Pain from intra-articular NGF or joint injury in the rat requires contributions from peptidergic joint afferents

The Biology of Pain: Through the Rheumatology Lens

Chronic pain is a major socioeconomic burden globally. The most frequent origin of chronic pain is musculoskeletal. In inflammatory musculoskeletal diseases such as rheumatoid arthritis (RA), chronic pain is a primary determinant of deleterious quality of life. The pivotal role of peripheral inflammation in the initiation and perpetuation of nociceptive pain is well-established among patients with musculoskeletal diseases. However, the persistence of pain, even after the apparent resolution of peripheral inflammation, alludes to the coexistence of different pain states. Recent advances in neurobiology have highlighted the importance of nociplastic pain mechanisms. In this review we aimed to explore the biology of pain with a particular focus on nociplastic pain in RA.

Basic Mechanisms of Pain in Osteoarthritis: Experimental Observations and New Perspectives

The specific changes in the peripheral neuronal pathways underlying joint pain in osteoarthritis are the focus of this review. The plasticity of the nociceptive system in osteoarthritis and how this involves changes in the structural, physiologic, and genetic properties of neurons in pain pathways are discussed. The role of the neurotrophin, nerve growth factor, in these pathogenic processes is discussed. Finally, how neuronal pathways are modified by interaction with the degenerating joint tissues they innervate and with the innate immune system is considered. These extensive cellular interactions provide a substrate for identification of targets for osteoarthritis pain.

Opioid-Induced Hyperalgesic Priming in Single Nociceptors

Clinical µ-opioid receptor (MOR) agonists produce hyperalgesic priming, a form of maladaptive nociceptor neuroplasticity, resulting in pain chronification. We have established an in vitro model of opioid-induced hyperalgesic priming (OIHP), in male rats, to identify nociceptor populations involved and its maintenance mechanisms. OIHP was induced in vivo by systemic administration of fentanyl and confirmed by prolongation of prostaglandin E₂ (PGE₂) hyperalgesia. Intrathecal cordycepin, which reverses Type I priming, or the combination of Src and mitogen-activated protein kinase (MAPK) inhibitors, which reverses Type II priming, both partially attenuated OIHP. Parallel in vitro experiments were performed on small-diameter (<30 µm) dorsal root ganglion (DRG) neurons, cultured from fentanyl-primed rats, and rats with OIHP treated with agents that reverse Type I or Type II priming. Enhancement of the sensitizing effect of a low concentration of PGE₂ (10 nm), another characteristic feature of priming, measured as reduction in action potential (AP) rheobase, was found in weakly isolectin B4 (IB4)-positive and IB4-negative (IB4-) neurons. In strongly IB4-positive (IB4+) neurons, only the response to a higher concentration of PGE₂ (100 nm) was enhanced. The sensitizing effect of 10 nm PGE₂ was attenuated in weakly IB4+ and IB4- neurons cultured from rats whose OIHP was reversed in vivo Thus, in vivo administration of fentanyl induces neuroplasticity in weakly IB4+ and IB4- nociceptors that persists in vitro and has properties of Type I and Type II priming. The mechanism underlying the enhanced sensitizing effect of 100 nm PGE₂ in strongly IB4+ nociceptors, not attenuated by inhibitors of Type I and Type II priming, remains to be elucidated.SIGNIFICANCE STATEMENT Commonly used clinical opioid analgesics, such as fentanyl and morphine, can produce hyperalgesia and chronification of pain. To uncover the nociceptor population mediating opioid-induced hyperalgesic priming (OIHP), a model of pain chronification, and elucidate its underlying mechanism, at the cellular level, we established an in vitro model of OIHP. In dorsal root ganglion (DRG) neurons cultured from rats primed with fentanyl, robust nociceptor population-specific changes in sensitization by prostaglandin E₂ (PGE₂) were observed, when compared with nociceptors from opioid naive rats. In DRG neurons cultured from rats with OIHP, enhanced PGE₂-induced sensitization was observed in vitro, with differences identified in non-peptidergic [strongly isolectin B4 (IB4)-positive] and peptidergic [weakly IB4-positive (IB4+) and IB4-negative (IB4-)] nociceptors.

Targeting neurotrophic factors: Novel approaches to musculoskeletal pain

Chronic pain represents a substantial unmet medical need globally. In recent years, the quest for a new generation of novel, safe, mechanism-based analgesic treatments has focused on neurotrophic factors, a large group of secreted proteins that control the growth and survival of different populations of neurons, but that postnatally are involved in the genesis and maintenance of pain, with biological activity in both the periphery and the central nervous system. In this narrative review, we discuss the two families of neurotrophic proteins that have been extensively studied for their role in pain: first, the neurotrophins, nerve growth factor (NGF) and brain-derived growth factor (BDNF), and secondly, the GDNF family of ligands (GFLs). We provide an overview of the pain pathway, and the pain-producing effects of these different proteins. We summarize accumulating preclinical and clinical findings with a focus on musculoskeletal pain, and on osteoarthritis in particular, because the musculoskeletal system is the most prevalent source of chronic pain and of disability, and clinical testing of these novel agents - often biologics- is most advanced in this area.

Concentration-Dependent Effects of Fibroblast-Like Synoviocytes on Collagen Gel Multiscale Biomechanics and Neuronal Signaling: Implications for Modeling Human Ligamentous Tissues

Abnormal loading of a joint's ligamentous capsule causes pain by activating the capsule's nociceptive afferent fibers, which reside in the capsule's collagenous matrix alongside fibroblast-like synoviocytes (FLS) and transmit pain to the dorsal root ganglia (DRG). This study integrated FLS into a DRG-collagen gel model to better mimic the anatomy and physiology of human joint capsules; using this new model, the effect of FLS on multiscale biomechanics and cell physiology under load was investigated. Primary FLS cells were co-cultured with DRGs at low or high concentrations, to simulate variable anatomical FLS densities, and failed in tension. Given their roles in collagen degradation and nociception, matrix-metalloproteinase (MMP-1) and neuronal expression of the neurotransmitter substance P were probed after gel failure. The amount of FLS did not alter (p > 0.3) the gel failure force, displacement, or stiffness. FLS doubled regional strains at both low (p < 0.01) and high (p = 0.01) concentrations. For high FLS, the collagen network showed more reorganization at failure (p < 0.01). Although total MMP-1 and neuronal substance P were the same regardless of FLS concentration before loading, protein expression of both increased after failure, but only in low FLS gels (p ≤ 0.02). The concentration-dependent effect of FLS on microstructure and cellular responses implies that capsule regions with different FLS densities experience variable microenvironments. This study presents a novel DRG-FLS co-culture collagen gel system that provides a platform for investigating the complex biomechanics and physiology of human joint capsules, and is the first relating DRG and FLS interactions between each other and their surrounding collagen network.

Role of Nociceptor Toll-like Receptor 4 (TLR4) in Opioid-Induced Hyperalgesia and Hyperalgesic Priming

In addition to analgesia, opioids produce opioid-induced hyperalgesia (OIH) and neuroplasticity characterized by prolongation of inflammatory-mediator-induced hyperalgesia (hyperalgesic priming). We evaluated the hypothesis that hyperalgesia and priming induced by opioids are mediated by similar nociceptor mechanisms. In male rats, we first evaluated the role of nociceptor Toll-like receptor 4 (TLR4) in OIH and priming induced by systemic low-dose morphine (LDM, 0.03 mg/kg). Intrathecal oligodeoxynucleotide antisense to TLR4 mRNA (TLR4 AS-ODN) prevented OIH and prolongation of prostaglandin E₂ hyperalgesia (priming) induced by LDM. In contrast, high-dose morphine (HDM, 3 mg/kg) increased nociceptive threshold (analgesia) and induced priming, neither of which was attenuated by TLR4 AS-ODN. Protein kinase C ε (PKCε) AS-ODN also prevented LDM-induced hyperalgesia and priming, whereas analgesia and priming induced by HDM were unaffected. Treatment with isolectin B4 (IB4)-saporin or SSP-saporin (which deplete IB4⁺ and peptidergic nociceptors, respectively), or their combination, prevented systemic LDM-induced hyperalgesia, but not priming. HDM-induced priming, but not analgesia, was markedly attenuated in both saporin-treated groups. In conclusion, whereas OIH and priming induced by LDM share receptor and second messenger mechanisms in common, action at TLR4 and signaling via PKCε, HDM-induced analgesia, and priming are neither TLR4 nor PKCε dependent. OIH produced by LDM is mediated by both IB4⁺ and peptidergic nociceptors, whereas priming is not dependent on the same population. In contrast, priming induced by HDM is mediated by both IB4⁺ and peptidergic nociceptors. Implications for the use of low-dose opioids combined with nonopioid analgesics and in the treatment of opioid use disorder are discussed.SIGNIFICANCE STATEMENT Opioid-induced hyperalgesia (OIH) and priming are common side effects of opioid agonists such as morphine, which acts at μ-opioid receptors. We demonstrate that OIH and priming induced by systemic low-dose morphine (LDM) share action at Toll-like receptor 4 (TLR4) and signaling via protein kinase C ε (PKCε) in common, whereas systemic high-dose morphine (HDM)-induced analgesia and priming are neither TLR4 nor PKCε dependent. OIH produced by systemic LDM is mediated by isolectin B4-positive (IB4⁺) and peptidergic nociceptors, whereas priming is dependent on a different class of nociceptors. Priming induced by systemic HDM is, however, mediated by both IB4⁺ and peptidergic nociceptors. Our findings may provide useful information for the use of low-dose opioids combined with nonopioid analgesics to treat pain and opioid use disorders.

Role of GPCR (mu-opioid)-receptor tyrosine kinase (epidermal growth factor) crosstalk in opioid-induced hyperalgesic priming (type II)

Repeated stimulation of mu-opioid receptors (MORs), by an MOR-selective agonist DAMGO induces type II priming, a form of nociceptor neuroplasticity, which has 2 components: opioid-induced hyperalgesia (OIH) and prolongation of prostaglandin-E2 (PGE2)-induced hyperalgesia. We report that intrathecal antisense knockdown of the MOR in nociceptors, prevented the induction of both components of type II priming. Type II priming was also eliminated by SSP-saporin, which destroys the peptidergic class of nociceptors. Because the epidermal growth factor receptor (EGFR) participates in MOR signaling, we tested its role in type II priming. The EGFR inhibitor, tyrphostin AG 1478, prevented the induction of prolonged PGE2-induced hyperalgesia, but not OIH, when tested out to 30 days after DAMGO. However, even when repeatedly injected, an EGFR agonist did not induce hyperalgesia or priming. A phosphopeptide, which blocks the interaction of Src, focal adhesion kinase (FAK), and EGFR, also prevented DAMGO-induced prolongation of PGE2 hyperalgesia, but only partially attenuated the induction of OIH. Inhibitors of Src and mitogen-activated protein kinase (MAPK) also only attenuated OIH. Inhibitors of matrix metalloproteinase, which cleaves EGF from membrane protein, markedly attenuated the expression, but did not prevent the induction, of prolongation of PGE2 hyperalgesia. Thus, although the induction of prolongation of PGE2-induced hyperalgesia at the peripheral terminal of peptidergic nociceptor is dependent on Src, FAK, EGFR, and MAPK signaling, Src, FAK, and MAPK signaling is only partially involved in the induction of OIH.

Incidence and risk factors of facet joint violation following percutaneous kyphoplasty for osteoporotic vertebral compression fractures

BACKGROUND

Percutaneous kyphoplasty (PKP) has been widely used to osteoporotic vertebral compression fractures (OVCFs). No previous investigations have reported the incidence and risk factors of facet joint violation (FJV) caused by PKP.

PURPOSE

To determine the incidence and risk factors of FJV following PKP in patients with OVCFs.

MATERIAL AND METHODS

We reviewed a total of 153 patients who underwent bilateral PKP. Postoperative computed tomography (CT) scans were assessed to determine the degree of FJV owing to invasion by a puncture trocar. Clinical outcomes, including visual analogue scale (VAS) scores and Oswestry Disability Index (ODI) scores, were collected from all patients. Clinical and radiological data were analyzed to identify the risk factors for FJV.

RESULTS

FJV caused by PKP affected 18.9% of patients and 9.6% of facet joints; approximately 3.9% and 5.7% of facet joints were considered to have grade 1 and grade 2 violations, respectively. There were significant differences between the FJV and non-FJV groups in VAS and ODI scores after surgery. Significant differences were found with respect to the facet joint angle (FJA), the pedicle diameter (PD), and the distance from the entry point to the facet joint space (DEF). Multiple logistic regression analysis indicated that an FJA > 55°, a PD < 5 mm, and a DEF < 5 mm were independent risk factors for FJV.

CONCLUSION

The placement of a puncture trocar can cause FJV in patients with OVCFs and impact clinical outcomes after PKP. Special attention should be given to patients with an FJA > 55°, a PD < 5 mm, and a DEF < 5 mm.

Physiologic facet capsule stretch can induce pain & upregulate matrix metalloproteinase-3 in the dorsal root ganglia when preceded by a physiological mechanical or nonpainful chemical exposure

BACKGROUND

Neck pain from cervical facet loading is common and induces inflammation and upregulation of nerve growth factor (NGF) that can sensitize the joint afferents. Yet, the mechanisms by which these occur and whether afferents can be pre-conditioned by certain nonpainful stimuli are unknown. This study tested the hypothesis that a nonpainful mechanical or chemical insult predisposes a facet joint to generate pain after a later exposure to typically nonpainful distraction.

METHODS

Rats were exposed to either a nonpainful distraction or an intra-articular subthreshold dose of NGF followed by a nonpainful distraction two days later. Mechanical hyperalgesia was measured daily and C6 dorsal root ganglia (DRG) tissue was assayed for NGF and matrix metalloproteinase-3 (MMP-3) expression on day 7.

FINDINGS

The second distraction increased joint displacement and strains compared to its first application (p = 0.0011). None of the initial exposures altered behavioral sensitivity in either of the groups being pre-conditioned or in controls; but, sensitivity was established in both groups receiving a second distraction within one day that lasted until day 7 (p < 0.024). NGF expression in the DRG was increased in both groups undergoing a pre-conditioning exposure (p < 0.0232). Similar findings were observed for MMP-3 expression, with a pre-conditioning exposure increasing levels after an otherwise nonpainful facet distraction.

INTERPRETATION

These findings suggest that nonpainful insults to the facet joint, when combined, can generate painful outcomes, possibly mediated by upregulation of MMP-3 and mature NGF.

Pre-treatment with Meloxicam Prevents the Spinal Inflammation and Oxidative Stress in DRG Neurons that Accompany Painful Cervical Radiculopathy

Painful neuropathic injuries are accompanied by robust inflammatory and oxidative stress responses that contribute to the development and maintenance of pain. After neural trauma the inflammatory enzyme cyclooxygenase-2 (COX-2) increases concurrent with pain onset. Although pre-treatment with the COX-2 inhibitor, meloxicam, before a painful nerve root compression prevents the development of pain, the pathophysiological mechanisms are unknown. This study evaluated if pre-treatment with meloxicam prior to painful root injury prevents pain by reducing spinal inflammation and peripheral oxidative stress. Glial activation and expression of the inflammatory mediator secreted phospholipase A2 (sPLA2) in the spinal cord were assessed at day 7 using immunohistochemistry. The extent of oxidative damage was measured using the oxidative stress marker, 8-hydroxyguanosine (8-OHG) and localization of 8-OHG with neurons, microglia and astrocytes in the spinal cord and peripherally in the dorsal root ganglion (DRG) at day 7. In addition to reducing pain, meloxicam reduced both spinal microglial and astrocytic activation at day 7 after nerve root compression. Spinal sPLA2 was also reduced with meloxicam treatment, with decreased production in neurons, microglia and astrocytes. Oxidative damage following nerve root compression was found predominantly in neurons rather than glial cells. The expression of 8-OHG in DRG neurons at day 7 was reduced with meloxicam. These findings suggest that meloxicam may prevent the onset of pain following nerve root compression by suppressing inflammation and oxidative stress both centrally in the spinal cord and peripherally in the DRG.

The Impact of Facet Joint Violation on Clinical Outcomes After Percutaneous Kyphoplasty for Osteoporotic Vertebral Compression Fractures

OBJECTIVE

To determine whether the facet joint violation (FJV) impacts clinical outcomes after percutaneous kyphoplasty (PKP) in patients with osteoporotic vertebral compression fractures.

METHODS

Patients undergoing bilateral PKP were enrolled and divided into FJV and non-FJV groups depending on their postoperative axial computed tomography scans. Radiologic parameters, visual analogue scale (VAS), and Oswestry Disability Index (ODI) scores were obtained preoperatively and each follow-up time postoperatively (1day, 1 week, 1 month, 3 months, 6 months, and 1 year). Finally, patient satisfaction surveys also were noted.

RESULTS

A total of 157 patients completed the 1-year follow-up. The incidence of FJV resulting from puncture trocars was 15.9% (25/157). Approximately 5.7% (9/157) and 10.2% (16/157) of patients were considered to have grade 1 and grade 2 violation, respectively. PKP decreased the VAS and ODI scores compared with the preoperative values in both groups. Patients with FJV had much greater VAS and ODI scores than did those without FJV at 1-day, 1-week, and 1-month follow-up. No significant differences were found between groups at 3-month, 6-month, and 1-year follow up. Patients in the FJV group were less satisfied with the surgical outcomes than were those without FJV at the last survey.

CONCLUSIONS

Approximately 15.9% of patients were diagnosed with FJV by postoperative computed tomography scans. FJV had a negative influence on clinical outcomes after PKP in osteoporotic vertebral compression fractures, primarily at short-term follow-up. In addition, FJV was also a risk factor in the long-term surgical satisfaction.

Fentanyl Induces Rapid Onset Hyperalgesic Priming: Type I at Peripheral and Type II at Central Nociceptor Terminals

Systemic fentanyl induces hyperalgesic priming, long-lasting neuroplasticity in nociceptor function characterized by prolongation of inflammatory mediator hyperalgesia. To evaluate priming at both nociceptor terminals, we studied, in male Sprague Dawley rats, the effect of local administration of agents that reverse type I (protein translation) or type II [combination of Src and mitogen-activated protein kinase (MAPK)] priming. At the central terminal, priming induced by systemic, intradermal, or intrathecal fentanyl was reversed by the combination of Src and MAPK inhibitors, but at the peripheral terminal, it was reversed by the protein translation inhibitor. Mu-opioid receptor (MOR) antisense prevented fentanyl hyperalgesia and priming. To determine whether type I and II priming occur in the same population of neurons, we used isolectin B4-saporin or [Sar⁹, Met(O₂)¹¹]-substance P-saporin to deplete nonpeptidergic or peptidergic nociceptors, respectively. Following intrathecal fentanyl, central terminal priming was prevented by both saporins, whereas that in peripheral terminal was not attenuated even by their combination. However, after intradermal fentanyl, priming in the peripheral terminal requires both peptidergic and nonpeptidergic nociceptors, whereas that in the central terminal is dependent only on peptidergic nociceptors. Pretreatment with dantrolene at either terminal prevented fentanyl-induced priming in both terminals, suggesting communication between central and peripheral terminals mediated by intracellular Ca²⁺ signaling. In vitro application of fentanyl increased cytoplasmic Ca²⁺ concentration in dorsal root ganglion neurons, which was prevented by pretreatment with dantrolene and naloxone. Therefore, acting at MOR in the nociceptor, fentanyl induces hyperalgesia and priming rapidly at both the central (type II) and peripheral (type I) terminal and this is mediated by Ca²⁺ signaling.SIGNIFICANCE STATEMENT Fentanyl, acting at the μ-opioid receptor (MOR), induces hyperalgesia and hyperalgesic priming at both the central and peripheral terminal of nociceptors and this is mediated by endoplasmic reticulum Ca²⁺ signaling. Priming in the central terminal is type II, whereas that in the peripheral terminal is type I. Our findings may provide useful information for the design of drugs with improved therapeutic profiles, selectively disrupting individual MOR signaling pathways, to maintain an adequate long-lasting control of pain.

Collagen organization regulates stretch-initiated pain-related neuronal signals in vitro: Implications for structure-function relationships in innervated ligaments

Injury to the spinal facet capsule, an innervated ligament with heterogeneous collagen organization, produces pain. Although mechanical facet joint trauma activates embedded afferents, it is unclear if, and how, the varied extracellular microstructure of its ligament affects sensory transduction for pain from mechanical inputs. To investigate the effects of macroscopic deformations on afferents in collagen matrices with different organizations, an in vitro neuron-collagen construct (NCC) model was used. NCCs with either randomly organized or parallel aligned collagen fibers were used to mimic the varied microstructure in the facet capsular ligament. Embryonic rat dorsal root ganglia (DRG) were encapsulated in the NCCs; axonal outgrowth was uniform and in all directions in random NCCs, but parallel in aligned NCCs. NCCs underwent uniaxial stretch (0.25 ± 0.06 strain) corresponding to sub-failure facet capsule strains that induce pain. Macroscopic NCC mechanics were measured and axonal expression of phosphorylated extracellular signal-regulated kinase (pERK) and the neurotransmitter substance P (SP) was assayed at 1 day to assess neuronal activation and nociception. Stretch significantly upregulated pERK expression in both random and aligned gels (p < 0.001), with the increase in pERK being significantly higher (p = 0.013) in aligned than in random NCCs. That increase likely relates to the higher peak force (p = 0.025) and stronger axon alignment (p < 0.001) with stretch direction in the aligned NCCs. In contrast, SP expression was greater in stretched NCCs (p < 0.001) regardless of collagen organization. These findings suggest that collagen organization differentially modulates pain-related neuronal signaling and support structural heterogeneity of ligament tissue as mediating sensory function. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:770-777, 2018.

A Nociceptive Role for Integrin Signaling in Pain After Mechanical Injury to the Spinal Facet Capsular Ligament

Integrins modulate chemically-induced nociception in a variety of inflammatory and neuropathic pain models. Yet, the role of integrins in mechanically-induced pain remains undefined, despite its well-known involvement in cell adhesion and mechanotransduction. Excessive spinal facet capsular ligament stretch is a common injury that induces morphological and functional changes in its innervating afferent neurons and can lead to pain. However, the local mechanisms underlying the translation from tissue deformation to pain signaling are unclear, impeding effective treatment. Therefore, the involvement of the integrin subunit β1 in pain signaling from facet injury was investigated in complementary in vivo and in vitro studies. An anatomical study in the rat identified expression of the integrin subunit β1 in dorsal root ganglion (DRG) neurons innervating the facet, with greater expression in peptidergic than non-peptidergic DRG neurons. Painful facet capsule stretch in the rat upregulated the integrin subunit β1 in small- and medium-diameter DRG neurons at day 7. Inhibiting the α2β1 integrin in a DRG-collagen culture prior to its stretch injury prevented strain-induced increases in axonal substance P (SP) in a dose-dependent manner. Together, these findings suggest that integrin subunit β1-dependent pathways may contribute to SP-mediated pain from mechanical injury of the facet capsular ligament.

The Interface of Mechanics and Nociception in Joint Pathophysiology: Insights From the Facet and Temporomandibular Joints

Chronic joint pain is a widespread problem that frequently occurs with aging and trauma. Pain occurs most often in synovial joints, the body's load bearing joints. The mechanical and molecular mechanisms contributing to synovial joint pain are reviewed using two examples, the cervical spinal facet joints and the temporomandibular joint (TMJ). Although much work has focused on the macroscale mechanics of joints in health and disease, the combined influence of tissue mechanics, molecular processes, and nociception in joint pain has only recently become a focus. Trauma and repeated loading can induce structural and biochemical changes in joints, altering their microenvironment and modifying the biomechanics of their constitutive tissues, which themselves are innervated. Peripheral pain sensors can become activated in response to changes in the joint microenvironment and relay pain signals to the spinal cord and brain where pain is processed and perceived. In some cases, pain circuitry is permanently changed, which may be a potential mechanism for sustained joint pain. However, it is most likely that alterations in both the joint microenvironment and the central nervous system (CNS) contribute to chronic pain. As such, the challenge of treating joint pain and degeneration is temporally and spatially complicated. This review summarizes anatomy, physiology, and pathophysiology of these joints and the sensory pain relays. Pain pathways are postulated to be sensitized by many factors, including degeneration and biochemical priming, with effects on thresholds for mechanical injury and/or dysfunction. Initiators of joint pain are discussed in the context of clinical challenges including the diagnosis and treatment of pain.

Ablation of IB4 non-peptidergic afferents in the rat facet joint prevents injury-induced pain and thalamic hyperexcitability via supraspinal glutamate transporters

The facet joint is a common source of neck pain, particularly after excessive stretch of its capsular ligament. Peptidergic afferents have been shown to have an important role in the development and maintenance of mechanical hyperalgesia, dysregulated nociceptive signaling, and spinal hyperexcitability that develop after mechanical injury to the facet joint. However, the role of non-peptidergic isolectin-B4 (IB4) cells in mediating joint pain is unknown. Isolectin-B4 saporin (IB4-SAP) was injected into the facet joint to ablate non-peptidergic cells, and the facet joint later underwent a ligament stretch known to induce pain. Behavioral sensitivity, thalamic glutamate transporter expression, and thalamic hyperexcitability were evaluated up to and at day 7. Administering IB4-SAP prior to a painful injury prevented the development of mechanical hyperalgesia that is typically present. Intra-articular IB4-SAP also prevented the upregulation of the glutamate transporters GLT-1 and EAAC1 in the ventral posterolateral nucleus of the thalamus and reduced thalamic neuronal hyperexcitability at day 7. These findings suggest that a painful facet injury induces changes extending to supraspinal structures and that IB4-positive afferents in the facet joint may be critical for the development and maintenance of sensitization in the thalamus after a painful facet joint injury.

The Physiological Basis of Cervical Facet-Mediated Persistent Pain: Basic Science and Clinical Challenges

Synopsis Chronic neck pain is a common condition and a primary clinical symptom of whiplash and other spinal injuries. Loading-induced neck injuries produce abnormal kinematics between the vertebrae, with the potential to injure facet joints and the afferent fibers that innervate the specific joint tissues, including the capsular ligament. Mechanoreceptive and nociceptive afferents that innervate the facet have their peripheral terminals in the capsule, cell bodies in the dorsal root ganglia, and terminal processes in the spinal cord. As such, biomechanical loading of these afferents can initiate nociceptive signaling in the peripheral and central nervous systems. Their activation depends on the local mechanical environment of the joint and encodes the neural processes that initiate pain and lead to its persistence. This commentary reviews the complex anatomical, biomechanical, and physiological consequences of facet-mediated whiplash injury and pain. The clinical presentation of facet-mediated pain is complex in its sensory and emotional components. Yet, human studies are limited in their ability to elucidate the physiological mechanisms by which abnormal facet loading leads to pain. Over the past decade, however, in vivo models of cervical facet injury that reproduce clinical pain symptoms have been developed and used to define the complicated and multifaceted electrophysiological, inflammatory, and nociceptive signaling cascades that are involved in the pathophysiology of whiplash facet pain. Integrating the whiplash-like mechanics in vivo and in vitro allows transmission of pathophysiological mechanisms across scales, with the hope of informing clinical management. Yet, despite these advances, many challenges remain. This commentary further describes and highlights such challenges. J Orthop Sports Phys Ther 2017;47(7):450-461. Epub 16 Jun 2017. doi:10.2519/jospt.2017.7255.

Nociceptor plasticity: A closer look

Nociceptors are receptors specifically involved in detecting a tissue damage and transducing it in an electrical signal. Nociceptor activation provoked by any kind of acute lesion is related to the release of several mediators of inflammation, within the framework of a process defined as "peripheral sensitization." This results in an exaggerated response to the painful stimulus, clinically defined as "primary hyperalgesia." The concept of "neuroplasticity" may explain the adaptive mechanisms carried out by the Nervous System in relation to a "harmful" damage; also, neuroplasticity mechanisms are also fundamental for rehabilitative intervention protocols. Here we review several studies that addressed the role of different receptors and ionic channels discovered on nociceptor surface and their role in pain perception. The changes in expression, distribution, and functioning of receptors and ionic channels are thought to be a part of the neuroplasticity property, through which the Nervous System constantly adapts to external stimuli. Moreover, some of the reviewed mediators are also been associated to "central sensitization," a process that results in pain chronicization when the painful stimulation is particularly prolonged or intense, and lastly leads to the memorization of the uncomfortable painful perception.

Gi-protein-coupled 5-HT1B/D receptor agonist sumatriptan induces type I hyperalgesic priming

We have recently described a novel form of hyperalgesic priming (type II) induced by agonists at two clinically important Gi-protein-coupled receptors (Gi-GPCRs), mu-opioid and A1-adenosine. Like mu-opioids, the antimigraine triptans, which act at 5-HT1B/D Gi-GPCRs, have been implicated in pain chronification. We determined whether sumatriptan, a prototypical 5-HT1B/D agonist, produces type II priming. Characteristic of hyperalgesic priming, intradermal injection of sumatriptan (10 ng) induced a change in nociceptor function such that a subsequent injection of prostaglandin-E2 (PGE2) induces prolonged mechanical hyperalgesia. However, onset to priming was delayed 3 days, characteristic of type I priming. Also characteristic of type I priming, a protein kinase Cε, but not a protein kinase A inhibitor attenuated the prolongation phase of PGE2 hyperalgesia. The prolongation of PGE2 hyperalgesia was also permanently reversed by intradermal injection of cordycepin, a protein translation inhibitor. Also, hyperalgesic priming did not occur in animals pretreated with pertussis toxin or isolectin B4-positive nociceptor toxin, IB4-saporin. Finally, as observed for other agonists that induce type I priming, sumatriptan did not induce priming in female rats. The prolongation of PGE2 hyperalgesia induced by sumatriptan was partially prevented by coinjection of antagonists for the 5-HT1B and 5-HT1D, but not 5-HT7, serotonin receptors and completely prevented by coadministration of a combination of the 5-HT1B and 5-HT1D antagonists. Moreover, the injection of selective agonists, for 5-HT1B and 5-HT1D receptors, also induced hyperalgesic priming. Our results suggest that sumatriptan, which signals through Gi-GPCRs, induces type I hyperalgesic priming, unlike agonists at other Gi-GPCRs, which induce type II priming.

Marked sexual dimorphism in 5-HT1 receptors mediating pronociceptive effects of sumatriptan

Amongst the side effects of triptans, a substantial percentage of patients experience injection site pain and tenderness, the underlying mechanism of which is unknown. We found that the dose range from 10fg to 1000ng (intradermal) of sumatriptan induced a complex dose-dependent mechanical hyperalgesia in male rats, with distinct peaks, at 1pg and 10ng, but no hyperalgesia at 1ng. In contrast, in females, there was 1 broad peak. The highest dose (1000ng) did not produce hyperalgesia in either sex. We evaluated the receptors mediating sumatriptan hyperalgesia (1pg, 1 and 10ng). In males, the injection of an antagonist for the serotonin (5-HT) receptor subtype 1B (5-HT_1B), but not 5-HT_1D, markedly inhibited sumatriptan (1pg)-induced hyperalgesia, at 10ng a 5-HT_1D receptor antagonist completely eliminated hyperalgesia. In contrast, in females, the 5-HT_1D, but not 5-HT_1B, receptor antagonist completely blocked sumatriptan (1pg and 10ng) hyperalgesia and both 5-HT_1B and 5-HT_1D receptor antagonists attenuated hyperalgesia (1ng) in females, which is GPR30 estrogen receptor dependent. While selective 5-HT_1D or 5-HT_1B, agonists produce robust hyperalgesia in female and male rats, respectively, when co-injected the hyperalgesia induced in both sexes was attenuated. Mechanical hyperalgesia induced by sumatriptan (1pg and 10ng) is dependent on the G-protein α_i subunit and protein kinase A (PKA), in IB4-positive and negative nociceptors. Understanding the mechanisms responsible for the complex dose dependence for triptan hyperalgesia may provide useful information for the design of anti-migraine drugs with improved therapeutic profiles.

Upper Cervical Spine Loading Simulating a Dynamic Low-Speed Collision Significantly Increases the Risk of Pain Compared to Quasi-Static Loading With Equivalent Neck Kinematics

Dynamic cervical spine loading can produce facet capsule injury. Despite a large proportion of neck pain being attributable to the C2/C3 facet capsule, potential mechanisms are not understood. This study replicated low-speed frontal and rear-end traffic collisions in occiput-C3 human cadaveric cervical spine specimens and used kinematic and full-field strain analyses to assess injury. Specimens were loaded quasi-statically in flexion and extension before and after dynamic rotation of C3 at 100 deg/s. Global kinematics in the sagittal plane were tracked at 1 kHz, and C2/C3 facet capsule full-field strains were measured. Dynamic loading did not alter the kinematics from those during quasi-static (QS) loading, but maximum principal strain (MPS) and shear strain (SS) were significantly higher (p = 0.028) in dynamic flexion than for the same quasi-static conditions. The full-field strain analysis demonstrated that capsule strain was inhomogeneous, and that the peak MPS generally occurred in the anterior aspect and along the line of the C2/C3 facet joint. The strain magnitude in dynamic flexion continued to rise after the rotation of C3 had stopped, with a peak MPS of 12.52 ± 4.59% and a maximum SS of 5.34 ± 1.60%. The peak MPS in loading representative of rear-end collisions approached magnitudes previously shown to induce pain in vivo, whereas strain analysis using linear approaches across the facet joint was lower and may underestimate injury risk compared to full-field analysis. The time at which peak MPS occurred suggests that the deceleration following a collision is critical in relation to the production of injurious strains within the facet capsule.

Whole-body vibration induces pain and lumbar spinal inflammation responses in the rat that vary with the vibration profile

Whole-body vibration (WBV) is linked epidemiologically to neck and back pain in humans, and to forepaw mechanical allodynia and cervical neuroinflammation in a rodent model of WBV, but the response of the low back and lumbar spine to WBV is unknown. A rat model of WBV was used to determine the effect of different WBV exposures on hind paw behavioral sensitivity and neuroinflammation in the lumbar spinal cord. Rats were exposed to 30 min of WBV at either 8 or 15 Hz on days 0 and 7, with the lumbar spinal cord assayed using immunohistochemistry at day 14. Behavioral sensitivity was measured using mechanical stimulation of the hind paws to determine the onset, persistence, and/or recovery of allodynia. Both WBV exposures induce mechanical allodynia 1 day following WBV, but only the 8 Hz WBV induces a sustained decrease in the withdrawal threshold through day 14. Similarly, increased activation of microglia, macrophages, and astrocytes in the superficial dorsal horn of the lumbar spinal cord is only evident after the painful 8 Hz WBV. Moreover, extracellular signal-regulated kinase (ERK)-phosphorylation is most robust in neurons and astrocytes of the dorsal horn, with the most ERK phosphorylation occurring in the 8 Hz group. These findings indicate that a WBV exposure that induces persistent pain also induces a host of neuroimmune cellular activation responses that are also sustained. This work indicates there is an injury-dependent response that is based on the vibration parameters, providing a potentially useful platform for studying mechanisms of painful spinal injuries. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 34:1439-1446, 2016.