Ultrastructural organization of lamprey reticulospinal synapses in three dimensions.

The giant reticulospinal synapse in lamprey provides a unique model to study synaptic vesicle traffic. The axon permits microinjections, and the active zones are often separated from each other, which makes it possible to track vesicle cycling at individual release sites. However, the proportion of reticulospinal synapses with individual active zones ("simple synapses") is unknown and a quantitative description of their organization is lacking. Here, we report such data obtained by serial section analysis, intermediate-voltage electron microscopy, and electron tomography. The simple synapse was the most common type (78%). It consisted of one active zone contacting one dendritic process. The remaining synapses were "complex," mostly containing one vesicle cluster and two to three active zones synapsing with distinct dendritic shafts. Occasional axosomatic synapses with multiple active zones forming synapses with the same cell were also observed. The vast majority of active zones in all synapse types contained both chemical and electrotonic synaptic specializations. Quantitative analysis of simple synapses showed that the majority had active zones with a diameter of 0.8-1.8 microm. The number of synaptic vesicles and the height of the vesicle cluster in middle sections of serially cut synapses correlated with the active zone length within, but not above, this size range. Electron tomography of simple synapses revealed small filaments between the clustered synaptic vesicles. A single vesicle could be in contact with up to 12 filaments. Another type of filament, also associated with synaptic vesicles, emerged from dense projections. Up to six filaments could be traced from one dense projection.

Impaired recycling of synaptic vesicles after acute perturbation of the presynaptic actin cytoskeleton.

Actin is an abundant component of nerve terminals that has been implicated at multiple steps of the synaptic vesicle cycle, including reversible anchoring, exocytosis, and recycling of synaptic vesicles. In the present study we used the lamprey reticulospinal synapse to examine the role of actin at the site of synaptic vesicle recycling, the endocytic zone. Compounds interfering with actin function, including phalloidin, the catalytic subunit of Clostridium botulinum C2 toxin, and N-ethylmaleimide-treated myosin S1 fragments were microinjected into the axon. In unstimulated, phalloidin-injected axons actin filaments formed a thin cytomatrix adjacent to the plasma membrane around the synaptic vesicle cluster. The filaments proliferated after stimulation and extended toward the vesicle cluster. Synaptic vesicles were tethered along the filaments. Injection of N-ethylmaleimide-treated myosin S1 fragments caused accumulation of aggregates of synaptic vesicles between the endocytic zone and the vesicle cluster, suggesting that vesicle transport was inhibited. Phalloidin, as well as C2 toxin, also caused changes in the structure of clathrin-coated pits in stimulated synapses. Our data provide evidence for a critical role of actin in recycling of synaptic vesicles, which seems to involve functions both in endocytosis and in the transport of recycled vesicles to the synaptic vesicle cluster.