Blocking the glial function suppresses subcutaneous formalin-induced nociceptive behavior in the rat

N-methyl-D-aspartate Receptor Subunits 2A and 2B Mediate Connexins and Pannexins in the Trigeminal Ganglion Involved in Orofacial Inflammatory Allodynia during Temporomandibular Joint Inflammation

Temporomandibular joint osteoarthritis (TMJOA) is a severe form of temporomandibular joint disorders (TMD), and orofacial inflammatory allodynia is one of its common symptoms which lacks effective treatment. N-methyl-D-aspartate receptor (NMDAR), particularly its subtypes GluN2A and GluN2B, along with gap junctions (GJs), are key players in the mediation of inflammatory pain. However, the precise regulatory mechanisms of GluN2A, GluN2B, and GJs in orofacial inflammatory allodynia during TMJ inflammation still remain unclear. Here, we established the TMJ inflammation model by injecting Complete Freund's adjuvant (CFA) into the TMJ and used Cre/loxp site-specific recombination system to conditionally knock out (CKO) GluN2A and GluN2B in the trigeminal ganglion (TG). Von-frey test results indicated that CFA-induced mechanical allodynia in the TMJ region was relieved in GluN2A and GluN2B deficient mice. In vivo, CFA significantly up-regulated the expression of GluN2A and GluN2B, Gjb1, Gjb2, Gjc2 and Panx3 in the TG, and GluN2A and GluN2B CKO played different roles in mediating the expression of Gjb1, Gjb2, Gjc2 and Panx3. In vitro, NMDA up-regulated the expression of Gjb1, Gjb2, Gjc2 and Panx3 in satellite glial cells (SGCs) as well as promoted the intercellular communication between SGCs, and GluN2A and GluN2B knocking down (KD) altered the expression and function differently. NMDAR regulated Gjb1 and Panx3 through ERK1/2 pathway, and mediated Gjb2 and Gjc2 through MAPK, PKA, and PKC intracellular signaling pathways. These findings shed light on the distinct functions of GluN2A and GluN2B in mediating peripheral sensitization induced by TMJ inflammation in the TG, offering potential therapeutic targets for managing orofacial inflammatory allodynia.

Astrocytes in the spinal cord contributed to acute stress-induced gastric damage via the gap junction protein CX43

Rat restraint water-immersion stress (RWIS) is a compound stress of high intensity and is widely used to study the pathological mechanisms of stress gastric ulcers. The spinal cord, as a part of the central nervous system, plays a dominant role in the gastrointestinal tract, but whether the spinal cord is involved in rat restraint water-immersion stress (RWIS)-induced gastric mucosal damage has not been reported. In this study, we examined the expression of spinal astrocytic glial fibrillary acidic protein (GFAP), neuronal c-Fos, connexin 43 (Cx43), and p-ERK1/2 during RWIS by immunohistochemistry and Western blotting. In addition, we intrathecally injected the astrocytic toxin L-a-aminoadipate (L-AA), gap junction blocker carbenoxolone (CBX), and ERK1/2 signaling pathway inhibitor PD98059 to explore the role of astrocytes in the spinal cord in RWIS-induced gastric mucosal damage and its possible mechanism in rats. The results showed that the expression of GFAP, c-Fos, Cx43, and p-ERK1/2 was significantly elevated in the spinal cord after RWIS. Intrathecal injection of both the astrocyte toxin L-AA and the gap junction blocker CBX significantly attenuated RWIS-induced gastric mucosal damage and decreased the activation of astrocytes and neurons induced in the spinal cord. Meanwhile, the ERK1/2 signaling pathway inhibitor PD98059 significantly inhibited gastric mucosal damage, gastric motility and RWIS-induced activation of spinal cord neurons and astrocytes. These results suggest that spinal astrocytes may regulate the RWIS-induced activation of neurons via CX43 gap junctions and play a critical role in RWIS-induced gastric mucosa damage through the ERK1/2 signaling pathway.

Peripheral Purinergic Modulation in Pediatric Orofacial Inflammatory Pain Affects Brainstem Nitroxidergic System: A Translational Research

Physiology of orofacial pain pathways embraces primary afferent neurons, pathologic changes in the trigeminal ganglion, brainstem nociceptive neurons, and higher brain function regulating orofacial nociception. The goal of this study was to investigate the nitroxidergic system alteration at brainstem level (spinal trigeminal nucleus), and the role of peripheral P2 purinergic receptors in an experimental mouse model of pediatric inflammatory orofacial pain, to increase knowledge and supply information concerning orofacial pain in children and adolescents, like pediatric dentists and pathologists, as well as oro-maxillo-facial surgeons, may be asked to participate in the treatment of these patients. The experimental animals were treated subcutaneously in the perioral region with pyridoxalphosphate-6-azophenyl-2′,4′-disulphonic acid (PPADS), a P2 receptor antagonist, 30 minutes before formalin injection. The pain-related behavior and the nitroxidergic system alterations in the spinal trigeminal nucleus using immunohistochemistry and western blotting analysis have been evaluated. The local administration of PPADS decreased the face-rubbing activity and the expression of both neuronal and inducible nitric oxide (NO) synthase isoforms in the spinal trigeminal nucleus. These results underline a relationship between orofacial inflammatory pain and nitroxidergic system in the spinal trigeminal nucleus and suggest a role of peripheral P2 receptors in trigeminal pain transmission influencing NO production at central level. In this way, orofacial pain physiology should be elucidated and applied to clinical practice in the future.

Electrical Synapses are Involved in Orofacial Neuropathic Pain

Accumulated evidences suggest important roles of glial GAP-junctions in pain. However, only a few studies have explored the role of neuronal GAP-junctions or electrical synapses in neuropathic pain (NP). Therefore, the present study explores the role of connexin 36 (Cx36) in NP using the chronic constriction injury of the infraorbital nerve (CCI-IoN) model in rat. A significant increase in Cx36 labeling was observed in the medullary dorsal horn (MDH) of CCI-IoN-lesioned compared to sham rats. The expression of Cx36 in CCI-IoN-lesioned rats revealed a rostroventral gradient of punctuate labeling within lamina IIo of the MDH. Cx36-positive somata and processes were also observed in MDH laminae IIi and III-V. These somata were mostly of the Gamma aminobutyric acid (GABA) and occasionally Glycine transporter 2 (GlyT2) cell subtypes. Moreover the GABA cell subtypes are highly coupled in lamina IIo as revealed by the intense Cx36 staining in this lamina. Pharmacological Cx36 blockade by intracisternal administration of mefloquine decreased significantly the mechanical allodynia observed in CCI-IoN-lesioned rats. Altogether, our findings demonstrated that Cx36 play an important role in mechanical allodynia by coupling GABA cells. Increasing cell coupling by enhancing Cx36 expression favors neuropathic pain while disrupting this coupling alleviates it. This mechanism may constitute a novel target for the treatment of orofacial mechanical allodynia.

Pyruvate dehydrogenase kinase 2 and 4 gene deficiency attenuates nociceptive behaviors in a mouse model of acute inflammatory pain

Pyruvate dehydrogenase (PDH) kinases (PDKs) 1-4, expressed in peripheral and central tissues, regulate the activity of the PDH complex (PDC). The PDC is an important mitochondrial gatekeeping enzyme that controls cellular metabolism. The role of PDKs in diverse neurological disorders, including neurometabolic aberrations and neurodegeneration, has been described. Implications for a role of PDKs in inflammation and neurometabolic coupling led us to investigate the effect of genetic ablation of PDK2/4 on nociception in a mouse model of acute inflammatory pain. Deficiency in Pdk2 and/or Pdk4 in mice led to attenuation of formalin-induced nociceptive behaviors (flinching, licking, biting, or lifting of the injected paw). Likewise, the pharmacological inhibition of PDKs substantially diminished the nociceptive responses in the second phase of the formalin test. Furthermore, formalin-provoked paw edema formation and mechanical and thermal hypersensitivities were significantly reduced in Pdk2/4-deficient mice. Formalin-driven neutrophil recruitment at the site of inflammation, spinal glial activation, and neuronal sensitization were substantially lessened in the second or late phase of the formalin test in Pdk2/4-deficient animals. Overall, our results suggest that PDK2/4 can be a potential target for the development of pharmacotherapy for the treatment of acute inflammatory pain. © 2016 Wiley Periodicals, Inc.

CD200R1 agonist attenuates glial activation, inflammatory reactions, and hypersensitivity immediately after its intrathecal application in a rat neuropathic pain model

Background

Interaction of CD200 with its receptor CD200R has an immunoregulatory role and attenuates various types of neuroinflammatory diseases.

Methods

Immunofluorescence staining, western blot analysis, and RT-PCR were used to investigate the modulatory effects of CD200 fusion protein (CD200Fc) on activation of microglia and astrocytes as well as synthesis of pro- (TNF, IL-1β, IL-6) and anti-inflammatory (IL-4, IL-10) cytokines in the L4–L5 spinal cord segments in relation to behavioral signs of neuropathic pain after unilateral sterile chronic constriction injury (sCCI) of the sciatic nerve. Withdrawal thresholds for mechanical hypersensitivity and latencies for thermal hypersensitivity were measured in hind paws 1 day before operation; 1, 3, and 7 days after sCCI operation; and then 5 and 24 h after intrathecal application of artificial cerebrospinal fluid or CD200Fc.

Results

Seven days from sCCI operation and 5 h from intrathecal application, CD200Fc reduced mechanical and thermal hypersensitivity when compared with control animals. Simultaneously, CD200Fc attenuated activation of glial cells and decreased proinflammatory and increased anti-inflammatory cytokine messenger RNA (mRNA) levels. Administration of CD200Fc also diminished elevation of CD200 and CD200R proteins as a concomitant reaction of the modulatory system to increased neuroinflammatory reactions after nerve injury. The anti-inflammatory effect of CD200Fc dropped at 24 h after intrathecal application.

Conclusions

Intrathecal administration of the CD200R1 agonist CD200Fc induces very rapid suppression of neuroinflammatory reactions associated with glial activation and neuropathic pain development. This may constitute a promising and novel therapeutic approach for the treatment of neuropathic pain.

Assessment of incising ethology in the absence and presence of jaw muscle hyperalgesia in a mouse home cage environment

INTRODUCTION

Assessment of oral motor behavior in a mouse is challenging due to the lack of currently available techniques that are non-invasive and allow long-term assessment in a home cage environment. The purpose of this study was to evaluate incising behavior using mouse chow attached to a three-dimensional force transducer that was mounted on the existing home cage. In addition, a persistent hyperalgesia condition was introduced to evaluate the sensitivity of the technique to identify incising behavioral changes.

METHODS

Incising activity of CD-1 male and female mice (n=48) was evaluated over a 24 hour recording session during four baseline and six longitudinal hyperalgesia assessment sessions using custom written software. A pre-clinical persistent pain model was used to induce hyperalgesia in the masseter muscle by repetitive acidic saline injections. Sex and age differences were evaluated for multiple incising variables during both light and dark cycles during baseline and hyperalgesia conditions.

RESULTS

Significant sex differences were found for multiple incising variables but not for age. Discrete incising frequencies were identified in the range of 4.6-10.4 Hz and were reproducibly found in both female and male mice. A significant shift to lower incising frequencies was observed after repetitive acidic saline injections compared to neutral saline injections. This shift to lower frequencies of incising returned to baseline levels after approximately four weeks but was statistically longer in female compared to male mice. Significant differences were also found for chow intake (reduced) and weight change during the hyperalgesia condition. No significant differences were found for total number of incisions or number of incising episodes per day or incising force.

CONCLUSIONS

The findings from this study support the use of recording three dimensional incising forces as a sensitive measure of incising behavior. This novel technique allowed the identification of specific incising variables that were differentially affected in female and male mice during a persistent hyperalgesia. The data were collected in the home cage environment with minimal bias such as experimenter interaction. Similar to other dental pain studies, mice were able to maintain normal incising activity levels per day (total incisions, total number of incising episodes) even in the presence of hyperalgesia.

Chronic Pain and Decreased Opioid Efficacy: An Inflammatory Link

Chronic pain is a devastating amalgam of symptoms that affects millions of Americans at tremendous cost to our healthcare system and, more importantly, to patients' quality of life. Literature and research demonstrate that neuroimmune cells called glia are not only responsible for initiating and maintaining part of the chronic pain disease process, but also release inflammatory molecules responsible for decreasing the efficacy of one of the most prominent treatments for pain, opioid analgesia. This article describes chronic pain as a disease process that has ineffective treatment modalities, explores the mechanisms of glial cell activation and inflammatory responses that lead to chronic pain and decreased opioid treatment efficacy, and hypothesizes novel chronic pain treatment modalities based on the glial cell inactivation and anti-inflammatory pathways.

Involvement of connexin43 in the acute hyperosmotic stimulus‑induced synthesis and release of vasopressin in the supraoptic nucleus of rats

There is evidence that astroglial connexin43 (Cx43) in the supraoptic nucleus (SON) is required for the hyperosmolarity‑induced increase in Fos protein expression in magnocellular neurosecretory cells (MNCs). In the present study, the role of astroglial Cx43 in the synthesis and release of vasopressin (VP) by MNCs in the SON subjected to hyperosmotic stimulus was examined. The results revealed that the VP levels in the SON and plasma were increased following acute hyperosmotic stimulus. Treatment of MNCs with Cx43‑specific antisense oligodeoxynucleotides (ASODN), which temporarily reduced Cx43 protein production, limited the VP synthesis and release induced by a hyperosmotic stimulus. Similarly, the addition of gap junction and Cx43 hemichannel blockers also attenuated the VP synthesis and release induced by an acute hyperosmotic stimulus. A high extracellular [Ca2+]([Ca2+]o) has been demonstrated to reduce the gap junction activity or opening probability of Cx54 hemichannels. Notably, it was identified that high [Ca2+]o attenuated the VP synthesis and release induced by acute hyperosmotic stimulus, while low [Ca2+]o had a weak or no effect. These results suggested that Cx43 participates in the VP synthesis and release induced by hyperosmotic stimulation in the SON.

Glia and pain: is chronic pain a gliopathy?

Activation of glial cells and neuro-glial interactions are emerging as key mechanisms underlying chronic pain. Accumulating evidence has implicated 3 types of glial cells in the development and maintenance of chronic pain: microglia and astrocytes of the central nervous system (CNS), and satellite glial cells of the dorsal root and trigeminal ganglia. Painful syndromes are associated with different glial activation states: (1) glial reaction (ie, upregulation of glial markers such as IBA1 and glial fibrillary acidic protein (GFAP) and/or morphological changes, including hypertrophy, proliferation, and modifications of glial networks); (2) phosphorylation of mitogen-activated protein kinase signaling pathways; (3) upregulation of adenosine triphosphate and chemokine receptors and hemichannels and downregulation of glutamate transporters; and (4) synthesis and release of glial mediators (eg, cytokines, chemokines, growth factors, and proteases) to the extracellular space. Although widely detected in chronic pain resulting from nerve trauma, inflammation, cancer, and chemotherapy in rodents, and more recently, human immunodeficiency virus-associated neuropathy in human beings, glial reaction (activation state 1) is not thought to mediate pain sensitivity directly. Instead, activation states 2 to 4 have been demonstrated to enhance pain sensitivity via a number of synergistic neuro-glial interactions. Glial mediators have been shown to powerfully modulate excitatory and inhibitory synaptic transmission at presynaptic, postsynaptic, and extrasynaptic sites. Glial activation also occurs in acute pain conditions, and acute opioid treatment activates peripheral glia to mask opioid analgesia. Thus, chronic pain could be a result of "gliopathy," that is, dysregulation of glial functions in the central and peripheral nervous system. In this review, we provide an update on recent advances and discuss remaining questions.

Activation of microglial cells in the trigeminal subnucleus caudalis evoked by inflammatory stimulation of the oral mucosa

To study the inflammatory hyperplasia induced by an acute noxious stimulation of oral mucosa with 5% formalin (5%FOR), we performed an immunohistochemical study on the expression of TNFá in the intermolar region of the dorsal lingual eminence (IDLE), and Iba1 and phosphorylated (phospho)- p38 MAPK involved with central nervous system microglial activation in the trigeminal subnucleus caudalis (Vc). The present study observed significantly increased expression of TNFá at either 1h or 24h of 5%FOR nociception, as well as sustained TNFá immunoreactivity in the IDLE. On the other hand, at either 1h or 24h 5%FOR nociception, Iba1- immunoreactive (IR) cells in the Vc were significantly increased after inflammatory stimulation of the IDLE; the increase was more evident at 24h 5%FOR nociception. By using the double-label immunofluorescence technique, the findings in particular demonstrated a significant increase in the number of phospho-p38 MAPK- and Iba1-IR coexpressed cells in the Vc at 24h 5%FOR nociception. The results suggest that 24h persistent microglial activation in subnuclei zonalis and gelatinosus of the Vc is evoked by 5%FOR noxious stimulation of the IDLE oral mucosa, thereby the present study indicates that the MAPK expression plays important roles in microglial activation related with central sensitization and inflammatory hyperalgesia.

Spinal astrocyte gap junctions contribute to oxaliplatin-induced mechanical hypersensitivity

Spinal glial cells contribute to the development of many types of inflammatory and neuropathic pain. Here the contribution of spinal astrocytes and astrocyte gap junctions to oxaliplatin-induced mechanical hypersensitivity was explored. The expression of glial fibrillary acidic protein (GFAP) in spinal dorsal horn was significantly increased at day 7 but recovered at day 14 after oxaliplatin treatment, suggesting a transient activation of spinal astrocytes by chemotherapy. Astrocyte-specific gap junction protein connexin 43 (Cx43) was significantly increased in dorsal horn at both day 7 and day 14 following chemotherapy, but neuronal (connexin 36 [Cx36]) and oligodendrocyte (connexin 32 [Cx32]) gap junction proteins did not show any change. Blockade of astrocyte gap junction with carbenoxolone (CBX) prevented oxaliplatin-induced mechanical hypersensitivity in a dose-dependent manner and the increase of spinal GFAP expression, but had no effect once the mechanical hypersensitivity induced by oxaliplatin had fully developed. These results suggest that oxaliplatin chemotherapy induces the activation of spinal astrocytes and this is accompanied by increased expression of astrocyte-astrocyte gap junction connections via Cx43. These alterations in spinal astrocytes appear to contribute to the induction but not the maintenance of oxaliplatin-induced mechanical hypersensitivity. Combined, these results suggest that targeting spinal astrocyte/astrocyte-specific gap junction could be a new therapeutic strategy to prevent oxaliplatin-induced neuropathy.

PERSPECTIVE

Spinal astrocytes but not microglia were recently shown to be recruited in paclitaxel-related chemoneuropathy. Here, spinal astrocyte gap junctions are shown to play an important role in the induction of oxaliplatin neuropathy.

Importance of glial activation in neuropathic pain

Glia plays a crucial role in the maintenance of neuronal homeostasis in the central nervous system. The microglial production of immune factors is believed to play an important role in nociceptive transmission. Pain may now be considered a neuro-immune disorder, since it is known that the activation of immune and immune-like glial cells in the dorsal root ganglia and spinal cord results in the release of both pro- and anti-inflammatory cytokines, as well as algesic and analgesic mediators. In this review we presented an important role of cytokines (IL-1alfa, IL-1beta, IL-2, IL-4, IL-6, IL-10, IL-15, IL-18, TNFalpha, IFNgamma, TGF-beta 1, fractalkine and CCL2); complement components (C1q, C3, C5); metaloproteinases (MMP-2,-9) and many other factors, which become activated on spinal cord and DRG level under neuropathic pain. We discussed the role of the immune system in modulating chronic pain. At present, unsatisfactory treatment of neuropathic pain will seek alternative targets for new drugs and it is possible that anti-inflammatory factors like IL-10, IL-4, IL-1alpha, TGF-beta 1 would fulfill this role. Another novel approach for controlling neuropathic pain can be pharmacological attenuation of glial and immune cell activation. It has been found that propentofylline, pentoxifylline, minocycline and fluorocitrate suppress the development of neuropathic pain. The other way of pain control can be the decrease of pro-nociceptive agents like transcription factor synthesis (NF-kappaB, AP-1); kinase synthesis (MEK, p38MAPK, JNK) and protease activation (cathepsin S, MMP9, MMP2). Additionally, since it is known that the opioid-induced glial activation opposes opioid analgesia, some glial inhibitors, which are safe and clinically well tolerated, are proposed as potential useful ko-analgesic agents for opioid treatment of neuropathic pain. This review pointed to some important mechanisms underlying the development of neuropathic pain, which led to identify some possible new approaches to the treatment of neuropathic pain, based on the more comprehensive knowledge of the interaction between the nervous system and glial and immune cells.

Contribution of the spinal microglia to bee venom-induced inflammatory pain in conscious rats

It is well known that spinal glia plays a key role in the pathogenesis of pain. The present study was designed to determine the roles of spinal microglia in bee venom-induced persistent spontaneous nociception (PSN), mechanical hyperalgesia and inflammation. We determined the effects of microglia inhibitor minocycline on BV-induced PSN, mechanical hyperalgesia and inflammatory swelling. Pre-treatment with intrathecal administration of minocyline at different doses significantly inhibited BV-induced PSN and mechanical hyperalgesia, but had no effect on BV-induced inflammatory swelling. These data suggest that the activation of spinal microglia may play a key role in BV-induced nociception, but not inflammation.

D-Amino acid oxidase-mediated increase in spinal hydrogen peroxide is mainly responsible for formalin-induced tonic pain

BACKGROUND AND PURPOSE

Spinal reactive oxygen species (ROS) are critically involved in chronic pain. D-Amino acid oxidase (DAAO) oxidizes D-amino acids such as D-serine to form the byproduct hydrogen peroxide without producing other ROS. DAAO inhibitors are specifically analgesic in tonic pain, neuropathic pain and cancer pain. This study examined the role of spinal hydrogen peroxide in pain and the mechanism of the analgesic effects of DAAO inhibitors.

EXPERIMENTAL APPROACH

Formalin-induced pain behaviours and spinal hydrogen peroxide levels were measured in rodents.

KEY RESULTS

Formalin injected into the paw increased spinal hydrogen peroxide synchronously with enhanced tonic pain; both were effectively prevented by i.t. fluorocitrate, a selective astrocyte metabolic inhibitor. Given systemically, the potent DAAO inhibitor CBIO (5-chloro-benzo[d]isoxazol-3-ol) blocked spinal DAAO enzymatic activity and specifically prevented formalin-induced tonic pain in a dose-dependent manner. Although CBIO maximally inhibited tonic pain by 62%, it completely prevented the increase in spinal hydrogen peroxide. I.t. catalase, an enzyme specific for decomposition of hydrogen peroxide, completely depleted spinal hydrogen peroxide and prevented formalin-induced tonic pain by 65%. Given systemically, the ROS scavenger PBN (phenyl-N-tert-butylnitrone) also inhibited formalin-induced tonic pain and increase in spinal hydrogen peroxide. Formalin-induced tonic pain was potentiated by i.t. exogenous hydrogen peroxide. CBIO did not increase spinal D-serine level, and i.t. D-serine did not alter either formalin-induced tonic pain or CBIO's analgesic effect.

CONCLUSIONS AND IMPLICATIONS

Spinal hydrogen peroxide is specifically and largely responsible for formalin-induced pain, and DAAO inhibitors produce analgesia by blocking spinal hydrogen peroxide production rather than interacting with spinal D-serine.

Localization of neurones expressing the gap junction protein Connexin45 within the adult spinal dorsal horn: a study using Cx45-eGFP reporter mice

Connexin (Cx) proteins localized to neuronal and glial syncytia provide the ultrastructural components for intercellular communication via gap junctions. In this study, a Cx45 reporter mouse model in which the Cx45 coding sequence is substituted for enhanced green fluorescent protein (eGFP) was used to characterize Cx45 expressing neurones within adult mouse spinal cord. eGFP-immunoreactive (eGFP-IR) cells were localized at all rostro-caudal levels to laminae I-III of the dorsal horn (DH), areas associated with nociception. The neuronal rather than glial phenotype of these cells in DH was confirmed by co-localisation of eGFP-IR with the neuronal marker NeuN. Further immunohistochemical studies revealed that eGFP-IR interneurones co-express the calcium-binding protein calbindin, and to a lesser extent calretinin. In contrast, eGFP-IR profiles did not co-localize with either parvalbumin or GAD-67, both of which are linked to inhibitory interneurones. Staining with the primary afferent markers isolectin-B4 (IB4) and calcitonin gene-related peptide revealed that eGFP-IR somata within laminae I-III receive close appositions from the former, presumed non-peptidergic nociceptive afferents of peripheral origin. The presence of 5-HT terminals in close apposition to eGFP-IR interneuronal somata suggests modulation via descending pathways. These data demonstrate a highly localized expression of Cx45 in a population of interneurones within the mouse superficial dorsal horn. The implications of these data in the context of the putative role of Cx45 and gap junctions in spinal somatosensory processing and pain are discussed.

Quantitative proteomic analysis of membrane proteins involved in astroglial differentiation of neural stem cells by SILAC labeling coupled with LC-MS/MS

Membrane proteins play a critical role in the process of neural stem cell self-renewal and differentiation. Here, we apply the SILAC (stable isotope labeling by amino acids in cell culture) approach to quantitatively compare the membrane proteome of the self-renewing and the astroglial differentiating cells. High-resolution analysis on a linear ion trap-Orbitrap instrument (LTQ-Orbitrap) at sub-ppm mass accuracy resulted in confident identification and quantitation of more than 700 distinct membrane proteins during the astroglial differentiation. Of the 735 quantified proteins, seven cell surface proteins display significantly higher expression levels in the undifferentiated state membrane compared to astroglial differentiating membrane. One cell surface protein transferrin receptor protein 1 may serve as a new candidate for NSCs surface markers. Functional clustering of differentially expressed proteins by Ingenuity Pathway Analysis revealed that most of overexpressed membrane proteins in the astroglial differentiation neural stem cells are involved in cellular growth, nervous system development, and energy metabolic pathway. Taken together, this study increases our understanding of the underlying mechanisms that modulate complex biological processes of neural stem cell proliferation and differentiation.

Suppressive effects of glycyrrhetinic acid derivatives on tachykinin receptor activation and hyperalgesia

Glycyrrhetinic acid (GA), an aglycone of glycyrrhizin, isolated from the licorice root (Glycyrrhizia), and its semi-synthetic derivatives have a wide range of pharmacological effects. To investigate whether GA derivatives may be used as a new class of analgesics, we examined the effects of these compounds on human tachykinin receptors expressed in CHO-K1 cells. Among the GA derivatives examined, the disodium salt of olean-11,13(18)-dien-3β,30-O-dihemiphthalate inhibited the mobilization of [Ca(2+)](i) induced by substance P, neurokinin A, and neurokinin B in CHO-K1 cells expressing the human NK(1), NK(2), and NK(3) tachykinin receptors, respectively. In an inflammatory pain model, Compound 5 suppressed the capsaicin-induced flinching behavior in a dose-dependent manner. Compound 5 was also effective in suppressing pain-related behaviors in the late phase of the formalin test and reducing thermal hyperalgesia in the neuropathic pain state caused by sciatic nerve injury. Collectively, Compound 5 may be an analgesic candidate via tachykinin receptor antagonism.

Modulation of astroglial glutamine synthetase activity affects nociceptive behaviour and central sensitization of medullary dorsal horn nociceptive neurons in a rat model of chronic pulpitis

Previous studies indicate that the astroglial glutamate-glutamine shuttle may be involved in acute pulpal inflammatory pain by influencing central sensitization induced in nociceptive neurons in the trigeminal subnucleus caudalis [the medullary dorsal horn (MDH)] by application of an inflammatory irritant to the rat tooth pulp. The aim of this study was to test if intrathecal application to the rat medulla of the astroglial glutamine synthetase inhibitor methionine sulfoximine (MSO) can influence the central sensitization of MDH nociceptive neurons and the animal's associated behaviour that are manifested in a model of chronic pulpitis pain induced by exposure of a mandibular molar pulp. This model was found to be associated with nocifensive behaviour and enhanced reflex activity evoked by mechanical stimulation of the rat's facial skin and with immunocytochemical evidence of astroglial activation in the MDH. These features were apparent for up to 28 days post-operatively. During this post-operative period, the nocifensive behaviour and enhanced reflex activity were significantly attenuated by intrathecal application of MSO (5 μL, 10 mM) but not by vehicle application. In electrophysiological recordings of nociceptive neuronal activity in the MDH, central sensitization was also evident in pulp-exposed rats but not in intact rats and could be significantly attenuated by MSO application but not by vehicle application. These behavioural and neuronal findings suggest that the astroglial glutamate-glutamine shuttle is responsible for the maintenance of inflammation-induced nocifensive behavioural changes and the accompanying central sensitization in MDH nociceptive neurons in this chronic pulpitis pain model.

Effects of vagotomy, splanchnic nerve lesion, and fluorocitrate on the transmission of acute hyperosmotic stress signals to the supraoptic nucleus

The response to hyperosmotic stresses in the abdominal cavity is regulated, in part, by vasopressin (VP)-secreting neurons in the supraoptic nucleus (SON). How osmotic stress signals are transmitted to the brain is incompletely understood, and whether the transmission routes for osmotic stress signals differ between acute and chronic stresses is unknown. Here we investigated the role of the vagus, splanchnic nerves, and astrocytes in the SON in transducing acute hyperosmotic-stress signals from the abdominal cavity. We found that acute administration of hyperosmotic saline triggered the activation of neurons as well as astrocytes in the SON and the adjoining ventral glia limitans (SON-VGL). Severing the subdiaphragmatic vagal nerve (SDV) prevented the normal response of cells in the SON to HS treatment and attenuated the release of VP into the bloodstream. Lesioning the splanchnic nerves (SNL) diminished HS-induced release of VP, but to a much lesser extent than SDV. Furthermore, SNL did not significantly affect the up-regulation of Fos in SON neurons or the up-regulation of Fos and GFAP in SON and SON-VGL astrocytes that normally occurred in response to HS and did not affect HS-induced expansion of the SON-VGL. Inhibiting astrocytes with fluorocitrate (FCA) prevented the response of the SON to HS and attenuated the release of VP, similarly to SDV surgery. These results suggest that the vagus is the principle route for the transmission of hyperosmotic signals to the brain and that astrocytes in the SON region are necessary for the activation of SON neurons and the release of VP into the bloodstream.

Targeting astrocyte signaling for chronic pain

Clinical management of chronic pain after nerve injury (neuropathic pain) and tumor invasion (cancer pain) is a real challenge due to our limited understanding of the cellular mechanisms that initiate and maintain chronic pain. It has been increasingly recognized that glial cells, such as microglia and astrocytes in the CNS play an important role in the development and maintenance of chronic pain. Notably, astrocytes make very close contacts with synapses and astrocyte reaction after nerve injury, arthritis, and tumor growth is more persistent than microglial reaction, and displays a better correlation with chronic pain behaviors. Accumulating evidence indicates that activated astrocytes can release pro-inflammatory cytokines (e.g., interleukin [IL]-1β) and chemokines (e.g., monocyte chemoattractant protein-1 [MCP-1]/also called CCL2) in the spinal cord to enhance and prolong persistent pain states. IL-1β can powerfully modulate synaptic transmission in the spinal cord by enhancing excitatory synaptic transmission and suppressing inhibitory synaptic transmission. IL-1β activation (cleavage) in the spinal cord after nerve injury requires the matrix metalloprotease-2. In particular, nerve injury and inflammation activate the c-Jun N-terminal kinase in spinal astrocytes, leading to a substantial increase in the expression and release of MCP-1. The MCP-1 increases pain sensitivity via direct activation of NMDA receptors in dorsal horn neurons. Pharmacological inhibition of the IL-1β, c-Jun N-terminal kinase, MCP-1, or matrix metalloprotease-2 signaling via spinal administration has been shown to attenuate inflammatory, neuropathic, or cancer pain. Therefore, interventions in specific signaling pathways in astrocytes may offer new approaches for the management of chronic pain.

Acute hyperosmotic stimulus-induced Fos expression in neurons depends on activation of astrocytes in the supraoptic nucleus of rats

Acute hyperosmolarity induced a time-dependent expression of Fos protein in both neurons and astrocytes of the rat supraoptic nucleus, with peak Fos expression occurring at 45 min in astrocytes and at 90 min in neurons after hypertonic stimulation in vivo. To determine whether the two cell types were activated separately or in an integrated manner, animals were pretreated with fluorocitrate, a glial metabolic blocker or carbenoxolone, a gap junction blocker followed by an acute hypertonic stimulation similar to that of the controls. Antibodies against glial fibrillary acidic protein, connexin 43, vasopressin, and oxytocin were used in serial sections to identify the cellular elements of the supraoptic nucleus. It was found that interruption of astrocyte metabolism with fluorocitrate significantly reduced Fos protein expression in both astrocytes and neurons, whereas blockage of gap junctions with carbenoxolone clearly reduced Fos protein expression in neurons, but not in astrocytes. These results indicate that both neurons and astrocytes in the rat supraoptic nucleus are involved in regulating osmolarity. Astrocytes are activated first, whereas connexin 43 functional hemichannels in SON astrocytes are required for the subsequent activation of the neurons.

Dexmedetomidine blocks thermal hyperalgesia and spinal glial activation in rat model of monoarthritis

AIM

To investigate the effect of systemic administration dexmedetomidine, a selective alpha 2 adrenergic receptor (alpha(2)AR) agonist, on thermal hyperalgesia and spinal glial activation evoked by monoarthritis (MA).

METHODS

MA was induced by an intra-articular injection of complete Freund's adjuvant (CFA). Thermal hyperalgesia was measured by Hargreaves' test. The spinal glial activation status was analyzed by GFAP (an astrocytic marker) and Iba-1 (a microglial marker) immunohistochemistry or immunoblotting.

RESULTS

Unilateral intra-articular injection of CFA produced a robust glial activation of astrocytes and microglia in the spinal cord, which was associated with the development and maintenance of thermal hyperalgesia. Intraperitoneal (ip) injection of dexmedetomidine (2.5 and 10 microg/kg) was repeatedly given once daily for 5 days with the first injection 60 min before intra-articular CFA. At the dose of 10 microg/kg, dexmedetomidine significantly attenuated MA-induced ipsilateral hyperalgesia from day 2 to day 5. MA-induced up-regulation of GFAP expression on both sides of the spinal dorsal horn was significantly suppressed by day 5 post-MA following dexmedetomidine application, whereas MA-induced Iba-1 up-regulation was only partially suppressed.

CONCLUSION

Systemic dexmedetomidine inhibits the activation of spinal glia, which is possibly associated with its antihyperalgesia in monoarthritic rats.

Gliopathic pain: when satellite glial cells go bad

Neurons in sensory ganglia are surrounded by satellite glial cells (SGCs) that perform similar functions to the glia found in the CNS. When primary sensory neurons are injured, the surrounding SGCs undergo characteristic changes. There is good evidence that the SGCs are not just bystanders to the injury but play an active role in the initiation and maintenance of neuronal changes that underlie neuropathic pain. In this article the authors review the literature on the relationship between SGCs and nociception and present evidence that changes in SGC potassium ion buffering capacity and glutamate recycling can lead to neuropathic pain-like behavior in animal models. The role that SGCs play in the immune responses to injury is also considered. We propose the term gliopathic pain to describe those conditions in which central or peripheral glia are thought to be the principal generators of principal pain generators.

Focal Inflammation Causes Carbenoxolone-Sensitive Tactile Hypersensitivity in Mice

A focal and transitory inflammation induced by injection of complete Freund's adjuvant (CFA) in the submandibular skin of mice elicits pain behavior that persists for several weeks after the initial inflammation has resolved. Chronic pain, assessed as tactile hypersensitivity to stimulation with von Frey filaments, was evident from 1-7 weeks following CFA injection, although inflammation at the injection site was resolved by 3-4 weeks. In contrast, there were no changes in tactile sensitivity in the paw (un-injected site for comparison), no alterations in open field behavior and no differences in a functional observation battery evident in CFA-treated mice compared to controls (saline-injected) or to baseline (before CFA injection). Neither strain (Balb/c vs. C57BL/6) nor sex differences in baseline tactile threshold were significant in the submandibular skin. CFA-induced tactile hypersensitivity was also not a function of strain or sex. A single intraperitoneal injection of the gap junction blocker carbenoxolone (CBX) restored normal tactile thresholds in CFA-treated mice when administered at the peak of inflammation (1 week), after significant resolution of inflammation (3 weeks) or after total resolution of inflammation (4 and 5 weeks) without altering the tactile threshold of control subjects, tactile threshold in the paw or open field behavior. Thus, in this novel model of post-inflammatory pain, transitory inflammation induced persistent sex- and strain-independent behavioral hypersensitivity that was reversed by the gap junction blocker CBX, suggesting neuronal and/or glial plasticity as a major component of the chronic pain.

Spinal glia modulate both adaptive and pathological processes

Recent research indicates that glial cells control complex functions within the nervous system. For example, it has been shown that glial cells contribute to the development of pathological pain, the process of long-term potentiation, and the formation of memories. These data suggest that glial cell activation exerts both adaptive and pathological effects within the CNS. To extend this line of work, the present study investigated the role of glia in spinal learning and spinal learning deficits using the spinal instrumental learning paradigm. In this paradigm rats are transected at the second thoracic vertebra (T2) and given shock to one hind limb whenever the limb is extended (controllable shock). Over time these subjects exhibit an increase in flexion duration that reduces net shock exposure. However, when spinalized rats are exposed to uncontrollable shock or inflammatory stimuli prior to testing with controllable shock, they exhibit a learning deficit. To examine the role of glial in this paradigm, spinal glial cells were pharmacologically inhibited through the use of fluorocitrate. Our results indicate that glia are involved in the acquisition, but not maintenance, of spinal learning. Furthermore, the data indicate that glial cells are involved in the development of both shock and inflammation-induced learning deficits. These findings are consistent with prior research indicating that glial cells are involved in both adaptive and pathological processes within the spinal cord.

Differential activation of spinal microglial and astroglial cells in a mouse model of peripheral neuropathic pain

The pharmacological attenuation of glial activation represents a novel approach for controlling neuropathic pain, but the role of microglial and astroglial cells is not well established. To better understand the potential role of two types of glial cells, microglia and astrocytes, in the pathogenesis of neuropathic pain, we examined markers associated with them by quantitative RT-PCR, western blot and immunohistochemical analyses in the dorsal horn of the lumbar spinal cord 7days after chronic constriction injury (CCI) to the sciatic nerve in mice. The mRNA and protein of microglial cells were labeled with C1q and OX42(CD11b/c), respectively. The mRNA and protein of astrocytes were labeled with GFAP. The RT-PCR results indicated an increase in C1q mRNA that was more pronounced than the increased expression of GFAP mRNA ipsilateral to the injury in the dorsal spinal cord. Similarly, western blot and immunohistochemical analyses demonstrated an ipsilateral upregulation of OX42-positive cells (72 and 20%, respectively) and no or little (8% upregulation) change in GFAP-positive cells in the ipsilateral dorsal lumbar spinal cord. We also found that chronic intraperitoneal injection of the minocycline (microglial inhibitor) and pentoxifylline (cytokine inhibitor) attenuated CCI-induced activation of microglia, and both, but not fluorocitrate (astroglial inhibitor), diminished neuropathic pain symptoms and tactile and cold sensitivity. Our findings indicate that spinal microglia are more activated than astrocytes in peripheral injury-induced neuropathic pain. These findings implicate a glial regulation of the pain response and suggest that pharmacologically targeting microglia could effectively prevent clinical pain syndromes in programmed and/or anticipated injury.

Tramadol and flurbiprofen depress the cytotoxicity of cisplatin via their effects on gap junctions

PURPOSE

Cancer patients are often concurrently treated with analgesics and antineoplastic drugs, yet the influence of analgesic agents on therapeutic activity of antineoplastic drugs is largely unexplored. This study investigates the effects of three commonly used analgesics, which produce analgesia by different mechanisms, on cytotoxicity induced by cisplatin, a widely used antitumor agent, and the relation between those effects and modulation of gap junction function by the analgesics.

EXPERIMENTAL DESIGN

The role of gap junctions in the modulation of cisplatin toxicity is explored by manipulation of connexin expression, and gap junction presence and function, using clinically relevant concentrations of the analgesics and cisplatin.

RESULTS

Short-term exposure of transformed cells to cisplatin reduced the clonogenic survival in low-density cultures (without gap junction formation) and in high density (with gap junction formation), but the toxic effect was greater at high density. In the absence of connexin expression or with block of connexin channels, cell density had no effect on cisplatin toxicity. Tramadol and flurbiprofen, but not morphine, significantly reduced cisplatin cytotoxicity, but this effect required functional gap junctions between the cells. Tramadol and flurbiprofen inhibited dye-coupling through gap junctions, but morphine did not.

CONCLUSIONS

The results suggest that the density dependence of cisplatin toxicity is mediated by gap junctions. They further indicate that tramadol and flurbiprofen depress cisplatin cytotoxicity through inhibition of gap junction activity, and more generally, that agents that depress junctional communication can counteract the effects of antitumor agents.

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.

Evidence for a role of connexin 43 in trigeminal pain using RNA interference in vivo

The importance of glial cells in the generation and maintenance of neuropathic pain is becoming widely accepted. We examined the role of glial-specific gap junctions in nociception in the rat trigeminal ganglion in nerve-injured and -uninjured states. The connexin 43 (Cx43) gap-junction subunit was found to be confined to the satellite glial cells (SGCs) that tightly envelop primary sensory neurons in the trigeminal ganglion and we therefore used Cx43 RNA interference (RNAi) to alter gap-junction function in SGCs. Using behavioral evaluation, together with immunocytochemical and Western blot monitoring, we show that Cx43 increased in the trigeminal ganglion in rats with a chronic constriction injury (CCI) of the infraorbital nerve. Reducing Cx43 expression using RNAi in CCI rats reduced painlike behavior, whereas in non-CCI rats, reducing Cx43 expression increased painlike behavior. The degree of painlike behavior in CCI rats and intact, Cx43-silenced rats was similar. Our results support previous suggestions that increases in glial gap junctions after nerve injury increases nociceptive behavior but paradoxically the reduction of gap junctions in normal ganglia also increases nociceptive behavior, possibly a reflection of the multiple functions performed by glia.

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.

Spinal glial activation contributes to pathological pain states

Chronic pain, a pathological state, affects millions of people worldwide. Despite decades of study on the neuronal processing of pain, mechanisms underlying the creation and maintenance of enhanced pain states after injury or inflammation remain far from clear. In the last decade, however, the discovery that glial activation amplifies pain has challenged classic neuronal views of "pain". This review focuses on recent developments in understanding that spinal cord glia are involved in pathological pain. We overview the action of spinal glia (both microglia and astrocytes) in several persistent pain models, and provide new evidence that spinal glia activation contributes to the development and maintenance of arthritic pain facilitation. We also attempt to discuss some critical questions, such as how signals are conveyed from primary afferents to spinal glia following peripheral nerve injury and inflammation. What causes glia to become activated after peripheral/central injury/inflammation? And how the activated glia alter neuronal sensitivity and pain processing? Answers to these questions might open a new approach for treatment of pathological pain.

Astroglial glutamate-glutamine shuttle is involved in central sensitization of nociceptive neurons in rat medullary dorsal horn

Growing evidence suggests that astroglia are involved in pain states, but no studies have tested their possible involvement in modulating the activity of nociceptive neurons per se. This study has demonstrated that the central sensitization induced in functionally identified nociceptive neurons in trigeminal subnucleus caudalis (the medullary dorsal horn) by application of an inflammatory irritant to the rat's tooth pulp can be significantly attenuated by continuous intrathecal superfusion of methionine sulfoximine (MSO; 0.1 mM), an inhibitor of the astroglial enzyme glutamine synthetase that is involved in the glutamate-glutamine shuttle. Simultaneous superfusion of MSO and glutamine (0.25 mM) restored the irritant-induced central sensitization. In control experiments, superfusion of either MSO or glutamine alone, or vehicle, did not produce any significant changes in neuronal properties. These findings suggest that the astroglial glutamate-glutamine shuttle is essential for the initiation of inflammation-induced central sensitization but that inhibition of astroglial function may not affect normal nociceptive processing.