Advanced ageing is associated with hippocampal deterioration and mild cognitive decline. The hippocampal subregion CA3 stratum lucidum (CA3-SL) receives neuronal inputs from the giant mossy fibre boutons of the dentate gyrus, but relatively little is known about the integrity of this synaptic connection with ageing. Using serial electron microscopy and unbiased stereology, we examined age-related changes in mossy fibre synapses on CA3 thorny excrescences within the CA3-SL of young adults (4-month-old), middle-aged (12-month-old), and old-aged (28-month-old) Wistar rats. Our data show that while there is an increase in CA3 volume with ageing, there is a significant (40–45%) reduction in synaptic density within the CA3-SL of 12- and 28-month-old animals compared with 4-month-old animals. We also present preliminary data showing that the CA3 neuropil in advanced ageing was conspicuously full of lipofuscin and phagolysosome positive, activated microglial cellular processes, and altered perivascular pathology. These data suggest that synaptic density in the CA3-SL is significantly impaired in ageing, accompanied by underlying prominent ultrastructural glial and microvascular changes. 1. Introduction Age-related hippocampal deterioration and deficits in cognitive function (spatial memory) [1, 2] are correlated with ultrastructural changes in the CA3 hippocampal subregion, including (i) loss of synapses and integral synaptic proteins (CA3-SR) [3–11], (ii) significant microglia activation [12], (iii) changes in synaptic plasticity [13], and (iv) altered synaptic-glial interactions [9]. The functional integrity of mossy fibre axons whose large giant boutons project to the dendritic spines (thorny excrescences) located in the CA3 stratum lucidum (SL) is essential in both long-term potentiation (LTP) and storage and recall of spatial representation on modifiable synapses of recurrent collaterals of the CA3 pyramidal cells [14]. Electron microscopic studies have reported marked ultrastructural plasticity of synaptic connections in the CA3-SL, in correlation with neurobehavioral performance [4]. Although a large body of data has reported significant age-related ultrastructural changes in the CA3, very little is currently known about the integrity of these mossy fibre synaptic connections in the CA3-SL with advanced ageing. Our aim was to use unbiased stereological methodology to determine whether subtle age-related morphological changes could be demonstrated in the relatively unexplored mossy fibre synapses which form on CA3 thorny excrescences in Wistar rats. The
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