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 (P < 0.001). The expression of p65 and S100 increased significantly with increasing concentrations of BTX-A (P < 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.