S100 and p65 expression are increased in the masseter muscle after botulinum toxin-A injection.

[BACKGROUND] The purpose of this study was to compare the expression levels of p65 and S100 in the rat masseter muscle after the injection of different concentrations of botulinum toxin-A (BTX-A).

[METHODS] We injected either 5 or 10 U of BTX-A into both masseter muscle of rats. As a control group, the same volume of saline was injected. After 14 days, the animals were sacrificed. Subsequently, a biopsy and immunohistochemical staining of the samples were performed using a p65 or S100 antibody.

[RESULTS] The cross-sectional area of each myofibril was significantly reduced by BTX-A injection ( < 0.001). The expression of p65 and S100 increased significantly with increasing concentrations of BTX-A ( < 0.001).

[CONCLUSIONS] The injection of BTX-A into the masseter muscle induced muscle atrophy. Subsequently, p65 and S100 expression in myoblasts were increased for the protection of muscle cells.