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Calcium Dynamics, Buffering, and Buffer Saturation in the Boutons



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1612 The Journal of Neuroscience March 1 2003 23 5 1612 1621 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 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 1 M 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 m 2 Action potential induced Ca 2 changes varied with dye concentration in a manner consistent with E 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 500 nM and a concentration of 130 M This is similar to the dissociation constant of calbindinD28K 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 Key words nerve terminals calcium dynamics calcium buffers hippocampus dentate gyrus calbindin D28K mossy fibers



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