Initial thermal heat hypoalgesia and delayed hyperalgesia in a murine model of bone cancer pain

Experiences and reflections about behavioral pain assays in laboratory animals

Pharmacological assays based on the measurement of nociceptive responses in laboratory animals are a fundamental tool to assess analgesic strategies. During our experience with this type of experiments, we have been repeatedly challenged by different concerns related to their interpretation or relevance. Although these subjects are frequently discussed in our lab, they do not usually find a place in research articles with original data, in which the focus on results seems mandatory. In the present manuscript we try to discuss as central issues some of these aspects that often cross transversally our research. We have gathered them in five topics inspired by the results obtained in our laboratory. The two initial sections are devoted to the influence of the behavioral method used to assess nociception on the results achieved, as well as to the possibility that data may be more easily accepted when obtained with standard methods than with alternative ones. The third topic is related to the difficulties encountered when working with a molecule that may evoke dual effects, acting as pronociceptive or antinociceptive depending on the dose. The fourth point deals with the situation in which a particular hyperalgesic reaction is related to several molecules but the single inhibition of only one of them can completely prevent it. Finally, the last issue is addressed to comment the impact in the progress of pain research of experiments performed in animal models of pathological settings.

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.

Effect and Mechanism of Endothelin Receptor A Inhibitor BQ-123 Combined with Electroacupuncture on Tibia Cancer Pain in Rats

Objective

To research the impact and mechanism of endothelin receptor A inhibitor BQ-123 combined with electroacupuncture on tibia cancer pain in rats.

Methods

Sprague-Dawley (SD) rats were randomly divided into sham group (SHAM group) and bone cancer pain model group (BCP group). The behavior of SD rats was measured. The histology of the right tibia was observed by hematoxylin-eosin (HE) staining. The remaining rats were randomly divided into model, BQ-123, electroacupuncture, and BQ-123+ electroacupuncture group. Behavioral tests were performed, and mechanical pain threshold (MWT) and thermal pain threshold (TWL) were measured. The expressions of α-smooth muscle actin (αSMA), ETAR (endothelin A receptor), ETB (End of Transmission Block), P-Phosphatidylinositol 3-kinase (PI3K), and P-Protein kinase B (Akt) were detected by real-time fluorescence quantitative PCR and western blot.

Results

In the BCP group, bone structure was severely damaged, local tissue swelling was obvious, bone trabecula was missing, and bone cortex was discontinuous. The optical density of Glial fibrillary acidic protein (GFAP) and CD11b immunoreactive signal in BCP group was significantly increased, and most of the ETAR of endothelin receptor was comapped with NeuN, and a small part of GFAP was comapped with CD11b, but no comapped with CD11b. The AS score of BQ-123+ electroacupuncture group was significantly lower than that of BQ-123 group and electroacupuncture group (P < 0.05), whereas the MWT and TWL values were significantly higher than that of the BQ-123 group and electroacupuncture group (P < 0.05). The mRNA expression of α-SMA and ETAR in BQ-123+ electroacupuncture group was lower than that in BQ-123 and electroacupuncture group, and the protein expression of P-PI3K and P-Akt in BQ-123+ electroacupuncture group was lower as well.

Conclusion

BQ-123 may inhibit the activation of PI3K/Akt signal path combined with electroacupuncture to alleviate the effects of tibia cancer pain in rats.

Peripheral nerve injury and sensitization underlie pain associated with oral cancer perineural invasion

Cancer invading into nerves, termed perineural invasion (PNI), is associated with pain. Here, we show that oral cancer patients with PNI report greater spontaneous pain and mechanical allodynia compared with patients without PNI, suggesting that unique mechanisms drive PNI-induced pain. We studied the impact of PNI on peripheral nerve physiology and anatomy using a murine sciatic nerve PNI model. Mice with PNI exhibited spontaneous nociception and mechanical allodynia. Perineural invasion induced afterdischarge in A high-threshold mechanoreceptors (HTMRs), mechanical sensitization (ie, decreased mechanical thresholds) in both A and C HTMRs, and mechanical desensitization in low-threshold mechanoreceptors. Perineural invasion resulted in nerve damage, including axon loss, myelin damage, and axon degeneration. Electrophysiological evidence of nerve injury included decreased conduction velocity, and increased percentage of both mechanically insensitive and electrically unexcitable neurons. We conclude that PNI-induced pain is driven by nerve injury and peripheral sensitization in HTMRs.

A low pKa ligand inhibits cancer-associated pain in mice by activating peripheral mu-opioid receptors

The newly designed fentanyl derivative [( ±)-N-(3-fluoro-1-phenethylpiperidine-4-yl)-N-phenyl propionamide] (NFEPP) was recently shown to produce analgesia selectively via peripheral mu-opioid receptors (MOR) at acidic pH in rat inflamed tissues. Here, we examined the pH-dependency of NFEPP binding to brain MOR and its effects on bone cancer-induced pain in mice. The IC50 of NFEPP to displace bound [3H]-DAMGO was significantly higher compared to fentanyl at pH 7.4, but no differences were observed at pH 5.5 or 6.5. Intravenous NFEPP (30-100 nmol/kg) or fentanyl (17-30 nmol/kg) inhibited heat hyperalgesia in mice inoculated with B16-F10 melanoma cells. The peripherally-restricted opioid receptor antagonist naloxone-methiodide reversed the effect of NFEPP (100 nmol/kg), but not of fentanyl (30 nmol/kg). The antihyperalgesic effect of NFEPP was abolished by a selective MOR- (cyprodime), but not delta- (naltrindole) or kappa- (nor-binaltorphimine) receptor antagonists. Ten-fold higher doses of NFEPP than fentanyl induced maximal antinociception in mice without tumors, which was reversed by the non-restricted antagonist naloxone, but not by naloxone-methiodide. NFEPP also reduced heat hyperalgesia produced by fibrosarcoma- (NCTC 2472) or prostate cancer-derived (RM1) cells. These data demonstrate the increased affinity of NFEPP for murine MOR at low pH, and its ability to inhibit bone cancer-induced hyperalgesia through peripheral MOR. In mice, central opioid receptors may be activated by ten-fold higher doses of NFEPP.

Effect of Peritumoral Bupivacaine on Primary and Distal Hyperalgesia in Cancer-Induced Bone Pain

Background

Cancer-induced bone pain (CIBP) is a debilitating chronic pain condition caused by injury to bone nerve terminals due to primary or metastasized bone tumors. Pain manifests as enhanced sensitivity, not only over the affected bone site but also at distal areas that share common nerve innervation with the tumor. In this study, we aim to understand how tumor-induced primary and distal pain sensitivities are affected by bupivacaine-induced block of bone nerve endings in a rat model of CIBP.

Methods

MRMT-1 breast cancer cells were injected into the proximal segment of tibia in female Sprague–Dawley rats. Radiograms and micro-CT images were obtained to confirm tumor growth. Bupivacaine was injected peritumorally at day 7 or day 14 post-tumor induction, and withdrawal thresholds in response to pressure and punctate mechanical stimulus were recorded from the knee and hind-paw, respectively. Immunohistochemical studies for the determination of ATF3 and GFAP expression in DRG and spinal cord sections were performed.

Results

Rats developed primary and distal hyperalgesia after MRMT-1 administration that was sustained for 2 weeks. Peritumoral administration of bupivacaine in 7-day post-tumor-induced (PTI) rats resulted in a reversal of both primary and distal hyperalgesia for 20–30 mins. However, bupivacaine failed to reverse distal hyperalgesia in 14 day-PTI rats. ATF3 and GFAP expression were much enhanced in 14 day-PTI animals, compared to 7 day-PTI group.

Conclusion

Results from this study strongly suggest that distal hyperalgesia of late-stage CIBP demonstrates differential characteristics consistent with neuropathic pain as compared to early stage, which appears more inflammatory in nature.

Transcriptomic characterisation of the optimised rat model of Walker 256 breast cancer cell-induced bone pain

In patients with breast cancer, metastases of cancer cells to the axial skeleton may cause excruciating pain, particularly in the advanced stages. The current drug treatments available to alleviate this debilitating pain condition often lack efficacy and/or produce undesirable side effects. Preclinical animal models of cancer-induced bone pain are key to studying the mechanisms that cause this pain and for the success of drug discovery programs. In a previous study conducted in our laboratory, we validated and characterised the rat model of Walker 256 cell-induced bone pain, which displayed several key resemblances to the human pain condition. However, gene level changes that occur in the pathophysiology of cancer-induced bone pain in this preclinical model are unknown. Hence, in this study, we performed the transcriptomic characterisation of the Walker 256 cell line cultured in vitro to predict the molecular genetic profile of this cell line. We also performed transcriptomic characterisation of the Walker 256 cell-induced bone pain model in rats using the lumbar spinal cord and lumbar dorsal root ganglia tissues. Here we show that the Walker 256 cell line resembles the basal-B molecular subtype of human breast cancer cell lines. We also identify several genes that may underpin the progression of pain hypersensitivities in this condition, however, this needs further confirmatory studies. These transcriptomic insights have the potential to direct future studies aimed at identifying various mechanisms underpinning pain hypersensitivities in this model that may also assist in discovery of novel pain therapeutics for breast cancer-induced bone pain.

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.

Cancer pain and neuropathic pain are associated with A β sensory neuronal plasticity in dorsal root ganglia and abnormal sprouting in lumbar spinal cord

Evidence suggests that there are both nociceptive and neuropathic components of cancer-induced pain. We have observed that changes in intrinsic membrane properties and excitability of normally non-nociceptive Aβ sensory neurons are consistent in rat models of peripheral neuropathic pain and cancer-induced pain. This has prompted a comparative investigation of the intracellular electrophysiological characteristics of sensory neurons and of the ultrastructural morphology of the dorsal horn in rat models of neuropathic pain and cancer-induced pain. Neuropathic pain model rats were induced with a polyethylene cuff implanted around a sciatic nerve. Cancer-induced pain model rats were induced with mammary rat metastasis tumour-1 rat breast cancer or MATLyLu rat prostate cancer cells implanted into the distal epiphysis of a femur. Behavioural evidence of nociception was detected using von Frey tactile assessment. Aβ-fibre low threshold mechanoreceptor neurons in both cancer-induced pain and neuropathic pain models exhibited slower dynamics of action potential genesis, including a wider action potential duration and lower action potential amplitude compared to those in control animals. Enhanced excitability of Aβ-fibre low threshold mechanoreceptor neurons was also observed in cancer-induced pain and neuropathic pain models. Furthermore, both cancer-induced pain and neuropathic pain models showed abundant abnormal axonal sprouting in bundles of myelinated axons in the ipsilateral spinal laminae IV and V. The patterns of changes show consistency between rat models of cancer-induced pain and neuropathic pain. These findings add to the body of evidence that animal models of cancer-induced pain and neuropathic pain share features that may contribute to the peripheral and central sensitization and tactile hypersensitivity in both pain states.

Optimization and In Vivo Profiling of a Refined Rat Model of Walker 256 Breast Cancer Cell-Induced Bone Pain Using Behavioral, Radiological, Histological, Immunohistochemical and Pharmacological Methods

In the majority of patients with advanced breast cancer, there is metastatic spread to bones resulting in pain. Clinically available drug treatments for alleviation of breast cancer-induced bone pain (BCIBP) often produce inadequate pain relief due to dose-limiting side-effects. A major impediment to the discovery of novel well-tolerated analgesic agents for the relief of pain due to bony metastases is the fact that most cancer-induced bone pain models in rodents relied on the systemic injection of cancer cells, causing widespread formation of cancer metastases and poor general animal health. Herein, we have established an optimized, clinically relevant Wistar Han female rat model of breast cancer induced bone pain which was characterized using behavioral assessments, radiology, histology, immunohistochemistry and pharmacological methods. In this model that is based on unilateral intra-tibial injection (ITI) of Walker 256 carcinoma cells, animals maintained good health for at least 66 days post-ITI. The temporal development of hindpaw hypersensitivity depended on the initial number of Walker 256 cells inoculated in the tibiae. Hindpaw hypersensitivity resolved after approximately 25 days, in the continued presence of bone tumors as evidenced by ex vivo histology, micro-computed tomography scans and immunohistochemical assessments of tibiae. A possible role for the endogenous opioid system as an internal factor mediating the self-resolving nature of BCIBP was identified based upon the observation that naloxone, a non-selective opioid antagonist, caused the re-emergence of hindpaw hypersensitivity. Bolus dose injections of morphine, gabapentin, amitriptyline and meloxicam all alleviated hindpaw hypersensitivity in a dose-dependent manner. This is a first systematic pharmacological profiling of this model by testing standard analgesic drugs from four important diverse classes, which are used to treat cancer induced bone pain in the clinical setting. Our refined rat model more closely mimics the pathophysiology of this condition in humans and hence is well-suited for probing the mechanisms underpinning breast cancer induced bone pain. In addition, the model may be suitable for efficacy profiling of new molecules from drug discovery programs with potential to be developed as novel agents for alleviation of intractable pain associated with disseminated breast cancer induced bony metastases.

Synergistic combinations of the dual enkephalinase inhibitor PL265 given orally with various analgesic compounds acting on different targets, in a murine model of cancer-induced bone pain

BACKGROUND

The first line pharmacological treatment of cancer pain is morphine and surrogates but a significant pain relief and a reduction of the side-effects of these compounds makes it necessary to combine them with other drugs acting on different targets. The aim of this study was to measure the antinociceptive effect on cancer-induced bone pain resulting from the association of the endogenous opioids enkephalin and non-opioid analgesic drugs. For this purpose, PL265 a new orally active single dual inhibitor of the two degrading enkephalins enzymes, neprilysin (NEP) and aminopeptidase N (APN) was used. It strictly increased the levels of enkephalin at their sites of releases. The selected non-opioid compounds are: gabapentin, A-317491 (P2X₃ receptor antagonist), ACEA (CB1 receptor antagonist), AM1241 (CB2 receptor antagonist), JWH-133 (CB2 receptor antagonist), URB937 (FAAH inhibitor), and NAV26 (Nav1.7 channel blocker).

METHODS

Experiments. Experiments were performed in 5-6 weeks old (26-33g weight) C57BL/6 mice. Cell culture and cell inoculation. B16-F10 melanoma cells were cultured and when preconfluent, treated and detached. Finally related cells were resuspended to obtain a concentration of 2×10⁶ cells/100μL. Then 10⁵ cells were injected into the right tibial medullar cavity. Control mice were treated by killed cells by freezing. Behavioural studies. Thermal withdrawal latencies were measured on a unilatered hot plate (UHP) maintained at 49±0.2°C. Mechanical threshold values were obtained by performing the von Frey test using the "up and down" method. To evaluate the nature (additive or synergistic) of the interactions between PL265 and different drugs, an isobolographic analysis following the method described by Tallarida was performed.

RESULTS

The results demonstrate the ability of PL265, a DENKI that prevents the degradation of endogenous ENKs, to counteract cancer-induced bone thermal hyperalgesia in mice, by exclusively stimulating peripheral opioid receptors as demonstrated by used of an opioid antagonist unable to enter the brain. The development of such DENKIs, endowed with druggable pharmacokinetic characteristics, such as good absorption by oral route, can be considered as an important step in the development of much needed novel antihyperalgesic drugs. Furthermore, all the tested combinations resulted in synergistic antihyperalgesic effects. As shown here, the greatest synergistic antinociceptive effect (doses could be lowered by 70%) was produced by the combination of PL265 with the P2X₃ receptor antagonist (A-317491), cannabinoid CB1 receptor agonist (exogenous, ACEA and endogenous URB937-protected-AEA) and Na_v1.7 blocker (NAV26) whose mechanism of action involves the direct activation of the enkephalinergic system.

CONCLUSIONS

These multi-target-based antinociceptive strategies using combinations of non-opioid drugs with dual inhibitors of enkephalin degrading enzymes may bring therapeutic advantages in terms of efficacy and safety by allowing the reduction of doses of one of the compounds or of both, which is of the utmost interest in the chronic treatment of cancer pain.

IMPLICATIONS

This article presents synergistic antinociceptive effect produced by the combination of PL265 with non-opioid analgesic drugs acting via unrelated mechanisms. These multi-target-based antinociceptive strategies may bring therapeutic advantages by allowing the reduction of doses, which is of great interest in the chronic treatment of cancer pain.

Welfare Assessment following Heterotopic or Orthotopic Inoculation of Bladder Cancer in C57BL/6 Mice

Few studies have assessed whether mice used as cancer models experience pain. Despite this possibility, the usual practice is to withhold analgesics as these are generally viewed as confounding. However, pain also alters cancer progression, so preventing it might not only be beneficial to welfare but also to study validity. Establishing the extent to which different cancer models result in pain is an important first step towards their refinement. We used conditioned place preference (CPP) testing and body-weight and behaviour analyses to evaluate the assumption that heterotopically implanted tumours result in less pain and fewer welfare concerns than those implanted orthotopically. C57Bl/6 mice received MB49^Luc luciferase expressing bladder cancer cells or saline implanted subcutaneously or into the bladder. These tumour-bearing or control groups underwent 2 daily 45 minute conditioning trials to saline or morphine (2mg/kg) and then a 15 minute drug-free preference test on day 3 of a 3 day cycle, continuing until the study ended. Tumours were imaged and behaviour data obtained following preference tests. Development of preference for the morphine-paired chamber (morphine-seeking) was determined over time. Heterotopic tumour development had no effect on morphine-seeking, and although the restraint used for heterotopic inoculation caused greater initial weight losses than anaesthesia, these mice steadily gained weight and behaved comparatively normally throughout the study. Orthotopic tumour inoculation caused no initial weight losses, but over the final 7 days these mice became less active and lost more body weight than cancer-free controls. This indicated orthotopic implantation probably caused a more negative impact on welfare or conceivably pain; but only according to the current test methods. Pain could not be confirmed because morphine-seeking in the tumour-bearing groups was similar to that seen in controls. Imaging was not found to be an effective method of monitoring tumour development surpassing manual tumour inspection.

Cancer pain physiology

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

Use of Animal Models in Understanding Cancer-induced Bone Pain

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

Downregulation of spinal endomorphin-2 correlates with mechanical allodynia in a rat model of tibia cancer

The endogenous tetrapeptide endomorphin-2 (EM2) participates in pain modulation by binding to pre- and/or post-synaptic μ opioid receptor (MOR). In the present study, pathological expression and antinociceptive effects of EM2 at the spinal level were investigated in a rat model of bone cancer pain. The model was established by introducing Walker 256 mammary gland carcinoma cells into the tibia medullary cavity. Immunohistochemical staining for EM2 showed a markedly reduced EM2-immunoreactivity in the ipsilateral spinal dorsal horn on days 6, 12 and 18 post Walker 256 inoculation (p < 0.05). Intrathecal injection (i.t.) of EM2 significantly attenuated cancer-induced mechanical allodynia (p < 0.05) which could be blocked by β-funaltrexamine (β-FNA), the μ receptor antagonist (p < 0.05). Furthermore, topical application of EM2 dose-dependently inhibited the electrically evoked C-fiber responses and postdischarge of wide dynamic range (WDR) neurons within the spinal cord (p < 0.05), and pretreatment with β-FNA abolished the hyperactivity of these neurons. Compared with the antinociception of morphine which took effect from 40 min to 100 min post application, the analgesic action of EM2 was characterized by quick onset and short-lived efficacy (p < 0.05), being most potent at 10 min and lasting about 20 min. These findings indicate that the down-regulated spinal EM2 is an important contributor to the neuropathological process of bone cancer pain and enhancing activation of EM2/μ receptor signaling might provide a therapeutic alternative to optimizing the treatment of cancer-induced bone pain.

Increased expression of protease-activated receptor 2 and 4 within dorsal root ganglia in a rat model of bone cancer pain

In an effort to understand the underlying mechanisms of cancer-induced bone pain, we investigated the presence of two protease-activated receptors, protease-activated receptor 2 (PAR2), and protease-activated receptor 4 (PAR4), in dorsal root ganglia (DRGs) neurons in an animal model of bone cancer pain. Female Wistar rats were randomized into three groups: tumor-bearing animals killed after 14 days (D14) and tumor-bearing animals killed after 21 days (D21) group and a sham operation group. After establishment of the Walker 256 carcinoma bone cancer pain model, behavioral tests were carried out to determine both the spontaneous nocifensive behavior and the paw withdrawal threshold (PWT) of mechanical and thermal hyperalgesia in these rats. Subsequently, real-time RT-PCR, Western bolt, and immunofluorescence were used to determine the messenger RNA (mRNA) and protein expression of PAR2 and PAR4 in the ipsilateral lumbar 4-5 DRG neurons. Rats in the D21 treatment group displayed a significant increase in spontaneous nocifensive behavior scores compared with the sham group as well as a considerably decreased withdrawal threshold in mechanical allodynia and thermal stimulation. Compared to sham group, the relative mRNA and protein expression of PAR2 and PAR4 was significantly upregulated in the D14 group and D21 groups, concurrent with tumor growth and proliferation. In addition, we identified the co-expression of PAR2 and PAR4 in the DRG neurons. The upregulation of mRNA and protein levels as well as the co-localization of PAR2 and PAR4 in DRG neurons suggests their novel involvement in the development and maintenance of bone 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.

The chemokine CCL5 induces CCR1-mediated hyperalgesia in mice inoculated with NCTC 2472 tumoral cells

Although the expression of the chemokine receptor CCR1 has been demonstrated in several structures related to nociception, supporting the nociceptive role of chemokines able to activate it, the involvement of CCR1 in neoplastic pain has not been previously assessed. We have assayed the effects of a CCR1 antagonist, J113863, in two murine models of neoplastic hyperalgesia based on the intratibial injection of either NCTC 2472 fibrosarcoma cells, able to induce osteolytic bone injury, or B16-F10 melanoma cells, associated to mixed osteolytic/osteoblastic bone pathological features. The systemic administration of J113863 inhibited thermal and mechanical hyperalgesia but not mechanical allodynia in mice inoculated with NCTC 2472 cells. Moreover, in these mice, thermal hyperalgesia was counteracted following the peritumoral (10-30μg) but not spinal (3-5μg) administration of J113863. In contrast, hyperalgesia and allodynia measured in mice inoculated with B16-F10 cells remained unaffected after the administration of J113863. The inoculation of tumoral cells did not modify the levels of CCL3 at tumor or spinal cord. In contrast, although the concentration of CCL5 remained unmodified in mice inoculated with B16-F10 cells, increased levels of this chemokine were measured in tumor-bearing limbs, but not the spinal cord, of mice inoculated with NCTC 2472 cells. Increased levels of CCL5 were also found following the incubation of NCTC 2472, but not B16-F10, cells in the corresponding culture medium. The intraplantar injection of CCL5 (0.5ng) to naïve mice evoked thermal hyperalgesia prevented by the coadministration of J113863 or the CCR5 antagonist, d-Ala-peptide T-amide (DAPTA), demonstrating that CCL5 can induce thermal hyperalgesia in mice through the activation of CCR1 or CCR5. However, contrasting with the inhibitory effect evoked by J113863, the systemic administration of DAPTA did not prevent tumoral hyperalgesia. Finally, the peritumoral administration of an anti-CCL5 antibody completely inhibited thermal hyperalgesia evoked by the inoculation of NCTC 2472 cells.

Involvement of protease-activated receptor 2 in nociceptive behavior in a rat model of bone cancer

Treatment for bone cancer pain remains a clinical challenge due to a poor understanding of the underlying mechanisms. Protease-activated receptor 2 (PAR2), a receptor for inflammatory proteases, has been implicated in nociceptive signaling under both normal and pathologic pain states. However, little is known of the role of PAR2 in cancer-induced bone pain. Here we investigated the potential role of PAR2 in a rat model of bone cancer pain. The model of bone cancer pain was induced by inoculating Walker 256 into the tibia bone cavity of rats and verified by X-ray imaging, pathology, and behavior assessments. The rats with bone cancer exhibited marked mechanical allodynia, thermal hyperalgesia, and signs of spontaneous nocifensive behavior. Subcutaneous administration of the PAR2 antagonist FSLLRY-NH2 almost completely abolished mechanical allodynia and thermal hyperalgesia but had no effects on spontaneous pain behavior in the rats with bone cancer. Immunohistochemical study revealed that the expression of PAR2 was significantly increased in large- and medium-sized dorsal root ganglia (DRG) neurons but not in small-sized neurons after Walker 256 inoculation. These results suggest that the increased expression of PAR2 in the DRG may contribute to the development of mechanical allodynia and thermal hyperalgesia associated with bone cancer rats. PAR2 might become a novel target for the treatment of pain in patients with bone cancer.

The stimulation of A(3) adenosine receptors reduces bone-residing breast cancer in a rat preclinical model

Amongst cancers with poor prognosis those originating from breast commonly metastasise to the skeleton for the high affinity of breast cancer cells to bone. A(3) adenosine receptor (A(3)AR) agonists were found to be potent anti-tumour agents even if their effect on bone-residing breast cancer has not yet been investigated. An animal model of surgery-induced metastasis was used to mimic the human condition in an attempt to develop a novel effective treatment strategy. Sprague-Dawley rats receiving intra-tibial injections of syngeneic MRMT-1 rat mammary gland carcinoma cells developed cancer-associated osteolytic lesions and structural damage that were monitored by microcomputed tomography imaging and histological analysis. To address the involvement of A(3)ARs in tumour-related signalling pathway, A(3)AR expression and functional role were analysed in MRMT-1 cells. The effect of chronic treatment with an A(3)AR agonist, 2-chloro-N(6)-(3-iodobenzyl)-adenosine-5'-N-methyl-uronamide (Cl-IB-MECA) in comparison with cisplatin, was evaluated on rat tumour growth and bone cancer pain. A(3)ARs were expressed in MRMT-1 cells and their activation reduced NF-kB, increased p53 expression and apoptosis, inhibited tumour cell proliferation and migration. In vivo Cl-IB-MECA administration, started on day 1 after tumour cell injection, produced a significant reduction in tumour growth and cancer pain. Cl-IB-MECA treatment, performed on days 5 and 10 after the tumour cell inoculation, revealed the capability of A(3)AR stimulation to partially reduce tumour progression. Our findings highlighted the effectiveness of A(3)AR stimulation in the inhibition of breast tumour-derived bone metastasis growth strongly suggesting that targeting A(3)ARs may have promising therapeutic value in the treatment of bone-residing breast cancer.

Activation of c-jun N-terminal kinase in spinal cord contributes to breast cancer induced bone pain in rats

BACKGROUND

The most frequent pain in patients with metastatic breast and prostate cancer is bone pain, which can be severe and difficult to treat. The mechanisms underlying this pain remain unclear. Here we investigated the role of c-jun N-terminal kinase (JNK) pathway in the spinal cord in cancer-induced bone pain (CIBP).

RESULTS

In this study, we used an established rat CIBP model to investigate the possible role of JNK activation in the spinal cord. After intra-tibial inoculation with Walker 256 rat mammary gland carcinoma cells, the rats displayed mechanical allodynia on day 5, which lasted to day 16. The activation of JNK in neurons and astrocytes in the spinal cord was found on day 12 and day 16 after intra-tibial inoculation with carcinoma cells. A single intrathecal injection with JNK inhibitor SP600125 by lumbar puncture attenuated mechanical allodynia on day 12, and repeated intrathecal injection of SP600126 from day 10 to day 14 had a cumulative analgesic effect on CIBP.

CONCLUSIONS

Taken together, our results demonstrated for the first time that JNK activation in the spinal cord is required in the maintenance of CIBP. Inhibition of the spinal JNK pathway may provide a new therapy for CIBP management.

An inhibitor of neuronal exocytosis (DD04107) displays long-lasting in vivo activity against chronic inflammatory and neuropathic pain

Small peptides patterned after the N terminus of the synaptosomal protein of 25 kDa, a member of the protein complex implicated in Ca(2+)-dependent neuronal exocytosis, inhibit in vitro the release of neuromodulators involved in pain signaling, suggesting an in vivo analgesic activity. Here, we report that compound DD04107 (palmitoyl-EEMQRR-NH(2)), a 6-mer palmitoylated peptide that blocks the inflammatory recruitment of ion channels to the plasma membrane of nociceptors and the release of calcitonin gene-related peptide from primary sensory neurons, displays potent and long-lasting in vivo antihyperalgesia and antiallodynia in chronic models of inflammatory and neuropathic pain, such as the complete Freund's adjuvant, osteosarcoma, chemotherapy, and diabetic neuropathic models. Subcutaneous administration of the peptide produced a dose-dependent antihyperalgesic and antiallodynic activity that lasted ≥24 h. The compound showed a systemic distribution, characterized by a bicompartmental pharmacokinetic profile. Safety pharmacology studies indicated that the peptide is largely devoid of side effects and substantiated that the in vivo activity is not caused by locomotor impairment. Therefore, DD04107 is a potent and long-lasting antinociceptive compound that displays a safe pharmacological profile. These findings support the notion that neuronal exocytosis of receptors and neuronal algogens pivotally contribute to chronic inflammatory and neuropathic pain and imply a central role of peptidergic nociceptor sensitization to the pathogenesis of pain.

Cancer Pain: A Critical Review of Mechanism-based Classification and Physical Therapy Management in Palliative Care

Mechanism-based classification and physical therapy management of pain is essential to effectively manage painful symptoms in patients attending palliative care. The objective of this review is to provide a detailed review of mechanism-based classification and physical therapy management of patients with cancer pain. Cancer pain can be classified based upon pain symptoms, pain mechanisms and pain syndromes. Classification based upon mechanisms not only addresses the underlying pathophysiology but also provides us with an understanding behind patient's symptoms and treatment responses. Existing evidence suggests that the five mechanisms – central sensitization, peripheral sensitization, sympathetically maintained pain, nociceptive and cognitive-affective – operate in patients with cancer pain. Summary of studies showing evidence for physical therapy treatment methods for cancer pain follows with suggested therapeutic implications. Effective palliative physical therapy care using a mechanism-based classification model should be tailored to suit each patient's findings, using a biopsychosocial model of pain.

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.

Antinociceptive and anti-exudative synergism between dexketoprofen and tramadol in a model of inflammatory pain in mice

Preclinical studies have demonstrated antinociceptive synergism between dexketoprofen (DEX) and tramadol (TRM) in acute animal models of nociception. The aim of the present study was to investigate the type of interaction between DEX and TRM in a chronic musculoskeletal pain model in mice, which fairly replicates the characteristics of chronic osteoarticular pain in humans. Inflammation was induced by a subplantar injection of complete Freund's adjuvant (CFA) in male CF1 mice. Nociceptive thresholds were evaluated using the hot plate, the nocifensive spontaneous behavior and the acetone tests, while plasma extravasation (PE) was assessed with Evan's blue. We used the following experimental groups: control (no inflammation), acute (1 day after CFA injection), and chronic inflammation (7 days after CFA). Dose-response curves for DEX and TRM, individually and combined in a 1 : 1 proportion based on their potency were obtained, and the doses that produced a 50% inhibition calculated. The isobolographic analysis revealed that in all groups of study (no inflammation, acute, and chronic inflammation), the combination of DEX : TRM was synergistic, for both the inhibition of nociception and the PE. The results suggest that the DEX : TRM (1 : 1) combination could be useful in the management of acute and chronic inflammatory musculoskeletal pains in humans; in addition, the synergistic interaction between the drugs observed both during acute and chronic inflammation suggests that less doses would be required of each drug to obtain effective analgesia.

Spinal and peripheral analgesic effects of the CB2 cannabinoid receptor agonist AM1241 in two models of bone cancer-induced pain

BACKGROUND AND PURPOSE

The activation of CB(2) receptors induces analgesia in experimental models of chronic pain. The present experiments were designed to study whether the activation of peripheral or spinal CB(2) receptors relieves thermal hyperalgesia and mechanical allodynia in two models of bone cancer pain.

EXPERIMENTAL APPROACH

NCTC 2472 osteosarcoma or B16-F10 melanoma cells were intratibially inoculated to C3H/He and C57BL/6 mice. Thermal hyperalgesia was assessed by the unilateral hot plate test and mechanical allodynia by the von Frey test. AM1241 (CB(2) receptor agonist), AM251 (CB(1) receptor antagonist), SR144528 (CB(2) receptor antagonist) and naloxone were used. CB(2) receptor expression was measured by Western blot.

KEY RESULTS

AM1241 (0.3-10 mg.kg(-1)) abolished thermal hyperalgesia and mechanical allodynia in both tumour models. The antihyperalgesic effect was antagonized by subcutaneous, intrathecal or peri-tumour administration of SR144528. In contrast, the antiallodynic effect was inhibited by systemic or intrathecal, but not peri-tumour, injection of SR144528. The effects of AM1241 were unchanged by AM251 but were prevented by naloxone. No change in CB(2) receptor expression was found in spinal cord or dorsal root ganglia.

CONCLUSIONS AND IMPLICATIONS

Spinal CB(2) receptors are involved in the antiallodynic effect induced by AM1241 in two neoplastic models while peripheral and spinal receptors participate in the antihyperalgesic effects. Both effects were mediated by endogenous opiates. The use of drugs that activate CB(2) receptors could be a useful strategy to counteract bone cancer-induced pain symptoms.

Selective inhibition of JNK with a peptide inhibitor attenuates pain hypersensitivity and tumor growth in a mouse skin cancer pain model

Cancer pain significantly affects the quality of cancer patients, and current treatments for this pain are limited. C-Jun N-terminal kinase (JNK) has been implicated in tumor growth and neuropathic pain sensitization. We investigated the role of JNK in cancer pain and tumor growth in a skin cancer pain model. Injection of luciferase-transfected B16-Fluc melanoma cells into a hindpaw of mouse induced robust tumor growth, as indicated by increase in paw volume and fluorescence intensity. Pain hypersensitivity in this model developed rapidly (<5 days) and reached a peak in 2 weeks, and was characterized by mechanical allodynia and heat hyperalgesia. Tumor growth was associated with JNK activation in tumor mass, dorsal root ganglion (DRG), and spinal cord and a peripheral neuropathy, such as loss of nerve fibers in the hindpaw skin and induction of ATF-3 expression in DRG neurons. Repeated systemic injections of D-JNKI-1 (6 mg/kg, i.p.), a selective and cell-permeable peptide inhibitor of JNK, produced an accumulative inhibition of mechanical allodynia and heat hyperalgesia. A bolus spinal injection of D-JNKI-1 also inhibited mechanical allodynia. Further, JNK inhibition suppressed tumor growth in vivo and melanoma cell proliferation in vitro. In contrast, repeated injections of morphine (5 mg/kg), a commonly used analgesic for terminal cancer, produced analgesic tolerance after 1 day and did not inhibit tumor growth. Our data reveal a marked peripheral neuropathy in this skin cancer model and important roles of the JNK pathway in cancer pain development and tumor growth. JNK inhibitors such as D-JNKI-1 may be used to treat cancer pain.

Decreased motivational properties of morphine in mouse models of cancerous- or inflammatory-chronic pain: implication of supraspinal neuropeptide FF(2) receptors

Our main purpose was to evaluate the influence of cancer pain on the rewarding properties of morphine. Opioids are very addictive when used by healthy persons, conversely the occurrence of an opioid addiction seems very low when patients suffering from cancer are treated with morphine. We investigated the reinforcing properties of morphine in the place preference paradigm on a new model of mice suffering from a cancer pain induced by syngenic melanoma cells injected in the hind paw. These data were compared with mice suffering either from a short-term- or a chronic-inflammatory pain induced respectively by injection of carrageenan or complete Freund's adjuvant. Remarkably, mice suffering from cancer pain or chronic inflammatory pain did not develop any preference for the environment associated with the injection of morphine. In mice injected with melanoma cells, the specific binding of [(125)I]EYWSLAAPQRF-NH(2), an agonist of neuropeptide FF(2) receptors, was increased in several brain areas involved in the rewarding properties of opiates, including the shell of the nucleus accumbens, the major islands of Calleja, the ventral endopiriform nucleus and the amygdaloid area. Our study is the first to reveal a modification of morphine rewarding properties under cancer pain in rodents. We postulate that anti-opioid neuropeptides might contribute to the suppression of morphine rewarding effects in this murine model of cancer pain.

Tumor-evoked sensitization of C nociceptors: a role for endothelin

Primary and metastatic cancers that effect bone are frequently associated with pain. Sensitization of primary afferent C nociceptors innervating tissue near the tumor likely contributes to the chronic pain and hyperalgesia accompanying this condition. This study focused on the role of the endogenous peptide endothelin-1 (ET-1) as a potential peripheral algogen implicated in the process of cancer pain. Electrophysiological response properties, including ongoing activity and responses evoked by heat stimuli, of C nociceptors were recorded in vivo from the tibial nerve in anesthetized control mice and mice exhibiting mechanical hyperalgesia following implantation of fibrosarcoma cells into and around the calcaneus bone. ET-1 (100 microM) injected into the receptive fields of C nociceptors innervating the plantar surface of the hind paw evoked an increase in ongoing activity in both control and tumor-bearing mice. Moreover, the selective ETA receptor antagonist, BQ-123 (3 mM), attenuated tumor-evoked ongoing activity in tumor-bearing mice. Whereas ET-1 produced sensitization of C nociceptors to heat stimuli in control mice, C nociceptors in tumor-bearing mice were sensitized to heat, and their responses were not further increased by ET-1. Importantly, administration of BQ-123 attenuated tumor-evoked sensitization of C nociceptors to heat. We conclude that ET-1 at the tumor site contributes to tumor-evoked excitation and sensitization of C nociceptors through an ETA receptor mediated mechanism.

Inhibition of osteosarcoma-induced thermal hyperalgesia in mice by the orally active dual enkephalinase inhibitor PL37. Potentiation by gabapentin

We have previously shown that stimulation of peripheral opioid receptors by exogenous opiates counteracts the thermal hyperalgesia elicited by a tibial osteosarcoma due to intraosteal inoculation of NCTC 2472 cells to mice. Aiming to study whether pheripheral endogenous enkephalins could also counteract this painful symptom, we assayed in this model the effects of PL37, an orally active dual inhibitor of enkephalin inactivating enzymes. Oral administration of PL37 (25 mg/kg) completely supressed osteosarcoma-induced thermal hyperalgesia through the activation of micro-opioid receptors, since the administration of cyprodime (1 mg/kg) inhibited its antihyperalgesic effect. Neither naltrindole (0.1 mg/kg) nor nor-binaltorphimine (10 mg/kg) modified this PL37-induced antihyperalgesic effect. Moreover, the inhibition of the antihyperalgesic effect induced by PL37 after the administration of naloxone-methiodide (2 mg/kg), a non selective opioid antagonist that does not cross the blood-brain barrier, demonstrates the involvement of peripheral opioid receptors. In contrast, centrally mediated effects may be detected when assaying a higher dose of PL37 (50 mg/kg). Besides, the administration of gabapentin (6.25-25 mg/kg, i.p.) dose-dependently inhibited osteosarcoma-induced thermal hyperalgesia. Interestingly, the combined administration of subeffective doses of PL37 and gabapentin completely prevented this type of thermal hyperalgesia. An isobolographic analysis of this interaction demonstrated a synergistic interaction between both drugs.

Local loperamide inhibits thermal hyperalgesia but not mechanical allodynia induced by intratibial inoculation of melanoma cells in mice

The stimulation of peripheral opioid receptors counteracts thermal hyperalgesia produced by the intratibial inoculation of NCTC 2472 cells in mice, through the activation of the nitric oxide/cGMP/ATP-sensitive K+-channels (NO/cGMP/K(+) (ATP)) cascade (Menéndez et al. 2007, Neuropharmacology 53:71-80). We aimed to elucidate whether this peripheral opioid antihyperalgesic effect is exclusive to this model or might also occur in other types of bone neoplastic processes. In C57BL/6 mice intratibially inoculated with B16-F10 melanoma cells, the progressive tumoral damage was accompanied by the establishment of thermal hyperalgesia (unilateral hot plate test) and mechanical allodynia (von Frey test). Intraplantar administration of loperamide (15 microg, 30 min before) inhibited thermal hyperalgesia, but did not modify the intense mechanical allodynia. The fact that the coadministration of naloxone-methiodide (5 microg) completely suppressed the thermal antihyperalgesic effect induced by loperamide indicates its production through the stimulation of peripheral opioid receptors. Furthermore, its prevention by the coadministration of the non-selective inhibitor of the NO synthase, N(G)-monomethyl-L-arginine (L-NMMA, 10 microg), the selective inhibitor of neural NOS, N-omega-propyl-L-arginine (1-10 microg), or the K+ (ATP) channel blocker, glibenclamide (10 microg) demonstrated the involvement of the NO/cGMP/K(+) (ATP) pathway in the antihyperalgesic effect induced by loperamide. Overall, the present results show that the intratibial inoculation of B16-F10 cells to C57BL/6 mice evokes thermal hyperalgesia and mechanical allodynia and that, as occurred in the osteosarcoma model, the stimulation of peripheral opioid receptors is not effective in modifying neoplastic allodynia but completely inhibits thermal hyperalgesia through the activation of the NO/cGMP/K+ (ATP) cascade.

Tumor-evoked hyperalgesia and sensitization of nociceptive dorsal horn neurons in a murine model of cancer pain

Pain associated with cancer, particularly when tumors metastasize to bone, is often severe and debilitating. Better understanding of the neurobiological mechanisms underlying cancer pain will likely lead to the development of more effective treatments. The aim of this study was to characterize changes in response properties of nociceptive dorsal horn neurons following implantation of fibrosarcoma cells into and around the calcaneus bone, an established model of cancer pain. Extracellular electrophysiological recordings were made from wide dynamic range (WDR) and high threshold (HT) dorsal horn neurons in mice with tumor-evoked hyperalgesia and control mice. WDR and HT neurons were examined for ongoing activity and responses to mechanical, heat, and cold stimuli applied to the plantar surface of the hind paw. Behavioral experiments showed that mice exhibited hyperalgesia to mechanical and heat stimuli applied to their tumor-bearing hind paw. WDR, but not HT, nociceptive dorsal horn neurons in tumor-bearing mice exhibited sensitization to mechanical, heat, and cold stimuli and may contribute to tumor-evoked hyperalgesia. Specifically, the proportion of WDR neurons that exhibited ongoing activity and their evoked discharge rates were greater in tumor-bearing than in control mice. In addition, WDR neurons exhibited lower response thresholds for mechanical and heat stimuli, and increased responses to suprathreshold mechanical, heat, and cold stimuli. Our findings show that sensitization of WDR neurons contributes to cancer pain and supports the notion that the mechanisms underlying cancer pain differ from those that contribute to inflammatory and neuropathic pain.

Morphine treatment accelerates sarcoma-induced bone pain, bone loss, and spontaneous fracture in a murine model of bone cancer

Metastatic bone cancer causes severe pain that is primarily treated with opioids. A model of bone cancer pain in which the progression of cancer pain and bone destruction is tightly controlled was used to evaluate the effects of sustained morphine treatment. In cancer-treated mice, morphine enhanced, rather than diminished, spontaneous, and evoked pain; these effects were dose-dependent and naloxone-sensitive. SP and CGRP positive DRG cells did not differ between sarcoma or control mice, but were increased following morphine in both groups. Morphine increased ATF-3 expression only in DRG cells of sarcoma mice. Morphine did not alter tumor growth in vitro or tumor burden in vivo but accelerated sarcoma-induced bone destruction and doubled the incidence of spontaneous fracture in a dose- and naloxone-sensitive manner. Morphine increased osteoclast activity and upregulated IL-1 beta within the femurs of sarcoma-treated mice suggesting enhancement of sarcoma-induced osteolysis. These results indicate that sustained morphine increases pain, osteolysis, bone loss, and spontaneous fracture, as well as markers of neuronal damage in DRG cells and expression of pro-inflammatory cytokines. Morphine treatment may result in "add-on" mechanisms of pain beyond those engaged by sarcoma alone. While it is not known whether the present findings in this model of osteolytic sarcoma will generalize to other cancers or opioids, the data suggest a need for increased understanding of neurobiological consequences of prolonged opioid exposure which may allow improvements in the use of opiates in the effective management of cancer pain.

Chemical interactions between fibrosarcoma cancer cells and sensory neurons contribute to cancer pain

In an experimental model of cancer pain, the hyperalgesia that occurs with osteolytic tumor growth is associated with the sensitization of nociceptors. We examined functional and molecular changes in small-diameter dorsal root ganglion (DRG) neurons to determine cellular mechanisms underlying this sensitization. The occurrence of a Ca2+ transient in response to either KCl (25 mM) or capsaicin (500 nM) increased in small neurons isolated from murine L3-L6 DRGs ipsilateral to fibrosarcoma cell tumors. The increased responses were associated with increased mRNA levels for the Ca2+ channel subunit alpha2delta1 and TRPV1 receptor. Pretreatment with gabapentin, an inhibitor of the alpha2delta1 subunit, blocked the increased response to KCl in vitro and the mechanical hyperalgesia in tumor-bearing mice in vivo. Similar increases in neuronal responsiveness occurred when DRG neurons from naive mice and fibrosarcoma cells were cocultured for 48 h. The CC chemokine ligand 2 (CCL2) may contribute to the tumor cell-induced sensitization because CCL2 immunoreactivity was present in tumors, high levels of CCL2 peptide were present in microperfusates from tumors, and treatment of DRG neurons in vitro with CCL2 increased the amount of mRNA for the alpha2delta1 subunit. Together, our data provide strong evidence that the chemical mediator CCL2 is released from tumor cells and evokes phenotypic changes in sensory neurons, including increases in voltage-gated Ca2+ channels that likely underlie the mechanical hyperalgesia in the fibrosarcoma cancer model. More broadly, this study provides a novel in vitro model to resolve the cellular and molecular mechanisms by which tumor cells drive functional changes in nociceptors.

Antihyperalgesic effects induced by the IL-1 receptor antagonist anakinra and increased IL-1β levels in inflamed and osteosarcoma-bearing mice

Based on the well established involvement of IL-1beta in inflammatory hyperalgesia, we have assessed the possible role played by IL-1beta in a murine model of bone cancer-induced pain. With this aim, we measured IL-1beta levels at the region of the tibia and the spinal cord in mice bearing a tibial osteosarcoma induced by the inoculation of NCTC 2472 cells, and we tested whether the IL-1 receptor antagonist, anakinra, inhibits some hypernociceptive reactions evoked by the neoplastic injury. Parallel experiments were performed in mice with a chronic inflammatory process (intraplantar injection of complete Freund's adjuvant, CFA). IL-1beta levels were increased in the tibial region of osteosarcoma-bearing mice and in the paws of inflamed mice. To a lesser extent, the content of IL-1beta in the spinal cord was also augmented in both situations. Osteosarcoma-induced thermal hyperalgesia was inhibited by 30 and 100 mg/kg of systemic anakinra, but only 300 mg/kg prevented inflammatory thermal hyperalgesia. Mechanical hyperalgesia induced by the osteosarcoma was blocked by 100 and 300 mg/kg of anakinra, whereas a partial reversion of inflammatory mechanical hyperalgesia was induced by 300 mg/kg. Anakinra, intrathecally administered (1 and 10 microg) did not modify hyperalgesia of either origin. Besides, both tumoral and inflammatory mechanical allodynia remained unaltered after the administration of anakinra. In conclusion, some hyperalgesic symptoms observed in this model of bone cancer are mediated by the peripheral release of IL-1beta and may be inhibited by antagonists of type I IL-1 receptors with a similar or greater potency than symptoms produced by inflammation.

Walker 256 Tumor-Bearing Rats as a Model to Study Cancer Pain

An animal model of cancer pain induced by injection of Walker 256 carcinoma cells into the plantar surface of rat hind paw is described. Tumor growth and the occurrence of metastasis were investigated by histopathological analysis. Tumor cell growth was also analyzed plethysmographically by the increase in paw volume. For characterization of pain symptoms, hyperalgesia, allodynia, and spontaneous pain were evaluated 5 to 8 days after cell injection. The volume of the inoculated paw started to increase on day 2 after inoculation, being 40% higher on day 5 after injection. At this time, there was a marked proliferation of tumor cells, with the presence of anaplastic and pleomorphic cells, nucleoli, and atypical mitotic features. On days 7 and 8 after injection, histopathological analysis of popliteal lymph nodes showed the presence of tumor cells. The intraplantar injection of Walker 256 cells caused hyperalgesia at day 5 after cell inoculation. Low-threshold mechanical allodynia was significant 2 days after cell injection, being increased on day 5. In addition, inoculation of tumor cells induced gross behavior, characterized by a significant increase in licking and lifting of the injected paw 5 days after injection. The pain-enhancing effect caused by cell inoculation was partially inhibited by indomethacin on day 2 after cell injection, whereas morphine blocked allodynia on days 2 and 5. These results indicate that intraplantar injection of Walker 256 cells cause pain symptoms characteristic of cancer pain. This experimental model can then be used to investigate new analgesic or anti-tumor drugs.

PERSPECTIVE

This article presents a new animal model for studying cancer pain and metastasis. This model could help in understanding the mechanisms involved in cancer pain symptoms and may be used for the investigation of new analgesic or anti-tumor drugs.

Involvement of nitric oxide in the inhibition of bone cancer-induced hyperalgesia through the activation of peripheral opioid receptors in mice

Experiments were designed to elucidate the involvement of nitric oxide (NO) in the antihyperalgesic effect induced by the activation of peripheral mu-opioid receptors on osteosarcoma-induced thermal hyperalgesia in mice. Since this pathway has previously been shown to be involved in the antihyperalgesic effect induced by some drugs--including opiates--on inflammatory pain, experiments were also performed in inflamed mice. The intraplantar administration of loperamide (15 microg) abolishes the thermal hyperalgesia that appears 4 weeks after the intratibial inoculation of NCTC 2472 cells in C3H/HeJ mice. The blockade of this effect by coadministering a peripheral opioid receptor antagonist (naloxone methiodide), a nitric oxide synthase (NOS) inhibitor (L-NMMA), a soluble guanylyl cyclase inhibitor (ODQ), a PKG inhibitor (KT-5823) or a K(+)(ATP)-channel blocker (glibenclamide) shows the involvement of a NO/cGMP/K(+)(ATP)-channel pathway. Accordingly the administration of loperamide produced, in osteosarcoma-bearing mice, an increase in the concentrations of NO metabolites, nitrites and nitrates, extracted from paws. The selective inhibitor of eNOS L-NIO, but not the inhibitors of nNOS (N-omega-propyl-L-arginine) or iNOS (1400w), blocked the effect of loperamide on osteosarcoma-induced hyperalgesia and also the endogenous opioid peripheral hypoalgesia that appears during the initial stages of the development of this osteosarcoma. Although this pathway also participates in the inhibitory effect of loperamide on the thermal hyperalgesia induced by administration of complete Freund's adjuvant, only selective inhibitors of nNOS or iNOS antagonized this effect. Our results demonstrate that the activation of a NO/cGMP/K(+)(ATP)-channel triggered by eNOS participates in the peripheral antihyperalgesic of loperamide on osteosarcoma-induced thermal hyperalgesia.

Acute and chronic administration of the cannabinoid receptor agonist CP 55,940 attenuates tumor-evoked hyperalgesia

Patients with cancer frequently report pain that can be difficult to manage. This study examined the antihyperalgesic effects of a cannabinoid receptor agonist, CP 55,940, in a murine model of cancer pain. Implantation of fibrosarcoma cells into and around the calcaneous bone in mice produced mechanical hyperalgesia (decreased paw withdrawal thresholds and increased frequency of paw withdrawals). On day 13 after implantation, mechanical hyperalgesia, nociception, and catalepsy were assessed. Mice were randomly assigned to receive CP 55,940 (0.01-3 mg/kg, i.p.) or vehicle and behavioral measures were redetermined. CP 55,940 dose-dependently attenuated tumor-evoked mechanical hyperalgesia. To examine the effect of catalepsy on the antihyperalgesic effect of CP 55,940, mice with tumor-evoked hyperalgesia were pretreated with the dopamine agonist apomorphine prior to administration of CP 55,940. Apomorphine attenuated the cataleptic effect of CP 55,940 but did not attenuate its antihyperalgesic effect. In a separate group of mice with tumor-evoked hyperalgesia, administration of the dopamine antagonist spiperone produced catalepsy that was approximately 2.5 fold greater than that produced by CP 55,490. Whereas this dose of CP 55,940 completely reversed tumor-evoked mechanical hyperalgesia, spiperone only attenuated mechanical hyperalgesia by approximately 35%. Thus, the cataleptic effects of CP 55,940 did not fully account for its antihyperalgesic effect. The antihyperalgesic effect of CP 55,940 was mediated via the cannabinoid CB1 but not CB2 receptor. Finally, repeated administration of CP 55,940 produced a short-term and a longer-term attenuation of tumor-evoked hyperalgesia. These results suggest that cannabinoids may be a useful alternative or adjunct therapy for treating cancer pain.

Endogenous beta-endorphin induces thermal analgesia at the initial stages of a murine osteosarcoma

Transient thermal, but not mechanical, hypoalgesia appears at the early stages of the development of an hyperalgesic murine osteosarcoma. This hypoalgesia is suppressed by the administration of naloxone, its peripherally acting analog naloxone methiodide, the mu- and delta-opioid receptor antagonists cyprodime and naltrindole, or the CRF receptor antagonist, alpha-helical CRF (9-41). When immunohistochemical assays were performed with an anti-beta-endorphin antibody, whose in vivo administration suppressed the analgesia, labeled mononuclear immune cells appeared both inside and surrounding the tumoral tissue. In conclusion, the peripheral action of beta-endorphin, released in response to the osteosarcoma seems responsible for the observed thermal analgesia.

Heat and mechanical hyperalgesia in mice model of cancer pain

We developed a mouse model of cancer pain to investigate its underlying mechanisms. SCC-7, squamous cell carcinoma (SCC) derived from C3H mice, was inoculated subcutaneously into either the plantar region or thigh in male C3H/Hej mice. Heat and mechanical sensitivity as well as spontaneous behavior were measured at the plantar surface of the ipsilateral hind paw after the inoculation. Inoculated sites were histologically examined, and the expression of capsaicin receptors (TRPV1) was examined in the dorsal root ganglia (DRG) to clarify their potential contribution to pain sensitivity. Inoculation of cancer cells induced marked heat hyperalgesia and mechanical allodynia in the ipsilateral hind paw for two weeks in both plantar- and thigh-inoculation models. Signs of spontaneous pain, such as lifting, licking and flinching of the paw were also observed. However, further growth of the tumor reversed the mechanical allodynia in both plantar- and thigh-inoculation models, and heat hyperalgesia in thigh-inoculation models. Histologically, no infiltration of the tumor cells into the nerve was observed. TRPV1 immunoreactive cells increased in the L5 DRG on day 7, but returned to the control level on day 15 post-inoculation. Intraperitoneal administration of the competitive TRPV1 antagonist capsazepine inhibited hyperalgesia induced by tumor cell-inoculation in either plantar- or thigh-inoculated animals. This study indicated that inoculation of SCC resulted in spontaneous pain, heat hyperalgesia and mechanical allodynia. The altered expression of TRPV1 in the DRG may be involved in behavioral changes in this model.

Effects of the local administration of selective mu-, delta-and kappa-opioid receptor agonists on osteosarcoma-induced hyperalgesia

The stimulation of peripheral opioid receptors yields analgesic responses in a model of bone cancer-induced pain in mice. In order to know the type(s) of peripheral opiate receptors involved, the paw thermal withdrawal latencies were measured in C3H/HeJ mice bearing a tibial osteosarcoma, after administering selective agonists of mu-,delta-and kappa-opiate receptors. The peritumoral administration of DAGO (0.6-6 microg) inhibited the osteosarcoma-induced hyperalgesia at doses ineffective in healthy animals, the highest one even increasing the withdrawal latencies over the control values. Naloxone-methiodide (2 mg/kg) and cyprodime (1 mg/kg), but not naltrindole (0.1 mg/kg) nor nor-binaltorphimine (10 mg/kg), antagonized DAGO-induced analgesic effects, these therefore probably being mediated through peripheral mu-opioid receptors. The peritumoral injection of DPDPE (100 microg) induced analgesia which was inhibited by naloxone-methiodide and naltrindole but not by nor-binaltorphimine. Cyprodime partially antagonized the analgesia induced by 100 microg of DPDPE, but did not modify the effect induced by 30 microg of this agonist-a dose that restores the hyperalgesic latencies up to the control values. The antihyperalgesic effect induced by the peritumoral administration of U-50,488H (1 microg) was antagonized by naloxone-methiodide and nor-binaltorphimine, but not by cyprodime nor naltrindole, thus suggesting the involvement of peripheral kappa-opioid receptors. In conclusion, the stimulation of peripheral mu-, delta- and kappa-opioid receptors is a pharmacological strategy useful for relieving this experimental type of bone cancer-induced pain, the greatest analgesic effect being achieved by stimulating peripheral mu-opioid receptors.

Analgesic effects of capsazepine and resiniferatoxin on bone cancer pain in mice

In the present paper, we describe the analgesic effects induced by the transient receptor potential vanilloid type 1 (TRPV1) antagonist, capsazepine, and the TRPV1 agonist, resiniferatoxin, on the thermal hyperalgesia induced by the presence of a tibial osteosarcoma or an inflammatory process in mice. The administration of capsazepine abolished the osteosarcoma-induced hyperalgesia at a dose range (3-10 mg/kg; s.c.) ineffective to inhibit the hyperalgesia elicited by the intraplantar administration of complete Freund's adjuvant (CFA). In contrast, the administration of resiniferatoxin (0.01-0.1 mg/kg; s.c.) inhibited both the osteosarcoma- and the CFA-induced hyperalgesia. Remarkably, a single dose of resiniferatoxin abolished the osteosarcoma-induced hyperalgesia for several days and completely prevented the instauration of thermal hyperalgesia when administered at the initial stages of osteosarcoma development. The potential of drugs acting through TRPV1 for the management of some types of bone cancer pain is proposed.