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

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Abstract

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 0.3 mm-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-HT2A receptor (5-HT2AR) 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-HT2AR 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-HT2AR 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.

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