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

Contributions of central and peripheral TRPV1 receptors to mechanically evoked and spontaneous firing of spinal neurons in inflamed rats

TRPV1 receptors are activated and/or modulated by noxious heat, capsaicin, protons and other endogenous agents released following tissue injury. There is a growing appreciation that this molecular integrator may also have a role in mechanosensation. To further understand this role, we investigated the systemic and site-specific effects of a selective TRPV1 receptor antagonist, A-889425, on low-intensity mechanical stimulation in inflamed rats. Systemic administration of A-889425 (30 and 100 micromol/kg po) reduced mechanical allodynia in complete Freund's adjuvant (CFA)-inflamed rats. Systemic A-889425 (3 and 10 micromol/kg iv) also decreased the responses of spinal wide dynamic range (WDR) neurons to low-intensity mechanical stimulation in CFA-inflamed but not uninjured rats. This effect of A-889425 was likely mediated via multiple sites since local injection of A-889425 into the spinal cord (1-3 nmol), ipsilateral hindpaw (200 nmol), and cerebral ventricles (30-300 nmol) all attenuated WDR responses to low-intensity mechanical stimulation. In addition to an effect on mechanotransmission, systemic administration of A-889425 reduced the spontaneous firing of WDR neurons in inflamed but not uninjured rats. Spontaneous firing is elevated after injury and may reflect ongoing pain in the animal. Local injection experiments indicated that this effect of A-889425 on spontaneous firing was mainly mediated via TRPV1 receptors in the spinal cord. Thus the current data demonstrate that TRPV1 receptors have an enhanced role after an inflammatory injury, impacting both low-intensity mechanotransmission and possibly spontaneous pain. Furthermore this study delineates the differential contribution of central and peripheral TRPV1 receptors to affect spontaneous or mechanically evoked firing of WDR neurons.

TRP channels and ASICs mediate mechanical hyperalgesia in models of inflammatory muscle pain and delayed onset muscle soreness

The roles of ion channels in sensory neurons were examined in experimental models of muscle pain in the rat. Rats were injected with 50 microl of 4% carrageenan or subjected to an eccentric exercise (ECC) of the gastrocnemius muscle (GM). The Randall-Selitto and von Frey tests were performed on the calves to evaluate mechanical hyperalgesia of the muscle. The changes in expression of four genes and proteins of ion channels in dorsal root ganglia were examined using quantitative PCR and immunohistochemistry, respectively. Effects of antagonists to transient receptor potential (TRP) channels and acid sensing ion channels (ASICs) on the mechanical hyperalgesia induced by carrageenan injection or ECC were evaluated. The mechanical hyperalgesia was observed 6-24h after carrageenan injection and 1-3 days after ECC in the Randall-Selitto test. Infiltrations of the inflammatory cells in the GM were seen in carrageenan-injected animals but not in those subjected to ECC. Expressions of genes and proteins in sensory neurons showed no changes. Intramuscular injection of antagonists to TRPV1 showed an almost complete suppressive effect on ECC-induced muscle hyperalgesia but not a carrageenan-induced one. Antagonists to TRP channels and ASICs showed suppressive effects for both carrageenan- and ECC-induced muscle hyperalgesia. The carrageenan injection and ECC models are useful models of acute inflammatory pain and delayed onset muscle soreness (DOMS), respectively, and the time course and underlying etiology might be different. TRP channels and ASICs are closely related to the development of muscle mechanical hyperalgesia, and TRPV1 is involved in ECC-induced DOMS.

The pharmacological challenge to tame the transient receptor potential vanilloid-1 (TRPV1) nocisensor

The transient receptor potential vanilloid-1 (TRPV1) cation channel is a receptor that is activated by heat (>42 degrees C), acidosis (pH<6) and a variety of chemicals among which capsaicin is the best known. With these properties, TRPV1 has emerged as a polymodal nocisensor of nociceptive afferent neurones, although some non-neuronal cells and neurones in the brain also express TRPV1. The activity of TRPV1 is controlled by a multitude of regulatory mechanisms that either cause sensitization or desensitization of the channel. As many proalgesic pathways converge on TRPV1 and this nocisensor is upregulated and sensitized by inflammation and injury, TRPV1 is thought to be a central transducer of hyperalgesia and a prime target for the pharmacological control of pain. As a consequence, TRPV1 agonists causing defunctionalization of sensory neurones and a large number of TRPV1 blockers have been developed, some of which are in clinical trials. A major drawback of many TRPV1 antagonists is their potential to cause hyperthermia, and their long-term use may carry further risks because TRPV1 has important physiological functions in the peripheral and central nervous system. The challenge, therefore, is to pharmacologically differentiate between the physiological and pathological implications of TRPV1. There are several possibilities to focus therapy specifically on those TRPV1 channels that contribute to disease processes. These approaches include (i) site-specific TRPV1 antagonists, (ii) modality-specific TRPV1 antagonists, (iii) uncompetitive TRPV1 (open channel) blockers, (iv) drugs interfering with TRPV1 sensitization, (v) drugs interfering with intracellular trafficking of TRPV1 and (vi) TRPV1 agonists for local administration.

Body-temperature maintenance as the predominant function of the vanilloid receptor TRPV1

Agonists of the transient receptor potential vanilloid type 1 (TRPV1), such as capsaicin, cause pain and a drop in body temperature (hypothermia). Conversely, antagonists of TRPV1 block pain behaviors in rodent models of inflammation, osteoarthritis and cancer. Efforts that evaluate TRPV1 antagonists in on-target challenge models have uncovered that TRPV1 blockade elicits an increase in body temperature (hyperthermia) from rodents to primates, revealing the intimate relationship between the role of TRPV1 in pain and body-temperature maintenance. This evolutionarily conserved function of TRPV1 in body-temperature maintenance became a hurdle for clinical development of one antagonist, AMG 517. However, several other TRPV1 antagonists are currently being evaluated in the clinic and soon-to-be-published results should shed light on the potential of managing antagonist-induced hyperthermia while developing them as therapeutics.

Mice lacking acid-sensing ion channels (ASIC) 1 or 2, but not ASIC3, show increased pain behaviour in the formalin test

Extracellular acidification is a component of the inflammatory process and may be a factor driving the pain accompanying it. Acid-sensing ion channels (ASICs) are neuronal proton sensors and evidence suggests they are involved in signalling inflammatory pain. The aims of this study were to (1) clarify the role of ASICs in nociception and (2) confirm their involvement in inflammatory pain and determine whether this was subunit specific. This was achieved by (1) direct comparison of the sensitivity of ASIC1, ASIC2, ASIC3 and TRPV1 knockout mice versus wildtype littermates to acute thermal and mechanical noxious stimuli and (2) studying the behavioural responses of each transgenic strain to hind paw inflammation with either complete Freund's adjuvant (CFA) or formalin. Naïve ASIC1(-/-) and ASIC2(-/-) mice responded normally to acute noxious stimuli, whereas ASIC3(-/-) mice were hypersensitive to high intensity thermal stimuli. CFA injection decreased mechanical and thermal withdrawal thresholds for up to 8 days. ASIC2(-/-) mice had increased mechanical sensitivity on day 1 post-CFA compared to wildtype controls. TRPV1(-/-) mice had significantly reduced thermal, but not mechanical, hyperalgesia on all days after inflammation. Following formalin injection, ASIC1(-/-) and ASIC2(-/-), but not ASIC3(-/-) or TRPV1(-/-), mice showed enhanced pain behaviour, predominantly in the second phase of the test. These data suggest that whilst ASICs may play a role in mediating inflammatory pain, this role is likely to be modulatory and strongly dependent on channel subtype.

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.

Pharmacological blockade of the vanilloid receptor TRPV1 elicits marked hyperthermia in humans

The vanilloid receptor TRPV1 has been identified as a molecular target for the treatment of pain associated with inflammatory diseases and cancer. Hence, TRPV1 antagonists have been considered for therapeutic evaluation in such diseases. During Phase I clinical trials with AMG 517, a highly selective TRPV1 antagonist, we found that TRPV1 blockade elicited marked, but reversible, and generally plasma concentration-dependent hyperthermia. Similar to what was observed in rats, dogs, and monkeys, hyperthermia was attenuated after repeated dosing of AMG 517 (at the highest dose tested) in humans during a second Phase I trial. However, AMG 517 administered after molar extraction (a surgical cause of acute pain) elicited long-lasting hyperthermia with maximal body temperature surpassing 40 degrees C, suggesting that TRPV1 blockade elicits undesirable hyperthermia in susceptible individuals. Mechanisms of AMG 517-induced hyperthermia were then studied in rats. AMG 517 caused hyperthermia by inducing tail skin vasoconstriction and increasing thermogenesis, which suggests that TRPV1 regulates vasomotor tone and metabolic heat production. In conclusion, these results demonstrate that: (a) TRPV1-selective antagonists like AMG 517 cannot be developed for systemic use as stand alone agents for treatment of pain and other diseases, (b) individual susceptibility influences magnitude of hyperthermia observed after TRPV1 blockade, and (c) TRPV1 plays a pivotal role as a molecular regulator for body temperature in humans.

ThermoTRP channels in nociceptors: taking a lead from capsaicin receptor TRPV1

Nociceptors with peripheral and central projections express temperature sensitive transient receptor potential (TRP) ion channels, also called thermoTRP's. Chemosensitivity of thermoTRP's to certain natural compounds eliciting pain or exhibiting thermal properties has proven to be a good tool in characterizing these receptors. Capsaicin, a pungent chemical in hot peppers, has assisted in the cloning of the first thermoTRP, TRPV1. This discovery initiated the search for other receptors encoding the response to a wide range of temperatures encountered by the body. Of these, TRPV1 and TRPV2 encode unique modalities of thermal pain when exposed to noxious heat. The ability of TRPA1 to encode noxious cold is presently being debated. The role of TRPV1 in peripheral inflammatory pain and central sensitization during chronic pain is well known. In addition to endogenous agonists, a wide variety of chemical agonists and antagonists have been discovered to activate and inhibit TRPV1. Efforts are underway to determine conditions under which agonist-mediated desensitization of TRPV1 or inhibition by antagonists can produce analgesia. Also, identification of specific second messenger molecules that regulate phosphorylation of TRPV1 has been the focus of intense research, to exploit a broader approach to pain treatment. The search for a role of TRPV2 in pain remains dormant due to the lack of suitable experimental models. However, progress into TRPA1's role in pain has received much attention recently. Another thermoTRP, TRPM8, encoding for the cool sensation and also expressed in nociceptors, has recently been shown to reduce pain via a central mechanism, thus opening a novel strategy for achieving analgesia. The role of other thermoTRP's (TRPV3 and TRPV4) encoding for detection of warm temperatures and expressed in nociceptors cannot be excluded. This review will discuss current knowledge on the role of nociceptor thermoTRPs in pain and therapy and describes the activator and inhibitor molecules known to interact with them and modulate their activity.

Vanilloid receptor 1-positive neurons mediate thermal hyperalgesia and tactile allodynia

BACKGROUND CONTEXT

The vanilloid receptor 1 (VR1) is expressed by the type II A-delta and C-fiber neurons, functioning as a molecular integrator for nociception. VR1 can be selectively ablated by resiniferatoxin (RTX), an ultra-potent excitotoxic agonist, when injected into sensory ganglia.

PURPOSE

To evaluate the role of the VR1-positive neurons in neuropathic pain.

STUDY DESIGN

Photochemical injury to rat sciatic nerve (Gazelius model).

METHODS

Two groups of rats underwent the photochemical injury and RTX treatment. RTX was injected in the dorsal root ganglia (DRGs) of the L3, L4, L5, and L6 nerve roots, either after or before the nerve injury. The animals were tested for thermal hyperalgesia (noxious heat stimuli) and mechanical allodynia (von Frey filaments). Immunohistochemical analysis of the DRGs was performed after euthanasia.

RESULTS

In the tactile allodynic rats, RTX injection in the DRGs improved the average withdrawal threshold from 1.62 g to 5.68 g. Immunohistochemical labeling showed that almost all VR1-positive neurons were eliminated. When RTX was administrated into the ipsilateral DRGs before the nerve injury, this treatment prevented the development of tactile allodynia in 12 out of 14 rats. Immunohistochemical staining revealed that the VR1-positive neurons were eliminated in the rats that did not develop tactile allodynia, whereas they were still present in the allodynic rats.

CONCLUSIONS

VR1-positive neurons are essential for the development of mechanical allodynia. In rats already exhibiting neuropathic pain, the VR1-positive neurons mediate the most sensitive part of mechanical allodynia. RTX injection in sensory ganglia may represent a novel treatment for neuropathic pain.

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.

Sensory neural targets for the treatment of cough

1. Cough is a primary defensive reflex that protects the airways from potentially harmful stimuli. 2. During many respiratory diseases, the cough reflex threshold is lowered and coughing becomes excessive. 3. Currently available therapeutics are mostly ineffective at suppressing excessive coughing. 4. In the present review, we describe the sensory neural pathways involved in cough, how these pathways may become dysfunctional in airway disease and the most recent advances that have been made in identifying future targets for cough suppression.

Short- and long-term plasticity of the endocannabinoid system in neuropsychiatric and neurological disorders

The activity of the endocannabinoid system, in terms of the levels of the endocannabinoids and of cannabinoid receptors, or of the functional coupling of the latter to a biological response, undergoes to remodelling during pathological conditions. In the CNS, these changes, depending also on the nature of the disorder, can be transient or long-lasting, occur only in those tissues involved in the pathological condition and usually aim at restoring the physiological homeostasis by reducing excitotoxicity, inflammation and neuronal death. However, during chronic disorders, prolonged activation of the endocannabinoid system might also contribute to the symptoms of the pathology. Whilst acute changes of the tissue levels of the endocannabinoids reflect the "on demand" nature of their biosynthesis and release, and hence are effected mostly through regulation of the biosynthetic enzymes, chronic changes seem to be mostly due to longer-lasting alterations in the expression of anabolic and catabolic enzymes. The possibility of obtaining therapeutic advantage from endocannabinoid plasticity in neuropsychiatric and neurological disorders is discussed in this review article.

Use of the novel Contact Heat Evoked Potential Stimulator (CHEPS) for the assessment of small fibre neuropathy: correlations with skin flare responses and intra-epidermal nerve fibre counts

Background

The Contact Heat Evoked Potential Stimulator (CHEPS) rapidly stimulates cutaneous small nerve fibres, and resulting evoked potentials can be recorded from the scalp. We have studied patients with symptoms of sensory neuropathy and controls using CHEPS, and validated the findings using other objective measures of small nerve fibres i.e. the histamine-induced skin flare response and intra-epidermal fibres (IEF), and also quantitative sensory testing (QST), a subjective measure.

Methods

In patients with symptoms of sensory neuropathy (n = 41) and healthy controls (n = 9) we performed clinical examination, QST (monofilament, vibration and thermal perception thresholds), nerve conduction studies, histamine-induced skin flares and CHEPS. Skin punch biopsies were immunostained using standard ABC immunoperoxidase for the nerve marker PGP 9.5 or the heat and capsaicin receptor TRPV1. Immunoreactive IEF were counted per length of tissue section and epidermal thickness recorded.

Results

Amplitudes of Aδ evoked potentials (μV) following face, arm or leg stimulation were reduced in patients (e.g. for the leg: mean ± SEM – controls 11.7 ± 1.95, patients 3.63 ± 0.85, p = 0.0032). Patients showed reduced leg skin flare responses, which correlated with Aδ amplitudes (r_s = 0.40, p = 0.010). In patient leg skin biopsies, PGP 9.5- and TRPV1-immunoreactive IEF were reduced and correlated with Aδ amplitudes (PGP 9.5, r_s = 0.51, p = 0.0006; TRPV1, r_s = 0.48, p = 0.0012).

Conclusion

CHEPS appears a sensitive measure, with abnormalities observed in some symptomatic patients who did not have significant IEF loss and/or QST abnormalities. Some of the latter patients may have early small fibre dysfunction or ion channelopathy. CHEPS provides a clinically practical, non-invasive and objective measure, and can be a useful additional tool for the assessment of sensory small fibre neuropathy. Although further evaluation is required, the technique shows potential clinical utility to differentiate neuropathy from other chronic pain states, and provide a biomarker for analgesic development.

TRP channels: Targets for the relief of pain

Patients with inflammatory or neuropathic pain experience hypersensitivity to mechanical, thermal and/or chemical stimuli. Given the diverse etiologies and molecular mechanisms of these pain syndromes, an approach to developing successful therapies may be to target ion channels that contribute to the detection of thermal, mechanical and chemical stimuli and promote the sensitization and activation of nociceptors. Transient Receptor Potential (TRP) channels have emerged as a family of evolutionarily conserved ligand-gated ion channels that contribute to the detection of physical stimuli. Six TRPs (TRPV1, TRPV2, TRPV3, TRPV4, TRPM8 and TRPA1) have been shown to be expressed in primary afferent nociceptors, pain sensing neurons, where they act as transducers for thermal, chemical and mechanical stimuli. This short review focuses on their contribution to pain hypersensitivity associated with peripheral inflammatory and neuropathic pain states.

[3H]A-778317 [1-((R)-5-tert-butyl-indan-1-yl)-3-isoquinolin-5-yl-urea]: a novel, stereoselective, high-affinity antagonist is a useful radioligand for the human transient receptor potential vanilloid-1 (TRPV1) receptor

1-((R)-5-tert-butyl-indan-1-yl)-3-isoquinolin-5-yl-urea (A-778317) is a novel, stereoselective, competitive antagonist that potently blocks transient receptor potential vanilloid-1 (TRPV1) receptor-mediated changes in intracellular calcium concentrations (pIC50 = 8.31 +/- 0.13). The (S)-stereoisomer, 1-((S)-5-tert-butyl-indan-1-yl)-3-isoquinolin-5-yl-urea (A-778316), is 6.8-fold less potent (pIC50 = 7.47 +/- 0.07). A-778317 also potently blocks capsaicin and acid activation of native rat TRPV1 receptors in dorsal root ganglion neurons. A-778317 was tritiated ([3H]A-778317; 29.3 Ci/mmol) and used to study recombinant human TRPV1 (hTRPV1) receptors expressed in Chinese ovary cells (CHO) cells. [3H]A-778317 labeled a single class of binding sites in hTRPV1-expressing CHO cell membranes with high affinity (KD = 3.4 nM; Bmax = 4.0 pmol/mg protein). Specific binding of 2 nM [3H]A-778317 to hTRPV1-expressing CHO cell membranes was reversible. The rank-order potency of TRPV1 receptor antagonists to inhibit binding of 2 nM [3H]A-778317 correlated well with their functional potencies in blocking TRPV1 receptor activation. The present data demonstrate that A-778317 blocks polymodal activation of the TRPV1 receptor by binding to a single high-affinity binding site and that [3H]A-778317 possesses favorable binding properties to facilitate further studies of hTRPV1 receptor pharmacology.

Repeated administration of vanilloid receptor TRPV1 antagonists attenuates hyperthermia elicited by TRPV1 blockade

Capsaicin, the active ingredient in some pain-relieving creams, is an agonist of a nonselective cation channel known as the transient receptor potential vanilloid type 1 (TRPV1). The pain-relieving mechanism of capsaicin includes desensitization of the channel, suggesting that TRPV1 antagonism may be a viable pain therapy approach. In agreement with the above notion, several TRPV1 antagonists have been reported to act as antihyperalgesics. Here, we report the in vitro and in vivo characterization of a novel and selective TRPV1 antagonist, N-(4-[6-(4-trifluoromethyl-phenyl)-pyrimidin-4-yloxy]-benzothiazol-2-yl)-acetamide I (AMG 517), and compare its pharmacology with that of a closely related analog, tert-butyl-2-(6-([2-(acetylamino)-1,3-benzothiazol-4-yl]oxy)pyrimidin-4-yl)-5-(trifluoromethyl)phenylcarbamate (AMG8163). Both AMG 517 and AMG8163 potently and completely antagonized capsaicin, proton, and heat activation of TRPV1 in vitro and blocked capsaicin-induced flinch in rats in vivo. To support initial clinical investigations, AMG 517 was evaluated in a comprehensive panel of toxicology studies that included in vivo assessments in rodents, dogs, and monkeys. The toxicology studies indicated that AMG 517 was generally well tolerated; however, transient increases in body temperature (hyperthermia) were observed in all species after AMG 517 dosing. To further investigate this effect, we tested and showed that the antipyretic, acetaminophen, suppressed the hyperthermia caused by TRPV1 blockade. We also showed that repeated administration of TRPV1 antagonists attenuated the hyperthermia response, whereas the efficacy in capsaicin-induced flinch model was maintained. In conclusion, these studies suggest that the transient hyperthermia elicited by TRPV1 blockade may be manageable in the development of TRPV1 antagonists as therapeutic agents. However, the impact of TRPV1 antagonist-induced hyperthermia on their clinical utility is still unknown.

Differential involvement of TRPV1 receptors at the central and peripheral nerves in CFA-induced mechanical and thermal hyperalgesia

Transient receptor potential vanilloid 1 (TRPV1) antagonists are known to attenuate two typical symptoms of inflammatory hyperalgesia: thermal and mechanical. However, it is not clear whether the sites of participation of TRPV1 for each symptom are different. In this study, we clarified the difference between the site of TRPV1 involvement in both symptoms by analysing the anti-hyperalgesic activity of two kinds of TRPV1 antagonists given locally (i.e. intraplantarly and intrathecally) in rats with CFA (complete Freund's adjuvant)-induced inflammation. TRPV1 antagonists BCTC (N-(4-tertiarybutylphenyl)-4-(3-cholorphyridin-2-yl) tetrahydropyrazine-1(2H)-carbox-amide, 1-300 microg) and SB-366791 (N-(3-methoxyphenyl)-4-chlorocinnamide, 30-300 microg) administered intraplantarly in a dose-dependent manner inhibited CFA-induced thermal hyperalgesia. In addition, CFA-induced thermal hyperalgesia was significantly reversed by intrathecal administration of 1-100 microg of BCTC and SB-366791. While intraplantar BCTC (1-300 microg) and SB-366791 (30-300 microg) did not reverse CFA-induced mechanical hyperalgesia, 1-100 microg of intrathecally administered BCTC and SB-366791 dose-dependently reduced mechanical hyperalgesia. Regression analysis showed that a correlation exists between the inhibitory effects on thermal hyperalgesia and mechanical hyperalgesia after intrathecal administration (correlation factor = 0.6521), but not after intraplantar administration (correlation factor = 0.0215). These data suggest that TRPV1 in the peripheral endings of the primary afferents plays a key role in thermal hyperalgesia, but it makes only a minor contribution in CFA-induced mechanical hyperalgesia. Furthermore, it is suggested that the spinal TRPV1 is critical in the development of both types of hyperalgesia.

α-Methylation at benzylic fragment of N-aryl-N′-benzyl ureas provides TRPV1 antagonists with better pharmacokinetic properties and higher efficacy in inflammatory pain model

SAR studies for N-aryl-N'-benzyl urea class of TRPV1 antagonists have been extended to cover alpha-benzyl alkylation. Alkylated compounds showed weaker in vitro potencies in blocking capsaicin activation of TRPV1 receptor, but possessed improved pharmacokinetic properties. Further structural manipulations that included replacement of isoquinoline core with indazole and isolation of single enantiomer led to TRPV1 antagonists like (R)-16a with superior pharmacokinetic properties and greater potency in animal model of inflammatory pain.

Novel Vanilloid Receptor-1 Antagonists: 1. Conformationally Restricted Analogues of trans-Cinnamides

The vanilloid receptor-1 (VR1 or TRPV1) is a member of the transient receptor potential (TRP) family of ion channels and plays a role as an integrator of multiple pain-producing stimuli. From a high-throughput screening assay, measuring calcium uptake in TRPV1-expressing cells, we identified an N-aryl trans-cinnamide (AMG9810, compound 9) that acts as a potent TRPV1 antagonist. We have demonstrated the antihyperalgesic properties of 9 in vivo and have also reported the discovery of novel, orally bioavailable cinnamides derived from this lead. Herein, we expand our investigations and describe the synthesis and biological evaluation of a series of conformationally constrained analogues of the s-cis conformer of compound 9. These investigations resulted in the identification of 4-amino- and 4-oxopyrimidine cores as suitable isosteric replacements for the trans-acrylamide moiety. The best examples from this series, pyrimidines 79 and 74, were orally bioavailable and exhibited potent antagonism of both rat (IC50 = 4.5 and 0.6 nM, respectively) and human TRPV1 (IC50 = 7.4 and 3.7 nM, respectively). In addition, compound 74 was shown to be efficacious at blocking a TRPV1-mediated physiological response in vivo in the capsaicin-induced hypothermia model in rats; however, it was ineffective at preventing thermal hyperalgesia induced by complete Freund's adjuvant in rats.

Application of Large-Scale Transiently Transfected Cells to Functional Assays of Ion Channels: Different Targets and Assay Formats

Cell-based functional assays are increasingly being utilized for ion channels and other targets in drug discovery. However, development of functional assays is often hampered by problems related to stable expression of ion channels in host cell lines, such as variability in channel activity, cell line degeneration, toxicity associated with gene expression, and time and expense of maintaining the cells in culture. In a previous study, we showed that constitutive expression of the transient receptor potential ankyrin-1 (TRPA1) channel led to cellular toxicity and cell line degeneration. This problem could be circumvented by utilizing large-scale transiently transfected (LSTT) cells, which could be prepared in large quantity and kept frozen at -80 degrees C until needed. LSTT cells from a single preparation were successfully applied toward development of a Ca(2+) influx assay for TRPA1 and a high throughput screening of a >700,000 compound library. In the current study, we extended the application of LSTT cells to Ca(2+) influx assays for transient receptor potential vanilloid-1 (TRPV1), transient receptor potential melastatin-8, and transient receptor potential vanilloid-4 channels. In addition, we found that cryopreserved LSTT cells expressing TRPV1 exhibited the same pharmacology as a TRPV1 stable cell line in the Ca(2+) influx assay. Moreover, by using LSTT cells expressing TRPA1, we successfully developed a membrane potential assay, which gave comparable results to the Ca(2+) influx assay. Hence, the utilization of LSTT cells could reduce the need for stable cell lines, and enable development of functional assays in a more timely and economic fashion for different ion channels and different assay formats.

The vanilloid receptor TRPV1: 10 years from channel cloning to antagonist proof-of-concept

The clinical use of TRPV1 (transient receptor potential vanilloid subfamily, member 1; also known as VR1) antagonists is based on the concept that endogenous agonists acting on TRPV1 might provide a major contribution to certain pain conditions. Indeed, a number of small-molecule TRPV1 antagonists are already undergoing Phase I/II clinical trials for the indications of chronic inflammatory pain and migraine. Moreover, animal models suggest a therapeutic value for TRPV1 antagonists in the treatment of other types of pain, including pain from cancer. We argue that TRPV1 antagonists alone or in conjunction with other analgesics will improve the quality of life of people with migraine, chronic intractable pain secondary to cancer, AIDS or diabetes. Moreover, emerging data indicate that TRPV1 antagonists could also be useful in treating disorders other than pain, such as urinary urge incontinence, chronic cough and irritable bowel syndrome. The lack of effective drugs for treating many of these conditions highlights the need for further investigation into the therapeutic potential of TRPV1 antagonists.

The vanilloid receptor TRPV1 is tonically activated in vivo and involved in body temperature regulation

The vanilloid receptor TRPV1 (transient receptor potential vanilloid 1) is a cation channel that serves as a polymodal detector of pain-producing stimuli such as capsaicin, protons (pH <5.7), and heat. TRPV1 antagonists block pain behaviors in rodent models of inflammatory, neuropathic, and cancer pain, suggesting their utility as analgesics. Here, we report that TRPV1 antagonists representing various chemotypes cause an increase in body temperature (hyperthermia), identifying a potential issue for their clinical development. Peripheral restriction of antagonists did not eliminate hyperthermia, suggesting that the site of action is predominantly outside of the blood-brain barrier. Antagonists that are ineffective against proton activation also caused hyperthermia, indicating that blocking capsaicin and heat activation of TRPV1 is sufficient to produce hyperthermia. All TRPV1 antagonists evaluated here caused hyperthermia, suggesting that TRPV1 is tonically activated in vivo and that TRPV1 antagonism and hyperthermia are not separable. TRPV1 antagonists caused hyperthermia in multiple species (rats, dogs, and monkeys), demonstrating that TRPV1 function in thermoregulation is conserved from rodents to primates. Together, these results indicate that tonic TRPV1 activation regulates body temperature.

Characterization of SB-705498, a potent and selective vanilloid receptor-1 (VR1/TRPV1) antagonist that inhibits the capsaicin-, acid-, and heat-mediated activation of the receptor

Vanilloid receptor-1 (TRPV1) is a nonselective cation channel, predominantly expressed by sensory neurons, which plays a key role in the detection of noxious painful stimuli such as capsaicin, acid, and heat. TRPV1 antagonists may represent novel therapeutic agents for the treatment of a range of conditions including chronic pain, migraine, and gastrointestinal disorders. Here we describe the in vitro pharmacology of N-(2-bromophenyl)-N'-[((R)-1-(5-trifluoromethyl-2-pyridyl)pyrrolidin-3-yl)]urea (SB-705498), a novel TRPV1 antagonist identified by lead optimization of N-(2-bromophenyl)-N'-[2-[ethyl(3-methylphenyl)amino]ethyl]urea (SB-452533), which has now entered clinical trials. Using a Ca(2+)-based fluorometric imaging plate reader (FLIPR) assay, SB-705498 was shown to be a potent competitive antagonist of the capsaicin-mediated activation of the human TRPV1 receptor (pK(i) = 7.6) with activity at rat (pK(i) = 7.5) and guinea pig (pK(i) = 7.3) orthologs. Whole-cell patch-clamp electrophysiology was used to confirm and extend these findings, demonstrating that SB-705498 can potently inhibit the multiple modes of receptor activation that may be relevant to the pathophysiological role of TRPV1 in vivo: SB-705498 caused rapid and reversible inhibition of the capsaicin (IC(50) = 3 nM)-, acid (pH 5.3)-, or heat (50 degrees C; IC(50) = 6 nM)-mediated activation of human TRPV1 (at -70 mV). Interestingly, SB-705498 also showed a degree of voltage dependence, suggesting an effective enhancement of antagonist action at negative potentials such as those that might be encountered in neurons in vivo. The selectivity of SB-705498 was defined by broad receptor profiling and other cellular assays in which it showed little or no activity versus a wide range of ion channels, receptors, and enzymes. SB-705498 therefore represents a potent and selective multimodal TRPV1 antagonist, a pharmacological profile that has contributed to its definition as a suitable drug candidate for clinical development.

Antinociceptive pharmacology of N-(4-chlorobenzyl)-N'-(4-hydroxy-3-iodo-5-methoxybenzyl) thiourea, a high-affinity competitive antagonist of the transient receptor potential vanilloid 1 receptor

The transient receptor potential vanilloid 1 receptor (TRPV1) is expressed predominantly in a subset of primary afferent nociceptors. Due to its specific anatomical location and its pivotal role as a molecular integrator for noxious thermal and chemical stimuli, there is considerable interest to develop TRPV1 antagonists for the treatment of pain. Recently, N-(4-chlorobenzyl)-N'-(4-hydroxy-3-iodo-5-methoxybenzyl) thiourea (IBTU) was synthesized, and it was found in vitro to be a high-affinity competitive antagonist of cytoplasmic, but not intracellular, TRPV1. In this study, we examined the in vivo antinociceptive activity of IBTU in several acute and inflammatory pain models in mice. Our emphasis was on nociceptive pathways that are likely mediated by TRPV1, including capsaicin-, noxious heat-, and proton (including inflammation)-induced nociception tests. Capsazepine was used as a positive control in these experiments. IBTU dose-dependently blocked the capsaicin-induced nociception, confirming its antagonism at TRPV1 in vivo. By itself, IBTU produced significant antinociception, because it significantly prolonged the tail-flick latency in a dose-dependent manner. IBTU also blocked both early and late phases of the formalin-induced flinching response as well as acetic acid-induced writhing behavior. Moreover, IBTU inhibited the complete Freund's adjuvant-induced persistent hyperalgesia. Taken together, these data demonstrate that IBTU acts as a TRPV1 antagonist in vivo, and they suggest that it may be of therapeutic use for the treatment of pain.

Capsaicin causes protein synthesis inhibition and microtubule disassembly through TRPV1 activities both on the plasma membrane and intracellular membranes

TRPV1 is a non-selective cationic channel that is activated by capsaicin, acidic pH and thermal stimuli. Sustained TRPV1 channel activation causes severe cytotoxicity that leads to cell death. In this study, we investigated the mechanisms of capsaicin-induced cytotoxicity in HEK293 cells stably expressing TRPV1 with a focus on protein synthesis regulation and cytoskeleton reorganization. Capsaicin inhibited protein synthesis in TRPV1-expressing HEK cells with an IC(50) of 15.6nM and depolymerized microtubules within 10min after exposure. These effects were completely blocked by pretreatment of cells with the TRPV1 antagonist A-425619, both in the presence and absence of extracellular calcium. Protein synthesis inhibition induced by capsaicin was not a result of eIF2alpha hyperphosphorylation, but rather closely correlated with cytosolic calcium elevation caused by calcium flux through cell surface and intracellular TRPV1, and/or ER calcium depletion through intracellular TRPV1. Microtubule dependent cell process shrinkage may serve as a mechanism for rapid alteration of the neurotransmission network upon TRPV1 activation. Taken together, the present studies demonstrate that intracellular pool of TRPV1 plays an important role in regulating cell morphology and viability upon receptor activation.

Actions of the FAAH inhibitor URB597 in neuropathic and inflammatory chronic pain models

While cannabinoid receptor agonists have analgesic activity in chronic pain states, they produce a spectrum of central CB(1) receptor-mediated motor and psychotropic side effects. The actions of endocannabinoids, such as anandamide are terminated by removal from the extracellular space, then subsequent enzymatic degradation by fatty-acid amide hydrolase (FAAH). In the present study, we compared the effect of a selective FAAH inhibitor, URB597, to that of a pan-cannabinoid receptor agonist HU210 in rat models of chronic inflammatory and neuropathic pain. Systemic administration of URB597 (0.3 mg kg(-1)) and HU210 (0.03 mg kg(-1)) both reduced the mechanical allodynia and thermal hyperalgesia in the CFA model of inflammatory pain. In contrast, HU210, but not URB597, reduced mechanical allodynia in the partial sciatic nerve-ligation model of neuropathic pain. HU210, but not URB597, produced a reduction in motor performance in unoperated rats. The effects of URB597 in the CFA model were dose dependent and were reduced by coadministration with the cannabinoid CB1 antagonist AM251 (1 mg kg(-1)), or the CB2 and SR144528 (1 mg kg(-1)). Coadministration with AM251 plus SR144528 completely reversed the effects of URB597. These findings suggest that the FAAH inhibitor URB597 produces cannabinoid CB1 and CB2 receptor-mediated analgesia in inflammatory pain states, without causing the undesirable side effects associated with cannabinoid receptor activation.

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.

ASIC3 in muscle mediates mechanical, but not heat, hyperalgesia associated with muscle inflammation

Peripheral initiators of muscle pain are virtually unknown, but likely key to development of chronic pain after muscle insult. The current study tested the hypothesis that ASIC3 in muscle is necessary for development of cutaneous mechanical, but not heat, hyperalgesia induced by muscle inflammation. Using mechanical and heat stimuli, we assessed behavioral responses in ASIC3-/- and ASIC3+/+ mice after induction of carrageenan muscle inflammation. ASIC3-/- mice did not develop cutaneous mechanical hyperalgesia after muscle inflammation when compared to ASIC3+/+ mice; heat hyperalgesia developed similarly between groups. We then tested if the phenotype could be rescued in ASIC3-/- mice by using a recombinant herpes virus vector to express ASIC3 in skin (where testing occurred) or muscle (where inflammation occurred). Infection of mouse DRG neurons with ASIC3-encoding virus resulted in functional expression of ASICs. Injection of ASIC3-encoding virus into muscle or skin of ASIC3-/- mice resulted in ASIC3 mRNA in DRG and protein expression in DRG and the peripheral injection site. Injection of ASIC3-encoding virus into muscle, but not skin, resulted in development of mechanical hyperalgesia similar to that observed in ASIC3+/+ mice. Thus, ASIC3 in primary afferent fibers innervating muscle is critical to development of hyperalgesia that results from muscle insult.

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.

TRPV1 receptors in the CNS play a key role in broad-spectrum analgesia of TRPV1 antagonists

Vanilloid receptor type 1 (TRPV1) is a ligand-gated nonselective cation channel that is considered to be an important integrator of various pain stimuli such as endogenous lipids, capsaicin, heat, and low pH. In addition to expression in primary afferents, TRPV1 is also expressed in the CNS. To test the hypothesis that the CNS plays a differential role in the effect of TRPV1 antagonists in various types of pain, the analgesic effects of two TRPV1 antagonists with similar in vitro potency but different CNS penetration were compared in vivo. Oral administration of either A-784168 (1-[3-(trifluoromethyl)pyridin-2-yl]-N-[4-(trifluoromethylsulfonyl)phenyl]-1,2,3,6-tetrahydropyridine-4-carboxamide) (good CNS penetration) or A-795614 (N-1H-indazol-4-yl-N'-[(1R)-5-piperidin-1-yl-2,3-dihydro-1H-inden-1-yl]urea) (poor CNS penetration) blocked capsaicin-induced acute pain with the same potency. In complete Freund's adjuvant (CFA)-induced chronic inflammatory pain, oral administration of either compound blocked thermal hyperalgesia with similar potency. Furthermore, intraplantar or intrathecal administration of A-784168 blocked CFA-induced thermal hyperalgesia, suggesting that both peripheral and CNS TRPV1 receptors may play a role in inflammatory thermal hyperalgesia. The effects of the two TRPV1 antagonists were further assessed in models presumably mediated by central sensitization, including CFA- and capsaicin-induced mechanical allodynia and osteoarthritic pain. In these models, the potency of the two compounds was similar after intrathecal administration. However, when administered orally, A-784168, with good CNS penetration, was much more potent than A-795614. Together, these results demonstrate that TRPV1 receptors in the CNS play an important role in pain mediated by central sensitization. In addition, these results demonstrate that significant CNS penetration is necessary for a TRPV1 antagonist to produce broad-spectrum analgesia.

Design and Characterization of a Noncompetitive Antagonist of the Transient Receptor Potential Vanilloid Subunit 1 Channel With In Vivo Analgesic and Anti-inflammatory Activity

Vanilloid receptor subunit 1 (TRPV1) is an integrator of physical and chemical stimuli in the peripheral nervous system. This receptor plays a key role in the pathophysiology of inflammatory pain. Thus, the identification of receptor antagonists with analgesic and anti-inflammatory activity in vivo is an important goal of current neuropharmacology. Here, we report that [L-arginyl]-[N-[2,4-dichlorophenethyl]glycyl]-N-(2,4-dichlorophenethyl) glycinamide (H-Arg-15-15C) is a channel blocker that abrogates capsaicin and pH-evoked TRPV1 channel activity with submicromolar activity. Compound H-Arg-15-15C preferentially inhibits TRPV1, showing marginal block of other neuronal receptors. Compound H-Arg-15-15C acts as a noncompetitive capsaicin antagonist with modest voltage-dependent blockade activity. The compound inhibited capsaicin-evoked nerve activity in afferent fibers without affecting mechanically activated activity. Notably, administration of compound H-Arg-15-15C prevented the irritant activity of a local administration of capsaicin and formalin and reversed the thermal hyperalgesia evoked by injection of complete Freund's adjuvant. Furthermore, it attenuated carrageenan-induced paw inflammation. Compound H-Arg-15-15C specifically decreased inflammatory conditions without affecting normal nociception. Taken together, these findings demonstrate that compound H-Arg-15-15C is a channel blocker of TRPV1 with analgesic and anti-inflammatory activity in vivo at clinically useful doses and substantiate the tenet that TRPV1 plays an important role in the etiology of chronic inflammatory pain.

PERSPECTIVE

This study reports the design of a potent TRPV1 noncompetitive antagonist that exhibits anti-inflammatory and analgesic activity in preclinical models of acute and chronic pain. This compound is a lead for analgesic drug development.

TRPV1: a therapeutic target for novel analgesic drugs?

The vanilloid receptor TRPV1 is now recognized as a molecular integrator of painful stimuli ranging from noxious heat to endovanilloids in inflammation. Pharmacological blockade of TRPV1 represents a new strategy in pain relief. TRPV1 antagonists are expected to prevent pain by silencing receptors where pain is generated rather than stopping the propagation of pain, as most-traditional pain killers do. This hypothesis has already being tested in the clinic by administering small molecule TRPV1 antagonists (e.g. GlaxoSmithKline SB-705498) for migraine and dental pain. Paradoxically, in some murine models of chronic pain, TRPV1-deficient mice exhibit more pain-related behavior than their wild-type littermates, indicating that the understanding of TRPV1 in pain is still incomplete. Moreover, there is mounting evidence to suggest the existence of functional TRPV1 both in the brain and in various non-neuronal tissues. The biological role of these receptors remains elusive, but their tissue distribution clearly indicates that they are involved in many more functions than just pain perception. Here, we review the potential therapeutic indications and adverse effects of TRPV1 antagonists.

The TRPV1 receptor and nociception

The capsaicin receptor TRPV1 is an emerging target for the treatment of pain with a unique expression profile in peripheral nociceptors and the ability to show polymodal activation, TRPV1 is an important integrator of responses to inflammatory mediators. Sensitization of TRPV1 during chronic pain is believed to contribute to the transduction of noxious signaling for normally innocuous stimuli and consequently the search for novel TRPV1 therapeutics is intense. The current understanding of the physiological role the receptor, as well as the potential therapeutic utility and emerging liabilities of TRPV1 modulators are discussed.

Transient receptor potential ion channels as participants in thermosensation and thermoregulation

Living organisms must evaluate changes in environmental and internal temperatures to mount appropriate physiological and behavioral responses conducive to survival. Classical physiology has provided a wealth of information regarding the specialization of thermosensory functions among subclasses of peripheral sensory neurons and intrinsically thermosensitive neurons within the hypothalamus. However, until recently, the molecular mechanisms by which these cells carry out thermometry have remained poorly understood. The demonstration that certain ion channels of the transient receptor potential (TRP) family can be activated by increases or decreases in ambient temperature, along with the recognition of their heterogeneous expression patterns and heterogeneous temperature sensitivities, has led investigators to evaluate these proteins as candidate endogenous thermosensors. Much of this work has involved one specific channel, TRP vanilloid 1 (TRPV1), which is both a receptor for capsaicin and related pungent vanilloid compounds and a "heat receptor," capable of directly depolarizing neurons in response to temperatures >42 degrees C. Evidence for a contribution of TRPV1 to peripheral thermosensation has come from pharmacological, physiological, and genetic approaches. In contrast, although capsaicin-sensitive mechanisms clearly influence core body temperature regulation, the specific contribution of TRPV1 to this process remains a matter of debate. Besides TRPV1, at least six additional thermally sensitive TRP channels have been identified in mammals, and many of these also appear to participate in thermosensation. Moreover, the identification of invertebrate TRP channels, whose genetic ablation alters thermally driven behaviors, makes it clear that thermosensation represents an evolutionarily conserved role of this ion channel family.

Comparison of antinociceptive actions of standard analgesics in attenuating capsaicin and nerve-injury-induced mechanical hypersensitivity

Intradermal capsaicin injection produces immediate spontaneous pain behaviors, and a secondary mechanical hypersensitivity (SMH) that is employed in the clinic as a model potentially predictive of human neuropathic pain. Presently, we have characterized capsaicin-induced SMH in rats, and compared pharmacological actions of standard analgesics in this and two nerve injury models, the L5/L6 spinal nerve ligation (SNL) and sciatic nerve chronic constriction injury (CCI) models. Intraplantar capsaicin produced dose-related SMH (enhanced paw withdrawal response to von Frey monofilament stimulation at an area away from injection site) that lasted for over 4 h. While pretreatment with a potent selective transient receptor potential vanilloid receptor-1 (TRPV1) antagonist A-425619 (1-isoquinolin-5-yl-3-(4-trifluoromethyl-benzyl)-urea) prevented development of acute nocifensive (flinching) behavior immediately following capsaicin injection (ED(50)=4.9 mg/kg), the compound failed to attenuate the SMH when administered 2 h following capsaicin (10 microg/10 microl). Additional standard analgesics were also tested 3 h following intraplantar capsaicin in the SMH model. Comparison of their potencies in attenuating mechanical hypersensitivity in capsaicin, SNL and CCI models revealed similar ED(50)s for morphine (2.3 mg/kg, 1.6 mg/kg and 3.2 mg/kg, respectively), gabapentin (33.1 mg/kg, 33.9 mg/kg and 26.3 mg/kg, respectively) and lamotrigine (9.1 mg/kg, 8.9 mg/kg and 15.5 mg/kg, respectively). Duloxetine produced 50-65% effect at the highest tested dose (50 mg/kg), whereas the highest tested doses of morphine (10 mg/kg), gabapentin (85.5 mg/kg) and lamotrigine (30 mg/kg) all produced >70% efficacy in capsaicin SMH, SNL and CCI models. In contrast, celecoxib and ibuprofen showed weak effects in all three models. All standard analgesics generally had weak efficacy in attenuating capsaicin-induced immediate acute flinching behavior when administered before capsaicin. These results provide further support to the suggestions that distinct pharmacological mechanisms underlie capsaicin-induced acute nocifensive and SMH behaviors, and certain neuronal mechanisms underlying neuropathic pain states are also contributory to capsaicin-induced SMH.

Analgesic strategies beyond the inhibition of cyclooxygenases

Blocking the formation of prostaglandins with cyclooxygenase (COX) inhibitors has been the treatment of choice for inflammatory pain for more than a century. Although these agents provide profound pain relief, their long-term use is hampered by severe side-effects, mainly ulceration of the upper gastrointestinal tract. The development of COX-2-selective inhibitors ("coxibs") has significantly reduced gastrointestinal toxicity, but evidence from controlled clinical trials and experimental studies indicates that the use of coxibs has a significant cardiovascular risk. Recently, signalling elements downstream of COX-2 inhibition have been identified, which offer a great diversity of possible targets. This review focuses on prostaglandin E synthases, prostaglandin receptors and downstream effectors of prostaglandins in the PNS and CNS, including transient receptor potential channels, tetrodotoxin-resistant Na(+) channels and inhibitory glycine receptors. These novel targets should enable inflammatory pain to be treated with improved specificity and, possibly, fewer side-effects.

A Polyclonal Antibody to the Prepore Loop of Transient Receptor Potential Vanilloid Type 1 Blocks Channel Activation

Transient receptor potential vanilloid type 1 (TRPV1) can be activated by multiple chemical and physical stimuli such as capsaicin, anandamide, protons, and heat. Capsaicin interacts with the binding pocket constituted by transmembrane regions 3 and 4, whereas protons act through residues in the prepore loop of TRPV1. Here, we report on characterization of polyclonal and monoclonal antibodies to the prepore loop of TRPV1. A rabbit anti-rat TRPV1 polyclonal antibody (Ab-156H) acted as a full antagonist of proton activation (IC(50) values for pH 5 and 5.5 were 364.68 +/- 29.78 and 28.31 +/- 6.30 nM, respectively) and as a partial antagonist of capsaicin, heat, and pH 6 potentiated chemical ligand (anandamide and capsaicin) activation (50-79% inhibition). Ab-156H antagonism of TRPV1 is not affected by the conformation of the capsaicin-binding pocket because it is equally potent at wild-type (capsaicin-sensitive) rat TRPV1 and its T550I mutant (capsaicin-insensitive). With the goal of generating monoclonal antagonist antibodies to the prepore region of human TRPV1, we used a recently developed rabbit immunization protocol. Although rabbit polyclonal antiserum blocked human TRPV1 activation, rabbit monoclonal antibodies (identified on the basis of selective binding to Chinese hamster ovary cells expressing human TRPV1) did not block activation by either capsaicin or protons. Thus, rabbit polyclonal antibodies against rat and human TRPV1 prepore region seem to partially lock or stabilize the channel in the closed state, whereas rabbit anti-human TRPV1 monoclonal antibodies bind to the prepore region but do not lock or stabilize the channel conformation.