Cellular and Network Mechanisms Underlying Spontaneous Sharp Wave–Ripple Complexes in Mouse Hippocampal Slices

Abstract

The mammalian hippocampus displays a peculiar pattern of fast (≈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.