Alzheimer’s disease (AD) is a neurodegenerative disorder, characterised by severe memory loss and the accumulation of amyloid-beta (Aβ) and tau pathology within neocortex and medial temporal lobe (MTL) structures. Episodic memory impairment is a defining feature of early AD. The hippocampal formation (HF), a major network involved in both memory formation and retrieval is one of the first areas affected by AD pathology. However, the aetiology of AD is unknown; specifically how Aβ and tau pathologies cause memory impairment and how the physiological function of HF is affected. In this thesis, the 3xTgAD mouse was used as a high fidelity model of human AD pathological progression to study the function of HF during early (intracellular Aβ) and more progressive (extracellular plaque and hyperphosphorylated tau pathology) AD stages, referred to as ‘young’ and ‘old’ respectively. Specifically we: i) applied the hippocampal-dependent What-Where-Which (WWWhich) task to study the onset and progression of episodic-like memory decline (previously uncharacterised in the 3xTgAD mouse); ii) examined allocentric spatial memory in radial arm water maze (RAWM) and spontaneous alternation (SA) behaviour in T-Maze to discern whether cognitive differences exist between spontaneous and negatively reinforced tasks (the latter could be influenced by an exaggerated stress response); and iii) performed electrophysiological recordings in vivo from the HF of urethane-anaesthetised 3xTgAD and control mice to study basic synaptic connectivity, short-term synaptic plasticity and neuronal reverberation across the CA1-DG axis using a multi-site electrode. Our results showed an early and specific deficit for WWWhich episodic-like memory in the 3xTgAD model, with a decline in performance witnessed in mice as young as 3 months. In contrast, 3xTgAD component memory comprising single or dual associations of ‘What’, ‘Where’, ‘Which’ and ‘When’ remained intact suggesting the episodic impairment was due to dysfunction during the association of three component information streams within hippocampus (Chapters 3 and 4). 3xTgAD mice were equally impaired for allocentric spatial memory in RAWM and in their SA behaviour, suggesting no inherent advantage of examining cognition in paradigms which elicit behavioural distress (Chapter 5). We witnessed the development of subtle synaptic abnormalities in young 3xTgAD mice in the form of enhanced short-term paired pulse facilitation in CA1 and DG, however, a paucity of response facilitation in CA1 in response to train stimulation. In contrast, we saw intact basic synaptic function (fibre integrity and synaptic connectivity) in 3xTgAD mice of both young and old ages, suggesting gross hippocampal circuitry remained in place (Chapter 6). Finally, we saw an effect of normal ageing on cognition in the WWWhich and spatial tasks (Chapters 4 and 5), and a decline in neuronal reverberation with age in control and 3xTgAD mice. Dysfunction in these two parameters (behavioural and electrophysiological) coincided with the onset of intracellular Aβ accumulation within HF in 3xTgAD mice. This suggests a role of intracellular Aβ in impairing the physiological function of HF in AD which translates as cognitive decline in hippocampal-dependent forms of memory. Episodic memory was found to be especially sensitive to AD-related pathology and impairment, thus the WWWhich task may be applied to faithfully study the onset of cognitive decline in other AD mouse models. Further examination of the relative contribution of Aβ to hippocampal dysfunction in the 3xTgAD model is required.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:555525 |
| Date | January 2012 |
| Creators | Davis, Katherine |
| Contributors | Gigg, John |
| Publisher | University of Manchester |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | https://www.research.manchester.ac.uk/portal/en/theses/hippocampal-dysfunction-in-the-3xtgad-mouse-model-of-alzheimers-disease(ba2d4704-9e22-4213-8bc7-a1c55a0e0ccd).html |
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