Novel stress increases hypothalamic-pituitary-adrenal activity in mice with a raised bite

Nutrient insufficiencies and deficiencies involved in the pathogenesis of bruxism (Review)

Stress has been well-documented to have a significant role in the etiopathogenesis of bruxism. Activation of the hypothalamic-pituitary-adrenal axis (HPA) and subsequent release of corticosteroids lead to increased muscle activity. Neurological studies have demonstrated that chronic stress exposure induces neurodegeneration of important neuronal structures and destabilization of the mesocortical dopaminergic pathway. These disruptions impair the abilities to counteract the overactivity of the HPA axis and disinhibit involuntary muscle activity, while at the same time, there is activation of the amygdala. Recent evidence shows that overactivation of the amygdala under stressful stimuli causes rhythmic jaw muscle activity by over activating the mesencephalic and motor trigeminal nuclei. The present review aimed to discuss the negative effects of certain vitamin and mineral deficiencies, such as vitamin D, magnesium, and omega-3 fatty acids, on the central nervous system. It provides evidence on how such insufficiencies may increase stress sensitivity and neuromuscular excitability and thereby reduce the ability to effectively respond to the overactivation of the sympathetic nervous system, and also how stress can in turn lead to these insufficiencies. Finally, the positive effects of individualized supplementation are discussed in the context of diminishing anxiety and oxidative stress, neuroprotection and in the reversal of neurodegeneration, and also in alleviating/reducing neuromuscular symptoms.

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.

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.

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.

Vulnerability to stress in mouse offspring is ameliorated when pregnant dams are provided a chewing stick during prenatal stress

OBJECTIVE

To investigate whether maternal chewing during prenatal stress alters the responsivity of young offspring to novel stress, we examined the expression of hippocampal glucocorticoid receptors and mineralocorticoid receptors, and the levels of hypothalamic corticotropin-releasing hormone in young adult mouse offspring of dams exposed to restraint stress during pregnancy.

DESIGN

To induce stress, the dams were placed in a ventilated restraint tube for 45 min each day from day 12 of pregnancy through parturition. While restrained in the tube, one group of dams was provided a wooden stick for chewing. Hippocampal expression of glucocorticoid receptor and mineralocorticoid receptor messenger ribonucleic acid was assessed in 1-month-old pups. Hypothalamic expression of corticotropin-releasing hormone messenger ribonucleic acid was examined before and after exposing the offspring to a novel stressor.

RESULTS

Prenatal stress significantly decreased hippocampal expression of both glucocorticoid receptor and mineralocorticoid receptor messenger ribonucleic acid in the offspring, and increased the expression of corticotropin-releasing hormone messenger ribonucleic acid in the hypothalamic paraventricular nucleus in the offspring after novel stress exposure. Maternal chewing during exposure to prenatal stress attenuated the decreased hippocampal expression of both glucocorticoid receptor and mineralocorticoid receptor messenger ribonucleic acid, and the increased corticotropin-releasing hormone messenger ribonucleic acid expression in the hypothalamic paraventricular nucleus in the offspring.

CONCLUSIONS

Downregulation of hippocampal glucocorticoid receptor and mineralocorticoid receptor expression in offspring due to prenatal stress, which may be associated with increased susceptibility to novel stress in adulthood, are attenuated by allowing the dams to chew on a wooden stick.

Association between Mastication, the Hippocampus, and the HPA Axis: A Comprehensive Review

Mastication is mainly involved in food intake and nutrient digestion with the aid of teeth. Mastication is also important for preserving and promoting general health, including hippocampus-dependent cognition. Both animal and human studies indicate that mastication influences hippocampal functions through the end product of the hypothalamic-pituitary-adrenal (HPA) axis, glucocorticoid (GC). Epidemiologic studies suggest that masticatory dysfunction in aged individuals, such as that resulting from tooth loss and periodontitis, acting as a source of chronic stress, activates the HPA axis, leading to increases in circulating GCs and eventually inducing various physical and psychological diseases, such as cognitive impairment, cardiovascular disorders, and osteoporosis. Recent studies demonstrated that masticatory stimulation or chewing during stressful conditions suppresses the hyperactivity of the HPA axis via GCs and GC receptors within the hippocampus, and ameliorates chronic stress-induced hippocampus-dependent cognitive deficits. Here, we provide a comprehensive overview of current research regarding the association between mastication, the hippocampus, and HPA axis activity. We also discuss several potential molecular mechanisms involved in the interactions between mastication, hippocampal function, and HPA axis activity.