Synaptogenesis of Craniofacial Muscles Is Affected by Occlusal Activity

Objectives: It is unknown what extent the occlusal activity affects synaptogenesis of craniofacial muscles. Since teeth do not erupt in microphthalmic mice (mi/mi ) with a mutation in the mitf gene, mi/mi mice cannot perform occlusal activity. By using the mi/mi mouse as a model system, we have shown that occlusal activity is essential for normal synaptogenesis of masseter muscle. The purpose of this study is to elucidate effects of occlusal activity on the synaptogenesis in the other craniofacial muscles such as temporalis and digastric muscles during postnatal development using mi/mi mice.

Methods: We analyzed the mRNA expressions of nicotinic acetylcholine receptor (AChR) δ-, γ-, and ε-subunits as indicators of synaptogenesis in the masseter, temporalis, and digastric muscles of 8 week-old mi/mi and control (+/+) mice using quantitative RT-PCR. The AChR δ-subunit is commonly expressed in both the embryonic and adult type of AChR, whereas the γ- and ε-subunits are expressed only in the embryonic and adult type of AChR, respectively.

Results: In the masseter muscle, the mRNA levels of δ- subunit (common type) and γ-subunit (embryonic type) in mi/mi mice were approximately 6 and 18-fold greater than those in the control mice respectively (p<0.05), while no significant difference in the mRNA levels of ε-subunit (adult type) was found between mi/mi and control mice. In the digastric muscle, the mRNA levels of δ-subunit and ε-subunit in mi/mi mice were approximately 3 and 5-fold greater than those in the control mice respectively (p<0.05), while no significant difference in the mRNA levels of γ-subunit was found. In the temporalis muscle, there was no significant difference in the mRNA levels of the 3 subunits studied between mi/mi and control mice.

Conclusions: These results suggest that the contribution of occlusal activity to the synaptogenesis vary among the craniofacial muscles.