Glial involvement in trigeminal central sensitization

Neurophysiology of corneal neuropathic pain and emerging pharmacotherapeutics

The altered activity generated by corneal neuronal injury can result in morphological and physiological changes in the architecture of synaptic connections in the nervous system. These changes can alter the sensitivity of neurons (both second-order and higher-order projection) projecting pain signals. A complex process involving different cell types, molecules, nerves, dendritic cells, neurokines, neuropeptides, and axon guidance molecules causes a high level of sensory rearrangement, which is germane to all the phases in the pathomechanism of corneal neuropathic pain. Immune cells migrating to the region of nerve injury assist in pain generation by secreting neurokines that ensure nerve depolarization. Furthermore, excitability in the central pain pathway is perpetuated by local activation of microglia in the trigeminal ganglion and alterations of the descending inhibitory modulation for corneal pain arriving from central nervous system. Corneal neuropathic pain may be facilitated by dysfunctional structures in the central somatosensory nervous system due to a lesion, altered synaptogenesis, or genetic abnormality. Understanding these important pathways will provide novel therapeutic insight.

SHED-derived exosomes attenuate trigeminal neuralgia after CCI of the infraorbital nerve in mice via the miR-24-3p/IL-1R1/p-p38 MAPK pathway

BACKGROUND

Microglial activation in the spinal trigeminal nucleus (STN) plays a crucial role in the development of trigeminal neuralgia (TN). The involvement of adenosine monophosphate-activated protein kinase (AMPK) and N-methyl-D-aspartate receptor 1 (NMDAR1, NR1) in TN has been established. Initial evidence suggests that stem cells from human exfoliated deciduous teeth (SHED) have a potential therapeutic effect in attenuating TN. In this study, we propose that SHED-derived exosomes (SHED-Exos) may alleviate TN by inhibiting microglial activation. This study sought to assess the curative effect of SHED-Exos administrated through the tail vein on a unilateral infraorbital nerve chronic constriction injury (CCI-ION) model in mice to reveal the role of SHED-Exos in TN and further clarify the potential mechanism.

RESULTS

Animals subjected to CCI-ION were administered SHED-Exos extracted by differential ultracentrifugation. SHED-Exos significantly alleviated TN in CCI mice (increasing the mechanical threshold and reducing p-NR1) and suppressed microglial activation (indicated by the levels of TNF-α, IL-1β and IBA-1, as well as p-AMPK) in vivo and in vitro. Notably, SHED-Exos worked in a concentration dependent manner. Mechanistically, miR-24-3p-upregulated SHED-Exos exerted a more significant effect, while miR-24-3p-inhibited SHED-Exos had a weakened effect. Bioinformatics analysis and luciferase reporter assays were utilized for target gene prediction and verification between miR-24-3p and IL1R1. Moreover, miR-24-3p targeted the IL1R1/p-p38 MAPK pathway in microglia was increased in CCI mice, and participated in microglial activation in the STN.

CONCLUSIONS

miR-24-3p-encapsulated SHED-Exos attenuated TN by suppressing microglial activation in the STN of CCI mice. Mechanistically, miR-24-3p blocked p-p38 MAPK signaling by targeting IL1R1. Theoretically, targeted delivery of miR-24-3p may offer a potential strategy for TN.

Twenty-Three Months Repetitive Transcranial Magnetic Stimulation of the Primary Motor Cortex for Refractory Trigeminal Neuralgia: A Single-Case Study

Treatment refractory or recurrent trigeminal neuralgia (TN) is a severe chronic pain illness. Single-session repetitive transcranial magnetic stimulation (rTMS) has been shown to elicit analgesic effects in several craniofacial pain syndromes, including TN. However, the safety and long-term effect of multi-session rTMS for TN have yet to be fully explored. In this study, we present a case of a patient with medical treatment-refractory TN after microvascular decompression. The patient volunteered to undergo 73 sessions of 10 Hz rTMS over 23 months. Neurovagination was used for precise localization and stimulation of the hand and face representation at the left motor cortex. The numeric pain intensity scores derived using the visual analog scale served as a daily index of treatment efficacy. The patient experienced a significant weekly reduction in pain scores, cumulating in 70.89% overall pain relief. The medication dosages were reduced and then discontinued toward the end of the intervention period. No severe adverse events were reported. From our results, we can conclude that the longitudinal multi-session application of rTMS over the hand and face area of M1 is a safe and effective method for producing long-lasting pain relief in TN. Using rTMS may thus prove helpful as an adjunct to conventional methods for treating pain in TN.

The routines of women with fibromyalgia and an interdisciplinary challenge to promote self-care

OBJECTIVE

To describe the daily lives of women living with fibromyalgia through the implementation of an interdisciplinary group intervention and verify its benefits to the participants' health.

METHOD

This descriptive, exploratory, qualitative study was conducted with 12 women who participanted in an interdisciplinary health education group at the State University of Rio de Janeiro. Data were collected in 2016 through semi-structured interviews, which were organized and analyzed according to Bardin's content analysis. This study was approved by the Institutional Review Board (CAAE 16413013013100005259), in accordance with the legal and ethical guidelines estabished by Resolution 466/2012.

RESULTS

Two categories emerged from the analytical corpus, namely: Expression of everyday life and Repercussions of the interdisciplinary group.

CONCLUSION

The actions implemented by the interdisciplinary group resulted in numerous physical, psychological and social benefits for the women.

Optogenetic exploration and modulation of pain processing

Intractable pain is the single most common cause of disability, affecting more than 20% of the population world-wide. There is accordingly a global effort to decipher how changes in nociceptive processing in the peripheral and central nervous systems contribute to the onset and maintenance of chronic pain. The past several years have brought rapid progress in the adaptation of optogenetic approaches to study and manipulate the activity of sensory afferents and spinal cord neurons in freely behaving animals, and to investigate cortical processing and modulation of pain responses. This review discusses methodological advances that underlie this recent progress, and discusses practical considerations for the optogenetic modulation of nociceptive sensory processing.

Central Role of Protein Kinase A in Promoting Trigeminal Nociception in an In Vivo Model of Temporomandibular Disorders

AIMS

To investigate cellular changes in the spinal trigeminal nucleus (STN) and trigeminal ganglion (TG) associated with trigeminal nociception mediated by inflammation in the temporomandibular joint (TMJ).

METHODS

Male Sprague-Dawley rats (n = 86) were utilized to investigate cellular and behavioral responses to prolonged TMJ inflammation caused by bilateral injection of Complete Freund's Adjuvant (CFA) in the TMJ capsules. To investigate the cellular effects of protein kinase A (PKA) in the STN, rats were injected intrathecally with the selective PKA inhibitor KT5720 prior to injection of CFA into both TMJ capsules. Levels of calcitonin gene-related peptide (CGRP), active PKA, and ionized calcium-binding adapter molecule 1 (Iba1) in the STN and expression of phosphorylated extracellular regulated kinases (p-ERK) in the TG were determined with immunohistochemistry (n ≥ 3 experiments per test condition). Nocifensive head withdrawal responses to mechanical stimulation of the cutaneous tissue over the TMJ were monitored following CFA injection in the absence or presence of KT5720 (n = 7). Statistical analysis was performed using parametric analysis of variance (ANOVA) tests.

RESULTS

Intrathecal injection of KT5720 significantly inhibited the stimulatory effect of CFA on levels of CGRP, PKA, and Iba1 in the STN. In addition, administration of KT5720 decreased the average number of CFA-induced nocifensive withdrawal responses to mechanical stimulation and the CFA-mediated increase in p-ERK expression in the ganglion.

CONCLUSION

These findings provide evidence that elevated PKA activity in the STN promotes cellular events temporally associated with trigeminal nociception caused by prolonged TMJ inflammation.

Diverse Physiological Roles of Calcitonin Gene-Related Peptide in Migraine Pathology: Modulation of Neuronal-Glial-Immune Cells to Promote Peripheral and Central Sensitization

The neuropeptide calcitonin gene-related peptide (CGRP) is implicated in the underlying pathology of migraine by promoting the development of a sensitized state of primary and secondary nociceptive neurons. The ability of CGRP to initiate and maintain peripheral and central sensitization is mediated by modulation of neuronal, glial, and immune cells in the trigeminal nociceptive signaling pathway. There is accumulating evidence to support a key role of CGRP in promoting cross excitation within the trigeminal ganglion that may help to explain the high co-morbidity of migraine with rhinosinusitis and temporomandibular joint disorder. In addition, there is emerging evidence that CGRP facilitates and sustains a hyperresponsive neuronal state in migraineurs mediated by reported risk factors such as stress and anxiety. In this review, the significant role of CGRP as a modulator of the trigeminal system will be discussed to provide a better understanding of the underlying pathology associated with the migraine phenotype.

Elevated levels of calcitonin gene-related peptide in upper spinal cord promotes sensitization of primary trigeminal nociceptive neurons

Orofacial pain conditions including temporomandibular disorder (TMD) and migraine are characterized by peripheral and central sensitization of trigeminal nociceptive neurons. The goal of this study was to investigate the role of calcitonin gene-related peptide (CGRP) in promoting bidirectional signaling within the trigeminal system to mediate sensitization of primary nociceptive neurons. Adult male Sprague-Dawley rats were injected intercisternally with CGRP or co-injected with the receptor antagonist CGRP_8-37 or KT 5720, a protein kinase A (PKA) inhibitor. Nocifensive head withdrawal response to mechanical stimulation was investigated using von Frey filaments. Expression of PKA, glial fibrillary acidic protein (GFAP), and ionized calcium-binding adapter molecule 1 (Iba1) in the spinal cord and phosphorylated extracellular signal-regulated kinase (P-ERK) in the ganglion was studied using immunohistochemistry. Some animals were co-injected with CGRP and Fast Blue dye and the ganglion was imaged using fluorescent microscopy. CGRP increased nocifensive responses to mechanical stimulation when compared to control. Co-injection of CGRP_8-37 or KT 5720 with CGRP inhibited the nocifensive response. CGRP stimulated PKA and GFAP expression in the spinal cord, and P-ERK in ganglion neurons. Seven days post injection, Fast Blue was observed in ganglion neurons and satellite glial cells. Our results demonstrate that elevated levels of CGRP in the upper spinal cord promote sensitization of primary nociceptive neurons via a mechanism that involves activation of PKA centrally and P-ERK in ganglion neurons. Our findings provide evidence of bidirectional signaling within the trigeminal system that facilitate increased neuron-glia communication within the ganglion associated with trigeminal sensitization.

Glia and TRPM2 Channels in Plasticity of Central Nervous System and Alzheimer's Diseases

Synaptic plasticity refers to the ability of neurons to strengthen or weaken synaptic efficacy in response to activity and is the basis for learning and memory. Glial cells communicate with neurons and in this way contribute in part to plasticity in the CNS and to the pathology of Alzheimer's disease (AD), a neurodegenerative disease in which impaired synaptic plasticity is causally implicated. The transient receptor potential melastatin member 2 (TRPM2) channel is a nonselective Ca²⁺-permeable channel expressed in both glial cells (microglia and astrocytes) and neurons. Recent studies indicated that TRPM2 regulates synaptic plasticity as well as the activation of glial cells. TRPM2 also modulates oxidative stress and inflammation through interaction with glial cells. As both oxidative stress and inflammation have been implicated in AD pathology, this suggests a possible contribution of TRPM2 to disease processes. Through modulating the homeostasis of glutathione, TRPM2 is involved in the process of aging which is a risk factor of AD. These results potentially point TRPM2 channel to be involved in AD through glial cells. This review summarizes recent advances in studying the contribution of TRPM2 in health and in AD pathology, with a focus on contributions via glia cells.

Dual orexin receptor antagonist 12 inhibits expression of proteins in neurons and glia implicated in peripheral and central sensitization

Sensitization and activation of trigeminal nociceptors is implicated in prevalent and debilitating orofacial pain conditions including temporomandibular joint (TMJ) disorders. Orexins are excitatory neuropeptides that function to regulate many physiological processes and are reported to modulate nociception. To determine the role of orexins in an inflammatory model of trigeminal activation, the effects of a dual orexin receptor antagonist (DORA-12) on levels of proteins that promote peripheral and central sensitization and changes in nocifensive responses were investigated. In adult male Sprague-Dawley rats, mRNA for orexin receptor 1 (OX₁R) and receptor 2 (OX₂R) were detected in trigeminal ganglia and spinal trigeminal nucleus (STN). OX₁R immunoreactivity was localized primarily in neuronal cell bodies in the V3 region of the ganglion and in laminas I-II of the STN. Animals injected bilaterally with complete Freund's adjuvant (CFA) in the TMJ capsule exhibited increased expression of P-p38, P-ERK, and lba1 in trigeminal ganglia and P-ERK and lba1 in the STN at 2 days post injection. However, levels of each of these proteins in rats receiving daily oral DORA-12 were inhibited to near basal levels. Similarly, administration of DORA-12 on days 3 and 4 post CFA injection in the TMJ effectively inhibited the prolonged stimulated expression of protein kinase A, NFkB, and Iba1 in the STN on day 5 post injection. While injection of CFA mediated a nocifensive response to mechanical stimulation of the orofacial region at 2h and 3 and 5 days post injection, treatment with DORA-12 suppressed the nocifensive response on day 5. Somewhat surprisingly, nocifensive responses were again observed on day 10 post CFA stimulation in the absence of daily DORA-12 administration. Our results provide evidence that DORA-12 can inhibit CFA-induced stimulation of trigeminal sensory neurons by inhibiting expression of proteins associated with sensitization of peripheral and central neurons and nociception.

Traumatic brain injury, neuroinflammation, and post-traumatic headaches

Concussions following head and/or neck injury are common, and although most people with mild injuries recover uneventfully, a subset of individuals develop persistent post-concussive symptoms that often include headaches. Post-traumatic headaches vary in presentation and may progress to become chronic and in some cases debilitating. Little is known about the pathogenesis of post-traumatic headaches, although shared pathophysiology with that of the brain injury is suspected. Following primary injury to brain tissues, inflammation rapidly ensues; while this inflammatory response initially provides a defensive/reparative function, it can persist beyond its beneficial effect, potentially leading to secondary injuries because of alterations in neuronal excitability, axonal integrity, central processing, and other changes. These changes may account for the neurological symptoms often observed after traumatic brain injury, including headaches. This review considers selected aspects of the inflammatory response following traumatic brain injury, with an emphasis on the role of glial cells as mediators of maladaptive post-traumatic inflammation.

Inclusion of cocoa as a dietary supplement represses expression of inflammatory proteins in spinal trigeminal nucleus in response to chronic trigeminal nerve stimulation

SCOPE

Central sensitization is implicated in the pathology of temporomandibular joint disorder and other types of orofacial pain. We investigated the effects of dietary cocoa on expression of proteins involved in the development of central sensitization in the spinal trigeminal nucleus (STN) in response to inflammatory stimulation of trigeminal nerves.

METHODS AND RESULTS

Male Sprague-Dawley rats were fed either a control diet or an isocaloric diet consisting of 10% cocoa powder 14 days prior to bilateral injection of complete Freund's adjuvant (CFA) into the temporomandibular joint to promote prolonged activation of trigeminal ganglion neurons and glia. While dietary cocoa stimulated basal expression of glutamate-aspartate transporter and mitogen-activated protein kinase phosphatase-1 when compared to animals on a normal diet, cocoa suppressed basal calcitonin gene-related peptide levels in the STN. CFA-stimulated levels of protein kinase A, P2X3 , P-p38, glial fibrillary-associated protein, and OX-42, whose elevated levels in the STN are implicated in central sensitization, were repressed to near control levels in animals on a cocoa-enriched diet. Similarly, dietary cocoa repressed CFA-stimulated inflammatory cytokine expression.

CONCLUSION

Based on our findings, we speculate that cocoa-enriched diets could be beneficial as a natural therapeutic option for temporomandibular joint disorder and other chronic orofacial pain conditions.

New directions in migraine

Migraine is a highly prevalent neurological disorder imparting a major burden on health care around the world. The primary pathology may be a state of hyperresponsiveness of the nervous system, but the molecular mechanisms are yet to be fully elucidated. We could now be at a watershed moment in this respect, as the genetic loci associated with typical forms of migraine are being revealed. The genetic discoveries are the latest step in the evolution of our understanding of migraine, which was initially considered a cerebrovascular condition, then a neuroinflammatory process and now primarily a neurogenic disorder. Indeed, the genetic findings, which have revealed ion channels and transporter mutations as causative of migraine, are a powerful argument for the neurogenic basis of migraine. Modulations of ion channels leading to amelioration of the migraine 'hyperresponsive' brain represent attractive targets for drug discovery. There lies ahead an exciting and rapidly progressing phase of migraine translational research, and in this review we highlight recent genetic findings and consider how these may affect the future of migraine neurobiology and therapy.

Cerebral cortex modulation of pain

Pain is a complex experience encompassing sensory-discriminative, affective-motivational and cognitiv e-emotional components mediated by different mechanisms. Contrary to the traditional view that the cerebral cortex is not involved in pain perception, an extensive cortical network associated with pain processing has been revealed using multiple methods over the past decades. This network consistently includes, at least, the anterior cingulate cortex, the agranular insular cortex, the primary (SI) and secondary somatosensory (SII) cortices, the ventrolateral orbital cortex and the motor cortex. These cortical structures constitute the medial and lateral pain systems, the nucleus submedius-ventrolateral orbital cortex-periaqueductal gray system and motor cortex system, respectively. Multiple neurotransmitters, including opioid, glutamate, GABA and dopamine, are involved in the modulation of pain by these cortical structures. In addition, glial cells may also be involved in cortical modulation of pain and serve as one target for pain management research. This review discusses recent studies of pain modulation by these cerebral cortical structures in animals and human.