Prion diseases are characteristically accompanied by extensive synaptic pathology that can occur during the preclinical phase of the disease and, in animal models, correlates with the first decline of hippocampus-dependent cognitive functions. This pathology is defined by abnormally shaped synapses in which the postsynaptic membrane modifies its curvature and potentially engulfs the juxtaposed presynaptic membrane. Using the intrahippocampally injected ME7 prion model, we further detailed the structural alterations of the population of ostensibly intact synaptic compartments within the hippocampus during this period of extensive synaptic loss. A disease stage-dependent increase in the average PSD (postsynaptic density) area, the average length of the active zone and the average number of synaptic vesicles indicated that the synapses that were visualized as the animal progressed to end-stage disease were undergoing hypertrophy. Similar findings in samples from AD (Alzheimer's disease) patients, aged and senile individuals, and animal models of neurodegenerative diseases suggest synaptic swelling as synaptic loss is initiated and/or compensatory reaction to counteract the synaptic loss.

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