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

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 (Delta [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 similar to 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 (less than or equal to 50 mum) from the soma in some neurons. At distances greater than 50 mum 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 also 'copy' the pattern of the axon's electrical activity 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.