Increased densities of resting and activated microglia in the dentate gyrus follow senile plaque formation in the CA1 subfield of the hippocampus in the triple transgenic model of Alzheimer's disease

Alzheimer's disease (AD) is an irreversible neurodegenerative disease that is characterised by the presence of β-amyloid (Aβ) plaques, neurofibrillary tangles (NFTs) and synaptic loss specifically in brain regions involved in learning and memory such as the neocortex and the hippocampus. Aβ depositions in the form of neuritic plaques trigger activation of microglia that is believed to be a common neuropathological feature of AD brains. As an integral part of the hippocampus, the dentate gyrus (DG) plays an important role in cognitive function. Although post-mortem studies suggest later involvement of the DG into the AD progression, changes in microglia have not been studied in this subfield of the hippocampus. In the present study the numerical density (Nv, #/mm3) of both resting (identified by tomato lectin staining) and activated (identified by Mac-1 immunoreactivity) microglia was analysed in the molecular layer (ML) of the DG in the triple transgenic (3xTg-AD) mouse model of AD at different ages (9, 12 and 18 months). The 3xTg-AD mouse model of AD showed a significant increase in the Nv of resting (by 75%) and activated (by 67%) at 18 months of age compared to non-Tg controls. These results indicate a complex microglial remodelling during AD progression.