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The Journal of Neuroscience, March 1, 2003, 23(5):1612

Calcium Dynamics, Buffering, and Buffer Saturation in the Boutons of Dentate Granule-Cell Axons in the Hilus

Meyer B. Jackson and Stephen J. Redman

Division of Neuroscience, John Curtin School of Medical Research, Canberra, ACT 0200, Australia

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 _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.

Key words: nerve terminals; calcium dynamics; calcium buffers; hippocampus; dentate gyrus; calbindin-D28K; mossy fibers

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Copyright © 2003 Society for Neuroscience  0270-6474/03/2351612-10$05.00/0

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