Comparaison des propriétés des neurones d'engramme CA3 au cours des périodes initiales d'encodage et de la première phase de consolidation

Abstract

Forming new memories after a one-time experience requires rapid initial encoding then consolidation over time. During learning, multimodal information converges into the hippocampus, activating sparse neuronal assemblies. Activated neurons are believed to form a memory representation through concerted activity and synaptic interconnectivity. Computational and behavioral studies point at the hippocampal CA3 region as a key structure involved in multimodal information integration and fast memory storage. In this study, properties of activated CA3 neurons and their local network following one-trial learning of an episodic-like memory are explored. Virally-delivered constructs based on the immediate-early gene cFos are employed to identify and track CA3 assemblies ex-vivo at different time points in their emergence and development. We describe the use of a novel cFos-based construct allowing activity-dependent transitory expression of a destabilized fluorescent marker ZsGreen rapidly after one-trial learning (few hours). In parallel, the Robust Activity Marker (RAM) system, that provides activity-dependent labelling 24 hours following a novel experience, is used to characterize neuronal assemblies after an initial phase of consolidation. Electrophysiological properties of CA3 pyramidal cells labeled with these techniques are reported by means of patch-clamp recordings in ex vivo hippocampal slices. We analyze the intrinsic properties and synaptic connectivity of CA3 neurons activated by a one-trial memory task, in comparison with non-activated neighbor pyramidal neurons, providing information on the changes of these properties over the initial period of encoding and the early phase of consolidation.