Vanilloid receptor-1 is essential for inflammatory thermal hyperalgesia

Measuring pain in the (knockout) mouse: big challenges in a small mammal

The increasing popularity of the mouse as a subject in basic science studies of pain can largely be attributed to the development of transgenic "knockout" technology in this species only. To take advantage of this biological technique, many investigators are rushing to adapt to the mouse experimental protocols that were designed for the rat. However, the myriad physiological and behavioral differences between these two rodent species render such adaptations non-trivial and in many cases seriously problematic. In this article we review the basic nociceptive assays used in behavioral pain research (thermal, mechanical, electrical and chemical), and highlight how species differences affect their proper application. In addition, some of the issues specifically pertaining to the interpretation of such data in knockout studies are addressed.

Nonsteroid anti-inflammatory drugs inhibit both the activity and the inflammation-induced expression of acid-sensing ion channels in nociceptors

Nonsteroid anti-inflammatory drugs (NSAIDs) are major drugs against inflammation and pain. They are well known inhibitors of cyclooxygenases (COXs). However, many studies indicate that they may also act on other targets. Acidosis is observed in inflammatory conditions such as chronic joint inflammation, in tumors and after ischemia, and greatly contributes to pain and hyperalgesia. Administration of NSAIDs reduces low-pH-induced pain. The acid sensitivity of nociceptors is associated with activation of H(+)-gated ion channels. Several of these, cloned recently, correspond to the acid-sensing ion channels (ASICs) and others to the vanilloid receptor family. This paper shows (1) that ASIC mRNAs are present in many small sensory neurons along with substance P and isolectin B4 and that, in case of inflammation, ASIC1a appears in some larger Abeta fibers, (2) that NSAIDs prevent the large increase of ASIC expression in sensory neurons induced by inflammation, and (3) that NSAIDs such as aspirin, diclofenac, and flurbiprofen directly inhibit ASIC currents on sensory neurons and when cloned ASICs are heterologously expressed. These results suggest that the combined capacity to block COXs and inhibit both inflammation-induced expression and activity of ASICs present in nociceptors is an important factor in the action of NSAIDs against pain.

Gastrointestinal afferents as targets of novel drugs for the treatment of functional bowel disorders and visceral pain

An intricate surveillance network consisting of enteroendocrine cells, immune cells and sensory nerve fibres monitors the luminal and interstitial environment in the alimentary canal. Functional bowel disorders are characterized by persistent alterations in digestive regulation and gastrointestinal discomfort and pain. Visceral hyperalgesia may arise from an exaggerated sensitivity of peripheral afferent nerve fibres and/or a distorted processing and representation of gut signals in the brain. Novel strategies to treat these sensory bowel disorders are therefore targeted at primary afferent nerve fibres. These neurons express a number of molecular traits including transmitters, receptors and ion channels that are specific to them and whose number and/or behaviour may be altered in chronic visceral pain. The targets under consideration comprise vanilloid receptor ion channels, acid-sensing ion channels, sensory neuron-specific Na(+) channels, P2X(3) purinoceptors, 5-hydroxytryptamine (5-HT), 5-HT(3) and 5-HT(4) receptors, cholecystokinin CCK(1) receptors, bradykinin and prostaglandin receptors, glutamate receptors, tachykinin and calcitonin gene-related peptide receptors as well as peripheral opioid and cannabinoid receptors. The utility of sensory neuron-targeting drugs in functional bowel disorders will critically depend on the compounds' selectivity of action for afferent versus enteric or central neurons.

Mechanisms of pain

Persistent or chronic pain is the primary reason people seek medical care, yet current therapies are either inadequate for certain types of pain or cause intolerable side effects. Recently, pain neurobiologists have identified a number of cellular and molecular processes that lead to the initiation and maintenance of pain. Understanding these underlying mechanisms has given significant promise for the development of more effective, more specific pain therapies in the near future.

Noxious heat-induced CGRP release from rat sciatic nerve axons in vitro

Noxious heat may act as an endogenous activator of the ionotropic capsaicin receptor (VR1) and of its recently found homologue VRL1, expressed in rat dorsal root ganglion cells and present along their nerve fibres. We have previously reported that capsaicin induces receptor-mediated and Ca++-dependent calcitonin gene-related peptide (CGRP) release from axons of the isolated rat sciatic nerve. Here we extended the investigation to noxious heat stimulation and the transduction mechanisms involved. Heat stimulation augmented the CGRP release from desheathed sciatic nerves in a log-linear manner with a Q10 of approximately 15 and a threshold between 40 and 42 degrees C. The increases were 1.75-fold at 42 degrees C, 3.8-fold at 45 degrees C and 29.1-fold at 52 degrees C; in Ca++-free solution these heat responses were abolished or reduced by 71 and 92%, respectively. Capsazepine (10 microm) and Ruthenium Red (1 microm) used as capsaicin receptor/channel antagonists did not significantly inhibit the heat-induced release. Pretreatment of the nerves with capsaicin (100 microm for 30 min) caused complete desensitization to 1 microm capsaicin, but a significant heat response remained, indicating that heat sensitivity is not restricted to capsaicin-sensitive fibres. The sciatic nerve axons responded to heat, potassium and capsaicin stimulation with a Ca++-dependent CGRP release. Blockade of the capsaicin receptor/channels had little effect on the heat-induced neuropeptide release. We conclude therefore that other heat-activated ion channels than VR1 and VRL1 in capsaicin-sensitive and -insensitive nerve fibres may cause excitation, axonal Ca++ influx and subsequent CGRP release.

Molecular targets for cannabidiol and its synthetic analogues: effect on vanilloid VR1 receptors and on the cellular uptake and enzymatic hydrolysis of anandamide

1. (-)-Cannabidiol (CBD) is a non-psychotropic component of Cannabis with possible therapeutic use as an anti-inflammatory drug. Little is known on the possible molecular targets of this compound. We investigated whether CBD and some of its derivatives interact with vanilloid receptor type 1 (VR1), the receptor for capsaicin, or with proteins that inactivate the endogenous cannabinoid, anandamide (AEA). 2. CBD and its enantiomer, (+)-CBD, together with seven analogues, obtained by exchanging the C-7 methyl group of CBD with a hydroxy-methyl or a carboxyl function and/or the C-5' pentyl group with a di-methyl-heptyl (DMH) group, were tested on: (a) VR1-mediated increase in cytosolic Ca(2+) concentrations in cells over-expressing human VR1; (b) [(14)C]-AEA uptake by RBL-2H3 cells, which is facilitated by a selective membrane transporter; and (c) [(14)C]-AEA hydrolysis by rat brain membranes, which is catalysed by the fatty acid amide hydrolase. 3. Both CBD and (+)-CBD, but not the other analogues, stimulated VR1 with EC(50)=3.2 - 3.5 microM, and with a maximal effect similar in efficacy to that of capsaicin, i.e. 67 - 70% of the effect obtained with ionomycin (4 microM). CBD (10 microM) desensitized VR1 to the action of capsaicin. The effects of maximal doses of the two compounds were not additive. 4. (+)-5'-DMH-CBD and (+)-7-hydroxy-5'-DMH-CBD inhibited [(14)C]-AEA uptake (IC(50)=10.0 and 7.0 microM); the (-)-enantiomers were slightly less active (IC(50)=14.0 and 12.5 microM). 5. CBD and (+)-CBD were also active (IC(50)=22.0 and 17.0 microM). CBD (IC(50)=27.5 microM), (+)-CBD (IC(50)=63.5 microM) and (-)-7-hydroxy-CBD (IC(50)=34 microM), but not the other analogues (IC(50)>100 microM), weakly inhibited [(14)C]-AEA hydrolysis. 6. Only the (+)-isomers exhibited high affinity for CB(1) and/or CB(2) cannabinoid receptors. 7. These findings suggest that VR1 receptors, or increased levels of endogenous AEA, might mediate some of the pharmacological effects of CBD and its analogues. In view of the facile high yield synthesis, and the weak affinity for CB(1) and CB(2) receptors, (-)-5'-DMH-CBD represents a valuable candidate for further investigation as inhibitor of AEA uptake and a possible new therapeutic agent.

Capsaicin as a source for painful stimulation in functional MRI

Functional magnetic resonance imaging (fMRI) was used to examine the brain processing of capsaicin-induced painful stimulation in the alpha-chloralose anesthetized rat. Experiments were performed on a 9.4-T magnet (Magnex, UK) with Avance console (Bruker, Germany) using a surface coil tuned to 400.5 MHz centred over the rat forebrain. Gradient-echo images of two slices, with an echo time of 25 msec, repetition time of 70 msec, and 50 repetitions, were acquired per experiment. These images were analyzed using a fuzzy cluster analysis technique (EvIdent). Activation of areas of the brain known to be associated with the processing of pain, namely the anterior cingulate (bilateral), frontal cortex (bilateral), and sensory motor cortex (contralateral), was found in all animals (N = 6) following injection of 25 microl of capsaicin (128 microg/mL in 7.5% dimethylsulfoxide [DMSO]) into the dorsal forepaw. It is possible to reproduce the pain response in a given animal several times throughout the course of an experiment, provided that sufficient time is allowed between capsaicin injections. This acute phase of capsaicin-induced pain involving stimulation of C polymodal nociceptors was examined by functional imaging. There was a substantial initial increase in activation in regions of the brain associated with pain and there was a trend towards increasing activation with repeated stimulations. Treatment with morphine (3 mg/kg, intravenously) was found to substantially reduce, if not completely eliminate, the areas of functional activation associated with physiologic pain (anterior cingulate and frontal cortex) after C-nociceptor stimulation with capsaicin (N = 6). FMRI involving capsaicin-induced painful stimulation could prove to be an effective tool for the study of novel analgesics and the central nervous system processing of pain.

PKC regulates capsaicin-induced currents of dorsal root ganglion neurons in rats

Capsaicin activates a non-specific cation conductance in a subset of dorsal root ganglion (DRG) neurons. The inward current and membrane potential of acutely isolated DRG neurons were examined using whole-cell patch recording methods. We report here that the current and voltage responses activated by capsaicin were markedly increased by phorbol 12-myristate 13-acetate (PMA), an activator of protein kinase C (PKC). The mean current, after application of 0.3 microM PMA, was 153.5+/-5.7% of control (n=32) in Ca(2+)-free external solution and 181.6+/-6.8% of control (n=15) in standard external solution. Under current-clamp conditions, 0.3 microM PMA facilitated capsaicin-induced depolarization and action potential generation. Bindolylmaleimide I (BIM), a specific inhibitor of PKC activity, abolished the effect of PMA. In addition, capsaicin-evoked current was attenuated to 68.3+/-5.0% of control (n=13) by individual administration of 1 microM BIM in standard external solution, while 0.3 microM BIM did not have this effect. These data suggest that PKC can directly regulate the capsaicin response in DRG neurons, which could increase nociceptive sensory transmission and contribute to hyperalgesia.

The vanilloid receptor: a molecular gateway to the pain pathway

The detection of painful stimuli occurs primarily at the peripheral terminals of specialized sensory neurons called nociceptors. These small-diameter neurons transduce signals of a chemical, mechanical, or thermal nature into action potentials and transmit this information to the central nervous system, ultimately eliciting a perception of pain or discomfort. Little is known about the proteins that detect noxious stimuli, especially those of a physical nature. Here we review recent advances in the molecular characterization of the capsaicin (vanilloid) receptor, an excitatory ion channel expressed by nociceptors, which contributes to the detection and integration of pain-producing chemical and thermal stimuli. The analysis of vanilloid receptor gene knockout mice confirms the involvement of this channel in pain sensation, as well as in hypersensitivity to noxious stimuli following tissue injury. At the same time, these studies demonstrate the existence of redundant mechanisms for the sensation of heat-evoked pain.

Molecular mechanisms of nociception

The sensation of pain alerts us to real or impending injury and triggers appropriate protective responses. Unfortunately, pain often outlives its usefulness as a warning system and instead becomes chronic and debilitating. This transition to a chronic phase involves changes within the spinal cord and brain, but there is also remarkable modulation where pain messages are initiated - at the level of the primary sensory neuron. Efforts to determine how these neurons detect pain-producing stimuli of a thermal, mechanical or chemical nature have revealed new signalling mechanisms and brought us closer to understanding the molecular events that facilitate transitions from acute to persistent pain.

Regulation of melastatin, a TRP-related protein, through interaction with a cytoplasmic isoform

The TRP (transient receptor potential) superfamily includes a group of subfamilies of channel-like proteins mediating a multitude of physiological signaling processes. The TRP-melastatin (TRPM) subfamily includes the putative tumor suppressor melastatin (MLSN) and is a poorly characterized group of TRP-related proteins. Here, we describe the identification and characterization of an additional TRPM protein TRPM4. We reveal that TRPM4 and MLSN each mediate Ca(2+) entry when expressed in HEK293 cells. Furthermore, we demonstrate that a short form of MLSN (MLSN-S) interacts directly with and suppresses the activity of full-length MLSN (MLSN-L). This suppression seems to result from the inhibition of translocation of MLSN-L to the plasma membrane. We propose that control of translocation through interaction between MLSN-S and MLSN-L represents a mode for regulating ion channel activity.

Peripheral capsaicin receptors increase in the inflamed rat hindpaw: a possible mechanism for peripheral sensitization

The vanilloid receptor-1 (VR1) is activated by capsaicin, heat and protons and is localized on primary sensory neurons. The present study investigates whether VR1 increases in the inflamed hindpaw thereby contributing to the peripheral sensitization and heat hyperalgesia that characterizes the inflamed state. Forty-eight hours after intraplantar injection of Complete Freund's Adjuvant into one hindpaw, there is a significant increase in the proportion of VR1-labeled unmyelinated axons in digital nerves in the inflamed (32.8 +/- 5.9%) compared to normal (17.1 +/- 2.6%) hindpaws (t-test, P<0.01). A few, small diameter myelinated axons are labeled in normal and inflamed rats with no change in percentages following inflammation. The data suggest that an increase in number of unmyelinated sensory axons expressing VR1 may be one mechanism underlying peripheral sensitization of nociceptors in inflammation.

Inhibition of neurogenic inflammation by the Amazonian herbal medicine sangre de grado

This study was designed to determine if the Amazonian medicinal sangre de grado, confers benefit by suppressing the activation of sensory afferent nerves.

METHODS

(i) vasorelaxation of rat mesenteric arteries in response to calcitonin gene-related peptide; (ii) rat paw edema in response to protease- activating peptide receptor 2-activating peptide; (iii) rat paw hyperalgesia in response to low-dose protease-activating peptide receptor 2-activating peptide or prostaglandin E2; (iv) gastric hyperemia in response luminal capsaicin; (v) a clinical trial of a sangre de grado balm in pest control workers. The parent botanical was fractionated for evaluation of potential active components. In preconstricted rat mesenteric arteries, highly diluted sangre de grado (1:10,000) caused a shift to the right of the calcitonin gene-related peptide dose-response curve (p < 0.01). Paw edema in response to protease-activating peptide receptor 2-activating peptide (500 microg) was reduced by as single topical administration sangre de grado balm (1% concentration, p < 0.01) for at least 6 h. Hyperalgesia induced by either low-dose protease-activating peptide receptor 2-activating peptide (50 microg) or prostaglandin E2 was prevented by sangre de grado balm. A fraction possessing analgesic and capsaicin antagonistic properties was isolated and high-performance liquid chromatography and gas chromatography-mass spectrometry analysis indicated that it was a proanthocyandin oligomer. In pest control workers, sangre de grado balm (Zangrado) was preferred over placebo, for the relief of itching, pain, discomfort, edema, and redness in response to wasps, fire ants, mosquitoes, bees, cuts, abrasions, and plant reactions. Subjects reported relief within minutes. We conclude that sangre de grado is a potent inhibitor of sensory afferent nerve mechanisms and supports its ethnomedical use for disorders characterized by neurogenic inflammation.

Protein kinase C activation potentiates gating of the vanilloid receptor VR1 by capsaicin, protons, heat and anandamide

1. The effects of activation of protein kinase C (PKC) on membrane currents gated by capsaicin, protons, heat and anandamide were investigated in primary sensory neurones from neonatal rat dorsal root ganglia (DRG) and in HEK293 cells (human embryonic kidney cell line) transiently or stably expressing the human vanilloid receptor hVR1. 2. Maximal activation of PKC by a brief application of phorbol 12-myristate 13-acetate (PMA) increased the mean membrane current activated by a low concentration of capsaicin by 1.65-fold in DRG neurones and 2.18-fold in stably transfected HEK293 cells. Bradykinin, which activates PKC, also enhanced the response to capsaicin in DRG neurones. The specific PKC inhibitor RO31-8220 prevented the enhancement caused by PMA. 3. Activation of PKC did not enhance the membrane current at high concentrations of capsaicin, showing that PKC activation increases the probability of channel opening rather than unmasking channels. 4. Application of PMA alone activated an inward current in HEK293 cells transiently transfected with VR1. The current was suppressed by the VR1 antagonist capsazepine. PMA did not, however, activate a current in the large majority of DRG neurones nor in HEK293 cells stably transfected with VR1. 5. Removing external Ca(2+) enhanced the response to a low concentration of capsaicin 2.40-fold in DRG neurones and 3.42-fold in HEK293 cells. Activation of PKC in zero Ca(2+) produced no further enhancement of the response to capsaicin in either DRG neurones or HEK293 cells stably transfected with VR1. 6. The effects of PKC activation on the membrane current gated by heat, anandamide and low pH were qualitatively similar to those on the capsaicin-gated current. 7. The absence of a current activated by PMA in most DRG neurones or in stably transfected HEK293 cells suggests that activation of PKC does not directly open VR1 channels, but instead increases the probability that they will be activated by capsaicin, heat, low pH or anandamide. Removal of calcium also potentiates activation, and PKC activation then has no further effect. The results are consistent with a model in which phosphorylation of VR1 by PKC increases the probability of channel gating by agonists, and in which dephosphorylation occurs by a calcium-dependent process.

The genomic organization of the gene encoding the vanilloid receptor: evidence for multiple splice variants

Vanilloid receptor subtype-1 (VR1) is a nonselective cation channel that is expressed in sensory neurons and is activated by multiple noxious stimuli. Rat Vr1, stretch-inactivated channel (SIC), and vanilloid receptor 5' splice variant (VR.5'sv) have been hypothesized to be derived from a common VR gene. Characterization of the genomic structure encoding the 5' portion of rat Vr1 confirmed that VR.5'sv is derived from the VR gene; however, SIC seemed to be derived from two related but independent genes. We also deduced the genomic organization of the human gene VR1. Comparative studies of rat and human VR genes showed substantial conservation in genomic organization. The splice site flanking exon-intron 7 in rat and human VR1 diverged from the expected consensus sequence; this may help to explain the skipping of exon 7 within VR.5'sv and other VR splice variants.

Physiology, phylogeny, and functions of the TRP superfamily of cation channels

The transient receptor potential (TRP) protein superfamily consists of a diverse group of Ca(2+) permeable nonselective cation channels that bear structural similarities to Drosophila TRP. TRP-related proteins play important roles in nonexcitable cells, as demonstrated by the recent finding that a mammalian TRPC protein is expressed in endothelial cells and functions in vasorelaxation. However, an emerging theme is that many TRP-related proteins are expressed predominantly in the nervous system and function in sensory physiology. The TRP superfamily can be divided into six subfamilies, the first of which is composed of the "classical TRPs" (TRPC subfamily). These proteins all share the common features of three to four ankryin repeats, >/=30% amino acid homology over >/=750 amino acids, and a gating mechanism that operates through phospholipase C. Some classical TRPs may be store-operated channels (SOCs), which are activated by release of Ca(2+) from internal stores. The mammalian TRPC proteins are also expressed in the central nervous system, and several are highly enriched in the brain. One TRPC protein has been implicated in the pheromone response. The archetypal TRP, Drosophila TRP, is predominantly expressed in the visual system and is required for phototransduction. Many members of a second subfamily (TRPV) function in sensory physiology. These include VR1 and OSM-9, which respond to heat, osmolarity, odorants, and mechanical stimuli. A third subfamily, TRPN, includes proteins with many ankyrin repeats, one of which, NOMPC, participates in mechanotransduction. Among the members of a fourth subfamily, TRPM, is a putative tumor suppressor termed melastatin, and a bifunctional protein, TRP-PLIK, consisting of a TRPM channel fused to a protein kinase. PKD2 and mucolipidin are the founding members of the TRPP and TRPML subfamilies, respectively. Mutations in PKD2 are responsible for polycystic kidney disease, and mutations in mucolipidin result in a severe neurodegenerative disorder. Recent studies suggest that alterations in the activities of SOC and TRP channels may be at the heart of several additional neurodegenerative diseases. Thus, TRP channels may prove to be important new targets for drug discovery.

Temperature-dependent activation of recombinant rat vanilloid VR1 receptors expressed in HEK293 cells by capsaicin and anandamide

Capsaicin activates vanilloid (VR1) receptors found on sensory neurons. These ligand-gated ion channels are also sensitive to low pH, elevated temperature and the endocannabinoid, anandamide. In this study, we have measured capsaicin- and anandamide-induced elevations in intracellular calcium concentrations ([Ca(2+)](i)) in fura-2 loaded HEK293 cells stably expressing the rat VR1 receptor at 22, 37 and 50 degrees C. Both capsaicin and anandamide produced a concentration-dependent elevation in [Ca(2+)](i) at all temperatures. pEC(50) values were 7.74 and 5.69 at 22 degrees C and 6.90 and 5.15 at 37 degrees C for capsaicin and anandamide, respectively. At 50 degrees C, the pEC(50) value for capsaicin was 6.36 but the response to anandamide did not saturate. Responses to both agonists were sensitive to ruthenium red and capsazepine at all temperatures. This temperature-dependent reduction in potency may result from desensitization.

Induction by carrageenan inflammation of prepronociceptin mRNA in VR1-immunoreactive neurons in rat dorsal root ganglia

Nociceptin (orphanin FQ) may act on primary afferents and be involved in the regulation of nociceptive processing. We have shown, using reverse transcription-polymerase chain reaction (RT-PCR), that carrageenan-produced peripheral inflammation induces the expression of prepronociceptin (PPN) mRNA in the dorsal root ganglia (DRG). The present experiments were conducted to determine the localization of PPN mRNA in primary sensory neurons after peripheral inflammation, using in situ hybridization. An intraplantar injection of carrageenan induced the expression of PPN mRNA in small and medium sized neurons in the DRG; the effect peaked 0.5 h after carrageenan and subsided by 6 h. All neurons positive for PPN mRNA were positive for vanilloid receptor subtype 1 (VR1)-like immunoreactivity and some VR1-immunoreactive neurons were negative for PPN mRNA. The results suggest that peripheral inflammation induces the production of nociceptin in a sub-population of VR1-positive primary sensory neurons and support the idea that nociceptin produced there is involved in the regulation of nociceptive processing.

N-(3-acyloxy-2-benzylpropyl)-N'-dihydroxytetrahydrobenzazepine and tetrahydroisoquinoline thiourea analogues as vanilloid receptor ligands

The vanilloid receptor represents a promising target for drug development. Building on our previous strategies which have generated potent agonists for VR1, we now describe a series of novel N-(3-acyloxy-2-benzylpropyl)-N'-dihydroxytetrahydrobenzazepine and tetrahydroisoquinoline thiourea analogues, several of which are potent VR1 antagonists. We report here the rationale for the design, the synthesis, and the in vitro characterization of activity in assays for [(3)H]resiniferatoxin binding and (45)Ca influx using heterologously expressed rat VR1.

Bradykinin and nerve growth factor release the capsaicin receptor from PtdIns(4,5)P2-mediated inhibition

Tissue injury generates endogenous factors that heighten our sense of pain by increasing the response of sensory nerve endings to noxious stimuli. Bradykinin and nerve growth factor (NGF) are two such pro-algesic agents that activate G-protein-coupled (BK2) and tyrosine kinase (TrkA) receptors, respectively, to stimulate phospholipase C (PLC) signalling pathways in primary afferent neurons. How these actions produce sensitization to physical or chemical stimuli has not been elucidated at the molecular level. Here, we show that bradykinin- or NGF-mediated potentiation of thermal sensitivity in vivo requires expression of VR1, a heat-activated ion channel on sensory neurons. Diminution of plasma membrane phosphatidylinositol-4,5-bisphosphate (PtdIns(4,5)P2) levels through antibody sequestration or PLC-mediated hydrolysis mimics the potentiating effects of bradykinin or NGF at the cellular level. Moreover, recruitment of PLC-gamma to TrkA is essential for NGF-mediated potentiation of channel activity, and biochemical studies suggest that VR1 associates with this complex. These studies delineate a biochemical mechanism through which bradykinin and NGF produce hypersensitivity and might explain how the activation of PLC signalling systems regulates other members of the TRP channel family.

The effects of capsaicin and acidity on currents generated by noxious heat in cultured neonatal rat dorsal root ganglion neurones

1. The effects of capsaicin, acidic pH, ATP, kainate and GABA on currents generated by noxious heat were studied in cultured dorsal root ganglion (DRG) neurones (< 20 microm in diameter) isolated from neonatal rats. The patch clamp technique was used to record membrane currents or changes of membrane potential. 2. In agreement with previous results, inward membrane currents (I(heat)) induced by a 3 s ramp of increasing temperature from room temperature (approximately 23 degrees C) to over 42 degrees C varied greatly between cells (-100 pA to -2.4 nA at 48 degrees C) and had a temperature coefficient (Q(10)) > 10 over the range of 43-52 degrees C. 3. Capsaicin potentiated the heat-induced current even when capsaicin, at room temperature, had little or no effect on its own. In cells in which capsaicin induced no or very small membrane current at room temperature (< 50 pA), I(heat) exhibited detectable activation above 40 degrees C and increased 5.1 +/- 1.1 (n = 37) and 6.3 +/- 2.0 (n = 18) times at 0.3 and 1 microM capsaicin, respectively. 4. A rapid decrease in extracellular pH from 7.3 to 6.8, 6.3 or 6.1 produced an inward current which inactivated in ~5 s either completely (pH 6.8 or 6.3) or leaving a small current (approximately 50 pA) for more than 2 min (pH 6.1). After inactivation of the initial low pH-induced current, I(heat) at 48 degrees C increased 2.3 +/- 0.4 times at pH 6.8, 4.0 +/- 0.6 times at pH 6.3 and 4.8 +/- 0.8 times at pH 6.1 with a Q(10) > 10 (n = 16). 5. ATP (n = 22), kainate (n = 7) and GABA (n = 8) at 100 microM, produced an inactivating inward current in all heat-sensitive DRG neurones tested. During inactivation and in the presence of the drug, I(heat) was increased slightly with ATP and unaffected with kainate and GABA. These agents apparently do not directly affect the noxious heat receptor. 6. The results indicate a novel class of capsaicin-sensitive cells, in which capsaicin evokes no or very small inward current but nevertheless increases sensitivity to noxious heat.

Anandamide excites central terminals of dorsal root ganglion neurons via vanilloid receptor-1 activation

Recently, the cannabinoid (CB) receptor agonist anandamide (AEA) has been shown to excite perivascular terminals of primary sensory neurons via activation of the vanilloid receptor-1 (VR-1). To determine whether AEA stimulates central terminals of these neurons, via VR-1 activation, we studied the release of calcitonin gene-related peptide (CGRP)- and substance P (SP)-like immunoreactivities (LI) from slices of rat dorsal spinal cord. Mobilization of Ca(2+) in rat dorsal root ganglion (DRG) neurons in culture was also studied. AEA (0.1-10 micrometer) increased the outflow of CGRP-LI and SP-LI from slices of the rat dorsal spinal cord in a Ca(2+)-dependent manner and increased [Ca(2+)](i) in capsaicin-sensitive cultured DRG neurons. Both effects of AEA were abolished by capsaicin pretreatment and by the VR-1 antagonist capsazepine but not affected by the CB receptor antagonists AM281 or AM630. Both neuropeptide release and Ca(2+) mobilization induced by electrical field stimulation (EFS) were inhibited by a low concentration of AEA (10 nm). Inhibition by AEA of EFS-induced responses was reversed by AM281 and AM630, but was not affected by capsazepine. Results indicate that stimulation of VR-1 with high concentrations of AEA excites central terminals of capsaicin-sensitive DRG neurons, thus causing neuropeptide release in the dorsal spinal cord. This novel activity opposes the CB receptor-mediated inhibitory action of low concentrations AEA. However, only if large amounts of endogenous AEA could be produced at the level of the dorsal spinal cord, they may not inhibit, but rather activate, nociceptive sensory neurons.

Toward an understanding of the molecules that sense myocardial ischemia

Cardiac afferent neurons are activated in the setting of myocardial ischemia and mediate the sensation of angina. However, the precise stimuli and receptive molecules responsible are not completely understood. To further investigate the molecular components involved, cardiac afferents were isolated in dissociated culture and patch-clamp experiments were performed on these cells. It was found that acidic pH evoked large inward currents in almost all cardiac sympathetic afferents. By comparison, the responses to other potential chemical mediators were inconsistent and much smaller. The biophysical properties of the acid-evoked currents in cardiac afferents match the acid-sensing ion channel 3 (ASIC3).

VR1 protein expression increases in undamaged DRG neurons after partial nerve injury

Changes in phenotype or connectivity of primary afferent neurons following peripheral nerve injury may contribute to the hyperalgesia and allodynia associated with neuropathic pain conditions. Although earlier studies using partial nerve injury models have focused on the role of damaged fibres in the generation of ectopic discharges and pain, it is now thought that remaining undamaged fibres may be equally important. We have examined the expression of the sensory neuron-specific cation channel Vanilloid Receptor 1 (VR1), an important transducer of noxious stimuli, in three models of nerve injury in the rat, using anatomical separation or fluorescent retrograde tracers to identify damaged or undamaged sensory neurons. After total or partial sciatic nerve transection, or spinal nerve ligation, VR1-immunoreactivity (IR) was significantly reduced in the somata of all damaged dorsal root ganglion (DRG) neuronal profiles, compared to controls. However, after partial transection or spinal nerve ligation, VR1 expression was greater in the undamaged DRG somata than in controls. Unexpectedly, after L5 spinal nerve ligation, VR1-IR of the A-fibre somata increased approximately 3-fold in the uninjured L4 DRG compared to controls; a much greater increase than seen in the somata with C-fibres. Furthermore, we found that VR1-IR persisted in the transected sciatic nerve proximal to the lesion, despite its down-regulation in the damaged neuronal somata. This persistence in the nerve proximal to the lesion after nerve section, together with increased VR1 in DRG neurons left undamaged after partial nerve injury, may be crucial to the development or maintenance of neuropathic pain.

Potentiation of capsaicin receptor activity by metabotropic ATP receptors as a possible mechanism for ATP-evoked pain and hyperalgesia

The capsaicin (vanilloid) receptor, VR1, is a sensory neuron-specific ion channel that serves as a polymodal detector of pain-producing chemical and physical stimuli. It has been proposed that ATP, released from different cell types, initiates the sensation of pain by acting predominantly on nociceptive ionotropic purinoceptors located on sensory nerve terminals. In this study, we examined the effects of extracellular ATP on VR1. In cells expressing VR1, ATP increased the currents evoked by capsaicin or protons through activation of metabotropic P2Y(1) receptors in a protein kinase C-dependent pathway. The involvement of G(q/11)-coupled metabotropic receptors in the potentiation of VR1 response was confirmed in cells expressing both VR1 and M1 muscarinic acetylcholine receptors. In the presence of ATP, the temperature threshold for VR1 activation was reduced from 42 degrees C to 35 degrees C, such that normally nonpainful thermal stimuli (i.e., normal body temperature) were capable of activating VR1. This represents a novel mechanism through which the large amounts of ATP released from damaged cells in response to tissue trauma might trigger the sensation of pain.

Comparison of intracellular calcium signals evoked by heat and capsaicin in cultured rat dorsal root ganglion neurons and in a cell line expressing the rat vanilloid receptor, VR1

The cloning of the receptor for capsaicin, vanilloid receptor 1, has shown it to be non-selective cation channel with a high calcium permeability which can be opened by noxious heat as well as capsaicin. Here we compare the calcium signals produced by native and recombinant capsaicin receptors when activated by either heat or capsaicin by imaging intracellular calcium levels ([Ca2+](i)) in rat dorsal root ganglion neurons and Chinese hamster ovary cells transfected with the rat vanilloid receptor, vanilloid receptor 1. Vanilloid receptor 1 transfected cells and a subset of dorsal root ganglion neurons responded to both capsaicin and to heating to 50 degrees C with rapid, substantial and reversible rises in [Ca2+](i). All except one of the dorsal root ganglion neurons responsive to capsaicin also showed sensitivity to heat, and most, but not all, heat-sensitive neurons also responded to capsaicin. Both capsaicin and heat responses were dependent on the presence of extracellular Ca2+. Non-transfected Chinese hamster ovary cells and non-responsive dorsal root ganglion neurons showed only small rises in [Ca2+](i) in response to heat which did not depend on the presence of external Ca2+. Responsive dorsal root ganglion neurons and vanilloid receptor 1 transfected cells showed a clear temperature threshold, above which [Ca2+](i) increased rapidly. This was estimated to be 42.6+/-0.3 degrees C for vanilloid receptor 1 transfected cells and 42.0+/-0.6 degrees C for dorsal root ganglion neurons. The competitive capsaicin antagonist capsazepine (10microM) abolished [Ca2+](i) increases stimulated by capsaicin in both dorsal root ganglion neurons and vanilloid receptor 1 transfected cells. However, responses to heat of a similar magnitude in the same cells were inhibited by only 37% by capsazepine (10microM). In vanilloid receptor 1 transfected cells, Ruthenium Red (10microM) blocked responses to both capsaicin and heat. These results demonstrate that imaging of [Ca2+](i) can identify dorsal root ganglion neurons which are responsive to both heat and capsaicin. They show that heat and capsaicin responses mediated by native and recombinant capsaicin receptors are similar with respect to the characteristics and pharmacology examined, suggesting that expression of recombinant vanilloid receptor 1 in cell lines accurately reproduces the properties of the native receptor.

Analysis of the native quaternary structure of vanilloid receptor 1

Vanilloid receptor subtype 1 (VR1) is a ligand-gated channel that can be activated by capsaicin and other vanilloids as well as by protons and heat. In the present study, we have analyzed the oligomeric state of VR1. Co-immunoprecipitation of differently tagged VR1 molecules indicated that VR1 can form oligomers. Using two different heterologous VR1 expression systems as well as endogenous VR1 expressed in dorsal root ganglion cells, we analyzed oligomer formation using perfluoro-octanoic acid polyacrylamide gel electrophoresis. Results were confirmed both with chemical cross-linking agents as well as through endogenous cross-linking mediated by transglutaminase. Our results clearly show that VR1 forms multimers in each of the expression systems with a homotetramer as a predominant form. The oligomeric structure of VR1 may contribute to the complexity of VR1 pharmacology. Finally, differences in glycosylation between the systems were observed, indicating the need for caution in the use of the heterologous expression systems for analysis of VR1 properties.

Antinociceptive activity of the endogenous fatty acid amide, palmitylethanolamide

The endogenous fatty acid ethanolamide, palmitylethanolamide, alleviated, in a dose-dependent manner, pain behaviors elicited in mice by injections of formalin (5%, intraplantar), acetic acid (0.6%, 0.5 ml per animal, intraperitoneal, i.p.), kaolin (2.5 mg per animal, i.p.), and magnesium sulfate (120 mg per kg, i.p.). The antinociceptive effects of palmitylethanolamide were prevented by the cannabinoid CB2 receptor antagonist SR144528 [N-([1s]-endo-1.3.3-trimethylbicyclo[2.3.1]heptan-2-yl)-5-(4-chloro-3-methylphenyl)-1-(4-methylbenzyl)-pyrazole-3-carboxamide], not by the cannabinoid CB1 receptor antagonist SR141716A [N-(piperidin-1-yl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide x HCl]. By contrast, palmitylethanolamide had no effect on capsaicin-evoked pain behavior or thermal nociception. The endogenous cannabinoid, anandamide (arachidonylethanolamide), alleviated nociception in all tests (formalin, acetic acid, kaolin, magnesium sulfate, capsaicin and hot plate). These effects were prevented by the cannabinoid CB1 receptor antagonist SR141716A, not the cannabinoid CB2 receptor antagonist SR141716A. Additional fatty acid ethanolamides (oleylethanolamide, myristylethanolamide, palmitoleylethanolamide, palmitelaidylethanolamide) had little or no effect on formalin-evoked pain behavior, and were not investigated in other pain models. These results support the hypothesis that endogenous palmitylethanolamide participates in the intrinsic control of pain initiation. They also suggest that the putative receptor site activated by palmitylethanolamide may provide a novel target for peripherally acting analgesic drugs.

Vanilloid receptor 1 immunoreactivity in inflamed human bowel

Vanilloid receptor 1 (VR1) is expressed by sensory neurons. Once activated, these neurons evoke the sensation of burning pain and release neuropeptides that induce neurogenic inflammation. We used immunoblotting and immunostaining to estimate the density of VR1 in colonic tissues of patients with inflammatory bowel disease and of controls. Our study results indicate that VR1 immunoreactivity is greatly increased in colonic nerve fibres of patients with active inflammatory bowel disease. Thus, the discovery of new drugs that can bind the VR1 receptor, or antagonise endogenous inflammatory substances that activate this receptor, could lead to new therapies for pain and dysmotility.

Characterisation using FLIPR of human vanilloid VR1 receptor pharmacology

A full pharmacological characterisation of the recently cloned human vanilloid VR1 receptor was undertaken. In whole-cell patch clamp studies, capsaicin (10 microM) elicited a slowly activating/deactivating inward current in human embryonic kidney (HEK293) cells stably expressing human vanilloid VR1 receptor, which exhibited pronounced outward rectification (reversal potential -2.1+/-0.2 mV) and was abolished by capsazepine (10 microM). In FLIPR-based Ca(2+) imaging studies the rank order of potency was resiniferatoxin>olvanil>capsaicin>anandamide, and all were full agonists. Isovelleral and scutigeral were inactive (1 nM-30 microM). The potencies of capsaicin, olvanil and resiniferatoxin, but not anandamide, were enhanced 2- to 7-fold at pH 6.4. Capsazepine, isovelleral and ruthenium red inhibited the capsaicin (100 nM)-induced Ca(2+) response (pK(B)=6.58+/-0.02, 5.33+/-0.03 and 7.64+/-0.03, respectively). In conclusion, the recombinant human vanilloid VR1 receptor stably expressed in HEK293 cells acted as a ligand-gated, Ca(2+)-permeable channel with similar agonist and antagonist pharmacology to rat vanilloid VR1 receptor, although there were some subtle differences.

Ligand-induced dynamic membrane changes and cell deletion conferred by vanilloid receptor 1

The real time dynamics of vanilloid-induced cytotoxicity and the specific deletion of nociceptive neurons expressing the wild-type vanilloid receptor (VR1) were investigated. VR1 was C-terminally tagged with either the 27-kDa enhanced green fluorescent protein (eGFP) or a 12-amino acid epsilon-epitope. Upon exposure to resiniferatoxin, VR1eGFP- or VR1epsilon-expressing cells exhibited pharmacological responses similar to those of cells expressing the untagged VR1. Within seconds of vanilloid exposure, the intracellular free calcium ([Ca(2+)](i)) was elevated in cells expressing VR1. A functional pool of VR1 also was localized to the endoplasmic reticulum that, in the absence of extracellular calcium, also was capable of releasing calcium upon agonist treatment. Confocal imaging disclosed that resiniferatoxin treatment induced vesiculation of the mitochondria and the endoplasmic reticulum ( approximately 1 min), nuclear membrane disruption (5-10 min), and cell lysis (1-2 h). Nociceptive primary sensory neurons endogenously express VR1, and resiniferatoxin treatment induced a sudden increase in [Ca(2+)](i) and mitochondrial disruption which was cell-selective, as glia and non-VR1-expressing neurons were unaffected. Early hallmarks of cytotoxicity were followed by specific deletion of VR1-expressing cells. These data demonstrate that vanilloids disrupt vital organelles within the cell body and, if administered to sensory ganglia, may be employed to rapidly and selectively delete nociceptive neurons.

Sustained sensitization and recruitment of rat cutaneous nociceptors by bradykinin and a novel theory of its excitatory action

Excitation and sensitization to heat of nociceptors by bradykinin (BK) were examined using an isolated rat skin-saphenous nerve preparation. A total of 52 C-fibres was tested: 42 were mechano-heat sensitive (CMH) and 40% of them were excited and sensitized to heat by BK superfusion (10-5 M, 5 min) of their receptive fields; heat responses were augmented by more than five times and heat thresholds dropped to 36.4 degrees C, on average. Sixty per cent of the CMH did not respond to BK itself, but 3/4 of these units showed an increase in their heat responses by more than 100% following BK exposure. Ten high-threshold mechanosensitive C-fibres did not discharge upon BK application but following this five of them responded to heat in a well-graded manner. In all fibres, the sensitizing effect of BK was abolished within 9 min or less of wash-out, and it could be reproduced several times at equal magnitude, whereas the excitatory effect of BK regularly showed profound tachyphylaxis. Sustained superfusion (20 min) of BK induced a desensitizing excitatory response while superimposed heat responses showed constant degrees of sensitization. The large extent and high prevalence of BK-induced sensitization (almost 80% of all fibres tested) and de novo recruitment of heat sensitivity suggest a prominent role of BK not only in hyperalgesia but also in sustained inflammatory pain which may be driven by body or even lower local temperatures acting on sensitized nociceptors. Based on the latter assumption, a hypothesis is put forward that excludes a direct excitatory effect of BK on nociceptors, but assumes a temperature-controlled activation as a result of rapid and profound sensitization.

Peripheral PAR-2 triggers thermal hyperalgesia and nociceptive responses in rats

Protease-activated receptor-2 (PAR-2), a member of the G protein-coupled, seven trans-membrane domain receptor family, is activated by trypsin/tryptase and present in various tissues including the primary sensory neurons, playing a role in development of neurogenic inflammation. The present study examined if activation of peripheral PAR-2 could modulate nociception in the rat. Expression of mRNA for PAR-2 was confirmed in the L4-6 dorsal root ganglia, but not spinal cord. The PAR-2-activating peptide SLIGRL-NH2 administered by the intraplantar (i.pl.) route, produced thermal, but not mechanical, hyperalgesia in the rat, although the PAR-2-inactive control peptide LSIGRL-NH2 had no effect. Not only the PAR-2-activating but also inactive peptides elicited nociceptive behavior (licking/biting) in the intact rats, whereas only the former peptide produced such behavior in the rats that had received repeated administration of compound 48/80 for mast cell depletion. These data provide novel evidence that activation of peripheral PAR-2 is pro-nociceptive, producing thermal hyperalgesia and also triggering pain sensation, by itself, independently of mast cell degranulation.

Pharmacological differences between the human and rat vanilloid receptor 1 (VR1)

Vanilloid receptors (VR1) were cloned from human and rat dorsal root ganglion libraries and expressed in Xenopus oocytes or Chinese Hamster Ovary (CHO) cells. Both rat and human VR1 formed ligand gated channels that were activated by capsaicin with similar EC(50) values. Capsaicin had a lower potency on both channels, when measured electrophysiologically in oocytes compared to CHO cells (oocytes: rat=1.90+/-0.20 microM; human=1.90+/-0.30 microM: CHO cells: rat=0.20+/-0.06 microM; human=0.19+/-0.08 microM). In CHO cell lines co-expressing either rat or human VR1 and the calcium sensitive, luminescent protein, aequorin, the EC(50) values for capsaicin-induced responses were similar in both cell lines (rat=0.35+/-0.06 microM, human=0.53+/-0.03 microM). The threshold for activation by acidic solutions was lower for human VR1 channels than that for rat VR1 (EC(50) pH 5.49+/-0.04 and pH 5.78+/-0.09, respectively). The threshold for heat activation was identical (42 degrees C) for rat and human VR1. PPAHV was an agonist at rat VR1 (EC(50) between 3 and 10 microM) but was virtually inactive at the human VR1 (EC(50)>10 microM). Capsazepine and ruthenium red were both more potent at blocking the capsaicin response of human VR1 than rat VR1. Capsazepine blocked the human but not the rat VR1 response to low pH. Capsazepine was also more effective at inhibiting the noxious heat response of human than of rat VR1.

The tissue distribution and functional characterization of human VR1

The irritant action of capsaicin is mediated by the vanilloid receptor, VR1, which is expressed in sensory neurons termed nociceptors. Capsaicin also desensitizes nociceptors and, thus, is useful clinically as an analgesic. Given the potential importance of VR1 in pain, we have cloned the human capsaicin receptor, hVR1, from a human dorsal root ganglia (DRG) cDNA library. Human VR1 protein is 85% identical to the rat VR1 and many of the amino acid differences are concentrated at the amino and carboxyl termini. VR1 is expressed in DRG as an approximately 4.2 kilobase RNA, and is also expressed in the central nervous system and in the kidney. Capsaicin (EC(50) = 853 nM), low pH (<5.5), and noxious heat (44 degrees C) activate hVR1 expressed in Xenopus oocytes. Subthreshold pH (6.4) sensitizes VR1 to capsaicin (EC(50) = 221 nM). This study demonstrates the similarity of human and rat VR1 in integrating multiple noxious stimuli.

Nociceptors for the 21st century

This review summarizes recent developments in the context of the neurochemical classification of nociceptors and explores the relationships between functionally and neurochemically defined subgroups. Although the complete picture is not yet available, several lines of intriguing evidence suggest that despite the complexity and diversity of nociceptor properties, a relatively "simple" neurochemical classification fits well with several recently identified molecular characteristics.

Intracellular ATP increases capsaicin-activated channel activity by interacting with nucleotide-binding domains

Capsaicin (CAP)-activated ion channel plays a key role in generating nociceptive neural signals in sensory neurons. Here we present evidence that intracellular ATP upregulates the activity of capsaicin receptor channel. In inside-out membrane patches isolated from sensory neurons, application of CAP activated a nonselective cation channel (i(cap)). Further addition of ATP to the bath caused a significant increase in i(cap), with a K(1/2) of 3.3 mm. Nonhydrolyzable analogs of ATP, adenylimidodiphosphate and adenosine 5'-O-(3-thio)-triphosphate, also increased i(cap). Neither Mg(2+)-free medium nor inhibitors of various kinases blocked the increase in i(cap) induced by ATP. The enhancing effect of ATP was also observed in inside-out patches of oocytes expressing vanilloid receptor 1, a cloned capsaicin receptor. Single point mutations (D178N, K735R) within the putative Walker type nucleotide-binding domains abolished the effect of ATP. These results show that ATP increases i(cap) in sensory neurons by direct interaction with the CAP channel without involvement of phosphorylation.

Differences in sensitivity of vanilloid receptor 1 transfected to human embryonic kidney cells and capsaicin-activated channels in cultured rat dorsal root ganglion neurons to capsaicin receptor agonists

Heterologously expressed vanilloid receptor 1 (VR1), a cloned cDNA encoding for capsaicin (CAP)-sensitive currents, resembles the native CAP channels in cultured sensory neurons in channel property. But, the pharmacological profile of VR1 to various CAP analogs is not known. The stable expression of VR1 in human embryonic kidney (HEK) cells was generated and confirmed by reverse transcription-polymerase chain reaction and Western blots. VR1 expressed in HEK cells retained single-channel properties similar to those of the native channels. When concentration-response relationships were compared, CAP and DA-5018.HCl, a synthetic analog of CAP, exhibited a greater potency in activating VR1 than the native channels in sensory neurons. In contrast, resiniferatoxin and its analog, phorbol 12-phenylacetate 13-acetate 20-homovanillate, was more potent in activating the CAP-activated channels in cultured sensory neurons than VR1. Thus, the difference in pharmacological profiles of VR1 and the native channels suggests the possible presence of subtypes of the CAP receptor or regulatory mechanisms associated with VR1.

The effects of pH on the interaction between capsaicin and the vanilloid receptor in rat dorsal root ganglia neurons

1. The vanilloid receptor of sensory neurons is a polymodal nociceptor sensitive to capsaicin, protons, heat and anandamide. Although it is known that interaction occurs between these different mediators the mechanism by which this occurs is poorly understood. In this study capsaicin elicited currents were recorded from vanilloid receptors found in adult rat isolated dorsal root ganglia (DRG) neurons under conditions of varying pH and the mechanism whereby protons can modulate this capsaicin response investigated. 2. Under whole-cell voltage clamp, modulating extracellular pH shifted the position of the capsaicin log(concentration)-response curve. Acidification from pH 9.0 to pH 5.5 lowered the EC50 values from 1150+/-250 nM to 5+/-2 nM with coincident change in the mean apparent slope factor from 2.3+/-0.3 to 0.9+/-0.2 and no change in maximal response. 3. The magnitude of the potentiation seen on reducing extracellular pH was not significantly affected by changes in extracellular calcium and magnesium concentration. 4. The response to capsaicin was not potentiated by a reduction in intracellular pH suggesting a site of action more accessible from the extracellular than the intracellular side of the membrane. 5. Potentiation by low pH was voltage independent indicating a site of action outside the membrane electric field. 6. At the single channel level, reducing extracellular pH increased channel open probability but had no significant effect on single channel conductance or open time. 7. These results are consistent with a model in which, on reducing extracellular pH, the vanilloid receptor in rat DRG neurons, changes from a state with low affinity for capsaicin to one with high affinity, coincident with a loss of cooperativity. This effect, presumed to be proton mediated, appears to involve one or more sites with pK(a) value 7.4-7.9, outside the membrane electrical field on an extracellularly exposed region of the receptor protein.

The molecular dynamics of pain control

Pain is necessary for survival, but persistent pain can result in anxiety, depression and a reduction in the quality of life. The discriminative and affective qualities of pain are both thought to be regulated in an activity-dependent fashion. Recent studies have identified cells and molecules that regulate pain sensitivity and the parallel pathways that distribute nociceptive information to limbic or sensory areas of the forebrain. Here, we emphasize the cellular and neurobiological consequences of pain, especially those that are involved in the generation and maintenance of chronic pain. These new insights into pain processing will significantly alter our approach to pain control and the development of new analgesics.

Capsaicin-sensitive afferents and their role in gastroprotection: an update

The pivotal role of capsaicin-sensitive peptidergic sensory fibers in the maintenance of gastric mucosal integrity against injurious interventions was suggested by the authors 20 years ago. Since then substantial evidence has accumulated for the local sensory-efferent function of the released CGRP, tachykinins and NO in this gastroprotective mechanism. This overview outlines some recent achievements which shed light on new aspects and further horizons in this field. (1) Cloning the capsaicin VR-1 receptor (an ion channel-coupled receptor) and raising the VR-1 knockout mice provided a definite molecular background for the existence of capsaicin-sensitive afferents with both sensory and mediator releasing functions in the stomach. This cation channel is also sensitive to hydrogen ions. (2) VR-1 agonists (capsaicin, resiniferatoxin, piperine) protect against gastric ulcer of the rat parallel with their sensory stimulating potencies. (3) Antidromic stimulation of capsaicin-sensitive vagal and somatic afferents results in the release of CGRP, tachykinins, NO and somatostatin. Somatostatin with gastroprotective effect is released from D cells and sensory nerve endings. (4) The recent theory for the existence of spinal afferents without sensory function [P. Holzer, C.A. Maggi, Dissociation of dorsal root ganglion neurons into afferent and efferent-like neurons, Neuroscience 86 (1998) 389-398] is discussed. Data proposed to support this theory are interpreted here on the basis of a dual sensory-efferent function of VR-1 positive afferents, characterized by a frequency optimum of discharges for release vasodilatory neuropeptides below the nociceptive threshold. (5) Recent data on the effect of capsaicin in healthy human stomach are summarized. These results indicate that the gastroprotective effect of capsaicin in the human stomach involves additional mechanisms to those already revealed in the rat.

The ligand that came from within

Recent evidence suggests that the endogenous activators of neuronal vanilloid (capsaicin) receptors should be sought inside, not outside, the neuron. 12-Lipoxygenase metabolites of arachidonic acid, produced by pain-inducing mediators such as bradykinin, might trigger vanilloid receptor activation by interacting with an intracellular site on the receptor. Are these lipid-like second messengers the long-sought endogenous vanilloids?

Induction of vanilloid receptor channel activity by protein kinase C

Capsaicin or vanilloid receptors (VRs) participate in the sensation of thermal and inflammatory pain. The cloned (VR1) and native VRs are non-selective cation channels directly activated by harmful heat, extracellular protons and vanilloid compounds. However, considerable attention has been focused on identifying other signalling pathways in VR activation; it is known that VR1 is also expressed in non-sensory tissue and may mediate inflammatory rather than acute thermal pain. Here we show that activation of protein kinase C (PKC) induces VR1 channel activity at room temperature in the absence of any other agonist. We also observed this effect in native VRs from sensory neurons, and phorbol esters induced a vanilloid-sensitive Ca2+ rise in these cells. Moreover, the pro-inflammatory peptide, bradykinin, and the putative endogenous ligand, anandamide, respectively induced and enhanced VR activity, in a PKC-dependent manner. These results suggest that PKC may link a range of stimuli to the activation of VRs.

Vanilloid (capsaicin) receptors influence inflammatory sensitivity in response to particulate matter

The signs of airway inflammation and hyperresponsiveness that occur in animals exposed to air pollutants are often strain- and species-specific. To investigate the underlying causes of this phenomenon, BALB/c and C57bl/6 mice were exposed intratracheally to residual oil fly ash (ROFA, 3 mg/kg) and examined after 24 h for signs of airway inflammation. BALB/c showed significantly higher numbers of neutrophils and increased airway hyperresponsiveness in response to methacholine challenge, whereas B6 mice showed no significant change in either inflammatory endpoint. To determine the underlying cause of this strain specificity, cultures of dorsal root ganglion (DRG) sensory neurons, which innervate the upper airways in situ, were explanted from both BALB/c and B6 fetal mice. After 5-7 days in culture, they were exposed to ROFA, other urban and industrial particulate matter (PM; e.g., oil fly ash, woodstove, Mt. St. Helen, St. Louis, Ottawa, coal fly ash) or to prototype irritants (e.g., capsaicin 3-10 microM, pH 5.0 and 6.5). In all instances (except for woodstove), DRG neurons from BALB/c mice released significantly higher levels of the pro-inflammatory cytokine IL-6 into their nutrient media relative to neurons from B6 mice. This cytokine release could be significantly reduced for all PM treated cultures (except woodstove) by pretreatment of cultures with capsazepine (CPZ), a competitive antagonist of vanilloid receptors. DRG neurons, cultured from BALB/c and B6 neonates, were loaded with Fluo-3 AM and exposed to the prototype irritants, acid pH (5.0, 6.5), or capsaicin (3, 10 microM). Analysis of their increases in intracellular calcium showed that significantly higher numbers of BALB/c neurons responded to these prototype irritants, relative to B6 neurons. Morphometric analysis of BALB/c neurons, histochemically stained with cobalt to label neurons bearing capsaicin-sensitive receptors, showed a significantly higher level of stained neurons relative to B6 neurons. Finally, semiquantitative RT-PCR showed a higher expression of VR1 receptor mRNA in DRG and spinal cord taken from neonatal BALB/c mice relative to B6 mice. Taken together, these data suggest that capsaicin and acid-sensitive irritant receptors, located on somatosensory cell bodies and their nerve fiber terminals, subserve PM-induced airway inflammation and are quantitatively different in responsive and nonresponsive mouse strains.

Transgenic gene knock-outs: functional genomics and therapeutic target selection

The completion of the first draft of the human genome presents both a tremendous opportunity and enormous challenge to the pharmaceutical industry since the whole community, with few exceptions, will soon have access to the same pool of candidate gene sequences from which to select future therapeutic targets. The commercial imperative to select and pursue therapeutically relevant genes from within the overall content of the genome will be particularly intense for those gene families that currently represent the chemically tractable or 'drugable' gene targets. As a consequence the emphasis within exploratory research has shifted towards the evaluation and adoption of technology platforms that can add additional value to the gene selection process, either through functional studies or direct/indirect measures of disease alignment e.g., genetics, differential gene expression, proteomics, tissue distribution, comparative species data etc. The selection of biological targets for the development of potential new medicines relies, in part, on the quality of the in vivo biological data that correlates a particular molecular target with the underlying pathophysiology of a disease. Within the pharmaceutical industry, studies employing transgenic animals and, in particular, animals with specific gene deletions are playing an increasingly important role in the therapeutic target gene selection, drug candidate selection and product development phases of the overall drug discovery process. The potential of phenotypic information from gene knock-outs to contribute to a high-throughput target selection/validation strategy has hitherto been limited by the resources required to rapidly generate and characterise a large number of knock-out transgenics in a timely fashion. The offerings of several companies that provide an opportunity to overcome these hurdles, albeit at a cost, are assessed with respect to the strategic business needs of the pharmaceutical industry.