Regulatory mechanism for gene expression during the development of mouse masseter muscle

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There are no published studies on synaptogenesis focusing on the elimination of the nicotinic acetylcholine receptor (nAChR) outside the neuromuscular junction and the nAChR subunit switch from the embryonic-type (α_2βγδ subunits) to the adult-type (α_2βεδ subunits). We analyzed the expression level of δ subunit mRNA, which was present through development, to identify the time course of the nAChR elimination, and the expression level of the ε and γ subunit mRNAs to identify the time course of the nAChR subunit switch in the rat masseter muscle. Rats from 0 to 72 days of age were used, kept in the same cages as their mothers by 18 days of age and then fed a pellet diet thereafter. The mRNA amounts were measured by competitive polymerase chain reaction in combination with reverse transcription. The expression profile of these subunit mRNAs in that rat masseter muscle indicated that the nAChR was mainly eliminated between 7 and 28 days of age and that the nAChR mostly switches from the embryonic to adult types between 1 and 21 days of age. Since the rat feeding behavior seems to switch from suckling to biting after 19 days of age and masseter muscle is known to play an important role in the biting movement of jaw, the completion of the nAChR elimination and switch in the masseter muscle may be related to the switch of the feeding behavior during the postnatal development.