Hippocampal Ripples are Correlated with Subthreshold Up/Down Activity in Mouse Posterior Parietal Cortical Neurons

Abstract

Memories acquired during wakefulness are consolidated during sleep. During non-rapid eye movement (NREM) sleep, ripples occur in the hippocampus, while slow oscillations (SOs) occur in the neocortex. The coupling of ripples and SOs is believed to play a key role in memory consolidation. The posterior parietal cortex (PPC) is one of the regions that exhibit reactivation around the occurrence of ripples. Recent studies have revealed subthreshold membrane potential dynamics of PPC neurons after ripples. However, it remains unclear whether these membrane potential dynamics depend on the timing of ripples relative to the phase of SOs. We performed whole-cell recordings from neurons in the PPC and simultaneous LFP recordings from hippocampal CA1. Membrane potentials during SOs were classified into UP and DOWN states, and ripples were detected from the LFP recording. Analyses of the timing of ripples relative to the phases of SOs revealed that they preferentially occurred near the transition between UP and DOWN states. The timing of the ripples relative to the transitions was correlated with the duration and amplitude of the UP and DOWN states. Therefore, the phase of SOs may modulate the effects of ripples on the neocortex.