Identification of regions of high acetylcholine receptor density in tannic acid-fixed postsynaptic membranes from electric tissue.

Several distinct membrane specializations in electric tissue of the torpedine ray, Narcine brasiliensis, are rendered visible in thin section electron microscopy by the use of tannic acid during sample preparation. The acetylcholine receptor-containing regions of the postsynaptic membrane have a morphology shared by no other membrane in the tissue: a thick outer lamina, previously ascribed by others to the receptor protein itself, and a thick (approximately 8 nm) inner lamina which is precisely co-extensive with the thick outer lamina. Both of these features are found in biochemical preparations of the post-synaptic membrane upon fixation in the presence of tannic acid. Other membrane specializations having thick outer laminae can be distinguished from that due to the receptor. Thus, receptor-containing regions in isolated membrane fragments can be unambiguously identified by inspection. This provides an important practical tool for the study of these membranes. In addition, the data presented here and elsewhere suggest a model for the receptor-containing membrane in which material, presumably protein, is roughly symmetrically distributed on the two sides of the membrane.