Retrieval and recycling of synaptic vesicle membrane in pinched-off nerve terminals (synaptosomes).

Abstract

The morphological features of pinched-off presynaptic nerve terminals (synaptosomes) from rat brain were examined with EM techniques; in many experiments, an extracellular marker (horseradish peroxidase or colloidal thorium dioxide) was included in the incubation media. When incubated in physiological saline, most terminals appeared approximately spherical, and were filled with small (.apprx. 400-.ANG. diameter) synaptic vesicles; mitochondria were also present in many of the terminals. In a number of instances the region of synaptic contact, with adhering portions of the postsynaptic cell membrane and postsynaptic density, could be readily discerned. About 20-30% of the terminals in the preparations exhibited clear evidence of damage, as indicated by diffuse distribution of extracellular markers in the cytoplasm; the markers appeared to be excluded from the intraterminal vesicles under these circumstances. The markers were excluded from the cytoplasm in .apprx. 70-80% of the terminals, which may imply that these terminals have intact plasma membranes. When the terminals were treated with depolarizing agents (veratridine or K-rich media), in the presence of Ca, many new, large (600-900-.ANG. diameter) vesicles and some coated vesicles and new vacuoles appeared. When the media contained an extracellular marker, the newly formed structures frequently were labeled with the marker. If the veratridine-depolarized terminals were subsequently treated with tetrodotoxin (to repolarize the terminals) and allowed to recover for 60-90 min, most of the large marker-containing vesicles disappeared, and numerous small (.apprx. 400-.ANG. diameter) marker-containing vesicles appeared. Pinched-off presynaptic terminals evidently contain all of the machinery necessary for vesicular exocytosis and for the retrieval and recycling of synaptic vesicle membrane. The vesicle membrane appears to be retrieved primarily in the form of large diameter vesicles which are subsequently reprocessed to form new typical small-diameter synaptic vesicles.