Effects of the capsaicin analogue resiniferatoxin on spinal nociceptive mechanisms in the rat: behavioral, electrophysiological and in situ hybridization studies

Transient receptor potential vanilloid 1: A potential therapeutic target for the treatment of osteoarthritis and rheumatoid arthritis

This study aims to determine the molecular mechanisms and analgesic effects of transient receptor potential vanilloid 1 (TRPV1) in the treatments of osteoarthritis (OA) and rheumatoid arthritis (RA). We summarize and analyse current studies regarding the biological functions and mechanisms of TRPV1 in arthritis. We search and analyse the related literature in Google Scholar, Web of Science and PubMed databases from inception to September 2023 through the multi-combination of keywords like 'TRPV1', 'ion channel', 'osteoarthritis', 'rheumatoid arthritis' and 'pain'. TRPV1 plays a crucial role in regulating downstream gene expression and maintaining cellular function and homeostasis, especially in chondrocytes, synovial fibroblasts, macrophages and osteoclasts. In addition, TRPV1 is located in sensory nerve endings and plays an important role in nerve sensitization, defunctionalization or central sensitization. TRPV1 is a non-selective cation channel protein. Extensive evidence in recent years has established the significant involvement of TRPV1 in the development of arthritis pain and inflammation, positioning it as a promising therapeutic target for arthritis. TRPV1 likely represents a feasible therapeutic target for the treatment of OA and RA.

Neuroanatomical characterization of the G protein-coupled receptor activity evoked by galanin-related ligands

Galanin neuropeptide is distributed throughout the mammalian nervous system modulating a plethora of diverse physiological functions, including nociception, cognition and neuroendocrine regulation. The regulation of the galaninergic system is an interesting approach for the treatment of different diseases associated to those systems. Nevertheless, the pharmacological selectivity and activities of some galanin receptor (GalR) ligands are still in discussion and seem to depend on the dose, the receptor subtype and the second messengers to which they are coupled at different brain areas. The activity of different GalR ligands on G_i/o proteins, was evaluated by the guanosine 5'-(γ-[³⁵S]thio)triphosphate ([³⁵S]GTPγS) autoradiography in vitro assay applied to rat brain tissue slices in the presence of galanin, M15, M35, M40, gal(2-11) or galnon. The enhancement of the [³⁵S]GTPγS binding induced by the chimerical peptides M15, M35 and M40 was similar to that produced by Gal in those brain areas showing the highest stimulations, such as dorsal part of the olfactory nucleus and ventral subiculum. In contrast to these peptides, using gal(2-11) no effect was measured on G_i/o protein coupling in areas of the rat brain with high GalR₁ density such as posterior hypothalamic nucleus and amygdala, indicating low selectivity for GalR₁ receptors. The effects evoked by the non-peptide ligand, galnon, were different from those induced by galanin, behaving as agonist or antagonist depending on the brain area, but the stimulations were always blocked by M35. Thus, the activity of most used GalR ligands on G_i/o protein mediated signalling is complex and depends on the brain area. More selective and potent GalR ligands are necessary to develop new treatments aimed to modulate the galaninergic system.

A New Gal in Town: A Systematic Review of the Role of Galanin and Its Receptors in Experimental Pain

Galanin is a neuropeptide expressed in a small percentage of sensory neurons of the dorsal root ganglia and the superficial lamina of the dorsal horn of the spinal cord. In this work, we systematically reviewed the literature regarding the role of galanin and its receptors in nociception at the spinal and supraspinal levels, as well as in chronic pain conditions. The literature search was performed in PubMed, Web of Science, Scopus, ScienceDirect, OVID, TRIP, and EMBASE using "Galanin" AND "pain" as keywords. Of the 1379 papers that were retrieved in the initial search, we included a total of 141 papers in this review. Using the ARRIVE guidelines, we verified that 89.1% of the works were of good or moderate quality. Galanin shows a differential role in pain, depending on the pain state, site of action, and concentration. Under normal settings, galanin can modulate nociceptive processing through both a pro- and anti-nociceptive action, in a dose-dependent manner. This peptide also plays a key role in chronic pain conditions and its antinociceptive action at both a spinal and supraspinal level is enhanced, reducing animals' hypersensitivity to both mechanical and thermal stimulation. Our results highlight galanin and its receptors as potential therapeutic targets in pain conditions.

TRPV1-Mediated Microglial Autophagy Attenuates Alzheimer’s Disease-Associated Pathology and Cognitive Decline

Autophagy is a major regulator of the ageing process of the central nervous system and neurodegeneration. Autophagy dysfunction has been implicated in the pathogenesis of Alzheimer’s disease (AD). TRPV1 was reported to regulate autophagy to protect against foam cell formation and reduce the release of inflammatory factors in atherosclerosis. In this study, pharmacological activation of TRPV1 with the TRPV1 agonist capsaicin induced autophagy in a TRPV1-dependent manner in both primary microglia and BV2 cells. TRPV1-mediated autophagy regulated glycolysis and oxidative phosphorylation by controlling the expression of genes required for aerobic glycolysis and mitochondrial respiration in primary microglia. TRPV1 agonist capsaicin decreased amyloid and phosphorylated tau pathology and reversed memory deficits by promoting microglia activation, metabolism, and autophagy in 3xTg mice. These results indicate that TRPV1 was a potential therapeutic target for AD, which suggests that capsaicin should be further assessed as a possible treatment for AD.

The Mysteries of Capsaicin-Sensitive Afferents

A fundamental subdivision of nociceptive sensory neurons is named after their unique sensitivity to capsaicin, the pungent ingredient in hot chili peppers: these are the capsaicin-sensitive afferents. The initial excitation by capsaicin of these neurons manifested as burning pain sensation is followed by a lasting refractory state, traditionally referred to as "capsaicin desensitization," during which the previously excited neurons are unresponsive not only to capsaicin but a variety of unrelated stimuli including noxious heat. The long sought-after capsaicin receptor, now known as TRPV1 (transient receptor potential cation channel, subfamily V member 1), was cloned more than two decades ago. The substantial reduction of the inflammatory phenotype of Trpv1 knockout mice has spurred extensive efforts in the pharmaceutical industry to develop small molecule TRPV1 antagonists. However, adverse effects, most importantly hyperthermia and burn injuries, have so far prevented any compounds from progressing beyond Phase 2. There is increasing evidence that these limitations can be at least partially overcome by approaches outside of the mainstream pharmaceutical development, providing novel therapeutic options through TRPV1. Although ablation of the whole TRPV1-expressing nerve population by high dose capsaicin, or more selectively by intersectional genetics, has allowed researchers to investigate the functions of capsaicin-sensitive afferents in health and disease, several "mysteries" remain unsolved to date, including the molecular underpinnings of "capsaicin desensitization," and the exact role these nerves play in thermoregulation and heat sensation. This review tries to shed some light on these capsaicin mechanisms.

Use of Capsaicin to Treat Pain: Mechanistic and Therapeutic Considerations

Capsaicin is the pungent ingredient of chili peppers and is approved as a topical treatment of neuropathic pain. The analgesia lasts for several months after a single treatment. Capsaicin selectively activates TRPV1, a Ca²⁺-permeable cationic ion channel that is enriched in the terminals of certain nociceptors. Activation is followed by a prolonged decreased response to noxious stimuli. Interest also exists in the use of injectable capsaicin as a treatment for focal pain conditions, such as arthritis and other musculoskeletal conditions. Recently injection of capsaicin showed therapeutic efficacy in patients with Morton’s neuroma, a painful foot condition associated with compression of one of the digital nerves. The relief of pain was associated with no change in tactile sensibility. Though injection evokes short term pain, the brief systemic exposure and potential to establish long term analgesia without other sensory changes creates an attractive clinical profile. Short-term and long-term effects arise from both functional and structural changes in nociceptive terminals. In this review, we discuss how local administration of capsaicin may induce ablation of nociceptive terminals and the clinical implications.

The effect of intra-articular vanilloid receptor agonists on pain behavior measures in a murine model of acute monoarthritis

Arthritis is the most common cause of disability in the US, and the primary manifestation of arthritis is joint pain that leads to progressive physical limitation, disability, morbidity, and increased health care utilization. Capsaicin (CAP) is a vanilloid agonist that causes substance P depletion by interacting with vanilloid receptor transient receptor potential V1 on small unmyelinated C fibers. It has been used topically for analgesia in osteoarthritis with variable success. Resiniferatoxin (RTX) is an ultra potent CAP analog. The aim of this study was to measure the analgesic effects of intra-articular (IA) administration of CAP and RTX in experimental acute inflammatory arthritis in mice. Evoked pain score (EPS) and a dynamic weight bearing (DWB) device were used to measure nociceptive behaviors in a murine model of acute inflammatory monoarthritis. A total of 56 C57B16 male mice underwent EPS and DWB testing – 24 nonarthritic controls and 32 mice with carrageenan-induced arthritis. The effects of pretreatment with 0.1% CAP, 0.0003% RTX, or 0.001% RTX were measured. Nociception was reproducibly demonstrated by increased EPS and reduced DWB measures in the affected limb of arthritic mice. Pretreatment with 0.001% RTX resulted in statistically significant improvement in EPS and DWB measures when compared with those observed in carrageenan-induced arthritis animals. Pretreatment with IA 0.0003% RTX and IA 0.01% CAP resulted in improvement in some but not all of these measures. The remaining 24 mice underwent evaluation following treatment with 0.1% CAP, 0.0003% RTX, or 0.001% RTX, and the results obtained were similar to that of naïve, nonarthritic mice.

The efficacy of resiniferatoxin in prevention of catheter related bladder discomfort in patients after TURP - a pilot, randomized, open study

Background

Resiniferatoxin (RTX) has been shown to have variable efficacy in the treatment of intractable lower urinary tract symptoms and symptoms associated with neurogenic detrusor overactivity. It has also been used successfully in treating overactive bladder (OAB). In order to evaluate the potential and efficiency of RTX in treatment of catheter related bladder discomfort (CRBD) over post-operative period of transurethral resection of the prostate (TURP), we conducted the current pilot, randomized open study.

Methods

The study was comprised of 48 patients undergoing TURP in a single institute from September 2007 to September 2008. Patients were randomly divided into Group-RTX and Group-control. CRBD was classified into 4 degrees: degree 0, 1, 2, and 3. Patients with the most severed CRBD (degree-3) were treated with tolterodine.

Results

There were no statistical difference of patients' age, prostate volume, IPSS score, storage score and void score in IPSS, and QOL score between the two study groups(P=0.213, 0.356, 0.471, 0.554, 0.055 and 0.380, respectively). RTX pre-treatment reduced incidence of CRBD. 25% of the patients in RTX group had CRBD, at 6 hours/1 day and 3 days after TURP, significantly lower than the control group in which 75% of the patients during the same period (P=0.001). During the three days post-operative period, RTX also reduced severity of CRBD, compared with the control group. No patient in Group RTX sufferred CRBD of degree 3, a clear contrast to the control group in which 7 patients had degree 3 CRBD, during three days post-operative. Finally, less patients in RTX group required tolterodine, compared with control, P=0.009. Interestingly, the difference of CRBD incidence between Group RTX (50%) and control (75%) diminished 5 days after TURP, P=0.135.

Conclusions

Pretreatment with intravesical resiniferatoxin significantly reduces the incidence and severity of catheter related bladder discomfort in patients after TURP.

Differential regulation of peripheral IL-1β-induced mechanical allodynia and thermal hyperalgesia in rats

This study examined the differential mechanisms of mechanical allodynia and thermal hyperalgesia after injection of interleukin (IL) 1β into the orofacial area of male Sprague-Dawley rats. The subcutaneous administration of IL-1β produced both mechanical allodynia and thermal hyperalgesia. Although a pretreatment with iodoresiniferatoxin (IRTX), a transient receptor potential vanilloid 1 (TRPV1) antagonist, did not affect IL-1β-induced mechanical allodynia, it significantly abolished IL-1β-induced thermal hyperalgesia. On the other hand, a pretreatment with D-AP5, an N-methyl-d-aspartate (NMDA) receptor antagonist, and NBQX, an α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor antagonist, blocked IL-1β-induced mechanical allodynia. Pretreatment with H89, a protein kinase A (PKA) inhibitor, blocked IL-1β-induced mechanical allodynia but not thermal hyperalgesia. In contrast, pretreatment with chelerythrine, a protein kinase C (PKC) inhibitor, inhibited IL-1β-induced thermal hyperalgesia. Subcutaneous injections of 2% lidocaine, a local anesthetic agent, blocked IL-1β-induced thermal hyperalgesia but not IL-1β-induced mechanical allodynia. In the resiniferatoxin (RTX)-pretreated rats, a subcutaneous injection of IL-1β did not produce thermal hyperalgesia due to the depletion of TRPV1 in the primary afferent fibers. Double immunofluorescence revealed the colocalization of PKA with neurofilament 200 (NF200) and of PKC with the calcitonin gene-related peptide (CGRP) in the trigeminal ganglion. Furthermore, NMDA receptor 1 (NR1) and TRPV1 predominantly colocalize with PKA and PKC, respectively, in the trigeminal ganglion. These results suggest that IL-1β-induced mechanical allodynia is mediated by sensitized peripheral NMDA/AMPA receptors through PKA-mediated signaling in the large-diameter primary afferent nerve fibers, whereas IL-1β-induced thermal hyperalgesia is mediated by sensitized peripheral TRPV1 receptors through PKC-mediated signaling in the small-diameter primary afferent nerve fibers.

[TRPV1 in neuropathic pain: from animal models to therapeutical prospects]

Since its cloning in 1997, functional and structural studies of TRPV1 have led to an improvement in our understanding of the mechanisms that underlie the transduction of noxious thermal and mechanical stimuli by sensory neurons. Because of its role in inflammatory processes and nociceptive pathways, TRPV1 has become an important target for neuropathic pain relief. Models of painful small-fiber sensory neuropathy were developed and several laboratories have progressed in the conception of TRPV1 agonists and antagonists. Patch and cream containing capsaicin, the most famous TRPV1 agonist, are commercialized to relieve neuropathic pain. Others agonists and TRPV1 antagonists are tested in clinical trials and new agents, "TRPV1 modulators", with fewer side effects are currently developed in experimental studies.

Electroacupuncture improves thermal and mechanical sensitivities in a rat model of postherpetic neuralgia

Background

Electroacupuncture (EA) is effective in relieving pain in patients with postherpetic neuralgia (PHN). However, the mechanism underlying the therapeutic effect of EA in PHN is still unclear. Systemic injection of resiniferatoxin (RTX), an ultrapotent analog of TRPV1 agonist, in adult rats can reproduce the clinical symptoms of PHN by ablating TRPV1-expressing sensory neurons. In this study, we determined the beneficial effect of EA and the potential mechanisms in this rat model of PHN.

Methods

PHN was induced in rats by a single injection of RTX. Thermal hyperalgesia was tested with a radiant heat stimulus, and mechanical allodynia was quantified with von Frey filaments. TRPV1 receptors were shown by using immunofluorescence labeling. The ultrastructural changes of the sciatic nerve were assessed by electron microscopic examination. The sprouting of myelinated primary afferent terminals into the spinal dorsal horn was mapped by using the transganglionic tracer cholera toxin B-subunit (CTB).

Results

RTX injection diminished thermal sensitivity and gradually induced tactile allodynia within 3 weeks. EA applied to GB30 and GB34 at 2 and 15 Hz, but not 100 Hz, significantly increased the thermal sensitivity 4 weeks after treatment and decreased the tactile allodynia 2 weeks after treatment in RTX-treated rats. EA treatment at 2 and 15 Hz recovered the loss of TRPV1-positive dorsal root ganglion neurons and their central terminals of afferent fibers in the spinal superficial dorsal horn of RTX-treated rats. Moreover, EA significantly reduced the loss of unmyelinated fibers and the damage of the myelinated nerve fibers of RTX-treated rats. Furthermore, EA at 2 and 15 Hz inhibited the sprouting of myelinated primary afferent terminals into the spinal lamina II of RTX-treated rats.

Conclusions

EA treatment improves thermal perception by recovering TRPV1-positive sensory neurons and nerve terminals damaged by RTX. EA Also reduces RTX-induced tactile allodynia by attenuating the damage of myelinated afferent nerves and their abnormal sprouting into the spinal lamina II. Our study provides new information about the mechanisms of the therapeutic actions of EA in the treatment of PHN.

Anatomical and functional characterization of a duodeno-pancreatic neural reflex that can induce acute pancreatitis

Neural cross talk between visceral organs may play a role in mediating inflammation and pain remote from the site of the insult. We hypothesized such a cross talk exists between the duodenum and pancreas, and further it induces pancreatitis in response to intraduodenal toxins. A dichotomous spinal innervation serving both the duodenum and pancreas was examined, and splanchnic nerve responses to mechanical stimulation of these organs were detected. This pathway was then excited on the duodenal side by exposure to ethanol followed by luminal mustard oil to activate transient receptor potential subfamily A, member 1 (TRPA1). Ninety minutes later, pancreatic inflammation was examined. Ablation of duodenal afferents by resiniferatoxin (RTX) or blocking TRPA1 by Chembridge (CHEM)-5861528 was used to further investigate the duodeno-pancreatic neural reflex via TRPA1. ~40% of dorsal root ganglia (DRG) from the spinal cord originated from both duodenum and pancreas via dichotomous peripheral branches; ~50% splanchnic nerve single units responded to mechanical stimulation of both organs. Ethanol sensitized TRPA1 currents in cultured DRG neurons. Pancreatic edema and myeloperoxidase activity significantly increased after intraduodenal ethanol followed by mustard oil (but not capsaicin) but significantly decreased after ablation of duodenal afferents by using RTX or blocking TRPA1 by CHEM-5861528. We found the existence of a neural cross talk between the duodenum and pancreas that can promote acute pancreatitis in response to intraduodenal chemicals. It also proves a previously unexamined mechanism by which alcohol can induce pancreatitis, which is novel both in terms of the site (duodenum), process (neurogenic), and receptor (TRPA1).

The precursor of resolvin D series and aspirin-triggered resolvin D1 display anti-hyperalgesic properties in adjuvant-induced arthritis in rats

BACKGROUND AND PURPOSE

Resolution of inflammation is mediated by endogenous molecules with anti-inflammatory and pro-resolving activities and they have generated new possibilities for the treatment of inflammatory diseases. Here, we have investigated the possible anti-hyperalgesic effects of two lipids, aspirin-triggered resolvin D1 (AT-RvD1) and its precursor, 17(R)-hydroxy-4Z,7Z,10Z,13Z,15E,17R,19Z-docosahexaenoic acid (17(R)HDoHE).

EXPERIMENTAL APPROACH

The anti-hyperalgesic effects of both lipid mediators were evaluated, using mechanical and thermal stimuli, at different time-points in adjuvant-induced arthritis in rats. Cytokine levels were measured, and immunohistochemistry and real-time PCR for pro-inflammatory mediators were also performed.

KEY RESULTS

The precursor of resolvin D series, 17(R)HDoHE, given systemically, inhibited the development and the maintenance of mechanical hyperalgesia in acute inflammation. Such effects were likely to be associated with modulation of both NF-κB and COX-2 in dorsal root ganglia and spinal cord. 17(R)HDoHE was also effective against sub-chronic pain. Unexpectedly, repeated treatment with 17(R)HDoHE did not modify paw and joint oedema in the sub-chronic model, while joint stiffness was prevented. Notably, AT-RvD1 exhibited marked anti-hyperalgesic effects in acute inflammation when given systemically. The efficacy of long-term treatment with either 17(R)HDoHE or AT-RvD1 was partly related to decreased production of TNF-α and IL-1β in rat hind paw.

CONCLUSIONS AND IMPLICATIONS

Our findings provide fresh evidence for the anti-hyperalgesic properties of 17(R)HDoHE and its pro-resolution metabolite AT-RvD1. Such lipid mediators might be useful for treating pain associated with acute or chronic inflammation. LINKED ARTICLE This article is commented on by Xu and Ji, pp. 274-277 of this issue. To view this commentary visit http://dx.doi.org/10.1111/j.1476-5381.2011.01348.x.

Transient inflammation-induced ongoing pain is driven by TRPV1 sensitive afferents

Background

Neuropathic pain is a chronic disease resulting from dysfunction within the "pain matrix". The basolateral amygdala (BLA) can modulate cortical functions and interactions between this structure and the medial prefrontal cortex (mPFC) are important for integrating emotionally salient information. In this study, we have investigated the involvement of the transient receptor potential vanilloid type 1 (TRPV1) and the catabolic enzyme fatty acid amide hydrolase (FAAH) in the morphofunctional changes occurring in the pre-limbic/infra-limbic (PL/IL) cortex in neuropathic rats.

Results

The effect of N-arachidonoyl-serotonin (AA-5-HT), a hybrid FAAH inhibitor and TPRV1 channel antagonist, was tested on nociceptive behaviour associated with neuropathic pain as well as on some phenotypic changes occurring on PL/IL cortex pyramidal neurons. Those neurons were identified as belonging to the BLA-mPFC pathway by electrical stimulation of the BLA followed by hind-paw pressoceptive stimulus application. Changes in their spontaneous and evoked activity were studied in sham or spared nerve injury (SNI) rats before or after repeated treatment with AA-5-HT. Consistently with the SNI-induced changes in PL/IL cortex neurons which underwent profound phenotypic reorganization, suggesting a profound imbalance between excitatory and inhibitory responses in the mPFC neurons, we found an increase in extracellular glutamate levels, as well as the up-regulation of FAAH and TRPV1 in the PL/IL cortex of SNI rats. Daily treatment with AA-5-HT restored cortical neuronal activity, normalizing the electrophysiological changes associated with the peripheral injury of the sciatic nerve. Finally, a single acute intra-PL/IL cortex microinjection of AA-5-HT transiently decreased allodynia more effectively than URB597 or I-RTX, a selective FAAH inhibitor or a TRPV1 blocker, respectively.

Conclusion

These data suggest a possible involvement of endovanilloids in the cortical plastic changes associated with peripheral nerve injury and indicate that therapies able to normalize endovanilloid transmission may prove useful in ameliorating the symptoms and central sequelae associated with neuropathic pain.

Transient inflammation-induced ongoing pain is driven by TRPV1 sensitive afferents

BACKGROUND

Tissue injury elicits both hypersensitivity to evoked stimuli and ongoing, stimulus-independent pain. We previously demonstrated that pain relief elicits reward in nerve-injured rats. This approach was used to evaluate the temporal and mechanistic features of inflammation-induced ongoing pain.

RESULTS

Intraplantar Complete Freund's Adjuvant (CFA) produced thermal hyperalgesia and guarding behavior that was reliably observed within 24 hrs and maintained, albeit diminished, 4 days post-administration. Spinal clonidine produced robust conditioned place preference (CPP) in CFA treated rats 1 day, but not 4 days following CFA administration. However, spinal clonidine blocked CFA-induced thermal hyperalgesia at both post-CFA days 1 and 4, indicating different time-courses of ongoing and evoked pain. Peripheral nerve block by lidocaine administration into the popliteal fossa 1 day following intraplantar CFA produced a robust preference for the lidocaine paired chamber, indicating that injury-induced ongoing pain is driven by afferent fibers innervating the site of injury. Pretreatment with resiniferatoxin (RTX), an ultrapotent capsaicin analogue known to produce long-lasting desensitization of TRPV1 positive afferents, fully blocked CFA-induced thermal hypersensitivity and abolished the CPP elicited by administration of popliteal fossa lidocaine 24 hrs post-CFA. In addition, RTX pretreatment blocked guarding behavior observed 1 day following intraplantar CFA. In contrast, administration of the selective TRPV1 receptor antagonist, AMG9810, at a dose that reversed CFA-induced thermal hyperalgesia failed to reduce CFA-induced ongoing pain or guarding behavior.

CONCLUSIONS

These data demonstrate that inflammation induces both ongoing pain and evoked hypersensitivity that can be differentiated on the basis of time course. Ongoing pain (a) is transient, (b) driven by peripheral input resulting from the injury, (c) dependent on TRPV1 positive fibers and (d) not blocked by TRPV1 receptor antagonism. Mechanisms underlying excitation of these afferent fibers in the early post-injury period will offer insights for development of novel pain relieving strategies in the early post-traumatic period.

Pre-treatment with capsaicin in a rat osteoarthritis model reduces the symptoms of pain and bone damage induced by monosodium iodoacetate

A rat model of osteoarthritis was used to investigate the effect of pre-treatment with capsaicin on the symptoms of osteoarthritis induced by the injection of monosodium iodoacetate. This model mimics both histopathology and symptoms associated of human osteoarthritis. Injection of monosodium iodoacetate, an inhibitor of glycolysis, into the femorotibial joints of rodents promotes loss of articular trabecular bone and invokes pain symptoms similar to those noted in human osteoarthritis. Twenty rats were divided in two groups either receiving placebo or monosodium iodoacetate. Each group was subdivided in two groups either receiving pre-treatment with capsaicin two weeks before monosodium iodoacetate injection or not, resulting in four groups of five rats each. The impact of a single intra-articular administration of capsaicin (0.5%) on the generation of evoked mechanical pain (hind limb weight bearing, automated von Frey monofilament and RotaRod tests) and bone lesions (micro-CT scan radiographic analyses of bone structure) following monosodium iodoacetate-induced osteoarthritis in rats was determined. Evoked mechanical pain as monitored over a period of 4 weeks after monosodium iodoacetate injection was abolished in capsaicin pre-treated animals and pain values are comparable to those of capsaicin controls. Chronic joint pathological changes such as bone erosion and trabecular damage were significantly reduced by pre-treatment with a single administration of capsaicin. Decrease of bone volume was considerably ameliorated and trabecular connectivity was substantially better in capsaicin pre-treated animals. Capsaicin, an agonist activator of the vanilloid nociceptors (TRPV1), appears to be effective in protecting bone from arthritic damage. The present results support the hypothesis that capsaicin-sensitive sensory neurons contribute to bone lesions in the monosodium iodoacetate-induced osteoarthritis rat model.

Vanilloid-induced conduction analgesia: selective, dose-dependent, long-lasting, with a low level of potential neurotoxicity

Vanilloid agonists (capsaicin, resiniferatoxin, [RTX]) applied to the peripheral nerves provide conduction blockade. In contrast to the analgesic component of conduction anesthesia produced by local anesthetics, vanilloid agonists provide conduction analgesia not associated with suppression of motor or sensory functions not related to pain. Vanilloid agonists provide conduction analgesia selectively because their effect on the nerve trunks is limited to C- and ADelta-fibers. RTX is much more potent than capsaicin and has a wider therapeutic window. In rat experiments, perineural RTX produced a long-lasting thermal and mechanical hypoalgesia with a very wide separation between effective concentrations (from 0.00003% to 0.001%) providing an effect lasting from several hours to several weeks. A nerve block with RTX prevented the development of thermal and mechanical hyperalgesia as well as pain behavior in a model of incisional pain. RTX-induced conduction blockade has an inherent drawback of TRPV1 agonists, the initial excitation (pain); therefore, a local anesthetic should be injected to prevent it. When RTX was applied to the rat's sciatic nerve in doses necessary to provide conduction analgesia, the frequency of unmyelinated fiber degeneration was more than an order of magnitude lower than that with the therapeutic concentration of lidocaine. These promising results should be confirmed by experiments in species other than rodents (pigs, sheep). Taken together, the data indicate possible clinical applicability of vanilloid-induced conduction analgesia.

Functional depletion of capsaicin-sensitive primary afferent fibers attenuates rat pain-related behaviors and paw edema induced by the venom of scorpion Buthus martensi Karch

The role of capsaicin-sensitive primary afferent fibers in rat pain-related behaviors and paw edema induced by scorpion Buthus martensi Karch (BmK) venom was investigated in this study. It was found that functional depletion of capsaicin-sensitive primary afferent fibers with a single systemic injection of resiniferatoxin (RTX) dramatically decreased spontaneous nociceptive behaviors, prevented the development of primary mechanical and thermal hyperalgesia as well as mirror-image mechanical hyperalgesia. RTX treatment significantly attenuated BmK venom-induced c-Fos expression in all laminaes of bilateral L4-L5 lumbar spinal cord, especially in superficial laminaes. Moreover, RTX treatment markedly reduced the early paw edema induced by BmK venom. Thus, the results indicate that capsaicin-sensitive primary afferent fibers play a critical role in various pain-related behaviors and paw edema induced by BmK venom in rats.

Capsaicin (TRPV1 Agonist) therapy for pain relief: farewell or revival?

OBJECTIVE

In this review, we explain our current understanding of the molecular basis for pain relief by capsaicin and other transient receptor potential vanilloid subfamily, member 1 (TRPV1) agonists. We summarize disease-related changes in TRPV1 expression and its implications for therapy and potential adverse effects. Last, we provide an overview of the current clinical uses of topical and injectable TRPV1 agonist preparations in both oncologic and nononcologic populations.

METHOD

Search of MEDLINE and other databases.

RESULTS

The capsaicin receptor TRPV1 is a polymodal nociceptor exhibiting a dynamic threshold of activation that could be lowered under inflammatory conditions. Consistent with this model, TRPV1 knock-out mice are devoid of post-inflammatory thermal hyperalgesia. TRPV1 desensitization of primary sensory neurons is a powerful approach to relieve symptoms of nociceptive behavior in animal models of chronic pain. However, over-the-counter capsaicin creams have shown moderate to poor analgesic efficacy. This is in part related to low dose, poor skin absorption, and compliance factors. Recently developed site-specific capsaicin therapy with high-dose patches and injectable preparations seem to be safe and reportedly provide long-lasting analgesia with rapid onset.

CONCLUSIONS

We argue that TRPV1 agonists and antagonists are not mutually exclusive but rather complimentary pharmacologic approaches for pain relief and we predict a "revival" for capsaicin and other TRPV1 agonists in the clinical management of pain associated with inflammation, metabolic imbalances (eg, diabetes), infections (HIV), and cancer, despite the current focus of the pharmaceutical industry on TRPV1 antagonists.

Perineural resiniferatoxin selectively inhibits inflammatory hyperalgesia

Resiniferatoxin (RTX) is an ultrapotent capsaicin analog that binds to the transient receptor potential channel, vanilloid subfamily member 1 (TRPV1). There is a large body of evidence supporting a role for TRPV1 in noxious-mediated and inflammatory hyperalgesic responses. In this study, we evaluated low, graded, doses of perineural RTX as a method for regional pain control. We hypothesized that this approach can provide long-term, but reversible, blockade of a portion of nociceptive afferent fibers within peripheral nerves when given at a site remote from the neuronal perikarya in the dorsal root ganglia. Following perineural RTX application to the sciatic nerve, we demonstrated a significant inhibition of inflammatory nociception that was dose- and time-dependent. At the same time, treated animals maintained normal proprioceptive sensations and motor control, and other nociceptive responses were largely unaffected. Using a range of mechanical and thermal algesic tests, we found that the most sensitive measure following perineural RTX administration was inhibition of inflammatory hyperalgesia. Recovery studies showed that physiologic sensory function could return as early as two weeks post-RTX treatment, however, immunohistochemical examination of the DRG revealed a partial, but significant reduction in the number of the TRPV1-positive neurons. We propose that this method could represent a beneficial treatment for a range of chronic pain problems, including neuropathic and inflammatory pain not responding to other therapies.

The role of capsaicin-sensitive primary afferents in experimental sciatica induced by disc herniation in rats

STUDY DESIGN

The topical capsaicin treatment of the sciatic nerve, which was proved to destroy capsaicin-sensitive primary afferent (CSPA) fibers, was performed to determine the effect on decreases in paw withdrawal mechanical threshold (PWMT) and changes in spatial expression pattern of spinal c-Fos protein induced by the direct compression of L5 nerve root with autologous disc.

OBJECTIVE

To investigate the role of CSPA fibers in the development of mechanical hyperalgesia in the new sciatica model.

SUMMARY OF BACKGROUND DATA

To date, CSPA fibers have been shown to be involved in development of thermal hyperalgesia in various pain models. But the controversy still exists as to whether CSPA fibers are involved in the development of mechanical hyperalgesia in different pain models. To our best knowledge, the role of CSPA in sciatica was not investigated. Therefore, the present study was designed to determine the role of CSPA fibers in the newly developed sciatica model.

METHODS

All surgeries were performed in Sprague-Dawley rats. PWMT was measured at the different time points postsurgery and presurgery. The changes in spatial expression pattern of c-Fos protein in the spinal cord were also determined at 3 weeks when PWMT decreased to the peak.

RESULTS

The pretreatment with capsaicin produced a complete prevention of mechanical hyperalgesia induced by disc compression. The direct compression of L5 nerve root produced an obvious expression of Fos-like immunoreactivity neurons in the dorsal horn of the spinal cord, which was significantly decreased by pretreatment with capsaicin.

CONCLUSION

We conclude that CSPA fibers, which mainly terminated in superficial layers of dorsal horn, may play a key role in mechanical hyperalgesia in the new sciatica model.

Roles of capsaicin-sensitive primary afferents in differential rat models of inflammatory pain: a systematic comparative study in conscious rats

To characterize the role of capsaicin-sensitive primary afferents in inflammatory pain, the effects of subcutaneous (s.c.) injection of 0.15% capsaicin on different chemical irritants-induced pathological nociception including persistent spontaneous nociception, primary thermal and mechanical hyperalgesia, and inflammatory response were systematically investigated in unanesthetized conscious rats. Four different animal models of inflammatory pain: the bee venom (BV) test, the formalin test, the carrageenan model, and the complete Freund's adjuvant (CFA) model, were employed and compared. Local pre-treatment with capsaicin produced a significant inhibition on the s.c. BV and formalin induced long-lasting persistent spontaneous nociception. However, this capsaicin-induced inhibitory effect on spontaneous nociception in the BV test was only found within the late phase (tonic nociception; 11-60 min), but not the early phase (acute nociception; 0-10 min). A complete preventing effect of capsaicin on the decreased thermal paw withdrawal latency was found in the BV, carrageenan, and CFA models. Nevertheless, pre-treatment with capsaicin only produced complete blocking effects on the decreased mechanical paw withdrawal threshold in the BV and carrageenan models, but not in the CFA model. For inflammatory response, a significant inhibition of the BV-elicited paw swelling was found following capsaicin treatment. In marked contrast, capsaicin did not produce any effects on the paw inflammation during exposure to carrageenan, CFA, and formalin. These data suggest that capsaicin-sensitive primary afferents may play differential roles in the induction and development of pathological nociception in differential inflammatory pain models. In contrast to other chemical irritants, BV-induced long-term spontaneous nociception, facilitated nociceptive behavior, and inflammation are modulated by peripheral capsaicin-sensitive afferents.

Activity-dependent heterogeneous populations of nitric oxide synthase neurons in the rat dorsal raphe nucleus

The brainstem dorsal raphe nucleus (DRN) contains an abundant distribution of nitric oxide (NO) synthase (NOS)-containing neuronal profiles in two distinct populations: faint- and intense-immunoreactive cells in midline (ventromedial and dorsomedial) and lateral wing subregions, respectively. This study tested the hypothesis that different functional dynamics underlie the topography of NOS-containing cells in the DRN rostrocaudal and mediolateral neuraxis by using a capsaicin challenge paradigm (50 mg/kg, subcutaneous). Compared with vehicle, capsaicin significantly and preferentially increased nicotinamide adenine dinucleotide phosphate diaphorase (NADPH-d, an index of constitutive NOS) reactivity in the rostral midline and caudal lateral wing subregions. Furthermore, capsaicin activated more Fos-positive cells than vehicle within all subregions of the DRN but with a caudal versus rostral predominance in activation pattern. In addition, a high proportion of capsaicin-induced Fos cells in the midline but almost none in lateral wing stained for NADPH-d. These observations suggest the existence of two functionally distinct populations of NOS neurons in the DRN. Furthermore, capsaicin increased galanin immunoreactivity with predominant staining in cell soma and fiber processes in midline and lateral wing subregions of the nucleus, respectively. The total capsaicin-induced galanin immunoreactivity was higher in rostral versus caudal DRN, and a high proportion of galanin-positive cells in the midline also contained NADPH-d and neuronal NOS, thus suggesting a potential NO-galanin interaction in these neurons. The differential pattern of Fos/NADPH-d colocalization across the nucleus suggests that midline and lateral wing NOS neurons of the DRN express their neuromodulatory actions on discrete efferent targets via different intracellular mechanisms.

Spinal ventral root after-discharges as a pain index: Involvement of NK-1 and NMDA receptors

Nociceptive signals are transmitted to the spinal dorsal horn via primary afferent fibers, and the signals induce withdrawal reflexes by activating spinal motoneurons in the ventral horn. Therefore, nociceptive stimuli increase motoneuronal firing and ventral root discharges. This study was aimed to develop a method for the study of pain mechanisms and analgesics by recording ventral root discharges. Spinalized rats were laminectomized in the lumbo-sacral region. The fifth lumbar ventral root was sectioned and placed on a pair of wire electrodes. Multi unit efferent discharges from the ventral root were increased by mechanical stimulation using a von Frey hair applied to the plantar surface of the hindpaw. The low-intensity mechanical stimuli increased the discharges during stimulation (during-discharges) without increasing the discharges after cessation of stimulation (after-discharges), and the high-intensity mechanical stimuli increased both during- and after-discharges. Pretreatment with resiniferatoxin, an ultrapotent analogue of capsaicin, halved during-discharges and eliminated after-discharges, suggesting that after-discharges are generated by heat- and mechanosensitive polymodal nociceptors. Ezlopitant, a neurokinin-1 (NK-1) receptor antagonist, but not its inactive enantiomer, selectively reduced the after-discharges. Ketamine, an N-methyl-D-aspartate (NMDA) receptor antagonist, preferentially reduced the after-discharges, demonstrating that NK-1 and NMDA receptors mediate the after-discharges. Morphine reduced the after-discharges without affecting during-discharges. By contrast, mephenesin, a centrally acting muscle relaxant, reduced both during- and after-discharges. There results suggest that simultaneous recordings of during- and after-discharges are useful to study pain mechanisms and analgesics as well as to discriminate the analgesic effects from the side effects such as muscle relaxant effects.

Loss of TRPV1-expressing sensory neurons reduces spinal mu opioid receptors but paradoxically potentiates opioid analgesia

Systemic administration of resiniferatoxin (RTX), an ultrapotent capsaicin analogue, removes transient receptor potential vanilloid type 1 (TRPV1)-expressing afferent neurons and impairs thermal but not mechanical nociception in adult animals. In this study, we determined how loss of TRPV1-expressing sensory neurons alters the antinociceptive effect of mu opioids and mu opioid receptors in the spinal cord. The effect of morphine and (D-Ala2,N-Me-Phe4,Gly-ol5)-enkephalin (DAMGO) was measured by testing the paw mechanical withdrawal threshold in rats treated with RTX or vehicle. RTX treatment deleted TRPV1-immunoreactive dorsal root ganglion neurons and nerve terminals in the spinal dorsal horn. Also the mu opioid receptor immunoreactivity was markedly reduced in the superficial dorsal horn of RTX-treated rats. However, RTX treatment did not affect the dorsal horn neurons labeled with both TRPV1- and mu opioid receptor-immunoreactivity. Surprisingly, intrathecal morphine or DAMGO produced a greater increase in the withdrawal threshold in RTX- than in vehicle-treated rats. The duration of the effect of intrathecal morphine and DAMGO in RTX-treated rats was also profoundly increased. Furthermore, the antinociceptive effect of systemic morphine was significantly potentiated in RTX-treated rats. The B(MAX) (but not K(D)) of [3H]-DAMGO binding and DAMGO-stimulated [35S]GTPgammaS activity in the dorsal spinal cord were significantly reduced in the RTX group. This study provides novel information that loss of TRPV1 afferent neurons eliminates presynaptic mu opioid receptors present on TRPV1-expressing afferent neurons but paradoxically potentiates the analgesic effect of mu opioid agonists. Mechano-nociception, transmitted through non-TRPV1 sensory neurons, is subject to potent modulation by mu opioid agonists.

The effects of intraarticular resiniferatoxin in experimental knee-joint arthritis

In this study we sought to determine whether an intraarticular administration of a vanilloid agonist resiniferatoxin (RTX) produces an analgesic effect in experimental arthritis. Knee joint inflammation was induced in rats by intraarticular carrageenan (2%, 30 microL). Pain score and left/right hind leg weight distribution ratio were used to assess pain behavior. Changes in knee dimensions were evaluated by measuring external circumference and intraarticular area (ultrasound scanning). The intraarticular administration of RTX (0.0003% or 0.003%, 30 microL) provided a significant analgesic effect. Twenty-four hours after RTX administration, the pain score was reduced from 15.1 +/- 4.7 to 6.9 +/- 4.4 (P < 0.01) with 0.0003% and was abolished (P < 0.0001) with 0.003%. The improvement in weight distribution ratio lasted for several days after the RTX administration. Reduction in knee circumference demonstrated that intraarticular RTX suppressed the carrageenan-induced edema by at least one third. Ultrasound scanning revealed no RTX-induced decrease of the intraarticular area. The experiments demonstrated that intraarticular RTX inhibits pain behavior in knee-joint arthritis and that this effect is dose-dependent. These results suggest a new direction for peripheral analgesia.

Sensory nerve desensitization by resiniferatoxin improves glucose tolerance and increases insulin secretion in Zucker Diabetic Fatty rats and is associated with reduced plasma activity of dipeptidyl peptidase IV

Sensory nerve desensitization by capsaicin has been shown to improve the diabetic condition in Zucker Diabetic Fatty rats. However, administration of capsaicin to adult rats is associated with an increased mortality. Therefore, in this experiment, we examined the influence of resiniferatoxin, a tolerable analogue of capsaicin suitable for in vivo use, on the diabetic condition of Zucker Diabetic Fatty rats. A single subcutaneous injection of resiniferatoxin (0.01 mg/kg) to these rats was tolerable, with no mortality. When administered to early diabetic rats at 15 weeks of age, the further deterioration of glucose homeostasis was prevented by resiniferatoxin. Further, when administered to overtly diabetic rats at 19 weeks of age, resiniferatoxin markedly improved glucose tolerance at two weeks after administration and this was accompanied by an increased insulin response to oral glucose as well as a reduction in the plasma levels of dipeptidyl peptidase IV. Therefore, resiniferatoxin is a safe alternative to capsaicin for further investigations of the role of the sensory nerves in experimental diabetes.

Sensory nerve inactivation by resiniferatoxin improves insulin sensitivity in male obese Zucker rats

Recent studies have suggested that sensory nerves may influence insulin secretion and action. The present study investigated the effects of resiniferatoxin (RTX) inactivation of sensory nerves (desensitization) on oral glucose tolerance, insulin secretion and whole body insulin sensitivity in the glucose intolerant, hyperinsulinemic, and insulin-resistant obese Zucker rat. After RTX treatment (0.05 mg/kg RTX sc given at ages 8, 10, and 12 wk), fasting plasma insulin was reduced (P < 0.0005), and oral glucose tolerance was improved (P < 0.005). Pancreas perfusion showed that baseline insulin secretion (7 mM glucose) was lower in RTX-treated rats (P = 0.01). Insulin secretory responsiveness to 20 mM glucose was enhanced in the perfused pancreas of RTX-treated rats (P < 0.005) but unaffected in stimulated, isolated pancreatic islets. At the peak of spontaneous insulin resistance in the obese Zucker rat, insulin sensitivity was substantially improved after RTX treatment, as evidenced by higher glucose infusion rates (GIR) required to maintain euglycemia during a hyperinsulinemic euglycemic (5 mU.kg(-1).min(-1)) clamp (GIR(60-120min): 5.97 +/- 0.62 vs. 11.65 +/- 0.83 mg.kg(-1).min(-1) in RTX-treated rats, P = 0.003). In conclusion, RTX treatment and, hence, sensory nerve desensitization of adult male obese Zucker rats improved oral glucose tolerance by enhancing insulin secretion, and, in particular, by improving insulin sensitivity.

Perineural resiniferatoxin prevents hyperalgesia in a rat model of postoperative pain

Resiniferatoxin (RTX) is a vanilloid agonist with a unique spectrum of activities. Vanilloids bind to the transient receptor potential ion channel subtype 1, a nonselective cation ionophore important in the integration of different noxious signals. Vanilloid agonists selectively decrease sensitivity to noxious stimuli. In this study, we sought to determine whether perineural RTX prevents hyperalgesia in a model of incisional pain. In a rat model, RTX was administered percutaneously to the sciatic and saphenous nerves before the plantar incision. The withdrawal response to von Frey filaments, the struggle response to pressure on the paw, and pain scoring based on weight bearing were measured before RTX and at various intervals for 8 days after RTX. A percutaneous injection of RTX (0.0003%) to the sciatic (0.1 mL) and saphenous (0.05 mL) nerves completely prevented incisional hyperalgesia. Two hours after incision, the withdrawal threshold was 51 mN without and 456 mN with RTX (P < 0.0001). RTX also prevented the incision-induced decrease in struggle threshold and abolished the pain behavior associated with weight bearing. We conclude that RTX provides a type of neural blockade when postoperative pain is abolished and that nonpainful sensations and motor functions are preserved.

Lipopolysaccharide fever is initiated via a capsaicin-sensitive mechanism independent of the subtype-1 vanilloid receptor

As pretreatment with intraperitoneal capsaicin (8-methyl-N-vanillyl-6-nonenamide, CAP), an agonist of the vanilloid receptor known as VR1 or transient receptor potential channel-vanilloid receptor subtype 1 (TRPV-1), has been shown to block the first phase of lipopolysaccharide (LPS) fever in rats, this phase is thought to depend on the TRPV-1-bearing sensory nerve fibers originating in the abdominal cavity. However, our recent studies suggest that CAP blocks the first phase via a non-neural mechanism. In the present work, we studied whether this mechanism involves the TRPV-1. Adult Long-Evans rats implanted with chronic jugular catheters were used. Pretreatment with CAP (5 mg kg(-1), i.p.) 10 days before administration of LPS (10 microg kg(-1), i.v.) resulted in the loss of the entire first phase and a part of the second phase of LPS fever. Pretreatment with the ultrapotent TRPV-1 agonist resiniferatoxin (RTX; 2, 20, or 200 microg kg(-1), i.p.) 10 days before administration of LPS had no effect on the first and second phases of LPS fever, but it exaggerated the third phase at the highest dose. The latter effect was presumably due to the known ability of high doses of TRPV-1 agonists to cause a loss of warm sensitivity, thus leading to uncontrolled, hyperpyretic responses. Pretreatment with the selective competitive TRPV-1 antagonist capsazepine (N-[2-(4-chlorophenyl)ethyl]-1,3,4,5-tetrahydro-7,8-dihydroxy-2H-2-benzazepine-2-carbothioamidem, CPZ; 40 mg kg(-1), i.p.) 90 min before administration of LPS (10 microg kg(-1), i.v.) or CAP (1 mg kg(-1), i.p.) did not affect LPS fever, but blocked the immediate hypothermic response to acute administration of CAP. It is concluded that LPS fever is initiated via a non-neural mechanism, which is CAP-sensitive but RTX- and CPZ-insensitive. The action of CAP on this mechanism is likely TRPV-1-independent. It is speculated that this mechanism may be the production of prostaglandin E(2) by macrophages in LPS-processing organs.

Resiniferatoxin reversibly blocks adjuvant-induced thermal hyperalgesia in the rat

The role of capsaicin-sensitive sensory afferents and mast cells in complete Freund's adjuvant (CFA)-induced thermal hyperalgesia and edema was investigated in rats. A single systemic injection of resiniferatoxin produced a reversible prevention of adjuvant-induced thermal hyperalgesia which lasted several days. In addition, resiniferatoxin markedly reduced the early edema. Chronic degranulation of mast cells with compound 48/80 also reduced the thermal hyperalgesia and edema, especially in the early phase of inflammation. Co-pretreatment with resiniferatoxin and compound 48/80 induced effects similar to those of resiniferatoxin alone. The data support the involvement of capsaicin-sensitive fibers in the adjuvant-induced inflammation.

Peripherally induced resiniferatoxin analgesia

Selective blockade of nociceptive pathways represents a mechanism-based approach that has attracted a large variety of pharmacological and molecular investigations. A potential site for selective intervention is the primary afferent nociceptive nerve terminal. Binding of resiniferatoxin (RTX) to the vanilloid-1 receptor (VR1) stimulates and then inactivates heat and vanilloid-responsive nerve endings involved in heat and inflammatory pain signaling which can progress to localized degeneration of the peripheral ending followed by regeneration. Application of RTX directly to peripheral nerve endings produces a long term, reversible attenuation of nociceptive transmission. Heat hyperalgesia and mechanical allodynia were assessed prior to injection of RTX into the hindpaw (baseline) and at acute (minutes-hours) and more chronic (days-weeks) times after injection. Acutely, an inverse dose-to-pain response (guarding, licking) for RTX (0.0625-2.0 microg) occurs, followed by selective attenuation of peripheral pain transmission. Thermal nociception was decreased in a concentration-dependent fashion and lasted up to 21 days, without impairing motor function. Administration of RTX blocked both inflammation-induced hyperalgesia and spinal c-Fos induction. The results demonstrate the efficacy and therapeutic potential of reversible, peripheral C-fiber 'inactivation' for intermediate duration pain control.

Selective and long-lasting neural blockade with resiniferatoxin prevents inflammatory pain hypersensitivity

Capsaicin can produce a selective and long-lasting neural blockade. Resiniferatoxin (RTX) is an ultrapotent vanilloid agonist with a unique spectrum of activities different from that of capsaicin. We sought to determine whether a single application of RTX to a peripheral nerve could completely prevent the long-lasting mechanical hyperalgesia caused by carrageenan injection. In rat experiments, RTX (0.001%) was administered percutaneously to the sciatic and saphenous nerves before the intraplantar injection of carrageenan. Responses to noxious mechanical (pressure on the paw) and thermal (hot plate) stimulations and changes in paw circumference were measured at various time intervals for 8 days after treatment. The administration of RTX resulted in mechanical and thermal hypoalgesia (for 2 and 8 days, respectively). Inflammatory hyperalgesia was completely prevented by the precarrageenan injection of RTX. Inflammatory enhancement of paw circumference was reduced by RTX (12.0 +/- 2.4 mm versus 6.9 +/- 3.4 mm, P < 0.005). We suggest that the selective nature of the effect of vanilloid agonists on nociception could provide an opportunity for prolonged neural blockade when early mobilization and/or preservation of protective sensation are required.

IMPLICATIONS

We report that an ultrapotent vanilloid agonist resiniferatoxin can provide a selective and long-lasting neural blockade. Applied to the sciatic and saphenous nerves, it completely prevented pain hypersensitivity caused by prolonged inflammatory process (injection of carrageenan into the paw).

Vanilloid receptor and detrusor instability

Very recently, a membrane receptor (vanilloid receptor type 1 [VR-1]) sensitive to capsaicin or resiniferatoxin (RTX) was identified in small- and medium-sized dorsal root ganglion neurons that give rise to most unmyelinated sensory fibers. After vanilloid binding to VR-1, these neurons remain transiently desensitized; that is, less reactive to natural stimuli. It is this effect of vanilloid substances that is being investigated for its potential therapeutic utility. In the urinary bladder, VR-1-expressing fibers are extremely abundant in the mucosa and in the muscular layer. In the latter, VR-1 fibers are intimately apposed to smooth muscle cells. The demonstration, several years ago, that these fibers were involved in detrusor hyperreflexia of spinal origin and in bladder pain processing, justified the clinical application of intravesical capsaicin or RTX in humans with these bladder diseases. More recently, the experimental and clinical evidence that the same type of bladder sensory fibers were also involved in detrusor instability made a strong case for intravesical RTX assay in patients with idiopathic detrusor instability.

Metabotropic glutamate receptor 5 upregulation in A-fibers after spinal nerve injury: 2-methyl-6-(phenylethynyl)-pyridine (MPEP) reverses the induced thermal hyperalgesia

Metabotropic glutamate receptor 5 (mGluR5) protein increased after sciatic nerve section in ipsilateral L4 and L5 DRG neuronal profiles, with most of the increase occurring in myelinated A-fiber somata. mGluR5 also increased in lamina II of the ipsilateral spinal cord and the proximal sciatic nerve stump in this model. After L5 spinal nerve ligation, mGluR5 immunoreactivity increased dramatically not only in damaged L5 but also in the neighboring undamaged L4. Interestingly, after partial sciatic nerve section, mGluR5 expression did not change in either L4 or L5 DRG neuronal profiles. Both spinal nerve ligation and sciatic nerve partial section produced significant mechanical and thermal hyperalgesia and tactile allodynia. After partial sciatic nerve section, the mGluR5-specific antagonist 2-methyl-6-(phenylethynyl)-pyridine (MPEP) had no effect on any of these behaviors. However, after L5 spinal nerve ligation, although MPEP failed to alter the induced tactile allodynia or mechanical hyperalgesia, it dose dependently reversed the developed thermal hyperalgesia. Therefore, reversal of thermal hyperalgesia by MPEP correlates with increased mGluR5 in lumbar DRG A-fiber somata after nerve injury. Furthermore, A-fibers in the uninjured L4 DRG after L5 spinal nerve ligation that have increased mGluR5 are the same A-fibers that newly express vanilloid receptor 1 after such injury. Together, these results suggest that, after L5 spinal nerve injury, mGluR5 expression on A-fibers is essential to the development of thermal hyperalgesia. After partial nerve section, however, it is unlikely that thermal responses are mediated through mGluR5 because no such increase in mGluR5 is detected in this model and MPEP is ineffective.

Capsaicin-sensitive primary afferents mediate responses to cold in rats with a peripheral mononeuropathy

Persistent sensitivity to noxious and innocuous somatic stimuli results from peripheral nerve injury. Following chronic constriction injury (CCI) of the sciatic nerve in the rat, the hind paw ipsilateral to the injury displays significantly decreased response latencies to a noxious heat stimulus (thermal hyperalgesia), compared with the contralateral uninjured paw. The ligated paw also shows increased lifting and duration of lifting from a cooled (4 +/- 1 degrees C) surface. To characterize the peripheral nerve component of increased sensitivity to cold, CCI rats were systemically injected with the potent capsaicin analog resiniferatoxin (RTX). Twenty-four hours following RTX injection response latencies to noxious heat were significantly increased for both the ligated and unligated hind paws. In addition, increased responsiveness of the ligated paw to the cold surface was significantly attenuated. The results demonstrate that the enhanced responsiveness to cold and heat following a CCI are mediated in part by RTX-sensitive primary afferents.

Pivotal role of capsaicin-sensitive primary afferents in development of both heat and mechanical hyperalgesia induced by intraplantar bee venom injection

To investigate the roles of primary afferent fibers in development of the bee venom (BV)-induced persistent spontaneous nociception (PSN) and hyperalgesia (HA), the sciatic nerve or both the sciatic and saphenous nerves of rats were topically treated with capsaicin respectively under pentobarbital anesthesia to destroy the capsaicin-sensitive primary afferent (CSPA) fibers. Effect of the sciatic nerve capsaicin on the formalin-induced PSN was also evaluated. Destruction of the CSPA fibers of the sciatic nerve or both the sciatic and saphenous nerves only produced 34 or 69% inhibition of the mean total number of 1 h BV-induced paw flinches. However, the total number of 1 h formalin-induced paw flinches was inhibited by 90% (85% for phase 1 and 91% for phase 2). In naïve rats, destruction of the CSPA fibers of the sciatic nerve caused 237 and 60% increase in paw withdrawal thermal latency (PWTL) to radiant heat in the injection site (paw pad) and at the heel of the treated hind paw compared to the baseline values. However, it was without significant influence upon the PWTL in the non-treated side or the paw withdrawal mechanical threshold (PWMT) to von Frey filament stimuli in both hind paws. In the BV-treated rats, the CSPA fiber destruction of the sciatic nerve completely blocked development of the heat and mechanical HA in the BV injection site. However, the reduction in either PWTL (drop to baseline level) or PWMT (drop by 56% from the baseline level) at the heel of the BV-treated side was not affected by this treatment. However, destruction of the CSPA fibers of both the sciatic and saphenous nerves was able to block development of both heat and mechanical HA in the whole BV-treated hind paw and heat hyperalgesia in the non-injected hind paw. Taken together, we conclude that: (1) the CSPA (C- and A delta-) fibers play a pivotal role in mediation of either the heat or the mechanical hyperalgesia induced by s.c. BV; (2) the CSPA fibers may play a crucial role in mediation of the formalin-induced PSN, but play a partial role in the BV-induced nociceptive process; (3) in addition to the sciatic nerve, the saphenous nerve is also involved in mediation of the BV-induced PSN as well as heat and mechanical hyperalgesia, while it is not likely to be involved in the formalin-induced nociception.

Nerve injury-induced tactile allodynia is mediated via ascending spinal dorsal column projections

Peripheral nerve injury produces signs of neuropathic pain including tactile allodynia and thermal hyperalgesia, sensory modalities which may be associated with different neuronal pathways. Studies of spinally-transected, nerve-injured rats have led to suggestions that thermal hyperalgesia may be mediated predominately through local spinal circuitry whereas ascending input to supraspinal sites is critical to the manifestation of tactile allodynia. Here, the nature of ascending spinal input mediating tactile allodynia was explored using selective spinal lesions. Male Sprague-Dawley rats received L(5)/L(6) spinal nerve ligation (SNL) and ipsilateral or contralateral (relative to the SNL side) lesions including spinal hemisections and bilateral and unilateral dorsal column lesions. The rats were maintained in a sling and monitored for tactile allodynia by measuring withdrawal thresholds to probing with von Frey filaments 24 h after the hemisection. Rats receiving dorsal column lesions demonstrated no motor deficits while rats receiving spinal hemisection showed paralysis of the paw which nevertheless responded to strong noxious stimulation. Spinal hemisection ipsilateral, but not contralateral, to SNL completely abolished tactile allodynia while maintaining spinal nocifensive reflexes to noxious pinch. Bilateral and ipsilateral dorsal column lesions blocked tactile allodynia while contralateral dorsal column lesions did not. Administration of lidocaine into the nucleus gracilis ipsilateral to SNL also blocked tactile allodynia, but did not alter thermal hyperalgesia in SNL rats or increase thermal nociceptive responses in sham-operated rats. Lidocaine microinjected into the contralateral nucleus gracilis produced no changes in responses to tactile or thermal stimuli in either group. These results indicate that tactile allodynia after peripheral nerve injury is dependent upon inputs to supraspinal sites. Furthermore, it is apparent that afferent signals interpreted as tactile allodynia course through the ipsilateral dorsal columns and are relayed through the nucleus gracilis. This neuronal pathway is consistent with the interpretation that tactile allodynia pursuant to peripheral nerve injury is transmitted to the central nervous system by means of large diameter, myelinated fibers.

Mediation of spinal nerve injury induced tactile allodynia by descending facilitatory pathways in the dorsolateral funiculus in rats

Evidence exists to indicate that tactile allodynia arising from peripheral nerve injury is integrated predominately at supraspinal, rather than spinal, sites. In the present experiments, the possibility that disruption of descending pathways through the dorsolateral funiculus (DLF) might alter expression of nerve-injury induced tactile allodynia was explored. Male, Sprague-Dawley rats received L(5)/L(6) spinal nerve ligation (SNL). Lesions to the DLF were made ipsilateral or contralateral to SNL. Tactile allodynia was determined by measuring withdrawal thresholds to probing with von Frey filaments. Rats with DLF lesions presented no apparent motor deficits and did not alter sensory threshold in sham-SNL operated rats. DLF lesions made ipsilateral to SNL completely blocked tactile allodynia in SNL rats. Contralateral DLF lesions and sham surgery did not have any effect on SNL-induced allodynia. These results indicate that tactile allodynia after peripheral nerve injury is dependent upon tonic activation of net descending facilitation from supraspinal sites and support the hypothesis of tonic activation of descending facilitation as a basis for chronic pain.

Increased spinal cholecystokinin activity after systemic resiniferatoxin: electrophysiological and in situ hybridization studies

The present study assessed the effect of a single subcutaneous injection of resiniferatoxin (RTX), an ultrapotent capsaicin analogue, on the activity of spinal cholecystokinin (CCK) systems, by using electrophysiological and in situ hybridization techniques. Subcutaneous RTX at 0.3 mg/kg, but not vehicle, produced marked thermal hypoalgesia in rats on the hot plate and tail flick tests. Partial recovery from hypoalgesia occurred in some (<50%), but not all, RTX-treated rats after 2 weeks. The flexor reflex in response to activation of high threshold afferents was recorded 15-35 days after RTX- or vehicle-treatment. There was no obvious difference between RTX- and vehicle-treated rats in the baseline flexor reflex. Intravenous morphine at 1 mg/kg caused a depression of the flexor reflex in vehicle- and in RTX-treated rats. The reflex depressive effect of morphine was significantly briefer in RTX-treated, non-recovered rats than vehicle-treated rats. Furthermore, CI-988, a high affinity antagonist of CCKB receptors, caused a minor depression of the reflex in vehicle- and RTX-treated rats that had partially recovered, whereas the reflex depressive effect of CI-988 was significantly enhanced in RTX-treated, non-recovered rats. In situ hybridization showed that RTX treatment caused a marked and significant increase in the number of dorsal root ganglion (DRG) neurone profiles expressing CCKB receptor mRNA, whereas only a small increase was observed for CCKA receptor mRNA expressing neurone profiles. Significantly more DRG neurone profiles expressed CCKB receptor mRNA in RTX-treated, non-recovered rats compared to partially recovered rats. RTX-treatment did not influence the expression of CCK mRNA in DRGs. Since CCK functions as a physiological antagonist of morphine, it is suggested that RTX treatment enhances the activity of spinal CCK systems, leading to the reduced effect of morphine and increased effect of the CCKB receptor antagonist CI-988. This may mainly be due to upregulation of CCKB receptors in DRG neurones.

Effects of systemic resiniferatoxin treatment on substance P mRNA in rat dorsal root ganglia and substance P receptor mRNA in the spinal dorsal horn

Capsaicin depletes the sensory neuropeptide substance P (SP) in the rat due to a combination of neuron loss and decreased synthesis in the surviving cells. Resiniferatoxin (RTX) mimics most, but not all, capsaicin actions. In the present study, the effects of RTX (300 microg/kg, s.c.) were examined on mRNA levels for SP and its receptor in the adult rat. The percentage of dorsal root ganglia (DRG) neuronal profiles showing an in situ hybridization signal for preprotachykinin mRNAs encoding SP was not altered following RTX treatment (up to 8 weeks), though the signal became perceptibly weaker. In accord, 2 weeks after RTX administration a 60% decrease was observed in the steady-state levels of SP-encoding mRNAs using Northern blot analysis, leaving the ratio of beta- and gamma-preprotachykinin mRNAs unchanged. No change was, however, observed in mRNA levels encoding tachykinins NK-1 receptors in the dorsal horn, the spinal targets for SP. The present findings suggest that RTX does not kill SP-positive DRG neurons, though it suppresses the synthesis of SP. Since RTX treatment does not alter NK-1 receptor expression, this reduced SP synthesis is likely to play a central role in the analgesic actions of RTX.

The induction of pain: an integrative review

The highly disagreeable sensation of pain results from an extraordinarily complex and interactive series of mechanisms integrated at all levels of the neuroaxis, from the periphery, via the dorsal horn to higher cerebral structures. Pain is usually elicited by the activation of specific nociceptors ('nociceptive pain'). However, it may also result from injury to sensory fibres, or from damage to the CNS itself ('neuropathic pain'). Although acute and subchronic, nociceptive pain fulfils a warning role, chronic and/or severe nociceptive and neuropathic pain is maladaptive. Recent years have seen a progressive unravelling of the neuroanatomical circuits and cellular mechanisms underlying the induction of pain. In addition to familiar inflammatory mediators, such as prostaglandins and bradykinin, potentially-important, pronociceptive roles have been proposed for a variety of 'exotic' species, including protons, ATP, cytokines, neurotrophins (growth factors) and nitric oxide. Further, both in the periphery and in the CNS, non-neuronal glial and immunecompetent cells have been shown to play a modulatory role in the response to inflammation and injury, and in processes modifying nociception. In the dorsal horn of the spinal cord, wherein the primary processing of nociceptive information occurs, N-methyl-D-aspartate receptors are activated by glutamate released from nocisponsive afferent fibres. Their activation plays a key role in the induction of neuronal sensitization, a process underlying prolonged painful states. In addition, upon peripheral nerve injury, a reduction of inhibitory interneurone tone in the dorsal horn exacerbates sensitized states and further enhance nociception. As concerns the transfer of nociceptive information to the brain, several pathways other than the classical spinothalamic tract are of importance: for example, the postsynaptic dorsal column pathway. In discussing the roles of supraspinal structures in pain sensation, differences between its 'discriminative-sensory' and 'affective-cognitive' dimensions should be emphasized. The purpose of the present article is to provide a global account of mechanisms involved in the induction of pain. Particular attention is focused on cellular aspects and on the consequences of peripheral nerve injury. In the first part of the review, neuronal pathways for the transmission of nociceptive information from peripheral nerve terminals to the dorsal horn, and therefrom to higher centres, are outlined. This neuronal framework is then exploited for a consideration of peripheral, spinal and supraspinal mechanisms involved in the induction of pain by stimulation of peripheral nociceptors, by peripheral nerve injury and by damage to the CNS itself. Finally, a hypothesis is forwarded that neurotrophins may play an important role in central, adaptive mechanisms modulating nociception. An improved understanding of the origins of pain should facilitate the development of novel strategies for its more effective treatment.