Differential activation of p38 and extracellular signal-regulated kinase in spinal cord in a model of bee venom-induced inflammation and hyperalgesia

A Single Injection of rAAV-shmTOR in Peripheral Nerve Persistently Attenuates Nerve Injury-Induced Mechanical Allodynia

Activation of mammalian target of rapamycin (mTOR) has been known as one of the contributing factors in nociceptive sensitization after peripheral injury. Its activation followed by the phosphorylation of downstream effectors causes hyperexcitability of primary sensory neurons in the dorsal root ganglion. We investigated whether a single injection of rAAV-shmTOR would effectively downregulate both complexes of mTOR in the long-term and glial activation as well. Male SD rats were categorized into shmTOR (n = 29), shCON (n = 23), SNI (n = 13), and Normal (n = 8) groups. Treatment groups were injected with rAAV-shmTOR or rAAV-shCON, respectively. DRG tissues and sciatic nerve were harvested for Western blot and immunohistochemical analyses. Peripheral sensitization was gradually attenuated in the shmTOR group, and it reached a peak on PID 21. Western blot analysis showed that both p-mTORC1 and p-mTORC2 were downregulated in the DRG compared to shCON and SNI groups. We also found decreased expression of phosphorylated p38 and microglial activation in the DRG. We first attempted a therapeutic strategy for neuropathic pain with a low dose of AAV injection by interfering with the mTOR signaling pathway, suggesting its potential application in pain treatment.

Analgesic Effects of Fisetin, Peimine, Astaxanthin, Artemisinin, Bardoxolone Methyl and 740 Y-P and Their Influence on Opioid Analgesia in a Mouse Model of Neuropathic Pain

Treatment of neuropathic pain remains a challenge for modern medicine due to the insufficiently understood molecular mechanisms of its development and maintenance. One of the most important cascades that modulate the nociceptive response is the family of mitogen-activated protein (MAP) kinases and phosphatidylinositol-3-kinase (PI3K), as well as nuclear factor erythroid 2-related factor 2 (Nrf2). The aim of this study was to determine the effect of nonselective modulators of MAP kinases-fisetin (ERK1/2 and NFκB inhibitor, PI3K activator), peimine (MAPK inhibitor), astaxanthin (MAPK inhibitor, Nrf2 activator) and artemisinin (MAPK inhibitor, NFκB activator), as well as bardoxolone methyl (selective activator of Nrf2) and 740 Y-P (selective activator of PI3K)-in mice with peripheral neuropathy and to compare their antinociceptive potency and examine their effect on analgesia induced by opioids. The study was performed using albino Swiss male mice that were exposed to chronic constriction injury of the sciatic nerve (CCI model). Tactile and thermal hypersensitivity was measured using von Frey and cold plate tests, respectively. Single doses of substances were administered intrathecally on day 7 after CCI. Among the tested substances, fisetin, peimine, and astaxanthin effectively diminished tactile and thermal hypersensitivity in mice after CCI, while artemisinin did not exhibit analgesic potency in this model of neuropathic pain. Additionally, both of the activators tested, bardoxolone methyl and 740 Y-P, also showed analgesic effects after intrathecal administration in mice exposed to CCI. In the case of astaxanthin and bardoxolone methyl, an increase in analgesia after combined administration with morphine, buprenorphine, and/or oxycodone was observed. Fisetin and peimine induced a similar effect on tactile hypersensitivity, where analgesia was enhanced after administration of morphine or oxycodone. In the case of 740 Y-P, the effects of combined administration with each opioid were observed only in the case of thermal hypersensitivity. The results of our research clearly indicate that substances that inhibit all three MAPKs provide pain relief and improve opioid effectiveness, especially if they additionally block NF-κB, such as peimine, inhibit NF-κB and activate PI3K, such as fisetin, or activate Nrf2, such as astaxanthin. In light of our research, Nrf2 activation appears to be particularly beneficial. The abovementioned substances bring promising results, and further research on them will broaden our knowledge regarding the mechanisms of neuropathy and perhaps contribute to the development of more effective therapy in the future.

Rapamycin reduces orofacial nociceptive responses and microglial p38 mitogen-activated protein kinase phosphorylation in trigeminal nucleus caudalis in mouse orofacial formalin model

The mammalian target of rapamycin (mTOR) plays a role in various cellular phenomena, including autophagy, cell proliferation, and differentiation. Although recent studies have reported its involvement in nociceptive responses in several pain models, whether mTOR is involved in orofacial pain processing is currently unexplored. This study determined whether rapamycin, an mTOR inhibitor, reduces nociceptive responses and the number of Fos-immunoreactive (Fos-ir) cells in the trigeminal nucleus caudalis (TNC) in a mouse orofacial formalin model. We also examined whether the glial cell expression and phosphorylated p38 (p-p38) mitogen-activated protein kinases (MAPKs) in the TNC are affected by rapamycin. Mice were intraperitoneally given rapamycin (0.1, 0.3, or 1.0 mg/kg); then, 30 min after, 5% formalin (10 µl) was subcutaneously injected into the right upper lip. The rubbing responses with the ipsilateral forepaw or hindpaw were counted for 45 min. High-dose rapamycin (1.0 mg/kg) produced significant antinociceptive effects in both the first and second phases of formalin test. The number of Fos-ir cells in the ipsilateral TNC was also reduced by high-dose rapamycin compared with vehicle-treated animals. Furthermore, the number of p-p38-ir cells the in ipsilateral TNC was significantly decreased in animals treated with high-dose rapamycin; p-p38 expression was co-localized in microglia, but not neurons and astrocytes. Therefore, the mTOR inhibitor, rapamycin, reduces orofacial nociception and Fos expression in the TNC, and its antinociceptive action on orofacial pain may be associated with the inhibition of p-p38 MAPK in the microglia.

Spinophilin modulates pain through suppressing dendritic spine morphogenesis via negative control of Rac1-ERK signaling in rat spinal dorsal horn

Both spinophilin (SPN, also known as neurabin 2) and Rac1 (a member of Rho GTPase family) are believed to play key roles in dendritic spine (DS) remodeling and spinal nociception. However, how SPN interacts with Rac1 in the above process is unknown. Here, we first demonstrated natural existence of SPN-protein phosphatase 1-Rac1 complex in the spinal dorsal horn (DH) neurons by both double immunofluorescent labeling and co-immunoprecipitation, then the effects of SPN over-expression and down-regulation on mechanical and thermal pain sensitivity, GTP-bound Rac1-ERK signaling activity, and spinal DS density were studied. Over-expression of SPN in spinal neurons by intra-DH pAAV-CMV-SPN-3FLAG could block both mechanical and thermal pain hypersensitivity induced by intraplantar bee venom injection, however it had no effect on the basal pain sensitivity. Over-expression of SPN also resulted in a significant decrease in GTP-Rac1-ERK activities, relative to naive and irrelevant control (pAAV-MCS). In sharp contrast, knockdown of SPN in spinal neurons by intra-DH pAAV-CAG-eGFP-U6-shRNA[SPN] produced both pain hypersensitivity and dramatic elevation of GTP-Rac1-ERK activities, relative to naive and irrelevant control (pAAV-shRNA [NC]). Moreover, knockdown of SPN resulted in increase in DS density while over-expression of it had no such effect. Collectively, SPN is likely to serve as a regulator of Rac1 signaling to suppress DS morphogenesis via negative control of GTP-bound Rac1-ERK activities at postsynaptic component in rat DH neurons wherein both mechanical and thermal pain sensitivity are controlled.

p38 mitogen-activated protein kinase and pain

Inflammatory and neuropathic pain is initiated by tissue inflammation and nerve injury, respectively. Both are characterized by increased activity in the peripheral and central nervous system, where multiple inflammatory cytokines and other active molecules activate different signaling pathways that involve in the development and/or maintenance of pain. P38 mitogen-activated protein kinase (MAPK) is one member of the MAPK family, which is activated in neurons and glia and contributes importantly to inflammatory and neuropathic pain. The aim of this review is to summarize the latest advances made about the implication of p38 MAPK signaling cascade in pain. It can deepen our understanding of the molecular mechanisms of pain and may help to offer new targets for pain treatment.

Inhibition Of Monocarboxylate Transporter 1 In Spinal Cord Horn Significantly Reverses Chronic Inflammatory Pain

Purpose

Chronic inflammatory pain is a common condition in the clinic, and the underlying mechanism is not being completely understood. Various studies have demonstrated that central and peripheral sensitization and synaptic plasticity could play crucial functions in chronic inflammatory pain. Moreover, families of monocarboxylate transporters (MCTs) are closely related to cellular metabolism and synaptic plasticity, and it is also reported that MCTs participate in chronic inflammatory pain. Nevertheless, there is a probability of the engaging role of MCT 1 is in chronic inflammatory pain, but its specific cellular level mechanism is yet to be investigated. In our study, we hypothesized that MCT 1 in the spinal dorsal horn plays an important part in chronic inflammatory pain.

Methods

In experiment A, rats were gone through nociceptive behavioral testing at 1 d day before and 1 d, 3 d, and 7 d after completing complete Freund’s adjuvant (CFA) injection. The specimens collected for detecting MCT 1 by Western blotting. In experiment B, rats were randomly divided into four groups. Intrathecal injection of MCT 1 inhibitor and nociceptive behavioral tests were performed 1 d day before and 1 d, 3 d, 7 d, 14 d, and 21 d after CFA injection. MCT 1 and p-ERK levels in spinal dorsal horn were measured by Western blotting, and GFAP in spinal dorsal horn was detected by immunofluorescence.

Results

The expression of MCT 1 in the spinal dorsal horn was increased during chronic inflammatory pain in rats. The intrathecal injection of an MCT 1 inhibitor evidently diminished the expression of MCT 1 and GFAP in the spinal dorsal horn, and the behavioral nociceptive responses were also attenuated. Meanwhile, the expression of p-ERK was also decreased by the intrathecal injection of an MCT 1 inhibitor.

Conclusion

Our results indicate that MCT 1 very likely play a critical role in regulating chronic inflammatory pain and may influence the regulation of synaptic plasticity via ERK in the spinal dorsal horn of rats.

Electroacupuncture Inhibits the Activation of p38MAPK in the Central Descending Facilitatory Pathway in Rats with Inflammatory Pain

The mitogen-activated protein kinases (MAPKs), especially p38MAPK, play a pivotal role in chronic pain. Electroacupuncture (EA) relieves inflammatory pain underlying the descending pathway, that is, the periaqueductal gray (PAG), the rostral ventromedial medulla (RVM), and the spinal cord dorsal horn (SCDH). However, whether EA antagonizes inflammatory pain through regulation of p38MAPK in this descending facilitatory pathway is unclear. Complete Freund's adjuvant (CFA) was injected into the hind paw of rats to establish inflammatory pain model. EA was administrated for 30 min at Zusanli and Kunlun acupoints at 0.5, 24.5, 48.5, and 72.5 h, respectively. The paw withdrawal threshold (PWT), paw edema, and Phosphor-p38MAPK-Immunoreactivity (p-p38MAPK-IR) cells were measured before (0 h) and at 1, 3, 5, 7, 25, and 73 h after CFA or saline injection. EA increased PWT at 1, 3, 25, and 73 h and inhibited paw edema at 25 and 73 h after CFA injection. Moreover, the increasing number of p-p38MAPK-IR cells which was induced by CFA was suppressed by EA stimulation in PAG and RVM at 3 and 5 h and in SCDH at 5, 7, 25, and 73 h. These results suggest that EA suppresses inflammation-induced hyperalgesia probably through inhibiting p38MAPK activation in the descending facilitatory pathway.

ERK potentiates p38 in central sensitization induced by traumatic occlusion

This study was to investigate the role of p38 activation via ERK1/2 phosphorylation in neurons and microglia of the spinal trigeminal subnucleus caudalis (Vc) in the promotion of orofacial hyperalgesia induced by unilateral anterior crossbite (UAC) traumatic occlusion in adult rats. U0126, a p-ERK1/2 inhibitor, was injected intracisternally before UAC implant. The effects of the U0126 injection were compared to those following the injection of SB203580, a p-p38 inhibitor. Mechanical hyperalgesia was evaluated via pressure pain threshold measurements. Brain stem tissues were processed for a Western blot analysis to evaluate the activation of ERK1/2 and p38. Double immunofluorescence was also performed to observe the expression of p-ERK1/2 and p-p38 in neurons (labeled by NeuN) and microglia (labeled by OX42). The data showed that UAC caused orofacial hyperalgia ipsilaterally on d1 to d7, peaking on d3 (P<0.05). An upregulation of p-ERK1/2 was observed in the ipsilateral Vc on d1 to d3, peaking on d1. An upregulation of p-p38 was also observed on d1 to d7, peaking on d3 (P<0.05). p-ERK1/2 primarily co-localized with NeuN and, to a lesser extent, with OX42, while p-p38 co-localized with both NeuN and OX42. Pretreatment with U0126 prevented the upregulation of both p-ERK1/2 and p-p38. Similarly to an intracisternal injection of SB203580, U0126 pretreatment attenuated the UAC-induced orofacial hyperalgesia. These data indicate that UAC caused orofacial hyperalgesia by inducing central sensitization via the activation of ERK1/2 and p38 in both neurons and microglia in the Vc, potentially impacting the effects of p-ERK1/2 during p38 activation.

Involvement of Rac1 signalling pathway in the development and maintenance of acute inflammatory pain induced by bee venom injection

BACKGROUND AND PURPOSE

The Rho GTPase, Rac1, is involved in the pathogenesis of neuropathic pain induced by malformation of dendritic spines in the spinal dorsal horn (sDH) neurons. In the present study, the contribution of spinal Rac1 to peripheral inflammatory pain was studied.

EXPERIMENTAL APPROACH

Effects of s.c. bee venom (BV) injection on cellular localization of Rac1 in the rat sDH was determined with double labelling immunofluorescence. Activation of Rac1 and its downstream effector p21-activated kinase (PAK), ERKs and p38 MAPK in inflammatory pain states was evaluated with a pull-down assay and Western blotting. The preventive and therapeutic analgesic effects of intrathecal administration of NSC23766, a selective inhibitor of Rac1, on BV-induced spontaneous nociception and pain hypersensitivity were investigated.

KEY RESULTS

Rac1 labelling was mainly localized within neurons in both the superficial and deep layers of the sDH in rats of naïve, vehicle-treated and inflamed (BV injected) groups. GTP-Rac1-PAK and ERKs/p38 were activated following s.c. BV injection. Post-treatment with intrathecal NSC23766 significantly inhibited GTP-Rac1 activity and phosphorylation of Rac1-PAK, ERKs and p38 MAPK in the sDH. Both pre-treatment and post-treatment with intrathecal NSC23766 dose-dependently attenuated the paw flinches, primary thermal and mechanical hyperalgesia and the mirror-image thermal hyperalgesia induced by BV injection, but without affecting the baseline pain sensitivity and motor coordination.

CONCLUSIONS AND IMPLICATIONS

The spinal GTP-Rac1-PAK-ERK/p38MAPK signalling pathway is involved in both the development and maintenance of peripheral inflammatory pain and can be used as a potential molecular target for developing a novel therapeutic strategy for clinical pain.

Transient receptor potential ankyrin 1 that is induced in dorsal root ganglion neurons contributes to acute cold hypersensitivity after oxaliplatin administration

BACKGROUND

Peripheral cold neuropathic pain is a serious side effect of oxaliplatin treatment. However, the mechanism of oxaliplatin-induced cold hyperalgesia is unknown. In the present study, we investigated the effects of oxaliplatin on transient receptor potential ankyrin 1 (TRPA1) in dorsal root ganglion (DRG) neurons of rats.

RESULTS

Behavioral assessment using the acetone spray test showed that 3 and 6 mg/kg oxaliplatin (i.p.) induced acute cold hypersensitivity after 1, 2, 4, and 7 days. Real-time PCR showed that oxaliplatin (6 mg/kg) significantly increased TRPA1 mRNA expression in DRGs at days 1, 2, and 4. Western blotting revealed that oxaliplatin significantly increased TRPA1 protein expression in DRGs at days 2, 4, and 7. Moreover, in situ hybridization histochemistry revealed that most TRPA1 mRNA-labeled neurons in the DRGs were small in size. Oxaliplatin significantly increased co-localization of TRPA1 expression and isolectin B4 binding in DRG neurons. Oxaliplatin induced a significant increase in the percent of TRPA1 mRNA-positive small neurons in DRGs at days 1, 2, and 4. In addition, we found that intrathecal administration of TRPA1 antisense, but not TRPA1 mismatched oligodeoxynucleotides, knocked down TRPA1 expression and decreased oxaliplatin-induced cold hyperalgesia. Double labeling showed that p-p38 mitogen-activated protein kinase (MAPK) was co-expressed in TRPA1 mRNA-labeled neurons at day 2 after oxaliplatin administration. Intrathecal administration of the p38 MAPK inhibitor, SB203580, significantly decreased oxaliplatin-induced acute cold hypersensitivity.

CONCLUSIONS

Together, these results demonstrate that TRPA1 expression via activation of p38 MAPK in DRG neurons, at least in part, contributes to the development of oxaliplatin-induced acute cold hyperalgesia.

ERK1/2: Function, signaling and implication in pain and pain-related anxio-depressive disorders

Despite the increasing knowledge regarding pain modulation, the understanding of the mechanisms behind a complex and pathologic chronic pain condition is still insufficient. These knowledge gaps might result in ineffective therapeutic approaches to relieve painful sensations. As a result, severe untreated chronic pain frequently triggers the onset of new disorders such as depression and/or anxiety, and therefore, both the diagnosis and treatment of patients suffering from chronic pain become seriously compromised, prompting a self-perpetuating cycle of symptomatology. The extracellular signal-regulated kinases 1 and 2 (ERK1/2) are molecules strongly implicated in the somatic component of pain at the spinal cord level and have been emerging as mediators of the emotional-affective component as well. Although these molecules might represent good biomarkers, their use as pharmacological targets is still open to discussion as paradoxical information has been obtained. Here we review the current scientific literature regarding ERK1/2 signaling in the modulation of pain, depression and anxiety, including the emotional-affective spheres of the pain experience.

The effects of bee venom acupuncture on the central nervous system and muscle in an animal hSOD1G93A mutant

Amyotrophic lateral sclerosis (ALS) is caused by the degeneration of lower and upper motor neurons, leading to muscle paralysis and respiratory failure. However, there is no effective drug or therapy to treat ALS. Complementary and alternative medicine (CAM), including acupuncture, pharmacopuncture, herbal medicine, and massage is popular due to the significant limitations of conventional therapy. Bee venom acupuncture (BVA), also known as one of pharmacopunctures, has been used in Oriental medicine to treat inflammatory diseases. The purpose of this study is to investigate the effect of BVA on the central nervous system (CNS) and muscle in symptomatic hSOD1^G93A transgenic mice, an animal model of ALS. Our findings show that BVA at ST36 enhanced motor function and decreased motor neuron death in the spinal cord compared to that observed in hSOD1^G93A transgenic mice injected intraperitoneally (i.p.) with BV. Furthermore, BV treatment at ST36 eliminated signaling downstream of inflammatory proteins such as TLR4 in the spinal cords of symptomatic hSOD1^G93A transgenic mice. However, i.p. treatment with BV reduced the levels of TNF-α and Bcl-2 expression in the muscle hSOD1^G93A transgenic mice. Taken together, our findings suggest that BV pharmacopuncture into certain acupoints may act as a chemical stimulant to activate those acupoints and subsequently engage the endogenous immune modulatory system in the CNS in an animal model of ALS.

Acupuncture: Emerging evidence for its use as an analgesic (Review)

Acupuncture is an ancient Chinese technique, developed over >3,000 years, in which 'acupoints' are stimulated with the aim of treating various diseases. A number of previous studies have indicated that acupuncture may play a role in inducing analgesia. Acupuncture-induced analgesia has been hypothesized to act on various parts of the central nervous system, including the spinal cord, brain stem, cerebral ganglia and cerebral cortex. The mechanisms underlying the effects of acupuncture have been purported to include neurohumors and neurotransmitters, such as opioids and γ-aminobutyric acid, signaling pathways and the immune response, which are all involved in the induction of analgesia.

Selective prevention of mechanical hyperalgesia after incision by spinal ERK1/2 inhibition

BACKGROUND

Activation of extracellular signal-regulated kinases (ERK1/2) has been shown to play an important role in several pain states. Here we investigated the ERK1/2 contribution to non-evoked and evoked pain-like behaviour in rats after surgical incision.

METHODS

Spinal phosphorylation of ERK1 and ERK2 was assessed 15 min, 4 h, 24 h and 5 days after plantar incision and sham incision. The effect of PD98059, a specific inhibitor of ERK1/2 activation, administered intrathecally (IT) 1 h before or 2 h after incision on spinal ERK1 and ERK2 phosphorylation was assessed. In behavioural experiments, the effect of PD98059 administered 1 h before or after incision on non-evoked pain behaviour and mechanical and heat hyperalgesia was assessed.

RESULTS

Phosphorylated ERK1 and ERK2 were rapidly increased in the ipsilateral dorsal horn from rats after incision post-operatively. This increased ERK1 and ERK2 phosphorylation were blocked by PD98059 administered before incision. In congruence, IT administration of PD98059 before incision delayed mechanical hyperalgesia after incision; however, administration after incision had only a modest effect on mechanical hyperalgesia. In addition, PD98059 did not affect non-evoked pain behaviour or heat hyperalgesia after incision.

CONCLUSION

The results suggest that spinal ERK1 and ERK2 are involved in regulation of pain after incision differentially with regard to the pain modality. Furthermore, blockade of ERK1/2 activation was most effective in a preventive manner, a condition which is rare after incision. Spinal ERK1/2 inhibition could therefore be a very useful tool to manage selectively movement-evoked pain after surgery in the future.

Angiotensin (1-7) prevents angiotensin II-induced nociceptive behaviour via inhibition of p38 MAPK phosphorylation mediated through spinal Mas receptors in mice

BACKGROUND

We have recently demonstrated that intrathecal (i.t.) administration of angiotensin II (Ang II) induces nociceptive behaviour in mice accompanied by a phosphorylation of p38 mitogen-activated protein kinase (MAPK) mediated through Ang II type 1 (AT1 ) receptors. The N-terminal fragment of Ang II, Ang (1-7), plays a pivotal role in counterbalancing many of the well-established actions induced by Ang II. However, the role of Ang (1-7) in spinal nociceptive transmission remains unclear. Therefore, we examined whether i.t. administration of Ang (1-7) can inhibit the Ang II-induced nociceptive behaviour in mice.

METHODS

In the behavioural experiments, the accumulated response time of nociceptive behaviour consisting of scratching, biting and licking in conscious mice was determined during a 25-min period starting after i.t. injection. The distribution and localization of AT1 or Mas receptors were analysed using a MapAnalyzer and confocal microscope, respectively. Phosphorylation of p38 MAPK in the dorsal spinal cord was measured by Western blotting.

RESULTS

The nociceptive behaviour induced by Ang II was dose-dependently inhibited by the co-administration of Ang (1-7). The inhibitory effect of Ang (1-7) was reversed by the co-administration of A779, a Mas receptor antagonist. Western blot analysis showed that the increase in spinal p38 MAPK phosphorylation following the i.t. administration of Ang II was also inhibited by Ang (1-7), and the Ang (1-7) induced-inhibition was prevented by A779.

CONCLUSIONS

Our data show that the i.t. administration of Ang (1-7) attenuates an Ang II-induced nociceptive behaviour and is accompanied by the inhibition of p38 MAPK phosphorylation mediated through Mas receptors.

Chronic tooth pulp inflammation induces persistent expression of phosphorylated ERK (pERK) and phosphorylated p38 (pp38) in trigeminal subnucleus caudalis

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Chronic inflammation of tooth pulp activates pERK and pp38 in the trigeminal nucleus

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Activation is persistent and bilateral, and further increased by acute stimulation

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This altered signaling may be relevant in the development of chronic pulpitic pain

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pERK and pp38 are more sensitive markers of central change than Fos expression

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Sequential activation in different cell types may be linked to pain progression

Background

Extracellular signal-regulated kinase (ERK) and p38 mitogen-activated protein kinase are transiently phosphorylated (activated) in the spinal cord and trigeminal nucleus by acute noxious stimuli. Acute stimulation of dental pulp induces short-lived ERK activation in trigeminal subnucleus caudalis (Vc), and p38 inhibition attenuates short-term sensitization in Vc induced by acute pulpal stimulation. We have developed a model to study central changes following chronic inflammation of dental pulp that induces long-term sensitization. Here, we examine the effects of chronic inflammation and acute stimulation on the expression of phosphorylated ERK (pERK), phosphorylated p38 (pp38) and Fos in Vc.

Results

Chronic inflammation alone induced bilateral expression of pERK and pp38 in Vc, but did not induce Fos expression. Stimulation of both non-inflamed and inflamed pulps significantly increased pERK and pp38 bilaterally; expression was greatest in inflamed, stimulated animals, and was similar following 10-min and 60-min stimulation. Stimulation for 60 min, but not 10 min, induced Fos in ipsilateral Vc; Fos expression was significantly greater in inflamed, stimulated animals. pERK was present in both neurons and astrocytes; pp38 was present in neurons and other non-neuronal, non-astrocytic cell types.

Conclusions

This study provides the first demonstration that chronic inflammation of tooth pulp induces persistent bilateral activation of ERK and p38 within Vc, and that this activation is further increased by acute stimulation. This altered activity in intracellular signaling is likely to be linked to the sensitization that is seen in our animal model and in patients with pulpitis. Our data indicate that pERK and pp38 are more accurate markers of central change than Fos expression. In our model, localization of pERK and pp38 within specific cell types differs from that seen following acute stimulation. This may indicate specific roles for different cell types in the induction and maintenance of pulpitic and other types of pain.

Blockade of peripheral P2Y1 receptors prevents the induction of thermal hyperalgesia via modulation of TRPV1 expression in carrageenan-induced inflammatory pain rats: involvement of p38 MAPK phosphorylation in DRGs

Although previous reports have suggested that P2Y1 receptors (P2Y1Rs) are involved in cutaneous nociceptive signaling, it remains unclear how P2Y1Rs contribute to peripheral sensitization. The current study was designed to delineate the role of peripheral P2Y1Rs in pain and to investigate potential linkages to mitogen-activated protein kinase (MAPK) in DRGs and Transient Receptor Potential Vanilloid 1 (TRPV1) expression in a rodent inflammatory pain model. Following injection of 2% carrageenan into the hind paw, expressions of P2Y1 and TRPV1 and the phosphorylation rates of both p38 MAPK and ERK but not JNK were increased and peaked at day 2 post-injection. Blockade of peripheral P2Y1Rs by the P2Y1R antagonist, MRS2500 injection (i.pl, D0 to D2) significantly reduced the induction of thermal hyperalgesia, but not mechanical allodynia. Simultaneously, MRS2500 injections suppressed upregulated TRPV1 expression and DRG p38 phosphorylation, while pERK signaling was not affected. Furthermore, inhibition of p38 activation in the DRGs by SB203580 (a p38 inhibitor, i.t, D0 to D2) prevented the upregulation of TRPV1 and a single i.t injection of SB203580 reversed the established thermal hyperalgesia, but not mechanical allodynia. Lastly, to identify the mechanism of action of P2Y1Rs, we repeatedly injected the P2Y1 agonist, MRS2365 into the naïve rat's hind paw and observed a dose-dependent increase in TRPV1 expression and p38 MAPK phosphorylation. These data demonstrate a sequential role for P2Y1R, p38 MAPK and TRPV1 in inflammation-induced thermal hyperalgesia; thus, peripheral P2Y1Rs activation modulates p38 MAPK signaling and TRPV1 expression, which ultimately leads to the induction of thermal hyperalgesia.

Angiotensin II produces nociceptive behavior through spinal AT1 receptor-mediated p38 mitogen-activated protein kinase activation in mice

Background

It has been demonstrated that angiotensin II (Ang II) participates in either the inhibition or the facilitation of nociceptive transmission depending on the brain area. Neuronal Ang II is locally synthesized not only in the brain, but also in the spinal cord. Though the spinal cord is an important area for the modulation of nociception, the role of spinal Ang II in nociceptive transmission remains unclear. Therefore, in order to elucidate the role of Ang II in nociceptive transmission in the spinal cord, we examined the effect of intrathecal (i.t.) administration of Ang II into mice.

Results

I.t. administration of Ang II produced a behavioral response in mice mainly consisting of biting and/or licking of the hindpaw and the tail along with slight hindlimb scratching directed toward the flank. The behavior induced by Ang II (3 pmol) was dose-dependently inhibited by intraperitoneal injection of morphine (0.1-0.3 mg/kg), suggesting that the behavioral response is related to nociception. The nociceptive behavior was also inhibited dose-dependently by i.t. co-administration of losartan (0.3-3 nmol), an Ang II type 1 (AT₁) receptor antagonist, and SB203580 (0.1-1 nmol), a p38 MAPK inhibitor. However, the Ang II type 2 (AT₂) receptor antagonist PD123319, the upstream inhibitor of ERK1/2 phosphorylation U0126, and the JNK inhibitor SP600125 had no effect on Ang II-induced nociceptive behavior. Western blot analysis showed that the i.t. injection of Ang II induced phosphorylation of p38 MAPK in the lumbar dorsal spinal cord, which was inhibited by losartan, without affecting ERK1/2 and JNK. Furthermore, we found that AT₁ receptor expression was relatively high in the lumbar superficial dorsal horn.

Conclusions

Our data show that i.t. administration of Ang II induces nociceptive behavior accompanied by the activation of p38 MAPK signaling mediated through AT₁ receptors. This observation indicates that Ang II may act as a neurotransmitter and/or neuromodulator in the spinal transmission of nociceptive information.

Intervention of electroacupuncture on spinal p38 MAPK/ATF-2/VR-1 pathway in treating inflammatory pain induced by CFA in rats

Background

Previous studies have demonstrated that p38 MAPK signal transduction pathway plays an important role in the development and maintenance of inflammatory pain. Electroacupuncture (EA) can suppress the inflammatory pain. However, the relationship between EA effect and p38 MAPK signal transduction pathway in inflammatory pain remains poorly understood. It is our hypothesis that p38 MAPK/ATF-2/VR-1 and/or p38 MAPK/ATF-2/COX-2 signal transduction pathway should be activated by inflammatory pain in CFA-injected model. Meanwhile, EA may inhibit the activation of p38 MAPK signal transduction pathway. The present study aims to investigate that anti-inflammatory and analgesic effect of EA and its intervention on the p38 MAPK signal transduction pathway in a rat model of inflammatory pain.

Results

EA had a pronounced anti-inflammatory and analgesic effect on CFA-induced chronic inflammatory pain in rats. EA could quickly raise CFA-rat’s paw withdrawal thresholds (PWTs) and maintain good and long analgesic effect, while it subdued the ankle swelling of CFA rats only at postinjection day 14. EA could down-regulate the protein expressions of p-p38 MAPK and p-ATF-2, reduced the numbers of p-p38 MAPK-IR cells and p-ATF-2-IR cells in spinal dorsal horn in CFA rats, inhibited the expressions of both protein and mRNA of VR-1, but had no effect on the COX-2 mRNA expression.

Conclusions

The present study indicates that inhibiting the activation of spinal p38 MAPK/ATF-2/VR-1 pathway may be one of the main mechanisms via central signal transduction pathway in the process of anti-inflammatory pain by EA in CFA rats.

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.

Inhibition of p38-MAPK signaling pathway attenuates breast cancer induced bone pain and disease progression in a murine model of cancer-induced bone pain

BACKGROUND

Mechanisms driving cancer-induced bone pain are poorly understood. A central factor implicated to be a key player in the process of tumorigenesis, osteoclastogenesis and nociception is p38 MAPK. We determined the role of p38 MAPK in a mouse model of breast cancer induced bone pain in which mixed osteolytic and osteoblastic remodeling occurs.

RESULTS

In cancer-treated mice, acute as well as chronic inhibition of p38 MAPK with SB203580 blocked flinching and guarding behaviors in a dose-dependent manner whereas no effect on thresholds to tactile stimuli was observed. Radiographic analyses of bones demonstrated that chronic inhibition of p38 MAPK reduced bone loss and incidence of spontaneous fracture in cancer-treated mice. Histological analysis of bones collected from mice treated with the p38 MAPK inhibitor showed complete absence of osteoblastic growth in the intramedullary space as well as significantly reduced tumor burden.

CONCLUSIONS

Blockade of non-evoked pain behaviors but not hypersensitivity suggests differences in the underlying mechanisms of specific components of the pain syndrome and a possibility to individualize aspects of pain management. While it is not known whether the role of p38 MAPK signaling can be expanded to other cancers, the data suggest a need for understanding molecular mechanisms and cellular events that initiate and maintain cancer-induced bone pain for effective management for both ongoing pain as well as breakthrough pain.

Effect of Electroacupuncture on Activation of p38MAPK in Spinal Dorsal Horn in Rats with Complete Freund's Adjuvant-Induced Inflammatory Pain

Activation of mitogen-activated protein kinases (MAPKs), especially p38 MAPK, plays an important role in the development of central sensitization related to persistent inflammatory pain. Electroacupuncture (EA) is well known to relieve persistent inflammatory pain. However, little is known about relationship between EA and p38 MAPK. Inflammatory pain rat model was induced by intraplantar injection of complete Freund's adjuvant (CFA). Male adult SD rats were randomly divided into the saline group, CFA group, and CFA + EA group. EA (constant saquare wave, 2 Hz and 100 Hz alternating frequencies, intensities ranging from 1 to 2 mA) was applied to bilateral “Zusanli” (ST 36) and “Kunlun” acupoints (BL 60) for 30 min, once per day. The paw edema and paw withdrawal threshold (PWT) were measured at preinjection and days postinjection 1, 3, and 14. Spinal p-p38MAPK- immunoreactivty (p-p38MAPK-IR) cells were detected by immunohistochemistry at postinjection day 3 and 14. EA significantly inhibited paw edema at postinjection days 14 and increased PWT at postinjection days 3 and 14. Moreover, the increasing number of spinal p-p38MAPK-IR cells which was induced by CFA injection was suppressed by EA stimulation. These results indicate that anti-inflammatory and analgesic effect of EA might be associated with its inhibition of spinal p38 MAPK activation and thereby provide a potential mechanism for the treatment of inflammatory pain by EA.

The nociceptive and anti-nociceptive effects of bee venom injection and therapy: a double-edged sword

Bee venom injection as a therapy, like many other complementary and alternative medicine approaches, has been used for thousands of years to attempt to alleviate a range of diseases including arthritis. More recently, additional theraupeutic goals have been added to the list of diseases making this a critical time to evaluate the evidence for the beneficial and adverse effects of bee venom injection. Although reports of pain reduction (analgesic and antinociceptive) and anti-inflammatory effects of bee venom injection are accumulating in the literature, it is common knowledge that bee venom stings are painful and produce inflammation. In addition, a significant number of studies have been performed in the past decade highlighting that injection of bee venom and components of bee venom produce significant signs of pain or nociception, inflammation and many effects at multiple levels of immediate, acute and prolonged pain processes. This report reviews the extensive new data regarding the deleterious effects of bee venom injection in people and animals, our current understanding of the responsible underlying mechanisms and critical venom components, and provides a critical evaluation of reports of the beneficial effects of bee venom injection in people and animals and the proposed underlying mechanisms. Although further studies are required to make firm conclusions, therapeutic bee venom injection may be beneficial for some patients, but may also be harmful. This report highlights key patterns of results, critical shortcomings, and essential areas requiring further study.

Peripheral formalin injury induces 2 stages of microglial activation in the spinal cord

The formalin test produces 2 well-known acute phases of nociceptive behavior. Recently, we have shown that this same formalin test produces a third phase of nociceptive behavior consisting of prolonged thermal and mechanical hyperalgesia beginning days after formalin injection and lasting for at least 3 weeks. Here we investigated the activity of 3 MAPKs (p38, ERK and JNK) in the spinal dorsal horn following 5% formalin injection into rat hind paw. The p38 MAPK was rapidly activated in the spinal microglia minutes after injection and the activation persisted for 1 hour. In addition, this same injury induced a secondary increase of phospho-p38 expression in spinal microglia that was maximal 3 to 7 days postinjection. Intrathecal administration of p38 inhibitor SB203580 not only inhibited the early acute spontaneous nociceptive behaviors, but also inhibited the long-term formalin injury-induced mechanical hyperalgesia. Our results suggest that peripheral formalin injection induces 2 stages of microglial activation, and p38 activation in spinal microglia plays key roles in central pain modulation in formalin test respectively for the early acute phases and the late secondary long-term pain state as well.

PERSPECTIVE

This article presents unique properties of spinal microglial activation in a pain animal model. This finding could potentially help clinicians to further understand the contributions of spinal microglia to acute and chronic pain state.

p38 mediates mechanical allodynia in a mouse model of type 2 diabetes

BACKGROUND

Painful Diabetic Neuropathy (PDN) affects more than 25% of patients with type 2 diabetes; however, the pathogenesis remains unclear due to lack of knowledge of the molecular mechanisms leading to PDN. In our current study, we use an animal model of type 2 diabetes in order to understand the roles of p38 in PDN. Previously, we have demonstrated that the C57BLK db/db (db/db) mouse, a model of type 2 diabetes that carries the loss-of-function leptin receptor mutant, develops mechanical allodynia in the hind paws during the early stage (6-12 wk of age) of diabetes. Using this timeline of PDN, we can investigate the signaling mechanisms underlying mechanical allodynia in the db/db mouse.

RESULTS

We studied the role of p38 in lumbar dorsal root ganglia (LDRG) during the development of mechanical allodynia in db/db mice. p38 phosphorylation was detected by immunoblots at the early stage of mechanical allodynia in LDRG of diabetic mice. Phosphorylated p38 (pp38) immunoreactivity was detected mostly in the small- to medium-sized LDRG neurons during the time period of mechanical allodynia. Treatment with an antibody against nerve growth factor (NGF) significantly inhibited p38 phosphorylation in LDRG of diabetic mice. In addition, we detected higher levels of inflammatory mediators, including cyclooxygenase (COX) 2, inducible nitric oxide synthases (iNOS), and tumor necrosis factor (TNF)-alpha in LDRG neurons of db/db mice compared to non-diabetic db+ mice. Intrathecal delivery of SB203580, a p38 inhibitor, significantly inhibited the development of mechanical allodynia and the upregulation of COX2, iNOS and TNF-alpha.

CONCLUSIONS

Our findings suggest that NGF activated-p38 phosphorylation mediates mechanical allodynia in the db/db mouse by upregulation of multiple inflammatory mediators in LDRG.

Roles of the hippocampal formation in pain information processing

Pain is a complex experience consisting of sensory-discriminative, affective-motivational, and cognitive-evaluative dimensions. Now it has been gradually known that noxious information is processed by a widely-distributed, hierarchically- interconnected neural network, referred to as neuromatrix, in the brain. Thus, identifying the multiple neural networks subserving these functional aspects and harnessing this knowledge to manipulate the pain response in new and beneficial ways are challenging tasks. Albeit with elaborate research efforts on the cortical responses to painful stimuli or clinical pain, involvement of the hippocampal formation (HF) in pain is still a matter of controversy. Here, we integrate previous animal and human studies from the viewpoint of HF and pain, sequentially representing anatomical, behavioral, electrophysiological, molecular/biochemical and functional imaging evidence supporting the role of HF in pain processing. At last, we further expound on the relationship between pain and memory and present some unresolved issues.

Effects of estrogens and bladder inflammation on mitogen-activated protein kinases in lumbosacral dorsal root ganglia from adult female rats

Background

Interstitial cystitis is a chronic condition associated with bladder inflammation and, like a number of other chronic pain states, symptoms associated with interstitial cystitis are more common in females and fluctuate during the menstrual cycle. The aim of this study was to determine if estrogens could directly modulate signalling pathways within bladder sensory neurons, such as extracellular signal-related kinase (ERK) and p38 mitogen-activated protein (MAP) kinases. These signalling pathways have been implicated in neuronal plasticity underlying development of inflammatory somatic pain but have not been as extensively investigated in visceral nociceptors. We have focused on lumbosacral dorsal root ganglion (DRG) neurons projecting to pelvic viscera (L1, L2, L6, S1) of adult female Sprague-Dawley rats and performed both in vitro and in vivo manipulations to compare the effects of short- and long-term changes in estrogen levels on MAPK expression and activation. We have also investigated if prolonged estrogen deprivation influences the effects of lower urinary tract inflammation on MAPK signalling.

Results

In studies of isolated DRG neurons in short-term (overnight) culture, we found that estradiol and estrogen receptor (ER) agonists rapidly stimulated ER-dependent p38 phosphorylation relative to total p38. Examination of DRGs following chronic estrogen deprivation in vivo (ovariectomy) showed a parallel increase in total and phosphorylated p38 (relative to β-tubulin). We also observed an increase in ERK1 phosphorylation (relative to total ERK1), but no change in ERK1 expression (relative to β-tubulin). We observed no change in ERK2 expression or phosphorylation. Although ovariectomy increased the level of phosphorylated ERK1 (vs. total ERK1), cyclophosphamide-induced lower urinary tract inflammation did not cause a net increase of either ERK1 or ERK2, or their phosphorylation. Inflammation did, however, cause an increase in p38 protein levels, relative to β-tubulin. Prior ovariectomy did not alter the response to inflammation.

Conclusions

These results provide new insights into the complex effects of estrogens on bladder nociceptor signalling. The diversity of estrogen actions in these ganglia raises the possibility of developing new ways to modulate their function in pelvic hyperactivity or pain states.

Imbalance between excitatory and inhibitory amino acids at spinal level is associated with maintenance of persistent pain-related behaviors

Although the postsynaptic events responsible for development of pathological pain have been intensively studied, the relative contribution of presynaptic neurotransmitters to the whole process remains less elucidated. In the present investigation, we sought to measure temporal changes in spinal release of both excitatory amino acids (EAAs, glutamate and aspartate) and inhibitory amino acids (IAAs, glycine, ?-aminobutyric acid and taurine) in response to peripheral inflammatory pain state. The results showed that following peripheral chemical insult induced by subcutaneous bee venom (BV) injection, there was an initial, parallel increase in spinal release of both EAAs and IAAs, however, the balance between them was gradually disrupted when pain persisted longer, with EAAs remaining at higher level but IAAs at a level below the baseline. Moreover, the EAAs-IAAs imbalance at the spinal level was dependent upon the ongoing activity from the peripheral injury site. Intrathecal blockade of ionotropic (NMDA and non-NMDA) and metabotropic (mGluRI, II, III) glutamate receptors, respectively, resulted in a differential inhibition of BV-induced different types of pain (persistent nociception vs. hyperalgesia, or thermal vs. mechanical hyperalgesia), implicating that spinal antagonism of any specific glutamate receptor subtype fails to block all types of pain-related behaviors. This result provides a new line of evidence emphasizing an importance of restoration of EAAs-IAAs balance at the spinal level to prevent persistence or chronicity of pain.

Microarray analysis of gene expression profiles in response to treatment with bee venom in lipopolysaccharide activated RAW 264.7 cells

AIM OF THE STUDY

The therapeutic application of bee venom (BV) has been used in traditional medicine to treat diseases such as arthritis, rheumatism and pain. Macrophages produce molecules that are known to play roles in inflammatory responses.

MATERIAL AND METHODS

We performed microarray analysis to evaluate the global gene expression profiles of RAW264.7 macrophage cells treated with BV. In addition, six genes were subjected to real-time PCR to confirm the results of the microarray. The cells were treated with lipopolysaccharide (LPS) or BV plus LPS for 30 min or 1h.

RESULTS

124 genes were found to be up-regulated and 158 were found to be down-regulated in cells that were treated with BV plus LPS for 30 min, whereas 211 genes were up-regulated and 129 were down-regulated in cells that were treated with BV plus LPS for 1h when compared with cells that were treated with LPS alone. Furthermore, the results of real-time PCR were similar to those of the microarray. BV inhibited the expression of specific inflammatory genes that were up-regulated by nuclear factor-kappa B in the presence of LPS, including mitogen-activated protein kinase kinase kinase 8 (MAP3K8), TNF, TNF-alpha-induced protein 3 (TNFAIP3), suppressor of cytokine signaling 3 (SOCS3), TNF receptor-associated factor 1 (TRAF1), JUN, and CREB binding protein (CBP).

CONCLUSIONS

These results demonstrate the potent activity of BV as a modulator of the LPS-mediated nuclear factor-kappaB (NF-kappaB)/MAPK pathway in activated macrophages. In addition, these results can be used to understand other effects of BV treatment.

Differential roles of peripheral mitogen-activated protein kinase signal transduction pathways in bee venom-induced nociception and inflammation in conscious rats

Intraplantar injection of bee venom (BV) produces persistent spontaneous nociception (PSN) and hyperalgesia, as well as obvious inflammatory swelling, in the paws of injected rats. The present study was designed to determine the peripheral roles of mitogen-activated protein kinase (MAPK) signal transduction pathways in BV-induced nociception and inflammation. We examined the effect of intraplantar injection of an ERK1/2 inhibitor, PD98059, and a p38 inhibitor, SB202190, on BV-induced PSN, mechanical hyperalgesia, and inflammatory swelling. We found that (1) pretreatment with SB202190 (0.1 to 10 microg) had no effect on BV-induced PSN, whereas pretreatment with PD98059 (0.1 to 100 microg) produced a significant and dose-dependent inhibition of BV-induced PSN; (2) pretreatment with PD98059 (0.1 to 100 microg) had no effect on BV-induced decreases in paw withdrawal mechanical threshold (PWMT), while pretreatment with SB202190 (0.1 to 10 microg) produced an obvious prevention of the BV-induced decrease in PWMT; and (3) pretreatment with PD98059 (0.1 to 100 microg) had no effect on BV-induced increase in paw volume (PV), whereas pretreatment with SB202190 (0.1 to 10 microg) produced a dose-related inhibition of BV-induced increases in PV. No contralateral drug treatments, even at the highest dose, had any effect on BV-induced PSN, PWMT or PV, ruling out the systemic effect of these drugs. These results suggest that peripheral MAPK signal transduction pathways may play differential roles in bee venom-induced nociception and inflammation. Targeting specific peripheral MAPKs might prove effective in the treatment of persistent pain and inflammation.

PERSPECTIVE

The present article showed that intraplantar injection of different MAPK inhibitors produced differential effects on bee venom-induced nociception and inflammation, suggesting that the peripheral MAPK signal transduction pathways have differential roles. Targeting specific peripheral MAPKs might prove effective in the treatment of persistent pain and inflammation.

Effects of general anesthetics on visceral pain transmission in the spinal cord

Current evidence suggests an analgesic role for the spinal cord action of general anesthetics; however, the cellular population and intracellular mechanisms underlying anti-visceral pain by general anesthetics still remain unclear. It is known that visceral nociceptive signals are transmited via post-synaptic dorsal column (PSDC) and spinothalamic tract (STT) neuronal pathways and that the PSDC pathway plays a major role in visceral nociception. Animal studies report that persistent changes including nociception-associated molecular expression (e.g. neurokinin-1 (NK-1) receptors) and activation of signal transduction cascades (such as the protein kinase A [PKA]-c-AMP-responsive element binding [CREB] cascade)-in spinal PSDC neurons are observed following visceral pain stimulation. The clinical practice of interruption of the spinal PSDC pathway in patients with cancer pain further supports a role of this group of neurons in the development and maintenance of visceral pain. We propose the hypothesis that general anesthetics might affect critical molecular targets such as NK-1 and glutamate receptors, as well as intracellular signaling by CaM kinase II, protein kinase C (PKC), PKA, and MAP kinase cascades in PSDC neurons, which contribute to the neurotransmission of visceral pain signaling. This would help elucidate the mechanism of antivisceral nociception by general anesthetics at the cellular and molecular levels and aid in development of novel therapeutic strategies to improve clinical management of visceral pain.