Novel stress increases hypothalamic-pituitary-adrenal activity in mice with a raised bite
Introduction
Occlusal disharmony resulting from tooth loss and periodontitis, or an inadequate vertical dimension of crowns, bridges, or dentures has physical, psychologic, and social effects on quality of life (Bernabé, Sheiham, & de Oliveira, 2009; Christensen, 1970; Uemoto, Tsukiyama, & Koyano, 1998). Ekuni et al. (2011) reported that malocclusion contributes to psychologic stress in young Japanese adults. Occlusal disharmony induced by raising the bite in rodents increases plasma corticosterone levels, which leads to spatial learning deficits, and concomitant decreases in pyramidal cells and cell proliferation in the hippocampus (Ichihashi et al., 2007, Kubo et al., 2007; Mori, Katayama, Miyake, Fujiwara, & Kubo, 2013), which is consistent with previous chronic stress-induced hippocampal changes (Fuchs & Flugge, 1998; Sapolsky, Krey, & McEwen, 1985). These findings suggest that chronic stress resulting from occlusal disharmony induces hippocampal behavioral and morphologic changes.
Corticotropin releasing hormone (CRH) and arginine vasopressin (AVP) are secreted by parvocellular neurons in the paraventricular hypothalamus nucleus (PVN), and regulate adrenocorticotropic hormone (ACTH) secretion in the pituitary corticotroph, which stimulates glucocorticoid release from the adrenal cortex (Antoni, 1993). AVP alone only weakly induces the secretion of ACTH, but it has synergistic actions with CRH and helps to sustain pituitary responsiveness during chronic stress (Aguilera, 1998, Antoni, 1993). The PVN contains two populations of CRH neurons, one containing CRH alone and the other in the parvocellular region of the PVN that colocalizes with AVP (Whitnall, Smyth, & Gainer, 1987).
Secretion of both CRH and AVP is stimulated by acute stress (Ma & Lightman, 1998). In repeated or chronic stress conditions, however, cells in which CRH and AVP colocalize preferentially secrete VP (Bartanusz et al., 1993, Ma and Aguilera, 1999, Ma and Lightman, 1998). Chronic stress causes the hypothalamo-pituitary-adrenal (HPA) axis to desensitize to the homotypic stressor (Dallman et al., 1992, Ma and Lightman, 1998). On the other hand, exposure to a novel acute stress in a chronically stressed animal induces greater ACTH response than that observed in naive control animals (Aguilera, 1998, Bhatnager and Dallman, 1998). The relative contribution of the CRH and AVP to acute stress with chronic stress due to occlusal disharmony is unclear. Therefore, our objective of this study is to clarify the response of both CRH and AVP with occlusal disharmony induced by raising the bite in mice. We also examined the CRH alteration induced by novel acute stress in the animals with a raised bite.
Section snippets
Materials and methods
We used 8-month-old male senescence-accelerated prone 8 (SAMP8) mice (n = 68). The detailed characteristics of this strain were previously described (Flood & Morely, 1998). Briefly, SAMP8 mice mature normally up to the age of 6 months, and then exhibit accelerated aging (median life span of 12 months compared with 2–3 years for other strains). The SAMP8 mouse is a proposed experimental model of human senile dementia (Flood & Morely, 1998). The ethics committee of Asahi University School of
Results
Expression of CRH mRNA signals in the PVN in the bite-raised mice is shown in Fig. 2. CRH mRNA expression in the PVN differed significantly between the control and bite-raised mice [F(1, 127) = 252.7186, P < 0.001]. CRH mRNA expression differed significantly between 90 min and 14 days after the bite-raising procedure [F(1, 127) = 218.1070, P < 0.001]. CRH mRNA expression in bite-raised mice at 90 min after the bite-raising procedure was 330% higher than that in control mice (P < 0.05), but expression
Discussion
Occlusal disharmony occurs as a result of alterations in the vertical, sagittal, or transverse plane. General dentists frequently encounter patients with occlusal disharmony induced by various conditions such as tooth loss, and inappropriate vertical dimensions of crowns, bridges, or dentures. Occlusal disharmony in rodents induced by raising the bite results in increased plasma corticosterone levels (Ichihashi et al., 2007, Kubo et al., 2007). Therefore, we induced occlusal disharmony in mice
Conflict of interest
The authors declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the research reported.
Acknowledgement
This work was supported in part by Grant-in Aid Scientific Research from the Ministry of Education, Culture, Sports, Science, and Technology-Japan (KAKENHI 23890225).
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