Blockade of nerve sprouting and neuroma formation markedly attenuates the development of late stage cancer pain

A C-terminal cysteine residue is required for peptide-based inhibition of the NGF/TrkA interaction at nM concentrations: implications for peptide-based analgesics

Inhibition of the NGF/TrkA interaction presents an interesting alternative to the use of non-steroidal anti-inflammatories and/or opioids for the control of inflammatory, chronic and neuropathic pain. Most prominent of the current approaches to this therapy is the antibody Tanezumab, which is a late-stage development humanized monoclonal antibody that targets NGF. We sought to determine whether peptides might similarly inhibit the NGF/TrkA interaction and so serve as future therapeutic leads. Starting from two peptides that inhibit the NGF/TrkA interaction, we sought to eliminate a cysteine residue close to the C-terminal of both sequences, by an approach of mutagenic analysis and saturation mutagenesis of mutable residues. Elimination of cysteine from a therapeutic lead is desirable to circumvent manufacturing difficulties resulting from oxidation. Our analyses determined that the cysteine residue is not required for NGF binding, but is essential for inhibition of the NGF/TrkA interaction at pharmacologically relevant peptide concentrations. We conclude that a cysteine residue is required within potential peptide-based therapeutic leads and hypothesise that these peptides likely act as dimers, mirroring the dimeric structure of the TrkA receptor.

Mechanisms of acute and chronic pain after surgery: update from findings in experimental animal models

PURPOSE OF REVIEW

Management of postoperative pain is still a major issue and relevant mechanisms need to be investigated. In preclinical research, substantial progress has been made, for example, by establishing specific rodent models of postoperative pain. By reviewing most recent preclinical studies in animals related to postoperative, incisional pain, we outline the currently available surgical-related pain models, discuss assessment methods for pain-relevant behavior and their shortcomings to reflect the clinical situation, delineate some novel clinical-relevant mechanisms for postoperative pain, and point toward future needs.

RECENT FINDINGS

Since the development of the first rodent model of postoperative, incisional pain almost 20 years ago, numerous variations and some procedure-specific models have been emerged including some conceivably relevant for investigating prolonged, chronic pain after surgery. Many mechanisms have been investigated by using these models; most recent studies focussed on endogenous descending inhibition and opioid-induced hyperalgesia. However, surgical models beyond the classical incision model have so far been used only in exceptional cases, and clinical relevant behavioral pain assays are still rarely utilized.

SUMMARY

Pathophysiological mechanisms of pain after surgery are increasingly discovered, but utilization of pain behavior assays are only sparsely able to reflect clinical-relevant aspects of acute and chronic postoperative pain in patients.

Crosstalk Between Sensory Nerves and Cancer in Bone

PURPOSE OF REVIEW

Sensory nerves (SNs) richly innervate bone and are a component of bone microenvironment. Cancer metastasis in bone, which is under the control of the crosstalk with bone microenvironment, induces bone pain via excitation of SNs innervating bone. However, little is known whether excited SNs in turn affect bone metastasis.

RECENT FINDINGS

Cancer cells colonizing bone promote neo-neurogenesis of SNs and excite SNs via activation of the acid-sensing nociceptors by creating pathological acidosis in bone, evoking bone pain. Denervation of SNs or inhibition of SN excitation decreases bone pain and cancer progression and increases survival in preclinical models. Importantly, patients with cancers with increased SN innervation complain of cancer pain and show poor outcome. SNs establish the crosstalk with cancer cells to contribute to bone pain and cancer progression in bone. Blockade of SN excitation may have not only analgesic effects on bone pain but also anti-cancer actions on bone metastases.

Mechanisms that drive bone pain across the lifespan

Disorders of the skeleton are frequently accompanied by bone pain and a decline in the functional status of the patient. Bone pain occurs following a variety of injuries and diseases including bone fracture, osteoarthritis, low back pain, orthopedic surgery, fibrous dysplasia, rare bone diseases, sickle cell disease and bone cancer. In the past 2 decades, significant progress has been made in understanding the unique population of sensory and sympathetic nerves that innervate bone and the mechanisms that drive bone pain. Following physical injury of bone, mechanotranducers expressed by sensory nerve fibres that innervate bone are activated and sensitized so that even normally non-noxious loading or movement of bone is now being perceived as noxious. Injury of the bone also causes release of factors that; directly excite and sensitize sensory nerve fibres, upregulate proalgesic neurotransmitters, receptors and ion channels expressed by sensory neurons, induce ectopic sprouting of sensory and sympathetic nerve fibres resulting in a hyper-innervation of bone, and central sensitization in the brain that amplifies pain. Many of these mechanisms appear to be involved in driving both nonmalignant and malignant bone pain. Results from human clinical trials suggest that mechanism-based therapies that attenuate one type of bone pain are often effective in attenuating pain in other seemingly unrelated bone diseases. Understanding the specific mechanisms that drive bone pain in different diseases and developing mechanism-based therapies to control this pain has the potential to fundamentally change the quality of life and functional status of patients suffering from bone pain.

Anti-nerve growth factor does not change physical activity in normal young or aging mice but does increase activity in mice with skeletal pain

Anti-nerve growth factor (anti-NGF) therapy has shown significant promise in attenuating several types of skeletal pain. However, whether anti-NGF therapy changes the level of physical activity in individuals with or without skeletal pain is largely unknown. Here, automated day/night activity boxes monitored the effects of anti-NGF treatment on physical activity in normal young (3 months old) and aging (18-23 months old) mice and mice with bone fracture pain. Although aging mice were clearly less active and showed loss of bone mass compared with young mice, anti-NGF treatment had no effect on any measure of day/night activity in either the young or aging mice. By contrast, in mice with femoral fracture pain, anti-NGF treatment produced a clear increase (10%-27%) in horizontal activity, vertical rearing, and velocity of travel compared with the Fracture + Vehicle group. These results suggest, just as in humans, mice titrate their level of physical activity to their level of skeletal pain. The level of skeletal pain may in part be determined by the level of free NGF that seems to rise after injury but not normal aging of the skeleton. In terms of bone healing, animals that received anti-NGF showed an increase in the size of calcified callus but no increase in the number of displaced fractures or time to cortical union. As physical activity is the best nondrug treatment for many patients with skeletal pain, anti-NGF may be useful in reducing pain and promoting activity in these patients.

Anti-nerve growth factor monoclonal antibodies for the control of pain in dogs and cats

Nerve growth factor (NGF) is essential for the survival of sensory and sympathetic neurons during development. However, in the adult, NGF and its interaction with tropomyosin receptor kinase A receptor (TrkA) has been found to play a critical role in nociception and nervous system plasticity in pain conditions. Thus, various monoclonal antibody (mAb) therapies targeting this pathway have been investigated in the development of new pharmacotherapies for chronic pain. Although none of the mAbs against NGF are yet approved for use in humans, they look very promising for the effective control of pain. Recently, species-specific anti-NGF mAbs for the management of osteoarthritis (OA)-associated pain in dogs and cats has been developed, and early clinical trials have been conducted. Anti-NGF therapy looks to be both very effective and very promising as a novel therapy against chronic pain in dogs and cats. This review outlines the mechanism of action of NGF, the role of NGF in osteoarthritis, research in rodent OA models and the current status of the development of anti-NGF mAbs in humans. Furthermore, we describe and discuss the recent development of species-specific anti-NGF mAbs for the treatment of OA-associated pain in veterinary medicine.

Fulranumab as Adjunctive Therapy for Cancer-Related Pain: A Phase 2, Randomized, Double-Blind, Placebo-Controlled, Multicenter Study

This randomized, double-blind (DB), placebo-controlled, phase 2 study assessed the efficacy and safety of fulranumab as a pain therapy adjunctive to opioids in terminally ill cancer patients. Ninety-eight patients were randomized (2:1) to receive one subcutaneous injection of fulranumab (9 mg) or placebo in the 4-week DB phase. Seventy-one (72%) patients entered the 48-week open-label extension phase and were administered 9 mg of fulranumab every 4 weeks. The study failed to demonstrated efficacy at the end of the DB phase (primary endpoint, mean [SD] change in average cancer-related pain intensity was -.8 (1.26) for fulranumab and -.7 (1.56) for placebo; P = .592). However, potential benefit is suggested based on secondary endpoints (30% responder rate [P = .020], Brief Pain Inventory-Short Form [BPI-SF] pain intensity subscale [P = .003], and pain interference subscale [P = .006]). The most commonly reported treatment-emergent adverse events were (fulranumab vs placebo): asthenia (16% vs 10%), decreased appetite (12% vs 6%), fatigue (10% vs 0%), and malignant neoplasm progression (10% vs 0%). Although no differences were seen between fulranumab and placebo groups on the primary endpoint, improvements in BPI-SF pain subscale scores and responder rates support further research of anti-nerve growth factor therapy in cancer-related pain. PERSPECTIVE: Efficacy and safety of fulranumab as adjunctive pain therapy in terminally ill cancer patients were assessed. Results suggest that anti-NGF agents may prove to be novel additions in helping to optimize pain relief in cancer patients who fail to respond adequately to opioids and other common co-analgesics.

Functional effects of TrkA inhibition on system xC--mediated glutamate release and cancer-induced bone pain

Breast cancer cells release the signalling molecule glutamate via the system x_C⁻ antiporter, which is upregulated to exchange extracellular cystine for intracellular glutamate to protect against oxidative stress. Here, we demonstrate that this antiporter is functionally influenced by the actions of the neurotrophin nerve growth factor on its cognate receptor tyrosine kinase, TrkA, and that inhibiting this complex may reduce cancer-induced bone pain via its downstream actions on xCT, the functional subunit of system x_C⁻. We have characterized the effects of the selective TrkA inhibitor AG879 on system x_C⁻ activity in murine 4T1 and human MDA-MB-231 mammary carcinoma cells, as well as its effects on nociception in our validated immunocompetent mouse model of cancer-induced bone pain, in which BALB/c mice are intrafemorally inoculated with 4T1 murine carcinoma cells. AG879 decreased functional system x_C⁻ activity, as measured by cystine uptake and glutamate release, and inhibited nociceptive and physiologically relevant responses in tumour-bearing animals. Cumulatively, these data suggest that the activation of TrkA by nerve growth factor may have functional implications on system x_C⁻-mediated cancer pain. System x_C⁻-mediated TrkA activation therefore presents a promising target for therapeutic intervention in cancer pain treatment.

The Changing Sensory and Sympathetic Innervation of the Young, Adult and Aging Mouse Femur

Although bone is continually being remodeled and ultimately declines with aging, little is known whether similar changes occur in the sensory and sympathetic nerve fibers that innervate bone. Here, immunohistochemistry and confocal microscopy were used to examine changes in the sensory and sympathetic nerve fibers that innervate the young (10 days post-partum), adult (3 months) and aging (24 months) C57Bl/6 mouse femur. In all three ages examined, the periosteum was the most densely innervated bone compartment. With aging, the total number of sensory and sympathetic nerve fibers clearly declines as the cambium layer of the periosteum dramatically thins. Yet even in the aging femur, there remains a dense sensory and sympathetic innervation of the periosteum. In cortical bone, sensory and sympathetic nerve fibers are largely confined to vascularized Haversian canals and while there is no significant decline in the density of sensory fibers, there was a 75% reduction in sympathetic nerve fibers in the aging vs. adult cortical bone. In contrast, in the bone marrow the overall density/unit area of both sensory and sympathetic nerve fibers appeared to remain largely unchanged across the lifespan. The preferential preservation of sensory nerve fibers suggests that even as bone itself undergoes a marked decline with age, the nociceptors that detect injury and signal skeletal pain remain relatively intact.

Targeting the S1P/S1PR1 axis mitigates cancer-induced bone pain and neuroinflammation

Metastatic bone pain is the single most common form of cancer pain and persists as a result of peripheral and central inflammatory, as well as neuropathic mechanisms. Here, we provide the first characterization of sphingolipid metabolism alterations in the spinal cord occurring during cancer-induced bone pain (CIBP). Following femoral arthrotomy and syngenic tumor implantation in mice, ceramides decreased with corresponding increases in sphingosine and the bioactive sphingolipid metabolite, sphingosine 1-phosphate (S1P). Intriguingly, de novo sphingolipid biosynthesis was increased as shown by the elevations of dihydro-ceramides and dihydro-S1P. We next identified the S1P receptor subtype 1 (S1PR1) as a novel target for therapeutic intervention. Intrathecal or systemic administration of the competitive and functional S1PR1 antagonists, TASP0277308 and FTY720/Fingolimod, respectively, attenuated cancer-induced spontaneous flinching and guarding. Inhibiting CIBP by systemic delivery of FTY720 did not result in antinociceptive tolerance over 7 days. FTY720 administration enhanced IL-10 in the lumbar ipsilateral spinal cord of CIBP animals and intrathecal injection of an IL-10 neutralizing antibody mitigated the ability of systemic FTY720 to reverse CIBP. FTY720 treatment was not associated with alterations in bone metabolism in vivo. Studies here identify a novel mechanism to inhibit bone cancer pain by blocking the actions of the bioactive metabolites S1P and dihydro-S1P in lumbar spinal cord induced by bone cancer and support potential fast-track clinical application of the FDA-approved drug, FTY720, as a therapeutic avenue for CIBP.

Characterization and comparison of rat monosodium iodoacetate and medial meniscal tear models of osteoarthritic pain

Osteoarthritis (OA) is a degenerative form of arthritis that can result in loss of joint function and chronic pain. The pathological pain state that develops with OA disease involves plastic changes in the peripheral and central nervous systems, however, the cellular mechanisms underlying OA are not fully understood. We characterized the medial meniscal tear (MMT) surgical model and the intra-articular injection of monosodium iodoacetate (MIA) chemical model of OA in rats. Both models produced histological changes in the knee joint and associated bones consistent with OA pathology. Both models also increased p38 activation in the L3, but not L4 dorsal root ganglia (DRG), increased tyrosine hydroxylase immunostaining in the L3 DRG indicating sympathetic sprouting, and increased phosphorylated (p)CREB in thalamic neurons. In MIA-OA, but not MMT-OA rats, p38 and pERK were increased in the spinal cord, and pCREB was enhanced in the prefrontal cortex. Using in vivo electrophysiology, elevated spontaneous activity and increased responsiveness of wide dynamic range neurons to stimulation of the knee was found in both models. However, a more widespread sensitization was observed in the MIA-OA rats as neurons with paw receptive fields spontaneously fired at a greater rate in MIA-OA than MMT-OA rats. Taken together, the MIA and MMT models of OA share several common features associated with histopathology and sensitization of primary somatosensory pathways, but, observed differences between the models highlights unique consequences of the related specific injuries, and these differences should be considered when choosing an OA model and when interpreting data outcomes. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res.

Angiotensin-(1-7)/Mas receptor as an antinociceptive agent in cancer-induced bone pain

Many cancerous solid tumors metastasize to the bone and induce pain (cancer-induced bone pain [CIBP]). Cancer-induced bone pain is often severe because of enhanced inflammation, rapid bone degradation, and disease progression. Opioids are prescribed to manage this pain, but they may enhance bone loss and increase tumor proliferation, further compromising patient quality of life. Angiotensin-(1-7) (Ang-(1-7)) binds and activates the Mas receptor (MasR). Angiotensin-(1-7)/MasR activation modulates inflammatory signaling after acute tissue insult, yet no studies have investigated whether Ang-(1-7)/MasR play a role in CIBP. We hypothesized that Ang-(1-7) inhibits CIBP by targeting MasR in a murine model of breast CIBP. 66.1 breast cancer cells were implanted into the femur of BALB/cAnNHsd mice as a model of CIBP. Spontaneous and evoked pain behaviors were assessed before and after acute and chronic administration of Ang-(1-7). Tissues were collected from animals for ex vivo analyses of MasR expression, tumor burden, and bone integrity. Cancer inoculation increased spontaneous pain behaviors by day 7 that were significantly reduced after a single injection of Ang-(1-7) and after sustained administration. Preadministration of A-779 a selective MasR antagonist prevented this reduction, whereas pretreatment with the AT2 antagonist had no effect; an AT1 antagonist enhanced the antinociceptive activity of Ang-(1-7) in CIBP. Repeated Ang-(1-7) administration did not significantly change tumor burden or bone remodeling. Data here suggest that Ang-(1-7)/MasR activation significantly attenuates CIBP, while lacking many side effects seen with opioids. Thus, Ang-(1-7) may be an alternative therapeutic strategy for the nearly 90% of patients with advanced-stage cancer who experience excruciating pain.

Breast cancer induced nociceptor aberrant growth and collateral sensory axonal branching

The tumour and neuron interaction has a significant impact upon disease progression and the patients quality of life. In breast cancer patients, it is known that there is an interaction between the tumour microenvironment and the sensory neurons to influence the progression of cancer as well as pain, though these mechanisms still need to be clearly defined. Here it is demonstrated that in a rodent orthotopic model of breast cancer (MDA MB 231) there was an increase in nerve fibre innervation into the tumour microenvironment (protein gene product 9.5), which were calcitonin gene related peptide positive C fibre nociceptors. In contrast, there was a reduction in myelinated nerve fibres (NF200). A sensory neuronal cell line was cultured in response to conditioned media from MDA MB231 and MCF7 as well as vascular endothelial growth factor-A (VEGF-A). All these experimental conditions induced sensory neuronal growth, with increased formation of collateral axonal branches. Furthermore, it was demonstrated that MDA MB231 and VEGF-A induced sensory neuronal sensitisation in response to capsaicin a TRPV1 agonist. MDA MB231 induced neuronal growth was suppressed by VEGFR2 inhibition (ZM323881 and neutralising antibody DC101), in addition both MDA MB231 and VEGF-A induced neurite growth was attenuated by the inhibition of ARP2/3 complex through co-treatment with CK666. This demonstrates that breast cancer can interact with the sensory nervous system to drive neuritogenesis through a VEGF-A/VEGFR2/ARP2/3 mediated pathway.

Mice with cancer-induced bone pain show a marked decline in day/night activity

INTRODUCTION

Cancer-induced bone pain (CIBP) is the most common type of pain with cancer. In humans, this pain can be difficult to control and highly disabling. A major problem with CIBP in humans is that it increases on weight-bearing and/or movement of a tumor-bearing bone limiting the activity and functional status of the patient. Currently, there is less data concerning whether similar negative changes in activity occur in rodent models of CIBP.

OBJECTIVES

To determine whether there are marked changes in activity in a rodent model of CIBP and compare this to changes in skin hypersensitivity.

METHODS

Osteosarcoma cells were injected and confined to 1 femur of the adult male mouse. Every 7 days, spontaneous horizontal and vertical activities were assessed over a 20-hour day and night period using automated activity boxes. Mechanical hypersensitivity of the hind paw skin was assessed using von Frey testing.

RESULTS

As the tumor cells grew within the femur, there was a significant decline in horizontal and vertical activity during the times of the day/night when the mice are normally most active. Mice also developed significant hypersensitivity in the skin of the hind paw in the tumor-bearing limb.

CONCLUSION

Even when the tumor is confined to a single load-bearing bone, CIBP drives a significant loss of activity, which increases with disease progression. Understanding the mechanisms that drive this reduction in activity may allow the development of therapies that allow CIBP patients to better maintain their activity and functional status.

Developing drugs in cancer-related bone pain

INTRODUCTION

Cancer-related bone pain is a frequent and important key problem for metastatic patients that may reduce quality of life, with related limitations in daily activities and morbidity. Often traditional approach to pain may fail given the complex pathophysiology of this phenomenon.

METHODS

The aim of this review is to describe promising therapies for cancer-related bone pain, from the pathophysiology to the clinical trials currently ongoing. Moreover, any new evidence for better approach to cancer-related bone pain with the traditional drugs is also considered.

CONCLUSIONS

In clinical practice opioids remain the most important pharmacologic treatment for severe pain related to bone cancer. Regard developing drugs, anti-NGF and anti-TrkA are the most investigated new drug in this setting, but a future role in clinical practice is still uncertain.

How Schwann cells facilitate cancer progression in nerves

Recent studies have demonstrated a critical role for nerves in enabling tumor progression. The association of nerves with cancer cells is well established for a variety of malignant tumors, including pancreatic, prostate and the head and neck cancers. This association is often correlated with poor prognosis. A strong partnership between cancer cells and nerve cells leads to both cancer progression and expansion of the nerve network. This relationship is supported by molecular pathways related to nerve growth and repair. Peripheral nerves form complex tumor microenvironments, which are made of several cell types including Schwann cells. Recent studies have revealed that Schwann cells enable cancer progression by adopting a de-differentiated phenotype, similar to the Schwann cell response to nerve trauma. A detailed understanding of the molecular and cellular mechanisms involved in the regulation of cancer progression by the nerves is essential to design strategies to inhibit tumor progression.

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.

Bone Pain Induced by Multiple Myeloma Is Reduced by Targeting V-ATPase and ASIC3

Multiple myeloma patients experience severe bone pain (MMBP) that is undertreated and poorly understood. In this study, we studied MMBP in an intratibial mouse xenograft model that employs JJN3 human multiple myeloma cells. In this model, mice develop MMBP associated in bone with increased sprouting of calcitonin gene-related peptide-positive (CGRP⁺) sensory nerves and in dorsal root ganglia (DRG) with upregulation of phosphorylated ERK1/2 (pERK1/2) and pCREB, two molecular indicators of neuron excitation. We found that JJN3 cells expressed a vacuolar proton pump (V-ATPase) that induced an acidic bone microenvironment. Inhibition of JJN3-colonized bone acidification by a single injection of the selective V-ATPase inhibitor, bafilomycin A1, decreased MMBP, CGRP⁺ sensory neuron sprouting, and pERK1/2 and pCREB expression in DRG. CGRP⁺ sensory nerves also expressed increased levels of the acid-sensing nociceptor ASIC3. Notably, a single injection of the selective ASIC3 antagonist APETx2 dramatically reduced MMBP in the model. Mechanistic investigations in primary DRG neurons cocultured with JJN3 cells showed increased neurite outgrowth and excitation inhibited by bafilomycin A1 or APETx2. Furthermore, combining APETx2 with bafilomycin A1 reduced MMBP to a greater extent than either agent alone. Finally, combining bafilomycin A1 with the osteoclast inhibitor zoledronic acid was sufficient to ameliorate MMBP, which was refractory to zoledronic acid. Overall, our results show that osteoclasts and multiple myeloma cooperate to induce an acidic bone microenvironment that evokes MMBP as a result of the excitation of ASIC3-activated sensory neurons. Furthermore, they present a mechanistic rationale for targeting ASIC3 on neurons along with the multiple myeloma-induced acidic bone microenvironment as a strategy to relieve MMBP in patients. Cancer Res; 77(6); 1283-95. ©2017 AACR.

Role of extracellular calcitonin gene-related peptide in spinal cord mechanisms of cancer-induced bone pain

Severe pain is a common and debilitating complication of metastatic bone cancer. Current analgesics provide insufficient pain relief and often lead to significant adverse effects. In models of cancer-induced bone pain, pathological sprouting of sensory fibers at the tumor-bone interface occurs concomitantly with reactive astrocytosis in the dorsal horn of the spinal cord. We observed that calcitonin gene-related peptide (CGRP)-fiber sprouting in the bone was associated with an increase in CGRP content in sensory neuron cell bodies in the dorsal root ganglia (DRG) and increased basal and activity-evoked release of CGRP from their central terminals in the dorsal horn. Intrathecal administration of a peptide antagonist (α-CGRP8-37) attenuated referred allodynia in the hind paw ipsilateral to bone cancer. CGRP receptor components (CLR and RAMP1) were up-regulated in dorsal horn neurons and expressed by reactive astrocytes. In primary cultures of astrocytes, CGRP incubation led to a concentration-dependent increase of forskolin-induced cAMP production, which was attenuated by pretreatment with CGRP8-37. Furthermore, CGRP induced ATP release in astrocytes, which was inhibited by CGRP8-37. We suggest that the peripheral increase in CGRP content observed in cancer-induced bone pain is mirrored by a central increase in the extracellular levels of CGRP. This increase in CGRP not only may facilitate glutamate-driven neuronal nociceptive signaling but also act on astrocytic CGRP receptors and lead to release of ATP.

Can Stopping Nerves, Stop Cancer?

The nervous system is viewed as a tissue affected by cancer and as a conduit for the transmission of cancer pain and perineural invasion. Here, we review recent studies that indicate a more direct role. Several studies have shown that reducing stress or suppressing sympathetic drive correlates with improved outcomes and prolonged survival. Recent studies using animal models of visceral and somatic cancer further support a role for the nervous system in cancer progression. Specifically, nerve ablation had a profound impact on disease progression, including delayed development of precancerous lesions, and decreased tumor growth and metastasis. In this review, we summarize new evidence and discuss how future studies may address the role of neural signaling in the modulation of tumorigenesis.

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.

Targeting glia for bone cancer pain

INTRODUCTION

Bone cancer pain (BCP) remains to be a clinical challenge with limited pharmaceutical interventions. Therefore, novel therapeutic targets for the management of BCP are in desperate need. Recently, a growing body of evidence has suggested that glial cells may play a pivotal role in the pathogenesis of BCP. Areas covered: This review summarizes the recent progress in the understanding of glia in BCP and reveals the potential therapeutic targets in glia for BCP treatment. Expert opinion: Pharmacological interventions inhibiting the activation of glial cells, suppressing glia-derived proinflammatory cytokines, cell surface receptors, and the intracellular signaling pathways may be beneficial for the pain management of advanced cancer patients. However, these pharmacological interventions should not disrupt the normal function of glia cells since they play a vital supportive and protective role in the central nervous system.

Anti-nerve growth factor in pain management: current evidence

There continues to be an unmet need for safe and effective pain medications. Opioids and nonsteroidal anti-inflammatory drugs (NSAIDs) dominate the clinical landscape despite limited effectiveness and considerable side-effect profiles. Although significant advancements have identified myriad potential pain targets over the past several decades, the majority of new pain pharmacotherapies have failed to come to market. The discovery of nerve growth factor (NGF) and its interaction with tropomyosin receptor kinase A (trkA) have been well characterized as important mediators of pain initiation and maintenance, and pharmacotherapies targeting this pathway have the potential to be considered promising methods in the treatment of a variety of nociceptive and neuropathic pain conditions. Several methodologic approaches, including sequestration of free NGF, prevention of NGF binding and trkA activation, and inhibition of trkA function, have been investigated in the development of new pharmacotherapies. Among these, NGF-sequestering antibodies have exhibited the most promise in clinical trials. However, in 2010, reports of rapid joint destruction leading to joint replacement prompted the US Food and Drug Administration (FDA) to place a hold on all clinical trials involving anti-NGF antibodies. Although the FDA has since lifted this hold and a number of new trials are under way, the long-term efficacy and safety profile of anti-NGF antibodies are yet to be established.

Nerve growth factor & TrkA as novel therapeutic targets in cancer

In the past 20years, nerve growth factor (NGF) and its receptors TrkA & p75NTR were recognized to be overexpressed in the overwhelming majority of human solid cancers. Recent studies discovered the presence of overactive TrkA signaling due to TrkA rearrangements or TrkA fusion products in frequent cancers like colorectal cancer, thyroid cancer, or acute myeloid leukemia. Thus, targeting TrkA/NGF via selective small-molecule-inhibitors or antibodies has gained enormous attention in the drug discovery sector. Clinical studies on the anti-cancer impact of NGF-blocking antibodies are likely to be accelerated after the recent removal of clinical holds on these agents by regulatory authorities. Based on these current developments, the present review provides not only a broad overview of the biological effects of NGF-TrkA-p75NTR on cancer cells and their microenvironment, but also explains why NGF and its receptors are going to evoke major interest as promising therapeutic anti-cancer targets in the coming decade.

Fracture pain-Traveling unknown pathways

An increase of fracture incidence is expected for the next decades, mostly due to the undeniable increase of osteoporotic fractures, associated with the rapid population ageing. The rise in sports-related fractures affecting the young and active population also contributes to this increased fracture incidence, and further amplifies the economical burden of fractures. Fracture often results in severe pain, which is a primary symptom to be treated, not only to guarantee individual's wellbeing, but also because an efficient management of fracture pain is mandatory to ensure proper bone healing. Here, we review the available data on bone innervation and its response to fracture, and discuss putative mechanisms of fracture pain signaling. In addition, the common therapeutic approaches to treat fracture pain are discussed. Although there is still much to learn, research in fracture pain has allowed an initial insight into the mechanisms involved. During the inflammatory response to fracture, several mediators are released and will putatively activate and sensitize primary sensory neurons, in parallel, intense nerve sprouting that occurs in the fracture callus area is also suggested to be involved in pain signaling. The establishment of hyperalgesia and allodynia after fracture indicates the development of peripheral and central sensitization, still, the underlying mechanisms are largely unknown. A major concern during the treatment of fracture pain needs to be the preservation of proper bone healing. However, the most common therapeutic agents, NSAIDS and opiates, can cause significant side effects that include fracture repair impairment. The understanding of the mechanisms of fracture pain signaling will allow the development of mechanisms-based therapies to effectively and safely manage fracture pain.

The Neurobiology of Cancer Pain

Oral cancers are often severely painful and clinically difficult to manage. Few researchers have investigated the neurobiologic factors responsible for cancer pain; however, the study of oral cancer pain might inform us about the fundamental biology of cancer. The purpose of the present report was to summarize the clinical challenges inherent in oral cancer pain management, oral cancer pain mechanisms and mediators, and the convergence of the investigation of carcinogenesis and pain.

Ablation of sensory neurons in a genetic model of pancreatic ductal adenocarcinoma slows initiation and progression of cancer

Pancreatic ductal adenocarcinoma (PDAC) is characterized by an exuberant inflammatory desmoplastic response. The PDAC microenvironment is complex, containing both pro- and antitumorigenic elements, and remains to be fully characterized. Here, we show that sensory neurons, an under-studied cohort of the pancreas tumor stroma, play a significant role in the initiation and progression of the early stages of PDAC. Using a well-established autochthonous model of PDAC (PKC), we show that inflammation and neuronal damage in the peripheral and central nervous system (CNS) occurs as early as the pancreatic intraepithelial neoplasia (PanIN) 2 stage. Also at the PanIN2 stage, pancreas acinar-derived cells frequently invade along sensory neurons into the spinal cord and migrate caudally to the lower thoracic and upper lumbar regions. Sensory neuron ablation by neonatal capsaicin injection prevented perineural invasion (PNI), astrocyte activation, and neuronal damage, suggesting that sensory neurons convey inflammatory signals from Kras-induced pancreatic neoplasia to the CNS. Neuron ablation in PKC mice also significantly delayed PanIN formation and ultimately prolonged survival compared with vehicle-treated controls (median survival, 7.8 vs. 4.5 mo; P = 0.001). These data establish a reciprocal signaling loop between the pancreas and nervous system, including the CNS, that supports inflammation associated with oncogenic Kras-induced neoplasia. Thus, pancreatic sensory neurons comprise an important stromal cell population that supports the initiation and progression of PDAC and may represent a potential target for prevention in high-risk populations.

Randall Selitto pressure algometry for assessment of bone-related pain in rats

BACKGROUND

Deep pain is neglected compared with cutaneous sources. Pressure algometry has been validated in the clinic for assessment of bone-related pain in humans. In animal models of bone-related pain, we have validated the Randall Selitto behavioural test for assessment of acute and pathological bone pain and compared the outcome with more traditional pain-related behaviour measures.

METHODS

Randall Selitto pressure algometry was performed over the anteromedial part of the tibia in naïve rats, sham-operated rats, and rats inoculated with MRMT-1 carcinoma cells in the left tibia, and the effect of morphine was investigated. Randall Selitto measures of cancer-induced bone pain were supplemented by von Frey testing, weight-bearing and limb use test. Contribution of cutaneous nociception to Randall Selitto measures were examined by local anaesthesia.

RESULTS

Randall Selitto pressure algometry over the tibia resulted in reproducible withdrawal thresholds, which were dose-dependently increased by morphine. Cutaneous nociception did not contribute to Randall Selitto measures. In cancer-bearing animals, compared with sham, significant differences in pain-related behaviours were demonstrated by the Randall Selitto test on day 17 and 21 post-surgery. A difference was also demonstrated by von Frey testing, weight-bearing and limb use tests.

CONCLUSION

Our results indicate that pressure applied by the Randall Selitto algometer on a region, where the bone is close to the skin, may offer a way to measure bone-related pain in animal models and could provide a supplement to the traditional behavioural tests and a means to study deep pain.

What pain tells us about cancer

Cancer pain sends a message. It is frightening to the patient. It heralds progression or recurrence to the oncologist. It is a biological readout of the cancer-nerve interaction for the scientist. Nerves have been considered bystanders within the cancer microenvironment. However, emerging information suggests that nerves are recruited and participate in the carcinogenic process. These newly formed fibers respond to mediators secreted by constituents of the cancer microenvironment. In this manner, these nerves serve as bellwethers and sensors embedded within the cancer. When we rigorously assess patients' cancer pain, we gain insight into the action of cancer. An enhanced understanding of cancer pain offers biological questions that if answered might not only provide relief from cancer pain but might also improve survival.

Semaphorin 3A: A Potential Target for Low Back Pain

Low back pain is a common disorder. Pathological innervation and intervertebral disc degeneration are two major factors associated with this disease. Semaphorin 3A, originally known for its potent inhibiting effect on axonal outgrowth, is recently found to correlate with disease activity and histological features in some skeletal disorders. Based on its effects on innervation and vascularization, as well as enzyme secretion, we presume that semaphorin 3A may act as a potential target for low back 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.

Orthopedic surgery and bone fracture pain are both significantly attenuated by sustained blockade of nerve growth factor

The number of patients suffering from postoperative pain due to orthopedic surgery and bone fracture is projected to dramatically increase because the human life span, weight, and involvement in high-activity sports continue to rise worldwide. Joint replacement or bone fracture frequently results in skeletal pain that needs to be adequately controlled for the patient to fully participate in needed physical rehabilitation. Currently, the 2 major therapies used to control skeletal pain are nonsteroidal anti-inflammatory drugs and opiates, both of which have significant unwanted side effects. To assess the efficacy of novel therapies, mouse models of orthopedic and fracture pain were developed and evaluated here. These models, orthopedic surgery pain and bone fracture pain, resulted in skeletal pain-related behaviors that lasted 3 weeks and 8 to 10 weeks, respectively. These skeletal pain behaviors included spontaneous and palpation-induced nocifensive behaviors, dynamic weight bearing, limb use, and voluntary mechanical loading of the injured hind limb. Administration of anti-nerve growth factor before orthopedic surgery or after bone fracture attenuated skeletal pain behaviors by 40% to 70% depending on the end point being assessed. These data suggest that nerve growth factor is involved in driving pain due to orthopedic surgery or bone fracture. These animal models may be useful in developing an understanding of the mechanisms that drive postoperative orthopedic and bone fracture pain and the development of novel therapies to treat these skeletal pains.

Targeting cells of the myeloid lineage attenuates pain and disease progression in a prostate model of bone cancer

Tumor cells frequently metastasize to bone where they can generate cancer-induced bone pain (CIBP) that can be difficult to fully control using available therapies. Here, we explored whether PLX3397, a high-affinity small molecular antagonist that binds to and inhibits phosphorylation of colony-stimulating factor-1 receptor, the tyrosine-protein kinase c-Kit, and the FMS-like tyrosine kinase 3, can reduce CIBP. These 3 targets all regulate the proliferation and function of a subset of the myeloid cells including macrophages, osteoclasts, and mast cells. Preliminary experiments show that PLX3397 attenuated inflammatory pain after formalin injection into the hind paw of the rat. As there is an inflammatory component in CIBP, involving macrophages and osteoclasts, the effect of PLX3397 was explored in a prostate model of CIBP where skeletal pain, cancer cell proliferation, tumor metastasis, and bone remodeling could be monitored in the same animal. Administration of PLX3397 was initiated on day 14 after prostate cancer cell injection when the tumor was well established, and tumor-induced bone remodeling was first evident. Over the next 6 weeks, sustained administration of PLX3397 attenuated CIBP behaviors by approximately 50% and was equally efficacious in reducing tumor cell growth, formation of new tumor colonies in bone, and pathological tumor-induced bone remodeling. Developing a better understanding of potential effects that analgesic therapies have on the tumor itself may allow the development of therapies that not only better control the pain but also positively impact disease progression and overall survival in patients with bone cancer.

NGF/TrkA Signaling as a Therapeutic Target for Pain

Nerve growth factor (NGF) was first discovered approximately 60 years ago by Rita Levi-Montalcini as a protein that induces the growth of nerves. It is now known that NGF is also associated with Alzheimer's disease and intractable pain, and hence, it, along with its high-affinity receptor, tropomyosin receptor kinase (Trk) A, is considered to be 1 of the new targets for therapies being developed to treat these diseases. Anti-NGF antibody and TrkA inhibitors are known drugs that suppress NGF/TrkA signaling, and many drugs of these classes have been developed thus far. Interestingly, local anesthetics also possess TrkA inhibitory effects. This manuscript describes the development of an analgesic that suppresses NGF/TrkA signaling, which is anticipated to be 1 of the new methods to treat intractable pain.

Use of Animal Models in Understanding Cancer-induced Bone Pain

Many common cancers have a propensity to metastasize to bone. Although malignancies often go undetected in their native tissues, bone metastases produce excruciating pain that severely compromises patient quality of life. Cancer-induced bone pain (CIBP) is poorly managed with existing medications, and its multifaceted etiology remains to be fully elucidated. Novel analgesic targets arise as more is learned about this complex and distinct pain state. Over the past two decades, multiple animal models have been developed to study CIBP’s unique pathology and identify therapeutic targets. Here, we review animal models of CIBP and the mechanistic insights gained as these models evolve. Findings from immunocompromised and immunocompetent host systems are discussed separately to highlight the effect of model choice on outcome. Gaining an understanding of the unique neuromolecular profile of cancer pain through the use of appropriate animal models will aid in the development of more effective therapeutics for CIBP.

Neural regulation of hematopoiesis, inflammation, and cancer

Although the function of the autonomic nervous system (ANS) in mediating the flight-or-fight response was recognized decades ago, the crucial role of peripheral innervation in regulating cell behavior and response to the microenvironment has only recently emerged. In the hematopoietic system, the ANS regulates stem cell niche homeostasis and regeneration and fine-tunes the inflammatory response. Additionally, emerging data suggest that cancer cells take advantage of innervating neural circuitry to promote their progression. These new discoveries outline the need to redesign therapeutic strategies to target this underappreciated stromal constituent. Here, we review the importance of neural signaling in hematopoietic homeostasis, inflammation, and cancer.

Acidic microenvironment and bone pain in cancer-colonized bone

Solid cancers and hematologic cancers frequently colonize bone and induce skeletal-related complications. Bone pain is one of the most common complications associated with cancer colonization in bone and a major cause of increased morbidity and diminished quality of life, leading to poor survival in cancer patients. Although the mechanisms responsible for cancer-associated bone pain (CABP) are poorly understood, it is likely that complex interactions among cancer cells, bone cells and peripheral nerve cells contribute to the pathophysiology of CABP. Clinical observations that specific inhibitors of osteoclasts reduce CABP indicate a critical role of osteoclasts. Osteoclasts are proton-secreting cells and acidify extracellular bone microenvironment. Cancer cell-colonized bone also releases proton/lactate to avoid intracellular acidification resulting from increased aerobic glycolysis known as the Warburg effect. Thus, extracellular microenvironment of cancer-colonized bone is acidic. Acidosis is algogenic for nociceptive sensory neurons. The bone is densely innervated by the sensory neurons that express acid-sensing nociceptors. Collectively, CABP is evoked by the activation of these nociceptors on the sensory neurons innervating bone by the acidic extracellular microenvironment created by bone-resorbing osteoclasts and bone-colonizing cancer cells. As current treatments do not satisfactorily control CABP and can elicit serious side effects, new therapeutic interventions are needed to manage CABP. Understanding of the cellular and molecular mechanism by which the acidic extracellular microenvironment is created in cancer-colonized bone and by which the expression and function of the acid-sensing nociceptors on the sensory neurons are regulated would facilitate to develop novel therapeutic approaches for the management of CABP.

Role of the autonomic nervous system in tumorigenesis and metastasis

Convergence of multiple stromal cell types is required to develop a tumorigenic niche that nurtures the initial development of cancer and its dissemination. Although the immune and vascular systems have been shown to have strong influences on cancer, a growing body of evidence points to a role of the nervous system in promoting cancer development. This review discusses past and current research that shows the intriguing role of autonomic nerves, aided by neurotrophic growth factors and axon cues, in creating a favorable environment for the promotion of tumor formation and metastasis.

Contribution of acidic extracellular microenvironment of cancer-colonized bone to bone pain

Solid and hematologic cancer colonized bone produces a number of pathologies. One of the most common complications is bone pain. Cancer-associated bone pain (CABP) is a major cause of increased morbidity and diminishes the quality of life and affects survival. Current treatments do not satisfactorily control CABP and can elicit adverse effects. Thus, new therapeutic interventions are needed to manage CABP. However, the mechanisms responsible for CABP are poorly understood. The observation that specific osteoclast inhibitors can reduce CABP in patients indicates a critical role of osteoclasts in the pathophysiology of CABP. Osteoclasts create an acidic extracellular microenvironment by secretion of protons via vacuolar proton pumps during bone resorption. In addition, bone-colonized cancer cells also release protons and lactate via plasma membrane pH regulators to avoid intracellular acidification resulting from increased aerobic glycolysis known as the Warburg effect. Since acidosis is algogenic for sensory neurons and bone is densely innervated by sensory neurons that express acid-sensing nociceptors, the acidic bone microenvironments can evoke CABP. Understanding of the mechanism by which the acidic extracellular microenvironment is created in cancer-colonized bone and the expression and function of the acid-sensing nociceptors are regulated should facilitate the development of novel approaches for management of CABP. Here, the contribution of the acidic microenvironment created in cancer-colonized bone to elicitation of CABP and potential therapeutic implications of blocking the development and recognition of acidic microenvironment will be described. This article is part of a Special Issue entitled: Membrane channels and transporters in cancers.

CXCR3: latest evidence for the involvement of chemokine signaling in bone cancer pain

Growing evidence indicates that chemokines participate in the generation and maintenance of bone cancer pain (BCP). Recent work in Exp Neurol by Guan et al. (2015) demonstrated the involvement of spinal chemokine receptor CXCR3 and its downstream PI3K/Akt and Raf/MEK/ERK signaling pathways in BCP. This work provides new evidence to support that chemokines participate in central sensitization in BCP condition. Reviewed evidence suggests that few chemokines have been proved to be related to cancer pain. The underlying relationship between CXCR3 signaling and BCP condition requires further study.

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.

Expression of nociceptive ligands in canine osteosarcoma

Background

Canine osteosarcoma (OS) is associated with localized pain as a result of tissue injury from tumor infiltration and peritumoral inflammation. Malignant bone pain is caused by stimulation of peripheral pain receptors, termed nociceptors, which reside in the localized tumor microenvironment, including the periosteal and intramedullary bone cavities. Several nociceptive ligands have been determined to participate directly or indirectly in generating bone pain associated with diverse skeletal abnormalities.

Hypothesis

Canine OS cells actively produce nociceptive ligands with the capacity to directly or indirectly activate peripheral pain receptors residing in the bone tumor microenvironment.

Animals

Ten dogs with appendicular OS.

Methods

Expression of nerve growth factor, endothelin‐1, and microsomal prostaglandin E synthase‐1 was characterized in OS cell lines and naturally occurring OS samples. In 10 dogs with OS, circulating concentrations of nociceptive ligands were quantified and correlated with subjective pain scores and tumor volume in patients treated with standardized palliative therapies.

Results

Canine OS cells express and secrete nerve growth factor, endothelin‐1, and prostaglandin E2. Naturally occurring OS samples uniformly express nociceptive ligands. In a subset of OS‐bearing dogs, circulating nociceptive ligand concentrations were detectable but failed to correlate with pain status. Localized foci of nerve terminal proliferation were identified in a minority of primary bone tumor samples.

Conclusions and Clinical Importance

Canine OS cells express nociceptive ligands, potentially permitting active participation of OS cells in the generation of malignant bone pain. Specific inhibitors of nociceptive ligand signaling pathways might improve pain control in dogs with OS.

Exuberant sprouting of sensory and sympathetic nerve fibers in nonhealed bone fractures and the generation and maintenance of chronic skeletal pain

Skeletal injury is a leading cause of chronic pain and long-term disability worldwide. While most acute skeletal pain can be effectively managed with nonsteroidal anti-inflammatory drugs and opiates, chronic skeletal pain is more difficult to control using these same therapy regimens. One possibility as to why chronic skeletal pain is more difficult to manage over time is that there may be nerve sprouting in nonhealed areas of the skeleton that normally receive little (mineralized bone) to no (articular cartilage) innervation. If such ectopic sprouting did occur, it could result in normally nonnoxious loading of the skeleton being perceived as noxious and/or the generation of a neuropathic pain state. To explore this possibility, a mouse model of skeletal pain was generated by inducing a closed fracture of the femur. Examined animals had comminuted fractures and did not fully heal even at 90+days post fracture. In all mice with nonhealed fractures, exuberant sensory and sympathetic nerve sprouting, an increase in the density of nerve fibers, and the formation of neuroma-like structures near the fracture site were observed. Additionally, all of these animals exhibited significant pain behaviors upon palpation of the nonhealed fracture site. In contrast, sprouting of sensory and sympathetic nerve fibers or significant palpation-induced pain behaviors was never observed in naïve animals. Understanding what drives this ectopic nerve sprouting and the role it plays in skeletal pain may allow a better understanding and treatment of this currently difficult-to-control pain state.

Increase of TRPV1-immunoreactivity in dorsal root ganglia neurons innervating the femur in a rat model of osteoporosis

PURPOSE

Transient receptor potential vanilloid 1 (TRPV1) is a ligand-gated nonselective cation channel, which can be activated by capsaicin and other noxious stimuli. Recently, an association between bone pain and TRPV1 has been reported. However, the influence of osteoporosis on TRPV1 in the sensory system innervating the femur has not been reported.

MATERIALS AND METHODS

TRPV1-immunoreactive (ir) in dorsal root ganglia (DRG) neurons labeled with neurotracer [Fluoro-Gold (FG)] innervating the femurs of Sprague Dawley rats were examined in control, sham, and ovariectomized (OVX) rats. We evaluated osteoporosis in the femurs and compared the proportion of TRPV1-ir DRG neurons innervating femur between the 3 groups of rats.

RESULTS

OVX rats showed osteoporotic cancellous bone in the femur. FG labeled neurons were distributed from L1 to L6 DRG, but there was no significant difference in the proportion of labeled neurons between the 3 groups (p>0.05). The proportions of FG labeled TRPV1-ir DRG neurons were 1.7%, 1.7%, and 2.8% of DRG neurons innervating the femur, in control, sham-operated, and OVX rats, respectively. The proportion of TRPV1-ir neurons in DRG innervating the femur in OVX rats was significantly higher than that in control and sham-operated rats (p<0.05).

CONCLUSION

Under physiological conditions, DRG neurons innervating femurs in rats contain TRPV1. Osteoporosis increases the numbers of TRPV1-ir neurons in DRG innervating osteoporotic femurs in rats. These findings suggest that TRPV1 may have a role in sensory perception of osteoporotic femurs.

NGF blockade at early times during bone cancer development attenuates bone destruction and increases limb use

Studies in animals and humans show that blockade of nerve growth factor (NGF) attenuates both malignant and nonmalignant skeletal pain. While reduction of pain is important, a largely unanswered question is what other benefits NGF blockade might confer in patients with bone cancer. Using a mouse graft model of bone sarcoma, we demonstrate that early treatment with an NGF antibody reduced tumor-induced bone destruction, delayed time to bone fracture, and increased the use of the tumor-bearing limb. Consistent with animal studies in osteoarthritis and head and neck cancer, early blockade of NGF reduced weight loss in mice with bone sarcoma. In terms of the extent and time course of pain relief, NGF blockade also reduced pain 40% to 70%, depending on the metric assessed. Importantly, this analgesic effect was maintained even in animals with late-stage disease. Our results suggest that NGF blockade immediately upon detection of tumor metastasis to bone may help preserve the integrity and use, delay the time to tumor-induced bone fracture, and maintain body weight.

Pro-neurotrophins, sortilin, and nociception

Nerve growth factor (NGF) signaling is important in the development and functional maintenance of nociceptors, but it also plays a central role in initiating and sustaining heat and mechanical hyperalgesia following inflammation. NGF signaling in pain has traditionally been thought of as primarily engaging the classic high-affinity receptor tyrosine kinase receptor TrkA to initiate sensitization events. However, the discovery that secreted proforms of nerve NGF have biological functions distinct from the processed mature factors raised the possibility that these proneurotrophins (proNTs) may have distinct function in painful conditions. ProNTs engage a novel receptor system that is distinct from that of mature neurotrophins, consisting of sortilin, a type I membrane protein belonging to the VPS10p family, and its co-receptor, the classic low-affinity neurotrophin receptor p75NTR. Here, we review how this new receptor system may itself function with or independently of the classic TrkA system in regulating inflammatory or neuropathic pain.