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

Non-coding RNA alterations in occlusal disharmony-induced anxiety-like behaviour

BACKGROUND

Occlusal disharmony (OD) may induce anxiety-like behaviours; however, the underlying mechanism remains unclear. Herein, we explored the expression profiles of non-coding RNAs (ncRNAs), along with their biological function and regulatory network, in anxiety-like behaviour induced by OD.

MATERIALS AND METHODS

Occlusal disharmony was produced by anterior crossbite of C57BL/6 mice. Behavioural tests, corticosterone (CORT) and serotonin (5-HT) levels were used to measure anxiety. In addition, RNA sequencing was used to screen all differentially expressed (DE) ncRNAs. Moreover, the RNA-binding proteins interacting with ncRNAs were predicted by the ENCORI database and confirmed using western blots.

RESULTS

The significant differences in behavioural tests and CORT suggested the successful induction of anxiety-like behaviour by OD. In OD mice, ncRNAs were significantly dysregulated. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analyses suggested that the DE ncRNAs were enriched in anxiety-related pathways. CircRNA10039 was upregulated, and PTBP1 was predicted to interact with circRNA10039. In addition, KEGG pathway analysis showed that PTBP1 may be associated with messenger RNA biogenesis and spliceosomes.

CONCLUSION

OD induced by anterior crossbite can lead to the anxiety-like behaviours. During this process, ncRNA also changes. CircRNA10039 and PTBP1 may play a role in OD-induced anxiety-like behaviours.

Occlusal disharmony promotes anxiety-like behaviours by suppressing Sirt1

BACKGROUND

Previous studies have indicated that occlusal disharmony (OD) can promote anxiety-like behaviours. However, the specific molecules involved in the development of anxiety-like behaviours and their underlying mechanisms remain unknown.

METHODS

OD was produced by anterior crossbite of female mice. We measured the anxiety levels of mice in each group and screened the hippocampal mRNA expression profiles of mice in the control group and OD group. The role of target mRNA in OD-induced anxiety-like behaviours was evaluated and we preliminarily explored the possible downstream pathways.

RESULTS

The results suggested that OD can induce and promote anxiety-like behaviours with/without chronic unpredictable mild stress. We found that Sirt1 was significantly downregulated within the hippocampus in OD mice. In addition, the downregulation of Sirt1 within the hippocampus in OD and control mice promoted anxiety-like behaviours, increased acetylated histone H3 expression and decreased Dnah12 transcription levels. In contrast, in OD mice subjected to an injection of resveratrol, there was a remission of anxiety-like behaviours and an upregulation of Sirt1 in the hippocampus, the effects of which were accompanied by decreased acetylated histone H3 expression and increased Dnah12 transcription levels.

CONCLUSIONS

OD leads to increased sensitivity to chronic stress in mice, resulting in anxiety-like behaviours. During this process, Sirt1 acts as an effective factor in the regulation of OD-induced anxiety-like behaviours.

CLINICAL RELEVANCE

OD, as a stressor, could induce anxiety-like behaviours. It investigates the impact of OD (a stressor) on the molecular genetic of the pathophysiology of major neuropsychiatric disorders.

Effects of the angiotensin-converting enzyme inhibitor captopril on occlusal-disharmony-induced cardiac dysfunction in mice

Occlusal disharmony is known to affect not only the oral cavity environment, but also the autonomic nervous system in the heart. Since the renin-angiotensin system (RAS) inhibitor captopril (Cap) is one of the first-line drugs for preventing cardiac remodeling in patients with heart failure, we hypothesized that Cap might prevent cardiac dysfunction induced by occlusal disharmony. Here, to test this idea, we used our bite-opening (BO) mouse model, which was developed by cementing a suitable appliance onto the mandibular incisor. Mice were divided into four groups: (1) Control, (2) BO, (3) Cap, and (4) BO + Cap. After 2 weeks, we evaluated cardiac function by echocardiography and confirmed that cardiac function was significantly decreased in the BO group compared to the control, while Cap ameliorated the dysfunction. Cardiac fibrosis, myocyte apoptosis and oxidative stress-induced myocardial damage in the BO group were significantly increased versus the control, and these increases were suppressed by Cap. Cardiac dysfunction induced by BO was associated with dual phosphorylation on PKCδ (Tyr-311/Thr-505), leading to activation of CaMKII with increased phosphorylation of RyR2 and phospholamban. Our results suggest that the RAS might play an important role in the development of cardiac diseases induced by occlusal anomalies.

Trigeminal Stimulation and Visuospatial Performance: The Struggle between Chewing and Trigeminal Asymmetries

Chewing improves visuospatial performance through locus coeruleus (LC) activation. The effects of bilateral and unilateral mastication were investigated in subjects showing different degrees of asymmetry in masseter electromyographic (EMG) activity during clenching and in pupil size at rest (anisocoria), which is a proxy of LC imbalance. Correlations between performance changes and asymmetry values were found in males, but not in females. Among males, subjects with low asymmetry values (balanced-BAL) were more sensitive than those with high asymmetry values (imbalanced-IMB) to bilateral and unilateral chewing on the side with higher EMG activity (hypertonic). The opposite was true for hypotonic side chewing. BAL subjects were sensitive to unilateral chewing on both sides, while in IMB subjects, hypertonic side chewing did not influence performance in either males or females. Bilateral chewing elicited larger effects in BAL subjects than in IMB subjects, exceeding the values predicted from unilateral chewing in both groups. Finally, pupil size and anisocoria changes elicited by chewing were correlated with asymmetry values, independent of sex. Data confirmed the facilitation of visuospatial performance exerted by chewing. Trigeminal asymmetries modulate the chewing effects, making occlusal rebalancing an appropriate strategy to improve performance.

The effect of occlusal disharmony on a chronic stress-induced animal model of gut microbiota dysbiosis

BACKGROUND AND OBJECTIVES

Chronic stress (CS) is closely related to intestinal health. Occlusal disharmony (OD) is a risk factor for hypersensitivity to novel stress, and the relationship between OD and the intestinal system with or without other chronic stresses remains unclear. Therefore, the purpose of this study was to investigate whether OD affects the gut microbiota and the intestinal barrier in a CS-exposed animal model.

METHODS

OD was induced by making a 0.5-mm-thick incision on the right maxillary molar. CS involved exposure to one stressor per day for 35 days. Sprague-Dawley rats were randomly divided into an untreated control group and OD-, CS- and OD + CS-treated groups. The behavioural tests, serum corticosterone level, gut microbiota composition and tight junction protein expression in colon tissue were measured on the 56th day to elucidate the effect of OD on animals under CS.

RESULTS

Significant differences in performance on behavioural tests and serum corticosterone concentrations were observed on day 56 in the OD + CS group compared with the control group. Exposure to occlusal disharmony or chronic stress resulted in a change in the composition of the gut microbiota of rats. Differences in the expression of the tight junction proteins zonula occludens-1 and junctional adhesion molecule-A were observed in colon tissue from the OD + CS group compared with the control group.

CONCLUSIONS

We concluded that the significant changes in performance on behavioural tests, serum corticosterone concentrations and microbiota dysbiosis and tight junction protein levels induced by OD with CS may indicate that OD is a potential factor promoting gut microbiota dysbiosis.

Activation of the VpdmVGLUT1-VPM pathway contributes to anxiety-like behaviors induced by malocclusion

Occlusal disharmony has a negative impact on emotion. The mesencephalic trigeminal nucleus (Vme) neurons are the primary afferent nuclei that convey proprioceptive information from proprioceptors and low-threshold mechanoreceptors in the periodontal ligament and jaw muscles in the cranio-oro-facial regions. The dorsomedial part of the principal sensory trigeminal nucleus (Vpdm) and the ventral posteromedial nucleus (VPM) of thalamus have been proven to be crucial relay stations in ascending pathway of proprioception. The VPM sends numerous projections to primary somatosensory areas (SI), which modulate emotion processing. The present study aimed to demonstrate the ascending trigeminal-thalamic-cortex pathway which would mediate malocclusion-induced negative emotion. Unilateral anterior crossbite (UAC) model created by disturbing the dental occlusion was applied. Tract-tracing techniques were used to identify the existence of Vme-Vpdm-VPM pathway and Vpdm-VPM-SI pathway. Chemogenetic and optogenetic methods were taken to modulate the activation of Vpdm^VGLUT1 neurons and the Vpdm-VPM pathway. Morphological evidence indicated the involvement of the Vme-Vpdm-VPM pathway, Vpdm-VPM-SI pathway and Vpdm^VGLUT1-VPM pathway in orofacial proprioception in wild-type mice and vesicular glutamate transporter 1 (^VGLUT1): tdTomato mice, respectively. Furthermore, chemogenetic inhibition of Vpdm^VGLUT1 neurons and the Vpdm-VPM pathway alleviated anxiety-like behaviors in a unilateral anterior crossbite (UAC) model, whereas chemogenetic activation induced anxiety-like behaviors in controls and did not aggravate these behaviors in UAC mice. Finally, optogenetic inhibition of the Vpdm^VGLUT1-VPM pathway in VGLUT1-IRES-Cre mice reversed UAC-induced anxiety comorbidity. In conclusion, these results suggest that the Vpdm^VGLUT1-VPM neural pathway participates in the modulation of malocclusion-induced anxiety comorbidity. These findings provide new insights into the links between occlusion and emotion and deepen our understanding of the impact of occlusal disharmony on brain dysfunction.

Occlusal Disharmony-A Potential Factor Promoting Depression in a Rat Model

Objectives: Patients with occlusal disharmony (OD) may be susceptible to depression. The hypothalamus−pituitary−adrenal axis, 5-HT and 5HT2AR in the prefrontal cortex (PFC), amygdala, and hippocampus are involved in the modulation of emotion and depression. This study investigated whether OD affects the HPA axis and 5-HT system and, subsequently, produces depression-like behaviors in rats. Materials and methods: OD was produced by removing 0.5 and 0.25 mm of hard tissue from the cusps of the maxillary molars in randomly selected sides of Sprague−Dawley rats. CUS involved exposure to 2 different stressors per day for 35 days. OD-, CUS-, and OD + CUS-treated groups and an untreated control group were compared in terms of behavior, endocrine status and brain histology. Results: There were significant differences among the four groups in the behavior tests (p < 0.05), especially in the sucrose preference test, where there was a significant decrease in the OD group compared to the control group. ACTH and CORT concentrations were significantly higher in the OD + CUS group than the control group (p < 0.05). Expression of GR and 5-HT2AR in the PFC, amygdala and hippocampal CA1 was significantly higher in the OD, CUS and OD + CUS groups than the control group (p < 0.05). Conclusion: OD promotes depression-like behaviors through peripheral and central pathways via the HPA axis, GR and 5-HT system.

The Sedentary Lifestyle and Masticatory Dysfunction: Time to Review the Contribution to Age-Associated Cognitive Decline and Astrocyte Morphotypes in the Dentate Gyrus

As aging and cognitive decline progresses, the impact of a sedentary lifestyle on the appearance of environment-dependent cellular morphologies in the brain becomes more apparent. Sedentary living is also associated with poor oral health, which is known to correlate with the rate of cognitive decline. Here, we will review the evidence for the interplay between mastication and environmental enrichment and assess the impact of each on the structure of the brain. In previous studies, we explored the relationship between behavior and the morphological features of dentate gyrus glial fibrillary acidic protein (GFAP)-positive astrocytes during aging in contrasting environments and in the context of induced masticatory dysfunction. Hierarchical cluster and discriminant analysis of GFAP-positive astrocytes from the dentate gyrus molecular layer revealed that the proportion of AST1 (astrocyte arbors with greater complexity phenotype) and AST2 (lower complexity) are differentially affected by environment, aging and masticatory dysfunction, but the relationship is not straightforward. Here we re-evaluated our previous reconstructions by comparing dorsal and ventral astrocyte morphologies in the dentate gyrus, and we found that morphological complexity was the variable that contributed most to cluster formation across the experimental groups. In general, reducing masticatory activity increases astrocyte morphological complexity, and the effect is most marked in the ventral dentate gyrus, whereas the effect of environment was more marked in the dorsal dentate gyrus. All morphotypes retained their basic structural organization in intact tissue, suggesting that they are subtypes with a non-proliferative astrocyte profile. In summary, the increased complexity of astrocytes in situations where neuronal loss and behavioral deficits are present is counterintuitive, but highlights the need to better understand the role of the astrocyte in these conditions.

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.

Gut Microbiota Are Associated With Psychological Stress-Induced Defections in Intestinal and Blood-Brain Barriers

Altered gut microbiota has been identified during psychological stress, which causes severe health issues worldwide. The integrity of the intestinal barrier and blood-brain barrier regulates the process of bacterial translocation and can supply the nervous system with real-time information about the environment. However, the association of gut microbiota with psychological stress remains to be fully interpreted. In this study, we established a psychological stress model using an improved communication box and compared the expression of tight junction proteins in multiple regions of the intestinal (duodenum, jejunum, ileum) and blood-brain (amygdala, hippocampus) barriers between model and control rats. We also conducted fecal microbiota analysis using 16S rRNA gene sequencing. Expression levels of the stress-related indicators adrenocorticotropic hormone, NR3C1,2, and norepinephrine were increased in the model group compared to control group. Psychological stress reduced brain and intestinal levels of tight junction proteins, including claudin5, occludin, α-actin, and ZO-1. Microbiota analysis revealed elevated microbial diversity and fecal proportions of Intestinimonas, Catenisphaera, and Globicatella in the model group. Further analysis indicated a negative correlation of Allisonella and Odoribacter, as well as a positive correlation of norank_f__Peptococcaceae, Clostridium_sensu_stricto_1, and Coprococcus_2, with claudin5, occludin, α-actin, and ZO-1. Our use of a rodent model to explore the association between compromised intestinal and blood-brain barriers and altered fecal microbiota under psychological stress improves our understanding of the gut-brain axis. Here, cues converge to control basic developmental processes in the intestine and brain such as barrier function. This study provides new directions for investigating the pathogenesis of emotional disorders and the formulation of clinical treatment.

Adverse effects of the bite-raised condition in animal studies: A systematic review

OBJECTIVE

To provide a systematic review of the effects of the bite-raised condition in animal models, a widespread technique in modern orthodontics.

DESIGN

A systematic review of the literature was conducted. Original articles were searched through Pubmed, Cochrane Central database and Embase until December 2018.

RESULTS

242 articles were identified through database searching. After removing the duplicates, 198 articles were screened by reviewing the abstracts. 27 full text articles were assessed for eligibility and, after 7 exclusions, 20 articles were included in the review process. Studies selected by the review process concerned animal models. Histological, molecular, biochemical and electromyographical studies were evaluated. The results, with a high level of agreement in different animals, showed that the bite-raised condition is a source of stress, inducing increased plasma corticosterone, urinary cortisol and HPA axis alterations; it predisposes the organism to react to subsequent stressful stimulation with a significantly greater incretion of glucocorticoids, thus inducing hypersensitivity to novel forms of stress; it affects the structure of the hippocampus, reducing the number of neurons, increasing the number of glial cells and worsening memory and spatial orientation; it alters the electromyographical activity of masticatory muscles.

CONCLUSIONS

The results of research conducted on animal models do not necessarily apply directly to human beings. More clinical research, with special attention to adolescent patients, is necessary to clarify whether, in humans, the bite-raised condition is accompanied by adverse effects comparable to those observed in animals.

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.