Activity-independent modulation of acetylcholine receptor levels in rat skeletal muscle following neonatal denervation

Following denervation of adult muscle, levels of acetylcholine receptor (AChR) increase; normal, low levels are restored only after muscle reinnervation. After neonatal denervation, we found a large initial increase in AChR levels during the first days postsurgery, as in adult denervated muscle. However, 1 week after denervation, total AChR levels decreased in the absence of any sign of reinnervation. By 3 weeks after surgery, near-normal levels of AChR were restored and extrajunctional AChR had disappeared. Thus, in sharp contrast to adult muscle, in young denervated muscle a down-regulation of AChR occurs without recovery of innervation and normal muscle contractile activity. These results suggest that different mechanisms regulate the levels of AChR in developing and adult skeletal muscle.