can be modulated by cations, neuropeptides, proteases, etc…. ASICs are not the only pH sensors in the PNS – tissue acidification can modulate function of other channels on the peripheral terminal and along axon. Defining muscle afferent populations using stimuli response and gene expression properties

Ex vivo preparation with forepaw muscle, median and ulna nerves, T2-C6 DRG and spinal cord.

Record response of group III and group IV DRG afferents to thermal, mechanical and then metabolite stimuli. Immunolabel characterized neurons.

Comprehensive phenotyping of group III and IV muscle afferents Metabolite conditions: in mouse. Jankowski, …Koerber. (2013) J. Neurophys. Low metabolite – 15mM lactate, 1mM ATP, pH7 High metabolite – 50mM lactate, 5mM ATP, pH6.6

Human metabolite conc in resting muscles = 1mM lactate, 300nM ATP, pH7.3

Concentrations used based on studies showing high and low metabolite responders (A. Light, 2000) 34% group III and IV afferents were mechanosensitive 32% were thermosensitive 52% were chemosensitive (mainly group IV)

18 fibers neurochemically identified:

ASIC3 TRPV1

- low metabolite responders (exercise evoked levels) - these fibers lack ASIC3 and TRPV1, have P2X3 - high metabolite responders (ischemic contraction evoked levels) - majority of these fibers were ASIC3 and/or TRPV1+, have P2X3

 Two chemosensitive populations: Metabo-receptors (low responders) and metabo- nociceptors (high responders) Defining muscle afferent populations using single-cell PCR of backlabeled muscle afferents reveals diversity Group IV Group III Proprioceptors

Adelman et al (2019). 28 genes analyzed, 7 major groups. Possible group III myelinated afferents: High Nefh, mid-high ASIC3, low TrpV1, high Calca, no Tac1 Possible group IV unmyelinated afferents: High Trpv1, Trpa1, Gfra3, Tac1 Injury evoked phenotypic switching occurs in muscle afferents.

Ischemic/reperfusion injury – elicits behavioral mechanical hyperalgesia, allodynia - decreased mechanical threshold and increased responses to mechanical stimuli in group III/IV afferents. - decreases the number of metabo-receptors (low metabolite responders) - increases afferents responsive to low and high metabolites - alters gene expression in DRG

Increase in ASIC3 P2X3 ASIC3+P2X3 coexpressers TRPV1 Queme, Ross and Jankowski. 2017. Peripheral mechanisms of ischemic myalgia. P2Y1 Frontiers in Cellular Neuroscience. P2Y4 ASIC1 Immune cells • How do different group III and IV afferents contribute to the development of altered cardiovascular reflexes or musculoskeletal pain after damage?

• Does muscle fatigue (inability to contract muscles) relate to the functional properties of group III and IV fibers?

• Do physiological concentrations of metabolite combinations evoke sensations of fatigue and pain when injected into human muscle?

• Do changes in group III and IV fiber properties underlie muscle weakness /pathophysiology, e.g., following ischemia or in chronic fatigue syndrome? Flex Pharma – biotech company developed HOTSHOT – a blend of ginger, cinnamon and capsaicin that prevents muscle cramping by stimulating Trp receptors in the mouth, esophagus and stomach.

“Targets the nerve. not the muscle. Its science.” •The only SCIENTIFICALLY PROVEN TREATMENT for muscle cramp relief and post-workout recovery •An organic spicy blend of GINGER, CINNAMON and CAPSAICIN stops cramps and soreness where they start •Use to PREVENT / TREAT / RECOVER from cramps and soreness in legs, calves, feet and more •The ALL-NATURAL AND SAFE formula is NSF Certified for Sport and Non-GMO Project Verified •Trusted by over 600 collegiate, professional and champion teams for CRAMPING and SORE MUSCLES $3.19 / Fl Oz)

Company tried developing treatments to control cramping and spasticity associated with MS, Charcot-Marie-Tooth (CMT) and ALS. Phase 2 trial failed (oral tolerability concerns).

Merger with Salarius Pharmaceuticals. Drinking HOTSHOT stimulates sensory neurons in the mouth, esophagus, and stomach.

Stimulated neurons send impulses to the spinal cord.

These impulses overpower and inhibit repetitive signals coming to and from the cramped muscle.

This stops repetitive signals and prevents and/or treats the cramp.