The density of cholinergic receptors at the endplate postsynaptic membrane: ultrastructural studies in two mammalian species.

Electron-microscope autoradiography of diaphragm endplates of the American brown bat, labeled to saturation with tritiated alpha-bungarotoxin, has been used as a means to localize and quantitate the acetylcholine receptor there. Analysis of the grain distribution in these autoradiographs reveals that the receptor sites in this endplate are located on the postsynaptic membrane at an average density of 8,800/mu2. The sites are distributed asymmetrically along that membrane, being concentrated at the crests of the postjunctional folds--that portion nearest to the presynaptic membrane. The receptor site density at these regions of the postsynaptic membrane is estimated to be 20,000--25,000/mu2 of membrane surface. A comparison of these membrane site densities with those of endplates of red and white fibers of the mouse reveals a close similarity. On this basis, it is suggested that the receptor site density at the crests of the folds may be a characteristic feature of endplates of vertebrates. In contrast to the acetylcholine receptor sites, cholinesterase sites (determined autoradiographically in 3H-diisopropylfluorophosphate-labeled endplates) are largely distributed in a uniform manner over the postjunctional folds. The function of the secondary folds is, therefore, reassessed. Ultrastructural evidence available from other laboratories on the spatial characteristics of transmitter release and of postsynaptic dense particles is in accord with a model drawn for this molecular architecture at the vertebrate endplate.