Localization of acetylcholine receptor by 125I-labeled alpha-bungarotoxin binding at mouse motor endplates

Exposed sternomastoid muscles of anaesthetized mice were bathed in (125)I-labeled alpha-bungarotoxin until all neurally evoked muscle contractions were eliminated. The distribution of label was then determined by electron microscope autoradiography. It was found that the label was localized at the top of the junctional folds, i.e., at the postjunctional membrane nearest the axon. Since the alpha-bungarotoxin had fully eliminated the physiological muscle response, these results indicate that the active acetylcholine receptor occupies a limited area of the junctional folds and is not distributed uniformly throughout this membrane. Specialized membrane densities seem to coincide with the labeled regions.