Early effect of Botox-A injection into the masseter muscle of rats: functional and histological evaluation.

[BACKGROUND] The purpose of this study was to evaluate the change of food intake after different dosages of botulinum toxin A (BTX) injection in the animal model. Additionally, the dimensional and histological change at 14 days after BTX injection was also evaluated.

[METHODS] The comparative study was performed using the BTX injection model in rats ( = 5 for each group). Group 1 was the saline-injected group. Group 2 was the 5-unit BTX-injection group to each masseter muscle. Group 3 was the 10-unit BTX-injection group to each masseter muscle. Food intake rates and body weight were checked daily before and after BTX injection until 10 days. All animals were sacrificed at 14 days after BTX injection, and the specimens underwent hematoxylin and eosin stain and immunohistochemical staining for myosin type II (MYH2).

[RESULTS] The recovery of food intake in groups 2 and 3 decreased significantly compared with group 1 from day 2 to day 7 and day 9 after injection ( < 0.05). The BTX-treated masseter muscles were significantly smaller than those in group 1 ( = 0.015). The immunohistochemical findings demonstrated that the expression of MYH2 was significantly higher in group 3 compared to groups 1 and 2 ( < 0.001).

[CONCLUSIONS] BTX injection to the masseter muscle in rats demonstrated short food-intake-rate reduction with recovery until 10 days after injection. The thickness of the masseter muscle and MYH2 expression were significantly changed according to the injected dose of BTX.