The amygdala between sensation and affect: a role in pain

Alcohol and cannabis use for pain management: Translational findings of relative risks, benefits, and interactions

Chronic pain affects over 20% of the global population and contributes to the vast burden of psychiatric illness. While effective treatments for chronic pain remain limited, both alcohol and cannabis have been used for centuries to manage pain and closely associated negative affective symptoms. However, persistent misuse of alcohol and/or cannabis in such a negative reinforcement fashion is hypothesized to increase the risk of severity of substance use disorders (SUDs). The current review describes neurobiological evidence for the analgesic efficacy of alcohol and primary cannabis constituents and how use or co-use of these substances may influence SUD risk.

Forebrain networks driving brainstem pain modulatory circuits during nocebo hyperalgesia in healthy humans

ABSTRACT

Prior experiences, conditioning cues, and expectations of improvement are essential for nocebo hyperalgesia expression. The neural circuits that communicate with brainstem pain modulatory nuclei during nocebo hyperalgesia responsivity are underexplored. In this study, we employed a classical conditioning and expectation model in 25 healthy human participants and measured brain activity using ultra-high field functional magnetic resonance imaging. We assessed forebrain activity changes as well as noxious stimulus-independent and -dependent connectivity changes with the lateral midbrain periaqueductal gray matter (lPAG). We found hyperalgesia-related signal increases in the orbitofrontal cortex, insula, and amygdala. In addition, we found stimulus-dependent lPAG connectivity changes with the orbitofrontal, anterior cingulate, and dorsolateral prefrontal cortices and stimulus-independent lPAG connectivity with the anterior cingulate cortex, dorsolateral prefrontal cortex, and nucleus accumbens during hyperalgesia. Whilst these connectivity changes are all associated with hyperalgesia, dynamic causal modelling analysis revealed that the dorsolateral prefrontal cortex was principally responsible for driving the lPAG. Overall, our results show that there is a complex relationship between forebrain activation and connectivity with brainstem pain modulation circuitry that results in the behavioural expression of nocebo hyperalgesia.

Repeated Administrations of Polyphenolic Extracts Prevent Chronic Reflexive and Non-Reflexive Neuropathic Pain Responses by Modulating Gliosis and CCL2-CCR2/CX3CL1-CX3CR1 Signaling in Spinal Cord-Injured Female Mice

Neuropathic pain after spinal cord injury lacks any effective treatments, often leading to chronic pain. This study tested whether the daily administration of fully characterized polyphenolic extracts from grape stalks and coffee could prevent both reflexive and non-reflexive chronic neuropathic pain in spinal cord-injured mice by modulating the neuroimmune axis. Female CD1 mice underwent mild spinal cord contusion and received intraperitoneal extracts in weeks one, three, and six post-surgery. Reflexive pain responses were assessed weekly for up to 10 weeks, and non-reflexive pain was evaluated at the study's end. Neuroimmune crosstalk was investigated, focusing on glial activation and the expression of CCL2/CCR2 and CX3CL1/CX3CR1 in supraspinal pain-related areas, including the periaqueductal gray, rostral ventromedial medulla, anterior cingulate cortex, and amygdala. Repeated treatments prevented mechanical allodynia and thermal hyperalgesia, and also modulated non-reflexive pain. Moreover, they reduced supraspinal gliosis and regulated CCL2/CCR2 and CX3CL1/CX3CR1 signaling. Overall, the combination of polyphenols in these extracts may offer a promising pharmacological strategy to prevent chronic reflexive and non-reflexive pain responses by modifying central sensitization markers, not only at the contusion site but also in key supraspinal regions implicated in neuropathic pain. Overall, these data highlight the potential of polyphenolic extracts for spinal cord injury-induced chronic neuropathic pain.

Opioidergic pain relief in humans is mediated by beta and high-gamma modulation in limbic regions

The nature of the neurophysiological effects of opioids, especially those responsible for their analgesic properties, are unknown, hindering efforts to develop non-addictive alternatives. Fentanyl and hydromorphone were administered to patients experiencing semi-chronic, clinically-relevant pain after surgical implantation of electrodes for the localization of seizure onset. Opioids suppressed beta oscillations in lateral amygdala, ventral and dorsolateral prefrontal cortices, and increased beta in medial amygdala and hippocampus. Opioids also suppressed high gamma oscillations in insula and lateral amygdala, and increased high gamma in cingulate cortex and hippocampus. The amplitude of these beta effects in the ventral prefrontal cortex, medial amygdala and hippocampus, and of gamma effects in the insula, were positively correlated with the magnitude of pain relief in response to a constant dose. These findings identify electrophysiological events in a network of limbic structures that may participate in opioidergic pain relief through nociceptive gating and a decreased concerned fixation on pain, providing insights into the neural basis of pain relief and suggesting possible biomarkers for developing non-addictive opioid alternatives.

BDNF Signaling and Pain Modulation

Brain-derived neurotrophic factor (BDNF) is an important neuromodulator of nervous system functions and plays a key role in neuronal growth and survival, neurotransmission, and synaptic plasticity. The effects of BDNF are mainly mediated by the activation of tropomyosin receptor kinase B (TrkB), expressed in both the peripheral and central nervous system. BDNF has been implicated in several neuropsychiatric conditions such as schizophrenia and anxio-depressive disorders, as well as in pain states. This review summarizes the evidence for a critical role of BDNF throughout the pain system and describes contrasting findings of its pro- and anti-nociceptive effects. Different cellular sources of BDNF, its influence on neuroimmune signaling in pain conditions, and its effects in different cell types and regions are described. These and endogenous BDNF levels, downstream signaling mechanisms, route of administration, and approaches to manipulate BDNF functions could explain the bidirectional effects in pain plasticity and pain modulation. Finally, current knowledge gaps concerning BDNF signaling in pain are discussed, including sex- and pathway-specific differences.

Involvement of a central amygdaloid nucleus-lateral habenular nucleus pathway in the processing of formalin-induced pain

The central amygdaloid nucleus (CeA) and the lateral habenular nucleus (LHb) are essential nuclei playing modulatory roles in encoding noxious stimuli. Their interaction has recently been demonstrated in chronic pain-induced depression. However, little is known about the CeA-LHb pathway in a formalin-induced pain model. In the present study, we aimed to clarify whether the CeA-LHb pathway modulates the formalin-induced pain model using a neuroanatomical tracing method combined with a designer receptor exclusively activated by a designer drugs strategy (DREADD). The results revealed that the CeA predominantly sends projections to vesicular glutamate transporter-2 (VGluT2)-expressing neurons of the LHb, and inhibition of LHb function exhibits an analgesic effect in the formalin-induced pain model. Furthermore, activating the CeA-LHb pathway significantly attenuates pain sensation only in phase 2 of formalin-induced pain in mice. The present results indicate the participation of the LHb in inflammatory pain sensation and reveal a CeA-LHbVGluT2 pathway that displays analgesic effects in a formalin pain model.

Fibromyalgia and the painful self: A meta-analysis of resting-state fMRI data

Fibromyalgia (FM) is a complex medical condition. The nested hierarchical model of self and its extension to the pain matrix could represent an integrated theoretical framework that might comprehensively captures FM clinical feautres. A multi-level meta-analysis was conducted. Resting-state functional connectivity (RS-FC) studies that compared patients with FM and healthy controls (HCs) were included. The association between RS-FC among self-related brain regions and pain intensity was also explored in the FM group. Eleven studies were eligible for meta-analytic procedures. Patients with FM, compared to HCs, were characterized by an increased RS-FC between the default mode network (DMN) and areas ascribed to interoceptive (e.g., insula) and exteroceptive (e.g., premotor, visual/auditory cortices) self layers. The clinical group also showed a reduced RS-FC among regions of the pain matrix (i.e., periaqueductal gray matter, somatosensory areas) involved in pain modulation. An increased RS-FC within DMN together with a heightened RS-FC between DMN and interoceptive self areas were positively associated to pain intensity reported by patients with FM. The nested hierarchical model of self and its extension to the pain matrix might represent comprehensive neurobiological backgrounds for clarifying core mind-body clinical features of FM.

Metabotropic Glutamate Receptor 5: A Potential Target for Neuropathic Pain Treatment

Neuropathic pain, a multifaceted and incapacitating disorder, impacts a significant number of individuals globally. Despite thorough investigation, the development of efficacious remedies for neuropathic pain continues to be a formidable task. Recent research has revealed the potential of metabotropic glutamate receptor 5 (mGlu5) as a target for managing neuropathic pain. mGlu5 is a receptor present in the central nervous system that has a vital function in regulating synaptic transmission and the excitability of neurons. This article seeks to investigate the importance of mGlu5 in neuropathic pain pathways, analyze the pharmacological approach of targeting mGlu5 for neuropathic pain treatment, and review the negative allosteric mGlu5 modulators used to target mGlu5. By comprehending the role of mGlu5 in neuropathic pain, we can discover innovative treatment approaches to ease the distress endured by persons afflicted with this incapacitating ailment.

Dualism, Materialism, and the relationship between the brain and the mind in experiencing pain

Characterizing the relationship between the brain and the mind is essential, both for understanding how we experience sensations and for attempts to create machine-based artificial intelligence. Materialists argue that the brain and the mind are both physical/material in nature whereas Cartesian dualists posit that the brain is material, the mind is non-material, and that they are separate. Recent investigations into the mechanisms responsible for pain can resolve this issue. Pain from an injury requires both the induction of a long-term potentiation (LTP) in a subset of pyramidal neurons in the anterior cingulate cortex and the creation of electromagnetic waves in the surrounding area. The LTP sensitizes synaptic transmission and, by activating enzyme cascades, changes the phenotype of the pyramidal neurons. The changes sustain the generation of the waves and the pain. The waves rapidly disseminate information about the pain to distant areas of the brain and studies using Transcranial Stimulation show that EM waves can influence the induction of LTP. According to leading contemporary theories, the waves will communicate with the mind, which is where the painfulness is experienced. The material brain and immaterial mind are therefore separate and we can no longer attribute painfulness solely to the activities of the brain. This is a radical departure from the contemporary view of brain functions and supports Cartesian Dualism. Consequently, consciousness and higher mental functions cannot be duplicated by mimicking the activities of the brain.

Differential activation of lateral parabrachial nuclei and their limbic projections during head compared with body pain: A 7-Tesla functional magnetic resonance imaging study

Pain is a complex experience that involves sensory, emotional, and motivational components. It has been suggested that pain arising from the head and orofacial regions evokes stronger emotional responses than pain from the body. Indeed, recent work in rodents reports different patterns of activation in ascending pain pathways during noxious stimulation of the skin of the face when compared to noxious stimulation of the body. Such differences may dictate different activation patterns in higher brain regions, specifically in those areas processing the affective component of pain. We aimed to use ultra-high field functional magnetic resonance imaging (fMRI at 7-Tesla) to determine whether noxious thermal stimuli applied to the surface of the face and body evoke differential activation patterns within the ascending pain pathway in awake humans (n=16). Compared to the body, noxious heat stimulation to the face evoked more widespread signal changes in prefrontal cortical regions and numerous brainstem and subcortical limbic areas. Moreover, facial pain evoked significantly different signal changes in the lateral parabrachial nucleus, substantia nigra, paraventricular hypothalamus, and paraventricular thalamus, to those evoked by body pain. These results are consistent with recent preclinical findings of differential activation in the brainstem and subcortical limbic nuclei and associated cortices during cutaneous pain of the face when compared with the body. The findings suggest one potential mechanism by which facial pain could evoke a greater emotional impact than that evoked by body pain.

Synaptic sensitization in the anterior cingulate cortex sustains the consciousness of pain via synchronized oscillating electromagnetic waves

A recent report showed that experiencing pain requires not only activities in the brain, but also the generation of electric fields in a defined area of the anterior cingulate cortex (ACC). The present manuscript presents evidence that electromagnetic (EM) waves are also necessary. Action potentials (APs) encoding information about an injury stimulate thousands synapses on pyramidal neurons within the ACC resulting in the generation of synchronized oscillating (EM) waves and the activation of NMDA receptors. The latter induces a long-term potentiation (LTP) in the pyramidal dendrites that is necessary to experience both neuropathic and visceral pain. The LTP sensitizes transmission across the synapses that sustains the duration of the waves and the pain, EM waves containing information about the injury travel throughout the brain and studies using transcranial stimulation indicate that they can induce NMDA-mediated LTP in distant neuronal circuits. What is ultimately experienced as pain depends on the almost instantaneous integration of information from numerous neuronal centers, such as the amygdala, that are widely separated in the brain. These centers also generate EM waves and I propose that the EM waves from these centers interact to rapidly adjust the intensity of the pain to accommodate past and present circumstances. Where the waves are transformed into a consciousness of pain is unknown. One possibility is the mind which, according to contemporary theories, is where conscious experiences arise. The hypothesis can be tested directly by blocking the waves from the ACC. If correct, the waves would open new avenues of research into the relationship between the brain, consciousness, and the mind.

Ionic mechanisms involved in arginine vasopressin-mediated excitation of auditory cortical and thalamic neurons

The axons containing arginine vasopressin (AVP) from the hypothalamus innervate a variety of structures including the cerebral cortex, thalamus, hippocampus and amygdala. A plethora amount of evidence indicates that activation of the V1a subtype of the vasopressin receptors facilitates anxiety-like and fear responses. As an essential structure involved in fear and anxiety responses, the amygdala, especially the lateral nucleus of amygdala (LA), receives glutamatergic innervations from the auditory cortex and auditory thalamus where high density of V1a receptors have been detected. However, the roles and mechanisms of AVP in these two important areas have not been determined, which prevents the understanding of the mechanisms whereby V1a activation augments anxiety and fear responses. Here, we used coronal brain slices and studied the effects of AVP on neuronal activities of the auditory cortical and thalamic neurons. Our results indicate that activation of V1a receptors excited both auditory cortical and thalamic neurons. In the auditory cortical neurons, AVP increased neuronal excitability by depressing multiple subtypes of inwardly rectifying K+ (Kir) channels including the Kir2 subfamily, the ATP-sensitive K+ channels and the G protein-gated inwardly rectifying K+ (GIRK) channels, whereas activation of V1a receptors excited the auditory thalamic neurons by depressing the Kir2 subfamily of the Kir channels as well as activating the hyperpolarization-activated cyclic nucleotide-gated (HCN) channels and a persistent Na+ channel. Our results may help explain the roles of V1a receptors in facilitating fear and anxiety responses. Categories: Cell Physiology.

Neonatal amygdala volumes, procedural pain and the association with social-emotional development in children born very preterm

Very preterm birth (< 32 weeks' gestational age) is associated with later social and emotional impairments, which may result from enhanced vulnerability of the limbic system during this period of heightened vulnerability. Evidence suggests that early procedural pain may be a key moderator of early brain networks. In a prospective cohort study, neonates born very preterm (< 30 weeks' gestation) underwent MRI scanning at term-equivalent age (TEA) and clinical data were collected (mechanical ventilation, analgesics, sedatives). Procedural pain was operationalized as the number of skin breaking procedures. Amygdala volumes were automatically extracted. The Strengths and Difficulties questionnaire was used to assess social-emotional outcomes at 5 years of age (mean age 67.5 months). General linear models were employed to examine the association between neonatal amygdala volumes and social-emotional outcomes and the timing and amount of procedural pain exposure (early within the first weeks of life to TEA) as a moderator, adjusting for biological sex, gestational age, 5-year assessment age, days of mechanical ventilation and total cerebral volumes. A total of 42 preterm infants participated. Right amygdala volumes at TEA were associated with prosocial behaviour at age 5 (B = -0.010, p = 0.005). Procedural pain was found to moderate the relationship between right amygdala volumes in the neonatal period and conduct problems at 5 years, such that early skin breaking procedures experienced within the first few weeks of life strengthened the association between right amygdala volumes and conduct problems (B = 0.005, p = 0.047). Late skin breaking procedures, experienced near TEA, also strengthened the association between right amygdala volumes and conduct problems (B = 0.004, p = 0.048).

Activation of CRF/CRFR1 Signaling in the Central Nucleus of the Amygdala Contributes to Chronic Stress-Induced Exacerbation of Neuropathic Pain by Enhancing GluN2B-NMDA Receptor-Mediated Synaptic Plasticity in Adult Male Rats

Exacerbation of pain by chronic stress and comorbidity of pain with stress-related disorders such as depression and post-traumatic stress disorder, represent significant clinical challenges. Previously we have documented that chronic forced swim (FS) stress exacerbates neuropathic pain in spared nerve injury (SNI) rats, associated with an up-regulation of GluN2B-containing N-methyl-D-aspartate receptors (GluN2B-NMDARs) in the central nucleus of the amygdala (CeA). However, the molecular mechanisms underlying chronic FS stress (CFSS)-mediated exacerbation of pain sensitivity in SNI rats still remain unclear. In this study, we demonstrated that exposure of CFSS to rats activated the corticotropin-releasing factor (CRF)/CRF receptor type 1 (CRFR1) signaling in the CeA, which was shown to be necessary for CFSS-induced depressive-like symptoms in stressed rats, and as well, for CFSS-induced exacerbation of pain hypersensitivity in SNI rats exposed to chronic FS stress. Furthermore, we discovered that activation of CRF/CRFR1 signaling in the CeA upregulated the phosphorylation of GluN2B-NMDARs at tyrosine 1472 (pGluN2BY1472) in the synaptosomal fraction of CeA, which is highly correlated to the enhancement of synaptic GluN2B-NMDARs expression that has been observed in the CeA in CFSS-treated SNI rats. In addition, we revealed that activation of CRF/CRFR1 signaling in the CeA facilitated the CFSS-induced reinforcement of long-term potentiation as well as the enhancement of NMDAR-mediated excitatory postsynaptic currents in the basolateral amygdala (BLA)-CeA pathway in SNI rats. These findings suggest that activation of CRF/CRFR1 signaling in the CeA contributes to chronic stress-induced exacerbation of neuropathic pain by enhancing GluN2B-NMDAR-mediated synaptic plasticity in rats subjected to nerve injury. PERSPECTIVE: Our present study provides a novel mechanism for elucidating stress-induced hyperalgesia and highlights that the CRF/CRFR1 signaling and the GluN2B-NMDAR-mediated synaptic plasticity in the CeA may be important as potential therapeutic targets for chronic stress-induced pain exacerbation in human neuropathic pain. DATA AVAILABILITY: The data that support the findings of this study are available from the corresponding author upon reasonable request.

Brain compensatory mechanisms in depression and memory complaints in fibromyalgia: the role of theta oscillatory activity

BACKGROUND

The different clinical presentations of fibromyalgia syndrome (FMS) might play independent roles in the unclear etiology of cognitive impairments and depressive symptoms seen in patients with FMS. Understanding how these clinical presentations are associated with the clinical and neurophysiological aspects of FMS is important for the development of effective treatments.

AIM

To explore the relationship of memory complaints and depressive symptoms with the different clinical and neurophysiological characteristics of FMS.

METHODS

Cross-sectional data analysis from a randomized clinical trial. Baseline demographics and data on physical fitness, sleep, anxiety, depression, cortical excitability, and pain (clinical and mechanistic) from 63 subjects with FMS were used. Multiple linear and logistic association models were constructed.

RESULTS

Final regression models including different sets of predictions were statistically significant (P < .001), explaining approximately 50% of the variability in cognitive complaints and depression status. Older subjects had higher levels of anxiety, poorer sleep quality, lower motor threshold, and higher relative theta power in the central area and were more likely to have clinical depression. Higher anxiety, pain, and theta power were associated with a higher likelihood of memory complaints.

CONCLUSION

Depression symptoms seem to be associated with transcranial magnetic stimulation-indexed motor threshold and psychosocial variables, whereas memory complaints are associated with pain intensity and higher theta oscillations. These mechanisms might be catalyzed or triggered by some behavioral and clinical features, such as older age, sleep disruption, and anxiety. The correlation with clinical variables suggests that the increasing of theta oscillations is a compensatory response in patients with FMS, which can be explored in future studies to improve the treatment of FMS.

TRIAL REGISTRATION

ClinicalTrials.gov ID NCT03371225.

Chronic Neuropathic Pain and Comorbid Depression Syndrome: From Neural Circuit Mechanisms to Treatment

Chronic neuropathic pain and comorbid depression syndrome (CDS) is a major worldwide health problem that affects the quality of life of patients and imposes a tremendous socioeconomic burden. More than half of patients with chronic neuropathic pain also suffer from moderate or severe depression. Due to the complex pathogenesis of CDS, there are no effective therapeutic drugs available. The lack of research on the neural circuit mechanisms of CDS limits the development of treatments. The purpose of this article is to provide an overview of the various circuits involved in CDS. Notably, activating some neural circuits can alleviate pain and/or depression, while activating other circuits can exacerbate these conditions. Moreover, we discuss current and emerging pharmacotherapies for CDS, such as ketamine. Understanding the circuit mechanisms of CDS may provide clues for the development of novel drug treatments for improved CDS management.

Pain in Parkinson's disease: a neuroanatomy-based approach

Parkinson's disease is a progressive neurodegenerative disorder characterized by the deposition of misfolded alpha-synuclein in different regions of the central and peripheral nervous system. Motor impairment represents the signature clinical expression of Parkinson's disease. Nevertheless, non-motor symptoms are invariably present at different stages of the disease and constitute an important therapeutic challenge with a high impact for the patients' quality of life. Among non-motor symptoms, pain is frequently experienced by patients, being present in a range of 24-85% of Parkinson's disease population. Moreover, in more than 5% of patients, pain represents the first clinical manifestation, preceding by decades the exordium of motor symptoms. Pain implies a complex biopsychosocial experience with a downstream complex anatomical network involved in pain perception, modulation, and processing. Interestingly, all the anatomical areas involved in pain network can be affected by a-synuclein pathology, suggesting that pathophysiology of pain in Parkinson's disease encompasses a 'pain spectrum', involving different anatomical and neurochemical substrates. Here the various anatomical sites recruited in pain perception, modulation and processing are discussed, highlighting the consequences of their possible degeneration in course of Parkinson's disease. Starting from peripheral small fibres neuropathy and pathological alterations at the level of the posterior laminae of the spinal cord, we then describe the multifaceted role of noradrenaline and dopamine loss in driving dysregulated pain perception. Finally, we focus on the possible role of the intertwined circuits between amygdala, nucleus accumbens and habenula in determining the psycho-emotional, autonomic and cognitive experience of pain in Parkinson's disease. This narrative review provides the first anatomically driven comprehension of pain in Parkinson's disease, aiming at fostering new insights for personalized clinical diagnosis and therapeutic interventions.

Deciphering the functional role of insular cortex stratification in trigeminal neuropathic pain

Trigeminal neuropathic pain (TNP) is a major concern in both dentistry and medicine. The progression from normal to chronic TNP through activation of the insular cortex (IC) is thought to involve several neuroplastic changes in multiple brain regions, resulting in distorted pain perception and associated comorbidities. While the functional changes in the insula are recognized contributors to TNP, the intricate mechanisms underlying the involvement of the insula in TNP processing remain subjects of ongoing investigation. Here, we have overviewed the most recent advancements regarding the functional role of IC in regulating TNP alongside insights into the IC's connectivity with other brain regions implicated in trigeminal pain pathways. In addition, the review examines diverse modulation strategies that target the different parts of the IC, thereby suggesting novel diagnostic and therapeutic management of chronic TNP in the future.

Chemogenetic Manipulation of Amygdala Kappa Opioid Receptor Neurons Modulates Amygdala Neuronal Activity and Neuropathic Pain Behaviors

Neuroplasticity in the central nucleus of the amygdala (CeA) plays a key role in the modulation of pain and its aversive component. The dynorphin/kappa opioid receptor (KOR) system in the amygdala is critical for averse-affective behaviors in pain conditions, but its mechanisms are not well understood. Here, we used chemogenetic manipulations of amygdala KOR-expressing neurons to analyze the behavioral consequences in a chronic neuropathic pain model. For the chemogenetic inhibition or activation of KOR neurons in the CeA, a Cre-inducible viral vector encoding Gi-DREADD (hM4Di) or Gq-DREADD (hM3Dq) was injected stereotaxically into the right CeA of transgenic KOR-Cre mice. The chemogenetic inhibition of KOR neurons expressing hM4Di with a selective DREADD actuator (deschloroclozapine, DCZ) in sham control mice significantly decreased inhibitory transmission, resulting in a shift of inhibition/excitation balance to promote excitation and induced pain behaviors. The chemogenetic activation of KOR neurons expressing hM3Dq with DCZ in neuropathic mice significantly increased inhibitory transmission, decreased excitability, and decreased neuropathic pain behaviors. These data suggest that amygdala KOR neurons modulate pain behaviors by exerting an inhibitory tone on downstream CeA neurons. Therefore, activation of these interneurons or blockade of inhibitory KOR signaling in these neurons could restore control of amygdala output and mitigate pain.

Altered volume of the amygdala subregions in patients with chronic low back pain

Background

Neuroimaging studies have suggested a pivotal role for the amygdala involvement in chronic low back pain (CLBP). However, the relationship between the amygdala subregions and CLBP has not yet been delineated. This study aimed to analyze whether the amygdala subregions were linked to the development of CLBP.

Methods

A total of 45 patients with CLBP and 45 healthy controls (HCs) were included in this study. All subjects were asked to complete a three-dimensional T1-weighted magnetic resonance imaging (3D-T1 MRI) scan. FreeSurfer 7.3.2 was applied to preprocess the structural MRI images and segment the amygdala into nine subregions. Afterwards, comparisons were made between the two groups in terms of the volumes of the amygdala subregions. Correlation analysis is utilized to examine the relationship between the amygdala subregion and the scale scores, as well as the pain duration in patients with CLBP. Additionally, logistic regression was used to explore the risk of the amygdala and its subregions for CLBP.

Results

In comparison to HCs, patients with CLBP exhibited a significant enlargement of the left central nucleus (Ce) and left cortical nucleus (Co). Furthermore, the increased volume of the left Ce was associated with a higher risk of CLBP.

Conclusion

Our study suggests that the left Ce and left Co may be involved in the pathophysiological processes of CLBP. Moreover, the volume of the left Ce may be a biomarker for detecting the risk of CLBP.

Impaired amygdala astrocytic signaling worsens neuropathic pain-associated neuronal functions and behaviors

Introduction: Pain is a clinically relevant health care issue with limited therapeutic options, creating the need for new and improved analgesic strategies. The amygdala is a limbic brain region critically involved in the regulation of emotional-affective components of pain and in pain modulation. The central nucleus of amygdala (CeA) serves major output functions and receives nociceptive information via the external lateral parabrachial nucleus (PB). While amygdala neuroplasticity has been linked causally to pain behaviors, non-neuronal pain mechanisms in this region remain to be explored. As an essential part of the neuroimmune system, astrocytes that represent about 40-50% of glia cells within the central nervous system, are required for physiological neuronal functions, but their role in the amygdala remains to be determined for pain conditions. In this study, we measured time-specific astrocyte activation in the CeA in a neuropathic pain model (spinal nerve ligation, SNL) and assessed the effects of astrocyte inhibition on amygdala neuroplasticity and pain-like behaviors in the pain condition. Methods and Results: Glial fibrillary acidic protein (GFAP, astrocytic marker) immunoreactivity and mRNA expression were increased at the chronic (4 weeks post-SNL), but not acute (1 week post-SNL), stage of neuropathic pain. In order to determine the contribution of astrocytes to amygdala pain-mechanisms, we used fluorocitric acid (FCA), a selective inhibitor of astrocyte metabolism. Whole-cell patch-clamp recordings were performed from neurons in the laterocapsular division of the CeA (CeLC) obtained from chronic neuropathic rats. Pre-incubation of brain slices with FCA (100 µM, 1 h), increased excitability through altered hyperpolarization-activated current (Ih) functions, without significantly affecting synaptic responses at the PB-CeLC synapse. Intra-CeA injection of FCA (100 µM) had facilitatory effects on mechanical withdrawal thresholds (von Frey and paw pressure tests) and emotional-affective behaviors (evoked vocalizations), but not on facial grimace score and anxiety-like behaviors (open field test), in chronic neuropathic rats. Selective inhibition of astrocytes by FCA was confirmed with immunohistochemical analyses showing decreased astrocytic GFAP, but not NeuN, signal in the CeA. Discussion: Overall, these results suggest a complex modulation of amygdala pain functions by astrocytes and provide evidence for beneficial functions of astrocytes in CeA in chronic neuropathic pain.

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.

Mapping Brain Structure Variability in Chronic Pain: The Role of Widespreadness and Pain Type and Its Mediating Relationship With Suicide Attempt

BACKGROUND

Chronic pain affects nearly 20% of the U.S.

POPULATION

It is a leading cause of disability globally and is associated with a heightened risk for suicide. The role of the central nervous system in the perception and maintenance of chronic pain has recently been accepted, but specific brain circuitries involved have yet to be mapped across pain types in a large-scale study.

METHODS

We used data from the UK Biobank (N = 21,968) to investigate brain structural alterations in individuals reporting chronic pain compared with pain-free control participants and their mediating effect on history of suicide attempt.

RESULTS

Chronic pain and, more notably, chronic multisite pain was associated with, on average, lower surface area throughout the cortex after adjusting for demographic, clinical, and neuropsychiatric confounds. Only participants with abdominal pain showed lower subcortical volumes, including the amygdala and brainstem, and lower cerebellum volumes. Participants with chronic headaches showed a widespread thicker cortex compared with control participants. Mediation analyses revealed that precuneus thickness mediated the relationship of chronic multisite pain and history of suicide attempt. Mediating effects were also identified specific to localized pain, with the strongest effect being amygdala volume in individuals with chronic abdominal pain.

CONCLUSIONS

Results support a widespread effect of chronic pain on brain structure and distinct brain structures underlying chronic musculoskeletal pain, visceral pain, and headaches. Mediation effects of regions in the extended ventromedial prefrontal cortex subsystem suggest that exacerbated negative internal states, negative self-referencing, and impairments in future planning may underlie suicidal behaviors in individuals with chronic pain.

Chemogenetic Modulation of Posterior Insula CaMKIIa Neurons Alters Pain and Thermoregulation

The posterior insular cortex (PIC) is well positioned to perform somatosensory-limbic integration; yet, the function of neuronal subsets within the PIC in processing the sensory and affective dimensions of pain remains unclear. Here, we employ bidirectional chemogenetic modulation to characterize the function of PIC CaMKIIa-expressing excitatory neurons in a comprehensive array of sensory, affective, and thermoregulatory behaviors. Excitatory pyramidal neurons in the PIC were found to be sensitized under inflammatory pain conditions. Chemogenetic activation of excitatory CaMKIIa-expressing PIC neurons in non-injured conditions produced an increase in reflexive and affective pain- and anxiety-like behaviors. Moreover, activation of PIC CaMKIIa-expressing neurons during inflammatory pain conditions exacerbated hyperalgesia and decreased pain tolerance. However, Chemogenetic activation did not alter heat nociception via hot plate latency or body temperature. Conversely, inhibiting CaMKIIa-expressing neurons did not alter either sensory or affective pain-like behaviors in non-injured or under inflammatory pain conditions, but it did decrease body temperature and decreased hot plate latency. Our findings reveal that PIC CaMKIIa-expressing neurons are a critical hub for producing both sensory and affective pain-like behaviors and important for thermoregulatory processing. PERSPECTIVE: The present study reveals that activation of the posterior insula produces hyperalgesia and negative affect, and has a role in thermal tolerance and thermoregulation. These findings highlight the insula as a key player in contributing to the multidimensionality of pain.

The parabrachial to central amygdala pathway is critical to injury-induced pain sensitization in mice

The spino-ponto-amygdaloid pathway is a major ascending circuit relaying nociceptive information from the spinal cord to the brain. Potentiation of excitatory synaptic transmission in the parabrachial nucleus (PBN) to central amygdala (CeA) pathway has been reported in rodent models of persistent pain. However, the functional significance of this pathway in the modulation of the somatosensory component of pain was recently challenged by studies showing that spinal nociceptive neurons do not target CeA-projecting PBN cells and that manipulations of this pathway have no effect on reflexive-defensive somatosensory responses to peripheral noxious stimulation. Here, we showed that activation of CeA-projecting PBN neurons is critical to increase both stimulus-evoked and spontaneous nociceptive responses following an injury in male and female mice. Using optogenetic-assisted circuit mapping, we confirmed a functional excitatory projection from PBN→CeA that is independent of the genetic or firing identity of CeA cells. We then showed that peripheral noxious stimulation increased the expression of the neuronal activity marker Fos in CeA-projecting PBN neurons and that chemogenetic inactivation of these cells decreased behavioral hypersensitivity in models of neuropathic and inflammatory pain without affecting baseline nociception. Lastly, we showed that chemogenetic activation of CeA-projecting PBN neurons is sufficient to induced bilateral hypersensitivity without injury. Together, our results indicate that the PBN→CeA pathway is a key modulator of pain-related behaviors that can increase reflexive-defensive and affective-motivational responses to somatosensory stimulation in injured states without affecting nociception under normal physiological conditions.

Opioid Receptor Mu 1 Gene (OPRM1) A118G Polymorphism and Emotional Modulation of Pain

Purpose

The A118G polymorphism in the opioid receptor mu 1 gene (OPRM1) is associated with decreased opioid receptor availability, altered emotion, and increased pain. Given that emotions modulate pain (positive emotions inhibit pain, negative emotions enhance pain), we predicted that G allele carriers would experience impaired emotional modulation of pain compared to non-G allele carriers.

Patients and Methods

Emotional pictures (ie, erotica, neutral, attack) from the International Affective Picture System were used by permission from the authors to experimentally manipulate emotions in 64 adult participants while painful electrocutaneous stimulations were delivered in a cross-sectional study. Ratings of arousal and valence/pleasure were made in response to pictures, and pain ratings and a physiological measure of spinal nociception (ie, nociceptive flexion reflex, NFR) were collected in response to painful stimulations. Secondary analyses were conducted to examine the relationship between the A118G polymorphism and emotional modulation of pain/NFR.

Results

Exposure to emotional pictures elicited similar changes in valence, but G-carriers rated erotic pictures as more arousing. In non-carriers, pain was facilitated by attack pictures and pain and NFR were inhibited by erotic pictures relative to neutral pictures. Among G-carriers, pain was facilitated by negative emotional pictures but there was no pain or NFR inhibition by positive emotional pictures.

Conclusion

The altered response to pleasant stimuli further supports the role of opioids in appetitive behavior and describes how the A118G polymorphism may prevent carriers from inhibiting pain during pleasure.

Leptin excites basolateral amygdala principal neurons and reduces food intake by LepRb-JAK2-PI3K-dependent depression of GIRK channels

Leptin is an adipocyte-derived hormone that modulates food intake, energy balance, neuroendocrine status, thermogenesis, and cognition. Whereas a high density of leptin receptors has been detected in the basolateral amygdala (BLA) neurons, the physiological functions of leptin in the BLA have not been determined yet. We found that application of leptin excited BLA principal neurons by activation of the long form leptin receptor, LepRb. The LepRb-elicited excitation of BLA neurons was mediated by depression of the G protein-activated inwardly rectifying potassium (GIRK) channels. Janus Kinase 2 (JAK2) and phosphoinositide 3-kinase (PI3K) were required for leptin-induced excitation of BLA neurons and depression of GIRK channels. Microinjection of leptin into the BLA reduced food intake via activation of LepRb, JAK2, and PI3K. Our results may provide a cellular and molecular mechanism to explain the physiological roles of leptin in vivo.

Neural adaptation of the reward system in primary dysmenorrhea

Introduction: The brain's reward system (RS) reacts differently to pain and its alleviation. This study examined the correlation between RS activity and behavior during both painful and pain-free periods in individuals with primary dysmenorrhea (PDM) to elucidate their varying responses throughout the menstrual cycle. Methods: Ninety-two individuals with PDM and 90 control participants underwent resting-state functional magnetic resonance imaging (rsfMRI) scans during their menstrual and peri-ovulatory phases. Regional homogeneity (ReHo) and amplitude of low-frequency fluctuation (ALFF) analyses were used to evaluate RS responses. Psychological evaluations were conducted using the McGill Pain Questionnaire and the Pain Catastrophizing Scale. Results: ReHo analysis showed higher values in the left putamen and right amygdala of the PDM group during the peri-ovulatory phase compared to the menstrual phase. ALFF analysis revealed lower values in the putamen of the PDM group compared to controls, regardless of phase. ReHo and ALFF values in the putamen, amygdala, and nucleus accumbens were positively correlated with pain scales during menstruation, while ALFF values in the ventral tegmental area inversely correlated with pain intensity. Those with severe PDM (pain intensity ≥7) displayed distinct amygdala ALFF patterns between pain and pain-free phases. PDM participants also had lower ReHo values in the left insula during menstruation, with no direct correlation to pain compared to controls. Discussion: Our study highlights the pivotal role of the RS in dysmenorrhea management, exhibiting varied responses between menstrual discomfort and non-painful periods among individuals with PDM. During menstruation, the RS triggers mechanisms for pain avoidance and cognitive coping strategies, while it transitions to processing rewards during the peri-ovulatory phase. This demonstrates the flexibility of the RS in adapting to the recurring pain experienced by those with PDM.

Projections from infralimbic medial prefrontal cortex glutamatergic outputs to amygdala mediates opioid induced hyperalgesia in male rats

Repeated use of opioid analgesics may cause a paradoxically exacerbated pain known as opioid-induced hyperalgesia (OIH), which hinders effective clinical intervention for severe pain. Currently, little is known about the neural circuits underlying OIH modulation. Previous studies suggest that laterocapsular division of the central nucleus of amygdala (CeLC) is critically involved in the regulation of OIH. Our purpose is to clarify the role of the projections from infralimbic medial prefrontal cortex (IL) to CeLC in OIH. We first produced an OIH model by repeated fentanyl subcutaneous injection in male rats. Immunofluorescence staining revealed that c-Fos-positive neurons were significantly increased in the right CeLC in OIH rats than the saline controls. Then, we used calcium/calmodulin-dependent protein kinase IIα (CaMKIIα) labeling and the patch-clamp recordings with ex vivo optogenetics to detect the functional projections from glutamate pyramidal neurons in IL to the CeLC. The synaptic transmission from IL to CeLC, shown in the excitatory postsynaptic currents (eEPSCs), inhibitory postsynaptic currents (eIPSCs) and paired-pulse ratio (PPR), was observably enhanced after fentanyl administration. Moreover, optogenetic activation of this IL-CeLC pathway decreased c-Fos expression in CeLC and ameliorated mechanical and thermal pain in OIH. On the contrary, silencing this pathway by chemogenetics exacerbated OIH by activating the CeLC. Combined with the electrophysiology results, the enhanced synaptic transmission from IL to CeLC might be a cortical gain of IL to relieve OIH rather than a reason for OIH generation. Scaling up IL outputs to CeLC may be an effective neuromodulation strategy to treat OIH.

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.

Altered amygdala volumes and microstructure in focal epilepsy patients with tonic-clonic seizures, ictal, and post-convulsive central apnea

OBJECTIVES

Sudden unexpected death in epilepsy (SUDEP) is a leading cause of death for patients with epilepsy; however, the pathophysiology remains unclear. Focal-to-bilateral tonic-clonic seizures (FBTCS) are a major risk factor, and centrally-mediated respiratory depression may increase the risk further. Here, we determined the volume and microstructure of the amygdala, a key structure that can trigger apnea in people with focal epilepsy, stratified by the presence or absence of FBTCS, ictal central apnea (ICA), and post-convulsive central apnea (PCCA).

METHODS

Seventy-three patients with focal impaired awareness seizures without FBTC seizures (FBTCneg group) and 30 with FBTCS (FBTCpos group) recorded during video electroencephalography (VEEG) with respiratory monitoring were recruited prospectively during presurgical investigations. We acquired high-resolution T1-weighted anatomic and multi-shell diffusion images, and computed neurite orientation dispersion and density imaging (NODDI) metrics in all patients with epilepsy and 69 healthy controls. Amygdala volumetric and microstructure alterations were compared between three groups: healthy subjects, FBTCneg and FBTCpos groups. The FBTCpos group was further subdivided by the presence of ICA and PCCA, verified by VEEG.

RESULTS

Bilateral amygdala volumes were significantly increased in the FBTCpos cohort compared to healthy controls and the FBTCneg group. Patients with recorded PCCA had the highest increase in bilateral amygdala volume of the FBTCpos cohort. Amygdala neurite density index (NDI) values were decreased significantly in both the FBTCneg and FBTCpos groups relative to healthy controls, with values in the FBTCpos group being the lowest of the two. The presence of PCCA was associated with significantly lower NDI values vs the non-apnea FBTCpos group (p = 0.004).

SIGNIFICANCE

Individuals with FBTCpos and PCCA show significantly increased amygdala volumes and disrupted architecture bilaterally, with greater changes on the left side. The structural alterations reflected by NODDI and volume differences may be associated with inappropriate cardiorespiratory patterns mediated by the amygdala, particularly after FBTCS. Determination of amygdala volumetric and architectural changes may assist identification of individuals at risk.

Infant pain vs. pain with parental suppression: Immediate and enduring impact on brain, pain and affect

BACKGROUND

In the short term, parental presence while a human infant is in pain buffers the immediate pain responses, although emerging evidence suggests repeated social buffering of pain may have untoward long-term effects.

METHODS/FINDING

To explore the short- and long-term impacts of social buffering of pain, we first measured the infant rat pup's [postnatal day (PN) 8, or 12] response to mild tail shock with the mother present compared to shock alone or no shock. Shock with the mother reduced pain-related behavioral activation and USVs of pups at both ages and reduced Fos expression in the periaqueductal gray, hypothalamic paraventricular nucleus, and the amygdala at PN12 only. At PN12, shock with the mother compared to shock alone differentially regulated expression of several hundred genes related to G-protein-coupled receptors (GPCRs) and neural development, whereas PN8 pups showed a less robust and less coherent expression pattern. In a second set of experiments, pups were exposed to daily repeated Shock-mother pairings (or controls) at PN5-9 or PN10-14 (during and after pain sensitive period, respectively) and long-term outcome assessed in adults. Shock+mother pairing at PN5-9 reduced adult carrageenan-induced thermal hyperalgesia and reduced Fos expression, but PN10-14 pairings had minimal impact. The effect of infant treatment on adult affective behavior showed a complex treatment by age dependent effect. Adult social behavior was decreased following Shock+mother pairings at both PN5-9 and PN10-14, whereas shock alone had no effect. Adult fear responses to a predator odor were decreased only by PN10-14 treatment and the infant Shock alone and Shock+mother did not differ.

CONCLUSIONS/SIGNIFICANCE

Overall, integrating these results into our understanding of long-term programming by repeated infant pain experiences, the data suggest that pain experienced within a social context impacts infant neurobehavioral responses and initiates an altered developmental trajectory of pain and affect processing that diverges from experiencing pain alone.

Modulation of visceral pain by brain nuclei and brain circuits and the role of acupuncture: a narrative review

Visceral pain is a complex and heterogeneous pain condition that is often associated with pain-related negative emotional states, including anxiety and depression, and can exert serious effects on a patient's physical and mental health. According to modeling stimulation protocols, the current animal models of visceral pain mainly include the mechanical dilatation model, the ischemic model, and the inflammatory model. Acupuncture can exert analgesic effects by integrating and interacting input signals from acupuncture points and the sites of pain in the central nervous system. The brain nuclei involved in regulating visceral pain mainly include the nucleus of the solitary tract, parabrachial nucleus (PBN), locus coeruleus (LC), rostral ventromedial medulla (RVM), anterior cingulate cortex (ACC), paraventricular nucleus (PVN), and the amygdala. The neural circuits involved are PBN-amygdala, LC-RVM, amygdala-insula, ACC-amygdala, claustrum-ACC, bed nucleus of the stria terminalis-PVN and the PVN-ventral lateral septum circuit. Signals generated by acupuncture can modulate the central structures and interconnected neural circuits of multiple brain regions, including the medulla oblongata, cerebral cortex, thalamus, and hypothalamus. This analgesic process also involves the participation of various neurotransmitters and/or receptors, such as 5-hydroxytryptamine, glutamate, and enkephalin. In addition, acupuncture can regulate visceral pain by influencing functional connections between different brain regions and regulating glucose metabolism. However, there are still some limitations in the research efforts focusing on the specific brain mechanisms associated with the effects of acupuncture on the alleviation of visceral pain. Further animal experiments and clinical studies are now needed to improve our understanding of this area.

Psychological characteristics associated with the brain volume of patients with fibromyalgia

Fibromyalgia (FM) is a disease characterized by chronic widespread pain concomitant with psychiatric symptoms such as anxiety and depression. It has been reported that FM patients engage in pain catastrophizing. In this study, we investigated characteristics of the brain volume of female FM patients and the association between psychological indices and brain volume. Thirty-nine female FM patients and 25 female healthy controls (HCs) were recruited for the study, and five FM patients were excluded due to white matter lesions. The following analyses were performed: (1) T1-weighted MRI were acquired for 34 FM patients (age 41.6 ± 7.4) and 25 HCs (age 39.5 ± 7.4). SPM12 was used to compare their gray and white matter volumes. (2) Data from anxiety and depression questionnaires (State-Trait Anxiety Inventory and Hospital Anxiety and Depression Scale), the Pain Catastrophizing Scale (subscales rumination, helplessness, magnification), and MRI were acquired for 34 FM patients (age 41.6 ± 7.4). Correlation analysis was done of the psychological indices and brain volume. We found that (1) The white matter volume of the temporal pole was larger in the FM patient group than in the HC group. (2) Correlation analysis of the psychological indices and gray matter volume showed a negative correlation between trait anxiety and the amygdala. For the white matter volume, positive correlations were found between depression and the brainstem and between magnification and the postcentral gyrus. Changes in the brain volume of female FM patients may be related to anxiety, depression, and pain catastrophizing.

Stimulation of lateral parabrachial (LPB) to central amygdala (CeA) pituitary adenylate cyclase-activating polypeptide (PACAP) neurons induces anxiety-like behavior and mechanical allodynia

BACKGROUND

Anxiety disorders are the most prevalent psychiatric disorders, and they are highly comorbid with chronic pain conditions. The central nucleus of the amygdala (CeA) is known not only for its role in the regulation of anxiety but also as an important site for the negative affective dimension of pain. Pituitary adenylate cyclase activating polypeptide (PACAP), a neuropeptide whose terminals are abundant in the CeA, is strongly implicated in the stress response as well as in pain processing. Here, using Cre-dependent viral vectors, we explored in greater detail the role of the PACAP projection to the CeA that originates in the lateral parabrachial nucleus (LPB).

METHODS

We first performed a circuit mapping experiment by injecting an anterograde Cre-dependent virus expressing a fluorescent reporter in the LPB of PACAP-Cre mice and observing their projections. Then, we used a chemogenetic approach (a Cre-dependent Designer Receptors Activated by Designer Drugs, DREADDs) to assess the effects of the direct stimulation of the PACAP LPB to CeA projection on general locomotor activity, anxiety-like behavior (using a defensive withdrawal test), and mechanical pain sensitivity (using the von Frey test).

RESULTS

We found that the CeA, together with other areas, is one of the major downstream projection targets of PACAP neurons originating in the lateral parabrachial nucleus (LPB). In the DREADD experiment, we then found that the selective activation of this neuronal pathway is sufficient to increase both anxiety-like behavior and mechanical pain sensitivity in mice, without affecting general locomotor activity.

CONCLUSION

In conclusion, our data suggest that the dysregulation of this circuit may contribute to a variety of anxiety disorders and chronic pain states, and that PACAP may represent an important therapeutic target for the treatment of these conditions.

Exercise therapy for chronic pain: How does exercise change the limbic brain function?

We are exposed to various external and internal threats which might hurt us. The role of taking flexible and appropriate actions against threats is played by "the limbic system" and at the heart of it there is the ventral tegmental area and nucleus accumbens (brain reward system). Pain-related fear causes excessive excitation of amygdala, which in turn causes the suppression of medial prefrontal cortex, leading to chronification of pain. Since the limbic system of chronic pain patients is functionally impaired, they are maladaptive to their situations, unable to take goal-directed behavior and are easily caught by fear-avoidance thinking. We describe the neural mechanisms how exercise activates the brain reward system and enables chronic pain patients to take goal-directed behavior and overcome fear-avoidance thinking. A key to getting out from chronic pain state is to take advantage of the behavioral switching function of the basal nucleus of amygdala. We show that exercise activates positive neurons in this nucleus which project to the nucleus accumbens and promote reward behavior. We also describe fear conditioning and extinction are affected by exercise. In chronic pain patients, the fear response to pain is enhanced and the extinction of fear memories is impaired, so it is difficult to get out of "fear-avoidance thinking". Prolonged avoidance of movement and physical inactivity exacerbate pain and have detrimental effects on the musculoskeletal and cardiovascular systems. Based on the recent findings on multiple bran networks, we propose a well-balanced exercise prescription considering the adherence and pacing of exercise practice. We conclude that therapies targeting the mesocortico-limbic system, such as exercise therapy and cognitive behavioral therapy, may become promising tools in the fight against chronic pain.

Hmgb1 Silencing in the Amygdala Inhibits Pain-Related Behaviors in a Rat Model of Neuropathic Pain

Chronic pain presents a therapeutic challenge due to the highly complex interplay of sensory, emotional-affective and cognitive factors. The mechanisms of the transition from acute to chronic pain are not well understood. We hypothesized that neuroimmune mechanisms in the amygdala, a brain region involved in the emotional-affective component of pain and pain modulation, play an important role through high motility group box 1 (Hmgb1), a pro-inflammatory molecule that has been linked to neuroimmune signaling in spinal nociception. Transcriptomic analysis revealed an upregulation of Hmgb1 mRNA in the right but not left central nucleus of the amygdala (CeA) at the chronic stage of a spinal nerve ligation (SNL) rat model of neuropathic pain. Hmgb1 silencing with a stereotaxic injection of siRNA for Hmgb1 into the right CeA of adult male and female rats 1 week after (post-treatment), but not 2 weeks before (pre-treatment) SNL induction decreased mechanical hypersensitivity and emotional-affective responses, but not anxiety-like behaviors, measured 4 weeks after SNL. Immunohistochemical data suggest that neurons are a major source of Hmgb1 in the CeA. Therefore, Hmgb1 in the amygdala may contribute to the transition from acute to chronic neuropathic pain, and the inhibition of Hmgb1 at a subacute time point can mitigate neuropathic pain.

Intrinsic brain connectivity alterations despite intact pain inhibition in subjects with neuropathic pain after spinal cord injury: a pilot study

Endogenous pain modulation in humans is frequently investigated with conditioned pain modulation (CPM). Deficient pain inhibition is a proposed mechanism that contributes to neuropathic pain (NP) after spinal cord injury (SCI). Recent studies have combined CPM testing and neuroimaging to reveal neural correlates of CPM efficiency in chronic pain. This study investigated differences in CPM efficiency in relation to resting-state functional connectivity (rsFC) between 12 SCI-NP subjects and 13 age- and sex-matched healthy controls (HC). Twelve and 11 SCI-NP subjects were included in psychophysical and rsFC analyses, respectively. All HC were included in the final analyses. Psychophysical readouts were analysed to determine CPM efficiency within and between cohorts. Group differences of rsFC, in relation to CPM efficiency, were explored with seed-to-voxel rsFC analyses with pain modulatory regions, e.g. ventrolateral periaqueductal gray (vlPAG) and amygdala. Overall, pain inhibition was not deficient in SCI-NP subjects and was greater in those with more intense NP. Greater pain inhibition was associated with weaker rsFC between the vlPAG and amygdala with the visual and frontal cortex, respectively, in SCI-NP subjects but with stronger rsFC in HC. Taken together, SCI-NP subjects present with intact pain inhibition, but can be differentiated from HC by an inverse relationship between CPM efficiency and intrinsic connectivity of supraspinal regions. Future studies with larger cohorts are necessary to consolidate the findings in this study.

Differential Structural Brain Changes Between Responders and Nonresponders After Physical Exercise Therapy for Chronic Nonspecific Neck Pain

OBJECTIVES

Physical exercise therapy is effective for some people with chronic nonspecific neck pain but not for others. Differences in exercise-induced pain-modulatory responses are likely driven by brain changes. We investigated structural brain differences at baseline and changes after an exercise intervention. The primary aim was to investigate changes in structural brain characteristics after physical exercise therapy for people with chronic nonspecific neck pain. The secondary aims were to investigate (1) baseline differences in structural brain characteristics between responders and nonresponders to exercise therapy, and (2) differential brain changes after exercise therapy between responders and nonresponders.

MATERIALS AND METHODS

This was a prospective longitudinal cohort study. Twenty-four participants (18 females, mean age 39.7 y) with chronic nonspecific neck pain were included. Responders were selected as those with ≥20% improvement in Neck Disability Index. Structural magnetic resonance imaging was obtained before and after an 8-week physical exercise intervention delivered by a physiotherapist. Freesurfer cluster-wise analyses were performed and supplemented with an analysis of pain-specific brain regions of interest.

RESULTS

Various changes in grey matter volume and thickness were found after the intervention, for example, frontal cortex volume decreased (cluster-weighted P value = 0.0002, 95% CI: 0.0000-0.0004). We found numerous differences between responders and nonresponders, most notably, after the exercise intervention bilateral insular volume decreased in responders, but increased in nonresponders (cluster-weighted P value ≤ 0.0002).

DISCUSSION

The brain changes found in this study may underpin clinically observed differential effects between responders and nonresponders to exercise therapy for people with chronic neck pain. Identification of these changes is an important step toward personalized treatment approaches.

Neuromedin B excites central lateral amygdala neurons and reduces cardiovascular output and fear-potentiated startle

Neuromedin B (NMB) and gastrin-releasing peptide (GRP) are the two mammalian analogs in the bombesin peptide family that exert a variety of actions including emotional processing, appetitive behaviors, cognition, and tumor growth. The bombesin-like peptides interact with three receptors: the NMB-preferring bombesin 1 (BB1) receptors, the GRP-preferring bombesin 2 (BB2) receptors and the orphan bombesin 3 (BB3) receptors. Whereas, injection of bombesin into the central amygdala reduces satiety and modulates blood pressure, the underlying cellular and molecular mechanisms have not been determined. As administration of bombesin induces the expression of Fos in the lateral nucleus of the central amygdala (CeL) which expresses BB1 receptors, we probed the effects of NMB on CeL neurons using in vitro and in vivo approaches. We showed that activation of the BB1 receptors increased action potential firing frequency recorded from CeL neurons via inhibition of the inwardly rectifying K+ (Kir) channels. Activities of phospholipase Cβ and protein kinase C were required, whereas intracellular Ca2+ release was unnecessary for BB1 receptor-elicited potentiation of neuronal excitability. Application of NMB directly into the CeA reduced blood pressure and heart rate and significantly reduced fear-potentiated startle. We may provide a cellular and molecular mechanism whereby bombesin-like peptides modulate anxiety and fear responses in the amygdala.

Identification of brain-to-spinal circuits controlling the laterality and duration of mechanical allodynia in mice

Mechanical allodynia (MA) represents one prevalent symptom of chronic pain. Previously we and others have identified spinal and brain circuits that transmit or modulate the initial establishment of MA. However, brain-derived descending pathways that control the laterality and duration of MA are still poorly understood. Here we report that the contralateral brain-to-spinal circuits, from Oprm1 neurons in the lateral parabrachial nucleus (lPBNOprm1), via Pdyn neurons in the dorsal medial regions of hypothalamus (dmHPdyn), to the spinal dorsal horn (SDH), act to prevent nerve injury from inducing contralateral MA and reduce the duration of bilateral MA induced by capsaicin. Ablating/silencing dmH-projecting lPBNOprm1 neurons or SDH-projecting dmHPdyn neurons, deleting Dyn peptide from dmH, or blocking spinal κ-opioid receptors all led to long-lasting bilateral MA. Conversely, activation of dmHPdyn neurons or their axonal terminals in SDH can suppress sustained bilateral MA induced by lPBN lesion.

The basolateral amygdala-anterior cingulate pathway contributes to depression-like behaviors and comorbidity with chronic pain behaviors in male mice

While depression and chronic pain are frequently comorbid, underlying neuronal circuits and their psychopathological relevance remain poorly defined. Here we show in mice that hyperactivity of the neuronal pathway linking the basolateral amygdala to the anterior cingulate cortex is essential for chronic pain-induced depression. Moreover, activation of this pathway in naive male mice, in the absence of on-going pain, is sufficient to trigger depressive-like behaviors, as well as transcriptomic alterations that recapitulate core molecular features of depression in the human brain. These alterations notably impact gene modules related to myelination and the oligodendrocyte lineage. Among these, we show that Sema4a, which was significantly upregulated in both male mice and humans in the context of altered mood, is necessary for the emergence of emotional dysfunction. Overall, these results place the amygdalo-cingulate pathway at the core of pain and depression comorbidity, and unravel the role of Sema4a and impaired myelination in mood control.

Sex-specific effects of psychedelic drug exposure on central amygdala reactivity and behavioral responding

Psilocybin and its active metabolite psilocin have been shown to elicit rapid and long-lasting symptom improvements in a variety of affective psychiatric illnesses. However, the region-specific alterations underlying these therapeutic effects remain relatively unknown. The central amygdala (CeA) is a primary output region within the extended amygdala that is dysregulated in affective psychiatric disorders. Here, we measured CeA activity using the activity marker c-Fos and CeA reactivity using fiber photometry paired with an aversive air-puff stimulus. We found that psilocin administration acutely increased CeA activity in both males and females and increased stimulus specific CeA reactivity in females, but not males. In contrast, psilocin produced time-dependent decreases in reactivity in males, but not in females, as early as 2 days and lasting to 28 days post administration. We also measured behavioral responses to the air-puff stimulus and found sex-dependent changes in threat responding but not exploratory behavior or general locomotion. Repeated presentations of the auditory component of the air-puff were also performed and sex-specific effects of psilocin on CeA reactivity to the auditory-alone stimulus were also observed. This study provides new evidence that a single dose of psilocin produces sex-specific, time-dependent, and enduring changes in CeA reactivity and behavioral responding to specific components of an aversive stimulus.

Emerging role of microglia and astrocyte in the affective-motivational response induced by a rat model of persistent orofacial pain

Few studies are approaching the neural basis underlying the aggregation of emotional disorders in orofacial pain despite the stress, depression, and anxiety are some of the most commonly reported risk factors. Using a persistent orofacial pain rat model induced by complete Freund's adjuvant (CFA) injection into the temporomandibular joint, we have investigated the plasticity astrocytes and microglia key brain regions for the affective-emotional component of pain. We measured the expression and morphologic pattern of reactivation of glial fibrillary acidic protein (GFAP, astrocyte marker) and Iba-1 (microglial marker) by western blotting and immunohistochemistry analysis. The results showed no alterations on motor activity during inflammatory pain, indicating an exclusive effect of nociceptive behavior on the plasticity of limbic regions. CFA-induced temporomandibular inflammation changed GFAP and Iba-1 expression in distinct regions related to emotional behavior in a time-dependent manner. A significant increase in GFAP and Iba-1 expression was observed in the central nucleus of the amygdala, hippocampus and periaqueductal grey matter from day 3 to day 10 post-CFA injection. Moreover, a positive correlation between GFAP and Iba-1 upregulation and an increased mechanical hypersensitivity was observed. Conversely, no change on GFAP and Iba-1 expression was observed in the hypothalamus and colliculus during orofacial inflammatory pain. Our data suggest an important role for glial cells in the affective-motivational dimension of orofacial pain beyond their well-explored role in the traditional nociceptive transmission circuits.

Amygdala and anterior insula control the passage from nociception to pain

Activation of the spinothalamic system does not always result in a subjective pain perception. While the cerebral network processing nociception is relatively well known, the one underlying its transition to conscious pain remains poorly described. We used intracranial electroencephalography in epileptic patients to investigate whether the amplitudes and functional connectivity of posterior and anterior insulae (PI and AI) and amygdala differ according to the subjective reports to laser stimuli delivered at a constant intensity set at nociceptive threshold. Despite the constant intensity of stimuli, all patients reported variable subjective perceptions from one stimulus to the other. Responses in the sensory PI remained stable throughout the experiment, hence reflecting accurately the stability of the stimulus. In contrast, both AI and amygdala responses showed significant enhancements associated with painful relative to nonpainful reports, in a time window corresponding to the conscious integration of the stimulus. Functional connectivity in the gamma band between these two regions increased significantly, both before and after stimuli perceived as painful. While the PI appears to transmit faithfully the actual stimulus intensity received via the spinothalamic tract, the AI and the amygdala appear to play a major role in the transformation of nociceptive signals into a painful perception.

Altered Amygdala Volumes and Microstructure in Focal Epilepsy Patients with Tonic-Clonic Seizures, Ictal and Post-Ictal Central Apnea

Objectives

Sudden unexpected death in epilepsy (SUDEP) is a leading cause of death for patients with epilepsy; however, the pathophysiology remains unclear. Focal-to-bilateral tonic-clonic seizures (FBTCS) are a major risk factor, and centrally-mediated respiratory depression may increase the risk further. Here, we determined volume and microstructure of the amygdala, a key structure that can trigger apnea in people with focal epilepsy, stratified by presence or absence of FBTCS, ictal central apnea (ICA) and post-ictal central apnea (PICA).

Methods

73 patients with only-focal seizures and 30 with FBTCS recorded during video EEG (VEEG) with respiratory monitoring were recruited prospectively during presurgical investigations. We acquired high-resolution T1-weighted anatomical and multi-shell diffusion images, and computed neurite orientation dispersion and density imaging (NODDI) metrics in all epilepsy patients and 69 healthy controls. Amygdala volumetric and microstructure alterations were compared between healthy subjects, and patients with only-focal seizures or FBTCS The FBTCS group was further subdivided by presence of ICA and PICA, verified by VEEG.

Results

Bilateral amygdala volumes were significantly increased in the FBTCS cohort compared to healthy controls and the focal cohort. Patients with recorded PICA had the highest increase in bilateral amygdala volume of the FBTCS cohort.Amygdala neurite density index (NDI) values were significantly decreased in both the focal and FBTCS groups relative to healthy controls, with values in the FBTCS group being the lowest of the two. The presence of PICA was associated with significantly lower NDI values vs the non-apnea FBTCS group (p=0.004).

Significance

Individuals with FBTCS and PICA show significantly increased amygdala volumes and disrupted architecture bilaterally, with greater changes on the left side. The structural alterations reflected by NODDI and volume differences may be associated with inappropriate cardiorespiratory patterns mediated by the amygdala, particularly after FBTCS. Determination of amygdala volumetric and architectural changes may assist identification of individuals at risk.

Epigenetic mechanisms underlying stress-induced visceral pain: Resilience versus vulnerability in a two-hit model of early life stress and chronic adult stress

BACKGROUND

Women with a history of early life stress (ELS) have a higher risk of developing irritable bowel syndrome (IBS). In addition, chronic stress in adulthood can exacerbate IBS symptoms such as abdominal pain due to visceral hypersensitivity. We previously showed that sex and the predictability of ELS determine whether rats develop visceral hypersensitivity in adulthood. In female rats, unpredictable ELS confers vulnerability and results in visceral hypersensitivity, whereas predictable ELS induces resilience and does not induce visceral hypersensitivity in adulthood. However, this resilience is lost after exposure to chronic stress in adulthood leading to an exacerbation of visceral hypersensitivity. Evidence suggests that changes in histone acetylation at the promoter regions of glucocorticoid receptor (GR) and corticotrophin-releasing factor (CRF) in the central nucleus of the amygdala (CeA) underlie stress-induced visceral hypersensitivity. Here, we aimed to investigate the role of histone acetylation in the CeA on visceral hypersensitivity in a two-hit model of ELS followed by chronic stress in adulthood.

METHODS

Male and female neonatal rats were exposed to unpredictable, predictable ELS, or odor only (no stress control) from postnatal days 8 to 12. In adulthood, rats underwent stereotaxic implantation of indwelling cannulas. Rats were exposed to chronic water avoidance stress (WAS, 1 h/day for 7 days) or SHAM stress and received infusions of vehicle, the histone deacetylase inhibitor trichostatin A (TSA) or the histone acetyltransferase inhibitor garcinol (GAR) after each WAS session. 24 h after the final infusion, visceral sensitivity was assessed and the CeA was removed for molecular experiments.

RESULTS

In the two-hit model (ELS + WAS), female rats previously exposed to predictable ELS, showed a significant reduction in histone 3 lysine 9 (H3K9) acetylation at the GR promoter and a significant increase in H3K9 acetylation at the CRF promoter. These epigenetic changes were associated with changes in GR and CRF mRNA expression in the CeA and an exacerbation of stress-induced visceral hypersensitivity in female animals. TSA infusions in the CeA attenuated the exacerbated stress-induced visceral hypersensitivity, whereas GAR infusions only partially ameliorated ELS+WAS induced visceral hypersensitivity.

CONCLUSION

The two-hit model of ELS followed by WAS in adulthood revealed that epigenetic dysregulation occurs after exposure to stress in two important periods of life and contributes to the development of visceral hypersensitivity. These aberrant underlying epigenetic changes may explain the exacerbation of stress-induced abdominal pain in IBS patients.

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.

The parabrachial to central amygdala circuit is a key mediator of injury-induced pain sensitization

The spino-ponto-amygdaloid pathway is a major ascending circuit relaying nociceptive information from the spinal cord to the brain. Potentiation of excitatory synaptic transmission in the parabrachial nucleus (PbN) to central amygdala (CeA) pathway has been reported in rodent models of persistent pain. At the behavioral level, the PbN→CeA pathway has been proposed to serve as a general alarm system to potential threats that modulates pain-related escape behaviors, threat memory, aversion, and affective-motivational (but not somatosensory) responses to painful stimuli. Increased sensitivity to previously innocuous somatosensory stimulation is a hallmark of chronic pain. Whether the PbN→CeA circuit contributes to heightened peripheral sensitivity following an injury, however, remains unknown. Here, we demonstrate that activation of CeA-projecting PbN neurons contributes to injury-induced behavioral hypersensitivity but not baseline nociception in male and female mice. Using optogenetic assisted circuit mapping, we confirmed a functional excitatory projection from PbN→CeA that is independent of the genetic or firing identity of CeA cells. We then showed that peripheral noxious stimulation increases the expression of the neuronal activity marker c-Fos in CeA-projecting PbN neurons and chemogenetic inactivation of these cells reduces behavioral hypersensitivity in models of neuropathic and inflammatory pain without affecting baseline nociception. Lastly, we show that chemogenetic activation of CeA-projecting PbN neurons is sufficient to induce bilateral hypersensitivity without injury. Together, our results demonstrate that the PbN→CeA pathway is a key modulator of pain-related behaviors that can amplify responses to somatosensory stimulation in pathological states without affecting nociception under normal physiological conditions.

Significance Statement

Early studies identified the spino-ponto-amygdaloid pathway as a major ascending circuit conveying nociceptive inputs from the spinal cord to the brain. The functional significance of this circuit to injury-induced hypersensitivity, however, remains unknown. Here, we addressed this gap in knowledge using viral-mediated anatomical tracers, ex-vivo electrophysiology and chemogenetic intersectional approaches in rodent models of persistent pain. We found that activation of this pathway contributes to injury-induced hypersensitivity, directly demonstrating a critical function of the PbN→CeA circuit in pain modulation.

Electrophysiology as a Tool to Decipher the Network Mechanism of Visceral Pain in Functional Gastrointestinal Disorders

Abdominal pain, including visceral pain, is prevalent in functional gastrointestinal (GI) disorders (FGIDs), affecting the overall quality of a patient's life. Neural circuits in the brain encode, store, and transfer pain information across brain regions. Ascending pain signals actively shape brain dynamics; in turn, the descending system responds to the pain through neuronal inhibition. Pain processing mechanisms in patients are currently mainly studied with neuroimaging techniques; however, these techniques have a relatively poor temporal resolution. A high temporal resolution method is warranted to decode the dynamics of the pain processing mechanisms. Here, we reviewed crucial brain regions that exhibited pain-modulatory effects in an ascending and descending manner. Moreover, we discussed a uniquely well-suited method, namely extracellular electrophysiology, that captures natural language from the brain with high spatiotemporal resolution. This approach allows parallel recording of large populations of neurons in interconnected brain areas and permits the monitoring of neuronal firing patterns and comparative characterization of the brain oscillations. In addition, we discussed the contribution of these oscillations to pain states. In summary, using innovative, state-of-the-art methods, the large-scale recordings of multiple neurons will guide us to better understanding of pain mechanisms in FGIDs.