BACKGROUND:Cigarette smoking can seriously damage the periodontal tissues and root, and the nicotine in tobacco accelerates the progression of periodontal diseases. OBJECTIVE: To investigate the effect of different doses of nicotine on the expression of cyclooxygenase-2 mRNA in the periodontal tissue during orthodontic tooth movement. METHODS: Totaly 110 Sprague-Dawley rats were randomly divided into blank control group (n=10), normal saline group (n=25), 0.5 mg/kg nicotine group (n=25), 0.75 mg/kg nicotine group (n=25), 1 mg/kg nicotine group (n=25). Rats in the normal saline groups were injected intraperitonealy with 0.1 mL normal saline, and those in the three nicotine groups were respectively injected with 0.5, 0.75, 1 mg/kg nicotine tartrate solution. Except the blank control group, the unilateral maxilary first molars of rats in the other four groups were exposed to 50 g force. At 1, 3, 5, 7, 14 days under force, the rats were sacrificed to take the maxilary tissues. Hematoxylin-eosin staining was used to observe the changes of periodontal tissues, immunohistochemical staining was employed to count positive cels, andin situ hybridization staining was adopted to detect the mean absorbance value of cyclooxygenase-2 in the periodontal tissues. RESULTTS AND CONCLUSION:The number of odontoclasts and the expression of cyclooxygenase-2 in the nicotine groups were higher than those in the non-nicotine groups. With the increasing dose of nicotine, the number of odontoclasts gradualy increased, and the difference was statisticaly significant (P < 0.05). At 7 days under force, the number of odontoclasts reached the peak. With the increasing dose of nicotine, the positive expression intensity of cyclooxygenase-2 was also increased, and the difference was statisticaly significant (P < 0.05). The expression of cyclooxygenase-2 reached peak at 5 days under force. These findings indicate that with the increasing dose of nicotine, the number of odontoclasts and the expression intensity of cyclooxygenase-2 are both increased at the same time point under force. During the orthodontic tooth movement, the intake of nicotine can damage the periodontal tissue, and the dose of nicotine can directly influence the severity of damage to the periodontal tissue.