Objective: Circadian dentin increments are universally found in mammalian dentin. We previously reported that the circadian dentin increments in rats were abolished when the suprachiasmatic nucleus (SCN) in hypothalamus was lesioned. It has been reported that multiple regulatory genes including Clock are involved in a core circadian clock mechanism in the SCN and in peripheral circadian organization. The present study was undertaken to determine whether there is a circadian rhythm in the expression of dentin matrix protein in murine teeth, and whether disruption of circadian timing due to mutation of the clock gene would affect the circadian dentin increments. Materials and Methods: Wistar rats, homozygous Clock mutant (Clk/Clk) and wild-type (WT) mice were acclimated on a 12-h light/dark cycle for more than 2 wks. They were killed at Zeitgeber time (ZT) 02, 06, 10, 14, 18, and 22 over a 24-hour period. Maxillary incisors were dissected and subjected for RT-PCR and Northern blot analyses for mRNA expressions of collagen type 1 (Col-1alpha), dentin-matrix-protein 1 (Dmp1), and clock genes (Per2 and Bmal1). The transverse frozen incisor-dentin sections were prepared and stained with hematoxylin to histologically examine the expression of circadian growth lines in dentin. Results: RT-PCR and northern blot analyses showed that all mRNA examined expressed according to a diurnal rhythm with a peak at ZT06 (light period) for Col-1alpha, Dmp1 and Bmal1, and at ZT18 (dark period) for Per2 in both rats and WT mice. But these diurnal rhythms became weak or disappeared in the Clk/Clk mouse. In addition, circadian incremental lines in dentin which found clearly in rats and WT mice were not observed in Clk/Clk mice. Conclusion: The Clock gene is involved in the circadian organization of dentin increments. Disruption of the clock gene suppresses the generation and maintenance of circadian dentin increments.

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