Figure 1. Action potential-evoked synaptic release critically depends on basal intracellular Ca²⁺ concentration.

(A) Left: Illustration of the cellular connectivity of the cMFB to GC synapse during simultaneous pre- and postsynaptic patch-clamp recording. The presynaptic terminal was loaded with an intracellular solution having either low or high free basal Ca²⁺ concentration (top and bottom, respectively). Right: Comparison of the average free Ca²⁺ concentration in the presynaptic patch pipette (quantified by two-photon Ca²⁺ imaging) for the intracellular solutions with low and high basal Ca²⁺ (n = 4 each). (B) Example two-photon microscopic image of a cMFB and a GC in the paired whole-cell configuration. (C) Example traces of a paired cMFB-GC recording with current injection (I_cMFB) (top) eliciting an action potential in the cMFB (middle) and an EPSC in the postsynaptic GC (bottom). Black and blue color code corresponds to low and high free basal Ca²⁺ concentration in the presynaptic solution, respectively. The decay of the EPSC was fitted with a bi-exponential function (magenta line). (D) Comparison of the properties of presynaptic action potentials and EPSCs evoked after eliciting an action potential in the presynaptic terminal using solutions having either low (black) or high (blue) free Ca²⁺ concentration. From left to right: peak amplitude of the EPSC, weighted decay time constant of the EPSC, 10-to-90% rise time of the EPSC, amplitude of the presynaptic action potential, and action potential half-duration (n = 8 and 8 paired cells for the conditions with low and high resting Ca²⁺ concentration, respectively). Boxplots show median and 1st/3rd quartiles with whiskers indicating the whole data range. Values of individual experiments are superimposed as circles. The numbers above the boxplots represent p-values of Mann-Whitney U tests.