The impact of bone cancer on the peripheral encoding of mechanical pressure stimuli

An interdisciplinary perspective on peripheral drivers of pain in rheumatoid arthritis

Pain is one of the most debilitating symptoms of rheumatoid arthritis (RA), and yet remains poorly understood, especially when pain occurs in the absence of synovitis. Without active inflammation, experts most often attribute joint pain to central nervous system dysfunction. However, advances in the past 5 years in both immunology and neuroscience research suggest that chronic pain in RA is also driven by a variety of abnormal interactions between peripheral neurons and mediators produced by resident cells in the local joint environment. In this Review, we discuss these novel insights from an interdisciplinary neuro-immune perspective. We outline a potential working model for the peripheral drivers of pain in RA, which includes autoantibodies, resident immune and mesenchymal cells and their interactions with different subtypes of peripheral sensory neurons. We also offer suggestions for how future collaborative research could be designed to accelerate analgesic drug development.

Evaluation of pain related behaviors and disease related outcomes in an immunocompetent mouse model of prostate cancer induced bone pain

Cancer-induced bone pain (CIBP) is the most common and devastating symptom of bone metastatic cancer that substantially disrupts patients' quality of life. Currently, there are few effective analgesic treatments for CIBP other than opioids which come with severe side effects. In order to better understand the factors and mechanisms responsible for CIBP it is essential to have clinically relevant animal models that mirror pain-related symptoms and disease progression observed in patients with bone metastatic cancer. In the current study, we characterize a syngeneic mouse model of prostate cancer induced bone pain. We transfected a prostate cancer cell line (RM1) with green fluorescent protein (GFP) and luciferase reporters in order to visualize tumor growth longitudinally in vivo and to assess the relationship between sensory neurons and tumor cells within the bone microenvironment. Following intra-femoral injection of the RM1 prostate cancer cell line into male C57BL/6 mice, we observed a progressive increase in spontaneous guarding of the inoculated limb between 12 and 21 days post inoculation in tumor bearing compared to sham operated mice. Daily running wheel performance was evaluated as a measure of functional impairment and potentially movement evoked pain. We observed a progressive reduction in the distance traveled and percentage of time at optimal velocity between 12 and 21 days post inoculation in tumor bearing compared to sham operated mice. We utilized histological, radiographic and μCT analysis to examine tumor induced bone remodeling and observed osteolytic lesions as well as extra-periosteal aberrant bone formation in the tumor bearing femur, similar to clinical findings in patients with bone metastatic prostate cancer. Within the tumor bearing femur, we observed reorganization of blood vessels, macrophage and nerve fibers within the intramedullary space and periosteum adjacent to tumor cells. Tumor bearing mice displayed significant increases in the injury marker ATF3 and upregulation of the neuropeptides SP and CGRP in the ipsilateral DRG as well as increased measures of central sensitization and glial activation in the ipsilateral spinal cord. This immunocompetent mouse model will be useful when combined with cell type selective transgenic mice to examine tumor, immune cell and sensory neuron interactions in the bone microenvironment and their role in pain and disease progression associated with bone metastatic prostate cancer.

Nerve terminals in the tumor microenvironment as targets for local infiltration analgesia

Nerve terminals within the tumor microenvironment as potential pain-mitigating targets for local infiltration analgesia is relatively less explored. In this study, we examine the role of key analgesics administered as local infiltration analgesia in a model of cancer-induced bone pain (CIBP). CIBP was induced by administration of allogenic MRMT1 breast cancer cells in the proximal tibia of rats, and tumor mass characterized using radiogram, micro-CT, and histological analysis. In vitro responsiveness to key analgesics δ-opioid receptor agonist (DOPr), Ca2+ channel and TRPV1 antagonists was assessed using ratiometric Ca2+ imaging in sensory neurons innervating the tumor site. Effectiveness of locally infiltrated analgesics administered independently or in combination was assessed by quantifying evoked limb withdrawal thresholds at two distinct sites for up to 14 days. CIBP animals demonstrated DOPr, N-, and L-type and TRPV1 expression in lumbar dorsal root ganglion neurons (DRG), comparable to controls. Evoked Ca2+ transients in DRG neurons from CIBP animals were significantly reduced in response to treatment with compounds targeting DOPr, N-, L-type Ca2+ channels and TRPV1 proteins. Behaviourally, evoked hyperalgesia at the tumor site was strongly mitigated by peritumoral injection of the DOPr agonist and T-type calcium antagonist, via its activity on bone afferents. Results from this study suggest that nerve terminals at tumor site could be utilized as targets for specific analgesics, using local infiltration analgesia.

In Vivo Calcium Imaging of Neuronal Ensembles in Networks of Primary Sensory Neurons in Intact Dorsal Root Ganglia

Ca2+ imaging can be used as a proxy for cellular activity, including action potentials and various signaling mechanisms involving Ca2+ entry into the cytoplasm or the release of intracellular Ca2+ stores. Pirt-GCaMP3-based Ca2+ imaging of primary sensory neurons of the dorsal root ganglion (DRG) in mice offers the advantage of simultaneous measurement of a large number of cells. Up to 1,800 neurons can be monitored, allowing neuronal networks and somatosensory processes to be studied as an ensemble in their normal physiological context at a populational level in vivo. The large number of neurons monitored allows the detection of activity patterns that would be challenging to detect using other methods. Stimuli can be applied to the mouse hindpaw, allowing the direct effects of stimuli on the DRG neuron ensemble to be studied. The number of neurons producing Ca2+ transients as well as the amplitude of Ca2+ transients indicates sensitivity to specific sensory modalities. The diameter of neurons provides evidence of activated fiber types (non-noxious mechano vs. noxious pain fibers, Aβ, Aδ, and C fibers). Neurons expressing specific receptors can be genetically labeled with td-Tomato and specific Cre recombinases together with Pirt-GCaMP. Therefore, Pirt-GCaMP3 Ca2+ imaging of DRG provides a powerful tool and model for the analysis of specific sensory modalities and neuron subtypes acting as an ensemble at the populational level to study pain, itch, touch, and other somatosensory signals.

Activation of GDNF-ERK-Runx1 signaling contributes to P2X3R gene transcription and bone cancer pain

Bone cancer pain is a common symptom in cancer patients with bone metastases and its underlying mechanisms remain unknown. Here, we report that Runx1 directly upregulates the transcriptional activity of P2X3 receptor (P2X3R) gene promoter in PC12 cells. Knocking down Runx1 in dorsal root ganglion (DRG) neurons suppresses the functional upregulation of P2X3R, attenuates neuronal hyperexcitability and pain hypersensitivity in tumor-bearing rats, whereas overexpressing Runx1 promotes P2X3R gene transcription in DRG neurons, induces neuronal hyperexcitability and pain hypersensitivity in naïve rats. Activation of GDNF-GFRα1-Ret-ERK signaling is required for Runx1-mediated P2X3R gene transcription in DRG neurons, and contributes to neuronal hyperexcitability and pain hypersensitivity in tumor-bearing rats. These findings indicate that the Runx1-mediated P2X3R gene transcription resulted from activation of GDNF-GFRα1-Ret-ERK signaling contributes to the sensitization of DRG neurons and pathogenesis of bone cancer pain. Our findings identify a potentially targetable mechanism that may cause bone metastasis-associated pain in cancer patients.

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Runx1 directly upregulates the transcriptional activity of P2X3R gene promoter

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Upregulation of Runx1-mediated P2X3R gene transcription underlies bone cancer pain

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Involvement of GDNF-Ret-ERK signaling in Runx1-mediated P2X3R gene transcription

Meclizine and Metabotropic Glutamate Receptor Agonists Attenuate Severe Pain and Ca2+ Activity of Primary Sensory Neurons in Chemotherapy-Induced Peripheral Neuropathy

Chemotherapy-induced peripheral neuropathy (CIPN) affects ∼68% of patients undergoing chemotherapy, causing debilitating neuropathic pain and reducing quality of life. Cisplatin is a commonly used platinum-based chemotherapeutic drug known to cause CIPN, possibly by causing oxidative stress damage to primary sensory neurons. Metabotropic glutamate receptors (mGluRs) are widely hypothesized to be involved in pain processing and pain mitigation. Meclizine is an H1 histamine receptor antagonist known to have neuroprotective effects, including an anti-oxidative effect. Here, we used a mouse model of cisplatin-induced CIPN using male and female mice to test agonists of mGluR8 and Group II mGluR as well as meclizine as interventions to reduce cisplatin-induced pain. We performed behavioral pain tests, and we imaged Ca2+ activity of the large population of dorsal root ganglia (DRG) neurons in vivo For the latter, we used a genetically-encoded Ca2+ indicator, Pirt-GCaMP3, which enabled us to monitor different drug interventions at the level of the intact DRG neuronal ensemble. We found that CIPN increased spontaneous Ca2+ activity in DRG neurons, increased number of Ca2+ transients, and increased hyper-responses to mechanical, thermal, and chemical stimuli. We found that mechanical and thermal pain caused by CIPN was significantly attenuated by the mGluR8 agonist, (S)-3,4-DCPG, the Group II mGluR agonist, LY379268, and the H1 histamine receptor antagonist, meclizine. DRG neuronal Ca2+ activity elevated by CIPN was attenuated by LY379268 and meclizine, but not by (S)-3,4-DCPG. Furthermore, meclizine and LY379268 attenuated cisplatin-induced weight loss. These results suggest that Group II mGluR agonist, mGluR8 agonist, and meclizine are promising candidates as new treatment options for CIPN, and studies of their mechanisms are warranted.SIGNIFICANCE STATEMENT Chemotherapy-induced peripheral neuropathy (CIPN) is a painful condition that affects most chemotherapy patients and persists several months or longer after treatment ends. Research on CIPN mechanism is extensive but has produced only few clinically useful treatments. Using in vivo GCaMP Ca2+ imaging in live animals over 1800 neurons/dorsal root ganglia (DRG) at once, we have characterized the effects of the chemotherapeutic drug, cisplatin and three treatments that decrease CIPN pain. Cisplatin increases sensory neuronal Ca2+ activity and develops various sensitization. Metabotropic glutamate receptor (mGluR) agonist, LY379268 or the H1 histamine receptor antagonist, meclizine decreases cisplatin's effects on neuronal Ca2+ activity and reduces pain hypersensitivity. Our results and experiments provide insights into cellular effects of cisplatin and drugs preventing CIPN pain.

Calcium imaging for analgesic drug discovery

Somatosensation and pain are complex phenomena involving a rangeofspecialised cell types forming different circuits within the peripheral and central nervous systems. In recent decades, advances in the investigation of these networks, as well as their function in sensation, resulted from the constant evolution of electrophysiology and imaging techniques to allow the observation of cellular activity at the population level both in vitro and in vivo. Genetically encoded indicators of neuronal activity, combined with recent advances in DNA engineering and modern microscopy, offer powerful tools to dissect and visualise the activity of specific neuronal subpopulations with high spatial and temporal resolution. In recent years various groups developed in vivo imaging techniques to image calcium transients in the dorsal root ganglia, the spinal cord and the brain of anesthetised and awake, behaving animals to address fundamental questions in both the physiology and pathophysiology of somatosensation and pain. This approach, besides giving unprecedented details on the circuitry of innocuous and painful sensation, can be a very powerful tool for pharmacological research, from the characterisation of new potential drugs to the discovery of new, druggable targets within specific neuronal subpopulations. Here we summarise recent developments in calcium imaging for pain research, discuss technical challenges and advances, and examine the potential positive impact of this technique in early preclinical phases of the analgesic drug discovery process.

Activation of the spinal EGFR signaling pathway in a rat model of cancer-induced bone pain with morphine tolerance

Cancer-induced bone pain (CIBP) is considered to be one of the most difficult pain conditions to treat. Morphine, an analgesic drug, is widely used in clinical practice, and long-term use of morphine can lead to drug tolerance. Recent reports have suggested that inhibitors of epidermal growth factor receptor (EGFR) may have analgesic effects in cancer patients suffering from pain. Therefore, we sought to determine whether EGFR signaling was involved in morphine tolerance (MT) in a rat model of cancer-induced bone pain. In this study, Walker 256 mammary gland carcinoma cells were inoculated into the tibias of rats to provoke cancer-induced bone pain. Then, morphine was intrathecally administered twice daily for seven consecutive days to induce drug tolerance. We observed sustained increased in the protein levels of EGFR, p-EGFR, ERK1/2, and p-ERK1/2 during the development of morphine tolerance in rats with cancer-induced bone pain by western blotting. The EGFR level was significantly increased in the MT and CIBP + MT groups, and EGFR was colocalized with markers of microglia and neurons in the spinal cords of rats. Inhibition of EGFR by a small molecule inhibitor markedly attenuated the degree of morphine tolerance and decreased the number of microglia, and the protein levels of EGFR, p-EGFR, ERK1/2, and p-ERK1/2 were also reduced. In summary, our results suggest that the activation of the EGFR signaling pathway in spinal microglia plays an important modulatory role in the development of morphine tolerance and that inhibition of EGFR may provide a new therapeutic option for cancer-induced bone pain.

Challenges and opportunities in translational pain research - An opinion paper of the working group on translational pain research of the European pain federation (EFIC)

For decades, basic research on the underlying mechanisms of nociception has held promise to translate into efficacious treatments for patients with pain. Despite great improvement in the understanding of pain physiology and pathophysiology, translation to novel, effective treatments for acute and chronic pain has however been limited, and they remain an unmet medical need. In this opinion paper bringing together pain researchers from very different disciplines, the opportunities and challenges of translational pain research are discussed. The many factors that may prevent the successful translation of bench observations into useful and effective clinical applications are reviewed, including interspecies differences, limited validity of currently available preclinical disease models of pain, and limitations of currently used methods to assess nociception and pain in non-human and human models of pain. Many paths are explored to address these issues, including the backward translation of observations made in patients and human volunteers into new disease models that are more clinically relevant, improved generalization by taking into account age and sex differences, and the integration of psychobiology into translational pain research. Finally, it is argued that preclinical and clinical stages of developing new treatments for pain can be improved by better preclinical models of pathological pain conditions alongside revised methods to assess treatment-induced effects on nociception in human and non-human animals. Significance: For decades, basic research of the underlying mechanisms of nociception has held promise to translate into efficacious treatments for patients with pain. Despite great improvement in the understanding of pain physiology and pathophysiology, translation to novel, effective treatments for acute and chronic pain has however been limited, and they remain an unmet medical need.

Sensitization of Cutaneous Primary Afferents in Bone Cancer Revealed by In Vivo Calcium Imaging

Simple Summary

Cancer-induced bone pain severely impairs the quality of life of cancer patients, many of whom suffer from inadequate pain relief. The development of new analgesic therapies depends on the identification of the cells and mechanisms involved in cancer-induced bone pain. Bone marrow innervating sensory neurons have been proposed to contribute to this debilitating disease, but their role remains unexplored. Here we used in vivo calcium imaging to determine the functional role of bone innervating and skin innervating neurons in contributing to pain at an advanced stage of bone cancer. Our results indicate increased excitability of skin innervating neurons, while those innervating bone are unaffected. Our data suggests skin-innervating neurons become hyperexcitable in cancer-induced bone pain and are a potential target for pain relief.

Abstract

Cancer-induced bone pain (CIBP) is a complex condition, comprising components of inflammatory and neuropathic processes, but changes in the physiological response profiles of bone-innervating and cutaneous afferents remain poorly understood. We used a combination of retrograde labelling and in vivo calcium imaging of bone marrow-innervating dorsal root ganglia (DRG) neurons to determine the contribution of these cells in the maintenance of CIBP. We found a majority of femoral bone afferent cell bodies in L3 dorsal root ganglia (DRG) that also express the sodium channel subtype Na_v1.8—a marker of nociceptive neurons—and lack expression of parvalbumin—a marker for proprioceptive primary afferents. Surprisingly, the response properties of bone marrow afferents to both increased intraosseous pressure and acid were unchanged by the presence of cancer. On the other hand, we found increased excitability and polymodality of cutaneous afferents innervating the ipsilateral paw in cancer bearing animals, as well as a behavioural phenotype that suggests changes at the level of the DRG contribute to secondary hypersensitivity. This study demonstrates that cutaneous afferents at distant sites from the tumour bearing tissue contribute to mechanical hypersensitivity, highlighting these cells as targets for analgesia.

The Stage-Specific Plasticity of Descending Modulatory Controls in a Rodent Model of Cancer-Induced Bone Pain

Simple Summary

The mechanisms that underlie pain resulting from metastatic bone disease remain elusive. This translates to a clinical and socioeconomic burden—targeted therapy is not possible, and patients do not receive adequate analgesic relief. The heterogeneous nature of metastatic bone disease complicates matters. Early stage cancers are molecularly very different to their late stage counterparts and so is the pain associated with early stage and advanced tumours. Thus, analgesic approaches should differ according to disease stage. In this article, we demonstrate that a unique form of brain inhibitory control responsible for the modulation of incoming pain signals at the level of the spinal cord changes with the progression of bone tumours. This corresponds with the degree of damage to the primary afferents innervating the cancerous tissue. Plasticity in the modulation of spinal neuronal activity by descending control pathways reveals a novel opportunity for targeting bone cancer pain in a stage-specific manner.

Abstract

Pain resulting from metastatic bone disease is a major unmet clinical need. Studying spinal processing in rodent models of cancer pain is desirable since the percept of pain is influenced in part by modulation at the level of the transmission system in the dorsal horn of the spinal cord. Here, a rodent model of cancer-induced bone pain (CIBP) was generated following syngeneic rat mammary gland adenocarcinoma cell injection in the tibia of male Sprague Dawley rats. Disease progression was classified as “early” or “late” stage according to bone destruction. Even though wakeful CIBP rats showed progressive mechanical hypersensitivity, subsequent in vivo electrophysiological measurement of mechanically evoked deep dorsal horn spinal neuronal responses revealed no change. Rather, a dynamic reorganization of spinal neuronal modulation by descending controls was observed, and this was maladaptive only in the early stage of CIBP. Interestingly, this latter observation corresponded with the degree of damage to the primary afferents innervating the cancerous tissue. Plasticity in the modulation of spinal neuronal activity by descending control pathways reveals a novel opportunity for targeting CIBP in a stage-specific manner. Finally, the data herein have translational potential since the descending control pathways measured are present also in humans.

Activation of the descending pain modulatory system using cuff pressure algometry: Back translation from man to rat

BACKGROUND

Diffuse noxious inhibitory controls (DNIC) as measured in rat and conditioned pain modulation (CPM), the supposed psychophysical paradigm of DNIC measured in humans, are unique manifestations of an endogenous descending modulatory pathway that is activated by the application of a noxious conditioning stimulus. The predictive value of the human CPM processing is crucial when deliberating the translational worth of the two phenomena.

METHODS

For CPM or DNIC measurement, test and conditioning stimuli were delivered using a computer-controlled cuff algometry system or manual inflation of neonate blood pressure cuffs, respectively. In humans (n = 20), cuff pain intensity (for pain detection and pain tolerance thresholds) was measured using an electronic visual analogue scale. In isoflurane-anaesthetized naïve rats, nociception was measured by recording deep dorsal horn wide dynamic range (WDR) neuronal firing rates (n = 7) using in vivo electrophysiology.

RESULTS

A painful cuff-pressure conditioning stimulus on the leg increased pain detection and pain tolerance thresholds recorded by cuff stimulation on the contralateral leg in humans by 32% ± 3% and 24% ± 2% (mean ± SEM) of baseline responses, respectively (p < .001). This finding was back-translated by revealing that a comparable cuff-pressure conditioning stimulus (40 kPa) on the hind paw inhibited the responses of WDR neurons to noxious contralateral cuff test stimulation to 42% ± 9% of the baseline neuronal response (p = .003).

CONCLUSIONS

These data substantiate that the noxious cuff pressure paradigm activates the descending pain modulatory system in rodent (DNIC) and man (CPM), respectively. Future back and forward translational studies using cuff pressure algometry may reveal novel mechanisms in varied chronic pain states.

SIGNIFICANCE

This study provides novel evidence that a comparable noxious cuff pressure paradigm activates a unique form of endogenous inhibitory control in healthy rat and man. This has important implications for the forward translation of bench and experimental pain research findings to the clinical domain. If translatable mechanisms underlying dysfunctional endogenous inhibitory descending pathway expression (previously evidenced in painful states in rat and man) were revealed using cuff pressure algometry, the identification of new analgesic targets could be expedited.