The hyperpolarization-activated current Ih disconnects somatic and dendritic spike initiation zones in layer V pyramidal neurons

Layer V pyramidal cells of the somatosensory cortex operate with two spike initiation zones. Sub-threshold depolarizations are strongly attenuated along the apical dendrite linking the somatic and distal dendritic spike initiation zones. Sodium action potentials, on the other hand, are actively back-propagating from the axon hillock into the apical tuft. There they can interact with local excitatory input leading to the generation of calcium action potentials. We investigated if and how back-propagating sodium action potentials alone, without concomitant excitatory dendritic input, can initiate calcium action potentials in the distal dendrite. In acute slices of the rat somatosensory cortex, layer V pyramidal cells were studied under current-clamp with simultaneous recordings from the soma and the apical dendrite. A train of four somatic action potentials had to reach high frequencies, in order to induce calcium action potentials in the dendrite (“critical frequency”, CF ~ 100 Hz). Depolarization in the dendrite reduced the CF, while hyperpolarization increased it. The CF depended on the presence of the hyperpolarizationactivated current Ih: blockade with 20 μM ZD7288 reduced the CF to 68 % of control. If the neurons were stimulated with noisy current injections, leading to in vivo-like irregular spiking, no calcium action potentials were induced in the dendrite. However, after Ih channel blockade calcium action potentials were frequently seen. These data suggest that Ih prevents initiation of the dendritic calcium action potential by proximal input alone. Dendritic calcium action potentials may therefore represent a unique signature for coincident somatic and dendritic activation.