Influence of medial entorhinal cortex on CA1 population bursts

The hippocampal formation is believed to play an irreplaceable role in the processes of formation and retrieval of episodic memories. Located at the end of the pallial areas in the mammalian brain, it receives streams of pre-processed sensory information as well as higher order cognitive signals. The hippocampus is an essential brain structure responsible for preserving and recalling episodic memories. Without it, we would have serious problems preserving the integrity of our actions in a meaningful way. One of the main tasks of the hippocampus is to help us with keeping track of the ordinality of events. It has been shown that the hippocampal place cells would preserve the order of their activity for a significant amount of time. They usually engage in coordinated bursts of activity lasting between 50 to 500 ms. During these bursts hippocampal neurons present rather stereotypical patterns of population activity. This is believed to reflect the replay of different episodes of recent experiences for long term consolidation. This phenomena has been extensively observed among place cells of the hippocampus, specially in the area CA1. Another brain structure with a high number of spatially modulated cell is the medial entorhinal cortex (MEC). Since entorhinal cortex is the main input/output gate of hippocampus, the interaction between these two structures must play an immense role in the dynamics and content of hippocampal population bursts and in term the quality of memory consolidation and memory recall. To investigate the effect of this interaction on hippocampal replays, I analysed resting activity in rats with bilateral MEC lesions. I found that during the course of awake immobility periods in a new spatial experience, sequence replay in MEC-lesioned rats was reduced compared to control animals. However, the reduction in replay is no longer detectable during rest sessions following behaviour. MEC inputs thereby seem to facilitate plasticity of population burst activity as quantified by co-activation analysis and the participation of place cells in sequence replay. Moreover, in both animal groups there is only a minor increase of pattern activation due to a novel spatial experience, indicative of a strong intrinsic network structure that is similar prior and subsequent to the behavioural esion