Facial pain induces the alteration of transient receptor potential vanilloid receptor 1 expression in rat trigeminal ganglion

Prediction of post-ESD esophageal stricture by a nomogram and risk factor analysis of ineffective oral steroids prophylaxis

BACKGROUND AND AIMS

Several risk models for esophageal stricture after endoscopic submucosal dissection have been developed. However, some of them did not include the use of steroids in the risk analysis. Glucocorticoid sensitivity mediated by glucocorticoid receptor expression has not been discussed in this condition.

METHODS

Clinical and endoscopic characteristics were included in the logistic regression model to establish a nomogram for stenosis prediction. The score for each risk factor was estimated. Risk factors of ineffective oral steroid prophylaxis were analyzed and glucocorticoid receptor expressions were detected by immunohistochemistry.

RESULTS

Three hundred fourteen patients of endoscopic submucosal dissection for esophageal superficial neoplasms were included to develop the nomogram. The circumferential range(≤ 3/4, 3/4-1 or the whole circumference), longitudinal diameter reached 4 cm (yes or not) and lesion location (the cervical and upper thoracic part, the middle thoracic part or the lower thoracic part) consisted of the nomogram. Patients have a high risk of esophageal stricture if they have a total point greater than 36. In the simplified risk score model, the corresponding cutoff score was 1. 92 patients with oral steroid prophylaxis were separately analyzed and the circumferential mucosal defect involving 7/8 or more was an independent risk factor of ineffective prevention (OR 12.2, 95%CI 5.27-28.11). The expression of glucocorticoid receptor β was higher in the stricture group (p = 0.042 for AOD; p = 0.016 for the scoring system).

CONCLUSIONS

We established a nomogram for esophageal stricture prediction. Depending on the characteristics of lesions, it is possible to estimate the risk of stricture under routine post-ESD treatments (no steroids or oral steroids). Alternative treatments should be considered if the risk is extremely high, especially for patients with mucosal defects involving 7/8 or more of circumference in which oral steroid treatment tends to be ineffective. The higher glucocorticoid receptor β may indicate potential glucocorticoid resistance.

Assessing Orofacial Pain Behaviors in Animal Models: A Review

Orofacial pain refers to pain occurring in the head and face, which is highly prevalent and represents a challenge to clinicians, but its underlying mechanisms are not fully understood, and more studies using animal models are urgently needed. Currently, there are different assessment methods for analyzing orofacial pain behaviors in animal models. In order to minimize the number of animals used and maximize animal welfare, selecting appropriate assessment methods can avoid repeated testing and improve the reliability and accuracy of research data. Here, we summarize different methods for assessing spontaneous pain, evoked pain, and relevant accompanying dysfunction, and discuss their advantages and disadvantages. While the behaviors of orofacial pain in rodents are not exactly equivalent to the symptoms displayed in patients with orofacial pain, animal models and pain behavioral assessments have advanced our understanding of the pathogenesis of such pain.

BmK DKK13, A Scorpion Toxin, Alleviates Pain Behavior in a Rat Model of Trigeminal Neuralgia by Modulating Voltage-Gated Sodium Channels and MAPKs/CREB Pathway

BmK DKK13 (DKK13) is a mutated recombinant peptide, which has a significant antinociception in a rat model of the inflammatory pain. The purpose of this study was to evaluate the antinociceptive effect of DKK13 on trigeminal neuralgia (TN) in rats. Male Sprague-Dawley (SD) rats were treated with the chronic constriction injury of the infraorbital nerve (IoN-CCI) model to induce stable symptoms of TN. DKK13 (1.0 mg/kg, 2.0 mg/kg and 4.0 mg/kg, i.v.) or morphine (4.0 mg/kg, i.v.) was administered by tail vein once on day 14 after IoN-CCI injury. Behavioral tests, electrophysiology and western blotting were performed to investigate the role and underlying mechanisms of DKK13 on IoN-CCI model. Behavioral test results showed that DKK13 could significantly increase the mechanical pain and thermal radiation pain thresholds of IoN-CCI rats and inhibit the asymmetric spontaneous pain scratching behavior. Electrophysiological results showed that DKK13 could significantly reduce the current density of Nav1.8 in the ipsilateral side of trigeminal ganglion (TG) neurons in IoN-CCI rats, and the steady-state activation and inactivation curves of Nav1.8 shifted, respectively, to the direction of hyperpolarization and depolarization. Western blotting results showed that DKK13 significantly reduced the expression of Nav1.8 and the phosphorylation levels of key proteins of MAPKs/CREB pathway in TG tissues of IoN-CCI rats. In brief, DKK13 has a significant antinociceptive effect on IoN-CCI rats, which may be achieved by changing the dynamic characteristics of Nav1.8 channel and regulating the protein phosphorylation in MAPKs/CREB pathway.

Enhanced Ocular Surface and Intraoral Nociception via a Transient Receptor Potential Vanilloid 1 Mechanism in a Rat Model of Obstructive Sleep Apnea

Obstructive sleep apnea (OSA), characterized by low arterial oxygen saturation during sleep, is associated with an increased risk of orofacial pain. In this study, we simulated chronic intermittent hypoxia (CIH) during the sleep/rest phase (light phase) to determine the role of transient receptor potential vanilloid 1 (TRPV1) in mediating enhanced orofacial nocifensive behavior and trigeminal spinal subnucleus caudalis (Vc) neuronal responses to capsaicin (a TRPV1 agonist) stimulation in a rat model of OSA. Rats were subjected to CIH (nadir O₂, 5%) during the light phase for 8 or 16 consecutive days. CIH yielded enhanced behavioral responses to capsaicin after application to the ocular surface and intraoral mucosa, which was reversed under normoxic conditions. The percentage of TRPV1-immunoreactive trigeminal ganglion neurons was greater in CIH rats than in normoxic rats and recovered under normoxic conditions after CIH. The ratio of large-sized TRPV1-immunoreactive trigeminal ganglion neurons increased in CIH rats. The density of TRPV1 positive primary afferent terminals in the superficial laminae of Vc was higher in CIH rats. Phosphorylated extracellular signal-regulated kinase (pERK)-immunoreactive cells intermingled with the central terminal of TRPV1-positive afferents in the Vc. The number of pERK-immunoreactive cells following low-dose capsaicin (0.33 µM) application to the tongue was significantly greater in the middle portion of the Vc of CIH rats than of normoxic rats and recovered under normoxic conditions after CIH. These data suggest that CIH during the sleep (light) phase is sufficient to transiently enhance pain on the ocular surface and intraoral mucosa via TRPV1-dependent mechanisms.

The mechanism of Annexin A1 to modulate TRPV1 and nociception in dorsal root ganglion neurons

BACKGROUND

Annexin A1 (ANXA1) exerts anti-nociceptive effect through ANXA1 receptor formyl peptide receptor 2 (FPR2/ALX (receptor for lipoxin A4), FPR2) at the dorsal root ganglia (DRG) level. However, the mechanisms remain elucidated. By using radiant heat, hot/cold plate, tail flick, von Frey, and Randall-Selitto tests to detect nociception in intact and chemical (capsaicin, menthol, mustard oil, formalin or CFA) injected AnxA1 conditional knockout (AnxA1^-/-) mice, applying calcium imaging and patch clamp recordings in cultured DRG neurons to measure neuronal excitability, conducting immunofluorescence and western blotting to detect the protein levels of TRPV1, FPR2 and its downstream molecules, and performing double immunofluorescence and co-immunoprecipitation to investigate the interaction between Calmodulin (CaM) and TRPV1; we aim to uncover the molecular and cellular mechanisms of ANXA1's role in antinociception.

RESULTS

AnxA1^-/- mice exhibited significant sensitivity to noxious heat (mean ± SD, 6.2 ± 1.0 s vs. 9.9 ± 1.6 s in Hargreaves test; 13.6 ± 1.5 s vs. 19.0 ± 1.9 s in hot plate test; n = 8; P < 0.001), capsaicin (101.0 ± 15.3 vs. 76.2 ± 10.9; n = 8; P < 0.01), formalin (early phase: 169.5 ± 32.8 s vs. 76.0 ± 21.9 s; n = 8; P < 0.05; late phase: 444.6 ± 40.1 s vs. 320.4 ± 33.6 s; n = 8; P < 0.01) and CFA (3.5 ± 0.8 s vs. 5.9 ± 1.4 s; n = 8; P < 0.01). In addition, we found significantly increased capsaicin induced Ca²⁺ response, TRPV1 currents and neuronal firing in AnxA1 deficient DRG neurons. Furthermore, ANXA1 mimic peptide Ac2-26 robustly increased intracellular Ca²⁺, inhibited TRPV1 current, activated PLCβ and promoted CaM-TRPV1 interaction. And these effects of Ac2-26 could be attenuated by FPR2 antagonist Boc2.

CONCLUSIONS

Selective deletion of AnxA1 in DRG neurons enhances TRPV1 sensitivity and deteriorates noxious heat or capsaicin induced nociception, while ANXA1 mimic peptide Ac2-26 desensitizes TRPV1 via FPR2 and the downstream PLCβ-Ca²⁺-CaM signal. This study may provide possible target for developing new analgesic drugs in inflammatory pain.

Effect of environmental tobacco smoke on COX-2 and SHP-2 expression in a periodontitis rat model

OBJECTIVES

To investigate the effects of environmental tobacco smoke (ETS) on the inflammatory process of periodontitis by evaluating bone loss and the expression of cyclooxygenase-2 (COX-2) and Src homology phosphotyrosine phosphatase 2 (SHP-2).

MATERIALS AND METHODS

Eighty 6-month-old male SD rats were randomized into four groups (10 rats/group/per time point): (a) normal group, (b) ETS group, (c) ligature-induced periodontitis group, and (d) ligature-induced periodontitis + ETS group. After treatment with ligature and/or ETS for 8 and 12 weeks, the levels of alveolar bone resorption and the expressions of COX-2 and SHP-2 in periodontal tissue were analyzed using histology and immunohistochemistry.

RESULTS

The ligature-induced periodontitis group displayed increased bone resorption and elevated expression of COX-2 and SHP-2 in periodontal tissues compared to the normal and ETS groups at 8 and 12 weeks. Furthermore, bone resorption and COX-2 and SHP-2 levels in the ligature-induced periodontitis + ETS group were significantly increased compared to those in the normal and ligature-induced periodontitis groups at both 8 and 12 weeks.

CONCLUSION

Environmental tobacco smoke increased alveolar bone loss in periodontitis with enhanced expression of COX-2 and SHP-2 in periodontal tissues. Further investigation is needed to explore the role of COX-2 and SHP-2 in ETS-associated periodontitis.

Transient receptor potential melastatin-3 in the rat sensory ganglia of the trigeminal, glossopharyngeal and vagus nerves

Transient receptor potential melastatin-3 (TRPM3) is a nonselective cation channel, has permeability of Ca²⁺, and probably participates in thermosensitive nociception. In this study, immunohistochemistry for TRPM3 was conducted in the rat trigeminal, glossopharyngeal and vagal sensory ganglia. TRPM3-immunoreactivity was expressed by half of sensory neurons in the trigeminal (TG), petrosal (PG) and jugular ganglia (JG), and by about 80% of sensory neurons in the nodose ganglion (NG). They mostly had small to medium-sized cell bodies. A trichrome immunofluorescence method showed co-existence of TRPM3 with TRP vanilloid 1 (TRPV1) and calcitonin gene-related peptide (CGRP). Approximately 70% of TRPM3-immunoreactive (-IR) neurons contained TRPV1-immunoreactivity in all the examined ganglia. More than 40% of TRPM3-IR neurons exhibited CGRP-immunoreactivity in the TG, PG and JG. Only a few sensory neurons co-expressed TRPM3- and CGRP-immunoreactivity in the NG. In addition, more than 40% of TRPM3-IR neurons bound to isolectin B4 in all the examined ganglia. By combination of retrograde tracing method and immunohistochemistry, half of TG neurons innervating the facial skin and incisive papilla expressed TRPM3-immunoreactivity whereas approximately 20% of those innervating the tooth pulp contained TRPM3-immunoreactivity. Co-expression of TRPM3-immunoreactivity with TRPV1- or CGRP-immunoreactivity was common among cutaneous and papillary TG neurons but not among pulpal TG neurons. More than 60% of PG and JG neurons innervating the external ear canal skin and circumvallate papilla contained TRPM3-immunoreactivity. Co-expression of TRPM3 with TRPV1 or CGRP was common among PG and JG neurons innervating the external ear canal skin. However, a smaller number of TRPM3-IR neurons co-expressing TRPV1- or CGRP-immunoreactivity innervate the circumvallate papilla in the PG. The present study suggests that expression of TRPM3 and its co-existence with TRPV1 and CGRP in sensory neurons depend on the variety of their peripheral targets in the trigeminal, glossopharyngeal and vagal nervous systems.

A moldable sustained release bupivacaine formulation for tailored treatment of postoperative dental pain

A moldable and biodegradable dental material was designed for customized placement and sustained delivery of bupivacaine (BP) within an extraction cavity. Microparticles comprising poly(lactic-co-glycolic acid) (PLGA) containing BP were generated via solvent-evaporation and combined with absorbable hemostat Gelfoam®. Kinetics of drug release were evaluated by in vitro dialysis assays, showing higher release within the first 24 hours, with subsequent tapering of release kinetics. Formulations of Gelfoam® and BP-PLGA microparticles (GelBP), with three targeted dosing profiles (0.25, 0.5, and 1 mg/kg/day), were evaluated alongside acute subcutaneous BP injections (2 mg/kg) to determine analgesic efficacy in a rat model of tooth extraction pain. Molar extraction resulted in mechanical and thermal cold hyperalgesia in male and female rats. GelBP outperformed acute BP in blocking post-surgical dental pain, with the 0.25 mg/kg GelBP dose preventing hypersensitivity to mechanical (p < 0.01) and thermal cold stimuli (p = 0.05). Molar extraction also resulted in decreased food consumption and weight. Males receiving acute BP and 0.25 mg/kg GelBP maintained normal food consumption (p < 0.002) and weight (p < 0.0001) throughout 7 days. Females, receiving 0.25 mg/kg GelBP maintained weight on days 5-7 (p < 0.04). Customized, sustained release formulation of anesthetic within a tooth extraction cavity holds potential to eliminate post-operative dental pain over several days.

Regulation of transient receptor potential vanilloid 1 expression in trigeminal ganglion neurons via methyl-CpG binding protein 2 signaling contributes tongue heat sensitivity and inflammatory hyperalgesia in mice

Background

Pain hypoalgesia has been reported in Rett syndrome patients, a severe neurodevelopmental disorder which can be attributed to mutations in the methyl-CpG binding protein 2 (MeCP2). Here, we examined the role of MeCP2 signaling in tongue heat sensitivity in the normal and inflamed state using Mecp2 heterozygous (Mecp2^+/−) mice.

Results

Heat hypoalgesia of the tongue occurred in Mecp2^+/− mice and submucosal injection of complete Freund’s adjuvant into the tongue produced a long-lasting heat hyperalgesia at the inflamed site in wild-type mice but not in Mecp2^+/− mice. Transient receptor potential vanilloid 1 was expressed in a large number of MeCP2-immunoreactive trigeminal ganglion neurons innervating the tongue in both wild-type and Mecp2^+/− mice (70.9% in wild type; 72.1% in Mecp2^+/−). The number of transient receptor potential vanilloid 1-immunoreactive trigeminal ganglion neurons innervating the tongue was smaller in Mecp2^+/− mice relative to wild-type mice (30.5% in wild type; 20.2% in Mecp2^+/−). Following complete Freund’s adjuvant injection, the number of transient receptor potential vanilloid 1- and MeCP2-immunoreactive trigeminal ganglion neurons innervating the tongue, as well as MeCP2 protein expression in trigeminal ganglion, was significantly increased in wild-type mice but not in Mecp2^+/− mice. Additionally, tongue heat hyperalgesia following complete Freund’s adjuvant injection was completely suppressed by the administration of SB366791, a transient receptor potential vanilloid 1 antagonist, in the tongue.

Conclusions

These findings indicate that tongue heat sensitivity and hypersensitivity are dependent on the expression of transient receptor potential vanilloid 1 which is regulated via MeCP2 signaling in trigeminal ganglion neurons innervating the tongue.

The evaluation of neural and vascular hyper-reactivity for sensitive skin

BACKGROUND

The impaired barrier function has been studied comprehensively but few about the heighted neural and vascular reaction for the pathogenesis of sensitive skin.

METHODS

Lactic acid stinging test (LAST) was used to identify sensitive subjects in selection phase. In the subsequent test phase, the baseline value of the blood flow (BF) and the current perception threshold (CPT) was measured by non-invasive instruments firstly. Then, the 0.001% capsaicin was applied to the nasolabial fold for 5 min. After the capsaicin test (CAT), the BF (immediately after the CAT) and CPT (1 h later after the CAT) were measured again. Blood sample were collected for genetic analysis of four TRPV1 gene single nucleotide polymorphisms between the positive-group and the negative-group.

RESULT

The positive-group had lower baseline value of CPT at 5 and 250 Hz compared with the negative-group, but no difference in baseline value of BF. After the CAT, significant variation in CPT at 5 and 250 Hz values and the BF were found in positive-group but not in negative-group. The genotype frequencies of AG/GG in RS224534 and AC/CC in RS4790523 in positive-group were higher than that of negative-group.

CONCLUSION

The sensitive subjects were prone to be stimulated by capsaicin to trigger neural and vascular hyper-reactivity. The genetic variation of TRPV1 and the unpleasant sensation demonstrate that TRPV1 play an important role in the pathogenesis of sensitive skin. Our study supports that sensory irritation inhibitors and anti-inflammatory compounds should be considered to be added in cosmetics to reduce the heighted neural and vascular reaction of sensitive skin.

Role of TRPV1 and ASIC3 channels in experimental occlusal interference-induced hyperalgesia in rat masseter muscle

BACKGROUND

Masticatory muscle pain may occur following immediate occlusal alteration by dental treatment. The underlying mechanisms are poorly understood. Transient receptor potential vanilloid-1 (TRPV1) and acid-sensing ion channel-3 (ASIC3) mediate muscle hyperalgesia under various pathologic conditions. We have developed a rat model of experimental occlusal interference (EOI) that consistently induces mechanical hyperalgesia in jaw muscles. Whether TRPV1 and ASIC3 mediate this EOI-induced hyperalgesia is unknown.

METHODS

Rat model of EOI-induced masseter hyperalgesia was established. Real-time polymerase chain reaction, Western blot and retrograde labelling combined with immunofluorescence were performed to evaluate the modulation of TRPV1 and ASIC3 expression in trigeminal ganglia (TGs) and masseter afferents of rats after EOI. The effects of intramuscular administration of TRPV1 and ASIC3 antagonists on the EOI-induced hyperalgesia in masseter muscle were examined.

RESULTS

After EOI, gene expressions and protein levels of TRPV1 and ASIC3 in bilateral TGs were up-regulated. The percentage of ASIC3- (but not TRPV1-) positive neurons in masseter afferents increased after EOI. More small-sized and small to medium-sized masseter afferents expressed TRPV1 and ASIC3 separately following EOI. These changes peaked at day 7 and then returned to original status within 10 days after EOI. Intramuscular administration of the TRPV1 antagonist AMG-9810 partially reversed this mechanical hyperalgesia in masseter muscle. No improvement was exhibited after administration of the ASIC3 antagonist APETx2. Co-injection of AMG-9810 and APETx2 enhanced the effect of AMG-9810 administration alone.

CONCLUSIONS

Peripheral TRPV1 and ASIC3 contribute to the development of the EOI-induced mechanical hyperalgesia in masseter muscle.

Sensitization of the trigeminovascular system following environmental irritant exposure

BACKGROUND

Air pollution is linked to increased emergency room visits for headache, and migraine patients frequently cite chemicals or odors as headache triggers, but the association between air pollutants and headache is not well understood. We previously reported that nasal administration of environmental irritants acutely increases meningeal blood flow via a TRPA1-dependent mechanism involving the trigeminovascular system. Here, we examine whether chronic environmental irritant exposure sensitizes the trigeminovascular system.

METHODS

Male rats were exposed to acrolein, a TRPA1 agonist, or room air by inhalation for four days prior to meningeal blood flow measurements. Some animals were injected daily with a TRPA1 antagonist, AP-18, or vehicle prior to inhalation exposure. Trigeminal ganglia were isolated following blood flow measurements for immunocytochemistry and/or qPCR determination of TRPV1, TRPA1 and CGRP levels.

RESULTS

Acrolein inhalation exposure potentiated blood flow responses both to TRPA1 and TRPV1 agonists compared to room air. Acrolein exposure did not alter TRPV1 or TRPA1 mRNA levels or TRPV1 or CGRP immunoreactive cell counts in the trigeminal ganglion. Acrolein sensitization of trigeminovascular responses to a TRPA1 agonist was attenuated by pre-treatment with AP-18.

INTERPRETATION

These results suggest trigeminovascular sensitization as a mechanism for enhanced headache susceptibility after chemical exposure.

Comparative analysis of P2X1, P2X2, P2X3, and P2X4 receptor subunits in rat nodose ganglion neurons

Nodose ganglion (NG) neurons are visceral primary sensory neurons. The transmission and regulation of visceral sensation is mediated mainly by the P2X purinoceptor (P2X receptor). Although the characteristics of different P2X receptor subunits in the NG have been studied previously, comprehensive analyses have not been performed. In this study, we used immunohistochemistry, immunocytochemistry, and whole cell patch clamp techniques to compare the expression and function of P2X1, P2X2, P2X3, and P2X4 receptor subunits in adult rat NG neurons. Polyclonal antibodies against the four P2X subunits labeled different subpopulations of NG neurons. P2X1 and P2X3 were expressed mainly in small-to-medium sized NG neurons, whereas P2X2 and P2X4 were located mostly in medium- and larger-sized NG neurons. Over 36% of NG neurons were P2X3 positive, which was higher than the other three P2X subunits. In addition, different types of currents were recorded from neurons expressing different P2X subunits. The fast type of ATP current was recorded from neurons containing P2X1–4 subunits, the intermediate type of current was recorded from neurons containing the P2X1, P2X3, and P2X4 subunits, the slow type was recorded from neurons expressing P2X1–3, and/or P2X4 subunits, whereas the very slow type was recorded from neurons containing the P2X2 and P2X3 subunits. These comparative results provide an anatomical verification of the different subunits in NG neurons, and offer direct support for the idea that various functional NG populations have distinct responses to ATP, which might be in part due to the different expression profiles of diverse P2X subunits.

Annexin 1: a glucocorticoid-inducible protein that modulates inflammatory pain

Annexin 1, a glucocorticoid (GC)-inducible protein, can play an important role via formyl peptide receptor like 1 (FPR2/ALX, also known as FPRL1) in inflammatory pain modulation. The aim of this review is to analyze different lines of evidence for the role of ANXA1 with different mechanisms on inflammatory pain and describe the profile of ANXA1 as a potential analgesic. A Medline (PUBMED) search using the terms 'Annexin 1 distribution OR expression, FPR2/ALX distribution OR expression, Annexin 1 AND pain, Annexin 1 AND FPR2/ALX AND pain' was performed. Articles with a publication date up to Nov. 1st, 2012 were included. The antinociception of ANXA1 has been evaluated in diverse pain models. It has been suggested that ANXA1 may exerts its action via: (1) inhibiting vital cytokines involved in pain transmission, (2) inhibiting neutrophil accumulation through preventing transendothelial migration via an interaction with formyl peptide receptors, (3) facilitating tonic opioid release from neutrophil in inflammatory site, (4) interrupting the peripheral nociceptive transmission by suppressing neuronal excitability. In general, ANXA1 is a potential mediator for anti-nociception and the role with its receptor constitute attractive targets for developing anesthesia and analgesic drugs, and their interaction may prove to be a useful strategy to treat inflammatory pain.

Formaldehyde up-regulates TRPV1 through MAPK and PI3K signaling pathways in a rat model of bone cancer pain

OBJECTIVE

Our previous study showed that tumor tissue-derived formaldehyde at low concentrations plays an important role in bone cancer pain through activating transient receptor potential vanilloid subfamily member 1 (TRPV1). The present study further explored whether this tumor tissue-derived endogenous formaldehyde regulates TRPV1 expression in a rat model of bone cancer pain, and if so, what the possible signal pathways are during the development of this type of pain.

METHODS

A rat model of bone cancer pain was established by injecting living MRMT-1 tumor cells into the tibia. The formaldehyde levels were determined by high performance liquid chromatography, and the expression of TRPV1 was examined with Western blot and RT-PCR. In primary cultured dorsal root ganglion (DRG) neurons, the expression of TRPV1 was assessed after treatment with 100 µmol/L formaldehyde with or without pre-addition of PD98059 [an inhibitor for extracellular signal-regulated kinase], SB203580 (a p38 inhibitor), SP600125 [an inhibitor for c-Jun N-terminal kinase], BIM [a protein kinase C (PKC) inhibitor] or LY294002 [a phosphatidylinositol 3-kinase (PI3K) inhibitor].

RESULTS

In the rat model of bone cancer pain, formaldehyde concentration increased in blood plasma, bone marrow and the spinal cord. TRPV1 protein expression was also increased in the DRG. In primary cultured DRG neurons, 100 μmol/L formaldehyde significantly increased the TRPV1 expression level. Pre-incubation with PD98059, SB203580, SP600125 or LY294002, but not BIM, inhibited the formaldehyde-induced increase of TRPV1 expression.

CONCLUSION

Formaldehyde at a very low concentration up-regulates TRPV1 expression through mitogen-activated protein kinase and PI3K, but not PKC, signaling pathways. These results further support our previous finding that TRPV1 in peripheral afferents plays a role in bone cancer pain.

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.

Putative neuronal mechanisms of sensitive skin

According to epidemiological studies, up to 50% of adults report facial sensitivity with various distinctive symptoms, such as prickling, burning, tingling, pain or itching. This is termed sensitive skinand represents a syndrome of physiological reactions rather than a disease entity. In this review, we discuss the currently available literature on this syndrome and describe the possible underlying neuronal pathomechanisms. The sensory receptors expressed on unmyelinated nerve fibres and keratinocytes involved in nociception, such as TRPV1 and endothelin receptors, are hypothesized to play a role in the induction of sensitive skin. Furthermore, we discuss the role of neurotrophins and the influence of stress on sensitive skin.

Emerging peripheral receptor targets for deep-tissue craniofacial pain therapies

While effective therapies are available for some types of craniofacial pain, treatments for deep-tissue craniofacial pain such as temporomandibular disorders are less efficacious. Several ion channels and receptors which are prominent in craniofacial nociceptive mechanisms have been identified on trigeminal primary afferent neurons. Many of these receptors and channels exhibit unusual distributions compared with extracranial regions. For example, expression of the ATP receptor P2X(3) is strongly implicated in nociception and is more abundant on trigeminal primary afferent neurons than analogous extracranial neurons, making them potentially productive targets specifically for craniofacial pain therapies. The initial part of this review therefore focuses on P2X(3) as a potential therapeutic target to treat deep-tissue craniofacial pain. In the trigeminal ganglion, P2X(3) receptors are often co-expressed with the nociceptive neuropeptides CGRP and SP. Therefore, we discuss the role of CGRP and SP in deep-tissue craniofacial pain and suggest that neuropeptide antagonists, which have shown promise for the treatment of migraine, may have wider therapeutic potential, including the treatment of deep-tissue craniofacial pain. P2X(3), TRPV1, and ASIC3 are often co-expressed in trigeminal neurons, implying the formation of functional complexes that allow craniofacial nociceptive neurons to respond synergistically to altered ATP and pH in pain. Future therapeutics for craniofacial pain thus might be more efficacious if targeted at combinations of P2X(3), CGRP, TRPV1, and ASIC3.

Alterations of AQP2 expression in trigeminal ganglia in a murine inflammation model

Aquaporins (AQPs) are small membrane channel proteins involved in osmoregulation. To date, only AQP1, AQP2, AQP4 and AQP9 have been found in the nervous system. Generally, they are involved in water movement in nervous tissue, nevertheless, recent data would suggest the involvement of AQPs in neurotransmission. In this work, we have evaluated the expression of AQP1 and AQP2 in the trigeminal ganglia of mice in an animal model of perioral acute inflammatory pain using immunohistochemistry and immunoblotting analysis. Our data have shown for the first time, the alteration of AQP2 expression in trigeminal ganglia in acute inflammatory pain showing increased and intracellular redistribution of AQP2 mainly in small-sized neurons and Schwann cells. Apart from this, the AQP1 expression remained unaltered. On the whole, these data support the hypothesis that AQP2 is involved in pain transmission in the peripheral nervous system.

Regulatory effect of nerve growth factor on release of substance P in cultured dorsal root ganglion neurons of rat

OBJECTIVE

To investigate the regulatory effects of nerve growth factor (NGF) on basal and capsaicin-induced release of neuropeptide substance P (SP) in primary cultured embryonic rat dorsal root ganglion (DRG) neurons.

METHODS

DRGs were dissected from 15-day-old embryonic Wistar rats. DRG neurons were dissociated and cultured, and then exposed to different concentrations of NGF (10 ng/mL, 30 ng/mL, or 100 ng/mL) for 72 h. The neurons cultured in media without NGF served as control. RT-PCR were used for detecting the mRNAs of SP and vanilloid receptor 1 (VR1) in the DRG neurons. The SP basal and capsaicin (100 nmol/L)-induced release in the culture were measured by radioimmunoassay (RIA).

RESULTS

SP mRNA and VR1 mRNA expression increased in primary cultured DRG neurons in a dose-dependent manner of NGF. Both basal release and capsaicin-evoked release of SP increased in NGF-treated DRG neurons compared with in control group. The capsaicin-evoked release of SP also increased in a dose-dependent manner of NGF.

CONCLUSION

NGF may promote both basal release and capsaicin-evoked release of SP. NGF might increase the sensitivity of nociceptors by increasing the SP mRNA or VR1 mRNA.