Marozzi, EC; (2014) The Cellular and Molecular Characterisation of Context Representation in Entorhinal Cortex. Doctoral thesis , UCL (University College London).

Abstract

The hippocampus forms a representation of spatial context. Hippocampal place cells respond to contextual changes by “remapping”. Co-recorded place cells respond heterogeneously, with the majority responding in a conditional manner, whereby the response to one contextual cue (e.g. colour) is modulated by the presence of another (e.g. odour), suggesting contextual information is combined upstream. This thesis explored whether this combination occurs upstream in grid cells in medial entorhinal cortex. These cells have multiple receptive fields that span the environment in a triangular array and are thought to encode distance information. In Experiment 1, grid cells were recorded while rats explored geometrically equivalent boxes that differed only in their colour and odour. Results revealed grid cells remap to contextual changes via a translation but not a rotation of the grid fields. The majority of grid cells remapped conditionally, suggesting contextual cues are combined elsewhere. Unlike place cells, co-recorded grid cells responded in a homogeneous manner. Experiment 2 explored where colour and odour information is combined in the brain using two related phenomena. First, the time course of nuclear versus cytoplasmic Arc RNA accumulation is distinct, allowing neurons activated at two distinct time epochs to be visualised. Second, the decay of Arc, independent of electrophysiological activity, back to baseline levels after 2 hours in a familiar environment, known as electro-transcriptional decoupling (ETD), was also exploited. Animals were placed in boxes and experienced a colour change followed by an odour change (or vice versa). Arc activity was measured in the hippocampus and entorhinal cortex. Surprisingly results showed the time course of ETD was different in the hippocampus and entorhinal cortex than expected. Nevertheless, both structures were responsive to contextual changes compared to home cage controls. Overall, it is clear that the entorhinal cortex responds to non-spatial, contextual information, in additional to spatial information.

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