Distribution of Components of the SNARE Complex in Relation to Transmitter Release Sites at the Frog Neuromuscular Junction

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Abstract
At the frog neuromuscular junction, neurotransmitter release sites are regularly spaced at 1 μm intervals along the nerve terminal, directly facing postsynaptic folds which contain a high density of acetylcholine receptors. Immunostaining and laser confocal scanning microscopy were used to compare the distribution of presynaptic proteins implicated in exocytosis with that of fluorescent α‐bungarotoxin. Syntaxin, synaptosome‐associated 25 kDa protein and calcium channels were located predominantly at release sites. Synaptobrevin (vesicle‐associated membrane protein) was distributed in the cytoplasm of the nerve terminal, presumably in the packets of microvesicles associated with each active zone. N‐Ethylmaleimide‐sensitive fusion protein (NSF) and soluble NSF attachment proteins (αβSNAP) displayed a diffuse distribution throughout the terminal cytoplasm and also colocalized in distinct concentrated zones adjacent to the presynaptic membrane.