The role of peptides in central sensitization

Description of Novel Molecular Factors in Lumbar DRGs and Spinal Cord Factors Underlying Development of Neuropathic Pain Component in the Animal Model of Osteoarthritis

Osteoarthritis (OA) is one of the most common joint disorder, with pain accompanied by functional impairment, as the most pronounced clinical symptom. Currently used pharmacotherapy involves symptomatic treatment that do not always provide adequate pain relief. This may be due to concomitance of central sensitization and development of neuropathic features in OA patients. Here we performed studies in the animal model of OA to investigate of the neuropathic component. Intraarticular injection of monoiodoacetate (MIA, 1 mg) was used to induce OA in Wistar male rats. Development of pain phenotype was assessed by behavioral testing (PAM test and von Frey's test), while corresponding changes in dorsal root ganglia (DRGs L3-L5) and spinal cord (SC) gene expression were assessed by means of qRT-PCR technique. We also performed microtomography of OA-affected knee joints to correlate the level of bone degradation with observed behavioral and molecular changes. We observed gradually developing remote allodynia after MIA treatment, indicating the presence of neuropathic component. Our results showed that, among DRGs innervating knee joint, development of central sensitization is most likely due to peripheral input of stimuli through DRG L5. In SC, development of secondary hypersensitivity correlated with increased expression of TAC1 and NPY. Our studies provided molecular records on abnormal activation of pain transmission markers in DRG and SC during development of OA that are responsible for the manifestation of neuropathic features. The obtained results increase insight into molecular changes occurring in the neuronal tissue during OA development and may contribute to readdressing treatment paradigms.

Changes in the central nervous system in diabetic neuropathy

One of the most common chronic complications arising from diabetes is diabetic peripheral neuropathy. Depending on research statistics, approximately half of the people who have diabetes will suffer from diabetic peripheral neuropathy over time, which manifests as abnormal sensations in the distal extremities, and about 25%-50% of these patients have symptoms of neuralgia, called painful diabetic neuropathy. These patients often exhibit adverse emotional conditions, like anxiety or depression, which can reduce their quality of life. The pathogenesis of diabetic peripheral neuropathy is complex, and although persistent hyperglycemia plays a central role in the development of diabetic peripheral neuropathy, strict glycemic control does not eliminate the risk of diabetic peripheral neuropathy. This suggests the need to understand the role of the central nervous system in the development of diabetic peripheral neuropathy to modulate treatment regimens accordingly. Magnetic resonance imaging not only allows for the noninvasive detection of structural and functional alterations in the central nervous system, but also provides insight into the processing of abnormal information such as pain by the central nervous system, and most importantly, contributes to the development of more effective pain relief protocols. Therefore, this article will focus on the mechanisms and related imaging evidence of central alterations in diabetic peripheral neuropathy, especially in painful diabetic neuropathy.

CGRP, Migraine, and Brain MRI in CADASIL: A Pilot Study

BACKGROUND

Migraine is associated with neuroimaging differences in cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy (CADASIL). However, it is unknown if migraine-related disability (MRD) or if calcitonin gene-related peptide (CGRP), a vasoactive peptide important in migraine pathology, have radiographic implications. The aims of this study were to identify whether MRD or interictal serum CGRP levels impacted neuroimaging findings for those with CADASIL.

MATERIALS AND METHODS

A cross-sectional analysis was performed. The primary outcomes were neuroimaging differences associated with MRD among those with migraine or interictal serum CGRP levels of those with and without migraine. MRD was defined by 2 migraine disability scales (Migraine Disability Assessment, Headache Impact Test-6). Retrospective brain magnetic resonance imaging was reviewed (average 1.7 ± 2.0 y before enrollment). Rank-sum and χ 2 tests were used.

RESULTS

Those with migraine (n=25, vs. n=14 without) were younger [median 49 (25 to 82) y vs. 60 (31 to 82) y, P <0.007], had fewer cerebral microbleeds (0 to 31 vs. 0 to 50, P =0.02) and less frequently had anterior temporal lobe T2 hyperintensities [68% (17/25) vs 100% (14/14), P =0.02]. MRD scale outcomes had no significant radiographic associations. Interictal serum CGRP did not differ (migraine: n=18, 27.0±9.6 pg/mL vs. no migraine: n=10, 26.8±15.7 pg/mL, P =0.965).

CONCLUSIONS

Migraine may forestall microangiopathy in CADASIL, though possibly independent of severity as measured by MRD. Interictal serum CGRP did not differ in our cohort suggesting CGRP may not be vital to migraine pathophysiology in CADASIL. Larger studies are needed to account for age differences.

Pain Biomarkers in Fibromyalgia Syndrome: Current Understanding and Future Directions

Fibromyalgia is a complex and heterogeneous clinical syndrome, mainly characterized by the presence of widespread pain, possibly associated with a variety of other symptoms. Fibromyalgia can have an extremely negative impact on the psychological, physical and social lives of people affected, sometimes causing patients to experience dramatically impaired quality of life. Nowadays, the diagnosis of fibromyalgia is still clinical, thus favoring diagnostic uncertainties and making its clear identification challenging to establish, especially in primary care centers. These difficulties lead patients to undergo innumerable clinical visits, investigations and specialist consultations, thus increasing their stress, frustration and even dissatisfaction. Unfortunately, research over the last 25 years regarding a specific biomarker for the diagnosis of fibromyalgia has been fruitless. The discovery of a reliable biomarker for fibromyalgia syndrome would be a critical step towards the early identification of this condition, not only reducing patient healthcare utilization and diagnostic test execution but also providing early intervention with guideline-based treatments. This narrative article reviews different metabolite alterations proposed as possible biomarkers for fibromyalgia, focusing on their associations with clinical evidence of pain, and highlights some new, promising areas of research in this context. Nevertheless, none of the analyzed metabolites emerge as sufficiently reliable to be validated as a diagnostic biomarker. Given the complexity of this syndrome, in the future, a panel of biomarkers, including subtype-specific biomarkers, could be considered as an interesting alternative research area.

Autonomic responses during bladder hydrodistention under general versus spinal anaesthesia in patients with interstitial cystitis/bladder pain syndrome: a randomized clinical trial

Blocking the abrupt increase in systolic blood pressure associated with autonomic response during bladder hydrodistention in patients with interstitial cystitis/bladder pain syndrome (IC/BPS) is essential for patient safety. We conducted this study to compare autonomic responses during bladder hydrodistention in patients with IC/BPS under general and spinal anaesthesia. Thirty-six patients were randomly allocated to a general anaesthesia (GA, n = 18) or a spinal anaesthesia (SA, n = 18) group. Blood pressure and heart rate were measured continuously and ΔSBP, defined as maximum increases in SBP during bladder hydrodistention from baseline, was compared between groups. Heart rate variability was analysed using electrocardiograms. The post-anaesthesia care unit assessed postoperative pain using a numeric (0-10) rating scale. Our analyses yield a significantly greater ΔSBP (73.0 [26.0-86.1] vs. 2.0 [- 4.0 to 6.0] mmHg), a significantly lower root-mean-square of successive differences in heart rate variability after bladder hydrodistention (10.8 [7.7-19.8] vs. 20.6 [15.1-44.7] ms), and significantly higher postoperative pain scores (3.5 [0.0-5.5] vs. 0.0 [0.0-0.0]) in the GA compared to the SA group. These findings suggest that SA has advantages over GA for bladder hydrodistention in preventing an abrupt increase in SBP and postoperative pain in IC/BPS patients.

CGRP physiology, pharmacology, and therapeutic targets: migraine and beyond

Calcitonin gene-related peptide (CGRP) is a neuropeptide with diverse physiological functions. Its two isoforms (α and β) are widely expressed throughout the body in sensory neurons as well as in other cell types, such as motor neurons and neuroendocrine cells. CGRP acts via at least two G protein-coupled receptors that form unusual complexes with receptor activity-modifying proteins. These are the CGRP receptor and the AMY1 receptor; in rodents, additional receptors come into play. Although CGRP is known to produce many effects, the precise molecular identity of the receptor(s) that mediates CGRP effects is seldom clear. Despite the many enigmas still in CGRP biology, therapeutics that target the CGRP axis to treat or prevent migraine are a bench-to-bedside success story. This review provides a contextual background on the regulation and sites of CGRP expression and CGRP receptor pharmacology. The physiological actions of CGRP in the nervous system are discussed, along with updates on CGRP actions in the cardiovascular, pulmonary, gastrointestinal, immune, hematopoietic, and reproductive systems and metabolic effects of CGRP in muscle and adipose tissues. We cover how CGRP in these systems is associated with disease states, most notably migraine. In this context, we discuss how CGRP actions in both the peripheral and central nervous systems provide a basis for therapeutic targeting of CGRP in migraine. Finally, we highlight potentially fertile ground for the development of additional therapeutics and combinatorial strategies that could be designed to modulate CGRP signaling for migraine and other diseases.

Zebrafish (Danio rerio) as a model to assess the effects of cocaine as a drug of abuse and its environmental implications

Cocaine (COC) use concerns are on the increase for both authorities and civil society. Despite this, it is important to investigate COC effects or those of its main metabolite, belzoylecgonine (BE), in consolidated aquatic model organisms, such as the zebrafish (Danio rerio). This (mini) review consists in an assessment regarding toxicological studies carried out employing zebrafish (embryos, larvae or adults) exposed to COC and/or BE indexed at the SCOPUS and Web of Science databases. Ten different endpoints were analyzed in both embryos and larvae, whereas only four were analyzed in adults. Of the 23 studies, only five investigated COC and/or BE effects following an environmental approach when exposing zebrafish, while most (18 studies) analyzed COC effects under a drug of abuse approach. Cocaine exposure was noted as altering the expression of several genes, such as those linked to COC transport proteins, dopamine receptors, SP substance production, the tachykinin system, and the tyrosine hydroxylase enzyme. BE exposure resulted in more oxidative and proteomic effects than COC in embryos. Cocaine abstinence resulted in hyperactivity associated with stereotypy in adult fish, in addition to reduced responses to visual stimuli to red light and neuronal development pattern alterations. Cocaine was noted as accumulating in zebrafish eyes, possibly due to melanin binding, and causing dose-response cardiac effects in both embryos and adults. Despite the different effects addressed by our survey, we emphasize the lack of COC and BE exposure assessments in zebrafish employing an environmental point of view.

Estradiol Treatment Enhances Behavioral and Molecular Changes Induced by Repetitive Trigeminal Activation in a Rat Model of Migraine

A migraine is a neurological condition that can cause multiple symptoms. It is up to three times more common in women than men, thus, estrogen may play an important role in the appearance attacks. Its exact pathomechanism is still unknown; however, the activation and sensitization of the trigeminal system play an essential role. We aimed to use an animal model, which would better illustrate the process of repeated episodic migraine attacks to reveal possible new mechanisms of trigeminal pain chronification. Twenty male (M) and forty ovariectomized (OVX) female adult rats were used for our experiment. Male rats were divided into two groups (M + SIF, M + IS), while female rats were divided into four groups (OVX + SIF, OVX + IS, OVX + E2 + SIF, OVX + E2 + IS); half of the female rats received capsules filled with cholesterol (OVX + SIF, OVX + IS), while the other half received a 1:1 mixture of cholesterol and 17β-estradiol (OVX + E2 + SIF, OVX + E2 + IS). The animals received synthetic interstitial fluid (SIF) (M + SIF, OVX + SIF, OVX + E2 + SIF) or inflammatory soup (IS) (M + IS, OVX + IS, OVX + E2 + IS) treatment on the dural surface through a cannula for three consecutive days each week (12 times in total). Behavior tests and immunostainings were performed. After IS application, a significant decrease was observed in the pain threshold in the M + IS (0.001 < p < 0.5), OVX + IS (0.01 < p < 0.05), and OVX + E2 + IS (0.001 < p < 0.05) groups compared to the control groups (M + SIF; OVX + SIF, OVX + E2 + SIF). The locomotor activity of the rats was lower in the IS treated groups (M + IS, 0.01 < p < 0.05; OVX + IS, p < 0.05; OVX + E2 + IS, 0.001 < p < 0.05), and these animals spent more time in the dark room (M + IS, p < 0.05; OVX + IS, 0.01 < p < 0.05; OVX + E2 + IS, 0.001 < p < 0.01). We found a significant difference between M + IS and OVX + E2 + IS groups (p < 0.05) in the behavior tests. Furthermore, IS increased the area covered by calcitonin gene-related peptide (CGRP) immunoreactive (IR) fibers (M + IS, p < 0.01; OVX + IS, p < 0.01; OVX + E2 + IS, p < 0.001) and the number of neuronal nitric oxide synthase (nNOS) IR cells (M + IS, 0.001< p < 0.05; OVX + IS, 0.01 < p < 0.05; OVX + E2 + IS, 0.001 < p < 0.05) in the caudal trigeminal nucleus (TNC). There was no difference between M + IS and OVX + IS groups; however, the area was covered by CGRP IR fibers (0.01 < p < 0.05) and the number of nNOS IR cells was significantly higher in the OVX + E2 + IS (p < 0.05) group than the other two IS- (M + IS, OVX + IS) treated animals. Overall, repeated administration of IS triggers activation and sensitization processes and develops nociceptive behavior changes. CGRP and nNOS levels increased significantly in the TNC after IS treatments, and moreover, pain thresholds and locomotor activity decreased with the development of photophobia. In our model, stable high estradiol levels proved to be pronociceptive. Thus, repeated trigeminal activation causes marked behavioral changes, which is more prominent in rats treated with estradiol, also reflected by the expression of the sensitization markers of the trigeminal system.

Involvement of the cerebellum in migraine

Migraine is a disabling neurological disease characterized by moderate or severe headaches and accompanied by sensory abnormalities, e.g., photophobia, allodynia, and vertigo. It affects approximately 15% of people worldwide. Despite advancements in current migraine therapeutics, mechanisms underlying migraine remain elusive. Within the central nervous system, studies have hinted that the cerebellum may play an important sensory integrative role in migraine. More specifically, the cerebellum has been proposed to modulate pain processing, and imaging studies have revealed cerebellar alterations in migraine patients. This review aims to summarize the clinical and preclinical studies that link the cerebellum to migraine. We will first discuss cerebellar roles in pain modulation, including cerebellar neuronal connections with pain-related brain regions. Next, we will review cerebellar symptoms and cerebellar imaging data in migraine patients. Lastly, we will highlight the possible roles of the neuropeptide calcitonin gene-related peptide (CGRP) in migraine symptoms, including preclinical cerebellar studies in animal models of migraine.

High-frequency electrical stimulation attenuates neuronal release of inflammatory mediators and ameliorates neuropathic pain

BACKGROUND

Neuroinflammation is an important driver of acute and chronic pain states. Therefore, targeting molecular mediators of neuroinflammation may present an opportunity for developing novel pain therapies. In preclinical models of neuroinflammatory pain, calcitonin gene-related peptide (CGRP), substance P and high mobility group box 1 protein (HMGB1) are molecules synthesized and released by sensory neurons which activate inflammation and pain. High-frequency electrical nerve stimulation (HFES) has achieved clinical success as an analgesic modality, but the underlying mechanism is unknown. Here, we reasoned that HFES inhibits neuroinflammatory mediator release by sensory neurons to reduce pain.

METHODS

Utilizing in vitro and in vivo assays, we assessed the modulating effects of HFES on neuroinflammatory mediator release by activated sensory neurons. Dorsal root ganglia (DRG) neurons harvested from wildtype or transgenic mice expressing channelrhodopsin-2 (ChR2) were cultured on micro-electrode arrays, and effect of HFES on optogenetic- or capsaicin-induced neuroinflammatory mediator release was determined. Additionally, the effects of HFES on local neuroinflammatory mediator release and hyperalgesia was assessed in vivo using optogenetic paw stimulation and the neuropathic pain model of chronic constriction injury (CCI) of the sciatic nerve.

RESULTS

Light- or capsaicin-evoked neuroinflammatory mediator release from cultured transgenic DRG sensory neurons was significantly reduced by concurrent HFES (10 kHz). In agreement with these findings, elevated levels of neuroinflammatory mediators were detected in the affected paw following optogenetic stimulation or CCI and were significantly attenuated using HFES (20.6 kHz for 10 min) delivered once daily for 3 days.

CONCLUSION

These studies reveal a previously unidentified mechanism for the pain-modulating effect of HFES in the setting of acute and chronic nerve injury. The results support the mechanistic insight that HFES may reset sensory neurons into a less pro-inflammatory state via inhibiting the release of neuroinflammatory mediators resulting in reduced inflammation and pain.

Multifactorial pathways in burn injury-induced chronic pain: novel targets and their pharmacological modulation

Burn injuries are among the highly prevalent medical conditions worldwide that occur mainly in children, military veterans and victims of fire accidents. It is one of the leading causes of temporary as well as permanent disabilities in patients. Burn injuries are accompanied by pain that persists even after recovery from tissue damage which puts immense pressure on the healthcare system. The pathophysiology of burn pain is poorly understood due to its complex nature and lack of considerable preclinical and clinical shreds of evidence, that creates a substantial barrier to the development of new analgesics. Burns damage the skin layers supplied with nociceptors such as NAV1.7, TRPV1, and TRPA1. Burn injury-mediated co-localization and simultaneous activation of TRPA1 and TRPV1 in nociceptive primary afferent C-fibers which contributes to the development and maintenance of chronic pain. Burn injuries are accompanied by central sensitization, a key feature of pain pathophysiology mainly driven by a series of cascades involving aberrations in the glutamatergic system, microglial activation, release of neuropeptides, cytokines, and chemokines. Activation of p38 mitogen-activated protein kinase, altered endogenous opioid signaling, and distorted genomic expression are other pathophysiological factors responsible for the development and maintenance of burn pain. Here we discuss comprehensive literature on molecular mechanisms of burn pain and potential targets that could be translated into near future therapeutics.

Modulating the tachykinin: Role of substance P and neurokinin receptor expression in ocular surface disorders

Substance P (SP) is a tachykinin expressed by various cells in the nervous and immune systems. SP is predominantly released by neurons and exerts its biological and immunological effects through the neurokinin receptors, primarily the neurokinin-1 receptor (NK1R). SP is essential for maintaining ocular surface homeostasis, and its reduced levels in disorders like diabetic neuropathy disrupt the corneal tissue. It also plays an essential role in promoting corneal wound healing by promoting the migration of keratocytes. In this review, we briefly discuss the structure, expression, and function of SP and its principal receptor NK1R. In addition, SP induces pro-inflammatory effects through autocrine or paracrine action on the immune cells in various ocular surface pathologies, including dry eye disease, herpes simplex virus keratitis, and Pseudomonas keratitis. We provide an in-depth review of the pathogenic role of SP in various ocular surface diseases and several new approaches developed to counter the immune-mediated effects of SP either through modulating its production or blocking its target receptor.

Tear film and ocular surface neuropeptides: Characteristics, synthesis, signaling and implications for ocular surface and systemic diseases

Ocular surface neuropeptides are vital molecules primarily involved in maintaining ocular surface integrity and homeostasis. They also serve as communication channels between the nervous system and the immune system, maintaining the homeostasis of the ocular surface. Tear film and ocular surface neuropeptides have a role in disease often due to abnormalities in their synthesis (either high or low production), signaling through defective receptors, or both. This creates imbalances in otherwise normal physiological processes. They have been observed to be altered in many ocular surface and systemic diseases including dry eye disease, ocular allergy, keratoconus, LASIK-induced dry eye, pterygium, neurotrophic keratitis, corneal graft rejection, microbial keratitis, headaches and diabetes. This review examines the characteristics of neuropeptides, their synthesis and their signaling through G-protein coupled receptors. The review also explores the types of neuropeptides within the tears and ocular surface, and how they change in ocular and systemic diseases.

Inhibition of Nociception in a Preclinical Episodic Migraine Model by Dietary Supplementation of Grape Seed Extract Involves Activation of Endocannabinoid Receptors

Migraine is associated with peripheral and central sensitization of the trigeminal system and dysfunction of descending pain modulation pathways. Recently, dietary inclusion of grape seed extract (GSE) was shown to inhibit mechanical nociception in a preclinical model of chronic temporomandibular joint disorder, a condition often comorbid with migraine, with the antinociceptive effect mediated, in part, by activation of 5-HT3/7 and GABAB receptors. This study further investigated the mechanisms by which GSE inhibits mechanical nociception in a preclinical model of episodic migraine. Hyperalgesic priming of female and male Sprague Dawley rats was induced by three consecutive daily two-hour episodes of restraint stress. Seven days after the final restraint stress, rats were exposed to pungent odors from an oil extract that contains the compound umbellulone, which stimulates CGRP release and induces migraine-like pain. Some animals received dietary supplementation of GSE in their drinking water beginning one week prior to restraint stress. Changes in mechanical sensitivity in the orofacial region and hindpaw were determined using von Frey filaments. To investigate the role of the endocannabinoid receptors in the effect of GSE, some animals were injected intracisternally with the CB1 antagonist AM 251 or the CB2 antagonist AM 630 prior to odor inhalation. Changes in CGRP expression in the spinal trigeminal nucleus (STN) in response to stress, odor and GSE supplementation were studied using immunohistochemistry. Exposure of stress-primed animals to the odor caused a significant increase in the average number of withdrawal responses to mechanical stimulation in both the orofacial region and hindpaw, and the effect was significantly suppressed by daily supplementation with GSE. The anti-nociceptive effect of GSE was inhibited by intracisternal administration of antagonists of CB1 and CB2 receptors. GSE supplementation inhibited odor-mediated stimulation of CGRP expression in the STN in sensitized animals. These results demonstrate that GSE supplementation inhibits trigeminal pain signaling in an injury-free model of migraine-like pain via activation of endocannabinoid receptors and repression of CGRP expression centrally. Hence, we propose that GSE may be beneficial as a complementary migraine therapeutic.

Multimodal cerebral imaging study on the effects of "Adjust Zang Dredge Meridian" electroacupuncture on cerebral central sensitization in PDPN patients: a study protocol for a sham-controlled, randomized trial

BACKGROUND

Painful diabetic peripheral neuropathy (PDPN) has a great impact on an individual's quality of life. The current researchers' previous trial suggested that acupuncture was a promising adjunctive treatment for PDPN. However, the underlying mechanism of action of acupuncture treatment for PDPN is still unclear, especially its effects at the cerebral level. The aim of this trial will be to explore how acupuncture works in treating PDPN by using multimodal cerebral imaging.

METHOD AND DESIGN

This will be a randomized controlled trial. A total of 150 participants will be recruited and assigned to one of three groups: the healthy group, the DM without PDPN group and the DM with PDPN group. Participants in the DM without PDPN and the DM with PDPN groups will each be further divided between an electroacupuncture group and a sham electroacupuncture group. Participants will receive six treatment sessions per week for 4 weeks. Multimodal cerebral imaging includes resting-state functional magnetic resonance imaging (rs-fMRI), diffusion tensor imaging (DTI) and magnetic resonance spectroscopy (MRS); this neurophysiological testing will be the primary outcome measure. Subjective pain scales and blood analysis will be a secondary outcome measure and will be used to assess the clinical efficacy of the intervention. Multimodal cerebral imaging will be used to detect cerebral activity changes in each treatment group. The clinical data and fMRI data will be analysed for all the groups. Multiple correlation regression analyses will be used to assess the association between changes in cerebral functional activity and the improvement of clinical outcomes after acupuncture treatment.

DISCUSSION

This study is based on the results of the researchers' previous study, and using combined clinical and cerebral function changes, it will help evaluate the effects of acupuncture on PDPN. The investigation of acupuncture's central mechanism of action will further expand the understanding of acupuncture treatment of PDPN.

TRIAL REGISTRATION

Chinese Clinical Trial Registry ChiCTR1900024109 . Registered on 26 June 2019.

Molecular mechanism in trigeminal nerve and treatment methods related to orthodontic pain

BACKGROUND

Orthodontic treatment is the main treatment approach for malocclusion. Orthodontic pain is an inevitable undesirable adverse reaction during orthodontic treatment. It is reported orthodontic pain has become one of the most common reason that patients withdraw from orthodontic treatment. Therefore, understanding the underlying mechanism and finding treatment of orthodontic pain are in urgent need.

AIMS

This article aims to sort out the mechanisms and treatments of orthodontic pain, hoping to provide some ideas for future orthodontic pain relief.

MATERIALS

Tooth movement will cause local inflammation. Certain inflammatory factors and cytokines stimulating the trigeminal nerve and further generating pain perception, as well as drugs and molecular targeted therapy blocking nerve conduction pathways, will be reviewed in this article.

METHOD

We review and summaries current studies related to molecular mechanisms and treatment approaches in orthodontic pain control.

RESULTS

Orthodontics pain related influencing factors and molecular mechanisms has been introduced. Commonly used clinical methods in orthodontic pain control has been evaluated.

DISCUSSION

With the clarification of more molecular mechanisms, the direction of orthodontic pain treatment will shift to targeted drugs.

Potential Nociceptive Role of the Thoracolumbar Fascia: A Scope Review Involving In Vivo and Ex Vivo Studies

Nociceptive innervation of the thoracolumbar fascia (TLF) has been investigated over the past few decades; however, these studies have not been compiled or collectively appraised. The purpose of this scoping review was to assess current knowledge regarding nociceptive innervation of the TLF to better inform future mechanistic and clinical TLF research targeting lower back pain (LBP) treatment. PubMed, ScienceDirect, Cochrane, and Embase databases were searched in January 2021 using relevant descriptors encompassing fascia and pain. Eligible studies satisfied the following: (a) published in English; (b) preclinical and clinical (in vivo and ex vivo) studies; (c) original data; (d) included quantification of at least one TLF nociceptive component. Two-phase screening procedures were conducted by a pair of independent reviewers, after which data were extracted and summarized from eligible studies. The search resulted in 257 articles of which 10 met the inclusion criteria. Studies showed histological evidence of nociceptive nerve fibers terminating in lower back fascia, suggesting a TLF contribution to LBP. Noxious chemical injection or electrical stimulation into fascia resulted in longer pain duration and higher pain intensities than injections into subcutaneous tissue or muscle. Pre-clinical and clinical research provides histological and functional evidence of nociceptive innervation of TLF. Additional knowledge of fascial neurological components could impact LBP treatment.

Neuropathic Pain: Biomolecular Intervention and Imaging via Targeting Microglia Activation

Many diseases, including cancer, can lead to neuropathic pain (NP). NP is one of the accompanying symptoms of suffering in many conditions and the life quality of NP patient is seriously affected. Due to complex causes, the effects of clinical treatments have been very unsatisfactory. Many experts have found that neuron-microglia interaction plays an essential role in NP occurrence and development. Therefore, the activation of microglia, related inflammatory mediators and molecular and cellular signaling pathways have become the focus of NP research. With the help of modern functional imaging technology, advanced pre-and clinical studies have been carried out and NP interventions have been attempted by using the different pharmaceuticals and the extracted active components of various traditional herbal medicines. In this communication, we review the mechanism of microglia on NP formation and treatment and molecular imaging technology's role in the clinical diagnosis and evaluation of NP therapies.

Improving NKCC1 Function Increases the Excitability of DRG Neurons Exacerbating Pain Induced After TRPV1 Activation of Primary Sensory Neurons

Background Our aim was to investigate the effects of the protein expression and the function of sodium, potassium, and chloride co-transporter (NKCC1) in the dorsal root ganglion (DRG) after activation of transient receptor potential vanilloid 1 receptor (TRPV1) in capsaicin-induced acute inflammatory pain and the possible mechanism of action.

Methods Male Sprague–Dawley rats were randomly divided into control, capsaicin, and inhibitor groups. The expression and distribution of TRPV1 and NKCC1 in rat DRG were observed by immunofluorescence. Thermal radiation and acetone test were used to detect the pain threshold of heat and cold noxious stimulation in each group. The expressions of NKCC1 mRNA, NKCC1 protein, and p-NKCC1 in the DRG were detected by PCR and western blotting (WB). Patch clamp and chloride fluorescent probe were used to observe the changes of GABA activation current and intracellular chloride concentration. After intrathecal injection of protein kinase C (PKC) inhibitor (GF109203X) or MEK/extracellular signal-regulated kinase (ERK) inhibitor (U0126), the behavioral changes and the expression of NKCC1 and p-ERK protein in L_4–6 DRG were observed.

Result: TRPV1 and NKCC1 were co-expressed in the DRG. Compared with the control group, the immunofluorescence intensity of NKCC1 and p-NKCC1 in the capsaicin group was significantly higher, and the expression of NKCC1 in the nuclear membrane was significantly higher than that in the control group. The expression of NKCC1 mRNA and protein of NKCC1 and p-NKCC1 in the capsaicin group were higher than those in the control group. After capsaicin injection, GF109203X inhibited the protein expression of NKCC1 and p-ERK, while U0126 inhibited the protein expression of NKCC1. In the capsaicin group, paw withdrawal thermal latency (WTL) was decreased, while cold withdrawal latency (CWL) was prolonged. Bumetanide, GF109203X, or U0126 could reverse the effect. GABA activation current significantly increased in the DRG cells of the capsaicin group, which could be reversed by bumetanide. The concentration of chloride in the DRG cells of the capsaicin group increased, but decreased after bumetanide, GF109203X, and U0126 were administered.

Conclusion Activation of TRPV1 by exogenous agonists can increase the expression and function of NKCC1 protein in DRG, which is mediated by activation of PKC/p-ERK signaling pathway. These results suggest that DRG NKCC1 may participate in the inflammatory pain induced by TRPV1.

Altered sensory innervation and pain hypersensitivity in a model of young painful arthritic joints: short- and long-term effects

BACKGROUND

Early life experience can cause long-term alterations in the nociceptive processes underlying chronic pain, but the consequences of early life arthritic joint inflammation upon the sensory innervation of the joint is not known. Here, we measure pain sensitivity and sensory innervation in a young, juvenile and adult rodent model of arthritic joints and test the consequences of joint inflammation in young animals upon adult arthritic pain and joint innervation.

METHODS

Unilateral ankle joint injections of complete Freund's adjuvant (CFA) (6-20 µl) were performed in young, postnatal day (P)8, adolescent (P21) and adult (P40) rats. A separate cohort of animals were injected at P8, and again at P40. Hindpaw mechanical sensitivity was assessed using von Frey monofilaments (vF) for 10 days. Nerve fibres were counted in sections through the ankle joint immunostained for calcitonin gene-related peptide (CGRP) and neurofilament 200 kDa (NF200).

RESULTS

Ankle joint CFA injection increased capsular width at all ages. Significant mechanical pain hypersensitivity and increased number of joint CGRP + ve sensory fibres occurred in adolescent and adult, but not young, rats. Despite the lack of acute reaction, joint inflammation at a young age resulted in significantly increased pain hypersensitivity and CGRP+ fibre counts when the rats were re-inflamed as adults.

CONCLUSIONS

Joint inflammation increases the sensory nociceptive innervation and induces acute pain hypersensitivity in juvenile and adult, but not in young rats. However, early life joint inflammation 'primes' the joint such that adult inflammatory pain behaviour and nociceptive nerve endings in the joint are significantly increased. Early life joint inflammation may be an important factor in the generation and maintenance of chronic arthritic pain.

Hypersensitivity to calcitonin gene-related peptide in chronic migraine

OBJECTIVE

To investigate if calcitonin gene-related peptide infusion induces migraine-like attacks in chronic migraine patients.

METHODS

Fifty-eight patients with chronic migraine, either with or without headache on the experimental day, were assessed for the incidence of migraine-like attacks after an intravenous infusion with calcitonin gene-related peptide 1.5 µg/min over 20 min. The primary endpoint was the incidence of migraine-like attacks after calcitonin gene-related peptide. Exploratory endpoints were the association between the incidence of migraine-like attacks and presence of headache on the experimental day, and headache frequency in the past month. Migraine-like attack data was compared to a historic cohort of 91 episodic migraine patients without headache on the experimental day. Total tenderness score, pressure-pain threshold and supra-threshold pressure pain at baseline were investigated in relation to incidence of migraine-like attacks and presence of headache on the experimental day.

RESULTS

In total, 83% of the 58 chronic migraine patients developed migraine-like attacks after calcitonin gene-related peptide infusion. Migraine-like attacks were found in 92% of chronic migraine patients with headache on the experimental day compared to 65% of chronic migraine patients without headache on the experimental day (p = 0.035). No differences were observed in total tenderness score and pressure-pain threshold between chronic migraine patients with and without headache on the experimental day. The incidence of migraine-like attacks following calcitonin gene-related peptide in chronic migraine patients without headache (65%) was equal to the historic cohort of 91 episodic migraine patients without headache (67%) on the experimental day.

CONCLUSIONS

Chronic migraine patients are hypersensitive to calcitonin gene-related peptide. The potency of calcitonin gene-related peptide as a migraine inductor is increased in chronic migraine patients with ongoing headache. We suggest that calcitonin gene-related peptide, besides being a migraine trigger also acts as a modulator of nociceptive transmission in the trigeminal system.

Ocular surface inflammation induces de novo expression of substance P in the trigeminal primary afferents with large cell bodies

We evaluated the changes in substance P (SP)-expressing trigeminal neurons (TNs) innervating the cornea following ocular surface inflammation. Ocular surface inflammation was induced in Sprague-Dawley rats using 0.1% benzalkonium chloride (BAK). The corneal staining score, corneal epithelial apoptosis, conjunctival goblet cells, and density of corneal subbasal nerve plexus (SNP) were assessed, and the mRNA levels of SP, interleukin (IL)-1β, IL-6, and tumour necrosis factor-α were measured in corneas and ipsilateral trigeminal ganglia (TG). SP-immunoreactivity (IR) was measured in corneal intraepithelial nerves and TNs. The cell size of corneal TNs in the TG was calculated. All parameters were observed immediately (BAK group), at 1 week (1 w group), and 2 months (2 m group) after 2 weeks of BAK application. BAK caused an increase in the corneal staining score and the number of apoptotic cells, loss of conjunctival goblet cells, reduced density of corneal SNP, and upregulated expression of SP and inflammatory cytokines in both the cornea and TG in the BAK group but those changes were not observed in the 2 m group. On the other hand, SP-IR% and mean cell size of corneal TNs increased significantly in the BAK, 1 w, and 2 m groups, compared to the control. Our data suggest that following ocular surface inflammation, large-sized corneal TNs which normally do not express SP, expressed it and this phenotype switching lasted even after the inflammation disappeared. Long-lasting phenotypic switch, as well as changes in the expression level of certain molecules should be addressed in future studies on the mechanism of corneal neuropathic pain.

PAR2, Keratinocytes, and Cathepsin S Mediate the Sensory Effects of Ciguatoxins Responsible for Ciguatera Poisoning

Ciguatera fish poisoning is caused by the consumption of fish contaminated with ciguatoxins (CTXs). The most distressing symptoms are cutaneous sensory disturbances, including cold dysesthesia and itch. CTXs are neurotoxins known to activate voltage-gated sodium channels, but no specific treatment exists. Peptidergic neurons have been critically involved in ciguatera fish poisoning sensory disturbances. Protease-activated receptor-2 (PAR2) is an itch- and pain-related G protein‒coupled receptor whose activation leads to a calcium-dependent neuropeptide release. In this study, we studied the role of voltage-gated sodium channels, PAR2, and the PAR2 agonist cathepsin S in the cytosolic calcium increase and subsequent release of the neuropeptide substance P elicited by Pacific CTX-2 (P-CTX-2) in rat sensory neurons and human epidermal keratinocytes. In sensory neurons, the P-CTX-2‒evoked calcium response was driven by voltage-gated sodium channels and PAR2-dependent mechanisms. In keratinocytes, P-CTX-2 also induced voltage-gated sodium channels and PAR2-dependent marked calcium response. In the cocultured cells, P-CTX-2 significantly increased cathepsin S activity, and cathepsin S and PAR2 antagonists almost abolished P-CTX-2‒elicited substance P release. Keratinocytes synergistically favored the induced substance P release. Our results demonstrate that the sensory effects of CTXs involve the cathepsin S-PAR2 pathway and are potentiated by their direct action on nonexcitable keratinocytes through the same pathway.

Stimulation of Posterior Thalamic Nuclei Induces Photophobic Behavior in Mice

OBJECTIVE

A hallmark of migraine is photophobia. In mice, photophobia-like behavior is induced by calcitonin gene-related peptide (CGRP), a neuropeptide known to be a key player in migraine. In this study, we sought to identify sites within the brain from which CGRP could induce photophobia.

DESIGN

We focused on the posterior thalamic region, which contains neurons responsive to both light and dural stimulation and has CGRP binding sites. We probed this area with both optogenetic stimulation and acute CGRP injections in wild-type mice. Since the light/dark assay has historically been used to investigate anxiety-like responses in animals, we measured anxiety in a light-independent open field assay and asked if stimulation of a brain region, the periaqueductal gray, that induces anxiety would yield similar results to posterior thalamic stimulation. The hippocampus was used as an anatomical control to ensure that light-aversive behaviors could not be induced by the stimulation of any brain region.

RESULTS

Optogenetic activation of neuronal cell bodies in the posterior thalamic nuclei elicited light aversion in both bright and dim light without an anxiety-like response in an open field assay. Injection of CGRP into the posterior thalamic region triggered similar light-aversive behavior without anxiety. In contrast to the posterior thalamic nuclei, optogenetic stimulation of dorsal periaqueductal gray cell bodies caused both light aversion and an anxiety-like response, while CGRP injection had no effect. In the dorsal hippocampus, neither optical stimulation nor CGRP injection affected light aversion or open field behaviors.

CONCLUSION

Stimulation of posterior thalamic nuclei is able to initiate light-aversive signals in mice that may be modulated by CGRP to cause photophobia in migraine.

High-flow oxygen and pro-serotonin agents for non-interventional treatment of post-dural-puncture headache

OBJECTIVE

Post dural puncture headache (PDPH) is a common complication in patients following diagnostic or therapeutic lumbar puncture, procedures requiring epidural access, and spinal surgery. Epidural blood patch (EBP), the gold standard for the treatment of this pathology requires training not provided to emergency physicians. In addition, the presence of concomitant pathology and abnormal laboratory values are contraindications to perform EBP. In presence of these limitations, we sought for a non-interventional management of PDPH utilizing high-flow oxygen and pro-serotonin agents. We reviewed the mechanism of action of this therapy METHODS: To illustrate our proposal, we report a series of twelve consecutive patients with PDPH treated with high-flow oxygen therapy at 12 L/min via a non-rebreathing mask and intravenous metoclopramide.

RESULTS

All patients were treated with this conservative therapy, no adverse reactions were observed. After the intervention, the headache resolved without further indications for PDPH.

CONCLUSION

Our series suggests that combining high-flow oxygen and pro-serotonin agents such metoclopramide in the ED might be a feasible option as effective as the invasive methods used in treating PDPH. This therapy appears to be efficient and to minimize risk, cost and side effects. It presents an easily accessible alternative that should be considered when PDPH is not a viable option.

Dietary supplementation with grape seed extract prevents development of trigeminal sensitization and inhibits pain signaling in a preclinical chronic temporomandibular disorder model

BACKGROUND

The risk factors neck muscle tension, prolonged jaw opening, and female gender are associated with developing temporomandibular disorders (TMD), which are characterized by persistent sensitization of trigeminal neurons and enhanced pain signaling. Dietary supplementation with a grape seed extract (GSE) can modulate expression of proteins that decrease neuronal excitability and trigeminal sensitization.

METHODS

Mechanical nocifensive thresholds over the masseter were determined using von Frey filaments in male and female adult Sprague Dawley rats. To promote trigeminal sensitization, animals were injected with complete Freund's adjuvant in the upper trapezius. After 8 days, animals were subjected to near maximal jaw opening and head withdrawal responses were determined for 28 days. Some animals received continuous supplementation with 0.5% GSE in their drinking water two weeks prior to trapezius injections.

RESULTS

Prolonged jaw opening increased the average number of nocifensive responses to mechanical stimuli for 14 days in males and females. However, trapezius inflammation prior to jaw opening promoted persistent mechanical sensitivity up to 28 days post-jaw opening in females, while in males nociceptive levels were still elevated at day 21. Supplementation with GSE, which is enriched in polyphenols and exhibits antioxidant and COX-2 activity, inhibited trigeminal nociception in response to jaw opening in both male and female sensitized animals.

CONCLUSIONS

Our findings provide evidence that multiple risk factors contribute to the development of a prolonged state of trigeminal sensitization that is more severe in females and provide preclinical evidence that supplementation with GSE could be beneficial in the management of TMD.

Tryptophan Metabolites Along the Microbiota-Gut-Brain Axis: An Interkingdom Communication System Influencing the Gut in Health and Disease

The ‘microbiota-gut-brain axis’ plays a fundamental role in maintaining host homeostasis, and different immune, hormonal, and neuronal signals participate to this interkingdom communication system between eukaryota and prokaryota. The essential aminoacid tryptophan, as a precursor of several molecules acting at the interface between the host and the microbiota, is fundamental in the modulation of this bidirectional communication axis. In the gut, tryptophan undergoes 3 major metabolic pathways, the 5-HT, kynurenine, and AhR ligand pathways, which may be directly or indirectly controlled by the saprophytic flora. The importance of tryptophan metabolites in the modulation of the gastrointestinal tract is suggested by several preclinical and clinical studies; however, a thorough revision of the available literature has not been accomplished yet. Thus, this review attempts to cover the major aspects on the role of tryptophan metabolites in host-microbiota cross-talk underlaying regulation of gut functions in health conditions and during disease states, with particular attention to 2 major gastrointestinal diseases, such as irritable bowel syndrome (IBS) and inflammatory bowel disease (IBD), both characterized by psychiatric disorders. Research in this area opens the possibility to target tryptophan metabolism to ameliorate the knowledge on the pathogenesis of both diseases, as well as to discover new therapeutic strategies based either on conventional pharmacological approaches or on the use of pre- and probiotics to manipulate the microbial flora.

The preemptive analgesia of pre-electroacupuncture in rats with formalin-induced acute inflammatory pain

Background

Electroacupuncture has been elicited to effectively alleviate the pain sensation. Preemptive analgesic effect of pre-electroacupuncture has also been suggested in recent studies, while the underlying analgesic mechanism of pre-electroacupuncture requires further investigation. This study aimed to explore the preemptive analgesia of pre-electroacupuncture in formalin-induced acute inflammatory pain model.

Methods

Forty rats were randomly divided into control, model, pre-electroacupuncture, and post-electroacupuncture group. Inflammatory pain model was induced via injecting 50 µl 5% formalin into the plantar surface of right hind paw, while the equal volume of saline injection in the control group. Rats in the pre-electroacupuncture group were treated with electroacupuncture at ipsilateral Zusanli (ST36) and Weizhong (BL40) acupoints (2 Hz, 1 mA) for 30 min before formalin injection, while received the same electroacupuncture treatment immediately after formalin injection in the post-electroacupuncture group. Flinching number and licking time were recorded during 60 min after formalin injection. Immunofluorescence and Western blot were used to detect the expression of ionized calcium binding adapter molecule 1 (Iba1) and c-fos in spinal cord. Moreover, enzyme-linked immunosorbent assay was applied to measure the secretion of IL-6, IFN-γ, IL-4, substance P, and calcitonin gene-related peptide in spinal cord.

Results

Paw flinching and licking were obviously induced by formalin injection. Iba1, c-fos, proinflammatory cytokines (IL-6 and IFN-γ), and pain neurotransmitters (substance P and calcitonin gene-related peptide) were dramatically increased in the L4-5 spinal cord after formalin injection, while anti-inflammatory cytokine IL-4 was decreased. Pre-electroacupuncture and post-electroacupuncture administration significantly attenuated formalin-induced nociceptive effects, spinal microglia and neurons activation, proinflammatory cytokines and pain neurotransmitters upregulation, and upregulated the anti-inflammatory cytokine. Furthermore, these effects of pre-electroacupuncture were more significant than that of post-electroacupuncture.

Conclusions

This study illustrates the potential therapeutic effect of pre-electroacupuncture against acute inflammatory pain and reveals the mechanism underlying pre-electroacupuncture mediated analgesia, thus providing a novel preemptive analgesic treatment.

Spinal Mobilization Prevents NGF-Induced Trunk Mechanical Hyperalgesia and Attenuates Expression of CGRP

Introduction

Low back pain (LBP) is a complex and growing global health problem in need of more effective pain management strategies. Spinal mobilization (SM) is a non-pharmacological approach recommended by most clinical guidelines for LBP, but greater utilization and treatment optimization are hampered by a lack of mechanistic knowledge underlying its hypoalgesic clinical effects.

Methods

Groups of female Sprague-Dawley rats received unilateral trunk (L5 vertebral level) injections (50 μl) of either vehicle (phosphate-buffer solution, PBS; VEH) or nerve growth factor (NGF; 0.8 μM) on Days 0 and 5 with or without daily L5 SM (VEH, NGF, VEH + SM, VEH + SM). Daily passive SM (10 min) was delivered by a feedback motor (1.2 Hz, 0.9N) from Days 1 to 12. Changes in pain assays were determined for mechanical and thermal reflexive behavior, exploratory behavior (open field events) and spontaneous pain behavior (rat grimace scale). On Day 12, lumbar (L1–L6) dorsal root ganglia (DRG) were harvested bilaterally and calcitonin gene-related peptide (CGRP) positive immunoreactive neurons were quantified from 3 animals (1 DRG tissue section per segmental level) per experimental group.

Results

NGF induced bilateral trunk (left P = 0.006, right P = 0.001) mechanical hyperalgesia and unilateral hindpaw allodynia (P = 0.006) compared to the vehicle group by Day 12. Additionally, we found for the first time that NGF animals demonstrated decreased exploratory behaviors (total distance traveled) and increased grimace scale scoring compared to the VEH group. Passive SM prevented this development of local (trunk) mechanical hyperalgesia and distant (hindpaw) allodynia, and normalized grimace scale scores. NGF increased CGRP positive immunoreactive neurons in ipsilateral lumbar DRGs compared to the VEH group ([L1]P = 0.02; [L2]P = 0.007) and SM effectively negated this increase in pain-related neuropeptide CGRP expression.

Conclusion

SM prevents the development of local (trunk) NGF-induced mechanical hyperalgesia and distant (hindpaw) allodynia, in part, through attenuation of CGRP expression in lumbar DRG sensory neurons. NGF decreases rat exploratory behavior and increases spontaneous pain for which passive SM acts to mitigate these pain-related behavioral changes. These initial study findings suggest that beginning daily SM soon after injury onset might act to minimize or prevent the development of LBP by reducing production of pain-related neuropeptides.

Metabolomics in Central Sensitivity Syndromes

Central sensitization syndromes are a collection of frequently painful disorders that contribute to decreased quality of life and increased risk of opiate abuse. Although these disorders cause significant morbidity, they frequently lack reliable diagnostic tests. As such, technologies that can identify key moieties in central sensitization disorders may contribute to the identification of novel therapeutic targets and more precise treatment options. The analysis of small molecules in biological samples through metabolomics has improved greatly and may be the technology needed to identify key moieties in difficult to diagnose diseases. In this review, we discuss the current state of metabolomics as it relates to central sensitization disorders. From initial literature review until Feb 2020, PubMed, Embase, and Scopus were searched for applicable studies. We included cohort studies, case series, and interventional studies of both adults and children affected by central sensitivity syndromes. The majority of metabolomic studies addressing a CSS found significantly altered metabolites that allowed for differentiation of CSS patients from healthy controls. Therefore, the published literature overwhelmingly supports the use of metabolomics in CSS. Further research into these altered metabolites and their respective metabolic pathways may provide more reliable and effective therapeutics for these syndromes.

Neuroprotective Effect of Enriched Chicken Bone Broth as a Dietary Supplement in a Model of Migraine Mediated by Early Life Stress

Early life stress is a risk factor for development of migraine, a prevalent painful neurological disease characterized by sensitization and activation of trigeminal neurons. Secondary early life stress was previously shown to cause increased expression of neuronal proteins implicated in peripheral and central sensitization. Recently, dietary supplementation of chicken bone broth was shown to attenuate trigeminal nociception in an orofacial pain model. Accordingly, the focus of this study was to determine the effects of early life stress and dietary inclusion of bone broth on trigeminal nociceptor sensitization and activation in a model of episodic migraine. Adult Sprague-Dawley male sender rats subjected to primary traumatic stress were placed next to breeding or pregnant female rats that served as receiver rats (secondary traumatic stress) and in proximity to the offspring until weaning. Unstressed and stressed young adult offspring were tested for mechanical nocifensive response after exposure to a pungent odor known to be a migraine trigger, and in response to daily supplementation of bone broth. Early life stress promoted a primed state of trigeminal nociceptors that were activated by the pungent odor in both genders. Female animals exhibited a higher basal sensitization level and prolonged nociception compared with males. Supplementation of bone broth beginning at the time of weaning inhibited basal and triggered trigeminal mechanical sensitivity. Early life stress caused development of a sensitized trigeminal system that is implicated in migraine pathology and dietary supplementation with bone broth suppressed trigeminal sensitization, and thus may provide neuroprotective activity for reducing migraine risk.

Hemokinin-1 Gene Expression Is Upregulated in Trigeminal Ganglia in an Inflammatory Orofacial Pain Model: Potential Role in Peripheral Sensitization

A large percentage of primary sensory neurons in the trigeminal ganglia (TG) contain neuropeptides such as tachykinins or calcitonin gene-related peptide. Neuropeptides released from the central terminals of primary afferents sensitize the secondary nociceptive neurons in the trigeminal nucleus caudalis (TNC), but also activate glial cells contributing to neuroinflammation and consequent sensitization in chronic orofacial pain and migraine. In the present study, we investigated the newest member of the tachykinin family, hemokinin-1 (HK-1) encoded by the Tac4 gene in the trigeminal system. HK-1 had been shown to participate in inflammation and hyperalgesia in various models, but its role has not been investigated in orofacial pain or headache. In the complete Freund’s adjuvant (CFA)-induced inflammatory orofacial pain model, we showed that Tac4 expression increased in the TG in response to inflammation. Duration-dependent Tac4 upregulation was associated with the extent of the facial allodynia. Tac4 was detected in both TG neurons and satellite glial cells (SGC) by the ultrasensitive RNAscope in situ hybridization. We also compared gene expression changes of selected neuronal and glial sensitization and neuroinflammation markers between wild-type and Tac4-deficient (Tac4^-/-) mice. Expression of the SGC/astrocyte marker in the TG and TNC was significantly lower in intact and saline/CFA-treated Tac4^-/- mice. The procedural stress-related increase of the SGC/astrocyte marker was also strongly attenuated in Tac4^-/- mice. Analysis of TG samples with a mouse neuroinflammation panel of 770 genes revealed that regulation of microglia and cytotoxic cell-related genes were significantly different in saline-treated Tac4^-/- mice compared to their wild-types. It is concluded that HK-1 may participate in neuron-glia interactions both under physiological and inflammatory conditions and mediate pain in the trigeminal system.

Microglia P2X4R-BDNF signalling contributes to central sensitization in a recurrent nitroglycerin-induced chronic migraine model

Background

According to our previous study, microglia P2X4 receptors (P2X4Rs) play a pivotal role in the central sensitization of chronic migraine (CM). However, the molecular mechanism that underlies the crosstalk between microglia P2X4Rs and neurons of the trigeminal nucleus caudalis (TNC) is not fully understood. Therefore, the aim of this study is to examine the exact P2X4Rs signalling pathway in the development of central sensitization in a CM animal model.

Methods

We used an animal model with recurrent intermittent administration of nitroglycerin (NTG), which closely mimics CM. NTG-induced basal mechanical and thermal hypersensitivity were evaluated using a von Frey filament test and an increasing-temperature hot plate apparatus (IITC). We detected P2X4Rs, brain-derived neurotrophic factor (BDNF) and phosphorylated p38 mitogen-activated protein kinase (p-p38-MAPK) expression profiles in the TNC. We investigated the effects of a P2X4R inhibitor (5-BDBD) and an agonist (IVM) on NTG-induced hyperalgesia and neurochemical changes as well as on the expression of p-p38-MAPK and BDNF. We also detected the effects of a tropomyosin-related kinase B (TrkB) inhibitor (ANA-12) on the CM animal model in vivo. Then, we evaluated the effect of 5-BDBD and SB203580 (a p38-MAPK inhibitors) on the release and synthesis of BDNF in BV2 microglia cells treated with 50 μM adenosine triphosphate (ATP).

Results

Chronic intermittent administration of NTG resulted in chronic mechanical and thermal hyperalgesia, accompanied by the upregulation of P2X4Rs and BDNF expression. 5-BDBD or ANA-12 prevented hyperalgesia induced by NTG, which was associated with a significant inhibition of the NTG-induced increase in phosphorylated extracellular regulated protein kinases (p-ERK) and calcitonin gene related peptide (CGRP) release in the TNC. Repeated administration of IVM produced sustained hyperalgesia and significantly increased the levels of p-ERK and CGRP release in the TNC. Activating P2X4Rs with ATP triggered BDNF release and increased BDNF synthesis in BV2 microglia, and these results were then reduced by 5-BDBD or SB203580.

Conclusions

Our results indicated that the P2X4R contributes to the central sensitization of CM by releasing BDNF and promoting TNC neuronal hyper-excitability. Blocking microglia P2X4R-BDNF signalling may have an effect on the prevention of migraine chronification.

Translational research of temporomandibular joint pathology: a preliminary biomarker and fMRI study

BACKGROUND

The temporomandibular joint (TMJ) is well innervated by braches of the trigeminal nerve. The temporomandibular joint disorders (TMD) can cause neural-inflammation in the peripheral nervous system (PNS) at the site of injury, or compression, and may have systemic effects on the central nervous system (CNS). Neural-inflammation causes elevations in cytokine expression and microglia activation. When the site of injury, or compression is treated, or relieved, neural inflammation is reduced. These changes can be seen and measured with fMRI brain activities.

METHODS

For this study, patients with comorbid TMD and systemic/neurologic conditions were compared using clinical diagnostic markers, inflammatory, pain, tissue destruction enzymatic biomarkers, and functional magnetic resonance imaging (fMRI) activity of the brain, with and without a custom-made dental orthotic.

RESULTS

Our results showed a correlation between the clinical diagnosis of the pathological TMJ, biomarkers and the fMRI study. There was a marked elevation of biomarkers in samples taken from TMJ of patients who were clinically diagnosed with TMD. The fMRI study of TMD patients showed an abnormal hyper-connected salience network and a diminished blood flow to the anterior frontal lobes when they did not wear their customized dental orthotics.

CONCLUSIONS

Our findings highlight the importance of TMJ-CNS connections and use of fMRI as an investigative tool for understanding TMD and its related neurological pathologies.

Activation of the RAS/B-RAF-MEK-ERK pathway in satellite glial cells contributes to substance p-mediated orofacial pain

The cross talk between trigeminal ganglion (TG) neurons and satellite glial cells (SGCs) is crucial for the regulation of inflammatory orofacial pain. Substance P (SP) plays an important role by activating neurokinin (NK)-I receptors in this cross talk. The activation of extracellular signal-regulated kinase (ERK) 1/2, protein kinase A (PKA) and protein kinase C (PKC) in neurons and SGCs of peripheral ganglions by peripheral inflammation is associated with inflammatory hypersensitivity. This study tested the hypothesis that SP evoked SP-NK-I receptor positive feedback via the Renin-Angiotensin System/B-Protein Kinase A-Rapidly Accelerates Fibrosarcoma-MEK-Extracellular Signal-Regulated Kinase (RAS/PKA-RAF-MEK-ERK) pathway, which is involved in pain hypersensitivity. Inflammatory models were induced in vivo by injecting Complete Freund's adjuvant (CFA) into the whisker pad of rats. SP was administrated to SGCs in vitro for investigating, whether SP regulates the expression of NK-I receptor in the SGC nucleus. The effects of RAS-RAF-MEK, PKA and PKC pathways in this process were measured by co-incubating SGCs with respective Raf, PKA, PKC and MEK inhibitors in vitro and by pre-injecting these inhibitors into the TG in vivo. SP significantly upregulated NK-I receptor, p-ERK1/2, Ras, B-Raf, PKA and PKC in SGCs under inflammatory conditions. In addition, L703,606 (NK-I receptor antagonist), U0126 (MEK inhibitor), Sorafenib (Raf inhibitor) and H892HCL (PKA inhibitor) but not chelerythrine chloride (PKC inhibitor) significantly decreased NK-I mRNA and protein levels induced by SP. The allodynia-related behavior evoked by CFA was inhibited by pre-injection of L703,606, U0126, Sorafenib and H892HCL into the TG. Overall, SP upregulates NK-I receptor in TG SGCs via PKA/RAS-RAF-MEK-ERK pathway activation, contributing to a positive feedback of SP-NK-I receptor in inflammatory orofacial pain.

Plasma Levels of Oxidative Stress Markers, before and after BoNT/A Treatment, in Chronic Migraine

The pathophysiological mechanisms of migraine transformation are debated. Modifications of plasma oxidative stress biomarkers have been described in chronic migraine. OnabotulintoxinA (BoNT/A) treatment, approved for chronic migraine prophylaxis, possibly reduces pain neurotransmitters release and oxidative stress products. Aims of our study were to investigate differences in the levels of selected plasmatic oxidative stress biomarkers (Advanced Oxidation Protein Products (AOPP), Ferric Reducing Antioxidant Power (FRAP), Thiolic Groups (SH)) comparing chronic migraineurs (CM) and healthy controls (HC). We also explored possible clinical and biochemical modifications in the CM group after six months of treatment with BoNT/A. At the baseline, we found higher values of AOPP (p < 0.001), and lower values of SH (p < 0.001) and FRAP (p = 0.005) in the CM group. At the six-month follow-up we found a reduction of AOPP (p < 0.001) and an increase of FRAP (p < 0.001) and SH (p = 0.023) within the CM group. BoNT/A treatment improved migraine symptoms in the CM group. We confirmed previous reports of imbalanced antioxidant mechanisms in chronic migraine showing lower antioxidant capacities in patients than controls. BoNT/A improved the levels of plasma oxidative stress biomarkers and confirmed its role as an effective prophylactic treatment for CM. Other studies should investigate the potential antioxidant properties of BoNT/A treatment.

A Prospective Observational Cohort Study on Pharmacological Habitus, Headache-Related Disability and Psychological Profile in Patients with Chronic Migraine Undergoing OnabotulinumtoxinA Prophylactic Treatment

Chronic Migraine (CM) is a disabling neurologic condition with a severe impact on functioning and quality of life. Successful therapeutic management of patients with CM is complex, and differences in therapeutic response could be attributable to genetically determined factors, sensitivity to pharmacological treatment, psychosocial and relational factors affecting the patient’s compliance and approach on the therapeutic treatment. The aim of this prospective observational study was to explore self-efficacy, coping strategies, psychological distress and headache-related disability in a cohort of 40 patients with CM (mean age: 46.73; standard deviation 13.75) treated with OnabotulinumtoxinA and the relationship between these clinical and psychological aspects and acute medication consumption during OnabotulinumtoxinA prophylactic treatment. Patients presented an overall significant reduction in the Headache Index (HI) (p < 0.001), HI with severe intensity (p = 0.009), and total analgesic consumption (p = 0.003) after the prophylactic treatment. These results are in line with the literature. Despite this, higher nonsteroidal anti-inflammatory drugs consumption was associated with higher psychological distress, higher HI with severe and moderate intensity, and worse quality of life. Conversely, triptans consumption was correlated with HI of mild intensity, and problem-focused coping strategies. To conclude, the psychological profile, and in particular, the psychological distress and specific coping strategies might influence the self-management of acute medication.

Berberine prevents stress-induced gut inflammation and visceral hypersensitivity and reduces intestinal motility in rats

BACKGROUND

Irritable bowel syndrome (IBS) is a common chronic non-organic disease of the digestive system. Berberine (BBR) has been used to treat patients with IBS, but the underlying therapeutic mechanism is little understood. We believe that BBR achieves its therapeutic effect on IBS by preventing stress intestinal inflammation and visceral hypersensitivity and reducing bowel motility.

AIM

To test the hypothesis that BBR achieves its therapeutic effect on IBS by preventing subclinical inflammation of the intestinal mucosa and reducing visceral hypersensitivity and intestinal motility.

METHODS

IBS was induced in rats via water avoidance stress (WAS). qRT-PCR and histological analyses were used to evaluate the levels of cytokines and mucosal inflammation, respectively. Modified ELISA and qRT-PCR were used to evaluate the nuclear factor kappa-B (NF-κB) signal transduction pathway. Colorectal distention test, gastrointestinal transit measurement, Western blot, and qRT-PCR were used to analyze visceral sensitivity, intestinal motility, the expression of C-kit (marker of Cajal mesenchymal cells), and the expression of brain derived neurotrophic factor (BDNF) and its receptor TrkB.

RESULTS

WAS led to mucosal inflammation, visceral hyperalgesia, and high intestinal motility. Oral administration of BBR inhibited the NF-κB signal transduction pathway, reduced the expression of pro-inflammatory cytokines [interleukin (IL)-1β, IL-6, interferon-γ, and tumor necrosis factor-α], promoted the expression of anti-inflammatory cytokines (IL-10 and transforming growth factor-β), and improved the terminal ileum tissue inflammation. BBR inhibited the expression of BDNF, TrkB, and C-kit in IBS rats, leading to the reduction of intestinal motility and visceral hypersensitivity. The therapeutic effect of BBR at a high dose (100 mg/kg) was superior to than that of the low-dose (25 mg/kg) group.

CONCLUSION

BBR reduces intestinal mucosal inflammation by inhibiting the intestinal NF-κB signal pathway in the IBS rats. BBR reduces the expression of BDNF, its receptor TrkB, and C-kit. BBR also reduces intestinal motility and visceral sensitivity to achieve its therapeutic effect on IBS.

Electrical stimulation of the superior sagittal sinus suppresses A-type K+ currents and increases P/Q- and T-type Ca2+ currents in rat trigeminal ganglion neurons

BACKGROUND

Migraine is a debilitating neurological disorder involving abnormal trigeminovascular activation and sensitization. However, the underlying cellular and molecular mechanisms remain unclear.

METHODS

A rat model of conscious migraine was established through the electrical stimulation (ES) of the dural mater surrounding the superior sagittal sinus. Using patch clamp recording, immunofluorescent labelling, enzyme-linked immunosorbent assays and western blot analysis, we studied the effects of ES on sensory neuronal excitability and elucidated the underlying mechanisms mediated by voltage-gated ion channels.

RESULTS

The calcitonin gene-related peptide (CGRP) level in the jugular vein blood and the number of CGRP-positive neurons in the trigeminal ganglia (TGs) were significantly increased in rats with ES-induced migraine. The application of ES increased actional potential firing in both small-sized IB₄-negative (IB₄^-) and IB₄⁺ TG neurons. No significant changes in voltage-gated Na⁺ currents were observed in the ES-treated groups. ES robustly suppressed the transient outward K⁺ current (I_A) in both types of TG neurons, while the delayed rectifier K⁺ current remained unchanged. Immunoblot analysis revealed that the protein expression of Kv4.3 was significantly decreased in the ES-treated groups, while Kv1.4 remained unaffected. Interestingly, ES increased the P/Q-type and T-type Ca²⁺ currents in small-sized IB₄^- TG neurons, while there were no significant changes in the IB₄⁺ subpopulation of neurons.

CONCLUSION

These results suggest that ES decreases the I_A in small-sized TG neurons and increases P/Q- and T-type Ca²⁺ currents in the IB₄^- subpopulation of TG neurons, which might contribute to neuronal hyperexcitability in a rat model of ES-induced migraine.

Targeting Neuropathic Pain: Pathobiology, Current Treatment and Peptidomimetics as a New Therapeutic Opportunity

There is a huge need for pharmaceutical agents for the treatment of chronic Neuropathic Pain (NP), a complex condition where patients can suffer from either hyperalgesia or allodynia originating from central or peripheral nerve injuries. To date, the therapeutic guidelines include the use of tricyclic antidepressants, serotonin-noradrenaline reuptake inhibitors and anticonvulsants, beside the use of natural compounds and non-pharmacological options. Unfortunately, these drugs suffer from limited efficacy and serious dose-dependent adverse effects. In the last decades, the heptapeptide SP1-7, the major bioactive metabolite produced by Substance P (SP) cleavage, has been extensively investigated as a potential target for the development of novel peptidomimetic molecules to treat NP. Although the physiological effects of this SP fragment have been studied in detail, the mechanism behind its action is not fully clarified and the target for SP1-7 has not been identified yet. Nevertheless, specific binding sites for the heptapeptide have been found in brain and spinal cord of both mouse and rats. Several Structure-Affinity Relationship (SAR) studies on SP1-7 and some of its synthetic analogues have been carried out aiming to developing more metabolically stable and effective small molecule SP1-7-related amides that could be used as research tools for a better understanding of the SP1-7 system and, in a longer perspective, as potential therapeutic agents for future treatment of NP.

Revisiting Tramadol: A Multi-Modal Agent for Pain Management

Tramadol-an atypical opioid analgesic-has a unique pharmacokinetic and pharmacodynamic profile, with opioidergic, noradrenergic, and serotonergic actions. Tramadol has long been used as a well-tolerated alternative to other drugs in moderate pain because of its opioidergic and monoaminergic activities. However, cumulative evidence has been gathered over the last few years that supports other likely mechanisms and uses of tramadol in pain management. Tramadol has modulatory effects on several mediators involved in pain signaling, such as voltage-gated sodium ion channels, transient receptor potential V1 channels, glutamate receptors, α₂-adrenoceptors, adenosine receptors, and mechanisms involving substance P, calcitonin gene-related peptide, prostaglandin E₂, and proinflammatory cytokines. Tramadol also modifies the crosstalk between neuronal and non-neuronal cells in peripheral and central sites. Through these molecular effects, tramadol could modulate peripheral and central neuronal hyperexcitability. Given the broad spectrum of molecular targets, tramadol as a unimodal analgesic relieves a broad range of pain types, such as postoperative, low back, and neuropathic pain and that associated with labor, osteoarthritis, fibromyalgia, and cancer. Moreover, tramadol has anxiolytic, antidepressant, and anti-shivering activities that could improve pain management outcomes. The aim of this review was to address these issues in the context of maladaptive physiological and psychological processes that are associated with different pain types.

Motor cortex and pain control: exploring the descending relay analgesic pathways and spinal nociceptive neurons in healthy conscious rats

Motor cortex stimulation (MCS) is an effective therapy for refractory neuropathic pain. MCS increases the nociceptive threshold in healthy rats via endogenous opioids, inhibiting thalamic nuclei and activating the periaqueductal gray. It remains unclear how the motor cortex induces top-down modulation of pain in the absence of persistent pain. Here, we investigated the main nuclei involved in the descending analgesic pathways and the spinal nociceptive neurons in rats that underwent one session of MCS and were evaluated with the paw pressure nociceptive test. The pattern of neuronal activation in the dorsal raphe nucleus (DRN), nucleus raphe magnus (NRM), locus coeruleus (LC), and dorsal horn of the spinal cord (DHSC) was assessed by immunoreactivity (IR) for Egr-1 (a marker of activated neuronal nuclei). IR for serotonin (5HT) in the DRN and NRM, tyrosine hydroxylase (TH) in the LC, and substance P (SP) and enkephalin (ENK) in the DHSC was also evaluated. MCS increased the nociceptive threshold of the animals; this increase was accompanied by activation of the NRM, while DRN activation was unchanged. However, cortical stimulation induced an increase in 5HT-IR in both serotonergic nuclei. MCS did not change the activation pattern or TH-IR in the LC, and it inhibited neuronal activation in the DHSC without altering SP or ENK-IR. Taken together, our results suggest that MCS induces the activation of serotonergic nuclei as well as the inhibition of spinal neurons, and such effects may contribute to the elevation of the nociceptive threshold in healthy rats. These results allow a better understanding of the circuitry involved in the antinociceptive top-down effect induced by MCS under basal conditions, reinforcing the role of primary motor cortex in pain control.

Glutamatergic Signaling Along The Microbiota-Gut-Brain Axis

A complex bidirectional communication system exists between the gastrointestinal tract and the brain. Initially termed the “gut-brain axis” it is now renamed the “microbiota-gut-brain axis” considering the pivotal role of gut microbiota in maintaining local and systemic homeostasis. Different cellular and molecular pathways act along this axis and strong attention is paid to neuroactive molecules (neurotransmitters, i.e., noradrenaline, dopamine, serotonin, gamma aminobutyric acid and glutamate and metabolites, i.e., tryptophan metabolites), sustaining a possible interkingdom communication system between eukaryota and prokaryota. This review provides a description of the most up-to-date evidence on glutamate as a neurotransmitter/neuromodulator in this bidirectional communication axis. Modulation of glutamatergic receptor activity along the microbiota-gut-brain axis may influence gut (i.e., taste, visceral sensitivity and motility) and brain functions (stress response, mood and behavior) and alterations of glutamatergic transmission may participate to the pathogenesis of local and brain disorders. In this latter context, we will focus on two major gut disorders, such as irritable bowel syndrome and inflammatory bowel disease, both characterized by psychiatric co-morbidity. Research in this area opens the possibility to target glutamatergic neurotransmission, either pharmacologically or by the use of probiotics producing neuroactive molecules, as a therapeutic approach for the treatment of gastrointestinal and related psychiatric disorders.

CGRP-based Migraine Therapeutics: How Might They Work, Why So Safe, and What Next?

Migraine is a debilitating neurological condition that involves the neuropeptide calcitonin gene-related peptide (CGRP). An exciting development is the recent FDA approval of the first in an emerging class of CGRP-targeted drugs designed to prevent migraine. Yet despite this efficacy, there are some fundamental unanswered questions, such as where and how CGRP works in migraine. Preclinical data suggest that CGRP acts via both peripheral and central mechanisms. The relevance of peripheral sites is highlighted by the clinical efficacy of CGRP-blocking antibodies, even though they do not appreciably cross the blood-brain barrier. The most likely sites of action are within the dura and trigeminal ganglia. Furthermore, it would be foolish to ignore perivascular actions in the dura since CGRP is the most potent vasodilatory peptide. Ultimately, the consequence of blocking CGRP or its receptor is reduced peripheral neural sensitization. Underlying their efficacy is the question of why the antibodies have such an excellent safety profile so far. This may be due to the presence of a second CGRP receptor and vesicular release of a large bolus of peptides. Finally, despite the promise of these drugs, there are unmet gaps because they do not work for all patients; so what next? We can expect advances on several fronts, including CGRP receptor structures that may help development of centrally-acting antagonists, combinatorial treatments that integrate other therapies, and development of drugs that target other neuropeptides. This is truly an exciting time for CGRP and the migraine field with many more discoveries on the horizon.

Plasma Calcitonin Gene-Related Peptide: A Potential Biomarker for Diagnosis and Therapeutic Responses in Pediatric Migraine

Background: Plasma calcitonin gene-related peptide (CGRP) plays a key role in the migraine pathophysiology. This study aimed to investigate its role in predicting diagnosis and outcome of pharmacotherapy in pediatric migraine.

Methods: We prospectively recruited 120 subjects, who never took migraine-preventive agents in a pediatric clinic, including 68 patients with migraine, 30 with non-migraine headache (NM), and 22 non-headache (NH) age-matched controls. Short-term therapeutic response was measured for at least 2 weeks after the start of therapy. Responders were defined with >50% headache reduction. Plasma CGRP concentrations were measured by ELISA.

Results: In the migraine group, more patients required acute therapy, as compared to the NM group (62/68, 91% vs. 5/30, 15%, p = 0.001). The mean plasma CGRP level in migraineurs either during (291 ± 60 pg/ml) or between (240 ± 48) attacks was higher than in NM patients (51 ± 5 pg/ml, p = 0.006 and 0.018, respectively) and NH controls (53 ± 6 pg/ml, p = 0.016 and 0.045, respectively). Forty-seven patients (69%) needed preventive treatments and had higher plasma CGRP levels (364 ± 62 pg/ml, n = 47) than those not (183 ± 54 pg/ml, n = 21) (p = 0.031). Topiramate responders had higher plasma CGRP levels than non-responders (437 ± 131 pg/ml, n = 14 vs. 67 ± 19 pg/ml, n = 6, p = 0.021). Survival curves of plasma CGRP levels also showed those with higher CGRP levels responded better to topiramate. Differences were not found in the other preventives.

Conclusion: The plasma CGRP level can differentiate migraine from non-migraine headache. It may also serve as a reference for the therapeutic strategy since it is higher in patients requiring migraine prevention and responsive to short-term topiramate treatment. These results are clinically significant, especially for the young children who cannot clearly describe their headache symptoms and may provide new insights into the clinical practice for the diagnosis and treatment of pediatric migraine.

The effects of Bisphenol A (BPA) on sympathetic nerve fibers in the uterine wall of the domestic pig

Bisphenol A (BPA), used in the production of plastic, shows multidirectional negative effects on the living organism. BPA may affect the reproductive and nervous systems; however, its influence on the nerves supplying the uterus has not been studied. During the present study, the impact of BPA on the sympathetic nerves in the uterus was investigated using a double immunofluorescence technique. The results have shown that even low doses of BPA may change the neurochemical characterization of uterine sympathetic nerves, and the severity of these changes depends on the part of the uterus and the dose of the toxic substance. Probably the changes observed during the present study resulted from the neurotoxic and/or pro-inflammatory activity of BPA, but the exact mechanism for the observed fluctuation still remains unknown. The fluctuations of the neurochemical characterization of the uterine intramural nerves may be the first subclinical signs of harmful exposure to BPA.

Erenumab-aooe

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Central nervous system neuroplasticity and the sensitization of hypertension

The causes of essential hypertension remain an enigma. Interactions between genetic and external factors are generally recognized to act as aetiological mechanisms that trigger the pathogenesis of high blood pressure. However, the questions of which genes and factors are involved, and when and where such interactions occur, remain unresolved. Emerging evidence indicates that the hypertensive response to pressor stimuli, like many other physiological and behavioural adaptations, can become sensitized to particular stimuli. Studies in animal models show that, similarly to other response systems controlled by the brain, hypertensive response sensitization (HTRS) is mediated by neuroplasticity. The brain circuitry involved in HTRS controls the sympathetic nervous system. This Review outlines evidence supporting the phenomenon of HTRS and describes the range of physiological and psychosocial stressors that can produce a sensitized hypertensive state. Also discussed are the cellular and molecular changes in the brain neural network controlling sympathetic tone involved in long-term storage of information relating to stressors, which could serve to maintain a sensitized state. Finally, this Review concludes with a discussion of why a sensitized hypertensive response might previously have been beneficial and increased biological fitness under some environmental conditions and why today it has become a health-related liability.

Expression of ASIC3 in the Trigeminal Nucleus Caudalis Plays a Role in a Rat Model of Recurrent Migraine

Acid-sensing ion channel 3 (ASIC3) is abundant in the trigeminal nervous system and is most sensitive to a slight pH decrease. Recent studies have indicated that ASIC3 in the peripheral trigeminal ganglia is likely involved in the pathogenesis of migraine pain. However, it is unclear whether this receptor plays a role in recurrent migraine, namely, migraine chronicity. Here, we aimed to investigate the role of ASIC3 in an animal model of recurrent migraine (RM). In this study, we established a rat model of RM through repeated administration of inflammatory soup (IS) onto the dura. Then, we tested the mechanical pain thresholds of the face and hindpaws by von Frey filaments. qRT-PCR, Western blot and immunofluorescence labelling were used to detect the expression and localization of ASIC3 in the trigeminal nucleus caudalis (TNC). The protein levels of calcitonin gene-related peptide (CGRP), its receptor component receptor activity modifying protein 1 (RAMP1) and c-Fos were analysed following treatment with the ASIC3 inhibitor APETx2 and activator 2-guanidine-4-methylquinazoline (GMQ). We found decreased pain thresholds after repeated dural inflammatory stimulation, which suggested the establishment of an RM model. Based on this model, we observed elevated expression of ASIC3 in the TNC group compared to that in the Sham group. ASIC3 was primarily expressed in neurons but not in astrocytes of the TNC. Moreover, APETx2 attenuated tactile allodynia and significantly decreased the expression of c-Fos, CGRP and RAMP1, while GMQ aggravated these effects compared to those observed in the IS + vehicle group. These findings indicate a critical role of ASIC3 channels in the pathophysiology of RM, and ASIC3 might represent a potential therapeutic target to prevent the progression of migraine.