"Cholinergic" postsynaptic membranes of bullfrog sympathetic ganglia: electron microscopy of thin sections and freeze-fracture replicas.

"Cholinergic" synapses of the bullfrog sympathetic ganglion cells were investigated with thin sectioning, complementary freeze-fracturing, and deep-etching methods after glutaraldehyde fixation. The protoplasmic (-fracture) face (PF) of the postsynaptic membrane was characterized by intramembranous particles (IMPs), 3,500/micron 2 in density, consisting of larger particles, 10-12 nm in diameter, and smaller ones, 8-9 nm; the complementary exoplasmic (-fracture) face (EF) contained larger and smaller IMPs, 750/micron 2 in density, and numbers of pits. By close inspection of the sections and freeze-fracture replicas at high magnification and with deep-etching in particular, it was concluded that aggregated IMPs might represent transmembranous components and that the particulate entities existing in the postsynaptic active zones might be larger in number than those exposed to view and counted here in the "cholinergic" synapses. An individual IMP often appeared to consist of five or six subunits arranged in a rosette with a central pit. These findings suggest that the aggregated IMPs, particularly the larger ones, may be closely related to the structure of the nicotinic ACh receptor-ion channel complex.