Selective stimulation of either tumor necrosis factor receptor differentially induces pain behavior in vivo and ectopic activity in sensory neurons in vitro

Tumor necrosis factor-α increases brain-derived neurotrophic factor expression in trigeminal ganglion neurons in an activity-dependent manner

Many chronic trigeminal pain conditions, such as migraine or temporo-mandibular disorders, are associated with inflammation within peripheral endings of trigeminal ganglion (TG) sensory neurons. A critical role in mechanisms of neuroinflammation is attributed to proinflammatory cytokines, such as interleukin-1β and tumor necrosis factor-α (TNFα) that also contribute to mechanisms of persistent neuropathic pain resulting from nerve injury. However, the mechanisms of cytokine-mediated synaptic plasticity and nociceptor sensitization are not completely understood. In the present study, we examined the effects of TNFα on neuronal expression of brain-derived neurotrophic factor (BDNF), whose role in synaptic plasticity and sensitization of nociceptive pathways is well documented. We show that 4- and 24-h treatment with TNFα increases BDNF mRNA and protein, respectively, in neuron-enriched dissociated cultures of rat TG. TNFα increases the phosphorylated form of the cyclic AMP-responsive element binding protein (CREB), a transcription factor involved in regulation of BDNF expression in neurons, and activates transcription of BDNF exon IV (former exon III) and, to a lesser extent, exon VI (former exon IV), but not exon I. TNFα-mediated increase in BDNF expression is accompanied by increase in calcitonin gene-related peptide (CGRP), which is consistent with previously published studies, and indicates that both peptides are similarly regulated in TG neurons by inflammatory mediators. The effect of TNFα on BDNF expression is dependent on sodium influx through TTX-sensitive channels and on p38-mitogen-activated protein kinase. Moreover, electrical stimulation and forskolin, known to increase intracellular cAMP, potentiate the TNFα-mediated upregulation of BDNF expression. This study provides new evidence for a direct action of proinflammatory cytokines on TG primary sensory neurons, and reveals a mechanism through which TNFα stimulates de novo synthesis of BDNF in these neurons. Thus, TNFα should be considered in mechanisms of BDNF-dependent neuronal plasticity.

Blockade of TNF-α rapidly inhibits pain responses in the central nervous system

There has been a consistent gap in understanding how TNF-α neutralization affects the disease state of arthritis patients so rapidly, considering that joint inflammation in rheumatoid arthritis is a chronic condition with structural changes. We thus hypothesized that neutralization of TNF-α acts through the CNS before directly affecting joint inflammation. Through use of functional MRI (fMRI), we demonstrate that within 24 h after neutralization of TNF-α, nociceptive CNS activity in the thalamus and somatosensoric cortex, but also the activation of the limbic system, is blocked. Brain areas showing blood-oxygen level-dependent signals, a validated method to assess neuronal activity elicited by pain, were significantly reduced as early as 24 h after an infusion of a monoclonal antibody to TNF-α. In contrast, clinical and laboratory markers of inflammation, such as joint swelling and acute phase reactants, were not affected by anti-TNF-α at these early time points. Moreover, arthritic mice overexpressing human TNF-α showed an altered pain behavior and a more intensive, widespread, and prolonged brain activity upon nociceptive stimuli compared with wild-type mice. Similar to humans, these changes, as well as the rewiring of CNS activity resulting in tight clustering in the thalamus, were rapidly reversed after neutralization of TNF-α. These results suggest that neutralization of TNF-α affects nociceptive brain activity in the context of arthritis, long before it achieves anti-inflammatory effects in the joints.

Role for peroxynitrite in sphingosine-1-phosphate-induced hyperalgesia in rats

Sphingosine-1-phosphate (S1P) is an important mediator of inflammation recently shown in in vitro studies to increase the excitability of small-diameter sensory neurons, at least in part, via activation of the S1P(1) receptor subtype. Activation of S1PR(1) has been reported to increase the formation of nicotinamide adenine dinucleotide phosphate (NADPH) oxidase-derived superoxide (O(2)(·-)) and nitric oxide synthase (NOS)-derived nitric oxide (NO). This process favors the formation of peroxynitrite (ONOO(-) [PN]), a potent mediator of hyperalgesia associated with peripheral and central sensitization. The aims of our study were to determine whether S1P causes peripheral sensitization and thermal hyperalgesia via S1PR(1) activation and PN formation. Intraplantar injection of S1P in rats led to a time-dependent development of thermal hyperalgesia that was blocked by the S1PR(1) antagonist W146, but not its inactive enantiomer W140. The hyperalgesic effects of S1P were mimicked by intraplantar injection of the well-characterized S1PR(1) agonist SEW2871. The development of S1P-induced hyperalgesia was blocked by apocynin, a NADPH oxidase inhibitor; N(G)-nitro-l-arginine methyl ester, a nonselective NOS inhibitor; and by the potent PN decomposition catalysts (FeTM-4-PyP(5+) and MnTE-2-PyP(5+)). Our findings provide mechanistic insight into the signaling pathways engaged by S1P in the development of hyperalgesia and highlight the contribution of the S1P(1) receptor-to-PN signaling in this process. Sphingosine-1-phosphate (S1P)-induced hyperalgesia is mediated by S1P1 receptor activation and mitigated by inhibition or decomposition of peroxynitrite, providing a target pathway for novel pain management strategies.

Systemic chemical desensitization of peptidergic sensory neurons with resiniferatoxin inhibits experimental periodontitis

Background and objective:

The immune system is an important player in the pathophysiology of periodontitis. The brain controls immune responses via neural and hormonal pathways, and brain-neuro-endocrine dysregulation may be a central determinant for pathogenesis. Our current knowledge also emphasizes the central role of sensory nerves. In line with this, we wanted to investigate how desensitization of peptidergic sensory neurons influences the progression of ligature-induced periodontitis, and, furthermore, how selected cytokine and stress hormone responses to Gram-negative bacterial lipopolysaccharide (LPS) stimulation are affected.

Material and methods:

Resiniferatoxin (RTX; 50 μg/kg) or vehicle was injected subcutaneously on days 1, 2, and 3 in stress high responding and periodontitis-susceptible Fischer 344 rats. Periodontitis was induced 2 days thereafter. Progression of the disease was assessed after the ligatures had been in place for 20 days. Two h before decapitation all rats received LPS (150 μg/kg i.p.) to induce a robust immune and stress response.

Results:

Desensitization with RTX significantly reduced bone loss as measured by digital X-rays. LPS provoked a significantly higher increase in serum levels of the pro-inflammatory cytokine tumour necrosis factor (TNF)-α, but lower serum levels of the anti-inflammatory cytokine interleukin (IL)-10 and the stress hormone corticosterone.

Conclusions:

In this model RTX-induced chemical desensitization of sensory peptidergic neurons attenuated ligature-induced periodontitis and promoted a shift towards stronger pro-inflammatory cytokine and weaker stress hormone responses to LPS. The results may partly be explained by the attenuated transmission of immuno-inflammatory signals to the brain. In turn, this may weaken the anti-inflammatory brain-derived pathways.

TNF-α contributes to spinal cord synaptic plasticity and inflammatory pain: distinct role of TNF receptor subtypes 1 and 2

Tumor necrosis factor-alpha (TNF-α) is a key proinflammatory cytokine. It is generally believed that TNF-α exerts its effects primarily via TNF receptor subtype-1 (TNFR1). We investigated the distinct roles of TNFR1 and TNFR2 in spinal cord synaptic transmission and inflammatory pain. Compared to wild-type (WT) mice, TNFR1- and TNFR2-knockout (KO) mice exhibited normal heat sensitivity and unaltered excitatory synaptic transmission in the spinal cord, as revealed by spontaneous excitatory postsynaptic currents in lamina II neurons of spinal cord slices. However, heat hyperalgesia after intrathecal TNF-α and the second-phase spontaneous pain in the formalin test were reduced in both TNFR1- and TNFR2-KO mice. In particular, heat hyperalgesia after intraplantar injection of complete Freund's adjuvant (CFA) was decreased in the early phase in TNFR2-KO mice but reduced in both the early and later phase in TNFR1-KO mice. Consistently, CFA elicited a transient increase of TNFR2 mRNA levels in the spinal cord on day 1. Notably, TNF-α evoked a drastic increase in spontaneous excitatory postsynaptic current frequency in lamina II neurons, which was abolished in TNFR1-KO mice and reduced in TNFR2-KO mice. TNF-α also increased N-methyl-d-aspartate (NMDA) currents in lamina II neurons, and this increase was abolished in TNFR1-KO mice but retained in TNFR2-KO mice. Finally, intrathecal injection of the NMDA receptor antagonist MK-801 prevented heat hyperalgesia elicited by intrathecal TNF-α. Our findings support a central role of TNF-α in regulating synaptic plasticity (central sensitization) and inflammatory pain via both TNFR1 and TNFR2. Our data also uncover a unique role of TNFR2 in mediating early-phase inflammatory pain. TNF-α is shown to play a critical role in regulating spinal cord synaptic plasticity and central sensitization, and TNFR1 and TNFR2 play a distinct role in regulating different phases of inflammatory pain.

Tumor necrosis factor-α induces sensitization of meningeal nociceptors mediated via local COX and p38 MAP kinase actions

The proinflammatory cytokine TNF-α has been shown to promote activation and sensitization of primary afferent nociceptors. The downstream signaling processes that play a role in promoting this neuronal response remain however controversial. Increased TNF-α plasma levels during migraine attacks suggest that local interaction between this cytokine and intracranial meningeal nociceptors plays a role in promoting the headache. Here, using in vivo single unit recording in the trigeminal ganglia of anesthetized rats, we show that meningeal TNF-α action promotes a delayed mechanical sensitization of meningeal nociceptors. Using immunohistochemistry, we provide evidence for non-neuronal localization of the TNF receptors TNFR1 to dural endothelial vascular cells and TNFR2 to dural resident macrophages as well as to some CGRP-expressing dural nerve fibers. We also demonstrate that meningeal vascular TNFR1 is co-localized with COX-1 while the perivascular TNFR2 is co-expressed with COX-2. We further report here for the first time that TNF-α evoked sensitization of meningeal nociceptors is dependent upon local action of cyclooxygenase (COX). Finally, we show that local application of TNF-α to the meninges evokes activation of the p38 MAP kinase in dural blood vessels that also express TNFR1 and that pharmacological blockade of p38 activation inhibits TNF-α evoked sensitization of meningeal nociceptors. Our study suggests that meningeal action of TNF-α could play an important role in the genesis of intracranial throbbing headaches such as migraine through a mechanism that involves at least part activation of non-neuronal TNFR1 and TNFR2 and downstream activation of meningeal non-neuronal COX and the p38 MAP kinase.

AMPA-receptor trafficking and injury-induced cell death

AMPA receptors (AMPARs) are critical for synaptic plasticity, and are subject to alterations based on subunit composition and receptor trafficking to and from the plasma membrane. One of the most potent regulators of AMPAR trafficking is the pro-inflammatory cytokine tumor necrosis factor (TNF)α, which is involved in physiological regulation of synaptic strength (Beattie et al., (2002) Science, 295, 2282-2285; Stellwagen and Malenka, (2006) Nature, 440, 1054-1059) and is also present at high concentrations after CNS injury. Here, we review evidence that TNF can rapidly alter the surface expression of AMPARs so that the proportion of Ca(++) -permeable receptors is increased and that this increase, in combination with increased levels of extracellular glutamate after injury, plays an important role in enhancing excitotoxic cell death after CNS injury. Thus, the pathophysiological hijacking of a critical regulator of synaptic plasticity and homeostasis by the secondary injury cascade may represent a new therapeutic target for neuroprotection.

A novel cell-cell signaling by microglial transmembrane TNFα with implications for neuropathic pain

Neuropathic pain is accompanied by neuroimmune activation in dorsal horn of spinal cord. We have observed that in animal models this activation is characterized by an increased expression of transmembrane tumor necrosis factor α (mTNFα) without the release of soluble tumor necrosis factor α (sTNFα). Herein we report that the pain-related neurotransmitter peptide substance P (SP) increases the expression of mTNFα without the release of sTNFα from primary microglial cells. We modeled this interaction using an immortalized microglial cell line; exposure of these cells to SP also resulted in the increased expression of mTNFα but without any increase in the expression of the TNF-cleaving enzyme (TACE) and no release of sTNFα. In order to evaluate the biological function of uncleaved mTNFα, we transfected COS-7 cells with a mutant full-length TNFα construct resistant to cleavage by TACE. Coculture of COS-7 cells expressing the mutant TNFα with microglial cells led to microglial cell activation indicated by increased OX42 immunoreactivity and release of macrophage chemoattractant peptide 1 (CCL2) by direct cell-cell contact. These results suggest a novel pathway through which the release of SP by primary afferents activates microglial expression of mTNFα, establishing a feed-forward loop that may contribute to the establishment of chronic pain.

TNF-alpha and neuropathic pain--a review

Tumor necrosis factor alpha (TNF-α) was discovered more than a century ago, and its known roles have extended from within the immune system to include a neuro-inflammatory domain in the nervous system. Neuropathic pain is a recognized type of pathological pain where nociceptive responses persist beyond the resolution of damage to the nerve or its surrounding tissue. Very often, neuropathic pain is disproportionately enhanced in intensity (hyperalgesia) or altered in modality (hyperpathia or allodynia) in relation to the stimuli. At time of this writing, there is as yet no common consensus about the etiology of neuropathic pain - possible mechanisms can be categorized into peripheral sensitization and central sensitization of the nervous system in response to the nociceptive stimuli. Animal models of neuropathic pain based on various types of nerve injuries (peripheral versus spinal nerve, ligation versus chronic constrictive injury) have persistently implicated a pivotal role for TNF-α at both peripheral and central levels of sensitization. Despite a lack of success in clinical trials of anti-TNF-α therapy in alleviating the sciatic type of neuropathic pain, the intricate link of TNF-α with other neuro-inflammatory signaling systems (e.g., chemokines and p38 MAPK) has indeed inspired a systems approach perspective for future drug development in treating neuropathic pain.

Sensitization of voltage activated calcium channel currents for capsaicin in nociceptive neurons by tumor-necrosis-factor-alpha

It is known that application of tumor-necrosis-factor-alpha (TNF-alpha) sensitizes neuronal calcium channels for heat stimuli in rat models of neuropathic pain. This study examines whether TNF-alpha modulates the capsaicin-induced effects after transient receptor potential vanilloid (TRPV)-1 receptor activation on voltage activated calcium channel currents (I(Ca(V))). TRPV-1 receptors are activated by heat and play an important role in the pathogenesis of thermal hyperalgesia in neuropathic pain syndromes, while voltage activated channels are essential for transmission of neuronal signals. Eliciting I(Ca(V)) in DRG neurons of rats by a depolarization from the resting potential to 0 mV, TNF-alpha (100 ng/ml) reduces I(Ca(V)) by 16.9+/-2.2%, while capsaicin (0.1 microM) decreases currents by 27+/-4.3%. Pre-application of TNF-alpha (100 ng/ml) for 24h results in a sensitization of I(Ca(V)) to capsaicin (0.1 microM) with a reduction of 42.8+/-4.4% mediated by TRPV-1. While L-type (36.6+/-5.2%) and P/Q-type currents (35.6+/-4.1%) are also sensitized by TRPV-1 activation, N-type channel currents are most sensitive (74.5+/-7.3%). The capsaicin-induced shift towards the hyperpolarizing voltage range does not occur when TNF-alpha is applied. Summarizing, TNF-alpha sensitizes nociceptive neurons for capsaicin.

Rheumatic manifestations of sleep disorders

PURPOSE OF REVIEW

We have long assumed that rheumatic pain causes sleep problems, fatigue, and functional disability. This paper reviews the accumulating evidence from human and animal experimental research studies that show a bidirectional relationship of disordered sleep to pain and fatigue.

RECENT FINDINGS

The studies demonstrate that both disturbances of sleep and sleep restriction result in increased sensitivity to noxious stimuli and musculoskeletal pain symptoms. The notion of central nervous system hypersensitivity affecting widespread pain in patients with fibromyalgia syndrome is the result of a reduction in neurophysiologic inhibition of perception of noxious stimuli that is provoked by disordered sleep. Clinical and epidemiological studies show that sleep disturbances directly influence musculoskeletal pain, fatigue, mood, and overall well-being. Indeed, the interrelationships of the sleeping/waking brain with cytokine and cellular immune functions have important implications for the understanding of rheumatic disease pathology and management with disease-modifying antirheumatic drugs.

SUMMARY

The determination of how disordered sleep affects musculoskeletal pain, fatigue, mood, and behavior is important in the assessment and management of patients with rheumatic illness. The high prevalence of obstructive sleep apnea and restless legs syndromes requires more research to determine whether treatments of these sleep disorders will benefit the symptoms of rheumatic diseases.

Mechanical but not painful electrical stimuli trigger TNF alpha release in human skin

Pro-inflammatory cytokines-in particular tumor necrosis factor (TNF)-alpha-play an important role in pain and hyperalgesia. The stimuli inducing TNF-alpha release in humans and the time course of this release are largely unknown. We performed dermal microdialysis in healthy subjects (n=36) during three experimental conditions: The first condition (control) was microdialysis without stimulation, the second condition was 30 min of electrical current stimulation (1 Hz, 20 mA, moderately painful), the third condition was 30 min of repetitive mechanical stimulation via an impact stimulator (bullet 0.5 g; velocity 11 m/s, minimally painful). TNF-alpha was quantified in the samples collected at the end of the baseline perfusion (about 1 h of saline perfusion), at the end of stimulation period (exactly 30 min after stimulation commenced) and at the end of the experiment (exactly 90 min after stimulation commenced) using a commercial enzyme-linked immunosorbent assay. The C-fiber-related flare was quantified with a laser-Doppler imager. ANOVA revealed that TNF-alpha levels increased during the eluate sampling period. At 90 min TNF-alpha in the eluate of the mechanical stimulation condition was significantly increased as compared to electrical current or control condition. Flare intensity was highest in the electrical current stimulation condition and only marginally different from control in mechanical stimulation. Our results show that minimal mechanical trauma is sufficient to induce significant TNF-alpha release in the skin. These results may be relevant to the treatment of posttraumatic pain disorders.

Behavioral models of pain states evoked by physical injury to the peripheral nerve

Physical injury or compression of the root, dorsal root ganglion, or peripheral sensory axon leads to well-defined changes in biology and function. Behaviorally, humans report ongoing painful dysesthesias and aberrations in function, such that an otherwise innocuous stimulus will yield a pain report. These behavioral reports are believed to reflect the underlying changes in nerve function after injury, wherein increased spontaneous activity arises from the neuroma and dorsal root ganglion and spinal changes increase the response of spinal projection neurons. These pain states are distinct from those associated with tissue injury and pose particular problems in management. To provide for developing an understanding of the underlying mechanisms of these pain states and to promote development of therapeutic agents, preclinical models involving section, compression, and constriction of the peripheral nerve or compression of the dorsal root ganglion have been developed. These models give rise to behaviors, which parallel those observed in the human after nerve injury. The present review considers these models and their application.

TNFalpha mechanically sensitizes masseter muscle afferent fibers of male rats

Behavioral evidence in rats indicates that injection of tumor necrosis factor alpha (TNFalpha) into skeletal muscle results in a prolonged mechanical sensitization without gross inflammation. To investigate whether a peripheral mechanism could underlie this effect, in the present study, TNFalpha (1 or 0.1 microg) was injected into the rat masseter muscle to assess its effect on the excitability and mechanical threshold (MT) of muscle nociceptors as well as on inflammation. Expression of TNFR1 (P55 receptors) and TNFR2 (P75 receptors) by the masseter muscle and trigeminal ganglion neurons that innervate that muscle was determined by Western blot and immunohistochemistry, respectively. The Evans blue dye technique was used at the end of the TNFalpha experiments to assess for plasma protein extravasation. In subsequent experiments to confirm the involvement of receptor activation in TNFalpha-induced effects, P55 or P75 receptor antibody was co-injected with TNFalpha. Intramuscular injection of 1 microg TNFalpha did not excite nociceptors but did significantly decrease MT compared with vehicle control. There was no evidence of gross inflammation 3 h after injection of TNFalpha. Co-injection of TNFalpha with P55 or P75 receptor antibodies attenuated TNFalpha-induced mechanical sensitization. P55 and P75 receptors were expressed by 29 and 62% of masseter nociceptors, respectively. These findings indicate that TNFalpha induces mechanical sensitization of masseter nociceptors that is mediated through activation of peripheral P55 and P75 receptors. These results support the hypothesis that a peripheral receptor mechanism could contribute to TNFalpha-induced noninflammatory mechanical sensitization of skeletal muscle previously reported in behaving rats.

Mode of action of cytokines on nociceptive neurons

Cytokines are pluripotent soluble proteins secreted by immune and glial cells and are key elements in the induction and maintenance of pain. They are categorized as pro-inflammatory cytokines, which are mostly algesic, and anti-inflammatory cytokines, which have analgesic properties. Progress has been made in understanding the mechanisms underlying the action of cytokines in pain. To date, several direct and indirect pathways are known that link cytokines with nociception or hyperalgesia. Cytokines may act via specific cytokine receptors inducing downstream signal transduction cascades, which then modulate the function of other receptors like the ionotropic glutamate receptor, the transient vanilloid receptors, or sodium channels. This receptor activation, either through amplification of the inflammatory reaction, or through direct modulation of ion channel currents, then results in pain sensation. Following up on results from animal experiments, cytokine profiles have recently been investigated in human pain states. An imbalance of pro- and anti-inflammatory cytokine expression may be of importance for individual pain susceptibility. Individual cytokine profiles may be of diagnostic importance in chronic pain states, and, in the future, might guide the choice of treatment.