Intravenous paclitaxel administration in the rat induces a peripheral sensory neuropathy characterized by macrophage infiltration and injury to sensory neurons and their supporting cells

Monocytes expressing CX3CR1 orchestrate the development of vincristine-induced pain

A major dose-limiting side effect associated with cancer-treating antineoplastic drugs is the development of neuropathic pain, which is not readily relieved by available analgesics. A better understanding of the mechanisms that underlie pain generation has potential to provide targets for prophylactic management of chemotherapy pain. Here, we delineate a pathway for pain that is induced by the chemotherapeutic drug vincristine sulfate (VCR). In a murine model of chemotherapy-induced allodynia, VCR treatment induced upregulation of endothelial cell adhesion properties, resulting in the infiltration of circulating CX3CR1⁺ monocytes into the sciatic nerve. At the endothelial-nerve interface, CX3CR1⁺ monocytes were activated by the chemokine CX3CL1 (also known as fractalkine [FKN]), which promoted production of reactive oxygen species that in turn activated the receptor TRPA1 in sensory neurons and evoked the pain response. Furthermore, mice lacking CX3CR1 exhibited a delay in the development of allodynia following VCR administration. Together, our data suggest that CX3CR1 antagonists and inhibition of FKN proteolytic shedding, possibly by targeting ADAM10/17 and/or cathepsin S, have potential as peripheral approaches for the prophylactic treatment of chemotherapy-induced pain.

Chemotherapy-induced peripheral neuropathy in adults: a comprehensive update of the literature

Commonly used chemotherapeutic agents in oncology/hematology practice, causing toxic peripheral neuropathy, include taxanes, platinum compounds, vinca alkaloids, proteasome inhibitors, and antiangiogenic/immunomodulatory agents. This review paper intends to put together and discuss the spectrum of chemotherapy-induced peripheral neuropathy (CIPN) characteristics so as to highlight areas of future research to pursue on the topic. Current knowledge shows that the pathogenesis of CIPN still remains elusive, mostly because there are several sites of involvement in the peripheral nervous system. In any case, it is acknowledged that the dorsal root ganglia of the primary sensory neurons are the most common neural targets of CIPN. Both the incidence and severity of CIPN are clinically under- and misreported, and it has been demonstrated that scoring CIPN with common toxicity scales is associated with significant inter-observer variability. Only a proportion of chemotherapy-treated patients develop treatment-emergent and persistent CIPN, and to date it has been impossible to predict high-and low-risk subjects even within groups who receive the same drug regimen. This issue has recently been investigated in the context of pharmacogenetic analyses, but these studies have not implemented a proper methodological approach and their results are inconsistent and not really clinically relevant. As such, a stringent approach has to be implemented to validate that information. Another open issue is that, at present, there is insufficient evidence to support the use of any of the already tested chemoprotective agents to prevent or limit CIPN. The results of comprehensive interventions, including clinical, neurophysiological, and pharmacogenetic approaches, are expected to produce a consistent advantage for both doctors and patients and thus allow the registration and analysis of reliable data on the true characteristics of CIPN, eventually leading to potential preventive and therapeutic interventions.

Current methods for the assessment and management of taxane-related neuropathy

Taxane-induced peripheral neuropathy (TIPN) affects a number of patients with breast cancer. To properly manage these patients, nurses must be able to identify and assess TIPN, as well as educate patients on TIPN as a side effect of taxane therapy. This article provides practical suggestions regarding how nurses can incorporate clinically feasible measurement approaches into practice and includes examples of grading TIPN that illustrate the limitations of the current tools and techniques for assessment. For example, a shortened and revised version of the Total Neuropathy Score and the Functional Assessment of Cancer Therapy/Gynecologic Oncology Group-Neurotoxicity subscale should be considered for future use. In addition, neuropathy-related results from numerous phase III trials in breast cancer are discussed, and the latest evidence regarding pharmacologic interventions for TIPN is briefly summarized.

Tumor necrosis factor-α-dependent infiltration of macrophages into the dorsal root ganglion in a rat disc herniation model

STUDY DESIGN

A prospective molecular mechanism of macrophages infiltration in experimental disc herniation.

OBJECTIVE

To investigate the mechanisms of macrophages infiltration into the dorsal root ganglion (DRG) in a rat model of disc herniation.

SUMMARY OF BACKGROUND DATA

Macrophages infiltrate the DRG after application of nucleus pulposus (NP) on the DRG, and may play an important role in radiculopathy. However, the mechanisms of macrophages infiltration after NP application remain poorly understood.

METHODS

After experimental disc herniation in this study, we investigated changes in the expression of ED1 (a marker of macrophages) and vascular cell adhesion molecule-1 (VCAM-1) in DRG using immunofluorescence. We also investigated the expression of ED1 and VCAM-1 in DRG by treatment with tumor necrosis factor-α (TNF-α) inhibitor at the time of surgery.

RESULTS

We found a massive ED1-positive macrophages infiltrated the DRG, and VCAM-1-like immunoreactivity vessels became evident after NP application. Furthermore, both macrophage infiltration and VCAM-1 expression were prevented by treatment with TNF-α inhibitor at the time of surgery.

CONCLUSION

These findings indicated that macrophages infiltration into the DRG was TNF-α-dependent, and might be partly mediated by VCAM-1 in the early stage of experimental lumbar disc herination. Taken together, this study provides important preliminary data suggesting that TNF-α plays an important role in the macrophage infiltration.

LEVEL OF EVIDENCE

N/A.

Morphologic features and glial activation in rat oxaliplatin-dependent neuropathic pain

Neurotoxicity is the limiting side effect of the anticancer agent oxaliplatin. A tangled panel of symptoms, sensory loss, paresthesia, dysesthesia, and pain may be disabling for patients and adversely affect their quality of life. To elucidate the morphologic and molecular alterations that occur in the nervous system during neuropathy, rats were daily injected with 2.4 mg kg(-1) oxaliplatin intraperitoneally. A progressive decrease in the pain threshold and hypersensitivity to noxious and nonnoxious stimuli were evidenced during the treatment (7, 14, 21 days). On day 21, morphometric alterations were detectable exclusively in the dorsal root ganglia, whereas the activating transcription factor 3 and neurofilament (heavy-chain) expression changed dramatically in both the nerves and ganglia. Inflammatory features were not highlighted. Interestingly, satellite cells exhibited signs of activation. Glial modulation was characterized in the spinal cord and brain areas involved in pain signaling. On the 21st day, spinal astrocytes increased numerically whereas the microglial population was unaltered. The number of glial cells in the brain differed according to the zone and treatment time points. In particular, on day 21, a significant astrocyte increase was measured in the anterior cingulate cortex, somatosensory area 1, neostriatum, ventrolateral periaqueductal gray, and nucleus raphe magnus.

PERSPECTIVES

These data highlight the relevance of glial cells in chemotherapy-induced neurotoxicity as part of the investigation of the role that specific brain areas play in neuropathy.

Satellite glial cell proliferation in the trigeminal ganglia after chronic constriction injury of the infraorbital nerve

We have examined satellite glial cell (SGC) proliferation in trigeminal ganglia following chronic constriction injury of the infraorbital nerve. Using BrdU labeling combined with immunohistochemistry for SGC specific proteins we positively confirmed proliferating cells to be SGCs. Proliferation peaks at approximately 4 days after injury and dividing SGCs are preferentially located around neurons that are immunopositive for ATF-3, a marker of nerve injury. After nerve injury there is an increase GFAP expression in SGCs associated with both ATF-3 immunopositive and immunonegative neurons throughout the ganglia. SGCs also express the non-glial proteins, CD45 and CD163, which label resident macrophages and circulating leukocytes, respectively. In addition to SGCs, we found some Schwann cells, endothelial cells, resident macrophages, and circulating leukocytes were BrdU immunopositive.

Local and remote immune-mediated inflammation after mild peripheral nerve compression in rats

After experimental nerve injuries that extensively disrupt axons, such as chronic constriction injury, immune cells invade the nerve, related dorsal root ganglia (DRGs), and spinal cord, leading to hyperexcitability, raised sensitivity, and pain. Entrapment neuropathies, such as carpal tunnel syndrome, involve minimal axon damage, but patients often report widespread symptoms. To understand the underlying pathology, a tube was placed around the sciatic nerve in 8-week-old rats, leading to progressive mild compression as the animals grew. Immunofluorescence was used to examine myelin and axonal integrity, glia, macrophages, and T lymphocytes in the nerve, L5 DRGs, and spinal cord after 12 weeks. Tubes that did not constrict the nerve when applied caused extensive and ongoing loss of myelin, together with compromise of small-, but not large-, diameter axons. Macrophages and T lymphocytes infiltrated the nerve and DRGs. Activated glia proliferated in DRGs but not in spinal cord. Histologic findings were supported by clinical hyperalgesia to blunt pressure and cold allodynia. Tubes that did not compress the nerve induced only minor local inflammation. Thus, progressive mild nerve compression resulted in chronic local and remote immune-mediated inflammation depending on the degree of compression. Such neuroinflammation may explain the widespread symptoms in patients with entrapment neuropathies.

Neuroinflammation and the generation of neuropathic pain

Inflammation is the process by which an organism responds to tissue injury involving both immune cell recruitment and mediator release. Diverse causes of neuropathic pain are associated with excessive inflammation in both the peripheral and central nervous system which may contribute to the initiation and maintenance of persistent pain. Chemical mediators, such as cytokines, chemokines, and lipid mediators, released during an inflammatory response have the undesired effect of sensitizing and stimulating nociceptors, their central synaptic targets or both. These changes can promote long-term maladaptive plasticity resulting in persistent neuropathic pain. This review aims to provide an overview of inflammatory mechanisms at differing levels of the sensory neuroaxis with a focus on neuropathic pain. We will compare and contrast neuropathic pain states such as traumatic nerve injury which is associated with a vigorous inflammatory response and chemotherapy induced pain in which the inflammatory response is much more modest. Targeting excessive inflammation in neuropathic pain provides potential therapeutic opportunities and we will discuss some of the opportunities but also the clinical challenges in such an approach.

Tubulin: an example of targeted chemotherapy

Anticancer drugs directed against the microtubule, including taxanes and vinca alkaloids, have been the backbone of many chemotherapy regimes for decades. These drugs have, however, significant limitations, which have prompted the development of novel microtubule targeting agents (MTAs). This article will discuss MTAs for anticancer therapies and recent debates regarding their mechanisms of action. Furthermore, the limitations of taxanes, including hypersensitivity reactions, neurotoxicity, drug resistance and lack of validated biomarkers to guide therapy will be discussed, all of which have driven the development of novel agents. The mechanisms of action and drug development of new generations of MTAs will also be outlined. Agents demonstrating utility in Phase III clinical trials, including eribulin, ixabepilone, cabazitaxel and trastuzumab-DM1 will be highlighted, as well as novel agents currently in development and future directions for MTAs.

Breast cancer survivorship issues

Survivors of breast cancer are confronted with a plethora of cancer treatment-related long-term symptoms, the most common being fatigue, hot flashes, sexual dysfunction, arthralgias, neuropathy, and cognitive dysfunction. Survivors of breast cancer also face cancer treatment-related disease states, such as osteoporosis, cardiac dysfunction, obesity, infertility, and secondary cancers. Evidence-based recommendations for screening, prevention, and early intervention should be implemented to improve quality of life and decrease comorbidities in this population.

Bioenergetic deficits in peripheral nerve sensory axons during chemotherapy-induced neuropathic pain resulting from peroxynitrite-mediated post-translational nitration of mitochondrial superoxide dismutase

Many of the widely used anticancer drugs induce dose-limiting peripheral neuropathies that undermine their therapeutic efficacy. Animal models of chemotherapy-induced painful peripheral neuropathy (CIPN) evoked by a variety of drug classes, including taxanes, vinca alkaloids, platinum-complexes, and proteasome-inhibitors, suggest that the common underlying mechanism in the development of these neuropathies is mitotoxicity in primary nerve sensory axons (PNSAs) arising from reduced mitochondrial bioenergetics [eg adenosine triphosphate (ATP) production deficits due to compromised respiratory complex I and II activity]. The causative mechanisms of this mitotoxicity remain poorly defined. However, peroxynitrite, an important pro-nociceptive agent, has been linked to mitotoxicity in several disease states and may also drive the mitotoxicity associated with CIPN. Our findings reveal that the development of mechano-hypersensitivity induced by paclitaxel, oxaliplatin, and bortezomib was prevented by administration of the peroxynitrite decomposition catalyst Mn(III) 5,10,15,20-tetrakis(N-n-hexylpyridinium-2-yl)porphyrin (MnTE-2-PyP(5+)) without interfering with their anti-tumor effects. Peak CIPN was associated with the nitration and inactivation of superoxide dismutase in the mitochondria, but not in the cytosol, as well as a significant decrease in ATP production within the PNSAs; all of these events were attenuated by MnTE-2-PyP(5+). Our results provide continued support for the role of mitotoxicity in the development of CIPN across chemotherapeutic drug classes, and identify peroxynitrite as a key mediator in these processes, thereby providing the rationale towards development of "peroxynitrite-targeted" therapeutics for CIPN.

Spinal CCL2 and microglial activation are involved in paclitaxel-evoked cold hyperalgesia

The antineoplastic paclitaxel induces a sensory neuropathy that involves the spinal release of neuroinflammatory mediators and activation of glial cells. Although the chemokine CCL2 can evoke glial activation and its participation in neuropathic pain has been demonstrated in other models, its involvement in paclitaxel-evoked neuropathy has not been previously explored. Paclitaxel-evoked cold hypernociception was assessed in mice by the unilateral cold plate test and the effects on cold hyperalgesia of the CCR2 antagonist RS 504393, the CCR1 antagonist J113863, the microglial inhibitor minocycline or an anti-CCL2 antibody were tested. Furthermore, ELISA measurements of CCL2 concentration and immunohistochemical assays of Iba-1 and GFAP, markers of microglial and astroglial cells respectively, were performed in the lumbar spinal cord. Cold hypernociception measured 3 days after the administration of paclitaxel (10mg/kg) was inhibited by the s.c. (0.3-3mg/kg) or i.t. (1-10 μg) administration of RS 504393 but not of J113863 (3-30 mg/kg). CCL2 levels measured by ELISA in the lumbar spinal cord were augmented in mice treated with paclitaxel and the i.t. administration of an anti-CCL2 antibody completely suppressed paclitaxel-evoked cold hyperalgesia, strongly suggesting that CCL2 is involved in the hypernociception evoked by this taxane. Besides, the implication of microglial activation is supported by the increase in the immunolabelling of Iba-1, but not GFAP, in the spinal cord of paclitaxel-treated mice and by the inhibition of cold hyperalgesia produced by the i.t. administration of the microglial inhibitor minocycline (1-10 nmol). Finally, the neutralization of spinal CCL2 by the i.t. administration of a selective antibody for 3 days almost totally inhibited paclitaxel-evoked microglial activation. In conclusion, our results indicate that paclitaxel-evoked cold hypernociception depends on the activation of CCR2 due to the spinal release of CCL2 and the subsequent microglial activation.

The role of TLR4 in the paclitaxel effects on neuronal growth in vitro

Paclitaxel (Pac) is an antitumor agent that is widely used for treatment of solid cancers. While being effective as a chemotherapeutic agent, Pac in high doses is neurotoxic, specifically targeting sensory innervations. In view of these toxic effects associated with conventional chemotherapy, decreasing the dose of Pac has been recently suggested as an alternative approach, which might limit neurotoxicity and immunosuppression. However, it remains unclear if low doses of Pac retain its neurotoxic properties or might exhibit unusual effects on neuronal cells. The goal of this study was to analyze the concentration-dependent effect of Pac on isolated and cultured DRG neuronal cells from wild-type and TLR4 knockout mice. Three different morphological parameters were analyzed: the number of neurons which developed neurites, the number of neurites per cell and the total length of neurites per cell. Our data demonstrate that low concentrations of Pac (0.1 nM and 0.5 nM) do not influence the neuronal growth in cultures in both wild type and TLR4 knockout mice. Higher concentrations of Pac (1–100 nM) had a significant effect on DRG neurons from wild type mice, affecting the number of neurons which developed neurites, number of neurites per cell, and the length of neurites. In DRG from TLR4 knockout mice high concentrations of Pac showed a similar effect on the number of neurons which developed neurites and the length of neurites. At the same time, the number of neurites per cell, indicating the process of growth cone initiation, was not affected by high concentrations of Pac. Thus, our data showed that Pac in high concentrations has a significant damaging effect on axonal growth and that this effect is partially mediated through TLR4 pathways. Low doses of Pac are devoid of neuronal toxicity and thus can be safely used in a chemomodulation mode.

A clinically relevant rodent model of the HIV antiretroviral drug stavudine induced painful peripheral neuropathy

HIV-associated sensory neuropathy is the most frequent manifestation of HIV disease, afflicting 40-50% of patients whose HIV disease is otherwise controlled by antiretroviral therapy. It often presents with significant neuropathic pain and is consistently associated with previous exposure to nucleoside reverse transcriptase inhibitors including stavudine (d4T), which is widely used in resource-limited settings. Here we investigated complex pain-related behaviours associated with d4T treatment using ethologically relevant thigmotaxis and burrowing behaviours in adult rats. Detailed neuropathological response was also examined using neurochemistry, electron microscopy, and proteomics. After 2 intravenous injections of d4T (50 mg/kg, 4 days apart), rats developed hind paw mechanical hypersensitivity, which plateaued at 21 days after initial d4T injection, a time that these animals also had significant changes in thigmotaxis and burrowing behaviours when compared to the controls; reductions in hind paw intraepidermal nerve fibre density and CGRP/IB4 immunoreactivity in L5 spinal dorsal horn, suggesting injury to both the peripheral and central terminals of L5 dorsal root ganglion neurons; and increases in myelinated and unmyelinated axon diameters in the sural nerve, suggesting axonal swelling. However, no significant glial and inflammatory cell response to d4T treatment was observed. Sural nerve proteomics at 7 days after initial d4T injection revealed down-regulated proteins associated with mitochondrial function, highlighting distal axons vulnerability to d4T neurotoxicity. In summary, we have reported complex behavioural changes and a distinctive neuropathology in a clinically relevant rat model of d4T-induced sensory neuropathy that is suitable for further pathophysiological investigation and preclinical evaluation of novel analgesics.

Sodium channel Na(v)1.7 is essential for lowering heat pain threshold after burn injury

Marked hypersensitivity to heat and mechanical (pressure) stimuli develop after a burn injury, but the neural mechanisms underlying these effects are poorly understood. In this study, we establish a new mouse model of focal second-degree burn injury to investigate the molecular and cellular basis for burn injury-induced pain. This model features robust injury-induced behavioral effects and tissue-specific altered cytokine profile, but absence of glial activation in spinal dorsal horn. Three voltage-gated sodium channels, Na(v)1.7, Na(v)1.8, and Na(v)1.9, are preferentially expressed in peripheral somatosensory neurons of the dorsal root ganglia (DRGs) and have been implicated in injury-induced neuronal hyperexcitability. Using knock-out mice, we provide evidence that Na(v)1.7 selectively contributes to burn-induced hypersensitivity to heat, but not mechanical, stimuli. After burn model injury, wild-type mice display increased sensitivity to heat stimuli, and a normally non-noxious warm stimulus induces activity-dependent Fos expression in spinal dorsal horn neurons. Strikingly, both effects are absent in Na(v)1.7 conditional knock-out (cKO) mice. Furthermore, burn injury increases density and shifts activation of tetrodotoxin-sensitive currents in a hyperpolarized direction, both pro-excitatory properties, in DRG neurons from wild-type but not Na(v)1.7 cKO mice. We propose that, in sensory neurons damaged by burn injury to the hindpaw, Na(v)1.7 currents contribute to the hyperexcitability of sensory neurons, their communication with postsynaptic spinal pain pathways, and behavioral thresholds to heat stimuli. Our results offer insights into the molecular and cellular mechanisms of modality-specific pain signaling, and suggest Na(v)1.7-blocking drugs may be effective in burn patients.

The contribution of satellite glial cells to chemotherapy-induced neuropathic pain

BACKGROUND

Chemotherapy-induced peripheral neuropathy is a serious side effect in cancer treatment, a major manifestation being neuropathic pain that can be debilitating and can reduce the quality of life of the patient. Oxaliplatin and taxol are common anti-cancer drugs that induce neuropathic pain by an unknown mechanism. We tested the hypothesis that satellite glial cells in dorsal root ganglia (DRGs) are altered in chemotherapy-induced peripheral neuropathy models and contribute to neuropathic pain.

METHODS

Mice were injected with either oxaliplatin or taxol and examined at 7-30 days. Glial fibrillary acidic protein (glial activation marker) expression was determined by immunohistochemistry. Satellite glial cells in isolated DRG were injected with the fluorescent dye Lucifer yellow and the incidence of dye coupling among these cells that surround different neurons was quantified.

RESULTS

Taxol or oxaliplatin increased glial fibrillary acidic protein expression in satellite glial cells. Gap junction-mediated coupling between satellite glial cells was increased by up to fivefold after oxaliplatin and by up to twofold after taxol. This is consistent with work on other pain models showing that augmented satellite glial cell coupling contributes to chronic pain. Administration of the gap junction blocker carbenoxolone to chemotherapy-treated mice produced an analgesic-like effect.

CONCLUSIONS

We propose that increased coupling by gap junctions is part of satellite glial cell activation, and that augmented coupling contributes to the lowering of pain threshold in oxaliplatin- and taxol-treated mice. We further propose that gap junction blockers may have potential in treating chemotherapy-induced neuropathic pain.

The role of the immune system in the generation of neuropathic pain

Persistent pain is a sequela of several neurological conditions with a primary immune basis, such as Guillain-Barré syndrome and multiple sclerosis. Additionally, diverse forms of injury to the peripheral or the central nervous systems--whether traumatic, metabolic, or toxic--result in substantial recruitment and activation of immune cells. This response involves the innate immune system, but evidence also exists of T-lymphocyte recruitment, and in some patient cohorts antibodies to neuronal antigens have been reported. Mediators released by immune cells, such as cytokines, sensitise nociceptive signalling in the peripheral and central nervous systems. Preclinical data suggest an immune pathogenesis of neuropathic pain, but clinical evidence of a central role of the immune system is less clear. An important challenge for the future is to establish to what extent this immune response initiates or maintains neuropathic pain in patients and thus whether it is amenable to therapy.

Effect of synthetic eel calcitonin, elcatonin, on cold and mechanical allodynia induced by oxaliplatin and paclitaxel in rats

Oxaliplatin and paclitaxel are commonly used anti-cancer drugs, but they frequently cause peripheral neuropathic pain. In this study, we investigated the effect of elcatonin, a synthetic eel calcitonin, on oxaliplatin- and paclitaxel-induced neuropathy in rats. The rats were treated with a single dose of oxaliplatin (6 mg/kg, i.p.) or repeated doses of paclitaxel (2 mg/kg, i.p.) on 4 alternate days. Both treatments resulted in cold and mechanical allodynia. We assessed the anti-allodynic effects of subcutaneously administered elcatonin (20 U/kg/day) by using a newly developed method to provide cold stimulation (8°C) directly to the hind paw of the rats and by using the von Frey test. Elcatonin almost completely reversed the effects of both cold and mechanical allodynia. To determine the mechanism of this anti-allodynic effect, we examined the effect of elcatonin on neuropathy induced by intraplantar injection of two organic compounds: allyl isothiocyanate (1 nmol/paw), which activates transient receptor potential ankyrin-1 channels, and menthol (1.28 μmol/paw), which activates transient receptor potential ankyrin-1 and melastatin-8. Pre-administration of elcatonin almost completely prevented cold and mechanical allodynia from being induced by both compounds. These results suggest that elcatonin attenuates oxaliplatin- and paclitaxel-induced neuropathic pain by inhibiting the cellular signaling related to transient receptor potential ankyrin-1 and melastatin-8. Thus, we conclude that administration of elcatonin may improve the quality of life of cancer patients receiving chemotherapy.

Electrophysiological features of the mouse tail nerves and their changes in chemotherapy induced peripheral neuropathy (CIPN)

Electrophysiology of tail nerves in rodents has been demonstrated a reliable method to investigate models of peripheral neuropathies. Nevertheless, data concerning mouse models are lacking. We assessed the normal features of sensory and motor conduction of tail nerves in adult mice. We found that, as in rats, a sensory compound action potential and motor responses could be recorded with the non invasive and highly reliable technique proposed, especially if bipolar derivations were used. We also investigated the changes related to chemotherapy induced peripheral neuropathy (CIPN) after paclitaxel treatment (times 1 and 2), compared to pre-treatment (time 0) and to controls. It was found that only the sensory compound action potential was involved in CIPN, with decrease in amplitude and conduction velocity, suggesting a significant reduction in number of fast conducting fibres and a correspondent increase in the number of slow conducting ones, although the total amount of active myelinated fibres was deemed to be unchanged through time 0, time 1 and time 2. The results obtained in CIPN provide new functional evidence about the involvement of sensory fibres and may help in better understanding the underlying mechanisms.

Targeting the overproduction of peroxynitrite for the prevention and reversal of paclitaxel-induced neuropathic pain

Chemotherapy-induced peripheral neuropathy (CIPN) accompanied by chronic neuropathic pain is a major dose-limiting side effect of a large number of antitumoral agents including paclitaxel (Taxol). Thus, CIPN is one of most common causes of dose reduction and discontinuation of what is otherwise a life-saving therapy. Neuropathological changes in spinal cord are linked to CIPN, but the causative mediators and mechanisms remain poorly understood. We report that formation of peroxynitrite (PN) in response to activation of nitric oxide synthases and NADPH oxidase in spinal cord contributes to neuropathological changes through two mechanisms. The first involves modulation of neuroexcitatory and proinflammatory (TNF-α and IL-1β) and anti-inflammatory (IL-10 and IL-4) cytokines in favor of the former. The second involves post-translational nitration and modification of glia-derived proteins known to be involved in glutamatergic neurotransmission (astrocyte-restricted glutamate transporters and glutamine synthetase). Targeting PN with PN decomposition catalysts (PNDCs) not only blocked the development of paclitaxel-induced neuropathic pain without interfering with antitumor effects, but also reversed it once established. Herein, we describe our mechanistic study on the role(s) of PN and the prevention of neuropathic pain in rats using known PNDCs (FeTMPyP(5+) and MnTE-2-PyP(5+)). We also demonstrate the prevention of CIPN with our two new orally active PNDCs, SRI6 and SRI110. The improved chemical design of SRI6 and SRI110 also affords selectivity for PN over other reactive oxygen species (such as superoxide). Our findings identify PN as a critical determinant of CIPN, while providing the rationale toward development of superoxide-sparing and "PN-targeted" therapeutics.

Paclitaxel-induced neuropathic pain is age dependent and devolves on glial response

BACKGROUND

Paclitaxel is an antimitotic antitumour drug highly effective against a broad range of cancers considered refractory to conventional chemotherapy. One of the main serious side effects of paclitaxel treatment is the induction of peripheral neuropathic pain that often diminishes the patient's quality of life. In this study, we evaluated the severity of the neuropathy induced by paclitaxel and the inflammatory reaction in the dorsal horn of the spinal cord in young, adult and aged male CD1 mice.

METHOD

Hyperalgesia to noxious thermal stimulus and allodynia to non-noxious mechanical stimulus were evaluated using the plantar test and the von Frey filament model, respectively. Spinal cord microglia and astrocytes expression was assessed using Iba1 and glial fibrillary acidic protein immunofluorescence staining, respectively.

RESULTS

All groups of mice showed a higher nociceptive reaction to thermal noxious (hyperalgesia) and mechanical non-noxious (allodynia) stimuli after paclitaxel treatment. However, these signs of neuropathy were enhanced in young mice followed by aged animals. Additionally, paclitaxel evoked a marked microglial and astrocytic response in the spinal cord of young and aged mice, whereas this enhanced reactivity was less important in adult mice. Indeed, the most severe glial activation observed in juvenile animals correlated well with major signs of neuropathy in this group of age.

CONCLUSION

Our results demonstrate that paclitaxel-induced neuropathy in mice is an age-dependent phenomenon whose severity devolves on glial response.

Functional changes in muscle afferent neurones in an osteoarthritis model: implications for impaired proprioceptive performance

Background

Impaired proprioceptive performance is a significant clinical issue for many who suffer osteoarthritis (OA) and is a risk factor for falls and other liabilities. This study was designed to evaluate weight-bearing distribution in a rat model of OA and to determine whether changes also occur in muscle afferent neurones.

Methodology/Principal Findings

Intracellular recordings were made in functionally identified dorsal root ganglion neurones in acute electrophysiological experiments on the anaesthetized animal following measurements of hind limb weight bearing in the incapacitance test. OA rats but not naïve control rats stood with less weight on the ipsilateral hind leg (P = 0.02). In the acute electrophysiological experiments that followed weight bearing measurements, action potentials (AP) elicited by electrical stimulation of the dorsal roots differed in OA rats, including longer AP duration (P = 0.006), slower rise time (P = 0.001) and slower maximum rising rate (P = 0.03). Depolarizing intracellular current injection elicited more APs in models than in naïve muscle afferent neurones (P = 0.01) indicating greater excitability. Axonal conduction velocity in model animals was slower (P = 0.04).

Conclusions/Significance

The present study demonstrates changes in hind limb stance accompanied by changes in the functional properties of muscle afferent neurones in this derangement model of OA. This may provide a possible avenue to explore mechanisms underlying the impaired proprioceptive performance and perhaps other sensory disorders in people with OA.

Discovering cytokines as targets for chemotherapy-induced painful peripheral neuropathy

Chemotherapy-induced peripheral neuropathy (CIPN), a dose-limiting neurotoxic effect of chemotherapy, is the most common reason for early cessation of cancer treatment. This can result in an increased risk of recurrence and decreased survival rate. Inflammatory cascade activation, proinflammatory cytokine upregulation, and neuro-immune communication pathways play essential roles in the initiation and progression of CIPN. Most notably, TNF-α, IL-1β, IL-6, and CCL2 are involved in neuropathic pain. Further elucidation of the role of these cytokines could lead to their development and use as biomarkers for predicting the onset of painful peripheral neuropathy and early axonal damage. In this review, we provide evidence for the involvement of cytokines in CIPN, the possible underlying mechanisms, and their use as potential therapeutic targets and biomarkers to prevent and improve the painful peripheral neuropathy related to chemotherapeutic agents.

Characterisation of a peripheral neuropathic component of the rat monoiodoacetate model of osteoarthritis

Joint degeneration observed in the rat monoiodoacetate (MIA) model of osteoarthritis shares many histological features with the clinical condition. The accompanying pain phenotype has seen the model widely used to investigate the pathophysiology of osteoarthritis pain, and for preclinical screening of analgesic compounds. We have investigated the pathophysiological sequellae of MIA used at low (1 mg) or high (2 mg) dose. Intra-articular 2 mg MIA induced expression of ATF-3, a sensitive marker for peripheral neuron stress/injury, in small and large diameter DRG cell profiles principally at levels L4 and 5 (levels predominated by neurones innervating the hindpaw) rather than L3. At the 7 day timepoint, ATF-3 signal was significantly smaller in 1 mg MIA treated animals than in the 2 mg treated group. 2 mg, but not 1 mg, intra-articular MIA was also associated with a significant reduction in intra-epidermal nerve fibre density in plantar hindpaw skin, and produced spinal cord dorsal and ventral horn microgliosis. The 2 mg treatment evoked mechanical pain-related hypersensitivity of the hindpaw that was significantly greater than the 1 mg treatment. MIA treatment produced weight bearing asymmetry and cold hypersensitivity which was similar at both doses. Additionally, while pregabalin significantly reduced deep dorsal horn evoked neuronal responses in animals treated with 2 mg MIA, this effect was much reduced or absent in the 1 mg or sham treated groups. These data demonstrate that intra-articular 2 mg MIA not only produces joint degeneration, but also evokes significant axonal injury to DRG cells including those innervating targets outside of the knee joint such as hindpaw skin. This significant neuropathic component needs to be taken into account when interpreting studies using this model, particularly at doses greater than 1 mg MIA.

Further data supporting that paclitaxel-associated acute pain syndrome is associated with development of peripheral neuropathy: North Central Cancer Treatment Group trial N08C1

BACKGROUND

Paclitaxel causes an acute pain syndrome (P-APS), occurring within days after each dose and usually abating within days. Paclitaxel also causes a more classic peripheral neuropathy, which steadily increases in severity with increasing paclitaxel total doses. Little detail is available regarding the natural history of these 2 syndromes, or any relationship between them, although a recent publication does provide natural history data about weekly paclitaxel, supporting an association between the severity of P-APS and eventual peripheral neuropathy symptoms.

METHODS

Patients entering this study were about to receive paclitaxel and carboplatin every 3 weeks. Daily questionnaires were completed for the first week after every chemotherapy dose, and European Organization for Research and Treatment of Cancer, Quality of Life Questionnaire, Chemotherapy-Induced Peripheral Neuropathy 20-item instruments were completed weekly.

RESULTS

The P-APS severity peaked on day 4 after the initial chemotherapy dose, with 12%, 29%, 23%, and 36% of patients having maximal pain scores of 0, 1 to 4, 5 or 6, or 7 to 10 during the first week after the first dose of therapy, respectively. Patients with P-APS scores of 0 to 4 with the first dose of chemotherapy had less eventual sensory neuropathy than did patients with P-APS scores of 5 to 10 (P = 0.001). With regard to the more peripheral neuropathy, sensory neuropathy was more problematic than was either motor or autonomic neuropathy. Numbness and tingling were more common components of the sensory neuropathy than was pain.

CONCLUSIONS

Patients with worse P-APS severities appear to have more eventual chemotherapy-induced peripheral neuropathy. This provides support for the concept that P-APS is a form of nerve pathology.

Prevention of paclitaxel-induced neuropathy through activation of the central cannabinoid type 2 receptor system

BACKGROUND

Peripheral neuropathy is a major dose-limiting toxicity of chemotherapy, especially after multiple courses of paclitaxel. The development of paclitaxel-induced neuropathy is associated with the activation of microglia followed by the activation and proliferation of astrocytes, and the expression and release of proinflammatory cytokines in the spinal dorsal horn. Cannabinoid type 2 (CB(2)) receptors are expressed in the microglia in neurodegenerative disease models.

METHODS

To explore the potential of CB(2) agonists for preventing paclitaxel-induced neuropathy, we designed and synthesized a novel CB(2)-selective agonist, namely, MDA7. The effect of MDA7 in preventing paclitaxel-induced allodynia was assessed in rats and in CB(2)(+/+) and CB(2)(-/-) mice. We hypothesized that the CB(2) receptor functions in a negative-feedback loop and that early MDA7 administration can blunt the neuroinflammatory response to paclitaxel and prevent mechanical allodynia through interference with specific signaling pathways.

RESULTS

We found that MDA7 prevents paclitaxel-induced mechanical allodynia in rats and mice in a dose- and time-dependent manner without compromising paclitaxel's antineoplastic effect. MDA7's neuroprotective effect was absent in CB(2)(-/-) mice and was blocked by CB(2) antagonists, suggesting that MDA7's action directly involves CB(2) receptor activation. MDA7 treatment was found to interfere with early events in the paclitaxel-induced neuroinflammatory response as evidenced by relatively reduced toll-like receptor and CB(2) expression in the lumbar spinal cord, reduced levels of extracellular signal-regulated kinase 1/2 activity, reduced numbers of activated microglia and astrocytes, and reduced secretion of proinflammatory mediators in vivo and in in vitro models.

CONCLUSIONS

Our findings suggest an innovative therapeutic approach to prevent chemotherapy-induced neuropathy and may permit more aggressive use of active chemotherapeutic regimens with reduced long-term sequelae.

Cannabinoid agonist WIN 55,212-2 prevents the development of paclitaxel-induced peripheral neuropathy in rats. Possible involvement of spinal glial cells

Spinal glial activation contributes to the development and maintenance of chronic pain states, including neuropathic pain of diverse etiologies. Cannabinoid compounds have shown antinociceptive properties in a variety of neuropathic pain models and are emerging as a promising class of drugs to treat neuropathic pain. Thus, the effects of repeated treatment with WIN 55,212-2, a synthetic cannabinoid agonist, were examined throughout the development of paclitaxel-induced peripheral neuropathy. Painful neuropathy was induced in male Wistar rats by intraperitoneal (i.p.) administration of paclitaxel (1mg/kg) on four alternate days. Paclitaxel-treated animals received WIN 55,212-2 (1mg/kg, i.p.) or minocycline (15 mg/kg, i.p.), a microglial inhibitor, daily for 14 days, simultaneous with the antineoplastic. The development of hypersensitive behaviors was assessed on days 1, 7, 14, 21 and 28 following the initial administration of drugs. Both the activation of glial cells (microglia and astrocytes) at day 29 and the time course of proinflammatory cytokine release within the spinal cord were also determined. Similar to minocycline, repeated administration of WIN 55,212-2 prevented the development of thermal hyperalgesia and mechanical allodynia in paclitaxel-treated rats. WIN 55,212-2 treatment also prevented spinal microglial and astrocytic activation evoked by paclitaxel at day 29 and attenuated the early production of spinal proinflammatory cytokines (interleukin (IL)-1β, IL-6 and tumor necrosis factor (TNF)-α). Our results confirm changes in the reactivity of glial cells during the development of peripheral neuropathy induced by paclitaxel and support a preventive effect of WIN 55,212-2, probably via glial cells reactivity inactivation, on the development of this neuropathy.

Dorsal root ganglion - a potential new therapeutic target for neuropathic pain

A regional approach can protect our patients from often unacceptable adverse effects produced by systematically applied drugs. Regional therapeutic approaches, as well as interventions at the level of the peripheral nervous system and particularly the dorsal root ganglion (DRG), represent an alternative to the systemic application of therapeutic agents. This article provides an overview of DRG anatomical peculiarities, explains why the DRG is an important therapeutic target, and how animal models of targeted drug delivery can help us in the translation of basic research into clinical practice.

Activating transcription factor 3 and the nervous system

Activating transcription factor 3 (ATF3) belongs to the ATF/cyclic AMP responsive element binding family of transcription factors and is often described as an adaptive response gene whose activity is usually regulated by stressful stimuli. Although expressed in a number of splice variants and generally recognized as a transcriptional repressor, ATF3 has the ability to interact with a number of other transcription factors including c-Jun to form complexes which not only repress, but can also activate various genes. ATF3 expression is modulated mainly at the transcriptional level and has markedly different effects in different types of cell. The levels of ATF3 mRNA and protein are normally very low in neurons and glia but their expression is rapidly upregulated in response to injury. ATF3 expression in neurons is closely linked to their survival and the regeneration of their axons following axotomy, and that in peripheral nerves correlates with the generation of a Schwann cell phenotype that is conducive to axonal regeneration. ATF3 is also induced by Toll-like receptor (TLR) ligands but acts as a negative regulator of TLR signaling, suppressing the innate immune response which is involved in immuno-surveillance and can enhance or reduce the survival of injured neurons and promote the regeneration of their axons.

Mitochondrial abnormality in sensory, but not motor, axons in paclitaxel-evoked painful peripheral neuropathy in the rat

The dose-limiting side effect of the anti-neoplastic agent, paclitaxel, is a chronic distal symmetrical peripheral neuropathy that produces sensory dysfunction (hypoesthesia and neuropathic pain) but little or no distal motor dysfunction. Similar peripheral neuropathies are seen with chemotherapeutics in the vinca alkaloid, platinum-complex, and proteasome inhibitor classes. Studies in rats suggest that the cause is a mitotoxic effect on axonal mitochondria. If so, then the absence of motor dysfunction may be due to mitotoxicity that affects sensory axons but spares motor axons. To investigate this, paclitaxel exposure levels in the dorsal root, ventral root, dorsal root ganglion, peripheral nerve, and spinal cord were measured, and the ultrastructure and the respiratory function of mitochondria in dorsal roots and ventral roots were compared. Sensory and motor axons in the roots and nerve had comparably low exposure to paclitaxel and exposure in the spinal cord was negligible. However, sensory neurons in the dorsal root ganglion had a very high and remarkably persistent (up to 10 days or more after the last injection) exposure to paclitaxel. Paclitaxel evoked a significant increase in the incidence of swollen and vacuolated mitochondria in the myelinated and unmyelinated sensory axons of the dorsal root (as seen previously in the peripheral nerve) but not in the motor axons of the ventral root. Stimulated mitochondrial respiration in the dorsal root was significantly depressed in paclitaxel-treated animals examined 2-4 weeks after the last injection, whereas respiration in the ventral root was normal. We conclude that the absence of motor dysfunction in paclitaxel-evoked peripheral neuropathy may be due to the absence of a mitotoxic effect in motor neuron axons, whereas the sensory dysfunction may be due to a mitotoxic effect resulting from the primary afferent neuron's cell body being exposed to high and persistent levels of paclitaxel.

The mechanisms of microgliosis and pain following peripheral nerve injury

Microglia are the resident macrophages in the central nervous system (CNS). Any insult to the CNS homeostasis will induce a rapid change in microglia morphology, gene expression profile and functional behaviour. These responses of microglia have been collectively known as 'microgliosis'. Interestingly, damage to the nervous system outside the CNS, such as axotomy of a peripheral nerve, can lead to microgliosis in the spinal cord. There is a variation in the degree of microgliosis depending on the model of nerve injury employed for instance this response is more marked following traumatic nerve injury than in models of chemotherapy induced neuropathy. Following peripheral nerve injury nociceptive inputs from sensory neurons appear to be critical in triggering the development of spinal microgliosis. A number of signalling pathways including growth factors such as Neuregulin-1, matrix metalloproteases such as MMP-9 and multiple chemokines enable direct communication between injured primary afferents and microglia. In addition, we describe a group of mediators which although not demonstrably shown to be released from neurons are known to modulate microglial phenotype. There is a great functional diversity of the microglial response to peripheral nerve injury which includes: Cellular migration, proliferation, cytokine release, phagocytosis, antigen presentation and recruitment of T cells. It should also be noted that in certain contexts microglia may have a role in the resolution of neuro-inflammation. Although there is still no direct evidence demonstrating that spinal microglia have a role in neuropathic pain in humans, these patients present a pro-inflammatory cytokine profile and it is a reasonable hypothesis that these cells may contribute to this inflammatory response. Modulating microglial functions offers a novel therapeutic opportunity following nerve injury which ideally would involve reducing the pro-inflammatory nature of these cells whilst retaining their potential beneficial functions.

Terminal arbor degeneration--a novel lesion produced by the antineoplastic agent paclitaxel

The antineoplastic agent paclitaxel causes a dose-limiting distal, symmetrical, sensory peripheral neuropathy that is often accompanied by a neuropathic pain syndrome. In a low-dose model of paclitaxel-evoked painful peripheral neuropathy in the rat, we have shown that the drug causes degeneration of intraepidermal nerve fibers (IENFs), i.e. the fibers which give rise to the sensory afferent's terminal receptor arbor. However, we did not find any evidence for axonal degeneration in samples taken at the mid-nerve level. Here we aimed to determine whether the absence of degenerating peripheral nerve axons was due to sampling a level that was too proximal. We used electron microscopy to study the distal-most branches of the nerves innervating the hind paw glabrous skin of normal and paclitaxel-treated rats. We confirmed that we sampled at a time when IENF degeneration was prominent. Because degeneration might be easier to detect with higher paclitaxel doses, we examined a four-fold cumulative dose range (8-32 mg/kg). We found no evidence of degeneration in the superficial subepidermal axon bundles (sSAB) that are located just a few microns below the epidermal basal lamina. Specifically, for all three dose groups there was no change in the number of sSAB per millimeter of epidermal border, no change in the number of axons per sSAB and no change in the diameter of sSAB axons. We conclude that paclitaxel produces a novel type of lesion that is restricted to the afferent axon's terminal arbor; we name this lesion 'terminal arbor degeneration'.

Natural history of paclitaxel-associated acute pain syndrome: prospective cohort study NCCTG N08C1

PURPOSE

The characteristics and natural history of the paclitaxel-acute pain syndrome (P-APS) and paclitaxel's more chronic neuropathy have not been well delineated.

METHODS

Patients receiving weekly paclitaxel (70 to 90 mg/m(2)) completed daily questionnaires and weekly European Organisation for Research and Treatment of Cancer (EORTC) Chemotherapy-Induced Peripheral Neuropathy (CIPN) -20 instruments during the entire course of therapy.

RESULTS

P-APS symptoms peaked 3 days after chemotherapy. Twenty percent of patients had pain scores of 5 to 10 of 10 with the first dose of paclitaxel. Sensory neuropathy symptoms were more prominent than were motor or autonomic neuropathy symptoms. Of the sensory neuropathy symptoms, numbness and tingling were more prominent than was shooting or burning pain. Patients with higher P-APS pain scores with the first dose of paclitaxel appeared to have more chronic neuropathy.

CONCLUSION

These data support that the P-APS is related to nerve pathology as opposed to being arthralgias and/or myalgias. Numbness and tingling are more prominent chronic neuropathic symptoms than is shooting or burning pain.

The response of spinal microglia to chemotherapy-evoked painful peripheral neuropathies is distinct from that evoked by traumatic nerve injuries

Painful peripheral neuropathies produced by nerve trauma are accompanied by substantial axonal degeneration and by a response in spinal cord microglia that is characterized by hypertrophy and increased expression of several intracellular and cell-surface markers, including ionizing calcium-binding adapter molecule 1 (Iba1) and Cd11b (a complement receptor 3 antigen recognized by the OX42 antibody). The microglia response has been hypothesized to be essential for the pathogenesis of the neuropathic pain state. In contrast, the painful peripheral neuropathies produced by low doses of cancer chemotherapeutics do not produce degeneration of axons in the peripheral nerve, although they do cause partial degeneration of the sensory axons' distal-most tips, that is the intraepidermal nerve fibers that form the axons' terminal receptor arbors. The question thus arises as to whether the relatively minor and distal axonal injury characterizing the chemotherapy-evoked neuropathies is sufficient to evoke the microglial response that is seen after traumatic nerve injury. We examined the lumbar spinal cord of rats with painful peripheral neuropathies due to the anti-neoplastic agents, paclitaxel, vincristine, and oxaliplatin, and the anti-retroviral agent, 2',3'-dideoxycytidine (ddC), and compared them to rats with a complete sciatic nerve transection and the partial sciatic nerve injury produced in the chronic constriction injury model (CCI). As expected, microglia hypertrophy and increased expression of Iba1 were pronounced in the nerve transection and CCI animals. However, there was no microglia hypertrophy or increased Iba1 staining in the animals treated with paclitaxel, vincristine, oxaliplatin, or ddC. These results suggest that the mechanisms that produce neuropathic pain after exposure to chemotherapeutics may be fundamentally different than those operating after nerve trauma.

Contralateral neuropathology in dorsal root ganglia in a rat model of noncompressive disc herniation

Some cases of lumbar disc herniation with contralateral radiculopathy have been reported. Many investigators explained this clinical syndrome from several aspects. However, to our best knowledge, there is no explanation on the basis of molecular changes in the contralateral dorsal root ganglia (DRG) until now. We firstly explored the expression of activating transcription factor 3 (ATF3, a marker of nerve injury), glial fibrillary acidic protein (GFAP, a marker of satellite cells activation) in bilateral L5 DRG and spinal cord using immunohistochemistry after nucleus pulposus (NP) application onto the left L5 DRG exposed by unilateral facetectomy. Immunoblotting was used to detect the expression of tumor necrosis factor-alpha (TNF-alpha) in bilateral L5 DRG. We tested that ATF3-immunoreactive (IR) neurons, GFAP-IR satellite cells and TNF-alpha expression in the contralateral DRG increased significantly after NP application. In the spinal cord, ATF3-IR motor neurons increased significantly after surgery, but GFAP-IR astrocytes were not significant. These results suggested that NP application on the unilateral DRG could induce nerve injury, satellite cells activation and upregulation of TNF-alpha expression in the contralateral DRG. In addition, our results indicated that motor neurons injury might play a significant role in contralateral changes.

Neuronal FLT1 receptor and its selective ligand VEGF-B protect against retrograde degeneration of sensory neurons

Even though VEGF-B is a homologue of the potent angiogenic factor VEGF, its angiogenic activities have been controversial. Intrigued by findings that VEGF-B may also affect neuronal cells, we assessed the neuroprotective and vasculoprotective effects of VEGF-B in the skin, in which vessels and nerves are functionally intertwined. Although VEGF-B and its FLT1 receptor were prominently expressed in dorsal root ganglion (DRG) neurons innervating the hindlimb skin, they were not essential for nerve function or vascularization of the skin. However, primary DRG cultures lacking VEGF-B or FLT1 exhibited increased neuronal stress and were more susceptible to paclitaxel-induced cell death. Concomitantly, mice lacking VEGF-B or a functional FLT1 developed more retrograde degeneration of sensory neurons in a model of distal neuropathy. On the other hand, the addition of the VEGF-B isoform, VEGF-B(186), to DRG cultures antagonized neuronal stress, maintained the mitochondrial membrane potential and stimulated neuronal survival. Mice overexpressing VEGF-B(186) or FLT1 selectively in neurons were protected against the distal neuropathy, whereas exogenous VEGF-B(186), either delivered by gene transfer or as a recombinant factor, was protective by directly affecting sensory neurons and not the surrounding vasculature. Overall, this indicates that VEGF-B, instead of acting as an angiogenic factor, exerts direct neuroprotective effects through FLT1. These findings also suggest a clinically relevant role for VEGF-B in preventing distal neuropathies.

Prevention of paclitaxel-induced allodynia by minocycline: Effect on loss of peripheral nerve fibers and infiltration of macrophages in rats

Background

Although paclitaxel is a frontline antineoplastic agent for treatment of solid tumors, the paclitaxel-evoked pain syndrome is a serious problem for patients. There is currently no valid drug to prevent or treat the paclitaxel-induced allodynia, partly due to lack of understanding regarding the cellular mechanism. Studies have shown that minocycline, an inhibitor of microglia/macrophage, prevented neuropathic pain and promoted neuronal survival in animal models of neurodegenerative disease. Recently, Cata et al also reported that minocycline inhibited allodynia induced by low-dose paclitaxel (2 mg/kg) in rats, but the mechanism is still unclear.

Results

Here, we investigate by immunohistochemistry the change of intraepidermal nerve fiber (IENF) in the hind paw glabrous skin, expression of macrophage and activating transcription factor 3 (ATF3) in DRG at different time points after moderate-dose paclitaxel treatment (cumulative dose 24 mg/kg; 3 × 8 mg/kg) in rats. Moreover, we observe the effect of minocycline on the IENF, macrophages and ATF3. The results showed that moderate-dose paclitaxel induced a persisted, gradual mechanical allodynia, which was accompanied by the loss of IENF in the hind paw glabrous skin and up-regulation of macrophages and ATF3 in DRG in rats. The expressions of ATF3 mainly focus on the NF200-positive cells. More importantly, we observed that pretreatment of minocycline at dose of 30 mg/kg or 50 mg/kg, but not 5 mg/kg, prevented paclitaxel-evoked allodynia. The evidence from immunohistochemistry showed that 30 mg/kg minocycline rescued the degeneration of IENF, attenuated infiltration of macrophages and up-regulation of ATF3 induced by paclitaxel treatment in rats.

Conclusions

Minocycline prevents paclitaxel-evoked allodynia, likely due to its inhibition on loss of IENF, infiltration of macrophages and up-regulation of ATF3 in rats. The finding might provide potential target for preventing paclitaxel-induced neuropathic pain.

Vasodilatation in the rat dorsal hindpaw induced by activation of sensory neurons is reduced by paclitaxel

Peripheral neuropathy is a major side effect following treatment with the cancer chemotherapeutic drug paclitaxel. Whether paclitaxel-induced peripheral neuropathy is secondary to altered function of small diameter sensory neurons remains controversial. To ascertain whether the function of the small diameter sensory neurons was altered following systemic administration of paclitaxel, we injected male Sprague Dawley rats with 1mg/kg paclitaxel every other day for a total of four doses and examined vasodilatation in the hindpaw at day 14 as an indirect measure of calcitonin gene related peptide (CGRP) release. In paclitaxel-treated rats, the vasodilatation induced by either intradermal injection of capsaicin into the hindpaw or electrical stimulation of the sciatic nerve was significantly attenuated in comparison to vehicle-injected animals. Paclitaxel treatment, however, did not affect direct vasodilatation induced by intradermal injection of methacholine or CGRP, demonstrating that the blood vessels' ability to dilate was intact. Paclitaxel treatment did not alter the compound action potentials or conduction velocity of C-fibers. The stimulated release of CGRP from the central terminals in the spinal cord was not altered in paclitaxel-injected animals. These results suggest that paclitaxel affects the peripheral endings of sensory neurons to alter transmitter release, and this may contribute to the symptoms seen in neuropathy.

Increased interleukin-6 activity associated with painful chemotherapy-induced peripheral neuropathy in women after breast cancer treatment

Accumulating evidence suggests that neural-immune interactions are involved in the development of painful chemotherapy-induced peripheral neuropathy, particularly through the increased release of proinflammatory cytokines. The purpose of this study was used to evaluate levels of interleukin [IL]-6 and IL-6 receptors in women with breast cancer after the conclusion of chemotherapy who either had painful symptoms of chemotherapy-induced peripheral neuropathy (CIPN group, N = 20) or did not experience CIPN symptoms (Comparison group, N = 20). CIPN participants had significantly higher levels of IL-6 and soluble IL-6R (sIL-6R) compared to women without CIPN symptoms (P < .001 for both). In addition, soluble gp130, which blocks the IL-6/sIL-6R complex from binding to gp130 within the cellular membrane, was significantly lower (P < .01). Circulating concentrations of sIL-6R were inversely correlated with the density of IL-6R on the cell surface of monocytes in the total sample (r = −.614, P = .005). These findings suggest that IL-6 transsignaling may be an important biological mechanism associated with the persistence of painful CIPN symptoms, with potential implications for symptom management and research.

Glial cells and chronic pain

Over the past few years, the control of pain exerted by glial cells has emerged as a promising target against pathological pain. Indeed, changes in glial phenotypes have been reported throughout the entire nociceptive pathway, from peripheral nerves to higher integrative brain regions, and pharmacological inhibition of such glial reactions reduces the manifestation of pain in animal models. This complex interplay between glia and neurons relies on various mechanisms depending both on glial cell types considered (astrocytes, microglia, satellite cells, or Schwann cells), the anatomical location of the regulatory process (peripheral nerve, spinal cord, or brain), and the nature of the chronic pain paradigm. Intracellularly, recent advances have pointed to the activation of specific cascades, such as mitogen-associated protein kinases (MAPKs) in the underlying processes behind glial activation. In addition, given the large number of functions accomplished by glial cells, various mechanisms might sensitize nociceptive neurons including a release of pronociceptive cytokines and neurotrophins or changes in neurotransmitter-scavenging capacity. The authors review the conceptual advances made in the recent years about the implication of central and peripheral glia in animal models of chronic pain and discuss the possibility to translate it into human therapies in the future.

Skin incision induces expression of axonal regeneration-related genes in adult rat spinal sensory neurons

Skin incision and nerve injury both induce painful conditions. Incisional and postsurgical pain is believed to arise primarily from inflammation of tissue and the subsequent sensitization of peripheral and central neurons. The role of axonal regeneration-related processes in development of pain has only been considered when there has been injury to the peripheral nerve itself, even though tissue damage likely induces injury of resident axons. We sought to determine if skin incision would affect expression of regeneration-related genes such as activating transcription factor 3 (ATF3) in dorsal root ganglion (DRG) neurons. ATF3 is absent from DRG neurons of the normal adult rodent, but is induced by injury of peripheral nerves and modulates the regenerative capacity of axons. Image analysis of immunolabeled DRG sections revealed that skin incision led to an increase in the number of DRG neurons expressing ATF3. RT-PCR indicated that other regeneration-associated genes (galanin, GAP-43, Gadd45a) were also increased, further suggesting an injury-like response in DRG neurons. Our finding that injury of skin can induce expression of neuronal injury/regeneration-associated genes may impact how clinical postsurgical pain is investigated and treated.

PERSPECTIVE

Tissue injury, even without direct nerve injury, may induce a state of enhanced growth capacity in sensory neurons. Axonal regeneration-associated processes should be considered alongside nerve signal conduction and inflammatory/sensitization processes as possible mechanisms contributing to pain, particularly the transition from acute to chronic pain.

Activation of satellite glial cells in rat trigeminal ganglion after upper molar extraction

The neurons in the trigeminal ganglion (TG) are surrounded by satellite glial cells (SGCs), which passively support the function of the neurons, but little is known about the interactions between SGCs and TG neurons after peripheral nerve injury. To examine the effect of nerve injury on SGCs, we investigated the activation of SGCs after neuronal damage due to the extraction of the upper molars in rats. Three, 7, and 10 days after extraction, animals were fixed and the TG was removed. Cryosections of the ganglia were immunostained with antibodies against glial fibrillary acidic protein (GFAP), a marker of activated SGCs, and ATF3, a marker of damaged neurons. After tooth extraction, the number of ATF3-immunoreactive (IR) neurons enclosed by GFAP-IR SGCs had increased in a time-dependent manner in the maxillary nerve region of the TG. Although ATF3-IR neurons were not detected in the mandibular nerve region, the number of GFAP-IR SGCs increased in both the maxillary and mandibular nerve regions. Our results suggest that peripheral nerve injury affects the activation of TG neurons and the SGCs around the injured neurons. Moreover, our data suggest the existence of a neuronal interaction between maxillary and mandibular neurons via SGC activation.

Involvement of inflammatory mediators in neuropathic pain caused by vincristine

Elucidation of the mechanism of neuropathic pain caused by vincristine is required because long-term treatment with this anticancer agent often causes neuropathic pain. We refer to the involvement of inflammatory mediators in vincristine-induced neuropathic pain in this review. Several reports using rodents have shown that long-lasting neuropathic pain (mechanical allodynia) is caused by repeated systemic injection of vincristine. Vincristine damaged Schwann cells and DRG neurons in this model. Vincristine-induced macrophage infiltration in the peripheral nervous system (PNS) and macrophage-derived IL-6 elicited mechanical allodynia. These findings proved that inhibition of IL-6 function prevented neuropathic pain caused by vincristine. In the central nervous system (CNS), activation of microglia and astrocytes in the spinal cord were demonstrated after long-term vincristine treatment. TNF-alpha was upregulated in activated microglia and astrocytes, and inhibition of TNF-alpha function attenuated neuropathic pain caused by vincristine. These results suggest that vincristine induces macrophage infiltration to the damaged PNS, and that macrophage-derived inflammatory cytokines such as IL-6 elicits neuroinflammation. Signal transduction of pain from the PNS to the CNS activates microglia and astrocytes, and these activated glial cells release inflammatory cytokines such as TNF-alpha. In the CNS, these inflammatory cytokines have an important role in the neuropathic pain caused by vincristine. Immune-modulating agents that prevent activation of immune cells and/or the inhibitory agents of inflammatory cytokines could prevent the neuropathic pain caused by vincristine. These agents could increase the tolerability of vincristine when used for the treatment of leukemia and lymphoma.