TRPV1 Receptor Antagonist Research Articles

Synergistic interactions between airway afferent nerve subtypes regulating the cough reflex in guinea‐pigs

Cough initiated from the trachea and larynx in anaesthetized guinea-pigs is mediated by capsaicin-insensitive, mechanically sensitive vagal afferent neurones. Tachykinin-containing, capsaicin-sensitive C-fibres also innervate the airways and have been implicated in the cough reflex. Capsaicin-sensitive nerves act centrally and synergistically to modify reflex bronchospasm initiated by airway mechanoreceptor stimulation. The hypothesis that polymodal mechanoreceptors and capsaicin-sensitive afferent nerves similarly interact centrally to regulate coughing was addressed in this study. Cough was evoked from the tracheal mucosa either electrically (16 Hz, 10 s trains, 1-10 V) or by citric acid (0.001-2 m). Neither capsaicin nor bradykinin evoked a cough when applied to the trachea of anaesthetized guinea-pigs, but they substantially reduced the electrical threshold for initiating the cough reflex. The TRPV1 receptor antagonist capsazepine prevented the increased cough sensitivity induced by capsaicin. These effects of topically applied capsaicin and bradykinin were not due to interactions between afferent nerve subtypes within the tracheal wall or a direct effect on the cough receptors, as they were mimicked by nebulizing 1 mg ml(-1) bradykinin into the lower airways and by microinjecting 0.5 nmol capsaicin into nucleus of the solitary tract (nTS). Citric acid-induced coughing was also potentiated by inhalation of bradykinin. The effects of tracheal capsaicin challenge on cough were mimicked by microinjecting substance P (0.5-5 nmol) into the nTS and prevented by intracerebroventricular administration (20 nmol h(-1)) of the neurokinin receptor antagonists CP99994 or SB223412. Tracheal application of these antagonists was without effect. C-fibre activation may thus sensitize the cough reflex via central mechanisms.

Anandamide elicits an acute release of nitric oxide through endothelial TRPV1 receptor activation in the rat arterial mesenteric bed

In the isolated rat mesenteric bed, the 1 min perfusion with 100 nm anandamide, a concentration that did not evoke vasorelaxation, elicited an acute release of 165.1 +/- 9.2 pmol nitric oxide (NO) that was paralleled by a 2-fold increase in cGMP tissue levels. The rise in NO released was mimicked by either (R)-(+)-methanandamide or the vanilloid receptor agonists resiniferatoxin and (E)-capsaicin but not by its inactive cis-isomer (Z)-capsaicin. The NO release elicited by either anandamide or capsaicin was reduced by the TRPV1 receptor antagonists 5'-iodoresiniferatoxin, SB 366791 and capsazepine as well as by the cannabinoid CB(1) receptor antagonists SR 141716A or AM251. The outflow of NO elicited by anandamide and capsaicin was also reduced by endothelium removal or NO synthase inhibition, suggesting the specific participation of endothelial TRPV1 receptors, rather than the novel endothelial TRPV4 receptors. Consistently, RT-PCR showed the expression of the mRNA coding for the rat TRPV1 receptor in the endothelial cell layer, in addition to its expression in sensory nerves. The participation of sensory nerves on the release of NO was precluded on the basis that neonatal denervation of the myenteric plexus sensory nerves did not modify the pattern of NO release induced by anandamide and capsaicin. We propose that low concentrations of anandamide, devoid of vasorelaxing effects, elicit an acute release of NO mediated predominantly by the activation of endothelial TRPV1 receptors whose physiological significance remains elusive.

Systemic and Site-Specific Effects of A-425619, a Selective TRPV1 Receptor Antagonist, on Wide Dynamic Range Neurons in CFA-Treated and Uninjured Rats

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

Capsazepine, a vanilloid receptor antagonist, inhibits allergen-induced tracheal contraction

To investigate the possible role of the vanilloid receptor-1 (TRPV1) in allergic airway responses, the effect of the specific TRPV1 receptor antagonist capsazepine was examined. Capsazepine significantly decreased the ovalbumin-induced contraction of isolated tracheal rings from ovalbumin-sensitized guinea pigs. This is the first report directly showing the involvement of the TRPV1 in experimental allergic airway responses.

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

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

Analysis of structure–activity relationships for the ‘B-region’ of N-(4- t-butylbenzyl)- N′-[4-(methylsulfonylamino)benzyl]-thiourea analogues as TRPV1 antagonists

The structure–activity relationships for the ‘B-region’ of N-(4- t-butylbenzyl)- N′-[4-(methylsulfonylamino)benzyl]thiourea analogues have been investigated as TRPV1 receptor antagonists. A docking model of potent antagonist 2 with the sensor region of TRPV1 is proposed.

Pharmacological characterization of the TRPV1 receptor antagonist JYL1421 (SC0030) in vitro and in vivo in the rat

The TRPV1 capsaicin receptor is an integrator molecule on primary afferent neurones participating in inflammatory and nociceptive processes. The present paper characterizes the effects of JYL1421 (SC0030), a TRPV1 receptor antagonist, on capsaicin-evoked responses both in vitro and in vivo in the rat. JYL1421 concentration-dependently (0.1–2 μM) inhibited capsaicin-evoked substance P, calcitonin gene-related peptide and somatostatin release from isolated tracheae, while only 2 μM resulted in a significant inhibition of electrically induced neuropeptide release. Capsazepine (0.1–2 μM), as a reference compound, similarly diminished both capsaicin-evoked and electrically evoked peptide release. JYL1421 concentration-dependently decreased capsaicin-induced Ca 2+ accumulation in cultured trigeminal ganglion cells, while capsazepine was much less effective. In vivo 2 mg/kg i.p. JYL1421, but not capsazepine, inhibited capsaicin-induced hypothermia, eye wiping movements and reflex hypotension (a component of the pulmonary chemoreflex or Bezold–Jarisch reflex). Based on these data JYL1421 is a more selective and in most models also a more potent TRPV1 receptor antagonist than capsazepine, therefore it may promote the assessment of the (patho)physiological roles of the TRPV1 receptor.

Effects of the novel TRPV1 receptor antagonist SB366791 in vitro and in vivo in the rat

The TRPV1 capsaicin receptor is a non-selective cation channel localized in the cell membrane of a subset of primary sensory neurons and functions as an integrator molecule in nociceptive/inflammatory processes. The present paper characterizes the effects of SB366791, a novel TRPV1 antagonist, on capsaicin-evoked responses both in vitro and in vivo using rat models. SB366791 (100 and 500 nM) significantly inhibited capsaicin-evoked release of the pro-inflammatory sensory neuropeptide substance P from isolated tracheae, while it did not influence electrically induced neuropeptide release. It also decreased capsaicin-induced Ca 2+ influx in cultured trigeminal ganglion cells in a concentration-dependent manner (0.5–10 μM) with an IC 50 of 651.9 nM. In vivo 500 μg/kg i.p. dose of SB366791 significantly inhibited capsaicin-induced hypothermia, wiping movements and vasodilatation in the knee joint, while 2 mg/kg capsazepine was ineffective, its effect lasted for 1 h. However, neither antagonist was able to inhibit capsaicin-evoked hypothermia in Balb/c mice. Based on these data SB366791 is a more selective and in vivo also a more potent TRPV1 receptor antagonist than capsazepine in the rat therefore, it may promote the assessment of the therapeutic utility of TRPV1 channel blockers.

The effects of Δ9‐tetrahydrocannabinol in rat mesenteric vasculature, and its interactions with the endocannabinoid anandamide

1 Delta9-tetrahydrocannabinol (THC) produces varying effects in mesenteric arteries: vasorelaxation (third-order branches, G3), modest vasorelaxation (G2), no effect (G1) and vasoconstriction (the superior mesenteric artery, G0). 2 In G3, vasorelaxation to THC was inhibited by pertussis toxin, but was unaffected by the CB1 receptor antagonist, AM251 (1 microM), incubation with the TRPV1 receptor agonist capsaicin (10 microM, 1 h), the TRPV1 receptor antagonist capsazepine (10 microM) or de-endothelialisation. 3 In G3, vasorelaxation to THC was inhibited by high K+ buffer, and by the following K+ channel inhibitors: charybdotoxin (100 nM), apamin (500 nM) and barium chloride (30 microM), but not by 4-aminopyridine, glibenclamide or tertiapin. 4 In G3, THC (10 and 100 microM) inhibited the contractile response to Ca2+ in a Ca2+-free, high potassium buffer, indicating that THC blocks Ca2+ influx. 5 In G0, the vasoconstrictor responses to THC were inhibited by de-endothelialisation and SR141716A (100 nM), but not by the endothelin (ET(A)) receptor antagonist FR139317 (1 microM).THC (1 and 10 microM) antagonised vasorelaxation to anandamide in G3 but not G0. THC did not antagonise the noncannabinoid verapamil, capsaicin or the CB1 receptor agonist CP55,940. 6 THC (10 and 100 microM) inhibited endothelium-derived relaxing factor (EDHF)-mediated responses to carbachol in a manner similar to the gap junction inhibitor 18alpha-glycyrrhetinic acid. 7 These data show that THC causes vasorelaxation through activation of K+ channels and inhibition of Ca2+ channels, and this involves non-CB1, non-TRPV1 but G-protein-coupled receptors. In G0, THC does not cause relaxation and at high concentrations causes contractions. Importantly, THC antagonises the effects of anandamide, possibly through inhibition of EDHF activity.

Peripheral, but not central effects of cannabidiol derivatives: Mediation by CB 1 and unidentified receptors

Delta-9 tetrahydrocannabinol (Δ 9-THC) and (−)-cannabidiol ((−)-CBD) are major constituents of the Cannabis sativa plant with different pharmacological profiles: (Δ 9-THC activates cannabinoid CB 1 and CB 2 receptors and induces psychoactive and peripheral effects. (−)-CBD possesses no, or very weak affinity for these receptors. We tested a series of (+)- and (−)-CBD derivatives for central and peripheral effects in mice. None of the (−)-CBD derivatives were centrally active, yet most inhibited intestinal motility. Of the five (+)-CBD derivatives, all with CB 1 receptor affinity, only (+)-7-OH-CBD-DMH (DMH=1,1-dimethylheptyl), acted centrally, while all five arrested defecation. The effects of (+)-CBD-DMH and (+)-7-OH-CBD-DMH were inhibited by the CB 1 receptor antagonist SR141716. The CB 2 receptor antagonist SR144528, and the vanilloid TRPV1 receptor antagonist capsazepine, had no influence. Further, the (−)-CBD derivatives (−)-7-COOH-CBD and (−)-7-COOH-CBD-DMH, displayed antiinflammatory activity. We suggest that (+)-CBD analogues have mixed agonist/antagonist activity in the brain. Second, (−)-CBD analogues which are devoid of cannabinoid receptor affinity but which inhibit intestinal motility, suggest the existence of a non-CB 1, non-CB 2 receptor. Therefore, such analogues should be further developed as antidiarrheal and/or antiinflammatory drugs. We propose to study the therapeutic potential of (−)- and (+)-CBD derivatives for complex conditions such as inflammatory bowel disease and cystic fibrosis.

Pain TRPs

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

Human brain endothelium: coexpression and function of vanilloid and endocannabinoid receptors

The arachidonic acid derivative, 2-arachidonoyl-glycerol (2-AG), was initially isolated from gut and brain; it is also produced and released from blood and vascular cells. Many of the 2-AG-induced cellular responses (i.e., neuromodulation, cytoprotection and vasodilation) are mediated by cannabinoid receptors CB 1 and CB 2. The findings presented here demonstrate the expression of CB 1, CB 2 and TRPV1 receptors on cerebromicrovascular endothelial cells (HBEC). The expression of TRPV1, CB 1 and CB 2 receptor mRNA and proteins were demonstrated by RT-PCR and polyclonal antibodies, respectively. The endocannabinoid 2-AG, and other related compounds [anandamide (ANA), methanandamide (m-ANA), N-(4-hydroxyphenyl-arachidonyl-ethanolamide) (AM404) and capsaicin] dose-dependently stimulated Ca 2+ influx in HBEC. The selective TRPV1 receptor antagonist (capsazepine), CB 1 receptor antagonist (SR141716A) and CB 2 receptor antagonist (SR144528) inhibited these responses. The effects of capsaicin, a specific agonist for TRPV1 receptors, were inhibited by capsazepine, but only weakly by CB 1 or CB 2 receptor antagonists. 2-AG also induced phosphorylation of vasodilator-stimulated phosphoprotein (VASP); this response was mediated by VR1 receptors. These studies clearly indicate that 2-AG and other related compounds may function as agonists on VR1 receptors, as well as CB 1 and CB 2 receptors, and implicated these factors in various HBEC functions.

Nociceptin inhibits vanilloid TRPV-1-mediated neurosensitization induced by fenoterol in human isolated bronchi.

Chronic exposure to beta(2)-adrenoceptor agonists, especially fenoterol, has been shown to increase smooth muscle contraction to endothelin-1 in human bronchi partly through tachykinin-mediated pathways. The purpose of this work was to further investigate the role of sensory nerves in fenoterol-induced sensitization of human airways and the effect of nociceptin, a nociceptin/orphanin FQ (NOP) receptor agonist, on the increase in contraction after fenoterol exposure. Human bronchi from 62 patients were sensitized to endothelin-1 by prolonged incubation with fenoterol (0.1 microM, 15 h). The sensitizing effect of fenoterol was inhibited by high concentration of capsaicin (10 microM, 30 min before fenoterol sensitization), which induces depletion of mediators from sensory nerves, or co-incubation of fenoterol and capsazepine (1 microM), a vanilloid TRPV-1 receptor antagonist. Moreover, short pretreatment of bronchi with capsaicin (10 microM) or capsazepine (1 microM) after sensitization by fenoterol decreased the rise in smooth muscle contraction to endothelin-1. Nociceptin (1 microM) also inhibited the increased contraction in fenoterol-sensitized bronchi. Tertiapin (10 microM), an inhibitor of the inward-rectifier K(+) channels, but not naloxone (0.1 microM), a DOP/KOP/MOP receptor antagonist, prevented the inhibitory effect of nociceptin. In conclusion, fenoterol induces sensitization of human isolated bronchi to endothelin-1 in part through the stimulation of the vanilloid TRPV-1 receptor on tachykininergic sensory nerves. Nociceptin inhibits airway hyperresponsiveness via NOP receptor activation. This effect involves inward-rectifier K(+) channels.

AM404 enhances the spontaneous release of l-glutamate in a manner sensitive to capsazepine in adult rat substantia gelatinosa neurones

In 84% of substantia gelatinosa (SG) neurones examined in adult rat spinal cord slices, an anandamide transport inhibitor, AM404, increased the frequency of spontaneous excitatory postsynaptic currents in a manner similar to that of capsaicin. AM404 was without actions in the presence of a vanilloid TRPV1 receptor antagonist, capsazepine. We conclude that AM404 enhances the spontaneous release of l-glutamate by activating TRPV1 receptors in the SG.

Effects of TRPV1 receptor antagonists on stimulated iCGRP release from isolated skin of rats and TRPV1 mutant mice

Capsaicin antagonists including ruthenium red, capsazepine and iodo-resiniferatoxin (I-RTX) have recently been shown to inhibit the activation by noxious heat of the capsaicin receptor (TRPV1) expressed in non-neuronal host cells, and natively, in cultured dorsal root ganglion cells. Noxious heat has been shown to release immunoreactive calcitonin gene-related peptide (iCGRP) from the isolated rat skin. In this model, ruthenium red, I-RTX as well as capsazepine 10 μM caused no alteration in iCGRP release at 32 °C by themselves whereas capsazepine 100 μM doubled it reversibly. In wild-type mice 100 μM capsazepine also stimulated iCGRP release while it was without effect in TRPV1 knockout littermates. In the rat skin, both ruthenium red and capsazepine (10/100 μM) reduced and abolished, respectively, capsaicin-induced iCGRP release while I-RTX (1/10 μM) was ineffective. Only ruthenium red 100 μM showed an unspecific effect inhibiting iCGRP release induced by KCl. Ruthenium red and capsazepine (10/100 μM) caused no significant alteration of iCGRP release induced by heat stimulation at 47 °C. Employing 45 °C stimulation intensity, capsazepine and I-RTX (in the higher concentrations) showed a significant facilitatory effect on the heat response suggesting a partial agonistic action of the compounds. It is concluded that noxious heat-induced iCGRP release in the isolated rat skin occurs through a mechanism that is not inhibited by TRPV1 antagonism reflecting a different pharmacological profile of noxious heat transduction in terminals of sensory neurons compared to that in cultured cell bodies and TRPV1-transfected host cells.

Sex-linked differences in the vasorelaxant effects of anandamide in vascular mesenteric beds: role of oestrogens

Anandamide (0.01 to 10 μM) caused greater concentration-dependent reductions of the contractile-induced responses to noradrenaline in female than in male mesenteric vascular beds isolated from adult Sprague–Dawley rats. Greater relaxant responses in females were also induced by the vanilloid TRPV1 receptor agonist capsaicin (0.01 to 10 μM), whereas no sex differences were observed for the relaxations caused by either acetylcholine or sodium nitroprusside. The effect of anandamide in either sex was reduced by the vanilloid TRPV1 receptor antagonist capsazepine but not by the cannabinoid CB 1 receptor antagonist N-piperidino-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-3-pyrazole-carboxamide (SR141716A). In males, the anandamide-induced relaxations were potentiated by in vitro exposure during 5 min to 0.5 μM 17β-oestradiol and unmodified by the protein synthesis inhibitor cycloheximide. The vasorelaxant effects of anandamide in female rats were decreased by ovariectomy. This decrease was prevented by in vivo treatment with 17β-oestradiol-3-benzoate (450 μg/kg i.m., once a week during 3 weeks) and counteracted by in vitro exposure to oestrogen. In vivo treatment with 17β-oestradiol also potentiated anandamide-induced responses in males. In conclusion, this study shows an oestrogen-dependent sensitivity to the vanilloid TRPV1 receptor-mediated vasorelaxant effects of anandamide in the mesenteric vasculature of Sprague–Dawley rats, that could be mediated by both genomic and non-genomic mechanisms.

Effects of TRPV1 receptor antagonists on stimulated iCGRP release from isolated skin of rats and TRPV1 mutant mice

Capsaicin antagonists including ruthenium red, capsazepine and iodo-resiniferatoxin (I-RTX) have recently been shown to inhibit the activation by noxious heat of the capsaicin receptor (TRPV1) expressed in non-neuronal host cells, and natively, in cultured dorsal root ganglion cells. Noxious heat has been shown to release immunoreactive calcitonin gene-related peptide (iCGRP) from the isolated rat skin. In this model, ruthenium red, I-RTX as well as capsazepine 10 μM caused no alteration in iCGRP release at 32 °C by themselves whereas capsazepine 100 μM doubled it reversibly. In wild-type mice 100 μM capsazepine also stimulated iCGRP release while it was without effect in TRPV1 knockout littermates. In the rat skin, both ruthenium red and capsazepine (10/100 μM) reduced and abolished, respectively, capsaicin-induced iCGRP release while I-RTX (1/10 μM) was ineffective. Only ruthenium red 100 μM showed an unspecific effect inhibiting iCGRP release induced by KCl. Ruthenium red and capsazepine (10/100 μM) caused no significant alteration of iCGRP release induced by heat stimulation at 47 °C. Employing 45 °C stimulation intensity, capsazepine and I-RTX (in the higher concentrations) showed a significant facilitatory effect on the heat response suggesting a partial agonistic action of the compounds. It is concluded that noxious heat-induced iCGRP release in the isolated rat skin occurs through a mechanism that is not inhibited by TRPV1 antagonism reflecting a different pharmacological profile of noxious heat transduction in terminals of sensory neurons compared to that in cultured cell bodies and TRPV1-transfected host cells.

Halogenation of a capsaicin analogue leads to novel vanilloid TRPV1 receptor antagonists.

1. The C-5 halogenation of the vanillyl moiety of resiniferatoxin, an ultrapotent agonist of vanilloid TRPV1 receptors, results in a potent antagonist for these receptors. Here, we have synthesized a series of halogenated derivatives of 'synthetic capsaicin' (nonanoyl vanillamide=nordihydrocapsaicin) differing for the nature (iodine, bromine-chlorine) and the regiochemistry (C-5, C-6) of the halogenation. 2. The activity of these compounds was investigated on recombinant human TRPV1 receptors overexpressed in HEK-293 cells. None of the six compounds exerted any significant agonist activity, as assessed by measuring their effect on TRPV1-mediated calcium mobilization. Instead, all compounds antagonized, to various extents, the effect of capsaicin in this assay. 3. All 6-halo-nordihydrocapsaicins behaved as competitive antagonists against human TRPV1 according to the corresponding Schild's plots, and were more potent than the corresponding 5-halogenated analogues. The iodo-derivatives were more potent than the bromo- and chloro-derivatives. 4. Using human recombinant TRPV1, 6-iodo-nordihydrocapsaicin (IC(50)=10 nM against 100 nM capsaicin) was about four times more potent than the prototypical TRPV1 antagonist, capsazepine, and was tested against capsaicin also on native TRPV1 in: (i) rat dorsal root ganglion neurons in culture; (ii) guinea-pig urinary bladder; and (iii) guinea-pig bronchi. In all cases, except for the guinea-pig bronchi, the compound was significantly more potent than capsazepine as a TRPV1 antagonist. 5. In conclusion, 6-iodo-nordihydrocapsaicin, a stable and easily prepared compound, is a potent TRPV1 antagonist and a convenient replacement for capsazepine in most of the in vitro preparations currently used to assess the activity of putative vanilloid receptor agonists.