A beta and perlecan in rat brain: glial activation, gradual clearance and limited neurotoxicity

Abstract

A beta1-40 and perlecan (A beta + perlecan) were infused into rat hippocampus for 1 week via osmotic pumps. At the end of the infusion a deposit of A beta immunoreactive material was found surrounding the infusion site. No neurons could be identified within this A beta deposit. The neuron-free area resulting from A beta + perlecan was significantly larger than that found after infusions of A beta40-1 and perlecan (reverse A beta + perlecan), perlecan alone or phosphate-buffered saline vehicle. Following infusion of A beta + perlecan, the glial cells segregated in a manner similar to that associated with compacted amyloid plaques in Alzheimer's disease (AD). Activated microglia/macrophages were prevalent within the A beta deposit while the perimeter of the deposit was delimited by reactive astrocytes. Thioflavin S and Congo red staining indicated a beta-pleated sheet conformation of the A beta deposits, implying formation of fibrils. Intact, apparently healthy neurons were found immediately adjacent to the A beta + perlecan deposit. In contrast, reverse A beta peptide did not form congophilic deposits despite the presence of perlecan. Apoptotic profiles visualized with bisbenzamide or TUNEL staining of fragmented DNA were not seen at any of the infusion sites, yet were readily seen in hippocampal sections from animals treated with kainic acid. At 8 weeks, A beta immunoreactivity, Thioflavin S and Congo red staining was reduced, indicating that A beta was being cleared. There also was no evidence of neuron loss by Nissl or TUNEL staining. The zone of apparent necrosis did not expand between 1 and 8 weeks, and in some instances appeared to contract. The consistency of the A beta + perlecan infusion method in producing reliable A beta amyloid deposits permits estimates of the rate at which fibrillar A beta amyloid can be removed from the brain, and may provide a useful model to study this process in vivo. However, the absence of clearly identifiable degenerating/dying neurons at the 1 or 8 week survival times suggests that either fibrillar A beta + perlecan slowly displaced the brain parenchyma during infusion, or neurons were killed very gradually during the process of clearing the A beta.