Effect of Botulinum Toxin (A) injections on locomotion and bone architecture in unloading mouse model
Nadine Lebcir, Mahmoud S. Moussa, Taylor Devet, Bettina M. Willie, Svetlana V. Komarova
Background – Botulinum toxin A (BTX-A) is widely used for aesthetic and therapeutic purposes, including treatment of neuropathic conditions. In preclinical models, BTX-A effects on the musculoskeletal system are used to mimic conditions of prolonged paralysis or microgravity. In these models, BTX-A induces significant bone loss, the mechanism by which BTX-A causes deterioration remains to be fully elucidated. This study aims to assess the effects of BTX-A on behavioral movement and bone structure in mouse tibia.
Methods – Ethics approval was attained from the Ã山ǿ¼é FACC (#2020-8192) for animal use. Twenty-six-week-old, female C57bl6/J mice (n=29) were randomly allocated to either receive an injection of BTX-A (2 units/100gm) or equivalent amount of saline in the left hindlimb. Degree of limb paralysis was assessed, and movement was evaluated prior to injection and for a 3-week period afterwards using a 3D-mounted camera on an Openfield box. Descriptive statistics and unpaired student t-test were performed to compare the level of activity between the two groups. In another subset of animals, bone architecture was evaluated prior to injection and after 3-weeks using a micro-CT scanner (Skyscan 1276) with a voxel size of 8μm. Bone morphometric parameters were calculated for mid-diaphysis and proximal metaphysis regions. A two-way ANOVA was performed to identify significant changes between limbs and injection group.
Results – Paralysis testing indicated that BTX-A induced maximum paralysis during the first week and slowly recovered during the remainder of the experiment. Openfield data analysis showed no significant differences in speed or total movement between BTX-A and saline injected mice. Interestingly, a significant and consistent decrease was present in the rearing count between groups, with the most prominent difference, 82.4% less rearing observed during the first week (p<0.01). Both the injected and non-injected limbs of the BTX-A group exhibited significant decreases in cortical thickness and cortical area to total tissue area at both tibial regions.
Conclusion – The observed reduction in rearing behavior of BTX-A animals indicates a decrease in frequency of vertical loading, affecting forces falling on tibia of both hindlimbs. Bone loss remains higher in the injected limb than non-injected and further studies are needed to investigate how BTX-A can directly influence bone homeostasis and other physiological tissues.