Place and behavioral modulation of hippocampal neurons during immobility.

Behavioral state fluctuations profoundly impact episodic memory processing. To explore the underlying mechanisms, we recorded CA1 place cells in head-fixed male mice and focused on awake immobility to capture spontaneous behavioral state fluctuations by facial motion, pupillometry, and local field potential (LFP) analysis. We found that during awake immobility, the duration of spontaneous whisker-pad motion events correlated with ongoing levels of arousal and modulated both the frequency and the power of theta oscillations. CA1 place cells continued to encode location during immobility, with the spatial code being primarily driven by a subset of behaviorally-modulated place cells which increased their firing upon behavioral state transitions. Single-cell stimulation during immobility was sufficient for the induction of place fields, indicating that plasticity mechanisms can be engaged even in the absence of locomotion. Altogether, these data indicate that behavioral state fluctuations might contribute to episodic memory processing by modulating theta oscillatory dynamics and hippocampal gain via the engagement of a discrete place-cell ensemble.