In vivo dendritic calcium dynamics in deep-layer cortical pyramidal neurons

Dendritic Ca2+ action potentials in neocortical pyramidal neurons have been characterized in brain slices, but their presence and role in the intact n eocortex remain unclear. Here we used two-photon microscopy to demonstrate Ca2+ electrogenesis in apical dendrites of deep-layer pyramidal neurons of rat barrel cortex in vive. During whisker stimulation, complex spikes recor ded intracellularly from distal dendrites and sharp waves in the electrocor ticogram were accompanied by large dendritic [Ca2+] transients; these also occurred during bursts of action potentials recorded from somata of identif ied layer 5 neurons. The amplitude of the [Ca2+] transients was largest pro ximal to the main bifurcation, where sodium action potentials produced litt le Ca2+ influx. In some cases, synaptic stimulation evoked [Ca2+] transient s without a concomitant action potential burst, suggesting variable couplin g between dendrite and soma.