Mechanisms operated by endothelin ETA and ETB receptors in the trigeminal ganglion contribute to orofacial thermal hyperalgesia induced by infraorbital nerve constriction in rats

Satellite glial cells in sensory ganglia play a wider role in chronic pain via multiple mechanisms

Satellite glial cells are unique glial cells that surround the cell body of primary sensory neurons. An increasing body of evidence suggests that in the presence of inflammation and nerve damage, a significant number of satellite glial cells become activated, thus triggering a series of functional changes. This suggests that satellite glial cells are closely related to the occurrence of chronic pain. In this review, we first summarize the morphological structure, molecular markers, and physiological functions of satellite glial cells. Then, we clarify the multiple key roles of satellite glial cells in chronic pain, including gap junction hemichannel Cx43, membrane channel Pannexin1, K channel subunit 4.1, ATP, purinergic P2 receptors, and a series of additional factors and their receptors, including tumor necrosis factor, glutamate, endothelin, and bradykinin. Finally, we propose that future research should focus on the specific sorting of satellite glial cells, and identify genomic differences between physiological and pathological conditions. This review provides an important perspective for clarifying mechanisms underlying the peripheral regulation of chronic pain and will facilitate the formulation of new treatment plans for chronic pain.

Sex differences in carbamazepine effects in a rat model of trigeminal neuropathic pain

Carbamazepine (CBZ) represents the first-line treatment for trigeminal neuralgia, a condition of facial pain that affects mainly women. The chronic constriction of the infraorbital nerve (CCI-ION) is a widely used model to study this condition, but most studies do not include females. Thus, this study aimed to characterize sensory and affective changes in female rats after CCI-ION and compare the effect of CBZ in both sexes. Mechanical allodynia was assessed 15 days after CCI-ION surgery in rats treated with CBZ (10 and 30 mg/kg, i.p.) or vehicle, together with the open-field test. Independent groups were tested on the Conditioned Place Preference (CPP) paradigm and ultrasonic vocalization (USV) analysis. Blood samples were collected for dosage of the main CBZ metabolite. CBZ at 30 mg/kg impaired locomotion of CCI-ION male and sham and CCI-ION female rats and resulted in significantly higher plasma concentrations of 10-11-EPX-CBZ in the latter. Only male CCI-ION rats showed increased facial grooming which was significantly reduced by CBZ at 10 mg/kg. CBZ at 10 mg/kg significantly reduced mechanical allodynia and induced CPP only in female CCI-ION rats. Also, female CCI-ION showed reduced emission of appetitive USV but did not show anxiety-like behavior. In conclusion, male and female CCI-ION rats presented differences in the expression of the affective-motivational pain component and CBZ was more effective in females than males. Further studies using both sexes in trigeminal neuropathic pain models are warranted for a better understanding of potential differences in the pathophysiological mechanisms and efficacy of pharmacological treatments.

From pain to tumor immunity: influence of peripheral sensory neurons in cancer

The nervous and immune systems are the primary sensory interfaces of the body, allowing it to recognize, process, and respond to various stimuli from both the external and internal environment. These systems work in concert through various mechanisms of neuro-immune crosstalk to detect threats, provide defense against pathogens, and maintain or restore homeostasis, but can also contribute to the development of diseases. Among peripheral sensory neurons (PSNs), nociceptive PSNs are of particular interest. They possess a remarkable capability to detect noxious stimuli in the periphery and transmit this information to the brain, resulting in the perception of pain and the activation of adaptive responses. Pain is an early symptom of cancer, often leading to its diagnosis, but it is also a major source of distress for patients as the disease progresses. In this review, we aim to provide an overview of the mechanisms within tumors that are likely to induce cancer pain, exploring a range of factors from etiological elements to cellular and molecular mediators. In addition to transmitting sensory information to the central nervous system, PSNs are also capable, when activated, to produce and release neuropeptides (e.g., CGRP and SP) from their peripheral terminals. These neuropeptides have been shown to modulate immunity in cases of inflammation, infection, and cancer. PSNs, often found within solid tumors, are likely to play a significant role in the tumor microenvironment, potentially influencing both tumor growth and anti-tumor immune responses. In this review, we discuss the current state of knowledge about the degree of sensory innervation in tumors. We also seek to understand whether and how PSNs may influence the tumor growth and associated anti-tumor immunity in different mouse models of cancer. Finally, we discuss the extent to which the tumor is able to influence the development and functions of the PSNs that innervate it.

Is Presurgical Pain Intensity Related to Postoperative Recurrence of Post-Traumatic Trigeminal Neuropathic Pain?

PURPOSE

Post-traumatic trigeminal neuropathic pain (PTTNp) is a challenging condition to treat, and equally as challenging is the identification of surgical outcome variables to guide treatment. The study purpose was to determine if preoperative pain intensity was related to postoperative recurrence of PTTNp.

MATERIALS AND METHODS

This retrospective cohort study assessed subjects with preoperative PTTNp of either the lingual or inferior alveolar nerves who underwent elective microneurosurgery at a single institution. Two cohorts were established as follows: No PTTNp at 6 months (group 1); presence of PTTNp at 6 months (group 2). The primary predictor variable was the preoperative visual analog scale (VAS) score. The primary outcome variable was PTTNp (recurrence or no recurrence at 6 months). The demographic and injury characteristics of the groups were compared to assess whether they were similar using Wilcoxon rank analysis. Two-tailed Student's t-test was performed to analyze the difference in preoperative mean VAS scores. Multivariate multiple linear regression models were used to determine the association between the covariates on the outcomes of the primary predictor variable and the primary outcome variable. A P value of <.05 was considered statistically significant.

RESULTS

Forty-eight patients were included in the final analysis. There were 20 patients with no pain at 6 months and 28 with recurrence at 6 months following surgery. There was a significant difference in mean preoperative pain intensity between the two groups (P value .04). The mean preoperative VAS score in group 1 was 6.31 (standard deviation, 2.65), while the mean preoperative VAS score in group 2 was 7.75 (standard deviation, 1.95). Regression analysis showed that one covariate, the type of nerve injured, explained some variability of preoperative VAS score, but by only 16% (P value .005). Regression analysis also showed that two covariates, Sunderland classification and time to surgery, explained some of the variability of PTTNp at 6 months, by approximately 30% (P value .001).

CONCLUSION

This study showed that presurgical pain intensity level was related to postoperative recurrence in the surgical treatment of PTTNp. In patients with recurrence, the preoperative pain intensity was higher. Other factors, including time interval from injury to surgery, were also related to recurrence.

Sex Dimorphism in Resolvin D5-induced Analgesia in Rat Models of Trigeminal Pain

Resolvin D5 (RvD5) is a specialized pro-resolving lipid mediator with potent anti-inflammatory and analgesic properties. Orofacial pain conditions, especially those that are chronic, present clinical challenges in terms of pharmacological management. Thus, new therapeutic options are clearly warranted. Herein, we investigated the antinociceptive effect of RvD5 in the chronic constriction injury of the infraorbital nerve (CCI-ION) model and in the orofacial formalin test in female and male Wistar rats. Our results indicated that repeated subarachnoid medullary injections of RvD5 at 10 ng resulted in a significant reduction of heat and mechanical hyperalgesia induced by the CCI-ION in male and female rats, but males were more sensitive to RvD5 effects. In addition, after CCI-ION, interleukin-6 (IL-6) level was increased in the trigeminal nucleus caudalis of male, but not female rats, which was reduced by RvD5 repeated treatment. No changes in the levels of IL-1β were found. Minocycline blocked the effect of RvD5 in male rats but failed to affect RvD5 antinociceptive effect in females. Moreover, a single medullary injection of RvD5 caused a significant reduction of formalin-induced facial grooming, in phases I and II of the test, but only in male rats. This study demonstrated for the first time the analgesic effect of RvD5 in trigeminal pain models, and corroborated previous evidence of sex dichotomy, with a greater effect in males. This article presents a translational potential of RvD5 for targeted therapies aiming at the control of acute and chronic trigeminal pain, but further studies are needed to elucidate its sex-related mechanisms. PERSPECTIVE: This study demonstrated that RvD5 may provide the benefits for trigeminal neuropathic pain treatment in male and female rats, but its effect on inflammatory orofacial pain seems to be restricted only to males. Also, it provided the evidence for sex dichotomy in the mechanisms related to the antinociceptive effect of RvD5.

Role of endothelin in the pathophysiology of migraine: A new view on an old player

There is cumulating evidence that endothelin-1 (ET-1) may play a role in migraine, however controversial findings still impede a conclusion to be drawn. Herein we tested the hypothesis that endothelin ETB receptors are major contributors to migraine-like responses. ET-1, IRL-1620 (selective ETB receptor agonist) or CGRP were injected into the trigeminal ganglion (TG) of female Wistar rats, and the development of periorbital mechanical allodynia was assessed hourly with von Frey hairs. Twenty-four hours later, rats were exposed to an aversive light for 1 h, after which the reactivation of periorbital mechanical allodynia (indicating photic sensitivity) was assessed up to 4 h. Moreover, the effect of systemic Bosentan (ETA/ETB receptors antagonist) or the selective antagonists of ETA (BQ-123) and ETB (BQ-788) receptors injected into the TG were evaluated against CGRP-induced responses. ET-1 and IRL-1620 injection into the TG induced periorbital mechanical allodynia and photic sensitivity. Bosentan attenuated periorbital mechanical allodynia but failed to affect photic sensitivity induced by CGRP. Selective blockade of ETB receptors in the TG fully prevented the development of periorbital mechanical allodynia and photic sensitivity induced by CGRP, but ETA receptor blockade caused only a slight reduction of periorbital mechanical allodynia without affecting photic sensitivity. ETB receptor-operated mechanisms in the TG may contribute to migraine-like responses in female rats.

Antinociceptive effects of bupivacaine and its sulfobutylether-β-cyclodextrin inclusion complex in orofacial pain

Bupivacaine hydrochloride (BVC) represents an option to produce long-lasting analgesia, and complexation in cyclodextrins has shown improvements in biopharmaceutical properties. This study aimed to characterize and test the cytotoxicity and antinociceptive effects of BVC complexed in sulfobutylether-β-cyclodextrin (SBEβCD). The kinetics and stoichiometry of complexation and BVC-SBEβCD association constant were evaluated by phase solubility study and Job's plot. Evidence of the BVC-SBEβCD complex formation was obtained from scanning electron microscopy (SEM), infrared spectroscopy (FTIR), and differential scanning calorimetry (DSC). The cytotoxicity was evaluated in keratinocyte (HaCaT) and neuroblastoma (SH-SY5Y). Antinociceptive effects were registered via orofacial pain models: the formalin test, carrageenan-induced hyperalgesia, and postoperative pain (intraoral incision). The complex formation occurred at a 1:1 BVC-SBEβCD molar ratio, with a low association constant (13.2 M^-1). SEM, DSC, and FTIR results demonstrated the host-guest interaction. The IC_50% values determined in SH-SY5Y were 216 µM and 149 µM for BVC and BVC-SBEβCD, respectively (p < 0.05). There was no difference in HaCaT IC_50%. In orofacial pain model, BVC-SBEβCD significantly prolonged antinociceptive effect, in about 2 h, compared to plain BVC. SBEβCD can be used as a drug delivery system for bupivacaine, whereas the complex showed long-lasting analgesic effects.

The opposing contribution of neurotrophin-3 and nerve growth factor to orofacial heat hyperalgesia in rats

It has been proposed that neurotrophin-3 acts in a manner that is opposed to nerve growth factor, especially in the modulation of heat hyperalgesia. Injury to the constriction of the infraorbital nerve (CION) is a well-established model of trigeminal neuropathic pain that leads to robust heat, cold, and mechanical hyperalgesia. Here, we assessed the effect of local neurotrophin-3 treatment on CION-induced hyperalgesia, and we examined some mechanisms related to the effect of neurotrophin-3. Neurotrophin-3 (1 µg/50 µl) injected into the upper lip of CION rats caused a significant and long-lasting reduction of CION-induced heat hyperalgesia, but failed to affect cold and mechanical hyperalgesia. Increased levels of neurotrophin-3 were detected in the injured nerve at the time point that represents the peak of heat hyperalgesia. The anti-hyperalgesic effect of neurotrophin-3 was markedly reduced in the presence of an antagonist of TrkA receptors (K-252a, 1 μg/50 μl). Moreover, association of lower doses of neurotrophin-3 with an antibody anti-nerve growth factor resulted in a synergistic anti-hyperalgesic effect in CION rats. Local injection of nerve growth factor (3 µg/50 µl) or the TRPV1 agonist capsaicin (1 μg/50 μl), but not neurotrophin-3 injection (1 µg/50 µl), resulted in long-lasting facial heat hyperalgesia, which was both significantly reduced by previous neurotrophin-3 local treatment. In conclusion, we suggest that neurotrophin-3 is a potent modulator of facial heat hyperalgesia, which may exert an inhibitory influence on the trkA pathway. Neurotrophin-3 treatment may represent a promising approach, especially in pain conditions associated with increased levels of nerve growth factor.

Botulinum toxin type A reduces the expression of transient receptor potential melastatin 3 and transient receptor potential vanilloid type 4 in the trigeminal subnucleus caudalis of a rat model of trigeminal neuralgia

This study was designed to investigate the expression of transient receptor potential melastatin 3 (TRPM3) and transient receptor potential vanilloid type 4 (TRPV4) in the trigeminal spinal subnucleus caudalis of a rat model of trigeminal neuralgia (TN). The influence of botulinum toxin type A (BTX-A) on the expression of these channels was also explored. In this study, a model was established involving chronic constriction injury to the infraorbital nerve (ION-CCI), inducing TN. To explore the effects of BTX-A and whether it was dose related, rats were divided randomly into four groups: a control group, an ION-CCI group, a 3 U group, and a 10 U group (which received 3 and 10 U/kg BTX-A injections, respectively). Von Frey hairs were used to determine the pain threshold of the rats. The expression of TRPM3 and TRPV4 in the trigeminal spinal subnucleus caudalis was detected using western blots and immunohistochemistry. The pain thresholds of rats decreased to a minimum 14 days after ION-CCI. Compared with the ION-CCI group, the pain thresholds of the 3 and 10 U groups were significantly higher 4 days after the subcutaneous injection of BTX-A (P<0.05). The expression of TRPM3 and TRPV4 in the ION-CCI group was significantly higher than that in the control group (P<0.05). TRPM3 and TRPV4 expression in the 3 and 10 U groups was significantly lower than that in the ION-CCI group (P<0.05). In conclusion, overexpression of TRPM3 and TRPV4 can jointly mediate the occurrence of mechanical hyperalgesia in TN. The analgesic effects of BTX-A may be related to the inhibition of TRPM3 and TRPV4 expression.

Oral Pain in the Cancer Patient

Oral pain due to cancer and associated treatments is common. The prevalence and severity of oral cancer is high. Painful oral mucositis develops in head and neck cancer patients following surgery and associated radiation therapy and/or chemotherapy. In addition, oral pain, including pain from mucositis, occurs in patients receiving chemotherapy for cancers of the hematopoietic system and cancers at other anatomic sites. Despite pain management practices that include high-dose opioid analgesics, patients rarely obtain relief from either head and neck cancer pain or mucositis pain. Because oral pain in cancer patients is likely due to both nociceptive and neuropathic mechanisms, effective management of pain requires treatments for both processes. As knowledge of the pathophysiology of oral pain in cancer patients increases, new approaches for the prevention and management are anticipated. This article focuses on the emerging evidence that supports the molecular mechanisms and the unique oral micro-neuroanatomy that in combination produce the severe oral pain experienced by cancer patients. In addition, this article summarizes the current state of clinical management of oral mucositis pain.

Comparison of antinociceptive effects of plain lidocaine versus lidocaine complexed with hydroxypropyl-β-cyclodextrin in animal models of acute and persistent orofacial pain

Herein, it was investigated whether a complex of lidocaine with 2-hydroxypropyl-β-cyclodextrin (HP-β-CD) would present a better antinociceptive profile in vivo when compared with plain lidocaine in models of orofacial pain. Plain lidocaine (LDC) and complexed lidocaine (LDC:HP-β-CD) were initially evaluated in vitro to determine the release rate of the two formulations. Subsequently, the effect of both formulations was evaluated in independent groups of rats submitted to the orofacial formalin test, induction of facial heat hyperalgesia by capsaicin and carrageenan, and induction of facial heat and mechanical hyperalgesia by constriction of the infraorbital nerve. LDC:HP-β-CD led to a reduction in the lidocaine release assessed in the in vitro release assay compared to plain LDC. Both formulations presented an antinociceptive effect in all models, but LDC:HP-β-CD showed a better effect in the second phase of the formalin response, in carrageenan-induced heat hyperalgesia, and in the heat hyperalgesia associated to infraorbital nerve constriction. Our results show that complexation improved in vivo antinociceptive effects of LDC, but further studies are necessary to elucidate what properties contribute to the better effect of the complexed formulation on this models and/or what characteristics of the pain model facilitate the action of the complexed formulation.

The role of satellite glial cells in orofacial pain

Some chronic pain conditions in the orofacial region are common, the mechanisms underlying which are unresolved. Satellite glial cells (SGCs) are the glial cells of the peripheral nervous system. In the sensory ganglia, each neuronal body is surrounded by SGCs forming distinct functional units. The unique structural organization enables SGCs to communicate with each other and with their enwrapped neurons via a variety of ways. There is a growing body of evidence that SGCs can influence the level of neuronal excitability and are involved in the development and/or maintenance of pain. The aim of this review was to summarize the latest advances made about the implication of SGCs in orofacial pain. It may offer new targets for the development of orofacial pain treatment.

Blockage of spinal endothelin A receptors attenuates bone cancer pain via regulation of the Akt/ERK signaling pathway in mice

Bone cancer pain (BCP) is a common source of pain in patients with advanced stage and metastatic cancer; however, existing treatment for this kind of pain remains deficient. Being closely related to sensory change and inflammatory pain in both the central and peripheral nervous systems, endothelin A receptor (ETAR) plays an essential role in pain processing. As a result, ETAR antagonist has been reported to alleviate both neuropathic and inflammatory pain. Thus far, the role of ETAR in the process of BCP is still ambiguous. In this study, by using a BCP mouse model, the analgesic effect and molecular mechanism of the ETAR antagonist BQ-123 was investigated. Pain sensation in the BCP mouse model was investigated by the number of spontaneous flinches (NSF) and pain withdrawal threshold (PWT), and the mechanism of BCP was assessed by measuring p-Akt, p-Akt/t-Akt, p-ERK-1/2 and p-ERK-1/2/t-ERK-1/2 levels in L_4-6 segments of the spinal cord. Our results demonstrated that BCP mice showed a higher NSF and a lower PWT score than Sham mice. In addition to the development of nociceptive sensitization, p-Akt, p-Akt/t-Akt, p-ERK-1/2 and p-ERK-1/2/t-ERK-1/2 were up-regulated correspondingly in L_4-6 segments of the spinal cord in BCP mice. BQ-123 treatment showed a promising analgesic effect, and the effect was correlated to the down-regulation of p-Akt, p-Akt/t-Akt, p-ERK-1/2 and p-ERK-1/2/t-ERK-1/2 in spinal cord cells. The results suggested that intrathecal administration of BQ-123 was able to relieve BCP in mice as a consequence of suppressing the Akt and ERK signalling pathways.

Facial hyperalgesia due to direct action of endothelin-1 in the trigeminal ganglion of mice

Objective

This study assessed the ability of endothelin-1 (ET-1) to evoke heat hyperalgesia when injected directly into the trigeminal ganglia (TG) of mice and determined the receptors implicated in this effect. The effects of TG ETA and ETB receptor blockade on alleviation of heat hyperalgesia in a model of trigeminal neuropathic pain induced by infraorbital nerve constriction (CION) were also examined.

Methods

Naive mice received an intraganglionar (i.g.) injection of ET-1 (0.3–3 pmol) or the selective ETBR agonist sarafotoxin S6c (3–30 pmol), and response latencies to ipsilateral heat stimulation were assessed before the treatment and at 1-h intervals up to 5 h after the treatment. Heat hyperalgesia induced by i.g. ET-1 or CION was assessed after i.g. injections of ETAR and ETBR antagonists (BQ-123 and BQ-788, respectively, each at 0.5 nmol).

Key findings

Intraganglionar ET-1 or sarafotoxin S6c injection induced heat hyperalgesia lasting 4 and 2 h, respectively. Heat hyperalgesia induced by ET-1 was attenuated by i.g. BQ-123 or BQ-788. On day 5 after CION, i.g. BQ-788 injection produced a more robust antihyperalgesic effect compared with BQ-123.

Conclusions

ET-1 injection into the TG promotes ETAR/ETBR-mediated facial heat hyperalgesia, and both receptors are clearly implicated in CION-induced hyperalgesia in the murine TG system.

Endothelin-1 Elicits TRP-Mediated Pain in an Acid-Induced Oral Ulcer Model

Oral ulcer is the most common oral disease and leads to pain during meals and speaking, reducing the quality of life of patients. Recent evidence using animal models suggests that oral ulcers induce cyclooxygenase-dependent spontaneous pain and cyclooxygenase-independent mechanical allodynia. Endothelin-1 is upregulated in oral mucosal inflammation, although it has not been shown to induce pain in oral ulcers. In the present study, we investigated the involvement of endothelin-1 signaling with oral ulcer-induced pain using our proprietary assay system in conscious rats. Endothelin-1 was significantly upregulated in oral ulcers experimentally induced by topical acetic acid treatment, while endothelin-1 production was suppressed by antibacterial pretreatment. Spontaneous nociceptive behavior in oral ulcer model rats was inhibited by swab applications of BQ-788 (ET_B receptor antagonist), ONO-8711 (prostanoid receptor EP₁ antagonist), and HC-030031 (TRPA1 antagonist). Prostaglandin E₂ production in the ulcers was suppressed by BQ-788. Mechanical allodynia in the model was inhibited not only by BQ-788 and HC-030031 but also by BQ-123 (ET_A receptor antagonist), SB-366791 (TRPV1 antagonist), and RN-1734 (TRPV4 antagonist). In naive rats, submucosal injection of endothelin-1 caused mechanical allodynia that was sensitive to HC-030031 and SB-366791 but not to RN-1734. These results suggest that endothelin-1 production following oral bacterial invasion via ulcerative regions elicits TRPA1-mediated spontaneous pain. This pain likely occurs through an indirect route that involves ET_B receptor-accelerated prostanoid production. Endothelin-1 elicits directly TRPA1- and TRPV1-mediated mechanical allodynia via both ET_A and ET_B receptors on nociceptive fibers. The TRPV4-mediated allodynia component seems to be independent of endothelin signaling. These findings highlight the potential of endothelin signaling blockers as effective analgesic approaches for oral ulcer patients.

Nerve growth factor induces facial heat hyperalgesia and plays a role in trigeminal neuropathic pain in rats

There is preclinical evidence that nerve growth factor (NGF) contributes toward inflammatory hyperalgesia in the orofacial region, but the mechanisms underlying its hyperalgesic effect as well as its role in trigeminal neuropathic pain require further investigation. This study investigated the ability of NGF to induce facial heat hyperalgesia and the involvement of tyrosine kinase receptor A, transient receptor potential vanilloid 1, and mast cells in NGF pronociceptive effects. In addition, the role of NGF in heat hyperalgesia in a model of trigeminal neuropathic pain was evaluated. NGF injection into the upper lip of naive rats induced long-lasting heat hyperalgesia. Pretreatment with an antibody anti-NGF, antagonists of tyrosine kinase receptor A, and transient receptor potential vanilloid 1 receptors or compound 48/80, to induce mast-cell degranulation, all attenuated NGF-induced hyperalgesia. In a rat model of trigeminal neuropathic pain, local treatment with anti-NGF significantly reduced heat hyperalgesia. In addition, increased NGF levels were detected in the ipsilateral infraorbital nerve branch at the time point that represents the peak of heat hyperalgesia. The results suggest that NGF is a prominent hyperalgesic mediator in the trigeminal system and it may represent a potential therapeutic target for the management of painful orofacial conditions, including trigeminal neuropathic pain.

Repeat low-level blast exposure increases transient receptor potential vanilloid 1 (TRPV1) and endothelin-1 (ET-1) expression in the trigeminal ganglion

Blast-associated sensory and cognitive trauma sustained by military service members is an area of extensively studied research. Recent studies in our laboratory have revealed that low-level blast exposure increased expression of transient receptor potential vanilloid 1 (TRPV1) and endothelin-1 (ET-1), proteins well characterized for their role in mediating pain transmission, in the cornea. Determining the functional consequences of these alterations in protein expression is critical to understanding blast-related sensory trauma. Thus, the purpose of this study was to examine TRPV1 and ET-1 expression in ocular associated sensory tissues following primary and tertiary blast. A rodent model of blast injury was used in which anesthetized animals, unrestrained or restrained, received a single or repeat blast (73.8 ± 5.5 kPa) from a compressed air shock tube once or daily for five consecutive days, respectively. Behavioral and functional analyses were conducted to assess blast effects on nocifensive behavior and TRPV1 activity. Immunohistochemistry and Western Blot were also performed with trigeminal ganglia (TG) to determine TRPV1, ET-1 and glial fibrillary associated protein (GFAP) expression following blast. Increased TRPV1, ET-1 and GFAP were detected in the TG of animals exposed to repeat blast. Increased nocifensive responses were also observed in animals exposed to repeat, tertiary blast as compared to single blast and control. Moreover, decreased TRPV1 desensitization was observed in TG neurons exposed to repeat blast. Repeat, tertiary blast resulted in increased TRPV1, ET-1 and GFAP expression in the TG, enhanced nociception and decreased TRPV1 desensitization.

Role of peripheral and central TRPV1 receptors in facial heat hyperalgesia in streptozotocin-induced diabetic rats

There is increasing evidence that diabetes may be related to sensory changes in the trigeminal system. Long lasting facial heat hyperalgesia has been described in diabetic rats, but the mechanisms remain to be elucidated. Herein, the contribution of peripheral and central TRPV1 receptors to facial heat hyperalgesia in diabeticrats was investigated. Diabetes was induced in male Wistar rats by streptozotocin (60mg/kg, i.p) and facial heat hyperalgesia was assessed once a week up to four weeks. The role of TRPV1 receptors in the heat hyperalgesia in diabetic rats was evaluated through: 1) the ablation of TRPV1 receptors by resiniferatoxin (RTX) treatment and 2) injection of the TRPV1 antagonist, capsazepine, into the upper lip, trigeminal ganglion or medullary subarachnoid space, at doses that completed prevented the heat hyperalgesia induced by capsaicin in naïve rats. Western blot was used to estimate the changes in TRPV1 expression in diabetic rats. Diabetic rats exhibited facial heat hyperalgesia from the first up to the fourth week after streptozotocin injection, which was prevented by insulin treatment. Ablation of TRPV1-expressing fibers prevented facial hyperalgesia in diabetic rats. Capsazepine injection in all sites resulted in significant reduction of facial heat hyperalgesia in diabetic rats. Diabetic rats exhibited a significant decrease in TRPV1 expression in the trigeminal nerve, increased expression in the trigeminal ganglion and no changes in subnucleus caudalis when compared to normoglycemic ones. In conclusion, our results suggest that facial heat hyperalgesia in diabetic rats is maintained by peripheral and central TRPV1 receptors activation.

The effects of endothelin-1 on satellite glial cells in peripheral ganglia

Endothelins (ET) are a family of highly active neuropeptides with manifold influences via ET receptors (ETR) in both the peripheral and central nervous systems. We have shown previously that satellite glial cells (SGCs) in mouse trigeminal ganglia (TG) are extremely sensitive to ET-1 in evoking [Ca²⁺]_in increase, apparently via ET_BR activation, but there is no functional information on ETR in SGCs of other peripheral ganglia. Here we tested the effects of ET-1 on SGCs in nodose ganglia (NG), which is sensory, and superior cervical ganglia (Sup-CG), which is part of the sympathetic nervous system, and further investigated the influence of ET-1 on SGCs in TG. Using calcium imaging we found that SGCs in intact, freshly isolated NG and Sup-CG are highly sensitive to ET-1, with threshold concentration at 0.1nM. Our results showed that [Ca²⁺]_in elevation in response to ET-1 was partially due to Ca²⁺ influx from the extracellular space and partially to Ca²⁺ release from intracellular stores. Using receptor selective ETR agonists and antagonists, we found that the responses were mediated by mixed ET_AR/ET_BR in SGCs of NG and predominantly by ET_BR in SGCs of Sup-CG. By employing intracellular dye injection we examined coupling among SGCs around different neurons in the presence of 5nM ET-1 and observed coupling inhibition in all the three ganglion types. In summary, our work showed that SGCs in mouse sensory and sympathetic ganglia are highly sensitive to ET-1 and that this peptide markedly reduces SGCs coupling. We conclude that ET-1, which may participate in neuron-glia communications, has similar functions in wide range of peripheral ganglia.

Potential functional and pathological side effects related to off-target pharmacological activity

Most pharmaceutical companies test their discovery-stage proprietary molecules in a battery of in vitro pharmacology assays to try to determine off-target interactions. During all phases of drug discovery and development, various questions arise regarding potential side effects associated with such off-target pharmacological activity. Here we present a scientific literature curation effort undertaken to determine and summarize the most likely functional and pathological outcomes associated with interactions at 70 receptors, enzymes, ion channels and transporters with established links to adverse effects. To that end, the scientific literature was reviewed using an on-line database, and the most commonly reported effects were summarized in tabular format. The resultant table should serve as a practical guide for research scientists and clinical investigators for the prediction and interpretation of adverse side effects associated with molecules interacting with components of this screening battery.

Evaluation of heat hyperalgesia and anxiety like-behaviors in a rat model of orofacial cancer

Pain and anxiety are commonly experienced by cancer patients and both significantly impair their quality of life. Some authors claim that there is a relationship between pain and anxiety, while others suggest that there is not a direct association. In any case, there is indeed a consensus that anxiety impairs the pain condition beyond be under diagnosed and undertreated in cancer pain patients. Herein we investigated if rats presenting heat hyperalgesia induced by orofacial cancer cell inoculation would display anxiety-like behaviors. In addition, we evaluated if pain blockade would result in alleviation of anxiety behaviors, as well as, if blockade of anxiety would result in pain relief. Orofacial cancer was induced in male Wistar rats by inoculation of Walker-256 cells into the right vibrissal pad. Heat facial hyperalgesia was assessed on day 6 after the inoculation, and on this time point rats were submitted to the elevated plus maze and the light-dark transition tests. The influence of lidocaine and midazolam on heat hyperalgesia and anxiety-like behaviors was assessed. The peak of facial heat hyperalgesia was detected 6 days after cancer cells inoculation, and at this time point, rats exhibited increased anxiety-like behaviors. Local treatment with lidocaine (2%/50μL) caused a marked reduction of heat hyperalgesia, but failed to affect the anxiety-like behaviors, while midazolam (0.5mg/kg, i.p.) treatment failed to change the heat threshold, but induced an anxiolytic-like effect. Altogether, our data demonstrated that rats with orofacial cancer present pain- and anxiety-like behaviors, but brief heat hyperalgesia relief does not affect the anxiety-like behaviors, and vice-versa, in our experimental conditions.

Periodontal CGRP contributes to orofacial pain following experimental tooth movement in rats

Calcitonin-related gene peptide (CGRP) plays an important role in orofacial inflammatory pain. The aim of this study was to determine whether periodontal CGRP contributes to orofacial pain induced by experimental tooth movement in rats. Male Sprague-Dawley rats were used in this study. Closed coil springs were used to deliver forces. Rats were euthanized on 0d, 1d, 3d, 5d, 7d, and 14d following experimental tooth movement. Then, alveolar bones were obtained for immunostaining of periodontal tissues against CGRP. Two hours prior to euthanasia on each day, orofacial pain levels were assessed through rat grimace scale. CGRP and olcegepant (CGRP receptor antagonist) were injected into periodontal tissues to verify the roles of periodontal CGRP in orofacial pain induced by experimental tooth movement. Periodontal CGRP expression levels and orofacial pain levels were elevated on 1d, 3d, 5d, and 7d following experimental tooth movement. The two indices were significantly correlated with each other and fitted into a dose-response model. Periodontal administration of CGRP could elevate periodontal CGRP expressions and exacerbate orofacial pain. Moreover, olcegepant administration could decrease periodontal CGRP expressions and alleviate orofacial pain. Therefore, periodontal CGRP plays an important role in pain transmission and modulation following experimental tooth movement. We suggest that it may participate in a positive feedback aiming to amplify orofacial pain signals.

Endothelin systems in the brain: involvement in pathophysiological responses of damaged nerve tissues

In addition to their potent vasoconstriction effects, endothelins (ETs) show multiple actions in various tissues including the brain. The brain contains high levels of ETs, and their production is stimulated in many brain disorders. Accumulating evidence indicates that activation of brain ET receptors is involved in several pathophysiological responses in damaged brains. In this article, the roles of brain ET systems in relation to brain disorders are reviewed. In the acute phase of stroke, prolonged vasospasm of cerebral arteries and brain edema occur, both of which aggravate brain damage. Studies using ET antagonists show that activation of ETA receptors in the brain vascular smooth muscle induces vasospasm after stroke. Brain edema is induced by increased activity of vascular permeability factors, such as vascular endothelial growth factor and matrix metalloproteinases. Activation of ETB receptors stimulates astrocytic production of these permeability factors. Increases in reactive astrocytes are observed in neurodegenerative diseases and in the chronic phase of stroke, where they facilitate the repair of damaged nerve tissues by releasing neurotrophic factors. ETs promote the induction of reactive astrocytes through ETB receptors. ETs also stimulate the production of astrocytic neurotrophic factors. Recent studies have shown high expression of ETB receptors in neural progenitors. Activation of ETB receptors in neural progenitors promotes their proliferation and migration, suggesting roles for ETB receptors in neurogenesis. Much effort has been invested in the pursuit of novel drugs to induce protection or repair of damaged nerve tissues. From these studies, the pharmacological significance of brain ET systems as a possible target of neuroprotective drugs is anticipated.

Vitamin B complex attenuated heat hyperalgesia following infraorbital nerve constriction in rats and reduced capsaicin in vivo and in vitro effects

Vitamins of the B complex attenuate some neuropathic pain sensory aspects in various animal models and in patients, but the mechanisms underlying their effects remain to be elucidated. Herein it was investigated if the treatment with a vitamin B complex (VBC) reduces heat hyperalgesia in rats submitted to infraorbital nerve constriction and the possibility that TRPV1 receptors represent a target for B vitamins. In the present study, the VBC refers to a combination of vitamins B1, B6 and B12 at low- (18, 18 and 1.8mg/kg, respectively) or high- (180, 180 and 18mg/kg, respectively) doses. Acute treatment of rats with either the low- or the high-doses combination reduced heat hyperalgesia after nerve injury, but the high-doses combination resulted in a long-lasting effect. Repeated treatment with the low-dose combination reduced heat hyperalgesia on day four after nerve injury and showed a synergist effect with a single injection of carbamazepine (3 or 10mg/kg), which per se failed to modify the heat threshold. In naïve rats, acute treatment with the high-dose of VBC or B1 and B12 vitamins independently reduced heat hyperalgesia evoked by capsaicin (3µg into the upper lip). Moreover, the VBC, as well as, each one of the B vitamins independently reduced the capsaicin-induced calcium responses in HEK 293 cells transiently transfected with the human TRPV1 channels. Altogether, these results indicate that B vitamins can be useful to control heat hyperalgesia associated with trigeminal neuropathic pain and that modulation of TRPV1 receptors may contribute to their anti-hyperalgesic effects.

Divergence in endothelin-1- and bradykinin-activated store-operated calcium entry in afferent sensory neurons

Endothelin-1 (ET-1) and bradykinin (BK) are endogenous peptides that signal through Gαq/11-protein coupled receptors (GPCRs) to produce nociceptor sensitization and pain. Both peptides activate phospholipase C to stimulate Ca²⁺ accumulation, diacylglycerol production, and protein kinase C activation and are rapidly desensitized via a G-protein receptor kinase 2-dependent mechanism. However, ET-1 produces a greater response and longer lasting nocifensive behavior than BK in multiple models, indicating a potentially divergent signaling mechanism in primary afferent sensory neurons. Using cultured sensory neurons, we demonstrate significant differences in both Ca²⁺ influx and Ca²⁺ release from intracellular stores following ET-1 and BK treatments. As intracellular store depletion may contribute to the regulation of other signaling cascades downstream of GPCRs, we concentrated our investigation on store-operated Ca²⁺ channels. Using pharmacological approaches, we identified transient receptor potential canonical channel 3 (TRPC3) as a dominant contributor to Ca²⁺ influx subsequent to ET-1 treatment. On the other hand, BK treatment stimulated Orai1 activation, with only minor input from TRPC3. Taken together, data presented here suggest that ET-1 signaling targets TRPC3, generating a prolonged Ca²⁺ signal that perpetuates nocifensive responses. In contrast, Orai1 dominates as the downstream target of BK receptor activation and results in transient intracellular Ca²⁺ increases and abridged nocifensive responses.

Intraganglionar resiniferatoxin prevents orofacial inflammatory and neuropathic hyperalgesia

Trigeminal ganglion C-fiber neurons bearing transient receptor potential vanilloid-1 (TRPV1) channels are selectively destroyed by resiniferatoxin (RTX), a potent capsaicin analogue. The current study assessed the effect of an RTX injection (200 ng/4 μl) into the trigeminal ganglion in inflammatory and neuropathic rat models of orofacial thermal hyperalgesia. Intraganglionar RTX injection resulted in trigeminal ganglion C-fiber deletion, which was confirmed by the capsaicin eye wipes test, performed 6 days after the injection. The nociceptive responses induced by 2.5% formalin injected into the orofacial region were unchanged by a previous intraganglionar RTX injection. However, orofacial heat and cold hyperalgesia, induced by carrageenan injected into the upper lip (50 µg/50 μl), was abolished by previous intraganglionar RTX treatment. In addition, the development of orofacial heat and cold hyperalgesia after constriction of the infraorbital nerve was prevented by previous RTX treatment. Thus, trigeminal ganglion neurons expressing TRPV1 are crucial for the development of orofacial inflammatory and neuropathic thermal hyperalgesia.

Review of preclinical studies on treatment of mucositis and associated pain

Oral mucositis is a significant problem in cancer patients treated with radiation or chemotherapy, often hindering definitive cancer treatment. For patients with oral mucositis, pain is the most distressing symptom, leading to loss of orofacial function and poor quality of life. While oral mucositis has been well-described, its pathophysiology is poorly understood. Oral health professionals treating patients with mucositis have almost no effective therapies to treat or prevent oral mucositis. The purpose of this review is to (1) describe the current preclinical models of oral mucositis and their contribution to the understanding of mucositis pathophysiology, (2) explore preclinical studies on therapies targeting mucositis and discuss the clinical trials that have resulted from these preclinical studies, and (3) describe the proposed pathophysiology of oral mucositis pain and preclinical modeling of oral mucositis pain.

Peripheral substance P and neurokinin-1 receptors have a role in inflammatory and neuropathic orofacial pain models

There is accumulating evidence that substance P released from peripheral sensory neurons participates in inflammatory and neuropathic pain. In this study it was investigated the ability of substance P to induce orofacial nociception and thermal and mechanical hyperalgesia, as well as the role of NK1 receptors on models of orofacial inflammatory and neuropathic pain. Substance P injected into the upper lip at 1, 10 and 100 μg/50 μL failed to induce nociceptive behavior. Also, substance P (0.1-10 μg/50 μL) injected into the upper lip did not evoke orofacial cold hyperalgesia and when injected at 1 μg/50 μL did not induce mechanical hyperalgesia. However, substance P at this latter dose induced orofacial heat hyperalgesia, which was reduced by the pre-treatment of rats with a non-peptide NK1 receptor antagonist (SR140333B, 3mg/kg). Systemic treatment with SR140333B (3 mg/kg) also reduced carrageenan-induced heat hyperalgesia, but did not exert any influence on carrageenan-induced cold hyperalgesia. Blockade of NK1 receptors with SR140333B also reduced by about 50% both phases of the formalin response evaluated in the orofacial region. Moreover, heat, but not cold or mechanical, hyperalgesia induced by constriction of the infraorbital nerve, a model of trigeminal neuropathic pain, was abolished by pretreatment with SR140333B. Considering that substance P was peripherally injected (i.e. upper lip) and the NK1 antagonist used lacks the ability to cross the blood-brain-barrier, our results demonstrate that the peripheral SP/NK1 system participates in the heat hyperalgesia associated with inflammation or nerve injury and in the persistent pain evoked by formalin in the orofacial region.

Endothelin receptor-mediated responses in trigeminal ganglion neurons

Recent evidence implicates endothelin in nociception, but it is unclear how endothelin activates trigeminal ganglion (TRG) neurons. In the present study, we investigated the expression of the endothelin receptors ETA and ETB and endothelin-induced responses in rat TRG neurons. Double-immunofluorescence studies demonstrated that ETA and ETB were expressed in TRG neurons and that 26% of ETA- or ETB-expressing neurons expressed both receptors. During whole-cell patch-clamp recording, endothelin-1 enhanced an induced current in response to capsaicin, a TRPV1 agonist, in approximately 20% of dissociated neurons. The enhancement was blocked by the PKC inhibitor chelerythrine and by the ETA antagonist BQ-123, but not by the ETB antagonist BQ-788. Ca(2+)-imaging showed that endothelin-1 increased the intracellular Ca(2+) concentration in more than 20% of the dissociated neurons. Importantly, unlike the effect of endothelin-1 on capsaicin-induced current, the Ca(2+) response was largely suppressed by BQ-788 but not by BQ-123. These results suggest that ETA-mediated TRPV1 hyperactivation via PKC activation and ETB-mediated Ca(2+) mobilization occurs in different subsets of TRG neurons. These endothelin-induced responses may contribute to the induction of orofacial pain. The ETB-mediated function in TRG neurons is a special feature in the trigeminal system because of no ETB expression in dorsal root ganglion neurons.

Orofacial sensory changes after streptozotocin-induced diabetes in rats

Peripheral neuropathy is a common complication of diabetes and is often accompanied by episodes of pain. There is evidence that diabetic neuropathy may affect the trigeminal nerve, altering the transmission of orofacial sensory information. Structural changes in the trigeminal ganglia may be involved in the development of these sensory alterations. Herein, we evaluate the development of orofacial sensory changes after streptozotocin-induced diabetes in rats, and their sensitivity to pregabalin and morphine treatments. Furthermore, stereological analysis of the trigeminal ganglia was performed. Diabetic rats showed similar responses to 1% formalin applied into the upper lip compared to normoglycemic rats on weeks 1, 2 and 4 after streptozotocin. Additionally, there was no difference in the facial mechanical threshold of normoglycemic and diabetic rats, on weeks 1 up to 5 after streptozotocin, while the paw mechanical threshold of diabetic rats was significantly reduced. In contrast, diabetic rats developed long-lasting orofacial heat and cold hyperalgesia. Moreover, stereological analyses revealed significant neuronal loss in the trigeminal ganglia of diabetic compared to normoglycemic rats. Pregabalin treatment (30mg/kg, p.o.) of diabetic rats resulted in marked and prolonged (up to 6h) reduction of heat and cold orofacial hyperalgesia. Likewise, morphine treatment (2.5mg/kg, s.c.) abolished orofacial heat and cold hyperalgesia, but its effect was significant only up to 1h after the administration. In conclusion, the results of the present study demonstrated that streptozotocin-treated rats developed long-lasting orofacial heat and cold hyperalgesia, which is more amenable to reduction by pregabalin than morphine.

Behavioral testing in rodent models of orofacial neuropathic and inflammatory pain

Orofacial pain conditions are often very debilitating to the patient and difficult to treat. While clinical interest is high, the proportion of studies performed in the orofacial region in laboratory animals is relatively low, compared with other body regions. This is partly due to difficulties in testing freely moving animals and therefore lack of reliable testing methods. Here we present a comprehensive review of the currently used rodent models of inflammatory and neuropathic pain adapted to the orofacial areas, taking into account the difficulties and drawbacks of the existing approaches. We examine the available testing methods and procedures used for assessing the behavioral responses in the face in both mice and rats and provide a summary of some pharmacological agents used in these paradigms to date. The use of these agents in animal models is also compared with outcomes observed in the clinic.

B vitamins relieve neuropathic pain behaviors induced by infraorbital nerve constriction in rats

AIMS

There is mounting evidence that use of B vitamins can help control neuropathic pain. This study investigated if treatment with B1, B6 and B12 vitamins, alone or in combination with carbamazepine, can ameliorate distinct nociceptive behaviors in a model of trigeminal neuropathic pain.

MAIN METHODS

Male Wistar rats were submitted to infraorbital nerve constriction or sham surgery and received a 5-day treatment with one of the B vitamins, a single carbamazepine injection or the association of both treatments and were tested for facial thermal and mechanical hyperalgesia at different time intervals.

KEY FINDINGS

Repeated treatment with B1 (thiamine), B6 (pyridoxine) and B12 (cyanocobalamin) vitamins (at 180, 180 and 18 mg/kg/day, respectively, for 5 days) prevented the development of heat hyperalgesia after infraorbital nerve injury, but only B12 and B6 treatments attenuated cold and mechanical hyperalgesia, respectively. A single injection of carbamazepine (30 mg/kg) significantly reduced thermal, but not mechanical, hyperalgesia after nerve injury. Combinations of lower doses of each B vitamin (B1 and B6 at 18 mg/kg/day and B12 at 1.8 mg/kg/day for 5 days) with carbamazepine (10mg/kg) markedly reduced heat hyperalgesia after infraorbital nerve injury. Treatment with B12 (1.8 mg/kg/day) combined with carbamazepine (10mg/kg) also synergized to attenuate cold hyperalgesia at some time points, but combination of B6 (18 mg/kg/day) with carbamazepine (30 mg/kg) failed to modify mechanical hyperalgesia.

SIGNIFICANCE

We suggest that B vitamins might constitute a relevant adjuvant to control some aspects of the pain afflicting patients suffering from trigeminal neuropathic pain.

Functional study of endothelin B receptors in satellite glial cells in trigeminal ganglia

There is immunohistochemical evidence for endothelin (ET) receptors in satellite glial cells in sensory ganglia, but there is no information on the function of these receptors. We used calcium imaging to study this question in isolated mouse trigeminal ganglia and found that satellite glial cells are highly sensitive to ET-1, with threshold at 0.05 nM. Responses displayed strong desensitization at ET-1 concentrations of more than 1 nM. A large component of the response persisted when Ca was deleted from the external medium, consistent with Ca release from internal stores. The use of receptor selective agents showed that the responses were mediated by ETB receptors. We conclude that satellite glial cells display endothelin receptors, which may participate in neuron-glia communications in the trigeminal ganglia.

ET-1 induced Elevation of intracellular calcium in clonal neuronal and embryonic kidney cells involves endogenous endothelin-A receptors linked to phospholipase C through Gα(q/11)

Endothelin-1 (ET-1) is a pain mediator, elevated in skin after injury, which potentiates noxious thermal and mechanical stimuli (hyperalgesia) through the activation of ET(A) (and, perhaps, ET(B)) receptors on pain fibers. Part of the mechanism underlying this effect has recently been shown to involve potentiation of neuronal TRPV1 by PKCɛ. However, the early steps of this pathway, which are recapitulated in HEK 293 cells co-expressing TRPV1 and ET(A) receptors, remain unexplored. To clarify these steps, we investigated the pharmacological profile and signaling properties of native endothelin receptors in immortalized cell lines including HEK 293 and ND7 model sensory neurons. Previously we showed that in ND7/104, a dorsal root ganglia-derived cell line, ET-1 elicits a rise in intracellular calcium ([Ca(2+)](in)) which is blocked by BQ-123, an ET(A) receptor antagonist, but not by BQ-788, an ET(B) receptor antagonist, suggesting that ET(A) receptors mediate this effect. Here we extend these findings to HEK 293T cells. Examination of the expression of ET(A) and ET(B) receptors by RT-PCR and [(125)I]-ET-1 binding experiments confirms the slight predominance of ET(A) receptor binding sites and messenger RNA in both ND7/104 and HEK 293T cells. In addition, selective agonists of the ET(B) receptor (sarafotoxin 6c, BQ-3020 or IRL-1620) do not induce a transient increase in [Ca(2+)](in). Furthermore, reduction of ET(B) mRNA levels by siRNA does not abrogate calcium mobilization by ET-1 in HEK 293T cells, corroborating the lack of an ET(B) receptor role in this response. However, in HEK 293 cells with low endogenous ET(A) mRNA levels, ET-1 does not induce a transient increase in [Ca(2+)](in). Observation of the [Ca(2+)](in) elevation in ND7/104 and HEK 293T cells in the absence of extracellular calcium suggests that ET-1 elicits a release of calcium from intracellular stores, and pretreatment of the cells with pertussis toxin or a selective inhibitor of phospholipase C (PLC) point to a mechanism involving Gαq/11 coupling. These results are consistent with the hypothesis that a certain threshold of ET(A) receptor expression is necessary to drive a transient [Ca(2+)](in) increase in these cells and that this process involves release of calcium from intracellular stores following Gαq/11 activation.

The involvement of TRPA1 channel activation in the inflammatory response evoked by topical application of cinnamaldehyde to mice

AIMS

In the present work, we characterize the inflammatory process induced by the topical application of cinnamaldehyde on the skin of mice and verify the participation of transient receptor potential A1 TRPA1 receptors in this process.

MAIN METHODS

We measured mouse ear edema and sensitization/desensitization after topical application of cinnamaldehyde or/and capsaicin. We also quantified cellular infiltration through myeloperoxidase (MPO) activity and histological and immunohistochemical analyses and evaluated the expression of TRPV1 and TRPA1 by western blot.

KEY FINDINGS

Cinnamaldehyde induced ear edema in mice (1-6μg/ear) with a maximum effect of 4μg/ear. Cinnamaldehyde promoted leukocyte infiltration as detected by increasing MPO activity and confirmed by histological analyses. The edema and cellular infiltration evoked by the application of 4μg/ear of cinnamaldehyde were prevented by topical application of ruthenium red, a non-selective TRP antagonist as well as camphor and HC030031, two TRPA1 receptor antagonists. Cinnamaldehyde-induced edema, but not cellular infiltration, was prevented by topical application of the tachykinin NK1 antagonist, aprepitant, indicating a neuropeptide release phenomenon in this process. Additionally, we observed that repeated topical applications of cinnamaldehyde did not induce changes in sensitization or desensitization with respect to the edema response. Interestingly, repeated treatment with the TRPV1 agonist, capsaicin, abrogated it edematogenic response, confirming the desensitization process and partially decreasing the cinnamaldehyde-induced edema, suggesting the involvement of capsaicin-sensitive fibers.

SIGNIFICANCE

Our data demonstrate that the topical application of cinnamaldehyde produces an inflammatory response that is dependent on TRPA1 receptor stimulation.

Endothelins as pronociceptive mediators of the rat trigeminal system: role of ETA and ETB receptors

The trigeminal nerve is comprised of three main divisions, ophthalmic, maxillary and mandibular, each providing somatosensory innervation to distinct regions of the head, face and oral cavity. Recently, a role for endothelins in nociceptive signaling in the trigeminal system has been proposed. The present study aimed to gain better insight into the participation of the endothelin system in trigeminal nociceptive transmission. Herein ET-1 and ET-3 mRNA was detected in the rats' trigeminal ganglion (TG). Fluorescent labeling of TG neurons revealed that ET(A) and ET(B) receptors are distributed along the entire TG, but ET(A) receptor expression slightly predominated within the three divisions. TRPV1 receptors were also detected throughout the entire TG, and a significant proportion of TRPV1-positive neurons (approximately 30%) co-expressed either ET(A) or ET(B) receptors. Our behavioral data showed that ET-1 (3 to 30 pmol/site) induced overt nociceptive responses after injection into the upper lip or temporomandibular joint (TMJ) and hyperalgesic actions when applied to the eye, while ET-3 and the selective ET(B) receptor agonist IRL-1620 (each at 3 to 30 pmol/site) showed only the first two effects. Injection of BQ-123, but not BQ-788 (ET(A) and ET(B) receptor antagonists, respectively, 10 nmol/site each, 30 min beforehand), into the ipsilateral upper lip abolished ET-1 induced facial grooming, but both antagonists markedly reduced the nociceptive responses induced by ET-1 injected into the TMJ. Taken together, these findings suggest that endothelins, acting through ET(A) and/or ET(B) receptors, may play an important role in mediating pain resulting from activation of most trigeminal nerve branches.

Kinin B(1) and B(2) receptors contribute to orofacial heat hyperalgesia induced by infraorbital nerve constriction injury in mice and rats

Mechanisms coupled to kinin B(1) and B(2) receptors have been implicated in sensory changes associated to various models of neuropathy. The current study aimed to investigate if kinins also participate in orofacial thermal hyperalgesia induced by constriction of the infraorbital nerve (CION), a model of trigeminal neuropathic pain which displays persistent hypersensitivity to orofacial sensory stimulation, in rats and mice. Male Swiss mice (30-35g) or Wistar rats (200-250g; n=6-10 per group in both cases) underwent CION or sham surgery and were submitted repeatedly to application of heat ( approximately 50 degrees C) to the ipsilateral or contralateral snout, delivered by a heat source placed 1cm from the vibrissal pad. Decreases in latency to display head withdrawal or vigorous snout flicking were considered indicative of heat hyperalgesia. CION caused long-lasting heat hyperalgesia which started on Day 2 after surgery in both species and lasted up to Day 17 in mice and Day 10 in rats. Administration of DALBK or HOE-140 (peptidic B(1) and B(2) receptor antagonists, respectively; each at 3nmol in 10microl) onto the exposed infraorbital nerve of mice at the moment of surgery delayed the development of the thermal hyperalgesia. Systemic treatment on Day 5 (mice) or Day 4 (rats) with Des-Arg(9), Leu(8)-Bradykinin (DALBK, B(1) receptor antagonist, 0.1-1micromol/kg, i.p.) or HOE-140 (B(2) receptor antagonist, 0.001-1micromol/kg, i.p.) transiently reduced heat hyperalgesia in both species. Due to the peptidic nature of DALBK and HOE-140, it is likely that their effects reported herein resulted from blockade of peripheral kinin receptors. Thus, mechanisms operated by kinin B(1) and B(2) receptors, contribute to orofacial heat hyperalgesia induced by CION in both mice and rats. Perhaps kinin B(1) and B(2) receptor antagonists might constitute effective preventive and curative treatments for orofacial thermal hyperalgesia induced by nerve injury.

Contribution of peripheral endothelin ETA and ETB receptors in neuropathic pain induced by spinal nerve ligation in rats

Endothelins (ETs) contribute to the sensory changes seen in animals models of inflammatory, cancer and diabetic neuropathic pain, but little is known about their nociceptive role following peripheral nerve injury. The current study evaluated mechanisms by which ETs can drive changes in nociceptive responses to thermal stimulation of the hind paw of rats induced by unilateral lumbar L5/L6 spinal nerve ligation (SNL) injury. SNL sensitizes rats to acetone-evoked cooling of and radiant heat application (Hargreaves test) to the ipsilateral hind paw (throughout 3-40 and 9-40 days after surgery, respectively). At 12 days after SNL, intraplantar (i.pl.) injection of endothelin-1 (ET-1, 10 pmol) induces greater overt nociception that was reduced only by treatment with the selective ET(A) peptidic antagonist (BQ-123, 10 nmol, i.pl), but unchanged by the selective ET(B) peptidic antagonist (BQ-788). Cold allodynia evoked by cooling the ipsilateral hind paw with acetone was reduced by i.pl. injection of both antagonists BQ-123 or BQ-788 (3 or 10 nmol). In contrast, heat hyperalgesia evaluated by Hargreaves method was reduced only by BQ-123. SNL enhanced the [Ca(+2)](i) increases induced by ET-1 (100 nM) in neurons from L5/L6 (injured) and L4 (intact) cultured dorsal root ganglion, but did not change the responses of non-neuronal cells. Furthermore, Western blot analysis revealed that SNL increased ET(A) and ET(B) receptor protein expression in spinal nerves. Thus, SNL induces marked hind paw hypersensitivity to thermal stimulation in part via up-regulation of peripheral sensory nerve pronociceptive ET(A) and ET(B) receptor-operated mechanisms.

Calcitonin gene-related peptide selectively relaxes contractile responses to endothelin-1 in rat mesenteric resistance arteries

We tested the hypothesis that endothelin-1 (ET-1) modulates sensory-motor nervous arterial relaxation by prejunctional and postjunctional mechanisms. Isolated rat mesenteric resistance arteries were investigated with immunohistochemistry, wire-myography, and pharmacological tools. ET(A)- and ET(B)-receptors could be visualized on the endothelium and smooth muscle and on periarterial fibers containing calcitonin gene-related peptide (CGRP). Arterial contractile responses to ET-1 (0.25-16 nM) were not modified by blockade of ET(B)-receptors, NO-synthase, and cyclooxygenase or desensitization of transient receptor potential cation channel, subfamily V, member 1 (TRPV1) with capsaicin. ET-1 reversed relaxing responses to CGRP in depolarized arteries. This effect was inhibited by ET(A)-antagonists. It was not selective because ET-1 also reversed relaxing responses to Na-nitroprusside (SNP) and because phenylephrine (PHE; 0.25-16 microM) similarly reversed relaxing responses to CGRP or SNP. Conversely, contractile responses to ET-1 were, compared with PHE, hypersensitive to the relaxing effects of the TRPV1-agonist capsaicin and to exogenous CGRP, but not to acetylcholine, forskolin, pinacidil, or SNP. In conclusion, ET-1 does not stimulate sensory-motor nervous arterial relaxation, but ET(A)-mediated arterial contractions are selectively sensitive to relaxation by the sensory neurotransmitter CGRP. This does not involve NO, cAMP, or ATP-sensitive K(+) channels.