Intracellular dynamics of hippocampal place cells during virtual navigation

Hippocampal place cells encode spatial information in rate and temporal codes. To examine the mechanisms underlying hippocampal coding, here we measured the intracellular dynamics of place cells by combining in vivo whole-cell recordings with a virtual-reality system. Head-restrained mice, running on a spherical treadmill, interacted with a computer-generated visual environment to perform spatial behaviours. Robust place-cell activity was present during movement along a virtual linear track. From whole-cell recordings, we identified three subthreshold signatures of place fields: an asymmetric ramp-like depolarization of the baseline membrane potential, an increase in the amplitude of intracellular theta oscillations, and a phase precession of the intracellular theta oscillation relative to the extracellularly recorded theta rhythm. These intracellular dynamics underlie the primary features of place-cell rate and temporal codes. The virtual-reality system developed here will enable new experimental approaches to study the neural circuits underlying navigation. Place cells know their place As we move around, spatial information is encoded and processed by 'place' cells in the hippocampus. Each of these neurons increases its firing rate when situated in its 'place', thus equating rate coding with location. The firing patterns of hippocampal place cells have been studied extensively over the past 40 years and theoretical models have been proposed to explain the mechanisms responsible for encoding place and time in neural circuits. Now the development of techniques allowing intracellular recording from place cells in awake, behaving mice navigating a virtual reality maze makes it possible to test these models. Initial results point to the existence of two subthreshold signatures of place fields: an increase in the amplitude of membrane potential oscillations at theta frequencies and a ramp-like depolarization of baseline membrane potential. As we navigate, spatial information is encoded in both rate and temporal codes by place cells located in the hippocampus. To investigate the origin of these codes, the intracellular dynamics of place cells are now measured in vivo in awake mice navigating a virtual-reality environment. Three subthreshold signatures of place fields are identified that underlie the primary features of place-cell rate and temporal codes.