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. Ca(2+) signaling was investigated in the boutons of these axons using confocal fluorescence imaging. Boutons were loaded with various concentrations of the Ca(2+) indicator Oregon Green BAPTA-1 by patch-clamping the cell bodies and allowing the dye to diffuse into the axon. Resting free [Ca(2+)] started at 74 nm, rose to approximately 1 microm 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 [Ca(2+)] rises were smaller in larger boutons, consistent with a size-independent Ca(2+) channel density of 45/microm(2). Action potential-induced [Ca(2+)] changes varied with dye concentration in a manner consistent with kappa(E) approximately 20 for the ratio of endogenous buffer-bound Ca(2+) to free Ca(2+). During trains of action potentials, [Ca(2+)] increments summed supralinearly by more than that expected from dye saturation. The amount of endogenous Ca(2+) buffering declined as [Ca(2+)] rose, and this saturation indicated a buffer with a dissociation constant of approximately 500 nm and a concentration of approximately 130 microm. This is similar to the dissociation constant of calbindin-D28K, a Ca(2+)-binding protein that is abundant in dentate granule cells. Thus, calbindin-D28K is a good candidate for the Ca(2+) buffer revealed by these experiments. The saturation of endogenous buffer can generate short-term facilitation by amplifying [Ca(2+)] 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.