Blockade of peripheral P2Y1 receptors prevents the induction of thermal hyperalgesia via modulation of TRPV1 expression in carrageenan-induced inflammatory pain rats: involvement of p38 MAPK phosphorylation in DRGs

Dual effect of anandamide on spinal nociceptive transmission in control and inflammatory conditions

Anandamide (AEA) is an important modulator of nociception in the spinal dorsal horn, acting presynaptically through Cannabinoid (CB1) and Transient receptor potential vanilloid (TRPV1) receptors. The role of AEA (1 µM, 10 µM, and 30 µM) application on the modulation of nociceptive synaptic transmission under control and inflammatory conditions was studied by recording miniature excitatory postsynaptic currents (mEPSCs) from neurons in spinal cord slices. Inhibition of the CB1 receptors by PF514273, TRPV1 by SB366791, and the fatty acid amide hydrolase (FAAH) by URB597 was used. Under naïve conditions, the AEA application did not affect the mEPSCs frequency (1.43±0.12 Hz) when all the recorded neurons were considered. The mEPSC frequency increased (180.0±39.2%) only when AEA (30 µM) was applied with PF514273 and URB597. Analysis showed that one sub-population of neurons had synaptic input inhibited (39.1% of neurons), the second excited (43.5%), whereas 8.7% showed a mixed effect and 8.7% did not respond to the AEA. With inflammation, the AEA effect was highly inhibitory (72.7%), while the excitation was negligible (9.1%), and 18.2% were not modulated. After inflammation, more neurons (45.0%) responded even to low AEA by mEPSC frequency increase with PF514273/URB597 present. AEA-induced dual (excitatory/inhibitory) effects at the 1st nociceptive synapse should be considered when developing analgesics targeting the endocannabinoid system. These findings contrast the clear inhibitory effects of the AEA precursor 20:4-NAPE application described previously and suggest that modulation of endogenous AEA production may be more favorable for analgesic treatments.

Peroxisome proliferator activated receptor-gamma (PPAR-γ) ligand, pioglitazone, increases analgesic and anti-inflammatory effects of naproxen

The aim of this study was the investigation of analgesic and anti-inflammatory activity of naproxen and pioglitazone following intra-plantar injection of carrageenan and assessment of the PPAR-γ receptor involvement in these effects. Rats were intra-plantarly injected with carrageenan (1%, 100 μl) to induce thermal hyperalgesia and paw inflammation. Different groups of rats were pre-treated intraperitoneally with naproxen (1 and 10 mg/kg) or pioglitazone (3 and 10 mg/kg) or GW9662 (a selective PPAR-γ antagonist, 100 μl/paw). The volume of the paw was evaluated using a plethysmometer, and the hot plate test was employed to assess the pain threshold in the animals. Finally, TNF-α, IL-1ß, IL-6, and myeloperoxidase (MPO) activity status were evaluated in the hind paw tissue. Naproxen and pioglitazone demonstrated analgesic and anti-inflammatory activity. Concurrent injection of an ineffective dose of naproxen (1 mg/kg) with an ineffective dose of pioglitazone (3 mg/kg) caused augmented analgesic and anti-inflammatory activity, significantly (p≤0.001 and p≤0.01, respectively). Additionally, intra-plantar injection of GW-9662 before naproxen or pioglitazone significantly suppressed their analgesic (p≤0.001) and anti-inflammatory activity (p≤0.01). Also, naproxen and pioglitazone (10 mg/kg) significantly (p≤0.001) reduced carrageenan-induced MPO activity and TNF-α, IL-6, and IL-1ß releasing. Furthermore, PPAR-γ blockade significantly prevented suppressive effects of naproxen and pioglitazone on the MPO activity and inflammatory cytokines. Pioglitazone significantly increased analgesic and anti-inflammatory effects of naproxen. This study proposes that concurrent treatment with naproxen and pioglitazone may be a substitute for overcome pain and inflammation clinically, in the future, particularly in patients with cardiovascular disorders and diabetes.

Alcohol-Induced Headache with Neuroinflammation: Recent Progress

Ethanol and other congeners in alcoholic beverages and foods are known triggers of alcohol-induced headaches (AIHs). Recent studies implicate AIHs as an important underlying factor for neuroinflammation. Studies show the relationship between alcoholic beverages, AIH agents, neuroinflammation, and the pathway they elicit. However, studies elucidating specific AIH agents’ pathways are scarce. Works reviewing their pathways can give invaluable insights into specific substances’ patterns and how they can be controlled. Hence, we reviewed the current understanding of how AIH agents in alcoholic beverages affect neuroinflammation and their specific roles. Ethanol upregulates transient receptor potential cation channel subfamily V member 1 (TRPV1) and Toll-like receptor 4 (TLR4) expression levels; both receptors trigger a neuroinflammation response that promotes AIH manifestation—the most common cause of AIHs. Other congeners such as histamine, 5-HT, and condensed tannins also upregulate TRPV1 and TLR4, neuroinflammatory conditions, and AIHs. Data elucidating AIH agents, associating pathways, and fermentation parameters can help reduce or eliminate AIH inducers and create healthier beverages.

Unveil the pain of endometriosis: from the perspective of the nervous system

Endometriosis is a chronic inflammatory disease with pelvic pain and uncharacteristic accompanying symptoms. Endometriosis-associated pain often persists despite treatment of the disease, thus it brings a deleterious impact on their personal lives as well as imposing a substantial economic burden on them. At present, mechanisms underlie endometriosis-associated pain including inflammatory reaction, injury, aberrant blood vessels and the morphological and functional anomaly of the peripheral and central nervous systems. The nerve endings are influenced by the physical and chemical factors surrounding the lesion, via afferent nerve to the posterior root of the spinal nerve, then to the specific cerebral cortex involved in nociception. However, our understanding of the aetiology and mechanism of this complex pain process caused by endometriosis remains incomplete. Identifying the pathogenesis of endometriosis is crucial to disease management, offering proper treatment, and helping patients to seek novel targets for the maintenance and contributors of chronic pain. The main aim of this review is to focus on every possible mechanism of pain related to endometriosis in both peripheral and central nervous systems, and to present related mechanisms of action from the interaction between peripheral lesions and nerves to the changes in transmission of pain, resulting in hyperalgesia and the corresponding alterations in cerebral cortex and brain metabolism.

Antinociceptive properties of losmapimod in two acute pain models in rats: behavioural analysis, immunohistochemistry, dose response, and comparison with usual analgesic drugs

Background

The p38 protein is a ubiquitous mitogen-activated protein kinase involved in the proinflammatory signalling pathway and in the pain response after various noxious stimuli. Many p38 inhibitors have been developed and shown to provide effective analgesia in animal models. They are, however, mainly administered intrathecally or intravenously. Our study aimed to evaluate losmapimod, a novel oral p38 inhibitor, in two murine acute pain models.

Methods

Losmapimod (12 mg kg-1) was compared with paracetamol, ketamine, and morphine using thermal and mechanical stimulation after carrageenan injection. A dose-effect study was also performed with this model. Behavioural testing was also performed in a plantar incision model to confirm the analgesic effect of losmapimod. Expression of activated p38 in neurones, microglia, and astrocytes was also investigated at 2, 15, and 24 h after carrageenan injection.

Results

Losmapimod was both antiallodynic and antihyperalgesic in the carrageenan pain model and provided an antinociceptive effect similar to that of morphine. The dose of 12 mg kg-1 was shown to be the ED78 and ED64 after thermal and mechanical stimulation, respectively. After plantar incision, losmapimod provided a significant antinociceptive effect. No life-threatening side-effect was observed in the behavioural study. Losmapimod prevented neurone and microglial activation at 2 and 15 h after carrageenan injection, respectively, but no effect was found on astrocytic activation.

Conclusion

Losmapimod appears to be a promising drug in severe acute pain conditions. Losmapimod could also be helpful for postoperative pain control, as suggested by its effect after plantar incision.

P2Y1 purinergic receptor inhibition attenuated remifentanil-induced postoperative hyperalgesia via decreasing NMDA receptor phosphorylation in dorsal root ganglion

BACKGROUND

Remifentanil-induced postoperative hyperalgesia is an intractable side effect of the clinical use of remifentanil, the mechanism of which remains obscure, especially in the peripheral nervous system. N-methyl-D-aspartate receptor (NMDAR) phosphorylation in dorsal root ganglion (DRG) plays a pronociceptive role in neuropathic pain. The contribution of the P2Y1 purinergic receptor (P2Y1R) in DRG to pain hypersensitivity derived from various origins and P2Y1R upregulation-induced NMDAR activation in neurons have also been uncovered. This study aimed to investigate whether P2Y1R participates in nociceptive processing in the DRG and spinal cord in remifentanil-induced postoperative hyperalgesia.

METHODS

Rats with remifentanil-induced postoperative hyperalgesia were intrathecally injected with NMDAR antagonist MK801 or P2Y1R antagonist MRS2179 at 10 min prior to remifentanil infusion. Mechanical allodynia, heat hyperalgesia, and cold hyperalgesia were measured at -24 h, 2 h, 6 h, 24 h, and 48 h following remifentanil infusion. The P2Y1R expression and NMDAR expression and phosphorylation in DRG ipsilateral to the incision were detected by Western blot and immunofluorescence.

RESULTS

Incision and remifentanil induced mechanical allodynia, heat hyperalgesia, and cold hyperalgesia accompanied by upregulated P2Y1R expression, increased NMDAR subunit NR1 expression and phosphorylation at Ser896, and NR2B expression and phosphorylation at Tyr1472 in DRG. Inhibition of NMDAR phosphorylation by MK801 effectively attenuated remifentanil-induced postoperative hyperalgesia. Furthermore, P2Y1R blockade by MRS2179 not only lessened remifentanil-evoked postoperative hypersensitivity to mechanical, heat, and cold stimuli, but also suppressed the increases in NR1 and NR2B expression and phosphorylation in DRG induced by incision and remifentanil.

CONCLUSION

The process by which P2Y1R mediates NMDAR expression and phosphorylation represents a mechanism of remifentanil-induced postoperative hyperalgesia in the DRG and/or spinal cord.

Activation of ATF3/AP-1 signaling pathway is required for P2X3-induced endometriosis pain

STUDY QUESTION

Does P2X ligand-gated ion channel 3 (P2X3) play a role in endometriosis pain?

SUMMARY ANSWER

Upregulation of P2X3 in dorsal root ganglia (DRG) tissues via the activating transcription factor 3 (ATF3)/activator protein (AP)-1 pathway contributed to endometriosis-associated hyperalgesia, which could be attenuated by the chitosan oligosaccharide stearic acid (CSOSA)/liposomes (LPs)/SP600125 delivery system.

WHAT IS KNOWN ALREADY

Infiltrating nerve fibers and elevated nociceptors in endometriotic lesions are associated with endometriosis pain. P2X3 has been demonstrated to play an important role in neuropathic pain.

STUDY DESIGN, SIZE, DURATION

A rat model of endometriosis was used to investigate the signaling pathways involved in P2X3-induced pain.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Degrees of hyperalgesia, endogenous adenosine 5'-triphosphate (ATP) contents and P2X3 expression levels in endometriotic lesions and DRG tissues were detected in a rat model of endometriosis. The expression levels of ATF3 and P2X3 were measured using qRT-PCR, western blot analysis and immunofluorescence analysis after adenosine 5'-diphosphate (ADP) exposure in DRG cells. Plasmids encoding ATF3 and its siRNA were used to investigate the role of ATF3 on ADP-induced P2X3 upregulation. The activity of ATF binding to the P2X3 promoter was evaluated by using chromatin immunoprecipitation (CHIP) and luciferase assays. SP600125, an inhibitor of c-JUN N-terminal kinase, was wrapped in CSOSA/LPs delivery system and its inhibitory effects on ADP-induced upregulation of P2X3 in DRG cells and endometriosis-induced hyperalgesia in rats were tested.

MAIN RESULTS AND THE ROLE OF CHANCE

The concentrations of endogenous ATP and expression levels of P2X3 were significantly increased in both endometriotic lesions and DRG tissues in endometriosis rat models and were found to be positively correlated with the severity of hyperalgesia. In DRG cells, P2X3 expression levels were elevated by ADP stimulation, but dramatically inhibited by blocking ATF3 with its siRNA and SP600125. CHIP and luciferase assay showed that ADP increased the binding of ATF3 to the P2X3 promoter, resulting in an increase in P2X3 expression levels. In the CSOSA/LPs/SP600125 delivery system, the drug could be effectively concentrated in endometriotic lesions, and it could alleviate endometriosis-induced hyperalgesia, reduce the size of endometriotic lesions and attenuate upregulated P2X3 expression levels in endometriosis rat models.

LARGE SCALE DATA

N/A.

LIMITATIONS, REASONS FOR CAUTION

Changes in the sensitivity and function of P2X3 caused by endometriosis need to be further investigated.

WIDER IMPLICATIONS OF THE FINDINGS

This study indicates that ATP and the P2X3 receptor are involved in endometriosis pain, thus providing a novel therapeutic approach for the treatment of endometriosis pain by targeting the P2X3 receptor.

STUDY FUNDING/COMPETING INTEREST(S)

This work was funded by National Key R&D Program of China (Grant No. 2017YFC1001202) and National Natural Science Foundation of China (Grant Nos. 81974225, 81671429 and 81471433). There are no competing interests.

SNI and CFA induce similar changes in TRPV1 and P2X3 expressions in the acute phase but not in the chronic phase of pain

Peripheral inflammation and nerve injury usually accompany each other. However, whether inflammatory and neuropathic pain share similar mechanisms at all stages is unknown. TRPV1 and P2X3 are two major ion channels in dorsal root ganglia (DRGs) and are involved in chronic pain. Here, their function and expression in DRGs at different phases of the two types of pain were investigated. Both the paw withdrawal threshold (PWT) and paw withdrawal latency were decreased in rats injected with complete Freud's adjuvant (CFA). However, only the PWT was decreased in rats with spared nerve injury (SNI). CFA increased the magnitude of the TRPV1-mediated Ca²⁺ response but not the P2X3-mediated Ca²⁺ response 14 days after injection. Consistent with this result, the P2X3 expression level in CFA rats was increased only at 3 days after injection. SNI surgery increased the magnitudes of the TRPV1- and P2X3-mediated Ca²⁺ responses and upregulated both TRPV1 and P2X3 expression in lumbar DRGs. The distributions of TRPV1 and P2X3 in DRGs after modeling were observed, and TRPV1 was found to be highly expressed mainly in the L4-L5 DRGs in CFA rats and in the L5-L6 DRGs in SNI rats. P2X3 was highly expressed in the L4-L6 DRGs in CFA rats 3 days after injection but was only highly expressed in the L4 DRG 14 days after modeling. On the other hand, SNI promoted the P2X3 expression L4-L5 DRGs 3 days after surgery, but only L6 DRG 14 days after modeling. All the results indicate that P2X3 and TPRV1 are involved in inflammatory and neuropathic pain by different expression levels and distributions in the lumbar DRG in the chronic stage.

Oxytocin-Dependent Regulation of TRPs Expression in Trigeminal Ganglion Neurons Attenuates Orofacial Neuropathic Pain Following Infraorbital Nerve Injury in Rats

We evaluated the mechanisms underlying the oxytocin (OXT)-induced analgesic effect on orofacial neuropathic pain following infraorbital nerve injury (IONI). IONI was established through tight ligation of one-third of the infraorbital nerve thickness. Subsequently, the head withdrawal threshold for mechanical stimulation (MHWT) of the whisker pad skin was measured using a von Frey filament. Trigeminal ganglion (TG) neurons innervating the whisker pad skin were identified using a retrograde labeling technique. OXT receptor-immunoreactive (IR), transient receptor potential vanilloid 1 (TRPV1)-IR, and TRPV4-IR TG neurons innervating the whisker pad skin were examined on post-IONI day 5. The MHWT remarkably decreased from post-IONI day 1 onward. OXT application to the nerve-injured site attenuated the decrease in MHWT from day 5 onward. TRPV1 or TRPV4 antagonism significantly suppressed the decrement of MHWT following IONI. OXT receptors were expressed in the uninjured and Fluoro-Gold (FG)-labeled TG neurons. Furthermore, there was an increase in the number of FG-labeled TRPV1-IR and TRPV4-IR TG neurons, which was inhibited by administering OXT. This inhibition was suppressed by co-administration with an OXT receptor antagonist. These findings suggest that OXT application inhibits the increase in TRPV1-IR and TRPV4-IR TG neurons innervating the whisker pad skin, which attenuates post-IONI orofacial mechanical allodynia.

Purinergic Signaling in Endometriosis-Associated Pain

Endometriosis is an estrogen-dependent gynecological disease, with an associated chronic inflammatory component, characterized by the presence of endometrial tissue outside the uterine cavity. Its predominant symptom is pain, a condition notably altering the quality of life of women with the disease. This review is intended to exhaustively gather current knowledge on purinergic signaling in endometriosis-associated pain. Altered extracellular ATP hydrolysis, due to changes in ectonucleotidase activity, has been reported in endometriosis; the resulting accumulation of ATP in the endometriotic microenvironment points to sustained activation of nucleotide receptors (P2 receptors) capable of generating a persistent pain message. P2X3 receptor, expressed in sensory neurons, mediates nociceptive, neuropathic, and inflammatory pain, and is enrolled in endometriosis-related pain. Pharmacological inhibition of P2X3 receptor is under evaluation as a pain relief treatment for women with endometriosis. The role of other ATP receptors is also discussed here, e.g., P2X4 and P2X7 receptors, which are involved in inflammatory cell–nerve and microglia–nerve crosstalk, and therefore in inflammatory and neuropathic pain. Adenosine receptors (P1 receptors), by contrast, mainly play antinociceptive and anti-inflammatory roles. Purinome-targeted drugs, including nucleotide receptors and metabolizing enzymes, are potential non-hormonal therapeutic tools for the pharmacological management of endometriosis-related pain.

p38 mitogen-activated protein kinase and pain

Inflammatory and neuropathic pain is initiated by tissue inflammation and nerve injury, respectively. Both are characterized by increased activity in the peripheral and central nervous system, where multiple inflammatory cytokines and other active molecules activate different signaling pathways that involve in the development and/or maintenance of pain. P38 mitogen-activated protein kinase (MAPK) is one member of the MAPK family, which is activated in neurons and glia and contributes importantly to inflammatory and neuropathic pain. The aim of this review is to summarize the latest advances made about the implication of p38 MAPK signaling cascade in pain. It can deepen our understanding of the molecular mechanisms of pain and may help to offer new targets for pain treatment.

Osthole alleviates neuropathic pain in mice by inhibiting the P2Y1-receptor-dependent JNK signaling pathway

There are many reports about natural products relieving neuralgia. Osthole is the main component of Angelica biserrata Yuan et Shan, a natural product that treats rheumatism through the elimination of inflammation and the alleviation of pain that has a long history in the clinic. The analgesic mechanism of osthole is complicated and confusing. Astrocytes have attracted increasing attention from pain researchers. Inhibitors targeting astrocytes are thought to be promising treatments for neuropathic pain. Whether osthole can alleviate neuropathic pain through astrocytes has not been elucidated in detail. In this study, CCI surgery was used to establish the neuropathic pain model in mice. The CCI mice were treated with osthole (5, 10, or 20 mg/kg/day) for 14 days in vivo. Mechanical allodynia and heat hyperalgesia were measured to evaluate the therapeutic effect of osthole. In mechanism research, the activation of astrocytes; the protein expression of P2Y₁R and p-JNK in astrocytes; the release of inflammatory factors; the variations in mEPSPs and eEPSPs; and the levels of GluA1, GluN2B, p-ERK, p-CREB and c-Fos in neurons were observed. The P2Y₁R inhibitor MRS2179 and the p-JNK inhibitor SP600125 were used to demonstrate how osthole works in neuropathic pain. In addition, astrocytes and neurons were used to estimate the direct effect of osthole on astrocyte-neuron interactions and signal transmission in vitro. Our findings suggest that osthole treatment obviously relieved mechanical allodynia and heat hyperalgesia in CCI mice. P2Y₁R is involved in CCI-induced pain hypersensitivity, and P2Y₁R is required for osthole-induced p-JNK downregulation in the spinal cord. Osthole inhibited astrocyte activation and reduced inflammatory factor expression. After osthole treatment, mEPSP frequency and eEPSP amplitude were decreased in spinal lamina I-II neurons. Downstream signaling molecules such as pGluA1, pGluN2B, p-ERK, p-CREB and c-Fos were also reduced very quickly in osthole-treated neuralgic mice. Our conclusion is that osthole alleviates neuropathic pain in mice via the P2Y₁-receptor-dependent JNK signaling pathway in spinal astrocytes, and osthole could be considered a potential pharmacotherapy to alleviate neuropathic pain.

A Targeted Mutation Disrupting Mitochondrial Complex IV Function in Primary Afferent Neurons Leads to Pain Hypersensitivity Through P2Y1 Receptor Activation

As mitochondrial dysfunction is evident in neurodegenerative disorders that are accompanied by pain, we generated inducible mutant mice with disruption of mitochondrial respiratory chain complex IV, by COX10 deletion limited to sensory afferent neurons through the use of an Advillin Cre-reporter. COX10 deletion results in a selective energy-deficiency phenotype with minimal production of reactive oxygen species. Mutant mice showed reduced activity of mitochondrial respiratory chain complex IV in many sensory neurons, increased ADP/ATP ratios in dorsal root ganglia and dorsal spinal cord synaptoneurosomes, as well as impaired mitochondrial membrane potential, in these synaptoneurosome preparations. These changes were accompanied by marked pain hypersensitivity in mechanical and thermal (hot and cold) tests without altered motor function. To address the underlying basis, we measured Ca²⁺ fluorescence responses of dorsal spinal cord synaptoneurosomes to activation of the GluK1 (kainate) receptor, which we showed to be widely expressed in small but not large nociceptive afferents, and is minimally expressed elsewhere in the spinal cord. Synaptoneurosomes from mutant mice showed greatly increased responses to GluK1 agonist. To explore whether altered nucleotide levels may play a part in this hypersensitivity, we pharmacologically interrogated potential roles of AMP-kinase and ADP-sensitive purinergic receptors. The ADP-sensitive P2Y₁ receptor was clearly implicated. Its expression in small nociceptive afferents was increased in mutants, whose in vivo pain hypersensitivity, in mechanical, thermal and cold tests, was reversed by a selective P2Y₁ antagonist. Energy depletion and ADP elevation in sensory afferents, due to mitochondrial respiratory chain complex IV deficiency, appear sufficient to induce pain hypersensitivity, by ADP activation of P2Y₁ receptors.

Upregulation of P2Y1 in neonatal nociceptors regulates heat and mechanical sensitization during cutaneous inflammation

The upregulation of various channels and receptors classically linked to sensory transduction from the periphery tightly correspond with changes in the responsiveness of specific subpopulations of primary afferents to mechanical and heat stimulation of the skin at different ages. Previous reports in adults have suggested that the purinergic adenosine diphosphate receptor, P2Y1 can specifically regulate sensory neuron responsiveness to heat stimuli in addition to neurochemical alterations in primary afferents during cutaneous inflammation. To determine if the upregulation of P2Y1 found in the dorsal root ganglia of neonatal mice with cutaneous inflammation initiated at postnatal day 7 (P7) was responsible for the specific alteration in heat sensitivity found in faster conducting (“A”-fiber) nociceptors, we assessed the response properties of cutaneous afferents using an ex vivo hairy hindpaw skin-saphenous nerve-dorsal root ganglion-spinal cord preparation in conjunction with nerve-targeted knockdown of P2Y1. We found that P2Y1 knockdown during neonatal cutaneous inflammation was sufficient to reduce the sensitization of “A”-fiber nociceptors to heat stimuli. Surprisingly, we also found that nerve-specific downregulation of P2Y1 could reduce the observed sensitization of these afferent subtypes to mechanical deformation of the skin. Immunocytochemical analysis of dorsal root ganglia showed that P2Y1 may mediate its effects through modulation of the injury-induced increase of transient receptor potential vanilloid type 1 receptor. This suggests that the upregulation of P2Y1 in cutaneous nociceptors during early life peripheral inflammation can regulate the sensitization of myelinated nociceptors to both mechanical and heat stimuli possibly through modulation of transient receptor potential vanilloid type 1 expression.

Role of TRPV1 in acupuncture modulation of reflex excitatory cardiovascular responses

We have shown that acupuncture, including manual and electroacupuncture (MA and EA), at the P5-6 acupoints stimulates afferent fibers in the median nerve (MN) to modulate sympathoexcitatory cardiovascular reflexes through central regulation of autonomic function. However, the mechanisms underlying acupuncture activation of these sensory afferent nerves and their cell bodies in the dorsal root ganglia (DRG) are unclear. Transient receptor potential vanilloid type 1 (TRPV1) is present in sensory nerve fibers distributed in the general region of acupoints like ST36 and BL 40 located in the hindlimb. However, the contribution of TRPV1 to activation of sensory nerves by acupuncture, leading to modulation of pressor responses, has not been studied. We hypothesized that TRPV1 participates in acupuncture's activation of sensory afferents and their associated cell bodies in the DRG to modulate pressor reflexes. Local injection of iodoresiniferatoxin (Iodo-RTX; a selective TRPV1 antagonist), but not 5% DMSO (vehicle), into the P6 acupoint on the forelimb reversed the MA's inhibition of pressor reflexes induced by gastric distension (GD). Conversely, inhibition of GD-induced sympathoexcitatory responses by EA at P5-6 was unchanged after administration of Iodo-RTX into P5-6. Single-unit activity of Group III or IV bimodal afferents sensitive to both mechanical and capsaicin stimuli responded to MA stimulation at P6. MA-evoked activity was attenuated significantly ( P < 0.05) by local administration of Iodo-RTX ( n = 12) but not by 5% DMSO ( n = 12) into the region of the P6 acupoint in rats. Administration of Iodo-RTX into P5-6 did not reduce bimodal afferent activity evoked by EA stimulation ( n = 8). Finally, MA at P6 and EA at P5-6 induced phosphorylation of extracellular signal-regulated kinases (ERK; an intracellular signaling messenger involved in cellular excitation) in DRG neurons located at C7-8 spinal levels receiving MN inputs. After TRPV1 was knocked down in the DRG at these spinal levels with intrathecal injection of TRPV1-siRNA, expression of phosphorylated ERK in the DRG neuron was reduced in MA-treated, but not EA-treated animals. These data suggest that TRPV1 in Group III and IV bimodal sensory afferent nerves contributes to acupuncture inhibition of reflex increases in blood pressure and specifically plays an important role during MA but not EA.

Interaction between calcitonin gene-related peptide-immunoreactive neurons and satellite cells via P2Y12 R in the trigeminal ganglion is involved in neuropathic tongue pain in rats

The P2Y₁₂ receptor expressed in satellite cells of the trigeminal ganglion is thought to contribute to neuropathic pain. The functional interaction between neurons and satellite cells via P2Y₁₂ receptors and phosphorylated extracellular signal-regulated kinase 1/2 (pERK1/2) underlying neuropathic pain in the tongue was evaluated in this study. Expression of P2Y₁₂ receptor was enhanced in pERK1/2-immunoreactive cells encircling trigeminal ganglion neurons after lingual nerve crush. The administration to lingual nerve crush rats of a selective P2Y₁₂ receptor antagonist, MRS2395, attenuated tongue hypersensitivity to mechanical and heat stimulation and suppressed the increase in the relative numbers of calcitonin gene-related peptide (CGRP)-immunoreactive neurons and neurons encircled by pERK1/2-immunoreactive cells. Administration of the P2Y_1,12,13 receptor agonist, 2-(methylthio)adenosine 5'-diphosphate trisodium salt hydrate (2-MeSADP), to naïve rats induced neuropathic pain in the tongue, as in lingual nerve crush rats. Co-administration of 2-MeSADP + MRS2395 to naïve rats did not result in hypersensitivity of the tongue. The relative number of CGRP-immunoreactive neurons increased following this co-administration, but to a lesser degree than observed in 2-MeSADP-administrated naïve rats, and the relative number of neurons encircled by pERK1/2-immunoreactive cells did not change. These results suggest that the interaction between activated satellite cells and CGRP-immunoreactive neurons via P2Y₁₂ receptors contributes to neuropathic pain in the tongue associated with lingual nerve injury.

Purinergic Signalling: Therapeutic Developments

Purinergic signalling, i.e., the role of nucleotides as extracellular signalling molecules, was proposed in 1972. However, this concept was not well accepted until the early 1990's when receptor subtypes for purines and pyrimidines were cloned and characterised, which includes four subtypes of the P1 (adenosine) receptor, seven subtypes of P2X ion channel receptors and 8 subtypes of the P2Y G protein-coupled receptor. Early studies were largely concerned with the physiology, pharmacology and biochemistry of purinergic signalling. More recently, the focus has been on the pathophysiology and therapeutic potential. There was early recognition of the use of P1 receptor agonists for the treatment of supraventricular tachycardia and A2A receptor antagonists are promising for the treatment of Parkinson's disease. Clopidogrel, a P2Y12 antagonist, is widely used for the treatment of thrombosis and stroke, blocking P2Y12 receptor-mediated platelet aggregation. Diquafosol, a long acting P2Y2 receptor agonist, is being used for the treatment of dry eye. P2X3 receptor antagonists have been developed that are orally bioavailable and stable in vivo and are currently in clinical trials for the treatment of chronic cough, bladder incontinence, visceral pain and hypertension. Antagonists to P2X7 receptors are being investigated for the treatment of inflammatory disorders, including neurodegenerative diseases. Other investigations are in progress for the use of purinergic agents for the treatment of osteoporosis, myocardial infarction, irritable bowel syndrome, epilepsy, atherosclerosis, depression, autism, diabetes, and cancer.

P2Y1R is involved in visceral hypersensitivity in rats with experimental irritable bowel syndrome

AIM

To evaluate the role of P2Y1R in visceral hypersensitivity in rats with experimental irritable bowel syndrome.

METHODS

A rat model of irritable bowel syndrome was generated by intra-colonic administration of acetic acid (AA) and assessed by histology and myeloperoxidase (MPO) activity assay. Then P2Y1R expression in the colonic tissue was detected by Western blot. In order to explore the regulatory role of P2Y1R in visceral hypersensitivity, an agonist (MRS2365) and an antagonist (MRS2179) of P2Y1R were intra-colonically administered and effects were tested through a colorectal distension test. The abdominal withdrawal reflex and abdominal electromyography were tested during the course.

RESULTS

Model assessment tests showed an obvious inflammatory reaction that appeared on the 2^nd d after the AA injection, and the inflammatory reaction gradually recovered and almost disappeared on the 7^th d. The model finished on day 8 and showed a clear feature of IBS that had no organic lesion. The average expression of P2Y1R was significantly higher in the AA group than in the naïve group (0.319 ± 0.02 vs 0.094 ± 0.016, P < 0.001). MRS2365 could effectively raise the colonic hypersensitivity status at intervention doses of 10 (AUC value from 0.30 ± 0.089 to 1.973 ± 0.127 mv·s, P < 0.01) and 100 μmol/L (AUC value from 0.290 ± 0.079 to 1.983 ± 0.195 mv·s, P < 0.01); MRS2179 could effectively reduce the hypersensitivity status at intervention dose of 100 μmol/L (from a mean baseline AUC value of 1.587 ± 0.099 mv·s to 0.140 ± 0.089 mv·s, P < 0.0001). Differences between the MRS2179 group (1.88 ± 1.45) and either the MRS2365 group (3.96 ± 0.19) or the combined treatment (MRS2179 and MRS2365) group (3.28 ± 0.11) were significant (P < 0.01).

CONCLUSION

P2Y1R plays a regulatory role in visceral hypersensitivity in rats with experimental IBS. Specific antagonists of P2Y1R may have potential therapeutic value in treating abdominal pain in IBS.

TRPV1 modulates morphine-induced conditioned place preference via p38 MAPK in the nucleus accumbens

Emerging evidence suggests that the transient receptor potential vanilloid type 1 channel (TRPV1) is a novel target for the treatment of drug addiction, such as cocaine and morphine. Previously we reported that TRPV1 inhibition reduced morphine reward in the dorsal striatum (DSt) of mice and morphine self-administration through a decrease in accumbal activity in rats. However, the role of TRPV1 on morphine-conditioned reward in addiction-related brain regions, such as the nucleus accumbens (NAc), has not been previously established. Here, we investigated the effects of TRPV1 on morphine conditioned place preference (CPP) and intracellular mechanisms of TRPV1 using Western blot analysis and immunohistochemistry (IHC) in morphine-administered mice. TRPV1 knockout mice did not exhibit morphine reward responses, and both i.p. and intra-NAc injections of SB366791, a selective TRPV1 antagonist, reduced morphine-induced CPP in wild-type mice. Furthermore, i.p. injection of SB203580, a selective p38 MAPK inhibitor, also dampened morphine-induced CPP. To determine the molecular mechanisms of the TRPV1/p38 MAPK pathway in morphine CPP, we investigated the expression of adenylyl cyclase type 1 (AC1) and phospho-p38 mitogen-activated protein kinase (MAPK) and nuclear factor-kappa B (NF-κB) in the NAc. Either SB366791 or SB203580 decreased the protein expression levels of phospho-p38 MAPK, phosphor-NF-κB, and AC1 in the NAc of morphine CPP mice. Taken together, our findings suggest that TRPV1 may modulate morphine-induced conditioned reward effects via the p38 MAPK signaling pathway in the NAc. Therefore, blockade of TRPV1 may provide a novel therapeutic approach for the prevention and treatment of opioid addiction.

Metformin Synergizes With Conventional and Adjuvant Analgesic Drugs to Reduce Inflammatory Hyperalgesia in Rats

BACKGROUND

Metformin is a widely used and safe antidiabetic drug that has recently been shown to possess analgesic properties in models of inflammatory pain. Because various arthritic inflammatory disorders are highly prevalent in diabetic patients, we aimed to examine the type of interaction between metformin and several conventional and adjuvant analgesic drugs (ibuprofen, aspirin, tramadol, and pregabalin) in a rat model of somatic inflammatory hyperalgesia.

METHODS

Inflammation of the rat hind paw was induced by an intraplantar injection of carrageenan (0.1 mL, 1%). The antihyperalgesic effects of metformin (intraperitoneally), analgesics (orally or intraperitoneally), and 2-drug metformin-analgesic combinations were assessed with an electronic Von Frey anesthesiometer, by measuring the change in paw withdrawal thresholds induced by carrageenan (n = 6 rats in drug/drug combination-treated groups). First, we determined the doses of individual drugs needed to produce an antihyperalgesic effect of 50% (ED50 values). In combination experiments, drugs were coadministered in fixed-dose fractions (1/16, 1/8, 1/4, and 1/2) of their individual ED50 values and the type of interaction between components was determined by isobolographic analysis.

RESULTS

Metformin (50-200 mg/kg) significantly and dose-dependently reduced carrageenan-induced hyperalgesia with a maximal antihyperalgesic effect (mean ± SEM) of 62 ± 6% (all P ≤ .024). Ibuprofen (25-150 mg/kg), aspirin (100-400 mg/kg), tramadol (0.5-5 mg/kg), and pregabalin (2.5-20 mg/kg) also produced significant and dose-dependent antihyperalgesic effects (all P ≤ .042) of similar magnitude to metformin (the maximal antihyperalgesic effects were 73 ± 4% for ibuprofen, 62 ± 4.2% for aspirin, 69 ± 5.9% for tramadol, and 56 ± 3.9% for pregabalin). In combination experiments, administration of 2-drug metformin-analgesic combinations led to a significant and dose-dependent reduction of carrageenan-induced hyperalgesia (all P ≤ .027). The isobolographic analysis revealed that metformin interacted synergistically with the examined analgesics (experimental ED50 values of 2-drug combinations were significantly lower than theoretical additive ED50 values; all P < .05) and that there was a similar, approximately 5-fold, reduction of doses of both drugs in all tested combinations.

CONCLUSIONS

Our results suggest that in patients who are already receiving metformin therapy, lower doses of ibuprofen/aspirin/tramadol/pregabalin might be sufficient for achieving satisfactory pain relief. Metformin-aspirin combination might be particularly useful because it may achieve multiple therapeutic goals (glucoregulation, pain relief, and cardioprotection).

Peripheral inflammation affects modulation of nociceptive synaptic transmission in the spinal cord induced by N-arachidonoylphosphatidylethanolamine

BACKGROUND AND PURPOSE

Endocannabinoids play an important role in modulating spinal nociceptive signalling, crucial for the development of pain. The cannabinoid CB₁ receptor and the TRPV1 cation channel are both activated by the endocannabinoid anandamide, a product of biosynthesis from the endogenous lipid precursor N-arachidonoylphosphatidylethanolamine (20:4-NAPE). Here, we report CB₁ receptor- and TRPV1-mediated effects of 20:4-NAPE on spinal synaptic transmission in control and inflammatory conditions.

EXPERIMENTAL APPROACH

Spontaneous (sEPSCs) and dorsal root stimulation-evoked (eEPSCs) excitatory postsynaptic currents from superficial dorsal horn neurons in rat spinal cord slices were assessed. Peripheral inflammation was induced by carrageenan. Anandamide concentration was assessed by mass spectrometry.

KEY RESULTS

Application of 20:4-NAPE increased anandamide concentration in vitro. 20:4-NAPE (20 μM) decreased sEPSCs frequency and eEPSCs amplitude in control and inflammatory conditions. The inhibitory effect of 20:4-NAPE was sensitive to CB₁ receptor antagonist PF514273 (0.2 μM) in both conditions, but to the TRPV1 antagonist SB366791 (10 μM) only after inflammation. After inflammation, 20:4-NAPE increased sEPSCs frequency in the presence of PF514273 and this increase was blocked by SB366791.

CONCLUSIONS AND IMPLICATIONS

While 20:4-NAPE treatment inhibited the excitatory synaptic transmission in both naive and inflammatory conditions, peripheral inflammation altered the underlying mechanisms. Our data indicate that 20:4-NAPE application induced mainly CB₁ receptor-mediated inhibitory effects in naive animals while TRPV1-mediated mechanisms were also involved after inflammation. Increasing anandamide levels for analgesic purposes by applying substrate for its local synthesis may be more effective than systemic anandamide application or inhibition of its degradation.

LINKED ARTICLES

This article is part of a themed section on Recent Advances in Targeting Ion Channels to Treat Chronic Pain. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.12/issuetoc.

Purinergic signaling and the functioning of the nervous system cells

Purinergic signaling in the nervous system has been the focus of a considerable number of studies since the 1970s. The P2X and P2Y receptors are involved in the initiation of purinergic signaling. They are very abundant in the central and peripheral nervous systems, where they are expressed on the surface of neurons and glial cells--microglia, astrocytes, oligodendrocytes and Schwann cells and the precursors of the latter two. Their ligands--extracellular nucleotides--are released in the physiological state by astrocytes and neurons forming synaptic connections, and are essential for the proper functioning of nervous system cells. Purinergic signaling plays a crucial role in neuromodulation, neurotransmission, myelination in the CNS and PNS, intercellular communication, the regulation of ramified microglia activity, the induction of the response to damaging agents, the modulation of synaptic activity and other glial cells by astrocytes, and the induction of astrogliosis. Understanding these mechanisms and the fact that P2 receptors and their ligands are involved in the pathogenesis of diseases of the nervous system may help in the design of drugs with different and more effective mechanisms of action.

P2Y1, P2Y2, and TRPV1 Receptors Are Increased in Diarrhea-Predominant Irritable Bowel Syndrome and P2Y2 Correlates with Abdominal Pain

BACKGROUND

Previous studies indicated that P2Y1 and P2Y2 receptors, which are widely distributed in the enteric nervous system, are related to pain, while TRPV1 may contribute to visceral pain and hypersensitivity states in irritable bowel syndrome (IBS). Other studies showed that ATP activates the capsaicin-sensitive TRPV1 channel via P2Y receptors.

AIM

To detect the expression of P2Y1, P2Y2, and TRPV1 receptors in diarrhea-predominant IBS (IBS-D) patients and analyze any correlations with abdominal pain and to investigate interactions between P2Y receptors and the TRPV1 receptor in IBS-D patients.

METHODS

Rectosigmoid biopsies were collected from patients with IBS-D (n = 36) and healthy controls (n = 15). Abdominal pain was scored using a 10-cm visual analogue scale. Expression levels of P2Y1, P2Y2, and TRPV1 receptors in rectosigmoid biopsies were determined by real-time PCR and double-labeling immunofluorescence with specific antibodies.

RESULTS

Both mRNA and protein expression levels of P2Y1, P2Y2, and TRPV1 receptors were increased in IBS-D compared with controls. Of these receptors, P2Y2 expression correlated with the maximum pain scores (p = 0.02, r = 0.63, Spearman correlation) in IBS-D patients. However, no relationships were detected between P2Y receptors and the TRPV1 receptor.

CONCLUSION

In the present study, we identified an increased expression of P2Y1 and P2Y2 receptors in the rectosigmoid mucosa of IBS-D patients, and P2Y2 correlated with abdominal pain. Furthermore, we identified an increase in TRPV1 expression; however, there were no correlations found between P2Y receptors and the TRPV1 receptor.

Serotonin type-1D receptor stimulation of A-type K(+) channel decreases membrane excitability through the protein kinase A- and B-Raf-dependent p38 MAPK pathways in mouse trigeminal ganglion neurons

Although recent studies have implicated serotonin 5-HT1B/D receptors in the nociceptive sensitivity of primary afferent neurons, the underlying molecular and cellular mechanisms remain unclear. In this study, we identified a novel functional role of the 5-HT1D receptor subtype in regulating A-type potassium (K(+)) currents (IA) as well as membrane excitability in small trigeminal ganglion (TG) neurons. We found that the selective activation of 5-HT1D, rather than 5-HT1B, receptors reversibly increased IA, while the sustained delayed rectifier K(+) current was unaffected. The 5-HT1D-mediated IA increase was associated with a depolarizing shift in the voltage dependence of inactivation. Blocking G-protein signaling with pertussis toxin or by intracellular application of a selective antibody raised against Gαo or Gβ abolished the 5-HT1D effect on IA. Inhibition of protein kinase A (PKA), but not of phosphatidylinositol 3-kinase or protein kinase C, abolished the 5-HT1D-mediated IA increase. Analysis of phospho-p38 (p-p38) revealed that activation of 5-HT1D, but not 5-HT1B, receptors significantly activated p38, while p-ERK and p-JNK were unaffected. The p38 MAPK inhibitor SB203580, but not its inactive analogue SB202474, and inhibition of B-Raf blocked the 5-HT1D-mediated IA response. Functionally, we observed a significantly decreased action potential firing rate induced by the 5-HT1D receptors; pretreatment with 4-aminopyridine abolished this effect. Taken together, these results suggest that the activation of 5-HT1D receptors selectively enhanced IA via the Gβγ of the Go-protein, PKA, and the sequential B-Raf-dependent p38 MAPK signaling cascade. This 5-HT1D receptor effect may contribute to neuronal hypoexcitability in small TG neurons.

Long-Term Activation of Group I Metabotropic Glutamate Receptors Increases Functional TRPV1-Expressing Neurons in Mouse Dorsal Root Ganglia

Damaged tissues release glutamate and other chemical mediators for several hours. These chemical mediators contribute to modulation of pruritus and pain. Herein, we investigated the effects of long-term activation of excitatory glutamate receptors on functional expression of transient receptor potential vaniloid type 1 (TRPV1) in dorsal root ganglion (DRG) neurons and then on thermal pain behavior. In order to detect the TRPV1-mediated responses in cultured DRG neurons, we monitored intracellular calcium responses to capsaicin, a TRPV1 agonist, with Fura-2. Long-term (4 h) treatment with glutamate receptor agonists (glutamate, quisqualate or DHPG) increased the proportion of neurons responding to capsaicin through activation of metabotropic glutamate receptor mGluR1, and only partially through the activation of mGluR5; engagement of these receptors was evident in neurons responding to allylisothiocyanate (AITC), a transient receptor potential ankyrin type 1 (TRPA1) agonist. Increase in the proportion was suppressed by phospholipase C (PLC), protein kinase C, mitogen/extracellular signal-regulated kinase, p38 mitogen-activated protein kinase or transcription inhibitors. Whole-cell recording was performed to record TRPV1-mediated membrane current; TRPV1 current density significantly increased in the AITC-sensitive neurons after the quisqualate treatment. To elucidate the physiological significance of this phenomenon, a hot plate test was performed. Intraplantar injection of quisqualate or DHPG induced heat hyperalgesia that lasted for 4 h post injection. This chronic hyperalgesia was attenuated by treatment with either mGluR1 or mGluR5 antagonists. These results suggest that long-term activation of mGluR1/5 by peripherally released glutamate may increase the number of neurons expressing functional TRPV1 in DRG, which may be strongly associated with chronic hyperalgesia.

Predominant role of spinal P2Y1 receptors in the development of neuropathic pain in rats

The role of P2X2/3, P2X3, P2X4 or P2X7 and P2Y2, P2Y6, and P2Y12 receptors in neuropathic pain has been widely studied. In contrast, the role of P2Y1 receptors is scarcely studied. In this study we assessed the role of P2Y1 receptors in several neuropathic pain models in the rat. Furthermore, we analyzed the expression of P2Y1 receptors in the ipsilateral dorsal root ganglia (DRG) and dorsal part of the spinal cord during the development and maintenance of neuropathic pain. We also determined the effect of the P2Y1 receptor antagonist on the expression of P2Y1 receptors. Chronic constriction injury (CCI), spared nerve injury (SNI) or spinal nerve ligation (SNL) produced tactile allodynia from 1 to 14 days after nerve injury. CCI, SNI and SNL enhanced expression of P2Y1 receptors in DRG but not in the dorsal part of the spinal cord at 1-3 days after injury. Intrathecal injection of the selective P2Y1 receptor antagonist MRS2500, but not vehicle, reduced tactile allodynia in rats 1-3 days after CCI, SNI, or SNL. Moreover, intrathecal injection of MRS2500 (at day 1 or 3) reduced neuropathy-induced up-regulation of P2Y1 receptors expression. Intrathecal injection of MRS2500 lost most of the antiallodynic effect when injected 14 days after injury. At this time, MRS2500 did not modify nerve-injury-induced P2Y1 receptors up-regulation. Our results suggest that P2Y1 receptors are localized in DRG, are up-regulated by nerve injury and play a pronociceptive role in development and, to a lesser extent, maintenance of neuropathic pain.

Chronic inflammatory pain upregulates expression of P2Y2 receptor in small-diameter sensory neurons

Roles of ionotropic purinergic (P2X) receptors in chronic pain have been intensively investigated. However, the contribution of metabotropic purinergic (P2Y) receptors to pathological pain is controversial. In the present study, using single cell RT-PCR (reverse transcription-polymerase chain reaction) and single cell nested-PCR techniques, we examined the expression of P2X(2), P2X(3), P2Y(1) and P2Y(2) mRNA transcripts in retrogradely labeled cutaneous sensory neurons from mouse lumber dorsal root ganglia (DRGs) following peripheral inflammation. The percentage of cutaneous sensory neurons expressing P2Y(2) mRNA transcripts increased after complete Freund's adjuvant (CFA) treatment. Particularly, the P2Y(2) mRNA transcripts were more frequently detected in small-diameter cutaneous neurons from CFA-treated mice than those from control mice. Coexpression of P2Y(2) and P2X (P2X(2) or P2X(3)) mRNAs was more frequently observed in cutaneous sensory neurons from CFA-treated mice relative to controls. Pain behavioral tests showed that the blockade of P2Y receptors by suramin attenuated mechanical allodynia evoked either by CFA or uridine triphosphate (UTP), an endogenous P2Y(2) and P2Y(4) agonist. These results suggest that chronic inflammatory pain enhances expression of P2Y(2) receptor in peripheral sensory neurons that innervate the injured tissue and the activation of P2Y receptors contributes to mechanical allodynia following inflammation.

p38 MAPK mediates glial P2X7R-neuronal P2Y1R inhibitory control of P2X3R expression in dorsal root ganglion neurons

Background

We have previously shown that endogenously active purinergic P2X7 receptors (P2X7Rs) in satellite glial cells of dorsal root ganglia (DRGs) stimulate ATP release. The ATP activates P2Y1Rs located in the enwrapped neuronal somata, resulting in down-regulation of P2X3Rs. This P2X7R-P2Y1-P2X3R inhibitory control significantly reduces P2X3R-mediated nociceptive responses. The underlying mechanism by which the activation of P2Y1Rs inhibits the expression of P2X3Rs remains unexplored.

Results

Examining the effect of the activation of p38 mitogen-activated protein kinase on the expression of P2X3Rs in DRGs, we found that the p38 activator, anisomycin (Anis), reduced the expression of P2X3Rs. Blocking the activity of SGCs by the glial Krebs cycle inhibitor, fluorocitrate, did not change the effect of Anis. These results suggest that neuronal p38 plays a major role in the inhibition of P2X3R expression. Western blotting analyses showed that inhibiting P2Y1Rs by MRS2179 (MRS) or blocking P2X7Rs by either oxATP or A740003 reduced pp38 and increased P2X3R expression in DRGs. These results are further supported by the immunohistochemical study showing that P2X7R and P2Y1R antagonists reduce the percentage of pp38-positive neurons. These observations suggest that activation of P2X7Rs and P2Y1Rs promotes p38 activity to exert inhibitory control on P2X3R expression. Since activation of p38 by Anis in the presence of either A740003 or MRS could overcome the block of P2X7R-P2Y1R inhibitory control, p38 in DRG neurons is downstream of P2Y1Rs. In addition, inhibition of p38 by SB202190 was found to prevent the P2X7R and P2Y1R block of P2X3R expression and increase P2X3R-mediated nociceptive flinch behaviors.

Conclusions

p38 in DRG neurons downstream of P2Y1R is necessary and sufficient for the P2X7R-P2Y1R inhibitory control of P2X3R expression.

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.

Effect of Intramuscular Protons, Lactate, and ATP on Muscle Hyperalgesia in Rats

Chronic muscle pain is a significant health problem leading to disability[1]. Muscle fatigue can exacerbate muscle pain. Metabolites, including ATP, lactate, and protons, are released during fatiguing exercise and produce pain in humans. These substances directly activate purinergic (P2X) and acid sensing ion channels (ASICs) on muscle nociceptors, and when combined, produce a greater increase in neuron firing than when given alone. Whether the enhanced effect of combining protons, lactate, and ATP is the sum of individual effects (additive) or more than the sum of individual effects (synergistic) is unknown. Using a rat model of muscle nociceptive behavior, we tested each of these compounds individually over a range of physiologic and supra-physiologic concentrations. Further, we combined all three compounds in a series of dilutions and tested their effect on muscle nociceptive behavior. We also tested a non-hydrolyzable form of ATP (α,β-meATP) alone and in combination with lactate and acidic pH. Surprisingly, we found no dose-dependent effect on muscle nociceptive behavior for protons, lactate, or ATP when given alone. We similarly found no effect after application of each two-metabolite combination. Only pH 4 saline and α,β-meATP produced hyperalgesia when given alone. When all 3 substances were combined, however, ATP (2.4μm), lactate (10mM), and acidic pH (pH 6.0) produced an enhanced effect greater than the sum of the effects of the individual components, i.e. synergism. α,β me ATP (3nmol), on the other hand, showed no enhanced effects when combined with lactate (10mM) or acidic pH (pH 6.0), i.e. additive. These data suggest that combining fatigue metabolites in muscle produces a synergistic effect on muscle nociception.

Molecular and cellular mechanisms that initiate pain and itch

Somatosensory neurons mediate our sense of touch. They are critically involved in transducing pain and itch sensations under physiological and pathological conditions, along with other skin-resident cells. Tissue damage and inflammation can produce a localized or systemic sensitization of our senses of pain and itch, which can facilitate our detection of threats in the environment. Although acute pain and itch protect us from further damage, persistent pain and itch are debilitating. Recent exciting discoveries have significantly advanced our knowledge of the roles of membrane-bound G protein-coupled receptors and ion channels in the encoding of information leading to pain and itch sensations. This review focuses on molecular and cellular events that are important in early stages of the biological processing that culminates in our senses of pain and itch.

Effects of Schisandra chinensis extracts on cough and pulmonary inflammation in a cough hypersensitivity guinea pig model induced by cigarette smoke exposure

Schisandra chinensis (S. chinensis) is a traditional Chinese medicine commonly used in prescription medications for the treatment of chronic cough. However, the material basis of S. chinensis in relieving cough has not been completely elucidated yet. This study established a guinea pig model of cough hypersensitivity induced by 14 days of cigarette smoke (CS) exposure, to evaluate the antitussive, antioxidant, and anti-inflammatory effects of three S. chinensis extracts. And then the function of four lignans in reducing expression of TRPV1 and TRPA1 was examined using A549 cells induced by cigarette smoke extract (CSE). The results demonstrated that both ethanol extract (EE) and ethanol-water extract (EWE) of S. chinensis, but not water extract (WE), significantly reduced the cough frequency enhanced by 0.4M citric acid solution in these cough hypersensitivity guinea pigs. Meanwhile, pretreatment with EE and EWE both significantly attenuated the CS-induced increase in infiltration of pulmonary neutrophils and total inflammatory cells, as well as pulmonary MDA, TNF-α, and IL-8, while remarkably increased activities of pulmonary SOD and GSH. According to H&E and immunofluorescence staining assays, airway epithelium hyperplasia, smooth muscle thickening, inflammatory cells infiltration, as well as expression of TRPV1 and TRPA1, were significantly attenuated in animals pretreatment with 1g/kg EE. Moreover, four lignans of EE, including schizandrin, schisantherin A, deoxyschizandrin and γ-schisandrin, significantly inhibited CSE-induced expression of TRPV1, TRPA1 and NOS3, as well as NO release in A549 cells. In conclusion, S. chinensis reduces cough frequency and pulmonary inflammation in the CS-induced cough hypersensitivity guinea pigs. Lignans may be the active components.

Participation of peripheral P2Y1, P2Y6 and P2Y11 receptors in formalin-induced inflammatory pain in rats

Metabotropic P2Y receptors subfamily consists of eight functional mammalian receptors. Specifically, P2Y1, P2Y6 and P2Y11 receptors have been described in the sensory nervous system, but their participation, at peripheral level, in behavioral pain models is scarcely understood. This study assessed the role of peripheral P2Y1, P2Y6 and P2Y11 receptors in formalin-induced inflammatory pain. Ipsilateral, but not contralateral peripheral pre-treatment with the endogenous P2Y1 (ADP, 100-1000nmol/paw), P2Y6 (UDP, 180-300nmol/paw) and P2Y11 (ATP, 100-1000nmol/paw), or selective P2Y1 (MRS2365, 0.1-10nmol/paw), P2Y6 (PSB0474, 0.1-0.10pmol/paw) and P2Y11 (NF546, 0.3-3nmol/paw) receptor agonists increased 0.5% formalin-induced flinching behavior. Concordantly, peripheral pre-treatment with the selective P2Y1 (MRS2500, 0.01-10pmol/paw), P2Y6 (MRS2578, 3-30nmol/paw) and P2Y11 (NF340, 1-10nmol/paw) receptor antagonists significantly decreased 1% formalin-induced flinching behavior. Furthermore, the pronociceptive effect of ADP (100nmol/paw) or MRS2365 (10nmol/paw), UDP (300nmol/paw) or PSB0474 (10pmol/paw) and ATP (1000nmol/paw) or NF546 (3nmol/paw) was blocked by the selective P2Y1 (MRS2500, 0.01nmol/paw), P2Y6 (MRS2578, 3nmol/paw), and P2Y11 (NF340, 1nmol/paw) receptor antagonists, respectively. Western blot analysis confirmed the presence of P2Y1 (66kDa), P2Y6 (36kDa) and P2Y11 (75kDa) receptors in dorsal root ganglia (DRG) and sciatic nerve. Results suggest that peripheral activation of P2Y1, P2Y6 and P2Y11 receptors plays a pronociceptive role in formalin-induced pain.