Sharp-wave ripples are transient oscillatory events in the hippocampus that are associated
with the reactivation of neuronal ensembles within specific circuits during memory
formation. Fast-spiking, parvalbumin-expressing interneurons (FS-PV INs) are thought
to provide fast integration in these oscillatory circuits by suppressing regenerative
activity in their dendrites. Here, using fast 3D two-photon imaging and a caged glutamate,
we challenge this classical view by demonstrating that FS-PV IN dendrites can generate
propagating Ca(2+) spikes during sharp-wave ripples. The spikes originate from dendritic
hot spots and are mediated dominantly by L-type Ca(2+) channels. Notably, Ca(2+) spikes
were associated with intrinsically generated membrane potential oscillations. These
oscillations required the activation of voltage-gated Na(+) channels, had the same
frequency as the field potential oscillations associated with sharp-wave ripples,
and controlled the phase of action potentials. Furthermore, our results demonstrate
that the smallest functional unit that can generate ripple-frequency oscillations
is a segment of a dendrite.
