Phosphorylation of extracellular signal-related protein kinase is required for rapid facilitation of heat-induced currents in rat dorsal root ganglion neurons

Transient receptor potential vanilloid 1: A potential therapeutic target for the treatment of osteoarthritis and rheumatoid arthritis

This study aims to determine the molecular mechanisms and analgesic effects of transient receptor potential vanilloid 1 (TRPV1) in the treatments of osteoarthritis (OA) and rheumatoid arthritis (RA). We summarize and analyse current studies regarding the biological functions and mechanisms of TRPV1 in arthritis. We search and analyse the related literature in Google Scholar, Web of Science and PubMed databases from inception to September 2023 through the multi-combination of keywords like 'TRPV1', 'ion channel', 'osteoarthritis', 'rheumatoid arthritis' and 'pain'. TRPV1 plays a crucial role in regulating downstream gene expression and maintaining cellular function and homeostasis, especially in chondrocytes, synovial fibroblasts, macrophages and osteoclasts. In addition, TRPV1 is located in sensory nerve endings and plays an important role in nerve sensitization, defunctionalization or central sensitization. TRPV1 is a non-selective cation channel protein. Extensive evidence in recent years has established the significant involvement of TRPV1 in the development of arthritis pain and inflammation, positioning it as a promising therapeutic target for arthritis. TRPV1 likely represents a feasible therapeutic target for the treatment of OA and RA.

Temporal and spatial summation of laser heat stimuli in cultured nociceptive neurons of the rat

We studied the efficacy of a near-infrared laser (1475 nm) to activate rat dorsal root ganglion (DRG) neurons with short punctate radiant heat pulses (55 µm diameter) and investigated temporal and spatial summation properties for the transduction process for noxious heat at a subcellular level. Strength-duration curves (10–80 ms range) indicated a minimum power of 30.2mW for the induction of laser-induced calcium transients and a chronaxia of 13.9 ms. However, threshold energy increased with increasing stimulus duration suggesting substantial radial cooling of the laser spot. Increasing stimulus duration demonstrated suprathreshold intensity coding of calcium transients with less than linear gains (Stevens exponents 0.29/35mW, 0.38/60mW, 0.46/70mW). The competitive TRPV1 antagonist capsazepine blocked responses to short near-threshold stimuli and significantly reduced responses to longer duration suprathreshold heat. Heating 1/3 of the soma of a neuron was sufficient to induce calcium transients significantly above baseline (p < 0.05), but maximum amplitude was only achieved by centering the laser over the entire neuron. Heat-induced calcium increase was highest in heated cell parts but rapidly reached unstimulated areas reminiscent of spreading depolarization and opening of voltage-gated calcium channels. Full intracellular equilibrium took about 3 s, consistent with a diffusion process. In summary, we investigated transduction mechanisms for noxious laser heat pulses in native sensory neurons at milliseconds temporal and subcellular spatial resolution and characterized strength duration properties, intensity coding, and spatial summation within single neurons. Thermal excitation of parts of a nociceptor spread via both membrane depolarization and intracellular calcium diffusion.

ERK Phosphorylation Regulates the Aml1/Runx1 Splice Variants and the TRP Channels Expression during the Differentiation of Glioma Stem Cell Lines

The identification of cancer stem cells in brain tumors paved the way for new therapeutic approaches. Recently, a role for the transcriptional factor Runx1/Aml1 and the downstream ion channel genes in brain cancer development and progression has been suggested. This study aimed to explore the expression and the role of Runx1/Aml1, its Aml1b and Aml1c splice variants and the downstream TRPA1 and TRPV1 ion channels in undifferentiated and day-14 differentiated neural stem cells (NSCs and D-NSCs) and glioblastoma stem cells (GSCs and D-GSCs) lines with different proneural (PN) or mesenchymal (MES) phenotype. Gene and protein expression were evaluated by qRT-PCR, cytofluorimetric, western blot and confocal microscopy analyses. Moreover, by western blot, we observed that ERK phosphorylation enhances the Aml1b and Aml1c protein expression during glioma differentiation. Furthermore, the agonists of TRPA1 and TRPV1 channels stimulated apoptosis/necrosis in GSCs and D-GSCs as evaluated by Annexin V and PI staining and cytofluorimetric analysis. Finally, by qRT-PCR, the modulation of Wnt/β catenin, FGF, and TGFβ/SMAD signaling pathways in PN- and MES-GSCs was reported. Overall, our results provide new evidence regarding Runx1/Aml1 isoform overexpression and modulation in TRP channel expression during gliomagenesis, thus offering new directions for glioblastoma therapy.

Systems and Circuits Linking Chronic Pain and Circadian Rhythms

Research over the last 20 years regarding the link between circadian rhythms and chronic pain pathology has suggested interconnected mechanisms that are not fully understood. Strong evidence for a bidirectional relationship between circadian function and pain has been revealed through inflammatory and immune studies as well as neuropathic ones. However, one limitation of many of these studies is a focus on only a few molecules or cell types, often within only one region of the brain or spinal cord, rather than systems-level interactions. To address this, our review will examine the circadian system as a whole, from the intracellular genetic machinery that controls its timing mechanism to its input and output circuits, and how chronic pain, whether inflammatory or neuropathic, may mediate or be driven by changes in these processes. We will investigate how rhythms of circadian clock gene expression and behavior, immune cells, cytokines, chemokines, intracellular signaling, and glial cells affect and are affected by chronic pain in animal models and human pathologies. We will also discuss key areas in both circadian rhythms and chronic pain that are sexually dimorphic. Understanding the overlapping mechanisms and complex interplay between pain and circadian mediators, the various nuclei they affect, and how they differ between sexes, will be crucial to move forward in developing treatments for chronic pain and for determining how and when they will achieve their maximum efficacy.

Modality-specific facilitation of noninjurious sharp mechanical pain by topical capsaicin

We had previously shown that a "blunt blade" stimulator can mimic the noninjurious strain phase of incisional pain, but not its sustained duration. Here, we tested whether acute sensitization of the skin with topical capsaicin can add the sustained phase to this noninvasive surrogate model of intraoperative pain. Altogether, 110 healthy volunteers (55 male and 55 female; 26 ± 5 years) participated in several experiments using the "blunt blade" (0.25 × 4 mm) on normal skin (n = 36) and on skin pretreated by a high-concentration capsaicin patch (8%, Qutenza; n = 36). These data were compared with an experimental incision (n = 40) using quantitative and qualitative pain ratings by numerical rating scale and SES Pain Perception Scale descriptors. Capsaicin sensitization increased blade-induced pain magnitude and duration significantly (both P < 0.05), but it failed to fully match the sustained duration of incisional pain. In normal skin, the SES pattern of pain qualities elicited by the blade matched incision in pain magnitude and pattern of pain descriptors. In capsaicin-treated skin, the blade acquired a significant facilitation only of the perceived heat pain component (P < 0.001), but not of mechanical pain components. Thus, capsaicin morphed the descriptor pattern of the blade to become more capsaicin-like, which is probably explained best by peripheral sensitization of the TRPV1 receptor. Quantitative sensory testing in capsaicin-sensitized skin revealed hyperalgesia to heat and pressure stimuli, and loss of cold and cold pain sensitivity. These findings support our hypothesis that the blade models the early tissue-strain-related mechanical pain phase of surgical incisions.

Gene therapy by lentivirus-mediated RNA interference targeting extracellular-regulated kinase alleviates neuropathic pain in vivo

BACKGROUNDS

Neuropathic pain is an abnormal pain, which is related to the activation of extracellular-regulated kinase (ERK) signaling. This study was to investigate the effects of ERK knockdown via lentivirus-mediated RNA interference on allodynia in rats with chronic compression of the dorsal root ganglia (DRG) and to uncover the potential mechanisms.

METHODS

The model of chronic compression of the dorsal root ganglia (CCD) was established in rats by surgery. Gene silence was induced by injecting rats with lentivirus expressing ERK short hairpin RNA (shRNA). Behavioral test was performed by calculating paw withdrawal mechanical threshold (PWMT) and thermal paw withdrawal latency (TPWL).

RESULTS

We firstly generated lentivirus expressing ERK shRNA to downregulate ERK gene expression both in vitro and in vivo by using Western blot analysis and quantitative reverse transcription polymerase chain reaction. In CCD, ERK mRNA, and protein levels in DRG neurons were dramatically increased, accompanied with decreased PWMT and TPWL. Lentivirus-mediated RNA interference decreased ERK gene expression in DRG neurons and normalized the PWMT and TPWL in CCD rats, but not in rats infected with lentivirus expressing negative control shRNA. Further, calcium responses of DRG neurons to the hypotonic solution and 4α-phorbol 12,13-didecanoate were enhanced in CCD rats, which were suppressed by lentivirus-mediated ERK gene silence. Finally, the levels of transient receptor potential vanilloid 4 gene expressions in DRG neurons and L4 to L5 spinal cord isolated from CCD rats were dramatically upregulated, which were reversed by lentivirus-mediated ERK gene knockdown.

CONCLUSION

Lentivirus-mediated RNA interference (RNAi) silencing targeting ERK might reverse CCD-induced neuropathic pain in rats through transient receptor potential vanilloid 4.

3-iodothyroacetic acid, a metabolite of thyroid hormone, induces itch and reduces threshold to noxious and to painful heat stimuli in mice

BACKGROUND AND PURPOSE

Itch is associated with increased sensitization to nociceptive stimuli. We investigated whether 3-iodothyroacetic acid (TA1), by releasing histamine, induces itch and increases sensitization to noxious and painful heat stimuli.

EXPERIMENTAL APPROACH

Itch was evaluated after s.c. administration of TA1 (0.4, 1.32 and 4 μg·kg(-1) ). Mice threshold to noxious (NHT) and to painful heat stimuli were evaluated by the increasing-temperature hot plate (from 45.5 to 49.5°C) or by the hot plate (51.5°C) test, respectively, 15 min after i.p. injection of TA1 (0.4, 1.32 and 4 μg·kg(-1) ). Itch, NHT and pain threshold evaluation were repeated in mice pretreated with pyrilamine. Itch and NHT were also measured in HDC(+/+) and HDC(-/-) following injection of saline or TA1 (1.32, 4 and 11 μg·kg(-1) ; s.c. and i.p.). pERK1/2 levels were determined by Western blot in dorsal root ganglia (DRG) isolated from CD1 mice 15 min after they received (i.p.): saline, saline and noxious heat stimulus (46.5°C), TA1 (0.1, 0.4, 1.32, 4 μg·kg(-1) ) or TA1 1.32 μg·kg(-1) and noxious heat stimulus.

KEY RESULTS

TA1 0.4 and 1.32 μg·kg(-1) induced itch and reduced NHT; pyrilamine pretreatment prevented both of these effects. TA1 4 μg·kg(-1) (i.p.) reduced pain threshold without inducing itch or modifying NHT. In HDC(-/-) mice, TA1 failed to induce itch and to reduce NHT. In DRG, pERK1/2 levels were significantly increased by noxious heat stimuli and by TA1 0.1, 0.4 and 1.32 μg·kg(-1) ; i.p.

CONCLUSIONS AND IMPLICATIONS

Increased TA1 levels induce itch and an enhanced sensitivity to noxious heat stimuli suggesting that TA1 might represent a potential cause of itch in thyroid diseases.

PACAP receptor pharmacology and agonist bias: analysis in primary neurons and glia from the trigeminal ganglia and transfected cells

BACKGROUND AND PURPOSE

A major challenge in the development of new medicines targeting GPCRs is the ability to quantify drug action in physiologically relevant models. Primary cell models that closely resemble the clinically relevant in vivo site of drug action are important translational tools in drug development. However, pharmacological studies in these models are generally very limited due to the methodology used.

EXPERIMENTAL APPROACH

We used a neuropeptide system to demonstrate the applicability of using highly sensitive signalling assays in primary cells. We quantified the action of pituitary adenylate cyclase-activating peptide (PACAP)-38, PACAP-27 and vasoactive intestinal polypeptide in primary cultures of neurons and glia derived from rat trigeminal ganglia (TG), comparing our observations to transfected cells.

KEY RESULTS

PACAP-responsive receptors in rat trigeminal neurons, glia and transfected PAC1n receptors were pharmacologically distinct. PACAP-38, but not PACAP-27, activated ERK in glia, while both forms stimulated cellular cAMP production. PACAP(6-38) also displayed cell-type-dependent, agonist-specific, antagonism.

CONCLUSIONS AND IMPLICATIONS

The complexity of PACAP pharmacology in the TG may help to direct, more effectively, the development of disease treatments targeting the PACAP receptor. We suggest that these methodologies are broadly applicable to other primary cell types of human or animal origin, and that our approach may allow more thorough characterization of ligand properties in physiologically relevant cell types.

TRPV1

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

Acetylsalicylic acid enhances tachyphylaxis of repetitive capsaicin responses in TRPV1-GFP expressing HEK293 cells

Since many years acetylsalicylic acid (ASA) is known for its antithrombotic, antiphlogistic and analgesic effects caused by irreversible acetylation of cyclooxygenase. ASA also inhibits capsaicin- and heat-induced responses in cultured dorsal root ganglia (DRG) neurons, suggesting TRPV1 (transient receptor potential channel of the vanilloid receptor family, subtype 1) to be an additional target of ASA. We now studied the effect of ASA on heterologously expressed rat TRPV1 using calcium microfluorimetry. Capsaicin dose-dependently increased intracellular calcium with an EC50 of 0.29 μM in rTRPV1 expressing HEK293 cells. During repetitive stimulation the second response to capsaicin was reduced (53.4 ± 8.3% compared to vehicle control; p<0.005; Student's unpaired t-test) by 1μM ASA, a concentration much below the one needed to inhibit cyclooxygenase (IC50 of 35 μM in thromboxane B2 production assay). In contrast, calcium transients induced by a single stimulus of 0.3 or 1 μM capsaicin were not significantly reduced by 0.3 or 1 μM ASA. These data suggest that ASA increases the tachyphylaxis of rTRPV1 channel activation. Mechanisms are unknown and may be direct by e.g. stabilization of the desensitized state or indirect via inhibition of intracellular signaling pathways e.g. of the mitogen-activated protein kinase family (MAPK/ERK).

Activation of microglial cells in the trigeminal subnucleus caudalis evoked by inflammatory stimulation of the oral mucosa

To study the inflammatory hyperplasia induced by an acute noxious stimulation of oral mucosa with 5% formalin (5%FOR), we performed an immunohistochemical study on the expression of TNFá in the intermolar region of the dorsal lingual eminence (IDLE), and Iba1 and phosphorylated (phospho)- p38 MAPK involved with central nervous system microglial activation in the trigeminal subnucleus caudalis (Vc). The present study observed significantly increased expression of TNFá at either 1h or 24h of 5%FOR nociception, as well as sustained TNFá immunoreactivity in the IDLE. On the other hand, at either 1h or 24h 5%FOR nociception, Iba1- immunoreactive (IR) cells in the Vc were significantly increased after inflammatory stimulation of the IDLE; the increase was more evident at 24h 5%FOR nociception. By using the double-label immunofluorescence technique, the findings in particular demonstrated a significant increase in the number of phospho-p38 MAPK- and Iba1-IR coexpressed cells in the Vc at 24h 5%FOR nociception. The results suggest that 24h persistent microglial activation in subnuclei zonalis and gelatinosus of the Vc is evoked by 5%FOR noxious stimulation of the IDLE oral mucosa, thereby the present study indicates that the MAPK expression plays important roles in microglial activation related with central sensitization and inflammatory hyperalgesia.

TRPV1 and TRPA1 antagonists prevent the transition of acute to chronic inflammation and pain in chronic pancreatitis

Visceral afferents expressing transient receptor potential (TRP) channels TRPV1 and TRPA1 are thought to be required for neurogenic inflammation and development of inflammatory hyperalgesia. Using a mouse model of chronic pancreatitis (CP) produced by repeated episodes (twice weekly) of caerulein-induced AP (AP), we studied the involvement of these TRP channels in pancreatic inflammation and pain-related behaviors. Antagonists of the two TRP channels were administered at different times to block the neurogenic component of AP. Six bouts of AP (over 3 wks) increased pancreatic inflammation and pain-related behaviors, produced fibrosis and sprouting of pancreatic nerve fibers, and increased TRPV1 and TRPA1 gene transcripts and a nociceptive marker, pERK, in pancreas afferent somata. Treatment with TRP antagonists, when initiated before week 3, decreased pancreatic inflammation and pain-related behaviors and also blocked the development of histopathological changes in the pancreas and upregulation of TRPV1, TRPA1, and pERK in pancreatic afferents. Continued treatment with TRP antagonists blocked the development of CP and pain behaviors even when mice were challenged with seven more weeks of twice weekly caerulein. When started after week 3, however, treatment with TRP antagonists was ineffective in blocking the transition from AP to CP and the emergence of pain behaviors. These results suggest: (1) an important role for neurogenic inflammation in pancreatitis and pain-related behaviors, (2) that there is a transition from AP to CP, after which TRP channel antagonism is ineffective, and thus (3) that early intervention with TRP channel antagonists may attenuate the transition to and development of CP effectively.

Different responses of galanin and calcitonin gene-related peptide to capsaicin stimulation on dorsal root ganglion neurons in vitro

Both galanin (Gal) and calcitonin gene-related peptide (CGRP) are sensory neuropeptides which expressed in dorsal root ganglion (DRG) neurons and are involved in nociceptive processing. Capsaicin (CAP) influences nociceptive processing via influencing the expression of sensory neuropeptides in primary sensory neurons. However, little is known about the alterations of Gal and CGRP expression at the same condition stimulated by CAP. In the present study, primary cultured DRG neurons were used to determine the different responses of Gal and CGRP to CAP stimulation. DRG neurons were cultured for 48 hours and then exposed to CAP (2 μmol/L), capsazepine (CPZ) (2 μmol/L) plus CAP (2 μmol/L), or extracellular signal-regulated kinase 1/2 (ERK1/2) inhibitor PD98059 (10 μmol/L) plus CAP (2 μmol/L) for an additional 24hours. The DRG neurons were continuously exposed to culture media as a control. After that, the levels of Gal mRNA and CGRP mRNA of DRG neurons were determined using real time-PCR analysis. Gal and CGRP expression in situ was detected by an immunofluorescent labeling technique. The levels of phosphorylated-ERK1/2 (pERK1/2) protein were detected using a Western blot assay. The results showed that CAP evoked increases of Gal and its mRNA and decreases of CGRP and its mRNA in DRG neurons. Administration of either CPZ or PD98059 blocked the effects of CAP. These data indicate that Gal and CGRP shared different responses to CAP stimulation. Gal and CGRP may have different effects in nociceptive processing during neurogenic inflammation.

Organization of pERK-immunoreactive cells in trigeminal spinal nucleus caudalis, upper cervical cord, NTS and Pa5 following capsaicin injection into masticatory and swallowing-related muscles in rats

Many phosphorylated extracellular signal-regulated kinase (pERK)-immunoreactive (IR) cells are expressed in the trigeminal spinal subnucleus caudalis (Vc), upper cervical spinal cord (C1-C2), nucleus tractus solitarii (NTS) and paratrigeminal nucleus (Pa5) after capsaicin injection into the whisker pad (WP), masseter muscle (MM), digastric muscle (DM) or sternohyoideus muscle (SM). The pERK-IR cells also showed NeuN immunoreactivity, indicating that ERK phosphorylation occurs in neurons. The pERK-IR cells were significantly reduced after intrathecal injection of MEK 1/2 inhibitor PD98059. The pERK-IR cells expressed bilaterally in the Vc and C1-C2 after capsaicin injection into the unilateral DM or SM, whereas unilaterally in the Vc and C1-C2 after unilateral WP or MM injection. After capsaicin injection into the WP or MM, the pERK-IR cell expression in the Vc was restricted rostrocaudally within a narrow area. However, the distribution of pERK-IR cells was more wide spread without a clear peak in the Vc and C1-C2 after capsaicin injection into the DM or SM. In the NTS, the unimodal pERK-IR cell expression peaked at 0-720μm rostral from the obex following capsaicin injection into WP, MM, DM or SM. In the ipsilateral Pa5, many pERK-IR cells were observed following capsaicin injection into the SM. The number of swallows elicited by distilled water administration was significantly smaller after capsaicin injection into the WP, MM or DM but not SM compared to that of vehicle-injected rats. Various noxious inputs due to the masticatory or swallowing-related muscle inflammation may be differentially involved in muscle pain and swallowing reflex activity.

Activation of CaMKII and ERK1/2 contributes to the time-dependent potentiation of Ca2+ response elicited by repeated application of capsaicin in rat DRG neurons

When capsaicin is applied repeatedly to dorsal root ganglion (DRG) neurons for brief periods (10-15 s) at short intervals (5-10 min), the evoked responses rapidly decline, a phenomenon termed tachyphylaxis. In addition to this phenomenon, the present study using Ca(2+) imaging revealed that repeated application of capsaicin to rat dissociated DRG neurons at longer intervals (20-40 min) or during multiple applications at short intervals elicited an enhancement of the responses, termed potentiation. The potentiation occurred in 50-60% of the capsaicin-responsive cells, on average representing a 20- to 30% increase in the peak amplitude of the Ca(2+) signal, and was maximal at a 40-min application interval. An analysis of the mechanisms underlying potentiation revealed that it was suppressed by block of Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) with 5 μM KN-93 or block of the activation of extracellular signal-regulated kinase (ERK) 1/2 with 2 μM U-0126. Lowering the extracellular Ca(2+) concentration from 2 to 1 mM or pretreatment with deltamethrin (1 μM), which blocks calcineurin and tachyphylaxis, enhanced potentiation. Potentiation was not affected by: 1) inhibition of protein kinase C or protein kinase A, 2) block of the three subtypes of neurokinin receptors, or 3) block of the trafficking of transient receptor potential V1 channel to the membrane. These results indicate that the potentiation is a slowly developing Ca(2+)-modulated process that is mediated by a complex intracellular signaling pathway involving activation of CaMKII and ERK1/2. Potentiation may be an important peripheral autosensitization mechanism that occurs independently of the pronociceptive effects of inflammatory mediators and neurotrophic factors.

Long-range regulatory synergy is required to allow control of the TAC1 locus by MEK/ERK signalling in sensory neurones

Changes in the expression of the neuropeptide substance P (SP) in different populations of sensory neurones are associated with the progression of chronic inflammatory disease. Thus, understanding the genomic and cellular mechanisms driving the expression of the TAC1 gene, which encodes SP, in sensory neurones is essential to understanding its role in inflammatory disease. We used a novel combination of computational genomics, primary-cell culture and mouse transgenics to determine the genomic and cellular mechanisms that control the expression of TAC1 in sensory neurones. Intriguingly, we demonstrated that the promoter of the TAC1 gene must act in synergy with a remote enhancer, identified using comparative genomics, to respond to MAPK signalling that modulates the expression of TAC1 in sensory neurones. We also reveal that noxious stimulation of sensory neurones triggers this synergy in larger diameter sensory neurones – an expression of SP associated with hyperalgesia. This noxious stimulation of TAC1 enhancer-promotor synergy could be strongly blocked by antagonism of the MEK pathway. This study provides a unique insight into the role of long-range enhancer-promoter synergy and selectivity in the tissue-specific response of promoters to specific signal transduction pathways and suggests a possible new avenue for the development of novel anti-inflammatory therapies.

Extracellular signal-regulated kinases in pain of peripheral origin

Activation of members of the family of enzymes known as extracellular signal-regulated kinases (ERKs) is now known to be involved in the development and/or maintenance of the pain associated with many inflammatory conditions, such as herniated spinal disc pain, chronic inflammatory articular pain, and the pain associated with bladder inflammation. Moreover, ERKs are implicated in the development of neuropathic pain signs in animals which are subjected to the lumbar 5 spinal nerve ligation model and the chronic constriction injury model of neuropathic pain. The position has now been reached where all scientists working on pain subjects ought to be aware of the importance of ERKs, if only because certain of these enzymes are increasingly employed as experimental markers of nociceptive processing. Here, we introduce the reader, first, to the intracellular context in which these enzymes function. Thereafter, we consider the involvement of ERKs in mediating nociceptive signalling to the brain resulting from noxious stimuli at the periphery which will be interpreted by the brain as pain of peripheral origin.

Genetic targeting of ERK1 suggests a predominant role for ERK2 in murine pain models

The extracellular signal-regulated kinase (ERK) isoforms, ERK1 and ERK2, are believed to be key signaling molecules in nociception and nociceptive sensitization. Studies using inhibitors targeting the shared ERK1/2 upstream activator, mitogen-activated protein kinase kinase (MEK), and transgenic mice expressing a dominant-negative form of MEK have established the importance of ERK1/2 signaling. However, these techniques do not discriminate between ERK1 and ERK2. To dissect the function of each isoform in pain, we used mice with a targeted genetic deletion of ERK1 [ERK1 knock-out (KO)] to test the hypothesis that ERK1 is required for behavioral sensitization in rodent pain models. Despite activation (phosphorylation) of ERK1 after acute noxious stimulation and in models of chronic pain, we found that ERK1 was not required for formalin-induced spontaneous behaviors, complete Freund's adjuvant-induced heat and mechanical hypersensitivity, and spared nerve injury-induced mechanical hypersensitivity. However, ERK1 deletion did delay formalin-induced long-term heat hypersensitivity, without affecting formalin-induced mechanical hypersensitivity, suggesting that ERK1 partially shapes long-term responses to formalin. Interestingly, ERK1 deletion resulted in elevated basal ERK2 phosphorylation. However, this did not appear to influence nociceptive processing, since inflammation-induced ERK2 phosphorylation and pERK1/2 immunoreactivity in spinal cord were not elevated in ERK1 KO mice. Additionally, systemic MEK inhibition with SL327 (alpha-[amino[(4-aminophenyl)thio]methylene]-2-(trifluoromethyl)benzeneacetonitrile) attenuated formalin-induced spontaneous behaviors similarly in wild-type and ERK1 KO mice, indicating that unrelated signaling pathways do not functionally compensate for the loss of ERK1. Together, these results suggest that ERK1 plays a limited role in nociceptive sensitization and support a predominant role for ERK2 in these processes.

Activation of transient receptor potential vanilloid subtype 1 increases secretion of the hypofunctional, transplanted submandibular gland

Hyposecretion occurs in most patients early after submandibular gland autotransplantation for severe keratoconjunctivitis sicca. Endogenous transient receptor potential vanilloid subtype 1 (TRPV1) has been recently demonstrated in rabbit submandibular glands, and activation of TRPV1 by capsaicin increases secretion in isolated glands, but the TRPV1-mediated secretory mechanism remains to be elucidated. The purpose of this study was to verify whether activation of TRPV1 by capsaicin could improve the secretion of transplanted gland and its underlying mechanism. The salivary flow of the transplanted glands was significantly decreased, and the mRNA and protein levels of TRPV1 and aquaporin 5 (AQP5) were downregulated in the transplanted glands. Topical capsaicin cream increased secretion and upregulated levels of TRPV1 and AQP5 in transplanted glands. Moreover, in cultured submandibular gland cells, capsaicin increased the mRNA expression of AQP5 and led to redistribution of AQP5 from the cytoplasm to the plasma membrane via TRPV1 activation. Capsaicin enhanced the phosphorylation of extracellular signal-regulated kinase (ERK). Preincubation of cells with PD98059, an inhibitor of ERK kinase, suppressed the capsaicin-induced mRNA expression of AQP5. In summary, the capsaicin-induced secretory mechanism involved activation of TRPV1 and upregulation of AQP5 in an ERK-dependent manner and promoted the redistribution of AQP5 in submandibular gland cells. Activation of TRPV1 may provide a new therapeutic strategy to improve submandibular gland hypofunction.

The ERK 1 and 2 pathway in the nervous system: from basic aspects to possible clinical applications in pain and visceral dysfunction

The extracellular signal-regulated kinases 1 and 2 (ERK) cascade, member of the mitogen-activated protein kinases superfamily of signalling pathways, is one of the best characterized pathways as many protein interactions and phosphorylation events have been systematically studied. Traditionally, ERK are associated with the regulation of proliferation and differentiation as well as survival of various cell types. Their activity is controlled by phosphorylation on specific aminoacidic residues, which is induced by a variety of external cues, including growth-promoting factors.

In the nervous system, ERK phosphorylation is induced by binding of neurotrophins to their specific tyrosine kinase receptors or by neuronal activity leading to glutamate release and binding to its ionotropic and metabotropic receptors. Some studies have provided evidence of its importance in neuroplastic events. In particular, ERK phosphorylation in the spinal cord was shown to be nociceptive-specific and its upregulation, occurring in cases of chronic inflammatory and neuropathic pain, seems to be of the utmost importance to behavioural changes observed in those conditions. In fact, experiments using specific inhibitors of ERK phosphorylation have proved that ERK directly contributes to allodynia and hyperalgesia caused by spinal cord injury or chronic pain. Additionally, spinal ERK phosphorylation regulates the micturition reflex in experimental models of bladder inflammation and chronic spinal cord transection.

In this review we will address the main findings that suggest that ERK might be a future therapeutic target to treat pain and other complications arising from chronic pain or neuronal injury.

Identification of perineal sensory neurons activated by innocuous heat

C-fiber sensory neurons comprise nociceptors and smaller populations of cells detecting innocuous thermal and light tactile stimuli. Markers identify subpopulations of these cells, aiding our understanding of their physiological roles. The transient receptor potential vanilloid 1 (TRPV1) cation channel is characteristic of polymodal C-fiber nociceptors and is sensitive to noxious heat, irritant vanilloids, and protons. By using immunohistochemistry, in situ hybridization, and retrograde tracing, we anatomically characterize a small subpopulation of C-fiber cells that express high levels of TRPV1 (HE TRPV1 cells). These cells do not express molecular markers normally associated with C-fiber nociceptors. Furthermore, they express a unique complement of neurotrophic factor receptors, namely, the trkC receptor for neurotrophin 3, as well as receptors for neurturin and glial cell line-derived neurotrophic factor. HE TRPV1 cells are distributed in sensory ganglia throughout the neuraxis, with higher numbers noted in the sixth lumbar ganglion. In this ganglion and others of the lumbar and sacral regions, 75% or more of such HE TRPV1 cells express estrogen receptor alpha, suggestive of their regulation by estrogen and a role in afferent sensation related to reproduction. Afferents from these cells provide innervation to the hairy skin of the perineal region and can be activated by thermal stimuli from 38 degrees C, with a maximal response at 42 degrees C, as indicated by induction of extracellular signal-regulated kinase phosphorylation. We hypothesize that apart from participating in normal thermal sensation relevant to thermoregulation and reproductive functions, HE TRPV1 cells may mediate burning pain in chronic pain syndromes with perineal localization.

Research progress in transient receptor potential vanilloid 1 of sensory nervous system

The transient receptor potential vanilloid subfamily member 1 (TRPV1) is a protein mainly expressed in sensory neurons and fibers, such as in trigeminal ganglion and dorsal root ganglion, and has been indicated to be involved in several physiological and pathological processes. Studies on thermal activation have revealed that phosphorylation is involved in TRPV1 activation and 2 putative phosphorylation sites, Ser residues 502 (Ser-502) and Ser residues 800 (Ser-800), have been recently confirmed to possess the capability of resensitizing TRPV1. In addition to acidification, alkalization has also been proved to be a highly effective stimulator for TRPV1. TRPV1 could be regulated by various physical and chemical modulators, as well as the chronic pain. TRPV1 plays a crucial role in the transmission of pain signals, especially under inflammation and the neoplasm conditions, and it can also modulate nociceptive afferents by reinforcing morphine tolerance. The present review mainly focused on the structural and functional complexities of TRPV1, together with its activation and modulation by a wide variety of physical and chemical stimuli. Its pharmacological manipulation (sensitization/desensitization) and therapeutical targets were also discussed.

Expression of TRPV4 in the stimulated rat oral mucous membrane--nociceptive mechanisms of lingual conical papillae

The study was supported by 2006-2007 Aid Program for Overseas Training of the Promotion and Mutual Aid Corporation for Private School of Japan and International Exchange Grant, Osaka Dental University. We studied the function of TRPV4 expression and its neuronal activation in response to noxious stimulation of oral mucosa. The intermolar region of dorsal lingual eminence (IDLE) of rats was stimulated with 10 microl of either normal saline or 5% formalin. Immunohistological studies of the TRPV4, pERK and serotonin (5HT) expression in designated regions of tongues and brainstems were performed for studying the descending pain modulatory system in response to nociception. Specimens of the experimental IDLE demonstrated a significant increase of TRPV4 activity in particular in stratum basale of conical papillae (p < 0.01). pERK-IR positive neurons were significantly increased in the RMg (p < 0.05), Sp5C (p < 0.05) and Md (p < 0.01); TRPV4-IR neurons were found to show a similar distribution with pERK-IR cells in the peripheral Sp5C (p < 0.05). A significant increase of 5HT expression was observed in the RMg (p < 0.01), RPa (p < 0.01) and ROb (p < 0.05). The results suggest that TRPV4 in the oral mucosa is nociceptor of peripheral hyperalgesia, and pERK expression in the Sp5C is closely related with central hyperalgesia of the nociception. Furthermore, pERK-IR cells of the central 5HT nervous system are activated to accelerate 5HT release for neuronal modulation of the descending pain modulatory system in response to nociception.

Vanilloid receptor TRPV1-mediated phosphorylation of ERK in murine adjuvant arthritis

OBJECTIVE

The vanilloid receptor transient receptor potential vanilloid 1 (TRPV1), expressed by sensory neurons that innervate joints, is implicated in arthritis but the mechanisms are not fully understood. One possibility is that downstream effects of activation of TRPV1 are mediated by the extracellularly-regulated kinase (ERK). ERK is phosphorylated (p-ERK) in sensory neurons in response to noxious stimuli and its inhibition has been found to be antinociceptive in several pain models. We here wanted to ascertain whether TRPV1 may contribute to the pain hypersensitivity and inflammation of arthritis via an ERK-mediated pathway.

METHODS

We used a model of adjuvant-induced arthritis (AIA) of the ankle and investigated the changes in expression of p-ERK in sensory afferent neurons in dorsal root ganglia (DRG) and spinal dorsal horn of TRPV1-knockout (KO) mice, compared to wild-type (WT) mice of the same genetic background, using multiple immunofluorescence.

RESULTS

Two to three weeks after inducing AIA in mice, the number of neurons in DRG and spinal cord that expressed p-ERK was significantly higher on the side of AIA than on the contralateral, vehicle-injected side. The fraction of p-ERK-positive neurons in the DRG that also expressed TRPV1 was increased, indicating that activation of ERK occurred preferentially in TRPV1-positive neurons. Moreover, TRPV1-KO mice had reduced activation of ERK in sensory neurons, compared to WT mice. These changes in expression of p-ERK correlated with changes in pain behavior and joint histopathology: TRPV1-KO mice had reduced nociceptive behavior and severity of arthritis, compared to WT mice.

CONCLUSION

Our results support the idea that activation of ERK in primary afferent neurons is mediated, at least in part, by TRPV1. In the absence of TRPV1, the signs of arthralgia and histopathology in the mouse model of AIA are reduced. We conclude that TRPV1, expressed by neurons in the articular afferent pathway, contributes to the pathogenesis of arthritis via an ERK-mediated pathway.

Effects of anandamide and noxious heat on intracellular calcium concentration in nociceptive drg neurons of rats

As an endogenous agonist at the cannabinoid receptor CB1 and the capsaicin-receptor TRPV1, anandamide may exert both anti- and pronociceptive actions. Therefore we studied the effects of anandamide and other activators of both receptors on changes in free cytosolic calcium ([Ca(2+)](i)) in acutely dissociated small dorsal root ganglion neurons (diameter: < or =30 microm). Anandamide (10 microM) increased [Ca(2+)](i) in 76% of the neurons. The EC(50) was 7.41 microM, the Hill slope was 2.15 +/- 0.43 (mean +/- SE). This increase was blocked by the competitive TRPV1-antagonist capsazepine (10 microM) and in Ca(2+)-free extracellular solution. Neither exclusion of voltage-gated sodium channels nor additional blockade of voltage-gated calcium channels of the L-, N-, and/or T-type, significantly reduced the anandamide-induced [Ca(2+)](i) increase or capsaicin-induced [Ca(2+)](i) transients (0.2 microM). The CB1-agonist HU210 (10 microM) inhibited the anandamide-induced rise in [Ca(2+)](i). Conversely, the CB1-antagonist AM251 (3 microM) induced a leftward shift of the concentration-response relationship by approximately 4 microM (P < 0.001; Hill slope, 2.17 +/- 0.75). Intracellular calcium transients in response to noxious heat (47 degrees C for 10 s) were highly correlated with the anandamide-induced [Ca(2+)](i) increases (r = 0.84, P < 0.001). Heat-induced [Ca(2+)](i) transients were facilitated by preincubation with subthreshold concentrations of anandamide (3 microM), an effect that was further enhanced by 3 microM AM251. Although anandamide acts on both TRPV1 and CB1 receptors in the same nociceptive DRG neurons, its pronociceptive effects dominate. Anandamide triggers an influx of calcium through TRPV1 but no intracellular store depletion. It facilitates the heat responsiveness of TRPV1 in a calcium-independent manner. These effects of anandamide differ from those of the classical exogenous TRPV1-agonist capsaicin and suggest a primarily modulatory mode of action of anandamide.

Heat sensitization in skin and muscle nociceptors expressing distinct combinations of TRPV1 and TRPV2 protein

Recordings were made from small and medium diameter dorsal root ganglia (DRG) neurons that expressed transient receptor potential (TRP) proteins. Physiologically characterized skin nociceptors expressed either TRPV1 (type 2) or TRPV2 (type 4) in isolation. Other nociceptors co-expressed both TRP proteins and innervated deep tissue sites (gastrocnemius muscle, distal colon; type 5, type 8) and skin (type 8). Subpopulations of myelinated (type 8) and unmyelinated (type 5) nociceptors co-expressed both TRPs. Cells that expressed TRPV1 were excellent transducers of intense heat. Proportional inward currents were obtained from a threshold of approximately 46.5 to approximately 56 degrees C. In contrast, cells expressing TRPV2 alone (52 degrees C threshold) did not reliably transduce the intensity of thermal events. Studies were undertaken to assess the capacity of skin and deep nociceptors to exhibit sensitization to repeated intense thermal stimuli [heat-heat sensitization (HHS)]. Only nociceptors that expressed TRPV2, alone or in combination with TRPV1, exhibited HHS. HHS was shown to be Ca(2+) dependent in either case. Intracellular Ca(2+) dependent pathways to HHS varied with the pattern of TRP protein expression. Cells co-expressing both TRPs modulated heat reactivity through serine/threonine phosphorylation or PLA(2)-dependent pathways. Cells expressing only TRPV2 may have relied on tyrosine kinases for HHS. We conclude that heat sensitization in deep and superficial capsaicin and capsaicin-insensitive C and Adelta nociceptors varies with the distribution of TRPV1 and TRPV2 proteins. The expression pattern of these proteins are specific to subclasses of physiologically identified C and A fiber nociceptors with highly restricted tissue targets.