Inflammation in pathogenesis of chronic pain: Foe and friend

Proposition of Two Subtypes of Patients at Risk of Suicide: Pain Hypersensitive Vs. Dissociative

PURPOSE OF REVIEW

The pain-suicide relationship is one of the most debated in recent literature, but theories and clinical evidence have often reached contrasting conclusions. Through a critical overview of theoretical, meta-analytical and empirical contributions, we aimed at advancing the conversation on the pain-suicide relationship by integrating research on related concepts, specifically inflammation and dissociation, and their effects on interoceptive processes and pain perception.

RECENT FINDINGS

Ideation-to-action theories consider increased pain tolerance a key risk factor for the transition from suicidal ideation to attempt. However, several meta-analytical findings suggest that suicidal thoughts and behaviors are associated with inflammation-induced pain sensitization. On the one hand, inflammation contributes to the development and maintenance of chronic pain conditions and mood disorders, and is associated with interoceptive hypervigilance and pain hypersensitivity. Moreover, a trait of increased pain tolerance does not seem to distinguish the individuals attempting suicide among those living with suicidal thoughts. On the other, temporary hypoalgesia is often activated by dissociative experiences. Highly dissociative individuals can indeed be exposed to frequent disintegration of interoceptive processes and transitory hyposensitivity to pain. In light of this, two different patterns of responses to stress (i.e. inflammation vs. dissociation) may characterize different kinds of patients at risk of suicide, associated with specific patterns of interoceptive functioning, pain sensitivity and possibly suicidal ideation. This proposition is partially supported by neuroimaging studies on post-traumatic stress disorder and psychodynamic perspectives on neurodevelopment, as well as alternative clustering models of suicidal behavior. Theoretical, meta-analytical and neurobiological evidence highlight two opposite directions in the pain-suicide relationship: hyper- vs. hyposensitivity. Such contrasts may be explained by the existence of two tendencies in stress-response, namely inflammation and dissociation, defining two different subtypes of patients at risk of suicide. We thus propose the existence of a hypersensitive subtype, defined by underlying neuroinflammatory processes, increased vulnerability to chronic pain and mood disorders, interoceptive hypervigilance, pain hypersensitivity and potentially more persistent suicidal ideation. We further hypothesize a dissociative subtype, characterized by greater trait dissociation, vulnerability to depersonalization and derealization, frequent disintegration of interoceptive processes, transient pain hyposensitivity and abrupt peaks in suicidal ideation.

Clinical Benefits of Ibuprofen Arginine: A Narrative Review

Ibuprofen arginine (IBA) combines well-established analgesic and anti-inflammatory properties with enhanced pharmacokinetics. The addition of arginine significantly improves solubility and absorption, leading to a faster onset of action compared to conventional ibuprofen. Clinical studies consistently demonstrate that IBA achieves meaningful pain relief within a shorter timeframe while maintaining a favorable safety profile. IBA's rapid action is particularly valuable in managing acute exacerbations of chronic pain and preventing central sensitization, thus improving patient comfort, adherence, and overall quality of life. By addressing both the inflammatory and nociceptive components of pain, IBA offers an effective and well-tolerated alternative in multimodal pain management strategies. This review explores the clinical benefits of IBA in pain management among various clinical settings.

Structure-activity relationship studies and pharmacological evaluation of 4-phenylthiazoles as dual soluble epoxide hydrolase/fatty acid amide hydrolase inhibitors

Forty-two 4-phenylthiazole analogs, organized in two libraries 4a-u and 6a-u, were prepared and biologically evaluated in human fatty acid amide hydrolase (FAAH), and human, rat and mouse soluble epoxide hydrolase (sEH) inhibition assays. This structure-activity relationship (SAR) study explores the impact of electronic and steric changes on the molecule's potency and binding affinity to better understand the structural features important for dual sEH/FAAH inhibition which will guide the development of novel treatments for pain and inflammation. Our SAR revealed that electron-donating groups on the aromatic ring of the 4-phenylthiazole moiety are particularly well tolerated by both enzymes when placed at the ortho, meta and para positions; however, the overall 3D shape of the molecule is very important for the potent FAAH inhibition, suggesting more restricted size of the FAAH binding pocket compared to sEH binding pocket. Two selected dual inhibitors, 4p and 4s, were tested in the rat liver microsomes stability assays and evaluated in vivo in the formalin test. Systemic administration of 4p and 4s via intraperitoneal injection decreased nociceptive behavior (i.e., licking of the injected paw) in male rats, and this effect was dose-dependent for both compounds. Two doses, 1 and 3 mg/kg of 4p, decreased nociceptive behavior to a similar extent to that of 30 mg/kg ketoprofen, a traditional nonsteroidal anti-inflammatory drug. However, only 3 mg/kg of 4s decreased nociceptive behavior compared to vehicle-treated animals, and this effect was comparable to ketoprofen-treated animals. Taken together, these findings reveal the antinociceptive potential of 4-phenylthiazole-based dual FAAH and sEH inhibitors and suggest pharmacodynamic differences within this class of inhibitors despite similar potencies in vitro.

Role of efferocytosis in chronic pain -- From molecular perspective

The complex nature of pain pathophysiology complicates the establishment of objective diagnostic criteria and targeted treatments. The heterogeneous manifestations of pain stemming from various primary diseases contribute to the complexity and diversity of underlying mechanisms, leading to challenges in treatment efficacy and undesirable side effects. Recent evidence suggests the presence of apoptotic cells at injury sites, the distal dorsal root ganglia (DRG), spinal cord, and certain brain regions, indicating a potential link between the ineffective clearance of dead cells and debris and pain persistence. This review highlights recent research findings indicating that efferocytosis plays a significant yet often overlooked role in lesion expansion while also representing a potentially reversible impairment that could be targeted therapeutically to mitigate chronic pain progression. We examine recent advances into how efferocytosis, a process by which phagocytes clear apoptotic cells without triggering inflammation, influences pain initiation and intensity in both human diseases and animal models. This review summarizes that efferocytosis contributes to pain progression from the perspective of defective and inefficient efferocytosis and its subsequent secondary necrocytosis, cascade inflammatory response, and the shift of phenotypic plasticity and metabolism. Additionally, we investigate the roles of newly discovered genetic alterations or modifications in biological signaling pathways in pain development and chronicity, providing insights into innovative treatment strategies that modulate efferocytosis, which are promising candidates and potential avenues for further research in pain management and prevention.

Further evidence that peritraumatic 17β-estradiol levels influence chronic posttraumatic pain outcomes in women, data from both humans and animals

ABSTRACT

Chronic posttraumatic pain (CPTP) is common after traumatic stress exposure (TSE) and disproportionately burdens women. We previously showed across 3 independent longitudinal cohort studies that, in women, increased peritraumatic 17β-estradiol (E2) levels were associated with substantially lower CPTP over 1 year. Here, we assessed this relationship in a fourth longitudinal cohort and also assessed the relationship between E2 and CPTP at additional time points post-TSE. Furthermore, we used a well-validated animal model of TSE to determine whether exogenous E2 administration protects against mechanical hypersensitivity. Using nested samples and data from the Advancing Understanding of RecOvery afteR traumA study (n = 543 samples, 389 participants), an emergency department-based prospective study of TSE survivors, we assessed the relationship between circulating E2 levels and CPTP in women and men using multivariate repeated-measures mixed modeling. Male and ovariectomized female Sprague Dawley rats were exposed to TSE and administered E2 either immediately after or 3 days post-TSE. Consistent with previous results, we observed an inverse relationship between peritraumatic E2 and longitudinal CPTP in women only (β = -0.137, P = 0.033). In animals, E2 protected against mechanical hypersensitivity in female ovariectomized rats only if administered immediately post-TSE. In conclusion, peritraumatic E2 levels, but not those at post-TSE time points, predict CPTP in women TSE survivors. Administration of E2 immediately post TSE protects against mechanical hypersensitivity in female rats. Together with previous findings, these data indicate that increased peritraumatic E2 levels in women have protective effects against CPTP development and suggest that immediate post-TSE E2 administration in women could be a promising therapeutic strategy for reducing risk of CPTP.

Targeting glial activation to mitigate mirror-image and extraterritorial neuropathic pain in a CCI model of neuropathic pain in male rats

Neuropathic pain (NP) arises from nerve damage or compression and often extends to the contralateral side of the body (mirror-image pain, MP) and adjacent non-injured areas (extraterritorial pain). This study investigates whether altered sensitivity in these contralateral and peripheral regions is mediated by glial cells, using the chronic constriction injury (CCI) model of NP. Thirty-two male Wistar rats were randomly assigned into four groups (8/group): sham, CCI + vehicle, CCI + minocycline (MIN;10 mg/kg), and CCI + pentoxifylline (PTX;8 mg/kg). The CCI model was employed for NP induction. MIN and PTX were administered intraperitoneally from postoperative days (POD)4 to POD14, once daily. Pain responses were assessed on POD0, 2, 6, 10, and14 using Hargreaves, von Frey, and Tail-flick tests. Western blot analysis was performed on POD14 to measure Iba1 and GFAP protein expression in the spinal cord hemispheres. Results revealed that post-injury treatment with MIN and PTX significantly reduced contralateral thermal hyperalgesia, mechanical allodynia, and tail-flick responses. Correspondingly, the contralateral spinal cord exhibited significantly decreased GFAP and Iba1 protein expression compared to the CCI + vehicle treated group. These findings suggest that post-injury glial cell inhibition effectively mitigates neuropathic pain and prevents the development of MP and extraterritorial pain. This highlights the potential for clinical applications targeting glial cells to manage NP even after nerve injury.

Inflammatory pain and electroacupuncture: how the P2X3 receptor can help modulate inflammation-a review of current literature

AIM

Inflammatory pain arises from tissue stress or injury and is initiated by signaling molecules that stimulate the immune and nervous systems. Evidence suggests that purinergic signaling pathways can modulate pain and inflammation through the activation of P1 and P2 purinergic receptors, such as the P2X3 receptor, which are stimulated by extracellular molecules like adenosine triphosphate (ATP). Electroacupuncture (EA) exhibits precise mechanisms that modulate inflammatory pain through the activation of the P2X3 receptor.

OBJECTIVE

This review analyzed evidence regarding the role of electroacupuncture and the purinergic system, particularly the P2X3 receptor, in modulating inflammation and pain.

MATERIALS AND METHODS

A search for the most relevant articles available in the SciVerse Scopus and MEDLINE/PubMed databases was conducted for publications from 1995 to 2024. Articles were initially selected by reading the title, abstract, and main text, respectively.

RESULTS

It was found that the P2X3 receptor, as well as the molecules activating purinergic receptors, such as ATP and adenosine, have the potential to regulate pain and inflammation. Additionally, EA can modulate the purinergic system in an anti-inflammatory response. EA may stimulate analgesia mainly through the conversion of ATP to adenosine, a crucial molecule in pain control.

CONCLUSION

The purinergic system directly influences inflammatory pain and controls inflammation. In this context, EA has the potential to orchestrate this system to control pain and inflammation.

Association of visceral adiposity index and chronic pain in US adults: a cross-sectional study

The Visceral Obesity Index (VAI) is utilized as a metric employed to assess the distribution of abdominal adipose tissue as well as the functional status of adipose tissue. Nevertheless, the interplay between VAI and persistent pain has yet to be investigated. This cross-sectional analysis investigated the relationship between VAI and persistent pain among 1357 American adults from NHANES data. A logarithmic transformation of VAI was performed to adjust for skewness. Following the adjustment for relevant variables, logistic regression analysis showed a noteworthy association between VAI and chronic pain, suggesting that higher VAI values may be linked to an increased prevalence of persistent pain. Curve fitting analysis revealed a nonlinear correlation, with a breakpoint at a VAI value of 0.18. For VAI values below this threshold, each unit increase was notably correlated with an elevated prevalence of persistent pain, while increases in VAI beyond this threshold did not show a significant impact on chronic pain prevalence. Subgroup analyses indicated that the VAI may serve as a relatively independent risk factor for persistent pain. These findings highlight the possibility of incorporating abdominal adipose modification into pain management approaches and emphasize the critical importance of monitoring visceral fat accumulation to better identify patients more susceptible to chronic pain.

Physical health correlates of co-occurring chronic pain and PTSD Among U.S. Veterans: Results from a nationally representative study

Co-occurring chronic pain and posttraumatic stress disorder (PTSD) affects between 4%-8% of the U.S. veteran population. A large body of research has documented the psychosocial burden of the co-occurrence, but fewer studies have examined physical health conditions and functioning. The current study aimed to address this gap by examining physical health conditions and functioning of co-occurring chronic pain and PTSD. Data were analyzed from the National Health and Resilience in Veterans Study, which surveyed a nationally representative sample of 4,069 (age=62.2 years) U.S. veterans in 2019-2020. The sample was predominantly male (N = 3,564; 90.2%) and non-Hispanic White (N = 3,318; 78.0%). PTSD diagnoses were determined using the PTSD Checklist for DSM-5 and chronic pain and other physical health conditions using self-report and validated measures. Logistic regression analyses revealed that veterans with co-occurring chronic pain and PTSD were generally more likely to have a physical health condition (ORs 2.79-9.53) or a cognitive disorder (OR=16.55) relative to controls (ORs 2.79-9.53). ANCOVA analyses revealed that relative to veterans with chronic pain or PTSD only, those with both chronic pain and PTSD had worse physical functioning across several domains (ds=0.32-1.13). Results underscore the importance of approaching co-occurring chronic pain and PTSD using a whole person approach to care that targets both physical and mental health concerns.

Botulinum toxin effects on biochemical biomarkers related to inflammation-associated head and neck chronic conditions: a systematic review of clinical research

Botulinum toxin type A (BoNT) has emerged as a potential alternative to conventional therapies to many debilitating chronic diseases characterised by inflammatory states. However, the biological rationale remains ambiguous. Our review aimed to systematically assessed which biochemical biomarkers have been reported in clinical research to evaluate BoNT analgesic and mood-lifting effects in head and neck chronic conditions related to inflammation. We searched databases and registries between inception and September 29, 2023. Of the nine included studies, there were concerns about risk of bias for six studies. The leading biomarker with five studies was the calcitonin gene-related peptide (CGRP), followed by serotonin with two studies. Oxidative stress biomarkers were only reported in one study. Several important players in inflammatory processes and different immune cell classes have been evaluated in four studies. There was only one trial measuring changes in beta Tubulin and SNAP-25, and another study evaluating cutaneous neuropeptide substance-P. After BoNT, a significant effect was reported in six studies, including decrease in plasma levels of CGRP in chronic migraine and trigeminal neuralgia; serotonin decrease when collected from human tears in refractory intractable dry eye disease and increase in peripheral blood platelets in painful cervical dystonia associated to depression and anxiety; decrease in plasma concentration of markers of oxidative damage to proteins and increase in biomarkers for antioxidant power; decrease in expression of gene sets involved in inflammatory pathways and immune cells classes in the periosteum and metalloproteinase-9 molecule in the tears. BoNT seems to affect some biomarkers present in chronic inflammatory conditions. However, the certainty evidence found was very low to moderate. This study is registered on PROSPERO (CRD42023432131).

Weight discrimination partially mediates the longitudinal relationship between Body Mass Index and pain

Pain is common among individuals with high Body Mass Index (BMI). This study investigated weight discrimination as a mediator of the longitudinal relationship between BMI and the presence of moderate/severe pain among adults from the English Longitudinal Study of Ageing (ELSA) cohort. ELSA is a longitudinal study of middle-aged and older adults living in England. Data were taken from three consecutive waves. Demographics, BMI, and self-reported pain were collected at wave 4 (2008-2009), perceived weight discrimination at wave 5 (2010-2011,) and pain (no/mild vs moderate/severe) at wave 6 (2012-2013). Generalized linear models estimated the association of BMI at wave 4 (assessed continuously and dichotomized to obesity status BMI ≥30) with pain at wave 6, and weight discrimination at wave 5 as a mediator of this relationship. Models were adjusted for demographics and baseline pain. Complete data were available for n=5362 individuals. Continuous BMI was associated with future moderate/severe pain (b=1.05 SE=0.01, p<.01) and weight discrimination partially mediated this effect (b=1.00 SE=0.002, p=.05). Obesity status was also significant (b=1.43 SE=0.11, p<.01) but weight discrimination did not mediate the relationship (p=.13). Post hoc analyses were conducted among individuals with no/mild pain at baseline (n=3979.) BMI and obesity status were both associated with pain and weight discrimination mediated both pathways (ps<.05.) Weight discrimination partially mediated the relationship between BMI and future moderate/severe pain, especially among individuals with no/mild pain initially. Weight discrimination may be an overlooked contributor to the transition to more severe pain among individuals of higher body weight. PERSPECTIVE: Weight discrimination may be an overlooked contributor to pain among individuals of higher body weight, particularly transition from lower to higher impact states. Post hoc analyses indicate the effect may be specific, as other forms of discrimination did not mediate the relationship.

Rare variant association studies: Significance, methods, and applications in chronic pain studies

Rare genetic variants, characterized by their low frequency in a population, have emerged as essential components in the study of complex disease genetics. The biology of rare variants underscores their significance, as they can exert profound effects on phenotypic variation and disease susceptibility. Recent advancements in sequencing technologies have yielded the availability of large-scale sequencing data such as the UK Biobank whole-exome sequencing (WES) cohort empowered researchers to conduct rare variant association studies (RVASs). This review paper discusses the significance of rare variants, available methodologies, and applications. We provide an overview of RVASs, emphasizing their relevance in unraveling the genetic architecture of complex diseases with special focus on chronic pain and Arthritis. Additionally, we discuss the strengths and limitations of various rare variant association testing methods, outlining a typical pipeline for conducting rare variant association. This pipeline encompasses crucial steps such as quality control of WES data, rare variant annotation, and association testing. It serves as a comprehensive guide for researchers in the field of chronic pain diseases interested in rare variant association studies in large-scale sequencing datasets like the UK Biobank WES cohort. Lastly, we discuss how the identified variants can be further investigated through detailed experimental studies in animal models to elucidate their functional impact and underlying mechanisms.

Astragalin inhibits neuronal excitability and activates neuronal autophagy in the ACC and LH of CFA mice to alleviate inflammatory pain and pain-related emotions

Astragalin (AST), a natural flavonoid, exhibits anti-inflammatory, anti-cancer, and antioxidant properties. However, its effects and molecular mechanisms in inflammatory pain remain unclear. Therefore, this study aims to investigate the impact of AST on a Complete Freund's Adjuvant (CFA)-induced inflammatory pain mouse model and to elucidate its potential mechanisms. We employed behavioral tests, including the paw withdrawal test (PWT) and open field test (OFT), to assess pain thresholds and emotional changes in mice, while ELISA was utilized to measure the expression of inflammatory factors. Western blot analysis was performed to evaluate the expression of autophagy-related proteins, c-Fos, and pathway-related proteins. Additionally, immunofluorescence staining was conducted to assess the co-localization of neurons with autophagy-related factors and c-Fos. Our findings indicate that AST significantly reduces pain sensitivity and anxiety-like behaviors in CFA mice, similar to the analgesic Naproxen (NAP). AST treatment inhibited the expression of c-Fos, a neuronal excitability marker, in the ACC and LH of CFA mice, while upregulating the expression of autophagy-related proteins. Furthermore,AST modulates the expression of proteins associated with the CXCR4-Beclin1/VPS34 signaling pathway. In conclusion, these results suggest that AST inhibits neuronal excitability and enhances autophagy by modulating the CXCR4-Beclin1/VPS34 signaling pathway in the ACC and LH of CFA mice, leading to a reduction in pain sensitivity and anxiety-like behaviors, thereby producing analgesic effects. This study reveals a novel mechanism for the potential use of AST in the treatment of inflammatory pain and pain-related emotions, offering a promising strategy for clinical applications.

How can Blood Flow Restriction Exercise be Utilised for the Management of Persistent Pain Following Complex Injuries in Military Personnel? A Narrative Review

BACKGROUND

Persistent pain is a complicated phenomenon associated with a wide array of complex pathologies and conditions (e.g., complex regional pain syndrome, non-freezing cold injury), leading to extensive disability and reduced physical function. Conventional resistance training is commonly contraindicated in load compromised and/or persistent pain populations, compromising rehabilitation progression and potentially leading to extensive pharmacological intervention, invasive procedures, and reduced occupational status. The management of persistent pain and utility of adjunct therapies has become a clinical and research priority within numerous healthcare settings, including defence medical services.

MAIN BODY

Blood flow restriction (BFR) exercise has demonstrated beneficial morphological and physiological adaptions in load-compromised populations, as well as being able to elicit acute hypoalgesia. The aims of this narrative review are to: (1) explore the use of BFR exercise to elicit hypoalgesia; (2) briefly review the mechanisms of BFR-induced hypoalgesia; (3) discuss potential implications and applications of BFR during the rehabilitation of complex conditions where persistent pain is the primary limiting factor to progress, within defence rehabilitation healthcare settings. The review found BFR application is a feasible intervention across numerous load-compromised clinical populations (e.g., post-surgical, post-traumatic osteoarthritis), and there is mechanistic rationale for use in persistent pain pathologies. Utilisation may also be pleiotropic in nature by ameliorating pathological changes while also modulating pain response. Numerous application methods (e.g., with aerobic exercise, passive application, or resistance training) allow practitioners to cater for specific limitations (e.g., passive, or contralateral application with kinesiophobia) in clinical populations. Additionally, the low-mechanical load nature of BFR exercise may allow for high-frequency use within residential military rehabilitation, providing a platform for conventional resistance training thereafter.

CONCLUSION

Future research needs to examine the differences in pain modulation between persistent pain and pain-free populations with BFR application, supporting the investigation of mechanisms for BFR-induced hypoalgesia, the dose-response relationship between BFR-exercise and pain modulation, and the efficacy and effectiveness of BFR application in complex musculoskeletal and persistent pain populations.

Chronic sleep deficiency and its impact on pain perception in healthy females

Acute sleep deprivation in experimental studies has been shown to induce pain hypersensitivity in females. However, the impact of natural sleep deficiency and fluctuations across the week on pain perception remains unclear. A sleep-monitoring headband and self-reports were utilized to assess objective and subjective sleep in longer (> 6 hr) and short sleepers (< 6 hr). Pain sensitivity measures including heat, cold, pressure pain thresholds, pain inhibition (conditioned pain modulation) and facilitation (tonic pain summation) were assessed on Mondays and Fridays. Forty-one healthy young (23.9 ± 0.74 years) women participated. Short sleepers slept on average 2 hr less than longer sleepers (297.9 ± 8.2 min versus 418.5 ± 10.9 min) and experienced impaired pain inhibitory response (mean = -21.14 ± 7.9°C versus mean = 15.39 ± 9.5°C; p = 0.005). However, no effect was observed in pain thresholds and pain summation (p > 0.05). Furthermore, pain modulatory responses differed between Mondays and Fridays. Chronic sleep deficiency (< 6 hr) compromises pain responses, notably on Mondays. Maintaining a consistent sleep pattern with sufficient sleep (> 6 hr) throughout the week may protect against pain sensitization and the development of chronic pain in females. Further research is needed, especially in patients with chronic pain.

Tangzu granule alleviate neuroinflammation in diabetic peripheral neuropathy by suppressing pyroptosis through P2X7R /NLRP3 signaling pathway

ETHNOPHARMACOLOGICAL RELEVANCE

Diabetic peripheral neuropathy (DPN) is a common complication of diabetes mellitus, mainly manifested as paresthesia. Tangzu granule (TZG) is derived from famous traditional Chinese medicine decoctions and optimized by long-term temporary practice. TZG has good efficacy in improving numbness, pain and pruritus of the lower extremities of DPN patients. However, the overall regulatory mechanisms underlying its effects on DPN remain unclear.

AIM OF THE STUDY

This study aims to explore the potential mechanism of TZG for treating DPN.

MATERIALS AND METHODS

Sprague-Dawley (SD) rats were used to establish an in vivo model of DPN with streptozotocin (STZ) injection and high-fat diet (HFD) feeding. Additionally, sciatic glial RSC96 cells were induced with high glucose in vitro. SD rats in intervention group received TZG treatment for 12 weeks. After 12 weeks of treatment, sciatic nerve function was evaluated by intelligent hot plate meter and neuro electrophysiology detector. The morphological changes of sciatic nerve cells were observed by hematoxylin-eosin staining and transmission electron microscope. IL-1β, IL-18 inflammatory cytokines, pyroptosis and P2X7R/NLRP3 signaling pathway were observed by Western blotting, immunofluorescence staining and ELISA.

RESULTS

TZG improved nerve conduction velocity and sciatic neuropathy rational structural changes in DPN rats. It also inhibited RSC96 inflammatory response and cell death that induced by high glucose. This may be related to TZG inhibiting P2X7R, decreasing the activation of NLRP3 inflammasomes, down-regulating the levels of pyroptosis proteins such as caspase-1, cleaved caspase-1, gasdermin D (GSDMD), and GSDMD-N, and inhibiting the release of interleuki (IL)-18 and IL-1β inflammatory cytokines.

CONCLUSIONS

TZG inhibited pyroptosis through P2X7R/NLRP3 signaling pathway, alleviated neuroinflammation, and showed protective effect in the treatment of DPN.

Exploring the therapeutic potential of TRPC channels in chronic pain: An investigation into their mechanisms, functions, and prospects

Transient Receptor Potential Canonical (TRPC) channels have received more attention in recent years for their role of in the pathophysiology of chronic pain. These non-selective cation channels, which are predominantly present on cell membranes, play a pivotal role in regulating both physiological and pathological processes. Research advances have shown the critical role of TRPC channels in a variety of chronic pain, including neuropathic, inflammatory, and visceral pain. Activation of TRPC channels increases neuronal excitability, amplifying and prolonging pain signals. Moreover, these channels collaborate with other ion channels and receptors to form complexes that augment the transmission and perception of pain. As research advances, our understanding of TRPC channels' regulation mechanisms and signaling pathways improves. An expanding variety of TRPC modulators has been identified as promising therapeutic agents for chronic pain, opening up novel treatment options. Nevertheless, the diversity and complexity of TRPC channels present challenges in drug development, highlighting the importance of full understanding of their unique properties and activities. This review aims to provide a thorough evaluation of recent breakthrough in TRPC channels research related to chronic pain, with a focus on their mechanisms, functions, and prospective therapeutic application. By integrating existing research findings, we seek to bring new viewpoints and approaches for chronic pain management.

A sustained-release gel alleviates neuropathic pain in SNI mice by reversing Glu/GABA imbalance and chloride efflux disorders

Impaired spinal GABAergic inhibitory neuronal system is one popular target for developing new drugs or procedures for treatment of neuropathic pain, but effective and transferable methods are still lacking. We designed an assembled, temperature sensitive and sustained releasing hydrogel to repair the impaired GABAergic neural system by reversing imbalance of glutamic acid (Glu) and γ-aminobutyric acid (GABA) and healing impaired Cl- extrusion capacity of neurons. Hydrogel solution is a mixture of pluronic F-127, recombinant glutamate decarboxylase 67 (rGAD67) protein and CLP257, a K+-Cl- cotransporter isoform 2 (KCC2) enhancer. The temperature sensitive properties, gel properties and slow-releasing properties of the drug system were determined in vitro. After intrathecal injected in sural spared nerve injury mice model, the hydrogel solution turned into gel, capturing Glu and transforming it into GABA. CLP257 released from gel reversed the suppressed expression of KCC2 in spinal cord, maintaining a low intracellular Cl- concentration in neurons and allowing the normal work of GABA receptors. Combination of rGAD67 and CLP257 showed synergistic effects in alleviating hyperalgesia, altering glia activation, and inhibiting cell apoptosis and inflammatory response. In conclusion, the in situ assembled gel is a long-term effective tool for repairing damaged GABAergic inhibitory system and alleviating neuropathic pain.

Frailty as a mediator of postoperative delirium in older adults undergoing orthopedic surgery: A causal mediation analysis

Background

Postoperative delirium has a high prevalence in hospitalized older adults. Frail older adults have an increased risk for developing it. Preoperative pain-a common symptom in older adults with orthopedic conditions-shows a connection with frailty through an inflammatory process. However, an association among preoperative pain, frailty, and postoperative delirium remains unexplored among older adults undergoing orthopedic surgery.

Objectives

To examine the magnitude, direction, and mediation effect of frailty on the association between preoperative pain and postoperative delirium among older adults undergoing orthopedic surgery.

Design

A secondary analysis of a prospective cohort study.

Setting and participants

A cohort of 200 older adults (60 years and older) who underwent major orthopedic surgery at a university hospital in Thailand was recruited for the study.

Methods

Participants responded to the Demographic and Health Information Form, the Reported Edmonton Frailty Scale-Thai version, and the Thai version of the 4 A's test. Analyses were conducted using a causal mediation analysis with 95 % confidence intervals.

Results

We found that 12.5 % of participants developed postoperative delirium. For the direct paths associated with postoperative delirium, statistically significant effects were observed for frailty, preoperative pain, and comorbidity. Considering frailty as a mediator of postoperative delirium, a significant positive indirect effect was identified from preoperative pain. Controlling for age and comorbidities, frailty mediated the association between preoperative pain and postoperative delirium in a statistically significant manner; the average direct effect was 0.014 (95 % confidence interval: 0.008-0.020), the average causal mediating effect was 0.002 (95 % confidence interval: 0.000-0.010), the total effects was 0.017 (95 % confidence interval: 0.010-0.020), and the probability mediation accounted for 14 % (95 % confidence interval: 0.031-0.300).

Conclusion

Integrating the concept of age-related decline and frailty assessment may offer opportunities to provide disease-specific care and strengthen precision perioperative care, which ultimately enhance quality of life in older adults undergoing orthopedic surgery.

Psilocybin as a novel treatment for chronic pain

Psychedelic drugs are under active consideration for clinical use and have generated significant interest for their potential as anti-nociceptive treatments for chronic pain, and for addressing conditions like depression, frequently co-morbid with pain. This review primarily explores the utility of preclinical animal models in investigating the potential of psilocybin as an anti-nociceptive agent. Initial studies involving psilocybin in animal models of neuropathic and inflammatory pain are summarised, alongside areas where further research is needed. The potential mechanisms of action, including targeting serotonergic pathways through the activation of 5-HT2A receptors at both spinal and central levels, as well as neuroplastic actions that improve functional connectivity in brain regions involved in chronic pain, are considered. Current clinical aspects and the translational potential of psilocybin from animal models to chronic pain patients are reviewed. Also discussed is psilocybin's profile as an ideal anti-nociceptive agent, with a wide range of effects against chronic pain and its associated inflammatory or emotional components.

Electroacupuncture may alleviate inflammatory pain by downregulating the expression of P2Y14 receptor in the primary somatosensory cortex

Increasing evidence indicated that purinergic signalling involved in electroacupuncture (EA)-induced analgesia. Whether purinergic P2Y14 receptor contributes to EA-mediated analgesia remains unclear. Here, we report that the expression of P2Y14 receptor in the hindlimb region of the primary somatosensory cortex (S1HL) was significantly upregulated on Complete Freund's Adjuvant (CFA)-induced pain model mice, while was downregulated after EA treatment (2 Hz frequency, 1 mA intensity, and 30 min duration) at "Zusanli" (also named ST36 acupoint). EA-mediated analgesia could be reversed by injection of P2RY14 agonist uridine diphosphate glucose (UDPG) into the bilateral S1HL, while prolonged by injection of P2RY14 antagonist pyridoxal phosphate-6-azophenyl-2',4'-disulfonic acid (PPTN). It suggested that EA may alleviate inflammatory pain by downregulating the expression of P2RY14 in the S1HL.

Anti-Inflammatory and Antinociceptive Properties of the Quercetin-3-Oleate AV2, a Novel FFAR1 Partial Agonist

Free fatty acid receptor 1 (FFAR1) has emerged as the most targeted isoform of the free fatty acid receptors because of its involvement in the modulation of energy balance and its potential role in the control of inflammatory and pain conditions. Quercetin-3-oleate (AV2), recognized as a new FFAR1 partial agonist, was investigated for its ability to modulate inflammation and nociception. Human immortal neuroblastoma SH and the murine macrophagic RAW 264.7 cells were used to evaluate cell viability, the potential cytoprotective activity, and the anti-inflammatory properties of AV2 in vitro. Paw edema, caused by zymosan-A, and the formalin test were used to assess the in vivo anti-inflammatory and antinociceptive effects in CD-1 mice. In vitro, AV2 was devoid of cytotoxicity, significantly reduced ROS in both cell types, and protected RAW 264.7 cells from lipopolysaccharide damage by reducing tumor necrosis factor-α production. Interestingly, AV2 induced a transient elevation of intracellular calcium that was reduced in cells, pre-incubated with the FFAR1 antagonist DC260126. In vivo, AV2 reduced formalin-induced nociception and zymosan A-induced paw edema, and both effects were reversed by the FFAR1 antagonist GW1100. In conclusion, these data strongly support the AV2-mediated antioxidant, anti-inflammatory, and antinociceptive activity. AV2 represents a promising molecule for the clinical management of inflammatory-related pain conditions.

Neuroinflammation in osteoarthritis: From pain to mood disorders

Osteoarthritis (OA) is the most common form of musculoskeletal disease, and its prevalence is increasing due to the aging of the population. Chronic pain is the most burdensome symptom of OA that significantly lowers patients' quality of life, also due to its frequent association with emotional comorbidities, such as anxiety and depression. In recent years, both chronic pain and mood alterations have been linked to the development of neuroinflammation in the peripheral nervous system, spinal cord and supraspinal brain areas. Thus, mechanisms at the basis of the development of the neuroinflammatory process may indicate promising targets for novel treatment for pain and affective comorbidities that accompany OA. In order to assess the key role of neuroinflammation in the maintenance of chronic pain and its potential involvement in development of psychiatric components, the monoiodoacetate (MIA) model of OA in rodents has been used and validated. In the present commentary article, we aim to summarize up-to-date results achieved in this experimental model of OA, focusing on glia activation and cytokine production in the sciatic nerve, dorsal root ganglia (DRGs), spinal cord and brain areas. The association of a neuroinflammatory state with the development of pain and anxiety- and depression-like behaviors are discussed. Results suggest that cells and molecules involved in neuroinflammation may represent novel targets for innovative pharmacological treatments of OA pain and mood comorbidities.

Targeting the soluble epoxide hydrolase pathway as a novel therapeutic approach for the treatment of pain

Chronic pain is a major burden and the complexities of chronic pain pathophysiology, including both peripheral and central sensitisation mechanisms, involves multiple cell types (neuronal, immune, neuroimmune, and vascular) which substantially complicates the development of new effective analgesic treatments. The epoxy fatty acids (EpFAs), including the epoxyeicosatrienoic acids (EETs), are derived from the metabolism of polyunsaturated fatty acids (PUFAs) via the cytochrome P450 enzymatic pathway and act to shut-down inflammatory signalling and provide analgesia. The EpFAs are rapidly metabolised by the enzyme soluble epoxide hydrolase (sEH) into their corresponding diol metabolites, which recent studies suggest are pro-inflammatory and pro-nociceptive. This review discusses clinical and mechanistic evidence for targeting the sEH pathway for the treatment of pain.

Role of l -arginine/nitric oxide/cyclic GMP/K ATP channel signaling pathway and opioid receptors in the antinociceptive effect of rutin in mice

The l -arginine ( l -Arg)/nitric oxide/cyclic GMP/potassium channel (K ATP ) pathway and opioid receptors are known to play critical roles in pain perception and the antinociceptive effects of various compounds. While there is evidence suggesting that the analgesic effects of rutin may involve nitric oxide modulation, the direct link between rutin and the l -Arg/nitric oxide/cyclic GMP/K ATP pathway in the context of pain modulation requires further investigation. The antinociceptive effect of rutin was studied in male NMRI mice using the formalin test. To investigate the role of the l -Arg/nitric oxide/cyclic GMP/K ATP pathway and opioid receptors, the mice were pretreated intraperitoneally with different substances. These substances included l -Arg (a precursor of nitric oxide), S-nitroso- N -acetylpenicillamine (SNAP, a nitric oxide donor), N(gamma)-nitro- l -arginine methyl ester (L-NAME, an inhibitor of nitric oxide synthase), sildenafil (an inhibitor of phosphodiesterase enzyme), glibenclamide (a K ATP channel blocker), and naloxone (an opioid receptor antagonist). All pretreatments were administered 20 min before the administration of the most effective dose of rutin. Based on our investigation, it was found that rutin exhibited a dose-dependent antinociceptive effect. The administration of SNAP enhanced the analgesic effects of rutin during both the initial and secondary phases. Moreover, L-NAME, naloxone, and glibenclamide reduced the analgesic effects of rutin in both the primary and secondary phases. In conclusion, rutin holds significant value as a flavonoid with analgesic properties, and its analgesic effect is directly mediated through the nitric oxide/cyclic GMP/K ATP channel pathway.

Asiatic acid reduces lipopolysaccharides-induced pulp inflammation through activation of nuclear factor erythroid 2-related factor 2 in rats

Background

Dental pulp inflammation, often initiated by Gram-negative microorganisms and lipopolysaccharides (LPS), can lead to pulpitis and, subsequently, dental pulp necrosis, compromising tooth structure and increasing susceptibility to fracture. Asiatic acid, derived from Centella asiatica, has demonstrated pharmacological properties, including anti-inflammatory and antioxidant effects, making it a potential candidate for mitigating LPS-induced pulp inflammation. This in vivo study aims to investigate the impact of Asiatic acid on the nuclear factor erythroid 2-related factor 2 (Nrf2) pathway in Rattus norvegicus with LPS-induced pulp inflammation.

Methods

This quasi-laboratory experimental in vivo study employed a post-test-only control group design to investigate the effects of Asiatic acid on LPS-induced pulp inflammation in Wistar rats. Thirty rats were randomly divided into six groups subjected to various interventions. LPS was administered to all groups for 6 h except the standard control group (CG, n = 5). The negative control group (NCG, n = 5) received only glass ionomer cement. The positive control group (PCG, n = 5) received Eugenol with glass ionomer cement. Intervention groups 1, 2, and 3 (IG1, IG2, IG3; n = 5 each) received Asiatic acid at concentrations of 0.5%, 1%, and 2%, respectively, with glass ionomer cement. Dental pulp inflammation was confirmed through immunological (tumor necrosis factor alpha (TNF-α) levels), histopathological (inflammatory parameters), and physiological (pain assessment using the rat grimace scale) analyses. Additionally, Nrf2 levels were examined using enzyme-linked immunosorbent assay (ELISA).

Results

Asiatic acid administration significantly influenced Nrf2 levels in rats with LPS-induced pulp inflammation. Nrf2 levels were significantly higher in groups treated with 0.5% (IG1) (8.810 ± 1.092 ng/mL; p = 0.047), 1.0% (IG2) (9.132 ± 1.285 ng/mL; p = 0.020), and 2.0% (IG3) (11.972 ± 1.888 ng/mL; p = 0.000) Asiatic acid compared to NCG (7.146 ± 0.706). Notably, Nrf2 levels were also significantly higher in the 2.0% Asiatic acid group (IG3) compared to the PCG treated with Eugenol (8.846 ± 0.888 ng/mL; p = 0.001), as well as IG1 (p = 0.001) and IG2 (p = 0.002). However, no significant difference was observed between administering 0.5% Asiatic acid (IG1), 1.0% Asiatic acid (IG2), and Eugenol (PCG).

Conclusion

This research showed that Asiatic acid significantly impacted the Nrf2 levels in rats with LPS-induced pulp inflammation. This suggests that it has the potential to be used as a therapeutic agent for reducing dental pulp inflammation. These findings support the need to further explore Asiatic acid as a promising intervention for maintaining dental pulp health.

The role of tryptophan and its derivatives in musculoskeletal pains: A systematic review and meta-analysis

BACKGROUND

Studies present ambiguous findings regarding the role of tryptophan and its metabolites, kynurenine and serotonin in chronic musculoskeletal pain. This systematic review aimed to investigate the expression of tryptophan and its metabolites, serotonin and kynurenine in patients with local and generalized chronic musculoskeletal pain in comparison with pain-free controls.

METHODS

An electronic search was conducted in the databases MEDLINE, CINAHL, EMBASE, the Cochrane Central Registry of Controlled Trials (CENTRAL) and Web of Science for clinical and observational trials from the beginning of each database to 21 April 2023. Out of 6734 articles, a total of 17 studies were included; 12 studies were used in the meta-analysis of serotonin, 3 regarding tryptophan and 2 studies for a narrative synthesis regarding kynurenine. Risk of bias was assessed using the quality assessment tool for observational cohort and cross-sectional studies of the National Heart, Lung, and Blood Institute, while the certainty of evidence was by GRADE.

RESULTS

All included studies showed a low risk of bias. The meta-analysis showed lower blood levels of tryptophan (p < .001; very low quality of evidence) and higher blood levels of serotonin (p < .001; very low-quality evidence) in patients with generalized musculoskeletal pain, when compared to pain-free individuals. In local chronic musculoskeletal pain, there were higher blood levels of serotonin (p=.251; very low quality of evidence) compared to pain-free individuals. Regarding kynurenine, the studies reported both higher and lower blood levels in generalized chronic musculoskeletal pain compared to pain-free individuals.

CONCLUSIONS

The blood levels of tryptophan and its metabolites serotonin and kynurenine seem to influence chronic musculoskeletal pain.

The Antinociceptive Role of Nrf2 in Neuropathic Pain: From Mechanisms to Clinical Perspectives

Neuropathic pain, a chronic condition resulting from nerve injury or dysfunction, presents significant therapeutic challenges and is closely associated with oxidative stress and inflammation, both of which can lead to mitochondrial dysfunction. The nuclear factor erythroid 2-related factor 2 (Nrf2) pathway, a critical cellular defense mechanism against oxidative stress, has emerged as a promising target for neuropathic pain management. Nrf2 modulators enhance the expression of antioxidant and cytoprotective genes, thereby reducing oxidative damage, inflammation, and mitochondrial impairment. This review explores the antinociceptive effects of Nrf2, highlighting how pharmacological agents and natural compounds may be used as potential therapeutic strategies against neuropathic pain. Although preclinical studies demonstrate significant pain reduction and improved nerve function through Nrf2 activation, several clinical challenges need to be addressed. However, emerging clinical evidence suggests potential benefits of Nrf2 modulators in several conditions, such as diabetic neuropathy and multiple sclerosis. Future research should focus on further elucidating the molecular role of Nrf2 in neuropathic pain to optimize its modulation efficacy and maximize clinical utility.

Anti-inflammatory and analgesic properties of Polyphyllin VI revealed by network pharmacology and RNA sequencing

Inflammatory pain, sustained by a complex network of inflammatory mediators, is a severe and persistent illness affecting many of the general population. We explore possible anti-inflammatory pathways of Polyphyllin VI (PPVI) based on our prior study, which showed that PPVI reduces inflammation in mice to reduce pain. Network pharmacology and RNA-Seq identified the contribution of the MAPK signaling pathway to inflammatory pain. In the LPS/ATP-induced RAW264.7 cell model, pretreatment with PPVI for 1 h inhibited the release of IL-6 and IL-8, down-regulated expression of the P2X7 receptor(P2X7R), and decreased phosphorylation of p38 and ERK1/2 components of the MAPK pathway. Moreover, PPVI decreased expression of IL-6 and IL-8 was observed in the serum of the inflammatory pain mice model and reduced phosphorylation of p38 and ERK1/2 in the dorsal root ganglia while the reductions of expression of IL-6 and phosphorylation of ERK1/2 were not observed after the pre-treatment with A740003 (an antagonist of the P2X7R). These results suggest that PPVI may inhibit the release of IL-8 by regulating P2X7R to reduce the phosphorylation of p38. However, the modulation of PPVI on the release of IL-6 and phosphorylation of ERK1/2 may mediated by other P2X7R-independent signals.

Recent update on cerebral sparganosis: A bibliometric analysis and scientific mapping

Human sparganosis, a parasitic infection prevalent in Asia, can progress to cerebral sparganosis, a severe condition with significant neurological symptoms. Diagnosis and treatment are challenging due to its clinical similarity to other infections, highlighting the need for improved detection and management strategies. The aim of this study was to observe research trends, key contributors, gaps in the existing knowledge, diagnosis challenges, effective treatment options, and prevention strategies, providing recommendations for future research directions and clinical practice improvements on cerebral sparganosis. A bibliometric analysis was conducted by extracting 139 documents from the Scopus database in June 2024. The retrieved data were analyzed using the R package's Bibliometrix (Biblioshiny) and VOSviewer. Spanning 97 different sources, the research exhibited an annual growth rate of 2.5%. Annual scientific production revealed fluctuating research activity with peaks in 2010 and 2011 and notable citation peaks in 1996 and 2005, indicating pivotal studies that significantly influenced subsequent research. Early studies focused on diagnosis and specific parasites, while recent studies (2010-2024) have increasingly addressed clinical outcomes, treatment strategies, and advanced diagnostic techniques. Trends revealed a shift towards clinical and diagnostic advancements, with recent emphasis on diagnostic imaging, immunoassays, and the relationship between cerebral sparganosis and brain tumors. In conclusion, the studies on cerebral sparganosis underscore the potential for enhancing clinical practice by improving diagnostic accuracy, informing treatment decisions, and implementing targeted screening efforts based on epidemiology and risk factors. Recommendation to further study needs to notify the cerebral sparganosis in high-risk countries with similar socioeconomic and cultural characteristics to endemic regions, including Indonesia.

Unraveling the Potential of Electroanalgesia: A Literature Review of Current Therapeutics

Neuropathic pain (NP), arising from dysfunction in the neurological system, poses a significant challenge in pain management due to its intricate origin and unpredictable response to conventional treatments. Electroanalgesia, a collection of techniques such as transcutaneous electric nerve stimulation (TENS), peripheral electrical nerve stimulation (PENS), spinal cord stimulation (SCS), deep brain stimulation (DBS), and electroacupuncture (EA), presents a potential alternative or complementary approach. This review brings together evidence from 56 studies to evaluate the effectiveness and safety of electroanalgesia in chronic NP. It discusses the mechanisms underlying NP, the indications for electroanalgesia, and the techniques utilized, emphasizing the diverse applications and potential benefits. However, despite its potential uses, electroanalgesia has its limitations, including variable effectiveness and potential adverse effects. Furthermore, the review recognizes the limitations of the methodology and the need for further research to refine treatment protocols and enhance the understanding of electroanalgesia's role in comprehensive pain management strategies.

Metabolic Dysregulation and Its Role in Postoperative Pain among Knee Osteoarthritis Patients

Knee osteoarthritis (KOA) is characterized by low-grade inflammation, loss of articular cartilage, subchondral bone remodeling, synovitis, osteophyte formation, and pain. Strong, continuous pain may indicate the need for joint replacement in patients with end-stage OA, although postoperative pain (POP) of at least a two-month duration persists in 10-40% of patients with OA.

STUDY PURPOSE

The inflammation observed in joint tissues is linked to pain caused by the production of proinflammatory cytokines. Since the biosynthesis of cytokines requires energy, their production is supported by extensive metabolic conversions of carbohydrates and fatty acids, which could lead to a disruption in cellular homeostasis. Therefore, this study aimed to investigate the association between POP development and disturbances in energy metabolic conversions, focusing on carbohydrate and fatty acid metabolism.

METHODS

Peripheral blood samples were collected from 26 healthy subjects and 50 patients with end-stage OA before joint replacement surgery. All implants were validated by orthopedic surgeons, and patients with OA demonstrated no inherent abnormalities to cause pain from other reasons than OA disease, such as malalignment, aseptic loosening, or excessive bleeding. Pain levels were assessed before surgery using the visual analogue scale (VAS) and neuropathic pain questionnaires, DN4 and PainDETECT. Functional activity was evaluated using the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC). Three and six months after surgery, pain indices according to a VAS of 30 mm or higher were considered. Total RNA isolated from whole blood was analyzed using quantitative real-time RT-PCR (qRT-PCR) for the expression of genes related to carbohydrate and fatty acid metabolism. Protein levels of the examined genes were measured using an ELISA in the peripheral blood mononuclear cells (PBMCs). We used qRT-PCR because it is the most sensitive and reliable method for gene expression analysis, while an ELISA was used to confirm our qRT-PCR results.

KEY FINDINGS

Among the study cohort, 17 patients who reported POP demonstrated significantly higher (p < 0.05) expressions of the genes PKM2, LDH, SDH, UCP2, CPT1A, and ACLY compared to pain-free patients with KOA. Receiver-operating characteristic (ROC) curve analyses confirmed the association between these gene expressions and pain development post-arthroplasty. A principle component analysis identified the prognostic values of ACLY, CPT1A, AMPK, SDHB, Caspase 3, and IL-1β gene expressions for POP development in the examined subjects.

CONCLUSION

These findings suggest that the disturbances in energy metabolism, as observed in the PBMCs of patients with end-stage KOA before arthroplasty, may contribute to POP development. An understanding of these metabolic processes could provide insights into the pathogenesis of KOA. Additionally, our findings can be used in a clinical setting to predict POP development in end-stage patients with KOA before arthroplasty.

The role of neuroinflammation in the transition of acute to chronic pain and the opioid-induced hyperalgesia and tolerance

Current evidence suggests that activation of glial and immune cells leads to increased production of proinflammatory mediators, creating a neuroinflammatory state. Neuroinflammation has been proven to be a fundamental mechanism in the genesis of acute pain and its transition to neuropathic and chronic pain. A noxious event that stimulates peripheral afferent nerve fibers may also activate pronociceptive receptors situated at the dorsal root ganglion and dorsal horn of the spinal cord, as well as peripheral glial cells, setting off the so-called peripheral sensitization and spreading neuroinflammation to the brain. Once activated, microglia produce cytokines, chemokines, and neuropeptides that can increase the sensitivity and firing properties of second-order neurons, upregulating the signaling of nociceptive information to the cerebral cortex. This process, known as central sensitization, is crucial for chronification of acute pain. Immune-neuronal interactions are also implicated in the lesser-known complex regulatory relationship between pain and opioids. Current evidence suggests that activated immune and glial cells can alter neuronal function, induce, and maintain pathological pain, and disrupt the analgesic effects of opioid drugs by contributing to the development of tolerance and dependence, even causing paradoxical hyperalgesia. Such alterations may occur when the neuronal environment is impacted by trauma, inflammation, and immune-derived molecules, or when opioids induce proinflammatory glial activation. Hence, understanding these intricate interactions may help in managing pain signaling and opioid efficacy beyond the classical pharmacological approach.

How Can Experimental Endotoxemia Contribute to Our Understanding of Pain? A Narrative Review

The immune system and the central nervous system exchange information continuously. This communication is a prerequisite for adaptive responses to physiological and psychological stressors. While the implicate relationship between inflammation and pain is increasingly recognized in clinical cohorts, the underlying mechanisms and the possibilities for pharmacological and psychological approaches aimed at neuro-immune communication in pain are not fully understood yet. This calls for preclinical models which build a bridge from clinical research to laboratory research. Experimental models of systemic inflammation (experimental endotoxemia) in humans have been increasingly recognized as an approach to study the direct and causal effects of inflammation on pain perception. This narrative review provides an overview of what experimental endotoxemia studies on pain have been able to clarify so far. We report that experimental endotoxemia results in a reproducible increase in pain sensitivity, particularly for pressure and visceral pain (deep pain), which is reflected in responses of brain areas involved in pain processing. Increased levels of blood inflammatory cytokines are required for this effect, but cytokine levels do not always predict pain intensity. We address sex-dependent differences in immunological responses to endotoxin and discuss why these differences do not necessarily translate to differences in behavioral measures. We summarize psychological and cognitive factors that may moderate pain sensitization driven by immune activation. Together, studying the immune-driven changes in pain during endotoxemia offers a deeper mechanistic understanding of the role of inflammation in chronic pain. Experimental endotoxemia models can specifically help to tease out inflammatory mechanisms underlying individual differences, vulnerabilities, and comorbid psychological problems in pain syndromes. The model offers the opportunity to test the efficacy of interventions, increasing their translational applicability for personalized medical approaches.

Nanomaterial-Based Drug Delivery Systems for Pain Treatment and Relief: From the Delivery of a Single Drug to Co-Delivery of Multiple Therapeutics

The use of nanomaterials in drug delivery systems for pain treatment is becoming increasingly common. This review aims to summarize how nanomaterial-based drug delivery systems can be used to effectively treat and relieve pain, whether via the delivery of a single drug or a combination of multiple therapeutics. By utilizing nanoformulations, the solubility of analgesics can be increased. Meanwhile, controlled drug release and targeted delivery can be realized. These not only improve the pharmacokinetics and biodistribution of analgesics but also lead to improved pain relief effects with fewer side effects. Additionally, combination therapy is frequently applied to anesthesia and analgesia. The co-encapsulation of multiple therapeutics into a single nanoformulation for drug co-delivery has garnered significant interest. Numerous approaches using nanoformulation-based combination therapy have been developed and evaluated for pain management. These methods offer prolonged analgesic effects and reduced administration frequency by harnessing the synergy and co-action of multiple targets. However, it is important to note that these nanomaterial-based pain treatment methods are still in the exploratory stage and require further research to be effectively translated into clinical practice.

The Role of Glial Cells in Different Phases of Migraine: Lessons from Preclinical Studies

Migraine is a complex and debilitating neurological disease that affects 15% of the population worldwide. It is defined by the presence of recurrent severe attacks of disabling headache accompanied by other debilitating neurological symptoms. Important advancements have linked the trigeminovascular system and the neuropeptide calcitonin gene-related peptide to migraine pathophysiology, but the mechanisms underlying its pathogenesis and chronification remain unknown. Glial cells are essential for the correct development and functioning of the nervous system and, due to its implication in neurological diseases, have been hypothesised to have a role in migraine. Here we provide a narrative review of the role of glia in different phases of migraine through the analysis of preclinical studies. Current evidence shows that astrocytes and microglia are involved in the initiation and propagation of cortical spreading depolarization, the neurophysiological correlate of migraine aura. Furthermore, satellite glial cells within the trigeminal ganglia are implicated in the initiation and maintenance of orofacial pain, suggesting a role in the headache phase of migraine. Moreover, microglia in the trigeminocervical complex are involved in central sensitization, suggesting a role in chronic migraine. Taken altogether, glial cells have emerged as key players in migraine pathogenesis and chronification and future therapeutic strategies could be focused on targeting them to reduce the burden of migraine.