Neurons of the hippocampus get ready to learn

The hippocampus contains a map of our environment formed by a specific subset of active cells (the place cells) among a larger population of silent neurons. But what determines the selection of active versus silent nerve cells in a given environment? Here, using whole-cell recordings in freely moving rats, we found that the odds for a neuron to be activated or to remain silent were determined by intrinsic factors that were set even before the new exploration began.

HFSP Long-Term Fellow Jérôme Epsztein and colleagues
authored on Fri, 13 May 2011

The hippocampus, a structure located in the inner temporal lobe of the brain, is critically involved in the formation of new memories of our everyday life events. In the rodent, some neurons in this structure (called the place cells) are specifically activated when an animal is in a particular place in its environment while the majority of them remain inactivated. Thus an environment is represented (and memorized) not only by the activity of hippocampal cells but also by the very identity of these neurons that are activated (place cells) or remain silent (silent cells). Similarly in humans, specific items are represented with unique and sparse subsets of active hippocampal cells among a larger population of silent neurons.

Despite the discovery of place cells over 40 years ago, the factors determining the selection of cells involved in mapping a given environment (and potentially memorizing it) are poorly understood. We addressed this question, together with my colleagues Michael Brecht and Albert Lee at the BCCN-Humboldt University Berlin,  using a newly developed technique to record the intracellular activity of hippocampal place and silent cells while rats were exploring an environment for the first time. Unlike extracellular recording techniques which only record the output of neurons in form of action potentials, this highly sensitive technique allows measurement of the synaptic inputs into the neurons as well as their fundamental intrinsic properties.

Our results highlight a critical role of intrinsic cellular properties such as the action potential threshold, in addition to synaptic inputs, in determining the initial division into place or silent cell.  Surprisingly, these differences in intrinsic properties could be detected even before the new exploration began. These results suggest that some hippocampal cells could be preconfigured to map and potentially memorize the next explored environment. This study enhances our understanding of the complex mechanism of storing information in the brain, showing a potentially decisive role of intrinsic properties in the selection of neurons involved in memory formation.

 

Reference:

Intracellular Determinants of Hippocampal CA1 Place and Silent Cell Activity in a Novel Environment. Epsztein J, Brecht M, Lee AK. Neuron. 2011 Apr 14;70(1):109-20.

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