Interactions between anterior thalamus and hippocampus during different behavioral states in the rat by Héctor Penagos.

Abstract

The anterior thalamus and hippocampus are part of an extended network of brain structures underlying cognitive functions such as episodic memory and spatial navigation. Earlier work in rodents has demonstrated that hippocampal cell ensembles re-express firing profiles associated with previously experienced spatial behavior. Such recapitulation occurs during periods of awake immobility, slow wave sleep (SWS) and rapid eye movement sleep (REM). Despite its close functional and anatomical association with the hippocampus, whether or how activity in the anterior thalamus is related to activity in the hippocampus during behavioral states characterized by hippocampal replay remains unknown. Here, we monitor and compare thalamic and hippocampal activities during epochs in which rats execute a simple alternation task on a circular maze as well as during sleep periods before and after track running. We employ a neural decoding algorithm to interpret spiking activity in terms of spatial correlates during wake and REM. We analyze multi unit activity (MUA) to characterize the organization of thalamic and hippocampal populations during SWS. Consistent with their role in spatial navigation, we show that during active locomotion ensembles of thalamic and hippocampal neurons represent the spatial behavior of the rat in a coordinated fashion. However, during periods of hippocampal awake replay their spatial representations become decoupled. During REM, we demonstrate that thalamic activity replicates broad activity patterns associated with awake behavior and that both hippocampus and anterior thalamus concurrently represent similar ambulatory states. During SWS, we establish that the activities in these two areas alternate between frames of elevated firing and periods of little or no activity. We show that there is a tendency for thalamic depolarized states to start and end ahead of hippocampal activity frames. These results may shed light on how information encoded by thalamic circuits could bias or be incorporated into hippocampal replay phenomena.