Increased calcitonin gene-related peptide in neuroma and invading macrophages is involved in the up-regulation of interleukin-6 and thermal hyperalgesia in a rat model of mononeuropathy

Strategies for Treating Traumatic Neuromas with Tissue-Engineered Materials

Neuroma, a pathological response to peripheral nerve injury, refers to the abnormal growth of nerve tissue characterized by disorganized axonal proliferation. Commonly occurring after nerve injuries, surgeries, or amputations, this condition leads to the formation of painful nodular structures. Traditional treatment options include surgical excision and pharmacological management, aiming to alleviate symptoms. However, these approaches often offer temporary relief without addressing the underlying regenerative challenges, necessitating the exploration of advanced strategies such as tissue-engineered materials for more comprehensive and effective solutions. In this study, we discussed the etiology, molecular mechanisms, and histological morphology of traumatic neuromas after peripheral nerve injury. Subsequently, we summarized and analyzed current nonsurgical and surgical treatment options, along with their advantages and disadvantages. Additionally, we emphasized recent advancements in treating traumatic neuromas with tissue-engineered material strategies. By integrating biomaterials, growth factors, cell-based approaches, and electrical stimulation, tissue engineering offers a comprehensive solution surpassing mere symptomatic relief, striving for the structural and functional restoration of damaged nerves. In conclusion, the utilization of tissue-engineered materials has the potential to significantly reduce the risk of neuroma recurrence after surgical treatment.

Navigating the blurred path of mixed neuroimmune signaling

Evolution has created complex mechanisms to sense environmental danger and protect tissues, with the nervous and immune systems playing pivotal roles. These systems work together, coordinating local and systemic reflexes to restore homeostasis in response to tissue injury and infection. By sharing receptors and ligands, they influence the pathogenesis of various diseases. Recently, a less-explored aspect of neuroimmune communication has emerged: the release of neuropeptides from immune cells and cytokines/chemokines from sensory neurons. This article reviews evidence of this unique neuroimmune interplay and its impact on the development of allergy, inflammation, itch, and pain. We highlight the effects of this neuroimmune signaling on vital processes such as host defense, tissue repair, and inflammation resolution, providing avenues for exploration of the underlying mechanisms and therapeutic potential of this signaling.

Biology and pathophysiology of symptomatic neuromas

ABSTRACT

Neuromas are a substantial cause of morbidity and reduction in quality of life. This is not only caused by a disruption in motor and sensory function from the underlying nerve injury but also by the debilitating effects of neuropathic pain resulting from symptomatic neuromas. A wide range of surgical and therapeutic modalities have been introduced to mitigate this pain. Nevertheless, no single treatment option has been successful in completely resolving the associated constellation of symptoms. While certain novel surgical techniques have shown promising results in reducing neuroma-derived and phantom limb pain, their effectiveness and the exact mechanism behind their pain-relieving capacities have not yet been defined. Furthermore, surgery has inherent risks, may not be suitable for many patients, and may yet still fail to relieve pain. Therefore, there remains a great clinical need for additional therapeutic modalities to further improve treatment for patients with devastating injuries that lead to symptomatic neuromas. However, the molecular mechanisms and genetic contributions behind the regulatory programs that drive neuroma formation-as well as the resulting neuropathic pain-remain incompletely understood. Here, we review the histopathological features of symptomatic neuromas, our current understanding of the mechanisms that favor neuroma formation, and the putative contributory signals and regulatory programs that facilitate somatic pain, including neurotrophic factors, neuroinflammatory peptides, cytokines, along with transient receptor potential, and ionotropic channels that suggest possible approaches and innovations to identify novel clinical therapeutics.

Nociceptor-Macrophage Interactions in Apical Periodontitis: How Biomolecules Link Inflammation with Pain

Periradicular tissues have a rich supply of peripheral afferent neurons, also known as nociceptive neurons, originating from the trigeminal nerve. While their primary function is to relay pain signals to the brain, these are known to be involved in modulating innate and adaptive immunity by initiating neurogenic inflammation (NI). Studies have investigated neuroanatomy and measured the levels of biomolecules such as cytokines and neuropeptides in human saliva, gingival crevicular fluid, or blood/serum samples in apical periodontitis (AP) to validate the possible role of trigeminal nociceptors in inflammation and tissue regeneration. However, the contributions of nociceptors and the mechanisms involved in the neuro-immune interactions in AP are not fully understood. This narrative review addresses the complex biomolecular interactions of trigeminal nociceptors with macrophages, the effector cells of the innate immune system, in the clinical manifestations of AP.

Control of myeloid cell functions by nociceptors

The immune system has evolved to protect the host from infectious agents, parasites, and tumor growth, and to ensure the maintenance of homeostasis. Similarly, the primary function of the somatosensory branch of the peripheral nervous system is to collect and interpret sensory information about the environment, allowing the organism to react to or avoid situations that could otherwise have deleterious effects. Consequently, a teleological argument can be made that it is of advantage for the two systems to cooperate and form an “integrated defense system” that benefits from the unique strengths of both subsystems. Indeed, nociceptors, sensory neurons that detect noxious stimuli and elicit the sensation of pain or itch, exhibit potent immunomodulatory capabilities. Depending on the context and the cellular identity of their communication partners, nociceptors can play both pro- or anti-inflammatory roles, promote tissue repair or aggravate inflammatory damage, improve resistance to pathogens or impair their clearance. In light of such variability, it is not surprising that the full extent of interactions between nociceptors and the immune system remains to be established. Nonetheless, the field of peripheral neuroimmunology is advancing at a rapid pace, and general rules that appear to govern the outcomes of such neuroimmune interactions are beginning to emerge. Thus, in this review, we summarize our current understanding of the interaction between nociceptors and, specifically, the myeloid cells of the innate immune system, while pointing out some of the outstanding questions and unresolved controversies in the field. We focus on such interactions within the densely innervated barrier tissues, which can serve as points of entry for infectious agents and, where known, highlight the molecular mechanisms underlying these interactions.

The Yin/Yang Balance of Communication between Sensory Neurons and Macrophages in Traumatic Peripheral Neuropathic Pain

Traumatic peripheral neuropathic pain is a complex syndrome caused by a primary lesion or dysfunction of the peripheral nervous system. Secondary to the lesion, resident or infiltrating macrophages proliferate and initiate a cross-talk with the sensory neurons, at the level of peripheral nerves and sensory ganglia. The neuron–macrophage interaction, which starts very early after the lesion, is very important for promoting pain development and for initiating changes that will facilitate the chronicization of pain, but it also has the potential to facilitate the resolution of injury-induced changes and, consequently, promote the reduction of pain. This review is an overview of the unique characteristics of nerve-associated macrophages in the peripheral nerves and sensory ganglia and of the molecules and signaling pathways involved in the neuro-immune cross-talk after a traumatic lesion, with the final aim of better understanding how the balance between pro- and anti-nociceptive dialogue between neurons and macrophages may be modulated for new therapeutic approaches.

Immunomodulatory Role of Neuropeptides in the Cornea

The transparency of the cornea along with its dense sensory innervation and resident leukocyte populations make it an ideal tissue to study interactions between the nervous and immune systems. The cornea is the most densely innervated tissue of the body and possesses both immune and vascular privilege, in part due to its unique repertoire of resident immune cells. Corneal nerves produce various neuropeptides that have a wide range of functions on immune cells. As research in this area expands, further insights are made into the role of neuropeptides and their immunomodulatory functions in the healthy and diseased cornea. Much remains to be known regarding the details of neuropeptide signaling and how it contributes to pathophysiology, which is likely due to complex interactions among neuropeptides, receptor isoform-specific signaling events, and the inflammatory microenvironment in disease. However, progress in this area has led to an increase in studies that have begun modulating neuropeptide activity for the treatment of corneal diseases with promising results, necessitating the need for a comprehensive review of the literature. This review focuses on the role of neuropeptides in maintaining the homeostasis of the ocular surface, alterations in disease settings, and the possible therapeutic potential of targeting these systems.

Genetic and functional evidence for gp130/IL6ST-induced transient receptor potential ankyrin 1 upregulation in uninjured but not injured neurons in a mouse model of neuropathic pain

ABSTRACT

Peripheral nerve injuries result in pronounced alterations in dorsal root ganglia, which can lead to the development of neuropathic pain. Although the polymodal mechanosensitive transient receptor potential ankyrin 1 (TRPA1) ion channel is emerging as a relevant target for potential analgesic therapies, preclinical studies do not provide unequivocal mechanistic insight into its relevance for neuropathic pain pathogenesis. By using a transgenic mouse model with a conditional depletion of the interleukin-6 (IL-6) signal transducer gp130 in Nav1.8 expressing neurons (SNS-gp130-/-), we provide a mechanistic regulatory link between IL-6/gp130 and TRPA1 in the spared nerve injury (SNI) model. Spared nerve injury mice developed profound mechanical hypersensitivity as indicated by decreased withdrawal thresholds in the von Frey behavioral test in vivo, as well as a significant increase in mechanosensitivity of unmyelinated nociceptive primary afferents in ex vivo skin-nerve recordings. In contrast to wild type and control gp130fl/fl animals, SNS-gp130-/- mice did not develop mechanical hypersensitivity after SNI and exhibited low levels of Trpa1 mRNA in sensory neurons, which were partially restored by adenoviral gp130 re-expression in vitro. Importantly, uninjured but not injured neurons developed increased responsiveness to the TRPA1 agonist cinnamaldehyde, and neurons derived from SNS-gp130-/- mice after SNI were significantly less responsive to cinnamaldehyde. Our study shows for the first time that TRPA1 upregulation is attributed specifically to uninjured neurons in the SNI model, and this depended on the IL-6 signal transducer gp130. We provide a solution to the enigma of TRPA1 regulation after nerve injury and stress its significance as an important target for neuropathic pain disorders.

A translational study: Involvement of miR-21-5p in development and maintenance of neuropathic pain via immune-related targets CCL5 and YWHAE

Neuropathic pain occurs in more than half of the patients suffering from peripheral neuropathies. We investigated the role of microRNA (miR)-21 in neuropathic pain using a murine-human translational approach. We applied the spared nerve injury (SNI) model at the sciatic nerve of mice and assessed the potential analgesic effect of perineurial miR-21-5p inhibitor application. Immune-related targets of miR-21-5p were determined by a qRT-PCR based cytokine and chemokine array. Bioinformatical analysis identified potential miR-21-5p targets interacting with CC-chemokine ligand (CCL)5. We validated CCL5 and tyrosine 3-monooxygenase/tryptophan 5-monooxygenase activation protein (YWHAE), an interaction partner of miR-21-5p and CCL5, by qRT-PCR in murine common peroneal and tibial nerves. Validated candidates were then investigated in white blood cell and sural nerve biopsy samples of patients with focal to generalized pain syndromes, i.e. small fiber neuropathy (SFN), polyneuropathy (PNP), and nerve lesion (NL). We showed that perineurial miR-21-5p inhibition reverses SNI-induced mechanical and heat hypersensitivity in mice and found a reduction of the SNI-induced increase of the pro-inflammatory mediators CCL5 (p < 0.01), CCL17 (p < 0.05), and IL-12ß (p < 0.05) in miR-21-5p inhibitor-treated mice. In silico analysis revealed several predicted and validated targets for miR-21-5p with CCL5 interaction. Among these, we found lower YWHAE gene expression in mice after SNI and perineurial injections of a scrambled oligonucleotide compared to naïve mice (p < 0.05), but this was not changed by miR-21-5p inhibition. Furthermore, miR-21-5p inhibition led to a further increase of the SNI-induced increase in TGFß (p < 0.01). Patient biomaterial revealed different systemic expression patterns of miR-21-5p, with higher expression in SFN and lower expression in NL. Further, we showed higher systemic expression of pro-inflammatory mediators in white blood cells of SFN patients compared to healthy controls. We have conducted a translational study comparing results from animal models to human patients with three different neuropathic pain syndromes. We identified CCL5 as a miR-21 dependent common player in the mouse SNI model and the human painful disease SFN.

Perineural high-mobility group box 1 induces mechanical hypersensitivity through activation of spinal microglia: Involvement of glutamate-NMDA receptor dependent mechanism in spinal dorsal horn

High mobility box 1 (HMGB1), a damage-associated molecular pattern, has crucial roles in induction of neuropathic pain. Upregulation of HMGB1 around the injured sciatic nerve contributes to mechanical hypersensitivity following partial sciatic nerve ligation (PSNL) of mice. However, central mechanisms mediating perineural HMGB1-induced nociceptive hypersensitivity, especially within the spinal dorsal horn, have not been determined. The current study shows that perineural treatment of naïve mice with recombinant HMGB1, which mimics increased HMGB1 around the injured sciatic nerve of PSNL mice, significantly induced activation of microglia, but not astrocytes, in the spinal dorsal horn. Intraperitoneal injection of minocycline, a microglial inhibitor, ameliorated perineural rHMGB1-induced mechanical hypersensitivity. In addition, blockade of spinal N-methyl-D-aspartate (NMDA) receptors significantly prevented perineural rHMGB1-induced mechanical hypersensitivity and microglial activation. In contrast, non-NMDA receptors, neurokinin 1 receptor, colony-stimulating factor 1 receptor and P2Y₁₂ receptor were not involved in perineural rHMGB1-induced mechanical hypersensitivity. Furthermore, repeated perineural treatment with an anti-HMGB1 antibody blocked activation of spinal microglia in PSNL mice. Collectively, the current findings demonstrate that increased HMGB1 around injured sciatic nerve might induce nociceptive hypersensitivity through activation of spinal microglia. Thus, HMGB1-dependent mechanisms between the injured sciatic nerve and spinal dorsal horn could be crucial in induction of neuropathic pain.

Pulsed Ultrasound Remedies Post-thoracotomy Hypersensitivity and Increases Spinal Anti-inflammatory Cytokine in Rats

The purpose of the experiment was to study the effect of pulsed ultrasound (PUS) on post-thoracotomy pain and local tissue temperature and to correlate the findings with the alteration in spinal anti-inflammatory and pro-inflammatory cytokines. Mechanical sensitivity, subcutaneous temperature and spinal interleukin-10 (IL-10), IL-6 or tumor necrosis factor-alpha (TNF-α) expression were examined in a rat model of experimental post-thoracotomy pain. Group 1 received a sham surgery where thoracotomy was performed except for rib retraction. Group 2 underwent thoracotomy with rib retraction (TRR). Group 3 received the TRR procedure followed by PUS. Group 4 underwent the TRR procedure followed by only the massage with the ultrasound turned off. Compared with group 1 (sham), groups 2-4 showed a decrease in mechanical withdrawal thresholds on postoperative days (PODs) 10 and 11. On PODs 16, 23 and 30, group 3 (TRR+PUS-1) displayed an increase in mechanical withdrawal thresholds compared with groups 2 and 4. Subcutaneous and body temperatures in group 3 were not prominently different from group 1, group 2 (TRR only) or group 4 (TRR+PUS-0). Compared with group 2, group 3 had an increase in spinal IL-10 level on POD 30 and a decrease in spinal IL-6 or TNF-α expression on PODs 16 and 30. We concluded that mechanical hypersensitivity after TRR is postponed by PUS, and its effect continues for 3 wk. A PUS dose not increase local tissue temperature. The beneficial effect of PUS appears related to upregulation of spinal anti-inflammatory cytokine and downregulation of spinal pro-inflammatory cytokines.

Functional Block of Interleukin-6 Reduces a Bone Pain Marker but Not Bone Loss in Hindlimb-Unloaded Mice

Interleukin-6 (IL-6) is widely accepted to stimulate osteoclasts. Our aim in this study was to examine whether the inhibitory effect of IL-6 on bone loss and skeletal pain associated with osteoporosis in hindlimb-unloaded (HU) mice in comparison with bisphosphonate. Eight-week-old male ddY mice were tail suspended for 2 weeks. Starting immediately after reload, vehicle (HU group), alendronate (HU-ALN group), or anti-IL-6 receptor antibody (HU-IL-6i group) was injected subcutaneously. After a 2-week treatment, pain-related behavior was examined using von Frey filaments. The bilateral distal femoral and proximal tibial metaphyses were analyzed three-dimensionally with micro-computed tomography. Calcitonin gene-related peptide (CGRP) expressions in dorsal root ganglion (DRG) neurons innervating the hindlimbs were examined using immunohistochemistry. HU mice with tail suspension developed bone loss. The HU mice showed mechanical hyperalgesia in the hindlimbs and increased CGRP immunoreactive neurons in the L3-5 DRG. Treatment with IL-6i and ALN prevented HU-induced mechanical hyperalgesia and upregulation of CGRP expressions in DRG neurons. Furthermore, ALN but not IL-6i prevented HU-induced bone loss. In summary, treatment with IL-6i prevented mechanical hyperalgesia in hindlimbs and suppressed CGRP expressions in DRG neurons of osteoporotic models. The novelty of this research suggests that IL-6 is one of the causes of immobility-induced osteoporotic pain regardless improvement of bone loss.

Red nucleus IL-6 mediates the maintenance of neuropathic pain by inducing the productions of TNF-α and IL-1β through the JAK2/STAT3 and ERK signaling pathways

We previously reported that interleukin (IL)-6 in the red nucleus (RN) is involved in the maintenance of neuropathic pain induced by spared nerve injury (SNI), and exerts a facilitatory effect via Janus-activated kinase 2/signal transducer and activator of transcription 3 (JAK2/STAT3) and extracellular signal-regulated kinase (ERK) signal transduction pathways. The present study aimed at investigating the roles of tumor necrosis factor-α (TNF-α) and IL-1β in RN IL-6-mediated maintenance of neuropathic pain and related signal transduction pathways. Being similar to the elevation of RN IL-6 three weeks after SNI, increased protein levels of both TNF-α and IL-1β were also observed in the contralateral RN three weeks after the nerve injury. The upregulations of TNF-α and IL-1β were closely correlative with IL-6 and suppressed by intrarubral injection of a neutralizing antibody against IL-6. Administration of either the JAK2 antagonist AG490 or the ERK antagonist PD98059 to the RN of rats with SNI remarkably increased the paw withdrawal threshold (PWT) and inhibited the up-regulations of local TNF-α and IL-1β. Further experiments indicated that intrarubral injection of exogenous IL-6 in naive rats apparently lowered the PWT of the contralateral hindpaw and boosted the local expressions of TNF-α and IL-1β. Pretreatment with AG490 could block IL-6-induced tactile hypersensitivity and suppress the up-regulations of both TNF-α and IL-1β. However, injection of PD98059 in advance only inhibited the upregulation of IL-1β, but not TNF-α. These findings indicate that RN IL-6 mediates the maintenance of neuropathic pain by inducing the productions of TNF-α and IL-1β. IL-6 induces the expression of TNF-α through the JAK2/STAT3 pathway, and the production of IL-1β through the JAK2/STAT3 and ERK pathways.

Optogenetic Inhibition of CGRPα Sensory Neurons Reveals Their Distinct Roles in Neuropathic and Incisional Pain

Cutaneous somatosensory neurons convey innocuous and noxious mechanical, thermal, and chemical stimuli from peripheral tissues to the CNS. Among these are nociceptive neurons that express calcitonin gene-related peptide-α (CGRPα). The role of peripheral CGRPα neurons (CANs) in acute and injury-induced pain has been studied using diphtheria toxin ablation, but their functional roles remain controversial. Because ablation permanently deletes a neuronal population, compensatory changes may ensue that mask the physiological or pathophysiological roles of CANs, particularly for injuries that occur after ablation. Therefore, we sought to define the role of intact CANs in vivo under baseline and injury conditions by using noninvasive transient optogenetic inhibition. We assessed pain behavior longitudinally from acute to chronic time points. We generated adult male and female mice that selectively express the outward rectifying proton pump archaerhodopsin-3 (Arch) in CANs, and inhibited their peripheral cutaneous terminals in models of neuropathic (spared nerve injury) and inflammatory (skin-muscle incision) pain using transdermal light activation of Arch. After nerve injury, brief activation of Arch reversed the chronic mechanical, cold, and heat hypersensitivity, alleviated the spontaneous pain, and reversed the sensitized mechanical currents in primary afferent somata. In contrast, Arch inhibition of CANs did not alter incision-induced hypersensitivity. Instead, incision-induced mechanical and heat hypersensitivity was alleviated by peripheral blockade of CGRPα peptide-receptor signaling. These results reveal that CANs have distinct roles in the time course of pain during neuropathic and incisional injuries and suggest that targeting peripheral CANs or CGRPα peptide-receptor signaling could selectively treat neuropathic or postoperative pain, respectively.SIGNIFICANCE STATEMENT The contribution of sensory afferent CGRPα neurons (CANs) to neuropathic and inflammatory pain is controversial. Here, we left CANs intact during neuropathic and perioperative incision injury by using transient transdermal optogenetic inhibition of CANs. We found that peripheral CANs are required for neuropathic mechanical, cold, and heat hypersensitivity, spontaneous pain, and sensitization of mechanical currents in afferent somata. However, they are dispensable for incisional pain transmission. In contrast, peripheral pharmacological inhibition of CGRPα peptide-receptor signaling alleviated the incisional mechanical and heat hypersensitivity, but had no effect on neuropathic pain. These results show that CANs have distinct roles in neuropathic and incisional pain and suggest that their targeting via novel peripheral treatments may selectively alleviate neuropathic versus incisional pain.

Interleukin-6 Inhibitor Suppresses Hyperalgesia Without Improvement in Osteoporosis in a Mouse Pain Model of Osteoporosis

The aim of this study was to evaluate skeletal pain associated with osteoporosis and examine the inhibitory effect of interleukin-6 (IL-6) on pain in ovariectomized (OVX) mice. The mechanism of osteoporotic pain in OVX mice was evaluated by examining pain-related behavior and immunohistochemistry. The effects of IL-6 receptor inhibitor (IL-6i) on these parameters were also assessed. Eight-week-old female ddY mice were ovariectomized and assigned to three groups: OVX mice treated with vehicle (OVX); OVX mice treated with alendronate (OVX-ALN); and OVX mice treated with anti-IL-6 receptor (anti-IL-6R) antibody (OVX-IL6i). Sham-operated mice were treated with vehicle. Immediately after surgery, vehicle, ALN, or anti-IL-6R antibody was injected subcutaneously. After a 4-week treatment, mechanical sensitivity was examined using von Frey filaments. The bilateral distal femoral metaphyses and proximal tibial metaphyses were analyzed three-dimensionally with micro-computed tomography. Calcitonin gene-related peptide (CGRP) expression in L3-L5 dorsal root ganglion (DRG) neurons was examined using immunohistochemistry. Ovariectomy induced bone loss and mechanical hyperalgesia in the hindlimbs with upregulation of CGRP expression in the DRG neurons innervating the hindlimbs. ALN treatment prevented bone loss, but anti-IL-6R antibody treatment had no effect on bone morphometry compared with that of the OVX group. However, mechanical hyperalgesia and CGRP expression were significantly decreased in the OVX-IL6i and OVX-ALN groups compared with those in the OVX group. Although anti-IL-6R antibody treatment had no effect on ovariectomy-induced bone loss, the treatment prevented ovariectomy-induced mechanical hyperalgesia in the hindlimbs and suppressed CGRP expression in DRG neurons. The results suggest that IL-6 is one of the causes of postmenopausal osteoporotic pain, and anti-IL-6R antibody might preserve bone health and decrease osteoporotic pain.

Inflammatory Macrophages in the Sciatic Nerves Facilitate Neuropathic Pain Associated with Type 2 Diabetes Mellitus

Despite the requirement for effective medication against neuropathic pain associated with type 2 diabetes mellitus (T2DM), mechanism-based pharmacotherapy has yet to be established. Given that long-lasting neuroinflammation, driven by inflammatory macrophages in the peripheral nerves, plays a pivotal role in intractable pain, it is important to determine whether inflammatory macrophages contribute to neuropathic pain associated with T2DM. To generate an experimental model of T2DM, C57BL/6J mice were fed a high-fat diet (HFD) ad libitum. Compared with control diet feeding, obesity and hyperglycemia were observed after HFD feeding, and the mechanical pain threshold evaluated using the von Frey test was found to be decreased, indicating the development of mechanical allodynia. The expression of mRNA markers for macrophages, inflammatory cytokines, and chemokines were significantly upregulated in the sciatic nerve (SCN) after HFD feeding. Perineural administration of saporin-conjugated anti-Mac1 antibody (Mac1-Sap) improved HFD-induced mechanical allodynia. Moreover, treatment of Mac1-Sap decreased the accumulation of F4/80⁺ macrophages and the upregulation of inflammatory mediators in the SCN after HFD feeding. Inoculation of lipopolysaccharide-activated peritoneal macrophages in tissue surrounding the SCN elicited mechanical allodynia. Furthermore, pharmacological inhibition of inflammatory macrophages by either perineural or systemic administration of TC-2559 [4-(5-ethoxy-3-pyridinyl)-N-methyl-(3E)-3-buten-1-amine difumarate], a α4β2 nicotinic acetylcholine receptor-selective agonist, relieved HFD-induced mechanical allodynia. Taken together, inflammatory macrophages that accumulate in the SCN mediate the pathophysiology of neuropathic pain associated with T2DM. Inhibitory agents for macrophage-driven neuroinflammation could be potential candidates for novel pharmacotherapy against intractable neuropathic pain.

Therapeutic opportunities for pain medicines via targeting of specific translation signaling mechanisms

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A common underlying cause of chronic pain is a phenotypic change in nociceptors in the peripheral nervous system.

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Translation regulation signaling pathways control gene expression changes that drive chronic pain.

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We focus on developments in pharmacology around translation regulation signaling that may yield new pain therapeutics.

As the population of the world ages and as more and more people survive diseases that used to be primary causes of mortality, the incidence of severe chronic pain in most of the world has risen dramatically. This type of pain is very difficult to treat and the opioid overdose epidemic that has become a leading cause of death in the United States and other parts of the world highlights the urgent need to develop new pain therapeutics. A common underlying cause of severe chronic pain is a phenotypic change in pain-sensing neurons in the peripheral nervous system called nociceptors. These neurons play a vital role in detecting potentially injurious stimuli, but when these neurons start to detect very low levels of inflammatory meditators or become spontaneously active, they send spurious pain signals to the brain that are significant drivers of chronic pain. An important question is what drives this phenotypic shift in nociceptors from quiescence under most conditions to sensitization to a broad variety of stimuli and spontaneous activity. The goal of this review is to discuss the critical role that specific translation regulation signaling pathways play in controlling gene expression changes that drive nociceptor sensitization and may underlie the development of spontaneous activity. The focus will be on advances in technologies that allow for identification of such targets and on developments in pharmacology around translation regulation signaling that may yield new pain therapeutics. A key advantage of pharmacological manipulation of these signaling events is that they may reverse phenotypic shifts in nociceptors that drive chronic pain thereby creating the first generation of disease modifying drugs for chronic pain.

Schwann cell TRPA1 mediates neuroinflammation that sustains macrophage-dependent neuropathic pain in mice

It is known that transient receptor potential ankyrin 1 (TRPA1) channels, expressed by nociceptors, contribute to neuropathic pain. Here we show that TRPA1 is also expressed in Schwann cells. We found that in mice with partial sciatic nerve ligation, TRPA1 silencing in nociceptors attenuated mechanical allodynia, without affecting macrophage infiltration and oxidative stress, whereas TRPA1 silencing in Schwann cells reduced both allodynia and neuroinflammation. Activation of Schwann cell TRPA1 evoked NADPH oxidase 1 (NOX1)-dependent H2O2 release, and silencing or blocking Schwann cell NOX1 attenuated nerve injury-induced macrophage infiltration, oxidative stress and allodynia. Furthermore, the NOX2-dependent oxidative burst, produced by macrophages recruited to the perineural space activated the TRPA1-NOX1 pathway in Schwann cells, but not TRPA1 in nociceptors. Schwann cell TRPA1 generates a spatially constrained gradient of oxidative stress, which maintains macrophage infiltration to the injured nerve, and sends paracrine signals to activate TRPA1 of ensheathed nociceptors to sustain mechanical allodynia.

Red nucleus interleukin-6 participates in the maintenance of neuropathic pain through JAK/STAT3 and ERK signaling pathways

We previously reported that interleukin-6 (IL-6) in the red nucleus (RN) is up-regulated at 3weeks after spared nerve injury (SNI), and plays facilitated role in the later maintenance of neuropathic pain. The current study aimed to reveal the roles of different signaling pathways, including Janus kinase/signal transducer and activator of transcription 3 (JAK/STAT3), extracellular signal-regulated kinase (ERK), c-Jun N-terminal kinase (JNK), p38 mitogen-activated protein kinase (MAPK) and phosphatidylinositide 3-kinase/protein kinase B (PI3K/AKT), in RN IL-6-mediated pain modulation. In accord with the increase of IL-6 in the RN following SNI, the protein levels of phospho-STAT3 (p-STAT3), p-ERK and p-JNK were also up-regulated in the RN contralateral to the nerve injury side at 3weeks after SNI. The increases of p-STAT3 and p-ERK (but not p-JNK) were associated with IL-6 and could be blocked by anti-IL-6 antibody. Microinjection of JAK2 inhibitor AG490, ERK inhibitor PD98059 and also JNK inhibitor SP600125 into the RN significantly increased the paw withdrawal threshold (PWT) and alleviated SNI-induced mechanical allodynia. Further studies showed that microinjection of recombinant rat IL-6 (rrIL-6, 20ng) into the RN of normal rats significantly decreased the PWT of rats and increased the local protein levels of p-STAT3 and p-ERK, but not p-JNK. Pre-treatment with AG490 and PD98059 could prevent IL-6-induced mechanical allodynia. Whereas, p-p38 MAPK and p-AKT did not show any expression changes in the RN of rats with SNI or rats treated with rrIL-6. These results suggest that RN IL-6 participates in the later maintenance of SNI-induced neuropathic pain and plays facilitated role through activating JAK/STAT3 and ERK signaling pathways.

Calcitonin Peptide Family Members Are Differentially Regulated by LPS and Inhibit Functions of Rat Alveolar NR8383 Macrophages

Members of the calcitonin peptide family—calcitonin gene-related peptide (CGRP), adrenomedullin (AM), and adrenomedullin2/intermedin (IMD)–exert modulatory effects upon monocytes and macrophages of various extrapulmonary origins. Utilizing the rat alveolar macrophage (AMφ) cell line NR8383, we here set out to determine to which extent these three peptides and their receptors are differentially regulated in AMφ and what specific effects they have on AMφ key functions. LPS treatment differentially up-regulated expression of the peptides and receptors. Among the three peptides, IMD mRNA content was lowest both in primary rat AMφ and NR8383 cells, whereas IMD peptide dominated in basal and LPS-stimulated secretion from NR8383 cells. Fcγ receptor-mediated phagocytosis and TNF-α production were inhibited by AM, IMD, and CGRP, whereas pro-IL-1β mRNA was slightly down-regulated exclusively by CGRP. Neither of these peptides affected IL-6 or IL-10 production. None increased intracellular calcium concentration, but AM significantly inhibited store-operated calcium entry. In conclusion, the rat AMφ cell line NR8383 is both a source and a target of the calcitonin peptide family members AM, IMD, and CGRP. Despite sharing proteins of the receptor complexes, AM, IMD, and CGRP each showed a characteristic pattern of effects and regulation, suggesting that these closely related peptides are not just redundant members of one common signaling pathway but act in concert by addressing parallel signaling cascades. Since peptide and receptor expression are up-regulated by LPS, these signaling pathways might act as inhibitory feedback mechanisms in pulmonary bacterial infection.

Therapeutic Ultrasound and Treadmill Training Suppress Peripheral Nerve Injury-Induced Pain in Rats

BACKGROUND

Although evidence suggests that therapeutic ultrasound (TU) in combination with treadmill training (TT) suppresses nerve injury-associated pain, the molecular mechanisms for this action are not clear.

OBJECTIVE

The purpose of this research was to study the possible beneficial effects of TU and TT, alone and in combination, on 2 clinical indicators of neuropathic pain and correlate these findings with changes in inflammatory mediators within the spinal cord. Our experimental model used the well-known chronic constriction injury (CCI) of the rat sciatic nerve.

DESIGN

This was an experimental study.

METHODS

Each group contained 10 rats. Group 1 underwent only the CCI procedure. Group 2 underwent a sham operation where the sciatic nerve was exposed but not ligated. Group 3 had the sham operation followed by both TT and TU. Groups 4, 5, and 6 underwent the CCI procedure followed by TT alone, TU alone, and both the TT and TU interventions, respectively. Heat and mechanical sensitivity, interleukin-6 (IL-6), interleukin-10 (IL-10), and ionized calcium binding adaptor molecule 1 (Iba1) were evaluated.

RESULTS

Compared with group 1 animals, TT or TU, or both, produced smaller decreases in mechanical withdrawal threshold and heat withdrawal latencies. The combination of TT and TU was more effective than either treatment alone. In addition, rats that received these treatments did not express the upregulation of IL-6 and Iba1 in their spinal cords on postoperative days 14 and 28, as was found in the group 1 animals.

LIMITATIONS

These experimental findings may not be generalizable to humans.

CONCLUSIONS

The combination of TU and TT reduces neuropathic pain more than either modality alone. This beneficial effect appears related to downregulation of proinflammatory IL-6 and Iba1, while upregulating the anti-inflammatory IL-10.

Perineural expression of high-mobility group box-1 contributes to long-lasting mechanical hypersensitivity via matrix metalloprotease-9 up-regulation in mice with painful peripheral neuropathy

High-mobility group box-1 (HMGB1) has been shown to be critical in the modulation of nociceptive transduction following a peripheral neuropathy. However, the precise role of peripherally expressed HMGB1 in neuropathic pain has yet to be fully elaborated. Following a partial sciatic nerve ligation (PSNL) in mice, a persistent ipsilateral up-regulation of HMGB1 was observed from 3 to 21 days after PSNL, in paralleled with a robust ipsilateral hind paw mechanical hypersensitivity. Increased HMGB1 was detected in both infiltrating macrophages and proliferating Schwann cells in the ipsilateral nerve 14 days following PSNL. Repeated perineural treatment with anti-HMGB1 antibody significantly ameliorated PSNL-induced mechanical hypersensitivity. Several pronociceptive molecules, including matrix metalloprotease-9 (MMP-9), tumor necrosis factor-α, interleukin-1β (IL-1β), and cyclooxygenase-2, were up-regulated in injured sciatic nerve 14 days following PSNL. Repeated perineural treatment with an anti-HMGB1 antibody significantly suppressed expression of MMP-9, but not other pronociceptive molecules. Perineural treatment with a selective MMP-9 inhibitor ameliorated PSNL-induced mechanical hypersensitivity. The current findings demonstrate that the maintenance of the neuropathic state following an injured nerve is dependent on the up-regulation of HMGB1 and MMP-9. Thus, blocking HMGB1 function in sciatic nerve could be a potent therapeutic strategy for the treatment of neuropathic pain. Increased peripheral high-mobility group box-1 (HMGB1) is involved in the modulation of nociceptive transduction following a peripheral neuropathy. Following nerve injury in mice, increased HMGB1 is detected in both infiltrating macrophages and proliferating Schwann cells in the ipsilateral nerve. Repeated perineural treatment with anti-HMGB1 antibody significantly ameliorates nerve injury-induced mechanical hypersensitivity, and suppresses expression of matrix metalloprotease-9 (MMP-9). The findings demonstrate that the maintenance of the neuropathic state following an injury nerve is dependent on the up-regulation of HMGB1 and MMP-9.

Peripheral interleukin-4 ameliorates inflammatory macrophage-dependent neuropathic pain

There is increasing evidence that inflammatory (M1-polarized) macrophages drive the nonresolving neuroinflammation that causes neuropathic pain after nerve injury. As interleukin-4 (IL-4) promotes the suppressive (M2-polarized) state in macrophages, we examined whether exploiting an IL-4-mediated pathway could ameliorate M1 macrophage-dependent neuropathic pain. The mRNA and protein expression of IL-4 receptor α chain (IL-4Rα) were upregulated in accumulating F4/80 macrophages in injured sciatic nerve (SCN). In mouse macrophage cell line J774A.1, IL-4 downregulated the mRNA expression of M1 macrophage-specific molecules (IL-1β, CC chemokine ligand 3, and CD86) normally provoked by lipopolysaccharide, while increasing the mRNA expression of M2 macrophage-specific molecules (arginase-1, IL-10, and CD206) through a STAT6-mediated pathway. In ex vivo SCN culture, M1 molecules were highly expressed in the injured SCN on day 7 after partial SCN ligation (PSL) but were decreased by IL-4 treatment. In contrast, M2 molecules were upregulated by IL-4. IL-4 also increased phosphorylated STAT6 (pSTAT6) expression and shifted IL-1β M1 macrophages toward a CD206 M2 phenotype. Perineural administration of IL-4 in mice subject to PSL ameliorated development and maintenance of tactile allodynia and thermal hyperalgesia. These effects of IL-4 were based on that IL-4 treatment increased the proportions of pSTAT6 and CD206 macrophages in injured SCN on day 14 after PSL. We found that neuropathic pain can be ameliorated by IL-4 treatment, which exerts its therapeutic effect on accumulating macrophages through a STAT6-dependent pathway. A shift in macrophage phenotype from the inflammatory to the suppressive phenotype, driven by IL-4R signaling, may have benefits in the treatment of neuropathic pain.

Cross-Excitation in Peripheral Sensory Ganglia Associated with Pain Transmission

Despite the absence of synaptic contacts, cross-excitation of neurons in sensory ganglia during signal transmission is considered to be chemically mediated and appears increased in chronic pain states. In this study, we modulated neurotransmitter release in sensory neurons by direct application of type A botulinum neurotoxin (BoNT/A) to sensory ganglia in an animal model of neuropathic pain and evaluated the effect of this treatment on nocifensive. Unilateral sciatic nerve entrapment (SNE) reduced the ipsilateral hindpaw withdrawal threshold to mechanical stimulation and reduced hindpaw withdrawal latency to thermal stimulation. Direct application of BoNT/A to the ipsilateral L4 dorsal root ganglion (DRG) was localized in the cell bodies of the DRG and reversed the SNE-induced decreases in withdrawal thresholds within 2 days of BoNT/A administration. Results from this study suggest that neurotransmitter release within sensory ganglia is involved in the regulation of pain-related signal transmission.

The novel CGRP receptor antagonist BIBN4096BS alleviates a postoperative intestinal inflammation and prevents postoperative ileus

BACKGROUND

Abdominal surgery results in neuronal mediator release and subsequent acute intestinal hypomotility. This phase is followed by a longer lasting inflammatory phase resulting in postoperative ileus (POI). Calcitonin gene-related peptide (CGRP) has been shown to induce motility disturbances and in addition may be a candidate mediator to elicit neurogenic inflammation. We hypothesized that CGRP contributes to intestinal inflammation and POI.

METHODS

The effect of CGRP in POI was tested in mice treated with the highly specific CGRP receptor antagonist BIBN4096BS and in CGRP receptor-deficient (RAMP-1(-/-) ) mice. POI severity was analyzed by cytokine expression, muscular inflammation and gastrointestinal (GI) transit. Peritoneal and muscularis macrophages and mast cells were analyzed for CGRP receptor expression and functional response to CGRP stimulation.

KEY RESULTS

Intestinal manipulation (IM) resulted in CGRP release from myenteric nerves, and a concurrent increased interleukin (IL)-6 and IL-1β transcription and leukocyte infiltration in the muscularis externa and increased GI transit time. CGRP potentiates IM-induced cytokine transcription within the muscularis externa and peritoneal macrophages. BIBN4096BS reduced cytokine levels and leukocyte infiltration and normalized GI transit. RAMP1(-/-) mice showed a significantly reduced leukocyte influx. CGRP receptor was expressed in muscularis and peritoneal macrophages but not mast cells. CGRP mediated macrophage activation but failed to induce mast cell degranulation and cytokine expression.

CONCLUSIONS & INFERENCES

CGRP is immediately released during abdominal surgery and induces a neurogenic inflammation via activation of abdominal macrophages. BIBN4096BS prevented IM-induced inflammation and restored GI motility. These findings suggest that CGRP receptor antagonism could be instrumental in the prevention of POI.

Peripheral alpha4beta2 nicotinic acetylcholine receptor signalling attenuates tactile allodynia and thermal hyperalgesia after nerve injury in mice

AIM

Neuropathic pain is often refractory to conventional analgesics including opioids and non-steroidal anti-inflammatory drugs. Evidence suggests nicotinic acetylcholine receptor ligands regulate pain transmission. Effects of α4β2 nicotinic acetylcholine receptor activation on pain behaviours after nerve injury were studied.

METHODS

Mice were subjected to partial sciatic nerve ligation (PSL). Nicotinic acetylcholine receptor α4 and β2 subunits localization in injured nerves were evaluated by immunohistochemistry. Neuropathic pain, assessed by tactile allodynia and thermal hyperalgesia, was examined by von Frey test and Hargreaves test respectively.

RESULTS

Nicotinic acetylcholine receptor α4 and β2 subunits were up-regulated in injured nerves and were expressed on F4/80-positive macrophages. When nicotine was perineurally administered daily for 4 days (day 7-10; maintenance phase) after nerve injury, pain behaviours were significantly alleviated. The inhibitory effects of nicotine were reversed by co-administration of mecamylamine (non-selective nicotinic acetylcholine receptor antagonist) and dihydro-β-erythroidine (selective α4β2 nicotinic acetylcholine receptor antagonist). Likewise, when α4β2 nicotinic acetylcholine receptor agonists (TC2559 or ABT418) were administered daily for 4 days (day 7-10) after nerve injury, pain behaviours were significantly attenuated. On the other hand, nicotine administered daily for 4 days (day 0-3; initiation phase) after nerve injury alleviated pain behaviours, which were antagonized by co-administration of dihydro-β-erythroidine. TC2559 administered daily for 4 days (day 0-3) also attenuated nerve injury-induced pain behaviours.

CONCLUSION

The activation of α4β2 nicotinic acetylcholine receptor expressed on infiltrating macrophages in injured nerves may participate in the relief of PSL-induced neuropathic pain during maintenance and initiation phases.

Variability in C-type lectin receptors regulates neuropathic pain-like behavior after peripheral nerve injury

INTRODUCTION

Neuropathic pain is believed to be influenced in part by inflammatory processes. In this study we examined the effect of variability in the C-type lectin gene cluster (Aplec) on the development of neuropathic pain-like behavior after ligation of the L5 spinal nerve in the inbred DA and the congenic Aplec strains, which carries seven C-type lectin genes originating from the PVG strain.

RESULTS

While both strains displayed neuropathic pain behavior early after injury, the Aplec strain remained sensitive throughout the whole study period. Analyses of several mRNA transcripts revealed that the expression of Interleukin-1β, Substance P and Cathepsin S were more up-regulated in the dorsal part of the spinal cord of Aplec rats compared to DA, indicating a stronger inflammatory response. This notion was supported by flow cytometric analysis revealing increased infiltration of activated macrophages into the spinal cord. In addition, macrophages from the Aplec strain stimulated in vitro displayed higher expression of inflammatory cytokines compared to DA cells. Finally, we bred a recombinant congenic strain (R11R6) comprising only four of the seven Aplec genes, which displayed similar clinical and immune phenotypes as the Aplec strain.

CONCLUSION

We here for the first time demonstrate that C-type lectins, a family of innate immune receptors with largely unknown functions in the nervous system, are involved in regulation of inflammation and development of neuropathic pain behavior after nerve injury. Further experimental and clinical studies are needed to dissect the underlying mechanisms more in detail as well as any possible relevance for human conditions.

Heat hyperalgesia and mechanical hypersensitivity induced by calcitonin gene-related peptide in a mouse model of neurofibromatosis

This study examined whether mice with a deficiency of neurofibromin, a Ras GTPase activating protein, exhibit a nociceptive phenotype and probed a possible contribution by calcitonin gene-related peptide. In the absence of inflammation, Nf1^+/− mice (B6.129S6 Nf1<tm1Fcr>/J) and wild type littermates responded comparably to heat or mechanical stimuli, except for a subtle enhanced mechanical sensitivity in female Nf1^+/− mice. Nociceptive phenotype was also examined after inflammation induced by capsaicin and formalin, which release endogenous calcitonin gene-related peptide. Intraplantar injection of capsaicin evoked comparable heat hyperalgesia and mechanical hypersensitivity in Nf1^+/− and wild type mice of both genders. Formalin injection caused a similar duration of licking in male Nf1^+/− and wild type mice. Female Nf1^+/− mice licked less than wild type mice, but displayed other nociceptive behaviors. In contrast, intraplantar injection of CGRP caused greater heat hyperalgesia in Nf1^+/− mice of both genders compared to wild type mice. Male Nf1^+/− mice also exhibited greater mechanical hypersensitivity; however, female Nf1^+/− mice exhibited less mechanical hypersensitivity than their wild type littermates. Transcripts for calcitonin gene-related peptide were similar in the dorsal root ganglia of both genotypes and genders. Transcripts for receptor activity-modifying protein-1, which is rate-limiting for the calcitonin gene-related peptide receptor, in the spinal cord were comparable for both genotypes and genders. The increased responsiveness to intraplantar calcitonin gene-related peptide suggests that the peripheral actions of calcitonin gene-related peptide are enhanced as a result of the neurofibromin deficit. The analgesic efficacy of calcitonin gene-related peptide receptor antagonists may therefore merit investigation in neurofibromatosis patients.

Activation profile of dorsal root ganglia Iba-1 (+) macrophages varies with the type of lesion in rats

The interactions between neurons, immune and immune-like glial cells can initiate the abnormal processes that underlie neuropathic pain. In the peripheral nervous system the resident macrophages may play an important role. In this study we investigated in experimental adult Sprague-Dawley rats how Iba-1 (ionized calcium binding adaptor molecule 1) (+) resident macrophages in the dorsal root ganglion (DRG) are activated after a spinal nerve ligation (SNL) or streptozotocin (STZ)-induced diabetes. The activation profile was defined by comparing the responses of resident macrophages against microglia in the spinal cord as they share a common origin. After SNL, the Iba-1 (+) macrophages in L5 DRG reached their activation peak 5 days later, clustered as satellite cells around large A-neurons, expressed the MHC-II marker, but did not show p-p38 and p-ERK1/2 activation and did not secrete IL-18. After STZ-induced diabetes, the Iba-1 (+) macrophages reached their activation peak 1 week later in L4 and L5 DRG, remained scattered between neurons, expressed the MHC-II marker only in L5 DRG, did not show p-p38 and p-ERK1/2 activation and did not secrete any of the investigated cytokines/chemokines. These responses suggest that depending on the type of lesion DRG Iba-1 (+) resident macrophages have different activation mechanisms, which are dissimilar to those in microglia.

The role of different methods of nerve ablation in prevention of neuroma

BACKGROUND

The aim of this study was to compare the incidence of neuroma formation and neuropathic pain following different techniques of nerve ablation in a rat sural nerve model.

METHODS

Rat sural nerve was subjected to four different techniques of ablation with standardized creation of a 1-cm gap (n = 15 in each group). These included nerve avulsion, transection and burying in muscle, transection and folding of nerve, and transection alone. Animals were killed after 3 months. Explanted nerves were sectioned and stained with Masson trichrome and S-100 stain against neural tissue. The maximal neural cross-sectional area and neural-to-connective tissue ratio was quantified. Quantitative reverse-transcriptase polymerase chain reaction (n = 5) was used to analyze relative mRNA expression of ciliary neurotrophic factor and calcitonin gene-related peptide.

RESULTS

Neural cross-sectional area was statistically increased (p < 0.05) compared with controls in folded, muscle buried, and transected specimens but decreased in avulsed specimens. The neural-to-connective tissue ratio was statistically decreased in the avulsed group. Relative mRNA expression of ciliary neurotrophic factor was lowest in muscle buried (4 percent of control) (p < 0.05) and avulsed specimens (15 percent of control) (p < 0.05) and higher in folded (52 percent of control) and transected specimens (75 percent of control). Relative mRNA expression of calcitonin gene-related peptide was highest in folded specimens (302 percent of control) (p < 0.05).

CONCLUSIONS

Folding and transection lead to increased histologic evidence of neuroma formation, whereas folding leads to neuropathic pain, assayed by calcitonin gene-related peptide expression. Avulsion and muscle burying are preferable techniques for nerve ablation and inhibit nerve regeneration, evidenced by decreased ciliary neurotrophic factor expression. Avulsion offers an alternative to muscle burying when there is no muscle in the vicinity to bury the transected nerve.

Activation of nicotinic acetylcholine receptors on bone marrow-derived cells relieves neuropathic pain accompanied by peripheral neuroinflammation

Emerging evidence indicates that chronic neuroinflammation plays a pivotal role in neuropathic pain. We explored whether activation of the nicotinic acetylcholine receptor (nAChRs) pathway on peripheral immune cells improves neuropathic pain. Mice were subjected to partial sciatic nerve ligation (PSL). Enhanced green fluorescent protein (EGFP)-chimeric mice were generated by transplantation of EGFP(+) bone marrow (BM) cells from EGFP-transgenic mice into wild-type mice. EGFP(+) BM-derived cells infiltrated the injured sciatic nerve (SCN) of EGFP-chimeric mice, and these cells were found to be F4/80(+) macrophages and Ly6G(+) neutrophils. The protein expression of nAChR subunit α4 and α7 were up-regulated in the injured SCN. Increased α4 and α7 subunits were localized on both BM-derived macrophages and neutrophils. When nicotine (20nmol) was perineurally administered once a day for 4days (days 0-3), PSL-induced tactile allodynia and thermal hyperalgesia were significantly prevented. Relieving effects of nicotine on neuropathic pain were reversed by co-administration of mecamylamine (20nmol), a non-selective antagonist for nAChRs. PSL-induced up-regulation of inflammatory cytokines and chemokines was suppressed by perineural administration of nicotine. Taken together, the expression of α4β2 and α7 subtypes of nAChRs may be increased on circulating macrophages and neutrophils in injured peripheral nerves. Activation of nAChRs on immune cells may relieve neuropathic pain accompanied by the suppression of neuroinflammation.

Heterogeneity of macrophages in injured trigeminal nerves: cytokine/chemokine expressing vs. phagocytic macrophages

BACKGROUND

Macrophages are important immune effector cells in both innate and adaptive immune responses. Injury to peripheral nerves triggers activation of resident macrophages and infiltration of haematogenous macrophages, which they play critical roles in Wallerian degeneration and neuropathic pain. As macrophages are able to change their phenotypes in response to environment cues, we attempt to identify distinct phenotypes of macrophages in injured nerves and to understand the potential contribution of each macrophage subpopulation to the genesis of neuropathic pain associated with nerve injury.

METHODS

Rat mental nerves (terminal branches of trigeminal nerve) were loosely ligated. Sensitivity to mechanical stimuli at the lower lip area was monitored using calibrated von Frey Hairs. We examined the expression pattern of Iba-1, MAC1 and ED1 which allow us to reveal the immunophenotypes of macrophages at different time points post-injury. Functional status of each macrophage subpopulation was further investigated by colocalization with cytokines/chemokines, myelin basic protein and MHC II antigen, which reflect respectively secretory, phagocytic and antigen presentation properties of activated macrophages.

RESULTS

Following nerve injury, a burst of Iba-1(+) macrophages was found in injured mental nerves. Among them, we detected two major immunophenotypes: MAC1(+) cytokines/chemokines secreting macrophages and ED1(+) phagocytic macrophages. Small, round shaped MAC1(+) macrophages were distributed essentially around the lesion site and existed only at early time points. Large, irregular and foamy ED1(+) macrophages were found among damaged nerve fibers and they persisted for at least 3 months post-injury. Although ED1(+) macrophages did not secrete inflammatory mediators, they were able to express neurotransmitter CGRP and MHC II at later time points. In parallel, we observed that mechanical allodynia developed after the nerve ligation was at its lowest level within 1 month. Although slightly increased afterwards, the head escape threshold maintained significantly lower than before injury until 3 months. We suggest that MAC1(+) macrophages contribute to the initiation of neuropathic pain by releasing cytokines/chemokines, and ED1(+) macrophages may contribute in maintaining the hypersensitivity under other mechanisms.

CONCLUSION

Our results highlighted the heterogeneity and the plasticity of macrophages in response to the injury and provided further information on their potential involvement in neuropathic pain. Exploring the full spectrum of macrophage phenotypes in injured nerve is necessary. Individual macrophage population may be selectively targeted by cell-specific intervention for an effective treatment of neuropathic pain.

Differential effects of calcitonin gene-related peptide receptor blockade by olcegepant on mechanical allodynia induced by ligation of the infraorbital nerve vs the sciatic nerve in the rat

Previous studies showed that 5-hydroxytryptamine (5-HT)(1B/1D) receptor stimulation by triptans alleviates neuropathic pain caused by chronic constriction injury to the infraorbital nerve (CCI-ION) but not the sciatic nerve (CCI-SN) in rats. To assess whether such differential effects in the cephalic vs extracephalic territories is a property shared by other antimigraine drugs, we used the same models to investigate the effects of olcegepant, which has an antimigraine action mediated through calcitonin gene-related peptide (CGRP) receptor blockade. Adult male rats underwent unilateral CCI to the ION or the SN, and subsequent allodynia and/or hyperalgesia were assessed in ipsilateral vibrissal territory or hindpaw, respectively, using von Frey filaments and validated nociceptive tests. c-Fos expression was quantified by immunohistochemistry and interleukin 6 and activating transcription factor 3 (ATF3) mRNAs by real-time quantitative reverse transcriptase-polymerase chain reaction. Like naratriptan (0.1 to 0.3mg/kg, subcutaneously), olcegepant (0.3 to 0.9mg/kg, intravenously) markedly reduced mechanical allodynia in CCI-ION rats. In contrast, in CCI-SN rats, mechanical allodynia was completely unaffected and hyperalgesia was only marginally reduced by these drugs. A supra-additive antiallodynic effect was observed in CCI-ION rats treated with olcegepant (0.3mg/kg intravenously) plus naratriptan (0.1mg/kg subcutaneously), whereas this drug combination remained inactive in CCI-SN rats. Olcegepant (0.6mg/kg, intravenously) significantly reduced the number of c-Fos immunolabeled cells in spinal nucleus of the trigeminal nerve and upregulation of ATF3 transcript (a marker of neuron injury) but not that of interleukin-6 in trigeminal ganglion of CCI-ION rats. These findings suggest that CGRP receptor blockade might be of potential interest to alleviate trigeminal neuropathic pain.

CC-chemokine ligand 4/macrophage inflammatory protein-1β participates in the induction of neuropathic pain after peripheral nerve injury

BACKGROUND

Neuropathic pain is caused by neural damage or dysfunction and neuropathic pain-related symptoms are resistant to conventional analgesics. Neuroinflammation due to the cytokine-chemokine network may play a pivotal role in neuropathic pain. We demonstrate that macrophage inflammatory protein-1β (MIP-1β) participates in neuropathic pain.

METHODS

Mice received partial sciatic nerve ligation (PSL), and tactile allodynia and thermal hyperalgesia were assessed by von Frey test and Hargreaves test, respectively. Agents were administered into the region surrounding the sciatic nerve (SCN).

RESULTS

Using reverse transcription polymerase chain reaction, the mRNA expressions of MIP-1β and its receptor (CC-chemokine receptor 5; CCR5) in the injured SCN were up-regulated after PSL. MIP-1β immunoreactivity was localized in macrophages and Schwann cells and increased in the injured SCN on day 1. PSL-induced tactile allodynia on days 4 to 7 was prevented by the administration of MIP-1β neutralizing antibody (anti-MIP-1β; on days 0, 3 and 6). PSL-induced up-regulations of inflammatory cytokine-chemokine mRNAs in the injured SCN were suppressed with anti-MIP-1β treatment on day 7. Administration of CCR5 antagonist, D-ala-peptide T-amide (on days 0, 3 and 6) prevented tactile allodynia and thermal hyperalgesia on days 4 to 14. Single administration of recombinant mouse MIP-1β (rmMIP-1β) elicited tactile allodynia. Moreover, rmMIP-1β increased the mRNA expression of inflammatory mediators in the SCN on day 1 after administration.

CONCLUSIONS

These results suggest that MIP-1β is a novel key mediator, and the peripheral MIP-1β-CCR5 axis contributes to neuropathic pain. Therefore, investigation of this cascade might be a validated approach for the elucidation of neuropathic pain mechanisms.

Prostaglandin E2 contributes to the synthesis of brain-derived neurotrophic factor in primary sensory neuron in ganglion explant cultures and in a neuropathic pain model

Brain-derived neurotrophic factor (BDNF) exists in small to medium size neurons in adult rat dorsal root ganglion (DRG) and serves as a modulator at the first synapse of the pain transmission pathway in the spinal dorsal horn. Peripheral nerve injury increases BDNF expression in DRG neurons, an event involved in the genesis of neuropathic pain. In the present study, we tested the hypothesis that prostaglandin E2 (PGE2) over-produced in injured nerves contributes to the up-regulation of BDNF in DRG neurons. Two weeks after partial sciatic nerve ligation (PSNL), BDNF levels in the ipsilateral L4-L6 DRG of injured rats were significantly increased compared to the contralateral side. Perineural injection of a selective cyclooxygenase (COX2) inhibitor or a PGE2 EP4 receptor antagonist not only dose-dependently relieved PSNL elicited mechanical hypersensitivity, but also suppressed the increased BDNF levels in DRG neurons. PSNL shifted BDNF expression in the ipsilateral DRG from small to medium and larger size injured neurons. BDNF is mainly co-expressed with the EP1 and EP4 while moderately with the EP2 and EP3 receptor subtypes in naïve and PSNL rats. PSNL also shifted the expression of EP1-4 receptors to a larger size population of DRG neurons. In DRG explant cultures, a stabilized PGE2 analog 16,16 dimethyl PGE2 (dmPGE2) or the agonists of EP1 and EP4 receptors significantly increased BDNF levels and the phosphorylated protein kinase A (PKA), extracellular signal-regulated kinase (ERK)/mitogen activated protein kinase (MAPK) and cAMP response element binding protein (CREB). The EP1 and EP4 antagonists, a sequester of nerve growth factor (NGF), the inhibitors of PKA and MEK as well as CREB small interfering RNA suppressed dmPGE2-induced BDNF. Taken together, EP1 and EP4 receptor subtypes, PKA, ERK/MAPK and CREB signaling pathways as well as NGF are involved in PGE2-induced BDNF synthesis in DRG neurons. Injured nerve derived-PGE2 contributes to BDNF up-regulation in DRG neurons following nerve injury. Facilitating the synthesis of BDNF in primary sensory neurons is a novel mechanism underlying the role of PGE2 in the genesis of neuropathic pain.

Epigenetic augmentation of the macrophage inflammatory protein 2/C-X-C chemokine receptor type 2 axis through histone H3 acetylation in injured peripheral nerves elicits neuropathic pain

Although there is growing evidence showing that the involvement of chemokines in the pathogenesis of neuropathic pain is associated with neuroinflammation, the details are unclear. We investigated the C-X-C chemokine ligand type 2 [macrophage inflammatory protein 2 (MIP-2)]/C-X-C chemokine receptor type 2 (CXCR2) axis and epigenetic regulation of these molecules in neuropathic pain after peripheral nerve injury. Expression of MIP-2 and CXCR2 were up-regulated and localized on accumulated neutrophils and macrophages in the injured sciatic nerve (SCN) after partial sciatic nerve ligation (PSL). Perineural injection of MIP-2-neutralizing antibody (anti-MIP-2) or the CXCR2 antagonist N-(2-bromophenyl)-N'-(2-hydroxy-4-nitrophenyl)urea (SB225002) prevented PSL-induced tactile allodynia and thermal hyperalgesia. Perineural injection of recombinant MIP-2 elicited neuropathic pain-like behaviors. Anti-MIP-2 suppressed neutrophil accumulation in the SCN after PSL. Neutrophil depletion by intraperitoneal injection of Ly6G antibody attenuated PSL-induced neuropathic pain. Both anti-MIP-2 and SB225002 suppressed up-regulation of inflammatory cytokines and chemokines in the injured SCN. In addition, acetylation of histone H3 [lysine (Lys9)-acetylated histone H3 (AcK9-H3)] on the promoter region of MIP-2 and CXCR2 was increased in the injured SCN after PSL. Expression of AcK9-H3 was observed in the nuclei of neutrophils and macrophages surrounding the epineurium. Administration of the histone acetyltransferase inhibitor anacardic acid suppressed the up-regulation of MIP-2 and CXCR2 in the SCN after PSL and resulted in the prevention of PSL-induced neuropathic pain. Taken together, these results show that augmentation of the MIP-2/CXCR2 axis by hyperacetylation of histone H3 on the promoter region of MIP-2 and CXCR2 located in the injured peripheral nerve elicits chronic neuroinflammation through neutrophil accumulation, leading to neuropathic pain.

Determination of specific neuropeptides modulation time course in a rat model of osteoarthritis pain by liquid chromatography ion trap mass spectrometry

Animal models are useful to evaluate pharmacological therapies to alleviate joint pain. The present study characterized central neuropeptides modulation in the monoiodoacetate (MIA) rat model. Animals receiving a single 3mg MIA injection were euthanized at 3, 7, 14, 21 and 28 days post injection. Spinal cords were analyzed by liquid chromatography ion trap mass spectrometry. Up-regulations of the calcitonin gene-related peptide and substance P were observed starting on days 7 and 28 respectively, whereas big dynorphin(₁₋₃₂) content decreased significantly on day 14 in comparison to control animals (P<0.05). Preclinical drug evaluations using this model should be conducted between 7 and 21 days post injection when the lesions resemble most to human osteoarthritis.

Differential regulation of the expression of neurotrophin receptors in rat extraocular motoneurons after lesion

Neurotrophins acting through high-affinity tyrosine kinase receptors (trkA, trkB, and trkC) play a crucial role in regulating survival and maintenance of specific neuronal functions after injury. Adult motoneurons supplying extraocular muscles survive after disconnection from the target, but suffer dramatic changes in morphological and physiological properties, due in part to the loss of their trophic support from the muscle. To investigate the dependence of the adult rat extraocular motoneurons on neurotrophins, we examined trkA, trkB, and trkC mRNA expression after axotomy by in situ hybridization. trkA mRNA expression was detectable at low levels in unlesioned motoneurons, and its expression was downregulated 1 and 3 days after injury. Expression of trkB and trkC mRNAs was stronger, and after axotomy a simultaneous, but inverse regulation of both receptors was observed. Thus, whereas a considerable increase in trkB expression was seen about 2 weeks after axotomy, the expression of trkC mRNA had decreased at the same post-lesion period. Injured extraocular motoneurons also experienced an initial induction in expression of calcitonin gene-related peptide and a transient downregulation of cholinergic characteristics, indicating a switch in the phenotype from a transmitter-specific to a regenerative state. These results suggest that specific neurotrophins may contribute differentially to the survival and regenerative responses of extraocular motoneurons after lesion.

Role of prostaglandin E2 in the synthesis of the pro-inflammatory cytokine interleukin-6 in primary sensory neurons: an in vivo and in vitro study

Following various types of nerve injury, cyclooxygenase 2 and prostaglandin E2 (PGE2) are universally and chronically up-regulated in injured nerves and contribute to the genesis of neuropathic pain. Persistent high levels of PGE2 likely exert chronic effects on nociceptive dorsal root ganglion (DRG) neurons. In the present study, we tested the hypothesis that injured nerve-derived PGE2 contributes to the up-regulation of the pro-inflammatory cytokine interleukin-6 (IL-6) in DRG neurons following partial sciatic nerve ligation. In naive adult rats, IL-6 was expressed in only a few small size DRG neurons which all co-expressed EP4 receptors. Partial sciatic nerve ligation increased and shifted IL-6 expression from small to medium and large size damaged DRG neurons. Perineural injection of a selective cyclooxygenase 2 inhibitor or a selective EP4 receptor antagonist significantly suppressed the up-regulation of IL-6 in DRG, suggesting that injured nerve derived PGE2 contributes to the de novo synthesis of IL-6 in DRG neurons through EP4 receptors. In cultured sensory ganglion explants, a stabilized PGE2 analog increased IL-6 mRNA and protein levels through the activation of EP4, protein kinase A, protein kinase C, extracellular regulated protein kinase/MAPK, cAMP response element binding protein and NFκB signalling pathways. Taken together, these data indicate that facilitating the de novo synthesis of pain-related cytokines in injured medium and large size DRG neurons is a novel mechanism underlying the role of injured nerve derived PGE2 in the genesis of neuropathic pain.

Transcriptional repression of the M channel subunit Kv7.2 in chronic nerve injury

Neuropathic pain is a severe health problem for which there is a lack of effective therapy. A frequent underlying condition of neuropathic pain is a sustained overexcitability of pain-sensing (nociceptive) sensory fibres. Therefore, the identification of mechanisms for such abnormal neuronal excitability is of utmost importance for understanding neuropathic pain. Despite much effort, an inclusive model explaining peripheral overexcitability is missing. We investigated transcriptional regulation of the Kcnq2 gene, which encodes the Kv7.2 subunit of membrane potential-stabilizing M channel, in peripheral sensory neurons in a model of neuropathic pain—partial sciatic nerve ligation (PSNL). We show that Kcnq2 is the major Kcnq gene transcript in dorsal root ganglion (DRG); immunostaining and patch-clamp recordings from acute ganglionic slices verified functional expression of Kv7.2 in small-diameter nociceptive DRG neurons. Neuropathic injury induced substantial downregulation of Kv7.2 expression. Levels of repressor element 1–silencing transcription factor (REST), which is known to suppress Kcnq2 expression, were upregulated in response to neuropathic injury identifying the likely mechanism of Kcnq2 regulation. Behavioural experiments demonstrated that neuropathic hyperalgesia following PSNL developed faster than the downregulation of Kcnq2 expression could be detected, suggesting that this transcriptional mechanism may contribute to the maintenance rather than the initiation of neuropathic pain. Importantly, the decrease in the peripheral M channel abundance could be functionally compensated by peripherally applied M channel opener flupirtine, which alleviated neuropathic hyperalgesia. Our work suggests a novel mechanism for neuropathic overexcitability and brings focus on M channels and REST as peripheral targets for the treatment of neuropathic pain.

Neuropathic injury induces transcriptional downregulation of the Kcnq2 potassium channel gene by the transcriptional suppressor repressor element 1–silencing transcription factor; this mechanism contributes to peripheral sensitization of the afferent fibres.

Macrophage inflammatory protein-1alpha mediates the development of neuropathic pain following peripheral nerve injury through interleukin-1beta up-regulation

In the present study, we investigated the role of the macrophage inflammatory protein-1alpha (MIP-1alpha) in the pathogenesis of neuropathic pain following partial sciatic nerve ligation (PSL) in mice. MIP-1alpha mRNA and its protein were dramatically up-regulated after PSL, and MIP-1alpha was localized on macrophages and Schwann cells in the injured sciatic nerve (SCN). PSL-induced long-lasting tactile allodynia and thermal hyperalgesia were prevented by the perineural injection of anti-MIP-1alpha (2ng). Intraneural (20ng) and perineural (100ng) injection of recombinant MIP-1alpha elicited tactile allodynia and thermal hyperalgesia in sham-operated limb. MIP-1alpha receptors (CCR1 and CCR5) mRNA and their proteins were also up-regulated in the SCN after PSL, and were localized on macrophages and Schwann cells. PSL-induced tactile allodynia was attenuated by perineural injection (0.2nmol) of siRNA against CCR1 and CCR5. On the other hand, PSL-induced thermal hyperalgesia was prevented by siRNA against CCR5, but not CCR1. Interleukin-1beta (IL-1beta) mRNA and its precursor protein in macrophages and Schwann cells were also up-regulated in the SCN after PSL, and PSL-induced neuropathic pain was prevented by the perineural injection of anti-IL-1beta (2ng). PSL-induced IL-1beta up-regulation was suppressed by anti-MIP-1alpha and siRNA against CCR1 and CCR5. Perineural injection of nicotine (20nmol), a macrophage suppressor, prevented PSL-induced neuropathic pain and suppressed MIP-1alpha and IL-1beta expressions. In conclusion, we propose a novel critical molecule MIP-1alpha derived from macrophages and Schwann cells that appears to play a crucial role in the development of neuropathic pain induced by PSL.

Lipopolysaccharide induces calcitonin gene-related peptide in the RAW264.7 macrophage cell line

SUMMARY

Calcitonin gene-related peptide (CGRP) is widely distributed and plays important roles in a wide array of biological functions. It is enriched in primary sensory neurons and hence involved in nociception and neurogenic inflammation. Recent studies have shown that CGRP can be produced by immune cells such as monocytes/macrophages following inflammatory stimulation, suggesting a role in innate immunity. However, it is unclear how CGRP is up-regulated in macrophages and if it plays a role in macrophage functions such as the production of cytokines and chemokines. Using enzyme-linked immunosorbent assay (ELISA) and multiplex ELISA, lipopolysaccharide (LPS) was found to induce CGRP in the RAW 264.7 macrophage cell line. LPS-induced inflammatory mediators such as nerve growth factor (NGF), interleukin-1beta (IL-1beta), IL-6, prostaglandin E(2) (PGE(2)) and nuclear factor-kappaB (NF-kappaB) signalling are involved in inducing CGRP, whereas the NGF receptor trkA and CGRP receptor signalling pathways are unexpectedly involved in suppressing LPS-induced CGRP, which leads to the fine-tune regulation of CGRP release. Exogenous CGRP and CGRP receptor antagonists, in a concentration-dependent manner, stimulated, inhibited or had no effect on basal or LPS-induced release of monocyte chemoattractant protein-1, IL-1beta, IL-6, tumour necrosis factor-alpha and IL-10 in RAW macrophages. The ligand-concentration-dependent regulation of the production of inflammatory mediators by CGRP receptor signalling is a novel mechanism underlying the stimulating and suppressing role of CGRP in immune and inflammatory responses. Together, our data suggest that monocytes/macrophages are an important source of CGRP. Inflammation-induced CGRP has a positive or negative reciprocal effect on the production of other pro- and anti-inflammatory mediators. Thereby CGRP plays both facilitating and suppressing roles in immune and inflammatory responses.