2020 Volume 29 Issue 4 Pages 231-238
Diurnal variations in bone remodeling, which are regulated by the central circadian clock in the suprachiasmatic nuclei, have previously been identified. Glucocorticoids induce a circadian rhythm in osteoclasts, causing a circadian rhythm in bone resorption. It has been reported that the extent of experimental tooth movement (ETM) also exhibits diurnal variations, but this has not been thoroughly investigated. The aim of this study was to examine the role of glucocorticoids in the diurnal variations in the extent of ETM. Male C57BL6/J mice were divided into three groups: the whole-day group (WDG); ETM was performed during both the light and dark periods, the light period group (LPG); ETM was performed during the light period (7:00–19:00), and the dark period group (DPG); ETM was performed during the dark period (19:00–7:00). Orthodontic force was applied for a total of 48 hr in all groups. A piece of an orthodontic elastic band was inserted between the right upper first and second molars (M1 and M2) according to the Waldo method. During the study period, the distance between M1 and M2 increased by 185.1±7.0 μm in the WDG, 186.0±6.6 μm in the LPG, and 152.7±9.9 μm in the DPG. The amount of ETM-induced tooth movement was significantly larger in the WDG and LPG than in the DPG. The osteoclast surface/bone surface (compressed side) and osteoclast number/bone surface (compressed side) ratios were also significantly larger in the WDG and LPG than in the DPG. Consistent with the results of osteoclast parameters, the immune reactivity of receptor activator of nuclear factor-κB ligand (RANKL) was higher in the WDG and LPG than in the DPG. Adrenalectomy eliminated the differences in osteoclast parameters, RANKL immune reactivity and the extent of ETM between the LPG and DPG. These differences were restored by the daily administration of the synthetic glucocorticoid dexamethasone to adrenalectomized mice. These results suggest that circulating glucocorticoids contribute to the diurnal variations in the osteoclast parameters which result in the diurnal variation of the tooth movement, and the diurnal variation of RANKL immune reactivity may contribute to it.
