The ultrastructural localization and quantitation of cholinergic receptors at the mouse motor endplate

Summary The snake venom toxin, -bungarotoxin, is known to bind specifically to the acetylcholine receptor at skeletal muscle endplates. In this study, tritiated -bungarotoxin has been used in conjunction with electron-microscope autoradiography to visualize and enumerate acetylcholine receptor sites at the neuromuscular junctions of the mouse diaphragm. From an analysis of the grain distribution, the receptor sites appear to be located specifically on the postjunctional membrane. The density there is about 8,500/² of membrane surface. For comparison purposes, cholinesterases and related active centers were labeled using [³H] diisopropylfluorophosphate; they were shown to be at this same concentration over the synaptic membranes (or along the cleft). The 11 relationship of the receptors to the cholinesterase type of site, found previously to hold in studies on whole endplates, is also true at the ultrastructural level in this case. In fact, this 11 relationship is believed to be a characteristic of the postsynaptic membranes of endplates in other muscles and other vertebrates.Based on the constant density value thus arrived at, the total surface areas of postsynaptic and of presynaptic membranes are at once obtained from the known total numbers of these sites per endplate, available from previous studies in this laboratory. Examples of such synaptic surface area values are given. These values are only reliable for a given muscle type if the approximate fiber size is defined.