OBJECTIVETo test the hypothesis that the CCL2/CCR2 signaling pathway plays an important role in pain induced by experimental tooth movement.

METHODSMale Sprague- Dawley rats weighing between 200 and 300 g were used in this study. Expression of CCL2/CCR2 in the trigeminal ganglion (TG) was determined by Western blotting 0 h, 4 h, 1 d, 3 d, 5 d, 7 d after tooth movement. Localization of the CCL2 was revealed by immunohistochemistry. Changes in body weight, nocifensive behaviors, and the effects of CCL2/CCR2 antagonists on these changes in pain behaviors were evaluated. Exogenous CCL2 was injected into periodontal tissues and added to TG neurons in culture and the resulting c-fos expression and pain responses were detected. In addition, the expression and cellular localization of CCL2 in the medullary dorsal horn (MDH) was determined by immunohistochemistry 3 d and 14 d after tooth movement.

RESULTSExperimental tooth movement led to a statistically significant increase in CCL2/CCR2 expression at the protein level from day 3 to 7 after application of force initiating tooth movement.When compared with control group (1.000 ± 0.000), CCL2 increased to (2.620 ± 0.128), (3.300 ± 0.197) and (1.740 ± 1.290) at day 3, 5 and 7 respectively, which were statistically significant (P < 0.05). CCR2 expression levels were (1.636 ± 0.061) and (1.766 ± 0.126) compared with that in control group (1.000 ± 0.000) at day 3 and 5 respectively with statistical significance (P < 0.05). Both of them peaked on day 5 (3.3 and 1.8 time compared to control group). Application of recombinant CCL2 led to the up-regulation of c-fos expression in vivo and in vitro, and triggered a corresponding nocifensive behavior in rats. The magnitude of the nocifensive behavior could be reduced by a CCR2 antagonist, and by CCL2 neutralizing antibody. Furthermore, we found a significant increase in the expression of CCL2, corresponding well to the up-regulation of the time spent on nocifensive behaviors after ETM. In addition, CCL2 was up-regulated in TG neurons and astrocytes in Vc.

CONCLUSIONSThe CCL2/CCR2 axis was modulated by experimental tooth movement and involved in the development of tooth movement pain, and thus palyed an important role in orthodontic pain mechanism.