Trigeminal Subnucleus Research Articles

Modulation of excitatory synaptic transmissions by TRPV1 in the spinal trigeminal subnucleus caudalis neurons of neuropathic pain rats

The present study examined contribution of the transient receptor potential vanilloid 1 channel (TRPV1) to the chronic orofacial pain. Bilateral partial nerve ligation (PNL) of the mental nerve, a branch of trigeminal nerve, was performed to induce neuropathic pain. The withdrawal threshold in response to mechanical stimulation of the lower lip skin was substantially reduced after the surgery in the PNL rats while it remained unchanged in the sham rats. This reduction in the PNL rats was alleviated by pregabalin injected intraperitoneally (10 mg/kg) and intracisternally (10, 30, 100 μg). Furthermore, an intracisternal injection of AMG9810, an antagonist of TRPV1, (1.5, 5.0 μg) attenuated the reduction of withdrawal threshold. Spontaneous and miniature excitatory postsynaptic currents (sEPSCs and mEPSCs) were recorded from the spinal trigeminal subnucleus caudalis (Vc) neurons in the brainstem slice, which receive the orofacial nociceptive signals. In the PNL rats, superfusion of capsaicin (0.03, 0.1 μM) enhanced their frequency without effect on the amplitude and the highest concentration (0.3 μM) increased both the frequency and amplitude. In the sham rats, only 0.3 μM capsaicin increased their frequency. Thus, capsaicin-induced facilitation of sEPSCs and mEPSCs in the PNL rats was significantly stronger than that in the sham rats. AMG9810 (0.1 μM) attenuated the capsaicin's effect. Capsaicin was ineffective on the trigeminal tract-evoked EPSCs in the PNL and sham rats. These results suggest that the chronic orofacial pain in the PNL model results from facilitation of the spontaneous excitatory synaptic transmission in the Vc region through TRPV1 at least partly.

Activation of parabrachial nucleus - ventral tegmental area pathway underlies the comorbid depression in chronic neuropathic pain in mice.

SummaryDepression symptoms are often found in patients suffering from chronic pain, a phenomenon that is yet to be understood mechanistically. Here, we systematically investigate the cellular mechanisms and circuits underlying the chronic-pain-induced depression behavior. We show that the development of chronic pain is accompanied by depressive-like behaviors in a mouse model of trigeminal neuralgia. In parallel, we observe increased activity of the dopaminergic (DA) neuron in the midbrain ventral tegmental area (VTA), and inhibition of this elevated VTA DA neuron activity reverses the behavioral manifestations of depression. Further studies establish a pathway of glutamatergic projections from the spinal trigeminal subnucleus caudalis (Sp5C) to the lateral parabrachial nucleus (LPBN) and then to the VTA. These glutamatergic projections form a direct circuit that controls the development of the depression-like behavior under the state of the chronic neuropathic pain.

IL-33 induces orofacial neuropathic pain through Fyn-dependent phosphorylation of GluN2B in the trigeminal spinal subnucleus caudalis

Orofacial neuropathic pain can cause considerable disruptions in patients' daily lives, especially because of a lack of effective medications as its underlying causative mechanisms are not fully understood. Here, we found neuron-specific expression of the interleukin (IL)-33 receptor in the trigeminal spinal subnucleus caudalis (Vc), distinct from the spinal dorsal horn. Reduction in head withdrawal threshold in response to von Frey filament stimulation of the whisker pad skin was inversely correlated with the upregulation of IL-33 in the Vc after infraorbital nerve injury (IONI). Neutralization of IL-33 in the Vc alleviated mechanical allodynia in the whisker pad skin after IONI; conversely, intracisternal administration of IL-33 elicited mechanical allodynia in the whisker pad skin, which was relieved by GluN2B antagonism. Moreover, IL-33 triggered the potentiation of GluN2B-containing N-methyl-D-aspartate receptor-mediated synaptic currents and phosphorylation of synaptosomal GluN2B in the Vc, whereas IONI-induced GluN2B phosphorylation was inhibited by neutralization of IL-33 in the Vc. IL-33-induced GluN2B phosphorylation was mediated by phosphorylation of Fyn kinase, and inhibition of the Fyn kinase pathway prevented the development of IL-33-induced mechanical allodynia. Our findings provide insights into a new mechanism by which IL-33 directly regulates synaptic transmission and suggest that IL-33 signaling could be a candidate target for therapeutic interventions for orofacial neuropathic pain.

Pannexin 1-Mediated ATP Signaling in the Trigeminal Spinal Subnucleus Caudalis Is Involved in Tongue Cancer Pain.

Pain is one of the most severe concerns in tongue cancer patients. However, the underlying mechanisms of tongue cancer pain are not fully understood. We investigated the molecular mechanisms of tongue cancer-induced mechanical allodynia in the tongue by squamous cell carcinoma (SCC) inoculation in rats. The head-withdrawal threshold of mechanical stimulation (MHWT) to the tongue was reduced following SCC inoculation, which was inhibited by intracisternal administration of 10Panx, an inhibitory peptide for pannexin 1 (PANX1) channels. Immunohistochemical analyses revealed that the expression of PANX1 was upregulated in the trigeminal spinal subnucleus caudalis (Vc) following SCC inoculation. The majority of PANX1 immunofluorescence was merged with ionized calcium-binding adapter molecule 1 (Iba1) fluorescence and a part of it was merged with glial fibrillary acidic protein (GFAP) fluorescence. Spike frequencies of Vc nociceptive neurons to noxious mechanical stimulation were significantly enhanced in SCC-inoculated rats, which was suppressed by intracisternal 10Panx administration. Phosphorylated extracellular signal-regulated kinase (pERK)-immunoreactive (IR) neurons increased significantly in the Vc after SCC inoculation, which was inhibited by intracisternal 10Panx administration. SCC inoculation-induced MHWT reduction and increased pERK-IR Vc neuron numbers were inhibited by P2X7 purinoceptor (P2X7R) antagonism. Conversely, these effects were observed in the presence of P2X7R agonist in SCC-inoculated rats with PANX1 inhibition. SCC inoculation-induced MHWT reduction was significantly recovered by intracisternal interleukin-1 receptor antagonist administration. These observations suggest that SCC inoculation causes PANX1 upregulation in Vc microglia and adenosine triphosphate released through PANX1 sensitizes nociceptive neurons in the Vc, resulting in tongue cancer pain.

Efferent and afferent connections of supratrigeminal neurons conveying orofacial muscle proprioception in rats.

The supratrigeminal nucleus (Su5) is a key structure for controlling jaw movements; it receives proprioceptive sensation from jaw-closing muscle spindles (JCMSs) and sends projections to the trigeminal motor nucleus (Mo5). However, the central projections and regulation of JCMS proprioceptive sensation are not yet fully understood. Therefore, we aimed to reveal the efferent and afferent connections of the Su5 using neuronal tract tracings. Anterograde tracer injections into the Su5 revealed that the Su5 sends contralateral projections (or bilateral projections with a contralateral predominance) to the Su5, basilar pontine nuclei, pontine reticular nucleus, deep mesencephalic nucleus, superior colliculus, caudo-ventromedial edge of the ventral posteromedial thalamic nucleus, parafascicular thalamic nucleus, zona incerta, and lateral hypothalamus, and ipsilateral projections (or bilateral projections with an ipsilateral predominance) to the intertrigeminal region, trigeminal oral subnucleus, dorsal medullary reticular formation, and hypoglossal nucleus as well as the Mo5. Retrograde tracer injections into the Su5 demonstrated that the Su5 receives bilateral projections with a contralateral predominance (or contralateral projections) from the primary and secondary somatosensory cortices, granular insular cortex, and Su5, and ipsilateral projections (or bilateral projections with an ipsilateral predominance) from the dorsal peduncular cortex, bed nuclei of stria terminalis, central amygdaloid nucleus, lateral hypothalamus, parasubthalamic nucleus, trigeminal mesencephalic nucleus, parabrachial nucleus, juxtatrigeminal region, trigeminal oral and caudal subnuclei, and dorsal medullary reticular formation. These findings suggest that the Su5, which receives JCMS proprioception, has efferent and afferent connections with multiple brain regions that are involved in emotional and autonomic functions as well as orofacial motor functions.

Diurnal Variation in Trigeminal Pain Sensitivity in Mice.

Management of time and circadian disruption is an extremely important factor in basic research on pain and analgesia. Although pain is known to vary throughout the day, the mechanism underlying this circadian variation remains largely unknown. In this study, we hypothesized that the process of pain transmission to the central nervous system (after receiving nociceptive stimuli from outside the body) would show day-night differences. Ten-week-old male mice were kept under a strict 12/12-h light/dark cycle for at least 10 days. Formalin was then injected into the second branch region of the trigeminal nerve and the duration of pain-related behaviors (PRBs) was assessed. Immunohistochemical staining was then performed, and the c-Fos-immunopositive cells in the trigeminal spinal tract subnucleus caudalis (Sp5C) were counted. The results showed that the duration of PRBs was longer and the number of c-Fos immunopositive cells in the Sp5C was higher at nighttime than during the day. In addition, the trigeminal ganglia (TG) were extracted from the mice and examined by quantitative real-time PCR to evaluate the daytime and nighttime expression of nociceptive receptors. The results showed that the mRNA expression of transient receptor potential ankyrin 1 in the TG was significantly higher at night than during the day. These results suggest that pain in the trigeminal nerve region is more intense at nighttime, when rodents are active, than during the daytime, partly due to differences in nociceptor expression.

Behavioral Responses and Expression of Nociceptin/Orphanin FQ and Its Receptor (N/OFQ-NOP System) during Experimental Tooth Movement in Rats.

Objective To determine behavioral testing after experimental tooth movement in rats and to explore the role of nociceptin/orphanin FQ and its receptor (the N/OFQ-NOP system) in pain induced by experimental tooth movement. Design The mouth-wiping behavior of rats was assessed by studying behavioral responses after experimental tooth movement. The distribution of N/OFQ in the periodontal ligament, the trigeminal ganglion (TG), and the caudal one-third of the trigeminal subnucleus caudalis (Vc) was assessed by immunohistochemistry. The variations in N/OFQ expression in the TG and Vc were measured by Western blotting. The ongoing changes in the gene expression of the prepronociceptin gene and opioid receptor-like 1 receptor were assessed in the TG and Vc by real-time polymerase chain reaction (RT-PCR). Results Overall, the mouth-wiping behavior increased significantly. The behavior first increased and then gradually decreased to a low level, showing cyclical variation. N/OFQ immunoreactivity increased in the periodontal ligament after tooth movement. ppN/OFQ mRNA and protein levels showed a time-dependent increase in the TG and were positively correlated with pain stimulus. NOP gene levels showed large fluctuations. In the Vc, the expression and changes in the N/OFQ-NOP system showed the opposite trend as those noted in TG and the periodontal membrane. Conclusion The N/OFQ system may have a complex regulatory function in the pain induced by tooth movement and may be related to inflammation caused by orthodontic tooth movement and periodontal damage. The specific mechanism remains to be further studied.

Anti-calcitonin gene-related peptide antibody attenuates orofacial mechanical and heat hypersensitivities induced by infraorbital nerve injury

Currently, limited information regarding the role of calcitonin gene-related peptide (CGRP) in neuropathic pain is available. Intracerebroventricular administrations of an anti-CGRP antibody were performed in rats with infraorbital nerve ligation. Anti-CGRP antibody administration attenuated mechanical and heat hypersensitivities induced by nerve ligation and decreased the phosphorylated extracellular signal-regulated kinase expression levels in the trigeminal spinal subnucleus caudalis (Vc) following mechanical or heat stimulation. An increased CGRP immunoreactivity in the Vc appeared after nerve ligation. A decreased CGRP immunoreactivity resulted from anti-CGRP antibody administration. Our findings suggest that anti-CGRP antibody administration attenuates the symptoms of trigeminal neuropathic pain by acting on CGRP in the Vc.

Sensory Effects of Nicotine and Tobacco.

Ingestion of nicotine by smoking, vaping, or other means elicits various effects including reward, antinociception, and aversion due to irritation, bitter taste, and unpleasant side effects such as nausea and dizziness. Here we review the sensory effects of nicotine and the underlying neurobiological processes. Nicotine elicits oral irritation and pain via the activation of neuronal nicotinic acetylcholine receptors (nAChRs) expressed by trigeminal nociceptors. These nociceptors excite neurons in the trigeminal subnucleus caudalis (Vc) and other brainstem regions in a manner that is significantly reduced by the nAChR antagonist mecamylamine. Vc neurons are excited by lingual application of nicotine and exhibit a progressive decline in firing to subsequent applications, consistent with desensitization of peripheral sensory neurons and progressively declining ratings of oral irritation in human psychophysical experiments. Nicotine also elicits a nAChR-mediated bitter taste via excitation of gustatory afferents. Nicotine solutions are avoided even when sweeteners are added. Studies employing oral self-administration have yielded mixed results: Some studies show avoidance of nicotine while others report increased nicotine intake over time, particularly in adolescents and females. Nicotine is consistently reported to increase human pain threshold and tolerance levels. In animal studies, nicotine is antinociceptive when delivered by inhalation of tobacco smoke or systemic infusion, intrathecally, and by intracranial microinjection in the pedunculopontine tegmentum, ventrolateral periaqueductal gray, and rostral ventromedial medulla. The antinociception is thought to be mediated by descending inhibition of spinal nociceptive transmission. Menthol cross-desensitizes nicotine-evoked oral irritation, reducing harshness that may account for its popularity as a flavor additive to tobacco products. Nicotine activates brain systems underlying reward and antinociception, but at the same time elicits aversive sensory effects including oral irritation and pain, bitter taste, and other unpleasant side effects mediated largely by nicotinic acetylcholine receptors (nAChRs). This review discusses the competing aversive and antinociceptive effects of nicotine and exposure to tobacco smoke, and the underlying neurobiology. An improved understanding of the interacting effects of nicotine will hopefully inform novel approaches to mitigate nicotine and tobacco use.

Evolution of mirror-image pain in temporomandibular joint osteoarthritis mouse model.

Mirror-image pain is a kind of pain that occurs on the contralateral side, but its pathogenesis remains unclear.ObjectiveTo develop an osteoarthritis mouse model for investigating mirror-image pain through observing nocifensive behaviors, histological changes, and nociceptive activity at days 3, 7, 14, 21, and 28 after the chemical induction of unilateral temporomandibular joint (TMJ) osteoarthritis.MethodologyWe randomly divided 6-week-old mice into sham and complete Freund adjuvant groups. To induce nocifensive behaviors, we applied 0.04 g of von Frey filament, 10 psi of air puff, and cold acetone on both sides of whisker pads at different days. The histology of TMJ on both sides was observed by hematoxylin/eosin staining and microcomputed tomography scanning. Furthermore, the nociceptive activity was evaluated using the phosphorylated cyclic AMP response element binding protein (pCREB) and a microglia marker at different days in the trigeminal subnucleus caudalis.ResultsNocifensive behaviors against mechanical and temperature stimuli on the contralateral side became stronger than the baseline on day 28, in agreement with the elevation of the pCREB and the microglia marker in the trigeminal subnucleus caudalis. Thus, hypernociception on the contralateral side occurred at day 28.ConclusionsClearly, the TMJ model with unilateral osteoarthritis exhibited mirror-image pain. Therefore, this model is useful in investigating the pathogenesis of pain and in developing treatments.

Microglial activation in the trigeminal spinal subnucleus interpolaris/caudalis modulates orofacial incisional mechanical pain hypersensitivity associated with orofacial injury in infancy

Infantile tissue injury induces sensory deficits in adulthood. Infantile facial incision (IFI) was reported to cause an enhancement of incision-induced mechanical hypersensitivity in adulthood due to acceleration of the trigeminal ganglion neuronal excitability. However, the effects of IFI on activation of microglia in the spinal trigeminal nucleus and its involvement in facial pain sensitivity is not well known. A facial skin incision was made in the left whisker pad in infant (IFI) and/or adult rats (AFI). Mechanical head withdrawal threshold and microglial activation in the trigeminal spinal nucleus were analyzed. Mechanical pain hypersensitivity induced by AFI was significantly exacerbated and prolonged by IFI. The number of Iba1-immunoreactive cells in the trigeminal spinal nucleus following AFI was increased by IFI, suggesting that IFI facilitates microglial hyperactivation following AFI. Intraperitoneal administration of minocycline, a microglial activation inhibitor, suppressed the facial incision-induced microglial hyperactivation in the trigeminal spinal nucleus and the exacerbation of the facial mechanical pain hypersensitivity induced by IFI. These results suggest that facial trauma in infants causes hyperactivation of microglia in the trigeminal spinal nucleus following AFI, leading to the prolongation of the facial mechanical pain hypersensitivity.

GABA- and Glycine-Mimetic Responses of Linalool on the Substantia Gelatinosa of the Trigeminal Subnucleus Caudalis in Juvenile Mice: Pain Management through Linalool-Mediated Inhibitory Neurotransmission.

Linalool, a major odorous constituent in essential oils extracted from lavender, is known to have a wide range of physiological effects on humans including pain management. The substantia gelatinosa (SG) of the trigeminal subnucleus caudalis (Vc) is involved in transmission of orofacial nociceptive responses through thin myelinated A[Formula: see text] and unmyelinated C primary afferent fibers. Up to date, the orofacial antinociceptive mechanism of linalool concerning SG neurons of the Vc has not been completely clarified yet. To fill this knowledge gap, whole-cell patch-clamp technique was used in this study to examine how linalool acted on SG neurons of the Vc in mice. Under a high chloride pipette solution, non-desensitizing and repeatable linalool-induced inward currents were preserved in the presence of tetrodotoxin (a voltage-gated Na[Formula: see text]channel blocker), CNQX (a non-NMDA glutamate receptor antagonist), and DL-AP5 (an NMDA receptor antagonist). However, linalool-induced inward currents were partially suppressed by picrotoxin (a GABA[Formula: see text] receptor antagonist) or strychnine (a glycine receptor antagonist). These responses were almost blocked in the presence of picrotoxin and strychnine. It was also found that linalool exhibited potentiation with GABA- and glycine-induced responses. Taken together, these data show that linalool has GABA- and glycine-mimetic effects, suggesting that it can be a promising target molecule for orofacial pain management by activating inhibitory neurotransmission in the SG area of the Vc.

Participation of D-serine and NR2 subunits in EphA4-mediated trigeminal neuropathic pain

The present study investigated the participation of D-serine and NR2 in antinociception produced by blockade of central erythropoietin-producing hepatocellular carcinoma (Eph) A4 (EphA4) signaling in rats with trigeminal neuropathic pain. Trigeminal neuropathic pain was modeled in male Sprague-Dawley rats using mal-positioned dental implants. The left mandibular second molar was extracted under anesthesia, and a miniature dental implant was placed to induce injury to the inferior alveolar nerve. Our current findings showed that nerve injury induced by malpositioned dental implants significantly produced mechanical allodynia; additionally, the inferior alveolar nerve injury increased the expression of D-serine and NR2 subunits in the ipsilateral medullary dorsal horn (trigeminal subnucleus caudalis). Intracisternal administration of EphA4-Fc, an EphA4 inhibitor, inhibited nerve injury-induced mechanical allodynia and upregulated the expression of D-serine and NR2 subunits. Moreover, intracisternal administration of D-amino acids oxidase, a D-serine inhibitor, inhibited trigeminal mechanical allodynia. These results show that D-serine and NR2 subunit pathways participate in central EphA4 signaling after an inferior alveolar nerve injury. Therefore, blockade of D-serine and NR2 subunit pathways in central EphA4 signaling provides a new therapeutic target for the treatment of trigeminal neuropathic pain.

Microglia-Astrocyte Communication via C1q Contributes to Orofacial Neuropathic Pain Associated with Infraorbital Nerve Injury.

Trigeminal nerve injury causes a distinct time window of glial activation in the trigeminal spinal subnucleus caudalis (Vc), which are involved in the initiation and maintenance phases of orofacial neuropathic pain. Microglia-derived factors enable the activation of astrocytes. The complement component C1q, which promotes the activation of astrocytes, is known to be synthesized in microglia. However, it is unclear whether microglia–astrocyte communication via C1q is involved in orofacial neuropathic pain. Here, we analyzed microglia-astrocyte communication in a rat model with infraorbital nerve injury (IONI). The orofacial mechanical hypersensitivity induced by IONI was significantly attenuated by preemptive treatment with minocycline. Immunohistochemical analyses revealed that minocycline inhibited the increase in c-Fos immune-reactive (IR) cells and the fluorescence intensity of both Iba1 and glial fibrillary acidic protein (GFAP) in the Vc following IONI. Intracisternal administration of C1q caused orofacial mechanical hypersensitivity and an increase in the number of c-Fos-IR cells and fluorescence intensity of GFAP. C1q-induced orofacial mechanical hypersensitivity was completely abrogated by intracisternal administration of fluorocitrate. The present findings suggest that the enhancement in the excitability of Vc nociceptive neurons is produced by astrocytic activation via the signaling of C1q released from activated microglia in the Vc following IONI, resulting in persistent orofacial neuropathic pain.

Inhibitory actions of borneol on the substantia gelatinosa neurons of the trigeminal subnucleus caudalis in mice.

The substantia gelatinosa (SG) of the trigeminal subnucleus caudalis (Vc) is the first relay site for the orofacial nociceptive inputs via the thin myelinated Aδ and unmyelinated C primary afferent fibers. Borneol, one of the valuable time-honored herbal ingredients in traditional Chinese medicine, is a popular treatment for anxiety, anesthesia, and antinociception. However, to date, little is known as to how borneol acts on the SG neurons of the Vc. To close this gap, the whole-cell patch-clamp technique was applied to elucidate the antinociceptive mechanism responding for the actions of borneol on the SG neurons of the Vc in mice. In the voltage-clamp mode, holding at –60 mV, the borneol-induced non-desensitizing inward currents were not affected by tetrodotoxin, a voltage-gated Na+ channel blocker, 6-cyano-7-nitro-quinoxaline-2,3-dione, a non-N-methyl-d-aspartate (NMDA) glutamate receptor antagonist and DL-2-amino-5-phosphonopentanoic acid, an NMDA receptor antagonist. However, borneol-induced inward currents were partially decreased in the presence of picrotoxin, a γ-aminobutyric acid (GABA)A receptor antagonist, or strychnine, a glycine receptor antagonist, and was almost suppressed in the presence of picrotoxin and strychnine. Though borneol did not show any effect on the glycine-induced inward currents, borneol enhanced GABA-mediated responses. Beside, borneol enhanced the GABA-induced hyperpolarization under the current-clamp mode. Altogether, we suggest that borneol contributes in part toward mediating the inhibitory GABA and glycine transmission on the SG neurons of the Vc and may serve as an herbal therapeutic for orofacial pain ailments.

Modulatory effect of botulinum toxin type A on the microglial P2X7/CatS/FKN activated-pathway in antigen-induced arthritis of the temporomandibular joint of rats

Analgesic mechanism of Botulinum toxin type A (BoNT/A) involves retrograde axonal transport to central nervous system, where it may interact with sensory neurons. Though, some authors suggested that BoNT/A antinociceptive action may also be associated with the inhibition intracellular factors and neuromodulators expressed by immune cells, especially by microglia. Antigen-induced arthritis in the temporomandibular joint (TMJ) of rats is signal by P2X7 receptor/Cathepsin S (CatS)/Fractalkine (FKN) microglia-activated pathway. Thus, we aimed to evaluate the possible modulatory effect of an intra-TMJ injection of BoNT/A on the P2X7/CatS/FKN microglia-activated pathway in the trigeminal subnucleus caudalis of rats with antigen-induced arthritis of the TMJ. A model of antigen-induced arthritis was used on Wistar rats (n = 40) by systemic injections of an emulsion containing complete Freund's adjuvant and methylated bovine serum albumin (mBSA) diluted in PBS. The arthritic condition was stablished by an intra-TMJ injection of mBSA (10 μg/TMJ/week) for 3 weeks. Then, animals were treated with an intra-TMJ injection of BoNT/A (onabotulinumtoxinA, Allergan®; 7U/kg) or vehicle saline. Animals were euthanized 24 h, 7 or 14 days after BoNT/A treatment and their trigeminal nucleus caudalis was harvested to evaluate the protein level of microglial purinergic P2X7 receptor and CX3 chemokine receptor 1 (CX3CR1) by Western blot, and to measure the protein level of microglial modulators CatS, FKN, and the pro-inflammatory cytokines tumor necrosis alfa (TNF-α) and interleukin 1β (IL-1β) by enzyme-linked immunosorbent assay (ELISA). The antigen-induced arthritis in the TMJ significantly increased the protein levels of P2X7, CatS, FKN, TNF-α and IL-1β in the trigeminal subnucleus caudalis (P < 0.05). The intra-TMJ injection of BoNT/A reduced the protein levels of P2X7 in all time points tested. Additionally, BoNT/A significantly reduced the protein levels of CatS, FKN, and TNF-α 14 days after treatment. However, IL-1β was significantly reduced just 24 h after the BoNT/A intra-TMJ treatment. Based on our results, we can suggest that the intra-TMJ injection of BoNT/A may promote a central effect by reducing the P2X7/CatS/FKN microglia-activated pathway in the trigeminal subnucleus caudalis.

Physical exercise enhances vulnerability to migraine headache associated with CGRP up-expression in trigeminal nucleus caudalis of stressed rats

ABSTRACT Objectives There is conflicting evidence on the effect of physical exercise on migraine development. Present study investigated the impact of treadmill exercise on migraine – associated symptoms and changes in calcitonin gene-related peptide (CGRP) expression in rats with and without maternal deprivation stress (MD). Methods Two days after birth, the male Wistar pups were randomly divided into four groups (n = 6) as follows: intact, exercise, MD, and MD plus exercise. The animals in the MD groups were separated from their dams 4 h per day for 2 weeks. At 8 weeks of age, the rats were exercised on a motor-driven treadmill for 4 weeks. Then, nitroglycerin (NTG) (5 mg/kg/IP) was used to induce migraine and pain-related symptoms were recorded for 90 min. NTG-related thermal hyperalgesia was measured by tail flick and hot plate methods. Finally, immunofluorescence staining of CGRP in trigeminal subnucleus caudalis (Vc) was performed. Results NTG – produced a significant headache symptoms and thermal hypersensitivity, which were aggravated following physical exercise in stressed or unstressed groups. Besides, NTG administration increased CGRP expression in the Vc of rats. Such effect was overpowered by treadmill running only in rats exposed to MD stress. Conclusion These findings highlight the worsening effects of treadmill exercise for migraine in rats with and without MD stress. However, inflammatory response can further exacerbate in stressed rats.

Early Blockade of EphA4 Pathway Reduces Trigeminal Neuropathic Pain.

BackgroundAlthough the Eph receptor plays an important role in the development of neuropathic pain following nerve injury, there has been no evidence of the participation of the ephrin A4 receptor (EphA4) in the development of trigeminal neuropathic pain. The present study investigated the role of EphA4 in central nociceptive processing in rats with inferior alveolar nerve injury.Materials and MethodsMale Sprague-Dawley rats were used in all our experiments. A rat model for trigeminal neuropathic pain was produced using malpositioned dental implants. The left mandibular second molar was extracted under anesthesia, followed by the placement of a miniature dental implant to injure the inferior alveolar nerve.ResultsOur current findings show that nerve injury induced by malpositioned dental implants evokes significant mechanical allodynia and up-regulation of EphA4 expression in the ipsilateral trigeminal subnucleus caudalis. Although daily treatment with EphA4-Fc, an EphA4 antagonist, did not produce prolonged anti-allodynic effects after the chronic neuropathic pain had been already established, an early treatment protocol with repeated EphA4-Fc administration significantly attenuated mechanical allodynia before initiation of chronic neuropathic pain. Finally, we confirmed the participation of the central EphA4 pathway in the development of trigeminal neuropathic pain by reducing EphA4 expression using EphA4 siRNA. This suppression of EphA4 produced significantly prolonged anti-allodynic effects.ConclusionThese results suggest that early blockade of central EphA4 signaling provides a new therapeutic target for the treatment of trigeminal neuropathic pain.

Analgesic and Neuroprotective Effects of Electroacupuncture in a Dental Pulp Injury Model-A Basic Research.

Irreversible pulpitis is an extremely painful condition and its consequence in the central nervous system (CNS) remains unclear. A mouse model of dental pulp injury (DPI) resembles the irreversible pulpitis profile in humans. This study sought to determine whether pain induced by DPI activates microglia and astrocytes in the trigeminal subnucleus caudalis (Vc), as well as increases levels of proinflammatory cytokines, and whether electroacupuncture (EA) can be a potential analgesic and neuroprotective therapy following DPI. Pain behavior was measured via head-withdrawal threshold (HWT) and burrowing behavior at days 1, 3, 7, 14 and 21 after DPI. A marked decrease in HWT and burrowing activity was observed from day 1 to 14 after DPI and no changes were seen on day 21. Microglial and astrocytes activation; along with high cytokine (TNFα, IL-1β, and IL-6) levels, were observed in the Vc at 21 days after DPI. These effects were attenuated by verum (local and distal) EA, as well as oral ibuprofen administration. The results suggest that DPI-induced pain and glial activations in the Vc and EA exert analgesic efficacy at both local and distal acupoints. Furthermore, verum (local and distal) EA might be associated with the modulations of microglial and astrocytes activation.

Dopaminergic Modulation of Orofacial Mechanical Hypersensitivity Induced by Infraorbital Nerve Injury.

While the descending dopaminergic control system is not fully understood, it is reported that the hypothalamic A11 nucleus is its principle source. To better understand the impact of this system, particularly the A11 nucleus, on neuropathic pain, we created a chronic constriction injury model of the infraorbital nerve (ION-CCI) in rats. ION-CCI rats received intraperitoneal administrations of quinpirole (a dopamine D2 receptor agonist). ION-CCI rats received microinjections of quinpirole, muscimol [a gamma-aminobutyric acid type A (GABAA) receptor agonist], or neurotoxin 6-hydroxydopamine (6-OHDA) into the A11 nucleus. A von Frey filament was used as a mechanical stimulus on the maxillary whisker pad skin; behavioral and immunohistochemical responses to the stimulation were assessed. After intraperitoneal administration of quinpirole and microinjection of quinpirole or muscimol, ION-CCI rats showed an increase in head-withdrawal thresholds and a decrease in the number of phosphorylated extracellular signal-regulated kinase (pERK) immunoreactive (pERK-IR) cells in the superficial layers of the trigeminal spinal subnucleus caudalis (Vc). Following 6-OHDA microinjection, ION-CCI rats showed a decrease in head-withdrawal thresholds and an increase in the number of pERK-IR cells in the Vc. Our findings suggest the descending dopaminergic control system is involved in the modulation of trigeminal neuropathic pain.