The nanoscale topographical arrangement of voltage-gated calcium channels (VGCC) and
synaptic vesicles (SVs) determines synaptic strength and plasticity, but whether distinct
spatial distributions underpin diversity of synaptic function is unknown. We performed
single bouton Ca2+ imaging, Ca2+ chelator competition, immunogold electron microscopic
(EM) localization of VGCCs and the active zone (AZ) protein Munc13-1, at two cerebellar
synapses. Unexpectedly, we found that weak synapses exhibited 3-fold more VGCCs than
strong synapses, while the coupling distance was 5-fold longer. Reaction-diffusion
modeling could explain both functional and structural data with two strikingly different
nanotopographical motifs: strong synapses are composed of SVs that are tightly coupled
(similar to 10 nm) to VGCC clusters, whereas at weak synapses VGCCs were excluded
from the vicinity (similar to 50 nm) of docked vesicles. The distinct VGCC-SV topographical
motifs also confer differential sensitivity to neuromodulation. Thus, VGCC-SV arrangements
are not canonical, and their diversity could underlie functional heterogeneity across
CNS synapses.
