resiniferatoxin and Neuralgia

Neuropathic pain exerts a global burden caused by the lesions in the somatosensory nerve system, including the central and peripheral nervous systems. The mechanisms of nerve injury-induced neuropathic pain involve multiple mechanisms, various signaling pathways, and molecules. Currently, poor efficacy is the major limitation of medications for treating neuropathic pain. Thus, understanding the detailed molecular mechanisms should shed light on the development of new therapeutic strategies for neuropathic pain. Several well-established in vivo pain models were used to investigate the detail mechanisms of peripheral neuropathic pain. Molecular mediators of pain are regulated differentially in various forms of neuropathic pain models; these regulators include purinergic receptors, transient receptor potential receptor channels, and voltage-gated sodium and calcium channels. Meanwhile, post-translational modification and transcriptional regulation are also altered in these pain models and have been reported to mediate several pain related molecules. In this review, we focus on molecular mechanisms and mediators of neuropathic pain with their corresponding transcriptional regulation and post-translational modification underlying peripheral sensitization in the dorsal root ganglia. Taken together, these molecular mediators and their modification and regulations provide excellent targets for neuropathic pain treatment.

Topics: Acrylamide; Animals; Diterpenes; Extracellular Signal-Regulated MAP Kinases; Ganglia, Spinal; Gene Expression Regulation; Guidelines as Topic; Humans; Neuralgia; Neurons; Protein Processing, Post-Translational; Spinal Cord; Spinal Nerves; Voltage-Gated Sodium Channels

Pulsed radiofrequency (PRF) is a safe method of treating neuropathic pain by generating intermittent electric fields at the needle tip. Resiniferatoxin (RTX) is an ultrapotent agonist of transient receptor potential vanilloid subtype-1 (TRPV1) receptors. We investigated the mechanism of PRF using a rat model of RTX-induced neuropathic pain. After administering RTX intraperitoneally, PRF was applied to the right sciatic nerve. We observed the changes in TRPV1, calcitonin gene-related peptide (CGRP), and brain-derived neurotrophic factor (BDNF) in the dorsal root ganglia by western blotting. Expressions of TRPV1 and CGRP were significantly lower in the contralateral (RTX-treated, PRF-untreated) tissue than in control rats (p<0.0001 and p<0.0001, respectively) and the ipsilateral tissues (p<0.0001 and p<0.0001, respectively). BDNF levels were significantly higher in the contralateral tissues than in the control rats (p<0.0001) and the ipsilateral tissues (p<0.0001). These results suggest that, while TRPV1 and CGRP are decreased by RTX-induced neuronal damage, increased BDNF levels result in pain development. PRF may promote recovery from neuronal damage with concomitant restoration of TRPV1 and CGRP, and exert its analgesic effect by reversing BDNF increase. Further research is required to understand the role of TRPV1 and CGRP restoration in improving mechanical allodynia.

Topics: Animals; Antineoplastic Agents; Brain-Derived Neurotrophic Factor; Calcitonin Gene-Related Peptide; Ganglia, Spinal; Neuralgia; Pulsed Radiofrequency Treatment; Rats; Sciatic Nerve; TRPV Cation Channels

Fifth lumbar (L5) nerve injury in rodent produces neuropathic manifestations in the corresponding hind paw. The aim of this study was to investigate the effect of cutaneous injection of resiniferatoxin (RTX), a TRPV1 receptor agonist, in the rat's hind paw on the neuropathic pain induced by L5 nerve injury. The results showed that intraplantar injection of RTX (0.002%, 100 µL) (1) completely reversed the development of chronic thermal and mechanical hypersensitivity; (2) completely prevented the development of nerve-injury-induced thermal and mechanical hypersensitivity when applied one week earlier; (3) caused downregulation of nociceptive pain markers, including TRPV1, IB4 and CGRP, and upregulation of VIP in the ipsilateral dorsal horn of spinal cord and dorsal root ganglion (DRG) immunohistochemically and a significant reduction in the expression of TRPV1 mRNA and protein in the ipsilateral DRG using Western blot and qRT-PCR techniques; (4) caused downregulation of PGP 9.5- and CGRP-immunoreactivity in the injected skin; (5) produced significant suppression of c-fos expression, as a neuronal activity marker, in the spinal neurons in response to a second intraplantar RTX injection two weeks later. This work identifies the ability of cutaneous injection of RTX to completely alleviate and prevent the development of different types of neuropathic pain in animals and humans.

Topics: Animals; Calcitonin Gene-Related Peptide; Diterpenes; Neuralgia; Rats; Rats, Sprague-Dawley; Trauma, Nervous System

Topics: Animals; Astrocytes; Capsaicin; Diterpenes; Ganglia, Spinal; Hyperalgesia; Intercellular Signaling Peptides and Proteins; Male; Neuralgia; Neurons; Rats, Sprague-Dawley; Spinal Cord

Treatment of neuropathic pain (NP) resulting from nerve injury is one of the most complicated and challenging in modern practice. Pharmacological treatments for NP are not fully effectively and novel approaches are requisite. Recently, transplantation of bone mesenchymal stem cells (BMSCs) has represented a promising approach for pain relief and neural repair, but how it produces beneficial effects on resiniferatoxin (RTX) induced nerve injury is still unclear. Here, we identified the BMSCs' analgesic effects and their potential mechanisms of microglial cells activation on RTX induced neuralgia. Immunostaining, biochemical studies demonstrated that microglia rather than astrocyte cells activation involved in RTX induced mechanical hyperalgesia, whereas the GFP-labeled BMSCs alleviated this mechanical hyperalgesia. Moreover, pain-related TRPA1, PKCδ, CaMKIIɑ (Calcium/calmodulin dependent protein kinase II), P38/MAPK (mitogen-activated protein kinase), p-P65 activation and expression in the spinal cord were significantly inhibited after BMSC administration. In addition, BMSCs treated RTX mice displayed a lower density of mushroom dendritic spines. Our research suggested that activation of PKCδ-CaMKIIɑ-P38/MAPK-p-P65 pathway and mushroom dendritic spines abnormal increase in the spinal cord is the main mechanism of RTX induced neuropathic pain, and transplant of BMSCs to the damaged nerve may offer promising approach for neuropathic pain.

Topics: Animals; Behavior, Animal; Bone Marrow Transplantation; Dendritic Spines; Diterpenes; Hyperalgesia; Macrophage Activation; Male; MAP Kinase Signaling System; Mesenchymal Stem Cell Transplantation; Mice; Mice, Inbred C57BL; Microglia; Neuralgia; Protein Kinase C-delta; Spinal Cord; Transcription Factor RelA; TRPA1 Cation Channel

Systemic treatment with resiniferatoxin (RTX) induces small-fiber sensory neuropathy by damaging TRPV1-expressing primary sensory neurons and causes distinct thermal sensory impairment and tactile allodynia, which resemble the unique clinical features of postherpetic neuralgia. However, the synaptic plasticity associated with RTX-induced tactile allodynia remains unknown. In this study, we found that RTX-induced neuropathy is associated with α2δ-1 upregulation in the dorsal root ganglion (DRG) and increased physical interaction between α2δ-1 and GluN1 in the spinal cord synaptosomes. RNAscope

Topics: Animals; Calcium Channels, L-Type; Diterpenes; Ganglia, Spinal; Glutamic Acid; Hyperalgesia; Male; Mice; Neuralgia; Neuralgia, Postherpetic; Neurotoxins; Rats; Rats, Sprague-Dawley; Receptors, N-Methyl-D-Aspartate; Sensory Receptor Cells; Up-Regulation

Topics: Analgesics; Animals; Calcium; Capsaicin; Disease Models, Animal; Diterpenes; Dose-Response Relationship, Drug; Flavonoids; Inflammation; Male; Mice; Neuralgia; Pain; Pain Measurement; Spinal Cord; Synaptosomes; Toxicity Tests; TRPV Cation Channels

The neurochemical effects of adenosine signaling in small-fiber neuropathy leading to neuropathic pain are yet to be explored in a direct manner. This study examined this system at the level of ligand (through the ectonucleotidase activity of prostatic acid phosphatase [PAP]) and adenosine A1 receptors (A1Rs) in resiniferatoxin (RTX) neuropathy, a peripheral neurodegenerative disorder that specifically affects nociceptive nerves expressing transient receptor potential vanilloid type 1 (TRPV1). We conducted immunohistochemistry on dorsal root ganglion (DRG) neurons, high-performance liquid chromatography for functional assays, and pharmacological interventions to alter PAP and A1Rs in mice with RTX neuropathy. In DRG of RTX neuropathy, PAP(+) neurons were reduced compared with vehicle-treated mice (P = 0.002). Functionally, PAP ectonucleotidase activity was consequently reduced (ie, the content of adenosine in DRG, P = 0.012). PAP(+) neuronal density was correlated with the degree of mechanical allodynia, which was reversed by intrathecal (i.t.) lumbar puncture injection of recombinant PAP with a dose-dependent effect. Furthermore, A1Rs were downregulated (P = 0.002), and this downregulation was colocalized with the TRPV1 receptor (31.0% ± 2.8%). Mechanical allodynia was attenuated in a dose-dependent response by i.t. injection of the A1R ligand, adenosine; however, no analgesia was evident when an exogenous adenosine was blocked by A1R antagonist. This study demonstrated dual mechanisms of neuropathic pain in TRPV1-induced neuropathy, involving a reduced adenosine system at both the ligand (adenosine) and receptor (A1Rs) levels.

Topics: Adenosine; Animals; Diterpenes; Down-Regulation; Ganglia, Spinal; Mice; Neuralgia; Neurons; Neurotoxins; Pain Measurement; Receptor, Adenosine A1; Small Fiber Neuropathy

Topics: Animals; Diterpenes; Drug Implants; Female; Hyperalgesia; Male; Neuralgia; Neuralgia, Postherpetic; Neurotoxins; Rats; Rats, Sprague-Dawley; Tetrodotoxin

Topics: Animals; Diterpenes; Hyperalgesia; Immunohistochemistry; Injections, Spinal; Male; Neuralgia; Rats; Rats, Sprague-Dawley; TRPV Cation Channels; Tumor Necrosis Factor-alpha

Injuries and/or dysfunctions in the somatosensory system can lead to neuropathic pain. Transient receptor potential vanilloid sub‑type 1 (TRPV1) play an important role in the development of allodynia and hyperalgesia following injury and the ensuing inflammatory conditions. Resiniferatoxin (RTX) is an ultrapotent synthetic TRPV1 agonist and many different administration routes are available for different mechanisms and different effects. RTX is used intraperitonially as a model of neuropathic pain or epidurally and topically to produce prolonged analgesic effects. However, the use of RTX is controversial because its neurotoxicity and margin of safety have not been addressed adequately. The present study evaluates the effect of intrathecal RTX on the induction and allodynia behavior of animals submitted to neuropathic pain by chronic constriction injury (CCI).. 160 Swiss mice were randomly distributed into two groups: intrathecal pre‑treatment group (PRE) aiming the effect in induction of allodynia and late intrathecal treatment group (POST) to evaluate the antiallodynic effect of the RTX on mechanical nociceptive threshold evaluated by the Von Frey hair filaments. Additionally, we evaluated the expression of TRPV1 in dorsal root ganglia (DRG) by western blotting after PRE‑ and POST‑treatment with RTX.. Our results showed that the CCI mice developed prolonged mechanical allodynia‑like behavior in ipsilateral paw after surgery up to 24 hours. The PRE‑ and POST‑treatment groups presented significant antiallodynic effects in ipsilateral paw for 24 hours. Only the POST‑treatment group showed a significant reduction of expression of the TRPV1 receptor after CCI.. The presented data demonstrated that both PRE‑ and POST‑treatment with RTX given intrathecally produced potent antiallodynic activities in CCI mice and that POST‑treatment can reduce TRPV1 expression in DRG, suggesting that POST‑treatment RTX can revert central sensitization and its associated allodynia.

Topics: Analgesics, Non-Narcotic; Analysis of Variance; Animals; Constriction; Disease Models, Animal; Diterpenes; Drug Administration Schedule; Ganglia, Spinal; Hyperalgesia; Injections, Spinal; Male; Mice; Neuralgia; Pain Measurement; Pain Threshold; Time Factors; TRPV Cation Channels

The medicinal plant generally known as monkey's comb (Amphilophium crucigerum) has been popularly described for the treatment of neuropathic and inflammatory pain, specially seeds preparations.. The goal of the present study was to evaluate the antinociceptive effect of the crude extract (Crd) and dichloromethane fraction (Dcm) of A. crucigerum seeds, and investigate the involvement of transient receptor potential vanilloid 1 (TRPV1) receptor in this effect.. Male Swiss mice were used in this study. The effects of Crd and Dcm was tested on capsaicin-induced Ca. The oral administration of Crd or Dcm resulted in an antinociceptive effect in the hot water tail-flick (48°C) and capsaicin intraplantar tests. Furthermore, these preparations exhibited antinociceptive and anti-inflammatory effects in a chronic inflammatory pain model, and antinociceptive effects in a neuropathic pain model. Moreover, Crd and Dcm reduced capsaicin-induced Ca. In conclusion, our results support the analgesic effect of A. crucigerum and suggest the presence of compounds that may act as TRPV1 antagonists.

Topics: Analgesics; Animals; Bignoniaceae; Binding, Competitive; Calcium Signaling; Capsaicin; Chronic Pain; Disease Models, Animal; Diterpenes; Dose-Response Relationship, Drug; Ethanol; Male; Methylene Chloride; Mice; Neuralgia; Nociception; Nociceptive Pain; Pain Measurement; Phytotherapy; Plant Extracts; Plants, Medicinal; Protein Binding; Seeds; Signal Transduction; Solvents; Spinal Cord; Synaptosomes; TRPV Cation Channels

Netrin-1 is a neuronal guidance molecule implicated in the development of spinal cord neurons and cortical neurons. In the adult spinal cord, UNC5H (repulsive receptor of netrin-1), but not deleted in colorectal cancer (DCC) (attractive receptor of netrin-1), constitutes a major mode of netrin-1 signal transduction, which may be involved in axon repulsion and inhibits neurite outgrowth. Abnormal sprouting of myelinated afferent fibers in the spinal dorsal horn can cause mechanical allodynia associated with postherpetic neuralgia (PHN, Shingles) and other neuropathic pains. However, whether netrin-1 participates in sprouting of myelinated afferent fibers and mechanical allodynia remains unknown. In an ultropotent TRPV1 agonist resiniferatoxin (RTX)-induced PHN-like model, RTX treatment for 6 weeks increased netrin-1 expression in dorsal horn neurons, including NK-1-positive projection neurons. In human neuroblastoma SH-SY5Y cells, we found that TRPV1 antagonist capsazepine antagonized RTX-induced upregulation of netrin-1. After RTX treatment, UNC5H2 expression was gradually decreased, whereas DCC expression was significantly increased. Silencing netrin-1 in the spinal dorsal horn significantly attenuated RTX-induced mechanical allodynia and sprouting of myelinated fibers into the spinal lamina II. Our results suggest that RTX treatment upregulates netrin-1 expression through activation of TRPV1 receptors and change UNC5H2-rich spinal dorsal horn into a growth-permissive environment by increasing DCC expression, thus enhancing the sprouting of myelinated afferent nerves. Netrin-1 may be targeted for reducing primary afferent sprouting and mechanical allodynia in PHN and other neuropathic pain conditions.

Topics: Animals; Biomarkers; Cell Line, Tumor; DCC Receptor; Diterpenes; Down-Regulation; Gene Silencing; Humans; Hyperalgesia; Lentivirus; Male; Myelin Sheath; Netrin-1; Neuralgia; Neurogenesis; Neurons, Afferent; Oligodendroglia; Posterior Horn Cells; Rats, Sprague-Dawley; Receptors, Cell Surface; RNA, Messenger; RNA, Small Interfering; Spinal Cord; Up-Regulation

Neuropathic pain in small-fiber neuropathy results from injury to and sensitization of nociceptors. Functional prostatic acid phosphatase (PAP) acts as an analgesic effector. However, the mechanism responsible for the modulation of PAP neuropathology, which leads to loss of the analgesic effect after small-fiber neuropathy, remains unclear.. We used a resiniferatoxin (RTX)-induced small-fiber neuropathy model to examine whether functional PAP(þ) neurons are essential to maintain the analgesic effect. PAP(þ) neurons were categorized into small to medium neurons (25th-75th percentile: 17.1-23.7 mm); these neurons were slightly reduced by RTX (p¼0.0003). By contrast, RTX-induced activating transcription factor 3 (ATF3), an injury marker, in PAP(þ) neurons (29.0%5.6% vs. 0.2%0.2%, p¼0.0043), indicating PAP neuropathology. Moreover, the high-affinity nerve growth factor (NGF) receptor (trkA) colocalized with PAP and showed similar profiles after RTX-induced neuropathy, and the PAP/trkA ratios correlated with the degree of mechanical allodynia (r¼0.62, p¼0.0062). The NGF inducer 4-methylcatechol (4MC) normalized the analgesic effects of PAP; specifically, it reversed the PAP and trkA profiles and relieved mechanical allodynia. Administering 2.5S NGF showed similar results to those of administering 4MC. This finding suggests that the analgesic effect of functional PAP is mediated by NGF-trkA signaling, which was confirmed by NGF neutralization.. This study revealed that functional PAP(þ) neurons are essential for the analgesic effect, which is mediated by NGF-trkA signaling.

Topics: Acid Phosphatase; Analgesics; Animals; Catechols; Diterpenes; Hyperalgesia; Mice; Models, Biological; Nerve Growth Factor; Neuralgia; Neurons; Phenotype; Receptor, trkA; Receptors, Purinergic P2X3; Signal Transduction

Resiniferatoxin (RTX) is an ultrapotent synthetic TRPV1 (transient receptor potential vanilloid subtype 1) agonist with significant initial transient hyperalgesia followed by a prolonged analgesic effect in response to thermal stimulus. Using a rat model of neuropathic pain, we evaluated the effect of pretreatment with clonidine-which has been shown to relieve intradermal capsaicin-induced hyperalgesia-on the initial hyperalgesic response and the thermal analgesic property of RTX. Thirty-six male rats were divided into 6 treatment groups (n=6 each):RTX 500 ng, RTX 1 μg, clonidine 20 μg (Cl), Cl＋RTX 500 ng, Cl＋RTX 1 μg, or normal saline 20 μL (control). We evaluated the short-term (180 min) and long-term (20 days) analgesic effects of RTX after thermal stimulation and mechanical stimulation. RTX had significant initial transient hyperalgesia followed by a prolonged analgesic effect in response to the thermal stimulus, but the RTX 500 ng and RTX 1 μg groups showed no initial short-term thermal hyperalgesic responses when pretreated with clonidine. The Cl＋RTX 1 μg rats' behavior scores indicated that they were more calm and comfortable compared to the RTX 1 μg rats. Even though we cannot precisely confirm that pretreatment with clonidine potentiates or adds to the analgesic effect of RTX, clonidine pretreatment with epidural RTX eliminated the initial RTX-associated hyperalgesic response and systemic toxicity in this neuropathic pain rat model.

Topics: Analgesia, Epidural; Analgesics; Animals; Clonidine; Diterpenes; Hyperalgesia; Injections, Epidural; Male; Models, Animal; Neuralgia; Rats; Rats, Sprague-Dawley; Stress, Mechanical; Time Factors; Treatment Outcome

Trigeminal ganglion C-fiber neurons bearing transient receptor potential vanilloid-1 (TRPV1) channels are selectively destroyed by resiniferatoxin (RTX), a potent capsaicin analogue. The current study assessed the effect of an RTX injection (200 ng/4 μl) into the trigeminal ganglion in inflammatory and neuropathic rat models of orofacial thermal hyperalgesia. Intraganglionar RTX injection resulted in trigeminal ganglion C-fiber deletion, which was confirmed by the capsaicin eye wipes test, performed 6 days after the injection. The nociceptive responses induced by 2.5% formalin injected into the orofacial region were unchanged by a previous intraganglionar RTX injection. However, orofacial heat and cold hyperalgesia, induced by carrageenan injected into the upper lip (50 µg/50 μl), was abolished by previous intraganglionar RTX treatment. In addition, the development of orofacial heat and cold hyperalgesia after constriction of the infraorbital nerve was prevented by previous RTX treatment. Thus, trigeminal ganglion neurons expressing TRPV1 are crucial for the development of orofacial inflammatory and neuropathic thermal hyperalgesia.

Topics: Animals; Capsaicin; Carrageenan; Cold Temperature; Disease Models, Animal; Diterpenes; Facial Pain; Formaldehyde; Hot Temperature; Hyperalgesia; Male; Neuralgia; Neurons; Nociception; Rats; Rats, Wistar; Sensory System Agents; Trigeminal Ganglion; TRPV Cation Channels

Small-fiber neuropathy was induced in young adult mice by intraperitoneal injection of resiniferatoxin (RTX), a TRPV1 agonist. At day 7, RTX induced significant thermal and mechanical hypoalgesia. At day 28, mechanical and thermal nociception were restored. No nerve degeneration in skin was observed and unmyelinated nerve fiber morphology and density in sciatic nerve were unchanged. At day 7, substance P (SP) was largely depleted in dorsal root ganglia (DRG) neurons, although calcitonin gene-related peptide (CGRP) was only moderately depleted. Three weeks after, SP and CGRP expression was restored in DRG neurons. At the same time, CGRP expression remained low in intraepidermal nerve fibers (IENFs) whereas SP expression had improved. In summary, RTX induced in our model a transient neuropeptide depletion in sensory neurons without nerve degeneration. We think this model is valuable as it brings the opportunity to study functional nerve changes in the very early phase of small fiber neuropathy. Moreover, it may represent a useful tool to study the mechanisms of action of therapeutic strategies to prevent sensory neuropathy of various origins.

Topics: Animals; Disease Models, Animal; Diterpenes; Epidermis; Ganglia, Spinal; Hot Temperature; Hyperalgesia; Male; Mice; Myelin Sheath; Nerve Degeneration; Nerve Fibers, Unmyelinated; Neuralgia; Nociception; Physical Stimulation; Sensory Receptor Cells; Touch; TRPV Cation Channels

Resiniferatoxin (RTX), an excitotoxic agonist for vanilloid receptor 1, is a promising candidate for intractable pain treatment.. We evaluated the effects of RTX, applied to dorsal root ganglia (DRG) at high doses (1200 ng), in sensory-motor function and nerve growth factor (NGF) alterations in a photochemical sciatic nerve injury rat model.. Following RTX injection into the L3-6 DRG at high doses and behavioral evaluation, the rats were sacrificed and the DRG were tested by immunohistochemistry and mRNA analysis for NGF and its' receptors, tyrosine kinase A (TrkA) and p75. The correlation between neuropathic pain and NGF, TrkA, and p75 expression was analyzed.. The treated rats had preserved touch, cold, pain, and high-heat sensations, and exhibited hypoalgesia to low-heat stimulation. After RTX treatment, TrkA and p75 altered their expressions from one neuronal type to another in the DRG. NGF and p75 expression changed from the small-size neurons in neuropathic rat DRG to the large- and medium-size neurons in non-neuropathic and RTX-treated animals, concomitantly with neuropathic pain suppression. TrkA was expressed in the small-size neurons in neuropathic rat DRG, and was drastically reduced in all size neurons after RTX treatment. NGF, TrkA, and p75 mRNA expression supported these phenotypic changes in the DRG.. The pathway of NGF-TrkA expressed in the small-size neurons, associated with neuropathic pain, was shifted to the NGF-p75 pathway expressed in the large-size neurons after RTX treatment, in association with neuropathic pain inhibition. These findings may play an important role in clinical trial designs.

Topics: Analgesics; Animals; Cells, Cultured; Diterpenes; Ganglia, Spinal; Nerve Growth Factor; Nerve Tissue Proteins; Neural Inhibition; Neuralgia; Pain Threshold; Rats; Rats, Sprague-Dawley; Receptor, trkA; Receptors, Nerve Growth Factor; Signal Transduction

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.

Topics: Adenosine Triphosphate; Animals; Apyrase; Disease Models, Animal; Diterpenes; Fluorescent Antibody Technique; Male; Mice; Mice, Inbred ICR; Neuralgia; Neurotoxins; Receptors, Purinergic P2X3; Signal Transduction

Patients suffering from sensory neuropathy due to skin denervation frequently have paradoxical manifestations of reduced nociception and neuropathic pain. However, there is a lack of satisfactory animal models to investigate these phenomena and underlying mechanisms. We developed a mouse system of neuropathy induced by resiniferatoxin (RTX), a capsaicin analog, and examined the functional significance of P2X3 receptor in neuropathic pain. From day 7 of RTX neuropathy, mice displayed mechanical allodynia (p<0.0001) and thermal hypoalgesia (p<0.0001). After RTX treatment, dorsal root ganglion (DRG) neurons of the peripherin type were depleted (p=0.012), while neurofilament (+) DRG neurons were not affected (p=0.62). In addition, RTX caused a shift in neuronal profiles of DRG: (1) increased in P2X3 receptor (p=0.0002) and ATF3 (p=0.0006) but (2) reduced TRPV1 (p=0.036) and CGRP (p=0.015). The number of P2X3(+)/ATF3(+) neurons was linearly correlated with mechanical thresholds (p=0.0017). The peripheral expression of P2X3 receptor in dermal nerves was accordingly increased (p=0.016), and an intraplantar injection of the P2X3 antagonists, A-317491 and TNP-ATP, relieved mechanical allodynia in a dose-dependent manner. In conclusion, RTX-induced sensory neuropathy with upregulation of P2X3 receptor for peripheral sensitization of mechanical allodynia, which provides a new therapeutic target for neuropathic pain after skin denervation.

Topics: Animals; Diterpenes; Ganglia, Spinal; Hyperalgesia; Male; Mice; Mice, Inbred ICR; Neuralgia; Neurons; Pain Measurement; Peripheral Nervous System Diseases; Receptors, Purinergic P2X3; Skin

Brain-derived neurotrophic factor (BDNF) and its cognate receptor, the tyrosine kinase B (TrkB), are normally expressed in neurons and implicated in multiple pathological conditions. Brain-derived neurotrophic factor is produced in the central nervous system microglia in response to noxious stimuli and appear to potentiate central sensitization. Resiniferatoxin (RTX) is an excitotoxic agonist of the vanilloid receptor 1 (VR1), a cation channel protein considered an integrator for nociception. Resiniferatoxin, administered into the dorsal root ganglia (DRG), selectively eliminates the VR1-positive neurons and improves tactile allodynia in a neuropathic pain rat model.. The goal of the present study was to evaluate the role of BDNF in RTX-induced neuropathic pain suppression.. The study design was a sciatic nerve injury animal model with intraganglionic RTX injection.. Resiniferatoxin was injected into the DRG of the L3-L6 spinal nerves after the rats displayed tactile allodynia and thermal hyperalgesia produced by a photochemical injury to the sciatic nerve. Behavioral testing and immunohistochemical and mRNA analysis of the DRG were performed to determine BDNF's role in pain modulation.. Brain-derived neurotrophic factor expression in the DRG of neuropathic rats was upregulated in the small- and medium-size neurons, whereas the upregulation was observed in the large-size neurons of non-neuropathic rat DRG. A high-dose RTX injection in the DRG of neuropathic rats led to elimination of both thermal hyperalgesia and tactile allodynia and also upregulated BDNF in the large-size neurons, similar to the nonallodynic rats. Tyrosine kinase B changes mirrored the BDNF ones.. Resiniferatoxin injection in the DRG of neuropathic rats upregulates BDNF expression in the same pattern as in the large-size neurons of non-neuropathic rats. Therefore, BDNF upregulation may have pain suppressive effects. These effects are likely mediated by TrkB.

Topics: Animals; Brain-Derived Neurotrophic Factor; Diterpenes; Ganglia, Spinal; Hyperalgesia; Immunohistochemistry; Male; Neuralgia; Neurons; Pain Measurement; Physical Stimulation; Rats; Rats, Sprague-Dawley; Sciatic Nerve; TRPV Cation Channels; Up-Regulation

Patients with postherpetic neuralgia often have an increased sensitivity to a tactile stimulus but impaired thermal sensitivity in the same affected dermatomes. We recently found that depletion of capsaicin-sensitive afferents by systemic treatment with a potent TRPV1 agonist, resiniferotoxin, in adult rats produces long-lasting paradoxical changes in mechanical and thermal sensitivities, which resemble the unique clinical features of postherpetic neuralgia. The anticonvulsant gabapentin is effective in reducing the subjective pain score in patients with postherpetic neuralgia. In this study, we quantified the potential effect of systemic and intrathecal gabapentin on tactile allodynia induced by resiniferotoxin in rats. Intraperitoneal injection of 200 microg/kg resiniferotoxin produced a rapid and sustained increase in the paw withdrawal latency to a radiant heat stimulus. Profound tactile allodynia developed in all the resiniferotoxin-treated rats within 3 weeks. Intraperitoneal injection of 30-60 mg/kg of gabapentin in resiniferotoxin-treated rats significantly increased the withdrawal threshold in response to von Frey filaments. Furthermore, intrathecal administration of 10-30 microg of gabapentin also produced a significant effect on the mechanical withdrawal threshold in all resiniferotoxin-treated rats. These data provide complementary new information that gabapentin administered systemically and spinally can effectively relieve tactile allodynia in this animal model of postherpetic neuralgia.

Topics: Amines; Analgesics; Animals; Cyclohexanecarboxylic Acids; Disease Models, Animal; Diterpenes; Gabapentin; gamma-Aminobutyric Acid; Hot Temperature; Injections, Intravenous; Injections, Spinal; Male; Mechanoreceptors; Neuralgia; Nociceptors; Pain Threshold; Rats; Rats, Sprague-Dawley; Touch

Resiniferatoxin (RTX), an ultrapotent analog of capsaicin, has been used as a tool to study the role of capsaicin-sensitive C fibers in pain. Recently, we found that RTX diminished the thermal sensitivity but unexpectedly increased the sensitivity to tactile stimulation in adult rats. In this study, we explored the potential mechanisms involved in RTX-induced changes in somatosensory function. An intraperitoneal injection of 200 microg/kg RTX, but not its vehicle, rapidly produced an increase in the paw withdrawal latency to a heat stimulus. Also, profound tactile allodynia developed in all the RTX-treated rats in 3 weeks. This paradoxical change in thermal and mechanical sensitivities lasted for at least 6 weeks. Electron microscopic examination of the sciatic nerve revealed a loss of unmyelinated fibers and extensive ultrastructural damage of myelinated fibers in RTX-treated rats. Immunofluorescence labeling showed a diminished vanilloid receptor 1 immunoreactivity in dorsal root ganglia neurons and the spinal dorsal horn of RTX-treated rats. Furthermore, two transganglionic tracers, horseradish peroxidase conjugates of cholera toxin B subunit (CTB) and isolectin-B(4) of Bandeiraea simplicifolia (IB(4)), were injected into the opposite sides of the sciatic nerve to trace myelinated and unmyelinated afferent terminations, respectively, in the spinal dorsal horn. In RTX-treated rats, IB(4)-labeled terminals in the dorsal horn were significantly reduced, and CTB-labeled terminals appeared to sprout into lamina II of the spinal dorsal horn. Thus, this study demonstrates that systemic RTX diminishes the thermal pain sensitivity by depletion of unmyelinated afferent neurons. The delayed tactile allodynia induced by RTX is likely attributable to damage to myelinated afferent fibers and their abnormal sprouting in lamina II of the spinal dorsal horn. These data provide new insights into the potential mechanisms of postherpetic neuralgia.

Topics: Afferent Pathways; Animals; Diterpenes; Fluorescent Antibody Technique; Ganglia, Spinal; Hot Temperature; Kinetics; Male; Nerve Fibers, Myelinated; Nerve Fibers, Unmyelinated; Neuralgia; Posterior Horn Cells; Rats; Rats, Sprague-Dawley; Receptors, Drug; Sciatic Nerve; Stress, Mechanical