Investigation of circuit mechanisms of spatial memory and navigation in virtual reality
Tennant, Sarah Anne
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Spatial memory and navigation relies on estimation of location. This can be achieved through several strategies, including the use of landmarks and by path integration. The latter involves inferring location from direction and distance moved relative to a known start point. The neural mechanisms of path integration are not well understood and implementation of experiments that dissociate path integration from alternative strategies is challenging. The roles of specific cell types are also unknown. Although grid cells in layer 2 of the medial entorhinal cortex (MEC) are theorised to be involved given their periodic and repeating firing fields that form a grid-like map that tiles the environment. Two excitatory cell populations have been identified in layer 2 of the MEC. Clusters of pyramidal cells that project to the CA1 are surrounded by dentate gyrus (DG) projecting stellate cells. Both populations have been shown to exhibit grid-like activity. The extent to which these cell types contribute to path integration or other strategies for solving spatial tasks is unknown. To investigate these issues, I developed a spatial memory task for mice, which uses virtual reality to generate sensitive measures of an animal’s ability to path integrate. In this task mice are trained to locate a reward zone marked with a visual cue within a virtual linear track. Use of path integration strategies can be tested in trials in which the reward zone is unmarked. In this task mice can locate the reward zone using either a local beaconing cue or path integration strategies. To assess whether self-motion derived motor information or visual feedback is used for path integration, I manipulated the translation between physical and virtual movement, putting optic and motor feedback in conflict. These manipulations suggest that mice use motor information to locate the reward zone on path integration trials. To test roles of stellate cells in the task I injected adeno-associated virus expressing the light chain of tetanus toxin, conditionally on the presence of Cre, into the MEC of mice expressing Cre specifically in stellate cells. This abolishes synaptic output from stellate cells therefore preventing them from influencing downstream neurons. I find mice with dorsal expression of the tetanus toxin virus in layer 2 stellate cells are unable to locate the reward zone using a local beaconing cue or path integration strategies. In contrast, mice with expression of green fluorescent protein (GFP) were able to locate the reward zone using both strategies. Locating the reward zone using path integration strategies first requires animal’s to learn the reward zone location, as denoted in trials with a beacon cue. To distinguish the role of stellate cells in learning versus execution of the tasks, I temporally modified the activity of stellate cells after mice had learnt to locate the reward zone using both strategies. Temporal control was achieved by use of cre-dependent adeno-associated viruses expressing mutant human muscarinic 4 receptor (hM4). When activated by clozapine - N - oxide (CNO), this receptor opens G-protein inwardly rectifying potassium (GIRK) channels and attenuates neuronal firing. Using this method, the activity of stellate cells can be temporally controlled during task execution and potentially distinguish their involvement in learning and execution of spatial memory tasks. No effect on behavioural performance was seen under these conditions. This may indicate stellate cells are required for learning but not execution of spatial memory tasks that require the use of local beaconing cues or path integration.