The contents of the mRNAs encoding the y-and E-subunits of the nicotinic acetylcholine receptor as well as the single-channel properties of the receptor have been assessed in innervated, denervated and reinnervated rat muscle. The changes in abundance of the ),-and E-subunit mRNAs correlate with the changes in relative density of two classes of acetylcholine receptor channels. The results support the view that a switch in the relative abundance of the y-and E-subunit mRNAs is a major mechanism in regulating the properties of acetylcholine receptor channels in muscle.

The muscular content of the mRNAs encoding the five subunits of the nicotinic acetylcholine receptor was measured during postnatal development in the rat. Subunit specific mRNAs show differchtial regulation. The levels of the a-, yand &subunit specific mRNAs decrease steadily after birth, while the & and a-subunit mRNAs increase transiently and then decrease. The adult pattern of subunit specific mRNA levels is reached at 46 weeks postnatally. The content of y-and e-subunit mRNA changes in a reciprocal fashion during the first 2 postnatal weeks, supporting the view that differential regulation of y-and s-subunit mRNA during development is one mechanism mediating the appearance of the adult, s-subunit containing, subtype of end-plate channel. Denervation of neonatal muscle increases the levels of all subunitspecific mRNAs during further development. It prevents the postnatal decrease in y-subunit mRNA and enhances the initial increase in a-subunit mRNA. This makes it appear that the e-subunit gene is less sensitive to regulation by the nerve in the postnatal period than the y-subunit gene.

The isolation and characterization of five clones carrying sequences of the alpha-, beta-, gamma-, delta- and epsilon-subunit precursors of the rat muscle acetylcholine receptor (AChR) are described. The deduced amino acid sequences indicate that these polypeptides contain 457-519 amino acids and reveal the structural characteristics common to subunits of ligand-gated ion channels. The pattern of subunit-specific mRNA levels in rat muscle shows characteristic changes during development and following denervation, suggesting that innervation of muscle reduces the expression of the alpha-, beta- and delta-subunit mRNAs, suppresses the expression of the gamma-subunit mRNA, and induces expression of epsilon-subunit mRNA. Subunit-specific cRNAs generated in vitro were injected into Xenopus laevis oocytes, resulting in the assembly of two functionally different AChR channel subtypes. The AChR gamma, composed of the alpha-, beta-, gamma- and delta-subunits, has functional properties similar to those of the native AChRs in fetal muscle. The AChR epsilon, composed of alpha-, beta-, delta- and epsilon-subunits, corresponds to the end-plate channel of the adult muscle. Thus in rat skeletal muscle the motor nerve regulates the expression of two functionally different AChR subtypes with different molecular composition by the differential expression of subunit-specific mRNAs.