Persistence of junctional acetylcholine receptors following denervation

The relative number of acetylcholine receptors at normal and denervated end plates in rat soleus muscles was studied using the binding of 125I-alpha-bungarotoxin as a quantitative assay. After short-term denervation (up to 2.4 weeks), the extrajunctional binding increased, but there was no change in specific binding to the motor end plate. Denervation for longer periods (up to 7 weeks) reduced binding sites at the end plate by as much as 60-70%. Direct electrical stimulation of these muscles for the entire period of denervation did not prevent the slow loss of junctional binding sites, even though it was adequate to abolish the increase in extrajunctional binding. Cross-innervation by a foreign nerve at a site distant from the original end plates was more effective than stimulation in preserving the old end plates; in cross-innervated muscles, many denervated end plates continued to bind normal amounts of toxin for over 4 months. In comparison with the rapid rate of appearance or disappearance of extrajunctional ACh receptors, however, the number of junctional receptors at all denervated end plates was quite stable, suggesting that the persistence of these receptors is not immediately dependent on the presence of a motor nerve terminal.