Expression of Nitric Oxide Synthases in Orthodontic Tooth Movement

Background: Nitric oxide (NO) is involved in orthodontic tooth movement(OTM), but the nitric oxide synthase (NOS) isoform and cell types producing NO in OTM is not known. Objective: To investigate differential expression of NOS isoforms in periodontal ligament (PDL)and bone in both tension and pressure side using a rat model of OTM. Methods: Immunohistochemistry (IHC) with antibodies directed against NOS isoforms (iNOS, eNOS and nNOS) was performed on horizontal sections of the right first maxillary molars subjected to 3 and 24 hours of OTM. The PDL and adjacent osteocytes of the disto-palatal root at the pressure and tension area of the orthodontically moved first molars were analyzed for the expression of these proteins. The contralateral molar served as a control. Results were analyzed with one-way ANOVA followed by the Tamhane and Games-Howell post hoc test as well as two-way ANOVA with interaction (SPSS, version 15.0). Results: Expression of all three isoforms was increased in thetension side. iNOS expression in the pressure side with cell free zone was decreased while in pressure side without cell free zone was increased at both time points. nNOS expression in pressure side with cell free zone was decreased while in pressure side without cell free zone was increased. Following force application the number of eNOS positive cells did not change, but the intensity of the staining was visibly increased in tension side. Duration of OTM only changed the pattern of nNOS pression. Osteocyte NOS expression did not change significantly in response to OTM. Conclusion: All three NOS isoforms are involved in OTM with different expression patterns between the tension and pressure sides, and with nNOS being more involved in early OTM events. NOS expression did not change in osteocytes suggesting that PDL cells, rather than osteocytes are the mechanosensors with respect to early OTM events.