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Back-propagating action potentials mediate calcium signalling in dendrites of bitufted interneurons in layer 2/3 of rat somatosensory cortex

Katharina M. M. Kaiser, Y. Zilberter * and B. Sakmann

1. Bitufted interneurons in layer 2/3 of the rat (P14) somatosensory cortex have elongated apical and basal dendritic arbors that can span the entire depth of the cortex. Simultaneous dendritic and somatic whole-cell voltage recordings combined with Ca²⁺ fluorescence measurements were made to quantify voltage and Ca²⁺ signalling in dendritic arbors of bitufted neurons.
2. Action potentials (APs) initiated close to the soma by brief current injection back-propagated into the apical and basal dendritic arbors and evoked a transient increase in volume-averaged dendritic Ca²⁺ concentration ([Ca²⁺]_i) of about 140 nM peak amplitude per AP. The AP evoked Ca²⁺ signal decayed with a time constant of about 200 ms.
3. A relatively high endogenous Ca²⁺ binding ratio of ~285 determines the comparatively small rise in [Ca²⁺]_i of bitufted cell dendrites evoked by a back-propagating AP.
4. The [Ca²⁺]_i transient evoked by back-propagating dendritic APs decreased with distance ( 50 µm) from the soma in some neurons. At distances greater than 50 µm transients did not show a spatial gradient between the proximal and distal dendritic branches.
5. During trains of APs the mean amplitude of the steady-state increase in dendritic [Ca²⁺]_i encoded the AP frequency linearly up to 40 Hz with a slope of 20 nM Hz^-1.
6. The results suggest that APs initiated in the axon of bitufted neurons back-propagate and 'copy' the pattern of the axon's electrical activity also to the dendritic arbor. The AP pattern is transduced into a transient rise of dendritic [Ca²⁺]_i which, presumably, can regulate the receptive properties of the dendritic arbor for synaptic input.

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