TRPA1/NOX in the soma of trigeminal ganglion neurons mediates migraine-related pain of glyceryl trinitrate in mice

The gut microbiome modulates nitroglycerin-induced migraine-related hyperalgesia in mice

BACKGROUND

Gut microbiota disturbance is increasingly suggested to be involved in the pathogenesis of migraine but this connection remains unsubstantiated. This study aimed to investigate whether the gut microbiome influences migraine-related hyperalgesia.

METHODS

Nitroglycerin-induced hyperalgesia was evaluated in mice with different gut microbiota statuses as follows: Specific pathogen-free mice; germ-free mice; specific pathogen-free mice treated with antibiotics to deplete the gut microbiome (ABX mice); and germ-free mice transplanted with the gut microbial profile from specific pathogen-free mice (GFC mice). Moreover, nitroglycerin-induced hyperalgesia was compared between recipient mice transplanted with gut microbiota from a patient with migraine and those that received gut microbiota from a sex- and age-matched healthy control.

RESULTS

In specific pathogen-free mice, a decreased mechanical threshold in the hind paw, increased grooming time, increased c-Fos expression level and decreased calcitonin gene-related peptide expression level as well as increased tumor necrosis factor-α concentration in the trigeminal nucleus caudalis were observed after nitroglycerin administration compared with saline treatment. However, increased basal sensitivity and higher basal concentrations of TNF-α in the trigeminal nucleus caudalis were observed in germ-free and ABX mice, while no significant difference in hyperalgesia was observed between the nitroglycerin group and saline group in germ-free and ABX mice. Moreover, significant hyperalgesia was induced by nitroglycerin administration in GFC mice. The mice transplanted with the gut microbial profile from a patient with migraine had more severe nitroglycerin-induced hyperalgesia than the mice receiving microbiota from a matched healthy control.

CONCLUSION

Our findings highlight the involvement of the gut microbiome in normal mechanical pain sensation and pathogenesis of migraine.

Higenamine Attenuates Neuropathic Pain by Inhibition of NOX2/ROS/TRP/P38 Mitogen-Activated Protein Kinase/NF-ĸB Signaling Pathway

Oxidative stress damage is known as one of the important factors that induce neuropathic pain (NP). Using antioxidant therapy usually achieves an obvious curative effect and alleviates NP. Previous pharmacological studies have shown that higenamine (Hig) performs to be antioxidant and anti-inflammatory. However, the protective effect and mechanism of Hig on NP are still unclear. This study mainly evaluated the changes in reactive oxygen species (ROS) level, lipid peroxidation, and antioxidant system composed of superoxide dismutase (SOD) and glutathione (GSH) through chronic constrict injury (CCI) model rats and t-BHP-induced Schwann cell (SC) oxidative stress model. The expressions of two inflammatory factors, tumor necrosis factor-alpha (TNF-α) and interleukin-6 (IL-6), were also assessed. The possible molecular mechanism of Hig in the treatment of NP was explored in conjunction with the expression of mitochondrial apoptosis pathway and NOX2/ROS/TRP/P38 mitogen-activated protein kinase (MAPK)/NF-ĸB pathway-related indicators. Hig showed substantial antioxidant and anti-inflammatory properties both in vivo and in vitro. Hig significantly reduced the upregulated levels of ROS, malondialdehyde (MDA), TNF-α, and IL-6 and increased the levels of SOD and GSH, which rebalanced the redox system and improved the survival rate of cells. In the animal behavioral test, it was also observed that Hig relieved the CCI-induced pain, indicating that Hig had a pain relief effect. Our research results suggested that Hig improved NP-induced oxidative stress injury, inflammation, and apoptosis, and this neuroprotective effect may be related to the NOX2/ROS/TRP/P38 MAPK/NF-ĸB signaling pathway.

Periorbital Nociception in a Progressive Multiple Sclerosis Mouse Model Is Dependent on TRPA1 Channel Activation

Headaches are frequently described in progressive multiple sclerosis (PMS) patients, but their mechanism remains unknown. Transient receptor potential ankyrin 1 (TRPA1) was involved in neuropathic nociception in a model of PMS induced by experimental autoimmune encephalomyelitis (PMS-EAE), and TRPA1 activation causes periorbital and facial nociception. Thus, our purpose was to observe the development of periorbital mechanical allodynia (PMA) in a PMS-EAE model and evaluate the role of TRPA1 in periorbital nociception. Female PMS-EAE mice elicited PMA from day 7 to 14 days after induction. The antimigraine agents olcegepant and sumatriptan were able to reduce PMA. The PMA was diminished by the TRPA1 antagonists HC-030031, A-967079, metamizole and propyphenazone and was absent in TRPA1-deficient mice. Enhanced levels of TRPA1 endogenous agonists and NADPH oxidase activity were detected in the trigeminal ganglion of PMS-EAE mice. The administration of the anti-oxidants apocynin (an NADPH oxidase inhibitor) or alpha-lipoic acid (a sequestrant of reactive oxygen species), resulted in PMA reduction. These results suggest that generation of TRPA1 endogenous agonists in the PMS-EAE mouse model may sensitise TRPA1 in trigeminal nociceptors to elicit PMA. Thus, this ion channel could be a potential therapeutic target for the treatment of headache in PMS patients.

CGRP-dependent signalling pathways involved in mouse models of GTN- cilostazol- and levcromakalim-induced migraine

BACKGROUND

Knowledge of exact signalling events during migraine attacks is lacking. Various substances are known to trigger migraine attacks in patients and calcitonin gene-related peptide antagonising drugs are effective against migraine pain. Here, we investigated the signalling pathways involved in three different mouse models of provoked migraine and relate them to calcitonin gene-related peptide and other migraine-relevant targets.

METHODS

In vivo mouse models of glyceryl trinitrate-, cilostazol- and levcromakalim-induced migraine were applied utilising tactile sensitivity to von Frey filaments as measuring readout. Signalling pathways involved in the three models were dissected by use of specific knockout mice and chemical inhibitors. In vivo results were supported by ex vivo wire myograph experiments measuring arterial dilatory responses and ex vivo calcitonin gene-related peptide release from trigeminal ganglion and trigeminal nucleus caudalis from mice.

RESULTS

Glyceryl trinitrate-induced hypersensitivity was dependent on both prostaglandins and transient receptor potential cation channel, subfamily A, member 1, whereas cilostazol- and levcromakalim-induced hypersensitivity were independent of both. All three migraine triggers activated calcitonin gene-related peptide signalling, as both receptor antagonism and antibody neutralisation of calcitonin gene-related peptide were effective inhibitors of hypersensitivity in all three models. Stimulation of trigeminal ganglia and brain stem tissue samples with cilostazol and levcromakalim did not result in release of calcitonin gene-related peptide, and vasodilation following levcromakalim stimulation was independent of CGRP receptor antagonism.

CONCLUSION

The mouse models of glyceryl trinitrate-, cilostazol- and levcromakalim- induced migraine all involve calcitonin gene-related peptide signalling in a complex interplay between different cell/tissue types. These models are useful in the study of migraine mechanisms.

Involvement of TRPM2 in the Neurobiology of Experimental Migraine: Focus on Oxidative Stress and Apoptosis

Excessive Ca²⁺ influx and mitochondrial oxidative stress (OS) of trigeminal ganglia (TG) have essential roles in the etiology of migraine headache and aura. The stimulation of TRPM2 channel via the generation of OS and ADP-ribose (ADPR) induces pain, inflammatory, and oxidative neurotoxicity, although its inhibition reduces the intensity of pain and neurotoxicity in several neurons. However, the cellular and molecular effects of TRPM2 in the TG of migraine model (glyceryl trinitrate, GTN) on the induction of pain, OS, apoptosis, and inflammation remain elusive. GTN-mediated increases of pain intensity, apoptosis, death, cytosolic reactive oxygen species (ROS), mitochondrial ROS, caspase -3, caspase -9, cytosolic Ca²⁺ levels, and cytokine generations (TNF-α, IL-1β, and IL-6) in the TG of TRPM2 wild-type mouse were further increased by the TRPM2 activation, although they were modulated by the treatments of GSH, PARP-1 inhibitors (PJ34 and DPQ), and TRPM2 blockers (ACA and 2APB). However, the effects of GTN were not observed in the TG of TRPM2 knockout mice. The current data indicate that the maintaining activation of TRPM2 is not only important for the quenching OS, inflammation, and neurotoxicity in the TG neurons of mice with experimental migraine but also equally critical to the modulation of GTN-induced pain.

Selective targeting of peripheral cannabinoid receptors prevents behavioral symptoms and sensitization of trigeminal neurons in mouse models of migraine and medication overuse headache

ABSTRACT

Migraine affects ∼15% of the world's population greatly diminishing their quality of life. Current preventative treatments are effective in only a subset of migraine patients, and although cannabinoids seem beneficial in alleviating migraine symptoms, central nervous system side effects limit their widespread use. We developed peripherally restricted cannabinoids (PRCBs) that relieve chronic pain symptoms of cancer and neuropathies, without appreciable central nervous system side effects or tolerance development. Here, we determined PRCB effectiveness in alleviating hypersensitivity symptoms in mouse models of migraine and medication overuse headache. Long-term glyceryl trinitrate (GTN, 10 mg/kg) administration led to increased sensitivity to mechanical stimuli and increased expression of phosphorylated protein kinase A, neuronal nitric oxide synthase, and transient receptor potential ankyrin 1 proteins in trigeminal ganglia. Peripherally restricted cannabinoid pretreatment, but not posttreatment, prevented behavioral and biochemical correlates of GTN-induced sensitization. Low pH-activated and allyl isothiocyanate-activated currents in acutely isolated trigeminal neurons were reversibly attenuated by PRCB application. Long-term GTN treatment significantly enhanced these currents. Long-term sumatriptan treatment also led to the development of allodynia to mechanical and cold stimuli that was slowly reversible after sumatriptan discontinuation. Subsequent challenge with a previously ineffective low-dose GTN (0.1-0.3 mg/kg) revealed latent behavioral sensitization and increased expression of phosphorylated protein kinase A, neuronal nitric oxide synthase, and transient receptor potential ankyrin 1 proteins in trigeminal ganglia. Peripherally restricted cannabinoid pretreatment prevented all behavioral and biochemical correlates of allodynia and latent sensitization. Importantly, long-term PRCB treatment alone did not produce any behavioral or biochemical signs of sensitization. These data validate peripheral cannabinoid receptors as potential therapeutic targets in migraine and medication overuse headache.

Targeting reactive nitroxidative species in preclinical models of migraine

BACKGROUND

Reactive nitroxidative species, such as nitric oxide but particularly peroxynitrite, have been strongly implicated in pain mechanisms. Targeting peroxynitrite is anti-nociceptive in pain models, but little is known about its role in migraine mechanisms. Given the need to validate novel targets for migraine headache, our objective was to study the potential of reactive nitroxidative species, particularly peroxynitrite, as novel targets for drug discovery and their role in migraine mechanisms.

METHODS

We recorded neuronal activity in rats with extracellular electrodes and examined the effects of targeting nitric oxide or peroxynitrite on ongoing and cranial-evoked firing rates of central trigeminocervical neurons. We injected calcitonin gene-related peptide (which produces migraine-like headache in migraineurs) and characterized neuronal responses to cranial stimulation and on behavioral responses to nociceptive periorbital stimulation and determined the effects of targeting reactive nitroxidative species on the mediated changes.

RESULTS

L-NAME (nitric oxide synthase inhibitor) and Fe(III)5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrinato chloride (FeTPPS; peroxynitrite decomposition catalyst) inhibited ongoing and dural-evoked responses of trigeminocervical neurons, without affecting normal facial-cutaneous responses. Calcitonin gene-related peptide caused activation and sensitization of dural-responsive trigeminovascular neurons with hypersensitivity to intracranial and extracranial stimulation, and reduction of periorbital withdrawal thresholds. Only the peroxynitrite decomposition catalyst prevented these neuronal and behavioral nociceptive responses.

DISCUSSION

The data support that calcitonin gene-related peptide mediates the underlying neurobiological mechanisms related to the development of migraine-like headache. They also confirm the role of nitric oxide and implicate peroxynitrite production along the trigeminovascular migraine pathway in these mechanisms. The data also support peroxynitrite as a novel and potentially effective target for migraine treatment. The current drug development focus on peroxynitrite decomposition catalysts for chronic pain disorders should therefore extend to migraine.

GPR151 in nociceptors modulates neuropathic pain via regulating P2X3 function and microglial activation

Neuropathic pain is a major health problem that affects up to 7-10% of the population worldwide. Currently, neuropathic pain is difficult to treat due to its elusive mechanisms. Here we report that orphan G protein-coupled receptor 151 (GPR151) in nociceptive sensory neurons controls neuropathic pain induced by nerve injury. GPR151 was mainly expressed in nonpeptidergic C-fiber dorsal root ganglion (DRG) neurons and highly upregulated after nerve injury. Importantly, conditional knockout of Gpr151 in adult nociceptive sensory neurons significantly alleviated chronic constriction injury (CCI)-induced neuropathic pain-like behavior but did not affect basal nociception. Moreover, GPR151 in DRG neurons was required for CCI-induced neuronal hyperexcitability and upregulation of colony-stimulating factor 1 (CSF1), which is necessary for microglial activation in the spinal cord after nerve injury. Mechanistically, GPR151 coupled with P2X3 ion channels and promoted their functional activities in neuropathic pain-like hypersensitivity. Knockout of Gpr151 suppressed P2X3-mediated calcium elevation and spontaneous pain behavior in CCI mice. Conversely, overexpression of Gpr151 significantly enhanced P2X3-mediated calcium elevation and DRG neuronal excitability. Furthermore, knockdown of P2X3 in DRGs reversed CCI-induced CSF1 upregulation, spinal microglial activation, and neuropathic pain-like behavior. Finally, the co-expression of GPR151 and P2X3 was confirmed in small-diameter human DRG neurons, indicating the clinical relevance of our findings. Together, our results suggest that GPR151 in nociceptive DRG neurons plays a key role in the pathogenesis of neuropathic pain and could be a potential target for treating neuropathic pain.

Single-Dose Toxicity Study on ML171, a Selective NOX1 Inhibitor, in Mice

Background

ML171 is a potent nicotinamide adenine dinucleotide phosphate oxidase (NOX) inhibitor with isoform selectivity only for NOX1. This study is aimed at investigating the safety of ML171 after a single intraperitoneal (IP) injection in mice.

Methods

The toxicity of a single dose of ML171 was evaluated in 6-week-old Institute of Cancer Research (ICR) mice in a good laboratory practice (GLP) laboratory. Twenty-five mice of each sex were assigned to five groups: negative control, vehicle control, and 125, 250, and 500 mg/kg of ML171. All mice were acclimatized for one week before beginning the study. Mice received an IP injection of ML171 or vehicle. The general condition and mortality of the animals were observed. The mice were sacrificed to evaluate histopathology 14 days after the administration of ML171 or vehicle.

Results

Bodyweights were not significantly different in any group. Three males and one female died due to ML171 administration in the 500 mg/kg dose group. Autopsies of the surviving mice did not reveal any significant abnormalities after the injection of 125 mg/kg of ML171. However, the anterior lobe edge of the liver was thickened and adhesions between the liver and adjacent organs were observed in mice treated with 250 or 500 mg/kg of ML171. In addition, hypertrophy of centrilobular hepatocytes and inflammatory cell infiltration were observed after injection of 250 and 500 mg/kg of ML171.

Conclusion

Our results indicate that the lethal IP injection dose of ML171 is 500 mg/kg for both males and females. Mortality were not observed for lower doses of ML171. The safe dose of single IP ML171 in ICR mice was 250 mg/kg or less. Further studies are needed to confirm the safety of ML171 in the human body.

Nox4-dependent upregulation of S100A4 after peripheral nerve injury modulates neuropathic pain processing

Previous studies suggested that reactive oxygen species (ROS) produced by NADPH oxidase 4 (Nox4) affect the processing of neuropathic pain. However, mechanisms underlying Nox4-dependent pain signaling are incompletely understood. In this study, we aimed to identify novel Nox4 downstream interactors in the nociceptive system. Mice lacking Nox4 specifically in sensory neurons were generated by crossing Advillin-Cre mice with Nox4^fl/fl mice. Tissue-specific deletion of Nox4 in sensory neurons considerably reduced mechanical hypersensitivity and neuronal action potential firing after peripheral nerve injury. Using a proteomic approach, we detected various proteins that are regulated in a Nox4-dependent manner after injury, including the small calcium-binding protein S100A4. Immunofluorescence staining and Western blot experiments confirmed that S100A4 expression is massively up-regulated in peripheral nerves and dorsal root ganglia after injury. Furthermore, mice lacking S100A4 showed increased mechanical hypersensitivity after peripheral nerve injury and after delivery of a ROS donor. Our findings suggest that S100A4 expression is up-regulated after peripheral nerve injury in a Nox4-dependent manner and that deletion of S100A4 leads to an increased neuropathic pain hypersensitivity.

G protein-coupled receptor GPR151 is involved in trigeminal neuropathic pain through the induction of Gβγ/extracellular signal-regulated kinase-mediated neuroinflammation in the trigeminal ganglion

ABSTRACT

Trigeminal nerve injury-induced neuropathic pain is a debilitating chronic orofacial pain syndrome but lacks effective treatment. G protein-coupled receptors (GPCRs), especially orphan GPCRs (oGPCRs) are important therapeutic targets in pain medicine. Here, we screened upregulated oGPCRs in the trigeminal ganglion (TG) after partial infraorbital nerve transection (pIONT) and found that Gpr151 was the most significantly upregulated oGPCRs. Gpr151 mRNA was increased from pIONT day 3 and maintained for more than 21 days. Furthermore, GPR151 was expressed in the neurons of the TG after pIONT. Global mutation or knockdown of Gpr151 in the TG attenuated pIONT-induced mechanical allodynia. In addition, the excitability of TG neurons was increased after pIONT in wild-type (WT) mice, but not in Gpr151-/- mice. Notably, GPR151 bound to Gαi protein, but not Gαq, Gα12, or Gα13, and activated the extracellular signal-regulated kinase (ERK) through Gβγ. Extracellular signal-regulated kinase was also activated by pIONT in the TG of WT mice, but not in Gpr151-/- mice. Gene microarray showed that Gpr151 mutation reduced the expression of a large number of neuroinflammation-related genes that were upregulated in WT mice after pIONT, including chemokines CCL5, CCL7, CXCL9, and CXCL10. The mitogen-activated protein kinase inhibitor (PD98059) attenuated mechanical allodynia and reduced the upregulation of these chemokines after pIONT. Collectively, this study not only revealed the involvement of GPR151 in the maintenance of trigeminal neuropathic pain but also identified GPR151 as a Gαi-coupled receptor to induce ERK-dependent neuroinflammation. Thus, GPR151 may be a potential drug target for the treatment of trigeminal neuropathic pain.

Transient receptor potential ankyrin 1 channel: An evolutionarily tuned thermosensor

The discovery of the role of the transient receptor potential ankyrin 1 (TRPA1) channel as a polymodal detector of cold and pain-producing stimuli almost two decades ago catalyzed the consequent identification of various vertebrate and invertebrate orthologues. In different species, the role of TRPA1 has been implicated in numerous physiological functions, indicating that the molecular structure of the channel exhibits evolutionary flexibility. Until very recently, information about the critical elements of the temperature-sensing molecular machinery of thermosensitive ion channels such as TRPA1 had lagged far behind information obtained from mutational and functional analysis. Current developments in single-particle cryo-electron microscopy are revealing precisely how the thermosensitive channels operate, how they might be targeted with drugs, and at which sites they can be critically regulated by membrane lipids. This means that it is now possible to resolve a huge number of very important pharmacological, biophysical and physiological questions in a way we have never had before. In this review, we aim at providing some of the recent knowledge on the molecular mechanisms underlying the temperature sensitivity of TRPA1. We also demonstrate how the search for differences in temperature and chemical sensitivity between human and mouse TRPA1 orthologues can be a useful approach to identifying important domains with a key role in channel activation.

Oxidative stress induced by NOX2 contributes to neuropathic pain via plasma membrane translocation of PKCε in rat dorsal root ganglion neurons

Background

Nicotinamide adenine dinucleotide phosphate oxidase 2 (NOX2)-induced oxidative stress, including the production of reactive oxygen species (ROS) and hydrogen peroxide, plays a pivotal role in neuropathic pain. Although the activation and plasma membrane translocation of protein kinase C (PKC) isoforms in dorsal root ganglion (DRG) neurons have been implicated in multiple pain models, the interactions between NOX2-induced oxidative stress and PKC remain unknown.

Methods

A spared nerve injury (SNI) model was established in adult male rats. Pharmacologic intervention and AAV-shRNA were applied locally to DRGs. Pain behavior was evaluated by Von Frey tests. Western blotting and immunohistochemistry were performed to examine the underlying mechanisms. The excitability of DRG neurons was recorded by whole-cell patch clamping.

Results

SNI induced persistent NOX2 upregulation in DRGs for up to 2 weeks and increased the excitability of DRG neurons, and these effects were suppressed by local application of gp91-tat (a NOX2-blocking peptide) or NOX2-shRNA to DRGs. Of note, the SNI-induced upregulated expression of PKCε but not PKC was decreased by gp91-tat in DRGs. Mechanical allodynia and DRG excitability were increased by ψεRACK (a PKCε activator) and reduced by εV1-2 (a PKCε-specific inhibitor). Importantly, εV1-2 failed to inhibit SNI-induced NOX2 upregulation. Moreover, the SNI-induced increase in PKCε protein expression in both the plasma membrane and cytosol in DRGs was attenuated by gp91-tat pretreatment, and the enhanced translocation of PKCε was recapitulated by H₂O₂ administration. SNI-induced upregulation of PKCε was blunted by phenyl-N-tert-butylnitrone (PBN, an ROS scavenger) and the hydrogen peroxide catalyst catalase. Furthermore, εV1-2 attenuated the mechanical allodynia induced by H₂O₂

Conclusions

NOX2-induced oxidative stress promotes the sensitization of DRGs and persistent pain by increasing the plasma membrane translocation of PKCε.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12974-021-02155-6.

Therapeutic targeting of nitroglycerin-mediated trigeminovascular neuronal hypersensitivity predicts clinical outcomes of migraine abortives

ABSTRACT

Cranial hypersensitivity is a prominent symptom of migraine, exhibited as migraine headache exacerbated with physical activity, and cutaneous facial allodynia and hyperalgesia. The underlying mechanism is believed to be, in part, activation and sensitization of dural-responsive trigeminocervical neurons. Validated preclinical models that exhibit this phenotype have great utility for understanding putative mechanisms and as a tool to screen therapeutics. We have previously shown that nitroglycerin triggers cranial allodynia in association with migraine-like headache, and this translates to neuronal cranial hypersensitivity in rats. Furthermore, responses in both humans and rats are aborted by triptan administration, similar to responses in spontaneous migraine. Here, our objective was to study the nitroglycerin model examining the effects on therapeutic targets with newly approved treatments, specifically gepants and ditans, for the acute treatment of migraine. Using electrophysiological methods, we determined changes to ongoing firing and somatosensory-evoked cranial sensitivity, in response to nitroglycerin, followed by treatment with a calcitonin gene-related peptide receptor antagonist, gepant (olcegepant), a 5-HT1F receptor agonist, ditan (LY344864), and an NK1 receptor antagonist (GR205171). Nitroglycerin induced activation of migraine-like central trigeminocervical neurons, and intracranial and extracranial neuronal hypersensitivity. These responses were aborted by olcegepant and LY344864. However, GR205171, which failed in clinical trial for both abortive and preventive treatment of migraine, had no effect. These data support the nitroglycerin model as a valid approach to study cranial hypersensitivity and putative mechanisms involved in migraine and as a screen to dissect potentially efficacious migraine therapeutic targets.

Increase in trigeminal ganglion neurons that respond to both calcitonin gene-related peptide and pituitary adenylate cyclase-activating polypeptide in mouse models of chronic migraine and posttraumatic headache

A large body of animal and human studies indicates that blocking peripheral calcitonin gene-related peptide (CGRP) and pituitary adenylate cyclase-activating polypeptide (PACAP) signaling pathways may prevent migraine episodes and reduce headache frequency. To investigate whether recurring migraine episodes alter the strength of CGRP and PACAP signaling in trigeminal ganglion (TG) neurons, we compared the number of TG neurons that respond to CGRP and to PACAP (CGRP-R and PACAP-R, respectively) under normal and chronic migraine-like conditions. In a mouse model of chronic migraine, repeated nitroglycerin (NTG) administration significantly increased the number of CGRP-R and PACAP-R neurons in TG but not dorsal root ganglia. In TG neurons that express endogenous αCGRP, repeated NTG led to a 7-fold increase in the number of neurons that respond to both CGRP and PACAP (CGRP-R&PACAP-R). Most of these neurons were unmyelinated C-fiber nociceptors. This suggests that a larger fraction of CGRP signaling in TG nociceptors may be mediated through the autocrine mechanism, and the release of endogenous αCGRP can be enhanced by both CGRP and PACAP signaling pathways under chronic migraine condition. The number of CGRP-R&PACAP-R TG neurons was also increased in a mouse model of posttraumatic headache (PTH). Interestingly, low-dose interleukin-2 treatment, which completely reverses chronic migraine-related and PTH-related behaviors in mouse models, also blocked the increase in both CGRP-R and PACAP-R TG neurons. Together, these results suggest that inhibition of both CGRP and PACAP signaling in TG neurons may be more effective in treating chronic migraine and PTH than targeting individual signaling pathways.

Brain Energy Deficit as a Source of Oxidative Stress in Migraine: A Molecular Basis for Migraine Susceptibility

People with migraine are prone to a brain energy deficit between attacks, through increased energy demand (hyperexcitable brain) or decreased supply (mitochondrial impairment). However, it is uncertain how this precipitates an acute attack. Here, the central role of oxidative stress is adduced. Specifically, neurons' antioxidant defenses rest ultimately on internally generated NADPH (reduced nicotinamide adenine dinucleotide phosphate), whose levels are tightly coupled to energy production. Mitochondrial NADPH is produced primarily by enzymes involved in energy generation, including isocitrate dehydrogenase of the Krebs (tricarboxylic acid) cycle; and an enzyme, nicotinamide nucleotide transhydrogenase (NNT), that depends on the Krebs cycle and oxidative phosphorylation to function, and that works in reverse, consuming antioxidants, when energy generation fails. In migraine aura, cortical spreading depression (CSD) causes an initial severe drop in level of NADH (reduced nicotinamide adenine dinucleotide), causing NNT to impair antioxidant defense. This is followed by functional hypoxia and a rebound in NADH, in which the electron transport chain overproduces oxidants. In migraine without aura, a similar biphasic fluctuation in NADH very likely generates oxidants in cortical regions farthest from capillaries and penetrating arterioles. Thus, the perturbations in brain energy demand and/or production seen in migraine are likely sufficient to cause oxidative stress, triggering an attack through oxidant-sensing nociceptive ion channels. Implications are discussed for the development of new classes of migraine preventives, for the current use of C57BL/6J mice (which lack NNT) in preclinical studies of migraine, for how a microembolism initiates CSD, and for how CSD can trigger a migraine.

The NLRP3 inflammasome: an emerging therapeutic target for chronic pain

Chronic pain affects the life quality of the suffering patients and posts heavy problems to the health care system. Conventional medications are usually insufficient for chronic pain management and oftentimes results in many adverse effects. The NLRP3 inflammasome controls the processing of proinflammatory cytokine interleukin 1β (IL-1β) and is implicated in a variety of disease conditions. Recently, growing number of evidence suggests that NLRP3 inflammasome is dysregulated under chronic pain condition and contributes to pathogenesis of chronic pain. This review provides an up-to-date summary of the recent findings of the involvement of NLRP3 inflammasome in chronic pain and discussed the expression and regulation of NLRP3 inflammasome-related signaling components in chronic pain conditions. This review also summarized the successful therapeutic approaches that target against NLRP3 inflammasome for chronic pain treatment.

The Role of TRPA1 in Skin Physiology and Pathology

The transient receptor potential ankyrin 1 (TRPA1), a member of the TRP superfamily of channels, acts as 'polymodal cellular sensor' on primary sensory neurons where it mediates the peripheral and central processing of pain, itch, and thermal sensation. However, the TRPA1 expression extends far beyond the sensory nerves. In recent years, much attention has been paid to its expression and function in non-neuronal cell types including skin cells, such as keratinocytes, melanocytes, mast cells, dendritic cells, and endothelial cells. TRPA1 seems critically involved in a series of physiological skin functions, including formation and maintenance of physico-chemical skin barriers, skin cells, and tissue growth and differentiation. TRPA1 appears to be implicated in mechanistic processes in various immunological inflammatory diseases and cancers of the skin, such as atopic and allergic contact dermatitis, psoriasis, bullous pemphigoid, cutaneous T-cell lymphoma, and melanoma. Here, we report recent findings on the implication of TRPA1 in skin physiology and pathophysiology. The potential use of TRPA1 antagonists in the treatment of inflammatory and immunological skin disorders will be also addressed.

Insulin-like growth factor-1 inhibits nitroglycerin-induced trigeminal activation of oxidative stress, calcitonin gene-related peptide and c-Fos expression

Migraineurs experience increased oxidative stress which drives the initiation and maintenance of migraine-related pain in animal models and, by extension, migraine in humans. Oxidative stress augments calcitonin gene-related peptide (CGRP) levels, a mediator of migraine pain. Insulin-like growth factor-1 (IGF-1), a neuroprotective growth factor, reduces susceptibility to spreading depression, a preclinical model of migraine, in cultured brain slices by blocking oxidative stress and neuroinflammation from microglia. Similarly, nasal delivery of IGF-1 inhibits spreading depression in vivo. After recurrent cortical spreading depression, nasal administration of IGF-1 also significantly reduces trigeminal ganglion oxidative stress and CGRP levels as well as trigeminocervical c-Fos activation. Here, we probed for the impact of nasal IGF-1 pretreatment on trigeminal system activation using a second well-established preclinical model of migraine, systemic nitroglycerin injection. Adult male rats were treated with one of three doses of IGF-1 (37.5, 75 or 150 μg) and the optimal dose found in males was subsequently used for treatment of female rats. One day later, animals received an intraperitoneal injection of nitroglycerin. Measurements taken two hours later after nitroglycerin alone showed increased surrogate markers of trigeminal activation - oxidative stress and CGRP in the trigeminal ganglion and c-Fos in the trigeminocervical complex compared to vehicle control. These effects were significantly reduced at all doses of IGF-1 for trigeminal ganglion metrics of oxidative stress and CGRP and only at the lowest dose in both males and females for c-Fos. The latter inverted U-shaped or hormetic response is seen in enzyme-targeting drugs. While the specific mechanisms remain to be explored, our data here supports the ability of IGF-1 to preserve mitochondrial and antioxidant pathway homeostasis as means to prevent nociceptive activation in the trigeminal system produced by an experimental migraine model.

Experimentally induced spinal nociceptive sensitization increases with migraine frequency: a single-blind controlled study

The comparison of low-frequency migraine with high-frequency migraine after nitroglycerin administration shows progression in the degree of derangement of spinal nociception processing.

The nitric-oxide donor nitroglycerin (NTG) administration induces a facilitation of nociceptive pathways in episodic migraine. This study aims to test the hypothesis that induced spinal sensitization could be more pronounced in patients affected by high-frequency migraine (HF-MIG) with respect to low-frequency migraine (LF-MIG). We enrolled 28 patients with LF-MIG (1-5 migraine days/month), 19 patients with HF-MIG (6-14 migraine days/month), and 21 healthy controls (HCs). Spinal sensitization was evaluated with the neurophysiological recording of the temporal summation threshold (TST) of the nociceptive withdrawal reflex at the lower limb. Temporal summation threshold was recorded at baseline and 30, 60, and 120 minutes after NTG administration (0.9 mg sublingual). Spinal sensitization was detected in LF-MIG at 60 (P = 0.010) and 120 minutes (P = 0.001) and in HF-MIG at 30 (P = 0.008), 60 (P = 0.001), and 120 minutes (P = 0.001) after NTG administration. Temporal summation threshold did not change in HC (P = 0.899). Moreover, TST reduction was more pronounced in HF-MIG with respect to LF-MIG (P = 0.002). The percentage of patients who developed a migraine-like headache after NTG was comparable in the 2 migraine groups (LF-MIG: 53.6%, HF-MIG: 52.6%, P = 0.284), whereas no subjects in the HC group developed a delayed-specific headache. Notably, the latency of headache onset was significantly shorter in the HF-MIG group when compared with the LF-MIG group (P = 0.015). Our data demonstrate a direct relationship between migraine frequency and both neurophysiological and clinical parameters, to suggest an increasing derangement of the nociceptive system control as the disease progresses, probably as a result of the interaction of genetic and environmental factors.

Implications of Transient Receptor Potential Cation Channels in Migraine Pathophysiology

Migraine is a common and debilitating headache disorder. Although its pathogenesis remains elusive, abnormal trigeminal and central nervous system activity is likely to play an important role. Transient receptor potential (TRP) channels, which transduce noxious stimuli into pain signals, are expressed in trigeminal ganglion neurons and brain regions closely associated with the pathophysiology of migraine. In the trigeminal ganglion, TRP channels co-localize with calcitonin gene-related peptide, a neuropeptide crucially implicated in migraine pathophysiology. Many preclinical and clinical data support the roles of TRP channels in migraine. In particular, activation of TRP cation channel V1 has been shown to regulate calcitonin gene-related peptide release from trigeminal nerves. Intriguingly, several effective anti-migraine therapies, including botulinum neurotoxin type A, affect the functions of TRP cation channels. Here, we discuss currently available data regarding the roles of major TRP cation channels in the pathophysiology of migraine and the therapeutic applicability thereof.

Pharmacological Characterization of Orofacial Nociception in Female Rats Following Nitroglycerin Administration

Rodent models of human disease can be valuable for understanding the mechanisms of a disease and for identifying novel therapies. However, it is critical that these models be vetted prior to committing resources to developing novel therapeutics. Failure to confirm the model can lead to significant losses in time and resources. One model used for migraine headache is to administer nitroglycerin to rodents. Nitroglycerin is known to produce migraine-like pain in humans and is presumed to do the same in rodents. It is not known, however, if the mechanism for nitroglycerin headaches involves the same pathological processes as migraine. In the absence of known mechanisms, it becomes imperative that the model not only translates into successful clinical trials but also successfully reverse translates by demonstrating efficacy of current therapeutics. In this study female rats were given nitroglycerin and nociception was evaluated in OPADs. Estrous was not monitored. Based on the ED₅₀ of nitroglycerin a dose of 10 mg/kg was used for experiments. Sumatriptan, caffeine, buprenorphine and morphine were administered to evaluate the reverse translatability of the model. We found that nitroglycerin did not produce mechanical allodynia in the face of the rats, which is reported to be a consequence of migraine in humans. Nitroglycerin reduced the animals’ participation in the assay. The reduced activity was verified using an assay to measure exploratory behavior. Furthermore, the effects of nitroglycerin were not reversed or prevented by agents that are effective acute therapies for migraine. Two interesting findings from this study, however, were that morphine and nitroglycerin interact to increase the rats’ tolerance of mechanical stimuli on their faces, and they work in concert to slow down the central motor pattern generator for licking on the reward bottle. These interactions suggest that nitroglycerin generated nitric oxide and mu opioid receptors interact with the same neuronal circuits in an additive manner. The interaction of nitroglycerin and morphine on sensory and motor circuits deserves additional examination. In conclusion, based on the results of this study the use of nitroglycerin at these doses in naïve female rats is not recommended as a model for migraine headaches.

Protective Effects of Hydrogen Sulfide Against the ATP-Induced Meningeal Nociception

We previously showed that extracellular ATP and hydrogen sulfide (H₂S), a recently discovered gasotransmitter, are both triggering the nociceptive firing in trigeminal nociceptors implicated in migraine pain. ATP contributes to meningeal nociception by activating the P2X3 subunit-containing receptors whereas H₂S operates mainly via TRP receptors. However, H₂S was also proposed as a neuroprotective and anti-nociceptive agent. This study aimed to test the effect of H₂S on ATP-mediated nociceptive responses in rat meningeal afferents and trigeminal neurons and on ATP-induced degranulation of dural mast cells. Electrophysiological recording of trigeminal nerve activity in meninges was supplemented by patch-clamp and calcium imaging studies of isolated trigeminal neurons. The H₂S donor NaHS induced a mild activation of afferents and fully suppressed the subsequent ATP-induced firing of meningeal trigeminal nerve fibers. This anti-nociceptive effect of H₂S was specific as an even stronger effect of capsaicin did not abolish the action of ATP. In isolated trigeminal neurons, NaHS decreased the inward currents and calcium transients evoked by activation of ATP-gated P2X3 receptors. Moreover, NaHS prevented ATP-induced P2X7 receptor-mediated degranulation of meningeal mast cells which emerged as triggers of migraine pain. Finally, NaHS decreased the concentration of extracellular ATP in the meningeal preparation. Thus, H₂S exerted the multiple protective actions against the nociceptive effects of ATP. These data highlight the novel pathways to reduce purinergic mechanisms of migraine with pharmacological donors or by stimulation production of endogenous H₂S.

TRPA1 as a therapeutic target for nociceptive pain

Introduction

Chronic pain affects approximatively 30–50% of the population globally. Pathologies such as migraine, diabetic neuropathy, nerve injury and treatment with chemotherapeutic agents, can induce chronic pain. Members of the transient receptor potential (TRP) channels, including the TRP ankyrin 1 (TRPA1), have a major role in pain.

Areas covered

We focus on TRPA1 as a therapeutic target for pain relief. The structure, localization, and activation of the channel and its implication in different pathways to signal pain are described. This paper underlines the role of pharmacological interventions on TRPA1 to reduce pain in numerous pain conditions. We conducted a literature search in PubMed up to and including July 2020.

Expert opinion

Our understanding of the molecular mechanisms underlying the sensitization of central and peripheral nociceptive pathways is limited. Preclinical evidence indicates that, in murine models of pain diseases, numerous mechanisms converge on the pathway that encompasses oxidative stress and Schwann cell TRPA1 to sustain chronic pain. Programs to identify and develop treatments to attenuate TRPA1-mediated chronic pain have emerged from this knowledge. Antagonists explored as a novel class of analgesics have a new and promising target in the TRPA1 expressed by peripheral glial cells.

TRPA1 mediates damage of the retina induced by ischemia and reperfusion in mice

Oxidative stress is implicated in retinal cell injury associated with glaucoma and other retinal diseases. However, the mechanism by which oxidative stress leads to retinal damage is not completely understood. Transient receptor potential ankyrin 1 (TRPA1) is a redox-sensitive channel that, by amplifying the oxidative stress signal, promotes inflammation and tissue injury. Here, we investigated the role of TRPA1 in retinal damage evoked by ischemia (1 hour) and reperfusion (I/R) in mice. In wild-type mice, retinal cell numbers and thickness were reduced at both day-2 and day-7 after I/R. By contrast, mice with genetic deletion of TRPA1 were protected from the damage seen in their wild-type littermates. Daily instillation of eye drops containing two different TRPA1 antagonists, an oxidative stress scavenger, or a NADPH oxidase-1 inhibitor also protected the retinas of C57BL/6J mice exposed to I/R. Mice with genetic deletion of the proinflammatory TRP channels, vanilloid 1 (TRPV1) or vanilloid 4 (TRPV4), were not protected from I/R damage. Surprisingly, genetic deletion or pharmacological blockade of TRPA1 also attenuated the increase in the number of infiltrating macrophages and in the levels of the oxidative stress biomarker, 4-hydroxynonenal, and of the apoptosis biomarker, active caspase-3, evoked by I/R. These findings suggest that TRPA1 mediates the oxidative stress burden and inflammation that result in murine retinal cell death. We also found that TRPA1 (both mRNA and protein) is expressed by human retinal cells. Thus, it is possible that inhibition of a TRPA1-dependent pathway could also attenuate glaucoma-related retinal damage.

Macrophages and Schwann cell TRPA1 mediate chronic allodynia in a mouse model of complex regional pain syndrome type I

Complex regional pain syndrome type I (CRPS-I) is characterized by intractable chronic pain. Poor understanding of the underlying mechanisms of CRPS-I accounts for the current unsatisfactory treatment. Antioxidants and antagonists of the oxidative stress-sensitive channel, the transient receptor potential ankyrin 1 (TRPA1), have been found to attenuate acute nociception and delayed allodynia in models of CRPS-I, evoked by ischemia and reperfusion (I/R) of rodent hind limb (chronic post ischemia pain, CPIP). However, it is unknown how I/R may lead to chronic pain mediated by TRPA1. Here, we report that the prolonged (day 1-15) mechanical and cold allodynia in the hind limb of CPIP mice was attenuated permanently in Trpa1^-/- mice and transiently after administration of TRPA1 antagonists (A-967079 and HC-030031) or an antioxidant (α-lipoic acid). Indomethacin treatment was, however, ineffective. We also found that I/R increased macrophage (F4/80⁺ cell) number and oxidative stress markers, including 4-hydroxynonenal (4-HNE), in the injured tibial nerve. Macrophage-deleted MaFIA (Macrophage Fas-Induced Apoptosis) mice did not show I/R-evoked endoneurial cell infiltration, increased 4-HNE and mechanical and cold allodynia. Furthermore, Trpa1^-/- mice did not show any increase in macrophage number and 4-HNE in the injured nerve trunk. Notably, in mice with selective deletion of Schwann cell TRPA1 (Plp1-Cre^ERT;Trpa1^fl/fl mice), increases in macrophage infiltration, 4-HNE and mechanical and cold allodynia were attenuated. In the present mouse model of CRPS-I, we propose that the initial oxidative stress burst that follows reperfusion activates a feed forward mechanism that entails resident macrophages and Schwann cell TRPA1 of the injured tibial nerve to sustain chronic neuroinflammation and allodynia. Repeated treatment one hour before and for 3 days after I/R with a TRPA1 antagonist permanently protected CPIP mice against neuroinflammation and allodynia, indicating possible novel therapeutic strategies for CRPS-I.

Receptor mechanisms mediating the anti-neuroinflammatory effects of endocannabinoid system modulation in a rat model of migraine

Calcitonin gene-related peptide (CGRP), substance P and dural mast cells are main contributors in neurogenic inflammation underlying migraine pathophysiology. Modulation of endocannabinoid system attenuates migraine pain, but its mechanisms of action remain unclear. We investigated receptor mechanisms mediating anti-neuroinflammatory effects of endocannabinoid system modulation in in vivo migraine model and ex vivo hemiskull preparations in rats. To induce acute model of migraine, a single dose of nitroglycerin was intraperitoneally administered to male rats. Moreover, isolated ex vivo rat hemiskulls were prepared to study CGRP and substance P release from meningeal trigeminal afferents. We used methanandamide (cannabinoid agonist), rimonabant (cannabinoid receptor-1 CB1 antagonist), SR144528 (CB2 antagonist) and capsazepine (transient receptor potential vanilloid-1 TRPV1 antagonist) to explore effects of endocannabinoid system modulation on the neurogenic inflammation, and possible involvement of CB1, CB2 and TRPV1 receptors during endocannabinoid effects. Methanandamide attenuated nitroglycerin-induced CGRP increments in in vivo plasma, trigeminal ganglia and brainstem and also in ex vivo hemiskull preparations. Methanandamide also alleviated enhanced number and degranulation of dural mast cells induced by nitroglycerin. Rimonabant, but not capsazepine or SR144528, reversed the attenuating effects of methanandamide on CGRP release in both in vivo and ex vivo experiments. Additionally, SR144528, but not rimonabant or capsazepine, reversed the attenuating effects of methanandamide on dural mast cells. However, neither nitroglycerin nor methanandamide changed substance P levels in both in vivo and ex vivo experiments. Methanandamide modulates CGRP release in migraine-related structures via CB1 receptors and inhibits the degranulation of dural mast cells through CB2 receptors. Selective ligands targeting CB1 and CB2 receptors may provide novel and effective treatment strategies against migraine.

Localization of TRPA1 channels and characterization of TRPA1 mediated responses in dural and pial arteries in vivo after intracarotid infusion of Na2S

BACKGROUND

The Transient Receptor Potential Ankyrin 1 (TRPA1) channel might play a role in migraine. However, different mechanisms for this have been suggested. The purpose of our study was to investigate the localization and significance of TRPA1 channels in rat pial and dural arteries.

METHODS

Immunofluorescence microscopy was used to localize TRPA1 channels in dural arteries, pial arteries, dura mater and trigeminal ganglion. The genuine closed cranial window model was used to examine the effect of Na₂S, a donor of the TRPA1 channel opener H₂S, on the diameter of pial and dural arteries. Further, we performed blocking experiments with TRPA1 antagonist HC-030031, calcitonin gene-related peptide (CGRP) receptor antagonist olcegepant and K_Ca3.1 channel blocker TRAM-34.

RESULTS

TRPA1 channels were localized to the endothelium of both dural and pial arteries and in nerve fibers in dura mater. Further, we found TRPA1 expression in the membrane of trigeminal ganglia neuronal cells, some of them also staining for CGRP. Na₂S caused dilation of both dural and pial arteries. In dural arteries, this was inhibited by HC-030031 and olcegepant. In pial arteries, the dilation was inhibited by TRAM-34, suggesting involvement of the K_Ca3.1 channel.

CONCLUSION

Na₂S causes a TRPA1- and CGRP-dependent dilation of dural arteries and a K_Ca3.1 channel-dependent dilation of pial arteries in rats.

Low-dose interleukin-2 reverses behavioral sensitization in multiple mouse models of headache disorders

Headache disorders are highly prevalent and debilitating, with limited treatment options. Previous studies indicate that many proinflammatory immune cells contribute to headache pathophysiology. Given the well-recognized role of regulatory T (Treg) cells in maintaining immune homeostasis, we hypothesized that enhancing Treg function may be effective to treat multiple headache disorders. In a mouse model of chronic migraine, we observed that repeated nitroglycerin (NTG, a reliable trigger of migraine in patients) administration doubled the number of CD3 T cells in the trigeminal ganglia without altering the number of Treg cells, suggesting a deficiency in Treg-mediated immune homeostasis. We treated mice with low-dose interleukin-2 (ld-IL2) to preferentially expand and activate endogenous Treg cells. This not only prevented the development of NTG-induced persistent sensitization but also completely reversed the established facial skin hypersensitivity resulting from repeated NTG administration. The effect of ld-IL2 was independent of mouse sex and/or strain. Importantly, ld-IL2 treatment did not alter basal nociceptive responses, and repeated usage did not induce tolerance. The therapeutic effect of ld-IL2 was abolished by Treg depletion and was recapitulated by Treg adoptive transfer. Furthermore, treating mice with ld-IL2 1 to 7 days after mild traumatic brain injury effectively prevented as well as reversed the development of behaviors related to acute and chronic post-traumatic headache. In a model of medication overuse headache, Ld-IL2 completely reversed the cutaneous hypersensitivity induced by repeated administration of sumatriptan. Collectively, this study identifies ld-IL2 as a promising prophylactic for multiple headache disorders with a mechanism distinct from the existing treatment options.

An Update: Pathophysiology of Migraine

Migraine is the most common disabling primary headache globally. Attacks typically present with unilateral throbbing headache and associated symptoms including, nausea, multisensory hypersensitivity, and marked fatigue. In this article, the authors address the underlying neuroanatomical basis for migraine-related headache, associated symptomatology, and discuss key clinical and preclinical findings that indicate that migraine likely results from dysfunctional homeostatic mechanisms. Whereby, abnormal central nervous system responses to extrinsic and intrinsic cues may lead to increased attack susceptibility.

No central action of CGRP antagonising drugs in the GTN mouse model of migraine

Introduction

Clinically, calcitonin gene-related peptide antagonising drugs are recognized as effective in migraine treatment, but their site of action is debated. Only a small fraction of these compounds pass the blood-brain barrier and accesses the central nervous system. Regardless, it has been argued that the central nervous system is the site of action. Here, we test this hypothesis by bypassing the blood-brain barrier through intracerebroventricular injection of calcitonin gene-related peptide antagonising drugs.

Methods

We used the glyceryl trinitrate (GTN) mouse model, which is well validated by its response to specific migraine drugs. The calcitonin gene-related peptide receptor antagonist olcegepant and the calcitonin gene-related peptide monoclonal antibody ALD405 were administered either intraperitoneally or intracerebroventricularly. The outcome measure was cutaneous mechanical allodynia.

Results

Mice given olcegepant intraperitoneally + GTN on day 1 had a mean 50% withdrawal threshold of 1.2 g in contrast to mice receiving placebo + GTN, which had a threshold of 0.3 g ( p < 0.001). Similarly, in the ALD405 + GTN group, mice had thresholds of 1.2 g versus 0.2 g in the placebo + GTN group ( p < 0.001). However, both drugs were ineffective when delivered intracerebroventricularly, as control and active groups had identical mechanical sensitivity thresholds, 0.2 g versus 0.1 g and 0.1 g versus 0.1 g for olcegepant and ALD405, respectively ( p > 0.99 in both cases).

Discussion

The site of action of olcegepant and of the monoclonal antibody ALD405 is outside the blood-brain barrier in this mouse model of migraine. It is likely that these results can be generalised to all gepants and all antibodies and that the results are relevant for human migraine.

Oxidation of Reduced Graphene Oxide via Cellular Redox Signaling Modulates Actin-Mediated Neurotransmission

Neurotransmission is the basis of brain functions, and controllable neurotransmission tuning constitutes an attractive approach for interventions in a wide range of neurologic disorders and for synapse-based therapeutic treatments. Graphene-family nanomaterials (GFNs) offer promising advantages for biomedical applications, particularly in neurology. Our study suggests that reduced graphene oxide (rGO) serves as a neurotransmission modulator and reveals that the cellular oxidation of rGO plays a crucial role in this effect. We found that rGO could be oxidized via cellular reactive oxygen species (ROS), as evidenced by an increased number of oxygen-containing functional groups on the rGO surface. Cellular redox signaling, which involves NADPH oxidases and mitochondria, was initiated and subsequently intensified rGO oxidation. The study further shows that the blockage of synaptic vesicle docking and fusion induced through a disturbance of actin dynamics is the underlying mechanism through which oxidized rGO exerts depressant effects on neurotransmission. Importantly, this depressant effect could be modulated by restricting the cellular ROS levels and stabilizing the actin dynamics. Taken together, our results identify the complicated biological effects of rGO as a controlled neurotransmission modulator and can provide helpful information for the future design of graphene materials for neurobiological applications.

Acid-sensing ion channel 3 blockade inhibits durovascular and nitric oxide-mediated trigeminal pain

Background and Purpose

There is a major unmet need to develop new therapies for migraine. We have previously demonstrated the therapeutic potential of the acid‐sensing ion channel (ASIC) blockade in migraine, via an ASIC1 mechanism. ASIC3 is expressed in the trigeminal ganglion and its response is potentiated by NO that can trigger migraine attacks in patients. Thus we sought to explore the potential therapeutic effect of ASIC3 blockade in migraine.

Experimental Approach

To investigate this, we utilised validated electrophysiological and behavioural rodent preclinical models. In rats, ASIC3 blockade using APETx2 (50 or 100 μg·kg⁻¹, i.v.) was measured by using durovascular and NO‐evoked trigeminal nociceptive responses along with cortical spreading depression models. In mice, we sought to determine if periorbital mechanical sensitivity, induced by acute nitroglycerin (10 mg·kg⁻¹, i.p.), was attenuated by APETx2 (230 μg·kg⁻¹, i.p.), as well as latent sensitisation induced by bright light stress in a chronic nitroglycerin model.

Key Results

Here, we show that the ASIC3 blocker APETx2 inhibits durovascular‐evoked and NO‐induced sensitisation of trigeminal nociceptive responses in rats. In agreement, acute and chronic periorbital mechanosensitivity induced in mice by nitroglycerin and subsequent bright light stress‐evoked latent sensitivity as a model of chronic migraine are all reversed by APETx2.

Conclusion and Implications

These results support the development of specific ASIC3 or combined ASIC1/3 blockers for migraine‐related pain and point to a potential role for ASIC‐dependent NO‐mediated attack triggering. This has key implications for migraine, given the major unmet need for novel therapeutic targets.

Cross-talk signaling in the trigeminal ganglion: role of neuropeptides and other mediators

The trigeminal ganglion with its three trigeminal nerve tracts consists mainly of clusters of sensory neurons with their peripheral and central processes. Most neurons are surrounded by satellite glial cells and the axons are wrapped by myelinating and non-myelinating Schwann cells. Trigeminal neurons express various neuropeptides, most notably, calcitonin gene-related peptide (CGRP), substance P, and pituitary adenylate cyclase-activating polypeptide (PACAP). Two types of CGRP receptors are expressed in neurons and satellite glia. A variety of other signal molecules like ATP, nitric oxide, cytokines, and neurotrophic factors are released from trigeminal ganglion neurons and signal to neighboring neurons or satellite glial cells, which can signal back to neurons with same or other mediators. This potential cross-talk of signals involves intracellular mechanisms, including gene expression, that can modulate mediators of sensory information, such as neuropeptides, receptors, and neurotrophic factors. From the ganglia cell bodies, which are outside the blood–brain barrier, the mediators are further distributed to peripheral sites and/or to the spinal trigeminal nucleus in the brainstem, where they can affect neural transmission. A major question is how the sensory neurons in the trigeminal ganglion differ from those in the dorsal root ganglion. Despite their functional overlap, there are distinct differences in their ontogeny, gene expression, signaling pathways, and responses to anti-migraine drugs. Consequently, drugs that modulate cross-talk in the trigeminal ganglion can modulate both peripheral and central sensitization, which may potentially be distinct from sensitization mediated in the dorsal root ganglion.

NADPH oxidase1 inhibition leads to regression of central sensitization during formalin induced acute nociception via attenuation of ERK1/2-NFκB signaling and glial activation

Role of NADPH oxidase1 in the development of inflammatory pain has been demonstrated by gene knockout studies. Nevertheless, pharmacological inhibition of NOX1 is a requisite approach for therapeutic utility. Recently, we have reported the anti-nociceptive effect of newly identified NOX1 specific inhibitor ML171 (2-acetylphenothiazine). Inhibition of NOX1 resulted in attenuation of nociceptive sensitization during acute inflammatory pain via inhibition of ROS generation and its downstream ERK1/2 activation. However, glial activation accompanying inflammation is closely related to the initiation and maintenance of pain. Peripheral nociceptive inputs activate the primary afferents via release of various chemical mediators which are potentially capable of mediating signals from neuron to glia in DRG and subsequently in spinal cord dorsal horn. The subsequent interactions between neuron and glia contribute to pain hypersensitivity. Thus, the present study was focused to investigate the effect of ML171 on ERK1/2 signaling, glial activation, and crosstalk between neuron and glia in a mouse model of formalin induced acute nociception. Thus, the present study was focused to investigate the effect of ML171 on ERK1/2 signaling, glial activation, and crosstalk between neuron and glia in DRG and dorsal horn of the spinal cord of lumbar region (L3-L5) in a mouse model of formalin induced acute nociception. Intraperitoneal administration of ML171 decreased nociceptive behavioral responses, i.e. the flinch and lick counts, in formalin induced nociceptive mice. Immunofluorescence and Western blot analysis demonstrated decreased levels of nociceptive mediators like p-ERK1/2, p-NFκB p65, Iba1 and GFAP in DRG as well as in spinal cord dorsal horn; supporting anti-nociceptive potential of ML171. Further, co-localization studies showed the neuron-glia crosstalk in tissue dependent manner. ERK1/2 was found to be activated in glia and NFκB in neurons in DRG; whereas in case of spinal cord ERK1/2 was activated in neurons and NFκB in astrocytes. Decrease in nociceptive behavioral response and activation of nociceptive mediators after intraperitoneal administration of ML171 strongly advocate anti-nociceptive potential of ML171. This is the first report demonstrating modulation of ERK1/2-NFκB signaling pathway, glial activation and regulation of neuron-glia crosstalk by NADPH oxidase1 inhibition towards its anti-nociceptive action.

Schwann cells expressing nociceptive channel TRPA1 orchestrate ethanol-evoked neuropathic pain in mice

Excessive alcohol consumption is associated with spontaneous burning pain, hyperalgesia, and allodynia. Although acetaldehyde has been implicated in the painful alcoholic neuropathy, the mechanism by which the ethanol metabolite causes pain symptoms is unknown. Acute ethanol ingestion caused delayed mechanical allodynia in mice. Inhibition of alcohol dehydrogenase (ADH) or deletion of transient receptor potential ankyrin 1 (TRPA1), a sensor for oxidative and carbonyl stress, prevented allodynia. Acetaldehyde generated by ADH in both liver and Schwann cells surrounding nociceptors was required for TRPA1-induced mechanical allodynia. Plp1-Cre Trpa1fl/fl mice with a tamoxifen-inducible specific deletion of TRPA1 in Schwann cells revealed that channel activation by acetaldehyde in these cells initiates a NADPH oxidase-1-dependent (NOX1-dependent) production of hydrogen peroxide (H2O2) and 4-hydroxynonenal (4-HNE), which sustains allodynia by paracrine targeting of nociceptor TRPA1. Chronic ethanol ingestion caused prolonged mechanical allodynia and loss of intraepidermal small nerve fibers in WT mice. While Trpa1-/- or Plp1-Cre Trpa1fl/fl mice did not develop mechanical allodynia, they did not show any protection from the small-fiber neuropathy. Human Schwann cells express ADH/TRPA1/NOX1 and recapitulate the proalgesic functions of mouse Schwann cells. TRPA1 antagonists might attenuate some symptoms of alcohol-related pain.

Euphorbia bicolor (Euphorbiaceae) Latex Extract Reduces Inflammatory Cytokines and Oxidative Stress in a Rat Model of Orofacial Pain

Recent studies have reported that the transient receptor potential V1 ion channel (TRPV1), a pain generator on sensory neurons, is activated and potentiated by NADPH oxidase-generated reactive oxygen species (ROS). ROS are increased by advanced oxidation protein products (AOPPs), which activate NADPH oxidase by upregulating Nox4 expression. Our previous studies reported that Euphorbia bicolor (Euphorbiaceae) latex extract induced peripheral analgesia, partly via TRPV1, in hindpaw-inflamed male and female rats. The present study reports that E. bicolor latex extract also can evoke analgesia via reduction of oxidative stress biomarkers and proinflammatory cytokines/chemokines in a rat model of orofacial pain. Male and female rats were injected with complete Freund's adjuvant (CFA) into the left vibrissal pad to induce orofacial inflammation, and mechanical allodynia was measured by the von Frey method. Twenty-four hours later, rats received one injection of E. bicolor latex extract or vehicle into the inflamed vibrissal pad. Mechanical sensitivity was reassessed at 1, 6, 24, and/or 72 hours. Trigeminal ganglia and trunk blood were collected at each time point. In the trigeminal ganglia, ROS were quantified using 2',7'-dichlorodihydrofluorescein diacetate dye, Nox4 protein was quantified by Western blots, and cytokines/chemokines were quantified using a cytokine array. AOPPs were quantified in trunk blood using a spectrophotometric assay. E. bicolor latex extract significantly reduced orofacial mechanical allodynia in male and female rats at 24 and 72 hours, respectively. ROS, Nox4, and proinflammatory cytokines/chemokines were significantly reduced in the trigeminal ganglia, and plasma AOPP was significantly reduced in the trunk blood of extract-treated compared to vehicle-treated rats. In vitro assays indicate that E. bicolor latex extract possessed antioxidant activities by scavenging free radicals. Together our data indicate that the phytochemicals in E. bicolor latex may serve as novel therapeutics for treating oxidative stress-induced pain conditions.

Emerging Perspectives on Pain Management by Modulation of TRP Channels and ANO1

Receptor-type ion channels are critical for detection of noxious stimuli in primary sensory neurons. Transient receptor potential (TRP) channels mediate pain sensations and promote a variety of neuronal signals that elicit secondary neural functions (such as calcitonin gene-related peptide [CGRP] secretion), which are important for physiological functions throughout the body. In this review, we focus on the involvement of TRP channels in sensing acute pain, inflammatory pain, headache, migraine, pain due to fungal infections, and osteo-inflammation. Furthermore, action potentials mediated via interactions between TRP channels and the chloride channel, anoctamin 1 (ANO1), can also generate strong pain sensations in primary sensory neurons. Thus, we also discuss mechanisms that enhance neuronal excitation and are dependent on ANO1, and consider modulation of pain sensation from the perspective of both cation and anion dynamics.

Liposomal Curcumin Enhances the Effect of Naproxen in a Rat Model of Migraine

BACKGROUND Curcumin is an antioxidant that reduces inflammation and pain. This study aimed to assess the effect of pretreatment with naproxen and liposomal curcumin compared with naproxen and curcumin solution on oxidative stress parameters and pain in a rat model of migraine. MATERIAL AND METHODS Sixty-three male Wistar rats included a control group (n=9) and a rat model of migraine (n=54) induced by intraperitoneal injection of nitroglycerin (1 mg/0.1 kg). The rat model group was divided into an untreated control group (n=9), a group pretreated with naproxen alone (2.8 mg/kg) (n=9), a group pretreated with naproxen (2.8 mg/kg) combined with curcumin solution (1 mg/0.1 kg) (n=9), a group pretreated with naproxen (2.8 mg/kg) combined with curcumin solution (2 mg/0.1 kg) (n=9), a group pretreated with naproxen (2.8 mg/kg) combined with liposomal curcumin solution (1 mg/0.1 kg) (n=9) a group pretreated with naproxen (2.8 mg/kg) combined with liposomal curcumin solution (2 mg/0.1 kg) (n=9). Spectroscopy measured biomarkers of total oxidative status and nociception was tested using an injection of 1% of formalin into the rat paw. RESULTS Expression of biomarkers of oxidative stress and enhanced nociception were significantly increased following pretreatment with combined naproxen and liposomal curcumin compared with curcumin solution or naproxen alone (P<0.001). Combined curcumin solution and naproxen were more effective at a concentration of 2 mg/0.1kg for the first nociceptive phase (P<0.005). CONCLUSIONS In a rat model of migraine, combined therapy with liposomal curcumin and naproxen showed an improved antioxidant effect and anti-nociceptive effect.

TRP Channels and Migraine: Recent Developments and New Therapeutic Opportunities

Migraine is the second-most disabling disease worldwide, and the second most common neurological disorder. Attacks can last many hours or days, and consist of multiple symptoms including headache, nausea, vomiting, hypersensitivity to stimuli such as light and sound, and in some cases, an aura is present. Mechanisms contributing to migraine are still poorly understood. However, transient receptor potential (TRP) channels have been repeatedly linked to the disorder, including TRPV1, TRPV4, TRPM8, and TRPA1, based on their activation by pathological stimuli related to attacks, or their modulation by drugs/natural products known to be efficacious for migraine. This review will provide a brief overview of migraine, including current therapeutics and the link to calcitonin gene-related peptide (CGRP), a neuropeptide strongly implicated in migraine pathophysiology. Discussion will then focus on recent developments in preclinical and clinical studies that implicate TRP channels in migraine pathophysiology or in the efficacy of therapeutics. Given the use of onabotulinum toxin A (BoNTA) to treat chronic migraine, and its poorly understood mechanism, this review will also cover possible contributions of TRP channels to BoNTA efficacy. Discussion will conclude with remaining questions that require future work to more fully evaluate TRP channels as novel therapeutic targets for migraine.

ROS/TRPA1/CGRP signaling mediates cortical spreading depression

Objectives

The transient receptor potential ankyrin A 1 (TRPA1) channel and calcitonin gene-related peptide (CGRP) are targets for migraine prophylaxis. This study aimed to understand their mechanisms in migraine by investigating the role of TRPA1 in cortical spreading depression (CSD) in vivo and exploring how reactive oxygen species (ROS)/TRPA1/CGRP interplay in regulating cortical susceptibility to CSD.

Methods

Immunohistochemistry was used for detecting TRPA1 expression. CSD was induced by K⁺ on the cerebral cortex, monitored using electrophysiology in rats, and intrinsic optical imaging in mouse brain slices, respectively. Drugs were perfused into contralateral ventricle of rats. Lipid peroxidation (malondialdehyde, MDA) analysis was used for indicating ROS level.

Results

TRPA1 was expressed in cortical neurons and astrocytes of rats and mice. TRPA1 deactivation by an anti-TRPA1 antibody reduced cortical susceptibility to CSD in rats and decreased ipsilateral MDA level induced by CSD. In mouse brain slices, H₂O₂ facilitated submaximal CSD induction, which disappeared by the antioxidant, tempol and the TRPA1 antagonist, A-967079; Consistently, TRPA1 activation reversed prolonged CSD latency and reduced magnitude by the antioxidant. Further, blockade of CGRP prolonged CSD latency, which was reversed by H₂O₂ and the TRPA1 agonist, allyl-isothiocyanate, respectively.

Conclusions

ROS/TRPA1/CGRP signaling plays a critical role in regulating cortical susceptibility to CSD. Inhibition ROS and deactivation of TRPA1 channels may have therapeutic benefits in preventing stress-triggered migraine via CGRP.

Migraine-provoking substances evoke periorbital allodynia in mice

Background

Administration of endogenous mediators or exogenous chemicals in migraine patients provoke early headaches and delayed migraine-like attacks. Although migraine provoking substances are normally vasodilators, dilation of arterial vessels does not seem to be the sole contributing factor, and the underlying mechanisms of the delayed migraine pain are mostly unknown. Sustained mechanical allodynia is a common response associated with the local administration of various proalgesic substances in experimental animals and humans. Here, we investigated the ability of a series of endogenous mediators which provoke or do not provoke migraine in patients, to cause or not cause mechanical allodynia upon their injection in the mouse periorbital area.

Methods

Mechanical allodynia was assessed with the von Frey filament assay. Stimuli were given by subcutaneous injection in the periorbital area of C57BL/6J mice; antagonists were administered by local and systemic injections.

Results

Calcitonin gene related peptide (CGRP), but not adrenomedullin and amylin, pituitary adenylyl cyclase activating peptide (PACAP), but not vasoactive intestinal polypeptide (VIP), histamine, prostaglandin E₂ (PGE₂) and prostacyclin (PGI₂), but not PGF_2α, evoked a dose-dependent periorbital mechanical allodynia. The painful responses were attenuated by systemic or local (periorbital) administration of antagonists for CGRP (CLR/RAMP1), PACAP (PAC-1), histamine H₁, PGE₂ (EP₄), and PGI₂ (IP) receptors, respectively.

Conclusions

The correspondence between substances that provoke (CGRP; PACAP, histamine, PGE₂, PGI₂), or do not provoke (VIP and PGF_2α), migraine-like attacks in patients and periorbital allodynia in mice suggests that the study of allodynia in mice may provide information on the proalgesic mechanisms of migraine-provoking agents in humans. Results underline the ability of migraine-provoking substances to initiate mechanical allodynia by acting on peripheral terminals of trigeminal afferents.

Nitroglycerine triggers triptan-responsive cranial allodynia and trigeminal neuronal hypersensitivity

Cranial allodynia associated with spontaneous migraine is reported as either responsive to triptan treatment or to be predictive of lack of triptan efficacy. These conflicting results suggest that a single mechanism mediating the underlying neurophysiology of migraine symptoms is unlikely. The lack of a translational approach to study cranial allodynia reported in migraine patients is a limitation in dissecting potential mechanisms. Our objective was to study triptan-responsive cranial allodynia in migraine patients, and to develop an approach to studying its neural basis in the laboratory. Using nitroglycerine to trigger migraine attacks, we investigated whether cranial allodynia could be triggered experimentally, observing its response to treatment. Preclinically, we examined the cephalic response properties of central trigeminocervical neurons using extracellular recording techniques, determining changes to ongoing firing and somatosensory cranial-evoked sensitivity, in response to nitroglycerine followed by triptan treatment. Cranial allodynia was triggered alongside migraine-like headache in nearly half of subjects. Those who reported cranial allodynia accompanying their spontaneous migraine attacks were significantly more likely to have symptoms triggered than those that did not. Patients responded to treatment with aspirin or sumatriptan. Preclinically, nitroglycerine caused an increase in ongoing firing and hypersensitivity to intracranial-dural and extracranial-cutaneous (noxious and innocuous) somatosensory stimulation, reflecting signatures of central sensitization potentially mediating throbbing headache and cranial allodynia. These responses were aborted by a triptan. These data suggest that nitroglycerine can be used as an effective and reliable method to trigger cranial allodynia in subjects during evoked migraine, and the symptom is responsive to abortive triptan treatments. Preclinically, nitroglycerine activates the underlying neural mechanism of cephalic migraine symptoms, central sensitization, also predicting the clinical outcome to triptans. This supports a biological rationale that several mechanisms can mediate the underlying neurophysiology of migraine symptoms, with nitrergic-induced changes reflecting one that is relevant to spontaneous migraine in many migraineurs, whose symptoms of cranial allodynia are responsive to triptan treatment. This approach translates directly to responses in animals and is therefore a relevant platform to study migraine pathophysiology, and for use in migraine drug discovery.

Ion Channel Pharmacology for Pain Modulation

A large series of different ion channels have been identified and investigated as potential targets for new medicines for the treatment of a variety of human diseases, including pain. Among these channels, the voltage gated calcium channels (VGCC) are inhibited by drugs for the treatment of migraine, neuropathic pain or intractable pain. Transient receptor potential (TRP) channels are emerging as important pain transducers as they sense low pH media or oxidative stress and other mediators and are abundantly found at sites of inflammation or tissue injury. Low pH may also activate acid sensing ion channels (ASIC) and mechanical forces stimulate the PIEZO channels. While potent agonists of TRP channels due to their desensitizing action on pain transmission are used as topical applications, the potential of TRP antagonists as pain therapeutics remains an exciting field of investigation. The study of ASIC or PIEZO channels in pain signaling is in an early stage, whereas antagonism of the purinergic P2X₃ channels has been reported to provide beneficial effects in chronic intractable cough. The present chapter covers these intriguing channels in great detail, highlighting their diverse mechanisms and broad potential for therapeutic utility.

TRPA1 Antagonists for Pain Relief

Here, we review the literature assessing the role of transient receptor potential ankyrin 1 (TRPA1), a calcium-permeable non-selective cation channel, in various types of pain conditions. In the nervous system, TRPA1 is expressed in a subpopulation of nociceptive primary sensory neurons, astroglia, oligodendrocytes and Schwann cells. In peripheral terminals of nociceptive primary sensory neurons, it is involved in the transduction of potentially harmful stimuli and in their central terminals it is involved in amplification of nociceptive transmission. TRPA1 is a final common pathway for a large number of chemically diverse pronociceptive agonists generated in various pathophysiological pain conditions. Thereby, pain therapy using TRPA1 antagonists can be expected to be a superior approach when compared with many other drugs targeting single nociceptive signaling pathways. In experimental animal studies, pharmacological or genetic blocking of TRPA1 has effectively attenuated mechanical and cold pain hypersensitivity in various experimental models of pathophysiological pain, with only minor side effects, if any. TRPA1 antagonists acting peripherally are likely to be optimal for attenuating primary hyperalgesia (such as inflammation-induced sensitization of peripheral nerve terminals), while centrally acting TRPA1 antagonists are expected to be optimal for attenuating pain conditions in which central amplification of transmission plays a role (such as secondary hyperalgesia and tactile allodynia caused by various types of peripheral injuries). In an experimental model of peripheral diabetic neuropathy, prolonged blocking of TRPA1 has delayed the loss of nociceptive nerve endings and their function, thereby promising to provide a disease-modifying treatment.