Long-term potentiation in the anterior cingulate cortex and chronic pain

Enhanced long-term potentiation in the anterior cingulate cortex of tree shrew

Synaptic plasticity is a key cellular model for learning, memory and chronic pain. Most previous studies were carried out in rats and mice, and less is known about synaptic plasticity in non-human primates. In the present study, we used integrative experimental approaches to study long-term potentiation (LTP) in the anterior cingulate cortex (ACC) of adult tree shrews. We found that glutamate is the major excitatory transmitter and α-amino-3-hydroxy-5-methyl-4-isoxazole-propionicacid (AMPA) receptors mediate postsynaptic responses. LTP in tree shrews was greater than that in adult mice and lasted for at least 5 h. N-methyl-d-aspartic acid (NMDA) receptors, Ca2+ influx and adenylyl cyclase 1 (AC1) contributed to tree shrew LTP. Our results suggest that LTP is a major form of synaptic plasticity in the ACC of primate-like animals. This article is part of a discussion meeting issue 'Long-term potentiation: 50 years on'.

Long-range inhibition from prelimbic to cingulate areas of the medial prefrontal cortex enhances network activity and response execution

It is well established that the medial prefrontal cortex (mPFC) exerts top-down control of many behaviors, but little is known regarding how cross-talk between distinct areas of the mPFC influences top-down signaling. We performed virus-mediated tracing and functional studies in male mice, homing in on GABAergic projections whose axons are located mainly in layer 1 and that connect two areas of the mPFC, namely the prelimbic area (PrL) with the cingulate area 1 and 2 (Cg1/2). We revealed the identity of the targeted neurons that comprise two distinct types of layer 1 GABAergic interneurons, namely single-bouquet cells (SBCs) and neurogliaform cells (NGFs), and propose that this connectivity links GABAergic projection neurons with cortical canonical circuits. In vitro electrophysiological and in vivo calcium imaging studies support the notion that the GABAergic projection neurons from the PrL to the Cg1/2 exert a crucial role in regulating the activity in the target area by disinhibiting layer 5 output neurons. Finally, we demonstrated that recruitment of these projections affects impulsivity and mechanical responsiveness, behaviors which are known to be modulated by Cg1/2 activity.

The Pathway-Selective Dependence of Nitric Oxide for Long-Term Potentiation in the Anterior Cingulate Cortex of Adult Mice

Nitric oxide (NO) is a key diffusible messenger in the mammalian brain. It has been proposed that NO may diffuse in retrograde into presynaptic terminals, contributing to the induction of hippocampal long-term potentiation (LTP). Here, we present novel evidence that NO is selectively required for the synaptic potentiation of the interhemispheric projection in the anterior cingulate cortex (ACC). Unilateral low-frequency stimulation (LFS) induced a short-term synaptic potentiation on the contralateral ACC through the corpus callosum (CC). The use of the antagonists of the NMDA receptor (NMDAR), or the inhibitor of the L-type voltage-dependent Ca2+ channels (L-VDCCs), blocked the induction of this ACC-ACC potentiation. In addition, the inhibitor of NO synthase, or inhibitors for its downstream signaling pathway, also blocked this ACC-ACC potentiation. However, the application of the NOS inhibitor blocked neither the local electric stimulation-induced LTP nor the stimulation-induced recruitment of silent responses. Our results present strong evidence for the pathway-selective roles of NO in the LTP of the ACC.

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.

Cortical synaptic basis of consciousness

Consciousness is one of final questions for humans to tackle in neuroscience. Due to a lack of understanding of basic brain networks and mechanisms of functions, our knowledge of consciousness mainly stays at a theoretical level. Recent studies using brain imaging in humans and modern neuroscience techniques in animal studies reveal the basic brain network for consciousness. The projection from the thalamus to different cortical regions forms a network of activities to maintain consciousness in humans and animals. These feedback and feedforward circuits maintain consciousness even in certain brain injury conditions. Pterions and ion channels that contribute to these circuit neural activities are targets for drugs and manipulations that affect consciousness such as anesthetic agents. Synaptic plasticity that trains synapses during learning and information recall modified the circuits and contributes to a high level of consciousness in a certain population.

Effects of Embodiment in Virtual Reality for Treatment of Chronic Pain: Pilot Open-Label Study

BACKGROUND

Chronic pain has long been a major health burden that has been addressed through numerous forms of pharmacological and nonpharmacological treatment. One of the tenets of modern medicine is to minimize risk while providing efficacy. Further, because of its noninvasive nature, virtual reality (VR) provides an attractive platform for potentially developing novel therapeutic modalities.

OBJECTIVE

The purpose of this study was to determine the feasibility of a novel VR-based digital therapy for the treatment of chronic pain.

METHODS

An open-label study assessed the feasibility of using virtual embodiment in VR to treat chronic pain. In total, 24 patients with chronic pain were recruited from local pain clinics and completed 8 sessions of a novel digital therapeutic that combines virtual embodiment with graded motor imagery to deliver functional rehabilitation exercises over the course of 4 weeks. Pain intensity as measured by a visual analog scale before and after each virtual embodiment training session was used as the primary outcome measure. Additionally, a battery of patient-reported pain questionnaires (Fear-Avoidance Beliefs Questionnaire, Oswestry Low Back Pain Disability Questionnaire, Pain Catastrophizing Scale, and Patient Health Questionnaire) were administered before and after 8 sessions of virtual embodiment training as exploratory outcome measures to assess if the measures are appropriate and warrant a larger randomized controlled trial.

RESULTS

A 2-way ANOVA on session × pre- versus postvirtual embodiment training revealed that individual virtual embodiment training sessions significantly reduced the intensity of pain as measured by the visual analog scale (P<.001). Perceived disability due to lower back pain as measured by the Oswestry Low Back Pain Disability Questionnaire significantly improved (P=.003) over the 4-week course of virtual embodiment regimen. Improvement was also observed on the helplessness subscale of the Pain Catastrophizing Scale (P=.02).

CONCLUSIONS

This study provides evidence that functional rehabilitation exercises delivered in VR are safe and may have positive effects on alleviating the symptoms of chronic pain. Additionally, the virtual embodiment intervention may improve perceived disability and helplessness of patients with chronic pain after 8 sessions. The results support the justification for a larger randomized controlled trial to assess the extent to which virtual embodiment training can exert an effect on symptoms associated with chronic pain.

TRIAL REGISTRATION

ClinicalTrials.gov NCT04060875; https://clinicaltrials.gov/ct2/show/NCT04060875.

Oxytocin Modulation in Mindfulness-Based Pain Management for Chronic Pain

In the context of chronic pain management, opioid-based treatments have been heavily relied upon, raising concerns related to addiction and misuse. Non-pharmacological approaches, such as Mindfulness-Based Pain Management, offer alternative strategies. We conducted a mechanistic clinical study to investigate the impact of an 8-week Mindfulness-Based Pain Management intervention on chronic pain, the modulation of inflammatory markers, stress physiology, and oxytocin, and their interplay with clinical pain symptoms and perception, in comparison to a patient wait-list active control. A total of 65 participants, including 50 chronic pain patients and 15 healthy controls, underwent salivary assays to assess endocrine markers, oxytocin, interleukin (IL)-1b, IL-6, IL-8, tumor necrosis factor (TNF)-a, and dehydroepiandrosterone sulphate (DHEA-S). Psychological assessments were also conducted to evaluate aspects of pain perception, mindfulness, mood, and well-being. Findings revealed significant differences between chronic pain patients and healthy controls in various clinical metrics, highlighting the psychological distress experienced by patients. Following Mindfulness-Based Pain Management, oxytocin levels significantly increased in chronic pain patients, that was not observed in the patient wait-list control group. In contrast, cytokine and DHEA-S levels decreased (not to statistically significant margins) supporting anti-inflammatory effects of Mindfulness-Based Pain Management. The fact DHEA-S levels, a marker of stress, did attenuate but not to statistically meaningful levels, suggests that pain reduction was not solely related to stress reduction, and that oxytocin pathways may be more salient than previously considered. Psychological assessments demonstrated substantial improvements in pain perception and mood in the intervention group. These results contribute to the growing body of evidence regarding the effectiveness of mindfulness-based interventions in chronic pain management and underscore oxytocin's potential role as a therapeutic target.

An ACC-VTA-ACC positive-feedback loop mediates the persistence of neuropathic pain and emotional consequences

The central mechanisms underlying pain chronicity remain elusive. Here, we identify a reciprocal neuronal circuit in mice between the anterior cingulate cortex (ACC) and the ventral tegmental area (VTA) that mediates mutual exacerbation between hyperalgesia and allodynia and their emotional consequences and, thereby, the chronicity of neuropathic pain. ACC glutamatergic neurons (ACCGlu) projecting to the VTA indirectly inhibit dopaminergic neurons (VTADA) by activating local GABAergic interneurons (VTAGABA), and this effect is reinforced after nerve injury. VTADA neurons in turn project to the ACC and synapse to the initial ACCGlu neurons to convey feedback information from emotional changes. Thus, an ACCGlu-VTAGABA-VTADA-ACCGlu positive-feedback loop mediates the progression to and maintenance of persistent pain and comorbid anxiodepressive-like behavior. Disruption of this feedback loop relieves hyperalgesia and anxiodepressive-like behavior in a mouse model of neuropathic pain, both acutely and in the long term.

Effects of Intrathecal Ketamine on Cerebrospinal Fluid Levels of Brain-Derived Neurotrophic Factor and Mechanical Allodynia in a Rat Model of Mild Traumatic Brain Injury

BACKGROUND Ketamine, a compelling candidate for neuropathic pain management, has attracted interest for its potential to elevate brain-derived neurotrophic factor (BDNF) levels. We aimed to assess the effects of intrathecally administered ketamine on the cerebrospinal fluid (CSF) levels of BDNF(c-BDNF) and allodynia in a rat model of traumatic brain injury (TBI). MATERIAL AND METHODS Forty-five rats were divided into 3 groups: sham operation (Group S), untreated TBI (Group T), and ketamine-treated TBI (Group K), with 15 rats in each group. Rats were anesthetized, and their skulls were secured in a stereotactic frame before undergoing craniotomy. A controlled cortical impact (CCI) was induced, followed by injection of ketamine (3.41 µg/g) into the CSF in Group K. In Group T, no drug was injected after CCI delivery. On postoperative days (POD) 1, 7, and 14, the 50% mechanical withdrawal threshold (50% MWT) and c-BDNF levels were assessed. RESULTS Groups T and K exhibited a significantly lower 50% MWT than Group S on POD 1(6.6 [5.7, 8.7] g, 10.0 [6.8, 11.6] g, and 18.7 [11.6, 18.7] g, respectively; P<0.001). The c-BDNF levels in Group K were significantly higher than those in Groups S and T on POD 1 (18.9 [16.1, 23.0] pg/ml, 7.3 [6.0, 8.8] pg/ml, and 11.0 [10.6, 12.3] pg/ml, respectively; P=0.006). CONCLUSIONS Intrathecal ketamine administration did not exhibit anti-allodynic effects following mild TBI. c-BDNF level is a promising potential indicator for predicting the expression of allodynia after mild TBI.

Long-term plasticity of NMDA GluN2B (NR2B) receptor in anterior cingulate cortical synapses

The anterior cingulate cortex (ACC) is a key cortical area for pain perception, emotional fear and anxiety. Cortical excitation is thought to be the major mechanism for chronic pain and its related emotional disorders such as anxiety and depression. GluN2B (or called NR2B) containing NMDA receptors play critical roles for such excitation. Not only does the activation of GluN2B contributes to the induction of the postsynaptic form of LTP (post-LTP), long-term upregulation of GluN2B subunits through tyrosine phosphorylation were also detected after peripheral injury. In addition, it has been reported that presynaptic NMDA receptors may contribute to the modulation of the release of glutamate from presynaptic terminals in the ACC. It is believed that inhibiting subtypes of NMDA receptors and/or downstream signaling proteins may serve as a novel therapeutic mechanism for future treatment of chronic pain, anxiety, and depression.

Exploring altered oscillatory activity in the anterior cingulate cortex after nerve injury: Insights into mechanisms of neuropathic allodynia

While neural oscillations play a critical role in sensory perception, it remains unclear how these rhythms function under conditions of neuropathic allodynia. Recent studies demonstrated that the anterior cingulate cortex (ACC) is associated with the affective-aversive component of pain, and plasticity changes in this region are closely linked to abnormal allodynic sensations. Here, to study the mechanisms of allodynia, we recorded local field potentials (LFPs) in the bilateral ACC of awake-behaving rats and compared the spectral power and center frequency of brain oscillations between healthy and CCI (chronic constriction injury) induced neuropathic pain conditions. Our results indicated that activation of the ACC occurs bilaterally in the presence of neuropathic pain, similar to the healthy condition. Furthermore, CCI affects both spontaneous and stimulus-induced activity of ACC neurons. Specifically, we observed an increase in spontaneous beta activity after nerve injury compared to the healthy condition. By stimulating operated or unoperated paws, we found more intense event-related desynchronization (ERD) responses in the theta, alpha, and beta frequency bands and faster alpha center frequency after CCI compared to before CCI. Although the behavioral manifestation of allodynia was more pronounced in the operated paw than the unoperated paw following CCI, there was no significant difference in the center frequency and ERD responses observed in the ACC between stimulation of the operated and unoperated limbs. Our findings offer evidence supporting the notion that aberrancies in ACC oscillations may contribute to the maintenance and development of neuropathic allodynia.

CCL2 Potentiates Inflammation Pain and Related Anxiety-Like Behavior Through NMDA Signaling in Anterior Cingulate Cortex

Previous studies have shown that the C-C motif chemokine ligand 2 (CCL2) is widely expressed in the nervous system and involved in regulating the development of chronic pain and related anxiety-like behaviors, but its precise mechanism is still unclear. This paper provides an in-depth examination of the involvement of CCL2-CCR2 signaling in the anterior cingulate cortex (ACC) in intraplantar injection of complete Freund's adjuvant (CFA) leading to inflammatory pain and its concomitant anxiety-like behaviors by modulation of glutamatergic N-methyl-D-aspartate receptor (NMDAR). Our findings suggest that local bilateral injection of CCR2 antagonist in the ACC inhibits CFA-induced inflammatory pain and anxiety-like behavior. Meanwhile, the expression of CCR2 and CCL2 was significantly increased in ACC after 14 days of intraplantar injection of CFA, and CCR2 was mainly expressed in excitatory neurons. Whole-cell patch-clamp recordings showed that the CCR2 inhibitor RS504393 reduced the frequency of miniature excitatory postsynaptic currents (mEPSC) in ACC, and CCL2 was involved in the regulation of NMDAR-induced current in ACC neurons in the pathological state. In addition, local injection of the NR2B inhibitor of NMDAR subunits, Ro 25-6981, attenuated the effects of CCL2-induced hyperalgesia and anxiety-like behavior in the ACC. In summary, CCL2 acts on CCR2 in ACC excitatory neurons and participates in the regulation of CFA-induced pain and related anxiety-like behaviors through upregulation of NR2B. CCR2 in the ACC neuron may be a potential target for the treatment of chronic inflammatory pain and pain-related anxiety.

Combined metabolomics and transcriptomics analysis of rats under neuropathic pain and pain-related depression

Neuropathic pain often leads to negative emotions, which in turn can enhance the sensation of pain. This study aimed to investigate the molecular mechanisms mediating neuropathic pain and negative emotions. Chronic constriction injury (CCI) rats were used as model animals and behavioral tests were conducted to assess pain and negative emotions. Then, the rat anterior cingulate cortex (ACC) was analyzed using UPLC-MS/MS and subsequently integrated with our previously published transcriptome data. Metabolomics analysis revealed that 68 differentially expressed metabolites (DEMs) were identified, mainly in amino acid metabolites and fatty acyls. Combined with our previously published transcriptome data, we predicted two genes that potentially exhibited associations with these metabolites, respectively Apolipoprotein L domain containing 1 (Apold1) and WAP four-disulfide core domain 1 (Wfdc1). Taken together, our results indicated that peripheral nerve injury contributing to neuropathic pain and pain-related depression may be associated with these metabolites and genes. This research provides new insights into the molecular regulatory mechanism, which could serve as a reference for the treatment of neuropathic pain and pain-related depression.

Presynaptic glutamate receptors in nociception

Chronic pain is a frequent, distressing and poorly understood health problem. Plasticity of synaptic transmission in the nociceptive pathways after inflammation or injury is assumed to be an important cellular basis for chronic, pathological pain. Glutamate serves as the main excitatory neurotransmitter at key synapses in the somatosensory nociceptive pathways, in which it acts on both ionotropic and metabotropic glutamate receptors. Although conventionally postsynaptic, compelling anatomical and physiological evidence demonstrates the presence of presynaptic glutamate receptors in the nociceptive pathways. Presynaptic glutamate receptors play crucial roles in nociceptive synaptic transmission and plasticity. They modulate presynaptic neurotransmitter release and synaptic plasticity, which in turn regulates pain sensitization. In this review, we summarize the latest understanding of the expression of presynaptic glutamate receptors in the nociceptive pathways, and how they contribute to nociceptive information processing and pain hypersensitivity associated with inflammation / injury. We uncover the cellular and molecular mechanisms of presynaptic glutamate receptors in shaping synaptic transmission and plasticity to mediate pain chronicity, which may provide therapeutic approaches for treatment of chronic pain.

Cingulate cGMP-dependent protein kinase I facilitates chronic pain and pain-related anxiety and depression

ABSTRACT

Patients with chronic pain often experience exaggerated pain response and aversive emotion, such as anxiety and depression. Central plasticity in the anterior cingulate cortex (ACC) is assumed to be a critical interface for pain perception and emotion, which has been reported to involve activation of NMDA receptors. Numerous studies have documented the key significance of cGMP-dependent protein kinase I (PKG-I) as a crucial downstream target for the NMDA receptor-NO-cGMP signaling cascade in regulating neuronal plasticity and pain hypersensitivity in specific regions of pain pathway, ie, dorsal root ganglion or spinal dorsal horn. Despite this, whether and how PKG-I in the ACC contributes to cingulate plasticity and comorbidity of chronic pain and aversive emotion has remained elusive. Here, we uncovered a crucial role of cingulate PKG-I in chronic pain and comorbid anxiety and depression. Chronic pain caused by tissue inflammation or nerve injury led to upregulation of PKG-I expression at both mRNA and protein levels in the ACC. Knockdown of ACC-PKG-I relieved pain hypersensitivity as well as pain-associated anxiety and depression. Further mechanistic analysis revealed that PKG-I might act to phosphorylate TRPC3 and TRPC6, leading to enhancement of calcium influx and neuronal hyperexcitability as well as synaptic potentiation, which results in the exaggerated pain response and comorbid anxiety and depression. We believe this study sheds new light on the functional capability of ACC-PKG-I in modulating chronic pain as well as pain-associated anxiety and depression. Hence, cingulate PKG-I may represent a new therapeutic target against chronic pain and pain-related anxiety and depression.

Up-regulation of LCN2 in the anterior cingulate cortex contributes to neural injury-induced chronic pain

Chronic pain caused by disease or injury affects more than 30% of the general population. The molecular and cellular mechanisms underpinning the development of chronic pain remain unclear, resulting in scant effective treatments. Here, we combined electrophysiological recording, in vivo two-photon (2P) calcium imaging, fiber photometry, Western blotting, and chemogenetic methods to define a role for the secreted pro-inflammatory factor, Lipocalin-2 (LCN2), in chronic pain development in mice with spared nerve injury (SNI). We found that LCN2 expression was upregulated in the anterior cingulate cortex (ACC) at 14 days after SNI, resulting in hyperactivity of ACC glutamatergic neurons (ACC^Glu) and pain sensitization. By contrast, suppressing LCN2 protein levels in the ACC with viral constructs or exogenous application of neutralizing antibodies leads to significant attenuation of chronic pain by preventing ACC^Glu neuronal hyperactivity in SNI 2W mice. In addition, administering purified recombinant LCN2 protein in the ACC could induce pain sensitization by inducing ACC^Glu neuronal hyperactivity in naïve mice. This study provides a mechanism by which LCN2-mediated hyperactivity of ACC^Glu neurons contributes to pain sensitization, and reveals a new potential target for treating chronic pain.

Somatically evoked cough responses help to identify patients with difficult-to-treat chronic cough: a six-month observational cohort study

BACKGROUND

Recently we identified in patients with chronic cough a sensory dysregulation via which the urge-to-cough (UTC) or coughing are evoked mechanically from "somatic points for cough" (SPCs) in the neck and upper trunk. We investigated the prevalence and the clinical relevance of SPCs in an unselected population of patients with chronic cough.

METHODS

From 2018 to 2021, symptoms of 317 consecutive patients with chronic cough (233 females) were collected on four visits (V1-V4) 2 months apart at the Cough Clinic of the University Hospital in Florence (I). Participants rated the disturbance caused by the cough (0-9 modified Borg Scale). We attempted to evoke coughing and/or UTC using mechanical actions in all participants who were subsequently categorised as responsive (somatic point for cough positive, SPC+) or unresponsive (SPC-) to these actions. An association was established between chronic cough and its commonest causes; treatments were administered accordingly.

FINDINGS

169 patients were SPC+ and had a higher baseline cough score (p < 0.01). In most of the patients, the treatments reduced (p < 0.01) cough-associated symptoms. All patients reported a decrease (p < 0.01) in cough score at V2 (from 5.70 ± 1.4 to 3.43 ± 1.9 and from 5.01 ± 1.5 to 2.74 ± 1.7 for SPC+ and SPC- patients respectively). However, whilst in SPC- patients the cough score continued to decrease indicating virtually complete cough disappearance at V4 (0.97 ± 0.8), in SPC+ patients this variable remained close to V2 values during the entire follow-up.

INTERPRETATION

Our study suggests that the assessment of SPCs may identify patients whose cough is unresponsive and are eligible for specific treatments.

FUNDING

This work was funded by an unrestricted grant from Merck (Italy).

Toward the unknown: consciousness and pain

Studies of consciousness are hindered by the complexity of the brain, but it is possible to study the consciousness of a sensation, namely pain. Three systems are necessary to experience pain: the somatosensory system conveys information about an injury to the thalamus where an awareness of the injury but not the painfulness emerges. The thalamus distributes the information to the affective system, which modulates the intensity of the pain, and to the cognitive system that imparts attention to the pain. Imaging of patients in pain and those experiencing placebo and hypnosis-induced analgesia shows that two essential cortical circuits for pain and attention are located within the anterior cingulate cortex. The circuits are activated when a high-frequency input results in the development of a long-term potentiation (LTP) at synapses on the apical dendrites of pyramidal neurons. The LTP acts via α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and N-methyl-D-aspartate (NMDA) receptors, and an anterior cingulate cortex-specific type-1 adenylate cyclase is necessary for both the LTP and the pain. The apical dendrites form an extensive network such that the input from serious injuries results in the emergence of a local field potential. Using mouse models, I propose experiments designed to test the hypothesis that the local field potential is necessary and sufficient for the consciousness of pain.

Extensive sensorimotor training enhances nociceptive cortical responses in healthy individuals

BACKGROUND

Prolonged and repeated sensorimotor training is a crucial driver for promoting use-dependent plasticity, but also a main risk factor for developing musculoskeletal pain syndromes, yet the neural underpinnings that link repetitive movements to abnormal pain processing are unknown.

METHODS

Twenty healthy musicians, one of the best in vivo models to study use-dependent plasticity, and 20 healthy non-musicians were recruited. Perceptual thresholds, reaction times (RTs) and event-related potentials (ERPs) were recorded using nociceptive intra-epidermal and non-nociceptive transcutaneous electrical stimulation.

RESULTS

In response to comparable stimulus intensities, musicians compared to non-musicians showed larger non-nociceptive N140 (associated with higher activation of regions within the salience network), higher nociceptive N200 ERPs (associated with higher activation of regions within the sensorimotor network) and faster RTs to both stimuli. Non-musicians showed larger non-nociceptive P200 ERP. Notably, a similar P200 component prominently emerged during nociceptive stimulation in non-musicians. Across participants, larger N140 and N200 ERPs were associated with RTs, whereas the amount of daily practice in musicians explained non-nociceptive P200 and nociceptive P300 ERPs.

CONCLUSIONS

These novel findings indicate that the mechanisms by which extensive sensorimotor training promotes use-dependent plasticity in multisensory neural structures may also shape the neural signatures of nociceptive processing in healthy individuals.

SIGNIFICANCE

Repetitive sensorimotor training may increase the responsiveness of nociceptive evoked potentials. These novel data highlight the importance of repetitive sensorimotor practice as a contributing factor to the interindividual variability of nociceptive-related potentials.

Decreased ALFF and Functional Connectivity of the Thalamus in Vestibular Migraine Patients

BACKGROUND

The thalamus has been reported to be associated with pain modulation and processing. However, the functional changes that occur in the thalamus of vestibular migraine (VM) patients remain unknown.

METHODS

In total, 28 VM patients and 28 healthy controls who were matched for age and sex underwent resting-state functional magnetic resonance imaging. They also responded to standardized questionnaires aimed at assessing the clinical features associated with migraine and vertigo. Differences in the amplitude of low-frequency fluctuation (ALFF) were analyzed and brain regions with altered ALFF in the two groups were used for further analysis of whole-brain functional connectivity (FC). The relationship between clusters and clinical features was investigated by correlation analyses.

RESULTS

The ALFF in the thalamus was significantly decreased in the VM group versus the control group. In the VM group, the ALFF in the left thalamus negatively correlated with VM episode frequency. Furthermore, the left thalamus showed significantly weaker FC than both regions of the medial prefrontal cortex, both regions of the anterior cingulum cortex, the left superior/middle temporal gyrus, and the left temporal pole in the VM group.

CONCLUSIONS

The thalamus plays an important role in VM patients and it is suggested that connectivity abnormalities of the thalamocortical region contribute to abnormal pain information processing and modulation, transmission, and multisensory integration in patients with VM.

Multiple modulatory roles of serotonin in chronic pain and injury-related anxiety

Chronic pain is long-lasting pain that often persists during chronic diseases or after recovery from disease or injury. It often causes serious side effects, such as insomnia, anxiety, or depression which negatively impacts the patient's overall quality of life. Serotonin (5-HT) in the central nervous system (CNS) has been recognized as an important neurotransmitter and neuromodulator which regulates various physiological functions, such as pain sensation, cognition, and emotions-especially anxiety and depression. Its widespread and diverse receptors underlie the functional complexity of 5-HT in the CNS. Recent studies found that both chronic pain and anxiety are associated with synaptic plasticity in the anterior cingulate cortex (ACC), the insular cortex (IC), and the spinal cord. 5-HT exerts multiple modulations of synaptic transmission and plasticity in the ACC and the spinal cord, including activation, inhibition, and biphasic actions. In this review, we will discuss the multiple actions of the 5-HT system in both chronic pain and injury-related anxiety, and the synaptic mechanisms behind them. It is likely that the specific 5-HT receptors would be new promising therapeutic targets for the effective treatment of chronic pain and injury-related anxiety in the future.

Synaptic Plasticity in the Pain-Related Cingulate and Insular Cortex

Cumulative animal and human studies have consistently demonstrated that two major cortical regions in the brain, namely the anterior cingulate cortex (ACC) and insular cortex (IC), play critical roles in pain perception and chronic pain. Neuronal synapses in these cortical regions of adult animals are highly plastic and can undergo long-term potentiation (LTP), a phenomenon that is also reported in brain areas for learning and memory (such as the hippocampus). Genetic and pharmacological studies show that inhibiting such cortical LTP can help to reduce behavioral sensitization caused by injury as well as injury-induced emotional changes. In this review, we will summarize recent progress related to synaptic mechanisms for different forms of cortical LTP and their possible contribution to behavioral pain and emotional changes.

Identification of a Glutamatergic Claustrum-Anterior Cingulate Cortex Circuit for Visceral Pain Processing

Chronic visceral pain is a major challenge for both patients and health providers. Although the central sensitization of the brain is thought to play an important role in the development of visceral pain, the detailed neural circuits remain largely unknown. Using a well-established chronic visceral hypersensitivity model induced by neonatal maternal deprivation (NMD) in male mice, we identified a distinct pathway whereby the claustrum (CL) glutamatergic neuron projecting to the anterior cingulate cortex (ACC) is critical for visceral pain but not for CFA-evoked inflammatory pain. By a combination of in vivo circuit-dissecting extracellular electrophysiological approaches and visceral pain related electromyographic (EMG) recordings, we demonstrated that optogenetic inhibition of CL glutamatergic activity suppressed the ACC neural activity and visceral hypersensitivity of NMD mice whereas selective activation of CL glutamatergic activity enhanced the ACC neural activity and evoked visceral pain of control mice. Further, optogenetic studies demonstrate a causal link between such neuronal activity and visceral pain behaviors. Chemogenetic activation or inhibition of ACC neural activities reversed the effects of optogenetic manipulation of CL neural activities on visceral pain responses. Importantly, molecular detection showed that NMD significantly enhances the expression of NMDA receptors and activated CaMKIIα in the ACC postsynaptic density (PSD) region. Together, our data establish a functional role for CL→ACC glutamatergic neurons in gating visceral pain, thus providing a potential treatment strategy for visceral pain.SIGNIFICANCE STATEMENT Studies have shown that sensitization of anterior cingulate cortex (ACC) plays an important role in chronic pain. However, it is as yet unknown whether there is a specific brain region and a distinct neural circuit that helps the ACC to distinguish visceral and somatic pain. The present study demonstrates that claustrum (CL) glutamatergic neurons maybe responding to colorectal distention (CRD) rather than somatic stimulation and that a CL glutamatergic projection to ACC glutamatergic neuron regulates visceral pain in mice. Furthermore, excessive NMDA receptors and overactive CaMKIIα in the ACC postsynaptic density (PSD) region were observed in mice with chronic visceral pain. Together, these findings reveal a novel neural circuity underlying the central sensitization of chronic visceral pain.

NMDARs mediate peripheral and central sensitization contributing to chronic orofacial pain

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

Low-Intensity Focused Ultrasound Alleviates Chronic Neuropathic Pain-Induced Allodynia by Inhibiting Neuroplasticity in the Anterior Cingulate Cortex

Low-intensity focused ultrasound (LIFU) is a potential noninvasive method to alleviate allodynia by modulating the central nervous system. However, the underlying analgesic mechanisms remain unexplored. Here, we assessed how LIFU at the anterior cingulate cortex (ACC) affects behavior response and central plasticity resulting from chronic constrictive injury (CCI). The safety of LIFU stimulation was assessed by hematoxylin and eosin (H&E) and Fluoro-Jade C (FJC) staining. A 21-day ultrasound exposure therapy was conducted from day 91 after CCI surgery in mice. We assessed the 50% mechanical withdrawal threshold (MWT₅₀) using Von Frey filaments (VFFs). The expression levels of microtubule-associated protein 2 (MAP2), growth-associated protein 43 (GAP43), and tau were determined via western blotting (WB) and immunofluorescence (IF) staining to evaluate the central plasticity in ACC. The regions of ACC were activated effectively and safely by LIFU stimulation, which significantly increased the number of c-fos-positive cells (P < 0.05) with no bleeding, coagulative necrosis, and neuronal loss. Under chronic neuropathic pain- (CNP-) induced allodynia, MWT₅₀ decreased significantly (P < 0.05), and overexpression of MAP2, GAP43, and tau was also observed. After 3 weeks of treatment, significant increases in MWT₅₀ were found in the CCI+LIFU group compared with the CCI group (P < 0.05). WB and IF staining both demonstrated a significant reduction in the expression levels of MAP2, GAP43, and tau (P < 0.05). LIFU treatment on ACC can effectively attenuate CNP-evoked mechanical sensitivity to pain and reverse aberrant central plasticity.

Multi-Region Local Field Potential Signatures in Response to the Formalin-induced Inflammatory Stimulus in Male Rats

Pain can be ignited by noxious chemical (e.g., acid), mechanical (e.g., pressure), and thermal (e.g., heat) stimuli and generated by the activation of sensory neurons and their axonal terminals called nociceptors in the periphery. Nociceptive information transmitted from the periphery is projected to the central nervous system (thalamus, somatosensory cortex, insular, anterior cingulate cortex, amygdala, periaqueductal grey, prefrontal cortex, etc.) to generate a unified experience of pain. Local field potential (LFP) recording is one of the neurophysiological tools to investigate the combined neuronal activity, ranging from several hundred micrometers to a few millimeters (radius), located around the embedded electrode. The advantage of recording LFP is that it provides stable simultaneous activities in various brain regions in response to external stimuli. In this study, differential LFP activities from the contralateral anterior cingulate cortex (ACC), ventral tegmental area (VTA), and bilateral amygdala in response to peripheral noxious formalin injection were recorded in anesthetized male rats. The results indicated increased power of delta, theta, alpha, beta, and gamma bands in the ACC and amygdala but no change of gamma-band in the right amygdala. Within the VTA, intensities of the delta, theta, and beta bands were only enhanced significantly after formalin injection. It was found that the connectivity (i.t. the coherence) among these brain regions reduced significantly under the formalin-induced nociception, which suggests a significant interruption within the brain. With further study, it will sort out the key combination of structures that will serve as the signature for pain state.

Gray matter volume reduction with different disease duration in trigeminal neuralgia

PURPOSE

Structural magnetic resonance imaging is widely used to explore brain gray and white matter structure in trigeminal neuralgia (TN) but has yielded conflicting findings. This study investigated the relationship between disease duration as a clinical feature of TN and changes in brain structure.

METHODS

We divided 49 TN patients into three groups (TN1-TN3) based on disease duration (TN1 = 1.1 ± 0.7 (0-2) years, TN2 = 4.8 ± 1.5 (3-7) years, TN3 = 15.1 ± 5.5 (10-30) years). We used voxel-based morphometry (VBM) to compare the gray matter volume (GMV) across groups and between TN patients and 18 matched healthy control subjects.

RESULTS

The TN1 group showed reduced GMV of pain-related regions in the cerebellum; the TN2 group showed reduced GMV in the thalamus and the motor/sensory cortex; and the TN3 group showed reduced GMV in the emotional and reward circuits compared with healthy controls. Similar brain regions, including bilateral hippocampi, caudate, left insular cortex, and medial superior frontal cortex, were affected in TN2 and TN3 compared with TN1.

CONCLUSION

Disease duration can explain differences in structural alterations-especially in pain-related brain regions-in TN. These results highlight the advanced structural neuroimaging method that are valuable tools to assess the trigeminal system in TN and may further our current understanding of TN pathology.

Synaptic potentiation of anterior cingulate cortex contributes to chronic pain of Parkinson's disease

Parkinson’s disease (PD) is a multi-system neurodegenerative disorder. Patients with PD often suffer chronic pain. In the present study, we investigated motor, sensory and emotional changes in three different PD mice models. We found that 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treatment caused significant changes in all measurements. Mechanical hypersensitivity of PD model induced by MPTP peaked at 3 days and persisted for at least 14 days. Using Fos transgenic mice, we found that neurons in the anterior cingulate cortex (ACC) were activated after MPTP treatment. Inhibiting ACC by bilateral microinjection of muscimol significantly reduced mechanical hypersensitivity and anxiety-like responses. By contrast, MPTP induced motor deficit was not affected, indicating ACC activity is mostly responsible for sensory and emotional changes. We also investigated excitatory synaptic transmission and plasticity using brain slices of MPTP treated animals. While L-LTP was blocked or significantly reduced. E-LTP was not significantly affected in slices of MPTP treated animals. LTD induced by repetitive stimulation was not affected. Furthermore, we found that paired-pulse facilitation and spontaneous release of glutamate were also altered in MPTP treated animals, suggesting presynaptic enhancement of excitatory transmission in PD. Our results suggest that ACC synaptic transmission is enhanced in the animal model of PD, and cortical excitation may play important roles in PD related pain and anxiety.

Brain-derived neurotrophic factor produced long-term synaptic enhancement in the anterior cingulate cortex of adult mice

Previous studies have demonstrated that brain-derived neurotrophic factor (BDNF) is one of the diffusible messengers for enhancing synaptic transmission in the hippocampus. Less information is available about the possible roles of BDNF in the anterior cingulate cortex (ACC). In the present study, we used 64-electrode array field recording system to investigate the effect of BDNF on ACC excitatory transmission. We found that BDNF enhanced synaptic responses in a dose-dependent manner in the ACC in C57/BL6 mice. The enhancement was long-lasting, and persisted for at least 3 h. In addition to the enhancement, BDNF also recruited inactive synaptic responses in the ACC. Bath application of the tropomyosin receptor kinase B (TrkB) receptor antagonist K252a blocked BDNF-induced enhancement. L-type voltage-gated calcium channels (L-VGCC), metabotropic glutamate receptors (mGluRs), but not NMDA receptors were required for BDNF-produced enhancement. Moreover, calcium-stimulated adenylyl cyclase subtype 1 (AC1) but not AC8 was essential for the enhancement. A selective AC1 inhibitor NB001 completely blocked the enhancement. Furthermore, BDNF-produced enhancement occluded theta burst stimulation (TBS) induced long-term potentiation (LTP), suggesting that they may share similar signaling mechanisms. Finally, the expression of BDNF-induced enhancement depends on postsynaptic incorporation of calcium-permeable AMPA receptors (CP-AMPARs) and protein kinase Mζ (PKMζ). Our results demonstrate that cortical BDNF may contribute to synaptic potentiation in the ACC.

NMDA receptors and synaptic plasticity in the anterior cingulate cortex

The anterior cingulate cortex (ACC) plays an important role in pain modulation, and pain-related emotional disorders. In the ACC, two major forms of long-term potentiation (LTP) coexist in excitatory synapses and lay the basis of chronic pain and pain-related emotional disorders. The induction of postsynaptic LTP is dependent on the activation of postsynaptic NMDA receptors (NMDARs), while the presynaptic LTP is NMDAR-independent. Long-term depression (LTD) can also be divided into two types according to the degree of sensitivity to the inhibition of NMDARs. NMDAR heteromers containing GluN2A and GluN2B act as key molecules in both the NMDAR-dependent postsynaptic LTP and LTD. Additionally, NMDARs also exist in presynaptic terminals and modulate the evoked and spontaneous transmitter release. From a translational point of view, inhibiting subtypes of NMDARs and/or downstream signaling proteins may provide potential drug targets for chronic pain and its related emotional disorders.

Extracellular matrix protein laminin β1 regulates pain sensitivity and anxiodepression-like behaviors in mice

Patients with neuropathic pain often experience comorbid psychiatric disorders. Cellular plasticity in the anterior cingulate cortex (ACC) is assumed to be a critical interface for pain perception and emotion. However, substantial efforts have thus far been focused on the intracellular mechanisms of plasticity rather than the extracellular alterations that might trigger and facilitate intracellular changes. Laminin, a key element of the extracellular matrix (ECM), consists of one α-, one β-, and one γ-chain and is implicated in several pathophysiological processes. Here, we showed in mice that laminin β1 (LAMB1) in the ACC was significantly downregulated upon peripheral neuropathy. Knockdown of LAMB1 in the ACC exacerbated pain sensitivity and induced anxiety and depression. Mechanistic analysis revealed that loss of LAMB1 caused actin dysregulation via interaction with integrin β1 and the subsequent Src-dependent RhoA/LIMK/cofilin pathway, leading to increased presynaptic transmitter release probability and abnormal postsynaptic spine remodeling, which in turn orchestrated the structural and functional plasticity of pyramidal neurons and eventually resulted in pain hypersensitivity and anxiodepression. This study sheds new light on the functional capability of ECM LAMB1 in modulating pain plasticity and identifies a mechanism that conveys extracellular alterations to intracellular plasticity. Moreover, we identified cingulate LAMB1/integrin β1 signaling as a promising therapeutic target for the treatment of neuropathic pain and associated anxiodepression.

Neocortical circuits in pain and pain relief

The sensory, associative and limbic neocortical structures play a critical role in shaping incoming noxious inputs to generate variable pain perceptions. Technological advances in tracing circuitry and interrogation of pathways and complex behaviours are now yielding critical knowledge of neocortical circuits, cellular contributions and causal relationships between pain perception and its abnormalities in chronic pain. Emerging insights into neocortical pain processing suggest the existence of neocortical causality and specificity for pain at the level of subdomains, circuits and cellular entities and the activity patterns they encode. These mechanisms provide opportunities for therapeutic intervention for improved pain management.

Pelvic Pain Alters Functional Connectivity Between Anterior Cingulate Cortex and Hippocampus in Both Humans and a Rat Model

The anterior cingulate cortex (ACC) and hippocampus (HIPP) are two key brain regions associated with pain and pain-related affective processing. However, whether and how pelvic pain alters the neural activity and connectivity of the ACC and HIPP under baseline and during social pain, and the underlying cellular and molecular mechanisms, remain unclear. Using functional magnetic resonance imaging (fMRI) combined with electrophysiology and biochemistry, we show that pelvic pain, particularly, primary dysmenorrhea (PDM), causes an increase in the functional connectivity between ACC and HIPP in resting-state fMRI, and a smaller reduction in connectivity during social exclusion in PDM females with periovulatory phase. Similarly, model rats demonstrate significantly increased ACC-HIPP synchronization in the gamma band, associating with reduced modulation by ACC-theta on HIPP-gamma and increased levels of receptor proteins and excitation. This study brings together human fMRI and animal research and enables improved therapeutic strategies for ameliorating pain and pain-related affective processing.

Inhibition of calcium-stimulated adenylyl cyclase subtype 1 (AC1) for the treatment of neuropathic and inflammatory pain in adult female mice

Cortical long-term potentiation (LTP) serves as a cellular model for chronic pain. As an important subtype of adenylyl cyclases (ACs), adenylyl cyclase subtype 1 (AC1) is critical for the induction of cortical LTP in the anterior cingulate cortex (ACC). Genetic deletion of AC1 or pharmacological inhibition of AC1 blocked behavioral allodynia in animal models of neuropathic and inflammatory pain. Our previous experiments have identified a lead candidate AC1 inhibitor, NB001, which is highly selective for AC1 over other AC isoforms, and found that NB001 is effective in inhibiting behavioral allodynia in animal models of chronic neuropathic and inflammatory pain. However, previous experiments were carried out in adult male animals. Considering the potential gender difference as an important issue in researches of pain and analgesia, we investigated the effect of NB001 in female chronic pain animal models. We found that NB001, when administered orally, has an analgesic effect in female animal models of neuropathic and inflammatory pain without any observable side effect. Genetic deletion of AC1 also reduced allodynia responses in models of neuropathic pain and chronic inflammation pain in adult female mice. In brain slices of adult female mice, bath application of NB001(20 μM) blocked the induction of LTP in ACC. Our results indicate that calcium-stimulated AC1 is required for injury-related cortical LTP and behavioral allodynia in both sexes of adult animals, and NB001 can be used as a potential therapeutic drug for treating neuropathic and inflammatory pain in man and woman.

Modulatory effects of photobiomodulation in the anterior cingulate cortex of diabetic rats

Anterior Cingulate Cortex (ACC) has a crucial contribution to higher order pain processing. Photobiomodulation (PBM) has being used as integrative medicine for pain treatment and for a variety of nervous system disorders. This study evaluated the effects of PBM in the ACC of diabetic rats. Type 1 diabetes was induced by a single dose of streptozotocin (85 mg/Kg). A total of ten sessions of PBM (pulsed gallium-arsenide laser, 904 nm, 9500 Hz, 6.23 J/cm²) was applied to the rat peripheral nervous system. Glial fibrillary acidic protein (GFAP), mu-opioid receptor (MOR), glutamate receptor 1 (GluR1), and glutamic acid decarboxylase (GAD65/67) protein level expression were analyzed in the ACC of diabetic rats treated with PBM. Our data revealed that PBM decreased 79.5% of GFAP protein levels in the ACC of STZ rats. Moreover, STZ + PBM rats had protein levels of MOR increased 14.7% in the ACC. Interestingly, STZ + PBM rats had a decrease in 70.7% of GluR1 protein level in the ACC. Additionally, PBM decreased 45.5% of GAD65/67 protein levels in the ACC of STZ rats.

Nociceptor-localized cGMP-dependent protein kinase I is a critical generator for central sensitization and neuropathic pain

Patients with neuropathic pain often experience exaggerated pain and anxiety. Central sensitization has been linked with the maintenance of neuropathic pain and may become an autonomous pain generator. Conversely, emerging evidence accumulated that central sensitization is initiated and maintained by ongoing nociceptive primary afferent inputs. However, it remains elusive what mechanisms underlie this phenomenon and which peripheral candidate contributes to central sensitization that accounts for pain hypersensitivity and pain-related anxiety. Previous studies have implicated peripherally localized cGMP-dependent protein kinase I (PKG-I) in plasticity of nociceptors and spinal synaptic transmission as well as inflammatory hyperalgesia. However, whether peripheral PKG-I contributes to cortical plasticity and hence maintains nerve injury-induced pain hypersensitivity and anxiety is unknown. Here, we demonstrated significant upregulation of PKG-I in ipsilateral L3 dorsal root ganglia (DRG), no change in L4 DRG, and downregulation in L5 DRG upon spared nerve injury. Genetic ablation of PKG-I specifically in nociceptors or post-treatment with intervertebral foramen injection of PKG-I antagonist, KT5823, attenuated the development and maintenance of spared nerve injury-induced bilateral pain hypersensitivity and anxiety. Mechanistic analysis revealed that activation of PKG-I in nociceptors is responsible for synaptic potentiation in the anterior cingulate cortex upon peripheral neuropathy through presynaptic mechanisms involving brain-derived neurotropic factor signaling. Our results revealed that PKG-I expressed in nociceptors is a key determinant for cingulate synaptic plasticity after nerve injury, which contributes to the maintenance of pain hypersensitivity and anxiety. Thereby, this study presents a strong basis for opening up a novel therapeutic target, PKG-I, in nociceptors for treatment of comorbidity of neuropathic pain and anxiety with least side effects.

Functional connectivity of the anterior cingulate cortex with pain-related regions in children with post-traumatic headache

Introduction:

Post-traumatic headaches (PTH) are common following mild traumatic brain injury (mTBI). There is evidence of altered central pain processing in adult PTH; however, little is known about how children with PTH process pain. The anterior cingulate cortex (ACC) plays a critical role in descending central pain modulation. In this study, we explored whether the functional connectivity (FC) of the ACC is altered in children with PTH.

Methods:

In this case-control study, we investigated resting-state FC of 5 ACC seeds (caudal, dorsal, rostral, perigenual, and subgenual) in children with PTH ( n = 73) and without PTH ( n = 29) following mTBI, and healthy controls ( n = 27). Post-concussion symptoms were assessed using the Post-Concussion Symptom Inventory and the Child Health Questionnaire. Resting-state functional Magnetic Resonance Imaging (fMRI) data were used to generate maps of ACC FC. Group-level comparisons were performed within a target mask comprised of pain-related regions using FSL Randomise.

Results:

We found decreased FC between the right perigenual ACC and the left cerebellum, and increased FC between the right subgenual ACC and the left dorsolateral prefrontal cortex in children with PTH compared to healthy controls. The ACC FC in children without PTH following mTBI did not differ from the group with PTH or healthy controls. FC between rostral and perigenual ACC seeds and the cerebellum was increased in children with PTH with pre-injury headaches compared to those with PTH without pre-injury headaches. There was a positive relationship between PTH severity and rostral ACC FC with the bilateral thalamus, right hippocampus and periaqueductal gray.

Conclusions:

Central pain processing is altered in children with PTH. Pre-existing headaches help to drive this process.

Trial registration:

The PlayGame Trial was registered in ClinicalTrials.gov database ( ClinicalTrials.gov Identifier: NCT01874847).

Contributions of aversive environmental stress to migraine chronification: Research update of migraine pathophysiology

Clinical studies have suggested that internal and/or external aversive cues may produce a negative affective-motivational component whereby maladaptive responses (plasticity) of dural afferent neurons are initiated contributing to migraine chronification. However, pathophysiological processes and neural circuitry involved in aversion (unpleasantness)-producing migraine chronification are still evolving. An interdisciplinary team conducted this narrative review aimed at reviewing neuronal plasticity for developing migraine chronicity and its relevant neurocircuits and providing the most cutting-edge information on neuronal mechanisms involved in the processing of affective aspects of pain and the role of unpleasantness evoked by internal and/or external cues in facilitating the chronification process of migraine headache. Thus, information presented in this review promotes the understanding of the pathophysiology of chronic migraine and contribution of unpleasantness (aversion) to migraine chronification. We hope that it will bring clinicians’ attention to how the maladaptive neuroplasticity of the emotion brain in the aversive environment produces a significant impact on the chronification of migraine headache, which will in turn lead to new therapeutic strategies for this type of pain.

NMDA GluN2C/2D receptors contribute to synaptic regulation and plasticity in the anterior cingulate cortex of adult mice

INTRODUCTION

N-Methyl-D-aspartate receptors (NMDARs) play a critical role in different forms of plasticity in the central nervous system. NMDARs are always assembled in tetrameric form, in which two GluN1 subunits and two GluN2 and/or GluN3 subunits combine together. Previous studies focused mainly on the hippocampus. The anterior cingulate cortex (ACC) is a key cortical region for sensory and emotional functions. NMDAR GluN2A and GluN2B subunits have been previously investigated, however much less is known about the GluN2C/2D subunits.

RESULTS

In the present study, we found that the GluN2C/2D subunits are expressed in the pyramidal cells of ACC of adult mice. Application of a selective antagonist of GluN2C/2D, (2R*,3S*)-1-(9-bromophenanthrene-3-carbonyl) piperazine-2,3-dicarboxylic acid (UBP145), significantly reduced NMDAR-mediated currents, while synaptically evoked EPSCs were not affected. UBP145 affected neither the postsynaptic long-term potentiation (post-LTP) nor the presynaptic LTP (pre-LTP). Furthermore, the long-term depression (LTD) was also not affected by UBP145. Finally, both UBP145 decreased the frequency of the miniature EPSCs (mEPSCs) while the amplitude remained intact, suggesting that the GluN2C/2D may be involved in presynaptic regulation of spontaneous glutamate release.

CONCLUSIONS

Our results provide direct evidence that the GluN2C/2D contributes to evoked NMDAR mediated currents and mEPSCs in the ACC, which may have significant physiological implications.

Electrical stimulation of the posterior insula induces mechanical analgesia in a rodent model of neuropathic pain by modulating GABAergic signaling and activity in the pain circuitry

The insula has emerged as a critical target for electrical stimulation since it influences pathological pain states. We investigated the effects of repetitive electrical stimulation of the insular cortex (ESI) on mechanical nociception, and general locomotor activity in rats subjected to chronic constriction injury (CCI) of the sciatic nerve. We also studied neuroplastic changes in central pain areas and the involvement of GABAergic signaling on ESI effects. CCI rats had electrodes implanted in the left agranular posterior insular cortex (pIC), and mechanical sensitivity was evaluated before and after one or five daily consecutive ESIs (15 min each, 60 Hz, 210 μs, 1 V). Five ESIs (repetitive ESI) induced sustained mechanical antinociception from the first to the last behavioral assessment without interfering with locomotor activity. A marked increase in Fos immunoreactivity in pIC and a decrease in the anterior and mid-cingulate cortex, periaqueductal gray and hippocampus were noticed after five ESIs. The intrathecal administration of the GABAA receptor antagonist bicuculline methiodide reversed the stimulation-induced antinociception after five ESIs. ESI increased GAD65 levels in pIC but did not interfere with GABA, glutamate or glycine levels. No changes in GFAP immunoreactivity were found in this work. Altogether, the results indicate the efficacy of repetitive ESI for the treatment of experimental neuropathic pain and suggest a potential influence of pIC in regulating pain pathways partially through modulating GABAergic signaling.

Targeting spinal TRAF6 expression attenuates chronic visceral pain in adult rats with neonatal colonic inflammation

Background

Irritable bowel syndrome is one of the most common gastrointestinal disorders. It is featured by abdominal pain in conjunction with altered bowel habits. However, the pathophysiology of the syndrome remains largely unknown. Tumor necrosis factor receptor-associated factor 6 (TRAF6) has been reported to be involved in neuropathic pain. The aim of this study was to investigate roles and mechanisms of TRAF6 in the chronic visceral hypersensitivity.

Methods

Visceral hypersensitivity was induced by neonatal colonic inflammation and was identified by colorectal distention. The protein level, RNA level, and cellular distribution of TRAF6 and its related molecules were detected with Western blot, quantitative polymerase chain reaction, and immunofluorescence. In vitro spinal cord slice recording technique was performed to determine the synaptic transmission activities.

Results

Neonatal colonic inflammation rats displayed visceral hypersensitivity at the age of six weeks. The expression of TRAF6 was obviously upregulated in spinal cord dorsal horn of neonatal colonic inflammation rats at the age of six weeks. Immunofluorescence study showed that TRAF6 was dominantly expressed in spinal astrocytes. Intrathecal injection of TRAF6 small interfering RNA (siRNA) significantly reduced the amplitude of spontaneous excitatory postsynaptic currents at the spinal dorsal horn level. Furthermore, knockdown of TRAF6 led to a significant downregulation of cystathionine β synthetase expression in the spinal dorsal horn of neonatal colonic inflammation rats. Importantly, intrathecal injection of TRAF6 siRNA remarkably alleviated visceral hypersensitivity of neonatal colonic inflammation rats.

Conclusions

Our results suggested that the upregulation of TRAF6 contributed to visceral pain hypersensitivity, which is likely mediated by regulating cystathionine β synthetase expression in the spinal dorsal horn. Our findings suggest that TRAF6 might act as a potential target for the treatment of chronic visceral pain in irritable bowel syndrome patients.

Repetitive non-invasive prefrontal stimulation reverses neuropathic pain via neural remodelling in mice

Chronic neuropathic pain presents a major challenge to pharmacological therapy and neurostimulation-based alternatives are gaining interest. Although invasive and non-invasive motor cortex stimulation has been the focus of several studies, very little is known about the potential of targeting the prefrontal cortex. This study was designed to elucidate the analgesic potential of prefrontal stimulation in a translational context and to uncover the neural underpinnings thereof. Here, we report that non-invasive, repetitive direct anodal current transcranial stimulation (tDCS) of the prefrontal cortex exerted analgesia in mice with neuropathic pain for longer than a week. When applied at chronic stages of neuropathic pain, prefrontal tDCS reversed established allodynia and suppressed aversion and anxiety-related behaviours. Activity mapping as well as in vivo electrophysiological analyses revealed that although the cortex responds to acute tDCS with major excitation, repetitive prefrontal tDCS brings about large-scale silencing of cortical activity. Different classes of different classes of GABAergic interneurons and classes of excitatory neurons differs dramatically between single, acute vs and repetitive tDCS. Repetitive prefrontal tDCS alters basal activity as well as responsivity of a discrete set of distant cortical and sub-cortical areas to tactile stimuli, namely the rostral anterior cingulate cortex, the insular cortex, the ventrolateral periaqueductal grey and the spinal dorsal horn. This study thus makes a strong case for harnessing prefrontal cortical modulation for non-invasive transcranial stimulation paradigms to achieve long-lasting pain relief in established neuropathic pain states and provides valuable insights gained on neural mechanistic underpinnings of prefrontal tDCS in neuropathic pain.

Histamine and Corticosterone Modulate Acid Sensing Ion Channels (ASICs) Dependent Long-term Potentiation at the Mouse Anterior Cingulate Cortex

Increase in proton concentration [H⁺] or decrease in local and global extracellular pH occurs in both physiological and pathological conditions. Acid-sensing ion channels (ASICs), belonging to the ENaC/Deg superfamily, play an important role in signal transduction as proton sensor. ASICs and in particular ASIC1a (one of the six ASICs subunits) which is permeable to Ca²⁺, are involved in many physiological processes including synaptic plasticity and neurodegenerative diseases. Activity-dependent long-term potentiation (LTP) is a major type of long-lasting synaptic plasticity in the CNS, associated with learning, memory, development, fear and persistent pain. Neurons in the anterior cingulate cortex (ACC) play critical roles in pain perception and chronic pain and express ASIC1a channels. During synaptic transmission, acidification of the synaptic cleft presumably due to the co-release of neurotransmitter and H⁺ from synaptic vesicles activates postsynaptic ASIC1a channels in ACC of mice. This generates ASIC1a synaptic currents that add to the glutamatergic excitatory postsynaptic currents (EPSCs). Here we report that modulators like histamine and corticosterone, acting through ASIC1a regulate synaptic plasticity, reducing the threshold for LTP induction of glutamatergic EPSCs. Our findings suggest a new role for ASIC1a mediating the neuromodulator action of histamine and corticosterone regulating specific forms of synaptic plasticity in the mouse ACC.

Pra-C exerts analgesic effect through inhibiting microglial activation in anterior cingulate cortex in complete Freund's adjuvant-induced mouse model

Chronic pain is highly prevalent worldwide and severely affects daily lives of patients and family members. Praeruptorin C (Pra-C) is a main active ingredient derived from Peucedanum praeruptorum Dunn, traditionally used as antibechic, anti-bronchitis and anti-hypertension drug. Here, we evaluated the effects of Pra-C in a chronic inflammatory pain mouse model induced by complete Freund's adjuvant (CFA) injection. Pra-C (3 mg/kg) treatment for just 3 days after CFA challenge relieved CFA-induced mechanical allodynia and hindpaw edema in mice. In the anterior cingulate cortex (ACC), Pra-C treatment inhibited microglia activation and reduced levels of proinflammatory cytokines, TNF-α and IL-1β, and suppressed upregulation of glutamate receptors caused by CFA injection. In addition, Pra-C attenuated neuronal hyperexcitability in ACC of CFA-injected mice. In vitro studies confirmed the analgesic effect of Pra-C was due to its inhibitory ability on microglial activation. In conclusion, Pra-C administration had a certain effect on relieving chronic pain by inhibiting microglial activation, attenuating proinflammatory cytokine releasing and regulating excitatory synaptic proteins in the ACC of the CFA-injected mice.

Electroacupuncture Attenuates CFA-Induced Inflammatory Pain by Regulating CaMKII

Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) is a multifunctional serine/threonine kinase that is ubiquitously distributed in the central and peripheral nervous systems. Moreover, its phosphorylated protein (P-CaMKII) is involved in memory, mood, and pain regulation in the anterior cingulate cortex (ACC). Electroacupuncture (EA) is a traditional Chinese therapeutic technique that can effectively treat chronic inflammatory pain. However, the CaMKII-GluA1 role in EA analgesia in the ACC remains unclear. This study investigated the role of P-CaMKII and P-GluA1 in a mouse model of inflammatory pain induced by complete Freund's adjuvant (CFA). There were increased P-CaMKII and P-GluA1 levels in the ACC. We found that intracerebroventricular injection of KN93, a CaMKII inhibitor, as well as EA stimulation, attenuated complete Freund's adjuvant-induced pain behavior. Further, EA increased pCaMKII-PICK1 complex (abbreviated as C-P complex) levels. Our findings demonstrate that EA inhibits inflammatory pain by inhibiting CaMKII-GluA1 phosphorylation. P-CaMKII is involved in EA analgesia as the pCaMKII-PICK1 complex.

Effect of experimental orthodontic pain on gray and white matter functional connectivity

Aim

Over 90% of patients receiving orthodontic treatment experience clinically significant pain. However, little is known about the neural correlates of orthodontic pain and which has therefore been investigated in the present study of healthy subjects using an experimental paradigm.

Methods

Resting‐state functional magnetic resonance imaging (rsfMRI) was performed in 44 healthy subjects 24 hours after an elastic separator had been introduced between the first and the second molar on the right side of the lower jaw and in 49 age‐ and sex‐matched healthy control (HC) subjects. A K‐means clustering algorithm was used to identify functional gray matter (GM) and white matter (WM) resting‐state networks, and differences in functional connectivity (FC) of GM and WM between the group of subjects with experimental orthodontic pain and HC were analyzed.

Results

Twelve GM networks and 14 WM networks with high stability were identified. Compared with HC, subjects with orthodontic pain showed significantly increased FC between WM12, which includes posterior thalamic radiation and posterior cingulum bundle, and most GM networks. Besides, the WM12 network showed significant differences in FC with three GM‐WM loops involving the default mode network, dorsal attention network, and salience network, respectively.

Conclusions

Orthodontic pain is shown to produce an alteration of FC in networks relevant to pain processing, which may be mediated by a WM network relevant to emotion perception and cognitive processing.

Embodiment in Virtual Reality for the Treatment of Chronic Low Back Pain: A Case Series

Purpose

We describe two case studies that use embodiment in virtual reality as a treatment for chronic low back pain. The purpose of this case series was to determine the feasibility of a novel virtual reality-based digital therapeutic for the treatment of chronic pain.

Patients and Methods

Two patients with chronic low back pain received seven sessions, two sessions per week, of a novel digital therapeutic that combines virtual embodiment with graded motor imagery to deliver functional rehabilitation exercises using an off-the-shelf virtual reality system. Pain intensity was measured using a visual analog scale before and after each session to get an indication whether individual sessions of virtual embodiment training decrease pain intensity. Pain catastrophizing scale was assessed before the first session and after the seventh session to determine the extent to which virtual embodiment training can improve psychological symptoms of chronic low back pain.

Results

In both patients, pain intensity was improved after individual sessions of virtual embodiment training as measured by a paired t-test: (Patient A: t = 2.890, P < 0.05) and (Patient B: t = 5.346, P < 0.005). This indicates that individual sessions of virtual embodiment training decrease pain intensity. In both patients, improvements were observed in three subscales of the pain catastrophizing scale (rumination, magnification, and helplessness). This indicates that virtual embodiment training may have benefits for chronic pain symptoms such as pain intensity, pain-related mobility impairment, and disability.

Conclusion

This case series provides evidence that embodiment in virtual reality improves symptoms of persistent chronic low back pain. We propose a mechanism by which virtual embodiment may improve chronic pain symptoms by recontextualizing sensory feedback from the body as patients engage in functional rehabilitation exercises while in virtual reality.

Effect of Electroacupuncture on Pain Perception and Pain-Related Affection: Dissociation or Interaction Based on the Anterior Cingulate Cortex and S1

Electroacupuncture (EA) can effectively modulate pain perception and pain-related negative affect; however, we do not know whether the effect of EA on sensation and affect is parallel, or dissociated, interactional. In this study, we observed the effects of the anterior cingulate cortex (ACC) lesion and the primary somatosensory cortex (S1) activation on pain perception, pain-related affection, and neural oscillation in S1. ACC lesions did not affect pain perception but relieved pain-paired aversion. S1 activation increased pain perception and anxious behavior. EA can mitigate pain perception regardless of whether there is an ACC lesion. Chronic pain may increase the delta and theta band oscillatory activity in the S1 brain region and decrease the oscillatory activity in the alpha, beta, and gamma bands. EA intervention may inhibit the oscillatory activity of the alpha and beta bands. These results suggest that EA may mitigate chronic pain by relieving pain perception and reducing pain-related affection through different mechanisms. This evidence builds upon findings from previous studies of chronic pain and EA treatment.

D1 receptors in the anterior cingulate cortex modulate basal mechanical sensitivity threshold and glutamatergic synaptic transmission

The release of dopamine (DA) into target brain areas is considered an essential event for the modulation of many physiological effects. While the anterior cingulate cortex (ACC) has been implicated in pain related behavioral processes, DA modulation of synaptic transmission within the ACC and pain related phenotypes remains unclear. Here we characterized a Crispr/Cas9 mediated somatic knockout of the D1 receptor (D1R) in all neuronal subtypes of the ACC and find reduced mechanical thresholds, without affecting locomotion and anxiety. Further, the D1R high-efficacy agonist SKF 81297 and low efficacy agonist (±)-SKF-38393 inhibit α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic receptor (AMPAR) currents in the ACC. Paradoxically, the D1R antagonists SCH-23390 and SCH 33961 when co-applied with D1R agonists produced a robust short-term synergistic depression of AMPAR currents in the ACC, demonstrating an overall inhibitory role for D1R ligands. Overall, our data indicate that absence of D1Rs in the ACC enhanced peripheral sensitivity to mechanical stimuli and D1R activation decreased glutamatergic synaptic transmission in ACC neurons.