Re‐innervation of rat skeleton muscle in the presence of alpha‐bungarotoxin.

1. The possible role of acetylcholine (ACh) receptors in controlling neuromuscular synapse formation was studied by blocking the receptors with alpha‐bungarotoxin during re‐innervationf the rat diaphragm. Anaesthetized and artificially respirated rats were perfused with toxin throughout the 3‐day period when most re‐innervation was shown to take place (from day 4 to day 6 after a nerve crush). 2. The toxin treatment blocked about 99.9% of both junctional and extrajunctional ACh receptors, thereby eliminating virtually all end‐plate potentials. However, a low level of ACh sensitivity returned to the muscle after several hours of washing, and small end‐plate potentials were then recorded from the majority of fibres on the side that previously had been denervated. The degree of re‐innervation was nearly as great as that seen in control denervated muscles that had not been treated with toxin. 3. The presence of bungarotoxin during re‐innervation did not significantly affect either the appearance of newly formed synapses in the light microscope or the amount of transmitter they released during nerve stimulation. 4. Double innervation, which was not seen in normal muscles, was found in about 10–15% of re‐innervated fibres in both the untreated and toxin‐treated preparations. This suggests that some of the synapses formed during re‐innervation may have been made over what previously had been extrajunctional membrane. 5. It is therefore unlikely that the ACh receptor, or at least that part of the receptor to which bungarotoxin binds, plays a direct role in controlling the process of synapse formation in mammalian skeletal muscle...