The role of nitric oxide in orthodontic tooth movement in rats.

Nitric oxide (NO) is involved in second messenger formation, osteoblast and osteoclast function, and pulpal blood flow. This raises the question of whether or not altered NO production interferes with orthodontic tooth movement (OTM) by influencing the bone remodeling cycle. To investigate the role of NO in OTM, a rat model was established and 48 rats were divided into four study groups of 12 rats each. A 5 mm nickel-titanium closed-coil spring was ligated between the right maxillary incisor and first molar of each rat to deliver an initial force of 60 g. A saline group received subperiosteal injections of normal saline (50 microL/kg), an L-arginine (L-arg) group received L-arginine (NO precursor) injections (200 mg/kg), and a, L-NAME group received N(G)-nitro-L-arginine methyl ester (nitric oxide synthase inhibitor)(10 mg/kg) injections. All injections were given in the upper right first molar mucosa from the first through the 11th day of force application at 48-hour intervals. A control group received no injections. Tooth movement measurements were done at the time of injections. Animals were sacrificed 13 days after appliance insertion and final OTMs were measured at the time of sacrifice. From the third day till the end of the experiment, the L-arg group showed a significant increase in tooth movements, whereas the L-NAME group showed a significant decrease in tooth movements compared to the control and saline group (P < .001). Histopathologic studies revealed that the number of osteoclasts was significantly higher in the L-arg group smears, while the number of osteoclasts in the L-NAME group was significantly lower as compared to the control group (P < .001). Scanning electron microscope analysis showed that the force-induced root resorption in the L-arg group was less than the control group. This study suggests a role for NO in the bone remodeling cycle.