Glial-cytokine-neuronal interactions underlying the mechanisms of persistent pain

TNF activates astrocytes and catecholaminergic neurons in the solitary nucleus: implications for autonomic control

Tumor necrosis factor [TNF] produces a profound anorexia associated with gastrointestinal stasis. Our work suggests that the principal site of action of TNF to cause this change in gastric function is via vagal afferents within the nucleus of the solitary tract [NST]. Excitation of these afferents presumably causes gastric stasis by activating downstream NST neurons that, in turn, suppress gastric motility via action on neurons in the dorsal motor nucleus of the vagus that project to the stomach. Results from our parallel studies on gastric vago-vagal reflexes suggest that noradrenergic neurons in the NST are particularly important to the generation of reflex gastroinhibition. Convergence of these observations led us to hypothesize that TNF action in the NST may preferentially affect putative noradrenergic neurons. The current study confirms our observations of a dose-dependent TNF activation of cells [as indicated by cFOS production] in the NST. The phenotypic identity of these TNF-activated neurons in the NST was approximately 29% tyrosine hydroxylase [TH]-positive [i.e., presumably noradrenergic neurons]. In contrast, less than 10% of the nitrergic neurons were activated after TNF exposure. Surprisingly, another 54% of the cFOS-activated cells in the NST were phenotypically identified to be astrocytes. Taken together with previous observations, the present results suggest that intense or prolonged vagal afferent activity [induced by visceral pathway activity, action of gut hormones or cytokines such as TNF] can alter local astrocyte immediate early gene expression that, in turn, can provoke long-term, perhaps permanent changes in the sensitivity of vagal-reflex circuitry.

The liberation of fractalkine in the dorsal horn requires microglial cathepsin S

Understanding of the sequence and nature of the events that govern neuron-microglia communication is critical for the discovery of new mechanisms and targets for chronic pain treatment. The neuronal chemokine fractalkine (FKN) and its microglial receptor CX3CR1 may mediate such a function in the dorsal horn of the spinal cord after cleavage of the extracellular domain of this transmembrane chemokine by a protease. Here we report that in neuropathic rat dorsal horn, with dorsal root-attached preparations, soluble FKN (sFKN) contents are increased in the superfusates collected after noxious-like electrical stimulation of ipsilateral primary afferent fibers. The increase of sFKN is prevented by morpholinurea-leucine-homophenylalanine-vinyl sulfone-phenyl (LHVS), an irreversible inhibitor of cathepsin S (CatS) whose proteolytic activity is also increased in the superfusates. The source of CatS activity is microglial cells activated by the peripheral nerve injury and secreting the enzyme, as a result of primary afferent fiber stimulation. Indeed, the acute activation of dorsal horn microglia by lipopolysaccharide results in increased CatS activity in the superfusates, followed by increased sFKN contents. Consistent with these observations ex vivo, the levels of both sFKN and CatS activity in CSF samples increased significantly after peripheral nerve injury, associated with spinal microglial activation. Finally, because we found that both FKN immunoreactivity and mRNA are confined to dorsal horn neurons, we suggest that under neuropathic conditions, noxious stimulation of primary afferent fibers induces release of CatS from microglia, which liberates FKN from dorsal horn neurons, thereby contributing to the amplification and maintenance of chronic pain.

Experimental occlusal interference induces long-term masticatory muscle hyperalgesia in rats

Temporomandibular joint or related masticatory muscle pain represents the most common chronic orofacial pain condition. Patients frequently report this kind of pain after dental alterations in occlusion. However, lack of understanding of the mechanisms of occlusion-related temporomandibular joint and muscle pain prevents treating this problem successfully. To explore the relationship between improper occlusion (occlusal interference) and masticatory muscle pain, we created an occlusal interference animal model by directly bonding a crown to a maxillary molar to raise the masticating surface of the tooth in rats. We raised the occlusal surface to three different heights (0.2, 0.4, and 0.6mm), and for one month we quantitatively measured mechanical nociceptive thresholds of the temporal and masseter muscles on both sides. Results showed a stimulus-response relationship between the height of occlusal interference and muscle hyperalgesia. Removal of the crown 6 days after occlusal interference showed that the removal at this time could not terminate the 1 month duration of mechanical hyperalgesia in the masticatory muscles. Lastly, we systemically administered NMDA antagonist MK801 (0.2, 0.1, and 0.05 mg/kg) to the treated rats and found that MK801 dose dependently attenuated the occlusal interference-induced hyperalgesia. These findings suggest that occlusal interference is directly related to masticatory muscle pain, and that central sensitization mechanisms are involved in the maintenance of the occlusal interference-induced mechanical hyperalgesia.

The role of enhanced cutaneous IL-1beta signaling in a rat tibia fracture model of complex regional pain syndrome

Tibia fracture in rats initiates a syndrome resembling the complex regional pain syndrome type I. Accumulating evidence indicates that IL-1beta is involved in the modulation of nociceptive information and it acts as an intermediate inflammatory mediator via up-regulation of NGF. We hypothesized that IL-1beta signaling might mediate the development of the CRPS-like changes after tibial fracture, either directly or by stimulating NGF expression. Rats underwent distal tibia fracture and casting for 4 weeks and were chronically treated with an IL-1 receptor antagonist (IL-1ra). Nociceptive testing and assessment of edema and hindpaw warmth were performed at baseline and after cast removal. Bone microarchitecture was evaluated by micro-computed tomography. Confocal immunofluorescence and in situ hybridization techniques were used to evaluate changes in the cutaneous expression of IL-1beta at 4 weeks post-fracture. The nociceptive and vascular effects of intraplantar IL-1beta injections were evaluated in intact rats at different time points after injection. We found that: (1) IL-1ra reduced fracture-induced nociceptive sensitization, but did not decrease hindpaw edema or warmth, (2) fracture chronically up-regulated IL-1beta mRNA and protein expression in hindpaw skin keratinocytes, (3) IL-1beta intraplantar injection induced mechanical allodynia in a dose-dependent manner and stimulated keratinocyte NGF expression in the hindpaw skin, and (4) intraplantar injection of NGF-induced nociceptive sensitization. Collectively, these results indicate that cutaneous IL-1beta signaling can contribute to chronic regional nociceptive sensitization after fracture, possibly by stimulating NGF over-expression in keratinocytes. Our data also highlight the importance of the keratinocyte as the primary source of post-traumatic IL-1beta over-expression.

Activation of the neuronal extracellular signal-regulated kinase 2 in the spinal cord dorsal horn is required for complete Freund's adjuvant-induced pain hypersensitivity

Extracellular signal-regulated kinase 1 (ERK1) and ERK2 signaling in the spinal cord dorsal horn (SCDH) has been implicated in injury-induced pain hypersensitivity. Available ERK pathway inhibitors cannot distinguish between ERK1 and ERK2, nor can they differentially target the expression of neuronal or glial ERK1/2. We selectively inhibited the expression of ERK2 in neurons of the adult mouse SCDH by use of an ERK2 small interfering RNA (siRNA) delivered by a neurotropic adenoassociated viral vector. In situ hybridization revealed a siRNA vector-induced decrease in ERK2 mRNA in the ipsilateral SCDH. Immunohistochemistry showed a decreased neuronal phospho-ERK1/2 (pERK1/2), and Western blot analysis revealed that both ERK2 expression and phosphorylation were reduced by the siRNA vector. In contrast, basal ERK1 expression was not affected, although pERK1 was slightly increased. The siRNA vector-induced knockdown of ERK2 expression in the SCDH did not alter the baseline mechanical or thermal paw withdrawal thresholds. Hindpaw intraplantar injection of complete Freund's adjuvant (CFA) produced peripheral inflammation, mechanical allodynia, and thermal hyperalgesia in vector control animals that persisted for at least 96 h. It also caused an increase in SCDH ERK1 and ERK2 levels at 96 h and pERK1 and pERK2 levels at 1 and 96 h. The ERK2 siRNA vector prevented changes in ERK1, ERK2, and pERK2. In addition, the siRNA vector protected the animals from developing mechanical allodynia and thermal hyperalgesia throughout the 96 h after CFA. These findings indicate that ERK2 in the SCDH neurons is critical for the development of inflammatory pain hypersensitivity.

Characterization of NF-kB-mediated inhibition of catechol-O-methyltransferase

BACKGROUND

Catechol-O-methyltransferase (COMT), an enzyme that metabolizes catecholamines, has recently been implicated in the modulation of pain. Specifically, low COMT activity is associated with heightened pain perception and development of musculoskeletal pain in humans as well as increased experimental pain sensitivity in rodents.

RESULTS

We report that the proinflammatory cytokine tumor necrosis factor alpha (TNFalpha) downregulates COMT mRNA and protein in astrocytes. Examination of the distal COMT promoter (P2-COMT) reveals a putative binding site for nuclear factor kappaB (NF-kappaB), the pivotal regulator of inflammation and the target of TNFalpha. Cell culture assays and functional deletion analyses of the cloned P2-COMT promoter demonstrate that TNFalpha inhibits P2-COMT activity in astrocytes by inducing NF-kappaB complex recruitment to the specific kappaB binding site.

CONCLUSION

Collectively, our findings provide the first evidence for NF-kappaB-mediated inhibition of COMT expression in the central nervous system, suggesting that COMT contributes to the pathogenesis of inflammatory pain states.

Remote neuroimmune signaling: a long-range mechanism of nociceptive network plasticity

Chronic pain secondary to neuronal injury is actively and continuously modulated at multiple locations along the sensory neuraxis. Here, we describe how nociceptive neurons of the spinal cord and thalamus process and communicate nociceptive information in terms of precisely calibrated firing patterns. We then discuss how several cell types with immunogenic properties (e.g. blood cells and glia) cause system-wide interference in nociceptive processing through novel signaling schema, thus contributing to nociceptive network plasticity and chronic pain.

Microinjection of IL-1β into the trigeminal transition zone produces bilateral NMDA receptor-dependent orofacial hyperalgesia involving descending circuitry

Our recent studies indicate that the prototypic proinflammatory cytokine IL-1β is upregulated in astroglial cells in the trigeminal interplolaris/caudalis (Vi/Vc) transition zone, a region of the spinal trigeminal complex involved in trigeminal pain processing, after masseter muscle inflammation. Here we investigated the effect of microinjection of IL-1β into the Vi/Vc transition zone on orofacial nociception. The mechanical sensitivity of the orofacial site was assessed with von Frey microfilaments. The EF(50) values, defined as the von Frey filament force (g) that produces a 50% response frequency, were derived and used as a measure of mechanical sensitivity. A significant reduction in EF(50) indicates the occurrence of mechanical hyperalgesia/allodynia. Unilateral intra-Vi/Vc IL-1β (0.016-160 fmol) produced hyperalgesia/allodynia dose-dependently, which appeared at bilateral facial sites. The hyperalgesia was detectable as early as 30 min and lasted for 2-6 h (n=6, p<0.01). Intra-Vi/Vc pretreatment with an IL-1receptor antagonist (1 nmol) attenuated the IL-1β-induced hyperalgesia (p<0.01). Pre-injection of AP-5 (10 pmol) and MK-801 (20 pmol), two NMDA receptor antagonists, significantly attenuated IL-1β-induced hyperalgesia (p<0.05). Pretreatment with glial inhibitors fluorocitrate (120 pmol), minocycline (200 pmol) and propentofylline (10 pmol) did not attenuate IL-1β-induced hyperalgesia. Excitotoxic lesions of the rostral ventromedial medulla with ibotenic acid (2 μg) abolished IL-1β-induced contralateral hyperalgesia, suggesting a contribution of descending facilitatory drive. These results suggest that the IL-1β-produced effect on nociception was downstream to glial activation and involves interaction with NMDA receptors.

Mechanism of allodynia evoked by intrathecal morphine-3-glucuronide in mice

Morphine-3-glucuronide (M3G), a main metabolite of morphine, has been proposed as a responsible factor when patients present with the neuroexcitatory side effects (allodynia, hyperalgesia, and myoclonus) observed following systemic administration of large doses of morphine. Indeed, both high-dose morphine (60 nmol/5 microl) and M3G (3 nmol/5 microl) elicit allodynia when administered intrathecally (i.t.) into mice. The allodynic behaviors are not opioid receptor mediated. This chapter reviews the potential mechanism of spinally mediated allodynia evoked by i.t. injection of M3G in mice. We discuss a possible presynaptic release of nociceptive neurotransmitters/neuromodulators such as substance P, glutamate, and dynorphin in the primary afferent fibers following i.t. M3G. It is possible to speculate that i.t. M3G injection could activate indirectly both NK(1) receptor and glutamate receptors that lead to the release of nitric oxide (NO) in the dorsal spinal cord. The NO plays an important role in M3G-induced allodynia. The phosphorylation of extracellular signal-regulated protein kinase (ERK) in the dorsal spinal cord evoked via NO/cGMP/PKG pathway contributes to i.t. M3G-induced allodynia. Furthermore, the increased release of NO observed after i.t. injection of M3G activates astrocytes and induces the release of the proinflammatory cytokine, interleukin-1beta. Taken together, these findings suggest that M3G may induce allodynia via activation of NO-ERK pathway, while maintenance of the allodynic response may be triggered by NO-activated astrocytes in the dorsal spinal cord. The demonstration of the cellular mechanisms of neuronal-glial interaction underlying M3G-induced allodynia provides a fruitful strategy for improved pain management with high doses of morphine.

Regulation of the trigeminal NR1 subunit expression induced by inflammation of the temporomandibular joint region in rats

Expression of the N-methyl-d-aspartate (NMDA) receptor in trigeminal nuclei has been shown to play a role in the mechanisms of trigeminal pain. Here, we examined the hypothesis that the upregulation of the NR1 subunit of the NMDA receptor (NR1) in the trigeminal subnucleus caudalis (Sp5c) following inflammation of the temporomandibular joint (TMJ) region would be regulated by interleukin-6 (IL-6) and the nuclear factor-kappa B (NF-kappaB). Inflammation of a unilateral TMJ region was produced in rats by injecting 50mul of complete Freund's adjuvant (CFA) into a TMJ and adjacent tissues, which resulted in persistent pain behavior as assessed using algometer before (baseline) and on days 1, 3, and 7 after the CFA injection. The CFA injection also induced a significant upregulation of NR1 and NF-kappaB on days 3 and 7, and of IL-6 on days 1, 3, and 7, within the ipsilateral Sp5c, as compared with the sham TMJ injection group. Once daily intracisternal injection of an IL-6 antiserum or NF-kappaB inhibitor (PDTC) for 6 days, beginning on day 1 immediately after the CFA injection, prevented both the upregulation of NR1 in the ipsilateral Sp5C and pain behavior. Moreover, once daily intracisternal IL-6 administration for 6 days in naïve rats induced the NR1 upregulation and pain behavior similar to that after TMJ inflammation. These results indicate that the upregulation of IL-6 and NF-kappaB after inflammation of the unilateral TMJ region is a critical regulatory mechanism for the expression of NR1 in the ipsilateral Sp5c, which contributed to the development of TMJ pain behavior in rats.

Role of interleukin-1beta during pain and inflammation

The cytokine cascade in pain and inflammatory processes is a tremendously complex system, involving glial, immune, and neuronal cell interactions. IL-1beta is a pro-inflammatory cytokine that has been implicated in pain, inflammation and autoimmune conditions. This review will focus on studies that shed light on the critical role of IL-1beta in various pain states, including the role of the intracellular complex, the inflammasome, which regulates IL-1beta production. Evidence will be presented demonstrating the importance of IL-1beta in both the induction of pain and in the maintenance of pain in chronic states, such as after nerve injury. Additionally, the involvement of IL-1beta as a key mediator in the interaction between glia and neurons in pain states will be discussed. Taken together, the evidence presented in the current review showing the importance of IL-1beta in animal and human pain states, suggests that blockade of IL-1beta be considered as a therapeutic opportunity.

Spinal interleukin-1beta in a mouse model of arthritis and joint pain

OBJECTIVE

Pain from arthritis has been associated with peripheral sensitization of primary sensory afferents and the development of inflammation at the dorsal horns. This study was undertaken to determine whether the role of spinal interleukin-1beta (IL-1beta) in central processing of pain is important in the development of arthritis.

METHODS

Col1-IL-1betaXAT mice and GFAP-IL-1betaXAT mice were injected with the feline immunodeficiency virus (FIV) (Cre) vector in the right and left temporomandibular joints (TMJs), or in the cisterna magna, respectively, to induce IL-1beta expression in the dorsal horns of the spinal horn. To inhibit intrathecal IL-1 receptor type I (IL-1RI) signaling, FIV(IL-1Ra) vector was injected into the cisterna magna of Col1-IL-1betaXAT mice. The effects of IL-1RI receptor inhibition in GFAP-IL-1betaXAT mice were studied in the GFAP-IL-1betaXAT-IL-1RI(-/-) compound mouse model. Neuroinflammatory, sensory, and behavioral changes were evaluated in conjunction with arthritic changes in the TMJ, assessed by histopathologic and immunohistochemical analyses.

RESULTS

Induction of an osteoarthritis-like condition in the TMJ in the Col1-IL-1betaXAT mouse model resulted in up-regulation of murine IL-1beta at the dorsal horns. Moreover, intrathecal inhibition of IL-1RI in Col1-IL-1betaXAT mice with arthritis led to amelioration of joint pathology and attenuation of the attendant joint pain. Overexpression of spinal IL-1beta in the recently developed GFAP-IL-1betaXAT somatic mosaic model of neuroinflammation led to development of arthritis-like pathology accompanied by increased pain-like behavior.

CONCLUSION

Our results indicate that joint pathology and pain are dependent on spinal IL-1beta, and suggest the presence of a bidirectional central nervous system-peripheral joints crosstalk that may contribute to the development, expansion, and exacerbation of arthritis.

Supraspinal glial-neuronal interactions contribute to descending pain facilitation

Spinal glial reaction and proinflammatory cytokine induction play an important role in the development of chronic pain states after tissue and nerve injury. The present study investigated the cellular and molecular mechanisms underlying descending facilitation of neuropathic pain with an emphasis on supraspinal glial-neuronal relationships. An early and transient reaction of microglia and prolonged reaction of astrocytes were found after chronic constriction injury (CCI) of the rat infraorbital nerve in the rostral ventromedial medulla (RVM), a major component of brainstem descending pain modulatory circuitry. There were prolonged elevations of cytokines tumor necrosis factor-alpha (TNF-alpha) and interleukin-1beta (IL-1beta) after CCI, and they were expressed in RVM astrocytes at 14 d after injury. Intra-RVM injection of microglial and astrocytic inhibitors attenuated mechanical hyperalgesia and allodynia at 3 and 14 d after CCI, respectively. Moreover, TNFR1 and IL-1R, receptors for TNF-alpha and IL-1beta, respectively, were expressed primarily in RVM neurons exhibiting immunoreactivity to the NMDA receptor (NMDAR) subunit NR1. CCI increased TNFR1 and IL-1R levels and NR1 phosphorylation in the RVM. Neutralization of endogenous TNF-alpha and IL-1beta in the RVM significantly reduced CCI-induced behavioral hypersensitivity and attenuated NR1 phosphorylation. Finally, intra-RVM administration of recombinant TNF-alpha or IL-1beta upregulated NR1 phosphorylation and caused a reversible and NMDAR-dependent allodynia in normal rats, further suggesting that TNF-alpha and IL-1beta couple glial hyperactivation with NMDAR function. These studies have addressed a novel contribution of supraspinal astrocytes and associated cytokines as well as central glial-neuronal interactions to the enhancement of descending facilitation of neuropathic pain.

Neuron-glia crosstalk gets serious: role in pain hypersensitivity

PURPOSE OF REVIEW

Recent studies show that peripheral injury activates both neuronal and nonneuronal or glial components of the peripheral and central cellular circuitry. The subsequent neuron-glia interactions contribute to pain hypersensitivity. This review will briefly discuss novel findings that have shed light on the cellular mechanisms of neuron-glia interactions in persistent pain.

RECENT FINDINGS

Two fundamental questions related to neuron-glia interactions in pain mechanisms have been addressed: what are the signals that lead to central glial activation after injury and how do glial cells affect central nervous system neuronal activity and promote hyperalgesia?

SUMMARY

Evidence indicates that central glial activation depends on nerve inputs from the site of injury and release of chemical mediators. Hematogenous immune cells may migrate to/infiltrate the brain and circulating inflammatory mediators may penetrate the blood-brain barrier to participate in central glial responses to injury. Inflammatory cytokines such as interleukin-1beta released from glia may facilitate pain transmission through its coupling to neuronal glutamate receptors. This bidirectional neuron-glia signaling plays a key role in glial activation, cytokine production and the initiation and maintenance of hyperalgesia. Recognition of the contribution of the mutual neuron-glia interactions to central sensitization and hyperalgesia prompts new treatment for chronic pain.

The effects of surgery and anesthesia on memory and cognition

This chapter describes current findings from the research into postoperative cognitive dysfunction (POCD) following cardiac and non-cardiac surgery in older adults. The evidence suggests that a significant proportion of patients show POCD in the early weeks following surgery and anesthesia. Specific domains of cognition are affected, especially memory. Much less evidence supports the presence of POCD several months or years after surgery, suggesting that POCD may be transient. However, several methodological issues make it difficult to compare findings across studies. Increasing age is among the most consistently reported patient-related risk factor. Other factors more directly related to the surgery and anesthesia are likely to contribute to the pathogenesis of POCD, including inflammatory processes triggered by the surgical procedure. Animal studies have provided valuable findings otherwise not possible in human studies; these include a correlation between the inflammatory response in the hippocampus and the development of POCD in rodents.

Roles of TRESK, a novel two-pore domain K+ channel, in pain pathway and general anesthesia

TRESK is the most recently reported two-pore domain K+ channel, and different from other two-pore domain channels in gene, molecular structure, electrophysiological and pharmacological properties. Although the current knowledge of this potassium channel is inadequate, researches have demonstrated that TRESK is remarkablely linked to acute and chronic pain by activation of calcineurin. The fact that TRESK is sensitive to volatile anesthetics and localization in central nerve system implies that TRESK may play a very important role in the mechanism mediating general anesthesia. The further research of TRESK may contribute to explore the underlying mechanism of some pathological conditions and yield novel treatments for some diseases.

Cytokine mechanisms of central sensitization: distinct and overlapping role of interleukin-1beta, interleukin-6, and tumor necrosis factor-alpha in regulating synaptic and neuronal activity in the superficial spinal cord

Central sensitization, increased sensitivity in spinal cord dorsal horn neurons after injuries, plays an essential role in the induction and maintenance of chronic pain. However, synaptic mechanisms underlying central sensitization are incompletely known. Growing evidence suggests that proinflammatory cytokines (PICs), such as interleukin-1beta (IL-1beta), interleukin-6 (IL-6), and tumor necrosis factor-alpha (TNFalpha), are induced in the spinal cord under various injury conditions and contribute to pain hypersensitivity. Using patch-clamp recordings in lamina II neurons of isolated spinal cord slices, we compared the effects of IL-1beta, IL-6, and TNFalpha on excitatory and inhibitory synaptic transmission. Whereas TNFalpha enhanced the frequency of spontaneous EPSCs (sEPSCs), IL-6 reduced the frequency of spontaneous IPSCs (sIPSCs). Notably, IL-1beta both enhanced the frequency and amplitude of sEPSCs and reduced the frequency and amplitude of sIPSCs. Consistently, TNFalpha and IL-1beta enhanced AMPA- or NMDA-induced currents, and IL-1beta and IL-6 suppressed GABA- and glycine-induced currents. Furthermore, all the PICs increased cAMP response element-binding protein (CREB) phosphorylation in superficial dorsal horn neurons and produced heat hyperalgesia after spinal injection. Surprisingly, soluble IL-6 receptor (sIL-6R) produced initial decrease of sEPSCs, followed by increase of sEPSCs and CREB phosphorylation. Spinal injection of sIL-6R also induced heat hyperalgesia that was potentiated by coadministration with IL-6. Together, our data have demonstrated that PICs induce central sensitization and hyperalgesia via distinct and overlapping synaptic mechanisms in superficial dorsal horn neurons either by increasing excitatory synaptic transmission or by decreasing inhibitory synaptic transmission. PICs may further induce long-term synaptic plasticity through CREB-mediated gene transcription. Blockade of PIC signaling could be an effective way to suppress central sensitization and alleviate chronic pain.

Inhibition of NMDA-induced outward currents by interleukin-1beta in hippocampal neurons

There is increasing evidence that a functional interaction exists between interleukin-1beta (IL-1beta) and N-methyl-D-aspartate (NMDA) receptors. The present study attempted to elucidate the effect of IL-1beta on the NMDA-induced outward currents in mechanically dissociated hippocampal neurons using a perforated patch recording technique. IL-1beta (30-100 ng/ml) inhibited the mean amplitude of the NMDA-induced outward currents that were mediated by charybdotoxin (ChTX)-sensitive Ca(2+)-activated K(+) (K(Ca)) channels. IL-1beta (100 ng/ml) also significantly increased the mean ratio of the NMDA-induced inward current amplitudes measured at the end to the beginning of a 20-s application of NMDA. In hippocampal neurons from acute slice preparations, IL-1beta significantly inhibited ChTX-sensitive K(Ca) currents induced by a depolarizing voltage-step. IL-1 receptor antagonist antagonized effects of IL-1beta. These results strongly suggest that IL-1beta increases the neuronal excitability by inhibition of ChTX-sensitive K(Ca) channels activated by Ca(2+) influx through both NMDA receptors and voltage-gated Ca(2+) channels.

Glial involvement in trigeminal central sensitization

Recent studies have indicated that trigeminal neurons exhibit central sensitization, an increase in the excitability of neurons within the central nervous system to the extent that a normally innocuous stimulus begins to produce pain after inflammation or injury, and that glial activities play a vital role in this central sensitization. The involvement of glial cells in trigeminal central sensitization contains multiple mechanisms, including interaction with glutamatergic and purinergic receptors. A better understanding of the trigeminal central sensitization mediated by glial cells will help to find potential therapeutic targets and lead to developing new analgesics for orofacial-specific pain with higher efficiency and fewer side-effects.

Contralateral neuropathic pain and neuropathology in dorsal root ganglion and spinal cord following hemilateral nerve injury in rats

STUDY DESIGN

The contralateral pain-related behavioral and immunohistochemical changes after hemilateral spinal nerve injury in rats were investigated.

OBJECTIVES

We evaluated the longitudinal changes in contralateral mechanical allodynia, expression of tumor necrosis factor (TNF)-alpha and glial fibrillary acidic protein (GFAP)-positive satellite cells in the contralateral dorsal root ganglion (DRG), and expression of astrocytes and microglia in the contralateral spinal dorsal horn after hemilateral spinal nerve injury in rats.

SUMMARY OF BACKGROUND DATA

In previous studies, hemilateral nerve injury has sometimes induced contralateral neuropathic pain. TNF-alpha expression and glial cell reactions in the DRG and spinal cord play an important role in the neuropathic pain state, and TNF-alpha is released from glial cells in the nervous system.

METHODS

Adult male Sprague-Dawley rats were used. The spinal L5 nerve distal to the DRG was crushed once for 3 seconds. At days 2, 7, 14, and 21 after surgery, mechanical allodynia was determined in bilateral hind paws by the von Frey test. Expression of TNF-alpha and GFAP in bilateral L5 DRGs and expression of GFAP and ionized calcium-binding adaptor molecule-1 (Iba-1) in bilateral L5 spinal dorsal horns were studied using immunohistochemistry and immunoblotting.

RESULTS

Mechanical withdrawal threshold of the ipsilateral hind paw was significantly decreased for 21 days. Conversely, mechanical withdrawal threshold of the contralateral hind paw was significantly decreased from 5 to 10 g for 7 days, and was <5 g at days 14 and 21. TNF-alpha expression and GFAP-positive satellite cells in the contralateral DRG significantly increased from day 7 to day 21. In the contralateral spinal dorsal horn, GFAP-positive astrocytes significantly increased for 21 days, but Iba-1 was not significant.

CONCLUSION

These results suggest that contralateral mechanical allodynia induced by hemilateral spinal nerve injury is associated with upregulation of satellite cells and TNF-alpha in the contralateral DRG. In addition, our results suggest that spinal astrocytes play an important role in these contralateral changes.

A comparison of spinal Iba1 and GFAP expression in rodent models of acute and chronic pain

The treatment of acute and chronic pain is still deficient. The modulation of glial cells may provide novel targets to treat pain. We hypothesize that astrocytes and microglia participate in the initiation and maintenance of both, acute surgical and chronic neuropathic pain. Rats underwent paw incision, L5 nerve exposure or L5 nerve transection surgery. Behavioral mechanical allodynia was assessed using von Frey filaments. Immunohistochemistry was performed using anti-ionized calcium binding adaptor protein, Iba-1 (microglia), and anti-Glial Fibrillary Acidic Protein, GFAP (astrocytes) on day 1, 4 and 7 after surgery. Following paw incision and at spinal L5 segment GFAP expression was increased in laminae I-II and Iba1 in deep laminae on day 1, in the entire dorsal horn on day 4 and dissipated on day 7 after paw incision in parallel with the allodynia. L5 nerve transection induced mechanical allodynia from day 1 to 7 which correlated with Iba-1 increases on day 1, 4 (entire dorsal horn) and day 7 after nerve injury (deep laminae of the dorsal horn) at spinal L5 segment. Conversely, GFAP increased at later time points from day 4 (deep laminae) and on day 7 (entire dorsal horn). Our data demonstrates that astrocytes (GFAP expression) play a role in the initiation of acute pain and the maintenance of chronic pain while Iba-1 increases closely correlated with the early phase of neuropathic pain. Iba1 and GFAP increased rostrally, at L3 segment, after paw incision (day 4) and only Iba1 increased following L5 nerve transection (day 7).

Interleukin 1beta facilitates bone cancer pain in rats by enhancing NMDA receptor NR-1 subunit phosphorylation

It has been shown that interleukin-1beta (IL-1beta) facilitates nociception during neuropathic and inflammatory pain, but its involvement in bone cancer pain and its mechanisms have not previously been established. This study is an investigation of IL-1beta spinal expression and the N-methyl-D-aspartate (NMDA) receptor (NMDAR) NR1 subunit phosphorylation during cancer pain, co-localization of IL-1 receptor type I (IL-1RI) and NMDAR in the spinal cord, and the effects of IL-1 receptor antagonist (IL-1ra) on NMDAR1 (NR1) phosphorylation and hyperalgesia in a rat model of bone cancer pain. Cancer was induced by injecting AT-3.1 prostate cancer cells into the tibia of the male Copenhagen rat. Phosphorylation of NR1, an essential subunit of the NMDAR, is known to modulate NMDAR activity and facilitate pain. Mechanical hyperalgesia, established by a decrease in paw withdrawal pressure threshold (PWPT), was measured at baseline and 2 h after IL-1ra treatment. IL-1ra was given (i.t.) daily for 7 days between days 13 and 19 after the cancer cell inoculation. Spinal cords were removed for Western blot to measure IL-1beta and NR1 phosphorylation and for double immunostaining of IL-1RI and NR1. The data showed that 1) spinal IL-1beta was up-regulated and NR1 phosphorylation was increased, 2) IL-1ra at 0.1 mg/rat significantly (P<0.05) inhibited mechanical hyperalgesia, increasing PWPT on day 14 from 71.1+/-3.1-85.3+/-4.6 g and on day 19 from 73.5.0+/-3.5-87.1+/-3.7 g, and inhibited NR1 phosphorylation compared with saline control, and 3) IL-1RI is localized in NR1-immunoreactive neurons within the spinal cord. The results suggest that spinal IL-1beta enhances NR1 phosphorylation to facilitate bone cancer pain.

Interleukin-1 alpha has antiallodynic and antihyperalgesic activities in a rat neuropathic pain model

Nerve injury and the consequent release of interleukins (ILs) are processes implicated in pain transmission. To study the potential role of IL-1 in the pathogenesis of allodynia and hyperalgesia, IL-1alpha and comparative IL-1beta, IL-6, and IL-10 mRNA levels were quantified using competitive RT-PCR of the lumbar spinal cord and dorsal root ganglia (DRG; L5-L6) three and seven days after chronic constriction injury (CCI) in rats. Microglial and astroglial activation in the ipsilateral spinal cord and DRG were observed after injury. In naive and CCI-exposed rats, IL-1alpha mRNA and protein were not detected in the spinal cord. IL-1beta and IL-6 mRNAs were strongly ipsilaterally elevated on day seven after CCI. In the ipsilateral DRG, IL-1alpha, IL-6, and IL-10 mRNA levels were increased on days three and seven; IL-1beta was elevated only on day seven. Western blot analysis revealed both the presence of IL-1alpha proteins (45 and 31 kDa) in the DRG and the down-regulation of these proteins after CCI. Intrathecal administration of IL-1alpha (50-500 ng) in naive rats did not influence nociceptive transmission, but IL-1beta (50-500 ng) induced hyperalgesia. In rats exposed to CCI, an IL-1alpha or IL-1 receptor antagonist dose-dependently attenuated symptoms of neuropathic pain; however, no effect of IL-1beta was observed. In sum, the first days after CCI showed a high abundance of IL-1alpha in the DRG. Together with the antiallodynic and antihyperalgesic effects observed after IL-1alpha administration, this finding indicates an important role for IL-1alpha in the development of neuropathic pain symptoms.

Persistent changes in spinal cord gene expression after recovery from inflammatory hyperalgesia: a preliminary study on pain memory

BACKGROUND

Previous studies found that rats subjected to carrageenan injection develop hyperalgesia, and despite complete recovery in several days, they continue to have an enhanced hyperalgesic response to a new noxious challenge for more than 28d. The study's aim was to identify candidate genes that have a role in the formation of the long-term hyperalgesia-related imprint in the spinal cord. This objective was undertaken with the understanding that the long-lasting imprint of acute pain in the central nervous system may contribute to the transition of acute pain to chronicity.

RESULTS

To analyze changes in gene expression when carrageenan-induced hyperalgesia has disappeared but propensity for the enhanced hyperalgesic response is still present, we determined the gene expression profile using oligo microarray in the lumbar part of the spinal cord in three groups of rats: 28d after carrageenan injection, 24h after injection (the peak of inflammation), and with no injection (control group). Out of 17,000 annotated genes, 356 were found to be differentially expressed compared with the control group at 28d, and 329 at 24h after carrageenan injection (both groups at p < 0.01). Among differentially expressed genes, 67 (39 in 28d group) were identified as being part of pain-related pathways, altered in different models of pain, or interacting with proteins involved in pain-related pathways. Using gene ontology (GO) classification, we have identified 3 functional classes deserving attention for possible association with pain memory: They are related to cell-to-cell interaction, synaptogenesis, and neurogenesis.

CONCLUSION

Despite recovery from inflammatory hyperalgesia, persistent changes in spinal cord gene expression may underlie the propensity for the enhanced hyperalgesic response. We suggest that lasting changes in expression of genes involved in the formation of new synapses and neurogenesis may contribute to the transition of acute pain to chronicity.

Calcitonin gene-related peptide stimulation of nitric oxide synthesis and release from trigeminal ganglion glial cells

Clinical and basic science data support an integral role of calcitonin gene-related peptide (CGRP) in migraine pathology. Following trigeminal nerve activation, afferent release of CGRP causes vasodilation while efferent release leads to pain. Although CGRP can also be secreted from cell bodies of trigeminal neurons located within the ganglion, the function of CGRP released in the ganglion is poorly understood. Initially, we showed that SNAP-25, a protein required for CGRP release, was localized in cell bodies of trigeminal ganglia neurons. We also found that satellite glial cells in the ganglia express the CGRP1 receptor protein RAMP1. To determine whether CGRP could directly activate glial cells, primary cultures of rat trigeminal ganglia were utilized to study the effects of CGRP on glial nitric oxide (NO) synthesis and release. Under our culture conditions, >95% of the cells expressed glial fibrillary acidic protein and RAMP1. While weak iNOS staining was observed in glia under basal conditions, CGRP treatment greatly increased glial iNOS expression and NO release. This stimulatory effect was blocked by the CGRP1 receptor antagonist, CGRP(8-37) peptide. Treatment of glial cultures with forskolin or cAMP also increased iNOS expression and stimulated NO release to levels similar to CGRP. To our knowledge, this is the first evidence that activation of CGRP1 receptors regulates glial iNOS and NO release. We propose that following trigeminal nerve activation, CGRP secretion from neuronal cell bodies activates satellite glial cells that release NO and initiate inflammatory events in the ganglia that contribute to peripheral sensitization in migraine.

"Listening" and "talking" to neurons: implications of immune activation for pain control and increasing the efficacy of opioids

It is recently become clear that activated immune cells and immune-like glial cells can dramatically alter neuronal function. By increasing neuronal excitability, these non-neuronal cells are now implicated in the creation and maintenance of pathological pain, such as occurs in response to peripheral nerve injury. Such effects are exerted at multiple sites along the pain pathway, including at peripheral nerves, dorsal root ganglia, and spinal cord. In addition, activated glial cells are now recognized as disrupting the pain suppressive effects of opioid drugs and contributing to opioid tolerance and opioid dependence/withdrawal. While this review focuses on regulation of pain and opioid actions, such immune-neuronal interactions are broad in their implications. Such changes in neuronal function would be expected to occur wherever immune-derived substances come in close contact with neurons.

Mice overexpressing chemokine ligand 2 (CCL2) in astrocytes display enhanced nociceptive responses

Recent findings demonstrate that chemokines, and more specifically CC chemokine ligand 2 (CCL2 or monocyte chemoattractant protein-1), play a major role in pain processing. In the present study, we assess nociceptive responses of mice that overexpressed CCL2 under control of glial fibrillary acidic protein promoter (CCL2 tg). In models of acute nociception CCL2 tg mice demonstrated significantly enhanced nociceptive behavior relative to wild-type controls in responses to both thermal (hot plate) and chemical (formalin test) stimulus modalities. There were no differences in mechanical allodynia in the partial sciatic nerve ligation model, in terms of either magnitude or duration of the allodynic response; however, both groups responded to the maximal extent measurable. In a model of inflammatory pain, elicited by intraplantar administration of complete Freund's adjuvant (CFA), CCL2 tg mice displayed both greater edema and thermal hyperalgesia compared with control mice. In control mice, edema and hyperalgesia returned to baseline values 5-7 days post CFA. However, in CCL2 tg mice, thermal hyperalgesia was significantly different from baseline up to 3 weeks post CFA. Parallel to these enhanced behavioral responses CCL2 serum levels were significantly greater in CCL2 overexpressing mice and remained elevated 7 days post CFA. Consequently, proinflammatory cytokine mRNA expression (IL-1beta, IL-6, and TNFalpha) levels were greater in skin, dorsal root ganglia (DRG), and spinal cord, whereas the anti-inflammatory cytokine (IL-10) level was lower in skin and DRG in CCL2 overexpressing mice than in control mice. Taken together with data from CCR2-deficient mice, these present data confirm a key role of CCL2/CCR2 axis in pain pathways and suggest that inhibiting this axis may result in novel pain therapies.