A novel technique of delivering viral vectors to trigeminal ganglia in rats

Development of a novel three-dimensional injection guide for trigeminal ganglia

BACKGROUND

Trigeminal ganglion (TG) plays an important role in the process of orthodontic pain. It's necessary to design an accurate, precise and minimally invasive trigeminal ganglion injection guide plate to study TG.

METHODS

Micro-CT was used to obtain the Dicom format data, and three-dimensional (3D) software (mimics and magics23.03) was used to reconstruct 3D head models. Design and modifications of the TG injection guide plate were performed in Magic 23.03 software, and the guide plate was produced by a 3D stereolithography printer. X-ray, micro-CT, Evans blue, and virus transduction were used to demonstrate the accuracy of the guide-assisted injection. Pain levels were evaluated after using the injection guide by a bite force test and Von Frey test.

RESULTS

X-ray and micro-CT tests confirmed that the injection needle reached the bilateral trigeminal ganglia fossa. The Evans blue test and virus transduction proved that the injected drug could be accurately injected into the bilateral trigeminal ganglion and the lentivirus could be successfully transfected. The percentage of accurate injection was 10/10 (bilateral trigeminal ganglia). Orofacial pain induced by the trigeminal ganglion injection was mild and returned to baseline within seven days.

CONCLUSION

The injection guide described in this study is viable and reliable for the delivery of drugs and virus transduction into the trigeminal ganglia.

Role of Neurotrophins in Orofacial Pain Modulation: A Review of the Latest Discoveries

Orofacial pain represents a multidisciplinary biomedical challenge involving basic and clinical research for which no satisfactory solution has been found. In this regard, trigeminal pain is described as one of the worst pains perceived, leaving the patient with no hope for the future. The aim of this review is to evaluate the latest discoveries on the involvement of neurotrophins in orofacial nociception, describing their role and expression in peripheral tissues, trigeminal ganglion, and trigeminal nucleus considering their double nature as "supporters" of the nervous system and as "promoters" of nociceptive transmission. In order to scan recent literature (last ten years), three independent researchers referred to databases PubMed, Embase, Google Scholar, Scopus, and Web of Science to find original research articles and clinical trials. The researchers selected 33 papers: 29 original research articles and 4 clinical trials. The results obtained by the screening of the selected articles show an interesting trend, in which the precise modulation of neurotrophin signaling could switch neurotrophins from being a "promoter" of pain to their beneficial neurotrophic role of supporting the nerves in their recovery, especially when a structural alteration is present, as in neuropathic pain. In conclusion, neurotrophins could be interesting targets for orofacial pain modulation but more studies are necessary to clarify their role for future application in clinical practice.

Trigeminal ganglion itself can be a viable target to manage trigeminal neuralgia

Excruciating trigeminal neuralgia (TN) management is very difficult and severely affects the patient's quality of life. Earlier studies have shown that the trigeminal ganglion (TG) comprises several receptors and signal molecules that are involved in the process of peripheral sensitization, which influences the development and persistence of neuropathic pain. Targeting TG can modulate this sensitization pathway and mediate the pain-relieving effect. So far,there are few studies in which modulation approaches to TG itself have been suggested so far. "Trigeminal ganglion modulation" and "trigeminal neuralgia" were used as search phrases in the Scopus Index and PubMed databases to discover articles that were pertinent to the topic. In this review, we address the role of the trigeminal ganglion in TN and underlying molecules and neuropeptides implicated in trigeminal pain pathways in processing pathological orofacial pain. We also reviewed different modulation approaches in TG for TN management. Furthermore, we discuss the prospect of targeting trigeminal ganglion to manage such intractable pain.

Differential roles of NMDAR subunits 2A and 2B in mediating peripheral and central sensitization contributing to orofacial neuropathic pain

The spinal N-methyl-d-aspartate receptor (NMDAR), particularly their subtypes NR2A and NR2B, plays pivotal roles in neuropathic and inflammatory pain. However, the roles of NR2A and NR2B in orofacial pain and the exact molecular and cellular mechanisms mediating nervous system sensitization are still poorly understood. Here, we exhaustively assessed the regulatory effect of NMDAR in mediating peripheral and central sensitization in orofacial neuropathic pain. Von-Frey filament tests showed that the inferior alveolar nerve transection (IANX) induced ectopic allodynia behavior in the whisker pad of mice. Interestingly, mechanical allodynia was reversed in mice lacking NR2A and NR2B. IANX also promoted the production of peripheral sensitization-related molecules, such as interleukin (IL)-1β, tumor necrosis factor (TNF)-α, brain-derived neurotrophic factor (BDNF), and chemokine upregulation (CC motif) ligand 2 (CCL2), and decreased the inward potassium channel (Kir) 4.1 on glial cells in the trigeminal ganglion, but NR2A conditional knockout (CKO) mice prevented these alterations. In contrast, NR2B CKO only blocked the changes of Kir4.1, IL-1β, and TNF-α and further promoted the production of CCL2. Central sensitization-related c-fos, glial fibrillary acidic protein (GFAP), and ionized calcium-binding adaptor molecule 1 (Iba-1) were promoted and Kir4.1 was reduced in the spinal trigeminal caudate nucleus by IANX. Differential actions of NR2A and NR2B in mediating central sensitization were also observed. Silencing of NR2B was effective in reducing c-fos, GFAP, and Iba-1 but did not affect Kir4.1. In contrast, NR2A CKO only altered Iba-1 and Kir4.1 and further increased c-fos and GFAP. Gain-of-function and loss-of-function approaches provided insight into the differential roles of NR2A and NR2B in mediating peripheral and central nociceptive sensitization induced by IANX, which may be a fundamental basis for advancing knowledge of the neural mechanisms' reaction to nerve injury.

NGF-Induced Upregulation of CGRP in Orofacial Pain Induced by Tooth Movement Is Dependent on Atp6v0a1 and Vesicle Release

The nerve growth factor (NGF) and calcitonin gene-related peptide (CGRP) play a crucial role in the regulation of orofacial pain. It has been demonstrated that CGRP increases orofacial pain induced by NGF. V-type proton ATPase subunit an isoform 1 (Atp6v0a1) is involved in the exocytosis pathway, especially in vesicular transport in neurons. The objective was to examine the role of Atp6v0a1 in NGF-induced upregulation of CGRP in orofacial pain induced by experimental tooth movement. Orofacial pain was elicited by ligating closed-coil springs between incisors and molars in Sprague–Dawley rats. Gene and protein expression levels were determined through real-time polymerase chain reaction, immunostaining, and fluorescence in situ hybridization. Lentivirus vectors carrying Atp6v0a1 shRNA were used to knockdown the expression of Atp6v0a1 in TG and SH-SY5Y neurons. The release of vesicles in SH-SY5Y neurons was observed by using fluorescence dye FM1-43, and the release of CGRP was detected by Enzyme-Linked Immunosorbent Assy. Orofacial pain was evaluated through the rat grimace scale. Our results revealed that intraganglionic administration of NGF and Atp6v0a1 shRNA upregulated and downregulated CGRP in trigeminal ganglia (TG) and trigeminal subnucleus caudalis (Vc), respectively, and the orofacial pain was also exacerbated and alleviated, respectively, following administration of NGF and Atp6v0a1 shRNA. Besides, intraganglionic administration of NGF simultaneously caused the downregulation of Atp6v0a1 in TG. Moreover, the release of vesicles and CGRP in SH-SY5Y neurons was interfered by NGF and Atp6v0a1 shRNA. In conclusion, in the orofacial pain induced by experimental tooth movement, NGF induced the upregulation of CGRP in TG and Vc, and this process is dependent on Atp6v0a1 and vesicle release, suggesting that they are involved in the transmission of nociceptive information in orofacial pain.

Mechanism of Catechol-O-methyltransferase Regulating Orofacial Pain Induced by Tooth Movement

OBJECTIVE

To explore the mechanism of catechol-O-methyltransferase (COMT) in tooth movement pain.

METHODS

The experimental groups were randomly allocated into the healthy control, sham operation, model, model+shCOMT experimental, model+shCOMT control, and model+COMT antagonist groups. A tooth movement pain model was established. The pain stimulation and behavior test were performed. The duration of grooming behavior was determined. The appropriate experimental force and duration for application were selected. COMT shRNA vector was constructed and packaged as adenovirus. The shCOMT adenovirus was injected into the left infraorbital foramen. Seven days later, the trigeminal ganglia of all treatment groups were obtained. The COMT and IL-17 expressions were detected by western blot. The appropriate COMT antagonist concentration was selected. The pathological results of each group were detected by HE staining. The tooth movement distance was determined. The COMT gene expression was detected by FISH. The COMT and IL-17 expressions in the right trigeminal ganglion tissue of each group were detected by western blot.

RESULTS

The 60 g force and 14-day duration required the lowest stimulus intensity, the duration of grooming behavior was the longest, and the effect on COMT and IL-17 was the most significant. In the model group, formation of digestive cavity was seen in the trigeminal ganglion tissue, with infiltration of inflammatory cells, upregulation of the COMT and IL-17 expressions, and significant increase in the tooth movement distance. Compared with the model group, the shCOMT experimental group and the COMT antagonist group significantly improved the trigeminal ganglion tissue injury, significantly decreased the tooth movement distance, and significantly inhibited the COMT and IL-17 expressions.

CONCLUSION

The efficiency of tooth movement can be influenced by interfering the COMT-related gene expression. This proves that the COMT system can regulate the orthodontic tooth movement pain.

Retrograde nerve growth factor signaling modulates tooth mechanical hyperalgesia induced by orthodontic tooth movement via acid-sensing ion channel 3

Orthodontic tooth movement elicits alveolar bone remodeling and orofacial pain that is manifested by tooth mechanical hyperalgesia. Nerve growth factor (NGF) is upregulated in periodontium and may modulate tooth mechanical hyperalgesia. The objectives were to examine the role of NGF in tooth mechanical hyperalgesia and to elucidate the underlying mechanisms. Tooth mechanical hyperalgesia was induced by ligating closed coil springs between incisors and molars in Sprague-Dawley rats. Retrograde labeling was performed by periodontal administration of fluor-conjugated NGF and the detection of fluorescence in trigeminal ganglia (TG). Lentivirus vectors carrying NGF shRNA were employed to knockdown the expression of NGF in TG. The administration of agonists, antagonists, and virus vectors into TG and periodontium was conducted. Tooth mechanical hyperalgesia was examined through the threshold of biting withdrawal. Our results revealed that tooth movement elicited tooth mechanical hyperalgesia that could be alleviated by NGF neutralizing antibody and that NGF was upregulated in periodontium (mainly in periodontal fibroblasts) and TG. Retrograde labeling revealed that periodontal NGF was retrogradely transported to TG after day 1. Acid-sensing ion channel 3 (ASIC3) and NGF were co-expressed in trigeminal neurons and the percentage of co-expression was significantly higher following tooth movement. The administration of NGF and NGF neutralizing antibody into TG could upregulate and downregulate the expression of ASIC3 in TG, respectively. NGF aggravated tooth mechanical hyperalgesia that could be alleviated by ASIC3 antagonist (APETx2). Moreover, NGF neutralizing antibody mitigated tooth mechanical hyperalgesia that could be recapitulated by ASIC3 agonist (GMQ). NGF-based gene therapy abolished tooth mechanical hyperalgesia and downregulated ASIC3 expression. Taken together, in response to force stimuli, periodontal fibroblasts upregulated the expressions of NGF that was retrogradely transported to TG, where NGF elicited tooth mechanical hyperalgesia through upregulating ASIC3. NGF-based gene therapy is a viable method in alleviating tooth-movement-induced mechanical hyperalgesia.

Nerve Growth Factor Enhances Tooth Mechanical Hyperalgesia Through C-C Chemokine Ligand 19 in Rats

The nerve growth factor (NGF) plays an important role in the regulation of neuropathic pain. It has been demonstrated that calcitonin gene-related peptide (CGRP), a well-known contributor to neurogenic inflammation, increases neuroinflammatory pain induced by NGF. The inflammatory mediator that NGF most strongly induces is C-C chemokine ligand 19 (CCL19), which can recruit inflammatory cells by binding to the receptor CCR7 followed by promoting the response of neuroinflammation. However, the regulatory mechanism of NGF and CCL19 in tooth movement orofacial pain and the interaction between both are still unclear. In this study, male Sprague–Dawley rats were used to study the modulation of NGF on orofacial pain through CCL19 and the role of each in tooth movement pain in rats. The expression levels of CCL19 mRNA and protein were determined by real-time PCR and immunofluorescence, respectively. Pain levels were assessed by measuring the rats' bite force, which drops as pain rises. Meanwhile, by verifying the relationship between CGRP and CCL19, it was laterally confirmed that NGF could modulate tooth movement-induced mechanical hyperalgesia through CCL19. The results showed that the expression level of CCL19 rose with the increased NGF, and neurons expressing CGRP can express stronger CCL19. Compared with the baseline level, the bite force for all rats dropped sharply on day 1, reached its lowest level on day 3, and recovered gradually on day 5. All results indicated that NGF played an important role in tooth movement orofacial pain via positively regulating CCL19 expression in the trigeminal ganglia of rats. Additionally, CCL19 increased the sensitivity to experimental tooth movement orofacial pain. NGF can regulate CCL19 expression, although it may regulate other inflammatory pathways as well. This is the first report on the interactions and modulations of tooth movement orofacial pain by NGF through CCL19 in rats.

N/OFQ modulates orofacial pain induced by tooth movement through CGRP-dependent pathways

BACKGROUND

Nociceptin/orphanin FQ (N/OFQ) has been revealed to play bidirectional roles in orofacial pain modulation. Calcitonin gene-related peptide (CGRP) is a well-known pro-nociceptive molecule that participates in the modulation of orofacial pain. We aimed to determine the effects of N/OFQ on the modulation of orofacial pain and on the release of CGRP.

METHODS

Orofacial pain model was established by ligating springs between incisors and molars in rats for the simulation of tooth movement. The expression level of N/OFQ was determined and pain level was scored in response to orofacial pain. Both agonist and antagonist of N/OFQ receptor were administered to examine their effects on pain and the expression of CGRP in trigeminal ganglia (TG). Moreover, gene therapy based on the overexpression of N/OFQ was delivered to validate the modulatory role of N/OFQ on pain and CGRP expression.

RESULTS

Tooth movement elicited orofacial pain and an elevation in N/OFQ expression. N/OFQ exacerbated orofacial pain and upregulated CGRP expression in TG, while UFP-101 alleviated pain and downregulated CGRP expression. N/OFQ-based gene therapy was successful in overexpressing N/OFQ in TG, which resulted in pain exacerbation and elevation of CGRP expression in TG.

CONCLUSIONS

N/OFQ exacerbated orofacial pain possibly through upregulating CGRP.

Botulinum toxin A alleviates orofacial nociception induced by orthodontic tooth movement through nociceptin/orphanin-FQ pathway in rats

OBJECTIVES

To investigate the effect and mechanism of botulinum neurotoxin type A (BoNT/A) in the modulation of orofacial nociception induced by orthodontic tooth movement in rats.

METHODS

An orofacial nociception model was established in male Sprague-Dawley rats by ligating closed-coil springs between incisors and ipsilateral molars. There were two group sets of animals. For the first group set, 120 rats were randomly divided into four groups: no-force group (n = 30), force + saline group (n = 30), force + low dose BoNT/A group (1U/6 μL, n = 30), and force + high dose BoNT/A group (1U/6 μL, n = 30). BoNT/A and saline were injected into periodontal ligament to explore the nociceptive effect of BoNT/A. Ipsilateral trigeminal ganglia (TG) were harvested for detecting the expression levels of nociceptin/orphanin-FQ (N/OFQ). For the second group set, 36 rats were randomly divided into three force groups: BoNT/A + saline group (n = 12), BoNT/A + UFP-101 group (n = 12), and saline + UFP-101 group (n = 12). A potent N/OFQ receptor (NOP) antagonist (UFP-101) was used to examine the role of N/OFQ in BoNT/A-induced antinociception. Tooth-movement nociception level of all groups was evaluated by bite force and rat grimace scale (RGS) at baseline, day 1, day 3, day 5, day 7, day 14.

RESULTS

The behavioral assessments showed the orofacial nociception level in the force + low dose BoNT/A group and force + high dose BoNT/A group were lower than that in the force + saline group. No significant difference was observed in orofacial nociception among no-force group, force + low dose and force + high dose group. The expression levels of N/OFQ in TG were elevated from day 1 and maintained a high level, presenting in descending order among the force + high dose, force + low dose, force + saline and no-force group, respectively. The nociception level of the BoNT/A + UFP-101 group was higher than that of the BoNT/A + saline group. No significant difference was observed between the BoNT/A + UFP-101 group and the saline + UFP-101 group.

CONCLUSIONS

BoNT/A can exert an antinociceptive effect on orofacial nociception induced by tooth movement by stimulating the expression of N/OFQ in TG.

CGRP Modulates Orofacial Pain through Mediating Neuron-Glia Crosstalk

Calcitonin gene-related peptide (CGRP) plays a crucial role in the modulation of orofacial pain, and we hypothesized that CGRP mediated a neuron-glia crosstalk in orofacial pain. The objective of this study was to elucidate the mechanisms whereby CGRP mediated trigeminal neuron-glia crosstalk in modulating orofacial pain. Orofacial pain was elicited by ligating closed-coil springs between incisors and molars. Trigeminal neurons and satellite glial cells (SGCs) were cultured for mechanistic exploration. Gene and protein expression were determined through immunostaining, polymerase chain reaction, and Western blot. Orofacial pain was evaluated through the rat grimace scale. Our results revealed that the expressions of CGRP were elevated in both trigeminal neurons and SGCs following the induction of orofacial pain. Intraganglionic administration of CGRP and olcegepant exacerbated and alleviated orofacial pain, respectively. The knockdown of CGRP through viral vector-mediated RNA interference was able to downregulate CGRP expressions in both neurons and SGCs and to alleviate orofacial pain. CGRP upregulated the expression of inducible nitric oxide synthase through the p38 signaling pathway in cultured SGCs. In turn, L-arginine (nitric oxide donor) was able to enhance orofacial pain by upregulating CGRP expressions in vivo. In cultured trigeminal neurons, L-arginine upregulated the expression of CGRP, and this effect was diminished by cilnidipine (N-type calcium channel blocker) while not by mibefradil (L-type calcium channel blocker). In conclusion, CGRP modulated orofacial pain through upregulating the expression of nitric oxide through the p38 signaling pathway in SGCs, and the resulting nitric oxide in turn stimulated CGRP expression through N-type calcium channel in neurons, building a CGRP-mediated positive-feedback neuron-glia crosstalk.

The development and use of facial grimace scales for pain measurement in animals

The measurement of pain in animals is surprisingly complex, and remains a critical issue in veterinary care and biomedical research. Based on the known utility of pain measurement via facial expression in verbal and especially non-verbal human populations, "grimace scales" were first developed a decade ago for use in rodents and now exist for 10 different mammalian species. This review details the background context, historical development, features (including duration), psychometric properties, modulatory factors, and impact of animal grimace scales for pain.

The Role of Acid-sensing Ion Channel 3 in the Modulation of Tooth Mechanical Hyperalgesia Induced by Orthodontic Tooth Movement

This study aimed to explore the role of acid-sensing ion channel 3 (ASIC3) in the modulation of tooth mechanical hyperalgesia induced by orthodontic tooth movement. In male Sprague-Dawley rats, closed coil springs were ligated between mandibular incisors and molars to mimic orthodontic tooth movement. Bite force was assessed to evaluate tooth mechanical hyperalgesia. The alveolar bone, trigeminal ganglia, and trigeminal nucleus caudalis underwent immunohistochemical staining and immunoblotting for ASIC3. The inferior alveolar nerves were transected to explore the interaction between the periodontal sensory endings and trigeminal ganglia. The role of ASIC3 in trigeminal ganglia was further explored with lentivirus-mediated ASIC3 ribonucleic acid interference. Results showed that ASIC3 was expressed in the periodontal Ruffini endings and expression of ASIC3 protein was elevated in periodontal tissues, trigeminal ganglia, and trigeminal nucleus caudalis, following orthodontic tooth movement. ASIC3 agonists and antagonists significantly aggravated and mitigated tooth mechanical hyperalgesia, respectively. ASIC3 expression decreased after inferior alveolar nerve transection in periodontal tissues. Both in vitro and vivo, the lentivirus vector carrying ASIC3 shRNA inhibited ASIC3 expression and relieved tooth mechanical hyperalgesia. To conclude, ASIC3 is important in the modulation of tooth mechanical hyperalgesia induced by orthodontic tooth movement. Further, the role of ASIC3 in the modulation of pain in periodontal tissues is regulated by trigeminal ganglia. An adjuvant analgesic therapy targeting ASIC3 could alleviate orthodontic movement-associated mechanical hyperalgesia in rats.

Calcitonin gene-related peptide facilitates sensitization of the vestibular nucleus in a rat model of chronic migraine

Background

Vestibular migraine has recently been recognized as a novel subtype of migraine. However, the mechanism that relate vestibular symptoms to migraine had not been well elucidated. Thus, the present study investigated vestibular dysfunction in a rat model of chronic migraine (CM), and to dissect potential mechanisms between migraine and vertigo.

Methods

Rats subjected to recurrent intermittent administration of nitroglycerin (NTG) were used as the CM model. Migraine- and vestibular-related behaviors were analyzed. Immunofluorescent analyses and quantitative real-time polymerase chain reaction were employed to detect expressions of c-fos and calcitonin gene-related peptide (CGRP) in the trigeminal nucleus caudalis (TNC) and vestibular nucleus (VN). Morphological changes of vestibular afferent terminals was determined under transmission electron microscopy. FluoroGold (FG) and CTB-555 were selected as retrograde tracers and injected into the VN and TNC, respectively. Lentiviral vectors comprising CGRP short hairpin RNA (LV-CGRP) was injected into the trigeminal ganglion.

Results

CM led to persistent thermal hyperalgesia, spontaneous facial pain, and prominent vestibular dysfunction, accompanied by the upregulation of c-fos labeling neurons and CGRP immunoreactivity in the TNC (c-fos: vehicle vs. CM = 2.9 ± 0.6 vs. 45.5 ± 3.4; CGRP OD: vehicle vs. CM = 0.1 ± 0.0 vs. 0.2 ± 0.0) and VN (c-fos: vehicle vs. CM = 2.3 ± 0.8 vs. 54.0 ± 2.1; CGRP mRNA: vehicle vs. CM = 1.0 ± 0.1 vs. 2.4 ± 0.1). Furthermore, FG-positive neurons was accumulated in the superficial layer of the TNC, and the number of c-fos+/FG+ neurons were significantly increased in rats with CM compared to the vehicle group (vehicle vs. CM = 25.3 ± 2.2 vs. 83.9 ± 3.0). Meanwhile, CTB-555+ neurons dispersed throughout the VN. The structure of vestibular afferent terminals was less pronounced after CM compared with the peripheral vestibular dysfunction model. In vivo knockdown of CGRP in the trigeminal ganglion significantly reduced the number of c-fos labeling neurons (LV-CGRP vs. LV-NC = 9.9 ± 3.0 vs. 60.0 ± 4.5) and CGRP mRNA (LV-CGRP vs. LV-NC = 1.0 ± 0.1 vs. 2.1 ± 0.2) in the VN, further attenuating vestibular dysfunction after CM.

Conclusions

These data demonstrates the possibility of sensitization of vestibular nucleus neurons to impair vestibular function after CM, and anti-CGRP treatment to restore vestibular dysfunction in patients with CM.

The P2Y14 receptor in the trigeminal ganglion contributes to the maintenance of inflammatory pain

P2Y purinergic receptors expressed in neurons and satellite glial cells (SGCs) of the trigeminal ganglion (TG) contribute to inflammatory and neuropathic pain. P2Y₁₄ receptor expression is reported in the spinal cord, dorsal root ganglion (DRG), and TG. In present study, the role of P2Y₁₄ receptor in the TG in inflammatory orofacial pain of Sprague-Dawley (SD) rats was investigated. Peripheral injection of complete Freund's adjuvant (CFA) induced mechanical hyperalgesia with the rapid upregulation of P2Y₁₄ receptor, glial fibrillary acidic protein (GFAP), interleukin-1β (IL-1β), tumor necrosis factor-α (TNF-α), C-C chemokine CCL2, phosphorylated extracellular signal-regulated kinase 1/2 (p-ERK1/2), and phosphorylated p38 (p-p38) proteins in the TG. Furthermore, immunofluorescence staining confirmed the CFA-induced upregulation of P2Y₁₄ receptor. Double immunostaining showed that P2Y₁₄ receptor colocalized with glutamine synthetase (GS) and neuronal nuclei (NeuN). Finally, trigeminal injection of a selective antagonist (PPTN) of P2Y₁₄ receptor attenuated CFA-induced mechanical hyperalgesia. PPTN also decreased the upregulation of the GFAP, IL-1β, TNF-α, CCL2, p-ERK1/2, and p-p38 proteins. Our findings showed that P2Y₁₄ receptor in TG may contribute to orofacial inflammatory pain via regulating SGCs activation, releasing cytokines (IL-1β, TNF-α, and CCL2), and phosphorylating ERK1/2 and p38.

Activation of mitogen-activated protein kinases in satellite glial cells of the trigeminal ganglion contributes to substance P-mediated inflammatory pain

Inflammatory orofacial pain, in which substance P (SP) plays an important role, is closely related to the cross-talk between trigeminal ganglion (TG) neurons and satellite glial cells (SGCs). SGC activation is emerging as the key mechanism underlying inflammatory pain through different signalling mechanisms, including glial fibrillary acidic protein (GFAP) activation, phosphorylation of mitogen-activated protein kinase (MAPK) signalling pathways, and cytokine upregulation. However, in the TG, the mechanism underlying SP-mediated orofacial pain generated by SGCs is largely unknown. In this study, we investigated whether SP is involved in inflammatory orofacial pain by upregulating interleukin (IL)-1β and tumour necrosis factor (TNF)-α from SGCs, and we explored whether MAPK signalling pathways mediate the pain process. In the present study, complete Freund’s adjuvant (CFA) was injected into the whisker pad of rats to induce an inflammatory model in vivo. SP was administered to SGC cultures in vitro to confirm the effect of SP. Facial expression analysis showed that pre-injection of L703,606 (an NK-1 receptor antagonist), U0126 (an inhibitor of MAPK/extracellular signal-regulated kinase [ERK] kinase [MEK] 1/2), and SB203580 (an inhibitor of P38) into the TG to induce targeted prevention of the activation of the NK-1 receptor and the phosphorylation of MAPKs significantly suppressed CFA-induced inflammatory allodynia. In addition, SP promoted SGC activation, which was proven by increased GFAP, p-MAPKs, IL-1β and TNF-α in SGCs under inflammatory conditions. Moreover, the increase in IL-1β and TNF-α was suppressed by L703, 606, U0126 and SB203580 in vivo and in vitro. These present findings suggested that SP, released from TG neurons, activated SGCs through the ERK1/2 and P38 pathways and promoted the production of IL-1β and TNF-α from SGCs, contributing to inflammatory orofacial pain associated with peripheral sensitization.

Effect of endomorphin-2 on orofacial pain induced by orthodontic tooth movement in rats

Endomorphin-2 demonstrates potent antinociceptive effects in various pain models. The objectives of the present study were to explore the role of endomorphin-2 in the modulation of orofacial pain induced by orthodontic tooth movement in rats. An orthodontic pain model was established in male Sprague-Dawley rats by ligating coiled springs to mimic orthodontic force (40 g). On days 0, 1, 3, 5, 7, and 14 following orthodontic tooth movement, bite force was recorded as a surrogate measure of orthodontic pain. Ipsilateral trigeminal ganglia, trigeminal nucleus caudalis, and periodontal tissues were harvested for immunostaining. Endomorphin-2, endomorphin-2 + naloxone (a non-selective opioid receptor antagonist), naloxone, and saline were injected into trigeminal ganglia and periodontal tissues to explore the role of endomorphin-2 on orthodontic pain. The results showed that following orthodontic tooth movement, endomorphin-2 expression levels in trigeminal ganglia were elevated on days 1, 3, 5, and 7. Orthodontic pain levels were increased on days 1, 3, and 5. The administration of endomorphin-2 into both trigeminal ganglia and periodontal tissues alleviated orthodontic pain. Moreover, the effects of endomorphin-2 could be blocked by naloxone completely in trigeminal ganglia but only partially in periodontal tissues. Therefore, endomorphin-2 plays an important role in the modulation of orthodontic pain both centrally and peripherally, probably through different pathways.

GABAB receptor activation attenuates inflammatory orofacial pain by modulating interleukin-1β in satellite glial cells: Role of NF-κB and MAPK signaling pathways

Orofacial inflammation could activate satellite glial cells (SGCs) in the trigeminal ganglion (TG) to produce interleukin 1β (IL-1β) which plays crucial roles in the development of inflammatory pain. Recent studies have shown that gamma-amino butyric acid-B (GABA_B) receptor could modulate the expression of inflammatory cytokines in microglia and astrocytes in the spinal cord. The objective of this study was to investigate whether GABA_B receptors in TG SGCs attenuate inflammatory facial pain via mediating IL-1β following inflammation and its mechanisms. Complete Freund's adjuvant (CFA) was injected into the whisker pad of rats to induce inflammation in vivo. Lipopolysaccharide (LPS) was added to culture medium to activate SGCs in vitro. Behavioral measures showed that microinjection of baclofen (a selective GABA_B receptor agonist) into the TG ameliorated the mechanical allodynia of CFA-treated rats. Interestingly, baclofen pretreatment inhibited SGC activation and IL-1β production, however, preserved the decreased expression of GABA_B receptors in SGCs activated by CFA in vivo and LPS in vitro. In addition, baclofen suppressed the increased expression of p-NF- κ B p65, p-I κ Bα, and p-p38 MAPK, while reversed the decreased production of I κ Bα, and further enhanced the increased expression of p-ERK(1/2) in LPS-treated SGCs in vitro. Finally, those effects of baclofen were abolished by saclofen (a specific GABA_B receptor antagonist) co-administration. Altogether, these results demonstrated for the first time that activation of GABA_B receptor might inhibit IL-1β production by suppressing NF- κ B and p38 MAPK signaling pathway activation and restore GABA_B receptor expression in SGCs to attenuate inflammatory facial pain.

Bite force measurements for objective evaluations of orthodontic tooth movement-induced pain in rats

OBJECTIVE

To examine the reliability of bite force for evaluating orthodontic tooth movement-induced pain in rats.

DESIGN

Orthodontic tooth movement-induced pain was induced by mounting springs (40 g) between incisors and ipsilateral molars in male Sprague-Dawley rats. Five group sets of animals were used: for the first group set, 20 rats were randomly divided into a force group (n = 10) and a sham group (n = 10); for the second group set, 20 rats were divided into a 20-g group and a 80-g group; for the third group set, 20 rats were randomly divided into either a CFA group (complete freund's adjuvant) (n = 10) receiving periodontal injections of CFA at baseline or a control group (n = 10) receiving periodontal injections of saline at baseline; for the forth group set, 24 rats were randomly divided into the following four groups: force + saline, control + saline, force + antiNGF and control + NGF (NGF: nerve growth factor). Rats in the fifth group set were used for immunostaining against CGRP. Bite force and bite frequency were measured at baseline (day 0) and following interventions (day 1, day 3, day 5, day 7 and day 14). Two-way ANOVA with repeated measures was used for statistical analysis and a p value less than 0.05 was considered statistical significance.

RESULTS

Our results revealed that bite force was significantly smaller in the force group than in the sham group at all time points (p < .001). As compared to the control group, periodontal injections of CFA significantly decreased bite force on the 3rd day (p < .01). Moreover, bite force was significantly higher in the force + antiNGF group than in the force + saline group (p = .01 < .05) while significantly smaller in the control + NGF group than in the control + saline group (p < .05). Bite force was similar between the force + antiNGF group and the control + saline group (p = .71 > .05) and between the control + NGF group and the force + saline group (p = .58 > .05). Similar results were found for bite frequency.

Migraine-Associated TRESK Mutations Increase Neuronal Excitability through Alternative Translation Initiation and Inhibition of TREK

It is often unclear why some genetic mutations to a given gene contribute to neurological disorders and others do not. For instance, two mutations have previously been found to produce a dominant negative for TRESK, a two-pore-domain K+ channel implicated in migraine: TRESK-MT, a 2-bp frameshift mutation, and TRESK-C110R. Both mutants inhibit TRESK, but only TRESK-MT increases sensory neuron excitability and is linked to migraine. Here, we identify a new mechanism, termed frameshift mutation-induced alternative translation initiation (fsATI), that may explain why only TRESK-MT is associated with migraine. fsATI leads to the production of a second protein fragment, TRESK-MT2, which co-assembles with and inhibits TREK1 and TREK2, two other two-pore-domain K+ channels, to increase trigeminal sensory neuron excitability, leading to a migraine-like phenotype in rodents. These findings identify TREK1 and TREK2 as potential molecular targets in migraine and suggest that fsATI should be considered as a distinct class of mutations.