Cellular and neurochemical remodeling of the spinal cord in bone cancer pain

Behavioral Animal Models and Neural-Circuit Framework of Depressive Disorder

Depressive disorder is a chronic, recurring, and potentially life-endangering neuropsychiatric disease. According to a report by the World Health Organization, the global population suffering from depression is experiencing a significant annual increase. Despite its prevalence and considerable impact on people, little is known about its pathogenesis. One major reason is the scarcity of reliable animal models due to the absence of consensus on the pathology and etiology of depression. Furthermore, the neural circuit mechanism of depression induced by various factors is particularly complex. Considering the variability in depressive behavior patterns and neurobiological mechanisms among different animal models of depression, a comparison between the neural circuits of depression induced by various factors is essential for its treatment. In this review, we mainly summarize the most widely used behavioral animal models and neural circuits under different triggers of depression, aiming to provide a theoretical basis for depression prevention.

Systemic administration of Resolvin D1 reduces cancer-induced bone pain in mice: Lack of sex dependency in pain development and analgesia

AIMS

Bone cancer produces severe pain that is treated with opioids, but serious side effects limit opioid utilization. There is therefore a need to develop effective and safe non-opioid alternatives. The lipid mediator, Resolvin D1 (RvD1), could be a prospective candidate for cancer pain treatment. To assess RvD1 and other potential candidates, appropriate animal models that recapitulate clinical features must be used. Although several preclinical models of cancer pain have been developed, the influence of sex on the development of cancer pain and the effectiveness of RvD1 have not been studied.

RESULTS

Using a mouse model of fibrosarcoma growth in and around the calcaneus bone, we demonstrated that the mechanical hyperalgesia in the tumor-bearing hind paw develops independently of sex, except that it developed a little sooner in female mice. A single intravenous injection of RvD1 (0.001-10 μg/kg) decreased hyperalgesia in both sexes with similar potency (ED50 = 0.0015 μg/kg) and efficacy. Repeated daily administration of 10 μg/kg RvD1 prolonged the analgesic effect and completely abolished hyperalgesia. This was also independent of sex.

CONCLUSION

In this preclinical mouse model of bone cancer pain, the development of pain and the analgesic effectiveness of RvD1 are not influenced by sex.

Mechanisms of cancer pain

Personalised and targeted interventions have revolutionised cancer treatment and dramatically improved survival rates in recent decades. Nonetheless, effective pain management remains a problem for patients diagnosed with cancer, who continue to suffer from the painful side effects of cancer itself, as well as treatments for the disease. This problem of cancer pain will continue to grow with an ageing population and the rapid advent of more effective therapeutics to treat the disease. Current pain management guidelines from the World Health Organisation are generalised for different pain severities, but fail to address the heterogeneity of mechanisms in patients with varying cancer types, stages of disease and treatment plans. Pain is the most common complaint leading to emergency unit visits by patients with cancer and over one-third of patients that have been diagnosed with cancer will experience under-treated pain. This review summarises preclinical models of cancer pain states, with a particular focus on cancer-induced bone pain and chemotherapy-associated pain. We provide an overview of how preclinical models can recapitulate aspects of pain and sensory dysfunction that is observed in patients with persistent cancer-induced bone pain or neuropathic pain following chemotherapy. Peripheral and central nervous system mechanisms of cancer pain are discussed, along with key cellular and molecular mediators that have been highlighted in animal models of cancer pain. These include interactions between neuronal cells, cancer cells and non-neuronal cells in the tumour microenvironment. Therapeutic targets beyond opioid-based management are reviewed for the treatment of cancer pain.

Calcium Signalling in Breast Cancer Associated Bone Pain

Calcium (Ca²⁺) is involved as a signalling mediator in a broad variety of physiological processes. Some of the fastest responses in human body like neuronal action potential firing, to the slowest gene transcriptional regulation processes are controlled by pathways involving calcium signalling. Under pathological conditions these mechanisms are also involved in tumoral cells reprogramming, resulting in the altered expression of genes associated with cell proliferation, metastatisation and homing to the secondary metastatic site. On the other hand, calcium exerts a central function in nociception, from cues sensing in distal neurons, to signal modulation and interpretation in the central nervous system leading, in pathological conditions, to hyperalgesia, allodynia and pain chronicization. It is well known the relationship between cancer and pain when tumoral metastatic cells settle in the bones, especially in late breast cancer stage, where they alter the bone micro-environment leading to bone lesions and resulting in pain refractory to the conventional analgesic therapies. The purpose of this review is to address the Ca²⁺ signalling mechanisms involved in cancer cell metastatisation as well as the function of the same signalling tools in pain regulation and transmission. Finally, the possible interactions between these two cells types cohabiting the same Ca²⁺ rich environment will be further explored attempting to highlight new possible therapeutical targets.

Pain in the Cancer Survivor

Recent decades have demonstrated significant strides in cancer screening, diagnostics and therapeutics. As such there have been dramatic changes in survival following a diagnosis of cancer.

The Study of Pain in Rats and Mice

Pain is a clinical syndrome arising from a variety of etiologies in a heterogeneous population, which makes successfully treating the individual patient difficult. Organizations and governments recognize the need for tailored and specific therapies, which drives pain research. This review summarizes the different types of pain assessments currently being used and the various rodent models that have been developed to recapitulate the human pain condition.

The endocannabinoid system: Novel targets for treating cancer induced bone pain

Treating Cancer-induced bone pain (CIBP) continues to be a major clinical challenge and underlying mechanisms of CIBP remain unclear. Recently, emerging body of evidence suggested the endocannabinoid system (ECS) may play essential roles in CIBP. Here, we summarized the current understanding of the antinociceptive mechanisms of endocannabinoids in CIBP and discussed the beneficial effects of endocannabinoid for CIBP treatment. Targeting non-selective cannabinoid 1 receptors or selective cannabinoid 2 receptors, and modulation of peripheral AEA and 2-AG, as well as the inhibition the function of fatty acid amide hydrolase (FAAH) and monoacylglycerol lipase (MAGL) have produced analgesic effects in animal models of CIBP. Management of ECS therefore appears to be a promising way for the treatment of CIBP in terms of efficacy and safety. Further clinical studies are encouraged to confirm the possible translation to humans of the very promising results already obtained in the preclinical studies.

Adjuvant Use and the Intensification of Pharmacologic Management for Pain in Nursing Home Residents with Cancer: Data from a US National Database

OBJECTIVES

Our objective was to describe the prevalence of adjuvants to opioid therapy and changes in these agents for pharmacologic management in nursing home residents with cancer.

METHODS

We included Medicare beneficiaries with cancer and documented opioid use at nursing home admission in 2011-2013 (N = 3268). The Minimum Data Set 3.0 provided information on sociodemographic and clinical characteristics. Part D claims provided information on opioid and adjuvant use during the 7 days after admission and 90 days later. Proportions of changes in these agents were estimated. Separate logistic models estimated associations between resident characteristics and (1) use of adjuvants at admission and (2) intensification of pharmacologic management at 90 days.

RESULTS

Nearly 20% of patients received adjuvants to opioids at admission, with gabapentin the most common adjuvant (34.4%). After 90 days, approximately 25% had maintained or intensified pharmacologic management. While advanced age (≥ 85 vs. 65-74 years, adjusted odds ratio [aOR] 0.80; 95% confidence interval [CI] 0.63-1.02) and comorbidities, including dementia (aOR 0.65; 95% CI 0.53-0.82) and depression (aOR 1.55; 95% CI 1.29-1.87), were associated with adjuvant use at admission, worse cognitive impairment (severe vs. no/mild, aOR 0.80; 95% CI 0.64-0.99) and presence of more severe pain (moderate/severe vs. no pain, aOR 1.60; 95% CI 1.26-2.03) were associated with intensification of drug regimen.

CONCLUSION

Given aging-related changes and the presence of comorbid conditions in older adults, safety studies of these practices are warranted.

Antinociceptive effects of Ginsenoside Rb1 in a rat model of cancer-induced bone pain

Ginsenoside Rb1 (GRb1) is a major ingredient of ginseng, a traditional medicine that has been used for thousands of years. Previous studies have reported that GRb1 had anti-inflammatory, antioxidant and neuroprotective effects. The current study aimed to evaluate the antinociceptive effects of GRb1 in a rat model of cancer-induced bone pain (CIBP) established by intratibial injection of Walker 256 cells. Intraperitoneal injection (i.p.) of GRb1 (5 and 10 mg/kg, but not 1 mg/kg) partially and transiently reversed the mechanical allodynia and thermal hyperalgesia in CIBP rats at 14 days following surgery when the pain behavior is established. Furthermore, repeated administration of GRb1 demonstrated persistent analgesic effect. Additionally, the protein expression and immunoreactivity of iba1, which is the maker of microglia, was significantly suppressed in CIBP rats treated with GRb1 (i.p., 10 mg/kg) from day 12 for three consecutive days compared with CIBP rats treated with a vehicle. Furthermore, upregulation of spinal interleukin (IL)-1β, IL-6 and tumor necrosis factor-α were also significantly inhibited by the treatment of GRb1 (i.p., 10 mg/kg) from day 12 for three consecutive days. Together, these results indicated that GRb1 may attenuate CIBP via inhibiting the activation of microglia and glial-derived proinflammatory cytokines.

Mechanisms of, and Adjuvants for, Bone Pain

Metastatic bone pain is a complex, poorly understood process. Understanding the unique mechanisms causing cancer-induced bone pain may lead to potential therapeutic targets. This article discusses the effects of osteoclast overstimulation within the tumor microenvironment; the role of inflammatory factors at the tumor-nociceptor interface; the development of structural instability, causing mechanical nerve damage; and, ultimately, the neuroplastic changes in the setting of sustained pain. Several adjuvant therapies are available to attenuate metastatic bone pain. This article discusses the role of pharmacologic therapies, surgery, kyphoplasty, vertebroplasty, and radiofrequency ablation.

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.

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.

Randomized Double-Blind Trial of Pregabalin Versus Placebo in Conjunction With Palliative Radiotherapy for Cancer-Induced Bone Pain

PURPOSE

Cancer-induced bone pain (CIBP) affects one third of patients with cancer. Radiotherapy remains the gold-standard treatment; however, laboratory and clinical work suggest that pregabalin may be useful in treating CIBP. The aim of this study was to examine pregabalin in patients with CIBP receiving radiotherapy.

PATIENTS AND METHODS

A multicenter, double-blind randomized trial of pregabalin versus placebo was conducted. Eligible patients were age ≥ 18 years, had radiologically proven bone metastases, were scheduled to receive radiotherapy, and had pain scores ≥ 4 of 10 (on 0-to-10 numeric rating scale). Before radiotherapy, baseline assessments were completed, followed by random assignment. Doses of pregabalin and placebo were increased over 4 weeks. The primary end point was treatment response, defined as a reduction of ≥ 2 points in worst pain by week 4, accompanied by a stable or reduced opioid dose, compared with baseline. Secondary end points assessed average pain, interference of pain with activity, breakthrough pain, mood, quality of life, and adverse events.

RESULTS

A total of 233 patients were randomly assigned: 117 to placebo and 116 to pregabalin. The most common cancers were prostate (n = 88; 38%), breast (n = 77; 33%), and lung (n = 42; 18%). In the pregabalin arm, 45 patients (38.8%) achieved the primary end point, compared with 47 (40.2%) in the placebo arm (adjusted odds ratio, 1.07; 95% CI, 0.63 to 1.81; P = .816). There were no statistically significant differences in average pain, pain interference, or quality of life between arms. There were differences in mood (P = .031) and breakthrough pain duration (P = .037) between arms. Outcomes were compared at 4 weeks.

CONCLUSION

Our findings do not support the role of pregabalin in patients with CIBP receiving radiotherapy. The role of pregabalin in CIBP with a clinical neuropathic pain component is unknown.

Advances in cancer pain from bone metastasis

With the technological advances in cancer diagnosis and treatment, the survival rates for patients with cancer are prolonged. The issue of figuring out how to improve the life quality of patients with cancer has become increasingly prominent. Pain, especially bone pain, is the most common symptom in malignancy patients, which seriously affects the life quality of patients with cancer. The research of cancer pain has a breakthrough due to the development of the animal models of cancer pain in recent years, such as the animal models of mouse femur, humerus, calcaneus, and rat tibia. The establishment of several kinds of animal models related to cancer pain provides a new platform in vivo to investigate the molecular mechanisms of cancer pain. In this review, we focus on the advances of cancer pain from bone metastasis, the mechanisms involved in cancer pain, and the drug treatment of cancer pain in the animal models.

Use of non-opioid analgesics as adjuvants to opioid analgesia for cancer pain management in an inpatient palliative unit: does this improve pain control and reduce opioid requirements?

BACKGROUND

Cancer pain is complex, and despite the introduction of the WHO cancer pain ladder, few studies have looked at the prevalence of adjuvant medication use in an inpatient palliative medicine unit. In this study, we evaluate the use of adjuvant pain medications in patients admitted to an inpatient palliative care unit and whether their use affects pain scores or opiate dosing.

METHODS

In this retrospective observational study, patients admitted to the inpatient palliative care unit over a 3-month period with a diagnosis of cancer on opioid therapy were selected. Data pertaining to demographics, diagnosis, oral morphine dose equivalent of the opioid at the time of discharge, adjuvant analgesics, nonsteroidal anti-inflammatory drugs (NSAIDs), and pain scores as reported by nurses and physicians were collected.

RESULTS

Seventy-seven patients were eligible over a 3-month period, out of which 65 (84 %) were taking an adjuvant medication. The most commonly prescribed adjuvant was gabapentin (70 %). Fifty-seven percent were taking more than one adjuvant. There were more women in the group receiving adjuvants (57 vs. 17%, p = 0.010). Those without adjuvants compared with those on adjuvants did not have worse pain scores on discharge as reported by physicians (0.8 ± 0.8 vs. 1.0 ± 0.7, p = 0.58) or nurses (2.0 ± 2.7 vs. 2.1 ± 2.6, p = 0.86). There was no difference in morphine equivalent doses of the opioid in both groups (median (min, max); 112 (58, 504) vs. 200 (30, 5,040)) at the time of discharge; 75-80 % of patients had improvement in pain scores as measured by a two-point reduction in numerical rating scale (NRS).

DISCUSSION

This study shows that adjuvant medications are commonly used for treating pain in patients with cancer. More than half of study population were on two adjuvants or an adjuvant plus NSAID along with an opioid. We did not demonstrate any benefit in terms of improved pain scores or opioid doses with adjuvants, but this could reflect confounding variables and physician choice. Larger prospective studies are needed to define the opioid-sparing effects of adjuvants.

CONCLUSION

Adjuvant agents are used in over 80 % of those treated for cancer pain.

Pain and nociception: mechanisms of cancer-induced bone pain

Cancer pain, especially pain caused by metastasis to bone, is a severe type of pain, and unless the cause and consequences can be resolved, the pain will become chronic. As detection and survival among patients with cancer have improved, pain has become an increasing challenge, because traditional therapies are often only partially effective. Until recently, knowledge of cancer pain mechanisms was poor compared with understanding of neuropathic and inflammatory pain states. We now view cancer-induced bone pain as a complex pain state involving components of both inflammatory and neuropathic pain but also exhibiting elements that seem unique to cancer pain. In addition, the pain state is often unpredictable, and the intensity of the pain is highly variable, making it difficult to manage. The establishment of translational animal models has started to reveal some of the molecular components involved in cancer pain. We present the essential pharmacologic and neurobiologic mechanisms involved in the generation and continuance of cancer-induced bone pain and discuss these in the context of understanding and treating patients. We discuss changes in peripheral signaling in the area of tumor growth, examine spinal cord mechanisms of sensitization, and finally address central processing. Our aim is to provide a mechanistic background for the sensory characteristics of cancer-induced bone pain as a basis for better understanding and treating this condition.

Pathobiology and management of prostate cancer-induced bone pain: recent insights and future treatments

Prostate cancer (PCa) has a high propensity for metastasis to bone. Despite the availability of multiple treatment options for relief of PCa-induced bone pain (PCIBP), satisfactory relief of intractable pain in patients with advanced bony metastases is challenging for the clinicians because currently available analgesic drugs are often limited by poor efficacy and/or dose-limiting side effects. Rodent models developed in the past decade show that the pathobiology of PCIBP comprises elements of inflammatory, neuropathic and ischemic pain arising from ectopic sprouting and sensitization of sensory nerve fibres within PCa-invaded bones. In addition, at the cellular level, PCIBP is underpinned by dynamic cross talk between metastatic PCa cells, cellular components of the bone matrix, factors associated with the bone microenvironment as well as peripheral components of the somatosensory system. These insights are aligned with the clinical management of PCIBP involving use of a multimodal treatment approach comprising analgesic agents (opioids, NSAIDs), radiotherapy, radioisotopes, cancer chemotherapy agents and bisphosphonates. However, a major drawback of most rodent models of PCIBP is their short-term applicability due to ethical concerns. Thus, it has been difficult to gain insight into the mal(adaptive) neuroplastic changes occurring at multiple levels of the somatosensory system that likely contribute to intractable pain at the advanced stages of metastatic disease. Specifically, the functional responsiveness of noxious circuitry as well as the neurochemical signature of a broad array of pro-hyperalgesic mediators in the dorsal root ganglia and spinal cord of rodent models of PCIBP is relatively poorly characterized. Hence, recent work from our laboratory to develop a protocol for an optimized rat model of PCIBP will enable these knowledge gaps to be addressed as well as identification of novel targets for drug discovery programs aimed at producing new analgesics for the improved relief of intractable PCIBP.

Flupirtine enhances the anti-hyperalgesic effects of morphine in a rat model of prostate bone metastasis

OBJECTIVE

Current treatments for cancer pain are often inadequate, particularly when metastasis to bone is involved. The addition to the treatment regimen of another drug that has a complementary analgesic effect may increase the overall analgesia without the necessity to increase doses, thus avoiding dose-related side effects. This project investigated the synergistic effect of the addition of the potassium channel (KCNQ2-3) modulator flupirtine to morphine treatment in a rat model of prostate cancer-induced bone pain.

DESIGN

Syngeneic prostate cancer cells were injected into the right tibia of male Wistar rats under anesthesia. This led to expanding tumor within the bone in 2 weeks, together with the concurrent development of hyperalgesia to noxious heat. Paw withdrawal thresholds from noxious heat were measured before and after the maximum non-sedating doses of morphine and flupirtine given alone and in combinations. Dose-response curves for morphine (0.13-5.0 mg/kg ip) and flupirtine (1.25-10.0 mg/kg ip) given alone and in fixed-dose combinations were plotted and subjected to an isobolographic analysis.

RESULTS

Both morphine (ED₅₀ = 0.74 mg/kg) and flupirtine (ED₅₀ = 3.32 mg/kg) caused dose-related anti-hyperalgesia at doses that did not cause sedation. Isobolographic analysis revealed that there was a synergistic interaction between flupirtine and morphine. Addition of flupirtine to morphine treatment improved morphine anti-hyperalgesia, and resulted in the reversal of cancer-induced heat hyperalgesia.

CONCLUSIONS

These results suggest that flupirtine in combination with morphine may be useful clinically to provide better analgesia at lower morphine doses in the management of pain caused by tumors growing in bone.

Contributions of spinal D-amino acid oxidase to bone cancer pain

D-Amino acid oxidase (DAAO), a FAD-dependent peroxisomal flavoenzyme that catalyzes oxidation of D-amino acids to hydrogen peroxide, is distributed in the spinal cord almost exclusively expressed within astrocytes. The present study aims to explore potential contributions of spinal DAAO to the development of bone cancer pain and morphine tolerance to analgesia. Tibia inoculation of carcinoma cells produced mechanical allodynia (but not heat hyperalgesia), in synchronous with induction of DAAO expression and DAAO enzymatic activity, as well as activation of spinal astrocytes marked by GFAP. Subcutaneous and intrathecal injection of the specific DAAO inhibitor CBIO (5-chloro-benzo[d]isoxazol-3-ol) blocked mechanical allodynia in a dose- and time-dependent manner in tumor-bearing rats, with maximum inhibition of 40-50 %. Multi-daily intrathecal injections of the DAAO gene silencer siRNA/DAAO also yielded anti-allodynic effects by approximately 40 % and the analgesia remained for at least 6 days. Subcutaneous injection of CBIO suppressed the production of spinal hydrogen peroxide and GFAP expression. 7-Day multiple bi-daily injections of CBIO produced anti-allodynia without inducing self-tolerance to analgesia or cross-tolerance to morphine, and concurrent injections of CBIO with morphine produced apparent additive anti-allodynia and completely prevented morphine tolerance in behaviors and spinal expression of μ-opioid receptors. Our results provide the first evidence that spinal DAAO contributes to the development of morphine tolerance to analgesia and bone cancer pain accounting for 40-50 % pain status, probably via production of hydrogen peroxide leading to activation of astrocytes. The unique characterizations of DAAO inhibitors make them a potential for the treatment of cancer pain when they are administered alone or in combination with morphine.

Pharmacological treatment of neuropathic cancer pain: a comprehensive review of the current literature

Neuropathic cancer pain (NCP), commonly encountered in clinical practice, may be cancer-related, namely resulting from nervous system tumor invasion, surgical nerve damage during tumor removal, radiation-induced nerve damage and chemotherapy-related neuropathy, or may be of benign origin, unrelated to cancer. A neuropathic component is evident in about 1/3 of cancer pain cases. Although from a pathophysiological perspective NCP may differ from chronic neuropathic pain (NP), such as noncancer-related pain, clinical practice, and limited publications have shown that these two pain entities may share some treatment modalities. For example, co-analgesics have been well integrated into cancer pain-management strategies and are often used as First-Line options for the treatment of NCP. These drugs, including antidepressants and anticonvulsants, are recommended by evidence-based guidelines, whereas, others such as lidocaine patch 5%, are supported by randomized, controlled, clinical data and are included in guidelines for restricted conditions treatment. The vast majority of these drugs have already been proven useful in the management of benign NP syndromes. Treatment decisions for patients with NP can be difficult. The intrinsic difficulties in performing randomized controlled trials in cancer pain have traditionally justified the acceptance of drugs already known to be effective in benign NP for the management of malignant NP, despite the lack of relevant high quality data. Interest in NCP mechanisms and pharmacotherapy has increased, resulting in significant mechanism-based treatment advances for the future. In this comprehensive review, we present the latest knowledge regarding NCP pharmacological management.

Differential expression of microRNAs in mouse pain models

BACKGROUND

MicroRNAs (miRNAs) are short non-coding RNAs that inhibit translation of target genes by binding to their mRNAs. The expression of numerous brain-specific miRNAs with a high degree of temporal and spatial specificity suggests that miRNAs play an important role in gene regulation in health and disease. Here we investigate the time course gene expression profile of miR-1, -16, and -206 in mouse dorsal root ganglion (DRG), and spinal cord dorsal horn under inflammatory and neuropathic pain conditions as well as following acute noxious stimulation.

RESULTS

Quantitative real-time polymerase chain reaction analyses showed that the mature form of miR-1, -16 and -206, is expressed in DRG and the dorsal horn of the spinal cord. Moreover, CFA-induced inflammation significantly reduced miRs-1 and -16 expression in DRG whereas miR-206 was downregulated in a time dependent manner. Conversely, in the spinal dorsal horn all three miRNAs monitored were upregulated. After sciatic nerve partial ligation, miR-1 and -206 were downregulated in DRG with no change in the spinal dorsal horn. On the other hand, axotomy increases the relative expression of miR-1, -16, and 206 in a time-dependent fashion while in the dorsal horn there was a significant downregulation of miR-1. Acute noxious stimulation with capsaicin also increased the expression of miR-1 and -16 in DRG cells but, on the other hand, in the spinal dorsal horn only a high dose of capsaicin was able to downregulate miR-206 expression.

CONCLUSIONS

Our results indicate that miRNAs may participate in the regulatory mechanisms of genes associated with the pathophysiology of chronic pain as well as the nociceptive processing following acute noxious stimulation. We found substantial evidence that miRNAs are differentially regulated in DRG and the dorsal horn of the spinal cord under different pain states. Therefore, miRNA expression in the nociceptive system shows not only temporal and spatial specificity but is also stimulus-dependent.

Behavioral, medical imaging and histopathological features of a new rat model of bone cancer pain

Pre-clinical bone cancer pain models mimicking the human condition are required to respond to clinical realities. Breast or prostate cancer patients coping with bone metastases experience intractable pain, which affects their quality of life. Advanced monitoring is thus required to clarify bone cancer pain mechanisms and refine treatments. In our model of rat femoral mammary carcinoma MRMT-1 cell implantation, pain onset and tumor growth were monitored for 21 days. The surgical procedure performed without arthrotomy allowed recording of incidental pain in free-moving rats. Along with the gradual development of mechanical allodynia and hyperalgesia, behavioral signs of ambulatory pain were detected at day 14 by using a dynamic weight-bearing apparatus. Osteopenia was revealed from day 14 concomitantly with disorganization of the trabecular architecture (µCT). Bone metastases were visualized as early as day 8 by MRI (T₁-Gd-DTPA) before pain detection. PET (Na¹⁸F) co-registration revealed intra-osseous activity, as determined by anatomical superimposition over MRI in accordance with osteoclastic hyperactivity (TRAP staining). Pain and bone destruction were aggravated with time. Bone remodeling was accompanied by c-Fos (spinal) and ATF3 (DRG) neuronal activation, sustained by astrocyte (GFAP) and microglia (Iba1) reactivity in lumbar spinal cord. Our animal model demonstrates the importance of simultaneously recording pain and tumor progression and will allow us to better characterize therapeutic strategies in the future.

Tibia tumor-induced cancer pain involves spinal p38 mitogen-activated protein kinase activation via TLR4-dependent mechanisms

Molecular mechanisms underlying bone cancer pain are poorly understood. Recently, p38 mitogen-activated protein kinase (MAPK) activation was shown to play a major role not only in the production of proinflammatory cytokines but also in the progression of inflammatory and neuropathic pain. We have demonstrated that tactile allodynia and spontaneous pain of female rats with tibia tumors were correlated with the increase of both phosphorylated-p38MAPK (p-p38MAPK) and proinflammatory cytokines (IL-1beta and TNF-alpha) in the spinal cord 6 days after Walker 256 cells' inoculation. This change was specific to bone cancer pain because rats without tibia tumors failed to show such an increase. On the other hand, a 3-day administration [4 microg/rat/day, intrathecally (i.t.)] of 4-(4-fluorophenyl)-2-(4-methylsulfinylphenyl)-5-(4-pyridyl)1H-imidazole (SB203580), an inhibitor of p38MAPK, could suppress tactile allodynia and spontaneous pain of the bone cancer pain rats and decrease the phosphorylation of p38 as well as the expression of IL-1beta and TNF-alpha. To characterize the cellular events upstream of p38MAPK, we have examined the role of the toll-like receptor 4 (TLR4), which had been suggested to be involved in pain hypersensitivity. We found that prolonged knockdown of TLR4 during the 3-day administration of TLR4 small interfering RNA (siRNA; 2 microg/rat/day, i.t.) could attenuate hyperalgesia developed by Walker 256 cells' inoculation and decrease the phosphorylation of p38 as well as the increase of IL-1beta and TNF-alpha expression. These results demonstrate that TLR4-dependent phosphorylation of p38MAPK in spinal cord of rats might contribute to the development and maintenance of bone cancer pain, and p38MAPK and TLR4 would possibly be the potential targets for pain therapy.

Ascorbic acid accelerates osteoclast formation and death

Ascorbic acid (AA) plays a key role in bone formation. However controversy remains about the effect of AA on cells responsible for bone destruction, osteoclasts. We investigated the effect of AA on osteoclastogenesis using primary mouse bone marrow cultures and monocytic RAW 264.7 cells treated with osteoclastogenic factors RANKL and MCSF. Treatment with AA resulted in significant increase in osteoclast number, size and nucleation. To assess osteoclast oxidative stress level, a ratio of reduced (GSH) to oxidized (GSSG) glutathione and the total glutathione content (GSH(t)) were evaluated. Osteoclast differentiation was associated with a decrease in GSH/GSSG and GSH(t). AA induced further decrease in both parameters, and resulted in significant production of H(2)O(2), indicating its pro-oxidant action. At low concentration, H(2)O(2) induced similar effects to AA, although less potently, and catalase partially inhibited AA-induced osteoclastogenesis. To assess the modification in osteoclast metabolism, the mitochondrial activity was evaluated using JC-1 and the ATP levels were assessed. Osteoclast formation was associated with the increase in mitochondrial activity and ATP concentration, which were further increased in the presence of AA. Importantly, the stimulatory effect of AA was only evident at early phase of osteoclastogenesis, whereas at the late stage AA significantly accelerated osteoclast death. Thus, during osteoclastogenesis AA acts as an oxidant, first stimulating osteoclast formation, but later limiting osteoclast lifespan. This duality of AA action allows reconciling the stimulatory action of AA on osteoclastogenesis observed in vitro with an overall attenuation of bone resorption in the presence of AA observed in vivo.

A novel approach to the use of animals in studies of pain: validation of the canine brief pain inventory in canine bone cancer

OBJECTIVE

To validate the Canine Brief Pain Inventory (CBPI), which is based on the human Brief Pain Inventory (BPI), in a canine model of spontaneous bone cancer.

DESIGN AND PARTICIPANTS

One hundred owners of dogs with bone cancer self-administered the CBPI on three occasions to test the reliability, validity, and responsiveness of the measure.

OUTCOME MEASURES

Factor analysis, internal consistency, convergent validity, and an extreme group validation assessment were completed using the responses from the first administration of the CBPI. Test-retest reliability was evaluated using two administrations of the instrument, 1 week apart. Responsiveness was tested by comparing responses 3 weeks apart.

RESULTS

The "severity" and "interference" factors hypothesized based on the BPI were demonstrated in the CBPI in dogs with bone cancer. Internal consistency was high (Cronbach's alpha, 0.95 and 0.93), as was test-retest reliability (kappa, 0.73 and 0.65). Convergent validity was demonstrated with respect to quality of life (r = 0.49 and 0.63). Extreme group validation against normal dogs showed significantly higher factor scores (P < 0.001 for both).

CONCLUSIONS

The CBPI reliably measures the same pain constructs in the companion canine model of spontaneous bone cancer as the BPI does in people with bone cancer. This innovative approach to preclinical outcomes development, validating a preclinical outcome measure that directly corresponds to an outcome measure routinely used in clinical research, applied to a readily available animal model of spontaneous disease could transform the predictive ability of preclinical pain studies.

Intravenous ibandronate rapidly reduces pain, neurochemical indices of central sensitization, tumor burden, and skeletal destruction in a mouse model of bone cancer

Over half of all chronic cancer pain arises from metastases to bone and bone cancer pain is one of the most difficult of all persistent pain states to fully control. Currently, bone pain is treated primarily by opioid-based therapies, which are frequently accompanied by significant unwanted side effects. In an effort to develop nonopioid-based therapies that could rapidly attenuate tumor-induced bone pain, we examined the effect of intravenous administration of the bisphosphonate, ibandronate, in a mouse model of bone cancer pain. Following injection and confinement of green fluorescent protein-transfected murine osteolytic 2472 sarcoma cells into the marrow space of the femur of male C3H/HeJ mice, ibandronate was administered either as a single dose (300 microg/kg), at Day 7 post-tumor injection, when tumor-induced bone destruction and pain were first evident, or in three consecutive doses (100 microg/kg/day) at Days 7, 8, and 9 post-tumor injection. Intravenous ibandronate administered once or in three consecutive doses reduced ongoing and movement-evoked bone cancer pain-related behaviors, neurochemical markers of central sensitization, tumor burden, and tumor-induced bone destruction. These results support limited clinical trials that suggest the potential of ibandronate to rapidly attenuate bone pain and illuminate the mechanisms that may be responsible for limiting pain and disease progression.

Endogenous tumor necrosis factor alpha (TNFalpha) requires TNF receptor type 2 to generate heat hyperalgesia in a mouse cancer model

To provide a tool to investigate the mechanisms inducing and maintaining cancer-related pain and hyperalgesia, a soft tissue tumor/metastasis model was developed that is applicable in C57BL/6J wild-type and transgenic mice. We show that the experimental tumor-induced heat hyperalgesia and nociceptor sensitization were prevented by systemic treatment with the tumor necrosis factor alpha (TNFalpha) antagonist etanercept. In naive mice, exogenous TNFalpha evoked heat hyperalgesia in vivo and sensitized nociceptive nerve fibers to heat in vitro. TNFalpha enhanced the expression of the nociceptor-specific heat transducer ion channel transient receptor potential vanilloid 1 (TRPV1) and increased the amplitudes of capsaicin and heat-activated ionic currents via p38/MAP (mitogen-activated protein) kinase and PKC (protein kinase C). Deletion of the tumor necrosis factor receptor type 2 (TNFR2) gene attenuated heat hyperalgesia and prevented TRPV1 upregulation in tumor-bearing mice, whereas TNFR1 gene deletion played a minor role. We propose endogenous TNFalpha as a key player in cancer-related heat hyperalgesia and nociceptor sensitization that generates TRPV1 upregulation and sensitization via TNFR2.

Translational medicine: cancer pain mechanisms and management

Cancer-induced bone pain (CIBP) is a major clinical problem with up to 85% of patients with bony metastases having pain, often associated with anxiety and depression, reduced performance status, and a poor quality of life. Malignant bone disease creates a chronic pain state through sensitization and synaptic plasticity within the spinal cord that amplifies nociceptive signals and their transmission to the brain. Fifty per cent of patients are expected to gain adequate analgesia from palliative radiotherapy within 4-6 weeks of treatment. Opioid analgesia does make a useful contribution to the management of CIBP, especially in terms of suppressing tonic background pain. However, CIBP remains a clinical challenge because the spontaneous and movement-related components are more difficult to treat with opioids and commonly used analgesic drugs, without unacceptable side-effects. Recently developed laboratory models of CIBP, which show congruency with the clinical syndrome, are contributing to an improved understanding of the neurobiology of CIBP. This chronic pain syndrome appears to be unique and distinct from other chronic pain states, such as inflammatory or neuropathic pain. This has clear implications for treatment and development of future therapies. A translational medicine approach, using a highly iterative process between the clinic and the laboratory, may allow improved understanding of the underlying mechanisms of CIBP to be rapidly translated into real clinical benefits in terms of improved pain management.

Pharmacological evaluation of opioid and non-opioid analgesics in a murine bone cancer model of pain

The intramedulary injection of osteosarcoma cells in the mouse femur has served as a laboratory model to study bone cancer pain. However, the efficacy of different classes of analgesics has not fully been analyzed in this model. Therefore, the acute antinociceptive properties of different classes of drugs were evaluated on post-inoculation day 15 when the degrees of spontaneous pain and mechanical hypersensitivity in the ipsilateral inoculated hind paw reached almost their maximal effects. At high doses, the opioids fentanyl, morphine, and tramadol had full efficacies for all pain parameters tested. Antagonism experiments with naloxone (10 mg/kg s.c.) or its peripheral analogue methylnaltrexone (10 mg/kg s.c.), suggest that the analgesic effects of fentanyl were predominantly mediated by centrally located mu-opiate receptors. Acetaminophen, the non-steroidal anti-inflammatory drug indomethacin, and the COX-2-inhibitor celecoxib did not significantly improve pain behavior. The tricyclic antidepressants amitriptyline and desipramine significantly reduced spontaneous pain behavior but this only at sedative doses; the serotonin reuptake inhibitor fluoxetine had limited efficacy. Also with the anticonvulsants lamotrigine, topiramate, and gabapentin limited or no efficacies were found. In conclusion, the present study provided integrated information about the tumor-induced bone pain in mice, and clarified acute efficacies of different categories of analgesics for the spontaneous lifting, limb-use impairment, and mechanical hypersensitivity. Moreover, the finding that bone cancer-pain behaviors are attenuated by various established compounds further supports the validity of the murine bone cancer model for the study of bone cancer pain and its use for the identification of novel treatments.

Biology of Bone Cancer Pain

Bone cancer pain is a devastating manifestation of metastatic cancer. Unfortunately, current therapies can be ineffective, and when they are effective, the duration of the patient's survival typically exceeds the duration of pain relief. New, mechanistically based therapies are desperately needed. Study of experimental animal models has provided insight into the mechanisms that drive bone cancer pain and provides an opportunity for developing targeted therapies. Mechanisms that drive bone cancer pain include tumor-directed osteoclast-mediated osteolysis, tumor cells themselves, tumor-induced nerve injury, stimulation of transient receptor potential vanilloid type 1 ion channel, endothelin A, and host cell production of nerve growth factor. Current and future therapies include external beam radiation, osteoclast-targeted inhibiting agents, anti-inflammatory drugs, transient receptor potential vanilloid type 1 antagonists, and antibody therapies that target nerve growth factor or tumor angiogenesis. It is likely that a combination of these therapies will be superior to any one therapy alone.

Advances in understanding bone cancer pain

Experimental animal models of bone cancer pain have emerged and findings have provided a unique glimpse into unraveling the mechanism that drives this debilitating condition. Key contributors to the generation and maintenance of bone cancer pain are tumor-induced osteolysis, tumor itself, and production of nociceptive mediators in the bone-tumor microenvironment.

The analgesic effect of low dose focal irradiation in a mouse model of bone cancer is associated with spinal changes in neuro-mediators of nociception

Despite the widespread use of radiotherapy to treat painful bone metastases, the mechanism underlying the analgesic effect of low dose ionizing radiation is unknown. Bone cancer pain is mostly associated with an inflammatory response dominated by local activation of osteoclasts and by astrogliosis in the spinal cord. We determined the effects of a 6 Gy irradiation given focally on osteolytic sarcoma cells inoculated in humeri of mice. Pain behavior was assessed using the rota-rod and the grip force test. Seven days post-irradiation (day 17 post-tumor implantation) the performance of mice markedly improved on the rotarod (non-irradiated, 67+/-16s vs irradiated, 223 +/- 22 s; P = 0.0005), and the grip force test (non-irradiated, 34 +/- 4 g vs irradiated, 55 +/- 2 g; P = 0.001). This improvement was similar to the analgesia achieved with 30 mg/kg of the cyclooxygenase (COX) inhibitor ketorolac (Rota-rod, 67 +/- 16 s vs 178 +/- 35 s; P = 0.01: grip force test, 34 +/- 4 g, vs 60 +/- 5 g; P = 0.003). Following irradiation, the tumor mass and the number of osteoclasts did not decrease while the expression of two pro-inflammatory cytokines (monocyte chemoattractant protein (MCP)-1 and tumor necrosis factor (TNF)-alpha) increased. Tumor irradiation led to clear differences in the spinal cord. These include a decrease in glial activity (astrocytes and microglial cells) as well as pain mediators such as dynorphin, COX-2 and chemotactic cytokine receptor (CCR2). We conclude that the analgesic effect of low dose irradiation of bone cancer is associated with the alteration of nociceptive transmission in the central nervous system.

Evidence of a role for descending serotonergic facilitation in a rat model of cancer-induced bone pain

Descending modulation of spinal processing plays an important role in chronic pain states. Monoamine pathways comprise a major component of descending controls from the brainstem to the spinal cord. Recent emphasis has been on facilitatory actions mediated by the 5-HT3 receptor. We investigated the effects of spinally administered ondansetron, a selective 5-HT3 receptor antagonist, on electrical- and natural-evoked dorsal horn (DH) neuronal responses in a rat model of cancer-induced bone pain (CIBP). Injection of MRMT-1 cells into the tibiae of Sprague-Dawley rats was used to model CIBP, whilst sham-operated rats were injected with the cell medium alone. Behavioural testing at regular intervals monitored the development of mechanical allodynia, cold allodynia, and ambulatory-evoked pain. In vivo electrophysiology experiments were carried out 15-17 days after surgery, when there were significant behavioural and neuronal alterations in the cancer animals. Spinally administered ondansetron (10, 50, and 100 microg) had no effect on electrical-evoked neuronal responses, but significantly reduced mechanical- and thermal-evoked responses in both the groups of animals. Furthermore, the effects of ondansetron were significantly greater in cancer animals compared to shams. These results therefore suggest a role for descending serotonergic facilitation in CIBP.

Evaluation of pain-related behavior, bone destruction and effectiveness of fentanyl, sufentanil, and morphine in a murine model of cancer pain

The present study was conducted to evaluate the pain development and bone destruction during bone cancer growth in a murine model of bone cancer pain and to evaluate the analgesic efficacy of fentanyl, sufentanil, and morphine in this model. C3H/HeNCrl mice were inoculated into the intramedullary space of the femur with osteolytic NCTC 2472 fibrosarcoma cells, and followed during a 3-week period to assess pain behaviors (spontaneous lifting and limb-use during forced ambulation on rotarod) and bone destruction (parameters indicative of bone lesions determined by microCT-scans of the tumor-bearing bones) during bone cancer growth. The results showed that in this murine model of cancer-induced bone pain, behavioural manifestations of pain emerge in parallel with the progression of bone destruction. The subcutaneous administration of fentanyl (0.025-0.64 mg/kg), sufentanil (0.005-0.04 mg/kg), and morphine (2.5-40 mg/kg) on the test days 15 and 22 post-inoculation reduced pain-related behaviors in a dose dependent manner. A complete relief from pain-related behaviors was achieved with the following doses: > or =0.16 mg/kg fentanyl, 0.02 mg/kg sufentanil, and 20 mg/kg morphine. In conclusion, the results showed a clear link between tumor growth-induced bone destruction and behavioral pain manifestations, the latter was effectively controlled by the opioids fentanyl, sufentanil, and morphine.

Emerging analgesics in cancer pain management

Cancer pain is one of the most frequent symptoms in malignant disease, severely impairing the patients' quality of life. The recommendations of the World Health Organization will provide adequate pain relief for the vast majority of cancer patients. However, some patients will suffer from inadequate analgesia or intolerable side effects. Cyclooxygenase-2 (COX-2)-selective non-steroidal anti-inflammatory drugs (NSAIDs), new anticonvulsants, cannabinoids and NMDA receptor antagonists are being developed for these patients. NSAIDs with nitric oxide-releasing moieties are an interesting addition, as this new class of analgesics combines improved analgesic efficacy with higher tolerability. Conotoxins and other drugs such as nicotinic acetylcholinergic receptor agonists will be advantageous only for a few patients in the near future, as side-effect profile and risk of complications, as well as the burden on the patient, often are not worth the additional analgesic benefit.

Underlying mechanisms of pronociceptive consequences of prolonged morphine exposure

The opioid analgesics, commonly exemplified by morphine, represent the best option for the treatment of severe pain and for the management of chronic pain states, of both malignant and nonmalignant origin. It is well recognized that the prolonged use of opioids is associated with a requirement for ever-increasing doses in order to maintain pain relief at an acceptable and consistent level. This phenomenon is termed analgesic tolerance. While the concept that tolerance can develop as a result of cellular adaptations to the presence of the opioid has been proposed, it is now becoming abundantly clear that tolerance may also be related to a state of hyperalgesia that results from exposure to the opioid itself. Patients who receive long-term opioid therapy sometimes develop unexpected, abnormal pain. Similar paradoxical opioid-induced pain has been confirmed in a number of animal studies, even during the period of continuous opioid delivery. A number of recent studies have demonstrated that such pain may be secondary to neuroplastic changes that occur in the brain and spinal cord. One such change may be the activation of descending pain facilitation mechanisms arising from the rostral ventromedial medulla (RVM) elicited in part by increased activity of cholecystokinin (CCK) in the RVM. A cascade of pronociceptive events may follow, such as opioid-induced upregulation of spinal dynorphin levels that promotes enhanced input from primary afferent nociceptors. This mechanism appears to depend on intact descending pathways from the RVM, since interrupting this pathway abolishes enhanced abnormal pain. Furthermore, extended opioid exposure also can elicit increased calcitonin gene related peptide (CGRP) and substance P expression in the dorsal root ganglia. It is probable that increased pain elicited by opioids is a critical factor in the behavioral manifestation of opioid tolerance because the same manipulations that block abnormal pain also block antinociceptive tolerance. Taken together, such studies show that opioids elicit systems-level adaptations resulting in pain due to descending facilitation, upregulation of spinal dynorphin, and enhanced, evoked release of excitatory transmitters from primary afferents. These adaptive changes in response to sustained exposure to opioids indicate the need for the evaluation of the clinical consequences of long-term opioid administration. Additionally, these findings suggest a need for novel chemistry involving design of agents that may counteract opiate-induced neuroplastic adaptations resulting in pain relief without analgesic tolerance.

Bone cancer pain: the effects of the bisphosphonate alendronate on pain, skeletal remodeling, tumor growth and tumor necrosis

Patients with metastatic breast, lung or prostate cancer frequently have significant bone cancer pain. In the present report we address, in a single in vivo mouse model, the effects the bisphosphonate alendronate has on bone cancer pain, bone remodeling and tumor growth and necrosis. Following injection and confinement of green fluorescent protein-transfected murine osteolytic tumor cells into the marrow space of the femur of male C3H/HeJ mice, alendronate was administered chronically from the time the tumor was established until the bone cancer pain became severe. Alendronate therapy reduced ongoing and movement-evoked bone cancer pain, bone destruction and the destruction of sensory nerve fibers that innervate the bone. Whereas, alendronate treatment did not change viable tumor burden, both tumor growth and tumor necrosis increased. These data emphasize that it is essential to utilize a model where pain, skeletal remodeling and tumor growth can be simultaneously assessed, as each of these can significantly impact patient quality of life and survival.

Bone cancer pain model in mice: evaluation of pain behavior, bone destruction and morphine sensitivity

The primary aim of the study was to correlate pain development during bone cancer growth with objectively obtained tumor-induced changes in bone morphology. Additionally morphine sensitivity of this bone pain was evaluated. Mice were injected into the femur with osteolytic NCTC2472 cells, and behaviorally followed during a 3-week period. During the observation period increasing pain behavior was observed in tumor-bearing animals. Tumor mice exhibited spontaneous and movement-evoked lifting, the latter evoked through non-noxious palpation of the tumor. Limb use during forced ambulation on a rotarod decreased to substantial non-use of the affected limb by day 23. On day 23, micro-computer tomography scans of the tumor-bearing bones were evaluated for bone destruction. Different bone parameters indicative of osteolysis or fragmentation were significantly correlated with pain behavior. In a separate group of mice the effects of different morphine doses on pain behavior were evaluated on days 17 and 21 of tumor growth. Spontaneous lifting and movement-evoked lifting were sensitive to morphine treatment, although stress-induced analgesia due to repeated restraint might minimize movement-evoked lifting in mice. Limb use during forced ambulation was only slightly ameliorated by high morphine doses.

A preclinical post laminectomy rat model mimics the human post laminectomy syndrome

Chronic low back pain with sciatica complicating post laminectomy surgery is poorly understood. It is likely that some aspects of persistent pain of the syndrome results from spinal facilitation in which there is lowering of pain excitation levels. A small animal preclinical model is needed that mimics the clinical condition to permit detailed studies of the underlying altered neurochemistry of the sensory pathways. We propose herein a rat laminectomy model containing the elements required for study of the neurobiology of the condition. The model consists of a surgical laminectomy that includes L5 spinal nerve manipulation and disc injury, elements necessarily employed in human disc herniation surgery. At 8 weeks post laminectomy the proposed model demonstrates paraspinous muscle spasm, tail contracture, behavioral pain behavior, tactile allodynia, epidural and nerve root scarring, and nerve root adherence by scar to the underlying disc and adjacent pedicle. Two underlying pain facilitation states are invoked in the clinical condition: (1) an inflammatory state required to achieve wound healing; and (2) a nerve injury state resulting from nerve manipulation and subsequent epidural scarring, spinal nerve scarring, and spinal nerve tethering to the adjacent disc and pedicle. Both pain facilitation states are active in the model.

The pathophysiology of cancer-induced bone pain: current understanding

Cancer-induced bone pain (CIBP) is a common clinical problem. Although treatment has been revolutionised in the past 10 years with the introduction of bisphosphonates, pain arising spontaneously or from movement, remains a leading cause of unresolved pain in many patients. Until recently little was understood about the peripheral and central mechanisms of bone pain. Insight into the mechanisms of osteoblast and osteoclast activation, via receptor activator for nuclear factor KB (RANK) dependent and independent mechanisms and a re-evaluation of primary afferent terminals within bone have led to a suggestion that CIBP may be a mixture of inflammatory and neuropathic stimuli. The recently published animal model of localised but progressive bone destruction has allowed greater insight into the peripheral and dorsal horn pathophysiology, which hitherto was precluded. Immunocytochemical markers of neurotransmitters and receptors indicate that CIBP has unique characteristics, unlike neuropathy or inflammation. Evidence for an increased excitability within the dorsal horn, and especially Lamina I, and possible mechanisms underlying this unique pain state will be discussed.