Highlights d Number of presynaptic calcium channels (Ca V ) does not correlate with synaptic strength d Weak synapses are more sensitive to competition with exogenous Ca 2+ chelators d EM immunogold labeling of Ca V 2.1 and Munc13-1 shows synapse-specific nanotopographies d Different nanoscale Ca V -synaptic vesicle arrangements explain functional differences

Target cell type-dependent differences in presynaptic release probability (P r ) and short-term plasticity are intriguing features of cortical microcircuits that increase the computational power of neuronal networks. Here, we tested the hypothesis that different voltage-gated Ca 2ϩ channel densities in presynaptic active zones (AZs) underlie different P r values. Two-photon Ca 2ϩ imaging, triple immunofluorescent labeling, and 3D electron microscopic (EM) reconstruction of rat CA3 pyramidal cell axon terminals revealed ϳ1.7-1.9 times higher Ca 2ϩ inflow per AZ area in high P r boutons synapsing onto parvalbumin-positive interneurons (INs) than in low P r boutons synapsing onto mGluR1␣-positive INs. EM replica immunogold labeling, however, demonstrated only 1.15 times larger Cav2.1 and Cav2.2 subunit densities in high P r AZs. Our results indicate target cell type-specific modulation of voltage-gated Ca 2ϩ channel function or different subunit composition as possible mechanisms underlying the functional differences. In addition, high P r synapses are also characterized by a higher density of docked vesicles, suggesting that a concerted action of these mechanisms underlies the functional differences.

Potassium channels comprise the most diverse family of ion channels and play critical roles in a large variety of physiological and pathological processes. In addition to their molecular diversity, variations in their distributions and densities on the axo-somato-dendritic surface of neurons are key parameters in determining their functional impact. Despite extensive electrophysiological and anatomical investigations, the exact location and densities of most K+ channels in small subcellular compartments are still unknown. Here we aimed at providing a quantitative surface map of two delayed-rectifier (Kv1.1 and Kv2.1) and one G-protein-gated inwardly rectifying (Kir3.2) K+ channel subunits on hippocampal CA1 pyramidal cells (PCs). Freeze-fracture replica immunogold labelling was employed to determine the relative densities of these K+ channel subunits in 18 axo-somato-dendritic compartments. Significant densities of the Kv1.1 subunit were detected on axon initial segments (AISs) and axon terminals, with an approximately eight-fold lower density in the latter compartment. The Kv2.1 subunit was found in somatic, proximal dendritic and AIS plasma membranes at approximately the same densities. This subunit has a non-uniform plasma membrane distribution; Kv2.1 clusters are frequently adjacent to, but never overlap with, GABAergic synapses. A quasi-linear increase in the Kir3.2 subunit density along the dendrites of PCs was detected, showing no significant difference between apical dendritic shafts, oblique dendrites or dendritic spines at the same distance from the soma. Our results demonstrate that each subunit has a unique cell-surface distribution pattern, and predict their differential involvement in synaptic integration and output generation at distinct subcellular compartments.