Theta oscillations coincide with sustained hyperpolarization in CA3 pyramidal cells, underlying decreased firing

Brain-state fluctuations modulate membrane potential dynamics of neurons, influencing the functional repertoire of the network. Pyramidal cells (PCs) in hippocampal CA3 are necessary for rapid memory encoding, preferentially occurring during exploratory behavior in the high-arousal theta state. However, the relationship between the membrane potential dynamics of CA3 PCs and theta has not been explored. Here, we characterize the changes in the membrane potential of  PCs in relation to theta using electrophysiological recordings in awake mice. During theta, most PCs behave in a stereotypical manner, consistently hyperpolarizing time-locked to the duration of  theta. Additionally, PCs display lower membrane potential variance and reduced firing rate. In contrast, during large irregular activity, a low-arousal state, PCs show heterogeneous changes in membrane potential. This suggests coordinated hyperpolarization of PCs during theta, possibly caused by increased inhibition. This could lead ...