Central sensitization of nociceptive neurons in rat medullary dorsal horn involves purinergic P2X7 receptors

Possible Interaction of Suramin with Thalamic P2X Receptors and NLRP3 Inflammasome Activation Alleviates Reserpine-Induced Fibromyalgia-Like Symptoms

The high pain sensitivity in fibromyalgia (FM) is processed by the thalamus that presents as a key component in the pain pathway in FM patients. Noteworthy, Purinergic receptors, specifically P2X, are implicated in pain signaling and neuroinflammation via inflammasome signaling. However, there is no available data on the impact of pharmacological intervention on the P2X receptor in thalamic pain transmission in FM. To investigate this aspect, the clinically tested P2X inhibitor, Suramin (SURM), was utilized. FM was induced over three days using Reserpine (1 mg/kg/day, s.c.), followed by a single dose of SURM (100 mg/kg, i.p.). At the molecular level, SURM countered the overexpression of P2X7 and P2X4 receptors accompanied by reduced NLRP3 inflammasome complex and pyroptotic markers like gasdermin-D. This was associated with the suppression of the p38-MAPK and NF-κB pathways, along with a decrease in pro-inflammatory cytokines and tumor necrosis factor-α as observed by increased CD86 expression on M1 microglia phenotype, a neuroinflammatory marker. Concurrently, blocking the P2X receptor shifted microglia polarization towards the M2 phenotype, marked by elevated CD163 expression, as a neuroprotective mechanism. This was outlined by increased neurotrophic and anti-inflammatory IL-10 with normalization of disturbed neurotransmitters. Behaviorally, SURM ameliorated the heightened pain processing, as observed in mechanical and thermal pain tests. Furthermore, it lowered Reserpine-induced motor impairment in the rotarod and open-field tests. This improvement in the somatosensory experience was reflected in alleviating depressive-like behavior in the forced swimming test. These findings highlight the therapeutic potential of blocking thalamic P2X receptors in alleviating fibromyalgia symptoms.

Enhanced exosome secretion regulated by microglial P2X7R in the medullary dorsal horn contributes to pulpitis-induced pain

BACKGROUND

Pulpitis is a prevalent oral disease characterized by severe pain. The activation of microglia in the medullary dorsal horn (MDH) is reportedly essential for the central sensitization mechanism associated with pulpitis. The P2X7 receptor (P2X7R) on microglia can trigger the secretion of exosomes enriched with IL-1β, which is involved in inflammation. Thus, we hypothesized that the enhanced exosome secretion regulated by microglial P2X7R in the MDH contributes to pulpitis-induced pain.

METHODS

An experimental pulpitis model was established in male SD rats to observe pain behaviors. Immunofluorescence staining, western blotting and quantitative real-time PCR were used to analyze the expression of IL-1β and Rab27a, a key protein secreted by exosomes during nociceptive processes. The effects of the exosome inhibitor GW4869 and the P2X7R antagonist Brilliant Blue G (BBG) on microglial P2X7R, exosome secretion and inflammation in the pulpitis model were analyzed. In vitro, microglial cells were cultured to collect exosomes, and stimulation with lipopolysaccharide (LPS), oxidized ATP (oxATP) and GW4869 altered the secretion of exosomes containing IL-1β.

RESULTS

In the experimental pulpitis model, the microglial exosome secretion and inflammatory factor release in the MDH were both correlated with the extent of pulpitis-induced pain, with the highest expression occurring on the 7th day. GW4869 and BBG inhibited Rab27a and IL-1β expression, reducing pulpitis-induced pain. In addition, exosomes were successfully extracted by ultracentrifugation in vitro, wherein LPS treatment promoted exosome secretion but GW4869 had the opposite effects on the secretion of exosomes and the IL-1β. Moreover, P2X7R inhibition by oxATP diminished exosome secretion, leading to a reduction in inflammatory responses.

CONCLUSION

This study highlights the regulatory role of microglial P2X7R in increased exosome secretion, indicating the potential utility of P2X7R as a promising target for pulpitis therapy. Our research highlights a new pulpitis mechanism in which exosomes enriched with IL-1β contribute to pulpitis-induced pain, suggesting the crucial roles of exosomes as pain biomarkers and harmful signaling molecules during pulpitis.

P2X7 receptors: a bibliometric review from 2002 to 2023

For many years, there has been ongoing research on the P2X7 receptor (P2X7R). A comprehensive, systematic, and objective evaluation of the scientific output and status of P2X7R will be instrumental in guiding future research directions. This study aims to present the status and trends of P2X7R research from 2002 to 2023. Publications related to P2X7R were retrieved from the Web of Science Core Collection database. Quantitative analysis and visualization tools were Microsoft Excel, VOSviewer, and CiteSpace software. The analysis content included publication trends, literature co-citation, and keywords. 3282 records were included in total, with the majority of papers published within the last 10 years. Based on literature co-citation and keyword analysis, neuroinflammation, neuropathic pain, gastrointestinal diseases, tumor microenvironment, rheumatoid arthritis, age-related macular degeneration, and P2X7R antagonists were considered to be the hotspots and frontiers of P2X7R research. Researchers will get a more intuitive understanding of the status and trends of P2X7R research from this study.

Blockage of the fractalkine pathway reduces hyperalgesia and prevents morphological glial alterations-Comparison between inflammatory and neuropathic orofacial pain in male rats

This study aimed to evaluate the effects of inhibitors of the fractalkine pathway in hyperalgesia in inflammatory and neuropathic orofacial pain in male rats and the morphological changes in microglia and satellite glial cells (SGCs). Rats were submitted to zymosan-induced arthritis of the temporomandibular joint or infraorbital nerve constriction, and treated intrathecally with a P2 X7 antagonist, a cathepsin S inhibitor or a p-38 mitogen-activated protein kinase (MAPK) inhibitor. Mechanical hyperalgesia was evaluated 4 and 6 h following arthritis induction or 7 and 14 days following nerve ligation. The expression of the receptor CX3 CR1 , phospho-p-38 MAPK, ionized calcium-binding adapter molecule-1 (Iba-1), and glutamine synthetase and the morphological changes in microglia and SGCs were evaluated by confocal microscopy. In both inflammatory and neuropathic models, untreated animals presented a higher expression of CX3 CR1 and developed hyperalgesia and up-regulation of phospho-p-38 MAPK, which was prevented by all drugs (p < .05). The number of microglial processes endpoints and the total branch length were lower in the untreated animals, but the overall immunolabeling of Iba-1 was altered only in neuropathic rats (p < .05). The mean area of SGCs per neuron was significantly altered only in the inflammatory model (p < .05). All morphological alterations were reverted by modulating the fractalkine pathway (p < .05). In conclusion, the blockage of the fractalkine pathway seemed to be a possible therapeutic strategy for inflammatory and neuropathic orofacial pain, reducing mechanical hyperalgesia by impairing the phosphorylation of p-38 MAPK and reverting morphological alterations in microglia and SGCs.

Involvement of microglial P2X7 receptor in pain modulation

BACKGROUND

Pain is a rapid response mechanism that compels organisms to retreat from the harmful stimuli and triggers a repair response. Nonetheless, when pain persists for extended periods, it can lead to adverse changes into in the individual's brain, negatively impacting their emotional state and overall quality of life. Microglia, the resident immune cells in the central nervous system (CNS), play a pivotal role in regulating a variety of pain-related disorders. Specifically, recent studies have shed light on the central role that microglial purinergic ligand-gated ion channel 7 receptor (P2X7R) plays in regulating pain. In this respect, the P2X7R on microglial membranes represents a potential therapeutic target.

AIMS

To expound on the intricate link between microglial P2X7R and pain, offering insights into potential avenues for future research.

METHODS

We reviewed 140 literature and summarized the important role of microglial P2X7R in regulating pain, including the structure and function of P2X7R, the relationship between P2X7R and microglial polarization, P2X7R-related signaling pathways, and the effects of P2X7R antagonists on pain regulation.

RESULTS

P2X7R activation is related to M1 polarization of microglia, while suppressing P2X7R can transfer microglia from M1 into M2 phenotype. And targeting the P2X7R-mediated signaling pathways helps to explore new therapy for pain alleviation. P2X7R antagonists also hold potential for translational and clinical applications in pain management.

CONCLUSIONS

Microglial P2X7R holds promise as a potential novel pharmacological target for clinical treatments due to its distinctive structure, function, and the development of antagonists.

Descending serotonergic modulation from rostral ventromedial medulla to spinal trigeminal nucleus is involved in experimental occlusal interference-induced chronic orofacial hyperalgesia

BACKGROUND

Dental treatment associated with unadaptable occlusal alteration can cause chronic primary myofascial orofacial pain. The serotonin (5-HT) pathway from the rostral ventromedial medulla (RVM) exerts descending modulation on nociceptive transmission in the spinal trigeminal nucleus (Sp5) and facilitates chronic pain. The aim of this study was to investigate whether descending 5-HT modulation from the RVM to the Sp5 is involved in the maintenance of primary myofascial orofacial hyperalgesia after persistent experimental occlusal interference (PEOI) or after delayed removal of experimental occlusal interference (REOI).

METHODS

Expressions of 5-HT3A and 5-HT3B receptor subtypes in the Sp5 were assessed by immunofluorescence staining and Western blotting. The release and metabolism of 5-HT in the Sp5 were measured by high-performance liquid chromatography. Changes in the pain behavior of these rats were examined after specific pharmacologic antagonism of the 5-HT3 receptor, chemogenetic manipulation of the RVM 5-HT neurons, or selective down-regulation of 5-HT synthesis in the RVM.

RESULTS

Upregulation of the 5-HT3B receptor subtype in the Sp5 was found in REOI and PEOI rats. The concentration of 5-HT in Sp5 increased significantly only in REOI rats. Intrathecal administration of Y-25130 (a selective 5-HT3 receptor antagonist) dose-dependently reversed the hyperalgesia in REOI rats but only transiently reversed the hyperalgesia in PEOI rats. Chemogenetic inhibition of the RVM 5-HT neurons reversed the hyperalgesia in REOI rats; selective down-regulation of 5-HT in advance also prevented the development of hyperalgesia in REOI rats; the above two manipulations did not affect the hyperalgesia in PEOI rats. However, chemogenetic activation of the RVM 5-HT neurons exacerbated the hyperalgesia both in REOI and PEOI rats.

CONCLUSIONS

These results provide several lines of evidence that the descending pathway from 5-HT neurons in the RVM to 5-HT3 receptors in the Sp5, plays an important role in facilitating the maintained orofacial hyperalgesia after delayed EOI removal, but has a limited role in that after persistent EOI.

Ligand-gated ion channel P2X7 regulates hypoxia-induced factor-1α mediated pain induced by dental pulpitis in the medullary dorsal horn

Dental pulpitis often induces severe pain, and the molecular immune response is remarkable in both peripheral and central nervous system. Accumulating evidence indicates that activated microglia in the medullary dorsal horn (MDH) contribute to dental pulpitis induced pain. The P2X7 receptor plays an important role in driving pain and inflammatory processes, and its downstream target hypoxia-induced factor-1α (HIF-1α) has a crucial role in maintaining inflammation. However, the relationship between P2X7 and HIF-1α in dental inflammatory pain remains unclear. This study demonstrated that the degree of inflammation in the dental pulp tissue became more severe in a time-dependent manner by establishing a rat dental pulpitis model via pulp exposure. Meanwhile, the expression of P2X7, HIF-1α, IL-1β, and IL-18 in the MDH increased most on the seventh day when the pain threshold was the lowest in the dental pulpitis model. Furthermore, lipopolysaccharides (LPS) increased P2X7-mediated HIF-1α expression in microglia. Notably, the suppression of P2X7 caused less IL-1β and IL-18 release and lower HIF-1α expression, and P2X7 antagonist Brilliant Blue G (BBG) could alleviate pain behaviors of the dental pulpitis rats. In conclusion, our results provide further evidence that P2X7 is a key molecule, which regulates HIF-1α expression and inflammation in dental pulpitis-induced pain.

Descending control of nociception in insects?

Modulation of nociception allows animals to optimize chances of survival by adapting their behaviour in different contexts. In mammals, this is executed by neurons from the brain and is referred to as the descending control of nociception. Whether insects have such control, or the neural circuits allowing it, has rarely been explored. Based on behavioural, neuroscientific and molecular evidence, we argue that insects probably have descending controls for nociception. Behavioural work shows that insects can modulate nocifensive behaviour. Such modulation is at least in part controlled by the central nervous system since the information mediating such prioritization is processed by the brain. Central nervous system control of nociception is further supported by neuroanatomical and neurobiological evidence showing that the insect brain can facilitate or suppress nocifensive behaviour, and by molecular studies revealing pathways involved in the inhibition of nocifensive behaviour both peripherally and centrally. Insects lack the endogenous opioid peptides and their receptors that contribute to mammalian descending nociception controls, so we discuss likely alternative molecular mechanisms for the insect descending nociception controls. We discuss what the existence of descending control of nociception in insects may reveal about pain perception in insects and finally consider the ethical implications of these novel findings.

Effects of high-frequency near infrared laser irradiation on experimental tooth movement-induced pain in rats

Discomfort and dull pain are known side effects of orthodontic treatment. Pain is expected to be reduced by near-infrared (NIR) lasers; however, the mechanism underlying effects of short-pulse NIR lasers in the oral and maxillofacial area remains unclear. This study aimed to examine the effects of high-frequency NIR diode laser irradiation on pain during experimental tooth movement (ETM) on 120 J. NIR laser with 910 nm wavelength, 45 W maximum output power, 300 mW average output power, and 200 ns pulse width (Lumix 2; (Lumix 2; Fisioline, Verduno CN, Italy) was used for the experiment. A nickel-titanium-closed coil was used to apply a 50-gf force between the maxillary left-side first molar and incisor in 7-week-old Sprague-Dawley rats (280-300 g) to induce ETM. We measured facial-grooming frequency and vacuous chewing movement (VCM) period between laser-irradiation and ETM groups. We performed immunofluorescent histochemistry analysis to quantify levels of Iba-1, astrocytes, and c-fos protein-like immunoreactivity (Fos-IR) in the trigeminal spinal nucleus caudalis (Vc). Compared with the ETM group, the laser irradiation group had significantly decreased facial-grooming frequency (P = 0.0036), VCM period (P = 0.043), Fos-IR (P = 0.0028), Iba-1 levels (P = 0.0069), and glial fibrillary acidic protein (GFAP) levels (P = 0.0071). High-frequency NIR diode laser irradiation appears to have significant analgesic effects on ETM-induced pain, which involve inhibiting neuronal activity, microglia, and astrocytes, and it inhibits c-fos, Iba-1, and GFAP expression, reducing ETM-induced pain in rats. High-frequency NIR diode laser application could be applied to reduce pain during orthodontic tooth movement.

Involvement of P2X7 receptors in chronic pain disorders

Chronic pain is caused by cellular damage with an obligatory inflammatory component. In response to noxious stimuli, high levels of ATP leave according to their concentration gradient, the intracellular space through discontinuities generated in the plasma membrane or diffusion through pannexin-1 hemichannels, and activate P2X7Rs localized at peripheral and central immune cells. Because of the involvement of P2X7Rs in immune functions and especially the initiation of macrophage/microglial and astrocytic secretion of cytokines, chemokines, prostaglandins, proteases, reactive oxygen, and nitrogen species as well as the excitotoxic glutamate/ATP, this receptor type has a key role in chronic pain processes. Microglia are equipped with a battery of pattern recognition receptors that detect pathogen-associated molecular patterns (PAMPs) such as lipopolysaccharide (LPS) from bacterial infections or danger associated molecular patterns (DAMPs) such as ATP. The co-stimulation of these receptors leads to the activation of the NLRP3 inflammasome and interleukin-1β (IL-1β) release. In the present review, we invite you to a journey through inflammatory and neuropathic pain, primary headache, and regulation of morphine analgesic tolerance, in the pathophysiology of which P2X7Rs are centrally involved. P2X7R bearing microglia and astrocyte-like cells playing eminent roles in chronic pain will be also discussed.

P2X7R in Mast Cells is a Potential Target for Salicylic Acid and Aspirin in Treatment of Inflammatory Pain

Background

Mast cells are well known for their role in inflammatory pain. P2X7 receptor (P2X7R) has attracted much attention due to its prominent role in inflammatory diseases. Salicylates are commonly used anti-inflammatory and analgesic drugs. Until now, little has been known about whether P2X7R in mast cells is involved in inflammatory pain and whether it is a potential target for salicylates.

Methods

First, the expression of P2X receptors in mouse peritoneal mast cells was detected by using RT-PCR, immunofluorescence, calcium imaging and electrophysiological technique. In addition, the functions of P2X receptors, especially P2X7R, in mast cells were studied by using QPCR, ELISA and behavioral tests. Furthermore, P2X7R was used as a target to screen for some anti-inflammatory monomers that could inhibit its activity. At last, the effect of salicylic acid (SA) and aspirin (ASA) on the activity of P2X7R was studied by using calcium imaging, electrophysiological technique, ELISA, real-time PCR, behavioral tests, immunofluorescence and molecular docking.

Results

We found that P2X1, P2X3, P2X4 and P2X7 receptors were expressed in mouse peritoneal mast cells. The functions of different P2X receptors were various. Activation of P2X7R in mouse mast cells induced the release of inflammatory mediators, such as histamine, IL-1β, and CCL3. In addition, inflammation pain induced by high concentrations of ATP could be alleviated by P2X7R blockers or mast cell defects. Interestingly, SA or ASA could reduce high concentrations of ATP-induced inward current, P2X7R upregulation, mediators release, and inflammatory pain. SA or ASA also inhibited the inward current evoked by P2X7R agonist, BZATP. Molecular docking showed that SA or ASA had affinity for the cytoplasmic GDP-binding region of P2X7R.

Conclusion

P2X7R in mast cells was involved in inflammation pain by releasing inflammatory mediators, and P2X7R might be a potential target for SA and ASA analgesia.

Inhibition of P2X7 Purinergic Receptor Ameliorates Fibromyalgia Syndrome by Suppressing NLRP3 Pathway

Fibromyalgia is a chronic condition characterized by persistent widespread pain that significantly reduces quality of life in patients. The purinergic P2X7 receptor (P2X7R) seems to be involved in different pain states and neuroinflammation. The purpose of this study is to investigate the positive effects of P2X7R inhibition by the antagonist Brilliant Blue G (BBG) in a rat model of reserpine-induced fibromyalgia. Sprague-Dawley male rats were injected with 1 mg/kg of reserpine for three consecutive days. Later, animals were administered BBG (50 mg/kg) intraperitoneally for seven days. Reserpine injections induced a significant increase in pain pro-inflammatory mediators as well as a significant increase in neuroinflammation. Chronic pain, in turn, led to depressive-like symptoms and reduced neurogenesis. Blockage of P2X7R by BBG administrations is able to attenuate the behavioral deficits, pain mediators and microglial activation induced by reserpine injection. Additionally, BBG prevents NLRP3 inflammasome activation and consequently the release of active interleukin (IL)-1 and IL-18, involved in the activation of nociceptors. In conclusion, these results suggest that inhibition of P2X7R should be further investigated to develop a potential approach for the management of fibromyalgia.

P2X receptors and trigeminal neuralgia

There is currently no effective cure for trigeminal neuralgia (TN) - a relatively common disease that causes long-term pain in patients. Previous research has shown that ionotropic ATP signaling through excitatory and calcium-permeable P2X receptor channels plays a critical role in pathological pain generation and maintenance. In this paper, we review several hypotheses on the pathogenic mechanisms underlying TN. We further discuss pathways or agents that can target P2X expression in TN, thereby affecting pain induction and maintenance.

Contribution of intraganglionic CGRP to migraine-like responses in male and female rats

Objective

To evaluate whether intraganglionic calcitonin gene-related peptide induced differential migraine-like responses in male and female rats.

Methods

Calcitonin gene-related peptide was injected in the trigeminal ganglion of male and female rats followed by assessment of periorbital mechanical allodynia with von Frey hairs. The influence of systemic treatment with sumatriptan or intraganglionic treatment with minocycline and propentofylline was determined on the calcitonin gene-related peptide-induced mechanical allodynia in male and female rats. One additional group was exposed to an aversive light 24 h after calcitonin gene-related peptide priming, followed by evaluation of periorbital mechanical threshold, and another group was tested in the elevated-plus maze.

Results

Intraganglionar calcitonin gene-related peptide-induced periorbital mechanical allodynia in female (0.5 to 6 h) and male rats (0.5 to 4 h). Systemic sumatriptan briefly attenuated the mechanical allodynia, but intraganglionar minocycline or propentofylline injection was effective only in male rats. Calcitonin gene-related peptide induced photic sensitivity in female and male rats (lasting 4 h and 1 h, respectively), as well as anxiety-like behavior.

Conclusions

Intraganglionar calcitonin gene-related peptide may play a major role in migraine-like responses, including periorbital mechanical allodynia, light sensitivity and anxiety like-behavior. Female rats are likely to be more susceptible to calcitonin gene-related peptide effects and a better understanding of the sexual dimorphism in calcitonin gene-related peptide signaling may help to improve migraine therapy.

Neuronal P2X7 receptor-induced reactive oxygen species production contributes to nociceptive behavior in mice

ATP can activate a variety of pathways through P2 purinoreceptors, leading to neuroprotection and pathology in the CNS. Among all P2X receptors, the P2X7 receptor (P2X7R) is a well-defined therapeutic target for inflammatory and neuropathic pain. Activation of P2X7R can generate reactive oxygen species (ROS) in macrophages and microglia. However, the role of ROS in P2X7R–induced pain remains unexplored. Here, we investigated the downstream effects of neuronal P2X7R activation in the spinal cord. We found that ATP induces ROS production in spinal cord dorsal horn neurons, an effect eliminated by ROS scavenger N-tert-butyl-α-phenylnitrone (PBN) and P2X7R antagonist A438079. A similar effect was observed with a P2X7R agonist, BzATP, and was attenuated by a NADPH oxidase inhibitor apocynin. Intrathecal administration of BzATP resulted in ROS production in the spinal cord and oxidative DNA damage in dorsal horn neurons. BzATP also induced robust biphasic spontaneous nociceptive behavior. Pre-treatment with A438079 abolished all BzATP-induced nociceptive behaviors, while ROS scavengers dose-dependently attenuated the secondary response. Here, we provide evidence that neuronal P2X7R activation leads to ROS production and subsequent nociceptive pain in mice. Together, the data indicate that P2X7R-induced ROS play a critical role in the P2X7R signaling pathway of the CNS.

ERK potentiates p38 in central sensitization induced by traumatic occlusion

This study was to investigate the role of p38 activation via ERK1/2 phosphorylation in neurons and microglia of the spinal trigeminal subnucleus caudalis (Vc) in the promotion of orofacial hyperalgesia induced by unilateral anterior crossbite (UAC) traumatic occlusion in adult rats. U0126, a p-ERK1/2 inhibitor, was injected intracisternally before UAC implant. The effects of the U0126 injection were compared to those following the injection of SB203580, a p-p38 inhibitor. Mechanical hyperalgesia was evaluated via pressure pain threshold measurements. Brain stem tissues were processed for a Western blot analysis to evaluate the activation of ERK1/2 and p38. Double immunofluorescence was also performed to observe the expression of p-ERK1/2 and p-p38 in neurons (labeled by NeuN) and microglia (labeled by OX42). The data showed that UAC caused orofacial hyperalgia ipsilaterally on d1 to d7, peaking on d3 (P<0.05). An upregulation of p-ERK1/2 was observed in the ipsilateral Vc on d1 to d3, peaking on d1. An upregulation of p-p38 was also observed on d1 to d7, peaking on d3 (P<0.05). p-ERK1/2 primarily co-localized with NeuN and, to a lesser extent, with OX42, while p-p38 co-localized with both NeuN and OX42. Pretreatment with U0126 prevented the upregulation of both p-ERK1/2 and p-p38. Similarly to an intracisternal injection of SB203580, U0126 pretreatment attenuated the UAC-induced orofacial hyperalgesia. These data indicate that UAC caused orofacial hyperalgesia by inducing central sensitization via the activation of ERK1/2 and p38 in both neurons and microglia in the Vc, potentially impacting the effects of p-ERK1/2 during p38 activation.

Brilliant Blue Dyes in Daily Food: How Could Purinergic System Be Affected?

Dyes were first obtained from the extraction of plant sources in the Neolithic period to produce dyed clothes. At the beginning of the 19th century, synthetic dyes were produced to color clothes on a large scale. Other applications for synthetic dyes include the pharmaceutical and food industries, which are important interference factors in our lives and health. Herein, we analyzed the possible implications of some dyes that are already described as antagonists of purinergic receptors, including special Brilliant Blue G and its derivative FD&C Blue No. 1. Purinergic receptor family is widely expressed in the body and is critical to relate to much cellular homeostasis maintenance as well as inflammation and cell death. In this review, we discuss previous studies and show purinergic signaling as an important issue to be aware of in food additives development and their correlations with the physiological functions.

Neuronal NTPDase3 Mediates Extracellular ATP Degradation in Trigeminal Nociceptive Pathway

ATP induces pain via activation of purinergic receptors in nociceptive sensory nerves. ATP signaling is terminated by ATP hydrolysis mediated by cell surface-localized ecto-nucleotidases. Using enzymatic histochemical staining, we show that ecto-ATPase activity is present in mouse trigeminal nerves. Using immunofluorescence staining, we found that ecto-NTPDase3 is expressed in trigeminal nociceptive neurons and their projections to the brainstem. In addition, ecto-ATPase activity and ecto-NTPDase3 are also detected in the nociceptive outermost layer of the trigeminal subnucleus caudalis. Furthermore, we demonstrate that incubation with anti-NTPDase3 serum reduces extracellular ATP degradation in the nociceptive lamina of both the trigeminal subnucleus caudalis and the spinal cord dorsal horn. These results are consistent with neuronal NTPDase3 activity modulating pain signal transduction and transmission by affecting extracellular ATP hydrolysis within the trigeminal nociceptive pathway. Thus, disruption of trigeminal neuronal NTPDase3 expression and localization to presynaptic terminals during chronic inflammation, local constriction and injury may contribute to the pathogenesis of orofacial neuropathic pain.

A Comparison of the Sensitivity of Brush Allodynia and Semmes-Weinstein Monofilament Testing in the Detection of Allodynia Within Regions of Secondary Hyperalgesia in Humans

BACKGROUND

Two of the most common Quantitative Sensory Techniques (QST) employed to detect allodynia include mechanical brush allodynia and Semmes-Weinstein monofilaments. However, their relative sensitivity at detecting allodynia is poorly understood. The purpose of this study was to compare the sensitivity of brush allodynia against Semmes-Weinstein monofilament technique for detecting allodynia within regions of secondary hyperalgesia in humans.

METHODS

Twenty subjects (10 males, 10 females; 21.1 ± 0.9 years) were recruited and randomly allocated to allodynia or monofilament groups. Topical capsaicin (Zostrix 0.075%) was applied to a target region defined by C4-C7 dermatomes. Allodynia testing was performed at 0- (baseline) and 10 minutes postcapsaicin. The Semmes-Weinstein group assessed changes in skin sensitivity 8 cm inferior to target region and 2 cm lateral to the spinous process, while brush allodynia was employed to detect the point inferior to the target region where subjects reported changes in skin sensitivity. The distance (cm) from this point to the inferior border of the target region was termed the Allodynia Score.

RESULTS

Statistically significant increases in the Allodynia Score were observed at 10 minutes postcapsaicin compared to baseline (P < 0.001). No differences in monofilament scores were observed between 10 minutes postcapsaicin and baseline (P = 0.125). Brush allodynia also demonstrated superior sensitivity, detecting allodynia in 100% of cases compared to 60% in the Semmes-Weinstein group.

CONCLUSION

Brush allodynia is more sensitive than Semmes-Weinstein monofilaments for detecting mechanical allodynia in regions of secondary hyperalgesia. Brush allodynia may be preferred over Semmes-Weinstein monofilaments for clinical applications requiring reliable detection of allodynia.

Minocycline enhances inhibitory transmission to substantia gelatinosa neurons of the rat spinal dorsal horn

Minocycline, a second-generation tetracycline, is well known for its antibiotic, anti-inflammatory, and antinociceptive effects. Modulation of synaptic transmission is one of the analgesic mechanisms of minocycline. Although it has been reported that minocycline may suppress excitatory glutamatergic synaptic transmission, it remains unclear whether it could affect inhibitory synaptic transmission, which also plays a key role in modulating pain signaling. To examine the effect of minocycline on synaptic transmission in rat spinal substantia gelatinosa (SG) neurons, we recorded spontaneous inhibitory postsynaptic currents (sIPSCs) using whole-cell patch-clamp recording at a holding potential of 0 mV. Bath application of minocycline significantly increased the frequency but not the amplitude of sIPSCs in a reversible and concentration-dependent manner with an EC50 of 85. The enhancement of inhibitory synaptic transmission produced by minocycline was not affected by the glutamate receptor antagonists CNQX and D-APV or by the voltage-gated sodium channel blocker tetrodotoxin (TTX). Moreover, the potency of minocycline for facilitating sIPSC frequency was the same in both glycinergic and GABAergic sIPSCs without changing their decay phases. However, the facilitatory effect of minocycline on sIPSCs was eliminated in a Ca(2+)-free Krebs solution or by co-administration with calcium channel blockers. In summary, our data demonstrate that baseline inhibitory synaptic transmission in SG neurons is markedly enhanced by minocycline. This may function to decrease the excitability of SG neurons, thus leading to a modulation of nociceptive transmission.

P2X ion channel receptors and inflammation

Neuroinflammation limits tissue damage in response to pathogens or injury and promotes repair. There are two stages of inflammation, initiation and resolution. P2X receptors are gaining attention in relation to immunology and inflammation. The P2X7 receptor in particular appears to be an essential immunomodulatory receptor, although P2X1 and P2X4 receptors also appear to be involved. ATP released from damaged or infected cells causes inflammation by release of inflammatory cytokines via P2X7 receptors and acts as a danger signal by occupying upregulated P2X receptors on immune cells to increase immune responses. The purinergic involvement in inflammation is being explored for the development of novel therapeutic strategies.

Purinergic Mechanisms and Pain

There is a brief introductory summary of purinergic signaling involving ATP storage, release, and ectoenzymatic breakdown, and the current classification of receptor subtypes for purines and pyrimidines. The review then describes purinergic mechanosensory transduction involved in visceral, cutaneous, and musculoskeletal nociception and on the roles played by receptor subtypes in neuropathic and inflammatory pain. Multiple purinoceptor subtypes are involved in pain pathways both as an initiator and modulator. Activation of homomeric P2X3 receptors contributes to acute nociception and activation of heteromeric P2X2/3 receptors appears to modulate longer-lasting nociceptive sensitivity associated with nerve injury or chronic inflammation. In neuropathic pain activation of P2X4, P2X7, and P2Y12 receptors on microglia may serve to maintain nociceptive sensitivity through complex neural-glial cell interactions and antagonists to these receptors reduce neuropathic pain. Potential therapeutic approaches involving purinergic mechanisms will be discussed.

Diverse effects of Brilliant Blue G administration in models of trigeminal activation in the rat

Activation of the trigeminal system plays an important role in the pathomechanism of headaches. A better understanding of trigeminal pain processing is expected to provide information helping to unravel the background of these diseases. ATP, a key modulator of nociceptive processing, acts on ligand-gated P2X receptors. Antagonists of the P2X7 receptors, such as Brilliant Blue G (BBG), have proved effective in several models of pain. We have investigated the effects of BBG after electrical stimulation of the trigeminal ganglion and in the orofacial formalin test in the rat. The right trigeminal ganglion of male rats was stimulated either with 5 Hz, 0.5 mA pulses for 5 min (mild procedure) or with 10 Hz, 0.5 mA pulses for 30 min (robust procedure), preceded by 50 mg/kg i.v. BBG. The animals were processed for c-Fos and calcitonin gene-related peptide (CGRP) immunohistochemistry. In the orofacial formalin test, 50 μL of 1.5 % formalin was injected into the right whisker pad of awake rats, following the pre-treatment with BBG. Behaviour was monitored for 45 min, and c-Fos and CGRP immunohistochemistry was performed. BBG attenuated the increase in c-Fos-positive cells in the caudal trigeminal nucleus (TNC) after robust stimulation, but not after mild stimulation. No alterations in CGRP levels were found with either methodology. BBG did not mitigate either the behaviour or the increase in c-Fos-positive cells in the TNC during the orofacial formalin test. These results indicate that P2X7 receptors may have a role in the modulation of nociception in the trigeminal system.

P2X7 receptor: an emerging target in central nervous system diseases

The ATP-sensitive homomeric P2X7 receptor (P2X7R) has received particular attention as a potential drug target because of its widespread involvement in inflammatory diseases as a key regulatory element of the inflammasome complex. However, it has only recently become evident that P2X7Rs also play a pivotal role in central nervous system (CNS) pathology. There is an explosion of data indicating that genetic deletion and pharmacological blockade of P2X7Rs alter responsiveness in animal models of neurological disorders, such as stroke, neurotrauma, epilepsy, neuropathic pain, multiple sclerosis (MS), amyotrophic lateral sclerosis (ALS), Alzheimer's disease, Parkinson's disease, and Huntington's disease. Moreover, recent studies suggest that P2X7Rs regulate the pathophysiology of psychiatric disorders, including mood disorders, implicating P2X7Rs as drug targets in a variety of CNS pathology.

The P2X7 receptor channel: recent developments and the use of P2X7 antagonists in models of disease

The P2X7 receptor is a trimeric ATP-gated cation channel found predominantly, but not exclusively, on immune cells. P2X7 activation results in a number of downstream events, including the release of proinflammatory mediators and cell death and proliferation. As such, P2X7 plays important roles in various inflammatory, immune, neurologic and musculoskeletal disorders. This review focuses on the use of P2X7 antagonists in rodent models of neurologic disease and injury, inflammation, and musculoskeletal and other disorders. The cloning and characterization of human, rat, mouse, guinea pig, dog, and Rhesus macaque P2X7, as well as recent observations regarding the gating and permeability of P2X7, are discussed. Furthermore, this review discusses polymorphic and splice variants of P2X7, as well as the generation and use of P2X7 knockout mice. Recent evidence for emerging signaling pathways downstream of P2X7 activation and the growing list of negative and positive modulators of P2X7 activation and expression are also described. In addition, the use of P2X7 antagonists in numerous rodent models of disease is extensively summarized. Finally, the use of P2X7 antagonists in clinical trials in humans and future directions exploring P2X7 as a therapeutic target are described.

Trigeminal nerve injury-induced thrombospondin-4 up-regulation contributes to orofacial neuropathic pain states in a rat model

BACKGROUND

Injury to the trigeminal nerve often results in the development of chronic pain states including tactile allodynia, or hypersensitivity to light touch, in orofacial area, but its underlying mechanisms are poorly understood. Peripheral nerve injury has been shown to cause up-regulation of thrombospondin-4 (TSP4) in dorsal spinal cord that correlates with neuropathic pain development. In this study, we examined whether injury-induced TSP4 is critical in mediating orofacial pain development in a rat model of chronic constriction injury to the infraorbital nerve.

METHODS

Orofacial sensitivity to mechanical stimulation was examined in a unilateral infraorbital nerve ligation rat model. The levels of TSP4 in trigeminal ganglia and associated spinal subnucleus caudalis and C1/C2 spinal cord (Vc/C2) from injured rats were examined at time points correlating with the initiation and peak orofacial hypersensitivity. TSP4 antisense and mismatch oligodeoxynucleotides were intrathecally injected into injured rats to see if antisense oligodeoxynucleotide treatment could reverse injury-induced TSP4 up-regulation and orofacial behavioural hypersensitivity.

RESULTS

Our data indicated that trigeminal nerve injury induced TSP4 up-regulation in Vc/C2 at a time point correlated with orofacial tactile allodynia. In addition, intrathecal treatment with TSP4 antisense, but not mismatch, oligodeoxynucleotides blocked both injury-induced TSP4 up-regulation in Vc/C2 and behavioural hypersensitivity.

CONCLUSIONS

Our data support that infraorbital nerve injury leads to TSP4 up-regulation in trigeminal spinal complex that contributes to orofacial neuropathic pain states. Blocking this pathway may provide an alternative approach in management of orofacial neuropathic pain states.

Physiological roles and potential therapeutic applications of the P2X7 receptor in inflammation and pain

The P2X7 receptor (P2X7R) is a nonselective cation channel that is activated by extracellular ATP and triggers the secretion of several proinflammatory substances, such as IL-1β, IL-18, TNF-α, and nitric oxide. Recently, several preclinical studies have demonstrated that this receptor participates in inflammation and pain mechanisms. Taken together, these results indicate that P2X7R is a promising pharmacological target, and compounds that modulate the function of this receptor show potential as new anti-inflammatory medicines. In this review, we discuss aspects of P2X7R pharmacology and the participation of this protein in inflammation and pain and provide an overview of some promising compounds that have been tested as antagonists of P2X7R, with clinical applicability.

Cellular and molecular mechanisms of dental nociception

Due, in part, to the unique structure of the tooth, dental pain is initiated via distinct mechanisms. Here we review recent advances in our understanding of inflammatory tooth pain and discuss 3 hypotheses proposed to explain dentinal hypersensitivity: The first hypothesis, supported by functional expression of temperature-sensitive transient receptor potential channels, emphasizes the direct transduction of noxious temperatures by dental primary afferent neurons. The second hypothesis, known as hydrodynamic theory, attributes dental pain to fluid movement within dentinal tubules, and we discuss several candidate cellular mechanical transducers for the detection of fluid movement. The third hypothesis focuses on the potential sensory function of odontoblasts in the detection of thermal or mechanical stimuli, and we discuss the accumulating evidence that supports their excitability. We also briefly update on a novel strategy for local nociceptive anesthesia via nociceptive transducer molecules in dental primary afferents with the potential to specifically silence pain fibers during dental treatment. Further understanding of the molecular mechanisms of dental pain would greatly enhance the development of therapeutics that target dental pain.

Central sensitization and MAPKs are involved in occlusal interference-induced facial pain in rats

We previously developed a rat dental occlusal interference model of facial pain that was produced by bonding a crown onto the right maxillary first molar and was reflected in sustained facial hypersensitivity that was suggestive of the involvement of central sensitization mechanisms. The aim of the present study was to investigate potential central mechanisms involved in the occlusal interference-induced facial hypersensitivity. A combination of behavioral, immunohistochemical, Western blot, and electrophysiological recording procedures was used in 98 male adult Sprague Dawley rats that either received the occlusal interference or were sham-operated or naive rats. Immunohistochemically labeled astrocytes and microglia in trigeminal subnucleus caudalis (Vc) showed morphological changes indicative of astrocyte and microglial activation after the occlusal interference. Prolonged upregulation of p38 mitogen-activated protein kinase (MAPK) and extracellular signal-regulated kinase (ERK) was also documented in Vc after placement of the occlusal interference and was expressed in both neurons and glial cells at time points when rats showed peak mechanical facial hypersensitivity. The intrathecal administration of the p38 MAPK inhibitor SB203580 to the medulla significantly inhibited the occlusal interference-induced hypersensitivity, and the ERK inhibitor PD98059 produced an even stronger effect. Central sensitization of functionally identified Vc nociceptive neurons following placement of the occlusal interference was also documented by extracellular electrophysiological recordings, and intrathecal administration of PD98059 could reverse the neuronal central sensitization. These novel findings suggest that central mechanisms including central sensitization of trigeminal nociceptive neurons and non-neuronal processes involving MAPKs play significant roles in the production of occlusal interference-induced facial pain.

PERSPECTIVE

Central mechanisms including trigeminal nociceptive neuronal sensitization, non-neuronal processes involving glial activation, and MAPKs play significant roles in occlusal interference-induced facial pain. These mechanisms may be involved in clinical manifestations of facial pain that have been reported in patients with an occlusal interference.

Central α-adrenoceptors contribute to mustard oil-induced central sensitization in the rat medullary dorsal horn

Our previous studies have demonstrated that application of the inflammatory irritant mustard oil (MO) to the tooth pulp produces trigeminal central sensitization that includes increases in mechanoreceptive field size and responses to noxious stimuli and decrease in activation threshold in brainstem nociceptive neurons of trigeminal subnucleus caudalis (the medullary dorsal horn, MDH). The aim of the present study was to test if central noradrenergic processes are involved in the central sensitization of MDH neurons and if α1-adrenoceptors or α2-adrenoceptors or both are involved. In urethane/α-chloralose-anesthetized rats, the activity of extracellularly recorded and functionally identified single nociceptive neurons in the MDH was studied. Continuous intrathecal (i.t.) superfusion of the adrenergic modulator guanethidine and α-adrenoceptor blocker phentolamine or selective α1-adrenoceptor antagonist prazosin over the medulla strongly attenuated all three MO-induced parameters of central sensitization in the MDH nociceptive neurons, compared to phosphate-buffered saline (as vehicle control). In contrast, i.t. superfusion of the selective α2-adrenoceptor antagonist yohimbine had little effect on the mechanoreceptive field expansion and the decreased mechanical activation threshold, and indeed facilitated responses to noxious stimuli of sensitized nociceptive neurons. Superfusion of each of the four chemicals alone did not affect baseline nociceptive neuronal properties. These findings provide the first documentation of the involvement of central noradrenergic processes in MDH in the development of the central sensitization, and that α1- and α2-adrenoceptors may be differentially involved.

Microglia and mast cells: two tracks on the road to neuroinflammation

One of the more important recent advances in neuroscience research is the understanding that there is extensive communication between the immune system and the central nervous system (CNS). Proinflammatory cytokines play a key role in this communication. The emerging realization is that glia and microglia, in particular, (which are the brain's resident macrophages), constitute an important source of inflammatory mediators and may have fundamental roles in CNS disorders from neuropathic pain and epilepsy to neurodegenerative diseases. Microglia respond also to proinflammatory signals released from other non-neuronal cells, principally those of immune origin. Mast cells are of particular relevance in this context. These immunity-related cells, while resident in the CNS, are capable of migrating across the blood-spinal cord and blood-brain barriers in situations where the barrier is compromised as a result of CNS pathology. Emerging evidence suggests the possibility of mast cell-glia communications and opens exciting new perspectives for designing therapies to target neuroinflammation by differentially modulating the activation of non-neuronal cells normally controlling neuronal sensitization, both peripherally and centrally. This review aims to provide an overview of recent progress relating to the pathobiology of neuroinflammation, the role of microglia, neuroimmune interactions involving mast cells, in particular, and the possibility that mast cell-microglia crosstalk may contribute to the exacerbation of acute symptoms of chronic neurodegenerative disease and accelerate disease progression, as well as promote pain transmission pathways. We conclude by considering the therapeutic potential of treating systemic inflammation or blockade of signaling pathways from the periphery to the brain in such settings.

P2X4 purinoceptor signaling in chronic pain

ATP, acting via P2 purinergic receptors, is a known mediator of inflammatory and neuropathic pain. There is increasing evidence that the ATP-gated P2X4 receptor (P2X4R) subtype is a locus through which activity of spinal microglia and peripheral macrophages instigate pain hypersensitivity caused by inflammation or by injury to a peripheral nerve. The present article highlights the recent advances in our understanding of microglia-neuron interactions in neuropathic pain by focusing on the signaling and regulation of the P2X4R. We will also develop a framework for understanding converging lines of evidence for involvement of P2X4Rs expressed on macrophages in peripheral inflammatory pain.

P2X7 receptor-stimulation causes fever via PGE2 and IL-1β release

Prostaglandins (PGs) are important lipid mediators involved in the development of inflammatory associated pain and fever. PGE2 is a well-established endogenous pyrogen activated by proinflammatory cytokine interleukin (IL)-1β. P2X7 receptors (P2X7Rs) expressed by inflammatory cells are stimulated by the danger signal extracellular ATP to activate the inflammasome and release IL-1β. Here we show that P2X7R activation is required for the release of PGE2 and other autacoids independent of inflammasome activation, with an ATP EC(50) for PGE2 and IL-1β release of 1.58 and 1.23 mM, respectively. Furthermore, lack of P2X7R or specific antagonism of P2X7R decreased the febrile response in mice triggered after intraperitoneal LPS or IL-1β inoculation. Accordingly, LPS inoculation caused intraperitoneal ATP accumulation. Therefore, P2X7R antagonists emerge as novel therapeutics for the treatment for acute inflammation, pain and fever, with wider anti-inflammatory activity than currently used cyclooxygenase inhibitors.-Barberà-Cremades, M., Baroja-Mazo, A., Gomez, A. I., Machado, F., Di Virgilio, F., Pelegrín, P. P2X7 receptor-stimulation causes fever via PGE2 and IL-1β release.

Functional properties of tooth pulp neurons responding to thermal stimulation

The response properties of tooth pulp neurons that respond to noxious thermal stimulation of the dental pulp have been not well-studied. The present study was designed to characterize the response properties of tooth pulp neurons to noxious thermal stimulation of the dental pulp. Experiments were conducted on 25 male ferrets, and heat stimulation was applied by a computer-controlled thermode. Only 15% of tooth pulp neurons (n = 39) responded to noxious thermal stimulation of the teeth. Tooth pulp neurons were found in both the superficial and deep nuclear regions of the subnucleus caudalis (Vc) and in the interface between the nucleus caudalis and interpolaris (Vc/Vi). Thirty-seven neurons had cutaneous receptive fields and were classified as either NS (16) or WDR (21) neurons. Repeated heat stimulation of the dental pulp sensitized and increased the number of electrically evoked potentials of tooth pulp neurons. These results provide evidence that both the Vc and Vc/Vi regions contain neurons that respond to noxious thermal stimulation of the dental pulp, and that these cells may contribute to the sensitization process associated with symptomatic pulpitis.

A decerebrate adult mouse model for examining the sensorimotor control of locomotion

As wild-type and genetically modified mice are progressively becoming the predominant models for studying locomotor physiology, the technical ability to record sensory and motor components from adult mice, in vivo, are expected to contribute to a better understanding of sensorimotor spinal cord networks. Here, specific technical and surgical details are presented on how to produce an adult decerebrate mouse preparation that can reliably produce sustained bouts of stepping, in vivo, in the absence of anesthetic drugs. Data are presented demonstrating the ability of this preparation to produce stepping during treadmill locomotion, adaptability in its responses to changes in the treadmill speed, and left-right alternation. Furthermore, intracellular recordings from motoneurons and interneurons in the spinal cord are presented from preparations where muscle activity was blocked. Intraaxonal recordings are also presented demonstrating that individual afferents can be recorded using this preparation. These data demonstrate that the adult decerebrate mouse is a tractable preparation for the study of sensorimotor systems.