VR1 protein expression increases in undamaged DRG neurons after partial nerve injury

Transcription of rat TRPV1 utilizes a dual promoter system that is positively regulated by nerve growth factor

The capsaicin receptor, also known as 'transient receptor potential vanilloid receptor subtype 1' (TRPV1, VR1), is an ion channel subunit expressed in primary afferent nociceptors, which plays a critical role in pain transduction and thermal hyperalgesia. Increases in nociceptor TRPV1 mRNA and protein are associated with tissue injury-inflammation. As little is understood about what controls TRPV1 RNA transcription in nociceptors, we functionally characterized the upstream portion of the rat TRPV1 gene. Two functional rTRPV1 promoter regions and their transcription initiation sites were identified. Although both promoter regions directed transcriptional activity in nerve growth factor (NGF) treated rat sensory neurons, the upstream Core promoter was the most active in cultures enriched in sensory neurons. Because NGF is a key modulator of inflammatory pain, we examined the effect of NGF on rTRPV1 transcription in PC12 cells. NGF positively regulated transcriptional activity of both rTRPV1 promoter regions in PC12 cells. We propose that the upstream regulatory region of the rTRPV1 gene is composed of a dual promoter system that is regulated by NGF. These findings support the hypothesis that NGF produced under conditions of tissue injury and/or inflammation directs an increase of TRPV1 expression in nociceptors in part through a transcription-dependent mechanism.

Mechanisms of neuropathic pain

Neuropathic pain refers to pain that originates from pathology of the nervous system. Diabetes, infection (herpes zoster), nerve compression, nerve trauma, "channelopathies," and autoimmune disease are examples of diseases that may cause neuropathic pain. The development of both animal models and newer pharmacological strategies has led to an explosion of interest in the underlying mechanisms. Neuropathic pain reflects both peripheral and central sensitization mechanisms. Abnormal signals arise not only from injured axons but also from the intact nociceptors that share the innervation territory of the injured nerve. This review focuses on how both human studies and animal models are helping to elucidate the mechanisms underlying these surprisingly common disorders. The rapid gain in knowledge about abnormal signaling promises breakthroughs in the treatment of these often debilitating disorders.

Vanilloid receptor 1 (TRPV1) expression in lingual nerve neuromas from patients with or without symptoms of burning pain

The lingual nerve, a peripheral branch of the trigeminal nerve, can be damaged during the surgical removal of lower third molar teeth. This damage can lead to the development of dysaesthesia, with some patients complaining of burning pain. We investigated the hypothesis that vanilloid receptor 1 (TRPV1), a transducer of noxious heat stimuli, was involved in the development of this burning pain. Neuroma specimens were obtained from patients undergoing microsurgical repair of a damaged lingual nerve. Repair was undertaken where there was little evidence of spontaneous recovery, 7-41 months after the initial injury. Preoperatively the incidence of dysaesthesia was determined by reported symptoms and using visual analogue scales (VAS) for pain, tingling and discomfort. Nine neuromas were studied from patients with burning dysaesthesia and six from patients with a sensory deficit but no dysaesthesia. Indirect immunofluorescence for protein gene product (PGP) 9.5 and TRPV1 was used to quantify the percentage area of PGP 9.5 positive neuronal tissue that also expressed TRPV1. The results showed no significant difference between the mean percentage area of TRPV1 expression in neuromas from patients with or without burning dysaesthesia. Furthermore, there was no correlation between TRPV1 expression and the VAS scores for pain, tingling or discomfort. However, if data from all patients was pooled, there was a negative correlation between the level of TRPV1 expression and the time after initial injury. These data do not rule out involvement of TRPV1 in the aetiology of burning dysaesthesia following lingual nerve injury but suggest that TRPV1 at the injury site does not play a primary role.

Mechanical allodynia and thermal hyperalgesia induced by experimental squamous cell carcinoma of the lower gingiva in rats

We developed a rat model of oral cancer pain by inoculating cancer cells into the lower gingiva. A squamous cell carcinoma (SCC) derived from Fisher rats, SCC-158, was inoculated into the subperiosteal tissue on the lateral side of the lower gingiva in male Fisher rats. Inoculation of cancer cells induced marked mechanical allodynia and thermal hyperalgesia in the ipsilateral maxillary and mandibular nerve area. Infiltration of the tumor cells into the mandible and the completely encompassed inferior alveolar nerve was observed. Calcitonin gene-related peptide (CGRP)-, substance P (SP)-, ATP receptor (P2X(3))-, and capsaicin receptor (TRPV1)-immunoreactive cells strikingly increased in the small-cell group of trigeminal ganglia (TGs) after tumor cell inoculation. The TRPV1-immunoreactive cells also increased in the medium- and large-cell groups. Retrograde tracing combined with immunofluorescence techniques revealed the increased expression of peptides and the receptors in maxillary nerve afferent neurons. These results suggest that inoculation of SCC cells into the lower gingiva produces mechanical allodynia and thermal hyperalgesia, indicating the establishment of a novel rat model of oral cancer pain. Increased expression of CGRP, SP, P2X(3), and TRPV1 in the TG may be involved in the behavioral changes in this model.

PERSPECTIVE

To clarify the mechanisms of oral cancer pain, we examined the expression of calcitonin gene-related peptide, substance P, ATP receptor P2X(3), and capsaicin receptor TRPV1 in trigeminal ganglia. Characterizations of these molecular systems which mediate pain perception are important to develop novel clinical tools for promoting relief of oral cancer pain.

Expression of capsaicin receptor immunoreactivity in human peripheral nervous system and in painful neuropathies

We describe the expression of the capsaicin receptor (TRPV1) in human peripheral nervous system (PNS) and its changes in sural nerve and skin nerve fibers of patients with painful neuropathy. Dorsal root ganglion (DRG), root, and spinal cord autopsy specimens from subjects without PNS diseases were immunoassayed with anti-TRPV1 antibodies. Bright-field and confocal microscope studies using anti-TRPV1, protein gene product 9.5 (PGP 9.5), and unique-beta-tubulin (TuJ1) antibodies were performed in skin biopsies from 15 healthy subjects and 10 painful neuropathies. The density of intraepidermal nerve fiber (IENF) labeled by each antibody was quantified. Sural nerve biopsies from three patients with painful, one patient with nonpainful diabetic neuropathy, and two patients with multifocal motor neuropathy used as controls were immunoassayed with anti-TRPV1 antibodies and investigated by immunoelectron microscopy. TRPV1 strongly labeled laminae I and II of dorsal horns, most small-size and some medium-size DRG neurons, and small-diameter axons of dorsal roots. In sural nerve, TRPV1 was expressed within the cytoplasm of most unmyelinated and some small myelinated axons, in the muscular lamina of epineural vessels, and in the endothelium of endoneurial vessels. The density of IENF labeled by TRPV1, PGP 9.5, and TuJ1 did not differ. TRPV1 colocalized with TuJ1 in all IENF and dermal nerve bundles. Painful neuropathies showed a diffuse loss of TRPV1-positive axons both in the sural nerve and in the skin. Our findings demonstrated that TRPV1 is normally expressed throughout the nociceptive pathway of PNS and that TRPV1-positive peripheral nerve fibers degenerate in painful neuropathies.

Antinociceptive effect of antisense oligonucleotides against the vanilloid receptor VR1/TRPV1

To examine the role of the vanilloid receptor TRPV1 in neuropathic pain, we assessed the effects of the receptor antagonist thioxo-BCTC and antisense oligonucleotides against the TRPV1 mRNA in a rat model of spinal nerve ligation. In order to identify accessible sites on the mRNA of TRPV1, the RNase H assay was used, leading to the successful identification of binding sites for antisense oligonucleotides. Cotransfection studies using Cos-7 cells were employed to identify the most effective antisense oligonucleotide efficiently inhibiting the expression of a fusion protein consisting of TRPV1 and the green fluorescent protein in a specific and concentration-dependent manner. In an in vivo rat model of spinal nerve ligation, intravenous application of the TRPV1 antagonist thioxo-BCTC reduced mechanical hypersensitivity yielding an ED(50) value of 10.6mg/kg. Intrathecal administration of the antisense oligonucleotide against TRPV1, but not the mismatch oligonucleotide or a vehicle control, reduced mechanical hypersensitivity in rats with spinal nerve ligation in a similar manner. Immunohistochemical analysis revealed neuropathy- and antisense-associated regulation of TRPV1 protein expression in spinal cord and dorsal root ganglia. Our data demonstrate comparative analgesic effects of a TRPV1 anatagonist and a rationally designed TRPV1 antisense oligonucleotide in a spinal nerve ligation model of neuropathic pain and thus, lend support to the validation of TRPV1 as a promising target for the treatment of neuropathic pain.

Activity-dependent slowing of conduction velocity in uninjured L4 C fibers increases after an L5 spinal nerve injury in the rat

Growing evidence suggests that uninjured afferents may play an important role in neuropathic pain following nerve injury. The excitability of nociceptive neurons in the L4 spinal nerve appears to be enhanced following an injury to the adjacent L5 spinal nerve. In this study, we investigated whether the action-potential conduction properties of unlesioned, unmyelinated fibers are also altered. A teased-fiber technique was used to record from single C fibers from the L4 spinal nerve of the rat in vitro. Repeated electrical stimulation of the tibial nerve was used to investigate activity-dependent slowing of conduction velocity. Twin pulse stimulation at a 50 ms interpulse interval allowed investigation of supranormal conduction velocity. Blinded experiments were performed 8-10 days after sham surgery and after an L5 spinal nerve ligation (L5 SNL). Activity-dependent slowing revealed two populations of C fibers, a "nociceptor" population with a large degree of activity-dependent slowing and a "non-nociceptor" population with a smaller degree of activity-dependent slowing. Both populations showed enhanced activity-dependent slowing of conduction velocity and enhanced supranormal conduction velocities in lesioned animals compared to sham animals. Activity-dependent slowing was also enhanced after an L5 SNL in the mouse. These alterations in conduction velocity may reflect changes in expression of ion channels responsible for the membrane excitability. These data provide additional evidence that a nerve injury leads to persistent alterations in the properties of adjacent uninjured, unmyelinated fibers.

Distinct ASIC currents are expressed in rat putative nociceptors and are modulated by nerve injury

The H(+)-gated acid-sensing ion channels (ASICs) are expressed in dorsal root ganglion (DRG) neurones. Studies with ASIC knockout mice indicated either a pro-nociceptive or a modulatory role of ASICs in pain sensation. We have investigated in freshly isolated rat DRG neurones whether neurones with different ASIC current properties exist, which may explain distinct cellular roles, and we have investigated ASIC regulation in an experimental model of neuropathic pain. Small-diameter DRG neurones expressed three different ASIC current types which were all preferentially expressed in putative nociceptors. Type 1 currents were mediated by ASIC1a homomultimers and characterized by steep pH dependence of current activation in the pH range 6.8-6.0. Type 3 currents were activated in a similar pH range as type 1, while type 2 currents were activated at pH < 6. When activated by acidification to pH 6.8 or 6.5, the probability of inducing action potentials correlated with the ASIC current density. Nerve injury induced differential regulation of ASIC subunit expression and selective changes in ASIC function in DRG neurones, suggesting a complex reorganization of ASICs during the development of neuropathic pain. In summary, we describe a basis for distinct cellular functions of different ASIC types in small-diameter DRG neurones.

Silencing of vanilloid receptor TRPV1 by RNAi reduces neuropathic and visceral pain in vivo

RNA interference (RNAi) has proven to be a powerful technique to study the function of genes by producing knock-down phenotypes. Here, we report that intrathecal injection of an siRNA against the transient receptor potential vanilloid receptor 1 (TRPV1) reduced cold allodynia of mononeuropathic rats by more than 50% over a time period of approximately 5 days. A second siRNA targeted to a different region of the TRPV1 gene was employed and confirmed the analgesic action of a TRPV1 knock-down. Furthermore, siRNA treatment diminished spontaneous visceral pain behavior induced by capsaicin application to the rectum of mice. The analgesic effect of siRNA-mediated knockdown of TRPV1 in the visceral pain model was comparable to that of the low-molecular weight receptor antagonist BCTC. Our data demonstrate that TRPV1 antagonists, including TRPV1 siRNAs, have potential in the treatment of both, neuropathic and visceral pain.

Mechanisms of disease: neuropathic pain--a clinical perspective

Neuropathic pain syndromes-pain after a lesion or disease of the peripheral or central nervous system-are clinically characterized by spontaneous and evoked types of pain, which are underpinned by various distinct pathophysiological mechanisms in the peripheral and central nervous systems. In some patients, the nerve lesion triggers molecular changes in nociceptive neurons, which become abnormally sensitive and develop pathological spontaneous activity. Inflammatory reactions of the damaged nerve trunk can induce ectopic nociceptor activity, causing spontaneous pain. The hyperactivity in nociceptors induces secondary changes in processing neurons in the spinal cord and brain, so that input from mechanoreceptive A-fibers is perceived as pain. Neuroplastic changes in the central pain modulatory systems can lead to further hyperexcitability. The treatment of neuropathic pain is still unsatisfactory, and a new hypothetical concept has been proposed, in which pain is analyzed on the basis of underlying mechanisms. The increased knowledge of pain-generating mechanisms and their translation into symptoms and signs might eventually allow a dissection of the mechanisms that operate in each patient. If a precise clinical phenotypic characterization of the neuropathic pain is combined with a selection of drugs that act on those mechanisms, it should ultimately be possible to design optimal treatments for individuals. This review discusses the conceptual framework of the novel mechanism-based classification, encouraging the reader to see neuropathic pain as a clinical entity rather than a compilation of single disease states.

Upregulation of brain-derived neurotrophic factor in the sensory pathway by selective motor nerve injury in adult rats

Selective motor nerve injury by lumbar 5 ventral root transection (L5 VRT) induces neuropathic pain, but the underlying mechanisms remain unknown. Previously, increased expression and secretion of brain-derived neurotrophic factor (BDNF) had been implicated in injury-induced neuropathic pain in the sensory system. In this study, as a step to examine potential roles of BDNF in L5 VRT-induced neuropathic pain, we investigated BDNF gene and protein expression in adult rats with L5 VRT. L5 VRT induced a dramatic upregulation of BDNF mRNA in intact sensory neurons in the ipsilateral L5 dorsal root ganglia (DRG), in non-neuronal cells in the ipsilateral sciatic nerve, and in motoneurons in the ipsilateral spinal cord. L5 VRT also induced de novo synthesis of BDNF mRNA in spinal dorsal horn neurons and in glial cells in the white matter of the ipsilateral spinal cord. Consistent with the mRNA expression pattern, BDNF protein was also mainly upregulated in all populations of sensory neurons in the ipsilateral L5 DRG and in spinal neurons and glia. Quantitative analysis by ELISA showed that the BDNF content in the DRG and sciatic nerve peaked on day 1 and remained elevated 14 days after L5 VRT. These results suggest that increased BDNF expression in intact primary sensory neurons and spinal cord may be an important factor in the induction of neuropathic pain without axotomy of sensory neurons.

Site-specific increases in peripheral cannabinoid receptors and their endogenous ligands in a model of neuropathic pain

Selective activation of the peripheral cannabinoid receptor 1 (CB1R) has been shown to suppress neuropathic pain symptoms in rodents. However, relatively little is known about changes in CB1R and its endogenous ligands during development or maintenance of neuropathic pain. Using immunohistochemistry, Western blot, real-time reverse transcription polymerase chain reaction, as well as liquid chromatography/mass spectrometry, we studied the changes in CB1Rs and endocannabinoids N-arachidonoylethanolamine/anandamide (AEA) and 2-arachidonoylglycerol (2-AG) in rat lumbar (L4 and L5) dorsal root ganglia (DRG) after neuropathic pain induction (L5 spinal nerve ligation: SNL). Immunohistochemistry revealed that in control rats, CB1R is expressed in the majority (76-83%) of nociceptive neurons as indicated by co-labeling with isolectin B4 (IB4) or antibodies recognizing transient receptor potential vanilloid (TRPV1), calcitonin gene related peptide (CGRP), and the NR2C/2D subunits of the N-methyl-D-aspartate receptor. After L5 SNL, CB1R mRNA and protein increases in the ipsilateral uninjured L4 DRG whereas the percentages of CB1R immunoreactive (CB1R-ir) neurons remain unchanged in L4 and L5 DRG. However, for these CB1R-ir neurons, we observe significant increases in percentage of TRPV1-ir cells in ipsilateral L4 DRG, and decreases in percentage of IB4- and CGRP-co-labeled cells in ipsilateral L5 DRG. Levels of both AEA and 2-AG increase significantly only in the ipsilateral L5 DRG. These results are consistent with the preserved analgesic effects of cannabinoids in neuropathic pain and provide a rational framework for the development of peripherally acting endocannabinoid-based therapeutic interventions for neuropathic pain.

Design of Potent, Orally Available Antagonists of the Transient Receptor Potential Vanilloid 1. Structure−Activity Relationships of 2-Piperazin-1-yl-1H-benzimidazoles

The vanilloid receptor-1 (VR1 or TRPV1) is a membrane-bound, nonselective cation channel that is predominantly expressed by peripheral neurons sensing painful stimuli. TRPV1 antagonists produce antihyperalgesic effects in animal models of inflammatory and neuropathic pain. Herein, we describe the synthesis and the structure-activity relationships of a series of 2-(4-pyridin-2-ylpiperazin-1-yl)-1H-benzo[d]imidazoles as novel TRPV1 antagonists. Compound 46ad was among the most potent analogues in this series. This compound was orally bioavailable in rats and was efficacious in blocking capsaicin-induced flinch in rats in a dose-dependent manner. Compound 46ad also reversed thermal hyperalgesia in a model of inflammatory pain, which was induced by complete Freund's adjuvant (CFA).

Pain behaviors produced by capsaicin: influence of inflammatory mediators and nerve injury

The present study was undertaken to characterize spontaneous (ie, nonevoked) pain behaviors (flinching, biting/licking) produced by local injections of capsaicin into the rat hindpaw as a model of chemogenic pain, and to determine effects of inflammatory mediators and nerve injury on such behaviors. Capsaicin antagonists are a potential class of novel topical analgesics, and this model may be of value for preclinical screening of novel compounds. Local injections of capsaicin (0.1-30 microg) into the hindpaw produced flinching and biting/licking behaviors over 5 min, and these were reduced by capsazepine, a competitive antagonist for capsaicin at the TRPV1 receptor. Coadministration of noradrenaline (NA), prostaglandin E(2) (PGE(2)), and 5-hydroxytryptamine (5-HT) augmented capsaicin-evoked responses primarily by extending the duration of behaviors. Partial sciatic nerve ligation decreased flinching produced by capsaicin alone, by capsaicin in combination with each of NA, PGE(2), and 5-HT, and by formalin. Tibial nerve injury also reduced capsaicin-evoked flinching, and responses to formalin, but spinal nerve ligation did not affect either. These results indicate that (1) spontaneous pain behaviors occur as a result of TRPV1 receptor activation with a different time course than evoked responses, (2) inflammatory mediators augment capsaicin-evoked pain behaviors, and (3) various forms of nerve injury produce different effects on capsaicin-evoked pain behaviors.

PERSPECTIVE

The VR1 receptor is a potential target for development of novel topical analgesics. This study characterized pain behaviors produced by local injections of capsaicin in the presence of inflammatory mediators and following various forms of nerve injury. Results are of interest for the preclinical screening of novel VR1 receptor antagonists.

Inhibitory M2 muscarinic receptors are upregulated in both axotomized and intact small diameter dorsal root ganglion cells after peripheral nerve injury

Acetylcholine reduces nociceptive input in part by activating inhibitory M2 muscarinic receptors on primary sensory neurons, and acetylcholinesterase inhibitors and muscarinic agonists produce analgesia in humans and animals. M2 muscarinic receptors are upregulated in animals with diabetic neuropathy, but their level of expression and function after peripheral nerve injury has not been previously examined. This study tested, using intracellular Ca(2+) response to membrane depolarization, the effect of the M2 muscarinic receptor agonist bethanechol on individual dorsal root ganglion cells from normal and L5-6 spinal nerve-ligated rats, followed by M2 muscarinic receptor immunostaining. We also examined functional transient receptor potential for vanilloids-1 activity by determining intracellular Ca(2+) response evoked by capsaicin in M2 muscarinic receptor immunoreactive cells. In normal dorsal root ganglion cells, bethanechol inhibited the Ca(2+) response in a concentration-related fashion, and this inhibition was blocked by the M2 muscarinic receptor antagonist gallamine. Cells expressing M2 muscarinic receptors by immunostaining were significantly inhibited by bethanechol, whereas those lacking positive staining were not. The proportion of studied dorsal root ganglion neurons with positive M2 muscarinic receptor staining increased significantly in the injured ipsilateral L5-6 and the uninjured ipsilateral L4 ganglia, but not in the contralateral dorsal root ganglion neurons compared with normals. In contrast, the proportion of neurons responding to capsaicin significantly decreased in the injured ipsilateral L5-6 dorsal root ganglion cells. These results suggest that inhibitory M2 muscarinic receptors are upregulated in small- and medium-sized axotomized dorsal root ganglion neurons and their uninjured neighbors following nerve injury, and may represent an appropriate target for analgesia in this setting.

Identification and biological characterization of 6-aryl-7-isopropylquinazolinones as novel TRPV1 antagonists that are effective in models of chronic pain

Vanilloid receptor 1 (VR1, TRPV1) is a cation-selective ion channel that is expressed on primary afferent neurons and is upregulated following inflammation and nerve damage. Blockers of this channel may have utility in the treatment of chronic nociceptive and neuropathic pain. Here, we describe the optimization from a high throughput screening hit, of a series of 6-aryl-7-isopropylquinazolinones that are TRPV1 antagonists in vitro. We also demonstrate that one compound is active in vivo against capsaicin-induced hyperalgesia and in models of neuropathic and nociceptive pain in the rat.

Assessment and analysis of mechanical allodynia-like behavior induced by spared nerve injury (SNI) in the mouse

Experimental models of peripheral nerve injury have been developed to study mechanisms of neuropathic pain. In the spared nerve injury (SNI) model in rats, the common peroneal and tibial nerves are injured, producing consistent and reproducible pain hypersensitivity in the territory of the spared sural nerve. In this study, we investigated whether SNI in mice is also a valid model system for neuropathic pain. SNI results in a significant decrease in withdrawal threshold in SNI-operated mice. The effect is very consistent between animals and persists for the four weeks of the study. We also determined the relative frequency of paw withdrawal for each of a series of 11 von Frey hairs. Analysis of response frequency using a mixed-effects model that integrates all variables (nerve injury, paw, gender, and time) shows a very stable effect of SNI over time and also reveals subtle divergences between variables, including gender-based differences in mechanical sensitivity. We tested two variants of the SNI model and found that injuring the tibial nerve alone induces mechanical hypersensitivity, while injuring the common peroneal and sural nerves together does not induce any significant increase in mechanical sensitivity in the territory of the spared tibial nerve. SNI induces a mechanical allodynia-like response in mice and we believe that our improved method of assessment and data analysis will reveal additional internal and external variability factors in models of persistent pain. Use of this model in genetically altered mice should be very effective for determining the mechanisms involved in neuropathic pain.

Antisense knock down of TRPA1, but not TRPM8, alleviates cold hyperalgesia after spinal nerve ligation in rats

Patients with neuropathic pain frequently experience hypersensitivity to cold stimulation. However, the underlying mechanisms of this enhanced sensitivity to cold are not well understood. After partial nerve injury, the transient receptor potential ion channel TRPV1 increases in the intact small dorsal root ganglion (DRG) neurons in several neuropathic pain models. In the present study, we precisely examined the incidence of cold hyperalgesia and the changes of TRPA1 and TRPM8 expression in the L4 and L5 DRG following L5 spinal nerve ligation (SNL), because it is likely that the activation of two distinct populations of TRPA1- and TRPM8-expressing small neurons underlie the sensation of cold. We first confirmed that L5 SNL rats developed cold hyperalgesia for more than 14 days after surgery. In the nearby uninjured L4 DRG, TRPA1 mRNA expression increased in trkA-expressing small-to-medium diameter neurons from the 1st to 14th day after the L5 SNL. This upregulation corresponded well with the development and maintenance of nerve injury-induced cold hyperalgesia of the hind paw. In contrast, there was no change in the expression of the TRPM8 mRNA/protein in the L4 DRG throughout the 2-week time course of the experiment. In the injured L5 DRG, on the other hand, both TRPA1 and TRPM8 expression decreased over 2 weeks after ligation. Furthermore, intrathecal administration of TRPA1, but not TRPM8, antisense oligodeoxynucleotide suppressed the L5 SNL-induced cold hyperalgesia. Our data suggest that increased TRPA1 in uninjured primary afferent neurons may contribute to the exaggerated response to cold observed in the neuropathic pain model.

Changes in vanilloid receptor 1 (TRPV1) expression following lingual nerve injury

We have investigated a possible role for vanilloid receptor 1 (TRPV1), a transducer of noxious stimuli, in the development of neuropathic pain following injury to a peripheral branch of the trigeminal nerve. In nine adult ferrets the left lingual nerve was sectioned and recovery permitted for 3 days, 3 weeks or 3 months (3 ferrets per group). A retrograde tracer, fluorogold, was injected into the damaged nerve to identify associated cell bodies in the trigeminal ganglion. Three further ferrets, receiving only tracer injection, served as uninjured controls. Indirect immunofluorescence for TRPV1 and image analysis was used to quantify the percentage area of staining (PAS) of TRPV1 in the left and right lingual nerves. Additionally, the proportion of fluorogold positive and fluorogold negative cells expressing TRPV1 in the ganglion was determined. TRPV1 expression increased significantly at the injury site of damaged nerves 3 days after injury and this was matched by a reduction in the proportion of fluorogold positive cells expressing TRPV1 in the ganglion. At 3 weeks TRPV1 expression at the injury site was still high, while in the ganglion was significantly greater than in the controls. In the 3-month recovery group TRPV1 expression in both nerve fibres and ganglion cells, was not significantly different from controls and there were no changes in expression in the fluorogold negative cells in the ganglion at any time point studied. These data suggest that after injury there is an increase in the axonal transport of TRPV1 from the cell bodies to the damaged axons and this is followed by an increase in synthesis in the ganglion. These changes in expression may be involved in development of sensory disturbances or dysaesthesia after injury.

BDNF in sensory neurons and chronic pain

Neurotrophic factors, which support neuronal survival and growth during development of the nervous system, have been shown to play significant roles in the transmission of physiologic and pathologic pain. Brain-derived neurotrophic factor (BDNF), synthesized in the primary sensory neurons, is anterogradely transported to the central terminals of the primary afferents in the spinal dorsal horn, where it is involved in the modulation of painful stimuli. In models of inflammatory and neuropathic pain, BDNF synthesis is greatly increased in different populations of dorsal root ganglion (DRG) neurons. Furthermore, it is now known that the activation of mitogen-activated protein kinases occurs in these sensory neurons and contributes to persistent inflammatory and neuropathic pain by regulating BDNF expression. The recent discovery that BDNF upregulation in the DRG and spinal cord contributes to chronic pain hypersensitivity indicates that blocking BDNF in sensory neurons could provide a fruitful strategy for the development of novel analgesics.

Heat and mechanical hyperalgesia in mice model of cancer pain

We developed a mouse model of cancer pain to investigate its underlying mechanisms. SCC-7, squamous cell carcinoma (SCC) derived from C3H mice, was inoculated subcutaneously into either the plantar region or thigh in male C3H/Hej mice. Heat and mechanical sensitivity as well as spontaneous behavior were measured at the plantar surface of the ipsilateral hind paw after the inoculation. Inoculated sites were histologically examined, and the expression of capsaicin receptors (TRPV1) was examined in the dorsal root ganglia (DRG) to clarify their potential contribution to pain sensitivity. Inoculation of cancer cells induced marked heat hyperalgesia and mechanical allodynia in the ipsilateral hind paw for two weeks in both plantar- and thigh-inoculation models. Signs of spontaneous pain, such as lifting, licking and flinching of the paw were also observed. However, further growth of the tumor reversed the mechanical allodynia in both plantar- and thigh-inoculation models, and heat hyperalgesia in thigh-inoculation models. Histologically, no infiltration of the tumor cells into the nerve was observed. TRPV1 immunoreactive cells increased in the L5 DRG on day 7, but returned to the control level on day 15 post-inoculation. Intraperitoneal administration of the competitive TRPV1 antagonist capsazepine inhibited hyperalgesia induced by tumor cell-inoculation in either plantar- or thigh-inoculated animals. This study indicated that inoculation of SCC resulted in spontaneous pain, heat hyperalgesia and mechanical allodynia. The altered expression of TRPV1 in the DRG may be involved in behavioral changes in this model.

Altered expression and function of sodium channels in large DRG neurons and myelinated A-fibers in early diabetic neuropathy in the rat

Differential alterations of sodium channels in small nociceptive C-fiber DRG neurons have been implicated in diabetic neuropathy. In this study, we investigated sodium currents and the expression of sodium channels in large A-fiber DRG neurons in diabetic rats. Compared with controls, large neurons from diabetic rats showed significant increases in both total and TTX-S sodium currents and approximately -15mV shifts in their voltage-dependent activation kinetics. TTX-R Na(v)1.9 sodium current was also significantly increased, whereas no alteration of TTX-R Na(v)1.8 current was observed in neurons from diabetic rats. Sodium current induced by fast- or slow-voltage ramps increased markedly in the diabetic neurons as well. Immunofluorescence studies showed significant increases in the levels and number of large DRG neurons from diabetic rats expressing Na(v)1.2, Na(v)1.3, Na(v)1.7, and Na(v)1.9 whereas Na(v)1.8 decreased. We also observed a decrease in the number of nodes of Ranvier expressing Na(v)1.8 and in staining intensity of Na(v)1.6 and Na(v)1.8 at nodes. Our results suggest that alterations of sodium channels occur in large DRG neurons and A-fibers, and may play an important role in diabetic sensory neuropathy.

Hyperexcitability of axotomized and neighboring unaxotomized sensory neurons is reduced days after perineural clonidine at the site of injury

Hyperexcitability after peripheral nerve injury occurs in axotomized and neighboring unaxotomized dorsal root ganglion (DRG) neurons and contributes to hypersensitivity. Previous studies have focused on proximal nerve injury and have not examined unaxotomized neurons innervating the site of sensory testing. The current study used a distal nerve injury (partial sciatic nerve ligation [PSNL]), and identified, using fluorescent tracers, axotomized and unaxotomized neurons innervating the site of hypersensitivity. We hypothesized that reduced hypersensitivity after perineural clonidine was associated with reduced hyperexcitability of DRG neurons. Rats underwent sham or PSNL surgery, followed 2 wk later by a single injection at the injury site of clonidine or saline. PSNL, but not sham surgery, reduced hindpaw mechanical withdrawal threshold, and clonidine, but not saline, partially reversed this effect 3 days after injection. Intracellular recording of neurons in whole DRG demonstrated similar changes in membrane properties and excitability in unaxotomized and axotomized neurons after PSNL compared with sham surgery, primarily depolarized resting membrane potential, reduced rheobase, presence of oscillations, and capability to fire repetitively. Most of these changes were present in small-, medium-, and large-diameter neurons. Perineural clonidine 3 days later significantly reversed many of these effects, whereas saline was without effect. We speculate that perineural clonidine reduces signals, likely proinflammatory cytokines and prostaglandins produced during Wallerian degeneration after nerve injury, which drive changes in ion channel expression in DRG somata leading to hyperexcitability and hypersensitivity.

Regulation of calcitonin gene-related peptide and TRPV1 in a rat model of osteoarthritis

Pain in osteoarthritis (OA) remains an intractable problem in a majority of patients, with many of the commonly prescribed analgesics providing insufficient relief and considerable side effects. However, the structural or mechanistic cause of OA pain is still unknown. Animal models to address this issue have only recently been established, with much of the research to date focused on tissue pathology rather than pain. We have previously compared the surgically induced partial medial meniscectomy and chemically induced intra-articular iodoacetate injection rat models of OA in the rat, with reference to pain behaviour. This demonstrated relevant tissue pathology in both models, but greater evidence of pain related behaviour in the iodoacetate induced model. Here we further investigate the iodoacetate model using Fast Blue backlabelling from the articular joint space to identify the cell bodies of primary sensory afferents from the knee at the L4 dorsal root ganglion. Expression of calcitonin gene-related peptide (CGRP) and the vanilloid receptor TRPV1 was quantified in these backlabelled cells and was enriched in the knee afferents in all animals studied, compared to the expression in neurons across the whole dorsal root ganglia (DRG). Analysis of the backlabelled population in the osteoarthritis model and controls showed an increase in both CGRP and TRPV1 expression in the iodoacetate model compared with control animals. Therefore, there is a potential role for CGRP and TRPV1 in the manifestation of pain behaviour accompanied by OA changes in the knee in the iodoacetate induced model.

Extensive co-localization and heteromultimer formation of the vanilloid receptor-like protein TRPV2 and the capsaicin receptor TRPV1 in the adult rat cerebral cortex

The capsaicin receptor TRPV1, a member of the transient receptor potential (TRP) family of calcium-selective ion channels, responds to noxious stimuli and is predominantly expressed in nociceptive neurons. The homologous receptor TRPV2 shows wide tissue distribution including some sensory neurons, where it is proposed to function as a heat sensor or a growth-factor-activated channel. Members of the TRP family of channels have been shown to interact, resulting in hybrid channels with new properties. We examined the possibility of multimer formation between TRPV1 and TRPV2, using biochemical techniques. We present evidence that TRPV1 and TRPV2 can heteromultimerize efficiently in vitro. By using immunohistochemistry we detected co-localization of the two receptors in rat dorsal root ganglia. TRPC4 transcripts are also detected in capsaicin-sensitive dorsal root ganglia neurons. We extended the search for TRPV1-TRPV2 co-localization in the brain, where we detected extensive co-expression of the two receptors in the IV, V and VI layer neurons of the adult rat cerebral cortex. Co-immunoprecipitation experiments confirmed the interaction of the two receptors in vivo, indicating heteromultimer formation in native tissue. Formation of heteromultimers between vanilloid receptors may increase the functional diversity of this receptor family.

Morphological and electrophysiological changes in mouse dorsal root ganglia after partial colonic obstruction

There is evidence that sensitization of neurons in dorsal root ganglia (DRG) may contribute to pain induced by intestinal injury. We hypothesized that obstruction-induced pain is related to changes in DRG neurons and satellite glial cells (SGCs). In this study, partial colonic obstruction was induced by ligation. The neurons projecting to the colon were traced by an injection of 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate into the colon wall. The electrophysiological properties of DRG neurons were determined using intracellular electrodes. Dye coupling was examined with an intracellular injection of Lucifer yellow (LY). Morphological changes in the colon and DRG were examined. Pain was assessed with von Frey hairs. Partial colonic obstruction caused the following changes. First, coupling between SGCs enveloping different neurons increased 18-fold when LY was injected into SGCs near neurons projecting to the colon. Second, neurons were not coupled to other neurons or SGCs. Third, the firing threshold of neurons projecting to the colon decreased by more than 40% (P < 0.01), and the resting potential was more positive by 4-6 mV (P < 0.05). Finally, the number of neurons displaying spontaneous spikes increased eightfold, and the number of neurons with subthreshold voltage oscillations increased over threefold. These changes are consistent with augmented neuronal excitability. The pain threshold to abdominal stimulation decreased by 70.2%. Inflammatory responses were found in the colon wall. We conclude that obstruction increased neuronal excitability, which is likely to be a major factor in the pain behavior observed. The augmented dye coupling between glial cells may contribute to the neuronal hyperexcitability.

Inhibitory effects of CB1 and CB2 receptor agonists on responses of DRG neurons and dorsal horn neurons in neuropathic rats

Cannabinoid 2 (CB2) receptor mediated antinociception and increased levels of spinal CB2 receptor mRNA are reported in neuropathic Sprague-Dawley rats. The aim of this study was to provide functional evidence for a role of peripheral, vs. spinal, CB2 and cannabinoid 1 (CB1) receptors in neuropathic rats. Effects of the CB2 receptor agonist, JWH-133, and the CB1 receptor agonist, arachidonyl-2-chloroethylamide (ACEA), on primary afferent fibres were determined by calcium imaging studies of adult dorsal root ganglion (DRG) neurons taken from neuropathic and sham-operated rats. Capsaicin (100 nm) increased [Ca2+]i in DRG neurons from sham and neuropathic rats. JWH-133 (3 microm) or ACEA (1 microm) significantly (P<0.001) attenuated capsaicin-evoked calcium responses in DRG neurons in neuropathic and sham-operated rats. The CB2 receptor antagonist, SR144528, (1 microm) significantly inhibited the effects of JWH-133. Effects of ACEA were significantly inhibited by the CB1 receptor antagonist SR141716A (1 microm). In vivo experiments evaluated the effects of spinal administration of JWH-133 (8-486 ng/50 microL) and ACEA (0.005-500 ng/50 microL) on mechanically evoked responses of neuropathic and sham-operated rats. Spinal JWH-133 attenuated mechanically evoked responses of spinal neurons in neuropathic, but not sham-operated rats. These inhibitory effects were blocked by SR144528 (0.001 microg/50 microL). Spinal ACEA inhibited mechanically evoked responses of neuropathic and sham-operated rats, these effects were blocked by SR141716A (0.01 microg/50 microL). Our data provide evidence for a functional role of CB2, as well as CB1 receptors on DRG neurons in sham and neuropathic rats. At the level of the spinal cord, CB2 receptors have inhibitory effects in neuropathic, but not sham-operated rats suggesting that spinal CB2 may be an important analgesic target.

Inhibition of peripheral vanilloid TRPV1 receptors reduces noxious heat-evoked responses of dorsal horn neurons in naïve, carrageenan-inflamed and neuropathic rats

The vanilloid TRPV1 receptor, present on primary afferent fibres, is activated by noxious heat, low pH and endogenous vanilloids. Changes in the function or distribution of TRPV1 receptors may play an important role in pain induced by inflammation or neuropathy. The aim of the present study was to evaluate the role of peripheral TRPV1 receptors in thermal nociception in rat models of inflammatory and neuropathic pain. Here, we have determined the effects of peripheral administration of the potent TRPV1 receptor antagonist iodoresiniferatoxin (IRTX) on noxious heat (45 degrees C)-evoked responses of spinal wide dynamic range (WDR) neurons in naïve, carrageenan-inflamed, sham-operated and L5/6 spinal nerve-ligated (SNL) anaesthetized rats in vivo. In addition, effects of peripheral administration of IRTX on mechanically evoked responses of WDR neurons were determined in sham-operated and SNL rats. Carrageenan inflammation significantly (P<0.05) increased the 45 degrees C-evoked responses of WDR neurons. Intraplantar injection of the lower dose of IRTX (0.004 microg) inhibited (P<0.05) 45 degrees C-evoked responses of WDR neurons in carrageenan-inflamed, but not in naïve, rats. The higher dose of IRTX (0.4 microg) significantly (P<0.05) inhibited 45 degrees C-evoked responses in both inflamed and naïve rats. In sham-operated and SNL rats, IRTX (0.004 and 0.4 microg) significantly (P<0.05) inhibited 45 degrees C-evoked, but had no effect on mechanically evoked responses of WDR neurons. These data support the role of peripheral TRPV1 receptors in noxious thermal transmission in naïve, inflamed and neuropathic rats, and suggest that there is an increased functional contribution of peripheral TRPV1 receptors following acute inflammation.

Systemic and site-specific effects of A-425619, a selective TRPV1 receptor antagonist, on wide dynamic range neurons in CFA-treated and uninjured rats

Systemic administration of A-425619, a potent and selective TRPV1 receptor antagonist that does not readily enter the CNS, produces antinociception in several rat models of pathological nociception, including complete Freund's adjuvant (CFA)-induced thermal hyperalgesia. To further understand the peripheral mechanisms of TRPV1-related antinociception, we examined the effects of systemic and site-specific injections of A-425619 on evoked and spontaneous firing of spinal wide dynamic range (WDR) neurons in uninjured rats and rats with peripheral inflammation (CFA; 48 h). In uninjured rats, capsaicin-evoked (1 microg) WDR activity was completely blocked by intraplantar administration of A-425619 (3-100 nmol). Systemic injection of A-425619 (3-30 micromol/kg, iv) reduced WDR responses to thermal stimulation in both CFA-inflamed (47 degrees C) and uninjured (52 degrees C) rats. However, the efficacy of A-425619 to attenuate thermal-evoked WDR activity was significantly greater (P < 0.01) in CFA-treated rats. Both intradorsal root ganglion (DRG; L5; 20 nmol) and intraplantar (30-300 nmol) injection of A-425619 reduced WDR responses to thermal stimulation. While the effectiveness of A-425619 was similar between CFA-inflamed and uninjured rats after intraplantar injection, the effects of A-425619 after intra-DRG injection were enhanced in the inflamed rats (compared with the uninjured rats). Spontaneous WDR discharges were unaltered by systemic or site-specific injections of A-425619. Thus noxious thermal stimulation triggers the transmission of TRPV1-related signals to spinal WDR neurons in both inflamed and uninjured animals. The apparent increase in TRPV1 signaling to WDR neurons after injury may be the result of changes to the distribution/sensitization of peripheral TRPV1 receptors.

Therapeutic potential of cannabinoid receptor agonists as analgesic agents

Increasing data emerging from controlled clinical trials support an analgesic activity of cannabinoids. However, the psychotropic side effects associated with tetrahydrocannabinol or synthetic derivatives essentially puts a brake on their use, possibly limiting the degree of analgesia that can be achieved as well as providing regulatory hurdles. Animal studies show that although these side effects are mediated via central cannabinoid type 1 (CB(1)) receptors, the analgesic activity in chronic pain states may be mediated via spinal CB(1) and potentially CB(2) receptors, as well as peripheral CB(1) and CB2 receptors on sensory nerves or immune cells. The design of novel compounds that either specifically target peripheral CB(1) receptors or display high selectivity for CB(2) receptors may offer avenues for harnessing the analgesic effect of CB receptor agonists while avoiding the central adverse events seen with cannabinoid structures. Clinical trials with such compounds are required to determine whether either approach can provide the level of analgesia required to fulfil the unmet medical need left by current therapies for chronic pain.

Involvement of an increased spinal TRPV1 sensitization through its up-regulation in mechanical allodynia of CCI rats

Vanilloid receptor 1 (TRPV1) antagonists are known to attenuate the neuropathic pain symptoms in peripheral nerve injury models, but the mechanism(s) of their effect remains unclear. At the same time, the role of spinal TRPV1 in pain transduction system has not been fully understood. In this study, the role of spinal TRPV1 in mechanical allodynia in rat chronic constriction injury (CCI) model was investigated. Intrathecal administration of a selective TRPV1 antagonist, N-(4-tertiarybutylphenyl)-4-(3-cholorphyridin-2-yl)tetrahydropryazine-1(2H)-carbox-amide (BCTC) significantly attenuated mechanical allodynia in CCI rats at 100 and 300 nmol. In vitro, BCTC inhibited capsaicin (300 nM)-induced releases of calcitonin gene-related peptide-like immunoreactivity (CGRP-LI) and substance P-like immunoreactivity (SP-LI) from the rat spinal cord slice preparations with IC(50)s of 37.0 and 36.0 nM, respectively, confirming that BCTC potently inhibits TRPV1 function in the rat spinal cord. TRPV1 expression levels in the spinal cord following CCI were quantified in by Western blot analysis. TRPV1 protein levels were significantly increased in the ipsilateral side of the lumbar spinal cord at 7 and 14 days following CCI surgery, but not in the contralateral side. Furthermore, capsaicin (300 nM)-evoked release of CGRP-LI was significantly higher in the ipsilateral spinal cord of CCI rats (14 days after surgery) than that of sham-operated rats. These findings suggest that an increased sensitization of the spinal TRPV1 through its up-regulation is involved in the development and/or maintenance of mechanical allodynia in rat CCI model.

The role of the vanilloid (capsaicin) receptor (TRPV1) in physiology and pathology

The cloning of the vanilloid receptor 1 opened a floodgate for discoveries regarding the function of this complex molecule. It has been found that, in addition to heat, protons and vanilloids, this receptor also responds to various endogenous ligands. Furthermore, it has been also emerged that, through associations with other molecules, the vanilloid receptor 1 plays an important role in the integration of various stimuli and modulation of cellular excitability. Although, originally, the vanilloid receptor 1 was associated with nociceptive primary afferent fibres, it has been gradually revealed that it is broadly expressed in the brain, epidermis and visceral cells. The expression pattern of the vanilloid receptor 1 indicates that it could be involved in various physiological functions and in the pathomechanisms of diverse diseases. Here, we summarise the molecular, pharmacological and physiological characteristics, and putative functions, of the vanilloid receptor 1, and discuss the therapeutic potential of this molecule.

Role of mitogen-activated protein kinase activation in injured and intact primary afferent neurons for mechanical and heat hypersensitivity after spinal nerve ligation

To investigate whether activation of mitogen-activated protein kinase (MAPK) in damaged and/or undamaged primary afferents participates in neuropathic pain after partial nerve injury, we examined the phosphorylation of extracellular signal-regulated protein kinase (ERK), p38 MAPK, and c-Jun N-terminal kinase (JNK) in the L4 and L5 dorsal root ganglion (DRG) in the L5 spinal nerve ligation (SNL) model. We first confirmed, using activating transcription factor 3 and neuropeptide Y immunoreactivity, that virtually all L4 DRG neurons are spared from axotomy in this model. In the injured L5 DRG, the L5 SNL induced the activation of ERK, p38, and JNK in different populations of DRG neurons. In contrast, in the uninjured L4 DRG, the L5 SNL induced only p38 activation in tyrosine kinase A-expressing small- to medium-diameter neurons. Intrathecal ERK, p38, and JNK inhibitor infusions reversed SNL-induced mechanical allodynia, whereas only p38 inhibitor application attenuated SNL-induced thermal hyperalgesia. Furthermore, the L5 dorsal rhizotomy did not prevent SNL-induced thermal hyperalgesia. We therefore hypothesized that p38 activation in the uninjured L4 DRG might be involved in the development of heat hypersensitivity in the L5 SNL model. In fact, the treatment of the p38 inhibitor and also anti-nerve growth factor reduced SNL-induced upregulation of brain-derived neurotrophic factor and transient receptor potential vanilloid type 1 expression in the L4 DRG. Together, our results demonstrate that the L5 SNL induces differential activation of MAPK in injured and uninjured DRG neurons and, furthermore, that MAPK activation in the primary afferents may participate in generating pain hypersensitivity after partial nerve injury.

Expression of the vanilloid receptor TRPV1 in rat dorsal root ganglion neurons supports different roles of the receptor in visceral and cutaneous afferents

A combination of tracing and multiple color immunofluorescence revealed that 69% of rat dorsal root ganglion (DRG) neurons innervating the urinary bladder expressed the vanilloid receptor TRPV1. In contrast, only 32% of DRG neurons innervating the skin of the L6 dermatome expressed TRPV1. However, a similar fraction of visceral (60-62%) and of cutaneous (59-60%) TRPV1-positive DRG neurons expressed the peptidergic markers substance P and calcitonin gene-related peptide, while the fraction of TRPV1-positive neurons that was labeled by the non-peptidergic marker Isolectin B4 was 58% for cutaneous and only 24% for visceral afferents. These results underscore differences of expression of functional markers in visceral and cutaneous afferents and support different mechanisms of activation of TRPV1 in viscera and in skin.

A-425619 [1-isoquinolin-5-yl-3-(4-trifluoromethyl-benzyl)-urea], a novel transient receptor potential type V1 receptor antagonist, relieves pathophysiological pain associated with inflammation and tissue injury in rats

The vanilloid receptor 1 (VR1, TRPV1), which is a member of the transient receptor potential (TRP) superfamily, is highly localized on peripheral and central processes of nociceptive afferent fibers. Activation of TRPV1 contributes to the pronociceptive effects of capsaicin, protons, heat, and various endogenous lipid agonists such as anandamide and N-arachidonoyl-dopamine. A-425619 [1-isoquinolin-5-yl-3-(4-trifluoromethyl-benzyl)urea] is a novel potent and selective antagonist at both human and rat TRPV1 receptors. In vivo, A-425619 dose dependently reduced capsaicin-induced mechanical hyperalgesia (ED50 = 45 micromol/kg p.o.). A-425619 was also effective in models of inflammatory pain and postoperative pain. A-425619 potently reduced complete Freund's adjuvant-induced chronic inflammatory pain after oral administration (ED50 = 40 micromol/kg p.o.) and was also effective after either i.t. administration or local injection into the inflamed paw. Furthermore, A-425619 maintained efficacy in the postoperative pain model after twice daily dosing p.o. for 5 days. A-425619 also showed partial efficacy in models of neuropathic pain. A-425619 did not alter motor performance at the highest dose tested (300 micromol/kg p.o.). Taken together, the present data indicate that A-425619, a potent and selective antagonist of TRPV1 receptors, effectively relieves acute and chronic inflammatory pain and postoperative pain.

Pain TRPs

Transient receptor potential (TRP) ion channels are molecular gateways in sensory systems, an interface between the environment and the nervous system. Several TRPs transduce thermal, chemical, and mechanical stimuli into inward currents, an essential first step for eliciting thermal and pain sensations. Precise regulation of the expression, localization, and function of the TRP channels is crucial for their sensory role in nociceptor terminals, particularly after inflammation, when they contribute to pain hypersensitivity by undergoing changes in translation and trafficking as well as diverse posttranslational modifications.

NGF and GDNF differentially regulate TRPV1 expression that contributes to development of inflammatory thermal hyperalgesia

The transient receptor potential ion channel, TRPV1 plays an essential role in the development of inflammatory thermal hyperalgesia. We investigated the dependence of inflammatory TRPV1 induction on neurotrophic factor. Rat dorsal root ganglia (DRG) neurons were classified according to immunostaining for trk-A and IB4 and the effects of antibodies against NGF or GDNF on TRPV1 expression within the groups were then analysed by immunohistochemical means. The data were compared with the time course of trophic factor expression and the effects of their antibodies on thermal hyperalgesia against radiant heat after inflammation. Although the levels of both NGF and GDNF were increased by inflammation, NGF rapidly and transiently increased whereas GDNF increased gradually over a period of approximately one week. TRPV1 expression was increased within both trk-A positive and IB4 positive neurons after inflammation. Increased TRPV1 expression within trk-A positive neurons was prevented by anti-NGF but not by anti-GDNF, whereas TRPV1 induction within the IB4 positive group was blocked by anti-GDNF but not by anti-NGF. Both antibodies prevented the short latency of withdrawing an inflamed paw from radiant heat. These results suggest that inflammation differentially increases both NGF and GDNF, which facilitate TRPV1 expression within distinctive neurons to induce thermal hyperalgesia.

Changes in transient receptor potential cation channel superfamily V (TRPV) mRNA expression in the mouse inner ear ganglia after kanamycin challenge

The transient receptor potential cation channel subfamily V (TRPV) is a non-specific cation ion channel receptor family that is gated by heat, protons, low extracellular osmolarity and arachidonic acid derivatives. Since some of these endogenous agonists of TRPV receptors are reactive oxygen intermediates produced by lipoxygenases, it has been hypothesized that some members of the TRPV family may respond to challenges by reactive oxygen species. This study used real-time PCR to quantitatively track changes in TRPV1-4 mRNA expression in the spiral, vestibular, and trigeminal ganglia and the kidney from kanamycin (KM)-treated mice. TRPV1, TRPV2, TRPV3 and TRPV4 mRNAs were expressed in spiral and vestibular ganglia, and TRPV2 and TRPV1 mRNAs were most predominant in control mice. After KM (700 mg/kg s.c. b.i.d., 14 days), TRPV1 mRNA and protein expression were significantly up-regulated both in the spiral and vestibular ganglia, but expression was unaffected in the trigeminal ganglion and kidney. Real-time PCR also demonstrated a significant down-regulation in TRPV4 mRNA expression in the inner ear ganglia and kidney after KM treatment. All these mRNA and protein expression changes were eliminated by simultaneous administration of dihydroxybenzoate (300 mg/kg s.c. b.i.d., 14 days), an anti-oxidant that blocks KM ototoxicity. It is proposed that up-regulated TRPV1 expression during KM exposure may promote ganglion cell survival by contributing to neuronal depolarization, with KM-induced tinnitus and dizziness as consequences.

Medium and large injured dorsal root ganglion cells increase TRPV-1, accompanied by increased α2C-adrenoceptor co-expression and functional inhibition by clonidine

Some electrophysiologic studies demonstrate new, excitatory alpha2-adrenoceptors on peripheral nociceptors and their dorsal root ganglion (DRG) cell bodies after nerve injury, yet administration of alpha2-adrenoceptor agonists at these sites reduces hypersensitivity rather than worsens it. Since TRPV-1 expressing nociceptor afferents are important in many pain states, we examined the expression of this channel and its co-expression with alpha2C-adrenoceptors in injured DRG cell bodies and the ability of alpha2-adrenoceptors to inhibit responses to stimulation. Rats underwent tight ligation of the left L5 and L6 spinal nerves, followed by behavioral testing, removal of L5 and L6 DRGs, and either immunostaining for TRPV-1 channels and alpha2C-adrenoceptors or intracellular calcium videomicroscopy in response to electrical field stimulation before and after perfusion with clonidine and capsaicin. Spinal nerve ligation produced tactile allodynia. In normal and sham controls, about one-third of DRG neurons were TRPV-1-immunoreactive (IR), one half were alpha2C-adrenoceptor-IR and one-fourth co-expressed both. After nerve ligation there was a reduction in the number of small, strongly TRPV-1-IR or alpha2C-adrenoceptor-IR neurons, but an increase in medium and large, lightly stained cells and in their co-expression. The proportion of clonidine inhibited cells which responded to capsaicin increased 5 fold after injury. We conclude that TRPV-1 and alpha2C-adrenoceptors are up-regulated in some injured medium and large size neurons after nerve ligation. Increased co-expression by immunocytochemistry, and increased proportion of cells inhibited by clonidine and expressing functional TRPV-1 channels suggest that these cells may play an important role in the analgesic effects of alpha2-adrenoceptor agonists in neuropathic pain.

Neurotrophin-3 suppresses thermal hyperalgesia associated with neuropathic pain and attenuates transient receptor potential vanilloid receptor-1 expression in adult sensory neurons

Neurotrophin-3 (NT-3) negatively modulates nerve growth factor (NGF) receptor expression and associated nociceptive phenotype in intact neurons, suggesting a beneficial role in treating aspects of neuropathic pain mediated by NGF. We report that NT-3 is effective at suppressing thermal hyperalgesia associated with chronic constriction injury (CCI); however, NT-3 does not alter the mechanical hypersensitivity that also develops with CCI. Thermal hyperalgesia is critically linked to expression and activation of the capsaicin receptor, transient receptor potential vanilloid receptor-1 (TRPV1). Thus, its modulation by NT-3 after CCI was examined. CCI results in elevated TRPV1 expression at both the mRNA and protein levels in predominantly small-to-medium neurons, with the percentage of neurons expressing TRPV1 remaining unchanged at approximately 56%. Attenuation of thermal hyperalgesia mediated by NT-3 correlates with decreased TRPV1 expression such that only approximately 26% of neurons ipsilateral to CCI expressed detectable TRPV1 mRNA. NT-3 effected a decrease in expression of the activated component of the signaling pathway linked to regulation of TRPV1 expression, phospho-p38 MAPK (Ji et al., 2002), in neurons ipsilateral to CCI. Exogenous NT-3 could both prevent the onset of thermal hyperalgesia and reverse established thermal hyperalgesia and elevated TRPV1 expression 1 week after CCI. Continuous infusion is required for suppression of both thermal hyperalgesia and TRPV1 expression, because removal of NT-3 resulted in a prompt reestablishment of the hyperalgesic state and corresponding CCI-associated TRPV1 phenotype. In conclusion, although NGF drives inflammation-associated thermal hyperalgesia via its regulation of TRPV1 expression, NT-3 is now identified as a potent negative modulator of this state.

Chemical mediators enhance the excitability of unmyelinated sensory axons in normal and injured peripheral nerve of the rat

Ectopic excitation of nociceptive axons by chemical mediators may contribute to symptoms in neuropathic pain. In this study, we have measured the excitability of unmyelinated rat C-fiber axons in isolated segments of sural nerves under different experimental conditions. (1) We demonstrate in normal rats that several mediators including ATP, serotonin (5-HT), 1-(3-chlorophenyl)biguanide (5-HT3 receptor agonist), norepinephrine, acetylcholine and capsaicin alter electrophysiological parameters of C-fibers which indicate an increase of axonal excitability. Other mediators such as histamine, glutamate, prostaglandin E(2) and the cytokines tumor necrosis factor alpha, interleukin-1beta and interleukin-6 did not produce such effects. (2) The effects of several mediators were tested after peripheral nerve injury (partial ligation or spared nerve injury). Sural nerves from such animals did not show significant changes when compared with controls. (3) We tested whether the effects of chemical mediators on axonal excitability are due to actions on the sensory C-fiber afferents or the postganglionic sympathetic efferents. In order to distinguish these effects, we performed surgical sympathectomy of the lumbar sympathetic chain, including the L3, L4 and L5 ganglia. Sympathectomy did not markedly influence the effects of mediators on axonal excitability (except that the norepinephrine effect was significantly diminished). In conclusion, our data suggest a constitutive rather than inducible expression of axonal receptors for some chemical mediators on the axonal membrane of unmyelinated fibers. Most of the changes in axonal excitability take place in sensory C-fiber afferents rather than in postganglionic sympathetic efferents. Thus, it is possible that certain immune and glial cell mediators released in or around the nerve following injury or inflammation influence the excitability of intact nociceptive fibers. This mechanism could contribute to ectopic excitation of axons in neuropathic pain.

Discovery of Potent, Orally Available Vanilloid Receptor-1 Antagonists. Structure−Activity Relationship of N-Aryl Cinnamides

The vanilloid receptor-1 (TRPV1 or VR1) is a member of the transient receptor potential (TRP) family of ion channels and plays a role in regulating the function of sensory nerves. A growing body of evidence demonstrates the therapeutic potential of TRPV1 modulators, particularly in the management of pain. As a result of our screening efforts, we identified (E)-3-(4-tert-butylphenyl)-N-(2,3-dihydrobenzo[b][1,4]dioxin-6-yl)acrylamide (1), an antagonist that blocks the capsaicin-induced and pH-induced uptake of (45)Ca(2+) in TRPV1-expressing Chinese hamster ovary cells with IC(50) values of 17 +/- 5 and 150 +/- 80 nM, respectively. In this report, we describe the synthesis and structure-activity relationship of a series of N-aryl cinnamides, the most potent of which (49a and 49b) exhibit good oral bioavailability in rats (F(oral) = 39% and 17%, respectively).

Thermosensation and pain

We feel a wide range of temperatures spanning from cold to heat. Within this range, temperatures over about 43 degrees C and below about 15 degrees C evoke not only a thermal sensation, but also a feeling of pain. In mammals, six thermosensitive ion channels have been reported, all of which belong to the TRP (transient receptor potential) superfamily. These include TRPV1 (VR1), TRPV2 (VRL-1), TRPV3, TRPV4, TRPM8 (CMR1), and TRPA1 (ANKTM1). These channels exhibit distinct thermal activation thresholds (>43 degrees C for TRPV1, >52 degrees C for TRPV2, > approximately 34-38 degrees C for TRPV3, > approximately 27-35 degrees C for TRPV4, < approximately 25-28 degrees C for TRPM8 and <17 degrees C for TRPA1), and are expressed in primary sensory neurons as well as other tissues. The involvement of TRPV1 in thermal nociception has been demonstrated by multiple methods, including the analysis of TRPV1-deficient mice. TRPV2, TRPM8, and TRPA1 are also very likely to be involved in thermal nociception, because their activation thresholds are within the noxious range of temperatures.

Loss of functional neuronal nicotinic receptors in dorsal root ganglion neurons in a rat model of neuropathic pain

Recent evidence has suggested that the anti-allodynic effect of neuronal acetylcholine receptor (nAChR) agonists may have a peripheral component [L.E. Rueter, K.L. Kohlhaas, P. Curzon, C.S. Surowy, M.D. Meyer, Peripheral and central sites of action for A-85380 in the spinal nerve ligation model of neuropathic pain, Pain 103 (2003) 269-276]. In further studies of the peripheral anti-allodynic mechanisms of nAChR agonists, we investigated the function of nAChRs in acutely isolated dorsal root ganglion (DRG) neurons from allodynic [L5-L6 spinal nerve ligation (SNL)] and naive adult rats. Following determination of cell diameter and membrane capacitance, responses to rapid applications of nAChR agonists were recorded under whole cell patch clamp. nAChR inward currents were observed in approximately 60% of naive neurons, across small, medium, and large diameter cells. Evoked nAChR currents could be clustered into three broad classes: fast transient, biphasic, and slow desensitizing currents, consistent with multiple subtypes of nAChR expressed in DRG [J.R. Genzen, W. Van Cleve, D.S. McGehee, Dorsal root ganglion neurons express multiple nicotinic acetylcholine receptor subtypes, J. Neurophysiol. 86 (2001) 1773-1782]. In contrast, in neurons from allodynic animals, the occurrence and amplitude of responses to nAChR agonists were significantly reduced. Reduced responsiveness to nAChR agonists covered the range of DRG neuron sizes. The decrease in the responsiveness to nAChR agonists was not seen in neighboring uninjured L4 neurons. The significant decrease in the number of cells with nAChR agonist responses, compounded with the significant decrease in response amplitude, indicates that there is a marked down regulation of functional nAChRs in DRG somata associated with SNL.

Cannabinoids blocks tactile allodynia in diabetic mice without attenuation of its antinociceptive effect

Diabetic neuropathic pain is one of the most commonly encountered neuropathic pain syndromes. However, the treatment of diabetic neuropathic pain is challenging because of partial effectiveness of currently available pain relievers. It is well known that diabetic animals are less sensitive to the analgesic effect of morphine, and opioids are found to be ineffective in the treatment of diabetic neuropathic pain. Cannabinoids are promising drugs and they share a similar pharmacological properties with opioids. It has been reported that cannabinoid analgesia remained intact and to be effective in some models of nerve injury. Thus, we investigated antinociceptive efficacy and the effects of cannabinoids on behavioral sign of diabetic neuropathic pain in diabetic mice by using WIN 55, 212-2, a cannabinoid receptor agonist. Diabetes was induced by streptozotocin (STZ) (200mg/kg) and animals were tested between 45 and 60 days after onset of diabetes. Antinociception was assessed using the radiant tail-flick test. Mechanical and thermal sensitivities were measured by Von Frey filaments and hot-plate test, respectively. Tactile allodynia, but not thermal hyperalgesia developed in diabetic mice. Systemic WIN 55, 212-2 (1, 5 and 10mg/kg) produced a dose-dependent antinociception both in diabetic and control mice. WIN 55, 212-2-induced antinociception were found to be similar in diabetic mice when compared to controls suggesting efficacy of cannabinoid antinociception was not diminished in diabetic mice. WIN 55, 212-2 also produced a dose-dependent antiallodynic effect in diabetic mice. This study suggests that cannabinoids have a potential beneficial effect on experimental diabetic neuropathic pain.