Peripheral inflammation increases Scya2 expression in sensory ganglia and cytokine and endothelial related gene expression in inflamed tissue

Interactions and Trends of Interleukins, PAI-1, CRP, and TNF-α in Inflammatory Responses during the Perioperative Period of Joint Arthroplasty: Implications for Pain Management-A Narrative Review

Inflammation during the perioperative period of joint arthroplasty is a critical aspect of patient outcomes, influencing both the pathophysiology of pain and the healing process. This narrative review comprehensively evaluates the roles of specific cytokines and inflammatory biomarkers in this context and their implications for pain management. Inflammatory responses are initiated and propagated by cytokines, which are pivotal in the development of both acute and chronic postoperative pain. Pro-inflammatory cytokines play essential roles in up-regulating the inflammatory response, which, if not adequately controlled, leads to sustained pain and impaired tissue healing. Anti-inflammatory cytokines work to dampen inflammatory responses and promote resolution. Our discussion extends to the genetic and molecular influences on cytokine production, which influence pain perception and recovery rates post-surgery. Furthermore, the role of PAI-1 in modulating inflammation through its impact on the fibrinolytic system highlights its potential as a therapeutic target. The perioperative modulation of these cytokines through various analgesic and anesthetic techniques, including the fascia iliac compartment block, demonstrates a significant reduction in pain and inflammatory markers, thus underscoring the importance of targeted therapeutic strategies. Our analysis suggests that a nuanced understanding of the interplay between pro-inflammatory and anti-inflammatory cytokines is required. Future research should focus on individualized pain management strategies.

Interactions between Cytokines and the Pathogenesis of Prion Diseases: Insights and Implications

Transmissible Spongiform Encephalopathies (TSEs), including prion diseases such as Bovine Spongiform Encephalopathy (Mad Cow Disease) and variant Creutzfeldt-Jakob Disease, pose unique challenges to the scientific and medical communities due to their infectious nature, neurodegenerative effects, and the absence of a cure. Central to the progression of TSEs is the conversion of the normal cellular prion protein (PrPC) into its infectious scrapie form (PrPSc), leading to neurodegeneration through a complex interplay involving the immune system. This review elucidates the current understanding of the immune response in prion diseases, emphasizing the dual role of the immune system in both propagating and mitigating the disease through mechanisms such as glial activation, cytokine release, and blood-brain barrier dynamics. We highlight the differential cytokine profiles associated with various prion strains and stages of disease, pointing towards the potential for cytokines as biomarkers and therapeutic targets. Immunomodulatory strategies are discussed as promising avenues for mitigating neuroinflammation and delaying disease progression. This comprehensive examination of the immune response in TSEs not only advances our understanding of these enigmatic diseases but also sheds light on broader neuroinflammatory processes, offering hope for future therapeutic interventions.

Mechanistic insights into the role of the chemokine CCL2/CCR2 axis in dorsal root ganglia to peripheral inflammation and pain hypersensitivity

BACKGROUND

Pain is reported as the leading cause of disability in the common forms of inflammatory arthritis conditions. Acting as a key player in nociceptive processing, neuroinflammation, and neuron-glia communication, the chemokine CCL2/CCR2 axis holds great promise for controlling chronic painful arthritis. Here, we investigated how the CCL2/CCR2 system in the dorsal root ganglion (DRG) contributes to the peripheral inflammatory pain sensitization.

METHODS

Repeated intrathecal (i.t.) administration of the CCR2 antagonist, INCB3344 was tested for its ability to reverse the nociceptive-related behaviors in the tonic formalin and complete Freund's adjuvant (CFA) inflammatory models. We further determined by qPCR the expression of CCL2/CCR2, SP and CGRP in DRG neurons from CFA-treated rats. Using DRG explants, acutely dissociated primary sensory neurons and calcium mobilization assay, we also assessed the release of CCL2 and sensitization of nociceptors. Finally, we examined by immunohistochemistry following nerve ligation the axonal transport of CCL2, SP, and CGRP from the sciatic nerve of CFA-treated rats.

RESULTS

We first found that CFA-induced paw edema provoked an increase in CCL2/CCR2 and SP expression in ipsilateral DRGs, which was decreased after INCB3344 treatment. This upregulation in pronociceptive neuromodulators was accompanied by an enhanced nociceptive neuron excitability on days 3 and 10 post-CFA, as revealed by the CCR2-dependent increase in intracellular calcium mobilization following CCL2 stimulation. In DRG explants, we further demonstrated that the release of CCL2 was increased following peripheral inflammation. Finally, the excitation of nociceptors following peripheral inflammation stimulated the anterograde transport of SP at their peripheral nerve terminals. Importantly, blockade of CCR2 reduced sensory neuron excitability by limiting the calcium mobilization and subsequently decreased peripheral transport of SP towards the periphery. Finally, pharmacological inhibition of CCR2 reversed the pronociceptive action of CCL2 in rats receiving formalin injection and significantly reduced the neurogenic inflammation as well as the stimuli-evoked and movement-evoked nociceptive behaviors in CFA-treated rats.

CONCLUSIONS

Our results provide significant mechanistic insights into the role of CCL2/CCR2 within the DRG in the development of peripheral inflammation, nociceptor sensitization, and pain hypersensitivity. We further unveil the therapeutic potential of targeting CCR2 for the treatment of painful inflammatory disorders.

Long-term pain relief in canine osteoarthritis by a single intra-articular injection of resiniferatoxin, a potent TRPV1 agonist

The translational potential of analgesic approaches emerging from basic research can be augmented by client-owned dog trials. We report on a peripheral interventional approach that uses intra-articular injection of the ultrapotent TRPV1 agonist resiniferatoxin (RTX) to produce a selective long-term chemoinactivation of nociceptive primary afferent nerve endings for pain control in naturally occurring canine osteoarthritis. A single injection of 10 µg of RTX, produced suppression of pain, improvement in gait, weight bearing, and improvement in the dog's activities of daily living lasting 4 months or longer. Two to 3 years after the injection, there are no alterations to suggest that removal of inflammatory pain caused accelerated joint degeneration (Charcot joint) in any of the dogs. To amplify the effective use of canine subjects in translational analgesia research, we report a high-quality canine dorsal root ganglion transcriptome. Some targets for analgesia are highly conserved both in protein sequence and level of expression within a target tissue while others diverge substantially from the human. This knowledge is especially important for development of analgesics aimed at peripheral molecular targets and provides a template for informed translational research. The peripheral site of action, long duration of analgesia, apparent safety, and retention of coordination, all resulting from a single dose suggest that intra-articular RTX may be an effective intervention for osteoarthritis pain with few or no side effects and lead to an improved quality of life.

Analgesia by Deletion of Spinal Neurokinin 1 Receptor Expressing Neurons Using a Bioengineered Substance P-Pseudomonas Exotoxin Conjugate

Cell deletion approaches to pain directed at either the primary nociceptive afferents or second-order neurons are highly effective analgesic manipulations. Second-order spinal neurons expressing the neurokinin 1 (NK1) receptor are required for the perception of many types of pain. To delete NK1+ neurons for the purpose of pain control, we generated a toxin–peptide conjugate using DTNB-derivatized (Cys₀) substance P (SP) and a N-terminally truncated Pseudomonas exotoxin (PE35) that retains the endosome-release and ADP-ribosylation enzymatic domains but with only one free sulfhydryl side chain for conjugation. This allowed generation of a one-to-one product linked by a disulfide bond (SP-PE35). In vitro, Chinese hamster ovary cells stably transfected with the NK1 receptor exhibited specific cytotoxicity when exposed to SP-PE35 (IC₅₀ = 5 × 10⁻¹¹M), whereas the conjugate was nontoxic to NK2 and NK3 receptor-bearing cell lines. In vivo studies showed that, after infusion into the spinal subarachnoid space, the toxin was extremely effective in deleting NK1 receptor-expressing cells from the dorsal horn of the spinal cord. The specific cell deletion robustly attenuated thermal and mechanical pain sensations and inflammatory hyperalgesia but did not affect motoric capabilities. NK1 receptor cell deletion and antinociception occurred without obvious lesion of non–receptor-expressing cells or apparent reorganization of primary afferent innervation. These data demonstrate the extraordinary selectivity and broad-spectrum antinociceptive efficacy of this ligand-directed protein therapeutic acting via receptor-mediated endocytosis. The loss of multiple pain modalities including heat and mechanical pinch, transduced by different populations of primary afferents, shows that spinal NK1 receptor-expressing neurons are critical points of convergence in the nociceptive transmission circuit. We further suggest that therapeutic end points can be effectively and safely achieved when SP-PE35 is locally infused, thereby producing a regionally defined analgesia.

Local Resiniferatoxin Induces Long-Lasting Analgesia in a Rat Model of Full Thickness Thermal Injury

Objective

Opioid-based analgesics are a major component of the lengthy pain management of burn patients, including military service members, but are problematic due to central nervous system-mediated side effects. Peripheral analgesia via targeted ablation of nociceptive nerve endings that express the transient receptor potential vanilloid channel 1 (TRPV1) may provide an improved approach. We hypothesized that local injection of the TRPV1 agonist resiniferatoxin (RTX) would produce long-lasting analgesia in a rat model of pain associated with burn injury.

Methods

Baseline sensitivities to thermal and mechanical stimuli were measured in male and female Sprague-Dawley rats. Under anesthesia, a 100 °C metal probe was placed on the right hind paw for 30 seconds, and sensitivity was reassessed 72 hours following injury. Rats received RTX (0.25 μg/100 μL; ipl) into the injured hind paw, and sensitivity was reassessed across three weeks. Tissues were collected from a separate group of rats at 24 hours and/or one week post-RTX for pathological analyses of the injured hind paw, dorsal spinal cord c-Fos, and primary afferent neuropeptide immunoreactivity.

Results

Local RTX reversed burn pain behaviors within 24 hours, which lasted through recovery at three weeks. At one week following RTX, decreased c-Fos and primary afferent neuropeptide immunoreactivities were observed in the dorsal horn, while plantar burn pathology was unaltered.

Conclusions

These results indicate that local RTX induces long-lasting analgesia in a rat model of pain associated with burn. While opioids are undesirable in trauma patients due to side effects, RTX may provide valuable long-term, nonopioid analgesia for burn patients.

Signatures of Altered Gene Expression in Dorsal Root Ganglia of a Fabry Disease Mouse Model

Fabry disease is an X-linked lysosomal storage disorder with involvement of the nervous system. Accumulation of glycosphingolipids within peripheral nerves and/or dorsal root ganglia results in pain due to small-fiber neuropathy, which affects the majority of patients already in early childhood. The α-galactosidase A deficient mouse proved to be an adequate model for Fabry disease, as it shares many symptoms including altered temperature sensitivity and pain perception. To characterize the signatures of gene expression that might underlie Fabry disease-associated sensory deficits and pain, we performed one-color based hybridization microarray expression profiling of DRG explants from adult α-galactosidase A deficient mice and age-matched wildtype controls. Protein-protein interaction (PPI) and pathway analyses were performed for differentially regulated mRNAs. We found 812 differentially expressed genes between adult α-galactosidase A deficient mice and age-matched wildtype controls, 506 of them being upregulated, and 306 being downregulated. Among the enriched pathways and processes, the disease-specific pathways “lysosome” and “ceramide metabolic process” were identified, enhancing reliability of the current analysis. Novel pathways that we identified include “G-protein coupled receptor signaling” and “retrograde transport” for the upregulated genes. From the analysis of downregulated genes, immune-related pathways, autoimmune, and infection pathways emerged. The current analysis is the first to present a differential gene expression profile of DRGs from α-galactosidase A deficient mice, thereby providing knowledge on possible mechanisms underlying neuropathic pain related symptoms in Fabry patients. Therefore, the presented data provide new insights into the development of the pain phenotype and might lead to new treatment strategies.

W. A. H, Lewis RJ, Vetter I. Transcriptomics in pain research: insights from new and old technologies

Physiological and pathological pain involves a complex interplay of multiple cell types and signaling pathways.

Hyperalgesic and hypoalgesic mechanisms evoked by the acute administration of CCL5 in mice

We show here that the intraplantar administration of CCL5 in mice produces hyperalgesia at low doses but activates compensatory antinociceptive mechanisms at doses slightly higher. Thus, the injection of 3-10ng of CCL5 evoked thermal hyperalgesia through the activation of CCR1 and CCR5 receptors, as demonstrated by the inhibitory effect exerted by the selective antagonists J113863 (0.01-0.1μg) and DAPTA (0.3-3μg), respectively. The prevention of this hyperalgesia by diclofenac (1-10μg), the inhibitors of COX-1 SC-560 (0.1-1μg) or COX-2 celecoxib (1-5μg), the TRPV1 antagonist capsazepine (0.03-0.3μg) or the TRPA1 antagonist HC030031 (10-50μg) demonstrates the involvement of prostaglandin synthesis and TRP sensitization in CCL5-evoked hyperalgesia. Doses of CCL5 higher than 17μg did not evoke hyperalgesia. However, this effect was restored by the administration of naloxone-methiodide (5μg), nor-binaltorphimine (10mg/kg) or an anti-dynorphin A antibody (0.62-2.5ng). The administration of 30ng of CCL5 also induced hyperalgesia in mice with reduced number of circulating white blood cells in response to cyclophosphamide or with selective neutrophil depletion induced by an anti-Ly6G antibody. In fact, the number of neutrophils present in paws treated with 30ng of CCL5 was greater than in paws receiving the administration of the hyperalgesic dose of 10ng. Finally, the expression of the endogenous opioid peptide dynorphin A was demonstrated by double immunofluorescence assays in these neutrophils attracted by CCL5. These results support previous data describing the hyperalgesic properties of CCL5 and constitute the first indication that a chemokine of the CC group can activate endogenous analgesic mechanisms.

Bone morphogenetic protein-2-mediated pain and inflammation in a rat model of posterolateral arthrodesis

BACKGROUND

Bone morphogenetic protein-2 (BMP-2) is a pleiotropic, secreted molecule with diverse effects. The potent ability of BMP-2 to stimulate bone growth prompted its widespread clinical use for arthrodesis (spine fusion). However, elevated post-operative pain in patients treated with BMP-2 has been increasingly reported. Determining whether BMP-2 induces pain directly or whether it induces neuroinflammation, which could lower the threshold for pain, is important for developing therapeutic interventions. We therefore modeled the clinical use of BMP-2 for posterior lumbar fusion by implanting absorbable collagen sponges soaked with either recombinant human BMP-2 (rhBMP-2) or vehicle above the L4-L5 transverse processes of rat spine.

RESULTS

Using microarray analysis we found that implantation of rhBMP-2-soaked absorbable collagen sponges resulted in altered expression of numerous pro-inflammatory genes in the adjacent dorsal root ganglia (DRG) showing that implantation of rhBMP-2/absorbable collagen sponges triggers potent neuroinflammatory responses in the DRG-2. Interestingly, direct BMP-2 treatment of DRG explants resulted in changes in gene expression that were not specifically pro-inflammatory. Rats implanted with rhBMP-2 in absorbable collagen sponges also exhibited a transient change in thermal and mechanical sensitivity indicating that rhBMP-2 applied to the lumbar spine could increase pain sensitivity. Immunohistochemical analysis indicated macrophage infiltration in the DRG and spinal nerve in rats implanted with rhBMP-2/absorbable collagen sponges or absorbable collagen sponges alone, but not in rats that underwent surgery without implantation of the absorbable collagen sponges suggesting that the sponges contributed to the biological response. Indeed, analysis of DRGs taken from rats implanted with absorbable collagen sponges without rhBMP-2 showed a significant change in gene expression distinct from DRGs from rats undergoing surgery only.

CONCLUSIONS

Our data indicate that implantation of rhBMP-2/absorbable collagen sponges on the lumbar spine triggers potent neuroinflammatory responses in the DRG. Importantly, however, these BMP-2 effects may be partially mediated through a response to the absorbable collagen sponges.

Early life inflammatory pain induces long-lasting deficits in hippocampal-dependent spatial memory in male and female rats

The present experiment tested the hypothesis that neonatal injury disrupts adult hippocampal functioning and that normal aging or chronic stress during adulthood, which are known to have a negative impact on hippocampal function, exacerbate these effects. Male and female Sprague-Dawley rats were given an intraplantar injection of the inflammatory agent carrageenan (1%) on the day of birth and their memory was tested in the hippocampal-dependent spatial water maze in adulthood and again in middle age. We found that neonatal injury impaired hippocampal-dependent memory in adulthood, that the effects of injury on memory were more pronounced in middle-aged male rats, and that chronic stress accelerated the onset of these memory deficits. Neonatal injury also decreased glucocorticoid receptor mRNA in the dorsal CA1 area of middle-aged rats, a brain region critical for spatial memory. Morphine administration at the time of injury completely reversed injury-induced memory deficits, but neonatal morphine treatments in the absence of injury produced significant memory impairments in adulthood. Collectively, these findings are consistent with our hypothesis that neonatal injury produces long-lasting disruption in adult hippocampal functioning.

Molecular signatures of mouse TRPV1-lineage neurons revealed by RNA-Seq transcriptome analysis

Disorders of pain neural systems are frequently chronic and, when recalcitrant to treatment, can severely degrade the quality of life. The pain pathway begins with sensory neurons in dorsal root or trigeminal ganglia, and the neuronal subpopulations that express the transient receptor potential cation channel, subfamily V, member 1 (TRPV1) ion channel transduce sensations of painful heat and inflammation and play a fundamental role in clinical pain arising from cancer and arthritis. In the present study, we elucidate the complete transcriptomes of neurons from the TRPV1 lineage and a non-TRPV1 neuroglial population in sensory ganglia through the combined application of next-gen deep RNA-Seq, genetic neuronal labeling with fluorescence-activated cell sorting, or neuron-selective chemoablation. RNA-Seq accurately quantitates gene expression, a difficult parameter to determine with most other methods, especially for very low and very high expressed genes. Differentially expressed genes are present at every level of cellular function from the nucleus to the plasma membrane. We identified many ligand receptor pairs in the TRPV1 population, suggesting that autonomous presynaptic regulation may be a major regulatory mechanism in nociceptive neurons. The data define, in a quantitative, cell population-specific fashion, the molecular signature of a distinct and clinically important group of pain-sensing neurons and provide an overall framework for understanding the transcriptome of TRPV1 nociceptive neurons.

PERSPECTIVE

Next-gen RNA-Seq, combined with molecular genetics, provides a comprehensive and quantitative measurement of transcripts in TRPV1 lineage neurons and a contrasting transcriptome from non-TRPV1 neurons and cells. The transcriptome highlights previously unrecognized protein families, identifies multiple molecular circuits for excitatory or inhibitory autocrine and paracrine signaling, and suggests new combinatorial approaches to pain control.

Neuronal CC chemokines: the distinct roles of CCL21 and CCL2 in neuropathic pain

The development of neuropathic pain in response to peripheral nerve lesion for a large part depends on microglia located at the dorsal horn of the spinal cord. Thus the injured nerve initiates a response of microglia, which represents the start of a cascade of events that leads to neuropathic pain development. For long it remained obscure how a nerve injury in the periphery would initiate a microglia response in the dorsal horn of the spinal cord. Recently, two chemokines have been suggested as potential factors that mediate the communication between injured neurons and microglia namely CCL2 and CCL21. This assumption is based on the following findings. Both chemokines are not found in healthy neurons, but are expressed in response to neuronal injury. In injured dorsal root ganglion cells CCL2 and CCL21 are expressed in vesicles in the soma and transported through the axons of the dorsal root into the dorsal horn of the spinal cord. Finally, microglia in vitro are known to respond to CCL2 and CCL21. Whereas the microglial chemokine receptor involved in CCL21-induced neuropathic pain is not yet defined the situation concerning the receptors for CCL2 in microglia in vivo is even less clear. Recent results obtained in transgenic animals clearly show that microglia in vivo do not express CCR2 but that peripheral myeloid cells and neurons do. This suggests that CCL2 expressed by injured dorsal root neurons does not act as neuron-microglia signal in contrast to CCL21. Instead, CCL2 in the injured dorsal root ganglia (DRG) may act as autocrine or paracrine signal and may stimulate first or second order neurons in the pain cascade and/or attract CCR2-expressing peripheral monocytes/macrophages to the spinal cord.

Grafting of neural stem and progenitor cells to the hippocampus of young, irradiated mice causes gliosis and disrupts the granule cell layer

Ionizing radiation persistently reduces the pool of neural stem and progenitor cells (NSPCs) in the dentate gyrus (DG) of the hippocampus, which may explain some of the learning deficits observed in patients treated with radiotherapy, particularly pediatric patients. A single dose of 8 Gy irradiation (IR) was administered to the brains of postnatal day 14 (P14) C57BL/6 mice and 1.0 × 10⁵ bromodeoxyuridine-labeled, syngeneic NSPCs were injected into the hippocampus 1 day, 1 week or 6 weeks after IR. Cell survival and phenotype were evaluated 5 weeks after grafting. When grafted 1 day post-IR, survival and neuronal differentiation of the transplanted NSPCs were lower in irradiated brains, whereas the survival and cell fate of grafted cells were not significantly different between irradiated and control brains when transplantation was performed 1 or 6 weeks after IR. A young recipient brain favored neuronal development of grafted cells, whereas the older recipient brains displayed an increasing number of cells developing into astrocytes or unidentified cells. Injection of NSPCs, but not vehicle, induced astrogliosis and reduced thickness of the dorsal blade of the GCL after 5 months. In summary, we demonstrate that age and interval between IR and grafting can affect survival and differentiation of grafted NSPCs. The observed long-term gliosis and degeneration warrant caution in the context of NSPC grafting for therapeutical purposes.

Resiniferatoxin for Pain Treatment: An Interventional Approach to Personalized Pain Medicine

This review examines existing preclinical and clinical studies related to resiniferatoxin (RTX) and its potential uses in pain treatment. Like capsaicin, RTX is a vanilloid receptor (TRPV1) agonist, only more potent. This increased potency confers both quantitative and qualitative advantages in terms of drug action on the TRPV1 containing nerve terminal, which result in an increased efficacy and a long duration of action. RTX can be delivered by a central route of administration through injection into the subarachnoid space around the lumbosacral spinal cord. It can also be administered peripherally into a region of skin or deep tissue where primary afferents nerves terminate, or directly into a nerve trunk or a dorsal root ganglion. The central route is currently being evaluated as a treatment for intractable pain in patients with advanced cancer. Peripheral administration offers the possibility to treat a wide diversity of pain problems because of the ability to bring the treatment to the site of the pain (the peripheral generator). While not all pain disorders are appropriate for RTX, tailoring treatment to an individual patient's needs via a selective and local intervention that chemically targets a specific population of nerve terminals provides a new capability for pain therapy and a simplified and effective approach to personalized pain medicine.

Positive allosteric modulation of TRPV1 as a novel analgesic mechanism

Background

The prevalence of long-term opiate use in treating chronic non-cancer pain is increasing, and prescription opioid abuse and dependence are a major public health concern. To explore alternatives to opioid-based analgesia, the present study investigates a novel allosteric pharmacological approach operating through the cation channel TRPV1. This channel is highly expressed in subpopulations of primary afferent unmyelinated C- and lightly-myelinated Aδ-fibers that detect low and high rates of noxious heating, respectively, and it is also activated by vanilloid agonists and low pH. Sufficient doses of exogenous vanilloid agonists, such as capsaicin or resiniferatoxin, can inactivate/deactivate primary afferent endings due to calcium overload, and we hypothesized that positive allosteric modulation of agonist-activated TRPV1 could produce a selective, temporary inactivation of nociceptive nerve terminals in vivo. We previously identified MRS1477, a 1,4-dihydropyridine that potentiates vanilloid and pH activation of TRPV1 in vitro, but displays no detectable intrinsic agonist activity of its own. To study the in vivo effects of MRS1477, we injected the hind paws of rats with a non-deactivating dose of capsaicin, MRS1477, or the combination. An infrared diode laser was used to stimulate TRPV1-expressing nerve terminals and the latency and intensity of paw withdrawal responses were recorded. qRT-PCR and immunohistochemistry were performed on dorsal root ganglia to examine changes in gene expression and the cellular specificity of such changes following treatment.

Results

Withdrawal responses of the capsaicin-only or MRS1477-only treated paws were not significantly different from the untreated, contralateral paws. However, rats treated with the combination of capsaicin and MRS1477 exhibited increased withdrawal latency and decreased response intensity consistent with agonist potentiation and inactivation or lesion of TRPV1-containing nerve terminals. The loss of nerve endings was manifested by an increase in levels of axotomy markers assessed by qRT-PCR and colocalization of ATF3 in TRPV1⁺ cells visualized via immunohistochemistry.

Conclusions

The present observations suggest a novel, non-narcotic, selective, long-lasting TRPV1-based approach for analgesia that may be effective in acute, persistent, or chronic pain disorders.

The vanilloid agonist resiniferatoxin for interventional-based pain control

The idea of selectively targeting nociceptive transmission at the level of the peripheral nervous system is attractive from multiple perspectives, particularly the potential lack of non-specific (non-targeted) CNS side effects. Out of the multiple TRP channels involved in nociception, TRPV1 is a strong candidate based on its biophysical conductance properties and its expression in inflammation-sensitive dorsal root ganglion neurons and their axons and central and peripheral nerve terminals. While TRPV1 antagonists have undergone extensive medicinal chemical and pharmacological investigation, for TRPV1 agonists nature has provided an optimized compound in RTX. RTX is not suitable for systemic administration, but it is highly adaptable to a variety of pain problems when used by local administration. This can include routes as diverse as subcutaneous, intraganglionic or intrathecal (CSF space around the spinal cord). The present review focuses on the molecular and preclinical animal experiments that form the underpinnings of our clinical trial of intrathecal RTX for pain in advanced cancer. As such this represents a new approach to pain control that emerges from a long line of research on capsaicin and other vanilloids, their physiological actions, and the molecular biology of the capsaicin receptor TRPV1.

The expression of spinal methyl-CpG-binding protein 2, DNA methyltransferases and histone deacetylases is modulated in persistent pain states

Background

DNA CpG methylation is carried out by DNA methyltransferases and induces chromatin remodeling and gene silencing through a transcription repressor complex comprising the methyl-CpG-binding protein 2 (MeCP2) and a subset of histone deacetylases. Recently, we have found that MeCP2 activity had a crucial role in the pattern of gene expression seen in the superficial dorsal horn rapidly after injection of Complete Freund's Adjuvant (CFA) in the rat ankle joint. The aim of the present study was to analyse the changes in expression of MeCP2, DNA methyltransferases and a subset of histone deacetylases in the superficial dorsal horn during the maintenance phase of persistent pain states. In this process, the cell specific expression of MeCP2 was also investigated.

Results

Using immunohistochemistry, we found that neurones, oligodendrocytes and astrocytes expressed MeCP2. Microglia, oligodendrocyte precursor cells and Schwann cells never showed any positive stain for MeCP2. Quantitative analyses showed that MeCP2 expression was increased in the superficial dorsal horn 7 days following CFA injection in the ankle joint but decreased 7 days following spared nerve injury. Overall, the expression of DNA methyltransferases and a subset of histone deacetylases followed the same pattern of expression. However, there were no significant changes in the expression of the MeCP2 targets that we had previously shown are regulated in the early time points following CFA injection in the ankle joint. Finally, the expression of MeCP2 was also down regulated in damaged dorsal root ganglion neurones following spared nerve injury.

Conclusion

Our results strongly suggest that changes in chromatin compaction, regulated by the binding of MeCP2 complexes to methylated DNA, are involved in the modulation of gene expression in the superficial dorsal horn and dorsal root ganglia during the maintenance of persistent pain states.

Cytokines and brain excitability

Cytokines are molecules secreted by peripheral immune cells, microglia, astrocytes and neurons in the central nervous system. Peripheral or central inflammation is characterized by an upregulation of cytokines and their receptors in the brain. Emerging evidence indicates that pro-inflammatory cytokines modulate brain excitability. Findings from both the clinical literature and from in vivo and in vitro laboratory studies suggest that cytokines can increase seizure susceptibility and may be involved in epileptogenesis. Cellular mechanisms that underlie these effects include upregulation of excitatory glutamatergic transmission and downregulation of inhibitory GABAergic transmission.

CCL2 has similar excitatory effects to TNF-α in a subgroup of inflamed C-fiber axons

Peripheral nerve inflammation can cause neuronal excitability changes that have been implicated in the pathogenesis of chronic pain. Although the neuroimmune interactions that lead to such physiological changes are unclear, in vitro studies suggest that the chemokine CCL2 may be involved. This in vivo study examines the effects of CCL2 on untreated and inflamed neurons and compares its effects with those of TNF-α. Extracellular recordings were performed in the anesthetized rat on isolated neurons with C-fiber axons. On untreated neurons, CCL2, as well as TNF-α, had negligible effects. Following neuritis, both cytokines transiently caused the firing of action potentials in 27–30% of neurons, which were either silent or had background (ongoing) activity. The neurons with ongoing activity, which responded to either cytokine, had significantly slower baseline firing rates {median = 3.0 spikes/min [interquartile range (IQR) 3.0]} compared with the nonresponders [median = 24.4 spikes/min (IQR 24.6); P < 0.001]. In an additional group, 26–27% of neurons, which were sensitized due to repeated noxious mechanical stimulation of the periphery, also responded to the effects of both cytokines. Neither cytokine caused axons to become mechanically sensitive. Immunohistochemistry confirmed that the cognate CCL2 receptor, CCR2, is mainly expressed on glia and is therefore not likely to be an axonal target for CCL2 following inflammation. In contrast, the cognate TNF-α receptor (TNFR), TNFR1, was present on untreated and inflamed neurons. In summary, CCL2 can excite inflamed C-fiber neurons with similar effects to TNF-α, although the underlying mechanisms may be different. The modulatory effects of both cytokines are limited to a subgroup of neurons, which may be subtly inflamed.

Expression of monocyte chemoattractant protein-1 and its induction by tumor necrosis factor receptor 1 in sensory neurons in the ventral rhizotomy model of neuropathic pain

The expression and role of monocyte chemoattractant protein-1 (MCP-1) in the rat dorsal root ganglion (DRG) and spinal cord was evaluated in the lumbar 5 ventral rhizotomy (L5 VR) model of neuropathic pain. MCP-1 protein expression in the L4/L5 DRG neurons following L5 VR peaked after 3 days, and then declined. Immunohistochemistry showed that no MCP-1 immunoreactivity was observed in the spinal cord after L5 VR, while enzyme-linked immunosorbent assay (ELISA) revealed a small but significant increase in MCP-1 protein content. L5 VR resulted in robust and prolonged mechanical allodynia and thermal hyperalgesia. Administration of anti-MCP-1 neutralizing antibody before and at early time points after L5 VR resulted in a significant attenuation of mechanical allodynia and thermal hyperalgesia, while post-treatment had a weaker effect on established neuropathic pain. Extensive colocalization of tumor necrosis factor receptor 1 (TNFR1) and MCP-1 was observed in the L5 DRG following L5 VR, and treatment with TNFR1 antisense oligonucleotide reduced L5 VR-induced MCP-1 expression in L5 DRG neurons and neuropathic pain behaviors. MCP-1/chemokine (C-C motif) receptor 2 signaling has been proposed as a major regulator of macrophage trafficking. In contrast to the effect on pain behaviors, however, intrathecal administration of anti-MCP-1 neutralizing antibody had no effect on the L5 VR-induced increase in ED-1-immunoreactive macrophages in the L5 DRG and the distal stump of the transected L5 ventral root. These data indicate that increased MCP-1 in DRG neurons might participate in the initiation, rather than the maintenance, of neuropathic pain induced by L5 VR. Furthermore, increased MCP-1 in the DRG is induced by TNF-α/TNFR1 and has no effect on the infiltration of macrophages into the DRG following L5 VR.

Sciatic nerve injury induces functional pro-nociceptive chemokine receptors in bladder-associated primary afferent neurons in the rat

Visceral sensory afferents during disease or following injury often produce vague, diffuse body sensations, and pain referred to somatic targets. Alternatively, injury due to trauma or disease of somatic nerve targets can also lead to referred pain in visceral targets via a somatovisceral reflex. Both phenomenons are thought to be due to convergence of visceral and somatic afferents within the spinal cord. To investigate a potential peripheral influence for referred pain in visceral targets following somatic nerve injury, we examined whether a sciatic nerve injury known to produce hindpaw tactile hyperalgesia alters the frequency of micturition and the sensitivity of bladder-associated sensory neurons to pro-nociceptive chemokines. Adult female Sprague-Dawley rats received injections of cholera toxin B subunit conjugated to 555 into urinary bladder wall to retrogradely label visceral primary afferent neurons. After 7 days, the right sciatic nerve of these animals was subjected to a lysophosphatidylcholine (LPC)-induced focal demyelination injury. Pre- and post-injury tactile sensitivity in the hind paw and micturition frequency were assayed. Animals were allowed to survive for 14-28 days. Lumbosacral and lumbar dorsal root ganglia (DRG) ipsilateral to the nerve injury were acutely dissociated from sham and nerve injured animals. Bladder wall-associated sensory neurons identified via the retrograde marker were assayed for fluxes in intracellular calcium following administration of pro-nociceptive chemokines. The assayed chemokines included monocyte chemoattractant protein-1 (MCP1/CCL2) and stromal cell derived factor-1 alpha (SDF1/CXCL12). LPC nerve injured animals exhibited tactile hyperalgesia and increased micturition frequency for at least 28 days. Focal demyelination of the sciatic nerve also increased the number of injured L₄L₅ and non-injured L₆-S₂ bladder-associated sensory neurons that responded to MCP1 and SDF1 when compared with sensory neurons derived from uninjured naïve and sham-injured control animals. Taken together, these data suggest that some visceral hypersensitivity states may have a somatic origin. More importantly, nociceptive somatovisceral sensation may be mediated by upregulation of chemokine signaling in visceral sensory neurons.

Patterns of pain: meta-analysis of microarray studies of pain

Existing microarray gene expression profiling studies of tonic/chronic pain were subjected to meta-analysis to identify genes found to be regulated by these pain states in multiple, independent experiments. Twenty studies published from 2002 to 2008 were identified, describing the statistically significant regulation of 2254 genes. Of those, a total of 79 genes were found to be statistically significant "hits" in 4 or more independent microarray experiments, corresponding to a conservative P<0.01 overall. Gene ontology-based functional annotation clustering analyses revealed strong evidence for regulation of immune-related genes in pain states. A multi-gene quantitative real-time polymerase chain reaction experiment was run on dorsal root ganglion (DRG) and spinal cord tissue from rats and mice given nerve (sciatic chronic constriction; CCI) or inflammatory (complete Freund's adjuvant) injury. We independently confirmed the regulation of 43 of these genes in the rat-CCI-DRG condition; the genetic correlates in all other conditions were largely and, in some cases, strikingly, independent. However, a handful of genes were identified whose regulation bridged etiology, anatomical locus, and/or species. Most notable among these were Reg3b (regenerating islet-derived 3 beta; pancreatitis-associated protein) and Ccl2 (chemokine [C-C motif] ligand 2), which were significantly upregulated in every condition in the rat.

Ablation of rat TRPV1-expressing Adelta/C-fibers with resiniferatoxin: analysis of withdrawal behaviors, recovery of function and molecular correlates

Background

Ablation of TRPV1-expressing nociceptive fibers with the potent capsaicin analog resiniferatoxin (RTX) results in long lasting pain relief. RTX is particularly adaptable to focal application, and the induced chemical axonopathy leads to analgesia with a duration that is influenced by dose, route of administration, and the rate of fiber regeneration. TRPV1 is expressed in a subpopulation of unmyelinated C- and lightly myelinated Adelta fibers that detect changes in skin temperature at low and high rates of noxious heating, respectively. Here we investigate fiber-type specific behaviors, their time course of recovery and molecular correlates of axon damage and nociception using infrared laser stimuli following an RTX-induced peripheral axonopathy.

Results

RTX was injected into rat hind paws (mid-plantar) to produce thermal hypoalgesia. An infrared diode laser was used to stimulate Adelta fibers in the paw with a small-diameter (1.6 mm), high-energy, 100 msec pulse, or C-fibers with a wide-diameter (5 mm), long-duration, low-energy pulse. We monitored behavioral responses to indicate loss and regeneration of fibers. At the site of injection, responses to C-fiber stimuli were significantly attenuated for two weeks after 5 or 50 ng RTX. Responses to Adelta stimuli were significantly attenuated for two weeks at the highest intensity stimulus, and for 5 weeks to a less intense Adelta stimulus. Stimulation on the toe, a site distal to the injection, showed significant attenuation of Adelta responses for 7- 8 weeks after 5 ng, or 9-10 weeks after 50 ng RTX. In contrast, responses to C-fiber stimuli exhibited basically normal responses at 5 weeks after RTX. During the period of fiber loss and recovery, molecular markers for nerve regeneration (ATF3 and galanin) are upregulated in the dorsal root ganglia (DRG) when behavior is maximally attenuated, but markers of nociceptive activity (c-Fos in spinal cord and MCP-1 in DRG), although induced immediately after RTX treatment, returned to normal.

Conclusion

Behavioral recovery following peripheral RTX treatment is linked to regeneration of TRPV1-expressing Adelta and C-fibers and sustained expression of molecular markers. Infrared laser stimulation is a potentially valuable tool for evaluating the behavioral role of Adelta fibers in pain and pain control.

Evaluation of phenoxybenzamine in the CFA model of pain following gene expression studies and connectivity mapping

Background

We have previously used the rat 4 day Complete Freund's Adjuvant (CFA) model to screen compounds with potential to reduce osteoarthritic pain. The aim of this study was to identify genes altered in this model of osteoarthritic pain and use this information to infer analgesic potential of compounds based on their own gene expression profiles using the Connectivity Map approach.

Results

Using microarrays, we identified differentially expressed genes in L4 and L5 dorsal root ganglia (DRG) from rats that had received intraplantar CFA for 4 days compared to matched, untreated control animals. Analysis of these data indicated that the two groups were distinguishable by differences in genes important in immune responses, nerve growth and regeneration. This list of differentially expressed genes defined a "CFA signature". We used the Connectivity Map approach to identify pharmacologic agents in the Broad Institute Build02 database that had gene expression signatures that were inversely related ('negatively connected') with our CFA signature. To test the predictive nature of the Connectivity Map methodology, we tested phenoxybenzamine (an alpha adrenergic receptor antagonist) - one of the most negatively connected compounds identified in this database - for analgesic activity in the CFA model. Our results indicate that at 10 mg/kg, phenoxybenzamine demonstrated analgesia comparable to that of Naproxen in this model.

Conclusion

Evaluation of phenoxybenzamine-induced analgesia in the current study lends support to the utility of the Connectivity Map approach for identifying compounds with analgesic properties in the CFA model.

Insights into the regulation of chemokine receptors by molecular signaling pathways: functional roles in neuropathic pain

Inflammation plays a central role in the manner that the nervous system responds to injury. These effects include vasodilatation, increased vascular permeability, plasma extravasation, cell migration, and pain. Extracellular signals associated with inflammation may also lead to increased levels of pro-nociceptive chemokines/receptors that directly contribute to persistent or chronic pain behavior. To date, research focused on improving the treatment of chronic pain has largely ignored the role of inflammation-associated transcription factors such as nuclear transcription factor in activated T cells (NFAT). Herein we discuss the idea that activation of this transcription factor may be responsible for the production of chemokines receptors in both neuronal and non-neuronal cells of the peripheral nervous system. Taken together, a better understanding of the transcription of these pro-nociceptive genes may lead to the development of novel analgesic targets.

RT-PCR analysis of pain genes: use of gel-based RT-PCR for studying induced and tissue-enriched gene expression

Frequently, it is important to ascertain whether a molecule that is involved in one model of pain is also involved in other models of pain. Similarly, it may be important to determine whether a molecule involved in nociception in one tissue is also expressed in other tissues and to ascertain the degree of enrichment. Additionally, before initiating a complex set of experiments or purchasing an expensive immunoassay kit, it may be useful to obtain initial supporting evidence to justify the time and money. Is the transcript for the target receptor, protein, or peptide precursor present in, for example, the dorsal root ganglion? And, if present, how abundant is it? Here is where the power of PCR can be applied to obtain a quick but informative answer. In this chapter, we mainly detail the use of gel-based RT-PCR and also provide suggestions on tissue dissection and interpretation of results. The use of gel-based RT-PCR can address many of the questions of abundance or tissue specificity with a minimum of expense and time.

Cytokine gene expression after total hip arthroplasty: surgical site versus circulating neutrophil response

BACKGROUND

After surgery, cytokines and chemokines are released at the surgical wound site, which can contribute to postoperative pain, local inflammation, and tissue repair. Multiple cell types are present that can release cytokines/chemokines at the wound site and, thus, the exact cellular source of these molecules is unclear. We sought to better understand the contribution of neutrophils to cytokine/chemokine gene expression at the surgical wound site during the initial postsurgery phase of total hip arthroplasty (THA).

METHODS

Hip drain fluid was collected at 24 h postsurgery from six patients undergoing standardized THA. In addition, venous blood was collected presurgery and 24 h postsurgery. Neutrophils were isolated, total RNA extracted, and a biotinylated cRNA probe generated. The probes were hybridized with a cDNA microarray containing approximately 100 oligonucleotide sequences representing various human cytokines/chemokines or receptor genes. Changes in gene expression seen in the microarray were verified by reverse transcription polymerase chain reaction.

RESULTS

In the microarray analysis of hip drain neutrophils, interleukin-1 receptor antagonist (IL1RN), interleukin-18 receptor 1 (IL18R1), macrophage migration inhibitory factor (MIF), and macrophage inflammatory protein 3alpha (CCL20) were upregulated, whereas interleukin-8 receptor beta (IL8RB/CXCR2) was consistently downregulated, compared with presurgery blood neutrophils. All of these changes were confirmed by reverse transcription polymerase chain reaction.

CONCLUSION

There is a distinct cytokine gene expression profile in neutrophils at the THA surgical wound site at 24 h postsurgery when compared with that found in presurgery circulating neutrophils. Understanding these changes may allow us to knowledgeably manipulate neutrophil activity to reduce postoperative pain and inflammation without impairing wound healing.

Molecular Roles of Cdk5 in Pain Signaling

Injury and inflammation trigger activation of several critical cellular pathways in nociceptive signaling in the peripheral nervous system, but their precise molecular mechanisms have not been clearly defined. Cyclin-dependent kinase 5 (Cdk5), a serine/threonine kinase, is mainly expressed in the post-mitotic neurons, and has many important roles in the development, functions and pathophysiology of diseases of the nervous system. Although many functional roles of Cdk5 have been identified in neurons, its precise role in pain signaling has not been well determined. Experimental inflammation in the hind paws of mice resulted in increased mRNA and protein levels of Cdk5 and its activator p35, as well as the Cdk5 activity in nociceptive neurons (Pareek et al., 2006). Furthermore, we also identified that Cdk5 phosphorylates transient receptor potential vanilloid 1 (TRPV1), a key receptor that modulates agonist-induced calcium influx in the neurons (Pareek et al., 2007). We subsequently demonstrated that inflammation triggers increase in Cdk5 activity through activation of early growth response 1 (Egr-1) and p35 expression by tumor necrosis factor alpha (TNF-α) (Utreras et al., 2009). These findings suggest that Cdk5 plays an important role in pain signaling and therefore Cdk5 and its activators are potentially important drug targets for development of novel analgesics to treat neuropathic pain.

Visualization of chemokine receptor activation in transgenic mice reveals peripheral activation of CCR2 receptors in states of neuropathic pain

CCR2 chemokine receptor signaling has been implicated in the generation of diverse types of neuropathology, including neuropathic pain. For example, ccr2 knock-out mice are resistant to the establishment of neuropathic pain, and mice overexpressing its ligand, monocyte chemoattractant protein-1 (MCP1; also known as CCL2), show enhanced pain sensitivity. However, whether CCR2 receptor activation occurs in the central or peripheral nervous system in states of neuropathic pain has not been clear. We developed a novel method for visualizing CCR2 receptor activation in vivo by generating bitransgenic reporter mice in which the chemokine receptor CCR2 and its ligand MCP1 were labeled by the fluorescent proteins enhanced green fluorescent protein and monomeric red fluorescent protein-1, respectively. CCR2 receptor activation under conditions such as acute inflammation and experimental autoimmune encephalomyelitis could be faithfully visualized by using these mice. We examined the status of CCR2 receptor activation in a demyelination injury model of neuropathic pain and found that MCP1-induced CCR2 receptor activation mainly occurred in the peripheral nervous system, including the injured peripheral nerve and dorsal root ganglia. These data explain the rapid antinociceptive effects of peripherally administered CCR2 antagonists under these circumstances, suggesting that CCR2 antagonists may ameliorate pain by inhibiting CCR2 receptor activation in the periphery. The method developed here for visualizing CCR2 receptor activation in vivo may be extended to G-protein-coupled receptors (GPCRs) in general and will be valuable for studying intercellular GPCR-mediated communication in vivo.

Progress in genetic studies of pain and analgesia

Interindividual variability in pain sensitivity and the response to analgesic manipulations remains a considerable clinical challenge as well as an area of intense scientific investigation. Techniques in this field have matured rapidly so that much relevant data have emerged only in the past few years. Our increasing understanding of the genetic mediation of these biological phenomena have nonetheless revealed their surprising complexity. This review provides a comprehensive picture and critical analysis of the field and its prospects.

Transient inflammation in neurogenic regions after irradiation of the developing brain

We characterized the inflammatory response after a single dose of 8 Gy to the brains of postnatal day 9 rats. Affymetrix gene chips revealed activation of multiple inflammatory mechanisms in the acute phase, 6 h after irradiation. In the subacute phase, 7 days after irradiation, genes related to neurogenesis and cell cycle were down-regulated, but glial fibrillary acidic protein (GFAP) was up-regulated. The concentrations of 14 different cytokines and chemokines were measured using a microsphere-based xMAP technology. CCL2, Gro/KC and IL-1alpha were the most strongly up-regulated 6 h after irradiation. CCL2 was expressed in astrocytes and microglia in the dentate gyrus and the subventricular zone (SVZ). Hypertrophy, but not hyperplasia, of astrocytes was demonstrated 7 days after irradiation. In summary, we found transient activation of multiple inflammatory mechanisms in the acute phase (6 h) after irradiation and activation of astrocytes in the subacute phase (7 days) after irradiation. It remains to be elucidated whether these transient changes are involved in the persistent effects of radiation observed on neurogenesis and cognition in rodents.

Global gene expression analysis in the mouse brainstem after hyperalgesia induced by facial carrageenan injection--evidence for a form of neurovascular coupling?

The present study was carried out to examine global gene expression in the brainstem, in a mouse facial carrageenan injection model of orofacial pain. Mice that received facial carrageenan injection showed increased mechanical allodynia, demonstrated by increased responses to von Frey hair stimulation of the face. The brainstem was harvested at 3 days post-injection, corresponding to the time of peak responses, and analyzed by Affymetrix Mouse Genome 430 2.0 microarrays. We sought to identify common genes that are changed in the respective sides of the brainstem after either right- or left-sided facial carrageenan injection. The result is a relatively small list of genes (22 genes), which were then classified using DAVID software. Many of them fell into the categories of "response to stress", "defence response", "response to biotic stimulus", "cell adhesion" and "leukocyte adhesion". Of these, increased expression of P-selectin, ICAM-1 and CCL12 after carrageenan injection could be verified by real-time RT-PCR on both the right and left sides, and increased in P-selectin and ICAM-1 further verified by Western blot analysis. P-selectin and ICAM-1 were immunolocalized to endothelial cells, and were double labelled with von Willebrand factor. Intraperitoneal injection of the P-selectin inhibitor KF38789 significantly reduced mechanical allodynia in the facial carrageenan-injected mice. P-selectin mediates the capturing of leukocytes from the bloodstream and rolling of leukocytes along the endothelial surface. We hypothesize that increased nociceptive input to the brainstem could attract circulating macrophages into the brain, resulting in neuroinflammation and pain.

Tumor necrosis factor-alpha regulates cyclin-dependent kinase 5 activity during pain signaling through transcriptional activation of p35

Cyclin-dependent kinase 5 (Cdk5) is a proline-directed serine/threonine kinase. We have previously reported that Cdk5 participates in the regulation of nociceptive signaling, and the expression of Cdk5 and its activator, p35, are up-regulated in nociceptive neurons during peripheral inflammation. The aim of our current study was to identify the proinflammatory molecules that regulate Cdk5/p35 activity in response to inflammation. We constructed a vector that contains the mouse p35 promoter driving luciferase expression. We transiently transfected this vector in PC12 cells to test the effect of several cytokines on p35 transcriptional activity and Cdk5 activity. Our results indicate that tumor necrosis factor-alpha (TNF-alpha) activates p35 promoter activity in a dose- and time-dependent manner and concomitantly up-regulates Cdk5 activity. Because TNF-alpha is known to activate ERK1/2, p38 MAPK, JNK, and NF-kappaB signaling pathways, we examined their involvement in the activation of p35 promoter activity. MEK inhibitor, which inhibits ERK activation, decreased p35 promoter activity, whereas the inhibitors of p38 MAPK, JNK, and NF-kappaB increased p35 promoter activity, indicating that these pathways regulate p35 expression differently. The mRNA and protein levels of Egr-1, a transcription factor, were increased by TNF-alpha treatment, and this increase was dependent on ERK signaling. In a mouse model of inflammation-induced pain in which carrageenan injection into the hind paw causes hypersensitivity to heat stimuli, TNF-alpha mRNA was increased at the site of injection. These findings suggest that TNF-alpha-mediated regulation of Cdk5 activity plays an important role in inflammation-induced pain signaling.

Expression of monocyte chemoattractant protein-1 in rat dorsal root ganglia and spinal cord in experimental models of neuropathic pain

In this study, we evaluated the expression of MCP-1 in the rat dorsal root ganglion (DRG) and spinal cord following axotomy and chronic constriction injury (CCI) of the sciatic nerve and L5 spinal nerve ligation (L5 SNL) using an immunohistochemical approach. MCP-1 expression in the DRG peaked and declined before the full onset of pain hypersensitivity following nerve injury. Spinal expression of MCP-1 peaked when mechanical allodynia was maximal, but then declined rapidly despite the remarkable persistence of mechanical allodynia. The results suggest that MCP-1 may participate in the initiation of neuropathic pain, rather than in its maintenance. Despite increased MCP-1 in small and large DRG neurons, a remarkable increase in MCP-1-IR terminals was observed in the spinal superficial laminae following CCI and L5SNL, but not following axotomy; however, in the deeper laminae, a considerable increase in MCP-1-IR terminals, which may originate from the large and injured L5 DRG neurons, was found after L5 SNL. Our results demonstrate that MCP-1 synthesized in DRG neurons may or may not be transported to the spinal cord depending on the type of peripheral nerve injury. Additionally, increased MCP-1 in both intact L4 and injured L5 DRG neurons may contribute to neuropathic pain hypersensitivity following L5 SNL.

Monocyte chemoattractant protein-1 in the choroid plexus: a potential link between vascular pro-inflammatory mediators and the CNS during peripheral tissue inflammation

During peripheral tissue inflammation, inflammatory processes in the CNS can be initiated by blood-borne pro-inflammatory mediators. The choroid plexus, the site of cerebrospinal fluid (CSF) production, is a highly specialized interface between the vascular system and CNS, and thus, this structure may be an important element in communication between the vascular compartment and the CNS during peripheral tissue inflammation. We investigated the potential participation of the choroid plexus in this process during peripheral tissue inflammation by examining expression of the small inducible cytokine A2 (SCYA2) gene which codes for monocyte chemoattractant protein-1 (MCP-1). MCP-1 protein was previously reported to be induced in a variety of cells during peripheral tissue inflammation. In the basal state, SCYA2 is highly expressed in the choroid plexus as compared with other rat CNS tissues. During hind paw inflammation, SCYA2 expression was significantly elevated in choroid plexus, whereas it remained unchanged in a variety of brain regions. The SCYA2-expressing cells were strongly associated with the choroid plexus as vascular depletion of blood cells by whole-body saline flush did not significantly alter SCYA2 expression in the choroid plexus. In situ hybridization suggested that the SCYA2-expressing cells were localized to the choroid plexus stroma. To elucidate potential molecular mechanisms of SCYA2 increase, we examined genes in the nuclear factor-kappa B (NF-kappaB) signaling cascade including tumor necrosis factor-alpha (TNF-alpha), interleukin-1beta (IL-1beta) and inhibitor of kappa B alpha (IkappaBalpha) in choroid tissue. Given that we also detected increased levels of MCP-1 protein by ELISA, we sought to identify potential downstream targets of MCP-1 and observed altered expression levels of mRNAs encoding tight junction proteins TJP2 and claudin 5. Finally, we detected a substantial up-regulation of the transcript encoding endothelial leukocyte adhesion molecule 1 (E-selectin), a molecule which could participate in leukocyte recruitment to the choroid plexus along with MCP-1. Together, these results suggest that profound changes occur in the choroid plexus during peripheral tissue inflammation, likely initiated by blood-borne inflammatory mediators, which may modify events in CNS.

Localization of S100A8 and S100A9 expressing neutrophils to spinal cord during peripheral tissue inflammation

Investigation of hyperalgesia at the spinal transcriptome level indicated that carrageenan-induced inflammation of rat hind paws leads to a rapid but sustained increase in S100A8 and S100A9 expression, two genes implicated in the pathology of numerous inflammatory diseases including rheumatoid arthritis and gout. In situ hybridization revealed that the elevation occurred in neutrophils that migrate to the spinal cord vasculature during peripheral inflammation, not in spinal neurons or glial cells. Immunohistochemical analysis suggests, but does not prove, that these neutrophils abundantly release S100A8 and S100A9. Consistent with this, we detected an increase in ICAM and VCAM, both indicators of endothelial activation, a known trigger for secretion of S100A8 and S100A9. Migration of S100A8- and S100A9-expressing neutrophils to spinal cord is selective, since MCP-1- and CD68-expressing leukocytes do not increase in spinal cord vasculature during hind paw inflammation. Examination of many neutrophil granule mediators in spinal cord indicated that they are not regulated to the same degree as S100A8 and S100A9. Neutrophil migration also occurs in the vasculature of brain and pituitary gland during peripheral inflammation. Together, these findings suggest an interaction between a subpopulation of leukocytes and the CNS during peripheral tissue inflammation, as implied by an apparent release and possible diffusion of S100A8 and S100A9 through the endothelial blood-brain barrier. Although the present findings do not establish the neurophysiological or behavioral relevance of these observations to nociceptive processing, the data raise the possibility that selective populations of leukocytes may communicate the presence of disease or tissue damage from the periphery to cells in the central nervous system.