A comparison of synaptic protein localization in hippocampal mossy fiber terminals and neurosecretory endings of the neurohypophysis using the cryo-immunogold technique

In central synapses synaptic vesicle docking and exocytosis occurs at morphologically specialized sites (active zones) and requires the interaction of specific proteins in the formation of a SNARE complex. In contrast, neurosecretory terminals lack active zones. Using the cryo-immunogold technique we analyzed the localization of synaptic vesicle proteins and of proteins of the docking complex at active zones. This was compared to the localization of the identical proteins in neurosecretory terminals. In addition we compared the vesicular and granular localization of the proteins investigated. Synaptic vesicles in rat hippocampal mossy fiber synapses and microvesicles in the neurosecretory terminals of the neurohypophysis contained in common the proteins VAMP II (a v-SNARE), SV2, rab3A, and N-type Ca2+ channels. Only minor immunolabeling for these proteins was observed at neurosecretory granules. These results support the notion of a close functional identity of microvesicles from neurosecretory endings of the neurohypophysis and of synaptic vesicles. The vesicular pool of N-type Ca2+ channels may serve their stimulation-induced translocation into the plasma membrane. We find increased labeling for VAMP II, SNAP-25, N-type Ca2+ channels and of rab3A at the active zones of mossy fiber synapses. Labeling at release sites is by far highest for Bassoon, a high molecular weight protein of the active zone. The labeling pattern implies an association of Bassoon with presynaptic dense projections. Bassoon is absent from neurosecretory terminals and VAMP II, SNAP-25, rab3A, and N-type Ca2+ channels reveal a scattered distribution over the plasma membrane. The competence of the presynaptic active zone for selective vesicle docking may not primarily result from its contents in SNARE proteins but rather from the preformation of presynaptic dense projections as structural guides for vesicle exocytosis.