Persistent Mechanical Hyperalgesia Research Articles

Scorpion Neurotoxin: An Analgesic Antitumor Peptide Down-Regulates VGSCs and Exhibits Local Anesthetic Potency during Sciatic Nerve Block

Background: Some neurotoxins inhibit nerves strongly. Saxitoxins and tetrodotoxin exhibit strong affinity for various voltage-gated sodium channels (VGSCs), including Nav 1.7, but low affinity for Nav 1.8. Emerging data suggests AGAP mediate analgesia by binding voltage-independently to sodium channel site-3. Objective: This study examined AGAP's mechanism and analgesic efficacy during sciatic nerve block. Methods: In rats, partial sciatic nerve ligation caused rapid and persistent mechanical hyperalgesia. Mechanical hyperalgesia after medication delivery was assessed with von Frey filaments. Western blot and immunohistochemical staining assessed VGSC expression following drug treatment. Results: The data showed that lidocaine and AGAP increased paw withdrawal threshold compared to control rats. Compared to the control group, lidocaine or AGAP inhibited VGSCs. Conclusion: This study suggested that AGAP inhibits VGSCs to elicit lidocaine-like analgesia.

CDDO regulates central and peripheral sensitization to attenuate post-herpetic neuralgia by targeting TRPV1/PKC-δ/p-Akt signals.

Postherpetic neuralgia (PHN) is a notorious neuropathic pain featuring persistent profound mechanical hyperalgesia with significant negative impact on patients' life quality. CDDO can regulate inflammatory response and programmed cell death. Its derivative also protects neurons from damages by modulating microglia activities. As a consequence of central and peripheral sensitization, applying neural blocks may benefit to minimize the risk of PHN. This study aimed to explore whether CDDO could generate analgesic action in a PHN-rats' model. The behavioural test was determined by calibrated forceps testing. The number of apoptotic neurons and degree of glial cell reaction were assessed by immunofluorescence assay. Activation of PKC-δ and the phosphorylation of Akt were measured by western blots. CDDO improved PHN by decreasing TRPV1-positive nociceptive neurons, the apoptotic neurons, and reversed glial cell reaction in adult rats. It also suppressed the enhanced PKC-δ and p-Akt signalling in the sciatic nerve, dorsal root ganglia (DRG) and spinal dorsal horn. Our research is the promising report demonstrating the analgesic and neuroprotective action of CDDO in a PHN-rat's model by regulating central and peripheral sensitization targeting TRPV1, PKC-δ and p-Akt. It also is the first study to elucidate the role of oligodendrocyte in PHN.

Microglia Promote Inhibitory Synapse Phagocytosis in the Spinal Cord Dorsal Horn and Modulate Pain-Like Behaviors in a Murine Cancer-Induced Bone Pain Model.

The microglial activation has been implicated in cancer-induced bone pain. Recent studies have revealed that microglia mediate synaptic pruning in the central nervous system, where the cluster of differentiation 47-signal regulatory protein α (CD47-SIRPα) axis creates a "don't eat me" signal and elicits an antiphagocytic effect to protect synapses against elimination. To date, the synaptic phagocytosis in microglia has never been investigated in the murine cancer-induced bone pain model. The present experiments sought to explore whether microglia phagocytize synapses in mice with bone cancer pain as well as the possible mechanisms. Male C3H/HeN mice were used to induce bone cancer pain. Minocycline and S-ketamine were injected into D14. The number of spontaneous flinches (NSF) and paw withdrawal mechanical thresholds (PWMT) were measured on D0, D4, D7, D10, D14, D21, and D28. Hematoxylin and eosin staining presented bone lesions. Western blotting examined the Gephyrin, CD47, and SIRPα expression. Flow cytometry evaluated the proportion of SIRPα + cells in the spine. Immunofluorescence and 3-dimensional reconstruction showed the Gephyrin puncta inside microglial lysosomes. Mice embedded with tumor cells induced persistent spontaneous pain and mechanical hyperalgesia. Hematoxylin and eosin staining revealed bone destruction and tumor infiltration in marrow cavities. Microglia underwent a responsive and proliferative burst (t = -16.831, P < .001). Western blotting manifested lowered Gephyrin expression in the tumor group (D4, D7, D10, D14, D21, and D28: P < .001). Immunofluorescence and 3-dimensional reconstruction showed larger volumes of Gephyrin puncta inside microglial lysosomes (t = -23.273, P < .001; t = -27.997, P < .001). Treatment with minocycline or S-ketamine exhibited pain relief and antiphagocytic effects (t = -6.191, P < .001, t = -7.083, P < .001; t = -20.767, P < .001, t = -17.080, P < .001; t = 11.789, P < .001, t = 16.777, P < .001; t = 8.868, P < .001, t = 21.319, P < .001). Last but not least, the levels of CD47 and SIRPα proteins were downregulated (D10: P = .004, D14, D21, and D28: P < .001; D10, D14, D21, and D28: P < .001). Flow cytometry and immunofluorescence substantiated reduced microglial SIRPα (t = 11.311, P < .001; t = 12.189, P < .001). Microglia-mediated GABAergic synapse pruning in the spinal cord dorsal horn in bone cancer pain mice, which might be associated with the declined CD47-SIRPα signal. Our research uncovered an innovative mechanism that highlighted microglia-mediated synaptic phagocytosis in a murine cancer-induced bone pain model.

Serjania marginata Casar. Hydroalcoholic Extract Reduced Cytokine and Inflammatory Parameters in Experimental Models of Inflammation and Infection in Mice

Pharmacognosy Research,2022,14,2,127-134.DOI:10.5530/pres.14.2.18Published:April 2022Type:Original ArticleAuthors:Maicon Matos Leitão, Joyce Alencar Santos Radai, Luis Fernando Benitez Macorini, Thiago Leite Fraga, Silvia Cristina Heredia-Viira, Claudia Andrea Lima Cardoso, Arielle Cristina Arena, and Candida Aparecida Leite Kassuya Author(s) affiliations:Maicon Matos Leitão1,2,*, Joyce Alencar Santos Radai1, Luis Fernando Benitez Macorini1, Thiago Leite Fraga1, Silvia Cristina Heredia-Vieira3, Claudia Andrea Lima Cardoso4, Arielle Cristina Arena5, Candida Aparecida Leite Kassuya1 1School of Health Sciences, Federal University of Grande Dourados, Dourados, Mato Grosso do Sul State, BRAZIL. 2School of Health Sciences, Unigran Capital University Center, Campo Grande, Mato Grosso do Sul State, BRAZIL. 3Center of Health Sciences, Anhanguera- Uniderp University, Campo Grande, Mato Grosso do Sul State, BRAZIL. 4Center of Studies on Natural Resources, Mato Grosso do Sul State University, Dourados, Mato Grosso do Sul State, BRAZIL. 5Department of Morphology, Institute of Biosciences of Botucatu, Paulista State University, Botucatu, SP, BRAZIL. Abstract:Objectives: This study investigated the antimycobacterial, anti-inflammatory and antihyperalgesic effects of hydroalcoholic extract from leaves of S. marginata (EESM) in in vitro and in vivo models. Methods and Results: EESM (0.98–1000 μg/ml) was evaluated in in vitro against Mycobacterium tuberculosis, M. bovis, Klebsiella pneumoniae, Pseudomonas aeruginosa, Staphylococcus epidermidis. The EESM oral administration (p.o.) (30, 100 and 300 mg/kg) and dexamethasone subcutaneous injection (s.c.) (1 mg/kg) were tested against the carrageenan-induced inflammatory paw edema and pleurisy in Swiss mice. The EESM (30 and 100 mg/kg, p.o.) and dexamethasone (1 mg/kg, s.c.) were tested against the CFA-induced paw inflammation and M. bovis (bacillus Calmette-Guerin - BCG)-induced pleurisy in C57bL6 mice. The minimum inhibitory concentration (MIC) of EESM in the presence of M. tuberculosis was 62.4 μg/ml. The values of MIC of EESM in the presence S. epidermidis, K. pneumoniae were 1000 μg/mL while EESM did not interfere against P. aeruginosa growth. EESM significantly inhibited paw edema/mechanical hyperalgesia in carrageenan induced paw inflammation and leukocytes migration/proteins exudation in carrageenan-induced pleurisy model. In the BCGinduced pleurisy model, the daily treatment for 7 days, with EESM inhibited the levels of IL-1β in blood and in pleural exudate. The EESM did not alter the mycobacterial growth in the cell culture from pleural lavage, spleen and liver samples collected from BCG-treated animals. The EESM significantly inhibited the persistent edema and mechanical hyperalgesia induced by CFA. Conclusion: This study confirms the EESM anti-inflammatory property and showed that EESM has high potency in inducing inhibition of mycobaterial growth and low potency or no effects in relation to other microorganisms. Key words: Mycobacterium tuberculosis, Bacillus Calmette-Guerin (BCG), Cipó-timbó, Inflammation, Tuberculosis. View:PDF (455.97 KB)

Effects of intrathecal injection of IRF8 SiRNA on pain threshold and activation of spinal cord microglia in rats with postoperative persistent pain

Objective: To investigate the effects of intrathecal injection of IRF8 SiRNA on the pain threshold and activation of spinal cord microglia in rats with postoperative persistent pain. Methods: One hundred and twenty male Sprague-Dawley rats were randomly divided into sham group (SH, n=12), SMIR group (SM, n=48), SMIR + DEPC group (SD, n=12) and SMIR + irf8 SiRNA group (SS, n=48). In the SM group, the persistent postsurgical pain(PPsP) model was established according to the skin/muscle incision and retraction (SMIR), and the SH group was only incised without retracted. The SD group and SS group received intrathecal catheterization one week before SMIR, the SS group was injected with 20 μl of IRF8 SiRNA solution (dissolved in DEPC-treated water, 150 pmol) intrathecally on the 5th and 6th day after SMIR, and the SD group was injected with the same amount of DEPC-treated water. The paw withdrawal threshold (PWT) of each group was measured and recorded before SMIR and on the 1st, 3rd, 7th, 12th, 22nd and 33rd days after SMIR. Western blot was used to detect the expression of Iba-1 in the dorsal horn of spinal cord on the 12th days after SMIR, and the saphenous nerves in the SH group and SM group were collected to observe their ultrastructural changes under electron microscope. The flow cytometry was used to detect the activation of microglia in spinal cord dorsal horn before SMIR and on the 1st, 3rd, 7th, 12th, 22nd and 33rd days after SMIR in the SM group and SS group. Results: Compared with D0, the PWT of SM group was decreased on the 1th to 22nd day after SMIR (P＜0.05 or P＜0.01), and returned to normal level on the 33rd day after SMIR (P＞ 0.05). Compared with the SH group, the PWT of the SM group was decreased on the 1th to 22nd day after SMIR (P＜0.05 or P＜ 0.01). However, compared with the SD group, the PWT of the SS group was increased on the 7th to 22nd day after SMIR (P＜0.05 or P＜0.01). Compared with SH group, the PWT of SS group was decreased on the 7th to 22nd day after SMIR (P＜0.05 or P＜0.01). The average thickness of saphenous nerve myelin was (377.0 3±69.60) nm in the SH group and (369.50±73.26) nm in the SM group, and there was no significant difference between the two groups (P＞0.05). Compared with the SH group, the expression level of Iba-1 was increased significantly (P＜0.01) in the SM group. Compared with the SD group, the expression of Iba-1 was inhibited (P＜0.05) in the SS group, and compared with the SH group, the expression of Iba-1 was also statistically different (P＜0.05) in the SS group, while the expression of Iba-1 was not statistically significant between the SM group and the SD group (P＞0.05). Compared with D0, the activation ratio of microglia was increased significantly on the 3rd to 22nd day after SMIR (P＜0.01) in the SM group , while the activation of microglia reached a peak on 3rd day after SMIR (P＜0.01) in the SS group. After intrathecal administration, the activation rate of microglia in the spinal dorsal horn of the SS group was decreased significantly, and compared with the SM group, it was decreased significantly on the 7th to 12th day after SMIR (P＜0.01). Conclusion: The significant and persistent mechanical hyperalgesia in PPsP induced by SMIR was caused non-obvious peripheral nerve injury, which may be mediated by the activation of microglia in the dorsal horn of the spinal cord. IRF8 SiRNA administrated by intrathecal injection could inhibit the activation of microglia and reverse SMIR-induced hyperalgesia.

Exchange proteins directly activated by cyclic adenosine monophosphate inhibitor reverses mechanical allodynia via the modification of astrocytes activity in the spinal cord.

BackgroundGliosis and inflammation are pivotal in the development of acute and chronic pain. Here, we demonstrated a previously unidentified molecular mechanism in which the activation of exchange proteins directly activated by cyclic adenosine monophosphate (Epac)1 accelerated the activation of astrocytes in the spinal cord, thereby promoting chronic postsurgical pain (CPSP).MethodsWe established a rat model of CPSP induced by skin/muscle incision and retraction (SMIR). Pain behaviors were assessed using mechanical withdrawal threshold (MWT) at different times. The lumbosacral enlargement of the spinal cord was isolated to detect the expression of Epac1 and the activity of astrocytes. They were assessed using western blot and immunofluorescence staining.ResultsSMIR induced persistent mechanical hyperalgesia after surgery. This hyperalgesia response was prolonged to more than 21 d after surgery. The time course of spinal Epac1 upregulation was correlated with SMIR-induced pain behaviors. Meanwhile, Epac1 immunoreactivity was colocalized primarily with astrocytes but not with microglial cells or neurons on 7 d after surgery. Intrathecal injection of Epac1 inhibitor CE3F4 significantly suppressed SMIR-induced mechanical allodynia and activation of astrocytes in the spinal cord. This analgesic effect of single-dose administration of CE3F4 lasted up to 6 h and wore off at 12 h after injection.ConclusionsSpinal Epac1-mediated activation of astrocytes may facilitate CPSP. Inhibition of Epac1 may effectively prevent CPSP.

TNF-Alpha as an Initiator of Allodynia and Anxiety-Like Behaviors in a Preclinical Model of PTSD and Comorbid Pain.

Post-Traumatic Stress Disorder (PTSD) is a debilitating mental health disorder that occurs after exposure to a traumatic event. Patients with comorbid chronic pain experience affective distress, worse quality of life, and poorer responses to treatments for pain or PTSD than those with either condition alone. FDA-approved PTSD treatments are often ineffective analgesics, requiring additional drugs to treat co-morbid symptoms. Therefore, development of new treatment strategies necessitate a better understanding of the pathophysiology of PTSD and comorbid pain. The single prolonged stress (SPS) model of PTSD induces the development of persistent mechanical allodynia and thermal hyperalgesia. Increased Nociceptin/Orphanin FQ (N/OFQ) levels in serum and CSF accompany these exaggerated nociceptive responses, as well as increased serum levels of the pro-inflammatory cytokine tumor necrosis factor (TNF-α). Therefore, the primary goal was to determine the role of TNF-α in the development of SPS-induced allodynia/hyperalgesia and elevated serum and CNS N/OFQ using two approaches: TNF-α synthesis inhibition, and blockade with anti-TNF-α antibody that acts primarily in the periphery. Administration of TNF-α synthesis blocker, thalidomide (THL), immediately after SPS prevented increased TNF-α and development of allodynia and hyperalgesia. The THL effect lasted at least 21 days, well after thalidomide treatment ended (day 5). THL also prevented SPS-induced increases in serum N/OFQ and reversed regional N/OFQ mRNA expression changes in the CNS. Serum TNF-α increases detected at 4 and 24 h post SPS were not accompanied by blood brain barrier disruption. A single injection of anti-TNF-α antibody to male and female rats during the SPS procedure prevented the development of allodynia, hyperalgesia, and elevated serum N/OFQ, and reduced SPS-induced anxiety-like behaviors in males. Anti-TNFα treatment also blocked development of SPS-induced allodynia in females, and blocked increased hypothalamic N/OFQ in males and females. This suggests that a peripheral TNF-α surge is necessary for the initiation of allodynia associated with SPS, as well as the altered central and peripheral N/OFQ that maintains nociceptive sensitivity. Therefore, early alleviation of TNF-α provides new therapeutic options for investigation as future PTSD and co-morbid pain treatments.

Regular swimming exercise prevented the acute and persistent mechanical muscle hyperalgesia by modulation of macrophages phenotypes and inflammatory cytokines via PPARγ receptors

Physically active individuals are less likely to develop chronic pain, and physical exercise is an established strategy to control inflammatory diseases. Here, we hypothesized that 1) peripheral pro-inflammatory macrophages phenotype contribute to predisposition of the musculoskeletal to chronic pain, and that 2) activation of PPARγ receptors, modulation of macrophage phenotypes and cytokines through physical exercise would prevent persistent muscle pain. We tested these hypotheses using swimming exercise, pharmacological and immunochemical techniques in a rodent model of persistent muscle hyperalgesia. Swimming prevented the persistent mechanical muscle hyperalgesia most likely through activation of PPARγ receptors, as well as activation of PPARγ receptors by 15d-PGJ2 and depletion of muscle macrophages in sedentary animals. Acute and persistent muscle hyperalgesia were characterized by an increase in pro-inflammatory macrophages phenotype, and swimming and the 15d-PGJ2 prevented this increase and increased anti-inflammatory macrophages phenotype. Finally, IL-1β concentration in muscle increased in the acute phase, which was also prevented by PPARγ receptors activation through swimming. Besides, swimming increased muscle concentration of IL-10 in both acute and chronic phases, but only in the persistent phase through PPARγ receptors. Our findings suggest physical exercise activates PPARγ receptors and increases anti-inflammatory responses in the muscle tissue by modulating macrophages phenotypes and cytokines, thereby preventing the establishment of persistent muscle hyperalgesia. These results further highlight the potential of physical exercise to prevent chronic muscle pain.

Analgesic effect of voluntary exercise in a rat model of persistent pain via suppression of microglial activation in the spinal cord.

In this study, we employed a rodent model for persistent allodynia and hyperalgesia to determine whether voluntary exercise could exert analgesic effects on these pain symptoms. Rats were subcutaneously injected with formalin into the plantar surface of the right hind paw to induce mechanical allodynia and hyperalgesia. We assessed the analgesic effects of a voluntary wheel running (VWR) using the von Frey test and investigated microglial proliferation in the dorsal horn of the spinal cord. We also determined the effect of formalin and VWR on the protein expression levels of brain-derived neurotrophic factor (BDNF), its receptor TrkB, and K+-Cl- cotransporter 2 (KCC2), which play a key role in inducing allodynia and hyperalgesia. Rats with access to the running wheels showed beneficial effects on persistent formalin-induced mechanical allodynia and hyperalgesia. The effects of VWR were elicited through the suppression of formalin-induced microglial proliferation, TrkB up-regulation, and KCC2 down-regulation in the spinal cord. BDNF, however, might not contribute to the beneficial effects of VWR. Our results show an analgesic effect of voluntary physical exercise in a rodent model with persistent pain, possibly through the regulation of microglial proliferation and TrkB and KCC2 expression in the spinal cord.

A role for gut microbiota in early-life stress-induced widespread muscle pain in the adult rat.

Adult rats that experienced neonatal limited bedding (NLB), a form of early-life stress, experience persistent muscle mechanical hyperalgesia. Since there is a growing recognition that the gut microbiome regulates pain and nociception, and that early-life stress produces a long-lasting impact on the gut microbiome, we tested the hypothesis that persistent muscle hyperalgesia seen in adult NLB rats could be ameliorated by interventions that modify the gut microbiome. Adult NLB rats received probiotics, either Lactobacillus rhamnosus GG (10 billion CFU/150 ml) or De Simone Formulation (DSF) (112.5 billion CFU/150 ml mixture of 8 bacterial species), in their drinking water, or non-absorbable antibiotics, rifaximin or neomycin, admixed with cookie dough, to provide 50 mg/kg. Mechanical nociceptive threshold in the gastrocnemius muscle was evaluated before and at several time points after administration of probiotics or antibiotics. Adult NLB rats fed probiotics L. Rhamnosus or DSF, antibiotics, as well as rats fed non-absorbable antibiotics rifaximin or neomycin, had markedly attenuated muscle mechanical hyperalgesia. We hypothesize that persistent skeletal muscle hyperalgesia produced by NLB stress may be, at least in part, due to a contribution of the gut microbiome, and that modulation of gut microbiome using probiotics or non-absorbable antibiotics, may be novel therapeutic approaches for the treatment of chronic musculoskeletal pain.

Running wheel exercise induces therapeutic and preventive effects on inflammatory stimulus-induced persistent hyperalgesia in mice.

Chronic pain affects significant portion of the world's population and physical exercise has been extensively indicated as non-pharmacological clinical intervention to relieve symptoms in chronic pain conditions. In general, studies on pain chronification and physical exercise intervention have focused on neuropathic pain, although chronic pain commonly results from an original inflammatory episode. Based on this, the objective of the present study was to investigate the therapeutic and preventive effect of the running wheel exercise on the persistent hyperalgesia induced by repetitive inflammatory stimulus, a rodent model that simulates clinical conditions of chronic pain that persist even with no more inflammatory stimulus present. To evaluate the therapeutic effect of physical exercise, we first induced persistent hyperalgesia through 14 days of PGE2 hind paw injections and, after that, mice have access to the regular voluntary running wheel. To evaluate the preventive effect of physical exercise, we first left the mice with access to the regular voluntary running wheel and, after that, we performed 14 days of PGE2 hind paw injection. Our results showed that voluntary running wheel exercise reduced persistent mechanical and chemical hyperalgesia intensity induced by repetitive inflammatory stimulus. In addition, we showed that this therapeutic effect is long-lasting and is observed even if started belatedly, i.e. two weeks after the development of hyperalgesia. Also, our results showed that voluntary running wheel exercise absolutely prevented persistent mechanical and chemical hyperalgesia induction. We can conclude that physical exercise has therapeutic and preventive effect on inflammatory stimulus-induced persistent hyperalgesia. Our data from animal experiments bypass placebo effects bias of the human studies and reinforce physical exercise clinical recommendations to treat and prevent chronic pain.

Pain-related behaviors associated with persistence of mechanical hyperalgesia after antigen-induced arthritis in rats.

Upon transient musculoskeletal diseases, some patients develop persistent pain while others recover from pain. Here, we studied whether such heterogeneity also occurs in rats after recovery from unilateral antigen-induced arthritis (AIA) in the knee joint, and which pain phenotype may predict the course of pain. Typically, inflammatory swelling lasts about 3 weeks. Pain-related behaviors were monitored for 84 days after AIA induction. Unbiased cluster analysis of intragroup differences at day 84 of AIA revealed that about one-third of the rats (cluster 1) showed persistent mechanical hyperalgesia at the injected knee joint, whereas the other rats (cluster 2) had recovered from pain. Retrograde analysis of pain-related behaviors revealed that cluster 1 rats exhibited more severe mechanical hyperalgesia at the injected knee from day 3 of AIA and mechanical hyperalgesia at the contralateral knee. Cluster 1 and 2 rats did not show different inflammatory swelling, secondary mechanical and thermal hyperalgesia at the ipsilateral hindpaw, guarding score, and asymmetry of weight bearing during AIA. Thus, in particular, early severe mechanical hyperalgesia in the inflamed joint and segmental contralateral mechanical hyperalgesia seem to be a risk factor for the development of persistent mechanical hyperalgesia pointing to the importance of spinal mechanisms. However, none of the rats showed an expression of ATF3 in dorsal root ganglion neurons, nor morphological spinal microglia activation thus not suggesting development of neuropathic pain. Both clusters showed a persistent upregulation of pCREB in dorsal root ganglion neurons, inversely correlated with mechanical hyperalgesia at the knee. The role of pCREB needs to be further explored.

Early high-frequency spinal cord stimulation treatment inhibited the activation of spinal mitogen-activated protein kinases and ameliorated spared nerve injury-induced neuropathic pain in rats

BackgroundNeuromodulation therapies offer a treatment option that has minimal side effects and is relatively safe and potentially reversible. Spinal cord stimulation (SCS) has been used to treat various pain conditions for many decades. High-frequency SCS (HFSCS) involves the application of a single waveform at 10,000 Hz at a subthreshold level, therefore providing pain relief without any paresthesia. MethodsWe tested whether early HFSCS treatment attenuated spared nerve injury (SNI)-induced neuropathic pain. The phosphorylation profile of mitogen-activated protein kinases (MAPKs), i.e., extracellular signal-regulated kinases (ERKs), c-Jun N-terminal kinases (JNKs), and p38, was evaluated to elucidate the potential underlying mechanism. ResultsSNI of rat unilateral sciatic nerves induced mechanical hyperalgesia in the ipsilateral hind paws. Rats were assigned to SCS sessions with HFSCS (frequency 10 kHz; pulse width 30 μs; pulse shape of charge-balanced, current controlled; delivered continuously for 72 h), or sham stimulation immediately after SNI. Tissue samples were examined at 1, 3, 7, and 14 days after SNI. Behavioral studies showed that HFSCS applied to the T10/T11 spinal cord significantly attenuated SNI-induced mechanical hyperalgesia compared with the sham stimulation group. Moreover, western blotting revealed a significant attenuation of the activation of ERK1, ERK2, JNK1, and p38 in the dorsal root ganglia and the spinal dorsal horn. ConclusionApplication of HFSCS provides an effective treatment for SNI-induced persistent mechanical hyperalgesia by attenuating ERK, JNK, and p38 activation in the dorsal root ganglia and the spinal dorsal horn.

SFKs/p38 Pathway is Involved in Radicular Pain by Promoting Spinal Expression of Pro-Inflammatory Cytokines in a Rat Model of Lumbar Disc Herniation.

A controlled, randomized, animal study. The aim of this study was to investigate the role of src-family kinases/p38 pathway in a rat model of lumbar disc herniation (LDH). LDH always generates radicular pain, and the mechanism remains unclear. We have reported that spinal src-family kinases (SFKs) may be involved in the process, but the downstream mechanism needs further investigation. LDH was induced by implantation of autologous nucleus pulposus (NP), harvest from the tail, in lumbar 4/5 spinal nerve roots of rat. Von Frey filaments and radiant heat tests were performed to determine mechanical and thermal pain threshold respectively. Basso, Beattie, and Bresnahan (BBB) scale was assessed to test the locomotor function. The protein level of p-SFKs, t-SFKs, p-p38, t-p38 in spinal cord was examined by western blotting analysis. Cellular location of p-p38 was determined by immunochemistry staining. Spinal tumor necrosis factor-alpha (TNF-α), interleukin (IL)-1β, and IL-6 levels were detected by enzyme-linked immunosorbent assay (ELISA). Rats with NP implantation showed persistent ipsilateral mechanical allodynia and thermal hyperalgesia, which manifested as obvious decrease of paw withdrawal threshold (PWT) and paw withdrawal latency (PWL). BBB scale indicated the locomotor function of hindpaws in rats with NP implantation kept intact. Western blotting and immunohistochemistry staining revealed that phosphorylated SFKs (p-SFKs) and phosphorylated p38 MAPK (p-p38) were sequentially upregulated in ipsilateral spinal dorsal horn, but not in contralateral side of rats with NP. Intrathecal delivery of SFKs inhibitor reduced spinal p-p38 expression. Both SFKs and p38 inhibitors alleviated pain behaviors in a dose-responsive manner without disturbing locomotor function and reduced spinal expression of TNF-α, IL-1β, and IL-6 in rats with NP. Spinal SFKs contribute to radicular pain by activation of p38 MAPK and increasing pro-inflammatory cytokines expression in rats with NP implantation. Targeting SFKs/p38 pathway may be helpful for alleviating radicular pain. N/A.

Assessment of anti-nociceptive effect of allopurinol in a neuropathic pain model

BackgroundThis study aimed to investigate the antinociceptive effect of allopurinol, a xanthine oxidase inhibitor, in the chronic constriction injury (CCI) to sciatic nerve rat model of neuropathic pain. MethodsAllopurinol administration (30, 60, 90 mg/kg, i.p.) was started at the time of nerve injury, and given for 14 continuous days. Behavioural tests (von Frey filaments, acetone drop, hot plate) were conducted on days 0, 3, 7, 10 and 14. Reverse transcriptase polymerase chain reaction (RT-PCR) analysis was performed on the spinal cord of CCI animals on day 14. The contribution of adenosine (A) receptors was tested using the methylxanthine theophylline, a non-selective A receptor antagonist and 1,3-dipropyl-8-cyclopentylxanthine (DPCPX), a selective A1 receptor antagonist, administered 30 min before allopurinol on day 10. ResultsCCI of the sciatic nerve resulted in a persistent mechanical allodynia, cold allodynia, and heat hyperalgesia, together with increased iNOS, bax/bcl2, iba-1 and TNF-α expression in the lumbar spinal cord of animals. The highest-dose group of allopurinol (90 mg/kg) attenuated pain-like behaviors compared with the normal saline treated group, and this was accompanied by normalization of iNOS, bax/bcl2, caspase 3, iba-1 and TNF-α gene expression changes. DPCPX and theophylline reversed the thermal anti-hyperalgesic effect of allopurinol. In contrast, the mechanical anti-allodynic effect was only prevented by theophylline. ConclusionAllopurinol through interacting with different aspects of neuropathic pain, via anti-oxidant effects, protection against neuroinflammation, and activating adenosine receptors, could be useful in the treatment of patients with neuropathic pain.

Priming of Adult Incision Response by Early-Life Injury: Neonatal Microglial Inhibition Has Persistent But Sexually Dimorphic Effects in Adult Rats.

Neonatal hindpaw incision primes developing spinal nociceptive circuitry, resulting in enhanced hyperalgesia following reinjury in adulthood. Spinal microglia contribute to this persistent effect, and microglial inhibition at the time of adult reincision blocks the enhanced hyperalgesia. Here, we pharmacologically inhibited microglial function with systemic minocycline or intrathecal SB203580 at the time of neonatal incision and evaluated sex-dependent differences following adult reincision. Incision in adult male and female rats induced equivalent hyperalgesia and spinal dorsal horn expression of genes associated with microglial proliferation (Emr1) and transformation to a reactive phenotype (Irf8). In control adults with prior neonatal incision, the enhanced degree and duration of incision-induced hyperalgesia and spinal microglial responses to reincision were equivalent in males and females. However, microglial inhibition at the time of the neonatal incision revealed sex-dependent effects: the persistent mechanical and thermal hyperalgesia following reincision in adulthood was prevented in males but unaffected in females. Similarly, reincision induced Emr1 and Irf8 gene expression was downregulated in males, but not in females, following neonatal incision with minocycline. To evaluate the distribution of reincision hyperalgesia, prior neonatal incision was performed at different body sites. Hyperalgesia was maximal when the same paw was reincised, and was increased following prior incision at ipsilateral, but not contralateral, sites, supporting a segmentally restricted spinal mechanism. These data highlight the contribution of spinal microglial mechanisms to persistent effects of early-life injury in males, and sex-dependent differences in the ability of microglial inhibition to prevent the transition to a persistent pain state span developmental stages.SIGNIFICANCE STATEMENT Following the same surgery, some patients develop persistent pain. Contributory mechanisms are not fully understood, but early-life experience and sex/gender may influence the transition to chronic pain. Surgery and painful procedural interventions in vulnerable preterm neonates are associated with long-term alterations in somatosensory function and pain that differ in males and females. Surgical injury in neonatal rodents primes the developing nociceptive system and enhances reinjury response in adulthood. Neuroimmune interactions are critical mediators of persistent pain, but sex-dependent differences in spinal neuroglial signaling influence the efficacy of microglial inhibitors following adult injury. Neonatal microglial inhibition has beneficial long-term effects on reinjury response in adult males only, emphasizing the importance of evaluating sex-dependent differences at all ages in preclinical studies.

Anterior nucleus of paraventricular thalamus mediates chronic mechanical hyperalgesia.

Pain-related diseases are the top leading causes of life disability. Identifying brain regions involved in persistent neuronal changes will provide new insights for developing efficient chronic pain treatment. Here, we showed that anterior nucleus of paraventricular thalamus (PVA) plays an essential role in the development of mechanical hyperalgesia in neuropathic and inflammatory pain models in mice. Increase in c-Fos, phosphorylated extracellular signal-regulated kinase, and hyperexcitability of PVA neurons were detected in hyperalgesic mice. Direct activation of PVA neurons using optogenetics and pharmacological approaches were sufficient to induce persistent mechanical hyperalgesia in naive animals. Conversely, inhibition of PVA neuronal activity using DREADDs (designer receptors exclusively activated by designer drugs) or inactivation of PVA extracellular signal-regulated kinase at the critical time window blunted mechanical hyperalgesia in chronic pain models. At the circuitry level, PVA received innervation from central nucleus of amygdala, a known pain-associated locus. As a result, activation of right central nucleus of amygdala with blue light was enough to induce persistent mechanical hyperalgesia. These findings support the idea that targeting PVA can be a potential therapeutic strategy for pain relief.

&lt;p&gt;Characterization of novel lnc RNAs in the spinal cord of rats with lumbar disc herniation&lt;/p&gt;

BackgroundRadicular pain, caused by a lesion or autologous nucleus pulposus (NP) implantation, is associated with alteration in gene expression of the pain-signaling pathways. lncRNAs have been shown to play critical roles in neuropathic pain. However, the mechanistic function of lncRNAs in lumbar disc herniation (LDH) remains largely unknown. Identifying different lncRNA expression under sham and NP-implantation conditions in the spinal cord is important for understanding the molecular mechanisms of radicular pain.MethodsLDH was induced by implantation of autologous nucleus pulposus (NP), harvested from rat tail, in lumbar 5 and 6 spinal nerve roots. The mRNA and lncRNA microarray analyses demonstrated that the expression profiles of lncRNAs and mRNAs between the LDH and sham groups were markedly altered at 7 days post operation. The expression patterns of several mRNAs and lncRNAs were further proved by qPCR.ResultsLDH produced persistent mechanical and thermal hyperalgesia. A total of 19 lncRNAs was differentially expressed (>1.5-folds), of which 13 was upregulated and 6 was downregulated. In addition, a total of 103 mRNAs was markedly altered (>1.5-folds), of which 40 was upregulated and 63 downregulated. Biological analyses of these mRNAs further demonstrated that the most significantly upregulated genes in LDH included chemotaxis, immune response, and positive regulation of inflammatory responses, which might be important mechanisms underlying radicular neuropathic pain. These 19 differentially expressed lncRNAs have overlapping mRNAs in the genome, which are related to glutamatergic synapse, cytokine-cytokine receptor interaction, and the oxytocin-signalling pathway.ConclusionOur findings revealed the alteration of expression patterns of mRNAs and lncRNAs in the spinal cord of rats in a radicular pain model of LDH. These mRNAs and lncRNAs might be potential therapeutic targets for the treatment of radicular pain.

Pain chronification and chronic pain impair a defensive behavior, but not the ability of acute pain to facilitate it, through the activation of an endogenous analgesia circuit.

The endogenous ability to decrease pain perception during life-threatening situations is crucial to the prevention of recuperative behaviors and to leave the subject free to engage in appropriated defensive responses. We have previously shown that acute pain activates the ascending nociceptive control-an endogenous analgesia circuit dependent on opioid mechanisms within nucleus accumbens-to facilitate the tonic immobility response, an innate defensive behavior. Now we asked whether chronic pain and pain chronification impairs either the tonic immobility response or the ability of acute pain to facilitate it by activating the ascending nociceptive control. We found a significant decrease in the duration of the tonic immobility response in rats during the induction and maintenance phases of the persistent mechanical hyperalgesia. This finding suggests that chronic pain and its development impair defensive responses. However, during the induction and maintenance phases of persistent hyperalgesia, the ascending nociceptive control activation, by a forepaw capsaicin injection, increased the tonic immobility response, an effect prevented by the blockade of μ-opioid receptors within nucleus accumbens. This finding suggests that pain chronification and chronic pain do not prevent the ability of acute pain to facilitate the defensive behavior of tonic immobility by activating the ascending nociceptive control. Therefore, although chronic pain states decrease the ability to engage in a defensive behavior, they may not prevent the expression of defensive behaviors during life-threatening situations accompanied by acute pain. The biological purpose of such a mechanism may be to increase the chances of survival of a wounded subject exposed to acute pain in a novel life-threatening situation. (PsycINFO Database Record (c) 2018 APA, all rights reserved).

Involvement of Transient Receptor Potential Cation Channel Member A1 activation in the irritation and pain response elicited by skin-lightening reagent hydroquinone

Hydroquinone (HQ) is one of the most frequently used and effective skin-lightening products to treat skin hyperpigmentation disorders, including postinflammatory hyperpigmentation, melasma and solar lentigines. HQ is also widely used in cosmetic products for skin whitening. However, HQ treatment can evoke substantial skin irritation, a side effect that remains poorly understood. Here we demonstrate that HQ is an activator of the peripheral irritant receptor transient receptor potential (TRP) cation channel member A1 (TRPA1). HQ failed to activate TRPV1, TRPV4 or TRPM8. HQ-induced TRPA1 activation was dependent on essential redox-sensitive cysteine and lysine residues within N-terminus of channel protein. HQ elicited Ca2+ influx in a subpopulation of mouse sensory neurons sensitive to the TRPA1 agonist, mustard oil. HQ-induced neuronal responses were significantly reduced by TRPA1 inhibitors, and reduced in neurons isolated from Trpa1-deficient mice. In mice, intraplantar injection of HQ at clinically relevant concentrations elicited both acute pain and persistent mechanical hyperalgesia which were almost completely abolished by TRPA1 inhibitors. These findings identify TRPA1 as a molecular target for HQ and provide insights into the mechanism of HQ-induced skin irritation. These findings also suggest that selective TRPA1 antagonists may be useful to counteract HQ-induced skin irritation.