Altered ATP release and metabolism in dorsal root ganglia of neuropathic rats

Photobiomodulation and vitamin B treatment alleviate both thermal and mechanical orofacial pain in rats

PURPOSE

The present study investigates the efficacy of Photobiomodulation (PBM) and Vitamin B Complex (VBC) to relieve pain, both in separately and combined (PBM and VBC).

METHODS

Rats with chronic constriction injury of the right infraorbital nerve (CCI-IoN) or Sham surgery were used. PBM was administered at a wavelength of 904 nm and energy density of 6.23 J/cm2 and VBC (containing B1, B6 and B12) subcutaneously, both separately and combined. Behavioral tests were performed to assess mechanical and thermal hypersensitivity before and after CCI and after PBM, VBC, or PBM + VBC. The expression of inflammatory proteins in the trigeminal ganglion and the immunohistochemical alterations of Periaqueductal Gray (PAG) astrocytes and microglia were examined following CCI and treatments.

RESULTS

All testeds treatments reversed the painful behavior. The decrease in pain was accompanied by a decrease of Glial Fibrillary Acidic Protein (GFAP), a specific astrocytic marker, and Ionized calcium-binding adaptor molecule 1 (Iba-1), a marker of microglia, and decreased expression of Transient Receptor Potential Vanilloid 1 (TRPV1), Substance P, and Calcitonin Gene-Related Peptide (CGRP) induced by CCI-IoN in PAG and Trigeminal ganglion. Furthermore, both treatments showed a higher expression of Cannabinoid-type 1 (CB1) receptor in the trigeminal ganglion compared to CCI-IoN rats. Our results show that no difference was observed between groups.

CONCLUSION

We showed that PBM or VBC regulates neuroinflammation and reduces inflammatory protein expression. However, the combination of PBM and VBC did not enhance the effectiveness of both therapies alone.

Neuronal activity-dependent ATP enhances the pro-growth effect of repair Schwann cell extracellular vesicles by increasing their miRNA-21 loading

Functional recovery after peripheral nerve injuries is critically dependent on axonal regeneration. Several autonomous and non-cell autonomous processes regulate axonal regeneration, including the activation of a growth-associated transcriptional program in neurons and the reprogramming of differentiated Schwann cells (dSCs) into repair SCs (rSCs), triggering the secretion of neurotrophic factors and the activation of an inflammatory response. Repair Schwann cells also release pro-regenerative extracellular vesicles (EVs), but is still unknown whether EV secretion is regulated non-cell autonomously by the regenerating neuron. Interestingly, it has been described that nerve activity enhances axonal regeneration by increasing the secretion of neurotrophic factors by rSC, but whether this activity modulates pro-regenerative EV secretion by rSC has not yet been explored. Here, we demonstrate that neuronal activity enhances the release of rSC-derived EVs and their transfer to neurons. This effect is mediated by activation of P2Y receptors in SCs after activity-dependent ATP release from sensory neurons. Importantly, activation of P2Y in rSCs also increases the amount of miRNA-21 present in rSC-EVs. Taken together, our results demonstrate that neuron to glia communication by ATP-P2Y signaling regulates the content of SC-derived EVs and their transfer to axons, modulating axonal elongation in a non-cell autonomous manner.

Acupuncture modulates extracellular ATP levels in peripheral sensory nervous system during analgesia of ankle arthritis in rats

As an ancient analgesia therapy, acupuncture has been practiced worldwide nowadays. A good understanding of its mechanisms will offer a promise for its rational and wider application. As the first station of pain sensation, peripheral sensory ganglia express pain-related P2X receptors that are involved in the acupuncture analgesia mechanisms transduction pathway. While the role of their endogenous ligand, extracellular ATP (eATP), remains less studied. This work attempted to clarify whether acupuncture modulated eATP levels in the peripheral sensory nerve system during its analgesia process. Male Sprague-Dawley rats underwent acute inflammatory pain by injecting Complete Freund's Adjuvant in the unilateral ankle joint for 2 days. A twenty-minute acupuncture was applied to ipsilateral Zusanli acupoint. Thermal hyperalgesia and tactile allodynia were assessed on bilateral hind paws to evaluate the analgesic effect. eATP of bilateral isolated lumbar 4-5 dorsal root ganglia (DRGs) and sciatic nerves were determined by luminescence assay. Nucleotidases NTPDase-2 and -3 in bilateral ganglia and sciatic nerves were measured by real-time PCR to explore eATP hydrolysis process. Our results revealed that acute inflammation induced bilateral thermal hyperalgesia and ipsilateral tactile allodynia, which were accompanied by increased eATP levels and higher mechano-sensitivity of bilateral DRGs and decreased eATP levels of bilateral sciatic nerves. Acupuncture exerted anti-nociception on bilateral hind paws, reversed the increased eATP and mechanosensitivity of bilateral DRGs, and restored the decreased eATP of bilateral sciatic nerves. NTPDase-2 and -3 in bilateral ganglia and sciatic nerves were inconsistently modulated during this period. These observations indicate that eATP metabolism of peripheral sensory nerve system was simultaneously regulated during acupuncture analgesia, which might open a new frontier for acupuncture research.

IL-10 and CXCL2 in trigeminal ganglia in neuropathic pain

Many trigeminal neuropathic pain patients suffer severe chronic pain. The neuropathic pain might be related with cross-excitation of the neighboring neurons and satellite glial cells (SGCs) in the sensory ganglia and increasing the pain signals from the peripheral tissue to the central nervous system. We induced trigeminal neuropathic pain by infraorbital nerve constriction injury (IONC) in Sprague-Dawley rats. We tested cytokine (CXCL2 and IL-10) levels in trigeminal ganglia (TGs) after trigeminal neuropathic pain induction, and the effect of direct injection of the anti-CXCL2 and recombinant IL-10 into TG. We found that IONC induced pain behavior. Additionally, IONC induced satellite glial cell activation in TG and cytokine levels of TGs were changed after IONC. CXCL2 levels increased on day 1 of neuropathic pain induction and decreased gradually, with IL-10 levels showing the opposite trend. Recombinant IL-10 or anti-CXCL2 injection into TG decreased pain behavior. Our results show that IL-10 or anti-CXCL2 are therapy options for neuropathic pain.

CGRP Induces Differential Regulation of Cytokines from Satellite Glial Cells in Trigeminal Ganglia and Orofacial Nociception

Neuron-glia interactions contribute to pain initiation and sustainment. Intra-ganglionic (IG) secretion of calcitonin gene-related peptide (CGRP) in the trigeminal ganglion (TG) modulates pain transmission through neuron-glia signaling, contributing to various orofacial pain conditions. The present study aimed to investigate the role of satellite glial cells (SGC) in TG in causing cytokine-related orofacial nociception in response to IG administration of CGRP. For that purpose, CGRP alone (10 μL of 10⁻⁵ M), Minocycline (5 μL containing 10 μg) followed by CGRP with one hour gap (Min + CGRP) were administered directly inside the TG in independent experiments. Rats were evaluated for thermal hyperalgesia at 6 and 24 h post-injection using an operant orofacial pain assessment device (OPAD) at three temperatures (37, 45 and 10 °C). Quantitative real-time PCR was performed to evaluate the mRNA expression of IL-1β, IL-6, TNF-α, IL-1 receptor antagonist (IL-1RA), sodium channel 1.7 (NaV 1.7, for assessment of neuronal activation) and glial fibrillary acidic protein (GFAP, a marker of glial activation). The cytokines released in culture media from purified glial cells were evaluated using antibody cytokine array. IG CGRP caused heat hyperalgesia between 6–24 h (paired-t test, p < 0.05). Between 1 to 6 h the mRNA and protein expressions of GFAP was increased in parallel with an increase in the mRNA expression of pro-inflammatory cytokines IL-1β and anti-inflammatory cytokine IL-1RA and NaV1.7 (one-way ANOVA followed by Dunnett’s post hoc test, p < 0.05). To investigate whether glial inhibition is useful to prevent nociception symptoms, Minocycline (glial inhibitor) was administered IG 1 h before CGRP injection. Minocycline reversed CGRP-induced thermal nociception, glial activity, and down-regulated IL-1β and IL-6 cytokines significantly at 6 h (t-test, p < 0.05). Purified glial cells in culture showed an increase in release of 20 cytokines after stimulation with CGRP. Our findings demonstrate that SGCs in the sensory ganglia contribute to the occurrence of pain via cytokine expression and that glial inhibition can effectively control the development of nociception.

The inflammasome as a target for pain therapy

The interleukin-1 family of cytokines are potent inducers of inflammation and pain. Proteolytic activation of this family of cytokines is under the control of several innate immune receptors that coordinate to form large multiprotein signalling platforms, termed inflammasomes. Recent evidence suggests that a wide range of inflammatory diseases, cancers, and metabolic and autoimmune disorders, in which pain is a common complaint, may be coordinated by inflammasomes. Activation of inflammasomes results in cleavage of caspase-1, which subsequently induces downstream initiation of several potent pro-inflammatory cascades. Therefore, it has been proposed that targeting inflammasome activity may be a novel and effective therapeutic strategy for these pain-related diseases. The purpose of this narrative review article is to provide the reader with an overview of the activation and regulation of inflammasomes and to investigate the potential therapeutic role of inflammasome inhibition in the treatment of diseases characterized by pain, including the following: complex regional pain syndrome, gout, rheumatoid arthritis, inflammatory pain, neuropathic pain, chronic prostatitis, chronic pelvic pain syndrome, and fibromyalgia. We conclude that the role of the inflammasome in pain-associated diseases is likely to be inflammasome subtype and disease specific. The currently available evidence suggests that disease-specific targeting of the assembly and activity of the inflammasome complex may be a novel therapeutic opportunity for the treatment of refractory pain in many settings.

ATP metabolizing enzymes ENPP1, 2 and 3 are localized in sensory neurons of rat dorsal root ganglion

In dorsal root ganglion (DRG) neurons, ATP is an important neurotransmitter in nociceptive signaling through P2 receptors (P2Rs) such as P2X2/3R, and adenosine is also involved in anti-nociceptive signaling through adenosine A1R. Thus, the clearance system for adenine nucleotide/nucleoside plays a critical role in regulation of nociceptive signaling, but there is little information on it, especially ectoenzyme expression profiles in DRG. In this study, we examined expression and localization of ecto-nucleotide pyrophosphatase/phosphodiesterases (ENPPs), by which ATP is metabolized to AMP, in rat DRG. The mRNA expression levels of ENPP2 were greater than those of ENPP1 and ENPP3 in rat DRGs. On immunohistochemical analysis, ENPP1, 2 and 3 were found in soma of DRG neurons. Immunopositive rate of ENPP3 was greater than that of ENPP1 and ENPP2 in all DRG neurons. ENPP3, as compared with ENPP1 and ENPP2, was expressed mainly by isolectin B4-positive cells, and slightly by neurofilament 200-positive ones. In this way, the expression profile of ENPP1, 2 and 3 was different in DRGs, and they were mainly expressed in small/medium-sized DRG neurons. Moreover, ENPP1-, 2- and 3-immunoreactivities were colocalized with P2X2R, P2X3R and prostatic acid phosphatase (PAP), as an ectoenzyme for metabolism from AMP to adenosine. Additionally, PAP-immunoreactivity was colocalized with equilibrative nucleoside transporter (ENT) 1, as an adenosine uptake system. These results suggest that the clearance system consisted of ENPPs, PAP and ENT1 plays an important role in regulation of nociceptive signaling in sensory neurons.

Aberrant plasticity of peripheral sensory axons in a painful neuropathy

Neuronal cells express considerable plasticity responding to environmental cues, in part, through subcellular mRNA regulation. Here we report on the extensive changes in distribution of mRNAs in the cell body and axon compartments of peripheral sensory neurons and the 3' untranslated region (3'UTR) landscapes after unilateral sciatic nerve entrapment (SNE) injury in rats. Neuronal cells dissociated from SNE-injured and contralateral L4 and L5 dorsal root ganglia were cultured in a compartmentalized system. Axonal and cell body RNA samples were separately subjected to high throughput RNA sequencing (RNA-Seq). The injured axons exhibited enrichment of mRNAs related to protein synthesis and nerve regeneration. Lengthening of 3'UTRs was more prevalent in the injured axons, including the newly discovered alternative cleavage and polyadenylation of NaV1.8 mRNA. Alternative polyadenylation was largely independent from the relative abundance of axonal mRNAs; but they were highly clustered in functional pathways related to RNA granule formation in the injured axons. These RNA-Seq data analyses indicate that peripheral nerve injury may result in highly selective mRNA enrichment in the affected axons with 3'UTR alterations potentially contributing to the mechanism of neuropathic pain.

Peripherally Selective Cannabinoid 1 Receptor (CB1R) Agonists for the Treatment of Neuropathic Pain

Alleviation of neuropathic pain by cannabinoids is limited by their central nervous system (CNS) side effects. Indole and indene compounds were engineered for high hCB1R affinity, peripheral selectivity, metabolic stability, and in vivo efficacy. An epithelial cell line assay identified candidates with <1% blood-brain barrier penetration for testing in a rat neuropathy induced by unilateral sciatic nerve entrapment (SNE). The SNE-induced mechanical allodynia was reversibly suppressed, partially or completely, after intraperitoneal or oral administration of several indenes. At doses that relieve neuropathy symptoms, the indenes completely lacked, while the brain-permeant CB1R agonist HU-210 (1) exhibited strong CNS side effects, in catalepsy, hypothermia, and motor incoordination assays. Pharmacokinetic findings of ∼0.001 cerebrospinal fluid:plasma ratio further supported limited CNS penetration. Pretreatment with selective CB1R or CB2R blockers suggested mainly CB1R contribution to an indene's antiallodynic effects. Therefore, this class of CB1R agonists holds promise as a viable treatment for neuropathic pain.

The scaffold protein calcium/calmodulin-dependent serine protein kinase controls ATP release in sensory ganglia upon P2X3 receptor activation and is part of an ATP keeper complex

P2X3 receptors, gated by extracellular ATP, are expressed by sensory neurons and are involved in peripheral nociception and pain sensitization. The ability of P2X3 receptors to transduce extracellular stimuli into neuronal signals critically depends on the dynamic molecular partnership with the calcium/calmodulin-dependent serine protein kinase (CASK). The present work used trigeminal sensory neurons to study the impact that activation of P2X3 receptors (evoked by the agonist α,β-meATP) has on the release of endogenous ATP and how CASK modulates this phenomenon. P2X3 receptor function was followed by ATP efflux via Pannexin1 (Panx1) hemichannels, a mechanism that was blocked by the P2X3 receptor antagonist A-317491, and by P2X3 silencing. ATP efflux was enhanced by nerve growth factor, a treatment known to potentiate P2X3 receptor function. Basal ATP efflux was not controlled by CASK, and carbenoxolone or Pannexin silencing reduced ATP release upon P2X3 receptor function. CASK-controlled ATP efflux followed P2X3 receptor activity, but not depolarization-evoked ATP release. Molecular biology experiments showed that CASK was essential for the transactivation of Panx1 upon P2X3 receptor activation. These data suggest that P2X3 receptor function controls a new type of feed-forward purinergic signaling on surrounding cells, with consequences at peripheral and spinal cord level. Thus, P2X3 receptor-mediated ATP efflux may be considered for the future development of pharmacological strategies aimed at containing neuronal sensitization. P2X3 receptors are involved in sensory transduction and associate to CASK. We have studied in primary sensory neurons the molecular mechanisms downstream P2X3 receptor activation, namely ATP release and partnership with CASK or Panx1. Our data suggest that CASK and P2X3 receptors are part of an ATP keeper complex, with important feed-forward consequences at peripheral and central level.

Spinal Astrocytic Thrombospondin-4 Induced by Excitatory Neuronal Signaling Mediates Pain After Facet Capsule Injury

Thrombospondin-4 (TSP4) is a synaptogenic molecule that is upregulated in the spinal cord after painful facet joint injury and may contribute to spinal hyperexcitability. However, the mechanisms leading to increased spinal TSP4 are unclear. Because primary afferent activity is critical in the development of spinal hyperexcitability after facet joint injury, this study evaluated the role of afferent firing in the increase of spinal TSP4 and excitatory synapses. Intra-articular bupivacaine was administered immediately or 4 days after painful facet joint injury in male Holtzman rats, and TSP4 and excitatory synapses were quantified in the spinal cord at day 7. Immediate, but not delayed bupivacaine treatment, prevents the injury-induced increase in TSP4 and excitatory synapses in the dorsal horn (p < 0.0001). Preliminary in vitro experiments suggest that the excitatory signaling molecules ATP and glutamate may stimulate astrocytic TSP4 expression (p ≤ 0.04). Collectively, these results suggest that afferent activity early after facet joint injury is critical for the induction of spinal TSP4. This study advances the understanding of the timing and role of afferent activity in TSP4 expression after injury, which is critical for the therapeutic targeting of TSP4 to treat persistent pain conditions.

TNAP and Pain Control

Chronic pain is one of the most debilitating and expensive diseases, yet current therapies are often insufficient in bringing about long-term relief. Further, many treatments for chronic pain also carry significant side effects. The molecule adenosine has long been identified as a potent inhibitor of nociceptive circuits in the spinal cord; however, the widespread expression of adenosine receptors in many organ systems has limited its use as an analgesic. Recently several 5' ectonucleotidases, including tissue non-specific alkaline phosphatase (TNAP), have been characterized for their ability to generate endogenous adenosine in nociceptive circuitry of the dorsal spinal cord. These ectonucleotidases have the ability to hydrolyze the endogenous pronociceptive nucleotides like adenosine triphosphate (ATP) into the antinociceptive nucleoside adenosine. This chapter discusses the role of TNAP and other ectonucleotidases in nociceptive circuits, and their potential as future targets of new therapeutics to treat chronic pain.

Cross-Excitation in Peripheral Sensory Ganglia Associated with Pain Transmission

Despite the absence of synaptic contacts, cross-excitation of neurons in sensory ganglia during signal transmission is considered to be chemically mediated and appears increased in chronic pain states. In this study, we modulated neurotransmitter release in sensory neurons by direct application of type A botulinum neurotoxin (BoNT/A) to sensory ganglia in an animal model of neuropathic pain and evaluated the effect of this treatment on nocifensive. Unilateral sciatic nerve entrapment (SNE) reduced the ipsilateral hindpaw withdrawal threshold to mechanical stimulation and reduced hindpaw withdrawal latency to thermal stimulation. Direct application of BoNT/A to the ipsilateral L4 dorsal root ganglion (DRG) was localized in the cell bodies of the DRG and reversed the SNE-induced decreases in withdrawal thresholds within 2 days of BoNT/A administration. Results from this study suggest that neurotransmitter release within sensory ganglia is involved in the regulation of pain-related signal transmission.

Progress in Sympathetically Mediated Pathological Pain

Aim of review

Many chronic pain conditions remain difficult to treat, presenting a high burden to society. Conditions such as complex regional pain syndrome may be maintained or exacerbated by sympathetic activity. Understanding the interactions between sympathetic nervous system and sensory system will help to improve the effective management of pathological pain including intractable neuropathic pain and persistent inflammatory pain.

Method

We first described the discovery of abnormal connections between sympathetic and sensory neurons. Subsequently, the functional roles of sympathetic sprouting in altered neuronal excitability and increased pain sensitivity were discussed. The mechanisms of the sympathetic sprouting were focusing on its relationship with neurotrophins, local inflammation, and abnormal spontaneous activity. Finally, we discussed clinical implications and conflicting findings in the laboratory and clinical research with respect to the interaction between sympathetic system and sensory system.

Recent findings

The findings that sprouting of sympathetic fibers into the sensory ganglia (dorsal root ganglion) after peripheral nerve injury, offers a possible explanation of the sympathetic involvement in pain. It is also suggested that releases of adenosine triphosphate (ATP), in addition to norepinephrine, from sympathetic nerve endings play important roles in sympathetic-mediated pain. New evidence indicates the importance of sympathetic innervation in local inflammatory responses.

Summary

Hopefully, this review will reinvigorate the study of sympathetic-sensory interactions in chronic pain conditions, and help to better understand how sympathetic system contributes to this serious clinical problem.

A bioengineered peripheral nerve construct using aligned peptide amphiphile nanofibers

Peripheral nerve injuries can result in lifelong disability. Primary coaptation is the treatment of choice when the gap between transected nerve ends is short. Long nerve gaps seen in more complex injuries often require autologous nerve grafts or nerve conduits implemented into the repair. Nerve grafts, however, cause morbidity and functional loss at donor sites, which are limited in number. Nerve conduits, in turn, lack an internal scaffold to support and guide axonal regeneration, resulting in decreased efficacy over longer nerve gap lengths. By comparison, peptide amphiphiles (PAs) are molecules that can self-assemble into nanofibers, which can be aligned to mimic the native architecture of peripheral nerve. As such, they represent a potential substrate for use in a bioengineered nerve graft substitute. To examine this, we cultured Schwann cells with bioactive PAs (RGDS-PA, IKVAV-PA) to determine their ability to attach to and proliferate within the biomaterial. Next, we devised a PA construct for use in a peripheral nerve critical sized defect model. Rat sciatic nerve defects were created and reconstructed with autologous nerve, PLGA conduits filled with various forms of aligned PAs, or left unrepaired. Motor and sensory recovery were determined and compared among groups. Our results demonstrate that Schwann cells are able to adhere to and proliferate in aligned PA gels, with greater efficacy in bioactive PAs compared to the backbone-PA alone. In vivo testing revealed recovery of motor and sensory function in animals treated with conduit/PA constructs comparable to animals treated with autologous nerve grafts. Functional recovery in conduit/PA and autologous graft groups was significantly faster than in animals treated with empty PLGA conduits. Histological examinations also demonstrated increased axonal and Schwann cell regeneration within the reconstructed nerve gap in animals treated with conduit/PA constructs. These results indicate that PA nanofibers may represent a promising biomaterial for use in bioengineered peripheral nerve repair.

Deletion of ENTPD3 does not impair nucleotide hydrolysis in primary somatosensory neurons or spinal cord

Ectonucleotidases are membrane-bound or secreted proteins that hydrolyze extracellular nucleotides.  Recently, we identified three ectonucleotidases that hydrolyze extracellular adenosine 5'-monophosphate (AMP) to adenosine in primary somatosensory neurons.  Currently, it is unclear which ectonucleotidases hydrolyze ATP and ADP in these neurons.  Ectonucleoside triphosphate diphosphohydrolases (ENTPDs) comprise a class of enzymes that dephosphorylate extracellular ATP and ADP.  Here, we found that ENTPD3 (also known as NTPDase3 or CD39L3) was located in nociceptive and non-nociceptive neurons of the dorsal root ganglion (DRG), in the dorsal horn of the spinal cord, and in free nerve endings in the skin.  To determine if ENTPD3 contributes directly to ATP and ADP hydrolysis in these tissues, we generated and characterized an Entpd3 knockout mouse.  This mouse lacks ENTPD3 protein in all tissues examined, including the DRG, spinal cord, skin, and bladder.  However, DRG and spinal cord tissues from Entpd3 (-/-) mice showed no reduction in histochemical staining when ATP, ADP, AMP, or UTP were used as substrates.  Additionally, using fast-scan cyclic voltammetry (FSCV), adenosine production was not impaired in the dorsal spinal cord of Entpd3 (-/-) mice when the substrate ADP was applied.  Further, Entpd3 (-/-) mice did not differ in nociceptive behaviors when compared to wild-type mice, although Entpd3 (-/-) mice showed a modest reduction in β-alanine-mediated itch.  Taken together, our data indicate that deletion of Entpd3 does not impair ATP or ADP hydrolysis in primary somatosensory neurons or in dorsal spinal cord.  Moreover, our data suggest there could be multiple ectonucleotidases that act redundantly to hydrolyze nucleotides in these regions of the nervous system.

ATP release mechanisms of endothelial cell-mediated stimulus-dependent hyperalgesia

Endothelin-1 (ET-1) acts on endothelial cells to enhance mechanical stimulation-induced release of adenosine triphosphate (ATP), which in turn can act on sensory neurons innervating blood vessels to contribute to vascular pain, a phenomenon we have referred to as stimulus-dependent hyperalgesia (SDH). In the present study, we evaluated the role of the major classes of ATP release mechanisms to SDH: vesicular exocytosis, plasma membrane-associated ATP synthase, ATP-binding cassette transporters, and ion channels. Inhibitors of vesicular exocytosis (ie, monensin, brefeldin A, and bafilomycin), plasma membrane-associated ATPase (ie, oligomycin and pigment epithelium-derived factor peptide 34-mer), and connexin ion channels (carbenoxolone and flufenamic acid) but not ATP-binding cassette transporter (ie, dipyridamole, nicardipine, or CFTRinh-172) attenuated SDH. This study reports a role of ATP in SDH and suggests novel targets for the treatment of vascular pain syndromes.

PERSPECTIVE

ET-1 acts on endothelial cells to produce mechanical stimulation-induced hyperalgesia. Inhibitors of 3 different ATP release mechanisms attenuated this SDH. This study provides support for a role of ATP in SDH and suggests novel targets for the treatment of vascular pain syndromes.

ATP release through lysosomal exocytosis from peripheral nerves: the effect of lysosomal exocytosis on peripheral nerve degeneration and regeneration after nerve injury

Studies have shown that lysosomal activation increases in Schwann cells after nerve injury. Lysosomal activation is thought to promote the engulfment of myelin debris or fragments of injured axons in Schwann cells during Wallerian degeneration. However, a recent interpretation of lysosomal activation proposes a different view of the phenomenon. During Wallerian degeneration, lysosomes become secretory vesicles and are activated for lysosomal exocytosis. The lysosomal exocytosis triggers adenosine 5′-triphosphate (ATP) release from peripheral neurons and Schwann cells during Wallerian degeneration. Exocytosis is involved in demyelination and axonal degradation, which facilitate nerve regeneration following nerve degeneration. At this time, released ATP may affect the communication between cells in peripheral nerves. In this review, our description of the relationship between lysosomal exocytosis and Wallerian degeneration has implications for the understanding of peripheral nerve degenerative diseases and peripheral neuropathies, such as Charcot-Marie-Tooth disease or Guillain-Barré syndrome.

Anti-hyperalgesic and anti-allodynic activities of capillarisin via suppression of inflammatory signaling in animal model

ETHNOPHARMACOLOGICAL RELEVANCE

Artemisia capillaris has widespread traditional and pharmacological applications such as analgesic, anti-inflammatory, anti-pyretic, enhance immunity and anti-tumor activity properties. To evaluate the pharmacological activities of this plant, capillarisin, one of the potent constituent of Artemisia capillaris was studied based on anti-hyperalgesic and anti-allodynic effects with detailed mechanism. It can be assumed that measurement of anti-nociceptive effects of capillarisin is one of the parameter for the evaluation of this herb. Capillarisin has extensive pharmacological properties and has been considered to have promising ant-inflammatory and anti-nociceptive activities. The aim of the current study is to investigate the effect of capillarisin and underlying molecular mechanisms of action in preventing acute and subchronic inflammatory pain.

MATERIALS AND METHODS

The inflammatory pain was induced after 40 min or 1h of administration of vehicle, 70% EtOH extract of Artemisia capillaris (100mg/kg) or capillarisin (20 and 80 mg/kg) by intraplantar (i.p.l.) injections of CFA and carrageenan in ICR mice, respectively. Mechanical hyperalgesia and allodynia were evaluated in both acute and subchronic models. Further analysis was performed in CFA-induced mice exploring various molecular and signaling pathways such as NF-κB, AP-1, and ERK-CREB involved in the persistent pain sensations.

RESULTS

In acute model, mechanical hyperalgesia and allodynia were evaluated after every 2h until 6h of CFA and after 4h of carrageenan injections. Whereas, in subchronic inflammatory pain model, mechanical hyperalgesia and paw edema were measured after 4h of CFA injection and every day after 4h of daily treatment until 5 days with interval of day four in order to assess the tolerance effect of capillarisin. Further analysis was performed in CFA-induced mice exploring various molecular and signaling pathways such as NF-κB, AP-1 and ERK-CREB involved in the persistent of pain sensations. Pre-treatment of capillarisin strongly inhibited NF-κB mediated genes (iNOS, COX-2), involved in pain. The plasma leading nitrite production was significantly reduced by capillarisin. Moreover, i.p. administration of capillarisin markedly suppressed the adenosine 5׳-triphosphate (ATP) in plasma and substance P in CFA-induced paw tissue.

CONCLUSIONS

The present study indicates that capillarisin possessed promising anti-hyperalgesic and anti-allodynic effects through the inhibition of various inflammatory pain signaling, suggesting that capillarisin constitutes a significant component for the treatment of inflammatory pain.

Glia and pain: is chronic pain a gliopathy?

Activation of glial cells and neuro-glial interactions are emerging as key mechanisms underlying chronic pain. Accumulating evidence has implicated 3 types of glial cells in the development and maintenance of chronic pain: microglia and astrocytes of the central nervous system (CNS), and satellite glial cells of the dorsal root and trigeminal ganglia. Painful syndromes are associated with different glial activation states: (1) glial reaction (ie, upregulation of glial markers such as IBA1 and glial fibrillary acidic protein (GFAP) and/or morphological changes, including hypertrophy, proliferation, and modifications of glial networks); (2) phosphorylation of mitogen-activated protein kinase signaling pathways; (3) upregulation of adenosine triphosphate and chemokine receptors and hemichannels and downregulation of glutamate transporters; and (4) synthesis and release of glial mediators (eg, cytokines, chemokines, growth factors, and proteases) to the extracellular space. Although widely detected in chronic pain resulting from nerve trauma, inflammation, cancer, and chemotherapy in rodents, and more recently, human immunodeficiency virus-associated neuropathy in human beings, glial reaction (activation state 1) is not thought to mediate pain sensitivity directly. Instead, activation states 2 to 4 have been demonstrated to enhance pain sensitivity via a number of synergistic neuro-glial interactions. Glial mediators have been shown to powerfully modulate excitatory and inhibitory synaptic transmission at presynaptic, postsynaptic, and extrasynaptic sites. Glial activation also occurs in acute pain conditions, and acute opioid treatment activates peripheral glia to mask opioid analgesia. Thus, chronic pain could be a result of "gliopathy," that is, dysregulation of glial functions in the central and peripheral nervous system. In this review, we provide an update on recent advances and discuss remaining questions.

Mechanism underlying anti-hyperalgesic and anti-allodynic properties of anomalin in both acute and chronic inflammatory pain models in mice through inhibition of NF-κB, MAPKs and CREB signaling cascades

The numerous mediators of pain and inflammation are products of injury-induced gene expression that lead to changes in the nervous system and immune responses. These multiple molecules and mechanisms suggest novel strategies that could be used for analgesic drug development. The present study investigated the possible anti-hyperalgesic effects of anomalin in complete Freund's adjuvant (CFA)-induced acute and chronic inflammatory pain models. Acute pretreatment of mice with anomalin (10 and 50mg/kg, i.p.) produced a significant anti-nociceptive effect against CFA- and carrageenan-induced mechanical hyperalgesia and allodynia. In a chronic pain model, administration of anomalin inhibited CFA-induced hyperalgesia, and it did not cause any apparent toxicity. Another set of experiments observed that anomalin inhibited CFA- and carrageenan-induced paw edema in acute and chronic models. To elucidate the molecular mechanism underlying the anti-nociceptive effect of anomalin, the various pain signaling pathways [NF-κB, cAMP response element-binding protein (CREB), and mitogen activated protein kinase (MAPKs)/AP-1] that are involved were examined. Intraperitoneal (i.p.) pretreatment of anomalin exhibited potent inhibitory effects on direct mediators of hyperalgesia (iNOS and COX-2). The release of CFA-induced plasma nitrite and paw tissue hyperalgesic cytokine (TNF-α) was reduced remarkably. In addition, the adenosine 5'-triphosphate (ATP) in plasma and substance P (SP) in paw tissue were markedly suppressed by anomalin. These results demonstrate that anomalin exhibits an analgesic effect in a consistent manner and that its mechanisms involve the inhibition of the NF-κB, CREB, and MAPKs/AP-1 signaling pathways.

Estrogen modulation of visceral nociceptors

A large body of literature supports the idea that estrogen modulates nociceptive responses in pelvic pain syndromes; however, whether this hormone is pro- or anti-nociceptive remains unresolved. The dorsal root ganglion (DRG) is an important site of visceral afferent convergence and cross-sensitization. Within the context of our hypothesis visceral nociception and nociceptor sensitization appear to be regulated by purinergic P2X₃ and vanilloid TRPV1 receptors and 17β-estradiol modulates DRG neuron response to ATP (P2X agonist) and capsaicin (TRPV1 agonist) suggesting that visceral afferent nociceptors are modulated by estrogen in the DRG. 17-β estradiol (E2), the most common form of estrogen, acts on functional properties of P2X₃ and TRPV1 receptors in DRG neurons in vitro. The localization of estrogen receptors (ER) in DRG neurons and the attenuation of ATP/capsaicin-induced intracellular calcium concentration [Ca2⁺]_i strongly suggest that E2 modulates visceral pain processing peripherally. Moreover, E2 appears to have different actions on nociceptive signaling depending on the input. Based on our data we propose that E2 can gate primary afferent response to increase or decrease nociception.

Enhancement of purinergic signalling by excessive endogenous ATP in resiniferatoxin (RTX) neuropathy

ATP is a ligand of P2X family purinoceptors, and exogenous ATP administration evokes pain behaviors. To date, there is a lack of systematic studies to address relationships between endogenous ATP and neuropathic pain. In this report, we took advantage of a mouse model of resiniferatoxin (RTX)-induced neuropathic pain to address the role of endogenous ATP in neuropathic pain. After RTX administration, endogenous ATP markedly increased in dorsal root ganglia (DRGs) (p < 0.01) and skin tissues (p < 0.001). The excessive endogenous ATP was removed by apyrase, an ATP hydrolyzing enzyme, administration via either a lumbar puncture route (p < 0.001) or an intraplantar injection (p < 0.001), which led to the normalization of neuropathic pain. In addition, intraplantar treatment with apyrase caused mechanical analgesia. Linear analyses showed that the densities of P2X3(+) neurons (r = -0.72, p < 0.0001) and P2X3(+) dermal nerves (r = -0.72, p < 0.0001) were inversely correlated with mechanical thresholds. Moreover, the contents of endogenous ATP in skin tissues were linearly correlated with P2X3(+) dermal nerves (r = 0.80, p < 0.0001) and mechanical thresholds (r = -0.80, p < 0.0001). In summary, this study demonstrated that enhanced purinergic signalling due to an increase in endogenous ATP after RTX-induced nerve injury contributed to the development of neuropathic pain. The data in this report provide a new therapeutic strategy for pain control by targeting the endogenous ligand of purinergic signalling.

Possible ATP release through lysosomal exocytosis from primary sensory neurons

The adenosine triphosphate (ATP) plays important roles under physiological and pathological conditions such as traumatic brain injury, neuroinflammation and neuropathic pain. In the present study, we set out to study the role of lysosomal vesicles on ATP release from the dorsal root ganglion neurons. We found that the lysosomal vesicles, which contain the quinacrine-positive fluorescence and express the vesicular nucleotide transporter (VNUT), were localized within the soma and growth cone of the cultured dorsal root ganglion neurons. In addition, the number of the quinacrine staining was decreased by application of lysosomal exocytosis activators, and this decrease was suppressed by the metformin and vacuolin-1, which suppressed lysosomal exocytosis. Thus, these findings suggest that ATP release via the lysosomal exocytosis may be one of the pathways for ATP release in response to stimulation.

Purinergic transmission and transglial signaling between neuron somata in the dorsal root ganglion

Most dorsal root ganglion neuronal somata (NS) are isolated from their neighbours by a satellite glial cell (SGC) sheath. However, some NS are associated in pairs, separated solely by the membrane septum of a common SGC to form a neuron-glial cell-neuron (NGlN) trimer. We reported that stimulation of one NS evokes a delayed, noisy and long-duration inward current in both itself and its passive partner that was blocked by suramin, a general purinergic antagonist. Here we test the hypothesis that NGlN transmission involves purinergic activation of the SGC. Stimulation of the NS triggered a sustained current noise in the SGC. Block of transmission through the NGlN by reactive blue 2 or thapsigargin, a Ca(2+) store-depletion agent, implicated a Ca(2+) store discharge-linked P2Y receptor. P2Y2 was identified by simulation of the NGlN-like transmission by puffing UTP onto the SGC and by immunocytochemical localization to the SGC membrane septum. Block of the UTP effect by BAPTA, an intracellular Ca(2+) scavenger, supported the involvement of SGC Ca(2+) stores in the signaling pathway. We infer that transmission through the NGlN trimer involves secretion of ATP from the NS and triggering of SGC Ca(2+) store discharge via P2Y2 receptors. Presumably, cytoplasmic Ca(2+) elevation leads to the release of an as-yet unidentified second transmitter from the glial cell to complete transmission. Thus, the two NS of the NGlN trimer communicate via a 'sandwich synapse' transglial pathway, a novel signaling mechanism that may contribute to information transfer in other regions of the nervous system.

Differential expression and pharmacology of native P2X receptors in rat and primate sensory neurons

Evidence suggesting the involvement of P2X2 and P2X3 in chronic pain has been obtained mostly from rodent models. Here we show that rodents may be poor predictors of P2X3 pharmacology in human. We demonstrate that monkey and human dorsal root ganglion (DRG) neurons do not express appreciable levels of P2X2 subunit, contrary to rat sensory neurons. Additionally, we report functional P2X3 activity in monkey DRG neurons and confirm the absence of functional P2X2/3 receptors. Interestingly, native P2X3 receptors in rat and monkey DRGs show similar agonist potency, but different antagonist potencies for TNP-ATP [2-O-(2,4,6-trinitrophenyl)-ATP] and RO51. This unexpected difference in antagonist potency was confirmed by comparing rat and human P2X3 receptors in HEK293 cells. Mutagenesis studies reveal that two extracellular residues, A197 and T202, are synergistically responsible for the potency drop in primate P2X3 receptors. These results uncover species-specific P2X3 pharmacology and identify key mechanisms impacting the translatability of potential analgesics targeting P2X3 receptors.

Microglial cathepsin B contributes to the initiation of peripheral inflammation-induced chronic pain

Interleukin (IL)-1β and IL-18 play critical roles in the induction of chronic pain hypersensitivity. Their inactive forms are activated by caspase-1. However, little is known about the mechanism underlying the activation of pro-caspase-1. There is increasing evidence that cathepsin B (CatB), a typical lysosomal cysteine protease, is involved in the pro-caspase-1 activation and the subsequent maturation of IL-1β and IL-18. In this context, CatB is considered to be an important molecular target to control chronic pain. However, no information is currently available about the role of CatB in chronic pain hypersensitivity. We herein show that CatB deficiency or the intrathecal administration of CA-074Me, a specific CatB inhibitor, significantly inhibited the induction of complete Freund's adjuvant-induced tactile allodynia in mice without affecting peripheral inflammation. In contrast, CatB deficiency did not affect the nerve injury-induced tactile allodynia. Furthermore, CatB deficiency or CA-074Me treatment significantly inhibited the maturation and secretion of IL-1β and IL-18 by cultured microglia following treatment with the neuroactive glycoprotein chromogranin A (CGA), but not with ATP. Moreover, the IL-1β expression in spinal microglia and the induction of tactile allodynia following the intrathecal administration of CGA depended on CatB, whereas those induced by the intrathecal administration of ATP or lysophosphatidic acid were CatB independent. These results strongly suggest that CatB is an essential enzyme for the induction of chronic inflammatory pain through its activation of pro-caspase-1, which subsequently induces the maturation and secretion of IL-1β and IL-18 by spinal microglia. Therefore, CatB-specific inhibitors may represent a useful new strategy for treating inflammation-associated pain.

Astrocytic CX43 hemichannels and gap junctions play a crucial role in development of chronic neuropathic pain following spinal cord injury

Chronic neuropathic pain is a frequent consequence of spinal cord injury (SCI). Yet despite recent advances, upstream releasing mechanisms and effective therapeutic options remain elusive. Previous studies have demonstrated that SCI results in excessive ATP release to the peritraumatic regions and that purinergic signaling, among glial cells, likely plays an essential role in facilitating inflammatory responses and nociceptive sensitization. We sought to assess the role of connexin 43 (Cx43) as a mediator of CNS inflammation and chronic pain. To determine the extent of Cx43 involvement in chronic pain, a weight-drop SCI was performed on transgenic mice with Cx43/Cx30 deletions. SCI induced robust and persistent neuropathic pain including heat hyperalgesia and mechanical allodynia in wild-type control mice, which developed after 4 weeks and was maintained after 8 weeks. Notably, SCI-induced heat hyperalgesia and mechanical allodynia were prevented in transgenic mice with Cx43/Cx30 deletions, but fully developed in transgenic mice with only Cx30 deletion. SCI-induced gliosis, detected as upregulation of glial fibrillary acidic protein in the spinal cord astrocytes at different stages of the injury, was also reduced in the knockout mice with Cx43/Cx30 deletions, when compared with littermate controls. In comparison, a standard regimen of post-SCI treatment of minocycline attenuated neuropathic pain to a significantly lesser degree than Cx43 deletion. These findings suggest Cx43 is critically linked to the development of central neuropathic pain following acute SCI. Since Cx43/Cx30 is expressed by astrocytes, these findings also support an important role of astrocytes in the development of chronic pain.

Slow chemical transmission between dorsal root ganglion neuron somata

Somatic sensory neuron somata are located within the dorsal root ganglia (DRG) and are mostly ensheathed by individual satellite glial cell sheets. It has been noted, however, that a subpopulation of these DRG somata are intimately associated, separated only by a single thin satellite glial cell membrane septum. We set out to test whether such neuron-glial cell-neuron trimers (NGlNs) are also linked functionally. The presence of NGlNs in chick DRGs was confirmed by electron microscopy. Selective satellite glial cell immunostains were identified and were used to image the inter-neuron septa in DRG frozen sections. We used a gentle, dispase-based enzymatic method to isolate chick and rat NGlNs in vitro for double patch clamp recordings. In the majority of pairs tested, an action potential-like stimulus train delivered to one soma resulted in a delayed, noisy and long-duration response in its idle partner. The response to a second stimulus train given minutes later was markedly facilitated. Both bidirectional and unidirectional transmission was observed between the paired neurons. Transmission was chemical and block by the general purinergic blocker suramin implicated ATP as a neurotransmitter. We conclude that the two neuronal somata in the NGlN can communicate by chemical transmission, which may involve a transglial, bi-synaptic pathway. This novel soma-to-soma transmission reflects a novel form of processing that may play a role in sensory disorders in the DRG and interneuron communication in the central nervous system.

Relationship of axonal voltage-gated sodium channel 1.8 (NaV1.8) mRNA accumulation to sciatic nerve injury-induced painful neuropathy in rats

Painful peripheral neuropathy is a significant clinical problem; however, its pathological mechanism and effective treatments remain elusive. Increased peripheral expression of tetrodotoxin-resistant voltage-gated sodium channel 1.8 (NaV1.8) has been shown to associate with chronic pain symptoms in humans and experimental animals. Sciatic nerve entrapment (SNE) injury was used to develop neuropathic pain symptoms in rats, resulting in increased NaV1.8 mRNA in the injured nerve but not in dorsal root ganglia (DRG). To study the role of NaV1.8 mRNA in the pathogenesis of SNE-induced painful neuropathy, NaV1.8 shRNA vector was delivered by subcutaneous injection of cationized gelatin/plasmid DNA polyplex into the rat hindpaw and its subsequent retrograde transport via sciatic nerve to DRG. This in vivo NaV1.8 shRNA treatment reversibly and repeatedly attenuated the SNE-induced pain symptoms, an effect that became apparent following a distinct lag period of 3-4 days and lasted for 4-6 days before returning to pretreatment levels. Surprisingly, apparent knockdown of NaV1.8 mRNA occurred only in the injured nerve, not in the DRG, during the pain alleviation period. Levels of heteronuclear NaV1.8 RNA were unaffected by SNE or shRNA treatments, suggesting that transcription of the Scn10a gene encoding NaV1.8 was unchanged. Based on these data, we postulate that increased axonal mRNA transport results in accumulation of functional NaV1.8 protein in the injured nerve and the development of painful neuropathy symptoms. Thus, targeted delivery of agents that interfere with axonal NaV1.8 mRNA may represent effective neuropathic pain treatments.

PAP and NT5E inhibit nociceptive neurotransmission by rapidly hydrolyzing nucleotides to adenosine

Background

Prostatic acid phosphatase (PAP) and ecto-5'-nucleotidase (NT5E, CD73) produce extracellular adenosine from the nucleotide AMP in spinal nociceptive (pain-sensing) circuits; however, it is currently unknown if these are the main ectonucleotidases that generate adenosine or how rapidly they generate adenosine.

Results

We found that AMP hydrolysis, when measured histochemically, was nearly abolished in dorsal root ganglia (DRG) neurons and lamina II of spinal cord from Pap/Nt5e double knockout (dKO) mice. Likewise, the antinociceptive effects of AMP, when combined with nucleoside transport inhibitors (dipyridamole or 5-iodotubericidin), were reduced by 80-100% in dKO mice. In addition, we used fast scan cyclic voltammetry (FSCV) to measure adenosine production at subsecond resolution within lamina II. Adenosine was maximally produced within seconds from AMP in wild-type (WT) mice but production was reduced >50% in dKO mice, indicating PAP and NT5E rapidly generate adenosine in lamina II. Unexpectedly, we also detected spontaneous low frequency adenosine transients in lamina II with FSCV. Adenosine transients were of short duration (<2 s) and were reduced (>60%) in frequency in Pap^-/-, Nt5e^-/- and dKO mice, suggesting these ectonucleotidases rapidly hydrolyze endogenously released nucleotides to adenosine. Field potential recordings in lamina II and behavioral studies indicate that adenosine made by these enzymes acts through the adenosine A₁ receptor to inhibit excitatory neurotransmission and nociception.

Conclusions

Collectively, our experiments indicate that PAP and NT5E are the main ectonucleotidases that generate adenosine in nociceptive circuits and indicate these enzymes transform pulsatile or sustained nucleotide release into an inhibitory adenosinergic signal.

Imaging single photons and intrinsic optical signals for studies of vesicular and non-vesicular ATP release from axons

The temporal and spatial dynamics of neurotransmitter release are fundamental to understanding activity-dependent signaling between axons and other cells, including neurons, glia, and vascular cells. A microscopic imaging technique is described that enables studying release of the neurotransmitter ATP from axons in response to action potentials. The method combines imaging single-photons, intrinsic optical signal imaging, and high magnification time-lapse microcopy to enable investigations of action potential-induced ATP release together with cell morphology and activity-dependent axon swelling. ATP released from axons catalyzes a chemiluminescent reaction between luciferin and luciferase that generates single photons that can be imaged individually. In addition to vesicular release, ATP release through membrane channels activated by axon swelling was monitored simultaneously with intrinsic optical signals. Repeated emissions of photons were observed from localized 15 μm regions of axons, with a frequency distribution that differed from a normal distribution and from the frequency of emissions outside these localized regions.

Pain-relieving prospects for adenosine receptors and ectonucleotidases

Adenosine receptor agonists have potent antinociceptive effects in diverse preclinical models of chronic pain. By contrast, the efficacy of adenosine and adenosine receptor agonists in treating pain in humans is unclear. Two ectonucleotidases that generate adenosine in nociceptive neurons were recently identified. When injected spinally, these enzymes have long-lasting adenosine A(1) receptor-dependent antinociceptive effects in inflammatory and neuropathic pain models. Furthermore, recent findings indicate that spinal adenosine A(2A) receptor activation can enduringly inhibit neuropathic pain symptoms. Collectively, these studies suggest the possibility of treating chronic pain in humans by targeting specific adenosine receptor subtypes in anatomically defined regions with agonists or with ectonucleotidases that generate adenosine.

Expression and contribution of satellite glial cells purinoceptors to pain transmission in sensory ganglia: an update

The role of adenosine-5'-triphosphate (ATP) and of the ligand-gated P2X3 receptor in neuronal dorsal root ganglia (DRG) pain transmission is relatively well established. Much less is known about the purinergic system in trigeminal ganglia (TG), which are involved in certain types of untreatable neuropathic and inflammatory pain, as well as in migraine. Emerging data suggest that purinergic metabotropic P2Y receptors on both neurons and satellite glial cells (SGCs) may also participate in both physiological and pathological pain development. Here, we provide an updated literature review on the role of purinergic signaling in sensory ganglia, with special emphasis on P2Y receptors on SGCs. We also provide new original data showing a time-dependent downregulation of P2Y2 and P2Y4 receptor expression and function in purified SGCs cultures from TG, in comparison with primary mixed neuron-SGCs cultures. These data highlight the importance of the neuron-glia cross-talk in determining the SGCs phenotype. Finally, we show that, in mixed TG cultures, both adenine and guanosine induce intracellular calcium transients in neurons but not in SGCs, suggesting that also these purinergic-related molecules can participate in pain signaling. These findings may have relevant implications for the development of new therapeutic strategies for chronic pain treatment.

Substance P sensitizes P2X3 in nociceptive trigeminal neurons

Peripheral inflammation produces pain hypersensitivity by sensitizing nociceptors. Potentiation of P2X3 receptor activity in nociceptors may play an important role in this peripheral sensitization. However, we do not fully understand how P2X3 activity is elevated in inflammation. Thus, we investigated whether P2X3 activity in trigeminal nociceptive neurons is regulated by the neurokinin-1 (NK-1) receptor that is activated by an inflammatory mediator, substance P. Single-cell RT-PCR and immunohistochemistry revealed that NK-1 in nociceptive neurons was mainly co-expressed with P2X3. Ca(2+) imaging and whole-cell patch-clamp recordings indicated that both substance P and Sar-substance P, a selective NK-1 agonist, significantly potentiated α,β-meATP-induced currents and [Ca(2+)](i) responses in nociceptive neurons. These potentiating effects were completely blocked by GR82334, a specific NK-1 antagonist. Our results demonstrate that substance P sensitizes P2X3 receptor through the activation of NK-1, thus warranting these receptors as possible targets for pain therapy in the orofacial region.

ABBREVIATIONS

α,β-methylene adenosine 5'-triphosphate (ATP), α,β-meATP; neurokinin-1, NK-1; single-cell reverse-transcription polymerase chain-reaction, single-cell RT-PCR; [Sar(9),Met(O(2))(11)]-substance P, Sar-substance P.

Basal release of ATP: an autocrine-paracrine mechanism for cell regulation

Cells release adenosine triphosphate (ATP), which activates plasma membrane-localized P2X and P2Y receptors and thereby modulates cellular function in an autocrine or paracrine manner. Release of ATP and the subsequent activation of P2 receptors help establish the basal level of activation (sometimes termed "the set point") for signal transduction pathways and regulate a wide array of responses that include tissue blood flow, ion transport, cell volume regulation, neuronal signaling, and host-pathogen interactions. Basal release and autocrine or paracrine responses to ATP are multifunctional, evolutionarily conserved, and provide an economical means for the modulation of cell, tissue, and organismal biology.

Increased peripheral nerve excitability and local NaV1.8 mRNA up-regulation in painful neuropathy

BACKGROUND

Neuropathic pain caused by peripheral nerve injury is a chronic disorder that represents a significant clinical challenge because the pathological mechanisms have not been fully elucidated. Several studies have suggested the involvement of various sodium channels, including tetrodotoxin-resistant NaV1.8, in affected dorsal root ganglion (DRG) neurons. We have hypothesized that altered local expression of NaV1.8 in the peripheral axons of DRG neurons could facilitate nociceptive signal generation and propagation after neuropathic injury.

RESULTS

After unilateral sciatic nerve entrapment injury in rats, compound action potential amplitudes were increased in both myelinated and unmyelinated fibers of the ipsilateral sciatic nerve. Tetrodotoxin resistance of both fiber populations and sciatic nerve NaV1.8 immunoreactivity were also increased. Further analysis of NaV1.8 distribution revealed that immunoreactivity and mRNA levels were decreased and unaffected, respectively, in the ipsilateral L4 and L5 DRG; however sciatic nerve NaV1.8 mRNA showed nearly an 11-fold ipsilateral increase. Nav1.8 mRNA observed in the sciatic nerve was likely of axonal origin since it was not detected in non-neuronal cells cultured from nerve tissue. Absence of changes in NaV1.8 mRNA polyadenylation suggests that increased mRNA stability was not responsible for the selective peripheral mRNA increase. Furthermore, mRNA levels of NaV1.3, NaV1.5, NaV1.6, NaV1.7, and NaV1.9 were not significantly different between ipsilateral and contralateral nerves. We therefore propose that selective NaV1.8 mRNA axonal transport and local up-regulation could contribute to the hyperexcitability of peripheral nerves in some neuropathic pain states.

CONCLUSION

Cuff entrapment injury resulted in significantly elevated axonal excitability and increased NaV1.8 immunoreactivity in rat sciatic nerves. The concomitant axonal accumulation of NaV1.8 mRNA may play a role in the pathogenesis of this model of neuropathic pain.