Glial-cytokine-neuronal interactions underlying the mechanisms of persistent pain

Implication of system xc- in complete Freund's adjuvant-induced peripheral inflammation and associated nociceptive sensitization

BACKGROUND

Persistent inflammation leading to neuronal sensitization in pain pathways, are key features of chronic inflammatory pain. Alike macrophages in the periphery, glial cells exacerbate hypersensitivity by releasing proalgesic mediators in the central nervous system. Expressed by peripheral and central immune cells, the cystine-glutamate antiporter system xc- plays a significant role in inflammatory responses, but its involvement in chronic inflammatory pain remains underexplored. We herein investigated the contribution of this exchanger in nociceptive hypersensitivity triggered by a peripheral inflammatory insult.

METHODS

Complete Freund's adjuvant (CFA) was injected into the left hind paw of wild-type C57Bl/6 female mice, of xCT-deficient mice (specific subunit of system xc-) and of mice receiving the system xc- inhibitor sulfasalazine. Paw edema was measured over three weeks and pain-associated behaviors were evaluated. Additionally, pro-inflammatory cytokine levels were assessed in blood samples.

RESULTS

CFA injection led to a persistent increase in paw edema and hypersensitivity to mechanical and thermal stimuli, which were less pronounced in xCT-deficient mice. This reduced sensitivity was accompanied by lower systemic pro-inflammatory cytokine levels in xCT-deficient mice. Accordingly, pharmacological inhibition of system xc- with sulfasalazine, either before or after pain induction, efficiently reduced the algesic and inflammatory responses to CFA in wild-type mice.

CONCLUSION

Our findings reveal a critical role for system xc- in the pathophysiology of inflammatory pain. xCT deficiency reduces pain behaviors and peripheral inflammation, positioning system xc- as a promising therapeutic target for alleviating chronic inflammatory pain.

GPR37L1 identifies spinal cord astrocytes and protects neuropathic pain after nerve injury

Astrocytes in the spinal cord dorsal horn (SDH) play a pivotal role in synaptic transmission and neuropathic pain. However, the precise classification of SDH astrocytes in health and disease remains elusive. Here, we reveal Gpr37l1 as a marker and functional regulator of spinal astrocytes. Through single-nucleus RNA sequencing, we identified Gpr37l1 as a selective G-protein-coupled receptor (GPCR) marker for spinal cord astrocytes. Notably, SDH displayed reactive astrocyte phenotypes and exacerbated neuropathic pain following nerve injury combined with Gpr37l1 deficiency. In naive animals, Gpr37l1 knockdown in SDH astrocytes induces astrogliosis and pain hypersensitivity, while Gpr37l1-/- mice fail to recover from neuropathic pain. GPR37L1 activation by maresin 1 increased astrocyte glutamate transporter 1 (GLT-1) activity and reduced spinal EPSCs and neuropathic pain. Selective overexpression of Gpr37l1 in SDH astrocytes reversed neuropathic pain and astrogliosis after nerve injury. Our findings illuminate astrocyte GPR37l1 as an essential negative regulator of pain, which protects against neuropathic pain through astrocyte signaling in SDH.

Electroacupuncture Inhibits NLRP3-Mediated Microglial Pyroptosis to Ameliorate Chronic Neuropathic Pain in Rats

Background

Patients with neuropathic pain (NP), caused by injury or disease of the somatosensory nervous system, usually suffer from severe pain. Our previous studies revealed that electroacupuncture (EA) stimulation could effectively improve NP. However, the underlying mechanisms of EA have not been fully clarified. This study aimed to investigate the specific mechanisms of EA in alleviating NP, focusing on the pyroptosis.

Materials and Methods

Chronic Constriction Injury (CCI) model was established on the male Sprague-Dawley rats. CCI rats were treated with EA at acupoints GV20 and ST36 or/with the NOD-like receptor protein 3 (NLRP3) antagonist MCC950. EA treatment was administered for successive 14 days 7 days after the CCI surgery. The mechanical withdrawal threshold (MWT) and paw withdrawal latency (PWL) were performed during the experiment. At the end of the experiment, spinal cord segments and serum of rats were collected, ELISA detected the expression of inflammatory factors, immunofluorescence detected the microglia and neuron cells with pyroptosis biomarkers, and Western blot detected the NLRP3 pathway.

Results

EA treatment significantly alleviated pain hypersensitivity by increasing the MWT and PWL. Moreover, EA reduced levels of pro-inflammatory cytokines IL-1β and TNF-α in the spinal tissue. Mechanistically, the pyroptosis-related proteins, including NLRP3, N-GSDMD, Cleaved Caspase-1, IL-18 as well as IL-1β were downregulated by EA, indicating that EA attenuated the pyroptosis phenotype in NP rats. In particular, EA reduced the co-expression of NLRP3, Caspase-1 and N-GSDMD in microglia rather than in neuronal or astrocytic cells within the spinal cord of CCI rats. Pharmacological inhibition of NLRP3 inflammasome by MCC950 alleviates CCI-induced pain hypersensitivity while blocking EA's effect on anti-pyroptosis in CCI rats.

Conclusion

These findings demonstrate the EA ameliorates the neuroinflammation and pyroptosis to relieve chronic NP by suppressing NLRP3 inflammasome activation in microglia. EA may serve as a viable treatment therapy for chronic NP.

Molecular Anatomy of Synaptic and Extrasynaptic Neurotransmission Between Nociceptive Primary Afferents and Spinal Dorsal Horn Neurons

Sensory signals generated by peripheral nociceptors are transmitted by peptidergic and nonpeptidergic nociceptive primary afferents to the superficial spinal dorsal horn, where their central axon terminals establish synaptic contacts with secondary sensory spinal neurons. In the case of suprathreshold activation, the axon terminals release glutamate into the synaptic cleft and stimulate postsynaptic spinal neurons by activating glutamate receptors located on the postsynaptic membrane. When overexcitation is evoked by peripheral inflammation, neuropathy or pruritogens, peptidergic nociceptive axon terminals may corelease various neuropeptides, neurotrophins and endomorphin, together with glutamate. However, in contrast to glutamate, neuropeptides, neurotrophins and endomorphin are released extrasynaptically. They diffuse from the site of release and modulate the function of spinal neurons via volume transmission, activating specific extrasynaptic receptors. Thus, the released neuropeptides, neurotrophins and endomorphin may evoke excitation, disinhibition or inhibition in various spinal neuronal populations, and together with glutamate, induce overall overexcitation, called central sensitization. In addition, the synaptic and extrasynaptic release of neurotransmitters is subjected to strong retrograde control mediated by various retrogradely acting transmitters, messengers, and their presynaptic receptors. Moreover, the composition of this complex chemical apparatus is heavily dependent on the actual patterns of nociceptive primary afferent activation in the periphery. This review provides an overview of the complexity of this signaling apparatus, how nociceptive primary afferents can activate secondary sensory spinal neurons via synaptic and volume transmission in the superficial spinal dorsal horn, and how these events can be controlled by presynaptic mechanisms.

mGluR5-mediated astrocytes hyperactivity in the anterior cingulate cortex contributes to neuropathic pain in male mice

Astrocytes regulate synaptic transmission in healthy and pathological conditions, but their involvement in modulating synaptic transmission in chronic pain is unknown. Our study demonstrates that astrocytes in the anterior cingulate cortex (ACC) exhibit abnormal calcium signals and induce the release of glutamate in male mice. This leads to an elevation in extracellular glutamate concentration, activation of presynaptic kainate receptors, and an increase in synaptic transmission following neuropathic pain. We discovered that the abnormal calcium signals are caused by the reappearance of metabotropic glutamate receptor type 5 (mGluR5) in astrocytes in male mice. Importantly, when we specifically inhibit the Gq pathway using iβARK and reduce the expression of mGluR5 in astrocytes through shRNA, we observe a restoration of astrocytic calcium activity, normalization of synaptic transmission and extracellular concentration of glutamate, and improvement in mechanical allodynia in male mice. Furthermore, the activation of astrocytes through chemogenetics results in an overabundance of excitatory synaptic transmission, exacerbating mechanical allodynia in mice with neuropathic pain, but not in sham-operated male mice. In summary, our findings suggest that the abnormal calcium signaling in astrocytes, mediated by mGluR5, plays a crucial role in enhancing synaptic transmission in ACC and contributing to mechanical allodynia in male mice.

Polyethylene terephthalate nanoplastics-induced neurotoxicity in adult male Swiss albino mice with amelioration of betaine: a histopathological, neurochemical, and molecular investigation

Medicines, food packaging, personal care products, and cosmetics extensively use polyethylene terephthalate nanoplastics (PET-NaPs). However, they also have harmful impacts on several organs. Betaine demonstrates potent antioxidant and anti-inflammatory characteristics. Our goal was to investigate the detrimental impact of PET-NaPs on the mouse brain and evaluate the neuroprotective properties of betaine. We allocated 40 completely mature male Swiss albino mice into four distinct groups: control group, betaine group, PET-NaPs group, and betaine-co-treated group. Following a 30-day duration, euthanasia was performed on the mice, and analyzed tissue samples were obtained from the cerebrum, cerebellum, and hippocampus. PET-NaPs resulted in an elevated level of malondialdehyde and upregulated cyclooxygenase-2 and interleukin-1 beta (IL-1β) expression while significantly reducing the levels of glutathione and downregulating acetylcholinesterase. The PET-NPs also caused significant changes in the histopathology of the brain tissue, and there was a demonstrable rise in the immunostaining of IL-1β and glial fibrillary acidic proteins. Consequently, betaine effectively alleviated the negative consequences of PET-NaPs. Therefore, betaine possesses the capacity to mitigate the neurotoxic consequences induced by PET-NaPs.

Evidence for glial reactivity using positron-emission tomography imaging of translocator Protein-18 kD [TSPO] in both sham and nerve-injured rats in a preclinical model of orofacial neuropathic pain

Chronic neuropathic pain is a debilitating condition that results from damage to the nervous system. Current treatments are largely ineffective, with limited understanding of the underlying mechanisms hindering development of effective treatments. Preclinical models of neuropathic pain have revealed that non-neural changes are important for the development of neuropathic pain, although these data are derived almost exclusively from post-mortem histological analyses. Although these static snapshots have provided valuable data, they cannot provide insights into non-neural cell changes that could be also assessed in human patients with chronic pain. In this study we used translocator protein 18 kDa (TSPO) PET imaging with [18F]PBR06 to visualise in-vivo, the activity of macrophages and microglia in a rodent preclinical model of trigeminal neuropathic pain. Using chronic constriction injury of the infraorbital nerve (ION-CCI) we compared temporal changes in TSPO binding in male rats, prior to, and up to 28 days after ION-CCI compared with both sham-injured and naïve counterparts. Unexpectedly, we found significant increases in TSPO signal in both ION-CCI and sham-injured rats within the trigeminal ganglion, spinal trigeminal nucleus and paratrigeminal nucleus during the initial phase following surgery and/or nerve injury. This increased TSPO binding returned to control levels by day 28. Qualitative histological appraisal of macrophage accumulation and glial reactivity in both ION-CCI and sham-injured rats indicated macrophage accumulation in the trigeminal ganglion and microglial reactivity in the brainstem trigeminal complex. These findings show, glial changes in the peripheral nerve and brain in both nerve-injured and sham-injured rats in a preclinical model of neuropathic pain which provides a platform for translation into human patients.

Neuroglia in substance use disorders

Substance use disorders (SUD) remain a major public health concern in which individuals are unable to control their use of substances despite significant harm and negative consequences. Drugs of abuse dysregulate major brain and behavioral functions. Glial cells, primarily microglia and astrocytes, play a crucial role in these drug-induced molecular and behavioral changes. This review explores preclinical and clinical studies of how neuroglia and their associated neuroinflammatory responses contribute to SUD and reward-related properties. We evaluate preclinical and clinical evidence for targeting neuroglia as therapeutic interventions. In addition, we evaluate the literature on the gut microbiome and its role in SUD. Clinical treatments are most effective for reducing drug cravings, and some have yielded promising results in other measures of drug use. N-Acetylcysteine, through modulation of cysteine-glutamate antiporter of glial cells, shows encouraging results across a variety of drug classes. Neuroglia and gut microbiome interactions are important factors to consider with regard to SUD and could lead to novel therapeutic avenues. Age- and sex-dependent properties of neuroglia, gut microbiome, and drug use behaviors are important areas in need of further investigation.

Pharmacological Network Analysis of the Functions and Mechanism of Quercetin From Jisuikang (JSK) in Spinal Cord Injury (SCI)

Neuroinflammation, especially microglia/macrophage activation, is a hallmark of spinal cord injury (SCI). Jisuikang (JSK) is a clinical experiential Chinese herbal formula for SCI therapy containing Huangqi (Astragali Radix), Danggui (Angelica sinensis Radix), Chishao (Paeoniae Radix Rubra), Dilong (earthworm, Pheretima aspergillum), Chuanxiong (Chuanxiong Rhizoma), Taoren (Persicae Seman) and Honghua (Carthami Flos). Eighteen active ingredients in 6 herbs of JSK were found to be correlated with inflammation, spinal injury and other diseases. These 18 active ingredients target 5464 genes according to the PubChem database. Through comparing differentially expressed genes between SCI and normal samples using GSE datasets, 50 hub genes were identified. These hub-genes were enriched in oxidative stress response and inflammation response. The herb-compound-target, herb-compound-signalling and compound-target-signalling networks were generated and quercetin was identified as the hub compound. A concentration of 25 μM quercetin showed no cytotoxicity but significantly protected microglial cells from LPS-induced inhibition of cell viability. LPS stimulation elevated the levels of iNOS, IL-1β and TNF-α but decreased IL-10 levels, whereas quercetin significantly attenuated LPS-induced alterations in these factors. Moreover, quercetin targeted gene, IL1R1 was reduced by quercetin as predicted. Overexpression of IL1R1 further increased LPS-induced inflammation, which could be partly reversed by quercetin treatment. In vivo, quercetin improved histopathological alterations, inflammation and promoted M2 macrophage polarisation post-injury, whereas IL1R1 overexpression partially attenuated the beneficial effects of quercetin on the rat SCI model. Collectively, quercetin, the main ingredient compound of JSK, protects against LPS-induced cell viability inhibition and cellular inflammation, which could be partially attenuated by IL1R1 overexpression.

Spinal astrocyte-derived interleukin-17A promotes pain hypersensitivity in bone cancer mice

Spinal microglia and astrocytes are both involved in neuropathic and inflammatory pain, which may display sexual dimorphism. Here, we demonstrate that the sustained activation of spinal astrocytes and astrocyte-derived interleukin (IL)-17A promotes the progression of mouse bone cancer pain without sex differences. Chemogenetic or pharmacological inhibition of spinal astrocytes effectively ameliorates bone cancer-induced pain-like behaviors. In contrast, chemogenetic or optogenetic activation of spinal astrocytes triggers pain hypersensitivity, implying that bone cancer-induced astrocytic activation is involved in the development of bone cancer pain. IL-17A expression predominantly in spinal astrocytes, whereas its receptor IL-17 receptor A (IL-17RA) was mainly detected in neurons expressing VGLUT2 and PAX2, and a few in astrocytes expressing GFAP. Specific knockdown of IL-17A in spinal astrocytes blocked and delayed the development of bone cancer pain. IL-17A overexpression in spinal astrocytes directly induced thermal hyperalgesia and mechanical allodynia, which could be rescued by CaMKIIα inhibitor. Moreover, selective knockdown IL-17RA in spinal Vglut2 + or Vgat +neurons, but not in astrocytes, significantly blocked the bone cancer-induced hyperalgesia. Together, our findings provide evidence for the crucial role of sex-independent astrocytic signaling in bone cancer pain. Targeting spinal astrocytes and IL-17A/IL-17RA-CaMKIIα signaling may offer new gender-inclusive therapeutic strategies for managing bone cancer pain.

Up-regulation of IL-1β and sPLA2-III in the medial prefrontal cortex contributes to orofacial and somatic hyperalgesia induced by malocclusion via glial-neuron crosstalk

The medial prefrontal cortex (mPFC) has been identified as a key brain region involved in the modulation of chronic pain. Our recent study demonstrated that unilateral anterior crossbite (UAC) developed the comorbidity model of temporomandibular disorders (TMD) and fibromyalgia syndrome (FMS), which was characterized by both orofacial and somatic hyperalgesia. In the present study, UAC rats exhibited significant changes in gene expression in the mPFC. Enrichment analysis revealed that the significantly involved pathways were cytokines-cytokine receptor interaction and immune response. The expression of group III secretory phospholipase A2 (sPLA2-III) was significantly increased in the mPFC of UAC rats. Silencing sPLA2-III expression in the mPFC blocked the orofacial and somatic hyperalgesia. Immunofluorescence showed that sPLA2-III was mainly localized in neurons. The expression of interleukin-1β (IL-1β) in the mPFC significantly increased after UAC. Injection of IL-1β antibody into the mPFC blocked orofacial and somatic hyperalgesia. IL-1β was mainly localized in microglia cells. Furthermore, injection of IL-1β antibody significantly reduced the expression of sPLA2-III. These results indicate that neuroinflammatory cascade responses induced by glial-neuron crosstalk in the mPFC may contribute to the development of TMD and FMS comorbidity, and IL-1β and sPLA2-III are identified as novel potential therapeutic targets for the treatment of chronic pain in the comorbidity of TMD and FMS.

Sigma-1 receptor targeting inhibits connexin 43 based intercellular communication in chronic neuropathic pain

BACKGROUND AND OBJECTIVE

Neuropathic pain is a chronic condition characterized by aberrant signaling within the somatosensory system, affecting millions of people worldwide with limited treatment options. Herein, we aim at investigating the potential of a sigma-1 receptor (σ1R) antagonist in managing neuropathic pain.

METHODS

A Chronic Constriction Injury (CCI) model was used to induce neuropathic pain. The potential of (+)-MR200 was evaluated following daily subcutaneous injections of the compound. Its mechanism of action was confirmed by administration of a well-known σ1R agonist, PRE084.

RESULTS

(+)-MR200 demonstrated efficacy in protecting neurons from damage and alleviating pain hypersensitivity in CCI model. Our results suggest that (+)-MR200 reduced the activation of astrocytes and microglia, cells known to contribute to the neuroinflammatory process, suggesting that (+)-MR200 may not only address pain symptoms but also tackle the underlying cellular mechanism involved. Furthermore, (+)-MR200 treatment normalized levels of the gap junction (GJ)-forming protein connexin 43 (Cx43), suggesting a reduction in harmful intercellular communication that could fuel the chronicity of pain.

CONCLUSIONS

This approach could offer a neuroprotective strategy for managing neuropathic pain, addressing both pain symptoms and cellular processes driving the condition. Understanding the dynamics of σ1R expression and function in neuropathic pain is crucial for clinical intervention.

FTY720/Fingolimod mitigates paclitaxel-induced Sparcl1-driven neuropathic pain and breast cancer progression

Paclitaxel is among the most active chemotherapy drugs for the aggressive triple negative breast cancer (TNBC). Unfortunately, it often induces painful peripheral neuropathy (CIPN), a major debilitating side effect. Here we demonstrate that in naive and breast tumor-bearing immunocompetent mice, a clinically relevant dose of FTY720/Fingolimod that targets sphingosine-1-phosphate receptor 1 (S1PR1), alleviated paclitaxel-induced neuropathic pain. FTY720 also significantly attenuated paclitaxel-stimulated glial fibrillary acidic protein (GFAP), a marker for activated astrocytes, and expression of the astrocyte-secreted synaptogenic protein Sparcl1/Hevin, a key regulator of synapse formation. Notably, the formation of excitatory synapses containing VGluT2 in the spinal cord dorsal horn induced by paclitaxel was also inhibited by FTY720 treatment, supporting the involvement of astrocytes and Sparcl1 in CIPN. Furthermore, in this TNBC mouse model that mimics human breast cancer, FTY720 administration also enhanced the anti-tumor effects of paclitaxel, leading to reduced tumor progression and lung metastasis. Taken together, our findings suggest that targeting the S1P/S1PR1 axis with FTY720 is a multipronged approach that holds promise as a therapeutic strategy for alleviating both CIPN and enhancing the efficacy of chemotherapy in TNBC treatment.

Targeting connexins: possible game changer in managing neuropathic pain?

Neuropathic pain is a chronic debilitating condition caused by nerve injury or a variety of diseases. At the core of neuropathic pain lies the aberrant neuronal excitability in the peripheral and/or central nervous system (PNS and CNS). Enhanced connexin expression and abnormal activation of connexin-assembled gap junctional channels are prominent in neuropathic pain along with reactive gliosis, contributing to neuronal hypersensitivity and hyperexcitability. In this review, we delve into the current understanding of how connexin expression and function contribute to the pathogenesis and pathophysiology of neuropathic pain and argue for connexins as potential therapeutic targets for neuropathic pain management.

Osteosarcoma-Induced Pain Is Mediated by Glial Cell Activation in the Spinal Dorsal Horn, but Not Capsaicin-Sensitive Nociceptive Neurons: A Complex Functional and Morphological Characterization in Mice

Bone cancer and its related chronic pain are huge clinical problems since the available drugs are often ineffective or cannot be used long term due to a broad range of side effects. The mechanisms, mediators and targets need to be identified to determine potential novel therapies. Here, we characterize a mouse bone cancer model induced by intratibial injection of K7M2 osteosarcoma cells using an integrative approach and investigate the role of capsaicin-sensitive peptidergic sensory nerves. The mechanical pain threshold was assessed by dynamic plantar aesthesiometry, limb loading by dynamic weight bearing, spontaneous pain-related behaviors via observation, knee diameter with a digital caliper, and structural changes by micro-CT and glia cell activation by immunohistochemistry in BALB/c mice of both sexes. Capsaicin-sensitive peptidergic sensory neurons were defunctionalized by systemic pretreatment with a high dose of the transient receptor potential vanilloid 1 (TRPV1) agonist resiniferatoxin (RTX). During the 14- and 28-day experiments, weight bearing on the affected limb and the paw mechanonociceptive thresholds significantly decreased, demonstrating secondary mechanical hyperalgesia. Signs of spontaneous pain and osteoplastic bone remodeling were detected both in male and female mice without any sex differences. Microglia activation was shown by the increased ionized calcium-binding adapter molecule 1 (Iba1) immunopositivity on day 14 and astrocyte activation by the enhanced glial fibrillary acidic protein (GFAP)-positive cell density on day 28 in the ipsilateral spinal dorsal horn. Interestingly, defunctionalization of the capsaicin-sensitive afferents representing approximately 2/3 of the nociceptive fibers did not alter any functional parameters. Here, we provide the first complex functional and morphological characterization of the K7M2 mouse osteosarcoma model. Bone-cancer-related chronic pain and hyperalgesia are likely to be mediated by central sensitization involving neuroinflammation via glial cell activation in the spinal dorsal horn, but not the capsaicin-sensitive sensory neuronal system.

Astrocytes in preoptic area regulate acute nociception-induced hypothermia through adenosine receptors

AIMS

The preoptic area (POA) of the hypothalamus, crucial in thermoregulation, has long been implicated in the pain process. However, whether nociceptive stimulation affects body temperature and its mechanism remains poorly studied.

METHODS

We used capsaicin, formalin, and surgery to induce acute nociceptive stimulation and monitored rectal temperature. Optical fiber recording, chemical genetics, confocal imaging, and pharmacology assays were employed to confirm the role and interaction of POA astrocytes and extracellular adenosine. Immunofluorescence was utilized for further validation.

RESULTS

Acute nociception could activate POA astrocytes and induce a decrease in body temperature. Manipulation of astrocytes allowed bidirectional control of body temperature. Furthermore, acute nociception and astrocyte activation led to increased extracellular adenosine concentration within the POA. Activation of adenosine A1 or A2A receptors contributed to decreased body temperature, while inhibition of these receptors mitigated the thermo-lowering effect of astrocytes.

CONCLUSION

Our results elucidate the interplay between acute nociception and thermoregulation, specifically highlighting POA astrocyte activation. This enriches our understanding of physiological responses to painful stimuli and contributes to the analysis of the anatomical basis involved in the process.

TRPV1: Receptor structure, activation, modulation and role in neuro-immune interactions and pain

In the 1990s, the identification of a non-selective ion channel, especially responsive to capsaicin, revolutionized the studies of somatosensation and pain that were to follow. The TRPV1 channel is expressed mainly in neuronal cells, more specifically, in sensory neurons responsible for the perception of noxious stimuli. However, its presence has also been detected in other non-neuronal cells, such as immune cells, β- pancreatic cells, muscle cells and adipocytes. Activation of the channel occurs in response to a wide range of stimuli, such as noxious heat, low pH, gasses, toxins, endocannabinoids, lipid-derived endovanilloid, and chemical agents, such as capsaicin and resiniferatoxin. This activation results in an influx of cations through the channel pore, especially calcium. Intracellular calcium triggers different responses in sensory neurons. Dephosphorylation of the TRPV1 channel leads to its desensitization, which disrupts its function, while its phosphorylation increases the channel's sensitization and contributes to the channel's rehabilitation after desensitization. Kinases, phosphoinositides, and calmodulin are the main signaling pathways responsible for the channel's regulation. Thus, in this review we provide an overview of TRPV1 discovery, its tissue expression as well as on the mechanisms by which TRPV1 activation (directly or indirectly) induces pain in different disease models.

Changes in pain during a depressive episode and relationship to cytokine levels in major depressive disorder

BACKGROUND

Depressed patients have an increased incidence of pain. A pathophysiological connection between depression and pain is still not revealed. Immunological activation has been found in both depression and pain. There are few studies of pain and immune activation in patients with depression, without inflammatory and autoimmune disorders.

METHODS

This is a naturalistic follow-up study of 50 patients with a major depressive disorder (MDD) depressive episode, without any inflammatory or autoimmune conditions. We have previously reported on the relationship between depression and cytokine levels. In this study, we obtained data of depression, pain and cytokine levels before and after 12 weeks of depression treatment. All patients were medication-free at inclusion.

RESULTS

At inclusion three out of four patients experienced pain, and the pain scores correlated with the depression scores. After treatment, as depression was relieved, the pain scores dropped significantly and were no longer correlated to the depression scores. There were no correlations between pain scores and cytokine levels. Pain level at inclusion did not correlate with depression treatment outcome.

CONCLUSION

Our findings indicate that pain is a feature of depression. Pain levels and cytokine values didn't correlate. Pain at inclusion did not predict depression treatment outcome.

Imbalance of Th1 and Th2 Cytokines and Stem Cell Therapy in Pathological Pain

The pathophysiological importance of T helper 1 (Th1) and Th2 cell cytokines in pathological pain has been highly debated in recent decades. However, the analgesic strategy targeting individual cytokines still has a long way to go for clinical application. In this review, we focus on the contributions of Th1 cytokines (TNF-α, IFN-γ, and IL-2) and Th2 cytokines (IL-4, IL-5, IL-10, and IL-13) in rodent pain models and human pain-related diseases. A large number of studies have shown that Th1 and Th2 cytokines have opposing effects on pain modulation. The imbalance of Th1 and Th2 cytokines might determine the final effect of pain generation or inhibition. However, increasing evidence indicates that targeting the individual cytokine is not sufficient for the treatment of pathological pain. It is practical to suggest a promising therapeutic strategy against the combined effects of Th1 and Th2 cytokines. We summarize the current advances in stem cell therapy for pain-related diseases. Preclinical and clinical studies show that stem cells inhibit proinflammatory cytokines and release enormous Th2 cytokines that exhibit a strong analgesic effect. Therefore, a shift of the imbalance of Th1 and Th2 cytokines induced by stem cells will provide a novel therapeutic strategy against intractable pain. It is extremely important to reveal the cellular and molecular mechanisms of stem cell-mediated analgesia. The efficiency and safety of stem cell therapy should be carefully evaluated in animal models and patients with pathological pain.

Regulation of Pain Perception by Microbiota in Parkinson Disease

Pain perception involves current stimulation in peripheral nociceptive nerves and the subsequent stimulation of postsynaptic excitatory neurons in the spinal cord. Importantly, in chronic pain, the neural activity of both peripheral nociceptors and postsynaptic neurons in the central nervous system is influenced by several inflammatory mediators produced by the immune system. Growing evidence has indicated that the commensal microbiota plays an active role in regulating pain perception by either acting directly on nociceptors or indirectly through the modulation of the inflammatory activity on immune cells. This symbiotic relationship is mediated by soluble bacterial mediators or intrinsic structural components of bacteria that act on eukaryotic cells, including neurons, microglia, astrocytes, macrophages, T cells, enterochromaffin cells, and enteric glial cells. The molecular mechanisms involve bacterial molecules that act directly on neurons, affecting their excitability, or indirectly on non-neuronal cells, inducing changes in the production of proinflammatory or anti-inflammatory mediators. Importantly, Parkinson disease, a neurodegenerative and inflammatory disorder that affects mainly the dopaminergic neurons implicated in the control of voluntary movements, involves not only a motor decline but also nonmotor symptomatology, including chronic pain. Of note, several recent studies have shown that Parkinson disease involves a dysbiosis in the composition of the gut microbiota. In this review, we first summarize, integrate, and classify the molecular mechanisms implicated in the microbiota-mediated regulation of chronic pain. Second, we analyze the changes on the commensal microbiota associated to Parkinson disease and propose how these changes affect the development of chronic pain in this pathology. SIGNIFICANCE STATEMENT: The microbiota regulates chronic pain through the action of bacterial signals into two main locations: the peripheral nociceptors and the postsynaptic excitatory neurons in the spinal cord. The dysbiosis associated to Parkinson disease reveals increased representation of commensals that potentially exacerbate chronic pain and reduced levels of bacteria with beneficial effects on pain. This review encourages further research to better understand the signals involved in bacteria-bacteria and bacteria-host communication to get the clues for the development of probiotics with therapeutic potential.

Integration of microbiota and metabolomics reveals the analgesic mechanisms of emodin against neuropathic pain

BACKGROUND AND OBJECTIVE

Neuropathic pain (NeP) induced dysbiosis of intestinal microbiota in chronic constriction injury (CCI) rats. Emodin has analgesic effect but the detailed mechanism is not clear at the present time. This study aims to explore the underling mechanism of action of emodin against NeP with in CCI model.

METHODS

Male SD rats (180-220 g) were randomly divided into three groups: sham group, CCI group, and emodin group. Behavioral tests were performed to evaluate the therapeutic effects of emodin on CCI model. Feces and spinal cords of all rats were collected 15 days after surgery. 16S rDNA sequencing, untargeted metabolomics, qPCR and ELISA were performed.

RESULTS

Mechanical withdrawal thresholds (MWT), thermal withdrawal latency (TWL) and Sciatic functional index (SFI) in emodin group were significantly higher than CCI group (P < 0.05). Emodin not only inhibited the expression of pro-inflammatory cytokines in the spinal cords and colonic tissue, but also increased the expression of tight junction protein in colonic tissue. 16S rDNA sequencing showed that emodin treatment changed the community structure of intestinal microbiota in CCI rats. Untargeted metabolomics analysis showed that 33 differential metabolites were screened out between CCI group and emodin group. After verification, we found that emodin increased the level of S-adenosylmethionine (SAM) and Histamine in the spinal cord of CCI rats.

CONCLUSION

Emodin was effective in relieving neuropathic pain, which is linked to inhibition inflammatory response, increasing the proportion of beneficial bacteria and beneficial metabolites.

URB937 Prevents the Development of Mechanical Allodynia in Male Rats with Trigeminal Neuralgia

Cannabinoids are proposed for alleviating neuropathic pain, but their use is limited by cannabimimetic side effects. The inhibition of the fatty acid amide hydrolase (FAAH), the degrading enzyme of the endocannabinoid anandamide, has received attention as an alternative to cannabinoids in the treatment of neuropathic pain. Here, we investigated the effect of URB937, a blood-brain barrier impermeant FAAH inhibitor, on experimentally induced mechanical allodynia in an animal model of trigeminal neuralgia. Male Sprague-Dawley rats were subjected to chronic constriction injury of the infraorbital nerve (IoN-CCI); operated animals were treated sub-chronically with URB937 (1 mg/kg, i.p.) or vehicle before or after trigeminal mechanical allodynia establishment. We also assayed mRNA expression levels of the pain neuropeptide calcitonin gene-related peptide (CGRP) and cytokines in the medulla, cervical spinal cord, and trigeminal ganglion ipsilateral to IoN-CCI using rt-PCR. URB937 treatment prevented the development of mechanical allodynia and IoN-CCI-induced changes in mRNA expression levels of CGRP and cytokines in the evaluated areas. When administered after allodynia development, URB937 prevented IoN-CCI-induced changes in CGRP and cytokine gene expression; this was not associated with a significant abrogation of the mechanical allodynia. These findings suggest that URB937 may counteract, but not reverse, the development of allodynia in trigeminal neuralgia. Further research is needed to elucidate the underlying mechanisms.

Peroxisome proliferator-activated receptor gamma agonist pioglitazone alleviates hemorrhage-induced thalamic pain and neuroinflammation

BACKGROUND

Thalamic pain frequently occurs after stroke and is a challenging clinical issue. However, the mechanisms underlying thalamic pain remain unclear. Neuroinflammation is a key determining factor in the occurrence and maintenance of hemorrhage-induced thalamic pain. Pioglitazone is an agonist of peroxisome proliferator-activated receptor gamma (PPARγ) and shows anti-inflammatory effects in multiple diseases. The present work focused on exploring whether PPARγ is related to hemorrhage-induced thalamic pain.

METHODS

Immunostaining was conducted to analyze the cellular localization of PPARγ and co-localization was evaluated with NeuN, ionized calcium-binding adapter molecular 1 (IBA1), and glia fibrillary acidic protein (GFAP). Western blot analyses were used to evaluate MyD88, pNF-κB/NF-κB, pSTAT6/STAT6, IL-1β, TNF-α, iNOS, Arg-1, IL-4, IL-6, and IL-10 expression. Behavioral tests in mice were conducted to evaluate continuous pain hypersensitivity.

RESULTS

We found that pioglitazone appeared to mitigate the contralateral hemorrhage-induced thalamic pain while inhibiting inflammatory responses. Additionally, Pioglitazone induced phosphorylation of STAT6 and suppressed the phosphorylation NF-κB in our model of thalamic pain. These effects could be partially reversed with the PPARγ antagonist GW9662.

CONCLUSION

The PPARγ agonist pioglitazone can mitigate mechanical allodynia by suppressing the NF-κB inflammasome while activating the STAT6 signal pathway, which are well-known to be associated with inflammation.

BDNF/TrkB Signaling Inhibition Suppresses Astrogliosis and Alleviates Mechanical Allodynia in a Partial Crush Injury Model

Neuropathic pain presents a formidable clinical challenge due to its persistent nature and limited responsiveness to conventional analgesic treatments. While significant progress has been made in understanding the role of spinal astrocytes in neuropathic pain, their contribution and functional changes following a partial crush injury (PCI) remain unexplored. In this study, we investigated structural and functional changes in spinal astrocytes during chronic neuropathic pain, employing a partial crush injury model. This model allowes us to replicate the transition from initial nociceptive responses to persistent pain, highlighting the relevance of astrocytes in pain maintenance and sensitization. Through the examination of mechanical allodynia, a painful sensation in response to innocuous stimuli, and the correlation with increased levels of brain-derived neurotrophic factor (BDNF) along with reactive astrocytes, we identified a potential mechanistic link between astrocytic activity and BDNF signaling. Ultimately, our research provides evidence that inhibiting astrocyte activation through a BDNF/TrkB inhibitor alleviates mechanical allodynia, underscoring the therapeutic potential of targeting glial BDNF-related pathways for pain management. These findings offer critical insights into the cellular and molecular dynamics of neuropathic pain, paving the way for innovative and targeted treatment strategies for this challenging condition.

Pathogenic mechanisms of human immunodeficiency virus (HIV)-associated pain

Chronic pain is a prevalent neurological complication among individuals living with human immunodeficiency virus (PLHIV) in the post-combination antiretroviral therapy (cART) era. These individuals experience malfunction in various cellular and molecular pathways involved in pain transmission and modulation, including the neuropathology of the peripheral sensory neurons and neurodegeneration and neuroinflammation in the spinal dorsal horn. However, the underlying etiologies and mechanisms leading to pain pathogenesis are complex and not fully understood. In this review, we aim to summarize recent progress in this field. Specifically, we will begin by examining neuropathology in the pain pathways identified in PLHIV and discussing potential causes, including those directly related to HIV-1 infection and comorbidities, such as antiretroviral drug use. We will also explore findings from animal models that may provide insights into the molecular and cellular processes contributing to neuropathology and chronic pain associated with HIV infection. Emerging evidence suggests that viral proteins and/or antiretroviral drugs trigger a complex pathological cascade involving neurons, glia, and potentially non-neural cells, and that interactions between these cells play a critical role in the pathogenesis of HIV-associated pain.

Repeated closed-head mild traumatic brain injury-induced inflammation is associated with nociceptive sensitization

BACKGROUND

Individuals who have experienced mild traumatic brain injuries (mTBIs) suffer from several comorbidities, including chronic pain. Despite extensive studies investigating the underlying mechanisms of mTBI-associated chronic pain, the role of inflammation in long-term pain after mTBIs is not fully elucidated. Given the shifting dynamics of inflammation, it is important to understand the spatial-longitudinal changes in inflammatory processes following mTBIs and their effects on TBI-related pain.

METHODS

We utilized a recently developed transgenic caspase-1 luciferase reporter mouse model to monitor caspase-1 activation through a thinned skull window in the in vivo setting following three closed-head mTBI events. Organotypic coronal brain slice cultures and acutely dissociated dorsal root ganglion (DRG) cells provided tissue-relevant context of inflammation signal. Mechanical allodynia was assessed by mechanical withdrawal threshold to von Frey and thermal hyperalgesia withdrawal latency to radiant heat. Mouse grimace scale (MGS) was used to detect spontaneous or non-evoked pain. In some experiments, mice were prophylactically treated with MCC950, a potent small molecule inhibitor of NLRP3 inflammasome assembly to inhibit injury-induced inflammatory signaling. Bioluminescence spatiotemporal dynamics were quantified in the head and hind paws, and caspase-1 activation was confirmed by immunoblot. Immunofluorescence staining was used to monitor the progression of astrogliosis and microglial activation in ex vivo brain tissue following repetitive closed-head mTBIs.

RESULTS

Mice with repetitive closed-head mTBIs exhibited significant increases of the bioluminescence signals within the brain and paws in vivo for at least one week after each injury. Consistently, immunoblotting and immunofluorescence experiments confirmed that mTBIs led to caspase-1 activation, astrogliosis, and microgliosis. Persistent changes in MGS and hind paw withdrawal thresholds, indicative of pain states, were observed post-injury in the same mTBI animals in vivo. We also observed enhanced inflammatory responses in ex vivo brain slice preparations and DRG for at least 3 days following mTBIs. In vivo treatment with MCC950 significantly reduced caspase-1 activation-associated bioluminescent signals in vivo and decreased stimulus-evoked and non-stimulus evoked nociception.

CONCLUSIONS

Our findings suggest that the inflammatory states in the brain and peripheral nervous system following repeated mTBIs are coincidental with the development of nociceptive sensitization, and that these events can be significantly reduced by inhibition of NLRP3 inflammasome activation.

Chinese Tuina remodels the synaptic structure in neuropathic pain rats by downregulating the expression of N-methyl D-aspartate receptor subtype 2B and postsynaptic density protein-95 in the spinal cord dorsal horn

OBJECTIVE

To investigate whether the Chinese massage system, Tuina, exerts analgesic effects in a rat model of chronic constriction injury (CCI) by remodeling the synaptic structure in the spinal cord dorsal horn (SCDH).

METHODS

Sixty-nine male Sprague-Dawley rats were randomly and evenly divided into the normal group, sham group, CCI group, CCI + Tuina group, CCI + MK-801 [an -methyl D-aspartate receptor subtype 2B (NR2B) antagonist] group, and CCI + MK-801 + Tuina group. The neuropathic pain model was established using CCI with right sciatic nerve ligation. Tuina was administered 4 d after CCI surgery, using pressing manipulation for 10 min, once daily. Motor function was observed with the inclined plate test, and pain behaviors were observed by the Von Frey test and acetone spray test. At 19 d after surgery, the L3-L5 spinal cord segments were removed. Glutamate, interleukin 1β (IL-1β), and tumor necrosis factor-α (TNF-α) levels were detected by enzyme-linked immunosorbent assay. The protein expression levels of NR2B and postsynaptic density protein-95 (PSD-95) were detected by Western blot, and the synaptic structure was observed by transmission electron microscopy (TEM).

RESULTS

CCI reduced motor function and caused mechanical and cold allodynia in rats, increased glutamate concentration and TNF-α and IL-1β levels, and increased expression of synapse-related proteins NR2B and PSD-95 in the SCDH. TEM revealed that the synaptic structure of SCDH neurons was altered. Most of these disease-induced changes were reversed by Tuina and intrathecal injection of MK-801 ( < 0.05 or < 0.01). For the majority of experiments, no significant differences were found between the CCI + MK-801 and CCI + MK-801 + Tuina groups.

CONCLUSIONS

Chinese Tuina can alleviate pain by remodeling the synaptic structure, and NR2B and PSD-95 receptors in the SCDH may be among its targets.

Immunotherapies in chronic pain through modulation of neuroimmune interactions

It is generally believed that immune activation can elicit pain through production of inflammatory mediators that can activate nociceptive sensory neurons. Emerging evidence suggests that immune activation may also contribute to the resolution of pain by producing distinct pro-resolution/anti-inflammatory mediators. Recent research into the connection between the immune and nervous systems has opened new avenues for immunotherapy in pain management. This review provides an overview of the most utilized forms of immunotherapies (e.g., biologics) and highlight their potential for immune and neuronal modulation in chronic pain. Specifically, we discuss pain-related immunotherapy mechanisms that target inflammatory cytokine pathways, the PD-L1/PD-1 pathway, and the cGAS/STING pathway. This review also highlights cell-based immunotherapies targeting macrophages, T cells, neutrophils and mesenchymal stromal cells for chronic pain management.

Pathophysiology, Clinical Implications and Management of Orofacial Neuropathic Pain- with special attention to Trigeminal neuralgia: A Narrative Review

Background: It is a widely held belief that if the trigeminal nerve is damaged, the victim would experience agonising and unrelenting external pain. A lesion to the trigeminal nerve may have a wide-reaching effect, such as on one side of the face in particular, or it might have a more localised effect, such as on some or all of your gums. The risk of damage increases the likelihood that it will be difficult to speak and swallow. This nerve provides sensation to a part of your face that may be constantly aching or tingling for some people. However, the trigeminal nerve injury-related persistent orofacial pain might be brought on by a wide variety of unknown triggers. Aim: In this study investigate the clinical manifestations of chronic orofacial pain brought on by a damage to the trigeminal nerve, as well as the diagnostic and therapeutic approaches available to treat this condition. Methodology Through the use of search phrases such as "Trigeminal nerve injury," "Trigeminal ganglion," "Trigeminal spinal subnucleus caudalis," "Craniofacial pain," "Oral prognosis," and "treatment," the computerised databases for the last twenty years have been investigated. There are now two hundred objects in total that have been accumulated. There have been around fifty of them that are pertinent to the discussion that is going on in this work. Majority of the patients fair enough with the pharmacology treatment/drugs like the carbamazepine & oxcarbazepine which forms the first line treatment options followed by lamotrigine & baclofen encompassing the second line of drugs along with adjuvant drug support of topiramate, levetiracetam, gabapentin, pregabalin. As the field of science has explored &advanced for the latest treatment options include microvascular decompression, gamma knife radiosurgery, percutaneous rhizotomies variable based on the evidences & guidelines 54 Conclusion: New diagnostic criteria and treatment alternatives have become available for people who suffer from trigeminal neuropathy and orofacial neuropathic pain as a result of recent developments in fundamental animal research that have led to their development. Despite the results, more research needs to investigate a greater variety of distinct non-neuronal cell feature approaches.

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.

CaMKII and CaV3.2 T-type calcium channel mediate Connexin-43-dependent inflammation by activating astrocytes in vincristine-induced neuropathic pain

Vincristine (VCR), an alkaloid isolated from vinca, is a commonly used chemotherapeutic drug. However, VCR therapy can lead to dose-dependent peripheral neurotoxicity, mainly manifesting as neuropathic pain, which is one of the dominant reasons for limiting its utility. Experimentally, we discovered that VCR-induced neuropathic pain (VINP) was accompanied by astrocyte activation; the upregulation of phospho-CaMKII (p-CaMKII), CaV3.2, and Connexin-43 (Cx43) expression; and the production and release of inflammatory cytokines and chemokines in the spinal cord. Similar situations were also observed in astrocyte cultures. Interestingly, these alterations were all reversed by intrathecal injection of KN-93 (a CaMKII inhibitor) or L-Ascorbic acid (a CaV3.2 inhibitor). In addition, KN-93 and L-Ascorbic acid inhibited the increase in [Ca2+]i associated with astrocyte activation. We also verified that knocking down or inhibiting Cx43 level via intrathecal injection of Cx43 siRNA or Gap27 (a Cx43 mimetic peptide) relieved pain hypersensitivity and reduced the release of inflammatory factors; however, they did not affect astrocyte activation or p-CaMKII and CaV3.2 expression. Besides, the overexpression of Cx43 through the transfection of the Cx43 plasmid did not affect p-CaMKII and CaV3.2 expressions in vitro. Therefore, CaMKII and CaV3.2 may activate astrocytes by increasing [Ca2+]i, thereby mediating Cx43-dependent inflammation in VINP. Moreover, we demonstrated that the CaMKII signalling pathway was involved in VCR-induced inflammation, apoptosis, and mitochondrial damage. Collectively, our findings show a novel mechanism by which CaMKII and CaV3.2 mediate Cx43-dependent inflammation by activating astrocytes in neuropathic pain induced by VCR.

Correlative increasing expressions of KIF5b and Nav1.7 in DRG neurons of rats under neuropathic pain conditions

Nav1.7, one of tetrodotoxin-sensitive voltage-gated sodium channels, mainly expressed in the small diameter dorsal root ganglion (DRG) neurons. The expression and accumulation on neuronal membrane of Nav1.7 increased following peripheral tissue inflammation or nerve injury. However, the mechanisms for membrane accumulation of Nav1.7 remained unclear. We report that KIF5b, a highly expressed member of the kinesin-1 family in DRGs, promoted the translocation of Nav1.7 to the plasma membrane in DRG neurons of the rat. Following nociceptive behaviors in rats induced by peripheral spared nerve injury (SNI), synchronously increased KIF5b and Nav1.7 expressions were observed in DRGs. Immunohistochemistry staining demonstrated the co-expressions of KIF5b and Nav1.7 in the same DRG neurons. Immunoprecipitation experiments further confirmed the interactions between KIF5b and Nav1.7. Moreover, intrathecal injections of KIF5b shRNA moderated the SNI-induced both mechanical and thermal hyperalgesia. The rescued analgesic effects also alleviated SNI-induced anxiety-like behaviors. In sum, KIF5b was required for the membrane localizations of Nav1.7, which suggests a novel mechanism for the trafficking of Nav1.7 involved in neuropathic pain.

The Biology of Pain: Through the Rheumatology Lens

Chronic pain is a major socioeconomic burden globally. The most frequent origin of chronic pain is musculoskeletal. In inflammatory musculoskeletal diseases such as rheumatoid arthritis (RA), chronic pain is a primary determinant of deleterious quality of life. The pivotal role of peripheral inflammation in the initiation and perpetuation of nociceptive pain is well-established among patients with musculoskeletal diseases. However, the persistence of pain, even after the apparent resolution of peripheral inflammation, alludes to the coexistence of different pain states. Recent advances in neurobiology have highlighted the importance of nociplastic pain mechanisms. In this review we aimed to explore the biology of pain with a particular focus on nociplastic pain in RA.

Excitatory and inhibitory neuronal signaling in inflammatory and diabetic neuropathic pain

Pain, although unpleasant, is an essential warning mechanism against injury and damage of the organism. An intricate network of specialised sensors and transmission systems contributes to reception, transmission and central sensitization of pain. Here, we briefly introduce some of the main aspects of pain signal transmission, including nociceptors and nociceptive signals, mechanisms of inflammatory and neuropathic pain, and the situation of diabetes-associated neuropathic pain. The role of glia-astrocytes, microglia, satellite glia cells-and their specific channels, transporters and signaling pathways is described. A focus is on the contribution of inhibitory synaptic signaling to nociception and a possible role of glycine receptors in glucose-mediated analgesia and treatment-induced diabetic neuropathy. Inhibitory receptors such as GABA_A- and glycine receptors are important contributors to nociceptive signaling; their contribution to altered pain sensation in diabetes may be of clinical relevance, and they could be promising therapeutic targets towards the development of novel analgesics.

Anti-Inflammatory Activity of Synaptamide in the Peripheral Nervous System in a Model of Sciatic Nerve Injury

N-docosahexaenoylethanolamine (DHEA), or synaptamide, is an endogenous metabolite of docosahexaenoic acid (DHA) that exhibits synaptogenic and neurogenic effects. In our previous studies, synaptamide administration inhibited the neuropathic pain-like behavior and reduced inflammation in the central nervous system following sciatic nerve injury. In the present study, we examine the effect of synaptamide on the peripheral nervous system in a neuropathic pain condition. The dynamics of ionized calcium-binding adapter molecule 1 (iba-1), CD68, CD163, myelin basic protein, and the production of interleukin 1β and 6 within the sciatic nerve, as well as the neuro-glial index and the activity of iba-1, CD163, glial fibrillary acidic protein (GFAP), neuronal NO synthase (nNOS), substance P (SP), activating transcription factor 3 (ATF3) in the dorsal root ganglia (DRG), are studied. According to our results, synaptamide treatment (4 mg/kg/day) (1) decreases the weight-bearing deficit after nerve trauma; (2) enhances the remyelination process in the sciatic nerve; (3) shows anti-inflammatory properties in the peripheral nervous system; (4) decreases the neuro-glial index and GFAP immunoreactivity in the DRG; (5) inhibits nNOS- and SP-ergic activity in the DRG, which might contribute to neuropathic pain attenuation. In general, the current study demonstrates the complex effect of synaptamide on nerve injury, which indicates its high potential for neuropathic pain management.

Gut Region-Specific Interleukin 1β Induction in Different Myenteric Neuronal Subpopulations of Type 1 Diabetic Rats

Interleukin 1β (IL1β) is a pro-inflammatory cytokine that may play a crucial role in enteric neuroinflammation in type 1 diabetes. Therefore, our goal is to evaluate the effects of chronic hyperglycemia and insulin treatment on IL1β immunoreactivity in myenteric neurons and their different subpopulations along the duodenum-ileum-colon axis. Fluorescent immunohistochemistry was used to count IL1β expressing neurons as well as the neuronal nitric oxide synthase (nNOS)- and calcitonin gene-related peptide (CGRP)-immunoreactive myenteric neurons within this group. Tissue IL1β level was measured by ELISA in muscle/myenteric plexus-containing homogenates. IL1β mRNA was detected by RNAscope in different intestinal layers. The proportion of IL1β-immunoreactive myenteric neurons was significantly higher in the colon than in the small intestine of controls. In diabetics, this proportion significantly increased in all gut segments, which was prevented by insulin treatment. The proportion of IL1β-nNOS-immunoreactive neurons only increased in the diabetic colon, while the proportion of IL1β-CGRP-immunoreactive neurons only increased in the diabetic ileum. Elevated IL1β levels were also confirmed in tissue homogenates. IL1β mRNA induction was detected in the myenteric ganglia, smooth muscle and intestinal mucosa of diabetics. These findings support that diabetes-related IL1β induction is specific for the different myenteric neuronal subpopulations, which may contribute to diabetic motility disturbances.

Modulating neuroinflammation through molecular, cellular and biomaterial-based approaches to treat spinal cord injury

The neuroinflammatory response that is elicited after spinal cord injury contributes to both tissue damage and reparative processes. The complex and dynamic cellular and molecular changes within the spinal cord microenvironment result in a functional imbalance of immune cells and their modulatory factors. To facilitate wound healing and repair, it is necessary to manipulate the immunological pathways during neuroinflammation to achieve successful therapeutic interventions. In this review, recent advancements and fresh perspectives on the consequences of neuroinflammation after SCI and modulation of the inflammatory responses through the use of molecular-, cellular-, and biomaterial-based therapies to promote tissue regeneration and functional recovery will be discussed.

Sulfatide-selectin signaling in the spinal cord induces mechanical allodynia

Sulfatide is a sulfated glycosphingolipid that is present abundantly in myelin sheaths of the brain and spinal cord. It is synthesized by a cerebroside sulfotransferase encoded by Gal3st1, which catalyzes the transfer of sulfate from 3'-phosphoadenylylsulfate to galactosylceramide. We previously reported that Gal3st1 gene expression in the spinal cord is up-regulated 1 day after intraplantar injection of complete Freund's adjuvant (CFA), indicating that sulfatide is involved in inflammatory pain. In the present study, we found that intrathecal injection of sulfatide led to mechanical allodynia. Sulfatide caused levels of glial fibrillary acidic protein (GFAP) and nitric oxide in the spinal cord to increase. Mechanical allodynia induced by intrathecal injection of sulfatide was blocked by nitric oxide synthase inhibitors and by suppression of astrocyte activation by L-α-aminoadipate. These results suggest that sulfatide-induced mechanical allodynia involved glial activation and nitric oxide production. Blocking selectin, a sulfatide-binding protein, with bimosiamose attenuated sulfatide-induced allodynia and ameliorated CFA-induced mechanical allodynia during inflammatory pain. Finally, elevated levels of sulfatide concentration in the spinal cord were observed during CFA-induced inflammatory pain. The elevated sulfatide levels enhanced selectin activation in the spinal cord, resulting in mechanical allodynia. Our data suggest that sulfatide-selectin interaction plays a key role in inflammatory pain.

Astrocytic and microglial interleukin-1β mediates complement C1q-triggered orofacial mechanical allodynia

Glial cells, such as microglia and astrocytes, in the trigeminal spinal subnucleus caudalis (Vc) are activated after trigeminal nerve injury and interact with Vc neurons to contribute to orofacial neuropathic pain. Complement C1q released from microglia has been reported to activate astrocytes and causes orofacial mechanical allodynia. However, how C1q-induced phenotypic alterations in Vc astrocytes are involved in orofacial pain remains to be elucidated. Intracisternal administration of C1q caused mechanical allodynia in the whisker pad skin and concurrent significant upregulation of glial fibrillary acidic protein and ionized calcium-binding adapter molecule 1 in the Vc. Immunohistochemical analyses clarified that C1q induces a significant increase in the cytokine interleukin (IL)-1β, predominantly in Vc astrocytes and partially in Vc microglia. The number of c-Fos-positive neurons in the Vc increased significantly in response to C1q. IL-1 receptor antagonist (IL-1Ra) was used to analyze the involvement of IL-1β in C1q-induced mechanical allodynia. Intracisternal administration of IL-1Ra ameliorated C1q-induced orofacial mechanical allodynia. The present findings suggest that IL-1β released from activated astrocytes and microglia in the Vc mediates C1q-induced orofacial pain.

Pharmacological activation of mediodorsal thalamic GABA-A receptors modulates morphine/cetirizine-induced changes in the prefrontal cortical GFAP expression in a rat model of neuropathic pain

The present study investigated the involvement of mediodorsal thalamic (MD) GABA-A receptors in cetirizine/morphine-induced anti-allodynia using a rat model of neuropathic pain. To assess the importance of the prefrontal cortex (PFC) for chronic pain processing, its expression level changes of glial fibrillary acidic protein (GFAP) were measured following drug treatments. Each animal was subjected to chronic constriction of the sciatic nerve surgery simultaneously with the MD cannulation under stereotaxic surgery. The results showed that the administration of morphine (3-5 mg/kg) or cetirizine (1-3 mg/kg) produced significant analgesia in neuropathic rats. Systemic administration of cetirizine (2.5 and 3 mg/kg) potentiated the analgesic response to a low and intolerance dose of morphine (3 mg/kg). Intra-MD microinjection of muscimol, a selective GABA-A receptor agonist (0.005-0.01 μg/rat), increased the cetirizine/morphine-induced anti-allodynia, while muscimol by itself did not affect neuropathic pain. The neuropathic pain was associated with the increased PFC expression level of GFAP, suggesting the impact of chronic pain on PFC glial management. Interestingly, the anti-allodynia was associated with a decrease in the PFC expression level of GFAP under the drugs' co-administration. Thus, cetirizine has a significant potentiating effect on morphine response in neuropathic pain via interacting with the MD GABA-A receptors. It seems that neuropathic pain affects the prefrontal cortex GFAP signaling pathway. In clinical studies, these findings can be considered to create a combination therapy with low doses of GABA-A receptor agonist plus cetirizine and morphine to manage neuropathic pain.

Mechanisms of Transmission and Processing of Pain: A Narrative Review

Knowledge about the mechanisms of transmission and the processing of nociceptive information, both in healthy and pathological states, has greatly expanded in recent years. This rapid progress is due to a multidisciplinary approach involving the simultaneous use of different branches of study, such as systems neurobiology, behavioral analysis, genetics, and cell and molecular techniques. This narrative review aims to clarify the mechanisms of transmission and the processing of pain while also taking into account the characteristics and properties of nociceptors and how the immune system influences pain perception. Moreover, several important aspects of this crucial theme of human life will be discussed. Nociceptor neurons and the immune system play a key role in pain and inflammation. The interactions between the immune system and nociceptors occur within peripheral sites of injury and the central nervous system. The modulation of nociceptor activity or chemical mediators may provide promising novel approaches to the treatment of pain and chronic inflammatory disease. The sensory nervous system is fundamental in the modulation of the host's protective response, and understanding its interactions is pivotal in the process of revealing new strategies for the treatment of pain.

Therapeutic potential of progesterone in spinal cord injury-induced neuropathic pain: At the crossroads between neuroinflammation and N-methyl-D-aspartate receptor

In recent decades, an area of active research has supported the notion that progesterone promotes a wide range of remarkable protective actions in experimental models of nervous system trauma or disease, and has also provided a strong basis for considering this steroid as a promising molecule for modulating the complex maladaptive changes that lead to neuropathic pain, especially after spinal cord injury. In this review, we intend to give the readers a brief appraisal of the main mechanisms underlying the increased excitability of the spinal circuit in the pain pathway after trauma, with particular emphasis on those mediated by the activation of resident glial cells, the subsequent release of proinflammatory cytokines and their impact on N-methyl-D-aspartate receptor function. We then summarize the available preclinical data pointing to progesterone as a valuable repurposing molecule for blocking critical cellular and molecular events that occur in the dorsal horn of the injured spinal cord and are related to the development of chronic pain. Since the treatment and management of neuropathic pain after spinal injury remains challenging, the potential therapeutic value of progesterone opens new traslational perspectives to prevent central pain.

Hevin/Sparcl1 drives pathological pain through spinal cord astrocyte and NMDA receptor signaling

High endothelial venule protein/SPARC-like 1 (hevin/Sparcl1) is an astrocyte-secreted protein that regulates synapse formation in the brain. Here we show that astrocytic hevin signaling plays a critical role in maintaining chronic pain. Compared with WT mice, hevin-null mice exhibited normal mechanical and heat sensitivity but reduced inflammatory pain. Interestingly, hevin-null mice have faster recovery than WT mice from neuropathic pain after nerve injury. Intrathecal injection of WT hevin was sufficient to induce persistent mechanical allodynia in naive mice. In hevin-null mice with nerve injury, adeno-associated-virus-mediated (AAV-mediated) re-expression of hevin in glial fibrillary acidic protein-expressing (GFAP-expressing) spinal cord astrocytes could reinstate neuropathic pain. Mechanistically, hevin is crucial for spinal cord NMDA receptor (NMDAR) signaling. Hevin-potentiated N-Methyl-D-aspartic acid (NMDA) currents are mediated by GluN2B-containing NMDARs. Furthermore, intrathecal injection of a neutralizing Ab against hevin alleviated acute and persistent inflammatory pain, postoperative pain, and neuropathic pain. Secreted hevin that was detected in mouse cerebrospinal fluid (CSF) and nerve injury significantly increased CSF hevin abundance. Finally, neurosurgery caused rapid and substantial increases in SPARCL1/HEVIN levels in human CSF. Collectively, our findings support a critical role of hevin and astrocytes in the maintenance of chronic pain. Neutralizing of secreted hevin with monoclonal Ab may provide a new therapeutic strategy for treating acute and chronic pain and NMDAR-medicated neurodegeneration.

A neuron-to-astrocyte Wnt5a signal governs astrogliosis during HIV-associated pain pathogenesis

Chronic pain is the most common neurological disorder of HIV patients. Multiple neuropathologies were identified in the pain pathway. Among them is the prominent astrocytic reaction (also know an astrogliosis). However, the pathogenic role and mechanism of the astrogliosis are unclear. Here, we show that the astrogliosis is crucial for the pain development induced by a key neurotoxic HIV protein gp120 and that a neuron-to-astrocyte Wnt5a signal controls the astrogliosis. Ablation of astrogliosis blocked the development of gp120-induced mechanical hyperalgesia, and concomitantly the expression of neural circuit polarization in the spinal dorsal horn. We demonstrated that conditional knockout of either Wnt5a in neurons or its receptor ROR2 in astrocytes abolished not only gp120-induced astrogliosis but also hyperalgesia and neural circuit polarization. Furthermore, we found that the astrogliosis promoted expression of hyperalgesia and NCP via IL-1β regulated by a Wnt5a-ROR2-MMP2 axis. Our results shed light on the role and mechanism of astrogliosis in the pathogenesis of HIV-associated 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.

NMDARs mediate peripheral and central sensitization contributing to chronic orofacial pain

Peripheral and central sensitizations of the trigeminal nervous system are the main mechanisms to promote the development and maintenance of chronic orofacial pain characterized by allodynia, hyperalgesia, and ectopic pain after trigeminal nerve injury or inflammation. Although the pathomechanisms of chronic orofacial pain are complex and not well known, sufficient clinical and preclinical evidence supports the contribution of the N-methyl-D-aspartate receptors (NMDARs, a subclass of ionotropic glutamate receptors) to the trigeminal nociceptive signal processing pathway under various pathological conditions. NMDARs not only have been implicated as a potential mediator of pain-related neuroplasticity in the peripheral nervous system (PNS) but also mediate excitatory synaptic transmission and synaptic plasticity in the central nervous system (CNS). In this review, we focus on the pivotal roles and mechanisms of NMDARs in the trigeminal nervous system under orofacial neuropathic and inflammatory pain. In particular, we summarize the types, components, and distribution of NMDARs in the trigeminal nervous system. Besides, we discuss the regulatory roles of neuron-nonneuronal cell/neuron-neuron communication mediated by NMDARs in the peripheral mechanisms of chronic orofacial pain following neuropathic injury and inflammation. Furthermore, we review the functional roles and mechanisms of NMDARs in the ascending and descending circuits under orofacial neuropathic and inflammatory pain conditions, which contribute to the central sensitization. These findings are not only relevant to understanding the underlying mechanisms, but also shed new light on the targeted therapy of chronic orofacial pain.

Study protocol for a phase II, double-blind, randomised controlled trial of cannabidiol (CBD) compared with placebo for reduction of brain neuroinflammation in adults with chronic low back pain

INTRODUCTION

Chronic pain is a debilitating medical problem that is difficult to treat. Neuroinflammatory pathways have emerged as a potential therapeutic target, as preclinical studies have demonstrated that glial cells and neuroglial interactions play a role in the establishment and maintenance of pain. Recently, we used positron emission tomography (PET) to demonstrate increased levels of 18 kDa translocator protein (TSPO) binding, a marker of glial activation, in patients with chronic low back pain (cLBP). Cannabidiol (CBD) is a glial inhibitor in animal models, but studies have not assessed whether CBD reduces neuroinflammation in humans. The principal aim of this trial is to evaluate whether CBD, compared with placebo, affects neuroinflammation, as measured by TSPO levels.

METHODS AND ANALYSIS

This is a double-blind, randomised, placebo-controlled, phase II clinical trial. Eighty adults (aged 18-75) with cLBP for >6 months will be randomised to either an FDA-approved CBD medication (Epidiolex) or matching placebo for 4 weeks using a dose-escalation design. All participants will undergo integrated PET/MRI at baseline and after 4 weeks of treatment to evaluate neuroinflammation using [11C]PBR28, a second-generation radioligand for TSPO. Our primary hypothesis is that participants randomised to CBD will demonstrate larger reductions in thalamic [11C]PBR28 signal compared with those receiving placebo. We will also assess the effect of CBD on (1) [11C]PBR28 signal from limbic regions, which our prior work has linked to depressive symptoms and (2) striatal activation in response to a reward task. Additionally, we will evaluate self-report measures of cLBP intensity and bothersomeness, depression and quality of life at baseline and 4 weeks.

ETHICS AND DISSEMINATION

This protocol is approved by the Massachusetts General Brigham Human Research Committee (protocol number: 2021P002617) and FDA (IND number: 143861) and registered with ClinicalTrials.gov. Results will be published in peer-reviewed journals and presented at conferences.

TRIAL REGISTRATION NUMBER

NCT05066308; ClinicalTrials.gov.

The Role of Neuro-Immune Interactions in Chronic Pain: Implications for Clinical Practice

Chronic pain remains a public health problem and contributes to the ongoing opioid epidemic. Current pain management therapies still leave many patients with poorly controlled pain, thus new or improved treatments are desperately needed. One major challenge in pain research is the translation of preclinical findings into effective clinical practice. The local neuroimmune interface plays an important role in the initiation and maintenance of chronic pain and is therefore a promising target for novel therapeutic development. Neurons interface with immune and immunocompetent cells in many distinct microenvironments along the nociceptive circuitry. The local neuroimmune interface can modulate the activity and property of the neurons to affect peripheral and central sensitization. In this review, we highlight a specific subset of many neuroimmune interfaces. In the central nervous system, we examine the interface between neurons and microglia, astrocytes, and T lymphocytes. In the periphery, we profile the interface between neurons in the dorsal root ganglion with T lymphocytes, satellite glial cells, and macrophages. To bridge the gap between preclinical research and clinical practice, we review the preclinical studies of each neuroimmune interface, discuss current clinical treatments in pain medicine that may exert its action at the neuroimmune interface, and highlight opportunities for future clinical research efforts.

Targeting G protein coupled receptors for alleviating neuropathic pain

Pain sensation is a normal physiological response to alert and prevent further tissue damage. It involves the perception of external stimuli by somatosensory neurons, then transmission of the message to various other types of neurons present in the spinal cord and brain to generate an appropriate response. Currently available analgesics exhibit very modest efficacy, and that too in only a subset of patients with chronic pain conditions, particularly neuropathic pain. The G protein-coupled receptors (GPCRs) are expressed on presynaptic, postsynaptic terminals, and soma of somatosensory neurons, which binds to various types of ligands to modulate neuronal activity and thus pain sensation in both directions. Fundamentally, neuropathic pain arises due to aberrant neuronal plasticity, which includes the sensitization of peripheral primary afferents (dorsal root ganglia and trigeminal ganglia) and the sensitization of central nociceptive neurons in the spinal cord or trigeminal nucleus or brain stem and cortex. Owing to the expression profiles of GPCRs in somatosensory neurons and other neuroanatomical regions involved in pain processing and transmission, this article shall focus only on four families of GPCRs: 1- Opioid receptors, 2-Cannabinoid receptors, 3-Adenosine receptors, and 4-Chemokine receptors.