The polyene antibiotic filipin reacts specifically with membrane cholesterol and produces distinctive membrane lesions. We treated frog cutaneous and sartorius muscles with 0.04% filipin in a glutaraldehyde solution with or without prefixation with glutaraldehyde. Freeze- fracture of these muscles revealed numerous 19 to 38-nm protuberances and depressions (filipin-sterol complexes) in most areas of muscle, axon, and Schwann cell membranes. In the presynaptic membrane, however, these filipin-sterol complexes were absent from active zones consisting of ridges bordered with double rows of particles. In the postsynaptic membrane, filipin-sterol complexes were also virtually absent from the areas occupied by aggregates of large particles representing acetylcholine receptors. These results suggest that the membrane regions of active zones and acetylcholine receptor aggregates have a low cholesterol content
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