Intracellular dynamics of CA3 pyramidal cells supporting sparse coding during theta and large irregular activity

The hippocampal local field potential (LFP) displays large irregular activity (LIA) during quiet wakefulness and theta oscillations during exploratory behaviors. The hippocampal CA3 subregion is important for rapid memory encoding and integrates multimodal information processed in the entorhinal cortex. We have explored the intracellular dynamics of CA3 pyramidal cells (PCs) in awake mice using whole-cell and LFP recordings in awake head-fixed mice combined with measurements of pupil diameter and running speed. We have examined how single properties contribute to network processing across brain states. LIA coincides with higher synaptic and network noise and in some cells, starts with a transient increase in Vm and firing. During theta, most CA3 PCs hyperpolarize, reduce firing, and have lower Vm variance. However, a small subpopulation of CA3 PCs depolarizes and increases firing. These intracellular modulations during theta and LIA allow sparse coding in CA3 across different brain states.

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