Differential expression patterns of five acetylcholine receptor subunit genes in rat muscle during development

The spatial and temporal expression patterns of five genes which encode the alpha-, beta-, gamma-, delta- and epsilon-subunits of the nicotinic acetylcholine receptor in skeletal muscle were followed during development in the rat by in situ hybridization analysis. Three major developmental phases, characterized by specific expression patterns, could be distinguished. (i) During myogenic differentiation alpha-, beta-, gamma- and delta-subunit genes are activated and transcripts are expressed in muscle precursor cells at embryonic day 12 (E12) and during subsequent cell fusion. (ii) Following innervation of myotubes at approximately E15-E17 the mRNA of the alpha-, beta-, gamma- and delta-subunit genes accumulate in synaptic and decrease in extrasynaptic fibre regions during early synaptogenesis. The mRNA of the epsilon-subunit gene becomes detectable first in subsynaptic nuclei 2-3 days after innervation has occurred. (iii) During postnatal development alpha-, beta- and delta- subunit transcript levels are reduced predominantly in extrasynaptic fibre segments and show significant differences in distribution depending on the muscle subtype whereas the gamma-subunit mRNA disappears completely within the first postnatal week in all muscles. In contrast, the epsilon-subunit gene is transcribed only in subsynaptic myonuclei throughout development and in the adult muscle.