Differential effects of NMDA and group I mGluR antagonists on both nociception and spinal cord protein kinase C translocation in the formalin test and a model of neuropathic pain in rats

Glial Glutamate Transporter Modulation Prevents Development of Complete Freund's Adjuvant-Induced Hyperalgesia and Allodynia in Mice

Glial glutamate transporter (GLT-1) modulation in the hippocampus and anterior cingulate cortex (ACC) is critically involved in nociceptive pain. The objective of the study was to investigate the effects of 3-[[(2-methylphenyl) methyl] thio]-6-(2-pyridinyl)-pyridazine (LDN-212320), a GLT-1 activator, against microglial activation induced by complete Freund's adjuvant (CFA) in a mouse model of inflammatory pain. Furthermore, the effects of LDN-212320 on the protein expression of glial markers, such as ionized calcium-binding adaptor molecule 1 (Iba1), cluster of differentiation molecule 11b (CD11b), mitogen-activated protein kinases (p38), astroglial GLT-1, and connexin 43 (CX43), were measured in the hippocampus and ACC following CFA injection using the Western blot analysis and immunofluorescence assay. The effects of LDN-212320 on the pro-inflammatory cytokine interleukin-1β (IL-1β) in the hippocampus and ACC were also assessed using an enzyme-linked immunosorbent assay. Pretreatment with LDN-212320 (20 mg/kg) significantly reduced the CFA-induced tactile allodynia and thermal hyperalgesia. The anti-hyperalgesic and anti-allodynic effects of LDN-212320 were reversed by the GLT-1 antagonist DHK (10 mg/kg). Pretreatment with LDN-212320 significantly reduced CFA-induced microglial Iba1, CD11b, and p38 expression in the hippocampus and ACC. LDN-212320 markedly modulated astroglial GLT-1, CX43, and, IL-1β expression in the hippocampus and ACC. Overall, these results suggest that LDN-212320 prevents CFA-induced allodynia and hyperalgesia by upregulating astroglial GLT-1 and CX43 expression and decreasing microglial activation in the hippocampus and ACC. Therefore, LDN-212320 could be developed as a novel therapeutic drug candidate for chronic inflammatory pain.

Artesunate Alleviates Paclitaxel-Induced Neuropathic Pain in Mice by Decreasing Metabotropic Glutamate Receptor 5 Activity and Neuroinflammation in Primary Sensory Neurons

Experimental studies on the pathogenetic process of paclitaxel-induced neuropathic pain (PINP) have been initially carried out, but PINP still has no effective therapy. Recently reported studies have highlighted the involvement of glutamate receptors and neuroinflammation in peripheral and central nociceptive transmission in PINP. Artesunate is a first-line antimalarial drug with established efficacy in alleviating pain in a variety of pathologies. The current work assessed whether artesunate inhibits PINP by modulating metabotropic glutamate receptor 5 (mGluR5) and neuroinflammation in mice. The anti-hyperalgesic effect of artesunate was verified by assessing mechanical frequency and thermal latency in the paw withdrawal test as well as spontaneous pain. The expression levels of mGluR5, pain-related receptors and neuroinflammatory markers in dorsal root ganglion (DRG) were examined. In addition, treatment with CHPG and 2-methyl-6-(phenyl ethynyl) pyridine (MPEP) (mGluR5 agonist and antagonist, respectively) was performed to determine mGluR5’s role in the anti-hyperalgesic properties of artesunate. We demonstrated artesunate prevented PINP in a dose-dependent manner, while exerting a clear anti-hyperalgesic effect on already existing PINP. Artesunate normalized paclitaxel-related expression changes in DRG mGluR5, NR1, and GluA2, as well as six paclitaxel related neuroinflammation markers. Intrathecal application of MPEP treated PINP by reversing NR1 and GluA2 expression changes but had no effects on chemokines and inflammatory factors. Furthermore, artesunate treatment reversed acute pain following CHPG application. In conclusion, this study revealed that artesunate alleviates paclitaxel-induced hyperalgesia and spontaneous pain by decreasing DRG mGluR5 expression and neuroinflammation in the mouse model of PINP.

Evaluation of acute and chronic nociception in subchronically administered MK-801-induced rat model of schizophrenia

Patients diagnosed with schizophrenia have been reported to exhibit atypically low pain sensitivity and to vary in their experience of chronic pain. To the best of our knowledge, there has yet to be an animal study that provides information concerning the relationship between models of schizophrenia and pain. In the present study, we investigated several distinct nociceptive behaviors in a translational rat model of schizophrenia (0. 5 mg/kg MK-801, twice a day for 7 days followed by a 7-day washout period). The presence of the expected cognitive deficit was confirmed with novel object recognition (NOR) paradigm prior to nociception testing. MK-801-treated rats with lack of novelty interest in NOR testing showed: hyposensitivity to thermal and mechanical stimuli; short-term hypoalgesia followed by augmented hyperalgesia in response to formalin-induced spontaneous nociception and increased thermal and mechanical hyperalgesia in the complete Freund's adjuvant (CFA) induced chronic pain model. In conclusion, MK-801 induced antinociception effects for thermal stimuli in rats that were consistent with the decreased pain sensitivity observed in schizophrenia patients. Additionally, the amplified biphasic response exhibited by the MK-801 group in the formalin-induced spontaneous nociception test affirms the suitability of the test as a model of acute to delayed pain transition.

The Search for New Anticonvulsants in a Group of (2,5-Dioxopyrrolidin-1-yl)(phenyl)Acetamides with Hybrid Structure-Synthesis and In Vivo/In Vitro Studies

Epilepsy belongs to the most common and debilitating neurological disorders with multifactorial pathophysiology and a high level of drug resistance. Therefore, with the aim of searching for new, more effective, and/or safer therapeutics, we discovered a focused series of original hybrid pyrrolidine-2,5-dione derivatives with potent anticonvulsant properties. We applied an optimized coupling reaction yielding several hybrid compounds that showed broad-spectrum activity in widely accepted animal seizure models, namely, the maximal electroshock (MES) test and the psychomotor 6 Hz (32 mA) seizure model in mice. The most potent anticonvulsant activity and favorable safety profile was demonstrated for compound 30 (median effective dose (ED₅₀) MES = 45.6 mg/kg, ED₅₀ 6 Hz (32 mA) = 39.5 mg/kg, median toxic dose (TD₅₀) (rotarod test) = 162.4 mg/kg). Anticonvulsant drugs often show activity in pain models, and compound 30 was also proven effective in the formalin test of tonic pain, the capsaicin-induced pain model, and the oxaliplatin (OXPT)-induced neuropathic pain model in mice. Our studies showed that the most plausible mechanism of action of 30 involves inhibition of calcium currents mediated by Cav_1.2 (L-type) channels. Importantly, 30 revealed high metabolic stability on human liver microsomes, negligible hepatotoxicity, and relatively weak inhibition of CYP3A4, CYP2D6, and CYP2C9 isoforms of cytochrome P450, compared to reference compounds. The promising in vivo activity profile and drug-like properties of compound 30 make it an interesting candidate for further preclinical development.

Involvement of glutamate receptors of the paragigantocellularis lateralis nucleus in the pain modulatory effect of 17β-estradiol in male rats

The pain modulatory role of the paragigantocellularis lateralis nucleus (LPGi) and the 17β-estradiol has thoroughly been probed. This study investigates the contribution of ionotropic glutamate receptors in pain modulatory effect of intra-LPGi injection of 17β-estradiol. For this purpose, the LPGi nucleus cannulation was performed and drugs were injected into this nucleus, 15 min prior to the formalin test. The duration of formalin-induced flexing and licking behaviors was recorded for 60 min immediately after formalin injection. The results showed that the flexing behavior is significantly decreased by intra-LPGi injection of 0.8 µmol 17β-estradiol duringboth phases of formalin test (P < 0.001). However, 17β-estradiol attenuated the licking duration only in the second phase (P < 0.001). Interestingly, NMDA and AMPA/kainate receptor antagonists (AP5 and CNQX, respectively) significantly counteracted the analgesic effect of intra-LPGi injection of 17β-estradiol in both phases of the formalin test (P < 0.001). Consequently, the revealing results showed that the analgesic effect of intra-LPGi injection of 17β-estradiol on acute inflammatory pain might be mediated via the activation of ionotropic glutamate receptors.

Effect of single and repeated administration of amitriptyline on neuropathic pain model in rats: Focus on glutamatergic and upstream nitrergic systems

AIMS

Few studies have compared the interaction of single and repeated administration of amitriptyline (amit) with the nitrergic system and glutamatergic system in the experimental model of neuropathic pain. We aimed to evaluate the antinociceptive effect of single and repeated administration of amit and to assess whether glutamate preceded inducible nitric oxide synthase (iNOS) inhibition as a mechanism of the analgesic effect of amit in the neuropathic model of pain.

MATERIALS AND METHODS

Male Wistar rats were subjected to left sciatic nerve ligation. The effect of single (25 mg kg^-1) and repeated (10 mg kg^-1 daily for 3 weeks) administration of amit intraperitoneally (i.p.) alone or in combination with aminoguanidine (AG i.p., 100 mg kg^-1 for 3 days, a selective iNOS inhibitor) and MK-801 (0.05 mg kg^-1 i.p., NMDA antagonist) on resting paw posture and mechanical hyperalgesia were studied. Glutamate level and iNOS protein expression in hippocampus were detected.

KEY FINDINGS

Single and repeated administration of amit alone or in combination with AG or MK-801 demonstrated a significant decrease in resting pain score and increase in the pain threshold. Both glutamate and nitrite levels decreased in the hippocampi of single and repeated amit + MK-801 groups. Immunohistochemistry showed a marked decrease in iNOS immunoreactivity in rats treated with single and repeated amit + MK-801.

SIGNIFICANCE

Our results suggest that glutamate-dependent mechanisms are involved in the analgesic responses to amit administration. Importantly, glutamatergic system and its upstream nitrergic system play an important role in the antinociceptive action of amit.

Differential expression of mGluRs in rat spinal dorsal horns and their modulatory effects on nocifensive behaviors

Glutamate is a neurotransmitter present in most excitatory synapses in the nervous system. It also plays a key role in the spinal cord’s physiological excitatory circuit and is involved in pathological neurotransmissions such as those observed in inflammatory and neuropathic pain conditions. The actions of glutamate are mediated by different types of ionotropic glutamate receptors (iGluRs) and metabotropic glutamate receptors (mGluRs). Although expressions of iGluRs are well studied, those of mGluRs are not fully elucidated in the spinal cord. In this study, we examined the expressions of mGluRs (mGluR1-8) and investigated which mGluR subtypes can modulate pain transmission in the dorsal horn of the spinal cord using an inflammatory pain model. Reverse transcription-polymerase chain reaction revealed that mGluR mRNAs, except for mGluR2 and 6, were detected in the spinal cord. Double labeling analysis, in situ hybridization histochemistry with immunohistochemistry, was used to examine the distribution of each mGluR in neurons or glial cells in the lamina I–II of the spinal dorsal horn. mGluR1, 5, and 7 were generally, and 4 and 8 were frequently, expressed in neurons. mGluR3 was expressed not only in neurons but also in oligodendrocytes. We next examined the distribution of mGluR4 and 8 were expressed in excitatory or inhibitory neurons. Both mGluR4 and 8 were preferentially expressed in inhibitory neurons rather than in excitatory neurons. Furthermore, intrathecal delivery of CPPG((RS)-α-cyclopropyl-4-phosphonophenylglycine), an antagonist for mGluR 4 and 8, attenuated nocifensive behaviors and the increase in fos-positive-excitatory neurons of the dorsal horn induced by intraplantar injection of formalin. These findings suggest that mGluR4 and 8, which are preferentially expressed in inhibitory neurons, may play roles in the modulation of pain transmission in the spinal dorsal horn.

SIRT1 Activation Attenuates Bone Cancer Pain by Inhibiting mGluR1/5

Bone cancer pain (BCP), which is induced by primary or metastatic bone cancer, remains a clinically challenging problem due to the poor understanding of its mechanisms. Sirtuin 1 (SIRT1) plays an important role in various pain models. Intrathecal administration of SRT1720, a SIRT1 activator, attenuates BCP in a rat model. However, the expression and activity of SIRT1 during the development and maintenance of BCP remain unknown. Furthermore, the underlying mechanism of SIRT1 in BCP remains ambiguous. In this study, we detected the time course of SIRT1 expression and activity in the spinal cord of mice with BCP and examined whether SRT1720 alleviated BCP by inhibiting metabotropic glutamatergic receptor (mGluR) 1/5 expression. In addition, we downregulated spinal SIRT1 expression in normal mice through an intrathecal injection of AAV-SIRT1-shRNA and then assessed pain behavior and mGluR1/5 expression. Mice with BCP developed significant mechanical allodynia and spontaneous flinching, accompanied by decreased levels of the SIRT1 protein, mRNA, and activity in the spinal cord. The SRT1720 treatment produced an analgesic effect on tumor-bearing mice and decreased the spinal levels of the mGluR1/5 protein and mRNA. In contrast, the AAV-SIRT1-shRNA treatment induced pain behavior in normal mice and increased the spinal levels of the mGluR1/5 protein and mRNA. The results suggested a critical role for SIRT1 in the development and maintenance of BCP and further indicated that activation of SIRT1 in the spinal cord by SRT1720 functionally reverses BCP in mice by inhibiting mGluR1/5.

PI3K/Akt Pathway is Required for Spinal Central Sensitization in Neuropathic Pain

Phosphatidylinositol-3-kinase (PI3K) has been identified in the expression of central sensitization after noxious inflammatory stimuli. However, its contribution in neuropathic pain remains to be determined. Here we address the role of PI3K signaling in central sensitization in a model of neuropathic pain, and propose a novel potential drug target for neuropathic pain. Chronic constriction injury (CCI) rat model was used in the study as the model for neuropathic pain. Western blotting, whole-cell patch clamp, and von Frey assay were performed to study biochemical, electrical, and behavioral changes in CCI rats, respectively. A steroid metabolite of the fungi (wortmannin) was used to block PI3K signaling and its effects on CCI rats were tested. PI3K/Akt signaling increased in the spinal cord L4-L6 sections in the CCI rats. CCI also facilitated miniature excitatory postsynaptic potential of dorsal horn substantia gelatinosa neurons, increased phosphorylation of glutamate receptor subunit GluA1 and synapsin at the synapse, and induced mechanic allodynia. Wortmannin reversed biochemical, electrical, and behavioral changes in CCI rats. This study is the first to show PI3K/Akt signaling is required for spinal central sensitization in the CCI neuropathic pain model.

Protein Kinase C γ Contributes to Central Sensitization in a Rat Model of Chronic Migraine

Protein kinase C γ (PKCγ) is a critical regulator of central sensitization and is widely recognized to be involved in the pathogenesis of chronic migraine (CM). However, the function of PKCγ in CM remains unknown. This study investigated the role of PKCγ on pathogenesis of CM. We repeated infusions of inflammatory soup (IS) on the intact dura of conscious rats to model recurrent trigeminovascular or dural nociceptor activation assumed to occur in patients with CM. The von Frey test was then used to detect changes in pain threshold. QT-PCR, western blotting, and double immunofluorescence staining were performed to detect the expression and location of PKCγ in the trigeminal nucleus caudalis (TNC) and the expressions of calcitonin gene-related peptide (CGRP), c-Fos, and phosphorylation level of GluR1 subunit at serine 831. Chelerythrine chloride (CHE) and phorbol 12-myristate 13-acetate (PMA) were administrated to investigate the role of PKCγ in central sensitization. We found that repeated infusions of IS induced mechanical allodynia. PKCγ was significantly increased in TNC after CM. Furthermore, inhibition of PKCγ by CHE relieved allodynia and reduced the expression of CGRP and c-Fos. Activation of PKCγ by PMA aggravated allodynia and increased the expression of CGRP and c-Fos. In addition, inhibition of PKCγ reduced the phosphorylation level of GluR1; in contrast, activation of PKCγ increased the phosphorylation level of GluR1. These results suggest PKCγ-induced GluR1 phosphorylation might participate in central sensitization in a rat model of CM. We suggest that PKCγ is a potential therapeutic target for CM.

GABA acting on GABAB receptors located in a medullary pain facilitatory area enhances nociceptive behaviors evoked by intraplantar formalin injection

The dorsal reticular nucleus (DRt) plays a key role in facilitation of nociceptive transmission at the spinal cord. In this study, we evaluated the mechanisms involved in GABA-mediated control of the DRt focusing on the role of local GABAB receptors. First, we used in vivo microdialysis to study the release of GABA in the DRt during the course of the formalin test. An increase of GABA levels in comparison with baseline values was detected in the second phase of the test. Because we previously showed that GABAB receptors are expressed by opioidergic DRt neurons, which respond to nociceptive stimuli and inhibit spinally projecting DRt neurons involved in descending pronociception, we then interfered with local GABAB receptors using gene transfer and pharmacological approaches. Lentiviral-mediated knockdown of GABAB1a expression decreased nociceptive responses during the second phase of the test. Local administration of the GABAB receptor antagonist CGP 35348 also decreased nociceptive responses in the second phase of the test, whereas the opposite was detected after injection of the GABAB agonist baclofen. Finally, we determined the GABAergic afferents of the DRt, namely those arising from its main brain afferents, which are located at the telencephalon and diencephalon. For that purpose, we combined retrograde tract-tracing from the DRt with immunodetection of glutamate decarboxylase, the GABA-synthesizing enzyme. The higher numbers of retrogradely labelled glutamate decarboxylase-immunoreactive neurons were located at insular, somatosensory, and motor cortices. Collectively, the results suggest that GABA acting on GABAB receptors may enhance pain facilitation from the DRt during inflammatory pain.

Endovanilloid control of pain modulation by the rostroventromedial medulla in an animal model of diabetic neuropathy

The involvement of transient receptor vanilloid type-1 (TRPV1) channels in pain modulation by the brain remains understudied. The rostroventromedial medulla (RVM) plays a key role in conveying to the spinal cord pain modulatory influences triggered in higher brain centres, with co-existence of inhibitory (antinociceptive) and facilitatory (pronociceptive) effects. In spite of some reports of TRPV1 expression in the RVM, it remains unknown if endovanilloid signalling plays a direct role in local pain modulation. Here we used a model of diabetic neuropathy, the streptozotocin (STZ)-diabetic rat, to study the role of endovanilloid signalling in RVM-mediated pain modulation during chronic pain. Four weeks after diabetes induction, the levels of TRPV1 mRNA and fatty acid amide hydrolase (FAAH), a crucial enzyme for endovanilloid catabolism, in the RVM of STZ-diabetic rats were higher than control. The RVM of STZ-diabetic rats presented decreased levels of several TRPV1 endogenous ligands, namely anandamide (AEA), palmitoylethanolamide (PEA) and oleoylethanolamide (OEA). Administration of capsaicin (a TRPV1 agonist) into the RVM decreased nociceptive behavioural responses in the inflammatory phase of the formalin test (phase 2). These findings suggest that diabetic neuropathy induces plastic changes of RVM endovanilloid signalling, indicating that TRPV1 may be a putative target for pain modulation in this chronic pain condition.

Liver X Receptor β Is Involved in Formalin-Induced Spontaneous Pain

Increasing evidence indicates that the liver X receptor(LXR) β modulates inflammatory pain. However, the molecular mechanisms through which LXRβ modulates pain are unclear. Here, we found that LXRβ-null mice responded more strongly to acute noxious stimuli than wild-type (WT) littermates (in the hot plate and Hargreaves tests) and had augmented tonic inflammatory pain (in the formalin test). This increased reactivity to inflammatory pain was accompanied by enhanced formalin-evoked Fos and pERK staining of second-order nociceptive neurons. Immunohistochemistry showed that the expression of CGRP, SP, and IB4 was increased in the lamina I-II of the lumbar dorsal horns in formalin-injected LXRβ knockout (KO) mice compared with the WT controls. In addition, LXRβ deletion in the mice enhanced the formalin-induced inflammation with more activated microglia and astrocytes in the spinal cord. Furthermore, the levels of pro-inflammatory cytokines (IL-1β ,TNF-α) as well as NFκB in the formalin-injected paw were elevated by the loss of LXRβ. Taken together, these data indicate that LXRβ is involved in acute as well as inflammatory pain, and thus, it may be considered as a new target for the development of analgesics.

Analgesic effects and possible mechanisms of iridoid glycosides from Lamiophlomis rotata (Benth.) Kudo in rats with spared nerve injury

ETHNOPHARMACOLOGICAL RELEVANCE

Lamiophlomis rotata (Benth.) Kudo (L. rotata) is a medical plant that has been traditionally used for centuries for the treatment of pain, such as bone and muscle pain, joint pain and dysmenorrhea. Although iridoid glycosides of L. rotata (IGLR) are the major active components of it according to reports, it still remains poorly understood about the molecular mechanisms underlying analgesic effects of IGLR. The aim of the present study was to investigate the analgesic effect of IGLR on a spared nerve injury (SNI) model of neuropathic pain.

MATERIALS AND METHODS

The SNI model in rats was established by complete transection of the common peroneal and tibial distal branches of the sciatic nerve, leaving the sural branch intact. Then SNI rats were treated with IGLR for 14 days, using normal saline as the negative control. The paw withdrawal mechanical threshold (PMWT) in response to mechanical stimulation was measured by von Frey filaments on day 1 before operation and on days 1, 3, 5, 7, 9, 11, 13 and 14 after operation, respectively. After 14 days, the levels of nitric oxide (NO), nitric oxide synthase (NOS), tumor necrosis factor-α (TNF-α), interleukin-1β (IL-1β), interleukin-10 (IL-10) and cyclic guanosine monophosphate (cGMP) in the spinal dorsal horn were measured by the corresponding kits, mRNA expression of inducible NOS (iNOS) and protein kinase G type I (PKGI) of spinal cord were analyzed by reverse-transcription polymerase chain reaction (RT-PCR). The expression of N-methyl-D-aspartate receptor (NMDAR) and protein kinase C (PKCγ) of the spinal dorsal horn was performed by Western blot. Before all the experiments, motor coordination performance and locomotor activity had been tested.

RESULTS

Our results showed that remarkable mechanical allodynia was observed on day 1 after operation in the SNI model, which was accompanied by a decrease in PMWT. Treatment with IGLR (200, 400, 800mg/kg) significantly alleviated SNI-induced mechanical allodynia, markedly decreased the levels of NO, NOS, TNF-α, IL-1β and cGMP, and increased the level of IL-10. Meanwhile, IGLR (200, 400, 800mg/kg) also inhibited the protein expression of NMDAR, PKCγ and the mRNA expression of iNOS and PKGΙ in the spinal cord. In addition, gavage with the IGLR aqueous extract (800mg/kg) did not signifiantly alter motor coordination or locomotor activity.

CONCLUSIONS

These results indicated IGLR could produce an anti-neuropathic pain effect that might partly be related to the inhibition of the NO/cGMP/PKG and NMDAR/PKC pathways and the level of TNF-α, IL-1β as well as to the increase of the level of IL-10 in spinal cord.

Interleukin-1 beta enhances endocytosis of glial glutamate transporters in the spinal dorsal horn through activating protein kinase C

Excessive activation of glutamate receptors in spinal dorsal horn neurons is a key mechanism leading to abnormal neuronal activation in pathological pain conditions. Previous studies have shown that activation of glutamate receptors in the spinal dorsal horn is enhanced by impaired glial glutamate transporter functions and proinflammatory cytokines including interleukin-1 beta (IL-1β). In this study, we for the first time revealed that spinal glial glutamate transporter activities in the neuropathic animals are attenuated by endogenous IL-1β. Specifically, we demonstrated that nerve injury results in an increased expression of IL-1β and activation of PKC in the spinal dorsal horn as well as suppression of glial glutamate uptake activities. We provided evidence that the nerve-injury induced suppression of glial glutamate uptake is at least in part ascribed to endogenous IL-1β and activation of PKC in the spinal dorsal horn. IL-1β reduces glial glutamate transporter activities through enhancing the endocytosis of both GLT-1 and GLAST glial glutamate transporters. The IL-1β induced trafficking of glial glutamate transporters is through the calcium/PKC signaling pathway, and the dynamin-dependent endocytosis, which is dependent on the integrity of actin filaments. The signaling pathway regulating glial glutamate transporters revealed in this study provides novel targets to attenuate aberrant activation of glutamate receptors in the spinal dorsal horn, which could ultimately help the development of analgesics.

Decreased expression and role of GRK6 in spinal cord of rats after chronic constriction injury

Nerve injury and inflammation can both induce neuropathic pain via the production of pro-inflammatory cytokines. In the process, G protein-coupled receptors (GPCRs) were involved in pain signal transduction. GPCR kinase (GRK) 6 is a member of the GRK family that regulates agonist-induced desensitization and signaling of GPCRs. However, its expression and function in neuropathic pain have not been reported. In this study, we performed a chronic constriction injury (CCI) model in adult male rats and investigated the dynamic change of GRK6 expression in spinal cord. GRK6 was predominantly expressed in the superficial layers of the lumbar spinal cord dorsal horn neurons and its expression was decreased bilaterally following induction of CCI. The changes of GRK6 were mainly in IB4 and P substrate positive areas in spinal cord dorsal horn. And over-expression of GRK6 in spinal cord by lentivirus intrathecal injection attenuated the pain response induced by CCI. In addition, the level of TNF-α underwent the negative pattern of GRK6 in spinal cord. And neutralized TNF-α by antibody intrathecal injection up-regulated GRK6 expression and attenuated the mechanical allodynia and heat hyperalgesia in CCI model. All the data indicated that down-regulation of neuronal GRK6 expression induced by cytokine may be a potential mechanism that contributes to increasing neuronal signaling in neuropathic pain.

Time-dependent cross talk between spinal serotonin 5-HT2A receptor and mGluR1 subserves spinal hyperexcitability and neuropathic pain after nerve injury

Emerging evidence implicates serotonergic descending facilitatory pathways from the brainstem to the spinal cord in the maintenance of pathologic pain. Upregulation of the serotonin receptor 2A (5-HT(2A)R) in dorsal horn neurons promotes spinal hyperexcitation and impairs spinal μ-opioid mechanisms during neuropathic pain. We investigated the involvement of spinal glutamate receptors, including metabotropic receptors (mGluRs) and NMDA, in 5-HT(2A)R-induced hyperexcitability after spinal nerve ligation (SNL) in rat. High-affinity 5-HT(2A)R agonist (4-bromo-3,6-dimethoxybenzocyclobuten-1-yl)methylamine hydrobromide (TCB-2) enhanced C-fiber-evoked dorsal horn potentials after SNL, which was prevented by mGluR1 antagonist AIDA [(RS)-1-aminoindan-1,5-dicarboxylic acid] but not by group II mGluR antagonist LY 341495 [(2S)-2-amino-2-[(1S,2S)-2-carboxycycloprop-1-yl]-3-(xanth-9-yl)propanoic acid] or NMDA antagonist d-AP5 [D-(-)-2-amino-5-phosphonopentanoic acid]. 5-HT(2A)R and mGluR1 were found to be coexpressed in postsynaptic densities in dorsal horn neurons. In the absence of SNL, pharmacological stimulation of 5-HT(2A)R with TCB-2 both induced rapid bilateral upregulation of mGluR1 expression in cytoplasmic and synaptic fractions of spinal cord homogenates, which was attenuated by PKC inhibitor chelerythrine, and enhanced evoked potentials during costimulation of mGluR1 with 3,5-DHPG [(RS)-3,5-dihydroxyphenylglycine]. SNL was followed by bilateral upregulation of mGluR1 in 5-HT(2A)R-containing postsynaptic densities. Upregulation of mGluR1 in synaptic compartments was partially prevented by chronic administration of selective 5-HT(2A)R antagonist M100907 [(R)-(+)-α-(2,3-dimethoxyphenyl)-1-[2-(4-fluorophenyl)ethyl]-4-pipidinemethanol], confirming 5-HT(2A)R-mediated control of mGluR1 upregulation triggered by SNL. Changes in thermal and mechanical pain thresholds following SNL were increasingly reversed over the days after injury by chronic 5-HT(2A)R blockade. These results emphasize a role for 5-HT(2A)R in hyperexcitation and pain after nerve injury and support mGluR1 upregulation as a novel feedforward activation mechanism contributing to 5-HT(2A)R-mediated facilitation.

Central nociceptive sensitization vs. spinal cord training: opposing forms of plasticity that dictate function after complete spinal cord injury

The spinal cord demonstrates several forms of plasticity that resemble brain-dependent learning and memory. Among the most studied form of spinal plasticity is spinal memory for noxious (nociceptive) stimulation. Numerous papers have described central pain as a spinally-stored memory that enhances future responses to cutaneous stimulation. This phenomenon, known as central sensitization, has broad relevance to a range of pathological conditions. Work from the spinal cord injury (SCI) field indicates that the lumbar spinal cord demonstrates several other forms of plasticity, including formal learning and memory. After complete thoracic SCI, the lumbar spinal cord can be trained by delivering stimulation to the hindleg when the leg is extended. In the presence of this response-contingent stimulation the spinal cord rapidly learns to hold the leg in a flexed position, a centrally mediated effect that meets the formal criteria for instrumental (response-outcome) learning. Instrumental flexion training produces a central change in spinal plasticity that enables future spinal learning on both the ipsilateral and contralateral leg. However, if stimulation is given in a response-independent manner, the spinal cord develops central maladaptive plasticity that undermines future spinal learning on both legs. The present paper tests for interactions between spinal cord training and central nociceptive sensitization after complete spinal cord transection. We found that spinal training alters future central sensitization by intradermal formalin (24 h post-training). Conversely intradermal formalin impaired future spinal learning (24 h post-injection). Because formalin-induced central sensitization has been shown to involve NMDA receptor activation, we tested whether pre-treatment with NMDA would also affect spinal learning in manner similar to formalin. We found intrathecal NMDA impaired learning in a dose-dependent fashion, and that this effect endures for at least 24 h. These data provide strong evidence for an opposing relationship between nociceptive plasticity and use-dependent learning in the spinal cord. The present work has clinical implications given recent findings that adaptive spinal training improves recovery in humans with SCI. Nociception below the SCI may undermine this rehabilitation potential.

Peripheral nerve injury increases glutamate-evoked calcium mobilization in adult spinal cord neurons

BACKGROUND

Central sensitization in the spinal cord requires glutamate receptor activation and intracellular Ca2+ mobilization. We used Fura-2 AM bulk loading of mouse slices together with wide-field Ca2+ imaging to measure glutamate-evoked increases in extracellular Ca2+ to test the hypotheses that: 1. Exogenous application of glutamate causes Ca2+ mobilization in a preponderance of dorsal horn neurons within spinal cord slices taken from adult mice; 2. Glutamate-evoked Ca2+ mobilization is associated with spontaneous and/or evoked action potentials; 3. Glutamate acts at glutamate receptor subtypes to evoked Ca2+ transients; and 4. The magnitude of glutamate-evoked Ca2+ responses increases in the setting of peripheral neuropathic pain.

RESULTS

Bath-applied glutamate robustly increased [Ca2+]i in 14.4 ± 2.6 cells per dorsal horn within a 440 x 330 um field-of-view, with an average time-to-peak of 27 s and decay of 112 s. Repeated application produced sequential responses of similar magnitude, indicating the absence of sensitization, desensitization or tachyphylaxis. Ca2+ transients were glutamate concentration-dependent with a Kd = 0.64 mM. Ca2+ responses predominantly occurred on neurons since: 1) Over 95% of glutamate-responsive cells did not label with the astrocyte marker, SR-101; 2) 62% of fura-2 AM loaded cells exhibited spontaneous action potentials; 3) 75% of cells that responded to locally-applied glutamate with a rise in [Ca2+]i also showed a significant increase in AP frequency upon a subsequent glutamate exposure; 4) In experiments using simultaneous on-cell recordings and Ca2+ imaging, glutamate elicited a Ca2+ response and an increase in AP frequency. AMPA/kainate (CNQX)- and AMPA (GYKI 52466)-selective receptor antagonists significantly attenuated glutamate-evoked increases in [Ca2+]i, while NMDA (AP-5), kainate (UBP-301) and class I mGluRs (AIDA) did not. Compared to sham controls, peripheral nerve injury significantly decreased mechanical paw withdrawal threshold and increased glutamate-evoked Ca2+ signals.

CONCLUSIONS

Bulk-loading fura-2 AM into spinal cord slices is a successful means for determining glutamate-evoked Ca2+ mobilization in naïve adult dorsal horn neurons. AMPA receptors mediate the majority of these responses. Peripheral neuropathic injury potentiates Ca2+ signaling in dorsal horn.

The antinociceptive activity of harmicine on chemical-induced neurogenic and inflammatory pain models in mice

Harmicine is a β-carboline alkaloid isolated and identified as a major active compound present in many plant species and marine invertebrates. This alkaloid exhibits a wide spectrum of pharmacological activities, including antispasmodic, antipyretic, and anticancer properties. This report described the antinociceptive properties of harmicine by means of chemical experimental models in order to evaluate the use for pain relief. The results demonstrating the potential analgesic properties of harmicine administered intraperitoneally were shown with the writhing test, reducing writhes around 60% (1 mg/kg), and in the formalin test, where harmicine was more effective toward neurogenic (reducing reaction time around 60%, 1 mg/kg) than inflammatory (68% reduction, 10 mg/kg) pain responses. Furthermore, these effects may operate via vanilloid receptors as revealed by the capsaicin test (41% reduction, with 3 mg/kg), as well as via peripheral glutamate receptors as shown by the glutamate test (50% reduction, with 1 mg/kg). Moreover, the opioid antagonist naloxone hydrochloride did not interfere in the antinociceptive properties of harmicine in the writhing test, revealing that this effect may not have a relationship with the opioid systems. Concluding, this report highlights harmicine as a new candidate to be used as analgesic in the future. Therefore, further studies are being undertaken in order to understand the exact mechanisms involved with the antinociceptive properties of harmicine.

Somatosensory scaffolding structures

Dynamic changes in somatosensory perception occur as a result of multiple signaling events. In many instances, over-activation of sensory receptors results in the desensitization and subsequent increased threshold for activation of receptors. In other cases, receptor sensitization can occur following tissue injury and/or inflammation. In both cases, signaling mechanisms that control alterations in receptor activities can significantly affect organism response to sensory stimuli, including thermal, mechanical, and chemical. Due to the homeostatic nature of somatosensory recognition, dynamic changes in receptor response can negatively affect an individual's way of life, as well as alert individuals to tissue damage. Here, we will focus on scaffolding structures that regulate somatosensory neuronal excitability.

Involvement of subtypes γ and ε of protein kinase C in colon pain induced by formalin injection

Protein kinase C (PKC) has been widely reported to participate in somatic pain; however, its role in visceral pain remains largely unclear. Using a colon inflammatory pain model by intracolonic injection of formalin in rats, the present study was to examine the role of PKC in visceral pain and determine which subtypes may be involved. The colon pain behavior induced by formalin injection could be enhanced by intrathecal pretreatment with a PKC activator (PMA), and alleviated by a PKC inhibitor (H-7). Wide dynamic range (WDR) neurons in the L6-S1 spinal dorsal horn that were responsive to colorectal distension were recorded extracellularly. It was found that neuronal activity was greatly increased following formalin injection. Microdialysis of PMA near the recorded neuron in the spinal dorsal horn facilitated the enhanced responsive activity induced by formalin injection, while H-7 inhibited significantly the enhanced response induced by formalin injection. Western blot analysis revealed that membrane translocation of PKC-γ and PKC-∊, but not other subtypes, in the spinal cord was obviously increased following formalin injection. Therefore, our findings suggest that PKC is actively involved in the colon pain induced by intracolonic injection of formalin. PKC-γ and PKC-∊ subtypes seem to significantly contribute to this process.

A diminished response to formalin stimulation reveals a role for the glutamate transporters in the altered pain sensitivity of mice with experimental autoimmune encephalomyelitis (EAE)

Multiple Sclerosis (MS) is an inflammatory demyelinating disease of the central nervous system (CNS) in which neuropathic pain is now recognized as a major symptom. To date, few studies have examined the underlying mechanisms of neuropathic pain in MS. Recently we showed that in a chronic-relapsing animal model of MS, experimental autoimmune encephalomyelitis (EAE), characteristic neuropathic behaviours develop. However, responses to persistent noxious stimuli in EAE remain unexplored. We, therefore set out to characterize the changes in pain sensitivity in our EAE model to subcutaneous injection of formalin. We show here that female C57BL/6 mice immunized with myelin oligodendrocyte glycoprotein (MOG(35-55)) display a significant decrease in elicited pain behaviours in response to formalin injection. These effects were found to involve dysregulation of the glutamatergic system in EAE. We show here that these effects are mediated by decreased glutamate transporter expression associated with EAE. Our findings demonstrate that dysregulation of glutamate transporter function in EAE mice is an important mechanism underlying the abnormal pain sensitivity in response to persistent noxious stimulation of mice with EAE and also sheds light on a potential mechanism underlying neuropathic pain behaviours in this model.

Sensory stimulation prior to spinal cord injury induces post-injury dysesthesia in mice

Chronic pain and dysesthesias are debilitating conditions that can arise following spinal cord injury (SCI). Research studies frequently employ rodent models of SCI to better understand the underlying mechanisms and develop better treatments for these phenomena. While evoked withdrawal tests can assess hypersensitivity in these SCI models, there is little consensus over how to evaluate spontaneous sensory abnormalities that are seen in clinical SCI subjects. Overgrooming (OG) and biting after peripheral nerve injury or spinal cord excitotoxic lesions are thought to be one behavioral demonstration of spontaneous neuropathic pain or dysesthesia. However, reports of OG after contusion SCI are largely anecdotal and conditions causing this response are poorly understood. The present study investigated whether repeated application of sensory stimuli to the trunk prior to mid-thoracic contusion SCI would induce OG after SCI in mice. One week prior to SCI or laminectomy, mice were subjected either to nociceptive and mechanical stimulation, mechanical stimulation only, the testing situation without stimulation, or no treatment. They were then examined for 14 days after surgery and the sizes and locations of OG sites were recorded on anatomical maps. Mice subjected to either stimulus paradigm showed increased OG compared with unstimulated or uninjured mice. Histological analysis showed no difference in spinal cord lesion size due to sensory stimulation, or between mice that overgroomed or did not overgroom. The relationship between prior stimulation and contusion injury in mice that display OG indicates a critical interaction that may underlie one facet of spontaneous neuropathic symptoms after SCI.

MK-801 changes the role of glutamic acid on modulation of algesia in nucleus accumbens

Dizocilpine maleate (MK-801) causes the blockage of the glutamic acid (Glu) receptors in the central nervous system that are involved in pain transmission. However, the mechanism of action of MK-801 in pain-related neurons is not clear, and it is still unknown whether Glu is involved in the modulation of this processing. This study examines the effect of MK-801, Glu on the pain-evoked response of pain-excitation neurons (PENs) and pain-inhibition neurons (PINs) in the nucleus accumbens (NAc) of rats. The trains of electric impulses applied to the sciatic nerve were used as noxious stimulation. The electrical activities of PENs or PINs in NAc were recorded by a glass microelectrode. Our results revealed that the lateral ventricle injection of Glu increased the discharged frequency and shortened the discharged latency of PEN, and decreased the discharged frequency and prolonged the discharged inhibitory duration (ID) of PIN in NAc of rats evoked by the noxious stimulation, while intra-NAc administration of MK-801 produced the opposite response. On the basis of above findings we can deduce that Glu, MK-801 and N-methyl-D-aspartate (NMDA) receptor are involved in the modulation of nociceptive information transmission in NAc.

Involvement of the spinal NMDA receptor/PKCγ signaling pathway in the development of bone cancer pain

N-methyl-d-aspartate (NMDA) receptor and protein kinase C (PKC) play important roles in the induction and maintenance of central sensitization during pain states. It has been shown that spinal NMDA receptor-dependent activation of PKCgamma facilitates nociception during neuropathic and inflammatory pain, but its involvement in bone cancer pain has not previously been established. The aim of this study was to examine the potential role of the spinal NMDA receptor/PKCgamma signaling pathway in the development of bone cancer pain. Osteosarcoma NCTC 2472 cells were implanted into the intramedullary space of the right femurs of C3H/HeJ mice to induce ongoing bone cancer-related pain behaviors. At day 7, 10 and 14 after operation, the expression of PKCgamma mRNA in the spinal cord was higher in tumor-bearing mice compared to the sham mice. At day 14, intrathecal administration of 5 microg of NR2B subunit-specific NMDA receptor antagonist ifenprodil attenuated the up-regulation of PKCgamma mRNA in the spinal cord as well as bone cancer-evoked thermal hyperalgesia and mechanical allodynia. Furthermore, intrathecal injection of 10 microg of PKC inhibitor H-7 attenuated cancer-evoked thermal hyperalgesia and mechanical allodynia at day 14. These results suggest that the NMDA receptor/PKCgamma signaling pathway may participate in the development of bone cancer pain, and ifenprodil may be a useful alternative or adjunct therapy for bone cancer pain.

Analysis of the mechanism of antinociceptive action of niga-ichigoside F1 obtained from Rubus imperialis (Rosaceae)

We have previously verified that niga-ichigoside F1 (NI), a triterpene isolated from Rubus imperialis, exhibits significant and potent antinociceptive action when evaluated in some pharmacological models of pain in mice. This effect was confirmed in other experimental models and also the mechanism of action has been evaluated. The antinociception caused by NI (60 mg kg−1) in both phases of the formalin test was significantly attenuated by intraperitoneal injection of mice with haloperidol (a dopaminergic antagonist, 0.20 mg kg−1) and L-arginine (precursor of nitric oxide, 600 mg kg−1). Regarding the cholinergic system, atropine (a cholinergic antagonist 60 mg kg−1) reverted only the second phase. The effect of NI was not affected by treatment of mice with yohimbine (an alpha2-adrenoceptor antagonist, 0.15 mg kg−1). The same pharmacological profile was observed for the administration of naloxone (an opioid receptor antagonist, 1 mg kg−1). On the other hand, intraperitoneal injection caused dose-related and significant effects against glutamate- and capsaicin-induced pain, respectively. In conclusion, the marked antinociception of NI appears to be related to the dopaminergic, cholinergic, glutamatergic, tachykininergic and oxinitrergic systems, supporting the ethnomedical use of Rubus imperialis (Rosaceae).

Antinociceptive action of limonexic acid obtained from Raulinoa echinata

The antinociceptive effect of the limonexic acid isolate of Raulinoa echinata Cowan in four models of pain in mice is described. When evaluated against acetic acid-induced abdominal constrictions, limonexic acid (10, 30 and 60 mg kg−1, i.p.) produced dose-related inhibition of the number of constrictions, with a mean ID50 value of 43 (2.3–79) μmol kg−1, and was more potent than some standard drugs. In the formalin test, limonexic acid inhibited both the first and second phases of formalin-induced pain. Furthermore, the effect was more pronounced in the second phase, with a mean ID50 value of 13.66 (9.35–19.61) μmol kg−1, and had a pharmacological profile that was similar to standard drugs such as acetaminophen and acetyl salicylic acid. Limonexic acid also produced dose-related inhibition of glutamate- and capsaicin-induced pain, with mean ID50 values of 11.67 (8.51–16.0) μmol kg−1 and 47.17 (36.51–60.93) μmol kg−1, respectively. The mechanism of action is not completely understood, but seems to involve direct interaction with the GABAergic and nitroxidergic pathways.

Anti-inflammatory and antinociceptive effects of Baccharis dracunculifolia DC (Asteraceae) in different experimental models

AIM OF THE STUDY

The aerial parts of Baccharis dracunculifolia D.C., popularly known as "alecrim do campo", are used in folk medicine as anti-inflammatory. The aim of the present study was to evaluate the anti-inflammatory and antinociceptive activities of the crude hydroalcoholic extract obtained from leaves of Baccharis dracunculifolia (BdE), which have not been reported.

MATERIALS AND METHODS

BdE was analyzed by HPLC and in vivo evaluated (doses ranging from 50 to 400mg/kg, p.o.) by using the acetic acid-induced abdominal constrictions, paw oedema induced by carrageenan or histamine, overt nociception models using capsaicin, glutamate or phorbol myristate acetate (PMA), formalin-induced nociception and mechanical hypernociception induced by carrageenan or complete Freund adjuvant (CFA). As positive controls it was used paracetamol in both acetic acid and formalin tests; dipyrone in capsaicin, glutamate and PMA-induced nociception; indomethacin in CFA and carrageenan-induced hypernociception models. In addition, the in vitro effects of BdE on COX-2 activity and on the activation of NF-kappaB were also evaluated.

RESULTS

BdE (50-400mg/kg, p.o.) significantly diminished the abdominal constrictions induced by acetic acid, glutamate and CFA. Furthermore, BdE also inhibited the nociceptive responses in both phases of formalin-induced nociception. BdE, administered orally, also produced a long-lasting anti-hypernociceptive effect in the acute model of inflammatory pain induced by carrageenan. It was also observed the inhibition of COX-2 activity by BdE.

CONCLUSION

In summary, the data reported in this work confirmed the traditional anti-inflammatory indications of Baccharis dracunculifolia leaves and provided biological evidences that Baccharis dracunculifolia, like Brazilian green propolis, possess antinociceptive and anti-inflammatory activities.

Central sensitization: a generator of pain hypersensitivity by central neural plasticity

Central sensitization represents an enhancement in the function of neurons and circuits in nociceptive pathways caused by increases in membrane excitability and synaptic efficacy as well as to reduced inhibition and is a manifestation of the remarkable plasticity of the somatosensory nervous system in response to activity, inflammation, and neural injury. The net effect of central sensitization is to recruit previously subthreshold synaptic inputs to nociceptive neurons, generating an increased or augmented action potential output: a state of facilitation, potentiation, augmentation, or amplification. Central sensitization is responsible for many of the temporal, spatial, and threshold changes in pain sensibility in acute and chronic clinical pain settings and exemplifies the fundamental contribution of the central nervous system to the generation of pain hypersensitivity. Because central sensitization results from changes in the properties of neurons in the central nervous system, the pain is no longer coupled, as acute nociceptive pain is, to the presence, intensity, or duration of noxious peripheral stimuli. Instead, central sensitization produces pain hypersensitivity by changing the sensory response elicited by normal inputs, including those that usually evoke innocuous sensations.

PERSPECTIVE

In this article, we review the major triggers that initiate and maintain central sensitization in healthy individuals in response to nociceptor input and in patients with inflammatory and neuropathic pain, emphasizing the fundamental contribution and multiple mechanisms of synaptic plasticity caused by changes in the density, nature, and properties of ionotropic and metabotropic glutamate receptors.

Contralateral high or a combination of high- and low-frequency transcutaneous electrical nerve stimulation reduces mechanical allodynia and alters dorsal horn neurotransmitter content in neuropathic rats

The purpose of the study was to examine the effect of 3 different application strategies for transcutaneous electrical nerve stimulation (TENS) on neuropathy-induced allodynia and dorsal horn neurotransmitter content. Rats were treated with high-frequency, low-frequency, or a combination of high and low-frequency stimulation. TENS was delivered through self-adhesive electrodes daily for 1 hour to rats with a right-sided chronic constriction injury (CCI). Stimulation was delivered to skin or acupuncture points on the left and mechanical and thermal pain thresholds were assessed in the right hind paw. Neurotransmitter content was assessed bilaterally in the dorsal horn of the spinal cord. Daily, high-frequency or a combination of high- and low-frequency TENS reduced mechanical (P < .001), but not thermal allodynia in the right hind paw when compared with untreated CCI rats. Daily high frequency TENS elevated the dorsal horn synaptosomal content of GABA bilaterally (P < .014) and a combination of high- and low-frequency TENS elevated the dorsal horn content of aspartate (P < .001), glutamate (P < .001) and glycine (P < .001) bilaterally over that seen in untreated CCI rats. The present findings support a contralateral approach to the application of TENS and suggest that distinct strategies for TENS application may differentially alter neurotransmission in the central nervous system.

PERSPECTIVE

Because CCI rats are reminiscent of humans with neuropathy, daily high or a combination of high- and low-frequency TENS may reduce mechanical allodynia in humans with neuropathic pain. Because the 2 intervention strategies produce distinctive alterations in spinal cord neurotransmitter content, each may represent a distinctive option for treatment.

Expression and distribution of 'high affinity' glutamate transporters GLT1, GLAST, EAAC1 and of GCPII in the rat peripheral nervous system

l-Glutamate is one of the major excitatory neurotransmitters in the mammalian central nervous system, but recently it has been shown to have a role also in the transduction of sensory input at the periphery, and in particular in the nociceptive pathway. An excess of glutamate is implicated in cases of peripheral neuropathies as well. Conventional therapeutic approaches for treating these diseases have focused on blocking glutamate receptors with small molecules or on reducing its synthesis of the receptors through the inhibition of glutamate carboxypeptidase II (GCPII), the enzyme that generates glutamate. In vivo studies have demonstrated that the pharmacological inhibition of GCPII can either prevent or treat the peripheral nerve changes in both BB/Wor and chemically induced diabetes in rats. In this study, we characterized the expression and distribution of glutamate transporters GLT1, GLAST, EAAC1 and of the enzyme GCPII in the peripheral nervous system of female Wistar rats. Immunoblotting results demonstrated that all glutamate transporters and GCPII are present in dorsal root ganglia (DRG) and the sciatic nerve. Immunofluorescence localization studies revealed that both DRG and sciatic nerves were immunopositive for all glutamate transporters and for GCPII. In DRG, satellite cells were positive for GLT1 and GCPII, whereas sensory neurons were positive for EAAC1. GLAST was localized in both neurons and satellite cells. In the sciatic nerve, GLT1 and GCPII were expressed in the cytoplasm of Schwann cells, whereas GLAST and EAAC1 stained the myelin layer. Our results give for the first time a complete characterization of the glutamate transporter system in the peripheral nervous system. Therefore, they are important both for understanding glutamatergic signalling in the PNS and for establishing new strategies to treat peripheral neuropathies.

Spinal Metabotropic Glutamate Receptors (mGluRs) are Involved in the Melittin-induced Nociception in Rats

Intraplantar injection of melittin has been known to induce sustained decrease of mechanical threshold and increase of spontaneous flinchings. The present study was undertaken to investigate how the melittin-induced nociceptive responses were modulated by changes of metabotropic glutamate receptor (mGluR) activity. Changes in paw withdrawal threshold (PWT), number of flinchings and paw thickness were measured at a given time point after injection of melittin (10 microg/paw) into the mid-plantar area of rat hindpaw. To observe the effects of mGluRs on the melittin-induced nociceptions, group I mGluR (AIDA, 100 microg and 200 microg), mGluR(1) (LY367385, 50 microg and 100 microg) and mGluR(5) (MPEP, 200 microg and 300 microg) antagonists, group II (APDC, 100 microg and 200 microg) and III (L-SOP, 100 microg and 200 microg) agonists were intrathecally administered 20 min before melittin injection. Intraplantar injection of melittin induced a sustained decrease of mechanical threshold, spontaneous flinchings and edema. The effects of melittin to reduce mechanical threshold and to induce spontaneous flinchings were significantly suppressed following intrathecal pre-administration of group I mGluR, mGluR(1) and mGluR(5) antagonists, group II and III mGluR agonists. Group I mGluR antagonists and group II and III mGluR agonists had no significant effect on melittin-induced edema. These experimental findings indicate that multiple spinal mGluRs are involved in the modulation of melittin-induced nociceptive responses.

Quercetin: further investigation of its antinociceptive properties and mechanisms of action

The antinociceptive action of quercetin, a common bioactive flavonoid present in many medicinal plants, was assessed in different models of chemical and thermal nociception in mice. Quercetin (10-60 mg/kg, i.p. or 100-500 mg/kg, p.o.) dose-dependently inhibited nociceptive behavior in the acetic acid-induced pain test. Moreover, quercetin (10-60 mg/kg, i.p.) inhibited both phases of formalin-induced pain, with ID50 values of 374.1 (68.0-402.0) mmol/kg and 103.0 (45.0-201.0) mmol/kg, for the neurogenic and inflammatory phases, respectively. Quercetin (10-60 mg/kg) also inhibited the nociception induced by glutamate and capsaicin by 68.2% and 75.5%, respectively. Its analgesic action was significantly reversed by p-chlorophenylalanine methyl ester, katanserin, methysergide, a GABA(A) antagonist (bicuculline), or a GABA(B) antagonists (baclofen). Its action was also modulated by tachykinins, but was not affected by adrenal-gland hormones. Furthermore, the antinociceptive effects did not result from muscle-relaxant or sedative action. Together, these results indicate that quercetin produces dose-related anti-nociception in several models of chemical pain, through mechanisms that involve interaction with L-arginine-nitric oxide, serotonin, and GABAergic systems. These results confirm and extend other investigations on the analgesic effect of quercetin and its mechanisms of action.

Spread of excitation across modality borders in spinal dorsal horn of neuropathic rats

Under physiological conditions, nociceptive information is mainly processed in superficial laminae of the spinal dorsal horn, whereas non-nociceptive information is processed in deeper laminae. Neuropathic pain patients often suffer from touch-evoked pain (allodynia), suggesting that modality borders are disrupted in their nervous system. We studied whether excitation evoked in deep dorsal horn neurons either via stimulation of primary afferent Abeta-fibres, by direct electrical stimulation or via glutamate microinjection leads to activation of neurons in the superficial dorsal horn. We used Ca(2+)-imaging in transversal spinal cord slices of neuropathic and control animals to monitor spread of excitation from the deep to the superficial spinal dorsal horn. In neuropathic but not control animals, a spread of excitation occurred from the deep to the superficial dorsal horn. The spread of excitation was synaptically mediated as it was blocked by the AMPA receptor antagonist CNQX. In contrast, block of NMDA receptors was ineffective. In control animals, the violation of modality borders could be reproduced by bath application of GABA(A) and glycine receptor antagonists. Furthermore, we could show that neuropathic animals were more prone to synchronous network activity than control animals. Thus, following peripheral nerve injury, excitation generated in dorsal horn areas which process non-nociceptive information can invade superficial dorsal horn areas which normally receive nociceptive input. This may be a spinal mechanism of touch-evoked pain.

IL-1 beta signaling is required for mechanical allodynia induced by nerve injury and for the ensuing reduction in spinal cord neuronal GRK2

Many neurotransmitters involved in pain perception transmit signals via G protein-coupled receptors (GPCRs). GPCR kinase 2 (GRK2) regulates agonist-induced desensitization and signaling of multiple GPCRs and interacts with downstream molecules with consequences for signaling. In general, low GRK2 levels are associated with increased responses to agonist stimulation of GPCRs. Recently, we reported that in mice with reduced GRK2 levels, inflammation-induced mechanical allodynia was increased. In addition, mice with impaired interleukin (IL)-1 beta signaling did not develop mechanical allodynia after L5 spinal nerve transection (SNT). We hypothesized that in the L5 SNT model mechanical allodynia would be associated with reduced neuronal GRK2 levels in the spinal cord dorsal horn and that IL-1 beta signaling would be required to induce both the decrease in GRK2 and mechanical allodynia. We show here that in wild type (WT) mice L5 SNT induces a bilateral decrease in neuronal GRK2 expression in the lumbar spinal cord dorsal horn, 1 and 2 weeks after L5 SNT. No changes in GRK2 were observed in the thoracic segments. Moreover, spinal cord GRK2 expression was not decreased in IL-1R(-/-) mice after L5 SNT. These data show that IL-1 beta signaling is not only required for the development of mechanical allodynia, but also to reduce neuronal GRK2 expression. These results suggest a functional relation between the L5 SNT-induced IL-1 beta-mediated decrease in GRK2 and development of mechanical allodynia.

Commonly used preclinical models of pain

Pain cuts across gender, age, and disease and is the most common reason people seek health-related treatment. Certain pain states do not respond to standard therapies, leaving nurses with few options to successfully care for patients in pain. Preclinical studies use many models to investigate the mechanisms and treatments for pain states similar to those encountered in humans. Within Cervero and Laird's conceptual framework of experimental pain, the authors present several commonly used preclinical models. Phase 1 pain models measure responses to a brief, controlled stimulus that minimizes tissue damage. Phase 2 pain models use topical or injected irritants, which cause inflammation and persistent pain that change peripheral and central neuronal responses. Phase 3 pain models produce neuropathic pain through partial or complete ligation of peripheral nerves. The use of preclinical models of pain can lead to the development of better pharmacological and nonpharmacological therapies that enhance nursing practice.

Activin acts with nerve growth factor to regulate calcitonin gene-related peptide mRNA in sensory neurons

Calcitonin gene-related peptide (CGRP) increases in sensory neurons after inflammation and plays an important role in abnormal pain responses, but how this neuropeptide is regulated is not well understood. Both activin A and nerve growth factor (NGF) increase in skin after inflammation and induce CGRP in neurons in vivo and in vitro. This study was designed to understand how neurons integrate these two signals to regulate the neuropeptide important for inflammatory pain. In adult dorsal root ganglion neurons, NGF but not activin alone produced a dose-dependent increase in CGRP mRNA. When added together with NGF, activin synergistically increased CGRP mRNA, indicating that sensory neurons combine these signals. Studies were then designed to learn if that combination occurred at a common receptor or shared intracellular signals. Studies with activin IB receptor or tyrosine receptor kinase A inhibitors suggested that each ligand required its cognate receptor to stimulate the neuropeptide. Further, activin did not augment NGF-initiated intracellular mitogen-activated protein kinase signals but instead stimulated Smad phosphorylation, suggesting these ligands initiated parallel signals in the cytoplasm. Activin synergy required several NGF intracellular signals to be present. Because activin did not further stimulate, but did require NGF intracellular signals, it appears that activin and NGF converge not in receptor or cytoplasmic signals, but in transcriptional mechanisms to regulate CGRP in rat sensory neurons after inflammation.

Enhanced 3,5-dihydroxyphenylglycine-induced sustained nociceptive behaviors in rats with neuropathy or chronic inflammation

Sustained nociceptive behaviors (SNBs) are an important but under-studied component of chronic pain conditions. The group I metabotropic glutamate receptor (mGluR) agonist (R,S)-3,5-dihydroxyphenylglycine (DHPG) produces SNBs when injected intrathecally, and group I mGluR antagonists are effective at reducing symptoms of neuropathic and inflammatory pain. The present experiments examined whether rats with sciatic nerve injury or persistent inflammation exhibit greater SNBs following intrathecal DHPG compared with control animals. SNBs were observed following intrathecal injection of DHPG (25 nmol) between the L4 and L5 vertebrae. We used a behavioral observation scoring system that allowed for assessment of specific behaviors in the hind paws. When DHPG was injected intrathecally in rats with chronic constriction injury (CCI) of the sciatic nerve, they showed increased paw stamping behavior compared to DHPG-injected sham controls. Rats treated with complete Freund's adjuvant (CFA)-induced inflammation failed to demonstrate a significant increase in paw stamping behavior. However, both CCI and CFA rats showed increased paw licking and biting of the neuropathic/inflamed hind paw after intrathecal DHPG injection. These results provide evidence for behaviorally relevant contributions of group I mGluRs to SNBs in models of neuropathic and inflammatory pain.