Figure 1.

Distribution of Acetylcholinesterase (AChE) sites within the synaptic cleft. (A) EM-autoradiogram of mouse neuromuscular junction (NMJ) labeled with ¹²⁵I-Fas. Mouse sternomastoid muscles were incubated with ¹²⁵I-Fas (0.4 mM, 55 Ci/mmole) to inhibit AChE activity. They were then rinsed, fixed and processed for EM-autoradiography. Grains appeared almost exclusively at the NMJs. N, nerve; M, muscle. Arrows point to the primary cleft (PC) and to postsynaptic junctional folds (PJFs). Bar, 1 μm. (B) Radioactivity is distributed over the full depth of the PJFs. Grain density histogram (grains per unit area of autoradiogram) as a function of distance (in resolution units of HD = 80 nm; see, Salpeter et al., 1977) on either side of the axonal membrane (at 0), of 81 NMJs, three mice: left side into axon; right side into muscle. The bottom of the PJFs is on the average 11 HD (arrow), but with a large range; consequently, it is not as sharp a boundary as that provided by the axonal membrane. The reduced grain densities on both sides of the histogram are consistent with the radiation sources being within the PC and PJF, the variability in the depths of the PJFs and the actual spread of radiation under the experimental conditions (>3 HD). *, see text in “Results” section for detailed statistical parameter.