Mechanisms and therapeutic potential of vanilloids (capsaicin-like molecules)

Evaluation of Antinociceptive Profile of Chalcone Derivative (3-(2,5-dimethoxyphenyl)-1-(5-methylfuran-2-yl) prop-2-en-1-one (DMPF-1) in vivo

Introduction

Pain is a major global health issue, where its pharmacotherapy prompts unwanted side effects; hence, the development of effective alternative compounds from natural derivatives with lesser side effects is clinically needed. Chalcone; the precursors of flavonoid, and its derivatives have been widely investigated due to its pharmacological properties.

Objective

This study addressed the therapeutic effect of 3-(2,5-dimethoxyphenyl)-1-(5-methyl furan-2-yl) prop-2-en-1-one (DMPF-1); synthetic chalcone derivative, on antinociceptive activity in vivo.

Materials and Methods

The antinociceptive profile was evaluated using acetic-acid-induced abdominal writhing, hot plate, and formalin-induced paw licking test. Capsaicin, phorbol 12-myristate 12 acetate (PMA), and glutamate-induced paw licking test were carried out to evaluate their potential effects toward different targets.

Results

It was shown that the doses of 0.1, 0.5, 1, and 5 mg/kg of DMPF-1 given via intraperitoneal injection showed significant reduction in writhing responses and increased the latency time in hot-plate test where reduced time spent on licking the injected paw in formalin and dose contingency inhibition was observed. The similar results were observed in capsaicin, PMA, and glutamate-induced paw licking test. In addition, the challenge with nonselective opioid receptor antagonist (naloxone) aimed to evaluate the involvement of the opioidergic system, which showed no reversion in analgesic profile in formalin and hot-plate test.

Conclusion

Collectively, this study showed that DMPF-1 markedly inhibits both peripheral and central nociception through the mechanism involving an interaction with vanilloid and glutamatergic system regardless of the activation of the opioidergic system.

Catalytic Oxidation of Lignins into the Aromatic Aldehydes: General Process Trends and Development Prospects

This review discusses principal patterns that govern the processes of lignins' catalytic oxidation into vanillin (3-methoxy-4-hydroxybenzaldehyde) and syringaldehyde (3,5-dimethoxy-4-hydroxybenzaldehyde). It examines the influence of lignin and oxidant nature, temperature, mass transfer, and of other factors on the yield of the aldehydes and the process selectivity. The review reveals that properly organized processes of catalytic oxidation of various lignins are only insignificantly (10-15%) inferior to oxidation by nitrobenzene in terms of yield and selectivity in vanillin and syringaldehyde. Very high consumption of oxygen (and consequentially, of alkali) in the process-over 10 mol per mol of obtained vanillin-is highlighted as an unresolved and unexplored problem: scientific literature reveals almost no studies devoted to the possibilities of decreasing the consumption of oxygen and alkali. Different hypotheses about the mechanism of lignin oxidation into the aromatic aldehydes are discussed, and the mechanism comprising the steps of single-electron oxidation of phenolate anions, and ending with retroaldol reaction of a substituted coniferyl aldehyde was pointed out as the most convincing one. The possibility and development prospects of single-stage oxidative processing of wood into the aromatic aldehydes and cellulose are analyzed.

Novel Radiolabeled Vanilloid with Enhanced Specificity for Human Transient Receptor Potential Vanilloid 1 (TRPV1)

Transient receptor potential vanilloid 1 (TRPV1) has emerged as a promising therapeutic target. While radiolabeled resiniferatoxin (RTX) has provided a powerful tool for characterization of vanilloid binding to TRPV1, TRPV1 shows 20-fold weaker binding to the human TRPV1 than to the rodent TRPV1. We now describe a tritium radiolabeled synthetic vanilloid antagonist, 1-((2-(4-(methyl-[³H])piperidin-1-yl-4-[³H])-6-(trifluoromethyl)pyridin-3-yl)methyl)-3-(3-oxo-3,4-dihydro-2H-benzo[b][1,4]oxazin-8-yl)urea ([³H]MPOU), that embodies improved absolute affinity for human TRPV1 and improved synthetic accessibility.

Anti-inflammatory and antinociceptive effects of Sterculia striata A. St.-Hil. & Naudin (Malvaceae) in rodents

The present work reports the anti-inflammatory and antinociceptive activities of the ethanol extract obtained from the stem bark of Sterculia striata A. St.-Hil. & Naudin (Ss-EtOH) in the experimental models of edema induced by carrageenan, dextran, or histamin and nociception induced by chemical stimuli, such as acetic acid, formalin, capsaicin, or glutamate. The Ss-EtOH (50 mg/kg) promoted a marked inhibition on the hind paw edema induced by carrageenan or dextran (30% and 73%, respectively). Besides, Ss-EtOH (25 mg/kg) exhibited a slight activity (30%) on the hind paw edema induced by histamin. The Ss-EtOH (12.5 and 25 mg/kg) showed the antinociceptive activity on chemical stimuli induced by acetic acid (65.59% and 38.37%, respectively), formalin, in the initial (35.08% and 31.5%, respectively) and late phases (44.09% and 83.57%, respectively), capsaicin (43.77% and 51.31%, respectively), or glutamate (36.6% and 52.12%, respectively). Regarding the possible mechanism involved in the antinociceptive effect, Ss-EtOH (12.5 mg/kg) showed a decrease in the antinociceptive effect (65.8%) in the acetic acid model after pretreatment with naloxone. Thus, opioid mechanisms might be underlying this response.

Quantitative determination of capsaicin, a transient receptor potential channel vanilloid 1 agonist, by liquid chromatography quadrupole ion trap mass spectrometry: evaluation of in vitro metabolic stability

Capsaicin is the most abundant pungent molecule present in red peppers and it is widely used for food flavoring, in pepper spray in self-defense devices and more recently in ointments for the relief of neuropathic pain. Capsaicin is a selective agonist of transient receptor potential channel, vanilloid subfamily member 1. A selective and sensitive quantitative method for the determination of capsaicin by LC-ESI/MS/MS was developed. The method consisted of a protein precipitation extraction followed by analysis using liquid chromatography electrospray quadrupole ion trap mass spectrometry. The chromatographic separation was achieved using a 100 x 2 mm C(18) Waters Symmetry column combined with a gradient mobile phase composed of acetonitrile and 0.1% formic acid aqueous solution at a flow rate of 220 microL/min. The mass spectrometer was operating in full-scan MS/MS mode using two-segment analysis. An analytical range of 10-5000 ng/mL was used in the calibration curve constructed in rat plasma. The interbatch precision and accuracy observed were 6.5, 6.7, 5.3 and 101.2, 102.7, 103.5% at 50, 500 and 5000 ng/mL, respectively. An in vitro metabolic stability study was performed in rat, dog and mouse liver microsomes and the novel analytical method was adapted and used to determine intrinsic clearance of capsaicin. Results suggest very rapid degradation with T(1/2) ranging from 2.3 to 4.1 min and high clearance values suggesting that drug bioavailability will be considerably reduced, consequently affecting drug response and efficacy.

Antinociceptive properties of the hydroalcoholic extract and the flavonoid rutin obtained from Polygala paniculata L. in mice

The present study examined the antinociceptive effects of a hydroalcoholic extract of Polygala paniculata in chemical and thermal behavioural models of pain in mice. The antinociceptive effects of hydroalcoholic extract was evaluated in chemical (acetic-acid, formalin, capsaicin, cinnamaldehyde and glutamate tests) and thermal (tail-flick and hot-plate test) models of pain or by biting behaviour following intratecal administration of both ionotropic and metabotropic agonists of excitatory amino acids receptors glutamate and cytokines such as interleukin-1beta (IL-1beta) and tumour necrosis factor-alpha (TNF-alpha) in mice. When given orally, hydroalcoholic extract (0.001-10 mg/kg), produced potent and dose-dependent inhibition of acetic acid-induced visceral pain. In the formalin test, the hydroalcoholic extract (0.0001-0.1 mg/kg orally) also caused significant inhibition of both the early (neurogenic pain) and the late (inflammatory pain) phases of formalin-induced licking. However, it was more potent and efficacious in relation to the late phase of the formalin test. The capsaicin-induced nociception was also reduced at a dose of only 1.0 mg/kg orally. The hydroalcoholic extract significantly reduced the cinnamaldehyde-induced nociception at doses of 0.01, 0.1 and 1.0 mg/kg orally. Moreover, the hydroalcoholic extract (0.001-1.0 mg/kg orally) caused significant and dose-dependent inhibition of glutamate-induced pain. However, only rutin, but not phebalosin or aurapten, isolated from P. paniculata, administered intraperitoneally to mice, produced dose-related inhibition of glutamate-induced pain. Furthermore, the hydroalcoholic extract (0.1-100 mg/kg orally) had no effect in the tail-flick test. On the other hand, the hydroalcoholic extract caused a significant increase in the latency to response at a dose of 10 mg/kg orally, in the hot-plate test. The hydroalcoholic extract (0.1 mg/kg orally) antinociception, in the glutamate test, was neither affected by intraperitoenal treatment of animals with l-arginine (precursor of nitric oxide, 600 mg/kg) and naloxone (opioid receptor antagonist, 1 mg/kg) nor associated with non-specific effects such as muscle relaxation or sedation. In addition, oral administration of hydroalcoholic extract produced a great inhibition of the pain-related behaviours induced by intrathecal injection of glutamate, N-methyl-D-aspartate (NMDA), IL-1beta and TNF-alpha, but not by alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid (AMPA), kainate or trans-1-amino-1.3-cyclopentanediocarboxylic acid (trans-ACPD). Together, our results suggest that inhibition of glutamatergic ionotropic receptors, may account for the antinociceptive action reported for the hydroalcoholic extract from P. paniculata in models of chemical pain used in this study.

The functional regulation of TRPV1 and its role in pain sensitization

Transient receptor potential V1 (TRPV1) is specifically expressed in the nociceptive receptors and can detect a variety of noxious stimuli, thus potentiating pain sensitization. While peripheral delivery of capsaicin causes the desensitization of sensory neurons, thus alleviating pain. Therefore capsaicin is used in the clinical treatment of various types of pain; however, these treatments will bring many side effects, such as a strong burning pain in the early stages of treatment which hampers the further use of capsaicin. Thus, the studies of the functional regulation of TRPV1 are mainly focused on two aspects: to develop more potent analogues of capsaicin with less side effects; or to elucidate the mechanisms of TRPV1 in pain sensitivity, especially of that TRPV1 as a target of various protein kinases such as PKD1 and Cdk5 is involved pain hypersensitivity. Thus we would summarize the progress of these two aspects in this mini review.

Metabolism of capsaicinoids by P450 enzymes: a review of recent findings on reaction mechanisms, bio-activation, and detoxification processes

Capsaicinoids are botanical irritants present in chili peppers. Chili pepper extracts and capsaicinoids are common dietary constituents and important pharmaceutical agents. Use of these substances in modern consumer products and medicinal preparations occurs worldwide. Capsaicinoids are the principals of pepper spray self-defense weapons and several over-the-counter pain treatments as well as the active component of many dietary supplements. Capsaicinoids interact with the capsaicin receptor (a.k.a., VR1 or TRPV1) to produce acute pain and cough as well as long-term analgesia. Capsaicinoids are also toxic to many cells via TRPV1-dependent and independent mechanisms. Chemical modifications to capsaicinoids by P450 enzymes decreases their potency at TRPV1 and reduces the pharmacological and toxicological phenomena associated with TRPV1 stimulation. Metabolism of capsaicinoids by P450 enzymes also produces reactive electrophiles capable of modifying biological macromolecules. This review highlights data describing specific mechanisms by which P450 enzymes convert the capsaicinoids to novel products and explores the relationship between capsaicinoid metabolism and its effects on capsaicinoid pharmacology and toxicology.

Antinociceptive properties of the ethanolic extract and of the triterpene 3β,6β,16β-trihidroxilup-20(29)-ene obtained from the flowers of Combretum leprosum in mice

The present study examined the antinociceptive effects of the ethanolic extract (EE) and of the triterpene 3beta,6beta,16beta-trihidroxilup-20(29)-ene obtained from the flowers of Combretum leprosum in chemical and thermal behavioural models of pain in mice. The EE (10-1000 mg/kg) given orally (p.o.), 1 h prior to testing, produced dose-dependent inhibition of acetic acid-induced visceral pain, with mean ID50 value of 131.9 mg/kg. In the formalin test, the EE (10-300 mg/kg, p.o.) also caused significant inhibition of both the early (neurogenic pain) and the late (inflammatory pain) phases of formalin-induced licking, however, it was more potent and efficacious in relation to the late phase of the formalin test, with mean ID50 values for the neurogenic and the inflammatory phases of approximately 300 and 88.8 mg/kg, respectively. The EE (10-1000 mg/kg, p.o.) also caused significant and dose-dependent inhibition of capsaicin- and glutamate-induced pain, with mean ID50 values of 160.5 and 38.3 mg/kg, respectively. Furthermore, the triterpene 3beta,6beta,16beta-trihidroxilup-20(29)-ene (1-30 mg/kg), given p.o., 1 h prior to testing, also produced dose-related inhibition of glutamate-induced pain, with a mean ID50 value of 5.6 mg/kg. When assessed in a thermal model of pain, the EE (10-300 mg/kg, p.o.) and fentanyl (100 microg/kg, s.c.) caused a significant and marked increase in the latency response on the hot-plate test (50 degrees C). The antinociception caused by EE (100 mg/kg, p.o.) in the glutamate test was significantly attenuated by intraperitoneal (i.p.) treatment of mice with naloxone (opioid receptor antagonist, 1 mg/kg), pindolol (a 5-HT 1A/1B receptor/beta adrenoceptor antagonist, 1 mg/kg), WAY100635 (a 5-HT 1A receptor antagonist, 0.7 mg/kg) or ketanserin (a 5-HT 2A receptor antagonist, 0.3 mg/kg). In contrast, EE (100 mg/kg, p.o.) antinociception was affected neither by L-arginine (precursor of nitric oxide, 600 mg/kg) nor by ondansetron (a 5-HT3 receptor antagonist, 0.5 mg/kg) i.p. treatment. It was not associated with non-specific effects such as muscle relaxation or sedation. Together, these results indicate that EE produces dose-related antinociception in several models of chemical and thermal pain through mechanisms that involve an interaction with opioid and serotonergic (i.e., through 5-HT 1A/1B and 5-HT 2A receptors) systems.

Mechanisms involved in the antinociception caused by agmatine in mice

The present study examined the antinociceptive effects of agmatine in chemical behavioural models of pain. Agmatine (1-30 mg/kg), given by i.p. route, 30 min earlier, produced dose-dependent inhibition of acetic acid-induced visceral pain, with mean ID50 value of 5.6 mg/kg. Given orally, 60 min earlier, agmatine (10-300 mg/kg) also produced dose-related inhibition of the visceral pain caused by acetic acid, with mean ID50 value of 147.3 mg/kg. Agmatine (3-100 mg/kg, i.p.) also caused significant and dose-dependent inhibition of capsaicin- and glutamate-induced pain, with mean ID50 values of 43.7 and 19.5 mg/kg, respectively. Moreover, agmatine (1-100 mg/kg, i.p.) caused marked inhibition of both phases of formalin-induced pain, with mean ID50 values for the neurogenic and the inflammatory phases of 13.7 and 5.6 mg/kg, respectively. The antinociception caused by agmatine in the acetic acid test was significantly attenuated by i.p. treatment of mice with L-arginine (precursor of nitric oxide, 600 mg/kg), naloxone (opioid receptor antagonist, 1 mg/kg), p-chlorophenylalanine methyl ester (PCPA, an inhibitor of serotonin synthesis, 100 mg/kg once a day for 4 consecutive days), ketanserin (a 5-HT2A receptor antagonist, 0.3 mg/kg), ondansetron (a 5-HT3 receptor antagonist, 0.5 mg/kg), yohimbine (an alpha2-adrenoceptor antagonist, 0.15 mg/kg) or by efaroxan (an I1 imidazoline/alpha2-adrenoceptor antagonist, 1 mg/kg). In contrast, agmatine antinociception was not affected by i.p. treatment of animals with pindolol (a 5-HT1A/1B receptor antagonist, 1 mg/kg) or idazoxan (an I2 imidazoline/alpha2-adrenoceptor antagonist, 3 mg/kg). Likewise, the antinociception caused by agmatine was not affected by neonatal pre-treatment with capsaicin. Together, these results indicate that agmatine produces dose-related antinociception in several models of chemical pain through mechanisms that involve an interaction with opioid, serotonergic (i.e., through 5-HT2A and 5-HT3 receptors) and nitrergic systems, as well as via an interaction with alpha2-adrenoceptors and imidazoline I1 receptors.

Structure-activity relationships of simplified resiniferatoxin analogues with potent VR1 agonism elucidates an active conformation of RTX for VR1 binding

We previously described a series of N-(3-acyloxy-2-benzylpropyl) homovanillate and N'-(4-hydroxy-3-methoxybenzyl) thiourea derivatives that were potent VR1 agonists with high-affinities and excellent analgesic profiles. The design of these simplified RTX analogues was based on our RTX-derived pharmacophore model which incorporates the 4-hydroxy-3-methoxyphenyl (A-region), C(20)-ester (B-region), orthophenyl (C1-region) and C(3)-keto (C2-region) groups of RTX. For the purpose of optimizing the spatial arrangement of the four principal pharmacophores on the lead agonists (1-4), we have modified the distances in the parent C-region, 3-acyloxy-2-benzylpropyl groups, by lengthening or shortening one carbon to vary the distances between the pharmacophores. We find that two of the amides, 4 and 19, possess EC(50) values <1 nM for induction of calcium influx in the VR1-CHO cells. As observed previously, the structure-activity relations for inhibition of RTX binding to VR1 and for induction of calcium uptake were distinct, presumably reflecting both intrinsic and methodological factors. In order to find the active conformation of VR1 ligands, the energy-minimized conformations of seven selected agonists were determined and the positions of their four pharmacophores were matched with those of five low energy RTX conformations. The rms values for the overlaps in the pharmacophores were calculated and correlated with the measured binding affinities (K(i)) and calcium influx (EC(50)) values. The binding affinities of the agonists correlated best with the RMS values derived from RTX conformation E (r(2)=0.92), predicting a model of the active conformation of RTX and related vanilloids for binding to VR1. Poorer correlation was obtained between any of the conformations and the EC(50) values for calcium influx.

Nicotine inhibits voltage-dependent sodium channels and sensitizes vanilloid receptors

Nicotine is an alkaloid that is used by large numbers of people. When taken into the body, it produces a myriad of physiological actions that occur primarily through the activation of neuronal nicotinic acetylcholine receptors (nAChRs). We have explored its ability to modulate TRPV1 receptors and voltage-gated sodium channels. The reason for investigating nicotine's effect on sodium channels is to obtain a better understanding of its anti-nociceptive properties. The reasons for investigating its effects on capsaicin-activated TRPV1 channels are to understand how it may modulate this channel that is involved in pain, inflammation, and gustatory physiology. Whole cell patch-clamp recordings from rat trigeminal ganglion (TG) nociceptors revealed that nicotine exhibited anesthetic properties by decreasing the number of evoked action potentials and by inhibiting tetrodotoxin-resistant sodium currents. This anesthetic property can be produced without the necessity of activating nAChRs. Nicotine also modulates TRPV1 receptors inducing a several-fold increase in capsaicin-activated currents in both TG neurons and in cells with heterologously expressed TRPV1 receptors. This sensitizing effect does not require the activation of nAChRs. Nicotine did not alter the threshold temperature (approximately 41 degrees C) of heat-activated currents in TG neurons that were attributed to arise from the activation of TRPV1 receptors. In this regard, its effect on TRPV1 receptors differs from those of ethanol that has been shown to increase the capsaicin-activated current but decrease the threshold temperature. These studies document several new effects of nicotine on channels involved in nociception and indicate how they may impact physiological processes involving pain and gustation.

Differential effects of intraplantar capsazepine and ruthenium red on capsaicin-induced desensitization in mice

Intraplantar injection of capsaicin (1.6 microg/paw) into the mouse hindpaw produced an acute paw-licking/biting response. This study was designed (1) to investigate the antinociceptive effects of intraplantar administration of capsazepine, a competitive vanilloid receptor antagonist, and ruthenium red, a noncompetitive antagonist, in the nociceptive licking/biting response induced by intraplantar injection of capsaicin, and (2) to determine whether these compounds were able to prevent capsaicin-induced desensitization in mice. Both capsazepine and ruthenium red produced a dose-dependent reduction in the capsaicin-induced nociceptive response. In licking/biting response to intraplantar capsaicin, ruthenium red was more potent than capsazepine in producing antinociceptive activity as assayed by the capsaicin test. The first injection of capsaicin induced a profound desensitization to the second and third injections of capsaicin at the interval of 15 or 30 min. The capsaicin-induced desensitization was prevented dose-dependently by antinociceptive doses of capsazepine, whereas ruthenium red in doses exhibiting antinociceptive activity was without effect on capsaicin-induced desensitization. The present results suggest that both capsazepine and ruthenium red can produce a local peripheral antinociceptive action, which may be mediated by inhibiting the membrane ion channel activated by capsaicin. In addition, these data suggest that capsazepine may act in the mechanism clearly different from ruthenium red in the capsaicin-induced nociceptive desensitization.

Pharmacological characterization of vanilloid receptor located in the brain

Specific [3H]resiniferatoxin (RTX) binding detects the vanilloid receptor type I (VR1). In the present study we demonstrate specific, high-affinity, saturable [3H]RTX binding in various areas of monkey brain not known to be innervated by primary afferent neurons as well as in spinal cord and dorsal root ganglion neurons of the same origin. Detailed pharmacological characterization and comparison revealed no major difference in binding affinities between the peripheral and the central sites as measured by K(d)/K(i) values. In general, lower receptor density was measured in selected brain areas than in the periphery. Areas with higher receptor density were detected in the locus ceruleus, preoptic area, and medial basal hypothalamus of the brain. Both capsaicin and the competitive antagonist capsazepine inhibited the specific binding of [3H]RTX to membrane preparations of the dorsal horn of the spinal cord and dorsal root ganglia with K(i) values of 4.3+/-0.32 microM and 2.7+/-0.33 microM, respectively. Inhibition was observed in the central areas (hypothalamus) with K(i) values of 0.95+/-0.1 microM for capsaicin and 0.86+/-0.11 microM for capsazepine. Previous biological and pharmacological evidence suggested that vanilloid receptors were present in the brain. Our results demonstrate that the pharmacological properties of both the peripheral and central receptor sites display appropriate pharmacological similarity to represent the same receptor class. The modest differences in ligand affinities for the vanilloid receptor expressed in the brain nuclei and the dorsal root ganglion neurons may correspond to differences in sequence, modification or associated proteins.

Naturally occurring antinociceptive substances from plants

Despite the progress that has occurred in recent years in the development of therapy, there is still a need for effective and potent analgesics, especially for the treatment of chronic pain. One of the most important analgesic drugs employed in clinical practice today continues to be the alkaloid morphine. In this review, emphasis will be given to the important contribution and the history of Papaver somniferum, Salix species, Capsicum species and Cannabis sativa in the development of new analgesics and their importance in the understanding of the complex pathways related to electrophysiological and molecular mechanisms associated with pain transmission. Recently discovered antinociceptive substances include alkaloids, terpenoids and flavonoid. Plant-derived substances have, and will certainly continue to have, a relevant place in the process of drug discovery, particularly in the development of new analgesic drugs.

One-way cross-desensitization between P2X purinoceptors and vanilloid receptors in adult rat dorsal root ganglion neurones

1. Capsaicin and ATP can activate ligand-gated cation channels in nociceptive rat dorsal root ganglion (DRG) neurones. We have studied cross-desensitization between these two agents in rat isolated DRG neurones using the whole-cell voltage-clamp technique. 2. ATP (10 microM) activated an inward current in DRG neurones at a holding potential of -60 mV. ATP evoked 'fast' responses that underwent rapid activation and desensitization, 'slow' responses that activated and desensitized more slowly, or responses that displayed a mixture of these two characteristics. The time course of the response to ATP was not related obviously to capsaicin sensitivity. 3. Prior application of capsaicin (0.5 microM) increased the proportion of cells displaying only fast responses to ATP (10 microM) suggesting that cross-desensitization had occurred between capsaicin and the slow component of the ATP response. Prior desensitization to ATP had no apparent effect on the inward current response to capsaicin (0.5 microM). 4. Cross-desensitization between capsaicin and ATP was Ca2+ dependent. 5. Changing the membrane holding potential (Vh) to +40 mV for brief period before applying ATP at -60 mV had a similar effect to capsaicin, i.e. the proportion of cells displaying only fast responses to ATP was increased significantly. This effect of depolarization was not Ca2+ dependent. 6. The heterogenity of responses to ATP is probably due to co-expression of homomeric P2X3 receptors and heteromeric receptors comprising P2X3 subunits with other P2X subunits. We propose that the change in time course of the ATP response produced by prior desensitization to capsaicin is due to selective cross-desensitization with the heteromeric P2X receptors.

Capsaicin-sensitive afferent sensory nerves in modulating gastric mucosal defense against noxious agents

In the rat stomach, evidence has been provided that capsaicin-sensitive sensory nerves (CSSN) are involved in a local defense mechanism against gastric ulcer. In the present study capsaicin or resiniferatoxin (RTX), a more potent capsaicin analogue, was used to elucidate the role of these sensory nerves in gastric mucosal protection, mucosal permeability, gastric acid secretion and gastrointestinal blood flow in the rat. In the rat stomach and jejunum, intravenous RTX or topical capsaicin or RTX effected a pronounced and long-lasting enhancement of the microcirculation at these sites, measured by laser Doppler flowmetry technique. Introduction of capsaicin into the rat stomach in very low concentrations of ng-microg x mL(-1) range protected the gastric mucosa against damage produced by topical acidified aspirin, indomethacin, ethanol or 0.6 N HCl. Resiniferatoxin exhibited acute gastroprotective effect similar to that of capsaicin and exerted marked protective action on the exogenous HCl, or the secretagogue-induced enhancement of the indomethacin injury. The ulcer preventive effect of both agents was not prevented by atropine or cimetidine treatment. Capsaicin given into the stomach in higher desensitizing concentrations of 6.5 mM markedly enhanced the susceptibility of the gastric mucosa and invariably aggravated gastric mucosal damage evoked by later noxious challenge. Such high desensitizing concentrations of capsaicin, however, did not reduce the cytoprotective effect of prostacyclin (PGI2) or beta-carotene. Capsaicin or RTX had an additive protective effect to that of atropine or cimetidine. In rats pretreated with cysteamine to deplete tissue somatostatin, capsaicin protected against the indomethacin-induced mucosal injury. Gastric acid secretion of the pylorus-ligated rats was inhibited with capsaicin or RTX given in low non-desensitizing concentrations, with the inhibition being most marked in the first hour following pylorus-ligation. Low intragastric concentrations of RTX reduced gastric hydrogen ion back-diffusion evoked by topical acidified salicylates. It is concluded that the gastropotective effect of capsaicin-type agents involves primarily an enhancement of the microcirculation effected through local release of mediator peptides from the sensory nerve terminals. A reduction in gastric acidity may contribute to some degree in the gastric protective action of capsaicin-type agents. The vasodilator and gastroprotective effects of capsaicin-type agents do not depend on vagal efferents or sympathetic neurons, involve prostanoids, histaminergic or cholinergic pathways.

Epidural resiniferatoxin induced prolonged regional analgesia to pain

Adequate treatment of cancer pain remains a significant clinical problem. To reduce side effects of treatment, intrathecal and epidural routes of administration have been used where appropriate to reduce the total dose of agent administered while achieving regional control. Resiniferatoxin (RTX), an ultrapotent capsaicin analog, gives long-term desensitization of nociception via C-fiber sensory neurons. We evaluate here the analgesic effect on rats of epidurally administered RTX, using latency of response to a thermal stimulus in unrestrained animals. Results were compared with those for systemically administered RTX. Vehicle or graded doses of RTX were injected subcutaneously (s.c.) or through an indwelling lumbar (L4) epidural catheter as a single dose. Both routes of application of RTX produced profound thermal analgesia, reaching a plateau within 4-6 h and showing no restoration of pain sensitivity over 7 days. Vehicle was without effect. For the epidural route, the effect was selective as expected for the targeted spinal cord region, whereas the subcutaneous administration of RTX had a generalized analgesic effect. At doses yielding a tripling of back paw withdrawal latency, epidural treatment was 25-fold more effective than the subcutaneous route of application. Consistent with the regional selectivity of the lumbar epidural route, the front paws showed no more effect than by systemic RTX treatment. Binding experiments with [3H]RTX provided further evidence of the segmental desensitization induced by epidural RTX. We conclude that epidural administration of RTX at the lumbar spinal level produces profound, long-lasting, segmental analgesia to C-fiber mediated pain in the rat.

A study of the voltage dependence of capsaicin-activated membrane currents in rat sensory neurones before and after acute desensitization

1. Responses to capsaicin in isolated sensory neurones have been shown to desensitize in a Ca2+- and voltage-dependent manner. We have studied desensitization of capsaicin-activated currents in cultured adult rat dorsal root ganglion (DRG) neurones over a range of membrane potentials using whole-cell patch-clamp techniques. 2. Acute desensitization of responses to capsaicin (0.5 microM) was significantly less when the holding potential (Vh) was +40 mV rather than -60 mV. This was not due only to reduced Ca2+ entry as the response to capsaicin was desensitized by the same amount whether prior exposure to capsaicin was at -60 or +40 mV. The I-V relationship for capsaicin-induced current, determined using a voltage step protocol, was outwardly rectifying and during the acute phase of desensitization the degree of outward rectification increased. 3. Acute desensitization and the increase in outward rectification that accompanied desensitization were inhibited when cells were dialysed with the rapid Ca2+ chelator BAPTA. Addition of a pseudosubstrate inhibitor of the Ca2+-calmodulin-dependent enzyme calcineurin (CI, 100 microM) prevented the increase in outward rectification although it did not cause a significant decrease of acute desensitization. 4. Removal of external Ca2+ or Mg2+ did not reverse the increase in outward rectification of capsaicin-activated current after Ca2+-dependent desensitization had occurred. This indicates that a voltage-dependent block of the capsaicin-activated ion channel by Ca2+ or Mg2+ was not responsible for the observed changes in the properties of the capsaicin-activated conductance.

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.

Conformational studies on resiniferatoxin (RTX), an ultrapotent vanilloid agonist

In polar solution, NOE studies show a pronounced clustering of the aromatic moieties (9,13,14-phenylacetate orthoester and 20-homovanillate) of the ultrapotent vanilloid agonist resiniferatoxin (RTX). This clustering is absent in nonpolar solution. Low energy clustered structures from molecular dynamics simulations account for the observed NOEs. These results suggest that the phenylorthoacetate moiety can assist the attainment of specific alignments between the terpenoid core and the vanillyl moiety, possibly preorganizing them for ideal receptor binding.

A new pharyngitis model using capsaicin in rats

1. Application of capsaicin solution onto the rat pharyngeal mucosa caused a well-reproducible increase in vascular permeability in the pharynx. 2. Capsaicin-induced pharyngeal inflammation was unaffected by a histamine H1 blocker and non-steroidal anti-inflammatory agents, whereas dexamethasone was effective in its inhibition. 3. FK224, a dual antagonist of tachykinin NK1 and NK2 receptors, and FK888, a selective antagonist of NK1 receptor, significantly inhibited capsaicin-induced plasma exudation in the pharynx. 4. In capsaicinized animals, the application of capsaicin solution in the pharyngeal mucosa did not induce pharyngitis. 5. These results suggest that the mechanism of the capsaicin-induced pharyngitis primarily involves tachykinins.

Pharmacological characterization of the vanilloid receptor in the rat dorsal spinal cord

1. In the present study a novel 96-well plate assay system was used to characterize pharmacologically the vanilloid receptor in the dorsal spinal cord of the rat. When activated, this receptor stimulates release of calcitonin gene-related peptide (CGRP) from the central terminals of the afferent nerves. 2. Capsaicin, resiniferatoxin (RTX) and olvanil each evoked a concentration-dependent increase in CGRP release with pEC50 values of 6.55 +/- 0.07, 7.90 +/- 0.24 and 6.19 +/- 0.15 respectively. RTX and olvanil were partial agonists with respect to capsaicin. All concentration-effect curves were bell-shaped. 3. The vanilloid receptor antagonist, capsazepine (10 microM) had no effect on basal peptide release but inhibited the CGRP release evoked by all 3 agonists to a similar extent. These results suggest that the antagonistic effects of capsazepine were agonist-independent. 4. The capsaicin-sensitive cation channel blocker, ruthenium red (10 microM) had no effect on basal CGRP release, but antagonized the peptide release evoked by capsaicin, olvanil and RTX. 5. The pharmacology of the vanilloid receptor in the rat dorsal spinal cord is not identical to that previously found in other systems. The reason for these differences is unclear, but the possibility of multiple classes of receptor cannot at this stage be ruled out.

Effect of capsaicin on airway responsiveness to hypertonic saline challenge in asthmatic and non-asthmatic children

Recurrent cough and asthma are common problems in children. In the evaluation of children with recurrent cough, the sequential measurements of airway responsiveness (AR) and capsaicin cough receptor sensitivity may be useful. However, the effect of capsaicin on AR induced by an indirect stimulus such as hypertonic saline (HS) is not known. Current evidence suggests that a common pathway is involved in both capsaicin and HS challenges. This study was designed to determine whether inhalation of capsaicin for the cough receptor sensitivity test before HS challenge will alter AR of asthmatic and non-asthmatic children to that challenge. Twenty-one children (12 asthmatics, 9 non-asthmatics; mean age, 11.3 years) performed the HS challenge alone or 2 min after capsaicin inhalation on 2 different days in random order. The end point of the capsaicin inhalation was when > or = 5 coughs were stimulated from a single inhalation. The power of the study was > 90% at a significance level of 0.05. Capsaicin inhalation prior to HS challenge did not alter the AR of normal children. In the asthmatic group, the PD15 (provocation dose causing a fall in forced expiratory volume in 1 s of > or = 15% from the baseline) without prior inhalation of capsaicin (mean, 2.44 +/- SEM 1.21 ml) was not significantly different from that when HS challenge was performed after capsaicin inhalation (mean, 2.19 +/- SEM 0.83 ml). The mean of the difference in log PD15 of the HS challenge with and without capsaicin was -0.02 (95% CI, -0.16, 0.12), i.e. within the equivalence range of the HS challenge in children with asthma. We conclude that in normal and asthmatic children, capsaicin inhalation does not alter AR to HS; consequently the capsaicin cough sensitivity test can be performed validly before an HS challenge.

Capsaicin and the stomach. A review of experimental and clinical data

Capsaicin, the pungent principle of hot pepper, because of its ability to excite and later defunctionalize a subset of primary afferent neurons, has been extensively used as a probe to elucidate the function of these sensory neurons in a number of physiological processes. In the rat stomach, experimental data provided clear evidence that capsaicin-sensitive (CS) sensory nerves are involved in a local defense mechanism against gastric ulcer. Stimulation of CS sensory nerves with low intragastric concentrations of capsaicin protected the rat gastric mucosa against injury produced by different ulcerogenic agents. High local desensitizing concentrations of capsaicin or systemic neurotoxic doses of the agent markedly enhanced the susceptibility of the rat gastric mucosa to later noxious challenge. Resiniferatoxin, a potent analogue of capsaicin possesses an acute gastroprotective effect similar to that of capsaicin in the stomach. The gastroprotective effect of capsaicin-type agents involves an enhancement of the microcirculation effected through the release of mediator peptides from the sensory nerve terminals with calcitonin gene-related peptide being the most likely candidate implicated. They do not depend on vagal efferent or sympathetic neurons or involve prostanoids. The gastric mucosal protective effect of prostacyclin is retained after systemic or topical capsaicin desensitization. Capsaicin-sensitive fibers are involved in the repair mechanisms of the gastric mucosa. A protective role for CS sensory nerves has also been demonstrated in the colon. In most studies, capsaicin given into the stomach of rats or cats inhibited gastric acid secretion. In humans, although recent studies provide evidence in favor of a beneficial effect of capsaicin on the gastric mucosa, an exact concentration-related assessment of the effect of the agent is still lacking.

Vanilloid receptors: new insights enhance potential as a therapeutic target

Compounds related to capsaicin and its ultrapotent analog, resiniferatoxin (RTX), collectively referred to as vanilloids, interact at a specific membrane recognition site (vanilloid receptor), expressed almost exclusively by primary sensory neurons involved in nociception and neurogenic inflammation. Desensitization to vanilloids is a promising therapeutic approach to mitigate neuropathic pain and pathological conditions (e.g. vasomotor rhinitis) in which neuropeptides released from primary sensory neurons play a major role. Capsaicin-containing preparations are already commercially available for these purposes. The use of capsaicin, however, is severely limited by its irritancy, and the synthesis of novel vanilloids with an improved pungency/desensitization ratio is an on-going objective. This review highlights the emerging evidence that the vanilloid receptor is not a single receptor but a family of receptors, and that these receptors recognize not simply RTX and capsaicin structural analogs but are broader in their ligand-binding selectivity. We further focus on ligand-induced messenger plasticity, a recently discovered mechanism underlying the analgesic actions of vanilloids. Lastly, we give a brief overview of the current clinical uses of vanilloids and their future therapeutic potential. The possibility is raised that vanilloid receptor subtype-specific drugs may be synthesized, devoid of the undesirable side-effects of capsaicin.

The stimulation of capsaicin-sensitive neurones in a vanilloid receptor-mediated fashion by pungent terpenoids possessing an unsaturated 1,4-dialdehyde moiety

1. The irritant fungal terpenoid isovelleral caused protective eye-wiping movements in the rat upon intraocular instillation and showed cross-tachyphylaxis with capsaicin, the pungent principle in hot pepper. 2. Isovelleral induced a dose-dependent calcium uptake by rat dorsal root ganglion neurones cultured in vitro with an EC50 of 95 nM, which was fully inhibited by the competitive vanilloid receptor antagonist capsazepine. 3. Isovelleral inhibited specific binding of [3H]-resiniferatoxin (RTX), an ultrapotent capsaicin analogue, to rat trigeminal ganglion or spinal cord preparations with an IC50 of 5.2 microM; in experiments in which the concentration of [3H]-RTX was varied, isovelleral changed both the apparent affinity (from 16 pM to 37 pM) and the co-operativity index (from 2.1 to 1.5), but not the Bmax. 4. The affinity of isovelleral for inducing calcium uptake or inhibiting RTX binding was in very good agreement with the threshold dose (2.2. nmol) at which it provoked pungency on the human tongue. 5. For a series of 14 terpenoids with an unsaturated 1,4-dialdehyde, a good correlation was found between pungency on the human tongue and affinity for vanilloid receptors on the rat spinal cord. 6. The results suggest that isovelleral-like compounds produce their irritant effect by interacting with vanilloid receptors on capsaicin-sensitive sensory neurones. Since these pungent diterpenes are structurally distinct from the known classes of vanilloids, these data provide new insights into structure-activity relations and may afford new opportunities for the development of drugs targeting capsaicin-sensitive pathways.

Vanilloid receptor loss is independent of the messenger plasticity that follows systemic resiniferatoxin administration

Resiniferatoxin (RTX) depletes vanilloid (capsaicin) receptors from lumbar dorsal root ganglia (DRG) of the rat. In addition, RTX causes changes in neuropeptide and nitric oxide synthase expression in lumbar DRG neurons, similar to those described following axotomy; this latter phenomenon is referred to as messenger plasticity. These findings suggested that vanilloid receptor loss may be part of the plasticity that follows RTX treatment. Here we show that vanilloid receptor expression, as detected by [3H]RTX autoradiography, is not changed in lumbar DRGs of axotomized rats, nor is it altered in a rat model (chronic constriction injury) of neuropathic pain. Thus, the in vivo expression of vanilloid receptors detected by specific [3H]RTX binding does not require the presence of intraaxonally transported trophic factors such as nerve growth factor. We conclude that messenger plasticity and vanilloid receptor loss are mediated by distinct mechanisms.

A novel agonist, phorbol 12-phenylacetate 13-acetate 20-homovanillate, abolishes positive cooperativity of binding by the vanilloid receptor

Capsaicin binds to a specific recognition site, referred to as the vanilloid receptor, which it shares with the natural, ultrapotent agonist resiniferatoxin and with the competitive antagonist capsazepine. Upon binding to its receptor, capsaicin opens a cation channel leading to Ca2+ influx. The binding of capsaicin or resiniferatoxin by the vanilloid receptor follows a sigmoidal saturation curve, indicative of positive cooperativity. The biological significance of this positive cooperative behaviour is unknown, as is the mechanism responsible for it. We have developed a novel ligand, phorbol 12-phenylacetate 13-acetate 20-homovanillate (PPAHV), which binds to cultured rat sensory neurons (with a Ki of 3.1 +/- 0.4 microM), and induces Ca2+ uptake by them (with an ED50 of 1.8 +/- 0.3 microM) with similar affinities and in a non-cooperative manner (Hill coefficients are 0.99 and 1.06 for binding and Ca2+ uptake, respectively). The behaviour of PPAHV thus contrasts with resiniferatoxin or capsaicin not only in the lack of cooperativity but also in the relative potencies for resiniferatoxin binding versus Ca2+ uptake (resiniferatoxin is less potent and capsaicin is more potent for induction of Ca2+ uptake than for binding). In further experiments in which the concentration of [3H]resiniferatoxin was varied, 1 microM PPAHV likewise reduced the cooperativity index that characterizes resiniferatoxin binding to rat spinal cord membranes from 2.3 +/- 0.1 to 1.1 +/- 0.2; in parallel experiments, neither capsaicin nor capsazepine (both at a concentration of 2 microM) affected binding cooperativity. Moreover, PPAHV (1 microM) turned the bi-phasic dissociation curve of resiniferatoxin into a monophasic curve, eliminating the second, slow-dissociation phase. The present results suggest that positive cooperativity is a ligand-induced feature rather than an inherent property of vanilloid receptors. A comparison of the spectrum of biological activity of ligands which bind to vanilloid receptors with different degrees of cooperativity may provide an approach to explore the functional significance of this binding behaviour.

Binding of neuroleptic drugs (trifluoperazine and rimcazole) to vanilloid receptors in porcine dorsal horn

Neuroleptic drugs were reported to modulate [3H]resiniferatoxin binding to vanilloid receptors in the spinal cord, with marked differences between rat and man. In the present study, we have used a [3H]resiniferatoxin binding assay using porcine dorsal horn membranes to explore further species differences in the interaction of neuroleptic drugs at spinal vanilloid receptors. Specific binding of 13 pM [3H]resiniferatoxin to porcine dorsal horn membranes (corresponding to a 7% fractional receptor occupancy) was affected by trifluoperazine in a bi-phasic fashion, with an initial 90% enhancement of binding preceding inhibition: a fit to the modified Hill equation yielded a cooperativity index of 1.8 and a Ki of 5 microM. Under similar conditions, rimcazole, by contrast, had a monophasic effect: it enhanced but, up to 100 microM, did not inhibit [3H]resiniferatoxin binding. These results are in accord with previous findings in human spinal cord but contrast with those in the rat. In experiments in which the concentration of [3H]resiniferatoxin was varied, 20 microM trifluoperazine reduced the Bmax by 33% (from 181 +/- 9 fmol/mg protein to 121 +/- 5 fmol/mg protein) without a measurable change in affinity or cooperativity. In parallel experiments, by contrast, neither capsaicin nor capsazepine (both at a concentration of 10 microM) affected the Bmax or cooperativity but, as expected, reduced the affinity from 61 +/- 8 pM to 120 +/- 11 pM or to 101 +/- 7 pM, respectively. Whereas vanilloid receptor agonists (resiniferatoxin and capsaicin) affected [3H]resiniferatoxin binding at low (approximately 7%) fractional receptor occupancies by the radioligand in a bi-phasic fashion, the competitive vanilloid receptor antagonist capsazepine failed to induce the initial binding enhancement. Thus, capsazepine appears to bind to vanilloid receptors in a non-cooperative fashion, or at least with much reduced positive cooperativity in this system. The mechanism by which neuroleptic drugs modulate resiniferatoxin binding is yet to be clarified and is clearly complicated as well as species-dependent; nonetheless, the reduced Bmax at higher concentrations suggests that it may at least in part be non-competitive.

Capsaicin activated currents in rat dorsal root ganglion cells

Capsaicin is a pungent-tasting compound produced by plants in the Capsium family that activates a subset of primary afferent neurons associated with pain and thermoreception. Previous studies from dorsal root ganglion (DRGs) neurons suggest that many of capsaicin's physiological responses are a consequence of its activating a cation-selective current. To further characterize the responses to capsaicin whole-cell patch-clamp measurements were performed on rat DRGs to which 0.1-10 microM capsaicin was continuously applied. The capsaicin-activated currents exhibited marked variability in their thresholds, amplitude (to 15 nA), rates of desensitization, and the number of distinct maxima in the evoked current. Similar responses were found in rat trigeminal ganglion cells. The heterogeneity in the magnitude of the currents evoked by 0.1 microM capsaicin likely reflects different types of capsaicin-sensitive neurons; a result consistent with in vitro extracellular recordings from capsaicin-sensitive sensory afferents (Seno and Dray 1993).

Differences in action of topical and systemic cysteamine on gastric blood flow, gastric acid secretion and gastric ulceration in the rat

The effect of cysteamine on gastric blood flow and on the indomethacin-induced gastric mucosal damage was studied. In anesthetized rats, cysteamine (280 mg/kg) given subcutaneously (s.c.) decreased gastric blood flow measured by the laser Doppler flowmetry technique. In contrast, cysteamine (1-60 mg/ml) applied topically to the serosal surface of the stomach evoked a concentration-dependent and long-lasting increase in gastric blood flow. At 60 mg/ml, cysteamine increased blood flow by 166.8 +/- 26.1% of predrug control value. Pretreatment with indomethacin (20 mg/kg, s.c.), intravenous (i.v.) atropine (1 mg/kg), propranolol (1 mg/kg, i.v.), combined H1 and H2-blockade or bilateral cervical vagotomy alone or combined with i.v. guanethidine (8 mg/kg), or pretreatment with the capsaicin analogue resiniferatoxin did not reduce the vasodilator response to cysteamine. The vasodilator response to topical capsaicin, was not reduced after s.c. cysteamine (280 mg/kg) pretreatment. In conscious pylonus-ligated rats, s.c. cysteamine (100 or 280 mg/kg) given simultaneously with indomethacin inhibited gastric acid output but had variable effects on the indomethacin-induced gastric mucosal damage. Cysteamine (100 or 280 mg/kg) administered s.c. 4 h prior to indomethacin enhanced gastric injury by s.c. indomethacin, but did not prevent the gastroprotective action of capsaicin. In contrast, orally administered cysteamine (60 mg/ml) reduced gastric injury induced by s.c. indomethacin plus intragastric HCl. These data provide the first evidence for the effect of cysteamine on gastric microcirculation in the rat and suggest a direct vasodilator effect for topical cysteamine. The microvascular effects of cysteamine are largely responsible for the different effects of this agent on experimental gastric injury.

Distinct structure-activity relations for stimulation of 45Ca uptake and for high affinity binding in cultured rat dorsal root ganglion neurons and dorsal root ganglion membranes

The [3H]resiniferatoxin (RTX) binding assay using membrane preparations has been used to identify and characterize the vanilloid receptors in the central and peripheral nervous system of different species. In the present study, using cultured adult rat dorsal root ganglion neurons either in suspension or attached to the tissue culture plates, we developed an assay to measure specific [3H]RTX binding by the intact cells. We were able to characterize the vanilloid binding characteristics of the neurons and compared those to the properties of vanilloid binding sites present in rat dorsal root ganglia membrane preparations. We found that [3H]RTX bound with similar affinity and positive cooperativity to attached neurons (cultured for 5 days before being assayed), neurons in suspension (using a filtration assay) and dorsal root ganglion membrane preparations. Dissociation constants obtained in the three assays were 47.6 +/- 3.5 pM, 38.4 +/- 3.1 pM and 42.6 +/- 3.1 pM, respectively. The cooperativity indexes determined by fitting the data to the Hill equation were 1.73 +/- 0.11, 1.78 +/- 0.12 and 1.78 +/- 0.09, respectively. The maximal binding capacity was 0.218 +/- 0.026 fmol/10(3) cells and 0.196 +/- 0.021 fmol/10(3) cells in the case of the attached cells and cells in suspension, respectively. Nonradioactive RTX, capsaicin, capsazepine and resiniferonol 20-homovanillylamide fully displaced specifically bound [3H]RTX from cells in suspension with Ki and Hill coefficient values of 42.5 +/- 5.3 pM, 2.06 +/- 0.16 microM, 3.16 +/- 0.21 microM and 32.4 +/- 4.1 nM and 1.79 +/- 0.17, 1.68 +/- 0.06, 1.72 +/- 0.11 and 1.81 +/- 0.12, respectively. Structure-activity analysis of different vanilloid derivatives revealed that the various compounds have distinct potencies for receptor binding and inducing 45Ca uptake in rat dorsal root ganglion neurons. Affinities for receptor binding and stimulation of 45Ca uptake of RTX, resiniferonol 20-homovanillylamide, RTX-thiourea, tinyatoxin, phorbol 12,13-dibenzoate 20-homovanillylamide and capsaicin were 38.5 +/- 2.9 pM, 25.7 +/- 3.0 nM, 68.5 +/- 3.8 nM, 173 +/- 25 pM, 7.98 +/- 0.83 microM and 4.93 +/- 0.35 microM as compared to 0.94 +/- 0.12 nM, 26.5 +/- 3.5 nM, 149 +/- 30 nM, 1.46 +/- 0.25 nM, 1.41 +/- 0.48 microM and 340 +/- 57 nM. Computer fitting of the data yielded Hill coefficient values indicating positive cooperativity of receptor binding; however, stimulation of 45Ca uptake appeared to follow a non-cooperative mechanism of action. The competitive capsaicin antagonist capsazepine inhibited specific binding of [3H]RTX by rat dorsal root ganglion membrane preparations with Ki and Hill coefficient values of 3.89 +/- 0.38 microM and 1.74 +/- 0.11. On the other hand it inhibited the induction of 45Ca uptake into the cells induced by capsaicin and RTX in a non-cooperative fashion with Ki values of 271 +/- 29 nM and 325 +/- 47 nM. Our results show that the membrane binding assay relates to the reality of receptor function in the intact, cultured neurons, both in terms of affinity and positive cooperativity. However the different vanilloid derivatives displayed markedly distinct structure-activity relations for high affinity receptor binding and stimulation of 45Ca uptake into rat dorsal root ganglion neurons. Among various explanations for this discrepancy, we favor the possibility that the two assays detect distinct classes of the vanilloid (capsaicin) receptor present in primary sensory neurons.

Quantitative structure-agonist activity relationship of capsaicin analogues

The MULTIple Computer Automated Structure Evaluation (MULTICASE) methodology has been used to study the quantitative structure-agonist activity relationship of a series of capsaicin agonists. A number of substructures and physiochemical properties of capsaicin analogues were identified as being responsible for high agonist potency. The optimal log P value for the agonist potency as estimated from QSAR analysis is 5.12. It was also found that a cluster of inactive molecules in the database have lipophilicity values below 2.94. Molecular modeling was employed to elucidate the detailed structural features of the pharmacophore of capsaicin analogues. Systematic conformational analysis has shown that the activity of capsaicin analogues strongly depends upon their ability to reach the required conformational profile. Based upon these observations, a three-dimensional pharmacophore model for the capsaicin-receptor interactions is proposed.

Long-lasting desensitization of bladder afferents following intravesical resiniferatoxin and capsaicin in the rat

The present study was conducted to determine whether long-lasting desensitization of bladder afferents could be achieved using a single local application of the capsaicin (CAP)-like irritant resiniferatoxin (RTX), and to compare the effects of RTX and CAP on behavioral and histological endpoints. While rats were anesthetized, vehicle (VEH), RTX (10-100 nmol) or CAP (10-100 mumol) was instilled in the bladder (intravesical, i.ves.) via a cannula surgically implanted into the bladder dome. Beginning 1 week after treatment, once per week for 4 weeks, rats were tested behaviorally for desensitization to i.ves. RTX (10 nmol) using the abdominal lick test. Rats pretreated with low doses of RTX and CAP were partially desensitized at week 1; desensitization diminished over weeks 2-3. In contrast, rats pretreated with high doses of RTX or CAP were more completely desensitized at week 1, and desensitization did not diminish by week 4. Separate groups of rats tested 8 weeks after treatment showed substantial recovery. Rats pretreated with RTX but tested only with VEH for the first 3 weeks showed desensitization at week 4 approximately equivalent to that of RTX-treated rats tested with RTX every week. Sensitivity of corneal afferents to RTX (1.0 microgram/ml) at week 4 was not different between VEH- and RTX- or CAP-treated rats. Gross and histological examination of bladder tissue indicated that both RTX and CAP produced inflammation, which diminished in a dose- and time-dependent manner (1-8 weeks post-treatment).(ABSTRACT TRUNCATED AT 250 WORDS)

Capsaicin, a double-edged sword: toxicity, metabolism, and chemopreventive potential

Capsaicin (8-methyl-N-vanillyl-6-nonenamide) is a primary pungent and irritating principle present in chilies and red peppers which are widely used as spices. Because of its selective effects on the functions of a defined subpopulation of sensory neurons, capsaicin is currently used as a versatile tool for the study of pain mechanisms and also for pharmacotherapy to treat several pain disorders. Considering the frequent consumption of capsaicin as a food additive and its current medicinal use, correct assessment of hazardous effects of this compound is important. Mutagenic and carcinogenic activities of capsaicin and chili extracts have been studied, but results are conflicting. Mammalian metabolism of capsaicin has been also reported. Capsaicin appears to interact with xenobiotic metabolizing enzymes, particularly microsomal cytochrome P450-dependent monooxygenases which are involved in activation as well as detoxification of various chemical carcinogens and mutagens. Recent studies have shown that hepatic cytochrome P450 2E1 catalyzes the conversion of capsaicin to reactive species such as the phenoxy radical intermediate capable of covalently binding to the active site of the enzyme as well as tissue macromolecules. While covalent modification of protein and nucleic acids leads to toxicity including necrosis, mutagenesis, and carcinogenesis, suicidal inhibition of microsomal cytochrome P450 may prohibit further activation of capsaicin and also of other toxic xenobiotics. Results from recent studies indicate that capsaicin possesses the chemoprotective activity against some chemical carcinogens and mutagens.

Metabolism of capsaicinoids: evidence for aliphatic hydroxylation and its pharmacological implications

A new metabolic oxidation pathway of capsaicin (N-[(4-hydroxy-3-methoxyphenyl)-methyl]-8-methyl-(E)-6 -nonenamide), a major pungent and pharmacologically active principle of hot peppers, was investigated. Incubation of capsaicin with phenobarbital-induced rat liver postmitochondrial supernatant enriched with NADPH-generating system produced N-(4,5-dihydroxy-3-methoxybenzyl)-(E)-6 -nonenylamide and a more polar metabolite. The latter metabolite was spectrophotometrically and chromatographically identical to authentic omega-hydroxycapsaicin. This new metabolite was also detected in the urine of rabbits given capsaicin by gastric intubation. Other analogs of capsaicin, such as dihydrocapsaicin and nonivamide, also formed similar metabolites via aliphatic hydroxylation. When tested for antinociceptive activity as well as pungency, the above polar metabolites were found to be inactive while their parent compounds exhibited strong sensory effects. Capsaicin interacted irreversibly with heptic drug metabolizing enzymes, thereby inhibiting their activity as indicated by prolongation of pentobarbital sleeping time in rats. Such inhibition of drug metabolism was not observed with omega-hydroxycapsaicin. These findings suggest that metabolism of capsaicinoids via hydroxylation of their side chains plays an important role in the detoxification of these pharmacologically active substances.

[3H]resiniferatoxin binding by the human vanilloid (capsaicin) receptor

We report here that we were able to detect the human vanilloid receptor in all three major central endings of primary afferent neurons--in the dorsal horn of the spinal cord, in the cuneate and gracile nuclei and in the spinal nucleus of the trigeminal nerve--and to characterize the binding properties of the receptor in the dorsal horn. Specific [3H]resiniferatoxin (RTX) binding is thought to represent the vanilloid (capsaicin) receptor. [3H]RTX binding to membranes obtained from total human spinal cord and dorsal horn followed sigmoidal saturation kinetics indicating apparent positive cooperativity. The cooperativity index determined by fitting the data to the Hill equation was 1.37 +/- 0.02 in the total spinal cord and 1.77 +/- 0.16 in the dorsal horn. The apparent dissociation constants in whole spinal cord and dorsal horn membranes were 915 +/- 12 and 532 +/- 27 pM; the receptor densities were 140 +/- 6 and 227 +/- 15 fmol/mg protein, respectively. Membrane preparations from the spinal nucleus of the trigeminal nerve and the cuneate and gracile nuclei also bound [3H]RTX in a similar fashion. In parallel experiments, rat spinal cord membranes bound [3H]RTX with 20- to 40-fold higher affinity, somewhat greater positive cooperativity, but at a 3-fold lower receptor density. As predicted by the modified Hill equation, non-radioactive RTX at low receptor occupancy produced biphasic competition curves.(ABSTRACT TRUNCATED AT 250 WORDS)

The vanilloid (capsaicin) receptor: receptor types and species differences

1. Capsaicin was postulated to exert its pharmacological actions by interacting at a specific recognition site (receptor) expressed predominantly by primary afferent neurons. 2. The actual existence of this long-sought "capsaicin-receptor" has recently been demonstrated by the specific binding of [3H]resiniferatoxin (RTX), an ultrapotent capsaicin analog with a unique spectra of actions. 3. Since homovanillic acid is the key structural motif shared by capsaicin and RTX, their recognition site appears to be best termed the vanilloid receptor. 4. Central (sensory ganglia and spinal cord) vanilloid receptors of the rat bind RTX with high affinity in a cooperative fashion; moreover, they recognize capsaicin with higher affinity than the competp6ive antagonist, capsazepine. Peripheral (urinary bladder, urethra, airways, colon) vanilloid receptors, by contrast, bind RTX with lower affinity in a noncooperative manner. An opposite affinity for capsazepine relative to capsaicin appears to distinguish vanilloid receptors in the urinary bladder from those present in the airways or colon. These findings imply heterogeneity in the properties of vanilloid receptors. 5. The affinity of [3H]RTX binding in vitro is influenced by reducing agents, suggesting an in vivo modulatory role for endogenous reducing agents in vanilloid receptor functions. 6. The size of central vanilloid receptors (270 kDa) as measured by radiation inactivation and the cooperative binding both suggest a receptor cluster with cooperating subunits. 7. RTX binds to vanilloid receptors with orders of magnitude higher affinity than capsaicin; its ability to induce cooperative binding is also more pronounced. These differences in receptor binding along with the pharmacokinetical differences in tissue equilibration and in plasma binding may form a rational basis to explain the peculiar spectrum of actions of RTX. 8. Guinea pig spinal cord and airway membranes bind RTX with lower affinity than rat tissues. The receptor density is, however, higher in the guinea pig in keeping with the marked sensitivity of this species to vanilloid actions. 9. The apparently low level of specific [3H]RTX binding sites in the hamster and rabbit is in accord with the resistance of these species to vanilloid actions. 10. In post-mortem human spinal cord specific [3H]RTX binding sites can be detected; their binding parameters are similar to those determined in guinea pig spinal cord. 11. The vanilloid receptor appears to display both intraspecies heterogeneity and marked interspecies differences. 12. As yet, it is known whether the vanilloid receptor is operated by endogenous ligands. It is not known either which receptor superfamily (if any) it belongs to. The [3H]RTX binding assay has, however, the potential of answering these questions.(ABSTRACT TRUNCATED AT 400 WORDS)