Calcium dynamics, buffering, and buffer saturation in the boutons of dentate granule-cell axons in the hilus

The axons of dentate gyrus granule cells form synapses in the hilus. Ca²⁺ signaling was investigated in the boutons of these axons using confocal fluorescence imaging. Boutons were loaded with various concentrations of the Ca²⁺ indicator Oregon Green BAPTA-1 by patch-clamping the cell bodies and allowing the dye to diffuse into the axon. Resting free [Ca²⁺] started at 74 nM, rose to ∼1 μM immediately after an action potential, and then decayed to rest with a time constant of 43 msec (all extrapolated to a dye concentration of zero). Action potential-induced [Ca²⁺] rises were smaller in larger boutons, consistent with a size-independent Ca²⁺ channel density of 45/μm². Action potential-induced [Ca²⁺] changes varied with dye concentration in a manner consistent with Κ_E∼20 for the ratio of endogenous buffer-bound Ca²⁺ to free Ca²⁺. During trains of action potentials, [Ca²⁺] increments summed supralinearly by more than that expected from dye saturation. The amount of endogenous Ca²⁺ buffering declined as [Ca²⁺] rose, and this saturation indicated a buffer with a dissociation constant of ∼500 nM and a concentration of ∼130 μM. This is similar to the dissociation constant of calbindin-D28K, a Ca²⁺-binding protein that is abundant in dentate granule cells. Thus, calbindin-D28K is a good candidate for the Ca²⁺ buffer revealed by these experiments. The saturation of endogenous buffer can generate short-term facilitation by amplifying [Ca²⁺] changes during repetitive activity. Buffer saturation may also be relevant to the presynaptic induction of long-term potentiation at synapses formed by dentate granule cells.