Staging of cognitive deficits and neuropathological and ultrastructural changes in streptozotocin-induced rat model of Alzheimer’s disease

Sporadic Alzheimer's disease (sAD) is the most common form of dementia. Rats injected intracerebroventricularly with streptozotocin (STZ-icv) develop insulin-resistant brain state and represent a non-transgenic sAD model with a number of AD-like cognitive and neurochemical features. We explored cognitive, structural and ultrastructural changes in the brain of the STZ-icv rat model over a course of 9 months. Cognitive functions were measured in the STZ- icv- (0.3, 1 and 3 mg/kg) and age-matched control rats by passive avoidance test. Structural changes were assessed by Nissl and Bielschowsky silver staining. Immunohistochemistry and electron microscopy analysis were used to detect amyloid β- (Aβ(1- 42)) and hyperphosphorylated tau (AT8) accumulation and ultrastructural changes in the brain. Memory decline was time- (≤3 months/acute, ≥3 months/progressive) and STZ- icv dose-dependent. Morphological changes were manifested as thinning of parietal cortex (≥1 month) and corpus callosum (9 months), and were more pronounced in the 3 mg/kg STZ group. Early neurofibrillary changes (AT8) were detected from 1 month onward in the neocortex, and progressed after 3 months to the hippocampus. Intracellular Aβ(1-42) accumulation was found in the neocortex at 3 months following STZ-icv treatment, while diffuse Aβ(1-42)-positive plaque-like formations were found after 6 months in the neocortex and hippocampus. Ultrastructural changes revealed enlargement of Golgi apparatus, pyknotic nuclei, and time- dependent increase in lysosome size, number, and density. Our data provide a staging of cognitive, structural/ultrastructural, and neuropathological markers in the STZ-icv rat model that in many aspects seems to be generally comparable to stages seen in human sAD.