Diffusion of Ca from Small Boutons en Passant into the Axon Shapes AP-Evoked Ca Transients.

Not only the amplitude but also the time course of a presynaptic Ca transient determine multiple aspects of synaptic transmission. In small bouton-type synapses, the mechanisms underlying the Ca decay kinetics have not been fully investigated. Here, factors that shape an action-potential-evoked Ca transient were quantitatively studied in synaptic boutons of neocortical layer 5 pyramidal neurons. Ca transients were measured with different concentrations of fluorescent Ca indicators and analyzed based on a single-compartment model. We found a small endogenous Ca-binding ratio (7 ± 2) and a high activity of Ca transporters (0.64 ± 0.03 ms), both of which enable rapid clearance of Ca from the boutons. However, contrary to predictions of the single-compartment model, the decay time course of the measured Ca transients was biexponential and became prolonged during repetitive stimulation. Measurements of [Ca] along the adjoining axon, together with an experimentally constrained model, showed that the initial fast decay of the Ca transients predominantly arose from the diffusion of Ca from the boutons into the axon. Therefore, for small boutons en passant, factors like terminal volume, axon diameter, and the concentration of mobile Ca-binding molecules are critical determinants of Ca dynamics and thus Ca-dependent processes, including short-term synaptic plasticity.