Contribution of occlusal activity to synaptogenesis in masticatory muscles.

Synaptogenesis in the neuromuscular junction involves a nicotinic acetylcholine receptor (nAChR) switch and elimination. The microphthalmic mouse (mi/mi) with a mutation in the mitf gene cannot perform occlusal activity, because its teeth do not erupt. The present study attempted to elucidate the contribution of occlusal activity to synaptogenesis in masticatory muscles. In the masseter of the mi/mi, the nAChR elimination initiated, but did not progress normally, after 3 weeks of age, when the occlusal activity emerged in the +/+ mouse, whereas the nAChR switch progressed normally during the entire period of synaptogenesis. The mRNA expression patterns of nAChR subunits in the temporalis and digastric of the mi/mi differed from those in its masseter. These findings suggest that, in the masseter, occlusal activity is essential for the completion of nAChR elimination, but not for the nAChR switch, and that the contribution of occlusal activity to synaptogenesis varies among the masticatory muscles.