Upregulation of Spinal Voltage-Dependent Anion Channel 1 Contributes to Bone Cancer Pain Hypersensitivity in Rats

Microglial voltage-dependent anion channel 1 signaling modulates sleep deprivation-induced transition to chronic postsurgical pain

STUDY OBJECTIVES

This study verified that sleep deprivation before and after skin/muscle incision and retraction (SMIR) surgery increased the risk of chronic pain and investigated the underlying roles of microglial voltage-dependent anion channel 1 (VDAC1) signaling.

METHODS

Adult mice received 6 hours of total sleep deprivation from 1 day prior to SMIR until the third day after surgery. Mechanical and heat-evoked pain was assessed before and within 21 days after surgery. Microglial activation and changes in VDAC1 expression and oligomerization were measured. Minocycline was injected to observe the effects of inhibiting microglial activation on pain maintenance. The VDAC1 inhibitor 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS) and oligomerization inhibitor VBIT-4 were used to determine the roles of VDAC1 signaling on microglial adenosine 5' triphosphate (ATP) release, inflammation (IL-1β and CCL2), and chronicity of pain.

RESULTS

Sleep deprivation significantly increased the pain duration after SMIR surgery, activated microglia, and enhanced VDAC1 signaling in the spinal cord. Minocycline inhibited microglial activation and alleviated sleep deprivation-induced pain maintenance. Lipopolysaccharide (LPS)-induced microglial activation was accompanied by increased VDAC1 expression and oligomerization, and more VDAC1 was observed on the cell membrane surface compared with control. DIDS and VBIT-4 rescued LPS-induced microglial ATP release and IL-1β and CCL2 expression. DIDS and VBIT-4 reversed sleep loss-induced microglial activation and pain chronicity in mice, similar to the effects of minocycline. No synergistic effects were found for minocycline plus VBIT-4 or DIDS.

CONCLUSIONS

Perioperative sleep deprivation activated spinal microglia and increases the risk of chronic postsurgical pain in mice. VDAC1 signaling regulates microglial activation-related ATP release, inflammation, and chronicity of pain.

Toll-like receptor 2 and 4 antagonism for the treatment of experimental autoimmune encephalomyelitis (EAE)-related pain

Neuropathic pain is a major symptom of multiple sclerosis (MS) with up to 92% of patients reporting bodily pain, and 85% reporting pain severe enough to cause functional disability. None of the available therapeutics target MS pain. Toll-like receptors 2 and 4 (TLR2/TLR4) have emerged as targets for treating a wide array of autoimmune disorders, including MS, as well as having demonstrated success at suppressing pain in diverse animal models. The current series of studies tested systemic TLR2/TLR4 antagonists in males and females in a low-dose Myelin oligodendrocyte glycoprotein (MOG) experimental autoimmune encephalomyelitis (EAE) model, with reduced motor dysfunction to allow unconfounded testing of allodynia through 50+ days post-MOG. The data demonstrated that blocking TLR2/TLR4 suppressed EAE-related pain, equally in males and females; upregulation of dorsal spinal cord proinflammatory gene expression for TLR2, TLR4, NLRP3, interleukin-1β, IkBα, TNF-α and interleukin-17; and upregulation of dorsal spinal cord expression of glial immunoreactivity markers. In support of these results, intrathecal interleukin-1 receptor antagonist reversed EAE-induced allodynia, both early and late after EAE induction. In contrast, blocking TLR2/TLR4 did not suppress EAE-induced motor disturbances induced by a higher MOG dose. These data suggest that blocking TLR2/TLR4 prevents the production of proinflammatory factors involved in low dose EAE pathology. Moreover, in this EAE model, TLR2/TLR4 antagonists were highly effective in reducing pain, whereas motor impairment, as seen in high dose MOG EAE, is not affected.

Modulation of Rat Cancer-Induced Bone Pain is Independent of Spinal Microglia Activity

The dissemination of cancer to bone can cause significant cancer-induced bone pain (CIBP), severely impairing the patient's quality of life. Several rodent models have been developed to explore the nociceptive mechanisms of CIBP, including intratibial inoculation of breast carcinoma cells in syngeneic Sprague Dawley rats. Using this model, we investigated whether resident spinal microglial cells are involved in the transmission and modulation of CIBP, a long-debated disease feature. Immunohistochemical staining of ionizing calcium-binding adaptor molecule 1 (Iba-1) and phosphorylated p38-mitogen-activated protein kinase (P-p38 MAPK) showed no spinal microglial reaction in cancer-bearing rats, independently of disease stage, sex, or carcinoma cell line. As a positive control, significant upregulation of both Iba-1 and P-p38 was observed in a rat model of neuropathic pain. Additionally, intrathecal administration of the microglial inhibitor minocycline did not ameliorate pain-like behaviors in cancer-bearing rats, in contrast to spinal morphine administration. Our results indicate that microglial reaction is not a main player in CIBP, adding to the debate that even within the same models of CIBP, significant variations are seen in disease features considered potential drug targets. We suggest that this heterogeneity may reflect the clinical landscape, underscoring the need for understanding the translational value of CIBP models.

Overexpression of Purinergic P2X4 Receptors in Hippocampus Rescues Memory Impairment in Rats with Type 2 Diabetes

Purinergic receptors have been reported to be involved in brain disorders. In this study, we explored their roles and mechanisms underlying the memory impairment in rats with type 2 diabetes mellitus (T2DM). T2DM rats exhibited a worse performance in the T-maze and Morris water maze (MWM) than controls. Microglia positive for P2X purinoceptor 4 (P2X4R) in the hippocampus were reduced and activated microglia were increased in T2DM rats. Long Amplicon PCR (LA-PCR) showed that DNA amplification of the p2x4r gene in the hippocampus was lower in T2DM rats. Minocycline significantly reduced the number of activated microglia and the mean distance traveled by T2DM rats in the MWM. Most importantly, P2X4R overexpression suppressed the activated microglia and rescued the memory impairment of T2DM rats. Overall, T2DM led to excessive activation of microglia in the hippocampus, partly through the DNA damage-mediated downregulation of P2X4Rs, thus contributing to memory impairment.

Upregulation of lncRNA-NONRATT021203.2 in the dorsal root ganglion contributes to cancer-induced pain via CXCL9 in rats

Cancer-induced pain (CIP) is a kind of chronic pain that occurs during cancer progression over time. However, the mechanisms are largely unknown, and clinical treatment remains challenging. LncRNAs have been reported to play critical roles in various biological processes, including chronic pain. The aim of our study was to investigate whether lncRNAs participate in the development of CIP by regulating the expression levels of some molecules related to pain modulation. The CIP model was established by injecting Walker 256 mammary gland tumor cells into the tibial canal of rats. In this study, we found that lncRNA-NONRATT021203.2 was increased in the CIP rats and that lncRNA-NONRATT021203.2-siRNA could relieve hyperalgesia in these rats. For elucidation of the underlying mechanism, we showed that lncRNA-NONRATT021203.2 could target C-X-C motif chemokine ligand 9 (CXCL9), which was increased in the CIP rats, and that CXCL9-siRNA could relieve hyperalgesia. At the same time, silencing lncRNA-NONRATT021203.2 expression decreased the mRNA and protein levels of CXCL9. Immunofluorescence analysis showed that CXCL9 was mainly expressed in the CGRP-positive and IB4-positive DRG neurons. Further research showed that lncRNA-NONRATT021203.2 and CXCL9 were colocalized in the DRG neurons. Our data suggested that lncRNA-NONRATT021203.2 participated in the CIP in rats and likely mediates the upregulation of CXCL9. The present study provided us with a new potential target for the clinical treatment of cancer-induced pain.

Blockade of c-Src Within the Paraventricular Nucleus Attenuates Inflammatory Cytokines and Oxidative Stress in the Mechanism of the TLR4 Signal Pathway in Salt-Induced Hypertension

Toll-like receptor 4 (TLR4) and cellular Src (c-Src) are closely associated with inflammatory cytokines and oxidative stress in hypertension, so we designed this study to explore the exact role of c-Src in the mechanism of action of the TLR4 signaling pathway in salt-induced hypertension. Salt-sensitive rats were given a high salt diet for 10 weeks to induce hypertension. This resulted in higher levels of TLR4, activated c-Src, pro-inflammatory cytokines, oxidative stress, and arterial pressure. Infusion of a TLR4 blocker into the hypothalamic paraventricular nucleus (PVN) decreased the activated c-Src, while microinjection of a c-Src inhibitor attenuated the PVN levels of nuclear factor-kappa B, pro-inflammatory cytokines, and oxidative stress. Our findings suggest that a long-term high-salt diet increases TLR4 expression in the PVN and this promotes the activation of c-Src, which upregulates the expression of pro-inflammatory cytokines and results in the overproduction of reactive oxygen species. Therefore, inhibiting central c-Src activity may be a new target for treating hypertension.

Downregulation of glucose-6-phosphate dehydrogenase contributes to diabetic neuropathic pain through upregulation of toll-like receptor 4 in rats

Background and aim

Diabetic neuropathic pain is a refractory and disabling complication of diabetes mellitus. The pathogenesis of the diabetic neuropathic pain is still unclear, and treatment is insufficient. The aim of this study is to investigate the roles of glucose-6-phosphate dehydrogenase (G6PD) and toll-like receptor 4 (TLR4) in neuropathic pain in rats with diabetes.

Methods

Type 1 diabetes model was induced by intraperitoneal injection of streptozotocin (STZ, 75 mg/kg) in adult female Sprague-Dawley rats. Paw withdrawal threshold and paw withdrawal latency of rats were measured by von Frey filaments and thermal radiation, respectively. The expressions of G6PD and TLR4 in L4-L6 dorsal root ganglions (DRGs) were measured by western blotting and quantitative real-time polymerase chain reaction analysis. Fluorescent immunohistochemistry was employed to detect expressions of G6PD and TLR4 and co-location of G6PD with TLR4.

Results

The mRNA and protein expression levels of G6PD in DRGs were significantly decreased in diabetic rats when compared with age-matched control rats. Upregulation of G6PD by intrathecal injection of G6PD overexpression adenovirus markedly attenuated hindpaw pain hypersensitivity of diabetic rats. The mRNA and protein expression levels of TLR4 in DRGs of diabetic rats were significantly increased when compared with control rats. Intrathecal injection of TLR4-selective inhibitor CLI-095 attenuated diabetic pain in dose- and time-dependent manners. Furthermore, G6PD and TLR4 were co-localized in DRG neurons. Intrathecal injection of G6PD overexpression adenovirus greatly reduced TLR4 expression, while intrathecal injection of CLI-095 had no significant effect on G6PD expression in diabetic rats.

Conclusions

Our results suggest that decrease in G6PD expression was involved in diabetic peripheral neuropathic pain, which was most likely through upregulation of TLR4 expression in the DRGs of rats.

Decreased miR-325-5p Contributes to Visceral Hypersensitivity Through Post-transcriptional Upregulation of CCL2 in Rat Dorsal Root Ganglia

Chronic visceral hypersensitivity is an important type of chronic pain with unknown etiology and pathophysiology. Recent studies have shown that epigenetic regulation plays an important role in the development of chronic pain conditions. However, the role of miRNA-325-5p in chronic visceral pain remains unknown. The present study was designed to determine the roles and mechanism of miRNA-325-5p in a rat model of chronic visceral pain. This model was induced by neonatal colonic inflammation (NCI). In adulthood, NCI led to a significant reduction in the expression of miRNA-325-5p in colon-related dorsal root ganglia (DRGs), starting to decrease at the age of 4 weeks and being maintained to 8 weeks. Intrathecal administration of miRNA-325-5p agomir significantly enhanced the colorectal distention (CRD) threshold in a time-dependent manner. NCI also markedly increased the expression of CCL2 (C-C motif chemokine ligand 2) in colon-related DRGs at the mRNA and protein levels relative to age-matched control rats. The expression of CXCL12, IL33, SFRS7, and LGI1 was not significantly altered in NCI rats. CCL2 was co-expressed in NeuN-positive DRG neurons but not in glutamine synthetase-positive glial cells. Furthermore, CCL2 was mainly expressed in isolectin B4-binding- and calcitonin gene-related peptide-positive DRG neurons but in few NF-200-positive cells. More importantly, CCL2 was expressed in miR-325-5p-positive DRG neurons. Intrathecal injection of miRNA-325-5p agomir remarkably reduced the upregulation of CCL2 in NCI rats. Administration of Bindarit, an inhibitor of CCL2, markedly raised the CRD threshold in NCI rats in a dose- and time-dependent manner. These data suggest that NCI suppresses miRNA-325-5p expression and enhances CCL2 expression, thus contributing to visceral hypersensitivity in adult rats.

Characterization of novel lnc RNAs in the spinal cord of rats with lumbar disc herniation

Background

Radicular 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.

Methods

LDH 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.

Results

LDH 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.

Conclusion

Our 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.

Neonatal Maternal Deprivation Followed by Adult Stress Enhances Adrenergic Signaling to Advance Visceral Hypersensitivity

The pathophysiology of visceral pain in patients with irritable bowel syndrome remains largely unknown. Our previous study showed that neonatal maternal deprivation (NMD) does not induce visceral hypersensitivity at the age of 6 weeks in rats. The aim of this study was to determine whether NMD followed by adult stress at the age of 6 weeks induces visceral pain in rats and to investigate the roles of adrenergic signaling in visceral pain. Here we showed that NMD rats exhibited visceral hypersensitivity 6 h and 24 h after the termination of adult multiple stressors (AMSs). The plasma level of norepinephrine was significantly increased in NMD rats after AMSs. Whole-cell patch-clamp recording showed that the excitability of dorsal root ganglion (DRG) neurons from NMD rats with AMSs was remarkably increased. The expression of β₂ adrenergic receptors at the protein and mRNA levels was markedly higher in NMD rats with AMSs than in rats with NMD alone. Inhibition of β₂ adrenergic receptors with propranolol or butoxamine enhanced the colorectal distention threshold and application of butoxamine also reversed the enhanced hypersensitivity of DRG neurons. Overall, our data demonstrate that AMS induces visceral hypersensitivity in NMD rats, in part due to enhanced NE-β₂ adrenergic signaling in DRGs.

Prenatal maternal stress induces visceral hypersensitivity of adult rat offspring through activation of cystathionine-β-synthase signaling in primary sensory neurons

Irritable bowel syndrome is a disorder of unknown etiology characterized by widespread, chronic abdominal pain associated with altered bowel movements. Increasing amounts of evidence indicate that stressors presented during gestational periods could have long-term effects on the offspring's tissue structure and function, which may predispose to gastrointestinal diseases. The aim of the present study is to determine whether prenatal maternal stressis a adverse factor affecting gastrointestinal sensitivity and to investigate possible mechanisms underlying prenatal maternal stress-induced visceral hypersensitivity in adult offspring. Prenatal maternal stress was induced in pregnant Sprague-Dawley rats by exposure to heterotypic intermitent stress from gestational day 7 to delivery. Prenatal maternal stress significantly increased visceromotor response to colorectal distention in adult offspring from the age of 6 weeks to 10 weeks. Prenatal maternal stress also enhanced neuronal excitability including depolarization of resting membrane potentials, reduction in rheobase, and an increase in the number of action potentials evoked by 2× and 3× rheobase current stimultion of colon-specific dorsal root ganglion neurons. Prenatal maternal stress remarkably enhanced expression of cystathionine-β-synthase and Nav1.7 in T13-L2 thoracolumbar dorsal root ganglions both at protein and mRNA levels. Intraperitoneal injection of aminooxyacetic acid, an inhibitor of cystathionine-β-synthase, attenuated prenatal maternal stress-induced visceral hypersensitivity in a dose-dependent manner. A consecutive seven-day administration of aminooxyacetic acid reversed the hyperexcitability of colon-specific dorsal root ganglion neurons and markedly reduced Nav1.7 expression. These results indicate that the presence of multiple psychophysical stressors during pregnancy is associated with visceral hypersensitivity in offspring, which is likely mediated by an upregualtion of cystathionine-β-synthase and Nav1.7 expression. Prenatal maternal stress might be a significant contributor to irritable bowel syndrome, and cystathionine-β-synthase might be a potential target for treatment for chronic visceral hypersensitivity in patients with irritable bowel syndrome.