Stomatin-like protein 3 modulates the responses of Aδ, but not C fiber bone afferent neurons to noxious mechanical stimulation in an animal model of acute experimental bone pain.

STOML3 is a membrane bound scaffolding protein that has been shown to facilitate the opening of mechanically sensitive ion channels and contribute to noxious mechanical sensation, allodynia and hyperalgesia. In this study, we aimed to determine the role of STOML3 in noxious mechanical sensitivity of bone afferent neurons and carrageenan-induced acute inflammation in the bone. An in vivo, electrophysiological bone-nerve preparation was used to make recordings of the activity and sensitivity of bone afferent neurons that innervate the tibial marrow cavity in anaesthetised rats, in response to noxious mechanical stimuli delivered to the marrow cavity, before and after injection of either the STOML3 oligomerisation inhibitor OB-1 or vehicle, in either naïve animals or animals with carrageenan-induced inflammation of the marrow cavity. A dynamic weight-bearing apparatus was used to measure weight bearing in response to inflammatory pain before and after injection of OB-1 or saline into the tibial marrow cavity in the presence of carrageenan-induced inflammation. Electrophysiological recordings revealed that Aδ, but not C bone afferent neurons have a reduced discharge frequency in response to mechanical stimulation, and that carrageenan-induced sensitisation of Aδ, but not C bone afferent neurons was attenuated by inhibition of STOML3 oligomerisation with OB-1. Animals treated with OB-1 spent a significantly greater amount of time on the limb injected with carrageenan than animals treated with saline. Our findings demonstrate that inhibition of STOML3 oligomerisation reduces inflammatory bone pain by reducing the sensitivity of Aδ bone afferent neurons to mechanical stimulation. Targeting STOML3 may be an effective approach to reduce pain from noxious pressure and/or painful inflammatory pathology in bone.

Chemogenetic Silencing of NaV1.8-Positive Sensory Neurons Reverses Chronic Neuropathic and Bone Cancer Pain in FLEx PSAM4-GlyR Mice.

Drive from peripheral neurons is essential in almost all pain states, but pharmacological silencing of these neurons to effect analgesia has proved problematic. Reversible gene therapy using long-lived chemogenetic approaches is an appealing option. We used the genetically activated chloride channel PSAM4-GlyR to examine pain pathways in mice. Using recombinant AAV9-based delivery to sensory neurons, we found a reversal of acute pain behavior and diminished neuronal activity using in vitro and in vivo GCaMP imaging on activation of PSAM4-GlyR with varenicline. A significant reduction in inflammatory heat hyperalgesia and oxaliplatin-induced cold allodynia was also observed. Importantly, there was no impairment of motor coordination, but innocuous von Frey sensation was inhibited. We generated a transgenic mouse that expresses a CAG-driven FLExed PSAM4-GlyR downstream of the Rosa26 locus that requires Cre recombinase to enable the expression of PSAM4-GlyR and tdTomato. We used NaV1.8 Cre to examine the role of predominantly nociceptive NaV1.8+ neurons in cancer-induced bone pain (CIBP) and neuropathic pain caused by chronic constriction injury (CCI). Varenicline activation of PSAM4-GlyR in NaV1.8-positive neurons reversed CCI-driven mechanical, thermal, and cold sensitivity. Additionally, varenicline treatment of mice with CIBP expressing PSAM4-GlyR in NaV1.8+ sensory neurons reversed cancer pain as assessed by weight-bearing. Moreover, when these mice were subjected to acute pain assays, an elevation in withdrawal thresholds to noxious mechanical and thermal stimuli was detected, but innocuous mechanical sensations remained unaffected. These studies confirm the utility of PSAM4-GlyR chemogenetic silencing in chronic pain states for mechanistic analysis and potential future therapeutic use.

Periodic hypoxia, intermittent pain, and caffeine in male and female neonatal rats: corticosterone, insulin resistance, and hepatic gene expression.

Preterm infants experience multiple stressors including periodic neonatal hypoxia, maternal/caregiver separation, and acute pain from clinical procedures. Although neonatal hypoxia or interventional pain are associated with sexually dimorphic effects that may last into adulthood, the interaction of these common preterm stressors and caffeine pretreatment remains unknown. We hypothesize that an interaction of acute neonatal hypoxia, isolation, and pain modeling the experience of the preterm infant will augment the acute stress response and that caffeine routinely given to preterm infants will alter this response. Male and female rat pups were isolated and exposed to six cycles of periodic hypoxia (10% O2) or normoxia (room air control) and/or intermittent pain by administering needle pricks (or touch control) to the paw on postnatal (PD) days 1-4. An additional set of rat pups was pretreated with caffeine citrate (80 mg/kg ip) and studied on PD1. Plasma corticosterone, fasting glucose, and insulin were measured to calculate homeostatic model assessment for insulin resistance (HOMA-IR) (index of insulin resistance). Glucocorticoid-, insulin-, and caffeine-sensitive gene mRNAs were analyzed in the PD1 liver and hypothalamus to evaluate downstream markers of glucocorticoid action. Acute pain with periodic hypoxia led to a large increase in plasma corticosterone, which was attenuated by pretreatment with caffeine. Pain with periodic hypoxia led to a 10-fold increase in hepatic Per1 mRNA expression in males, which was attenuated with caffeine. The augmentation of corticosterone and HOMA-IR at PD1 after periodic hypoxia with pain suggests early intervention to attenuate the stress response may mitigate the programming effects of neonatal stress.

Modulatory role of intra-accumbal dopamine receptors in the restraint stress-induced antinociceptive responses.

Stress contributes to pain sensation by affecting several neural pathways, including mesolimbic-cortical dopamine neurons. Nucleus accumbens, an essential element of the mesolimbic dopaminergic pathway, plays a fundamental role in modulating pain and is differentially influenced by stressful events. Since we previously demonstrated the marked association of intra-NAc dopamine receptors with forced swim stress-evoked analgesia in acute pain state, this research was conducted to consider the contribution of intra-accumbal D1- and D2-like dopamine receptors to modulating effects of exposure to restraint stress in pain-related behaviors during the tail-flick test. Stereotaxic surgery was executed to implant a guide cannula within the NAc in male Wistar rats. On the test day, different concentrations of SCH23390 and Sulpiride as D1- and D2-like dopamine receptor antagonists, respectively, were unilaterally microinjected within the NAc. The vehicle animals received saline or 12 % DMSO (0.5 µl) instead of SCH23390 or Sulpiride into the NAc, respectively. Five minutes following receiving drug or vehicle, animals were restrained for 3 h and then their acute nociceptive threshold was measured for a 60-min period by the tail-flick test. Our data revealed that RS considerably enhanced antinociceptive reaction in acute pain states. The analgesia evoked by RS dramatically declined following blocking either D1- or D2-like dopamine receptors in the NAc, an effect was more noticeable by D1-like dopamine receptor antagonist. These findings indicated that intra-NAc dopamine receptors are considerably mediated in the RS-produced analgesia in acute pain states, suggesting their possible role in psychological stress and disease.

HDAC2 in Primary Sensory Neurons Constitutively Restrains Chronic Pain by Repressing α2δ-1 Expression and Associated NMDA Receptor Activity.

α2δ-1 (encoded by the Cacna2d1 gene) is a newly discovered NMDA receptor-interacting protein and is the therapeutic target of gabapentinoids (e.g., gabapentin and pregabalin) frequently used for treating patients with neuropathic pain. Nerve injury causes sustained α2δ-1 upregulation in the dorsal root ganglion (DRG), which promotes NMDA receptor synaptic trafficking and activation in the spinal dorsal horn, a hallmark of chronic neuropathic pain. However, little is known about how nerve injury initiates and maintains the high expression level of α2δ-1 to sustain chronic pain. Here, we show that nerve injury caused histone hyperacetylation and diminished enrichment of histone deacetylase-2 (HDAC2), but not HDAC3, at the Cacna2d1 promoter in the DRG. Strikingly, Hdac2 knockdown or conditional knockout in DRG neurons in male and female mice consistently induced long-lasting mechanical pain hypersensitivity, which was readily reversed by blocking NMDA receptors, inhibiting α2δ-1 with gabapentin or disrupting the α2δ-1-NMDA receptor interaction at the spinal cord level. Hdac2 deletion in DRG neurons increased histone acetylation levels at the Cacna2d1 promoter, upregulated α2δ-1 in the DRG, and potentiated α2δ-1-dependent NMDA receptor activity at primary afferent central terminals in the spinal dorsal horn. Correspondingly, Hdac2 knockdown-induced pain hypersensitivity was blunted in Cacna2d1 knockout mice. Thus, our findings reveal that HDAC2 functions as a pivotal transcriptional repressor of neuropathic pain via constitutively suppressing α2δ-1 expression and ensuing presynaptic NMDA receptor activity in the spinal cord. HDAC2 enrichment levels at the Cacna2d1 promoter in DRG neurons constitute a unique epigenetic mechanism that governs acute-to-chronic pain transition.SIGNIFICANCE STATEMENT Excess α2δ-1 proteins produced after nerve injury directly interact with glutamate NMDA receptors to potentiate synaptic NMDA receptor activity in the spinal cord, a prominent mechanism of nerve pain. Because α2δ-1 upregulation after nerve injury is long lasting, gabapentinoids relieve pain symptoms only temporarily. Our study demonstrates for the first time the unexpected role of intrinsic HDAC2 activity at the α2δ-1 gene promoter in limiting α2δ-1 gene transcription, NMDA receptor-dependent synaptic plasticity, and chronic pain development after nerve injury. These findings challenge the prevailing view about the role of general HDAC activity in promoting chronic pain. Restoring the repressive HDAC2 function and/or reducing histone acetylation at the α2δ-1 gene promoter in primary sensory neurons could lead to long-lasting relief of nerve pain.

Genetic variants of PKLR are associated with acute pain in sickle cell disease.

Acute pain, the most prominent complication of sickle cell disease (SCD), results from vaso-occlusion triggered by sickling of deoxygenated red blood cells (RBCs). Concentration of 2,3-diphosphoglycerate (2,3-DPG) in RBCs promotes deoxygenation by preferentially binding to the low-affinity T conformation of HbS. 2,3-DPG is an intermediate substrate in the glycolytic pathway in which pyruvate kinase (gene PKLR, protein PKR) is a rate-limiting enzyme; variants in PKLR may affect PKR activity, 2,3-DPG levels in RBCs, RBC sickling, and acute pain episodes (APEs). We performed a candidate gene association study using 2 cohorts: 242 adult SCD-HbSS patients and 977 children with SCD-HbSS or SCD-HbSß0 thalassemia. Seven of 47 PKLR variants evaluated in the adult cohort were associated with hospitalization: intron 4, rs2071053; intron 2, rs8177970, rs116244351, rs114455416, rs12741350, rs3020781, and rs8177964. All 7 variants showed consistent effect directions in both cohorts and remained significant in weighted Fisher's meta-analyses of the adult and pediatric cohorts using P < .0071 as threshold to correct for multiple testing. Allele-specific expression analyses in an independent cohort of 52 SCD adults showed that the intronic variants are likely to influence APE by affecting expression of PKLR, although the causal variant and mechanism are not defined.

Small extracellular vesicles from iPSC-derived mesenchymal stem cells ameliorate tendinopathy pain by inhibiting mast cell activation.

Aim: This study aimed to explore the effect of small extracellular vesicles from induced pluripotent stem cell-derived mesenchymal stem cells (iMSC-sEVs) on acute pain and investigate the underlying mechanisms. Materials & methods: The pathology of tendons was accessed by hematoxylin and eosin staining, immunohistochemical and immunofluorescent staining. The pain degree was measured by pain-related behaviors. In vitro, we performed ß-hexosaminidase release assay, RT-qPCR, toluidine blue staining, ELISA and RNA sequencing. Results: iMSC-sEVs effectively alleviated acute pain in tendinopathy as well as inhibiting activated mast cell infiltration and interactions with nerve fibers in vivo. In vitro, iMSC-sEVs reduced the degranulation of mast cells and the expression of proinflammatory cytokines and genes involved in the HIF-1 signaling pathway. Conclusion: This study demonstrated that iMSC-sEVs relieved tendinopathy-related pain through inhibiting mast cell activation via the HIF-1 signaling pathway.

α1-Antitrypsin derived SP16 peptide demonstrates efficacy in rodent models of acute and neuropathic pain.

SP16 is an innovative peptide derived from the carboxyl-terminus of α1-Antitrypsin (AAT), corresponding to residues 364-380, and contains recognition sequences for the low-density lipoprotein receptor-related protein-1 (LRP1). LRP1 is an endocytic and cell-signaling receptor that regulates inflammation. Deletion of Lrp1 in Schwann cells increases neuropathic pain; however, the role of LRP1 activation in nociceptive and neuropathic pain regulation remains unknown. Herein, we show that SP16 is bioactive in sensory neurons in vitro. Neurite length and regenerative gene expression were increased by SP16. In PC12 cells, SP16 activated Akt and ERK1/2 cell-signaling in an LRP1-dependent manner. When formalin was injected into mouse hind paws, to model inflammatory pain, SP16 dose-dependently attenuated nociceptive pain behaviors in the early and late phases. In a second model of acute pain using capsaicin, SP16 significantly reduced paw licking in both male and female mice (p < .01) similarly to enzymatically inactive tissue plasminogen activator, a known LRP1 interactor. SP16 also prevented development of tactile allodynia after partial nerve ligation and this response was sustained for nine days (p < .01). Immunoblot analysis of the injured nerve revealed decreased CD11b (p < .01) and Toll-like receptor-4 (p < .005). In injured dorsal root ganglia SP16 reduced CD11b+ cells (p < .05) and GFAP (p < .005), indicating that inflammatory cell recruitment and satellite cell activation were inhibited. In conclusion, administration of SP16 blocked pain-related responses in three distinct pain models, suggesting efficacy against acute nociceptive, inflammatory, and neuropathic pain. SP16 also attenuated innate immunity in the PNS. These studies identify SP16 as a potentially effective treatment for pain.

Further exploration of the structure-activity relationship of dual soluble epoxide hydrolase/fatty acid amide hydrolase inhibitors.

Fatty acid amide hydrolase (FAAH) is a membrane protein that hydrolyzes endocannabinoids, and its inhibition produces analgesic and anti-inflammatory effects. The soluble epoxide hydrolase (sEH) hydrolyzes epoxyeicosatrienoic acids (EETs) to dihydroxyeicosatetraenoic acids. EETs have anti-inflammatory and inflammation resolving properties, thus inhibition of sEH consequently reduces inflammation. Concurrent inhibition of both enzymes may represent a novel approach in the treatment of chronic pain. Drugs with multiple targets can provide a superior therapeutic effect and a decrease in side effects compared to ligands with single targets. Previously, microwave-assisted methodologies were employed to synthesize libraries of benzothiazole analogs from which high affinity dual inhibitors (e.g. 3, sEH IC50 = 9.6 nM; FAAH IC50 = 7 nM) were identified. Here, our structure-activity relationship studies revealed that the 4-phenylthiazole moiety is well tolerated by both enzymes, producing excellent inhibition potencies in the low nanomolar range (e.g. 6o, sEH IC50 = 2.5 nM; FAAH IC50 = 9.8 nM). Docking experiments show that the new class of dual inhibitors bind within the catalytic sites of both enzymes. Prediction of several pharmacokinetic/pharmacodynamic properties suggest that these new dual inhibitors are good candidates for further in vivo evaluation. Finally, dual inhibitor 3 was tested in the Formalin Test, a rat model of acute inflammatory pain. The data indicate that 3 produces antinociception against the inflammatory phase of the Formalin Test in vivo and is metabolically stable following intraperitoneal administration in male rats. Further, antinociception produced by 3 is comparable to that of ketoprofen, a traditional nonsteroidal anti-inflammatory drug. The results presented here will help toward the long-term goal of developing novel non-opioid therapeutics for pain management.

The active fragments of ghrelin cross the blood-brain barrier and enter the brain to produce antinociceptive effects after systemic administration.

G (1-5)-NH2, G (1-7)-NH2, and G (1-9) are the active fragments of ghrelin. The aim of this study was to investigate the antinociceptive effects, their ability to cross the blood-brain barrier, and the receptor mechanism(s) of these fragments using the tail withdrawal test in male Kunming mice. The antinociceptive effects of these fragments (2, 6, 20, and 60 nmol/mouse) were tested at 5, 10, 20, 30, 40, 50, and 60 min after intravenous (i.v.) injection. These fragments induced dose- and time-related antinociceptive effects relative to saline. Using the near infrared fluorescence imaging experiments, our results showed that these fragments could cross the brain-blood barrier and enter the brain. The antinociceptive effects of these fragments were completely antagonized by naloxone (intracerebroventricular, i.c.v.); however, naloxone methiodide (intraperitoneal, i.p.), which is the peripheral restricted opioid receptor antagonist, did not antagonize these antinociceptive effects. Furthermore, the GHS-R1α antagonist [D-Lys3]-GHRP-6 (i.c.v.) completely antagonized these antinociceptive effects, too. These results suggested that these fragments induced antinociceptive effects through central opioid receptors and GHS-R1α. In conclusion, our studies indicated that these active fragments of ghrelin could cross the brain-blood barrier and enter the brain and induce antinociceptive effects through central opioid receptors and GHS-R1α after intravenous injection.

Acute Visceral Pain in Rats: Vagal Nerve Block Compared to Bupivacaine Administered Intramuscularly.

BACKGROUND: Visceral and parietal peritoneum layers have different sensory innervations. Most visceral peritoneum sensory information is conveyed via the vagus nerve to the nucleus of the solitary tract (NTS). We already showed in animal models that intramuscular (i.m.) injection of local anesthetics decreases acute somatic and visceral pain and general inflammation induced by aseptic peritonitis. The goal of the study was to compare the effects of parietal block, i.m. bupivacaine, and vagotomy on spinal cord and NTS stimulation induced by a chemical peritonitis. METHODS: We induced peritonitis in rats using carrageenan and measured cellular activation in spinal cord and NTS under the following conditions, that is, a parietal nerve block with bupivacaine, a chemical right vagotomy, and i.m. microspheres loaded with bupivacaine. Proto-oncogene c-Fos (c-Fos), cluster of differentiation protein 11b (CD11b), and tumor necrosis factor alpha (TNF-α) expression in cord and NTS were studied. RESULTS: c-Fos activation in the cord was inhibited by nerve block 2 hours after peritoneal insult. Vagotomy and i.m. bupivacaine similarly inhibited c-Fos activation in NTS. Forty-eight hours after peritoneal insult, the number of cells expressing CD11b significantly increased in the cord (P = .010). The median difference in the effect of peritonitis compared to control was 30 cells (CI95, 13.5-55). TNF-α colocalized with CD11b. Vagotomy inhibited this microglial activation in the NTS, but not in the cord. This activation was inhibited by i.m. bupivacaine both in cord and in NTS. The median difference in the effect of i.m. bupivacaine added to peritonitis was 29 cells (80% increase) in the cord and 18 cells (75% increase) in the NTS. Our study underlines the role of the vagus nerve in the transmission of an acute visceral pain message and confirmed that systemic bupivacaine prevents noxious stimuli by inhibiting c-Fos and microglia activation. CONCLUSIONS: In rats receiving intraperitoneal carrageenan, i.m. bupivacaine similarly inhibited c-Fos and microglial activation both in cord and in the NTS. Vagal block inhibited activation only in the NTS. Our study underlines the role of the vagus nerve in the transmission of an acute visceral pain message and confirmed that systemic bupivacaine prevents noxious stimuli. This emphasizes the effects of systemic local anesthetics on inflammation and visceral pain.

Electroacupuncture alleviates the transition from acute to chronic pain through the p38 MAPK/TNF-α signalling pathway in the spinal dorsal horn.

BACKGROUND: Hyperalgesic priming (HP) is a model of the transition from acute to chronic pain. Electroacupuncture (EA) could inhibit pain development through the peripheral dorsal root ganglia; however, it is unclear whether it can mitigate the transition from acute to chronic pain by attenuating protein expression in the p38 MAPK (mitogen-activated protein kinase)/tumour necrosis factor alpha (TNF-α) pathway in the spinal dorsal horn. AIMS: We aimed to determine whether EA could prevent the transition from acute to chronic pain by affecting the p38 MAPK/TNF-α pathway in the spinal dorsal horn in a rat model established using HP. METHODS: We first randomly subdivided 30 male Sprague-Dawley (SD) rats into 5 groups (n = 6 per group): control (N), sham HP (Sham-HP), HP, HP + SB203580p38 MAPK (HP+SB203580), and HP + Lenalidomide (CC-5013) (HP+Lenalidomide). We then randomly subdivided a further 30 male SD rats into 5 groups (n = 6 per group): Sham-HP, HP, sham EA (Sham EA), EA (EA), and EA + U-46619 p38 MAPK agonist (EA+U-46619). We assessed the effects of EA on the mechanical paw withdrawal threshold and p38 MAPK/TNF-α expression in the spinal dorsal horn of rats subjected to chronic inflammatory pain. RESULTS: Rats in the EA group had reduced p38 MAPK and TNF-α expression and had significantly reduced mechanical hyperalgesia compared with rats in the other groups. CONCLUSION: Our findings indicate that EA could increase the mechanical pain threshold in rats and inhibit the transition from acute pain to chronic pain. This mechanism could involve reduced p38 MAPK/TNF-α expression in the spinal dorsal horn.

Carbenoxolone has the potential to ameliorate acute incision pain in rats.

Carbenoxolone (CBX) is primarily used to relieve various types of neuropathic and inflammatory pain. However, little is known concerning the role of CBX in acute pain and its functional mechanisms therein and this was investigated in the present study. Rats underwent toe incision and behavioral tests were performed to assess mechanical hypersensitivity. The expression levels of pannexin 1 (Px1) and connexin 43 (Cx43) were detected using western blot analysis 2, 4, 6 or 24 h after toe incision, and the expression of TNF­α, IL­1ß and P substance (SP) was determined by ELISA; Px1 and Cx43 expression was also examined by immunofluorescence staining. At 2, 6 and 12 h post­toe incision, the postoperative pain threshold was significantly reduced, which was subsequently recovered at 2 and 6 h post­surgery following pretreatment with CBX or pannexin 1 mimetic inhibitory peptide. CBX reduced Px1 levels at 4 and 24 h post­incision. However, Cx43 levels were reduced by CBX as little as 2 h post­surgery. Furthermore, CBX not only distinctly decreased the levels of Px1 and Cx43, but also reduced the co­localization of Px1 or Cx43 with glial fibrillary acidic protein, 2 h after incision. It was also observed that the protein levels of inflammatory makers (IL­1ß, SP and TNF­α) showed a tendency to decline at 2, 4, 6 and 24 h after incision. Collectively, the expression of Px1 and Cx43 in astrocytes may be involved in pain behaviors diminished by CBX, and CBX potentially reduces acute pain by decreasing Px1 and Cx43 levels. Px1 and Cx43 from spinal astrocytes may serve important roles in the early stages and maintenance of acute pain, while preoperative injection of CBX has the potential to relieve hyperalgesia.

Deletion of MyD88 adaptor in nociceptor alleviates low-dose formalin-induced acute pain and persistent pain in mice.

The myeloid differentiation factor 88 (MyD88) adaptor mediates signaling by Toll-like receptors and some interleukins (ILs) in neural and non-neuronal cells. Recently, MyD88 protein was found to express in primary sensory neurons and be involved in the maintenance of persistent pain induced by complete Freund's adjuvant, chronic constriction injury and chemotherapy treatment in rodents. However, whether MyD88 in nociceptive neurons contributes to persistent pain induced by intraplantar injection of formalin remains elusive. Here, using conditional knockout (CKO) mice, we found that selective deletion of Myd88 in Nav1.8-expressing primary nociceptive neurons led to reduced pain response in the recovery phase of 1% formalin-induced mechanical pain and impaired the persistent thermal pain. Moreover, CKO mice exhibited reduced phase II pain response in 1%, but not 5%, formalin-induced acute inflammatory pain. Finally, nociceptor MyD88 deletion resulted in less neuronal c-Fos activation in spinal dorsal horns following 1% formalin stimulation. These data suggest that MyD88 in nociceptive neurons is not only involved in persistent mechanical pain but also promotes the transition from acute inflammatory pain to persistent thermal hyperalgesia induced by low-dose formalin stimulation.

Ultra-purified alginate gel implantation decreases inflammatory cytokine levels, prevents intervertebral disc degeneration, and reduces acute pain after discectomy.

Lumbar intervertebral disc (IVD) herniation causes severe low back pain (LBP), which results in substantial financial and emotional strains. Despite the effectiveness of discectomy, there is no existing treatment for post-operative LBP induced by progressive IVD degeneration. Two key factors of LBP are intradiscal inflammation, indicated by tumour necrosis factor alpha (TNF-α) and interleukin-6 (IL-6), and sensory nerve ingrowth into the inner layer of the annulus fibrosus, triggered by nerve growth factor/high-affinity tyrosine kinase A (TrkA) signalling. In an animal models of discectomy, the bioresorbable ultra-purified alginate (UPAL) gel with an extremely low-toxicity has been effective in acellular tissue repair. We aimed to investigate whether UPAL gel can alleviate LBP using a rat nucleus pulposus (NP) punch model and a rabbit NP aspirate model. In both models, we assessed TNF-α and IL-6 production and TrkA expression within the IVD by immunohistochemistry. Further, histological analysis and behavioural nociception assay were conducted in the rat model. UPAL gel implantation suppressed TNF-α and IL-6 production, downregulated TrkA expression, inhibited IVD degeneration, and reduced nociceptive behaviour. Our results suggest the potential of UPAL gel implantation as an innovative treatment for IVD herniation by reducing LBP and preventing IVD degeneration after discectomy.

Selective activation of metabotropic glutamate receptor 7 blocks paclitaxel-induced acute neuropathic pain and suppresses spinal glial reactivity in rats.

RATIONALE: Paclitaxel-induced acute pain syndrome (P-APS), characterized by deep muscle aches and arthralgia, occurs in more than 70% of patients who receive paclitaxel. P-APS can be debilitating for patients and lead to reductions and discontinuation of potentially curable therapy. Despite being relatively common in clinical practice, no clear treatment exists for P-APS and the underlying mechanisms remain poorly defined. Regulation of glutamatergic transmission by metabotropic glutamate receptors (mGluRs) has received growing attention with respect to its role in neuropathic pain. To our knowledge, no study has been conducted on alterations and functions of group III mGluR7 signaling in P-APS. OBJECTIVES: In the present study, we determined whether a single administration of paclitaxel induces glutamatergic alterations and whether mGluR7 activation blocks paclitaxel-induced neuropathic pain by suppressing glial reactivity in the spinal cord. RESULTS: A single paclitaxel injection dose-dependently induced acute mechanical and thermal hypersensitivity, and was associated with increased glutamate level accompanied by reduction in mGluR7 expression in the spinal cord. Selective activation of mGluR7 by its positive allosteric modulator, AMN082, blocked the development of paclitaxel-induced acute mechanical and thermal hypersensitivity, without affecting the normal pain behavior of control rats. Moreover, activation of mGluR7 by AMN082 inhibited glial reactivity and decreased pro-inflammatory cytokine release during P-APS. Abortion of spinal glial reaction to paclitaxel alleviated paclitaxel-induced acute mechanical and thermal hypersensitivity. CONCLUSIONS: There results support the hypothesis that spinal mGluR7 signaling plays an important role in P-APS; Selective activation of mGluR7 by its positive allosteric modulator, AMN082, blocks P-APS in part by reducing spinal glial reactivity and neuroinflammatory process.

Preoperative Chronic and Acute Pain Affects Postoperative Cognitive Function Mediated by Neurotransmitters.

The effective prevention of postoperative cognitive dysfunction (POCD) needs to be explored, and the effect of preoperative pain on POCD remains unclear. We established a chronic pain model induced by chronic constriction injury (CCI) and models of acute pain and anxiety without pain in mice that were subsequently subjected to partial hepatectomy surgery. Morris water maze (MWM) tests were performed to evaluate the learning and memory abilities of the mice. ELISA was used to measure IL-1ß, IL-6, and TNF-α in serum, and HPLC-MS was used to detect neurotransmitters in the prefrontal cortices and hippocampi of the mice. The results indicated that chronic pain induced by CCI might have significantly impaired the learning and memory abilities of mice, while acute pain and anxiety without pain only affected the memory abilities of mice. Perioperative acute pain increased the level of IL-1ß in serum, and CCI might have increased the level of IL-6. CCI and acute pain increased dopamine (DA) levels in the cortex, similar to anxiety. Like anxiety, CCI increased 5-hydroxytryptamine (5-HT) levels in the prefrontal cortex and hippocampus. Acute pain led to a decrease in the acetylcholine (ACH) level in the hippocampus. Our results suggest that acute pain and CCI-induced chronic pain might aggravate postoperative cognitive dysfunction via neurotransmitters and by changing the levels of inflammatory factors such as IL-1ß and IL-6.

Topical and systemic use of Joannesia princeps vell. LC seed oil in acute pain and inflammation induced by different agents.

ETHNOPHARMACOLOGICAL RELEVANCE: Joannesia princeps (SOJP) has been used in folk medicine as anthelmintic treatment and cutaneous wound healing. AIM OF THE STUDY: The purpose of this study is to evaluate the pharmacological activity of seed oil of Joannesia princeps, administered systemically and topically, on acute pain and inflammation. MATERIALS AND METHODS: Male swiss mice were treated orally and topically with seed oil of Joannesia princeps in models of acute pain (acetic acid-induced abdominal writhing, formalin-induced licking behaviour and tail flick tests) and acute inflammation (carrageenan- and histamine-induced paw oedema; arachidonic acid-, capsaicin- and croton oil-induced ear oedema and air pouch tests), besides the open field model in the motor performance evaluation. RESULTS: Seed oil of Joannesia princeps showed systemic action against acute pain in abdominal writhing test (37% and 56% inhibition in the number of writhes at doses of 30 and 100 mg/kg, respectively) and in the second phase of formalin-induced licking behaviour test (29%, 47 and 52% inhibition in the licking time at doses of 10, 30 and 100 mg/kg, respectively), as well as reducing croton oil-induced ear oedema by 72%, leukocyte recruitment and production of TNF-α and IL-6 in the air pouch tests. In addition, topical administration of SOJP inhibited carrageenan-induced paw oedema by 39% at dose of 500 µg/paw and inhibited histamine-induced oedema by 43 and 52% at doses of 300 and 500 µg/paw, respectively. SOJP also decreased croton oil-induced ear oedema by 67% at dose of 500 µg/paw and arachidonic acid-induced ear oedema by 63% at dose of 500 µg/paw, reducing the production of TNF-α, IL-1ß and MIP2 in both. In addition, no adverse effects were observed at doses up to 2000 mg/kg. CONCLUSIONS: Seed oil of Joannesia princeps presents antinociceptive and anti-inflammatory actions through its topical and systemic administration, promoted by inhibition of leukocyte recruitment and cytokine production (TNF-α, IL-1ß, IL-6 and MIP-2).

Role of orexin receptors within the dentate gyrus in antinociception induced by chemical stimulation of the lateral hypothalamus in an animal model of inflammatory pain.

Pain is a complex experience consisting of sensory, affective-motivational, and cognitive dimensions. Hence, identifying the multiple neural pathways subserving these functional aspects is a valuable task. The role of dentate gyrus (DG) as a relay station of neocortical afferents in the hippocampal formation (HF) in persistent pain is still controversial. The lateral hypothalamus (LH)-HF neural circuits are involved in numerous situations such as anxiety-like behavior, reward processing, feeding, orofacial as well as acute pain. Nonetheless, to our knowledge, the involvement of the LH-DG neural circuit in persistent pain has already remained unexplored. Adult male Wistar rats weighing 220-250 g were undergone stereotaxic surgery for unilateral implantation of two separate cannulae into the LH and DG. Intra-DG administration of the orexin-1 (OX1) and orexin-2 (OX2) receptor antagonists, SB334867 and TCS OX2 29, respectively, was performed 5 min before intra-LH microinjection of carbachol. Animals were then undergone the formalin test using 50 µl formalin injection (2.5%) into the plantar surface of the hind paw. Microinjection of SB334867 or TCS OX2 29 into the DG region attenuated the antinociceptive effect produced by carbachol microinjection into the LH. The preventive effect of SB334867 and TCS OX2 29 on intra-LH carbachol-induced antinociception was approximately equal in both early and late phases of formalin nociception. The results suggest a neural pathway from the LH to the DG, which contributes to the modulation of formalin-induced inflammatory pain through the recruitment of OX1 and OX2 receptors within the DG.

P2X3 receptors contribute to transition from acute to chronic muscle pain.

This study aimed to evaluate whether the development and/or maintenance of chronic-latent muscle hyperalgesia is modulated by P2X3 receptors. We also evaluate the expression of P2X3 receptors and PKCε of dorsal root ganglions during these processes. A mouse model of chronic-latent muscle hyperalgesia, induced by carrageenan and evidenced by PGE2, was used. Mechanical muscle hyperalgesia was measured by Randall-Selitto analgesimeter. The involvement of P2X3 receptors was analyzed by using the selective P2X3 receptors antagonist A-317491 by intramuscular or intrathecal injections. Expression of P2X3 and PKCε in dorsal root ganglion (L4-S1) were evaluated by Western blotting. Intrathecal blockade of P2X3 receptors previously to carrageenan prevented the development and maintenance of acute and chronic-latent muscle hyperalgesia, while intramuscular blockade of P2X3 receptors previously to carrageenan only reduced the acute muscle hyperalgesia and had no effect on chronic-latent muscle hyperalgesia. Intrathecal, but not intramuscular, blockade of P2X3 receptors immediately before PGE2, in animals previously sensitized by carrageenan, reversed the chronic-latent muscle hyperalgesia. There was an increase in total and phosphorylated PKCε 48 h after the beginning of acute muscle hyperalgesia, and in P2X3 receptors at the period of chronic muscle hyperalgesia. P2X3 receptors expressed on spinal cord dorsal horn contribute to transition from acute to chronic muscle pain. We also suggest an interaction of PKCε and P2X3 receptors in this process. Therefore, we point out P2X3 receptors of the spinal cord dorsal horn as a pharmacological target to prevent the development or reverse the chronic muscle pain conditions.