Induction Of Pain Research Articles

Implication of system xc- in Complete Freund's Adjuvant-induced peripheral inflammation and associated nociceptive sensitization.

Persistent inflammation leading to neuronal sensitization in pain pathways, are key features of chronic inflammatory pain. Alike macrophages in the periphery, glial cells exacerbate hypersensitivity by releasing proalgesic mediators in the central nervous system. Expressed by peripheral and central immune cells, the cystine-glutamate antiporter system xc- plays a significant role in inflammatory responses, but its involvement in chronic inflammatory pain remains underexplored. We herein investigated the contribution of this exchanger in nociceptive hypersensitivity triggered by a peripheral inflammatory insult. Complete Freund's adjuvant (CFA) was injected into the left hind paw of wild-type C57Bl/6 female mice, of xCT-deficient mice (specific subunit of system xc-) and of mice receiving the system xc- inhibitor sulfasalazine. Paw edema was measured over three weeks and pain-associated behaviors were evaluated. Additionally, pro-inflammatory cytokine levels were assessed in blood samples. CFA injection led to a persistent increase in paw edema and hypersensitivity to mechanical and thermal stimuli, which were less pronounced in xCT-deficient mice. This reduced sensitivity was accompanied by lower systemic pro-inflammatory cytokine levels in xCT-deficient mice. Accordingly, pharmacological inhibition of system xc- with sulfasalazine, either before or after pain induction, efficiently reduced the algesic and inflammatory responses to CFA in wild-type mice. Our findings reveal a critical role for system xc- in the pathophysiology of inflammatory pain. xCT deficiency reduces pain behaviors and peripheral inflammation, positioning system xc- as a promising therapeutic target for alleviating chronic inflammatory pain.

The Intrinsic Neuronal Activation of the CXCR4 Signaling Axis Is Associated with a Pro-Regenerative State in Cervical Primary Sensory Neurons Conditioned by a Sciatic Nerve Lesion.

CXCL12 and CXCR4 proteins and mRNAs were monitored in the dorsal root ganglia (DRGs) of lumbar (L4-L5) and cervical (C7-C8) spinal segments of naïve rats, rats subjected to sham operation, and those undergoing unilateral complete sciatic nerve transection (CSNT) on post-operation day 7 (POD7). Immunohistochemical, Western blot, and RT-PCR analyses revealed bilaterally increased levels of CXCR4 protein and mRNA in both lumbar and cervical DRG neurons after CSNT. Similarly, CXCL12 protein levels increased, and CXCL12 mRNA was upregulated primarily in lumbar DRGs ipsilateral to the nerve lesion. Intrathecal application of the CXCR4 inhibitor AMD3100 following CSNT reduced CXCL12 and CXCR4 protein levels in cervical DRG neurons, as well as the length of afferent axons regenerated distal to the ulnar nerve crush. Furthermore, treatment with the CXCR4 inhibitor decreased levels of activated Signal Transducer and Activator of Transcription 3 (STAT3), a critical transforming factor in the neuronal regeneration program. Administration of IL-6 increased CXCR4 levels, whereas the JAK2-dependent STAT3 phosphorylation inhibitor (AG490) conversely decreased CXCR4 levels. This indicates a link between the CXCL12/CXCR4 signaling axis and IL-6-induced activation of STAT3 in the sciatic nerve injury-induced pro-regenerative state of cervical DRG neurons. The role of CXCR4 signaling in the axon-promoting state of DRG neurons was confirmed through in vitro cultivation of primary sensory neurons in a medium supplemented with CXCL12, with or without AMD3100. The potential involvement of conditioned cervical DRG neurons in the induction of neuropathic pain is discussed.

Reorganization of lateral habenula neuronal connectivity underlies pain-related impairment in spatial memory encoding.

Dysfunctional hyperactivity of the lateral habenula nucleus (LHb) has emerged as a critical marker for pain-related mood impairments. Acting as a central hub, the LHb filters and disseminates pertinent information to other brain structures during learning. However, it is not well understood how intra-LHb activity is altered during cognitive demand under neuropathic pain conditions. To address this gap, we implanted an optrode structure to record neuronal activity in adult male CD (rat strain without definition) rats during the execution of a delayed nonmatch-to-sample (DNMS) spatial working memory (WM) task. We selectively modulated intra-LHb network activity by optogenetically inhibiting local LHb CaMKIIα (calcium calmodulin-dependent protein kinase II alpha)-expressing neurons during the delay phase of the DNMS task. Behavioral assessments were conducted using a persistent rodent model of neuropathic pain-spared nerve injury. Our results showed that the induction of neuropathic pain disrupted WM encoding accuracy and intra-LHb functional neuronal connectivity. This disruption was reversed by optogenetic inhibition of LHb CaMKIIα-expressing neurons, which also produced antinociceptive effects. Together, our findings provide insight into how intra-LHb networks reorganize information to support different task contexts, suggesting that the abnormal pain-related intra-LHb dynamic segregation of information may contribute to poor cognitive accuracy in male rodents during pain experiences.

Programmed Cell Death Protein 1 Contributes to Oral Cancer Pain via Regulating Tumor Necrosis Factor Alpha in the Spinal Trigeminal Nucleus Caudalis.

Oral cancer causes intense pain at the primary site, and such pain can impair oral functions. However, the underlying mechanisms for oral cancer pain are still not fully understood. In the present study, it is investigated whether programmed cell death protein 1 (PD-1) is involved in the development of oral cancer pain. RMP1-14, a specific anti-PD-1 antibody, was injected into spinal trigeminal nucleus caudalis (Sp5C) and measured pain behaviors using von Frey filaments and dolognawmeter. Western blotting and immunofluorescence staining were performed to analyze the expression of PD-1 and tumor necrosis factor alpha (TNFα) in the Sp5C. It was observed that the PD-1 antibody significantly inhibited mechanical hypersensitivity and functional allodynia in our oral cancer pain mouse model. Moreover, we found that TNFα was highly upregulated in the Sp5C following the induction of oral cancer pain and that intra-Sp5C injection of the PD-1 antibody diminished the upregulation of TNFα. It was found that genetic deletion of TNFα or its receptor antagonism synergized the analgesic effect of PD-1 antibody on oral cancer pain. Our results suggest that PD-1 in the Sp5C contributes to oral cancer pain by altering TNFα signaling in the trigeminal nociceptive system, and PD-1 could be targeted to develop a novel approach for oral cancer pain management.

Tell me what to expect: how instructions affect the pain response of patients with chronic myofascial pain with referral.

The aims of the study are to analyze the influence of pain and no pain expectations on the physiological (electromyography (EMG) and pupillometry) and cognitive (Numerical Rating Scale (NRS)) response to pain. Pain expectation and no pain expectation situations were induced by employing instructional videos. The induction of pain was performed by palpating the masseter with an algometer in a sample of 2 groups: 30 healthy participants (control group) and 30 patients (Temporomandibular disorders (TMD) group) with chronic myofascial pain with referral in the masseter muscle (Diagnostic Criteria for Temporomandibular Dissorders (DC/TMD)). Used a mixed design all participants were exposed to pain and no pain conditions in the same session, but the order of the presentation was counterbalanced across participants to control its possible influence. A significantly larger pupillary diameter was observed in the pain expectation relative to the no pain expectation condition in both groups. The TMD group presented larger EMG activity and larger scores in anxiety, somatization, catastrophizing and central sensitization than the control group. In the NRS, the TMD group also showed a significantly higher score than the control group. The TMD group presented similar NRS scores in the expectation condition compared to the no pain expectation condition, while the control group presented higher scores for pain expectation than for no pain expectation. Pain expectation modulated the pain cognitive pain assessment and pupil diameter in controls. The cognitive pain assessment was altered in the TMD group compared to the control group, particularly in the no pain expectation condition, this may be due to a negative reappraisal of pain due to past experiences, as pointed out by the observed level of catastrophizing. Pain expectations did not influence the EMG, significantly higher EMG activity was found in the TMD group compared to the control group regardless of expectation type.

Mechanism of Action of Mindfulness-Based Interventions for Pain Relief-A Systematic Review.

Background: Currently, no systematic evidence synthesis of the mechanism of action of mindfulness-based approaches exists for pain conditions. Aim: To identify and synthesize experimental and clinical studies examining aspects of the mechanism of action of mindfulness for pain relief. Methods: The following databases and search interfaces were searched: Embase (via Embase.com) and Medline (via PubMed). Additional references were identified via bibliographies of included studies. The following were the inclusion criteria applied: (1) original studies published in peer-reviewed journals, (2) in adult populations that (3) examined the mechanism of action of mindfulness meditation on pain outcomes or (4) provided conclusions regarding the potential mechanism of action of mindfulness meditation. The studies were selected by two independent reviewers. Discrepancies were resolved by discussion. Results: A total of 21 studies published in English met the inclusion criteria, of which 5 studies were clinical studies, which included patients with chronic pain, and 16 studies used experimental pain induction. The investigation into brain mechanisms through functional magnetic resonance imaging and diffusion tensor imaging revealed mindfulness meditation's ability to modulate brain activity, particularly in the anterior cingulate cortex, anterior insula, and orbitofrontal cortex, and to enhance structural and functional connectivity in regions associated with pain perception. Regarding the role of opioids, findings across five studies indicated that the analgesic effects of mindfulness are maintained even when opioid receptors are blocked, suggesting a nonopioidergic pathway for pain modulation. Pain perception studies highlighted that mindfulness practices foster pain acceptance and modify pain control beliefs, serving as key mediators in improving pain outcomes. For experienced versus novice mindfulness practitioners, results demonstrated that long-term practice enhances pain threshold and reduces pain unpleasantness through increased activity in salience and attentional control regions. Conclusion: This systematic review highlights mindfulness meditation as a multifaceted approach to pain management, utilizing mechanisms such as cognitive and emotional reappraisal, nonopioidergic pathways, and enhanced attention in control regions. It emphasizes the role of mindfulness in fostering pain acceptance and altering pain control perceptions, showcasing its broad impact on the neurological and experiential dimensions of pain. However, the predominance of studies on healthy subjects and methodological variations across experiments necessitates careful interpretation of the findings. The review calls for further research to explore the mechanisms of mindfulness in chronic pain populations more deeply, distinguishing the specific effects of mindfulness from nonspecific effects and expanding its applicability in clinical settings for chronic pain management.

Anti-allodynic effect of intrathecal antibodies against macrophage-inducible C-type lectin in spinal nerve ligation model in rat

IntroductionMacrophage-inducible C-type lectin (Mincle) has emerged as a potential contributor to neuropathic pain induction and neuroinflammatory responses within the spinal cord. Moreover, evidence suggests a close association between toll-like receptor (TLR) and Mincle expression in myeloid cells. This study evaluated the effectiveness of Mincle antibodies in neuropathic pain and identified the epitope of these antibodies. In addition, the mode of interaction between Mincle and TLR inhibition was explored using isobolographic analysis. MethodsThree different Mincle antibodies and a specific TLR4 inhibitor (TAK-242) were intrathecally administered, and mechanical allodynia was evaluated using the von Frey test in a rat model of spinal nerve ligation (SNL). Isobolographic analysis was conducted on the effect of combination of TAK-242 and Mincle Ab. Microarray analysis examined the specific region of Mincle targeted by the antibodies. ResultsAll Mincle antibodies and TAK-242 significantly alleviated mechanical allodynia in a dose-dependent manner. However, the maximal possible effects (MPE) produced by the antibodies ranged widely from 37.1% to 91.8%, comparable to that of TAK-242 (88.7%). The combination of TAK-242 and the antibody with the highest MPE resulted in an additive interaction for their anti-allodynic effects. Epitope mapping revealed that each antibody targeted the extracellular domain, with epitope lengths ranging from 5 to 15 amino acids. ConclusionsThe current study demonstrates the anti-allodynic effect of Mincle antibodies and additive interaction with TLR4 inhibition in spinal nerve ligation model, suggesting the potential of blocking of Mincle signaling with its antibodies as a novel treatment strategy for neuropathic pain.

The effects of chronic neuropathic pain states on the discriminative stimulus effect of fentanyl and other MOR agonists.

Pleasant subjective effects of drugs (e.g., euphoria) have been demonstrated to contribute to their abuse potential. In humans, there is some evidence that acute pain states may decrease the positive subjective effects of opioids; however, no studies have directly tested the impact of a long-lasting pain state. Therefore, the goal of this study was to directly evaluate the discriminative stimulus of mu opioid receptor (MOR) agonist, fentanyl, or the non-opioid drug of abuse, cocaine, in the presence or absence of spared nerve injury (SNI) induced chronic neuropathic pain. Prior to surgery, MOR agonists (fentanyl, morphine, nalbuphine) dose-dependently increased % fentanyl-like responding, as expected; surprisingly, after surgery, we saw small, significant rightward shifts in the fentanyl and morphine dose response curves in both sham and SNI groups suggesting that the observed shifts were not due to chronic pain. In both sham and SNI groups, there was an increase in the generalization of nalbuphine to the fentanyl-discriminative stimulus. There was no change in the discriminative stimulus of cocaine (or amphetamine substitutions) over 4 months of SNI-induced chronic neuropathic pain or sham states, suggesting that the SNI model failed to alter the discriminative stimuli of fentanyl and cocaine. Following induction of chronic neuropathic pain, there was an observed increase in quinpirole-induced generalization to the cocaine discriminative stimulus. In the future, studies should directly examine the abuse potential of low efficacy MOR agonists and dopaminergic agonists in the presence and absence of chronic pain states.

Optimizing Genomic DNA Extraction from Avian Feathers: A Modified Phenol-Chloroform Approach for Enhanced Efficiency and Cost-Effectiveness.

The procurement of blood and tissue samples for DNA extraction in avian species intended for molecular studies is associated with the induction of discomfort and pain in the subjects, compounded by practical challenges in application and ethical considerations. Consequently, feathers have emerged as a more prevalent source for molecular investigations, particularly in the fields of poultry and ornithology. However, the effective extraction of DNA from feathers necessitates the breakdown of the hard keratinized tissue within the feather structure. This study aimed to devise a highly efficient, cost-effective, and easily adaptable Modified Phenol-Chloroform (MPC) approach for genomic DNA extraction from feathers, addressing shortcomings identified in previous studies on feather-based DNA isolation. The MPC method was employed to extract genomic DNA from feather samples obtained from six distinct avian species (chicken, guinea fowl, canary, pigeon, emu, and goose). Comparative evaluation of DNA isolation efficiency was conducted by employing two different commercial DNA kits alongside the MPC method. The results showed significantly higher DNA concentrations (ng/ml) from chicken feathers using the MPC method compared to those obtained with commercial kits (p < 0.05), along with high DNA purity (1.83 ± 0.11). Subsequent PCR experiments, employing nuclear and mitochondrial DNA-specific primers, illustrated the effective amplification of short and long fragments from MPC-isolated DNA samples. In contrast to commercial kits, the findings underscore the successful application of the MPC method in isolating high-quality genomic DNA from feathers characterized by elevated keratin content.

Neuroinflammatory Pathways Associated with Chronic Post-Thoracotomy Pain: A Review of Current Literature.

Chronic post-thoracotomy pain (CPTP) is a major clinical problem that affects up to 35-55% of patients undergoing thoracic incisions. Evidence suggests that multiple cellular signaling pathways and neuro-inflammatory mediators may play an essential role in the pathogenesis of CPTP. In this comprehensive review, we present the current evidence on the cellular signaling pathways and inflammatory changes associated with the initiation and maintenance of CPTP, focusing on the potential application of these findings in the clinical setting. An electronic search of Medline, EMBASE, Cochrane, Google Scholar, and ClinicalTrials.gov was performed, and 3652 abstracts were identified. After an initial abstract screening, 131 studies underwent a full-text review, and nine papers were eventually included in this review. Studies were included if they assessed the cellular signaling pathways or inflammatory processes associated with the induction and/or maintenance of CPTP. All the identified studies were pre-clinical studies conducted on animal models. Our search identified seven cellular pathways (NK-1 receptor (NK-1), Glutaminase 1, Toll-like receptor 4 (TLR4), Resolvins, Ror-2, Sonic hedgehog signaling (Shh), and Wnt5a/Wnts) and six cytokines (IL-1β, IL-6, IL-8, IL-10, IFN-γ, and TNF-α) that were investigated in the context of CPTP. Multiple cellular signaling pathways and inflammatory cytokines may play an important role in the neuroinflammatory changes associated with the induction and maintenance of chronic post-thoracotomy pain in animal models. However, the clinical impact and therapeutic utility of these neuroinflammatory changes in routine clinical practice have yet to be demonstrated.

Impact of Experimentally Induced Pain on Logical Reasoning and Underlying Attention-Related Psychophysiological Mechanisms.

Pain is known to negatively impact attention, but its influence on more complex cognitive abilities, such as logical reasoning, remains inconsistent. This may be due to compensatory mechanisms (e.g., investing additional resources), which might not be detectable at the behavioral level but can be observed through psychophysiological measures. In this study, we investigated whether experimentally induced pain affects logical reasoning and underlying attentional mechanisms, using both behavioral and electroencephalographic (EEG) measures. A total of 98 female participants were divided into a pain-free control group (N = 47) and a pain group (N = 51). Both groups completed the Advanced Progressive Matrices (APM) task, with EEG recordings capturing task-related power (TRP) changes in the upper alpha frequency band (10-12 Hz). We used a mixed design where all participants completed half of the APM task in a pain-free state (control condition); the second half was completed under pain induction by the pain group but not the pain-free group (experimental condition). Logical reasoning performance, as measured by APM scores and response times, declined during the experimental condition, compared to the control condition for both groups, indicating that the second part of the APM was more difficult than the first part. However, no significant differences were found between the pain and pain-free groups, suggesting that pain did not impair cognitive performance at the behavioral level. In contrast, EEG measures revealed significant differences in upper alpha band power, particularly at fronto-central sites. In the pain group, the decrease in TRP during the experimental condition was significantly smaller compared to both the control condition and the pain-free group. Pain did not impair task performance at the behavioral level but reduced attentional resources, as reflected by changes in upper alpha band activity. This underscores the importance of incorporating more sensitive psychophysiological measures alongside behavioral measures to better understand the impact of pain on cognitive processes.

Decoding pain: prediction under different emotional contexts through physiological signals

AbstractPain recognition is a challenging yet crucial problem. Common methods used to measure pain are based on self-reported tools; however, not every person can communicate it. Therefore, automatic pain detection methods have emerged. Since pain is related to the emotional state of an individual, this variable must be considered. Thus, this work proposes pain prediction under different emotional contexts. For this purpose, data were collected during a protocol designed for pain induction with previous emotional elicitation. Emotions were elicited through videos composed of excerpts of documentaries, horror and comedy films, while the pain was induced through a Cold Pressor Test. Physiological signals, such as electrocardiogram, electrodermal activity and surface electromyogram, were collected during the protocol. Furthermore, several questionnaires were answered and pain reports were also registered. Two problems were addressed: pain classification and estimation of the Pain Tolerance score. The algorithm with the best performance for each problem was found using only data from the neutral session and nested cross-validation strategy. Using only physiological data from the neutral session, a F1-score of 99.32% was obtained for pain recognition and a mean absolute error (MAE) of 0.29 was obtained for Pain Tolerance estimation. When considering all the emotional sessions, the physiological data were merged with scores of the Visual Analogue Scale questionnaire, achieving a F1-score of 98.60% and a MAE of 0.41, for the first and second problems, respectively. These results are promising and stress out the key role that the emotional context of the individuals plays in pain prediction.

Chronic inflammatory pain reduces fentanyl intake during early acquisition of fentanyl self-administration, but does not change motivation to take fentanyl in male and female rats

The co-occurrence of chronic pain and opioid misuse has led to numerous preclinical investigations of pain-opioid interactions to examine how pain manipulations alter the reinforcing properties of opioids. However, preclinical investigations of chronic pain effects on opioid drug self-administration have produced inconsistent results. Our previous work demonstrated that established fentanyl self-administration is resistant to change by induction of chronic inflammatory pain (Complete Freund's Adjuvant; CFA) in male and female rats, while other laboratories have shown that CFA increased fentanyl self-administration in male but not female rats when pain induction precedes self-administration, which may be a critical factor in determining the effects of chronic pain on self-administration. The present study was designed similarly to Higginbotham et al. (2022) to test the effects of CFA on fentanyl self-administration in rats that underwent pain prior to acquisition of fentanyl self-administration. Male and female rats treated with hindpaw CFA or saline were trained to intravenously self-administer (IVSA) fentanyl for 3 weeks under limited access to fentanyl (2 h per day) conditions. After 3 weeks of fentanyl IVSA acquisition, we tested motivation to take fentanyl using progressive ratio testing and dose-response testing. CFA male and female rats self-administered less fentanyl than saline-treated controls during week 1 of acquisition, but not during weeks 2–3 of acquisition. Intra-session analysis of week 1 data demonstrated that chronic inflammatory pain suppressed fentanyl intake towards the end of week 1 and at the end of each operant session. We also report no effects of chronic inflammatory pain on motivation to take fentanyl. We discuss potential methodological explanations for differences between these results and prior reports. Our findings demonstrate that CFA temporarily suppresses fentanyl IVSA in animals without changing motivation to take fentanyl or promoting escalation of opioid use, suggesting that chronic inflammatory pain is unlikely to promote long-term risk of opioid misuse.

Neural mechanisms underlying placebo and nocebo effects in tonic muscle pain

Pain is a highly subjective and multidimensional experience, significantly influenced by various psychological factors. Placebo analgesia and nocebo hyperalgesia exemplify this influence, where inert treatments result in pain relief or exacerbation, respectively. While extensive research has elucidated the psychological and neural mechanisms behind these effects, most studies have focused on transient pain stimuli. To explore these mechanisms in the context of tonic pain, we conducted a study using a 15-minute tonic muscle pain induction procedure, where hypertonic saline was infused into the left masseter of healthy participants. We collected real-time Visual Analogue Scale (VAS) scores and functional magnetic resonance imaging (fMRI) data during the induction of placebo analgesia and nocebo hyperalgesia via conditioned learning. Our findings revealed that placebo analgesia was more pronounced and lasted longer than nocebo hyperalgesia. Real-time pain ratings correlated significantly with neural activity in several brain regions. Notably, the putamen was implicated in both effects, while the caudate and other regions were differentially involved in placebo and nocebo effects. These findings confirm that the tonic muscle pain paradigm can be used to investigate the mechanisms of placebo and nocebo effects and indicate that placebo analgesia and nocebo hyperalgesia may have more distinct than common neural bases.

Upregulation of Phosphodiesterase 7A Contributes to Concurrent Pain and Depression via Inhibition of cAMP-PKA-CREB-BDNF Signaling and Neuroinflammation in the Hippocampus of Mice.

Phosphodiesterases (PDEs) are enzymes that catalyze the hydrolysis of cyclic adenosine monophosphate AMP (cAMP) and/or cyclic guanosine monophosphate (cGMP). PDE inhibitors can mitigate chronic pain and depression when these disorders occur individually; however, there is limited understanding of their role in concurrent chronic pain and depression. We aimed to evaluate the mechanisms of action of PDE using 2 mouse models of concurrent chronic pain and depression. C57BL/6J mice were subjected to partial sciatic nerve ligation (PSNL) to induce chronic neuropathic pain or injected with complete Freund's adjuvant (CFA) to induce inflammatory pain, and both animals showed depression-like behavior. First, we determined the change in PDE expression in both animal models. Next, we determined the effect of PDE7 inhibitor BRL50481 or hippocampal PDE7A knockdown on PSNL- or CFA-induced chronic pain and depression-like behavior. We also investigated the role of cAMP-protein kinase A (PKA)-cAMP response element binding protein (CREB)-brain-derived neurotrophic factor (BDNF) signaling and neuroinflammation in the effect of PDE7A inhibition on PSNL- or CFA-induced chronic pain and depression-like behavior. This induction of chronic pain and depression in the 2 animal models upregulated hippocampal PDE7A. Oral administration of PDE7 inhibitor, BRL50481, or hippocampal PDE7A knockdown significantly reduced mechanical hypersensitivity and depression-like behavior. Hippocampal PDE7 inhibition reversed PSNL- or CFA-induced downregulation of cAMP and BDNF and the phosphorylation of PKA, CREB, and p65. cAMP agonist forskolin reversed these changes and caused milder behavioral symptoms of pain and depression. BRL50481 reversed neuroinflammation in the hippocampus in PSNL mice. Hippocampal PDE7A mediated concurrent chronic pain and depression in both mouse models by inhibiting cAMP-PKA-CREB-BDNF signaling. Inhibiting PDE7A or activating cAMP-PKA-CREB-BDNF signaling are potential strategies to treat concurrent chronic pain and depression.

Cross-Species Convergence of Functional Connectivity Changes in Thalamic Pain Across Human Patients and Model Macaques

Thalamic pain can be understood as a network reorganization disorder. This study aimed to investigate functional connectivity (FC) in human patients and a macaque model of thalamic pain. In humans, resting-state FC was compared between patients with thalamic pain and healthy individuals. Furthermore, resting-state FC was compared in macaques, before and after the induction of thalamic pain in the same individuals. FC between the amygdala of the unaffected hemisphere and the brainstem was significantly higher in patients with thalamic pain. More specifically, a significantly higher FC was observed between the basolateral amygdala and the ventral tegmental area, which also significantly predicted the value of a visual analog scale of pain intensity in individual patients. The macaque model of thalamic pain also exhibited a significantly higher FC between the amygdala of the unaffected hemisphere and the brainstem, particularly between the basolateral amygdala and the midbrain. Furthermore, the previously reported significantly higher FC between the amygdala and the mediodorsal nucleus of the thalamus in macaques with thalamic pain was also reproduced in the human patients. Therefore, the present results suggest that the FC changes in the regions associated with emotion, memory, motivation, and reward are part of the underlying mechanisms of thalamic pain onset present in both human patients and model macaques. This cross-species convergence provides new insights into the neurological mechanisms underlying thalamic pain, paving the way for further studies and the development of therapeutic strategies. PerspectiveThis article presents that the FC changes in the regions associated with emotion, motivation, and reward are part of the underlying mechanisms of thalamic pain in humans and macaques.

RNA-binding protein SYNCRIP contributes to neuropathic pain through stabilising CCR2 expression in primary sensory neurones

BackgroundNerve injury-induced changes in gene expression in the dorsal root ganglion (DRG) contribute to the genesis of neuropathic pain. SYNCRIP, an RNA-binding protein, is critical for the stabilisation of gene expression. Whether SYNCRIP participates in nerve injury-induced alterations in DRG gene expression and nociceptive hypersensitivity is unknown. MethodsThe expression and distribution of SYNCRIP in mouse DRG after chronic constriction injury (CCI) of the unilateral sciatic nerve were assessed. Effect of microinjection of Syncrip small interfering RNA into the ipsilateral L3 and L4 DRGs on the CCI-induced upregulation of chemokine (C-C motif) receptor 2 (CCR2) and nociceptive hypersensitivity were examined. Additionally, effects of microinjection of adeno-associated virus 5 expressing full length Syncrip mRNA (AAV5-Syncrip) on basal DRG CCR2 expression and nociceptive thresholds were observed. ResultsSYNCRIP is expressed predominantly in DRG neurones, where it co-exists with CCR2. Levels of Syncrip mRNA and SYNCRIP protein in injured DRG increased time-dependently on days 3–14 after CCI. Blocking this increase through microinjection of Syncrip small interfering RNA into injured DRG attenuated CCI-induced upregulation of DRG CCR2 and development and maintenance of nociceptive hypersensitivities. Mimicking this increase through DRG microinjection of AAV5-Syncrip elevated CCR2 expression in microinjected DRGs, enhanced the responses to mechanical, heat, and cold stimuli, and induced ongoing pain in naive mice. Mechanistically, SYNCRIP bound to 3-UTR of Ccr2 mRNA and stabilised its expression in DRG neurones. ConclusionsSYNCRIP contributes to the induction and maintenance of neuropathic pain likely through stabilising expression of CCR2 in injured DRG. SYNCRIP may be a potential target for treating this disorder.

Pain research in a petri dish? Advantages and limitations of neuro-glial primary cell cultures from structures of the nociceptive system

How can we learn more about pain without causing pain in humans or animals? This short review focuses on neuro-glial primary cell cultures as models to study neuro-immune interactions in the context of pain and discusses their advantages and limitations.The field of basic pain research places scientists in an ethical dilemma. We aim to understand underlying mechanisms of pain for an improved pain therapy for humans and animals. At the same time, this regularly includes the induction of pain in model animals. Within the field of psychoneuroimmunology, the examination of the complexity of neuro-immune interactions in health and disease as well as the bi-directional communication between the brain and the periphery make animal experiments an inevitable part of pain research. To address ethical and legal considerations as well as the growing societal awareness for animal welfare, scientists push for the identification and characterization of complementary methods to implement the 3R principle of Russel and Burch. As such, methods to replace animal studies, reduce the number of animals used, and refine experiments are tested. Neuro-glial primary cell cultures of structures of the nociceptive system, such as dorsal root ganglia (DRG) or the spinal dorsal horn (SDH) represent useful in vitro tools, when research comes to a cellular and molecular level. They allow for studying mechanisms of neuronal sensitization, glial cell activation, or the role of specific inflammatory mediators and intracellular signaling cascades involved in the development of inflammatory and neuropathic pain. Moreover, DRG/SDH-cultures provide the opportunity to test novel strategies for interventions, such as pharmaceuticals or cell-based therapies targeting neuroinflammatory processes. Thereby, in vitro models contribute to a better understanding of neuron-glia-immune communication in the context of pain and in the advancement of pain therapies. However, this can only be one piece in a large puzzle. Our knowledge about the complexity of pain will depend on studies in humans and animals applied in vitro and in vivo and will benefit from clear and open-minded interdisciplinary communication and transparency in public outreach.

Spinal cord microglia drive sex differences in ethanol-mediated PGE2-induced allodynia

The mechanisms of how long-term alcohol use can lead to persistent pain pathology are unclear. Understanding how earlier events of short-term alcohol use can lower the threshold of non-painful stimuli, described as allodynia could prove prudent to understand important initiating mechanisms. Previously, we observed that short-term low-dose alcohol intake induced female-specific allodynia and increased microglial activation in the spinal cord dorsal horn. Other literature describes how chronic ethanol exposure activates Toll-like receptor 4 (TLR4) to initiate inflammatory responses. TLR4 is expressed on many cell types, and we aimed to investigate whether TLR4 on microglia is sufficient to potentiate allodynia during a short-term/low-dose alcohol paradigm. Our study used a novel genetic model where TLR4 expression is removed from the entire body by introducing a floxed transcriptional blocker (TLR4-null background (TLR4LoxTB)), then restricted to microglia by breeding TLR4LoxTB animals with Cx3CR1:CreERT2 animals. As previously reported, after 14 days of ethanol administration alone, we observed no increased pain behavior. However, we observed significant priming effects 3 hrs post intraplantar injection of a subthreshold dose of prostaglandin E2 (PGE2) in wild-type and microglia-TLR4 restricted female mice. We also observed a significant female-specific shift to pro-inflammatory phenotype and morphological changes in microglia of the lumbar dorsal horn. Investigations in pain priming-associated neuronal subtypes showed an increase of c-Fos and FosB activity in PKCγ interneurons in the dorsal horn of female mice directly corresponding to increased microglial activity. This study uncovers cell- and female-specific roles of TLR4 in sexual dimorphisms in pain induction among non-pathological drinkers.

Modulatory Effects of Morphine and Xylopia aethioica Extract on Kappa Opiod Receptors (KOR), Delta Opioid Receptor (DOR), Pain Hypersensitivity and Motor Functions in Wistar Rats

This study investigates the modulatory effects of morphine and Xylopia aethiopica extract on kappa opioid receptors (KOR), delta opioid receptors (DOR), pain hypersensitivity, and motor functions in Wistar rats. We utilized three experimental groups: a control group receiving distilled water, a morphine group receiving either low (5 mg/kg) or high (10 mg/kg) doses after inducing pain, Xylopia aethiopica group received either 25 mg/kg or 50 mg/kg of hydromethanolic extract following similar pain induction. Pain perception was quantified using the tail flick test and Analgesy-Meter while motor functions were assessed through the Rotarod and Climbing/Beam Walk tests. Additionally, molecular docking studies were performed on selected compounds from Xylopia aethiopica to determine their binding affinities to opioid receptors using Vina. Results demonstrated that morphine and Xylopia aethiopica significantly increased tail flick response times, indicating notable analgesic effects, while improving motor functions particularly in animals treated with higher doses of Xylopia aethiopica. Molecular analysis revealed potential interactions between bioactive compounds and opioid receptors, suggesting further therapeutic applications. These findings highlight the potential of Xylopia aethiopica as a natural analgesic and its implications in managing pain and associated motor deficits, the findings further revealed that Morphine and not Xylopia aethiopica is implicated in pain hypersensitivity after long term exposure. Further research should pay attention on improving the pharmacological profiles of the identified compounds for clinical use.