The arginine-rich hexapeptide R4W2 is a stereoselective antagonist at the vanilloid receptor 1: a Ca2+ imaging study in adult rat dorsal root ganglion neurons

BAC transgenic mice to study the expression of P2X2 and P2Y1 receptors

Extracellular purines are important signaling molecules involved in numerous physiological and pathological processes via the activation of P2 receptors. Information about the spatial and temporal P2 receptor (P2R) expression and its regulation remains crucial for the understanding of the role of P2Rs in health and disease. To identify cells carrying P2X2Rs in situ, we have generated BAC transgenic mice that express the P2X2R subunits as fluorescent fusion protein (P2X2-TagRFP). In addition, we generated a BAC P2Y₁R TagRFP reporter mouse expressing a TagRFP reporter for the P2RY1 gene expression. We demonstrate expression of the P2X2R in a subset of DRG neurons, the brain stem, the hippocampus, as well as on Purkinje neurons of the cerebellum. However, the weak fluorescence intensity in our P2X2R-TagRFP mouse precluded tracking of living cells. Our P2Y₁R reporter mice confirmed the widespread expression of the P2RY1 gene in the CNS and indicate for the first time P2RY1 gene expression in mouse Purkinje cells, which so far has only been described in rats and humans. Our P2R transgenic models have advanced the understanding of purinergic transmission, but BAC transgenic models appeared not always to be straightforward and permanent reliable. We noticed a loss of fluorescence intensity, which depended on the number of progeny generations. These problems are discussed and may help to provide more successful animal models, even if in future more versatile and adaptable nuclease-mediated genome-editing techniques will be the methods of choice.

Cationic Arginine-Rich Peptides (CARPs): A Novel Class of Neuroprotective Agents With a Multimodal Mechanism of Action

There are virtually no clinically available neuroprotective drugs for the treatment of acute and chronic neurological disorders, hence there is an urgent need for the development of new neuroprotective molecules. Cationic arginine-rich peptides (CARPs) are an expanding and relatively novel class of compounds, which possess intrinsic neuroprotective properties. Intriguingly, CARPs possess a combination of biological properties unprecedented for a neuroprotective agent including the ability to traverse cell membranes and enter the CNS, antagonize calcium influx, target mitochondria, stabilize proteins, inhibit proteolytic enzymes, induce pro-survival signaling, scavenge toxic molecules, and reduce oxidative stress as well as, having a range of anti-inflammatory, analgesic, anti-microbial, and anti-cancer actions. CARPs have also been used as carrier molecules for the delivery of other putative neuroprotective agents across the blood-brain barrier and blood-spinal cord barrier. However, there is increasing evidence that the neuroprotective efficacy of many, if not all these other agents delivered using a cationic arginine-rich cell-penetrating peptide (CCPPs) carrier (e.g., TAT) may actually be mediated largely by the properties of the carrier molecule, with overall efficacy further enhanced according to the amino acid composition of the cargo peptide, in particular its arginine content. Therefore, in reviewing the neuroprotective mechanisms of action of CARPs we also consider studies using CCPPs fused to a putative neuroprotective peptide. We review the history of CARPs in neuroprotection and discuss in detail the intrinsic biological properties that may contribute to their cytoprotective effects and their usefulness as a broad-acting class of neuroprotective drugs.

Calcium signals that determine vascular resistance

Small arteries in the body control vascular resistance, and therefore, blood pressure and blood flow. Endothelial and smooth muscle cells in the arterial walls respond to various stimuli by altering the vascular resistance on a moment to moment basis. Smooth muscle cells can directly influence arterial diameter by contracting or relaxing, whereas endothelial cells that line the inner walls of the arteries modulate the contractile state of surrounding smooth muscle cells. Cytosolic calcium is a key driver of endothelial and smooth muscle cell functions. Cytosolic calcium can be increased either by calcium release from intracellular stores through IP3 or ryanodine receptors, or the influx of extracellular calcium through ion channels at the cell membrane. Depending on the cell type, spatial localization, source of a calcium signal, and the calcium-sensitive target activated, a particular calcium signal can dilate or constrict the arteries. Calcium signals in the vasculature can be classified into several types based on their source, kinetics, and spatial and temporal properties. The calcium signaling mechanisms in smooth muscle and endothelial cells have been extensively studied in the native or freshly isolated cells, therefore, this review is limited to the discussions of studies in native or freshly isolated cells. This article is categorized under: Biological Mechanisms > Cell Signaling Laboratory Methods and Technologies > Imaging Models of Systems Properties and Processes > Mechanistic Models.

Sustained relief of ongoing experimental neuropathic pain by a CRMP2 peptide aptamer with low abuse potential

Uncoupling the protein-protein interaction between collapsin response mediator protein 2 (CRMP2) and N-type voltage-gated calcium channel (CaV2.2) with an allosteric CRMP2-derived peptide (CBD3) is antinociceptive in rodent models of inflammatory and neuropathic pain. We investigated the efficacy, duration of action, abuse potential, and neurobehavioral toxicity of an improved mutant CRMP2 peptide. A homopolyarginine (R9)-conjugated CBD3-A6K (R9-CBD3-A6K) peptide inhibited the CaV2.2-CRMP2 interaction in a concentration-dependent fashion and diminished surface expression of CaV2.2 and depolarization-evoked Ca influx in rat dorsal root ganglia neurons. In vitro studies demonstrated suppression of excitability of small-to-medium diameter dorsal root ganglion and inhibition of subtypes of voltage-gated Ca channels. Sprague-Dawley rats with tibial nerve injury had profound and long-lasting tactile allodynia and ongoing pain. Immediate administration of R9-CBD3-A6K produced enhanced dopamine release from the nucleus accumbens shell selectively in injured animals, consistent with relief of ongoing pain. R9-CBD3-A6K, when administered repeatedly into the central nervous system ventricles of naive rats, did not result in a positive conditioned place preference demonstrating a lack of abusive liability. Continuous subcutaneous infusion of R9-CBD3-A6K over a 24- to 72-hour period reversed tactile allodynia and ongoing pain, demonstrating a lack of tolerance over this time course. Importantly, continuous infusion of R9-CBD3-A6K did not affect motor activity, anxiety, depression, or memory and learning. Collectively, these results validate the potential therapeutic significance of targeting the CaV-CRMP2 axis for treatment of neuropathic pain.

TRPV1: A Potential Drug Target for Treating Various Diseases

Transient receptor potential vanilloid 1 (TRPV1) is an ion channel present on sensory neurons which is activated by heat, protons, capsaicin and a variety of endogenous lipids termed endovanilloids. As such, TRPV1 serves as a multimodal sensor of noxious stimuli which could trigger counteractive measures to avoid pain and injury. Activation of TRPV1 has been linked to chronic inflammatory pain conditions and peripheral neuropathy, as observed in diabetes. Expression of TRPV1 is also observed in non-neuronal sites such as the epithelium of bladder and lungs and in hair cells of the cochlea. At these sites, activation of TRPV1 has been implicated in the pathophysiology of diseases such as cystitis, asthma and hearing loss. Therefore, drugs which could modulate TRPV1 channel activity could be useful for the treatment of conditions ranging from chronic pain to hearing loss. This review describes the roles of TRPV1 in the normal physiology and pathophysiology of selected organs of the body and highlights how drugs targeting this channel could be important clinically.

Functionally important amino acid residues in the transient receptor potential vanilloid 1 (TRPV1) ion channel--an overview of the current mutational data

This review aims to create an overview of the currently available results of site-directed mutagenesis studies on transient receptor potential vanilloid type 1 (TRPV1) receptor. Systematization of the vast number of data on the functionally important amino acid mutations of TRPV1 may provide a clearer picture of this field, and may promote a better understanding of the relationship between the structure and function of TRPV1. The review summarizes information on 112 unique mutated sites along the TRPV1, exchanged to multiple different residues in many cases. These mutations influence the effect or binding of different agonists, antagonists, and channel blockers, alter the responsiveness to heat, acid, and voltage dependence, affect the channel pore characteristics, and influence the regulation of the receptor function by phosphorylation, glycosylation, calmodulin, PIP2, ATP, and lipid binding. The main goal of this paper is to publish the above mentioned data in a form that facilitates in silico molecular modelling of the receptor by promoting easier establishment of boundary conditions. The better understanding of the structure-function relationship of TRPV1 may promote discovery of new, promising, more effective and safe drugs for treatment of neurogenic inflammation and pain-related diseases and may offer new opportunities for therapeutic interventions.

Divalent heavy metal cations block the TRPV1 Ca(2+) channel

Transient receptor potential vanilloid 1 (TRPV1) is a non-selective cation channel involved in pain sensation and in a wide range of non-pain-related physiological and pathological conditions. The aim of the present study was to explore the effects of selected heavy metal cations on the function of TRPV1. The cations ranked in the following sequence of pore-blocking activity: Co(2+) [half-maximal inhibitory concentration (IC(50)) = 13 μM] > Cd(2+) (I (50) = 38 μM) > Ni(2+) (IC(50) = 62 μM) > Cu(2+) (IC(50) = 200 μM). Zn(2+) proved to be a weak (IC(50) = 27 μM) and only partial inhibitor of the channel function, whereas Mg(2+), Mn(2+) and La(3+) did not exhibit any substantial effect. Co(2+), the most potent channel blocker, was able not only to compete with Ca(2+) but also to pass with it through the open channel of TRPV1. In response to heat activation or vanilloid treatment, Co(2+) accumulation was verified in TRPV1-transfected cell lines and in the TRPV1+ dorsal root ganglion neurons. The inhibitory effect was also demonstrated in vivo. Co(2+) applied together with vanilloid agonists attenuated the nocifensive eye wipe response in mice. Different rat TRPV1 pore point mutants (Y627W, N628W, D646N and E651W) were created that can validate the binding site of previously used channel blockers in agonist-evoked (45)Ca(2+) influx assays in cells expressing TRPV1. The IC(50) of Co(2+) on these point mutants were determined to be reasonably comparable to those on the wild type, which suggests that divalent cations passing through the TRPV1 channel use the same negatively charged amino acids as Ca(2+).

Suppression of pain-related behavior in two distinct rodent models of peripheral neuropathy by a homopolyarginine-conjugated CRMP2 peptide

The N-type voltage-gated calcium channel (CaV2.2) is a clinically endorsed target in chronic pain treatments. As directly targeting the channel can lead to multiple adverse side effects, targeting modulators of CaV2.2 may prove better. We previously identified ST1-104, a short peptide from the collapsin response mediator protein 2 (CRMP2), which disrupted the CaV2.2-CRMP2 interaction and suppressed a model of HIV-related neuropathy induced by anti-retroviral therapy but not traumatic neuropathy. Here, we report ST2-104 -a peptide wherein the cell-penetrating TAT motif has been supplanted with a homopolyarginine motif, which dose-dependently inhibits the CaV2.2-CRMP2 interaction and inhibits depolarization-evoked Ca(2+) influx in sensory neurons. Ca(2+) influx via activation of vanilloid receptors is not affected by either peptide. Systemic administration of ST2-104 does not affect thermal or tactile nociceptive behavioral changes. Importantly, ST2-104 transiently reduces persistent mechanical hypersensitivity induced by systemic administration of the anti-retroviral drug 2',3'-dideoxycytidine (ddC) and following tibial nerve injury (TNI). Possible mechanistic explanations for the broader efficacy of ST2-104 are discussed.

Substance P release in response to cardiac ischemia from rat thoracic spinal dorsal horn is mediated by TRPV1

Spinal cord stimulation (SCS) inhibits substance P (SP) release and decreases the expression of the transient receptor potential vanilloid 1 (TRPV1) in the spinal cord at thoracic 4 (T4) during cardiac ischemia in rat models (Ding et al., 2007). We hypothesized that activation of TRPV1 in the T4 spinal cord segment by intermittent occlusion of the left anterior descending coronary artery (CoAO) mediates spinal cord SP release. Experiments were conducted in urethane-anesthetized Sprague-Dawley male rats using SP antibody-coated microprobes to measure SP release at the central terminal endings of cardiac ischemic-sensitive afferent neurons (CISAN) in the spinal T4 dorsal horns. Vehicle, capsaicin (CAP; TRPV1 agonist) and capsazepine (CZP; TRPV1 antagonist) were injected into the left T4 prior to stimulation of CISAN by intermittent CoAO (with or without upper cervical SCS). CAP induced endogenous SP release from laminae I and II in the T4 spinal cord above baseline. Conversely, CZP injections significantly inhibited SP release from laminae I-VII in the T4 spinal cord segment below baseline. CZP also attenuated CoAO-induced SP release, while T4 injections of CZP with SCS completely restored SP release to basal levels during CoAO activation. CAP increased the number of c-Fos (a marker for CISAN activation) positive T4 dorsal horn neurons compared to sham-operated animals, while CZP (alone or during CoAO and SCS+CoAO) significantly reduced the number of c-Fos positive neurons. These results suggest that spinal release of the putative nociceptive transmitter SP occurs, at least in part, via a TRPV1 mechanism.

Physiology and pharmacology of the vanilloid receptor

The identification and cloning of the vanilloid receptor 1 (TRPV1) represented a significant step for the understanding of the molecular mechanisms underlying the transduction of noxious chemical and thermal stimuli by peripheral nociceptors. TRPV1 is a non-selective cation channel gated by noxious heat, vanilloids and extracellular protons. TRPV1 channel activity is remarkably potentiated by pro-inflammatory agents, a phenomenon that is thought to underlie the peripheral sensitisation of nociceptors that leads to thermal hyperalgesia. Cumulative evidence is building a strong case for the involvement of this receptor in the etiology of both peripheral and visceral inflammatory pain, such as inflammatory bowel disease, bladder inflammation and cancer pain. The validation of TRPV1 receptor as a key therapeutic target for pain management has thrust intensive drug discovery programs aimed at developing orally active antagonists of the receptor protein. Nonetheless, the real challenge of these drug discovery platforms is to develop antagonists that preserve the physiological activity of TRPV1 receptors while correcting over-active channels. This is a condition to ensure normal pro-prioceptive and nociceptive responses that represent a safety mechanism to prevent tissue injury. Recent and exciting advances in the function, dysfunction and modulation of this receptor will be the focus of this review.

Metabotropic P2Y receptors inhibit P2X3 receptor-channels via G protein-dependent facilitation of their desensitization

BACKGROUND AND PURPOSE

The aim of the present study was to investigate whether the endogenous metabotropic P2Y receptors modulate ionotropic P2X(3) receptor-channels.

EXPERIMENTAL APPROACH

Whole-cell patch-clamp experiments were carried out on HEK293 cells permanently transfected with human P2X(3) receptors (HEK293-hP2X(3) cells) and rat dorsal root ganglion (DRG) neurons.

KEY RESULTS

In both cell types, the P2Y(1,12,13) receptor agonist, ADP-beta-S, inhibited P2X(3) currents evoked by the selective agonist, alpha,beta-methylene ATP (alpha,beta-meATP). This inhibition could be markedly counteracted by replacing in the pipette solution the usual GTP with GDP-beta-S, a procedure known to block all G protein heterotrimers. P2X(3) currents evoked by ATP, activating both P2Y and P2X receptors, caused a smaller peak amplitude and desensitized faster than those currents evoked by the selective P2X(3) receptor agonist alpha,beta-meATP. In the presence of intracellular GDP-beta-S, ATP- and alpha,beta-meATP-induced currents were identical. Recovery from P2X(3) receptor desensitization induced by repetitive ATP application was slower than the recovery from alpha,beta-meATP-induced desensitization. When G proteins were blocked by intracellular GDP-beta-S, the recovery from the ATP- and alpha,beta-meATP-induced desensitization were of comparable speed.

CONCLUSIONS AND IMPLICATIONS

Our results suggest that the activation of P2Y receptors G protein-dependently facilitates the desensitization of P2X(3) receptors and suppresses the recovery from the desensitized state. Hence, the concomitant stimulation of P2X(3) and P2Y receptors of DRG neurons by ATP may result both in an algesic effect and a partly counterbalancing analgesic activity.

Antinociceptive effect of antisense oligonucleotides against the vanilloid receptor VR1/TRPV1

To examine the role of the vanilloid receptor TRPV1 in neuropathic pain, we assessed the effects of the receptor antagonist thioxo-BCTC and antisense oligonucleotides against the TRPV1 mRNA in a rat model of spinal nerve ligation. In order to identify accessible sites on the mRNA of TRPV1, the RNase H assay was used, leading to the successful identification of binding sites for antisense oligonucleotides. Cotransfection studies using Cos-7 cells were employed to identify the most effective antisense oligonucleotide efficiently inhibiting the expression of a fusion protein consisting of TRPV1 and the green fluorescent protein in a specific and concentration-dependent manner. In an in vivo rat model of spinal nerve ligation, intravenous application of the TRPV1 antagonist thioxo-BCTC reduced mechanical hypersensitivity yielding an ED(50) value of 10.6mg/kg. Intrathecal administration of the antisense oligonucleotide against TRPV1, but not the mismatch oligonucleotide or a vehicle control, reduced mechanical hypersensitivity in rats with spinal nerve ligation in a similar manner. Immunohistochemical analysis revealed neuropathy- and antisense-associated regulation of TRPV1 protein expression in spinal cord and dorsal root ganglia. Our data demonstrate comparative analgesic effects of a TRPV1 anatagonist and a rationally designed TRPV1 antisense oligonucleotide in a spinal nerve ligation model of neuropathic pain and thus, lend support to the validation of TRPV1 as a promising target for the treatment of neuropathic pain.

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

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

Regulation of human recombinant P2X3 receptors by ecto-protein kinase C

The whole-cell patch-clamp technique was used to record current responses to nucleotides and nucleosides in human embryonic kidney HEK293 cells transfected with the human purinergic P2X3 receptor. When guanosine 5'-O-(3-thiodiphosphate) was included into the pipette solution, UTP at concentrations that did not alter the holding current facilitated the alpha,beta-methylene ATP (alpha,beta-meATP)-induced current. ATP and GTP, but not UDP or uridine, had an effect similar to that of UTP. Compounds known to activate protein kinase C (PKC) acted like the nucleoside triphosphates investigated, whereas various PKC inhibitors invariably reduced the effects of both PKC activators and UTP. The substitution by Ala of Ser/Thr residues situated within PKC consensus sites of the P2X3 receptor ectodomain either abolished (PKC2 and PKC3; T134A, S178A) or did not alter (PKC4 and PKC6; T196A, S269A) the UTP-induced potentiation of the alpha,beta-meATP current. Both the blockade of ecto-protein kinase C activity and the substitution of Thr-134 or Ser-178 by Ala depressed the maximum of the concentration-response curve for alpha,beta-meATP without altering the EC50 values. Molecular simulation of the P2X3 receptor structure indicated no overlap between assumed nucleotide binding domains and the relevant phosphorylation sites PKC2 and PKC3. alpha,beta-meATP-induced currents through native homomeric P2X3 receptors of rat dorsal root ganglia were also facilitated by UTP. In conclusion, it is suggested that low concentrations of endogenous nucleotides in the extracellular space may prime the sensitivity of P2X3 receptors toward the effect of subsequently applied (released) higher agonistic concentrations. The priming effect of nucleotides might be attributable to a phosphorylation of PKC sites at the ectodomain of P2X3 receptors.

Metabotropic P2Y1 receptors inhibit P2X3 receptor-channels in rat dorsal root ganglion neurons

Whole-cell patch-clamp recordings from cultured rat dorsal root ganglion neurons demonstrated that the P2Y1 receptor agonists adenosine 5'-O-2-thiodiphosphate (ADP-beta-S) and 2-methylthio adenosine 5'-diphosphate (2-MeSADP) inhibit the alpha,beta-methylene adenosine 5'-triphosphate (alpha,beta-meATP)-induced P2X3 receptor-currents. This effect could be antagonized by the wide-spectrum G protein blocker GDP-beta-S and the P2Y(1) receptor antagonist MRS 2179. The P2Y12,13 receptor antagonist AR-C6993MX and pertussis toxin, a blocker of Galphai/o, did not interact with the effect of ADP-beta-S. Hence, the results indicate that ADP-sensitive P2Y1 receptors of rat dorsal root ganglion neurons inhibit ionotropic P2X3 receptors via G protein-activation.

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

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

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.

Analysis of structure–activity relationships for the ‘B-region’ of N -(3-acyloxy-2-benzylpropyl)- N ′ -[4-(methylsulfonylamino)benzyl]thiourea analogues as vanilloid receptor antagonists: discovery of an N -hydroxythiourea analogue with potent analgesic activity

The structural modifications on the B-region of the potent and high affinity vanilloid receptor (VR1) lead ligand N-(3-acyloxy-2-benzylpropyl)-N(')-[4-(methylsulfonylamino)benzyl]thiourea were investigated by the replacement of the thiourea with diverse isosteric functional groups. Structure-activity analysis indicated that the A-region in this series was the primary factor in determining the agonistic/antagonistic activities regardless of the B-region. The N(C)-hydroxy thiourea analogues (12, 13) showed excellent analgesic activities in the acetic acid writhing assay compared to the parent thiourea analogues.

Inhibition of N-type voltage-activated calcium channels in rat dorsal root ganglion neurons by P2Y receptors is a possible mechanism of ADP-induced analgesia

Patch-clamp recordings from small-diameter rat dorsal root ganglion (DRG) neurons maintained in culture demonstrated preferential inhibition by ATP of high-voltage-activated, but not low-voltage-activated, Ca2+ currents (I(Ca)). The rank order of agonist potency was UTP > ADP > ATP. ATP depressed the omega-conotoxin GVIA-sensitive N-type current only. Pyridoxal-5-phosphate-6-azophenyl-2',4'-disulphonic acid (PPADS) and 2'-deoxy-N6-methyladenosine 3',5'-bisphosphate tetraammonium, two P2Y1 receptor antagonists, almost abolished the ATP-induced inhibition. Both patch-clamp recordings and immunocytochemistry coupled with confocal laser microscopy indicated a colocalization of functional P2X3 and P2Y1 receptors on the same DRG neurons. Because the effect of ATP was inhibited by intracellular guanosine 5'-O-(2-thiodiphosphate) or by applying a strongly depolarizing prepulse, P2Y1 receptors appear to block I(Ca) by a pathway involving the betagamma subunit of a G(q/11) protein. Less efficient buffering of the intracellular Ca2+ concentration ([Ca2+]i) by reducing the intrapipette EGTA failed to interfere with the ATP effect. Fura-2 microfluorimetry suggested that ATP raised [Ca2+]i by a Galpha-mediated release from intracellular pools and simultaneously depressed the high external potassium concentration-induced increase of [Ca2+]i by inhibiting I(Ca) via Gbetagamma. Adenosine 5'-O-(2-thiodiphosphate) inhibited dorsal root-evoked polysynaptic population EPSPs in the hemisected rat spinal cord and prolonged the nociceptive threshold on intrathecal application in the tail-flick assay. These effects were not antagonized by PPADS. Hence, P2Y receptor activation by ADP, which is generated by enzymatic degradation of ATP, may decrease the release of glutamate from DRG terminals in the spinal cord and thereby partly counterbalance the algogenic effect of ATP.

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

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

Subsensitivity of P2X but not vanilloid 1 receptors in dorsal root ganglia of rats caused by cyclophosphamide cystitis

The application of cyclophosphamide to rats was used to induce interstitial cystitis. Behavioural studies indicated a strong pain reaction that developed within 2 h and levelled off thereafter causing a constant pain during the following 18 h. Neurons prepared from L6/S1 dorsal root ganglia innervating the urinary bladder responded to the application of capsaicin or alpha,beta-methylene ATP (alpha,beta-meATP) with an increase of intracellular Ca2+ ([Ca2+]i). The [Ca2+]i responses to capsaicin were identical in the dorsal root ganglion cells of cyclophosphamide- and saline-treated rats, whereas alpha,beta-meATP induced less increase in [Ca2+]i in the cyclophosphamide-treated animals than in their saline-treated counterparts. Hence, alpha,beta-meATP-sensitive P2X3 and/or P2X2/3 receptors of L6/S1 dorsal root ganglion neurons were functionally downregulated during subacute pain caused by experimental cystitis. In contrast, capsaicin-sensitive vanilloid 1 receptors did not react to the same procedure. Thoracal dorsal root ganglia, not innervating the urinary bladder, were also unaltered in their responsiveness to alpha,beta-meATP by cyclophosphamide treatment.