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 neocortex 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 vivo. During whisker stimulation, complex spikes recorded intracellularly from distal dendrites and sharp waves in the electrocorticogram were accompanied by large dendritic [Ca²⁺] transients; these also occurred during bursts of action potentials recorded from somata of identified layer 5 neurons. The amplitude of the [Ca²⁺] transients was largest proximal to the main bifurcation, where sodium action potentials produced little Ca²⁺ influx. In some cases, synaptic stimulation evoked [Ca²⁺] transients without a concomitant action potential burst, suggesting variable coupling between dendrite and soma.