Target Cell Type-Dependent Differences in Ca²⁺Channel Function Underlie Distinct Release Probabilities at Hippocampal Glutamatergic Terminals

Abstract

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.

Distinct nanoscale calcium channel and synaptic vesicle topographies contribute to the diversity of synaptic function

“…Similar results were obtained using the low affinity Ca 2+ indicator OGB-5N (Figure 1F). The larger average sAPCaT in SC boutons contrasts previous reports that sAPCaT amplitudes predict synaptic strength (Eltes et al, 2017; Holderith et al, 2012; Koester and Johnston, 2005).…”

“…Previous analysis of gold particle labelling of Cav2.1 channels in CNS synapses suggested that VGCCs form clusters (Eltes et al, 2017; Holderith et al, 2012, Nakamura et al, 2015; Miki et al, 2017). We therefore performed a spatial point pattern analysis that test for deviations in the density of particles over various spatial scales, using the Ripley’s H-function, an analysis method that is particularly sensitive to particle clustering (down to two points per cluster, for observed NNDs; Figure S3A-C).…”

Synaptic Ribbon Active Zones in Cone Photoreceptors Operate Independently from One Another

“…Significant differences in Ca 2+ influx arising from local differences in the function or subunit composition of Ca 2+ channels at different hippocampal synapses have also been observed (Éltes et al, 2017). Voltage dependence might also vary at different synapses as a consequence of local differences in the ionic environment and neuromodulatory influence.…”

Improved spike inference accuracy by estimating the peak amplitude of unitary [Ca2+] transients in weakly GCaMP6f‐expressing hippocampal pyramidal cells

“…; Eltes et al . ). To normalize data across batches of dyes, G max / R values were measured by imaging a sealed (tip melted and closed by heating) pipette filled with intracellular solution containing 10 mM CaCl 2 for each cell.…”

“…For the measurement of variability in unitary peak [Ca 2+ ] transient amplitudes, data were collected from the following 5 min. AP-evoked changes in fluorescence were quantified as G/R(t) = (F green(t) -F rest, green )/(F red -I dark, red ) where F green(t) represents the green fluorescence signal as a function of time, F rest, green is the green fluorescence before stimulation, and I dark, red is the dark current in the red channel (Holderith et al 2012;Eltes et al 2017). To normalize data across batches of dyes, G max /R values were measured by imaging a sealed (tip melted and closed by heating) pipette filled with intracellular solution containing 10 mM CaCl 2 for each cell.…”