A new model of sciatic inflammatory neuritis (SIN): induction of unilateral and bilateral mechanical allodynia following acute unilateral peri-sciatic immune activation in rats

Prevention and reversal of neuropathic pain by near-infrared photobiomodulation therapy in male and female rats

Pathological nociception arising from peripheral nerve injury impacts quality of life. Current therapeutics are generally ineffective. However, photobiomodulation therapy (PBMT) has shown promise in addressing this issue. We aimed to assess the potential anti-allodynic effects of 2 p.m. protocols, each applied transcutaneously over the peripheral nerve injury. In addition to evaluating nociceptive behavior, we also conducted morphological analysis using electron microscopy (EM) to investigate potential ultrastructural changes at the cellular level. We sought to determine, using the chronic constriction injury (CCI) model, whether our parameters could alleviate established allodynia and/or dampen allodynia development. Adult male and female rats with CCI or sham were treated with PBMT (850-nm wavelength) for 2 min, 3 times a week over three or four weeks across three studies, where PBMT began either before or after CCI. Allodynia was assessed prior to surgery and across weeks and, at the conclusion of the third study, sciatic nerve was processed for EM and histomorphometrically evaluated. The results showed that PBMT before versus after CCI injury yielded similar behaviors, effectively decreasing allodynia. Interestingly, these positive effects of PBMT do not appear to be accounted by protection of the sciatic injury site, based on EM. CCI reliably decreased axon size and the number of myelinated axons present in both PBMT and control groups. While PBMT reduced the number of C-fibers in CCI samples, no improvement in any measure was observed in response to PBMT.

Mirror-Image Pain Update: Complex Interactions Between Central and Peripheral Mechanisms

The appearance of contralateral effects after unilateral injury has been shown in various experimental pain models, as well as in clinics. They consist of a diversity of phenomena in contralateral peripheral nerves, sensory ganglia, or spinal cord: from structural changes and altered gene or protein expression to functional consequences such as the development of mirror-image pain (MP). Although MP is a well-documented phenomenon, the exact molecular mechanism underlying the induction and maintenance of mirror-like spread of pain is still an unresolved challenge. MP has generally been explained by central sensitization mechanisms leading to facilitation of pain impulse transfer through neural connections between the two sides of the central nervous system. On the contrary, the peripheral nervous system (PNS) was usually regarded unlikely to evoke such a symmetrical phenomenon. However, recent findings provided evidence that events in the PNS could play a significant role in MP induction. This manuscript provides an updated and comprehensive synthesis of the MP phenomenon and summarizes the available data on the mechanisms. A more detailed focus is placed on reported evidence for peripheral mechanisms behind the MP phenomenon, which were not reviewed up to now.

Post-Traumatic Trigeminal Neuropathy: Neurobiology and Pathophysiology

Painful traumatic trigeminal neuropathy (PTTN) is a chronic neuropathic pain that may develop following injury to the trigeminal nerve. Etiologies include cranio-orofacial trauma that may result from dental, surgical, or anesthetic procedures or physical trauma, such as a motor vehicle accident. Following nerve injury, there are various mechanisms, including peripheral and central, as well as phenotypic changes and genetic predispositions that may contribute to the development of neuropathic pain. In this article, we review current literature pertaining to the cellular processes that occur following traumatic damage to the trigeminal nerve, also called cranial nerve V, that results in chronic neuropathic pain. We examine the neurobiology and pathophysiology based mostly on pre-clinical animal models of neuropathic/trigeminal pain.

Interleukin-1beta and inflammasome expression in spinal cord following chronic constriction injury in male and female rats

Females represent a majority of chronic pain patients and show greater inflammatory immune responses in human chronic pain patient populations as well as in animal models of neuropathic pain. Recent discoveries in chronic pain research have revealed sex differences in inflammatory signaling, a key component of sensory pathology in chronic neuropathic pain, inviting more research into the nuances of these sex differences. Here we use the chronic constriction injury (CCI) model to explore similarities and differences in expression and production of Inflammatory cytokine IL-1beta in the lumbar spinal cord, as well as its role in chronic pain. We have discovered that intrathecal IL-1 receptor antagonist reverses established pain in both sexes, and increased gene expression of inflammasome NLRP3 is specific to microglia and astrocytes rather than neurons, while IL-1beta is specific to microglia in both sexes. We report several sex differences in the expression level of the genes coding for IL-1beta, as well as the four inflammasomes responsible for IL-1beta release: NLRP3, AIM2, NLRP1, and NLRC4 in the spinal cord. Total mRNA, but not protein expression of IL-1beta is greater in females than males after CCI. Also, while CCI increases all four inflammasomes in both sexes, there are sex differences in relative levels of inflammasome expression. NLRP3 and AIM2 are more highly expressed in females, whereas NLRP1 expression is greater in males.

Voluntary wheel running prevents formation of membrane attack complexes and myelin degradation after peripheral nerve injury

Regular aerobic activity is associated with a reduced risk of chronic pain in humans and rodents. Our previous studies in rodents have shown that prior voluntary wheel running can normalize redox signaling at the site of peripheral nerve injury, attenuating subsequent neuropathic pain. However, the full extent of neuroprotection offered by voluntary wheel running after peripheral nerve injury is unknown. Here, we show that six weeks of voluntary wheel running prior to chronic constriction injury (CCI) reduced the terminal complement membrane attack complex (MAC) at the sciatic nerve injury site. This was associated with increased expression of the MAC inhibitor CD59. The levels of upstream complement components (C3) and their inhibitors (CD55, CR1 and CFH) were altered by CCI, but not increased by voluntary wheel running. Since MAC can degrade myelin, which in turn contributes to neuropathic pain, we evaluated myelin integrity at the sciatic nerve injury site. We found that the loss of myelinated fibers and decreased myelin protein which occurs in sedentary rats following CCI was not observed in rats with prior running. Substitution of prior voluntary wheel running with exogenous CD59 also attenuated mechanical allodynia and reduced MAC deposition at the nerve injury site, pointing to CD59 as a critical effector of the neuroprotective and antinociceptive actions of prior voluntary wheel running. This study links attenuation of neuropathic pain by prior voluntary wheel running with inhibition of MAC and preservation of myelin integrity at the sciatic nerve injury site.

Modulation of the spinal N13 SEP component by high- and low-frequency electrical stimulation. Experimental pain models matter

OBJECTIVE

The N13 component of somatosensory evoked potential (N13 SEP) represents the segmental response of cervical dorsal horn neurons. Neurophysiological studies in healthy participants showed that capsaicin-induced central sensitization causes an increase of the N13 SEP amplitude. Consequently, in human research, this spinal component may serve as a valuable readout of central sensitization. In this study, we wanted to verify if the sensitivity of the N13 SEP for detecting central sensitization is consistent across different experimental pain models inducing central sensitization and secondary hyperalgesia, namely high and low-frequency electrical stimulation (HFS and LFS).

METHODS

In 18 healthy participants, we recorded SEP after bilateral ulnar nerve stimulation before and after secondary hyperalgesia was induced through HFS and LFS applied on the ulnar nerve territory of the hand of one side. The area of secondary hyperalgesia was mapped with a calibrated 128-mN pinprick probe, and the mechanical pain sensitivity with three calibrated 16-64-256-mN pinprick probes.

RESULTS

Although both HFS and LFS successfully induced secondary hyperalgesia only LFS increased the amplitude of the N13 SEP.

CONCLUSIONS

These findings suggest that the sensitivity of the N13 SEP for detecting dorsal horn excitability changes may critically depend on the different experimental pain models.

SIGNIFICANCE

Our results indicate that LFS and HFS could trigger central sensitization at the dorsal horn level through distinct mechanisms, however this still needs confirmation by replication studies.

Macrophages and microglia in inflammation and neuroinflammation underlying different pain states

Pain is a main symptom in inflammation, and inflammation induces pain via inflammatory mediators acting on nociceptive neurons. Macrophages and microglia are distinct cell types, representing immune cells and glial cells, respectively, but they share similar roles in pain regulation. Macrophages are key regulators of inflammation and pain. Macrophage polarization plays different roles in inducing and resolving pain. Notably, macrophage polarization and phagocytosis can be induced by specialized pro-resolution mediators (SPMs). SPMs also potently inhibit inflammatory and neuropathic pain via immunomodulation and neuromodulation. In this review, we discuss macrophage signaling involved in pain induction and resolution, as well as in maintaining physiological pain. Microglia are macrophage-like cells in the central nervous system (CNS) and drive neuroinflammation and pathological pain in various inflammatory and neurological disorders. Microglia-produced inflammatory cytokines can potently regulate excitatory and inhibitory synaptic transmission as neuromodulators. We also highlight sex differences in macrophage and microglial signaling in inflammatory and neuropathic pain. Thus, targeting macrophage and microglial signaling in distinct locations via pharmacological approaches, including immunotherapies, and non-pharmacological approaches will help to control chronic inflammation and chronic pain.

Effects of Low-Level Laser Therapy and Purified Natural Latex (Hevea brasiliensis) Protein on Injured Sciatic Nerve in Rodents: Morpho-Functional Analysis

The present study analyzed the effects of low-level laser therapy (LLLT) and the purified natural latex protein (Hevea brasiliensis, F1 protein) on the morpho-function of sciatic nerve crush injuries in rats. One-hundred and eight male Wistar rats were randomly allocated to six groups (n = 18): 1. Control; 2. Exposed (nerve exposed); 3. Injury (injured nerve without treatment); 4. LLLT (injured nerve irradiated with LLLT (15 J/cm2, 780 nm)); 5. F1 (injured nerve treated with F1 protein (0.1%)); and 6. LLLT + F1 (injured nerve treated with LLLT and F1). On the 1st, 7th, 14th, and 56th days after injury, a functional sensory analysis of mechanical allodynia and mechanical hyperalgesia and a motor analysis of grip strength and gait were performed. After 3, 15, and 57 days, the animals were euthanized for morphometric/ultrastructural analyses. The treatments applied revealed improvements in morphometric/ultrastructural parameters compared to the injured group. Sensory analyses suggested that the improvements observed were associated with time progression and not influenced by the treatments. Motor analyses revealed significant improvements in grip strength from the 7th day in the LLLT group and in gait from the 56th day in all treated groups. We concluded that even though the morphological analyses showed improvements with the treatments, they did not influence sensory recovery, and LLLT improved motor recovery.

Neuroimmune Mechanisms Underlying Post-acute Sequelae of SARS-CoV-2 (PASC) Pain, Predictions from a Ligand-Receptor Interactome

PURPOSE OF REVIEW

Individuals with post-acute sequelae of SARS-CoV-2 (PASC) complain of persistent musculoskeletal pain. Determining how COVID-19 infection produces persistent pain would be valuable for the development of therapeutics aimed at alleviating these symptoms.

RECENT FINDINGS

To generate hypotheses regarding neuroimmune interactions in PASC, we used a ligand-receptor interactome to make predictions about how ligands from PBMCs in individuals with COVID-19 communicate with dorsal root ganglia (DRG) neurons to induce persistent pain. In a structured literature review of -omics COVID-19 studies, we identified ligands capable of binding to receptors on DRG neurons, which stimulate signaling pathways including immune cell activation and chemotaxis, the complement system, and type I interferon signaling. The most consistent finding across immune cell types was an upregulation of genes encoding the alarmins S100A8/9 and MHC-I. This ligand-receptor interactome, from our hypothesis-generating literature review, can be used to guide future research surrounding mechanisms of PASC-induced pain.

Hmgb1 Silencing in the Amygdala Inhibits Pain-Related Behaviors in a Rat Model of Neuropathic Pain

Chronic pain presents a therapeutic challenge due to the highly complex interplay of sensory, emotional-affective and cognitive factors. The mechanisms of the transition from acute to chronic pain are not well understood. We hypothesized that neuroimmune mechanisms in the amygdala, a brain region involved in the emotional-affective component of pain and pain modulation, play an important role through high motility group box 1 (Hmgb1), a pro-inflammatory molecule that has been linked to neuroimmune signaling in spinal nociception. Transcriptomic analysis revealed an upregulation of Hmgb1 mRNA in the right but not left central nucleus of the amygdala (CeA) at the chronic stage of a spinal nerve ligation (SNL) rat model of neuropathic pain. Hmgb1 silencing with a stereotaxic injection of siRNA for Hmgb1 into the right CeA of adult male and female rats 1 week after (post-treatment), but not 2 weeks before (pre-treatment) SNL induction decreased mechanical hypersensitivity and emotional-affective responses, but not anxiety-like behaviors, measured 4 weeks after SNL. Immunohistochemical data suggest that neurons are a major source of Hmgb1 in the CeA. Therefore, Hmgb1 in the amygdala may contribute to the transition from acute to chronic neuropathic pain, and the inhibition of Hmgb1 at a subacute time point can mitigate neuropathic pain.

Intrathecal non-viral interleukin-10 gene therapy ameliorates neuropathic pain as measured by both classical static allodynia and a novel supra-spinally mediated pain assay, the Two-Arm Rodent Somatosensory (TARS) task

Intrathecal delivery of interleukin-10 (IL-10) gene therapy has been reported to be effective in suppressing pain enhancement in a variety of rodent models. However, all publications that have tested this treatment have relied upon measures of static allodynia (von Frey test) and thermal hyperalgesia (Hargreaves test). As this plasmid DNA IL-10 (pDNA-IL10) therapeutic approach is now in human clinical trials for multiple pain indications, including intrathecal delivery for human neuropathic pain, it is important to consider the recent concerns raised in the pain field that such tests reflect spinal rather than supraspinal processing of, and responsivity to, noxious stimuli. Consequently, this raises the question of whether intrathecal pDNA-IL10 can reverse established neuropathic pain when assessed by a test requiring supraspinal, rather than solely spinal, mediation of the behavioral response. The present study utilizes the rat sciatic chronic constriction injury (CCI) model of neuropathic pain to compare the expression of static allodynia with that of cognitively controlled choice behavior in a two-arm maze, adapted from Hayashida et al. (2019). This modification, termed the Two-Arm Rodent Somatosensory (TARS) task, provides rats free choice to reach a desired goal box via a short "arm" of the maze with tactile probes as flooring versus a longer "arm" of the maze with a smooth surface. Here we demonstrate that static allodynia and avoidance of the nociceptive flooring in TARS develop in parallel over time, and that both behaviors also resolve in parallel following intrathecal pDNA-IL10 gene therapy. Details for the construction and use of this new maze design are also provided. Together, this study documents both: (a) the important finding that intrathecal IL-10 gene therapy does indeed resolve neuropathic pain as measured by a supraspinally-mediated behavioral task, and (b) a new, supraspinally-mediated task that allows behavioral assessments across weeks and allows the analysis of both development and resolution of neuropathic pain by therapeutic interventions. As such, the TARS operant behavior task is an improvement over other approaches such as the mechanical conflict-avoidance system which have difficulties demonstrating development and reversal of pain behavior in a within-subject design.

Pathophysiology of Post-Traumatic Trigeminal Neuropathic Pain

Trigeminal nerve injury is one of the causes of chronic orofacial pain. Patients suffering from this condition have a significantly reduced quality of life. The currently available management modalities are associated with limited success. This article reviews some of the common causes and clinical features associated with post-traumatic trigeminal neuropathic pain (PTNP). A cascade of events in the peripheral and central nervous system function is involved in the pathophysiology of pain following nerve injuries. Central and peripheral processes occur in tandem and may often be co-dependent. Due to the complexity of central mechanisms, only peripheral events contributing to the pathophysiology have been reviewed in this article. Future investigations will hopefully help gain insight into trigeminal-specific events in the pathophysiology of the development and maintenance of neuropathic pain secondary to nerve injury and enable the development of new therapeutic modalities.

Preconditioning by voluntary wheel running attenuates later neuropathic pain via nuclear factor E2-related factor 2 antioxidant signaling in rats

ABSTRACT

Animal and human studies have shown that exercise prior to nerve injury prevents later chronic pain, but the mechanisms of such preconditioning remain elusive. Given that exercise acutely increases the formation of free radicals, triggering antioxidant compensation, we hypothesized that voluntary running preconditioning would attenuate neuropathic pain by supporting redox homeostasis after sciatic nerve injury in male and female rats. We show that 6 weeks of voluntary wheel running suppresses neuropathic pain development induced by chronic constriction injury across both sexes. This attenuation was associated with reduced nitrotyrosine immunoreactivity-a marker for peroxynitrite-at the sciatic nerve injury site. Our data suggest that prior voluntary wheel running does not reduce the production of peroxynitrite precursors, as expression levels of inducible nitric oxide synthase and NADPH oxidase 2 were unchanged. Instead, voluntary wheel running increased superoxide scavenging by elevating expression of superoxide dismutases 1 and 2. Prevention of neuropathic pain was further associated with the activation of the master transcriptional regulator of the antioxidant response, nuclear factor E2-related factor 2 (Nrf2). Six weeks of prior voluntary wheel running increased Nrf2 nuclear translocation at the sciatic nerve injury site; in contrast, 3 weeks of prior wheel running, which failed to prevent neuropathic pain, had no effect on Nrf2 nuclear translocation. The protective effects of prior voluntary wheel running were mediated by Nrf2, as suppression was abolished across both sexes when Nrf2 activation was blocked during the 6-week running phase. This study provides insight into the mechanisms by which physical activity may prevent neuropathic pain. Preconditioning by voluntary wheel running, terminated prior to nerve injury, suppresses later neuropathic pain in both sexes, and it is modulated through the activation of Nrf2-antioxidant signaling.

Involvement of TLR2-TLR4, NLRP3, and IL-17 in pain induced by a novel Sprague-Dawley rat model of experimental autoimmune encephalomyelitis

Up to 92% of patients suffering from multiple sclerosis (MS) experience pain, most without adequate treatment, and many report pain long before motor symptoms associated with MS diagnosis. In the most commonly studied rodent model of MS, experimental autoimmune encephalomyelitis (EAE), motor impairments/disabilities caused by EAE can interfere with pain testing. In this study, we characterize a novel low-dose myelin-oligodendrocyte-glycoprotein (MOG)-induced Sprague-Dawley (SD) model of EAE-related pain in male rats, optimized to minimize motor impairments/disabilities. Adult male SD rats were treated with increasing doses of intradermal myelin-oligodendrocyte-glycoprotein (MOG1-125) (0, 4, 8, and 16 μg) in incomplete Freund's adjuvant (IFA) vehicle to induce mild EAE. Von Frey testing and motor assessments were conducted prior to EAE induction and then weekly thereafter to assess EAE-induced pain and motor impairment. Results from these studies demonstrated that doses of 8 and 16 μg MOG1-125 were sufficient to produce stable mechanical allodynia for up to 1 month in the absence of hindpaw motor impairments/disabilities. In the follow-up studies, these doses of MOG1-125, were administered to create allodynia in the absence of confounded motor impairments. Then, 2 weeks later, rats began daily subcutaneous injections of the Toll-like receptor 2 and 4 (TLR2-TLR4) antagonist (+)-naltrexone [(+)-NTX] or saline for an additional 13 days. We found that (+)-NTX also reverses EAE-induced mechanical allodynia in the MOG-induced SD rat model of EAE, supporting parallels between models, but now allowing a protracted timecourse to be examined completely free of motor confounds. Exploring further mechanisms, we demonstrated that both spinal NOD-like receptor protein 3 (NLRP3) and interleukin-17 (IL-17) are necessary for EAE-induced pain, as intrathecal injections of NLRP3 antagonist MCC950 and IL-17 neutralizing antibody both acutely reversed EAE-induced pain. Finally, we show that spinal glial immunoreactivity induced by EAE is reversed by (+)-NTX, and that spinal demyelination correlates with the severity of motor impairments/disabilities. These findings characterize an optimized MOG-induced SD rat model of EAE for the study of pain with minimal motor impairments/disabilities. Finally, these studies support the role of TLR2-TLR4 antagonists as a potential treatment for MS-related pain and other pain and inflammatory-related disorders.

Opportunities and challenges of pain-related myocardial ischemia-reperfusion injury

Pain is one of the most serious problems plaguing human health today. Pain is not an independent pathophysiological condition and is associated with a high impact on elevated disability and organ dysfunction. Several lines of evidence suggested the associations of pain with cardiovascular diseases, especially myocardial ischemia-reperfusion (I/R) injury, while the role of pain in I/R injury and related mechanisms are not yet comprehensively assessed. In this review, we attempted to explore the role of pain in myocardial I/R injury, and we concluded that acute pain protects myocardial ischemia-reperfusion injury and chronic pain aggravates cardiac ischemia-reperfusion injury. In addition, the construction of different pain models and animal models commonly used to study the role of pain in myocardial I/R injury were discussed in detail, and the potential mechanism of pain-related myocardial I/R injury was summarized. Finally, the future research direction was prospected. That is, the remote regulation of pain to cardiac function requires peripheral pain signals to be transmitted from the peripheral to the cardiac autonomic nervous system, which then affects autonomic innervation during cardiac ischemia-reperfusion injury and finally affects the cardiac function.

Targeting Chemokines and Chemokine GPCRs to Enhance Strong Opioid Efficacy in Neuropathic Pain

Neuropathic pain (NP) originates from an injury or disease of the somatosensory nervous system. This heterogeneous origin and the possible association with other pathologies make the management of NP a real challenge. To date, there are no satisfactory treatments for this type of chronic pain. Even strong opioids, the gold-standard analgesics for nociceptive and cancer pain, display low efficacy and the paradoxical ability to exacerbate pain sensitivity in NP patients. Mounting evidence suggests that chemokine upregulation may be a common mechanism driving NP pathophysiology and chronic opioid use-related consequences (analgesic tolerance and hyperalgesia). Here, we first review preclinical studies on the role of chemokines and chemokine receptors in the development and maintenance of NP. Second, we examine the change in chemokine expression following chronic opioid use and the crosstalk between chemokine and opioid receptors. Then, we examine the effects of inhibiting specific chemokines or chemokine receptors as a strategy to increase opioid efficacy in NP. We conclude that strong opioids, along with drugs that block specific chemokine/chemokine receptor axis, might be the right compromise for a favorable risk/benefit ratio in NP management.

How Is Peripheral Injury Signaled to Satellite Glial Cells in Sensory Ganglia?

Injury or inflammation in the peripheral branches of neurons of sensory ganglia causes changes in neuronal properties, including excessive firing, which may underlie chronic pain. The main types of glial cell in these ganglia are satellite glial cells (SGCs), which completely surround neuronal somata. SGCs undergo activation following peripheral lesions, which can enhance neuronal firing. How neuronal injury induces SGC activation has been an open question. Moreover, the mechanisms by which the injury is signaled from the periphery to the ganglia are obscure and may include electrical conduction, axonal and humoral transport, and transmission at the spinal level. We found that peripheral inflammation induced SGC activation and that the messenger between injured neurons and SGCs was nitric oxide (NO), acting by elevating cyclic guanosine monophosphate (cGMP) in SGCs. These results, together with work from other laboratories, indicate that a plausible (but not exclusive) mechanism for neuron-SGCs interactions can be formulated as follows: Firing due to peripheral injury induces NO formation in neuronal somata, which diffuses to SGCs. This stimulates cGMP synthesis in SGCs, leading to their activation and to other changes, which contribute to neuronal hyperexcitability and pain. Other mediators such as proinflammatory cytokines probably also contribute to neuron-SGC communications.

Frailty and pain, human studies and animal models

The hypothesis that pain can predispose to frailty development has been recently investigated in several clinical studies suggesting that frailty and pain may share some mechanisms. Both pain and frailty represent important clinical and social problems and both lack a successful treatment. This circumstance is mainly due to the absence of in-depth knowledge of their pathological mechanisms. Evidence of shared pathways between frailty and pain are preliminary. Indeed, many clinical studies are observational and the impact of pain treatment, and relative pain-relief, on frailty onset and progression has never been investigated. Furthermore, preclinical research on this topic has yet to be performed. Specific researches on the pain-frailty relation are needed. In this narrative review, we will attempt to point out the most relevant findings present in both clinical and preclinical literature on the topic, with particular attention to genetics, epigenetics and inflammation, in order to underline the existing gaps and the potential future interventional strategies. The use of pain and frailty animal models discussed in this review might contribute to research in this area.

Parenchymal neuroinflammatory signaling and dural neurogenic inflammation in migraine

Background

Pain is generally concomitant with an inflammatory reaction at the site where the nociceptive fibers are activated. Rodent studies suggest that a sterile meningeal inflammatory signaling cascade may play a role in migraine headache as well. Experimental studies also suggest that a parenchymal inflammatory signaling cascade may report the non-homeostatic conditions in brain to the meninges to induce headache. However, how these signaling mechanisms function in patients is unclear and debated. Our aim is to discuss the role of inflammatory signaling in migraine pathophysiology in light of recent developments.

Body

Rodent studies suggest that a sterile meningeal inflammatory reaction can be initiated by release of peptides from active trigeminocervical C-fibers and stimulation of resident macrophages and dendritic/mast cells. This inflammatory reaction might be needed for sustained stimulation and sensitization of meningeal nociceptors after initial activation along with ganglionic and central mechanisms. Most migraines likely have cerebral origin as suggested by prodromal neurologic symptoms. Based on rodent studies, a parenchymal inflammatory signaling cascade has been proposed as a potential mechanism linking cortical spreading depolarization (CSD) to meningeal nociception. A recent PET/MRI study using a sensitive inflammation marker showed the presence of meningeal inflammatory activity in migraine with aura patients over the occipital cortex generating the visual aura. These studies also suggest the presence of a parenchymal inflammatory activity, supporting the experimental findings. In rodents, parenchymal inflammatory signaling has also been shown to be activated by migraine triggers such as sleep deprivation without requiring a CSD because of the resultant transcriptional changes, predisposing to inadequate synaptic energy supply during intense excitatory transmission. Thus, it may be hypothesized that neuronal stress created by either CSD or synaptic activity-energy mismatch could both initiate a parenchymal inflammatory signaling cascade, propagating to the meninges, where it is converted to a lasting headache with or without aura.

Conclusion

Experimental studies in animals and emerging imaging findings from patients warrant further research to gain deeper insight to the complex role of inflammatory signaling in headache generation in migraine.

Relationship between Neuromuscular Mechanosensitivity and Chronic Neck Pain in Guitarists: A Cross-Sectional Study

Musicians frequently complain of musculoskeletal pain due to high mechanical demands, with the cervical spine being the most affected. Increased neuromuscular mechanosensitivity due to repetitive mechanical stress over time has been described in neck pain patients. Nevertheless, the association between musculoskeletal pain and neuromuscular mechanosensitivity in musicians is unknown. Therefore, the aim of this study was to analyze the relationship between neuromuscular tissue mechanosensitivity and neck pain in guitarists. Guitarists with chronic neck pain (n = 70) and without pain (n = 70) were enrolled. Pain and disability were measured by the visual analogue scale and the Neck Disability Index, respectively. The pressure pain threshold (PPT) was bilaterally measured for the upper trapezius and median nerve. Finally, the Upper limb neural test one (ULNT1) was bilaterally measured. The analyses included a 2-by-2 mixed analysis of variance, pairwise comparisons with Bonferroni correction, linear regression model, and multiple linear regression. Our data showed that chronic neck pain guitarists have a lower PPT at all locations compared to healthy guitarists. They also showed a bilateral main effect for pain for ULNT1 compared to healthy guitarists. These results were not affected by the mediator variables. Finally, a relationship between upper trapezius PPT and median nerve PPT was found.

Toll-like receptor 2 and 4 antagonism for the treatment of experimental autoimmune encephalomyelitis (EAE)-related pain

Neuropathic pain is a major symptom of multiple sclerosis (MS) with up to 92% of patients reporting bodily pain, and 85% reporting pain severe enough to cause functional disability. None of the available therapeutics target MS pain. Toll-like receptors 2 and 4 (TLR2/TLR4) have emerged as targets for treating a wide array of autoimmune disorders, including MS, as well as having demonstrated success at suppressing pain in diverse animal models. The current series of studies tested systemic TLR2/TLR4 antagonists in males and females in a low-dose Myelin oligodendrocyte glycoprotein (MOG) experimental autoimmune encephalomyelitis (EAE) model, with reduced motor dysfunction to allow unconfounded testing of allodynia through 50+ days post-MOG. The data demonstrated that blocking TLR2/TLR4 suppressed EAE-related pain, equally in males and females; upregulation of dorsal spinal cord proinflammatory gene expression for TLR2, TLR4, NLRP3, interleukin-1β, IkBα, TNF-α and interleukin-17; and upregulation of dorsal spinal cord expression of glial immunoreactivity markers. In support of these results, intrathecal interleukin-1 receptor antagonist reversed EAE-induced allodynia, both early and late after EAE induction. In contrast, blocking TLR2/TLR4 did not suppress EAE-induced motor disturbances induced by a higher MOG dose. These data suggest that blocking TLR2/TLR4 prevents the production of proinflammatory factors involved in low dose EAE pathology. Moreover, in this EAE model, TLR2/TLR4 antagonists were highly effective in reducing pain, whereas motor impairment, as seen in high dose MOG EAE, is not affected.

Nonopioid GTS-21 Mitigates Burn Injury Pain in Rats by Decreasing Spinal Cord Inflammatory Responses

BACKGROUND

Burn injury (BI) pain consists of inflammatory and neuropathic components and activates microglia. Nicotinic alpha 7 acetylcholine receptors (α7AChRs) expressed in microglia exhibit immunomodulatory activity during agonist stimulation. Efficacy of selective α7AChR agonist GTS-21 to mitigate BI pain and spinal pain-mediators was tested.

METHODS

Anesthetized rats after hind-paw BI received intraperitoneal GTS-21 or saline daily. Allodynia and hyperalgesia were tested on BI and contralateral paw for 21 days. Another group after BI receiving GTS-21 or saline had lumbar spinal cord segments harvested (day 7 or 14) to quantify spinal inflammatory-pain transducers or microglia activation using fluorescent marker, ionized calcium-binding adaptor protein (Iba1).

RESULTS

BI significantly decreased allodynia withdrawal threshold from baseline of ~9-10 to ~0.5-1 g, and hyperalgesia latency from ~16-17 to ~5-6 seconds by day 1. Both doses of GTS-21 (4 or 8 mg/kg) mitigated burn-induced allodynia from ~0.5-1 to ~2-3 g threshold (P = .089 and P = .010), and hyperalgesia from ~5-6 to 8-9 seconds (P < .001 and P < .001) by day 1. The GTS-21 group recovered to baseline pain threshold by day 15-17 compared to saline-treated, where the exaggerated nociception persisted beyond 15-17 days. BI significantly (P < .01) increased spinal cord microgliosis (identified by fluorescent Iba1 staining), microglia activation (evidenced by the increased inflammatory cytokine), and pain-transducer (protein and/or messenger RNA [mRNA]) expression (tumor necrosis factor-α [TNF-α], interleukin-1β [IL-1β], nuclear factor-kappa B [NF-κB], interleukin-6 [IL-6], Janus-associated kinase signal transducer and activator of transcription 3 [JAK-STAT3], and/or N-methyl-D-aspartate receptor [NMDAR]). GTS-21 mitigated pain-transducer changes. The α7AChR antagonist methyllycaconitine nullified the beneficial effects of GTS-21 on both increased nociception and pain-biomarker expression.

CONCLUSIONS

Nonopioid, α7AChR agonist GTS-21 elicits antinociceptive effects at least in part by decreased activation spinal-cord pain-inducers. The α7AChR agonist GTS-21 holds promise as potential therapeutic adjunct to decrease BI pain by attenuating both microglia changes and expression of exaggerated pain transducers.

Role of HMGB1 in Chemotherapy-Induced Peripheral Neuropathy

Chemotherapy-induced peripheral neuropathy (CIPN), one of major dose-limiting side effects of first-line chemotherapeutic agents such as paclitaxel, oxaliplatin, vincristine, and bortezomib is resistant to most of existing medicines. The molecular mechanisms of CIPN have not been fully understood. High mobility group box 1 (HMGB1), a nuclear protein, is a damage-associated molecular pattern protein now considered to function as a pro-nociceptive mediator once released to the extracellular space. Most interestingly, HMGB1 plays a key role in the development of CIPN. Soluble thrombomodulin (TMα), known to degrade HMGB1 in a thrombin-dependent manner, prevents CIPN in rodents treated with paclitaxel, oxaliplatin, or vincristine and in patients with colorectal cancer undergoing oxaliplatin-based chemotherapy. In this review, we describe the role of HMGB1 and its upstream/downstream mechanisms in the development of CIPN and show drug candidates that inhibit the HMGB1 pathway, possibly useful for prevention of CIPN.

Emotional and cognitive impairments in the peripheral nerve chronic constriction injury model (CCI) of neuropathic pain: A systematic review

BACKGROUND AND OBJECTIVE

Emotional and cognitive impairments are common comorbidities of chronic neuropathic pain that significantly impact the quality of life of patients. While the nociceptive components of the peripheral nerve chronic constriction injury (CCI) animal model have been extensively analyzed, data related to the development of mood and cognitive disorders, and especially its impact on female rats remains fragmented. We systematically reviewed the literature analyzing the methods used to induce and evaluate the development of emotional- and cognitive-like impairments and sex-specific differences in the CCI model.

DATABASES AND DATA TREATMENT

We searched PubMed, Google Scholar and Web of Science from inception to September 30th, 2019, and a total of 44 papers were considered eligible for inclusion. We included animal studies assessing nociception, locomotion, anxious-like, depressive-like and cognitive behaviours after the CCI induction.

RESULTS

The overall quality of the studies was considered moderate to high. Overall, the induction of CCI leads to the development of emotional impairments, namely anxiety- and depressive-like behaviours, as well as cognitive impairments. With the majority of the studies using male subjects, the lack of evidence on female animals prevents the evaluation of sex-specific differences.

CONCLUSIONS

This review supports the development of an anxiodepressive-like phenotype, associated with cognitive impairments, in CCI-induced animals. These results support the use of this animal model for the study of the mechanisms underlying these comorbidities, as well as a screening tool for the development/repurposing of drugs that tackle both the neuropathy-induced nociceptive and emotional impairments, such as tricyclic antidepressants. Importantly, our review also highlights the need for studies performed in female rodents as these are almost non-existent.

Disturbance of somatotopic spatial cognition and extra-territorial pain in carpal tunnel syndrome

BACKGROUND

Several studies on carpal tunnel syndrome have reported pain that exists beyond the median nerve territory of the affected hand. However, the mechanism is unknown.

PURPOSE

We investigated the cause of extra-territorial pain by the analysis of clinical assessments and cortical activity using magnetoencephalography.

METHODS

To compare patients with and without extra-territorial pain, fourteen patients with carpal tunnel syndrome were assessed using clinical examination, such as patients' profile, paresthesia, physical tests, and psychological tests. The physical assessment included tactile threshold and static and moving two-point discrimination sensations on digital pulp. Neural activation in the cerebral cortex was also measured using z-scores calculated by magnetoencephalography.

RESULTS

Among fourteen patients, ten patients had pain in the affected median nerve territory only and four patients had extra-territorial pain. When comparing the groups, the static and moving two-point discrimination sensation values in patients with extra-territorial pain were larger than those of patients without the pain (p < 0.05). The supra-marginal gyrus, mid-part of the precentral sulcus, angular gyrus in the left hemisphere, bilateral sensorimotor areas for legs, and bilateral isthmus-cingulate areas showed larger z-scores in patients with extra-territorial pain than in patients without the pain (p < 0.05).

CONCLUSIONS

The static and moving two-point discrimination sensations signify the ability of tactile spatial acuity. Bilateral sensorimotor areas were activated in sites that were not the hand. Furthermore, the inferior parietal lobule in the left hemisphere, which synthesizes and integrates multiple sensations showed high activation. Our findings suggested that the mechanism of extra-territorial pain was associated with dysfunction of spatial cognition.

Oxycodone, fentanyl, and morphine amplify established neuropathic pain in male rats

Opioids are widely prescribed for chronic pain, including neuropathic pain, despite growing evidence of long-term harm. Previous preclinical studies have documented exacerbation of nociceptive hypersensitivity, including that induced by peripheral nerve injury, by morphine. The present series of behavioral studies sought to replicate and extend our prior research, which demonstrated a multimonth exacerbation of nociceptive hypersensitivity by a 5-day course of morphine initiated 10 days after nerve injury. The current studies demonstrate that enduring exacerbation of nociceptive hypersensitivity is not restricted to morphine, but rather is also created by the clinically relevant opioids fentanyl and oxycodone when these are likewise administered for 5 days beginning 10 days after nerve injury. Furthermore, enduring exacerbation of nociceptive hypersensitivity is also observed when the same dosing regimen for either morphine, fentanyl, or oxycodone begins 1 month after nerve injury. Finally, a striking result from these studies is that no such exacerbation of nociceptive hypersensitivity occurs when either morphine, fentanyl, or oxycodone dosing begins at the time of nerve injury. These results extend our previous findings that morphine exacerbates nociceptive hypersensitivity to the clinically relevant opioids fentanyl and oxycodone when administered after the development of nociceptive hypersensitivity, while also providing possible clinically relevant insight into when these opioids can be safely administered and not exacerbate neuropathic pain.

Electroacupuncture Alleviates Spared Nerve Injury-Induced Neuropathic Pain And Modulates HMGB1/NF-κB Signaling Pathway In The Spinal Cord

Background

Neuropathic pain with complications greatly affects patients worldwide. High mobility group box 1 (HMGB1) has been shown to contribute to the pathogenesis of neuropathic pain; thus, suppression of HMGB1 may provide a novel therapeutic option for neuropathic pain. Electroacupuncture (EA) has been indicated to be effective in attenuating neuropathic pain, but the underlying mechanism remains to be fully clarified. We aim to explore whether 2Hz EA stimulation regulates the spinal HMGB1/NF-κB signaling in neuropathic pain induced by spared nerve injury (SNI).

Materials and methods

Paw withdrawal threshold and CatWalk gait analysis were used to assess the effect of 2Hz EA on pain-related behaviors in SNI rats. Administration of 2Hz EA to SNI rats once every other day lasting for 21 days. Expression of spinal protein molecules were detected using Western blot and immunofluorescence staining.

Results

It was found that SNI significantly induced mechanical hypersensitivity and decrease of gait parameters, and subsequently increased the levels of HMGB1, TLR4, MyD88, and NF-κB p65 protein expression. 2Hz EA stimulation led to remarkable attenuation of mechanical hypersensitivity, upregulation of spinal HMGB1, TLR4, MyD88, and NF-κB p65 protein expressions induced by SNI, and significant improvement in gait parameters. Furthermore, immunofluorescence staining also confirmed that 2Hz EA obviously suppressed the co-expression of microglia activation marker CD11b and TLR4 or MyD88, as well as the activation of NF-κB p65 in SNI rats.

Conclusion

This study suggested that blockade of HMGB1/NF-κB signaling in the spinal cord may be a promising therapeutic approach for 2Hz EA management of SNI-induced neuropathic pain.

Animal Models of Peripheral Pain: Biology Review and Application for Drug Discovery

Pain is a complex constellation of cognitive, unpleasant sensory, and emotional experiences that primarily serves as a survival mechanism. Pain arises in the peripheral nervous system and pain signals synapse with nerve tracts extending into the central nervous system. Several different schemes are used to classify pain, including the underlying mechanism, tissues primarily affected, and time-course. Numerous animal models of pain, which should be employed with appropriate Institutional Animal Care and Use approvals, have been developed to elucidate pathophysiology mechanisms and aid in identification of novel therapeutic targets. The variety of available models underscores the observations that pain phenotypes are driven by several distinct mechanisms. Pain outcome measurement encompasses both reflexive (responses to heat, cold, mechanical and electrical stimuli) and nonreflexive (spontaneous pain responses to stimuli) behaviors. However, the question of translatability to human pain conditions and potential treatment outcomes remains a topic of continued scrutiny. In this review we discuss the different types of pain and their mechanisms and pathways, available rodent pain models with an emphasis on type of pain stimulations and pain outcome measures and discuss the role of pathologists in assessing and validating pain models.

A single peri-sciatic nerve administration of the adenosine 2A receptor agonist ATL313 produces long-lasting anti-allodynia and anti-inflammatory effects in male rats

Neuropathic pain is a widespread problem which remains poorly managed by currently available therapeutics. Peripheral nerve injury and inflammation leads to changes at the nerve injury site, including activation of resident and recruited peripheral immune cells, that lead to neuronal central sensitization and pain amplification. The present series of studies tested the effects of peri-sciatic nerve delivery of single doses of adenosine 2A receptor (A_2aR) agonists on pain and neuroinflammation. The data provide converging lines of evidence supportive that A_2aR agonism at the site of peripheral nerve injury and inflammation is effective in suppressing ongoing neuropathic pain. After A_2aR agonism resolved neuropathic pain, a return of pain enhancement (allodynia) was observed in response to peri-sciatic injection of H-89, which can inhibit protein kinase A, and by peri-sciatic injection of neutralizing antibody against the potent anti-inflammatory cytokine interleukin-10. A_2aR agonist actions at the nerve injury site suppress neuroinflammation, as reflected by decreased release of interleukin-1β and nitric oxide, as well as decreased sciatic expression of markers of monocytes/macrophages and inducible nitric oxide synthase. Taken together, the data are supportive that A_2aR agonists, acting at the level of peripheral nerve injury, may be of therapeutic value in treating chronic pain of neuroinflammatory origin.

Protraction of neuropathic pain by morphine is mediated by spinal damage associated molecular patterns (DAMPs) in male rats

We have recently reported that a short course of morphine, starting 10days after sciatic chronic constriction injury (CCI), prolonged the duration of mechanical allodynia for months after morphine ceased. Maintenance of this morphine-induced persistent sensitization was dependent on spinal NOD-like receptor protein 3 (NLRP3) inflammasomes-protein complexes that proteolytically activate interleukin-1β (IL-1β) via caspase-1. However, it is still unclear how NLRP3 inflammasome signaling is maintained long after morphine is cleared. Here, we demonstrate that spinal levels of the damage associated molecular patterns (DAMPs) high mobility group box 1 (HMGB1) and biglycan are elevated during morphine-induced persistent sensitization in male rats; that is, 5weeks after cessation of morphine dosing. We also show that HMGB1 and biglycan levels are at least partly dependent on the initial activation of caspase-1, as well as Toll like receptor 4 (TLR4) and the purinergic receptor P2X7R-receptors responsible for priming and activation of NLRP3 inflammasomes. Finally, pharmacological attenuation of the DAMPs HMGB1, biglycan, heat shock protein 90 and fibronectin persistently reversed morphine-prolonged allodynia. We conclude that after peripheral nerve injury, morphine treatment results in persistent DAMP release via TLR4, P2X7R and caspase-1, which are involved in formation/activation of NLRP3 inflammasomes. These DAMPs are responsible for maintaining persistent allodynia, which may be due to engagement of a positive feedback loop, in which NLRP3 inflammasomes are persistently activated by DAMPs signaling at TLR4 and P2X7R.

Evidence for Increased Magnetic Resonance Imaging Signal Intensity and Morphological Changes in the Brachial Plexus and Median Nerves of Patients With Chronic Arm and Neck Pain Following Whiplash Injury

Background Whiplash following a motor vehicle accident can result in chronic neck and arm pain. Patients frequently present with cutaneous hypersensitivities and hypoesthesia, but without obvious clinical signs of nerve injury. T2-weighted magnetic resonance imaging (MRI) has previously been used to identify nerve pathology. Objectives To determine whether there are signs of peripheral nerve pathology on MRI in patients with chronic arm and neck pain following whiplash injury. Methods This cross-sectional study used T2-weighted MRI to examine the brachial plexus and median nerve in patients and age-matched, healthy control subjects. Clinical examination included tests of plexus and nerve trunk mechanical sensitivity. Results The T2 signal intensity was greater in the brachial plexus and median nerve at the wrist in the patient group (mean intensity ratio = 0.52 ± 0.13 and 2.09 ± 0.33, respectively) compared to the control group (mean intensity ratio = 0.45 ± 0.07 and 1.38 ± 0.31, respectively; P<.05). Changes in median nerve morphology were also observed, which included an enlargement (mean area: patient group, 8.05 ± 1.29 mm²; control group, 6.52 ± 1.08 mm²; P<.05) and flattening (mean aspect ratio: patient group, 2.46 ± 0.53; control group, 1.62 ± 0.30; P<.05) at the proximal carpal row. All patients demonstrated signs of nerve trunk mechanical sensitivity. Conclusion These findings suggest that patients with chronic whiplash may have inflammatory changes and/or mild neuropathy, which may contribute to symptoms. J Orthop Sports Phys Ther 2018;48(7):523-532. Epub 24 Apr 2018. doi:10.2519/jospt.2018.7875.

The Somatostatin Receptor-4 Agonist J-2156 Alleviates Mechanical Hypersensitivity in a Rat Model of Breast Cancer Induced Bone Pain

In the majority of patients with breast cancer in the advanced stages, skeletal metastases are common, which may cause excruciating pain. Currently available drug treatments for relief of breast cancer-induced bone pain (BCIBP) include non-steroidal anti-inflammatory drugs and strong opioid analgesics along with inhibitors of osteoclast activity such as bisphosphonates and monoclonal antibodies such as denosumab. However, these medications often lack efficacy and/or they may produce serious dose-limiting side effects. In the present study, we show that J-2156, a somatostatin receptor type 4 (SST4 receptor) selective agonist, reverses pain-like behaviors in a rat model of BCIBP induced by unilateral intra-tibial injection of Walker 256 breast cancer cells. Following intraperitoneal administration, the ED₅₀ of J-2156 for the relief of mechanical allodynia and mechanical hyperalgesia in the ipsilateral hindpaws was 3.7 and 8.0 mg/kg, respectively. Importantly, the vast majority of somatosensory neurons in the dorsal root ganglia including small diameter C-fibers and medium-large diameter fibers, that play a crucial role in cancer pain hypersensitivities, expressed the SST4 receptor. J-2156 mediated pain relief in BCIBP-rats was confirmed by observations of a reduction in the levels of phosphorylated extracellular signal-regulated kinase (pERK), a protein essential for central sensitization and persistent pain, in the spinal dorsal horn. Our results demonstrate the potential of the SST4 receptor as a pharmacological target for relief of BCIBP and we anticipate the present work to be a starting point for further mechanism-based studies.

MicroRNA-124 and microRNA-146a both attenuate persistent neuropathic pain induced by morphine in male rats

We have recently reported that a short course of morphine, starting 10 days after sciatic chronic constriction injury (CCI), prolonged the duration of mechanical allodynia for months after morphine ceased. Maintenance of this morphine-induced persistent sensitization was dependent on microglial reactivity and Toll-like receptor 4 signaling. Given that microRNAs (miRNAs) such as miR-124 and miR-146a possess the ability to modulate such signaling, we directly compared their function in this model. We found that both miRNAs reversed established allodynia in our model of morphine-induced persistent sensitization. The efficacy of miR-124 and miR-146a were comparable, and in both cases allodynia returned within hours to days of miRNA dosing conclusion. Our findings demonstrate that miRNAs targeting Toll-like receptor signaling are effective in reversing neuropathic pain, which underscores the clinical potential of these non-coding RNAs.

Sustained reversal of central neuropathic pain induced by a single intrathecal injection of adenosine A2A receptor agonists

Central neuropathic pain is a debilitating outcome of spinal cord injury (SCI) and current treatments to alleviate this pain condition are ineffective. A growing body of literature suggests that activating adenosine A_2A receptors (A_2ARs) decreases the production of proinflammatory cytokines and increases the production of anti-inflammatory cytokines. Here, the effect of administering intrathecal A_2AR agonists on central neuropathic pain was measured using hindpaw mechanical allodynia in a rat model of SCI termed spinal neuropathic avulsion pain (SNAP). Other models of SCI cause extensive damage to the spinal cord, resulting in paralysis and health problems. SNAP rats with unilateral low thoracic (T13)/high lumbar (L1) dorsal root avulsion develop below-level bilateral allodynia, without concomitant motor or health problems. A single intrathecal injection of the A_2AR agonist 2-p-(2-carboxyethyl)phenethylamino-5'-N-ethylcarboxamido adenosine HCl (CGS21680) reversed SCI-induced allodynia for at least 6 weeks. The reversal is likely in part mediated by interleukin (IL)-10, as intrathecally administering neutralizing IL-10 antibodies 1 week after CGS21680 abolished the anti-allodynic effect of CGS21680. Dorsal spinal cord tissue from the ipsilateral site of SCI (T13/L1) was assayed 1 and 6 weeks after CGS21680 for IL-10, CD11b, and tumor necrosis factor (TNF) gene expression. CGS21680 treatment did not change IL-10 gene expression but did significantly decrease CD11b and TNF gene expression at both timepoints. A second A_2AR agonist, 4-(3-(6-amino-9-(5-cyclopropylcarbamoyl-3,4-dihydroxytetrahydrofuran-2-yl)-9H-purin-2-yl)prop-2-ynyl)piperidine-1-carboxylic acid methyl ester (ATL313), was also able to significantly prevent and reverse SCI-induced allodynia for several weeks after a single intrathecal injection, providing converging lines of evidence of A_2AR involvement. The enduring pain reversal after a single intrathecal injection of A_2AR agonists suggests that A_2AR agonists could be exciting new candidates for treating SCI-induced central neuropathic pain.

Time course of ongoing activity during neuritis and following axonal transport disruption

Local nerve inflammation (neuritis) leads to ongoing activity and axonal mechanical sensitivity (AMS) along intact nociceptor axons and disrupts axonal transport. This phenomenon forms the most feasible cause of radiating pain, such as sciatica. We have previously shown that axonal transport disruption without inflammation or degeneration also leads to AMS but does not cause ongoing activity at the time point when AMS occurs, despite causing cutaneous hypersensitivity. However, there have been no systematic studies of ongoing activity during neuritis or noninflammatory axonal transport disruption. In this study, we present the time course of ongoing activity from primary sensory neurons following neuritis and vinblastine-induced axonal transport disruption. Whereas 24% of C/slow Aδ-fiber neurons had ongoing activity during neuritis, few (<10%) A- and C-fiber neurons showed ongoing activity 1-15 days following vinblastine treatment. In contrast, AMS increased transiently at the vinblastine treatment site, peaking on days 4-5 (28% of C/slow Aδ-fiber neurons) and resolved by day 15. Conduction velocities were slowed in all groups. In summary, the disruption of axonal transport without inflammation does not lead to ongoing activity in sensory neurons, including nociceptors, but does cause a rapid and transient development of AMS. Because it is proposed that AMS underlies mechanically induced radiating pain, and a transient disruption of axonal transport (as previously reported) leads to transient AMS, it follows that processes that disrupt axonal transport, such as neuritis, must persist to maintain AMS and the associated symptoms. NEW & NOTEWORTHY Many patients with radiating pain lack signs of nerve injury on clinical examination but may have neuritis, which disrupts axonal transport. We have shown that axonal transport disruption does not induce ongoing activity in primary sensory neurons but does cause transient axonal mechanical sensitivity. The present data complete a profile of key axonal sensitivities following axonal transport disruption. Collectively, this profile supports that an active peripheral process is necessary for maintained axonal sensitivities.

Extracellular HMGB1 as a therapeutic target in inflammatory diseases

INTRODUCTION

High-mobility group box 1 (HMGB1) is a ubiquitous nuclear protein that promotes inflammation when released extracellularly after cellular activation, stress, damage or death. HMGB1 operates as one of the most intriguing molecules in inflammatory disorders via recently elucidated signal and molecular transport mechanisms. Treatments based on antagonists specifically targeting extracellular HMGB1 have generated encouraging results in a wide number of experimental models of infectious and sterile inflammation. Clinical studies are still to come. Areas covered: We here summarize recent advances regarding pathways for extracellular HMGB1 release, receptor usage, and functional consequences of post-translational modifications. The review also addresses results of preclinical HMGB1-targeted therapy studies in multiple inflammatory conditions and outlines the current status of emerging clinical HMGB1-specific antagonists. Expert opinion: Blocking excessive amounts of extracellular HMGB1, particularly the disulfide isoform, offers an attractive clinical opportunity to ameliorate systemic inflammatory diseases. Therapeutic interventions to regulate intracellular HMGB1 biology must still await a deeper understanding of intracellular HMGB1 functions. Future work is needed to create more robust assays to evaluate functional bioactivity of HMGB1 antagonists. Forthcoming clinical studies would also greatly benefit from a development of antibody-based assays to quantify HMGB1 redox isoforms, presently assessed by mass spectrometry methods.

Differential involvement of ipsilateral and contralateral spinal cord astrocyte D-serine in carrageenan-induced mirror-image pain: role of σ1 receptors and astrocyte gap junctions

BACKGROUND AND PURPOSE

Although we have recently demonstrated that spinal astrocyte gap junctions mediate the development of mirror-image pain (MIP), it is still unclear which astrocyte-derived factor is responsible for the development of MIP and how its production is controlled. In the present study, we focused on the role of ipsilateral versus contralateral D-serine in the development of MIP and investigated the possible involvement of σ1 receptors and gap junctions in astrocyte D-serine production.

EXPERIMENTAL APPROACH

Following carrageenan injection, mechanical allodynia was tested at various time points to examine the effect of individual drugs. Immunohistochemistry and Western blot analyses were performed to clarify the expression levels of spinal D-serine, serine racemase, σ1 receptors and connexin 43.

KEY RESULTS

The expression of ipsilateral D-serine was up-regulated during the early phase of inflammation, while contralateral D-serine increased during the later phase of inflammation. The pharmacological inhibition of D-serine during the early phase blocked the development of both ipsilateral and contralateral mechanical allodynia. However, the inhibition of D-serine during the later phase of inflammation blocked contralateral, but not ipsilateral mechanical allodynia. Furthermore, the inhibition of σ1 receptors during the earlier phase of inflammation inhibited the increase in ipsilateral D-serine. Conversely, the blockade of astrocyte gap junctions suppressed the up-regulation of contralateral D-serine during the later phase of inflammation.

CONCLUSION AND IMPLICATIONS

Spinal astrocyte D-serine plays an important role in the development of mirror-image pain. Furthermore, σ1 receptors and astrocyte gap junction signalling mediate ipsilateral and contralateral D-serine production respectively.

HMGB1 Activates Proinflammatory Signaling via TLR5 Leading to Allodynia

Infectious and sterile inflammatory diseases are correlated with increased levels of high mobility group box 1 (HMGB1) in tissues and serum. Extracellular HMGB1 is known to activate Toll-like receptors (TLRs) 2 and 4 and RAGE (receptor for advanced glycation endproducts) in inflammatory conditions. Here, we find that TLR5 is also an HMGB1 receptor that was previously overlooked due to lack of functional expression in the cell lines usually used for studying TLR signaling. HMGB1 binding to TLR5 initiates the activation of NF-κB signaling pathway in a MyD88-dependent manner, resulting in proinflammatory cytokine production and pain enhancement in vivo. Biophysical and in vitro results highlight an essential role for the C-terminal tail region of HMGB1 in facilitating interactions with TLR5. These results suggest that HMGB1-modulated TLR5 signaling is responsible for pain hypersensitivity.

MicroRNA-142-3p relieves neuropathic pain by targeting high mobility group box 1

MicroRNA (miRNA) are emerging as critical regulators of neuropathic pain development. Neuroinflammation contributes to the development of neuropathic pain. miR‑142‑3p has been characterized as an inflammation‑related miRNA in various pathological processes. However, little is known about the role of miR‑142‑3p in neuroinflammation and neuropathic pain. The present study aimed to investigate the function of miR‑142‑3p in neuropathic pain by creating a murine model using spinal nerve ligation (SNL). A significant reduction in miR‑142‑3p expression was observed in the dorsal root ganglion of mice with SNL (P<0.05) compared with control mice. Overexpression of miR‑142‑3p significantly inhibited neuropathic pain and neuroinflammation in mice with SNL (P<0.05). High mobility group box 1 (HMGB1) was identified as a direct target gene of miR‑142‑3p by bioinformatic analysis and dual‑luciferase reporter assays. Overexpression of miR‑142‑3p significantly reduced the mRNA and protein expression levels of HMGB1 in vitro and in vivo (P<0.05). In addition, HMGB1 mRNA expression and miR‑142‑3p expression were inversely correlated in mice with SNL. Furthermore, overexpression of HMGB1 significantly reversed the inhibitory effect of miR‑142‑3p on neuroinflammation and neuropathic pain development (P<0.05). Overall, these results suggest that miR‑142‑3p functions as a negative regulator of neuropathic pain development through the downregulation of HMGB1, indicating that miR‑142‑3p may serve as a potential therapeutic target for neuropathic pain.

Impact of shoulder internal rotation on normal sensory response during ulnar nerve-biased neurodynamic testing of asymptomatic individuals

OBJECTIVE

To determine if substitution of shoulder internal rotation for external rotation during the upper limb neurodynamic test (ULNT₃) evokes comparable ulnar nerve sensory responses in asymptomatic individuals.

METHODS

Range of motion, quality, quantity, and distribution of sensory responses in 50 asymptomatic individuals during the traditional ULNT₃ were compared to identical measures during an experimental maneuver using shoulder internal rotation. Quality and quantity of sensory responses were recorded using a 10-cm visual analog scale.

RESULTS

Means of sensory responses for traditional and experimental maneuvers, respectively, were as follows: stretching, 3.84 ± 8.85 and 5.38 ± 2.85 cm; burning, 1.82 ± 2.82 and 2.50 ± 3.10 cm; tingling, 2.13 ± 3.12 and 2.18 ± 2.97 cm; and numbness, 1.04 ± 2.17 and 1.01 ± 2.03 cm. A moderate to strong correlation (ICC = 0.51-0.86) was shown to exist between maneuvers; this relationship was significant (p = .001).

DISCUSSION

Results of this study provide evidence that there was no appreciable difference in sensory responses with regard to burning and tingling when substituting shoulder internal rotation for external rotation during the ULNT₃. The results also suggest that there were only marginal differences in the sensory responses of stretching and numbness during the same substitution.

CONCLUSION

Patients who have limited glenohumeral external rotation due to pain, instability, and/or articular limitation may benefit from this substitution when presenting with signs of ulnar nerve pathodynamics. Further research will be needed to validate this maneuver in a symptomatic population.

LEVEL OF EVIDENCE

Level 2b.

Optimization and In Vivo Profiling of a Refined Rat Model of Walker 256 Breast Cancer Cell-Induced Bone Pain Using Behavioral, Radiological, Histological, Immunohistochemical and Pharmacological Methods

In the majority of patients with advanced breast cancer, there is metastatic spread to bones resulting in pain. Clinically available drug treatments for alleviation of breast cancer-induced bone pain (BCIBP) often produce inadequate pain relief due to dose-limiting side-effects. A major impediment to the discovery of novel well-tolerated analgesic agents for the relief of pain due to bony metastases is the fact that most cancer-induced bone pain models in rodents relied on the systemic injection of cancer cells, causing widespread formation of cancer metastases and poor general animal health. Herein, we have established an optimized, clinically relevant Wistar Han female rat model of breast cancer induced bone pain which was characterized using behavioral assessments, radiology, histology, immunohistochemistry and pharmacological methods. In this model that is based on unilateral intra-tibial injection (ITI) of Walker 256 carcinoma cells, animals maintained good health for at least 66 days post-ITI. The temporal development of hindpaw hypersensitivity depended on the initial number of Walker 256 cells inoculated in the tibiae. Hindpaw hypersensitivity resolved after approximately 25 days, in the continued presence of bone tumors as evidenced by ex vivo histology, micro-computed tomography scans and immunohistochemical assessments of tibiae. A possible role for the endogenous opioid system as an internal factor mediating the self-resolving nature of BCIBP was identified based upon the observation that naloxone, a non-selective opioid antagonist, caused the re-emergence of hindpaw hypersensitivity. Bolus dose injections of morphine, gabapentin, amitriptyline and meloxicam all alleviated hindpaw hypersensitivity in a dose-dependent manner. This is a first systematic pharmacological profiling of this model by testing standard analgesic drugs from four important diverse classes, which are used to treat cancer induced bone pain in the clinical setting. Our refined rat model more closely mimics the pathophysiology of this condition in humans and hence is well-suited for probing the mechanisms underpinning breast cancer induced bone pain. In addition, the model may be suitable for efficacy profiling of new molecules from drug discovery programs with potential to be developed as novel agents for alleviation of intractable pain associated with disseminated breast cancer induced bony metastases.

Prior voluntary wheel running attenuates neuropathic pain

Exercise is known to exert a systemic anti-inflammatory influence, but whether its effects are sufficient to protect against subsequent neuropathic pain is underinvestigated. We report that 6 weeks of voluntary wheel running terminating before chronic constriction injury (CCI) prevented the full development of allodynia for the ∼3-month duration of the injury. Neuroimmune signaling was assessed at 3 and 14 days after CCI. Prior exercise normalized ipsilateral dorsal spinal cord expression of neuroexcitatory interleukin (IL)-1β production and the attendant glutamate transporter GLT-1 decrease, as well as expression of the disinhibitory P2X4R-BDNF axis. The expression of the macrophage marker Iba1 and the chemokine CCL2 (MCP-1), and a neuronal injury marker (activating transcription factor 3), was attenuated by prior running in the ipsilateral lumbar dorsal root ganglia. Prior exercise suppressed macrophage infiltration and/or injury site proliferation, given decreased presence of macrophage markers Iba1, iNOS (M1), and Arg-1 (M2; expression was time dependent). Chronic constriction injury-driven increases in serum proinflammatory chemokines were suppressed by prior running, whereas IL-10 was increased. Peripheral blood mononuclear cells were also stimulated with lipopolysaccharide ex vivo, wherein CCI-induced increases in IL-1β, nitrite, and IL-10 were suppressed by prior exercise. Last, unrestricted voluntary wheel running, beginning either the day of, or 2 weeks after, CCI, progressively reversed neuropathic pain. This study is the first to investigate the behavioral and neuroimmune consequences of regular exercise terminating before nerve injury. This study suggests that chronic pain should be considered a component of "the diseasome of physical inactivity," and that an active lifestyle may prevent neuropathic pain.

The Walker 256 Breast Cancer Cell- Induced Bone Pain Model in Rats

The majority of patients with terminal breast cancer show signs of bone metastasis, the most common cause of pain in cancer. Clinically available drug treatment options for the relief of cancer-associated bone pain are limited due to either inadequate pain relief and/or dose-limiting side-effects. One of the major hurdles in understanding the mechanism by which breast cancer causes pain after metastasis to the bones is the lack of suitable preclinical models. Until the late twentieth century, all animal models of cancer induced bone pain involved systemic injection of cancer cells into animals, which caused severe deterioration of animal health due to widespread metastasis. In this mini-review we have discussed details of a recently developed and highly efficient preclinical model of breast cancer induced bone pain: Walker 256 cancer cell- induced bone pain in rats. The model involves direct localized injection of cancer cells into a single tibia in rats, which avoids widespread metastasis of cancer cells and hence animals maintain good health throughout the experimental period. This model closely mimics the human pathophysiology of breast cancer induced bone pain and has great potential to aid in the process of drug discovery for treating this intractable pain condition.

Morphine paradoxically prolongs neuropathic pain in rats by amplifying spinal NLRP3 inflammasome activation

Opioid use for pain management has dramatically increased, with little assessment of potential pathophysiological consequences for the primary pain condition. Here, a short course of morphine, starting 10 d after injury in male rats, paradoxically and remarkably doubled the duration of chronic constriction injury (CCI)-allodynia, months after morphine ceased. No such effect of opioids on neuropathic pain has previously been reported. Using pharmacologic and genetic approaches, we discovered that the initiation and maintenance of this multimonth prolongation of neuropathic pain was mediated by a previously unidentified mechanism for spinal cord and pain-namely, morphine-induced spinal NOD-like receptor protein 3 (NLRP3) inflammasomes and associated release of interleukin-1β (IL-1β). As spinal dorsal horn microglia expressed this signaling platform, these cells were selectively inhibited in vivo after transfection with a novel Designer Receptor Exclusively Activated by Designer Drugs (DREADD). Multiday treatment with the DREADD-specific ligand clozapine-N-oxide prevented and enduringly reversed morphine-induced persistent sensitization for weeks to months after cessation of clozapine-N-oxide. These data demonstrate both the critical importance of microglia and that maintenance of chronic pain created by early exposure to opioids can be disrupted, resetting pain to normal. These data also provide strong support for the recent "two-hit hypothesis" of microglial priming, leading to exaggerated reactivity after the second challenge, documented here in the context of nerve injury followed by morphine. This study predicts that prolonged pain is an unrealized and clinically concerning consequence of the abundant use of opioids in chronic pain.