Involvement of temporomandibular joint P2X3 and P2X2/3 receptors in carrageenan-induced inflammatory hyperalgesia in rats

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.

Role and therapeutic target of P2X2/3 receptors in visceral pain

Visceral pain (VP) is caused by internal organ disease. VP is involved in nerve conduction and related signaling molecules, but its specific pathogenesis has not yet been fully elucidated. Currently, there are no effective methods for treating VP. The role of P2X2/3 in VP has progressed. After visceral organs are subjected to noxious stimulation, cells release ATP, activate P2X2/3, enhance the sensitivity of peripheral receptors and the plasticity of neurons, enhance sensory information transmission, sensitize the central nervous system, and play an important role in the development of VP. However, antagonists possess the pharmacological effect of relieving pain. Therefore, in this review, we summarize the biological functions of P2X2/3 and discuss the intrinsic link between P2X2/3 and VP. Moreover, we focus on the pharmacological effects of P2X2/3 antagonists on VP therapy and provide a theoretical basis for its targeted therapy.

Preclinical orofacial pain assays and measures and chronic primary orofacial pain research: where we are and where we need to go

Chronic primary orofacial pain (OFP) conditions such as painful temporomandibular disorders (pTMDs; i.e., myofascial pain and arthralgia), idiopathic trigeminal neuralgia (TN), and burning mouth syndrome (BMS) are seemingly idiopathic, but evidence support complex and multifactorial etiology and pathophysiology. Important fragments of this complex array of factors have been identified over the years largely with the help of preclinical studies. However, findings have yet to translate into better pain care for chronic OFP patients. The need to develop preclinical assays that better simulate the etiology, pathophysiology, and clinical symptoms of OFP patients and to assess OFP measures consistent with their clinical symptoms is a challenge that needs to be overcome to support this translation process. In this review, we describe rodent assays and OFP pain measures that can be used in support of chronic primary OFP research, in specific pTMDs, TN, and BMS. We discuss their suitability and limitations considering the current knowledge of the etiology and pathophysiology of these conditions and suggest possible future directions. Our goal is to foster the development of innovative animal models with greater translatability and potential to lead to better care for patients living with chronic primary OFP.

Folic acid relieves bone cancer pain by downregulating P2X2/3 receptors in rats

BACKGROUND

Bone cancer pain (BCP) remains a clinical challenge due to the limited and side effects of therapeutic methods. Folic acid has been known as an FDA approved dietary supplement and proved to have an analgesic effect in neuropathic pain. Here we investigate the role and mechanism of folic acid in bone cancer pain of a rat model.

METHODS

Walker 256 tumor cells were inoculated into the left tibia of rats to induce bone cancer pain model. Pain reflex were assessed by paw withdrawal threshold (PWT) response to Von Frey filaments and paw withdrawal latency (PWL) response to thermal stimulation. Folic acid was injected intraperitoneally to evaluate its analgesic effect in rats with bone cancer pain. Western blotting and qPCR were used to determine P2X2/3 receptor protein and mRNA levels in ipsilateral L4-6 dorsal root ganglion (DRG) and spinal dorsal horn (SDH).

RESULTS

The PWT and PWL of rats with bone cancer pain were obviously decreased compared to the naïve and sham rats. Interestingly, continuous folic acid treatment significantly increased the PWT and PWL of rats with bone cancer pain. P2X2 and P2X3 receptors were clearly upregulated at both mRNA and protein expression in L4-6 DRG and SDH of rats with bone cancer pain. P2X2 and P2X3 receptors were mainly localized with CGRP (calcitonin gene-related peptide) or IB4 (isolectin B4) positive neurons in L4-6 DRG of rats with bone cancer pain. Notably, continuous folic acid treatment significantly reduced the expression of P2X2 and P2X3 receptors in L4-6 DRG and SDH of rats with bone cancer pain. Finally, intrathecal injection of A317491 (a selective antagonist of P2X2/3 receptors) markedly elevated the PWT and PWL of rats with bone cancer pain.

CONCLUSION

These results suggest that folic acid has an effective antinociceptive effect on bone cancer pain, which is mediated by downregulating P2X2/3 receptors in L4-6 DRG and SDH of rats with bone cancer pain. Folic acid may be a novel therapeutic strategy in cancer patients for pain relief.

Physiologic roles of P2 receptors in leukocytes

Since their discovery in the 1970s, purinergic receptors have been shown to play key roles in a wide variety of biologic systems and cell types. In the immune system, purinergic receptors participate in innate immunity and in the modulation of the adaptive immune response. In particular, P2 receptors, which respond to extracellular nucleotides, are widely expressed on leukocytes, causing the release of cytokines and chemokines and the formation of inflammatory mediators, and inducing phagocytosis, degranulation, and cell death. The activity of these receptors is regulated by ectonucleotidases-expressed in these same cell types-which regulate the availability of nucleotides in the extracellular environment. In this article, we review the characteristics of the main purinergic receptor subtypes present in the immune system, focusing on the P2 family. In addition, we describe the physiologic roles of the P2 receptors already identified in leukocytes and how they can positively or negatively modulate the development of infectious diseases, inflammation, and pain.

Noise exposure and its relationship with postinfarction cardiac remodeling: implications for NLRP3 inflammasome activation

In recent years, high-decibel noise has emerged as a causative risk factor for ischemic heart disease. Massive noise overdose is associated with increased endocrine, neural, and immune stress responses. The NLRP3 (nucleotide-binding oligomerization domain, leucine-rich repeat, and pyrin domain containing 3) inflammasome, the most characterized supramolecular complex and a potent mediator of inflammatory signaling, has been reported to be a marker of increased ischemic heart disease vulnerability. Our study evaluated the association of noise exposure with postinfarction cardiac remodeling and its effect on NLRP3 inflammasome activation. Rats were exposed to a noisy environment (14 days, 24 h/per day, 70 ± 5 dB), and speck formation by the NLRP3 inflammasome scaffold protein ASC (apoptosis-associated speck-like protein) was assessed by confocal immunofluorescence. Echocardiography, pathological analysis, and in vivo electrophysiology were performed. Our results revealed the improved postinfarction cardiac function, mitigated fibrosis, and decreased arrhythmia vulnerability and sympathetic sprouting in low-environment noise groups. Moreover, western blotting of NLRP3, caspase-1, ASC, IL-1β, and IL-18 and confocal microscopy of ASC speck showed that the priming and activation of NLRP3 inflammasome were higher in the NE group than in the NI group. In conclusion, our findings reveal a previously unidentified association between NLRP3 inflammasome activation and noise exposure, underscoring the significance of effective noise prevention in improving postinfarction prognosis.

Photobiomodulation inhibits inflammation in the temporomandibular joint of rats

Photobiomodulation (PBM) has been applied as a non-invasive technique for treating temporomandibular joint symptoms, especially on painful condition's relief, however the anti-inflammatory mechanism underlying the effect of PBM remains uncertain. This study aims to evaluate the mechanisms of action of PBM (808 nm) in a carrageenan-induced inflammation on temporomandibular joint (TMJ) of rats. In this study male Wistar rats were pre-treated with irradiation of a low-power diode laser for 15 s on TMJ (infra-red 808 nm, 100 mW, 50 J/cm² and 1.5 J) 15 min prior an injection in the temporomandibular joint of carrageenan (100 μg/TMJ). 1 h after the TMJ treatments, the rats were terminally anesthetized for joint cavity wash and periarticular tissues collect. Samples analysis demonstrated that PBM inhibit leukocytes chemotaxis in the TMJ and significantly reduces amounts of TNF-α, IL-1β and CINC-1. In addition, Western blotting analysis demonstrated that PBM significantly decreased the protein levels of P2X3 and P2X7 receptors in the periarticular tissues. On the other hand, PBM was able to increase protein level of IL-10 (anti-inflammatory cytokine). In summary, it is possible to suggest that PBM inhibit inflammatory chemotaxis, modulation the balance of the pro- and anti-inflammatory characteristics of inflammatory cells.

The role of adenosine A1 receptor in the peripheral tramadol's effect in the temporomandibular joint of rats

Peripheral tramadol's delivery in the temporomandibular joint (TMJ) leads to significant analgesic outcomes and inflammatory process's resolvent actions. Mechanistically, these properties are apart from the opioid system. Nevertheless, the molecular mechanisms behind these effects are still unclear. Therefore, the present study investigated the hypothesis that adenosine A₁ receptors are involved in the tramadol-induced analgesic and anti-inflammatory effects in the TMJ. Animals were pretreated with an intra-TMJ injection of DPCPX (antagonist of A₁ receptor) or tramadol and subsequent nociceptive challenge with an intra-TMJ injection of 1.5% formalin. For over 45 min, the nociceptive behavior was quantitated, and by the end of this assessment, the animals were euthanized, and the periarticular tissue was collected. Lastly, an in vitro assay of BMDM (Bone Marrow-Derived Macrophages) was performed to investigate tramadol activity in macrophages. The intra-TMJ injection of tramadol ameliorates formalin-induced hypernociception along with inhibiting leukocyte migration. The tramadol's peripheral anti-inflammatory effect was mediated by the adenosine A₁ receptor and was associated with increased protein expression of α2a-adrenoceptor in the periarticular tissues (p < 0.05: ANOVA, Tukey's test). Also, tramadol inhibits formalin-induced leukocyte migration and protein expression of P2X7 receptors in the periarticular tissue (p < 0.05); however, DPCPX did not alter this effect (p > 0.05). Moreover, DPCPX significantly reduced the protein expression of the M2 macrophage marker, MRC1. In BMDM, tramadol significantly reduces inflammatory cytokines release, and DPCPX abrogated this effect (p < 0.05). We identify tramadol's peripheral effect is mediated by adenosine A₁ receptor, possibly expressed in macrophages in the TMJ tissue. We also determined an important discovery related to the activation of A₁R/α2_a receptors in the tramadol action.

The role of connexins and pannexins in orofacial pain

Trigeminal neuralgia is characterized by extensive spreading of pain, referred to as ectopic pain, which describes the phenomenon of the pain passing from the injured regions to uninjured regions. Patients with orofacial pain often show no response to commonly used analgesics, and the exact mechanism of ectopic pain remains unclear, which restricts the development of specific drugs. The present review aims to summarize the contribution of the two families of transmembrane proteins, connexins (Cxs) and pannexins (Panxs), to the induction and spreading of orofacial pain and to provide potential targets for orofacial pain treatment. Cxs and Panxs have recently been shown to play essential roles in intercellular signal propagation in sensory ganglia, and previous studies have provided evidence for the contribution of several subtypes of Cxs and Panxs in various orofacial pain models. Upregulation of the expression of Cxs and Panxs in the trigeminal ganglia is observed in most cases after trigeminal injury, and regulating their expression or activity can improve pain-like behaviors in animals. It is speculated that after trigeminal injury, pain-related signals are transmitted to adjacent neurons and satellite glial cells in the trigeminal ganglia directly through gap junctions and simultaneously through hemichannels and pannexons through both autocrine and paracrine mechanisms. This review highlights recent discoveries in the regulation of Cxs and Panxs in different orofacial pain models and presents a hypothetical mechanism of ectopic pain in trigeminal neuralgia. In addition, the existing problems in current research are discussed.

P2X3 receptors contribute to transition from acute to chronic muscle pain

This study aimed to evaluate whether the development and/or maintenance of chronic-latent muscle hyperalgesia is modulated by P2X3 receptors. We also evaluate the expression of P2X3 receptors and PKCε of dorsal root ganglions during these processes. A mouse model of chronic-latent muscle hyperalgesia, induced by carrageenan and evidenced by PGE₂, was used. Mechanical muscle hyperalgesia was measured by Randall-Selitto analgesimeter. The involvement of P2X3 receptors was analyzed by using the selective P2X3 receptors antagonist A-317491 by intramuscular or intrathecal injections. Expression of P2X3 and PKCε in dorsal root ganglion (L4-S1) were evaluated by Western blotting. Intrathecal blockade of P2X3 receptors previously to carrageenan prevented the development and maintenance of acute and chronic-latent muscle hyperalgesia, while intramuscular blockade of P2X3 receptors previously to carrageenan only reduced the acute muscle hyperalgesia and had no effect on chronic-latent muscle hyperalgesia. Intrathecal, but not intramuscular, blockade of P2X3 receptors immediately before PGE₂, in animals previously sensitized by carrageenan, reversed the chronic-latent muscle hyperalgesia. There was an increase in total and phosphorylated PKCε 48 h after the beginning of acute muscle hyperalgesia, and in P2X3 receptors at the period of chronic muscle hyperalgesia. P2X3 receptors expressed on spinal cord dorsal horn contribute to transition from acute to chronic muscle pain. We also suggest an interaction of PKCε and P2X3 receptors in this process. Therefore, we point out P2X3 receptors of the spinal cord dorsal horn as a pharmacological target to prevent the development or reverse the chronic muscle pain conditions.

P2X3 and P2X2/3 receptors activation induces articular hyperalgesia by an indirect sensitization of the primary afferent nociceptor in the rats' knee joint

We have previously shown that endogenous adenosine 5'-triphosphate (ATP), via P2X3 and P2X2/3 receptors, plays an essential role in carrageenan-induced articular hyperalgesia model in rats' knee joint. In the present study, we used the rat knee joint incapacitation test, Enzyme-Linked Immunosorbent Assay (ELISA), and myeloperoxidase enzyme activity assay, to test the hypothesis that the activation of P2X3 and P2X2/3 receptors by their agonist induces articular hyperalgesia mediated by the inflammatory mediators bradykinin, prostaglandin, sympathomimetic amines, pro-inflammatory cytokines and by neutrophil migration. We also tested the hypothesis that the activation of P2X3 and P2X2/3 receptors contributes to the articular hyperalgesia induced by the inflammatory mediators belonging to carrageenan inflammatory cascade. The non-selective P2X3 and P2X2/3 receptors agonist αβ-meATP induced a dose-dependent articular hyperalgesia, which was significantly reduced by the selective antagonists for P2X3 and P2X2/3 receptors (A-317491), bradykinin B₁- (DALBK) or B₂-receptors (bradyzide), β₁-(atenolol) or β₂-adrenoceptors (ICI-118,551), by the pre-treatment with cyclooxygenase inhibitor (indomethacin) or with the nonspecific selectin inhibitor (Fucoidan). αβ-meATP induced the release of pro-inflammatory cytokines TNFα, IL-1β, IL-6, and CINC-1, as well as the neutrophil migration. Moreover, the co-administration of A-317491 significantly reduced the articular hyperalgesia induced by bradykinin, prostaglandin E₂ (PGE₂), and dopamine. These findings suggest that peripheral P2X3 and P2X2/3 receptors activation induces articular hyperalgesia by an indirect sensitization of the primary afferent nociceptor of rats' knee joint through the release of inflammatory mediators. Further, they also indicate that the activation of these purinergic receptors by endogenous ATP mediates the bradykinin-, PGE₂-, and dopamine-induced articular hyperalgesia.

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.

P2X3 receptors contribute to muscle pain induced by static contraction by a mechanism dependent on neutrophil migration

P2X3 receptors are involved with several pain conditions. Muscle pain induced by static contraction has an important socioeconomic impact. Here, we evaluated the involvement of P2X3 receptors on mechanical muscle hyperalgesia and neutrophil migration induced by static contraction in rats. Also, we evaluated whether static contraction would be able to increase muscle levels of TNF-α and IL-1β. Male Wistar rats were pretreated with the selective P2X3 receptor antagonist, A-317491, by intramuscular or intrathecal injection and the static contraction-induced mechanical muscle hyperalgesia was evaluated using the Randall-Selitto test. Neutrophil migration was evaluated by measurement of myeloperoxidase (MPO) kinetic-colorimetric assay and the cytokines TNF-α and IL-1β by enzyme-linked immunosorbent assay. Intramuscular or intrathecal pretreatment with A-317491 prevented static contraction-induced mechanical muscle hyperalgesia. In addition, A-317491 reduced static contraction-induced mechanical muscle hyperalgesia when administered 30 and 60 min of the beginning of static contraction, but not after 30 and 60 min of the end of static contraction. Intramuscular A-317491 also prevented static contraction-induced neutrophil migration. In a period of 24 h, static contraction did not increase muscle levels of TNF-α and IL-1β. These findings demonstrated that mechanical muscle hyperalgesia and neutrophil migration induced by static contraction are modulated by P2X3 receptors expressed on the gastrocnemius muscle and spinal cord dorsal horn. Also, we suggest that P2X3 receptors are important to the development but not to maintenance of muscle hyperalgesia. Therefore, P2X3 receptors can be pointed out as a target to musculoskeletal pain conditions induced by daily or work-related activities.

Current understanding of trigeminal ganglion structure and function in headache

INTRODUCTION

The trigeminal ganglion is unique among the somatosensory ganglia regarding its topography, structure, composition and possibly some functional properties of its cellular components. Being mainly responsible for the sensory innervation of the anterior regions of the head, it is a major target for headache research. One intriguing question is if the trigeminal ganglion is merely a transition site for sensory information from the periphery to the central nervous system, or if intracellular modulatory mechanisms and intercellular signaling are capable of controlling sensory information relevant for the pathophysiology of headaches.

METHODS

An online search based on PubMed was made using the keyword "trigeminal ganglion" in combination with "anatomy", "headache", "migraine", "neuropeptides", "receptors" and "signaling". From the relevant literature, further references were selected in view of their relevance for headache mechanisms. The essential information was organized based on location and cell types of the trigeminal ganglion, neuropeptides, receptors for signaling molecules, signaling mechanisms, and their possible relevance for headache generation.

RESULTS

The trigeminal ganglion consists of clusters of sensory neurons and their peripheral and central axon processes, which are arranged according to the three trigeminal partitions V1-V3. The neurons are surrounded by satellite glial cells, the axons by Schwann cells. In addition, macrophage-like cells can be found in the trigeminal ganglion. Neurons express various neuropeptides, among which calcitonin gene-related peptide is the most prominent in terms of its prevalence and its role in primary headaches. The classical calcitonin gene-related peptide receptors are expressed in non-calcitonin gene-related peptide neurons and satellite glial cells, although the possibility of a second calcitonin gene-related peptide receptor in calcitonin gene-related peptide neurons remains to be investigated. A variety of other signal molecules like adenosine triphosphate, nitric oxide, cytokines, and neurotrophic factors are released from trigeminal ganglion cells and may act at receptors on adjacent neurons or satellite glial cells.

CONCLUSIONS

The trigeminal ganglion may act as an integrative organ. The morphological and functional arrangement of trigeminal ganglion cells suggests that intercellular and possibly also autocrine signaling mechanisms interact with intracellular mechanisms, including gene expression, to modulate sensory information. Receptors and neurotrophic factors delivered to the periphery or the trigeminal brainstem can contribute to peripheral and central sensitization, as in the case of primary headaches. The trigeminal ganglion as a target of drug action outside the blood-brain barrier should therefore be taken into account.

A cyclic pathway of P2 × 7, bradykinin, and dopamine receptor activation induces a sustained articular hyperalgesia in the knee joint of rats

OBJECTIVE

We investigated whether: (1) P2 × 7 receptor activation by its agonist (BzATP) induces articular hyperalgesia in the rat's knee joint via inflammatory mechanisms and (2) activation of P2 × 7 receptors by endogenous ATP contributes to the articular hyperalgesia induced by bradykinin, TNF-α, IL-1β, CINC-1, PGE_2, and dopamine.

METHODS

The articular hyperalgesia was quantified using the rat knee joint incapacitation test. The knee joint inflammation, characterized by the concentration of pro-inflammatory cytokines and by neutrophil migration, was quantified in the synovial lavage fluid by ELISA and myeloperoxidase enzyme activity assay, respectively.

RESULTS

BzATP induced a dose-dependent articular hyperalgesia in the rat's knee joint that was significantly reduced by the selective antagonists for P2 × 7, bradykinin B₁ or B₂ receptors, β₁ or β₂ adrenoceptors, and by pre-treatment with Indomethacin. BzATP induced a local increase of TNF-α, IL-1β, IL-6, and CINC-1 concentration and neutrophil migration into the knee joint. The co-administration of the selective P2 × 7 receptor antagonist A-740003 significantly reduced the articular hyperalgesia induced by bradykinin and dopamine, but not by TNF-α, IL-1β, CINC-1, and PGE₂.

CONCLUSIONS

P2 × 7 receptor activation induces articular hyperalgesia mediated by the previous inflammatory mediator release. P2 × 7 receptor-induced articular hyperalgesia is sustained by the involvement of this purinergic receptor in bradykinin and dopamine-induced hyperalgesia in the knee joint.

Alpha-lipoic Acid suppresses P2X receptor activities and visceral hypersensitivity to colorectal distention in diabetic rats

The present study was designed to investigate the roles of P2X3 receptors in dorsal root ganglion (DRG) neurons in colonic hypersensitivity and the effects of alpha-lipoic acid (ALA) on P2X3 receptor activity and colonic hypersensitivity of diabetic rats. Streptozotocin (STZ) was used to induce diabetic model. Abdominal withdrawal reflex (AWR) responding to colorectal distention (CRD) was recorded as colonic sensitivity. ATP-induced current density of colon-specific DRG (T13-L2 DRGs) neurons was measured with whole-cell patch clamp. The expression of P2X3Rs of T13-L2 DRGs was measured by western blot analysis. The results showed that AWR scores significantly increased after STZ injection. P2X3R expression and ATP current density of T13-L2 DRG neurons were enhanced in diabetic rats. Intraperitoneal injection with ALA once a day for 1 week remarkably reduced P2X3R expression and ATP current density in diabetic rats. Importantly, ALA treatment attenuated colonic hypersensitivity in diabetic rats. Our data suggest that STZ injection increases expression and function of P2X3 receptors of colon-specific DRG neurons, thus contributing to colonic hypersensitivity in diabetic rats. Administration of ALA attenuates diabetic colonic hypersensitivity, which is most likely mediated by suppressing expression and function of P2X3 receptors in DRGs of diabetic rats.

The role of P2X3 receptors in bilateral masseter muscle allodynia in rats

AIM

To determine the relationship between bilateral allodynia induced by masseter inflammation and P2X3 receptor expression changes in trigeminal ganglia (TRG) and the influence of intramasseteric P2X3 antagonist administration on bilateral masseter allodynia.

METHODS

To induce bilateral allodynia, rats received a unilateral injection of complete Freund's adjuvant (CFA) into the masseter muscle. Bilateral head withdrawal threshold (HWT) was measured 4 days later. Behavioral measurements were followed by bilateral masseter muscle and TRG dissection. Masseter tissue was evaluated histopathologically and TRG tissue was analyzed for P2X3 receptor mRNA expression by using quantitative real-time polymerase chain reaction (PCR) analysis. To assess the P2X3 receptor involvement in nocifensive behavior, two doses (6 and 60 μg/50 μL) of selective P2X3 antagonist A-317491 were administrated into the inflamed masseter muscle 4 days after the CFA injection. Bilateral HWT was measured at 15-, 30-, 60-, and 120-minute time points.

RESULTS

HWT was bilaterally reduced after the CFA injection (P<0.001). Intramasseteric inflammation was confirmed ipsilaterally to the CFA injection. Quantitative real-time PCR analysis demonstrated enhanced P2X3 expression in TRG ipsilaterally to CFA administration (P<0.01). In comparison with controls, the dose of 6 μg of A-317491 significantly increased bilateral HWT at 15-, 30-, and 60-minute time points after the A-317491 administration (P<0.001), whereas the dose of 60 μg of A-317491 was efficient at all time points ipsilaterally (P=0.004) and at 15-, 30-, and 60-minute time points contralaterally (P<0.001).

CONCLUSION

Unilateral masseter inflammation can induce bilateral allodynia in rats. The study provided evidence that P2X3 receptors can functionally influence masseter muscle allodynia and suggested that P2X3 receptors expressed in TRG neurons are involved in masseter inflammatory pain conditions.

Intra-Articular Blockade of P2X7 Receptor Reduces the Articular Hyperalgesia and Inflammation in the Knee Joint Synovitis Especially in Female Rats

Synovitis is a key factor in joint disease pathophysiology, which affects a greater proportion of women than men. P2X7 receptor activation contributes to arthritis, but whether it plays a role in articular inflammatory pain in a sex-dependent manner is unknown. We investigated whether the P2X7 receptor blockade in the knee joint of male and female rats reduces the articular hyperalgesia and inflammation induced by a carrageenan knee joint synovitis model. Articular hyperalgesia was quantified using the rat knee joint incapacitation test and the knee joint inflammation, characterized by the concentration of cytokines tumor necrosis factor-α, interleukin-1β, interleukin-6, and cytokine-induced neutrophil chemoattractant-1, and by neutrophil migration, was quantified using enzyme-linked immunosorbent assay and by myeloperoxidase enzyme activity measurement, respectively. P2X7 receptor blockade by the articular coadministration of selective P2X7 receptor antagonist A740003 with carrageenan significantly reduced articular hyperalgesia, pro-inflammatory cytokine concentrations, and myeloperoxidase activity induced by carrageenan injection into the knee joint of male and estrus female rats. However, a lower dose of P2X7 receptor antagonist was sufficient to significantly induce the antihyperalgesic and anti-inflammatory effects in estrus female but not in male rats. These results suggest that P2X7 receptor activation by endogenous adenosine 5'-triphosphate is essential to articular hyperalgesia and inflammation development in the knee joint of male and female rats. However, female rats are more responsive than male rats to the antihyperalgesic and anti-inflammatory effects induced by P2X7 receptor blockade.

PERSPECTIVE

P2X7 receptors could be promising therapeutic targets in the treatment of knee joint disease symptoms, especially in women, who are more affected than men by these conditions.

P2X3 and P2X2/3 Receptors Play a Crucial Role in Articular Hyperalgesia Development Through Inflammatory Mechanisms in the Knee Joint Experimental Synovitis

Osteoarthritis (OA) is a degenerative and progressive disease characterized by cartilage breakdown and by synovial membrane inflammation, which results in disability, joint swelling, and pain. The purinergic P2X3 and P2X2/3 receptors contribute to development of inflammatory hyperalgesia, participate in arthritis processes in the knee joint, and are expressed in chondrocytes and nociceptive afferent fibers innervating the knee joint. In this study, we hypothesized that P2X3 and P2X2/3 receptors activation by endogenous ATP (adenosine 5'-triphosphate) induces articular hyperalgesia in the knee joint of male and female rats through an indirect sensitization of primary afferent nociceptors dependent on the previous release of pro-inflammatory cytokines and/or on neutrophil migration. We found that the blockade of articular P2X3 and P2X2/3 receptors significantly attenuated carrageenan-induced hyperalgesia in the knee joint of male and estrus female rats in a similar manner. The carrageenan-induced knee joint inflammation increased the expression of P2X3 receptors in chondrocytes of articular cartilage. Further, the blockade of articular P2X3 and P2X2/3 receptors significantly reduced the increased concentration of TNF-α, IL-6, and CINC-1 and the neutrophil migration induced by carrageenan. These findings indicate that P2X3 and P2X2/3 receptors activation by endogenous ATP is essential to hyperalgesia development in the knee joint through an indirect sensitization of primary afferent nociceptors dependent on the previous release of pro-inflammatory cytokines and/or on neutrophil migration.

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.

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.

Peripheral P2X7 receptor-induced mechanical hyperalgesia is mediated by bradykinin

P2X7 receptors play an important role in inflammatory hyperalgesia, but the mechanisms involved in their hyperalgesic role are not completely understood. In this study, we hypothesized that P2X7 receptor activation induces mechanical hyperalgesia via the inflammatory mediators bradykinin, sympathomimetic amines, prostaglandin E2 (PGE2), and pro-inflammatory cytokines and via neutrophil migration in rats. We found that 2'(3')-O-(4-benzoylbenzoyl)adenosine 5'-triphosphate triethylammonium salt (BzATP), the most potent P2X7 receptor agonist available, induced a dose-dependent mechanical hyperalgesia that was blocked by the P2X7 receptor-selective antagonist A-438079 but unaffected by the P2X1,3,2/3 receptor antagonist TNP-ATP. These findings confirm that, although BzATP also acts at both P2X1 and P2X3 receptors, BzATP-induced hyperalgesia was mediated only by P2X7 receptor activation. Co-administration of selective antagonists of bradykinin B1 (Des-Arg(8)-Leu(9)-BK (DALBK)) or B2 receptors (bradyzide), β1 (atenolol) or β2 adrenoceptors (ICI 118,551), or local pre-treatment with the cyclooxygenase inhibitor indomethacin or the nonspecific selectin inhibitor fucoidan each significantly reduced BzATP-induced mechanical hyperalgesia in the rat hind paw. BzATP also induced the release of the pro-inflammatory cytokines tumor necrosis factor α (TNF-α), interleukin (IL)-1β, IL-6 and cytokine-induced neutrophil chemoattractant-1 (CINC-1), an effect that was significantly reduced by A-438079. Co-administration of DALBK or bradyzide with BzATP significantly reduced BzATP-induced IL-1β and CINC-1 release. These results indicate that peripheral P2X7 receptor activation induces mechanical hyperalgesia via inflammatory mediators, especially bradykinin, which may contribute to pro-inflammatory cytokine release. These pro-inflammatory cytokines in turn may mediate the contributions of PGE2, sympathomimetic amines and neutrophil migration to the mechanical hyperalgesia induced by local P2X7 receptor activation.

Effect of genetic deletion and pharmacological antagonism of P2X7 receptors in a mouse animal model of migraine

BACKGROUND

Purine receptors participate in peripheral and central sensitization and are associated with migraine headache. We investigated the role of P2X7 receptor (P2X7) in a nitroglycerin (NTG)-induced mouse model of migraine.

METHODS

Intraperitoneal NTG injection (15 mg/kg) triggered thermal hyperalgesia in the hindpaws of wild-type C57BL/6J mice, followed by the induction of c-fos in upper cervical spinal cord and trigeminal nucleus caudalis. The effect of genetic deletion of P2X7 and the selective P2X7 antagonist Brilliant Blue G (BBG) were examined on hyperalgesia and c-fos induction.

RESULTS

NTG decreased the paw withdrawal threshold in both wild-type and P2X7 knockout mice. Nevertheless, subacute BBG treatment (50 mg/kg/day i.p.) completely prevented the effect of NTG in wild-type, but not in knockout mice. Whereas P2X7 deficiency differentially affected the expression of c-fos, the average number of fos-immuno-reactive neurons in trigeminal nucleus caudalis, but not in upper cervical spinal cord was lower in BBG-treated wild-type mice after NTG treatment.

CONCLUSIONS

Our results show that P2X7 receptors might participate in the pathogenesis of migraine, although upregulation of other P2X receptors probably compensate for the loss of its action in knockout mice. The data also suggest the therapeutic potential of P2X7 antagonists for the treatment of migraine.

Anatomical and physiological factors contributing to chronic muscle pain

Chronic muscle pain remains a significant source of suffering and disability despite the adoption of pharmacologic and physical therapies. Muscle pain is mediated by free nerve endings distributed through the muscle along arteries. These nerves project to the superficial dorsal horn and are transmitted primarily through the spinothalamic tract to several cortical and subcortical structures, some of which are more active during the processing of muscle pain than other painful conditions. Mechanical forces, ischemia, and inflammation are the primary stimuli for muscle pain, which is reflected in the array of peripheral receptors contributing to muscle pain-ASIC, P2X, and TRP channels. Sensitization of peripheral receptors and of central pain processing structures are both critical for the development and maintenance of chronic muscle pain. Further, variations in peripheral receptors and central structures contribute to the significantly greater prevalence of chronic muscle pain in females.

Peripheral substance P and neurokinin-1 receptors have a role in inflammatory and neuropathic orofacial pain models

There is accumulating evidence that substance P released from peripheral sensory neurons participates in inflammatory and neuropathic pain. In this study it was investigated the ability of substance P to induce orofacial nociception and thermal and mechanical hyperalgesia, as well as the role of NK1 receptors on models of orofacial inflammatory and neuropathic pain. Substance P injected into the upper lip at 1, 10 and 100 μg/50 μL failed to induce nociceptive behavior. Also, substance P (0.1-10 μg/50 μL) injected into the upper lip did not evoke orofacial cold hyperalgesia and when injected at 1 μg/50 μL did not induce mechanical hyperalgesia. However, substance P at this latter dose induced orofacial heat hyperalgesia, which was reduced by the pre-treatment of rats with a non-peptide NK1 receptor antagonist (SR140333B, 3mg/kg). Systemic treatment with SR140333B (3 mg/kg) also reduced carrageenan-induced heat hyperalgesia, but did not exert any influence on carrageenan-induced cold hyperalgesia. Blockade of NK1 receptors with SR140333B also reduced by about 50% both phases of the formalin response evaluated in the orofacial region. Moreover, heat, but not cold or mechanical, hyperalgesia induced by constriction of the infraorbital nerve, a model of trigeminal neuropathic pain, was abolished by pretreatment with SR140333B. Considering that substance P was peripherally injected (i.e. upper lip) and the NK1 antagonist used lacks the ability to cross the blood-brain-barrier, our results demonstrate that the peripheral SP/NK1 system participates in the heat hyperalgesia associated with inflammation or nerve injury and in the persistent pain evoked by formalin in the orofacial region.

Purinergic mechanisms and pain--an update

There is a brief summary of the background literature about purinergic signalling. The review then considers purinergic mechanosensory transduction involved in visceral, cutaneous and musculoskeletal nociception and on the roles played by P2X3, P2X2/3, P2X4, P2X7 and P2Y₁₂ receptors in neuropathic and inflammatory pain. Current developments of compounds for the therapeutic treatment of both visceral and neuropathic pain are discussed.

Expression of P2X₁ and P2X₄ receptors in rat trigeminal ganglion neurons

Extracellular ATP, an essential pain mediator, is received by cell-surface ionotropic P2X and/or metabotropic P2Y receptors. Although the contribution of P2X₃ and/or P2X(2/3) receptors toward the pain mechanism is well described in trigeminal ganglion neurons, the expression of other subtypes of P2X receptor remains to be clarified. We examined expression of P2X receptor mRNA and measured intracellular free Ca²⁺ concentration ([Ca²⁺]i) by the activation of these receptors by fura-2 fluorescence in primary cultured rat trigeminal ganglion neurons. Real-time reverse transcription-PCR analysis revealed mRNA expression of P2X receptor subtype P2X₁, P2X₃, and P2X₄ in trigeminal ganglion neurons. In the presence of extracellular Ca²⁺, the application of P2X receptors agonists, ATP, α,β-methylene ATP or β,γ-methylene ATP induced Ca²⁺ influx significantly. The ATP-induced increase in [Ca²⁺]i was inhibited by a series of selective antagonists for P2X₁, P2X₃, or P2X₄ receptors. These results indicate that trigeminal ganglion neurons functionally express P2X₁, P2X₃, and P2X₄ receptors and that these receptors are involved in the mediation of not only nociceptive but also neuropathic pain in the orofacial area.

Involvement of ATP in noxious stimulus-evoked release of glutamate in rat medullary dorsal horn: a microdialysis study

Our electrophysiological studies have shown that both purinergic and glutamatergic receptors are involved in central sensitization of nociceptive neurons in the medullary dorsal horn (MDH). Here we assessed the effects of intrathecal administration of apyrase (a nucleotide degrading enzyme of endogenous adenosine 5-triphosphate [ATP]), a combination of apyrase and 1,3-dipropyl-8-cyclopentylxanthine (DPCPX, an adenosine A1 receptor antagonist), or 2,3-O-2,4,6-trinitrophenyl-adenosine triphosphate (TNP-ATP, a P2X1, P2X3, P2X2/3 receptor antagonist) on the release of glutamate in the rat MDH evoked by application of mustard oil (MO) to the molar tooth pulp. In vivo microdialysis was used to dialyse the MDH every 5 min, and included 3 basal samples, 6 samples after drug treatment and 12 samples following application of MO. Tooth pulp application of MO induced a significant increase in glutamate release in the MDH. Superfusion of apyrase or TNP-ATP alone significantly reduced the MO-induced glutamate release in the MDH, as compared to vehicle. Furthermore, the suppressive effects of apyrase on glutamate release were reduced by combining it with DPCPX. This study demonstrates that application of an inflammatory irritant to the tooth pulp induces glutamate release in the rat MDH in vivo that may be reduced by processes involving endogenous ATP and adenosine.

P2X3 antagonists: novel therapeutics for afferent sensitization and chronic pain

SUMMARY Despite decades of innovation and effort, the pharmaceutical needs of countless patients with chronic pain remain underserved. Effective and safe treatments must clearly come from novel approaches, yet targets and molecules selected hitherto have returned little benefit. Antagonism of P2X3 purinoceptors on pain-conveying nerves is a highly novel approach, and compounds from this class are advancing into patient studies. P2X3 channels are found in C- and Aδ-primary afferent neurons in most tissues, and are strikingly specific to pain detection. P2X3 antagonists block peripheral activation of these fibers via ATP, released from most cells by inflammation, injury, stress and distension, and clearly provide an alternative pharmacological mechanism to attenuate pain signals. P2X3 is also expressed presynaptically at central spinal terminals of afferent neurons, where ATP further sensitizes painful signals en route to the brain. The selectivity of P2X3 expression allows hope of a lower potential for adverse effects in brain, gut and cardiovascular tissues – limiting factors for most analgesics. P2X3 receptor-mediated sensitization has been implicated in rodent models in inflammatory, visceral, neuropathic and cancer pain states, as well as in airways hyper-reactivity, migraine and visceral organ irritability. Although we are often reminded that the effects of new medicines can translate poorly into clinical effectiveness, the broad efficacy seen following P2X3 inhibition in rodent models strengthens the prospect that an unprecedented mechanism to counter sensitization of afferent pathways may offer some merciful relief to millions of patients struggling daily with persistent discomfort and pain.

Intramuscular ketorolac inhibits activation of rat peripheral NMDA receptors

The nonsteroidal anti-inflammatory drug (NSAID) diclofenac has local anesthetic-like and peripheral N-methyl-d-aspartate (NMDA) receptor antagonist characteristics when administered at higher concentrations to masticatory muscle. It is not known if the ability to inhibit NMDA receptors is unique to diclofenac or shared by other NSAIDs. This study was undertaken to determine whether intramuscular injection of ketorolac or naproxen at concentrations that do not induce local anesthetic-like effects could attenuate jaw-closer muscle nociceptor discharge in anesthetized Sprague-Dawley rats. It was found that ketorolac (5 mM) inhibited hypertonic saline-evoked nociceptor discharge, which suggests that at this concentration, ketorolac has local anesthetic-like properties. A lower concentration of ketorolac (0.5 mM), which did not affect hypertonic saline-evoked discharge, did inhibit nociceptor discharge evoked by NMDA. In contrast, naproxen (5 mM) did not alter hypertonic saline- or NMDA-evoked nociceptor discharge. Subsequent experiments revealed that ketorolac (0.5 mM) had no effect on nociceptor discharge evoked by αβ-methylene ATP, 5-hydroxytryptamine, or AMPA. The inhibitory effect of ketorolac did not appear to be related to cyclooxygenase inhibition, because the concentration of prostaglandin E(2) in the masticatory muscles 10 min after injection of either NSAID was not significantly decreased. The present study indicates that in vivo, ketorolac, but not naproxen, can antagonize NMDA-evoked nociceptor discharge similarly to diclofenac. We speculate that structural similarities between ketorolac and diclofenac could account for the ability of these NSAIDs to inhibit NMDA-evoked nociceptor discharge. These properties may partly explain the analgesic effect of intramuscularly injected ketorolac in the clinic.

In pursuit of P2X3 antagonists: novel therapeutics for chronic pain and afferent sensitization

Treating pain by inhibiting ATP activation of P2X3-containing receptors heralds an exciting new approach to pain management, and Afferent's program marks the vanguard in a new class of drugs poised to explore this approach to meet the significant unmet needs in pain management. P2X3 receptor subunits are expressed predominately and selectively in so-called C- and Aδ-fiber primary afferent neurons in most tissues and organ systems, including skin, joints, and hollow organs, suggesting a high degree of specificity to the pain sensing system in the human body. P2X3 antagonists block the activation of these fibers by ATP and stand to offer an alternative approach to the management of pain and discomfort. In addition, P2X3 is expressed pre-synaptically at central terminals of C-fiber afferent neurons, where ATP further sensitizes transmission of painful signals. As a result of the selectivity of the expression of P2X3, there is a lower likelihood of adverse effects in the brain, gastrointestinal, or cardiovascular tissues, effects which remain limiting factors for many existing pain therapeutics. In the periphery, ATP (the factor that triggers P2X3 receptor activation) can be released from various cells as a result of tissue inflammation, injury or stress, as well as visceral organ distension, and stimulate these local nociceptors. The P2X3 receptor rationale has aroused a formidable level of investigation producing many reports that clarify the potential role of ATP as a pain mediator, in chronic sensitized states in particular, and has piqued the interest of pharmaceutical companies. P2X receptor-mediated afferent activation has been implicated in inflammatory, visceral, and neuropathic pain states, as well as in airways hyperreactivity, migraine, itch, and cancer pain. It is well appreciated that oftentimes new mechanisms translate poorly from models into clinical efficacy and effectiveness; however, the breadth of activity seen from P2X3 inhibition in models offers a realistic chance that this novel mechanism to inhibit afferent nerve sensitization may find its place in the sun and bring some merciful relief to the torment of persistent discomfort and pain. The development philosophy at Afferent is to conduct proof of concept patient studies and best identify target patient groups that may benefit from this new intervention.

Nerve growth factor contribution via transient receptor potential vanilloid 1 to ectopic orofacial pain

It is well known that oral inflammation causes tenderness in temporomandibular joints or masseter muscles. The exact mechanism of such an orofacial ectopic hyperalgesia remains unclear. Here, we investigated the functional significance of interaction of nerve growth factor (NGF) and transient receptor potential vanilloid 1 (TRPV1) in relation to heat hyperalgesia in the whisker pad skin caused by complete Freund's adjuvant (CFA) injection into the lower lip. CFA injection induced heat hyperalgesia of the ipsilateral whisker pad skin. Moreover, it leads to enhancement of spontaneous activity and heat responses in trigeminal ganglion (TG) neurons that was elicited by heat stimulation of the whisker pad skin. The heat hyperalgesia was dose-dependently reversed by intraperitoneal TRPV1 antagonist administration, also diminished by neutralizing anti-NGF antibody administration into the lower lip and intraganglionic administration of K252a, a tyrosine kinase receptor inhibitor. Nerve fibers in bundle of mandibular nerve and TG neurons that innervates the whisker pad skin and lower lip both expressed labeled NGF, which was administrated into the lower lip. Moreover, the NGF concentrations in ophthalmic-maxillary and mandibular divisions of the TG increased after CFA injection into the lower lip. The number of TRPV1-positive neurons that innervates the whisker pad skin and lower lip was increased after CFA injection into the lower lip, and this increase was annulled by anti-NGF administration. The present findings suggest that inflammation in the lower lip induces release of NGF that regulates TRPV1 expression in TG neurons. This TRPV1 overexpression may underlie ectopic heat hyperalgesia in the whisker pad skin.

A chronic pain: inflammation-dependent chemoreceptor adaptation in rat carotid body

Experiments in recent years have revealed labile electrophysiological and neurochemical phenotypes in primary afferent neurons exposed to specific stimulus conditions associated with the development of chronic pain. These studies collectively demonstrate that the mechanisms responsible for functional plasticity are primarily mediated by novel neuroimmune interactions involving circulating and resident immune cells and their secretory products, which together induce hyperexcitability in the primary sensory neurons. In another peripheral sensory modality, namely the arterial chemoreceptors, sustained stimulation in the form of chronic hypoxia (CH) elicits increased chemoafferent excitability from the mammalian carotid body. Previous studies which focused on functional changes in oxygen-sensitive type I cells in this organ have only partially elucidated the molecular and cellular mechanisms which initiate and control this adaptive response. Recent studies in our laboratory indicate a unique role for the immune system in regulating the chemo-adaptive response of the carotid body to physiologically relevant levels of hypoxia.