Ca2+ buffering and action potential-evoked Ca2+ signaling in dendrites of pyramidal neurons

The effect of the fluorescent Ca2+ indicator dye Fura-2 on Ca2+ dynamics was studied in proximal apical dendrites of neocortical layer V and hippocampal CA1 pyramidal neurons in rat brain slices using somatic whole-cell recording and a charge-coupled device camera. A single action potential evoked a transient increase of intradendritic calcium concentration ([Ca2+](i)) that was reduced in size and prolonged when the Fura-2 concentration was increased from 20 to 250 mu M. Extrapolation to zero Fura-2 concentration suggests that ''physiological'' transients at 37 degrees C have large amplitudes (150-300 nM) and fast decays (time constant <100 ms). Assuming a homogeneous compartment model for the dendrite, 0.5-1% of the total Ca2+ entering during an action potential was estimated to remain free. Washout of cytoplasmic Ca2+ buffers was not detectable, suggesting that they are relatively immobile. During trains of action potentials, [Ca2+](i) increased and rapidly reached a steady state (time constant <200 ms), fluctuating around a plateau level which depended linearly on the action potential frequency. Thus, the mean dendritic [Ca2+](i) encodes the action potential frequency during physiological patterns of electrical activity and may regulate Ca2+-dependent dendritic functions in an activity-dependent way.