Alterations in the gut microbiota and metabolite profiles in the context of neuropathic pain

Integrative review of the gut microbiome’s role in pain management for orthopaedic conditions

The gut microbiome, a complex ecosystem of microorganisms, has a significant role in modulating pain, particularly within orthopaedic conditions. Its impact on immune and neurological functions is underscored by the gut-brain axis, which influences inflammation, pain perception, and systemic immune responses. This integrative review examines current research on how gut dysbiosis is associated with various pain pathways, notably nociceptive and neuroinflammatory mechanisms linked to central sensitization. We highlight advancements in meta-omics technologies, such as metagenomics and metaproteomics, which deepen our understanding of microbiome-host interactions and their implications in pain. Recent studies emphasize that gut-derived short-chain fatty acids and microbial metabolites play roles in modulating neuroinflammation and nociception, contributing to pain management. Probiotics, prebiotics, synbiotics, and faecal microbiome transplants are explored as potential therapeutic strategies to alleviate pain through gut microbiome modulation, offering an adjunct or alternative to opioids. However, variability in individual microbiomes poses challenges to standardizing these treatments, necessitating further rigorous clinical trials. A multidisciplinary approach combining microbiology, immunology, neurology, and orthopaedics is essential to develop innovative, personalized pain management strategies rooted in gut health, with potential to transform orthopaedic pain care.

Gut Microbiota: A Modulator and Therapeutic Target for Chronic Pain

Chronic pain is a prevalent condition, impacting nearly one-fifth of the global population. Despite the availability of various clinical treatments, each comes with inherent limitations, and few offer a complete cure, resulting in a significant social and economic burden. Therefore, it is important to determine the pathogenesis and causes of chronic pain. Numerous studies have shown a close link between the intestinal microflora and chronic pain. The gut microbiota can exert their effects on chronic pain through both central and peripheral mechanisms and is able to communicate with the brain through its own components or metabolites. They also can regulate chronic pain by affecting pro- and anti-inflammatory cells. This review is aimed at reviewing the connection between gut flora and different types of chronic pain, including visceral pain, neuropathic pain, inflammatory pain, musculoskeletal pain, migraine, and chronic cancer pain; exploring the central and peripheral mechanisms of the influence of gut flora on chronic pain; and attempting to provide novel treatment options for chronic pain, that is, the gut microbiota can be regulated by probiotics, fecal microbial transplantation, and natural products to treat chronic pain. By examining the intricate relationship between gut flora and chronic pain, the review sought to pave the way for new treatment strategies that target the gut microbiota, offering hope for more effective pain management.

Using Integrated Network Pharmacology and Metabolomics to Reveal the Mechanisms of the Combined Intervention of Ligustrazine and Sinomenine in CCI-Induced Neuropathic Pain Rats

Neuropathic pain (NP) is a type of chronic pain resulting from injury or dysfunction of the nerves or spinal cord. Previous studies have shown that the combination of ligustrazine (LGZ) and sinomenine (SIN) exerts a synergistic antinociceptive effect in peripheral and central NP models. On this basis, a comprehensive analgesic evaluation was performed in a chronic constriction injury (CCI)-induced NP model in rats. Sciatic nerve histopathological changes were observed, and 22 cytokines and chemokines levels were analyzed. We also combined network pharmacology and metabolomics to explore their molecular mechanisms. Results showed that the combination of LGZ and SIN significantly alleviated the pain-like behaviors in CCI rats in a time- and dose-dependent manner, demonstrating superior therapeutic effects compared to LGZ or SIN alone. It also improved pathological damage to sciatic nerves and regulated inflammatory cytokine levels. Network pharmacology identified shared and distinct pain-related targets for LGZ and SIN, while metabolomics revealed 54 differential metabolites in plasma, and 17 differential metabolites in CSF were associated with the combined intervention of LGZ and SIN. Finally, through an integrated analysis of the core targets and differential metabolites, tyrosine metabolism, phenylalanine metabolism, and arginine and proline metabolism were identified as potential key metabolic pathways underlying the therapeutic effects of LGZ and SIN in CCI treatment. In conclusion, our study provides evidence to support the clinical application of LGZ and SIN in the treatment of NP.

Microbiome contributions to pain: a review of the preclinical literature

ABSTRACT

Over the past 2 decades, the microbiome has received increasing attention for the role that it plays in health and disease. Historically, the gut microbiome was of particular interest to pain scientists studying nociplastic visceral pain conditions given the anatomical juxtaposition of these microorganisms and the neuroimmune networks that drive pain in such diseases. More recently, microbiomes both inside and across the surface of the body have been recognized for driving sensory symptoms in a broader set of diseases. Microbiomes have never been a more popular topic in pain research, but to date, there has not been a systematic review of the preclinical microbiome pain literature. In this article, we identified all animal studies in which both the microbiome was manipulated and pain behaviors were measured. Our analysis included 303 unique experiments across 97 articles. Microbiome manipulation methods and behavioral outcomes were recorded for each experiment so that field-wide trends could be quantified and reported. This review specifically details the animal species, injury models, behavior measures, and microbiome manipulations used in preclinical pain research. From this analysis, we were also able to conclude how manipulations of the microbiome alter pain thresholds in naïve animals and persistent pain intensity and duration in cutaneous and visceral pain models. This review summarizes by identifying existing gaps in the literature and providing recommendations for how to best plan, implement, and interpret data collected in preclinical microbiome pain experiments.

Precision neurology

Over the past several decades, high-resolution brain imaging, blood and cerebrospinal fluid analyses, and other advanced technologies have changed diagnosis from an exercise depending primarily on the history and physical examination to a computer- and online resource-aided process that relies on larger and larger quantities of data. In addition, randomized controlled trials (RCT) at a population level have led to many new drugs and devices to treat neurological disease, including disease-modifying therapies. We are now at a crossroads. Combinatorially profound increases in data about individuals has led to an alternative to population-based RCTs. Genotyping and comprehensive "deep" phenotyping can sort individuals into smaller groups, enabling precise medical decisions at a personal level. In neurology, precision medicine that includes prediction, prevention and personalization requires that genomic and phenomic information further incorporate imaging and behavioral data. In this article, we review the genomic, phenomic, and computational aspects of precision medicine for neurology. After defining biological markers, we discuss some applications of these "-omic" and neuroimaging measures, and then outline the role of computation and ultimately brain simulation. We conclude the article with a discussion of the relation between precision medicine and value-based care.

Electroacupuncture Alleviates Neuropathic Pain and Negative Emotion in Mice by Regulating Gut Microbiota

Background

Neuropathic pain (NP) is a prevalent chronic condition frequently accompanied by adverse emotional states. Previous research has demonstrated the clinical efficacy of electroacupuncture (EA) in mitigating neuropathic pain and its associated mood disorders. Recent studies have underscored a correlation between gut microbiota and both NP and negative emotional states. Nevertheless, the relationship between the modulation of gut microbiota by EA and the amelioration of NP remains inadequately understood.

Methods

Mice were randomly assigned to one of the three groups: the Control (Con) group, the EA group, and the Chronic Constrictive Injury (CCI) group (n = 12 each). Starting from the 8th day post-CCI induction, the EA group underwent EA treatment once every two days, for a total of 20 sessions. To investigate the impact of gut microbiota on CCI mice, we employed a variety of methods, including various behavioral tests and 16S ribosomal DNA (rDNA) sequencing.

Results

The results indicated that EA significantly ameliorated mechanical allodynia and emotional dysfunction induced by CCI in mice. Analysis through 16S rDNA sequencing revealed that the gut microbiota of NP model mice exhibited a marked increase in diversity. However, EA could partially reverse changes in the diversity of gut microbiota. The abundance of Alloprevotella, A2, Roseburia, Muribaculum, Ruminiclostridium, and Rikenella was increased, and the abundance levels of Bacteroides were decreased at the genus level in CCI mice. Following EA treatment, the relative abundance of Alistipes, A2, Roseburia, and Rikenella was decreased, whereas the relative abundance of Alloprevotella and Parabacteroides was increased in EA group when compared with the CCI group.

Conclusion

These findings suggested that EA exerted a significant therapeutic effect on NP, potentially through modulation of the gut microbiota.

Diabetic neuropathy: understanding the nexus of diabetic neuropathy, gut dysbiosis and cognitive impairment

Objectives

Diabetic neuropathy is a traditional and one of the most prevalent complications of diabetes mellitus. The exact pathophysiology of diabetic neuropathy is not fully understood. However, oxidative stress and inflammation are proven to be one of the major underlying mechanisms of neuropathy which is described in detail. Gut dysbiosis is being studied for various neurological disorders and its impact on diabetic neuropathy is also explained. Diabetic neuropathy also causes loss in an individual's quality of life and one such adverse event is cognitive dysfunction. The interrelation between the neuropathy, cognitive dysfunction and gut is reviewed.

Methods

The exact mechanism is not understood but several hypotheses, cross-sectional studies and systematic reviews suggest a relationship between cognition and neuropathy. The review has collected data from various review and research publications that justifies this inter-relationship.

Results

The multifactorial etiology and pathophysiology of diabetic neuropathy is described with special emphasis on the role of gut dysbiosis. There might exist a correlation between the neuropathy and cognitive impairment caused simultaneously in diabetic patients.

Conclusions

This review summarizes the relationship that might exist between diabetic neuropathy, cognitive dysfunction and the impact of disturbed gut microbiome on its development and progression.

The gut microbiome and the brain

PURPOSE OF REVIEW

The importance of the gut microbiome for human health and well-being is generally accepted, and elucidating the signaling pathways between the gut microbiome and the host offers novel mechanistic insight into the (patho)physiology and multifaceted aspects of healthy aging and human brain functions.

RECENT FINDINGS

The gut microbiome is tightly linked with the nervous system, and gut microbiota are increasingly emerging as important regulators of emotional and cognitive performance. They send and receive signals for the bidirectional communication between gut and brain via immunological, neuroanatomical, and humoral pathways. The composition of the gut microbiota and the spectrum of metabolites and neurotransmitters that they release changes with increasing age, nutrition, hypoxia, and other pathological conditions. Changes in gut microbiota (dysbiosis) are associated with critical illnesses such as cancer, cardiovascular, and chronic kidney disease but also neurological, mental, and pain disorders, as well as chemotherapies and antibiotics affecting brain development and function.

SUMMARY

Dysbiosis and a concomitant imbalance of mediators are increasingly emerging both as causes and consequences of diseases affecting the brain. Understanding the microbiota's role in the pathogenesis of these disorders will have major clinical implications and offer new opportunities for therapeutic interventions.

Assessing the Genetic Causal Effects Between Blood Metabolites and Spinal Pain: A Bidirectional Two-Sample Mendelian Randomization Study

Background

Previous metabolomics studies have indicated a close association between blood metabolites and pain. However, the causal relationship between blood metabolites and spinal pain (SP) remains unclear. This study employs a bidirectional two-sample Mendelian randomization (MR) analysis to evaluate the causal relationship between 452 blood metabolites and SP.

Methods

We used bidirectional two-sample MR analysis to assess the causal relationship between blood metabolites and SP, including neck pain (NP), thoracic spine pain (TSP), low back pain (LBP), and back pain (BP). Genome-wide association studies (GWAS) data for 452 metabolites (7,824 participants) were used as exposure variables. Summary data for NP were obtained from the UK Biobank, for TSP from the FinnGen Biobank, and for LBP from both the UK Biobank and the FinnGen Biobank. Summary data for BP were obtained from the UK Biobank. Inverse-variance weighting (IVW) was used to estimate the causal relationships between metabolites and SP, complemented by various sensitivity analyses to account for pleiotropy and heterogeneity, ensuring robust results.

Results

The IVW analysis identified 155 metabolites associated with SP risk and 142 metabolites influenced by SP. No significant heterogeneity or horizontal pleiotropy was observed through other analytical methods.

Conclusion

This study demonstrates potential causal effects between blood metabolites and SP, providing new insights into the pathogenesis of SP. These findings lay a theoretical foundation for preventing and treating SP through targeted interventions on specific blood metabolites, potentially elucidating underlying biological mechanisms.

Immune cells mediate the effects of gut microbiota on neuropathic pain: a Mendelian randomization study

BACKGROUND

The gut microbiota may be involved in neuropathic pain. However, the causal association between gut microbiota and neuropathic pain remains unclear. Whether immune cells and inflammatory factors mediate the pathway from gut microbiota to neuropathic pain has not been elucidated.

METHODS

We obtained the summary data of 412 gut microbiota, 731 immune cells, 91 inflammatory factors, and five types of neuropathic pain (drug-induced neuropathy, postherpetic neuralgia, sciatica, trigeminal neuralgia, and unspecified neuralgia) from large-scale genome-wide association study (GWAS) datasets and the FinnGen database. We used bidirectional Mendelian randomization (MR) analysis to explore the causal association between gut microbiota and neuropathic pain. Additionally, we conducted a mediation analysis to identify whether immune cells and inflammatory factors act as mediators within these causal relationships.

RESULTS

Our study revealed 30 causal relationships between 26 gut bacterial taxa and five types of neuropathic pain, including four associated with drug-induced neuropathy, six with postherpetic neuralgia, five with sciatica, eight with trigeminal neuralgia, and seven with unspecified neuralgia. Moreover, we identified 35 gut bacterial pathway abundances causally involved in neuropathic pain. The reverse MR analysis showed no evidence of reverse causality from gut microbiota to neuropathic pain. Mediation analysis demonstrated that the immune cell phenotype "HLA-DR++ monocyte % leukocyte" mediated the causal relationship between p_Proteobacteria and sciatica with a mediation proportion of 36.15% (P = 0.038), whereas "CD11c on CD62L+ myeloid dendritic cell" mediated the causal pathway from assimilatory sulfate reduction to trigeminal neuralgia with a mediation proportion of 27.90% (P = 0.041).

CONCLUSION

This study identified the causal relationships between several specific gut microbiota and various neuropathic pain subtypes. Additionally, two immune cells may act as potential mediators in the pathways from gut microbiota to neuropathic pain.

Gastrointestinal Conditions Affect Chronic Pain and Quality of Life in Women

Pain is a chronic condition in many women; drugs used for its treatment are often accompanied by detrimental effects on many organs, including the gut. Once inflamed, the gut can affect pain processes. The aim of this study was to evaluate the general health of women suffering chronic pain, with particular attention to gastrointestinal (GI) conditions. The possibility to improve pain and quality of life through personalized nutritional advice was also tested. Forty women suffering from chronic pain were contacted for the administration of questionnaires to define their pain features and gastrointestinal conditions. Their psychological, clinical and reproductive states were also recorded. Pain scores were correlated with GI, psychological and clinical scores. Diet suggestions were given, and evaluation was repeated after 4 weeks. Thirty-eight women were included in the study: 32 suffered chronic widespread pain and had 6 pelvic pain. Pain had been present in all women for years; more than 80% of women reported various types of disorders related to the gut. Pain scores were worse in the women intolerant to milk and dairy products. The GI score was positively correlated with the pain score. The Dietary Inflammatory Index was very high in all subjects. Personalized nutritional advice followed by 26 subjects for 4 weeks resulted in a significant improvement of pain and quality of life parameters. We describe women with chronic pain as being particularly affected by GI alterations. The change in feeding habits had a beneficial effect on pain and other quality of life parameters.

Impact of Lumbar Fusion Internal Fixation on Lumbar Disc Herniation in Young Patients: A Retrospective Study

BACKGROUND Lumbar fusion and internal fixation techniques have shown promise in treating lumbar disc herniation (LDH), yet the impact on lumbar function in young patients remains unclear. This study aimed to investigate the impact of lumbar fusion on lumbar function in young patients. MATERIAL AND METHODS A retrospective analysis was conducted on 330 patients diagnosed with LDH admitted to our hospital. Patients were divided into 2 groups: a control group (n=264) that underwent a minimally invasive procedure with a keyhole lens, and a research group (n=66) that underwent lumbar fusion internal fixation. Clinical features and therapeutic outcomes were assessed using Oswestry Disability Index (ODI) and Japanese Orthopedic Association (JOA) Lumbar Scores before surgery and 12 months postoperatively. Additionally, intervertebral space height, degree of vertebral spondylolisthesis (grades I, II, and III), incidence of adverse effects, and treatment efficacy were measured pre-and post-surgically. RESULTS No significant difference in ODI and JOA scores was found between the groups before surgery (P>0.05). Postoperatively, the research group had lower ODI scores, higher JOA scores, and lower intervertebral space heights compared to the control group (P=0.001). While grade 1 and 2 spondylolisthesis showed slight improvement (P>0.05), a significant difference was observed in grade III spondylolisthesis between the 2 groups (P=0.001). Additionally, the research group had a lower incidence of adverse effects (P=0.049) and higher treatment efficacy, although the difference was not statistically significant (P>0.05). CONCLUSIONS Lumbar fusion with internal fixation produced better postoperative outcomes and fewer adverse effects than minimally invasive procedures in young patients with lumbar disc herniation.

Perturbed saliva microbiome is gender-specific in patients with oral lichen planus

OBJECTIVE

To understand the gender characteristics of oral lichen planus (OLP) by identifying the gender-specific salivary microbiome and its potential biomarkers.

METHODS

A gender-based study was undertaken, commencing with the collection of saliva samples, followed by 16S rRNA gene sequencing, to explore the differences in the composition of saliva microbiome in OLP patients (40 males and 56 females) and healthy controls (40 males and 56 females), respectively.

RESULTS

Both male and female OLP patients had significant differences in saliva microbiome composition from healthy controls, especially in female patients. Notably, Pseudomonas was only enriched in female patients. Rhodococcus (AUC: 0.91) and Pseudomonas (AUC: 0.97) had great potential as diagnostic biomarkers in male and female patients, respectively. The KEGG results showed metabolic dysfunction was more pronounced in female patients and a high level of microbial metabolism in diverse environments, ABC transporters, Quorum sensing and Two-component system. Capnocytophaga was negatively correlated with the erosion area in male patients. Neisseria was negatively correlated with the erosion area and Rothia was positively correlated with the pain level in female patients.

CONCLUSIONS

Our study revealed gender-specific perturbation in salivary microbiome within OLP patients, suggesting that the male and female patients with OLP may have different pathogenesis.

A deep learning framework combining molecular image and protein structural representations identifies candidate drugs for pain

Artificial intelligence (AI) and deep learning technologies hold promise for identifying effective drugs for human diseases, including pain. Here, we present an interpretable deep-learning-based ligand image- and receptor's three-dimensional (3D)-structure-aware framework to predict compound-protein interactions (LISA-CPI). LISA-CPI integrates an unsupervised deep-learning-based molecular image representation (ImageMol) of ligands and an advanced AlphaFold2-based algorithm (Evoformer). We demonstrated that LISA-CPI achieved ∼20% improvement in the average mean absolute error (MAE) compared to state-of-the-art models on experimental CPIs connecting 104,969 ligands and 33 G-protein-coupled receptors (GPCRs). Using LISA-CPI, we prioritized potential repurposable drugs (e.g., methylergometrine) and identified candidate gut-microbiota-derived metabolites (e.g., citicoline) for potential treatment of pain via specifically targeting human GPCRs. In summary, we presented that the integration of molecular image and protein 3D structural representations using a deep learning framework offers a powerful computational drug discovery tool for treating pain and other complex diseases if broadly applied.

Danggui Sini Decoction normalizes the intestinal microbiota and serum metabolite levels to treat sciatica

ETHNOPHARMACOLOGICAL RELEVANCE

Danggui Sini Decoction (DGSD), which is commonly used to treat sciatica, has been shown to have an analgesic effect, but the underlying mechanisms are unclear. Here, Danggui Sini Decoction was shown to normalize the intestinal microbiota and serum metabolite levels to exert an analgesic effect.

AIM OF THE STUDY

This study aimed to elucidate the therapeutic effects of DGSD on sciatica and the underlying mechanisms involved.

METHODS

In this study, we conducted chronic constriction injury (CCI) model. Mecobalamin and DGSD were administered to CCI rats. Behavioural tests were used to examine the therapeutic effects of the drugs. UHPLC was used to identify DGSD components. 16S rRNA gene sequencing analysis of the intestinal flora was used to analyse the effect of DGSD on the intestinal microbiota. UHPLC‒MS/MS was used to identify blood metabolites. KEGG pathway analysis of differentially abundant metabolites was subsequently conducted. ELISA was used to measure the serum inflammatory factor levels, and correlation analysis between the serum inflammatory factor levels and intestinal microbe abundance was conducted. PCR, western blotting, and immunohistochemical staining were used to validate the results of the KEGG pathway analysis.

RESULTS

After CCI, the rats exhibited obvious thermal hyperalgesia; disruption of sciatic nerve structure; increased IL1α, SP, CCL5, and PGE2 levels; decreased IL10 levels in the blood; increased IL1β, IL6, COX2, MMP9, nNOS, and p-NF-κB levels; and decreased IL4 levels in the sciatic nerve. In addition, CCI led to increased abundances of Peptostreptococcaceae, Leuconostocaceae, Christensenellaceae, Akkermansiaceae, Staphylococcaceae, Romboutsia, Marvinbryantia, Turicibacter, Weissella, UCG-005, Christensenellaceae_R-7_group, Akkermansia, Staphylococcus, Romboutsia_ilealis, Weissella_paramesenteroides, and Akkermansia_muciniphila and decreased abundances of Lactobacillaceae, Lactobacillus, Lactobacillus_murinus, and Lactobacillus_johnsonii. Correlation analysis indicated that Turicibacter abundance was most strongly related to IL1α, PGE2, IL10, and CCL5 levels, while norank_o_Coriobacteriales abundance had the weakest relationship with SP levels. KEGG pathway analysis of the differentially abundant metabolites revealed that the 'NF-kappa B signalling pathway' was involved in sciatica. DGSD reduced the levels of inflammatory factors, including IL1α, SP, CCL5, PGE2, IL6, COX2, and MMP9, in the blood and sciatic nerve and inhibited nNOS and NF-κB phosphorylation. DGSD improved the abundance of probiotics, including Lactobacillus and Blautia, and lowered the abundance of harmful bacteria, including Romboutsia, Turicibacter, and Weissella. DGSD promoted the repair of the injured sciatic nerve.

CONCLUSIONS

DGSD can treat sciatica by inhibiting intestinal microbiota disorders induced by CCI in rats, normalizing inflammatory factor levels, and promoting nerve repair.

Activation of GPER-1 Attenuates Traumatic Brain Injury-Induced Neurological Impairments in Mice

This study aimed to investigate the effects of G1-activated G protein-coupled estrogen receptor 1 (GPER1) on neurological impairments and neuroinflammation in traumatic brain injury (TBI) mice. The controlled cortical impingement (CCI) method was used to establish the TBI model. The mice were subjected to ovariectomy (OVX) for two weeks prior to modeling. GPER1 agonist G1 was administered by intracerebroventricular injection. Brain tissue water content was detected by wet/dry method, and blood-brain barrier damage was detected by Evans blue extravasation. The neurological impairments in mice were evaluated by open field test, Y-maze test, nest-building test, object location memory test and novel object recognition test. Ionized calcium-binding adapter molecule 1 (Iba1) staining was used to indicate the activation of microglia. Expression of M1/M2-type microglia markers and inflammatory factors were evaluated by ELISA and qRT-PCR. The G1 administration significantly reduced cerebral edema and Evans blue extravasation at injury ipsilateral cortex and basal ganglia in TBI mice. Activation of GPER1 by G1 improved the anxiety behavior and the cognitive dysfunction of mice induced by TBI. G1 administration significantly decreased Iba1-positive staining cells and the mRNA levels of CD86, macrophage cationic peptide 1 (Mcp-1), nitric oxide synthase 2 (Nos2), interleukin 1 beta (IL-1β), and macrophage inflammatory protein-2 (MIP-2), while increased the mRNA levels of interleukin 10 (IL-10), arginase1 (Arg-1) and CD206. Activation of GPER1 through G1 administration has the potential to ameliorate cognitive dysfunction induced by TBI in mice. It may also inhibit the activation of M1 microglia in cortical tissue resulting from TBI, while promoting the activation of M2 microglia and contributing to the regulation of inflammatory responses.

Gut microbiota promotes pain chronicity in Myosin1A deficient male mice

Chronic pain is a heavily debilitating condition and a huge socio-economic burden, with no efficient treatment. Over the past decade, the gut microbiota has emerged as an important regulator of nervous system's health and disease states. Yet, its contribution to the pathogenesis of chronic somatic pain remains poorly documented. Here, we report that male but not female mice lacking Myosin1a (KO) raised under single genotype housing conditions (KO-SGH) are predisposed to develop chronic pain in response to a peripheral tissue injury. We further underscore the potential of MYO1A loss-of-function to alter the composition of the gut microbiota and uncover a functional connection between the vulnerability to chronic pain and the dysbiotic gut microbiota of KO-SGH males. As such, parental antibiotic treatment modifies gut microbiota composition and completely rescues the injury-induced pain chronicity in male KO-SGH offspring. Furthermore, in KO-SGH males, this dysbiosis is accompanied by a transcriptomic activation signature in the dorsal root ganglia (DRG) macrophage compartment, in response to tissue injury. We identify CD206+CD163- and CD206+CD163+ as the main subsets of DRG resident macrophages and show that both are long-lived and self-maintained and exhibit the capacity to monitor the vasculature. Consistently, in vivo depletion of DRG macrophages rescues KO-SGH males from injury-induced chronic pain underscoring a deleterious role for DRG macrophages in a Myo1a-loss-of function context. Together, our findings reveal gene-sex-microbiota interactions in determining the predisposition to injury-induced chronic pain and point-out DRG macrophages as potential effector cells.

Huangqin decoction attenuates spared nerve injury (SNI)-induced neuropathic pain by modulating microglial M1/M2 polarization partially mediated by intestinal nicotinamide metabolism

BACKGROUND

The incidence of neuropathic pain is progressively increasing over time. The activation of M1-type microglia plays a crucial role in the initiation and progression of neuropathic pain. Huangqin Decoction (HQD) is traditionally used to alleviate dysentery and abdominal pain. However, it remains unclear whether HQD can effectively mitigate neuropathic pain and the underlying mechanisms.

PURPOSE

The present study aims to investigate the impact of HQD on neuropathic pain induced by spared nerve injury (SNI) in mice, and to elucidate whether the analgesic effect of HQD is associated with microglia polarization.

METHODS

The analgesic effect of HQD on SNI mice was investigated through assessments of mechanical pain threshold, thermal pain threshold, cold pain threshold, and motor ability. We elucidated the molecular mechanisms of HQD in alleviating SNI-induced neuropathic pain by focusing on microglia polarization and intestinal metabolite abnormalities. The expression levels of markers associated with microglia polarization (Iba-1, CD68, CD206, iNOS) was detected by immunofluorescence and Western blot, and the levels of inflammatory factors (IL-4, IL-10, IL-6, TNF-α) were assessed by ELISA. UPLC-QTOF-MS metabolomics was utilized to identify differential metabolites in the intestines of SNI mice. We screened the differential metabolites related to microglial polarization by correlation analysis, subsequently nicotinamide was selected for validation in LPS-induced BV-2 cells.

RESULTS

Our findings demonstrated that HQD (20 g/kg) significantly enhanced the mechanical pain threshold, thermal pain threshold, and cold pain threshold, and protected the injured DRG neurons of SNI mice. Moreover, HQD (20 g/kg) obviously suppressed the expression of microglia M1 polarization markers (Iba-1, CD68, iNOS, IL-6, TNF-α), and promoted the expression of microglia M2 polarization markers (CD206, IL-10, IL-4) in the spinal cord of SNI mice. Additionally, HQD (20 g/kg) prominently ameliorated intestinal barrier damage by upregulating Claudin 1 and Occludin expression in the colon of SNI mice. Furthermore, HQD (20 g/kg) rectified 19 metabolite abnormalities in the intestine. Notably, nicotinamide (100 μM), an amide derivative with anti-inflammatory property, effectively suppresses microglia activation and polarization in LPS-induced BV-2 cells by downregulating IL-6 level and CD68 expression while upregulating IL-4 level and CD206 expression.

CONCLUSION

In summary, HQD alleviates neuropathic pain in SNI mice by regulating the activation and polarization of microglia, partially mediated through intestinal nicotinamide metabolism.

The interplay between the microbiota and opioid in the treatment of neuropathic pain

Neuropathic pain (NP) is characterized by its complex and multifactorial nature and limited responses to opioid therapy; NP is associated with risks of drug resistance, addiction, difficulty in treatment cessation, and psychological disorders. Emerging research on gut microbiota and their metabolites has demonstrated their effectiveness in alleviating NP and augmenting opioid-based pain management, concurrently mitigating the adverse effects of opioids. This review addresses the following key points: (1) the current advances in gut microbiota research and the challenges in using opioids to treat NP, (2) the reciprocal effects and benefits of gut microbiota on NP, and (3) the interaction between opioids with gut microbiota, as well as the benefits of gut microbiota in opioid-based treatment of NP. Through various intricate mechanisms, gut microbiota influences the onset and progression of NP, ultimately enhancing the efficacy of opioids in the management of NP. These insights pave the way for further pragmatic clinical research, ultimately enhancing the efficacy of opioid-based pain management.

Combining fecal 16 S rRNA sequencing and spinal cord metabolomics analysis to explain the modulatory effect of PPARα on neuropathic pain

BACKGROUND

Existing evidence suggests that the composition of the gut microbiota is associated with neuropathic pain (NP), but the mechanistic link is elusive. Peroxisome proliferator-activated receptor α (PPARα) has been shown to be a pharmacological target for the treatment of metabolic disorders, and its expression is also involved in inflammatory regulation. The aim of this study was to investigate the important modulatory effects of PPARα on gut microbiota and spinal cord metabolites in mice subjected to chronic constriction injury.

METHODS

We analyzed fecal microbiota and spinal cord metabolic alterations in mice from the sham, CCI, GW7647 (PPARα agonist) and GW6471 (PPARα antagonist) groups by 16 S rRNA amplicon sequencing and untargeted metabolomics analysis. On this basis, the intestinal microbiota and metabolites that were significantly altered between treatment groups were analyzed in a combined multiomics analysis. We also investigated the effect of PPARα on the polarization fractionation of spinal microglia.

RESULTS

PPARα agonist significantly reduce paw withdrawal threshold and paw withdrawal thermal latency, while PPARα antagonist significantly increase paw withdrawal threshold and paw withdrawal thermal latency. 16 S rRNA gene sequencing showed that intraperitoneal injection of GW7647 or GW6471 significantly altered the abundance, homogeneity and composition of the gut microbiome. Analysis of the spinal cord metabolome showed that the levels of spinal cord metabolites were shifted after exposure to GW7647 or GW6471. Alterations in the composition of gut microbiota were significantly associated with the abundance of various spinal cord metabolites. The abundance of Licheniformes showed a significant positive correlation with nicotinamide, benzimidazole, eicosanoids, and pyridine abundance. Immunofluorescence results showed that intraperitoneal injection of GW7647 or GW6471 altered microglial activation and polarization levels.

CONCLUSION

Our study shows that PPARα can promote M2-type microglia polarization, as well as alter gut microbiota and metabolites in CCI mice. This study enhances our understanding of the mechanism of PPARα in the treatment of neuropathic pain.

Current understanding of the molecular mechanisms of chemotherapy-induced peripheral neuropathy

Chemotherapy-induced peripheral neuropathy (CIPN) is the most common off-target adverse effects caused by various chemotherapeutic agents, such as cisplatin, oxaliplatin, paclitaxel, vincristine and bortezomib. CIPN is characterized by a substantial loss of primary afferent sensory axonal fibers leading to sensory disturbances in patients. An estimated of 19-85% of patients developed CIPN during the course of chemotherapy. The lack of preventive measures and limited treatment options often require a dose reduction or even early termination of life-saving chemotherapy, impacting treatment efficacy and patient survival. In this Review, we summarized the current understanding on the pathogenesis of CIPN. One prominent change induced by chemotherapeutic agents involves the disruption of neuronal cytoskeletal architecture and axonal transport dynamics largely influenced by the interference of microtubule stability in peripheral neurons. Due to an ineffective blood-nerve barrier in our peripheral nervous system, exposure to some chemotherapeutic agents causes mitochondrial swelling in peripheral nerves, which lead to the opening of mitochondrial permeability transition pore and cytochrome c release resulting in degeneration of primary afferent sensory fibers. The exacerbated nociceptive signaling and pain transmission in CIPN patients is often linked the increased neuronal excitability largely due to the elevated expression of various ion channels in the dorsal root ganglion neurons. Another important contributing factor of CIPN is the neuroinflammation caused by an increased infiltration of immune cells and production of inflammatory cytokines. In the central nervous system, chemotherapeutic agents also induce neuronal hyperexcitability in the spinal dorsal horn and anterior cingulate cortex leading to the development of central sensitization that causes CIPN. Emerging evidence suggests that the change in the composition and diversity of gut microbiota (dysbiosis) could have direct impact on the development and progression of CIPN. Collectively, all these aspects contribute to the pathogenesis of CIPN. Recent advances in RNA-sequencing offer solid platform for in silico drug screening which enable the identification of novel therapeutic agents or repurpose existing drugs to alleviate CIPN, holding immense promises for enhancing the quality of life for cancer patients who undergo chemotherapy and improve their overall treatment outcomes.

Associations Between Brain-Gut Axis and Psychological Distress in Fibromyalgia: A Microbiota and Magnetic Resonance Imaging Study

An altered brain-gut axis is suspected to be one of the pathomechanisms in fibromyalgia (FM). This cross-sectional study investigated the associations among altered microbiota, psychological distress, and brain functional connectivity (FC) in FM. We recruited 25 FM patients and 25 healthy people in the present study. Psychological distress was measured using standardized questionnaires. Microbiota analysis was performed on the participants' stools. Functional magnetic resonance imaging data were acquired, and seed-based resting-state FC (rs-FC) analysis was conducted with the salience network nodes as seeds. Linear regression and mediation analyses evaluated microbiota, symptoms, and rs-FCs associations. We found altered microbiota diversity in FM, of which Phascolarctobacterium and Lachnoclostridium taxa increased the most and Faecalibacterium taxon decreased the most compared to controls. The Phascolarctobacterium abundance significantly predicted Beck depression inventory (BDI-II) scores in FM (β = 6.83; P = .033). Rs-FCs from salience network nodes were reduced in FM, of which rs-FCs from the right lateral rostral prefrontal cortex (RPFC) to the lateral occipital cortex, superior division right (RPFC-sLOC) could be predicted by BDI-II scores in patients (β = -.0064; P = .0054). In addition, the BDI-II score was a mediator in the association between Phascolarctobacterium abundance and rs-FCs of RPFC-sLOC (ab = -.06; 95% CI: -.16 to -9.10-3). In conclusion, microbial dysbiosis might be associated with altered neural networks mediated by psychological distress in FM, emphasizing the critical role of the brain-gut axis in FM's non-pain symptoms and supporting further analysis of mechanism-targeted therapies to reduce FM symptoms. PERSPECTIVE: Our study suggests microbial dysbiosis might be associated with psychological distress and the altered salience network, supporting the role of brain-gut axis dysfunction in fibromyalgia pathomechanisms. Further targeting therapies for microbial dysbiosis should be investigated to manage fibromyalgia patients in the future.

Ginger alleviates mechanical hypersensitivity and anxio-depressive behavior in rats with diabetic neuropathy through beneficial actions on gut microbiome composition, mitochondria, and neuroimmune cells of colon and spinal cord

The relationship among gut microbiota, mitochondrial dysfunction/neuroinflammation, and diabetic neuropathic pain (DNP) has received increased attention. Ginger has antidiabetic and analgesic effects because of its anti-inflammatory property. We examined the effects of gingerols-enriched ginger (GEG) supplementation on pain-associated behaviors, gut microbiome composition, and mitochondrial function and neuroinflammation of colon and spinal cord in DNP rats. Thirty-three male rats were randomly divided into 3 groups: control group, DNP group (high-fat diet plus single dose of streptozotocin at 35 mg/kg body weight, and GEG group (DNP+GEG at 0.75% in the diet for 8 weeks). Von Frey and open field tests were used to assess pain sensitivity and anxio-depressive behaviors, respectively. Colon and spinal cord were collected for gene expression analysis. 16S rRNA gene sequencing was done from cecal samples and microbiome data analysis was performed using QIIME 2. GEG supplementation mitigated mechanical hypersensitivity and anxio-depressive behavior in DNP animals. GEG supplementation suppressed the dynamin-related protein 1 protein expression (colon) and gene expression (spinal cord), astrocytic marker GFAP gene expression (colon and spinal cord), and tumor necrosis factor-α gene expression (colon, P < .05; spinal cord, P = .0974) in DNP rats. GEG supplementation increased microglia/macrophage marker CD11b gene expression in colon and spinal cord of DNP rats. GEG treatment increased abundance of Acinetobacter, Azospirillum, Colidextribacter, and Fournierella but decreased abundance of Muribaculum intestinale in cecal feces of rats. This study demonstrates that GEG supplementation decreased pain, anxio-depression, and neuroimmune cells, and improved the composition of gut microbiomes and mitochondrial function in rats with diabetic neuropathy.

The Gut Microbiota and Chronic Pain

PURPOSE OF REVIEW

To examine the effects and interactions between gut microbia and chronic pain.

RECENT FINDINGS

The gut microbiome has been an area of interest in both the scientific and general audience due to a growing body of evidence suggesting its influence in a variety of health and disease states. Communication between the central nervous system (CNS) and gut microbiome is said to be bidirectional, in what is referred to as the gut-brain axis. Chronic pain is a prevalent costly personal and public health burden and so, there is a vested interest in devising safe and efficacious treatments. Numerous studies, many of which are animal studies, have been conducted to examine the gut microbiome's role in the pathophysiology of chronic pain states, such as neuropathy, inflammation, visceral pain, etc. As the understanding of this relationship grows, so does the potential for therapeutic targeting of the gut microbiome in chronic pain.

Constipation and pain in Parkinson's disease: a clinical analysis

Parkinson's Disease (PD) is a neurodegenerative disorder characterized by both motor and non-motor symptoms (NMS). Among NMS, constipation and pain are both highly prevalent and debilitating affecting up to 80% of PD patients and impairing their quality of life. Here, we investigated the relationship between constipation and pain in PD patients. This is a retrospective study assessing the relationship between pain and constipation in a PD patient population from a clinical database of patients attending the outpatient clinic of the movement disorders division, Neurology Unit of Policlinico Tor Vergata, in Rome. Subjects were assessed with the Unified Parkinson's Disease Rating Scale (UPDRS) part III, Hoehn and Yahr (H&Y) stage, King's Parkinson's Disease Pain Scale (KPPS), Brief Pain Inventory (BPI), Non-Motor Symptoms Scale (NMSS) and Beck Depression Inventory (BDI). Patients were further divided in two groups (Group 1, 32 patients with constipation and Group 2, 35 PD patients without constipation) ANOVA and ANCOVA analysis were used to compare the two groups. PD patients with constipation had significantly higher pain severity and pain interference, as measured by the BPI scale and higher total KPPS score, fluctuation-related pain, nocturnal pain, and radicular pain when compared to PD patients without constipation. This study highlights for the first time a possible interplay between constipation and pain in PD that deserves further investigations.

Gut Microbiota Modulation and Its Implications on Neuropathic Pain: A Comprehensive Literature Review

Neuropathic pain (NP) is a chronic pain disorder arising from somatosensory nervous system impairment. Extensive evidence supports the notion that the gut microbiota (GM) is crucial in maintaining human health by performing vital tasks. At the same time, its disruption has been linked to the emergence and advancement of an expanding range of disorders, including NP, in which GM could play a role in its pathophysiology. The crosstalk between the nervous system and GM happens through immune mediators, metabolites, and nervous structures and involves both central and peripheral nervous systems. This literature review aims to thoroughly investigate the function of modulating GM in the treatment of NP. It will achieve this by integrating existing knowledge, identifying underlying mechanisms, and evaluating the possible clinical consequences of exploiting the gut-brain axis. We will cover the main therapeutic applications of the described GM-modulators, such as probiotics, faecal microbiota transplantation, dietary supplements and emotional support, to the main kinds of NP in which any evidence, even if only pre-clinical, has been unravelled in recent years. The explored NP areas include chemotherapy-induced peripheral neuropathy, diabetic neuropathy, trauma-induced neuropathic pain, trigeminal neuralgia, postherpetic neuralgia and low back pain.

The importance of the gut microbiome and its signals for a healthy nervous system and the multifaceted mechanisms of neuropsychiatric disorders

Increasing evidence links the gut microbiome and the nervous system in health and disease. This narrative review discusses current views on the interaction between the gut microbiota, the intestinal epithelium, and the brain, and provides an overview of the communication routes and signals of the bidirectional interactions between gut microbiota and the brain, including circulatory, immunological, neuroanatomical, and neuroendocrine pathways. Similarities and differences in healthy gut microbiota in humans and mice exist that are relevant for the translational gap between non-human model systems and patients. There is an increasing spectrum of metabolites and neurotransmitters that are released and/or modulated by the gut microbiota in both homeostatic and pathological conditions. Dysbiotic disruptions occur as consequences of critical illnesses such as cancer, cardiovascular and chronic kidney disease but also neurological, mental, and pain disorders, as well as ischemic and traumatic brain injury. Changes in the gut microbiota (dysbiosis) and a concomitant imbalance in the release of mediators may be cause or consequence of diseases of the central nervous system and are increasingly emerging as critical links to the disruption of healthy physiological function, alterations in nutrition intake, exposure to hypoxic conditions and others, observed in brain disorders. Despite the generally accepted importance of the gut microbiome, the bidirectional communication routes between brain and gut are not fully understood. Elucidating these routes and signaling pathways in more detail offers novel mechanistic insight into the pathophysiology and multifaceted aspects of brain disorders.

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.

Formate Might Be a Novel Potential Serum Metabolic Biomarker for Type 2 Diabetic Peripheral Neuropathy

Background

As one of the most frequent complications of type 2 diabetes mellitus (T2DM), diabetic peripheral neuropathy (DPN) shows a profound impact on 50% of patients with symptoms of neuropathic pain, numbness and other paresthesia. No valid serum biomarkers for the prediction of DPN have been identified in the clinic so far. This study is to investigate the potential serum biomarkers for DPN firstly based on 1H-Nuclear Magnetic Resonance (1H-NMR)-based metabolomics technique.

Methods

Thirty-six patients enrolled in this study were divided into two groups: 18 T2DM patients without DPN (T2DM group) and 18 T2DM patients with DPN (DPN group). Serum metabolites were measured via 1H-NMR spectroscopy. Bioinformatic approaches including principal component analysis (PCA), orthogonal partial least squares-discriminant analysis (OPLS-DA), independent sample t-test, Fisher's test, Pearson and Spearman correlation analysis, Stepwise multiple linear regression analysis and receiver operating characteristic (ROC) curve analysis were used to identify the potential altered serum biomarkers.

Results

A total of 20 metabolites were obtained and further analyzed. Formate was identified as the only potential biomarker that decreased in the DPN group with statistical significance after multiple comparisons (p<0.05). Formate also displayed a negative relationship with some elevated clinical markers in DPN. ROC curve analysis showed a good discriminative ability for formate in DPN with an area under the curve (AUC) value of 0.981.

Conclusion

Formate could be considered a potential serum metabolic biomarker for DPN. The reduced level of formate in DPN may be associated with mitochondrial dysfunction and gut microbiota alteration. Monitoring the level of serum formate would be an important strategy for the early diagnosis of DPN and a supplement of formate may be a promising treatment for DPN in the future.

Alterations of the gut microbiota in patients with postherpetic neuralgia

Postherpetic neuralgia (PHN) is a prevalent, intricate, and intractable form of neuropathic pain. The available evidence indicates that alterations in the gut microbiota are significant environmental determinants in the development of chronic neuropathic pain. Nevertheless, the correlation between the gut microbiota and PHN remains elusive. A cross-sectional study was performed on a cohort of 27 patients diagnosed with PHN and 27 matched healthy controls. Fecal samples were collected and subjected to microbiota analysis using 16S ribosomal RNA gene sequencing. Comparable levels of bacterial richness and diversity were observed in the gut microbiota of PHN patients and healthy controls. A significant difference was observed in 37 genera between the two groups. Furthermore, the LEfSe method revealed that the abundance levels of Escherichia-Shigella, Streptococcus, Ligilactobacillus, and Clostridia_UCG-014_unclassified were elevated in PHN patients, while Eubacterium_hallii_group, Butyricicoccus, Tyzzerella, Dorea, Parasutterella, Romboutsia, Megamonas, and Agathobacter genera were reduced in comparison to healthy controls. Significantly, the discriminant model utilizing the predominant microbiota exhibited efficacy in distinguishing PHN patients from healthy controls, with an area under the curve value of 0.824. Moreover, Spearman correlation analysis demonstrated noteworthy correlations between various gut microbiota and clinical symptoms, including disease course, anxiety state, sleep quality, heat pain, pain intensity, and itching intensity. Gut microbiota dysbiosis exists in PHN patients, microbiome differences could be used to distinguish PHN patients from normal healthy individuals with high sensitivity and specificity, and altered gut microbiota are related to clinical manifestations, suggesting potentially novel prevention and therapeutic directions of PHN.

Metabolomics profiling in predicting of post-herpetic neuralgia induced by varicella zoster

To explore potential metabolomics biomarkers in predicting post-herpetic neuralgia (PHN) induced by herpes zoster (HZ). A total of 90 eligible patients were prospectively enrolled and assigned into an acute pain (ACP) group and a PHN group. Serum samples were collected before clinical intervention to perform metabolomics profiling analyses using gas chromatography mass spectrometry (GC-MS). Key metabolites were identified using partial least squares discriminant analysis (PLS-DA). A binary logistic regression was used to build a combined biomarker model to predict PHN from ACP. The discriminating efficiency of the combined biomarker model was investigated and validated by internal validation. Six metabolites were identified as the key metabolites related to PHN. All these metabolites (N-Acetyl-5-hydroxytryptaMine, glucose, dehydroascorbic acid, isopropyl-beta-D-thiogalactopyranoside, 1,5-anhydro-D-sorbitol, and glutamic acid) were found elevated in the PHN group. Pathway analyses showed that glucose-alanine cycle, tryptophan metabolism, tyrosine metabolism, lactose degradation, malate-aspartate shuttle were top five metabolic pathways evolved in PHN. The AUC was 0.85 (95% CI 0.76-0.93) for the combined biomarker model, and was 0.91 (95% CI 0.84-1.00) for the internal validation data set to predict PHN. Metabolomics analyses of key metabolites could be used to predict PHN induced by HZ.

Fecal Microbiota Transplantation Alleviated Paclitaxel-Induced Peripheral Neuropathy by Interfering with Astrocytes and TLR4/p38MAPK Pathway in Rats

Purpose

Paclitaxel-induced peripheral neuropathy (PIPN) constitutes a refractory and progressive adverse consequence of paclitaxel treatment, causing pain and sensory anomalies in cancer survivors. Although the gut-brain axis is involved in multiple disorders including cancer, its impact on peripheral pain conditions remains elusive. Thus, we assessed the importance of gut microbiota and related mechanisms in PIPN.

Methods

By implementing fecal microbiota transplantation (FMT) in a rat PIPN model (ie, rats treated with paclitaxel; hereafter as PIPN rats), we explored the effect of gut microbiota on PIPN rats using multiple methods, including different behavioral tests, 16S ribosomal DNA (rDNA) sequencing, and biochemical techniques.

Results

Sequencing of 16S rDNA revealed that the abundance of genera Bacteroides and UCG-005 increased, while that of genera Turicibacter, Clostridium sensu stricto 1 and Corynebacterium decreased in the PIPN rats. However, when treated with FMT using fecal from normal rats, the mechanical allodynia and thermal hyperalgesia in PIPN rats were significantly alleviated. In addition, FMT treatment reduced the expression of toll-like receptor 4 (TLR4), phospho-p38 mitogen-activated protein kinase (p-p38MAPK), and the astrocytic marker glial fibrillary acidic protein in the colon and spinal dorsal horn. TAK242 (a TLR4 inhibitor) significantly alleviated the behavioral hypersensitivity of PIPN rats and inhibited the TLR4/p38MAPK pathway in astrocytes in these rats.

Conclusion

The gut microbiota played a critical role in PIPN. Future therapies treating PIPN should consider microbe-based treatment as an option.

The gut microbiota-neuroimmune crosstalk and neuropathic pain: a scoping review

Environmental stressors can disrupt the gut-brain relationship and alter the gut microbial composition, potentially leading to chronic pain, including neuropathic pain (NP). To understand this complex relationship, we conducted a systematic scoping review to examine the gut microbial-neuroimmune connection to NP and the potential therapeutic targets. The review includes English-language manuscripts in databases such as MEDLINE, Cochrane, and DOAJ between January 2000 and April 2022. Out of the 48 full texts examined, only 15 articles met the inclusion criteria. These included a randomised controlled trial involving 327 individuals, an in vitro, and 13 animal model studies. The findings suggest that the gut flora plays a role in the immunological, neurological, and metabolic signalling pathways associated with NP. Animal studies have been the primary focus in this area, indicating that an imbalanced-gut microbiome and subsequent activation of biochemical and neuro-immunologic pathways may influence the development of NP. This review provides a comprehensive summary of the gut microbiome-immune-NP axis and identifies potential therapeutic targets. However, since most of the evidence comes from animal studies, future research should include clinical trials to gain a better understanding of the role of gut microbiota in NP and discover new therapeutic strategies.

Hydrogen Sulfide Interacting with Cannabinoid 2 Receptors during Sciatic Nerve Injury-Induced Neuropathic Pain

Hydrogen sulfide (H₂S) donors make opioids more effective in inhibiting nociception during inflammatory and neuropathic pain. We examined whether the analgesic, anxiolytic and/or antidepressant actions of the cannabinoid 2 receptor (CB2R) agonist, JWH-133, might be improved by pretreatment with H₂S donors, DADS and GYY4137 in mice with sciatic nerve injury-provoked neuropathy (CCI). The reversion of the antinociceptive effects of these treatments with the CB2R antagonist, AM630, and the regulatory actions of H₂S in the phosphorylation of NF-κB inhibitor alpha (IKBα) and in the brain-derived neurotrophic factor (BDNF), CB2R, Nrf2 and heme oxygenase 1 (HO-1) levels in prefrontal cortex (PFC), ventral hippocampus (vHIP) and periaqueductal gray matter (PAG), were examined. Data showed that the analgesic effects of JWH-133, systemically and locally administered, were improved by the DADS or GYY4137 pretreatment. The co-treatment of GYY4137 with JWH-133 also stopped anxiodepressive-like activities that concur with neuropathy. Our data likewise showed that both H₂S donors normalized the inflammatory (p-IKBα), neurotrophic (BDNF) variations caused by CCI, increased the expression of CB2R and activated the Nrf2/HO-1 antioxidant pathway in PFC, v-HIP and/or PAG of animals with neuropathic pain. In addition, the blockade of the analgesia produced by high doses of DADS and GYY4137 with AM630 indicated the contribution of the endocannabinoid system in the effects of H₂S during neuropathic pain, thus supporting the positive interaction between H₂S and CB2R. Therefore, this study demonstrates the potential use of CB2R agonists combined with H₂S donors as a possible treatment for peripheral nerve injury-caused neuropathic pain and the associated emotional disturbances.

Alterations of endogenous pain-modulatory system of the cerebral cortex in the neuropathic pain

Neuropathic pain (NeP) remains a significant clinical challenge owing to insufficient awareness of its pathological mechanisms. We elucidated the aberrant metabolism of the cerebral cortex in NeP induced by the chronic constriction injury (CCI) using metabolomics and proteomics analyses. After CCI surgery, the values of MWT and TWL markedly reduced and maintained at a low level. CCI induced the significant dysregulation of 57 metabolites and 31 proteins in the cerebral cortex. Integrative analyses showed that the differentially expressed metabolites and proteins were primarily involved in alanine, aspartate and glutamate metabolism, GABAergic synapse, and retrograde endocannabinoid signaling. Targeted metabolomics and western blot analysis confirmed the alterations of some key metabolites and proteins in endogenous pain-modulatory system. In conclusion, our study revealed the alterations of endocannabinoids system and purinergic system in the CCI group, and provided a novel perspective on the roles of endogenous pain-modulatory system in the pathological mechanisms of NeP.

Proteomic and metabolomic approaches elucidate the molecular mechanism of emodin against neuropathic pain through modulating the gamma-aminobutyric acid (GABA)-ergic pathway and PI3K/AKT/NF-κB pathway

Neuropathic pain (NeP) is a major health concern. Due to the complex pathological mechanisms, management of NeP is challenging. Emodin, a natural anthraquinone derivative, exerts excellent analgesic effects. However, its mechanisms of action are still poorly understood. In this study, we investigated the mechanisms underlying pain-relief effects of emodin in the cerebral cortex using proteomic and metabolomic approaches. After 15 days of emodin administration, the mechanical withdrawal threshold (MWT) and thermal withdrawal latency (TWL) values in the emodin groups were significantly higher than those in the chronic constriction injury (CCI) group (p < .05), suggesting emodin treatment could reverse CCI-induced hyperalgesia. Emodin treatment evoked the expression alteration of 402 proteins (153 up-regulated and 249 down-regulated) in the CCI models, which were primarily involved in PI3K/AKT signaling pathway, gamma-aminobutyric acid (GABA) receptor signaling, complement and coagulation cascades, cGMP/PKG signaling pathway, MAPK signaling pathway, and calcium signaling pathway. In parallel, emodin intervention regulated the abundance alteration of 27 brain metabolites (20 up-regulated and 7 down-regulated) in the CCI rats, which were primarily implicated in carbon metabolism, biosynthesis of amino acids, pentose phosphate pathway, and glucagon signaling pathway. After a comprehensive analysis and western blot validation, we demonstrated that emodin alleviated NeP mainly through regulating GABAergic pathway and PI3K/AKT/NF-κB pathway.

Impact of Gut Microbiota on the Peripheral Nervous System in Physiological, Regenerative and Pathological Conditions

It has been widely demonstrated that the gut microbiota is responsible for essential functions in human health and that its perturbation is implicated in the development and progression of a growing list of diseases. The number of studies evaluating how the gut microbiota interacts with and influences other organs and systems in the body and vice versa is constantly increasing and several 'gut-organ axes' have already been defined. Recently, the view on the link between the gut microbiota (GM) and the peripheral nervous system (PNS) has become broader by exceeding the fact that the PNS can serve as a systemic carrier of GM-derived metabolites and products to other organs. The PNS as the communication network between the central nervous system and the periphery of the body and internal organs can rather be affected itself by GM perturbation. In this review, we summarize the current knowledge about the impact of gut microbiota on the PNS, with regard to its somatic and autonomic divisions, in physiological, regenerative and pathological conditions.

Microbiological and Physiological Effects of Pain

Pain is an important innate defense mechanism that can dramatically alter a person's quality of life. Understanding the microbiological and physiological effects of pain may be important in the pursuit of novel pain interventions. The three descriptors of pain recognized by the International Association for the Study of Pain are nociceptive, neuropathic, and nociplastic pain. Our review examined the current understanding of all three pain types, focusing on the key molecules involved in the manifestation of each type as well as physiological effects. Additionally, we compared the differences in painful and painless neuropathies and discussed the neuroimmune interaction involved in the manifestation of pain.

Pain-resolving immune mechanisms in neuropathic pain

Interactions between the immune and nervous systems are of central importance in neuropathic pain, a common and debilitating form of chronic pain caused by a lesion or disease affecting the somatosensory system. Our understanding of neuroimmune interactions in pain research has advanced considerably. Initially considered as passive bystanders, then as culprits in the pathogenesis of neuropathic pain, immune responses in the nervous system are now established to underpin not only the initiation and progression of pain but also its resolution. Indeed, immune cells and their mediators are well-established promoters of neuroinflammation at each level of the neural pain pathway that contributes to pain hypersensitivity. However, emerging evidence indicates that specific subtypes of immune cells (including antinociceptive macrophages, pain-resolving microglia and T regulatory cells) as well as immunoresolvent molecules and modulators of the gut microbiota-immune system axis can reduce the pain experience and contribute to the resolution of neuropathic pain. This Review provides an overview of the immune mechanisms responsible for the resolution of neuropathic pain, including those involved in innate, adaptive and meningeal immunity as well as interactions with the gut microbiome. Specialized pro-resolving mediators and therapeutic approaches that target these neuroimmune mechanisms are also discussed.

Brachial plexus avulsion induced changes in gut microbiota promotes pain related anxiety-like behavior in mice

Introduction

Brachial plexus avulsion (BPA) injury develops frequent and intense neuropathic pain, involving in both peripheral and central nervous systems. The incidence of anxiety or depression caused by BPA-induced neuropathic pain is high, but the underlying mechanism remains unclear.

Methods

We established a BPA mice model and assessed its negative emotions through behavioral tests. To further explore the role of the microbiota-gut-brain axis in the unique emotional behavior after BPA, we performed intestinal fecal 16s and metabolomics assays. Psychobiotics (PB) supplementation was administered to BPA mice to check the probiotics effects on BPA-induced anxiety behaviors.

Results

Pain related anxiety-like behavior was observed at the early stage after BPA (7 days), while no depression-like behavior was detected. Intriguingly, gut microbiota diversity was increased in BPA mice, and the most abundant probiotics, Lactobacillus, showed obvious changes. Lactobacillus_reuteri was significantly decreased in BPA mice. Metabolomics analysis showed that Lactobacillus_reuteri-related bile acid pathway and some neurotransmitter amino acids were significantly altered. Further PB (dominated by Lactobacillus_reuteri) supplementation could significantly relieve BPA-induced anxiety-like behaviors in mice.

Conclusion

Our study suggests that pathological neuralgia after BPA could alter intestinal microbiota diversity, especially Lactobacillus, and the changes in neurotransmitter amino acid metabolites may be the key reason for the onset of anxiety-like behaviors in BPA mice.

Altered serum bile acid profile in fibromyalgia is associated with specific gut microbiome changes and symptom severity

ABSTRACT

Alterations in the composition and function of the gut microbiome in women with fibromyalgia have recently been demonstrated, including changes in the relative abundance of certain bile acid-metabolizing bacteria. Bile acids can affect multiple physiological processes, including visceral pain, but have yet to be explored for association to the fibromyalgia gut microbiome. In this study, 16S rRNA sequencing and targeted metabolomic approaches were used to characterize the gut microbiome and circulating bile acids in a cohort of 42 women with fibromyalgia and 42 healthy controls. Alterations in the relative abundance of several bacterial species known to metabolize bile acids were observed in women with fibromyalgia, accompanied by significant alterations in the serum concentration of secondary bile acids, including a marked depletion of α-muricholic acid. Statistical learning algorithms could accurately detect individuals with fibromyalgia using the concentration of these serum bile acids. Serum α-muricholic acid was highly correlated with symptom severity, including pain intensity and fatigue. Taken together, these findings suggest serum bile acid alterations are implicated in nociplastic pain. The changes observed in the composition of the gut microbiota and the concentration of circulating secondary bile acids seem congruent with the phenotype of increased nociception and are quantitatively correlated with symptom severity. This is a first demonstration of circulating bile acid alteration in individuals with fibromyalgia, potentially secondary to upstream gut microbiome alterations. If corroborated in independent studies, these observations may allow for the development of molecular diagnostic aids for fibromyalgia as well as mechanistic insights into the syndrome.

Impact of gut-peripheral nervous system axis on the development of diabetic neuropathy

Diabetes is a chronic metabolic disease caused by a reduction in the production and/or action of insulin, with consequent development of hyperglycemia. Diabetic patients, especially those who develop neuropathy, presented dysbiosis, with an increase in the proportion of pathogenic bacteria and a decrease in the butyrate-producing bacteria. Due to this dysbiosis, diabetic patients presented a weakness of the intestinal permeability barrier and high bacterial product translocation to the bloodstream, in parallel to a high circulating levels of pro-inflammatory cytokines such as TNF-α. In this context, we propose here that dysbiosis-induced increased systemic levels of bacterial products, like lipopolysaccharide (LPS), leads to an increase in the production of pro-inflammatory cytokines, including TNF-α, by Schwann cells and spinal cord of diabetics, being crucial for the development of neuropathy.

An unexpected connection: A narrative review of the associations between Gut Microbiome and Musculoskeletal Pain

PURPOSE

Multiple diverse factors contribute to musculoskeletal pain, a major cause of physical dysfunction and health-related costs worldwide. Rapidly growing evidence demonstrates that the gut microbiome has overarching influences on human health and the body's homeostasis and resilience to internal and external perturbations. This broad role of the gut microbiome is potentially relevant and connected to musculoskeletal pain, though the literature on the topic is limited. Thus, the literature on the topic of musculoskeletal pain and gut microbiome was explored.

METHODS

This narrative review explores the vast array of reported metabolites associated with inflammation and immune-metabolic response, which are known contributors to musculoskeletal pain. Moreover, it covers known modifiable (e.g., diet, lifestyle choices, exposure to prescription drugs, pollutants, and chemicals) and non-modifiable factors (e.g., gut architecture, genetics, age, birth history, and early feeding patterns) that are known to contribute to changes to the gut microbiome. Particular attention is devoted to modifiable factors, as the ultimate goal of researching this topic is to implement gut microbiome health interventions into clinical practice.

RESULTS

Overall, numerous associations exist in the literature that could converge on the gut microbiome's pivotal role in musculoskeletal health. Particularly, a variety of metabolites that are either directly produced or indirectly modulated by the gut microbiome have been highlighted.

CONCLUSION

The review highlights noticeable connections between the gut and musculoskeletal health, thus warranting future research to focus on the gut microbiome's role in musculoskeletal conditions.

Common bacterial metabolite indole directly activates nociceptive neuron through transient receptor potential ankyrin 1 channel

ABSTRACT

Nociceptors are known to directly recognize bacterial cell wall components or secreted toxins, thereby leading to pain induced by bacterial infection. However, direct activation of nociceptors by bacterial metabolites remains unclear although bacteria produce numerous metabolites related to health and disease. In this study, we investigated whether and how a common bacterial metabolite, indole, which is produced by normal microflora of the gastrointestinal tract and oral cavity, can directly activate nociceptive sensory neurons. We found that indole elicits calcium response and evokes inward currents in subsets of dorsal root ganglia (DRG) neurons. Intraplantar (i.pl.) injection of indole produced nocifensive behaviors in adult mice, which were enhanced in complete Freund's adjuvant-induced chronic inflammatory condition. Indole increased calcitonin gene-related peptide release in DRG neurons, and i.pl. injection of indole increased hind paw thickness, suggesting its role in generation of neurogenic inflammation. These in vitro and in vivo indole-induced responses were pharmacologically blocked by transient receptor potential ankyrin 1 (TRPA1) antagonist, HC-030031, and significantly abolished in TRPA1 knockout (KO) mice, indicating that indole targets TRPA1 for its action in DRG neurons. Nocifensive licking behavior induced by the injection of live Escherichia coli was significantly decreased in tryptophanase mutant (TnaA KO) E. coli- injected mice that lack indole production, further supporting the idea that bacteria-derived indole can induce pain during infection. Identifying the mechanism of action of indole through TRPA1 provides insights into bacteria-neuron interactions and the role of bacterial metabolites in pain signaling, especially in inflammation-accompanied bacterial infection.

Hydrogen Sulfide Increases the Analgesic Effects of µ- and δ-Opioid Receptors during Neuropathic Pain: Pathways Implicated

Recent studies have revealed that hydrogen sulfide (H₂S) increases the analgesic actions of the δ-opioid receptor (DOR) in inflammatory pain. However, the possible improvement of the analgesia of μ-opioid receptor (MOR) and DOR agonists during neuropathic pain, through pretreatment with two slow-releasing H₂S donors—DADS (diallyl disulfide) and GYY4137 (morpholin-4-ium 4-methoxyphenyl(morpholino) phosphinodithioate dichloromethane complex)—is still unknown. In male C57BL/6J mice with neuropathic pain incited by chronic constriction of the sciatic nerve (CCI), we evaluated: (1) the influence of DADS (3.5 mg/kg) and GYY4137 (0.7 mg/kg) on the inhibition of the allodynia and hyperalgesia produced by the systemic or local administration of morphine (3 mg/kg or 65 µg) and UFP-512 (1 mg/kg or 12.5 µg); (2) the reversion of the antinociceptive actions of high doses of DADS (30 mg/kg) and GYY4137 (24 mg/kg) with MOR and DOR antagonists; and (3) the effects of H₂S donors on oxidative stress, apoptotic responses, and MOR and DOR expression in the medial septum (MS) and dorsal root ganglia (DRG). The results revealed that both DADS and GYY4137 improved the antiallodynic effects of morphine and UFP-512, possibly by up-regulating MOR and DOR expression in DRG. The administration of MOR and DOR antagonists blocked the analgesic properties of DADS and GYY4137, revealing the feasible participation of the endogenous opioid system in H₂S analgesic effects. Moreover, both H₂S donors inhibited oxidative stress and apoptosis generated by CCI in the MS and/or DRG. This study suggests the co-treatment of H₂S donors with MOR or DOR agonists as a potential therapy for neuropathic pain.

The pathogenesis and treatment mechanism of Parkinson's disease from the perspective of traditional Chinese medicine

BACKGROUND

Parkinson's disease (PD) is the second most common neurodegenerative disease with no treatment currently available to modify its progression. Traditional Chinese medicine (TCM) has gained attention for its unique theoretical basis and clinical effects. Many studies have reported on the clinical effects and pharmacological mechanisms of Chinese herbs in PD. However, few studies have focused on the treatment mechanisms of anti-PD TCM drugs from the perspective of TCM itself.

PURPOSE

To elaborate the treatment mechanisms of anti-PD TCM drugs in the perspective of TCM.

METHODS

We performed a literature survey using traditional books of Chinese medicine and online scientific databases including PubMed, Web of Science, Google Scholar, China National Knowledge Infrastructure (CNKI), and others up to July 2021.

RESULTS

TCM theory states that PD is caused by a dysfunction of the zang-fu organs (liver, spleen, kidney, and lung) and subsequent pathogenic factors (wind, fire, phlegm, and blood stasis). Based on the pathogenesis, removing pathogenic factors and restoring visceral function are two primary treatment principles for PD in TCM. The former includes dispelling wind, clearing heat, resolving phlegm, and promoting blood circulation, while the latter involves nourishing the liver and kidney and strengthening the spleen. The anti-PD mechanisms of the active ingredients of TCM compounds and herbs at different levels include anti-apoptosis, anti-inflammation, and anti-oxidative stress, as well as the restoration of mitochondrial function and the regulation of autophagy and neurotransmitters.

CONCLUSION

Chinese herbs and prescriptions can be used to treat PD by targeting multiple pharmacological mechanisms.

Role of gut microbiota in neuropathy and neuropathic pain states: A systematic preclinical review

Gut microbiota has implications in Central Nervous System (CNS) disorders. Our study systematically identified preclinical studies aimed to investigate the possible gut microbiota contribution in neuropathy and neuropathic pain. The systematic review is reported in accordance with PRISMA checklist and guidelines outlined updated to 2020. We included research articles reporting neuropathy-related behavioral evaluations and/or neurological scores coupled to gut microbiota analysis performed by high-throughput technologies in the last ten years. Two investigators performed a search through 3 electronic bibliographic databases for full-text articles (PubMed, Scopus, and EMBASE) and three registries (Prospero, SyRF, and bioRxiv), cross-references, and linear searches. We assessed the methodological quality via the CAMARADES checklist and appraised the heterogeneous body of evidence by narrative synthesis. In total, there were 19 eligible studies. The most of these reports showed significant changes in gut microbiota setting in neuropathy conditions. The major gut microbiome remodeling was through fecal microbiome transplantation. Mechanistic proof of the gut-CNS communication was achieved by measuring inflammatory mediators, metabolic products, or neurotransmitters. As a limitation, we found considerable heterogeneity across eligible studies. We conclude that the current understanding of preclinical findings suggested an association between neuropathy and/or neuropathic pain and gut microbiota modifications. Our analysis provides the basis for further studies targeting microbiota for managing symptoms of neuropathy or other neuroinflammation-based CNS disorders. The systematic review protocol was registered on the international database Prospero under the registration number (# 257628).

Intracellular and extracellular untargeted metabolomics reveal the effect of acute uranium exposure in HK-2 cells

Uranium exposure poses a serious threat to the health of occupational populations and the public. Although metabolomics is a promising research approach to study the toxicological mechanisms of uranium exposure, in vitro studies using human cells are scarce. Applying cultured cell metabolomics, we exhaustively analyzed the intracellular and extracellular differential metabolites upon uranium exposure and characterized the possible biological effects of uranium exposure on human kidney cells. Uranium exposure significantly induced disturbance in the amino acid biosynthesis and linoleic acid metabolism of the cells. Cells exposed to uranium produce excessive amounts of arachidonic acid, which has the potential to cause oxidative stress and damage cells. The results provide new evidence for an oxidative stress mechanism of uranium-induced renal cell injury. Cell metabolomics has proven to be a useful diagnostic tool to study the molecular mechanisms of uranium poisoning.

Gut Microbiome in Anesthesiology and Pain Medicine

The gut microbiome plays critical roles in human health and disease. Recent studies suggest it may also be associated with chronic pain and postoperative pain outcomes. In animal models, the composition of the gut microbiome changes after general anesthesia and affects the host response to medications, including anesthetics and opioids. In humans, the gut microbiome is associated with the development of postoperative pain and neurocognitive disorders. Additionally, the composition of the gut microbiome has been associated with pain conditions including visceral pain, nociplastic pain, complex regional pain syndrome, and headaches, partly through altered concentration of circulating bacterial-derived metabolites. Furthermore, animal studies demonstrate the critical role of the gut microbiome in neuropathic pain via immunomodulatory mechanisms. This article reviews basic concepts of the human gut microbiome and its interactions with the host and provide a comprehensive overview of the evidence linking the gut microbiome to anesthesiology, critical care, and pain medicine.