Maintained synaptophysin immunoreactivity in Tg2576 transgenic mice during aging: correlations with cognitive impairment.

Regional loss of synapses, particularly within the neocortex and hippocampus, is characteristic of Alzheimer's Disease (AD) and strongly correlated with extent of cognitive impairment. The Tg2576 transgenic mouse model of AD develops Abeta-containing neuritic plaques by 10-16 months of age and shows cognitive impairment in several tasks. In the present study, synaptophysin immunoreactivity (SYN-IR; a marker for synaptic terminals) was evaluated in the neocortex and hippocampus of behaviorally-tested Tg2576 transgenic (Tg+) mice aged 3, 9, 14, and 19 months of age. In control non-transgenic (Tg-) mice, SYN-IR in both neocortex and hippocampus tended to decrease with age, while SYN-IR in Tg+ mice was maintained with age. Thus, 19M Tg+ mice exhibited significantly greater synaptophysin immunostaining compared to 19M Tg- mice in both inner and outer neocortical regions, as well as in the dentate gyrus' outer molecular layer and polymorphic layer. Over all four age groups collectively, outer cortical SYN-IR was also greater in Tg+ compared to Tg- mice. Multiple factors could be responsible for maintained SYN-IR in aged Tg+ mice, including compensatory changes in synaptic morphology and staining of dystrophic neuritics associated with Abeta deposition. For all animals combined (Tg+ and Tg-), as well as for aged 19M animals alone, hippocampal SYN-IR was correlated with impaired acquisition and spatial reference memory in the Morris water maze task, suggestive that elevated hippocampal SYN-IR is a manifestation of pathophysiologic synaptic processing within the hippocampus. Also for 19M animals alone, hippocampal SYN-IR was highly correlated with impaired visible platform recognition, indicative that elevated SYN-IR is linked to visual agnosia. The results of this study are consistent with the premise that maintained SYN-IR in Tg2576 mice during aging is associated with impaired synaptic function, resulting in cognitive deficits.