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Temporomandibular disorder (TMD) is a common painful condition of the temporomandibular joint (TMJ) and associated structures. Stress is a significant risk factor for developing this painful condition that predominantly affects women. This study aimed to test the hypothesis that stress increases the risk of developing TMJ pain by facilitating inflammatory mechanisms in female and male rats. To test this hypothesis, we evaluated TMJ carrageenan-induced expression of pro-inflammatory cytokines and migration of inflammatory cells and TMJ formalin-induced nociception in female and male rats submitted to a repeated stress protocol induced by sound. We found that sound-induced repeated stress facilitates TMJ inflammation and contributes to TMJ nociception development equally in females and males. We conclude that stress is a risk factor for developing painful TMJ conditions in males and females, at least in part, by favoring the inflammatory process similarly in both sexes., (© 2023 Scandinavian Division of the International Association for Dental Research. Published by John Wiley & Sons Ltd.)

Peripheral diabetic neuropathy (DN) manifests in nearly 60% of diabetic patients, being pain its most debilitating symptom. Although electrophysiological and morphological aspects are well described, little is known about its development and progression, undermining effective therapies. Hyperglycemia and insulin signaling impairment are considered the triggering events of oxidative stress observed in the dying nerves, however there are still many gaps in the knowledge of intracellular plastic changes it generates., Aims: In this study we aimed to evaluate the early transcriptome changes in DN when the first symptoms of the disease start to appear., Main Methods: Next-Generation Sequencing of messenger RNA (RNA-Seq) of L4 and L5 dorsal root ganglia (DRG) four weeks post-diabetes induction in a rat model for type 1 diabetes., Key Findings: RNA sequencing found 66 transcripts differentially expressed between diabetic and control groups, related mainly to the following biological processes: inflammation, hyperalgesia/analgesia, cell growth and cell survival. Given their roles, the differentially expressed genes suggest an attempt to switch to a survival/regenerative program., Significance: Our results show that changes in the transcriptome profile start to appear early in the course of DN and might be related to secure cell homeostasis. Hence, the present data may indicate how DRG cells are responding to hyperglycemia in its early stages and which mechanisms first fail to respond, further leading to cell damage and cell death. Early screening of cell alterations in DN might lead to more concrete targets for pharmaceutical interventions, which could more efficiently delay cell damage., (Copyright © 2018 Elsevier Inc. All rights reserved.)

Background: Although TRPA1, SP, histamine and 5-hydroxytryptamine (5-HT) have recognized contribution to nociceptive mechanisms, little is known about how they interact with each other to mediate inflammatory pain in vivo. In this study we evaluated whether TRPA1, SP, histamine and 5-HT interact, in an interdependent way, to induce nociception in vivo., Methods and Results: The subcutaneous injection of the TRPA1 agonist allyl isothiocyanate (AITC) into the rat's hind paw induced a dose-dependent and short lasting behavioral nociceptive response that was blocked by the co-administration of the TRPA1 antagonist, HC030031, or by the pretreatment with antisense ODN against TRPA1. AITC-induced nociception was significantly decreased by the co-administration of selective antagonists for the NK1 receptor for substance P, the H1 receptor for histamine and the 5-HT 1A or 3 receptors for 5-HT. Histamine- or 5-HT-induced nociception was decreased by the pretreatment with antisense ODN against TRPA1. These findings suggest that AITC-induced nociception depends on substance P, histamine and 5-HT, while histamine- or 5-HT-induced nociception depends on TRPA1. Most important, AITC interact in a synergistic way with histamine, 5-HT or substance P, since their combination at non-nociceptive doses induced a nociceptive response much higher than that expected by the sum of the effect of each one alone. This synergistic effect is dependent on the H1, 5-HT 1A or 3 receptors., Conclusion: Together, these findings suggest a self-sustainable cycle around TRPA1, no matter where the cycle is initiated each step is achieved and even subeffective activation of more than one step results in a synergistic activation of the overall cycle.

We have previously demonstrated that activation of kappa-opioid receptor located in the temporomandibular joint (TMJ) of rats induces a significantly greater TMJ antinociception in diestrus females than in proestrus females (higher estradiol serum levels than diestrus) and males. These findings indicate that gonadal hormones decrease TMJ kappa-mediated antinociception. The aim of this study was to investigate some of the mechanisms by which gonadal hormones decrease TMJ kappa-mediated antinociception. Western blot analysis demonstrated a significantly lower kappa-opioid receptor expression in the trigeminal ganglia of intact males than in intact and ovariechtomized (OVX) females and orchidectomized (ORX) males. In females, kappa-opioid receptor expression in the trigeminal ganglia was significantly lower in proestrus than in diestrus and OVX females. Taken together these findings suggest that gonadal hormones, especially male gonadal hormones, down-regulate kappa-opioid receptor expression. Co-application of the NOS inhibitor L-NMMA or the NO-sensitive guanylyl cyclase inhibitor ODQ with the kappa-opioid receptor agonist U50,488 blocked TMJ kappa-mediated antinociception in males and females. These findings suggest that antinociception induced by activation of kappa opioid receptors in the TMJ region is mediated by the L-arginine/NO/cGMP pathway in both sexes. Despite the involvement of the L-arginine/NO/cGMP pathway in TMJ kappa-mediated antinociception in both sexes, gonadal hormones do not diminish the activity of this pathway to decrease TMJ kappa-mediated antinociception. Alternatively, they significantly reduce kappa-opioid receptor expression in the trigeminal ganglia.

Background and Purpose: Protein kinase (PK) A and the epsilon isoform of PKC (PKCepsilon) are involved in the development of hypernociception (increased sensitivity to noxious or innocuous stimuli) in several animal models of acute and persistent inflammatory pain. The present study evaluated the contribution of PKA and PKCepsilon to the development of prostaglandin E(2) (PGE(2))-induced mechanical hypernociception., Experimental Approach: Prostaglandin E(2)-induced mechanical hypernociception was assessed by constant pressure rat paw test. The activation of PKA or PKCepsilon was evaluated by radioactive enzymic assay in the dorsal root ganglia (DRG) of sensory neurons from the hind paws., Key Results: Hypernociception induced by PGE(2) (100 ng) by intraplantar (i.pl.) injection, was reduced by i.pl. treatment with inhibitors of PKA [A-kinase-anchoring protein St-Ht31 inhibitor peptide (AKAPI)], PKCepsilon (PKCepsilonI) or adenylyl cyclase. PKA activity was essential in the early phase of the induction of hypernociception, whereas PKC activity was involved in the maintenance of the later phase of hypernociception. In the DRG (L4-L5), activity of PKA increased at 30 min after injection of PGE(2) but PKC activity increased only after 180 min. Moreover, i.pl. injection of the catalytic subunit of PKA induced hypernociception which was markedly reduced by pretreatment with an inhibitor of PKCepsilon, while the hypernociception induced by paw injection of PKCepsilon agonist was not affected by an inhibitor of PKA (AKAPI)., Conclusions and Implications: Taken together, these findings are consistent with the suggestion that PKA activates PKCepsilon, which is a novel mechanism of interaction between these kinases during the development of PGE(2)-induced mechanical hypernociception.

In the present study, the participation of the Na(V)1.8 sodium channel was investigated in the development of the peripheral pro-nociceptive state induced by daily intraplantar injections of PGE(2) in rats and its regulation in vivo by protein kinase A (PKA) and protein kinase C epsilon (PKCvarepsilon) as well. In the prostaglandin E(2) (PGE(2))-induced persistent hypernociception, the Na(V)1.8 mRNA in the dorsal root ganglia (DRG) was up-regulated. The local treatment with dipyrone abolished this persistent hypernociception but did not alter the Na(V)1.8 mRNA level in the DRG. Daily intrathecal administrations of antisense Na(V)1.8 decreased the Na(V)1.8 mRNA in the DRG and reduced ongoing persistent hypernociception. Once the persistent hypernociception had been abolished by dipyrone, but not by Na(V)1.8 antisense treatment, a small dose of PGE(2) restored the hypernociceptive plateau. These data show that, after a period of recurring inflammatory stimuli, an intense and prolonged nociceptive response is elicited by a minimum inflammatory stimulus and that this pro-nociceptive state depends on Na(V)1.8 mRNA up-regulation in the DRG. In addition, during the persistent hypernociceptive state, the PKA and PKCvarepsilon expression and activity in the DRG are up-regulated and the administration of the PKA and PKCvarepsilon inhibitors reduce the hypernociception as well as the Na(V)1.8 mRNA level. In the present study, we demonstrated that the functional regulation of the Na(V)1.8 mRNA by PKA and PKCvarepsilon in the primary sensory neuron is important for the development of the peripheral pro-nociceptive state induced by repetitive inflammatory stimuli and for the maintenance of the behavioral persistent hypernociception.

Previous work from our group showed that intrathecal (i.t.) administration of substances such as glutamate, NMDA, or PGE(2) induced sensitization of the primary nociceptive neuron (PNN hypernociception) that was inhibited by a distal intraplantar (i.pl.) injection of either morphine or dipyrone. This pharmacodynamic phenomenon is referred to in the present work as "teleantagonism". We previously observed that the antinociceptive effect of i.t. morphine could be blocked by injecting inhibitors of the NO signaling pathway in the paw (i.pl.), and this effect was used to explain the mechanism of opioid-induced peripheral analgesia by i.t. administration. The objective of the present investigation was to determine whether this teleantagonism phenomenon was specific to this biochemical pathway (NO) or was a general property of the PNNs. Teleantagonism was investigated by administering test substances to the two ends of the PNN (i.e., to distal and proximal terminals; i.pl. plus i.t. or i.t. plus i.pl. injections). We found teleantagonism when: (i) inhibitors of the NO signaling pathway were injected distally during the antinociception induced by opioid agonists; (ii) a nonselective COX inhibitor was tested against PNN sensitization by IL-1beta; (iii) selective opioid-receptor antagonists tested against antinociception induced by corresponding selective agonists. Although the dorsal root ganglion seems to be an important site for drug interactions, the teleantagonism phenomenon suggests that, in PNNs, a local sensitization spreads to the entire cell and constitutes an intriguing and not yet completely understood pharmacodynamic property of this group of neurons.

Neutrophil migration is responsible for tissue damage observed in inflammatory diseases. Neutrophils are also implicated in inflammatory nociception, but mechanisms of their participation have not been elucidated. In the present study, we addressed these mechanisms in the carrageenan-induced mechanical hypernociception, which was determined using a modification of the Randall-Sellito test in rats. Neutrophil accumulation into the plantar tissue was determined by the contents of myeloperoxidase activity, whereas cytokines and PGE(2) levels were measured by ELISA and radioimmunoassay, respectively. The pretreatment of rats with fucoidin (a leukocyte adhesion inhibitor) inhibited carrageenan-induced hypernociception in a dose- and time-dependent manner. Inhibition of hypernociception by fucoidin was associated with prevention of neutrophil recruitment, as it did not inhibit the hypernociception induced by the direct-acting hypernociceptive mediators, PGE(2) and dopamine, which cause hypernociception, independent of neutrophils. Fucoidin had no effect on carrageenan-induced TNF-alpha, IL-1beta, and cytokine-induced neutrophil chemoattractant 1 (CINC-1)/CXCL1 production, suggesting that neutrophils were not the source of hypernociceptive cytokines. Conversely, hypernociception and neutrophil migration induced by TNF-alpha, IL-1beta, and CINC-1/CXCL1 was inhibited by fucoidin, suggesting that neutrophils are involved in the production of direct-acting hypernociceptive mediators. Indeed, neutrophils stimulated in vitro with IL-1beta produced PGE(2), and IL-1beta-induced PGE(2) production in the rat paw was inhibited by the pretreatment with fucoidin. In conclusion, during the inflammatory process, the migrating neutrophils participate in the cascade of events leading to mechanical hypernociception, at least by mediating the release of direct-acting hypernociceptive mediators, such as PGE(2). Therefore, the blockade of neutrophil migration could be a target to development of new analgesic drugs.

The 15-deoxy-Delta(12,14)-prostaglandin J(2) (15d-PGJ(2)) is an endogenous ligand of peroxisome proliferator-activated receptors gamma (PPAR-gamma) and is now recognized as a potent anti-inflammatory mediator. However, information regarding the influence of 15d-PGJ(2) on inflammatory pain is still unknown. In this study, we evaluated the effect of 15d-PGJ(2) upon inflammatory hypernociception and the mechanisms involved in this effect. We observed that intraplantar administration of 15d-PGJ(2) (30-300 ng/paw) inhibits the mechanical hypernociception induced by both carrageenan (100 mug/paw) and the directly acting hypernociceptive mediator, prostaglandin E(2) (PGE(2)). Moreover, 15d-PGJ(2) [100 ng/temporomandibular joint (TMJ)] inhibits formalin-induced TMJ hypernociception. On the other hand, the direct administration of 15d-PGJ(2) into the dorsal root ganglion was ineffective in blocking PGE(2)-induced hypernociception. In addition, the 15d-PGJ(2) antinociceptive effect was enhanced by the increase of macrophage population in paw tissue due to local injection of thioglycollate, suggesting the involvement of these cells on the 15d-PGJ(2)-antinociceptive effect. Moreover, the antinociceptive effect of 15d-PGJ(2) was also blocked by naloxone and by the PPAR-gamma antagonist 2-chloro-5-nitro-N-phenylbenzamide (GW9662), suggesting the involvement of peripheral opioids and PPAR-gamma receptor in the process. Similar to opioids, the 15d-PGJ(2) antinociceptive action depends on the nitric oxide/cGMP/protein kinase G (PKG)/K(ATP)(+) channel pathway because it was prevented by the pretreatment with the inhibitors of nitric-oxide synthase (N(G)-monomethyl-l-arginine acetate), guanylate cyclase]1H-(1,2,4)-oxadiazolo(4,2-alpha)quinoxalin-1-one[, PKG [indolo[2,3-a]pyrrolo[3,4-c]carbazole aglycone (KT5823)], or with the ATP-sensitive potassium channel blocker glibenclamide. Taken together, these results demonstrate for the first time that 15d-PGJ(2) inhibits inflammatory hypernociception via PPAR-gamma activation. This effect seems to be dependent on endogenous opioids and local macrophages.

A considerable amount of evidence suggests that temporomandibular joint (TMJ) pain associated with temporomandibular disorder results, at least in part, from an inflammatory episode. Although histamine can cause pain, it is not clear whether this mediator induces nociception in the TMJ. In this study, we investigated the contribution of endogenous histamine to formalin-induced nociception in the TMJ of rats. We also investigated whether the administration of histamine induces nociception in the TMJ and, if so, whether this effect is mediated by an indirect action on primary afferent nociceptors. Local administration of the H1-receptor antagonist pyrilamine prevented formalin-induced nociception in the TMJ in a dose-dependent manner. Local administration of histamine (250 microg) in the TMJ induced nociceptive behavior that was inhibited by co-administration of the lidocaine N-ethyl bromide quaternary salt QX-314 (2%) or the selective H1-receptor antagonist pyrilamine (400 microg). Nociception induced by histamine was also inhibited by pre-treatment with sodium cromoglycate (800 microg) and by co-administration of the 5-HT(3) receptor antagonist tropisetron (400 mug), while pyrilamine (400 mug) did not inhibit nociception induced by 5-hydroxytryptamine (5-HT, 250 microg) in the TMJ. Furthermore, histamine, in a dose that did not induce nociception by itself, strongly enhanced 5-HT-induced nociception. Finally, the administration of a sub-threshold dose of 5-HT (100 microg), but not of histamine (100 microg), elicited nociception in the TMJ previously challenged with the inflammatory agent carrageenan (100 microg). In conclusion, these data suggest that histamine induces TMJ nociception by an indirect mechanism involving endogenous release of 5-HT and activation of 5-HT(3) receptors on sensory afferents. It is proposed that histamine activates the H1 receptor to induce the release of 5-HT which depolarizes the nociceptor by activating 5-HT(3) receptor.

There is pre-clinical evidence that therapies targeting IL-18 might be beneficial in controlling arthropathies, which are accompanied by hypernociception (nociceptor sensitization). In the present study, we addressed the hypernociceptive role of IL-18 in a model of antigen-induced inflammation in mice and its mechanisms. In naïve mice, the intraplantar injection of IL-18 induced dose- and time-dependent mechanical hypernociception, which was inhibited in IFN-gamma deficient (-/-) mice, and by the pre-treatment with bosentan (dual endothelin [ET] receptor antagonist), BQ123 (ET(A) receptor antagonist) or indomethacin (cyclooxygenase inhibitor). IL-18 hypernociception was unaffected in TNFR1(-/-) mice or by the pre-treatment with sIL-15Ralpha (soluble form of IL-15 receptor), BQ788 (ET(B) receptor antagonist) or guanethidine (sympathetic blocker). The ovalbumin (OVA) challenge-induced mechanical hypernociception in immunized mice was inhibited by the pre-treatment with anti-IL-18 antibody or in IL-18(-/-) mice. Furthermore, IL-18 induced significant IFN-gamma production in the paw skin of naïve mice. The OVA challenge-induced IFN-gamma and ET-1 productions were inhibited in IL-18(-/-) immunized mice, as well as ET-1 production in IFN-gamma(-/-) immunized mice. In addition, significant PGE2 production was detected after IL-18 or ET-1 (via ET(A) receptors) injection in naïve mice. Taken together with previous data, these results suggest that IL-18 plays a significant role in antigen-induced inflammatory hypernociception via the production of IFN-gamma, ET-1 and PGE2. Thus, IL-18 and IL-18-downstream mediators demonstrated herein might constitute targets to inhibit antigen-induced inflammatory pain.

In this study, we tested the potential use of a lectin from Lonchocarpus sericeus seeds (LSL), to control neutrophil migration and inflammatory hypernociception (decrease of nociceptive threshold). Pretreatment of the animals intravenously (15 min before) with LSL inhibited neutrophil migration to the peritoneal cavity in a dose-dependent fashion confirmed by an inhibition of rolling and adhesion of leukocytes by intravital microscopy. We also tested the ability of the pretreatment with LSL to inhibit neutrophil migration on immunised mice, and it was observed that a strong inhibition of neutrophil migration induced by ovoalbumin in immunized mice. Another set of experiments showed that pretreatment of the animals with LSL, inhibited the mechanical hypernociception in mice induced by the i.pl. injection of OVA in immunized mice and of carrageenan in naïve mice, but not that induced by prostaglandin E(2) (PGE(2)) or formalin. This anti-nociceptive effect correlated with an effective blockade of neutrophil influx, as assessed by the hind paw tissue myeloperoxidase levels. In addition, we measured cytokines (TNF-alpha and IL-1beta) and chemokines (MIP-1alpha [CCL3] and KC [CXCL1]) from the peritoneal exudates and i.pl. tissue. Animals treated with LSL showed inhibition of cytokines and chemokines release in a dose-dependent manner. In conclusion, we demonstrated that the inhibitory effects of LSL on neutrophil migration and mechanical inflammatory hypernocicepetion are associated with the inhibition of the production of cytokines and chemokines.

Pain is one of the classical signs of the inflammatory process in which sensitization of the nociceptors is the common denominator. This sensitization causes hyperalgesia or allodynia in humans, phenomena that involve pain perception (emotional component+nociceptive sensation). As this review focuses mainly on animal models, which don't allow discrimination of the emotional component, the terms nociception and hypernociception are used to describe overt behavior induced by mechanical stimulation and increase of nociceptor sensitivity, respectively. Pro- and anti-inflammatory cytokines and chemokines are endogenous small protein mediators released by local or migrating cells whose balance modulates the intensity of inflammatory response. The inflammatory stimuli or tissue injuries stimulate the release of characteristic cytokine cascades, which ultimately trigger the release of final mediators responsible for inflammatory pain. These final mediators, such as prostanoids or sympathetic amines, act directly on the nociceptors to cause hypernociception, which results from the lowering of threshold due to modulation of specific voltage-dependent sodium channels. Furthermore, a direct effect of cytokines on nociceptors is also described. On the other hand, there are also anti-inflammatory cytokines, such as interleukin (IL)-10, IL-4 and IL-13, and IL-1 receptor antagonists (IL-1ra), which inhibit the production of hypernociceptive cytokines and/or the final hypernociceptive mediators, preventing the installation of or the increase in the hypernociception. This review highlights the importance of the direct and indirect actions of cytokines and chemokines in inflammatory and neuropathic hypernociception, emphasizing the evidence suggesting these molecules are potential targets to develop novel drugs and therapies for the treatment of pain.

The aim of this study was to test the hypothesis that 5-hydroxytryptamine induces nociception by an indirect action on the primary afferent nociceptor in addition to its previously described direct action. Injection of 5-hydroxytryptamine into the s.c. tissue of the hind paw of rats produced nociceptive flinch behavior and inflammatory cell migration, that were significantly reduced by the nonspecific selectin inhibitor fucoidan. 5-Hydroxytryptamine-induced nociception was also significantly reduced by local blockade of the 5-HT3 receptor by tropisetron, by the cyclooxygenase inhibitor indomethacin and by local blockade of the beta1-adrenergic receptor or of the D1 receptor by atenolol or SCH 23390, respectively. Neither guanethidine depletion of norepinephrine in the sympathetic terminals nor local blockade of the beta2-adrenergic receptor by ICI-118,551 significantly reduced 5-hydroxytryptamine-induced nociception. Taken together, these findings indicate that 5-hydroxytryptamine induces nociception by a novel, indirect and norepinephrine-independent mechanism mediated by neutrophil migration and local release of prostaglandin and dopamine. Furthermore, to test whether dopamine acts on beta1-adrenergic and/or D1 receptor to contribute to 5-hydroxytryptamine-induced nociception, dopamine was s.c. injected either alone or combined with atenolol or with SCH 23390. S.c.-injected dopamine also produced a dose-dependent nociceptive behavior that was significantly reduced by both SCH 23390 and atenolol. Based on that it is proposed that dopamine, once released, activates D1 and beta1-adrenergic receptors to contribute to 5-hydroxytryptamine-induced nociception.

The present study was carried out in Wistar rats, using the jaw-opening reflex and dental pulp stimulation, to investigate noradrenaline- and serotonin-mediated antinociceptive circuits. The effects of microinjections of bradykinin into the principal sensory trigeminal nucleus (PSTN) before and after neurochemical lesions of the locus coeruleus noradrenergic neurons were studied. Neuroanatomical experiments showed evidence for reciprocal neuronal pathways connecting the locus coeruleus (LC) to trigeminal sensory nuclei and linking monoaminergic nuclei of the pain inhibitory system to spinal trigeminal nucleus (STN). Fast blue (FB) injections in the locus coeruleus/subcoeruleus region retrogradely labeled neurons in the contralateral PSTN and LC. Microinjections of FB into the STN showed neurons labeled in both ipsilateral and contralateral LC, as well as in the ipsilateral Barrington's nucleus and subcoeruleus area. Retrograde tract-tracing with FB also showed that the mesencephalic trigeminal nucleus sends neural pathways towards the ipsilateral PSTN, with outputs from cranial and caudal aspects of the brainstem. In addition, neurons from the lateral and dorsolateral columns of periaqueductal gray matter also send outputs to the ipsilateral PSTN. Microinjections of FB in the interpolar and caudal divisions of the STN labeled neurons in the caudal subdivision of STN. Microinjections in the STN interpolar and caudal divisions also retrogradely labeled serotonin- and noradrenaline-containing nucleus of the brainstem pain inhibitory system. Finally, the gigantocellularis complex (nucleus reticularis gigantocellularis/paragigantocellularis), nucleus raphe magnus and nucleus raphe pallidus also projected to the caudal divisions of the STN. Microinjections of bradykinin in the PSTN caused a statistically significant long-lasting antinociception, antagonized by the damage of locus coeruleus-noradrenergic neuronal fibres with (N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine) (DSP4), a neurotoxin that specifically depleted noradrenaline from locus coeruleus terminal fields. These data suggest that serotonin- and noradrenaline-containing nuclei of the endogenous pain inhibitory system exert a key-role in the antinociceptive mechanisms of bradykinin and the locus coeruleus is crucially involved in this effect.

Background and Purpose: Although interferential therapy (IFT) is used widely in the management of many painful conditions, the effectiveness and the mechanism of action of IFT in animal models of inflammatory pain have not been evaluated. The aim of this study was to evaluate the effectiveness of IFT in reducing inflammatory pain and edema in rats., Subjects: Sixty-nine male Wistar rats were used in the study., Methods: The effect of IFT application (4,000-Hz carrier frequency, 140-Hz amplitude-modulated beat frequency, pulse duration=125 milliseconds, current intensity=5 mA) for 1 hour on the formalin-induced nociceptive response and edema and on carrageenan-induced mechanical hyperalgesia and edema was evaluated., Results: Interferential therapy significantly reduced the formalin-evoked nociceptive response when applied to the paw immediately after but not before the formalin injection. Interferential therapy application at 2 hours after the carrageenan injection significantly prevented a further increase in carrageenan-induced mechanical hyperalgesia only immediately after discontinuation of the electrical current application. The antinociception induced by IFT was not attributable to a reduction in inflammation because IFT did not significantly reduce the edema induced by either formalin or carrageenan., Discussion and Conclusion: The results suggest that, despite its short-duration effect, IFT is effective in reducing inflammatory pain and should be considered primarily for use in the control of acute inflammatory pain.

Interleukin-12 (IL-12) is an inflammatory Th1-driving cytokine that has been clinically used as immune therapy and vaccine adjuvant. Recently, it was reported that patients receiving IL-12 presented hyperalgesia. In the present study, we investigated the mechanical hyperalgesic effect of IL-12 in rats using two tests: 1) paw constant pressure and 2) electronic pressure-meter. In both tests, intraplantar administration of IL-12 (3-30 ng paw(-1)) caused a dose- and time-dependent mechanical hyperalgesia, which peaked between 3 to 5 h, remaining significantly different from control levels until 7 h and resolved 24 h postinjection. However, the same doses of IL-12 did not induce thermal hyperalgesia, determined using the Hargreaves test. Pretreatments with effective doses of indomethacin (2.5 mg kg(-1)), atenolol (1 mg kg(-1)), 3-[1-(p-chlorobenzyl)-5-(isopropyl)-3-t-butylthioindol-2-yl]-2,2-dimethylpropanoic acid, sodium (MK886) (5-lipoxygenase activating protein inhibitor; 1 mg kg(-1)), or cyclo[(D)Trp-(D)Asp-Pro-(D)Val-Leu] (BQ123) [endothelin (ET)(A) receptor antagonist; 30 nmol paw(-1)] did not inhibit IL-12-evoked mechanical hyperalgesia (10 ng paw(-1)). However, dexamethasone (2 mg kg(-1)), morphine (3-12 microg paw(-1)), and N-cys-2,6 dimethylpiperidinocarbonyl-L-gamma-methylleucyl-D-1-methoxycarboyl-d-norleucine (BQ788) (ET(B) receptor antagonist; 3-30 nmol paw(-1)) did inhibit IL-12 hyperalgesia. Furthermore, neither pretreatment with effective doses of antiserum against rat-TNF-alpha (50 microl paw(-1)) nor against IL-18 (10 microg paw(-1)) inhibited the IL-12-induced hyperalgesia. Likewise, antiserum against IL-12 (10 ng paw(-1)) did not alter IL-18-induced hyperalgesia. In conclusion, we demonstrated for the first time that IL-12 is a prohyperalgesic cytokine that induces mechanical hyperalgesia mediated by endothelin action on the ET(B) receptor. Therefore, endothelin receptor antagonism could be beneficial in controlling IL-12 therapy-induced pain or hyperalgesia.

We previously described an animal model of persistent inflammatory sensitization of nociceptors. In this model the hypernociception persists for more than 30 days after the cessation of 2 weeks of daily intraplantar treatment with prostaglandin E(2) (PGE(2)). The tetrodotoxin-resistant (TTX-R) voltage-gated sodium channel Na(V)1.8 is considered a characteristic of primary afferent nociceptive C fibers and plays an important role in acute hypernociception. In the present study, the relevance of the Na(V)1.8 channel was investigated in this model of persistent mechanical hypernociception in rats. In the PGE(2)-induced persistent hypernociception, but not in the single injection-induced acute hypernociception, the mRNA expression (RT-PCR) of Na(V)1.8 in dorsal root ganglia (DRG) was up-regulated. A similar increase of Na(V)1.8 mRNA was observed when DbcAMP was used to induce persistent hypernociception. Four daily intrathecal administrations of oligodeoxynucleotides (ODN) antisense against Na(V)1.8 decreased the mRNA encoding Na(V)1.8 in DRG. The intrathecal administration of ODN antisense prevented the PGE(2)-induced acute hypernociception and significantly reduced ongoing PGE(2)-induced persistent hypernociception. A parallel restoration of the persistent hypernociception and up-regulation of Na(V)1.8 mRNA was observed after the cessation of ODN antisense treatment. These results suggest the participation of Na(V)1.8 channels in the development and maintenance of chronic inflammatory hyperalgesia, and confirm their involvement in the acute inflammatory hypernociception.

Many painful conditions are associated with alterations in the extracellular matrix (ECM) of affected tissues. While several integrins, the receptors for ECM proteins, are present on sensory neurons that mediate pain, the possible role of these cell adhesion molecules in inflammatory or neuropathic pain has not been explored. We found that the intradermal injection of peptide fragments of domains of laminin and fibronectin important for adhesive signaling selectively inhibited the hyperalgesia caused by prostaglandin E2 (PGE2) and epinephrine (EPI), respectively. The block of EPI hyperalgesia was mimicked by other peptides containing the RGD integrin-binding sequence. Monoclonal antibodies (mAbs) against the alpha1 or alpha3 integrin subunits, which participate in laminin binding, selectively blocked PGE2 hyperalgesia, while a mAb against the alpha5 subunit, which participates in fibronectin binding, blocked only EPI-induced hyperalgesia. A mAb against the beta1 integrin subunit, common to receptors for both laminin and fibronectin, inhibited hyperalgesia caused by both agents, as did the knockdown of beta1 integrin expression by intrathecal injection of antisense oligodeoxynucleotides. The laminin peptide, but not the fibronectin peptides, also reversibly abolished the longer lasting inflammatory hyperalgesia induced by carrageenan. Finally, the neuropathic hyperalgesia caused by systemic administration of the cancer chemotherapy agent taxol was reversibly inhibited by antisense knockdown of beta1 integrin. These results strongly implicate specific integrins in the maintenance of inflammatory and neuropathic hyperalgesia.

Inhibition of tonic activity in spino-supraspinal projection neurons induces heterosegmental antinociception that is mediated by opioid receptors in nucleus accumbens. To investigate the origin of this tonic activity, we evaluated the ability of inhibiting neurotransmission in the spinal cord to produce heterosegmental antinociception in the trigeminal nociceptive jaw-opening reflex (JOR) in the rat. Spinal intrathecal administration of calcium channel blockers attenuated the JOR, suggesting that the tonic spinal activity depends on synaptic input. To identify the excitatory neurotransmitter receptors involved, selective antagonists for AMPA/kainate, mGluR1, NMDA or NK1 receptors were administered intrathecally to the spinal cord. The AMPA/kainate and mGluR1 receptor antagonists, but not the NMDA or NK1 receptor antagonists, induced antinociception, which was antagonized by intra-accumbens administration of the selective micro -opioid receptor antagonist CTOP. Thus, inhibition of tonic spinal glutamatergic activity resulted in supraspinally mediated antinociception. As this antinociception occurred in the absence of interventions that would produce a facilitated nociceptive state, this tonic glutamatergic activity is important in setting nociceptive threshold.

Temporomandibular joint (TMJ) pain conditions are poorly understood. Since formalin is a noxious stimulus widely used in animal behavioral experiments for studying pain mechanisms, the aim of this study was to develop a behavioral model to study the TMJ pain conditions by characterizing the nociceptive behavioral responses induced by the injection of formalin into the TMJ region of rats. NaCl (0.9%) or different concentrations of formalin (0.5, 1.5, 2.5 or 5%) were administrated into the TMJ region. The formalin-induced behavioral responses characterized by moving the mandible, rubbing the orofacial region and flinching the head quickly were quantified for 45 min. The TMJ injection of formalin significantly increased the asymmetrical orofacial rubbing and head flinching behaviors, but not the movement of the mandible with concentrations of 1.5% and above (P<0.05, Dunn's test) when compared with the NaCl (0.9%) injection. These responses were significantly reduced (P<0.05, Mann-Whitney test) by the co-application of lidocaine N-ethyl bromide quaternary salt, QX-314 (2%), and by the administration of intraperitoneal morphine (4 mg/kg) 30 min prior to the TMJ formalin injection. This study demonstrates that the injection of formalin into the TMJ region of rats produces quantitative nociceptive behaviors constituting a novel behavioral model for TMJ pain.

Formalin injected subcutaneously into the paw is a widely used model of pain. This procedure evokes a short-lasting period of flinching (phase 1) and a long-lasting period of intense flinching (phase 2) following a very short period of quiescence. Phase 2 has been extensively used to support the involvement of central (spinal cord) sensitization in inflammatory hyperalgesia. The present study evaluated the contribution of stimulation of peripheral nociceptors by the release of endogenous mediators at the site of lesion. The participation of histamine and 5-hydroxytryptamine was demonstrated by the treatment of the rat hindpaws with selective histamine H1 (pyrilamine and meclizine) and histamine H2 (cimetidine) receptor antagonists or selective 5-hydroxytryptamine(1A) (WAY100,135) and 5-hydroxytryptamine(4/3) (tropisetron) receptor antagonists. The co-administration of pyrilamine or meclizine with formalin (1%) significantly reduced phases 1 and 2, while cimetidine had no effect. Pyrilamine administration during the period of quiescence (10min after formalin administration) caused strong dose-related inhibition of phase 2. The co-administration of tropisetron with formalin caused a blockade of both phases, while with WAY100,135 caused only inhibition of the phase 2. In contrast, tropisetron administrated during the period of quiescence did not cause antinociception. Histamine and 5-hydroxytryptamine receptors could be strongly activated in naïve animals by administration of a mixture of both agonists or compound 48/80 (2microg/paw) which is known to release both mediators from mast cells. Pretreatment of the paws with a mast cell stabilizer, sodium cromoglycate, significantly reduced the second phase of the formalin injection model. From these results we suggest that phases 1 and 2 of the formalin test are dependent upon the ongoing afferent input. Furthermore, while histamine H1 participates in both phases, 5-hydroxytryptamine(4/3) participates in phase 1 and 5-hydroxytryptamine(1A) in phase 2.