In vivo pharmacology of SDZ 249-665, a novel, non-pungent capsaicin analogue

Anatomical Analysis of Transient Potential Vanilloid Receptor 1 (Trpv1+) and Mu-Opioid Receptor (Oprm1+) Co-expression in Rat Dorsal Root Ganglion Neurons

Primary afferent neurons of the dorsal root ganglia (DRG) transduce peripheral nociceptive signals and transmit them to the spinal cord. These neurons also mediate analgesic control of the nociceptive inputs, particularly through the μ-opioid receptor (encoded by Oprm1). While opioid receptors are found throughout the neuraxis and in the spinal cord tissue itself, intrathecal administration of μ-opioid agonists also acts directly on nociceptive nerve terminals in the dorsal spinal cord resulting in marked analgesia. Additionally, selective chemoaxotomy of cells expressing the TRPV1 channel, a nonselective calcium-permeable ion channel that transduces thermal and inflammatory pain, yields profound pain relief in rats, canines, and humans. However, the relationship between Oprm1 and Trpv1 expressing DRG neurons has not been precisely determined. The present study examines rat DRG neurons using high resolution multiplex fluorescent in situ hybridization to visualize molecular co-expression. Neurons positive for Trpv1 exhibited varying levels of expression for Trpv1 and co-expression of other excitatory and inhibitory ion channels or receptors. A subpopulation of densely labeled Trpv1+ neurons did not co-express Oprm1. In contrast, a population of less densely labeled Trpv1+ neurons did co-express Oprm1. This finding suggests that the medium/low Trpv1 expressing neurons represent a specific set of DRG neurons subserving the opponent processes of both transducing and inhibiting nociceptive inputs. Additionally, the medium/low Trpv1 expressing neurons co-expressed other markers implicated in pathological pain states, such as Trpa1 and Trpm8, which are involved in chemical nociception and cold allodynia, respectively, as well as Scn11a, whose mutations are implicated in familial episodic pain. Conversely, none of the Trpv1+ neurons co-expressed Spp1, which codes for osteopontin, a marker for large diameter proprioceptive neurons, validating that nociception and proprioception are governed by separate neuronal populations. Our findings support the hypothesis that the population of Trpv1 and Oprm1 coexpressing neurons may explain the remarkable efficacy of opioid drugs administered at the level of the DRG-spinal synapse, and that this subpopulation of Trpv1+ neurons is responsible for registering tissue damage.

Anti-Hyperuricemic, Anti-Inflammatory and Analgesic Effects of Siegesbeckia orientalis L. Resulting from the Fraction with High Phenolic Content

Background

The medicinal plant Siegesbeckia orientalis L. has been commonly used for the treatment of acute arthritis, rheumatism, and gout in Vietnam. However, pharmacological research of this plant associated with gout has not been reported. Anti-hyperuricemic and anti-inflammatory effects were evaluated and observed for the crude ethanol extract (CEE) of S. orientalis. Retention of these biological properties was found in a n-butanol-soluble fraction (BuOH fr.) of the extract, and therefore further biological and chemical investigations were undertaken on the BuOH fr. to support the medical relevance of this plant.

Methods

The aerial part of S. orientalis was obtained in the mountainous region of Vietnam. The crude ethanol extract (CEE) and its BuOH fr. were prepared from the plant materials. Anti-hyperuricemic activities of the CEE and BuOH fr. were tested in vivo using the model oxonate-induced hyperuricemia rats through determination of serum uric acid levels and inhibitory effects on xanthine oxidase (XO) in the rat liver. Anti-inflammatory activities of the BuOH fr. were also evaluated in vivo using carrageenan-induced paw edema and urate-induced synovitis in rats. Active components of the BuOH fr. were characterized by comparison of HPLC retention time (t_R) and spectroscopic data (UV, ¹H–NMR) with those of reference compounds.

Results

The CEE of S. orientalis displayed anti-hyperuricemic activity, and the BuOH fr. was found to be the most active portion of the extract. Further in vivo studies on this fraction showed 31.4% decrease of serum uric acid levels, 32.7% inhibition of xanthine oxidase (XO), 30.4% reduction of paw edema volume, symptomatic relief in urate-induced synovitis and significant analgesic effect at the dose of 120 mg/kg, as compared to the corresponding values of the control groups. Chemical analysis of the BuOH fr. revealed high phenolic content, identified as caffeic acid analogues and flavonones.

Conclusions

This study suggested that anti-hyperuricemic and anti-inflammatory mechanism of S. orientalis is related to XO inhibitory effect of the phenolic components. Our findings support the use of this plant as the treatment of gout and other inflammatory diseases.

Electronic supplementary material

The online version of this article (doi:10.1186/s12906-017-1698-z) contains supplementary material, which is available to authorized users.

Capsaicin, Nociception and Pain

Capsaicin, the pungent ingredient of the hot chili pepper, is known to act on the transient receptor potential cation channel vanilloid subfamily member 1 (TRPV1). TRPV1 is involved in somatic and visceral peripheral inflammation, in the modulation of nociceptive inputs to spinal cord and brain stem centers, as well as the integration of diverse painful stimuli. In this review, we first describe the chemical and pharmacological properties of capsaicin and its derivatives in relation to their analgesic properties. We then consider the biochemical and functional characteristics of TRPV1, focusing on its distribution and biological effects within the somatosensory and viscerosensory nociceptive systems. Finally, we discuss the use of capsaicin as an agonist of TRPV1 to model acute inflammation in slices and other ex vivo preparations.

Corneal sensitivity following lacrimal gland excision in the rat

PURPOSE

Dry eye disease (DED) produces ocular pain and irritation, yet a detailed characterization of ocular sensitivity in a preclinical model of DED is lacking. The aim of the present study was to assess nociceptive behaviors in an aqueous tear deficiency model of DED in the rat.

METHODS

Spontaneous blinking, corneal mechanical thresholds, and eye wipe behaviors elicited by hypertonic saline (5.0 M) were examined over a period of 8 weeks following the unilateral excision of either the exorbital lacrimal gland or of the exorbital and infraorbital lacrimal glands, and in sham surgery controls. The effect of topical proparacaine on spontaneous blinking and of systemic morphine (0.5-3.0 mg/kg, subcutaneous [SC]) on spontaneous blinking and eye wipe responses were also examined.

RESULTS

Lacrimal gland excision resulted in mechanical hypersensitivity and an increase in spontaneous blinking in the ipsilateral eye over an 8-week period that was more pronounced after infra- and exorbital gland excision. The time spent eye wiping was also enhanced in response to hypertonic saline (5.0 M) at both 1- and 8-week time-points, but only in infra- and exorbital gland excised animals. Morphine attenuated spontaneous blinking, and the response to hypertonic saline in dry eye animals and topical proparacaine application reduced spontaneous blinking down to control levels.

CONCLUSIONS

These results indicate that aqueous tear deficiency produces hypersensitivity in the rat cornea. In addition, the increase in spontaneous blinks and their reduction by morphine and topical anesthesia indicate the presence of persistent irritation elicited by the activation of corneal nociceptors.

The involvement of TRPV1 in emesis and anti-emesis

Diverse transmitter systems (e.g. acetylcholine, dopamine, endocannabinoids, endorphins, glutamate, histamine, 5-hydroxytryptamine, substance P) have been implicated in the pathways by which nausea and vomiting are induced and are targets for anti-emetic drugs (e.g. 5-hydroxytryptamine₃ and tachykinin NK₁ antagonists). The involvement of TRPV1 in emesis was discovered in the early 1990s and may have been overlooked previously as TRPV1 pharmacology was studied in rodents (mice, rats) lacking an emetic reflex. Acute subcutaneous administration of resiniferatoxin in the ferret, dog and Suncus murinus revealed that it had “broad–spectrum” anti-emetic effects against stimuli acting via both central (vestibular system, area postrema) and peripheral (abdominal vagal afferents) inputs. One of several hypotheses discussed here is that the anti-emetic effect is due to acute depletion of substance P (or another peptide) at a critical site (e.g. nucleus tractus solitarius) in the central emetic pathway. Studies in Suncus murinus revealed a potential for a long lasting (one month) effect against the chemotherapeutic agent cisplatin. Subsequent studies using telemetry in the conscious ferret compared the anti-emetic, hypothermic and hypertensive effects of resiniferatoxin (pungent) and olvanil (non-pungent) and showed that the anti-emetic effect was present (but reduced) with olvanil which although inducing hypothermia it did not have the marked hypertensive effects of resiniferatoxin. The review concludes by discussing general insights into emetic pathways and their pharmacology revealed by these relatively overlooked studies with TRPV1 activators (pungent an non-pungent; high and low lipophilicity) and antagonists and the potential clinical utility of agents targeted at the TRPV1 system.

TRPV1

TRPV1 is a well-characterised channel expressed by a subset of peripheral sensory neurons involved in pain sensation and also at a number of other neuronal and non-neuronal sites in the mammalian body. Functionally, TRPV1 acts as a sensor for noxious heat (greater than ~42 °C). It can also be activated by some endogenous lipid-derived molecules, acidic solutions (pH < 6.5) and some pungent chemicals and food ingredients such as capsaicin, as well as by toxins such as resiniferatoxin and vanillotoxins. Structurally, TRPV1 subunits have six transmembrane (TM) domains with intracellular N- (containing 6 ankyrin-like repeats) and C-termini and a pore region between TM5 and TM6 containing sites that are important for channel activation and ion selectivity. The N- and C- termini have residues and regions that are sites for phosphorylation/dephosphorylation and PI(4,5)P2 binding, which regulate TRPV1 sensitivity and membrane insertion. The channel has several interacting proteins, some of which (e.g. AKAP79/150) are important for TRPV1 phosphorylation. Four TRPV1 subunits form a non-selective, outwardly rectifying ion channel permeable to monovalent and divalent cations with a single-channel conductance of 50-100 pS. TRPV1 channel kinetics reveal multiple open and closed states, and several models for channel activation by voltage, ligand binding and temperature have been proposed. Studies with TRPV1 agonists and antagonists and Trpv1 (-/-) mice have suggested a role for TRPV1 in pain, thermoregulation and osmoregulation, as well as in cough and overactive bladder. TRPV1 antagonists have advanced to clinical trials where findings of drug-induced hyperthermia and loss of heat sensitivity have raised questions about the viability of this therapeutic approach.

Paradoxic effects of propofol on visceral pain induced by various TRPV1 agonists

Intraperitoneal injection of propofol inhibits subsequent acetic acid-induced writhing response in mice. Propofol increases the sensitivity of dorsal root ganglion neurons to capsaicin through transient receptor potential ankyrin subtype-1 (TRPA1) and protein kinase Cε (PKCε)-mediated phosphorylation of transient receptor potential vanilloid subtype-1 (TRPV1). Intraperitoneal co-injection of propofol may increase visceral nociception induced by TRPV1 agonists via sensitization of TRPV1. Therefore, we investigated the effects of intraperitoneal co-injection of propofol on nociception induced by acetic acid and capsaicin. The number of writhing movements induced by acetic acid or nociception time by capsaicin with or without propofol were counted. Neonatal capsaicin-treated mice were also used to demonstrate the role of TRPV1 in the effects of propofol on nociception, induced by TRPV1 agonists. Co-injection of propofol resulted in a pronociceptive effect on the writhing response induced by acetic acid, while the same dose of propofol ameliorated the response to capsaicin. The writhing response to intraperitoneal acetic acid was sharply inhibited following neonatal treatment with capsaicin. Co-injection with propofol reduced the number of writhing movements induced by acetic acid in neonatal capsaicin-treated mice. These results suggest that propofol binds to TRPV1 at the capsaicin-binding pocket.

Menthol activation of corneal cool cells induces TRPM8-mediated lacrimation but not nociceptive responses in rodents

PURPOSE

Stimulation to the cornea via noxious chemical and mechanical means evokes tearing, blinking, and pain. In contrast, mild cooling of the ocular surface has been reported to increase lacrimation via activation of corneal cool primary afferent neurons. The purpose of our study was to determine whether menthol induces corneal cool cell activity and lacrimation via the transient receptor potential melastatin-8 (TRPM8) channel without evoking nociceptive responses.

METHODS

Tear measurements were made using a cotton thread in TRPM8 wild type and knockout mice after application of menthol (0.05-50 mM) to the cornea. In additional studies, nocifensive responses (eye swiping and lid closure) were quantified following cornea menthol application. Trigeminal ganglion electrophysiologic single unit recordings were performed in rats to determine the effect of low and high concentrations of menthol on corneal cool cells.

RESULTS

At low concentrations, menthol increased tear production in TRPM8 wild type and heterozygous animals, but had no effect in TRPM8 knockout mice, while nocifensive responses remained unaffected. At the highest concentration, menthol (50 mM) increased tearing and nocifensive responses in TRPM8 wild type and knockout animals. A low concentration of menthol (0.1 mM) increased cool cell activity, yet a high concentration of menthol (50 mM) had no effect.

CONCLUSIONS

These studies indicated that low concentrations of menthol can increase lacrimation via TRPM8 channels without evoking nocifensive behaviors. At high concentrations, menthol can induce lacrimation and nocifensive behaviors in a TRPM8 independent mechanism. The increase in lacrimation is likely due to an increase in cool cell activity.

Transient receptor potential (TRP) channels as drug targets for diseases of the digestive system

Approximately 20 of the 30 mammalian transient receptor potential (TRP) channel subunits are expressed by specific neurons and cells within the alimentary canal. They subserve important roles in taste, chemesthesis, mechanosensation, pain and hyperalgesia and contribute to the regulation of gastrointestinal motility, absorptive and secretory processes, blood flow, and mucosal homeostasis. In a cellular perspective, TRP channels operate either as primary detectors of chemical and physical stimuli, as secondary transducers of ionotropic or metabotropic receptors, or as ion transport channels. The polymodal sensory function of TRPA1, TRPM5, TRPM8, TRPP2, TRPV1, TRPV3 and TRPV4 enables the digestive system to survey its physical and chemical environment, which is relevant to all processes of digestion. TRPV5 and TRPV6 as well as TRPM6 and TRPM7 contribute to the absorption of Ca²⁺ and Mg²⁺, respectively. TRPM7 participates in intestinal pacemaker activity, and TRPC4 transduces muscarinic acetylcholine receptor activation to smooth muscle contraction. Changes in TRP channel expression or function are associated with a variety of diseases/disorders of the digestive system, notably gastro-esophageal reflux disease, inflammatory bowel disease, pain and hyperalgesia in heartburn, functional dyspepsia and irritable bowel syndrome, cholera, hypomagnesemia with secondary hypocalcemia, infantile hypertrophic pyloric stenosis, esophageal, gastrointestinal and pancreatic cancer, and polycystic liver disease. These implications identify TRP channels as promising drug targets for the management of a number of gastrointestinal pathologies. As a result, major efforts are put into the development of selective TRP channel agonists and antagonists and the assessment of their therapeutic potential.

Chemosensory properties of the trigeminal system

The capacity of cutaneous, including trigeminal endings, to detect chemicals is known as chemesthesis or cutaneous chemosensation. This sensory function involves the activation of nociceptor and thermoreceptor endings and has a protective or defensive function, as many of these substances are irritants or poisonous. However, humans have also developed a liking for the distinct sharpness or pungency of many foods, beverages, and spices following activation of the same sensory afferents. Our understanding of the cellular and molecular mechanisms of chemosensation in the trigeminal system has experienced enormous progress in the past decade, following the cloning and functional characterization of several ion channels activated by physical and chemical stimuli. This brief review attempts to summarize our current knowledge in this field, including a functional description of various sensory channels, especially TRP channels, involved in trigeminal chemosensitivy. Finally, some of these new findings are discussed in the context of the pathophysiology of trigeminal chemosensation, including pain, pruritus, migraine, cough, airway inflammation, and ophthalmic diseases.

Acid sensing by visceral afferent neurones

Acidosis in the gastrointestinal tract can be both a physiological and pathological condition. While gastric acid serves digestion and protection from pathogens, pathological acidosis is associated with defective acid containment, inflammation and ischaemia. The pH in the oesophagus, stomach and intestine is surveyed by an elaborate network of acid-sensing mechanisms to maintain homeostasis. Deviations from physiological values of extracellular pH (7.4) are monitored by multiple acid sensors expressed by epithelial cells and sensory neurones. Protons evoke multiple currents in primary afferent neurones, which are carried by several acid-sensitive ion channels. Among these, acid-sensing ion channels (ASICs) and transient receptor potential (TRP) vanilloid-1 (TRPV1) ion channels have been most thoroughly studied. ASICs survey moderate decreases in extracellular pH whereas TRPV1 is activated only by severe acidosis resulting in pH values below 6. Other molecular acid sensors comprise TRPV4, TRPC4, TRPC5, TRPP2 (PKD2L1), epithelial Na(+) channels, two-pore domain K(+) (K₂(P)) channels, ionotropic purinoceptors (P2X), inward rectifier K(+) channels, voltage-activated K(+) channels, L-type Ca²(+) channels and acid-sensitive G-protein-coupled receptors. Most of these acid sensors are expressed by primary sensory neurones, although to different degrees and in various combinations. As upregulation and overactivity of acid sensors appear to contribute to various forms of chronic inflammation and pain, acid-sensitive ion channels and receptors are also considered as targets for novel therapeutics.

Drug-induced mild therapeutic hypothermia obtained by administration of a transient receptor potential vanilloid type 1 agonist

Background

The use of mechanical/physical devices for applying mild therapeutic hypothermia is the only proven neuroprotective treatment for survivors of out of hospital cardiac arrest. However, this type of therapy is cumbersome and associated with several side-effects. We investigated the feasibility of using a transient receptor potential vanilloid type 1 (TRPV1) agonist for obtaining drug-induced sustainable mild hypothermia.

Methods

First, we screened a heterogeneous group of TRPV1 agonists and secondly we tested the hypothermic properties of a selected candidate by dose-response studies. Finally we tested the hypothermic properties in a large animal. The screening was in conscious rats, the dose-response experiments in conscious rats and in cynomologus monkeys, and the finally we tested the hypothermic properties in conscious young cattle (calves with a body weight as an adult human). The investigated TRPV1 agonists were administered by continuous intravenous infusion.

Results

Screening: Dihydrocapsaicin (DHC), a component of chili pepper, displayed a desirable hypothermic profile with regards to the duration, depth and control in conscious rats. Dose-response experiments: In both rats and cynomologus monkeys DHC caused a dose-dependent and immediate decrease in body temperature. Thus in rats, infusion of DHC at doses of 0.125, 0.25, 0.50, and 0.75 mg/kg/h caused a maximal ΔT (°C) as compared to vehicle control of -0.9, -1.5, -2.0, and -4.2 within approximately 1 hour until the 6 hour infusion was stopped. Finally, in calves the intravenous infusion of DHC was able to maintain mild hypothermia with ΔT > -3°C for more than 12 hours.

Conclusions

Our data support the hypothesis that infusion of dihydrocapsaicin is a candidate for testing as a primary or adjunct method of inducing and maintaining therapeutic hypothermia.

In vitro TRPV1 activity of piperine derived amides

A series of natural and synthetic piperine amides were evaluated for activity on the human TRPV1 expressed in HEK293 cells. The agonistic effect of piperine amides was mainly dependent on the length of the carbon chain. Structural changes of double bonds and stereochemistry in the aliphatic chain of these compounds did not change their potency or efficacy, indicating that increased rigidity or planarity of the piperine structure does not affect the activity. The opening of the methylenedioxy ring or changes in the heterocyclic ring of the piperine molecule reduced or abolished activity. Furthermore, inactive compounds did not display functional antagonistic activity.

Effect of repeated, long term capsaicin ingestion on intestinal chemo- and mechanosensation in healthy volunteers

Repeated ingestion of capsaicin over a prolonged period reduces symptoms in functional dyspepsia, but initially induces upper abdominal symptoms. Sensitizing chemonociception might be the cause for this initial effect of capsaicin. The aim was to evaluate the effect of prolonged capsaicin ingestion on duodenal chemo- and mechanonociception. Healthy subjects ingested capsules containing either 0.25 mg capsaicin tid (n = 8) or placebo (n = 8) for 28 days. Before (day 0) and after (day 29) capsule ingestion the duodenum was distended with a balloon and perfused with a capsaicin solution. Mechanically and chemically induced sensation was evaluated by a graded questionnaire. Aggregate perception scores were calculated. Perception scores during balloon distensions with 12 and 18 mmHg were significantly lower after 4 weeks capsaicin when compared to baseline (P < 0.05). Balloon volumes to induce first sensation (63 +/- 14 mL (day 0) vs 92 +/- 22 mL (day 29); P < 0.05) and discomfort (101 +/- 12 mL vs 137 +/- 22 mL; P = 0.05) where significantly higher after 4 weeks capsaicin application; balloon pressures to induce sensations were not significantly different. Intraluminal capsaicin application induced first sensation after 3.4 +/- 1.5 min (day 0) and 7.5 +/- 4.6 min (day 29) (P < 0.05) and discomfort after 15.9 +/- 9.8 min and 22.4 +/- 7.3 min (P < 0.05). The quality of perception was not altered by repeated capsaicin ingestion. In the placebo group, mechano- and chemonociception remained unaltered at day 29. Four weeks ingestion of capsaicin desensitized both chemonociceptive and mechanonociceptive pathways in healthy volunteers. Symptom reduction after prolonged treatment with capsaicin in dyspeptic patients might be attributed to a dual desensitizing effect of capsaicin on chemonociceptors and mechanonociceptors.

Bladder hyperactivity and increased excitability of bladder afferent neurons associated with reduced expression of Kv1.4 alpha-subunit in rats with cystitis

Hyperexcitability of C-fiber bladder afferent pathways has been proposed to contribute to urinary frequency and bladder pain in chronic bladder inflammation including interstitial cystitis. However, the detailed mechanisms inducing afferent hyperexcitability after bladder inflammation are not fully understood. Thus, we investigated changes in the properties of bladder afferent neurons in rats with bladder inflammation induced by intravesical application of hydrochloric acid. Eight days after the treatment, bladder function and bladder sensation were analyzed using cystometry and an electrodiagnostic device of sensory function (Neurometer), respectively. Whole cell patch-clamp recordings and immunohistochemical staining were also performed in dissociated bladder afferent neurons identified by a retrograde tracing dye, Fast Blue, injected into the bladder wall. Cystitis rats showed urinary frequency that was inhibited by pretreatment with capsaicin and bladder hyperalgesia mediated by C-fibers. Capsaicin-sensitive bladder afferent neurons from sham rats exhibited high thresholds for spike activation and a phasic firing pattern, whereas those from cystitis rats showed lower thresholds for spike activation and a tonic firing pattern. Transient A-type K(+) current density in capsaicin-sensitive bladder afferent neurons was significantly smaller in cystitis rats than in sham rats, although sustained delayed-rectifier K(+) current density was not altered after cystitis. The expression of voltage-gated K(+) Kv1.4 alpha-subunits, which can form A-type K(+) channels, was reduced in bladder afferent neurons from cystitis rats. These data suggest that bladder inflammation increases bladder afferent neuron excitability by decreasing expression of Kv1.4 alpha-subunits. Similar changes in capsaicin-sensitive C-fiber afferent terminals may contribute to bladder hyperactivity and hyperalgesia due to acid-induced bladder inflammation.

Acid-sensitive ion channels and receptors

Acidosis is a noxious condition associated with inflammation, ischaemia or defective acid containment. As a consequence, acid sensing has evolved as an important property of afferent neurons with unmyelinated and thinly myelinated nerve fibres. Protons evoke multiple currents in primary afferent neurons, which are carried by several acid-sensitive ion channels. Among these, acid-sensing ion channels (ASICs) and transient receptor potential (TRP) vanilloid-1 (TRPV1) ion channels have been most thoroughly studied. ASICs survey moderate decreases in extracellular pH, whereas TRPV1 is activated only by severe acidosis resulting in pH values below 6. Two-pore-domain K(+) (K(2P)) channels are differentially regulated by small deviations of extra- or intracellular pH from physiological levels. Other acid-sensitive channels include TRPV4, TRPC4, TRPC5, TRPP2 (PKD2L1), ionotropic purinoceptors (P2X), inward rectifier K(+) channels, voltage-activated K(+) channels, L-type Ca(2+) channels, hyperpolarization-activated cyclic nucleotide gated channels, gap junction channels, and Cl(-) channels. In addition, acid-sensitive G protein coupled receptors have also been identified. Most of these molecular acid sensors are expressed by primary sensory neurons, although to different degrees and in various combinations. Emerging evidence indicates that many of the acid-sensitive ion channels and receptors play a role in acid sensing, acid-induced pain and acid-evoked feedback regulation of homeostatic reactions. The existence and apparent redundancy of multiple pH surveillance systems attests to the concept that acid-base regulation is a vital issue for cell and tissue homeostasis. Since upregulation and overactivity of acid sensors appear to contribute to various forms of chronic pain, acid-sensitive ion channels and receptors are considered as targets for novel analgesic drugs. This approach will only be successful if the pathological implications of acid sensors can be differentiated pharmacologically from their physiological function.

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

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

Hypersensitivity for capsaicin in patients with functional dyspepsia

The pathophysiology of functional dyspepsia is poorly understood, thus diagnostic and therapeutic options for this disease are limited. We assessed the relevance of a simple test for chemical hypersensitivity by applying an oral capsaicin load. After a preliminary dose-finding study, 61 healthy controls and 54 functional dyspepsia patients swallowed a capsule containing 0.75 mg capsaicin. A graded questionnaire evaluated severity of symptoms before and after capsule ingestion; an aggregate symptom score was calculated by adding all symptom scores. Controls developed moderate symptoms (symptom score: 6.0+/-4.1; median: 5.0). The 75% quartile (9.0) was considered the upper limit of normal. Functional dyspepsia patients had significantly higher symptom scores (10.0+/-6.5) than controls. About 54% of functional dyspepsia patients tested positive; clinically this group was not different from the group testing negative besides being on average younger and suffering more from bloating. In additional 13 patients with functional dyspepsia who tested positive (symptom score: 15.8+/-0.9), symptom response to placebo capsules (1.9+/-0.6) was similar to controls. In reliability testing, the Cronbach alpha-value of the capsaicin test was 0.86. The capsaicin test is a simple and non-invasive method to detect a subgroup of functional dyspepsia with chemical hypersensitivity.

Drug discovery and development with plant-derived compounds

An overview is given on current efforts in drug development based on plant-derived natural products. Emphasis is on projects which have advanced to clinical development. Therapeutic areas covered include cancer, viral infections including HIV, malaria, inflammatory diseases, nociception and vaccine adjuvants, metabolic disorders, and neurodegenerative diseases. Aspects which are specific to plant-based drug discovery and development are also addressed, such as supply issues in the commercial development, and the Convention on Biological Diversity.

Intestinal chemo- and mechano-sensitivity: selective modification of small intestinal sensitivity by lipids

BACKGROUND

Intestinal chemo-nociception is a recently described mechanism of perception of intraluminal stimuli that might involve pathways independent from mechano-nociception.

AIM

To investigate whether intraluminal lipids not only modify mechano-sensitivity, but also influence chemo-sensitivity.

METHOD

Nineteen healthy volunteers ingested an orojejunal tube assembly for mechanical and chemical stimulation of the proximal jejunum. Volume-controlled distension of a jejunal balloon and capsaicin perfusion were performed during simultaneous infusion of saline or a 10% lipid emulsion. A standard questionnaire evaluated quality and intensity of symptoms induced by intestinal stimulation.

RESULTS

Balloon volumes to induce perception thresholds were significantly reduced during lipid perfusion (P < 0.05), while balloon pressures remained unaltered (P > 0.05). Saline infusion (2.5 mL/min) did not alter thresholds of perception. Lipid infusion also increased intensity of perception during given distension volumes (P < 0.05). Lipid infusion did neither affect perception quality induced by capsaicin, nor perception thresholds, nor intensity of perception during capsaicin perfusion (P > 0.05). The quality of sensations induced by capsaicin perfusion was similar to sensations during distension, except of sensation of warmth, that was almost exclusively reported during capsaicin perfusion.

CONCLUSIONS

Intraluminal lipids selectively modify intestinal mechano-perception and do not alter chemo-perception. Intestinal chemo-nociception and mechano-nociception are two mechanisms of intestinal perception that involve distinct sensory pathways.

Paracetamol: new vistas of an old drug

Paracetamol (acetaminophen) is one of the most popular and widely used drugs for the treatment of pain and fever. It occupies a unique position among analgesic drugs. Unlike NSAIDs it is almost unanimously considered to have no antiinflammatory activity and does not produce gastrointestinal damage or untoward cardiorenal effects. Unlike opiates it is almost ineffective in intense pain and has no depressant effect on respiration. Although paracetamol has been used clinically for more than a century, its mode of action has been a mystery until about one year ago, when two independent groups (Zygmunt and colleagues and Bertolini and colleagues) produced experimental data unequivocally demonstrating that the analgesic effect of paracetamol is due to the indirect activation of cannabinoid CB(1) receptors. In brain and spinal cord, paracetamol, following deacetylation to its primary amine (p-aminophenol), is conjugated with arachidonic acid to form N-arachidonoylphenolamine, a compound already known (AM404) as an endogenous cannabinoid. The involved enzyme is fatty acid amide hydrolase. N-arachidonoylphenolamine is an agonist at TRPV1 receptors and an inhibitor of cellular anandamide uptake, which leads to increased levels of endogenous cannabinoids; moreover, it inhibits cyclooxygenases in the brain, albeit at concentrations that are probably not attainable with analgesic doses of paracetamol. CB(1) receptor antagonist, at a dose level that completely prevents the analgesic activity of a selective CB(1) receptor agonist, completely prevents the analgesic activity of paracetamol. Thus, paracetamol acts as a pro-drug, the active one being a cannabinoid. These findings finally explain the mechanism of action of paracetamol and the peculiarity of its effects, including the behavioral ones. Curiously, just when the first CB(1) agonists are being introduced for pain treatment, it comes out that an indirect cannabino-mimetic had been extensively used (and sometimes overused) for more than a century.

Preferential block of inactivation-deficient Na+ currents by capsaicin reveals a non-TRPV1 receptor within the Na+ channel

Capsaicin elicits burning pain via the activation of the vanilloid receptor (TRPV1). Intriguingly, several reports showed that capsaicin also inhibits Na+ currents but the mechanisms remain unclear. To explore this non-TRPV1 action we applied capsaicin to HEK293 cells stably expressing inactivation-deficient rat skeletal muscle Na+ mutant channels (rNav1.4-WCW). Capsaicin elicited a conspicuous time-dependent block of inactivation-deficient Na+ currents. The 50% inhibitory concentration (IC50) of capsaicin for open Na+ channels at +30 mV was measured 6.8+/-0.6 microM (n=5), a value that is 10-30 times lower than those for resting (218 microM) and inactivated (74 microM) wild-type Na+ channels. On-rate and off-rate constants for capsaicin open-channel block at +30 mV were estimated to be 6.37 microM(-1) s(-1) and 34.4 s(-1), respectively, with a calculated dissociation constant (KD) of 5.4 microM. Capsaicin at 30 microM produced approximately 70% additional use-dependent block of remaining rNav1.4-WCW Na+ currents during repetitive pulses at 1 Hz. Site-directed mutagenesis showed that the local anesthetic receptor was not responsible for the capsaicin block of the inactivation-deficient Na+ channel. Interestingly, capsaicin elicited little time-dependent block of batrachotoxin-modified rNav1.4-WCW Na+ currents, indicating that batrachotoxin prevents capsaicin binding. Finally, neuronal open Na+ channels endogenously expressed in GH3 cells were as sensitive to capsaicin block as rNav1.4 counterparts. We conclude that capsaicin preferentially blocks persistent late Na+ currents, probably via a receptor that overlaps the batrachotoxin receptor but not the local anesthetic receptor. Drugs that target such a non-TRPV1 receptor could be beneficial for patients with neuropathic pain.

Effect of repeated capsaicin ingestion on intestinal chemosensation and mechanosensation

BACKGROUND

Ingestion of capsaicin reduces symptoms in functional dyspepsia but induces upper abdominal symptoms initially.

AIMS

To evaluate the effects of one week of oral dosing with capsaicin on mechanonociception and chemonociception.

METHODS

Healthy subjects ingested capsules containing 0.5 mg capsaicin t.d.s (n = 8) or placebo (n = 5) for 7 days. Before (day 0) and after (day 8) capsule ingestion the jejunum was distended with a balloon and perfused with a capsaicin solution. A graded questionnaire evaluated distension- and capsaicin-induced sensation, aggregate perception scores were calculated.

RESULTS

Infusion of capsaicin induced sensations similar to distension. In subjects receiving capsaicin capsules, mean +/- S.E.M. perception scores at 24 mL distension volumes were 4.7 +/- 1.2 (day 0) and 3.2 +/- 1.3 (day 8, N.S.). Distension with 40 mL induced perception scores of 10.3 +/- 1.0 vs. 8.2 +/- 1.0 (P < 0.05). During capsaicin perfusion, first sensation was reported after 13.2 +/- 1.9 min (day 0) and 8.1 +/- 0.8 min (day 8, N.S.), discomfort thresholds after 44.1 +/- 6.0 and 31.2 +/- 5.7 min (P < 0.05). In the placebo group, mechano- and chemonociception remained unaltered on days 0 and 8.

CONCLUSIONS

One week of ingestion of capsaicin sensitized chemonociceptors, while sensitivity of mechanonociceptors for distension decreased. Painful sensations during the first week of treatment with capsaicin can be attributed to a sensitizing effect of capsaicin on vanilloid receptors.

Substantial role of locus coeruleus-noradrenergic activation and capsaicin-insensitive primary afferent fibers in bee venom's anti-inflammatory effect

Several lines of evidence indicate significant interactions between the immune and nervous systems. Our recent study reveals that 'bee venom (BV) induced anti-inflammatory effect' (BVAI) was produced by sympathetic preganglionic neuronal activation and subsequent adrenomedullary catecholamine release in a zymosan-induced inflammation model. However, the specific peripheral input and the supraspinal neuronal systems that are involved in this BVAI remain to be defined. Here we show that subcutaneous BV injection into left hind limb significantly reduces zymosan-induced leukocyte migration and that this effect is completely inhibited by denervation of the left sciatic nerve. This BVAI was not affected by the destruction of capsaicin-sensitive primary afferent fibers using either neonatal capsaicin or resiniferatoxin (RTX) pretreatment. BV injection into the left hind limb significantly increased Fos expression in the contralateral locus coeruleus (LC) in non-inflamed mice. In zymosan-inflamed mice, BV injection produced a further increase in LC Fos expression as compared with non-inflamed mice. This BV-induced Fos increase in the LC was not affected by RTX pretreatment. Pharmacological blockage of central noradrenergic activity by either central chemical sympathectomy (i.c.v. 6-hydroxydopamine) or alpha2 adrenoceptor antagonism (i.c.v. idazoxan) completely blocked BVAI. Taken together, these results suggest that BVAI is mediated by peripheral activation of capsaicin-insensitive primary afferent fibers and subsequent central noradrenergic activation including the LC.

TRPV1 recapitulates native capsaicin receptor in sensory neurons in association with Fas-associated factor 1

TRPV1, a cloned capsaicin receptor, is a molecular sensor for detecting adverse stimuli and a key element for inflammatory nociception and represents biophysical properties of native channel. However, there seems to be a marked difference between TRPV1 and native capsaicin receptors in the pharmacological response profiles to vanilloids or acid. One plausible explanation for this overt discrepancy is the presence of regulatory proteins associated with TRPV1. Here, we identify Fas-associated factor 1 (FAF1) as a regulatory factor, which is coexpressed with and binds to TRPV1 in sensory neurons. When expressed heterologously, FAF1 reduces the responses of TRPV1 to capsaicin, acid, and heat, to the pharmacological level of native capsaicin receptor in sensory neurons. Furthermore, silencing FAF1 by RNA interference augments capsaicin-sensitive current in native sensory neurons. We therefore conclude that FAF1 forms an integral component of the vanilloid receptor complex and that it constitutively modulates the sensitivity of TRPV1 to various noxious stimuli in sensory neurons.

Recent developments in transient receptor potential vanilloid receptor 1 agonist-based therapies

Capsaicin and other naturally occurring pungent molecules have been used for centuries as topical analgesics and rubefactants to treat a variety of chronically painful conditions. Recently, instillations of high-concentration capsaicin and resiniferatoxin solutions have been found to be useful for the management of persistent bladder pain or overactive bladder. However, only within the last 7 years has it been appreciated that the selective action of these compounds on a subset of sensory nerve fibres is mediated by agonist activity at a ligand-gated ion channel called the transient receptor potential vanilloid receptor 1 (TRPV1). Accordingly, this discovery has fueled intensive research and drug development efforts, mainly in a search for novel analgesic or anti-inflammatory therapies. Two different, but non-mutually exclusive, strategies are being pursued: optimisation of TRPV1 agonist-based therapies, which can functionally inactivate nociceptive nerve fibres, and identification of receptor antagonists, which would prevent nociceptive fibres from being activated by ongoing inflammatory stimuli. Available information on TRPV1 agonists in development and their biological rationale will be summarised in this review.

Conversion of acetaminophen to the bioactive N-acylphenolamine AM404 via fatty acid amide hydrolase-dependent arachidonic acid conjugation in the nervous system

Acetaminophen (paracetamol) is a popular domestic analgesic and antipyretic agent with a weak anti-inflammatory action and a low incidence of adverse effects as compared with aspirin and other non-steroidal anti-inflammatory drugs. Here we show that acetaminophen, following deacetylation to its primary amine, is conjugated with arachidonic acid in the brain and the spinal cord to form the potent TRPV1 agonist N-arachidonoylphenolamine (AM404). This conjugation is absent in mice lacking the enzyme fatty acid amide hydrolase. AM404 also inhibits purified cyclooxygenase (COX)-1 and COX-2 and prostaglandin synthesis in lipopolysaccharide-stimulated RAW264.7 macrophages. This novel metabolite of acetaminophen also acts on the endogenous cannabinoid system, which, together with TRPV1 and COX, is present in the pain and thermoregulatory pathways. These findings identify fatty acid conjugation as a novel pathway for drug metabolism and provide a molecular mechanism for the occurrence of the analgesic N-acylphenolamine AM404 in the nervous system following treatment with acetaminophen.

Oleoylethanolamide excites vagal sensory neurones, induces visceral pain and reduces short-term food intake in mice via capsaicin receptor TRPV1

Oleoylethanolamide (OEA) is an endogenous lipid that regulates feeding and body weight. Although the effects of OEA are believed to depend on activation of vagal sensory afferent neurones, the mechanisms involved in exciting these neurones are unclear. Here we show that OEA directly excited nodose ganglion neurones, the cell bodies of vagal afferents. OEA depolarized these neurones and evoked inward currents that were restricted to capsaicin-sensitive cells. These currents were fully blocked by the TRPV1 inhibitor, capsazepine, and no responses to OEA were observed in neurones cultured from TRPV1-null mice. Similarly, OEA induced a rise in Ca(+) concentration in wild-type but not TRPV1-deficient neurones, and responses to OEA were greater at 37 degrees C compared to room temperature. Significantly, OEA administration in mice induced visceral pain-related behaviours that were inhibited by capsazepine and absent in TRPV1-null animals. Further, OEA reduced 30-min food intake in wild-type but not in TRPV1-null mice. Thus, the acute behavioural effects of OEA may result from visceral malaise via the activation of TRPV1.

Nitric-oxide mediated effects of transdermal capsaicin patches on the ischemic threshold in patients with stable coronary disease

BACKGROUND

Capsaicin has been shown to exert direct vasodilating effects through increased calcitonin gene-related peptide (CGRP) release. However, no data exist on its effect following systemic administration in humans.

METHODS

Twelve male patients with stable coronary disease and a persistently positive exercise were selected for study. According to a double blind, placebo-controlled, cross-over study, patients were randomized to placebo or 3 g oleic capsaicin-containing patches, on 2 different days and with a 2-day interval between treatments. Patients performed treadmill exercise testing according to the Bruce protocol. Time to 1 mm ST segment depression and to peak exercise, maximal ST segment depression, and the number of ECG leads showing diagnostic changes were also measured. Blood samples for nitric oxide (NO) and CGRP were drawn at baseline, 2, 6, and 24 hours after exercise.

RESULTS

On placebo, all patients had a positive ECG during exercise test. Only 1 patient experienced angina, on both treatments. With capsaicin, 1 patient had a negative exercise, while 8 patients significantly increased time to 1 mm ST depression from 328 +/- 167 to 401 +/- 174 seconds (P = 0.01). Of the remaining patients, 1 did not show any changes and 2 showed a worse ischemic threshold when on capsaicin. CGRP levels were not significantly different between placebo and capsaicin treatment. Conversely, when on capsaicin, NO significantly increased at 6 hours.

CONCLUSIONS

Transdermal capsaicin may improve ischemic threshold in patients with stable coronary disease, probably through arteriolar vasodilation. Increased capsaicin-induced NO availability could represent the principal mechanism of action.

Synthesis and evaluation of pyridazinylpiperazines as vanilloid receptor 1 antagonists

A structurally biased chemical library of pyridazinylpiperazine analogs was prepared in an effort to improve the pharmaceutical and pharmacological profile of the lead compound N-(4-tertiarybutylphenyl)-4-(3-chloropyridin-2-yl)tetrahydropyrazine-1(2H)-carboxamide (BCTC). The library was evaluated for VR1 antagonist activity in capsaicin-induced (CAP) and pH5.5-induced (pH) FLIPR assays in a human VR1-expressing HEK293 cell line. The most potent VR1 antagonists were found to have IC(50) values in the range of 9-200nM with improved pharmaceutical and pharmacological profiles versus the lead BCTC. These compounds represent possible second-generation BCTC analogs.

Decreased pain sensitivity of Capsaicin-treated rats results from decreased VR1 expression

We investigated the neurotoxic effects of capsaicin (CAP) on pain sensitivity and on the expression of capsaicin receptor, the vanilloid receptor (VR1), in rats. High-dose application of CAP has been known to degenerate a large fraction of the sensory neurons. Although the neurotoxic effects of CAP are well documented, the effects of CAP on the vanilloid receptor (VR1) are not yet known. In this paper, we investigated the effects of high-dose application of CAP on the expression of VR1 in rats. Thermal and mechanical pain sensitivity was reduced when neonatal rats were treated with a high dose of CAP. This reduction of pain sensitivity was significantly decreased after initiating carrageenan-induced inflammation. The expression of VR1 in dorsal root ganglia (DRG) isolated from the CAP-treated rats was reduced compared to that from the vehicle-treated rats. Therefore, we can conclude that the neurotoxic effect of CAP is related to the decrease of VR1 expression.

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.

TRPV1 and the gut: from a tasty receptor for a painful vanilloid to a key player in hyperalgesia

Capsaicin, the pungent ingredient in red pepper, has been used since ancient times as a spice, despite the burning sensation associated with its intake. More than 50 years ago, Nikolaus Jancso discovered that capsaicin can selectively stimulate nociceptive primary afferent neurons. The ensuing research established that the neuropharmacological properties of capsaicin are due to its activation of the transient receptor potential ion channel of the vanilloid type 1 (TRPV1). Expressed by primary afferent neurons innervating the gut and other organs, TRPV1 is gated not only by vanilloids such as capsaicin, but also by noxious heat, acidosis and intracellular lipid mediators such as anandamide and lipoxygenase products. Importantly, TRPV1 can be sensitized by acidosis and activation of various pro-algesic pathways. Upregulation of TRPV1 in inflammatory bowel disease and the beneficial effect of TRPV1 downregulation in functional dyspepsia and irritable bladder make this polymodal nociceptor an attractive target of novel therapies for chronic abdominal pain.

Discovery of small molecule antagonists of TRPV1

Small molecule antagonists of the vanilloid receptor 1 (TRPV1, also known as VR1) are disclosed. Ureas such as 5 (SB-452533) were used to explore the structure activity relationship with several potent analogues identified. Pharmacological studies using electrophysiological and FLIPR Ca(2+) based assays showed compound 5 was an antagonist versus capsaicin, noxious heat and acid mediated activation of TRPV1. Study of a quaternary salt of 5 supports a mode of action in which compounds from this series cause inhibition via an extracellularly accessible binding site on the TRPV1 receptor.

The analgesic effect induced by capsaicin is enhanced in inflammatory states

Agonists of the vanilloid receptor type 1 (VR1), such as capsaicin, induce an analgesic effect following an initial excitatory response. It has been demonstrated that the vanilloid system plays an important role in inflammatory hyperalgesia. In accordance, we show that the VR1 antagonist capsazepine (30 microg; i.pl.) prevented the thermal hyperalgesia induced by carrageenan or complete Freund's adjuvant (CFA) in mice. Furthermore, we studied whether this inflammation-induced activation of the vanilloid system could enhance the analgesic properties of capsaicin. A single administration of capsaicin (10 microg; i.pl.) induced in control mice an analgesic effect that lasted for 2 days. In contrast, in carrageenan-treated animals, the analgesic effect of this dose of capsaicin lasted for 6 days and in CFA-treated mice for 30 days. This prolongation of capsaicin-induced analgesia during inflammation was mediated through VR1 since it was completely blocked by coadministration of capsazepine (10 microg). Licking behavior induced by capsaicin in carrageenan- and CFA-treated mice was greater than in control animals. However, although capsaicin induced a more prolonged analgesia in CFA-treated mice, the licking behavior was greater in the carrageenan-treated group, suggesting that the prolongation of analgesia is independent of the initial nociceptive input. Overall, these results show that the analgesic effects of capsaicin are importantly enhanced during inflammation, supporting the fact that the stimulation of VR1 could perhaps constitute a suitable strategy to avoid inflammatory hyperalgesia.

Modulation of trigeminal sensory neuron activity by the dual cannabinoid-vanilloid agonists anandamide, N-arachidonoyl-dopamine and arachidonyl-2-chloroethylamide

1. Peripheral cannabinoids have been shown to suppress nociceptive neurotransmission in a number of behavioral and neurophysiological studies. It is not known, however, whether cannabinoids exert this action through direct interactions with nociceptors in the periphery and/or if other processes are involved. To gain a better understanding of the direct actions of cannabinoid-vanilloid agonists on sensory neurons, we examined the effects of these compounds on trigeminal ganglion (TG) neurons in vitro. 2. AEA (EC(50)=11.0 microM), NADA (EC(50)=857 nM) and arachidonyl-2-chloroethylamide ACEA (EC(50)=14.0 microM) each evoked calcitonin gene-related peptide (CGRP) release from TG neurons. The TRPV1 antagonists iodo-resiniferatoxin (I-RTX) and capsazepine (CPZ) each obtunded AEA-, NADA-, ACEA- and capsaicin (CAP)-evoked CGRP release with individually equivalent IC(50)'s for each of the compounds (I-RTX IC(50) range=2.6-4.0 nM; CPZ IC(50) range=523-1140 microM). 3. The pro-inflammatory mediator prostaglandin E(2) significantly increased the maximal effect of AEA-evoked CGRP release without altering the EC(50). AEA, ACEA and CAP stimulated cAMP accumulation in TG neurons in a calcium- and TRPV1-dependent fashion. Moreover, the protein kinase inhibitor staurosporine significantly inhibited AEA- and CAP-evoked CGRP release. 4. The pungency of AEA, NADA, ACEA and CAP in the rat eye-wipe assay was also assessed. Interestingly, when applied intraocularly, NADA or CAP each produced nocifensive responses, while AEA or ACEA did not. 5. Finally, the potential inhibitory effects of these cannabinoids on TG nociceptors were evaluated. Neither AEA nor ACEA decreased CAP-evoked CGRP release. Furthermore, neither of the cannabinoid receptor type 1 antagonists SR141716A nor AM251 had any impact on either basal or CAP-evoked CGRP release. AEA also did not inhibit 50 mM K(+)-evoked CGRP release and did not influence bradykinin-stimulated inositol phosphate accumulation. 6. We conclude that the major action of AEA, NADA and ACEA on TG neurons is excitatory, while, of these, only NADA is pungent. These findings are discussed in relation to our current understanding of interactions between the cannabinoid and vanilloid systems and nociceptive processing in the periphery.

N-(3-acyloxy-2-benzylpropyl)-N'-[4-(methylsulfonylamino)benzyl]thiourea analogues: novel potent and high affinity antagonists and partial antagonists of the vanilloid receptor

Isosteric replacement of the phenolic hydroxyl group in potent vanilloid receptor (VR1) agonists with the alkylsulfonamido group provides a series of compounds which are effective antagonists to the action of the capsaicin on rat VR1 heterologously expressed in Chinese hamster ovary (CHO) cells. In particular, compound 61, N-[2-(3,4-dimethylbenzyl)-3-pivaloyloxypropyl]-N'-[3-fluoro-4-(methylsulfonylamino)benzyl]thiourea was a full antagonist against capsaicin, displayed a K(i) value of 7.8 nM (compared to 520 nM for capsazepine and 4 nM for 5-iodoRTX), and showed excellent analgesic activity in mice. Structure-activity analysis of the influence of modifications in the A- and C-regions of 4-methylsulfonamide ligands on VR1 agonism/antagonism indicated that 3-fluoro substitution in the A-region and a 4-tert-butylbenzyl moiety in the C-region favored antagonism, whereas a 3-methoxy group in the A-region and 3-acyloxy-2-benzylpropyl moieties in the C-region favored agonism.

TRPV1 activation and induction of nociceptive response by a non-pungent capsaicin-like compound, capsiate

Capsiate is a capsaicin-like ingredient of a non-pungent cultivar of red pepper, CH-19 sweet. To elucidate the mechanisms underlying the non-pungency of capsiate, we investigated whether capsiate activates the cloned capsaicin receptor, TRPV1 (VR1). In patch-clamp experiments, capsiate was found to activate TRPV1 expressed transiently in HEK293 cells with a similar potency as capsaicin. Capsiate induced nociceptive responses in mice when injected subcutaneously into their hindpaws with a similar dose dependency as capsaicin. These data indicate that the non-pungent capsiate is an agonist for TRPV1 and could excite peripheral nociceptors. In contrast to this, capsiate did not induce any significant responses when applied to the skin surface, eye or oral cavity of mice, suggesting that capsiate requires direct access to nerve endings to exhibit its effects. Capsiate was proved to have high lipophilicity and to be easily broken down in normal aqueous conditions, leading to less accessibility to nociceptors. Another highly lipophilic capsaicin analogue, olvanil, was similar to capsiate in that it did not produce irritant responses when applied to the skin surface, although it could activate TRPV1. Taken together, high lipophilicity and instability might be critical determinants for pungency and so help in understanding the effects of capsaicin-related compounds.

The VR1 antagonist capsazepine reverses mechanical hyperalgesia in models of inflammatory and neuropathic pain

Vanilloid receptor type 1 (VR1) (TRPV1) is a ligand-gated ion channel expressed on sensory nerves that responds to noxious heat, protons, and chemical stimuli such as capsaicin. Herein, we have examined the activity of the VR1 antagonist capsazepine in models of inflammatory and neuropathic pain in the rat, mouse, and guinea pig. In naïve animals, subcutaneous administration of capsazepine (10-100 mg/kg s.c.) did not affect withdrawal thresholds to noxious thermal or mechanical stimuli. However, pretreatment with capsazepine prevented the development of mechanical hyperalgesia induced by intraplantar injection of capsaicin, with a similar potency in all three species. Capsazepine (up to 100 mg/kg s.c.) did not affect mechanical hyperalgesia in the Freund's complete adjuvant (FCA)-inflamed hind paw of the rat or mouse. Strikingly, capsazepine (3-30 mg/kg s.c.) produced up to 44% reversal of FCA-induced mechanical hyperalgesia in the guinea pig. Capsazepine also produced significant reversal of carageenan-induced thermal hyperalgesia in the guinea pig at 30 mg/kg s.c., but was ineffective in the rat. Similarly, in the partial sciatic nerve ligation model of neuropathic pain, capsazepine was surprisingly effective in the guinea pig, producing up to 80% reversal of mechanical hyperalgesia (1-30 mg/kg s.c.) but had no effect in the rat or mouse. These data show that VR1 antagonists have antihyperalgesic activity in animal models of chronic inflammatory and neuropathic pain, and illustrate species differences in the in vivo pharmacology of VR1 that correlate with differences in pharmacology previously seen in vitro.

Sensory neurone responses to mucosal noxae in the upper gut: relevance to mucosal integrity and gastrointestinal pain

The digestive tract is supplied by extrinsic and intrinsic sensory neurones that, together with endocrine and immune cells, form a surveillance network that is essential to gut function. This article focuses on the responses of extrinsic afferent neurones to chemical insults of the gastrointestinal mucosa and their pathophysiological relevance to mucosal integrity and abdominal pain. Within the gastroduodenal region, spinal afferents subserve an emergency function because, in case of alarm by influxing acid, they stimulate mechanisms of mucosal protection via an efferent-like release of transmitters. Other sensory neurones signal chemical noxae to the brain, a task that is not confined to spinal afferents because vagal afferents communicate gastric acid and peripheral immune challenges to the brainstem and in this way elicit autonomic, endocrine, affective and behavioural reactions. Emerging evidence indicates that hypersensitivity of extrinsic afferent pathways to mechanical and chemical stimuli makes an important contribution to the abdominal hyperalgesia seen in functional dyspepsia and irritable bowel syndrome. Sensitization may be brought about by inflammatory processes that lead to up-regulation and functional alterations of receptors and ion channels on sensory neurones. Such sensory neurone-specific molecules, which include vanilloid (capsaicin) receptors, may represent important targets for novel drugs to treat abdominal pain.