Roles of peripheral mitogen-activated protein kinases in melittin-induced nociception and hyperalgesia

Induction of Endogenous Antimicrobial Peptides to Prevent or Treat Oral Infection and Inflammation

Antibiotics are often used to treat oral infections. Unfortunately, excessive antibiotic use can adversely alter oral microbiomes and promote the development of antibiotic-resistant microorganisms, which can be difficult to treat. An alternate approach could be to induce the local transcription and expression of endogenous oral antimicrobial peptides (AMPs). To assess the feasibility and benefits of this approach, we conducted literature searches to identify (i) the AMPs expressed in the oral cavity; (ii) the methods used to induce endogenous AMP expression; and (iii) the roles that expressed AMPs may have in regulating oral inflammation, immunity, healing, and pain. Search results identified human neutrophil peptides (HNP), human beta defensins (HBD), and cathelicidin AMP (CAMP) gene product LL-37 as prominent AMPs expressed by oral cells and tissues. HNP, HBD, and LL-37 expression can be induced by micronutrients (trace elements, elements, and vitamins), nutrients, macronutrients (mono-, di-, and polysaccharides, amino acids, pyropeptides, proteins, and fatty acids), proinflammatory agonists, thyroid hormones, and exposure to ultraviolet (UV) irradiation, red light, or near infrared radiation (NIR). Localized AMP expression can help reduce infection, inflammation, and pain and help oral tissues heal. The use of a specific inducer depends upon the overall objective. Inducing the expression of AMPs through beneficial foods would be suitable for long-term health protection. Additionally, the specialized metabolites or concentrated extracts that are utilized as dosage forms would maintain the oral and intestinal microbiome composition and control oral and intestinal infections. Inducing AMP expression using irradiation methodologies would be applicable to a specific oral treatment area in addition to controlling local infections while regulating inflammatory and healing processes.

Persistent severe muscle pain following mistakenly injected high-dose bee venom: A care-compliant case report

RATIONALE

In clinical practice, bee venom injection into acupuncture points is used to relieve various types of musculoskeletal pain. In the current study, we describe a patient who had persistent severe muscle pain caused by mistakenly injected high-dose bee venom.

PATIENT CONCERNS

A 63-year-old woman mistakenly received an injection of high-dose (2 mL; standard dose, 0.1 mL) bee venom (diluted in saline at a 1:2000 ratio). Immediately after the injection, extreme burning pain developed at the injection site, which persisted for 1 month with a mean pain score of 9 on the numeric rating scale. T1-weighted gadolinium-enhanced axial lumbar magnetic resonance imaging revealed increased intensity in the medial part of the left psoas muscle around the L4-5 intervertebral disc level.

DIAGNOSIS

This finding indicated the presence of inflammation in the left psoas muscle, which was thought to be associated with pain.

INTERVENTIONS

A buprenorphine transdermal patch (releasing 5 mcg/hours) was applied to alleviate the pain.

OUTCOMES

Six months after the bee venom injection, the pain completely resolved.

LESSONS

Bee venom has a strong toxic effect; therefore, only a minimal dose of diluted bee venom should be administered for musculoskeletal pain control. Special caution is required during bee venom injection to avoid excessive doses of bee venom.

Melittin administration ameliorates motor function, prevents apoptotic cell death and protects Purkinje neurons in the rat model of cerebellar ataxia induced by 3-Acetylpyridine

Cerebellar ataxia (CA) is a condition in which cerebellar dysfunction leads to movement disorders such as dysmetria, asynergy and dysdiadochokinesia. This study investigates the therapeutic effects of Melittin (MEL) on 3-acetylpyridine-induced (3-AP) cerebellar ataxia (CA) rat model. Initially, CA rat models were generated by 3-AP administration followed by the intraperitoneal injection of MEL. Then, motor performance and electromyography (EMG) activity were assessed. Afterwards, the pro-inflammatory cytokines were analyzed in the cerebellar tissue. Moreover, the anti-apoptotic role of MEL in CA and its relationship with the protection of Purkinje cells were explored. The findings showed that the administration of MEL in a 3-AP model of ataxia improved motor coordination (P < 0.001) and neuro-muscular activity (p < 0.05), prevented the cerebellar volume loss (P < 0.01), reduced the level of inflammatory cytokines (p < 0.05) and thwarted the degeneration of Purkinje cells against 3-AP toxicity (P < 0.001). Overall, the findings imply that the MEL attenuates the 3-AP-induced inflammatory response. As such, it could be used as a treatment option for CA due to its anti-inflammatory effects.

Activation of the spinal EGFR signaling pathway in a rat model of cancer-induced bone pain with morphine tolerance

Cancer-induced bone pain (CIBP) is considered to be one of the most difficult pain conditions to treat. Morphine, an analgesic drug, is widely used in clinical practice, and long-term use of morphine can lead to drug tolerance. Recent reports have suggested that inhibitors of epidermal growth factor receptor (EGFR) may have analgesic effects in cancer patients suffering from pain. Therefore, we sought to determine whether EGFR signaling was involved in morphine tolerance (MT) in a rat model of cancer-induced bone pain. In this study, Walker 256 mammary gland carcinoma cells were inoculated into the tibias of rats to provoke cancer-induced bone pain. Then, morphine was intrathecally administered twice daily for seven consecutive days to induce drug tolerance. We observed sustained increased in the protein levels of EGFR, p-EGFR, ERK1/2, and p-ERK1/2 during the development of morphine tolerance in rats with cancer-induced bone pain by western blotting. The EGFR level was significantly increased in the MT and CIBP + MT groups, and EGFR was colocalized with markers of microglia and neurons in the spinal cords of rats. Inhibition of EGFR by a small molecule inhibitor markedly attenuated the degree of morphine tolerance and decreased the number of microglia, and the protein levels of EGFR, p-EGFR, ERK1/2, and p-ERK1/2 were also reduced. In summary, our results suggest that the activation of the EGFR signaling pathway in spinal microglia plays an important modulatory role in the development of morphine tolerance and that inhibition of EGFR may provide a new therapeutic option for cancer-induced bone pain.

Bee Updated: Current Knowledge on Bee Venom and Bee Envenoming Therapy

Honey bees can be found all around the world and fulfill key pollination roles within their natural ecosystems, as well as in agriculture. Most species are typically docile, and most interactions between humans and bees are unproblematic, despite their ability to inject a complex venom into their victims as a defensive mechanism. Nevertheless, incidences of bee stings have been on the rise since the accidental release of Africanized bees to Brazil in 1956 and their subsequent spread across the Americas. These bee hybrids are more aggressive and are prone to attack, presenting a significant healthcare burden to the countries they have colonized. To date, treatment of such stings typically focuses on controlling potential allergic reactions, as no specific antivenoms against bee venom currently exist. Researchers have investigated the possibility of developing bee antivenoms, but this has been complicated by the very low immunogenicity of the key bee toxins, which fail to induce a strong antibody response in the immunized animals. However, with current cutting-edge technologies, such as phage display, alongside the rise of monoclonal antibody therapeutics, the development of a recombinant bee antivenom is achievable, and promising results towards this goal have been reported in recent years. Here, current knowledge on the venom biology of Africanized bees and current treatment options against bee envenoming are reviewed. Additionally, recent developments within next-generation bee antivenoms are presented and discussed.

Mitogen-activated protein kinase phosphatase-3 (MKP-3) in the surgical wound is necessary for the resolution of postoperative pain in mice

Mitogen-activated protein kinase (MAPK) phosphatase-3 (MKP-3) and its substrates (extracellular signal-regulated kinase [ERK] and p38) play an important role in pathophysiological mechanisms of acute postoperative and chronic neuropathic pain in the spinal cord. This study aimed to understand the role of MKP-3 and its target MAPKs at the site of surgical incision in nociceptive behavior. Wild-type (WT) and MKP-3 knockout (KO) mice underwent unilateral plantar hind paw incision. Mechanical allodynia was assessed by using von Frey filaments. Peripheral ERK-1/2 and p38 phosphorylation were measured by Western blot. Cell infiltration was determined using hematoxylin and eosin histological staining. Peripheral phosphorylated ERK-1/2 (p-ERK-1/2) inhibition was performed in MKP-3 KO mice. In WT mice, mechanical hypersensitivity was observed on postoperative day 1 (0.69±0.17 g baseline vs 0.13±0.08 g day 1), which resolved normally by postoperative day 12 (0.46±0.08 g, N=6). In MKP-3 KO mice, this hypersensitivity persisted at least 12 days after surgery (0.19±0.06 g; N=6). KO mice displayed higher numbers of infiltrating cells (51.4±6 cells/0.1 mm²) than WT mice (8.7±1.2 cells/0.1 mm²) on postoperative day 1 (vs 5–6 cells/0.1 mm² at baseline) that returned to baseline 12 days after surgery (10–12 cells/0.1 mm²). In WT mice, peripheral p-p38 and p-ERK-1/2 expression increased (5- and 3-fold, respectively) on postoperative days 1 and 5, and returned to basal levels 7–12 days after surgery (N=3 per group). Peripheral p-p38 levels in MKP-3 KO mice followed a similar expression pattern as WT mice. Peripheral p-ERK-1/2 levels in MKP-3 KO mice remained elevated 12 days after surgery (2.5-fold, N=3 per group). Administration of PD98059 (MEK inhibitor, N=8, vehicle N=9) reduced p-ERK-1/2 expression in the incised tissue and blocked hypersensitivity in MKP-3 KO mice (N=6). The findings of this study suggest that MKP-3 is pivotal for normal resolution of acute postoperative allodynia, through the regulation of peripheral p-ERK-1/2.

Melittin, the Major Pain-Producing Substance of Bee Venom

Melittin is a basic 26-amino-acid polypeptide that constitutes 40-60% of dry honeybee (Apis mellifera) venom. Although much is known about its strong surface activity on lipid membranes, less is known about its pain-producing effects in the nervous system. In this review, we provide lines of accumulating evidence to support the hypothesis that melittin is the major pain-producing substance of bee venom. At the psychophysical and behavioral levels, subcutaneous injection of melittin causes tonic pain sensation and pain-related behaviors in both humans and animals. At the cellular level, melittin activates primary nociceptor cells through direct and indirect effects. On one hand, melittin can selectively open thermal nociceptor transient receptor potential vanilloid receptor channels via phospholipase A2-lipoxygenase/cyclooxygenase metabolites, leading to depolarization of primary nociceptor cells. On the other hand, algogens and inflammatory/pro-inflammatory mediators released from the tissue matrix by melittin's pore-forming effects can activate primary nociceptor cells through both ligand-gated receptor channels and the G-protein-coupled receptor-mediated opening of transient receptor potential canonical channels. Moreover, subcutaneous melittin up-regulates Nav1.8 and Nav1.9 subunits, resulting in the enhancement of tetrodotoxin-resistant Na(+) currents and the generation of long-term action potential firing. These nociceptive responses in the periphery finally activate and sensitize the spinal dorsal horn pain-signaling neurons, resulting in spontaneous nociceptive paw flinches and pain hypersensitivity to thermal and mechanical stimuli. Taken together, it is concluded that melittin is the major pain-producing substance of bee venom, by which peripheral persistent pain and hyperalgesia (or allodynia), primary nociceptive neuronal sensitization, and CNS synaptic plasticity (or metaplasticity) can be readily induced and the molecular and cellular mechanisms underlying naturally-occurring venomous biotoxins can be experimentally unraveled.

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.

SDF1-CXCR4 signaling contributes to persistent pain and hypersensitivity via regulating excitability of primary nociceptive neurons: involvement of ERK-dependent Nav1.8 up-regulation

BACKGROUND

Pain is one critical hallmark of inflammatory responses. A large number of studies have demonstrated that stromal cell-derived factor 1 (SDF1, also named as CXCL12) and its cognate receptor C-X-C chemokine receptor type 4 (CXCR4) play an important role in immune reaction and inflammatory processes. However, whether and how SDF1-CXCR4 signaling is involved in inflammatory pain remains unclear.

METHODS

Under the intraplantar (i.pl.) bee venom (BV) injection-induced persistent inflammatory pain state, the changes of SDF1 and CXCR4 expression and cellular localization in the rat dorsal root ganglion (DRG) were detected by immunofluorescent staining. The role of SDF1 and CXCR4 in the hyperexcitability of primary nociceptor neurons was assessed by electrophysiological recording. Western blot analysis was used to quantify the DRG Nav1.8 and phosphorylation of ERK (pERK) expression. Behavioral tests were conducted to evaluate the roles of CXCR4 as well as extracellular signal-regulated kinase (ERK) and Nav1.8 in the BV-induced persistent pain and hypersensitivity.

RESULTS

We showed that both SDF1 and CXCR4 were dramatically up-regulated in the DRG in i.pl. BV-induced inflammatory pain model. Double immunofluorescent staining showed that CXCR4 was localized in all sizes (large, medium, and small) of DRG neuronal soma, while SDF1 was exclusively expressed in satellite glial cells (SGCs). Electrophysiological recording showed that bath application with AMD3100, a potent and selective CXCR4 inhibitor, could reverse the hyperexcitability of medium- and small-sized DRG neurons harvested from rats following i.pl. BV injection. Furthermore, we demonstrated that the BV-induced ERK activation and Nav1.8 up-regulation in the DRG could be blocked by pre-antagonism against CXCR4 in the periphery with AMD3100 as well as by blockade of ERK activation by intrathecal (i.t.) or intraplantar (i.pl.) U0126. At behavioral level, the BV-induced persistent spontaneous pain as well as primary mechanical and thermal hypersensitivity could also be significantly suppressed by blocking CXCR4 and Nav1.8 in the periphery as well as by inhibition of ERK activation at the DRG level.

CONCLUSIONS

The present results suggest that peripheral inflammatory pain state can trigger over release of SDF1 from the activated SGCs in the DRG by which SGC-neuronal cross-talk is mediated by SDF1-CXCR4 coupling that result in subsequent ERK-dependent Nav1.8 up-regulation, leading to hyperexcitability of tonic type of the primary nociceptor cells and development and maintenance of persistent spontaneous pain and hypersensitivity.

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.

Tolerance to the antinociceptive effects of chronic morphine requires c-Jun N-terminal kinase

BACKGROUND

Morphine and fentanyl are opioid analgesics in wide clinical use that act through the μ-opioid receptor (MOR). However, one limitation of their long-term effectiveness is the development of tolerance. Receptor desensitization has been proposed as a putative mechanism driving tolerance to G protein-coupled receptor (GPCR) agonists. Recent studies have found that tolerance to morphine is mediated by the c-Jun N-terminal Kinase (JNK) signaling pathway. The goal of the present study was to test the hypotheses that: 1) JNK inhibition will be antinociceptive on its own; 2) JNK inhibition will augment morphine antinociception and; 3) JNK mediates chronic tolerance for the antinociceptive effects of morphine using acute (hotplate and tail-flick), inflammatory (10 μl of formalin 2.5%) and chemotherapy (cisplatin 5 mg/kg ip once weekly)-induced neuropathic pain assays.

RESULTS

We found that JNK inhibition by SP600125 (3 mg/kg) produces a greater antinociceptive effect than morphine (6 mg/kg) alone in the formalin test. Moreover, co-administration of morphine (6 mg/kg) with SP600125 (3 mg/kg) produced a sub-additive antinociceptive effect in the formalin test. We also show that pre-treatment with SP600125 (3 or 10 mg/kg), attenuates tolerance to the antinociceptive effects of morphine (10 mg/kg), but not fentanyl (0.3 mg/kg), in the tail-flick and hotplate tests. Pre-treatment with SP600125 also attenuates tolerance to the hypothermic effects of both morphine and fentanyl. We also examined the role of JNK in morphine tolerance in a cisplatin-induced model of neuropathic pain. Interestingly, treatment with SP600125 (3 mg/kg) alone attenuated mechanical and cold allodynia in a chemotherapy-induced pain model using cisplatin. Strikingly, SP600125 (3 mg/kg) pre-treatment prolonged the anti-allodynic effect of morphine by several days (5 and 7 days for mechanical and cold, respectively).

CONCLUSIONS

These results demonstrate that JNK signaling plays a crucial role in mediating antinociception as well as chronic tolerance to the antinociceptive effects of morphine in acute, inflammatory, and neuropathic pain states. Thus, inhibition of JNK signaling pathway, via SP600125, represents an efficacious pharmacological approach to delay tolerance to the antinociceptive effects of chronic morphine in diverse pain models.

A Review of Signal Transduction of Endothelin-1 and Mitogen-activated Protein Kinase-related Pain for Nanophysiotherapy

[Purpose] An understanding of pain is very important in the study of nanophysiotherapy. In this review, we summarize the mechanisms of endothelin-1 (ET-1)- and mitogen-activated protein kinase (MAPK)-related pain, and suggest their applications in pain physiotherapy. [Method] This review focuses on the signal transduction of pain and its mechanisms. [Results] Our reviews show that mechanisms of ET-1- and MAPK-related pain exist. [Conclusions] In this review article, we carefully discuss the signal transduction in ET-1- and MAPK-related pain with reference to pain nanophysiotherapy from the perspective of nanoparticle-associated signal transduction.

Extracellular signal-regulated protein kinase activation in spinal cord contributes to pain hypersensitivity in a mouse model of type 2 diabetes

Painful peripheral neuropathy is a common complication of diabetes mellitus. The symptom of pain can become a major factor that decreases the quality of life of patients with diabetes, while effective treatment is lacking. In the present study, we aimed to investigate the changes of pain threshold in the early stage of diabetes in db/db mice, an animal model of type 2 diabetes mellitus, and the underlying molecular mechanisms. We found that (1) db/db mice (with a leptin receptor-null mutation and characterized by obesity and hyperglycemia) showed hypersensitivity to mechanical and thermal stimuli at the early stage of diabetes; (2) phosphorylated extracellular signal-regulated kinase (pERK), but not total ERK in the spinal cord and dorsal root ganglia in db/db mice significantly increased compared with wild-type mice. The increased pERK immunoreactivity occurred in both NeuN-expressing neurons and GFAP-expressing astrocytes, but not in Iba-1-expressing microglia; (3) both single and consecutive (for 5 days) intrathecal injections of U0126 (2 nmol per day), a selective MEK (an ERK kinase) inhibitor beginning at 8 weeks of age, attenuated the bilateral mechanical allodynia in the von-Frey test and heat hyperalgesia in Hargreave's test; and (4) db/db mice also displayed increased nocifensive behavior during the formalin test, and this was blocked by intrathecal injection of U0126. Also, the expression of pERK1 and pERK2 was upregulated following the formalin injection. Our results suggested that the activation of ERK in spinal neurons and astrocytes is correlated with pain hypersensitivity of the type 2 diabetes animal model. Inhibiting the ERK pathway may provide a new therapy for pain control in type 2 diabetes.

Activation of tetrodotoxin-resistant sodium channel NaV1.9 in rat primary sensory neurons contributes to melittin-induced pain behavior

Tetrodotoxin-resistant (TTX-R) sodium channels NaV1.8 and NaV1.9 in dorsal root ganglion (DRG) neurons play important roles in pathological pain. We recently reported that melittin, the major toxin of whole bee venom, induced action potential firings in DRG neurons even in the presence of a high concentration (500 nM) of TTX, indicating the contribution of TTX-R sodium channels. This hypothesis is fully investigated in the present study. After subcutaneous injection of melittin, NaV1.8 and NaV1.9 significantly upregulate mRNA and protein expressions, and related sodium currents also increase. Double immunohistochemical results show that NaV1.8-positive neurons are mainly medium- and small-sized, whereas NaV1.9-positive ones are only small-sized. Antisense oligodeoxynucleotides (AS ODNs) targeting NaV1.8 and NaV1.9 are used to evaluate functional significance of the increased expressions of TTX-R sodium channels. Behavioral tests demonstrate that AS ODN targeting NaV1.9, but not NaV1.8, reverses melittin-induced heat hypersensitivity. Neither NaV1.8 AS ODN nor NaV1.9 AS ODN affects melittin-induced mechanical hypersensitivity. These results provide previously unknown evidence that upregulation of NaV1.9, but not NaV1.8, in small-sized DRG neurons contributes to melittin-induced heat hypersensitivity. Furthermore, melittin-induced biological effect indicates a potential strategy to study properties of TTX-R sodium channels.

Effects of a non-selective TRPC channel blocker, SKF-96365, on melittin-induced spontaneous persistent nociception and inflammatory pain hypersensitivity

OBJECTIVE

Melittin is the main peptide in bee venom and causes both persistent spontaneous nociception and pain hypersensitivity. Our recent studies indicated that both transient receptor potential (TRP) vanilloid receptor 1 (TRPV1) and canonical TRPs (TRPCs) are involved in mediating the melittin-induced activation of different subpopulations of primary nociceptive cells. Here, we further determined whether TRPC channels are involved in melittin-induced inflammatory nociceptive responses in behavioral assays.

METHODS

The anti-nociceptive and anti-hyperalgesic effects of localized peripheral administration of three doses of the non-selective TRPC antagonist, SKF-96365 (1-{β-[3-(4-methoxyphenyl)propoxy]-4-methoxyphenyl}-1H-imidazole hydrochloride), were evaluated in melittin tests. Pain-related behaviors were rated by counting the number of paw flinches, and measuring paw withdrawal thermal latency (s) and paw withdrawl mechanical threshold (g), over a 1-h time-course.

RESULTS

Localized peripheral SKF-96365 given before melittin prevented, and given after melittin significantly suppressed, the melittin-evoked persistent spontaneous nociception. Pre-blockade and post-suppression of activation of primary nociceptive activity resulted in decreased hypersensitivity to both thermal and mechanical stimuli applied to the primary injury site of the ipsilateral hindpaw, despite dose-effect differences between thermal and mechanical hyperalgesia. However, local administration of SKF-96365 into the contralateral hindpaw had no significant effect on any pain-associated behaviors. In addition, SKF-96365 had no effect on baseline threshold for either thermal or mechanical sensitivity under normal conditions.

CONCLUSION

Besides TRPV1, SKF-96365-sensitive TRPC channels might also be involved in the pathophysiological processing of melittin-induced inflammatory pain and hypersensitivity. Therapeutically, SKF-96365 is equally effective in preventing primary thermal and mechanical hyperalgesia as well as persistent spontaneous nociception. However, this drug is likely to be more effective in the relief of thermal hyperalgesia than mechanical hyperalgesia when applied 5 min after establishment of primary afferent activation.

Metabotropic glutamate receptor 5 contributes to inflammatory tongue pain via extracellular signal-regulated kinase signaling in the trigeminal spinal subnucleus caudalis and upper cervical spinal cord

Background

In the orofacial region, limited information is available concerning pathological tongue pain, such as inflammatory pain or neuropathic pain occurring in the tongue. Here, we tried for the first time to establish a novel animal model of inflammatory tongue pain in rats and to investigate the roles of metabotropic glutamate receptor 5 (mGluR5)-extracellular signal-regulated kinase (ERK) signaling in this process.

Methods

Complete Freund’s adjuvant (CFA) was submucosally injected into the tongue to induce the inflammatory pain phenotype that was confirmed by behavioral testing. Expression of phosphorylated ERK (pERK) and mGluR5 in the trigeminal subnucleus caudalis (Vc) and upper cervical spinal cord (C1-C2) were detected with immunohistochemical staining and Western blotting. pERK inhibitor, a selective mGluR5 antagonist or agonist was continuously administered for 7 days via an intrathecal (i.t.) route. Local inflammatory responses were verified by tongue histology.

Results

Submucosal injection of CFA into the tongue produced a long-lasting mechanical allodynia and heat hyperalgesia at the inflamed site, concomitant with an increase in the pERK immunoreactivity in the Vc and C1-C2. The distribution of pERK-IR cells was laminar specific, ipsilaterally dominant, somatotopically relevant, and rostrocaudally restricted. Western blot analysis also showed an enhanced activation of ERK in the Vc and C1-C2 following CFA injection. Continuous i.t. administration of the pERK inhibitor and a selective mGluR5 antagonist significantly depressed the mechanical allodynia and heat hyperalgesia in the CFA-injected tongue. In addition, the number of pERK-IR cells in ipsilateral Vc and C1-C2 was also decreased by both drugs. Moreover, continuous i.t. administration of a selective mGluR5 agonist induced mechanical allodynia in naive rats.

Conclusions

The present study constructed a new animal model of inflammatory tongue pain in rodents, and demonstrated pivotal roles of the mGluR5-pERK signaling in the development of mechanical and heat hypersensitivity that evolved in the inflamed tongue. This tongue-inflamed model might be useful for future studies to further elucidate molecular and cellular mechanisms of pathological tongue pain such as burning mouth syndrome.

Cdk5: An Emerging Kinase in Pain Signaling

Pain is an important survival mechanism for an organism. It can turn into severe mental and physical disorder however, if the molecular and/or cellular pathways involved in pain signaling are altered. Chronic pain is characterized by an altered pain perception that includes allodynia (a response to a normally non-noxious stimulus) and hyperalgesia (an exaggerated response to a normally noxious stimulus). Past few years of pain research has been mainly focused on precise understanding of the molecular and cellular nociceptive signatures altered during chronic pain, so that more effective pain relievers can be developed. The importance of protein kinases in normal cellular homeostasis and disease pathogenesis has evolved rapidly in the past few decades. The recent advancement defining the role of multiple protein kinases in regulating neuronal plasticity and pain sensitization has gained enough attention of pharmaceutical industry to develop specific and selective kinase inhibitors as analgesics. Cyclin-dependent kinase 5 (Cdk5) is one such emerging kinase in pain biology. We will discuss here the recent advancement and therapeutic potential of Cdk5 in pain signaling.

Involvement of peripheral NMDA receptor in melittin-induced thermographic flare

Intradermal injection of an active compound of European honeybee toxin, melittin, into the forearm in humans produces temporary pain and evokes sustained increase of local skin temperature. This increase of skin temperature is suppressed by the pretreatment of a voltage gated sodium channel blocker, lidocaine, suggesting that neurogenic inflammation is involved in the skin temperature increase after the melittin treatment. In this study, we tested a hypothesis that the melittin-induced skin temperature increase is augmented by an N-methyl-D-aspartate (NMDA) glutamate receptor that is present on the peripheral terminals of cutaneous primary afferents. Skin temperature was examined after the local application of incremental doses of melittin by a computer-assisted-thermography in pentobarbital-anesthetized rats. Local subcutaneous glutamate was collected through a microdialysis probe and glutamate levels were measured by a high pressure liquid chromatography with electrochemical detection method. Intraplantar injection of melittin resulted in the increase of subcutaneous glutamate levels and the increase of local skin temperature, which was partially attenuated by co-injection of an NMDA receptor antagonist, MK-801. In addition, intraplantar injection of NMDA itself increased the local skin temperature. Our data suggest that melittin-induced increase of skin temperature is enhanced through the activation of peripheral NMDA receptors by locally released glutamate. We suggest that topical administration of NMDA receptor antagonists could be an effective treatment of neuro-inflammatory pain.

Activation of c-jun N-terminal kinase in spinal cord contributes to breast cancer induced bone pain in rats

BACKGROUND

The most frequent pain in patients with metastatic breast and prostate cancer is bone pain, which can be severe and difficult to treat. The mechanisms underlying this pain remain unclear. Here we investigated the role of c-jun N-terminal kinase (JNK) pathway in the spinal cord in cancer-induced bone pain (CIBP).

RESULTS

In this study, we used an established rat CIBP model to investigate the possible role of JNK activation in the spinal cord. After intra-tibial inoculation with Walker 256 rat mammary gland carcinoma cells, the rats displayed mechanical allodynia on day 5, which lasted to day 16. The activation of JNK in neurons and astrocytes in the spinal cord was found on day 12 and day 16 after intra-tibial inoculation with carcinoma cells. A single intrathecal injection with JNK inhibitor SP600125 by lumbar puncture attenuated mechanical allodynia on day 12, and repeated intrathecal injection of SP600126 from day 10 to day 14 had a cumulative analgesic effect on CIBP.

CONCLUSIONS

Taken together, our results demonstrated for the first time that JNK activation in the spinal cord is required in the maintenance of CIBP. Inhibition of the spinal JNK pathway may provide a new therapy for CIBP management.

The Anti-Arthritic Effects of Synthetic Melittin on the Complete Freund's Adjuvant-Induced Rheumatoid Arthritis Model in Rats

Bee venom (BV) has been used for millennia in Chinese traditional medicine to treat rheumatoid arthritis (RA). However, its components and mechanism remain unclear, which has hampered its development and application for the treatment of RA. In this study, we examined the anti-arthritis effects of melittin, which composes nearly 50% of the dry weight of whole BV, on the complete Freund's adjuvant-induced (CFA-induced) RA model in rats. The RA animal models were treated with solutions of BV, melittin, and saline by injection into a specific acupoint (Zusanli). The BV and melittin treatments statistically diminished the thickness of the arthroses in the injected side of the paw, compared to the saline treatment. Melittin therapy also significantly reduced arthritis-induced nociceptive behaviors, as assessed by the thermal hyperalgesia test. In addition, CFA-induced Fos expression in the superficial layer of the lumbar spinal cord was significantly suppressed by the BV and melittin treatments, compared to the saline treatment. These results indicate that melittin is an effective anti-arthritis component of whole bee venom, making it a promising candidate as an anti-arthritis drug.

Effects of SKF-96365, a TRPC inhibitor, on melittin-induced inward current and intracellular Ca2+ rise in primary sensory cells

OBJECTIVE

Melittin (MEL) is a major component of bee venom and can produce both persistent spontaneous nociception and pain hypersensitivity when injected subcutaneously in the periphery. The present study aimed to examine the roles of transient receptor potential canonical (TRPC) channels in mediation of MEL-induced activation of primary nociceptive cells.

METHODS

Whole-cell patch-clamp and laser scanning confocal calcium detection were used to evaluate the effects of SKF-96365, a TRPC inhibitor, applied on the acutely isolated dorsal root ganglion (DRG) cells of rat, on MEL-induced increase in intracellular calcium concentration ([Ca(2+)](i)) and inward current.

RESULTS

Under voltage-clamp mode, 43.9% (40/91) DRG cells were evoked to give rise to the inward current by 2 μmol/L MEL, which could be significantly suppressed by 3 doses of SKF-96365 (1, 5 and 10 μmol/L) in a dose-dependent manner. Of the other 210 cells, 67.6% responded to MEL with an intracellular Ca(2+) rise, as revealed by confocal calcium imaging. Of these MEL-sensitive cells, 46.5% (66/142) were suppressed by the highest dose of SKF-96365.

CONCLUSION

MEL-induced activation of small to medium-sized DRG cells can be suppressed by SKF-96365, suggesting the involvement of TRPC channels in the mediation of MEL-induced activation of primary nociceptive cells.

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.

The c-Jun N-terminal kinase 1 (JNK1) in spinal astrocytes is required for the maintenance of bilateral mechanical allodynia under a persistent inflammatory pain condition

Peripheral inflammation induces persistent central sensitization characterized by mechanical allodynia and heat hyperalgesia that are mediated by distinct mechanisms. Compared to well-demonstrated mechanisms of heat hyperalgesia, mechanisms underlying the development of mechanical allodynia and contralateral pain are incompletely known. In this study, we investigated the distinct role of spinal JNK in heat hyperalgesia, mechanical allodynia, and contralateral pain in an inflammatory pain model. Intraplantar injection of complete Freund's adjuvant (CFA) induced bilateral mechanical allodynia but unilateral heat hyperalgesia. CFA also induced a bilateral activation (phosphorylation) of JNK in the spinal cord, and the phospho JNK1 (pJNK1) levels were much higher than that of pJNK2. Notably, both pJNK and JNK1 were expressed in GFAP-positive astrocytes. Intrathecal infusion of a selective peptide inhibitor of JNK, D-JNKI-1, starting before inflammation via an osmotic pump, reduced CFA-induced mechanical allodynia in the maintenance phase but had no effect on CFA-induced heat hyperalgesia. A bolus intrathecal injection of D-JNKI-1 or SP600126, a small molecule inhibitor of JNK also reversed mechanical allodynia bilaterally. In contrast, peripheral (intraplantar) administration of D-JNKI-1 reduced the induction of CFA-induced heat hyperalgesia but did not change mechanical allodynia. Finally, CFA-induced bilateral mechanical allodynia was attenuated in mice lacking JNK1 but not JNK2. Taken together, our data suggest that spinal JNK, in particular JNK1 plays an important role in the maintenance of persistent inflammatory pain. Our findings also reveal a unique role of JNK1 and astrocyte network in regulating tactile allodynia and contralateral pain.

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.

Activation of ERK1/2 in the primary injury site is required to maintain melittin-enhanced wind-up of rat spinal wide-dynamic-range neurons

Peripheral modulation of wind-up enhancement induced by peripheral tissue injury is investigated in rat spinal wide-dynamic-range (WDR) neurons. After subcutaneous (s.c.) injection of melittin, a pain-related peptidergic component separated from bee venom, the responsiveness of spinal cord WDR neuron to repeated suprathreshold (1.5T, the intensity threshold) electrical stimuli is enhanced. Comparing with the less effects on early response (0-100 ms), melittin significantly increases late response (100 ms to the next stimulus artifact) and after-discharge (starting from 2s after the last stimulus artifact) with 189% and 546%, respectively. Peripheral administration of a specific MEK inhibitor, 1,4-diamino-2,3-dicyano-1,4-bis-[o-aminophenylmercapto] butadiene (U0126, 1 microg) gradually suppresses, but not completely blocks melittin-enhanced wind-up to the similar level of baseline. The inhibitions of U0126 are mainly on late response and after-discharge with 49% and 65%, respectively. Peripheral administration of three doses of U0126 (0.1, 1, 10 microg) has no effects on melittin-induced local paw edema regardless of either pre- or post-treatment of the drug. We conclude that peripheral ERKs pathway in the primary injury site is required to maintain melittin-enhanced wind-up of rat spinal cord wide-dynamic-range neurons.

Extracellular signal-regulated kinases mediate melittin-induced hypersensitivity of spinal neurons to chemical and thermal but not mechanical stimuli

Subcutaneous melittin injection causes central plasticity at the spinal level in wide-dynamic-range (WDR) neurons, which are hypersensitive to various nociceptive stimuli. Previous behavioral studies demonstrated that the mitogen-activated protein kinases (MAPKs) extracellular signal-regulated kinase 1/2(ERK1/2), p38 MAPK, and c-Jun N-terminal kinase are involved in both peripheral and spinal processing of melittin-induced nociception and hypersensitivity. Yet the functional roles of the three MAPKs vary among different stimulus modalities, and must be further studied at the cellular level in vivo. In this report, extracellular single unit recordings were performed to investigate whether activation of ERK1/2 in the primary injury site of melittin is essential to the establishment of a spinally sensitized state. Localized peripheral administration of a single dose of the MEK inhibitor U0126 (1 μg/10 μl) significantly suppressed neuronal hyper-responsiveness to thermal stimulus and chemical (melittin)-induced tonic firing of WDR neurons after full establishment of a spinally sensitized state. However, U0126 failed to affect mechanical hypersensitivity to both noxious and non-noxious stimuli. Melittin-induced enhancement of thermal hypersensitivity was also greatly inhibited by a single dose of capsazepine, a thermal nociceptor (TRPV1) blocker. These results suggest that activation of the ERK signaling pathway in the periphery is likely necessary for maintenance of a spinally sensitized state; activation of ERK1/2 in the primary injury site may regulate TRPV1, leading to dorsal horn hypersensitivity to thermal and chemical stimuli. ERK signaling pathways are not likely to be associated with melittin-induced dorsal horn hypersensitivity to mechanical stimuli.

Roles of peripheral P2X and P2Y receptors in the development of melittin-induced nociception and hypersensitivity

A recent report from our laboratory shows that subcutaneous (s.c.) injection of melittin could induce persistent spontaneous nociception (PSN) and primary thermal or mechanical hyperalgesia. However, the exact peripheral mechanisms underlying melittin-induced multiple pain-related behaviors remain unclear. In this study, behavioral tests combined with pharmacological manipulations were used to explore potential roles of local P2X and P2Y receptors in melittin-induced inflammatory pain and hyperalgesia. Post-treatment of the primary injury site with s.c. injection of A-317491 (a potent P2X(3)/P2X(2/3) receptor antagonist) and Reactive Blue 2 (a potent P2Y receptor antagonist) could significantly suppress the development of melittin-evoked PSN and hypersensitivity (thermal and mechanical). Our control experiments demonstrated that local administration of either antagonist into the contralateral hindpaw produced no significant effect on any kind of pain-associated behaviors. Taken together, these data indicate that activation of P2X and P2Y receptors might be essential to the maintenance of melittin-induced primary thermal and mechanical hyperalgesia as well as on-going pain.