Astrocytes--multitaskers in chronic pain

Astrocytes in the rostral ventromedial medulla mediate the analgesic effect of electroacupuncture in a rodent model of chemotherapy-induced peripheral neuropathic pain

ABSTRACT

Chemotherapy-induced peripheral neuropathic pain aggravates cancer survivors' life burden. Electroacupuncture (EA) has exhibited promising analgesic effects on neuropathic pain in previous studies. We investigated whether EA was effective in a paclitaxel-induced neuropathic pain mouse model. We further explored the functional role of astrocytes in the rostral ventromedial medulla (RVM), a well-established pain modulation center, in the process of neuropathic pain as well as the analgesic effect of EA. We found that paclitaxel induced mechanical allodynia, astrocytic calcium signaling, and neuronal activation in the RVM and spinal cord, which could be suppressed by EA treatment. Electroacupuncture effectively alleviated paclitaxel-induced mechanical allodynia, and the effect was attenuated by the chemogenetic activation of astrocytes in the RVM. In addition, inhibiting astrocytic calcium activity by using either IP3R2 knockout (IP3R2 KO) mice or microinjection of AAV-mediated hPMCA2 w/b into the RVM to reduce non-IP3R2-dependent Ca2+ signaling in astrocytes exhibited an analgesic effect on neuropathic pain, which mimicked the EA effect. The current study revealed the pivotal role of the RVM astrocytes in mediating the analgesic effects of EA on chemotherapy-induced peripheral neuropathic pain.

The Impact of Treadmill Training on Tissue Integrity, Axon Growth, and Astrocyte Modulation

Spinal cord injury (SCI) presents a complex challenge in neurorehabilitation, demanding innovative therapeutic strategies to facilitate functional recovery. This study investigates the effects of treadmill training on SCI recovery, emphasizing motor function enhancement, neural tissue preservation, and axonal growth. Our research, conducted on a rat model, demonstrates that controlled treadmill exercises significantly improve motor functions post-SCI, as evidenced by improved scores on the Basso, Beattie, and Bresnahan (BBB) locomotor rating scale and enhanced electromyography readings. Notably, the training facilitates the preservation of spinal cord tissue, effectively reducing secondary damage and promoting the maintenance of neural fibers in the injured area. A key finding is the significant stimulation of axonal growth around the injury epicenter in trained rats, marked by increased growth-associated protein 43 (GAP43) expression. Despite these advancements, the study notes a limited impact of treadmill training on motoneuron adaptation and highlights minimal changes in the astrocyte and neuron-glial antigen 2 (NG2) response. This suggests that, while treadmill training is instrumental in functional improvements post-SCI, its influence on certain neural cell types and glial populations is constrained.

Hyperexcitability of muscle spindle afferents in jaw-closing muscles in experimental myalgia: Evidence for large primary afferents involvement in chronic pain

The goals of this review are to improve understanding of the aetiology of chronic muscle pain and identify new targets for treatments. Muscle pain is usually associated with trigger points in syndromes such as fibromyalgia and myofascial syndrome, and with small spots associated with spontaneous electrical activity that seems to emanate from fibers inside muscle spindles in EMG studies. These observations, added to the reports that large-diameter primary afferents, such as those innervating muscle spindles, become hyperexcitable and develop spontaneous ectopic firing in conditions leading to neuropathic pain, suggest that changes in excitability of these afferents might make an important contribution to the development of pathological pain. Here, we review evidence that the muscle spindle afferents (MSAs) of the jaw-closing muscles become hyperexcitable in a model of chronic orofacial myalgia. In these afferents, as in other large-diameter primary afferents in dorsal root ganglia, firing emerges from fast membrane potential oscillations that are supported by a persistent sodium current (INaP ) mediated by Na+ channels containing the α-subunit NaV 1.6. The current flowing through NaV 1.6 channels increases when the extracellular Ca2+ concentration decreases, and studies have shown that INaP -driven firing is increased by S100β, an astrocytic protein that chelates Ca2+ when released in the extracellular space. We review evidence of how astrocytes, which are known to be activated in pain conditions, might, through their regulation of extracellular Ca2+ , contribute to the generation of ectopic firing in MSAs. To explain how ectopic firing in MSAs might cause pain, we review evidence supporting the hypothesis that cross-talk between proprioceptive and nociceptive pathways might occur in the periphery, within the spindle capsule.

The origins and dynamic changes of C3- and S100A10-positive reactive astrocytes after spinal cord injury

Accaumulating studies focus on the effects of C3-positive A1-like phenotypes and S100A10-positive A2-like phenotypes of reactive astrocytes on spinal cord injury (SCI), however the origins and dynamic changes of C3- and S100A10-positive reactive astrocytes after SCI remain poorly understood. Through transgenic mice and lineage tracing, we aimed to determine the origins of C3- and S100A10-positive reactive astrocytes. Meanwhile, the distribution and dynamic changes in C3- and S100A10-positive reactive astrocytes were also detected in juvenile and adult SCI mice models and cultured astrocytes. Combing with bulk RNA sequencing (RNA-seq), single-cell RNA sequencing (scRNA-seq) and bioinformatic analysis, we further explored the dynamic transcripts changes of C3- and S100A10-positive reactive astrocytes after SCI. We confirmed that resident astrocytes produced both C3- and S100A10-positive reactive astrocytes, whereas ependymal cells regenerated only S100A10-positive reactive astrocytes in lesion area. Importantly, C3-positive reactive astrocytes were predominantly activated in adult SCI mice, while S100A10-positive reactive astrocytes were hyperactivated in juvenile mice. Furthermore, we observed that C3- and S100A10-positive reactive astrocytes had a dynamic transformation process at different time in vitro and vivo, and a majority of intermediate states of C3- and S100A10-positive reactive astrocytes were found during transformation. RNA-seq and scRNA-seq results further confirmed that the transcripts of C3-positive reactive astrocytes and their lipid toxicity were gradually increased with time and age. In contrast, S100A10-positive reactive astrocytes transcripts increased at early time and then gradually decreased after SCI. Our results provide insight into the origins and dynamic changes of C3- and S100A10-positive reactive astrocytes after SCI, which would be valuable resources to further target C3- and S100A10-positive reactive astrocytes after SCI.

Soluble epoxide hydrolase inhibitor blockage microglial cell activation in subnucleus caudalis in a persistent model of arthritis

Rheumatoid arthritis (RA) is a chronic condition characterized by pain and infiltration of immune cells into the joint. Immune cells can be activated, producing inflammatory cytokines, leading to continuously degenerative and inflammatory reactions and the temporomandibular joint (TMJ) can be affected by RA. In this scenario, novel targets are needed to increase treatment efficacy with minimized side effects. The epoxy-eicosatrienoic acids (EETs), are endogenous signaling molecules, playing important roles in diminishing inflammation and pain but are promptly metabolized by soluble epoxide hydrolase (sEH), generating less-bioactive acids.Therefore, sEH inhibitors is an interest therapeutic target to enhance the beneficial effect of natural EETs. TPPU is a potent sEH inhibitor that is capable of dampening EETs hydrolysis. Thus, we aimed to assess the impact of pharmacological sEH inhibition on a persistent model of albumin-induced arthritis in the TMJ, in two scenarios: first, as post-treatment, in an installed arthritic condition, and second, the protective role, in preventing the development of an arthritic condition. In addition, we investigate the influence of sEH inhibition on microglia cell activation in the trigeminal subnucleus caudalis (TSC) and in vitro experiments. Finally, we examined the astrocyte phenotype. Oral administration of TPPU, acts in multiple pathways, in a protective and reparative post-treatment, ameliorating the preservation of the TMJ morphology, reducing the hypernociception, with an immunosuppressive action reducing neutrophil and lymphocytes and pro-inflammatory cytokines in the TMJ of rats. In TSC, TPPU reduces the cytokine storm and attenuates the microglia activated P2X7/Cathepsin S/Fractalkine pathway and reduces the astrocyte activation and glutamate levels. Collectively, our findings revealed that sEH inhibition mitigates hypersensitive nociception through the regulation of microglia activation and astrocyte modulation, demonstrating the potential use of sEH inhibitors as immunoresolvents in the treatment of autoimmune disorders.

Effects of prenatal hypoxia-ischemia on male rat periaqueductal gray matter: Hyperalgesia, astrogliosis and nitrergic system impairment

Prenatal hypoxic-ischemic insult (HI) may lead to a variety of neurological consequences that may persist throughout adulthood. In the most severe cases, HI is known to increase pain sensitivity which profoundly impacts quality of life. Periaqueductal gray matter (PAG) is a relevant region of the descending pain pathway and its function may be modulated by a complex network that includes nitrergic neurons and glial response, among other factors. Astrocytes, central players in pain modulation, are known to respond to HI by inducing hyperplasia, hypertrophy and increasing the number of their processes and the staining of glial fibrillary acidic protein (GFAP). In this work we investigated the effects of prenatal HI on touch and pain sensitivity, besides the distribution of the neuronal isoform of Nitric Oxide Synthase (nNOS) and GFAP in the PAG of young and adult male rats. At 18 days of gestation, rats had their uterine arteries clamped for 45 min (HI group). SHAM-operated animals were also generated (SHAM group). At post-natal day 30 (P30) or 90 (P90), the offspring was submitted to the behavioral tests of Von Frey and formalin or histological processing to perform immunohistochemistry for nNOS and GFAP. Although there was no significant difference between the groups concerning touch sensitivity, we observed an increase in pain sensitivity in HI P30 and HI P90. The number of nNOS + cells was reduced in HI adult animals in dlPAG and vlPAG. GFAP immunostaining was increased in HI P90 in dlPAG and dmPAG. Our results demonstrated for the first time an increase in pain sensitivity as a consequence of prenatal HI in an animal model. It reinforces the relevance of this model to mimic the effects of prenatal HI, as hyperalgesia.

Long-lasting reflexive and nonreflexive pain responses in two mouse models of fibromyalgia-like condition

Nociplastic pain arises from altered nociception despite no clear evidence of tissue or somatosensory system damage, and fibromyalgia syndrome can be highlighted as a prototype of this chronic pain subtype. Currently, there is a lack of effective treatments to alleviate both reflexive and nonreflexive pain responses associated with fibromyalgia condition, and suitable preclinical models are needed to assess new pharmacological strategies. In this context, although in recent years some remarkable animal models have been developed to mimic the main characteristics of human fibromyalgia, most of them show pain responses in the short term. Considering the chronicity of this condition, the present work aimed to develop two mouse models showing long-lasting reflexive and nonreflexive pain responses after several reserpine (RIM) or intramuscular acid saline solution (ASI) injections. To our knowledge, this is the first study showing that RIM6 and ASI mouse models show reflexive and nonreflexive responses up to 5-6 weeks, accompanied by either astro- or microgliosis in the spinal cord as pivotal physiopathology processes related to such condition development. In addition, acute treatment with pregabalin resulted in reflexive pain response alleviation in both the RIM6 and ASI models. Consequently, both may be considered suitable experimental models of fibromyalgia-like condition, especially RIM6.

Wnt10a/β-catenin signalling is involved in kindlin-1-mediated astrocyte activation in a chronic construction injury rat model

The activation of spinal astrocytes and release of neuroinflammatory mediators are important events in neuropathic pain (NP) pathogenesis. In this study, we investigated the role of Wnt10a/β-catenin signalling in kindlin-1-mediated astrocyte activation using a chronic constriction injury (CCI) NP rat model. Using kindlin-1 overexpression and knockdown plasmids, we assessed hyperalgesia, changes in spinal astrocyte activation and the release of inflammatory mediators in a NP rat model. We also performed coimmunoprecipitation, Western blotting and real-time polymerase chain reaction (PCR) to characterize the underlying mechanisms of kindlin-1 in astrocyte cultures in vitro. Kindlin-1 was significantly upregulated in CCI rats and promoted hyperalgesia. Moreover, we observed increased kindlin-1, Wnt10a and glial fibrillary acidic protein (GFAP; biomarker for astroglial injury) levels and the release of inflammatory mediators in NP rats (p < 0.05). Inhibiting GFAP in vitro led to decreased kindlin-1 levels, prevented astrocyte activation, decreased Wnt10a level and the release of inflammatory mediators (p < 0.05). Coimmunoprecipitation showed that kindlin-1 can interact with Wnt10a. We showed that kindlin-1-mediated astrocyte activation was associated with Wnt10a/β-catenin signalling and the downstream release of inflammatory mediators in a CCI NP rat model. Our findings provide novel insights into the molecular mechanisms of kindlin-1-mediated astrocyte activation after CCI.

Calcitonin gene-related peptide induces the histone H3 lysine 9 acetylation in astrocytes associated with neuroinflammation in rats with neuropathic pain

Aims

Calcitonin gene‐related peptide (CGRP) as a regulator of astrocyte activation may facilitate spinal nociceptive processing. Histone H3 lysine 9 acetylation (H3K9ac) is considered an important regulator of cytokine and chemokine gene expression after peripheral nerve injury. In this study, we explored the relationship between CGRP and H3K9ac in the activation of astrocytes, and elucidated the underlying mechanisms in the pathogenesis of chronic neuropathic pain.

Methods

Astroglial cells (C6) were treated with CGRP and differentially enrichments of H3K9ac on gene promoters were examined using ChIP‐seq. A chronic constriction injury (CCI) rat model was used to evaluate the role of CGRP on astrocyte activation and H3K9ac signaling in CCI‐induced neuropathic pain. Specific inhibitors were employed to delineate the involved signaling.

Results

Intrathecal injection of CGRP and CCI increased the number of astrocytes displaying H3K9ac in the spinal dorsal horn of rats. Treatment of CGRP was able to up‐regulate H3K9ac and glial fibrillary acidic protein (GFAP) expression in astroglial cells. ChIP‐seq data indicated that CGRP significantly altered H3K9ac enrichments on gene promoters in astroglial cells following CGRP treatment, including 151 gaining H3K9ac and 111 losing this mark, which mostly enriched in proliferation, autophagy, and macrophage chemotaxis processes. qRT‐PCR verified expressions of representative candidate genes (ATG12, ATG4C, CX3CR1, MMP28, MTMR14, HMOX1, RET) and RTCA verified astrocyte proliferation. Additionally, CGRP treatment increased the expression of H3K9ac, CX3CR1, and IL‐1β in the spinal dorsal horn. CGRP antagonist and HAT inhibitor attenuated mechanical and thermal hyperalgesia in CCI rats. Such analgesic effects were concurrently associated with the reduced levels of H3K9ac, CX3CR1, and IL‐1β in the spinal dorsal horn of CCI rats.

Conclusion

Our findings highly indicate that CGRP is associated with the development of neuropathic pain through astrocytes‐mediated neuroinflammatory responses via H3K9ac in spinal dorsa horn following nerve injury.

This study found that CGRP act on their astrocytic receptors and lead to H3K9 acetylation (H3K9ac), which are mainly associated with proliferation‐, autophagy‐, and inflammation‐related gene expression. The number of astrocytes with H3K9ac expression is increased after nerve injury. Inhibition of CGRP attenuates the development of neuropathic pain, which was accompanied by the suppression of H3K9ac, CX3CR1, and IL‐1β expression in CCI rats.

Astrocyte reactivity in spinal cord and functional impairment after tendon injury in rats

Astrocyte reactivity in the spinal cord may occur after peripheral neural damage. However, there is no data to report such reactivity after Achilles tendon injury. We investigate whether changes occur in the spinal cord, mechanical sensitivity and gait in two phases of repair after Achilles tendon injury. Wistar rats were divided into groups: control (CTRL, without rupture), 2 days post-injury (RUP2) and 21 days post-injury (RUP21). Functional and mechanical sensitivity tests were performed at 2 and 21 days post-injury (dpi). The spinal cords were processed, cryosectioned and activated astrocytes were immunostained by GFAP at 21 dpi. Astrocyte reactivity was observed in the L5 segment of the spinal cord with predominance in the white matter regions and decrease in the mechanical threshold of the ipsilateral paw only in RUP2. However, there was gait impairment in both RUP2 and RUP21. We conclude that during the acute phase of Achilles tendon repairment, there was astrocyte reactivity in the spinal cord and impairment of mechanical sensitivity and gait, whereas in the chronic phase only gait remains compromised.

Scorpion alleviates bone cancer pain through inhibition of bone destruction and glia activation

Background

Bone cancer pain is common in patients with advanced cancers as tumor metastasizes to bone. The inefficient clinical treatment severely reduces quality of life of bone cancer pain patients. During the pain status, activated spinal astrocytes and microglia release various inflammatory cytokines, resulting in spinal inflammation and the development of neuron sensitization. Scorpion is the dry body of Buthus martensii Karsch and is often used for various pain management in clinical practice. However, its function on bone cancer pain is unclear.

Methods

We investigated the effects of intragastric administration of scorpion on bone cancer pain induced by left tibial cavity injection of Walker 256 cells. Nociceptive behavior was measured using the von Frey filaments test and the spontaneous ambulatory pain score. The bone destruction was assessed by tibial radiographs. Expression of spinal cord astrocyte marker glial fibrillary acidic protein and microglial marker Iba1 was monitored by Western blot assay and immunofluorescence. Tumor necrosis factor-alpha (TNF-α), interleukin (IL)-6, and IL-1β was detected by real-time polymerase chain reaction. The proliferation of Walker 256 cells was evaluated by CCK8 assay.

Results

Intragastric administration of scorpion reduced bone cancer pain behavior and relieved bone destruction, accompanied by decreased expression of spinal glial fibrillary acidic protein and Iba1 protein level and TNF-α, IL-6, and IL-1β mRNA level. Besides, scorpion inhibited proliferation of Walker 256 cells in a dose- and time-dependent manner.

Conclusion

Our results demonstrate that scorpion produces an analgesic effect in a rat model of bone cancer pain via inhibiting bone destruction and activation of spinal cord astrocytes and microglia.

LncRNA KCNA2-AS regulates spinal astrocyte activation through STAT3 to affect postherpetic neuralgia

Objectives

Postherpetic neuralgia (PHN) is the most common complication of herpes zoster, but the mechanism of PHN is still unclear. Activation of spinal astrocytes is involved in PHN. Our study aims to explore whether lncRNA KCNA2 antisense RNA (KCNA2-AS) regulates spinal astrocytes in PHN through signal transducers and activators of transcription 3 (STAT3).

Methods

Varicella zoster virus (VZV)-infected CV-1 cells were injected into rats to construct a PHN model. Primary spinal cord astrocytes were activated using S-Nitrosoglutathione (GSNO). Glial fibrillary acidic protein (GFAP; marker of astrocyte activation), phosphorylated STAT3 (pSTAT3), and KCNA2-AS were analyzed by immunofluorescence and RNA fluorescence in situ hybridization. RNA pull-down and RNA immunoprecipitation were used to detect binding of KCNA2-AS to pSTAT3.

Results

KCNA2-AS was highly expressed in the spinal cord tissue of PHN model rats, and was positively correlated with GFAP expression. GFAP was significantly increased in GSNO-induced cells, but the knockdown of KCNA2-AS reversed this result. Meanwhile, pSTAT3 was significantly increased in GSNO-induced cells, but knockdown of KCNA2-AS reduced pSTAT3 within the nucleus while the total pSTAT3 did not change significantly. pSTAT3 bound to KCNA2-AS and this binding increased with GSNO treatment. Furthermore, knockdown of KCNA2-AS in PHN model rats relieved mechanical allodynia.

Conclusion

Down-regulation of KCNA2-AS alleviates PHN partly by reducing the translocation of pSTAT3 cytoplasm to the nucleus and then inhibiting the activation of spinal astrocytes.

An Investigation into Proteomic Constituents of Cerebrospinal Fluid in Patients with Chronic Peripheral Neuropathic Pain Medicated with Opioids- a Pilot Study

The pharmacodynamics of opioids for chronic peripheral neuropathic pain are complex and likely extend beyond classical opioid receptor theory. Preclinical evidence of opioid modulation of central immune signalling has not been identified in vivo in humans. Examining the cerebrospinal fluid (CSF) of patients medicated with opioids is required to identify potential pharmacodynamic mechanisms. We compared CSF samples of chronic peripheral neuropathic pain patients receiving opioids (n = 7) versus chronic peripheral neuropathic pain patients not taking opioids (control group, n = 13). Baseline pain scores with demographics were recorded. Proteome analysis was performed using mass spectrometry and secreted neuropeptides were measured by enzyme-linked immunosorbent assay. Based on Gene Ontology analysis, proteins involved in the positive regulation of nervous system development and myeloid leukocyte activation were increased in patients taking opioids versus the control group. The largest decrease in protein expression in patients taking opioids were related to neutrophil mediated immunity. In addition, notably higher expression levels of neural proteins (85%) and receptors (80%) were detected in the opioid group compared to the control group. This study suggests modulation of CNS homeostasis, possibly attributable to opioids, thus highlighting potential mechanisms for the pharmacodynamics of opioids. We also provide new insights into the immunomodulatory functions of opioids in vivo.

Astrocytes-induced neuronal inhibition contributes to depressive-like behaviors during chronic stress

Although emerging evidence has highlighted the heterogeneities of astrocytes under physiological versus pathological conditions, little is known regarding these processes in different brain regions during stress. Thus, the present study established a mouse model of chronic social defeat stress (CSDS) and isolated astrocytes from the medial prefrontal cortex (mPFC) and hippocampus. The results revealed dramatic A1-specific (neurotoxic phenotype) astrocytic responses, depressive-like behaviors, and significant inhibition of neuronal activities in both the mPFC and hippocampus according to electrophysiological data. Subsequently, astrocytes in the mPFC and hippocampus of CSDS mice were suppressed and this reversed the astrocytic responses and rescued depressive-like behaviors. Furthermore, when astrocytes were activated in the mPFC and hippocampus in healthy mice, there was a non-specific phenotypic activation of astrocytes in the absence of depressive-like behaviors. Next, microglia were depleted and the mice subsequently performed in the CSDS model; this reduced astrocyte responses and restored depressive-like behaviors. On the other hand, when microglia were depleted but astrocytes were activated in CSDS mice, this abolished the restoration of microglia depletion-induced depressive-like behaviors. Taken together, these results indicate that neuronal inhibition by astrocytes in the mPFC and hippocampus contributed to depressive-like behaviors mediated by activated microglia. This study provides evidence regarding the interaction of microglia and astrocytes during stress and how that relationship can trigger depressive-like behaviors.

Beneficial effects of (-)-epigallocatechin-3-O-gallate on diabetic peripheral neuropathy in the rat model

Diabetic neuropathic pain is characterized by spontaneous pain with hyperalgesia and allodynia. We investigated whether (-)-epigallocatechin-3-O-gallate could improve diabetic neuropathic pain development through hypoglycemic, hypolipidemic, antioxidant, and anti-inflammatory effects. Diabetes was induced in rats by streptozotocin (55 mg/kg/once) and treated with (-)-epigallocatechin-3-O-gallate (25 mg/kg/orally/once/daily/5 weeks). Diabetic rats showed an increase in serum levels of glucose, nitric oxide, triglyceride, total cholesterol, and low-density lipoprotein-cholesterol with a decrease in high-density lipoprotein-cholesterol and body weight. Also, there was an elevation in brain malondialdehyde with a marked reduction in brain levels of glutathione, superoxide dismutase, catalase, glutathione peroxidase, and glutathione-S-transferase. Furthermore, diabetic rats showed a clear reduction in plasma levels of insulin and an increase in plasma cytokines (interleukin-6 and tumor necrosis factor-α). Moreover, diabetic rats exhibited hyperalgesia as indicated by a hot plate, tail immersion, formalin, and carrageenan-induced edema tests as well as brain histopathological changes (neuron degeneration, gliosis, astrocytosis, congestion and hemorrhage). (-)-Epigallocatechin-3-O-gallate treatment ameliorated alterations in body weight, biochemical parameters, pain sensation, and histopathological changes in brain tissue. (-)-Epigallocatechin-3-O-gallate offers promising hypoglycemic, hypolipidemic, antioxidant and anti-inflammatory effects, which can prevent the development and progression of diabetic neuropathic pain.

Functional Consequences of Calcium-Dependent Synapse-to-Nucleus Communication: Focus on Transcription-Dependent Metabolic Plasticity

In the nervous system, calcium signals play a major role in the conversion of synaptic stimuli into transcriptional responses. Signal-regulated gene transcription is fundamental for a range of long-lasting adaptive brain functions that include learning and memory, structural plasticity of neurites and synapses, acquired neuroprotection, chronic pain, and addiction. In this review, we summarize the diverse mechanisms governing calcium-dependent transcriptional regulation associated with central nervous system plasticity. We focus on recent advances in the field of synapse-to-nucleus communication that include studies of the signal-regulated transcriptome in human neurons, identification of novel regulatory mechanisms such as activity-induced DNA double-strand breaks, and the identification of novel forms of activity- and transcription-dependent adaptations, in particular, metabolic plasticity. We summarize the reciprocal interactions between different kinds of neuroadaptations and highlight the emerging role of activity-regulated epigenetic modifiers in gating the inducibility of signal-regulated genes.

The GCs-SGK1-ATP Signaling Pathway in Spinal Astrocytes Underlied Presurgical Anxiety-Induced Postsurgical Hyperalgesia

BACKGROUND

Patients undergoing surgery often feel anxious. Accumulating evidence indicated that presurgical anxiety was related to the more severe postsurgical pain. An animal model was established that exposed Sprague-Dawley rats to a single-prolonged stress (SPS) procedure to induce presurgical anxiety-like behaviors. The experiment revealed that presurgical anxiety not only aggravated but also prolonged postsurgical pain. However, the underlying mechanisms were unknown.

METHODS

The rats in group C + Cort, group I + Cort, group A + Cort, and group AI + Cort were injected with corticosterone. The rats in group C + RU486, group I + RU486, group A + RU486, and group AI + RU486 were injected with mifepristone (RU486). The rats in group C + GSK650394 and group AI + GSK650394 were injected with GSK650394. The rats in group C + FC1 and group AI + FC1 were injected with fluorocitrate (FC) 30 minutes before SPS, 30 minutes before incision, and on postoperative days 1, 2, 3, 4, and 5. The rats in group C + FC2 and group AI + FC2 were injected with FC on postoperative days 7, 8, 9, 10, 11, 12, and 13. The paw withdrawal mechanical threshold was assessed 24 hours before SPS and from postoperative days 1 to 28. The level of corticosterone was determined by enzyme-linked immunosorbent assay. The expression of serum/glucocorticoid regulated kinase 1 (SGK1), interleukin-1β, and tumor necrosis factor-α was visualized by Western blot. The concentrations of adenosine triphosphate (ATP) were measured by ATP assay kit.

RESULTS

This study showed SPS elevated plasma glucocorticoids and ATP release from astrocytes, which meant the mechanical pain hypersensitivity in presurgical anxiety-induced postsurgical hyperalgesia was dependent on GCs-SGK1-ATP signaling pathway. SGK1 protein level in astrocytes was increased in response to the glucocorticoid stimuli and enhanced the extracellular release of ATP. Furthermore, spinal astrocytes played a key role in the maintenance. Targeting spinal astrocytes in maintenance phase prevented the pathological progression.

CONCLUSIONS

These data suggested an important signaling pathway that affected the pain sensitivity after operation caused by presurgical anxiety.

Role of bone morphogenetic protein-2/4 in astrocyte activation in neuropathic pain

Background

Bone morphogenetic protein-2/4 (BMP2/4) has been recognized as promoters of astrocyte activity. Substantial evidence suggests that BMP2/4 may be elevated and plays a critical role in astrocyte activation upon spinal cord injury. Although neuropathic pain is similarly associated with astrocyte activation, the participation of BMP2/4 in this regard still remains unclear.

Methods

A rat model of neuropathic pain achieved by spinal nerve ligation at L5 was used to evaluate the expression of glial fibrillary acidic protein and BMP2/4 in the spinal cord in days 1, 4, 7, 10, and 14. Next, normal rats received intrathecal exogenous BMP2/4 and the antagonist Noggin to assess the effect of BMP2/4 on astrocyte activation. In both experiments, von Frey filaments were used to evaluate the changes in paw withdrawal threshold. In addition, Western blotting and immunofluorescence were performed to assess the expression of glial fibrillary acidic protein, BMP2/4, p-Smad 1/5/8, and phospho-signal transducer and activator of transcription-3 (p-STAT3) in the spinal cord.

Results

Firstly, spinal nerve ligation caused a significant increase in the expression of BMP4, while BMP2 levels remained unchanged. Secondly, exogenous BMP4 but not BMP2 induced a significant decrease in paw withdrawal threshold, along with the upregulation of glial fibrillary acidic protein. Moreover, exogenous BMP4 stimulated both p-Smad 1/5/8 and p-STAT3, while BMP2 only upregulated p-Smad 1/5/8. Finally, exogenous Noggin alleviated the decrease in paw withdrawal threshold induced by BMP4 and reduced astrocyte activation, as well as p-STAT3 upregulation.

Conclusions

Our results indicate only BMP4—and not BMP2—intervened in allodynia in rats by eliciting glial activation probably through both p-Smad 1/5/8 and p-STAT3 signaling.

NMDAR and JNK Activation in the Spinal Trigeminal Nucleus Caudalis Contributes to Masseter Hyperalgesia Induced by Stress

It is commonly accepted that psychological stress is closely associated with the occurrence and development of chronic orofacial pain. However, the pathogenesis underlying this process has not been fully elucidated. In the present study, we explored the role of N-methyl-D-aspartate receptors (NMDARs) and Jun N-terminal kinase (JNK) mediated intercellular communication between neurons and astrocytes in the spinal trigeminal nucleus caudalis (Vc) in the induction of masseter hyperalgesia by psychological stress in rats. We found that subjecting rats to 14 days of restraint stress (8 h/d) caused a significant decrease in body weight gain, behavioral changes and marked masseter hyperalgesia in the rats. We also found that exposure to restraint stress for 14 days caused the expression of pJNK in astrocytes in the Vc to significantly increase, and intrathecally infusing a JNK inhibitor significantly prevented restraint stress-induced masseter hyperalgesia in the rats. In addition, after exposure to restraint stress for 14 days, the stressed group exhibited a noticeably increased expression level of pNR2B in neurons in the Vc. Then, we intrathecally injected MK-801 (an NMDAR inhibitor) and ifenprodil (a selective NR2B subunit antagonist) and observed that the two types of inhibitors not only alleviated masseter hyperalgesia but also significantly inhibited the phosphorylation of JNK in the Vc after restraint stress; this indicates that the effect of NMDAR antagonists may influence the activation of astrocytic JNK. Furthermore, inhibitors of neuronal nitric oxide synthase (nNOS) activation and guanylate cyclase (GC) inhibitor could not only inhibit the expression of pJNK in the Vc, but also effectively alleviate masseter hyperalgesia induced by restraint stress. Taken together, our results suggest that NMDAR activation could increase JNK phosphorylation in astrocytes after restraint stress, which may depend on the nNOS-GC pathway. The intercellular communication between neurons and astrocytes in the Vc may play a key role in the induction of masseter muscle hyperalgesia by psychological stress in rats.

Stimulation of nuclear receptor REV-ERBs suppresses production of pronociceptive molecules in cultured spinal astrocytes and ameliorates mechanical hypersensitivity of inflammatory and neuropathic pain of mice

The orphan nuclear receptors REV-ERBα and REV-ERBβ (REV-ERBs) are crucial in the regulation of inflammatory-related gene transcription in astroglioma cells, but their role in nociceptive transduction has yet to be elaborated. Spinal dorsal horn astrocytes contribute to the maintenance of chronic pain. Treatment of cultured spinal astrocytes with specific REV-ERBs agonists SR9009 or GSK4112 significantly prevented lipopolysaccharide (LPS)-induced mRNA upregulation of pronociceptive molecules interleukin-1β (IL-1β) mRNA, interleukin-6 (IL-6) mRNA and matrix metalloprotease-9 (MMP-9) mRNA, but not CCL2 mRNA expression. Treatment with SR9009 also blocked tumor necrosis factor-induced IL-1β mRNA, IL-6 mRNA and MMP-9 mRNA. In addition, treatment with SR9009 significantly blocked LPS-induced upregulation of IL-1β protein, IL-6 protein and MMP-9 activity. The inhibitory effects of SR9009 on LPS-induced expression of pronociceptive molecules were blocked by knockdown of REV-ERBs expression with short interference RNA, confirming that SR9009 exerts its effect through REV-ERBs. Intrathecal LPS treatment in male mice induces hind paw mechanical hypersensitivity, and upregulation of IL-1β mRNA, IL-6 mRNA and glial fibrillary acidic protein (GFAP) expression in spinal dorsal horn. Intrathecal pretreatment of SR9009 prevented the onset of LPS-induced mechanical hypersensitivity, cytokine expression and GFAP expression. Intrathecal injection of SR9009 also ameliorated mechanical hypersensitivity during the maintenance phase of complete Freund's adjuvant-induced inflammatory pain and partial sciatic nerve ligation-, paclitaxel-, and streptozotocin-induced neuropathy in mice. The current findings suggest that spinal astrocytic REV-ERBs could be critical in the regulation of nociceptive transduction through downregulation of pronociceptive molecule expression. Thus, spinal REV-ERBs could be an effective therapeutic target in the treatment of chronic pain.

Central inhibition of granulocyte-macrophage colony-stimulating factor is analgesic in experimental neuropathic pain

Supplemental Digital Content is Available in the Text.

GM-CSF is a proinflammatory cytokine that plays a role in central pain pathways through the modulation of spinal glial cells.

With less than 50% of patients responding to the current standard of care and poor efficacy and selectivity of current treatments, neuropathic pain continues to be an area of considerable unmet medical need. Biological therapeutics such as monoclonal antibodies (mAbs) provide better intrinsic selectivity; however, delivery to the central nervous system (CNS) remains a challenge. Granulocyte-macrophage colony-stimulating factor (GM-CSF) is well described in inflammation-induced pain, and early-phase clinical trials evaluating its antagonism have exemplified its importance as a peripheral pain target. Here, we investigate the role of this cytokine in a murine model of traumatic nerve injury and show that deletion of the GM-CSF receptor or treatment with an antagonizing mAb alleviates pain. We also demonstrate enhanced analgesic efficacy using an engineered construct that has greater capacity to penetrate the CNS. Despite observing GM-CSF receptor expression in microglia and astrocytes, the gliosis response in the dorsal horn was not altered in nerve injured knockout mice compared with wild-type littermate controls as evaluated by ionized calcium binding adapter molecule 1 (Iba1) and glial fibrillary acidic protein, respectively. Functional analysis of glial cells revealed that pretreatment with GM-CSF potentiated lipopolysaccharide-induced release of proinflammatory cytokines. In summary, our data indicate that GM-CSF is a proinflammatory cytokine that contributes to nociceptive signalling through driving spinal glial cell secretion of proinflammatory mediators. In addition, we report a successful approach to accessing CNS pain targets, providing promise for central compartment delivery of analgesics.

2-Bromopalmitate attenuates bone cancer pain via reversing mitochondrial fusion and fission imbalance in spinal astrocytes

Bone cancer pain is common in patients with advanced cancers as tumors metastasize to bone. Pathogenesis of bone cancer pain is complex and poorly understood which leads to inefficiency of clinical treatment. During pathological pain status, astrocytes are activated and release various inflammatory cytokines, which result in the development of peripheral and central sensitization. As energy factory, mitochondria undergo frequent fusion and fission and play essential role for astrocyte function. 2-bromopalmitate (2-BP) is an inhibitor of protein palmitoylation, and its function on bone cancer pain was unclear. In this article, we aimed to investigate the potential curative effects and mechanisms of 2-BP on bone cancer pain. Bone cancer pain rat model was established through intratibial inoculation of rat mammary gland carcinoma cells (MRMT-1) into the left tibia of Sprague–Dawley female rats. As a result, bone cancer pain rats exhibited bone destruction and sensitive nociceptive behavior. And increased leukocyte infiltration, activation of astrocytes, and imbalance of mitochondrial fission and fusion dynamics were observed in spinal cord of bone cancer pain rats. Intrathecal 2-BP administration significantly attenuated pain behavior of bone cancer pain rats. Meanwhile, 2-BP administration reduced spinal inflammation, reversed spinal mitochondrial fission and fusion dynamic imbalance, and further inhibited spinal mitochondrial apoptosis in bone cancer pain rats. In C6 cell level, 2-BP treatment decreased dynamin-related protein 1 expression and increased optic atrophy 1 expression in a dose-dependent manner and inhibited carbonyl cyanide 3-chlorophenylhydrazone (CCCP)-induced mitochondrial membrane potential change. These data illustrated that 2-BP attenuated bone cancer pain by reversing mitochondrial fusion and fission dynamic imbalance in spinal astrocytes.

Spinal IL-36γ/IL-36R participates in the maintenance of chronic inflammatory pain through astroglial JNK pathway

Emerging evidence indicates that spinal neuroinflammation contributes to the maintenance of chronic inflammatory pain. IL-36, as a novel member of the interleukin (IL)-1 super-family cytokines, plays an important role in inflammatory responses. The present study aimed to investigate the role of spinal IL-36 and IL-36 receptor (IL-36R) signaling in the pathology of chronic inflammatory pain. IL-36γ and IL-36R, but not IL-36α and IL-36β, were persistently upregulated in the spinal cord of mice with intraplantar injections of complete Freund's adjuvant (CFA). Intrathecal administration of both IL-36R antagonist (IL-36Ra) and IL-36γ siRNA significantly attenuated CFA-induced chronic inflammatory pain behaviors. Furthermore, CFA-induced IL-36γ expression was mainly observed in spinal neurons whereas IL-36R was primarily expressed in spinal astrocytes. Additionally, the intrathecal injection of IL-36γ was sufficient to induce pain hypersensitivity and astrocyte activation in naive mice, and these effects could be inhibited by blocking c-Jun N-terminal kinase (JNK) phosphorylation. In vitro experiments also demonstrated that the IL-36γ could induce astrocytic JNK activation and inflammatory cytokines release, which was mediated by IL-36R. Finally, intrathecal injection of IL-36γ-activated astrocytes in a pJNK-dependent manner induced mechanical allodynia and thermal hyperalgesia in naive mice. Collectively, these findings reveal that the neuronal/astrocytic interaction in the spinal cord by which neuronally produced IL-36γ activates astrocytes via IL-36R-mediated JNK pathway is crucial for the maintenance of chronic inflammatory pain. Thus, IL-36γ/IL-36R-mediated astrocyte signaling may be a suitable therapeutic target for chronic inflammatory pain.

Dexmedetomidine Reduces Diabetic Neuropathy Pain in Rats through the Wnt 10a/β-Catenin Signaling Pathway

Diabetic neuropathy pain (DNP), a spontaneous pain with hyperalgesia and allodynia, greatly compromises patients' quality of life. Our previous study suggested that dexmedetomidine (DEX) can relieve hyperalgesia in rats by inhibiting inflammation and apoptosis at the level of the spinal cord. In the present study, we aimed to evaluate the role of Wnt 10a/β-catenin signaling in DEX-induced alleviation of DNP in rats. Forty-eight rats were randomly allocated to four groups (n=12/group): control, DNP, DEX, and yohimbine groups. The DNP model was established by streptozotocin (STZ) injection. The effects of DEX with or without the α₂ adrenergic antagonist yohimbine were assessed by behavior tests (mechanical withdrawal threshold and thermal withdrawal latency). Spinal cord tissue was evaluated by immunofluorescence staining of astrocytes as well as for Wnt 10a and β-catenin expression, western blot analysis of Wnt 10a and β-catenin expression, and enzyme-linked immunosorbent assay measurement of proinflammatory cytokines (tumor necrosis factor-α and interleukin-1β). Rats with STZ-induced DNP had a decreased pain threshold, activated astrocytes, increased expression of Wnt 10a and β-catenin, and increased levels of proinflammatory cytokines compared to the control group, and these effects were ameliorated by treatment with DEX. Yohimbine administration partly abolished the protective effects of DEX in the DNP model rats. In conclusion, DEX alleviated DNP in rats by inhibiting inflammation and astrocyte activation, which may be attributed to downregulation of the Wnt 10a/β-catenin signaling pathway.

Zaprinast diminished pain and enhanced opioid analgesia in a rat neuropathic pain model

The mechanism of neuropathic pain is complex and unclear. Based on our results, we postulate that an intensification of the kynurenine pathway occurs as a consequence of nerve injury. The G protein-coupled receptor 35 (GPR35) is important for kynurenine pathway activation. Cyclic GMP-specific phosphodiesterase inhibitors have also been shown to have beneficial effects on neuropathic pain. Therefore, the aims of our research were to elucidate how a substance that acts as both an agonist of GPR35 and an inhibitor of phosphodiesterase influences neuropathic pain in a rat model. Here, we demonstrated that preemptive and repeated intrathecal (i.t.) administration (16 h and 1 h before injury and then after nerve ligation daily for 7 days) of zaprinast (1 μg/5 μl) significantly attenuated mechanical (von Frey test) and thermal (cold plate test) hypersensitivity measured on day 7 after chronic constriction injury, and the effect of even a single injection lasted up to 24 h. Our data indicate that zaprinast diminished the number of IBA1-positive cells and consequently attenuated the levels of IL-1beta, IL-6, IL-18, and NOS2 in the lumbar spinal cord and/or dorsal root ganglia. Our results also demonstrated that zaprinast potentiated the analgesic properties of morphine and buprenorphine. In summary, in a neuropathic pain model, zaprinast significantly reduced pain symptoms and enhanced the effectiveness of opioids. Our data provide new evidence that modulation of both GPR35 and phosphodiesterase could be an important strategy for innovative pharmacological treatments designed to decrease hypersensitivity evoked by nerve injury.

Spinal dorsal horn astrocytes release GABA in response to synaptic activation

KEY POINTS

GABA is an essential molecule for sensory information processing. It is usually assumed to be released by neurons. Here we show that in the dorsal horn of the spinal cord, astrocytes respond to glutamate by releasing GABA. Our findings suggest a novel role for astrocytes in somatosensory information processing.

ABSTRACT

Astrocytes participate in neuronal signalling by releasing gliotransmitters in response to neurotransmitters. We investigated if astrocytes from the dorsal horn of the spinal cord of adult red-eared turtles (Trachemys scripta elegans) release GABA in response to glutamatergic receptor activation. For this, we developed a GABA sensor consisting of HEK cells expressing GABA_A receptors. By positioning the sensor recorded in the whole-cell patch-clamp configuration within the dorsal horn of a spinal cord slice, we could detect GABA in the extracellular space. Puff application of glutamate induced GABA release events with time courses that exceeded the duration of inhibitory postsynaptic currents by one order of magnitude. Because the events were neither affected by extracellular addition of nickel, cadmium and tetrodotoxin nor by removal of Ca²⁺ , we concluded that they originated from non-neuronal cells. Immunohistochemical staining allowed the detection of GABA in a fraction of dorsal horn astrocytes. The selective stimulation of A∂ and C fibres in a dorsal root filament induced a Ca²⁺ increase in astrocytes loaded with Oregon Green BAPTA. Finally, chelating Ca²⁺ in a single astrocyte was sufficient to prevent the GABA release evoked by glutamate. Our results indicate that glutamate triggers the release of GABA from dorsal horn astrocytes with a time course compatible with the integration of sensory inputs.

Kindlin-1 Regulates Astrocyte Activation and Pain Sensitivity in Rats With Neuropathic Pain

BACKGROUND AND OBJECTIVES

Astrocyte activation has been implicated in the pathogenesis of neuropathic pain, but the involvement of kindlin-1 in astrocyte activation and neuropathic pain has not yet been illustrated. Using a chronic constriction injury (CCI) rat model of neuropathic pain, we investigated the expression levels of kindlin-1 during neuropathic pain and the influences of kindlin-1 on regulating pain sensitivity.

METHODS

Neuropathic pain was induced in rats by CCI of the sciatic nerve. Rats were randomly assigned to 4 groups: sham operation, CCI, CCI + kindlin-1 short hairpin RNA (shRNA), and CCI + kindlin-1 groups. Animals in the CCI + kindling-1 shRNA and CCI + kindlin-1 groups were given kindlin-1 shRNA or kindlin-1 virus infection to reduce or overexpress kindlin-1, respectively. Kindlin-1 expression was persistently increased in rats 10 days after CCI. A large proportion of glial fibrillary acidic protein (GFAP)-positive astrocytes expressed kindlin-1 in spinal cord tissues of rats after CCI.

RESULTS

Compared with the sham operation group, CCI animals exhibited increased GFAP expression and GFAP-positive astrocytes in the spinal cord. Down-regulation of kindlin-1 reduced the up-regulation of GFAP in the spinal cord, whereas overexpression of kindlin-1 promoted elevation of GFAP levels. Kindlin-1 silencing elevated the mechanical and thermal pain thresholds of CCI rats (P < 0.05). However, overexpression of kindlin-1 aggravated CCI-induced pain sensitivity.

CONCLUSIONS

Kindlin-1 may regulate pain sensitivity by affecting activated astrocytes in the spinal cord. Inhibition of kindlin-1 may provide a novel paradigm for the management of neuropathic pain.

Spinal activation of alpha7-nicotinic acetylcholine receptor attenuates posttraumatic stress disorder-related chronic pain via suppression of glial activation

The high prevalence of chronic pain in posttraumatic stress disorder (PTSD) individuals has been widely reported by clinical studies, which emphasized an urgent need to uncover the underlying mechanisms and identify potential therapeutic targets. Recent studies suggested that targeting activated glia and their pro-inflammatory products may provide a novel and effective therapy for the stress-related pain. In this study, we investigated whether activation of alpha-7 nicotinic acetylcholine receptor (α7 nAChR), a novel anti-inflammatory target, could attenuate PTSD-related chronic pain. The experiments were conducted in a rat model of single prolonged stress (SPS), an established model of PTSD-pain comorbidity. We found that SPS exposure produced persistent mechanical allodynia. Immunohistochemical and enzyme-linked immuno sorbent assay analysis showed that SPS also induced elevated activation of glia cells (including microglia and astrocytes) and accumulation of pro-inflammatory cytokines in spinal cord. In another experiment, we found that intrathecal injection of PHA-543613, a selective α7 nAchR agonist, attenuated the SPS-evoked allodynia in a dose dependent manner. However, this anti-hyperalgesic effect was blocked by pretreatment with methyllycaconitine (MLA), a selective α7 nAchR antagonist. Further analyses showed that PHA-543613 suppressed SPS-induced spinal glial activation and SPS-elevated spinal pro-inflammatory cytokines, and these were abolished by MLA. Taken together, the present study showed that spinal activation of α7 nAChR by PHA-543613 attenuated mechanical allodynia induced by PTSD-like stress, and the suppression of spinal glial activation may underlie this anti-hyperalgesic effect. Our study demonstrated the therapeutic potential of targeting α7 nAChR in the treatment of PTSD-related chronic pain.

Astrocyte contributes to pain development via MMP2-JNK1/2 signaling in a mouse model of complex regional pain syndrome

BACKGROUND

The activation of spinal glial cells (astrocyte and microglia) is reported in patient with complex regional pain syndrome (CRPS). However, the roles of spinal glial activities in the pathophysiology of CRPS are unclear. Here, we explored the roles of spinal astrocyte and microglia and the molecular mechanisms underlying CRPS using a mouse model of chronic post-ischemia pain (CPIP).

RESULTS

CPIP injury increased the level of glial fibrillary acidic protein (GFAP, reactive astrocyte biomarker), but had no significant impact on ionized calcium binding adaptor molecule 1 (IBA1, reactive microglia biomarker), in the ipsilateral dorsal horn on post-injury day (PID) 3 when the pain threshold started to reduce significantly. Astrocytic inhibition with fluorocitrate but not microglial inhibition with minocycline attenuated the development of allodynia in CPIP-injured mice, which was concomitant with increased spinal levels of phosphorylated c-jun N-terminal kinase 1/2 (pJNK1/2) on PID 3. Furthermore, the intrathecal administration of SP600125 (JNK inhibitor) prevented the development of allodynia in CPIP-injured mice. Double immunofluorescence staining showed that pJNK1/2 was mainly co-localized with GFAP. Subsequently, increased levels of pJNK1/2 were reversed by intrathecal fluorocitrate. Furthermore, the level of spinal matrix metalloproteinase-2 (MMP2) was increased and mainly expressed in NeuN (neuron biomarker) on PID 3 in the CPIP-injured mice, while intrathecal APR 100 (MMP2 inhibitor) delayed the development of allodynia and decreased spinal levels of GFAP and pJNK1/2 on PID 3.

CONCLUSION

This study shows that activation of astrocyte MMP2/JNK1/2 signaling pathway contributes to the pathogenesis of pain hypersensitivity in the CPIP model.

Glial contributions to visceral pain: implications for disease etiology and the female predominance of persistent pain

In the central nervous system, bidirectional signaling between glial cells and neurons ('neuroimmune communication') facilitates the development of persistent pain. Spinal glia can contribute to heightened pain states by a prolonged release of neurokine signals that sensitize adjacent centrally projecting neurons. Although many persistent pain conditions are disproportionately common in females, whether specific neuroimmune mechanisms lead to this increased susceptibility remains unclear. This review summarizes the major known contributions of glia and neuroimmune interactions in pain, which has been determined principally in male rodents and in the context of somatic pain conditions. It is then postulated that studying neuroimmune interactions involved in pain attributed to visceral diseases common to females may offer a more suitable avenue for investigating unique mechanisms involved in female pain. Further, we discuss the potential for primed spinal glia and subsequent neurogenic inflammation as a contributing factor in the development of peripheral inflammation, therefore, representing a predisposing factor for females in developing a high percentage of such persistent pain conditions.

Minocycline attenuates bone cancer pain in rats by inhibiting NF-κB in spinal astrocytes

AIM

To investigate the mechanisms underlying the anti-nociceptive effect of minocycline on bone cancer pain (BCP) in rats.

METHODS

A rat model of BCP was established by inoculating Walker 256 mammary carcinoma cells into tibial medullary canal. Two weeks later, the rats were injected with minocycline (50, 100 μg, intrathecally; or 40, 80 mg/kg, ip) twice daily for 3 consecutive days. Mechanical paw withdrawal threshold (PWT) was used to assess pain behavior. After the rats were euthanized, spinal cords were harvested for immunoblotting analyses. The effects of minocycline on NF-κB activation were also examined in primary rat astrocytes stimulated with IL-1β in vitro.

RESULTS

BCP rats had marked bone destruction, and showed mechanical tactile allodynia on d 7 and d 14 after the operation. Intrathecal injection of minocycline (100 μg) or intraperitoneal injection of minocycline (80 mg/kg) reversed BCP-induced mechanical tactile allodynia. Furthermore, intraperitoneal injection of minocycline (80 mg/kg) reversed BCP-induced upregulation of GFAP (astrocyte marker) and PSD95 in spinal cord. Moreover, intraperitoneal injection of minocycline (80 mg/kg) reversed BCP-induced upregulation of NF-κB, p-IKKα and IκBα in spinal cord. In IL-1β-stimulated primary rat astrocytes, pretreatment with minocycline (75, 100 μmol/L) significantly inhibited the translocation of NF-κB to nucleus.

CONCLUSION

Minocycline effectively alleviates BCP by inhibiting the NF-κB signaling pathway in spinal astrocytes.

Spinal astrocytic activation contributes to both induction and maintenance of pituitary adenylate cyclase-activating polypeptide type 1 receptor-induced long-lasting mechanical allodynia in mice

Background

Pituitary adenylate cyclase-activating polypeptide (PACAP) and its receptors are present in the spinal dorsal horn and dorsal root ganglia, suggesting an important role of PACAP–PACAP receptors signaling system in the modulation of spinal nociceptive transmission. We have previously reported that a single intrathecal injection of PACAP or a PACAP specific (PAC1) receptor selective agonist, maxadilan, in mice induced dose-dependent aversive behaviors, which lasted more than 30 min, and suggested that the maintenance of the nociceptive behaviors was associated with the spinal astrocytic activation.

Results

We found that a single intrathecal administration of PACAP or maxadilan also produced long-lasting hind paw mechanical allodynia, which persisted at least 84 days without affecting thermal nociceptive threshold. In contrast, intrathecal application of vasoactive intestinal polypeptide did not change mechanical threshold, and substance P, calcitonin gene-related peptide, or N-methyl-D-aspartate induced only transient mechanical allodynia, which disappeared within 21 days. Western blot and immunohistochemical analyses with an astrocytic marker, glial fibrillary acidic protein, revealed that the spinal PAC1 receptor stimulation caused sustained astrocytic activation, which also lasted more than 84 days. Intrathecal co-administration of L-α-aminoadipate, an astroglial toxin, with PACAP or maxadilan almost completely prevented the induction of the mechanical allodynia. Furthermore, intrathecal treatment of L-α-aminoadipate at 84 days after the PAC1 stimulation transiently reversed the mechanical allodynia accompanied by the reduction of glial fibrillary acidic protein expression level.

Conclusion

Our data suggest that spinal astrocytic activation triggered by the PAC1 receptor stimulation contributes to both induction and maintenance of the long-term mechanical allodynia.

Interactions of Opioids and HIV Infection in the Pathogenesis of Chronic Pain

Over 50% of HIV-1/AIDS patients suffer chronic pain. Currently, opioids are the cornerstone medications for treating severe pain in these patients. Ironically, emerging clinical data indicates that repeated use of opiate pain medicines might in fact heighten the chronic pain states in HIV patients. Both laboratory-based and clinical studies strongly suggest that opioids exacerbate the detrimental effects of HIV-1 infection on the nervous system, both on neurons and glia. The combination of opioids and HIV-1infection may promote the damage of neurons, including those in the pain sensory and transmission pathway, by activating both caspase-dependent and caspase-independent pro-apoptotic pathways. In addition, the opiate-HIV-1 interaction may also cause widespread disturbance of glial function and elicit glial-derived pro-inflammatory responses that dysregulate neuronal function. The deregulation of neuron-glia cross-talk that occurs with the combination of HIV-1 and opioids appears to play an important role in the development of the pathological pain state. In this article, we wish to provide an overview of the potential molecular and cellular mechanisms by which opioids may interact with HIV-1 to cause neurological problems, especially in the context of HIV-associated pathological pain. Elucidating the underlying mechanisms will help researchers and clinicians to understand how chronic use of opioids for analgesia enhances HIV-associated pain. It will also assist in optimizing therapeutic approaches to prevent or minimize this significant side effect of opiate analgesics in pain management for HIV patients.

Microglial Janus kinase/signal transduction and activator of transcription 3 pathway activity directly impacts astrocyte and spinal neuron characteristics

After peripheral nerve injury microglial reactivity change in the spinal cord is associated with an early activation of Janus kinase (JAK)/STAT3 transduction pathway whose blockade attenuates local inflammation and pain hypersensitivity. However, the consequences of microglial JAK/STAT3-mediated signaling on neighboring cells are unknown. Using an in vitro paradigm we assessed the impact of microglial JAK/STAT3 activity on functional characteristics of astrocytes and spinal cord neurons. Purified rat primary microglia was stimulated with JAK/STAT3 classical activator interleukin-6 in the presence or absence of a selective STAT3 inhibitor and rat primary astrocytes or spinal cord neurons were exposed to microglia conditioned media (CM). JAK/STAT3 activity-generated microglial CM modulated both astrocyte and neuron characteristics. Beyond inducing mRNA expression changes in various targets of interest in astrocytes and neurons, microglia CM activated c-Jun N-terminal kinase, STAT3 and NF-κB intracellular pathways in astrocytes and promoted their proliferation. Without modifying neuronal excitability or survival, CM affected the nerve processes morphology and distribution of the post-synaptic density protein 95, a marker of glutamatergic synaptic contacts. These findings show that JAK/STAT3 activity in microglia impacts the functional characteristics of astrocytes and neurons. This suggests its participation in spinal cord tissue plasticity and remodeling occurring after peripheral nerve injury. We show that the activity of JAK/STAT3 pathway in microglial cells confers them a specific signaling modality toward neighboring cells, promoting astrocyte proliferation and changes in neuronal morphology. These in vitro data suggest that the early JAK/STAT3 activation in spinal cord microglia, associated with peripheral nerve injury, participates in functional alteration of various cell populations and in spinal tissue remodeling.

Activity-triggered tetrapartite neuron-glial interactions following peripheral injury

Recent studies continue to support the proposition that non-neuronal components of the nervous system, mainly glial cells and associated chemical mediators, contribute to the development of neuronal hyperexcitability that underlies persistent pain conditions. In the event of peripheral injury, enhanced or abnormal nerve input is likely the most efficient way to activate simultaneously central neurons and glia. Injury induces phenotypic changes in glia and triggers signaling cascades that engage reciprocal interactions between presynaptic terminals, postsynaptic neurons, microglia and astrocytes. While some responses to peripheral injury may help the nervous system to adapt positively to counter the disastrous effect of injury, the net effect often leads to long-lasting sensitization of pain transmission pathways and chronic pain.

Astrocyte barriers to neurotoxic inflammation

Astrocytes form borders (glia limitans) that separate neural from non-neural tissue along perivascular spaces, meninges and tissue lesions in the CNS. Transgenic loss-of-function studies reveal that astrocyte borders and scars serve as functional barriers that restrict the entry of inflammatory cells into CNS parenchyma in health and disease. Astrocytes also have powerful pro-inflammatory potential. Thus, astrocytes are emerging as pivotal regulators of CNS inflammatory responses. This Review discusses evidence that astrocytes have crucial roles in attracting and restricting CNS inflammation, with important implications for diverse CNS disorders.

Oligodendrocyte ablation triggers central pain independently of innate or adaptive immune responses in mice

Mechanisms underlying central neuropathic pain are poorly understood. Although glial dysfunction has been functionally linked with neuropathic pain, very little is known about modulation of pain by oligodendrocytes. Here we report that genetic ablation of oligodendrocytes rapidly triggers a pattern of sensory changes that closely resemble central neuropathic pain, which are manifest before overt demyelination. Primary oligodendrocyte loss is not associated with autoreactive T- and B-cell infiltration in the spinal cord and neither activation of microglia nor reactive astrogliosis contribute functionally to central pain evoked by ablation of oligodendrocytes. Instead, light and electron microscopic analyses reveal axonal pathology in the spinal dorsal horn and spinothalamic tract concurrent with the induction and maintenance of nociceptive hypersensitivity. These data reveal a role for oligodendrocytes in modulating pain and suggest that perturbation of oligodendrocyte functions that maintain axonal integrity can lead to central neuropathic pain independent of immune contributions.

Whether oligodendrocytes have a role in the development of chronic pain is not clear. Here the authors show that oligodendrocyte depletion causes a neuropathic pain that sets in before demyelination and is independent of immune cell activation and infiltration.

Spinal astrocyte gap junction and glutamate transporter expression contributes to a rat model of bortezomib-induced peripheral neuropathy

There is increasing evidence implicating astrocytes in multiple forms of chronic pain, as well as in the specific context of chemotherapy-induced peripheral neuropathy (CIPN). However, it is still unclear what the exact role of astrocytes may be in the context of CIPN. Findings in oxaliplatin and paclitaxel models have displayed altered expression of astrocytic gap junctions and glutamate transporters as means by which astrocytes may contribute to observed behavioral changes. The current study investigated whether these changes were also generalizable to the bortezomib CIPN. Changes in mechanical sensitivity were verified in bortezomib-treated animals, and these changes were prevented by co-treatment with a glial activation inhibitor (minocycline), a gap junction decoupler (carbenoxolone), and by a glutamate transporter upregulator (ceftriaxone). Immunohistochemistry data at day 30 in bortezomib-treated animals showed increases in expression of glial fibrillary acidic protein (GFAP) and connexin 43 but a decrease in GLAST expression. These changes were prevented by co-treatment with minocycline. Follow-up Western blotting data showed a shift in connexin 43 from a non-phosphorylated state to a phosphorylated state, indicating increased trafficking of expressed connexin 43 to the cell membrane. These data suggest that increases in behavioral sensitivity to cutaneous stimuli may be tied to persistent synaptic glutamate resulting from increased calcium flow between spinal astrocytes.

Astrogliosis

In addition to their many functions in the healthy central nervous system (CNS), astrocytes respond to CNS damage and disease through a process called astrogliosis. For many decades, astrogliosis was sparsely studied and enigmatic. This article examines recent evidence supporting a definition of astrogliosis as a spectrum of heterogeneous potential changes in astrocytes that occur in a context-specific manner as determined by diverse signaling events that vary with the nature and severity of different CNS insults. Astrogliosis is associated with essential beneficial functions, but under specific circumstances can lead to harmful effects. Potential dysfunctions of astrocytes and astrogliosis are being identified that can contribute to, or be primary causes of, CNS disorders, leading to the notion of astrocytopathies. A conceptual framework is presented that allows consideration of normally occurring and dysfunctional astrogliosis and their different roles in CNS disorders.

Neuropathic pain and cytokines: current perspectives

Neuropathic pain represents a major problem in clinical medicine because it causes debilitating suffering and is largely resistant to currently available analgesics. A characteristic of neuropathic pain is abnormal response to somatic sensory stimulation. Thus, patients suffering peripheral neuropathies may experience pain caused by stimuli which are normally nonpainful, such as simple touching of the skin or by changes in temperature, as well as exaggerated responses to noxious stimuli. Convincing evidence suggests that this hypersensitivity is the result of pain remaining centralized. In particular, at the first pain synapse in the dorsal horn of the spinal cord, the gain of neurons is increased and neurons begin to be activated by innocuous inputs. In recent years, it has become appreciated that a remote damage in the peripheral nervous system results in neuronal plasticity and changes in microglial and astrocyte activity, as well as infiltration of macrophages and T cells, which all contribute to central sensitization. Specifically, the release of pronociceptive factors such as cytokines and chemokines from neurons and non-neuronal cells can sensitize neurons of the first pain synapse. In this article we review the current evidence for the role of cytokines in mediating spinal neuron–non-neuronal cell communication in neuropathic pain mechanisms following peripheral nerve injury. Specific and selective control of cytokine-mediated neuronal–glia interactions results in attenuation of the hypersensitivity to both noxious and innocuous stimuli observed in neuropathic pain models, and may represent an avenue for future therapeutic intervention.

Multiple roles for astrocytes as effectors of cytokines and inflammatory mediators

Astrocytes are increasingly recognized as exerting complex functions essential for normal neural activity in the healthy central nervous system (CNS). Because astrocytes also respond to all forms of CNS injury or disease, there is growing interest in how reactive astrogliosis might alter astrocyte functions and thereby affect neural functions. Reactive astrogliosis is heterogeneous and regulated in a context specific manner by different molecular signals. Prominent among astrocyte signaling mechanisms is the ability to respond to, as well as to produce, many different cytokines and inflammatory mediators. These signaling mechanisms enable astrocytes to interact with diverse cell types in ways that may contribute to crosstalk between immune/inflammatory and neural systems. Consistent with this notion is the increasing evidence that cytokines and inflammatory mediators modulate astrocyte signaling not only to influence immune and inflammatory activities in the CNS, but also to influence synaptic and neural functions in ways that may affect complex behaviors such as sickness behavior, pain, appetite, sleep, and mood.