Cacucci, Francesca; (2003) Studies on directional signalling in the hippocampal formation of the rat. Doctoral thesis (Ph.D.), University College London (United Kingdom).

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Abstract

This thesis examines the properties of a new category of cell in the rodent hippocampal formation: topodirectional (TD) cells. Cell activity was extracellularly recorded in freely moving rats. TD cell firing is modulated by three variables: the animal's allocentric position in the environment, its head direction in the horizontal plane, and the theta oscillation in the hippocampal formation. Thus TD cells combine properties of well characterised cell types within the hippocampal formation: place cells, head direction cells and theta cells. The thesis begins with a general introduction to the anatomy and physiology of the hippocampal-parahippocampal network. The methods section follows, with the detailed description of the recording protocol and analytical procedures. The results section opens with the histological findings, regarding the anatomical location of TD cells, and shows that TD cells were mostly found in the dorsal extent of presubiculum and parasubiculum. The extracellularly acquired waveforms were analysed: TD cells show shorter duration and smaller amplitude waveforms than head direction cells. Using several quantitative measures of locational and directional spatiality, TD cells were then compared to place cells and head direction cells. The directional signal of TD cells was found to be stronger and more robust than their locational signal. TD firing is strongly modulated by the theta oscillation, more so than that of place cells. The depth of theta modulation in TD cells is similar to that reported for cells in the septum, which are thought to drive hippocampal theta oscillation. TD cells preferentially fire around the trough of the locally recorded theta. TD cells were recorded in four environments. Environmental geometry selectively affected the locational properties of TD cells, while leaving directional properties unchanged. The changes in the locational properties were similar to those reported for place cells. TD cells were also studied under other environmental manipulations, including darkness. Discussion focuses on the particular contribution of TD cells to the hippocampal navigational system. It provides hypotheses of how the allocentric locational and directional properties of TD cells emerge from sensory and idiothetic information. It also suggests that TD cells may represent inputs to the hippocampal place cells.

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