A Behavioural, Molecular and Lesion Examination of Hippocampal Contributions to Morris Water Maze Acquisition
Diviney, Mairead (2011) A Behavioural, Molecular and Lesion Examination of Hippocampal Contributions to Morris Water Maze Acquisition. PhD thesis, National University of Ireland Maynooth.
The Morris water maze (MWM) is a widely known, simple and effective task in the examination of spatial learning and memory. Successful acquisition of the task is thought to rely on retained representations of allocentric spatial relations, whereby animals learn to associate the location of a hidden platform with surrounding distal cues and subsequently use this information to navigate towards the hidden goal. As the distal cues are critical in this process, features of the cues, such as location, are an important factor to consider in examining how the task is solved. It has also been well documented that the hippocampus is a critical structure in the processing of allocentric representations. However, there has been debate surrounding the exact nature of this involvement, with suggestions that hippocampal damage leads to deficiencies in navigational aspects of the task rather than purely spatial processing impairments. To assess this, we adopted novel methods of analyses which include sub-second monitoring of each individual animal’s behaviour as they navigate during a training trial. From this analysis we initially determine that positioning of the distal cues around the maze can impact on intact animals’ performance. Specifically, we noted that animals with cues positioned close to their goal are more efficient in reaching the target and use more view-dependent strategies, over animals whose cues are in a position further away, who, instead, are more reliant on view-independent behaviours in order to reach their goal. Molecular examinations of both groups of animals reveal higher BDNF expression in the dorsal hippocampus in the group whose cues are positioned further away from their goal, which we suggest reflects the Far cue groups need to infer their position more than the Near cue group. Following this, assessment of animal behaviour following lesions to the dorsal hippocampus indicated that both the Near and Far lesioned groups were significantly impaired in the MWM. Behavioural analysis highlighted lesioned animals’ deficits in accurately monitoring and adapting their motor movements in response to task demands, suggesting that the impairments seen in the maze are due deficits in integrating exploratory behaviours, rather than a purely spatial memory impairment. While there were few differences in performance of the Near and Far lesioned animals, further assessment of the intact hippocampus using immunohistochemical procedures revealed increased c-Fos expression in the Far cue group in area CA1 of the hippocampus. Further to this, subregional assessment using lesion and IEG methodologies led to the distinction that the dentate gyrus, in particular, is critical in performance in the water maze. Together, the behavioural, molecular and lesion data assessing hippocampal contributions to acquisition of the MWM are discussed in terms of models of navigation. From this, we suggest that the water maze task is solved using a vectormodel of navigation, rather than the widely reported, and accepted, cognitive mapping theory of spatial learning. The behavioural lesion data also supports a role for the hippocampus in this model, specifically as lesioned animals’ display clear impairments in the accurate judgement of distance and direction to their goal when in the maze; a critical feature of the vector-model
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