Temporal synchrony and gamma-to-theta power conversion in the dendrites of CA1 pyramidal neurons

Pyramidal cells have to integrate thousands of inputs across their expansive dendritic arbors. The spatial spread of these inputs should lead to intraneuronal propagation delays. Here the authors show that the distribution of HCN channels in hippocampal pyramidal cells normalizes these delays, particularly for inputs at theta and gamma frequencies. Timing is a crucial aspect of synaptic integration. For pyramidal neurons that integrate thousands of synaptic inputs spread across hundreds of microns, it is thus a challenge to maintain the timing of incoming inputs at the axo-somatic integration site. Here we show that pyramidal neurons in the rodent hippocampus use a gradient of inductance in the form of hyperpolarization-activated cation-nonselective (HCN) channels as an active mechanism to counteract location-dependent temporal differences of dendritic inputs at the soma. Using simultaneous multi-site whole-cell recordings complemented by computational modeling, we find that this intrinsic biophysical mechanism produces temporal synchrony of rhythmic inputs in the theta and gamma frequency ranges across wide regions of the dendritic tree. While gamma and theta oscillations are known to synchronize activity across space in neuronal networks, our results identify a new mechanism by which this synchrony extends to activity within single pyramidal neurons with complex dendritic arbors.