Regulation of the size and distribution of agrin-induced postsynaptic-likeapparatus in adult skeletal muscle by electrical muscle activity

We compared actylcholine receptor (AChR) aggregates induced by neural agrin released from transfected muscle fibers with ACI IR aggregates induced by transplanted axons in extrajunctional regions of denervated rat soleus musc les. Both neural agrin and transplanted axons induced multiple, irregularly distributed AChR aggregates on muscle fibers. Direct electrical muscle sti mulation of transfected muscles for up to 10 weeks removed all agrin-induce d AChR aggregates (the losers) except one (the winner) on many fibers. Axon -induced AChR aggregates underwent comparable selection of winners and lose rs. The results suggest that agrin and acetylcholine-driven muscle activity provided by transplanted axons are sufficient to elicit in a denervated ad ult muscle fiber processes that regulate the size and distribution of ectop ic neuromuscular junctions.