Unilateral Hypofunction of the Masseter Leads to Molecular and 3D Morphometric Signs of Atrophy in Ipsilateral Agonist Masticatory Muscles in Adult Mice

Abstract

Mice are commonly used to study mandibular dynamics due to their similarity in chewing cycle patterns with humans. Adult mice treated unilaterally with botulinum toxin type A (BoNTA) in the masseter exhibit atrophy of this muscle characterized by an increase in the gene expression of atrophy-related molecular markers, and a reduction in both muscle fiber diameter and muscle mass at 14d. However, the impact of this muscle imbalance on the non-treated masticatory muscles remains unexplored. Here, we hypothesize that the unilateral masseter hypofunction leads to molecular and 3D morphometric signs of atrophy of the masseter and its agonist masticatory muscles in adult mice. Twenty-three 8-week-old male BALB/c mice received a single injection of BoNTA in the right masseter, whereas the left masseter received the same volume of saline solution (control side). Animals were euthanized at 2d, 7d, and 14d, and the masticatory muscles were analyzed for mRNA expression. Five heads were harvested at 14d, fixed, stained with a contrast-enhanced agent, and scanned using X-ray microtomography. The three-dimensional morphometric parameters (the volume and thickness) from muscles in situ were obtained. Atrogin-1/MAFbx, MuRF-1, and Myogenin mRNA gene expression were significantly increased at 2 and 7d for both the masseter and temporalis from the BoNTA side. For medial pterygoid, increased mRNA gene expression was found at 7d for Atrogin-1/MAFbx and at 2d–7d for Myogenin. Both the volume and thickness of the masseter, temporalis, and medial pterygoid muscles from the BoNTA side were significantly reduced at 14d. In contrast, the lateral pterygoid from the BoNTA side showed a significant increase in volume at 14d. Therefore, the unilateral hypofunction of the masseter leads to molecular and morphological signs of atrophy in both the BoNTA-injected muscle and its agonistic non-injected masticatory muscles. The generalized effect on the mouse masticatory apparatus when one of its components is intervened suggests the need for more clinical studies to determine the safety of BoNTA usage in clinical dentistry.

Temporalis Muscle Changes Following Botulinum Toxin A Injections in Masseter Hypertrophy Patients: A Randomized Triple-Blinded Trial

“…Consequently, any alteration within masticatory system structures can reach implications for health across multiple domains, affecting, in addition, other components of the same system [18,20]. In addition, any disturbance in a masticatory muscle, such as a decrease in masseter muscle mechanical activity caused by the inherent paralytic effect of BoNT-A as an aesthetic treatment for MH, may alter the operation pattern of other masticatory muscles, leading to shifts in mandibular kinematics and masticatory coordination [21]. On the contrary, studies have shown that changes in muscular coordination with BoNT-A in chewing muscles can be advantageous in managing sialorrhoea (drooling) in patients with Parkinson's disease [22], as well as in reducing muscle hypertrophy and stiffness and enhancing the facial contours and aesthetics in people with MH [23,24].…”

“…By diminishing masseter muscle contraction due to BoNT-A injections, a potential increase in temporalis volume and thickness could be expected, due to an increased compensatory contraction activity in temporalis muscle. In addition, an animal study has proved that the anterior temporalis exhibits an increase in thickness due to a compensatory mechanism for reduced function of the masseter muscles [21].…”