Absence of [125I] alpha-bungarotoxin binding to motor nerve terminals of frog, lizard and mouse muscle.

The existence of nicotinic acetylcholine receptors (AChRs) on the motor nerve terminals of vertebrates has long been controversial. We have re-examined this issue by electron microscope autoradiography with [125I] alpha-bungarotoxin, following separation of nerve terminals from muscle fibers by collagenase and protease treatment. We found no label over nerve terminal membranes other than that due to background, and we calculate upper limits of less than 0.1% of the postsynaptic AChR density for nerve terminals in frogs, lizards, and mice. We conclude that there are essentially no presynaptic acetylcholine receptors that bind alpha-bungarotoxin at vertebrate neuromuscular junctions.