Descending pain modulation and chronification of pain

Correlations of The Central Sensitization Inventory, conditioned pain modulation, cognitions and psychological factors in individuals with chronic neck pain: A cross-sectional study

INTRODUCTION

Chronic neck pain (CNP) is a global public health problem, with high prevalence and absenteeism rates. Central sensitization (CS) as a basis for chronic pain may play an essential role in its development and progression. It is often comorbid with low conditioned pain modulation (CPM) effects, cognitions, and psychological problems.

OBJECTIVES

The purposes of this study were to (1) explore the relationship between pain-related cognitions and psychological factors, CPM effects, and the central sensitization inventory (CSI) scores; and (2) determine whether cognitions and psychological factors can predict CSI scores and CPM effects in individuals with CNP.

METHODS

Fifty-four individuals with CNP were recruited for this cross-sectional study. The following outcome measures were evaluated: The CSI (screening tool) was compared with the cold pressor test (CPT), which was the psychophysical test used to assess the CPM; neck pain intensity using the visual analogue scale (VAS), as well as pain-related cognitions (including kinesiophobia and pain catastrophization) and psychological states (including anxiety and depression) using self-report questionnaires.

RESULTS

CSI score was not associated with the CPM effect (r = 0.257, p > 0.05), and no cognitions or psychological factors were associated with CPM (p > 0.05), but CSI score was moderately positively correlated with kinesiophobia (r = 0.554, p < 0.01), lowly positively correlated with pain catastrophization (r = 0.332, p = 0.017) and anxiety (r = 0.492, p < 0.01), but not depression (r = 0.207, p = 0.132). Multiple linear regression analysis showed that kinesiophobia (B = 1.308, p < 0.01) and anxiety (B = 1.806, p = 0.02) were significant positive predictors of CSI score.

CONCLUSIONS

The findings confirm some of our hypotheses. Accordingly, the findings inferred that the CSI does not seem to respond to CPM effect in patients with CNP effectively. In addition, CSI score was associated with cognitions and psychological factors, of which kinesiophobia and anxiety were effective predictors. In clinical practice, pain-related cognitions and psychological factors should be fully considered to manage neck pain efficiently.

Age-related differences in functional connectivity associated with pain modulation

Growing evidence suggests that aging is associated with impaired endogenous pain modulation, and that this likely underlies the increased transition from acute to chronic pain in older individuals. Resting-state functional connectivity (rsFC) offers a valuable tool to examine the neural mechanisms behind these age-related changes in pain modulation. RsFC studies generally observe decreased within-network connectivity due to aging, but its relevance for pain modulation remains unknown. We compared rsFC within a set of brain regions involved in pain modulation between young and older adults and explored the relationship with the efficacy of distraction from pain. This revealed several age-related increases and decreases in connectivity strength. Importantly, we found a significant association between lower pain relief and decreased strength of three connections in older adults, namely between the periaqueductal gray and right insula, between the anterior cingulate cortex (ACC) and right insula, and between the ACC and left amygdala. These findings suggest that the functional integrity of the pain control system is critical for effective pain modulation, and that its function is compromised by aging.

Investigating the brain functional abnormalities underlying pain hypervigilance in chronic neck and shoulder pain: a resting-state fMRI study

PURPOSE

To investigate pain hypervigilance in individuals suffering from chronic neck and shoulder pain (CNSP) and its underlying brain mechanism.

METHODS

The evaluation of pain vigilance was conducted through the utilization of pain vigilance and awareness questionnaires. Voxel-wise regional homogeneity (ReHo) from 60 CNSP patients and 60 healthy controls (HCs) using resting-state fMRI data. Voxel-wise two-sample T-test was conducted to reveal the ReHo variations between CNSP and HC. Correlation analyses were utilized to reveal the connection between brain abnormalities and medical measurements. Furthermore, a mediation analysis was conducted to elucidate the pathway-linking changes in brain function with medical measurements.

RESULTS

Our present study revealed three main findings. Firstly, patients with CSNP demonstrated a heightened vigilance of pain in comparison to healthy adults, a common occurrence among individuals with chronic pain conditions. Secondly, we observed brain abnormalities in various brain regions in CSNP patients, and these alterations were associated with the extent of pain vigilance. Lastly, the pain hypervigilance impact on the severity of pain was found to be controlled by regional neural activity in the anterior cingulate cortex (ACC) in subjects with CSNP.

CONCLUSION

Our findings suggested that long-term repetitive nociceptive input caused by chronic pain further aggravates the pain intensity by impairing the vigilance-related pain processing within the anterior cingulate cortex in CNSP patients.

The rostral ventromedial medulla modulates pain and depression-related behaviors caused by social stress

ABSTRACT

The rostral ventromedial medulla (RVM) is a crucial structure in the descending pain modulatory system, playing a key role as a relay for both the facilitation and inhibition of pain. The chronic social defeat stress (CSDS) model has been widely used to study stress-induced behavioral impairments associated with depression in rodents. Several studies suggest that CSDS also causes changes related to chronic pain. In this study, we aimed to investigate the involvement of the RVM in CSDS-induced behavioral impairments, including those associated with chronic pain. We used chemogenetics to activate or inhibit the RVM during stress. The results indicated that the RVM is a vital hub influencing stress outcomes. Rostral ventromedial medulla activation during CSDS ameliorates all the stress outcomes, including social avoidance, allodynia, hyperalgesia, anhedonia, and behavioral despair. In addition, RVM inhibition in animals exposed to a subthreshold social defeat stress protocol induces a susceptible phenotype, facilitating all stress outcomes. Finally, chronic RVM inhibition-without any social stress stimulus-induces chronic pain but not depressive-like behaviors. Our findings provide insights into the comorbidity between chronic pain and depression by indicating the involvement of the RVM in establishing social stress-induced behavioral responses associated with both chronic pain and depression.

Low-frequency (5-Hz) stimulation of ventrolateral periaqueductal gray modulates the descending serotonergic system in the peripheral neuropathic pain

ABSTRACT

Neuropathic pain is a type of chronic pain that entails severe prolonged sensory dysfunctions caused by a lesion of the somatosensory system. Many of those suffering from the condition do not experience significant improvement with existing medications, resulting in various side effects. In this study, Sprague-Dawley male rats were used, and long-term deep brain stimulation of the ventrolateral periaqueductal gray was conducted in a rat model of spared nerve injury. We found that 5-Hz deep brain stimulation effectively modulated mechanical allodynia and induced neuronal activation in the rostral ventromedial medulla, restoring impaired descending serotonergic system. At the spinal level, glial cells were still activated but only the 5-HT1a receptor in the spinal cord was activated, implying its inhibitory role in mechanical allodynia. This study found that peripheral neuropathy caused dysfunction in the descending serotonergic system, and prolonged stimulation of ventrolateral periaqueductal gray can modulate the pathway in an efficient manner. This work would provide new opportunities for the development of targeted and effective treatments for this debilitating disease, possibly giving us lower chances of side effects from repeated high-frequency stimulation or long-term use of medication.

Descending facilitation from rostral ventromedial medulla mu opioid receptor-expressing neurons is necessary for maintenance of sensory and affective dimensions of chronic neuropathic pain

ABSTRACT

Pharmacological ablation of rostral ventromedial medulla (RVM) mu opioid receptor-expressing cells before peripheral nerve injury prevents the development of neuropathic pain. However, whether these neurons are required for the expression of established neuropathic pain is not known. Male Oprm1Cre heterozygous (MORCre) or wild-type (MORWT) mice received AAV8-hSyn-DIO-hM4D(Gi)-mCherry in the RVM. After partial sciatic nerve ligation (PSNL), we evaluated pain behaviors and descending control of nociception in response to acute or sustained chemogenetic inhibition of RVM-MOR cells expressing hM4D(Gi). A single systemic administration of hM4D(Gi) agonist clozapine-N-oxide (CNO) reversibly inhibited hind paw tactile allodynia and produced conditioned place preference only in MORCre mice with PSNL. Intrathecal CNO also reversibly inhibited PSNL-induced hind paw allodynia, suggesting that the spinal projections from these RVM-MOR cells are critical for manifestation of pain behaviors. Consistent with enhanced descending facilitation from RVM-MOR cells, MORCre-hM4D(Gi) mice with PSNL showed diminished descending control of nociception that was restored by systemic CNO. Sustained CNO in drinking water before PSNL prevented expression of chronic pain without affecting acute surgical pain; however, relief of chronic pain required sustained CNO treatment. Thus, in male mice, activity of spinally projecting RVM-MOR cells is required (1) for expression and manifestation of both sensory and affective dimensions of established neuropathic pain and (2) to promote descending facilitation that overcomes apparently intact descending inhibition to maintain chronic pain. Enhanced descending facilitation likely regulates the output signal from the spinal cord to the brain to shape the pain experience and may provide a mechanism for nonopioid management of pain.

Closed-Loop Infraslow Brain-Computer Interface can Modulate Cortical Activity and Connectivity in Individuals With Chronic Painful Knee Osteoarthritis: A Secondary Analysis of a Randomized Placebo-Controlled Clinical Trial

Introduction. Chronic pain is a percept due to an imbalance in the activity between sensory-discriminative, motivational-affective, and descending pain-inhibitory brain regions. Evidence suggests that electroencephalography (EEG) infraslow fluctuation neurofeedback (ISF-NF) training can improve clinical outcomes. It is unknown whether such training can induce EEG activity and functional connectivity (FC) changes. A secondary data analysis of a feasibility clinical trial was conducted to determine whether EEG ISF-NF training can significantly alter EEG activity and FC between the targeted cortical regions in people with chronic painful knee osteoarthritis (OA). Methods. A parallel, two-arm, double-blind, randomized, sham-controlled clinical trial was conducted. People with chronic knee pain associated with OA were randomized to receive sham NF training or source-localized ratio ISF-NF training protocol to down-train ISF bands at the somatosensory (SSC), dorsal anterior cingulate (dACC), and uptrain pregenual anterior cingulate cortices (pgACC). Resting state EEG was recorded at baseline and immediate post-training. Results. The source localization mapping demonstrated a reduction (P = .04) in the ISF band activity at the left dorsolateral prefrontal cortex (LdlPFC) in the active NF group. Region of interest analysis yielded significant differences for ISF (P = .008), slow (P = .007), beta (P = .043), and gamma (P = .012) band activities at LdlPFC, dACC, and bilateral SSC. The FC between pgACC and left SSC in the delta band was negatively correlated with pain bothersomeness in the ISF-NF group. Conclusion. The EEG ISF-NF training can modulate EEG activity and connectivity in individuals with chronic painful knee osteoarthritis, and the observed EEG changes correlate with clinical pain measures.

Spared nerve injury leads to reduced activity of neurons projecting from the ventrolateral periaqueductal gray to the locus coeruleus

The ventrolateral periaqueductal gray (vlPAG) serves as a central hub for descending pain modulation. It receives upstream projections from the medial prefrontal cortex (mPFC) and the ventrolateral orbitofrontal cortex (vlOFC), and projects downstream to the locus coeruleus (LC) and the rostroventral medulla (RVM). While much research has focused on upstream circuits and the LC-RVM connection, less is known about the PAG-LC circuit and its involvement in neuropathic pain. Here we examined the intrinsic electrophysiological properties of vlPAG-LC projecting neurons in Sham and spared nerve injury (SNI) operated mice. Injection of the retrotracer Cholera Toxin Subunit B (CTB-488) into the LC allowed the identification of LC-projecting neurons in the vlPAG. Electrophysiological recordings from CTB-488 positive cells revealed that both GABAergic and glutamatergic cells that project to the LC exhibited reduced intrinsic excitability after peripheral nerve injury. By contrast, CTB-488 negative cells did not exhibit alterations in firing properties after SNI surgery. An SNI-induced reduction of LC projecting cells was confirmed with c-fos labeling. Hence, SNI induces plasticity changes in the vlPAG that are consistent with a reduction in the descending modulation of pain signals.

An overview of the safety and efficacy of LX-9211 in treating neuropathic pain conditions

INTRODUCTION

LX-9211 is a drug designed to treat neuropathic pain conditions. It functions by inhibiting the adaptor-associated kinase 1 (AAK1) enzyme which promotes clathrin-dependent endocytosis. Preclinical studies have shown that LX-9211 does produce a reduction in nociceptive related behaviors and produces no major adverse effects in rats. Thus, LX-9211 has advanced to clinical trials to assess its safety and efficacy in humans. So far, phase 1 and phase 2 clinical trials involving patients with postherpetic neuralgia and diabetic peripheral neuropathic pain have been conducted with phase 3 trials planned in the future.

AREAS COVERED

This paper highlights preclinical studies involving LX-9211 in rodents. Additionally, phase 1 clinical trials examining the safety of LX-9211 in healthy subjects as well as phase 2 studies looking at the safety and efficacy of LX-9211 compared to placebo in patients with diabetic peripheral neuropathic pain and postherpetic neuralgia are also discussed.

EXPERT OPINION

In phase 1 and phase 2 clinical trials conducted so far, LX-9211 has been shown to produce few adverse effects as well as cause a significantly greater reduction in pain compared to placebo. However, more clinical studies are needed to further assess its effects in humans to ensure its safety.

Altered Corticobrainstem Connectivity during Spontaneous Fluctuations in Pain Intensity in Painful Trigeminal Neuropathy

Chronic neuropathic pain can result from nervous system injury and can persist in the absence of external stimuli. Although ongoing pain characterizes the disorder, in many individuals, the intensity of this ongoing pain fluctuates dramatically. Previously, it was identified that functional magnetic resonance imaging signal covariations between the midbrain periaqueductal gray (PAG) matter, rostral ventromedial medulla (RVM), and spinal trigeminal nucleus are associated with moment-to-moment fluctuations in pain intensity in individuals with painful trigeminal neuropathy (PTN). Since this brainstem circuit is modulated by higher brain input, we sought to determine which cortical sites might be influencing this brainstem network during spontaneous fluctuations in pain intensity. Over 12 min, we recorded the ongoing pain intensity in 24 PTN participants and classified them as fluctuating (n = 13) or stable (n = 11). Using a PAG seed, we identified connections between the PAG and emotional-affective sites such as the hippocampal and posterior cingulate cortices, the sensory-discriminative posterior insula, and cognitive-affective sites such as the dorsolateral prefrontal (dlPFC) and subgenual anterior cingulate cortices that were altered dependent on spontaneous high and low pain intensity. Additionally, sliding-window functional connectivity analysis revealed that the dlPFC-PAG connection anticorrelated with perceived pain intensity over the entire 12 min period. These findings reveal cortical systems underlying moment-to-moment changes in perceived pain in PTN, which likely cause dysregulation in the brainstem circuits previously identified, and consequently alter the appraisal of pain across time.

Cutaneous Wound Healing and the Effects of Cannabidiol

Cutaneous wounds, both acute and chronic, begin with loss of the integrity, and thus barrier function, of the skin. Surgery and trauma produce acute wounds. There are 22 million surgical procedures per year in the United States alone, based on data from the American College of Surgeons, resulting in a prevalence of 6.67%. Acute traumatic wounds requiring repair total 8 million per year, 2.42% or 24.2 per 1000. The cost of wound care is increasing; it approached USD 100 billion for just Medicare in 2018. This burden for wound care will continue to rise with population aging, the increase in metabolic syndrome, and more elective surgeries. To heal a wound, an orchestrated, evolutionarily conserved, and complex series of events involving cellular and molecular agents at the local and systemic levels are necessary. The principal factors of this important function include elements from the neurological, cardiovascular, immune, nutritional, and endocrine systems. The objectives of this review are to provide clinicians engaged in wound care and basic science researchers interested in wound healing with an updated synopsis from recent publications. We also present data from our primary investigations, testing the hypothesis that cannabidiol can alter cutaneous wound healing and documenting their effects in wild type (C57/BL6) and db/db mice (Type 2 Diabetes Mellitus, T2DM). The focus is on the potential roles of the endocannabinoid system, cannabidiol, and the important immune-regulatory wound cytokine IL-33, a member of the IL-1 family, and connective tissue growth factor, CTGF, due to their roles in both normal and abnormal wound healing. We found an initial delay in the rate of wound closure in B6 mice with CBD, but this difference disappeared with time. CBD decreased IL-33 + cells in B6 by 70% while nearly increasing CTGF + cells in db/db mice by two folds from 18.6% to 38.8% (p < 0.05) using a dorsal wound model. We review the current literature on normal and abnormal wound healing, and document effects of CBD in B6 and db/db dorsal cutaneous wounds. CBD may have some beneficial effects in diabetic wounds. We applied 6-mm circular punch to create standard size full-thickness dorsal wounds in B6 and db/db mice. The experimental group received CBD while the control group got only vehicle. The outcome measures were rate of wound closure, wound cells expressing IL-33 and CTGF, and ILC profiles. In B6, the initial rate of wound closure was slower but there was no delay in the time to final closure, and cells expressing IL-33 was significantly reduced. CTGF + cells were higher in db/bd wounds treated with CBD. These data support the potential use of CBD to improve diabetic cutaneous wound healing.

Centralizing the Knowledge and Interpretation of Pain in Chemotherapy-Induced Peripheral Neuropathy: A Paradigm Shift towards Brain-Centric Approaches

Chemotherapy-induced peripheral neuropathy (CIPN) is a side effect of cancer treatment, often linked with pain complaints. Patients report mechanical and thermal hypersensitivity that may emerge during chemotherapy treatment and may persist after cancer remission. Whereas the latter situation disturbs the quality of life, life itself may be endangered by the appearance of CIPN during cancer treatment. The causes of CIPN have almost entirely been ascribed to the neurotoxicity of chemotherapeutic drugs in the peripheral nervous system. However, the central consequences of peripheral neuropathy are starting to be unraveled, namely in the supraspinal pain modulatory system. Based on our interests and experience in the field, we undertook a review of the brain-centered alterations that may underpin pain in CIPN. The changes in the descending pain modulation in CIPN models along with the functional and connectivity abnormalities in the brain of CIPN patients are analyzed. A translational analysis of preclinical findings about descending pain regulation during CIPN is reviewed considering the main neurochemical systems (serotoninergic and noradrenergic) targeted in CIPN management in patients, namely by antidepressants. In conclusion, this review highlights the importance of studying supraspinal areas involved in descending pain modulation to understand the pathophysiology of CIPN, which will probably allow a more personalized and effective CIPN treatment in the future.

Fucoidan as a Promising Drug for Pain Treatment: Systematic Review and Meta-Analysis

Fucoidan is a polymer of L-fucose and L-fucose-4-sulphate naturally found in marine sources that inhibits p-selectin, preventing neutrophil recruitment to the site of injury. Fucoidan is employed in many studies as a tool to investigate the contribution of neutrophils to pain, showing analgesic effects. We performed a systematic review and meta-analysis to quantify the analgesic effects of pretreatment with fucoidan reported in the available preclinical studies. In addition, we summarized the articles which have studied the therapeutic effects of fucoidan in pathological pain at preclinical and clinical levels. The results of this systematic review reveal that pretreatment with fucoidan is a powerful tool which reduces neutrophil infiltration by 70-90% at early time points. This meta-analysis showed that preventative treatment with fucoidan produced a significant pain reduction. In addition, several preclinical studies have observed that fucoidan treatment reduces the pain that is associated with various pathologies. Finally, fucoidan has also been tested in several clinical trials, with some degree of analgesic efficacy, but they were mostly small pilot studies. Considering all the above information, it can be concluded that fucoidan is not only a preclinical tool for studying the role of neutrophils in pain but also a promising therapeutic strategy for pain treatment.

Assessing the Influence of Nonischemic A-Fiber Conduction Blockade on Offset Analgesia: An Experimental Study

Offset analgesia (OA) is believed to reflect the efficiency of the endogenous pain modulatory system. However, the underlying mechanisms are still being debated. Previous research suggested both, central and peripheral mechanisms, with the latter involving the influence of specific A-delta-fibers. Therefore, this study aimed to investigate the influence of a nonischemic A-fiber conduction blockade on the OA response in healthy participants. A total of 52 participants were recruited for an A-fiber conduction blockade via compression of the superficial radial nerve. To monitor fiber-specific peripheral nerve conduction capacity, quantitative sensory testing was performed continuously. Before, during, and after the A-fiber block, an individualized OA paradigm was applied to the dorsum of both hands (blocked and control sides were randomized). The pain intensity of each heat stimulus was evaluated by an electronic visual analog scale. A successful A-fiber conduction blockade was achieved in thirty participants. OA has been verified within time (before, during, and after blockade) and condition (blocked and control side) (P < .01, d > .5). Repeated measurements analysis of variance showed no significant interaction effects between OA within condition and time (P = .24, η²p = .05). Hence, no significant effect of A-fiber blockade was detected on OA during noxious heat stimulation. The results suggest that peripheral A-fiber afferents may play a minor role in OA compared with alternative central mechanisms or other fibers. However, further studies are needed to substantiate a central rather than peripheral influence on OA. PERSPECTIVE: This article presents the observation of OA before, during, and after a successful A-fiber conduction blockade in healthy volunteers. A better understanding of the mechanisms of OA and endogenous pain modulation, in general, may help to explain the underlying aspects of pain disorders.

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.

Intravenous recombinant cerebellin 1 treatment restores signalling by spinal glutamate delta 1 receptors and mitigates chronic pain

BACKGROUND AND PURPOSE

Chronic pain remains a major clinical problem that needs effective therapeutic agents. Glutamate delta 1 (GluD1) receptors and the protein cerebellin 1 (Cbln1) are down-regulated in the central amygdala (CeA) in models of inflammatory and neuropathic pain. One treatment with Cbln1, intracerebroventricularly (ICV) or in CeA, normalized GluD1 and reduced AMPA receptor expression, resulting in lasting (7-10 days) pain relief. Unlike many CNS-targeting biological agents, the structure of Cbln1 suggests potential blood-brain barrier penetration. Here, we have tested whether systemic administration of Cbln1 provides analgesic effects via action in the CNS.

EXPERIMENTAL APPROACH

Analgesic effects of intravenous recombinant Cbln1 was assessed in complete Freund's adjuvant inflammatory pain model in mice. GluD1 knockout and a mutant form of Cbln1 were used.

KEY RESULTS

A single intravenous injection of Cbln1 mitigated nocifensive and averse behaviour in both inflammatory and neuropathic pain models. This effect of Cbln1 was dependent on GluD1 receptors and required binding to the amino terminal domain of GluD1. Time course of analgesic effect was similar to previously reported ICV and intra-CeA injection. GluD1 in both spinal cord and CeA was down -regulated in the inflammatory pain model, whereas GluD1 expression in spinal cord but not in CeA, was partly normalized by intravenous Cbln1. Importantly, recombinant Cbln1 was detected in the synaptoneurosomes in spinal cord but not in the CeA.

CONCLUSIONS AND IMPLICATIONS

Our results describe a novel mechanism by which systemic Cbln1 induces analgesia potentially by central actions involving normalization of signalling by spinal cord GluD1 receptors.

EEG-Based Cortical Alterations in Individuals With Chronic Knee Pain Secondary to Osteoarthritis: A Cross-sectional Investigation

Chronic painful knee osteoarthritis (OA) is a disabling physical health condition. Alterations in brain responses to arthritic changes in the knee may explain persistent pain. This study investigated source localized, resting-state electroencephalography activity and functional connectivity in people with knee OA, compared to healthy controls. Adults aged 44 to 85 years with knee OA (n = 37) and healthy control (n = 39) were recruited. Resting-state electroencephalography was collected for 10 minutes and decomposed into infraslow frequency (ISF) to gamma frequency bands. Standard low-resolution electromagnetic brain tomography statistical nonparametric maps were conducted, current densities of regions of interest were compared between groups and correlation analyses were performed between electroencephalography (EEG) measures and clinical pain and functional outcomes in the knee OA group. Standard low-resolution electromagnetic brain tomography nonparametric maps revealed higher (P = .006) gamma band activity over the right insula (RIns) in the knee OA group. A significant (P < .0001) reduction in ISF band activity at the pregenual anterior cingulate cortex, whereas higher theta, alpha, beta, and gamma band activity at the dorsal anterior cingulate cortex, pregenual anterior cingulate cortex, the somatosensory cortex, and RIns in the knee OA group were identified. ISF activity of the dorsal anterior cingulate cortex was positively correlated with pain measures and psychological distress scores. Theta and alpha activity of RIns were negatively correlated with pain interference. In conclusion, aberrations in infraslow and faster frequency EEG oscillations at sensory discriminative, motivational-affective, and descending inhibitory cortical regions were demonstrated in people with chronic painful knee OA. Moreover, EEG oscillations were correlated with pain and functional outcome measures. PERSPECTIVE: This study confirms alterations in the rsEEG oscillations and its relationship with pain experience in people with knee OA. The study provides potential cortical targets and the EEG frequency bands for neuromodulatory interventions for managing chronic pain experience in knee OA.

Inputs to the locus coeruleus from the periaqueductal gray and rostroventral medulla shape opioid-mediated descending pain modulation

The supraspinal descending pain modulatory system (DPMS) shapes pain perception via monoaminergic modulation of sensory information in the spinal cord. However, the role and synaptic mechanisms of descending noradrenergic signaling remain unclear. Here, we establish that noradrenergic neurons of the locus coeruleus (LC) are essential for supraspinal opioid antinociception. While much previous work has emphasized the role of descending serotonergic pathways, we find that opioid antinociception is primarily driven by excitatory output from the ventrolateral periaqueductal gray (vlPAG) to the LC. Furthermore, we identify a previously unknown opioid-sensitive inhibitory input from the rostroventromedial medulla (RVM), the suppression of which disinhibits LC neurons to drive spinal noradrenergic antinociception. We describe pain-related activity throughout this circuit and report the presence of prominent bifurcating outputs from the vlPAG to the LC and the RVM. Our findings substantially revise current models of the DPMS and establish a supraspinal antinociceptive pathway that may contribute to multiple forms of descending pain modulation.

Re-examining the Mysterious Role of the Cerebellum in Pain

Pain is considered a multidimensional experience that embodies not merely sensation, but also emotion and perception. As is appropriate for this complexity, pain is represented and processed by an extensive matrix of cortical and subcortical structures. Of these structures, the cerebellum is gaining increasing attention. Although association between the cerebellum and both acute and chronic pain have been extensively detailed in electrophysiological and neuroimaging studies, a deep understanding of what functions are mediated by these associations is lacking. Nevertheless, the available evidence implies that lobules IV-VI and Crus I are especially pertinent to pain processing, and anatomical studies reveal that these regions connect with higher-order structures of sensorimotor, emotional, and cognitive function. Therefore, we speculate that the cerebellum exerts a modulatory role in pain via its communication with sites of sensorimotor, executive, reward, and limbic function. On this basis, in this review, we propose numerous ways in which the cerebellum might contribute to both acute and chronic pain, drawing particular attention to emotional and cognitive elements of pain. In addition, we emphasise the importance of advancing our knowledge about the relationship between the cerebellum and pain by discussing novel therapeutic opportunities that capitalize on this association.

Selective modification of ascending spinal outputs in acute and neuropathic pain states

Pain hypersensitivity arises from the plasticity of peripheral and spinal somatosensory neurons, which modifies nociceptive input to the brain and alters pain perception. We utilized chronic calcium imaging of spinal dorsal horn neurons to determine how the representation of somatosensory stimuli in the anterolateral tract, the principal pathway transmitting nociceptive signals to the brain, changes between distinct pain states. In healthy conditions, we identify stable, narrowly tuned outputs selective for cooling or warming, and a neuronal ensemble activated by intense/noxious thermal and mechanical stimuli. Induction of an acute peripheral sensitization with capsaicin selectively and transiently retunes nociceptive output neurons to encode low-intensity stimuli. In contrast, peripheral nerve injury-induced neuropathic pain results in a persistent suppression of innocuous spinal outputs coupled with activation of a normally silent population of high-threshold neurons. These results demonstrate the differential modulation of specific spinal outputs to the brain during nociceptive and neuropathic pain states.

Ayahuasca and its major component harmine promote antinociceptive effects in mouse models of acute and chronic pain

ETHNOPHARMACOLOGICAL RELEVANCE

Ayahuasca (AYA) is a psychedelic brew used in religious ceremonies. It is broadly used as a sacred medicine for treating several ailments, including pain of various origins.

AIM OF THE STUDY

To investigate the antinociceptive effects of AYA and its mechanisms in preclinical models of acute and chronic pain in mice, in particular during experimental neuropathy.

MATERIALS AND METHODS

The antinociceptive effects of AYA administered orally were assessed in the following models of pain: formalin test, Complete Freund's Adjuvant (CFA)-induced inflammation, tail flick test, and partial sciatic nerve ligation model of neuropathic pain. Antagonism assays and Fos immunohistochemistry in the brain were performed. AYA-induced toxicity was investigated. AYA was chemically characterized. The antinociceptive effect of harmine, the major component present in AYA, was investigated.

RESULTS

AYA (24-3000 μL/kg) dose-dependently reduced formalin-induced pain-like behaviors and CFA-induced mechanical allodynia but did not affect CFA-induced paw edema or tail flick latency. During experimental neuropathy, single treatments with AYA (24-3000 μL/kg) reduced mechanical allodynia; daily treatments once or twice a day for 14 days promoted consistent and sustained antinociception. The antinociceptive effect of AYA (600 μL/kg) was reverted by bicuculline (1 mg/kg) and methysergide (5 mg/kg), but not by naloxone (5 mg/kg), phaclofen (2 mg/kg), and rimonabant (10 mg/kg), suggesting the roles of GABAA and serotonergic receptors. AYA increased Fos expression in the ventrolateral periaqueductal gray and nucleus raphe magnus after 1 h, but not after 6 h or 14 days of daily treatments. AYA (600 μL/kg) twice a day for 14 days did not alter mice's motor function, spontaneous locomotion, body weight, food and water intake, hematological, biochemical, and histopathological parameters. Harmine (3.5 mg/kg) promoted consistent antinociception during experimental neuropathy.

CONCLUSIONS

AYA promotes consistent antinociceptive effects in different mouse models of pain without inducing detectable toxic effects. Harmine is at least partially accountable for the antinociceptive properties of AYA.

Personality systems interactions theory: an integrative framework complementing the study of the motivational and volitional dynamics underlying adjustment to chronic pain

In the endeavor to advance our understanding of interindividual differences in dealing with chronic pain, numerous motivational theories have been invoked in the past decade. As they focus on relevant, yet different aspects of the dynamic, multilevel processes involved in human voluntary action control, research findings seem fragmented and inconsistent. Here we present Personality Systems Interactions theory as an integrative meta-framework elucidating how different motivational and volitional processes work in concert under varying contextual conditions. PSI theory explains experience and behavior by the relative activation of four cognitive systems that take over different psychological functions during goal pursuit. In this way, it may complement existing content-related explanations of clinical phenomena by introducing a functional, third-person perspective on flexible goal management, pain acceptance and goal maintenance despite pain. In line with emerging evidence on the central role of emotion regulation in chronic pain, PSI theory delineates how the self-regulation of positive and negative affect impacts whether behavior is determined by rigid stimulus-response associations (i.e., habits) or by more abstract motives and values which afford more behavioral flexibility. Along with testable hypotheses, multimodal interventions expected to address intuitive emotion regulation as a central process mediating successful adaptation to chronic pain are discussed.

Chronic nonspecific multiple-sites pain [CNMSP] of unknown etiology: Biopsychosocial method of evaluation for the primary care level

Background

Understanding and dealing with chronic nonspecific pain (CNP) is the important entity at primary care hospital. Chronic nonspecific multiple-site pain [CNMSP] of unknown etiology creates diagnostic and therapeutic challenges for primary care physicians due to lack of guidance regarding evaluation and treatment.

Aims and Objectives

To classify and formulate the evaluation, treatment strategies, and prediction of prognosis of patients with CNMSP of unknown etiology.

Methods

Patients present with CNMSP of more than 3-month duration without any obvious medical cause. The biopsychosocial [BPS] model with 3P model was applied to see the biological, psychological, and social factors behind persistence. Finally, patients were classified into four groups for evaluation response to treatment and relapse rates in 12-month follow-up.

Results

Of the total 243 patients of CNMSP, 243 [96.3%] were females. Sixty [24.7%] patients had short duration, and 183 [75.3%] had long duration. Headache was in 115 [47%], low back pain ± leg pain in 96 [39.4%], cervical pain ± shoulder/arm pain in 83 [34.1%], and diffuse body pain in 50 [20.5%] in various combinations. A total of 155 [63.8%] patients had high somatization-sensitization index (SSI), and 144 [59.3%] had low ferritin level. Group 1 [high SSI and low ferritin] had 37.9% of patients, group 2 [high SSI and normal ferritin] had 25.9% of patients, group 3 [low to medium SSI with low ferritin] had 21.4% of patients, and group 4 [low to medium SSI with normal ferritin] had 14.8% of patients. Response to pain symptoms was better in group 1, and relapse rate was higher in group 2.

Conclusion

CNMSP of unknown etiology itself is a heterogeneous entity, and assessment based on the BPS model can be very useful to understand the treatment plan and outcome of these patients.

Association between myofascial trigger point therapy and conditioned pain modulation

INTRODUCTION

Myofascial trigger point therapy (MTrP) is a widely used therapeutic approach, although the underlying mechanisms remain unclear. Mechanisms discussed include peripheral involvement of muscles as well as central pain modulating processes such as the conditioned pain modulation (CPM). The aim of this study was to investigate whether the analgesic response of MTrP and the analgesic response of CPM correlate in asymptomatic participants in order to identify shared underlying mechanisms of MTrP and CPM.

METHOD

Both, CPM and MTrP protocols consisted of heat-based test stimuli (heat pain thresholds before and after the intervention) and pressure-based (conditioning) stimuli. Asymptomatic participants (n = 94) were randomly assigned to receive either mild, intense or no pressure stimuli (between-group design) to both the fingernail and the MTrP of the infraspinatus muscle (within-group design). Pressure stimuli at both locations (fingernail, MTrP) were applied with a pressure algometer for 120 s and continuously adjusted to maintain a constant pain intensity of mild or intense pain. All thermal stimuli were applied on the lower leg with a thermal stimulator.

RESULTS

A significant correlation was shown between the analgesic effect of CPM and MTrP therapy for mild (r = 0.53, p = 0.002) and intensive stimuli (r = 0.73, p < 0.001). 17.3% of the variance of the MTrP effect were explained by CPM after mild stimulation, and 47.1% after intense stimulation. Pain-related characteristics did not explain the variance within the analgesic response using a regression analysis.

CONCLUSIONS

Between the analgesic responses following MTrP and CPM paradigms, a moderate to strong correlation was observed, suggesting shared underlying mechanisms.

Sex differences in conditioned pain modulation effects and its associations with autonomic nervous system activities in healthy, younger individuals: a pilot study

Introduction

Sex differences in conditioned pain modulation (CPM) have not been sufficiently explored.

Objectives

This pilot study aimed to examine sex differences in CPM effects and associations between autonomic activities and CPM effects in healthy, younger individuals.

Methods

University students were recruited from February to March 2021 and divided by sex. They remained seated for 10 minutes as a rest period, then immersed their right hands in cold water for 2 minutes as a cold period. The pressure pain threshold (PPT) was measured after each period, presenting the CPM index (%) using the formula: (PPTcold/PPTrest) × 100. Autonomic nervous system variables were calculated using the formula-(autonomic variablecold/autonomic variablerest) × 100-and suffixed by "index" such as low-frequency/high-frequency (LF/HF) index. Some psychological questionnaires were self-recorded. Sex differences in the CPM index were statistically compared, and a simple linear regression analysis between the CPM and autonomic indices was conducted.

Results

Thirty-two participants were analyzed (14 women and 18 men; aged 21.1 ± 0.6 and 20.9 ± 0.3 years, respectively). Conditioned pain modulation effects were not different at 127.0 ± 19.1% in women and 124.0 ± 18.7% in men. The LF/HF index, LF normalized unit (nu) index (LFnu), and HFnu index had significant predictor variables for the CPM index across overall samples. The LF/HF index and LFnu index were significant predictor variables for the CPM index for women but not for men.

Conclusions

Conditioned pain modulation effects between groups seem to be similar. The LF/HF and LFnu indices in women were significant, indicating that descending pain modulations in women might be more associated with autonomic activities than those in men.

Gender Differences in Pain Threshold, Unpleasantness, and Descending Pain Modulatory Activation Across the Adult Life Span: A Cross Sectional Study

The neurobiological underpinnings of gender differences in pain perception, and how these differences may be modified by age, are incompletely understood, placing patients at risk of suboptimal pain management. Using functional magnetic resonance imaging, we examined brain responses in the descending pain modulatory system (DPMS, specifically, dorsolateral prefrontal cortex, anterior cingulate cortex, insula, hypothalamus, amygdala, and periaqueductal gray, during an evoked pain task. We investigated the interaction of age and gender in our sample of healthy adults (27 females, 32 males, 30-86 years) on DPMS response. In a perceptually matched thermal pain paradigm, we investigated pain unpleasantness and neural responses for 3 heat pain percepts: just noticeable pain, weak pain, and moderate pain (MP). Females reported just noticeable pain at a lower temperature, but reported less unpleasantness at weak pain and MP percepts, compared to males. There was a significant age-by-gender interaction during moderate pain in the right anterior cingulate cortex and bilateral insula, such that, males had a stronger positive relationship between DPMS response and age compared to females in these regions. Our results indicate that differences in DPMS responses may explain some gender differences in pain perception and that this effect may change across the adult lifespan. PERSPECTIVE: Gender differences in pain have been well-documented but the brain mechanisms for these differences are still unclear. This article describes potential differences in brain functioning during different levels of pain that could explain differences in pain responses between men and women across the adult lifespan.

Restoration of the Activity of the Prefrontal Cortex to the Nucleus Accumbens Core Pathway Relieves Fentanyl-Induced Hyperalgesia in Male Rats

Purpose

Functional connectivity between the prelimbic medial prefrontal cortex (PL-mPFC) and the core of the nucleus accumbens (NAc core) predicts pain chronification. Inhibiting the apoptosis of oligodendrocytes in the PL-mPFC prevents fentanyl-induced hyperalgesia in rats. However, the role of prefrontal cortex (PFC)-NAc projections in opioid-induced hyperalgesia (OIH) remains unclear. Herein, we explored the role of the PL-NAc core circuit in fentanyl-induced hyperalgesia.

Methods

An OIH rat model was established, and patch-clamp recording, immunofluorescence, optogenetics, and chemogenetic methods were employed for neuron excitability detection and nociceptive behavioral assessment.

Results

Our results showed decreased activity of the right PL-mPFC layer V output neurons in rats with OIH. Similarly, the excitability of the NAc core neurons receiving glutamatergic projections from the PL-mPFC decreased in OIH rats, observed by the light-evoked excitatory postsynaptic currents/light-excited inhibitory postsynaptic currents ratio (eEPSC/eIPSC ratio). Fentanyl-induced hyperalgesia was reversed by optogenetic activation of the PL-NAc core pathway, and chemogenetic suppression of this pathway induced hyperalgesia in control (saline-treated) rats. However, behavioral hyperalgesia was not aggravated by this chemogenetic suppression in OIH (fentanyl-treated) rats.

Conclusion

Our findings indicate that inactivation of the PL-NAc core pathway may be a cause of OIH and restoring the activity of this pathway may provide a strategy for OIH treatment.

Association between stress and catecholamines with painful TMD in children

BACKGROUND

Studying biomarkers in children with temporomandibular disorders (TMD) such as epinephrine, norepinephrine, and dopamine may reveal factors like screen time or sleep loss that affect these biomarkers and predict TMD-related pain, offering new research opportunities.

AIM

To determine the association between stress and catecholamines with myofascial pain and headache related to TMD in children.

DESIGN

Sixty-six 9- to 11-year-old children assisting at the clinics of Pediatric Dentistry of Universidad CES participated in the study. Myofascial pain and headache attributed to TMD were determined according to the Diagnostic Criteria for TMD (DC/TMD) Axis I. Stress was evaluated with the Perceived Stress Scale-Children (PSS-C), and a 24-h urine sample was analyzed using liquid chromatography to assay catecholamines. Single and multiple regression analyses were performed.

RESULTS

Children with a mean age of 10.3 years participated in the study. The mean score of stress was 29 ± 4. Perceived stress, dopamine, epinephrine, and norepinephrine were statistically significant predictors of myofascial pain and headache attributed to TMD in the single- and multiple variable logistic regression analyses.

CONCLUSION

Stressful states and its biological biomarkers increase the probability of developing myofascial pain and headache attributed to TMD in children.

Mechanisms of pain in aging and age-related conditions: Focus on caregivers

Pain is a complex, subjective experience that can significantly impact quality of life, particularly in aging individuals, by adversely affecting physical and emotional well-being. Whereas acute pain usually serves a protective function, chronic pain is a persistent pathological condition that contributes to functional deficits, cognitive decline, and emotional disturbances in the elderly. Despite substantial progress that has been made in characterizing age-related changes in pain, complete mechanistic details of pain processing mechanisms in the aging patient remain unknown. Pain is particularly under-recognized and under-managed in the elderly, especially among patients with Alzheimer's disease (AD), Alzheimer's disease-related dementias (ADRD), and other age-related conditions. Furthermore, difficulties in assessing pain in patients with AD/ADRD and other age-related conditions may contribute to the familial caregiver burden. The purpose of this article is to discuss the mechanisms and risk factors for chronic pain development and persistence, with a particular focus on age-related changes. Our article also highlights the importance of caregivers working with aging chronic pain patients, and emphasizes the urgent need for increased legislative awareness and improved pain management in these populations to substantially alleviate caregiver burden.

Exercise-induced hypoalgesia in chronic neck pain: A narrative review

Chronic neck pain (CNP) is a worldwide health problem with several risk factors. One of the most widely used treatments for managing this condition is therapeutic exercise, which could generate a response called exercise-induced hypoalgesia (EIH). There is no consensus on the best exercise modality to induce hypoalgesia. Therefore, this review aims to analyze and synthesize the state-of-the-art about the hypoalgesic effect of exercise in subjects with CNP. We included articles on EIH and CNP in patients older than 18 years, with pain for more than three months, where the EIH response was measured. Articles that studied CNP associated with comorbidities or measured the response to treatments other than exercise were excluded. The studies reviewed reported variable results. Exercise in healthy subjects has been shown to reduce indicators of pain sensitivity; however, in people with chronic pain, the response is variable. Some investigations reported adverse effects with increased pain intensity and decreased pain sensitivity, others found no clinical response, and some even reported EIH with decreased pain and increased sensitivity. EIH is an identifiable, stimulable, and helpful therapeutic response in people with pain. More research is still needed on subjects with CNP to clarify the protocols and therapeutic variables that facilitate the EIH phenomenon. In addition, it is necessary to deepen the knowledge of the intrinsic and extrinsic factors that influence EIH in people with CNP.

Pharmacological and Non-pharmacological Approaches for the Management of Neuropathic Pain in Multiple Sclerosis

Multiple sclerosis is a chronic inflammatory disease that affects the central nervous system and can cause various types of pain including ongoing extremity pain, Lhermitte's phenomenon, trigeminal neuralgia, and mixed pain. Neuropathic pain is a major concern for individuals with multiple sclerosis as it is directly linked to myelin damage in the central nervous system and the management of neuropathic pain in multiple sclerosis is challenging as the options available have limited efficacy and can cause unpleasant side effects. The literature search was conducted across two databases, PubMed, and Google Scholar. Eligible studies included clinical trials, observational studies, meta-analyses, systematic reviews, and narrative reviews. The objective of this article is to provide an overview of literature on pharmacological and non-pharmacological strategies employed in the management of neuropathic pain in multiple sclerosis. Pharmacological options include cannabinoids, muscle relaxants (tizanidine, baclofen, dantrolene), anticonvulsants (benzodiazepines, gabapentin, phenytoin, carbamazepine, lamotrigine), antidepressants (duloxetine, venlafaxine, tricyclic antidepressants), opioids (naltrexone), and botulinum toxin variants, which have evidence from various clinical trials. Non-pharmacological approaches for trigeminal neuralgia may include neurosurgical methods. Non-invasive methods, physical therapy, and psychotherapy (cognitive behavioral therapy, acceptance and commitment therapy and mindfulness-based stress reduction) may be recommended for patients with neuropathic pain in multiple sclerosis. The choice of treatment depends on the severity and type of pain as well as other factors, such as patient preferences and comorbidities. There is a pressing need for healthcare professionals and researchers to prioritize the development of better strategies for managing multiple sclerosis-induced neuropathic pain.

Psycho-Neuroendocrinology in the Rehabilitation Field: Focus on the Complex Interplay between Stress and Pain

Chronic stress and chronic pain share neuro-anatomical, endocrinological, and biological features. However, stress prepares the body for challenging situations or mitigates tissue damage, while pain is an unpleasant sensation due to nociceptive receptor stimulation. When pain is chronic, it might lead to an allostatic overload in the body and brain due to the chronic dysregulation of the physiological systems that are normally involved in adapting to environmental challenges. Managing stress and chronic pain (CP) in neurorehabilitation presents a significant challenge for healthcare professionals and researchers, as there is no definitive and effective solution for these issues. Patients suffering from neurological disorders often complain of CP, which significantly reduces their quality of life. The aim of this narrative review is to examine the correlation between stress and pain and their potential negative impact on the rehabilitation process. Moreover, we described the most relevant interventions used to manage stress and pain in the neurological population. In conclusion, this review sheds light on the connection between chronic stress and chronic pain and their impact on the neurorehabilitation pathway. Our results emphasize the need for tailored rehabilitation protocols to effectively manage pain, improve treatment adherence, and ensure comprehensive patient care.

How do opioids control pain circuits in the brainstem during opioid-induced disorders and in chronic pain? Implications for the treatment of chronic pain

ABSTRACT

Brainstem areas involved in descending pain modulation are crucial for the analgesic actions of opioids. However, the role of opioids in these areas during tolerance, opioid-induced hyperalgesia (OIH), and in chronic pain settings remains underappreciated. We conducted a revision of the recent studies performed in the main brainstem areas devoted to descending pain modulation with a special focus on the medullary dorsal reticular nucleus (DRt), as a distinctive pain facilitatory area and a key player in the diffuse noxious inhibitory control paradigm. We show that maladaptive processes within the signaling of the µ-opioid receptor (MOR), which entail desensitization and a switch to excitatory signaling, occur in the brainstem, contributing to tolerance and OIH. In the context of chronic pain, the alterations found are complex and depend on the area and model of chronic pain. For example, the downregulation of MOR and δ-opioid receptor (DOR) in some areas, including the DRt, during neuropathic pain likely contributes to the inefficacy of opioids. However, the upregulation of MOR and DOR, at the rostral ventromedial medulla, in inflammatory pain models, suggests therapeutic avenues to explore. Mechanistically, the rationale for the diversity and complexity of alterations in the brainstem is likely provided by the alternative splicing of opioid receptors and the heteromerization of MOR. In conclusion, this review emphasizes how important it is to consider the effects of opioids at these circuits when using opioids for the treatment of chronic pain and for the development of safer and effective opioids.

Subgrouping of facilitatory or inhibitory conditioned pain modulation responses in patients with chronic knee pain. Explorative analysis from a multicentre trial

BACKGROUND

Facilitatory and inhibitory conditioned pain modulation (CPM) responses are observed in healthy volunteers and chronic pain patients, but the clinical implications for phenotyping are unknown. This study aimed to subgroup and compare chronic knee pain patients according to their CPM responses.

METHODS

This explorative, cross-sectional study included 127 patients with chronic knee pain (osteoarthritis or following total knee arthroplasty). Individual CPM responses were categorized as facilitatory (test stimuli pain intensity increased when conditioning stimuli were applied), as inhibitory (test stimuli pain intensity decreased) or as no change (defined as less than 5.3% change in pain intensity). Outcomes were clinical pain intensities, temporal summation, widespread pain, self-reported physical function, PainDETECT questionnaire and Pain Quality Assessment Scale. Data were analysed as comparisons between the inhibitory and the facilitatory groups and using multivariate linear regression models.

RESULTS

Fifty-four patients had facilitatory CPM responses, 49 had inhibitory CPM responses, and 24 showed no change in CPM response. A between-group difference was observed for self-reported physical function, with the facilitatory CPM group reporting better function (54.4 vs. 46.0, p = 0.028) and the facilitatory CPM group reported more deep pain sensations (3.2 vs. 2.0, p = 0.021). The remaining outcomes showed no between-group differences. Higher clinical pain intensity and facilitated temporal summation were associated in the facilitated CPM group but not in the inhibitory CPM group.

CONCLUSION

These explorative findings indicated that quantitative clinical and experimental differences exist between facilitatory or inhibitory CPM responses in a chronic knee pain patient population. Differences in patients' CPM responses should be further investigated to unravel possible clinical importance.

SIGNIFICANCE

Our findings confirm that conditioned pain modulation consist of inhibitory and facilitatory responders among a patient population with chronic knee pain. This explorative study indicates that patients with either facilitatory or inhibitory conditioned pain modulation could exhibit differences in pain outcomes. Subgrouping of chronic pain patients depending on individual conditioned pain modulation responses could be considered in phenotyping patients prior to inclusion in clinical trials or used for personalizing the management regime.

Understanding the Biological Relationship between Migraine and Depression

Migraine is a highly prevalent neurological disorder. Among the risk factors identified, psychiatric comorbidities, such as depression, seem to play an important role in its onset and clinical course. Patients with migraine are 2.5 times more likely to develop a depressive disorder; this risk becomes even higher in patients suffering from chronic migraine or migraine with aura. This relationship is bidirectional, since depression also predicts an earlier/worse onset of migraine, increasing the risk of migraine chronicity and, consequently, requiring a higher healthcare expenditure compared to migraine alone. All these data suggest that migraine and depression may share overlapping biological mechanisms. Herein, this review explores this topic in further detail: firstly, by introducing the common epidemiological and risk factors for this comorbidity; secondly, by focusing on providing the cumulative evidence of common biological aspects, with a particular emphasis on the serotoninergic system, neuropeptides such as calcitonin-gene-related peptide (CGRP), pituitary adenylate cyclase-activating polypeptide (PACAP), substance P, neuropeptide Y and orexins, sexual hormones, and the immune system; lastly, by remarking on the future challenges required to elucidate the etiopathological mechanisms of migraine and depression and providing updated information regarding new key targets for the pharmacological treatment of these clinical entities.

Gastrointestinal symptoms in patients with postural orthostatic tachycardia syndrome in relation to hemodynamic findings and immunological factors

Gastrointestinal (GI) symptoms are common in postural orthostatic tachycardia syndrome (POTS). We aimed to explore the prevalence and severity of GI symptoms in POTS, and to investigate immunological factors, hemodynamic findings, and their possible association with GI symptoms in POTS. Forty-three patients (93% female, median age 30.6 (26.0-41.0) years), previously diagnosed with POTS and 74 healthy controls (78% female, median age 35.6 (28.8-41.7) years) were included. The participants completed a questionnaire including prevalence of GI symptoms, the irritable bowel syndrome severity scoring system (IBS-SSS), and visual analog scale for IBS (VAS-IBS). All POTS patients were previously examined by tilt test (2010-2021) and the vast majority with more recent active standing test (2017-2021), which included monitoring of heart rate (HR). ΔHR was calculated as difference between supine and upright position. Continuous variables from IBS-SSS and VAS-IBS were correlated to ΔHR. A microarray containing several autoantigens commonly targeted in systemic autoimmune disorders was used to assess prevalent autoantibodies in POTS and controls. Total IgE and S-tryptase were analyzed. GI symptoms were more prevalent and severe in POTS than in controls; nausea being the most prevalent (79.1% vs 4.9%, p < 0.001) and bloating and flatulence being the most severe (median 65 (25-88) vs 0 (0-14), p < 0.001). The median total IBS-SSS was 213 (135-319) in POTS vs 13 (0-54) in controls (p < 0.001). Total IBS-SSS was associated with low psychological wellbeing (r = 0.539, p < 0.001) in POTS. ΔHRmax correlated inversely with abdominal pain (r = -0.406, p = 0.007). After adjustments for psychological wellbeing, total IBS-SSS still associated inversely with ΔHR10min (β: 4.748; 95% CI: -9.172 to -0.324; p = 0.036). Similar results were seen with active standing test. The prevalence of autoantibodies did not differ between POTS and controls (29.4% vs 33.3%, p = 0.803). There was no association between GI symptoms and autoantibody status. Total IgE and tryptase were elevated in a few cases. This study confirms the high prevalence of GI symptoms in POTS. More pronounced tachycardia upon tilt table testing seems to be inversely correlated with severity of chronic GI symptoms in POTS. This study did not support the hypothesis that POTS is associated with immunological factors.

Pain-sensorimotor interactions: New perspectives and a new model

How pain and sensorimotor behavior interact has been the subject of research and debate for many decades. This article reviews theories bearing on pain-sensorimotor interactions and considers their strengths and limitations in the light of findings from experimental and clinical studies of pain-sensorimotor interactions in the spinal and craniofacial sensorimotor systems. A strength of recent theories is that they have incorporated concepts and features missing from earlier theories to account for the role of the sensory-discriminative, motivational-affective, and cognitive-evaluative dimensions of pain in pain-sensorimotor interactions. Findings acquired since the formulation of these recent theories indicate that additional features need to be considered to provide a more comprehensive conceptualization of pain-sensorimotor interactions. These features include biopsychosocial influences that range from biological factors such as genetics and epigenetics to psychological factors and social factors encompassing environmental and cultural influences. Also needing consideration is a mechanistic framework that includes other biological factors reflecting nociceptive processes and glioplastic and neuroplastic changes in sensorimotor and related brain and spinal cord circuits in acute or chronic pain conditions. The literature reviewed and the limitations of previous theories bearing on pain-sensorimotor interactions have led us to provide new perspectives on these interactions, and this has prompted our development of a new concept, the Theory of Pain-Sensorimotor Interactions (TOPSMI) that we suggest gives a more comprehensive framework to consider the interactions and their complexity. This theory states that pain is associated with plastic changes in the central nervous system (CNS) that lead to an activation pattern of motor units that contributes to the individual's adaptive sensorimotor behavior. This activation pattern takes account of the biological, psychological, and social influences on the musculoskeletal tissues involved in sensorimotor behavior and on the plastic changes and the experience of pain in that individual. The pattern is normally optimized in terms of biomechanical advantage and metabolic cost related to the features of the individual's musculoskeletal tissues and aims to minimize pain and any associated sensorimotor changes, and thereby maintain homeostasis. However, adverse biopsychosocial factors and their interactions may result in plastic CNS changes leading to less optimal, even maladaptive, sensorimotor changes producing motor unit activation patterns associated with the development of further pain. This more comprehensive theory points towards customized treatment strategies, in line with the management approaches to pain proposed in the biopsychosocial model of pain.

Suppression of neuropathic pain in the circadian clock-deficient Per2m/m mice involves up-regulation of endocannabinoid system

Neuropathic pain often results from injuries and diseases that affect the somatosensory system. Disruption of the circadian clock has been implicated in the exacerbation of the neuropathic pain state. However, in this study, we report that mice deficient in a core clock component Period2 (Per2m/m mice) fail to develop tactile pain hypersensitivity even following peripheral nerve injury. Similar to male wild-type mice, partial sciatic nerve ligation (PSL)-Per2m/m male mice showed activation of glial cells in the dorsal horn of the spinal cord and increased expression of pain-related genes. Interestingly, α1D-adrenergic receptor (α1D-AR) expression was up-regulated in the spinal cord of Per2m/m mice, leading to increased production of 2-arachidonoylglycerol (2-AG), an endocannabinoid receptor ligand. This increase in 2-AG suppressed the PSL-induced tactile pain hypersensitivity. Furthermore, intraspinal dorsal horn injection of adeno-associated viral vectors expressing α1D-AR also attenuated pain hypersensitivity in PSL-wild-type male mice by increasing 2-AG production. Our findings reveal an uncovered role of the circadian clock in neuropathic pain disorders and suggest a link between α1D-AR signaling and the endocannabinoid system.

Modulatory Processes in Craniofacial Pain States

Pain is a common symptom associated with many disorders affecting the craniofacial tissues that include the teeth and their supporting structures, the jaw, face and tongue muscles, and the temporomandibular joint. Most acute craniofacial pain states are easily recognized and readily treated, but chronic craniofacial pain states (e.g., temporomandibular disorders [TMD], trigeminal neuropathies, and some headaches) may be especially challenging to manage successfully. This chapter provides an overview of the processes that underlie craniofacial pain, with a focus on the pain-modulatory mechanisms operating in craniofacial tissues and in the central nervous system (CNS), including the role of endogenous chemical processes such as those involving opioids. The chapter outlines in particular findings from preclinical studies that have provided substantial information about the neural as well as nonneural (e.g., glial) processes involved in the initiation, transmission, and modulation of nociceptive signals in the trigeminal system, and also draws attention to their clinical correlates. The increased understanding gained from these preclinical studies of how nociceptive signals can be modulated will contribute to improvements in presently available therapeutic approaches to manage craniofacial pain as well as to the development of novel analgesic approaches.

Volumetric alteration of brainstem in female migraineurs with and without aura

BACKGROUND AND AIM

Brainstem descending modulatory circuits have been postulated to be involved in migraine. Differences in brainstem volume between migraineurs and healthy controls have been demonstrated in previous research, nevertheless, the effect of migraine aura on brainstem volume is still uncertain. The aim of this study was to investigate the brainstem volume in migraineurs and examine the effect of migraine aura on brainstem volume.

METHODS

Our study included 90 female migraine patients without white matter lesions. (29 migraine patients with aura (MwA) and 61 migraine patients without aura (MwoA) and 32 age-matched female healthy controls (HC). Using the FreeSurfer image analysis suite, the volumes of the entire brainstem and its subfields (medulla, pons, and midbrain) were measured and compared between migraine subgroups (MwA vs. MwoA) and the healthy control group. The possible effects of migraine characteristics (i.e., disease duration and migraine attack frequency) on brainstem volume were also investigated.

RESULTS

Migraineurs had greater medulla volume (MwoA 3552 ± 459 mm3, MwA 3424 ± 448 mm3) than healthy controls (3236 ± 411 mm3). Statistically, MwA vs. HC p = 0.040, MwoA vs. HC p = 0.002, MwA vs. MwoA p = 0.555. A significant positive correlation was found between disease duration and the volume of medulla in the whole migraine group (r = 0.334, p = 0.001). Neither the whole brainstem nor its subfields were significantly different in volume between migraine subgroups.

CONCLUSION

Brainstem volume changes in migraine are mainly localized to the medulla and not specific to the presence of aura.

Differential Effects of Thermal Stimuli in Eliciting Temporal Contrast Enhancement: A Psychophysical Study

Offset analgesia (OA) is observed when pain relief is disproportional to the reduction of noxious input and is based on temporal contrast enhancement (TCE). This phenomenon is believed to reflect the function of the inhibitory pain modulatory system. However, the mechanisms contributing to this phenomenon remain poorly understood, with previous research focusing primarily on painful stimuli and not generalizing to nonpainful stimuli. Therefore, the aim of this study was to investigate whether TCE can be induced by noxious as well as innocuous heat and cold stimuli. Asymptomatic subjects (n = 50) were recruited to participate in 2 consecutive experiments. In the first pilot study (n = 17), the parameters of noxious and innocuous heat and cold stimuli were investigated in order to implement them in the main study. In the second (main) experiment, subjects (n = 33) participated in TCE paradigms consisting of 4 different modalities, including noxious heat (NH), innocuous heat (IH), noxious cold (NC), and innocuous cold (IC). The intensity of the sensations of each thermal modality was assessed using an electronic visual analog scale. TCE was confirmed for NH (P < .001), NC (P = .034), and IC (P = .002). Conversely, TCE could not be shown for IH (P = 1.00). No significant correlation between TCE modalities was found (r < .3, P > .05). The results suggest that TCE can be induced by both painful and nonpainful thermal stimulation but not by innocuous warm temperature. The exact underlying mechanisms need to be clarified. However, among other potential mechanisms, this may be explained by a thermo-specific activation of C-fiber afferents by IH and of A-fiber afferents by IC, suggesting the involvement of A-fibers rather than C-fibers in TCE. More research is needed to confirm a peripheral influence. PERSPECTIVE: This psychophysical study presents the observation of temporal contrast enhancement during NH, NC, and innocuous cold stimuli but not during stimulation with innocuous warm temperatures in healthy volunteers. A better understanding of endogenous pain modulation mechanisms might be helpful in explaining the underlying aspects of pain disorders.

Examining the role of mood in pain-limited treadmill walking duration in young healthy individuals

PURPOSE

The purpose of the study was to examine the effects of acute mood modulation on treadmill walking duration during experimental pain application.

METHODS

This was a repeated measure, within-subject study design. 30 healthy individuals (Males: Females 16:14; age 22.9 ± 2.5 years; height 170.9 ± 9.5 cm; body mass 68.4 ± 14.6 kg) attended a familiarization session and three experimental sessions, whereby they simultaneously viewed emotionally evoking stimulus from the International Affective Picture System (IAPS) and performed two treadmill walking tests (maximum 10 min duration) with a fixed nociceptive input applied to the thigh (pressure cuff). The primary outcome was treadmill walking duration during pain application to achieve a fixed pain score. During walking, mood (Self-Assessment Manikin: SAM 0-9) and pain (numerical rating scale: NRS 0-10) were measured.

RESULTS

Mood valence scores were significantly different in all conditions (p < 0.001), negative (2.4 ± 0.3), neutral (4.9 ± 0.6) and positive (6.6 ± 0.3). There was a significant difference (p = 0.04) in the treadmill walking duration for different mood states. For the primary outcome, post hoc analysis found differences between the negative and positive mood conditions for the treadmill walking durations to reach pain scores of 3 (negative: 224 ± 49 s; positive: 259 ± 60s, effect size [ES]: 0.80), 4 (negative: 262 ± 59 s; positive: 326 ± 90s, ES: 0.92), 5 (negative: 313 ± 86 s; positive: 385 ± 113 s, ES: 0.90), 6 (negative: 367 ± 106 s; positive: 447 ± 113 s, ES: 1.04) and 7 (negative: 423 ± 114 s; positive: 521 ± 110 s, ES: 1.02). There was no significant difference in the treadmill walking duration between the neutral vs negative mood and neutral vs positive mood conditions.

CONCLUSION

These results highlight the potential psychophysiological impacts on the pain experience in healthy individuals and encourages pursuit in strategies to minimize pain-limited exercise, a highly prevalent issue in the chronic pain population.

SIGNIFICANCE

Walking-based rehabilitation, designed to improve physical activity, has been shown to improve pain and disability. However active participation and adherence in walking-based rehabilitation has shown to be jeopardized by pain and pain-related cognitive and behavioural adaptations. This study examined the effect of a shift in mood on pain perception and treadmill walking tolerance. We found that with a worse mood, individuals were less tolerant of pain and walked on the treadmill for a shorter duration. These results suggest that factors which improve mood should be combined with walking-based training to improve tolerance.

The Roles of Endogenous Opioids in Placebo and Nocebo Effects: From Pain to Performance to Prozac

Placebo and nocebo effects have been well documented for nearly two centuries. However, research has only relatively recently begun to explicate the neurobiological underpinnings of these phenomena. Similarly, research on the broader social implications of placebo/nocebo effects, especially within healthcare delivery settings, is in a nascent stage. Biological and psychosocial outcomes of placebo/nocebo effects are of equal relevance. A common pathway for such outcomes is the endogenous opioid system. This chapter describes the history of placebo/nocebo in medicine; delineates the current state of the literature related to placebo/nocebo in relation to pain modulation; summarizes research findings related to human performance in sports and exercise; discusses the implications of placebo/nocebo effects among diverse patient populations; and describes placebo/nocebo influences in research related to psychopharmacology, including the relevance of endogenous opioids to new lines of research on antidepressant pharmacotherapies.

The role of pain modulation pathway and related brain regions in pain

Pain is a multifaceted process that encompasses unpleasant sensory and emotional experiences. The essence of the pain process is aversion, or perceived negative emotion. Central sensitization plays a significant role in initiating and perpetuating of chronic pain. Melzack proposed the concept of the "pain matrix", in which brain regions associated with pain form an interconnected network, rather than being controlled by a singular brain region. This review aims to investigate distinct brain regions involved in pain and their interconnections. In addition, it also sheds light on the reciprocal connectivity between the ascending and descending pathways that participate in pain modulation. We review the involvement of various brain areas during pain and focus on understanding the connections among them, which can contribute to a better understanding of pain mechanisms and provide opportunities for further research on therapies for improved pain management.

Fibromyalgia is associated with hypersensitivity but not with abnormal pain modulation: evidence from QST trials and spinal fMRI

Widespread pain and hyperalgesia are characteristics of chronic musculoskeletal pain conditions, including fibromyalgia syndrome (FM). Despite mixed evidence, there is increasing consensus that these characteristics depend on abnormal pain augmentation and dysfunctional pain inhibition. Our recent investigations of pain modulation with individually adjusted nociceptive stimuli have confirmed the mechanical and thermal hyperalgesia of FM patients but failed to detect abnormalities of pain summation or descending pain inhibition. Furthermore, our functional magnetic resonance imaging evaluations of spinal and brainstem pain processing during application of sensitivity-adjusted heat stimuli demonstrated similar temporal patterns of spinal cord activation in FM and HC participants. However, detailed modeling of brainstem activation showed that BOLD activity during "pain summation" was increased in FM subjects, suggesting differences in brain stem modulation of nociceptive stimuli compared to HC. Whereas these differences in brain stem activation are likely related to the hypersensitivity of FM patients, the overall central pain modulation of FM showed no significant abnormalities. These findings suggest that FM patients are hyperalgesic but modulate nociceptive input as effectively as HC.

Elevated Serotonin in Mouse Spinal Dorsal Horn Is Pronociceptive

Serotonergic neurons in the rostral ventral medulla (RVM) contribute to bidirectional control of pain through modulation of spinal and trigeminal nociceptive networks. Deficits in this pathway are believed to contribute to pathologic pain states, but whether changes in serotonergic mechanisms are pro- or antinociceptive is debated. We used a combination of optogenetics and fiber photometry to examine these mechanisms more closely. We find that optogenetic activation of RVM serotonergic afferents in the spinal cord of naive mice produces mechanical hypersensitivity and conditioned place aversion (CPA). Neuropathic pain, produced by chronic constriction injury of the infraorbital nerve (CCI-ION), evoked a tonic increase in serotonin (5HT) concentrations within the spinal trigeminal nucleus caudalis (SpVc), measured with liquid chromatography-tandem mass spectroscopy (LC-MS/MS). By contract, CCI-ION had no effect on the phasic serotonin transients in SpVc, evoked by noxious pinch, and measured with fiber photometry of a serotonin sensor. These findings suggest that serotonin release in the spinal cord is pronociceptive and that an increase in sustained serotonin signaling, rather than phasic or event driven increases, potentiate nociception in models of chronic pain.

Photic sensitization is mediated by cortico-accumbens pathway in rats with trigeminal neuropathic pain

Exposure to light stimuli may trigger or exacerbate perception of pain, also known as a common yet debilitating symptom of photophobia in patient with chronic orofacial pain. Mechanism underlying this phenomenon of photic sensitization in neuropathic condition remains elusive. Here, we found that rats developed hypersensitivity to normal light illumination after establishment of chronic constriction injury of infraorbital nerve (ION-CCI) model, which can be attenuated by blocking the exposure of photic stimulation. Additionally, this behavioral phenotype of light-sensitivity impairment was associated with overexpression of anterior cingulate cortex (ACC) c-fos positive neurons, enhancement of neural excitability in the ACC neurons and its excitatory synaptic transmission between nucleus accumbens (NAc). Optogenetic and chemogenic silencing of ACC-NAc pathway improved trigeminal sensitization in responses to light stimuli by decreasing spontaneous pain-like episodes in ION-CCI animals. In contrast, selective activation of ACC-to-NAc circuits enhanced photic hypersensitivity in dark environment. Thus, our data provided novel role of ACC and its projection to NAc in bidirectional modulation of photic sensation, which may contribute to the understanding of photic allodynia in trigeminal neuropathic pain status.

Antinociceptive activity of doliroside B

CONTEXT

Dolichos trilobus Linn (Leguminosae) is often used in Yi ethnic medicine to treat pain, fracture, and rheumatism.

OBJECTIVE

To explore the therapeutic potential of doliroside B (DB) from D. trilobus and its disodium salt (DBDS) and the underlying mechanism in pain.

MATERIALS AND METHODS

In the writhing test, Kunming mice were orally treated with DB and DBDS at doses of 0.31, 0.62, 1.25, 2.5, and 5 mg/kg. Vehicle, morphine, indomethacin, and acetylsalicylic acid were used as negative and positive control on the nociception-induced models, respectively. In the hot plate test, mice were orally treated with DB and DBDS at doses of 2.5, 5, 10, and 20 mg/kg. In the formalin test, mice were orally treated with DB and DBDS at doses of 2.5, 5, 10, and 20 mg/kg. In the meanwhile, lipopolysaccharide-induced inflammatory model in RAW264.7 macrophages was adopted to study the mechanism of pain alleviation for DBDS.

RESULTS

DBDS (5 mg/kg) inhibited the writhing number by 80.2%, which exhibited the highest antinociceptive activity in pain models. DBDS could selectively inhibite the activity of COX-1. Meanwhile, it also reduced the production of NO, iNOS, and IL-6 by 55.8%, 69.0%, and 49.9% inhibition, respectively. It was found that DBDS also positively modulated the function of GABA_A1 receptor.

DISCUSSION AND CONCLUSIONS

DBDS displayed antinociceptive activity by acting on both the peripheral and central nervous systems, which may act on multitargets. Further work is warranted for developing DBDS into a potential drug for the treatment of pain.

Precision, integrative medicine for pain management in sickle cell disease

Sickle cell disease (SCD) is a prevalent and complex inherited pain disorder that can manifest as acute vaso-occlusive crises (VOC) and/or chronic pain. Despite their known risks, opioids are often prescribed routinely and indiscriminately in managing SCD pain, because it is so often severe and debilitating. Integrative medicine strategies, particularly non-opioid therapies, hold promise in safe and effective management of SCD pain. However, the lack of evidence-based methods for managing SCD pain hinders the widespread implementation of non-opioid therapies. In this review, we acknowledge that implementing personalized pain treatment strategies in SCD, which is a guideline-recommended strategy, is currently fraught with limitations. The full implementation of pharmacological and biobehavioral pain approaches targeting mechanistic pain pathways faces challenges due to limited knowledge and limited financial and personnel support. We recommend personalized medicine, pharmacogenomics, and integrative medicine as aspirational strategies for improving pain care in SCD. As an organizing model that is a comprehensive framework for classifying pain subphenotypes and mechanisms in SCD, and for guiding selection of specific strategies, we present evidence updating pain research pioneer Richard Melzack's neuromatrix theory of pain. We advocate for using the updated neuromatrix model to subphenotype individuals with SCD, to better select personalized multimodal treatment strategies, and to identify research gaps fruitful for exploration. We present a fairly complete list of currently used pharmacologic and non-pharmacologic SCD pain therapies, classified by their mechanism of action and by their hypothesized targets in the updated neuromatrix model.

Saffron (Crocus sativus) and its constituents for pain management: A review of current evidence

Pain can become a chronic and deliberating experience with a significant burden. In preclinical and clinical studies, Saffron (Crocus sativus L.) has shown analgesic activities. Considering the unsatisfactory results of current therapeutic management for chronic pain conditions, we aimed to review saffron's analgesic activity and underlying mechanisms. Saffron showed antinociceptive activities in formalin-, carrageenan-, and capsaicin-induced experimental pain models. Saffron analgesic activities affected several targets, including ion channels of nociceptors; the adrenergic system and central histaminic system; inhibition of inflammatory pathways, apoptotic pathways, and oxidative stress; regulation of NO pathway, and the endocannabinoid system. Clinical studies showed analgesia of Saffron in rheumatoid arthritis, after-pain following childbirth, dysmenorrhea, and fibromyalgia. Our literature review showed that saffron can be beneficial as an adjunct therapy to commonly used analgesics in practice, particularly in chronic pain conditions.