Cholinergic receptors at denervated mammalian motor end-plates.

The number of acetylcholine receptors at normal and denervated end-plated in rat soleus muscles was studied using the binding of [125A] alpha-bungarotoxin as a quantitative assay. Normal end-plates bound several thousand times as much toxin as equal areas of extra-synaptic muscle membrane. 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 up to 60-70%. Direct electrical stimulation of these muscles for the entire period of denervation did not prevent the loss of junctional binding sites even though it was adequate to abolish the increase in extrajunctional toxin binding. In contrast, denervated end-plates on muscle fibres cross-innervated by a foreign nerve at a distant location continued to bind normal amounts of toxin for over four months.