Astrocyte activation in the periaqueductal gray promotes descending facilitation to cancer-induced bone pain through the JNK MAPK signaling pathway

Dysfunctional astrocyte glutamate uptake in the hypothalamic paraventricular nucleus contributes to visceral pain and anxiety-like behavior in mice with chronic pancreatitis

Abdominal visceral pain is a predominant symptom in patients with chronic pancreatitis (CP); however, the underlying mechanism of pain in CP remains elusive. We hypothesized that astrocytes in the hypothalamic paraventricular nucleus (PVH) contribute to CP pain pathogenesis. A mouse model of CP was established by repeated intraperitoneal administration of caerulein to induce abdominal visceral pain. Abdominal mechanical stimulation, open field and elevated plus maze tests were performed to assess visceral pain and anxiety-like behavior. Fiber photometry, brain slice Ca2+ imaging, electrophysiology, and immunohistochemistry were used to investigate the underlying mechanisms. Mice with CP displayed long-term abdominal mechanical allodynia and comorbid anxiety, which was accompanied by astrocyte glial fibrillary acidic protein reactivity, elevated Ca2+ signaling, and astroglial glutamate transporter-1 (GLT-1) deficits in the PVH. Specifically, reducing astrocyte Ca2+ signaling in the PVH via chemogenetics significantly rescued GLT-1 deficits and alleviated mechanical allodynia and anxiety in mice with CP. Furthermore, we found that GLT-1 deficits directly contributed to the hyperexcitability of VGLUT2PVH neurons in mice with CP, and that pharmacological activation of GLT-1 alleviated the hyperexcitability of VGLUT2PVH neurons, abdominal visceral pain, and anxiety in these mice. Taken together, our data suggest that dysfunctional astrocyte glutamate uptake in the PVH contributes to visceral pain and anxiety in mice with CP, highlighting GLT-1 as a potential therapeutic target for chronic pain in patients experiencing CP.

Astroglial morphological changes in periaqueductal grey in different pain and itch mice models

BACKGROUND

The periaqueductal gray (PAG) plays a well-established pivotal role in the descending pain modulatory circuit. The objective of this study was to investigate morphological changes in the astroglia in models that are commonly used in pain and itch studies.

METHODS

Five different mouse models of pain, as well as two models of chronic itch, were established using complete Freund's adjuvant (CFA), spared nerve injury (SNI), bone cancer pain (BCP), cisplatin (CIS), and paclitaxel (PTX) for pain, and diphenylcyclopropenone (DCP) and acetone and diethyl ether followed by water (AEW) for chronic itch. von Frey tests and video recordings were employed to assess pain and itching behaviors. The immunofluorescence of S100β, pSTAT3, and glial fibrillary acidic protein (GFAP) was examined. Two- and three-dimensional studies were used to evaluate changes in astrocyte morphology.

RESULTS

Significant scratching was caused by DCP and AEW, whereas the administration of CFA, SNI, BCP, CIS, and PTX produced clear mechanical allodynia. The expression of GFAP in the lPAG/vlPAG was upregulated in CFA, SNI, BCP, CIS, PTX, and DCP mice but decreased in AEW mice. According to Sholl analysis, CFA, SNI, PTX, and BCP mice showed substantially higher astrocyte intersections in the vlPAG, whereas CFA, SNI, BCP, CIS, and DCP mice presented longer peak lengths. In three-dimensional analysis, CFA, SNI, PTX, and DCP mice showed increased astrocyte surface areas, while CIS and AEW mice showed both reduced surface areas and/or volumes of astrocytes.

CONCLUSION

The findings showed that different pain and itching conditions have different astrocyte morphologies, and these variations in morphological changes help to explain the pathophysiology of these conditions.

Inhibiting the JNK Signaling Pathway Attenuates Hypersensitivity and Anxiety-Like Behavior in a Rat Model of Non-specific Chronic Low Back Pain

Low back pain (LBP) has become a leading cause of disability worldwide. Astrocyte activation in the spinal cord plays an important role in the maintenance of latent sensitization of dorsal horn neurons in LBP. However, the role of spinal c-Jun N-terminal kinase (JNK) in astrocytes in modulating pain behavior of LBP model rats and its neurobiological mechanism have not been elucidated. Here, we investigate the role of the JNK signaling pathway on hypersensitivity and anxiety-like behavior caused by repetitive nerve growth factor (NGF) injections in male non-specific LBP model rats. LBP was produced by two injections (day 0, day 5) of NGF into multifidus muscle of the low backs of rats. We observed prolonged mechanical and thermal hypersensitivity in the low backs or hindpaws. Persistent anxiety-like behavior was observed, together with astrocyte, p-JNK, and neuronal activation and upregulated expression of monocyte chemoattractant protein-1 (MCP-1), and chemokine (C-X-C motif) ligand 1 (CXCL1) proteins in the spinal L2 segment. Second, the JNK inhibitor SP600125 was intrathecally administrated in rats from day 10 to day 12. It attenuated mechanical and thermal hypersensitivity of the low back or hindpaws and anxiety-like behavior. Meanwhile, SP600125 decreased astrocyte and neuronal activation and the expression of MCP-1 and CXCL1 proteins. These results showed that hypersensitivity and anxiety-like behavior induced by NGF in LBP rats could be attenuated by the JNK inhibitor, together with downregulation of spinal astrocyte activation, neuron activation, and inflammatory cytokines. Our results indicate that intervening with the spinal JNK signaling pathway presents an effective therapeutic approach to alleviating LBP.

Impaired amygdala astrocytic signaling worsens neuropathic pain-associated neuronal functions and behaviors

Introduction: Pain is a clinically relevant health care issue with limited therapeutic options, creating the need for new and improved analgesic strategies. The amygdala is a limbic brain region critically involved in the regulation of emotional-affective components of pain and in pain modulation. The central nucleus of amygdala (CeA) serves major output functions and receives nociceptive information via the external lateral parabrachial nucleus (PB). While amygdala neuroplasticity has been linked causally to pain behaviors, non-neuronal pain mechanisms in this region remain to be explored. As an essential part of the neuroimmune system, astrocytes that represent about 40-50% of glia cells within the central nervous system, are required for physiological neuronal functions, but their role in the amygdala remains to be determined for pain conditions. In this study, we measured time-specific astrocyte activation in the CeA in a neuropathic pain model (spinal nerve ligation, SNL) and assessed the effects of astrocyte inhibition on amygdala neuroplasticity and pain-like behaviors in the pain condition. Methods and Results: Glial fibrillary acidic protein (GFAP, astrocytic marker) immunoreactivity and mRNA expression were increased at the chronic (4 weeks post-SNL), but not acute (1 week post-SNL), stage of neuropathic pain. In order to determine the contribution of astrocytes to amygdala pain-mechanisms, we used fluorocitric acid (FCA), a selective inhibitor of astrocyte metabolism. Whole-cell patch-clamp recordings were performed from neurons in the laterocapsular division of the CeA (CeLC) obtained from chronic neuropathic rats. Pre-incubation of brain slices with FCA (100 µM, 1 h), increased excitability through altered hyperpolarization-activated current (Ih) functions, without significantly affecting synaptic responses at the PB-CeLC synapse. Intra-CeA injection of FCA (100 µM) had facilitatory effects on mechanical withdrawal thresholds (von Frey and paw pressure tests) and emotional-affective behaviors (evoked vocalizations), but not on facial grimace score and anxiety-like behaviors (open field test), in chronic neuropathic rats. Selective inhibition of astrocytes by FCA was confirmed with immunohistochemical analyses showing decreased astrocytic GFAP, but not NeuN, signal in the CeA. Discussion: Overall, these results suggest a complex modulation of amygdala pain functions by astrocytes and provide evidence for beneficial functions of astrocytes in CeA in chronic neuropathic pain.

Impaired Lactate Release in Dorsal CA1 Astrocytes Contributed to Nociceptive Sensitization and Comorbid Memory Deficits in Rodents

BACKGROUND

Memory deficits are a common comorbid disorder in patients suffering from neuropathic pain. The mechanisms underlying the comorbidities remain elusive. The hypothesis of this study was that impaired lactate release from dysfunctional astrocytes in dorsal hippocampal CA1 contributed to memory deficits.

METHODS

A spared nerve injury model was established to induce both pain and memory deficits in rats and mice of both sexes. von Frey tests, novel object recognition, and conditioned place preference tests were applied to evaluate the behaviors. Whole-cell recording, fiber photometry, Western blotting, and immunohistochemistry combined with intracranial injections were used to explore the underlying mechanisms.

RESULTS

Animals with spared sciatic nerve injury that had displayed nociception sensitization or memory deficit comorbidities demonstrated a reduction in the intrinsic excitability of pyramidal neurons, accompanied by reduced Ca2+ activation in astrocytes (ΔF/F, sham: 6 ± 2%; comorbidity: 2 ± 0.4%) and a decrease in the expression of glial fibrillary acidic protein and lactate levels in the dorsal CA1. Exogenous lactate supply or increasing endogenous lactate release by chemogenetic activation of astrocytes alleviated this comorbidity by enhancing the cell excitability (129 ± 4 vs. 88 ± 10 for 3.5 mM lactate) and potentiating N-methyl-d-aspartate receptor-mediated excitatory postsynaptic potentials of pyramidal neurons. In contrast, inhibition of lactate synthesis, blocking lactate transporters, or chemogenetic inhibition of astrocytes resulted in comorbidity-like behaviors in naive animals. Notably, β2-adrenergic receptors in astrocytes but not neurons were downregulated in dorsal CA1 after spared nerve injury. Microinjection of a β2 receptor agonist into dorsal CA1 or activation of the noradrenergic projections onto the hippocampus from the locus coeruleus alleviated the comorbidity, possibly by increasing lactate release.

CONCLUSIONS

Impaired lactate release from dysfunctional astrocytes, which could be rescued by activation of the locus coeruleus, led to nociception and memory deficits after peripheral nerve injury.

EDITOR’S PERSPECTIVE

Understanding the initiation, delivery and processing of bone cancer pain from the peripheral to the central nervous system

Bone cancer pain is a complex condition characterized by persistent, sudden, spontaneous pain accompanied by hyperalgesia that typically arises from bone metastases or primary bone tumors, causing severe discomfort and significantly diminishing cancer patients' quality of life and confidence in their ability to overcome the disease. It is widely known that peripheral nerves are responsible for detecting harmful stimuli, which are then transmitted to the brain via the spinal cord, resulting in the perception of pain. In the case of bone cancer, tumors and stromal cells within the bone marrow release various chemical signals, including inflammatory factors, colony-stimulating factors, chemokines, and hydrogen ions. Consequently, the nociceptors located at the nerve endings within the bone marrow sense these chemical signals, generating electrical signals that are then transmitted to the brain through the spinal cord. Subsequently, the brain processes these electrical signals in a complex manner to create the sensation of bone cancer pain. Numerous studies have investigated the transmission of bone cancer pain from the periphery to the spinal cord. However, the processing of pain information induced by bone cancer within the brain remains unclear. With the continuous advancements in brain science and technology, the brain mechanism of bone cancer pain would become more clearly understood. Herein, we focus on summarizing the peripheral nerve perception of the spinal cord transmission of bone cancer pain and provide a brief overview of the ongoing research regarding the brain mechanisms involved in bone cancer pain.

Astrocytes in Chronic Pain: Cellular and Molecular Mechanisms

Chronic pain is challenging to treat due to the limited therapeutic options and adverse side-effects of therapies. Astrocytes are the most abundant glial cells in the central nervous system and play important roles in different pathological conditions, including chronic pain. Astrocytes regulate nociceptive synaptic transmission and network function via neuron–glia and glia–glia interactions to exaggerate pain signals under chronic pain conditions. It is also becoming clear that astrocytes play active roles in brain regions important for the emotional and memory-related aspects of chronic pain. Therefore, this review presents our current understanding of the roles of astrocytes in chronic pain, how they regulate nociceptive responses, and their cellular and molecular mechanisms of action.

Macrophages and glial cells: Innate immune drivers of inflammatory arthritic pain perception from peripheral joints to the central nervous system

Millions of people suffer from arthritis worldwide, consistently struggling with daily activities due to debilitating pain evoked by this disease. Perhaps the most intensively investigated type of inflammatory arthritis is rheumatoid arthritis (RA), where, despite considerable advances in research and clinical management, gaps regarding the neuroimmune interactions that guide inflammation and chronic pain in this disease remain to be clarified. The pain and inflammation associated with arthritis are not isolated to the joints, and inflammatory mechanisms induced by different immune and glial cells in other tissues may affect the development of chronic pain that results from the disease. This review aims to provide an overview of the state-of-the-art research on the roles that innate immune, and glial cells play in the onset and maintenance of arthritis-associated pain, reviewing nociceptive pathways from the joint through the dorsal root ganglion, spinal circuits, and different structures in the brain. We will focus on the cellular mechanisms related to neuroinflammation and pain, and treatments targeting these mechanisms from the periphery and the CNS. A comprehensive understanding of the role these cells play in peripheral inflammation and initiation of pain and the central pathways in the spinal cord and brain will facilitate identifying new targets and pathways to aide in developing therapeutic strategies to treat joint pain associated with RA.

Ventrolateral Periaqueductal Gray Astrocytes Regulate Nociceptive Sensation and Emotional Motivation in Diabetic Neuropathic Pain

Diabetic neuropathic pain (DNP) is a diabetes complication experienced by many patients. Ventrolateral periaqueductal gray (vlPAG) neurons are essential mediators of the descending pain modulation system, yet the role of vlPAG astrocytes in DNP remains unclear. The present study applied a multidimensional approach to elucidate the role of these astrocytes in DNP. We verified the activation of astrocytes in different regions of the PAG in male DNP-model rats. We found that only astrocytes in the vlPAG exhibited increased growth. Furthermore, we described differences in vlPAG astrocyte activity at different time points during DNP progression. After the 14th day of modeling, vlPAG astrocytes exhibited obvious activation and morphologic changes. Furthermore, activation of Gq-designer receptors exclusively activated by a designer drug (Gq-DREADDs) in vlPAG astrocytes in naive male rats induced neuropathic pain-like symptoms and pain-related aversion, whereas activation of Gi-DREADDs in vlPAG astrocytes in male DNP-model rats alleviated sensations of pain and promoted pain-related preference behavior. Thus, bidirectional manipulation of vlPAG astrocytes revealed their potential to regulate pain. Surprisingly, activation of Gi-DREADDs in vlPAG astrocytes also mitigated anxiety-like behavior induced by DNP. Thus, our results provide direct support for the hypothesis that vlPAG astrocytes regulate diabetes-associated neuropathic pain and concomitant anxiety-like behavior.SIGNIFICANCE STATEMENT Many studies examined the association between the ventrolateral periaqueductal gray (vlPAG) and neuropathic pain. However, few studies have focused on the role of vlPAG astrocytes in diabetic neuropathic pain (DNP) and DNP-related emotional changes. This work confirmed the role of vlPAG astrocytes in DNP by applying a more direct and robust approach. We used chemogenetics to bidirectionally manipulate the activity of vlPAG astrocytes and revealed that vlPAG astrocytes regulate DNP and pain-related behavior. In addition, we discovered that activation of Gi-designer receptors exclusively activated by a designer drug in vlPAG astrocytes alleviated anxiety-like behavior induced by DNP. Together, these findings provide new insights into DNP and concomitant anxiety-like behavior and supply new therapeutic targets for treating DNP.

The Central Nervous Mechanism of Stress-Promoting Cancer Progression

Evidence shows that stress can promote the occurrence and development of tumors. In recent years, many studies have shown that stress-related hormones or peripheral neurotransmitters can promote the proliferation, survival, and angiogenesis of tumor cells and impair the body’s immune response, causing tumor cells to escape the “surveillance” of the immune system. However, the perception of stress occurs in the central nervous system (CNS) and the role of the central nervous system in tumor progression is still unclear, as are the underlying mechanisms. This review summarizes what is known of stress-related CNS-network activation during the stress response and the influence of the CNS on tumors and discusses available adjuvant treatment methods for cancer patients with negative emotional states, such as anxiety and depression.

Sex and dose-dependent antinociceptive effects of the JNK (c-Jun N-terminal kinase) inhibitor SU 3327 are mediated by CB2 receptors in female, and CB1/CB2 receptors in male mice in an inflammatory pain model

Activation of c-Jun N-terminal kinases (JNKs) has been implicated in the development and persistence of inflammatory and neuropathic pain in animal models. Moreover, JNKs have been involved in the maintenance of chronic pain, as well as development of tolerance to antinociceptive agents in the opioid and cannabinoid class of compounds. In this study, we evaluated the antinociceptive effects of the JNK inhibitor SU 3327 (0.3-30 mg/kg) in the formalin pain model with an emphasis on the sex-specific actions of this compound. In wild-type C57BL6J mice, SU 3327 produced strong antinociceptive effects in the formalin pain model which were mediated by CB₂ receptors in females, and both CB₁ and CB₂ receptors in males. SU 3327 at a dose of 10 mg/kg produced antinociception, hypothermia, motor impairment, and hypolocomotion to a similar extent in both males and females. The antinociceptive effects of SU 3327 were more potent in males at lower doses (1 and 3 mg/kg), while females were more sensitive to the hypothermic, and motor-suppression effects at lower (3 mg/kg) doses versus males. Analysis of spinal cords, using qPCR following SU 3327 administration in the formalin test, revealed changes in cannabinoid, tolerance and inflammatory markers in females only, and only in the high (10-30 mg/kg) dose conditions. Indeed, females showed an increase in mRNA levels of cannabinoid (CB₂), but a decrease in tolerance (β-arrestin 1) and inflammatory (TNF-α, IL-1β, IL-6)-associated markers. The differences between males and females, in this study, support sex as an important factor in nociception and antinociceptive responses mediated by JNK and the endocannabinoid system.

Rosiglitazone Alleviates Mechanical Allodynia of Rats with Bone Cancer Pain through the Activation of PPAR-γ to Inhibit the NF-κB/NLRP3 Inflammatory Axis in Spinal Cord Neurons

Bone cancer pain (BCP) is a serious clinical problem that affects the quality of life of cancer patients. However, the current treatment methods for this condition are still unsatisfactory. This study investigated whether intrathecal injection of rosiglitazone modulates the noxious behaviors associated with BCP, and the possible mechanisms related to this effect were explored. We found that rosiglitazone treatment relieved bone cancer-induced mechanical hyperalgesia in a dose-dependent manner, promoted the expression of peroxisome proliferator-activated receptor-γ (PPAR-γ) in spinal cord neurons, and inhibited the activation of the nuclear factor-kappa B (NF-κB)/nod-like receptor protein 3 (NLRP3) inflammatory axis induced by BCP. However, concurrent administration of the PPAR-γ antagonist GW9662 reversed these effects. The results show that rosiglitazone inhibits the NF-κB/NLRP3 inflammation axis by activating PPAR-γ in spinal neurons, thereby alleviating BCP. Therefore, the PPAR-γ/NF-κB/NLRP3 signaling pathway may be a potential target for the treatment of BCP in the future.

Sulforaphane alleviates hyperalgesia and enhances analgesic potency of morphine in rats with cancer-induced bone pain

Due to the efficacy and tolerability of the available drugs, the current treatment for cancer-induced bone pain (CIBP) is not considered ideal, and new drugs are required for better treatment results. This study investigated whether intrathecal injection of sulforaphane (SFN) can modulates the noxious behavior associated with CIBP and enhances the analgesic effects of morphine and the possible mechanisms related to these effects were investigated. Walker256 breast cancer cells were injected into the bone marrow cavity of rats to establish the CIBP model. When CIBP rats began to exhibit painful behavior (CIBP 6 days), SFN was injected intrathecally for 7 days. The results showed that SFN alleviated the painful behavioral hypersensitivity caused by cancer, accompanied by nuclear factor, erythroid 2 like 2 (Nrf2), Haem oxygenase 1 (HO-1) activation, nuclear factor kappa B (NF-κB) inhibition and inflammation-related factors (tumor necrosis factor-alpha (TNF-α), interleukin-1β (IL-β), interleukin-6 (IL-6), and inducible nitric oxide synthase (iNOS) reduction. In addition, SFN treatment inhibited the proliferation of Walker 256 cells in a dose-dependent manner, promoted mu-opioid receptor (MOR) expression in SH-SY5Y cells and enhanced the antihyperalgesic effects of morphine on CIBP rats by restoring the downregulation of MOR expression in the spinal cord. Interestingly, the antihyperalgesic effects of SFN were partially blocked by opioid receptor antagonists. This study showed that SFN combined with morphine might be a new way to treat CIBP.

The Modulatory Effect of Motor Cortex Astrocytes on Diabetic Neuropathic Pain

Diabetic neuropathic pain (DNP) is a common complication of diabetes characterized by persistent pain. Emerging evidence links astrocytes to mechanical nociceptive processing, and the motor cortex (MCx) is a cerebral cortex region that is known to play a key role in pain regulation. However, the association between MCx astrocytes and DNP pathogenesis remains largely unexplored. Here, we studied this association using designer receptors exclusively activated by designer drugs to specifically manipulate MCx astrocytes. We proved that the selective inhibition of MCx astrocytes reduced DNP in streptozocin (STZ)-induced DNP models and discovered a potential mechanism by which astrocytes release cytokines, including TNF-α and IL-1β, to increase neuronal activation in the MCx, thereby regulating pain. Together, these results demonstrate a pivotal role for MCx astrocytes in DNP pathogenesis and provide new insight into DNP treatment strategies.

Reactive Astrocytes: Critical Players in the Development of Chronic Pain

Chronic pain is associated with long term plasticity of nociceptive pathways in the central nervous system. Astrocytes can profoundly affect synaptic function and increasing evidence has highlighted how altered astrocyte activity may contribute to the pathogenesis of chronic pain. In response to injury, astrocytes undergo a shift in form and function known as reactive astrogliosis, which affects their release of cytokines and gliotransmitters. These neuromodulatory substances have been implicated in driving the persistent changes in central nociceptive activity. Astrocytes also release lactate which neurons can use to produce energy during synaptic plasticity. Furthermore, recent research has provided insight into lactate's emerging role as a signaling molecule in the central nervous system, which may be involved in directly modulating neuronal and astrocytic activity. In this review, we present evidence for the involvement of astrocyte-derived tumor necrosis factor alpha in pain-associated plasticity, in addition to research suggesting the potential involvement of gliotransmitters D-serine and adenosine-5'-triphosphate. We also discuss work implicating astrocyte-neuron metabolic coupling, and the possible role of lactate, which has been sparsely studied in the context of chronic pain, in supporting pathological changes in central nociceptive activity.

Recent Advances in Pain Management: Relevant Protein Kinases and Their Inhibitors

The purpose of this review is to underline the protein kinases that have been established, either in fundamental approach or clinical trials, as potential biological targets in pain management. Protein kinases are presented according to their group in the human kinome: TK (Trk, RET, EGFR, JAK, VEGFR, SFK, BCR-Abl), CMGC (p38 MAPK, MEK, ERK, JNK, ASK1, CDK, CLK2, DYRK1A, GSK3, CK2), AGC (PKA, PKB, PKC, PKMζ, PKG, ROCK), CAMK, CK1 and atypical/other protein kinases (IKK, mTOR). Examples of small molecule inhibitors of these biological targets, demonstrating an analgesic effect, are described. Altogether, this review demonstrates the fundamental role that protein kinase inhibitors could play in the development of new pain treatments.

Phosphoproteomic Profiling of Rat's Dorsal Root Ganglia Reveals mTOR as a Potential Target in Bone Cancer Pain and Electro-Acupuncture's Analgesia

Bone cancer pain (BCP) is a clinical refractory mixed pain involving neuropathic and inflammatory pain, with the underlying mechanisms remaining largely unknown. Electro-acupuncture (EA) can partly alleviate BCP according to previous research. We aim to explore the proteins and major pathways involved in BCP and EA treatment through phosphoproteomic profiling. BCP rat model was built by tibial inoculation of MRMT-1 mammary gland carcinoma cells. Mechanical hyperalgesia determined by paw withdrawal thresholds (PWTs) and bone destruction manifested on the radiographs confirmed the success of modeling, which were attenuated by EA treatment. The differentially expressed phosphorylated proteins (DEPs) co-regulated by BCP modeling and EA treatment in rat dorsal root ganglions (DRGs) were analyzed through PEX100 Protein microarray. Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis revealed that DEPs were significantly enriched in mammalian target of rapamycin (mTOR) signaling pathway. The phosphorylations of mTOR at Ser2448 and Thr2446 were increased in BCP and downregulated by EA. In addition, the phosphorylation of S6K and Akt, markers of the mTOR complex, were also increased in BCP and downregulated by EA. Inhibition of mTOR signaling alleviated the PWTs of BCP rats, while the mTOR agonist impaired the analgesic effect of EA. Thus, our study provided a landscape of protein phosphorylation changes in DRGs of EA-treated BCP rats and revealed that mTOR signaling can be potentially targeted to alleviate BCP by EA treatment.

Astrocytic Regulation of Neural Circuits Underlying Behaviors

Astrocytes, characterized by a satellite-like morphology, are the most abundant type of glia in the central nervous system. Their main functions have been thought to be limited to providing homeostatic support for neurons, but recent studies have revealed that astrocytes actually actively interact with local neural circuits and play a crucial role in information processing and generating physiological and behavioral responses. Here, we review the emerging roles of astrocytes in many brain regions, particularly by focusing on intracellular changes in astrocytes and their interactions with neurons at the molecular and neural circuit levels.

Selective Ablation of Descending Serotonin from the Rostral Ventromedial Medulla Unmasks Its Pro-Nociceptive Role in Chemotherapy-Induced Painful Neuropathy

Purpose

Chemotherapy-induced painful neuropathy (CIPN) is a severe adverse effect of many anti-neoplastic drugs that is difficult to manage. Serotonin (5-hydroxytryptamine, 5-HT) is an important neurotransmitter in the rostral ventromedial medulla (RVM), which modulates descending spinal nociceptive transmission. However, the influence of the descending 5-HT from the RVM on CIPN is poorly understood. We investigated the role of 5-HT released from descending RVM neurons in a paclitaxel-induced CIPN rat model.

Methods

CIPN rat model was produced by intraperitoneally injecting of paclitaxel. Pain behavioral assessments included mechanical allodynia and heat hyperalgesia. 5-HT content was analyzed by high-performance liquid chromatography (HPLC). Western blot and immunohistochemistry were used to determine tryptophan hydroxylase (Tph) and c-Fos expression. The inhibitors p-chlorophenylalanine (PCPA) and SB203580 were administrated by stereotaxical RVM microinjection. Ondansetron was injected through intrathecal catheterization.

Results

The results demonstrated that Tph, the rate-limiting enzyme in 5-HT synthesis, was significantly upregulated in the RVM, and that spinal 5-HT release was increased in CIPN rats. Intra-RVM microinjection of Tph inhibitor PCPA significantly attenuated mechanical and thermal pain behavior through Tph downregulation and decreased spinal 5-HT. Intra-RVM administration of p38 mitogen-activated protein kinase (p38 MAPK) inhibitor SB203580 alleviated paclitaxel-induced pain in a similar manner to PCPA. Intrathecal injection of ondansetron, a 5-HT3 receptor antagonist, partially reversed paclitaxel-induced pain, indicating that 5-HT3 receptors were involved in descending serotoninergic modulation of spinal pain processing.

Conclusion

The results suggest that activation of the p38 MAPK pathway in the RVM leads to increased RVM Tph expression and descending serotoninergic projection to the spinal dorsal horn and contributes to the persistence of CIPN via spinal 5-HT3 receptors.

Liquiritin Alleviates Pain Through Inhibiting CXCL1/CXCR2 Signaling Pathway in Bone Cancer Pain Rat

Bone cancer pain (BCP) is an intractable clinical problem, and lacked effective drugs for treating it. Recent research showed that several chemokines in the spinal cord are involved in the pathogenesis of BCP. In this study, the antinociceptive effects of liquiritin, which is an active component extracted from Glycyrrhizae Radix, were tested and the underlying mechanisms targeting spinal dorsal horn (SDH) were investigated. The BCP group displayed a significant decrease in the mechanical withdrawal threshold on days 6, 12, and 18 when compared with sham groups. Intrathecal administration of different doses of liquiritin alleviated mechanical allodynia in BCP rats. The results of immunofluorescent staining and western blotting showed that liquiritin inhibited BCP-induced activation of astrocytes in the spinal cord. Moreover, intrathecal administration of liquiritin effectively inhibited the activation of CXCL1/CXCR2 signaling pathway and production of IL-1β and IL-17 in BCP rats. In astroglial-enriched cultures, Lipopolysaccharides (LPS) elicited the release of chemokine CXCL1, and the release was decreased in a dose-dependent manner by liquiritin. In primary neurons, liquiritin indirectly reduced the increase of CXCR2 by astroglial-enriched-conditioned medium but not directly on the CXCR2 target site. These results suggested that liquiritin effectively attenuated BCP in rats by inhibiting the activation of spinal astrocytic CXCL1 and neuronal CXCR2 pathway. These findings provided evidence regarding the the antinociceptive effect of liquiritin on BCP.

MicroRNA-448 modulates the progression of neuropathic pain by targeting sirtuin 1

MicroRNAs (miRNAs) play crucial roles in the pathogenesis of neuropathic pain. The present study investigated the effects of miR-448 on the progression of neuropathic pain in a rat model of chronic constriction injury (CCI) of the sciatic nerve. Reverse-transcription quantitative polymerase chain reaction was conducted to detect the gene expression. The paw withdrawal threshold (PWT) and paw withdrawal latency (PWL) were used to assess the pain threshold. The protein expression levels of interleukin (IL)-6, IL-1β and tumor necrosis factor-α (TNF-α) were detected by ELISA. The target of miR-448 was predicted by TargetScan software. The Student's t-test or one-way ANOVA were used to identify statistical differences among groups. miR-448 was persistently upregulated in CCI rats, and both mechanical allodynia and thermal hyperalgesia in CCI rats were decreased following miR-448 downregulation. The expression levels of IL-1β, IL-6 and TNF-α were significantly increased in CCI rats compared with controls, and these effects were reversed following treatment with a miR-448 inhibitor. A luciferase reporter assay revealed that sirtuin 1 (SIRT1) was a target gene of miR-448. SIRT1 was found to abrogate the effect of miR-448 on neuropathic pain development. Collectively, the results of the present study revealed that miR-448 promoted neuropathic pain in CCI rats by regulating neuroinflammation via SIRT1. Therefore, SIRT1 may be considered as a novel biomarker for neuropathic pain.

The endocannabinoid system: Novel targets for treating cancer induced bone pain

Treating Cancer-induced bone pain (CIBP) continues to be a major clinical challenge and underlying mechanisms of CIBP remain unclear. Recently, emerging body of evidence suggested the endocannabinoid system (ECS) may play essential roles in CIBP. Here, we summarized the current understanding of the antinociceptive mechanisms of endocannabinoids in CIBP and discussed the beneficial effects of endocannabinoid for CIBP treatment. Targeting non-selective cannabinoid 1 receptors or selective cannabinoid 2 receptors, and modulation of peripheral AEA and 2-AG, as well as the inhibition the function of fatty acid amide hydrolase (FAAH) and monoacylglycerol lipase (MAGL) have produced analgesic effects in animal models of CIBP. Management of ECS therefore appears to be a promising way for the treatment of CIBP in terms of efficacy and safety. Further clinical studies are encouraged to confirm the possible translation to humans of the very promising results already obtained in the preclinical studies.

Astrocytes in chronic pain and itch

Astrocytes are critical for maintaining the homeostasis of the CNS. Increasing evidence suggests that a number of neurological and neuropsychiatric disorders, including chronic pain, may result from astrocyte 'gliopathy'. Indeed, in recent years there has been substantial progress in our understanding of how astrocytes can regulate nociceptive synaptic transmission via neuronal-glial and glial-glial cell interactions, as well as the involvement of spinal and supraspinal astrocytes in the modulation of pain signalling and the maintenance of neuropathic pain. A role of astrocytes in the pathogenesis of chronic itch is also emerging. These developments suggest that targeting the specific pathways that are responsible for astrogliopathy may represent a novel approach to develop therapies for chronic pain and chronic itch.