Electroacupuncture Zusanli (ST36) Relieves Somatic Pain in Colitis Rats by Inhibiting Dorsal Root Ganglion Sympathetic-Sensory Coupling and Neurogenic Inflammation.

Referred somatic pain triggered by hyperalgesia is common in patients with inflammatory bowel disease (IBD). It was reported that sprouting of sympathetic nerve fibers into the dorsal root ganglion (DGR) and neurogenic inflammation were related to neuropathic pain, the excitability of neurons, and afferents. The purpose of the study was to explore the potential and mechanism of electroacupuncture (EA) at Zusanli (ST36) for the intervention of colon inflammation and hyperalgesia. Sprague-Dawley (SD) was randomly divided into four groups, including control, model, EA, and sham-EA. Our results showed EA treatment significantly attenuated dextran sulfate sodium- (DSS-) induced colorectal lesions and inflammatory cytokine secretion, such as TNF-α, IL-1ß, PGE2, and IL-6. EA also inhibited mechanical and thermal pain hypersensitivities of colitis rats. Importantly, EA effectively abrogated the promotion effect of DSS on ipsilateral lumbar 6 (L6) DRG sympathetic-sensory coupling, manifested as the sprouting of tyrosine hydroxylase- (TH-) positive sympathetic fibers into sensory neurons and colocalization of and calcitonin gene-related peptide (CGRP). Furthermore, EA at Zusanli (ST36) activated neurogenic inflammation, characterized by decreased expression of substance P (SP), hyaluronic acid (HA), bradykinin (BK), and prostacyclin (PGI2) in colitis rat skin tissues corresponding to the L6 DRG. Mechanically, EA treatment reduced the activation of the TRPV1/CGRP, ERK, and TLR4 signaling pathways in L6 DRG of colitis rats. Taken together, we presumed that EA treatment improved colon inflammation and hyperalgesia, potentially by suppressing the sprouting of sympathetic nerve fibers into the L6 DGR and neurogenic inflammation via deactivating the TRPV1/CGRP, ERK, and TLR4 signaling pathways.

Spinal CCK1 Receptors Contribute to Somatic Pain Hypersensitivity Induced by Malocclusion via a Reciprocal Neuron-Glial Signaling Cascade.

Recent studies have shown that the incidence of chronic primary pain including temporomandibular disorders (TMD) and fibromyalgia syndrome (FMS) often exhibit comorbidities. We recently reported that central sensitization and descending facilitation system contributed to the development of somatic pain hypersensitivity induced by orofacial inflammation combined with stress. The purpose of this study was to explore whether TMD caused by unilateral anterior crossbite (UAC) can induce somatic pain hypersensitivity, and whether the cholecystokinin (CCK) receptor-mediated descending facilitation system promotes hypersensitivity through neuron-glia cell signaling cascade. UAC evoked thermal and mechanical pain hypersensitivity of the hind paws from day 5 to 70 that peaked at week 4 post UAC. The expression levels of CCK1 receptors, interleukin-18 (IL-18) and IL-18 receptors (IL-18R) were significantly up-regulated in the L4 to L5 spinal dorsal horn at 4 weeks post UAC. Intrathecal injection of CCK1 and IL-18 receptor antagonists blocked somatic pain hypersensitivity. IL-18 mainly co-localized with microglia, while IL-18R mainly co-localized with astrocytes and to a lesser extent with neurons. These findings indicate that the signaling transduction between neurons and glia at the spinal cord level contributes to the descending pain facilitation through CCK1 receptors during the development of the comorbidity of TMD and FMS. PERSPECTIVE: CCK1 receptor-dependent descending facilitation may mediate central mechanisms underlying the development of widespread somatic pain via a reciprocal neuron-glial signaling cascade, providing novel therapeutic targets for the clinical treatment of TMD and FMS comorbidities.

Coenzyme Q10 encapsulated in micelles ameliorates osteoarthritis by inhibiting inflammatory cell death.

BACKGROUND: Osteoarthritis (OA) is the most common degenerative joint disease and is characterized by breakdown of joint cartilage. Coenzyme Q10 (CoQ10) exerts diverse biological effects on bone and cartilage; observational studies have suggested that CoQ10 may slow OA progression and inflammation. However, any effect of CoQ10 on OA remains unclear. Here, we investigated the therapeutic utility of CoQ10-micelles. METHODS: Seven-week-old male Wistar rats were injected with monosodium iodoacetate (MIA) to induce OA. CoQ10-micelles were administered orally to MIA-induced OA rats; celecoxib served as the positive control. Pain, tissue destruction, and inflammation were measured. The expression levels of catabolic and inflammatory cell death markers were assayed in CoQ10-micelle-treated chondrocytes. RESULTS: Oral supplementation with CoQ10-micelles attenuated OA symptoms remarkably, including pain, tissue destruction, and inflammation. The expression levels of the inflammatory cytokines IL-1ß, IL-6, and MMP-13, and of the inflammatory cell death markers RIP1, RIP3, and pMLKL in synovial tissues were significantly reduced by CoQ10-micelle supplementation, suggesting that CoQ10-micelles might attenuate the synovitis of OA. CoQ10-micelle addition to cultured OA chondrocytes reduced the expression levels of catabolic and inflammatory cell death markers. CONCLUSIONS: CoQ10-micelles might usefully treat OA.

Low-intensity low-frequency pulsed ultrasound ameliorates sciatic nerve dysfunction in a rat model of cisplatin-induced peripheral neuropathy.

Chemotherapy-induced peripheral neuropathy is a neurological complication that frequently occurs during chemotherapeutic intervention, resulting in damaged myelin sheath, motor weakness and/or sensory impairment. This study aims to investigate the therapeutic efficiency of low-intensity pulsed low-frequency ultrasound on cisplatin-induced peripheral neuropathy. Rats were randomly divided into five experimental groups as control, cisplatin administration, 10 mg/kg melatonin treatment after cisplatin administration, 1 MHz frequency 0.5 W/cm2 pulsed ultrasound treatment after cisplatin administration and 1 MHz frequency 1.5 W/cm2 pulsed ultrasound treatment after cisplatin administration. Chemical neuropathy was induced by the injection of 3 mg/kg/week of cisplatin (i.p.) for 5 weeks. Afterwards, melatonin and pulsed ultrasound treatments were applied for 15 consecutive days. Cisplatin administration resulted in a decrease in nociceptive pain perception and nerve conduction velocities together with a decrease in myelin thickness and diameters of axons and myelinated fibers, indicating a dysfunction and degeneration in sciatic nerves. In addition, cisplatin administration led to a decrease, in superoxide dismutase activity, and an increase in malondialdehyde and IL-1ß levels together with an increase in caspase-3 protein expression levels and a decrease in Bcl-2 and Parkin levels. The ultrasound treatments resulted in an increase in nociceptive pain perception and sciatic nerve conduction; led to a decrease in oxidative stress and inflammation, restored nerve degeneration and regulated apoptosis and mitophagy. Taken together, low-intensity pulsed low-frequency ultrasound was efficient in restoring the alterations attributable to cisplatin-induced peripheral neuropathy, and warrants further investigations.

Lysophosphatidic acid receptor1/3 antagonist inhibits the activation of satellite glial cells and reduces acute nociceptive responses.

Lysophosphatidic acid (LPA) exerts various biological activities through six characterized G protein-coupled receptors (LPA1-6 ). While LPA-LPA1  signaling contributes toward the demyelination and retraction of C-fiber and induces neuropathic pain, the effects of LPA-LPA1  signaling on acute nociceptive pain is uncertain. This study investigated the role of LPA-LPA1  signaling in acute nociceptive pain using the formalin test. The pharmacological inhibition of the LPA-LPA1 axis significantly attenuated formalin-induced nociceptive behavior. The LPA1  mRNA was expressed in satellite glial cells (SGCs) in dorsal root ganglion (DRG) and was particularly abundant in SGCs surrounding large DRG neurons, which express neurofilament 200. Treatment with LPA1/3 receptor (LPA1/3 ) antagonist inhibited the upregulation of glial markers and inflammatory cytokines in DRG following formalin injection. The LPA1/3 antagonist also attenuated phosphorylation of extracellular signal-regulated kinase, especially in SGCs and cyclic AMP response element-binding protein in the dorsal horn following formalin injection. LPA amounts after formalin injection to the footpad were quantified by liquid chromatography/tandem mass spectrometry, and LPA levels were found to be increased in the innervated DRGs. Our results indicate that LPA produced in the innervated DRGs promotes the activation of SGCs through LPA1 , increases the sensitivity of primary neurons, and modulates pain behavior. These results facilitate our understanding of the pathology of acute nociceptive pain and demonstrate the possibility of the LPA1 on SGCs as a novel target for acute pain control.

Pinocembrin relieves hip fracture-induced pain by repressing spinal substance P signaling in aged rats.

Whether pinocembrin (PCN) could be used to alleviate hip fracture-induced pain is investigated in this research. Aged rats with hip fractures were treated with vehicle or 80 mg/kg/day PCN from week 3 to week 4. Then, hind paw mechanical allodynia, unweighting, warmth, and thickness were measured. The microglia and astrocytes activation and proliferation markers in the spinal dorsal horn were detected with real-time PCR and immunofluorescence staining. The relative expression of substance P and its receptor, tachykinin receptor 1 (Tacr1), was detected with enzyme-linked immunosorbent assay (ELISA) and Western blots. The antinociceptive effect of Tacr1 inhibitor LY303870 was also testified. PCN alleviated hip fracture-induced hind paw nociceptive (allodynia and unweighting) and vascular changes (warmth and thickness) in aged rats with diminished microglia and astrocytes activation and proliferation in the spinal dorsal horn. Upregulated substance P and Tacr1 were induced after hip fracture, which could be reversed by PCN treatment. Furthermore, LY303870 treatment partially reversed both spinal nociceptive sensitization and vascular changes after hip fracture. Substance P signaling contributes to the nociceptive and vascular changes observed in the hip fracture, which could be alleviated by PCN.NEW & NOTEWORTHY Substance P signaling contributes to the nociceptive and vascular changes observed in hip fracture, which could be alleviated by PCN.

Disrupted population coding in the prefrontal cortex underlies pain aversion.

The prefrontal cortex (PFC) regulates a wide range of sensory experiences. Chronic pain is known to impair normal neural response, leading to enhanced aversion. However, it remains unknown how nociceptive responses in the cortex are processed at the population level and whether such processes are disrupted by chronic pain. Using in vivo endoscopic calcium imaging, we identify increased population activity in response to noxious stimuli and stable patterns of functional connectivity among neurons in the prelimbic (PL) PFC from freely behaving rats. Inflammatory pain disrupts functional connectivity of PFC neurons and reduces the overall nociceptive response. Interestingly, ketamine, a well-known neuromodulator, restores the functional connectivity among PL-PFC neurons in the inflammatory pain model to produce anti-aversive effects. These results suggest a dynamic resource allocation mechanism in the prefrontal representations of pain and indicate that population activity in the PFC critically regulates pain and serves as an important therapeutic target.

IL-23/IL-17A/TRPV1 axis produces mechanical pain via macrophage-sensory neuron crosstalk in female mice.

Although sex dimorphism is increasingly recognized as an important factor in pain, female-specific pain signaling is not well studied. Here we report that administration of IL-23 produces mechanical pain (mechanical allodynia) in female but not male mice, and chemotherapy-induced mechanical pain is selectively impaired in female mice lacking Il23 or Il23r. IL-23-induced pain is promoted by estrogen but suppressed by androgen, suggesting an involvement of sex hormones. IL-23 requires C-fiber nociceptors and TRPV1 to produce pain but does not directly activate nociceptor neurons. Notably, IL-23 requires IL-17A release from macrophages to evoke mechanical pain in females. Low-dose IL-17A directly activates nociceptors and induces mechanical pain only in females. Finally, deletion of estrogen receptor subunit α (ERα) in TRPV1+ nociceptors abolishes IL-23- and IL-17-induced pain in females. These findings demonstrate that the IL-23/IL-17A/TRPV1 axis regulates female-specific mechanical pain via neuro-immune interactions. Our study also reveals sex dimorphism at both immune and neuronal levels.

Activity of Mitragyna speciosa ("Kratom") Alkaloids at Serotonin Receptors.

Kratom alkaloids have mostly been evaluated for their opioid activity but less at other targets that could contribute to their physiological effects. Here, we investigated the in vitro and in vivo activity of kratom alkaloids at serotonin receptors (5-HTRs). Paynantheine and speciogynine exhibited high affinity for 5-HT1ARs and 5-HT2BRs, unlike the principal kratom alkaloid mitragynine. Both alkaloids produced antinociceptive properties in rats via an opioid receptor-independent mechanism, and neither activated 5-HT2BRs in vitro. Paynantheine, speciogynine, and mitragynine induced lower lip retraction and antinociception in rats, effects blocked by a selective 5-HT1AR antagonist. In vitro functional assays revealed that the in vivo 5-HT1AR agonistic effects may be due to the metabolites 9-O-desmethylspeciogynine and 9-O-desmethylpaynantheine and not the parent compounds. Both metabolites did not activate 5-HT2BR, suggesting low inherent risk of causing valvulopathy. The 5-HT1AR agonism by kratom alkaloids may contribute to the mood-enhancing effects associated with kratom use.

Acid-sensing ion channels modulate bladder nociception.

Sensitization of neuronal pathways and persistent afferent drive are major contributors to somatic and visceral pain. However, the underlying mechanisms that govern whether afferent signaling will give rise to sensitization and pain are not fully understood. In the present report, we investigated the contribution of acid-sensing ion channels (ASICs) to bladder nociception in a model of chemical cystitis induced by cyclophosphamide (CYP). We found that the administration of CYP to mice lacking ASIC3, a subunit primarily expressed in sensory neurons, generates pelvic allodynia at a time point at which only modest changes in pelvic sensitivity are apparent in wild-type mice. The differences in mechanical pelvic sensitivity between wild-type and Asic3 knockout mice treated with CYP were ascribed to sensitized bladder C nociceptors. Deletion of Asic3 from bladder sensory neurons abolished their ability to discharge action potentials in response to extracellular acidification. Collectively, the results of our study support the notion that protons and their cognate ASIC receptors are part of a mechanism that operates at the nerve terminals to control nociceptor excitability and sensitization.NEW & NOTEWORTHY Our study indicates that protons and their cognate acid-sensing ion channel receptors are part of a mechanism that operates at bladder afferent terminals to control their function and that the loss of this regulatory mechanism results in hyperactivation of nociceptive pathways and the development of pain in the setting of chemical-induced cystitis.

The neuronal potassium current IA is a potential target for pain during chronic inflammation.

Voltage-gated ion channels play a key role in the action potential (AP) initiation and its propagation in sensory neurons. Modulation of their activity during chronic inflammation creates a persistent pain state. In this study, we sought to determine how peripheral inflammation caused by complete Freund's adjuvant (CFA) alters the fast sodium (INa ), L-type calcium (ICaL ), and potassium (IK ) currents in primary afferent fibers to increase nociception. In our model, intraplantar administration of CFA induced mechanical allodynia and thermal hyperalgesia at day 14 post-injection. Using whole-cell patch-clamp recording in dissociated small (C), medium (Aδ), and large-sized (Aß) rat dorsal root ganglion (DRG) neurons, we found that CFA prolonged the AP duration and increased the amplitude of the tetrodotoxin-resistant (TTX-r) INa in Aß fibers. In addition, CFA accelerated the recovery of INa from inactivation in C and Aδ nociceptive fibers but enhanced the late sodium current (INaL ) only in Aδ and Aß neurons. Inflammation similarly reduced the amplitude of ICaL in each neuronal cell type. Fourteen days after injection, CFA reduced both components of IK (IKdr and IA ) in Aδ fibers. We also found that IA was significantly larger in C and Aδ neurons in normal conditions and during chronic inflammation. Our data, therefore, suggest that targeting the transient potassium current IA represents an efficient way to shift the balance toward antinociception during inflammation, since its activation will selectively decrease the AP duration in nociceptive fibers. Altogether, our data indicate that complex interactions between IK , INa , and ICaL reduce pain threshold by concomitantly enhancing the activity of nociceptive neurons and reducing the inhibitory action of Aß fibers during chronic inflammation.

TREK-1 potassium channels participate in acute and long-lasting nociceptive hypersensitivity induced by formalin in rats.

TREK-1 channels are expressed in small nociceptive dorsal root ganglion (DRG) neurons where they participate in acute inflammatory and neuropathic pain. However, the role of TREK-1 in persistent pain is not well understood. The aim of this study was to investigate the local peripheral and spinal participation of TREK-1 in formalin-induced acute and long-lasting nociceptive hypersensitivity. Local peripheral or intrathecal pre-treatment with spadin, selective blocker of TREK-1, increased acute flinching behavior and secondary mechanical allodynia and hyperalgesia behavior observed 6 days after formalin injection. Local peripheral or intrathecal pre-treatment with BL-1249, selective opener of TREK-1, decreased long-lasting secondary mechanical allodynia and hyperalgesia induced by formalin. Pre-treatment with BL-1249 prevented the pro-nociceptive effect of spadin on acute nociception and long-lasting mechanical allodynia and hyperalgesia in rats. Pre-treatment with two recombinant channels that produce a high TREK-1 current, S300A and S333A (non-phosphorylated state of TREK-1), reduced formalin-induced acute pain and long-lasting mechanical allodynia and hyperalgesia. Besides, post-treatment with S300A, S333A or BL-1249 reversed long-lasting mechanical allodynia and hyperalgesia induced by formalin. Formalin increased TREK-1 expression at 1 and 6 days in DRG and dorsal spinal cord in rats, whereas that it increased c-fos expression at the DRG. Intrathecal repeated transfection of rats with S300A and S333A or injection with BL-1249 reduced formalin-induced enhanced c-fos expression. Data suggest that TREK-1 activity at peripheral and spinal sites reduces neuronal excitability in the process of acute and long-lasting nociception induced by formalin in rats.

Nociceptive sensory neurons promote CD8 T cell responses to HSV-1 infection.

Host protection against cutaneous herpes simplex virus 1 (HSV-1) infection relies on the induction of a robust adaptive immune response. Here, we show that Nav1.8+ sensory neurons, which are involved in pain perception, control the magnitude of CD8 T cell priming and expansion in HSV-1-infected mice. The ablation of Nav1.8-expressing sensory neurons is associated with extensive skin lesions characterized by enhanced inflammatory cytokine and chemokine production. Mechanistically, Nav1.8+ sensory neurons are required for the downregulation of neutrophil infiltration in the skin after viral clearance to limit the severity of tissue damage and restore skin homeostasis, as well as for eliciting robust CD8 T cell priming in skin-draining lymph nodes by controlling dendritic cell responses. Collectively, our data reveal an important role for the sensory nervous system in regulating both innate and adaptive immune responses to viral infection, thereby opening up possibilities for new therapeutic strategies.

Role of ß-arrestin-2 in short- and long-term opioid tolerance in the dorsal root ganglia.

G-protein-biased agonists with reduced ß-arrestin-2 activation are being investigated as safer alternatives to clinically-used opioids. ß-arrestin-2 has been implicated in the mechanism of opioid-induced antinociceptive tolerance. Opioid-induced analgesic tolerance is classically considered as centrally-mediated, but recent reports implicate nociceptive dorsal root ganglia neurons as critical mediators in this process. Here, we investigated the role of ß-arrestin-2 in the mechanism of opioid tolerance in dorsal root ganglia nociceptive neurons using ß-arrestin-2 knockout mice and the G-protein-biased µ-opioid receptor agonist, TRV130. Whole-cell current-clamp electrophysiology experiments revealed that 15-18-h overnight exposure to 10 µM morphine in vitro induced acute tolerance in ß-arrestin-2 wild-type but not knockout neurons. Furthermore, in wild-type neurons circumventing ß-arrestin-2 activation by overnight treatment with 200 nM TRV130 attenuated tolerance. Similarly, acute morphine tolerance in vivo in ß-arrestin-2 knockout mice was prevented in the warm-water tail-withdrawal assay. Treatment with 30 mg/kg TRV130 s.c. also inhibited acute antinociceptive tolerance in vivo in wild-type mice. Alternately, in ß-arrestin-2 knockout neurons tolerance induced by 7-day in vivo exposure to 50 mg morphine pellet was conserved. Likewise, ß-arrestin-2 deletion did not mitigate in vivo antinociceptive tolerance induced by 7-day exposure to 25 mg or 50 mg morphine pellet in both female or male mice, respectively. Consequently, these results indicated that ß-arrestin-2 mediates acute but not chronic opioid tolerance in dorsal root ganglia neurons and to antinociception in vivo. This suggests that opioid-induced antinociceptive tolerance may develop even in the absence of ß-arrestin-2 activation, and thus significantly affect the clinical utility of biased agonists.

Antinociceptive effect of intrathecal P7C3 via GABA in a rat model of inflammatory pain.

The recently identified molecule P7C3 has been highlighted in the field of pain research. We examined the effect of intrathecal P7C3 in tissue injury pain evoked by formalin injection and determined the role of the GABA system in the activity of P7C3 at the spinal level. Male Sprague-Dawley rats with intrathecal catheters implanted for experimental drug delivery were studied. The effects of intrathecal P7C3 and nicotinamide phosphoribosyltransferase (NAMPT) administered 10 min before the formalin injection were examined. Animals were pretreated with bicuculline, a GABA-A receptor antagonist; saclofen, a GABA-B receptor antagonist; L-allylglycine, a glutamic acid decarboxylase (GAD) blocker; and CHS 828, an NAMPT inhibitor; to observe involvement in the effects of P7C3. The effects of P7C3 alone and the mixture of P7C3 with GABA receptor antagonists on KCl-induced calcium transients were examined in rat dorsal root ganglion (DRG) neurons. The expression of GAD and the concentration of GABA in the spinal cord were evaluated. Intrathecal P7C3 and NAMPT produced an antinociceptive effect in the formalin test. Intrathecal bicuculline, saclofen, L-allylglycine, and CHS 828 reversed the antinociception of P7C3 in both phases. P7C3 decreased the KCl-induced calcium transients in DRG neurons. Both bicuculline and saclofen reversed the blocking effect of P7C3. The levels of GAD expression and GABA concentration decreased after formalin injection and were increased by P7C3. These results suggest that P7C3 increases GAD activity and then increases the GABA concentration in the spinal cord, which in turn may act on GABA receptors causing the antinociceptive effect against pain evoked by formalin injection.

Functional expression of glycine receptors in DRG neurons of mice.

Glycine receptor is one of the chloride-permeable ion channels composed of combinations of four α subunits and one ß subunit. In adult spinal cord, the glycine receptor α1 subunit is crucial for the generation of inhibitory neurotransmission. The reduced glycinergic inhibition is regarded as one of the key spinal mechanisms underlying pathological pain symptoms. However, the expression and function of glycine receptors in the peripheral system are largely unknown as yet. Here we found that glycine receptor α1 subunit was prevalent in the dorsal root ganglia (DRG) neurons as well as in the sciatic nerves of adult mice. Intraganglionar or intraplantar injection of glycine receptor antagonist strychnine caused the hypersensitivity to mechanical, thermal and cold stimuli, suggesting the functional importance of peripheral glycine receptors in the control of nociceptive signal transmission. Our data showed that peripheral inflammation induced by formalin decreased the expression of glycine receptor α1 subunit on the plasma membrane of DRG neurons, which was attributed to the activation of protein kinase C signaling. Intraplantar application of glycine receptor agonist glycine or positive modulator divalent zinc ion alleviated the first-phase painful behaviors induced by formalin. These data suggested that peripheral glycine receptor might serve as an effective target for pain therapy.

Selective blockade of transient receptor potential vanilloid 4 reduces cyclophosphamide-induced bladder pain in mice.

Transient receptor potential vanilloid 4 (TRPV4) is a non-selective cation channel activated by various physical stimuli such as cell swelling and shear stress. TRPV4 is expressed in bladder sensory nerves and epithelium, and its activation produces urinary dysfunction in rodents. However, there have been few reports regarding its involvement in bladder pain. Therefore, we investigated whether TRPV4 is involved in bladder pain in mouse cystitis model. Intraperitoneal injection of cyclophosphamide (CYP; 300 mg/kg) produced mechanical hypersensitivity in the lower abdomen associated with a severe inflammatory bladder in mice. The mechanical threshold was reversed significantly in Trpv4-knockout (KO) mice. Repeated injections of CYP (150 mg/kg) daily for 4 days provoked mild bladder inflammation and persistent mechanical hypersensitivity in mice. Trpv4-KO mice prevented a reduction of the mechanical threshold without an alteration in bladder inflammation. A selective TRPV4 antagonist also reversed the mechanical threshold in chronic cystitis mice. Although expression of Trpv4 was unchanged in the bladders of chronic cystitis mice, the level of phosphorylated TRPV4 was increased significantly. These results suggest involvement of TRPV4 in bladder pain of cystitis mice. A TRPV4 antagonist might be useful for patients with irritable bladder pain such as those with interstitial cystitis/painful bladder syndrome.

TNF-α, CXCL-1 and IL-1 ß as activators of the opioid system involved in peripheral analgesic control in mice.

Tissue injury results in the release of inflammatory mediators, including a cascade of nociceptive substances, which contribute to development of hyperalgesia. In addition, during this process endogenous analgesic substances are also peripherally released with the aim of controlling the hyperalgesia. Thus, the present study aimed to investigate whether inflammatory mediators TNF-α, IL-1ß, CXCL1, norepinephrine (NE) and prostaglandin E2 (PGE2) may be involved in the deflagration of peripheral endogenous modulation of inflammatory pain by activation of the opioid system. Thus, male Swiss mice and the paw withdrawal test were used. All substances were injected by the intraplantar route. Carrageenan, TNF-α, CXCL-1, IL1-ß, NE and PGE2 induced hyperalgesia. Selectives µ (clocinamox), δ (naltrindole) and κ (norbinaltorphimine, nor-BNI) and non-selective (naloxone) opioid receptor antagonists potentiated the hyperalgesia induced by carrageenan, TNF-α, CXCL-1 and IL1-ß. In contrast, when the enzyme N-aminopeptidase involved in the degradation of endogenous opioid peptides was inhibited by bestatin, the hyperalgesia was significantly reduced. In addition, the western blotting assay indicated that the expression of the opioid δ receptor was increased after intraplantar injection of carrageenan. The data obtained in this work corroborate the hypothesis that TNF-α, CXCL-1 and IL-ß cause, in addition to hyperalgesia, the release of endogenous substances such as opioid peptides, which in turn exert endogenous control over peripheral inflammatory pain.

The synergistic effects of opioid and neuropeptide B/W in rat acute inflammatory and neuropathic pain models.

The use of morphine is controversial due to the incidence of rewarding behavior, respiratory depression, and tolerance, leading to increased drug dose requirements, advancing to morphine addiction. To overcome these barriers, strategies have been taken to combine morphine with other analgesics. Neuropeptide B23 and neuropeptide W23 (NPB23 and NPW23) are commonly used to relieve inflammatory pain and neuropathic pain. As NPB23 and NPW23 system shares similar anatomical basis with opioid system at least in the spinal cord we hypothesized that NPB23 or NPW23 and morphine may synergistically relieve inflammatory pain and neuropathic pain. To test this hypothesis, we demonstrated that µ opioid receptor and NPBW1 receptor (receptor of NPB23 and NPW23) are colocalized in the superficial dorsal horn of the spinal cord. Secondly, co-administration of morphine witheitherNPB23 or NPW23 synergistically attenuated inflammatory and neuropathic pain. Furthermore, either NPB23 or NPW23 significantly reduced morphine-induced conditioned place preference (CPP) and constipation. We also found that phosphorylation of extracellular-regulated protein kinase (ERK1/2) following morphine was profoundly potentiated by the application of NPB23 or NPW23. Hence, combination of morphine with either NPB23 or NPW23 reduced dose of morphine required for pain relief in inflammatory and neuropathic pain, while effectively prevented some side-effects of morphine.

Involvement of TCF7L2 in generation of morphine-induced antinociceptive tolerance and hyperalgesia by modulating TLR4/ NF-κB/NLRP3 in microglia.

Morphine is an opioid agonist and a nonselective mu, kappa and delta receptor agonist. It is a commonly used analgesic drug for the treatment of acute and chronic pain as well as cancer pain. Morphine is particularly important to address the problem of morphine tolerance. Tcf7l2, known as a risk gene for schizophrenia and autism, encodes a member of the LEF1/TCF transcription factor family. TCF7L2 is an important transcription factor that is upregulated in neuropathic pain models. However, the relationship between TCF7L2 and morphine tolerance has not been reported. In this study, we found that morphine tolerance led to the upregulation of TCF7L2 in the spinal cord, and also led to the upregulation of TCF7L2 expression in glial cells, which promoted inflammation related signal, and activated TLR4 / NF-κB/NLRP3 pathway. In addition, TCF7L2 regulated microglial cell activation induced by chronic morphine treatment. Mechanically, we found that TCF7L2 transcriptionally regulated TLR4 expression, and the depletion of TCF7L2 alleviated morphine tolerance induced by chronic morphine treatment, and further alleviated pain hypersensitivity induced by chronic morphine treatment. We therefore suggested that TCF7L2 regulates the activation of TLR4/ NF-κB/NLRP3 pathway in microglia, and is involved in the formation of morphine tolerance. Our results provide a new idea for the regulation mechanism of morphine tolerance.