The responses of rat trigeminal ganglion neurons to capsaicin and two nonpungent vanilloid receptor agonists, olvanil and glyceryl nonamide

Cannabinoid Analgesia in Postoperative Pain Management: From Molecular Mechanisms to Clinical Reality

Postoperative pain (POP) is a challenging clinical phenomenon that affects the majority of surgical patients and demands effective management to mitigate adverse outcomes such as persistent pain. The primary goal of POP management is to alleviate suffering and facilitate a seamless return to normal function for the patient. Despite compelling evidence of its drawbacks, opioid analgesia remains the basis of POP treatment. Novel therapeutic approaches rely on multimodal analgesia, integrating different pharmacological strategies to optimize efficacy while minimizing adverse effects. The recognition of the imperative role of the endocannabinoid system in pain regulation has prompted the investigation of cannabinoid compounds as a new therapeutic avenue. Cannabinoids may serve as adjuvants, enhancing the analgesic effects of other drugs and potentially replacing or at least reducing the dependence on other long-term analgesics in pain management. This narrative review succinctly summarizes pertinent information on the molecular mechanisms, clinical therapeutic benefits, and considerations associated with the plausible use of various cannabinoid compounds in treating POP. According to the available evidence, cannabinoid compounds modulate specific molecular mechanisms intimately involved in POP. However, only two of the eleven clinical trials that evaluated the efficacy of different cannabinoid interventions showed positive results.

Carcinogenesis and Metastasis: Focus on TRPV1-Positive Neurons and Immune Cells

Both sensory neurons and immune cells, albeit at markedly different levels, express the vanilloid (capsaicin) receptor, Transient Receptor Potential, Vanilloid-1 (TRPV1). Activation of TRPV1 channels in sensory afferent nerve fibers induces local effector functions by releasing neuropeptides (most notably, substance P) which, in turn, trigger neurogenic inflammation. There is good evidence that chronic activation or inactivation of this inflammatory pathway can modify tumor growth and metastasis. TRPV1 expression was also demonstrated in a variety of mammalian immune cells, including lymphocytes, dendritic cells, macrophages and neutrophils. Therefore, the effects of TRPV1 agonists and antagonists may vary depending on the prominent cell type(s) activated and/or inhibited. Therefore, a comprehensive understanding of TRPV1 activity on immune cells and nerve endings in distinct locations is necessary to predict the outcome of therapies targeting TRPV1 channels. Here, we review the neuro-immune modulation of cancer growth and metastasis, with focus on the consequences of TRPV1 activation in nerve fibers and immune cells. Lastly, the potential use of TRPV1 modulators in cancer therapy is discussed.

Olvanil activates sensory nerve fibers, increases T cell response and decreases metastasis of breast carcinoma

BACKGROUND

Inactivation of sensory neurons expressing transient receptor potential vanilloid 1 (TRPV1) enhances breast cancer metastasis. Sensory neurons have profound effects on immune response to a wide range of diseases including cancer. Hence, activation of sensory nerves using feasible approaches such as specific TRPV1 agonists may inhibit breast cancer metastasis through neuroimmune pathways. TRPV1 agonists are considered for the treatment of pain and inflammatory diseases.

METHODS

We here first determined the effects of four different TRPV1 agonists on proliferation of three different metastatic breast carcinoma cells since TRPV1 is also expressed in cancer cells. Based on the results obtained under in-vitro conditions, brain metastatic breast carcinoma cells (4TBM) implanted orthotopically into the mammary-pad of Balb-c mice followed by olvanil treatment (i.p.). Changes in tumor growth, metastasis and immune response to cancer cells were determined.

RESULTS

Olvanil dose-dependently activated sensory nerve fibers and markedly suppressed lung and liver metastasis without altering the growth of primary tumors. Olvanil (5 mg/kg) systemically increased T cell count, enhanced intra-tumoral recruitment of CD8+ T cells and increased IFN-γ response to irradiated cancer cells and Con-A. Anti-inflammatory changes such as increased IL-10 and decrease IL-6 as well as S100A8+ cells were observed following olvanil treatment.

CONCLUSIONS

Our results show that anti-metastatic effects of olvanil is mainly due to activation of neuro-immune pathways since olvanil dose used here is not high enough to directly activate immune cells. Furthermore, olvanil effectively depletes sensory neuropeptides; hence, olvanil is a good non-pungent alternative to capsaicin.

Targeting Chemosensory Ion Channels in Peripheral Swallowing-Related Regions for the Management of Oropharyngeal Dysphagia

Oropharyngeal dysphagia, or difficulty in swallowing, is a major health problem that can lead to serious complications, such as pulmonary aspiration, malnutrition, dehydration, and pneumonia. The current clinical management of oropharyngeal dysphagia mainly focuses on compensatory strategies and swallowing exercises/maneuvers; however, studies have suggested their limited effectiveness for recovering swallowing physiology and for promoting neuroplasticity in swallowing-related neuronal networks. Several new and innovative strategies based on neurostimulation in peripheral and cortical swallowing-related regions have been investigated, and appear promising for the management of oropharyngeal dysphagia. The peripheral chemical neurostimulation strategy is one of the innovative strategies, and targets chemosensory ion channels expressed in peripheral swallowing-related regions. A considerable number of animal and human studies, including randomized clinical trials in patients with oropharyngeal dysphagia, have reported improvements in the efficacy, safety, and physiology of swallowing using this strategy. There is also evidence that neuroplasticity is promoted in swallowing-related neuronal networks with this strategy. The targeting of chemosensory ion channels in peripheral swallowing-related regions may therefore be a promising pharmacological treatment strategy for the management of oropharyngeal dysphagia. In this review, we focus on this strategy, including its possible neurophysiological and molecular mechanisms.

Combination of a Rapidly Penetrating Agonist and a Slowly Penetrating Antagonist Affords Agonist Action of Limited Duration at the Cellular Level

The capsaicin receptor TRPV1 (transient receptor potential vanilloid 1) has been an object of intense interest for pharmacological development on account of its critical role in nociception. In the course of structure activity analysis, it has become apparent that TRPV1 ligands may vary dramatically in the rates at which they interact with TRPV1, presumably reflecting differences in their abilities to penetrate into the cell. Using a fast penetrating agonist together with an excess of a slower penetrating antagonist, we find that we can induce an agonist response of limited duration and, moreover, the duration of the agonist response remains largely independent of the absolute dose of agonist, as long as the ratio of antagonist to agonist is held constant. This general approach for limiting agonist duration under conditions in which absolute agonist dose is variable should have more general applicability.

Hypersensitivity Induced by Activation of Spinal Cord PAR2 Receptors Is Partially Mediated by TRPV1 Receptors

Protease-activated receptors 2 (PAR2) and transient receptor potential vanilloid 1 (TRPV1) receptors in the peripheral nerve endings are implicated in the development of increased sensitivity to mechanical and thermal stimuli, especially during inflammatory states. Both PAR2 and TRPV1 receptors are co-expressed in nociceptive dorsal root ganglion (DRG) neurons on their peripheral endings and also on presynaptic endings in the spinal cord dorsal horn. However, the modulation of nociceptive synaptic transmission in the superficial dorsal horn after activation of PAR2 and their functional coupling with TRPV1 is not clear. To investigate the role of spinal PAR2 activation on nociceptive modulation, intrathecal drug application was used in behavioural experiments and patch-clamp recordings of spontaneous, miniature and dorsal root stimulation-evoked excitatory postsynaptic currents (sEPSCs, mEPSCs, eEPSCs) were performed on superficial dorsal horn neurons in acute rat spinal cord slices. Intrathecal application of PAR2 activating peptide SLIGKV-NH₂ induced thermal hyperalgesia, which was prevented by pretreatment with TRPV1 antagonist SB 366791 and was reduced by protein kinases inhibitor staurosporine. Patch-clamp experiments revealed robust decrease of mEPSC frequency (62.8 ± 4.9%), increase of sEPSC frequency (127.0 ± 5.9%) and eEPSC amplitude (126.9 ± 12.0%) in dorsal horn neurons after acute SLIGKV-NH₂ application. All these EPSC changes, induced by PAR2 activation, were prevented by SB 366791 and staurosporine pretreatment. Our results demonstrate an important role of spinal PAR2 receptors in modulation of nociceptive transmission in the spinal cord dorsal horn at least partially mediated by activation of presynaptic TRPV1 receptors. The functional coupling between the PAR2 and TRPV1 receptors on the central branches of DRG neurons may be important especially during different pathological states when it may enhance pain perception.

Multi-inhibitor prodrug constructs for simultaneous delivery of anti-inflammatory agents to mustard-induced skin injury

The molecular pathology of sulfur mustard injury is complex, with at least nine inflammation-related enzymes and receptors upregulated in the zone of the insult. A new approach wherein inhibitors of these targets have been linked by hydrolyzable bonds, either one to one or via separate preattachment to a carrier molecule, has been shown to significantly enhance the therapeutic response compared with the individual agents. This article reviews the published work of the authors in this drug development domain over the last 8 years.

Differential Activation of TRP Channels in the Adult Rat Spinal Substantia Gelatinosa by Stereoisomers of Plant-Derived Chemicals

Activation of TRPV1, TRPA1 or TRPM8 channel expressed in the central terminal of dorsal root ganglion (DRG) neuron increases the spontaneous release of l-glutamate onto spinal dorsal horn lamina II (substantia gelatinosa; SG) neurons which play a pivotal role in regulating nociceptive transmission. The TRP channels are activated by various plant-derived chemicals. Although stereoisomers activate or modulate ion channels in a distinct manner, this phenomenon is not fully addressed for TRP channels. By applying the whole-cell patch-clamp technique to SG neurons of adult rat spinal cord slices, we found out that all of plant-derived chemicals, carvacrol, thymol, carvone and cineole, increase the frequency of spontaneous excitatory postsynaptic current, a measure of the spontaneous release of l-glutamate from nerve terminals, by activating TRP channels. The presynaptic activities were different between stereoisomers (carvacrol and thymol; (-)-carvone and (+)-carvone; 1,8-cineole and 1,4-cineole) in the extent or the types of TRP channels activated, indicating that TRP channels in the SG are activated by stereoisomers in a distinct manner. This result could serve to know the properties of the central terminal TRP channels that are targets of drugs for alleviating pain.

1,8- and 1,4-cineole enhance spontaneous excitatory transmission by activating different types of transient receptor potential channels in the rat spinal substantia gelatinosa

Although transient receptor potential (TRP) channels expressed in the spinal substantia gelatinosa play a role in modulating nociceptive transmission, their properties have not been fully examined yet. In order to address this issue, the effects of 1,8-cineole and its stereoisomer 1,4-cineole on excitatory transmission were examined by applying the whole-cell patch-clamp technique to substantia gelatinosa neurons in adult rat spinal cord slices. Miniature excitatory postsynaptic current frequency was increased by 1,8- and 1,4-cineole. The cineole activities were repeated and resistant to voltage-gated Na⁺ -channel blocker tetrodotoxin. The 1,8-cineole activity was inhibited by TRP ankyrin-1 (TRPA1) antagonists (HC-030031 and mecamylamine) but not TRP vanilloid-1 (TRPV1) antagonists (capsazepine and SB-366791), whereas the 1,4-cineole activity was depressed by the TRPV1 but not TRPA1 antagonists. Although 1,8- and 1,4-cineole reportedly activate TRP melastatin-8 (TRPM8) channels, their activities were unaffected by TRPM8 antagonist 4-(3-chloro-2-pyridinyl)-N-[4-(1,1-dimethylethyl)phenyl]-1-piperazinecarboxamide. Monosynaptically evoked C-fiber, but not Aδ-fiber excitatory postsynaptic current amplitude, was reduced by 1,8- and 1,4-cineole. These results indicate that 1,8- and 1,4-cineole increase spontaneous l-glutamate release from nerve terminals by activating TRPA1 and TRPV1 channels, respectively, while inhibiting C-fiber but not Aδ-fiber evoked l-glutamate release. This difference between 1,8- and 1,4-cineole may serve to know the properties of TRP channels located in the central terminals of primary-afferent neurons. The spinal dorsal horn lamina II (substantia gelatinosa; SG) plays a pivotal role in regulating nociceptive transmission from the periphery. We found out in the SG that 1,4- and 1,8-cineole activate TRPV1 and TRPA1 channels, respectively, located in primary-afferent, possibly C-fiber, central terminals. This difference may serve to know the properties of TRP channels expressed in the central terminals.

Photoswitchable fatty acids enable optical control of TRPV1

Fatty acids (FAs) are not only essential components of cellular energy storage and structure, but play crucial roles in signalling. Here we present a toolkit of photoswitchable FA analogues (FAAzos) that incorporate an azobenzene photoswitch along the FA chain. By modifying the FAAzos to resemble capsaicin, we prepare a series of photolipids targeting the Vanilloid Receptor 1 (TRPV1), a non-selective cation channel known for its role in nociception. Several azo-capsaicin derivatives (AzCAs) emerge as photoswitchable agonists of TRPV1 that are relatively inactive in the dark and become active on irradiation with ultraviolet-A light. This effect can be rapidly reversed by irradiation with blue light and permits the robust optical control of dorsal root ganglion neurons and C-fibre nociceptors with precision timing and kinetics not available with any other technique. More generally, we expect that photolipids will find many applications in controlling biological pathways that rely on protein–lipid interactions.

Fatty acids are ancient lipids with numerous functions, from metabolic processes as a source of energy to structural and signalling roles within cell membranes. Here, the authors present azobenzene-modified fatty acids and their application as photoswitchable agonists of the Vanilloid Receptor 1.

Relationship between the acyl chain length of paradol analogues and their antiobesity activity following oral ingestion

6-Paradol is known to activate thermogenesis in brown adipose tissue (BAT), and paradol analogues with different acyl chain lengths possess different pungency thresholds. In this study, the influence of the acyl chain length on the antiobesity activity of the paradol analogues was investigated. The antiobesity activity of 6-paradol in mice fed a high-fat diet for 8 weeks was greater than that of dihydrocapsiate. A comparison of the antiobesity activities of zingerone and 6-paradol showed that the length of the acyl chain in the paradol analogue was important for strong activity. Furthermore, the antiobesity activities of 6-, 8-, and 12-paradol appeared to decrease in an acyl chain length-dependent manner. The mechanism of the antiobesity activity of 6-paradol was enhanced by increasing levels of energy metabolism in the BAT, as well as an increase in the expression of uncoupling proteins 1 via the activation of sympathetic nerve activity.

TRP Channels Involved in Spontaneous L-Glutamate Release Enhancement in the Adult Rat Spinal Substantia Gelatinosa

The spinal substantia gelatinosa (SG) plays a pivotal role in modulating nociceptive transmission through dorsal root ganglion (DRG) neurons from the periphery. TRP channels such as TRPV1 and TRPA1 channels expressed in the SG are involved in the regulation of the nociceptive transmission. On the other hand, the TRP channels located in the peripheral terminals of the DRG neurons are activated by nociceptive stimuli given to the periphery and also by plant-derived chemicals, which generates a membrane depolarization. The chemicals also activate the TRP channels in the SG. In this review, we introduce how synaptic transmissions in the SG neurons are affected by various plant-derived chemicals and suggest that the peripheral and central TRP channels may differ in property from each other.

TRP modulation by natural compounds

The use of medicinal plants or other naturally derived products to relieve illness can be traced back over several millennia, and these natural products are still extensively used nowadays. Studies on natural products have, over the years, enormously contributed to the development of therapeutic drugs used in modern medicine. By means of the use of these substances as selective agonists, antagonists, enzyme inhibitors or activators, it has been possible to understand the complex function of many relevant targets. For instance, in an attempt to understand how pepper species evoke hot and painful actions, the pungent and active constituent capsaicin (from Capsicum sp.) was isolated in 1846 and the receptor for the biological actions of capsaicin was cloned in 1997, which is now known as TRPV1 (transient receptor potential vanilloid 1). Thus, TRPV1 agonists and antagonists have currently been tested in order to find new drug classes to treat different disorders. Indeed, the transient receptor potential (TRP) proteins are targets for several natural compounds, and antagonists of TRPs have been synthesised based on the knowledge of naturally derived products. In this context, this chapter focuses on naturally derived compounds (from plants and animals) that are reported to be able to modulate TRP channels. To clarify and make the understanding of the modulatory effects of natural compounds on TRPs easier, this chapter is divided into groups according to TRP subfamilies: TRPV (TRP vanilloid), TRPA (TRP ankyrin), TRPM (TRP melastatin), TRPC (TRP canonical) and TRPP (TRP polycystin). A general overview on the naturally derived compounds that modulate TRPs is depicted in Table 1.

Zingerone enhances glutamatergic spontaneous excitatory transmission by activating TRPA1 but not TRPV1 channels in the adult rat substantia gelatinosa

Transient receptor potential (TRP) channels are thought to play a role in regulating nociceptive transmission to spinal substantia gelatinosa (SG) neurons. It remains to be unveiled whether the TRP channels in the central nervous system are different in property from those involved in receiving nociceptive stimuli in the peripheral nervous system. We examined the effect of the vanilloid compound zingerone, which activates TRPV1 channels in the cell body of a primary afferent neuron, on glutamatergic excitatory transmission in the SG neurons of adult rat spinal cord slices by using the whole cell patch-clamp technique. Bath-applied zingerone reversibly and concentration-dependently increased spontaneous excitatory postsynaptic current (EPSC) frequency. This effect was accompanied by an inward current at -70 mV that was resistant to glutamate receptor antagonists. These zingerone effects were repeated and persisted in Na(+)-channel blocker tetrodotoxin-, La(3+)-, or IP3-induced Ca(2+)-release inhibitor 2-aminoethoxydiphenyl borate-containing or Ca(2+)-free Krebs solution. Zingerone activity was resistant to the selective TRPV1 antagonist capsazepine but sensitive to the nonselective TRP antagonist ruthenium red, the TRPA1 antagonist HC-030031, and the Ca(2+)-induced Ca(2+)-release inhibitor dantrolene. TRPA1 agonist allyl isothiocyanate but not capsaicin inhibited the facilitatory effect of zingerone. On the other hand, zingerone reduced monosynaptically evoked EPSC amplitudes, as did TRPA1 agonists. Like allyl isothiocyanate, zingerone enhanced GABAergic spontaneous inhibitory transmission in a manner sensitive to tetrodotoxin. We conclude that zingerone presynaptically facilitates spontaneous excitatory transmission, probably through Ca(2+)-induced Ca(2+)-release mechanisms, and produces a membrane depolarization in SG neurons by activating TRPA1 but not TRPV1 channels.

Inhibition by capsaicin and its related vanilloids of compound action potentials in frog sciatic nerves

AIMS

Although capsaicin not only activates transient receptor potential vanilloid-1 (TRPV1) channels but also inhibits nerve conduction, the latter action has not yet been fully examined. The purpose of the present study was to know whether various vanilloids have an inhibitory action similar to that of capsaicin and further to compare their actions with that of local anesthetic procaine.

MAIN METHODS

Fast-conducting compound action potentials (CAPs) were recorded from frog sciatic nerve fibers by using the air-gap method.

KEY FINDINGS

Capsaicin reversibly and concentration-dependently reduced the peak amplitude of the CAP. TRPV1 antagonist capsazepine did not affect the capsaicin activity, and powerful TRPV1 agonist resiniferatoxin had no effect on CAPs, indicating no involvement of TRPV1 channels. Capsaicin analogs and other various vanilloids also inhibited CAPs in a concentration-dependent manner. An efficacy sequence of these inhibitions was capsaicin=dihydrocapsaicin>capsiate>eugenol>guaiacol≥zingerone≥vanillin>vanillylamine. Vanillic acid had almost no effect on CAPs; olvanil and curcumin appeared to be effective less than capsaicin. Capsaicin and eugenol were, respectively, ten- and two-fold effective more than procaine in CAP inhibition, while each of guaiacol, zingerone and vanillin was five-fold effective less than procaine.

SIGNIFICANCE

Various vanilloids exhibit CAP inhibition, the extent of which is determined by the property of the side chain bound to the vanillyl group, and some of them are more effective than procaine. These results may serve to unveil molecular mechanisms for capsaicin-induced conduction block and to develop antinociceptive drugs related to capsaicin.

Palvanil, a non-pungent capsaicin analogue, inhibits inflammatory and neuropathic pain with little effects on bronchopulmonary function and body temperature

N-Palmitoyl-vanillamide (palvanil) is a non-pungent capsaicinoid, found in low amounts in Capsicum and shown to rapidly desensitize transient receptor potential vanilloid type-1 (TRPV1) channels to the action of capsaicin and to exert analgesic effects after local administration. We have investigated here if systemic administration of palvanil to mice causes two typical adverse events of TRPV1 agonists, i.e. profound changes in body temperature and bronchoconstriction, and if it can still produce effective inhibition of inflammatory and chronic pain in different experimental models. Varying doses of palvanil were tested subcutaneously and acutely on body temperature in vivo or, or as a bolus, on bronchopulmunary function ex vivo, in comparison with capsaicin. Intraperitoneal palvanil was also tested against formalin-induced nocifensive behavior and carrageenan-induced oedema and thermal hyperalgesia, acutely, and against mechanical allodynia and thermal hyperalgesia in mice with spared nerve injury (SNI) of the sciatic nerve, after repeated administration over 7 days from SNI. Palvanil, at therapeutically relevant doses, produced significantly less hypothermia and bronchoconstriction than capsaicin. Palvanil (0.5-2.5 mg/kg) abolished formalin-induced nocifensive behavior and strongly attenuated SNI-induced mechanical allodynia and thermal hyperalgesia and carrageenan-induced oedema and thermal hyperalgesia. Systemic administration of the non-pungent capsaicinoid, palvanil, produces, at least in mice, much less of those side effects typical of TRPV1 agonists (hypothermia and bronchoconstriction), whilst being very effective at reducing pain and oedema. Thus, palvanil might be developed further as a novel pharmacological treatment for chronic abnormal pain.

Intracellular acidification evoked by moderate extracellular acidosis attenuates transient receptor potential V1 (TRPV1) channel activity in rat dorsal root ganglion neurons

Transient receptor potential V1 (TRPV1) has been suggested to play an important role in detecting decreases in extracellular pH (pH(o)). Results from recent in vivo studies, however, have suggested that TRPV1 channels play less of a role in sensing a moderately acidic pH(o) (6.0 < pH < 7.0) than predicted from the in vitro experiments. A clear explanation for this discrepancy between the in vitro and in vivo data has not yet been provided. We report here that intracellular acidification induced by a moderately low pH(o) (6.4) almost completely inhibited the effect of extracellular acidosis on TRPV1 activity. In our experiments, sodium acetate (20 mm), which was used to induce intracellular acidosis, attenuated the capsaicin-evoked TRPV1 current (I(CAP)) in a reversible manner in whole-cell patch-clamp mode and shifted the concentration-response curve to the right. Likewise, the concentration-response curve was significantly shifted to the right by lowering the pH of the pipette solution from 7.2 to 6.5. In addition, application of an acidic bath solution (pH 6.4) to the intracellular side also significantly suppressed I(CAP) in inside-out patch mode. In cell-attached patch mode, the single-channel activity of i(CAP) was significantly attenuated by intracellular acidosis that was induced by a decrease in pH(o) (6.4). These results suggested that intracellular acidification induced by a low pH(o) inhibited TRPV1 activity. When studied in perforated patch mode or by acidifying the intracellular pipette solution, potentiation or activation of TRPV1 by extracellular acidosis (pH 6.4) at 37 °C was almost completely inhibited. Likewise, enhancement of neuronal excitability by a moderately acidic pH(o) (6.4) at a physiological temperature (37 °C) was attenuated by lowering the pH of the pipette solution to 6.5 or using perforated patch mode. Taken together, these results suggest that extracellular acidosis of moderate intensity may not significantly modulate TRPV1 activity in physiological conditions at which intracellular pH can be readily affected by pH(o), and this phenomenon is due to attenuation of TRPV1 channel activity by low-pH(o)-induced intracellular acidification.

TRPV1 agonist piperine but not olvanil enhances glutamatergic spontaneous excitatory transmission in rat spinal substantia gelatinosa neurons

We examined the effects of TRPV1 agonists olvanil and piperine on glutamatergic spontaneous excitatory transmission in the substantia gelatinosa (SG) neurons of adult rat spinal cord slices with the whole-cell patch-clamp technique. Bath-applied olvanil did not affect the frequency and amplitude of spontaneous excitatory postsynaptic current (sEPSC), and unchanged holding currents at -70 mV. On the other hand, superfusing piperine reversibly and concentration-dependently increased sEPSC frequency (half-maximal effective concentration: 52.3 μM) with a minimal increase in its amplitude. This sEPSC frequency increase was almost repetitive at an interval of more than 20 min. Piperine at a high concentration produced an inward current in some neurons. The facilitatory effect of piperine was blocked by TRPV1 antagonist capsazepine. It is concluded that piperine but not olvanil activates TRPV1 channels in the central terminals of primary-afferent neurons, resulting in an increase in the spontaneous release of l-glutamate onto SG neurons.

Cannabinoid receptor antagonists AM251 and AM630 activate TRPA1 in sensory neurons

Cannabinoid receptor antagonists have been utilized extensively in vivo as well as in vitro, but their selectivity has not been fully examined. We investigated activation of sensory neurons by two cannabinoid antagonists - AM251 and AM630. AM251 and AM630 activated trigeminal (TG) sensory neurons in a concentration-dependent fashion (threshold 1 μM). AM251 and AM630 responses are mediated by the TRPA1 channel in a majority (90-95%) of small-to-medium TG sensory neurons. AM630 (1-100 μM), but not AM251, was a significantly more potent agonist in cells co-expressing both TRPA1 and TRPV1 channels. We next evaluated AM630 and AM251 effects on TRPV1- and TRPA1-mediated responses in TG neurons. Capsaicin (CAP) effects were inhibited by pre-treatment with AM630, but not AM251. Mustard oil (MO) and WIN55,212-2 (WIN) TRPA1 mediated responses were also inhibited by pre-treatment with AM630, but not AM251 (25 uM each). Co-treatment of neurons with WIN and either AM630 or AM251 had opposite effects: AM630 sensitized WIN responses, whereas AM251 inhibited WIN responses. WIN-induced inhibition of CAP responses in sensory neurons was reversed by AM630 pre-treatment and AM251 co-treatment (25 μM each), as these conditions inhibit WIN responses. Hindpaw injections of AM630 and AM251 did not produce nocifensive behaviors. However, both compounds modulated CAP-induced thermal hyperalgesia in wild-type mice and rats, but not TRPA1 null-mutant mice. AMs also partially regulate WIN inhibition of CAP-induced thermal hyperalgesia in a TRPA1-dependent fashion. In summary, these findings demonstrate alternative targets for the cannabinoid antagonists, AM251 and AM630, in peripheral antihyperalgesia which involve certain TRP channels.

N-palmitoyl-vanillamide (palvanil) is a non-pungent analogue of capsaicin with stronger desensitizing capability against the TRPV1 receptor and anti-hyperalgesic activity

N-acyl-vanillamide (NAVAM) analogues of the natural pungent principle of capsicum, capsaicin, were developed several years ago as potential non-pungent analgesic compounds. N-oleoyl-vanillamide (olvanil) and N-arachidonoy-vanillamide (arvanil), in particular, were described in several publications and patents to behave as potent anti-hyperalgesic compounds in experimental models of chronic and inflammatory pain, and to activate both "capsaicin receptors", i.e. the transient receptor potential of vanilloid type-1 (TRPV1) channel, and, either directly or indirectly, cannabinoid receptors of type-1. Here we report the biochemical and pharmacological characterization of a so far neglected NAVAM, N-palmitoyl-vanillamide (palvanil), and propose its possible use instead of capsaicin, as a possible topical analgesic. Palvanil exhibited a kinetics of activation of human recombinant TRPV1-mediated intracellular calcium elevation significantly slower than that of capsaicin (t(1/2)=21s and 8s, respectively at 1μM). Slow kinetics of TRPV1 agonists were previously found to be associated with stronger potencies as TRPV1 desensitizing agents, which in turn are usually associated with lower pungency and stronger anti-hyperalgesic activity. Accordingly, palvanil desensitized the human recombinant TRPV1 to the effect of capsaicin (10nM) with significantly higher potency than capsaicin (IC(50)=0.8nM and 3.8nM, respectively), this effect reaching its maximum more rapidly (50 and 250min, respectively). Palvanil was also more potent than capsaicin at desensitizing the stimulatory effect of TRPV1 by low pH together with anandamide, which mimics conditions occurring during inflammation. In the eye-wiping assay carried out in mice, palvanil was not pungent and instead caused a strong and long-lasting inhibition of capsaicin-induced eye-wiping. Finally, intraplantar palvanil inhibited the second phase of the nociceptive response to formalin in mice. In conclusion, palvanil appears to be a non-pungent analogue of capsaicin with stronger desensitizing effects on TRPV1 and hence potentially higher anti-hyperalgesic activity.

Electrophysiological and chemical properties in subclassified acutely dissociated cells of rat trigeminal ganglion by current signatures

In the present study, we subclassified acutely dissociated trigeminal ganglion (TRG) cells of rats using a current signature method in whole cell patch-clamp recordings. Using modified criteria for cell classification for the dorsal root ganglion (DRG), TRG cells were subclassified into nine cell types: 1-5, 7-9, and 13. Types 1, 3, and 7 were in the small cell groups (15-24 μm); types 4, 5, and 8-13 were in the medium cell groups (25-38 μm); and type 2 was a mixed group of both cell sizes. Types 1-3, 5, and 7 showed high-input resistance and types 1, 2, and 7 showed more depolarized resting membrane potentials. Types 1, 2, and 5-13 expressed long-duration action potentials (APs), but types 3 and 4 expressed short-duration APs. Sensitivities to capsaicin, protons, and adenosine 5'-triphosphate (ATP) in TRG cell types largely corresponded to DRG cell types. However, different from the matched DRG types, half of TRG type 1 cells were capsaicin insensitive, showing desensitizing proton-induced currents, and types 5, 7, and 9 exhibited slow-desensitizing ATP-induced currents. Types 4, 5, and 8-13 had nicotine sensitivity, but the other cell types were insensitive. These results indicate that the "current signatures" classification is a useful means to separate TRG cells into internally homogeneous subpopulations that were distinct from other cell types. Furthermore, the data suggest some specific differences in the chemical responsiveness of some cell types between the TRG and DRG.

Homozygous TRPV1 315C influences the susceptibility to functional dyspepsia

OBJECTIVES

Capsaicin/vanilloid (transient receptor potential vanilloid 1, (TRPV1) receptor has been shown to be expressed in gastrointestinal tract and play a role as a member of sensory ion channel superfamily. The G315C polymorphism affects the TRPV1 gene and alters its protein level. We aimed to investigate the effect of TRPV1 G315C polymorphism on functional dyspepsia (FD) in a Japanese population.

METHODS

TRPV1 G315C polymorphism was genotyped in 98 subjects with no upper abdominal symptoms and 109 patients with FD. Severity of 7 upper gastrointestinal symptoms was assessed during cold water, and cold carbonated water drinking for randomly selected 20 healthy subjects.

RESULTS

We found a significant inverse association between TRPV1 315CC genotype and FD [CC vs. others; odds ratio (OR)=0.40, 95% confidence interval (CI)=0.38-0.82]. We also found that the same genotype held a lower risk of both epigastric pain syndrome (OR=0.25, 95% CI=0.09-0.73), postprandial syndrome (OR=0.27, 95% CI=0.07-0.96) according to Rome III, and Helicobacter pylori positive FD (OR=0.28, 95% CI=0.10-0.79). The evolution of symptom severity scale of 7 total symptoms (P=0.004), and heavy feeling in stomach (P=0.02) during cold carbonated water drinking were significantly lower among 315CC genotypes compared with others.

CONCLUSIONS

Homozygous TRPV1 315C influences the susceptibility to FD through altering the upper gastrointestinal sensation.

Pharmacologic antagonism of the oral aversive taste-directed response to capsaicin in a mouse brief access taste aversion assay

Chemosensory signaling by the tongue is a primary determinant of ingestive behavior and is mediated by specific interactions between tastant molecules and G protein-coupled and ion channel receptors. The functional relationship between tastant and receptor should be amenable to pharmacologic methods and manipulation. We have performed a pharmacologic characterization of the taste-directed licking of mice presented with solutions of capsaicin and other transient receptor potential vanilloid-1 (TRPV1) agonists using a brief access taste aversion assay. Dose-response functions for lick-rate suppression were established for capsaicin (EC(50) = 0.5 microM), piperine (EC(50) = 2 muM), and resiniferatoxin (EC(50) = 0.02 microM). Little or no effect on lick rate was observed in response to the full TRPV1 agonist olvanil. Capsaicin lick rates of wild-type and transient receptor potential melastatin-5 (TRPM5) knockout mice were equivalent, indicating that TRPM5, a critical component of aversive signaling for many bitter tastants, did not contribute to the capsaicin taste response. The selective TRPV1 antagonists N-(4-tertiarybutylphenyl)-4-(3-chloropyridin-2-yl)tetrahydropyrazine-1(2H)-carbox-amide (10 microM) and (E)-3-(4-t-butylphenyl)-N-(2,3-dihydrobenzo[b][1,4]dioxin-6-yl)acrylamide (AMG9810) (10 microM) effectively blocked capsaicin- and piperine-mediated lick suppression. However, (E)-3-(4-chlorophenyl)-N-(3-methoxyphenyl)-N-phenylprop-2-enamide (SB 366791) and capsazepine, also TRPV1 antagonists, were without effect at test concentrations of up to 30 and 100 microM, respectively. Our results demonstrate that TRPV1-mediated oral aversiveness presents a pharmacologic profile differing from what has been reported previously for TRPV1 pain signaling and, furthermore, that aversive tastes can be evaluated and controlled pharmacologically.

TRPA1-mediated responses in trigeminal sensory neurons: interaction between TRPA1 and TRPV1

The transient receptor potential (TRP)A1 channel is involved in the transduction of inflammation-induced noxious stimuli from the periphery. Previous studies have characterized the properties of TRPA1 in heterologous expression systems. However, there is little information on the properties of TRPA1-mediated currents in sensory neurons. A capsaicin-sensitive subset of rat and mouse trigeminal ganglion sensory neurons was activated with TRPA1-specific agonists, mustard oil and the cannabinoid WIN55,212. Mustard oil- and WIN55,212-gated currents exhibited marked variability in their kinetics of activation and acute desensitization. TRPA1-mediated responses in neurons also possess a characteristic voltage dependency with profound outward rectification that is influenced by extracellular Ca(2+) and the type and concentration of TRPA1-specific agonists. Examination of TRPA1-mediated responses in TRPA1-containing cells indicated that the features of neuronal TRPA1 are not duplicated in cells expressing only TRPA1 and, instead, can be restored only when TRPA1 and TRPV1 channels are coexpressed. In summary, our results suggest that TRPA1-mediated responses in sensory neurons have distinct characteristics that can be accounted for by the coexpression of the TRPV1 and TRPA1 channels.

Role of ionotropic cannabinoid receptors in peripheral antinociception and antihyperalgesia

Despite the wealth of information on cannabinoid-induced peripheral antihyperalgesic and antinociceptive effects in many pain models, the molecular mechanism(s) for these actions remains unknown. Although metabotropic cannabinoid receptors have important roles in many pharmacological actions of cannabinoids, recent studies have led to the recognition of a family of at least five ionotropic cannabinoid receptors (ICRs). The known ICRs are members of the family of transient receptor potential (TRP) channels and include TRPV1, TRPV2, TRPV4, TRPM8 and TRPA1. Cannabinoid activation of ICRs can result in desensitization of the TRPA1 and TRPV1 channel activities, inhibition of nociceptors and antihyperalgesia and antinociception in certain pain models. Thus, cannabinoids activate both metabotropic and ionotropic mechanisms to produce peripheral analgesic effects. Here, we provide an overview of the pharmacology of TRP channels as ICRs.

Functional effects of nonsynonymous polymorphisms in the human TRPV1 gene

The prototypical member of the vanilloid-responsive-like subfamily of transient receptor potential (TRP) channels is TRPV1. TRPV1 mediates aspects of nociception and neurogenic inflammation; however, new roles are emerging in sensation of both luminal stretch and systemic tonicity. Although at least six nonsynonymous polymorphisms in the human TRPV1 gene have been identified, there has been no systematic investigation into their functional consequences. When heterologously expressed in HEK293 cells, all variants exhibited equivalent EC(50) for the classic agonist capsaicin. This agonist elicited a greater maximal response in TRPV1(I315M) and TRPV1(P91S) variants (relative to TRPV1(WT)), as did a second agonist, anandamide. Expression of these two variants in whole-cell lysates and at the cell surface was markedly greater than that of wild-type TRPV1, whereas expression at the mRNA level was either unchanged (TRPV1(P91S)) or only very modestly increased (TRPV1(I315M)). Incorporation of multiple nonsynonymous SNPs, informed by the population-specific haplotype block structure of the TRPV1 gene, did not lead to variant channels with unique features vis-à-vis capsaicin responsiveness. Recently, polymorphisms/mutations were identified in two highly conserved TRPV1 residues in the nonobese diabetic (NOD) murine model. Incorporation of these changes into human TRPV1 gave rise to a channel with a normal EC(50) for capsaicin, but with a markedly elevated Hill slope such that the variant channel was hyporesponsive to capsaicin at low doses (<10 nM) and hyperresponsive at high doses (>10 nM). In aggregate, these data underscore expression-level and functional differences among naturally occurring TRPV1 variants; the implications with respect to human physiology are considered.

Neuroprotective Effects of Glyceryl Nonivamide against Microglia-Like Cells and 6-Hydroxydopamine-Induced Neurotoxicity in SH-SY5Y Human Dopaminergic Neuroblastoma Cells

Glyceryl nonivamide (GLNVA), a vanilloid receptor (VR) agonist, has been reported to have calcitonin gene-related peptide-associated vasodilatation and to prevent subarachnoid hemorrhage-induced cerebral vasospasm. In this study, we investigated the neuroprotective effects of GLNVA on activated microglia-like cell mediated- and proparkinsonian neurotoxin 6-hydroxydopamine (6-OHDA)-induced neurotoxicity in human dopaminergic neuroblastoma SH-SY5Y cells. In coculture conditions, we used lipopolysaccharide (LPS)-stimulated BV-2 cells as a model of activated microglia. LPS-induced neuronal death was significantly inhibited by diphenylene iodonium (DPI), an inhibitor of NADPH oxidase. However, capsazepine, the selective VR1 antagonist, did not block the neuroprotective effects of GLNVA. GLNVA reduced LPS-activated microglia-mediated neuronal death, but it lacked protection in DPI-pretreated cultures. GLNVA also decreased LPS activated microglia induced overexpression of neuronal nitric-oxide synthase (nNOS) and glycoprotein 91 phagocyte oxidase (gp91(phox)) on SH-SY5Y cells. Pretreatment of BV-2 cells with GLNVA diminished LPS-induced nitric oxide production, overexpression of inducible nitric-oxide synthase (iNOS), and gp91(phox) and intracellular reactive oxygen species (iROS). GLNVA also reduced cyclooxygenase (COX)-2 expression, inhibitor of nuclear factor (NF)-kappaB (IkappaB)alpha/IkappaBbeta degradation, NF-kappaB activation, and the overproduction of tumor necrosis factor-alpha, interleukin (IL)-1beta, and prostaglandin E2 in BV-2 cells. However, GLNVA augmented anti-inflammatory cytokine IL-10 production on LPS-stimulated BV-2 cells. Furthermore, in 6-OHDA-treated SH-SY5Y cells, GLNVA rescued the changes in condensed nuclear and apoptotic bodies, prevented the decrease in mitochondrial membrane potential, and reduced cells death. GLNVA also suppressed accumulation of iROS and up-regulated heme oxygenase-1 expression. 6-OHDA-induced overexpression of nNOS, iNOS, COX-2, and gp91(phox) was also reduced by GLNVA. In summary, the neuroprotective effects of GLNVA are mediated, at least in part, by decreasing the inflammation- and oxidative stress-associated factors induced by microglia and 6-OHDA.

Transient receptor potential TRPA1 channel desensitization in sensory neurons is agonist dependent and regulated by TRPV1-directed internalization

The pharmacological desensitization of receptors is a fundamental mechanism for regulating the activity of neuronal systems. The TRPA1 channel plays a key role in the processing of noxious information and can undergo functional desensitization by unknown mechanisms. Here we show that TRPA1 is desensitized by homologous (mustard oil; a TRPA1 agonist) and heterologous (capsaicin; a TRPV1 agonist) agonists via Ca2+-independent and Ca2+-dependent pathways, respectively, in sensory neurons. The pharmacological desensitization of TRPA1 by capsaicin and mustard oil is not influenced by activation of protein phosphatase 2B. However, it is regulated by phosphatidylinositol-4,5-bisphosphate depletion after capsaicin, but not mustard oil, application. Using a biosensor, we establish that capsaicin, unlike mustard oil, consistently activates phospholipase C in sensory neurons. We next demonstrate that TRPA1 desensitization is regulated by TRPV1, and it appears that mustard oil-induced TRPA1 internalization is prevented by coexpression with TRPV1 in a heterologous expression system and in sensory neurons. In conclusion, we propose novel mechanisms whereby TRPA1 activity undergoes pharmacological desensitization through multiple cellular pathways that are agonist dependent and modulated by TRPV1.

Effect of olvanil and anandamide on vagal C-fiber subtypes in guinea pig lung

Certain fatty acid amides such as anandamide (AEA) and olvanil are agonists for the transient receptor potential, vanilloid-1 (TRPV1) receptor, but have been found to activate TRPV1-containing C-fibers in some tissues but not others. We used extracellular recording and whole-cell patch clamp techniques to investigate the effect of olvanil and AEA on different types of vagal C-fibers innervating the same tissue, namely jugular and nodose vagal C-fibers in guinea pig lungs. A 30 s exposure to AEA and olvanil caused action potential discharge in all nodose C-fiber innervating lung but failed to activate jugular C-fibers innervating lung and airways. The activation of nodose C-fibers was blocked by the TRPV1 antagonist iodo-resiniferatoxin. In whole-cell patch clamp recordings of dissociated nodose and jugular capsaicin-sensitive neurons labeled from lungs and airways, olvanil induced large TRPV1-dependent inward currents in cell bodies of both nodose and jugular ganglion neurons. Prolonged exposure (up to 5 min) to olvanil caused action potential discharge in jugular C-fiber innervating lung but the onset latency was four times longer in jugular than in nodose C-fibers. The onsets of capsaicin response in nodose and jugular C-fibers were not different. Decreasing the tissue temperature to 25 degrees C increased the onset latency of olvanil-induced activation of nodose C-fibers 2-3-fold, but did not effect the latency of the capsaicin response. Capsaicin, olvanil, and AEA stimulate jugular C-fibers leading to tachykinergic contractions of isolated bronchi. The time to reach half-maximum is more than four times longer for olvanil and AEA, as compared to capsaicin in evoking contractions. We conclude that brief exposure to certain fatty acid amides, such as AEA and olvanil activate nodose but not jugular C-fiber terminals in the lungs. We hypothesize that this is because the nodose C-fiber terminals are equipped with a temperature-dependent mechanism for effectively and rapidly transporting the TRPV1 agonists so that they gain access to the intracellular binding sites on TRPV1. This transport mechanism may be differently expressed in two distinct subtypes of pulmonary C-fiber terminals innervating the same tissue.

PAR-2 agonists activate trigeminal nociceptors and induce functional competence in the delta opioid receptor

The role of protease activated receptor-2 (PAR-2) activation in trigeminal nociception and in induction of functional competence in the delta opioid receptor (DOR) is not known. In this study, we evaluated whether agonists of PAR-2 activate the capsaicin-sensitive subclass of trigeminal nociceptors in a PLC-PKC-dependent manner and induce functional competence in the DOR. Adult male rat trigeminal ganglion (TG) cultured neurons were treated with the PAR-2 agonist (SL-NH2) or an enzyme activator of PAR (trypsin) and the activation of TG nociceptors was assessed using three independent methods: neuropeptide release, calcium influx, and whole cell patch-clamp. The specificity of SL-NH2 and trypsin responses was evaluated using TG cultures transfected with siRNA against PAR-2. The in vivo role of PAR-2 activation was determined measuring SL-NH2 and trypsin-evoked nocifensive behavior and increase in blood flow. Trigeminal neurons were treated with SL-NH2/vehicle and then the DOR agonist to determine DOR inhibition of evoked neuropeptide release and cAMP accumulation. The results showed that SL-NH2 (100 microM) and trypsin (1-600 nM) activate TG nociceptors, which is partly reversible by the PKC inhibitor bisindolylmaleimide (500 nM) and by ruthenium red (10 microM). In cultures treated with siRNA against PAR-2, both SL-NH2 and trypsin responses were significantly diminished. Both SL-NH2 and trypsin evoke nocifensive behavior and increases in blood flow in an orofacial pain model. Application of SL-NH2 rapidly produced functional competence of DOR for inhibiting nociceptor function. In inflamed tissue, endogenous proteases may activate TG nociceptors and generate pain. Moreover, activation of PAR-2 can also induce functional competence in DOR.

Capsaicin, transient receptor potential (TRP) protein subfamilies and the particular relationship between capsaicin receptors and small primary sensory neurons

A number of subfamilies of the capsaicin receptor, collectively called TRP, have been reported since the discovery of vanilloid receptor 1 (VR1). The term 'TRP' is derived from 'transient receptor potential', which means the transient and rapid defect of reaction following long stimulation with light in the photoreceptor cells of mutant Drosophila. The common features of TRP family members are the centrally situated six transmembrane domain, in which an ion channel is located, three to four ankirin repeats at the N-terminus and a TRP domain comprising 25 amino acids at the C-terminus. The TRP family members are present in animals, including invertebrates and vertebrates, and in the cells in various tissues in individual animals. During evolution, the original TRP seems to have acquired a wide variety of functions related to sensing the inner or outer environment (e.g. to sensing light (Drosophila), osmolarity, protons, temperature, ligands and mechanical force). In mammals, the TRPV subfamily is exclusively expressed in small- to medium-sized primary sensory neurons that also co-express some chemical markers (i.e. isolectin B4 (IB4), fluoride-resistant acid phosphatase (FRAP), the P2X3 purinoceptor (a receptor provoked by ATP-induced nociception) and Ret, a glial cell line-derived neurotrophic factor receptor). There is a paradox in that regardless of the marked or complete loss of noxious, small sensory neurons (polymodal nociceptors) in mice treated with capsaicin during the neonatal period, as well as in VR1 (TRPV1)-deficient knock-out mice, the responses to noxious heat are normal. Regarding the paradox in mice treated with capsaicin as neonates, our explanation is that although capsaicin probably reduces the number of a subgroup of small neurons (IB4-, VR1+), the remaining IB4+ (VR1-) neurons can sense noxious heat normally. One working hypothesis is that mice lacking TRPV1/2 can sense noxious heat under normal conditions, presumably via another still unknown pathway, and TRPV1 has been suggested to be involved in noxious heat transduction under pathological conditions, such as inflammation and tissue injury. Further studies will be required to clarify these complexities. Mice treated with capsaicin as neonates would provide a model to investigate the above paradoxes, as would TRPV1-knock-out mice, although different mechanisms may be operating in the two models.

TRPV1 is a novel target for omega-3 polyunsaturated fatty acids

Omega-3 (n-3) fatty acids are essential for proper neuronal function, and they possess prominent analgesic properties, yet their underlying signalling mechanisms are unclear. Here we show that n-3 fatty acids interact directly with TRPV1, an ion channel expressed in nociceptive neurones and brain. These fatty acids activate TRPV1 in a phosphorylation-dependent manner, enhance responses to extracellular protons, and displace binding of the ultrapotent TRPV1 ligand [3H]resiniferatoxin. In contrast to their agonistic properties, n-3 fatty acids competitively inhibit the responses of vanilloid agonists. These actions occur in mammalian cells in the physiological concentration range of 1-10 mum. Significantly, docosahexaenoic acid exhibits the greatest efficacy as an agonist, whereas eicosapentaenoic acid and linolenic acid are markedly more effective inhibitors. Similarly, eicosapentaenoic acid but not docosahexaenoic acid profoundly reduces capsaicin-evoked pain-related behaviour in mice. These effects are independent of alterations in membrane elasticity because the micelle-forming detergent Triton X-100 only minimally affects TRPV1 properties. Thus, n-3 fatty acids differentially regulate TRPV1 and this form of signalling may contribute to their biological effects. Further, these results suggest that dietary supplementation with selective n-3 fatty acids would be most beneficial for the treatment of pain.

The cannabinoid WIN 55,212-2 inhibits transient receptor potential vanilloid 1 (TRPV1) and evokes peripheral antihyperalgesia via calcineurin

Cannabinoids can evoke antihyperalgesia and antinociception at a peripheral site of action. However, the signaling pathways mediating these effects are not clearly understood. We tested the hypothesis that certain cannabinoids directly inhibit peripheral capsaicin-sensitive nociceptive neurons by dephosphorylating and desensitizing transient receptor potential vanilloid 1 (TRPV1) via a calcium calcineurin-dependent mechanism. Application of the cannabinoid WIN 55,212-2 (WIN) to cultured trigeminal (TG) neurons or isolated skin biopsies rapidly and significantly inhibited capsaicin-activated inward currents and neuropeptide exocytosis by a mechanism requiring the presence of extracellular calcium. The inhibitory effect did not involve activation of G protein-coupled cannabinoid receptors, because neither pertussis toxin nor GDPbetaS treatments altered the WIN effect. However, application of WIN-activated calcineurin, as measured by nuclear translocation of the nuclear factor of activated T cells (NFAT)c4 transcription factor, dephosphorylated TRPV1. The WIN-induced desensitization of TRPV1 was mediated by calcineurin, because the application of structurally distinct calcineurin antagonists (calcineurin autoinhibitory peptide and cyclosporine/cyclophilin complex) abolished WIN-induced inhibition of capsaicin-evoked inward currents and neuropeptide exocytosis. This mechanism also contributed to peripheral antinociceptive/antihyperalgesic effects of WIN because pretreatment with the calcineurin antagonist calcineurin autoinhibitory peptide (CAIP) significantly reduced peripherally mediated WIN effects in two behavioral models. Collectively, these data demonstrate that cannabinoids such as WIN directly inhibit TRPV1 functional activities via a calcineurin pathway that represents a mechanism of cannabinoid actions at peripheral sites.

Signaling mechanisms of down-regulation of voltage-activated Ca2+ channels by transient receptor potential vanilloid type 1 stimulation with olvanil in primary sensory neurons

Olvanil ((N-vanillyl)-9-oleamide), a non-pungent transient receptor potential vanilloid type 1 agonist, desensitizes nociceptors and alleviates pain. But its molecular targets and signaling mechanisms are little known. Calcium influx through voltage-activated Ca(2+) channels plays an important role in neurotransmitter release and synaptic transmission. Here we determined the effect of olvanil on voltage-activated Ca(2+) channel currents and the signaling pathways in primary sensory neurons. Whole-cell voltage-clamp recordings were performed in acutely isolated rat dorsal root ganglion neurons. Olvanil (1 microM) elicited a delayed but sustained inward current, and caused a profound inhibition (approximately 60%) of N-, P/Q-, L-, and R-type voltage-activated Ca(2+) channel current. Pretreatment with a specific transient receptor potential vanilloid type 1 antagonist or intracellular application of 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid abolished the inhibitory effect of olvanil on voltage-activated Ca(2+) channel current. Calmodulin antagonists (ophiobolin-A and calmodulin inhibitory peptide) largely blocked the effect of olvanil and capsaicin on voltage-activated Ca(2+) channel current. Furthermore, calcineurin (protein phosphatase 2B) inhibitors (deltamethrin and FK-506) eliminated the effect of olvanil on voltage-activated Ca(2+) channel current. Notably, 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid, calmodulin antagonists, and calcineurin inhibitors each alone significantly increased the amplitude of voltage-activated Ca(2+) channel current. In addition, double immunofluorescence labeling revealed that olvanil induced a rapid internalization of Ca(V)2.2 immunoreactivity from the membrane surface of dorsal root ganglion neurons. Collectively, this study suggests that stimulation of non-pungent transient receptor potential vanilloid type 1 inhibits voltage-activated Ca(2+) channels through a biochemical pathway involving intracellular Ca(2+)-calmodulin and calcineurin in nociceptive neurons. This new information is important for our understanding of the signaling mechanisms of desensitization of nociceptors by transient receptor potential vanilloid type 1 analogues and the feedback regulation of intracellular Ca(2+) and voltage-activated Ca(2+) channels in nociceptive sensory neurons.

Comparison of P2X and TRPV1 receptors in ganglia or primary culture of trigeminal neurons and their modulation by NGF or serotonin

BACKGROUND

Cultured sensory neurons are a common experimental model to elucidate the molecular mechanisms of pain transduction typically involving activation of ATP-sensitive P2X or capsaicin-sensitive TRPV1 receptors. This applies also to trigeminal ganglion neurons that convey pain inputs from head tissues. Little is, however, known about the plasticity of these receptors on trigeminal neurons in culture, grown without adding the neurotrophin NGF which per se is a powerful algogen. The characteristics of such receptors after short-term culture were compared with those of ganglia. Furthermore, their modulation by chronically-applied serotonin or NGF was investigated.

RESULTS

Rat or mouse neurons in culture mainly belonged to small and medium diameter neurons as observed in sections of trigeminal ganglia. Real time RT-PCR, Western blot analysis and immunocytochemistry showed upregulation of P2X(3) and TRPV1 receptors after 1-4 days in culture (together with their more frequent co-localization), while P2X(2) ones were unchanged. TRPV1 immunoreactivity was, however, lower in mouse ganglia and cultures. Intracellular Ca(2+) imaging and whole-cell patch clamping showed functional P2X and TRPV1 receptors. Neurons exhibited a range of responses to the P2X agonist alpha, beta-methylene-adenosine-5'-triphosphate indicating the presence of homomeric P2X(3) receptors (selectively antagonized by A-317491) and heteromeric P2X(2/3) receptors. The latter were observed in 16 % mouse neurons only. Despite upregulation of receptors in culture, neurons retained the potential for further enhancement of P2X(3) receptors by 24 h NGF treatment. At this time point TRPV1 receptors had lost the facilitation observed after acute NGF application. Conversely, chronically-applied serotonin selectively upregulated TRPV1 receptors rather than P2X(3) receptors.

CONCLUSION

Comparing ganglia and cultures offered the advantage of understanding early adaptive changes of nociception-transducing receptors of trigeminal neurons. Culturing did not prevent differential receptor upregulation by algogenic substances like NGF or serotonin, indicating that chronic application led to distinct plastic changes in the molecular mechanisms mediating pain on trigeminal nociceptors.

Kinetics of penetration influence the apparent potency of vanilloids on TRPV1

Evidence that the ligand binding site of TRPV1 lies on the inner face of the plasma membrane and that much of the TRPV1 itself is localized to internal membranes suggests that the rate of ligand entry into the cell may be an important determinant of the kinetics of ligand action. In this study, we synthesized a BODIPY TR-labeled fluorescent capsaicin analog (CHK-884) so that we could directly measure ligand entry. We report that CHK-884 penetrated only slowly into Chinese hamster ovary (CHO) cells expressing rat TRPV1, with a t1/2 of 30 +/- 4 min, and localized in the endoplasmic reticulum and Golgi. Although CHK-884 was only weakly potent for TRPV1 binding (Ki = 6400 +/- 230 nM), it was appreciably more potent when assayed by intracellular calcium imaging and was 3.2-fold more potent with a 1-h incubation time (37 nM) than with a 5-min incubation time. Olvanil, a highly lipophilic vanilloid, yielded an EC50 of 4.3 nM upon intracellular calcium imaging with an incubation time of 1 h, compared with an EC50 value of 29.5 nM for calcium imaging assayed at 5 min. Likewise, the antagonist 5-iodo-resiniferatoxin (5-iodo-RTX) displayed a Ki of 4.2 pM if incubated with CHO-TRPV1 cells for 2 h before addition of capsaicin compared with 1.5 nM if added simultaneously. We conclude that some vanilloids may have slow kinetics of uptake; this slow uptake may affect assessment of structure activity relations and may represent a significant factor for vanilloid drug design.

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

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

Sensory nerves, neurogenic inflammation and pain: missing components of alternative irritation strategies? A review and a potential strategy

The eyes and skin are highly innervated by sensory nerves; stimulation of these nerves by irritants may give rise to neurogenic inflammation, leading to sensory irritation and pain. Few in vitro models of neurogenic inflammation have been described in conjunction with alternative skin and eye irritation methods, despite the fact that the sensory innervation of these organs is well-documented. To date, alternative approaches to the Draize skin and eye irritation tests have proved largely successful at classifying severe irritants, but are generally poor at discriminating between agents with mild to moderate irritant potential. We propose that the development of in vitro models for the prediction of sensory stimulation will assist in the re-classification of the irritant potential of agents that are under-predicted by current in vitro strategies. This review describes the range of xenobiotics known to cause inflammation and pain through the stimulation of sensory nerves, as well as the endogenous mediators and receptor types that are involved. In particular, it focuses on the vanilloid receptor, its activators and its regulation, as these receptors function as integrators of responses to numerous noxious stimuli. Cell culture models and ex vivo preparations that have the potential to serve as predictors of sensory irritation are also described. In addition, as readily available sensory neuron cell line models are few in number, stem cell lines (with the capacity to differentiate into sensory neurons) are explored. Finally, a preliminary strategy to enable assessment of whether incorporation of a sensory component will enhance the predictive power of current in vitro eye and skin testing strategies is proposed.

Identification and characterisation of SB-366791, a potent and selective vanilloid receptor (VR1/TRPV1) antagonist

Vanilloid receptor-1 (TRPV1) is a non-selective cation channel, predominantly expressed by peripheral sensory neurones, which is known to play a key role in the detection of noxious painful stimuli, such as capsaicin, acid and heat. To date, a number of antagonists have been used to study the physiological role of TRPV1; however, antagonists such as capsazepine are somewhat compromised by non-selective actions at other receptors and apparent modality-specific properties. SB-366791 is a novel, potent, and selective, cinnamide TRPV1 antagonist isolated via high-throughput screening of a large chemical library. In a FLIPR-based Ca(2+)-assay, SB-366791 produced a concentration-dependent inhibition of the response to capsaicin with an apparent pK(b) of 7.74 +/- 0.08. Schild analysis indicated a competitive mechanism of action with a pA2 of 7.71. In electrophysiological experiments, SB-366791 was demonstrated to be an effective antagonist of hTRPV1 when activated by different modalities, such as capsaicin, acid or noxious heat (50 degrees C). Unlike capsazepine, SB-366791 was also an effective antagonist vs. the acid-mediated activation of rTRPV1. With the aim of defining a useful tool compound, we also profiled SB-366791 in a wide range of selectivity assays. SB-366791 had a good selectivity profile exhibiting little or no effect in a panel of 47 binding assays (containing a wide range of G-protein-coupled receptors and ion channels) and a number of electrophysiological assays including hippocampal synaptic transmission and action potential firing of locus coeruleus or dorsal raphe neurones. Furthermore, unlike capsazepine, SB-366791 had no effect on either the hyperpolarisation-activated current (I(h)) or Voltage-gated Ca(2+)-channels (VGCC) in cultured rodent sensory neurones. In summary, SB-366791 is a new TRPV1 antagonist with high potency and an improved selectivity profile with respect to other commonly used TRPV1 antagonists. SB-366791 may therefore prove to be a useful tool to further study the biology of TRPV1.

High-affinity partial agonists of the vanilloid receptor

The vanilloid receptor VR1 is a polymodal nociceptor sensitive to capsaicin, protons, and heat. Because VR1 represents an attractive therapeutic target for conditions ranging from long-term pain to bladder hyperreflexia, we and other groups have sought to develop novel ligands with enhanced potencies and novel pharmacological properties. Here, we characterize two compounds, N-[2-(3,4-dimethylbenzyl)-3-(pivaloyloxy)propyl]-N'-[4-(methylsulfonylamino)benzyl]thiourea (JYL827) and N-(4-tert-butylbenzyl)-N'-[3-methoxy-4-(methylsulfonylamino)benzyl]thiourea (JYL1511), that function as partial agonists for rat VR1 heterologously expressed in Chinese hamster ovary cells. Both compounds showed substantially enhanced potency, inhibiting [3H] resiniferatoxin binding with Ki values of 29.3 +/- 7.6 and 50.4 +/- 16.5 nM, respectively, compared with 1810 +/- 270 nM for capsaicin. The compounds showed different extents of partial agonism, 6.8 +/- 0.7% and 17.4 +/- 0.6%, respectively, and the expected corresponding degrees of partial antagonism (93.9 +/- 0.9 and 84.1 +/- 3.2%, respectively). Their IC50 values for antagonism of 45Ca2+ uptake in response to capsaicin were 67.3 +/- 24.9 nM and 3.4 +/- 0.5 nM, respectively. Protons, temperature, and protein kinase C all function as coactivators/modulators of rVR1. All enhanced the extent of partial agonism of JYL827 and JYL1511. Thus, at pH 5.5, for example, the extents of partial agonism increased to 54.9 +/- 2.5% and to 90.7 +/- 1.7%, respectively, relative to the response elicited by 300 nM capsaicin. The extents of partial antagonism decreased correspondingly. Compounds such as JYL827 and JYL1511 now permit exploration of the potential utility of partial agonists of rVR1 in animal models. Our results emphasize, moreover, the strong dependence of such partial agonists on other modulators of rVR1 and predict that their biological behavior will depend strongly on biological context.

Effect of olvanil on the afferent and efferent function of capsaicin-sensitive C-fibers in guinea pig airways

The aim of the present study was to examine the ability of the nonpungent vanilloid VR1 receptor agonist, olvanil, to activate the afferent and efferent function of capsaicin-sensitive C-fibers in guinea pig airways. We found that while capsaicin (10 nM-10 microM) and resiniferatoxin (0.1 nM-1.0 microM) evoked a robust contraction of the guinea pig trachea in vitro, olvanil (10 nM-10 microM) was a weak spasmogen. In addition, pretreatment with olvanil caused only a minor reduction of subsequent responses to capsaicin or resiniferatoxin. Using single fiber recording from guinea pig airway C-fibers, we found that olvanil (10 microM) did not evoke action potential discharge although these fibers responded vigorously to capsaicin after prolonged treatment with olvanil (10 microM). These findings are indicative of significant differences in the relative sensitivity of vanilloid VR1 receptor-transfected cells and the peripheral terminals of airway C-fibers to pungent and nonpungent vanilloid VR1 receptor agonists.

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

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

Pharmacology of vanilloids at recombinant and endogenous rat vanilloid receptors

This study compared the actions of members of five different chemical classes of vanilloid agonists at the recombinant rat vanilloid VR1 receptor expressed in HEK293 cells, and at endogenous vanilloid receptors on dorsal root ganglion cells and sensory nerves in the rat isolated mesenteric arterial bed. In mesenteric beds, vanilloids elicited dose-dependent vasorelaxation with the rank order of potency: resiniferatoxin>>capsaicin=olvanil>phorbol 12-phenyl-acetate 13-acetate 20-homovanillate (PPAHV)>isovelleral. Scutigeral was inactive. Responses were abolished by capsaicin pretreatment and inhibited by ruthenium red. In VR1-HEK293 cells and dorsal root ganglion neurones, Ca(2+) responses were induced by resiniferatoxin>capsaicin=olvanil>PPAHV; all four were full agonists. Isovelleral and scutigeral were inactive. The resiniferatoxin-induced Ca(2+) response had a distinct kinetic profile. Olvanil had a Hill coefficient of approximately 1 whilst capsaicin, resiniferatoxin and PPAHV had Hill coefficients of approximately 2 in VR1-HEK293 cells. The capsaicin-induced Ca(2+) response was inhibited in a concentration-dependent manner by ruthenium red>capsazepine>isovelleral. These data show that resiniferatoxin, capsaicin, olvanil and PPAHV, but not scutigeral and isovelleral, are agonists at recombinant rat VR1 receptors and endogenous vanilloid receptors on dorsal root ganglion neurones and in the rat mesenteric arterial bed. The vanilloids display the same relative potencies (resiniferatoxin>capsaicin=olvanil>PPAHV) in all of the bioassays.