In vitro measurement of spontaneous neuronal network activity
Sharp- wave ripples (SPW-Rs) are rapid bursts of synchronised neuronal activity elicited by the hippocampus. They are widely thought to play a critical role in the consolidation of episodic memory and 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. We used fast 3D AO two- photon imaging and a caged glutamate to challenge this classical view of the functional role of interneurons, by demonstrating a network activity- dependent dynamic switch in dendritic integration mode: that FS-PV IN dendrites can generate propagating Ca2+ spikes during SPW-Rs. The spikes originated from dendritic hot spots and were mediated dominantly by L-type Ca2+ channels. Ca2+ 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 SPW-Rs, and controlled the phase of action potentials. We also demonstrated that the smallest functional unit that can generate ripple- frequency oscillations is a segment of a dendrite. The main difficulty in imaging hot spot activity in complex dendritic arbors is the inadequate temporal and spatial resolution of currently available imaging technologies: our fast 3D scanning methods overcome these limitations, by providing a high temporal resolution up to tens of microseconds, which allows simultaneous measurement of even the fastest regenerative events in multiple dendritic segments of the thin distal dendritic arborization, with a high spatial discretization on the size scale of dendritic hot spots during SPW-Rs.