Direct phosphorylation of capsaicin receptor VR1 by protein kinase Cepsilon and identification of two target serine residues

Direct inhibition of the cold-activated TRPM8 ion channel by Gαq

Activation of the TRPM8 ion channel in sensory nerve endings produces a sensation of pleasant coolness. Here we show that inflammatory mediators such as bradykinin and histamine inhibit TRPM8 in intact sensory nerves, but do not do so via conventional signalling pathways. The G-protein subunit Ga_q instead binds to TRPM8 and when activated by a Gq-coupled receptor directly inhibits ion channel activity. Deletion of Ga_q largely abolished inhibition of TRPM8, and inhibition was rescued by a Ga_q chimera whose ability to activate downstream signalling pathways was completely ablated. Activated Ga_q protein, but not Gβγ, potently inhibits TRPM8 in excised patches. We conclude that Ga_q pre-forms a complex with TRPM8 and inhibits activation of TRPM8, following activation of G-protein coupled receptors, by a direct action. This signalling mechanism may underlie the abnormal cold sensation caused by inflammation.

Functional interactions between NMDA receptors and TRPV1 in trigeminal sensory neurons mediate mechanical hyperalgesia in the rat masseter muscle

The NMDA and TRPV1 receptors that are expressed in sensory neurons have been independently demonstrated to play important roles in peripheral pain mechanisms. In the present study, we investigated whether the 2 receptor-channel systems form a functional complex that provides the basis for the development of mechanical hyperalgesia. In the masseter muscle, direct application of NMDA induced a time-dependent increase in mechanical sensitivity, which was significantly blocked when the muscle was pretreated with a specific TRPV1 antagonist, AMG9810. The NR1 subunit of the NMDA receptor and TRPV1 were coexpressed in 32% of masseter afferents in trigeminal ganglia (TG). Furthermore, NR1 and NR2B formed protein-protein complexes with TRPV1 in TG as demonstrated by coimmunoprecipitation experiments. Calcium imaging analyses further corroborated that NMDA and TRPV1 receptors functionally interact. In TG culture, application of NMDA resulted in phosphorylation of serine, but not threonine or tyrosine, residues of TRPV1 in a time course similar to that of the development of NMDA-induced mechanical hyperalgesia. The NMDA-induced phosphorylation was significantly attenuated by CaMKII and PKC inhibitors, but not by a PKA inhibitor. Consistent with the biochemical data, the NMDA-induced mechanical hyperalgesia was also effectively blocked when the muscle was pretreated with a CaMKII or PKC inhibitor. Thus, NMDA receptors and TRPV1 functionally interact via CaMKII and PKC signaling cascades and contribute to mechanical hyperalgesia. These data offer novel mechanisms by which 2 ligand-gated channels in sensory neurons interact and reinforce the notion that TRPV1 functions as a signal integrator under pathological conditions.

Group III metabotropic glutamate receptors and transient receptor potential vanilloid 1 co-localize and interact on nociceptors

Several lines of evidence indicate group III metabotropic glutamate receptors (mGluRs) have systemic anti-hyperalgesic effects. We hypothesized this could occur through modulation of transient receptor potential vanilloid 1 (TRPV1) receptors on nociceptors. To address this question we performed anatomical studies to determine if group III mGluRs were expressed on cutaneous axons and if they co-localized with TRPV1. Immunostaining at the electron microscopic level demonstrated that 22% of unmyelinated axons labeled for mGluR8. Immunostaining at the light microscopic level in lumbar dorsal root ganglia (DRG) demonstrated that 80% and 28% of neurons labeled for mGluR8 or TRPV1, respectively. Of those neurons labeled for mGluR8, 25% labeled for TRPV1; of those labeled for TRPV1, 71% labeled for mGluR8. In behavior studies intraplantar injection of the group III mGluR agonist, L-(+)-2-amino-4-phosphonobutyric acid (L-AP-4: 0.1, 1.0, and 10.0 μM) had no effect on paw withdrawal latency (PWL) to heat in naïve rats but administration of 10 μM L-AP-4 prior to 0.05% capsaicin (CAP), significantly attenuated CAP-induced lifting/licking and reduced flinching behavior. The L-AP-4 effect was specific since administration of a group III antagonist α-methyl-3-methyl-4-phosphonophenylglycine (UBP1112) (100μM) blocked the L-AP-4 effect on CAP, resulting in behaviors similar to CAP alone. Intraplantar injection of UBP1112 alone did not result in nociceptive behaviors, indicating group III mGluRs are not tonically active. Finally, the anti-hyperalgesic effect of group III in this paradigm was local and not systemic since intraplantar administration of L-AP-4 in one hind paw did not attenuate nociceptive behaviors following CAP injection in the contralateral hind paw. Adenyl cyclase/cyclic AMP/PKA may be the second messenger pathway linking these two receptor families because intraplantar injection of forskolin (FSK, 10 μM) reduced PWL to heat and L-AP-4 reversed this FSK effect. Taken together, these results suggest group III mGluRs can negatively modulate TRPV1 through inhibition of adenyl cyclase and downstream intracellular activity, blocking TRPV1-induced activation of nociceptors.

N-glycosylation determines ionic permeability and desensitization of the TRPV1 capsaicin receptor

The balance of glycosylation and deglycosylation of ion channels can markedly influence their function and regulation. However, the functional importance of glycosylation of the TRPV1 receptor, a key sensor of pain-sensing nerves, is not well understood, and whether TRPV1 is glycosylated in neurons is unclear. We report that TRPV1 is N-glycosylated and that N-glycosylation is a major determinant of capsaicin-evoked desensitization and ionic permeability. Both N-glycosylated and unglycosylated TRPV1 was detected in extracts of peripheral sensory nerves by Western blotting. TRPV1 expressed in HEK-293 cells exhibited various degrees of glycosylation. A mutant of asparagine 604 (N604T) was not glycosylated but did not alter plasma membrane expression of TRPV1. Capsaicin-evoked increases in intracellular calcium ([Ca(2+)](i)) were sustained in wild-type TRPV1 HEK-293 cells but were rapidly desensitized in N604T TRPV1 cells. There was marked cell-to-cell variability in capsaicin responses and desensitization between individual cells expressing wild-type TRPV1 but highly uniform responses in cells expressing N604T TRPV1, consistent with variable levels of glycosylation of the wild-type channel. These differences were also apparent when wild-type or N604T TRPV1-GFP fusion proteins were expressed in neurons from trpv1(-/-) mice. Capsaicin evoked a marked, concentration-dependent increase in uptake of the large cationic dye YO-PRO-1 in cells expressing wild-type TRPV1, indicative of loss of ion selectivity, that was completely absent in cells expressing N604T TRPV1. Thus, TRPV1 is variably N-glycosylated and glycosylation is a key determinant of capsaicin regulation of TRPV1 desensitization and permeability. Our findings suggest that physiological or pathological alterations in TRPV1 glycosylation would affect TRPV1 function and pain transmission.

Generation of a pain memory in the primary afferent nociceptor triggered by PKCε activation of CPEB

Isolectin B(4)-positive [IB(4)(+)] primary afferent nociceptors challenged with an inflammatory or neuropathic insult develop a PKCε-dependent long-lasting hyperalgesic response to a subsequent challenge by the proinflammatory cytokine prostaglandin E(2) (PGE(2)), a phenomenon known as hyperalgesic priming. Here we demonstrate that the neuroplasticity underlying nociceptor priming requires 72 h to be established; rats that have been challenged with the inflammatory mediator TNFα 24 or 48 h ahead of PGE(2) do not show the enhanced and prolonged hyperalgesic response by which primed IB(4)(+)-nociceptors are being characterized. Moreover, as the underlying plasticity can be interrupted by the peripheral administration of the protein translation inhibitor anisomycin it is reflected by changes in the peripheral protein expression pattern. Finally, the induction of priming by the selective PKCε agonist, psi ε receptor for activated c kinase (ψεRACK) can be prevented, but not reversed by intrathecal injections of antisense oligodeoxynucleotides for the cytoplasmic polyadenylation element binding protein (CPEB) mRNA, a master regulator of protein translation that coimmunoprecipitated with PKCε and is almost exclusively expressed by IB(4)(+)-nociceptors. Our results suggest that CPEB is downstream of PKCε in the cellular signaling cascade responsible for the induction of priming, raising the intriguing possiblity that prion-like misfolding could be a responsible mechanism for the chronification of pain.

PKCε phosphorylation of the sodium channel NaV1.8 increases channel function and produces mechanical hyperalgesia in mice

Mechanical hyperalgesia is a common and potentially disabling complication of many inflammatory and neuropathic conditions. Activation of the enzyme PKCε in primary afferent nociceptors is a major mechanism that underlies mechanical hyperalgesia, but the PKCε substrates involved downstream are not known. Here, we report that in a proteomic screen we identified the NaV1.8 sodium channel, which is selectively expressed in nociceptors, as a PKCε substrate. PKCε-mediated phosphorylation increased NaV1.8 currents, lowered the threshold voltage for activation, and produced a depolarizing shift in inactivation in wild-type - but not in PKCε-null - sensory neurons. PKCε phosphorylated NaV1.8 at S1452, and alanine substitution at this site blocked PKCε modulation of channel properties. Moreover, a specific PKCε activator peptide, ψεRACK, produced mechanical hyperalgesia in wild-type mice but not in Scn10a-/- mice, which lack NaV1.8 channels. These studies demonstrate that NaV1.8 is an important, direct substrate of PKCε that mediates PKCε-dependent mechanical hyperalgesia.

Agonist-dependent potentiation of vanilloid receptor transient receptor potential vanilloid type 1 function by stilbene derivatives

Transient receptor potential vanilloid type 1 (TRPV1) is a nonselective cation channel activated by capsaicin, low pH, and noxious heat and plays a key role in nociception. Understanding mechanisms for functional modulation of TRPV1 has important implications. One characteristic of TRPV1 is that channel activity induced by either capsaicin or other activators can be sensitized or modulated by factors involving different cell signaling mechanisms. In this study, we describe a novel mechanism for the modulation of TRPV1 function: TRPV1 function is modulated by 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS) and its analogs. We found that, in rat dorsal root ganglion neurons, although DIDS did not induce the activation of TRPV1 per se but drastically increased the TRPV1 currents induced by either capsaicin or low pH. DIDS also blocked the tachyphylaxis of the low pH-induced TRPV1 currents. 4-Acetamido-4'-isothiocyanatostilbene-2,2'-disulfonic acid (SITS), a DIDS analog, failed to enhance the capsaicin-evoked TRPV1 current but increased the low pH-evoked TRPV1 currents, with an effect comparable with that of DIDS. SITS also blocked the low pH-induced tachyphylaxis. DIDS also potentiated the currents of TRPV1 channels expressed in human embryonic kidney 293 cells, with an effect of left-shifting the concentration-response curve of the capsaicin-induced TRPV1 currents. This study demonstrates that DIDS and SITS, traditionally used chloride channel blockers, can modify TRPV1 channel function in an agonist-dependent manner. The results provide new input for understanding TRPV1 modulation and developing new modulators of TRPV1 function.

The thermo-TRP ion channel family: properties and therapeutic implications

The thermo-transient receptor potentials (TRPs), a recently discovered family of ion channels activated by temperature, are expressed in primary sensory nerve terminals where they provide information about thermal changes in the environment. Six thermo-TRPs have been characterised to date: TRP vanilloid (TRPV) 1 and 2 are activated by painful levels of heat, TRPV3 and 4 respond to non-painful warmth, TRP melastatin 8 is activated by non-painful cool temperatures, while TRP ankyrin (TRPA) 1 is activated by painful cold. The thermal thresholds of many thermo-TRPs are known to be modulated by extracellular mediators, released by tissue damage or inflammation, such as bradykinin, PG and growth factors. There have been intensive efforts recently to develop antagonists of thermo-TRP channels, particularly of the noxious thermal sensors TRPV1 and TRPA1. Blockers of these channels are likely to have therapeutic uses as novel analgesics, but may also cause unacceptable side effects. Controlling the modulation of thermo-TRPs by inflammatory mediators may be a useful alternative strategy in developing novel analgesics.

Activation of TRPV1 by capsaicin induces functional kinin B(1) receptor in rat spinal cord microglia

Background

The kinin B₁ receptor (B₁R) is upregulated by pro-inflammatory cytokines and oxydative stress, which are enhanced by transient receptor potential vanilloid subtype 1 (TRPV1) activation. To examine the link between TRPV1 and B₁R in inflammatory pain, this study aimed to determine the ability of TRPV1 to regulate microglial B₁R expression in the spinal cord dorsal horn, and the underlying mechanism.

Methods

B₁R expression (mRNA, protein and binding sites) was measured in cervical, thoracic and lumbar spinal cord in response to TRPV1 activation by systemic capsaicin (1-50 mg/kg, s.c) in rats pre-treated with TRPV1 antagonists (capsazepine or SB-366791), the antioxidant N-acetyl-L-cysteine (NAC), or vehicle. B₁R function was assessed using a tail-flick test after intrathecal (i.t.) injection of a selective B₁R agonist (des-Arg⁹-BK), and its microglial localization was investigated by confocal microscopy with the selective fluorescent B₁R agonist, [N^α-bodipy]-des-Arg⁹-BK. The effect of i.t. capsaicin (1 μg/site) was also investigated.

Results

Capsaicin (10 to 50 mg/kg, s.c.) enhanced time-dependently (0-24h) B₁R mRNA levels in the lumbar spinal cord; this effect was prevented by capsazepine (10 mg/kg, i.p.; 10 μg/site, i.t.) and SB-366791 (1 mg/kg, i.p.; 30 μg/site, i.t.). Increases of B₁R mRNA were correlated with IL-1β mRNA levels, and they were significantly less in cervical and thoracic spinal cord. Intrathecal capsaicin (1 μg/site) also enhanced B₁R mRNA in lumbar spinal cord. NAC (1 g/kg/d × 7 days) prevented B₁R up-regulation, superoxide anion production and NF-kB activation induced by capsaicin (15 mg/kg). Des-Arg⁹-BK (9.6 nmol/site, i.t.) decreased by 25-30% the nociceptive threshold at 1 min post-injection in capsaicin-treated rats (10-50 mg/kg) while it was without effect in control rats. Des-Arg⁹-BK-induced thermal hyperalgesia was blocked by capsazepine, SB-366791 and by antagonists/inhibitors of B₁R (SSR240612, 10 mg/kg, p.o.), glutamate NMDA receptor (DL-AP5, 10 μg/site, i.t.), substance P NK-1 receptor (RP-67580, 10 μg/site, i.t.) and nitric oxide synthase (L-NNA, 10 μg/site, i.t.). The B₁R fluorescent agonist was co-localized with an immunomarker of microglia (Iba-1) in spinal cord dorsal horn of capsaicin-treated rats.

Conclusion

This study highlights a new mechanism for B₁R induction via TRPV1 activation and establishes a link between these two pro-nociceptive receptors in inflammatory pain.

Hyperalgesic activity of kisspeptin in mice

Background

Kisspeptin is a neuropeptide known for its role in the hypothalamic regulation of the reproductive axis. Following the recent description of kisspeptin and its 7-TM receptor, GPR54, in the dorsal root ganglia and dorsal horns of the spinal cord, we examined the role of kisspeptin in the regulation of pain sensitivity in mice.

Results

Immunofluorescent staining in the mouse skin showed the presence of GPR54 receptors in PGP9.5-positive sensory fibers. Intraplantar injection of kisspeptin (1 or 3 nmol/5 μl) induced a small nocifensive response in naive mice, and lowered thermal pain threshold in the hot plate test. Both intraplantar and intrathecal (0.5 or 1 nmol/3 μl) injection of kisspeptin caused hyperalgesia in the first and second phases of the formalin test, whereas the GPR54 antagonist, p234 (0.1 or 1 nmol), caused a robust analgesia. Intraplantar injection of kisspeptin combined with formalin enhanced TRPV1 phosphorylation at Ser800 at the injection site, and increased ERK1/2 phosphorylation in the ipsilateral dorsal horn as compared to naive mice and mice treated with formalin alone.

Conclusion

These data demonstrate for the first time that kisspeptin regulates pain sensitivity in rodents and suggest that peripheral GPR54 receptors could be targeted by novel drugs in the treatment of inflammatory pain.

Real-Time Translocation and Function of PKCβII Isoform in Response to Nociceptive Signaling via the TRPV1 Pain Receptor

Serine/threonine protein kinase C βII isoform (PKCβII) or the pain receptor transient receptor potential vanilloid 1 (TRPV1) have been separately implicated in mediating heat hyperalgesia during inflammation or diabetic neuropathy. However, detailed information on the role of PKC βII in nociceptive signaling mediated by TRPV1 is lacking. This study presents evidence for activation and translocation of the PKC βII isoform as a signaling event in nociception mediated by activation of TRPV1 by capsaicin. We show that capsaicin induces translocation of cytosolic PKCβII isoform fused with enhanced green fluorescence protein (PKCβII-EGFP) in dorsal root ganglion (DRG) neurons. We also show capsaicin-induced translocation in Chinese Hamster Ovarian (CHO) cells co-transfected with TRPV1 and PKCβII-EGFP, but not in CHO cells expressing PKCβII-EGFP alone. By contrast, the PKC activator phorbol-12-myristate-13-acetate (PMA) induced translocation of PKCβII-EGFP which was sustained and independent of calcium or TRPV1. In addition PMA-induced sensitization of TRPV1 to capsaicin response in DRG neurons was attenuated by PKCβII blocker CGP 53353. Capsaicin response via TRPV1 in the DRG neurons was confirmed by TRPV1 antagonist AMG 9810. These results suggested a novel and potential signaling link between PKCβII and TRPV1. These cell culture models provide a platform for investigating mechanisms of painful neuropathies mediated by nociceptors expressing the pain sensing gene TRPV1, and its regulation by the PKC isoform PKCβII.

Antinociceptive effect of the essential oil of Zingiber zerumbet in mice: possible mechanisms

ETHNOPHARMACOLOGICAL RELEVANCE

Zingiber zerumbet (L.) Smith, a wild edible ginger species or locally known as "lempoyang", commonly used in the Malays traditional medicine as an appetizer or to treat stomachache, toothache, muscle sprain and as a cure for swelling sores and cuts.

AIM

The present study was conducted to investigate the possible mechanism of actions underlying the systemic antinociception activity of the essential oil of Zingiber zerumbet (EOZZ) in chemical-induced nociception tests in mice.

MATERIALS AND METHODS

Acetic acid-induced abdominal constriction, capsaicin-, glutamate- and phorbol 12-myristate 13-acetate-induced paw licking tests in mice were employed in the study. In all experiments, EOZZ was administered systemically at the doses of 50, 100, 200 and 300 mg/kg.

RESULTS

It was shown that EOZZ given to mice via intraperitoneal and oral routes at 50, 100, 200 and 300 mg/kg produced significant dose dependent antinociception when assessed using acetic acid-induced abdominal writing test with calculated mean ID(50) values of 88.84 mg/kg (80.88-97.57 mg/kg) and 118.8 mg/kg (102.5-137.8 mg/kg), respectively. Likewise, intraperitoneal administration of EOZZ at similar doses produced significant dose dependent inhibition of neurogenic pain induced by intraplantar injection of capsaicin (1.6 μg/paw), glutamate (10 μmol/paw) and phorbol 12-myristate 13-acetate (1.6μg/paw) with calculated mean ID(50) of 128.8 mg/kg (118.6-139.9 mg/kg), 124.8 mg/kg (111.4-139.7 mg/kg) and 40.29 (35.39-45.86) mg/kg, respectively. It was also demonstrated that pretreatment with l-arginine (100mg/kg, i.p.), a nitric oxide precursor significantly reversed antinociception produced by EOZZ suggesting the involvement of l-arginine/nitric oxide pathway. In addition, methylene blue (20mg/kg, i.p.) significantly enhanced antinociception produced by EOZZ. Administration of glibenclamide (10mg/kg, i.p.), an ATP-sensitive K(+) channel antagonist significantly reversed antinociceptive activity induced by EOZZ.

CONCLUSION

Together, the present results suggested that EOZZ-induced antinociceptive activity was possibly related to its ability to inhibit glutamatergic system, TRPV1 receptors as well as through activation of l-arginine/nitric oxide/cGMP/protein kinase C/ATP-sensitive K(+) channel pathway.

Roles of reactive oxygen and nitrogen species in pain

Peroxynitrite (PN; ONOO⁻) and its reactive oxygen precursor superoxide (SO; O₂•⁻) are critically important in the development of pain of several etiologies including pain associated with chronic use of opiates such as morphine (also known as opiate-induced hyperalgesia and antinociceptive tolerance). This is now an emerging field in which considerable progress has been made in terms of understanding the relative contributions of SO, PN, and nitroxidative stress in pain signaling at the molecular and biochemical levels. Aggressive research in this area is poised to provide the pharmacological basis for development of novel nonnarcotic analgesics that are based upon the unique ability to selectively eliminate SO and/or PN. As we have a better understanding of the roles of SO and PN in pathophysiological settings, targeting PN may be a better therapeutic strategy than targeting SO. This is because, unlike PN, which has no currently known beneficial role, SO may play a significant role in learning and memory. Thus, the best approach may be to spare SO while directly targeting its downstream product, PN. Over the past 15 years, our team has spearheaded research concerning the roles of SO and PN in pain and these results are currently leading to the development of solid therapeutic strategies in this important area.

Sensing hot and cold with TRP channels

The past decade has witnessed the cloning of a new family of ion channels that are responsive to temperature. Six of these transient receptor potential (TRP) channels are proposed to be involved in thermosensation and are located in sensory nerves and skin. The TRPV1, TRPV2, TRPV3, and TRPV4 channels have incompletely overlapping functions over a broad thermal range from warm to hot. Deletion of the individual TRPV1, TRPV3, and TRPV4 channels in mice has established their physiological role in thermosensation. In all cases thermosensation is not completely abolished - suggesting some functional redundancy among the channels. Notably, the TRPV2 channel is responsive to hot temperatures in heterologous systems, but its physiological relevance in vivo has not been established. Cool and cold temperatures are sensed by TRPM8 and TRPA1 family members. Currently, the pharmaceutical industry is developing agonists and antagonists for the various TRP channels. For instance, TRPV1 receptor agonists produce hypothermia, while antagonists induce hyperthermia. Recent investigations have found that different regions of the TRPV1 receptor are responsive to temperature, nociceptive stimuli, and various chemical agents. With this information, it has been possible to develop a TRPV1 compound that blocks responses to capsaicin and acid while leaving temperature sensitivity intact. These channels have important implications for hyperthermia research and may help to identify previously unexplored mechanisms in different tissues that are responsive to thermal stress.

Involvement of subtypes γ and ε of protein kinase C in colon pain induced by formalin injection

Protein kinase C (PKC) has been widely reported to participate in somatic pain; however, its role in visceral pain remains largely unclear. Using a colon inflammatory pain model by intracolonic injection of formalin in rats, the present study was to examine the role of PKC in visceral pain and determine which subtypes may be involved. The colon pain behavior induced by formalin injection could be enhanced by intrathecal pretreatment with a PKC activator (PMA), and alleviated by a PKC inhibitor (H-7). Wide dynamic range (WDR) neurons in the L6-S1 spinal dorsal horn that were responsive to colorectal distension were recorded extracellularly. It was found that neuronal activity was greatly increased following formalin injection. Microdialysis of PMA near the recorded neuron in the spinal dorsal horn facilitated the enhanced responsive activity induced by formalin injection, while H-7 inhibited significantly the enhanced response induced by formalin injection. Western blot analysis revealed that membrane translocation of PKC-γ and PKC-∊, but not other subtypes, in the spinal cord was obviously increased following formalin injection. Therefore, our findings suggest that PKC is actively involved in the colon pain induced by intracolonic injection of formalin. PKC-γ and PKC-∊ subtypes seem to significantly contribute to this process.

Activation characteristics of transient receptor potential ankyrin 1 and its role in nociception

Transient receptor potential (TRP) ankyrin 1 (TRPA1) is a Ca(2+)-permeant, nonselective cationic channel. It is predominantly expressed in the C afferent sensory nerve fibers of trigeminal and dorsal root ganglion neurons and is highly coexpressed with the nociceptive ion channel transient receptor potential vanilloid 1 (TRPV1). Several physical and chemical stimuli have been shown to activate the channel. In this study, we have used electrophysiological techniques and behavioral models to characterize the properties of TRPA1. Whole cell TRPA1 currents induced by brief application of lower concentrations of N-methyl maleimide (NMM) or allyl isothiocyanate (AITC) can be reversed readily by washout, whereas continuous application of higher concentrations of NMM or AITC completely desensitized the currents. The deactivation and desensitization kinetics differed between NMM and AITC. TRPA1 current amplitude increased with repeated application of lower concentrations of AITC, whereas saturating concentrations of AITC induced tachyphylaxis, which was more pronounced in the presence of extracellular Ca(2+). The outward rectification exhibited by native TRPA1-mediated whole cell and single-channel currents was minimal as compared with other TRP channels. TRPA1 currents were negatively modulated by protons and polyamines, both of which activate the heat-sensitive channel, TRPV1. Interestingly, neither protein kinase C nor protein kinase A activation sensitized AITC-induced currents, but each profoundly sensitized capsaicin-induced currents. Current-clamp experiments revealed that AITC produced a slow and sustained depolarization as compared with capsaicin. TRPA1 is also expressed at the central terminals of nociceptors at the caudal spinal trigeminal nucleus. Activation of TRPA1 in this area increases the frequency and amplitude of miniature excitatory or inhibitory postsynaptic currents. In behavioral studies, intraplantar and intrathecal administration of AITC induced more pronounced and prolonged changes in nociceptive behavior than those induced by capsaicin. In conclusion, the characteristics of TRPA1 we have delineated suggest that it might play a unique role in nociception.

Chronic alteration in phosphatidylinositol 4,5-biphosphate levels regulates capsaicin and mustard oil responses

There is an agreement that acute (in minutes) hydrolysis and accumulation of phosphatidylinositol 4,5-bisphosphate (PIP(2) ) modulate TRPV1 and TRPA1 activities. Because inflammation results in PIP(2) depletion, persisting for long periods (hours to days) in pain models and in the clinic, we examined whether chronic depletion and accumulation of PIP(2) affect capsaicin (CAP) and mustard oil (MO) responses. In addition, we wanted to evaluate whether the effects of PIP(2) depend on TRPV1 and TRPA1 coexpression and whether the PIP(2) actions vary in expression cells vs. sensory neurons. Chronic PIP(2) production was stimulated by overexpression of phosphatidylinositol-4-phosphate-5-kinase, and PIP(2) -specific phospholipid 5'-phosphatase was selected to reduce plasma membrane levels of PIP(2) . Our results demonstrate that CAP (100 nM) responses and receptor tachyphylaxis are not significantly influenced by chronic changes in PIP(2) levels in wild-type (WT) or TRPA1 null-mutant sensory neurons as well as CHO cells expressing TRPV1 alone or with TRPA1. However, low concentrations of CAP (20 nM) produced a higher response after PIP(2) depletion in cells containing TRPV1 alone but not TRPV1 together with TRPA1. MO (25 μM) responses were also not affected by PIP(2) in WT sensory neurons and cells coexpressing TRPA1 and TRPV1. In contrast, PIP(2) reduction leads to pronounced tachyphylaxis to MO in cells with both channels. Chronic effect of PIP(2) on TRPA1 activity depends on presence of the TRPV1 channel and cell type (CHO vs. sensory neurons). In summary, chronic alterations in PIP(2) levels regulate magnitude of CAP and MO responses as well as MO tachyphylaxis. This regulation depends on coexpression profile of TRPA1 and TRPV1 and cell type.

AKAP150-mediated TRPV1 sensitization is disrupted by calcium/calmodulin

Background

The transient receptor potential vanilloid type1 (TRPV1) is expressed in nociceptive sensory neurons and is sensitive to phosphorylation. A-Kinase Anchoring Protein 79/150 (AKAP150) mediates phosphorylation of TRPV1 by Protein Kinases A and C, modulating channel activity. However, few studies have focused on the regulatory mechanisms that control AKAP150 association with TRPV1. In the present study, we identify a role for calcium/calmodulin in controlling AKAP150 association with, and sensitization of, TRPV1.

Results

In trigeminal neurons, intracellular accumulation of calcium reduced AKAP150 association with TRPV1 in a manner sensitive to calmodulin antagonism. This was also observed in transfected Chinese hamster ovary (CHO) cells, providing a model for conducting molecular analysis of the association. In CHO cells, the deletion of the C-terminal calmodulin-binding site of TRPV1 resulted in greater association with AKAP150, and increased channel activity. Furthermore, the co-expression of wild-type calmodulin in CHOs significantly reduced TRPV1 association with AKAP150, as evidenced by total internal reflective fluorescence-fluorescence resonance energy transfer (TIRF-FRET) analysis and electrophysiology. Finally, dominant-negative calmodulin co-expression increased TRPV1 association with AKAP150 and increased basal and PKA-sensitized channel activity.

Conclusions

the results from these studies indicate that calcium/calmodulin interferes with the association of AKAP150 with TRPV1, potentially extending resensitization of the channel.

Activity-dependent targeting of TRPV1 with a pore-permeating capsaicin analog

The capsaicin receptor TRPV1 is the principal transduction channel for nociception. Excessive TRPV1 activation causes pathological pain. Ideal pain mangement requires selective inhibition of hyperactive pain-sensing neurons, but sparing normal nociception. We sought to determine whether it is possible to use activity-dependent TRPV1 agonists to identify nerves with excessive TRPV1 activity, as well as exploit the TRPV1 pore to deliver charged anesthetics for neuronal silencing. We synthesized a series of permanently charged capsaicinoids and found that one, cap-ET, efficaciously evoked TRPV1-dependent entry of Ca(2+) or the large cationic dye YO-PRO-1 comparably to capsaicin, but far smaller electrical currents. Cap-ET-induced YO-PRO-1 transport required permeation of both the agonist and the dye through the TRPV1 pore and could be enhanced by kinase activation or oxidative covalent modification. Moreover, cap-ET reduced capsaicin-induced currents by a voltage-dependent block of the pore. A low dose of cap-ET elicited entry of permanently charged Na(+) channel blockers to effectively suppress Na(+) currents in sensory neurons presensitized with oxidative chemicals. These results implicate therapeutic potential of these unique TRPV1 agonists exhibiting activity-dependent ion transport but of minimal pain-producing risks.

Insight into pain-inducing and -related gene expression: a challenge for development of novel targeted therapeutic approaches

The multidimensional issue of pain in relation to the need for efficient treatment has been the focus of extensive research. Gaining insight into the molecular mechanisms of pain and identifying specific genes and proteins as possible drug targets is strongly required considering that not all patients can be adequately treated with the currently available drugs. This up-to-date review aimed to summarize the findings of recent proteomic and genomic approaches in different types of pain to comment on their potential role in pain signaling pathways and to evaluate their possible contribution to the development of novel and possibly more targeted pain therapeutic strategies. Although pain treatment strategies have been greatly improved during the past century, no ideal targeted pain treatment has been developed. The development of modern and accurate platforms of technology for the study of genetics and physiology of pain has led to the identification of an increased number of altered genes and proteins that are involved in pain-related pathways. Through genomics and proteomics, pain-related genes and proteins, respectively, may be identified as diagnostic markers or drug targets improving therapeutic strategies. Furthermore, such molecular mediators of pain may reveal novel strategies for individualized pain management. The utilization of unique experimental approaches (through specific animal models) as well as powered genetic association studies conducted on appropriate populations is more than essential.

The role of protein kinase C epsilon in neural signal transduction and neurogenic diseases

Protein kinase C epsilon (PKC ɛ) is one of major isoforms in novel PKC family. Although it has been extensively characterized in the past decade, the role of PKC ɛ in neuron is still not well understood. Advances in molecular biology have now removed significant barriers to the direct investigation of PKC ɛ functions in vivo, and PKC ɛ has been increasingly implicated in the neural biological functions and associated neurogenic diseases. Recent studies have provided important insights into the influence of PKC ɛ on cortical processing at both the single cell level and network level. These studies provide compelling evidence that PKC ɛ could regulate distinct aspects of neural signal transduction and suggest that the coordinated actions of a number of molecular signals contribute to the specification and differentiation of PKC ɛ signal pathway in the developing brain.

Identification of a tetrameric assembly domain in the C terminus of heat-activated TRPV1 channels

Transient receptor potential (TRP) channels as cellular sensors are thought to function as tetramers. Yet, the molecular determinants governing channel multimerization remain largely elusive. Here we report the identification of a segment comprising 21 amino acids (residues 752-772 of mouse TRPV1) after the known TRP-like domain in the channel C terminus that functions as a tetrameric assembly domain (TAD). Purified recombinant C-terminal proteins of TRPV1-4, but not the N terminus, mediated the protein-protein interaction in an in vitro pulldown assay. Western blot analysis combined with electrophysiology and calcium imaging demonstrated that TAD exerted a robust dominant-negative effect on wild-type TRPV1. When fused with the membrane-tethered peptide Gap43, the TAD blocked the formation of stable homomultimers. Calcium imaging and current recordings showed that deletion of the TAD in a poreless TRPV1 mutant subunit suppressed its dominant-negative phenotype, confirming the involvement of the TAD in assembly of functional channels. Our findings suggest that the C-terminal TAD in TRPV1 channels functions as a domain that is conserved among TRPV1-4 and mediates a direct subunit-subunit interaction for tetrameric assembly.

Complex regulation of TRPV1 and related thermo-TRPs: implications for therapeutic intervention

The capsaicin receptor TRPV1 (Transient Receptor Potential, Vanilloid family member 1), the founding member of the heat-sensitive TRP ("thermo-TRP") channel family, plays a pivotal role in pain transduction. There is mounting evidence that TRPV1 regulation is complex and is manifest at many levels, from gene expression through post-translational modification and formation of receptor heteromers to subcellular compartmentalization and association with regulatory proteins. These mechanisms are believed to be involved both in disease-related changes in TRPV1 expression, and the long-lasting refractory state, referred to as "desensitization", that follows TRPV1 agonist treatment. The signaling cascades that regulate TRPV1 and related thermo-TRP channels are only beginning to be understood. Here we review our current knowledge in this rapidly changing field. We propose that the complex regulation of TRPV1 may be exploited for therapeutic purposes, with the ultimate goal being the development of novel, innovative agents that target TRPV1 in diseased, but not healthy, tissues. Such compounds are expected to be devoid of the side-effects (e.g. hyperthermia and impaired noxious heat sensation) that plague the clinical use of existing TRPV1 antagonists.

Involvement of lysophosphatidic acid in bone cancer pain by potentiation of TRPV1 via PKCε pathway in dorsal root ganglion neurons

Background

It has been demonstrated that lysophosphatidic acid (LPA) released from injury tissue and transient receptor potential vanilloid 1 (TRPV1) receptor are implicated in the induction of chronic pain. In the present study we examined whether an interaction between LPA receptor LPA₁ and TRPV1 in dorsal root ganglion (DRG) neurons contributes to the development of bone cancer pain.

Results

Bone cancer was established by injection of mammary gland carcinoma cells into the rat tibia. Following the development of bone cancer pain, the TRPV1 expression and capsaicin-evoked currents were up-regulated in rat DRG neurons at L_4-6 segments. Immunohistochemistry staining revealed a high co-localization of LPA₁ with TRPV1 in DRG neurons. In isolated DRG neurons, whole-cell patch recording showed that capsaicin-induced currents were potentiated by LPA in a dose-dependent manner. The potentiation was blocked by either LPA₁ antagonist, protein kinase C (PKC) inhibitor or PKCϵ inhibitor, but not by protein kinase A (PKA) inhibitor or Rho inhibitor. In the behavioral tests, both mechanical allodynia and thermal hyperalgesia in bone cancer rats were attenuated by LPA₁ antagonist.

Conclusion

LPA potentiates TRPV1 current via a PKCϵ-dependent pathway in DRG neurons of rats with bone cancer, which may be a novel peripheral mechanism underlying the induction of bone cancer pain.

Hematopoietic colony-stimulating factors: new players in tumor-nerve interactions

A variety of cancers are accompanied by debilitating pain, which constitutes the primary reason for poor quality of life in cancer patients. There is an urgent demand for the development of specific mechanism-based therapies against cancer pain. Recently, important advances have been made in mechanisms contributing to cancer pain. A notable finding was that the tumor-derived hematopoietic growth factors, granulocyte- and granulocyte-macrophage-colony-stimulating factors (G-CSF/GM-CSF), subserve important functions in the generation of pain hypersensitivity in tumor-affected regions. In this context, their receptors were unexpectedly found on pain-sensing nerves and were observed to be functionally linked to nociceptive sensitization and tumor-induced pain. Here, we review evidence supporting a role for G-/GM-CSF in sensitization of pain-sensing nerves, the underlying signaling pathways and the cross-talk with other pronociceptive cytokines, peptides and modulators derived from immune cells, osteoclasts and tumor cells. These findings hold implications in the therapy of pain in disease states, such as cancer and rheumatoid arthritis.

Inactivation of Anandamide Signaling: A Continuing Debate

Since the first endocannabinoid anandamide was identified in 1992, extensive research has been conducted to characterize the elements of the tightly controlled endocannabinoid signaling system. While it was established that the activity of endocannabinoids are terminated by a two-step process that includes cellular uptake and degradation, there is still a continuing debate about the mechanistic role of these processes in inactivating anandamide signals.

Modulation of single-channel properties of TRPV1 by phosphorylation

Activation of TRPV1, the heat and capsaicin receptor, is known to be promoted by phosphorylation, but the molecular details are unclear. In the present study we recorded from single TRPV1 ion channels using the cell-attached patch clamp technique. The influence of capsaicin concentration on the time constants of open and closed states demonstrates the existence of at least four closed and three open states, and shows that channel opening can occur from partially liganded states. Activation of protein kinase C (PKC) promotes channel opening in some channels but not others, consistent with some channels being inaccessible to the kinase. The changes in open and closed state time constants following activation of PKC are equivalent to an increased affinity of capsaicin binding, but other arguments suggest that channel opening must be potentiated by downstream changes in channel activation rather than by a direct action of phosphorylation on the capsaicin binding site. Mutation of functionally important PKC phosphorylation sites on TRPV1, or application of staurosporine, a broad-spectrum kinase inhibitor, completely inhibited the effect of PKC in enhancing channel open time. Staurosporine also inhibited channel activity in the absence of overt PKC activation, showing that TRPV1 is partially phosphorylated at rest. This study elucidates the mechanism by which phosphorylation by PKC potentiates the activation of single TRPV1 ion channels.

TRPV1: a target for next generation analgesics

Transient Receptor Potential Vanilloid 1 (TRPV1) is a Ca²⁺ permeant non-selective cation channel expressed in a subpopulation of primary afferent neurons. TRPV1 is activated by physical and chemical stimuli. It is critical for the detection of nociceptive and thermal inflammatory pain as revealed by the deletion of the TRPV1 gene. TRPV1 is distributed in the peripheral and central terminals of the sensory neurons and plays a role in initiating action potentials at the nerve terminals and modulating neurotransmitter release at the first sensory synapse, respectively. Distribution of TRPV1 in the nerve terminals innervating blood vessels and in parts of the CNS that are not subjected to temperature range that is required to activate TRPV1 suggests a role beyond a noxious thermal sensor. Presently, TRPV1 is being considered as a target for analgesics through evaluation of different antagonists. Here, we will discuss the distribution and the functions of TRPV1, potential use of its agonists and antagonists as analgesics and highlight the functions that are not related to nociceptive transmission that might lead to adverse effects.

Inflammatory pain: the cellular basis of heat hyperalgesia

Injury or inflammation release a range of inflammatory mediators that increase the sensitivity of sensory neurons to noxious thermal or mechanical stimuli. The heat- and capsaicin-gated channel TRPV1, which is an important detector of multiple noxious stimuli, plays a critical role in the development of thermal hyperalgesia induced by a wide range of inflammatory mediators. We review here recent findings on the molecular mechanisms of sensitisation of TRPV1 by inflammatory mediators, including bradykinin, ATP, NGF and prostaglandins. We describe the signalling pathways believed to be involved in the potentiation of TRPV1, and our current understanding of how inflammatory mediators couple to these pathways.

Modulators of pain: Bv8 and prokineticins

Bv8 is a small protein secreted by frog skin. Mammalian homologues of Bv8, the prokineticins PK1 and PK2, and their G-protein coupled receptors PKR1 and PKR2 have been identified and linked to several biological effects. Bv8 elicits a dose-dependent reduction in nociceptive threshold to thermal and mechanical stimuli applied to the skin of tail and paw of rats and mice and increases the sensitivity to nociceptive mediators as capsaicin and prostaglandins. The receptors for Bv8/PKs are present in a fraction of peptidergic population of C-fibre neurons, and in a fraction of A myelinated-fibre neurons. In mouse and rat dorsal root ganglia, PKR-expressing neurons also express TRPV1 and the activation of PKRs sensitises TPRV1 to the action of capsaicin. Mice lacking PKR1 gene exhibit impaired Bv8-induced hyperalgesia, develop deficient responses to noxious heat, capsaicin and protons and show reduced thermal and mechanical hypersensitivity to paw inflammation, indicating a requirement for PKR1 signalling associated with activation and sensitisation of primary afferent fibres. PKs are highly expressed by neutrophils and other inflammatory cells and must be considered as new pronociceptive mediators in inflammatory tissues. Bv8-like hyperalgesic activity was demonstrated in extracts of rat inflammatory granulocytes. Bv8 stimulate macrophage and T lymphocyte to differentiate between an inflammatory and Th1 profile indicating that Bv8/PK proteins play a role in immuno-inflammatory responses. Blockade of PKRs may represent a novel therapeutic strategy in acute and inflammatory pain conditions.

The action of prostaglandins on ion channels

Prostaglandins, in particular PGE(2) and prostacyclin PGI(2) have diverse biological effects. Most importantly, they are involved in inflammation and pain. Prostaglandins in nano- and micromolar concentrations sensitize nerve cells, i.e. make them more sensitive to electrical or chemical stimuli. Sensitization arises from the effect of prostaglandins on ion channels and occurs both at the peripheral terminal of nociceptors at the site of tissue injury (peripheral sensitization) and at the synapses in the spinal cord (central sensitization). The first step is the binding of prostaglandins to receptors in the cell membrane, mainly EP and IP receptors. The receptors couple via G proteins to enzymes such as adenylate cyclase and phospholipase C (PLC). Activation of adenylate cyclase leads to increase of cAMP and subsequent activation of protein kinase A (PKA) or PKA-independent effects of cAMP, e.g. mediated by Epac (=exchange protein activated by cAMP). Activation of PLC causes increase of inositol phosphates and increase of cytosolic calcium. This article summarizes the effects of PGE(2), PGE(1), PGI2 and its stable analogues on non-selective cation channels and sodium, potassium, calcium and chloride channels. It describes the mechanism responsible for the facilitatory or inhibitory prostaglandin effects on ion channels. Understanding these mechanisms is essential for the development of useful new analgesics.

Hyperalgesic priming is restricted to isolectin B4-positive nociceptors

We have previously described a rat model for the contribution of neuroplastic changes in nociceptors to the transition from acute to chronic pain. In this model a prior injury activates protein kinase C epsilon (PKCepsilon), inducing a chronic state characterized by marked prolongation of the hyperalgesia induced by inflammatory cytokines, prototypically prostaglandin E(2) (PGE(2)), referred to as hyperalgesic priming. In this study we evaluated the population of nociceptors involved in priming, by lesioning isolectin B4-positive (IB4(+)) nociceptors with intrathecal administration of a selective neurotoxin, IB4-saporin. To confirm that the remaining, TrkA(+)/IB4(-), nociceptors are still functional, we evaluated if nerve growth factor (NGF) induced hyperalgesia. While pretreatment with IB4-saporin eliminated the acute mechanical hyperalgesia induced by glia-derived neurotrophic factor (GDNF), NGF and PsiepsilonRACK, a highly selective activator of PKCepsilon, induced robust hyperalgesia. After injection of NGF, GDNF or PsiepsilonRACK, at a time at which hyperalgesia induced by PGE(2) is markedly prolonged (hyperalgesic priming) in control rats, in IB4-saporin-pretreated rats PGE(2) failed to produce this prolonged hyperalgesia. Thus, while PKCepsilon is present in most dorsal root ganglion neurons, where it can contribute to acute mechanical hyperalgesia, priming is restricted to IB4(+)-nociceptors, including those that are TrkA(+). While PKCepsilon activation can induce acute hyperalgesia in the IB4(+) population, it fails to induce priming. We suggest that hyperalgesic priming occurs only in IB4(+) nociceptors, and that in the peripheral terminals of nociceptors separate intracellular pools of PKCepsilon mediate nociceptor sensitization and the induction of hyperalgesic priming.

Light-dependent phosphorylation of the drosophila transient receptor potential ion channel

The Drosophila phototransduction cascade terminates in the opening of an ion channel, designated transient receptor potential (TRP). TRP has been shown to become phosphorylated in vitro, suggesting regulation of the ion channel through posttranslational modification. However, except for one phosphorylation site, Ser(982), which was analyzed by functional in vivo studies (Popescu, D. C., Ham, A. J., and Shieh, B. H. (2006) J. Neurosci. 26, 8570-8577), nothing is known about the role of TRP phosphorylation in vivo. Here, we report the identification of 21 TRP phosphorylation sites by a mass spectrometry approach. 20 phosphorylation sites are located in the C-terminal portion of the channel, and one site is located near the N terminus. All 21 phosphorylation sites were also identified in the inaC(P209) mutant, indicating that phosphorylation of TRP at these sites occurred independently from the eye-enriched protein kinase C. Relative quantification of phosphopeptides revealed that at least seven phosphorylation sites were predominantly phosphorylated in the light, whereas one site, Ser(936), was predominantly phosphorylated in the dark. We show that TRP phosphorylated at Ser(936) was located in the rhabomere. Light-dependent changes in the phosphorylation state of this site occurred within minutes. The dephosphorylation of TRP at Ser(936) required activation of the phototransduction cascade.

Host-pathogen interactions mediating pain of urinary tract infection

BACKGROUND

Pelvic pain is a major component of the morbidity associated with urinary tract infection (UTI), yet the molecular mechanisms underlying UTI-induced pain remain unknown. UTI pain mechanisms probably contrast with the clinical condition of asymptomatic bacteriuria (ASB), characterized by significant bacterial loads without lack symptoms.

METHODS

A murine UTI model was used to compare pelvic pain behavior elicited by infection with uropathogenic Escherichia coli strain NU14 and ASB strain 83972.

RESULTS

NU14-infected mice exhibited pelvic pain, whereas mice infected with 83972 did not exhibit pain, similar to patients infected with 83972. NU14-induced pain was not dependent on mast cells, not correlated with bacterial colonization or urinary neutrophils. UTI pain was not influenced by expression of type 1 pili, the bacterial adhesive appendages that induce urothelial apoptosis. However, purified NU14 lipopolysaccharide (LPS) induced Toll-like receptor 4 (TLR4)-dependent pain, whereas 83972 LPS induced no pain. Indeed, 83972 LPS attenuated the pain of NU14 infection, suggesting therapeutic potential.

CONCLUSIONS

These data suggest a novel mechanism of infection-associated pain that is dependent on TLR4 yet independent of inflammation. Clinically, these findings also provide the rational for probiotic therapies that would minimize the symptoms of infection without reliance on empirical therapies that contribute to antimicrobial resistance.

The substrates and binding partners of protein kinase Cepsilon

The epsilon isoform of protein kinase C (PKCepsilon) has important roles in the function of the cardiac, immune and nervous systems. As a result of its diverse actions, PKCepsilon is the target of active drug-discovery programmes. A major research focus is to identify signalling cascades that include PKCepsilon and the substrates that PKCepsilon regulates. In the present review, we identify and discuss those proteins that have been conclusively shown to be direct substrates of PKCepsilon by the best currently available means. We will also describe binding partners that anchor PKCepsilon near its substrates. We review the consequences of substrate phosphorylation and discuss cellular mechanisms by which target specificity is achieved. We begin with a brief overview of the biology of PKCepsilon and methods for substrate identification, and proceed with a discussion of substrate categories to identify common themes that emerge and how these may be used to guide future studies.

HSV vector-mediated modification of primary nociceptor afferents: an approach to inhibit chronic pain

Chronic pain is a serious medical condition with millions of sufferers for whom long-term therapies are either lacking or inadequate. Here we review the use of herpes simplex virus vectors as therapeutic tools to treat chronic pain by gene therapy. We describe an approach to inhibit chronic pain signaling whereby vector-mediated genes transferred to sensory nerves will modify the primary afferent nociceptor to prevent pain signaling to second-order nerves in the spinal cord. This approach may be used to reverse the chronic pain state of the nociceptor and could affect downstream pain-related changes in the central nervous system.

The effects of sympathetic outflow on upregulation of vanilloid receptors TRPV(1) in primary afferent neurons evoked by intradermal capsaicin

The vanilloid receptor TRPV(1) is a key nociceptive molecule located in primary afferent nociceptive neurons in dorsal root ganglia (DRG) for initiating neurogenic inflammation and pain. Our recent study demonstrates that up-regulation of TRPV(1) receptors by intradermal injection of capsaicin is modulated by activation of the protein kinase C (PKC) cascade. Neurogenic inflammation and pain resulting from capsaicin injection are sympathetically dependent, responding to norepinephrine, adenosine 5'-triphosphate (ATP) and/or neuropeptide Y released from sympathetic efferents. In a rat model of acute neurogenic inflammatory pain produced by capsaicin injection, we used immunofluorescence and Western blots combined with pharmacology and surgical sympathectomies to analyze whether the capsaicin-evoked up-regulation of TRPV(1) in DRG neurons is affected by sympathetic outflow by way of activating the PKC cascade. Sympathetic denervation reduced significantly the capsaicin-evoked expressions of TRPV(1), calcitonin gene-related peptide and/or phosphorylated PKC and their co-expression. These reductions could be restored by exogenous pretreatment with an analog of ATP, alpha,beta-methylene ATP. Inhibition of PKC with chelerythrine chloride prevented the ATP effect. Consistent results were obtained from experiments in which capsaicin-evoked changes in cutaneous inflammation (vasodilation and edema) were examined after sympathetic denervation, and the effects of the above pharmacological manipulations were evaluated. Our findings suggest that the capsaicin-evoked up-regulation of TRPV(1) receptors in DRG neurons is modulated sympathetically by the action of ATP released from sympathetic efferents to activate the PKC cascade. Thus, this study proposes a potential new mechanism of sympathetic modulation of neurogenic inflammation.

A key role for gp130 expressed on peripheral sensory nerves in pathological pain

Interleukin-6 (IL-6) is a key mediator of inflammation. Inhibitors of IL-6 or of its signal transducing receptor gp130 constitute a novel class of anti-inflammatory drugs, which raise great hopes for improved treatments of painful inflammatory diseases such as rheumatoid arthritis. IL-6 and gp130 may enhance pain not only indirectly through their proinflammatory actions but also through a direct action on nociceptors (i.e., on neurons activated by painful stimuli). We found indeed that the IL-6/gp130 ligand-receptor complex induced heat hypersensitivity both in vitro and in vivo. This process was mediated by activation of PKC-delta via Gab1/2/PI(3)K and subsequent regulation of TRPV1, a member of the transient receptor potential (TRP) family of ion channels. To assess the relevance of this direct pain promoting effect of IL-6, we generated conditional knock-out mice, which lack gp130 specifically in nociceptors, and tested them in models of inflammatory and tumor-induced pain. These mice showed significantly reduced levels of inflammatory and tumor-induced pain but no changes in immune reactions or tumor growth. Our results uncover the significance of gp130 expressed in peripheral pain sensing neurons in the pathophysiology of major clinical pain disorders and suggest their use as novel pain relieving agents in inflammatory and tumor pain.

Nociceptor subpopulations involved in hyperalgesic priming

We have previously developed a model in the rat for the transition from acute to chronic pain, hyperalgesic priming, in which a long-lasting neuroplastic change in signaling pathways mediates a prolongation of proinflammatory cytokine-induced nociceptor sensitization and mechanical hyperalgesia, induced at the site of a previous inflammatory insult. Induction of priming is mediated by activation of protein kinase C(epsilon) (PKC(epsilon)) in the peripheral terminal of the primary afferent nociceptor. Given that hyperalgesic mediator-induced PKC(epsilon) translocation occurs in isolectin B4 (IB4)(+)-nonpeptidergic but not in receptor tyrosine kinase (TrkA)(+)-peptidergic nociceptors, we tested the hypothesis that hyperalgesic priming was restricted to the IB4(+) subpopulation of nociceptors. After recovery from nerve growth factor (NGF)- and GDNF-induced hyperalgesia, a proinflammatory cytokine, prostaglandin E(2) (PGE(2)) induced, PKC(epsilon)-dependent, markedly prolonged hyperalgesia, two features that define the development of the primed state. Thus, hyperalgesic priming occurs in both the IB4(+)-nonpeptidergic and TrkA(+)-peptidergic subpopulations of nociceptive afferents. Of note, however, while attenuation of PKC(epsilon) prevented NGF-induced priming, the hyperalgesia induced by NGF is PKC(epsilon) independent. We propose that separate intracellular pools of PKC(epsilon), in the peripheral terminals of nociceptors, mediate nociceptor sensitization and the induction of hyperalgesic priming.

Oxidative challenges sensitize the capsaicin receptor by covalent cysteine modification

The capsaicin receptor TRPV1, one of the major transduction channels in the pain pathway, integrates information from extracellular milieu to control excitability of primary nociceptive neurons. Sensitization of TRPV1 heightens pain sensation to moderately noxious or even innocuous stimuli. We report here that oxidative stress markedly sensitizes TRPV1 in multiple species' orthologs. The sensitization can be recapitulated in excised inside-out membrane patches, reversed by strong reducing agents, and blocked by pretreatment with maleimide that alkylates cysteines. We identify multiple cysteines required for full modulation of TRPV1 by oxidative challenges. Robust oxidative modulation recovers the agonist sensitivity of receptors desensitized by prolonged exposure to capsaicin. Moreover, oxidative modulation operates synergistically with kinase or proton modulations. Thus, oxidative modulation is a robust mechanism tuning TRPV1 activity via covalent modification of evolutionarily conserved cysteines and may play a role in pain sensing processes during inflammation, infection, or tissue injury.

Ablation of TrpV1 neurons reveals their selective role in thermal pain sensation

Here we make use of neural ablation to investigate the properties of the TrpV1-expressing neurons in the trigeminal and dorsal root ganglia of mice. Resiniferotoxin (RTX), a potent TrpV1 agonist, administered either by direct injection in the ganglion or intrathecally killed approximately 70% of TrpV1 cells and resulted in modest thermal analgesia. Interestingly, after carageenan injection in the hind paw, the analgesic effects of RTX were dramatically increased with mice now paradoxically showing far less response to heat applied at sites of inflammation. This additional carageenan and RTX-induced analgesia was transient, lasting less than 2 days, and likely resulted from deafferentation of remaining TrpV1 neurons. Remarkably, although RTX affected sensitivity to heat, mechanical sensitivity (both of normal and inflamed tissue) was completely unaltered by toxin-mediated silencing of the TrpV1 sensory input. Thus, our data demonstrate that TrpV1 neurons are selectively tuned nociceptors that mediate responses to thermal but not mechanical pain and insinuate a labeled line model for somatosensory coding.

Protease-activated receptor 2-mediated protection of myocardial ischemia-reperfusion injury: role of transient receptor potential vanilloid receptors

Activation of the protease-activated receptor 2 (PAR2) or the transient receptor potential vanilloid type 1 (TRPV1) channels expressed in cardiac sensory afferents containing calcitonin gene-related peptide (CGRP) and/or substance P (SP) has been proposed to play a protective role in myocardial ischemia-reperfusion (I/R) injury. However, the interaction between PAR2 and TRPV1 is largely unknown. Using gene-targeted TRPV1-null mutant (TRPV1(-/-)) or wild-type (WT) mice, we test the hypothesis that TRPV1 contributes to PAR2-mediated cardiac protection via increasing the release of CGRP and SP. Immunofluorescence labeling showed that TRPV1 coexpressed with PAR2, PKC-epsilon, or PKAc in cardiomyocytes, cardiac blood vessels, and perivascular nerves in WT but not TRPV1(-/-) hearts. WT or TRPV1(-/-) hearts were Langendorff perfused with the selective PAR2 agonist, SLIGRL, in the presence or absence of various antagonists, followed by 35 min of global ischemia and 40 min of reperfusion (I/R). The recovery rate of coronary flow, the maximum rate of left ventricular pressure development, left ventricular end-diastolic pressure, and left ventricular developed pressure were evaluated after I/R. SLIGRL improved the recovery of hemodynamic parameters, decreased lactate dehydrogenase release, and reduced the infarct size in both WT and TRPV1(-/-) hearts (P < 0.05). The protection of SLIGRL was significantly surpassed for WT compared with TRPV1(-/-) hearts (P < 0.05). CGRP(8-37), a selective CGRP receptor antagonist, RP67580, a selective neurokinin-1 receptor antagonist, PKC-epsilon V1-2, a selective PKC-epsilon inhibitor, or H-89, a selective PKA inhibitor, abolished SLIGRL protection by inhibiting the recovery of the rate of coronary flow, maximum rate of left ventricular pressure development, and left ventricular developed pressure, and increasing left ventricular end-diastolic pressure in WT but not TRPV1(-/-) hearts. Radioimmunoassay showed that SLIGRL increased the release of CGRP and SP in WT but not TRPV1(-/-) hearts (P < 0.05), which were prevented by PKC-epsilon V1-2 and H-89. Thus our data show that PAR2 activation improves cardiac recovery after I/R injury in WT and TRPV1(-/-) hearts, with a greater effect in the former, suggesting that PAR2-mediated protection is TRPV1 dependent and independent, and that dysfunctional TRPV1 impairs PAR2 action. PAR2 activation of the PKC-epsilon or PKA pathway stimulates or sensitizes TRPV1 in WT hearts, leading to the release of CGRP and SP that contribute, at least in part, to PAR2-induced cardiac protection against I/R injury.

A-kinase anchoring protein 150 controls protein kinase C-mediated phosphorylation and sensitization of TRPV1

Post-translational modifications on various receptor proteins have significant effects on receptor activation. For the Transient Receptor Potential family V type 1 (TRPV1) receptor, phosphorylation of certain serine/threonine amino acid residues sensitizes the receptor to activation by capsaicin and heat. Although Protein Kinase C (PKC) phosphorylates TRPV1 on certain serine/threonine residues, it is not completely understood how PKC functionally associates with TRPV1. Recent studies have reported that the A-kinase Anchoring Protein 150 (AKAP150) mediates PKA phosphorylation of TRPV1 in several nociceptive models. Here, we demonstrate that AKAP150 also mediates PKC-directed phosphorylation and sensitization of TRPV1. In cultured rat trigeminal ganglia, immunocytochemical analyses demonstrate co-localization of AKAP150 and PKC isoforms alpha, delta, epsilon, and gamma in TRPV1-positive neurons. Additional biochemical evidence supports immunocytochemical results, indicating that AKAP150 preferentially associates with certain PKC isoforms in rat trigeminal ganglia neurons. Employing siRNA-mediated knock-down of AKAP150 expression, we demonstrate that PKC-mediated phosphorylation of TRPV1 and sensitization to a capsaicin response is dependent upon functional expression of the AKAP150 scaffolding protein. Furthermore, PKC-induced sensitization to a thermal stimulus is abrogated in AKAP150 knock-out animals relative to wild-type. Collectively, the results from these studies indicate that the AKAP150 scaffolding protein functionally modulates PKC-mediated phosphorylation and sensitization of the TRPV1 receptor in rat sensory neurons, suggesting the scaffolding protein to be an integral regulator of peripheral inflammatory hyperalgesia.