The hippocampus is an important structure for spatial memory in rodents and episodic memory in humans. The hippocampus uses a sparse coding scheme where a given environment is represented by the place selective firing of a small group of cells, (called place cells) among a larger population of silent neurons. Thus a given environment is not only coded by the firing rate and timing of active cells but also by the very identity of these cells that fire or stay silent in that environment. Similarly, in humans, specific items or episodes are coded by the selective firing of particular cells in the temporal lobe among a larger population of silent neurons. Thus understanding the mechanisms involved in the selection of which cells will be active in a particular environment is one of the most important to understand the formation of spatial memories in rodents and episodic memories in humans. This question is at the core of our research project. Place cells have been extensively studied at the system level using extracellular recording which can only record the spiking output of neurons but not the intracellular mechanisms leading to that spiking. This is why I recently contributed to the development of a new technique allowing intracellular recordings in freely behaving animals. Using this technique we found an important role for intrinsic neuronal properties in the distinction between place and silent cells. Intriguingly, these differences were observed even before the new exploration began. Based on these findings we will address three objectives: 1) determine the role of intrinsic excitability in the initial selection of place cells, 2) test whether a similar coding scheme are valid for the other major hippocampal area for spatial coding: the CA3 area and last 3) determine whether these intrinsic mechanisms play a role in another major function of the hippocampus the remapping.
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