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 CAl 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 ,uM. Extrapolation to zero Fura-2 concentration suggests that "physiological" transients at 370C 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+], encodes the action potential frequency during physiological patterns of electrical activity and may regulate Ca2+-dependent dendritic functions in an activity-dependent way.