Experimental occlusal interference induces long-term masticatory muscle hyperalgesia in rats

Botulinum toxin type A inhibits hyperalgesia in the rat masseter muscle in a carrageenan model of myofascial pain

OBJECTIVE

Previous studies have shown that botulinum toxin type A (BoNT-A) attenuates nociception, but the underlying mechanisms remain unclear. Studies of experimental pain in humans have also shown conflicting results. Carrageenan is commonly used to produce short-term acute inflammation and hyperalgesia in animal models, and the effect of BoNT-A on carrageenan-induced pain in the masseter muscle has not been studied. This study evaluated the antinociceptive and anti-inflammatory effects of intramuscular injection of BoNT-A in an experimental model of inflammatory pain in the masseter muscle of rats.

DESIGN

Carrageenan (2 %) was injected into the masseters of sixty rats pretreated with three sessions of BoNT-A (3.5 U/kg) or daily with ibuprofen (40 mg/kg) for seven days. Masseter injected with saline was used as a control. An electronic von Frey anesthesiometer determined the head withdrawal threshold before carrageenan and at 5 h, 1, 3, and 7 days following administration. The masseters were processed for paraffin embedding and H&E staining and subjected to histomorphometric analysis 1 and 8 days after carrageenan administration.

RESULTS

Pretreatments with BoNT-A or ibuprofen significantly decreased carrageenan-induced hyperalgesia. BoNT-A did not inhibit inflammation and tissue damage induced by carrageenan.

CONCLUSIONS

These findings reveal that BoNT-A promotes antinociceptive effects in the masseter muscle during painful conditions independently of anti-inflammatory mechanisms.

Effect of occlusal interference on condylar position and trajectory of movement: a randomized crossover-controlled trial

OBJECTIVE

The occlusal interferences may alter the position of the condyles in the articular fossa and trigger abnormal mandibular movements, but the specific effects on condylar position and movement trajectory are currently unknown. The present study was designed as a randomized crossover-controlled trial to investigate the direct effects of distinguished types of occlusal interference on condylar position and movement trajectory.

METHODS

14 volunteers with healthy dentition were recruited. Each of them underwent four occlusal interference conditions: mediotrusive occlusal interference (MOI), laterotrusive occlusal interference (LOI), dummy occlusal interference (DOI), and free of interference. The sequence of interferences was randomized. Mandibular movements were recorded and measured by using jaw motion analysis system, during which the condylar trajectory and functional parameters, including sagittal condylar inclination (SCI) and Bennett Angle (BA) were analyzed to assess the condylar motion status. Furthermore, a descriptive analysis of condylar positional trends was conducted to measure the relative three-dimensional position of the condyle.

RESULTS

Affected by MOI, the condylar axis turned towards the anterior and superior directions. BA values exhibited a significant increase on the interference side and a decrease on the other side, accompanied by a reduction in SCI on the interference side. LOI caused a larger mandibular deviation angle towards the interference side, resulting in a notable increase in BA. The differences were all statistically significant (P<0.05).

CONCLUSION

Occlusal interferences can alter the condylar position and movement trajectory during the mandibular movement in laterotrusion and protrusion. Different occlusal interferences have different influences, manifested in the varying values of condylar guidance inclination.

TRIAL REGISTRATION

The study was registered at the Chinese Clinical Trial Registry on 01/05/2024 (Identification number: ChiCTR2400084150).

Genistein reverses the exacerbating effect of 17β-estradiol on experimental occlusal interference induced chronic masseter hyperalgesia through suppressing ERK1/2 signal pathway in spinal trigeminal nucleus of ovariectomized rats

BACKGROUND

Temporomandibular disorder (TMD) pain is more prevalent in females than in males, with high estrogen levels potentially being a risk factor. Research indicates that 17β-estradiol (E2) exacerbates experimental occlusal interference (EOI)-induced orofacial hyperalgesia, which can be reversed by genistein. This study aimed to explore the central mechanisms within the spinal trigeminal nucleus (Sp5) related to the pain-exacerbating effect of E2 and the antiestrogenic properties of genistein in a model of EOI-induced chronic masseter pain.

METHODS

Female rats underwent ovariectomy (OVX), followed by pretreatment with genistein or genistin (a control drug for genistein that does not inhibit protein tyrosine kinases (PTKs)), E2 replacement, and EOI application. The head withdrawal thresholds (HWTs) of the bilateral masseters were measured to evaluate pain sensitivity. Expression levels of p-ERK and two PTKs (Yes-associated protein, YAP; Src kinase, Src) in bilateral Sp5 were assessed through immunofluorescent staining and/or Western blotting. The ERK inhibitor PD98059 or vehicle was administered via intrathecal injection (i.t.) to inhibit the ERK1/2 signaling pathway.

RESULTS

E2 intensified EOI-induced masseter mechanical hyperalgesia in OVX rats, and upregulated the phosphorylation of ERK1/2 in bilateral Sp5. Blocking phosphorylation of ERK1/2 in Sp5 reversed the exacerbating effect of E2. Genistein partially reversed the masseter hyperalgesia induced by E2 combined with EOI, possibly through the inhibition of PTKs and p-ERK1/2 upregulation in bilateral Sp5.

CONCLUSION

Genistein alleviates the pain-exacerbating effect of E2 on EOI-induced chronic mechanical hyperalgesia by inhibiting YAP and Src tyrosine kinases as well as the downstream ERK1/2 signaling pathway in Sp5.

Up-regulation of IL-1β and sPLA2-III in the medial prefrontal cortex contributes to orofacial and somatic hyperalgesia induced by malocclusion via glial-neuron crosstalk

The medial prefrontal cortex (mPFC) has been identified as a key brain region involved in the modulation of chronic pain. Our recent study demonstrated that unilateral anterior crossbite (UAC) developed the comorbidity model of temporomandibular disorders (TMD) and fibromyalgia syndrome (FMS), which was characterized by both orofacial and somatic hyperalgesia. In the present study, UAC rats exhibited significant changes in gene expression in the mPFC. Enrichment analysis revealed that the significantly involved pathways were cytokines-cytokine receptor interaction and immune response. The expression of group III secretory phospholipase A2 (sPLA2-III) was significantly increased in the mPFC of UAC rats. Silencing sPLA2-III expression in the mPFC blocked the orofacial and somatic hyperalgesia. Immunofluorescence showed that sPLA2-III was mainly localized in neurons. The expression of interleukin-1β (IL-1β) in the mPFC significantly increased after UAC. Injection of IL-1β antibody into the mPFC blocked orofacial and somatic hyperalgesia. IL-1β was mainly localized in microglia cells. Furthermore, injection of IL-1β antibody significantly reduced the expression of sPLA2-III. These results indicate that neuroinflammatory cascade responses induced by glial-neuron crosstalk in the mPFC may contribute to the development of TMD and FMS comorbidity, and IL-1β and sPLA2-III are identified as novel potential therapeutic targets for the treatment of chronic pain in the comorbidity of TMD and FMS.

Three-dimensional assessment of temporomandibular joint in skeletal Class I malocclusion with variable degrees of overbite and overjet

OBJECTIVE

To three-dimensionally evaluate the position and morphology of the TMJ in skeletal Class I with variable degrees of overbite and overjet in comparison with normal peers.

METHODS

Pretreatment CBCT scans of 126 adults were retrieved. Based on the severity of overjet and overbite, the sample was divided into three main groups and six subgroups (18 patients each): the first group with a normal overbite and variable overjet degree and the second group with normal overjet and variable overbite degree. These six subgroups were compared with a third control group of normal overjet and overbite.

RESULTS

There were significant differences in vertical condylar position, vertical and anteroposterior condylar inclinations, condylar height, and significant posterior condylar positioning in severe deep overbite, compared with the control group.

CONCLUSION

There were significant changes in the TMJ components in severe deep overbite cases. Therefore, sever deep overbite could be considered a predisposing factor for temporomandibular disorders.

Nociceptive and histomorphometric evaluation of the effects of ozone therapy on the rat masseter muscle in a carrageenan model of myofascial pain

OBJECTIVE

This study evaluated the effects of intramuscular ozone therapy on nociception, inflammation, and tissue damage caused by the injection of carrageenan in the masseter muscle of rats.

DESIGN

Rat masseter muscles were injected with saline or carrageenan. Seventy-seven adult male rats were divided into six groups: Sal, saline; Car, carrageenan; Ibup + Sal, ibuprofen and saline; Ibup + Car, ibuprofen and carrageenan; O3 + Sal, ozone and saline; and O3 + Car, ozone and carrageenan. The mixture of 5% ozone and 95% oxygen (20 µg/mL) was administered three times in the course of a week. Nociceptive responses in the masseter muscles were measured using a head withdrawal threshold, determined by an electronic von Frey anesthesiometer. The animals were euthanized one or eight days after the carrageenan injection, and the masseters were submitted to histological and histomorphometric analyses.

RESULTS

Mechanical allodynia and inflammation levels were reduced in the Ibup + Car group compared to the other groups. Myonecrosis was similar among carrageenan-treated groups. Picrosirius red stained sections showed more collagen fibers and more regenerating myofibers in the O3 + Car group compared to the other groups. Eight days after carrageenan injection, the O3 + Car group showed neutrophils close to the regenerating myofibers.

CONCLUSIONS

Intramuscular ozone therapy did not alleviate mechanical allodynia, and it did not protect the masseter muscle against the deleterious effects produced by carrageenan, probably due to the mode of administration of this therapeutic agent.

Dissecting neural circuits from rostral ventromedial medulla to spinal trigeminal nucleus bidirectionally modulating craniofacial mechanical sensitivity

Chronic craniofacial pain is intractable and its mechanisms remain unclarified. The rostral ventromedial medulla (RVM) plays a crucial role in descending pain facilitation and inhibition. It is unclear how the descending circuits from the RVM to spinal trigeminal nucleus (Sp5) are organized to bidirectionally modulate craniofacial nociception. We used viral tracing, in vivo optogenetics, calcium signaling recording, and chemogenetic manipulations to investigate the structure and function of RVM-Sp5 circuits. We found that most RVM neurons projecting to Sp5 were GABAergic or glutamatergic and facilitated or inhibited craniofacial nociception, respectively. Both GABAergic interneurons and glutamatergic projection neurons in Sp5 received RVM inputs: the former were antinociceptive, whereas the latter were pronociceptive. Furthermore, we demonstrated activation of both GABAergic and glutamatergic Sp5 neurons receiving RVM inputs in inflammation- or dysfunction-induced masseter hyperalgesia. Activating GABAergic Sp5 neurons or inhibiting glutamatergic Sp5 neurons that receive RVM projections reversed masseter hyperalgesia. Our study identifies specific cell types and projections of RVM-Sp5 circuits involved in facilitating or inhibiting craniofacial nociception respectively. Selective manipulation of RVM-Sp5 circuits can be used as potential treatment strategy to relieve chronic craniofacial muscle pain.

Preclinical orofacial pain assays and measures and chronic primary orofacial pain research: where we are and where we need to go

Chronic primary orofacial pain (OFP) conditions such as painful temporomandibular disorders (pTMDs; i.e., myofascial pain and arthralgia), idiopathic trigeminal neuralgia (TN), and burning mouth syndrome (BMS) are seemingly idiopathic, but evidence support complex and multifactorial etiology and pathophysiology. Important fragments of this complex array of factors have been identified over the years largely with the help of preclinical studies. However, findings have yet to translate into better pain care for chronic OFP patients. The need to develop preclinical assays that better simulate the etiology, pathophysiology, and clinical symptoms of OFP patients and to assess OFP measures consistent with their clinical symptoms is a challenge that needs to be overcome to support this translation process. In this review, we describe rodent assays and OFP pain measures that can be used in support of chronic primary OFP research, in specific pTMDs, TN, and BMS. We discuss their suitability and limitations considering the current knowledge of the etiology and pathophysiology of these conditions and suggest possible future directions. Our goal is to foster the development of innovative animal models with greater translatability and potential to lead to better care for patients living with chronic primary OFP.

Descending serotonergic modulation from rostral ventromedial medulla to spinal trigeminal nucleus is involved in experimental occlusal interference-induced chronic orofacial hyperalgesia

BACKGROUND

Dental treatment associated with unadaptable occlusal alteration can cause chronic primary myofascial orofacial pain. The serotonin (5-HT) pathway from the rostral ventromedial medulla (RVM) exerts descending modulation on nociceptive transmission in the spinal trigeminal nucleus (Sp5) and facilitates chronic pain. The aim of this study was to investigate whether descending 5-HT modulation from the RVM to the Sp5 is involved in the maintenance of primary myofascial orofacial hyperalgesia after persistent experimental occlusal interference (PEOI) or after delayed removal of experimental occlusal interference (REOI).

METHODS

Expressions of 5-HT3A and 5-HT3B receptor subtypes in the Sp5 were assessed by immunofluorescence staining and Western blotting. The release and metabolism of 5-HT in the Sp5 were measured by high-performance liquid chromatography. Changes in the pain behavior of these rats were examined after specific pharmacologic antagonism of the 5-HT3 receptor, chemogenetic manipulation of the RVM 5-HT neurons, or selective down-regulation of 5-HT synthesis in the RVM.

RESULTS

Upregulation of the 5-HT3B receptor subtype in the Sp5 was found in REOI and PEOI rats. The concentration of 5-HT in Sp5 increased significantly only in REOI rats. Intrathecal administration of Y-25130 (a selective 5-HT3 receptor antagonist) dose-dependently reversed the hyperalgesia in REOI rats but only transiently reversed the hyperalgesia in PEOI rats. Chemogenetic inhibition of the RVM 5-HT neurons reversed the hyperalgesia in REOI rats; selective down-regulation of 5-HT in advance also prevented the development of hyperalgesia in REOI rats; the above two manipulations did not affect the hyperalgesia in PEOI rats. However, chemogenetic activation of the RVM 5-HT neurons exacerbated the hyperalgesia both in REOI and PEOI rats.

CONCLUSIONS

These results provide several lines of evidence that the descending pathway from 5-HT neurons in the RVM to 5-HT3 receptors in the Sp5, plays an important role in facilitating the maintained orofacial hyperalgesia after delayed EOI removal, but has a limited role in that after persistent EOI.

Temporomandibular Disorders and Dental Occlusion: What Do We Know so Far?

Throughout the recorded history in the literature of temporomandibular disorders (TMD) there have been a variety of opinions as to its primary cause. Those supporting an occlusal basis of TMD opined that occlusal dysfunction is either the primary cause for or a significant etiopathogenic factor in the causation of TMD. Most of the current literature, however, points to evidence in another direction and questions the causal role of occlusion and occlusal disharmony in TMD etiopathogenesis. Recognition of this evidence-based literature is paramount in eliminating and preventing the chances of overtreatment of patients with TMD.

Assessing Orofacial Pain Behaviors in Animal Models: A Review

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

Development of a model to investigate the effects of prolonged ischaemia on the muscles of mastication of male Sprague Dawley rats

OBJECTIVE

The aims of this study were to develop a novel rodent model of masticatory muscle ischaemia via unilateral ligation of the external carotid artery (ECA), and to undertake a preliminary investigation to characterize its downstream effects on mechanosensitivity and cellular features of the masseter and temporalis muscles.

DESIGN

The right ECA of 18 male Sprague-Dawley rats was ligated under general anaesthesia. Mechanical detection thresholds (MDTs) at the masseter and temporalis bilaterally were measured immediately before ECA ligation and after euthanasia at 10-, 20-, and 35-days (n = 6 rats/timepoint). Tissue samples from both muscles and sides were harvested for histological analyses and for assessing changes in the expression of markers of hypoxia and muscle degeneration (Hif-1α, VegfA, and Fbxo32) via real time PCR. Data were analyzed using mixed effect models and non-parametric tests. Statistical significance was set at p < 0.05.

RESULTS

MDTs were higher in the right than left hemiface (p = 0.009) after 20 days. Histological changes indicative of muscle degeneration and fibrosis were observed in the right muscles. Hif-1α, VegfA, and Fbxo32 were more highly expressed in the masseter than temporalis muscles (all p < 0.05). Hif-1α and, VegfA did not change significantly with time in all muscles (all p > 0.05). Fbxo32 expression gradually increased in the right masseter (p = 0.024) and left temporalis (p = 0.05).

CONCLUSIONS

ECA ligation in rats induced hyposensitivity in the homolateral hemiface after 20 days accompanied by tissue degenerative changes. Our findings support the use of this model to study pathophysiologic mechanisms of masticatory muscle ischaemia in larger investigations.

The chronification mechanism of orofacial inflammatory pain: Facilitation by GPER1 and microglia in the rostral ventral medulla

Background

Chronic orofacial pain is a common and incompletely defined clinical condition. The role of G protein-coupled estrogen receptor 1 (GPER1) as a new estrogen receptor in trunk and visceral pain regulation is well known. Here, we researched the role of GPER1 in the rostral ventral medulla (RVM) during chronic orofacial pain.

Methods and Results

A pain model was established where rats were injected in the temporomandibular joint with complete Freund's adjuvant (CFA) to simulate chronic orofacial pain. Following this a behavioral test was performed to establish pain threshold and results showed that the rats injected with CFA had abnormal pain in the orofacial regions. Additional Immunostaining and blot analysis indicated that microglia were activated in the RVM and GPER1 and c-Fos were significantly upregulated in the rats. Conversely, when the rats were injected with G15 (a GPER1 inhibitor) the abnormal pain the CFA rats were experiencing was alleviated and microglia activation was prevented. In addition, we found that G15 downregulated the expression of phospholipase C (PLC) and protein kinase C (PKC), inhibited the expression of GluA1, restores aberrant synaptic plasticity and reduces the overexpression of the synapse-associated proteins PSD-95 and syb-2 in the RVM of CFA rats.

Conclusion

The findings indicate that GPER1 mediates chronic orofacial pain through modulation of the PLC-PKC signal pathway, sensitization of the RVM region and enhancement of neural plasticity. These results of this study therefore suggest that GPER1 may serve as a potential therapeutic target for chronic orofacial pain.

Spinal CCK1 Receptors Contribute to Somatic Pain Hypersensitivity Induced by Malocclusion via a Reciprocal Neuron-Glial Signaling Cascade

Recent studies have shown that the incidence of chronic primary pain including temporomandibular disorders (TMD) and fibromyalgia syndrome (FMS) often exhibit comorbidities. We recently reported that central sensitization and descending facilitation system contributed to the development of somatic pain hypersensitivity induced by orofacial inflammation combined with stress. The purpose of this study was to explore whether TMD caused by unilateral anterior crossbite (UAC) can induce somatic pain hypersensitivity, and whether the cholecystokinin (CCK) receptor-mediated descending facilitation system promotes hypersensitivity through neuron-glia cell signaling cascade. UAC evoked thermal and mechanical pain hypersensitivity of the hind paws from day 5 to 70 that peaked at week 4 post UAC. The expression levels of CCK1 receptors, interleukin-18 (IL-18) and IL-18 receptors (IL-18R) were significantly up-regulated in the L4 to L5 spinal dorsal horn at 4 weeks post UAC. Intrathecal injection of CCK1 and IL-18 receptor antagonists blocked somatic pain hypersensitivity. IL-18 mainly co-localized with microglia, while IL-18R mainly co-localized with astrocytes and to a lesser extent with neurons. These findings indicate that the signaling transduction between neurons and glia at the spinal cord level contributes to the descending pain facilitation through CCK1 receptors during the development of the comorbidity of TMD and FMS. PERSPECTIVE: CCK1 receptor-dependent descending facilitation may mediate central mechanisms underlying the development of widespread somatic pain via a reciprocal neuron-glial signaling cascade, providing novel therapeutic targets for the clinical treatment of TMD and FMS comorbidities.

Celecoxib in the treatment of orofacial pain and discomfort in rats subjected to a dental occlusal interference model

Purpose:

To evaluate the effect of a selective cyclooxygenase 2 (COX-2) inhibitor on trigeminal ganglion changes and orofacial discomfort/nociception in rats submitted to an experimental model of dental occlusal interference (DOI).

Methods:

Female Wistar rats (180-200 g) were divided into five groups: a sham group (without DOI) (n=15); and four experimental groups with DOI treated daily with 0.1 mL/kg saline (DOI+SAL), 8, 16, or 32 mg/kg celecoxib (DOI+cel -8, -16, -32) (n=30/group). The animals were euthanized after one, three, and seven days. The bilateral trigeminal ganglia were analyzed histomorphometrically (neuron cell body area) and immunohistochemically (COX-2, nuclear factor-kappa B [NFkB], and peroxisome proliferator-activated receptor-y [PPARy]). A bilateral nociception assay of the masseter muscle was performed. The number of bites/scratches, weight, and grimace scale scores were determined daily. One-way/two-way analysis of variance (ANOVA)/Bonferroni post hoc tests were used (P < .05, GraphPad Prism 5.0).

Results:

DOI+SAL showed a reduction in neuron cell body area bilaterally, whereas DOI+cel-32 exhibited a significative increase in neuron cell body area compared with DOI+SAL group (P < 0.05). The ipsilateral (P=0.007 and P=0.039) and contralateral (P < 0.001 and P=0.005) overexpression of COX-2 and NFkB and downregulation of PPARy (P=0.016 and P < 0.001) occurred in DOI+SAL, but DOI+cel-32 reverted this alteration. DOI+SAL showed increase in isplateral (P < 0.001) and contralateral (P < 0.001) nociception, an increased number of bites (P=0.010), scratches (P < 0.001), and grimace scores (P=0.032). In the group of DOI+cel-32, these parameters were reduced.

Conclusions:

Celecoxib attenuated DOI-induced transitory nociception/orofacial discomfort resulting from trigeminal COX-2 overexpression.

[Hippocampus is involved in 17β-estradiol exacerbating experimental occlusal inter- ference-induced chronic masseter hyperalgesia in ovariectomized rats]

OBJECTIVE

To investigate the influence of chronic masseter hyperalgesia induced by 17β-estradiol (E2) and experimental occlusal interference (EOI) on underlying mechanism in hippocampus of ovariectomized (OVX) rats.

METHODS

In the study, 32 OVX rats were randomly divided into 4 groups (8 rats/group): The control group was OVX group, and 0 μg/d E2 (vehicle) injection was started 7 d after OVX without EOI; in the experimental group (1) OVX + E2 group, 80 μg/d E2 injection was started 7 d after OVX without EOI; in the experimental group (2) OVX + EOI group, vehicle injection was started 7 d after OVX and EOI was applied 17 d after OVX; in the experimental group (3) OVX + E2 + EOI group, 80 μg/d E2 injection was started 7 d after OVX and EOI was applied 17 d after OVX. Bilateral masseter muscle mechanical withdrawal thresholds were measured before OVX, 7 days after OVX (before E2 injection), 17 days after OVX (10 days after E2 injection and before EOI) and 24 days after OVX (7 days after EOI). Immunofluorescence staining was used to reveal phospho-extracellular signal regulated kinase 1/2 (p-ERK1/2)-positive neurons in CA3 of hippocampus. The protein expression of p-ERK1/2 in hippocampus was detected using Western Blot.

RESULTS

Compared with the control group [left side: (135.3±8.5) g, right side: (135.4±10.8) g], bilateral masseter muscle mechanical withdrawal thresholds of OVX+E2 group [left side: (113.3±5.6) g, right side: (112.5 ± 5.6) g] and OVX+EOI group [left side: (93.3±5.4) g, right side: 90.8±5.5) g] were decreased (P < 0.01). Bilateral masseter muscle mechanical withdrawal thresholds were significantly lower in OVX+E2+EOI group [left side: (81.2±6.2) g, right side: 79.8±7.7) g] than in the control, OVX+E2 and OVX+EOI groups (P < 0.05). The proportion of p-ERK1/2 positive neurons in the CA3 region of the hippocampus was increased in the control, OVX+E2, OVX+EOI and OVX+E2+EOI groups in turn, and the difference between the groups was statistically significant (P < 0.05). p-ERK1/2 protein expression was increased in the control, OVX+E2 and OVX+EOI groups in turn, but the difference was not statistically significant (P>0.05). p-ERK1/2 expression was significantly higher in OVX+E2+EOI group than in the other three groups (P < 0.05).

CONCLUSION

High concentration of E2 could exacerbated EOI-induced chronic masseter hyperalgesia in ovariectomized rats, and its central mechanism may be related to the upregulation of the phosphorylation of ERK1/2 in hippocampus.

Occlusal Trauma Induces Neuroimmune Crosstalk for a Pain State

Temporomandibular joint (TMJ) disorder caused by occlusal trauma is one of the most controversial topics in dentistry. Experimental traumatic occlusion (ETO) induced by metal crowns cemented to mandibular first molars in rats causes a long-lasting nociceptive response. This study aimed to elucidate whether ETO generates an increase in inflammatory mediators in the TMJ. In addition, the impact of ETO on trigeminal ganglia, neurotransmitter release, and satellite glial cell (SGC) activation was investigated. ELISA revealed enhanced inflammatory mediators, including TNF-α, IL-1β, IL-6, CX3CL1, and ADAM-17 by Western blotting, in periarticular TMJ tissue after 28 d of ETO. In the trigeminal ganglia, ETO groups increased the release of the neurotransmitters substance P and glutamate. Overexpression of the AMPA receptor and upregulation of NMDA were observed in the 0.4- and 0.7-mm ETO groups, respectively, highlighting enhanced neuronal excitation. Increased IL-1β and COX-2 mRNA levels in the 0.7-mm ETO group confirmed trigeminal ganglia SGC activation. Immunofluorescence and electrophoresis of SGC revealed increased pERK expression in the 0.7-mm ETO group. ERK phosphorylation was shown to be nociceptive specific, with its upregulation occurring in cases of chronic inflammatory pain. Increased PKA mRNA levels were observed in the 0.4-mm ETO group, while CREB mRNA levels were upregulated for both ETO groups. Electrophoresis showed overexpression of sodium channel Nav 1.7 in the 0.7-mm ETO group, while immunofluorescence revealed that Nav 1.7 is expressed in sensory trigeminal ganglia cells. The results of this study suggest that occlusal trauma induces neuroimmune crosstalk, with synthesis of proinflammatory/pronociceptive mediators, which increases neuronal activity in trigeminal ganglia via the activation of an inflammatory response cascade to develop a persistent neuroinflammatory state that leads to central sensitization.

Occlusal disharmony and chronic oro-facial pain: from clinical observation to animal study

BACKGROUND

Occlusion can be viewed as the most sensitive susceptor of the central nervous system in the oro-facial region. Its inalienable relationships to the temporomandibular joint, the muscles, the stomatognathic system and even the central nervous system are self-evident. Almost all the dental treatments inevitably change the occlusion, potentially or actually, locally or extensively, and immediately or gradually.

OBJECTIVE

The objective of this study was to present a narrative literature on occlusal disharmony and chronic oro-facial pain.

METHODS

Literature reviews focusing on clinical studies about the relationship between occlusal disharmony and myofascial oro-facial pain, and related preclinical studies about the animal models of, as well as the peripheral and central mechanisms underlying this condition related to, occlusal disharmony were used as starting point and guidelines to describe the topics mentioned. A search of the PubMed database was performed mainly with the following search terms: "occlusion," "occlusal interference," "occlusal disharmony," "occlusal change," "oro-facial pain" and "myofascial pain."

RESULTS

Relevant literature from the past 70 years until the present day was meticulously studied. The literature review together with three related characteristic clinical cases revealed an intimate association between occlusal disharmony and chronic oro-facial pain, involving pathological changes, extending from the peripheral tissues to the central nervous system. The patients suffered from psychological distress, sleep disturbance and poor life quality.

CONCLUSION

Occlusal disharmony-related oro-facial pain is a clinical problem that deserves attention, although there are no universally accepted clinical protocols. The existing literature provides some constructive suggestions, but further research is needed.

17β-Estradiol Exacerbated Experimental Occlusal Interference-Induced Chronic Masseter Hyperalgesia by Increasing the Neuronal Excitability and TRPV1 Function of Trigeminal Ganglion in Ovariectomized Rats

Pain symptoms in temporomandibular disorders (TMD) predominantly affect reproductive women, suggesting that estrogen regulates pain perception. However, how estrogen contributes to chronic TMD pain remains largely unclear. In the present study, we performed behavioral tests, electrophysiology, Western blot and immunofluorescence to investigate the role and underlying mechanisms of estrogen in dental experimental occlusal interference (EOI)-induced chronic masseter mechanical hyperalgesia in rats. We found that long-term 17β-estradiol (E2) replacement exacerbated EOI-induced masseter hyperalgesia in a dose-dependent manner in ovariectomized (OVX) rats. Whole-cell patch-clamp recordings demonstrated that E2 (100 nM) treatment enhanced the excitability of isolated trigeminal ganglion (TG) neurons in OVX and OVX EOI rats, and EOI increased the functional expression of transient receptor potential vanilloid-1 (TRPV1). In addition, E2 replacement upregulated the protein expression of TRPV1 in EOI-treated OVX rats. Importantly, intraganglionic administration of the TRPV1 antagonist AMG-9810 strongly attenuated the facilitatory effect of E2 on EOI-induced masseter mechanical sensitivity. These results demonstrate that E2 exacerbated EOI-induced chronic masseter mechanical hyperalgesia by increasing TG neuronal excitability and TRPV1 function. Our study helps to elucidate the E2 actions in chronic myogenic TMD pain and may provide new therapeutic targets for relieving estrogen-sensitive pain.

Genistein reverses the effect of 17β-estradiol on exacerbating experimental occlusal interference-induced chronic masseter hyperalgesia in ovariectomised rats

BACKGROUND

Oro-facial pain is more prevalent in women than in men, and oestrogen may underlie this sex difference. Genistein reversed the potentiation of 17β-estradiol (E2) on glutamate-induced acute masseter nociceptive behaviour, but its role in dental experimental occlusal interference (EOI)-induced chronic masseter hyperalgesia remains unclear.

OBJECTIVE

This study aimed to investigate sex differences, and to explore the role and underlying mechanisms of genistein in E2-potentiated EOI-induced chronic masseter hyperalgesia in rats.

METHODS

Female and male rats were prepared to compare the sex differences of masseter hyperalgesia induced by EOI using a 0.4-mm-thick metal crown. Female rats were ovariectomised (OVX) and treated with E2 and genistein, followed by EOI. The head withdrawal threshold (HWT) was examined to assess masseter sensitivity. The protein expression of transient receptor potential vanilloid-1 (TRPV1) in the trigeminal ganglion (TG) was detected using western blotting. Immunofluorescence staining was used to reveal the colocalisation of oestrogen receptors (ERs) with TRPV1 and the percentage of TRPV1-positive neurons in the TG.

RESULTS

To some extent, female rats displayed enhanced sensitivity to EOI-induced chronic masseter hyperalgesia compared with males. Female rats showed the lowest HWT in the pro-oestrus phase. Pre-treatment with genistein antagonised E2 potentiation in EOI-induced masseter hyperalgesia and blocked the effect of E2 by downregulating TRPV1 protein expression and the percentage of TRPV1-positive neurons in the TG.

CONCLUSION

Female rats showed greater masseter hyperalgesia than males under EOI. Genistein antagonised the facilitation of EOI-induced chronic masseter hyperalgesia by E2 probably through inhibiting TRPV1 in the TG.

Astrocytes in the rostral ventromedial medulla contribute to the maintenance of oro-facial hyperalgesia induced by late removal of dental occlusal interference

BACKGROUND

Astrocytes in the rostral ventromedial medulla (RVM) contribute to descending pain modulation, but their role in oro-facial pain induced by persistent experimental dental occlusal interference (PEOI) or following EOI removal (REOI) is unknown.

OBJECTIVE

To explore the involvement of RVM astrocytes in PEOI-induced oro-facial hyperalgesia or its maintenance following REOI.

METHODS

Male rats were randomly assigned into five groups: sham-EOI, postoperative day 6 and 14 of PEOI (PEOI 6 d and PEOI 14 d), postoperative day 6 following REOI on day 3 (REOI 3 d) and postoperative day 14 following REOI on day 8 (REOI 8 d). The nociceptive head withdrawal threshold (HWT) and activities of RVM ON- or OFF-cells were recorded before and after intra-RVM astrocyte gap junction blocker carbenoxolone (CBX) microinjection. RVM astrocytes were labelled immunohistochemically with glial fibrillary acidic protein (GFAP) and analysed semi-quantitatively.

RESULTS

Persistent experimental dental occlusal interference-induced oro-facial hyperalgesia, as reflected in decreased HWTs, was partially inhibited by REOI at day 3 but not at day 8 after EOI placement. Increased GFAP-staining area occurred only in REOI 8 d group in which CBX could inhibit the maintained hyperalgesia; CBX was ineffective in inhibiting hyperalgesia in PEOI 14 d group. OFF-cell activities showed no change, but the spontaneous activity and responses of ON-cells were significantly enhanced that could be suppressed by CBX in REOI 8 d group.

CONCLUSION

Rostral ventromedial medulla astrocytes may not participate in PEOI-induced oro-facial hyperalgesia or hyperalgesia inhibition by early REOI but are involved in the maintenance of oro-facial hyperalgesia by late REOI.

Adjustment of Occlusal Splint with Synchronized T-Scan III Digital Occlusal Analysis System and Bio-EMG III in a Patient with Sleep Bruxism

The occlusal splint has been frequently used as an effective treatment in sleep bruxism patients. In the adjustment procedure of occlusal splint with an optimal occlusion, clinicians usually use conventional methods; however, they cannot measure the surface area of contacts, amount of force, and contacting time sequence. Recently, two separate technologies have been synchronized together: T-Scan III and BioEMG III. In the present case report, an occlusal splint in a patient with sleep bruxism was adjusted with synchronized T-Scan computerized digital occlusal analysis system and BioEMG III. Optimal occlusal parameters were generated, and quantitative analyses of occlusal scheme and muscle activity level were performed before and after the adjustment of the occlusal splint.

In the present case report, occlusion of the occlusal splint in a bruxism patient was adjusted with T-Scan III to fulfill the requirement of the optimal occlusion criteria. Before adjustment of occlusal splint, digital evaluation revealed unbalanced force distribution, lengthy occlusion (1.51 s), and disocclusion time (0.09 s) in centric relation. EMG activity of muscles was lower than expected values in centric relation. After adjustment of occlusal splint, canine protected occlusion with appropriate occlusion (0.2 s) and disocclusion (0.4 s) time were performed. Increased electromyography activity of the right anterior temporalis muscle showed a decrease in all excursive movements. Modern occlusal adjustment procedures provide a new standard of verification for the validation of occlusal splint fabrication with optimal occlusion.

Nociceptive behavioural assessments in mouse models of temporomandibular joint disorders

Orofacial pain or tenderness is a primary symptom associated with temporomandibular joint (TMJ) disorders (TMDs). To understand the pathological mechanisms underlying TMDs, several mouse models have been developed, including mechanical stimulus-induced TMD and genetic mouse models. However, a lack of feasible approaches for assessing TMD-related nociceptive behaviours in the orofacial region of mice has hindered the in-depth study of TMD-associated mechanisms. This study aimed to explore modifications of three existing methods to analyse nociceptive behaviours using two TMD mouse models: (1) mechanical allodynia was tested using von Frey filaments in the mouse TMJ region by placing mice in specially designed chambers; (2) bite force was measured using the Economical Load and Force (ELF) system; and (3) spontaneous feeding behaviour tests, including eating duration and frequency, were analysed using the Laboratory Animal Behaviour Observation Registration and Analysis System (LABORAS). We successfully assessed changes in nociceptive behaviours in two TMD mouse models, a unilateral anterior crossbite (UAC)-induced TMD mouse model and a β-catenin conditional activation mouse model. We found that the UAC model and β-catenin conditional activation mouse model were significantly associated with signs of increased mechanical allodynia, lower bite force, and decreased spontaneous feeding behaviour, indicating manifestations of TMD. These behavioural changes were consistent with the cartilage degradation phenotype observed in these mouse models. Our studies have shown reliable methods to analyse nociceptive behaviours in mice and may indicate that these methods are valid to assess signs of TMD in mice.

Occlusion Heightened by Metal Crown Cementation is Aggressive for Periodontal Tissues

PURPOSE

To investigate the effect of experimental traumatic occlusion (ETO) induced by metal crowns on alveolar bone loss.

MATERIALS AND METHODS

Metal crowns were custom-made for the lower first molars with occlusal discrepancy of 0.4 and 0.7 mm from the maximum intercuspation. Thirty-six animals were randomly divided into three groups (n = 12 animals per group): 0.4-mm hyperocclusion group, 0.7-mm hyperocclusion group and the sham group (no metal crown). Twenty-eight days after crown cementation, the animals were euthanized and gingival tissue was collected to assess cytokine levels of IL-17, IL-6, and TNF-α using enzyme-linked immunosorbent assay (ELISA). Mandibles were stained with 1% methylene blue and alveolar bone levels were quantified. Western blotting was used to quantify the expression of receptor activator of nuclear factor κ B (RANK), and its ligand (RANKL), secreted osteoclastogenic factor of activated T cells (SOFAT) and TNF-α-converting enzyme (TACE). Also, mandibles were histologically processed and stained with hematoxylin and eosin, from which the presence of osteoclast-like cells, multinucleated cells containing ≥3 nuclei was counted at 100× magnification. The data were analyzed using one-way ANOVA and Tukey tests.

RESULTS

Experimental occlusal trauma for 28 consecutive days significantly increased alveolar bone loss and multinucleated cell counts (p < 0.05). RANK, RANKL, SOFAT, TACE, IL-6, and TNF-α were significantly higher in gingival tissues of ETO groups (p < 0.05). IL-17 titers were unchanged among the groups (p > 0.05).

CONCLUSION

Experimental traumatic occlusion activates and sustains bone resorption pathways in the periodontium inducing alveolar bone resorption. As the intensity of occlusal trauma increased, alternative osteoclastic pathways were activated, such as TACE and SOFAT.

Peripheral and central substance P expression in rat CFA-induced TMJ synovitis pain

Synovitis contributes to temporomandibular joint (TMJ) pain, nevertheless, the detailed nociceptive mechanism remains unclear. In this study, a rat model of TMJ synovitis was induced by intra-articular injection with complete Freund’s adjuvant (CFA). After CFA-induced synovitis, pain behaviors were observed. Then, TMJ, trigeminal ganglion, and trigeminal nucleus caudalis (TNC) tissues were collected, and immunohistochemistry was used to detect the expression of substance P (SP) and protein gene product 9.5 (PGP9.5) in the synovium tissue. Furthermore, the gene expression level of SP and PGP9.5 in synovium was detected by reverse transcription-polymerase chain reaction (RT-PCR). Afterwards, the expression of SP in the trigeminal ganglion and TNC and c-fos in the TNC was detected by immunohistochemistry. Compared with the control group, the expression of SP and PGP9.5 nerve fibers density and gene levels of them in the synovium tissue were significantly increased in CFA-induced TMJ synovitis rats. Similarly, SP expression in the trigeminal ganglion and TNC, and c-fos expression in the TNC were also obviously increased in CFA-induced TMJ synovitis rats. Collectively, CFA-induced rat TMJ synovitis resulted in obvious pain. This nociceptive reaction could be attributed to the augmented quantity of SP and PGP9.5 positive-stained nerve fibers distributed in the inflammatory synovium as well as enhanced SP expression in the trigeminal ganglion and TNC tissue. c-fos expression in the rat TNC illustrates CFA-induced TMJ synovitis can evoke the acute pain.

Tumor necrosis factor alpha mediates orofacial discomfort in an occlusal dental interference model in rats: The role of trigeminal ganglion inflammation

BACKGROUND

Tumor necrosis factor alpha (TNF-α) is a proinflammatory cytokine that plays an important role in the early stages of inflammation. In this study, we investigated its role in orofacial discomfort in rats subjected to occlusal dental interference (ODI).

METHODS

Female Wistar rats (180-200 g) were divided in three groups (n = 30/group): sham group, without ODI, and two experimental groups with ODI pre-treated with 0.1 mL/kg saline (ODI + SAL) or 5 mg/kg infliximab (ODI + INF) and treated every 3 days. The animals were euthanized after 1, 3, and 7 days. The number of bites and scratches and grimace scale scores were determined daily, and the bilateral trigeminal ganglion was histomorphometrically (neuronal body area) analyzed and submitted for immunohistochemistry for TNF-α, nitric oxide synthesis (NOS) neuronal (nNOS) and inducible (iNOS), peroxisome proliferator-activated receptors (PPAR) y (PPARy) and δ/β (PPARδ/β), and glial fibrillary acidic protein (GFAP). One-way/two-way ANOVA/Bonferroni tests were used (P < .05, GraphPad Prism 5.0).

RESULTS

ODI + SAL showed a large number of bites (P = .002), scratches (P = .002), and grimace scores (P < .001) in the firsts days, and ODI + INF partially reduced these parameters. The contralateral and ipsilateral neuronal body area was significantly reduced on day 1 in ODI + SAL, but returned to the basal size on days 3 and 7, by increase in TNF-α, nNOS, PPARy, PPARδ/β, and GFAP immunostaining. The infliximab treatment attenuated these alterations (P < .05). There was no iNOS immunostaining.

CONCLUSION

Occlusal dental interference induced transitory orofacial discomfort by trigeminal inflammatory mediator overexpression, and TNF-α blockage attenuated these processes.

[Electrophysiological monitoring of pain afferent pathway of the trigeminal nerve and its functional plasticity in response to occlusal interference in rats]

OBJECTIVE

To observe the effect of occlusal interference on the afferent pathway of the trigeminal nerve and neuronal excitability in the trigeminal subnucleus caudalis (SPVC) of rats by electrical stimulation of the trigeminal ganglion (TG) and extracellular recordings of SPVC activities.

METHODS

Twenty male Wistar rats were randomly divided into control group and model group (n=10). In the model group, occlusal interference for 30 consecutive days was induced using light-cured flowable resin on the right maxillary molars. During occlusal interference, the pain sensitivity was scored with von Frey Fibers in the masseter. Simultaneous recordings of electrical activities from the SPVC, electrocardiogram, body temperature and electromyogram of the breath muscles of the anesthetized rats were performed, and the responses evoked by electrical stimulation of the TG were analyzed.

RESULTS

Compared with the control rats, the rats in the model group showed significantly increased pain sensitivity scores (P < 0.05) and increased spontaneous discharge frequency of the SPVC (P < 0.05). The amplitude of the SPVC responses induced by electrical stimulation of the TG showed stimulus intensity-dependent changes (P < 0.05), and the amplitude evoked by 4 mA and 8 mA stimulation was similar between the model group and the control group (P>0.05). Train stimulation (0.2 ms, 1 mA, 30 s, 100 Hz) of the TG significantly increased the discharge frequency of the SPVC only in the rats in the model group (P < 0.05).

CONCLUSIONS

The functional activities of the pain afferent pathway of the trigeminal nerve can be electrophysiologically monitored by electrical stimulation of the TG and extracellular recordings of SPVC activities in rats. Occlusal interference can increase the excitability of the neurons in the SPVC and enhance their sensitivities to TG afferent activation, suggesting the neural plasticity of the pain afferent pathway.

Temporomandibular joint disorders contribute to anxiety in BalB/C mice

Despite a high comorbidity between these two disorders, the physiological association between temporomandibular joint disorders (TMDs) and anxiety remains unknown. This study aimed to investigate whether TMDs contribute to anxiety through the induction of oligodendrogenesis in the hippocampus using a mouse model of TMD. Forty 8-week-old male BalB/C mice were used in the experiments. The mice were randomly divided into 4 groups: (1) control group (N group); (2) elevated occlusion group (E group); (3) restriction group (R group); and (4) elevated occlusion and restriction group (ER group). The mice were subjected to behavior tests of open field tests and elevated plus maze analysis. The serum corticosterone levels and expression of mature oligodendrocyte marker MBP and the oligodendrocyte marker RIP were analyzed. All data were statistically analyzed using by one-way analysis of variance. The TMD group showed condylar degeneration compared with the control group. Additionally, exposure to chronic restraint stress for 3 weeks after TMD significantly exacerbated anxiety-like behavior and resulted in a significant increase in serum corticosterone levels and in the expression of MBP and RIP in the dentate gyrus (DG) and CA3 in the hippocampus. Taken together, these data suggest that TMD lead to increased oligodendrogenesis in the hippocampus, which contributes to the development of anxiety-like behavior. TMD could contribute to anxiety by inducing oligodendrogenesis in the hippocampus.

ACC Plasticity Maintains Masseter Hyperalgesia Caused by Occlusal Interference

Acute occlusal interference following improper occlusal alteration in dental practice can induce chronic masticatory muscle pain. The underlying mechanism has not been clarified. Synaptic plasticity in the anterior cingulate cortex (ACC) plays a key role in the chronic pain state. This study investigated the role of synaptic plasticity in the ACC in acute occlusal interference–induced chronic masticatory muscle pain. A rat model of experimental occlusal interference (EOI) was established. In vivo local field potential (LFP) recording was conducted to evaluate the change of synaptic strength and plasticity from the medial thalamus (MT) to the ACC after EOI application. The effects of microdialysis of antagonists of glutamate receptors into the ACC on synaptic transmission from the MT to the ACC were examined. Furthermore, the influence of inhibiting glutamate receptors in the ACC on EOI-induced mechanical hyperalgesia in the masseter muscles of rats was investigated. The amplitude of LFP in the ACC evoked by MT stimulation was significantly potentiated since 14 d of EOI application. Long-term potentiation of LFP in the ACC was reliably induced by theta burst stimulation to the MT in control rats but was occluded in 14-d EOI rats. Microdialysis of AMPA/kainate receptor antagonist CNQX into the ACC attenuated LFP in the ACC evoked by stimulating the MT in control and EOI rats. Administration of NMDA receptor subunit NR2B antagonist Ro 25-6981 into the ACC significantly alleviated the potentiation of MT stimulation-evoked LFP in the ACC of EOI rats without affecting that in control rats. EOI-induced hyperalgesia in the bilateral masseter muscles of rats was dose-dependently relieved after microdialysis of Ro 25-6981 into ACC. These findings provide direct evidence that prolonged acute occlusal interference potentiates synaptic transmission in the ACC, which in turn mediates chronic masticatory muscle pain.

[Effect of long-term resistance exercise on masseter muscle mechanical hyperalgesia in rats]

OBJECTIVE

To investigate the effect of long-term resistance exercise of hindlimb on mechanical hyperalgesia of bilateral masseter muscle in rats with or without occlusal interference.

METHODS

Six-teen male Sprague-Dawley rats (220-250 g) were randomly divided into four groups: the naive control group, naive exercise group, occlusal interference control group, and occlusal interference exercise group. The rats in occlusal interference groups (occlusal interference control group and occlusal interference exercise group) obtained occlusal interference with 0.4 mm-thick crowns bonded to the right maxillary first molars. The rats in exercise groups (naive exercise group and occlusal interference exercise group) performed squat-type resistance exercises for 30 minutes, once a day, 5 days/week, lasting for 14 weeks. Resistance exercise was recorded every day. Mechanical withdrawal thresholds of bilateral masseter muscle were tested per week by use of modified electronic von-frey anesthesiometer. The rats were weighed per week. After the 14-week exercise, the muscle strength of the hindlimb was tested with a grip strength meter. Muscle (gastrocnemius and soleus) weight of bilateral hindlimb and length of bilateral fibula of the rats were obtained. The muscle-mass/body-mass ratios and muscle-mass/fibula-length ratios were calculated.

RESULTS

Between the naive control group and naive exercise group, there was no significant difference in the mechanical withdrawal thresholds of bilateral masseter muscle for the 0-4 weeks (P>0.05). During the 5-14 weeks, the mechanical withdrawal thresholds of the rats in the naive exercise group were higher than those in the naive control group (P<0.05). Between the occlusal interference control group and occlusal interference exercise group, there was no significant difference in the mechanical withdrawal thresholds of bilateral masseter muscle for the 0-6 weeks (P>0.05). During the 7-14 weeks, the mechanical withdrawal thresholds of rats in the naive exercise group were higher than those in the occlusal interference control group (P<0.05). After the 14week exercise, the body mass of the rats in nonexercise group (the naive control group and occlusal interference control group) were larger than those in exercise group [(462±6) g vs. (418±14) g, P<0.05]. And the muscle strength of hindlimb of the rats in exercise group were bigger than those in non-exercise group [(6.75±0.13) N vs. (5.41±0.15) N, P<0.01].

CONCLUSION

long-term resistance exercise can increase mechanical withdrawal thresholds of the bilateral masseter muscle in rats with or without masseter muscle mechanical hyperalgesia.

Chronic orofacial pain animal models - progress and challenges

INTRODUCTION

Chronic orofacial pain is one of the most common pain conditions experienced by adults. Animal models are often selected as the most useful scientific methodology to explore the pathophysiology of the disorders that cause this disabling pain to facilitate the development of new treatments. The creation of new models or the improvement of existing ones is essential for finding new ways to approach the complex neurobiology of this type of pain. Areas covered: The authors describe and discuss a variety of animal models used in chronic orofacial pain (COFP). Furthermore, they examine in detail the mechanisms of action involved in orofacial neuropathic pain and orofacial inflammatory pain. Expert opinion: The use of animal models has several advantages in chronic orofacial pain drug discovery. Choosing an animal model that most closely represents the human disease helps to increase the chances of finding effective new therapies and is key to the successful translation of preclinical research to clinical practice. Models using genetically modified animals seem promising but have not yet been fully developed for use in chronic orofacial pain research. Although animal models have provided significant advances in the pharmacological treatment of orofacial pain, several barriers still need to be overcome for better treatment options.

Genistein Antagonizes 17β-Estradiol Effects on Glutamate-Evoked Masseter Muscle Hypernociception in Rats

Temporomandibular disorders (TMDs) predominantly affect women of reproductive ages, with pain as the main symptom. The aim of the present study was to examine the effects of 17β-estradiol (E2) on glutamate-evoked hypernociception of masseter muscle and to examine whether genistein could antagonize the effects of E2 in female rats. Injection of glutamate into the masseter muscle dose-dependently decreased head withdrawal thresholds, a parameter for mechanical hypernociception. Head withdrawal thresholds in ovariectomized rats also decreased with increasing doses of E2 replacement, and were further aggravated by injection of glutamate (1M, 40μL) into the masseters. Genistein at doses of 7.5 and 15 mg/kg antagonized E2-induced hypernociception of masseter muscle, and at doses of 7.5, 15, and 30 mg/kg also antagonized E2 potentiation of glutamate-evoked hypernociception of masseter muscle. Genistein produced optimal antagonistic effects of E2 on nociception behavior at a dose of 15 mg/kg. On the molecular level, tyrosine phosphorylation of the NR2B subunit of the N-methyl-D-aspartate receptor (pNR2B) and phosphorylated mitogen-activated protein kinase (pERK1/2) were significantly upregulated in the hippocampus following glutamate injection and were further potentiated by E2 replacement. Genistein at dose of 15 mg/kg partially reversed E2-potentiated glutamate-evoked upregulation of pNR2B and pERK1/2 expression in the hippocampus. These results indicated that moderate doses of genistein could antagonize E2 enhanced glutamate-evoked hypernociception of masseter muscle possibly via N-methyl-D-aspartate receptor and ERK1/2 signaling pathways in the hippocampus.

[Mitochondrial calcium overload in the masseter muscle of rats with occlusal interference: ionic changes and regulation by calmodulin kinase II]

OBJECTIVE

To investigate the changes in mitochondrial calcium and extracellular sodium concentrations in the masseter muscle of rats with occlusal interference and the regulatory mechanism of mitochondrial Ca²⁺ overload by calmodulin kinase II (CaMK II).

METHODS

SD rat models of occlusal interference were established by placing a stainless steel segments (0.8 mm in diameter) to raise the occlusal surface of the upper right first molar. At 3, 7, 14, and 21 days after occlusal interference and at 3 days after removal of occlusal interference, HE staining was used to observe the histomorphological changes of the masseter muscle. Mitochondrial calcium concentration in the masseter muscle was detected using fluorescence spectrophotometry, and direct turbidimetry with potassium pyroantimonate was used to detect the extracellular sodium concentration; the expression levels of masseter muscle p-CaMK II (Thr287) and CaMK II were detected using Western blotting.

RESULTS

Compared with those in the corresponding control groups, mitochondrial Ca²⁺ concentration in the masseter muscle on occlusal interference side increased significantly at 3, 7, 14 and 21 days after occlusal interference (P<0.05), but was significantly lowered at 3 days after removal of the interference (P<0.05). The concentration of extracellular Na⁺ increased progressively with time at 3, 7, 14 and 21 days after occlusal interference (P<0.05), and was significantly decreased at 3 days after interference removal (P<0.05). Occlusal interference for 3, 7 and 14 days resulted in significantly increased expressions of p-CaMK II (Thr287) and CaMK II (P<0.05), which was significantly decreased at 21 days compared with those in the control groups (P<0.05) and further decreased after removal of occlusal interference (P<0.05). Similar changes were also observed on the side without interference, but the changes on the interference side were more obvious (P<0.05).

CONCLUSION

Occlusal interference causes elevated mitochondrial Ca²⁺ and extracellular Na⁺ concentrations in the masseter muscle of rats to lead to calcium overload; the increase in mitochondrial Ca²⁺ concentration is correlated with the phosphorylation level of CaMK II signaling pathway, suggesting a negative feedback regulation mechanism by the CaMK II signal pathway.

Evaluation of the relationship between the occlusion parameters and symptoms of the temporomandibular joint disorder

Background.

The aetiology of temporomandibular joint disorders (TMD) is multifactorial, whereas occlusal disharmony is one of the predisposing factors. Researchers still discuss the relation between occlusion and TMD.

Objective.

The study aims to investigate the relation between static occlusal parameters and TMD clinical symptoms using T-Scan II analysis system.

Material and methods.

The sample consisted of 44 persons divided into the treatment group of 20 TMD patients and the control group of 24 subjects without TMD. The main task of T-Scan II computerized occlusal analysis system was to record every patient’s occlusion and estimate static occlusal parameters: centre of occlusal force, asymmetry index of maximum occlusal force and occlusion time. These results were compared between groups, data related to patients’ complaints and clinical symptoms. The analysis was carried out using Mann-Whitney U, Kruskal-Wallis and Chi-square tests.

Results.

Averages of the centre of occlusal force in TMD subjects were 6.55 ± 0.99 mm, in the control group – 5.88 ± 0.69 mm; the asymmetry index of maximum occlusal force averages: 15.90 ± 2.71 and 12.93 ± 1.88; occlusion time: 0.281 ± 0.036 s and 0.236 ± 0.022 s, respectively. There were no statistically significant differences between two groups but they were found in the centre of occlusal force and the asymmetry index in the two groups (p < 0.05).

Conclusions.

There exists a relation between complaints of patients with TMD and static occlusion parameters. Values of the centre of the occlusal force distance and the asymmetry index of occlusal force in TMD patients with pain in the temporomandibular joint (TMJ) were significantly higher than in the control group.

Experimental occlusal disharmony - A promoting factor for anxiety in rats under chronic psychological stress

BACKGROUND AND PURPOSE

Clinically, patients under chronic psychological stress (PS) appear to be more susceptible to occlusal disharmony (OD) compared with those without PS. OD was proved to introduce anxiety-like stress. Therefore, the purpose of the study was to investigate whether OD would affect psychological stress-induced anxiety and its underlying mechanisms.

METHODS

Chronic PS was induced by a communication box, and OD was produced by bonding a 0.3mm-thick crown on the right maxillary first molar of male Sprague-Dawley rats. Sixty-seven rats were randomly divided into 8 groups: (A) chronic PS plus OD group (n=6); (B) chronic PS plus sham OD group (n=6); (C) chronic PS only group (n=6); (D) OD group (n=6); (E) sham OD group (n=6); (F) control group (n=6); (G) naive group (n=6); (H) foot-shock group (n=25). Open-field test (OFT) and elevated plus maze test (EPM) were conducted on the 7th, 21th, 35th day to measure the anxiety level of each group except naive and foot-shock group. In addition, corticosterone (CORT) level in serum, 5-hydroxytryptamine (5-HT) and 5-HT_2A receptor (5-HT_2AR) expressions in prefrontal cortex (PFC), hippocampal CA1 and dentate gyrus (DG) areas were measured on the 35th day to elucidate the mechanism(s) by which the exacerbation occurred.

RESULTS

The significant differences in OFT and EPM tests on day 21 or day 35 between groups (p<0.01) indicated the successful establishment of animal model of PS or OD. And there was a significant increase in CORT concentration in serum (p<0.01), 5-HT expressions in PFC, hippocampal DG areas and 5-HT_2AR expressions in PFC, hippocampal CA1 areas (p<0.05) in group A, B, C, D compared with group F. Similar results were also found in group A, B, C, D when compared with group G (p<0.05) except 5-HT expression in DG area in group C and D (p>0.05), together with a gradual decrease in values of all the parameters mentioned above from group A to group G.

CONCLUSION

The significant changes in exploratory behaviors, serum CORT concentration, 5-HT and 5-HT_2AR expressions induced by OD in rats with or without chronic PS, and more obvious alterations in rats with chronic PS, may indicate that OD may be a promoting factor for anxiety through both peripheral and central pathways via the hypothalamus-pituitary-adrenal (HPA) axis and 5-HT system.

Effect of Toll-Like Receptor 4 on Synovial Injury of Temporomandibular Joint in Rats Caused by Occlusal Interference

Synovitis is an important disease that causes intractable pain in TMJ. Some investigations suggested that the increasing expression of IL-1β secreted by synovial lining cells plays an important role in synovial inflammation and cartilage destruction in TMJ. In our previous research, the results demonstrated that TLR4 is involved in the expression of IL-1β in SFs from TMJ with lipopolysaccharide stimulation. However, the inflammatory response that occurred in synovial membrane is not caused by bacterial infection. In the current study, we investigated whether or not TLR4 participates in the inflammatory responses and the expression of IL-1β in synovial membrane of rats induced by occlusal interference. The results showed that obvious inflammation changes were observed in the synovial membranes and the expression of TLR4 and IL-1β was increased at both mRNA and protein levels in the occlusal interference rats. In addition, the inflammation reactions and the increased expression of IL-1β could be restrained by treatment with TAK-242, a blocker of TLR4 signaling. The results prompted us that the activation of TLR4 may be involved in the inflammatory reactions and increased expression of IL-1β in patients with synovitis and participate in the mechanisms of the initiation and development of synovial injury by regulating the expression of inflammatory mediators like IL-1β in synovial membranes.

Association of occlusal interference-induced masseter muscle hyperalgesia and P2X3 receptors in the trigeminal subnucleus caudalis and midbrain periaqueductal gray

P2X3 receptor plays a role in nociception transmission of orofacial pain in temporomandibular disorder patients. A previous study found that P2X3 receptors in masseter muscle afferent neurons and the trigeminal ganglia were involved in masseter muscle pain induced by inflammation caused by chemical agents or eccentric muscle contraction. In this study, we attempted to investigate changes in P2X3 receptors in the trigeminal subnucleus caudalis (Vc) and midbrain periaqueductal gray (PAG) in relation to the hyperalgesia of masseter muscles induced by occlusal interference. Experimental occlusal interference by crown application was established in 30 rats and another 30 rats were treated as sham controls. On days 1, 3, 7, 14, and 28 after crown application, the mechanical pain threshold was examined by von-Frey filaments. The expression of the P2X3 receptor in Vc and PAG was investigated by immunohistochemistry and quantitative PCR. We found that mechanical pain threshold of bilateral masseter muscles decreased significantly after occlusal interference, which remained for the entire experimental period. The mRNA expression of the P2X3 receptor increased significantly and the number of P2X3R-positive neurons increased markedly in Vc and PAG accordingly. These results indicate that the upregulated expression of P2X3 receptors in Vc and PAG may contribute toward the development of orofacial pain induced by occlusal interference and P2X3 receptors in the PAG may play a key role in the supraspinal antiociception effect.

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

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Activation of satellite glial cells in the trigeminal ganglion contributes to masseter mechanical allodynia induced by restraint stress in rats

It is commonly accepted that psychological stress contributes to the development of chronic orofacial pain. However, the neural mechanism underlying this process has remained unclear. The present study was performed to determine the involvement of satellite glia cells (SGCs) in the trigeminal ganglion (TG) in stress-induced increases in masseter muscle allodynia in rats. Using a chronic restraint stress model, we found that exposure to a 14-day stress but not a 3-day stress (6 h/day) caused decreased body weight gain, behavioral changes and marked masseter allodynia in rats. SGCs were dramatically activated, and substance P (SP) expression was significantly increased in the TG. A further analysis was undertaken to investigate the contribution of SGCs; the expression of interleukin-1β (IL-1β) in SGCs and interleukin-1 receptor I (IL-1RI) in neurons was significantly increased after chronic restraint stress, whereas injection of L-α-aminoadipate (a SGC inhibitor, LAA) into the TG dramatically inhibited the overexpression of these proteins. In addition, LAA or interleukin-1 receptor antagonist (IL-1ra) administration into the TG could significantly attenuate the mechanical masseter allodynia and overexpression of SP in the TG induced by restraint stress. These results suggest that SGC activation in the TG may play a role in masseter allodynia induced by restraint stress. The over-release of IL-1β and excessive IL1-RI expressions have close relationship with the stress induced masseter allodynia.

Inflammatory pain memory facilitates occlusal interference-induced masticatory muscle hyperalgesia in rats

BACKGROUND

Patients with an orofacial pain history appear to be more susceptible to occlusal interference pain in dental practice for unknown reasons. Pain memory has a critical function in subsequent pain perception. This study aims to explore whether orofacial pain memory could affect the masticatory muscle pain perception for occlusal interference.

METHODS

Cross-injection of 2% carrageenan into bilateral masseters in male rats was carried out to establish the inflammatory pain memory model. The effects of pain memory on masseter muscle nociception were tested by applying crowns with heights beyond the occlusal plane by 0.2 or 0.4 mm onto a maxillary molar 2 weeks after inflammation in the right masseter. The 0.4-mm crowns were removed on day 2 or day 4 after application to further confirm the effects of pain memory. Moreover, memory impairment was established using ibotenic acid (IBO) infusion into the bilateral hippocampus, followed by behaviour tests, including the Morris water maze test and the locomotor activity test. The relationship between pain memory and occlusal interference-induced masseter muscle pain perception was subsequently re-examined. The head withdrawal thresholds of masseters on both sides were measured to reflect the perception.

RESULTS

Inflammatory pain memory aggravated the 0.2-mm crown-induced mechanical hyperalgesia of the masseters, but not in the 0.4-mm crown group. However, the recovery of the 0.4-mm crown-induced mechanical hyperalgesia was postponed. The effects of pain memory were reversed in rats with impaired mnemonic function of the hippocampus.

CONCLUSIONS

Inflammatory pain memory facilitated occlusal interference-induced masseter muscle pain.

Involvement of trigeminal astrocyte activation in masseter hyperalgesia under stress

It is commonly accepted that psychological stress contributes to the development of temporomandibular joint disorders, in which chronic orofacial pain is the main symptom. However, the central mechanism underlying the development of these disorders has remained unclear. The current study was performed to determine the involvement of the glia in the trigeminal spinal subnucleus caudalis in stress-induced increases in masseter muscle hyperalgesia in rats. After being subjected to chronic restraint stress, the animals showed decreased body weight gain, behavioral changes and marked masseter allodynia. We also found that astrocytes, but not microglia, in the trigeminal subnucleus caudalis (Vc) were dramatically activated. A further analysis was undertaken to investigate the contribution of the glia; we intrathecally injected l-α-aminoadipate (astrocyte-specific inhibitor) and/or minocycline (microglia-specific inhibitor) into the stressed rats. Our results showed that l-α-aminoadipate (LAA), but not minocycline, could significantly attenuate the mechanical masseter allodynia and behavioral changes induced by restraint stress. In addition, the expression of interleukin-1β (IL-1β) and phosphorylated N-methyl-d-aspartic acid receptor 1 (p-NR1) in the Vc was significantly increased after chronic restraint stress, whereas LAA dramatically inhibited the overexpression of IL-1β and p-NR1. Taken together, these results suggest that activated astrocytes in the Vc may be one of the most important factors in the pathophysiology of masseter hyperalgesia induced by restraint stress and the following overexpression of IL-1β and excessive NMDAR phosphorylation may ultimately contribute to masseter hyperalgesia. Thus, inhibiting spinal astrocytic activation may represent a novel therapeutic strategy for the treatment of orofacial pain induced by stress.

TRPV1 channel-mediated bilateral allodynia induced by unilateral masseter muscle inflammation in rats

Pain in masticatory muscles is among the most prominent symptoms of temperomandibular disorders (TMDs) that have diverse and complex etiology. A common complaint of TMD is that unilateral pain of craniofacial muscle can cause a widespread of bilateral pain sensation, although the underlying mechanism remains unknown. To investigate whether unilateral inflammation of masseter muscle can cause a bilateral allodynia, we generated masseter muscle inflammation induced by unilateral injection of complete Freund’s adjuvant (CFA) in rats, and measured the bilateral head withdrawal threshold at different time points using a von Frey anesthesiometer. After behavioral assessment, both right and left trigeminal ganglia (TRG) were dissected and examined for histopathology and transient receptor potential vanilloid 1 (TRPV1) mRNA expression using quantitative real-time PCR analysis. A significant increase in TRPV1 mRNA expression occurred in TRG ipsilateral to CFA injected masseter muscle, whereas no significant alteration in TRPV1 occurred in the contralateral TRG. Interestingly, central injection of TRPV1 antagonist 5-iodoresiniferatoxin into the hippocampus significantly attenuated the head withdrawal response of both CFA injected and non-CFA injected contralateral masseter muscle. Our findings show that unilateral inflammation of masseter muscle is capable of inducing bilateral allodynia in rats. Upregulation of TRPV1 at the TRG level is due to nociception caused by inflammation, whereas contralateral nocifensive behavior in masticatory muscle nociception is likely mediated by central TRPV1, pointing to the involvement of altered information processing in higher centers.

Digital Evaluation of Functional Occlusion Parameters and their Association with Temporomandibular Disorders

CONTEXT

Dental researchers are contradictory in their opinion on the role of occlusion in TMD. Occlusal evaluation of both conventional and digital methods in TMD patients will provide the accurate information about the factors accountable for occlusal instability. Identifying the factors responsible will facilitate precise diagnosis and treatment for TMD.

AIM

The AIM of the study was to determine the dynamic occlusal parameters strongly associated with the etiology of Temporomandibular disorders.

MATERIAL AND METHODS

Study group consisted of 100 patients; it included 50 patients with normal TMJ(Group I) and remaining 50 patients had a minimum of one positive sign or symptom of Temporomandibular disorder (GroupII). The patient's dynamic occlusal contacts were evaluated by both conventional and digital methods. The Articulating paper was utilized for conventional occlusion analysis. During conventional analysis centric, lateral and protrusive interferences were evaluated along with loss of vertical dimension. Digital occlusal analysis was performed with T-Scan III. Clusion time, disclusion times were recorded for both groups. Chi-square and Student't' statistical analyses were performed to ascertain the association and statistically significant difference between the groups using SPSS19.

RESULTS

Group II patients predominantly (66%)had Group-function occlusion compared to Group I subject. Centric slide more than 2 mm found to have strong influence (p value 0.008) on the etiology of TMD. Among the occlusal interferences evaluated balanced side interferences had a strong correlation with TMD with p-value of 0.003. Working side interferences, protrusive interferences had a p-value of 0.157, 0.826 respectively, indicating weak association. T-Scan analysis showed Group I had 0.689, 0.9136, 0.7952, 0.9794 seconds of clusion, left, right, protrusive disclusion time respectively compared to corresponding 1.862, 1.7995, 1.6978, 1.9296 seconds for Group II. Statistically significant difference (p≤0.05) was found between the mean values of both groups.

CONCLUSION

Among the dynamic occlusal parameters evaluated centric slide and balancing side interferences were found to be highly influential in TMD etiology. TMD patients had prolonged clusion and disclusion times compared to healthy TMJ patients.

Central sensitization and MAPKs are involved in occlusal interference-induced facial pain in rats

We previously developed a rat dental occlusal interference model of facial pain that was produced by bonding a crown onto the right maxillary first molar and was reflected in sustained facial hypersensitivity that was suggestive of the involvement of central sensitization mechanisms. The aim of the present study was to investigate potential central mechanisms involved in the occlusal interference-induced facial hypersensitivity. A combination of behavioral, immunohistochemical, Western blot, and electrophysiological recording procedures was used in 98 male adult Sprague Dawley rats that either received the occlusal interference or were sham-operated or naive rats. Immunohistochemically labeled astrocytes and microglia in trigeminal subnucleus caudalis (Vc) showed morphological changes indicative of astrocyte and microglial activation after the occlusal interference. Prolonged upregulation of p38 mitogen-activated protein kinase (MAPK) and extracellular signal-regulated kinase (ERK) was also documented in Vc after placement of the occlusal interference and was expressed in both neurons and glial cells at time points when rats showed peak mechanical facial hypersensitivity. The intrathecal administration of the p38 MAPK inhibitor SB203580 to the medulla significantly inhibited the occlusal interference-induced hypersensitivity, and the ERK inhibitor PD98059 produced an even stronger effect. Central sensitization of functionally identified Vc nociceptive neurons following placement of the occlusal interference was also documented by extracellular electrophysiological recordings, and intrathecal administration of PD98059 could reverse the neuronal central sensitization. These novel findings suggest that central mechanisms including central sensitization of trigeminal nociceptive neurons and non-neuronal processes involving MAPKs play significant roles in the production of occlusal interference-induced facial pain.

PERSPECTIVE

Central mechanisms including trigeminal nociceptive neuronal sensitization, non-neuronal processes involving glial activation, and MAPKs play significant roles in occlusal interference-induced facial pain. These mechanisms may be involved in clinical manifestations of facial pain that have been reported in patients with an occlusal interference.

New approaches to dental occlusion: a literature update

Because the study of occlusion is a basic area in dentistry, its components, physiology and integration with the stomatognathic system (SS) have been the subject of interest in the scientific literature. However, the focus given to this issue has changed substantially. Currently, new approaches have been proposed in order to update concepts and to demonstrate the full integration and functionality of this system within the human body. With this approach, the authors proposed the following literature review aimed at gathering recent papers (published from 2000 to 2010) with innovative study design, methodology and/or results. The authors' intention is to show the main trends in the study of occlusion and the SS. The literature review was conducted in the PubMed database, using initially the term "dental occlusion" as a key-word. As items of interest were found, papers were grouped by categories according to their main subject matter. Forty-seven articles were selected and the main categories obtained were: 1. functional magnetic resonance imaging (fMRI); 2. brain activation; 3. masticatory/occlusal function; 4. body function and physical performance; 5. osseoperception; 6. finite element models; and 7. occlusion and pain. Observing the current literature, the authors found that recent studies present different methodologies for the study of occlusion. These studies have allowed scientists to obtain detailed information about the physiology of occlusion and the SS, as well as about its integration in the body. Research in this area should be continued in order to clarify, in detail, the role of each component of the SS and its interaction with human physiology.