Activity-related redistribution of presynaptic proteins at the active zone

Immunogold labeling distributions of seven presynaptic proteins were quantitatively analyzed under control conditions and after high K+ depolarization in excitatory synapses from dissociated rat hippocampal cultures. Three parallel zones in presynaptic terminals were sampled: zones I and II, each about one synaptic vesicle wide extending from the active zone; and zone III, containing a distal pool of vesicles up to 200 nm from the presynaptic membrane. The distributions of SV2 and synaptophysin, two synaptic vesicle integral membrane proteins, generally followed the distribution of synaptic vesicles, which were typically evenly distributed under control conditions and had a notable depletion in zone III after stimulation. Labels of synapsin I and synuclein, two synaptic vesicle-associated proteins, were similar to each other; both were particularly sparse in zone I under control conditions but showed a prominent enrichment toward the active zone, after stimulation. Labels of Bassoon, Piccolo and RIM 1, three active zone proteins, had very different distribution profiles from one another under control conditions. Bassoon was enriched in zone II, Piccolo and RIM 1 in zone I. After stimulation, Bassoon and Piccolo remained relatively unchanged, but RIM 1 redistributed with a significant decrease in zone I, and increases in zones II and III. These results demonstrate that Bassoon and Piccolo are stable components of the active zone while RIM 1, synapsin I and synuclein undergo dynamic redistribution with synaptic activity.