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

摘要

class="enumerated" style="list-style-type:decimal">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.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 (Δ[Ca²⁺]i) of about 140 nM peak amplitude per AP. The AP evoked Ca²⁺ signal decayed with a time constant of about 200 ms.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.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.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.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.