A possible role for nerve growth factor in the augmentation of sodium channels in models of chronic pain

Cancer as a Channelopathy—Appreciation of Complimentary Pathways Provides a Different Perspective for Developing Treatments

Simple Summary

While improvements in technology have improved our ability to treat many forms of cancer when diagnosed at an early stage of the disease, the ability to improve survival and quality of life for patients with late stage disease has been limited, largely due to the ability of cancer cells to evade destruction when treatments block preferred paths for survival. Here, we review the role that ions and ion channels play in normal cell function, the development of disease and their role in the life and death of a cell. It is hoped that viewing cancer from the perspective of altered ion channel expression and ion balance may provide a novel approach for developing more effective treatments for this devastating disease.

Abstract

Life depends upon the ability of cells to evaluate and adapt to a constantly changing environment and to maintain internal stability to allow essential biochemical reactions to occur. Ions and ion channels play a crucial role in this process and are essential for survival. Alterations in the expression of the transmembrane proteins responsible for maintaining ion balance that occur as a result of mutations in the genetic code or in response to iatrogenically induced changes in the extracellular environment is a characteristic feature of oncogenesis and identifies cancer as one of a constellation of diseases known as channelopathies. The classification of cancer as a channelopathy provides a different perspective for viewing the disease. Potentially, it may expand opportunities for developing novel ways to affect or reverse the deleterious changes that underlie establishing and sustaining disease and developing tolerance to therapeutic attempts at treatment. The role of ions and ion channels and their interactions in the cell’s ability to maintain ionic balance, homeostasis, and survival are reviewed and possible approaches that mitigate gain or loss of ion channel function to contribute to new or enhance existing cancer therapies are discussed.

Spinal Cord Stimulation to Treat Unresponsive Cancer Pain: A Possible Solution in Palliative Oncological Therapy

(1) Background: Treatment of cancer-related pain is still challenging, and it can be managed by both medical and interventional therapies. Spinal Cord Stimulation (SCS) is a minimally invasive technique, and its use is rapidly increasing in the treatment of chronic pain. (2) Materials and Methods: Our study aims to perform a review of the pertinent literature about current evidences in cancer pain treatment by Spinal Cord Stimulation. Moreover, we created a database based on case reports or case series (18 studies) in the literature. We analyzed a clinical group of oncological patients affected by intractable pain undergoing SCS implantation, focusing on outcome. (3) Results: The analysis of the 18 included studies in our series has shown a reduction in painful symptoms in 48 out of 56 treated patients (87.51%); also 53 out of 56 patients (96.64%) have shown an improvement in their Quality of Life (QoL). (4) Conclusions: Spinal Cord Stimulation can be considered an efficient method in the treatment of cancer-related pain. However, literature regarding SCS for the treatment of cancer-related pain is largely represented by case reports and small case series, with no effective population studies or Randomized Controlled Trials demonstrating the efficacy and the level of evidence. Further prospective studies are needed.

Are there Clinical Ways to Assess Inflammation in Dry Eye Disease?

In the diagnostic process of dry eye disease, the detection of inflammatory activity is critical in order to evaluate the risk of progression and immunologic shift of the disease, to predict patient response to treatment, and to design an efficient therapeutic strategy, including artificial tear replacement, punctal occlusion or anti-inflammatory therapy.Even if it is difficult to quantify, some indicators of the presence of inflammation are collectible during the examination of the ocular surface in a first-line clinical setting. This review presents and critically discusses the assessment of inflammation in dry eye disease in clinical practice.

Overexpressed Na V 1.7 Channels Confer Hyperexcitability to in vitro Trigeminal Sensory Neurons of Ca V 2.1 Mutant Hemiplegic Migraine Mice

Trigeminal sensory neurons of transgenic knock-in (KI) mice expressing the R192Q missense mutation in the α1A subunit of neuronal voltage-gated Ca_V2.1 Ca²⁺ channels, which leads to familial hemiplegic migraine type 1 (FHM1) in patients, exhibit a hyperexcitability phenotype. Here, we show that the expression of Na_V1.7 channels, linked to pain states, is upregulated in KI primary cultures of trigeminal ganglia (TG), as shown by increased expression of its α1 subunit. In the majority of TG neurons, Na_V1.7 channels are co-expressed with ATP-gated P2X3 receptors (P2X3R), which are important nociceptive sensors. Reversing the trigeminal phenotype with selective Ca_V2.1 channel inhibitor ω-agatoxin IVA inhibited Na_V1.7 overexpression. Functionally, KI neurons revealed a TTX-sensitive inward current of larger amplitude that was partially inhibited by selective Na_V1.7 blocker Tp1a. Under current-clamp condition, Tp1a raised the spike threshold of both wild-type (WT) and KI neurons with decreased firing rate in KI cells. Na_V1.7 activator OD1 accelerated firing in WT and KI neurons, a phenomenon blocked by Tp1a. Enhanced expression and function of Na_V1.7 channels in KI TG neurons resulted in higher excitability and facilitated nociceptive signaling. Co-expression of Na_V1.7 channels and P2X3Rs in TGs may explain how hypersensitivity to local stimuli can be relevant to migraine.

Carvedilol Exerts Neuroprotective Effect on Rat Model of Diabetic Neuropathy

Diabetic neuropathy (DN) commonly occurs in diabetics, affecting approximately 50% of both type 1 and 2 diabetic patients. It is a leading cause of non-traumatic amputations. Oxidative stress could play a key role in the pathophysiology of DN. This study aimed to investigate the potential neuroprotective effect of carvedilol on STZ-induced DN in rats. Thirty male Sprague Dawley rats (weighing 200–250 g) were randomly divided into five groups (six/group), where group 1 (negative control) received only the vehicle (0.5% of carboxymethyl cellulose orally 1 ml/kg). DN was induced by a single injection of remaining rats with streptozotocin (STZ; 50 mg/kg, i.p.). After diabetes induction, group 2 served as the diabetic untreated animals; while groups 3 and 4 were treated with carvedilol (1 and 10 mg/kg/d, orally, respectively). Group 5 received a-lipoic acid as a reference neuroprotective (100 mg/kg/d, orally). All treatments were continued for 45 days after diabetes induction, followed by behavioural tests. After sacrificing the animals, dorsal root ganglia, and sciatic nerves were collected for histopathological examination and biochemical assessments. Briefly, STZ administration caused cold allodynia, induced oxidative stress, and increased nerve growth factor (NGF) concentration. Nevertheless, carvedilol improved the behavioural tests, ameliorated the oxidative imbalance as manifested by reducing malondialdehyde, restoring glutathione content, and superoxide dismutase activity. Carvedilol also decreased NGF concentration in DRG homogenate. In conclusion, this study demonstrates the neuroprotective effect of carvedilol in an experimentally induced DN rat model through–at least partly–its antioxidant effect and reduced NGF concentration in DRG.

Increased peptidergic fibers as a potential cutaneous marker of pain in diabetic small fiber neuropathy

ABSTRACT

Diabetic polyneuropathy (DPN) is a common complication of diabetes and is often associated with neuropathic pain. The mechanisms underlying development and maintenance of painful DPN are largely unknown, and quantification of intraepidermal nerve fiber density from skin biopsy, one of the neuropathological gold standard when diagnosing DPN, does not differentiate between patients with and without pain. Identification of possible pain pathophysiological biomarkers in patients with painful DPN may increase our knowledge of mechanisms behind neuropathic pain. Animal models of painful DPN have been shown to have an increased density of peptidergic nerve fibers (substance P and calcitonin gene-related peptide). In this study, we performed a detailed skin biopsy analysis in a well-characterized group of DPN patients with primarily small fiber involvement, with and without pain, and in healthy controls and test for correlation between skin biopsy findings and pain intensity and quantitative sensory testing. We found that although there was no difference in intraepidermal nerve fiber density using protein gene product 9.5 between patients with and without pain, patients with pain had increased density of dermal peptidergic fibers containing substance P and calcitonin gene-related peptide compared with patients with painless DPN and healthy controls. Peptidergic nerve fiber density correlated with pain ratings in patients with pain (R = 0.33; P = 0.019), but not with quantitative sensory testing results. In this article, we show, for the first time in humans, an increased density of dermal peptidergic fibers in painful DPN. These findings provide new insight in the pathophysiological mechanisms of pain in diabetes and open the research towards new therapeutic targets.

L5 Spinal Nerve Axotomy Induces Distinct Electrophysiological Changes in Axotomized L5- and Adjacent L4-Dorsal Root Ganglion Neurons in Rats In Vivo

Peripheral neuropathic pain (PNP) is a major health problem for which effective drug treatment is lacking. Its underlying neuronal mechanisms are still illusive, but pre-clinical studies using animal models of PNP including the L5-spinal nerve axotomy (L5-SNA) model, suggest that it is partly caused by excitability changes in dorsal root ganglion (DRG) neurons. L5-SNA results in two DRG neuronal groups: (1) axotomized/damaged neurons in L5- plus some in L4-DRGs, and (2) ipsilateral L4-neurons with intact/uninjured fibers intermingling with degenerating L5-fibers. The axotomized neurons are deprived of peripherally derived trophic factors and degenerate causing neuroinflammation, whereas the uninjured L4-neuorns are subject to increased trophic factors and neuroinflammation associated with Wallerian degeneration of axotomized L5-nerve fibers. Whether these two groups of DRG neurons exhibit similar or distinct electrophysiological changes after L5-SNA remains unresolved. Conflicting evidence for this may result from some studies assuming that all L4-fibers are undamaged. Here, we recorded somatic action potentials (APs) intracellularly from C- and A-fiber L4/L5 DRG neurons in vivo, to examine our hypothesis that L5-SNA would induce distinct electrophysiological changes in the two populations of DRG neurons. Consistent with this hypothesis, we found (7 days post-SNA), in SNA rats with established pain hypersensitivity, slower AP kinetics in axotomized L5-neurons and faster AP kinetics in L4-nociceptive neurons including decreased rise time in Aδ-and Aβ-fiber nociceptors, and after-hyperpolarization duration in Aβ-fiber nociceptors. We also found several changes in axotomized L5-neurons but not in L4-nociceptive neurons, and some changes in L4-nociceptive but not L5-neurons. The faster AP kinetics (decreased refractory period) in L4-nociceptive neurons that are consistent with their reported hyperexcitability may lead to repetitive firing and thus provide enhanced afferent input necessary for initiating and/or maintaining PNP development. The changes in axotomized L5-neurons may contribute to the central mechanisms of PNP via enhanced neurotransmitter release in the central nervous system (CNS).

Herpes Simplex Virus 2 Counteracts Neurite Outgrowth Repulsion during Infection in a Nerve Growth Factor-Dependent Manner

Herpes simplex virus 2 (HSV-2) is a prevalent human pathogen that establishes lifelong latency in neurons of the peripheral nervous system. Colonization of neurons is required for HSV-2 persistence and pathogenesis. The viral and cellular factors required for efficient infection of neurons are not fully understood. We show here that nonneuronal cells repel neurite outgrowth of sensory neurons, while HSV-2 infection overcomes this inhibition and, rather, stimulates neurite outgrowth. HSV-2 glycoprotein G and nerve growth factor contribute to this phenotype, which may attract neurites to sites of infection and facilitate virus spread to neurons. Understanding the mechanisms that modulate neurite outgrowth and facilitate HSV-2 infection of neurons might foster the development of therapeutics to reduce HSV-2 colonization of the nervous system and provide insights on neurite outgrowth and regeneration.

During primary infection, herpes simplex virus 2 (HSV-2) replicates in epithelial cells and enters neurites to infect neurons of the peripheral nervous system. Growth factors and attractive and repulsive directional cues influence neurite outgrowth and neuronal survival. We hypothesized that HSV-2 modulates the activity of such cues to increase neurite outgrowth. To test this hypothesis, we exposed sensory neurons to nerve growth factor (NGF) and mock- or HSV-2-infected HEK-293T cells, since they express repellents of neurite outgrowth. We show that HEK-293T cells secrete factors that inhibit neurite outgrowth, while infection with HSV-2 strains MS and 333 reduces this repelling phenotype, increasing neurite numbers. The HSV-2-mediated restoration of neurite outgrowth required the activity of NGF. In the absence of infection, however, NGF did not overcome the repulsion mediated by HEK-293T cells. We previously showed that recombinant, soluble glycoprotein G of HSV-2 (rSgG2) binds and enhances NGF activity, increasing neurite outgrowth. However, the effect of gG2 during infection has not been investigated. Therefore, we addressed whether gG2 contributes to overcoming neurite outgrowth repulsion. To do so, we generated viruses lacking gG2 expression and complemented them by exogenous expression of gG2. Overall, our results suggest that HSV-2 infection of nonneuronal cells reduces their repelling effect on neurite outgrowth in an NGF-dependent manner. gG2 contributed to this phenotype, but it was not the only factor. The enhanced neurite outgrowth may facilitate HSV-2 spread from epithelial cells into neurons expressing NGF receptors and increase HSV-2-mediated pathogenesis.

IMPORTANCE Herpes simplex virus 2 (HSV-2) is a prevalent human pathogen that establishes lifelong latency in neurons of the peripheral nervous system. Colonization of neurons is required for HSV-2 persistence and pathogenesis. The viral and cellular factors required for efficient infection of neurons are not fully understood. We show here that nonneuronal cells repel neurite outgrowth of sensory neurons, while HSV-2 infection overcomes this inhibition and, rather, stimulates neurite outgrowth. HSV-2 glycoprotein G and nerve growth factor contribute to this phenotype, which may attract neurites to sites of infection and facilitate virus spread to neurons. Understanding the mechanisms that modulate neurite outgrowth and facilitate HSV-2 infection of neurons might foster the development of therapeutics to reduce HSV-2 colonization of the nervous system and provide insights on neurite outgrowth and regeneration.

Vitamin D and Its Potential Interplay With Pain Signaling Pathways

About 50 million of the U.S. adult population suffer from chronic pain. It is a complex disease in its own right for which currently available analgesics have been deemed woefully inadequate since ~20% of the sufferers derive no benefit. Vitamin D, known for its role in calcium homeostasis and bone metabolism, is thought to be of clinical benefit in treating chronic pain without the side-effects of currently available analgesics. A strong correlation between hypovitaminosis D and incidence of bone pain is known. However, the potential underlying mechanisms by which vitamin D might exert its analgesic effects are poorly understood. In this review, we discuss pathways involved in pain sensing and processing primarily at the level of dorsal root ganglion (DRG) neurons and the potential interplay between vitamin D, its receptor (VDR) and known specific pain signaling pathways including nerve growth factor (NGF), glial-derived neurotrophic factor (GDNF), epidermal growth factor receptor (EGFR), and opioid receptors. We also discuss how vitamin D/VDR might influence immune cells and pain sensitization as well as review the increasingly important topic of vitamin D toxicity. Further in vitro and in vivo experimental studies will be required to study these potential interactions specifically in pain models. Such studies could highlight the potential usefulness of vitamin D either alone or in combination with existing analgesics to better treat chronic pain.

Nerve Growth Factor Signaling and Its Contribution to Pain

Nerve growth factor (NGF) is a neurotrophic protein essential for the growth, differentiation, and survival of sympathetic and sensory afferent neurons during development. A substantial body of evidence, based on both animal and human studies, demonstrates that NGF plays a pivotal role in modulation of nociception in adulthood. This has spurred development of a variety of novel analgesics that target the NGF signaling pathway. Here, we present a narrative review designed to summarize how NGF receptor activation and downstream signaling alters nociception through direct sensitization of nociceptors at the site of injury and changes in gene expression in the dorsal root ganglion that collectively increase nociceptive signaling from the periphery to the central nervous system. This review illustrates that NGF has a well-known and multifunctional role in nociceptive processing, although the precise signaling pathways downstream of NGF receptor activation that mediate nociception are complex and not completely understood. Additionally, much of the existing knowledge derives from studies performed in animal models and may not accurately represent the human condition. However, available data establish a role for NGF in the modulation of nociception through effects on the release of inflammatory mediators, nociceptive ion channel/receptor activity, nociceptive gene expression, and local neuronal sprouting. The role of NGF in nociception and the generation and/or maintenance of chronic pain has led to it becoming a novel and attractive target of pain therapeutics for the treatment of chronic pain conditions.

Conserved Expression of Nav1.7 and Nav1.8 Contribute to the Spontaneous and Thermally Evoked Excitability in IL-6 and NGF-Sensitized Adult Dorsal Root Ganglion Neurons In Vitro

Sensory neurons respond to noxious stimuli by relaying information from the periphery to the central nervous system via action potentials driven by voltage-gated sodium channels, specifically Nav1.7 and Nav1.8. These channels play a key role in the manifestation of inflammatory pain. The ability to screen compounds that modulate voltage-gated sodium channels using cell-based assays assumes that key channels present in vivo is maintained in vitro. Prior electrophysiological work in vitro utilized acutely dissociated tissues, however, maintaining this preparation for long periods is difficult. A potential alternative involves multi-electrode arrays which permit long-term measurements of neural spike activity and are well suited for assessing persistent sensitization consistent with chronic pain. Here, we demonstrate that the addition of two inflammatory mediators associated with chronic inflammatory pain, nerve growth factor (NGF) and interleukin-6 (IL-6), to adult DRG neurons increases their firing rates on multi-electrode arrays in vitro. Nav1.7 and Nav1.8 proteins are readily detected in cultured neurons and contribute to evoked activity. The blockade of both Nav1.7 and Nav1.8, has a profound impact on thermally evoked firing after treatment with IL-6 and NGF. This work underscores the utility of multi-electrode arrays for pharmacological studies of sensory neurons and may facilitate the discovery and mechanistic analyses of anti-nociceptive compounds.

An understanding of bone pain: A narrative review

Skeletal pathologies are often accompanied by bone pain, which has negative effects on the quality of life and functional status of patients. Bone pain can be caused by a wide variety of injuries and diseases including (poorly healed) fractures, bone cancer, osteoarthritis and also iatrogenic by skeletal interventions. Orthopedic interventions are considered to be the most painful surgical procedures overall. Two major groups of medication currently used to attenuate bone pain are NSAIDs and opioids. However, these systemic drugs frequently introduce adverse events, emphasizing the need for alternative therapies that are directed at the pathophysiological mechanisms underlying bone pain. The periosteum, cortical bone and bone marrow are mainly innervated by sensory A-delta fibers and C-fibers. These fibers are mostly present in the periosteum rendering this structure most sensitive to nociceptive stimuli. A-delta fibers and C-fibers can be activated upon mechanical distortion, acidic environment and increased intramedullary pressure. After activation, these fibers can be sensitized by inflammatory mediators, phosphorylation of acid-sensing ion channels and cytokine receptors, or by upregulation of transcription factors. This can result in a change of pain perception such that normally non-noxious stimuli are now perceived as noxious. Pathological conditions in the bone can produce neurotrophic factors that bind to receptors on A-delta fibers and C-fibers. These fibers then start to sprout and increase the innervation density of the bone, making it more sensitive to nociceptive stimuli. In addition, repetitive painful stimuli cause neurochemical and electrophysiological alterations in afferent sensory neurons in the spinal cord, which leads to central sensitization, and can contribute to chronic bone pain. Understanding the pathophysiological mechanisms underlying bone pain in different skeletal injuries and diseases is important for the development of alternative, targeted pain treatments. These pain mechanism-based alternatives have the potential to improve the quality of life of patients suffering from bone pain without introducing undesirable systemic effects.

Modern Trends for Peripheral Nerve Repair and Regeneration: Beyond the Hollow Nerve Guidance Conduit

Peripheral nerve repair and regeneration remains among the greatest challenges in tissue engineering and regenerative medicine. Even though peripheral nerve injuries (PNIs) are capable of some degree of regeneration, frail recovery is seen even when the best microsurgical technique is applied. PNIs are known to be very incapacitating for the patient, due to the deprivation of motor and sensory abilities. Since there is no optimal solution for tackling this problem up to this day, the evolution in the field is constant, with innovative designs of advanced nerve guidance conduits (NGCs) being reported every day. As a basic concept, a NGC should act as a physical barrier from the external environment, concomitantly acting as physical guidance for the regenerative axons across the gap lesion. NGCs should also be able to retain the naturally released nerve growth factors secreted by the damaged nerve stumps, as well as reducing the invasion of scar tissue-forming fibroblasts to the injury site. Based on the neurobiological knowledge related to the events that succeed after a nerve injury, neuronal subsistence is subjected to the existence of an ideal environment of growth factors, hormones, cytokines, and extracellular matrix (ECM) factors. Therefore, it is known that multifunctional NGCs fabricated through combinatorial approaches are needed to improve the functional and clinical outcomes after PNIs. The present work overviews the current reports dealing with the several features that can be used to improve peripheral nerve regeneration (PNR), ranging from the simple use of hollow NGCs to tissue engineered intraluminal fillers, or to even more advanced strategies, comprising the molecular and gene therapies as well as cell-based therapies.

Voltage gated sodium channels as therapeutic targets for chronic pain

Being maladaptive and frequently unresponsive to pharmacotherapy, chronic pain presents a major unmet clinical need. While an intact central nervous system is required for conscious pain perception, nociceptor hyperexcitability induced by nerve injury in the peripheral nervous system (PNS) is sufficient and necessary to initiate and maintain neuropathic pain. The genesis and propagation of action potentials is dependent on voltage-gated sodium channels, in particular, Nav1.7, Nav1.8 and Nav1.9. However, nerve injury triggers changes in their distribution, expression and/or biophysical properties, leading to aberrant excitability. Most existing treatment for pain relief acts through non-selective, state-dependent sodium channel blockage and have narrow therapeutic windows. Natural toxins and developing subtype-specific and molecular-specific sodium channel blockers show promise for treatment of neuropathic pain with minimal side effects. New approaches to analgesia include combination therapy and gene therapy. Here, we review how individual sodium channel subtypes contribute to pain, and the attempts made to develop more effective analgesics for the treatment of chronic pain.

(-)-Hardwickiic Acid and Hautriwaic Acid Induce Antinociception via Blockade of Tetrodotoxin-Sensitive Voltage-Dependent Sodium Channels

For an affliction that debilitates an estimated 50 million adults in the United States, the current chronic pain management approaches are inadequate. The Centers for Disease Control and Prevention have called for a minimization in opioid prescription and use for chronic pain conditions, and thus, it is imperative to discover alternative non-opioid based strategies. For the realization of this call, a library of natural products was screened in search of pharmacological inhibitors of both voltage-gated calcium channels and voltage-gated sodium channels, which are excellent targets due to their well-established roles in nociceptive pathways. We discovered (-)-hardwickiic acid ((-)-HDA) and hautriwaic acid (HTA) isolated from plants, Croton californicus and Eremocarpus setigerus, respectively, inhibited tetrodotoxin-sensitive sodium, but not calcium or potassium, channels in small diameter, presumptively nociceptive, dorsal root ganglion (DRG) neurons. Failure to inhibit spontaneous postsynaptic excitatory currents indicated a preferential targeting of voltage-gated sodium channels over voltage-gated calcium channels by these extracts. Neither compound was a ligand at opioid receptors. Finally, we identified the potential of both (-)-HDA and HTA to reverse chronic pain behavior in preclinical rat models of HIV-sensory neuropathy, and for (-)-HDA specifically, in chemotherapy-induced peripheral neuropathy. Our results illustrate the therapeutic potential for (-)-HDA and HTA for chronic pain management and could represent a scaffold, that, if optimized by structure-activity relationship studies, may yield novel specific sodium channel antagonists for pain relief.

Differential modulation of voltage-gated sodium channels by nerve growth factor in three major subsets of TrkA-expressing nociceptors

Nerve growth factor is an inflammatory mediator that induces long-lasting hyperalgesia, which can partially be attributed to nerve growth factor-induced sensitization of primary afferent nociceptors. It was shown that nerve growth factor increases the excitability of polymodal C-fibre nociceptors by modulating tetrodotoxin-sensitive and tetrodotoxin-resistant voltage-gated sodium channels, but hitherto only little is known about the effects of nerve growth factor on sodium currents in other nociceptor subtypes that express the nerve growth factor receptor TrkA. We previously characterized two reporter mouse lines that allow the unequivocal identification of two important subclasses of TrkA-expressing nociceptors – i.e. neuropeptide Y receptor type 2 (NPY2R⁺ ) Aδ-fibre nociceptors that mediate pinprick pain and nicotinic acetylcholine receptor alpha-3 subunit (CHRNA3⁺ ) silent nociceptors, which are the most abundant TrkA⁺ nociceptors in visceral organs and deep somatic tissues. Here, we utilized these mouse lines to investigate the expression patterns and the possible nerve growth factor-dependent modulation of sodium channels in these neurons using whole-cell patch-clamp recordings and quantitative real-time polymerase chain reaction. We demonstrate that NPY2R⁺ nociceptors, CHRNA3⁺ ‘silent’ nociceptors and polymodal C-fibre nociceptors express different combinations of sodium channel α- and β-subunits and accordingly exhibit functionally different sodium currents. Moreover, we demonstrate that nerve growth factor produces robust hyperpolarizing shifts in the half-activation voltage of tetrodotoxin-resistant currents in NPY2R⁺ nociceptors and polymodal C-fibre nociceptors and also shifts the half-activation of tetrodotoxin-sensitive currents in polymodal C-fibre nociceptors. In silent nociceptors, however, nerve growth factor solely increases the current density of the tetrodotoxin-resistant current but does not alter other sodium channel properties. Considering the different peripheral target tissues and the previously reported roles in different forms of pain of the nociceptor subpopulations that were examined here, our results suggest that nerve growth factor differentially contributes to the development visceral and cutaneous pain hypersensitivity and highlights the importance of developing different therapeutic strategies for different forms of pain.

Glial interleukin-1β upregulates neuronal sodium channel 1.7 in trigeminal ganglion contributing to temporomandibular joint inflammatory hypernociception in rats

Background

The proinflammatory cytokine interleukin-1β (IL-1β) drives pain by inducing the expression of inflammatory mediators; however, its ability to regulate sodium channel 1.7 (Nav1.7), a key driver of temporomandibular joint (TMJ) hypernociception, remains unknown. IL-1β induces cyclooxygenase-2 (COX-2) and prostaglandin E2 (PGE2). We previously showed that PGE2 upregulated trigeminal ganglionic Nav1.7 expression. Satellite glial cells (SGCs) involve in inflammatory pain through glial cytokines. Therefore, we explored here in the trigeminal ganglion (TG) whether IL-1β upregulated Nav1.7 expression and whether the IL-1β located in the SGCs upregulated Nav1.7 expression in the neurons contributing to TMJ inflammatory hypernociception.

Methods

We treated rat TG explants with IL-1β with or without inhibitors, including NS398 for COX-2, PF-04418948 for EP2, and H89 and PKI-(6-22)-amide for protein kinase A (PKA), or with adenylate cyclase agonist forskolin, and used real-time PCR, Western blot, and immunohistofluorescence to determine the expressions or locations of Nav1.7, COX-2, cAMP response element-binding protein (CREB) phosphorylation, and IL-1β. We used chromatin immunoprecipitation to examine CREB binding to the Nav1.7 promoter. Finally, we microinjected IL-1β into the TGs or injected complete Freund’s adjuvant into TMJs with or without previous microinjection of fluorocitrate, an inhibitor of SGCs activation, into the TGs, and evaluated nociception and gene expressions. Differences between groups were examined by one-way analysis of variance (ANOVA) or independent samples t test.

Results

IL-1β upregulated Nav1.7 mRNA and protein expressions in the TG explants, whereas NS398, PF-04418948, H89, or PKI-(6-22)-amide could all block this upregulation, and forskolin could also upregulate Nav1.7 mRNA and protein expressions. IL-1β enhanced CREB binding to the Nav1.7 promoter. Microinjection of IL-1β into the TGs or TMJ inflammation both induced hypernociception of TMJ region and correspondingly upregulated COX-2, phospho-CREB, and Nav1.7 expressions in the TGs. Moreover, microinjection of fluorocitrate into the TGs completely blocked TMJ inflammation-induced activation of SGCs and the upregulation of IL-1β and COX-2 in the SGCs, and phospho-CREB and Nav1.7 in the neurons and alleviated inflammation-induced TMJ hypernociception.

Conclusions

Glial IL-1β upregulated neuronal Nav1.7 expression via the crosstalk between signaling pathways of the glial IL-1β/COX-2/PGE2 and the neuronal EP2/PKA/CREB/Nav1.7 in TG contributing to TMJ inflammatory hypernociception.

Prostaglandin E2 Upregulated Trigeminal Ganglionic Sodium Channel 1.7 Involving Temporomandibular Joint Inflammatory Pain in Rats

Prostaglandin E₂ (PGE₂) is a key proinflammatory mediator that contributes to inflammatory hyperalgesia. Voltage-gated sodium channel 1.7 (Na_v1.7) plays an important role in inflammatory pain. However, the modulation of Na_v1.7 in inflammatory pain remains poorly understood. We hypothesized that PGE₂ might regulate Na_v1.7 expression in inflammatory pain. We here showed that treatment of rat trigeminal ganglion (TG) explants with PGE₂ significantly upregulated the mRNA and protein expressions of Na_v1.7 through PGE₂ receptor EP2. This finding was confirmed by studies on EP2-selective antagonist PF-04418948. We also demonstrated that Na_v1.7 and COX-2 expressions, as well as PGE₂ levels, were upregulated in the TG after induction of rats' temporomandibular joint (TMJ) inflammation. Correspondingly, hyperalgesia, as indicated by head withdrawal threshold, was observed. Moreover, TMJ inflammation-induced upregulation of Na_v1.7 expression and PGE₂ levels in the TG could be reversed by COX-2-selective inhibitor meloxicam given by oral gavage, and meanwhile, the hyperalgesia of inflamed TMJ was also mitigated. So we concluded that PGE₂ upregulated trigeminal ganglionic Na_v1.7 expression to contribute to TMJ inflammatory pain in rats. Our finding suggests that PGE₂ was an important regulator of Na_v1.7 in TMJ inflammatory pain, which may help increase understanding on the hyperalgesia of peripheral inflammation and develop a new strategy to address inflammatory pain.

Nerve growth factor induces facial heat hyperalgesia and plays a role in trigeminal neuropathic pain in rats

There is preclinical evidence that nerve growth factor (NGF) contributes toward inflammatory hyperalgesia in the orofacial region, but the mechanisms underlying its hyperalgesic effect as well as its role in trigeminal neuropathic pain require further investigation. This study investigated the ability of NGF to induce facial heat hyperalgesia and the involvement of tyrosine kinase receptor A, transient receptor potential vanilloid 1, and mast cells in NGF pronociceptive effects. In addition, the role of NGF in heat hyperalgesia in a model of trigeminal neuropathic pain was evaluated. NGF injection into the upper lip of naive rats induced long-lasting heat hyperalgesia. Pretreatment with an antibody anti-NGF, antagonists of tyrosine kinase receptor A, and transient receptor potential vanilloid 1 receptors or compound 48/80, to induce mast-cell degranulation, all attenuated NGF-induced hyperalgesia. In a rat model of trigeminal neuropathic pain, local treatment with anti-NGF significantly reduced heat hyperalgesia. In addition, increased NGF levels were detected in the ipsilateral infraorbital nerve branch at the time point that represents the peak of heat hyperalgesia. The results suggest that NGF is a prominent hyperalgesic mediator in the trigeminal system and it may represent a potential therapeutic target for the management of painful orofacial conditions, including trigeminal neuropathic pain.

Mechanisms of nerve growth factor signaling in bone nociceptors and in an animal model of inflammatory bone pain

Sequestration of nerve growth factor has been used successfully in the management of pain in animal models of bone disease and in human osteoarthritis. However, the mechanisms of nerve growth factor-induced bone pain and its role in modulating inflammatory bone pain remain to be determined. In this study, we show that nerve growth factor receptors (TrkA and p75) and some other nerve growth factor-signaling molecules (TRPV1 and Nav1.8, but not Nav1.9) are expressed in substantial proportions of rat bone nociceptors. We demonstrate that nerve growth factor injected directly into rat tibia rapidly activates and sensitizes bone nociceptors and produces acute behavioral responses with a similar time course. The nerve growth factor-induced changes in the activity and sensitivity of bone nociceptors we report are dependent on signaling through the TrkA receptor, but are not affected by mast cell stabilization. We failed to show evidence for longer term changes in expression of TrkA, TRPV1, Nav1.8 or Nav1.9 in the soma of bone nociceptors in a rat model of inflammatory bone pain. Thus, retrograde transport of NGF/TrkA and increased expression of some of the common nerve growth factor signaling molecules do not appear to be important for the maintenance of inflammatory bone pain. The findings are relevant to understand the basis of nerve growth factor sequestration and other therapies directed at nerve growth factor signaling, in managing pain in bone disease.

Molecular Mechanisms That Contribute to Bone Marrow Pain

Pain associated a bony pathology puts a significant burden on individuals, society, and the health-care systems worldwide. Pathology that involves the bone marrow activates sensory nerve terminal endings of peripheral bone marrow nociceptors, and is the likely trigger for pain. This review presents our current understanding of how bone marrow nociceptors are influenced by noxious stimuli presented in pathology associated with bone marrow. A number of ion channels and receptors are emerging as important modulators of the activity of peripheral bone marrow nociceptors. Nerve growth factor (NGF) sequestration has been trialed for the management of inflammatory bone pain (osteoarthritis), and there is significant evidence for interaction of NGF with bone marrow nociceptors. Activation of transient receptor potential cation channel subfamily V member 1 sensitizes bone marrow nociceptors and could contribute to increased sensitivity of patients to noxious stimuli in various bony pathologies. Acid-sensing ion channels sense changes to tissue pH in the bone marrow microenvironment and could be targeted to treat pathology that involves acidosis of the bone marrow. Piezo2 is a mechanically gated ion channel that has recently been reported to be expressed by most myelinated bone marrow nociceptors and might be a target for treatments directed against mechanically induced bone pain. These ion channels and receptors could be useful targets for the development of peripherally acting drugs to treat pain of bony origin.

NGF and Its Receptors in the Regulation of Inflammatory Response

There is growing interest in the complex relationship between the nervous and immune systems and how its alteration can affect homeostasis and result in the development of inflammatory diseases. A key mediator in cross-talk between the two systems is nerve growth factor (NGF), which can influence both neuronal cell function and immune cell activity. The up-regulation of NGF described in inflamed tissues of many diseases can regulate innervation and neuronal activity of peripheral neurons, inducing the release of immune-active neuropeptides and neurotransmitters, but can also directly influence innate and adaptive immune responses. Expression of the NGF receptors tropomyosin receptor kinase A (TrkA) and p75 neurotrophin receptor (p75NTR) is dynamically regulated in immune cells, suggesting a varying requirement for NGF depending on their state of differentiation and functional activity. NGF has a variety of effects that can be either pro-inflammatory or anti-inflammatory. This apparent contradiction can be explained by considering NGF as part of an endogenous mechanism that, while activating immune responses, also activates pathways necessary to dampen the inflammatory response and limit tissue damage. Decreases in TrkA expression, such as that recently demonstrated in immune cells of arthritis patients, might prevent the activation by NGF of regulatory feed-back mechanisms, thus contributing to the development and maintenance of chronic inflammation.

Bone Pain and Muscle Weakness in Cancer Patients

PURPOSE OF REVIEW

In this article, we will discuss the current understanding of bone pain and muscle weakness in cancer patients. We will describe the underlying physiology and mechanisms of cancer-induced bone pain (CIBP) and cancer-induced muscle wasting (CIMW), as well as current methods of diagnosis and treatment. We will discuss future therapies and research directions to help patients with these problems.

RECENT FINDINGS

There are several pharmacologic therapies that are currently in preclinical and clinical testing that appear to be promising adjuncts to current CIBP and CIMW therapies. Such therapies include resiniferitoxin, which is a targeted inhibitor of noceciptive nerve fibers, and selective androgen receptor modulators, which show promise in increasing lean mass. CIBP and CIMW are significant causes of morbidity in affected patients. Current management is mostly palliative; however, targeted therapies are poised to revolutionize how these problems are treated.

Permeability and distribution of nerve growth factor in the brain of neonatal rats by periphery venous injection in hypoxic-ischemic state

Objective

To investigate the permeability of β-NGF through blood–brain-barrier (BBB) in neonatal and adult rats, and the spatial distribution of β-NGF in different brain regions in hypoxic-ischemic (HI) and normal neonatal rats.

Methods

To investigate the overall permeability of β-NGF through BBB, β-NGF labeled with I¹²⁵ was injected into adult rats, neonatal rats and HI neonatal rats via tail vein. The radioactivity of brain tissue and blood was examined and analyzed 30 min after injection. Also, brain regions including the basal forebrain, frontal cortex, hippocampus, hypothalamus, cerebellum, bulbus olfactorius and hypophysis, of all the rats were dissected and radioactivity was examined to investigate the spatial specificity of NGF permeation through BBB.

Results

Statistically significant results were observed in I¹²⁵-β-NGF contents in brain tissues of adult rats group, neonatal rats group and HI neonatal rats group (P < 0.05). Compared to the HI neonatal rats’ brain with the highest I¹²⁵-β-NGF contents, normal neonatal rats ranks the second while the adult rats were the lowest. While for the spatial specificity examination part, I¹²⁵-β-NGF in both HI group and control group were widely distributed in basal forebrain, frontal cortex, hippocampus, cerebellum and bulbus olfactorius. But the radioactivity in frontal cortex, hippocampus and cerebellum of HI groups are statistically higher than control groups (P < 0.05).

Conclusion

β-NGF can more easily penetrate the BBB of newborn rats than adult rats via peripheral venous administration and this effect can be enhanced by HI insult. Also, this HI-induced permeation of β-NGF through BBB is more obvious in frontal cortex, hippocampus and cerebellum.

Emerging factors in the progression of cancer-related pain

Most cancer patients experience severe pain during their disease course, and the management of cancer pain is a major challenge for patients and the healthcare team. Many diverse translational models of cancer pain in recent years have improved our understanding of cancer-related pain. Cancer and associated cells in the cancer microenvironment may release various peripheral mediators, including ATP, formaldehyde, protons, proteases, endothelin, bradykinin, TNF and NGF, that result in the activation and/or sensitization of peripheral and central neurons, that contribute to the clinical manifestations of cancer-related pain. Identification of these mediators and the peripheral and central mechanisms by which they contribute to cancer-related pain may provide novel therapeutic targets to alleviate cancer patient suffering.

Changes in Neuronal Excitability by Activated Microglia: Differential Na(+) Current Upregulation in Pyramid-Shaped and Bipolar Neurons by TNF-α and IL-18

Microglia are activated during pathological events in the brain and are capable of releasing various types of inflammatory cytokines. Here, we demonstrate that the addition of 5% microglia activated by 1 μg/ml lipopolysaccharides (LPS) to hippocampal cultures upregulates Na⁺ current densities (I_NavD) of bipolar as well as pyramid-shaped neurons, thereby increasing their excitability. Deactivation of microglia by the addition of 10 ng/ml transforming growth factor-β (TGF-β) decreases I_NavD below control levels suggesting that the residual activated microglial cells influence neuronal excitability in control cultures. Preincubation of hippocampal cultures with 10 ng/ml tumor necrosis factor-α (TNF-α), a major cytokine released by activated microglia, upregulated I_NavD significantly by ~30% in bipolar cells, whereas in pyramid-shaped cells, the upregulation only reached an increase of ~14%. Incubation of the cultures with antibodies against either TNF-receptor 1 or 2 blocked the upregulation of I_NavD in bipolar cells, whereas in pyramid-shaped cells, increases in I_NavD were exclusively blocked by antibodies against TNF-receptor 2, suggesting that both cell types respond differently to TNF-α exposure. Since additional cytokines, such as interleukin-18 (IL-18), are released from activated microglia, we tested potential effects of IL-18 on I_NavD in both cell types. Exposure to 5–10 ng/ml IL-18 for 4 days increased I_NavD in both pyramid-shaped as well as bipolar neurons, albeit the dose–response curves were shifted to lower concentrations in bipolar cells. Our results suggest that by secretion of cytokines, microglial cells upregulate Na⁺ current densities in bipolar and pyramid-shaped neurons to some extent differentially. Depending on the exact cytokine composition and concentration released, this could change the balance between the activity of inhibitory bipolar and excitatory pyramid-shaped cells. Since bipolar cells show a larger upregulation of I_NavD in response to TNF-α as well as respond to smaller concentrations of IL-18, our results offer an explanation for the finding, that in certain conditions of brain inflammations periods of dizziness are followed by epileptic seizures.

PG110, A Humanized Anti-NGF Antibody, Reverses Established Pain Hypersensitivity in Persistent Inflammatory Pain, but not Peripheral Neuropathic Pain, Rat Models

BACKGROUND

Chronic inflammatory and peripheral neuropathic pain (PNP) is a major health problem for which effective drug treatment is lacking. The pathophysiology of these debilitating conditions is incompletely understood, but nerve growth factor (NGF) is believed to play a major role. NGF-antagonism has previously been shown to prevent pain hypersensitivity in rodent models of acute inflammatory pain and PNP, but most of those animal studies did not address the more clinically relevant issue of whether NGF-antagonism provides relief of established chronic pain behavior. Therefore, the aim of this study was to investigate whether blocking NGF actions with a humanized anti-NGF monoclonal antibody (PG110) would reverse/attenuate established pain hypersensitivity in rat models of chronic/persistent inflammatory pain and PNP.

METHODS

The complete Freund's adjuvant (CFA) rat model of persistent inflammatory pain, and the L5 spinal nerve axotomy (SNA) model of PNP, were used in the present study. The effect of a single intravenous injection (10, 30, and 300 µg/kg) of an anti-NGF antibody PG110 on heat and mechanical hypersensitivity was assessed 5 and 7 days after CFA and SNA, respectively.

RESULTS

Compared to vehicle treated group, PG110 dose dependently attenuated established heat and mechanical hypersensitivity induced by CFA, but not that induced by SNA. The anti-allodynic and anti-hyperalgesic effects of PG110 in the CFA model were similar to those of the positive control naproxen (30 mg/kg, i.v.).

CONCLUSION

These findings suggest that therapies that target NGF or its receptors may be effective for treatment of persistent/chronic inflammatory pain, but probably not PNP.

Post-translational modifications of voltage-gated sodium channels in chronic pain syndromes

In the peripheral sensory nervous system the neuronal expression of voltage-gated sodium channels (Navs) is very important for the transmission of nociceptive information since they give rise to the upstroke of the action potential (AP). Navs are composed of nine different isoforms with distinct biophysical properties. Studying the mutations associated with the increase or absence of pain sensitivity in humans, as well as other expression studies, have highlighted Nav1.7, Nav1.8, and Nav1.9 as being the most important contributors to the control of nociceptive neuronal electrogenesis. Modulating their expression and/or function can impact the shape of the AP and consequently modify nociceptive transmission, a process that is observed in persistent pain conditions. Post-translational modification (PTM) of Navs is a well-known process that modifies their expression and function. In chronic pain syndromes, the release of inflammatory molecules into the direct environment of dorsal root ganglia (DRG) sensory neurons leads to an abnormal activation of enzymes that induce Navs PTM. The addition of small molecules, i.e., peptides, phosphoryl groups, ubiquitin moieties and/or carbohydrates, can modify the function of Navs in two different ways: via direct physical interference with Nav gating, or via the control of Nav trafficking. Both mechanisms have a profound impact on neuronal excitability. In this review we will discuss the role of Protein Kinase A, B, and C, Mitogen Activated Protein Kinases and Ca++/Calmodulin-dependent Kinase II in peripheral chronic pain syndromes. We will also discuss more recent findings that the ubiquitination of Nav1.7 by Nedd4-2 and the effect of methylglyoxal on Nav1.8 are also implicated in the development of experimental neuropathic pain. We will address the potential roles of other PTMs in chronic pain and highlight the need for further investigation of PTMs of Navs in order to develop new pharmacological tools to alleviate pain.

Cancer pain physiology

Mechanisms of inflammatory and neuropathic pains have been elucidated and translated to patient care by the use of animal models of these pain states. Cancer pain has lagged behind since early animal models of cancer-induced bone pain were based on the systemic injection of carcinoma cells. This precluded systematic investigation of specific neuronal and pharmacological alterations that occur in cancer-induced bone pain. In 1999, Schwei et al. described a murine model of cancer-induced bone pain that paralleled the clinical condition in terms of pain development and bone destruction, confined to the mouse femur. This model prompted related approaches, and we can now state that cancer pain may include elements of inflammatory and neuropathic pains but also unique changes in sensory processing. Cancer-induced bone pain results in progressive bone destruction, elevated osteoclast activity and distinctive nocifensive behaviours (indicating the triad of ongoing, spontaneous and movement-induced hyperalgesia). In addition, cancer cells induce an inflammatory infiltrate and release growth factors, cytokines, interleukins, chemokines, prostanoids and endothelins, resulting in a reduction of pH to below 5 and direct deformation of primary afferents within bone. These peripheral changes, in turn, drive hypersensitivity of spinal cord sensory neurons, many of which project to the parts of the brain involved in the emotional response to pain. Within the spinal cord, a unique neuronal function reorganization within segments of the dorsal horn of the spinal cord receiving nociceptive input from the bone are discussed. Changes in certain neurotransmitters implicated in brain modulation of spinal function are also altered with implications for the affective components of cancer pain. Treatments are described in terms of mechanistic insights and in the case of opioids, which modulate pain transmission at spinal and supraspinal sites, their use can be compromised by opioid-induced hyperalgesia. We discuss evidence for how this comes about and how it may be treated.

Bone cancer pain: from mechanism to therapy

PURPOSE OF REVIEW

To review how common cancers such as breast, lung, and prostate cancers drive significant and frequently life-altering pain when the cells metastasize to bones.

RECENT FINDINGS

Similar to cancer, the factors that drive bone cancer pain evolve and change with disease progression. Bone cancer pain has both a nociceptive and neuropathic component. The nociceptive component is driven by the release of algogenic substances by tumor and their associated stromal cells, acidosis caused by bone-destroying osteoclasts, and mechanical destabilization and fracture of the bone. The neuropathic component is induced by tumor cell growth which injures and destroys the distal ends of nerve fibers that normally innervate the bone, as well as by inducing a highly pathological sprouting of both sensory and sympathetic nerve fibers.

SUMMARY

There is both a nociceptive and neuropathic component of bone cancer pain. In bone cancer pain, there is frequently a continual afferent drive of sensory nerve fibers that induces a peripheral and central sensitization. These mechanistic insights have begun to lead to advances in not only how we understand bone cancer pain but to the development of new therapies to treat bone cancer pain.

New insights of nociceptor sensitization in bone cancer pain

INTRODUCTION

Numerous studies have shown that an intact CNS is required for the conscious perception of cancer-induced bone pain (CIBP) and that changes in the CNS are clearly evident. Accordingly, the blockage of nociceptive stimulus into the CNS can effectively relieve or markedly attenuate CIBP, revealing the clinical implication of the blockage of ongoing peripheral inputs for the control of CIBP.

AREAS COVERED

In this review, the heterogeneity and excitability of nociceptors in bone are covered. Furthermore, their role in initiating and maintaining CIBP is also described.

EXPERT OPINION

Developing mechanistic therapies to treat CIBP is a challenge, but they have the potential to fundamentally change our ability to effectively block/relieve CIBP and increase the functional status and quality of life of patients with bone metastasis. Further studies are desperately needed at both the preclinical and clinical levels to determine whether the targets as mentioned in this review are viable and feasible for patient populations.

Nerve growth factor sensitizes adult sympathetic neurons to the proinflammatory peptide bradykinin

Levels of nerve growth factor (NGF) are elevated in inflamed tissues. In sensory neurons, increases in NGF augment neuronal sensitivity (sensitization) to noxious stimuli. Here, we hypothesized that NGF also sensitizes sympathetic neurons to proinflammatory stimuli. We cultured superior cervical ganglion (SCG) neurons from adult male Sprague Dawley rats with or without added NGF and compared their responsiveness to bradykinin, a proinflammatory peptide. The NGF-cultured neurons exhibited significant depolarization, bursts of action potentials, and Ca(2+) elevations after bradykinin application, whereas neurons cultured without NGF showed only slight changes in membrane potential and cytoplasmic Ca(2+) levels. The NGF effect, which requires trkA receptors, takes hours to develop and days to reverse. We addressed the ionic mechanisms underlying this sensitization. NGF did not alter bradykinin-induced M-current inhibition or phosphatidylinositol 4,5-bisphosphate hydrolysis. Maxi-K channel-mediated current evoked by depolarizations was reduced by 50% by culturing neurons in NGF. Application of iberiotoxin or paxilline, blockers of Maxi-K channels, mimicked NGF treatment and sensitized neurons to bradykinin application. A calcium channel blocker also mimicked NGF treatment. We found that NGF reduces Maxi-K channel opening by decreasing the activity of nifedipine-sensitive calcium channels. In conclusion, culture in NGF reduces the activity of L-type calcium channels, and secondarily, the calcium-sensitive activity of Maxi-K channels, rendering sympathetic neurons electrically hyper-responsive to bradykinin.

Plasticity of dorsal root ganglion neurons in a rat model of post-infectious gut dysfunction: potential implication of nerve growth factor

OBJECTIVE

Intestinal infections are suggested as a risk factor for the development of irritable bowel syndrome (IBS)-like visceral hypersensitivity. The mechanisms implicated might involve long-term changes in visceral afferents, with implication of nerve growth factor (NGF). We explored plastic changes in dorsal root ganglia (DRGs) receiving innervation from the gut and the potential implication of NGF in a rat model of IBS-like post-infectious gut dysfunction.

MATERIALS AND METHODS

Rats were infected with Trichinella spiralis larvae. Thirty days post-infection, inflammatory markers, including interleukins (ILs) and mucosal mast cell infiltration (rat mast cell protease II [RMCPII]), and NGF and TrkA expression was determined in the jejunum and colon (RT-qPCR). In the same animals, morphometry (neuronal body size) and NGF content (immunofluorescence) were assessed in thoracolumbar DRG neurons.

RESULTS

In infected animals, a low-grade inflammatory-like response, characterized by up-regulated levels of RMCPII and IL-6, was observed in the jejunum and colon. TrkA expression was increased in the jejunum, whereas the colon showed a slight reduction. NGF levels remained unaltered regardless the gut region. Overall, the mean cross-sectional area of DRG neurons was increased in T. spiralis-infected animals, with a reduction in both TrkA and NGF staining.

CONCLUSIONS

Results suggest that during T. spiralis infection in rats, there is a remodeling of sensory afferents that might imply a NGF-mediated mechanism. Plastic changes in sensory afferents might mediate the long-lasting functional alterations that characterize this model of IBS. Similar mechanisms might be operating in patients with post-infectious-IBS.

Differential axonal conduction patterns of mechano-sensitive and mechano-insensitive nociceptors--a combined experimental and modelling study

Cutaneous pain sensations are mediated largely by C-nociceptors consisting of both mechano-sensitive (CM) and mechano-insensitive (CMi) fibres that can be distinguished from one another according to their characteristic axonal properties. In healthy skin and relative to CMi fibres, CM fibres show a higher initial conduction velocity, less activity-dependent conduction velocity slowing, and less prominent post-spike supernormality. However, after sensitization with nerve growth factor, the electrical signature of CMi fibres changes towards a profile similar to that of CM fibres. Here we take a combined experimental and modelling approach to examine the molecular basis of such alterations to the excitation thresholds. Changes in electrical activation thresholds and activity-dependent slowing were examined in vivo using single-fibre recordings of CM and CMi fibres in domestic pigs following NGF application. Using computational modelling, we investigated which axonal mechanisms contribute most to the electrophysiological differences between the fibre classes. Simulations of axonal conduction suggest that the differences between CMi and CM fibres are strongly influenced by the densities of the delayed rectifier potassium channel (Kdr), the voltage-gated sodium channels NaV1.7 and NaV1.8, and the Na+/K+-ATPase. Specifically, the CM fibre profile required less Kdr and NaV1.8 in combination with more NaV1.7 and Na+/K+-ATPase. The difference between CM and CMi fibres is thus likely to reflect a relative rather than an absolute difference in protein expression. In support of this, it was possible to replicate the experimental reduction of the ADS pattern of CMi nociceptors towards a CM-like pattern following intradermal injection of nerve growth factor by decreasing the contribution of Kdr (by 50%), increasing the Na+/K+-ATPase (by 10%), and reducing the branch length from 2 cm to 1 cm. The findings highlight key molecules that potentially contribute to the NGF-induced switch in nociceptors phenotype, in particular NaV1.7 which has already been identified clinically as a principal contributor to chronic pain states such as inherited erythromelalgia.

[Neurotrophins and pain]

Neurotrophins are a family of trophic factors well known for their effects on neuronal survival, growth and neuronal differentiation. During the last decade, a large literature has shown in humans and in animal models that nerve growth factor (NGF) is a peripheral mediator of pain, especially in the states of inflammatory pain. NGF synthesis is indeed increased in a wide variety of inflammatory diseases and NGF neutralizing molecules are effective analgesic agents in these models of persistent pain. Therapeutical strategies targeting the sequestering of NGF did yield very encouraging results in clinical trials (stages II and III) but have been on hold since 2010 due to potential harmful effects in combination with non-steroid anti-inflammatory drug. NGF regulates the expression of a second neurotrophin, Brain Derived Neurotrophic Factor (BDNF), in nociceptors. BDNF is released where nociceptors are activated, and it acts as a modulator of pain in the central nervous system and is involved in central sensitization.

Genome-wide transcriptional profiling of skin and dorsal root ganglia after ultraviolet-B-induced inflammation

Ultraviolet-B (UVB)-induced inflammation produces a dose-dependent mechanical and thermal hyperalgesia in both humans and rats, most likely via inflammatory mediators acting at the site of injury. Previous work has shown that the gene expression of cytokines and chemokines is positively correlated between species and that these factors can contribute to UVB-induced pain. In order to investigate other potential pain mediators in this model we used RNA-seq to perform genome-wide transcriptional profiling in both human and rat skin at the peak of hyperalgesia. In addition we have also measured transcriptional changes in the L4 and L5 DRG of the rat model. Our data show that UVB irradiation produces a large number of transcriptional changes in the skin: 2186 and 3888 genes are significantly dysregulated in human and rat skin, respectively. The most highly up-regulated genes in human skin feature those encoding cytokines (IL6 and IL24), chemokines (CCL3, CCL20, CXCL1, CXCL2, CXCL3 and CXCL5), the prostanoid synthesising enzyme COX-2 and members of the keratin gene family. Overall there was a strong positive and significant correlation in gene expression between the human and rat (R = 0.8022). In contrast to the skin, only 39 genes were significantly dysregulated in the rat L4 and L5 DRGs, the majority of which had small fold change values. Amongst the most up-regulated genes in DRG were REG3B, CCL2 and VGF. Overall, our data shows that numerous genes were up-regulated in UVB irradiated skin at the peak of hyperalgesia in both human and rats. Many of the top up-regulated genes were cytokines and chemokines, highlighting again their potential as pain mediators. However many other genes were also up-regulated and might play a role in UVB-induced hyperalgesia. In addition, the strong gene expression correlation between species re-emphasises the value of the UVB model as translational tool to study inflammatory pain.

Regulation and pharmacological blockade of sodium-potassium ATPase: a novel pathway to neuropathy

Inflammation causes upregulation of NaV1.7 sodium channels in the associated dorsal root ganglia (DRG). The resultant increase in sodium influx must be countered to maintain osmotic homeostasis. The primary mechanism to pump sodium out of neurons is Na(+), K(+)-ATPase. To test whether there is a compensatory upregulation of Na(+), K(+)-ATPase after inflammation, rats received an injection of complete Freund's adjuvant (CFA) into one hindpaw and saline into the contralateral hindpaw. Three days later, L4-L6 DRGs were extracted and analyzed using gel electrophoresis and immunohistochemistry. Immunoreactivity for both the α-1 and α-3 subunits were increased in DRG associated with CFA-treatment, compared to saline-treatment. To test whether dysregulation of Na(+), K(+)-ATPase may cause cell death after inflammation, we produced a pharmacological blockade with ouabain (10mg/kg, s.c.) three days after CFA injection and paws were stimulated or not. Twenty-four hours later, DRG were removed and stained with cresyl violet. Greater cell death was seen in DRG from ouabain-treated animals on the CFA treated side than the saline-treated side. Paw stimulation doubled this difference. Control DRG showed little neuronal death. These results are evidence that regulation of Na(+), K(+)-ATPase during major inflammatory disease states is critical for homeostatic protection of primary afferent neurons.

Nerve growth factor and nociception: from experimental embryology to new analgesic therapy

Nerve growth factor (NGF) is central to the development and functional regulation of sensory neurons that signal the first events that lead to pain. These sensory neurons, called nociceptors, require NGF in the early embryo to survive and also for their functional maturation. The long road from the discovery of NGF and its roles during development to the realization that NGF plays a major role in the pathophysiology of inflammatory pain will be reviewed. In particular, we will discuss the various signaling events initiated by NGF that lead to long-lasting thermal and mechanical hyperalgesia in animals and in man. It has been realized relatively recently that humanized function blocking antibodies directed against NGF show remarkably analgesic potency in human clinical trials for painful conditions as varied as osteoarthritis, lower back pain, and interstitial cystitis. Thus, anti-NGF medication has the potential to make a major impact on day-to-day chronic pain treatment in the near future. It is therefore all the more important to understand the precise pathways and mechanisms that are controlled by NGF to both initiate and sustain mechanical and thermal hyperalgesia. Recent work suggests that NGF-dependent regulation of the mechanosensory properties of sensory neurons that signal mechanical pain may open new mechanistic avenues to refine and exploit relevant molecular targets for novel analgesics.