Cellular and network mechanisms underlying spontaneous sharp wave-ripple complexes in mouse hippocampal slices.

The mammalian hippocampus displays a peculiar pattern of fast (approximately 200 Hz) network oscillations superimposed on slower sharp waves. Such sharp wave-ripple complexes (SPW-R) have been implicated in memory consolidation. We have recently described a novel and unique method for studying SPW-R in naive slices of murine hippocampus. Here, we used this model to analyse network and cellular mechanisms of this type of network activity. SPW-R are usually generated within area CA3 but can also originate within the isolated CA1 region. Cellular synchronisation during SPW-R requires both excitatory and inhibitory synaptic transmission as well as electrical coupling, the latter being particularly important for the high-frequency component. Extracellular and intracellular recordings revealed a surprisingly strong inhibition of most CA1 pyramidal cells during SPW-R. A minority of active cells, however, increases action potential frequency and fires in strict synchrony with the field ripples. This strong separation between members and non-members of the network may serve to ensure a high signal-to-noise ratio in information processing during sharp wave-ripple complexes.